Synthesis, characterization and catalytic activity of saturated and unsaturated N-heterocyclic carbene iridium(i) complexes

Abstract

Both saturated and unsaturated N-benzyl substituted heterocyclic carbene (NHC) iridum(I) complexes were synthesized. The unsaturated carbene complex [(un-NHC-Bn)Ir(CO)₂Cl] in the cis form was prepared via the carbene transfer from the corresponding silver complex to [Ir(COD)₂Cl]₂ followed by ligand substitution with CO, whereas the saturated complex was obtained via the transfer from (sat-NHC-Bn)W(CO)₅. The treatment of phosphines with (NHC)Ir(CO)₂Cl complexes yielded the products with the phosphine ligand trans to the carbene moiety via substitution. X-Ray structural determination shows that distances of Ir–C_(carbene) in both (un-NHC-Bn)Ir(CO)(PR₃)Cl and (un-NHC-Bn)Ir(CO)(PR₃)Cl are essentially the same. Analyses of spectroscopic and crystal structural data of iridium complexes [(NHC)Ir(CO)(PR₃)Cl] and Vaska's complex show similar corresponding data in both types of complexes, suggesting that the studied NHC ligands and phosphines have similar bonding with Ir(I) metal center. All iridium complexes studied in this work illustrated their catalytically activity on N-alkylation of amine with alcohol viahydrogen transfer reduction. It appears no dramatic difference on the catalytic activity among these iridium carbene complexes; but the saturated carbene complex (sat-NHC-Bn)Ir(CO)(PR₃)Cl appears to be slightly more active. For example, the reaction of benzyl alcohol with aniline in the presence of catalyst (1 mol%) under basic conditions at 100 °C provided the secondary amine (N-benzylaniline) in 96% yield.