Electrocatalytic reduction of CO2 to CO by a series of organometallic Re(i)-tpy complexes

Abstract

A series of organometallic Re(I)(L)(CO)₃Br complexes with 4′-substituted terpyridine ligands (L) has been synthesised as electrocatalysts for CO₂ reduction. The complexes’ spectroscopic characterisation and computationally optimised geometry demonstrate a facial geometry around Re(I) with three cis COs and the terpyridine ligand coordinating in a bidentate mode. The effect of substitution on the 4′-position of terpyridine (Re1–5) on CO₂ electroreduction was investigated and compared with a known Lehn-type catalyst, Re(I)(bpy)(CO)₃Br (Re7). All complexes catalyse CO evolution in homogeneous organic media at moderate overpotentials (0.75–0.95 V) with faradaic yields of 62–98%. The electrochemical catalytic activity was further evaluated in the presence of three Brønsted acids to demonstrate the influence of the pK_a of the proton sources. The TDDFT and ultrafast transient absorption spectroscopy (TAS) studies showed combined charge transfer bands of ILCT and MLCT. Amongst the series, the Re-complex containing a ferrocenyl-substituted terpyridine ligand (Re5) shows an additional intra-ligand charge transfer band and was probed using UV-Vis spectroelectrochemistry.