Non-innocent behaviour of ancillary and bridging ligands in homovalent and mixed-valent ruthenium complexes [A(2)Ru(mu-L)RuA(2)](n), A=2,4-pentanedionato or 2-phenylazopyridine, L2-=2,5-bis(2-oxidophenyl)pyrazine

Abstract

Structurally characterised 2,5-bis(2-hydroxyphenyl)pyrazine (H2L) can be partially or fully deprotonated to form the complexes [(acac)(2)Ru(mu-L)Ru(acac)(2)], [1], acac(-) = acetylacetonato 2,4-pentanedionato, [(pap)(2)Ru(mu-L)Ru(pap)(2)](ClO4)(2), [2](ClO4)(2), pap = 2-phenylazopyridine, or [(pap)(2)Ru(HL)](ClO4), [3](ClO4). Several reversible oxidation and reduction processes were observed in each case and were analysed with respect to oxidation state alternatives through EPR and UV-VIS-NIR spectroelectrochemistry. In relation to previously reported compounds with 2,2'-bipyridine as ancillary ligands the complex redox system [1](n) is distinguished by a preference for metal-based electron transfer whereas the systems [2](n) and [3](n) favour an invariant Ru-II state. Accordingly, the paramagnetic forms of [1](n), n = -, 0, +, exhibit metal-centred spin whereas the odd-electron intermediates [2](+), [2](3+) and [3] show radical-type EPR spectra. A comparison with analogous complexes involving the 3,6-bis(2-oxidophenyl)-1,2,4,5-tetrazine reveals the diminished pi acceptor capability of the pyrazine-containing bridge.