Novel expanded ring N-heterocyclic carbenes; coordination and application in catalysis

The work presented in this thesis is concerned with the synthesis, characterisation, and application in catalysis of mono- and bis-expanded ring (including bicyclics) Nheterocyclic carbenes (NHCs) with emphasis on their coordination to palladium, silver, rhodium, gold, copper, and nickel. Chapter two provides the synthesis and characterisation of a series of bis-expanded ring NHC precursors along with the attempted synthesis of mixed 5- and 6-membered types. The attempted coordination of the bis-NHC precursors to palladium did not produce the desired complexes rather a rearrangement occurred to give nitrogen coordinated species through elimination of the linking group between the two heterocycles. Chapter three explores the synthesis and characterisation of a range of novel alkylated bicyclic NHC precursors including the dimethyl, diethyl, and diisopropyl derivatives, and their coordination to rhodium(I). The increased steric demand (iPr > Et > Me) of the exocyclic substituent leads to a larger NCN angle across the series. The rhodium complexes exist as a mixture of two isomers (syn-alkene and syn-chloride) in a 2:1 ratio resulting from restricted rotation about the Rh-CNHC bond. The complexes are active for the transfer hydrogenation of ketones with conversions up to 37 %. Chapter four discusses the coordination chemistry of monocyclic expanded-ring and bicyclic NHCs with copper(I). The expanded ring NHC copper(I) complexes of the type [Cu(NHC)X] show typical Ccarbene-Cu-X bond angles of around 180 °. However, the related complex of the mesityl bicyclic NHC shows an apparent non covalent interaction between the metal and the mesityl ring causing a deviation of the Ccarbene-Cu-X bond away from linearity. The catalytic activity of the expanded ring copper(I) halide complexes for hydrosilylation was explored but were shown to be inactive. Chapter five concerns gold(I) complexes of expanded-ring and bicyclic NHCs. A small crystallographic library of complexes enabled the percentage buried volumes to be determined. As expected, these were large for the aromatic substituted expanded ring complexes of type [Au(NHC)X] but appreciably smaller (more akin to the common 5- membered NHCs) for the bicyclic systems and the alkylated expanded ring systems. The catalytic activity of the complexes in the hydration of alkynes was explored with iv conversions of up to 100 % being observed. Selectivities were noticeably better than those reported for the [Au(6-DIPP)Cl] and [Au(7-DIPP)Cl] with values of around 30:70. Chapter six moves into the coordination and characterisation of a series of highly sensitive expanded ring nickel(I) complexes. Due to the paramagnetic nature of these compounds analysis using EPR was achieved. This showed that the nickel(I) complexes all exhibited orthorhombic g values

Twisting the arm: structural constraints in bicyclic expanded-ring N-heterocyclic carbenes

A series of diaryl, mono-aryl/alkyl and dialkyl mono- and bicyclic expanded-ring N-heterocyclic carbenes (ER-NHCs) have been prepared and their complexation to Au(I) investigated through the structural analysis of fifteen Au(NHC)X and/or [Au(NHC)2]X complexes. The substituted diaryl 7-NHCs are the most sterically encumbered with large buried volume (%VB) values of 40–50% with the less flexible six-membered analogues having %VB values at least 5% smaller. Although the bicyclic systems containing fused 6- and 7-membered rings (6,7-NHCs) are constrained with relatively acute NCN bond angles, they have the largest %VB values of the dialkyl derivatives reported here, a feature related to the fixed conformation of the heterocyclic rings and the compressional effect of a pre-set methyl substituent

Intramolecular formation of a CrI(bis-arene) species via TEA activation of [Cr(CO)4(Ph2P(C3H6)PPh2)]+: sn EPR and DFT investigation

Activation of the catalytically relevant complex [Cr(CO)4(1)] (1 = Ph2P(C3H6)PPh2) by Et3Al (TEA) leads to formation of the Cr(I) bis-arene complex [Cr(1-bis- 6-arene)]+, as revealed by EPR and DFT calculations. This bis-arene complex is formed by intramolecular rearrangement and coordination of Cr(I) to the ligand phenyl groups in aliphatic solvents following loss of CO, preventing release of Cr(I) into solution. By comparison in aromatic solvents (toluene), the [Cr(bis-tolyl)]+ complex is preferentially formed