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

Abstract

The present article deals with a newer class of ligand bridged asymmetric complexes incorporating ancillary ligands (AL) with varying electronic characteristics: [(bpy)₂Ru^II(μ-HL²⁻) Ru^II(bpy)₂](ClO₄)₂·([1](ClO₄)₂); [(pap)₂Ru^II(μ-HL²⁻)Ru^II(pap)₂](ClO₄)₂ ([2](ClO₄)₂); [(bpy)₂Ru^II(μ-HL²⁻)Ru^II(pap)₂](ClO₄)₂ ([3](ClO₄)₂); [(acac)₂Ru^III(μ-HL²⁻)Ru^III(acac)₂] (4) and [(bpy)₂Ru^II(μ-HL²⁻)Ru^III(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 Ru^IIIRu^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, Δg = g₁ − g₃ = 0.70) but without any half-field signal near g ∼ 4. The isolated mixed valent Ru^IIRu^III 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 Ru^II → Ru^III 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 K_c values of 10²–10⁵. The coulometrically generated mixed valent Ru^IIRu^III state in 1³⁺ 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 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.