Unveiling the origin of photo-induced enhancement of oxidation catalysis at Mo(vi) centres of Ru(ii)–Mo(vi) dyads

Abstract

hoto-induced oxidation-enhancement in biomimetic bridged Ru(II)–Mo(VI) photo-catalyst is unexpectedly photo-activated in ps timescales. One-photon absorption generates an excited state where both photo-oxidized and photo-reduced catalytic centres are activated simultaneously and independently. Graphical abstract: Unveiling the origin of photo-induced enhancement of oxidation catalysis at Mo(vi) centres of Ru(ii)–Mo(vi) dyads

Metalloenzymes often regenerate their active sites via efficient electron transfer (eT) between spatially separated eT units.1 Thus, the comprehension of electronic relays between these components in bioinspired redox catalysts is attracting increasing interest.2,3 Electrochemical approaches to mimic this process include for instance the attachment of reversible eT components to metalloenzyme mimics.4–6

Photo-induced eT processes have also been explored to harness solar energy and to accelerate redox catalysis. For example, ruthenium-based photosensitizers linked to manganese-based photosystem II models were used to mimic the four-electron oxidation of two water molecules by light-induced eT.7 Similarly, hydrogenase mimics with attached reversible photosensitizers have been investigated with the aim of catalysing the light-driven production of H2.8,9 Photoredox processes10–12 have also been used to catalyse oxygenation reactions with environmentally benign O-atom sources, in particular 3O2 or H2O.13 Enemark and Kirk et al. demonstrated that oxo-molybdenum(v) can be photoactivated via an antenna-mediated eT process by covalently linking the oxo–Mo(v) unit to porphyrin-Fe(iii) or Zn(ii) complexes.14,15

Recently, Duhme-Klair et al. developed biomimetic molybdenum complexes16 with appended ruthenium-based photoactive units to facilitate oxygen atom transfer (OAT) catalysis via photo-induced eT (Fig. 1). They found that the attachment of a bis(bipyridyl)–phenanthroline ruthenium complex to a cis-dioxo Mo(vi) thiosemicarbazone allows the OAT from dimethyl sulfoxide to triphenyl phosphine (PPh3) to be accelerated upon irradiation with visible light. To explain their findings, specifically the photo-induced oxidation-enhancing (PIOE) effect,17 they proposed the scheme illustrated in Fig. 1: the Ru(ii) is first photo-oxidized and one 2,2′-bipyridine(bpy) ligand is photo-reduced (1); the latter is then oxidized by an oxidizing agent (e.g. methyl viologen) (2), leading to the production of a highly reactive, one-electron oxidized catalytic Mo(vi) unit via an intramolecular eT toward the Ru(iii) (3). Eventually a reducing agent, as for instance PPh3, is oxidized via the OAT from the activated cis-dioxo molybdenum unit (4).