Remotely Bonded Bridging Dioxygen Ligands Enhance Hydrogen Transfer in a Silica-Supported Tetrairidium Cluster Catalyst.

A longstanding challenge in catalysis by noble metals has been to understand the origin of enhancements of rates of hydrogen transfer that result from the bonding of oxygen near metal sites. We investigated structurally well-defined catalysts consisting of supported tetrairidium carbonyl clusters with single-atom (apical iridium) catalytic sites for ethylene hydrogenation. Reaction of the clusters with ethylene and H2 followed by O2 led to the onset of catalytic activity as a terminal CO ligand at each apical Ir atom was removed and bridging dioxygen ligands replaced CO ligands at neighboring (basal-plane) sites. The presence of the dioxygen ligands caused a 6-fold increase in the catalytic reaction rate, which is explained by the electron-withdrawing capability induced by the bridging dioxygen ligands, consistent with the inference that reductive elimination is rate-determining. Electronic-structure calculations demonstrate an additional role of the dioxygen ligands, changing the mechanism of hydrogen transfer from one involving equatorial hydride ligands to that involving bridging hydride ligands. This mechanism is made evident by an inverse kinetic isotope effect observed in ethylene hydrogenation reactions with H2 and, alternatively, with D2 on the cluster incorporating the dioxygen ligands and is a consequence of quasi-equilibrated hydrogen transfer in this catalyst. The same mechanism accounts for rate enhancements induced by the bridging dioxygen ligands for the catalytic reaction of H2 with D2 to give HD. We posit that the mechanism involving bridging hydride ligands facilitated by oxygen ligands remote from the catalytic site may have some generality in catalysis by oxide-supported noble metals.

Cs-RHO Goes from Worst to Best as Water Enhances Equilibrium CO2 Adsorption via Phase Change.

The strong affinity of water to zeolite adsorbents has made adsorption of CO2 from humid gas mixtures such as flue gas nearly impossible under equilibrated conditions. Here, in this manuscript, we describe a unique cooperative adsorption mechanism between H2O and Cs+ cations on Cs-RHO zeolite, which actually facilitates the equilibrium adsorption of CO2 under humid conditions. Our data demonstrate that, at a relative humidity of 5%, Cs-RHO adsorbs 3-fold higher amounts of CO2 relative to dry conditions, at a temperature of 30 °C and CO2 pressure of 1 bar. A comparative investigation of univalent cation-exchanged RHO zeolites with H+, Li+, Na+, K+, Rb+, and Cs+ shows an increase of equilibrium CO2 adsorption under humid versus dry conditions to be unique to Cs-RHO. In situ powder X-ray diffraction indicates the appearance of a new phase with Im3̅m symmetry after H2O saturation of Cs-RHO. A mixed-cation exchanged NaCs-RHO exhibits similar phase transitions after humid CO2 adsorption; however, we found no evidence of cooperativity between Cs+ and Na+ cations in adsorption, in single-component H2O and CO2 adsorption. We hypothesize based on previous Rietveld refinements of CO2 adsorption in Cs-RHO zeolite that the observed phase change is related to solvation of extra-framework Cs+ cations by H2O. In the case of Cs-RHO, molecular modeling results suggest that hydration of these cations favors their migration from an original D8R position to S8R sites. We posit that this movement enables a trapdoor mechanism by which CO2 can interact with Cs+ at S8R sites to access the α-cage.

A Stable Silanol Triad in the Zeolite Catalyst SSZ-70.

Nests of three silanol groups are located on the internal pore surface of calcined zeolite SSZ-70. 2D 1 H double/triple-quantum single-quantum correlation NMR experiments enable a rigorous identification of these silanol triad nests. They reveal a close proximity to the structure directing agent (SDA), that is, N,N'-diisobutyl imidazolium cations, in the as-synthesized material, in which the defects are negatively charged (silanol dyad plus one charged SiO- siloxy group) for charge balance. It is inferred that ring strain prevents the condensation of silanol groups upon calcination and removal of the SDA to avoid energetically unfavorable three-rings. In contrast, tetrad nests, created by boron extraction from B-SSZ-70 at various other locations, are not stable and silanol condensation occurs. Infrared spectroscopic investigations of adsorbed pyridine indicate an enhanced acidity of the silanol triads, suggesting important implications in catalysis.

Bulky Calixarene Ligands Stabilize Supported Iridium Pair-Site Catalysts.

Although essentially molecular noble metal species provide active sites and highly tunable platforms for the design of supported catalysts, the susceptibility of the metals to reduction and aggregation and the consequent loss of catalytic activity and selectivity limit opportunities for their application. Here, we demonstrate a new construct to stabilize supported molecular noble-metal catalysts, taking advantage of sterically bulky ligands on the metal that serve as surrogate supports and isolate the active sites under conditions involving steady-state catalytic turnover in a reducing environment. The result is demonstrated with an iridium pair-site catalyst incorporating P-bridging calix[4]arene ligands dispersed on siliceous supports, chosen as prototypes because they offer weakly interacting surfaces on which metal aggregation is prone to occur. This catalyst was used for the hydrogenation of ethylene in a flow reactor. Atomic-resolution imaging of the Ir centers and spectra of the catalyst before and after use show that the metals resisted aggregation and deactivation, remaining atomically dispersed and accessible for catalysis. This strategy thus allows the stabilization of the catalysts even when they are weakly anchored to supports.

Spectroscopic Characterization of µ-η1:η1-Peroxo Ligands Formed by Reaction of Dioxygen with Electron-Rich Iridium Clusters.

Although oxygen is a common ligand in supported metal catalysts, its coordination has been challenging to elucidate. We now characterize a diiridium complex that has been previously shown by X-ray diffraction crystallography to incorporate a µ-η1:η1-peroxo ligand. We observe markedly enhanced intensity at 788 cm-1 in the Raman spectrum of this complex, which is a consequence of bonding of the peroxo ligand but does not shift upon 18O labeling. Electronic structure calculations at the density functional theory level suggest that this increase in Raman intensity results from bands associated with rocking of CH2 substituents directly attached to P(Ph)2 groups coupling with the O-O band. These results provide part of the foundation for understanding oxygen ligands on a silica-supported tetrairidium carbonyl cluster stabilized with bulky electron-donating phosphine ligands [p-tert-butyl-calix[4]arene(OPr)3(OCH2PPh2) (Ph = phenyl; Pr = propyl)]. Reaction of the cluster with O2 also led to the growing in of a Raman band at 788 cm-1, similar to that in the diiridium complex and also assigned to the bonding of a bridging peroxo ligand. Infrared spectra recorded as the supported cluster reacted in sequential exposures to (i) H2, (ii) O2, (iii) H2, and (iv) CO indicate that two bridging peroxo ligands were bonded irreversibly per tetrairidium cluster, replacing bridging carbonyl ligands without altering either the cluster frame or the phosphine ligands. X-ray absorption near edge and infrared spectra include isosbestic points signifying a stoichiometrically simple reaction of the cluster with O2, and mass spectra of the effluent gas show that CO2 formed by oxidation of one terminal CO ligand per cluster as H2 (and not H2O) formed, evidence that hydride ligands had been present on the cluster following treatment (i). The understanding of how O2 reacts with the metal polyhedron provides a foundation for understanding of how oxidation catalysis may proceed on the surfaces of noble metals.

Outer-Sphere Control of Catalysis on Surfaces: A Comparative Study of Ti(IV) Single-Sites Grafted on Amorphous versus Crystalline Silicates for Alkene Epoxidation.

The effect of outer-sphere environment on alkene epoxidation catalysis using an organic hydroperoxide oxidant is demonstrated for calix[4]arene-TiIV single-sites grafted on amorphous vs crystalline delaminated zeotype (UCB-4) silicates as supports. A chelating calix[4]arene macrocyclic ligand helps enforce a constant TiIV inner-sphere, as characterized by UV-visible and X-ray absorption spectroscopies, thus enabling the rigorous comparison of outer-sphere environments across different siliceous supports. These outer-sphere environments are characterized by solid-state 1H NMR spectroscopy to comprise proximally organized silanols confined within 12 membered-ring cups in crystalline UCB-4, and are responsible for up to 5-fold enhancements in rates of epoxidation by TiIV centers.

Role of N-Heterocyclic Carbenes as Ligands in Iridium Carbonyl Clusters.

The low-energy isomers of Irx(CO)y(NHC)z (x = 1, 2, 4) are investigated with density functional theory (DFT) and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The structures, relative energies, ligand dissociation energies, and natural charges are calculated. The energies of tetrairidium cluster are predicted at the CAM-B3LYP level that best fit the CCSD(T) results compared with the other four functionals in the benchmark calculations. The NHC's behave as stronger σ donors compared with CO's and have higher ligand dissociation energies (LDEs). For smaller isomers, the increase in the LDEs of the CO's and the decrease in the LDEs of the NHC's as more NHC's are substituted for CO's are due to π-back-bonding and electron repulsion, whereas the trend of how the LDEs change for larger isomers is not obvious. We demonstrate a µ3-CO resulting from the high electron density of the metal centers in these complexes, as the bridging CO's and the µ3-CO's can carry more negative charge and stabilize the isomers. Comparison of calculations for a mixed tetrairidum cluster consisting of two calixarene-phosphine ligands and a single calixarene-NHC ligand in the basal plane demonstrated good agreement in terms of both the ligand substitution symmetry (C3v derived), as well as the infrared spectra. Similar comparisons were also performed between calculations and experiment for novel monosubstituted calixarene-NHC tetrairidium clusters.

Catalytic consequences of open and closed grafted Al(III)-calix[4]arene complexes for hydride and oxo transfer reactions.

An approach for the control and understanding of supported molecular catalysts is demonstrated with the design and synthesis of open and closed variants of a grafted Lewis acid active site, consisting of Al(III)-calix[4]arene complexes on the surface of silica. The calixarene acts as a molecular template that enforces open and closed resting-state coordination geometries surrounding the metal active sites, due to its lower-rim substituents as well as site isolation by virtue of its steric bulk. These sites are characterized and used to elucidate mechanistic details and connectivity requirements for reactions involving hydride and oxo transfer. The consequence of controlling open versus closed configurations of the grafted Lewis acid site is demonstrated by the complete lack of observed activity of the closed site for Meerwein-Ponndorf-Verley (MPV) reduction; whereas, the open variant of this catalyst has an MPV reduction activity that is virtually identical to previously reported soluble molecular Al(III)-calix[4]arene catalysts. In contrast, for olefin epoxidation using tert-butyl-hydroperoxide as oxidant, the open and closed catalysts exhibit similar activity. This observation suggests that for olefin epoxidation catalysis using Lewis acids as catalyst and organic hydroperoxide as oxidant, covalent binding of the hydroperoxide is not required, and instead dative coordination to the Lewis acid center is sufficient for catalytic oxo transfer. This latter result is supported by density functional theory calculations of the transition state for olefin epoxidation catalysis, using molecular analogs of the open and closed catalysts.

[(Ph)3PBr][Br7], [(Bz)(Ph)3P]2[Br8], [(n-Bu)3MeN]2[Br20], [C4MPyr]2[Br20], and [(Ph)3PCl]2[Cl2I14]: extending the horizon of polyhalides via synthesis in ionic liquids.

The five polyhalides [(Ph)(3)PBr][Br(7)], [(Bz)(Ph)(3)P](2)[Br(8)], [(n-Bu)(3)MeN](2)[Br(20)], [C(4)MPyr](2)[Br(20)] ([C(4)MPyr] = N-butyl-N-methylpyrrolidinium), and [(Ph)(3)PCl](2)[Cl(2)I(14)] were prepared by the reaction of dibromine and iodine monochloride in ionic liquids. The compounds [(Ph)(3)PBr][Br(7)] and [(Bz)(Ph)(3)P](2)[Br(8)] contain discrete pyramidal [Br(7)](-) and Z-shaped [Br(8)](2-) polybromide anions. [(n-Bu)(3)MeN](2)[Br(20)] and [C(4)MPyr](2)[Br(20)] exhibit new infinite two- and three-dimensional polybromide networks and contain the highest percentage of dibromine ever observed in a compound. [(Ph)(3)PCl](2)[Cl(2)I(14)] also consists of a three-dimensional network and is the first example of an infinite polyiodine chloride. All compounds were obtained from ionic liquids as the solvent that, on the one hand, guarantees for a high stability against strongly oxidizing Br(2) and ICl and that, on the other hand, reduces the high volatility of the molecular halogens.

SSZ-70 borosilicate delamination without sonication: effect of framework topology on olefin epoxidation catalysis.

We report a scalable delamination procedure for a SSZ-70-framework layered-zeolite precursor, which for the first time does not involve either sonication or long-chain surfactants. Our approach instead relies on the mild heating of layered zeolite precursor B-SSZ-70(P) in an aqueous solution containing Zn(NO3)2 and tetrabutylammonium fluoride. Powder X-ray diffraction data are consistent with a loss of long-range order along the z-direction, while 29Si MAS NMR spectroscopy demonstrates preservation of the zeolite framework crystallinity during delamination. The resulting delaminated material, DZ-2, possesses 1.4-fold higher external surface area relative to the nondelaminated three-dimensional zeolite B-SSZ-70, based on N2 physisorption data at 77 K. DZ-2 was functionalized with cationic Ti heteroatoms to synthesize Ti-DZ-2 via exchange with framework B. Ti-DZ-2 contains isolated titanium centers in its crystalline framework, as shown by UV-Vis spectroscopy. The generality of the synthetic delamination approach and catalyst synthesis is demonstrated with the synthesis of delaminated material DZ-3, which is derived from layered zeolite precursor ERB-1(P) with MWW framework topology. Upon catalytic testing for the epoxidation of 1-octene with ethylbenzene hydroperoxide as oxidant, under harsh tail-end conditions that deactivate amorphous Ti-silica-based catalysts, Ti-DZ-2 exhibits the highest per-Ti-site activity, selectivity, and stability for 1-octene epoxidation of all catalysts investigated. This testing includes the prior benchmark delaminated zeolite catalyst in this area, Ti-UCB-4, which possesses similar external surface area to Ti-DZ-2 but requires sonication and long-chain surfactants for its synthesis. The synthesis of DZ-2 is the first example of an economical delamination of layered zeolite precursor SSZ-70(P) and opens up new doors to the development of delaminated zeolites as commercial catalysts.

Dialing in single-site reactivity of a supported calixarene-protected tetrairidium cluster catalyst.

A closed Ir4 carbonyl cluster, 1, comprising a tetrahedral metal frame and three sterically bulky tert-butyl-calix[4]arene(OPr)3(OCH2PPh2) (Ph = phenyl; Pr = propyl) ligands at the basal plane, was characterized with variable-temperature 13C NMR spectroscopy, which show the absence of scrambling of the CO ligands at temperatures up to 313 K. This demonstration of distinct sites for the CO ligands was found to extend to the reactivity and catalytic properties, as shown by selective decarbonylation in a reaction with trimethylamine N-oxide (TMAO) as an oxidant, which, reacting in the presence of ethylene, leads to the selective bonding of an ethyl ligand at the apical Ir site. These clusters were supported intact on porous silica and found to catalyze ethylene hydrogenation, and a comparison of the kinetics of the single-hydrogenation reaction and steady-state hydrogenation catalysis demonstrates a unique single-site catalyst-with each site having the same catalytic activity. Reaction orders in the catalytic ethylene hydrogenation reaction of approximately 1/2 and 0 for H2 and C2H4, respectively, nearly match those for conventional noble-metal catalysts. In contrast to oxidative decarbonylation, thermal desorption of CO from silica-supported cluster 1 occurred exclusively at the basal plane, giving rise to sites that do not react with ethylene and are catalytically inactive for ethylene hydrogenation. The evidence of distinctive sites on the cluster catalyst leads to a model that links to hydrogen-transfer catalysis on metals-involving some surface sites that bond to both hydrocarbon and hydrogen and are catalytically engaged (so-called "*" sites) and others, at the basal plane, which bond hydrogen and CO but not hydrocarbon and are reservoir sites (so-called "S" sites).

Selective molecular recognition by nanoscale environments in a supported iridium cluster catalyst.

The active sites of enzymes are contained within nanoscale environments that exhibit exquisite levels of specificity to particular molecules. The development of such nanoscale environments on synthetic surfaces, which would be capable of discriminating between molecules that would nominally bind in a similar way to the surface, could be of use in nanosensing, selective catalysis and gas separation. However, mimicking such subtle behaviour, even crudely, with a synthetic system remains a significant challenge. Here, we show that the reactive sites on the surface of a tetrairidium cluster can be controlled by using three calixarene-phosphine ligands to create a selective nanoscale environment at the metal surface. Each ligand is 1.4 nm in length and envelopes the cluster core in a manner that discriminates between the reactivities of the basal-plane and apical iridium atoms. CO ligands are initially present on the clusters and can be selectively removed from the basal-plane sites by thermal dissociation and from the apical sites by reactive decarbonylation with the bulky reactant trimethylamine-N-oxide. Both steps lead to the creation of metal sites that can bind CO molecules, but only the reactive decarbonylation step creates vacancies that are also able to bond to ethylene, and catalyse its hydrogenation.

Accessible gold clusters using calix[4]arene N-heterocyclic carbene and phosphine ligands.

We investigate the synthesis of accessible calix[4]arene-bound gold clusters consisting of open "coordinatively unsaturated" active sites, using a comparative approach that relies on calix[4]arene ligands with various upper- and lower-rim substituents. In contrast with a reported Au(I)-tert-butyl-calixarene phosphine complex, which exhibits a single cone conformer in solution, the H upper-rim analog exhibits multiple conformers in solution. This contrasts with observations of the tert-butyl upper-rim analog, which exhibits a single cone conformer in solution under similar conditions. In the solid state, as determined by single-crystal X-ray diffraction, both H and tert-butyl upper-rim analogs exhibit exclusively cone conformer. A detailed structural analysis of these two solid-state structures highlights a CH-π interaction involving a methoxy lower-rim substituent and phenyl substituent on P as the key feature that enforces a tight configuration of Au(I) atoms on the same side of the calix[4]arene lower-rim plane. We hypothesize that such a configuration promotes chelation of the ligand to a gold surface and facilitates the synthesis of small Au11-sized clusters after reduction of both complexes. The new cluster, like the one reported with the tert-butyl analog, has an extraordinary 25% of surface atoms that are open and accessible to a 2-NT (2-naphthalenethiol) probe in solution. We also investigated the effect of calix[4]arene lower-rim substituents that coordinate to the metal, by using N-heterocyclic carbene (NHC) functional groups rather than phosphines. Four small (<1.6 nm diameter) calix[4]arene NHC-bound gold clusters were synthesized, including three using novel calix[4]arene NHC ligands. The smallest calix[4]arene NHC-bound Au cluster consisted of a 1.2 nm gold core, and its number density of accessible and open surface sites was measured. This required development of a new titration method for open sites on gold clusters, using a SAMSA fluorescein dye molecule, which excites and emits at lower energy relative to the previously used 2-NT probe. The number density of open sites on the new calix[4]arene NHC-bound gold cluster measured by the SAMSA fluorescein probe strongly supports the generality of a mechanical model of accessibility, which does not depend on the functional group involved in binding to the gold surface and rather depends on the relative radii of curvature of bound ligands and the gold cluster core.

Stabilization of coordinatively unsaturated Ir4 clusters with bulky ligands: a comparative study of chemical and mechanical effects.

The synthesis and characterization of new cluster compounds represented by the series Ir(4)(CO)(12-x)L(x) (L = tert-butyl-calix[4]-arene(OPr)(3)(OCH(2)PPh(2)); x = 2 and 3) is reported using ESI mass spectrometry, NMR spectroscopy, IR spectroscopy and single-crystal X-ray diffraction. Thermally driven decarbonylation of the cluster compound series represented by x = 1-3 according to the formula above is followed via FTIR and NMR spectroscopies, and dynamic light scattering in toluene solution. The propensity of these clusters to decarbonylate in solution is shown to be directly correlated with number density of adsorbed calixarene phosphine ligands and controlled via Pauli repulsion between metal d and CO 5σ orbitals. The tendency for cluster aggregation unintuitively follows a trend that is exactly opposite to the cluster's propensity to decarbonylate. No cluster aggregation is observed for clusters consisting of x = 3, even after extensive decarbonylation via loss of all bridging CO ligands and coordinative unsaturation. Some of the CO lost during thermal treatment via decarbonylation can be rebound to the coordinatively unsaturated cluster consisting of x = 3. In contrast, the clusters consisting of x = 1 and x = 2 both aggregate into large nanoparticles when treated under identical conditions. Clusters in which the calixarene phosphine ligand is replaced with a sterically less demanding PPh(2)Me ligand 6 lead to significantly less coordinative unsaturation upon thermal treatment. Altogether, these data support a mechanical model of accessibility in coordinatively unsaturated metal clusters in solution, which hinges on having at least three sterically bulky organic ligands per Ir(4) core.

{[P(o-tolyl)3]Br}2[Cu2Br6](Br2)--an ionic compound containing molecular bromine.

Dark green cuboid-shaped crystals of the composition {[P(o-tolyl) 3]Br} 2[Cu 2Br 6](Br 2) are obtained by the reaction of P(o-tolyl) 3 and CuBr 2 with Br 2 in the ionic liquid [NMeBu 3][N(Tf) 2]. The bromocuprate crystallizes triclinic [space group P1; Z = 1; a = 10.667(2) A; b = 10.695(2) A; c = 11.582(2) A; alpha = 74.42(3) degrees ; beta = 75.64(3) degrees ; and gamma = 85.68(3) degrees ]. The title compound is constituted of {[P(o-tolyl) 3]Br} (+) cations and [Cu 2Br 6] (2-) anions and contains molecular dibromine [d Br-Br = 2.341(1) A]. The latter is verified by thermogravimetry and mass spectrometry.