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

Active Site Descriptors from 95Mo NMR Signatures of Silica-Supported Mo-Based Olefin Metathesis Catalysts

The olefin metathesis activity of silica-supported molybdenum oxides depends strongly on metal loading and preparation conditions, indicating that the nature and/or amounts of the active sites vary across compositionally similar catalysts. This is illustrated by comparing Mo-based (pre)catalysts prepared by impregnation (2.5-15.6 wt % Mo) and a model material (2.3 wt % Mo) synthesized via surface organometallic chemistry (SOMC). Analyses of FTIR, UV-vis, and Mo K-edge X-ray absorption spectra show that these (pre)catalysts are composed predominantly of similar isolated Mo dioxo sites. However, they exhibit different reaction properties in both liquid and gas-phase olefin metathesis with the SOMC-derived catalyst outperforming a classical catalyst of a similar Mo loading by ×1.5-2.0. Notably, solid-state ⁹⁵Mo NMR analyses leveraging state-of-the-art high-field (28.2 T) measurement conditions resolve four distinct surface Mo dioxo sites with distributions that depend on the (pre)catalyst preparation methods. The intensity of a specific deshielded ⁹⁵Mo NMR signal, which is most prominent in the SOMC-derived catalyst, is linked to reducibility and catalytic activity. First-principles calculations show that ⁹⁵Mo NMR parameters directly manifest the local strain and coordination environment: acute (SiO-Mo(O)₂-OSi) angles and low coordination numbers at Mo lead to highly deshielded ⁹⁵Mo chemical shifts and small quadrupolar coupling constants, respectively. Natural chemical shift analyses relate the ⁹⁵Mo NMR signature of strained species to low LUMO energies, which is consistent with their high reducibility and corresponding reactivity. The ⁹⁵Mo chemical shifts of supported Mo dioxo sites are thus linked to their specific electronic structures, providing a powerful descriptor for their propensity toward reduction and formation of active sites.

The promotional role of Mn in CO2 hydrogenation over Rh-based catalysts from a surface organometallic chemistry approach

Rh-based catalysts modified by transition metals have been intensively studied for CO₂ hydrogenation due to their high activity. However, understanding the role of promoters at the molecular level remains challenging due to the ill-defined structure of heterogeneous catalysts. Here, we constructed well-defined RhMn@SiO₂ and Rh@SiO₂ model catalysts via surface organometallic chemistry combined with thermolytic molecular precursor (SOMC/TMP) approach to rationalize the promotional effect of Mn in CO₂ hydrogenation. We show that the addition of Mn shifts the products from almost pure CH₄ to a mixture of methane and oxygenates (CO, CH₃OH, and CH₃CH₂OH) upon going from Rh@SiO₂ to RhMn@SiO₂. In situ X-ray absorption spectroscopy (XAS) confirms that the Mn^II is atomically dispersed in the vicinity of metallic Rh nanoparticles and enables to induce the oxidation of Rh to form the Mn-O-Rh interface under reaction conditions. The formed interface is proposed to be key to maintaining Rh⁺ sites, which is related to suppressing the methanation reaction and stabilizing the formate species as evidenced by in situ DRIFTS to promote the formation of CO and alcohols.

How Solid Surfaces Control Stability and Interactions of Supported Cationic CuI(dppf) Complexes─A Solid-State NMR Study

Organometallic complexes are frequently deposited on solid surfaces, but little is known about how the resulting complex-solid interactions alter their properties. Here, a series of complexes of the type Cu(dppf)(L_x)⁺ (dppf = 1,1'-bis(diphenylphosphino)ferrocene, L_x = mono- and bidentate ligands) were synthesized, physisorbed, ion-exchanged, or covalently immobilized on solid surfaces and investigated by ³¹P MAS NMR spectroscopy. Complexes adsorbed on silica interacted weakly and were stable, while adsorption on acidic γ-Al₂O₃ resulted in slow complex decomposition. Ion exchange into mesoporous Na-[Al]SBA-15 resulted in magnetic inequivalence of ³¹P nuclei verified by ³¹P-³¹P RFDR and ¹H-³¹P FSLG HETCOR. DFT calculations verified that a MeCN ligand dissociates upon ion exchange. Covalent immobilization via organic linkers as well as ion exchange with bidentate ligands both lead to rigidly bound complexes that cause broad ³¹P CSA tensors. We thus demonstrate how the interactions between complexes and functional surfaces determine and alter the stability of complexes. The applied Cu(dppf)(L_x)⁺ complex family members are identified as suitable solid-state NMR probes for investigating the influence of support surfaces on deposited inorganic complexes.

Site-Averaged Ab Initio Kinetics: Importance Learning for Multistep Reactions on Amorphous Supports

Single-atom centers on amorphous supports include catalysts for polymerization, partial oxidation, metathesis, hydrogenolysis, and more. The disordered environment makes each site different, and the kinetics exponentially magnifies these differences to make ab initio site-averaged kinetics calculations extremely difficult. This work extends the importance learning algorithm for efficient and precise site-averaged kinetics estimates to ab initio calculations and multistep reaction mechanisms. Specifically, we calculate site-averaged proton transfer relaxation rates on an ensemble of cluster models representing Brønsted acid sites on silica-alumina. We include direct and water-assisted proton transfer pathways and simultaneously estimate the water adsorption and activation enthalpies for forward and backward proton transfers. We use density functional theory (DFT) to obtain a site-averaged rate, somewhat like a turnover frequency, for the proton transfer relaxation rate. Finally, we show that importance learning can provide orders-of-magnitude acceleration over standard sampling methods for site-averaged rate calculations in cases where the rate is dominated by a few highly active sites.

Atomically Dispersed NiNx Site with High Oxygen Electrocatalysis Performance Facilely Produced via a Surface Immobilization Strategy

Nonprecious-metal heterogeneous catalysts with atomically dispersed active sites demonstrated high activity and selectivity in different reactions, and the rational design and large-scale preparation of such catalysts are of great interest but remain a huge challenge. Current approaches usually involve extremely high-temperature and tedious procedures. Here, we demonstrated a straightforward and scalable preparation strategy. In two simple steps, the atomically dispersed Ni electrocatalyst can be synthesized in a tens grams scale with quantitative yield under mild conditions, and the active Ni sites were produced by immobilizing preorganized NiN_x complex on the substrate surface via organic thermal reactions. This catalyst exhibits excellent catalysis performances in both oxygen evolution and reduction reactions. It also exhibited tunable catalysis activity, high catalysis reproducibility, and high stability. The atomically dispersed NiN_x sites are tolerant at high Ni concentration, as the random reactions and metal nanoparticle formation that generally occurred at high temperatures were avoided. This strategy illustrated a practical and green method for the industrial manufacture of nonprecious-metal single-site catalysts with a predictable structure.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

Effects of surface acidity on the structure of organometallics supported on oxide surfaces

Well-defined organometallics supported on high surface area oxides are promising heterogeneous catalysts. An important design factor in these materials is how the metal interacts with the functionalities on an oxide support, commonly anionic X-type ligands derived from the reaction of an organometallic M-R with an -OH site on the oxide. The metal can either form a covalent M-O bond or form an electrostatic M⁺⋯^-O ion-pair, which impacts how well-defined organometallics will interact with substrates in catalytic reactions. A less common reaction pathway involves the reaction of a Lewis site on the oxide with the organometallic, resulting in abstraction to form an ion-pair, which is relevant to industrial olefin polymerization catalysts. This Feature Article views the spectrum of reactivity between an organometallic and an oxide through the prism of Brønsted and/or Lewis acidity of surface sites and draws analogies to the molecular frame where Lewis and Brønsted acids are known to form reactive ion-pairs. Applications of the well-defined sites developed in this article are also discussed.

Computational investigation of α-SiO2 surfaces as a support for Pd

The properties of a supported metal catalyst depend crucially on the interaction between the active metal and the support. A case in point is Pd supported on silica, Pd/SiO₂, which is widely used in oxidation catalysis. There is a need for a broad range of computational models that describe the interaction of Pd with silica surfaces so that active site models can be proposed and tested. In this work, we create well-defined, reproducible, periodic models of SiO₂ surfaces and investigate their interaction with Pd using dispersion-corrected DFT. We use crystalline α-SiO₂ as a useful starting point for creating and estimating the adsorption properties of metals on SiO₂ surfaces, which can represent the specific isolated functional groups present on more complex amorphous silica surfaces. We have modelled α-SiO₂ (001), (100) and (101) surfaces containing isolated siloxane and silanol functional groups and estimated their affinity towards the adsorption of Pd atoms regarding an isolated gaseous Pd atom and the fcc Pd solid. This provides additional information on the ease with which Pd can be dispersed on the surfaces in question. From our model, we characterise the surface energies of the α-SiO₂ (hkl) surfaces and calculate the geometries of the Pd₁/α-SiO₂ (hkl) adsorption site on each surface. We estimate that Pd₁(g) will prefer to adsorb close to strained four-membered siloxane rings or on a vicinal silanol group of α-SiO₂ (101).

Surface Hydroxyl Chemistry of Titania- and Alumina-Based Supports: Quantitative Titration and Temperature Dependence of Surface Brønsted Acid-Base Parameters

Surface hydroxyl groups on metal oxides play significant roles in catalyst synthesis and catalytic reactions. Despite the importance of surface hydroxyls in broader material applications, quantitative measurements of surface acid-base properties are not regularly reported. Here, we describe direct methods to quantify fundamental properties of surface hydroxyls on several titania- and alumina-based supports. Comparing commercially available anatase, rutile, P25, and P90 titania, thermogravimetric analysis (TGA) indicated that the total surface hydroxyl density varied by a factor of 2, and each surface hydroxyl is associated with approximately one weakly adsorbed water molecule. Proton-exchange site densities, determined at 25 °C with slurry acid-base titrations, led to several conclusions: (i) the intrinsic acidity/basicity of surface hydroxyls were similar regardless of the titania source; (ii) differences in the surface isoelectric point (IEP) were primarily attributable to differences in the surface concentration of acid and base sites; (iii) rutile has a higher surface concentration of basic hydroxyls, leading to a higher IEP; and (iv) P25 and P90 titania have slightly higher surface concentrationsof acidic hydroxyls relative to anatase or rutile. Temperature effects on surface acid-base properties are rarely reported yet are significant: from 5 to 65 °C, IEP values change by roughly one pH unit. The IEP changes were associated with large changes to the intrinsic acid-base equilibrium constants over this temperature range, rather than changes in the composition or concentration of the surface sites.

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)₅Cl to form complexes of the type Re(bpy)(CO)₃Cl, which are related to known catalysts for CO₂ reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO₂ on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO₂ reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

Mechanistic Insights into Molecular Crystalline Organometallic Heterogeneous Catalysis through Parahydrogen-Based Nuclear Magnetic Resonance Studies

The heterogeneous solid-gas reactions of crystals of [Rh(L₂)(propene)][BAr^F₄] (1, L₂ = ^tBu₂PCH₂CH₂P^tBu₂) with H₂ and propene, 1-butene, propyne, or 1-butyne are explored by gas-phase nuclear magnetic resonance (NMR) spectroscopy under batch conditions at 25 °C. The temporal evolution of the resulting parahydrogen-induced polarization (PHIP) effects measures catalytic flux and thus interrogates the efficiency of catalytic pairwise para-H₂ transfer, speciation changes in the crystalline catalyst at the molecular level, and allows for high-quality single-scan ¹H, ¹³C NMR gas-phase spectra for the products to be obtained, as well as 2D-measurements. Complex 1 reacts with H₂ to form dimeric [Rh(L₂)(H)(μ-H)]₂[BAr^F₄]₂ (4), as probed using EXAFS; meanwhile, a single-crystal of 1 equilibrates NMR silent para-H₂ with its NMR active ortho isomer, contemporaneously converting into 4, and 1 and 4 each convert para-H₂ into ortho-H₂ at different rates. Hydrogenation of propene using 1 and para-H₂ results in very high initial polarization levels in propane (>85%). Strong PHIP was also detected in the hydrogenation products of 1-butene, propyne, and 1-butyne. With propyne, a competing cyclotrimerization deactivation process occurs to afford [Rh(^tBu₂PCH₂CH₂P^tBu₂)(1,3,4-Me₃C₆H₃)][BAr^F₄], while with 1-butyne, rapid isomerization of 1-butyne occurs to give a butadiene complex, which then reacts with H₂ more slowly to form catalytically active 4. Surprisingly, the high PHIP hydrogenation efficiencies allow hyperpolarization effects to be seen when H₂ is taken directly from a regular cylinder at 25 °C. Finally, changing the chelating phosphine to Cy₂PCH₂CH₂PCy₂ results in initial high polarization efficiencies for propene hydrogenation, but rapid quenching of the catalyst competes to form the zwitterion [Rh(Cy₂PCH₂CH₂PCy₂){η⁶-(CF₃)₂(C₆H₃)}BAr^F₃].

Titanium(IV) Surface Complexes Bearing Chelating Catecholato Ligands for Enhanced Band-Gap Reduction

Protonolysis reactions between dimethylamido titanium(IV) catecholate [Ti(CAT)(NMe₂)₂]₂ and neopentanol or tris(tert-butoxy)silanol gave catecholato-bridged dimers [(Ti(CAT)(OCH₂tBu)₂)(HNMe₂)]₂ and [Ti(CAT){OSi(OtBu)₃}₂(HNMe₂)₂]₂, respectively. Analogous reactions using the dimeric dimethylamido titanium(IV) (3,6-di-tert-butyl)catecholate [Ti(CATtBu₂-3,6)(NMe₂)₂]₂ yielded the monomeric Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂ and Ti(CATtBu₂-3,6)[OSi(OtBu)₃]₂(HNMe₂)₂. The neopentoxide complex Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂ engaged in further protonolysis reactions with Si-OH groups and was consequentially used for grafting onto mesoporous silica KIT-6. Upon immobilization, the surface complex [Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂]@[KIT-6] retained the bidentate chelating geometry of the catecholato ligand. This convergent grafting strategy was compared with a sequential and an aqueous approach, which gave either a mixture of bidentate chelating species with a bipodally anchored Ti(IV) center along with other physisorbed surface species or not clearly identifiable surface species. Extension of the convergent and aqueous approaches to anatase mesoporous titania (m-TiO₂) enabled optical and electronic investigations of the corresponding surface species, revealing that the band-gap reduction is more pronounced for the bidentate chelating species (convergent approach) than for that obtained via the aqueous approach. The applied methods include X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and solid-state UV/vis spectroscopy. The energy-level alignment for the surface species from the aqueous approach, calculated from experimental data, accounts for the well-known type II excitation mechanism, whereas the findings indicate a distinct excitation mechanism for the bidentate chelating surface species of the material [Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂]@[m-TiO₂].

Ethylene polymerization using heterogeneous multinuclear nickel catalysts supported by a crosslinked alpha diimine ligand network

A crosslinked alpha diimine ligand supporting a nickel metal center polymerizes ethylene to produce polyethylene with controlled microstructures, high activities, and can be removed from the product.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Small molecule binding to surface-supported single-site transition-metal reaction centres

Despite dominating industrial processes, heterogeneous catalysts remain challenging to characterize and control. This is largely attributable to the diversity of potentially active sites at the catalyst-reactant interface and the complex behaviour that can arise from interactions between active sites. Surface-supported, single-site molecular catalysts aim to bring together benefits of both heterogeneous and homogeneous catalysts, offering easy separability while exploiting molecular design of reactivity, though the presence of a surface is likely to influence reaction mechanisms. Here, we use metal-organic coordination to build reactive Fe-terpyridine sites on the Ag(111) surface and study their activity towards CO and C₂H₄ gaseous reactants using low-temperature ultrahigh-vacuum scanning tunnelling microscopy, scanning tunnelling spectroscopy, and atomic force microscopy supported by density-functional theory models. Using a site-by-site approach at low temperature to visualize the reaction pathway, we find that reactants bond to the Fe-tpy active sites via surface-bound intermediates, and investigate the role of the substrate in understanding and designing single-site catalysts on metallic supports.

Single-Site Heterogeneous Organometallic Ir Catalysts Embedded on Graphdiyne: Structural Manipulation Beyond the Carbon Support

Accurate control over the coordination circumstances of single-atom catalysts (SACs) is decisive to their intrinsic activity. Here, two single-site heterogeneous organometallic catalysts (SHOCs), Cp*Ir-L/GDY (L = OH^- and Cl^- ; Cp* = pentamethylcyclopentadienyl), with the fine-tuned local coordination and electronic structure of Ir sites, are constructed by anchoring Cp*Ir complexes on graphdiyne (GDY) matrix via a one-pot procedure. The spectroscopic studies and theoretical calculations indicate that the Ir atoms in Cp*Ir-Cl/GDY and Cp*Ir-OH/GDY have a much higher oxidation state than Ir in the SAC Ir/GDY. As a proof-of-principle demonstration, the GDY-supported SHOCs are used for formic acid dehydrogenation, which display a fivefold enhancement of catalytic activity compared with SAC Ir/GDY. The kinetic isotope effect and in situ Fourier-transform infrared studies reveal that the rate-limiting step is the β-hydride elimination process, and Cp* on the Ir site accelerates the β-hydride elimination reaction. The GDY-supported SHOCs integrate the merits of both SACs and molecular catalysts, wherein the isolated Ir anchored on GDY echoes with SACs' behavior, and the Cp* ligand enables precise structural and electronic regulation like molecular catalysts. The scheme of SHOCs adds a degree of freedom in accurate regulation of the local structure, the electronic property, and therefore the catalytic performance of single-atom catalysts.

Molecular Catalyst Synthesis Strategies to Prepare Atomically Dispersed Fe-N-C Heterogeneous Catalysts

We report a strategy to integrate atomically dispersed iron within a heterogeneous nitrogen-doped carbon (N-C) support, inspired by routes for metalation of molecular macrocyclic iron complexes. The N-C support, derived from pyrolysis of a ZIF-8 metal-organic framework, is metalated via solution-phase reaction with FeCl₂ and tributyl amine, as a Brønsted base, at 150 °C. Fe active sites are characterized by ⁵⁷Fe Mössbauer spectroscopy and aberration-corrected scanning transmission electron microscopy. The site density can be increased by selective removal of Zn²⁺ ions from the N-C support prior to metalation, resembling the transmetalation strategy commonly employed for the preparation of molecular Fe-macrocycles. The utility of this approach is validated by the higher catalytic rates (per total Fe) of these materials relative to established Fe-N-C catalysts, benchmarked using an aerobic oxidation reaction.

Pd single-atom-site stabilized by supported phosphomolybdic acid: design, characterizations and tandem Suzuki-Miyaura cross coupling/nitro hydrogenation reaction

Herein, a single-metal (Pd) site with high surface energy was stabilized and dispersed on a support (zirconia) via a stabilizing agent (phosphomolybdic acid) using a wet chemistry method. HRTEM and HAADF-STEM showed a highly uniform dispersion of Pd SASc on PMA/ZrO₂. The Pd SASc showed superior catalytic activity (>99% conversion) for the Suzuki-Miyaura cross-coupling reaction, which was further feasible for catalyzing mechanistically different nitro hydrogenation reactions in tandem fusion under mild reaction conditions. This catalyst showed outstanding activity (100% conversion and 99% selectivity) with a substrate/catalyst ratio of 927 and TON of 918 using a very low amount of Pd (0.94 × 10^-3 mmol) for the tandem Suzuki-Miyaura cross-coupling/nitro hydrogenation reaction. It also exhibited superior stability and reusability for up to three cycles without any change in its activity.

BCC-Cu nanoparticles: from a transient to a stable allotrope by tuning size and reaction conditions

Metallic copper generally adopts an FCC structure. In this work, we detect highly unusual BCC-structured Cu nanoparticles as a transient intermediate during the H₂ reduction of a Cu^I precursor, [Cu₄O^tBu₄], grafted onto the surface of partially dehydroxylated silica. The Cu BCC structure, assigned by in situ Cu K-edge XANES and EXAFS, as well as in situ synchrotron PXRD, converts upon heating into the most commonly found FCC allotrope. DFT calculations show that the BCC-Cu phase is in fact predicted to be more stable for small particles, and that their stability increases at lower H₂ concentrations. Using this knowledge, we show that it is possible to synthesize BCC-structured Cu nanoparticles as a stable allotrope by reduction of the same grafted precursor either in 10% H₂ diluted in Ar or 100% H₂ at low temperature.

S-Heptazine N-ligand based luminescent coordination materials: synthesis, structural and luminescent studies of lanthanide-cyamelurate networks

Various series of lanthanide metal-organic networks denoted Ln-Cy (Ln = La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), were synthesized under solvothermal conditions using potassium cyamelurate (K₃Cy) and lanthanide nitrate salts. All obtained materials were fully characterized, and their crystal structures were solved by single-crystal X-ray diffraction. Four types of coordination modes were elucidated for the Ln-Cy series with different Ln³⁺ coordination geometries. Structural studies were performed to compare the various coordination compounds of the Ln-Cy series. Moreover, the cyamelurate linkers of rich π-conjugated and uncoordinated Lewis basic sites were used as an absorbing chromophore to enhance the luminescence quantum efficiency, the band emission and the luminescence lifetime of the coordinated Ln metal centers. Solid-state UV-visible measurements combined with density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed to further explore luminescent features of the Ln-Cy series and their origins.

Molecular and Electronic Structure of Isolated Platinum Sites Enabled by the Expedient Measurement of 195Pt Chemical Shift Anisotropy

Techniques that can characterize the molecular structures of dilute surface species are required to facilitate the rational synthesis and improvement of Pt-based heterogeneous catalysts. ¹⁹⁵Pt solid-state NMR spectroscopy could be an ideal tool for this task because ¹⁹⁵Pt isotropic chemical shifts and chemical shift anisotropy (CSA) are highly sensitive probes of the local chemical environment and electronic structure. However, the characterization of Pt surface-sites is complicated by the typical low Pt loadings that are between 0.2 and 5 wt% and broadening of ¹⁹⁵Pt solid-state NMR spectra by CSA. Here, we introduce a set of solid-state NMR methods that exploit fast MAS and indirect detection using a sensitive spy nucleus (¹H or ³¹P) to enable the rapid acquisition of ¹⁹⁵Pt MAS NMR spectra. We demonstrate that high-resolution wideline ¹⁹⁵Pt MAS NMR spectra can be acquired in minutes to a few hours for a series of molecular and single-site Pt species grafted on silica with Pt loading of only 3-5 wt%. Low-power, long-duration, sideband-selective excitation, and saturation pulses are incorporated into t₁-noise eliminated dipolar heteronuclear multiple quantum coherence, perfect echo resonance echo saturation pulse double resonance, or J-resolved pulse sequences. The complete ¹⁹⁵Pt MAS NMR spectrum is then reconstructed by recording a series of 1D NMR spectra where the offset of the ¹⁹⁵Pt pulses is varied in increments of the MAS frequency. Analysis of the ¹⁹⁵Pt MAS NMR spectra yields the ¹⁹⁵Pt chemical shift tensor parameters. Zeroth order approximation density functional theory calculations accurately predict ¹⁹⁵Pt CS tensor parameters. Simple and predictive orbital models relate the CS tensor parameters to the Pt electronic structure and coordination environment. The methodology developed here paves the way for the detailed structural and electronic analysis of dilute platinum surface-sites.

A Robust and Efficient Propane Dehydrogenation Catalyst from Unexpectedly Segregated Pt2Mn Nanoparticles

The increasing demand for short chain olefins like propene for plastics production and the availability of shale gas make the development of highly performing propane dehydrogenation (PDH) catalysts, robust toward industrially applied harsh regeneration conditions, a highly important field of research. A combination of surface organometallic chemistry and thermolytic molecular precursor approach was used to prepare a nanometric, bimetallic Pt-Mn material (3 wt % Pt, 1.3 wt % Mn) supported on silica via consecutive grafting of a Mn and Pt precursor on surface OH groups present on the support surface, followed by a treatment under a H₂ flow at high temperature. The material exhibits a 70% fraction of the overall Mn as Mn^II single sites on the support surface; the remaining Mn is incorporated in segregated Pt₂Mn nanoparticles. The material shows great performance in PDH reaction with a low deactivation rate. In particular, it shows outstanding robustness during repeated regeneration cycles, with conversion and selectivity stabilizing at ca. 37 and 98%, respectively. Notably, a material with a lower Pt loading of only 0.05 wt % shows an outstanding catalytic performance─initial productivity of 4523 g_C3H6/g_Pt h and an extremely low k_d of 0.003 h^-1 under a partial pressure of H₂, which are among the highest reported productivities. A combined in situ X-ray absorption spectroscopy, scanning transmission electron microscopy, electron paramagnetic resonance, and metadynamics at the density functional theory level study could show that the strong interaction between the Mn^II-decorated support and the unexpectedly segregated Pt₂Mn particles is most likely responsible for the outstanding performance of the investigated materials.

A Heterogeneous Iridium Catalyst for the Hydroboration of Pyridines

Sulfated zirconium oxide (SZO) capped with silylium-like ions reacts with (cod)Ir(py)Cl (cod = 1,5-cyclooctadiene; py = pyridine) to form [Ir(cod)py][SZO] (1) and Me₃SiCl. 1 can also be formed in reactions of phosphonium functionalized SZO and [Ir(cod)(OSi(O^tBu)₃]₂, which forms [Ir(cod)P(^tBu)₂Ph][SZO] (2), followed by reaction with pyridine to form 1. FTIR and ¹⁵N{¹H} MAS NMR spectroscopy are consistent with coordination of pyridine in 1 to an electrophilic iridium. 1 is moderately active in the dearomative hydroboration of pyridine. The primary product of this reaction is 1,2-dihydropyridine, which converts to the 1,4-dihydropyridine product at long reaction times. 1 catalyzes the dearomative hydroboration of a variety of substituted pyridines and is also reactive toward pyrazines and N-methylimidazole.

Multiple Surface Site Three-Dimensional Structure Determination of a Supported Molecular Catalyst

The structural characterization of supported molecular catalysts is challenging due to the low density of active sites and the presence of several organic/organometallic surface groups resulting from the often complex surface chemistry associated with support functionalization. Here, we provide a complete atomic-scale description of all surface sites in an N-heterocyclic carbene based on iridium and supported on silica, at all stages of its synthesis. By combining a suitable isotope labeling strategy with the implementation of multinuclear dipolar recoupling DNP-enhanced NMR experiments, the 3D structure of the Ir-NHC sites, as well as that of the synthesis intermediates were determined. As a significant fraction of parent surface fragments does not react during the multistep synthesis, site-selective experiments were implemented to specifically probe proximities between the organometallic groups and the solid support. The NMR-derived structure of the iridium sites points to a well-defined conformation. By interpreting EXAFS spectroscopy and chemical analysis data augmented by computational studies, the presence of two coordination geometries is demonstrated: Ir-NHC fragments coordinated by a 1,5-cyclooctadiene and one Cl ligand, as well as, more surprisingly, a fragment coordinated by two NHC and two Cl ligands. This study demonstrates a unique methodology to disclose individual surface structures in complex, multisite environments, a long-standing challenge in the field of heterogeneous/supported catalysts, while revealing new, unexpected structural features of metallo-NHC-supported substrates. It also highlights the potentially large diversity of surface sites present in functional materials prepared by surface chemistry, an essential knowledge to design materials with improved performances.

Structure and Reactivity of Polynuclear Divalent Lanthanide Disiloxanediolate Complexes

Trinuclear molecular complexes of europium (II) and ytterbium(II) [Ln₃{(Ph₂SiO)₂O}₃(THF)₆], 1-Ln₃L₃ (Ln = Eu and Yb), supported by the dianionic tetraphenyl disiloxanediolate ligand, were synthesized via protonolysis of the [Ln{N(SiMe₃)₂}₂(THF)₂] complexes. In contrast, the reaction of [Sm{N(SiMe₃)₂}₂(THF)₂] with the (Ph₂SiOH)₂O ligand led to the isolation of the mixed-valent Sm(II)/Sm(III) complex [Sm₃{(Ph₂SiO)₂O}₃{N(SiMe₃)₂}(THF)₄], 2-Sm₃L₃, which was crystallographically characterized. The Eu(II) complex 1-Eu₃L₃ displays weak ferromagnetic coupling between the Eu(II) metal centers (J = 0.1035 cm^-1). The addition of 3 equiv of (Ph₂SiOK)₂O to 1-Eu₃L₃ resulted in the formation of the polynuclear Eu(II) dimer of dimers [K₄Eu₂{(Ph₂SiO)₂O}₄(Et₂O)₂]₂, 3-Eu₂L₄. Complexes 1-Ln₃L₃ (Ln = Eu and Yb) are stable in solution at room temperature, while 3-Eu₂L₄ shows higher reactivity and rapidly decomposes to give the mixed-valent Eu(II)/Eu(III) species [K₃Eu₂{(Ph₂SiO)₂O}₄], 4-Eu₂L₄. Complex 1-Yb₃L₃ affects the slow reductive disproportionation of carbon dioxide, but 1-Eu₃L₃ does not display any reactivity toward CO₂. However, the presence of one additional (Ph₂SiO^-)₂O per Eu(II) metal center in 3-Eu₂L₄ increases dramatically the reductive ability of the Eu(II) metal centers, affording the first example of carbon dioxide activation by an isolated divalent europium complex. The reduction of CO₂ by 3-Eu₂L₄ is immediate, and carbonate is formed selectively after the addition of a stoichiometric amount of CO_2.

Alkane C-H activation and ligand exchange on silica supported d0 metal alkylidenes: relevance to alkane metathesis

In this work, we study the ligand exchange process between an alkane and a series of silica supported metal alkylidenes, which may occur by different pathways: C-H addition, σ-bond metathesis, and α-H abstraction. The results indicate that the α-H abstraction pathway is the preferred one, regardless of the catalyst and ligands. This is in contrast to the expected preference for the C-H addition route. When looking for the origin of this preference, our calculations revealed that the α-H abstraction pathway is driven by entropy, which favors the initial dissociation of the alkyl ligand from the catalyst.