Metal-Tuned Ligand Reactivity Enables CX2 (X = O, S) Homocoupling with Spectator Cu Centers.

Abstract

Ligand non-innocence is ubiquitous in catalysis with ligands in synthetic complexes contributing as electron reservoirs or co-sites for substrate activation. The latter chemical non-innocence is manifested in H+ storage or relay at sites beyond the metal primary coordination sphere. Reaction of a competent CO2-to-oxalate reduction catalyst, namely, [K(THF)3](Cu3SL), where L3- is a tris(β-diketiminate) cyclophane, with CS2 affords tetrathiooxalate at long reaction times or at high CS2 concentrations, where otherwise an equilibrium is established between the starting species and a complex-CS2 adduct in which the CS2 is bound to the C atom on the ligand backbone. X-ray diffraction analysis of this adduct reveals no apparent metal participation, suggesting an entirely ligand-based reaction controlled by the charge state of the cluster. Thermodynamic parameters for the formation of the aforementioned Cligand-CS2 bond were experimentally determined, and trends with cation Lewis acidity were studied, where more acidic cations shift the equilibrium toward the adduct. Relevance of such an adduct in the reduction of CO2 to oxalate by this complex is supported by DFT studies, similar effects of countercation Lewis acidity on product formation, and the homocoupled heterocumulene product speciation as determined by isotopic labeling studies. Taken together, this system extends chemical non-innocence beyond H+ to effect catalytic transformations involving C-C bond formation and represents the rarest example of metal-ligand cooperativity, that is, spectator metal ion(s) and the ligand as the reaction center.