Rhodium complexes of NHC-based pincer ligands: catalysis and reactions through rings

N-heterocyclic carbenes (NHCs) are robust and versatile ancillary ligands for a wide range of metals. Incorporation of these strong σ-donors into mer-tridentate pincer frameworks confers greater thermal stability, representing an amalgamation of favourable ligand design principles that underpin many advances being made in contemporary organometallic chemistry and catalysis. This project aims to develop understanding of the mechanistic organometallic chemistry of terminal alkyne coupling reactions promoted by rhodium complexes of NHC-based pincer ligands, with the hope of exploiting this methodology in the formation of mechanically interlocked structures of macrocyclic variants of differing ring size. Following development of a mild copper-based transmetallation procedure, rhodium(I) ethylene complexes of a series of NHC-based pincer ligands were prepared (viz. 11-n; n = Me, 12, 14, 16). The acyclic complex 11-Me was found to be a highly effective pre-catalyst for the head-to-tail dimerisation of aryl alkynes to afford gem-enynes under mild conditions and curiously their subsequent annulation into bicyclo[4.2.0]octa-1,5,7-trienes. Through in situ reaction monitoring using NMR spectroscopy, kinetic studies and computational modelling, the mechanism and factors which influence the selectivity of the alkyne dimerisation reaction were probed. Terminal alkyne coupling of bulky alkynes through the macrocyclic annuli of 11-n (n = 12, 14, 16) are associated with increased selectivity for E-enynes products, which were formed exclusively in the case of n = 12 and 14, as a consequence of the unique ligand topology. Using these and related results the prospects for synthesising interlocked assemblies, comprising an NHC-based pincer macrocycle and an entrapped hydrocarbon axle, have been critically assessed