H-2 BINDING TO AND FLUXIONAL BEHAVIOR OF IR(H)2X(PTBU2R)2 (X=CL, BR, I R=ME, PH)

The molecules Ir(H)2X(PtBu2Ph)2 (X = Cl, Br, I) all show low-temperature H-1 NMR spectra consistent with a structure with inequivalent hydrides, one of which is trans to X and one trans to an empty coordination site. T1 measurements suggest that all three molecules have very similar hydride/hydride distances. Hydride rearrangement DELTAH(double dagger) values show little dependence on the halide identity. The decoalescence behavior is not observed for the analogous compounds with PtBu2Me. While PtBu2Ph compounds show only T1 and spin-saturation transfer evidence for H-2 binding, the PtBu2Me compounds show (at low temperature) signals for both H-2 adduct and five-coordinate species. Line shape analysis of the variable-temperature P-31{H-1} NMR spectra permits the determination of DELTAH-degrees, DELTAS-degrees, DELTAH(double dagger), and DELTAS(double dagger) H-2 dissociation. These parameters have been determined for Cl, Br, and I and (for the chloride case) when deuterium replaces all hydrogens on Ir. The transition state for H-2 dissociation is ''tight'' (based on DELTAS(double dagger)) and it is enthalpically most accessible for the best pi-donor halide, Cl. This same effect stabilizes the five-coordinate species most for chloride, and thus makes the activation enthalpy largest for H-2 binding. The DELTAH-degrees values for H-2 dissociation increase down the halogen group.