Determination of the Dominant Catalyst Derived from the Classic [RhCp*Cl₂]₂ Precatalyst System: Is it Single-Metal Rh₁Cp*-Based, Subnanometer Rh₄ Cluster-Based, or Rh(0)<i>_n</i> Nanoparticle-Based Cyclohexene Hydrogenation Catalysis at Room Temperature and Mild Pressures?

Determining the kinetically dominant catalyst in a given catalytic system is a forefront topic in catalysis. The [RhCp*Cl₂]₂ (Cp* = [η⁵-C₅(CH₃)₅]) system pioneered by Maitlis and co-workers is a classic precatalyst system from which homogeneous mononuclear Rh₁, subnanometer Rh₄ cluster, and heterogeneous polymetallic Rh(0)<i>_n</i> nanoparticle have all arisen as viable candidates for the true hydrogenation catalyst, depending on the precise substrate, H₂ pressure, temperature, and catalyst concentration conditions. Addressed herein is the question of whether the prior assignment of homogeneous, mononuclear Rh₁Cp*-based catalysis is correct, or are trace Rh₄ subnanometer clusters or possibly Rh(0)<i>_n</i> nanoparticles the dominant, actual cyclohexene hydrogenation catalyst at 22 °C and 2.7 atm initial H₂ pressure? The observation herein of Rh₄ species by in operando-X-ray absorption fine structure (XAFS) spectroscopy, at the only slightly more vigorous conditions of 26 °C and 8.3 atm H₂ pressure, and the confirmation of Rh₄ clusters by ex situ mass spectroscopy raises the question of the dominant, room temperature, and mild pressure cyclohexene hydrogenation catalyst derived from the classic [RhCp*Cl₂]₂ precatalyst pioneered by Maitlis and co-workers. Ten lines of evidence are provided herein to address the nature of the true room temperature and mild pressure cyclohexene hydrogenation catalyst derived from [RhCp*Cl₂]₂. Especially significant among those experiments are quantitative catalyst poisoning experiments, in the present case using 1,10-phenanthroline. Those poisoning studies allow one to distinguish mononuclear Rh₁, subnanometer Rh₄ cluster, and Rh(0)<i>_n</i> nanoparticle catalysis hypotheses. The evidence obtained provides a compelling case for a mononuclear, Rh₁Cp*-based cyclohexene hydrogenation catalyst at 22 °C and 2.7 atm H₂ pressure. The resultant methodology, especially the quantitative catalyst poisoning experiments in combination with in operando spectroscopy, is expected to be more broadly applicable to the study of other systems and the “what is the true catalyst?” question

Mononuclear Zeolite-Supported Iridium: Kinetic, Spectroscopic, Electron Microscopic, and Size-Selective Poisoning Evidence for an Atomically Dispersed True Catalyst at 22 °C

This work addresses the question of what is the true catalyst when beginning with a site-isolated, atomically dispersed precatalyst for the prototype catalytic reaction of cyclohexene hydrogenation in the presence of cyclohexane solvent: is the atomically dispersed nature of the zeolite-supported, [Ir­(C₂H₄)₂]/zeolite Y precatalyst retained, or are possible alternatives including Ir₄ subnanometer clusters or larger, Ir(0)_<i>n</i>, nanoparticles the actual catalyst? Herein we report the (a) kinetics of the reaction; (b) physical characterizations of the used catalyst, including extended X-ray absorption fine structure spectra plus images obtained by high-angle annular dark-field scanning transmission electron microscopy, demonstrating the mononuclearity and site-isolation of the catalyst; and the (c) results of poisoning experiments, including those with the size-selective poisons P­(C₆H₁₁)₃ and P­(OCH₃)₃ determining the location of the catalyst in the zeolite pores. Also reported are quantitative poisoning experiments showing that each added P­(OCH₃)₃ molecule poisons one catalytic site, confirming the single-metal-atom nature of the catalyst and the lack of leaching of catalyst into the reactant solution. The results (i) provide strong evidence that the use of a site-isolated [Ir­(C₂H₄)₂]/zeolite Y precatalyst allows a site-isolated [Ir₁]/zeolite Y hydrogenation catalyst to be retained even when in contact with solution, at least at 22 °C; (ii) allow a comparison of the solid–solution catalyst system with the equivalent one used in the solid–gas ethylene hydrogenation reaction at room temperature; and (iii) illustrate a methodology by which multiple, complementary physical methods, combined with kinetic, size-selective poisoning, and quantitative kinetic poisoning experiments, help to identify the catalyst. The results, to our knowledge, are the first identifying an atomically dispersed, supported transition-metal species as the catalyst of a reaction taking place in contact with solution

Agglomerative Sintering of an Atomically Dispersed Ir₁/Zeolite Y Catalyst: Compelling Evidence Against Ostwald Ripening but for Bimolecular and Autocatalytic Agglomeration Catalyst Sintering Steps

Agglomerative sintering of an atomically dispersed, zeolite Y-supported catalyst, Ir₁/zeolite Y, formed initially from the well-characterized precatalyst [Ir­(C₂H₄)₂]/zeolite Y and in the presence of liquid-phase reactants, was monitored over three cycles of 3800 turnovers (TTOs) of cyclohexene hydrogenation at 72 °C. The catalyst evolved and sintered during each cycle, even at the relatively mild temperature of 72 °C in the presence of the cyclohexene plus H₂ reactants and cyclohexane solvent. Post each of the three cycles of catalysis, the resultant sintered catalyst was characterized by extended X-ray absorption fine structure spectroscopy and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The results show that higher-nuclearity iridium species, Ir_<i>n</i>, are formed during each successive cycle. The progression from the starting mononuclear precursor, Ir₁, is first to Ir_∼4–6; then, on average, Ir_∼40; and finally, on average, Ir_∼70, the latter more accurately described as a bimodal dispersion of on-average Ir_∼40–50 and on-average Ir_∼1600 nanoparticles. The size distribution and other data disprove Ostwald ripening during the initial and final stages of the observed catalyst sintering. Instead, the diameter-dispersion data plus quantitative fits to the cluster or nanoparticle diameter vs time data provide compelling evidence for the underlying, pseudoelementary steps of bimolecular agglomeration, B + B → C, and autocatalytic agglomeration, B + C → 1.5C, where B represents the smaller, formally Ir(0) nanoparticles, and C is the larger (more highly agglomerated) nanoparticles (and where the 1.5 coefficient in the autocatalytic agglomeration of B + C necessarily follows from the definition, in the bimolecular agglomeration step, that 1C contains the Ir from 2B). These two specific, balanced chemical reactions are of considerable significance in going beyond the present state-of-the-art, but word-only, “mechanism”that is, actually and instead, just a collection of phenomenafor catalyst sintering of “Particle Migration and Coalescence”. The steps of bimolecular plus autocatalytic agglomeration provide two specific, balanced chemical equations useful for fitting sintering kinetics data, as is done herein, thereby quantitatively testing proposed sintering mechanisms. These two pseudoelementary reactions also define the specific words and concepts for sintering of bimolecular agglomeration and autocatalytic agglomeration. The results are also significant as the first quantitative investigation of the agglomeration and sintering of an initially atomically dispersed metal on a structurally well-defined (zeolite) support and in the presence of liquid reactants (cyclohexene substrate and cyclohexane solvent) plus H₂. A list of additional specific conclusions is provided in a summary 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

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