SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF LOW-VALENT GROUP V PHOSPHINE COMPLEXES

The compounds [Nb(CO)2(dmpe)2Cl] and [Ta(CO)2(dmpe)2Cl], where dmpe = 1,2-bis(dimethylphosphino)ethane, are important precursors in chemistry leading to the reductive coupling of carbon monoxide ligands to form bis(trialkylsiloxy)acetylenes. Convenient, high-yield syntheses of these seven-coordinate niobium(I) and tantalum(I) complexes have been devised by utilizing catalytic Mg(anthracene) to reduce either [NbCl4(dmpe)2] or [TaCl4-(dmpe)2] under an atmosphere of CO. With the use of Mg(anthracene).3THF in the absence of CO, the complexes [TaCl2(dmpe)2] (1) and [Ta(eta4-anthracene)(dmpe)2Cl] (2) were obtained, both of which have been structurally characterized by X-ray crystallography. The preparation of [V(CO)2(dMpe)2Me] and the crystal structure determinations of several related seven-coordinate [M(CO)2(dmpe)2X] complexes (M = V, X = Me (8); M = Nb, X = Cl (4), 1 (5); M = Ta, X = Me (7)) and [Ta(CO)2(depe)2Cl] (6), depe = 1,2-bis(diethylphosphino)ethane, having the capped trigonal prismatic geometry, are also described. These complexes all have acute OC-M-CO angles, low nu(CO) stretching frequencies, and close nonbonded C...C contacts, features which indicate their propensity to undergo reductive coupling reactions. The alkyl complexes [V(CO)2(dmpe)2Me] and [Ta(CO)2(dmpe)2Me] display abnormally long M-C sigma bond lengths, 2.31 (1) and 2.32 (1) angstrom, respectively. Crystal data: (1) monoclinic, P2(1)/n, a = 9.384 (1) angstrom, b = 12.863 (2) angstrom, c = 9.363 (1) angstrom, beta = 95.04 (1)-degrees, Z = 2, R = 0.030, R(w) = 0.039; (2) triclinic, P1BAR, a = 12.997 (6) angstrom, b = 13.495 (3) angstrom, c = 18.860 (4) angstrom, alpha = 75.38 (2)-degrees, beta = 84.75 (5)-degrees, gamma = 89.50 (5)-degrees, Z = 4, R = 0.043, R(w) = 0.063; (4) orthorhombic, Pna2(1), a = 16.106 (3) angstrom, b = 8.333 (1) angstrom, c = 33.881 (5) angstrom, Z = 8, R = 0.045, R(w) = 0.060; (5) orthorhombic, Pnma, a = 16.571 (2) angstrom, b = 16.632 (2) angstrom, c = 8.439 (1) angstrom, Z = 4, R = 0.038, R(w) = 0.046; (6) triclinic, P1BAR, a = 11.245 (1) angstrom, b = 13.325 (2) angstrom, c = 10.881 (2) angstrom, alpha = 99.58 (1)-degrees, beta = 112.44 (1)-degrees, gamma = 70.24 (1)-degrees, Z = 2, R = 0.018, R(w) = 0.023; (7) orthorhombic, Fdd2, a = 10.615 (1) angstrom, b = 33.123 (7) angstrom, c = 12.766 (2) angstrom, Z = 8, R = 0.029, R(w) = 0.037; (8) orthorhombic, P2(1)2(1)2(1), a = 13.509 (3) angstrom, b = 13.777 (2) angstrom, c = 11.719 (2) angstrom, Z = 4, R = 0.050, R(w) = 0.055.