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Reduction-Oxidation Properties of Organotransition-metal Complexes. Part 18. The Stabilisation of Paramagnetic Cyclopentadienylrhodium Compounds via o-Quinone Complexation: X-Ray Crystal Structure of
Connelly, Neil G.,Freeman, Mark J.,Manners, Ian,Orpen, A. Guy
, p. 2703 - 2712 (1984)
o-Chloranil (3,4,5,6-tetrachloro-o-benzoquinone) and in n-hexane give yellow (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 occured.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 metallacycle.Mean bond lengths are Rh-P 2.298(3), Rh-C (of metallacycle) 1.994(12), and Rh-O 2.079(6) Angstroem.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 (CO)(PPh3)(η-C5H5)>*, via cleavage of the bond linking the CO and catecholate group.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 (2; R = H, or Me).By contrast, the phosphine derivatives (1; L = PPh3) slowly decarbonylate to (3; R = H, or Me, L = PPh3); related complexes 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 * .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.
