Pendant-cation assisted spontaneous axial halide substitution in group 7 M(L)(CO)3X complexes

Abstract

The installation of cationic pendant groups proximal to catalyst active sites has resulted in improved performance for electrocatalytic CO₂ reduction (CO2R) by molecular catalysts. It is not well understood how pendant cationic groups, designed to stabilize anionic CO₂ reduction intermediates, interact with the axial halide ligands of M(diimine)(CO)₃X (M = Re, Mn; X = Cl, Br), an important class of molecular CO2R electrocatalysts. We observe that in [M(tmam)(CO)₃X]²⁺ complexes (1: M = Re, X = Cl; 2: M = Mn, X = Br, tmam²⁺ = 4,4′-methylenetrimethylammonium-2,2′-bipyridine) bearing the dicationic tmam²⁺ ligand, the cationic pendant groups facilitate the spontaneous exchange of the axial halide for solvent molecules (MeCN or H₂O), a highly unusual behavior for this class of complexes. We present a study of the thermodynamics and kinetics of these reactions under aqueous and non-aqueous conditions. The extent to which this reaction occurs was found to depend on the identity of the metal, with substitution being more favorable for Mn than Re, and the solvent and halide substitution was more favorable in H₂O than MeCN for both complexes. ¹H NMR kinetics experiments for axial chloride substitution for 1 in D₂O suggest a second-order dependence on complex concentration, indicating an intermolecular process. The halide-substitution reaction had little effect on the electrochemistry of the complexes in cyclic voltammetry experiments due to the reductive instability (deamination) of tmam²⁺. This study demonstrates that cationic pendant groups are noninnocent, actively modulating the coordination environment of the catalyst, which provides key insights into their potential roles in catalyst activation.