Reductive Trimerization of CO to the Deltate Dianion Using Activated Magnesium(I) Compounds

This study details syntheses of unsymmetrical magnesium(I)-adduct complexes, [(^ArNacnac)(D)Mg-Mg(^ArNacnac)] (^ArNacnac = [(ArNCMe)₂CH]^-), Ar = xylyl (Xyl), mesityl (Mes), 2,6-diethylphenyl (Dep), or 2,6-diisopropylphenyl (Dip); D = N-heterocyclic carbene or 4-dimethylaminopyridine, DMAP), which X-ray crystallographic studies show to have markedly elongated Mg-Mg bonds. Two of these highly reactive species are shown to reductively trimerize CO to yield rare crystallographically characterized examples of the planar, aromatic deltate dianion, incorporated in the complexes [{(^DipNacnac)(D)Mg(μ-C₃O₃)Mg(^DipNacnac)}₂] (D = DMAP or:C{N(Me)C(Me)}₂). DFT calculations suggest that these complexes form via stepwise two-electron reductions of three CO molecules, resulting in the formation of three C-C bonds within the cyclic deltate unit. This work highlights the utility of activated magnesium(I) adduct complexes as soluble organometallic models for the study of reductive C-C bond-forming events in, for example, the heterogeneously catalyzed Fischer-Tropsch process.