Catalytic Applications of Nickel and Copper Complexes Stabilised by Large Ring N-Heterocyclic Carbenes

Abstract

This thesis reports the effect of ring expansion of N-heterocyclic carbenes (NHCs) on complexes of copper and nickel in synthesis and catalysis. Chapters 2, 3 and 4 describe the influence of ring expansion on linear two-coordinate Cu complexes of the form (carbene)CuX, whereas chapter 5 focuses on a low coordinate Ni(I) species, Ni(6-Mes)(PPh3)Br (5.1), which has been stabilised by a ring-expanded NHC (RE-NHC).
Chapter 2 expands upon previous work describing the semihydrogenation of 1-phenyl-1-propyne by (6-Mes)CuOtBu (2.1). A series of (RE-NHC)CuOtBu complexes, synthesised through the protonolysis of (RE-NHC)CuMes with tBuOH, were then compared to 2.1 for the semihydrogenation of alkynes. Complex 2.1 was found to give the best results and upon optimisation of the conditions, proved active at 0.5 mol%. The hydroboration of alkynes was also investigated with (RE-NHC)CuOtBu species, and with a preliminary mechanistic study undertaken.
Chapter 3 reports the synthesis of six new examples of (carbene)CuF species (all structurally characterised) which were employed in the catalytic allylation of octanal. The bulky (ITr)CuF complex (3.3) was found to afford the best catalytic activity. Mechanistic experiments point to the involvement of Cu silicates.
Chapter 4 details the synthesis of (RE-NHC)CuX (X = Cl, Br and I) via a cuprate intermediate that avoided having to use a free carbene. A range of (RE-NHC)CuX complexes were synthesised employing this method, although increasing ring size, steric bulk or employment of a heavier halide resulted in more forcing conditions. The (RE-NHC)CuX complexes were also employed in the [3+2] cycloaddition of alkynes and azides. (6-Mes)CuF (3.2) and (6-Mes)CuI (4.3) were found to be the most active precursors, although variation of NHC ring size gave no clear trend.
Chapter 5 provides a mechanistic study of the catalytic hydrodehalogenation of aryl halides involving the low coordinate Ni(I) complex Ni(6-Mes)(PPh3)Br (5.1). Initial stoichiometric investigations with Ar-Br revealed formation of two Ni(II) species, Ni(6-Mes)(PPh3)Br2 (5.7) and Ni(6-Mes)(PPh3)(Ar)Br through a bimolecular oxidation, and formation of a series of Ni(I) alkoxide complexes with NaOR. The resulting complexes were characterised and spectroscopically studied, with EPR studies employed for Ni-OR. A complicated mechanism was revealed involving two cycles, one involving a postulated Ni(II)/Ni(0) cycle and the other a Ni(I) cycle.

Date of Award	28 Apr 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Michael Whittlesey (Supervisor) & Michael Hill (Supervisor)