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

Unintuitive Inverse Dependence of the Apparent Turnover Frequency on Precatalyst Concentration: A Quantitative Explanation in the Case of Ziegler-Type Nanoparticle Catalysts Made from [(1,5-COD)Ir(μ‑O₂C₈H₁₅)]₂ and AlEt₃

The Ziegler-type hydrogenation precatalyst dimer, [(1,5-COD)­Ir­(μ-O₂C₈H₁₅)]₂ (1,5-COD = 1,5-cyclooctadiene; O₂C₈H₁₅ = 2-ethylhexanoate) plus added AlEt₃ stabilizer has recently been shown to form AlEt₃-stabilized, Ziegler-type Ir(0)_∼4–15 nanoparticles initially, which then grow to larger Ziegler-type Ir(0)_∼40–50 nanoparticles during the catalytic hydrogenation of cyclohexene (Alley, W. M.; Hamdemir, I. K.; Wang, Q.; Frenkel, A. I.; Li, L.; Yang, J. C.; Menard, L. D.; Nuzzo, R. G.; Özkar, S.; Johnson, K. A.; Finke, R. G. <i>Inorg. Chem.</i> <b>2010</b>, 49, 8131–8147). An interesting observation for this Ziegler-type nanoparticle catalyst system is that the apparent TOF (TOF_app = <i>k</i>_obs/[Ir]) for cyclohexene hydrogenation <i>increases</i> with <i>decreasing</i> concentration of the precatalyst, [Ir] (defined as 2­[{(1,5-COD)­Ir­(μ-O₂C₈H₁₅)}₂], that is, twice the starting precatalyst concentration since that dimer contains 2 Ir). A perusal of the literature reveals that such an intuitively backward, inverse relationship between the apparent turnover frequency, TOF_app, and the concentration of precatalyst or catalyst has been seen at least eight times before in other, disparate systems in the literature. However, this effect has previously never been satisfactorily explained, nor have the mixed, sometimes opposite, explanations offered in the literature been previously tested by the disproof of all reasonable alternative explanations/mechanistic hypotheses. Herein, five alternative mechanistic explanations have been tested via kinetic studies, Z-contrast STEM microscopy of the nanoparticle product sizes, and other evidence. Four of the five possible explanations have been ruled out en route to the finding that the only mechanism of the five able to explain all the evidence, as well as to quantitatively curve-fit the inverse TOF_app vs [Ir] data, is a prior, dissociative equilibrium, in which <i>x</i> ≈ 3 equiv of the surface-bound, AlR₃-based nanocluster stabilizer is dissociated, Ir(0)_<i>n</i>·[AlEt₃]_<i>m</i> ⇄ <i>x</i>AlEt₃ + Ir(0)_<i>n</i>·[AlEt₃]_{<i>m</i>−<i>x</i>}, with the resulting, more coordinatively unsaturated Ir(0)_<i>n</i>·[AlEt₃]_{<i>m</i>−<i>x</i>} being the faster, kinetically dominant catalyst. The implication is that such unusual, inverse TOF_app vs [precatalyst or catalyst] concentration observations in the literature are, more generally, likely just unintentional, unwitting measurements of a component of the rate law for such systems. The results herein are significant (i) in providing the first quantitative, disproof-tested explanation for the inverse TOF_app vs [precatalyst or catalyst] observation; (ii) in providing precedent and, therefore, a plausible explanation for the eight prior examples of this phenomenon in the literature; and (iii) in demonstrating for one of those additional eight literature cases, a commercial cobalt-based polymer hydrogenation catalyst, that the prior dissociative equilibrium uncovered herein can also quantitatively fit the inverse TOF_app vs [precatalyst] data for that case, as well. The results herein are additionally significant (iv) in making apparent that the rigorous interpretation of any TOF requires that the rate law for the processes under study be known, a point that bears heavily on the confusion and current controversy in the literature over the proper use of the “TOF” concept; (v) in making apparent the usefulness and value of the TOF_app concept employed herein; and (vi) in uncovering the insight that the true, most active catalyst present in AlEt₃-stabilized, Ziegler-type Ir(0)_<i>n</i> nanoparticle catalysts is the more coordinatively unsaturated Ziegler-type Ir(0)_<i>n</i>·[AlEt₃]_{<i>m</i>−<i>x</i>} nanoparticle formed from the dissociative loss of ∼3 AlEt₃

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