REDUCTIVE COUPLING OF GROUP-5 DICARBONYLS TO DISILOXYACETYLENE COMPLEXES - RING FORMATION AND EFFECTS OF INCREASING STERIC DEMANDS

Convenient syntheses of dicarbonyl complexes of the type [M(CO)(2)(dep e)(2)Cl] (depe = 1,2-bis(diethylphosphino)ethane, M = Ta (1), Nb (3)) and [M(CO)(2)(dbpe)(2)Cl] (dbpe = 1,2-bis(dibutylphosphino)ethane, M = Ta (2), Nb (4)) having increased steric demands at the high coordinat e metal centers are described. Reductive coupling of the CO ligands of 1-4 occurs to provide products of the type [M(R'3SiOC double bond COS iR'(3))(R(2)PCH(2)CH(2)PR(2))(2)]X. Single crystal X-ray structural st udies were carried out for two reductively coupled products prepared w ith 1,2-bis(chlorodimethylsilyl)ethane as the electrophile, [V(Me(2)Si OC double bond COSiMe(2))(dmpe)(2)Cl] (monoclinic, space group C2/c, a = 9.349(2) Angstrom, b = 20.548(3) Angstrom, c = 16.146(4) Angstrom, beta = 104.79(1)degrees, and V = 2999(1) Angstrom(3)) and [Ta(Me(2)SiO C double bond COSiMe(2))(depe)(2)Cl] (monoclinic, space group Cc, a = 11.512(1) Angstrom, b = 18.311(3) Angstrom, c 18.493(3) Angstrom, beta = 97.322(7)degrees, and V = 3875(1) Angstrom(3)). In these complexes, the acetylene is contained within a newly formed eight-membered ring, and the ligands are arranged in a pentagonal bipyramid geometry compr ising two axial phosphorus atoms and five equatorial ligands, the coup led carbons, a trans chloride, and the remaining two phosphorus atoms.