Solution Studies of Ru2(O2CR)4n+Complexes (n= 0, 1; O2CR = Octanoate, Crotonate, Dimethylacrylate, Benzoate,p-Toluate) and Solid-State Structures of Ru2(O2C-p-tolyl)4(THF)2, [Ru2(O2C-p-tolyl)4(THF)2]+[BF4]-, and Ru2(O2C-p-tolyl)4(CH3CN)2: Investigations of the Axial Ligation of the Ru2Core

Abstract

Ru2(O2C(CH2)6CH3)4 (1a) is soluble in both coordinating (THF, CH3OH, CH3CN) and noncoordinating solvents (benzene, toluene, cyclohexane, CH2Cl2), allowing its solution properties to be investigated by 1H and 13C NMR spectroscopy, UV/visible spectroscopy, resonance Raman spectroscopy, and cyclic voltammetry. In noncoordinating solvents, 1a exists as an oligomer, presumably by way of axial intermolecular -(- -[Ru2]- -O- -)n- interactions. 1H NMR studies of 1a and [Ru2(O2C(CH2)6CH3)4]+[X]-([1a]+[X]-), where X = Cl, BF4, or O2C(CH2)6CH3, indicate that both dipolar and contact mechanisms contribute to the paramagnetic shifts of the protons. Resonances for axial and equatorial ligands are shifted upfield and downfield, respectively, by a dipolar mechanism. Aromatic ligands in the axial sites, e.g. pyridine and pyrazine, experience an enhanced upfield shift by direct π-delocalization. Comparison of the 1H NMR signals for M2(O2CR)4 compounds where M = Ru and O2CR = benzoate, toluate, butyrate, crotonate, and dimethylacrylate with those where M = Mo indicates that the equatorial carboxylate ligands in the diruthenium species also experience π-contact shifts. Variable-temperature studies and calculated estimates of dipolar shifts (using structural parameters taken from solid-state structures) indicate a significant zero-field splitting contribution to the dipolar shift. The arrangements of the toluate rings in Ru2(O2C-p-tolyl)4(THF)2, Ru2(O2C-p-tolyl)4(CH3CN)2, and [Ru2(O2C-p-tolyl)4(THF)2]+[BF4]- deviate by 15(1), 2.3(2), and 7.3°, respectively, from alignment with the Ru−Ru axis. The Ru−Ru distances for the two neutral and the cationic complexes are 2.27(1) Å, i.e. not significantly affected by the nature of the axial ligand (THF versus CH3CN) or by charge n+ (n = 0, 1). The cell parameters for Ru2(O2C-p-tolyl)4(THF)2·2THF at −154 °C are a = 10.730(5) Å, b = 12.335(6) Å, c = 9.193(4) Å, α = 105.15(2)°, β = 109.35(2)°, γ = 77.98(2)°, Z = 2 (asymmetric unit is RuC24H30O6), dcalcd = 1.559 g/cm3, and space group P1̄. The cell parameters for Ru2(O2C-p-tolyl)4(CH3CN)2·3CH3CN at −169 °C are a = 27.058(3) Å, b = 10.049(1) Å, c = 17.956(2) Å, β = 120.89(1)°, Z = 4, dcalcd = 1.465 g/cm3, and space group C2/c. The cell parameters for [Ru2(O2C-p-tolyl)4(THF)2]+[BF4]- at −172 °C are a = 13.056(4) Å, b = 21.358(6) Å, c = 9.199(2) Å, β = 111.28(1)°, Z = 2, dcalcd = 1.350 g/cm3, and space group C2/m.