Redox-active catecholate complexes of rhodium hydrotris(pyrazolyl)borates

Abstract

The complexes [Rh(CO)LTp′] {L = CO or PPh₃, Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} reacted with the ortho-quinone o-C₆Cl₄O₂ (o-chloranil; 3,4,5,6-tetrachloro-1,2-benzoquinone) to give [Rh{C(O)OC₆Cl₄O}LTp′] (L = CO, 1; L = PPh₃, 2). X-Ray structural studies on 2 reveal CO insertion into one Rh–O bond of a rhodium–catecholate ring. Loss of the inserted CO on UV irradiation (of 1) or thermolysis (of 2) gives [Rh(o-O₂C₆Cl₄)LTp′] (L = CO 3 or PPh₃4); thermal substitution of the CO ligand of [Rh(o-O₂C₆Cl₄)(CO)Tp′] with L provides a second route to [Rh(o-O₂C₆Cl₄)(PPh₃)Tp′] as well as [Rh(o-O₂C₆Cl₄)LTp′] {L = AsPh₃5, P(OPh)₃6 or py 7} which are oxidised by [NO]⁺ to the monocations [Rh(o-O₂C₆Cl₄)LTp′]⁺4⁺+–7⁺+. X-Ray structural studies on the redox pair [Rh(o-O₂C₆Cl₄)(PPh₃)Tp′]^z (z = 0, 4 or 1, 4⁺+) are consistent with catecholate ligand-based oxidation; the ESR spectra of the paramagnetic cations 4⁺+–7⁺+ suggest little delocalisation of unpaired electron density from the semiquinone ligand to the Rh^IIILTp′ unit.