Synthesis, NMR, and X‑ray Studies of Iridium Dihydride C,N and N,P Ligand Complexes

The iridium­(III) dihydride complexes
[Ir­(H)2(L1)­(6,6′-bi-2-picoline)]­BArF (5; L1 = (S)-1-[2-(2-adamantan-2-yl-4,5-dihydro­oxazol-4-yl)-ethyl]-3-(2,6-diisopropyl­phenyl)-1,2-dihydro­imidazol-2-ylidene,
BArF = tetrakis­[3,5-bis­(trifluoro­methyl)­phenyl]­borate)
and [Ir­(H)2(L2)­(6,6′-bi-2-picoline)]­BArF (6; L2 = (R)-2-((di-tert-butyl­phosphanyl)­methyl)-4-phenyl-4,5-dihydro­oxazole)
were prepared from the corresponding [Ir­(COD)­(L)]­BArF precursors by treatment with 6,6′-bi-2-picoline under H2 and characterized by 2D NMR spectroscopy and X-ray diffraction.
In solution, the N,P complex 6 exists as two isomeric
dihydride species (6a and 6b) that are in
rapid equilibrium at room temperature. Furthermore, the X-ray structures
for [Ir­(COD)­(L1)]­BArF (1) and [Ir­(COD)­(L2)]­BArF (2) are reported. The structural comparison
of the solid-state structures of the iridium­(I) precursor 1 and the iridium­(III) dihydride complex 5 revealed a
significant change in the backbone geometry of the C,N ligand. The
original U-shaped conformation of the ligand switches to an S-shaped
conformation, and therefore, the substituent in the oxazoline ring
occupies different quadrants in the iridium coordination sphere. Notable
in this context is the finding that a similar switch in the ligand
backbone was observed for the C,N iridium­(III) dihydride olefin species 3 ([Ir­(H)2[(E)-1-methyl-4-(1-phenylprop-1-en-2-yl)­benzene-D5]­(L1)]­BArF), which represents a catalytically
competent intermediate.