Synthesis and structural characterization of alkyne-functionalized N-heterocyclic carbene complexes of ruthenium, palladium and rhodium

Abstract

A series of novel ruthenium(II), palladium(II) and rhodium(I) complexes bearing N-heterocyclic carbene ligand functionalized by an alkyne-protected functional group (CCSiMe3) were synthesized by transmetalation from the silver-NHC intermediate or by deprotonation of the azolium salt using an external base. The complexes were fully characterized and X-ray diffraction was used for the structure determination. The donating properties of the NHC ligand were evaluated through the synthesis of the rhodium carbonyl complex. The reactivity of the functional group has been studied: deprotection of the alkyne on the ruthenium and rhodium species gave instable products whereas deprotection of the palladium compound was quantitative. However the resulting alkyne-functionalized species was incompatible with conditions for further post-functionalization such as metal-catalyzed 1,3-dipolar cycloaddition.