Effects of Hindrance in N‑Pyridyl Imidazolylidenes Coordinated to Iridium on Structure and Catalysis

The unhindered N-pyrid-2-yl imidazolidene NHC ligand has been shown to chelate to a Cp*Ir fragment (1). With the goal of weakening the coordination of the pyridyl substituent and enabling its role as pendant base or hemilabile ligand, a tert-butyl group at C-6 next to the pyridyl N was installed. Attempted coordination of the newly synthesized carbene ligand avoided N-coordination entirely, but led to unexpected C-metalation at C-3 by the Cp*Ir center. Successful formation of a weakly N-coordinated analogue was achieved by synthesizing a ligand with a second tert-butyl group at C-4. The complexes were studied using X-ray crystallography and NMR spectroscopy. The X-ray crystal structure of di-tert-butyl analogue 6 showed that in the solid the complex existed as a chloride-bridged dimer, with the pyridyl nitrogen uncoordinated. In solution, 15N chemical shift information revealed that 6 existed with the pyridyl substituent N-coordinated, presumably as a monomer, but that addition of an amine ligand readily opened the chelate. Finally, 6 was used as a catalyst for intramolecular hydroamination of primary and secondary alkenylamines. Comparing (heteroaryl)­NHC species, each with a tert-butyl group next to the nitrogen, which enables hydroamination, the rate differences are very modest but increase in the order imidazolyl tert-butyl)­pyridyl species 6 was shown to be much more effective compared to parent compound 1 without the tert-butyl groups, in which the chelating group was more tightly bound.