Silicon-Mediated Selective Homo- and Heterocoupling of Carbon Monoxide

While the transformation
of carbon monoxide to multicarbon compounds
(fuels and organic bulk chemicals) via reductive scission of the enormously
strong CO bond is dominated by transition metals, splitting and deoxygenative
reductive coupling of CO under nonmatrix conditions using silicon,
the second most abundant nonmetal of the earth’s crust, is
extremely scarce and mechanistically not well understood. Herein,
we report the selective deoxygenative homocoupling of carbon monoxide
by divalent silicon utilizing the (LSi:)2Xant 1a [Xant = 9,9-dimethyl-xanthene-4,5-diyl; PhC­(NtBu)­2] and (LSi:)2Fc 1b (Fc = 1,1′-ferrocenyl)
as four-electron reduction reagents under mild reaction conditions
(RT, 1 atm), affording the corresponding disilylketenes, Xant­(LSi)2(μ-O)­(μ-CCO) 2a and Fc­(LSi)2(μ-O)­(μ-CCO) 2b, respectively. However,
the dibenzofuran analogue of 1b, compound 1c, was unreactive toward CO due to the longer distance between the
two SiII atoms, which demonstrated the crucial role of
the Si···Si distance on cooperative CO binding and
activation. This is confirmed by density functional theory (DFT) calculations,
and further theoretical investigations on CO homocoupling with 1a and 1b revealed that the initial step of CO
binding and scission involved CO acting as a Lewis acid (four-electron
acceptor), in sharp contrast to CO activation mediated by transition
metals where CO serves as a Lewis base (two-electron donor). This
mechanism was strongly reinforced by the reaction of 1a with isocyanide Xyl-NC (Xyl = 2,6-Me2C6H3), isoelectronic with CO. Treatment of 1a with
one or two molecules of Xyl-NC furnished the unique (silyl)­(imido)­silene 3a and the CC coupled bis­(Xyl-NC) product 5, respectively, via the isolable doubly bridged Xant­(LSi)2(μ-XylNC)2 intermediate 4. Moreover,
compound 3a reacts with 1 molar equivalent of CO to give
the disilylketenimine Xant­(LSi)2(μ-O)­(μ-CCNR) 6, representing, for the first time, a selective heterocoupling
product of CO with isoelectronic isocyanide.