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We compare three methods for quantitatively distinguishing the location of noble metal (NM) particles in mesopores from those found on the external support surface. MCM-41 and SBA-15 with NM located in mesopores or on the external surface were prepared and characterized by TEM. 31 P MAS NMR spectroscopy was used to quantify arylphosphines in complexes with NM. Phosphine/NM ratios drop from 2.0 to 0.2 when increasing the probe diameter from 1.08 to 1.54 nm. The reaction between NM and triphenylphosphine (TPP) within 3.0 nm MCM-41 pores takes due to confinement effects multiple weeks. In contrast, external NM react with TPP instantly. A promising method is filling the pores by using the pore volume impregnation technique with tetraethylorthosilicate (TEOS). TPP loading revealed that 66 % of NMs are located on the external surface of MCM-41. The pore filling method can be used in association with any probe molecule, also for the quantification of acid sites.

Organometallics 40(11), 1751 - 1757 (2021). doi:10.1021/acs.organomet.1c00216, Platinum hydride species catalyze a number of interesting organic reactions. However, their reactions typically involve the use of high loadings of noble metal and are difficult to recycle, making them somewhat unsustainable. We have synthesized surface-immobilized Pt–H species via oxidative addition of surface OH groups to Pt(P$^t$Bu$_3$)$_2$ (1), a rarely used immobilization technique in surface organometallic chemistry. The hydride species thus made were characterized by infrared, magic-angle spinning nuclear magnetic resonance, and X-ray absorption spectroscopies and catalyzed both olefin isomerization and cycloisomerization of a 1,6 enyne (5) with a high selectivity and low Pt loading., Published by ACS Publ., Washington, DC

Hydrogen spillover is mechanistically poorly understood, due to the H˙ donor complexity. We show that HV(CO)4dppe (1) transfers H˙ to MoO3 in separate H+ and e− transfer steps. This is analogous to hydrogen spillover from Pt to MoO3 and suggests that 1 is a good model for H˙ adsorbed on Pt.

The influence of nuclearity and charge of chiral Rh diene complexes on the activity and enantioselectivity in catalytic asymmetric 1,2-additions of organoboron reagents to N-tosylimines and 1,4-add...

We compare three methods for quantitatively distinguishing the location of noble metal (NM) particles in mesopores from those found on the external support surface. MCM-41 and SBA-15 with NM located in mesopores or on the external surface were prepared and characterized by TEM.

We synthesized isolated Cr(III) sites on SiO2-Al2O3 and Al2O3 by grafting and subsequent controlled thermal treatment of Cr(OSi(OtBu)3)3(THF)2 and Cr(Al(OiPr)4)3 on alumina. CrOx/Al2O3 was obtained from incipient wetness impregnation of Al2O3 with CrO3 in H2O followed by calcination as carried out for the synthesis of industrial Cr based dehydrogenation catalysts. These materials were characterized by IR EPR XAS and by the adsorption of the probe molecules CO and pyridine and the results were compared to previously reported isolated Cr(III)/SiO2. All of these materials were active in propane dehydrogenation at 550 °C where higher TOFs were obtained for Cr(III)/SiO2 Al2O3 and Cr(III)/Al2O3 than for CrOx/Al2O3 and Cr(III)/SiO2. For mechanistic studies the reverse reaction propene hydrogenation was studied. Here the order of reactivity was opposite that of dehydrogenation with CrOx/Al2O3 and Cr(III)/SiO2 being more active in hydrogenation than Cr(III)/SiO2 Al2O3 and Cr(III)/Al2O3. Kinetic analysis and labeling studies with D2 showed that the rate law is in all cases first order in H2 partial pressure but had different dependence on propene partial pressure from catalyst to catalyst. We found small normal kinetic isotope effects of 1 = KIE = 2 activation enthalpies up to 40 kJ mol–1 and large negative activation entropies between -100 and -194 J K–1 mol–1 for the different Cr catalysts. We also performed parahydrogen induced polarization (PHIP) experiments which showed that H2 addition to propene proceeds at least in part via a pairwise mechanism. The experimental data for propene hydrogenation suggests adsorption/desorption pre equilibria of H2 (or D2) and propene followed by H2 activation and insertion of propene. DFT computations for propane dehydrogenation and propene hydrogenation on Cr(III) on a periodic alumina model show that a mechanism involving X–H activation (X = R H) is energetically feasible with activation enthalpies and entropies that are comparable to the experimentally determined values.

Improvement of the activity, stability, and chemoselectivity of alkyne-metathesis catalysts is necessary before this promising methodology can become a routine method to construct C≡C triple bonds. Herein, we show that grafting of the known molecular catalyst [MesC≡Mo(OtBuF6)3] (1, Mes=2,4,6-trimethylphenyl, OtBuF6=hexafluoro-tert-butoxy) onto partially dehydroxylated silica gave a well-defined silica-supported active alkyne-metathesis catalyst [(≡SiO)Mo(≡CMes)(OtBuF6)2] (1/SiO2-700). Both 1 and 1/SiO2-700 showed very high activity, selectivity, and stability in the self-metathesis of a variety of carefully purified alkynes, even at parts-per-million catalyst loadings. Remarkably, the lower turnover frequencies observed for 1/SiO2-700 by comparison to 1 do not prevent the achievement of high turnover numbers. We attribute the lower reactivity of 1/SiO2-700 to the rigidity of the surface Mo species owing to the strong interaction of the metal site with the silica surface.

Different phosphines (PR3, R = phenyl, 4-methoxyphenyl) are demonstrated to be useful probe molecules for the spatial location and quantification of noble metal (NM) atoms within mesoporous and microporous support materials, such as silica A200, mesoporous SBA-15, and different zeolites Y. For this purpose, the high NMR sensitivity of 31P nuclei (spin I = ½, abundance of 100%) and the characteristic chemical shifts of phosphine complexes formed with noble metals were utilized. A straightforward method of physically mixing the reduced noble metal-containing catalysts with the solid phosphine powders and heating under well-defined conditions is shown. Complexation of triphenylphosphine (PPh3) and tris(4-methoxyphenyl)phosphine, P(PhOMe)3, with Pt, Rh, Pd, and Ru on open surfaces and in mesopores leads to characteristic 31P MAS NMR signals at δ = 33–41 ppm. Because of the different molecular diameters of PPh3 (0.72 nm) and P(PhOMe)3 (0.91 nm), adsorption of these two probe molecules allows to distinguish between noble metal atoms located at the outer surface of zeolite particles and those in secondary mesopores and in supercages of zeolites Y. For the first time this method enables the characterization of noble metals directly using probe molecules and 31P MAS NMR spectroscopy.

Journal of the American Chemical Society, 139 (48), ISSN:0002-7863, ISSN:1520-5126

Heterogeneous catalysts consisting of isolated transition-metal sites dispersed on the surface of metal oxide supports are commonly used in the chemical industry. Often their reactivity relies on the Lewis acidity of the active sites on the surface of the catalyst. A recent report from our group showed that silica-supported Co(II) sites, prepared via surface organometallic chemistry, are active in both alkene hydrogenation and alkane dehydrogenation, possibly linked to the Lewis acidity of the Co(II) sites. Here we use molecular probes and analogues to both qualitatively and quantitatively model the Lewis acidity of the surface sites. Some sites do not bind probe molecules like carbon monoxide, tetrahydrofuran, and olefins, while others exhibit a continuum of Lewis acidities. This is consistent with variations in the coordination environment of Co. These results suggest that only the most Lewis acidic sites are involved in dehydrogenation and hydrogenation, consistent with catalyst poisoning studies.

A cobalt hydride has been proposed as an intermediate in many reactions of the Co(dmgBF2)2L2 system, but its observation has proven difficult. We have observed the UV-vis spectra of Co(dmgBF2)2L2 (1) in CH3CN under hydrogen pressures of up to 70 atm. A Co(I) compound (6a) with an exchangeable proton is eventually formed. We have determined the bond dissociation free energy and pK(a) of the new O-H bond in 6a to be 50.5 kcal/mol and 13.4, respectively, in CH3CN, matching previous reports.

The activation and conversion of hydrocarbons is one of the most important challenges in chemistry. Transition-metal ions (V, Cr, Fe, Co, etc.) isolated on silica surfaces are known to catalyze such processes. The mechanisms of these processes are currently unknown but are thought to involve C-H activation as the rate-determining step. Here, we synthesize well-defined Co(II) ions on a silica surface using a metal siloxide precursor followed by thermal treatment under vacuum at 500 °C. We show that these isolated Co(II) sites are catalysts for a number of hydrocarbon conversion reactions, such as the dehydrogenation of propane, the hydrogenation of propene, and the trimerization of terminal alkynes. We then investigate the mechanisms of these processes using kinetics, kinetic isotope effects, isotopic labeling experiments, parahydrogen induced polarization (PHIP) NMR, and comparison with a molecular analog. The data are consistent with all of these reactions occurring by a common mechanism, involving heterolytic C-H or H-H activation via a 1,2 addition across a Co-O bond.

Surface hydrides are ubiquitous in catalysis. However, their structures and properties are not as well-understood as those of their molecular counterparts, which have been extensively studied for the past 70 years. Hydrides isolated on surfaces have been characterized as stable entities on oxide surfaces or in zeolites. They have also been proposed as reaction intermediates in numerous catalytic processes (hydrogenation, hydrogenolysis, etc.). They have also been prepared via surface organometallic chemistry. In this review, we describe their key structural features and spectroscopic signatures. We discuss their reactivity and stability and also point out unexplored areas.

Transition-metal hydride radical cations (TMHRCs) are involved in a variety of chemical and biochemical reactions, making a more thorough understanding of their properties essential for explaining observed reactivity and for the eventual development of new applications. Generally, these species may be treated as the ones formed by one-electron oxidation of diamagnetic analogues that are neutral or cationic. Despite the importance of TMHRCs, the generally sensitive nature of these complexes has hindered their development. However, over the last four decades, many more TMHRCs have been synthesized, characterized, isolated, or hypothesized as reaction intermediates. This comprehensive review focuses on experimental studies of TMHRCs reported through the year 2014, with an emphasis on isolated and observed species. The methods used for the generation or synthesis of TMHRCs are surveyed, followed by a discussion about the stability of these complexes. The fundamental properties of TMHRCs, especially those pert...

Surface hydrides are ubiquitous in catalysis. However, their structures and properties are not as well-understood as those of their molecular counterparts, which have been extensively studied for the past 70 years. Hydrides isolated on surfaces have been characterized as stable entities on oxide surfaces or in zeolites. They have also been proposed as reaction intermediates in numerous catalytic processes (hydrogenation, hydrogenolysis, etc.). They have also been prepared via surface organometallic chemistry. In this review, we describe their key structural features and spectroscopic signatures. We discuss their reactivity and stability and also point out unexplored areas.

Gassy pyrotechnic delays composed of tungsten, antimony(III) oxide, potassium periodate, and various lubri- cants have been developed for use in the US Army hand- held signal. The new compositions were developed to re- place the current formulation, which contains potassium perchlorate and barium chromate, chemicals that are facing increasing scrutiny due to environmental regulation. The hand-held signal delay housing was used to demon- strate the burning rate tunability of the new compositions. The addition of 1-5 % of waxy lubricants (stearic acid or cal- cium stearate) was found to have a profound effect on burning rate. The effect of tungsten content and the Sb2O3/ KIO4 ratio on burning rate was also probed. A wide range of inverse burning rates (2 to 15 s cm 1 ) were demonstrat- ed, which encompasses the 7 to 8.5 s cm 1 range required by the hand-held signal. The W/KIO4 reaction produces I2, which was observed by visible spectroscopy in the vapor above a sample of combustion residue.

Several different pKa values for the complex (η5-C5H5)Fe(CO)2H (FpH) in acetonitrile are present in the literature, ranging over 7 orders of magnitude. As a result, the energy of its Fe–H bond is also in dispute, making it difficult to predict the reactivity of this complex. The pKa, bond dissociation enthalpy, bond dissociation free energy, and hydricity of FpH have been measured. The pKa values of analogous group 8 hydrides have been determined, and the energies of the M–H bonds and the hydricity of FpH have been established.

The elicitation of hydrolytic catalysts from a dynamic library of imine-zinc(II) complexes (and their precursor aldehydes and amines) via templating with pro-transition state analogs (pro-TSA) is described. pro-TSA (2-pyridyl)phosphonate 2 amplifies a benzimidazole-derived complex at the expense of an imidazole analogue; the amplified complex is also more active for the hydrolysis of the pyridyl ester 1a. Employing pro-TSA pyridyl hydrate 3 with libraries of Zn complexes of imines having nucleophilic side chains also perturbs the library in favor of imine-Zn complexes which prove to be the more active hydrolytic agents and catalysts. The catalytic hydrolyses exhibit both saturation kinetics and inhibition by the pro-TSA. This process for catalyst evolution is operationally simple, amenable to high throughput screening, potentially applicable to a wide variety of catalytic reactions and thus could offer a practical alternative to catalytic antibodies and imprinted polymers.

Transition-metal hydride radical cations (TMHRCs) are involved in a variety of chemical and biochemical reactions, making a more thorough understanding of their properties essential for explaining observed reactivity and for the eventual development of new applications. Generally, these species may be treated as the ones formed by one-electron oxidation of diamagnetic analogues that are neutral or cationic. Despite the importance of TMHRCs, the generally sensitive nature of these complexes has hindered their development. However, over the last four decades, many more TMHRCs have been synthesized, characterized, isolated, or hypothesized as reaction intermediates. This comprehensive review focuses on experimental studies of TMHRCs reported through the year 2014, with an emphasis on isolated and observed species. The methods used for the generation or synthesis of TMHRCs are surveyed, followed by a discussion about the stability of these complexes. The fundamental properties of TMHRCs, especially those pertaining to the M-H bond, are described, followed by a detailed treatment of decomposition pathways. Finally, reactions involving TMHRCs as intermediates are described.

We have investigated the effect of axial ligands on the ability of cobaloximes to catalyze the generation of transferable hydrogen atoms from hydrogen gas and have learned that the active catalyst contains one and only one axial ligand. We have, for example, shown that Co(dmgBF2)2 coordinates only one Ph3P and that the addition of additional Ph3P (beyond 1 equiv) to solvated Co(dmgBF2)2 does not affect its catalytic turnover for H• transfer from H2.

The reaction of CpCr(CO)(3)H with activated alkynes in benzene has been examined. The kinetics of these reactions have been studied with various alkynes, along with the stereochemistry with which the alkynes are hydrogenated. The hydrogenation of phenyl acetylene and diphenyl acetylene with CpCr(CO)(3)H has been shown to occur by a hydrogen atom transfer (HAT) mechanism. The reaction of CpCr(CO)(3)H with dimethyl acetylenedicarboxylate (DMAD) produced hydrogenated products as well as phenyl substitution from reaction with solvent. On the basis of kinetic data, it is thought that the reaction of DMAD may proceed via a single electron transfer (SET) as the rate-determining step. The radical anion of dimethylfumarate was observed by EPR spectroscopy during the course of the reaction, supporting this claim. The aromatic 1,6 eneyne (8) gave cyclized products in 78% yield under catalytic conditions (35 psi H(2)), presumably by the 5-exo-trig cyclization of the vinyl radical arising from H• transfer. Using a cobaloxime catalyst (12) hydrogenation was completely eliminated to yield 100% cyclized products.

Under H(2), the radical cyclization of appropriate dienes can be catalyzed by cobaloximes. H• can be abstracted from an intermediate (presumably a cobalt hydride) by trityl radicals (Ar(3)C•) or by TEMPO. The rate-determining step in these reactions is the uptake of H(2), which is second order in cobalt and first order in hydrogen; the third-order rate constant is 106(3) M(-2)·s(-1).

During many of its applications (especially as a catalyst support material), MoO3 acts as a medium for hydrogen storage via hydrogen spillover (H atom donation from proton and electron sources to a support), for which the energetics of H atoms on MoO3 are of importance. Despite the seeming simplicity of hydrogen spillover, previously reported ab initio results for the H adsorption on MoO3 contradict both experimental work and other ab initio results. In the present study, we resolve these discrepancies and provide a comprehensive ab initio understanding of H adsorption for MoO3 in the bulk and on the surface. To this end, we systematically investigate various exchange-correlation functionals and various H concentrations, and we carefully track the various relevant H positions. For a dilute H concentration, the asymmetric oxygen site (Oa) is found to be energetically the most favorable. With increasing H content, the difference of the H adsorption energies between the terminal (Ot) and the Oa sites becomes smaller. Previous contradictions are ascribed mostly to the disregard of the H position along the Oa-Oa zig-zag chains in the intrabilayer region. Using the modern non-empirical strongly-constrained and appropriately-normed (SCAN) meta-generalized gradient approximation (GGA), the dilute-limit H adsorption energies are obtained as -2.89 eV/(H atom) and -2.97 eV/(H atom) in the bulk and on the surface, respectively, and the activation energy of H diffusion between the Oa sites as 0.11-0.15 eV/(H atom), consistent with previous experiments.