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

• 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. Thevalence 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 fullyoxidised quinonediimine (bqdi0) character. DFT calculations of 1+ confirm the {RuII–bqdi0} versus the antiferromagneticallycoupled {RuIII–bqdi˙−} alternative. The 1H NMR spectra of [1]ClO4 in different solventsshow variations in chemical shift positions of the NH (bqdi) and CH (tpm) proton resonances due to theirdifferent degrees of acidity in different solvents. In CH3CN/0.1 mol dm−3 Et4NClO4, [1]ClO4 undergoesone reversible RuII ⇌ RuIII oxidation and two reductions, the reversible first electron uptake being bqdibased (bqdi0/bqdi˙−). The electrogenerated paramagnetic species {RuIII–bqdi0}(12+) and {RuII–Q˙−}(1)exhibit RuIII-type (12+: = 2.211/Δg = 0.580) and radical-type (1: g = 1.988) EPR signals, respectively, asis confirmed by calculated spin densities (Ru: 0.767 in 12+, bqdi: 0.857 in 1). The aqua complex [2](ClO4)2exhibits two one-electron oxidations at pH = 7, suggesting the formation of {RuIVvO} species. The electronicspectral 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 applicationpotential of the aqua complex 22+ as a pre-catalyst towards the epoxidation of olefins has beenexplored in the presence of the sacrificial oxidant PhI(OAc)2 in CH2Cl2 at 298 K, showing the desiredselectivity with a wide variety of alkenes. DFT calculations based on styrene as the model substratepredict that the epoxidation reaction proceeds through a concerted transition state pathway.IntroductionThe well recognized mixing of ruthenium dπ orbitals andπ orbitals of redox non-innocent quinonoid moieties introducesseveral manifestations with respect to the valence distributionat the metal–quinonoid interface, as depicted in Scheme 1.1This makes the electronic structure of such complexes sensitiveto the molecular frameworks, and in many occasions theexperimental results

Subject headings

NATURVETENSKAP  -- Kemi -- Oorganisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Inorganic Chemistry (hsv//eng)

Keyword

Chemistry with specialization in Inorganic Chemistry

Kemi med inriktning mot oorganisk kemi

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