Synthesis and characterization of the five-coordinate scandium dialkyl complexes ScR(2)[N(SiMe(2)CH(2)PPr(2)(i))(2)] (R=Me, Et, CH(2)SiMe(3))

The preparation of new scandium phosphine complexes that contain two hydrocarbyl groups is reported. Thus, reaction of the amido diphosphine ligand precursor LiN(SiMe(2)CH(2)PPr(2)(i))(2) with ScCl3(THF)(3) in toluene at 100 degrees C leads to the formation of ScCl2(THF)[N(SiMe(2)CH(2)PPr(2)(i))(2)] for which the THF molecule can be removed by pumping in the solid state. The X-ray crystal structure of this molecule shows it to be monomeric with a distorted octahedral geometry having trans-disposed chloride ligands and the tridentate ligand meridionally bound. The solution NMR spectra are also consistent with this geometry. Addition of the alkyllithium reagents RLi (where R = Me, Et, and CH(2)SiMe(3)) leads to the formation of bis(hydrocarbyl) derivatives of the formula ScR(2)[N(SiMe(2)CH(2)PPr(2)(i))(2)]. These are the first dialkyl-substituted scandium complexes that have been characterized. The solid-state X-ray structures of ScEt(2)[N(SiMe(2)CH(2)PPr(2)(i))(2)] and Sc(CH(2)SiMe(3))(2)[N(SiMe(2)CH(2)PPr(2)(i))(2)] show that these molecules are mononuclear in the solid state with distorted trigonal bipyramidal geometries. In solution, the NMR spectroscopic parameters are consistent with overall C-2v symmetry. A number of reactions were attempted such as addition of CO, H-2, CH3I, nitriles, isocyanides, and silanes: in all cases, complete decomposition to a mixture of unidentifiable products was observed. The reaction of ethylene with these bis(hydrocarbyl) species did result in the formation of polyethylene, but the nature of the catalytically active species could not be determined. For the reaction with CO2, insertion was observed to be competitive with decomposition. Molecular orbital calculations show that the frontier orbitals of the five-coordinate, symmetrized complex Sc(CH3)(2)[N(SiMe(2)CH(2)PH(2))(2)] (isopropyl groups at phosphorus replaced with H's) have a different symmetry than that of metallocene systems; in addition, the HOMO was found to be largely the amido nitrogen lone pair of the Sc-N interaction.