TABLE OF CONTENTS
Common Abbreviations ...... iv 1. Why Metathesis,Why Ruthenium? ...... 1 1.1. Introduction ...... 1 1.1.1. Background and History...... 1 1.1.2. Application of Metathesis Reactions and Catalysts ...... 3 1.2. Aim of Study...... 5 2. Current Knowledge on Metathesis ...... 7 2.1. Introduction ...... 7 2.2. Ruthenium...... 7 2.3. Organometallic Chemistry...... 8 2.3.1. Olefin Metathesis ...... 10 2.4. Ruthenium Complexes...... 15 2.5. Carbene Complexes ...... 15 2.5.1. Fischer Carbene Complexes...... 16 2.5.2. Schrock Carbene Complexes ...... 17 2.6. Catalysis...... 18 2.6.1. Heterogeneous Catalysis ...... 19 2.6.2. Homogenous Catalysis ...... 20 2.7. Ru Carbenes in Non-Metathesis Reactions ...... 20 2.8. Ru Carbenes in Metathesis Reactions...... 21 2.8.1. Synthesis of Ru Carbene Catalysts for Olefin Metathesis ...... 22 2.8.2. Ligand Effects on Catalytic Activity and Stability...... 24 2.8.3. Decomposition of Ruthenium Carbene Complexes ...... 33 2.9. Theoretical Studies of Olefin Metathesis...... 37 2.10. Phobcat: The Sasol Catalyst...... 38 3. Synthesis and Kinetics...... 43 3.1. Introduction ...... 43 3.1.1. Synthesis...... 44 3.1.2. Reaction Kinetics ...... 46 3.1.3. Theoretical Aspects of Reaction Kinetics...... 46
3.1.4. Reaction Kinetics of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]...... 57 3.1.5. Theoretical Aspects of Magnetisation Transfer...... 58
3.1.6. Coordination of Functionalized Olefins to Grubbs1-PCy3 ...... 64 3.1.7. Octene Metathesis ...... 65
i 3.2. Experimental Section ...... 65 3.2.1. Materials and Apparatus ...... 65 3.2.2. Attempted Synthesis of a Grubbs1-Type Catalyst ...... 66 3.2.3. Synthesis of the 2nd Generation Grubbs Type Catalyst
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh] ...... 70
3.2.4. Kinetic experiments of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] ...... 71
3.2.5. Coordination of Functionalized Olefins to Grubbs 1-PCy3 ...... 72 3.2.6. Octene Metathesis ...... 73 3.3. Results ...... 75
3.3.1. Reaction Kinetics of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]...... 75
3.3.2. MT Experiments on the exchange of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] and
PPh2Cy...... 77
3.3.3. Coordination of functionalized Olefins to Grubbs 1-PCy3...... 82 3.3.4. Octene Metathesis ...... 83 3.4. Discussion...... 86 3.4.1. Synthesis of 1st and 2nd Generation Grubbs Catalyst...... 86 3.4.2. Kinetic Reactions ...... 88 4. Theoretical Study ...... 92 4.1. Introduction ...... 92 4.1.1. Density Functional Theory ...... 92 4.1.2. The Frontier Orbital Theory...... 95 4.1.3. Computational Method ...... 96 4.2. Overlay of Rhodium Complexes ...... 98 4.2.1. Results ...... 98 4.3. Coordination of Functionalized Olefins to Ruthenium-Carbene Catalysts102 4.3.1. The Influence of Steric Interactions...... 104 4.3.2. Orbital Interactions ...... 120 4.3.3. Discussion...... 125 5. Relevance of Study ...... 129 5.1. Attainment of Goals...... 129 5.2. Future Studies...... 132 Summary ...... 133 Opsomming...... 136 Appendix...... 139 Appendix A ...... 139 Appendix B ...... 140
ii Appendix C ...... 142 Appendix D ...... 143
iii Common Abbreviations
AsPh3 Triphenylarsine BL Bond length CB Cyclobutane CM Cross Metathesis DCM Dichloromethane DFT Density Functional Theory El Electrophile FMO Frontier Molecular Orbitals G2 2nd Generation Grubbs Catalyst h Planck’s constant HOMO Highest Occupied Molecular Orbital I Spin quantum number of a nucleus IMes 1,3-dimesitylimidazol-2-ylidene
IMes H2 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene iPr 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene IR Infrared Spectroscopy K Equilibrium constant of a reaction k Rate constant of a reaction
kB Boltzmann’s constant L Neutral Two-electron Ligand LUMO Lowest Unoccupied Molecular Orbital M Metal
M0 Magnetization Vector MM Molecular Modelling MO Molecular Orbital NHC N-heterocyclic carbene NMR Nuclear Magnetic Resonance Nu Nucleophile
PCy3 Tricyclohexylphosphine
iv Common Abbreviations
PMe3 Trimethylphosphine
PPh2Cy Diphenylcyclohexylphosphine
PPh3 Triphenylphosphine
Ri Relaxation rate R Universal Gas Constant RF Radio Frequency S Angular Momentum T Relaxation time
T2* Effective transverse relaxation time
Ti Inversion time UV/Vis Ultra Violet/Visible region spectroscopy X Halogen ligand Angular Frequency
ii
1. Why Metathesis,Why Ruthenium?
1.1. Introduction
This introductory chapter defines the process of Alkene Metathesis and presents a brief account of the evolution of alkene metathesis catalysts. The theme of the chapter leads to the motive of the study: Knowledge and development of catalysts are closely linked to the application and potential of corresponding reactions. The specific aims of the project are given in section 1.2.
1.1.1. Background and History
Alkene metathesis is an equilibrium reaction in which double bonds are broken and restructured to form two new alkene products.1,2,3 The reaction describes evident interchange of carbon atoms between two pairs of bonds. The final result is that each half of the first olefin molecule becomes bound to either half of the second molecule. Figure 1.1 illustrates the olefin metathesis reaction.
H2C CH2 RHC CH2 + +
RHC CHR RHC CH2
Figure 1.1 – Olefin Metathesis
The initial metathesis catalytic systems often required the presence of a co- catalyst as well as a promoter.4 The most common systems were based on
Mo, Ru, W, Re, Os and Ir. Typical co-catalysts were EtAlCl2, R3Al and R4Sn while compounds containing oxygen such as O2, EtOH or PhOH were used as promoters. Evidence reveals that these catalysts lead to the formation of
1 CHAPTER 1 – Why Metathesis, Why Ruthenium? metal carbene complexes of the type [LM=CHR] which would represent the real catalytic form.
Over the past fifteen years, well-defined carbene complexes of late transition metals have been proposed. Two researchers were mainly responsible for catalyst development, namely Robert H. Grubbs at the California Institute of Technology and Richard R. Schrock at the Massachusetts Institute of Technology.
Grubbs and co-workers made a substantial contribution to the development of catalysts for olefin metathesis in the mid 1990’s by developing Ru-based 5a,b,c catalysts with the general structure [LL’X2Ru=CHR] shown in Figure 1.2.
L X Ru C-R X L' Figure 1.2 – General structure of Ruthenium Carbene Catalysts
The development of the “Grubbs catalyst” started in the 70’s. The first Grubb’s ruthenium catalysts, prepared in 1992,5c had good functional group tolerance but limited activity. Refinements led in 1996 to the catalyst 5b [(PCy3)2Cl2Ru=CHPh], today known as the first generation Grubbs catalyst. The even better second generation Grubbs catalyst came two years later.6
The use of ruthenium carbene-based olefin metathesis initiators5c,7 has recently gained wide acceptance in organic8 and polymer9 syntheses. Ruthenium-based catalysts exhibit greater tolerance towards functional groups and protic media (typical of late transition metals) as well as greatly enhanced air and water stability relative to other single component catalysts systems based on molybdenum and tungsten. Additionally the catalysts are easy to handle and can initiate all types of olefin metathesis directly without requiring the presence of a co-catalyst or promoter.4
2 CHAPTER 1 – Why Metathesis, Why Ruthenium?
Extensive synthetic, mechanistic and theoretical investigations have continually improved the performance of the catalysts and expanded their overall applicability.
1.1.2. Application of Metathesis Reactions and Catalysts
The catalysed olefin metathesis reaction represents one of the most important synthetic processes discovered in the last few decades. The reaction has become more and more important in recent years and has found considerable application in industry and academic laboratories.
Grubbs 17b has found extensive use in organic and polymer chemistry due to high reactivity with olefins in the presence of a diverse array of functional groups. Grubbs 2 ([(IMesH2)(PCy3)Cl2Ru=CHPh]) exhibited dramatically increased activity in ROMP (Ring Opening Metathesis)10, RCM (Ring Closing Metathesis)11 and CM (Cross Metathesis)12 reactions compared to the first generation catalyst.
Some of the Grubbs 1 catalyst applications are:
• The synthesis of epothilones • The preparation of carbohydrate-containing polymers which have significant biological activities • Cleavage of linkers in solid phase oligosaccharide synthesis • Catalysis of the key step in the total synthesis of the natural product ciguatoxin • The synthesis of pheromones, which are useful and economical as environmentally friendly pest-control agents
Grubbs 1 also found use in the Shell higher olefin process (SHOP) for the large-scale production of long-chain -olefins.1 Recent syntheses of a variety of natural and non-natural products are based on ring-closing metathesis to obtain macrocycles otherwise difficult to prepare.5a,b Metathesis of strained
3 CHAPTER 1 – Why Metathesis, Why Ruthenium? and low-strain cyclic olefins, exocyclic olefins and straight chain olefins have been done.
The Grubbs catalyst allows polymerisation to occur in the presence of fillers, additives and stabilisers in polymer formulation.13 Ruthenium technology produces castable or moldable polymer formulations which makes it easy to manufacture complex parts, ex. metathene for applications like bathroom fixtures as well as sporting goods and recreational equipment.
Olefin metathesis is a powerful reaction because it causes many reagents which are usually quite inert to react.13 By using the appropriate catalysts, new double bonds can even form at room temperature in aqueous media from starting materials that bear a variety of functional groups. Furthermore, the catalysts are commercially available.
Olefin metathesis uses inexpensive raw materials, such as low-molecular- weight olefins and plant oils to produce more valuable products.14 There has recently been a growing interest in utilising olefin metathesis for altering chain- lengths of fats and oils to form new products ranging from pharmaceuticals and cosmetics to polymers and fine chemicals. These materials are not chemically very different from petroleum and show promise of replacing the gradually diminishing crude oil reserves in the near future.
Using fats and oils instead of petroleum has many environmental advantages: it is produced from renewable resources, they are easily biodegradable and processing does not result in production of large amounts of CO2. These processes can therefore largely be used in “Green Chemistry” applications.
To summarise in the words of Dr. Marvey,14 a South African researcher: “In South Africa, interest in alkene metathesis research is growing. South Africa is in the forefront as world producer of Platinum group metals – Platinum, Rhodium, Ruthenium etc., from which active metathesis catalysts are derived. Africa also has hectares of lands and favourable climate for the production of
4 CHAPTER 1 – Why Metathesis, Why Ruthenium? oil crops and for stock farming (a source of animal fat). This is good news for the continent that is currently positioning itself to take a more proactive role in the global economy.”
1.2. Aim of Study
From the introduction it is evident that there are many potential applications of Grubbs type catalysts, however, at the same time it must be stated that there are still many question marks around the mechanism of how these catalysts operate. This lack of knowledge prevents the use and development of the catalyst in many areas.
With this in mind, the following chronological aims were set for this study:
1. Synthesis of a Grubbs type catalyst. 2. Phosphine exchange of the new synthesised complex. 3. Kinetic study covering the coordination of electron rich olefins to a ruthenium carbene catalyst. 4. Kinetic study of the cross metathesis reactions of 1-octene with Grubbs 1 and Phobcat as catalysts. 5. Comparison of experimentally observed rhodium complex crystal structures and theoretically calculated data preceding a theoretical study involving coordination of electron rich olefins to a ruthenium catalyst.
1 K.J. Ivin, J.C. Mol, Olefin Metathesis & Metathesis Polymerisation; Academic Press, NY, 1997. 2 T.J. Katz, Adv. Organomet. Chem. 1978, 16, 283. 3 N. Calderon, J.P. Lawrence, E.A. Ofstead, Adv. Organomet. Chem., 1979, 17, 449. 4 F. Bernardi, A. Bottoni, G.P Miscione, Organometallics, 2003, 22, 940. 5 (a) S.J. Miller, S. Kim, Z. Chen, R.H. Grubbs, J. Am. Chem. Soc., 1995, 117, 2108. (b) S.J. Miller, H.E. Blackwell, R.H. Grubbs, J. Am. Chem. Soc., 1996, 118, 9606. (c) S.T. Nguyen, L.K. Johnson, R.H. Grubbs, J. Am. Chem. Soc., 1992, 114, 3974.
5 CHAPTER 1 – Why Metathesis, Why Ruthenium?
6 T. Weskamp, W.C. Schattenmann, M. Spiegler, W.A. Herrmann, Angew. Chem. Int. Ed., 1998, 37 (18), 2490. 7 (a) S.T Nguyen, R. H. Grubbs, J. Am. Chem. Soc. 1993, 115, 9858. (b) P. Schwab, R. H. Grubbs, J.W. Ziller, J. Am. Chem. Soc. 1996, 118, 100. (c) A.W. Stumpf, Chem Commun., 1995, 1127. 8 R.H. Grubbs, S. Chang, Tetrahedron, 1998, 54, 4413. 9 F.J. Stelzer, J. Macromol. Sci,. Pure Appl. Chem., 1996, A33, 941. 10 C.B. Bielawski, R.H. Grubbs, Angew. Chem. Int Ed., 2000, 39, 2903. 11 M. Scholl, Org. Lett., 1999, 1, 953. 12 A.K. Chatterjee, J. Am. Chem. Soc., 2000, 122, 3783. 13 http://pubs.acs.org/cen/coverstory/8051/8051olefin.html. 14 www.scienceinafrica.co.za/2002/october/fats.htm.
6
2. Current Knowledge on Metathesis
2.1. Introduction
Chapter 2 encloses a bird eye’s view of broader themes in catalytic olefin metathesis. Included are general chemical aspects of the ruthenium metal centre (section 2.2) and complexes related to ruthenium carbenes (section 2.4), historic facts on organometallic chemistry (section 2.3) and principles of catalysis (section 2.6). However, the chapter also contains a microscopic view of the role and influence of distinctive components of these Grubbs type catalysts: the groups around the metal centre (section 2.8.2), the synthesis and decomposition of the catalysts (sections 2.8.1 and 2.8.3) as well as the mechanism (section 2.3.1.1) that is followed. Questions such as: “What influences the reactivity and stability of the catalysts?”; “Can metathesis catalysts only be used in olefin metathesis?”; “Of what value is theoretical studies?” and “What are the latest advances in this research area?” are also addressed from available academic and literature resources.
2.2. Ruthenium
Ruthenium is a late transition metal in Group 8 in the Periodic Table as illustrated in Figure 2.1.
7 8 9
Mn Fe Co
101 Tc 44 Ru Rh
Re Os Ir Pt
Figure 2.1 – The Periodic Table showing elements surrounding Ruthenium
7 CHAPTER 2 – Current Knowledge on Metathesis
The name “Ruthenium” was derived from the Latin Ruthenia for the “old name of Russia”. The element was discovered in a crude platinum ore by Russian chemist Gottfried Wilhelm Osann in 1828.15 Osann thought that he had found three new metals in the sample – pluranium, ruthenium and polonium, but later withdrew his claim of discovery. In 1844 another Russian chemist, Karl Klaus, was able to show that Osann’s mistake was due to the impurity of the sample and Klaus was able to isolate the metal and retained Osann’s original name of ruthenium.
Today the chief sources of ruthenium are nickel ores, but the metal can also be found in the pyroxinite deposits of South Africa.16
Ruthenium is a hard, white metal and has four crystal modifications. It is used in alloys of platinum and palladium for severe wear resistance, alloys of molybdenum to make a superconductor at 10.6 K and in alloys of titanium to improve corrosion resistance a hundredfold. Ruthenium is priced at about $30/g.
Compounds show a marked resemblance to those of cadmium and most common oxidation states are Ru(II), Ru(III) and Ru(IV); but Ru(VI) and Ru(VIII) complexes can also be found.
2.3. Organometallic Chemistry
The carbon-transition metal bond was postulated quite long ago, but most of its chemistry has been developed after the Second World War. The first transition metal-ethylene complex, K[PtCl3(C2H4)], was discovered by Danish pharmacist Zeise in 1827,17 however a whole century passed before the real significance of the compound was understood. The first industrial application of organometallic chemistry was that of tetra-carbonyl nickel, discovered by Langer and Mond in 1888,18 leading to The Mond Carbonylation-
8 CHAPTER 2 – Current Knowledge on Metathesis
Decarbonylation Process which has been used for the refinement of nickel for many decades.
19 In 1891 another metal carbonyl, [Fe(CO)5], was discovered, but it was only in 1919 when application of organometallic chemistry made further progress with the development of Hein’s reaction between chromium trichloride and [PhMgBr] producing the important compound polyphenylchromium.20
The next significant advance was in 1925 with the development of the Fischer-Tropsch process,21 which was later made famous by Sasol. This process allows synthesis gas (CO/H2) to be converted into a mixture of hydrocarbons and was used in Germany to convert coal into petrol during the last world war. Research into the mechanism of this transformation is still one of the most important areas of organometallic chemistry.
Roelen introduced the “oxo” process in 1938,22 which transforms olefins into aldehydes through the addition of CO/H2 to a double bond and today the process still has a world wide production of five million tons of aldehydes and derivatives per year.
British and American scientists overtook the Germans in the 1950’s with the first post-war breakthrough of “ferrocene” appearing in a famous Nature article published by Kealy and Pauson.23 They erroneously proposed a σ-complex (structure A in Figure Figure 2.2) where iron is bound to a single carbon of each ring. The correct formulation as a π-complex (structure B in Figure Figure 2.2), with iron equally bound to all five carbons in each ring, was established a year later by Wilkinson and Woodward.24
9 CHAPTER 2 – Current Knowledge on Metathesis
Fe Fe
(A) Kealy and Pauson (B) Wilkinson and Woodward Figure 2.2 – (A) First postulated structure for Ferrocene in 1951; (B) The structure that is generally accepted today
In 1955 Ziegler and Natta surprised the scientific world when they discovered that it is possible to polymerise olefins by using soluble titanium- and aluminium-based catalysts.25 From then on, discoveries accelerated to an almost continuous flow. Fischer discovered carbene complexes in 196426 and carbyne complexes in 1973,27 Banks discovered the metathesis of olefins in 1964,28 while Wilkinson described his famous catalyst the very next year29 and in 1971 Monsanto presented a new industrial synthesis for acetic acid.30 In the 80’s Bergman’s group described how transition metals activate C-H bonds in saturated hydrocarbons31 and Kubas showed for the first time that molecular hydrogen could coordinate to a transition metal without being cleaved.32
From the brief discussion above the richness of organometallic chemistry is clear and it is obvious that the history of molecular transition metal chemistry is intimately bound up with the discovery of new bond types, structures, reactions and industrial processes. In the following sections more specific aspects of olefin metathesis and its catalysts will be described.
2.3.1. Olefin Metathesis
The development of homogeneous catalysts for olefin polymerisation in the 1980’s revolutionised the field,33,34 since this development led to single-site catalysts that were well defined and offered more control over polymer properties than the heterogeneous Ziegler-Natta catalyst.35,36 These single site catalysts have provided chemists with the opportunity for rational catalyst
10 CHAPTER 2 – Current Knowledge on Metathesis engineering that began with initial single-site group 3 and 4 metallocenes37 and later led to the development of new organometallic catalysts involving almost all transition metals, some lanthanides, a few main-group elements and a large variety of ligands.38,39
2.3.1.1 The Mechanism of Olefin Metathesis
It is generally accepted that olefin metathesis occurs via the Herrison and Chauvin mechanism40 shown in Figure 2.3. This metal alkylidene chain mechanism proceeds via a metallacyclobutane intermediate and then forms a new metal carbene and new olefin.
R M M CH R M CH R CH + + CH R1 HC CH R2 R1 CH CH R2 CH R1
R2 Figure 2.3 – General mechanism of Olefin Metathesis
This mechanism proceeds as a series of equilibria. First the olefin coordinates to the metal adjacent to the carbene and then the reversible formation of a metallacycle takes place to form either the original or the metathesised carbene and olefin. Regeneration of the original olefin is referred to as non-productive, while formation of a new olefin and carbene is said to be productive.
The proposed mechanism stimulated the development of new homogeneous single-component metal catalysts with alkylidenes (M=CR) and metallacyclobutane groups. The mechanism of olefin metathesis catalysed by ruthenium alkylidene complexes has recently been the subject of intense investigations.41,81 All mechanisms that have so far been discussed in literature are given in Scheme 2.1.
11 CHAPTER 2 – Current Knowledge on Metathesis
These results indicate that one of the key steps in the catalytic cycle of the metathesis reaction is initial phosphine ligand substitution by an olefin substrate. The substitution may proceed via an associative or a dissociative pathway, but for Grubbs type ruthenium complexes the dissociative pathway (shown in Scheme 2.2) is generally agreed upon,41 although there is still much debate over details of the mechanism.
PR3 Cl PR3 Cl 1 4 Cl Ru Cl Ru + olefin . PR3 PR3 PR3 PR3 Cl 5 Cl Ru Ru . Cl Cl
- PR PR PR3 3 PR3 PR 3 Cl 3 Cl 2 6 Cl Cl Ru Ru Cl Ru Cl Ru - PR3 + olefin Cl Cl 9 . PR3
PR3 PR3 Cl Cl 7 Ru Ru Cl Cl .
PR3 - PR3 3 Cl 8 . Ru + PR + olefin 3 PR3 PR3 Cl Cl Cl PR3 R3P Ru R3P Ru Cl Cl . Scheme 2.1 - Postulated Mechanisms for Olefin Metathesis by Grubbs-type Complexes
12 CHAPTER 2 – Current Knowledge on Metathesis
L L L Cl Cl + R''HC=CH2 Cl - PR3 Ru CHR' Ru CHR' Ru CHR' Cl + PR3 Cl Cl PR3 R''HC=CH2
L L Cl (L)Ru Cl - R'HC=CH2 Cl 2 CHR' R''HC Ru R''HC Ru R'HC CH Cl Cl 2
R'HC=CH2
Scheme 2.2 – The Most Likely Mechanism for Olefin Metathesis by Grubbs-type Ruthenium Carbene Complexes
13 CHAPTER 2 – Current Knowledge on Metathesis
Scheme 2.3 shows the different classes of olefin metathesis.
ROMP
(Ring Opening M=CH2 Metathesis Polymerisation)
ADMET M=CH2 (Acyclic Diene + Metathesis
Polymerisation)
CM M=CH2 + R + (Cross Metathesis) R R R
ROCM R M=CH2 (Ring Opening Cross R +
Metathesis)
RORCM M=CH2 (Ring Opening Ring Closing Metathesis)
RCM M=CH2 (Ring Closing + Metathesis)
R' R + M=CH Enyne CM R' 2 R
M=CH2 Enyne RCM
Scheme 2.3 – Distinct modes of Olefin and Alkyne Metathesis
14 CHAPTER 2 – Current Knowledge on Metathesis
2.4. Ruthenium Complexes
“Complex” is the name given to a compound having ligands coordinated to the metal centre42 leading to an M-L interaction which is potentially weaker than conventional covalent bonds, but strong enough to cause redistribution of the metal electron density. This redistribution gives rise to new molecular orbitals with different chemical and magnetic properties. This weaker interaction in complexes is derived from the large difference between the ligand HOMO and metal valence orbital energy.
Ruthenium(II) complexes usually have two anionic ligands such as H-, X- or - 42 C5H5 present. Geometries are generally octahedral, but if large ligands such as PPh3 or PCy3 are present, coordination of a sixth ligand is prohibited.
Typical ML5 fragments have various geometries, as seen in Figure 2.4.
L L L L L L L M L M L M
L L L L L L
D 3h C4v D3h
Figure 2.4 – Different geometries for ML5 complexes
2.5. Carbene Complexes
In 1964 Fischer made a major breakthrough in organometallic chemistry by discovering carbene complexes, because this provided the first examples of metal-carbon multiple bonds.42 Today these complexes are implicated in many crucial processes, such as olefin metathesis and polymerisation.
15 CHAPTER 2 – Current Knowledge on Metathesis
Carbene complexes contain metal-carbon double bonds very similar to that of olefins (as seen from Figure 2.5), but in carbene complexes the metal must use a d-orbital rather than a p-orbital to form a π-bond with the carbon.
M C C C
Figure 2.5 – Comparison between carbene and olefin pi-bonding
Carbene complexes can be divided into two groups: Fischer carbenes have one or two highly electronegative heteroatoms such as O, N or S directly attached to the carbene carbon and show behaviour typical of electrophiles (C+). The second group of carbene complexes, discovered only ten years later, is called Schrock (alkylidene) complexes and confer nucleophilic properties (C-). Unlike in the case of Fischer carbenes, substituents attached to the carbene carbon contain exclusively carbon and/or hydrogen atoms.
2.5.1. Fischer Carbene Complexes
The most general synthesis for electrophilic carbene complexes comprises attack of a nucleophile upon a coordinated carbonyl or related ligand as illustrated in Scheme 2.4.
O E O-E M=C=O + Nu M=C M=C Nu Nu Scheme 2.4 - Mechanism for the formation of electrophilic carbene complexes (Fischer Carbenes) from metal carbonyls
An example of such a reaction is shown in Scheme 2.5 where the coordinated carbonyl is replaced by iso-electronic species like isonitriles.
16 CHAPTER 2 – Current Knowledge on Metathesis
SH [W(CO)5(CS)] + R2NH (OC)5W=C NR2 Scheme 2.5 - Example of the formation of an electrophilic carbene complex
The electrophilic character of the carbonic carbon in Fischer complexes facilitates the attack by nucleophiles. Consequently it is easy to convert a heteroatom-functionalized (Fischer) carbene complex to an alkylidene (Schrock) compound, as seen from Scheme 2.6.
OR OR' H Nu M=C + Nu M-C Nu M=C + R'OH R R R Scheme 2.6 – Conversion of Fischer type carbenes to Schrock type carbenes
The example below is typical a conversion.43
OMe 1) K[HB(OCHMe ) ], -78oC H (OC) W=C 2 3 5 (OC)5W=C o Ph 2) CF3CO2H, -78 C Ph Scheme 2.7 – Example of the conversion of Fischer carbenes to Schrock carbenes
This alkylidene complex is only stable at low temperature.
2.5.2. Schrock Carbene Complexes
In general, alkylidene complexes are obtained by -elimination from a metal dialkyl:
R R' R'
LnM C H LnM C + RH R'' R'' Scheme 2.8 – Mechanism for the formation of a Schrock carbene
17 CHAPTER 2 – Current Knowledge on Metathesis
The elimination is favoured by sterically hindered alkyl groups and small, basic phosphine co-ligands such as PMe3. Other requirements are that the alkyl groups R and CHR1R2 must be cis in the starting material and that the metals must be in high oxidation states, such as Ta(V) and W(VI), for elimination to occur easily. An example is given in Scheme.2.9.
H -30oC [TaCl2(CH2CMe3)2(Cp)] CpCl2Ta C + CMe4
CMe3 Scheme 2.9 – Example of the formation of a Schrock carbene
The complexes previously described find application as catalysts (discussed in succeeding sections), but first general catalytic aspects will be discussed to ensure necessary background.
2.6. Catalysis
Catalysts play an important role in both chemical and biological processes as external agents which cause chemical reactions to occur or improve, but are not consumed by the reactions.44 A catalyst affects only the reaction rate, changing neither the thermodynamics nor the equilibrium composition of the reaction.
The principal theme in catalysis is a desire to control the rate of a chemical reaction while the secondary theme is to understand the mechanism of control.
The modern basis for the understanding of catalysis is44 • Spectroscopy of catalysts and catalyst models • Kinetic data for catalytic reactions
18 CHAPTER 2 – Current Knowledge on Metathesis
• Quantum-chemical calculations for reactants, intermediates and products • Calculation of reagent, intermediate and product thermodynamics from measured spectra and quantum-chemical calculations • Micro-kinetic modelling
The requirements for a successful catalytic process are given below. • The reaction being catalysed must be thermodynamically favourable • The catalysed reaction must run at a reasonable rate • The catalyst must have an appropriate selectivity towards the desired product • The catalyst must have a lifetime long enough to be economically viable
Catalysis can be divided into heterogeneous catalysis and homogeneous catalysis.45
2.6.1. Heterogeneous Catalysis
Heterogeneous catalysts are present in different phases than reagents, whereas homogenous catalysts are in the same phase as reagents.
A simple model for heterogeneous catalysis involves the catalyst providing a surface on which reagents temporarily become adsorbed, bonds in the substrate become weakened sufficiently and new products can be created.
Heterogeneous catalysis is largely an empirical science.44 The application has been a necessity for the chemical industry for at least 150 years, while experimental techniques for investigation of catalysis at atomic level became routine less than 25 years ago. Computational techniques are even younger and have hardly become routine yet. Therefore a vast amount of empirical knowledge awaits systematic investigation.
19 CHAPTER 2 – Current Knowledge on Metathesis
2.6.2. Homogenous Catalysis
Homogenous catalysts generally react with one or more reagents to form a chemical intermediate that subsequently reacts to form the final reaction product and regenerate the catalyst.
The catalytic cycles of homogeneous catalysts are understood much better than those of heterogeneous catalysts, thus, knowledge provides the opportunity to modify electronic and steric properties of the homogeneous catalyst in order to optimise results, which is not the case in heterogeneous catalysis.
2.7. Ru Carbenes in Non-Metathesis Reactions
Several examples exists where Grubbs and related catalysts are used to initiate free-radical reactions, like the neutral ruthenium-alkylidene in reaction A of Scheme 2.10 used to initiate the Karachi reaction46 as well as atom transfer polymerisation.47 The first generation Grubbs catalyst is useful in the free radical polymerisation of acryl ate esters or vinyl acetate.48 Grubbs 1 catalyse several reactions like the conversion of O-allyl oxide to the nitride,49 (reaction B ofScheme 2.10) and hydrosilylation of aldehydes (reaction C ofScheme 2.10),50 while Grubbs 2 is effective for isomerisation of allylic ethers to enol ethers and allylic sulfonamides to N-sulfonyl enamides.51 The Grubbs catalyst can be classified as a “neutral non heteroatom-functionalized metal- carbene complex that is not a cumulene”.52
20 CHAPTER 2 – Current Knowledge on Metathesis
(A) Cl
OMe CCl4 Cl3C OMe
O O Mes N N Mes Cl Ru C O Ph N+ CH3
(B) N O NC O O Grubbs1 O O O O O O
(C)
Et3SiH/Grubbs1 CHO CH2OSiEt3
Scheme 2.10 – Ruthenium carbene complexes in non-metathesis reactions
2.8. Ru Carbenes in Metathesis Reactions
Figure 2.6 shows some of the most common catalysts used for olefin metathesis. Structure A is the first generation and B the second generation Grubbs catalyst. C is Nolan’s catalyst – identical to B except for the double bond in the five membered heterogeneous ring on the ligand; and D is the Schrock catalyst – the only non-ruthenium catalyst in the group. Many variations of especially the first and second generation Grubbs catalysts have seen the light in recent years.
21 CHAPTER 2 – Current Knowledge on Metathesis
Ar PCy3 Cl N N N N N Ph Mes Mes Mes Mes H3C CH3 Ru C OC(CH3)(CF3)2 Cl Cl Mo Cl Ph Ph Ph OC(CH )(CF ) Ru C Ru C 3 3 2 PCy3 Cl Cl PCy3 PCy3 Ar=2,6-diisopropylphenyl
(A) (B) (C) (D)
Figure 2.6 – Structures for Olefin Metathesis Catalysts: (A) Grubbs 1, (B) Grubbs 2, (C) Nolan’s Catalyst and (D) Schrock Catalyst
2.8.1. Synthesis of Ru Carbene Catalysts for Olefin Metathesis
Several synthetic routes to various types of neutral Grubbs complexes with 53-49 the general structure [(PR3)2Cl2Ru=CHR] have been developed. In most of these synthetic reactions trialkyl phosphine ligands (PR3) have been used due to higher olefin metathesis activity of the resulting complexes.
The Grubbs laboratory had proposed several methods for the synthesis of ruthenium carbenes of which the first was a multi step synthesis utilising 54 [RuCl2(PPh3)3] and diphenylcyclopropene. Unfortunately the instability of diphenylcyclopropene limited the availability of the catalyst.
[RuCl2(PPh3)3] was also reacted with diazo compounds to yield 55 [(PPh3)2Cl2Ru=CH-CH=CPh2], but this method was limited by the danger of handling diazo compounds.
In 1997 the Grubbs group presented a new method for the preparation of Grubbs 1 via the reaction of Ru(0)-compounds or Ru(0)-precursors with dihalo compounds56 and showed how this complex could be used as starting material for the synthesis of other ruthenium carbenes. This new method involved addition of PhCHCl2 to solutions of [Ru(COD)(COT)] and PCy3 in toluene at room temperature to produce Grubbs1 with a yield of 50%. The mechanism of the carbene formation shown in Scheme 2.11 involved two
22 CHAPTER 2 – Current Knowledge on Metathesis steps, namely oxidative addition of the alkyl dihalide to the Ru(0) species followed by -chloro elimination. This synthetic route presented two limitations, one being the fact that [Ru(COD)(COT)] was difficult to synthesise (although good yields have been reported) and the other being that the route could not be successfully applied to the synthesis of other carbenes.
2 PCy PCy3 3 Cl Ph PhCHCl2 Ru Ru C Cl toluene, r.t. days PCy3
Scheme 2.11 – Synthesis of the first generation Grubbs catalyst with [Ru(COD)(COT)] as starting material
However, methods have since been developed to such an extend that it is possible to use the first generation Grubbs catalyst as starting material to i obtain various derivatives via phosphine exchange. [(PCy3)( Pr)Cl2Ru=CHPh] can be synthesised by directly reacting Grubbs 1 and the iPr-ligand in hexane at 60oC57 (Scheme 2.12). The product takes the form of brown air-stable micro-crystals collected in moderate yield. Since the electron donating ability i of PCy3 and Pr are very similar, the driving force for the reaction is very small and the reaction does not take place at room temperature. Even in the i presence of excess Pr only one PCy3 ligand is substituted.
PCy3 PCy Cl . Cl . 3 Ph Ph Ru C + iPr Ru C + PCy H H 3 .Cl .Cl PCy3 IPr i Scheme 2.12 – Synthesis of [(PCy3)( Pr)Cl2Ru=CHPh] via phosphine exchange
Alkenylcarbene-ruthenium complexes can also be prepared by reacting 53,58 Wilkinson’s hydride, [(PPh3)3Ru(H)Cl], with 3-chloro-3-methyl-1-butyne as illustrated in Scheme 2.13.
23 CHAPTER 2 – Current Knowledge on Metathesis
Cl PPh3 Cl HC CH Ru CH3 (PPh3)3RuHCl + 3 CH2Cl2 Cl CH3 PPh 3 CH3
Scheme 2.13 – Synthesis of [(PPh3)2Cl2Ru=CH-CH=CR2]
Although the Grubbs-type complexes [(PPh3)2Cl2Ru=CHR] are less active catalysts, they are especially useful as precursors for ligand exchange reactions, since aryl phosphines are often more easily substituted than alkyl phosphines.53 The first generation Grubbs catalyst for instance can be obtained by adding PCy3 to [(PPh3)2Cl2Ru=CHPh]. Due to the relative instability of [(PPh3)2Cl2Ru=CHPh] in solution, a one-pot synthesis (illustrated 59 in Scheme 2.14) was developed to obtain [(PR3)2Cl2Ru=CHPh].
PR3 2 equiv. PhCHN2 Cl Ph CH2Cl2 2.2 equiv. PR3 Ru C [RuCl2(PPh3)3] o Cl -78oC - -50oC -50 C - RT PR3 3-5 min 30 min R = Cy, Cp, i-Pr
Scheme 2.14 – One-pot synthesis for [(PR3)2Cl2Ru=CHPh]
This method still finds wide application, as in the cases where i [(PPh3)2Cl2Ru=CH-CHCPh2] can result to [(P Pr2Ph)2Cl2Ru=CH-CHCPh2], i [(P Pr3)2Cl2Ru=CH-CHCPh2] or [(Cy2Ph)2Cl2Ru=CH-CHCPh2] by adding the appropriate phosphine ligand.60
2.8.2. Ligand Effects on Catalytic Activity and Stability
Figure 2.7 shows the groups surrounding the ruthenium-carbon bond, all influencing the activity and stability of the catalyst.
24 CHAPTER 2 – Current Knowledge on Metathesis
L X Ru C-R X L' Figure 2.7 – General Grubbs type catalyst, with L,L’ = Phosphine or Phosphine mimicking ligands; X = Halogen and R = -group on the carbene carbon
2.8.2.1 The Effect of Phosphine Ligands
Background on Phosphine Ligands
The role of organic phosphines in organometallic chemistry is indisputable. Phosphine ligands possess both the ability to stabilise transition metals in a low oxidation state and control the ligand coordination sterically and stereochemically,61 which are of great importance in catalysis.62
Nuclear magnetic resonance (NMR) is extensively applied to characterise organophosphorous ligands and to help gain insight into their role in catalysis. Thanks to the nuclear spin of ½ and the 100% natural abundance of the 31P isotope, the NMR technique can routinely be applied in organometallic chemistry.
Dissociation of a phosphine ligand is one of the most common ways to activate metal-phosphine complexes. Catalysts such as [(PPh3)3RhCl] in hydrogenation, [(PPh3)3(CO)RhH] in hydroformylation as well as
[(PCy3)2Cl2Ru=CHPh] in metathesis are well known to generate catalytically active species by means of phosphine dissociation.62
In the chemistry of PX3 ligands both electronic and steric effects are of great significance, but the steric factors are usually dominating when it comes to determination of stereochemistry and structure of compounds. Steric factors also influence the rate and equilibrium of a dissociative reaction.63 The stereochemistry of phosphine ligands is the prime factor in many highly
25 CHAPTER 2 – Current Knowledge on Metathesis selective catalytic reactions of phosphine complexes such as hydroformylation and asymmetric hydrogenation.
The number of PR3 ligands that can be arranged around a given central metal atom must depend on the size (or bulkiness) of the PR3 ligands. Tolman has devised a useful way of expressing the relative steric sizes of ligands
(particularly PR3 ligands) in terms of their cone angle, , as defined by Figure 2.8.64
P 228 pm Ni
Figure 2.8 – Tolman’s cone angle for PR3 groups as originally defined for nickel
The cone is defined in such a way to just enclose the van der Waals surface of all ligand atoms over all rotational orientations about the M-bond. Typical o o cone angle values range between 104 for PF3 to 184 for P(C6F5)3.
When only considering the cone angle of a phosphine compound it is to be expected that ligands with smaller cone angles would be more effective due to the sterically unhindered environment around the metal centre, but in the case of phosphines, these metal complexes would most likely be stronger bases.
Compounds with larger cone angles would be inclined to favour lower coordination numbers and less sterically crowded isomers. These compounds would also increase reaction rates and shift equilibria towards dissociative reaction mechanisms.
26 CHAPTER 2 – Current Knowledge on Metathesis
The choice of phosphine or phosphine substitute is critical for an effective and stable catalytic system,74 because even minor alterations in ligand properties can have significant consequences. The majority of PR3 ligands contain alkyl or aryl substituents, while phosphites contain alkoxy-groups (OR) and phosphoranes contain ylidene-type groups (R2C=PR3) with the aim to optimise the ligands to certain chemical functions.
Phosphine ligands are generally considered to be -donors due to their lone pair-to-metal donor capabilities.65 However, in complexes with electron rich metals (Ir, Os, Pt) phosphine ligands can act as π-acceptors by removing electron density from the metal d-orbital into P-C * or P 3d-orbitals. This electron movement is illustrated in Figure 2.9.
27 CHAPTER 2 – Current Knowledge on Metathesis
Figure 2.9 – Interaction of phosphine ligand orbitals with metal orbitals
Phosphine Ligands in Grubbs-type Catalysts
Dias et al.66 used the first generation Grubbs catalyst in the ring closing of dienes to study the effect different ligands have on catalyst activity and derived rate equation 2.1. When it is assumed that the formation of the 14- electron metallacyclobutane intermediate is the rate determining step of the
28 CHAPTER 2 – Current Knowledge on Metathesis olefin metathesis reaction, it can be seen from equation 2.1 that catalytic activity is directly proportional to K1, the equilibrium constant for olefin binding;
K2, the equilibrium constant for phosphine dissociation and k3, the rate constant for metallacyclobutane formation from the monophosphine olefin complex. See scheme 2.2 for the general reaction scheme.
≈ ’ d[diene] ∆ K1K 2 ÷ − = ∆k 3 ÷[catalyst].[diene] (2.1) dt « [PCy 3 ]◊
The Dias group determined the relative catalyst activities of a range of ruthenium based catalysts with the general formula [(PR3)2X2Ru=CH-CHCPh2] (shown in Figure 2.10) by monitoring the RCM of acyclic diethyl diallylmalonate.66 It was concluded that the catalytic activities increased as both the cone angle and the electron donating ability of the phosphines increased. The order of increasing activity was determined as PR3 = PPh3 << i i P Pr2Ph < PCy2Ph < P Pr3 < PCy3. This trend was explained on the basis of increasing trans-effect of larger and more basic phosphines, which was believed to accelerate dissociation of the second PR3 ligand and stabilise the Ru(IV) metallacyclobutane intermediate.
PR3 X Ru C Ph X Ph PR3
Figure 2.10 – General structure of ruthenium vinylcarbenes to study the influence of different ligands on catalyst activity
Using the N-heterocyclic carbene (NHC) ligands of the second generation Grubbs catalysts, which are significantly larger and more electron donating than the trialkylphosphines of the first generation catalysts, results in dramatically increased catalytic reactivity with olefinic substrates.41
29 CHAPTER 2 – Current Knowledge on Metathesis
31P NMR was used to examine the rates of phosphine exchange in the first and second generation Grubbs catalysts and it was found that exchange was relatively slow. Results also indicated an inverse relationship between the phosphine exchange rate and olefin metathesis activity71 and catalysis was 71 found to be independent of [PCy3] over a wide range of concentrations. An examination of the exchange rate constant as function of phosphine concentration established a dissociative mechanism for the reaction.
2.8.2.2 The Effect of Phosphine Mimicking Ligands
Mechanistic studies have shown that the presence of a bulky tertiary phosphine ligand is mandatory for stabilising reactive catalytic intermediates and/or preventing the decomposition of carbenes.67 However, tertiary phosphine ligands often undergo significant P-C degradation at higher temperatures which can result in the deactivation of the catalyst.68 Strong nucleophilic (electron-rich) ligands that form stable bonds with metals in order to stabilise the catalyst needed to be found. Carbene ligands like those shown in Figure 2.11 possess these properties and have proven to act as phosphine mimics.69
. .
N N N N ...... N N H H . . . H2IMes . i . . Pr
IMes
Figure 2.11 – Carbene ligands capable of acting as Phosphine Ligands
Substitution of one phosphine ligand in the first generation Grubbs catalyst with the IMes ligand produces the mixed ligand system
[(PCy3)(IMes)Cl2Ru=CHPh] (see Figure 2.6 for structures) and leads to significant improvement of activity and thermal stability compared to the parent complex Grubbs 1.70
30 CHAPTER 2 – Current Knowledge on Metathesis
57 i Nolan’s group compared the reaction enthalpies of PCy3, IMes and Pr by means of a calorimetric investigation of the reaction in equation 2.2.
THF(RT) [Cp * RuCl]4 + 4L → 4Cp * Ru(L)Cl (2.2)
Table 2.1 shows that all reactions were exothermic and that Ru-L stability i decreased in the order IMes > Pr > PCy3. IMes is the strongest binder with i Pr, electronically speaking, very similar to PCy3.
Table 2.1 – Comparison between reaction enthalpies of typical ligands for Grubbs type catalysts ∆H L (kcal/mol)
PCy3 -41.9 (0.2) IMes -62.6 (0.2) iPr -44.5 (0.2)
Despite iPr having an electronic disadvantage, the ligand is sterically more i demanding than both IMes and PCy3 and it can be predicted that Pr would be capable of stabilising the 14-electron ruthenium intermediate of the metathesis reaction (See Figure 2.3). It was also found that the thermal stability of the resulting carbene species, [(PCy3)(IMes)Cl2Ru=CHPh] and [(PCy3)( i Pr)Cl2Ru=CHPh] are very similar.
An independent study by the Grubbs group71 indicated that the higher activity of Grubbs 2 compared to Grubbs 1, which was previously attributed to its ability to promote phosphine dissociation, instead is due to its improved selectivity for binding π-acidic olefin substrates in the presence of -donating free phosphine.
31 CHAPTER 2 – Current Knowledge on Metathesis
2.8.2.3 Halogen Effect
Halide ligands have a significant impact on initiation rates of the catalysts
[L(PR3)X2Ru=CHR]. In both bis-phosphine complexes and H2IMes complexes, changing the X-type ligand from chloride to iodide leads to an approximately 250-fold increase in initiation rate.41 It is believed that the increase in initiation rate is predominantly due to the increase in steric bulk upon moving from chloride (ionic radius of Cl− = 167 pm) to iodide (ionic radius of I− = 206 pm). The larger size of iodide is expected to increase steric crowding at the ruthenium centre, thus promoting PR3 dissociation.
Electronics may also play a role in these systems, however cis electronic effects on dissociative ligand substitution reactions are generally relatively small.72 Tert-butoxides (OtBu) are even larger and more electron-donating than iodide ligands and are usually counted as XL ligands. Substitution of chlorides for tert-butoxides leads to an analogous system of
[PR3(OR)2Ru=CHR] and one free phosphine ligand, which shows that the right choice of X-type ligand can effectively promote complete phosphine dissociation.41
On the other hand it was found that the activity of catalysts with the general 41,60 formula [LL’X2Ru=CHR] increased in the order X = I < Br < Cl, signifying that smaller and more electron withdrawing halogens produce more active catalysts. This result is explained in terms of the cis- and trans-effect as well as steric crowding of the halogens.41,60
2.8.2.4 Effect of the -group on the Carbene Carbon
The -group on the carbene carbon has an effect on both the stability and activity of the catalyst. It was found that complexes containing an electron- donating group (Fischer complexes) were more stable than their bisphosphine analogues.73 In another study it was found that a bulky and electron-donating aliphatic group on the carbene carbon caused the rate of catalyst initiation to increase dramatically, because phosphine dissociation was favoured.41
32 CHAPTER 2 – Current Knowledge on Metathesis
2.8.3. Decomposition of Ruthenium Carbene Complexes
Grubb’s catalyst is moderately stabile in solid form compared to other metathesis catalysts like the tungsten, molybdenum, tantalum and titanium complexes.74 However, Grubbs 1 must still be handled using standard Schlenk techniques to prevent decomposition of the catalyst, i.e. loss of one phosphine ligand or oxidation in solution. Thus, the catalyst concentration decreases and low turnover numbers are obtained during the metathesis of olefins. These drawbacks deter the wide use of Grubb’s catalyst on a large scale for the metathesis of useful, high value olefins from ordinary olefin feedstock in industry.
Thermolytic decomposition limits the usefulness of the ruthenium system in many challenging reactions. Two methods of decomposition have been distinguished, namely unimolecular decomposition and bimolecular decomposition.
2.8.3.1 Unimolecular Decomposition
Although the benzylidene complex is used to initiate most metathesis reactions, the propagating species in the reaction is usually either an alkylidene or the methylidene as seen from the example of ring closing metathesis in Scheme 2.15.74
Ph
Ph . Ru CH2 Ru + Ru +
(A) (B) (C)
Scheme 2.15 – RCM Pathway showing the initial catalyst and the propagating species: (A) Benzylidene, (B) Alkylidene and (C) Methylidene74
33 CHAPTER 2 – Current Knowledge on Metathesis
Alkylidene decomposition is predominantly second order, requiring phosphine dissociation, while methylidene decomposition is primarily first order,74 but the exact nature of the inorganic decomposition products is not known. No bimolecular decomposition product (ethene) is observed from bis-phosphine methylidenes, but it has been reported that a ruthenium ethene complex was detected from attempted generation of a monophosphine bimetallic methylidene. This finding suggests that bimolecular decomposition can occur for methylidenes, but it is generally slower than the unimolecular decomposition pathway and as a result bimolecular methylidene decomposition is only observed for monophosphine methylidene complexes.
The fact that substrates that are difficult to cyclise require high catalyst loadings can be explained by the unimolecular decomposition of the propagating methylidene catalyst.74 Scheme 2.16 shows the postulated pathway for alkylidene decomposition.
PCy3 Cl Cl K Ru CHR Ru CHR + PCy3 Cl Cl PCy3 PCy3
Cl Ru CHR k 2 RHC=CHR + Inorganic Products Cl PCy3 Scheme 2.16 - Proposed Pathway for Alkylidene Decomposition74
Thermal decomposition of Grubbs 1 followed by treatment with two moles of phenylacetylene leads to the vinylidene-ruthenium complex (Scheme 2.17 (a)).75
34 CHAPTER 2 – Current Knowledge on Metathesis
PCy3 Cl Ph 55oC Ph Ph Ru C [Ru] [Ru] Cl H (a) PCy3 excess Ph Ph
H H + H [Ru] Ph Ph Ph [Ru] [Ru] (e) H H H (d) (c) (b) Ph Ph Ph Ph Scheme 2.17 – Decomposition of Grubbs 1 and the reaction of the decomposed product with phenylacetylene75
Although this complex could not be isolated in pure form, NMR studies support the assignment as a vinylidene complex. This complex was a moderately effective catalyst for the RCM of diethyl diallylmalonate and reaction with additional phenylacetylene led to the enyne (e) in low yield, however the yield was considerable higher when acetic acid was added. The mechanism for the formation of the enyne (e) involves formation of π-alkyne complex (b) followed by conversion to the alkynyl (c) which undergoes insertion to afford the enynyl complex (d). Protonation again affords the enyne (e). Acetic acid accelerates the process through protonation of the enynyl ligand. Grubbs 1 decomposition product, [Ru], also catalysed the addition of carboxylic acids to terminal alkynes.76
Ligands play an important role in stabilising the catalytic system.74 For most applications the utility of a catalyst is determined by the ratio of catalytic and decomposition rate. A ligand change that accelerates the rate of catalysis but also the rate of decomposition does not significantly improve the catalytic system. A catalyst can be very active, but when the propagating species is short-lived, the reaction can not be taken to completion. The second i generation Grubbs catalyst as well as [(PCy3)( Pr)Cl2Ru=CHPh] is much more thermally stable than the first generation catalyst (longer than 14 days vs. 1 hour; at 60oC).57 35 CHAPTER 2 – Current Knowledge on Metathesis
2.8.3.2 Bimolecular Deactivation
Dimerisation chemistry of the useful precursor complex [(PPh3)2Cl2Ru=CH- 77 CH=CMe2] was confirmed. The dinuclear alkylidenes products (shown in Figure 2.12) are easily formed and show very low activity, proving that bimolecular deactivation is a major catalyst deactivation pathway operative in this Grubbs catalyst.
CH3 H C Cl 3 Cl Cl
PPh Ph3P Ru Cl Ru PPh3 Cl 3 Ph P Cl 3 PPh3 Ph3P Ru Ru PPh3 H
Ph3P Cl Cl
H3C CH3
(A) (B)
Figure 2.12 – Deactivation products from the dimerization of
[(PPh3)2Cl2Ru=CH-CH=CMe2] with (A) being the primary product and (B) the by product
In a very recent article78 the mechanistic route for the thermal decomposition of the second generation Grubbs catalyst in benzene was proposed. The crystal structure of the decomposition product was obtained and it was shown that this complex could be responsible for competing isomerisation processes in certain olefin metathesis reactions. It was also postulated that the dissociated phosphine is involved in the decomposition of the catalyst. The structure is shown in Figure 2.13.
36 CHAPTER 2 – Current Knowledge on Metathesis
. . . N N Mes . . Mes Mes N N . Cl . Cl 0.023 M N Ph Ru CH PCy +Cl- Ru + 3 3 Ru C C6H6 o N Cl Cl 55 C H Cl PCy3 Mes Figure 2.13 – Decomposition of the second generation Grubbs catalyst78
2.9. Theoretical Studies of Olefin Metathesis
Computational calculations have proved to be very useful for clarifying various ambiguous experimental results. Orbital interactions and intermediate energy values are especially valuable information that are often unattainable by experimental techniques.
Many studies of olefin metathesis utilising DFT have been reported, with emphasis on the mechanism. A few results are discussed in the following paragraphs.79-83
DFT calculations have suggested that the dissociative mechanism is favoured over the associative mechanism for various complexes with the general 79 structure [LL’Cl2Ru=CH2]. In order to reduce calculation time, realistic ligands are usually simplified to L = PH3, PR3 or :C(NHCN=CHNH) and ethylene is chosen as reagent. The relative energies of the reactive intermediates are affected by the electron-donating ability of phosphine ligands, for example in [(PMe3)2Cl2Ru=CH2] phosphine dissociation is suppressed and olefin insertion is facilitated relative to the analogous PH3 complex.
Grubbs 1 has a high barrier midway through the reaction coordinate imposed by the necessity of rotation of the threefold symmetric phosphine ligand,80 but a comparable barrier is absent in Grubbs 2, because of twofold symmetry of
37 CHAPTER 2 – Current Knowledge on Metathesis the NHC ligand. This result was suggested as an important factor in the superior reactivity of Grubbs 2 versus Grubbs 1.
Mechanistic studies also investigated catalysts featuring either two phosphine ligands,81 one phosphine and one heterocyclic carbene ligand, or cis chelating phosphines. Results showed that phosphine ligands are more strongly bound in N-heterocyclic carbene-ligated species, since the bulky mesityl substituents exert steric pressure on the alkylidene group. The heterocyclic carbene ligands promote olefin coordination, lower the metathesis reaction barrier and stabilise metallacyclic intermediates, which account for the overall increase in activity of these catalysts.
More mechanistic studies have shown that the ruthenacyclobutane (illustrated in Figure 2.3) is a real intermediate and not a transition state as previously thought.82
Theoretical studies have also been used to give insight into substrate-induced catalyst decomposition.83
2.10. Phobcat: The Sasol Catalyst
To successfully apply homogeneous metathesis to industrial-scale production of commodity olefins, high turnover numbers and selectivity are required. Grubbs 1 does not comply with these demands because of poor thermal stability and a short lifetime (at low catalyst loadings) when exposed to Fischer-Tropsch olefin feedstock.84 Grubbs 2, although showing increased activity, has the drawback of significant isomerisation. A new catalyst had to be developed that is both stable and highly selective.
9-Cyclohexyl-9-phospha-9H-bycyclononane (Phoban) ligands are currently used industrially for the cobalt-catalysed hydroformylation reaction85 and have also found use for other catalytic processes.86 The ligand is convenient and
38 CHAPTER 2 – Current Knowledge on Metathesis cheap to prepare as a 3:1 mixture of [3.3.1]- and [4.2.1]-bridged isomers via radical addition of 1,5-cyclooctadiene to cyclohexylphosphine. A new ruthenium carbene catalyst containing a rigid bicyclic phosphine moiety have been prepared by phosphine exchange from [(PPh3)2Cl2Ru=CH-CH=CMe2] with Phoban. Figure 2.14 shows the phosphine exchange reaction that produced the Phobcat catalyst.
Cy Cy P P P L Cy Cl R R Cl Ru C Ru C Cl 3:1 Cl Cy L P
with L = PPh3, R = C=C(Me)2
or L = PCy3, R = Ph
Figure 2.14 – Synthesis of Phobcat via phosphine exchange from Grubbs 1
These isolated Ru alkylidene complexes are efficient catalysts for various metathesis reactions.84 Especially in the cases of self metathesis, ethenolysis and RCM the activity is significantly greater than Grubbs 1, while selectivity of above 95% is maintained.
15 http://www.nndc.bnl.gov/nndc/history/origindc.pdf. 16 http://www.scescape.net/~woods/elements/ruthenium.html. 17 W.C. Zeise, Pogg. Ann. Phys. Chem., 1827, 9, 632. 18 L. Mond, C. Langer, F. Quincke, J. Chem. Soc.,1891, 189, 749. 19 L. Mond, C. Langer, J. Chem. Soc., 1891, 189, 1090. 20 F. Hein, Chem. Ber., 1919, 52, 195. 21 F. Fischer, H. Tropsch, German patents 411416, 1922 & 484337, 1925. 22 O. Roelen, German patent 849548, 1938. 23 T.J. Kealy, P.L. Pauson, Nature, 1951, 168, 1039. 24 G. Wilkinson, J. Am. Chem. Soc., 1952, 74, 2125. 25 See K. Ziegler, Adv. Organomet. Chem., 1968, 6, 1.
39 CHAPTER 2 – Current Knowledge on Metathesis
26 E.O. Fischer, A. Maasbon, Angew. Chem., Int. Ed. Engl., 1964, 3, 580. 27 E.O. Fischer, Angew. Chem. Int. Ed. Engl., 1973, 12, 564. 28 R.L. Banks, G. C. Bailey, Ind. Eng. Chem. Res., 1964, 3, 580. 29 J.F. Yound, J.A. Osborn, F.H. Jardine, G. Wilkinson, J. Chem. Soc., Chem. Commun., 1965, 131. 30 J.F. Roth, J.H. Craddock, A. Hershman, F.E. Paulik, Chem. Tech., 1971, 1, 600. 31 A.H. Janowicz, R. G. Bergman, J. Am. Chem. Soc., 1982, 104, 352. 32 G.J. Kubas, R.R. Ryan, B.I. Swanson, P.J. Vergamini, H.J. Wasserman, J. Am. Chem. Soc., 1984, 106, 451. 33 H. Sinn, Angew. Chem. Int. Ed. Engl., 1980, 19, 380. 34 F.R.W.P. Wild, L. Zsolnai, G. Huttner, H.H. Brintzinger, J. Organomet. Chem., 1982, 232, 233. 35 K. Ziegler, Angew. Chem., 1955, 67, 541. 36 G. Natta, Angew. Chem., 1956, 68, 393. 37 A. Togni, R. L. Halterman, Metallocenes; Wiley: Weinheim. 1998. 38 H.H. Brintzinger, Angew Chem. Int. Ed. Engl., 1995, 34, 1143. 39 G.W. Coates, Chem. Rev., 2000, 100, 1223. 40 J.L. Hérisson, Y. Chauvin, Makromol. Chem., 1971, 141, 161. 41 M.S. Sanford, J.A. Love, R.H. Grubbs, J. Am. Chem. Soc., 2001, 123, 6543. 42 F. Mathey, A. Sevin, Molecular Chemistry of the Transition Elements, Wiley&Sons Ltd: Chichester, 1996, 11. 43 C.P. Casy, J. Am. Chem. Soc., 1979, 101, 7282. 44 http://www.aue.auc.dk/~stoltze/catal/book/intro/main.htm. 45 http://www.wordiq.com/definition/Catalyst. 46 B. De Clerq, F. Verpoort, Tetrahedron Lett., 2002, 43, 4687. 47 B. De Clerq, F. Verpoort, Macromolecules, 2002, 35, 8943. 48 F. Simal, Chem. Eur. J., 2002, 8, 3047. 49 A. Talukdar, Synth. Commun., 2002, 32, 3503. 50 S.V. Maifeld, R.L. Miller, D. Lee, Tetrahedron Lett., 2002, 43, 6363. 51 C. Cadot, P.I. Dalko, J. Cossy, Tetrahedron Lett., 2002, 43, 1839. 52 J.W. Herndon, Coordination Chemistry Reviews, 2004, 248, 2. 53 M.A.O. Volland, F. Rominger, F. Eisenträger, P. Hofmann, J. Organomet. Chem., 2002, 641, 220. 54 S.T. Nguyen, L.K. Johnson, R.H. Grubbs, J. Am. Chem. Soc., 1992, 114, 3974. 55 W.R. Roper, J. Organomet. Chem., 1986, 300, 167.
40 CHAPTER 2 – Current Knowledge on Metathesis
56 T.R. Belderrain, R.H. Grubbs, Organometallics, 1997, 16 (18), 4001. 57 L. Jafarpour, J. Organomet. Chem., 2000, 606, 49. 58 D. Amoroso, Synth. Catal., 2002, 344, 757. 59 P. Schwab, R.H. Grubbs, J.W. Ziller, J. Am. Chem. Soc., 1996, 118 (1), 100. 60 E.L. Dias, S.T. Nguyen, R.H. Grubbs, J. Am. Chem. Soc., 1997, 119, 3887. 61 F. Montilla, A. Inorg. Chem. 1999, 38, 4462. 62 L.H. Pignolet, Homogeneous Catalysis with Metal Phosphine Complexes 1st Ed.; Plenum Press: New York, 1983. 63 C.A. Tolman, Chem. Rev., 1977, 77, 313. 64 Tolman, Organometallics, 1983, 2, 1391. 65 Morris, R. J.; Girolami, G. S. Inorg. Chem., 1990, 29, 4167. 66 E.L Dias, S.T. Nguyen, R.H. Grubbs, J. Am. Chem. Soc., 1997, 119, 3887. 67 (a) E.L. Dias, S.T. Nguyen, R.H. Grubbs, J. Am. Chem. Soc., 1997, 119, 3887. (b) M. Ulman, R.H. Grubbs, Organometallics, 1998, 17, 2484. 68 J.P. Collman, Principles and Applications of Organotransition Metal Chemistry, 2nd Ed., University Science, Mill Valley, C.A., 1987. 69 M.F. Lappert, J. Organomet. Chem., 1988, 358, 185. 70 J. Huang, E.D. Stevens, S.P. Nolan, J.L. Peterson, J. Am. Chem. Soc., 1999, 121, 2674. 71 M.S. Sanford, M. Ulman, R.H. Grubbs, J. Am. Chem. Soc., 2001, 123, 749. 72 J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic Chemistry, Harper Collins; NY, 1993. 73 J. Louie, R.H. Grubbs, J. Am. Chem. Soc., 2001, 21, 2153. 74 M. Ulman, R.H. Grubbs, J. Org. Chem., 1999, 64, 7202. 75 K. Melis, D. De Vos, P. Jacobs, F. Verpoort, J. Organomet. Chem., 2002, 659, 159. 76 K, Melis, T. Opstal, F. Verpoort, Eur. J. Org. Chem., 2002, 2779. 77 D. Amoroso, G.P.A. Yap, D.E. Fogg, Organometallics, 2002, 21, 3335. 78 S.H. Holng, M.W. Day, R.H. Grubbs, J. Am. Chem. Soc., 2004, 126 (24), 7414. 79 SF. Vyboishchikov, M. Bühl, W. Thiel, Chem. Eur. J., 2002, 8, 1820. 80 C. Adlart, P. Chen, Angew. Chem. Int. Ed., 2002, 41, 4484. 81 L. Cavallo, J. Am. Chem. Soc., 2002, 124, 8965. 82 F. Bernardi, A. Bottoni, G.P. Miscione, Organometallics, 2003, 22, 940. 83 W. Janse van Rensburg, P.J. Steynberg, W.H. Meyer, M.M. Kirk, G.S. Forman, J. Am. Chem. Soc., 2004, 126, 14332.
41 CHAPTER 2 – Current Knowledge on Metathesis
84 G.S. Forman, A.E. McConnell, M.J. Hanton, A.M.Z. Slawin, R.P. Tooze, W. Janse van Rensburg, W.H. Meyer, C. Dwyer, M.M. Kirk, D.W. Serfontein, Organometallics, 2004, 23, 4824. 85 F. Wattimena (Shell Oil Co.) Netherlands Patent 6 604 094, 1966; Chem. Abstr. 1967, 66, 65101r. 86 B. Müller, Organometallics, 1994, 13, 2563.
42
3. Synthesis and Kinetics
3.1. Introduction
Chapter 3 accounts for the completion of the first four project aims discussed in section 1.2. Three catalysts were investigated:
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh], [(PCy3)2Cl2Ru=CHPh)] and 87 [(PhobCy)2Cl2Ru=CHPh]. These catalysts represent the first generation
Grubbs catalysts [(PR3)2Cl2Ru=CR’], second generation Grubbs catalysts
[(IMesH2)(PR3)Cl2Ru=CR’] and the modified first generation Grubbs catalysts 87 with a rigid bicyclic phosphine moiety [(PhobR)2Cl2Ru=CR’].
The first aim of the project was to prepare a novel Grubbs type catalyst. Initial efforts in this regard were focused on the preparation of a first generation catalyst (section 3.1.1.1), but being unsuccessful, the objective was instead accomplished with the synthesis of a second generation catalyst (section 3.1.1.2). The new catalyst was used in preliminary activity analysis (section 3.1.4) and mechanistic phosphine exchange studies (section 3.1.5). Olefin- catalyst coordination was investigated via kinetic case studies of oxygen- and nitrogen-substituted olefins (section 3.1.6). The Sasol catalyst, Phobcat 88 ([(PhobCy)2Cl2Ru=CHPh]), was compared to Grubbs1-PCy3
([(PCy3)2Cl2Ru=CHPh)]) to catalyse the self metathesis reaction of 1-octene (section 3.1.7). Since kinetics is the main theme in the second part of the experimental study and magnetisation transfer techniques which were used are generally not well known, some theoretical aspects of the two subjects were also addressed in this chapter (sections 3.1.3 and 3.1.5).
43 CHAPTER 3 – Synthesis and Kinetics
3.1.1. Synthesis
3.1.1.1 Attempted Synthesis of a First Generation Grubbs Type Catalyst
Many methods to prepare the first generation catalyst are known from literature (see section 2.8.1) of which a useful method was reported by Volland et. al.,89 who used the readily available and easy-to-handle
Wilkinson’s complex as starting material to prepare [(PPh3)2Cl2Ru=CH-
CH=CMe2]. This method to synthesise a first generation Grubbs type catalyst was preferred over other known routes involving diphenylcyclopropene,90 which are not readily available or diazo complexes which are potentially dangerous.91
Specifically, the synthesised [(PPh3)2Cl2Ru=CH-CH=CMe2] was chosen as starting material since it is known that the PPh3-ligands in this relatively unreactive catalyst can easily be exchanged with a variety of other phosphine ligands which gives access to a range of other Grubbs type catalysts.89 In addition these products with the general structure [LL’X2Ru=CHR] (with L =
PR3; L’ = PR3, PR’3, IMesH2, IMes) are likely to be suitable for phosphine exchange reactions – a helpful tool for mechanistic investigations and a subsequent aim of this study.92,101
Since the general first generation Grubbs catalyst contained PCy3, it was the aim of the study to replace PCy3 with PPh2Cy or PPhCy2 to very slightly alter the phosphine character and investigate the effect thereof.
The envisioned product was [(PPh2Cy)2Cl2Ru=CH-CH=CMe2]. This complex was chosen for the reason that various phosphine ligands have been used in
Grubbs catalysts, but the synthesis of [(PPh2Cy)2Cl2Ru=CH-CH=CMe2] has never been reported, even though this complex might give useful mechanistic information, being intermediate to [(PPh3)2Cl2Ru=CH-CH=CMe2], the parent complex, and [(PCy3)2Cl2Ru=CH-CH=CMe2], Grubbs 1, of which similar studies have previously been carried out.92
44 CHAPTER 3 – Synthesis and Kinetics
The preparation of the arsine-ligand analogue of the well-known phosphine- containing catalyst, [(AsPh3)2Cl2Ru=CH-CH=CMe2], was also attempted since no Grubbs type catalysts with arsine are known from literature, even though 93 examples of rhodium complexes stabilised by either PPh3 or AsPh3 and 94 ruthenium complexes containing AsPh3 can be found.
The different synthetic routes are discussed in more detail in the results section 3.2.1. Unfortunately, the efforts to synthesise the first generation catalyst were unsuccessful and the focus of the synthetic efforts turned towards the second generation Grubbs-type catalysts.
3.1.1.2 Synthesis of the 2nd Generation Grubbs Type Catalyst [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]
The simple method of phosphine exchange, reported by Stanford et. al.,101 proved to be very useful to obtain the second generation catalysts * [(IMesH2)(PCy2Ph)Cl2Ru=CHPh] and [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] as final products. Since the two complexes were expected to behave very similar because of closely related structures, only the latter was used in further studies.
Grubbs 2 ([(IMesH2)(PCy3)Cl2Ru=CHPh]) served as starting complex for the production of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]. The synthetic procedure involved the substitution of PCy3 in Grubbs 2 with (NC5H5)2 by means of pyridine addition to the parent complex. The (NC5H5)2–ligand in turn was substituted with PPh2Cy to yield the desired product. The synthetic route is illustrated in Scheme 3.1 and the detail is described in section 3.2.3.
* Not isolated, but based on NMR observations
45 CHAPTER 3 – Synthesis and Kinetics
N N N N N N N Cl Cl + PPh Cy Cl Ru Ru 2 Ru Cl H Cl H Cl H P P N N .
101 Scheme 3.1 – Synthetic route for RuCl2(IMesH2)(PPh2Cy)(=CHPh)
With the synthesis of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] being successfully completed, the focus of the remaining experimental investigation were on kinetic studies.
3.1.2. Reaction Kinetics
The remaining goals of the project involved kinetic investigations. To cover necessary background on kinetic principles and techniques which were applied during experimental work, general theoretical aspects of kinetics will now be discussed.
3.1.3. Theoretical Aspects of Reaction Kinetics
3.1.3.1 Overview
The rate of a chemical reaction can be described at two levels: dynamics and kinetics.95
Dynamics is the description of the rate of transformation for individual molecules. The molecule has a well-defined energy and may even start in a well-defined quantum state. There is no temperature, because temperature is a property applicable to a large number of molecules, not individual molecules.95
Kinetics, on the other hand, is the description of reaction rate for a large number of molecules. The molecules have a temperature value, although it
46 CHAPTER 3 – Synthesis and Kinetics may change during the course of the reaction. The energy is well-defined, but is a statistical average.
Chemical kinetics is reflected in the time-dependence of reagent and product concentrations.96
The rate of a chemical reaction is generally expressed as the change in concentration of a reagent or product per unit time.
For essentially all chemical reactions, the rate depends upon the same factors, namely: • Chemical nature of reagents and products • Concentrations of reagents and products • Temperature • Presence/concentration of a catalyst
Reaction rate generally decreases with time as composition approaches equilibrium. While the forward reaction rate of the net reaction may depend on reagents and products, the forward rate of each elementary step can only depend on the concentration of reagents for that step.
Reaction rates are studied for primarily two reasons.97 It is of practical importance to be able to predict how quickly a reaction mixture will approach equilibrium. If the rate depends on controllable variables, the rate can be optimised with the appropriate choice of conditions. However, an even more important reason for studying reaction rates is the possibility that it might lead to an understanding of the reaction mechanism (i.e. the analysis into a sequence of elementary steps).
Some goals of a kinetics study are given below. 1. Determine the rate law, i.e. to determine on which concentrations the rate depends and what the orders in these concentrations are.
47 CHAPTER 3 – Synthesis and Kinetics
2. Determine the value of the rate constant, k, at a particular known temperature. 3. Determine k at various temperatures to demonstrate the manner in which k varies as T changes. The rate constant (and therefore also the rate) of a reaction almost always increases with temperature elevation.
Before the rate law of the reaction can be determined, a general rate law needs to be derived as explained in the following section.
3.1.3.2 Rate Law Determination
Consider a reaction
A + B → C
At some instant the concentrations of reaction participants are [A], [B] and [C]. The measures for the reaction rate are the formation rate of C and the consumption rate of A or B.
The consumption rate of reagent A (or reagent B) is
[A] ν = −d (3.1) A dt
The formation rate of product C is
[C] ν = d (3.2) A dt
Both rates are positive. The negative sign in equation 3.1 indicates that the formation rate of C is equal to the consumption rate of A or B (whenever one molecule of C is formed, one molecule of A or B is destroyed).
48 CHAPTER 3 – Synthesis and Kinetics
Therefore, the reaction rate is given by
[A] [B] [C] m n ν = −d = −d = d = k[A] [B] (3.3) dt dt dt
The rate constant (or proportionality constant), k, implicitly contains within its magnitude dependence on the chemical nature of the reaction participants as well as temperature dependence of the rate. Temperature dependence can be independently measured by running the reaction at a series of temperatures. m or n is the reaction order with regard to the concentration of A or B. The order of the total equation is equal to m + n. Reaction order is the way in which the rate varies with a change in concentration of one or more of the reacting species.
Many experimental techniques to determine the rate law of a reaction exist, of which the pseudo-order method is useful and widely applicable. This technique will now be discussed.
3.1.3.3 The Pseudo-order Method
Using the pseudo-order method, m in equation 3.3 would be determined as follows: The initial concentration of A is chosen to give a reasonable value of the property being measured, while the initial concentration of B is chosen to be at least 10 times larger than that of A. This assures that over the course of the reaction, while the amount of A substantially decreases, the concentration of B remains essentially constant. In this case the rate law (equation 3.4) takes a simplified form in which the rate depends only on [A].
m n m ν = k[A] [B] = kobs [A] (3.4)
49 CHAPTER 3 – Synthesis and Kinetics in which
n kobs = k[B] (3.5)
Depending on the value of n, equation 3.4 can be integrated to produce one of three equations shown in Table 3.1. [A]0 signifies the initial concentration of A.
Table 3.1 – The characteristic rate laws for potential values of n (the reaction order) Integrated Rate Equation n Law [A] 3.5.1 1 ln = −kt [A]0 1 1 3.5.2 2 = − kt [A] [A]0 1 1 3.5.3 3 2 = 2 − kt [A] [A] 0
Reactions in which n has a value of 1 or 2 are by far the most common. When n is 1, the reaction is said to be pseudo-first-order in A,† and when n is 2 the reaction is pseudo second order in A.
To determine the order in [B] one repeats the study using a different, still large concentration of B. The values of kobs versus the concentration of [B] can then be plotted to determine the dependence of the reaction on [B].
The pseudo-order method can also be used to simplify second order reactions. If only one reaction takes place, the reaction is first order in B, m has a value of 1 and a graph of kobs against [B] will be a straight line with an intercept of 0.
† The prefix “pseudo” conveys that there may also be a rate dependence on [B], but that this is invisible because [B] is large.
50 CHAPTER 3 – Synthesis and Kinetics
If there is a second reaction taking place in parallel, the values of m and n will be 1 and the rate of the reaction will be
ν = k1[A][B] + k 2 [A] (3.6)
Under pseudo-first order conditions kobs is given by
kobs = k1[B] + k 2 (3.7)
Equation 3.7 is also used for equilibrium reactions where k2 denotes the reverse reaction. Equation 3.7 can be integrated from time = 0 to time = t to yield equation 3.8.
[C]t ln = kobs t (3.8) [C]0
In order to apply Equation 3.8 in practice a physical variable, which corresponds to the concentration of the reagent, is measured. The Beer- Lambert law is used in this regard.
The Beer-Lambert law states that
A = εcI (3.9) in which A is the absorbance value, is the molar extinction coefficient, c is concentration and I is the length of the light path
Also
[C]t A − At = ∞ (3.10) [C]0 A∞ − A0
51 CHAPTER 3 – Synthesis and Kinetics
Equation 3.9 and 3.10 is combined to give equation 3.11.
kobst At = A∞ − (A∞ − A0 )e (3.11)
in which At and A∞ are the absorbance at time t and at the end of the reaction respectively.
Equation 3.11 is valid and can be applied for various applications where slower reactions are followed, especially spectroscopic techniques such as
UV/VIS, IR and NMR. kobs is then obtained from the least-squares fit of the absorbance vs. time data for the first order reaction in this equation.
kobs is also used to determine the half-life of the reaction, defined as the time required for 50% of the reaction to occur.
For a first order reaction the half life is given in equation 3.12.
ln2 t 1 = (3.12) 2 kobs
The activation enthalpy and activation entropy of a reaction may possibly help to shed light on the nature of a reaction. The activation parameters are discussed in the following section.
3.1.3.4 Activation Enthalpy and Entropy
The Transition State Theory for Reaction Rates states that reagents pass through high-energy transition states before forming products. The transition state, or the activated complex, which is represented by (AB)≠ in equation 3.13, is in equilibrium with reagents before the reaction takes place. The ≠ equilibrium (with constant Kc ) is followed by the decomposition (with rate k) of the activated complex to yield the products.
52 CHAPTER 3 – Synthesis and Kinetics
≠ A + B←Kc →(AB)≠ k →C (3.13)
The rate of the reaction in equation 3.13 is given by equation 3.14.
k T k = b K ≠ (3.14) h c
-23 in which kB is Boltzmann’s constant (kB = 1.38 x 10 J/K.mol) and h is Plack’s constant (h = 6.63 x 1034 J.s).
It can be shown that
−∆G0≠ K ≠ = e RT (3.15) in which ∆G0≠ is the standard free energy change and R the universal gas constant (R = 8.314 J/K.mol).
≠ ≠ 0≠ ≠ ≠ With Kc being equal to K and ∆G = ∆H - T∆S , the Eyring equation (in exponential form) can be obtained as given by equation 3.16.
≈ ∆S ≠ ’ ≈ ∆H ≠ ’ ∆ ÷−∆ ÷ k ∆ R ÷ ∆ RT ÷ k = T b e« ◊ « ◊ (3.16) h
When equation 3.16 is expressed in the natural logarithmic form, equation 3.17 is obtained.
≈ ≠ ’ ≈ ≠ ’ ≈ k ’ ≈ kb ’ ∆S ∆H ln∆ ÷ = ln∆ ÷ + ∆ ÷ − ∆ ÷ (3.17) «T ◊ « h ◊ « R ◊ « RT ◊
The values of the activation parameters can be determined from the graph of
R ln (hk/kBT) vs. 1/T (or from the least-squares fit of the data in equation
53 CHAPTER 3 – Synthesis and Kinetics
3.16). The slope of this graph is equal to -∆H≠, which is the standard enthalpy change of activation. The Y-intercept yields ∆S≠, which is the standard entropy change of activation.
The substitution method of an entering group is another factor that influences the law of a reaction. The different substitution mechanisms are discussed in the following section.
3.1.3.5 Mechanisms of Substitution
Substitution mechanisms can be divided into three groups: dissociative (D), associative (A) and interchange (I) mechanisms.98 At the one extreme of dissociation, a departing ligand leaves the complex and a discernible intermediate with a lower coordination number is formed. At the other extreme of association, the entering ligand adds to the complex and a discernible intermediate with an increased coordination number can be detected. Between the two extremes is the interchange mechanism, in which the entering ligand is presumed to assist the reaction, but no detectable intermediates is observed.
Dissociation In a dissociative reaction, as illustrated in Scheme 3.2,‡ loss of a ligand to form an intermediate with a lower coordination number is followed by addition of a new ligand to the intermediate.
‡ In this chapter “X” will represent the leaving ligand, “Y” the entering ligand and L the unchanged ligand
54 CHAPTER 3 – Synthesis and Kinetics
k 1 rate-determining ML5X ML5 + X k-1
k2 fast ML5+ Y ML5Y
Scheme 3.2 – Dissociative Mechanism
By assuming the steady state approximation, the rate law for a dissociative mechanism, presented by equation 3.18, can be derived.
d[ML Y] k k [ML X][Y] 5 = 1 2 5 (3.18) dt k −1[X]+ k 2 [Y]
One of the criteria for the dissociative mechanism is that the intermediate,
ML5, should be detectable during the reaction, however, direct detection at low concentrations can be very difficult and there are very few clear-cut dissociative reactions. Usually, the evidence is indirect, but no intermediate has been found. Such reactions are classified as interchange mechanisms.
Interchange In an interchange reaction a rapid equilibrium between entering ligand and reagent is established, forming a loosely bound molecular combination (or ion pair). This species is not described as having an increased coordination number and can not be directly detected, but reacts to form the product (containing the initial entering ligand) and release the previously bound ligand. Scheme 3.3 shows the mechanism for interchange.
k1 . ML5X + Y ML5X Y fast k-1
k2 . rate-determining ML5X Y ML5Y + X
Scheme 3.3 – Interchange mechanism
55 CHAPTER 3 – Synthesis and Kinetics
Assuming the steady state approximation, the rate equation for an interchange mechanism can be derived to equation 3.19.
d[ML Y] k K [M] [Y] 5 = 2 1 0 0 (3.19) dt 1+ K1[Y]0
Two variations of the interchange mechanism are Id (dissociative interchange) and Ia (associative interchange). The distinction is made according to the degree of bond formation in the first step of the mechanism. If bonding between the entering ligand and the metal is more important, the mechanism is regarded as Ia. If breaking of the bond is more important, the mechanism is
Id. The difference between the two mechanisms is very subtle and careful experimental design is required to determine which description fits a given reaction best. The common rate equation can be modified to distinguish between Ia and Id.
Association In the fist step of an associative reaction (the rate determining step) an intermediate with increased coordination number is formed. In the subsequent, faster step the leaving ligand is lost. Scheme 3.4 shows the mechanism for an associative mechanism.
k 1 rate-determining ML5X + Y ML5XY k-1
k2 fast ML5XY ML5Y + X
Scheme 3.4 – Associative Mechanism
Again, the steady state approximation is used to derive the rate law, which for an associative mechanism is given by equation 3.20.
56 CHAPTER 3 – Synthesis and Kinetics
d[ML5 Y] k 2k1[ML5 X][Y] = = k[ML5 X][Y] (3.20) dt k −1 + k 2
As with the dissociative mechanism, very few clear cut associative mechanisms are known. Most reactions fit better between the two extremes, following the dissociative or associative interchange mechanism.
The specific kinetic reactions that were studied are discussed in the successive sections.
3.1.4. Reaction Kinetics of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]
The catalytic activity of the new complex, [(IMesH2)(PPh2Cy)Cl2Ru=CHPh], was tested for ring-closing metathesis (RCM) of commercially available diethyl diallylmalonate, illustrated in scheme 3.5.99 Diethyl diallylmalonate was chosen because RCM to diethyl cyclopentene dicarboxylate (the product) is relatively facile and the reaction is slow enough to be followed by 1H NMR (in
CH2Cl2), but also fast enough to be experimentally feasible. RCM has the advantage over ring opening metathesis (ROMP) that only one propagating species is observed.
O O O O [Ru] Catalyst O O O O
CD2Cl2
Diethyl diallylmalonate Diethyl cyclopentene dicarboxylate
Scheme 3.5 – Ring Closing Metathesis of diethyl diallylmalonate99
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh] was also used in mechanistic studies involving magnetisation transfer. Since magnetisation transfer techniques are
57 CHAPTER 3 – Synthesis and Kinetics generally not well known, some theoretical aspects are addressed in the following section.
3.1.5. Theoretical Aspects of Magnetisation Transfer
Magnetisation transfer (MT) methods have been reported to be an effective technique to study initial phosphine exchange of the metathesis reaction with 100,101 ruthenium carbene catalysts. This method is especially useful, in cases like the current study, where ligand displacement can not be investigated directly in solution, since no spectroscopic change is observed when the olefin coordinates to the complex.
3.1.5.1 NMR and the Definition of MT
NMR (Nuclear Magnetic Resonance) can be observed in all nuclei with unpaired protons and/or neutrons, resulting in a non-zero spin quantum number.102 Typical nuclei used in chemical applications are 1H, 31P and 13C which are present in most organic compounds.
Magnetisation transfer is an NMR method for determining kinetics of chemical exchange by perturbing magnetisation of nuclei in a particular site or sites and following the rate at which magnetic equilibrium is restored.103 The most common perturbation are saturation and inversion, furthermore the corresponding techniques are called “saturation transfer” and ”selective inversion-recovery”. Slow chemical exchange in NMR is best studied using relaxation-type experiments.104 Under the influence of both relaxation and exchange, magnetisation in the z-axis direction causes each peak to regain its equilibrium value. This process can be studied by using either one- or two- dimensional methods with each having their own advantages.
58 CHAPTER 3 – Synthesis and Kinetics
3.1.5.2 Magnetisation
When a non-zero nuclear spin is placed in an external magnetic field, the spin interacts with the field and causes the energy levels to split into 2I + 1 sub- quantum levels where 2I +1 = 2 for spin ½ nuclei.102 This process is called Zeeman splitting. At thermal equilibrium the lower level is slightly more populated. The ratio of spins is given by equation 3.21.
−hγB −∆E n−1 0 2 = e 2πkT = e kT (3.21) n 1 2
Equation 3.21 is called the Boltzmann distribution in which h (Planck constant), (the gyromagnetic ratio) and kB (Boltzmann constant) are constants. B0 is the external field strength, T represents temperature and ∆E is the energy difference between energy states.
The energy distribution of the system can be perturbed by introducing energy quanta of the size ∆E, which in practical terms means to induce electromagnetic radiation at a frequency value of B0. In nearly all NMR systems, these frequencies are in the radio frequency (RF) range stretching from a few MHz up to 1 GHz.
In both the higher and lower energy state, precession in B0 occurs at the 102 angular frequency 0 = 2B0 called the Larmor Frequency. The lower state precesses around the +z-axis and the higher state around the –z-axis. In a system with many non-synchronised spins, the average spin vector in the xy-plane is zero while along B0 there is a net vector M0 (magnetisation). M0 is a macroscopic quantity which is a more practical measure for quantum mechanical treatment of spin distribution.
In the laboratory frame of reference, magnetisation at thermal equilibrium is usually chosen to be parallel to +z.102 Manipulation of spin distribution via radio frequency results in rotation of the magnetisation towards the xy-plane or if energy is still brought into the system, towards –z. The RF pulse 59 CHAPTER 3 – Synthesis and Kinetics synchronises the spins, reaching a condition called phase coherence. The resulting net transverse magnetisation now rotates around the z-axis at the Larmor frequency. Whenever there is net transverse magnetisation in the xy- plane, an NMR signal can be detected through induction of current to a RF coil which is tuned to the frequency of the oscillation.
3.1.5.3 Relaxation
After perturbation of the Boltzmann distribution, NMR relaxation takes place, i.e. thermal equilibrium in the spin system is recovered.102 In order to reach this equilibrium, the spins must release their excess energy to the surrounding medium (also called the lattice). Only systems oscillating at the Larmor frequency are capable of absorbing this energy. The net energy of the spin system remains the same through the whole process. As relaxation rates (Ri
= 1/Ti) are more convenient in quantitative models, these values are often used instead of relaxation times (Ti).
Longitudinal Relaxation: T1 In longitudinal relaxation, the spins of the free phosphine (in the case of this project) interact with the complex (the lattice).102 This interaction requires oscillation of the xy-component of the local field at the Larmor frequency, which can be provided through several processes, such as thermal motion of complex molecules (rotation, translation and vibration). The correlation (exchange) times of different processes vary over a wide range. Depending on the chemical interaction, intramolecular correlation time can be a few picoseconds or in the time scale of seconds.
Measurement of Relaxation Times The traditional way of measuring longitudinal relaxation is by use of an inversion recovery sequence.102 Magnetisation is inverted to –z and allowed to relax for time Ti (the inversion time). The magnetisation is then rotated to
60 CHAPTER 3 – Synthesis and Kinetics
the xy-plane and measured. The signal behaviour as a function of Ti is given by equation 3.22.
−RiTi S(Ti ) = S(∞)(1− ke ) (3.22)
in which S is the angular momentum (or quantum number) of the atom and Ri
(1/Ti) is the longitudinal relaxation rate that needs to be determined. Ideally k should have a value of 2, but in practice k ≈ 1.95, because of pulse imperfections and other disturbances.
After the necessary measurements have been done, the exchange rates can be determined as described in the next section.
3.1.5.4 Determination of Exchange Rates
Exchange rates are measured by extracting the parameters of the mathematical model of the data.104 To obtain a good fit to experimental data, the model must converge on the best set of parameters, i.e. a global minimum in the error surface must be found, which can be a difficult task for a large number of parameters (usually more than 10). Furthermore, the parameter errors must be estimated and interpreted.
Rates can be determined via one-dimensional or two-dimensional methods. The one-dimensional method was used in the current study, because it has various advantages which include
• Areas (1D) are easier to measure than volumes (2D). • 1D methods require less spectrometer time and disk space. • 1D methods give more scope for controlling the experiment.
For many complex chemical exchange problems one-dimensional methods have been reported to give rate data more effectively than two-dimensional methods104 and with good experimental design and a good data-fitting
61 CHAPTER 3 – Synthesis and Kinetics program, values of parameters as well as associated errors can be reliably estimated.
3.1.5.5 Data Fitting and Error Analysis
McClung’s formulation105 has been used to set up the matrix which contains relaxation rates as well as exchange rates. The parameters in this model are these rates and the magnetisations at t0 (time zero) and t∞ (at equilibrium). Not all of these parameters are independent, since the exchange mechanism and the principle of detailed balance will determine relative sizes of magnetisations at equilibrium.
The analysis of a data set which follows coupled relaxation is usually done in one of two ways. One way is to guess at exchange rates along with other parameters and calculate a model set of data.106 From discrepancies between observations and the model, a new set of parameters can be calculated and iterated until it converges. Another way to analyse the set of data is to invert and directly solve the equation for the matrix elements.107 The first method was used in our investigation, because it has been reported to be more robust in the presence of noise and to give a good way of estimating parameters and associated errors in a non-linear system.104 CIFIT,104 a C-program based on McClung’s program SIFIT, was used to model the system and do iterations, which involves using McClung’s values for derivatives of the data with respect to the parameters and implementing the Marquardt algorithm108 with modifications due to Fletcher, 109 and to Meyer and Roth.109
The full set of errors for all parameters was approximated by the variance– covariance matrix of the fit to the data. The critical value of “badness of fit” is hard to define for a general non-linear case, however, the value for the linear case, given in equation 3.23, is often used in practice. The algorithm provides a general method for estimating the error for a single parameter.
62 CHAPTER 3 – Synthesis and Kinetics
» p ÿ critical _value = …1+ F (p,n − p)Ÿ (3.23) n − p 95% ⁄ in which n is the number of data points, p is the number of parameters and
F95% is the usual F statistic.
Specific aspects of the current study are presented in the following section.
3.1.5.6 MT experiments of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]
An understanding of initial ligand substitution in the olefin metathesis reaction is essential, because it is this reaction step that allows the catalyst to enter the catalytic cycle. Scheme 3.6 shows the phosphine exchange reaction of
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh], which was used as model for ligand substitution.
N N N N k Cl Cl PPh2Cy* + Ru Ru + PPh2Cy Cl H Cl H
PPh2Cy PPh2Cy* Scheme 3.6 – Exchange of free and bound phoshine
The model of the system under investigation consisted of 7 parameters: the free and bound phosphine each had a relaxation rate (1/T1) as well as values for magnetisation at the start (M0) and equilibrium (M∞) of the reaction. In addition the 7th parameter was the rate of exchange that needed to be determined.
31P NMR peak heights rather than the integrals were used as data, because it can be more accurately and conveniently measured, provided that well- shaped peaks are obtained and that the peak width does not change. Even
63 CHAPTER 3 – Synthesis and Kinetics
though M∞ should be the same for all the measured systems, these values are slightly different because of difference in width.
A series of selective inversions was performed on the free phosphine using the simple /2 - τ - /2 excitation sequence, in which both shaped pulses were soft.
The results of the MT experiments are discussed in section 3.2.4.2.
The MT experiments concluded studies involving
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh], but kinetic investigations were continued with the standard first generation Grubbs catalyst ([(PCy3)2Cl2Ru=CHPh)]) and a range of functionalized olefins, as discussed in the next section.
3.1.6. Coordination of Functionalized Olefins to Grubbs1- PCy3
Except for properties of the catalyst itself, the reagents in the metathesis reaction also influence the nature and efficiency of the reaction. Steric bulk, geometry differences as well as electronic effects play a major part. Bulkier olefins were found to react slower (with trans-[(PCy3)2Cl2Ru=CHR] initiators),110 as were trans-internal olefins compared to their cis-counterparts.
In a very recent theoretical study,111 reactions of ethyl vinylether and norborene (with Grubbs1-PCy3 and Grubbs2-PCy3) were exergonic by 8-15 kcal/mol and excess energy was released after passing through the metallacyclobutane structure, making the reactions irreversible. On the other hand thermoneutral reactions, as in the case of ethylene, are highly reversible and their reverse reaction rate accounts for their low reactivity, even though their reaction rates in the forward reactions may be very high.
In the current study coordination of olefins containing oxygen or nitrogen to the ruthenium carbene catalyst was evaluated and reported in section 3.2.5. This coordination is considered from a theoretical perspective in Chapter 4. 64 CHAPTER 3 – Synthesis and Kinetics
The last part of the kinetic study was to evaluate the Sasol metathesis catalyst, Phobcat, for the cross metathesis of 1-octene.
3.1.7. Octene Metathesis
Phobcat,88 the Sasol metathesis catalyst containing two rigid bicyclic phosphine ligands, has proved to be much more active, selective and stable than traditional first and second generation Grubbs catalysts (See section 2.10). To illustrate the enhanced activity and stability, this section reports the comparison of Phobcat ([(PhobCy)2Cl2Ru=CHPh]) and Grubbs1-PCy3
([(PCy3)2Cl2Ru=CHPh)]) for the cross metathesis (CM) reaction of 1-octene. Results are reported in section 3.2.6.
3.2. Experimental Section
3.2.1. Materials and Apparatus
Air and water sensitive compounds were manipulated using standard Schlenk techniques under an atmosphere of dry argon or in a nitrogen-filled vacuum atmosphere drybox.
All chemicals used were of analytical grade or better. Grubbs1-PCy3 was obtained from Fluka, Grubbs 2 from Sigma Aldrich and Phobcat88 was provided by Sasol. The olefins that were used in section 3.2.4.1, namely diethyl diallylmalonate (Sigma Aldrich); section 3.2.5, namely vinylacetate (Aldrich), allylacetate (Fluka) and allylcyanide (Fluka); and in section 3.2.6, namely 1-octene; were used as received from commercial sources. Dry solvents were used throughout the procedure and all glassware was oven dried.
NMR measurements were done on a Varian Unity INOVA 500 MHz 31 1 instrument. P spectra were referenced to H3PO4 (δ = 0 ppm) and H spectra
65 CHAPTER 3 – Synthesis and Kinetics
to benzene-d6 (δ= 7.15 ppm), unless otherwise stated. Each peak reported was categorised as a singlet (indicated by s), doublet (d), triplet (t) or multiplet (m). All peak shifts (δ) are given in parts per million (ppm) and coupling constants (J) are reported in Hertz.
3.2.2. Attempted Synthesis of a Grubbs1-Type Catalyst
3.2.2.1 Phosphine Addition
The synthesis of [(PPh2Cy)2Cl2Ru=CH-CH=CMe2] was first attempted by 89 following the method for the preparation of [(PPh3)2Cl2Ru=CH-CH=CMe2] according to literature procedure. Instead of using PPh3 however, PPh2Cy was added as ligand (see Scheme 3.7).
RuCl3.3H2O (0.1 g, 0.38 mmol) was dissolved in degassed methanol (25 ml) and refluxed under nitrogen for 10 minutes. The solution was cooled before
PPh2Cy (0.6 g, 2.2 mmol) was added.
Addition of PPh2Cy to the starting material resulted in decomposition after about an hour. A coal like black residue in the flask and a silver coating on the inner surface of the flask was the result, indicating lower oxidation states of ruthenium. The tar-like residue was insoluble and no NMR characterization was carried out.
The direct addition of PPh2Cy being unsuccessful, [(PPh3)2Cl2Ru=CH- 89 CH=CMe2] was synthesised according to the reported method to serve as starting material for the phosphine exchange reaction with PPh2Cy.
3.2.2.2 Phosphine Exchange of [(PPh3)2Cl2Ru=CH-CH=CMe2] and PPh2Cy
The synthesis of [(PPh3)2Cl2Ru=CH-CH=CMe2] was carried out in a three-step synthesis according to literature procedures (summarised in equations 3.24 – 3.26).
66 CHAPTER 3 – Synthesis and Kinetics
112 RuCl3.3H2O + n(C6H5)3P → [RuCl2{P(C6H5)3}3] (3.24)
113 RuCl2{P(C6H5)3}3] + H2 + base → [RuClH{P(C6H5)3}3] + base HCl (3.25)
89 [RuClH{P(C6H5)3}3] + CHC-CCl(CH3)2 → [(PPh3)2Cl2Ru=CH-CH=CMe2] (3.26)
112 [RuCl2PPh3] was prepared from [RuCl3.3H2O] and used as starting material 113 1 to produce the corresponding hydride, [RuClHPPh3], as confirmed by H
NMR. [RuClHPPh3] was treated with 3-chloro-3-methyl-1-butyne to yield the 89 1 final product [(PPh3)2Cl2Ru=CH-CH=CMe2] as confirmed by H NMR and 31P NMR. Each step is discussed individually in the following paragraphs.
Synthesis of Tris(Tripehenylphosphine)dichlororuthenium(II): 112 [RuCl2{P(C6H5)3}3]
RuCl3.3H2O (1 g, 3.8 mmol) was dissolved in 250 ml degassed methanol and refluxed under nitrogen for 10 minutes (instead of 5 min) because the solution decomposes easily upon addition of the phosphine. The solution was cooled before triphenylphosphine (6.125 g, 23.4 mmol) was added. In order to prevent decomposition, it is important that the solution is cooled sufficiently (to room temperature) before the ligand is added. The product was a red-brown compound (confirmed by the chemical dictionary and NMR results) with a yield of 2.7 g (74%), rather than shiny black crystals as the prescribed procedure suggests.
31 P NMR (C6D6): = -5.9 ppm (s, free PPh3), 41.7 (s, bound PPh3), 56.0 (s, bound PPh3). The two bound peaks are due to partial dissociation of the complex. These NMR results are consistent with literature findings.114
67 CHAPTER 3 – Synthesis and Kinetics
Synthesis of Hydridochlorotris(triphenylphosphine)ruthenium(II): 113 [RuClH{P(C6H5)3}3] Method A in the procedure of Hallman et.al.113 was followed, however weight values were reduced three times. [RuCl2{P(C6H5)3}3] (0.5 g, 0.5 mmol), trimethylamine (0.08 ml) and dry benzene (50 ml, distilled from sodium metal) were placed in a 100 ml Schlenk tube. The mixture was frozen with liquid nitrogen and the container was evacuated on a high vacuum line in order to reduce all oxygen. The container was refilled with hydrogen gas (for about 15 min), while sustaining the low temperature to condense as much gas as possible. The Schlenk tube was then closed off, left to thaw and stirred overnight. The Schlenk tube was refilled with hydrogen once (the following morning) to ensure the completion of the reaction. Upon stirring the colour of the mixture changed from violet-pink to violet-red within an hour. In the end of the reaction a dark pink mixture was obtained, which was reduced in volume under vacuum, washed three times with degassed ether and dried under vacuum. A dark pink product, [RuClH{P(C6H5)3}3], was obtained in a yield of 0.31 g (64.3%).
1 H-NMR (C6D6): = -17.6 ppm (q, 1H, RuClH), 0.2 – 2.8 (m, 11H, Ar-H), 6.8 – 7.8 (m, 34H, Ar-H)
89 Synthesis of the Ruthenium carbene [(PPh3)2Cl2Ru=CH-CH=CMe2]
The solution of [RuClH{P(C6H5)3}3] (0.2 g, 0.2 mmol) in dry CH2Cl2 (3 ml) was cooled with dry ice and acetone under vacuum. 3-chloro-3-methyl-1-butyne
(27 µl, 0.22 mmol) in dry CH2Cl2 (1 ml) was added via vacuum transfer within 5 minutes of stirring. The solution was allowed to warm while being stirred for 1.5 hours after which the volume was reduced slightly. Hexane (4 ml) was vacuum transferred into the container and the mixture was stirred for 20 min to produce the dark brown powder in a low yield of 0.025 g (15%).
1 H-NMR (CD2Cl2): = 0.99 ppm (s, 3H, CH3), 1.25 (s, 3H, CH3), 7.33 – 7.61
(m, 31H, Ar-H, CH=C(CH3)3), 18.10 (s, 1 H, Ru=CH);
68 CHAPTER 3 – Synthesis and Kinetics
31 P (CD2Cl2): = 27.1 ppm.
Phosphine exchange in [(PPh3)2Cl2Ru=CH-CH=CMe2] by PPh2Cy
In the phosphine exchange reaction of [(PPh3)2Cl2Ru=CH-CH=CMe2] and
PPh2Cy the bound ligand could not be substituted by the free phosphine. The reaction mixture was stirred overnight and heated slightly, but remained unchanged. 31P NMR-analysis confirmed that substitution did not occur.
3.2.2.3 First Generation Catalyst with Arsine Ligand
115 Synthesis of [RuClH(AsPh3)3]
In contrast with the synthetic route to yield [RuClH(PPh3)3], the analogous arsine containing hydride, [RuClH(AsPh3)3], could be synthesised directly from
[RuCl3.H2O], making this alternative more convenient and efficient.
RuCl3.3H2O (0.15 g, 0.6 mmol), AsPh3 (0.94 g, 3.0 mmol) and NaBH4 (0.23 g, 6.0 mmol) was stirred under vacuum in degassed ethanol (20 ml) for an hour to yield a green-brown product, which was washed consecutively with degassed ethanol, degassed water, degassed ethanol and degassed diethylether. A yield of 0.17 g (74%) was obtained. Characterisation was done via proton NMR.
1 H-NMR (C6D6): = -12.0 ppm (s, 1H, RuClH), 0.0-2.0 (m, 2H, Ar-H), 6.4-8.0 (m, 39H, Ar-H), 10-11 (broad s, 4H, Ar-H)
The hydride was treated with 3-chloro-3-methyl-1-butyne (19 µl, 0.2 mmol in 1 89 ml CH2Cl2, as recommended for [RuClHPPh3] ) with the purpose of yielding
[(AsPh3)2Cl2Ru=CH-CH=CMe2], but unfortunately this synthesis variation was unsuccessful and the reaction mixture decomposed to a black-brown residue.
1 H-NMR (CD2Cl2): = 0.8-2.0 (m),. 2.9 (s), 6.8-7.6 (m), 10-11 ppm (broad s)
69 CHAPTER 3 – Synthesis and Kinetics
Scheme 3.7 is a summary of the attempted synthetic procedures for a first 89,112,113,115 generation Grubbs catalyst.
[(AsPh3)Cl2Ru=CH-CH=CMe2]
Cl
HC CH3
CH3
[RuClH(AsPh ) ]115 3 3 [(PPh2Cy)Cl2Ru=CH-CH=CMe2]
+ H2 + AsPh3 + base + PPh2Cy Cl
HC CH3 + PPh + H + base [RuCl .3H O] 3 [RuCl (PPh ) ]112 2 113 CH3 89 3 2 2 3 3 [RuClH(PPh3)3] [(PPh3)Cl2Ru=CH-CH=CMe2]
+ PPh2Cy + PPh2Cy
[RuCl2(PPh2Cy)3] [RuCl2(PCy2Ph)3]
Scheme 3.7 – Attempted synthesis of Grubbs 1 derivatives
3.2.3. Synthesis of the 2nd Generation Grubbs Type Catalyst [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]
[(IMesH2)(C5H6N)2Cl2Ru=CHPh] was prepared from the second generation Grubbs catalyst according to literature procedure:101 Grubbs 2
([(IMesH2)(PCy3)2Cl2Ru=CHPh], 0.2 g, 0.2 mmol) was dissolved in a minimum amount of toluene under vacuum and pyridine (2 ml) was added. The solution was stirred for 10 minutes, during which the colour changed from purple to bright green. The solution was then cannula transferred into cold pentane (7.5 ml, 0 oC) under vigorous stirring. The green precipitate was filtered, washed four times with pentane and dried under vacuum. The isolated yield was 0.07 g, 71%.
[(IMesH2)(C5H5N)2Cl2Ru=CHPh] (0.15g, 0.21 mmol) was then stirred with diphenyl-cyclohexylphosphine (0.076 g, 0.28 mmol) in benzene (10 ml) for 10 minutes. The volume of the brown solution was then reduced to 2 ml after which 5 ml of pentane was added. The solution was cooled with ice to 0 oC
70 CHAPTER 3 – Synthesis and Kinetics and stirred vigorously for 30 minutes to precipitate the product,
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh], which was a light brown powder obtained in a yield of 0.10 g (53%).
1 H-NMR (C6D6, 298 K): ∂ = 0.6 – 1.22 ppm (m, 11 H, C6H11 on PPh2Cy), 1.5
– 2.2 (m, 18 H, CH3 on IMes), 5.8 – 6.5 (d, 4H, C6H6 of IMes, J = 21), 6.6 – 7.2
(m, 15H, C6H5 of PPh2Cy and =CPh), 19.233 (d, 1H, Ru=CH, J = 27); 31 P-NMR (C6D6, 298 K): ∂ = 39.4 ppm (s, PPh2Cy).
3.2.4. Kinetic experiments of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]
3.2.4.1 Reaction Kinetics
The ring closing metathesis (RCM) reaction of diethyl diallyl malonate (with a concentration of 0.2 M) was catalysed by [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]
(with a concentration of 0.02 M) in an NMR tube with CD2Cl2. The reaction was monitored via NMR at 303 K by monitoring the disappearance of the reagent peaks and the growth of the product peaks.
1 Diethyl diallylmalonate H(CD2Cl2): 1.2 ppm (t, 6H, CH3 on ethyl), 2.6 (d, 4H,
CH2 on allyl), 4.2 (m, 4H, CH2 on ethyl), 5.1 (m, 4H, =CH2 on allyl), 5.8 (m, 2H, =CH).
1 Diethyl cyclopentene dicarboxylate H(CD2Cl2): 1.2 ppm (t, 6H, CH3 on ethyl), 3.0 (s, 4H, CH2 on cyclopentene), 4.2 (m, 4H, CH2 on ethyl), 5.6 (s, 2H, =CH).
Change of the malonate peak at 2.6 ppm (d, 4H, CH2) was the easiest to monitor, since the peak was well defined and the corresponding product did not have a peak in the region 1.5 – 2.9 ppm. The growing product peak at 3.0 ppm (s, 4H, CH2) was used to evaluate product formation.
71 CHAPTER 3 – Synthesis and Kinetics
3.2.4.2 Magnetisation Transfer Experiments
Phosphine exchange rates were determined by 31P NMR magnetisation 104 transfer (MT) experiments at 353 K in C6D6. The free phosphine resonance was selectively inverted using shaped pulses, calculated by the Pbox program of VNMR, and the 31P NMR spectra were recorded after variable mixing times (ranging between 5 x 10-5 – 40 s) for a series of free phosphine concentrations (0.037 M – 0.137 M). The time-dependant magnetisation data were analysed using the computer program CIFIT,104 to obtain the rate constant, k, for the exchange between bound and free phosphine in each reaction.
3.2.5. Coordination of Functionalized Olefins to Grubbs 1- PCy3
Catalyst-olefin coordination reactions were measured on a UV/VIS spectrophotometer and a Varian Cary 100 spectrophotometer with 1.000 ± 0.001 cm path length tandem quartz cells. The spectrophotometer was equipped with constant temperature cell holders (accurate within 0.1 °C) and Jalabu MPV thermostatted water baths (accurate within 0.05 °C) fitted with circulators. All temperatures are reported to ± 0.1 °C accuracy. Measurements were done at a wavelength of 334.0 nm for allylacetate, 334.1 nm for vinylacetate and 340.0 nm for allylcyanide in dichloromethane at room temperature (298 K) with olefin concentrations varying from [olefin] = 0.002 M to 0.1 M, but catalyst concentration kept constant at [Ru] = 1.6 x 10-4 M.
After measurements were completed, the non-linear least squares program Scientist116 was used to do all fits and rate constant calculations. The absorbance vs. time data was fitted to the first order rate equation in order to obtain the kobs-value of each reaction. These kobs–values were plotted against olefin concentration and exhibited limiting kinetics discussed in section 3.3.3.
The rate constant of the second reaction, k2, and equilibrium constant for the initial coordination reaction, K1, could be determined.
72 CHAPTER 3 – Synthesis and Kinetics
A rate law similar to that in equation 3.19 was assumed and equation 3.24 was used for fitting data.
k k [Ru][olefin] = 1 2 kobs k −1[PR3 ] + [olefin]
k [Ru][olefin] (3.24) = 1 K1[PR3 ] + [olefin]
k −1 With K1 = k 2
3.2.6. Octene Metathesis
Gas Chromatography GC analyses were performed on a Shimadzu QP2010 instrument using the conditions given in Table 3.2.
Table 3.2 – Conditions for Gas Chromatographic analysis of reaction mixture samples Column ZB-1 (7HG-G001-11) Measurements 30 ml x 0.25 mm ID x 0.25 m FT S/N
Injection Temperature / oC 200 Injection Volume / l 0.5 Split Ratio 200:1 Pressure / kPa 120.1 Total flow / ml.min-1 342.4 Column Flow / ml.min-1 0.70 Linear Velocity / cm.s-1 36.6
Washing Solvent CH2Cl2
o Detector FID at 300 C
73 CHAPTER 3 – Synthesis and Kinetics
Makeup Gas / ml.min-1 Helium Makeup flow / ml.min-1 30.0 -1 H2 flow / ml.min 50.0 Air flow / ml.min-1 400.0
The following temperature program was used:
300 oC
30 oC/min 150 oC
40 oC 10 oC/min
Hold 5 min
Method The experimental set up and method that was used is illustrated in Figure 3.1. Nitrogen was passed through 1-octene while being stirred and heated in an oilbath kept at a constant temperature. The temperature inside the flask was measured with a thermometer and the upright neck of the flask was fitted with a reflux condenser to prevent loss of the reaction mixture. The catalyst was added as a solid (by quickly lifting the reflux condenser) and the clock was started at the same time. 0.5 ml samples were drawn from the catalytic mixture via syringe at appropriate time intervals and transferred to GC vials with 0.5 ml toluene, which served as internal standard, with a few drops of hydrogen peroxide added to quench the reaction. These samples were analysed by use of GC methods (see Table 3.2) and the data was calculated with the program GC Postrun Analysis.117
74 CHAPTER 3 – Synthesis and Kinetics
Cooling Water (~8-10oC) Thermometer
Oilbath
Heater/Stirrer 5 6 5 6 4 7 4 7 3 8 3 8 2 9 2 9 1 11 1 10
Figure 3.1 – Experimental setup for the catalysed Cross Metathesis of 1-octene
3.3. Results
3.3.1. Reaction Kinetics of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]
The catalysed RCM reaction was followed by monitoring product formation (and substrate disappearance) over time. The plots of both reagent consumption and product formation confirmed a first order reaction as represented by equation 3.25 (the integrated form of equation 3.5.1).
−kt [A] = [A 0 ]e (3.25)
Figure 3.2 shows the trend of reagent consumption. It was verified that reagent consumption could be correlated with product formation. Experimental and calculated values of data points are given in Appendix A.
75 CHAPTER 3 – Synthesis and Kinetics
140
120
100 t h g
i 80 e h
k a
e 60 P
40
20
0 0 100 200 300 400 500 t / s
Figure 3.2 – Kinetic results from NMR measurement of diethyl diallylmalonate consumption (by monitoring the peak at = 2.6 ppm (d, 4H, 2xCH2)) in the RCM reaction catalysed by [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]; represents the first -3 -1 order fit; k = (9 ± 0.09) x 10 s ; [diethyl malonate]0 = 0.2 mM, [Ru] = 0.01 mM;
T = 303 K; solv. = CD2Cl2
The kinetic study of the ring closing metathesis of diethyl diallyl malonate could successfully be monitored by NMR techniques. Results show that
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh] is an active catalyst for the RCM reaction, but the reaction is relatively slow with a rate constant of k = (9 ± 0.09) x 10-3 s-1. Similar RCM reactions of first generation ruthenium catalysts were reported by Sanford et. al.,101 but unfortunately rates could not be compared, since second order reaction rates were obtained in their study.
76 CHAPTER 3 – Synthesis and Kinetics
3.3.2. MT Experiments on the exchange of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] and PPh2Cy
3.3.2.1 NMR Measurements
Figure 3.3 shows an array of NMR-spectra for
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh] and free PPh2Cy with various mixing times ranging between 5.0x10-5 – 30 s, with specific periods of individual spectra being A = 5.0x10-5 s, B = 0.05 s, C = 0.1 s, D = 0.2 s, E = 0.5 s, F = 1 s, G = 2 s, H = 4 s, I = 8 s, J = 15 s, K = 25 s.
Three peaks are observed in all the spectra, namely that of the phosphine bound to the complex (at = 37 ppm) the phosphine oxide (at = 29 ppm) and the free phosphine (at = -4 ppm). Spectrum A (with a mixing time of t = 5 x 10-5 s) represents the first measurement in which the mixing time is very short in order to prevent any relaxation from taking place and allowing the peak of the free phosphine to remain totally inverted. As the mixing time increases with each spectrum (A to K), the peak of the free phosphine is turned towards the positive axis and in K (t = 25 s), the last spectrum, the mixing time is longest, exchange is complete and the free phosphine peak is totally relaxed. Ideally the absolute values of the first and last phosphine peak should be equal. While the free phosphine peak goes from –z to +z (minimum to maximum), the peak of the bound phosphine starts at a maximum, is reduced (in this case even to negative values) and then increased to a maximum again. Once more the first and last peak values must ideally be identical.
A phosphine oxide peak is also present in each spectrum, but as seen from Figure 3.3, MT had no effect on the peak and the oxide was regarded to be inert towards the system under consideration, in fact the phosphine oxide provided a well-defined internal reference. To compensate for the loss of phosphine due to the oxide, the integrated peak value of the oxide was subtracted from the integrated value of the free phosphine peak. The real free phosphine concentration was found to be 0.067 M.
77 CHAPTER 3 – Synthesis and Kinetics
-4 ppm
37 ppm 29 ppm
Figure 3.3 – Stacked 31P-NMR spectra from MT-experiments showing the change in phosphine peak heights representing free PPh2Cy ( = - 4 ppm) and bound PPh2Cy ( = 37 ppm) of the complex [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]; the phosphine oxide ( = 29 ppm) was left unaffected; [Ru] = 0.023 M, [free PPh2Cy] = 0.067 M; Mixing Times / s (A-K) = 5x10-5, 0.05, 0.1, 0.2, 0.5, 1, 2, 4, 8, 15, 25; solv. = C6D6; T = 373 K
The appropriate peak heights of each spectrum were measured, analysed and plotted. The data plot from the array of spectra in Figure 3.3 is shown in Figure 3.4. Values of individual data points are given in Appendix B. The process was repeated for various concentrations ranging from 0.027 M – 0.137 M.§
§ These values were corrected to compensate for the loss of free phosphine due to the formation of the phosphine oxide.
78 CHAPTER 3 – Synthesis and Kinetics
3.3.2.2 Data Fitting and the Rate Constant
Figure 3.4 shows the graphic presentation of the change in peak heights for the bound and free phosphine.
80
60
40
t 20 h
g i e h 0 k
a 0 5 10 15 20 25 e
P -20
-40
-60
-80 Mixing time (s)
Figure 3.4 – Change in NMR peak height with increasing mixing time for bound PPh2Cy of the complex [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] (indicated with ) and 104 free PPh2Cy (indicated with ∆), represents the fitted plot; Average k = 4.1 ± -1 0.9 s ; [Ru] = 0.023 M, [free PPh2Cy] =0.067 M; solv. = C6D6; T = 353 K
The rate constant for each concentration of free phosphine was calculated by the curve-fitting procedure. An average rate constant of k = 4.1 ± 0.9 s-1 was calculated, which was found to be independent of the free phosphine concentration. The same course of action was taken to determine the rate constant at different temperatures (343 – 373 K). Table 3.2 shows the rate constant at different concentrations of free phosphine.
79 CHAPTER 3 – Synthesis and Kinetics
Table 3.2 – Rate constants at different Concentration / M k / s-1 0.027 4.0 ± 0.1 0.067 5.0 ± 0.1 0.137 3.4 ± 0.1
For the peak height of the bound phosphine the graph shows an initial rapid decrease and a slower increase as the peak relaxes to its original value. For this peak, values at the beginning and end of the change are identical, showing that full phosphine exchange took place and that the NMR method used could measure the exchange successfully.
The graph also shows the change in free phosphine peak height – from being fully inverted to being fully relaxed. The absolute peak height values of the free phosphine were sufficiently similar at the beginning and end of the exchange, even though the absolute starting value was higher than the absolute value at full relaxation. This is most likely due to erroneous measurement at the beginning of the exchange, during which time inversion was taking place at a rapid rate. In addition, the first mixing time was very short, i.e. 5 x 10-5 s, and close to the lower limit of time intervals within the capacity of the spectrometer.
The experimental data fitted very well to the calculated plot and an error of ± -1 0.1 s on the value of rate constant was obtained for individual fits.
3.3.2.3 Activation Parameters
≠ Rln(hk/kBT) was plotted against 1/T (the Eyring plot) to obtain ∆H directly from the slope and ∆S≠ directly from the y-intercept. The Eyring plot is shown in Figure 3.5 and values of individual data points are given in Appendix C.
80 CHAPTER 3 – Synthesis and Kinetics
-215 0.00265 0.00270 0.00275 0.00280 0.00285 0.00290 0.00295
-220 1 - e l
o -225 m . 1 - y = -83672x + 3.885 K . 2 J
R = 0.9728 /
) -230 T B k / k h ( -235 n l
R
-240
-245
T -1 / K-1
Figure 3.5 – Eyring plot for PPh2Cy exchange of
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh]; [Ru] = 0.023 M, [free PPh2Cy] = 0.067 M; solv. =
C6D6; T = 343 K – 373 K
Table 3.3 contains the calculated rate constants obtained from the curve fitting procedure at the different temperatures.
Table 3.3 – Rate constants of the phosphine exchange reaction (shown in Scheme 3.6) at different temperatures T / K k / s-1 343 2.4 ± 0.1 353 4.1 ± 0.9 363 9.8 ± 1.0 373 27.4 ± 5.6
Table 3.4 contains the activation parameters as determined from the Eyring plot in Figure 3.5. These values are compared to activation parameters of similar complexes in literature in the discussion section 3.4.2.
81 CHAPTER 3 – Synthesis and Kinetics
Table 3.4 – Activation parameters obtained from the plot in Figure 3.5 ≠ ∆H / kJ.mol-1 83.7 ± 2.3 ≠ ∆S / J.K-1.mol-1 3.9 ± 0.1
3.3.3. Coordination of functionalized Olefins to Grubbs 1- PCy3
The coordination of vinyl acetate, allyl acetate and allyl cyanide to Grubbs1-
PCy3 was followed, analysed and is represented in Figure 3.6, showing the graph of kobs vs. olefin concentration for the different reactions. Values of individual data points are given in Appendix D.
0.050
0.040
0.030 1 - s
/
s b o k 0.020
0.010
0.000 0.000 0.020 0.040 0.060 0.080 0.100 [Olefin] / M
Figure 3.6 – kobs vs. [olefin] for the olefin-to-catalyst coordination of the catalyst [(PCy3)2Cl2Ru=CHPh] and olefins Allyl acetate (•) measured at = 334.0 nm, Vinyl acetate () at = 334.1 nm, Allyl cyanide () at = 340.0 nm;
116 k 1[Ru][olefin] represents the fitted plot to the equation k obs = ; [olefin] = K 1[PR 3 ] + [olefin] 0.02 – 0.10 M, [Ru] = 1.6 x 10-4 M; T = 298 K; solv. = DCM
82 CHAPTER 3 – Synthesis and Kinetics
The plot in Figure 3.6 shows saturation kinetics for the three olefins. Different plateaus for the different entering olefins were obtained, suggesting a two- step rate law which might indicate a rapid pre-equilibrium, followed by the second rate-determining step in which the olefin is fully coordinated and the phosphine ligand is eliminated. This mechanism is represented by equation 3.24.
Table 3.5 shows the rate and equilibrium constants for the reaction at 298 K.
Table 3.5– Rate and Equilibrium constants for olefin coordination to Grubbs1-
PCy3; values obtained from least-squares fit of the data in Figure 3.6; [olefin] = 0.02 – 0.10 M, [Ru] = 1.6 x 10-4 M; T = 298 K; solv. = DCM;
k 1[Ru][olefin] k obs = K 1[PR 3 ] + [olefin]
2 -1 -1 Olefins 10 k2 / s K / M Allyl acetate 6.2 ± 0.3 28 ± 3 Vinyl acetate 3.6 ± 0.2 35 ± 5 Allyl cyanide 2.7 ± 0.2 31 ± 5
The equilibrium constants for the initial (fast) coordination reactions are comparable, but the reaction rates for the second rate determining step, k2, differ somewhat. k2 decreases in the order allylacetate > vinylacetate > allylcyanide. Furthermore, relatively small error values were obtained in the fitting, validating the suitability of equation 3.24.
3.3.4. Octene Metathesis
Phobcat ([(PhobCy)2Cl2Ru=CHPh]) was evaluated against Grubbs1-PCy3
([(PCy3)2Cl2Ru=CHPh)]) in the cross metathesis reaction of 1-octene. Figure 3.8 (A) and (B) show the Gas Chromatography spectra of the cross metathesis reaction of 1-octene converting to tetradecene as catalysed by
Grubbs1-PCy3 and Phobcat respectively. The major peaks correlate to 1- octene (at a retention time of 6.1 min); the internal standard, i.e. toluene (at
83 CHAPTER 3 – Synthesis and Kinetics
7.1 min); and the isomers of tetradecene (in the region of 18 min). All peak positions have been confirmed by individual runs of the compounds.
6.0
5.0
t h
g 4.0 i e
H 3.0
k
a 2.0 e P 1.0
0.0
2.5 5.0 7.5 10.0 12.5 15.0 17.5 t / m in (A)
6.0
5.0
t
h 4.0 g i e 3.0 H
k
a 2.0 e P 1.0
0.0
2.5 5.0 7.5 10.0 12.5 15.0 17.5 t / m in (B)
Figure 3.7 – GC spectra of the cross metathesis reaction of 1-octene to tetradecene after 6 hours catalysed by (A) Grubbs1-PCy3
([(PCy3)2Cl2Ru=CHPh)]) and (B) Phobcat ([(PhobCy)2Cl2Ru=CHPh]); Retention times: 1-octene = 5.8 min, toluene (int. std.) = 7.0 min, cis-tetradecene = 17.73 min, trans-tetradecene = 17.77 min
84 CHAPTER 3 – Synthesis and Kinetics
Figure 3.8 presents a graphical comparison of the data collected from the GC analysis for the reactions catalysed by Grubbs1-PCy3 and Phobcat. The values of the individual data points are given in Appendix E.
100.0
90.0
80.0 %
/ 70.0
d l e i 60.0 Y
e
n 50.0 e c e
d 40.0 a r t
e 30.0 T 20.0
10.0
0.0 0 50 100 150 200 250 300 350 t / min
Figure 3.8 – Comparison between Phobcat ([(PhobCy)2Cl2Ru=CHPh]) and
Grubbs1-PCy3 ([(PCy3)2Cl2Ru=CHPh)]) for the cross metathesis reaction of 1- octene (tetradecene was assumed to be the only product): – total yield of tetradecene for Phobcat; – total yield of tetradecene for Grubbs1-PCy3; [Ru] = 0.014 M; T = 323 K; No additional solvent was used
The following observations were made by inspecting Figure 3.8: Grubbs yielded ± 10-30% tetradecene, while Phobcat performed much better with ± 70-90% conversion. The horisontal line for the largest part of the
Grubbs1-PCy3 curve indicates that the catalyst decomposed within 30 minutes, while the continuing slope of the Phobcat line shows that the catalyst was still active after 6 hours. The results are summarised in Table 3.6.
85 CHAPTER 3 – Synthesis and Kinetics
Table 3.6 – Comparison of the two cross metathesis reactions of 1-octene with
Phobcat ([(PhobCy)2Cl2Ru=CHPh]) and Grubbs1-PCy3 ([(PCy3)2Cl2Ru=CHPh)]) as catalysts respectively Grubbs 1 Phobcat
Total C14 yield 10-30% 70-90% Active Period < 30 min > 350 min
3.4. Discussion
3.4.1. Synthesis of 1st and 2nd Generation Grubbs Catalyst
Although various routes were attempted in order to synthesise a first generation Grubbs catalyst, none were successful. The different methods are discussed briefly.
Upon addition of PPh2Cy to [RuCl3.3H2O], the reaction mixture decomposed. A coal like black residue in the flask and a silver coating on the inner surface of the flask was the result, indicating lower oxidation states of ruthenium. The tar-like residue was insoluble and no NMR characterisation was carried out.
The attempted phosphine substitution reaction involving the complex
[(PPh3)2Cl2Ru=CH-CH=CMe2] and free phosphine, PPh2Cy, did not yield any products. The analogous complex [(PCy2Ph)2Cl2Ru=CH-CH=CPh2] is known from literature and was prepared via phosphine exchange from 92 [(PPh3)2Cl2Ru=CH-CH=CPh2]. However, it was reported that a large excess of phosphine had to be used and that exchange had to be repeated due to poor equilibrium of phosphine exchange between PCy2Ph and PPh3.
Considering this, it can be concluded that exchange between PPh2Cy and
PPh3 is even less likely, since the resemblance between the ligands is even closer and equilibrium even more unfavourable.
86 CHAPTER 3 – Synthesis and Kinetics
No success was achieved from the attempted synthesis of
[(AsPh3)2Cl2Ru=CH-CH=CMe2] either and NMR could not shed any light on the nature of the decomposed product.
1 H-NMR (CD2Cl2): = 0.8-2.0 (m),. 2.9 (s), 6.8-7.6 (m), 10-11 ppm (broad s)
The NMR shifts for [RuH(AsPh3)], from which (the attempted)
[(AsPh3)2Cl2Ru=CH-CH=CMe2] was synthesised, are given below.
1 H-NMR (C6D6): = -12.0 ppm (s, 1H, RuClH), 0.0-2.0 (m, 2H, Ar-H), 6.4-8.0 (m, 39H, Ar-H), 10-11 (broad s, 4H, Ar-H)
Comparing the two sets of NMR-data, it can be derived that the hydride was lost during the reaction, but all the aromatic peaks are still present, suggesting that the last product was a ruthenium complex with AsPh3-ligands, but not the desired carbene complex, neither the hydride starting complex.
A possible reason for the unsuccessful synthesis may be due to unsuitable electron donation ability of AsPh3. Compared to [(PCy3)2Cl2Ru=CH-
CH=CMe2], [(PPh3)2Cl2Ru=CH-CH=CMe2] is very unstable, among other reasons because of the weaker electron donating ability of the ligand. Since
As is a weaker electron donor than P, [(AsPh3)2Cl2Ru=CH-CH=CMe2] should be even more unstable than [(PPh3)2Cl2Ru=CH-CH=CMe2].
However, synthesis and characterisation of the second generation Grubbs catalyst, [(IMesH2)(PPh2Cy)Cl2Ru=CHPh], was accomplished successfully. Unlike in the case of the first generation catalyst (requiring phosphine exchange of PPh3 and PPh2Cy), poor equilibrium between the exchanging groups was not a problem. The ligand (C5H5N)2 was easily substituted by
PPh2Cy. However, reported methods to obtain the isolated product had to be adjusted, because [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] did not precipitate as readily as reported for [(IMesH2)(PPh3)Cl2Ru=CHPh],
87 CHAPTER 3 – Synthesis and Kinetics
101 [(IMesH2)(PBn3)Cl2Ru=CHPh] etc. This was also reflected in the yield (53%), which is lower than that of previously mentioned complexes.101
3.4.2. Kinetic Reactions
Results of all the kinetic experiments are summarised in the following paragraphs.
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh] was catalytically active for the ring closing metathesis of diethyl diallylmalonate. The reaction was first order with regard to the olefin. This observation is in contrast with second order RCM reactions of first generation catalysts previously reported.92 Preliminary experiments showed a dependence on phosphine concentration, but further experiments need to be done to determine the nature of both the influence of free phosphine and catalyst concentration.
Phosphine exchange studies were carried out successfully on the novel complex, [(IMesH2)(PPh2Cy)Cl2Ru=CHPh], which was synthesised in 3.2.3. The rate constant for phosphine exchange was independent of free phosphine concentration implying that the rate determining step of the metathesis reaction probably does not involve the phosphine ligand. This finding is consistent with the accepted dissociative mechanism for olefin metathesis.92,101,118,119 Table 3.7 gives a comparison of the determined rate constant and literature values.
88 CHAPTER 3 – Synthesis and Kinetics
Table 3.7 – Comparison of phosphine exchange rate constants of
[(IMesH2)(PPh2Cy)ClsRu=CHPh] (Current Study = CS) and catalysts reported in 101 literature; solv. = C6D6, [Ru] = 0.023 M, [P] = 0.03 M – 0.15 M, T = 353 K Catalyst k / s-1 ∆H≠/kJ.mol-1 ∆S≠/J.K-1.mol-1 Ref
[(IMesH2)(PCy3)Cl2Ru=CHPh] 0.13 ± 0.01 113 ± 8 54 ± 25 101
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh] 4.1 ± 0.9 84 ± 2 68 ± 9 CS
[(IMesH2)(PPh3)Cl2Ru=CHPh] 7.5 ± 0.5 88 ± 13 21 ± 38 101
[(PCy3)2Cl2Ru=CHPh] 9.6 ± 0.2 99 ± 2 50 ± 8 101
Sandford et. al.101 found that the rate constant of phosphine exchange for catalysts with the general structure [L(PR3)Ru=CHR’] ranged over six orders of magnitude with variation in ligand (X, L, R, R’).101 As expected the rate of phosphine exchange for the product complex in this study,
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh], was intermediate to that of
[(IMesH2)(PCy3)Cl2Ru=CHPh] and [(IMesH2)(PPh3)Cl2Ru=CHPh] due to the pKa-values of the phosphines involved. In particular, k was 32 times larger than the exchange rate of [(IMesH2)(PCy3)Cl2Ru=CHPh], but less than 2 times slower than the value of [(IMesH2)(PPh3)Cl2Ru=CHPh]. This result is to be expected because [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] resemble
[(IMesH2)(PPh3)Cl2Ru=CHPh] more closely than
[(IMesH2)(PCy3)Cl2Ru=CHPh]. It must be noted, though, that the catalyst concentration in the reported study101 was [Ru] = 0.04 M (compared to [Ru] =
0.023 M in the current study) and that toluene-d8 was used as solvent (instead of benzene-d6).
The activation parameters of the synthesised complex were also comparable to literature values, as illustrated inTable 3.7. All ∆S≠-values in Table 3.7 were positive in sign and mostly have a value above 2.4 J./K.mol, indicating a dissociative mechanism.101 Being relatively large (∆H≠ > 80 kJ/mol) and positive in sign, ∆H≠-values also suggested a dissociative mechanism.
Limiting kinetics were obtained for the catalyst-coordination of the three functionalized olefins. Different plateaus for the different entering olefins were 89 CHAPTER 3 – Synthesis and Kinetics obtained. This interchange mechanism is represented the equation 3.24 (given below).
k1[Ru][olefin] k obs = (3.24) K1[PR 3 ] + [olefin]
The equilibrium constants for the initial (fast) coordination reactions were comparable, but the reaction rates, k2, for the second rate determining step decreased in the order allylacetate > vinylacetate > allylcyanide. It was difficult to relate the rate laws to the electronic properties of the olefins, most likely due to the additional influence of steric effects.
It was shown that Phobcat [(PhobCy)2Cl2Ru=CHPh] was significantly more active than Grubbs-PCy3 ([(PCy)2Cl2Ru=CHPh]) and also decomposed much slower. Phobcat is thus more likely to give high product yields and exhibit longer catalytic life times when exposed to polluted feedstock of industrial processes.
87 C. Crause, L. Bennie, L. Damoense, C.L. Dwyer, C. Grove, N. Grimmer, W. Janse van Rensburg, M. M. Kirk, K.M. Mokheseng, S. Otto, P.J Steynberg, Dalton Trans., 2003, 2036. 88 G.S. Forman, A.E. McConnell, M.J. Hanton, A.M.Z. Slawin, R.P. Tooze, W. Janse van Rensburg, W.H. Meyer, C. Dwyer, M.M. Kirk, D.W. Serfontein, Organometallics, 2004, 23, 4824. 89 M.A.O. Volland, F. Rominger, F. Eisenträger, P. Hofmann, J. Orgmet. Chem., 2002, 641, 220. 90 S.T. Nguyen, L.K. Johnson. R.H. Grubbs, J. Am. Chem. Soc., 1992, 114, 3974. 91 W.R. Roper, J. Organomet. Chem., 1986, 300, 167. 92 E.L Dias, S.T. Nguyen, R.H. Grubbs, J. Am. Chem. Soc., 1997, 119, 3887. 93 A. Roodt, S. Otto, G. Steyl, Coord. Chem. Rev., 2003, 245, 121. 94 www.staff.ac.uk, C-C and C-H bond activation of N-heterocyclic carbenes. 95 http://www.aue.auc.dk/~stoltze/catal/book/kin/intro.htm. 96http://www.wpi.edu/Academics/Depts/Chemistry/Courses/CH1010/Stream1/ kinbenzalacetone.html. 90 CHAPTER 3 – Synthesis and Kinetics
97 P.W. Atkins, Physical Chemistry, 6th Ed., Oxford University Press: Oxford, 2000. 98 G.L. Miessler, D.A. Tarr, Inorganic Chemistry, 2nd Ed., Prentice Hall, Inc. Upper Saddle River, New Jersey, 1998. 99 S.T. Nguyen, R.H. Grubbs, J.W. Ziller, J. Am. Chem. Soc., 1993, 115, 9858. 100 M.S. Sanford, M. Ulman, R.H. Grubbs, J. Am. Chem. Soc., 2001, 123, 749. 101 M.S. Sanford, J.A. Love, R.H. Grubbs, J. Am. Chem. Soc., 2001, 123, 6543. 102 J. Silvennoinen, A Study of NMR Relaxation in Blood, Kuopio University Publications G-A. I. Virtanen Institute for Molecular Sciences 6: 1-61, 2002. 103 IUPAC Compendium of Chemical Terminology, 2nd Ed. 1997. 104 A.D. Bain, J.A. Kramer, J. Magn. Reson., 1996, 118A, 21. 105 D.R. Mulhandlram, R.E.D. McClung, J. Magn. Reson., 1987, 71, 187. 106 R. Willem, Prog. NMR Spectrosc., 1987, 20, 1. 107 R.E. Engler, J. Magn. Reson., 1988, 77, 377. 108 W.H. Press, Numerical Recipes in C. The Art of Scientific Computing, Cambridge Univ. Press, Cambridge, 1988. 109 M.A. Wolfe, Numerical Methods for Unconstrained Optimization, Van Nostrand & Reinhold, NY, 1978. 110 M. Ulman, R.H. Grubbs, Organometallics, 1998, 17, 2484. 111 C. Adlhart, P. Chen, J. Am. Chem. Soc., 2004, 126, 3496. 112 P.S. Hallman, T.A. Stephenson, G. Wilkinson, Inorg. Synth. 1970, 12, 237. 113 P.S. Hallman, B.R. McGarvey, G. Wilkinson, J. Chem. Soc A, 1968, 3143. 114 I. Bor, J. Org. Chem., 1980, 45, 4418 115 D. Amaroso, G.P.A. Yap, D.E. Fogg, Can. J. Chem., 2001, 79, 958 116 MicroMath Scientist for Windows, Version 2.01, MicroMath Inc., 1995. 117 GCsolution, Version 2.00.00 Su2, GCsolution Postrun © 2001 Shimadzu Corporation. 118 S.F. Vyboishchikov, M. Bhl, W. Thiel, Chem. Eur. J., 2002, 8, 3962. 119 S. Fomine, S.M. Vargas, M.A. Tlenkopatchev, Organometallics, 2003, 22, 93.
91 4. Theoretical Study
4.1. Introduction
In the not-so-distant past Molecular Modelling was accomplished by using plastic models.120 Today, with the help of powerful computational technology, Molecular Modelling allows access to molecular information useful for predicting molecular properties and possible reaction mechanisms. This tool helps chemists to visualise molecular geometry, demonstrate chemical properties and explain experimental observations.
In the current chapter such a study is presented in which computational modelling was applied to gain fundamental understanding and possible insight into experimental results. In section 4.2 the gas phase structures of two well- known rhodium complexes have been optimised and evaluated to determine the accuracy of the density functional theory (DFT) approach for predicting the geometries of second row transition metal complexes. In section 4.3 the discussion on functionalized olefins from Chapter 3 was continued with the modelling of the ruthenium catalyst-olefin interactions. Specifically, the geometries, coordination energy comparisons and orbital interactions of different coordination modes are reported. As in chapter 3, relevant theory is discussed to ensure necessary background knowledge: In section 4.1.1 the DFT quantum mechanical calculation method employed in the current study is presented and section 4.1.2 covers the frontier molecular orbital theory (FMO), which was used to interpret orbital interaction observations.
4.1.1. Density Functional Theory
Density functional theory (DFT) is one of the most popular and successful quantum mechanical approaches to many-body electronic structure calculations of molecular and condensed matter systems.121
CHAPTER 4 – Theoretical Study
The calculation reduces a practically unsolvable many-body problem of interacting electrons to a solvable problem of a single electron moving in an averaged effective force field. The effective force field can be represented by potential energy being created by all the other electrons as well as the atomic nuclei. The positions of the nuclei are fixed relative to the electron positions according to the Born-Oppenheimer approximation.‡‡
While the traditional Hartree-Fock theory is based on the complicated many- electron wavefunction, DFT correlates the total electronic energy of a system to the overall electron density based on the groundbreaking work of Hohenberg and Kohn.122 They showed that the ground-state energy and other properties of a system are uniquely defined by the electron density. The DFT approach is an exact theory for free electron gas, but approximations are used for extended atomic systems. According to the theorem of Hohenberg and Kohn122 the total energy of a system may be written as a function of its electron density, ρ, as illustrated in equation 4.1.
Et [ρ] = T [ρ]+ U[ρ]+ Exc [ρ] (4.1) in which T[] is the kinetic energy of a system of non-interacting particles; U[] is the classical electrostatic energy due to Coulombic interactions; and Exc[] includes all many-body contributions to the total energy, in particular exchange and correlation energies.
‡‡ The Born-Oppenheimer approximation is a technique used in both quantum chemistry and condensed matter physics. The approximation states that since the mass of the atomic nuclei is far greater than the mass of the electrons orbiting it for a given energy, the electrons move much faster than the nuclei and therefore the nuclei position can be considered to be fixed with respect to the electrons. Furthermore, the motion of the electrons can be considered decoupled from the motion of the nuclei, which leads to the elimination of several terms from the Schrödinger equation. This is a good approximation and has become a foundation for the physical study of molecular systems.
93 CHAPTER 4 – Theoretical Study
The last term in equation 4.1 requires some approximation for this method to be computationally tractable. The local density approximation (LDA) is based on the known exchange-correlation energy of a uniform electron gas.123 LDA functionals depend only on the value of the density at a specific point in space, explaining the local nature of this approach. However, a significant improvement on LDA is obtained when the gradient of change in electron density at each point is also considered. Such a functional is called the generalised gradient approximation (GGA) and has improved the accuracy of the self consistent total energy calculation. It is important to keep in mind that the use of LDA functionals usually results in the underestimation of bond distances, i.e. bond distances of calculated gas phase structures will generally be shorter than that of crystal structures. In contrast, application of GGA functionals usually leads to an overestimation of bond distances.
The major advantage of DFT above other quantum mechanical approaches is embedded in the explicit incorporation of electron correlation into the definition of the correlation functional for DFT. In contrast to the traditional Hartree- Fock theory,124 in which electron correlation is neglected all together, or the Möller-Plesset perturbation theory125 (e.g. MP2), where incorporation of electron correlation requires excessive hardware resources, DFT provides a favourable balance between accuracy and time required for calculations.
Unfortunately, in contrast to most post Hartree-Fock approaches, improvement of DFT methods is not variational, i.e. it can not be systematically improved, and therefore requires some degree of empirical manipulation to improve functionals. This being said, however, the development of accurate DFT functionals have currently reached a stage where for most applications accuracy of data is comparable to other electron correlation approaches at a fraction of the computational cost. DFT has been popular for calculations in solid state physics since the 1970’s,121 but was not considered accurate enough for calculations in quantum chemistry until the 1990’s, when the approximations were greatly refined. Today DFT is the leading method for electronic structure calculations in both fields.121 94 CHAPTER 4 – Theoretical Study
4.1.2. The Frontier Orbital Theory
HOMO and LUMO orbitals are called “frontier” orbitals because they are closest in energy to the Fermi level between the occupied and unoccupied orbitals.126
The frontier-orbital approach is based on the assumption that bonds are formed by a flow of electrons from the highest occupied molecular orbital (HOMO) of one reactant (or participating -bond) to the lowest unoccupied molecular orbital (LUMO) of another reactant (or -bond).127
Considering the mentioned assumptions it can be derived that 1. Any modification of reactants or transition state geometry that strengthens the HOMO(Nuceophile)-LUMO(Electrophile) interaction will stabilise the transition state and lead to a faster reaction, but any modification weakening the HOMO-LUMO interaction should destabilise the transition state. 2. Nucleophiles with "high energy" (less negative) HOMO’s tend to be more reactive. 3. Electrophiles with "low energy" (more negative) LUMO’s tend to be more reactive. 4. The preferred transition state for any reaction is the one that creates the best HOMO(Nu)-LUMO(El) overlap.
The distribution of the frontier molecular orbitals can be helpful in many respects to determine the reaction mechanism:127 1. A preferential reaction site may possibly be determined when examination of steric constraints does not provide an answer. 2. The extent of conjugation and the nature of extended -systems may well be elucidated. The shape of the HOMO’s or LUMO’s tend to reveal the localisation or delocalisation of charge in electron rich or electron poor species.
95 CHAPTER 4 – Theoretical Study
Furthermore, FMO’s can easily be calculated127 and even in cases where it does not provide a complete description of transition state orbitals (due to additional influences of other orbitals) the FMO’s are still likely to be important.
4.1.3. Computational Method
All geometry optimisations were performed with the DMol3 DFT code128 as implemented in the MaterialsStudioTM (Version 3.0) program package of Accelrys Inc.129 Structures and related energies were calculated by explicitly considering all electrons, with integral change of orbital electron occupation at the Fermi level (no smearing).
The convergence criteria for optimisations consisted of threshold values 2 × 10-5 Hartree (Ha), 0.00189 Ha/Å and 0.00529 Å for energy, gradient and displacement convergence, respectively, while a self consistent field (SCF) density convergence threshold value of 1 × 10-6 Ha was specified. DMol3 utilises a basis set of numeric atomic functions, which are exact solutions to the Kohn-Sham130 equations for the atoms. These basis sets are generally more complete than a comparable set of linearly independent Gaussian functions and have been demonstrated to have small basis set superposition errors.130 In the present study a polarised split valence basis set, termed double numeric polarised (DNP) basis set has been used. All geometry optimisations employed highly efficient delocalised internal coordinates.131 The use of delocalised coordinates instead of traditional Cartesian coordinates significantly reduces the number of geometry optimisation iterations needed to optimise larger molecules.
The DFT-computations on the rhodium structures (in section 4.2) were performed with the GGA functional PW91.132 The PW91 (Perdew and Wang) exchange correlation functional was derived by considering low- and high- density regimes and by enforcing various summation rules. The overlay of the crystallographic and modelled rhodium structures was done in the modelling program HyperChemTM.133 RMS and Overlay functions were used to
96 CHAPTER 4 – Theoretical Study determine and illustrate the RMS difference (in Ångstrom) between the two sets of coordinates.
Computations on ruthenium complexes (in section 4.3) were carried out with the RPBE134 (revised PBE) functional. The non-local PBE135 (Perdew, Burke and Enzerhof) is a GGA functional, in which all parameters except for LDA components are fundamental constants. PBE has a strong physical background and shows reliable numerical performance. The RPBE was proposed by Hammer, Hansen and Norskov in 1999134 who demonstrated that the method significantly enhances the accuracy of thermochemical calculations.
All energies reported for ruthenium structures are electronic energies, i.e. the energy of a static system at 0 K. This approach was (i) deemed sufficiently accurate for the comparative use of energies in the current study and (ii) allowed for a relatively high throughput of calculations which did not require the calculation of time-consuming second derivative Hessian matrices. However, if absolute accuracy should be required, corrections would have to be made for vibrational (∆Evib), rotational (∆Erot) and translational energy
(∆Etrans) contributions.
The role of steric and electronic effects was investigated by considering the simplified models of the ruthenium catalysts instead of the full structures. These strip-down models have significantly shorter calculation times and proved to be sufficient for geometry and energy comparisons. Previous studies on first and second generation Grubbs type catalyst have confirmed that these model systems are reliable imitations of full structures136,137,138,139 and binding energies were found to be in good quantitative agreement with experimental values.139 These model systems provided an elemental method for gaining understanding of the basic electronic features of the reaction intermediates.
97 CHAPTER 4 – Theoretical Study 4.2. Overlay of Rhodium Complexes
The first step for evaluating a molecular modelling (MM) method is to determine the accuracy of calculated versus experimentally determined geometrical parameters of appropriate crystal structures, as well as assessing the computational time needed to conduct these optimisations.
The crystal structures of the rhodium complexes that were studied, 140 141 Rh(acac)(CO)2 and Rh(acac)(CO)(PPh3), have long been known from literature. The first complex represents short period calculations in which only a few degrees of freedom need to be considered. The second larger complex has more degrees of freedom and calculation time was found to be longer. Rhodium is directly adjacent to ruthenium (considered in section 4.3) on the periodic table. It was anticipated that this MM approach would give a good indication of whether the DFT functionals considered are suitable for the prediction of second row transition metal geometries.
4.2.1. Results
4.2.1.1 Rh(acac)(CO)2
The overlayed structures of Rh(acac)(CO)2 is shown in Figure 4.1. Hydrogen atoms are not shown for the sake of clarity. Individual values of selected bond lengths and angles are given in Table 4.1.
98 CHAPTER 4 – Theoretical Study
O3 C5 C4 O2
C3 Rh
C2 C1 O1 O4
Figure 4.1 – Overlay of crystallographic (blue) and modelled (pink) structures for Rh(acac)(CO)2; individual bond lengths and angles are given in Table 4.1
Table 4.1 contains the bond lengths and angles of Rh(acac)(CO)2.
Table 4.1 – Comparison between crystallographic and gas phase (DMol3- calculated) bond lengths and angles for Rh(acac)(CO)2 as illustrated in Figure 4.1 Literature140 DMol3 129 Difference Rh-C5 1.76 (2) 1.898 0.14 Rh-C1 1.75 (2) 1.898 0.15
s Rh-O1 2.06 (1) 2.078 0.02 h t g
n Rh-O2 2.05 (1) 2.078 0.03 ) e
L ( C5-O3 1.21 (3) 1.154 0.06 d n o C1-O4 1.26 (3) 1.154 0.11 B C4-O2 1.27 (2) 1.285 0.02 C2-O1 1.29 (2) 1.285 0.01
C5-Rh-C1 85 (1.2) 90.8 5.8
s d
e l ) n o
g O1-Rh-O2 90 (0.4) 90.8 0.8 o ( n B A O2-Rh-C5 92 (0.9) 89.2 2.8
99 CHAPTER 4 – Theoretical Study
O1-Rh-C1 93 (0.9) 89.2 3.8 C2-C3-C4 127a 126.7 0.3 a No literature value was available140 and the measured cif-file value is presented
The calculation time for Rh(acac)(CO)2 was 412 minutes (6 hours and 42 minutes).
4.2.1.2 Rh(acac)(CO)(PPh3)
The overlayed structures of Rh(acac)(CO)(PPh3) are shown in Figure 4.2. Hydrogen atoms and phenyl rings are not shown for the sake of clarity. Individual values of selected bond lengths and angles are given in Table 4.2.
PP C4 O2
C3 Rh
C2 C1 O1 O3
Figure 4.2 – Overlay of crystallographic (blue) and modelled (yellow) structures for Rh(acac)(CO)(PPh3); individual bond lengths and angles are given in Table 4.2
100 CHAPTER 4 – Theoretical Study
Table 4.2 contains the bond lengths and angles of Rh(acac)(PPh3).
Table 4.2 – Comparison between crystallographic and gas phase (DMol3- calculated) bond lengths and angles for Rh(acac)(CO)(PPh3) as illustrated in Figure 4.2 Literature141 DMol3 129 Difference Rh-P 2.244(2) 2.307 0.063
s Rh-C1 1.801(8) 1.866 0.065 h t g
n Rh-O1 2.087(4) 2.109 0.022 ) e
L ( Rh-O2 2.029(5) 2.096 0.067 d n o C1-O3 1.153 (11) 1.162 0.009 B b P-CPh 1.834 (8) 1.845 0.011
P-Rh-C1 87.8(2) 93.2 5.4
s d
e l ) n o
g O1-Rh-O2 87.9(2) 89.9 2.0 o ( n B A C2-C3-C4 125.0(6) 125.8 0.8 b CPh is not shown in Figure
The calculation time for Rh(acac)(CO)(PPh3) was 8157.920 minutes (5 days and 16 hours).
4.2.1.3 Discussion
Computed bond lengths compared well with literature values, but bond angle values diverged more, since a greater range for change exists.
The RMS value for Rh(acac)(CO)2 was 0.076 Å and for Rh(acac)(CO)(PPh3) 0.142 Å, reflecting the smaller difference between calculated and literature values for bond lengths and angles of Rh(acac)(CO)2 compared to
Rh(acac)(CO)(PPh3). To understand this difference, it needs to be considered that calculated structures are in gas phase, but in the cases of larger molecules, literature presents values obtained from X-ray diffraction measurements of these molecules in solid state.142 The geometries of molecules in crystals are not necessarily the same as those of isolated (gas phase) molecules, since the former are influenced by intermolecular
101 CHAPTER 4 – Theoretical Study
interactions, i.e. crystal packing forces, absent in the gas phase.
Nevertheless, in general (as in the case of Rh(acac)(CO)(PPh3)) these differences are relatively small.
It can be concluded that the calculation method is well suited for late transition metal complexes.
The calculation time for Rh(acac)(CO)2 was reasonable (412 minutes), but increased almost exponentially with the number of atoms in the complex,
causing the calculation time for the larger complex, Rh(acac)(CO)(PPh3), to be much longer (5 days and 16 hours). For the succeeding ruthenium calculations, model systems were used, because as mentioned before, the systems are known to give sufficient results and excessively long calculation times would be avoided.
4.3. Coordination of Functionalized Olefins to Ruthenium-Carbene Catalysts
Molecular modelling has the advantage above experimental methods of being able to characterise and study reaction intermediates in a postulated mechanism. This can give information on reagent-catalyst system geometry, orbital interactions, energy of individual mechanistic steps as well as energy of transition states.
The purpose of the current theoretical study was to gain insight into the fundamental nature of olefin coordination to an unsaturated ruthenium complex by theoretically exploring the geometry and coordination energies of various olefin-catalyst interaction modes. To get a more complete picture, the orbital interactions of selected coordination modes were also investigated. The role of steric and electronic effects will be discussed on the basis of simplified model systems.
102 CHAPTER 4 – Theoretical Study
Four olefins, i.e. ethene, vinyl acetate, allyl acetate and allyl cyanide, were investigated of which three were the functionalized olefins studied in Chapter 3 and in addition ethene were included as an unfunctionalized olefin reference.
Michalak and Ziegler reported a DFT study143 on the polar monomer coordination modes of nickel and palladium copolymerisation catalysts to - olefins. Specifically, the coordination of methyl acrylate and vinyl acrylate to simplified models of the catalysts were studied to determine how electronic and steric effects influence catalytic properties. In practice, the palladium catalyst is an active copolymerisation catalyst, while the nickel analogue is inactive. - and -binding modes of olefin-catalyst interactions were compared to identify the factors responsible for the different behaviour. Results showed that for the active palladium catalyst the -complex was preferable, but the -complex of the inactive nickel analogue was more stable. This stable -interaction prevented polymerisation from taking place. Differences were mostly regulated by steric effects, while orbital-interactions were practically equal.
Figure 4.3 shows the model systems that were used in the reported study.
N C3H7 N C3H7 RC RC M M CH CH2 2 RC N RC N HC R O O R CH
M = Ni / Pd
(A) (B)
Figure 4.3 – Model catalyst systems used in the reported study of Michalak and Ziegler143 to determine the electronic and steric effects on catalytic activity of Pd and Ni catalysts; (A) The catalyst-olefin system with -interaction; (B) The catalyst-olefin system with -interaction
103 CHAPTER 4 – Theoretical Study
The same approach was taken in the current study to assess the influence of electronic effects in each of the functionalized olefin-systems. For all olefins two modes of π-coordination were considered, i.e. with the olefin double bond positioned in either an axial or equatorial position with regard to the Ru=C bond. It is known that the relative position of the catalyst’s methylidene (CH2) group can also influence the energy of the overall structure significantly and therefore two CH2-orientations were considered – with the
CH2 group in and out of the Cl-Ru-Cl plane. For functionalized olefins an additional heteroatom interaction is possible and a -bound complex needed to be considered for these complexes. In total four coordination modes for ethene (the unfunctionalized olefin) and five coordination modes for the remaining three (functionalized) olefins were studied.
The steric interactions of all the olefins were studied first and are discussed in the following section.
4.3.1. The Influence of Steric Interactions
For the sake of clarity, the atoms in figures were not labelled, but in order to distinguish between =C-atoms of the olefin, these were referred to as CL, CR,
CF and CB, with L = Left, R = Right, F = Front, B = Back with regard to the ruthenium atom from the reader’s viewpoint. To distinguish between related coordination modes (A, B, C, D, E) of the different olefins, the structures were labelled e (ethene), va (vinyl acetate), aa (allyl acetate) or ac (allyl cyanide).
4.3.1.1 The Pre-Catalyst and Active Catalyst
Structure A in Figure 4.4 shows the model system for the full first generation
Grubbs pre-catalyst with PMe3 as ligand. Structure B shows the active catalyst in the widely accepted dissociative mechanism of the metathesis reaction,137,144,145,146 i.e. the active intermediate after dissociation of one phosphine ligand. Reported energies of all structures in section 4.3.1 are the values relative to the energy of structure B (chosen to be E = 0.0 kcal/mol).
104 CHAPTER 4 – Theoretical Study
The coordination of each olefin to B was thus considered for calculating the reported coordination energies.
Relative energy chosen to be E = 0.0 kcal/mol
(A) (B) Figure 4.4 – Structure A is the optimised strip-down models of the Grubbs 1 pre-catalyst ([(PMe3)2Cl2Ru=CH2]) and structure B the corresponding active catalyst ([(PMe3)Cl2Ru=CH2]) after phosphine dissociation
In Table 4.3 the bond lengths of structures A and B (in Figure 4.3) are compared.
Table 4.3 – Selected bond lengths of the optimised strip-down models of the
Grubbs 1 pre-catalyst ([(PMe3)2Cl2Ru=CH2]) and the corresponding active catalyst ([(PMe3)Cl2Ru=CH2]) as illustrated in Figure 4.4 Bond lengths (Å) Ru-P Ru-Cl
[(PMe3)2Cl2Ru=CH2] 2.428 2.458
[(PMe3)Cl2Ru=CH2] 2.259 2.350
The pre-catalyst A in Figure 4.4 has a distorted square-pyramidal geometry with the methylidene group in the vertex and the Ru=CH2 moiety almost coplanar with respect to the Cl-atoms. Ru-P bond lengths in A were both equal to 2.428 Å and Ru-Cl bond lengths were 2.458 Å. These values are in 105 CHAPTER 4 – Theoretical Study good agreement with results from the DFT study of Fomine et.al.,137 reporting Ru-P and Ru-Cl distances of 2.410 Å and 2.450 Å respectively.
Dissociation of one PMe3 unit does not lead to a significant change in the remaining complex geometry; however, a general shortening of bond lengths, most likely caused by reduced electron density at the metal centre, did occur. In B Ru-P and Ru-Cl distances were 2.259 Å and 2.350 Å respectively. The methylidene group rotated 90o to be perpendicular to the Cl-Ru-Cl axis and the Cl-atoms also relaxed out of the original Cl-Ru-Cl plane, presumably to lessen steric strain.
4.3.1.2 Coordination of Ethene
In Figure 4.5 four optimised coordination modes of ethene are illustrated and Table 4.6 contains the corresponding coordination energies, bond distances and angles. Coordination of the olefin was trans to the PMe3 ligand and restored the square pyramidal geometry evident for the pre-catalyst. Ru--
Cethene (Ru--CL, R--CR, Ru--CF, Ru--CB) distances ranged between 2.4-2.6 Å.
For staggered structures, e-A and e-B, the Ru--CR distance was elongated compared to the distance of Ru--CL. This was presumably caused by unfavourable Ru=C π-interaction with the ethene π-electron cloud. For e-C and e-D the two Ru-Cethene distances were identical and ethene coordination resulted in the formation of symmetrical structures. In the case of the equatorially coordinated olefins (e-C and e-D) steric interaction caused the Cl- atoms to bend out of the Cl-Ru-Cl plane towards the phosphine ligand. C=C bond lengths (1.361-1.384 Å) were slightly longer than that of free ethene, which was calculated to be 1.343 Å. The same was found to be true for the optimised full Grubbs 1 structures reported by Cavallo.139 For the eclipsed structures e-A and e-C (i.e. structures with the methylidene group in plane) the Cl-Ru-Cl angle approached 170o, while this angle decreased to ca. 140o for staggered structures e-B and e-D (with out-of-plane methylidene group).
106 CHAPTER 4 – Theoretical Study
(e-A) (e-B) Axial coordination Axial olefin coordination
CH2 out of plane CH2 in plane ∆E = 3.3 kcal/mol ∆E = -3.7 kcal/mol
(e-C) (e-D) Equatorial coordination Equatorial coordination
CH2 out of plane CH2 in plane ∆E = 2.6 kcal/mol ∆E = -3.9 kcal/mol
Figure 4.5 – Coordination modes of ethene to active catalyst ∆ [(PMe3)Cl2Ru=CH2]; E-values are given relative to structure (B) in Figure 4.3
107 CHAPTER 4 – Theoretical Study
This phenomenon may possibly be correlated to unfavourable Ru=C π-cloud induced steric repulsion of the Cl p-orbitals which should be more pronounced for e-B and e-D compared to e-A and e-C. For the eclipsed structures e-A and e-C (i.e. structures with the methylidene group in plane) the Cl-Ru-Cl angle approached 170o, while this angle decreased to ca. 140o for staggered structures e-B and e-D (with out-of-plane methylidene group). This phenomenon may be correlated to possible unfavourable Ru=C π-cloud induced steric repulsion of the Cl p-orbitals which should be more pronounced for e-B and e-D compared to e-A and e-C.
The ethene coordination energies of e-B and e-D were found to be notably lower than that of e-A and e-C. This out-of-plane position of the methylidene group seemed to stabilise these complexes in the cases of all the olefins. In fact, intermediates with in-plane methylidene groups resulted from endothermic ethene coordination. B- and D-structures were significantly more stable compared to A- and C-structures. Table 4.4 contains the coordination energies, bond distances and angles illustrated in Figure 4.5.
Table 4.4 – Coordination energies as well as selected bond distances and angles of ethene coordinated to the active catalyst [(PMe3)Cl2Ru=CH2] as illustrated in Figure 4.5 ∆E Bond Distances (Å) and Angles (o) kcal.mol-1
Ru--CL Ru--CR Ru--CF Ru--CB Cethene=Cethene Cl-Ru-Cl e-A 3.3 2.428 2.554 - - 1.371 169.9 e-B -3.7 2.582 2.588 - - 1.361 146.1 e-C 2.6 - - 2.342 2.343 1.384 167.6 e-D -3.9 - - 2.476 2.477 1.368 139.6 Free e - - - - - 1.343 -
4.3.1.3 Coordination of Vinyl Acetate
Figure 4.6 shows the coordination modes of vinyl acetate and Table 4.5 contains the matching values of coordination energies, bond distances and
108 CHAPTER 4 – Theoretical Study angles. Geometries and bond lengths were found to be similar to results obtained for ethene coordination (Figure 4.5). However, unlike the ethene complexes, the Ru--Cvinyl acetate distances were unequal in all four coordination modes presumably due to a translated steric interaction of the olefin “tail” with the rest of the complex. Bond distances ranged between 2.4-3.5 Å, effectively indicating that relatively tight Ru/C=C coordination was not possible for va-B and va-D. The Cl-atoms in va-B relaxed slightly out of plane towards the olefin, resulting from reduced steric influence caused by the longer Ru/C=C distance. Coordination energies of staggered structures, va-B and va-D, were almost identical although the geometries were somewhat different, emphasising that no general trend could be established between axial and equatorial coordination modes. In addition to the Ru/C=C coordination modes (va-A, va-B, va-C and va-D) an additional σ-carbonyl coordination, illustrated in va-E, was considered. . At first glance the side–on Ru--OC interaction in va-E seemed to be caused by steric effects, but inspection of the olefin’s donating orbitals revealed that electronic effects played a part. This is discussed in section 4.3.2. The C=C bonds in va-A and va-C were longer than in the case of the free vinyl acetate, indicating the larger electronic effect of the catalyst being in close proximity. For the rest of the coordination modes which had longer Ru--Cvinyl acetate bond distances, C=C was comparable to that of the free olefin.
Again the formation energy of va-B and va-D were lower than that of va-A and va-C, but the corresponding energy of va-E, involving σ-carbonyl coordination, were significantly lower than that of any Ru/C=C coordination mode. The same trend was observed for all three functionalized olefins, indicating that σ- coordination is probably the most favourable Ru/olefin interaction for these complexes.
109 CHAPTER 4 – Theoretical Study
(va-A) (va-B) Axial coordination Axial coordination
CH2 in plane CH2 out of plane ∆E = 4.7 kcal/mol ∆E = -1.9 kcal/mol
(va-E)
CH2 out of plane ∆E = -6.2 kcal/mol
(va-C) (va-D) Equatorial coordination Equatorial coordination
CH2 out of plane CH2 out of plane ∆E = 6.7 kcal/mol ∆E = -1.6 kcal/mol
Figure 4.6 – Coordination modes of vinyl acetate to the active catalyst
[(PMe3)Cl2Ru=CH2]
110 CHAPTER 4 – Theoretical Study
Table 4.5 shows the values of coordination energies, bond distances and angles illustrated in Figure 4.6.
Table 4.5 - Coordination energies as well as selected bond distances and angles of vinyl acetate coordinated to the active catalyst [(PMe3)Cl2Ru=CH2] as illustrated in Figure 4.6 ∆E Bond Distances (Å) and Angles (o) kcal.mol-1
Ru--CL Ru--CR Ru--CF Ru--CB Ru--O Cva=Cva Cl-Ru-Cl va-A 4.7 2.461 2.733 - - - 1.367 170.1 va-B -1.9 3.257 3.464 - - - 1.341 147.2 va-C 6.7 - - 2.404 2.613 - 1.372 169.3 va-D -1.6 - - 2.986 3.223 - 1.347 141.4 va-E -6.2 - - - - 2.411 1.338 141.0 Free va ------1.339 -
4.3.1.4 Coordination of Allyl Acetate
Figure 4.7 shows the various coordination modes of allyl acetate to
[(PMe3)Cl2Ru=CH2] and Table 4.6 contains a summary of coordination energies, bond distances and bond angles illustrated in the figure. Ru--Callyl acetate distances ranged between 2.4-3.0 Å, with the exception of the very long
Ru--Callyl acetate distances of 4.0 Å in aa-D. This result shows that a formal coordination of allyl acetate was most likely not taking place, but rather a long distance electrostatic type interaction. The importance of this long range interaction is, however, reflected in the favourable coordination energy calculated for aa-D compared to aa-A and aa-B. In aa-B the steric interaction of methylidene and olefin acetate group caused the olefin to twist away from the catalyst. As a result the olefin in aa-B was further removed from the catalyst compared to the situation in aa-A. The structure of aa-C was different from the other modes – the double bond coordination of allyl acetate was not completely equatorial and the CO-group of the olefin turned towards the catalyst. The relaxation of the Cl-atoms towards the olefin in aa-D was most likely due to the reduction of steric strain caused by the long bond distances in the coordination mode.
111 CHAPTER 4 – Theoretical Study
(aa-A) (aa-B) Axial coordination Axial coordination
CH2 in plane CH2 out of plane ∆E = 2.8 kcal/mol ∆E = -2.8 kcal/mol
(aa-E)
CH2 out of plane ∆E = -8.7 kcal/mol
(aa-C) (aa-D) Equatorial coordination Equatorial coordination
CH2 in plane CH2 out of plane ∆E = 0.9 kcal/mol ∆E = -3.6 kcal/mol Figure 4.7 – Coordination modes of allyl acetate to the active catalyst
[(PMe3)Cl2Ru=CH2]
112 CHAPTER 4 – Theoretical Study
The -OC coordination of allyl acetate in aa-E seemed to be identical to that of vinyl acetate with regard to bond distances and geometry and was again attributed to orbital interactions (explained in section 4.3.2). C=C bond lengths for the closely coordinated olefins (in va-A, va-B and va-C) was about 0.01 – 0.03 Å longer than that of free vinyl acetate, but olefins in coordination modes with longer Ru--Cethene distances (va-D and va-E) had comparable bond lengths with the free olefin, because of the reduced electronic effect from the catalyst due to longer bond distances.
The relative coordination energy for each allyl acetate complex was lower than that of the analogous vinyl acetate complexes, with the largest gain being for the -OC coordination in aa-E.
Table 4.6 contains the values for the coordination energies, bond distances and angles illustrated in Figure 4.7.
Table 4.6 - Coordination energies as well as selected bond distances and angles of allyl acetate coordinated to the active catalyst [(PMe3)Cl2Ru=CH2] as illustrated in Figure 4.7 ∆E Bond Distances (Å) and Angles (o) kcal.mol-1
Ru--CL Ru--CR Ru--CF Ru--CB Ru--O Cva=Cva Cl-Ru-Cl aa-A 2.8 2.445 2.670 - - - 1.371 169.7 aa-B -2.8 2.794 2.966 - - - 1.354 145.0 aa-C 0.9 - - 2.482 2.496 - 1.376 176.1 aa-D -3.6 - - 4.005 4.019 - 1.344 140.6 aa-E -8.7 - - - - 2.371 1.342 142.1 Free aa ------1.342 -
4.3.1.5 Allyl Cyanide
Figure 4.8 shows the different coordination modes of allyl cyanide to
[(PMe3)Cl2Ru=CH2] and Table 4.7 contains a summary of coordination energies, bond distances and angles illustrated in the figure. Bond distances ranged between 2.1-3.0 Å. The Ru--Callyl cyanide distances in ac-B and ac-D
113 CHAPTER 4 – Theoretical Study were longer than that of the remaining coordination modes. The shortest Ru--
Callyl cyanide distance was that of the nitrogen interaction in ac-E. In contrast with the Ru-carbonyl coordination modes of the acetates which were sideways, this Ru-C≡N interaction was found to be linear and also explained in terms of electronic effects. Noteworthy is that all non-hydrogen atoms of the free allyl cyanide laid in the same plane, but in the coordinated state the geometry was notably altered. This is in contrast to va-E and aa-E where the acetates retained their free form geometries (see Figure 4.12 for free olefin geometries). In ac-C the double bond of the olefin was shifted slightly left, while Cl-atoms again moved out of the original Cl-Ru-Cl plain, away from the olefin due to its steric influence. The olefin double bond in ac-D was turned slightly towards an axial position and this preference of axial coordination could also be observed by the lower energy of the system, even though the steric interactions of the two systems were almost identical. For all - coordinated complexes the C=C interactions were about 0.01 – 0.04 Å longer than for free allyl cyanide, but the C=C bond length in ac-E was not effected.
The coordination energy of ac-E was lower than for any of the previously discussed structures even though the coordination energies of ac-A to ac-D were comparable to that of the corresponding allyl acetate structures. This was in contrast with allyl acetate where all four coordination modes showed a reduction in coordination energy with regard to vinyl acetate.
114 CHAPTER 4 – Theoretical Study
(ac-A) (ac-B) Axial coordination Axial coordination
CH2 in plane CH2 in plane ∆E = 2.8 kcal/mol ∆E = -3.8 kcal/mol
(ac-E)
CH2 out of plane ∆E = -11.0 kcal/mol
(ac-C) (ac-D) Equatorial coordination Equatorial coordination CH2 in plane CH2 out of plane ∆E = 2.4 kcal/mol ∆E = -2.2 kcal/mol Figure 4.8 – Coordination modes of allyl cyanide to the active catalyst
[(PMe3)Cl2Ru=CH2]
115 CHAPTER 4 – Theoretical Study
Table 4.7 contains a summary of coordination energies, bond distances and angles illustrated in Figure 4.8.
Table 4.7 - Coordination energies as well as selected bond distances and angles of allyl cyanide coordinated to the active catalyst [(PMe3)Cl2Ru=CH2] as illustrated in Figure 4.8 ∆E Bond Distances (Å) and Angles (o) kcal.mol-1
Ru--CL Ru--CR Ru--CF Ru--CB Ru--O Cva=Cva Cl-Ru-Cl ac-A 2.8 2.460 2.668 - - - 1.370 169.6 ac-B -3.8 2.743 2.917 - - - 1.354 145.7 ac-C 2.4 - - 2.480 2.388 - 1.386 166.9 ac-D -2.2 - - 2.869 2.786 - 1.353 141.5 ac-E -11.0 - - - - 2.167 1.342 142.6 Free ac - - - - - 1.342 -
4.3.1.6 Ruthenacyclobutane Intermediates
A classic metallacyclobutane ring is defined as a ring made up out of a metal centre and three carbon-atoms. M-C-C and C-C-C bond angles are close to 90o and C-C bond lengths approximately that of a single carbon bond (≈ 1.4 Å). An example of such a structure is the bisphosphine ruthenium complex reported in the work of Thiel et.al.150 (shown in Figure 4.9). The reported bond lengths and angles of the structure are presented in.Table 4.8.
116 CHAPTER 4 – Theoretical Study
150 Figure 4.9 - (PH3)2Cl2Ru(C3H6); an example of a complex containing a classic metallacyclobutane functionality; Table 4.8 contains selected bond lengths and angles of the structure
The ruthenacyclobutane intermediate of the metathesis reaction, however, is said to have a non-classical nature. In all calculated cases, the Ru--Cc distance (≈ 2.28 Å) was only 0.28 Å longer than the bond length of Ru-CR/L (≈ 2.0 Å). The bond angles of the ruthenacyclobutane intermediates were found to be approximately 77.0o for Ru-C-C (deviating 13o from the classic case) and 118.0o for C-C-C (deviating 28o from the classic structure). Olefin variation did not affect the properties of the metallacycle and the bond lengths and angles of all the complexes were approximately equal. These calculated values compared well with reported bond lengths and angles of the complex 136 (PH3)Cl2Ru=CH2(C2H2).
Figure 4.10 shows the functionalized metallacyclobutanes, i.e. the intermediates that succeed the coordinated complexes (of section 4.3.1.2 -
4.3.1.5) in the general dissociative metathesis mechanism. CC signifies the central carbon and as in the previous sections, CL and CR denote the carbon atoms on the left and right of ruthenium. This non-classical nature of the ruthenacyclobutane has been described as the equivalent of two -CC agostic interactions and could be an important factor in Grubbs type catalysts.136 This agostic interaction stabilises the ruthenacyclobutane, but also weakens both
117 CHAPTER 4 – Theoretical Study
-CC bonds to enhance the rupture of the C-C bond toward the double bond reordering process.
Ethene Vinyl Acetate ∆E = -2.4 kcal/mol ∆E = -0.5 kcal/mol
Allyl Acetate Allyl Cyanide ∆E = -1.3 kcal/mol ∆E = -3.7 kcal/mol
Figure 4.10 – Ruthenacyclobutane intermediates of the studied olefin-catalyst systems
118 CHAPTER 4 – Theoretical Study
Energy values of these intermediates are generally lower than that of its predecessors in which olefins are -bound. This trend is in agreement with the results of Cavallo.139
Table 4.8 contains a comparison between selected bond lengths and angles of the non-classic ruthenacyclobutane intermediates of the current study (CS) (shown in Figure 4.10) and the classic ruthenacyclobutane reported by Thiel et.al.150
Table 4.8 - Coordination energies as well as selected bond distances and angles of the non-classic ruthenacyclobutane intermediates of the current study (CS) (illustrated in Figure 4.9) compared to selected bond lengths and angles of (PH3)2Cl2Ru(C3H6) with classic ruthenacyclobutane structure from literature (illustrated in Figure 4.9)
C L R R C
f C C C C C - - e - - - - L C u u R u C C R R R
s h t g
Ethene n 2.006 2.000 2.280 1.589 1.595 CS ) e
L ( Vinyl Acetate 1.996 2.015 2.287 1.599 1.576 CS d n Allyl Acetate o 2.020 2.002 2.282 1.598 1.590 CS B Allyl Cyanide 2.004 2.013 2.284 1.602 1.584 CS
(PH3)2Cl2Ru(C3H6) − 2.197 − 1.511 − 150
1 C l -
R l C C C C
o -
- - C - C u R
E m L
. l C ∆ R C - C - - a l - L u c u C C k R R
s e l 77.8 CS Ethene -2.4 g 77.8 118.3 177.6
n ) o A
( CS
Vinyl Acetate -0.5 d 78.2 82.9 117.9 176.3 n o
Allyl Acetate -1.3 B 77.2 78.0 117.9 176.6 CS
Allyl Cyanide -3.7 77.7 77.8 117.4 175.4 CS
(PH3)2Cl2Ru(C3H6) − 97.3 − 97.6 − 150
Still continuing in line with the procedure followed by Michalak and Ziegler,143 the electronic interactions of the involved species were investigated in the
119 CHAPTER 4 – Theoretical Study following section. In addition the requirements for productive metathesis will be discussed.
4.3.2. Orbital Interactions
4.3.2.1 Requirements for Productive Metathesis
A decade ago Eisenstein, Hoffmann and Rossi151 reported some interesting electronic features of the intermediate stages of olefin metathesis based on extended Hckel calculations. They pointed out that the proper conformation of the olefin with respect to the carbene ligand is crucial for productive metathesis, i.e. formation of a new olefin and ruthenium-carbene intermediate. It was suggested that collinear conformation of the Ru=C fragment with regard to the olefin double bond results in optimal orbital interaction causing maximum -bonding around the metal centre and serving as the driving force for metathesis. Figure 4.11 shows various olefin-catalyst coordination modes.
L M. L M. L M. . n n n LnM . . . .
. . . . i ii iii iv
.
. .
. . .
a.
Figure 4.11 – (i) - (iv) Different catalyst-olefin coordination modes; (ii) – (iv) will not lead to productive metathesis, because M=C is not parallel to C=C and/or necessary d- and p-orbitals are not in plane; (i) will lead to productive metathesis, since the required orbital interaction, shown in (a), is achieved
120 CHAPTER 4 – Theoretical Study
.Considering the conformation requirements discussed above, it can be shown that structure (ii) in Figure 4.11 can not lead to productive metathesis despite the M=C being favourably parallel to C=C, because the M=C orientation causes the carbene p-orbital to be out of plane with regard to C=C p-orbitals. Structure (iii) and (iv) can not lead to productive metathesis either, because the olefin is twisted away from the carbene moiety, making interaction of at least the metal d-orbital and olefin p-orbital impossible. Structures (ii) - (iv) will thus all lead to unproductive metathesis. However, in structure (i) M=C is parallel to C=C and the necessary orbitals are in the same plane, hence requirements are met and productive metathesis can take place. The orbital interaction involved in the productive metathesis reaction is shown in (a).
Theoretical studies have shown that the staggered catalyst structure (i.e. a structure with out of plane methylidene moiety) does exist in the postulated mechanism of some systems, but during the transition prior to metallacyclobutane formation the methylidene-group rotates to the in-plane position ensuring the appropriate electronic interaction with the olefin.152
Frontier Molecular Orbitals of the Studied Complexes Figure 4.12 displays the prominent lobe of the LUMO (situated at the open coordination site of the catalyst) that takes part in olefin bonding. Maximum overlap of the LUMO (shown here) and the HOMO of the olefin (shown in Figure 4.12) would ensure optimum stability of the coordinated complex.
121 CHAPTER 4 – Theoretical Study
LUMO orbital involved in olefin bonding
Figure 4.12 – The LUMO orbital of the unsaturated catalyst involved in bonding with the olefin (shown from the front and side)
Figure 4.13 shows the molecular orbitals of all the olefins which were studied in section 4.3. Again e, va, aa, ac signifies the different olefins and H = HOMO, L = LUMO. Table 4.9 contains the corresponding orbital energy values. va-H & aa-H show the donating orbitals of vinyl and allyl acetate respectively. The coordinating orbitals of vinyl acetate (HOMO-1) and allyl acetate (HOMO) were identical. Both donating orbitals consisted of two lobes on each side of the carbonyl resulting from the lone pairs of electrons on the oxygen and causing the sideways -interaction observed in sections 4.3.1.3 and 4.3.1.4.
122 CHAPTER 4 – Theoretical Study
(e-H) HOMO of Ethene (e-L) LUMO of Ethene
(va-H) HOMO-1 of Vinyl Acetate (va-L) LUMO of Vinyl Acetate
(aa-H) HOMO of Allyl acetate (aa-L) LUMO of Allyl acetate
(ac-H) HOMO-3 of Allyl cyanide (ac-L) LUMO of Allyl cyanide
Figure 4.13 - HOMO- (left) and LUMO- (right) olefin orbitals that partake in catalyst bonding; the orbital energies of these olefins are listed in Table 4.9
123 CHAPTER 4 – Theoretical Study
Figure 4.13 ac-H shows that allyl cyanide interacts via its HOMO-3, which has a unique shape compared to the coordinating orbitals of the other olefins. In contrast with the vinyl acetate orbital (HOMO-1) in va-H and the allyl acetate orbital (HOMO) in aa-H, the HOMO-3 orbital of allyl cyanide does not consist of side lobes, but concentric lobes covering the cyanide ligand and permitting head-on interaction of CN with the catalyst. This orbital also has little electron density in the double bond region. Unlike the sideways interactions of the acetates (Figure 4.6 and Figure 4.7), the Ru-NC interaction of allyl cyanide was parallel to the Cl-Ru-Cl plane (Figure 4.8).
Table 4.9 shows the orbital energy values of the olefin orbitals involved in the olefin-catalyst coordination modes with the lowest energy, as calculated in section 4.3.1. For all functionalized olefins the modes which were - coordinated to the catalyst via the heteroatom had the lowest energy, and ethene (the unfunctionalized olefin) was the only -coordinated olefin in the range.
Table 4.9 – Orbital energy values of olefin orbitals partaking in coordination to the catalyst; HOMO and LUMO orbitals are illustrated in Figure 4.13 Orbital Energies (eV) HOMO LUMO Ethene - 6.613 - 0.888 Vinyl acetate - 6.163 - 1.590 Allyl acetate - 6.344 - 1.194 Allyl cyanide - 6.919 - 1.573
According to the FMO theory (discussed in section 4.1.2) the coordinating species with the higher relative HOMO energy will afford better electron donation to the catalyst, but conversely the coordinating species with lower LUMO energy will experience more pronounced back-donation of electron density from the metal. Both energy values must thus be considered for each olefin and if a general trend can be established, the electronic influence on the coordination energy can be established.
124 CHAPTER 4 – Theoretical Study
The information in Table 4.9 indicates that the HOMO energy of the olefins decreases in the order Vinyl Acetate > Allyl Acetate > Ethene > Allyl Cyanide. However, the LUMO energy increases in the order Vinyl Acetate < Allyl Cyanide < Allyl Acetate < Ethene. Thus, no clear cut conclusions could be made in this regard.
4.3.3. Discussion
In the current theoretical study computational modelling was applied to gain fundamental understanding and possible insight into experimental results. The gas phase structures of two well-known rhodium complexes were successfully optimised and results showed that accuracy of the density functional theory (DFT) approach for predicting the geometries of second row transition metal complexes was sufficient for calculations on ruthenium complexes. Ruthenium catalyst-olefin interactions were studied with regard to geometries, coordination energies and orbital interactions.
Geometry studies of the functionalized olefin intermediates showed resemblance with analogous ethene structures. Ru--Colefin distances of equatorial coordinated olefins were very comparable, while axial coordination was rather unsymmetrical.
The Cl-atoms in the axial coordination modes commonly moved out of plane towards the phosphine ligand. This was attributed to the steric strain of the axial coordinated olefin which was more pronounced in these coordination modes than in the cases of equatorial coordinated olefins.
Table 4.10 presents a summary of Figure 4.5 – Figure 4.8, comparing coordination energies of structures in the different olefin-to-catalyst coordination modes.
125 CHAPTER 4 – Theoretical Study
Table 4.10 - Bonding Energies for the different coordination modes for olefins studied; illustrated in Figure 4.5- Figure 4.8 Coordination Energy (∆E) / kcal.mol-1 (A) Axial (B) Axial (C) Equatorial (D) Equatorial (E) Hetero- Eclipsed Staggered Eclipsed Staggered atom e 3.3 -3.7 2.6 -3.9 - va 4.7 -1.9 6.7 -1.6 -6.2 aa 2.8 -2.8 0.9 -3.6 -8.7 ac 2.8 -3.8 2.4 -2.2 -11.0
Table 4.10 shows that structures with out-of-plane methylidene groups had a considerable energy advantage above analogues with in-plane methylidene groups. In all cases these intermediates were more stable than the corresponding eclipsed structures by about ∆E = 4.6-8.3 kcal/mol.
From the comparison in Table it is also clear that the preference of axial/equatorial coordination depended upon the olefin involved. However in all cases the -coordination via the hetero-atom (N or O) was preferred above -coordination. These -coordinated intermediates were about ∆E = 4.3-7.2 kcal/mol more stable than the -coordinated counterparts. The stability of this σ-coordination increased in the order vinyl acetate < allyl acetate < allyl cyanide.
The preference for the -coordination underlined the unlikelihood of these functionalized olefins to participate in metathesis. Considering that the metathesis mechanism requires a parallel Ru=C/C=C interaction for favourable orbital interaction (as discussed in the beginning of section 4.3.2.1), it can be said that the stable σ-coordination would be the most prominent interaction and most likely prevent the metathesis reaction from taking place in the cases of all the functionalized olefins.
Orbital studies illustrated some general electronic properties of the catalyst and olefins. Results showed that the geometry of the -coordination of vinyl
126 CHAPTER 4 – Theoretical Study acetate and allyl acetate was different from that of allyl cyanide because of differences in the donating orbitals of the olefins. No further trends could be recognised in this regard.
120 www.molecules.org. 121 www.wordiq.com/definition/Density_functional_theory. 122 P. Hohenberg, W. Kohn, W. Phys. Rev., 1964, B136, 864. 123 L. Hedin, B. I. Lundqvist, Phys. Rev. B., 1971, 136, 864. 124 A.R. Leach, Molecular Modelling: Principles and Applications,2nd Ed., 2001, Prentice Hall, London. 125 C. Møller, M.S. Plesset, Phys. Rev., 1934, 618. 126http://academic.reed.edu/chemistry/alan/201_202/lab_manual/Experiment_8/back ground.html. 127 http://www.cem.msu.edu/~reusch/VirtualText/special0.htm. 128 (a) B. Delley, J. Chem. Phys., 1990, 92, 508. (b) B. Delley, J. Phys. Chem., 1996, 100, 6107. (c) B. Delley, J. Chem. Phys., 2000, 113, 7756. 129 Accelrys Materials Studio, Version 3.0, Accelrys Inc. Copyright© 2003. 130 (a) B. Delley, Modern Density Functional Theory: A Tool for Chemistry. (b) J.M Seminario, P. Politzer, Theoretical and Computational Chemistry, Vol. 2, Elsevier: Amsterdam, The Netherlands, 1995. 131 J. Andzelm, R.D. King-Smith, G. Fitzgerald, Chem. Phys. Lett., 2001, 335, 321. 132 J.P. Perdew, Y. Wang, Phys. Rev., 1992, B54, 13244. 133 HyperChemTM, Release 6.03 for Windows Molecular Modelling System, Hypercube Inc. 134 B. Hammer, L.B. Hansen, J.K. Norskov, Phys. Rev., 1999, B 59, 7413. 135 J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865 136 C.H. Suresh, N. Koga, Organometallics, 2004, 23, 76. 137 S. Fomine, S.M. Vargas, M.A. Tlenkopatchev, Organometallics, 2003, 22, 93. 138 F. Bernardi, A. Bottoni, G.P. Miscione, Organometallics, 2000, 19, 5529. 139 L. Cavallo, J. Am. Chem. Soc., 2002, 124, 8965. 140 N.A. Bailey, E. Coates, G.B. Robertson, F. Bonati, R. Ugo, Chem. Comm., 1967, 1041.
127 CHAPTER 4 – Theoretical Study
141 J.G. Leipoldt, S.S. Basson, L.C.D. Bok, T.I.A. Gerber, Inorg. Chim. Acta, 1978, 26, L35. 142 W.J. Hehre, A Guide to Molecular Mechanics and Quantum Chemical Calculations, Wavefunction Inc., 2003. 143 T. Ziegler, A. Michalak, Organometallics, 2001, 20, 1521. 144 E.L. Dias, S.T. Nguyen, R.H. Grubbs, J. Am. Chem. Soc., 1997, 119, 3887. 145 S.F. Vyboishchikov, M. Bhl, W. Thiel, Chem. Eur. J., 2002, 8, 3962. 146 M.S. Sanford, J.A. Love, R.H. Grubbs, J. Am. Chem. Soc., 2001, 123, 6543. 150 S.F. Vyboishchikov, M. Buhl, W. Thiel, Chem. Eur. J., 2002, 8, 3962 151 O. Eisenstein, R. Hoffmann, A.R. Rossi, J. Am. Chem. Soc., 1981, 103, 5582. 152 C. Adlhart, P. Chen, J. Am. Chem. Soc., 2004, 126, 3496.
128
5. Relevance of Study
5.1. Attainment of Goals
In the current chapter, the relevance of the completed study will be discussed according to the aims of the project (section 1.2) and future work will be stipulated.
The first aim of the project was to synthesise a Grubbs type catalyst. Attempts to produce a novel first generation catalyst was unsuccessful, mainly due to unfavourable equilibrium between bound ligand PPh3 of
[(PPh3)2Cl2Ru=CH-CH=CMe2] and free ligand PPh2Cy. This result might have been anticipated at the beginning of the study, for the synthesis of
[(PCy2Ph)2Cl2Ru=CH-CH=CMe2] via phosphine exchange of 153 [(PPh3)2Cl2Ru=CH-CH=CPh2] and PCy2Ph was reported to be tedious compared to the phosphine exchange reactions of similar complexes involving i i P Pr3, P Pr2Ph and PCy3, which was accomplished much easier and in high yields (85-100%).
However, a novel second generation Grubbs type complex,
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh], could be synthesised successfully via the phosphine exchange method reported by Sanford et.al.154 The method was less complicated and time consuming than for first generation complexes and a yield of 71% was obtained which compared fairly well with similar complexes with typical yields of 65-75%.154 In addition, the product was relatively clean.
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh] proved to be active for the ring closing metathesis of diethyl diallyl malonate. Kinetic studies showed this metathesis reaction to be first order with regard to reagent consumption, however, the same reaction with first generation catalysts was reported to be second order
129 CHAPTER 5 – Relevance of Study with regard to reagent consumption. The investigation to find a reason for the difference was not continued, but is a subject for a prospective study.
The rate constant at 353 K (determined from [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] phosphine exchange studies) and activation parameters (determined from temperature studies) indicated that ligand exchange took place in a dissociative fashion, which is in agreement with literature.154,155,156 Determined values compared well with those of similar second generation catalysts.154 As expected, the rate constant of
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh] was intermediate to that of
[(IMesH2)(PPh3)Cl2Ru=CHPh] and [(IMesH2)(PCy3)Cl2Ru=CHPh].
A kinetic study was done on the coordination of functionalized olefins, namely vinyl acetate, allyl acetate and allyl cyanide, to the first generation Grubbs catalyst, [(PCy3)2Cl2Ru=CHPh]. A two-step rate law was determined, indicating an initial rapid equilibrium and a rate determining second step, similar to an interchange mechanism. It was difficult to relate the rate constants to the electronic properties of the olefins most likely due to the additional influence of steric factors.
The study on the functionalized olefins was continued from a theoretical viewpoint in order to gain fundamental understanding of the steric and electronic factors that influence the olefin-catalyst systems. It was determined that olefin coordination to the catalyst via its hetero-atom was most favourable in each case. This result indicates that metathesis for these olefins are unlikely, since double bond interaction (of the olefin) with the carbene moiety (of the catalyst) is suppressed due to the significantly more stable hetero-atom coordination.
It was shown that Phobcat, [(PhobCy)2Cl2Ru=CHPh], is significantly more active than the first generation Grubbs catalyst, [(PCy3)2Cl2Ru=CHPh], for the self metathesis reaction of 1-octene. Phobcat also gave better results in previous test runs with 1-decene at 338K (product yield of ± 80% compared to 157 <10%) and with Sasol C7 feedstock at standard conditions (with a product 130 CHAPTER 5 – Relevance of Study yield of ± 70% compared to ± 25%).157 This result is noteworthy because it presents the possibility of utilising the metathesis reaction to combine shorter chain products of the Fischer Tropsh reaction to longer chain olefins with higher value, where the traditional first and second generation catalysts failed to give good yields.
A very important advantage of Phobcat is that decomposition takes place at a considerable slower rate than for both first and second generation catalysts.157 Phoban phosphine ligands have been used with great success in hydroformylation applications158 and if the catalytic lifetime of Phobcat can be sufficiently long to give satisfactory yields, metathesis may well find industrial application. Results from this study showed that the catalytic lifetime of
Phobcat was more than five times longer than that of Grubbs1-PCy3. C7 feedstock studies showed that Grubbs1-PCy3 was poisoned almost instantaneously, while Phobcat decomposed normally after an amount of time.157 Nevertheless industrial application requires even better results.
From literature it is known that there exists a direct correlation between the type of phosphine ligand and lifetime of the catalyst. To gain insight into decomposition mechanisms and influences there of is of critical importance, since control of decomposition pathways would result in improved catalyst efficiency. However, few reported experimental studies focus on decomposition.159,160,161,162 Influences on and mechanisms of decomposition pathways are still quite unclear and therefore the novel bicyclic-containing structure of Phobcat poses a new area of investigation with the aim to establish a relationship between phosphine ligand and decomposition of the catalyst.
Another advantage of Phobcat is that the Phoban ligand is produced from very cheap starting materials, i.e. COD and cyclohexene, making it possible to produce a relatively cheap catalyst that shows significantly enhanced levels of activity compared to the traditional first generation catalyst.
131 CHAPTER 5 – Relevance of Study 5.2. Future Studies
Future investigations might include extensive studies on the activity of the complex [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] to determine catalyst efficiency and to obtain the complete rate equation (i.e. to determine reaction dependence upon catalyst concentration and phosphine concentration) which seems to be different from that of first generation catalysts.
Decomposition studies of Phobcat need to be continued in order to resolve questions about the mechanism of decomposition and possible control methods.
153 M.A.O. Volland, F. Rominger, F. Eisenträger, P. Hofmann, J. Orgmet. Chem., 2002, 641, 220. 154 M.S. Sanford, J.A. Love, R.H. Grubbs, J. Am. Chem. Soc., 2001, 123, 6543. 155 S. Fomine, S.M. Vargas, M.A. Tlenkopatchev, Organometallics, 2003, 22, 93. 156 C. Adlhart, C. Hinderling, H. Baumann, P. Chen, J. Am. Chem. Soc., 2000, 122, 8204. 157 G.S. Forman, A.E. McConnell, M.J. Hanton, A.M.Z. Slawin, R.P. Tooze, W. Janse van Rensburg, W.H. Meyer, C. Dwyer, M.M. Kirk, D.W. Serfontein, Organometallics, 2004, 23, 4824. 158 C. Crause, L. Bennie, L. Damoense, C.L. Dwyer, C. Grove, N. Grimmer, W. Janse van Rensburg, M.M. Kirk, K.M. Mokheseng, S. Otto, P.J. Steynberg, Dalton Trans., 2003, 2036 159 D. Amaroso, G.P.A. Yap, D.E. Fogg, Organometallics, 2002, 21, 3335. 160 D. Amaroso, G.P.A. Yap, D.E. Fogg, Can. J. Chem., 2001, 79, 958. 161 D. Bourgeois, A. Pancrazi, S.P. Nolan, J. Prunet, J. Am. Organometal. Chem., 2002, 643-644, 247. 162 S.H. Holng, M.W. Day, R.H. Grubbs, J. Am. Chem. Soc., 2004, 126 (24), 7414.
132
Summary
Ruthenium carbene complexes, with the general structure, [LL’Ru=CHR], are commonly known as Grubbs type catalysts, named after the discoverer of these metathesis catalysts. The discovery was quite revolutionary, since the catalysts proved to be easy to handle, tolerant towards various functional groups and more stable with regard to air and water than previous transition metal catalysts. Another important advantage was that all types of olefin metathesis reactions could be initiated without the help of co-catalysts or promoters.
Today Grubbs type catalysts find wide application in especially organic and synthetic chemistry. A well-known example is the SHOP-process which produces long chain -olefins, while other important applications include the synthesis of macro-cyclic and cyclic olefins.
The current study involved experimental and theoretical work to investigate various aspects comprising synthetic procedures, reactivity, kinetics, geometry and electronic properties of the complexes. Results are discussed briefly in the following paragraphs.
The first aim of the project was to synthesise a Grubbs type catalyst. Initial efforts were focused on the preparation of a first generation catalyst through various methods. This included modifying the reported method for the synthesis of [(PPh3)2Cl2Ru=CH-CH=CMe2] to yield [(PPh2Cy)2Cl2Ru=CH-
CH=CMe2] instead; a phosphine exchange reaction with the complex
[(PPh3)2Cl2Ru=CH-CH=CMe2] and free phosphine PPh2Cy; and utilising the analogue arsine ligand, AsPh3, to synthesise [(AsPh3)2Cl2Ru=CHCH=CMe2]; but unfortunately no success was achieved.
However, it was possible to synthesise a novel second generation Grubbs type catalyst, [(IMesH2)(PPh2Cy)Cl2Ru=CHPh], through the phosphine exchange reaction of [(IMesH2)(NC5H5)2Cl2Ru=CHPh] and PPh2Cy. 133 Summary
The new complex was tested in kinetic reaction studies and phosphine exchange reactions. Results showed that [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] was catalytically active for the ring closing metathesis of commercial diethyl diallylmalonate. The reaction was first order with regard to the olefin, contrary to the second order kinetic results reported for similar reactions catalysed by first generation Grubbs catalysts.
The phosphine exchange reactions were very successful and a rate constant could be determined. The rate constant was independent of the free phosphine concentration and activation parameters had relatively large, positive values; results indicative of a dissociative mechanism. These findings are in correlation with literature reports.
A kinetic investigation was done on the catalyst-olefin coordination involving the functionalized olefins vinyl acetate, allyl acetate and allyl cyanide; and the first generation Grubbs catalyst, [(PCy3)2Cl2Ru=CHPh]. A two-step rate law, similar to an interchange mechanism, was determined.
Phobcat, [(PhobCy)2Cl2Ru=CHPh], is modified first generation Grubbs type catalyst with rigid bicyclic phosphine rings which was recently developed by the Sasol Homogeneous Metathesis Group. In the current study Phobcat was compared to Grubbs1-PCy3 in the cross metathesis reaction of 1-octene. Results showed that Phobcat was up to 60% more active and had a 5 hour longer lifetime than Grubbs 1-PCy3.
Theoretical studies were done on the three functionalized olefins of the earlier experimental study to gain fundamental understanding of steric and electronic influences on these catalyst-olefin systems. Without exception, coordination via the heteroatom of the olefin was significantly more favourable than coordination via the double bond functionality. This result indicates that metathesis of these olefins is highly unlikely, since the stable heteroatom coordination will suppress the parallel Ru=C/C=C interaction which is compulsory for the metathesis reaction. Orbital studies highlighted the 134 Summary difference between coordination of acetate and cyanide, but no trend of an electronic nature could be recognised.
135
Opsomming
Ruthenium karbeen komplekse met die algemene struktuur, [LL’Ru=CHR], staan in die algemeen bekend as Grubbs katalisatore, vernoem na die ontdekker van hierdie revolusionêre metatese katalisatore wat maklik hanteerbaar is, ’n wye reeks funksionele groepe akkommodeer, stabieler in lug en water is as voorgangers en boonop alle olefien metatese reaksies inisieër sonder die hulp van ’n ko-katalisator of promoter.
Vandag vind Grubbs katalisatore toepassing in veral organiese en sintetiese chemie, waarvan ’n bekende voorbeeld die SHOP-proses is wat lang-ketting -olefiene vervaardig. Ander belangrike toepassings behels die vervaardiging van makrosikliese en kleiner sikliese olefiene.
Die huidige studie was van beide ’n eksperimentele en teoriese kant af benader om verskeie aspekte te ondersoek, insluitende sintetiese metodes, reaktiwiteit, kinetika, geometrie en elektroniese eienskappe van die ruthenium komplekse. Bevindings word vervolgens opgesom.
Die eerste doelwit van die studie was om ’n Grubbs katalisator te sintetiseer. Aanvanklike aanslae was gemik op die eerste generasie katalisatore en verskeie roetes is gevolg, onder andere gemodifiseerde metodes om
[(PPh2Cy)2Cl2Ru=CH-CH=CMe2] in stede van [(PPh3)2Cl2Ru=CH-CH=CMe2] te verkry, fosfien uitruiling tussen die kompleks [(PPh3)2Cl2Ru=CH-CH=CMe2] en vry fosfien PPh2Cy, en inkorperering van die analoë arseenbevattende ligand, AsPh3, om [(AsPh3)2Cl2Ru=CHCH=CMe2] te sintetiseer. Geen sukses is egter behaal nie.
Dit was wel moontlik om ‘n nuwe tweede generasie Grubbs katalisator,
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh], te gesintetiseer deur middel van fosfienuitruiling met [(IMesH2)(NC5H5)2Cl2Ru=CHPh] en PPh2Cy.
136 Opsomming
Die nuwe kompleks is in elementêre reaksie ondersoeke en fosfienuitruilingsreaksies getoets. Resultate toon dat
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh] wel katalities aktief is in die ringsluitingsmetatese reaksie van kommersiële diëtiel dialliel malonaat en dat die reaksie eerste orde is ten opsigte van die reagens, wat in kontras is met soortgelyke reaksies van eerste generasie katalisatore.
Die fosfienuitruilingsreaksies was baie suksesvol en die tempokonstante vir fosfienuitruiling kon bepaal word. Die berekende tempo konstante was onafhanklik van die vry fosfienkonsentrasie en die ooreenstemmende aktiveringsparameteters was relatief groot en positief wat beide op ‘n dissosiatiewe meganisme dui. Berekende tempo konstantes en gevolgtrekkings het goed vergelyk met inligting uit die literatuur.
Die koördinering van drie gesubstitueerde olefiene (vinielasetaat, allielasetaat en allielsianied) aan die katalisator (Grubbs1-PCy3) was die onderwerp van ’n kinetiese ondersoek. ’n Twee stap tempo wet is bepaal, wat dui op ’n aanvanklike vinnige ewewig waarin die olefien die katalisator nader, gevolg deur die snelheidsbepalende stap waarin ’n katalisator-olefien binding vorm en die fosfien-ligand geëlimineer word. Die meganisme is soortgelyk aan ’n uitruilingsmeganisme.
Phobcat, [(PhobCy)2Cl2Ru=CHPh], is ’n gemodifiseerde eerste generasie Grubbs katalisator met rigiede bi-sikliese fosfien ringe wat onlangs deur die Sasol Homogene Metatese groep ontwikkel is. In hierdie studie is Phobcat en
Grubbs1-PCy3 vergelyk in die kruismetatese reaksie van 1-okteen. Resultate het getoon dat Phobcat tot 60% meer aktief is en >5 ure langer leef as
Grubbs1-PCy3.
Teoretiese studies het gefokus op die steriese en elektroniese implikasies van olefien koödinering aan die aktiewe katalisator. Die drie gesubstitueerde olefiene wat vroeër in die eksperimentele hoofstuk ondersoek is, is gebruik in die modellering. Sonder uitsondering, was -koödinasie via die hetero-atoom van die olifien veel gunstiger as -koödinasie via die dubbelbinding. Hierdie 137 Opsomming bevindings dui daarop dat metatese van die betrokke olefiene onwaarskynlik is, omdat die stabiele hetero-atoom koödinasie die parallele Ru=C/C=C interaksie onderdruk wat juis deur die gepostuleerde metatese meganisme vereis word. Orbitaal studies het die verskil tussen sianied en asetaat koödinering onderstreep, maar geen tendens van elektroniese aard kon vasgestel word nie.
138
Appendix
Appendix A
Table A-1 – Experimental and Calculated values for the data points in figure 3.2: Kinetic results from NMR measurement of diethyl diallylmalonate consumption (by monitoring the reagent peak at = 2.6 ppm (d, 4H, CH2)) in the
RCM reaction catalysed by [RuCl2(IMesH2)(PPh2Cy)(=CHPh)]; -3 -1 k = (9 ± 0.09) x 10 s ; [diethyl malonate]0 = 0.2 mM, [Ru] = 0.01 mM; T = 303 K; solv. = CD2Cl2
Peak height Peak height Time / s Exp. (x) Calc. (__) 22.9 124.0 125.88 45.8 103.6 102.63 68.7 85.3 83.67 91.6 69.5 68.21 114.5 56.3 55.61 137.4 45.1 45.33 160.2 36.2 36.96 183.1 29.0 30.13 206.0 23.5 24.56 228.9 19.0 20.02 251.8 15.4 16.32 274.7 12.5 13.31 297.6 10.6 10.85 320.5 8.8 8.85 343.4 7.4 7.21 366.3 6.5 5.88 389.2 5.6 4.79 412.1 4.9 3.91 434.9 4.5 3.19 457.8 4.0 2.60
139 Appendix
Appendix B
Table B.1 – Experimental values of the data points in Figure 3.4: Change in
NMR peak heights with increasing mixing time for bound PPh2Cy of the complex [(IMesH2)(PPh2Cy)Cl2Ru=CHPh] (indicated with ) and free ligand
PPh2Cy (indicated with ∆); [Ru] = 0.023 M, [free PPh2Cy] = 0.067 M; solv. = C6D6; T = 353 K Peak height Peak height Mixingtime / s Free P (∆) Bound P () 5.00 x 10-5 -63.230 18.529 0.05 -52.030 11.595 0.10 -43.500 6.675 0.20 -31.520 0.958 0.50 -13.060 -2.708 1.00 4.530 0.518 2.00 28.960 7.137 4.00 55.140 14.086 8.00 70.400 18.166 15.00 72.930 18.741 25.00 72.860 18.854
Table B.2 - Calculated values for the fitted plot in Figure 3.4 (represented by ); Change in NMR peak height with increasing mixing time for free PPh2Cy and bound PPh2Cy of [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]; [Ru] = 0.023 M,
[free PPh2Cy] = 0.067 M, solv. = C6D6, T = 353 K
Mixingtime / Peak height Peak height s Free P (__) Bound P (__) 0.00 -62.668 19.139 1.00 4.718 0.736 2.00 29.226 7.203 3.00 44.938 11.377 4.00 55.025 14.057 5.00 61.502 15.778
140 Appendix
6.00 65.659 16.883 7.00 68.329 17.592 8.00 70.043 18.047 9.00 71.143 18.340 10.00 71.850 18.527 11.00 72.303 18.648 12.00 72.595 18.725 13.00 72.781 18.775 14.00 72.902 18.807 15.00 72.979 18.827 16.00 73.028 18.840 17.00 73.060 18.849 18.00 73.080 18.854 19.00 73.093 18.858 20.00 73.102 18.860 21.00 73.107 18.861 22.00 73.111 18.862 23.00 73.113 18.863 24.00 73.114 18.863 25.00 73.115 18.864 26.00 73.116 18.864 27.00 73.116 18.864 28.00 73.116 18.864 29.00 73.117 18.864 30.00 73.117 18.864
141 Appendix
Appendix C
Table C.1 – Data points of Figure 3.5: Eyring plot for phosphine exchange in
[(IMesH2)(PPh2Cy)Cl2Ru=CHPh]; [Ru] = 0.023 M, [free PPh2Cy] = 0.067 M;
T = 343 K – 373 K; solv. = C6D6 3 -1 -1 10 T / K R ln (hk / kBT)
2.92 -239
2.83 -235
2.76 -228
2.68 -219
142 Appendix
Appendix D
Table D.1 – Data points of Figure 3.6: kobs vs. [olefin] for the olefin-to-catalyst coordination of the catalyst [(PCy3)2Cl2Ru=CHPh] and olefins Allyl acetate (•) measured at = 334.0 nm, Vinyl acetate () at = 334.1 nm, Allyl cyanide () at
116 = 340.0 nm; represents the fitted plot to the equation k 2K1[PCy3 ] ; k obs = 1+ K1[PCy3 ] [olefin] = 0.02 – 0.10 M, [Ru] = 1.6 x 10-4 M; T = 298 K; solv. = DCM
kobs Allyl acetate Allyl cyanide Vinyl acetate a Exp. Exp.a Exp.a [Alkene] (•) Calc.b( ) ( ) Calc.b( ) ( ) Calc.b( )
0.002 0.0046 0.0033 0.0015 0.0016 0.0023 0.0024
0.004 0.0070 0.0063 0.0027 0.0029 0.0046 0.0045
0.006 0.0102 0.0090 0.0041 0.0042 0.0081 0.0063
0.010 0.0132 0.0137 0.0070 0.0063 0.0087 0.0095
0.015 0.0188 0.0185 0.0090 0.0085 0.0119 0.0126
0.030 0.0263 0.0285 0.0113 0.0128 0.0178 0.0187
0.060 0.0404 0.0390 0.0184 0.0173 0.0263 0.0246
0.100 0.0456 0.0458 0.0197 0.0200 0.0275 0.0283 a Experimental values as obtained from UV/Vis analysis b Calculated values for the plot fitted in the program MicroMath Scientist (See ref. 119 Chapter 3)
143 Appendix Appendix E
E.1 – Data points of Figure 3.8: Comparison between Phobcat and Grubbs1- PCy3 for the cross metathesis of 1-octene: – total yield of tetradecene for Phobcat, – total yield of tetradecene for Grubbs 1, [Ru] = 0.014 M; T = 323 K; 1-octene was the only solvent
Grubbs Phobcat
Total C14 cis-C14 trans-C14 Total C14 cis-C14 trans-C14 t / min () () () ( ) (∆) (∆)
0 0.00 0.00 0.00 0.00 0.00 0.00 2 6.03 2.70 3.33 0.97 0.34 0.62
4 12.68 5.67 7.02 8.25 3.05 5.21 9 18.35 8.14 10.22 18.94 6.81 12.13 15 23.51 10.35 13.16 30.83 10.63 20.20
21 25.68 11.27 14.41 40.44 13.26 27.18 30 26.65 11.65 15.00 50.00 15.40 34.61 45 27.03 11.85 15.18 61.32 17.10 44.22
60 27.08 11.84 15.24 68.18 17.49 50.70 90 27.26 11.84 15.42 76.32 17.38 58.94 120 27.24 11.94 15.31 80.13 17.45 62.67
180 27.52 12.00 15.51 84.89 17.35 67.53 240 27.59 12.03 15.56 87.43 17.26 70.17
360 27.50 12.00 15.51 90.44 17.12 73.32
144