SYNTHESIS AND EVALUATION OF THE BONDING PROPERTIES TOWARD RHODIUM(I) OF 2,5-BIS[3-(DIPHENYLPHOSPHINO)PROPYL]THIOPHENE - A NEW VERSATILE LIGAND WITH ETA(2), ETA(3), AND ETA(7) MODES OF BONDING

The new ligand 2,5-bis[3-(diphenylphosphino)propyl]thiophene (2) has been synthesized in four steps from thiophene, and its bonding capabilities with Rh(I) have been evaluated. With (COD)Rh(acac) in the presence of perchloric acid, the complex [(COD)Rh(2)][ClO4] (3) has been obtained but its low solubility suggests a polymeric structure.; The 1,5-cyclooctadiene ligand is displaced by CO, leading to complexes that are in equilibrium, depending on the partial pressure of CO. Under a CO atmosphere, only the complex [Rh(CO)(3)(2)][ClO4] (4) is present in solution and its spectroscopic data are consistent with a trigonal-bipyramidal structure in which the two phosphorus atoms are in axial positions. Progressive decarbonylation causes the formation of [Rh(CO)(2)(2)][ClO4] as a mixture of cis (5) and trans (6) isomers (ligand 2 being bidentate through the phosphorus atoms) and of [Rh(CO)(2)][ClO4] (7), in which 2 is tridentate binding through the two phosphorus atoms and the sulfur of the thiophene ring. Complete decarbonylation of 7 is achieved through refluxing in acetone solution and leads to [Rh(2)][ClO4] (8). The structure of [Rh(2)](+) has been established by an X-ray structure determination of the [Rh(2)][BPh(4)] salt (8'). Crystallographic data for 8': monoclinic C-2h(5)-P2(1)/(n), a = 11.261(2) Angstrom, b = 15.886(4) Angstrom, c = 29.986(4) Angstrom; beta = 96.70(2)degrees; V = 5149 Angstrom(3), Z = 4; R = 0.0527, Rw = 0.0586 for 4254 observations and 595 variable parameters. The cation consists of a rhodium atom pi bound to the thiophene ring and bound to the two phosphorus atoms of ligand 2.