Dihydrogen/Dihydride or Tetrahydride? An Experimental and Computational Investigation of Pincer Iridium Polyhydrides

The iridium pincer complexes (PCP)IrH4 (1; PCP = [κ3-1,3-(CH2PtBu2)2C6H3]) and (POCOP)IrH4 (2; POCOP = [κ3-1,3-(OPtBu2)2C6H3]) have proven to be effective catalyst precursors for dehydrogenation of alkanes. The complex (POCOP)IrH2 has also been applied successfully as a catalyst for release of H2 from ammonia borane. Investigation of the “tetrahydride” forms of these complexes by solution NMR methods suggests their formulation as dihydrogen/dihydride species. This is in contrast to the solid state structure of 1, determined by neutron diffraction (at 100 K), which indicates a compressed tetrahydride structure with only weak H−H interactions. Complex 1 (C24H47IrP2) crystallizes in the space group P42, tetragonal, (Z = 2) with a = 11.7006 (19) Å, c = 9.7008(27) Å, and V = 1328.1(5) Å3. Electronic structure calculations on 1 and 2 indicate that the global minima on the potential energy surfaces in the gas phase are tetrahydride structures; however, the dihydrogen/dihydride forms are only slightly higher in energy (1−3 kcal/mol). A dihydrogen/dihydride species is calculated to be the global minimum for 2 when in solution. The barriers to interconversion between the tetrahydride and dihydrogen/dihydride species are almost negligible.