SYNTHESIS AND STRUCTURE OF OS5(CO)N(PME3) (N = 18, 17 - N = 15, AXIAL AND EQUATORIAL ISOMERS)

The cluster Os5(CO)18(PMe3) (1) was prepared by the addition of OS(CO)4(PMe3) to Os4(CO)14 in CH2Cl2 at room temperature. Careful pyrolysis of 1 in solution at 43-degrees-C afforded Os5(CO)17(PMe3) (2). Pyrolysis of 2 at 63-degrees-C gave OS5(CO)15(ax-PMe3) (3) as the major product, whereas when the pyrolysis was carried out at 110-degrees-C Os5(CO)15(eq-PMe3) (4) was the major product. There was no spectroscopic evidence for Os5(CO)16(PMe3) in the pyrolysis of 2. The structures of 1-4 were determined by X-ray crystallography. The skeleton of 1 consists of a twisted bow-tie arrangement of metal atoms; the six Os-Os lengths are in the range 2.854(l)-2.941(1) angstrom. The PMe3 ligand occupies an equatorial site on an outer osmium atom. The structure of 2 has a raft arrangement of osmium atoms; the seven Os-Os lengths are in the range 2.827(l)-2.904(2) angstrom. As in 1, the phosphine ligand is in an equatorial position on a peripheral osmium atom. An unusual feature of the structure is the presence of a single bridging carbonyl. In 3 there is a trigonal bipyramidal arrangement of metal atoms, the nine Os-Os vectors ranging from 2.769(l) to 2.919(l) angstrom. The PMe3 substituent is coordinated to an apical osmium atom. Cluster 4 also has a trigonal bipyramidal configuration of metal atoms; the Os-Os distances range from 2.755(1) to 2.908(l) angstrom. Surprisingly, the trimethylphosphine group is in a sterically crowded site: it is coordinated to the osmium atom that has four ligands attached to it (this Os atom is also in the equatorial plane of the Os5 polyhedron). A preliminary study indicated that 3 and 4 are in dynamic equilibrium above 80-degrees-C, with 4 slightly preferred at 100-degrees-C. The unexpected stability of 4 is rationalized in terms of stabilization by the PMe3 ligand of the donor-acceptor Os-Os bonds between the osmium atom of the Os(CO)3(PMe3) unit and the apical osmium atoms. Crystallographic data: (compound 1-CH2Cl2) space group P2(1)/c, a = 15.204(2) angstrom, b = 9.612(l) angstrom, c = 23.942(3) angstrom, beta = 91.23(l)-degrees, Z = 4, R = 0.035,2730 observed reflections; (compound 2) space group P2(1)/c, a = 9.019(5) angstrom, b = 12.264(3) angstrom, c = 26.479(5) angstrom, beta = 91.00(3)-degrees, R = 0.040, 2187 observed reflections; (compound 3) space group P1BAR, a = 9.112(1) angstrom, b = 12.361(2) angstrom, c = 12.548(2) angstrom, alpha = 94.04(l)-degrees, beta = 98.34(l)-degrees, gamma = 91.96(l)-degrees, R = 0.031, 2768 observed reflections; (compound 4) space group P2(1)/n, a = 10.799(1) angstrom, b = 17.555(2) angstrom, c = 14.504(2) angstrom, beta = 93.68(1)-degrees, R = 0.033, 3578 observed reflections.