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

Abstract

Reactions of 9,10-phenanthrenequinone (PQ) in toluene with [M^II(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] (1_Me), trans-[Os(PQ)(PMe₃)₂Br₂] (2_Me), cis-[Rh(PQ)(PMe₃)₂Cl₂] (3_Me) and their oxidized and reduced analogues including trans-[Os(PQ)(PMe₃)₂(CO)(Br)]⁺ (4_Me⁺) 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-[M^III(PQ˙⁻)(PPh₃)₂X₂] with a minor contribution of the resonance form trans/cis-[M^II(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-[Rh^II(PQ)(PPh₃)₂Cl₂] in 3. DFT calculations established that the closed shell singlet (CSS) solutions of 1_Me and 2_Me are unstable due to open shell singlet (OSS) perturbation. However, the broken symmetry (BS) (1,1) M_s = 0 solutions of 1_Me and 2_Me 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-[M^III(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-[M^III(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-[Ru^III(PQ²⁻)(PPh₃)₂Cl₂]⁻ (g₁ = g₂ = 2.456; g₃ = 1.983) and trans-[Ru^II(PQ˙⁻)(PPh₃)₂X₂]⁻ (g_iso = 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 1_Me, 2_Me, 1_Me⁺ and 1_Me⁻.