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)

VERGARA, MÓNICA M.; GARCÍA POSSE, MÓNICA E.; FAGALDE, FLORENCIA; KATZ, NÉSTOR E.; FIEDLER, JAN; SARKAR, BIPRAJIT; SIEGER, MONIKA; KAIM, WOLFGANG

INORGANICA CHIMICA ACTA

Elsevier

Lugar: Frankfurt; Año: 2010 vol. 363 p. 163 - 167

0020-1693

New diruthenium complexes (PPN)4[(NC)4Ru(l-bptz)Ru(CN)4], (PPN)4 1, and [(bpy)2Ru(l-bptz)Ru(CN)4], 2, (PPN+ = bis(triphenylphospine)iminium; bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine; bpy = 2,20-bipyridine), were synthesised and characterised by spectroscopic and electrochemical techniques. The comproportionation constant Kc = 107.0 of the mixed-valent species [(NC)4Ru(l-bptz)Ru(CN)4]3_ as obtained by oxidation of 14_ 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 p-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 p acceptor capacity of the ancillary terminal ligands. In addition to the intense MLCT absorption at kmax = 624 nm, the main IVCT (intervalence charge transfer) band of 13_ was detected by spectroelectrochemistry at kmax = 1695 nm (in CH3CN; e = 3200 M_1 cm_1). The experimental band width at half-height, Dm1/2 = 2700 cm_1, is slightly smaller than the theoretical value Dm1/2 = 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 13_ was found to be EPR silent even at 4 K. The unsymmetrical mixed-valent complex [(bpy)2RuII(lbptz) 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 kmax = 1075 nm (e _ 1000 M_1 cm_1, Dm1/2 _ 4000 cm_1). In addition, the lifetime of the excited-state of the mononuclear precursor complex [Ru(bptz)(CN)4]2_ was measured in H2O by laser flash photolysis; the obtained value of s = 19.6 ns reveals that bptz induces a metal-to-ligand electronic delocalisation effect intermediate between that induced by bpy and bpz (bpz = 2,20-bipyrazine) in analogous tetracyanoruthenium complexes.