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The synthesis and reactivity of low-coordinate enamido-phosphinimine iron complexes

University of British Columbia

Unsymmetrical, bidentate ancillary ligand scaffolds containing enamido-phosphinimine donors (NpN) were developed for the synthesis of low-coordinate iron complexes. A variety of ligands varying in steric hindrance were synthesized through the modularity of the Staudinger reaction. Trigonal planar and dimeric tetrahedral iron bromide complexes were fully characterized and served as precursors for further reactivity. The reduction chemistry of NpN iron bromide complexes was investigated and a number of end-on bridging N2 complexes were characterized by X-ray crystallography. Analysis of the coordinated N2 bond length indicated that the NpN scaffold does not contribute to enhanced bond activation in comparison to well-established β-diketiminate iron N2 complexes. Additionally, an NpN ligand with an indene linker was synthesized with the intent of generating an anionic indenyl moiety that would contribute electron density to the Fe–N₂ backbonding interaction. Unfortunately, DFT results indicated that minimal enhancement to activation results from this effect. A reoccurring observation in the attempted synthesis of NpN dinitrogen complexes was that the P=N bond of the phosphinimine was susceptible to cleavage under reducing conditions. A low-coordinate, dimeric NpN iron hydride was synthesized and its reactivity with unsaturated substrates was explored. The products from hydride insertion into azobenzene, 3-hexyne, and 1-azidoadamantane were characterized and did not vary significantly from the analogous products reported for the reactivity of β-diketiminate iron hydrides. Surprisingly, the NpN iron hydride displays unprecedented reactivity towards hexafluorobenzene, affording an NpN iron fluoride complex and pentafluorobenzene as products. The NpN iron hydride is a precatalyst for catalytic hydrodefluorination of perfluorinated aromatics in the presence of silane. Kinetic studies indicated that the rate-determining step during catalysis involved silane. Enamine-phosphazide intermediates were isolated during the Staudinger reaction and were stable to deprotonation and coordination to Fe(II) and Co(II) halides. The enamido-phosphazide iron bromide complex displayed unusual reactivity upon treatment with potassium triethylborohydride, affording a dimeric phosphinimido species with an Fe–Fe bond. Isotopic labeling studies indicated that cleavage of the phosphazide N–C(aryl) bond occurred through a radical process.

Text

2017-05-29

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/61784

Chemistry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/