Novel Intramolecular C-Aryl-S Bond Activation by an Electron Rich, Ring-Expanded-NHC-Rh centre: A Combined Experimental and DFT Study

The reaction of (o-MeSPh)-N-functionalized tetrahydropyrimidinium salts with KN(SiMe3)2 and [Rh(COD)Cl]2 in THF leads to the formation of a novel dimeric RhIII bis-carbene complex. The reaction involves the unexpected cleavage/oxidative addition of the aryl-sulfur bond to give dimeric metallated RhIII with bridging MeS< moieties. This unusual reaction is probably a consequence of the sterically imposing structure and strong donor capacity of ring-expanded N-heterocyclic carbenes (RE-NHCs). An X-ray structure of the [(Ph,DIPP-NHC)Rh(Cl)(SMe)2] product complex has been obtained, and DFT studies were undertaken to gain an insight into the reaction pathway

Donor-Functionalised Expanded Ring N-Heterocyclic Carbenes: Highly Effective Ligands in Ir-Catalysed Transfer Hydrogenation

The performances of a number of RhI and IrI complexes of type [M(NHC)(COD)Cl] in the transfer hydrogenation of ketones were tested under a variety of reaction conditions, and with a variety of substrates, allowing comparison of Rh- and Ir-NHC complexes, and also comparison of the influence of the NHC ligand on catalytic performance. Notably, of the RhI and IrI complexes with symmetrically substituted NHCs only those bearing cyclohexyl substituents were active, with RhI complexes of saturated 5-, 6- and 7-NHCs with N-Mes substituents, [Rh(5,6,7-Mes)(COD)Cl], showing no activity in transfer hydrogenation under the test conditions. RhI and IrI complexes of unsymmetrical o-methoxyphenyl donor-functionalised NHCs (df-NHC) with differing carbene ring sizes were also tested in transfer hydrogenations, with the RhI complexes displaying no catalytic activity. However, the corresponding df-NHC IrI complexes were found to be extremely effective catalysts. Catalyst tests also demonstrated the excellent stability of these complexes

Expanded ring and functionalised expanded ring N-heterocyclic carbenes as ligands in catalysis

The synthesis of new functionalised 6- and 7-membered NHC (N-heterocyclic carbene) precursors bearing anisidyl or pyridine N-substituents, both symmetrically and non-symmetrically substituted is reported. Their corresponding rhodium(I) and iridium(I) complexes, M(COD)(NHC)Cl, were also prepared and characterised. The unusual Rh(III)/Rh(I) salt, [Rh(η2-NHC-py)2Cl2][Rh(COD)Cl2], was obtained with one of the pyridyl-functionalised NHC ligands. Single-crystal X-ray analyses have been obtained for the majority of the complexes and NHC salts. The activity of these complexes was tested in the hydrogenation of a range of substrates with molecular hydrogen, including 1-cyclooctadiene and 2-methyl styrene, where they show enhanced activity and stability in comparison to non-functionalised NHC analogues, operating under exceptionally mild conditions (ambient temperature and atmospheric pressure)