Mashhad of Inorganic Synthesis

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1 Ferdowasi Synthesis of Organometallic Compounds Mashhad • Organometallic compounds refer to thoseof containing at least one metal-carbon bond. • They are in an interdisciplinary area between inorganic and organic chemistry. University • Organometallic compounds have played a critical role in catalysis and organic synthesis, often leading to a more efficient use of

reagents, higher yields of products, and less use of energy. 2 Ferdowasi Synthesis of Organometallic Compounds Mashhad • Organometallic compounds have also beenof used as precursors in the preparation of nanomaterials and microelectronic materials such as thin films in integrated circuits

University • Species containing M-C bonds have been found in biology as well.

The vitamin B12 coenzyme: Co-C bond

3 Ferdowasi Mashhad • synthetic methods: of • (1) reactions between metal species and preformed ligands or ligand precursors

Grignard reagents, organolithiumUniversityreagents, M(CH2But)4, and MeCo(CO)4

4 Ferdowasi Mashhad • synthetic methods: of • (2) reactions of ligands in organometallic compounds yielding new ligand • the synthesis of Fischer carbeneUniversityand carbyne complexes containing M=C and M-C bonds ands.

5 Ferdowasi SYNTHETIC REACTIONS TO PREPARE ORGANOMETALLIC COMPLEXES Mashhad • ligand substitution of • oxidative addition and reductive elimination • insertion and elimination of ligands • nucleophilic and electrophilicUniversity attack on coordinated ligands

6 Ferdowasi SYNTHETIC REACTIONS Mashhad • LIGAND SUBSTITUTION of

Associative Substitution Dissociative Substitution University Interchange Type

7 Ferdowasi SYNTHETIC REACTIONS • LIGAND SUBSTITUTION Mashhad of • Ligand substitutions are often the first and last steps in a catalytic cycle which makes them a common reaction type in organometallic chemistry. • This type of reactions usuallyUniversity occurs at the metal with no overall change in oxidation state or coordination number.

8 Ferdowasi Mashhad • A typical reaction is the substitution of of a CO ligand by phosphine,

PR3. Here it is important to discuss the trans effect. The trans effect, often observed in square-planar complexes, directs where the ligand substitution takes place. In other words, certain ligands are capable of facilitating the departure of another ligand, trans, to the first. The more effective a ligand is atUniversity facilitating the departure, the higher its trans effect is.

9 Ferdowasi Mashhad • Ligands that form strong s bonds (hydridesof and alkyls) or are strong p

acceptors (CN, CO, PR3), bond strongly to the metal and cause preferential substitution of the trans metal-ligand bond. The 2- preparation of cisplatin PtCl2(NH3)2 from PtCl4 and 2 equiv. NH3 is a typical example. The second NH replaces the Cl ligand cis to the University3 first NH3, because Cl ligand has a larger trans effect than NH3

10 Ferdowasi Mashhad • There are two main types of ligand substitutionof reactions: associative (A) and dissociative (D).

Intermediate cases are also known. (I: Ia if closer to the associative or Id if closer to the dissociative Universitymechanism.)

11 Ferdowasi SYNTHETIC REACTIONS Mashhad • LIGAND SUBSTITUTION of Associative Substitution • In associative substitution, the incoming ligand first attacks, e.g., a 16e complex forming an 18e intermediate followed by a rapid University expulsion of a ligand to form a 16e product

12 Ferdowasi Mashhad of

• Associative substitution often takes place with d8, square planar, 16e species • It is possible for an 18e complexUniversity to undergo associative substitution as well, but one of the ligands is usually rearranged to avoid a 20e configuration.

13 Ferdowasi Mashhad • A good example is nitrosyls that change fromof their linear (3e) to bent (1e)

1e coordination University

• The departure of a CO ligand returns NO ligand to linear.

14 Ferdowasi SYNTHETIC REACTIONS

• LIGAND SUBSTITUTION Mashhad Dissociative Substitution of • In this type, the first step is a slow loss of CO to generate a vacant site at the metal. The incoming ligand then binds to the metal. This is most commonly observed in 18e carbonyl complexes University

15 Ferdowasi Mashhad • The overall rate: of dependent on the rate at which the CO ligand leaves independent of the concentration of the incoming ligand independent of the ligandUniversity type

16 Ferdowasi Mashhad of

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17 Ferdowasi SYNTHETIC REACTIONS Mashhad • LIGAND SUBSTITUTION of Interchange Type • There are cases in which it may be difficult to distinguish between associative or dissociative substitution. The difference is small in how University strongly the ligands bind to the metal in the intermediate. In Ia the

ligands bind strongly to the metal, and in Id the ligands bind weakly.

18 Ferdowasi Mashhad • OXIDATIVE ADDITION AND REDUCTIVE ELIMINATIONof • Oxidative addition (OA) and its reverse reaction, reductive elimination (RE), are important in many catalytic cycles and syntheses • OA consists: University

19 Ferdowasi • OA consists: • 1- breaking an AB ligand Mashhad • 2- adding the A and B moieties to a metal of

• Both A and B usually gain an electron in the process to become anionic A and B ligands. University • The oxidation state, coordination number, and electron count of the metal in the complex all increase by two during the addition.

20 Ferdowasi • RE is the reverse of OA, the pair of ligands, A and B, are removed from the metal to form A-B Mashhad of • The oxidation state, coordination number, and electron count of the metal are reduced by two in the reaction.

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21 Ferdowasi Oxidative Addition

• two ligands are added to the metal Mashhad of • The metal needs to have two open binding sites • Often the compound before the reaction is tetracoordinated with 16 valence electrons • It is also necessary that the metalUniversity at the n oxidation can reach its n + 2 oxidation state • OA is favored by strong donor ligands because they tend to stabilize the

oxidized state of the metal 22 Ferdowasi Mechanism 1 • Three-Center Concerted Additions Mashhad of • In a concerted addition, a nonpolar compound, such as H2, binds as a σ complex, forming a three-membered ring transition state. • Then the H-H bond is broken due in part to the strong back donation from the metal into the H-H σ* orbital, forming the final product. • Sometimes the transition stateUniversity is stable and the reaction stops there • Usually, however, the reaction proceeds to the end, yielding the additional product 23 Ferdowasi Mashhad of Vaska’s complex Ir(III) dihydride

oxidation

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trigonal bypyramid Ir(I) intermediate Several nonpolar reagents, including C-H or Si-H bonds and aryl halides, react through a concerted mechanism

24 Ferdowasi Mechanisms 2 • Nucleophilic Oxidative Addition of R-X Mashhad of • Nucleophilic OA is similar to SN2 reactions in organic chemistry involving a polar compound such as alkyl halide

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• second-order reactions with negative entropies of activation, and their rates are accelerated by polar solvents 25 Ferdowasi Mashhad • Another characteristic of nucleophilic OA is that the ofmore nucleophilic the metal is, the greater its reactivity is. Halide ions can increase the nucleophilicity of the metal and often enhance the rate of the addition.

• Such OA of MeI plays an important role in the catalytic carbonylation of methanol to University- acetic acid using, e.g., [RhI2(CO)2] and HI in the Monsanto Acetic Acid Process. In the

reaction, MeOH reacts with HI to form MeI and H2O. MeI then undergoes carbonylation to form Rh-C(=O)Me, followed by hydration to give MeCOOH.

26 Ferdowasi Mechanisms 3 Mashhad • Radical Mechanisms of • Radical OAs commonly occur in metals with an odd number of d electrons such as Co(II) and Rh(II). • kind of radical pathways: chainUniversity and nonchain .

27 Ferdowasi • chain mechanism is similar to other radical chain mechanisms with initiation, induction, and termination steps. Mashhad of

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28 Ferdowasi Mashhad • nonchain mechanism of • one electron is transferred from M to the RX σ* to form M+ and RX- . • transfer of X- to M+ to give and R University

29 Ferdowasi Mechanisms 4 Mashhad • Ionic Mechanisms of

• Acids, such as hydrogen halides and CF3COOH, usually dissociate in solution • Their anions and protons add Universityto the metal complex in separate steps

30 Ferdowasi • There are two types 1- the complex is basic enough to protonate, and then the anion binds to give the final product. This type is favored byMashhad basic ligands and low oxidation state metals. more common type of

2- the initial halide attack, followed by protonation of the intermediate. This is favored by electron-acceptorUniversity ligands and by a net positive charge on the complex. less common type

31 Ferdowasi Reductive Elimination

• RE is the reverse of OA Mashhad of • The oxidation state, coordination number, and electron count all decrease by two.

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32 Ferdowasi Mashhad • Certain groups are more easily eliminated thanof others. • Usually, complexes with bulky ligands are favored, since, with elimination, there is relief of steric hindrance and strain. • Complexes with high oxidation states and those with groups that will University stabilize the reduced metal after ligand loss are also favored. • In addition, reactions that involve hydride ligands

33 Ferdowasi Mashhad • are fast because the transition state energy isof lowered. The reaction is also efficient for intermediate oxidation states of late transition metals, including d8 square-planar metals: Ni(II), Pd(II), and Au(III), as well as d6 octahedral metals: Pt(IV), Pd(IV), Ir(III), and Rh(III). University

34 Ferdowasi • INSERTION AND ELIMINATION OF LIGANDS • Substitution and OA allows us to put ligands ontoMashhad metals. Insertion and its reverse reaction, elimination, rearrange, transform,of and combine these ligands intramolecularly. • There are two main types, commonly called 1,1- or 1,2-insertions.

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35 Ferdowasi 1,1-Insertion 2 Mashhad of

1 1 2

Mechanistic studies show that the inserted CO group is bound to the metal before the insertion occurs

University S : beginning compound L : added ligand

36 Ferdowasi • According to K magnitude These are the three possible situations: Mashhad • K is very small relative to k [L] )K << k [L]( -1 2 of -1 2

Since K-1 is small, the reaction stays in the intermediate phase, trapped by L, with the overall reaction based on k1, in a first-order reaction

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37 Ferdowasi • K is very large relative to k [L] )K >> k [L]( -1 2 Mashhad-1 2 • the intermediate goes back to the starting material,of and the attack by L governs the overall rate, giving a second-order reaction

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38 Ferdowasi • K is comparable to k [L] )K ~ k [L]( -1 2 -1 Mashhad2 causing the intermediate to be trapped by L atof a rate similar to the reverse reaction. The equation thus becomes significantly more complicated

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39 Ferdowasi 1,2-Insertion

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the two electrons involved in the C−H bond enter the empty d-orbital

The intermediate needs to be coplanar for the reaction to be reversible The stereochemistry of the insertion is usually synUniversity The reverse of this reaction is β elimination

40 Ferdowasi Other Insertion

• An unsaturated ligand can undergo insertions. Mashhad • SO ligand has been shown to undergo both 1,1- and 1,2-insertions 2 of • Other possible ligands for insertion include O2 and CO2

• An interesting insertion of SO2 is given in a Pd complex where the new ligand bridges two metal atoms University

41 Ferdowasi α, β, γ, δ Eliminations • The most common type of elimination is b elimination. Mashhad • It is the main decomposition pathway for alkylsof containing β-H atoms. In order for the reaction to occur, a vacant site on the metal is usually required. • A coplanar M-C-C-H arrangement brings the β-H atom closer to the metal center. University • If the species is an 18e complex, it first loses a ligand to open up a site for elimination. α β

β-H 42 Ferdowasi coplanar Mashhad • If there are no β-H atoms, a complex may go ofthrough α, γ, or δ elimination

α elimination University methylene hydride

43 Ferdowasi Mashhad • γ and δ eliminations give cyclic products, butof are also not common due to unfavorable sterics.

α β γ University

44 Ferdowasi