This thesis describes the development of nickel(I) complexes incorporating ring expanded N-heterocyclic carbene (RE NHC) ligands and examines their electronic characterisation, activation of O2, reactivity and catalytic applications. A series of three coordinate, paramagnetic Ni(I) complexes of the form Ni(RE NHC)(PPh3)Br (1 – 10) were prepared by comproportionation of Ni(COD)2 and Ni(PPh3)2Br2 in the presence of RE NHCs. The RE NHCs employed varied in the degree of ring expansion (6-, 7- and 8-membered), extent of N-substituent steric bulk (Mes, oTol, oAnis) and the donor/acceptor properties of the carbene (diamino vs. diamido). EPR and DFT electronic characterisation of 1 – 10 confirmed that the unpaired electron was located on the nickel ion in a mixed orbital of predominantly 3dz2 character.Yellow solutions of 1 or 6 (RE NHC = 6Mes and 7Mes respectively) immediately became purple in the presence of O2 due to O2 activation and incorporation of the oxygen atoms as bridging ligands resulting from C-H activation/oxygenation of an RE NHC N-substituent. This generated the dimeric Ni(II) complexes Ni(6/7Mes)Br(µ-OH)(µ-O-6/7Mes)’NiBr (6Mes = 13; 7Mes = 14). Mass spectrometry demonstrated that the doubly activated complexes [NiBr(µ-O-6/7Mes)’]2 (15 and 16 respectively) were also formed in the reactions. UV-vis spectroscopy revealed the reactions took place rapidly, even at 190 K. Contrasting reactivity was observed when 2 or 7, bearing the less sterically encumbered N-oTol substituents 6oTol and 7oTol respectively, were exposed to O2, which led to the ligand redistribution products NiII(6/7oTol)(PPh3)Br2 (17 and 18 respectively). The less electron rich diamido analogue containing 6MesDAC (5), underwent dissociation and oxidation of the RE NHC and PPh3 ligands. Attempts to abstract the bromide from 1 generated novel two and three coordinate Ni(I) products. Reaction with additional 6Mes produced the two coordinate cation [Ni(6Mes)2]+ (19), which could be reduced with KC8 to Ni(6Mes)2 (20). SQUID analysis of 19 revealed it to be the first example of a nickel containing mononuclear single molecular magnet (SMM). Addition of [Et3Si]+ to 1 followed by work up in toluene led to the isolation of the Ni(I)-(η2-toluene) complex [Ni(6Mes)(η2-C6H5CH3)]+ (21). Mesitylene generated the analogous [Ni(6Mes)(η2-C6H3(CH3)3)]+ (23), but neither 1,4-xylene nor naphthalene gave isolable products. In all cases, cocrystallisation of [6MesH]+…arene was observed in variable amounts, which compromised reaction studies of the Ni-arene complexes. Removal of bromide from 1 with TlPF6 in THF generated the solvent coordinated cationic species [Ni(6Mes)(PPh3)(THF)]+ (24). Attempts to remove the ligated THF molecule were unsuccessful, however, it could be directly substituted by CO to form [Ni(6Mes)(PPh3)(CO)]+ (26). Similarly to 1, complex 24 activated O2, generating a dimer analogous to the singly activated complex 13 (Ni(6Mes)(PPh3)(µ-OH)(µ-O-6Mes)’NiBr (25)). Reactivity of 1 with NaBH4 produced [Ni(6Mes)(κ2-BH4)]2 (28), a Ni(I) dimer bridged by two BH4 ligands.The catalytic efficiency of neutral 1 in Kumada cross-coupling of aryl halides and PhMgCl or MesMgBr was probed. Of note was the high activity towards both aryl chlorides and aryl fluorides. Comparisons with cationic 24, larger 7- (7) and 8-membered ring (8 and 9) variants and the Ni(II) complexes Ni(6Mes/6oTol/7oTol)(PPh3)Br2 (29, 17 and 18 respectively) revealed that 1 exhibited the highest reactivity of all the precursors.
|Date of Award||27 Apr 2015|
|Supervisor||Michael Whittlesey (Supervisor)|
- Ring Expanded N-Heterocyclic Carbenes
- N-Heterocyclic Carbenes
- Low coordinate