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

Abstract

The sodium amalgam reduction (2 Na per M) of the Group 5 metal chlorides [M 2Cl 10] (M=Nb, Ta) in the presence of bis(dicyclohexylphosphino)methane (dcpm) leads to the binuclear compounds [(dcpm)Cl 2M(μ 2-Cl) 2MCl 2(dcpm)] (M=Nb, 1; Ta, 2). Solution NMR spectroscopic properties of 1 and 2 indicate that the dcpm ligand does not bridge (binucleate) the di-metal unit but is chelated to a single metal center. A single crystal X-ray diffraction study 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) Å. The addition of dcpm to the tantalum trichloride [Ta(OC 6H 3Pr i 2-2,6) 2Cl 3] 2 initially leads to the adduct cis- mer-[Ta(OC 6H 3Pr i 2-2,6) 2Cl 3(dcpm)] ( 3). The molecular structure of 3 is found to contain an η 1-bound dcpm ligand. In the 31P NMR spectrum of 3 well resolved doublets are present for the coordinated and ‘dangling’ phosphorus atoms. The sodium amalgam reduction of 3 in hydrocarbon solvents does not lead to any isolable binuclear compounds. Instead the reaction leads to the compound [Ta(OC 6H 3Pr i-η 2-CMeCH 2)Cl(dcpm)] ( 4). Compound 4 contains an aryloxide ligand chelated to the metal via an η 2-interaction with a vinyl group formed by the dehydrogenation of an ortho-isopropyl group. The structural parameters are consistent with a metallacyclopropane bonding description for this interaction.