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

Abstract

The tppz-bridged diruthenium complex [{(L)ClRu^II}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)ClRu^II(tppz)]ClO₄, [2](ClO₄) have been synthesized. The 380 mV separation between successive Ru^II/Ru^III couples in [1]²⁺ leads to a comproportionation constant (K_c) of 2.7 × 10⁶. Consequently, the Ru^IIRu^III 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 (V_ab) 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.