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 < pyridyl < pyrimidyl, which may be
an effect of basicity but because of the similarity in rates is better
ascribed to counterbalancing of more than one factor, including hemilability.
The (4,6-di-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.