American Chemical Society
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New Mode of Coordination for the Dinitrogen Ligand:  Formation, Bonding, and Reactivity of a Tantalum Complex with a Bridging N2 Unit That Is Both Side-On and End-On

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journal contribution
posted on 2001-04-05, 00:00 authored by Michael D. Fryzuk, Samuel A. Johnson, Brian O. Patrick, Alberto Albinati, Sax A. Mason, Thomas F. Koetzle
The reaction of a mixture of 1 equiv of PhPH2 and 2 equiv of PhNHSiMe2CH2Cl with 4 equiv of BunLi followed by the addition of THF generates the lithiated ligand precursor [NPN]Li2·(THF)2 (where [NPN] = PhP(CH2SiMe2NPh)2). The reaction of [NPN]Li2·(THF)2 with TaMe3Cl2 produces [NPN]TaMe3, which reacts under H2 to yield the diamagnetic dinuclear Ta(IV) tetrahydride ([NPN]Ta)2(μ-H)4. This hydride reacts with N2 with the loss of H2 to produce ([NPN]Ta(μ-H))2(μ-η12-N2), which was characterized both in solution and in the solid state, and contains strongly activated N2 bound in the unprecedented side-on end-on dinuclear bonding mode. A density functional theory calculation on the model complex [(H3P)(H2N)2Ta(μ-H)]2(μ-η12-N2) provides insight into the molecular orbital interactions involved in the side-on end-on bonding mode of dinitrogen. The reaction of ([NPN]Ta(μ-H))2(μ-η12-N2) with propene generates the end-on bound dinitrogen complex ([NPN]Ta(CH2CH2CH3))2(μ-η11-N2), and the reaction of [NPN]Li2·(THF)2 with NbCl3(DME) generates the end-on bound dinitrogen complex ([NPN]NbCl)2(μ-η11-N2). These two end-on bound dinitrogen complexes provide evidence that the bridging hydride ligands are responsible for the unusual bonding mode of dinitrogen in ([NPN]Ta(μ-H))2(μ-η12-N2). The dinitrogen moiety in the side-on end-on mode is amenable to functionalization; the reaction of ([NPN]Ta(μ-H))2(μ-η12-N2) with PhCH2Br results in C−N bond formation to yield [NPN]Ta(μ-η12-N2CH2Ph)(μ-H)2TaBr[NPN]. Nitrogen-15 NMR spectral data are provided for all the tantalum−dinitrogen complexes and derivatives described.