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Ligand Substitution Mashhad of University FerdowasiInorganic Synthesis 1 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 ofa 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 University Ferdowasi 17 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. University 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 University 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 University • 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 University 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. University 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),
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