Stepwise construction of mono-, di- and tri-nuclear 2 ? 1, 1 ? 2, 2 ? 3 ligand ? mixed-metal complexes using a bis-tridentate bridging ligand

Abstract

Extended molecular structures have been constructed with the bis-tridentate ligand N,N,N′,N′-tetrakis(2-pyridylmethyl)benzene-1,4-diamine (tpbd), and the bis-tridentate, metal ‘complex’ ligand, [Ru(tpbd)2](PF6)2 (1) as building blocks. The ligand tpbd and complex 1 react analogously with Cu(II) salts to yield the corresponding di- and tri-nuclear aqua complexes, [Cu2(tpbd)(H2O)4](ClO4)4 (2a) and [Cu2Ru(tpbd)2(H2O)4](BF4)4(PF6)2·2H2O (3a). The labile water ligands in these complexes have been replaced by other solvents or 2,2′-bipyridine (bipy). In the latter case the mixed ligand complexes [Cu2(tpbd)(bipy)2](PF6)4·H2O (2b) and [Cu2Ru(BF4)2(tpbd)2(bipy)2](PF6)4·2H2O (3b) respectively are obtained. These reactions represent prototype substitutions for the controlled stepwise evolution of even larger molecular entities. Recrystallisation of 3a in 2% NaBF4 methanol/water solution, resulted in crystals of [Cu2Ru(tpbd)2(H2O)4](BF4)4(PF6)2·8H2O (3a′). Recrystallisation of 3b in 2% NaBF4 CH3CN/water solution gave [Cu2Ru(BF4)2(tpbd)2(bipy)2](BF4)2(PF6)2·H2O (3b′). The X-ray structures of the mononuclear and trinuclear ruthenium-containing complexes (1, 3a′, 3b′) show cis–fac six-coordinate geometries around the Ru atoms. As a consequence the phenylene groups of the tpbd ligands are approximately perpendicular to each other. In the trinuclear Ru–Cu2 complexes the copper atoms are located anti to each other about the [(tpbd)Ru(tpbd)]2+ building block. The flexibility of tpbd is demonstrated by the geometries of the copper coordinated ends of tpbd, which are coordinated meridionally in 3a′, but facially in 3b′. Solution spectroscopy, and the detection of appropriate ions by mass spectrometry, show that the dicopper and trinuclear copper–ruthenium complexes are present in solution. UV-Visible spectroscopy, EPR and cyclic voltammetry indicate that there is insignificant electronic communication between the metal centers in the di- and tri-nuclear complexes since salient metal-based features are additive.