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Dinitrogen complexes of titanium and vanadium stabilized by phosphine ligands Chow, Pauline Chang

Abstract

The goal of the work presented in this thesis is to investigate the preparation of dinitrogen complexes of titanium and vanadium from phosphine-based starting materials. The reactions of TiCl₄, TiCl₃(THF)₃, or VCl₂(TMEDA)₂ with LiN(SiMe₂CH₂PR₂)₂ (R = isopropyl or methyl) yielded the new phosphine complexes TiCl₃[N(SiMe₂CH₂PPri₂)₂] (1), TiCl₃[N(SiMe₂CH₂PMe₂)₂] (2), {TiCl₂[N(SiMe₂CH₂PPri₂)₂]}x (x = 1 or 2) (3), or [(Pri₂PCH₂SiMe₂)₂N]V(μ-Cl)₂Li(TMEDA) (4), respectively. NMR spectroscopy revealed that complexes 1 and 2 have C₂v symmetry. The titanium(III) species, 3, is likely to be a dimer as indicated by mass spectrometry. The structure of the vanadium mono(amido-diphosphine) complex, 4, is suggested to be a lithium-chloride adduct. The reduction of 1 with magnesium powder under dinitrogen resulted in the synthesis of the binuclear titanium dinitrogen complex, {[(Pri₂PCH₂SiMe₂)₂N]TiCl}₂(μ-η¹:η¹-N₂) (5). The unsuccessful reduction of 3 suggests that it is not an intermediate in the synthesis of 5. The most interesting structural feature of 5 is the end-on bound dinitrogen ligand with an N-N bond distance of 1.275 (7) Å, typical of other binuclear end-on bound dinitrogen complexes. The ¹H and ³¹P{¹H} NMR spectra indicate that this dinitrogen complex is fluxional at room temperature by a process that is proposed to involve dissociation of the phophorus atoms of the tridentate ligand and then rotation about the Ti-N (amide) bond, followed by reassociation. The low temperature ³¹P{¹H} NMR spectrum shows an AB quartet which is consistent with the solid state structure. Upon addition of HCl, 0.75 ± 0.04 equivalents of hydrazine and 0.29 ± 0.05 equivalents of dinitrogen were detected. The reaction between VCl₂(TMEDA)₂ and LiN(SiMe₂CH₂PRi₂)₂ under dinitrogen resulted in the formation of a similar vanadium dinitrogen complex, {[(Pri₂PCH₂SiMe₂)₂N]VCl}₂(μ-η¹:η¹-N₂) (7). An alternate synthetic route involves exposure of [(Pri₂PCH₂SiMe₂)₂N]V(μ-Cl)₂Li(TMEDA), 4, in the solid or solution state to one atmosphere of dinitrogen. Complex 7 was also characterized by X-ray crystallography; the end-on dinitrogen bond length is 1.257 (8) Å. Degradation of 7 with HCl produced 0.29 ± 0.04 equivalents of hydrazine.

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