Influence of ancillary ligands on the electronic structure and anion sensing features of ligand bridged diruthenium complexes

The present article deals with a newer class of ligand bridged asymmetric complexes incorporating ancillary ligands (AL) with varying electronic characteristics: [(bpy)(2)Ru-II(mu-HL2-) Ru-II(bpy)(2)](ClO4)(2)center dot([1](ClO4)(2)); [(pap)(2)Ru-II(mu-HL2-) Ru-II(pap)(2)](ClO4)(2) ([2](ClO4)(2)); [(bpy)(2)Ru-II(mu-HL2-)Ru-II(pap)(2)](ClO4)(2) ([3](ClO4)(2)); [(acac)(2)Ru-III(mu-HL2-)Ru-III(acac)(2)] (4) and [(bpy)(2)Ru-II(mu-HL2-)Ru-III(acac)(2)]ClO4 ([5]ClO4) (H3L: 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid, bpy = moderately pi-accepting 2,2'-bipyridine, pap = strongly pi-accepting 2-phenylazopyridine, acac(-) = sigma-donating acetylacetonate). The molecular identity of [1](ClO4)(2) was established by its single crystal X-ray structure. A large shift in Ru(II)/Ru(III) potential of 0.7-2.0 V took place on switching the ancillary ligands from AL = bpy to pap to acac(-), leading to the stabilisation of ruthenium(II) and ruthenium(III) states in 1(2+), 2(2+), 3(2+), 4 and 5(+), respectively. The detailed magnetic studies revealed that the paramagnetic (RuRuIII)-Ru-III state in 4 essentially behaves as a system with two independent S = 1/2 spins and it exhibits an anisotropic EPR at 77 K (< g > = 2.192, Delta g = g(1) - g(3) = 0.70) but without any half-field signal near g similar to 4. The isolated mixed valent (RuRuIII)-Ru-II state in 5(+) exhibits weak antiferromagnetic coupling of -0.25 cm(-1) and anisotropic EPR with < g > = 2.155, Delta g = 0.704 but fails to show a Ru-II -> Ru-III IVCT (intervalence charge transfer) transition in the near-IR region up to 2000 nm. The complexes 1(2+), 2(2+) and 4 encompassing identical metal fragments, exhibited weak to moderate electrochemical coupling at the intermediate mixed valent states with K-c values of 10(2)-10(5). The coulometrically generated mixed valent (RuRuIII)-Ru-II state in 1(3+) or 4(-) also failed to display any prominent absorption in the near-IR region, but exhibited Ru-III based rhombic EPR, implying valence localised situation with negligible intermetallic electronic coupling. The complexes 2(2+), 3(2+), 4 and 5(+) having one NH proton associated with the bridging ligand HL2- do not show any interaction with the anions F-, Cl-, Br-, I-, HSO4-, H2PO4-, OAc- and SCN- in CH3CN, though 1(2+) selectively recognises the fluoride ion.