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Amidophosphine complexes of zirconium and titanium for dinitrogen activation Morello, Lara

Abstract

The preparation and reactivity of zirconium and titanium complexes with the macrocyclic P₂N₂ ligand (where P₂N₂ = PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh) or the chelating NPN ligand (where NPN = PhP(CH₂SiMe₂NPh)₂) are presented. The zirconium dinitrogen complex ([P₂N₂]Zr)₂(μ-η²: η²-N₂) reacts with (p-methyl)phenylacetylene and (p-tert-butyl)phenylacetylene to afford ([P₂N₂]Zr)₂(μ-η²: η²-N₂CH=CH(p-Me-C₆H₄))(μ-C=C(p-Me-C₆H₄)) and ([P₂N₂]Zr)₂(μ-η²: η²-N₂CH=CH(p-[sup t]Bu- C₆H₄))(μ-C=C(p-[sup t]Bu- C₆H₄)), respectively. In both complexes, the bridging alkenylhydrazido unit has a trans-configuration across the C-C double bond. The formation of this new N-C bond is only observed for reactions between ([P₂N₂]Zr)₂(μ-η²: η²-N₂) and terminal arylalkynes. A simple kinetic analysis of the reaction indicates that the rate-determining step does not involve C-H bond cleavage or N-H bond formation. The reduction of [NPN]ZrCl₂(THF) under an atmosphere of N₂ with two equivalents of KC₈ gives ([NPN]ZrTHF)₂(μ-η²: η²-N₂). The THF ligand is labile and can be displaced in solution by pyridine, from which ([NPN]Zrpy)₂(μ-η²: η²-N₂) can be isolated. A third complex, ([NPN]ZrTHF)(μ-η²: η²-N₂)(pyZr[NPN]), is only observed in situ by ³¹P{¹H} and ¹H NMR spectroscopy. The reduction of [P₂N₂]TiCl₂ with two equivalents of KC₈ under an atmosphere of N₂ affords the dinitrogen complex ([P₂N₂]Ti)₂( μ-η²: η²-N₂). This complex does not react with hydrogen or (p-methyl)phenylacetylene. The reaction of [NPN]Li₂ (THF)₂ with TiCl₄ (THF)₂ or TiCl₃ (THF)₃ produces [NPN]TiCl2 or [NPN]TiCl(THF), respectively. Upon reduction of [NPN]TiCl(THF) or [NPN]TiCl₂ with the appropriate number of equivalents of KC₈ under N₂, a phosphinimide complex, ([N(PN)N]Ti)₂, can be isolated, suggesting that the N=N bond of N₂ is cleaved and a new N-P bond is formed as a result of the reduction reaction. This is supported by mass spectrometric data and ³¹P{¹H} NMR spectroscopic data obtained for the reduction of [NPN]TiCl₂ under an atmosphere of ¹⁵N₂.

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