Exploring the full potential of photocatalytic carbon dioxide reduction using a dinuclear Re2Cl2 complex assisted by various phhotosensitizers

Abstract

Good cooperation: The full potential of a xanthene-bridged homodinuclear rhenium(I) complex (Re2Cl2) for the light-driven reduction of CO2 to CO is enabled by addition of iridium(III) or copper(I) photosensitizers. The Re2Cl2 catalyst with two singly-reduced rhenium moieties is generated through photocatalytic induced potent reductants. This in turn enables bimetallic activation of CO2 and leads to an increase in catalytic activity. Photosensitizing units have already been applied to enable light-driven catalytic reduction of CO2 with mononuclear rhenium complexes. However, dinuclear catalytic systems that are able to activate CO2 in a cooperative bimetallic fashion have only rarely been combined with photosensitizers. We here present detailed studies on the influence of additional photosensitizers on the catalytic performance of a dirhenium complex (Re2Cl2) and present correlations with spectroscopic measurements, which shed light on the reaction mechanism. The use of [Ir(dFppy)3] (Ir, dFppy=2-(4,6-difluorophenyl)pyridine)) resulted in considerably faster CO2 to CO transformation than [Cu(xant)(bcp)]PF6 (Cu, xant=xantphos, bcp=bathocuproine). Emission quenching studies, transient absorption as well as IR spectroscopy provide information about the electron transfer paths of the intermolecular systems. It turned out that formation of double reduced species [Re2Cl2]2− along with an intermediate with a Re−Re bond ([ReRe]) can be taken as an indication of multi-electron storage capacity. Furthermore, under catalytic conditions a CO2-bridged intermediate was identified.