Iridium Hydride Complexes with Cyclohexyl-Based Pincer Ligands: Fluxionality and Deuterium Exchange

Two hydride compounds with aliphatic
pincer ligands, (PCyP)­IrH2 (PCyP = {cis-1,3-bis­[(di-tert-butylphosphino)­methyl]­cyclohexane}− (1) and (PCyP)­IrH4 (2), have been studied,
with emphasis on features where such systems differ from arene-based
analogues. Both compounds reveal relatively rapid exchange between
α-C–H and Ir–H, which can occur via formation
of carbene or through demetalation, with nearly equal barriers. This
observation is confirmed by deuterium incorporation into the α-C–H
position. Complex 1 can reversibly add an N2 molecule, which competes with the α-agostic bond for a coordination
site at iridium. The hydrogen binding mode in tetrahydride 2 is discussed on the basis of NMR and IR spectra, as well as DFT
calculations. While the interpretation of the data is somewhat ambiguous,
the best model seems to be a tetrahydride with minor contribution
from a dihydrido–dihydrogen complex. In addition, the catalytic
activity of 1 in deuterium exchange using benzene-d6 as a deuterium source is presented.