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

The reaction of a mixture of 1 equiv of PhPH2 and 2 equiv of PhNHSiMe2CH2Cl with 4 equiv of (BuLi)-Li-n followed by the addition of THF generates the lithiated ligand precursor [NPN]Li-2. (TKF)2(where [NPN] = PhP(CH2SiMe2NPh)(2)). The reaction of [NPN]Li-2. (THF)(2) with TaMe3Cl2 produces [NPN]TaMe3, which reacts under HZ to yield the diamagnetic dinuclear Ta(IV! tetrahydride ([NPN]Ta)(2)(mu -H)4 This hydride reacts with N-2 with the loss of H-2 to produce ([NPN]Ta(mu -H))(2)(mu-eta (1):eta (2)-N-2), which was characterized both in solution and in the solid state, and contains strongly activated Nz bound in the unprecedented side-on end-on dinuclear bonding mode. A density functional theory calculation on the model complex [(H3P)(H2N)(2)Ta(mu -H)](2)-mu-eta (1):eta (2)-N-2) provides insight into the molecular orbital interactions involved in the side-on end-on bonding mode of dinitrogen. The reaction of ([NPN]Ta(mu -H))(2)(mu-eta (1):eta (2)-N-2) with propene generates the end-on bound dinitrogen complex ([NPN]Ta(CH2CH2CH3))(2)(mu-eta (1):eta (1)-N-2), and the reaction of [NPN]Li-2. (THF)(2) with NbCl3-(DME) generates the end-on bound dinitrogen complex ([NPN]NbCl)(2)(mu-eta (1):eta (1)-N-2). 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(mu -H))(2)(mu-eta (1):eta (2)-N-2). The dinitrogen moiety in the side-on end-on mode is amenable to functionalization; the reaction of ([NPN]Ta(mu -H))(2)(mu-eta (1):eta (2)-N-2) with PhCH2Br results in C-N bond formation to yield [NPN]Ta(mu-eta (1):eta (2)-N2CH2Ph)(mu -H)(2)TaBr[NPN]. Nitrogen-15 NMR spectral data an provided for all the tantalum-dinitrogen complexes and derivatives described.