The role of [Re(η6-arene)2]+ substituents in electro- and photocatalytic water reduction with [Co(terpy)2]3+ catalysts

Abstract

In this report, we assess the electronic influence of [Re(η⁶-arene)₂]⁺ sandwich complexes conjugated to the water reducing catalyst [Co(terpy)₂]^2+/3+. We compare catalytic performances and catalytic activities of conjugates in which the catalyst and the sandwich complex are coupled via carbon–carbon bonds or via secondary amine linkers. For this purpose, the [Re(η⁶-arene)₂]⁺ sandwich was bound to the 4′-position of 2,2′:6′,2′′-terpyridine (terpy) and to the corresponding [Co^III(4′-R-terpy)₂]ⁿ⁺ complexes. Electro- and photocatalytic water reduction activities of the model catalysts indicated that direct conjugation of the two functional subunits has adverse effects on the catalytic performances, presumably due to irreversible reduction of the rhenium complexes. Connecting the two functional units through a secondary amine linker had essentially no impact on cyclic voltammogram shapes and the influence on the electrocatalytic hydrogen evolution was minimal. Such conjugates increased catalyst stabilities during photocatalytic hydrogen production, which was reflected by higher turnover numbers and longer catalytically active phases than the catalysts alone. Accordingly, a general approach of attaching catalysts to the [Re(η⁶-arene)₂]⁺ framework is demonstrated.