Ligand design and the synthesis of reactive organometallic complexes of tantalum for dinitrogen activation

The organometallic chemistry of tantalum with the macrocyclic [P ₂N₂] ligand (where [P₂N₂] = PhP(CH ₂SiMe₂NSiMe₂CH₂)₂Ph) and the acyclic tridentate [NPN] ligand (where [NPN] = PPh(CH₂SiMe ₂NPh)₂) is explored. The complex [P₂N₂]TaMe ₃ is accessed by the reaction of [P₂N₂]Li ₂(C₄H₈O₂) with TaMe₃Cl ₂. Photolysis with an UV source produces [P₂N₂]Ta=CH ₂(Me), with the elimination of methane. Attempts to deprotonate {lcub}[P ₂N₂]TaMe₂{rcub}+ do not provide a chemical route to [P₂N₂]Ta=CH₂(Me).; In the presence of ethylene [P₂N₂]Ta=CH₂(Me) is slowly converted to [P₂N₂]Ta(C₂H ₄)Et, with [P₂N₂]Ta(C₂H₄)Me observed as a minor product. Pure [P₂N₂]Ta(C₂H ₄)Et can be synthesized by the hydrogenation of [P₂N ₂]TaMe₃ in the presence of PMe₃, followed by the addition of ethylene. Crystallographic and NMR spectral data indicate the presence of a β-agostic interaction between the ethyl group and tantalum center in [P₂N₂]Ta(C₂H₄)Et. Partially deuterated analogues of [P₂N₂]Ta(C₂H ₄)Et show a large isotopic perturbation of resonance for both the β-protons and the α-protons of the ethyl group, indicative of an equilibrium between a β-agostic and an α-agostic interaction for the ethyl group in solution. An EXSY spectrum demonstrates that an additional fluxional process occurs that exchanges all of the 1H environments of the ethyl and ethylene ligands.; The hydrogenation of [P₂N₂]TaMe₃ affords the dinuclear Ta(IV) hydride ([P₂N₂]Ta)₂(μ-H) ₄. This diamagnetic tetrahydride fails to react with many reagents including ethylene and carbon monoxide; however, upon addition of iodomethane, {lcub}([P ₂N₂]Ta)₂(μ-H)₄{rcub} +I− is produced as a paramagnetic green crystalline solid. Ab initio calculations using density functional theory were performed in an attempt to further understand the influence of the macrocyclic ligand in the bonding in these complexes.; The reaction of [NPN]Li₂·(THF)₂ with TaMe ₃Cl₂ produces [NPN]TaMe₃. The hydrogenation of [NPN]TaMe₃ yields the tetrahydride ([NPN]Ta)₂(μ-H) ₄. This hydride reacts with dinitrogen with the loss of H₂ to produce ([NPN]Ta(μ-H))₂(μ:η1:η 2-N₂), which contains N₂ bound in the unprecedented side-on end-on dinuclear bonding mode. A prelimary study of the reactivity of this complex with B(C₆F₅)₃, AlMe ₃, HB(C₆F₅)₂, PhCH₂Br, 9-BBN, CIB(C₆F₅)₂ and PhC≡CH is reported.