Hydrocarbon-Soluble, Isolable Ziegler-Type Ir(0)_<i>n</i> Nanoparticle Catalysts Made from [(1,5-COD)Ir(μ-O₂C₈H₁₅)]₂ and 2–5 Equivalents of AlEt₃: Their High Catalytic Activity, Long Lifetime, and AlEt₃-Dependent, Exceptional, 200 °C Thermal Stability

Hydrocarbon-solvent-soluble, isolable, Ziegler-type Ir(0)_<i>n</i> nanoparticle hydrogenation catalysts made from the crystallographically characterized [(1,5-COD)­Ir­(μ-O₂C₈H₁₅)]₂ precatalyst and 2–5 equiv of AlEt₃ (≥2 equiv of AlEt₃ being required for the best catalysis and stability, vide infra) are scrutinized for their catalytic properties of (1) their isolability and then redispersibility without visible formation of bulk metal; (2) their initial catalytic activity of the isolated nanoparticle catalyst redispersed in cyclohexane; (3) their catalytic lifetime in terms of total turnovers (TTOs) of cyclohexene hydrogenation; and then also and unusually (4) their relative thermal stability in hydrocarbon solution at 200 °C for 30 min. These studies are of interest since Ir(0)_<i>n</i> nanoparticles are the currently best-characterized example, and a model/analogue, of industrial Ziegler-type hydrogenation catalysts made, for example, from Co­(O₂CR)₂ and ≥2 equiv of AlEt₃. Eight important insights result from the present studies, the highlights of which are that Ir(0)_<i>n</i> Ziegler-type nanoparticles, made from [(1,5-COD)­Ir­(μ-O₂C₈H₁₅)]₂ and AlEt₃, are (i) quite catalytically active and long-lived; (ii) thermally unusually stable nanoparticle catalysts at 200 °C, vide infra, a stability which requires the addition of at least 3 equiv of AlEt₃ (Al/Ir = 3), but where (iii) the Al/Ir = 5 Ir(0)_<i>n</i> nanoparticles are even more stable, for ≥30 min at 200 °C, and exhibit 100 000 TTOs of cyclohexene hydrogenation. The results also reveal that (iv) the observed nanoparticle catalyst stability at 200 °C appears to surpass that of any other demonstrated nanoparticle catalyst in the literature, those reports being limited to ≤130–160 °C temperatures; and reveal that (v) AlEt₃, or possibly surface derivatives of AlEt₃, along with [RCO₂·AlEt₃]⁻ formed from the first equiv of AlEt₃ per 1/2 equiv of [(1,5-COD)­Ir­(μ-O₂C₈H₁₅)]₂ are main components of the nanoparticle stabilizer system, consistent with previous suggestions from Shmidt, Goulon, Bönnemann, and others. The results therefore also (vi) imply that either (a) a still poorly understood mode of nanoparticle stabilization by alkyl Lewis acids such as AlEt₃ is present or, (b) that reactions between the Ir(0)_<i>n</i> and AlEt₃ occur to give initially surface species such as (Ir_surface)_<i>x</i>–Et plus (Ir_surface)_<i>x</i>–Al­(Et)₂Ir, where the number of surface Ir atoms involved, <i>x</i> = 1–4; and (vii) confirm the literature’s suggestion that the activity of Ziegler-type hydrogenation can be tuned by the Al/Ir ratio. Finally and perhaps most importantly, the results herein along with recent literature make apparent (viii) that isolable, hydrocarbon soluble, Lewis-acid containing, Ziegler-type nanoparticles are an underexploited, still not well understood type of high catalytic activity, long lifetime, and unusually if not unprecedentedly high thermal stability nanoparticles for exploitation in catalysis or other applications where their unusual hydrocarbon solubility and thermal stability might be advantageous

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

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