COUPLING OF 2 ETHYNE MOLECULES AT RHODIUM VERSUS COUPLING OF 2 RHODIUM ATOMS AT ETHYNE .1. ELECTRONIC CONTROL ON THE INTERCONVERSION OF (MU-ETA-2,ETA-2-ETHYNE)-DIRHODIUM AND (MU-ETA-1,ETA-1-ETHYNE)DIRHODIUM COMPLEXES

Depending on the metal to ethyne ratio, the reaction of [(triphos)RhCl(C2H4)] (1; triphos = MeC-(CH2PPh2)3) with ethyne forms either the binuclear complex [(triphos)Rh(mu-Cl)(mu-eta-2, eta-2-C2H2)Rh(triphos)]Cl (2) or the mononuclear rhodacyclopentadiene complex [(triphos)RhCl(eta-2-C4H4)] (3). The crystal structure of the complex cation [(triphos)Rh(mu-Cl)(mu-eta-2,eta-2-C2H2)Rh(triphos)]+ (2+) has been determined by X-ray methods. In the structure, a chloride ligand and a ethyne molecule bridge two (triphos)Rh fragments with a Rh-Rh separation (3.306 (2) angstrom) that corresponds to no formal metal-metal bond. The ethyne molecule is positioned in the mu-eta-2, eta-2 mode and formally serves as a four-electron donor (C-C bond length of 1.36 (2) angstrom). The cation 2+ is high fluxional in solution on the NMR time scale. The fluxional process involves two distinct mechanisms. Complex 2+ undergoes single-stepped two-electron oxidation by chemical or electrochemical methods, converting to the mu-eta-1, eta-1 derivative [(triphos)Rh(mu-Cl)2(mu-eta-1, eta-1-C2H2)Rh(triphos)]2+ (5(2+)), where ethyne bridges the two metals as a cis-dimetalated olefin. The reaction is reversible. Complex 2+ is regenerated by two one-electron-reduction steps from 5(2+). Controlled-potential macroelectrolysis at the potential of the first reduction step allows one to generate the paramagnetic species [(triphos)Rh(mu-Cl)2(mu-eta-1,eta-1-C2H2)Rh(triphos)]+ (5+), which has been characterized by X-band ESR spectroscopy. In the absence of external ligands, the cation 2+ undergoes two-electron oxidation, forming the mu-OH derivative [(triphos)Rh(mu-Cl)(mu-OH)(mu-eta-1, eta-1-C2H2)Rh(triphos)]2+ (4(2+)) through abstraction of a hydroxy group from adventitious water in solution. From preliminary X-ray analysis and detailed spectroscopic studies it is concluded that 5(2+) and 4(2+) share the same cis-dimetalated bonding mode of the ethyne bridge. Complex 4(2+) transforms into 5(2+) by reaction with gaseous HCl while water is eliminated. The reaction of the perchlorate salt of 4(2+) with CF3COOH yields H2O and the mu-carboxylate complex [(triphos)Rh(mu-O2CCF3)(mu-eta-1, eta-1-C2H2)Rh(triphos)](ClO4)2 (6). With the exception of the paramagnetic species 5+, all of the complexes have been isolated in the solid state as ClO4-, PF6-, and/or BPh4- salts. The redox properties of the relevant compounds in CH2Cl2 have been studied by electrochemical techniques. This has allowed us to shed some light on the mechanistic aspects of the chemically reversible pathway connected to the interconversion 2+/5(2+). In particular, we show that the oxidation of 2+ to 5(2+) most likely occurs via an EC(a)C(r)E mechanism, where E = electron transfer, C(a) = addition of chloride, and C(r) = stereochemical rearrangement of the ethyne bridge.