Dihydrogen/dihydride or tetrahydride? An experimental and computational investigation of pincer iridium polyhydrides.

The iridium pincer complexes (PCP)IrH(4) (1; PCP = [kappa(3)-1,3-(CH(2)P(t)Bu(2))(2)C(6)H(3)]) and (POCOP)IrH(4) (2; POCOP = [kappa(3)-1,3-(OP(t)Bu(2))(2)C(6)H(3)]) have proven to be effective catalyst precursors for dehydrogenation of alkanes. The complex (POCOP)IrH(2) has also been applied successfully as a catalyst for release of H(2) 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 (C(24)H(47)IrP(2)) crystallizes in the space group P4(2), tetragonal, (Z = 2) with a = 11.7006 (19) A, c = 9.7008(27) A, and V = 1328.1(5) A(3). 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.