LOW-COORDINATE Ni(I) COMPLEXES FOR MAGNETIC APPLICATIONS

Abstract

This thesis describes the synthesis and characterisation of a range of two- and three-coordinate mono- and dinuclear Ni(I) complexes featuring six- and seven membered ring-expanded N-heterocyclic carbene (RE-NHC) ligands, and examines their reactivity, electronic structure, and magnetic properties.
A series of trigonal planar three-coordinate Ni(RE-NHC)(PPh₃)Br compounds bearing N-aryl and N-alkyl six- and seven-membered RE-NHCs were prepared via the comproportionation of Ni(COD)₂ and Ni(PPh₃)₂Br₂ in the presence of the free RE-NHC. The reactivity of Ni(6Mes)(PPh₃)Br with halide abstracting agents gave T-shaped [Ni(6Mes)(PPh₃)(THF)]PF₆ and the mono-bromide bridged dimer [{Ni(6Mes)(PPh₃)}₂(µ-Br)]BAr^F₄, both of which reacted with CO to form T-shaped [Ni(6Mes)(PPh₃)(CO)]⁺. The use of an alternative synthetic route led to the formation of Ni(RE-NHC)(PCy₃)Br compounds. Through use of EPR spectroscopy and DFT calculations, the geometry of the three-coordinate Ni(I) complexes was shown to be determined by electronic effects rather than sterics, whereby a change in the d-orbital character of the SOMO caused a change in the observed g₂ values and structural geometries.
The three-coordinate Ni(I) compounds were used as precursors to form linear two coordinate bis-NHC complexes, either the homoleptic [Ni(RE-NHC)₂]Br species, or heteroleptic [Ni(RE-NHC)(NHC')]Br derivatives. Solution magnetic moment measurements gave values larger than those expected for a d⁹ ion. These unusual data were validated by SQUID analysis, which revealed room temperature magnetic susceptibilities larger than the predicted spin-only value. Computational studies predicted the electronic structure of linear Ni(I) complexes to have a low-lying excited state and two degenerate orbitals making up the HOMO. This orbital arrangement leads to unquenched orbital angular momentum, resulting in magnetic anisotropy and the large observed magnetic moment and susceptibility values, suggestive of single ion magnet (SIM) behaviour. The dynamic magnetic properties of the homoleptic and heteroleptic species were investigated with SQUID measurements. In all cases, field-induced slow magnetic relaxation was observed, and relaxation times were used to derive parameters for Orbach, Raman, direct, and quantum tunnelling of magnetisation (QTM) relaxation mechanisms.
Two-coordinate species of type [Ni(RE-NHC)(PCy₃)]⁺ were formed by halide abstraction from Ni(RE-NHC)(PCy₃)Br. The reactivity of the two-coordinate species was probed through the addition of P^tBu₃ (leading to PCy₃ substitution), and CO, which formed T-shaped Ni(I)-carbonyl compounds. Cyclic voltammetry on [Ni(RE-NHC)(PR₃)]⁺ showed irreversible oxidation and reduction waves, while magnetic moment measurements suggested magnetic anisotropy of the Ni(I) ion. SQUID analysis again revealed SIM behaviour, although the spin reversal barrier was lower than those in the bis-NHC complexes.
During investigations into Ni-NHC catalysed hydrophosphination of alkynes with secondary phosphines, it emerged that the NHCs themselves were able to perform catalysis in the absence of a metal with excellent conversions. DFT calculations provided a mechanism whereby the NHC acts as a Brønsted base to afford an imidazolium phosphide, which then undergoes nucleophilic attack at the terminal alkyne carbon. Extension of the study to the use of cyclic(alkyl)(amino)carbenes (CAAC) showed dependence of catalytic activity on substituent size, the smaller diethyl substituted CAAC proving inactive in contrast to bulkier analogues which displayed excellent activity.

Date of Award	19 Feb 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Andrew Johnson (Supervisor) & Michael Whittlesey (Supervisor)