Synthesis and mixed valence aspects of [{(L)ClRu} 2 (μ-tppz)] n+ incorporating 2,2′-dipyridylamine (L) as ancillary and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) as bridging ligand

The tppz-bridged diruthenium complex [{(L)ClRuII}2(μ-tppz)](ClO4)2, [1](ClO4)2 {tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, L = 2,2′-dipyridylamine} and its mononuclear counterpart [(L)ClRuII(tppz)]ClO4, [2](ClO4) have been synthesized. The 380 mV separation between successive RuII/RuIII couples in [1]2+ leads to a comproportionation constant (Kc) of 2.7 × 106. Consequently, the RuIIRuIII species [1]3+ exhibits a rather narrow intervalence charge transfer band at 1700 nm, suggesting a class III mixed-valence state, the electronic coupling constant (Vab) is calculated at 2940 cm−1. Complex [1]3+ displays a rhombic EPR spectrum at 4 K (g1 = 3.390, g2 = 2.278, g3 = 1.697), characteristic of ruthenium(III) in a distorted octahedral environment. Both complexes show two successive tppz-based reduction processes [(tppz)0/−1 and (tppz)−1/−2]. The one-electron reduced species [1]+ is a tppz radical anion species with an intense low-energy band at 1105 nm and an axial EPR signal at 4 K (g1 = 2.008, g2 = g3 = 1.994). [1]2+ and [2]+ exhibit moderately strong emissions at 740 nm and 668 nm, respectively, in EtOH–MeOH glass at 77 K.