Surface Organometallic and Coordination Chemistry toward Single

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Surface Organometallic and Coordination Chemistry toward SingleSite Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities Christophe Copéret,*,† Aleix Comas-Vives,† Matthew P. Conley,† Deven P. Estes,† Alexey Fedorov,† Victor Mougel,† Haruki Nagae,†,‡ Francisco Núñez-Zarur,† and Pavel A. Zhizhko†,§ †

Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1−5, CH-8093 Zürich, Switzerland Department of Chemistry, Graduate School of Engineering Science, Osaka University, CREST, Toyonaka, Osaka 560-8531, Japan § A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str. 28, 119991 Moscow, Russia ‡

S Supporting Information *

3.4. Stoichiometric Reactivity and Activation of Small Molecules on Well-Defined Surface Sites 3.4.1. Stoichiometric Ligand Exchange via Protonolysis Reactions with Brønsted Acids 3.4.2. Reactions with Lewis Bases 3.4.3. Coordination and Activation of N2 3.4.4. Activation of O2 and N2O 3.4.5. Activation of CO2 3.4.6. Stoichiometric Reactions with Ketones 3.4.7. Stoichiometric Reactions with Alkanes 4. Catalytic Activity and Applications 4.1. Hydrogenation of Alkenes, Alkynes, and Arenes 4.2. Metathesis of Alkenes and Alkynes 4.2.1. Alkene Metathesis Using Group 6−7 Silica-Supported Catalysts 4.2.2. Alkene Metathesis Using Silica-Supported Ru Alkylidenes 4.2.3. Alkene Metathesis with Re and W on Alumina-Containing Supports 4.2.4. Alkyne Metathesis 4.3. Oxo/Imido Heterometathesis 4.4. Oligomerization of Alkenes 4.4.1. Ethylene Dimerization and Trimerization 4.5. Direct Conversion of Ethylene to Propylene 4.6. Polymerization Reactions 4.6.1. Single-Site Models of Ziegler−Natta Type Catalysts 4.6.2. Single-Site Models for the Phillips Catalyst 4.6.3. Supported Late Transition Metal Catalysts for the Polymerization of Ethylene 4.6.5. Polymerization of Other Substrates 4.7. Alkane Homologation Processes and Related Reactions 4.7.1. Alkane Hydrogenolysis 4.7.2. Alkane Metathesis

CONTENTS 1. Introduction 2. Concepts, Definitions, Tools, and Practical Considerations 2.1. Concepts and Definitions 2.2. Toolbox of Characterization Methods 2.3. Practical Aspects 2.3.1. Preparation and Characterization of the Supports (Calcination, Dehydroxylation, and Passivation) 2.3.2. Grafting 2.3.3. Storage and Handling 3. Description of the Surface Chemistry on Oxides 3.1. General Concepts, Strategy, and Methods 3.2. Structure of Surface Species Resulting from Grafting of the Molecular Precursors 3.2.1. Silica-Supported Well-Defined Metal Sites 3.2.2. Alumina-Supported Metal Complexes 3.2.3. Silica−Alumina Supported Metal Complexes 3.2.4. Other Supports 3.3. Evolution of Well-Defined Surface Sites upon Post-Treatment and/or Functionalization 3.3.1. Calcination 3.3.2. Thermolysis under Vacuum or Inert Gas 3.3.3. Thermal Treatment under H2 3.3.4. Thermal Treatment under H2S © 2016 American Chemical Society

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Received: June 26, 2015 Published: January 7, 2016 323

DOI: 10.1021/acs.chemrev.5b00373 Chem. Rev. 2016, 116, 323−421

Chemical Reviews 4.7.3. Cross-Metathesis of Methane and Higher Alkanes 4.7.4. Nonoxidative Methane Coupling 4.8. Dehydrogenation of Alkanes 4.9. Epoxidation of Alkenes 4.10. Oxidation and Deperoxidation of Alkanes 4.11. Other Selected Catalytic Transformations 4.11.1. Hydroamination 4.11.2. Hydrosilylation 4.11.3. Other Reactions 5. Emerging Fields 5.1. Well-Defined Catalysts through Grafting on Advanced Functional Materials 5.2. Molecular Approach Beyond Well-Defined Heterogeneous Catalysts 6. Conclusions Associated Content Supporting Information Author Information Corresponding Author Notes Biographies Acknowledgments Dedication Abbreviation List References Note Added after ASAP Publication

Review

toward the generation of heterogeneous catalysts with welldefined active sites. One powerful approach, known as surface organometallic chemistry (SOMC),10−23 treats the surface of the catalyst support as a ligand and aims at controlling its reactivity toward molecular precursors, such as organometallic or coordination complexes. The grafted molecular complexes can be used directly in catalysis or transformed after grafting by a thermal post-treatment under vacuum, inert or reactive gas, or modified by a postreaction to incorporate new ancillary ligands. This approach permits well-defined surface species to be prepared and fully characterized with techniques complementary to those of solution chemistry. SOMC has been employed since the 1970s to generate single-site catalysts, supportedmetal nanoparticle catalysts, and even extended structures (e.g., WS2) from grafted species.14,24−32 The main advantage of SOMC lies in its ability to generate surface sites with a known coordination sphere, thus facilitating structure−activity relationship studies and rational design of heterogeneous catalysts. Research in SOMC has gained momentum in the past 20 years, spurred by the need to improve existing catalysts and the advances in spectroscopic and computational methods, which allow very detailed structural characterization, an essential step toward catalyst development through structure−activity relationship approach. Several reviews and books on SOMC have been published in the past 15 years.4,10−13,15−23,33−35 However, many new contributions have been reported in the past decade, creating a need for a comprehensive review covering these recent advances. Herein, we present the details of the development of SOMC. In particular, we describe the synthesis, characterization, and catalytic activity of well-defined metal species on the surface of oxide supports, discussing both strategies and methods. We concentrate the review on transition metal and lanthanide species because of their importance in catalytic processes; focusing mainly on data that appeared after 2000. Some main group element surface chemistry will be discussed in the context of support modification because of their use as solid cocatalyst for transition-metal complexes. Zeolites, which have different reactivity due to their microporosity, and supported metal clusters were recently reviewed and are not included.19,20,36−38 This review is divided into four separate sections, each discussing a different aspect of SOMC. In section 2, we introduce the concepts and offer important practical guidelines to chemists who would like to enter the field. In section 3, we provide a comprehensive list of molecular precursors used and the corresponding surface species formed upon their grafting. Section 3 is divided into subsections, each discussing a particular support type. Within each subsection in section 3, we classified molecular precursors based on the nature of the ligand set and the metal. We end section 3 with a discussion of the effect of post-treatment on the structure of surface species. In section 4, we discuss the catalytic properties of the aforementioned surface species in reactions such as hydrogenation, dehydrogenation, polymerization, metathesis, alkane conversion, and oxidation. In section 5, we give an outlook for the field with the appearance of novel functional materials and possible applications beyond the current scope of research activities.

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1. INTRODUCTION The chemical industry relies on catalytic processes, which involve mainly heterogeneous catalysts because they offer many advantages over their homogeneous counterparts such as easier separation of catalysts from reaction products, their applicability to continuous flow processes, and their recyclability (often by simply heating in the presence of air or H2).1,2 However, homogeneous catalysts display several advantages over heterogeneous catalysts. For example, homogeneous catalysts are often active at lower temperatures, have higher selectivities, and are chemically better defined; all active sites have ideally a known and uniform structure. In particular, the well-defined nature of their active sites, or at least of the precatalyst structures, allows for more rational catalyst development through structure−activity relationships. Heterogeneous catalysts are usually prepared via the dispersion of a metal salt/precursor onto a support. These materials are typically heated under a gas stream, e.g., air, H2, ..., giving dispersed metal ions with a variable number of M−O bonds between the support and the metal sites or supported metal particles. Such catalysts contain a broad distribution of metal coordination environments, and often only a small fraction of those are active in catalysis. These properties make characterization at the molecular level difficult, if not impossible, thereby preventing rational catalyst development. Thus, the active sites of many heterogeneous catalysts are matters of intense debate, e.g., the Phillips ethylene polymerization catalyst, Ziegler−Natta olefin polymerization catalyst, Catofin propane dehydrogenation catalyst, tungsten oxide and rhenium oxide olefin metathesis catalysts, propylene epoxidation catalyst, and vanadyl catalyst for oxidative propane dehydrogenation, to name but a few.3−9 To combine the advantages of both homogeneous and heterogeneous catalysis, research efforts have been directed 324


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Scheme 1. Dehydroxylation of a metal oxide surface and grafting LnMXx onto MS−OH

Table 1. Methods of Characterization and Chemical Information on Surface Sites method N2 adsorption transmission electron microscopy (TEM) and EDX X-ray diffraction elemental analysis (EA)

mass balance analysis (MB) IR spectroscopy

Raman spectroscopy UV−vis XPS, Auger spectroscopy Mössbauer spectroscopy XANES EXAFS EPR solid-state NMR

homogeneous molecular models computational chemistry

examples of information provided by each method

refs

surface area, porosity morphology−aggregation state−composition and distribution of atoms of the solid matrix

1 1

specific polymorph of the support and level of crystallinity metal and other elements loadings average composition of surface complexes stoichiometry of surface functionalization establishing the stoichiometry of grafting monitoring of the consumption of reactant and formation of released product evidence for the disappearance and/or appearance of surface functionalities, e.g. consumption of surface OH groups and appearance of specific surface sites (C−H, N−H, CC, M−H vibrations) quantification of acidic and basic sites by using basic probe molecules (CO, pyridine, MeCN) characterization of surface oxo and related species evaluation of the isolation or agglomeration of surface species metal oxidation state oxidation state and local geometry for specific elements such as Sn or Fe metal oxidation state and geometry of surface complexes coordination sphere of the metal: number, type and bond distance of atoms (ligands) bound to the metal sites characterization and titration of paramagnetic species detailed molecular-level structure types of ligands bound to the surface sites (usually via 1H, 13C, 15N, 31P NMR) reaction intermediates using labeled reactants measurement of acidity (phosphine oxide and pyridine adsorption) with 15N and 31P NMR discrimination of surface atoms in oxides (e.g., Al2O3, SiO2, etc. with 27Al and 29Si NMR) structural information on isoelectronic and structural molecular species (bond distances by X-ray and spectroscopic signatures) evaluation of structure stability prediction of spectroscopic signatures evaluation of reaction pathways

1

2. CONCEPTS, DEFINITIONS, TOOLS, AND PRACTICAL CONSIDERATIONS

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6,45,47,52,53 52−58 1 1 1,59,60 1,59,60 1,58 61−76

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analysis, and/or mass spectrometry, to name a few common techniques. Though these methods obviously cannot apply to well-defined heterogeneous catalysts, there are several spectroscopic tools that give molecular level information about the catalytically active site.

2.1. Concepts and Definitions

An important step to synthesize heterogeneous catalysts using a molecular strategy is the understanding of the reactivity of a support surface toward reactive inorganic species. Metal oxide supports are terminated by surface MS−OH and MS−O−MS groups. The types and densities of these groups can be modified by thermal treatment under vacuum. For example, treating silica at 700 °C under vacuum (10−5 mbar) forms SiO2−700 that contains ca. 0.8 SiOH nm−2.34,40−42 These surfaces react with inorganic complexes in a more predictable fashion than fully hydroxylated surfaces, as shown in Scheme 1. With a sufficiently reactive X anionic ligand, a generic metal complex LnMXx reacts with MS−OH to form MS−O−MLnXx−1 and HX. MS−O−MLnXx−1 can also interact or react with adjacent MS−O−MS or Lewis acid sites. MS−O−MLnXx−1 sites are often referred to as well-defined or single-sites. In molecular chemistry extensive characterization is required in order to establish a structure. This is often achieved by single-crystal X-ray diffraction, solution spectroscopy, elemental

2.2. Toolbox of Characterization Methods

A combination of techniques that describes the support and also addresses the molecular structure of the surface site is necessary to give a comprehensive understanding of welldefined heterogeneous catalysts (Table 1). Powder X-ray diffraction, microscopy, and gas absorption give information about the bulk structure, morphology, and surface area of the support. If grafting occurs on MS−OH sites, titration of these groups, which will be described below, can define the theoretical maximum loading/coverage of well-defined sites onto the support. Elemental analysis (EA) and mass balance (MB) give the stoichiometry of the reaction shown in Scheme 1. Spectroscopy provides information about the metal sites and the local environment in sufficient detail to often assign a structure to a well-defined site. Vibrational spectroscopies (IR, 325


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Figure 1. (a) Compacting and sieving the support. (b) Calcination of the support in static (left) or flow conditions (right). (c) Rehydroxylation of the support. (d) Dehydroxylation of the support (for Tdehydroxylation > 500 °C quartz reactors are necessary). (e) Titration of surface hydroxyls using MeMgBr solution in Et2O and GC quantification of released methane.

fine structure spectroscopy (EXAFS) provide information about the oxidation state, the geometry, and the distances between the metal and close neighbors. Though these methods can be quite powerful, they only provide average structural information when several sites are present.59,60 In some cases, EPR, XPS, Auger, and Mössbauer spectroscopies can also provide complementary information about the oxidation state and local symmetry of M−OMLn.1 In the past 20 years solid-state NMR spectroscopy (SSNMR) has emerged as one of the most powerful characterization tools for well-defined heterogeneous catalysts because it provides the most detailed information on the molecular structure and dynamics of surface sites.61,69,76 However, solid-state NMR typically suffers from an intrinsic lack of sensitivity that is exacerbated in well-defined heterogeneous catalysts because the surface sites are only a small fraction of the samples (few wt %).

Raman, resonance Raman) are critical to understand the chemistry occurring at the surface.6,39,47−49 For example, monitoring a grafting reaction by IR spectroscopy shows the consumption or interaction of MS−OH with LnM−X. IR spectroscopy can be used either in transmission mode using a self-supporting disk or in diffuse reflectance (DRIFT) mode directly on powdered samples. Raman spectroscopy is particularly useful for metal oxo surface species or surface (M−O)n rings.45,86,87 Vibrational spectroscopy also gives specific signatures when M−O−MLn is put into contact with probe molecules such as CO, CO 2 , or pyridine.43−46,50,51,58 Diffuse reflectance spectroscopy in UV and visible regions (DRUV−vis) is used to distinguish isolated sites from aggregates and to monitor active sites containing adsorbing/emitting ligand.8,52−57,88 X-ray adsorption near edge spectroscopy (XANES) and extended X-ray adsorption 326


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In addition, the surface sites are often present as a distribution of species due to different local environments, which leads to significant line broadening. Major advancements in signal enhancements have recently been achieved with application of polarization techniques, in particular dynamic nuclear polarization (DNP). In DNP an exogenous (organic) radical is added to the sample in a solution that is then frozen at 100 K. Irradiation of the EPR transition of the radical at this temperature at the resonant NMR frequency of the experiment leads to polarization transfer from the radical to the solvent.89,90 Cross-polarization from the solvent to the surface nuclei results in very large signal enhancements. This technique is referred to as “Surface Enhanced NMR spectroscopy” (DNPSENS).65,67,68,75,91 DNP-SENS decreases NMR experimental time by several orders of magnitude, currently up to ca. 250 000.92−94 This technique has readily been applied to a broad range of nuclei to characterize surfaces, e.g., 13C, 15N, 17 O, 27Al, 29Si, and 119Sn. Spectroscopy of surface species can also be accompanied by the preparation of soluble molecular analogues. These models are more easily characterized by solution techniques, and their spectroscopic signatures can be compared to those of surface species. Crystallographic parameters of molecular complexes can be used for building models for the interpretation of XAS data of the supported species. This approach is particularly powerful with silica-supported systems since numerous molecular analogues are available (see section 3.2.1.1). Computational chemistry is a complementary tool to obtain energetic profiles that can relate to structural preferences and particular spectroscopic signatures of surface species that may relate to catalytic activity. Computational methods can confirm or rule out possible structures and allow a deeper understanding of the relation between structure and reactivity.83−85 Taken together, the information obtained by combining complementary techniques in the characterization toolbox allows for an atomic-level description of surface sites. This is an essential step toward building structure−activity relationships that are at the heart of rational catalyst design.

of primary particles and surface properties of the solid according to N2 physisorption and microscopy. Metallic utensils such as spatula should be avoided at all times to prevent adventitious metal contamination. The support is then subjected to calcination conditions, heating to ca. 500 °C in air to remove organic impurities. Calcination can be carried out under static conditions in air for small quantities of supports or under flow conditions using synthetic air. Both static and flow calcination setups are shown in Figure 1b. If the calcination temperature is higher than the desired dehydroxylation temperature, a rehydroxylation step is needed. The calcined sample is exposed to a vapor pressure of degassed distilled/deionized water at room temperature and further heated to the desired temperature, typically 150−200 °C, to promote rehydroxylation. The rehydroxylated support is now ready for dehydroxylation. An experimental setup for this step is shown in Figure 1c. The dehydroxylation temperature determines the density of surface hydroxyl groups.40 A typical dehydroxylation setup uses a reactor tube that is 4.5 cm in diameter and ca. 50 cm in length equipped with a tap of large aperture, as shown in Figure 1d. In this setup the quantity of support should not exceed 5−10 cm3. Using larger amounts can result in partial dehydroxylation leading to higher than expected hydroxyl coverage. This is probably due to problems of heat transfer or pressure drop when large quantities of supports are used. The temperature ramp used to reach the dehydroxylation temperature should typically not exceed 5 °C min−1 to prevent sintering. This is particularly important for mesoporous supports that can undergo pore collapse. After the dehydroxylation temperature is reached for an appropriate amount of time (4−12 h), the reactor is cooled to room temperature under vacuum. Dehydroxylation can preferably be performed horizontally for better heat transfer. IR, 34,40 thermogravimetric analyses, 95,96 solid state NMR,67,70,73 deuterium exchange,40 titrations with organometallic reagents, 34,97−99 surface silylation followed by elemental analysis,100 and other methods were developed to identify the type and quantity of surface hydroxyl groups on metal oxides. Precise quantification can be performed by the reaction of a partially dehydroxylated oxide with a Grignard reagent and quantification of the protonated product by GC or solution NMR. The quantification of CH4 is a three-step procedure shown in Figure 1e. A large excess of MeMgBr in solution in Et2O is added to a known quantity of the support (typically 1 g) in an evacuated Schlenk flask and maintained at room temperature for 1 h. The liquid phase is vacuum transferred to a large (ca. 10 L) evacuated round-bottom flask. The large size is chosen to allow quantitative transfer of all contents from the Schlenk flask to a stable gas phase below the vapor pressure of the solvent. Quantification of CH4 by GC gives the OH coverage. Alternatively, the reaction of Mg(CH2Ph)2 with the oxide in C6D6 releases toluene that can be quantified in the presence of a ferrocene as internal standard.98,99 2.3.2. Grafting. Grafting a metal precursor onto oxide supports requires strict anhydrous and anaerobic conditions. This is due to the general sensitivity of many metal complexes to air and water in solution and the increased sensitivity of surface species supported on high surface area oxides. A basic set of glassware designed to carry out grafting in inert conditions using either a glovebox or a Schlenk line is shown in Figure 2. If the molecular precursor and solvents are

2.3. Practical Aspects

Specific glassware and procedures based on Schlenk and highvacuum techniques have been developed throughout the years to allow for easy and reproducible syntheses. These procedures require high vacuum (10−5 mbar), heating, and pristine storage capabilities. We will present an overview of relevant experimental setups and processes. 2.3.1. Preparation and Characterization of the Supports (Calcination, Dehydroxylation, and Passivation). Supports used in SOMC can be prepared and characterized using similar procedures. It is important to evaluate the structure of the support after each step of the preparation process because some oxides can readily change phase or sinter in the presence of water when heated. Oxides are typically purchased or prepared by sol−gel or alternative methods as fine powders. A trick of the trade is to agglomerate the fine powder into larger, easier to filter, particles by wetting with water followed by slow evaporation at 120 °C in an oven solely used for inorganic solids. For silica and alumina, the typical compaction steps are summarized in Figure 1a. A slurry of the oxide powder in distilled water is dried at ca. 120 °C for several days in an oven, resulting in the formation of large agglomerates. These agglomerates are sieved to select particles of 250−400 μm in size. This treatment does not change the size 327


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Figure 2. (a) Grafting in a Schlenk flask. (b) Grafting in a double-Schlenk flask. (c) Grafting by sublimation of the precursor using break-seal technique.

the two arms of the flask, as shown in Figure 2b. A solution of the organometallic reagent is placed in one side of the doubleSchlenk while the support is added to the other side. The solution is degassed by a freeze−pump−thaw cycle, added to the support through the filter, and the resulting suspension is slowly stirred. After the reaction, the supernatant is filtered into the other arm of the double-Schlenk. Avoiding the solid to go in contact with the glass filter is advised. To wash the functionalized oxide the solvent is transferred back into the other chamber of the double-Schlenk via vacuum distillation. The filtrate is then separated from the solids as in the previous step. This washing step is repeated 2−3 times depending on the solubility of the reactant and byproducts. The filtrate is collected and the product of the grafting reaction is quantified by solution NMR using an internal standard. Volatile molecular precursors can be grafted in the gas phase by sublimation of the compound onto the support using a break-seal tube connected to a reactor, shown in Figure 2c. In this case the excess reagent is sublimed back in the side tube, which can be sealed using a flame. For preliminary studies the steps described above can be performed on pressed pellet (self-supporting disk) for direct monitoring of the surface chemistry by IR spectroscopy as

compatible with their use in glovebox, grafting can be conveniently carried out in a Schlenk flask (Figure 2a). In this case, a solution of the molecular precursor is added to a slurry of the support in the same solvent. Stirring should be performed carefully since vigorous stirring tends to break apart the agglomerates to form a very fine powder. While a 3 h grafting time is typically sufficient to complete the reaction, it is best to monitor the consumption of surface hydroxyls by spectroscopic methods such as IR. After the reaction the supernatant is filtered away from the solid. The solid is washed with fresh solvent and the supernatant again removed. This rinsing step is repeated 2−3 times to remove any unreacted molecular precursor and the product of the grafting reaction (HX in Scheme 1). The combined filtrates contain the coproduct of the grafting reaction that is typically quantified by solution NMR using an internal standard. The functionalized oxide is dried on a high vacuum line prior to long-term storage in a glovebox. As mentioned above, the quantity and type of product(s) evolved during grafting, metal loading, and C, H, N elemental analyses give the stoichiometry of the grafting reaction (often referred to as mass balance analysis). Outside of a glovebox the grafting reaction is carried out in a double-Schlenk flask containing a sintered glass filter between 328


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Figure 3. Calcination, dehydroxylation and grafting on a pressed pellet of a support.

the, now MS−OH depleted, oxide surface. The reaction of the site-isolated MS−OH with molecular precursors forms welldefined surface species, which can then be post-treated to modify their environment. Below we describe this approach to form well-defined surface species on partially dehydroxylated silica (section 3.2.1), alumina (section 3.2.2), and silica− alumina (section 3.2.3). Section 3.2.4 discusses grafting on other oxide supports. In section 3.3 we describe post-treatment strategies to modify grafted species in order to probe surface site structure or to generate new surface sites.

shown in Figure 3. After pressing a sample of the support in air the pellet is placed in a holder designed to slide up and down a reactor. The head of the reactor contains IR transparent windows (CaF2, KBr, ZnSe, etc.) that allow acquisition of the spectra. The calcination and dehydroxylation are carried out in the reactor. After dehydroxylation the reactor is taken into a glovebox, and the pellet is submerged into a solution containing the molecular complex. Washing is accomplished by submerging the pellet in fresh solvent. If the reagent is volatile it can also be distilled/sublimed onto the pellet. 2.3.3. Storage and Handling. Because of their dispersion on a high surface area supports, most grafted complexes show an increased sensitivity to contaminants by comparison to their homogeneous analogues. For that reason, supported materials are best stored in ground glass storage tubes kept in solventfree gloveboxes.

3.2. Structure of Surface Species Resulting from Grafting of the Molecular Precursors

3.2.1. Silica-Supported Well-Defined Metal Sites. 3.2.1.1. General Considerations on Silica. Silica is typically an amorphous material with relatively high surface areas between 50 and 1000 m2 g−1.34,40,42 Quartz and cristobalite are crystalline, but the very low surface area of these materials (2.6 m2 g−1 and 2.1 m2 g−1, respectively)101 limits the number of surface sites per unit mass that restricts their application as supports for heterogeneous catalysts. The texture and shape of amorphous silica depends mainly on the preparation method. Flame pyrolysis of SiCl4 and H2/O2 above 1000 °C results in high purity, nonporous silica with moderate to large surface area (50−380 m2 g−1).102−104 Mesostructured materials such as MCM41 and SBA15 are prepared using sol−gel methods in the presence of a surfactant as a template to form large and ordered mesopores.105 These materials have surface areas up to 1000 m2 g−1. Note that treatment of mesoporous materials at high temperatures, above 500−700 °C, may result in the collapse of the porous network.42 The bulk of silica is composed of tetrahedral SiO4 units connected to each other to form siloxane rings of various sizes

3. DESCRIPTION OF THE SURFACE CHEMISTRY ON OXIDES 3.1. General Concepts, Strategy, and Methods

To restate, the key concept in SOMC is treating oxide surfaces as an isolated X-type anionic ligand as shown in Scheme 1. The controlled structure of the surface species, deduced from spectroscopic studies, and catalytic behavior allow building a structure−activity relationship and thereby more rational development of heterogeneous catalysts. Oxide supports are terminated by MS−O−MS, MS−OH, and in some cases MS− OH2 functionalities. Thermal treatment removes adsorbed water and condenses nearby MS−OH sites to form more water and MS−O−MS on the oxide surface. This process reduces the overall density of MS−OH groups that can be functionalized on 329


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Figure 4. (a) Types of silanols. (b) Infrared spectra of Aerosil-200 partially dehydroxylated at 200 and 700 °C (SiO2−200 and SiO2−700). (c) Siloxane rings. (d) Effect of temperature on surface OH density.

ranging from flexible 12-membered rings to strained 4membered rings (Figure 4). The silica surface exposes siloxane rings/bridges and is also terminated with different types of silanols (Figure 4a). Geminal and isolated silanols are present even at very high thermal treatment temperatures (>700 °C) and are readily observable using IR spectroscopy.34,106 Vicinal silanols are two silanol sites in close proximity on the surface that form hydrogen bonds with one another.40,42 Vicinal silanols appear in the IR spectrum as a broad red-shifted band centered at 3650 cm−1. Treating silica at temperatures above 150 °C under vacuum or in a stream of (inert) gas results in desorption of physisorbed water. Above this temperature vicinal silanols begin to condense to form siloxane bridges liberating more water. Using fumed Aerosil-200 (200 m2 g−1) as an example, the decrease of surface silanol concentration is nearly exponential with increasing temperature (Figure 4d). The  Si−OH density decreases from ca. 3 OH nm−2 to ca. 0.8 OH nm−2 for Aerosil-200 partially dehydroxylated at 200 and 700 °C. In silica partially dehydroxylated at 700 °C (SiO2−700) mostly isolated silanols are present, but 5−10% of the silanols are also geminal.70 As the dehydroxylation temperature increases more, strained siloxane bridges, such as 4-membered ring siloxanes, are formed. Cluster and periodic models of the silica surface have been reported.107 The cluster approach has been used extensively to model isolated geminal and vicinal silanols, and the reactivity of these sites with organometallic complexes. Siloxane cages with different Si−O rings sizes are of common use in the modeling of silica surfaces. Figure 5a shows typical siloxane cages used in the modeling of isolated silanols, whereas Figure 5b shows a typical model for a geminal silanol. Several periodic models have been also proposed for the silica surface. A periodic silica surface containing 26 SiO2 units was cleaved from an amorphous silica bulk structure108 and terminated with OH group,109 the final surface containing 5.8

Figure 5. (a) Typical cluster models of SiO2 for isolated silanols with different ring cages and (b) cluster model of a geminal silanol. (c) Top and side views of the amorphous model with OH coverage equal to 1.5 OH nm−2 from ref 110. (d) A crystalline models based on the (111) surface of β-cristoballite with OH coverage equal to 1.4 OH nm−2 from ref 112.

Si−OH nm−2. An alternative amorphous silica surface model with a Si−OH nm−2 coverage of 7.2 OH nm−2 was also constructed by annealing the structure of bulk β-cristoballite at very high temperatures using molecular dynamics.110 Removal of water molecules by condensing adjacent silanol pairs gave a surface containing silanol densities as low as 1.5 OH nm−2. The simulated IR spectrum of the model surface containing 4.5 OH nm−2 was in good agreement with experimental spectra. Other periodic models based on β-cristobalite were also reported, which closely match experimental data.111,112 A completely dehydroxylated periodic model originating from β-cristobalite contains strained 4-membered Si−O rings in the surface. These rings can be observed in the IR transparent window around 900 cm−1.113 330


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3.2.1.2. Grafting Molecular Precursors on Silica and Resulting Surface Species. Generalities. SiO2−700 contains mostly isolated silanols and low amount of strained siloxane bridges70 and is ideal for obtaining monografted (monosiloxy) surface species. The reaction of Si−OH with a generic LnMXx is shown in Scheme 2a. Most of the silica-supported

we also provide a summary of characterization methods for each well-defined structure. Designations for the supports in the tables are as follows: “SG” refers to any kind of silica gel, relevant details are given in parentheses in each case. The most commonly used Aerosil-200 (Degussa, 200 m2 g−1) is designated as “AS”. Less common Aerosil silicas with surface areas of 300 and 380 m2 g−1 are listed as “AS300” and “AS380″, respectively. Homoleptic Alkyl Complexes. Homoleptic complexes of the group 4−6 transition metals of the general formula MRx react with Si−OH to form grafted species and RH. The two driving forces for this reaction are the release of RH and the formation of a strong M−O bond. Early reports of this reaction date to the 1970s using mainly allyl24,25,115−119 or methyl complexes of group 4 and 6 metals.120,121 More recent studies have focused on methyl, neopentyl, neosilyl, and benzyl derivatives. Table 2a summarizes the reaction of homoleptic group 4 alkyl complexes, complexes of group 5 and 6, and later transition-metal alkyl complexes, respectively. The reaction of MRx with SiO2−700 forms the monografted (SiO)MRx−1 (Scheme 2a, X = R). In contrast, the reaction of MRx and silica partially dehydroxylated at lower temperatures, such as SiO2−200, have an average structure consistent with the formula (SiO)2MRx−2 (Scheme 2b, X = R). However, current spectroscopic methods are rarely sensitive enough to distinguish between (SiO)MRx−1, (SiO)2MRx−2, and ( SiO)3MRx−3 for this class of compounds, in particular since bisgrafted species are associated with a distribution of species that leads to broader spectral line width. With silica partially dehydroxylated at intermediate temperatures (300−500 °C), the reaction stoichiometry indicates that mixtures of ( SiO)MRx−1 and (SiO)2MRx−2 are obtained. Grafting also occurs with late-transition metal complexes M(allyl)2 (M = Ni, Pd, Pt),122−125 M(allyl)3 (M = Rh),126,127 and Fe(aryl)2 (aryl = mesityl or 2,4,6-tritertiobutylphenyl).128 In contrast, [Au(Mes)]5 does not react with surface silanols leading to physisorbed complex.129 The most recent examples are listed in Table 2a. Homoleptic and Related Amide Complexes. Metal amide complexes are widely available for most transition metals.154 The reactivity of metal amides with partially dehydroxylated silica is similar to homoleptic alkyl complexes. The polarized M−N bond in M(NR2)n reacts with Si−OH to form ( SiO)M(NR2)n‑1 and HNR2 (Scheme 2, X = NR2). Tables 2b and 2c give examples of grafting of d-block metal-amides and group 3/lanthanide amides on silica, respectively. The reaction of Ti(NEt2)4 with SiO2−500 forms ( SiO)Ti(NEt2)3 and HNEt2 (Table 2b, entry 1).155 Zr(NMe2)4 reacts with SiO2−700 in a similar fashion, though the intermediate (SiO)Zr(NMe2)3 undergoes β-H abstraction to form Zr-3m shown in Scheme 3 and Table 2b (entry 6).156 The analogous reaction with (SiO)2Ti(NR2)2 only occurs at elevated temperature (60 °C).157 W2(NMe3)6 contains a tungsten−tungsten triple bond that remains intact upon grafting onto SiO2−700 to form (SiO)W(NMe2)2(W(NMe2)3) (Table 2b, entry 8).158 Silyl amide derivatives M(N(SiMenH3‑n)2)x is perhaps the most studied class of metal amide complexes. The reaction of M(N(SiMenH3‑n)2)x with Si−OH forms (SiO)M(N(SiMenH3‑n)2)x-1 and HN(SiMenH3‑n)2. In this case HN(SiMenH3‑n)2 further reacts with Si−OH to form ( SiO)SiMenH3‑n (Scheme 4).18 This competitive passivation results in lower loadings that one would expect based on the

Scheme 2. Grafting of MLnXx Molecular Precursor on (a) SiO2‑700 and (b) SiO2‑200 and (c) Examples of Molecular Silanols Used To Prepare Molecular Analogues of Surface Species

species described below follow this general reaction path involving proton transfer to the M−X group to form ( SiO)MLnXx−1 surface species after the release of HX. There are, however, several exceptions to this rule, and we will discuss these below when appropriate. Silica prepared at lower dehydroxylation temperatures contains vicinal silanols, and bis-grafted (bis-siloxy) surface complexes are preferentially obtained.114 The reaction of metal complexes with SiO2−200 or SiO2−300 typically leads to a reaction stoichiometry (elemental analysis and mass balance) consistent with the formation of bisgrafted species (Scheme 2b). However, it is very important to obtain spectroscopic evidence for such species as stoichiometry can be misleading since it only provides an average structure (vide infra). In addition, characterization of the surface species can be complemented by preparing molecular analogues using molecular silanols (Scheme 2c).77−82 In this section we provide a comprehensive list of welldefined lanthanide, group 3, and transition-metal complexes supported on partially dehydroxylated silica. The sections and associated tables are divided according to the types of ligands found in the molecular precursors and by metal. In the tables 331


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Table 2. Well-Defined Silica-Supported Surface Complexes

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Scheme 3. Grafting of Zr(NMe2)4 on SiO2‑700

Scheme 4. (a) Grafting of Bis(trimethyl)silylamide Complexes on Silica and (b) Possible Surface Species with Lanthanide and Group 3 (Ln) Metal Complexes

Scheme 5. Grafting and Further Evolution of [Ti(OiPr)4]2 on SiO2‑500

dimeric [Ta(OMe)5]2 reacts with SiO2−700 to form the monomeric (SiO)Ta(OMe)4 (Table 2d, entry 4).186 In [Cu(OtBu)]4 grafting, the tetrameric structure is conserved (Table 2d, entry 6),187−189 and a tungsten dimer is intact in the reaction of [W(OEt)5]2 with SiO2−700. In the latter case bis- and tris-grafted species are obtained because of the reaction of adjacent siloxane bridges promoted by the presence of W (Table 2d, entry 5).190 A large number of complexes containing the −OSi(OtBu)3 ligand react with silica to form well-defined surface species (Table 2e). Grafting typically displaces a siloxy ligand, though in a few cases a −OtBu in the siloxide reacts with surface silanols (Table 2e, entries 2−5). Complexes containing the (tBuO)2Si(O)2 ligand react with silica by selective displacement of −OtBu groups (Table 2e, entry 6). M[OSi(OtBu)3]n complexes react with partially dehydroxylated silica to form well-defined surface species of Ti, Cr, Fe, Cu, and Zn. In the case of Cr(II) a dimeric siloxide remains dimeric when supported on silica (Table 2e, entry 10). THF-containing siloxides have similar reactivity with partially dehydroxylated silica as THF-free siloxide complexes. The reaction of related Ti(OGeiPr3)4 with partially dehydroxylated silica occurs exclusively at the Ti−O bond to form (SiO)Ti(OGeiPr3)3 (Table 2e, entry 7).191 Grafting of molecular complexes containing both alkoxy and siloxy ligands is not selective, even if displacement of the siloxy ligand is typically favored (Table 2e, entries 3−5 and 8). Metal Chlorides. MCln derivatives react with partially dehydroxylated silica to form (SiO)MCln‑1 and HCl. These structures are summarized in Table 2f. Compared to MRx,

quantity of surface silanols present on the partially dehydroxylated silica support. When M(N(SiMenH3‑n)2)x contains bridging ligands the grafting reaction typically forms the corresponding mononuclear grafted complex (Table 2c). In the specific case of [Ag(N(SiMe3)2)]4 the initial nuclearity was preserved (Table 2c, entry 10).159 With heteroleptic complexes having amide and silylamide ligands, the structures of the final surface species are probably complex due to the competitive reactivity of amide and silylamide toward surface silanols (Table 2c, entry 1). Molecules with coordinated solvents generally form the corresponding solvent adducts on the surface (Table 2c, entries 5 and 8). Ln(N(SiMenH3‑n)2)3 complexes follow qualitatively similar trends, these species are shown in Table 2c, entries 13−17. As for homoleptic MRx complexes discussed above, silica dehydroxylated at lower temperatures reacts with M(NR2)n complexes to form mixtures of surface species with greater fraction of bis-grafted species. Homoleptic Alkoxide Complexes. Metal alkoxide complexes react with partially dehydroxylated silica in similar fashion as described for alkyl and amide complexes. SiO2−700 gives primarily monografted products, and silicas with higher hydroxyl content give mixtures of surface species (Table 2d, entry 1). The trends to conserve or disrupt multimeric alkoxide structures are dependent on the metal. The reaction of dimeric [Ti(OiPr)4]2 with SiO2−500 is reported to give Ti2-6m, which loses one OiPr ligand through a γ-abstraction process generating propene and the μ2-O bridged species Ti2-6m′ (Scheme 5 and Table 2d, entries 2, 3).155,185 However, the 360


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(tBuO)3SiO−. This is also supported by the difference in buried volume between a surface siloxy in cristobalite and −OSi(OtBu)3.211 In contrast, when Rh-4, Rh-5, and Rh-6 are reacted with SiO2−350 the resulting mononuclear Rh-species are stabilized by an additional nearby silanol (Table 2h, entries 3− 5). The reaction of (COD)PtX2 with partially dehydroxylated silica depends on the nature of X ligand. With (COD)PtCl2 or (COD)PtMe2 very little grafting occurs. However, complexes containing amides or siloxides, such as (COD)Pt(N(SiMe3)2) (Me) or (COD)Pt(OSi(OtBu)3)Me, react cleanly with SiO2 to form monografted Pt−Me surface species with a selective displacement of the O- and N-bound ligand (Table 2h, entries 8−13). [Re(CO)3OH]4 grafts on silica with liberation of H2O as shown in Scheme 7.212

M(NR2)n, and M(OR)n, fewer MCln salts have been supported on silica to form well-defined sites. This is probably due to limitations in spectroscopic methods, which are major hurdles to support the structure of the proposed surface species. The presence of THF or Et2O in the coordination sphere of the metal chloride does not seem to affect grafting. Heteroleptic Complexes with Various Monoanionic Ligands. From a purely thermodynamic point of view, the reaction of MXx with dehydroxylated silica is most favorable when X = alkyl and less favorable when X = OR or Cl, particularly for early transition metals. From a kinetic perspective, in the specific case comparing the reaction of Zr(CH2tBu)4 and Zr(OtBu)4 with oxide supports, grafting of alkoxide (X = OR) is much faster than alkyl (X = CH2R). This may be due to faster proton transfer from the surface hydroxyl groups to the lone pair on oxygen in Zr−OR. This two step grafting process is evidently faster than direct proton transfer of the surface hydroxyl groups to Zr−R, involving a three center two electron intermediate (Scheme 6).207,208 Despite this

Scheme 7. Grafting [Re(CO)3OH]4 onto SiO2

Scheme 6. Interaction of Zr(OtBu)4 and Zr(CH2tBu)4 with Surface Hydroxyl Groups of Silica and Alumina

Metal Cyclopentadienyl Complexes. Grafting cyclopentadienyl complexes, CpyM(X)xLn (Cp = C5H5 or C5Me5) on partially dehydroxylated silica leads to protonolysis of the X ligand. These results are summarized in Table 2i. Note that for homoleptic M(Cp)n complexes grafting on silica forms ( SiO)M(Cp)x−1 with the release of CpH (Table 2i, entries 3, 4, 8, 9). Metal Aluminate Complexes. The reaction of lanthanide amides with AlMe3 species forms Ln(AlR4)3 complexes.229−236 These complexes react with partially dehydroxylated silica to give a mixture of surface species. In general, Ln(AlR4)3 is proposed to react with silica to form mono- and bis-grafted lanthanide species and alkylaluminum grafted species shown in Scheme 8.237,238 Grafting of some other aluminate-supported lanthanide derivatives was also studied and shows similar results (Table 2j).

kinetic difference in reactivity, the thermodynamic product is normally obtained when grafting heteroleptic complexes. There are only a few examples, shown in Table 2g, of grafting felement complexes containing different types of anionic ligands. With metal complexes containing alkyl and aryloxide ligands, the alkyl group is displaced. This behavior is expected based on thermodynamic considerations because in these cases the  SiO−M bond is stronger than the M−R bond. Similar observations were made when alkyl and halide ligands were present in the same compound. Metal Complexes Containing Neutral and Anionic Ligands. Metal complexes containing both anionic (X) and neutral (L) ligands typically react with partially dehydroxylated silica by protonolysis of the anionic ligands with the neutral ligand remaining bound to the surface complexes. Diolefin Metal Complexes and Other C-Centered Ligands. Dinuclear complexes like [(COD)M(OR)]2 (M = Rh and Ir) containing bridging alkoxide ligands react cleanly with SiO2−700 to generate monografted species [(COD)2M2(OSi(OtBu)3)(OSi)] while maintaining the dinuclear structure (Table 2h, entries 1−2 and 6−7). EXAFS analysis of [(COD)M(OSi(OtBu)3)]2 and [(COD)2M2(OSi(OtBu)3)(OSi)] showed that the surface complex has a much shorter M−O and M−Si distances than [(COD)M(OSi(OtBu)3)]2. This result indicates that surface siloxy ligand, SiO−, is a smaller ligand than

Scheme 8. Grafting Nd(AlMe4)3 onto Silica

Metal Complexes Containing N- and P-Donor Ligands. Complexes containing N- or P-donor ligands react in a similar fashion with partially dehydroxylated silica. In the family of Pd(X)(Y)(PR3)2 in which X = Me and Y = OTf, Cl, NO3 the methyl ligand is selectively replaced by surface siloxy (Tables 2k and 2l).239 In some cases cationic surface species were claimed.239 Metal Complexes Bearing O- and S-Donor Ligands. Acetylacetonate and Carbamate Derivatives. Despite the large number of metal acetylacetonate derivatives, only few examples of well-defined silica-supported species have been reported. This is probably related to the low reactivity (and solubility) of most of these acac derivatives. Y(fod)3,178 Zr(acac)4,253 and 361


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Scheme 9. Grafting (a) Ta(CHtBu)(CH2tBu)3, (b) Re( CtBu)(CHtBu)(CH2tBu)2, and (c) Os( CHtBu)2(CH2tBu)2 on SiO2‑700

Ir(acac)3254 generate the corresponding grafted species by protonolysis of the acac ligand to form monografted surface species (Table 2m, entries 1−3). Though Me2Au(acac) does not react with silica,255 the carbamate Au-4 does form Au-4m (Table 2m, entry 5).256 O- and S-Calixarene-Capped Metal Complexes. Calixarenes have been mainly used as tri- or tetra-anionic O-based ligands for group 4, 5, or 6 metal complexes.257−259 The reaction of calixarene complexes with partially dehydroxylated silica results in displacement of an anionic ligands, usually Cl or OR. This forms the calixarene-capped silica-supported metal complexes shown in Table 2n. The Ta calixarene complexes Ta-7/8 were grafted onto SiO2−300 to form Ta-7b/8b, containing a protonated arm of the calixarene ligand and two surface siloxy groups (Table 2n, entries 10−11). Metal Oxo and Imido Complexes with Monoanionic Ligands. Early transition metal imido and oxo derivatives grafted on silica are summarized in Table 2o and Table 2p. The oxo and imido ligands usually remain intact upon grafting onto silica. However, in certain cases characteristic N−H IR vibrations267,268 and NMR signatures268 of an amido ligand are present after grafting of imido complexes, indicating that protonation of the imido ligand occurs (Table 2o, entries 1−2). Currently available data, including 17O NMR studies of 17Olabeled surface oxo complexes,74,269 does not show evidence of the oxo ligand being involved in this transformation. As found in homoleptic complexes, oxo and imido molecular precursors generally react with SiO2−700 to form monografted complexes, but mixtures form on silica dehydroxylated at lower temperatures. In contrast, molybdenum bis-imides react with SiO2−200 and SiO2−500 to generate solely monografted products as the sole surface species (Table 2o, entries 4 and 5). There is currently no quantitative model on the relative reactivity of X ligands in heteroleptic oxo or imido complexes. While V(NtBu)(CH2tBu)2(OtBu) grafts on SiO2−500 to form (SiO)V(NtBu)(CH2tBu)(OtBu) (Table 2o, entry 2),267 W(O)(CH2tBu)3Cl reacts with SiO2−700 by selective W−Cl bond cleavage resulting in the formation of (SiO)W( O)(CH2tBu)3 as a sole product (Table 2p, entry 17).74 Among the most extensively studied is the reaction of VOCl3 with silica.270−276 Monografted species are formed on highly dehydroxylated silica (Tdehydrox > 500 °C).271−276 At lower dehydroxylation temperatures there is no consensus on the structure of the dominant surface species. Metal Alkylidene Complexes. The reaction of metal alkylidene complexes with partially dehydroxylated silica typically results in the formation of well-defined silicasupported metal alkylidenes (Table 2q−2s and Scheme 9a). 288 For example, the reaction of Ta(CHtBu) (CH 2 tBu) 3 with SiO 2−700 yields (SiO)Ta(CHtBu) (CH2tBu)2. In this case the alkylidene ligand participates in the grafting step (Scheme 9a). The reaction of Ta(CHtBu) (CH2tBu)3 with partially dehydroxylated deuterated silica forms (SiO)Ta(CDtBu)(CH2tBu)2 and (SiO)Ta(CHtBu) (CHDtBu)(CH2tBu) that incorporate deuterium. This result indicates that (SiO)Ta(CHDtBu)(CH2tBu)3 forms transiently and undergoes H-transfer to form the final surface species.225,289,290 Similar results were obtained for the grafting of Re(CtBu)(CHtBu)(CH2tBu)2 (Scheme 9b and Table 2q, entry 5).291 The extreme scenario is the reaction of Cp2Ta(CH2)Me with SiO2−450, which yields (SiO)TaCp2Me2. In this case, SiOH quantitatively protonates the TaCH2 group (Table 2q, entry 4).226

In sharp contrast, the reaction of Os(CHtBu)2(CH2tBu)2 with SiO2−700 yields the alkylidyne complex (SiO)Os( CtBu)(CH2tBu)2 (Scheme 9c, Table 2q, entry 6).292 ( SiO)Os(CtBu)(CH2tBu)2 forms by reaction of the silanol with Os(CtBu)(CH2tBu)3 as a result of isomerization of Os(CHtBu)2(CH2tBu)2 by α-H transfer upon coordination; this forms (SiOH)Os(CHtBu)(CH2tBu)3 that subsequently releases 2,2-dimethylpropane by α-H abstraction. DFT studies show that (SiO)Os(CtBu)(CH2tBu)2 adopts a butterfly geometry, which is favored over a tetrahedral geometry due to the presence of a weak siloxy ligand and the Os in d2 configuration. Grafting imido alkylidenes of the type M(NAr)( CHR)(X)2 (M = Mo or W, X = CH2R, OR or NR2) onto partially dehydroxylated silica yields the corresponding surface complexes in which one X ligand is replaced by a siloxy surface group (Table 2r). Protonation of the alkylidene ligand to form (SiO)M(NAr)(CH2R)(X)2 is observed in some cases. In contrast to Ta(CHtBu)(CH2tBu)3 or Re(CtBu)( CHtBu)(CH2tBu)2, (SiO)M(NAr)(CH2R)(X)2 is stable and does not form (SiO)M(NAr)(CHR)(X) upon heating up to 150 °C. The formation of (SiO)M( NAr)(CH2R)(X)2 can be suppressed by lowering the grafting temperature. For example, grafting W(NAr)(CHtBu)(OtBu)2 at −40 °C in place of room temperature forms ca. 30% less (SiO)W(NAr)(CH2R)(OtBu)2 than if the reaction is carried out at room temperature.211 The reaction of SiO2−200 and W(NAr)(CHtBu) (CH2tBu)2 forms ca. 2 equiv of 2,2-dimethylpropane per W on the surface, in agreement with the formation of bis-grafted (SiO)2W(NAr)(CHtBu). Analysis of this material by solid-state NMR showed that several products were present on the silica surface, as shown in Scheme 10.293 This reaction forms the monografted (SiO)W(NAr)(CHtBu)(CH2tBu), bis-grafted (SiO)2W(NAr)(CHtBu), the bis-grafted protonated alkylidene (SiO) 2 W(NAr)(CH2tBu)2 species along with other unidentified species 362


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Scheme 10. Identified Species upon Grafting of W( NAr)(CHtBu)(CH2tBu)2 on SiO2‑200

Scheme 12. Grafting Ruthenium Alkylidene Complexes on SiO2‑200

(Scheme 10). This result shows the degree of complexity that can be encountered when using silica dehydroxylated at lower temperatures where high concentrations of vicinal silanols are present. W(O)(CHR)(OAr)2 (OAr = 2,6-dimesitylaryloxide (HMTO) or 2,6-diadamantylaryloxide (dAdPO)) behaves similarly in the presence of partially dehydroxylated silica to form supported oxo alkylidene surface species (Table 2s). Protonation of the alkylidene does occur, but is dependent also on the size of the OAr ligand. For small W(O)( CHR)(OHMT)2 an 80:20 mixture of (SiO)W(O)( CHR)(OAr) and (SiO)W(O)(CH2R)(OAr)2 was obtained. The larger W(O)(CHR)(OdAdP)2 gives a 90:10 ratio of the grafted alkylidene complex. Solid-state NMR has proven particularly powerful in determining the structure and dynamics of alkylidene species supported on silica. The 1H MAS and 13C CP MAS spectrum of (SiO)Re(CtBu)(CHR)(CH2R) contains signals for both syn and anti isomers (Scheme 11).294 J-resolved solid state

yields (SiO)M(E)(X)2. As in the other cases mentioned before, reacting M(E)(X)3 with SiO2−200 forms mixtures of mono- and bis-grafted species. One exception to this reactivity pattern in the M(E)(X)3 family is Mo(N)(CH2tBu)3, which reacts with SiO2−700 to form (SiO)2Mo(NH)(CH2tBu)3 by protonation of the nitrido ligand (Scheme 13 Scheme 13. Grafting of Mo(N)(CH2tBu)3 on SiO2‑700

and Table 2u, entry 1).319,320 Heteroleptic alkylidyne complexes containing alkyl or aryloxy ligands react with partially dehydroxylated silica to give surface species containing intact alkylidyne ligands (Table 2t). 3.2.1.3. Modification of Silica Surfaces with Main Group Organometallics. In this section we provide a selection of reactions of main group organometallics with silica because of their relevance as cocatalysts (see section 4). Mg(CH2tBu)2(dioxane) reacts with SiO2−400 to give grafted magnesium surface species (Scheme 14a).327 With silica dehydroxylated at 620 °C Si−R fragments are also formed through the opening of siloxane bridges (Scheme 14b).328

Scheme 11. syn and anti Isomers of (SiO)Re(CtBu)( CHR)(CH2R)

Scheme 14. Reaction of Grignard Reagents with (a) Silanol Groups and (b) Siloxane Rings

NMR spectroscopy shows that the J(C−H) coupling constant associated with the alkylidene C−H bonds were 109 and 159 Hz, consistent with the assignment to syn and anti isomers, respectively. The low JCH obtained for the major syn isomer is consistent with the presence of an α-agostic interaction, which is known for similar isoelectronic molecular complexes.295,296 Isoelectronic silica-supported Mo- and W-imido as well as Woxo alkylidene complexes always form the syn isomer as the only detectable product. In addition, NMR studies of a large family of silica supported Ta, Mo, W, and Re alkylidene species revealed that Ta and Re are dynamic species (e.g., rotation around O−M bond), while the Mo- and W-imido species are static62 due to the interaction of the aromatic ring with the surface, which is also evidenced by IR spectroscopy.297 Silica-supported Ru-alkylidene chemistry is considerably less developed. The reaction of Ru-siloxy-alkylidenes Ru-4 or Ru-5 with SiO2−200 forms well-defined mono- and bis-grafed species according to EA and SSNMR study of 13C labeled complexes. In this case the bis-grafted species is the major product of the grafting reaction (Scheme 12).298 Metal Alkylidyne and Nitrido Complexes. The reaction of metal−alkylidyne and metal-nitrido complexes is summarized in Table 2t and 2u. The reaction of M(E)(X)3 (M = Mo or W, E = CtBu or N and X = CH2tBu, NR2 or OR) with SiO2−700

The reaction of alkylaluminums with high surface area mesoporous SBA15−500 yields a complex mixture of surface species. For example, the reaction of AlEt3 with SiO2−700 forms a mixture of dinuclear alkylaluminum species and tetrahedral aluminum sites that do not contain alkyl fragments according to 27 Al MAS NMR spectroscopy. The formation of these tetrahedral sites, mono-, bis-, and tris-alkylsilicon surface species, indicate that siloxane bridges are involved in the grafting process and show the complexity of grafting reactions of alkylaluminums onto silica surfaces (Scheme 15a).71 363


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SBA15−500 forms a complex mixture of aluminum species involving protonolysis of Al−Et groups and alkyl-transfer to the silica surface (Scheme 17).333−335 In this case most alkyl aluminum species contain a bridging μ2-Cl-ligand.

Scheme 15. Reaction of (a) AlEt3 and (b) GaMe3 with Dehydroxylated Silica

Scheme 17. Reaction of [Et2AlCl]2 with SBA15−500

The reaction of B(C6F5)3 with partially dehydroxylated silica was reported to give an active cocatalyst for the polymerization of olefins.336−340 However, this reaction leads to hydrogenbonded surface species (Scheme 18a).341 In the presence of

Tricoordinate (SiO)AlR2, which would result from grafting monomeric AlEt3 onto isolated silanols was not observed under these conditions. Similar dimeric species were also formed when GaMe3 was grafted onto partially dehydroxylated silica (Scheme 15b).329 The reaction of AliBu3, which is monomeric in solution, and SBA15−500 leads to even more complex surface chemistry than encountered in the grafting of AlEt3 onto partially dehydroxylated silica. In this case the bis-grafted dimeric ( SiO)4(AliBu)2 surface species is the sole alkylaluminum containing species on the surface. Significant amounts of tetrahedral aluminum and dimeric aluminum sites lacking alkyl fragments as well as alkylsilicon groups are also formed, shown in Scheme 16a.330 This behavior is likely due to the higher

Scheme 18. (a) Absorption of B(C6F5)3 onto Silica to Form Lewis Acid/Base Adducts; (b) Reaction of B(C6F5)3 Adsorbed on Silica and NEt2Ph to form [Et2NPh][( SiO)B(C6F5)3]; and (c) Grafting B(C6F5)3 onto Silica in the Presence of Water to Form (SiO)B(C6F5)2

Scheme 16. Grafting AliBu3 on (a) SBA15−500 and (b) SBA15−700

amine bases, such as Et2NPh, these hydrogen-bonded silanols react to form [Et 2 NHPh][(SiO)B(C 6 F 5 ) 3 ] ion-pairs (Scheme 18b).342,343 However, the reaction of B(C6F5)3 with partially dehydroxylated silica showed that grafting reactions only occur in the presence of trace water.344 This reaction forms the (SiO)B(C6F5)2 surface species (Scheme 18c) in which the two borane units are in close proximity. 3.2.2. Alumina-Supported Metal Complexes. 3.2.2.1. General Consideration on Alumina. Alumina(s) has a more complex surface chemistry than silica because of the greater variety of surface functionalities and types of allotropes. This oxide exists as pure phases or mixtures of α, δ, γ, η, and θ alumina.345,346 α-Alumina has low surface area (1−10 m2 g−1) and is mainly used for the preparation of reforming catalysts because it is stable above 1000 °C. Alumina contains a mixture of interconnected tetrahedral AlO4 and octahedral AlO6 units. The amount of tetrahedral aluminum decreases in the order of θ (50%) to δ (37%) to γ (25%) to α (0%).347−353 γ-Alumina can be obtained with relatively large surface areas (ca. 200 m2 g−1) and is kinetically stable up to ca. 700 °C when treated under vacuum for 4−12 h.354 Above this temperature oxygen

reactivity of monomeric AliBu3 in comparison to dimeric AlEt3. Although when SBA15−700 is used as a support grafting AliBu3331 or AliBu3·Et2O332 leads to the sole surface products: (SiO)2AliBu or (SiO)2AliBu·Et2O, respectively. The reaction was shown to involve siloxane bridge opening via either alkyl or β-H transfer, resulting in the formation of Si− iBu and Si−H coproducts (Scheme 16b). Similar to the reactions of AlEt3 and AliBu3 with partially dehydroxylated silica, the reaction of [Et 2 AlCl] 2 with 364


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Figure 6. (a) IR spectra of γ-Al2O3 treated at different temperatures. (b) Assignment of the peaks observed in the IR. (c) OH density (in OH nm−2) and surface area (in m2 g−1) as a function of temperature.

mobility increases, leading to a faster phase transition to δ/θalumina. The presence of H2O (steam) above 700 °C facilitates the phase transition.355 Mesostructured γ-alumina can be prepared by sol−gel process in the presence of structure directing agents and has surface area ranging from 600−800 m2 g−1 and 2 nm pore size.356−358 Alumina obtained by flame pyrolysis of AlCl3, H2, and O2, such as Aeroxide Alu C from Evonik, is a mixture of δ and θ phases that have a lower surface area of around 120 m2 g−1. Alu C is more stable and more IR transparent than pure γalumina, which can be helpful to monitor the evolution of surface sites, in particular because δ- and γ-alumina share similar bulk and surface properties. The presence of two types of Al-sites in the bulk reflects on the rich surface chemistry of γ-alumina surfaces. Surface aluminum sites can be hexa- (AlVI), penta- (AlV), tetra(AlIV), and tricoordinate (AlIII). The surface also contains terminal Al−OH group bound to AlIV, AlV, and AlIV as well as μ2- or μ3-OH bridging between Al sites. The OH vibration bands and the corresponding sites are summarized in Figure 6a,b, and beneath each structure is the νOH of each Al−OH. These diverse surface functionalities give rise to a complex IR spectrum of γ-alumina in the OH region (Figure 6a) and result in a difference of reactivity between each type of OH and Al site. Similar to silica, applying a thermal treatment to alumina under vacuum or inert gases leads to a decrease in the νOH intensity, indicating that the Al−OH density decreases (Figure 6c). For example, Al2O3−500 and Al2O3−700 have an OH density of 2.0 and 0.7 OH nm−2, and fully dehydroxylated alumina can be obtained at ca. 1000 °C (Figure 6c). However, at 700−800 °C γ-alumina transitions to δ and θ phases according to powder XRD diffraction. This transition is also accompanied by a slight loss of surface area (Figure 6c). In contrast to silica, the overall shapes of the νOH bands in IR are not dramatically affected as a function of temperature, indicating that the different Al−OH sites in Figure 6b are

present at all treatment temperatures. The dehydroxylation process is also accompanied by the formation of Lewis acidic Al(III) sites that were shown to correspond to highly reactive defect sites. These Al(III) sites appear in γ-alumina treated above 400 °C.354,359,360 The surface acidity of alumina is determined by the adsorption of probe molecules and studying their spectroscopic signatures by IR. The Lewis acidity depends on the number of oxygen atoms bonded to the Al site. AlVI, AlV, AlIV, and AlIII will all behave differently from one another depending on the probe molecule added. The most acidic sites in the alumina surfaces have been assigned to AlIII sites based on the adsorption of probe molecules such as pyridine, CO, H2 or CH4, and N2.354,359,361−364 Depending on the method of synthesis, γ-Al2O3 contains different dominant facets. Understanding the population of these facets is important to build periodic models that can lead to deeper understandings of the γ-alumina surface. Transmission electron microscopy (TEM) studies of precipitated (pseudo)boehmites usually show that lamellar and rhombic crystal shapes are dominant. The (110) and (100) facets are predominant for the lamellar particles, whereas the (110) and (111) facets are predominant for rhombic particles.346 Alumina from flamed pyrolysis also expose mainly the (110) facet (80%), but small amounts of the (100) and (111) facets are also present. For a more complete account of theoretical models of alumina surfaces a recent review on the topic appeared.346 Spinel type models contain 37.5% tetrahedral Al-sites and 62% octahedral Al-sites,365 values that are slightly different from experimental measurements made on γ-alumina. An alternative and now widely accepted model for the γ-alumina surface is based on the dehydration of boehmite. This bulk model of γAl2O3 consists of aluminum atoms in tetrahedral (25%) and octahedral (75%) coordination sites with nonspinel sites occupied. The unit cell of (110) termination at complete 365


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dehydroxylation (s0 surface) exposes one tricoordinate AlIII and two types of tetracoordinate AlIV sites, namely AlIVa and AlIVb (Figure 7). The AlIII site is trigonal planar, and the AlIVb sites are tetra-coordinated with a truncated octahedral geometry (Figure 7b). The experimentally less abundant (100) termination exposes only AlV sites that adopts a pentacoordinated square-base pyramidal geometry (AlVa− AlVd Figure 7a). The s0 fully dehydrated surface (Figure 7b) is not a realistic model because the γ-Al2O3 surface contains Al−OH groups at experimental pretreatment temperatures. In addition, thermodynamic calculations indicate that the surface energy of the fully dehydrated (110) termination is higher (less stable) than that of the (100) surface. This result indicates that the (110) facet of the s0 surface is metastable, and strongly stabilized by hydroxylation. These calculations also predict that s0 surface should exist only above ca. 900 °C,363 temperatures at which γAl2O3 phase is not stable.354 The low stability of the dehydrated surface combined with the higher mobility of O atoms at high temperatures explains the change of phase upon high temperature treatment.

Surface Sites and Probe Molecules. CO and pyridine are typical probe molecules, which provide information about the type and the strength of Lewis and Bronsted acid sites.346,361 Investigation of the adsorption of CO and pyridine on γ-Al2O3 by DFT calculations shows that AlIII and AlIV on the (110) surface and AlV sites on the (100) surface are Lewis acidic and follow the trend in strength of AlIII > AlIV > AlV.363,364 The strong Lewis acidity of AlIII sites present on the (110) surface of γ-Al2O3 is evidenced by its ability to coordinate N2. Combined with the Lewis basicity of surface oxygen atoms, this site readily promotes the heterolytic activation of CH4 and H2 (Figure 8). In these cases, the degree of hydroxylation of the surface plays a critical role in the reactivity of AlIII sites.354

Figure 8. Coordination of N2 and activation of CH4 and H2 occurring on the AlIII defect sites of the (110) surface of γ-Al2O3.

3.2.2.2. Grafting Molecular Precursors on Alumina and Resulting Surface Species. Generalities. Ballard25,118 and Burwell26 recognized that alumina supports often led to more active and stable single-site catalysts. Grafting on alumina can take place on surface Al−OH groups, or directly on Lewis acidic Al-sites. Often grafting involves more than one site on the alumina surface, which leads to a mixture of organometallic sites, making structural determination more complex than on silica. In addition, molecular analogues of alumina surface sites are not readily available. We describe a comprehensive list of grafted compounds and give a more detailed analysis on selected examples. Alumina-Supported Early Transition Metal (Group 4−6) Complexes. Grafting homoleptic MRn complexes on partially dehydroxylated alumina has been investigated since the early seventies.25 Seminal studies on alumina-supported lanthanide and actinide complexes were reported in the eighties.14 Zr(CH2tBu)4 is a representative example that shows the difference between organometallic sites supported on silica and alumina. The reaction of Zr(CH2tBu)4 with Al2O3−500 results in the mixture of surface species shown in Scheme 19a.370 Protonolysis of the alkyl ligand also occurs to form (AlSO)Zr(CH2tBu)3 and (AlSO)2Zr(CH2tBu)2, analogous to the reactivity of Zr(CH2tBu)4 with partially dehydroxylated silica. However, these surface species further react with adjacent Lewis acidic Al-sites by abstracting an alkyl group to yield cationic Zr surface species along with alkyl aluminate, [(AlSO)2ZrCH2tBu+][AlSCH2tBu−] (Table 3, entry 1). This reactivity behavior was observed in many Zr370 and Hf371,372 d0 complexes, including mono- or bis-cyclopentadienyl complexes222 (Scheme 19b,c and Table 3, entries 1−6). The product distribution obtained from the reaction of Taorganometallic complexes and partially dehydroxylated alumina depends strongly on the molecular precursor and the thermal

Figure 7. (a) (100) surface of γ-Al2O3 (2 × 1 unit cell) showing the different AlV sites, (b) (110) dehydrated surface of γ-Al2O3 (s0 surface), and (c) (110) hydrated surface of γ-Al2O3 (3.0 OH nm−2) with water adsorbed on AlIVa, on which the AlIII and AlIVb sites are available.

Different degrees of hydroxylation have been modeled for the (110) facet of the alumina surface with OH densities of 3.0, 6.0, and 9.0 OH nm−2. At low coverage (3.0 OH nm−2) the most stable surface has water adsorbed on AlIII sites. However, if the water molecule is split across two AlIVa sites, a relatively stable surface containing a free AlIII site is accessible (Figure 7c). This surface exposes the highly reactive AlIII defect site that binds N2 and readily reacts with H2 or CH4. Such Lewis acid sites are also able to convert CH3F and CH3OCH3 to olefins (e.g., isobutene) via carbon−carbon bond formation step involving adjacent Lewis acid sites.366,367 In contrast, the intrinsically more stable (100) termination is free of water at much lower temperatures (ca. 350 °C; Figure 7a). The (111) surface can be exposed, too, but its relevance depends on the synthesis conditions.368 Nanoscale facets of this surface have been proposed to form upon reconstruction of the (110) surfaces.369 366


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Scheme 19. Grafting Zirconium and Hafnium Complexes onto Al2O3

Scheme 22. Grafting by C−H bond activation

and Table 3. This is a result of direct transfer of the methyl group to Lewis acidic Al sites, which can presumably take place in the absence of AlSOH groups in highly dehydroxylated alumina (Table 3, entries 9−10).226 The less electrophilic W(CtBu)(CH2tBu)3 reacts with Al2O3−500 selectively on the most reactive AlIVOH sites to form [(OS)3AlIVO]W(CtBu)(CH2tBu)2 with the alkyl and alkylidyne fragments interacting with nearby AlSOH groups according to IR and NMR spectroscopies (Scheme 21a, Table 3, entry 11).370,374 With [W(OEt)5]2, grafting on Al2O3−500 generates the corresponding grafted dinuclear W(V) complex (Scheme 21b, Table 3, entry 12).190 Other W complexes have been adsorbed on Al2O3, but the final structure has not been studied in detail.375 Grafting MeReO3 on Alumina. MeReO3 is stable in acidic aqueous media and has a relatively acidic methyl group with a pKa of ca. 10.376 In contrast to most organometallic complexes, MeReO3 reacts with partially dehydroxylated alumina to form oxo-bound (85−90%) as well as μ-methylene surface species (10−15%) as shown in Scheme 22a. These assignments are based on extensive solid-state NMR, XAS, and computational studies.377,378 Grafting through the heterolytic activation of a C−H bond in MeReO3 is similar to what is observed for the reaction of H2 and CH4 on defect Al sites.354,359 Calculations indicate that the C−H bond activation involves two adjacent Al sites that generate the μ-methylene species. A similar grafting involving C−H bond activation was also proposed for CpMo(CO)3CH3 (Scheme 22b).379 Alumina-Supported Late Transition Metal Complexes. As found for silica, late transition metal complexes react with alumina by displacement of an anionic ligand as shown in Table 3. No evidence of the formation of cationic species has been reported so far. Grafting mononuclear and dinuclear Fe(II) aryl molecular complexes on Al2O3−500 generates the corresponding mono- and dinuclear monografted surface species for Fe (Table 3, entries 13−14).128 Although the corresponding gold complex [Au(Mes)]5 grafts on alumina, grafting is accompanied by spontaneous formation of nanoparticles, presumably because of the hydrolysis of the complex into AuOx, which then transforms into Au nanoparticles.129 Ni(allyl)2,125 Rh(allyl)3,126 and Ir(allyl)3380 also generate the corresponding monografted metal complexes on Al2O3 (Table 3, entries 15−17). While complexes containing the OSi(OtBu)3 ligand, as in [(COD)Ir(OSi(OtBu)3)]2, also graft on alumina, the HOSi(OtBu)3 formed upon grafting remains adsorbed at the surface, possibly due to a strong interaction of the silanol and Lewis acidic Alsites (Table 3, entries 18−19).381 3.2.3. Silica−Alumina Supported Metal Complexes. 3.2.3.1. General Considerations on Silica−Alumina. Silica− aluminas or aluminosilicates are much more complex materials because they exhibit the combined properties of silica and alumina. This class of materials also includes zeolites; welldefined complexes supported on zeolites have been comprehensively reviewed recently and will not be discussed here.19 Silica−alumina can be prepared by coprecipitation,383 sol−

Scheme 20. Grafting Tantalum Complexes onto Al2O3

Scheme 21. Grafting Tungsten Complexes onto Al2O3

pretreatment of alumina. Ta(CHtBu)(CH2tBu)3 reacts with the surface hydroxyls of Al2O3−500 to form mainly (AlSO)Ta( CHtBu)(CH2tBu)2 as shown in Scheme 20a (Table 3, entry 8).373 However, the reaction of Cp*Ta(CH3)4 and Cp2Ta(CH3)3 with Al2O3−1000 yields [Cp*TaMe3+][AlS−Me−] and [Cp2TaMe2+][AlS−Me−] ion-pairs as shown in Scheme 20b,c 367


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Table 3. Well-Defined Alumina-Supported Surface Complexes

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Table 3. continued

a

Degussa Aeroxide AluC (100 m2 g−1). bJohnson Matthey α-Boehmite monohydrate (200 m2 g−1). cAmerican Cyanamid, PHF (150 m2 g−1). SASOL SBa

d

gel,384−386 or thermolysis of molecular precursors.387−389 These materials contain Al and Si both in the bulk and on the surface. Silica−alumina surfaces can also be prepared by surface doping of the corresponding pure oxides with Al- or Si-precursors by ion deposition/exchange390 or grafting Al(OR)3 or Si(OR)4 followed by a thermal treatment.72,387−389 In general the surface is dominated by the presence of silanols that have similar IR signatures to those observed in silica, albeit with slightly broader bands. The broadening can be attributed to silanols interacting with, or in close proximity to, Al sites.384,391−393 The other possibility is that silanols are interacting with adjacent distorted Si Lewis acid sites.394 The surface also contains Lewis acidic Al-sites according to acetonitrile, pyridine, and CO

adsorption studies.51,391−393,395 In some cases OH groups bonded to Al centers are observed by IR spectroscopy.393,396 Their presence and relative amount highly depends on the mode of preparation of silica−alumina.383,391 A periodic model of silica−alumina was constructed by placing an epitaxial silica film on the dehydrated (100) surface of γ-Al2O3, which corresponds to a coverage of Si (θSi) of 6.4 nm−2.394,397 Annealing this model to 1023 K using force field molecular dynamics forms a mixed amorphous silica−alumina (ASA) surface. During the annealing some Al atoms are extracted from the γ-Al2O3. The extracted Al atoms are either AlV (II1, III3) or AlIV (III1, III2, III4) shown in Figure 9a. These results were in agreement with 27Al MAS NMR data.391,398−400 369


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Here we show idealized average structures based on available spectroscopic assignments. Using a combination of IR, computational and solid state NMR techniques, it was proposed that chemisorption of CH3ReO3 on silica−alumina at low rhenium loading affords a rhenium surface species grafted via Lewis acid/base interactions between rhenium and a surface oxide moiety and a rheniumoxo group and a surface Al (Scheme 23a).403 At higher rhenium loading, a second distinct species is formed, consisting of hydrogen bonded CH3ReO3 on the surface hydroxyl groups, this second species being inactive in alkene metathesis (Scheme 23b).404 Scheme 23. Proposed Surface Sites for CH3ReO3 on Silica− Alumina Systemsa

a

Sites (a) are solely observed at low rhenium loading; both (a) and (b) present at higher loading.

3.2.4. Other Supports. Other pure oxides, in particular MgO, TiO2, and CeO2, have been used to graft molecular precursors, though to a lesser extent. 3.2.4.1. Magnesia − MgO. The surface chemistry of MgO has been studied in some details. MgO is an ionic solid with terraces and irregularities on the (100) termination that impact its reactivity.411 The MgO surfaces have been modeled by DFT periodic calculations.412,413 Adsorption of water on edges is much stronger than on terraces, and two kinds of hydroxyls were assigned, appearing at 3749 and 3737−3690 cm−1, corresponding to terminal Mg−OH (Type A) or bridging multicoordinated Mg-(OH)-Mg (Type B) groups as shown in Figure 10a.414 The assignment is represented in Figure 10b. Subsequent computational analysis on a large variety of surface sites proposed a different assignment.413 The study showed that the most important parameter for determining the stretching frequency of a given OH group is hydrogen bonding: the signals showing the highest frequencies corresponded to terminal isolated OH groups of type C (present in kinks and step divacancies) and hydrogen-bond acceptor OH groups.413 The second parameter to take into account is the location of OH groups in concave or convex areas of the surface of MgO, resulting in various interactions of the hydroxyl with its environment. In contrast to previous studies, the coordination of the oxygen of the hydroxyl appears to be a less important parameter. Overall, the high-frequency side of the sharp band in the IR of MgO involves monocoordinated hydrogen-bond acceptors hydroxyls (type C), whereas its low frequency side involves multicoordinated isolated OH groups (type B) and dicoordinated hydroxyls of types A and C, labeled as A′ and C′ in Figure 10b. The broad band between 3200−3650 cm−1 is assigned to hydrogen-bond donor OH groups (type D). MeMn(CO)5 was reported to react with MgO−400 with release of methane (Table 5, entry 1). According to EXAFS, the MgO surface behaves as a bidentate ligand providing coordination of an additional oxygen atom. The latter is further evidenced by a loss of one CO ligand. Late transition metals could be also grafted onto MgO surface, although the

Figure 9. (a) Top view of the amorphous silica−alumina with an OH coverage equal to 5.4 OH nm−2 showing the pseudobridging silanols interacting either with Si or Al centers (PBS-Si and PBS-Al, respectively). (b) Side view of the amorphous silica−alumina surface.

When water molecules were added to the system the formation of silanols was energetically preferred over the formation of Al− OH groups, silanols being the only surface hydroxyl up to 4.3 OH nm−2. Increasing the OH coverage to 5.4 OH nm−2 is more representative of the real surface structure (Figure 9a). The surface contains silanols interacting either with Si or with Al centers. These “pseudo-bridging silanols” (PBS-Si and PBSAl if the interaction of the silanol is with a Si or Al center, respectively) could be responsible for the milder acidity of ASA compared to zeolites. The model presents one kind of PBS-Si between the U1 and V2 centers and three different of PBS-Al between the Z2-III2, Y2−III4 and V1−III3 centers; it was recently validated by DNP SENS.72 This surface was used in order to evaluate the Brønsted acidity of silica−alumina by means of DFT calculations using CO, pyridine, lutidine and ammonia as probe molecules.401,402 3.2.3.2. Grafting Molecular Precursors on Silica−Alumina. Generalities. Though the surface chemistry of silica−alumina is not fully understood, several organometallic complexes were supported on amorphous silica−alumina to form well-defined sites. The surface chemistry resembles that of silica; grafting occurs primarily on surface silanols. With silica−alumina partially dehydroxylated at 500 °C, the major species are typically monografted. The surface species have similar spectroscopic signatures as the corresponding silica-supported species. However, silica−alumina supported species can display significantly different reactivity than silica-supported organometallics, often closer to alumina supported species (see sections 3.4 and 4). These surface species are shown in Table 4. 370


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Table 4. Silica-Alumina-Supported Perhydrocarbyl Surface Species

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Table 4. continued

a Akzo-Nobel (HA-S-HPV, Si/Al 75/25, 480 m2 g−1). bKetjen (Si/Al 75/25, 375 m2 g−1). cEstimated value. dStructure proposed based on analogy to other surface species.

While most immobilized metal complexes on TiO2 were prepared by grafting through phosphonate-derived ligands,424−427 there are only few examples of a direct bond between the metal sites and the surface. Contradictory results were obtained on grafting Rh(allyl)3 (Rh-1) onto titania surface (Scheme 24). While bis-grafted species Rh-1Tb have been claimed on TiO2−200 on the basis of TPD of the grafted complex (Table 5, entry 7),420 quantification of propene evolved during grafting of Rh-1 on TiO2−250 is consistent with the formation of monografted species Rh-1Tm, analogously to what is observed on silica (Table 5, entry 6).126 Au(acac)Me2 was reported to react with TiO2−400.421 Although the detailed structure of the resulting surface complex was not elucidated, XAS study indicated presence of site-isolated mononuclear AuIII species on the surface. Surface species obtained via grafting V(NMe2)4 onto TiO2−180 were studied by EPR and elemental analysis, which supports the formation of tris-grafted complex (TiSO)3V(NMe2).163 3.2.4.3. Ceria − CeO2. CeO2 is a support with the ability of storage and release of oxygen due to its defective nature.428 Grafting Au(acac)Me2 onto CeO2−400 was studied by XANES/ EXAFS, which confirmed the formation of monografted complex (CeSO)(OS)AuMe2 with additional OS-atom coordinated to gold.422 3.2.4.4. Indium Tin Oxide (ITO) − In2O3:Sn. Reaction of Ir10 with conductive ITO is proposed to generate Ir-10ITO, a surface species able to catalyze the electrochemical water oxidation in acidic solutions with activities higher than the bulk IrOx (Scheme 25).429 The molecular identity of the surface layer was demonstrated indicating that choice of ligand could allow control over the activity. 3.2.4.5. Magnesium Chloride − MgCl2. The industrial Ziegler−Natta catalyst contains TiCl4 supported on MgCl2 that is activated with AlR3 reagents. To study well-defined models for this catalyst several groups have synthesized high surface area MgCl2 solvates of alcohols430−432 and ethers433,434 and contacted this material with TiCl4. Contacting MgCl2·THF1.5 with TiCl4 forms the TiCl4/MgCl2/THF precatalyst. Solid-state NMR analysis of TiCl4/MgCl2/THF showed that THF coordinated to Ti(IV) centers ring-opens to form the surface species shown in Scheme 26.435 Activation of this material by AlR3 reagents reduces the Ti(IV) center to Ti(III) according to XAS436 and EPR437 studies, but the structure of active sites remains to be understood.

Figure 10. (a) Proposed OH groups on MgO (A, B, C, and D types). (b) Assignment of the OH frequencies on MgO.413,414 A′ and C′ correspond to A and C types of OH groups in which Mg is bonded to dicoordinated oxygen atoms.

reactivity of partially dehydroxylated magnesia toward Rh(allyl)3, studied by IR spectroscopy and quantification of gases evolved, was found to be much lower compared to silica, alumina, and titania.126 This is in line with lower pKa values of surface Mg−OH groups. Thus, similarly to what is observed for silica, late metal alkyls display low reactivity toward surface hydroxyls and complexes bearing anionic O-ligands are preferred for grafting. More recently a series of well-defined magnesia-supported Rh, Ir, and Au complexes was reported (Table 5). Grafting of metal acetylacetonates leads to elimination of acac ligand and formation of isolated monografted surface species. In all cases interaction with an additional surface oxygen atom was evidenced by EXAFS. Although released Hacac reacts with magnesia and remains at the surface, and thus cannot be quantified, changes in IR vibrations and bond distances given by EXAFS confirm grafting. In contrast to single-site late transition metals on other supports, Rh and Ir magnesia-supported species stay welldispersed under H2 and CO, leading to bis-carbonyl species in the latter conditions. 3.2.4.2. Titania − TiO2. TiO2-anatase crystals are dominated by (101) surfaces as a major facet, the (001) being a minor one.423 The highest frequency in the IR of TiO2-anatase was assigned to TiV-μ1−OH groups located on the (001) surface, whereas the lowest one was assigned to TiVI−OH2 located on the (101) surface. 372


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Table 5. Well-Defined Surface Complexes on MgO, TiO2, and CeO2

a

Nominal value.

Scheme 24. Grafting Rh(allyl)3 onto TiO2

Scheme 26. Ring Opening of THF in TiCl4/MgCl2/THF

3.2.4.6. Sulfated Metal Oxides. Sulfated alumina and zirconia are prepared by exposing the parent oxide to aqueous solutions of sulfuric acid followed by calcination in air.438,439 Sulfated zirconia was reported to isomerize alkanes at lower temperatures than sulfuric acid.438,439 Several studies showed that the acidic sites on sulfated zirconia are less acidic than the bridging hydroxyls in zeolites.440 The catalytic reactivity of sulfated zirconia itself is probably not related to the acidity of these sites but rather to the redox activity and/or the noncoordinating nature of surface sulfate species of this support.441 Computational studies are consistent with the experimental data, showing that the reaction of the (101) and (001) surfaces of tetragonal ZrO2 with H2SO4 results in the protonatation of the oxide with H2SO4 to form the acidic sites shown in Figure 11.442 The most stable configurations contain

Scheme 25. Electrochemical Water Oxidation with a Heterogenized Ir Catalyst

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Scheme 27. (a) Organometallic Complexes and (b) Their Reaction with Sulfated Alumina

observations were made for organozirconium species supported on sulfated zirconia447 and sulfated tin oxide.448 In general, complexes supported on sulfated metal oxides have high quantities (>60%) of active sites according to quantitative poisoning studies, and in some cases 100% active site concentration was determined.446,449 DFT calculations were performed on a model of the sulfated alumina surface using the previously described (110) surface of γ-Al2O3.446 The sulfated alumina model was constructed by determining the lowest energy structures for the adsorption of H2SO4 on the (110) surface of alumina as an exchange/ condensation reaction with the OH groups on the γ-Al2O3 surface. The most stable structure was dehydrated at 550 °C as in the experimental synthesis of sulfated alumina. The model contains 3.0 H2SO4 per nm2 and a hydroxyl coverage of 4.4 OH per nm2. The surface presents two type of sulfate ions (SA and SB) adsorbed on the alumina surface, as shown in Figure 12. Grafting of Cp*ZrMe3 on sulfated alumina was evaluated by DFT calculations using the model in Figure 13. The most stable geometry for the Zr−Me protonolysis was favored on the acidic OH group coordinated to three Al atoms ((Al)3OH species). The organozirconium surface species was modeled by placing the cationic Cp*ZrMe2+ on the deprotonated sulfated alumina surface on either the SA or the SB surface sites. The interaction with SA forms a structure with a Zr···O distance of 2.24 Å. Grafting on the SB site results in the interaction of Cp*ZrMe2+ on two SO groups with a mean Zr···O distance equal to 2.36 Å. These calculations result in a geometry that is in good agreement with the EXAFS and NMR data. Benzene interacts with Cp*ZrMe2+ on either SA or SB sites to form Cp*ZrMe2(C6H6)+, in which the benzene inserts into the Zr··· O bond with a Zr−C(benzene) mean distance equal to 2.71 Å. This value is larger than the value obtained by EXAFS (2.35 Å). In this structure the Zr center is displaced away from the SA and SB surface sites.

Figure 11. Most stable configurations of the H2SO4 adsorption on the ZrO2 (101) and ZrO2 (001) surfaces. (a) Top and perspective views of the optimized structure of the 2H+ and SO42− species adsorbed on the ZrO2 (101) surface. (b) Top and perspective views of the optimized structure of H+, OH−, and SO3 species adsorbed on the ZrO2 (001) surface.

a sulfate anion (SO42−) that coordinates to three surface zirconium atoms and two protons adsorbed onto two surface oxygen atoms on the (101) facet (Figure 11a). The most favorable adsorption product on the ZrO2 (001) contain similar features, a 3-fold coordinate SO42− anion and two protons bound to surface oxygen atoms (Figure 11b). This study was extended by evaluating the adsorption of H2O and SO3 (or H2SO4) on the (101) surface of ZrO2 for higher coverages.443 Similar models have been developed for sulfated alumina (AlS, Figure 12).446

3.3. Evolution of Well-Defined Surface Sites upon Post-Treatment and/or Functionalization

Figure 12. Sulfated γ-Al2O3 model (AlS) based on DFT calculations presents two kinds of surface sites (SA and SB).446

3.3.1. Calcination. Calcination is a treatment at high temperatures (above 400 °C), typically performed under air. This step is common in the preparation and/or the regeneration of heterogeneous catalysts because this removes carbon-containing species, including organic ligands. Oxidation catalysts including single-site supported systems are often stabilized by a calcination step; prototypical examples are the calcination of grafted cyclopentadienyl21 and other162 Ti derivatives as a method to generate isolated Ti(IV) surface sites. By and large, surface sites remain isolated upon calcination, particularly for early and first row transition metals and when low surface loadings are used. This approach is general to −OSi(OtBu)3 complexes and provides isolated

The reaction of Cp*MR3 or M(CH2Ph)4 (M = Ti, Zr, Hf; R = Me, Ph, CH2Ph) with sulfated alumina forms weakly coordinated ion pairs with the surface as shown in Scheme 27.444−446 For example, the reaction of Cp*ZrMe3 with sulfated alumina forms the surface bound Cp*ZrMe2+ ion according to 13 C CPMAS NMR spectroscopy, which contains characteristic resonances associated with cationic Zr−Me+ complexes. In addition, X-ray absorption spectroscopy established that a long Zr−O contact was present in Cp*ZrMe3/AlS indicating that the surface acts as a weakly coordinating anion. Similar 374


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Figure 13. Cp*ZrMe2+ complex on the (a) SA-SB and (b) SB sites. (c) Insertion of benzene into the Zr−O bonds of the grafted Cp*ZrMe2+ species and displacement from the SA and SB sites.

3.3.2. Thermolysis under Vacuum or Inert Gas. Supported species containing −N(SiMe3)2, −OSi(OtBu)3, −OtBu, or −Cl ligands can readily lose their ligands when treated at high temperatures (>150−400 °C) under vacuum, while remaining isolated sites. This thermolysis step involves the reaction of the metal sites with adjacent siloxane bridges and generates more stable silicate species. For instance, silicasupported metal chloride evolve upon heating under vacuum to form bis- and tris-grafted species as shown in Scheme 29a.203,274 The silica-supported dimeric Cr(II) Cr-6m forms the corresponding dinuclear Cr(II) Cr-6T complexes at 300− 400 °C under vacuum (Scheme 29b).198 XANES of the

Ti(IV), Ta(V), and Fe(III) sites as shown in Scheme 28.193,199,450 Calcination at high temperatures can also lead to incorporation of the metal into the support matrix and/or to sintering to form metal oxide clusters.193,451 Scheme 28. Calcination of Grafted Surface Species

Scheme 29. Evolution of Surface Species upon Thermolysis under Oxygen-Free Conditions

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Scheme 30. (a) Hydrogenolysis of Supported Alkyl Complexes and (b) Surface Hydrides of Group 4−6 Metals on Various Supports

Table 6. Early Transition-Metal Hydrides (Group 4−6) and Their Characteristic IR Signatures compound

name

νM−H (cm−1)

refs

(SiO)3TiH (Si/AlO)3TiH (SiO)3ZrH (SiO)2ZrH2 (Si/AlO)3ZrH (AlO)3ZrH (SiO)3HfH (SiO)2HfH2 (SiO)Hf(Np)H2 (Si/AlO)3HfH (AlO)3HfH (SiO)2TaH (SiO)2TaH(PMe3) (AlO)2TaH (ZrO)(SiO)TaH (SiO)2WHn (n = 2−4) (Si/AlO)2WHn (n = 2−4) (AlO)2WHn (n = 2−4)

TiSiO2-H-1 TiASA-H-1 ZrSiO2-H-1 ZrSiO2-H-2 ZrASA-H-1 ZrAl2O3-H-1 HfSiO2-H-1 HfSiO2-H-2 HfSiO2-H-2-R-1 HfASA-H-1 HfAl2O3-H-1 TaSiO2-H-1 TaSiO2-H-1-P TaAl2O3-H-1 TaZrO2/SiO2-H-1 WSiO2-H-n WASA-H-n WAl2O3-H-n

1706,1692,1679,1647 1600−1725 1633 1649, 1622 1635(br) 1622 1701 1720, 1675 1685, 1651 1702, 1675(sh) 1670 1830,1815(sh), 1855(sh) 1687 1830 1800 1960, 1815 1948,1819 1903, 1804

132 405 136,455−457 457,458 406 459 460,461 461 461 142 462 299,463 454 373 464 465 409 373

3.3.3. Thermal Treatment under H2. The reaction of silica, alumina, and silica−alumina supported early transitionmetal homoleptic alkyl complexes discussed in sections 3.2.1.2 with H2 at 100−200 °C typically forms metal hydrides shown in Scheme 30a. The νM‑H vibrations are summarized in Table 6. At higher reaction temperatures under H2, the νM‑H band in the IR spectrum decays while the Si−H bands increase in intensity, indicating that hydrides are transferred to the surface and that the metal incorporates into the oxide support.452,453 For instance, (SiO)2Ta−H forms (SiO)3Ta and additional surface Si−H. (SiO)3Ta coordinates PMe3 and stoichiometrically reacts with O2.454

molecular Cr(II) dimer Cr-6 and Cr-6T are remarkably similar, indicating that Cr-6T remains Cr(II) and is dimeric after thermal treatment. This approach also applies to monomeric Cr(III) containing Cr-5, which forms the monomeric Cr(III) silicate Cr-5T shown in Scheme 29c.197 Contacting Cr-5T with CO results in two νCO bands in a 60:40 ratio, indicating that two species are present on the surface. These two species correspond to tricoordinate Cr(III) sites (60%) and tetracoordinate Cr(III) sites that interact with a nearby siloxane bridge. A similar thermolysis step under vacuum at 500 °C converts (SiO)Ln(N(SiMe3)2 into (SiO)3Ln as shown in Scheme 29d.88 376


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trimethylsilylamide complexes, also form particles in the presence of H2 and also passivate the regenerated OH groups of the support to yield supported nanoparticles on silica having no OH groups.159,168,216 There are a few examples of stable late metal hydrides supported on oxides. Rh249 and Ir23,250 surface complexes containing phosphine ligands generate the corresponding surface hydrides (Scheme 32b). In addition, some Ir metal complexes supported on TiO2 and CeO2 lead to isolated metal ions upon treatment under H2.417 [M(COD)(COT)] (M= Ru and Os) graft onto surface Si−H functionalities and react with H2 to generate discrete well-defined metal hydrides (Scheme 32c).469 3.3.4. Thermal Treatment under H2S. The reaction of H2S on well-defined W surface sites was explored in the eighties to prepare supported Ni-promotod W sulfide hydrodesulfurization (HDS) catalysts.470,471 [W(OEt)5]2 supported on silica− alumina undergoes sulfidation in the presence of H2S below 150 °C, a temperature much lower than needed to promote the sulfidation of supported polyoxometallates, forming hexagonal WS2 plates as shown in Figure 14. The formation of this phase is attributed to the milder sulfidation conditions, is probably kinetically controlled and yields the catalyst much more active in the hydrogenation of toluene.190

The formation of supported metal hydrides probably occurs through hydrogenolysis of M−C bonds to form highly reactive M−H species, which further react with the adjacent siloxane bridges in the case of silica or alternative μ2-O functionalities present in alumina and silica−alumina. This process creates new M−O bonds and transfers hydrides to the surface. For example, (SiO)Zr(CH2tBu)3 reacts with H2 to form ( SiO)3ZrH and (SiO)2ZrH2 in a 60:40 ratio with the concomitant formation of SiH and SiH2 species as shown in Scheme 30.457 Similar results were obtained for Ti405 and Hf.462 Though in the specific case of Ti, Ti(III) surface species are also formed.405 (SiO)Ta(CHtBu)(CH2tBu)2 reacts with H2 at 150 °C to form (SiO) 2 TaH and ( SiO)2TaH3.299 Further heating these species above 200 °C leads to loss of the hydride ligands and formation of ( SiO)3Ta (TaSiO2) (Scheme 31). Scheme 31. Thermal Treatment of TaSiO2-H-1

3.4. Stoichiometric Reactivity and Activation of Small Molecules on Well-Defined Surface Sites

Not all supported species form stable hydrides. The reaction of (SiO)W(CtBu)(CH2tBu)2 with H2 leads to recovery of surface silanols and formation of presumably W-containing surface aggregates.409 However, the reaction of (AlSO)W( CtBu)(CH2tBu)2 with H2 forms stable (AlSO)W(O)(H)3 species.466 Alumina-supported alkyl aluminum species also lead to the formation of aluminum hydride surface species upon reaction with H2 at 200 °C.467 Hydrides of Ti,405 Zr,406 Hf,142 Ta,373 and W hydrides409 are also formed on silica−alumina as evidenced by IR spectroscopy (Table 6), but their structures remain undetermined. Most late transition-metal species supported on SiO2, Al2O3, MgO, or TiO2 react with H2 to yield small metal nanoparticles (Scheme 32a).213,218,247,381,468 Supported late or coinage metal

3.4.1. Stoichiometric Ligand Exchange via Protonolysis Reactions with Brønsted Acids. Treatment of surface alkyls, amides, hydrides, and other complexes with Brønsted acids leads to substitution of the ligands via protonolysis and gives the corresponding ligand exchanged products. This methodology is widely applied to generate surface alkoxides (Scheme 33), carboxylates, and acetylacetonates from welldefined surface species.131,137,155,179,183,186,253,271,472−474 3.4.2. Reactions with Lewis Bases. Small molecules such as CO, CO2, and N2 are probes for post-treated surface sites.346 These probes have specific IR and NMR signatures that provide detailed information about the structure of the surface site. In addition, these signatures can be compared with literature precedents, and now be reliably calculated using computational methods.83,84,384 For instance, CO stretching, pyridine CC vibration361,384,475 and 15N chemical shifts476−478 provide direct information about the nature (Lewis, Brønsted vs H-bonding) and the strength of the acid sites. R3P or the oxide derivatives can also be similarly used as probe molecules.479 3.4.3. Coordination and Activation of N2. N2 is a very weak σ-donor ligand that binds to strong Lewis acid sites or metals that have strong back-bonding properties.480,481 N2 binds to Al(III) defect sites on the surface of Al2O3 and has a very distinctive IR signatures.360 N2 also binds (SiO)2TaH, and reacts in the presence of H2 to split N2 to form ( SiO)2Ta(NH)(NH2) at a single metal center as shown in Scheme 34.482,483 (SiO)2Ta(NH)(NH2) can be independently synthesized from reaction of (SiO)2TaH (TaSiO2H-1) with NH3.484 Computations suggest that coordination of H2 decreases the energy of activation of N2 by avoiding changes to the oxidation state of Ta.482 3.4.4. Activation of O2 and N2O. (SiO)3Ta (TaSiO2) reacts with 0.5 equiv of O2 to yield the corresponding Ta(V) oxo species, (SiO)3Ta(O) (TaSiO2-O) without the detection of reaction intermediates (Scheme 35a).452 ( SiO)3ZrH (ZrSiO2-H-1) and (SiO)2ZrH2 (ZrSiO2-H-2), react

Scheme 32. (a) General Synthesis of Supported Metal Particles from Isolated Metal Sites on Oxide Surfaces; (b) Isolated Late Metal Complexes That Are Stable Under H2; (c) Site Isolated Ru Hydride

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Figure 14. Formation of WS2 from well-defined grafted W sites: influence of the preparation method on the morphology and the hydrogenation activity.

Scheme 33. Stoichiometric Protonolysis of Surface Complexes

Scheme 35. (a) Reaction of TaSiO2 with O2; (b) Reaction of ZrSiO2-H-1/2 with N2O; and (c) Reaction of Cr-6T with N2O

Scheme 34. Nitrogen Activation by Silica-Supported Ta Hydrides

cleanly with N2O to yield (SiO)3Zr(OH) (ZrSiO2−OH-1) and (SiO)2Zr(OH)2 (ZrSiO2−OH-2) (Scheme 35b).457 For (SiO)4Cr2 (Cr-6T), the reaction with N2O at room temperature yields a dinuclear Cr(III) silicate (SiO)6Cr (Cr-6T-Ox) shown in Scheme 35c.198 This reactivity was confirmed by the reaction of the molecular [Cr(OSi(OtBu)3)2]2 with N2O, which forms an isolable Cr(III) siloxide dimer. The XANES of the Cr(III) siloxide dimer and Cr-6TOx are similar, indicating the similarities between both structures. 3.4.5. Activation of CO2. The reaction of CO2 with ( SiO)3ZrH (ZrSiO2-H-1) and (SiO)2ZrH2 (ZrSiO2-H-2) forms (SiO) 3 Zr(O 2 CH) (Zr SiO2 -O 2 CH-1) and (SiO) 2 Zr(O2CH)2 (ZrSiO2-O2CH-2) (Scheme 36).457 This reaction also yields some methoxy surface species, suggesting that either a small amount of a tris-hydride (SiO)ZrH3 is present or alternatively two Zr sites are close to each other on the surface. 3.4.6. Stoichiometric Reactions with Ketones. Ketones can also be used to probe surface sites, in particular surface alkylidene (Scheme 37a) and imido complexes (see section 4.3 for other examples). For example, di-t-butylketone reacts with (SiO)Ta(CHtBu)(CH2tBu)2 (Ta-12m) to form 2-t-butyl3,3-dimethyl-1-butene quantitatively (Scheme 37a).289 Fluo-

renone reacts with (SiO)Sm(N(SiMe3)2) by single electron transfer to form Sm-2m-Fluor (Scheme 37b).182 3.4.7. Stoichiometric Reactions with Alkanes. Alkanes react with (SiO)3MH and (SiO)2MH2 (M = Ti, Zr, Hf) to form (SiO)3M−R and (SiO)2M(R)2.455,485,486 This reaction takes place on d0 metal hydrides by σ-bond metathesis (Scheme 38a).487 The reaction of (SiO)2Ta−H and methane at 300 °C forms methyl, methylidene, and methylidyne surface 378


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evolves upon heating to produce additional H2 and ( SiO)2Ta−Cp complex (Scheme 38c).490

Scheme 36. Reaction of Silica-Supported Zirconium Hydrides with CO2

4. CATALYTIC ACTIVITY AND APPLICATIONS 4.1. Hydrogenation of Alkenes, Alkynes, and Arenes

Oxide-supported metal hydrides (or alkyls as precursors to hydrides) are efficient hydrogenation catalysts. Early examples include organoactinide complexes supported on γ-alumina,14,491 and group 5 homoleptic complexes supported on silica.492 These catalysts hydrogenate arenes to their saturated analogues at high H2 pressures. Catalysts containing organometallic Zrcomplexes on sulfated alumina were reported to hydrogenate arenes at 1 atm H2 pressure at 25 °C giving turnover frequencies up to 360 h−1.445,446,449 Titration studies showed that nearly 100% of the sites were active.446 The reaction of Cp*ZrMe3/AlS with H2 forms Cp*ZrH2/AlS that are the active species in this reaction. The proposed mechanism is shown in Scheme 39. Cp*ZrH2/AlS contains a weak ion pair with the AlS surface that allows for benzene coordination. Cp*ZrH2/AlS reacts with benzene to give the insertion products, which further react with H2 by turnover limiting σbond metathesis to give the cyclohexadiene complex. Subsequent hydride transfer and Zr−C hydrogenolysis steps give cyclohexane.446 The hydrogenation of arenes is sluggish with the isostructural Ti or Hf complexes, or tantalum organometallics, grafted onto sulfated supports. Late transition metals supported on oxides generally evolve to metal nanoparticles and hydrogenate a broader range of substrates (see section 3.3). However, late transition metal hydrides can be stabilized by additional ligands, and these species are active in the hydrogenation of olefins as shown in Scheme 40.23 For example, silica-supported Rh hydride Rh-9m catalyzes the hydrogenation of cis-2-butene to butane (Scheme 40a).127 The semihydrogenation of alkynes was also reported for supported Pd complexes (Scheme 40b), which presumably occurs through formation of Pd−H species.247 Silica-supported Ir hydride Ir-6m stabilized by pincer ligand hydrogenates ethylene without a detectable induction period (Scheme 40c).250 At elevated temperatures Zn-silicates hydrogenate propylene at 200 °C (Scheme 40d). This material was proposed to activate H2 on Zn−O sites that forms transient Zn−H intermediates active in propene hydrogenation.493

Scheme 37. Reactivity of Surface Complexes towards Ketones

species as shown in Scheme 38b.488 This reaction probably involves σ-bond metathesis that is accompanied by α-H transfer to give the final products.489 Analogous to the reactivity of the supported hydrides, Ta and W permethyl supported species (Ta-1m, Ta-1b, and W-1m) also react with alkanes to produce the corresponding metal alkyl species and methane.144 (SiO)2Ta−H reacts with cycloalkanes to yield the corresponding Ta cycloalkyl surface complex and hydrogen. With cyclopentane, (SiO)2Ta−C5H9

Scheme 38. Methane Activation on (a) ZrSiO2-H-1 and (b) by TaSiO2-H-1 and (c) Reaction of Cyclopentane with TaSiO2-H-1

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for alkene metathesis catalysts that operate at room temperature.500−505 Classical heterogeneous metathesis catalysts, prepared by calcination of metal oxide precursors at high temperatures, are typically activated first at high temperature and then exposed to olefins. They are generally incompatible with functionalized olefins, with the exception of Re2O7/Al2O7 activated by Me4Sn.8,506 With these ill-defined catalysts, only a small fraction of the metal sites are active and the mode of formation of the alkylidene remains unknown. The first well-defined heterogeneous metathesis catalyst was (SiO)Re(CtBu)( CHtBu)(CH2tBu) (Re2m).301,302 Silica-supported alkene metathesis catalysts belong to two distinct families, shown in Scheme 41: (i) well-defined Mo, W, and Re alkylidene species containing alkylidyne, oxo, or imido ligands, which initiate via cross-metathesis with the existing bulky alkylidene ligand [refs 211, 287, 293, 297, 301, 302, 306−311, 313, 316−318, and 320] and (ii) well-defined alkene metathesis “pre-catalysts” that form an alkylidene under reaction conditions. The precatalysts contain Mo and W alkyl complexes supported on silica.287,320,323,507 Re2m shows high metathesis activity in comparison to Re(CtBu)(CHtBu)(CH2tBu)2 in solution and classical heterogeneous catalysts known at the time.301,302 Computational studies revealed that the high reactivity of this complex resulted from the dissymmetry at the metal center that decreases the energy barrier for coordination/decoordination of the olefin (the key elementary step of olefin metathesis) and destabilized the metallacyclobutane intermediates (Scheme 42).508−511 This analysis extends to isoelectronic Mo and W imido/oxo complexes and applies to the corresponding homogeneous catalysts. 512−518 In fact, (SiO)M(NAr)(CHtBu) (CH2tBu) (M = Mo, W) display improved activity and stability in comparison to Re2m and the related molecular systems.293,297 Replacement of the neopentyl groups by amide and alkoxide ligands and tuning the imido ligand greatly improved the overall catalytic performances, in particular by avoiding the formation of olefin-isomerization byproducts.211,307−311,314 In addition, these silica-supported catalysts are also compatible with functionalized alkenes such as ethyl oleate; however, they have displayed poor performances in ring closing metathesis.309 (SiO)W(O)(CH2tBu)3 probably generates the alkylidene in situ by an α-H abstraction process.287 While less active than the well-defined tungsten imido alkylidene complex ( SiO)W(NAr)(CHtBu)(CH2tBu) (16,000 TON in propene metathesis at 30 °C after 24 h), (SiO)W(O)(CH2tBu)3 yields a more stable catalyst at high temperature, yielding 22 000 turnovers in 95 h for the self-metathesis of propene at 80 °C. The homogeneous oxo alkylidene W molecular complex W(O)(CHtBu)(OHMT)2 was recently described,519 and was used to prepare the corresponding silica-supported analogue, (SiO)W(O)(CHtBu)(OHMT).316 In cis-4-nonene metathesis (SiO)W(O)(CHtBu)(OHMT) has a turnover frequency at 3 min (TOF3 min) of 280 min−1. It is among the most active well-defined heterogeneous metathesis catalysts,316 with activity far greater than the homogeneous W(O)(CHtBu)(OHMT)2 precursor, which has TOF3 min = 5 min−1 under the same reaction conditions. This large difference in activity is probably due to the presence of two large aryloxy ligands in W(O)(CHtBu)(OHMT)2. Upon

Scheme 39. Hydrogenation of Benzene by Cp*ZrH2/AlS

Scheme 40. Transition Metal Hydrogenation Catalysts Supported on Silica

4.2. Metathesis of Alkenes and Alkynes

4.2.1. Alkene Metathesis Using Group 6−7 SilicaSupported Catalysts. Alkene metathesis was discovered 60 years ago using both homogeneous and heterogeneous catalysts.8,95,494−497 Chauvin proposed that metallocarbenes and metallacyclobutanes were the key reaction intermediates of olefin metathesis.498,499 This insight ultimately led to the development of homogeneous well-defined alkylidene catalysts 380


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Scheme 41. Well-Defined Supported Alkene Metathesis Catalysts

(CHtBu)(X) with 13C-dilabeled ethylene results in the formation of either the TBP or SP metallacyclobutane, and their ratio allows the σ-donation of the X-ligand to be ranked (Figure 15): the weaker σ-donor X-ligands favor the TBP over

Scheme 42. Elementary Steps in d0 Alkene Metathesis

grafting on silica one of these large groups is replaced by the relatively small SiO group in (SiO)W(O)(CHtBu) (OHMT). Larger aryloxy ligands give increased activity in olefin metathesis. Using the bulkier (SiO)W(O)(CHtBu) (dAdPO), TOF3 min of 356 min−1 can be reached in the selfmetathesis of cis-4-nonene.317 It also provides >75 000 turnovers in 1-hexene self-metathesis, when ethylene is vented from the reaction.317 (SiO)W(O)(CHtBu)(OAr) catalysts are active in ethyl oleate metathesis, in contrast to the classical WO3/SiO2 catalyst. This result shows that silica-supported Woxo alkylidenes are compatible with functional groups and that the incompatibility of WO3/SiO2 is probably related to the high reaction temperatures necessary to form active sites. Replacing the aryloxy ligand in (SiO)W(O)(CHtBu)(OHMT) by an arylthiolate in (SiO)W(O)(CHtBu)(SHMT) improves the catalytic activity in terminal olefin metathesis. This is probably due to a decrease in the stability of the metallacyclobutane as the strong σ-donor ArS ligand leads to a strong electronic dessymetrisation at the W center.318 The improved activities and stabilities of the silica-supported catalysts discussed above in comparison to their homogeneous analogues originate from two factors: (i) site isolation that prevents deactivation by bimolecular decomposition,520 (ii) the surface siloxy group is a sterically small, weak σ-donor ligand with electronic properties similar to OtBuF3.211 The sterics of the surface siloxy ligand was evaluated by its buried volume521 using an isolated silanol on the (111) surface of β-cristobalite. The buried volume is roughly 20.6%, compared with 36.8% for dAdPO.317 This result probably explains the lower activity of the molecular precursor in the W oxo series discussed above.522 In the (SiO)W(NAr)(CHtBu)(X) series, the electronic influence of X ligands (X = OtBu, OtBuF3, OtBuF6, OtBuF9, and OSi(OtBu)3) was evaluated in detail by assessing the TBP/SP ratio of the corresponding metallacyclobutane and its relation to the catalytic activity (TOF). Contacting (SiO)W(NAr)-

Figure 15. Formation and ratio of TBP/SP metallacyclobutanes for (SiO)W(NAr)(CHtBu)(X) complexes, determined by 13C CPMAS NMR.

the SP isomer (OtBuF9 < OtBuF6 < OtBuF3 < OSi(OtBu)3 < OtBu).211,314,511,523,524 In this series, catalytic activity (TOF) increases with decreasing σ-donation of the X-ligand. A systematic investigation on the influence of both the imido and X ligands in (SiO)W(NAryl)(CHCMe2R)(X) (Aryl = Ar, ArCl, ArCF3, and ArF5; X = OtBuF9, OtBuF6, OtBu, OSi(OtBu)3, Me2Pyr; and R = Me or Ph) was recently reported.314 The influence of each member of the ligand set was correlated to the TOF in cis-4-nonene self-metathesis using multivariate linear regression analysis tools.525 The analysis showed that the TOF of (SiO)W(NAryl)(CHCMe2R)(X) relates to the σ-donating character of the imido and X ligands, evaluated by the NBO charge of the nitrogen atom in ArylNH2 and the pKa of HX, respectively as well as the sterics of the X ligand evaluated from the Sterimol B5 parameter (eq 1).314 This quantitative structure−activity relationship analysis of the large series of well-defined heterogeneous catalysts showed that high activity could be optimized by combination of X and NAryl ligands of opposite electronic character and by a simple and straightforward evaluation of the electronics and sterics of the ligands from readily available parameters.314 TOF3min = 53.8 + 16.6(NBON,ArylNH2 /B5,HX ) + 33.6(NBON,ArylNH2 /pK a,HX) 381

(1)


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4.2.2. Alkene Metathesis Using Silica-Supported Ru Alkylidenes. In contrast to Schrock-type alkylidene complexes, only couple of examples of well-defined silica-supported Ru alkene metathesis catalysts have been reported. The silicasupported siloxide substituted Hoveyda-Grubbs catalysts Ru-4b and Ru-5b (Scheme 12) are active in alkene metathesis. While having a lower activity than homogeneous analogues, Ru-4b and Ru-5b could be recycled more than 15 times in the ring closing metathesis of diethyl diallyl malonate.298 4.2.3. Alkene Metathesis with Re and W on AluminaContaining Supports. Re2O7 supported on alumina is an active heterogeneous catalysts for alkene metathesis that is somewhat unusual because it is active at room temperature and tolerant to functional groups when activated with alkyltin compounds.526 In contrast to the well-defined silica supported rhenium perhydrocarbyl discussed above (Re2m), rhenium oxide complexes are inactive when supported on silica.527−530 Active Re oxo-based catalysts are always supported on Lewis acidic supports.531 For instance, chemisorption of methyltrioxorhenium (MTO) on acidic oxide supports leads to highly active catalysts.358,378,404,532−536 For CH3ReO3/γ-Al2O3 the minor species (10−15%) was proposed to be a reservoir of highly active alkylidene species, a process shown in Scheme 43.377 While alkylidene intermediates have not yet been

Scheme 44. Proposed Formation of Tungsten Alkylidene Species from Tungsten Hydride on Alumina in the Presence of Ethylene

45a).541 The similarities between alkene and alkyne metathesis led to similar mechanistic proposals. Alkyne metathesis involves Scheme 45. (a) Early Example of Alkyne Metathesis; (b) Proposed Mechanism and Representative Homogeneous Catalysts; and (c) Single Site Supported Alkyne Metathesis Catalysts

Scheme 43. Surface Species Obtained by Grafting CH3ReO3 on γ-Al2O3

spectroscopically identified, the exchange of the μ-methylene ligand upon reaction with 13C−labeled olefin is consistent with this proposal.377,536 More recently, a similar μ-methylene species was observed for Re2O7/γ-Al2O3 activated with Me4Sn, suggesting the presence of similar active sites in both systems.537 With other supports, it is not yet clear what are the active sites, even if the presence of Lewis acid sites are critical for activity.532,403,404 Passivation of the alumina surface with Me3Si groups in CH3ReO3/Al2O3 results in an increase of the activity and the Zselectivity in the metathesis of propene into ethylene and 2butenes.535 These increases of performances have been related to the change of adsorption properties of the supports, which allow for a fast desorption of olefins and hence the faster rate and the observation of the kinetic Z-olefinic products. Dispersing ZnCl 2 on silica−alumina support prior to CH3ReO3 grafting was shown to greatly enhance olefin metathesis activity.358 Chlorine atoms interacting with Re centers have been proposed as key to the increase of activity.538 Tungsten hydride species supported on alumina catalyze ethenolysis of 2-butenes into propene and the cross-metathesis of isobutene and trans-2-butene affording propylene and isopentene products (Scheme 44).539,540 The alkylidenes are proposed to be formed by the reaction of tris-hydrides and ethylene yielding the corresponding tris-ethyl intermediate, which further undergoes α-abstraction to form the W( CHMe)(Et) species shown in Scheme 44.540 4.2.4. Alkyne Metathesis. WO3/SiO2 catalyzes the disproportionation of 2-pentyne at 200−450 °C (Scheme

the [2 + 2] cycloaddition of a metal alkylidyne (A) and an alkyne to form the metallocyclobutadiene (B) shown in Scheme 45b.542 This mechanism is supported by the isolation of stable metallocyclobutadienes and the dissociation of these complexes into free alkyne and carbyne in solution (Scheme 45b).543,544 Numerous early transition metal carbyne complexes catalyze alkyne metathesis in solution, a few representative catalysts are shown in Scheme 45b.543−546 (SiO)Re(CtBu)(CHtBu)(CH2tBu) was the first reported well-defined alkyne metathesis catalyst (Scheme 45c). It catalyzes the disproportionation of 15 equiv of 2pentyne into an equilibrium mixture of 2-butyne, 2-pentyne, and 3-hexyne within 20 min.301 (SiO)Mo(CEt)(N(tBu)Ar′)2 (Mo-20m) catalyzes the metathesis of a variety of alkynes with TOF’s ranging from 0.3−0.05 molsub molcat−1 s−1 and catalyst loadings of 4.0−0.2 mol % (25−500 equiv of alkyne) without the formation of undesirable oligomers.322 However, 382


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(Scheme 47c).268,275,277 As oxo complexes of early transition metals tend to form O-bridged oligomeric structures, which terminates the catalytic cycle, silica-supported oxo and imido complexes were recently utilized as catalysts taking advantage of site-isolation that prevents oligomerization. This strategy afforded heterogeneous catalysts that outperform their homogeneous counterparts by almost an order of magnitude (Scheme 47d).268,275,277 Among the reactions studied are imidation of aldehydes, ketones, and DMF with N-sulfinylamines (Scheme 47e), and condensation of isocyanates or Nsulfinylamines into carbodiimides or sulfurdiimines, respectively (Scheme 47f).

the corresponding system grafted on silica partially dehydroxylated at 140−200 °C leads to unselective catalysts that catalyze both alkyne metathesis and polymerization.547 This catalyst is compatible with alkynes containing alkyl, aryl, benzoate, thiophene, and methoxy substituents and can be recycled. (SiO)W(CtBu)(OAr)2 (W-34m) is inactive in alkyne metathesis at room temperature but becomes active at 80 °C with initial TOF of 0.32 molsub molW−1 min−1.324 The related complex (SiO)W(CtBu)(CH2tBu)2 (W-33m) is inactive in alkyne metathesis.323 In analogy to the example above, ( SiO)W2(NMe2)5 (W-2m) is also inactive in alkyne metathesis, but its reaction with 5 equiv of tBuOH forms (SiO)W2 (NMe2)5‑n(OtBu)n (W-2m-OtBu) which equilibrates 4-nonyne (50 equiv) in less than 30 min.158 However, control experiments suggest that metathesis with this system probably occurs in solution. (SiO)Mo(N)(Py)(OSiMe3)(NMe2) (Mo-22m)548 catalyzes alkyne metathesis between 45 and 80 °C with a sizable induction period.325 This metal-nitrido complex probably reacts with an alkyne to form the alkylidyne in situ. The addition of B(C6F5)3 increases the initial rate of metathesis and reduces the induction period observed in the absence of borane (Scheme 46). B(C6F5)3 assists the conversion of the MoN to the

4.4. Oligomerization of Alkenes

4.4.1. Ethylene Dimerization and Trimerization. High surface area silica supports containing well-defined chloroalkylaluminum species are very efficient cocatalysts for the Nicatalyzed dimerization of ethylene, eq 2.552,553

Et2AlCl grafted onto silica reacts with nickel complexes to dimerize ethylene in the presence of BiPh3 with moderate selectivities to butenes; propylene dimerization gives dimers of the monomer in good yields.553 Et2AlCl grafted onto silica activates (n-Bu3P)2NiCl2 to form an ethylene dimerization catalyst with TOFs up to 498,000 molC2H4 molNi−1 h−1.552 In contrast, the corresponding silica-supported trialkylaluminum species are not co-catalysts for this reaction. Ta(III) species also catalyze the selective trimerization of ethylene in solution.554,555 The surface Ta(V) complex Ta-4m catalyzes the formation of 1-hexene with high selectivity and TON up to 330.210,556 Analysis of the volatiles during the trimerization reaction showed the formation of methane, ethane, propylene and butane prior to formation of 1-hexene. The presence of these compounds was rationalized by the ethylene insertion, reductive elimination, and β-hydride elimination reactions shown in Scheme 48 to form the active Ta(III) complex Ta-4m-X. Ta-4m-X oxidatively couples ethylene to form a metallacyclopentane intermediate that inserts one additional molecule of ethylene to form a metallacycloheptane, similarly to the reactivity of Ta(III) in solution.554,555 Ta-metallacycloheptane complexes are unstable and undergo β-hydride transfer to release 1-hexene, as shown in Scheme 48.554,555 (SiO)3Ta also trimerizes ethylene to give 1-hexene, though in this case polyethylene is also formed as a byproduct.557 Detailed computational studies on this system showed that trimerization is favored when a trigonal bipyramidal geometry is enforced at the tantalum center because this promotes β-hydride elimination of the metallacycloheptane intermediate (Scheme 49). Distortion of this species to square pyramidal environment opens a coordination

Scheme 46. Formation of Proposed Active Species in Alkyne Metathesis Catalyzed by Silica-Supported Molybdenum Nitride Mo-22m in the Presence of B(C6F5)3

alkylidyne ligand, probably by removing pyridine from the metal center. The addition of Lewis acid to this catalyst results in TOF’s as high as 11.9 mol mol−1 min−1. A summary of the catalytic activity of various alkyne metathesis catalysts is compared in Table 7, below. The TOF’s have been converted to comparable units for ease of comparison. 4.3. Oxo/Imido Heterometathesis

Imidation of oxo complexes with isocyanates and N-sulfinylamines is well-known in molecular chemistry549,550 and has been used to probe the structure of surface oxo complexes of Re,551 V,272,275 and Ta463 (Scheme 47a). Similarly, imido complexes also participate in imido-transfer reactions with aldehydes and other oxo-containing compounds, the stoichiometric reaction being demonstrated for supported V272,275 and Ta268 imido complexes (Scheme 47b). Being combined, these two transformations constitute a catalytic cycle referred to as oxo/imido heterometathesis in analogy to olefin metathesis

Table 7. Reactivity of Silica Supported Alkyne Metathesis Catalysts complex

substrate

TON

Re2m Mo-20mb W-34ma Mo-22mb Mo-22m + 2 equiv B(C6F5)3b

MeCCEt PhCCMe MeCCEt PhCCMe PhCCMe

15

a

a

18 460 321

TOF (mol mol−1 s−1)[T (°C)] − 0.051 0.005 − 0.2

[22] [24] [80] [45] [110]

Reaction performed in gas phase. bReaction performed in solution. 383


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Scheme 47. (a) Imidation of Vanadium Surface Oxo Complex; (b) Imido-Transfer Reaction of Tantalum Surface Imido Complex; (c) Catalytic Cycle Proposed for Oxo/ Imido Heterometathesis Reactions; (d) Supported Catalysts Used in Oxo/Imido Heterometathesis Reaction; (e) Heterometathetical Imidation of Organic Carbonyls with NSulfinylamines; and (f) Condensation of Isocyanates into Carbodiimides and N-Sulfinylamines into Sulfurdiimines

Scheme 48. (SiO)TaMe2Cl2-Catalyzed Trimerization of Ethylene to form 1-Hexene

Scheme 49. Reaction of (SiO)3Ta and Ethylene: Formation of Polyethylene vs 1-Hexene

butenes to form propylene.559 Deactivation of the WH3/Al2O3 catalyst is associated with ethylene insertions into W−alkyl bonds to form low density polyethylene that limits catalytic turnover.560 More recently, an ETP catalyst was prepared by treating (SiO)W(O)Cl3 with Me2Zn yielding the putative reaction intermediate (SiO)W(O)Me3. This ETP catalyst is able to reach 324 turnovers at 150 °C.561

site for ethylene, which leads to olefin insertion and polymeric byproducts. 4.5. Direct Conversion of Ethylene to Propylene

The direct ethylene-to-propylene transformation (the ETP reaction), originally discovered with Ni-doped mesoporous silica (MCM41),558 is also catalyzed by WH3/Al2O3. This system converts ethylene to propylene with >95% selectivity for propylene at 150 °C and 1 bar ethylene pressure in a continuous flow reactor, giving TON of 1120 after 120 h on stream.559 The proposed mechanism for the ETP reaction catalyzed by WH3/Al2O3 is presented in Scheme 50. WH3/ Al2O3 enters the catalytic cycle by forming the W(CHMe) (Et) alkylidene followed by insertion of ethylene and β-H elimination to give 1-butene and W(CHMe)(H). The latter isomerizes 1-butene to 2-butenes, and any of the alkylidenes in Scheme 50 can catalyze the metathesis of ethylene and 2-

4.6. Polymerization Reactions

The first catalysts implemented for the polymerization of ethylene were heterogeneous. The Ziegler−Natta catalyst contains TiCl4 supported on MgCl2 activated with AlR3 cocatalysts to generate active sites that polymerize ethylene. The Phillips catalyst contains chromium sites supported on silica and polymerizes ethylene in the absence of a cocatalyst. Several groups described methods to access well-defined heterogeneous catalysts inspired by the Ziegler−Natta and Phillips catalysts. 4.6.1. Single-Site Models of Ziegler−Natta Type Catalysts. The active sites in the Ziegler−Natta catalyst are 384


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Scheme 50. Postulated Mechanism for the ETP on WH3/Al2O3

from the borate anion, a common decomposition pathway in homogeneous catalysts,562 to form (SiO)Zr(Cp*)(C6F5)Me and MeB(C6F5)2. Grafting organometallic complexes on supports capable of abstracting alkyl groups also provides active polymerization catalysts with well-defined sites. As discussed above, the reaction of M(CH2tBu)4 (M = Zr, Hf) with alumina dehydroxylated at 500 °C forms a mixture of surface species: (AlO) 2 M(CH 2 tBu) 2 and [(AlO) 2 M(CH 2 tBu) + ][AlSCH2tBu−].370,371 The Zr- and Hf-derivatives were shown to have good activities in the polymerization of ethylene without the need for a cocatalyst (eq 3), consistent with the formation of zwitterionic [(AlO) 2 M(CH 2 tBu)][AlSCH2tBu].370,371,563 Similar results were obtained by grafting Cp*ZrMe3 on silica−alumina or alumina.222

commonly proposed to be highly electrophilic cationic organometallic species. This proposal is supported by extensive studies on homogeneous olefin polymerization catalysts. Welldefined heterogeneous catalysts containing similar active sites as proposed in the Ziegler−Natta catalyst have been reported. The polymerization properties are dependent on the support and the polymerization conditions, as this affects the formation and stability of the active site. Silica-supported catalysts generally need a cocatalyst to form the active site. For example, (SiO)ZrCp*Me2 is inactive in the polymerization of ethylene. Adding B(C6F5)3 to (SiO)ZrCp*Me2 probably forms the ion-pair [(SiO)ZrCp*Me][MeB(C6F5)3], which is active in the polymerization of ethylene (Scheme 51a).222 Scheme 51. (a) Activation of (SiO)ZrCp*Me2 with B(C6F5)3 to form [(SiO)ZrCp*Me][MeB(C6F5)3] and (b) Deactivation Pathways Encountered with [( SiO)ZrCp*Me][MeB(C6F5)3]

Complexes supported on sulfated metal oxides are also active in ethylene polymerization.4,14,15,444,449,564 As discussed above, the sulfated oxide support contains acidic OH functional groups that effectively behave as weakly coordinating anions after organometallic functionalization. In many cases the polyethylene generated with these catalysts cannot be extracted from the heterogeneous particle, even at 140 °C in 1,2,4trichlorobenzene.448,564 These results indicate that the polyethylene formed using these supported catalysts has ultrahigh molecular weights. 4.6.2. Single-Site Models for the Phillips Catalyst. A few years prior to the discovery of the Ziegler−Natta catalyst researchers at Phillips petroleum showed that silica containing reduced chromium sites polymerize ethylene.565 The Phillips precatalyst contains Cr(VI) sites, shown in Scheme 52, which are prepared by incipient wetness impregnation of silica with an aqueous solution of CrO3 or Cr(OAc)3 followed by calcination at high temperatures (>400 °C). High density polyethylene is formed when the Cr(VI) sites are contacted with ethylene. This catalyst is unique among ethylene polymerization catalysts because it does not contain a preformed Cr−C bond and polymerizes ethylene in the absence of an activator. Though the active site of the Phillips catalyst is unknown, it is formed in a complex series of redox events. It was shown that ethylene reacts with Cr(VI) sites on silica to form aldehydes, ketones, and mostly Cr(II) on the silica surface.566 Several groups showed that isolated Cr(II) sites are present on the silica surface after treatment with CO, a common model for the

Alternatively, cationic Zr-species supported on silica are accessible by the reaction of [HNEt2Ph][(SiO)B(C6F5)3] with Cp*ZrMe3 to form the [Cp*ZrMe2(NEt2Ph)][(SiO)B(C6F5)3],342 although this complex is unstable under polymerization conditions.224 Silica supported Cp*TiMe3 catalysts were also reported, though in this case AlMe3 cocatalysts were used to form active sites.219 These highly electrophilic Zr-species supported on silica are unstable under polymerization conditions and react with the silica surface to form inactive sites. For example, [(SiO)ZrCp*Me][MeB(C6F5)3] decomposes by two competitive processes shown in Scheme 51b.223,342 Alkyl transfer to the silica surface forms [(SiO)2ZrCp*][MeB(C6F5)3] and Si−Me; these species do not contain a Zr−Me group and cannot polymerize ethylene. Another decomposition process is C6F5 transfer 385


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β-hydride elimination571 to form Cr-alkyl-hydride surface species that can propagate polymer growth. The “ligandless” Cr-silicates discussed above contain welldefined sites in controlled oxidation states (see section 3.3, Scheme 29).197,198,572−574 However, (SiO)4Cr2 (Cr-6T) has poor polymerization activity, which was completely suppressed in the presence of 0.2 equiv of 4-picoline poison per chromium site. Cr-6T also contains a small amount of Cr(III) sites according to EPR spectroscopy, suggesting that Cr(III) sites may be active in polymerization. Ethylene polymerization experiments with (SiO)6Cr2 (Cr-6T-Ox, see section 3.4, Scheme 35) produces a polymer with large dispersity Đ = Mw/ Mn = 9.4 (with Mn = 5500 g mol−1 and Mw = 52 000 g mol−1) that is typical for polyethylene produced with the Phillips catalyst. This catalyst contains ca. 65% active sites by poisoning studies with 4-methylpicoline. Monomeric Cr(III) containing (SiO)3Cr (Cr-5T) also polymerizes ethylene with similar activity and produces polyethylene with similar properties as Cr-6T-Ox. Also similar to Cr-6T-Ox, roughly 60% of the sites in Cr-5T are active in polymerization according to poisoning studies with 4-methylpyridine. The higher polymerization activity of Cr(III) sites compared to Cr(II) containing materials is consistent with numerous examples in homogeneous chemistry,575,576 and reports of Cr(III) salts supported on silica.577,578 Cr-6T-Ox and Cr-5T have high concentrations of active sites on the silica surface, and both initiate polymerization without a noticeable induction period. Since the edge position of the XANES spectra of Cr-6T-Ox and Cr-5T is not altered upon exposure to ethylene, the oxidation state of the Cr-sites is conserved after catalysis. The formation of the first Cr−C bond was proposed to occur by the heterolytic cleavage of the C−H bond of ethylene to form Si-(μ−OH)-Cr-vinyl species shown in Scheme 54. Propagation of the polymer chain occurs by insertion polymerization, and termination is the microreverse of initiation.

Scheme 52. Reactivity of Cr(VI) Sites in the Phillips Catalyst with Ethylene and CO to form Cr(II) Reduced Sites

active Phillips catalyst.3,567,568 This has led to the general assumption that the isolated Cr(II) sites shown in Scheme 52 are active in polymerization using this catalyst. The Phillips catalyst contains only ca. 10% active sites,569 which has led to significant efforts to synthesize well-defined Cr-catalysts for the polymerization of ethylene. The Phillips catalyst likely contains organometallic intermediates, and initial well-defined studies used (SiO)2Cr(CH2tBu)2 and (SiO)2Cr(CHtBu) as models for active species in this system (Scheme 53a). At low ethylene pressures Scheme 53. (a) Formation of (SiO)2Cr(CHtBu) and Its Reactivity with Ethylene and (b) Polymerization Propagation Mechanism

Scheme 54. Proposed C−H Activation Step to form a Cr−C Bond in Well-Defined Cr(III) Silicates

(ca. 60 Torr) (SiO)2Cr(CH2tBu)2 does not initiate the polymerization of ethylene. However, under these conditions the putative (SiO)2Cr(CHtBu) catalyzes the formation of high density polyethylene. Detailed kinetic studies of this catalyst showed that the polymerization is first-order in chromium and follows pseudo first-order behavior in ethylene. In addition, polymerizations with ethylene-d4 gave a kinetic isotope of k(C2H4)/k(C2D4) = 1.29 ± 0.05. The small primary kinetic isotope effect is inconsistent with an alkylidene− metallacycle mechanism since this would require a 1,3hydrogen migration in the chromacycle to regenerate the alkylidene. However, this result is consistent with the olefin insertion propagation mechanism, shown in Scheme 53b.570 (SiO)2Cr(CHtBu) was proposed to initiate polymerization by [2 + 2] cycloaddition with ethylene to form the chromacyclobutane (Scheme 53a). The latter can insert ethylene to form expaned metallacycles that eventually undergo

This mechanism was investigated by DFT using the cluster models described in section 3.2.1.1. Using the B3LYP functional the heterolytic C−H activation of ethylene has a moderate barrier, and the overall thermodynamic driving force of the reaction is the insertion of ethylene into the Cr-vinyl bond to propagate polymerization.197 The insertion of ethylene into the Cr-alkyl bond is lower than chain transfer by 10 kcal 386


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mol−1 in the absence of ethylene and lower by 4 kcal mol−1 in the presence of coordinated ethylene.579 4.6.3. Supported Late Transition Metal Catalysts for the Polymerization of Ethylene. (SiO)Ni(α-diimine) (CH2SiMe3) (Ni-3m) is inactive in the polymerization of ethylene.246 Ni(II) olefin polymerization catalysts are cationic d8 square planar complexes containing an open coordination site.580 In Ni-3m the nickel center is saturated and cannot coordinate ethylene. Addition of gaseous BF3 to Ni-3m results in the formation of the ion pair [Ni(α-diimine)(CH2SiMe3)][(SiOBF3)] (Ni-3m-BF3) shown in Scheme 55a. Ni-3m-BF3

Scheme 56. (a) Polymerization of Methylmethacrylate, (b) β-Butyrolactone, (c) Styrene, and (d) Isoprene by Supported Complexes

Scheme 55. (a) Formation of [Ni(αdiimine)(CH2SiMe3)][(SiOBF3)] and (b) Formation of Ni-4SiO2

Scheme 57. Alkane Hydrogenolysis Catalysts

polymerizes ethylene with moderate activities (215 kgPE molNi−1 h−1 bar−1) that are lower than similar nickel complexes supported on alkylaluminum doped silica,581,582 or nickel complexes supported on alkylaluminum doped MgCl2.583 The reaction of (SiO)B(C6F5)2 with [N-phenyl-2-(phenylimino)propanamidato-κ 2 -N,N](η 1 -benzyl)-(trimethylphosphine)nickel(II) (Ni-4)584 forms the cationic nickel complex Ni-4SiO2 supported on silica shown in Scheme 55b.344 Ni-4SiO2 polymerizes ethylene with similar activities as homogeneous equivalents. The polyethylene produced by Ni-4SiO2 has a rather low dispersity Đ = Mw/Mn = 2.2, indicating that the active species in Ni-4SiO2 acts as a single-site catalyst. 4.6.5. Polymerization of Other Substrates. Ln[N(SiMe3)2]3 (Ln = La, Nd, Sm) supported on partially dehydroxylated silica are catalysts for the polymerization of methyl methacrylate (Scheme 56a).173,183 These catalysts produce isotactic methyl methacrylate polymers with narrow polydispersities. Ln(BH4)3 (Ln = La, Nd) supported on partially dehydroxylated silica catalyze the ring-opening polymerization of β-butyrolactone to give isotactic polylactones (Scheme 56b).585 Organocalcium reagents grafted onto silica polymerize styrene to give low-molecular weight syndiotactic polystyrene (Scheme 56c).586 Monophosphacyclopentadienyl bis(tetramethylaluminate) lanthanide complexes and Ln(AlR4)3 supported on high surface area SBA15 and MCM48 are active in the polymerization of isoprene to give cis-1,4-polyisoprene (Scheme 56d).236,237

linear and branched alkanes, with the exception of ethane, to form mixtures of methane and ethane as final products (see Scheme 58a for the hydrogenolysis of propane).132,139,456,460,461,486,587 Overall, comparing the catalytic properties of Zr, Hf, Ta, and W hydrides supported on silica− alumina shows that the catalysts follow the trend in activity of Zr > Hf > Ta > W and that Ta and W-hydrides are also able to catalyze the hydrogenolysis of ethane. The methane to ethane ratio depends on the initial alkane. The mechanism shown in Scheme 58b,c involves activation via σ-bond metathesis of CH3CH2R with M−Hx to form M− CH2CH2R species that undergo β-alkyl transfer to form M(CH2CH2)(R) species. This elementary step of carbon− carbon cleavage is consistent with the final ethane/methane

4.7. Alkane Homologation Processes and Related Reactions

4.7.1. Alkane Hydrogenolysis. The supported metal hydrides shown in Scheme 57 catalyze the low temperature hydrogenolysis of alkanes. In particular, group 4 metal hydrides (Ti, Zr and Hf, Scheme 57a) catalyze the hydrogenolysis of 387


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suggest that C−H bond activation corresponds occurs via a σbond metathesis,487 which is easier on dihydride than monohydride surface species (Scheme 58c).458 Hydrogenolysis of alkanes is also observed in the presence of H2 with Zr hydrides supported on alumina459 or aminemodified SBA15 (Scheme 57c,d).590,591 In this study the Zrdihydride linked through [N,O]-chelating surface ligands was proposed to be more active than the analogous system linked via [N,N]-ligands. (SiO)2TaHx (Scheme 57b) also catalyze the hydrogenolysis of alkanes, including ethane in contrast to group 4 metals.592 In this case methane is the final product for acyclic alkanes. (SiO) 2TaHx also cleave the C−C bond of cycloalkanes to yield acyclic and cyclic products. These results indicate that the hydrogenolysis mechanism must differ from that of supported group 4 hydrides. The proposed mechanism for (SiO)2TaHx with alkanes is shown in Scheme 59a. The

Scheme 58. Hydrogenolysis of Alkanes on Supported Metal Hydrides

Scheme 59. (a) Proposed Mechanism of Alkane Hydrogenolysis Catalyzed by Silica-Supported Ta Hydrides and (b) The Case of Cyclic Alkanes

key step of carbon−carbon cleavage is an α-alkyl transfer, which corresponds to the microreverse step of alkyl insertion to the TaC bond.593,594 The α-alkyl transfer mechanism would explain why all acyclic alkanes are converted to methane. Computational studies suggest that the (SiO)2TaH3 are the active species and that the rate-determining step is the α-alkyl transfer step.595,596 More recent studies suggest that β-alkyl transfer may also operate in the hydrogenolysis of higher alkanes.597 For cyclic alkanes the structure of the products suggest that a key step is γ-H abstraction followed via retrocyclization and metathesis like reaction (Scheme 59b).490

ratio of 1:1 for the hydrogenolysis of propane. The observation of methane to ethane ratio close to 1:1 in the hydrogenolysis of butane shows however that β-alkyl transfer from M-butyl and M-sec-butyl intermediates are equally probable (Scheme 58d).405,406 This mechanism is also consistent with the observation that cyclopentane and cyclohexane do not undergo hydrogenolysis because β-alkyl transfer requires a syn-coplanar arrangement of the M−C-Cβ and C−Cβ-C bonds (Scheme 58e). DFT calculations on cluster or extended models also agree with the mechanism in Scheme 58.456,588,589 These studies 388


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4.7.2. Alkane Metathesis. Alkane metathesis is a reaction that disproportionates an alkane into its lower and higher homologues. The reverse reaction (called alkane cross-metathesis) is also possible, in which larger and smaller alkanes comproportionate, yielding alkanes of intermediate size. This reaction was discovered by using heterogeneous dehydrogenation/hydrogenation and alkene metathesis catalysts in tandem.598 The tandem catalysis concept has also been used to develop homogeneous systems that catalyze alkane metathesis.599 Alkane metathesis is also catalyzed by well-defined group 5 and 6 supported metal-hydrides (Scheme 60). These heterogeneous catalysts are multifunctional because they perform dehydrogenation/hydrogenation and metathesis steps at a single metal site.

Scheme 61. (a) Ta Hydride on Zr Doped Silica and (b) Cross Metathesis of Ethane and Toluene

mediates metathesis, albeit with much lower TON (98 −e 20.0 −e 80g 98.4 98.9 45 24

62.5 176 643 116 633 132 83 957 64.8 2.6

185 615 193 617 263 266 617 326 199 243

a Total conversion of peroxide (%). bEpoxide yield based on peroxide (%). cSelectivity for epoxidation product (%). dTON = mol epoxidation product/mol metal. eno tabulated data available. fRcap = n-Bu. gepoxide + diol.

catalysts, cumene hydroperoxide (CHP) gives high activity and selectivity to the cyclohexene oxide in toluene at 65 °C (Table 8, entry 3). The catalytic activity of materials prepared by grafting molecular precursors with low Ti/Si ratio (Ti-8, Ti-10)

Ti-8, 9, and 10 on Aerosil silica, MCM41 or SBA15 partially dehydroxylated at 200 °C form mostly monografted tetrahedral Ti species Ti-8m, 9m, and 10m shown in Scheme 66.193 While H2O2 is ineffective in the epoxidation of cyclohexene with these 392


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Scheme 67. Capping Strategy for Improved Epoxidation Catalysts

Scheme 68. Proposed Epoxidation Mechanisms with Ta and Ti Catalysts

cyclohexene epoxidation at 60 °C in dry octane using CHP or TBHP as oxidant (Table 8, entry 5).263 The reaction pseudo first order rate constants and hot filtration experiments indicate that no deactivation, leaching or product inhibition took place. Activity of a related complex Ti-22m was lower than Ti-20m, demonstrating that the identity of the supporting calixarene ligand influences the catalytic properties of the material.261 The calixarene approach was extended to include Ta(V) complexes that upon grafting were proposed to give mono- and bipodal surface species, such as Ta-9b (Table 8, entry 6).265,266 Comparing Ta and Ti calixarene systems, Ta complexes provide higher selectivity to cyclohexene oxide when using H2O2 than respective Ti catalysts, although the Ta system gave high amounts of cyclohexane-1,2-diol. Capping of surface silanols with octanol allowed us to reduce this secondary hydrolysis pathway.266 Ta-3m and Ta-3b, obtained by grafting of (iPrO)2Ta[OSi(OtBu)3]3 on SBA15−130, enabled epoxidation of cyclohexene in good selectivities with organic peroxides (94% with CHP and 70% with TBHP) and poor selectivity with H2O2 (36%) in acetonitrile at 65 °C.196 Exposure of Ta-3m and Ta-3b to air at 200 °C followed by capping with (N,N-dimethylamino)trialkylsilane was proposed to give surface sites Ta-14t that featured improved catalytic performance with H2O2 (Table 8, entry 7).450 While capping has a modest effect on activity, the selectivity for epoxide was improved to 98%, by comparison with only ca. 32% with the surface unmodified catalyst. After 6 h, the epoxide yields produced by capped catalyst Ta-14t were ca. 5 times higher than with the unmodified catalyst, emphasizing that hydrophobic surface capping suppressed poisoning of the active Ta sites by water. It was also proposed that the siloxy group on Ta remains within its coordination sphere during the catalysis.450,619 This strategy to modify surface properties for improved catalysis was further extended to include related Ta-15t species with trimethylsilyl, -germyl, and -stannyl groups.620 Ta-15t has higher selectivity for cyclohexene oxide and 1,2-cyclohexene diol and slower rates of H2O2 decomposition than the unmodified catalyst. However, Ta-15t has a slower rates of cyclohexene epoxidation with H2O2 after 10 h compared to the unmodified catalyst.620 Ge

was improved, with turnover numbers up to 2,000.193 Ligandfree catalysts were prepared by calcination of Ti-8m, 9m, and 10m and showed reduced activity, possibly due to migration of the titanium centers into the support, making the catalytic sites less accessible.193 In order to evaluate the effect of a dimeric Ti precursor, [(tBuO)2Ti{μ-O2Si[OSi(OtBu)3]2}]2 was grafted on SBA15−130 to give Ti-11m epoxidation catalyst.195 Ti-11m has similar activity and selectivity for cyclohexene epoxidation with organic hydroperoxides as Ti-8m, 9m, and 10m. Several parameters were suggested to determine high catalytic activity of Ti catalysts including more Lewis acidic Ti sites formed from Si-rich molecular compounds, tetrahedral Ti(IV) sites, suppressed nucleation to octahedral TiO2 and high dispersion of Ti sites.193,195 Ti[OGe(iPr)3]4 on SBA15−120 (Ti-12m) was compared to the siloxide-only precursors for cyclohexene oxidation with TBHP and found to produce 2−3 times higher TONs after 9h under identical conditions.191 The activity and selectivity of Ti-8m and Ti-11m in epoxidation using H2O2 can be improved by capping surface OH groups with Me2N− SiMe2(R) (R = Me, n-Bu, or n-octyl, Scheme 67).616 When compared with uncapped Ti materials, species on modified surfaces such as Ti-27t exhibit up to 58% selectivity for cyclohexene oxide over the undesired allylic oxidation products (Table 8, entry 4).616,617 Treating Ti-10m capped with Me2N−SiMe2(R) at 200 °C under O2 exposes free Ti− OH.194 Modification of these titanols with phenols and carboxylic acids results in 10 to 50% higher conversions and selectivity of 1-octene epoxidation with TBHP or H2O2.194 Isolated Ti centers as well as oligomerized Ti species (TiO2 nanoclusters) could be prepared by grafting Ti(OiPr)4 on mesoporous aluminophosphate (AlPO) supports that contain reactive surface Al−OH and P−OH groups.618 The catalytic activity and selectivity of cyclohexene epoxidation with TiAlPO is slightly lower than that of Ti-SBA15 material with comparable Ti loadings and using TBHP; additionally, Ti-AlPO materials form more allylic oxidation products.618 The isolation of Ti sites could also be obtained by grafting molecular precursors containing a large calixarene ligand. The corresponding grafted complex Ti-20m displays a rigid pseudotetrahedral geometry, and TONs up to 633 for the 393


Chemical Reviews

Review

Scheme 69. (a) Industrial Synthesis of Adipic Acid; (b) Catalytically Active Species and Their Activity in Cycloalkane Oxidation; (c) Deperoxidation with M = Ti, R = tBu, H and M = Zr, Hf, R = tBu; (d) Benzylic Oxidation of Alkyl Aromatics; and (e) Methane Oxidation

entry 9).199 Fe-13m does not catalyze cyclohexene epoxidation with TBHP, it slowly forms cyclohexene oxide with H2O2 in acetonitrile with 7.3% conversion after 1 h and 24% selectivity (Table 8, entry 10).243,621 The poor selectivity is indicative of competing radical oxidation pathways.

and Si capped catalysts performed better in terms of rate and selectivity than Sn modified catalysts. Though the activity with Ta was modest, these results suggested that Ta(V) surface species are more water-tolerant than the more active Ti(IV) catalysts. It was proposed that oxygen transfer from Ti and Ta sites occurs via an electrophilic transfer to the alkene from a peroxide intermediate and that side products are formed via radical manifolds (Scheme 68).195,450,619 Mo-23m catalyzes olefin epoxidation with TBHP in refluxing DCM, and has higher activities than (tBuO)3Mo(N).326 However, Mo-23m deactivates during recycling due to ∼15% Mo leaching in each cycle (Table 8, entry 8). Mo-3m and W6m have activities and selectivities below 10% in cyclohexene epoxidation with TBHP (toluene, 65 °C); W-6m is more active than Mo-3m.284 These materials deactivate relatively fast, presumably due to the formation of water and alcohol side products. In contrast to Ti materials described above, calcination does not have a strong effect on the performances of Mo-3m and W-6m.284 Grafting Fe(OSi(OtBu)3)3(TMEDA) on SBA15−190 followed by calcination at 300 °C for 2h gives Fe-8b/t species that epoxidizes cyclohexene with H2O2 in acetonitrile at room temperature giving TON value of ca. 65 after 1 h (Table 8,

4.10. Oxidation and Deperoxidation of Alkanes

The conversion of cyclohexane to cyclohexanol via an intermediate cyclohexyl hydroperoxide is a key step in industrial process to make adipic acid shown in Scheme 69a.131 Homogeneous chromium-complexes catalyze this reaction to give cyclohexanol and cyclohexanone at higher than 100% selectivity, indicating that activation of the solvent (cyclohexane) by cyclohexyl hydroperoxide occurs. Isolated Fe(III) silicates oxidize cyclohexane to cyclohexanol in the presence of H2O2 at 60 °C with high selectivity.199,200 Longer reaction times lead to decreased selectivity due to competing cyclohexanone formation (Scheme 69b).199 Isolated Fe(III) silicates convert adamantane to a mixture of tertiary and secondary alcohols in a 3.1/1 ratio, as well as 2-adamantonone with an overall TON of 15. The ratio of tertiary to secondary alcohol products suggests that a nonradical oxidation pathway occurs in this reaction.199 394


Chemical Reviews

Review

conditions and give TON values similar to the Ti system (Table 9, entries 6−8).186 In contrast to homogeneous catalysts, the observed selectivities with Ti, Zr, Hf and Ta supported catalysts does not show evidence for cyclohexane oxidation by cyclohexyl hydroperoxide according to a pathway analogous to that presented in Scheme 69b.131,186 Co-3m enables selective benzylic oxidation of alkyl aromatics with TBHP in acetonitrile solution at 80 °C (Scheme 69d), presumably via a radical mechanism.245 Oxidation of ethylbenzene with Co-3m is rapid in the first hour (TOF up to 582) slowing down over the course of the reaction likely due to the catalyst poisoning with H2O. Single-site vanadium silicates (SiO)3VO, prepared from well-defined (SiO)V(O)(OR)2 grafted species (R = Si(OtBu)3 or tBu)279 catalyze the oxidation of methane to formaldehyde by molecular oxygen at 400 °C yielding ca. 2.4% formaldehyde at 8−15% methane conversion with a space-time yield of 5.84 kgald kgcat−1 h−1 (Scheme 69e).279

Well-defined heterogeneous catalysts of the general type ( SiO)3MOR (M = Ti, Zr, Hf; R = H, tBu) (Scheme 69c) also catalyze this reaction.131 (SiO)3MOtBu surface species show activity in the deperoxidation of cyclohexyl peroxide in the order Ti > Zr > Hf. (SiO)3TiOtBu reaches full conversion in 9 h with substrate/metal ratio =260.131 (SiO)3TiOH and ( SiO)TiNp3 (Ti-1m) show similar conversion vs time profiles and turn over numbers as compared to (SiO)3TiOtBu, indicating that (SiO)3 TiOH and (SiO)TiNp 3 are converted to catalytically active species (Table 9, entries 1− 5).131 Table 9. Comparison of the Activities and Selectivities of Silica-Supported Single-Site Catalysts for the Deperoxidation of Cyclohexyl Hydroperoxide at 84 °C131,186 entry

catalyst

S/Ca

time (h)

conv. (%)

1 2 3 4 5 6 7 8

(SiO)3HfOtBu (SiO)3ZrOtBu (SiO)3TiOtBu (SiO)3TiNp3 + O2 (SiO)3TiOH (SiO)Ta(OMe)4 (SiO)2Ta(OMe)3 (SiO)3Ta(OMe)2

260 350 260 312 252 270 250 240

6 6 6 6 6 6 9 8

30 40 88 77 87 88 66 88

sel.b

one/olc

4.11. Other Selected Catalytic Transformations 85 79 81 87 87 60

4.11.1. Hydroamination. Silylamides of Y, La, and Nd on SBA15−500 catalyze intramolecular hydroamination/cyclization of 2,2-dimethyl-4-pentene-1-amine in benzene at elevated temperatures.172 A mixture of Y-1m and Y-1b achieves high rates with 5.7% catalyst loading cleanly converting the amine substrate in

Surface Organometallic and Coordination Chemistry toward Single

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