Open Collections

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

University of British Columbia

The organometallic chemistry of tantalum with the macrocyclic [P2N2] ligand (where [P2N2] = PhP(CH2SiMe2NSiMe2CH2)2Ph)) and the acyclic tridentate [NPN] ligand (where [NPN] = PPh(CH2SiMe2NPh)2) is explored. The complex [P2N2]TaMe3 is accessed by the reaction of [P2N2]Li2(C4H802 ) with TaMe3Cl2. Photolysis with an UV source produces [P2N2]Ta=CH2(Me), with the elimination of methane. Attempts to deprotonate {[P2N2]TaMe2}+ 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(C2H4)Et, with [P2N2]Ta(C2H4)Me observed as a minor product. Pure [P2N2]Ta(C2H4)Et can be synthesized by the hydrogenation of [P2N2]TaMe3 in the presence of PMe3, followed by the addition of ethylene. Crystallographic and NMR spectral data indicate the presence of a (3-agostic interaction between the ethyl group and tantalum center in [P2N2]Ta(C2H4)Et. Partially deuterated analogues of [P2N2]Ta(C2H4)Et show a large isotopic perturbation of resonance for both the (3-protons and the a-protons of the ethyl group, indicative of an equilibrium between a (3-agostic and an a-agostic interaction for the ethyl group in solution. An EXSY spectrum demonstrates that an additional fluxional process occurs that exchanges all of the 'H environments of the ethyl and ethylene ligands. The hydrogenation of [P2N2]TaMe3 affords the dinuclear Ta(IV) hydride ([P2N2]Ta)2(fl-H)4. This diamagnetic tetrahydride fails to react with many reagents including ethylene and carbon monoxide; however, upon addition of iodomethane, {([P2N2]Ta)2(|i- H)4}+T 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 TaMe3Cl2 produces [NPN]TaMe3. The hydrogenation of [NPN]TaMe3 yields the tetrahydride ([NPN]Ta)2(|i-H)4. This hydride reacts with dinitrogen with the loss of H2 to produce ([NPN]Ta(u:-H))2(|>V:r)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, AlMe3, HB(C6F5)2, PhCH2Br, 9-BBN, C1B(C6F5)2 and PhO=CH is reported

Text

2009-12-23

For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.

http://hdl.handle.net/2429/17196

Chemistry

University of British Columbia

UBCV

DSpace