Synthesis and Characterization of [Ir(1,5-Cyclooctadiene)(μ-H)]₄: A Tetrametallic Ir₄H₄-Core, Coordinatively Unsaturated Cluster

Abstract

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 LiBEt₃H <i>in the presence of excess 1,5-COD</i> 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, <i>D</i>_2<i>d</i> symmetry Ir₄ 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 Ir₄-core structure, results which provide both considerable confidence in the XAFS methodology and set the stage for future XAFS in applications employing this Ir₄H₄ 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