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

The cyclopentadienyliridium hydrides Cp2Ir2Hn (Cp = eta(5)-C5H5; n = 6, 4, 2, 0), and CpIrHn (n = 4, 2) related to the experimentally known pentamethyl cyclopentadienyliridium hydrides Cp-2*Ir2Hn (Cp* = eta(5)-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 Ir-2 unit bridged by two hydrogen atoms. For the hexahydride Cp2Ir2H6, such doubly bridged Cp2Ir2(mu-H)(2)H-4 structures are bent, leading to trans and cis structures of similar energies with an unbridged Cp2Ir2H6 isomer lying only similar to 4 kcal/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-2*Ir2H6. 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-2*Ir2H4 in the Cp-2*Ir2Hn systems. The doubly bridged dihydride Cp2Ir2(mu-H)(2) is a particularly favorable species since it lies more than 18 kcal/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 2 sigma + 2 pi bond with no delta components and thus differs from the sigma + 2 pi + delta quadruple bond found in the long-known Re2Cl82. (C) 2015 Elsevier B.V. All rights reserved.