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

Abstract The cyclopentadienyliridium hydrides Cp 2 Ir 2 H n (Cp = η 5 -C 5 H 5 ; n  = 6, 4, 2, 0), and CpIrH n ( n  = 4, 2) related to the experimentally known pentamethyl cyclopentadienyliridium hydrides Cp ∗ 2 Ir 2 H n (Cp ∗  = η 5 -Me 5 C 5 ; n  = 6, 2) and Cp ∗ IrH 4 have been investigated by density functional theory. The lowest energy Cp 2 Ir 2 H n ( 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 Cp 2 Ir 2 H 6 , such doubly bridged Cp 2 Ir 2 (μ-H) 2 H 4 structures are bent, leading to trans and cis structures of similar energies with an unbridged Cp 2 Ir 2 H 6 isomer lying only ∼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 Ir 2 H 6 . The tetrahydride Cp 2 Ir 2 H 4 is predicted to undergo slightly exothermic disproportionation into Cp 2 Ir 2 H 6  + Cp 2 Ir 2 H 2 and thus not be a viable species. This is consistent with the failure to find any Cp ∗ 2 Ir 2 H 4 in the Cp ∗ 2 Ir 2 H n systems. The doubly bridged dihydride Cp 2 Ir 2 (μ-H) 2 is a particularly favorable species since it lies more than 18 kcal/mol in energy below any other isomer. Higher energy Cp 2 Ir 2 H n ( 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 Cp 2 Ir 2 . A higher energy Cp 2 Ir 2 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 σ  + 2π bond with no δ components and thus differs from the σ  + 2π +  δ quadruple bond found in the long-known Re 2 Cl 8 2− .