The mpz+/mpz•Pair as Organic Analogue of the NO+/NO•Ligand Redox System (mpz =N-Methylpyrazinium). A Combined Electrochemical and Spectroscopic Study (UV/Vis/NIR, IR, EPR) of Complexes [(mpz)M(CN)5]2-/3-(M = Fe, Ru, Os)

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Abstract
The oxidation and reduction behavior of complexes [(mpz)M(CN)5]2- between the group 8 pentacyanometalates(II) and the N-methylpyrazinium ion mpz+ was studied using spectroelectrochemical techniques (UV/vis/NIR, IR, EPR) in nonaqueous and, in part, aqueous media. As a further source of information on the electronic structure, we studied the variable solvatochromism of MLCT absorption bands in the precursor ions [(mpz)M(CN)5]2-. The mpz-centered one-electron reduction showed a tendency for concomitant loss of cyanide (EC process) to yield ions [(mpz)M(CN)4]2-, the lability increasing in the order M = Os << Ru < Fe. In contrast to the EPR spectra obtained from reduction with S2O42- in aqueous media, which showed no evidence for close mpz•/metal association, the intra muros electrolysis experiments in DMSO or acetonitrile yielded the following results: The EPR spectrum that was previously assigned to [(mpz)Fe(CN)5]3- is now ascribed to [(mpz)Fe(CN)4]2-. Whereas the dissociatively inert osmium radical complex [(mpz)Os(CN)5]3- is distinguished by a broad EPR signal with a rather large value giso = 2.0157, the EPR spectrum of [(mpz)Ru(CN)4]3- reveals well-resolved separate coupling with the 99Ru and 101Ru isotopes in natural abundance. Spectroelectrochemistry and reactivity patterns of the complex ions [(mpz)M(CN)5]2- are reminiscent of results for corresponding nitrosyl complexes [(NO)M(CN)5]2-, which suggests that the mpz+/mpz• ligand redox pair can function as an organic analogue of the NO+/NO• redox system.

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