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)2Ru2(m-L - H+)0 and (bpy)2Ru2(m-L - H+)4+ by spectroelectrochemistry and EPR
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Dalton Transactions (2005)

Crystallog. characterized 3,6-bis(2-pyridyl)pyridazine (L) forms complexes with (acac)2Ru or (bpy)2Ru2+ via one pyridyl-N/pyridazinyl-N chelate site in mononuclear RuII complexes (acac)2Ru(L), 1, and (bpy)2Ru(L)(ClO4)2, 3(ClO4)2. Coordination of a second metal complex fragment is accompanied by deprotonation at the pyridazinyl-C5 carbon L → (L - H+)- to yield cyclometalated, asym. bridged dinuclear complexes (acac)2RuIII(m-L - H+)RuIII(acac)2(ClO4), 2(ClO4), and (bpy)2RuII(m-L - H+)RuII(bpy)2(ClO4)3, 4(ClO4)3. The different electronic characteristics of the co-ligands, s donating acac- and p accepting bpy, cause a wide variation in metal redox potentials which facilitates the isolation of the diruthenium(III) form in 2(ClO4) with antiferromagnetically coupled RuIII centers (J = -11.5 cm-1) and of a luminescent diruthenium(II) species in 4(ClO4)3. The electrogenerated mixed-valent RuIIRuIII states 20 and 44+ with comproportionation consts. Kc \textgreater 108 are assumed to be localized with the RuIII ion bonded via the neg. charged pyridyl-N/pyridazinyl-C5 chelate site of the bridging (L - H+)- ligand. In spectroelectrochem. expts. they show similar intervalence charge transfer bands of moderate intensity around 1300 nm and comparable g anisotropies (g1-g3 $\approx$ 0.5) in the EPR spectra. However, the individual g tensor components are distinctly higher for the p acceptor ligated system 44+, signifying stabilized metal d orbitals. on SciFinder(R)
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