A Terminal Hydride of Hafnium Supported by a Triamidoamine Ligand

A mononuclear hafnium hydride supported by an aryl substituted triamidoamine ligand [HfH(thf)(Xy-N3N)] (Xy-N3N = {(3,5Me(2)C(6)H(3))NCH2CH2}(3)N-3(-)) was synthesized by either hydrogenolysis of the hydrocarbyl complex [Hf(R)(thf)(Xy-N3N)] (R = Me, CH2SiMe3, sl(3)(-) C3H5, CH2Ph) in tetrahydrofuran (THF) or, more conveniently, by salt metathesis of the chloro precursor [HfCl(thf)(Xy-N3N)] with NaBEt3H in methylcyclohexane. The structure of a terminal hydride with a distorted capped trigonal bipyramidal geometry was characterized by single crystal X-ray diffraction (d(Hf-H): 1.83(3) angstrom) and by 1H NMR spectroscopy. The unusually low field shifted 1H chemical shift for the hydride ligand was detected at (5 18.28 ppm. Dehydrogenative thermolysis of the hydride complex at 25 degrees C in benzene or toluene resulted in deprotonation of the a-hydrogen of one of the three XyNCH(2)CH(2) groups in the ligand by the hydride, followed by the combination of this deprotonated anionic complex with cationic complex [Hf(Xy-N3N)]+ to give an unsymmetric dinuclear complex bridged by mu-H, mu-NXy, and mu,KN,KC-XyNCHCH(2) groups. This decomposition product was characterized by single crystal X-ray diffraction and by H-1 and C-13 NMR spectroscopy and analyzed by density functional theory (DFT) calculations. Hydrogenation in THF at 25 degrees C reformed the hydride complex over a period of 48 h. The hydride complex reacted with olefins RHC=CH2 (R = H, Et, nHex, cHex, Ph) to give the corresponding anti-Markovnikov insertion products [Hf(CH2CH2R)(thf)n(Xy-N3N)]. Catalytic hydrogenation of olefins was achieved with 5 mol % of the hydride complex at 70 degrees C in THF or benzene.