Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.Ministerio de Ciencia e Innovación Projects CTQ2010–15833, CTQ2013-45011 - P and Consolider - Ingenio 2010 CSD2007 - 00006Junta de Andalucía FQM - 119, Projects P09 - FQM - 5117 and FQM - 2126EU 7th Framework Program, Marie Skłodowska - Curie actions C OFUND – Agreement nº 26722

Reactivity of Bis(imino)pyridine Cobalt Complexes in C-H Bond Activation and Catalytic C-C and C-Si Bond Formation

The reactivity of aryl-substituted bis(imino)pyridine (ArPDI = 2,6-(Ar=NCMe)2C5H3N) cobalt complexes was explored in C-H activation and catalytic C-C and C-Si bond formation. Thermolysis of cobalt azide compounds, in attempts to generate a cobalt nitride complex, resulted in formation of cyclometallated amide and imine products from the insertion of the putative nitride into the benzylic C-H bond of the alkyl group on the imine aryl substituent. Reaction of cobalt dinitrogen complexes with O2 or N2O also failed to furnish the corresponding cobalt oxo compound but resulted in formation of a cobalt hydroxide complex and a cobalt dinitrogen complex where one of the imine methyl groups has lost an equivalent of hydrogen. This reactivity is attributed to the presence of unpaired electrons in the partially filled pi* orbitals of the Co=X (X = N or O) moiety. The synthesis and characterization of cationic (ArPDI)Co alkyl complexes were targeted to model the active species in the (ArPDI)CoCl2/MAO-catalyzed (MAO = methylaluminoxane) ethylene polymerization. Experimental and computational studies established the electronic structure of this class of compounds as a low-spin Co(II) with a neutral chelate. Each of the cationic cobalt alkyl complexes prepared was an active single-component catalyst for ethylene polymerization and exhibited significantly improved activities over cationic cobalt(I) compounds previously proposed as the active species in the reaction. A series of (ArPDI)Co compounds was evaluated for catalytic C-Si bond formation and found to be active pre-catalysts for the dehydrogenative silylation of terminal olefins to generate allylsilanes. Using (MesPDI)CoMe (MesPDI = 2,6-(2,4,6-Me3-C6H2N=CMe)2C5H3N) as representative pre-catalyst, it was demonstrated that linear internal olefins can be isomerized and silylated to the terminal allylsilane. This type of reactivity was utilized in the crosslinking of liquid polysilylhydrides with -olefins to produce solid gels. Mechanistic investigations established that the pre-catalyst is activated by the silylhydride to generate a cobalt silyl complex. The olefin then inserts into the Co-Si bond and subsequent beta-hydrogen elimination liberates the allylsilane product. The resulting cobalt hydride reacts with a second equivalent of olefin and forms a cobalt alkyl which in turn reacts with the silylhydride to regenerate the cobalt-silyl complex

Reversible Carbon–Carbon Bond Formation Induced by Oxidation and Reduction at a Redox-Active Cobalt Complex

The electronic structure of the diamagnetic pyridine imine enamide cobalt dinitrogen complex, (^iPrPIEA)­CoN₂ (^iPrPIEA = 2-(2,6-ⁱPr₂–C₆H₃NCMe)-6-(2,6-ⁱPr₂–C₆H₃NCCH₂)­C₅H₃N), was determined and is best described as a low-spin cobalt­(II) complex antiferromagnetically coupled to an imine radical anion. Addition of potential radical sources such as NO, PhSSPh, or Ph₃Cl resulted in C–C coupling at the enamide positions to form bimetallic cobalt compounds. Treatment with the smaller halocarbon, PhCH₂Cl, again induced C–C coupling to form a bimetallic bis­(imino)­pyridine cobalt chloride product but also yielded a monomeric cobalt chloride product where the benzyl group added to the enamide carbon. Similar cooperative metal–ligand addition was observed upon treatment of (^iPrPIEA)­CoN₂ with CH₂CHCH₂Br, which resulted in allylation of the enamide carbon. Reduction of Coupled-(^iPrPDI)­CoCl (Coupled-(^iPrPDI)­CoCl = [2-(2,6-ⁱPr₂–C₆H₃NCMe)-C₅H₃N-6-(2,6-ⁱPr₂–C₆H₃NCCH₂−)­CoCl]₂) with NaBEt₃H led to quantitative formation of (^iPrPIEA)­CoN₂, demonstrating the reversibility of the C–C bond forming reactions. The electronic structures of each of the bimetallic cobalt products were also elucidated by a combination of experimental and computational methods

Bis(imino)pyridine Cobalt-Catalyzed Dehydrogenative Silylation of Alkenes: Scope, Mechanism, and Origins of Selective Allylsilane Formation

The aryl-substituted bis­(imino)­pyridine cobalt methyl complex, (^MesPDI)­CoCH₃ (^MesPDI = 2,6-(2,4,6-Me₃C₆H₂-NCMe)₂C₅H₃N), promotes the catalytic dehydrogenative silylation of linear α-olefins to selectively form the corresponding allyl­silanes with commercially relevant tertiary silanes such as (Me₃SiO)₂MeSiH and (EtO)₃SiH. Dehydrogenative silylation of internal olefins such as <i>cis</i>- and <i>trans</i>-4-octene also exclusively produces the allyl­silane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C–H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allyl­silane formation