Catalyst Control of Selectivity in CO₂ Reduction Using a Tunable Heterobimetallic Effect

Abstract

A tunable bimetallic effect on product selectivity in catalytic CO2 reduction was identified using N-heterocyclic carbene-ligated Cu complexes. While the monometallic Cu-only system catalyzes hydroboration of CO2 with pinacolborane to produce formate exclusively, introducing a bimetallic effect with analogous Cu-Fe, Cu-W, and Cu-Mo catalysts produces mixtures of formate and CO. Within a series of isosteric catalysts, the selectivity of CO versus formate was controlled by tuning the electronic nature of the Cu/M pairing, with high selectivity for CO being achieved using a Cu-Mo catalyst.

Selectivity Effects in Bimetallic Catalysis

“… Proposed modes of CO 2 binding and activation at a) a mononuclear Mo site of a formate dehydrogenase enzyme, and at b) Ni/Fe and c) Mo/Cu binuclear sites of carbon monoxide dehydrogenase enzymes. This figure was reproduced with permission from reference (copyright 2015 American Chemical Society).…”

“…These mononuclear systems give different products due to different active forms of the catalysts being generated by the different reductants: B 2 pin 2 gives [(NHC)Cu(Bpin)] catalysts that are active for CO 2 deoxygenation, while HBpin gives [(NHC)CuH] catalysts that are active for CO 2 hydroboration. In 2015, Bagherzadeh and Mankad compared the catalytic behavior, using the same reductant, of mononuclear [(NHC)Cu(O t Bu)] catalysts with that of binuclear [(NHC)Cu(M)] catalysts (M=FeCp(CO) 2 , WCp(CO) 3 , MoCp(CO) 3 ) . Using HBpin as the reductant under analogous conditions, CO 2 was reduced to HCO 2 Bpin by mononuclear [(NHC)Cu(O t Bu)] catalysts and was reduced predominantly to CO by many of the binuclear (NHC)Cu[M] catalysts.…”

“…Bottom: proposed auto‐tandem mechanism for bimetallic CO 2 reduction and formate decarbonylation. Portions of this figure were reproduced or adapted with permission from reference (copyright 2015 American Chemical Society).…”

“…The focus of this Concept article is to highlight the effect of catalyst nuclearity, as well as bimetallic pairings within binuclear systems themselves, on catalytic selectivity. Because bimetallic catalysis is underdeveloped compared to single‐site catalysis, in some existing cases bimetallic effects have been rationalized post hoc, in some others they still require careful ligand design to control nuclearity and intermetallic distances, and in others yet the apparent bimetallic effect was found to actually result from tandem monometallic reactions upon investigation . Nonetheless, understanding the selectivity effects operative in these systems serves to guide future efforts in designing bimetallic catalysts, in which nuclearity and bimetallic compatibility can be tuned rationally and independently of other parameters.…”

Selective Reduction of CO2 to a Formate Equivalent with Heterobimetallic Gold‐ ‐ ‐Copper Hydride Complexes

“…[46] Thef rustrated Lewis pair mode of activation of CO 2 and CO with transition-metal complexes has been posited before. One plausible mechanism for CO 2 reduction involves coordination of the C 1 reagent to the d 10 metal fragment followed by nucleophilic attack by either the gold or boron hydride species.…”