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

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)(2)Os-II(mu-L-1(2-))Os-II(bpy)(2))](ClO4)(2) ([3](ClO4)(2)) and [(bpy)(2)Os-II(mu-L-2(2-))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 (Delta Delta) 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 3(3+) and 4(3+), leading to the resonating formulation of {Os-II(mu-L2-)Os-III} {Os-II(mu-L-.)Os-II}, instead of a simple mixed valent situation {Os-II(mu-L2-)Os-III}. The dinuclear complexes 3(3+) and 4(3+) display one broad and moderately intense near-IR transition near 1000 nm corresponding to a mixed Os(d pi)/L(pi) -> Os(d pi)/L(pi(star)) MLLMCT (metal/ligand to ligand/metal charge transfer) transition. Different experimental studies have also established the interaction of 1(2+) and 2(2+) with the selective anions