Iridium Ziegler-Type Hydrogenation Catalysts Made from [(1,5-COD)Ir(μ-O₂C₈H₁₅)]₂ and AlEt₃: Spectroscopic and Kinetic Evidence for the Ir_n Species Present and for Nanoparticles as the Fastest Catalyst

Abstract

Ziegler-type hydrogenation catalysts, those made from a group 8-10 transition metal precatalyst and an AlR(3) cocatalyst, are often used for large scale industrial polymer hydrogenation; note that Ziegler-type hydrogenation catalysts are not the same as Ziegler-Natta polymerization catalysts. A review of prior studies of Ziegler-type hydrogenation catalysts (Alley et al. J. Mol. Catal. A: Chem. 2010, 315, 1-27) reveals that a approximately 50 year old problem is identifying the metal species present before, during, and after Ziegler-type hydrogenation catalysis, and which species are the kinetically best, fastest catalysts--that is, which species are the true hydrogenation catalysts. Also of significant interest is whether what we have termed "Ziegler nanoclusters" are present and what their relative catalytic activity is. Reported herein is the characterization of an Ir Ziegler-type hydrogenation catalyst, a valuable model (vide infra) for the Co-based industrial Ziegler-type hydrogenation catalyst, made from the crystallographically characterized [(1,5-COD)Ir(mu-O(2)C(8)H(15))](2) precatalyst plus AlEt(3). Characterization of this Ir model system is accomplished before and after catalysis using a battery of physical methods including Z-contrast scanning transmission electron microscopy (STEM), high resolution (HR)TEM, and X-ray absorption fine structure (XAFS) spectroscopy. Kinetic studies plus Hg(0) poisoning experiments are then employed to probe which species are the fastest catalysts. The main findings herein are that (i) a combination of the catalyst precursors [(1,5-COD)Ir(mu-O(2)C(8)H(15))](2) and AlEt(3) gives catalytically active solutions containing a broad distribution of Ir(n) species ranging from monometallic Ir complexes to nanometer scale, noncrystalline Ir(n) nanoclusters (up to Ir(approximately 100) by Z-contrast STEM) with the estimated mean Ir species being 0.5-0.7 nm, Ir(approximately 4-15) clusters considering the similar, but not identical results from the different analytical methods; furthermore, (ii) the mean Ir(n) species are practically the same regardless of the Al/Ir ratio employed, suggesting that the observed changes in catalytic activity at different Al/Ir ratios are primarily the result of changes in the form or function of the Al-derived component (and not due to significant AlEt(3)-induced changes in initial Ir(n) nuclearity). However (iii), during hydrogenation, a shift in the population of Ir species toward roughly 1.0-1.6 nm, fcc Ir(0)(approximately 40-150), Ziegler nanoclusters occurs with, significantly, (iv) a concomitant increase in catalytic activity. Importantly, and although catalysis by discrete subnanometer Ir species is not ruled out by this study, (v) the increases in activity with increased nanocluster size, plus Hg(0) poisoning studies, provide the best evidence to date that the approximately 1.0-1.6 nm, fcc Ir(0)(approximately 40-150), heterogeneous Ziegler nanoclusters are the fastest catalysts in this industrially related catalytic hydrogenation system (and ...

Fluxional behaviour of phosphole and phosphine ligands on triosmium clusters

“…In view of the importance of triosmium and triruthenium clusters, we have studied its reactivity with heterocyclic phosphines known as phospholes, such as 1-phenylphosphole, 3,4dimethyl-1-phenylphosphole and 3-methyl-1-phenylphosphole [18][19][20]. Herein, we report the reaction between the unsaturated cluster [Os 3 (µ-H) 2 In general, we have found that the binding mode of the -conjugated phospholes in the unsaturated [Os 3 (µ-H) 2 (CO) 10 ] cluster is restricted to the two-electron typical coordination of a tertiary phosphine, where electronic properties and steric effects of these phospholes must be playing a role in controlling the final product distribution. whereas the minor isomer contains two equivalent phosphines ( Figure 3).…”

“…Studies of electronic and coordinative deficiency in mononuclear complexes suggest that unsaturated transition metal clusters should exhibit unique patterns of chemical behavior with potential applications in catalysis [1][2][3][4][5]. The availability of multiple metallic binding sites at the heart of the cluster-surface can be an important factor influencing catalytic activity, occupying a position between traditional heterogeneous and homogeneous systems [6].…”

High variety of coordination modes of π-conjugated phospholes in dinuclear rhenium carbonyls. Fluxional behavior of σ,π-complexes

“…Carbonyl complexes that incorporate two or more metal centers are particularly attractive because of their ability to provide access to catalytic reaction pathways that might not be offered in mononuclear chemistry [25][26][27][28][29][30], such as reversible metal-metal bond cleavage, skeletal rearrangement without degradation and ligand activation via multisite coordination [31][32][33][34][35][36][37].…”

Monodisperse Iridium Clusters Protected by Phenylacetylene: Implication for Size-Dependent Evolution of Binding Sites

“…In this study, the synthesis and characterization of iridium clusters protected by a typical alkyne, phenylacetylene (PA), is reported. Recently, nanoparticles and clusters of Ir have attracted significant attention as active, selective catalysts for various reactions, e.g., CO 2 fixation, hydrogenation, and aerobic oxidation. − In these studies, Ir nanoparticles and clusters were synthesized using ligands (thiols, phosphines), polymers (polyvinylpyrrolidone (PVP), dendrimers), and surfactants (ethylene glycol (EG)) as stabilizers or zeolites as support. Examples of atomically precise synthesis are limited to Ir 4 , Ir 6 , and Ir 9 . − Motivated by the successful synthesis of Au analogues, e.g., Au 34 (PA) 16 and Au 54 (PA) 26 , − PA-protected Ir clusters (Ir:PA) were synthesized via the ligation of PA to the preformed Ir clusters stabilized by EG.…”

“…(ethylene glycol (EG)) as stabilizers or zeolites as support. Examples of atomically precise synthesis are limited to Ir 4 , Ir 6 , and Ir 9 .23−25 Motivated by the successful synthesis of Au analogues, e.g., Au 34 (PA)16 and Au 54 (PA) 26 , 1−3 PA-protected Ir clusters (Ir:PA) were synthesized via the ligation of PA to the preformed Ir clusters stabilized by EG. The structures of Ir:PA clusters thus prepared, including core size, atom packing structure, and interfacial structure, were investigated by means of a wide variety of spectroscopic methods.…”