Cambered Bipyridyl Ligand with Extended Aryl System Enables Electrochemical Reduction of Carbon Dioxide and Bicarbonate by Mn(bpy)(CO)3Br-type Catalyst Immobilized on Carbon Nanotubes.

Heterogeneous materials containing molecular catalytic sites show promise for electrocatalytic reduction of CO2 to energy-enriched carbon products. Interactions between the catalyst and the heterogeneous support increasingly are recognized as important in governing product selectivity and rate. Recent work on Mn(R-bpy)(CO)3Br type catalysts immobilized on multiwalled carbon nanotubes (MWCNT) demonstrated control of electrocatalytic behavior with steric modification of the molecular catalyst. Phenyl groups installed in the 4,4' positions of the bipyridine ligand (ph-bpy) maximized performance through π-π interactions with the MWCNT support. Herein we report the outcome of extending the ligand π system with Mn(nap-bpy)(CO)3Br (nap-bpy = 4,4'-di(naphthalen-1-yl)-2,2'-bipyridine) and Mn(pyr-bpy)(CO)3Br (pyr-bpy = 4,4'-di(pyren-1-yl)-2,2'-bipyridine) immobilized on MWCNT. We demonstrate exceptional electrocatalysis with Mn(nap-bpy)(CO)3Br/MWCNT (FECO > 92%; JCO = 16.5 mA/cm2) and find that this catalyst electrochemically reduces bicarbonate in the absence of deliberately added CO2 at a remarkable overall selectivity of >80% for carbon products (FEHCOO- = 52% and FECO = 29%). We show diminishing returns to simply adding aromatic character to the bipyridyl ligand with Mn(pyr-bpy)(CO)3Br/MWCNT and observe a unique cambering of the Mn(nap-bpy)(CO)3Br bipyridyl ligand that we believe enables selective catalysis. Mechanistic studies were carried out on Mn(nap-bpy)(CO)3Br/MWCNT using a novel thin-film infrared spectroelectrochemical (IR-SEC) technique. These experiments observe the immobilized Mn(nap-bpy)(CO)3Br undergo single electron reduction to a Mn-centered radical that binds CO2 in a reduction-coupled process.