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)2Ru}2(μ-L − H+)]0 and [{(bpy)2Ru}2(μ-L − H+)]4+ by spectroelectrochemistry and EPR

Abstract

Crystallographically characterised 3,6-bis(2′-pyridyl)pyridazine (L) forms complexes with {(acac)₂Ru} or {(bpy)₂Ru²⁺} via one pyridyl-N/pyridazyl-N chelate site in mononuclear Ru^II 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 pyridazyl-C⁵ carbon {L → (L − H⁺)⁻} to yield cyclometallated, asymmetrically bridged dinuclear complexes [(acac)₂Ru^III(μ-L − H⁺)Ru^III(acac)₂](ClO₄), [2](ClO₄), and [(bpy)₂Ru^II(μ-L − H⁺)Ru^II(bpy)₂](ClO₄)₃, [4](ClO₄)₃. The different electronic characteristics of the co-ligands, σ donating acac⁻ and π 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 Ru^III centres (J = −11.5 cm⁻¹) and of a luminescent diruthenium(II) species in [4](ClO₄)₃. The electrogenerated mixed-valent Ru^IIRu^III states 2 and [4]⁴⁺ with comproportionation constants K_c > 10⁸ are assumed to be localised with the Ru^III ion bonded via the negatively charged pyridyl-N/pyridazyl-C⁵ chelate site of the bridging (L − H⁺)⁻ ligand. In spectroelectrochemical experiments they show similar intervalence charge transfer bands of moderate intensity around 1300 nm and comparable g anisotropies (g₁ − g₃ ≈ 0.5) in the EPR spectra. However, the individual g tensor components are distinctly higher for the π acceptor ligated system [4]⁴⁺, signifying stabilised metal d orbitals.