Temperature- and solvent-dependent binding of dihydrogen in iridium pincer complexes

Mixtures of deuterium labeled complexes (p-XPOCOP)IrH2-xDx (1-6-d(0-2)) {POCOP = [C6H2-1,3-[OP(tBu)(2)](2)}; X = MeO (1), Me (2), H (3), F (4), C6F5 (5), and Ar-F = 3,5-(CF3)(2)-C6H3 (6)} have been generated by reaction of (p-XPOCOP)IrH2 complexes with HD gas in benzene followed by removal of the solvent under high vacuum. Spectroscopic analysis employing H-1 and D-2 NMR reveals significant temperature and solvent dependent isotopic shifts and HD coupling constants. Complexes 1-6-d(1) in toluene and pentane between 296 and 213 K exhibit coupling constants J(HD) of 3.8-9.0 Hz, suggesting the presence of an elongated H-2 ligand, which is confirmed by T-1(min) measurements of complexes 1, 3, and 6 in toluene-d(8). In contrast, complex 6-d(1) exhibits J(HD) = 0 Hz in CH2Cl2 or CDCl2F whereas isotopic shifts up to -4.05 ppm have been observed by lowering the temperature from 233 to 133 K in CDCl2F. The large and temperature-dependent isotope effects are attributed to nonstatistical occupation of two different hydride environments. The experimental observations are interpreted in terms of a two component model involving rapid equilibration of solvated Ir(III) dihydride and Ir(I) dihydrogen structures.