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

The complexes [Rh(CO)LTp'] {L=CO or PPh3, Tp'=hydrotris(3,5-dimethylpyrazol yl)borate} reacted with the ortho-quinone o-C6Cl4O2 (o-chloranil; 3,4,5,6-t etrachloro-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 (o f 1) or thermolysis (of 2) gives [Rh(o-O2C6Cl4)LTp'] (L=CO 3 or PPh3 4); th ermal substitution of the CO ligand of [Rh(o-O2C6Cl4)(CO)Tp'] with L provid es a second route to [Rh(o-O2C6Cl4)(PPh3)Tp'] as well as [Rh(o-O2C6Cl4)LTp' ] {L=AsPh3 5, P(OPh)(3) 6 or py 7} which are oxidised by [NO](+) to the mon ocations [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 w ith catecholate ligand-based oxidation; the ESR spectra of the paramagnetic cations 4(+)-7(+) suggest little delocalisation of unpaired electron densi ty from the semiquinone ligand to the Rh(III)LTp' unit.