Dynamic Behavior In Polyhydrogen Iridium Complexes

Abstract

In 1984, Kubas and co-workers determined the molecular structure of W(η2- H2)(L)3(PR3)2 by a single crystal neutron diffraction study. This seminal study established that molecular hydrogen could bind to a metal center in a “side-on” fashion without cleavage of the H-H bond forming the classical dihydride, M-(H)2. The terms “Kubas compound” and “Kubas-interaction” are now familiar to all scientists working in the areas of metal hydrides and physi-sorbed hydrogen. The bonding in a Kubas complex is described as involving a donation of electrons from the H2 σ orbital to a linear empty metal valence orbital and a back-bonding donation from a filled metal dπ orbital to the empty σ* orbital of the H2. In the Kubas interaction, the H2 ligand is primarily considered a σ base and a σ* acid. Alternatively, the bonding of an H2 ligand to a metal center could involve an “end-on” interaction, a 3-center 4-electron bond rather than a non-classical 3- center 2-electron bond (Kubas interaction). In the end-on bonding model, the H2 ligand is primarily considered a σ* acid. We recently determined the molecular structure of IrHx{2,6-C6H3-(OAs(tBu2))2} through a single crystal X-ray diffraction study. Surprisingly, upon refinement of both occupancy and position, the H2 ligand was found to be oriented in an “end-on” rather than “side-on” coordination motif. This defense will discuss the details of the M-(η1-H2) bonding mode and alternative characterization methods, most notably NMR spectroscopy. Through the studies of IrHx{2,6-C6H3-(OAs(tBu2))2}, and the possible discrete M-(η1-H2) coordination, an earlier project of the Jensen lab was revisited. In order to gain insight into the dynamic polyhydrogen bonding of M-(H)2 and M-(η2-H2) in an alternate iridium system, neutron diffraction data of IrBrHx(iPr3P)2·C10H8 and IrClHx(iPr3P)2 have been obtained and will be discussed. Surprisingly, upon refinement of IrClHx(iPr3P)2, the H2 ligand was found in an unusual eclipsed conformation of one of the complexes in the crystal structure.