Light-Induced Activation of a Molybdenum Oxotransferase Model within a Ru(II)–Mo(VI) Dyad

Abstract

Nature uses molybdenum-containing enzymes to catalyze oxygen atom transfer (OAT) from water to organic substrates. In these enzymes, the two electrons that are released during the reaction are rapidly removed, one at a time, by spatially separated electron transfer units. Inspired by this design, a Ru(II)-Mo(VI) dyad was synthesized and characterized, with the aim of accelerating the rate-determining step in the cis-dioxo molybdenum-catalyzed OAT cycle, the transfer of an oxo ligand to triphenyl phosphine, via a photo-oxidation process. The dyad consists of a photoactive bis(bipyridyl)-phenanthroline ruthenium moiety that is covalently linked to a bioinspired cis-dioxo molybdenum thiosemicarbazone complex. The quantum yield and luminescence lifetimes of the dyad [Ru(bpy)(L)MoO(solv)] were determined. The major component of the luminescence decay in MeCN solution (τ = 1149 ± 2 ns, 67%) corresponds closely to the lifetime of excited [Ru(bpy)(phen-NH)], while the minor component (τ = 320 ± 1 ns, 31%) matches that of [Ru(bpy)(H-L)]. In addition, the (spectro)electrochemical properties of the system were investigated. Catalytic tests showed that the dyad-catalyzed OAT from dimethyl sulfoxide to triphenyl phosphine proceeds significantly faster upon irradiation with visible light than in the dark. Methylviologen acts as a mediator in the photoredox cycle, but it is regenerated and hence only required in stoichiometric amounts with respect to the catalyst rather than sacrificial amounts. It is proposed that oxidative quenching of the photoexcited Ru unit, followed by intramolecular electron transfer, leads to the production of a reactive one-electron oxidized catalyst, which is not accessible by electrochemical methods. A significant, but less pronounced, rate enhancement was observed when an analogous bimolecular system was tested, indicating that intramolecular electron transfer between the photosensitizer and the catalytic center is more efficient than intermolecular electron transfer between the separate components.

Oxygen Atom Transfer Reactivity of Molybdenum(VI) Complexes Employing Pyrimidine- and Pyridine-2-thiolate Ligands

“…Molybdenum is the only second-row transition metal that is required by most living organisms, where it is found as a mononuclear metal center in the active site of many enzymes. , With the exception of nitrogenase and related proteins, the active site of all molybdoenzymes contains a pyranopterin-dithiolene cofactor in which the metal is coordinated by the dithiolene moiety. − The dimethyl sulfoxide reductase (DMSOR) superfamily is structurally and catalytically the largest and most diverse family of molybdoenzymes, and reactions catalyzed by its members frequently involve oxygen atom transfer (OAT). − With these naturally occurring systems as inspiration, dioxidomolybdenum­(VI) complexes have been extensively investigated in catalytic OAT reactions. − Since an aqueous environment has not been feasible for the majority of those complexes, a widespread model reaction, which was developed in the 1980s, is the OAT from Me 2 SO to PPh 3 , yielding Me 2 S and POPh 3 . − The ligands utilized in this model reaction are either dithiolene or non-dithiolene type systems. , Although bidentate non-dithiolene ligands with S,N, O,O, or N,O donor sets are structurally obviously different from the molybdopterin cofactor present in molybdoenzymes, they were found to be generally more suitable for molybdenum-catalyzed OAT reactions. − Even more different tri- and tetradentate ligands with various donor sets − have been successfully utilized, with the class of scorpionate ligands being outstanding. − Furthermore, various theoretical studies have been performed to assess the influence of the protein ligand , or charge differences and to compare active sites containing either molybdenum or tungsten. , Apart from sulfoxides, molybdenum complexes could also be applied in the deoxygenation of more challenging substrates: nitrate is reduced by naturally occurring nitrate reductases belon...…”

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

“…Indeed, in the experiments using MV 2+ and PPh 3 as oxidizing and reducing agents, respectively, the catalytic rate enhancement was observed only upon irradiation in the presence of MV 2+ and the OAT to the PPh 3 was an extremely slow process (B minutes). 17 This points to the conclusion that the oxidative quenching by MV 2+ prolongs the lifetime of the oxidized Mo unit beyond ms timescale, preventing the charge recombination at the Mo unit before the very slow OAT process occurs. Conversely, faster catalytic events at the Mo unit could lead to a different reaction path.…”

“…The difference TA spectrum ''200 ps-200 fs'' and the difference spectrum obtained during spectro-electrochemical oxidation of the catalytic control compound ([MoO 2 (L Me )MeOH]), 17 resemble each other closely and over an extended spectral range, after applying a shift of 0.65 eV (Fig. 3B).…”

“…To explain their findings, (1) the Ru(II) is photo-oxidized to Ru(III) and one of the bpy is reduced upon excitation to the lowest metal-to-ligand charge transfer excited state; (2) the photoreduced bpy À is oxidized by a diffusion-limited reaction with an oxidizing agent (OxA), (3) the Mo(VI) unit is activated by the back-reduction of Ru(III) to Ru(II); (4) a reducing agent (ReA) is oxidized via oxygen atom transfer (OAT) from the activated cis-dioxo molybdenum unit. 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 0 -bipyridine(bpy) ligand is photo-reduced (1); the latter is then oxidized by an oxidizing agent (e.g.…”

Redox‐Controlled Stabilization of an Open‐Shell Intermediate in a Bioinspired Enzyme Model

“…Well‐established bioinspired model complexes for molybdenum containing oxotransferases have been equipped with electron relays , , . Even a light‐induced single‐electron transfer activation of a molybdenum oxotransferase model has been recently reported by Duhme‐Klair . Remote redox changes also accelerated the oxygen‐atom transfer (OAT) from iodosobenzene to a manganese(II) metal site linked to three iron sites …”