Non-innocent behaviour of ancillary and bridging ligands in homovalent and mixed-valent ruthenium complexes [A2Ru(mu-L)RuA2]n, A = 2,4-pentanedionato or 2-phenylazopyridine, L(2-) = 2,5-bis(2-oxidophenyl)pyrazine

Structurally characterised 2,5-bis(2-hydroxyphenyl)pyrazine (H2L) can be partially or fully deprotonated to form the complexes (acac)2Ru(mu-L)Ru(acac)2], 1], acac = acetylacetonato = 2,4-pentanedionato, (pap)2Ru(mu-L)Ru(pap)2](ClO4)2, 2](ClO4)2, pap = 2-phenylazopyridine, or (pap)2Ru(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.