Diamido- and diaminocarbene complexes of the late d-block metals

Abstract

This Thesis is concerned with the synthesis and reactivity of N-heterocyclic carbene (NHC) complexes of copper and the implications of NHC electrophilicity for late d-block metal catalysed reductive processes. Comparison of two NHCs with different electronic, but similar steric properties (the diamidocarbene 6-MesDAC and its diaminocarbene analogue 6-Mes) reveals contrasting reactivity, with implications for the use of electrophilic NHCs in reductive catalytic processes.

The first diamidocarbene complexes of copper are synthesised, beginning with the isolation of a series of monomeric to tetrameric copper chloride complexes (6-MesDAC)CuCl (3) and (6-MesDAC)2(CuCl)n (1: n = 2; 2: n =3; 4: n = 4). Complexes 1 and 2, as well as the homoleptic complex [(6-MesDAC)2Cu][BF4] (6), are found to be less successful CuAAC catalysts than related diaminocarbene complexes [(NHC)2Cu][BF4] (7: NHC = 6-Mes; 8: NHC = 7-Mes; 9: NHC = 7-o-Tol). New methodology is developed for the synthesis of the NHC copper alkoxide complexes (6-MesDAC)CuOtBu (11) and (6-Mes)CuOtBu (20) by the protonolysis of the copper aryl complexes (6-MesDAC)CuMes (12) and (6-Mes)CuMes (19). Complex 11 undergoes a range of thermal and hydrolytic decomposition reactions by ring opening of 6-MesDAC.

The diaminocarbene complexes (6-Mes)ZnMe2 (25) and (7-Mes)ZnMe2 (26) are found to be active catalysts for the ROP of rac-lactide. In contrast, (6-MesDAC)MR2 (27: ZnEt2; 29: ZnMe2; 31: CdMe2) complexes undergo a facile migratory insertion process. This migratory insertion process also applies to copper hydride complexes, with both 6-MesDAC and 6-Mes found to be susceptible to copper hydride migratory insertion. Only the less electrophilic 6-Mes is able to support reductive catalysis (for example, alkyne semi-reduction and ketone hydrosilylation), and the hydride complex [(6-Mes)CuH]2 (38) is characterised at low temperature. The products of migratory insertion (6-MesDAC·H)Cu(P(p-Tol)3) (34) and (6-Mes·H)Cu(6-Mes) (35) are isolated and the mechanisms of their formation investigated through DFT calculations. Lewis acids are found to stabilise copper hydrides with respect to migratory insertion. The first mononuclear, Lewis acid stabilised, copper hydride complexes (6-Mes)CuHBEt3 (39) and (6-Mes)CuHB(C6F5)3 (42) are isolated and investigated by a QTAIM analysis.

Date of Award	12 Apr 2016
Original language	English
Awarding Institution	University of Bath
Supervisor	Michael Whittlesey (Supervisor)