Synthesis and structure of niobium and tantalum derivatives of bis(dicyclohexylphosphino)methane (dcpm)

The sodium amalgam reduction (2 Na per M) of the Group 5 metal chlorides [M 2Cl10] (M = Nh, Ta) in the presence of bis(dicyclohexylphosphino)methane (d cpm) leads to the binuclear compounds [(dcpm)Cl2M(mu(2)-Cl)(2)MCl2(dcpm)] ( M = Nh, 1; Ta, 2). Solution NMR spectroscopic properties of 1 and 2 indicat e that the dcpm ligand does not bridge (binucleate) the di-metal unit but i s chelated to a single metal center. A single crystal X-ray diffraction stu dy of 1 confirms this and shows two independent molecules both with an edge shared bis-octahedral geometry with Nb=Nb distances of 2.738(1) and 2.749( 1) A. The addition of dcpm to the tantalum trichloride [Ta(OC6H3Pr2i,6)(2)C l-3](2) initially leads to the adduct cis-mer-[Ta(OC6HPr2i-2,6)(2)Cl-3(dcpm )] (3). The molecular structure of 3 is found to contain an eta(1)-bound dc pm ligand. In the P-31 NMR spectrum of 3 well resolved doublets are present for the coordinated and 'dangling' phosphorus atoms. The sodium amalgam re duction of 3 in hydrocarbon solvents does not lead to any isolable binuclea r compounds. Instead the reaction leads to the compound [Ta(OC6H3Pri-eta(2) -CMe=CH2)Cl(dcpm)] (4). Compound 4 contains an aryloxide ligand chelated to the metal via an eta(2)-interaction with a vinyl group formed by the dehyd rogenation of an ortho-isopropyl group. The structural parameters are consi stent with a metallacyclopropane bonding description for this interaction. (C) 1999 Elsevier Science S.A. All rights reserved.