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Issue 18, 2013
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9,10-Phenanthrenesemiquinone radical complexes of ruthenium(III), osmium(III) and rhodium(III) and redox series

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Reactions of 9,10-phenanthrenequinone (PQ) in toluene with [MII(PPh3)3X2] at 298 K afford green complexes, trans-[M(PQ)(PPh3)2X2] (M = Ru, X = Cl, 1; M = Os, X = Br, 2) in moderate yields. Reaction of anhydrous RhCl3 with PQ and PPh3 in boiling ethanol affords the dark brown paramagnetic complex, cis-[Rh(PQ)(PPh3)2Cl2] (3) in good yields. Diffusion of iodine solution in n-hexane to the trans-[Os(PQ) (PPh3)2(CO)(Br)] solution in CH2Cl2 generates the crystals of trans-[Os(PQ)(PPh3)2(CO)(Br)]+I3, (4+I3), in lower yields. Single crystal X-ray structure determinations of 1·2toluene, 2·CH2Cl2 and 4+I3, UV-vis/NIR absorption spectra, EPR spectra of 3, electrochemical activities and DFT calculations on 1, 2, trans-[Ru(PQ)(PMe3)2Cl2] (1Me), trans-[Os(PQ)(PMe3)2Br2] (2Me), cis-[Rh(PQ)(PMe3)2Cl2] (3Me) and their oxidized and reduced analogues including trans-[Os(PQ)(PMe3)2(CO)(Br)]+ (4Me+) substantiated that 1–3 are the 9,10-phenanthrenesemiquinone radical (PQ˙) complexes of ruthenium(III), osmium(III) and rhodium(III) and are defined as trans/cis-[MIII(PQ˙)(PPh3)2X2] with a minor contribution of the resonance form trans/cis-[MII(PQ)(PPh3)2X2]. Two comparatively longer C–O (1.286(4) Å) and the shorter C–C lengths (1.415(7) Å) of the OO-chelate of 1·2toluene and 2·CH2Cl2 and the isotropic fluid solution EPR signal at g = 1.999 of 3 are consistent with the existence of the reduced PQ˙ ligand in 1–3 complexes. Anisotropic EPR spectra of the frozen glasses (g11 = g22 = 2.0046 and g33 = 1.9874) and solids (g11 = g22 = 2.005 and g33 = 1.987) instigate the contribution of the resonance form, cis-[RhII(PQ)(PPh3)2Cl2] in 3. DFT calculations established that the closed shell singlet (CSS) solutions of 1Me and 2Me are unstable due to open shell singlet (OSS) perturbation. However, the broken symmetry (BS) (1,1) Ms = 0 solutions of 1Me and 2Me are respectively 22.6 and 24.2 kJ mole−1 lower in energy and reproduced the experimental bond parameters well prompting the coordination of PQ˙ to the M(III) ions. The comparatively shorter C–O lengths, 1.268(4) and 1.266(5) Å and the longer C–C length, 1.466(6) Å, are consistent with the PQ chelation to osmium(II) ion in 4+. The reversible anodic waves at 0.22, 0.22, and 0.18 V of 1–3, referenced by the Fc+/Fc couple, are assigned to the PQ˙/PQ couple forming PQ complexes as trans/cis-[MIII(PQ)(PPh3)2X2]+ while the cathodic waves at −0.92 and −0.89 V of 2 and 3 are due to formations of PQ2− complexes as trans-[MIII(PQ2−)(PPh3)2X2]. 1 displays two overlapping cathodic waves at −0.72(89), −1.0(120) V. EPR spectrum of the frozen glass of 1 along with DFT calculations detected the contribution of both the valence tautomers, trans-[RuIII(PQ2−)(PPh3)2Cl2] (g1 = g2 = 2.456; g3 = 1.983) and trans-[RuII(PQ˙)(PPh3)2X2] (giso = 1.999) in the anion. The characteristic lower energy absorption bands of 1 and 2 at 700 nm were assigned to CSS–OSS perturbation MLCT those are absent in paramagnetic 3, 1+, 2+, 1, 2 and 4+ complexes, investigated by spectro-electrochemical measurements and time dependent (TD) DFT calculations on 1Me, 2Me, 1Me+ and 1Me.

Graphical abstract: 9,10-Phenanthrenesemiquinone radical complexes of ruthenium(iii), osmium(iii) and rhodium(iii) and redox series

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The article was received on 04 Jan 2013, accepted on 12 Feb 2013 and first published on 13 Feb 2013

Article type: Paper
DOI: 10.1039/C3DT00038A
Citation: Dalton Trans., 2013,42, 6538-6552
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    9,10-Phenanthrenesemiquinone radical complexes of ruthenium(III), osmium(III) and rhodium(III) and redox series

    M. K. Biswas, S. C. Patra, A. N. Maity, S. Ke, T. Weyhermüller and P. Ghosh, Dalton Trans., 2013, 42, 6538
    DOI: 10.1039/C3DT00038A

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