Reduction–oxidation properties of organotransition-metal complexes. Part 18. The stabilisation of paramagnetic cyclopentadienylrhodium compounds via o-quinone complexation : X-ray crystal structure of [Rh(PPh3){o-C(O)OC6Cl4O}(η-C5H5)]·0.25CH2Cl2

Abstract

o-Chloranil (3,4,5,6-tetrachloro-o-benzoquinone) and [Rh(CO)L(η-C₅R₅)] in n-hexane give yellow [[graphic omitted]}(η-C₅R₅)](1; R = H or Me, L = CO or PPh₃); an X-ray study of (1; R = H, L = PPh₃) as its CH₂Cl₂ solvate reveals that the linking of catecholate and carbonyl ligands has occurred. The Rh atom is therefore co-ordinated by an η-C₅H₅ ligand and a PPh₃ moiety, and in addition forms part of a six-membered [graphic omitted](O) metallacycle. Mean bond lengths are Rh–P 2.298(3), Rh–C (of metallacycle) 1.994(12), and Rh–O 2.079(6)Å. The metallacycle adopts a distorted boat conformation, partly because it is fused to the aryl C₆Cl₄ moiety. Complex (1; R = H, L = PPh₃) reacts with HBF₄·OEt₂ to give [Rh{o-OC₆Cl₄(OH)}(CO)(PPh₃)(η-C₅H₅)][BF₄], via cleavage of the bond linking the CO and catecholate groups. When dissolved in CH₂Cl₂, the carbonyls (1; L = CO) rapidly lose 2 equivalents of CO to give the deep blue or purple chelated catecholato-complexes [Rh(o-O₂C₆Cl₄)(η-C₅R₅)](2; R = H or Me). By contrast, the phosphine derivatives (1; L = PPh₃) slowly decarbonylate to [RhL(o-O₂C₆Cl₄)(η-C₅R₅)](3; R = H or Me, L = PPh₃); related complexes [3; R = H, L = PPh₃, AsPh₃, P(OPh)₃, or pyridine (py); R = Me, L = PPh₃ or AsPh₃] result from the reaction between (2) and L. Complexes (3) undergo reversible one-electron oxidation at a platinum electrode in CH₂Cl₂, and reaction with AgPF₆ yields the stable, paramagnetic salts [RhL(o-O₂C₆Cl₄)(η-C₅R₅)][PF₆][4; R = H, L = PPh₃, AsPh₃, P(OPh)₃, or py; R = Me, L = PPh₃ or AsPh₃]. The e.s.r. spectra of salts (4) suggest that the unpaired electron resides mainly on the semiquinone ligand, but hyperfine coupling to rhodium and to the donor atom (P or As) of L implies some delocalisation.