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

The iridium pincer complexes (PCP)IrH₄ (1; PCP = [ ³-1, 3-(CH₂P^tBu₂)₂C ₆H₃]) and (POCOP)IrH₄ (2; POCOP = [κ ³-1, 3-(OP^tBu₂)₂C₆H ₃]) 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 (C₂₄H ₄₇IrP₂) 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) Å 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.