3,6-bis(2′-pyridyl)pyridazine (L) and its deprotonated form (L-H+)- as ligands for {(acac)2Run+} or {(bpy)2Rum+}:: investigation of mixed valency in [{(acac)2Ru}2(μ-L-H+)]0 and [{(bpy)2Ru}2(μ-L-H+)]4+ by spectroelectrochemistry and EPR

Crystallographically characterised 3,6-bis(2'-pyridyl) pyridazine (L) forms complexes with {(acac)(2)Ru} or {(bpy)(2)Ru2+} via one pyridyl-N/pyridazyl-N chelate site in mononuclear Ru-II complexes (acac)(2)Ru(L), 1, and [(bpy)(2)Ru(L)] (ClO4)(2), [3] (ClO4)(2). Coordination of a second metal complex fragment is accompanied by deprotonation at the pyridazyl-C-5 carbon {L --> (L-H+)} to yield cyclometallated, asymmetrically bridged dinuclear complexes [(acac)(2)Ru-III (mu-L-H+) Ru-III (acac)(2)] (ClO4), [2] (ClO4), and [(bpy)(2)Ru-II (mu- L-H+) Ru-II (bpy)(2)] (ClO4)(3), [4] (ClO4)(3). The different electronic characteristics of the co-ligands, sigma donating acac(-) and pi accepting bpy, cause a wide variation in metal redox potentials which facilitates the isolation of the diruthenium(III) formin [2] (ClO4) with antiferromagnetically coupled Ru-III centres (J = 11.5 cm(-1)) and of a luminescent diruthenium(II) species in [4] (ClO4)(3). The electrogenerated mixed-valent (RuRuIII)-Ru-II states 2 and [4](4+) with comproportionation constants K-c > 10(8) are assumed to be localised with the Ru-III ion bonded via the negatively charged pyridyl-N/pyridazyl-C-5 chelate site of the bridging (L-H+)(-) ligand. In spectroelectrochemical experiments they show similar intervalence charge transfer bands of moderate intensity around 1300 nm and comparable g anisotropies (g(1)-g(3) approximate to 0.5) in the EPR spectra. However, the individual g tensor components are distinctly higher for the pi acceptor ligated system [4](4+), signifying stabilised metal d orbitals.