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 NH 3)

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 = 10 7.0 of the mixed-valent species [(NC) 4Ru(μ-bptz)Ru(CN) 4] 3− as obtained by oxidation of 1 4 − in CH 3CN is much lower than the K c = 10 15.0 previously detected for [(H 3N) 4Ru(bptz)Ru(NH 3) 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 K c and the diminishing effective π acceptor capacity of the ancillary terminal ligands. In addition to the intense MLCT absorption at λ max = 624 nm, the main IVCT (intervalence charge transfer) band of 1 3− was detected by spectroelectrochemistry at λ max = 1695 nm (in CH 3CN; ε = 3200 M −1 cm −1). The experimental band width at half-height, Δ ν 1/2 = 2700 cm −1, is slightly smaller than the theoretical value Δ ν 1/2 = 3660 cm −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 4 K. The unsymmetrical mixed-valent complex [(bpy) 2Ru II(μ-bptz)Ru III(CN) 4] +, obtained in situ by bromine oxidation of 2 in CH 3CN/H 2O, displays a broad NIR absorption originating from an IVCT transition at λ max = 1075 nm ( ε ≈ 1000 M −1 cm −1, Δ ν 1/2 ≈ 4000 cm −1). In addition, the lifetime of the excited-state of the mononuclear precursor complex [Ru(bptz)(CN) 4] 2− was measured in H 2O by laser flash photolysis; the obtained value of τ = 19.6 ns 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.