Bridging function mediated intermetallic coupling in diruthenium-bis(bipyridine) complexes

The interactions of potentially dinucleating bridging functionalities (I– VI) with the ruthenium-bis(bypyridine) precursor [RuII(bpy)2(EtOH)2]2+ have been explored. The bridging functions I, II and VI directly result in the expected dinuclear complexes of the type [(bpy)2RuII{Ln}RuII(bpy)2]z+ (1, 2, 7 and 8) (n = 0, z = 4 and n = –2, z = 2). The bridging ligand III undergoes N–N or N–C bond cleavage reaction on coordination to the RuII(bpy)2 core which eventually yields a mononuclear complex of the type [(bpy)2RuII(L)]+, 3, where L = –OC6H3(R)C(R)=N–H. However, the electrogenerated mononuclear ruthenium(III) congener, 3+ in acetonitrile dimerises to [(bpy)2RuIII {–OC6H3(R)C(R)=N–N=(R)C(R)C6H3O–}RuIII(bpy)2]4+ (4). In the presence of a slight amount of water content in the acetonitrile solvent the dimeric species (4) reduces back to the starting ruthenium(II) monomer (3). The preformed bridging ligand IV undergoes multiple transformations on coordination to the Ru(bpy)2 core, such as hydrolysis of the imine groups of IV followed by intermolecular head-to-tail oxidative coupling of the resultant amino phenol moieties, which in turn results in a new class of dimeric complex of the type [(bpy)2RuII {–OC6H4-N=C6H3(=NH)O–}RuII(bpy)2]2+ (5). In 5, the bridging ligand comprises of two N,O chelating binding sites each formally in the semiquinone level and there is a p-benzoquinonediimine bridge between the metal centres. In complex 6, the preformed bridging ligand, 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2-dihydro-1,2,4,5- tetrazine, H2L (V) undergoes oxidative dehydrogenation to aromatic tetrazine based bridging unit, 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine, L. The detailed spectroelectrochemical aspects of the complexes have been studied in order to understand the role of the bridging units towards the intermetallic electronic coupling in the dinuclear complexes.