Synthesis and complexes of imidazolinylidene-based CCC pincer ligands

Abstract

A series of imidazolinium-based CCC pro-ligands featuring N-Mes, Dipp, iPr and tBu substituents (1·2HCl) have been prepared. The corresponding free carbenes are readily generated through deprotonation by strong bases and, in addition to being characterised in situ by 1H and 13C NMR spectroscopy, were trapped through reaction with CuCl. Iridium pincer compounds of the N-Mes (5a) and N-Dipp (5b) substituted ligands, viz. [Ir(1)HCl(NCMe)], were obtained through reaction between the respective pro-ligand, [Ir(COE)2Cl]2, and Et3N in acetonitrile at ca. 80°C. Under similar conditions the N-iPr and N-tBu analogues were not formed. The new iridium pincer complexes 5a and 5b were fully characterised in solution, by NMR spectroscopy and ESI-MS, and in the solid-state by X-ray diffraction. Under relatively forcing reaction conditions neither 5a nor 5b, in combination with KOtBu, show any significant catalytic activity for the transfer dehydrogenation of cyclooctane to cyclooctene using tert-butylethylene as the sacrificial hydrogen acceptor (ca. 2 TONs, 150°C, 24h).