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

摘要

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-HL~(2-)) Ru~ (II)(bpy)_2](ClO_4)_2·([1] (ClO_4)_2); [(pap)_2Ru~(II)(μ-HL~ (2-))Ru~(II)(pap)_2](ClO_4)_2 ([2](ClO_4)_2); [(bpy)_2Ru~(II)(μ- HL~(2-))Ru~(II)(pap)_2](ClO_4)_2 ([3](ClO_4)_2); [(acac)_2Ru~(III)(μ- HL~(2-))Ru~(III)(acac)_2] (4) and [(bpy)_ 2Ru~(II)(μ-HL~(2-))RuIII(acac)_ 2]ClO_4 ([5]ClO_4) (H_3L: 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](ClO_4)_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 Ru~(III)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 ( = 2.192, Δg = g_1 - g_3 = 0.70) but without any half-field signal near g ～ 4. The isolated mixed valent Ru~ (II)Ru~(III) state in 5~+ exhibits weak antiferromagnetic coupling of -0.25 cm~(-1) and anisotropic EPR with = 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 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 Ru~(II)Ru~(III) 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 HL~(2-) do not show any interaction with the anions F~-, Cl~-, Br~-, I~-, HSO_4~ -, H_2PO_4~ -, OAc~- and SCN~- in CH_3CN, though 1~(2+) selectively recognises the fluoride ion.