Dihydrogen/Dihydride Or Tetrahydride? An Experimental And Computational Investigation Of Pincer Iridium Polyhydrides

The iridium pincer complexes (PCP)IrH4 (1; PCP = [kappa(3-)1,3-((CH2PBu2)-Bu-t)(2)C6H3]) and (POCOP)IrH4 (2; POCOP = [kappa(3)-1,3-((OPBu2)-Bu-t)(2)C6H3]) 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 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 (C24H47IrP2) crystallizes in the space group P4(2), tetragonal, (Z=2) with a = 11.7006 (19) angstrom, c = 9.7008(27) angstrom, and V=1328.1(5) angstrom(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.