Synthesis and characterization of [Ir(1,5-cyclooctadiene)(μ-H)] ₄: A tetrametallic Ir ₄H ₄-core, coordinatively unsaturated cluster

Reported herein is the synthesis of the previously unknown [Ir(1,5-COD)(μ-H)] ₄ (where 1,5-COD = 1,5-cyclooctadiene), from commercially available [Ir(1,5-COD)Cl] ₂ and LiBEt ₃H in the presence of excess 1,5-COD in 78% initial, and 55% recrystallized, yield plus its unequivocal characterization via single-crystal X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy, electrospray/atmospheric pressure chemical ionization mass spectrometry (ESI-MS), and UV-vis, IR, and nuclear magnetic resonance (NMR) spectroscopies. The resultant product parallels-but the successful synthesis is different from, vide infra-that of the known and valuable Rh congener precatalyst and synthon, [Rh(1,5-COD)(μ-H)] ₄. Extensive characterization reveals that a black crystal of [Ir(1,5-COD)(μ-H)] ₄ is composed of a distorted tetrahedral, D _2d symmetry Ir ₄ core with two long [2.90728(17) and 2.91138(17) Å] and four short Ir-Ir [2.78680 (12)-2.78798(12) Å] bond distances. One 1,5-COD and two edge-bridging hydrides are bound to each Ir atom; the Ir-H-Ir span the shorter Ir-Ir bond distances. XAFS provides excellent agreement with the XRD-obtained Ir ₄-core structure, results which provide both considerable confidence in the XAFS methodology and set the stage for future XAFS in applications employing this Ir ₄H ₄ and related tetranuclear clusters. The [Ir(1,5-COD)(μ-H)] ₄ complex is of interest for at least five reasons, as detailed in the Conclusions section.