Organometallic CHM 4317

between organic and inorganic Overview

• A real example: the Monsanto process • Introduction: what is organometallic chemistry, and why should you care? • : the basis for understanding structure and reactivity • An overview of Main-group and Transition chemistry • Main group metal chemistry and "Umpolung" • Intermezzo: characterization of organometallic compounds • chemistry: overview of common • substitution • Insertion and elimination • Oxidative insertion and • Applications in

Organometallic Chemistry Example: Acetic Acid synthesis

Acetic acid is an important industrial chemical.

The traditional synthesis uses bio-oxidation of ethanol obtained via fermentation:

C6H12O6 ® 2 C2H5OH + 2 CO2

C2H5OH + O2 ® CH3COOH + H2O This is not a clean and efficient process!

Industrial acetic acid synthesis:

CH3OH + CO ® CH3COOH Catalyzed by a rhodium complex. Acetic Acid synthesis

CH3COOH HI CH3OH Moderately complex :

CH3COI H2O CH3I

- Rh(CO)2I2

- - MeCORh(CO)2I3 MeRh(CO)2I3

CO - MeCORh(CO)I3 Acetic Acid synthesis

HI CH3 OH CH COOH This cycle is known 3 in considerable detail:

HO2

CH3 I CH COI 3 oxidative

- addition Rh(CO)22 I RhI 16 e To understand it, reductive elimination you need to be familiar RhIII RhIII 18 e with electron counting 18 e and common MeRh(CO) I - reaction types 23 - MeCORh(CO)23 I RhIII ligand 16 e insertion binding

CO - MeCORh(CO)I3 What is organometallic chemistry ?

Strictly speaking, the chemistry of compounds containing at least one metal- bond. Metal hydrides are often included, H being considered as the "smallest organic group" (as in propyl, ethyl, methyl, hydride). Metal-carbon bonds are often formed temporarily or potentially, so in practice many compounds are included that do not actually contain metal-carbon bonds.

H2

(Ph3P)3RhCl OMe OMe Li O O Li Why should you care ?

Organometallic chemistry is the basis of , which is the method of choice for clean and efficient synthesis of fine chemicals, pharmaceuticals and many larger-scale chemicals. Many plastics (polythene, polypropene, butadiene rubber, ...) and detergents are made via organometallic catalysis. Organometallic chemistry is also the basis for understanding important steps in reactions such as olefin and CO oxidation. Organometallic compounds are used on a large scale as precursors for generation of semiconductors (AlN, GaAs, etc). Silicone rubbers are one of the few classes of organometallic compounds used as "final products". Course Objectives

By the end of this course, you should be able to: • Make an educated guess about stability and reactivity of a given compound, based on (a.o.) electron counting rules • Propose reasonable mechanisms, based on "standard" organometallic reaction steps, for many metal-catalyzed reactions • Use steric and electronic arguments to predict how changes in reactants, metal or ligands affect the outcome of reactions • Read a current research literature paper, understand and explain its content and significance Useful Background Knowledge

Organometallic chemistry is a 4th-year course because it builds on: • : reaction mechanisms, primarily nucleophilic and electrophilic attack • : ; electron counting and stability; properties of (transition) • : orbitals and MO theory; free energy, enthalpy and entropy You will (now and then) need this background to understand the material or make assignments etc Between organic and inorganic...

Organic chemistry: • more or less covalent C-X bonds • rigid element environments • fixed oxidation states (better: valencies)

??Organometallic chemistry??

Inorganic chemistry: • primarily ionic M-X bonds, dative M-L bonds • variable and often fluxional environments • variable oxidation states

10 Organometallic reactivity

Since most organometallics are intermediates, the focus in organometallic chemistry is usually on understanding and tuning reactivity

This starts with analyzing reaction mechanisms in terms of elementary steps

The number of possible elementary steps is larger than in "pure organic" chemistry, but the ideas are similar

11 Organometallic structures

Knowledge of inorganic and coordination chemistry is useful to understand geometries, electron counts and oxidation states of organometallic compounds

Organometallics are more covalent and often less symmetric than coordination compounds, so orbital symmetry arguments are not as important

12 Trends in organometallic chemistry

Organometallic chemistry is concerned with all metals, in combination with all "organic" elements. there are many metals !

Generalization is important the chemistry of e.g. Fe is not much more complicated than that of C, but after that there are 80 more metals... we divide reactions in broader categories than organic do

We concentrate on the M side of the M-C bond, and on how to tune its reactivity

13 Elements of interest

Organic elements Main group metals Transition metals

14 Organic vs organometallic reactivity

Organic chemistry: - X Y X SN1 int (6-e) C-C / C-H : nearly covalent Cd+-Xd- : polar (partly ionic) X Y reactivity dominated by nucleophilic attack at C Y - X SN2 and SN1 like reactivity Y Organometallic chemistry: SN2 TS (10-e ?) C is the negative end of the M-C bond ("umpolung") reactivity dominated by electrophilic attack at C or nucleophilic attack at M dissociative associative and dissociative substitution at M (C2H4)PdCl2 - C2H4 CO (C2H4)2PdCl2 ? (C2H4)(CO)PdCl2

CO - C2H4 (C2H4)2(CO)PdCl2 associative

15 Main-group organometallics

Me Me Me s and p orbitals. Al Al Me 8-e rule, usually. Me Me 8-e with a lot of exceptions Cl 10-e Cl Sn Me More electropositive and larger: Me higher coordination numbers, O regardless of the number of electrons. N

“Early" groups and not very electropositive: lower coordination numbers. Me Zn Me 4-e

16 Main-group organometallics

Metal is the "d+" side of the M-C bond. Chemistry dominated by nucleophilic attack of Cd- at electrophiles. this is also the main application in organic synthesis

MeMgBr + Me2CO Me3COMgBr

note: this is a simplified picture of the

17 Main-group organometallics

M-M multiple bonds are fairly weak and rather reactive they are a curiosity and relatively unimportant, certainly compared to C-C multiple bonds Bond strengths in kcal/mol: C-C 85 C=C 150 N-N 40 N=N 100 P-P 50 P=P 75 Multiple-bonded compounds often have unusual geometries

"Me4Sn2" Me4C2

18 Transition-metal organometallics

s, p and d orbitals 18-e rule, sometimes 16-e Ni other counts relatively rare CO OC CO 18-e Cr OC CO Fe Et3P Me CO 18-e Pt Et3P Cl 18-e 16-e

19 Transition-metal organometallics

Lower electron counts if metals are sterically saturated: nor Me nor Me Co W nor Me Me nor Me Me 12-e nor =

13-e

Ti Me Me 16-e

20 Transition-metal organometallics

Often ligands capable of donating 2-8 electrons Preference for p-system ligands (good overlap with d-orbitals) cyclopentadienyl O olefin allyl C C

2 e 2 e 2 e 3 e 5 e

Bonding to neutral ligands (olefin//CO/) relatively weak Important for catalysis!

21 Transition-metal organometallics

An olefin complex: (Acac)Ir(NCMe)(C2H4)

donation

backdonation

22 Transition-metal organometallics

“Forbidden” reactions ?

? +

a Mo Mo +

No symmetry Very fast

Symmetry-forbidden Does not happen

23 Reactivity of the M-C bond

Polar Þ reactive towards e.g.

• Water: M-OH + H-C

Me3Al explodes with water; Me4Sn does not react.

• Oxygen: M-O-O-C

Me2Zn inflames in air; Me4Ge does not react.

• Carbonyl groups: M-O-C C

MeLi adds at -80°C, Me3Sb not even at +50°C.

24 Reactivity of the M-C bond

Oxidation and hydrolysis: large driving force

Bond strengths in kcal/mol: Al-C 65 As-C 55 Si-C 74 Al-O 119 As-O 72 Si-O 108 Al-Cl 100 Si-Cl 91

25 Organometallic reaction steps

Ligand dissociation / coordination

Me3Al + NMe3 Me3Al-NMe3 note: free Me Al 6e 8e 3 dimerizes to Me6Al2

Mo(CO)6 Mo(CO)5 + CO 18e 16e

26 Organometallic reaction steps

Insertion and b-elimination

MeMgBr + MeC N Me2C=NMgBr N N N N Fe Me N Fe Me N Fe N N N Me

N + N Fe Me = N N N N Ar = Fe Ar Ar

27 Organometallic reaction steps

Insertion and b-elimination P P P Pd P Pd H H

P P = Ph2P PPh2

28 Organometallic reaction steps

Oxidative addition / Reductive elimination H P Cl H2 P H Rh Rh "P" = PPh P P P P 3 Cl

29 Organometallic reaction steps

Oxidative addition / Reductive elimination Me O L Rh L = P(OPh)3 O L O I L MeI Rh L O L O I Rh L = PPh3 O Me L

30 Organometallic reaction steps

s-bond metathesis H3 *C *CH4 Lu CH3 Lu H Lu *CH3 C H3

14 e

31 Organometallic reaction steps

Redox reactions

- e- Fe Fe

Homolysis Et Hg Et Et Hg + Et

32 Organometallic reaction steps

Reactivity of coordinated ligands

33 Overview of Organometallic Chemistry Factors governing structure and reactivity of organometallic compounds

• M-C, M-X bond strengths

• Electronegativity of M (polarity of M-C etc bonds)

• Number of (d) electrons

• Coordination number

• Steric hindrance

34 Trends in the

Main group metals: • left and down: more electropositive • down: higher oxidation states less stable

Transition metals: • middle: strongest preference for 18-e • 2nd and 3rd row: strong preference for paired electrons (low-spin states) • down: higher oxidation states more stable

35 Working with organometallics

Synthesis and reactivity studies (inert atmosphere!): • Glove box • , specialized glassware

Characterization: • Xray diffraction Þ structure Þ bonding • NMR Þ structure en dynamic behaviour • (calculations)

• IR Not: • MS • GC • EPR • LC

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