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Group 4 complexes of an arene-bridged diamidophosphine ligand for nitrogen activation Maclachlan, Erin Alisa

Abstract

Molecular nitrogen comprises about 80% of the earth’s atmosphere, but is only used as a starling material in one industri2l reaction: NH₃ synthesis from N₂ and H₂ by the Haber Bosch process. Dinitrogen is generally unreactive, however, N₂ complexes of most transition metals and lanthanides are known, and the challenge remains to discover metal dinitrogen complexes that can functionalize N₂. This thesis describes a new ligand, [NPN]*, its coordination to Zr and Hf, and the use of [NPN]*Zr complexes to transform N₂. Arene-bridged diamidophosphine ligand [NPN]*Li₂ ([NPN]* = [{N-(2,4,6- Me₃C₆H₂)(2-N-5-Me-C₆H₃)}₂PPh]²) is synthesized from simple organic compounds and PhPCl₂ in high yield. Zr and Hf complexes, [NPN]*MCl₂ (M = Zr, Hf), are prepared via protonolysis from [NPN]*H₂, M(NMe₂)₄, and Me₃SiCl. Organometallic [NPN]*MR₂ (R = Me, CH₂Ph, CH₂SiMe₃) are prepared from [NPN]*MC₂ and Grignard or organolithium reagents. [NPN]*ZrR₂ (R = CH₂Ph, CH₂SiMe₃) are light- and heat-sensitive, and decompose to cyclometalated [NPNC] *ZrR ([NPNC] * = [(2-MesN-5-MeC₆H₃)P(Ph)(2-(NC₆H₂-2,4- CH₃-6-CH₂)-5-MeC₆H₃)]³⁻) and RH via an intramolecular σ-bond metathesis mechanism. A Zr-N 2 complex, {[NPN]*Zr(THF)}₂(μ-η²:η²-N₂), is synthesized in high yield from [NPN]*ZrCl₂ and 2.2 equiv. of KC₈ in THF under 4 atm of N₂. By single crystal X-ray analysis, N₂ has been reduced to N₂⁴⁻ and is side-on bound to two Zr atoms. Excess Py and PMe₂R (R = Me, Ph) reacts with the Zr-N₂ complex to furnish {[NpN]*Zr(Py)}₂(μ-η²:η²- N₂), and {[NPN]*Zr(PMe₂R) } (μ-η²:η²-N₂) {Zr[NPN] * }, respectively. The PMe₂R adducts react with H₂ to provide {[NPN]*Zr(PMe₂R)} (μ-η²:η²-NNH) {Zr[NPN]*} with a new N-H bond. A new N-Si bond forms upon addition of PhSiH₃ to the Py adduct to give {[NPN]*Zr(Py)} (μ-H)(μ-η¹:η²-NNSiH₂Ph)] {Zr[NPN]}* in high yield. The reaction of 4,4’- dimethylbenzophenone with the THF adduct provides NN=C(4-MeC₆H₄)₂) with a new N=C bond, and benzophenone imine reacts with the Py adduct to generate {[NPN]*Zr(N=CPh₂)}₂(μ-η²:η²-N₂H₂) with two new N-H bonds. The dilithium complex of Ph,AR[NPN]’ (Ar=4-PrC₆H₄) is prepared in two steps from commercially available reagents. Ph,Ar[NPN’Li₂(S) (S = THF, dioxane) is similar to [NPN]*Li₂(S) in solution and in the solid state, and is converted to Ph,Ar[NPN]’ZR(NMe₂)₂ via protonolysis. Future directions for research are also suggested.

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