A Tetracyclic Bis(imidazolindiylidene) Ligand and Its Diiridium and Dipalladium Complexes

Abstract

A bis(imidazolidine) with two butylene linkers between the nitrogen atoms of the adjacent heterocycles has been prepared. This tetracyclic molecule can be coordinated to iridium and palladium following two different methodologies. The coordination to iridium is achieved by double C(sp3)–H activation of the neutral tetracyclic bis(imidazolidine), which reacts with [IrCl(COD)]2 to afford the corresponding bis-NHC complex that contains two IrCl(COD) fragments. The reaction is facilitated by the addition of 1,5-cyclooctadiene, which is released from the reaction as cyclooctene. The reaction of the bis-NHC[IrCl(COD)] (bis-NHC = bis(imidazolindiylidene)) complex with CO allows the formation of the corresponding tetracarbonyl complex of diiridium. The molecular structures of the two iridium complexes have been determined by means of X-ray crystallography. The reaction of the tetracyclic bis(imidazolidine) with N-bromosuccinimide allows the formation of a tetracyclic bis(imidazolinium) salt, which serves as a traditional bis-NHC precursor. The reaction of this bis(imidazolinium) salt with Pd(OAc)2 in the presence of KI and in acetonitrile allows the formation of the bis-NHC[PdI2(CH3CN)]2 complex. The acetonitrile ligand can be easily replaced by reaction with 3-chloropyridine, affording the corresponding bis-NHC[PdI2(3-Clpyr)]2 complex. The two palladium complexes were tested in the acylation of aryl halides with hydrocinnamaldehyde, showing moderate to low activity.