Complexes of Rhodium and Iridium featuring cyclometallated NHC moieties : synthesis and catalysis

Abstract

The synthesis of a range of iridium- and rhodium-based N-heterocyclic carbenes (NHC) complexes were performed. To this end, a variety of imidazolium salts were synthesised and utilised in the formation of bidentate NHC complexes bound through a carbon-based tether. The synthesis of the imidazolium salts was done via the addition of the relevant alkyl halides to imidazole. Two metal precursors of rhodium, [Cp*RhCl2]2, and iridium, [Cp*IrCl2]2, were synthesised and subsequently applied to the synthesis of the novel complexes via a silver transmetallation method. This method requires the addition of the relevant imidazolium ligand precursors to silver(I) oxide in the absence of light to form silver-carbene intermediate complexes, followed by the addition of the metal (Ir or Rh) precursor to allow transmetallation to occur. The majority of the resulting products that formed were found to be bidentate complexes that formed through an unprecedented cyclometallation/isomerisation mechanism along with their by-products, which were isolated and characterised. In some cases, the by-products that formed were coordination complexes where de-alkylation of the carbon-based alkenyl substituent took place, which allowed for the coordination through the nitrogen atom on the imidazole ligand. Iridium-based by-products were more varied with one imidazolium ligand binding through the nitrogen atom, one where no cyclometallation occurred as well as one where dealkylation occurred. However, the C2-bonded NHC ligand featured a protonated nitrogen atom of the resulting neutral imidazolylidene ligand. Each novel complex was characterised using nuclear magnetic resonance (NMR) spectroscopy techniques, single crystal X-ray diffractometry (SCXRD), liquid chromatography-mass spectrometry (LC-MS) and elemental analysis (EA). The catalytic activity of each complex was evaluated in the hydrosilylation of internal alkynes, with the iridium complexes also being applied to transfer hydrogenation of aryl ketones/aldehydes utilizing iPrOH as the hydrogen source. The rhodium complexes performed the best in the hydrosilylation of internal alkynes reaction (conversions of up to 100% after 1 hour, TOF of up to 92.9 h-1), with the iridium complexes performing relatively poor (conversions of up to 59% after 1 hour, TOF of up to 14.8 h-1), however, still good when compared to literature for related Ir complexes. The iridium complexes performed better as transfer hydrogenation catalysts with conversions of up to 100% recorded after 18 hours (TOF of up to 5.5 h-1).