One- and two-electron photooxidation of a molybdenum(III) thiocyanate complex

Abstract

There has been continuing interest in the properties of excited states of transition-metal complexes and in their use as catalysts for redox reactions. However, photoredox processes have generally been confined to one-electron transfers, while many desirable redox reactions require the transfer of two or more electrons in a single substrate molecule. One-electron photoredox reactions, such as those of polypyridine complexes of chromium(III)1 and ruthenium(II),2and multielectron redox activity in ruthenium- and osmium-based systems,3 have been investigated separately. Our goal has been to generate multielectron oxidants photochemically by beginning with d3 complexes of the early transition metals: initial excited-state electron transfer is to be followed by a second thermal electron transfer, thus producing a net two-electron transformation. In addition to our recent work with vanadium(II),4 we have been interested in the heavier d3 ions, whose photoredox properties have not been studied before. We now report facile ground- and excited-state oxidation of the hexakis(thiocyanato-N)molybdate(3–) ion, Mo(NCS)63-, and the disproportionation of the initial electron-transfer product Mo(NCS)62-to generate the molybdenum(V) dimer (SCN)3Moo(μ-O)2Moo(Ncs)34-. These observations constitute the first photoredox reactions ever recorded in a molybdenum(III) complex. They indicate not only that a broad new class of transition-metal complexes can be used as redox sensitizers but that many such systems are likely to be useful in photoinitiated net two-electron-transfer processes. © 1986, American Chemical Society. All rights reserved.

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