Catalysis by group IV amido-pyridinate complexes for the reduction of carbon dioxide to methane

A highly attractive renewable energy technology involves the transformation of CO2 into fuel. Indeed, CO2 has not to be regarded as a waste product from the combustion of fossil fuel but rather as a chemical resource to be harvested and recycled into products of added value using the assistance of a catalyst. The metal-mediated CO2 reduction with silanes (hydrosilylation) is a thermodynamically favored chemical process and it can be conveniently applied to the transformation of this feedstock. Early transition-metal complexes stabilized by nitrogen-containing ligands have been identified as valuable candidates for a number of highly efficient and selective catalytic transformations. In particular, amidopyridinate Group-IV organometallics have shown excellent performance as catalysts precursors in olefins oligomerization, polymerization, and copolymerization as well as in the intramolecular hydroamination of primary and secondary aminoalkenes. In search for catalytic applications in the renewable energy field, we have focused on a new class of neutral dibenzyl ZrIV and HfIV complexes stabilized by a tridentate dianionic benzoimidazolyl-amidopyridinate ligand [(N-,N,N-)MIVBn2; M = Zr, Hf] as pre-catalysts for the CO2 hydrosylilation reaction. In this study, we have demonstrated that their in-situ activation with an equimolar amount of B(C6F5)3 leads to the generation of cationic monoalkyl species. In the presence of silanes, these cations promote the CO2 reduction to methane under mild reaction conditions (room temperature and 1 atm of CO2). 13C- and 13C{1H}-NMR experiments with isotopically enriched 13CO2 have been conducted to check the reaction course through the generation and conversion of all the reduction intermediates. A full account of the catalytic performance of these Group IV amido-pyridinate complexes in the reduction of carbon dioxide with various hydrosilanes will be discussed.