Electronic structure and catalytic aspects of [Ru(tpm)(bqdi)(Cl/H2O)]n, tpm = tris(1-pyrazolyl)methane and bqdi = o-benzoquinonediimine

Abstract

The diamagnetic complexes [Ru(tpm)(bqdi)(Cl)]ClO₄ ([1]ClO₄) (tpm = tris(1-pyrazolyl)methane, bqdi = o-benzoquinonediimine) and [Ru(tpm)(bqdi)(H₂O)](ClO₄)₂ ([2](ClO₄)₂) have been synthesized. The valence state-sensitive bond distances of coordinated bqdi [C–N: 1.311(5)/1.322(5) Å in [1]ClO₄; 1.316(7)/1.314(7) Å in molecule A and 1.315(6)/1.299(7) Å in molecule B of [2](ClO₄)₂] imply its fully oxidised quinonediimine (bqdi⁰) character. DFT calculations of 1⁺ confirm the {Ru^II–bqdi⁰} versus the antiferromagnetically coupled {Ru^III–bqdi˙⁻} alternative. The ¹H NMR spectra of [1]ClO₄ in different solvents show variations in chemical shift positions of the NH (bqdi) and CH (tpm) proton resonances due to their different degrees of acidity in different solvents. In CH₃CN/0.1 mol dm⁻³ Et₄NClO₄, [1]ClO₄ undergoes one reversible Ru^II ⇌ Ru^III oxidation and two reductions, the reversible first electron uptake being bqdi based (bqdi⁰/bqdi˙⁻). The electrogenerated paramagnetic species {Ru^III–bqdi⁰}(1²⁺) and {Ru^II–Q˙⁻}(1) exhibit Ru^III-type (1²⁺: <g> = 2.211/Δg = 0.580) and radical-type (1: g = 1.988) EPR signals, respectively, as is confirmed by calculated spin densities (Ru: 0.767 in 1²⁺, bqdi: 0.857 in 1). The aqua complex [2](ClO₄)₂ exhibits two one-electron oxidations at pH = 7, suggesting the formation of {Ru^IVO} species. The electronic spectral features of 1ⁿ (n = charge associated with the different redox states of the chloro complex: 2+, 1+, 0) in CH₃CN and of 2²⁺ in H₂O have been interpreted based on the TD-DFT calculations. The application potential of the aqua complex 2²⁺ as a pre-catalyst towards the epoxidation of olefins has been explored in the presence of the sacrificial oxidant PhI(OAc)₂ in CH₂Cl₂ at 298 K, showing the desired selectivity with a wide variety of alkenes. DFT calculations based on styrene as the model substrate predict that the epoxidation reaction proceeds through a concerted transition state pathway.