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

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 RhIIILTp′ unit.