The Synthesis and Reactivity of Zirconium and Hafnium Hydrazides

Lead Research Organisation: University of Oxford
Department Name: Oxford Chemistry

Abstract

It has been shown over the last decade in particular that Group 4 hydrazides of the type (L)M=NNR2 can undergo a variety of addition or insertion reactions of the Ti=N-alpha multiple bond with unsaturated substrates. A distinctive aspect of Group 4 hydrazido is the typically facile reductive cleavage of the N-alpha-alpha-N-beta bond that can also occur with oxidizable substrates such as CO, isocyanides and alkynes to form new N-element functional groups and/or organic products.
The chemistry of these hydrazides is tangentially connected with an established body of elegant dinitrogen activation and addition reactions of the early transition metals. Recent work in the Mountford group has investigated, both experimentally and computationally, E-H bond activation reactions of titanium hydrazides (E = C(sp1), B and Si) as well as other small molecule activation reactions. However, much of this chemistry with zirconium and hafnium hydrazides remains unexplored.
The use of zirconium and hafnium can alter reactivity, and allow for a drastically different ligand set (L in (L)M=NNR2) to be investigated. This enables intermediates on reaction pathways to be trapped and different reaction products to be isolated from analogous reactions. This project will investigate such reactions, as well as a range of ligand sets on the metal centre. Initial reactions carried out this month have already isolated an intermediate predicted by DFT studies for the reaction of Group 4 hydrazides with silanes - providing experimental proof of computational studies.
In addition to the reactivity studies of zirconium and hafnium hydrazides, the synthesis of previously unattainable hydrazides will be explored. Only zirconium and hafnium hydrazides of the type (L)Zr=NNPh2 have been synthesised and studied before. However, Zr and Hf hydrazides of the type (L)Zr=NNMe2 would be of much interest in reactivity and catalysis, as analogous dimethyl titanium hydrazides have shown increased reactivity and more favourable catalysis to diphenyl titanium hydrazides. Previously the synthesis of dimethyl zirconium and hafnium hydrazides has been attempted before, though the complex has dimerized to a complex that shows no reactivity to small molecules. However, it is thought that a change in ligand set may disfavour dimerization and allow the desired product to be isolated. This will be investigated in this project and, if successful, the novel dimethyl zirconium and hafnium hydrazides can then be tested with many small molecules for reactivity as well as for catalysis.
This proposed research project involves novel reactivity studies, as well as a novel synthesis route to a previously unattainable class of zirconium and hafnium hydrazides, which would allow for a series of unexplored reactivity and catalysis tests. These studies will be carried out both experimentally and computationally (within the group and with our collaborator Dr Eric Clot) to allow for a full understanding of the chemistry at play.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1811529 Studentship EP/N509711/1 01/10/2016 30/09/2019 Archibald Walter McNeillis
 
Description This research has discovered a new route to zirconocene silylamido complexes and the rich chemistry of these complexes has been explored and taken further than before. These complexes have been found to react with many small, common molecules such as carbon monoxide, dihydrogen, silanes and boranes. New catalytic processes using these complexes have also been discovered, such as the oligemerisation of ethylene - the conversion of ethylene to butane and hexane. While these catalytic process are not nearly comparable in efficiency to those used in industry, the catalysts used start a novel family which could be explored further.
Exploitation Route The complexes synthesised and rich chemistry shown could be taken to optimise the catalytic processes found in this project as well as discover new catalytric processes based on what has been found about the stoichiometric reactivty of the complexes.
Sectors Chemicals