Electrocatalytic reduction of CO to CO by a series of organometallic Re(I)-tpy complexes.

A series of organometallic Re(I)(L)(CO)Br complexes with 4'-substituted terpyridine ligands (L) has been synthesised as electrocatalysts for CO reduction. The complexes' spectroscopic characterisation and computationally optimised geometry demonstrate a facial geometry around Re(I) with three COs and the terpyridine ligand coordinating in a bidentate mode. The effect of substitution on the 4'-position of terpyridine (Re1-5) on CO electroreduction was investigated and compared with a known Lehn-type catalyst, Re(I)(bpy)(CO)Br (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 p 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.