A CuICoII cryptate for the visible light-driven reduction of CO2

Abstract

Among the rare bimetallic complexes known for the reduction of CO₂, Co^IICo^II and Zn^IICo^II hexamine cryptates are described as efficient photocatalysts. In close relation to the active sites of natural, CO₂-reducing enzymes, we recently reported the asymmetric cryptand {N^SN^N}_m ({N^SN^N}_m = N[(CH₂)₂SCH₂(m-C₆H₄)CH₂NH(CH₂)₂]₃N) comprising distinct sulphur- and nitrogen-rich binding sites and the corresponding Cu^IM^II (M^II = Co^II, Ni^II, Cu^II) complexes. To gain insight into the effect of metals in different oxidation states and sulphur-incorporation on the photocatalytic activity, we herein investigate the Cu^ICo^II complex of {N^SN^N}_m as catalyst for the visible light-driven reduction of CO₂. After 24 h irradiation with LED light of 450 nm, Cu^ICo^II-{N^SN^N}_m shows a high efficiency for the photocatalytic CO₂-to-CO conversion with 9.22 μmol corresponding to a turnover number of 2305 and a high selectivity of 98% over the competing H₂ production despite working in an acetonitrile/water (4 : 1) mixture. Experiments with mononuclear counterparts and computational studies show that the high activity can be attributed to synergistic catalysis between Cu and Co. Furthermore, it was shown that an increase of the metal distance results in the loss of synergistic effects and rather single-sited Co catalysis is observed.