A Pyrazolate-Bridged Bis(pentadentate) Ligand and Its Dinuclear Ruthenium Complex

Abstract

A new bis(pentadentate) pyrazole-bridged proligand HLbis5 providing two {N-5} binding pockets and its diruthenium complex (LRu2)-Ru-bis5(MeCN)(2)(3) (1) have been synthesized, and the structure of 1 has been elucidated by X-ray diffraction. The two exogenous MeCN ligands in 1 are both directed into the central pocket created by the two metal ion compartments and interfere with each other, which leads to some distortion of the dimetallic framework. According to an Eyring analysis of variable-temperature NMR spectroscopic data, interconversion of the resulting enantiomeric forms is rapid with a free activation energy of G = 43.7 kJmol(-1) at 298 K (H = 30.8 kJmol(-1), S = -43.2 Jmol(-1)K(-1)). Cyclic voltammetry of 1 shows two sequential reversible oxidations at ca. 0.85 and 1.25 V (vs. SCE), and the mixed-valent (RuRuII)-Ru-III species has been characterized by spectroelectrochemistry, UV/Vis/NIR and EPR spectroscopy. Complex 1 was found to be a poor catalyst for water oxidation: under standard conditions [triflic acid, pH 1, using cerium(IV) ammonium nitrate as oxidant] rapid degradation by oxidative ligand fragmentation at its benzylic-type CH2 groups was observed.