Binuclear cyclopentadienyliridium hydride chemistry: Terminal versus bridging hydride and cyclopentadienyl ligands

Abstract

The cyclopentadienyliridium hydrides Cp2Ir2Hn (Cp=η5-C5H5; n=6, 4, 2, 0), and CpIrHn (n=4, 2) related to the experimentally known pentamethyl cyclopentadienyliridium hydrides Cp∗2Ir2Hn (Cp∗=η5-Me5C5; n=6, 2) and Cp∗IrH4 have been investigated by density functional theory. The lowest energy Cp2Ir2Hn (n=6, 4, 2) structures are predicted to have terminal Cp rings with the central Ir2 unit bridged by two hydrogen atoms. For the hexahydride Cp2Ir2H6, such doubly bridged Cp2Ir2(μ-H)2H4 structures are bent, leading to trans and cis structures of similar energies with an unbridged Cp2Ir2H6 isomer lying only ∼4kcal/mol above the bridged structures. This suggests fluxional behavior consistent with experimental data on the temperature dependence of the proton NMR spectrum of the closely related Cp∗2Ir2H6. The tetrahydride Cp2Ir2H4 is predicted to undergo slightly exothermic disproportionation into Cp2Ir2H6+Cp2Ir2H2 and thus not be a viable species. This is consistent with the failure to find any Cp∗2Ir2H4 in the Cp∗2Ir2Hn systems. The doubly bridged dihydride Cp2Ir2(μ-H)2 is a particularly favorable species since it lies more than 18kcal/mol in energy below any other isomer. Higher energy Cp2Ir2Hn (n=4, 2) structures have one or two bridging Cp rings and exclusively terminal hydrogen atoms. Related structures are the lowest energy structures for the hydride-free Cp2Ir2. A higher energy Cp2Ir2 structure consists of two CpIr units linked solely by an Ir–Ir bond. Analysis of the frontier molecular orbitals indicates this Ir–Ir bond to be the quadruple bond required to give each iridium atom the favored 18-electron configuration. However, this quadruple bond is a 2σ+2π bond with no δ components and thus differs from the σ+2π+δ quadruple bond found in the long-known Re2Cl82−.