Importance of Ligand Exchange in the Modulation of Molecular Catalysis: Mechanism of the Electrochemical Reduction of Nitrous Oxide with Rhenium Bipyridyl Carbonyl Complexes

Molecular catalysis of electrochemical reactions involving transition-metal complexes as catalysts requires getting a free metal coordination site to bind the substrate. It implies that the generation of a strong coordinating ligand as a product or coproduct of the reaction might be detrimental for an efficient catalysis because it can bind the metal center and block or slow down the catalytic process. This self-modulation phenomenon is revealed and illustrated via a thorough spectro-electrochemical investigation of the mechanism of the electrochemical reduction of nitrous oxide with rhenium bipyridyl triscarbonyl complexes [Re(bpy)(CO)3X]n+ (X = CH3CN, Cl-, n = 0 or 1) as a catalyst. We show that the bi-reduced [Re0(bpy•-)(CO)3]-, electrogenerated from [Re(bpy)(CO)3X]n+, readily reacts with N2O and produces the hydroxo complex [ReI(bpy)(CO)3(OH)]. Because hydroxide, a product of the reaction, is a stronger coordinating ligand than acetonitrile or chloride, catalysis does not occur significantly at a potential where [Re0(bpy•-)(CO)3]- is generated from [Re(bpy)(CO)3X]n+. Substantial catalysis is only triggered at a potential corresponding to the second reduction of [ReI(bpy)(CO)3(OH)]. Nonetheless, we show that a slower innersphere reduction of N2O by the mono-reduced [ReI(bpy•-)(CO)3X](n-1)+ (X = CH3CN, Cl-) occurs due to the lability of acetonitrile and chloride in these species. Because hydroxide is less labile and cannot be displaced to create an open coordination position for N2O, only an even slower outersphere reduction of N2O by the mono-reduced [ReI(bpy•-)(CO)3(OH)]- takes place. However, we finally show that an excess of free chlorides is able to displace hydroxide and then open the way for a faster innersphere process. This remarkable example emphasizes the critical role of ligand exchange in modulating molecular catalysis of electrochemical reactions with transition-metal complexes as catalysts, a likely general phenomenon.