Electronic structure and catalytic aspects of [Ru(tpm)(bqdi)(Cl/H sub(2)O)] super(n), tpm = tris(1-pyrazolyl)methane and bqdi = o-benzoquinonediimine

• Published in: Dalton transactions : an international journal of inorganic chemistry 2013-02, Vol.42 (10), p.3721-3734
• Main Authors: Agarwala, Hemlata, Ehret, Fabian, Chowdhury, Abhishek Dutta, Maji, Somnath, Mobin, Shaikh M, Kaim, Wolfgang, Lahiri, Goutam Kumar
• Format: Article
• Language: English
• Subjects: Density; Electronics; Epoxidation; Mathematical models; Oxidation; Solvents; Spectra; Styrenes
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c2dt32402d

The diamagnetic complexes [Ru(tpm)(bqdi)(Cl)]ClO sub(4) ([1]ClO sub(4)) (tpm = tris(1-pyrazolyl)methane, bqdi = o-benzoquinonediimine) and [Ru(tpm)(bqdi)(H sub(2)O)](ClO sub(4)) sub(2) ([2](ClO sub(4)) sub(2)) have been synthesized. The valence state-sensitive bond distances of coordinated bqdi [C-N: 1.311(5)/1.322(5) Aa in [1]ClO sub(4); 1.316(7)/1.314(7) Aa in molecule A and 1.315(6)/1.299(7) Aa in molecule B of [2](ClO sub(4)) sub(2)] imply its fully oxidised quinonediimine (bqdi super(0)) character. DFT calculations of 1 super(+) confirm the {Ru super(II)-bqdi super(0)} versus the antiferromagnetically coupled {Ru super(III)-bqdi&z.rad; super(-)} alternative. The super(1)H NMR spectra of [1]ClO sub(4) 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 CH sub(3)CN/0.1 mol dm super(-3) Et sub(4)NClO sub(4), [1]ClO sub(4) undergoes one reversible Ru super(II) Ru super(III) oxidation and two reductions, the reversible first electron uptake being bqdi based (bqdi super(0)/bqdi&z.rad; super(-)). The electrogenerated paramagnetic species {Ru super(III)-bqdi super(0)}(1 super(2+)) and {Ru super(II)-Q&z.rad; super(-)}(1) exhibit Ru super(III)-type (1 super(2+): = 2.211/ Delta g= 0.580) and radical-type (1: g= 1.988) EPR signals, respectively, as is confirmed by calculated spin densities (Ru: 0.767 in 1 super(2+), bqdi: 0.857 in 1). The aqua complex [2](ClO sub(4)) sub(2) exhibits two one-electron oxidations at pH = 7, suggesting the formation of {Ru super(IV)&z.dbd; O} species. The electronic spectral features of 1 super(n) (n= charge associated with the different redox states of the chloro complex: 2+, 1+, 0) in CH sub(3)CN and of 2 super(2+) in H sub(2)O have been interpreted based on the TD-DFT calculations. The application potential of the aqua complex 2 super(2+) as a pre-catalyst towards the epoxidation of olefins has been explored in the presence of the sacrificial oxidant PhI(OAc) sub(2) in CH sub(2)Cl sub(2) 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.