Heterolytic and homolytic activation of dihydrogen at an unusual iridium (II) sulfide.

Two pathways are recognized for the activation of dihydrogen by metal catalysts, homolytic and heterolytic. Homolytic activation involves oxidative addition to metals usually in low oxidation states. Heterolytic activation, which occurs with no formal change in oxidation state, is associated with metals in higher oxidation states and gives rise to hydridic (H-, usually attached to a metal) and protic (H+) centers.1 Heterolytic activation is especially important for metal sulfides, being central to three topical catalytic processes, nitrogen fixation,2 hydrogen metabolism,3 and fossil fuel desulfurization (HDS).4 Soluble metal sulfido complexes are known to add H2, leading to the formation of SH ligands.5,6 In this report we describe the preparation of a very unusual metal sulfide that is of further interest because it reacts with 2 equiv of dihydrogen via both homolytic and heterolytic pathways.