Mixed-valency with cyanides as terminal ligands: Diruthenium(III,II) complexes with the 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine bridge and variable co-ligands (CN− vs. bpy or NH3)

Abstract: New diruthenium complexes (PPN)4[(NC)4Ru(μ-bptz)Ru(CN)4], (PPN)4 1, and [(bpy)2Ru(μ-bptz)Ru(CN)4], 2, (PPN+ =bis(triphenylphospine)iminium; bptz=3,6-bis(2-pyridyl)-1,2,4,5-tetrazine; bpy=2,2′-bipyridine), were synthesised and characterised by spectroscopic and electrochemical techniques. The comproportionation constant K c =107.0 of the mixed-valent species [(NC)4Ru(μ-bptz)Ru(CN)4]3− as obtained by oxidation of 1 4 − in CH3CN is much lower than the Kc =1015.0 previously detected for [(H3N)4Ru(bptz)Ru(NH3)4]5+, reflecting the competition between CN− and bptz for the π-electron density of the metals. Comparison with several other bptz-bridged diruthenium(II,III) complexes reveals an approximate correlation between Kc and the diminishing effective π acceptor capacity of the ancillary terminal ligands. In addition to the intense MLCT absorption at λ max =624nm, the main IVCT (intervalence charge transfer) band of 1 3− was detected by spectroelectrochemistry at λ max =1695nm (in CH3CN; ε =3200M−1 cm−1). The experimental band width at half-height, Δν 1/2 =2700cm−1, is slightly smaller than the theoretical value Δν 1/2 =3660cm−1, calculated from the Hush approximation for Class II mixed-valent species. In agreement with comparatively moderate metal–metal coupling, the mixed-valent intermediate 1 3− was found to be EPR silent even at 4K. The unsymmetrical mixed-valent complex [(bpy)2RuII(μ-bptz)RuIII(CN)4]+, obtained in situ by bromine oxidation of 2 in CH3CN/H2O, displays a broad NIR absorption originating from an IVCT transition at λ max =1075nm (ε ≈1000M−1 cm−1, Δν 1/2 ≈4000cm−1). In addition, the lifetime of the excited-state of the mononuclear precursor complex [Ru(bptz)(CN)4]2− was measured in H2O by laser flash photolysis; the obtained value of τ =19.6ns reveals that bptz induces a metal-to-ligand electronic delocalisation effect intermediate between that induced by bpy and bpz (bpz=2,2′-bipyrazine) in analogous tetracyanoruthenium complexes. [Copyright &y& Elsevier]