Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities

From single-site tantalum complexes to nanoparticles of Ta x N y and TaO x N y supported on silica: elucidation of synthesis chemistry by dynamic nuclear polarization surface enhanced NMR spectroscopy and X-ray absorption spectroscopy

Air-stable catalysts consisting of tantalum nitride nanoparticles represented as a mixture of Ta x N y and TaO x N y with diameters in the range of 0.5 to 3 nm supported on highly dehydroxylated silica were synthesized from TaMe5 (Me = methyl) and dimeric Ta2(OMe)10 with guidance by the principles of surface organometallic chemistry (SOMC). Characterization of the supported precursors and the supported nanoparticles formed from them was carried out by IR, NMR, UV-Vis, extended X-ray absorption fine structure, and X-ray photoelectron spectroscopies complemented with XRD and high-resolution TEM, with dynamic nuclear polarization surface enhanced NMR spectroscopy being especially helpful by providing enhanced intensities of the signals of 1H, 13C, 29Si, and 15N at their natural abundances. The characterization data provide details of the synthesis chemistry, including evidence of (a) O2 insertion into Ta-CH3 species on the support and (b) a binuclear to mononuclear transformation of species formed from Ta2(OMe)10 on the support. A catalytic test reaction, cyclooctene epoxidation, was used to probe the supported nanoparticles, with 30% H2O2 serving as the oxidant. The catalysts gave selectivities up to 98% for the epoxide at conversions as high as 99% with a 3.4 wt% loading of Ta present as Ta x N y /TaO x N y .

Metathesis Activity Encoded in the Metallacyclobutane Carbon-13 NMR Chemical Shift Tensors

Metallacyclobutanes are an important class of organometallic intermediates, due to their role in olefin metathesis. They can have either planar or puckered rings associated with characteristic chemical and physical properties. Metathesis active metallacyclobutanes have short M-C_α/α' and M···C_β distances, long C_α/α'-C_β bond length, and isotropic ¹³C chemical shifts for both early d⁰ and late d⁴ transition metal compounds for the α- and β-carbons appearing at ca. 100 and 0 ppm, respectively. Metallacyclobutanes that do not show metathesis activity have ¹³C chemical shifts of the α- and β-carbons at typically 40 and 30 ppm, respectively, for d⁰ systems, with upfield shifts to ca. -30 ppm for the α-carbon of metallacycles with higher d ⁿ electron counts (n = 2 and 6). Measurements of the chemical shift tensor by solid-state NMR combined with an orbital (natural chemical shift, NCS) analysis of its principal components (δ₁₁ ≥ δ₂₂ ≥ δ₃₃) with two-component calculations show that the specific chemical shift of metathesis active metallacyclobutanes originates from a low-lying empty orbital lying in the plane of the metallacyclobutane with local π*(M-C_α/α') character. Thus, in the metathesis active metallacyclobutanes, the α-carbons retain some residual alkylidene character, while their β-carbon is shielded, especially in the direction perpendicular to the ring. Overall, the chemical shift tensors directly provide information on the predictive value about the ability of metallacyclobutanes to be olefin metathesis intermediates.

Non-traditional Ziegler-Natta catalysis in α-olefin transformations: reaction mechanisms and product design

This paper describes our recent results in the field of zirconocene-catalyzed α-oltfin transformations, and focuses on questions regarding the reaction mechanism, rational design of zirconocene pre-catalysts, as well as prospective uses of α-olefin products. It has been determined that a wide range of α-olefin-based products, namely vinylidene dimers, oligomers and polymers, can be prepared via catalysis by zirconocene dichlorides, activated by a minimal (10–20 eq.) amount of MAO. We assumed that in the presence of minimal quantities of MAO, various types of zirconocene catalysts form different types of catalytic particles. In the case of bis-cyclopentadienyl complexes, the reactive center is formed under the influence of R2AlCl, which makes the chain termination via β-hydride elimination significantly easier, with α-olefin dimers being formed as the primary product. Bis-indenyl complexes and their heteroanalogues, form stable cationic catalytic particles and effectively catalyze the polymerization. Mono-indenyl and mono-substituted bis-cyclopentadienyl-ansa complexes catalyze α-olefin oligomerization. Effective catalysts of dimerization, oligomerization and polymerization of α-olefins in the presence of minimal MAO quantities are proposed. Prospects of using α-olefin dimers, oligomers and polymers in the synthesis of branched hydrocarbon functional derivatives (dimers), high quality, low viscosity motor oils (oligomers), and thickeners and absorbents (polymers) are examined.

Elucidation of 1H NMR Paramagnetic Features of Heterotrimetallic Lanthanide(III)/Manganese(III) 12-MC-4 Complexes

The paramagnetic one-dimensional ¹H NMR spectra of twelve Ln^IIINa^I(OAc)₄[12-MC_Mn^III(N)shi-4] complexes, where Ln^III is Pr^III-Yb^III (except Pm^III) and Y^III, are reported. Their solid-state isostructural nature is confirmed in methanol-d₄ solution, as a similar pattern in the ¹H NMR spectra is observed along the series. Notably, a relatively well-resolved spectrum is reported for the Gd^III complex. The chemical shift data are analyzed using the "all lanthanides" method, and the Fermi contact and pseudo-contact contributions are calculated for the lanthanide-induced shift (LIS). For the Tb^III-Yb^III complexes, the pseudo-contact contributions are typically 1 order of magnitude higher than the Fermi contact contributions; however, for the Gd^III complex, the Fermi contact is the main contribution to the paramagnetic chemical shift. For the methyl protons of the axial acetate (^-OAc) ligands, the LIS is opposite in sign, with respect to that of the aromatic salicylhydroximate (shi^3-) protons, because of structural rearrangements that occur upon dissociation of the Na^I cation in solution. The calculated crystal field parameters (B_Ln) for the Tb^III (360 cm^-1), Dy^III (250 cm^-1), Ho^III (380 cm^-1), Er^III (410 cm^-1), Tm^III (620 cm^-1), and Yb^III (380 cm^-1) complexes are not constant, likely as a consequence of the inaccuracy of the Bleaney's constants and, to a smaller extent, the small structural changes that occur in solution. Overall, the metallacrown scaffold retains structural integrity and similarity in solution for the entire series; however, small structural features, which do not affect the overall similarity, do likely occur.

Solid-state molecular organometallic chemistry. Single-crystal to single-crystal reactivity and catalysis with light hydrocarbon substrates

Solid-state molecular organometallic catalysis (SMOM-cat): synthetic routes, unique structural motifs, mobility in the solid-state and very active gas/solid isomerization catalysts.

Single-crystal to single-crystal solid/gas reactivity and catalysis starting from the precursor sigma-alkane complex [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F₄] (NBA = norbornane; Ar^F = 3,5-(CF₃)₂C₆H₃) is reported. By adding ethene, propene and 1-butene to this precursor in solid/gas reactions the resulting alkene complexes [Rh(Cy₂PCH₂CH₂PCy₂)(alkene)_x][BAr^F₄] are formed. The ethene (x = 2) complex, [Rh(Cy₂PCH₂CH₂PCy₂)(ethene)₂][BAr^F₄]-Oct, has been characterized in the solid-state (single-crystal X-ray diffraction) and by solution and solid-state NMR spectroscopy. Rapid, low temperature recrystallization using solution methods results in a different crystalline modification, [Rh(Cy₂PCH₂CH₂PCy₂)(ethene)₂][BAr^F₄]-Hex, that has a hexagonal microporous structure (P6₃22). The propene complex (x = 1) [Rh(Cy₂PCH₂CH₂PCy₂)(propene)][BAr^F₄] is characterized as having a π-bound alkene with a supporting γ-agostic Rh···H₃C interaction at low temperature by single-crystal X-ray diffraction, variable temperature solution and solid-state NMR spectroscopy, as well as periodic density functional theory (DFT) calculations. A fluxional process occurs in both the solid-state and solution that is proposed to proceed via a tautomeric allyl-hydride. Gas/solid catalytic isomerization of d₃-propene, H₂C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHCD₃, using [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F₄] scrambles the D-label into all possible positions of the propene, as shown by isotopic perturbation of equilibrium measurements for the agostic interaction. Periodic DFT calculations show a low barrier to H/D exchange (10.9 kcal mol^–1, PBE-D3 level), and GIPAW chemical shift calculations guide the assignment of the experimental data. When synthesized using solution routes a bis-propene complex, [Rh(Cy₂PCH₂CH₂PCy₂)(propene)₂][BAr^F₄], is formed. [Rh(Cy₂PCH₂CH₂PCy₂)(butene)][BAr^F₄] (x = 1) is characterized as having 2-butene bound as the cis-isomer and a single Rh···H₃C agostic interaction. In the solid-state two low-energy fluxional processes are proposed. The first is a simple libration of the 2-butene that exchanges the agostic interaction, and the second is a butene isomerization process that proceeds via an allyl-hydride intermediate with a low computed barrier of 14.5 kcal mol^–1. [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F₄] and the polymorphs of [Rh(Cy₂PCH₂CH₂PCy₂)(ethene)₂][BAr^F₄] are shown to be effective in solid-state molecular organometallic catalysis (SMOM-Cat) for the isomerization of 1-butene to a mixture of cis- and trans-2-butene at 298 K and 1 atm, and studies suggest that catalysis is likely dominated by surface-active species. [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F₄] is also shown to catalyze the transfer dehydrogenation of butane to 2-butene at 298 K using ethene as the sacrificial acceptor.

Density Functional Theory Study of the Reaction between d0 Tungsten Alkylidyne Complexes and H2O: Addition versus Hydrolysis

The reactions of early-transition-metal complexes with H₂O have been investigated. An understanding of these elementary steps promotes the design of precursors for the preparation of metal oxide materials or supported heterogeneous catalysts. Density functional theory (DFT) calculations have been conducted to investigate two elementary steps of the reactions between tungsten alkylidyne complexes and H₂O, i.e., the addition of H₂O to the W≡C bond and ligand hydrolysis. Four tungsten alkylidyne complexes, W(≡CSiMe₃)(CH₂SiMe₃)₃ (A-1), W(≡CSiMe₃)(CH₂^tBu)₃ (B-1), W(≡C^tBu)(CH₂^tBu)₃ (C-1), and W(≡C^tBu)(O^tBu)₃ (D-1), have been compared. The DFT studies provide an energy profile of the two competing pathways. An additional H₂O molecule can serve as a proton shuttle, accelerating the H₂O addition reaction. The effect of atoms at the α and β positions has also been examined. Because the lone-pair electrons of an O atom at the α position can interact with the orbital of the proton, the barrier of the ligand-hydrolysis reaction for D-1 is dramatically reduced. Both the electronic and steric effects of the silyl group at the β position lower the barriers of both the H₂O addition and ligand-hydrolysis reactions. These new mechanistic findings may lead to the further development of metal complex precursors.

Molecularly Tailored Nickel Precursor and Support Yield a Stable Methane Dry Reforming Catalyst with Superior Metal Utilization

Syngas production via the dry reforming of methane (DRM) is a highly endothermic process conducted under harsh conditions; hence, the main difficulty resides in generating stable catalysts. This can, in principle, be achieved by reducing coke formation, sintering, and loss of metal through diffusion in the support. [{Ni(μ²-OCHO)(OCHO)(tmeda)}₂(μ²-OH₂)] (tmeda = tetramethylethylenediamine), readily synthesized and soluble in a broad range of solvents, was developed as a molecular precursor to form 2 nm Ni(0) nanoparticles on alumina, the commonly used support in DRM. While such small nanoparticles prevent coke deposition and increase the initial activity, operando X-ray Absorption Near-Edge Structure (XANES) spectroscopy confirms that deactivation largely occurs through the migration of Ni into the support. However, we show that Ni loss into the support can be mitigated through the Mg-doping of alumina, thereby increasing significantly the stability for DRM. The superior performance of our catalytic system is a direct consequence of the molecular design of the metal precursor and the support, resulting in a maximization of the amount of accessible metallic nickel in the form of small nanoparticles while preventing coke deposition.

Silica-supported isolated gallium sites as highly active, selective and stable propane dehydrogenation catalysts

Single-site gallium centers on the surface of silica are prepared via grafting of [Ga(OSi(OtBu)3)3(THF)] on SiO2-700 followed by a thermolysis step. The resulting surface species corresponds to well-defined tetra-coordinate gallium single-sites, [([triple bond, length as m-dash]SiO)3Ga(XOSi[triple bond, length as m-dash])] (X = -H or [triple bond, length as m-dash]Si) according to IR, X-ray absorption near-edge structure and extended X-ray absorption fine structure analysis. These gallium sites show high activity, selectivity and stability for propane dehydrogenation with an initial turnover frequency of 20 per h per gallium center, propylene selectivity of ≥93% and remarkable stability over 20 h. The stability of the catalyst probably results from site-isolation of the active site on a non-reducible support such as silica, diminishing facile reduction typical of Ga2O3-based catalysts.

Magnetic Memory from Site Isolated Dy(III) on Silica Materials

Achieving magnetic remanence at single isolated metal sites dispersed at the surface of a solid matrix has been envisioned as a key step toward information storage and processing in the smallest unit of matter. Here, we show that isolated Dy(III) sites distributed at the surface of silica nanoparticles, prepared with a simple and scalable two-step process, show magnetic remanence and display a hysteresis loop open at liquid ⁴He temperature, in contrast to the molecular precursor which does not display any magnetic memory. This singular behavior is achieved through the controlled grafting of a tailored Dy(III) siloxide complex on partially dehydroxylated silica nanoparticles followed by thermal annealing. This approach allows control of the density and the structure of isolated, "bare" Dy(III) sites bound to the silica surface. During the process, all organic fragments are removed, leaving the surface as the sole ligand, promoting magnetic remanence.

Pairwise hydrogen addition in the selective semihydrogenation of alkynes on silica-supported Cu catalysts

Mechanistic insight into the semihydrogenation of 1-butyne and 2-butyne on Cu nanoparticles supported on partially dehydroxylated silica (Cu/SiO2-700) was obtained using parahydrogen. Hydrogenation of 1-butyne over Cu/SiO2-700 yielded 1-butene with ≥97% selectivity. The surface modification of this catalyst with tricyclohexylphosphine (PCy3) increased the selectivity to 1-butene up to nearly 100%, although at the expense of reduced catalytic activity. Similar trends were observed in the hydrogenation of 2-butyne, where Cu/SiO2-700 provided a selectivity to 2-butene in the range of 72-100% depending on the reaction conditions, while the catalyst modified with PCy3 again demonstrated nearly 100% selectivity. Parahydrogen-induced polarization effects observed in hydrogenation reactions catalyzed by copper-based catalysts demonstrate the viability of pairwise hydrogen addition over these catalysts. Contribution of pairwise hydrogen addition to 1-butyne was estimated to be at least 0.2-0.6% for unmodified Cu/SiO2-700 and ≥2.7% for Cu/SiO2-700 modified with PCy3, highlighting the effect of surface modification with the tricyclohexylphosphine ligand.

Heterogeneous catalase-like activity of gold(i)-cobalt(iii) metallosupramolecular ionic crystals

The heterogeneous catalase-like activity of ionic crystals consisting of Au^I₄Co^III₂ complex cations is studied along with their surface morphologies and oxidation states.

Unique heterogeneous catalase-like activity was observed for metallosupramolecular ionic crystals [Au^I₄Co^III₂(dppe)₂(d-pen)₄]X_n ([1]X_n; dppe = 1,2-bis(diphenylphosphino)ethane; d-pen = d-penicillaminate; X_n = (Cl^–)₂, (ClO₄^–)₂, (NO₃^–)₂ or SO₄^2–) consisting of Au^I₄Co^III₂ complex cations, [1]²⁺, and inorganic anions, X^– or X^2–. Treatment of the ionic crystals with an aqueous H₂O₂ solution led to considerable O₂ evolution with a high turnover frequency of 1.4 × 10⁵ h^–1 for the heterogeneous cobalt complexes, which was dependent on their size and shape as well as the arrangement of cationic and anionic species. These dependencies were rationalized by the presence of cobalt(ii) centers on the crystal surface and their efficient exposure on the (111) plane rather than the (100) plane based on morphological and theoretical studies.

A novel 2-step ALD route to ultra-thin MoS2 films on SiO2 through a surface organometallic intermediate

The lack of scalable-methods for the growth of 2D MoS₂ crystals, an identified emerging material with applications ranging from electronics to energy storage, is a current bottleneck against its large-scale deployment. We report here a two-step ALD route with new organometallic precursors, Mo(NMe₂)₄ and 1,2-ethanedithiol (HS(CH₂)₂SH) which consists in the layer-by-layer deposition of an amorphous surface Mo(iv) thiolate at 50 °C, followed by a subsequent annealing at higher temperature leading to ultra-thin MoS₂ nanocrystals (∼20 nm-large) in the 1-2 monolayer range. In contrast to the usual high-temperature growth of 2D dichalcogenides, where nucleation is the key parameter to control both thickness and uniformity, our novel two-step ALD approach enables chemical control over these two parameters, the growth of 2D MoS₂ crystals upon annealing being ensured by spatial confinement and facilitated by the formation of a buffer oxysulfide interlayer.

Efficient carbon-supported heterogeneous molybdenum-dioxo catalyst for chemoselective reductive carbonyl coupling

Selective carbonyl coupling, high symmetric ether selectivity.

Understanding surface site structures and properties by first principles calculations: an experimental point of view!

Computational Chemistry is key for the molecular-level understanding of active sites in heterogeneous catalysis paving the way to the rational design and development.

On the use of solid-state 45Sc NMR for structural investigations of molecular and silica-supported scandium amide catalysts

⁴⁵Sc NMR of molecular and silica-grafted scandium amido derivatives provides information about the metal coordination sphere with links to catalytic activity.

Spatial distribution of organic functional groups supported on mesoporous silica nanoparticles: a study by conventional and DNP-enhanced 29Si solid-state NMR

DNP-enhanced solid-state NMR determined spatial distributions of organic functionalities attached to surfaces of mesoporous silica nanoparticles via co-condensation and grafting.

Designing a heterogeneous Pd(ii)–NHC-based C–H activation catalyst on a self-supported coordination polymer platform

An extended Ru^II(terpy)₂-based coordination polymer provides a potential scaffold to design a self-supported, single-site Pd^II(NHC) oxidative catalyst for challenging reactions such as selective arene monohalogenation with enhanced catalytic activity compared to the homogeneous counterpart.

Organoindium-modified monodisperse ellipsoid-/platelet-like periodic mesoporous silicas

Indium-modified mondisperse ellipsoid-/platelet-like large-pore periodic mesoporous silica nanoparticles (MMSNs) SBA-15 have been prepared via molecular grafting of In[N(SiMe₃)₂]₃. Surface ligand exchange led to the formation of heteroleptic In species, and the resulting surface In species were converted into crystalline In₂O₃ nanoparticles by calcination.

Grand challenges for catalysis in the Science and Technology Roadmap on Catalysis for Europe: moving ahead for a sustainable future

This perspective discusses the general concepts that will guide future catalysis and related grand challenges based on the Science and Technology Roadmap on Catalysis for Europe prepared by the European Cluster on Catalysis.

Isolated, well-defined organovanadium(iii) on silica: single-site catalyst for hydrogenation of alkenes and alkynes

Well-defined, isolated, single-site organovanadium(iii) catalyst on SiO₂ for unprecedented liquid- and gas-phase hydrogenation of alkenes and alkynes under mild conditions.

Selective ethylene oligomerisation using supported tungsten mono-imido catalysts

Tungsten arylimido complexes grafted to the surface of solid polymethylaluminoxane are active for the selective dimerisation of ethylene.