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

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Abstract
The present article deals with a newer class of ligand bridged asymmetric complexes incorporating ancillary ligands (AL) with varying electronic characteristics: [(bpy)2RuII(μ-HL2−) RuII(bpy)2](ClO4)2·([1](ClO4)2); [(pap)2RuII(μ-HL2−)RuII(pap)2](ClO4)2 ([2](ClO4)2); [(bpy)2RuII(μ-HL2−)RuII(pap)2](ClO4)2 ([3](ClO4)2); [(acac)2RuIII(μ-HL2−)RuIII(acac)2] (4) and [(bpy)2RuII(μ-HL2−)RuIII(acac)2]ClO4 ([5]ClO4) (H3L: 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid, bpy = moderately π-accepting 2,2′-bipyridine, pap = strongly π-accepting 2-phenylazopyridine, acac− = σ-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 12+, 22+, 32+, 4 and 5+, respectively. The detailed magnetic studies revealed that the paramagnetic RuIIIRuIII 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, Δg = g1 − g3 = 0.70) but without any half-field signal near g ∼ 4. The isolated mixed valent RuIIRuIII state in 5+ exhibits weak antiferromagnetic coupling of −0.25 cm−1 and anisotropic EPR with <g> = 2.155, Δg = 0.704 but fails to show a RuII → RuIII IVCT (intervalence charge transfer) transition in the near-IR region up to 2000 nm. The complexes 12+, 22+ and 4 encompassing identical metal fragments, exhibited weak to moderate electrochemical coupling at the intermediate mixed valent states with Kc values of 102–105. The coulometrically generated mixed valent RuIIRuIII state in 13+ or 4− also failed to display any prominent absorption in the near-IR region, but exhibited RuIII based rhombic EPR, implying valence localised situation with negligible intermetallic electronic coupling. The complexes 22+, 32+, 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 12+ selectively recognises the fluoride ion.

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