The application of nickel complexes to halogen bonding and hydrofluoroarylation

The N-heterocyclic carbene complex, (1,5-hexadiene)Ni(IPr) (IPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-2H-imidazol-2-ylidene) was found to catalyse the hydrofluoroarylation reaction of alkynes at lower temperatures and more cleanly compared to the Ni(PR3)2 (R =Cyp and Cy) catalysts reported in the literature. Kinetic studies of the hydrofluoroarylation reaction using the above catalyst was carried out to obtain more information of the mechanism, which is addressed in chapters 2 and 3. The reaction followed in-situ by 1H-NMR spectroscopy using 4,4-dimethylpentyne and C6F5H as the substrates catalysed by (1,5-hexadiene)Ni(IPr), suggested saturation kinetics with respect to the alkyne and first order dependence on the fluoroarene and the catalyst. The results also suggests an initial coordination of the alkyne to nickel Ni(IPr)(alkyne), the active species that reacts with the fluoroarene. The mechanistic understanding gained from this study, is similar to that proposed in the literature based on the DFT calculations using Ni-PR3 as the catalyst.
Chapter 4 details the synthesis and study of two series of self-complementary nickel complexes Ni(X)(PEt3)2(y-C6F4I) (X=F-, Cl-, Br- and I- and y = ortho and para) exhibiting intermolecular halogen bonding interactions. X-ray crystallographic studies revealed the strength of the halogen bonding interaction decreases in the following order, with different XB acceptors; F->Cl-≈Br->I-. The XB interaction occurs at an angle of 180° for Ni-F…I and ranges from 139° to 147° for Ni-X…I, where X=Cl, Br and I). When PEt3 ligand were substituted with PEt2Ph ligand the halogen bonding interaction was observed to weaken. The higher number of fluorines ortho to the halogen bond donor strengthened the interaction. Investigation of the Ni-F…I interactions by solid state NMR spectroscopy upon the 19F nuclei showed a downfield shift in the fluoride resonance when compared to its analogous complex without the halogen bonding interaction. The thermodynamic study of the halogen and hydrogen bonding interactions of nickel fluoride complexes, ligated to N-heterocyclic carbene ligands are reported in chapter 5. Interactions of indole and C6F5I have been studied with Ni(F)(i-Pr2Im)2(C6F5) using 19F-NMR spectroscopic titrations (For C6F5I, K300 =7.2; change in S° =-50.2±1.2 J/(mol.K); change in H° =-23.9±0.3 kJ/mol and for indole K300 =33; change in S°= 53.6±1.7 J/ (mol.K); change in H° =-32.7±0.5 kJ/mol).