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

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.