Recognition of fractional non-innocent feature of osmium coordinated 2,2′-biimidazole or 2,2′-bis(4,5-dimethylimidazole) and their interactions with anions

Abstract

Mononuclear complexes [Os^II(bpy)₂(H₂L₁)](ClO₄)₂ ([1](ClO₄)₂) and [Os^II(bpy)₂(H₂L₂)](ClO₄)₂ ([2](ClO₄)₂) incorporating two free NH protons at the back face of the coordinated H₂L, and deprotonated L²⁻ bridged symmetric dinuclear complexes [(bpy)₂Os^II(μ-L₁²⁻)Os^II(bpy)₂)](ClO₄)₂ ([3](ClO₄)₂) and [(bpy)₂Os^II(μ-L₂²⁻)Os^II(bpy)₂)](ClO₄)₂ ([4](ClO₄)₂) (bpy = 2,2′-bipyridine, H₂L₁ = 2,2′-biimidazole and H₂L₂ = 2,2′-bis(4,5-dimethylimidazole)) have been characterised. Crystal structures of [1](ClO₄)₂ and the meso (ΔΛ) diastereomeric form of [3](ClO₄)₂ have been determined. The crystal structure of [1](ClO₄)₂ also reveals the hydrogen bonding interactions between its free acidic NH protons and the oxygen atoms of the perchlorate anion in the nearby asymmetric unit. Experimental and DFT/TD-DFT calculations have divulged the non-innocent feature of the doubly deprotonated L²⁻ in 3³⁺ and 4³⁺, leading to the resonating formulation of {Os^II(μ-L²⁻)Os^III} ↔ {Os^II(μ-L˙⁻)Os^II}, instead of a simple mixed valent situation {Os^II(μ-L²⁻)Os^III}. The dinuclear complexes 3³⁺ and 4³⁺ display one broad and moderately intense near-IR transition near 1000 nm corresponding to a mixed Os(dπ)/L(π) → Os(dπ)/L(π*) MLLMCT (metal/ligand to ligand/metal charge transfer) transition. Different experimental studies have also established the interaction of 1²⁺ and 2²⁺ with the selective anions.