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.