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Issue 10, 2013
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Electronic structure and catalytic aspects of [Ru(tpm)(bqdi)(Cl/H2O)]n, tpm = tris(1-pyrazolyl)methane and bqdi = o-benzoquinonediimine

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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 {RuII–bqdi0} versus the antiferromagnetically coupled {RuIII–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 RuII ⇌ RuIII oxidation and two reductions, the reversible first electron uptake being bqdi based (bqdi0/bqdi˙). The electrogenerated paramagnetic species {RuIII–bqdi0}(12+) and {RuII–Q˙}(1) exhibit RuIII-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 {RuIV[double bond, length as m-dash]O} 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.

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

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Publication details

The article was received on 10 Oct 2012, accepted on 07 Dec 2012 and first published on 11 Dec 2012


Article type: Paper
DOI: 10.1039/C2DT32402D
Citation: Dalton Trans., 2013,42, 3721-3734
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    Electronic structure and catalytic aspects of [Ru(tpm)(bqdi)(Cl/H2O)]n, tpm = tris(1-pyrazolyl)methane and bqdi = o-benzoquinonediimine

    H. Agarwala, F. Ehret, A. D. Chowdhury, S. Maji, S. M. Mobin, W. Kaim and G. K. Lahiri, Dalton Trans., 2013, 42, 3721
    DOI: 10.1039/C2DT32402D

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