PCP Pincer Iridium Chemistry – Coordination of Pyridines to [(tBuPCP)IrH(Cl)]

The complexes of [(tBuPCP)IrH(Cl)] {1, tBuPCP = 1,3-bis[(di-tert-butylphosphanyl)methyl]benzene} with pyridine (py, 2), 4-aminopyridine (4apy, 3) and 2-aminopyridine (2apy, 4) have been synthesized and fully characterized by variable-temperature spectroscopy (NMR, IR, and UV/Vis). In each case, the reaction yields a mixture of cis- and trans-hydrido–chlorido isomers. The stereochemistry of the latter was confirmed by single-crystal X-ray analysis of 2a and 3a, which showed that the pyridine ligands coordinated opposite the aryl ring do not produce an additional twist of the pincer ligand but lead to the elongation of all bonds involving the iridium atom. The chlorido ligand becomes substantially labilized owing to the strong trans effect of the hydrido ligand, and that in 3a it is prone to intermolecular N–H···Cl hydrogen bonding in the solid state. In the 2-aminopyridine complex 4, the intramolecular hydrogen bond to the chlorido ligand (isomer 4a′) is slightly more preferred over the N–H···H bond (isomer 4a). The structures of the cis-hydrido–chlorido isomers 2b–4b were established with the help of 15N-labeled pyridine. These isomers undergo fast ligand exchange on the NMR timescale that slows as the temperature decreases. The decoalescence temperature is the lowest for the 2-aminopyridine derivative 4b; therefore, it is the least stable. The repulsion between the tert-butyl groups of the pincer ligand and the o-NH2 group is the main reason for the lower stability of 4b.