Synthesis and Characterization
of [Ir(1,5-Cyclooctadiene)(μ-H)]<sub>4</sub>: A Tetrametallic
Ir<sub>4</sub>H<sub>4</sub>-Core, Coordinatively
Unsaturated Cluster
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Abstract
Reported herein is the synthesis of the previously unknown
[Ir(1,5-COD)(μ-H)]<sub>4</sub> (where 1,5-COD = 1,5-cyclooctadiene),
from commercially available
[Ir(1,5-COD)Cl]<sub>2</sub> and LiBEt<sub>3</sub>H <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 parallelsbut the successful
synthesis is different from, vide infrathat of the known and
valuable Rh congener precatalyst and synthon, [Rh(1,5-COD)(μ-H)]<sub>4</sub>. Extensive characterization reveals that a black crystal
of [Ir(1,5-COD)(μ-H)]<sub>4</sub> is composed of a distorted
tetrahedral, <i>D</i><sub>2<i>d</i></sub> symmetry
Ir<sub>4</sub> 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<sub>4</sub>-core structure, results which provide both considerable
confidence in the XAFS methodology and set the stage for future XAFS
in applications employing this Ir<sub>4</sub>H<sub>4</sub> and related
tetranuclear clusters. The [Ir(1,5-COD)(μ-H)]<sub>4</sub> complex
is of interest for at least five reasons, as detailed in the Conclusions
section