Scission of carbon monoxide using TaR3, R=(N(tBu)Ph) or OSi(tBu)3: a DFT investigation

The experimentally known reduction of carbon monoxide using a 3-coordinate [Ta(silox)(3)] (silox=OSi(tBu)(3)) complex initially forms a ketenylidene [(silox)(3)Ta-CCO], followed by a dicarbide [(silox)(3)Ta-CC-Ta(silox)(3)] structure. The mechanism for this intricate reaction has finally been revealed by using density functional theory, and importantly a likely structure for the previously unknown intermediate [(silox)(3)Ta-CO](2) has been identified. The analysis of the reaction pathway and the numerous intermediates has also uncovered an interesting pattern that results in CO cleavage, that being scission from a structure of the general form [(silox)(3)Ta-C(n)O] in which n is even. When n is odd, cleavage cannot occur. The mechanism has been extended to consider the effect of altering both the metal species and the ligand environment. Specifically, we predict that introducing electron-rich metals to the right of Ta in the periodic table to create mixed-metal dinuclear intermediates shows great promise, as does the ligand environment of the Cummins-style 3-coordinate amide structure. This latter environment has the added complexity of improved electron donation from amide rotation that can significantly increase the reaction exothermicity.