金属有机化学.ppt

Topic2 -elimination, -elimination Topic1 Alkylidenes, Alkylidynes, and CarbidesTopic2 Metalacycles Topic3 Synthesis of Metal Alkyl ComplexesLesson 2Decomposition Modes of Metal AlkylsTopic1 Empirical order of stabilitiesLesson 1Metal Alkyls and Aryls Topic3 Related bonded ligandsLesson 3Metal HydridesTopic2 Characterization:Tpoic3 SynthesisTopic1 The important role of the M HMetal alkyls and arylsNo development during the 1920s through 1940s.It was originally thought that such species were inherently unstable due to weak metal-carbon bonds. In fact, the issue is not their thermodynamic stability (M-C bond dissociation energies are typically 40-60 kcal/mol with 20-70 kcal/mol being a practical range), but their kinetic stability. Reagents:RMgX, the first source nucleophilic alkyl group (alcohols) ZnEt2, Grignard Reagents(1900) Pope and Peacheys Me3PtI(1909)Structure and BondingSimple alkyls are simple sigma donors, that can be considered to donate one or two electrons to the metal center depending on which electron counting formalism you use. For simple metal alkyls, the M-R bond distance is typically 190 to 220 pm. This is approximately the sum of the covalent radii of carbon and metal, rC = 77 pm and rM 120 pm. Realize that the first row transition metals are smaller, so any M-X bond distance will usually be smaller by 10-20 pm or so. MCRRR Bridging Alkyls and Related LigandsAlkyls can bridge two metal centers, something that is well known from aluminum-alkyl chemistry. For example, consider the condensed phase structure of these Al-alkyls (see Oliver et. al. Organometallics 1982, 1, 1307): AlAlMeCCH3H3MeMeMeAlAl -tol -tol -tolAlAlMeCCH3H3MeMeMeAlAl -tol -tol -tolOCCRRCp2ZrZrCp2OAlMe3py-pyAlMe3Cp2ZrCp2ZrRRCCCOOAlMeMeHHHIn this structure, the bridging Me group is nearly planar Another type of unusual alkyl group involves agostic interactions in which part of the alkyl substituent coordinates to the metal in addition to the M-C bond. TiPPClHClClMe2Me2-agostic interactionTiPPClHClClMe2Me2-agostic interaction Alkylidenes :The carbon atom of the group CR2 is able to form two normal covalencies, one to each atom. Carbene: (a special case of alkylidene) in this case the alkylidene acts as a terminal ligand, forming a double bond between carbon atom and metal atom.CHMMHAlkylidenes, Alkylidynes, and Carbides Alkylidynes: CR can bridge to three, or to two metals or act as a terminal group with an M-C triple bond. Carbides: a carbon atom bridge four metals.RCMMMCMRCHgOAcAcOHgHgOAcHgOAcRu(CO)3RuRuRuRuRu(CO)3(CO)3(CO)3(CO)3(CO)3C MetalacyclesvCyclic dialkyls are examples of metalacycles. CH2LnMCH2CH2CH2CH2CH2LnMLnMLnMCH2H2CCH2CH2H2CCH2CH2CH2vAryl, vinyl and acyl ligands can form stronger M-C bond , since they have empty * orbitals that can accept electron density from the metal. Such as pentahalophenyl ligands. Vinyl and acyl also have an alternative 2-bonding mode:Synthesis of Metal Alkyl ComplexesMetathetical exchange using a carbon nucleophile (R-). Common reagents are RLi, RMgX (or R2Mg), ZnR2,AlR3,BR3, and PbR4. vThis spans a range from strong to very weak as strong nucleophiles can sometimes result in undesired reduction. vMuch of this alkylation chemistry can be understood with Pearsons hard-soft principles. Here are just a few examples of nucleophilic routes; FeOCCpCOCl+RMgXCp(CO)2Fe-R+MgX2softhardsofthardsshhR reagent (nucleophilic attack on the metal):W C l6W M e6L iC l+L iM eN b C l5N b M e2C l3Z n C l2Z n M e2vnotice that some of the homoleptic alkyls are rather unstable. This is because they have low d-counts, are susceptible to alpha- and/or beta-hydride elimination, and lack good pi-donating ligands to stabilize their high oxidation states: vBeware of reactions where the hard-soft interactions are not so clear as these can represent equilibria instead of complete reactions vMCl + AlR3 M-R + AlR2Cl. Typical examples are a metal anion and alkyl halide (or pseudohalogen). for example: Metal-centered nucleophiles (i.e. using R+ as a reagent)NaFp + RX Fp-R + NaX Fp = Cp(CO)2FeMn(CO)5-MeIMeMn(CO)5 +I-NaFp + RX Fp-R + NaX Fp = Cp(CO)2FeMn(CO)5-MeIMeMn(CO)5 +I-vOne could propose an SN2 (associative mechanism) for this reaction, but a single electron transfer mechanism (SET) could also be postulated. vWhitesides and coworkers examined the above reaction using a stererochemical probe to show that this reaction proceeds with complete inversion of stereochemistry, consistent with an SN2 mechanism. See J. Am. Chem. Soc. 1974 , 96, 2814. However, the nature of the nucleophile is important, and under certain circumstances this can occur through the SET mechanism.Oxidative Addition. This requires a covalently unsaturated, low-valent complex (16 e- or less). A classic example: IrPPh3ClCOPh3PMeIIrClMePPh3COIPh3PVaskas complexC r2 +(a q )C rM e (H2O )M e I+52 +C rI(H2O )2 +5P tL4M e P tI L2M e IvInsertions such as this are involved in the hydrozirconation reaction. Insertion. To form an alkyl, this usually involves an ole