Isolated Surface Hydrides: Formation, Structure, and Reactivity

Review pubs.acs.org/CR

Isolated Surface Hydrides: Formation, Structure, and Reactivity Christophe Copéret,* Deven P. Estes, Kim Larmier, and Keith Searles Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1-5, CH-8093 Zürich, Switzerland ABSTRACT: Surface hydrides are ubiquitous in catalysis. However, their structures and properties are not as well-understood as those of their molecular counterparts, which have been extensively studied for the past 70 years. Hydrides isolated on surfaces have been characterized as stable entities on oxide surfaces or in zeolites. They have also been proposed as reaction intermediates in numerous catalytic processes (hydrogenation, hydrogenolysis, etc.). They have also been prepared via surface organometallic chemistry. In this review, we describe their key structural features and spectroscopic signatures. We discuss their reactivity and stability and also point out unexplored areas.

CONTENTS 1. Introduction 2. Supported Metal Hydrides Prepared via Surface Organometallic Chemistry 2.1. Supported Hydrides of Early-Transition Metals (Groups 4−7) 2.1.1. Group 4 Metal Hydrides Grafting on Amine-Modified Silica Surface Alumina-Supported Zr Hydrides Silica−Alumina-Supported Zirconium Hydrides 2.1.1.2. Titanium Hydrides Silica-Supported Titanium Hydrides Silica−Alumina Supported Titanium Hydrides 2.1.1.3. Hafnium Hydrides Silica- and Alumina-Supported Hafnium Hydrides 2.1.2. Group 5 Metal Hydrides 2.1.3. Group 6 Metal Hydrides (Cr, Mo, W) 2.1.4. Reactivity of Supported Early-TransitionMetal Hydrides 2.2. Late-Transition-Metal-Hydrides: Structure and Reactivity 2.2.1. Ruthenium 2.2.2. Rhodium 2.2.3. Iridium 2.3. Main Group Hydrides 2.3.1. Group 13 2.3.2. Group 14 2.3.3. Main Group Hydride Reactivity 2.4. Putative Supported Metal Hydrides 2.5. Concluding Remarks 3. Hydrides in Zeolites 3.1. Ga-Exchanged Zeolites 3.2. Zn-Exchanged Zeolites 4. Surface Hydrides on Bulk Oxides © 2016 American Chemical Society

4.1. Zinc Oxide 4.2. Group 2 Metal Oxides 4.3. Group 13 (Al2O3, Ga2O3, In2O3) 4.3.1. Al2O3 4.3.2. Ga2O3 4.3.3. In2O3 4.4. Group 14 (SiO2, GeO2, SnO2) 4.4.1. SiO2 4.4.2. GeO2 4.4.3. SnO2 4.5. Lanthanide, Group 3, and Transition-Metal Oxides 4.5.1. CeO2 4.5.2. TiO2 4.5.3. ZrO2 4.5.4. WO3 4.5.5. Others 4.6. Hydrides on Bulk Oxide Surfaces as Intermediates in Hydrogenation or Dehydrogenation Reactions 4.6.1. C−C and CC Bonds 4.6.2. C−O and CO Bonds 5. Conclusions and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

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Special Issue: Metal Hydrides Received: February 2, 2016 Published: July 11, 2016 8463

DOI: 10.1021/acs.chemrev.6b00082 Chem. Rev. 2016, 116, 8463−8505

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1. INTRODUCTION

Scheme 2. Synthetic Strategies for Supported Metal Hydrides

Hydrides are ubiquitous throughout molecular chemistry, surface science, and solid-state chemistry as either isolable or transient species. It is thus not surprising that hydrides are involved in many important processes, from H2 storage to catalytic processes, such as hydrogenation/dehydrogenation, hydrogenolysis (cracking), or reforming. Molecular hydrides, in particular transition-metal hydrides, have been studied in great detail because of their relevance to many homogeneous catalytic processes (hydrogenation, hydroformylation, hydrosilylation, etc.). These metal hydrides have been shown to react as hydride, hydrogen atom, or proton donors, depending on the coordination sphere of the metal (Scheme 1). They possess interesting properties and are useful in a broad range of applications. Hydrides on surfaces also play an important role in heterogeneous catalysis. However, their structures and reactivity are much less understood than those of their isolated molecular counterparts, due to the difficulty in analyzing structures in solids. Nevertheless, surface hydrides can be formed through the controlled functionalization of surfaces via surface organometallic chemistry.1−5 These surface hydrides retain most of the properties of the molecular complexes. However, they are typically present as a mixture of species because of the various local environments at the surface of oxide materials or the presence of different surface sites. However, they are often more thermally stable than their molecular counterparts due to site isolation on the support (preventing bimolecular decomposition). They are also usually more reactive than their molecular counterparts due to lower coordination number (vide infra). Hydrides are also obtained upon heterolytic dissociation of H2 on bulk surfaces (e.g., metals, bulk oxides, or zeolites). However, the surface of an oxide is often complex, exhibiting a variety of potential reactive surface sites on which the hydride can be formed. As a consequence, many different hydrides can be formed on such surfaces, making them more difficult to characterize. Hydrides on metallic surfaces are widely encountered in a number of relevant processes. These hydrides exhibit high surface mobility and borrow their reactivity from the entire ensemble. Thus, they cannot be considered isolated complexes. They constitute a very different category of hydrides that have been reviewed in several dedicated monographs and will not be discussed here.6 In this review, we give an overview of the synthesis, characterization and properties of isolated hydrides on surfaces. We divide this subject into three sections: metal hydrides (1) prepared by a surface organometallic approach, (2) in zeolites, and (3) in bulk oxides. Each section will describe the structure and spectroscopic signatures of the various hydrides, followed by discussing their reactivity and involvement as key intermediates in catalysis.

Scheme 3. Hydrogenolysis of Supported Metal Alkyls To Form a Metal Hydride on the Surface

2. SUPPORTED METAL HYDRIDES PREPARED VIA SURFACE ORGANOMETALLIC CHEMISTRY Synthesis of well-defined−so-called single-site−surface species can be accomplished through a technique known as surface organometallic chemistry (SOMC).1,2,5,7,8 SOMC relies on the generation of chemically uniform support surfaces and their subsequent controlled functionalization with well-defined molecular entities. This approach has been successfully applied to many supports, in particular SiO2 and Al2O3. The surface species can be characterized by IR, NMR, UV−vis, EPR, and X-ray absorption spectroscopies as well as complementary surface science and material chemistry techniques (N 2 adsorption, thermally programmed desorption/reaction, XP, etc.). Currently, several different strategies have been used to synthesize supported metal hydrides via SOMC (Scheme 2): (1) grafting of a molecular hydride complex onto a surface, (2) hydrogenolysis of supported metal alkyls, (3) ligand exchange by an external hydride source, and (4) oxidative addition of  SiOH to metal complexes. These will be explained further in the relevant sections below (vide infra). 2.1. Supported Hydrides of Early-Transition Metals (Groups 4−7)

Supported metal hydrides of early-transition metals can be synthesized by hydrogenation of surface metal alkyls. This strategy is the most widely used method in preparation of these species via SOMC. It involves grafting metal alkyl, alkylidene, or alkylidyne complexes onto a partially dehydroxylated oxide support followed by a subsequent reaction with H2 at high temperatures (100−200 °C, Scheme 3). This hydrogen treatment yields surface hydrides along with the free ligand as RH. However, the putative hydride analog of the alkyl complex (A) has never been observed. Instead, some of the hydride ligands further react with adjacent ES−O−ES bridges of the support to make new ES−H and E−O bonds. A general reaction

Scheme 1. Reactivity Modes of Metal Hydrides

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Table 1. Proposed Structures of Early-Transition-Metal Hydrides on Surfaces

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Table 1. continued

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Table 1. continued

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Table 1. continued

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Table 1. continued

producing Zr−D, and can be regenerated by a subsequent treatment with H2.9 These surface Zr−H’s also react with ketones to produce the corresponding alcohol upon hydrolysis.17 All of these observations support the assignment as Zr−H surface species. Various spectroscopies have been used to characterize the coordination sphere of these surface Zr−H species.12 For example, IR spectroscopy of the reaction of H2 with (SiO)Zr(CH2tBu)3 shows the loss of alkyl ligand bands from the surface and concomitant formation of new IR bands at 2300−2100 and 1625−1635 cm−1.9,16 Reaction of the resulting hydride species with D2 causes the disappearance of the band at 1625−35 cm−1, while the bands between 2300 and 2100 cm−1 remain unchanged.9,16 This observation ultimately led to the assignment of the latter as surface Si−H stretches, while the bands at 1625−1635 cm−1 were assigned to the Zr−H stretches. Solid-state NMR was also used to characterize the variety of Zr−H species on the surface. Resonances at δ = 8.4 and 10.1 ppm can be seen when SiO2−200 is the support, while resonances at δ = 10.1 and 12 ppm are seen with SiO2−500.12,18 These large, positive proton chemical shifts have been attributed to large spin−orbital coupling.19 These are similar to other molecular Zr−H complexes. Multiple quantum magic angle spinning (MQ MAS) 1H NMR showed that the peaks at δ 10.1 and 12 ppm belonged to Zr−H and ZrH2 groups, respectively. In addition, a broad peak at δ 4.4 ppm was observed that correlated to two different peaks at δ −40 and −80 ppm in the 29Si NMR and were assigned to Si−H and SiH2 groups. When Zr−H’s react with CO2, N2O, or H2O, the peaks at δ = 8.4, 10.1, and 12 ppm disappear, while the signal at δ = 4.4 ppm remains, supporting this assignment.18 Extended X-ray absorption fine structure (EXAFS) has also been useful in characterizing indirectly the presence of hydride ligands on supported species.12,20 Since the intensity of Zr−H scattering paths is very low, this technique cannot directly identify a hydride ligand. However, it can be used to determine the remaining coordination sphere of Zr. Use of SiO2−200 as Zr−

pathway is shown in Scheme 3. This approach has been mainly limited to groups 4−6, the specificity of which is discussed below. Major surface species and key spectral data are summarized in Table 1. The opening of oxygen bridges on the support, coupled with the complexity of the surface structures of some oxides (amorphous nature of silica; alumina has several different OH surface terminations), causes these surface hydrides to be less-well-defined than their parent metal alkyls or molecular analogues. Despite having multiple chemically inequivalent surface OH sites, surface hydrides on Al2O3 have similar polydispersity in their structures as those on SiO2. 2.1.1. Group 4 Metal Hydrides. Silica-Supported Zirconium Hydrides. The formation of surface zirconium hydrides was first reported in the 1970s and involves the reaction of Zr(C3H5)4 grafted on SiO2−450 followed by a treatment under H2 at elevated temperatures (100−200 °C).9 Since then a variety of Zr alkyl precursors (ZrR4) have been used, including R = α-Me-allyl,10 β-Me-allyl,10−12 −CH2Ph,13 and −CH2tBu.14−16 The hydrogenated ligand RH and the products of its hydrogenolysis (vide infra) are observed as byproducts of H2 treatment. The temperature of the H2 treatment also has an effect on the reaction and final surface species, with temperatures between 100 and 200 °C being optimal.10 Above this temperature, hydride ligands may be fully lost. Such terminal processes are dependent on the metal and the support. 2.1.1.1. Zirconium Hydrides. Zr−H surface species have been characterized by various methods. Initial characterization was mostly through chemical reactivity. For example, reacting Zr−H with D2O produces a Zr−OH/D species along with HD, which can be detected by mass spectrometry (MS).9 Quantitative analysis of this product by gas chromatography (GC) allows determination of the average number of hydride ligands per Zr. This number is dependent on the temperature at which SiO2 is partially dehydroxylated and the H2 treatment performed. It ranges between 0.5 and 2, with high dehydroxylation temperatures yielding low Zr:H ratios. The Zr−H also reacts with D2, 8469


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the corresponding Zr hydrides as discussed above.26 Two new hydride materials were made with νZr−H at 1627 and 1612 cm−1, respectively, along with the usual Si−H stretches. 1H NMR characterization of the species on the support showed three Zr− H resonances at 10, 12, and 14.5 ppm corresponding to the monohydride 4-Zr a (25%), dihydride 5-Zr a (45%), and the monohydride imido complex 6-Zr a (30%), respectively. However, the reaction on the diaminated support showed only two hydride resonances in the 1H NMR spectrum at 10 and 12 ppm. These signals were assigned as 4-Zr b (40%) and 5-Zr b (60%). Alumina-Supported Zr Hydrides. The formation of hydrides is possible from Zr alkyl surface species grafted on partially dehydroxylated Al2O3,27,28 which consist of cationic surfaces species like B (Table 1, entry 4).29 Reaction of B with H2 at 150 °C gives a mixture of Zr alkyl hydride (7-Zr, 70%) and Zr dihydride (8-Zr, 30%),27 which have been proposed to be cationic. In addition to νZr−H = 1622 cm−1, there is also νAl−H = 1919 and 1415 cm−1. These two stretches have been assigned to hydrides on four-coordinate and three-coordinate Al, respectively.28 Silica−Alumina-Supported Zirconium Hydrides. Zr−H can also be made on SiO2−Al2O3−500. Zr(CH2tBu)4 grafted on SiO2−Al2O3−500 reacts with hydrogen at 150 °C to make a material with νZr−H at 1635 cm−1.30 This band disappears in the presence of D2, ethylene, or propylene. It is thought that the proximity of Al atoms in the secondary coordination sphere makes the Zr more electrophilic. 2.1.1.2. Titanium Hydrides. Silica-Supported Titanium Hydrides. Titanium hydrides have also been synthesized by reaction of supported Ti alkyls with H2. Reaction of Ti(CH2Ph)4 supported on either SiO2−250 or SiO2−750 with H2 at 100−170 °C31 yielded surface species with roughly two or one H per Ti, respectively (Table 1, entries 8 and 9). The material shows new IR bands at 1560 cm−1 (νTi−H) that exchanged with D2.12 The EPR spectra are consistent with the presence of ca. 20% of Ti3+. Three different Ti3+−H centers were identified according to g anisotropy and proton hyperfine coupling (being as high as 200 G) attributed to monografted dihydride (4-Ti), bis-grafted monohydride (5-Ti), and dimeric hydride (6-Ti). Titanium hydride species were also obtained by the reaction of H2 with Ti(CH2tBu)4 grafted on SiO2−500.32 This material exhibited IR bands assigned to νTi−H at 1706, 1692, 1679, and 1647 cm−1 and νSi−H at 2263 and 2196 cm−1. The Ti−H bands exchanged with D2 gas. Silica−Alumina Supported Titanium Hydrides. Similarly, Ti(CH2tBu)4 grafted on SiO2−Al2O3−50033 treated under H2 for 4 h at 150 °C yielded a solid with IR bands at 1600−1725, 1926, 2195, and 2267 cm−1 (Table 1, entry 10).33 Similar to what was observed on silica, only the peaks at 1600−1725 cm−1 disappear upon exposure to D2. The peaks at 2267 and 2195 cm−1 were assigned as Si−H stretches, while the 1926 cm−1 band was assigned as an Al−H stretch. The amount of Ti3+ was ca. 14% according to EPR spectroscopy but was not attributed to specific surface species such as Ti3+−H or Ti3+ with no hydride ligand. The 1H NMR spectrum showed a broad resonance at 8.6 ppm that MQ MAS NMR showed to be composed of three overlapping signals at 8.6, 8.6, and 9.0 ppm. These correspond to monohydride species 1-Ti b (major), Ti(Me)−H species 2-Ti b, and dihydride species 3-Ti b, respectively. Overall 78% of surface Ti is present as 1-Ti b, 14% is present as Ti3+, 5% as 3-Ti b, and 2% is present as 2-Ti b.

H support produces a species having mainly two 2.10 Å Zr−O paths and a weak Zr−Zr path at >3.08 Å, indicating that some dimers are present on the surface. Species made with SiO2−500 do not exhibit this Zr−Zr path and have three Zr−O paths at 1.94 Å. EPR spectroscopy also found that, along with Zr−H’s, the surface contained some minor amounts of Zr3+ sites (ca. 1%).12,21 The signals of two different Zr3+ surface sites (g = 1.900 and 1.954) increase upon treatment of the hydrides with UV light. These sites can reversibly coordinate unsaturated organics such as benzene. All of this spectroscopic characterization suggests that the surface contains a mixture of various Zr−H species 1-, 2-, and 3Zr (Table 1, entry 1). 1-Zr is a monomeric Zr with one hydride and three surface oxygen ligands, 2-Zr is a monomeric Zr with two hydrides and two surface oxygen ligands, and 3-Zr is a Zr3+ with one hydride (3-Zr) and Zr dimer hydrides. The formation of Si−H bonds has been attributed to the reaction of the putative intermediate A with adjacent siloxane bridges of the surface of silica, which yields new Zr−O and Si−H bonds. When SiO2−500 is used as the support, 1-Zr is the major species, since the surface contains more strained siloxane bridges that are easy to open. On the other hand, using SiO2−200 mostly gives 2-Zr with two hydrides per Zr. 3-Zr is likely the product of Zr−H bond homolysis. Other kinds of precursor complexes have also been used to synthesize hydrides. For example, the azazirconaziridine complex C treated under H2 at 150 °C gives the hydride 9-Zr (Table 1, entry 5).22 Species 9-Zr display an IR band at 1587 cm−1 and a 1H NMR signal at 8.0 ppm attributed to Zr−H. More recently, Zr−H’s have been synthesized by the reaction of a silica-grafted tris-amido Zr species with H−Bpin (Table 1, entry 6).23 This species was characterized by various 1D and 2D NMR experiments, IR, D2 exchange, and mass balance. On the basis of its very low 1H NMR chemical shift, 6.9 ppm, it was assigned as Zr monohydride (10-Zr) with three surface bonds and one bond to the oxygen of the BPin ligand. Brintzinger and co-workers have proposed the formation of a supported Zr−H complex from rac−Me2Si(indenyl)2ZrCl2 supported on silica pretreated with methylalumoxane (MAO) and the subsequent reaction with HAl(iPr)2 (Table 1, entry 7, eq 1).24 The surface complex resulting from the grafting step is thought to be a contact ion pair formed by abstraction of a ligand from Zr to give D. Treatment of D with HAl(iBu)2(DIBAL) gave the hydride complex 11-Zr, having a UV−vis absorbance at 490 nm. This structure was assigned by comparison with analogous molecular species, which give UV−vis spectra identical to those of the supported ones on the surface. Grafting on Amine-Modified Silica Surface. Silica enriched with surface amine functionalities can also be used as a support. It is prepared by the reaction of NH3 with highly dehydroxylated SBA-15 (1100 °C) to give Si−NH2 and Si−OH groups in close proximity to each other (99.9%) for the

Table 2. Reactions Catalyzed by Early-Transition-Metal Surface Hydrides

hydride complexes of early-transition metals, late-transition metal hydride complexes can be directly generated on oxide surfaces via oxidative addition of a surface−OH bond to a lowvalent molecular precursor (Scheme 7a) or grafting of a molecular hydride complex possessing an additional anionic ligand (Scheme 7b). Furthermore, low-valent complexes grafted on an oxide surface are able to add substrates, such as H2, HX, and HSiR3, via an oxidative addition process (Scheme 7c). The following section describes the synthesis and characterization, as well as reactivity, of well-defined heterogeneous hydride complexes of groups 8 and 9. The formation and reactivity of

Scheme 8. Formation of Supported Ruthenium Hydrides

Scheme 7. Synthetic Strategies for Supported LateTransition-Metal Hydrides

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Table 3. Late-Transition-Metal Hydrides on Surfaces

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Table 3. continued

homogeneous analogues, the aforementioned immobilized complex is highly selective for the hydrogenation of benzylideneacetone to benzylacetone and benzonitrile to benzylidenebenzylamine (Scheme 9b). Although direct spectroscopic evidence of a hydride complex responsible for hydrogenation has not been reported, it is proposed that hydrogenation occurs via a dihydrogen complex, [(SiOH) X ( −O3S(C6H4 )CH 2C(CH2PPh2) 3)Ru(H 2)][(SiOH)XOTf] (3a-Ru), or a Ru dihydride, [(SiOH)X(−O3S(C6H4)CH2C(CH2PPh2)3)Ru(H)2][(SiOH)XOTf] (3b-Ru), intermediate. This proposal is based on molecular complexes extracted from

hydrogenation of styrene to ethylbenzene (Scheme 9a) due to competing side reactions in the former, which involve hydrogenation of the aromatic ring.87 Ruthenium hydrides have also been proposed as intermediates in hydrogenation reactions of a zwitterionic ruthenium complex immobilized on silica through hydrogen bonding.89 Exposure of the ruthenium complex [( − O 3 S(C 6 H 4 )CH 2 C(CH 2 PPh 2 ) 3 )Ru(NCMe) 3 ][OTf] (OTf = trifluoromethanesulfonate) to SiO2−300 results in the immobilized complex [(SiOH) X ( − O 3 S(C 6 H 4 )CH 2 C(CH2PPh2)3)Ru(NCMe)3][(SiOH)XOTf]. Compared to its 8475


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Scheme 9. Reactivity of Supported Ruthenium Hydrides

Scheme 11. Formation of Supported Rhodium Hydrides

the surface after catalytic studies or treatment of the surface species with molecular hydrogen. 2.2.2. Rhodium. [Rh(η3-C3H5)3] grafted on various supports (SiO2, TiO2, MgO, and Al2O3)90−96 has been treated with hydrogen, resulting in a band at 2048 cm−1 in the IR spectra that was originally assigned to νRh−H90,91,93,94,97 and has later been attributed to small quantities of carbon monoxide chemisorbed on rhodium particles, according to a variety of spectroscopic methods and isotopic labeling studies.92,95,98 The carbon monoxide that is ultimately bound to the particles has been proposed to result from the decarbonylation of allyl alcohol, formed upon the reductive elimination of a σ-allyl and siloxide ligand from rhodium, followed by further reaction with surface silanols (Scheme 10). On the other hand, when the silica-supported [(SiO)(SiOH)Rh(η3-C3H5)2] is first treated with an excess of P(CH3)3 followed by treatment with molecular hydrogen, formation of a cationic Rh hydride [(SiO)][Rh(H)2(P(CH3)3)4] (1-Rh) (Scheme 11a) (Table 3, entry 3) occurs.99 Using a nitromethane solution saturated with [Bun4N][Cl], the cationic fragment [Rh(H)2(P(CH3)3)4]+

was extracted from the surface siloxy anion [(SiO)], further supporting this assignment. Solid-state and solution 31P NMR spectra of the supported complex and extracted complex,

Scheme 10. Reduction of Grafted Rh(η3-C3H5)2 Surface Species with Hydrogen and Formation of Supported Nanoparticles

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to observe the νRh−H band in the IR spectrum of 3-Rh were hindered by the presence of an intense νCO band at 2066 cm−1. However, pretreatment of the precursor [(SiO)Rh(P(CH 3 ) 3 ) 2 (CO)] with 13 CO, yielding [(SiO)Rh(P(CH3)3)2(13CO)], followed by HCl resulted in a red-shifted ν13CO vibration (2018 cm−1), revealing a weak band at 2072 cm−1 attributed to the νRh−H vibration. Further authentication of the νRh−H stretch was achieved through treatment of [(SiO)Rh (P(CH3)3)2(CO)] with DCl. A new weak stretching vibration at 1496 cm−1 was observed and assigned as a νRh−D band, consistent with harmonic oscillator predictions. In addition to ion pairing (vide supra), hydrogen bonding has also been used for the immobilization of well-defined rhodium hydride precursors on silica. When reacted with SiO 2 − 3 0 0 , th e zwitt erion ic com plex [(SiOH) X ] [(−O3S(C6H4)CH2C(CH2PPh2)3)Rh(COD)] (COD = 1,5cyclooctadiene) was immobilized on the surface, which was attributed to hydrogen bonding between (SiOH) and the sulfonyl substituent of the tripodal phosphine ligand.101 Treatment of the aforementioned precursor with H2 at 150 °C resulted in hydrogenation of 1,5-cyclooctadiene and formation of the immobilized hydride complex [(SiOH) X ]( − O 3 S(C 6 H 4 )CH 2 C(CH 2 PPh 2 ) 3 )Rh(H) 2 ] (4-Rh) (Scheme 11d) (Table 3, entry 6).101 EXAFS analysis of 4-Rh indicated retention of the tripodal phosphine ligand with slightly elongated Rh−P distances (2.34 Å) in comparison to the molecular precursor (2.27 Å) and similar distances to the grafted hydride precursors (2.36 Å). Most notably, no Rh−Rh contacts were detected, suggesting site isolation of the Rh centers. Although the Rh−H ligands were not directly detected while immobilized on the surface, extraction of the surface-bound molecule using methanol yielded a molecular complex characterized as [(−O3S(C6H4)CH2C(CH2PPh2)3)Rh(H)(μH)2(H)Rh((PPh2CH2)3CCH2(C6H4)SO3−)]. IR spectroscopy of the extracted complex revealed a terminal νRh−H stretch at 1970 cm−1 reminiscent of a similar molecular complex, namely, [(triphos)Rh(H)(μ-H)2(H)Rh(triphos)]2+.102 The hydride complex 4-Rh was examined for the catalytic hydrogenation and hydroformylation of ethene and propene. At temperatures lower than 150 °C, no catalytic activity was observed due to retention of the COD ligand. However, above 150 °C, 4-Rh was active for the hydrogenation of ethene, propene, and styrene, resulting in formation of ethane, propane, and ethylbenzene, respectively (Scheme 12b). Although attempted hydroformylation of the gaseous alkenes resulted in formation of rhodium−carbonyl complexes, the hydroformylation of 1-hexene produced 2-methylhexanal (41.0%), heptanal (32.0%), and 2-ethylpentanal (15.5%) (Scheme 12b). Surface rhodium hydrides have also been described as intermediates in the catalytic hydrosilylation of olefins. The reaction of [Rh(μ-OSi(CH3)3)(COD)]2 with SiO2−350 results in formation of the grafted complex [(SiO)(SiOH)Rh(COD)],103 and in the presence of stoichiometric amounts of PR3 [R = −C6H11, −C6H5, and −CH(CH3)2], the [(SiO)Rh(COD)(PR3)]104 complexes are obtained. Both aforementioned complexes are active in the catalytic hydrosilylation of α-olefins (Scheme 12c).103−105 Only minimal leaching is observed in the presence of primary olefins during catalysis. Solid-state NMR studies performed on the reaction of these species with dimethylphenylsilane suggest formation of the hydride complex [(SiO)RhH(Si(CH3)2(C6H5))(COD)] (5-Rh) (Scheme 11e) (Table 3, entry 7).103,104 Oxidative addition of silane

respectively, indicated a cis-configuration of the hydride ligands gauged by two unique 31P resonances coupled to 103Rh, which could be resolved in solution measurements. The Rh−H stretch in the IR spectrum was identified through exchange reactions of 1-Rh with D2. These studies resulted in the disappearance of a broad vibrational band attributed to νRh−H at 1938 cm−1 accompanied by the formation of a new stretch at 1386 cm−1 assigned to the νRh−D stretch. When the bulkier phosphine P(iPr)3 was reacted with [(SiO)(SiOSi)Rh(η3-C3H5)2] the dimeric complex [(SiO-μ2)Rh(P(iPr)3)2]2 was formed on the surface of SiO2−500.100 Upon reaction with molecular hydrogen, the monomeric dihydride complex [(SiO)Rh(H)2(P(iPr)3)2] (2-Rh) (Scheme 11b) (Table 3, entry 4) is formed, exhibiting vibrational bands at 2162 and 2043 cm−1 in the IR spectrum. The assignment of these νRh−H stretching bands was confirmed by exposure of the dihydride complex to D2, resulting in the formation of two new vibrational bands at 1560 and 1488 cm−1 accompanied by disappearance of the former. In contrast to 1-Rh, the unsaturated dihydride 2-Rh complex is active in hydrogenation of cis-2-butene with no detectable amount of butane hydrogenolysis, supporting the absence of rhodium metal (Scheme 12a). The silica-bound complex [(SiO)Rh(P(CH3)3)2(CO)] generated from grafting of the molecular complex [Rh(CH3)(P(CH3)3)2(CO)] on SiO2−500, serves as a precursor to the monohydride complex [(SiO)Rh(H)Cl(P(CH3)3)2(CO)] (3-Rh) (Scheme 11c) (Table 3, entry 5) upon treatment with HCl.95 Initial attempts Scheme 12. Reactivity of Supported Rhodium Hydrides

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have been performed, the interaction of the variant [((tBu)2PO− POCOP)Ir(C2H4)] with γ-Al2O3 has been reported.108 The catalytic activity of 1-Ir for the hydrogenation of alkenes has been investigated (Scheme 14a).109 These studies showed that 1-Ir is competent for the hydrogenation of the gaseous alkenes ethylene and propene and the liquid alkenes 1-decene, trans-5-decene, cyclohexene, styrene, and 4-phenyl-1-butene at room temperature without the need of prior activation. In most cases, the grafted catalyst can be recovered after hydrogenation and reused for subsequent catalytic runs. However, in the case of ethylene, it has been reported that exposure of the silica-grafted complex 1-Ir to 1.5 bar of ethylene results in reductive elimination of silanol and partial formation of the olefin complex [(POCOP)Ir(C2H4)]. Interestingly, the reverse reaction of [(POCOP)Ir(C2H4 )] and SBA-15(400) results in partial formation of 1-Ir. Although this equilibrium exists with ethylene, 31 P NMR spectroscopy indicated no such equilibrium upon exposure of 1-Ir to propylene. Additionally, the reaction of SBA-15(400) with the propene adduct [(POCOP)Ir(C3H6)] results in quantitative formation of 1-Ir via oxidative addition of the surface silanols. The activities of the immobilized iridium hydride pincer complexes in the transfer dehydrogenation of cyclooctane to tert-butylethylene at 125 °C were evaluated and compared to that of their homogeneous analogues (Scheme 14b).91 In the case of 5H-Ir and 5OCH3-Ir, low catalytic performances were observed in comparison to the homogeneous analogues (4 vs 368 mM and 84 vs 352 mM of COE, respectively), which is attributed to catalyst decomposition. However, when bound to alumina, the more basic ester-substituted catalyst 5CO2CH3-Ir has higher activity in comparison to its homogeneous analogue (354 vs 258 mM COE), which is attributed to the diminished intermolecular decomposition known to occur under homogeneous conditions. The most basic substituent of the series, 5N(CH3)2-Ir, also outperforms the homogeneous analogue (285 vs 200 mM COE) and can be recycled several times with only a minimal decrease in performance per recycling. The 5N(CH3)2-Ir catalyst was also investigated for the transfer dehydrogenation of n-octane. Although the immobilized system produces more octene in comparison to the homogeneous complex (130 vs 98 mM octene), the heterogeneous system produces less α-olefin (4 vs 16 mM 1-octene), which is attributed to increased rates of isomerization. Overall, anchoring of the iridium complexes with

without evidence for siloxane elimination is of interest, as this differentiates the mechanism of hydrosilylation of the surface complex from that of the homogeneous analogues. 2.2.3. Iridium. Examples of well-defined molecular hydride complexes of iridium have also been used for grafting while a hydride ligand is retained. Reaction of the dihydride pincer complex [(POCOP)IrH 2 ] [POCOP = 2,6-bis(di-tertbutylphosphinito)phenyl] and the tetrahydride complex [(PCP)IrH4] [PCP = 1,3-bis((di-tert-butylphosphino)methyl)benzene] with SBA-15400 results in the grafted complexes [(POCOP)IrH(OSi)] (1-Ir) (Table 3, entry 8)106 and [(PCP)IrH(OSi)] (2-Ir) (Table 3, entry 9)107 concomitant with formation of molecular hydrogen (Scheme 13a). The hydride ligand of complex 1-Ir was observed in both the 1H solid-state MAS NMR and IR spectra at −31 ppm and 2120 cm−1, respectively. The 1H and 31P NMR chemical shifts of 1-Ir match well with the model molecular compounds and [(POCOP)IrH(OSiMe3)]106 [(POCOP)IrH(OSi8O12tBu7)].107 The pentacoordinate complex is susceptible to binding of carbon monoxide, resulting in the grafted complex [(POCOP)IrH(CO)(OSi)] (3-Ir) (Table 3, entry 10), which exhibits a hydride signal at −7.5 ppm in the 1H MAS NMR spectrum. The νIr−H and νCO bands were also observed in the IR spectrum occurring at 2205 and 2026 cm−1, respectively.107 While complex 3-Ir is spectroscopically observable, this complex readily undergoes reductive elimination of the hydride and surface siloxide ligands, resulting in [(POCOP)Ir(CO)] physisorbed to the surface. In contrast, the PCP carbonyl complex [(PCP)IrH(CO)(OSi)] (4-Ir) (Table 3, entry 11), formed under similar conditions, is less susceptible to reductive elimination due to the stronger electrondonating PCP ligand.107 The hydride ligand of 4-Ir can be observed in the solid-state 1H NMR at −6.9 ppm, while the νIr−H vibrational band can be located at 2167 cm−1 in the IR spectrum. In addition to silica surfaces, γ-Al2O3 has also been employed as a support for the immobilization of the iridium hydride pincer complexes [(X−PCP)Ir(H)2] [X = −H, −OCH3, −CO2CH3, and −N(CH3)2] (5X-Ir) (Scheme 13b) (Table 3, entry 12).91 It is proposed that the para-substituent of the central phenyl ring of these complexes interacts with the Lewis acidic sites of alumina. While no detailed structural studies of these PCP derivatives Scheme 13. Formation of Supported Iridium Hydrides

Scheme 14. Reactivity of Supported Iridium Hydrides

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observations are consistent with B−H and B−D bonds, respectively. Heating above 500 °C caused loss of the B−H species to form so-called “reactive boron” on the surface. Rather more-well-defined boron hydrides have been made by grafting BH3/B2H6 onto Al2O3.120 Absorption produces new IR bands at 2580, 2485, 2170, and 1470 cm−1, indicating the presence of both terminal and bridging B−H bonds. Three different structures were proposed for these BH groups, a monografted boron dihydride (1-B), a monografted diboron pentahydride (2-B), and BH4− coordinated to Lewis acidic Al centers on the surface (3-B) (Scheme 15a). Other well-defined boron hydride complexes can be made by grafting M(BH4)n complexes on the surface of SiO2 or Al2O3. These complexes are bound together through B−H−M bridging hydride interactions that react with surface OH groups upon contact to produce the grafted metal species (4-B) along with H2 and BH3 byproducts (Scheme 15b). Complexes grafted in this way include Ti(BH4)4,121 Zr(BH4)4,121−124 and Hf(BH4)4121 on SiO2 and Al2O3; Al(BH4)4 on Al2O3;120 La(BH4)3(THF)3 and Nd(BH4)3(THF)3 on SiO2−700.125 All the complexes had similar vibrational bands in the ranges 2600−2400 and 2200−2050 cm−1 attributed to terminal B−H and bridging M−H−B stretches. The grafted species (SiO)La(BH4)2(THF)2.2 has a 1 H NMR signal at 1.0 ppm that sharpens upon 11B decoupling and a 11B NMR signal at −23.2 ppm attributed to the BH4 units. The BH3 byproduct also grafts, as discussed above, to produce additional transition-metal-free B−H surface sites having 11B NMR signals between 20 and −7 ppm. Heating these borohydride complexes to above 200 °C produces the corresponding metal hydrides of Ti, Zr, and Hf. The hydrides produced by this method have the same properties as those discussed above. 2.3.2. Group 14. 2.3.2.1. Surface Silanes. The surface silicon hydrides are among the most common surface hydrides on oxides. They can be synthesized in high yield using the proper hydride molecular precursors, and they display relatively good stability in air and water at room temperature. Their reaction with water requires catalysis by a base or a metallic complex, while their decomposition in air into silanols occurs at temperatures higher than 400 °C.126,127 The formation and reactivity of surface hydrosilanes have been reviewed.128,129 This subsection summarizes the main conclusions regarding the formation and reactivity of such species, and the reader may refer to these works for additional details. One convenient way to prepare surface hydrosilanes consists of grafting molecular hydrosilane precursors on the surface of oxides such as SiO2. Chlorinated precursors were first studied.

more basic para-substituents of the PCP pincer ligand allows for prolonged catalytic activity. Late-transition-metal hydrides are also prevalent in zeolites, which is achieved through grafting of late-transition-metal complexes in the porosity of zeolites. The hydride is either already present on the complex or is obtained after treatment with H2. This has been reported with rhodium,110−112 ruthenium,113 and iridium.114 Rhodium hydride complexes can be obtained as single-site complexes from grafted Rh-carbonyl after H2 treatment, as evidenced by νRh−H stretching frequencies at 2164, 2156, 2143, and 2129 cm−1.111 Iridium ethylene Ir(C2H4) complexes can also be grafted in zeolites, forming iridium hydride complexes upon reductive treatment (νIr−H at 2068 cm−1). These iridium hydride complexes are able to catalyze the H2/D2 equilibration reaction and then react toward the formation of small metallic iridium clusters.114 However, changing the ligands of iridium to Ir(C2H4)(CO) led to grafted complexes that cannot form hydrides, and consequently, neither equilibrate the H2/D2 mixture nor evolve to the formation of clusters. 2.3. Main Group Hydrides

2.3.1. Group 13. Surface aluminum hydrides can also be generated by hydrogenolysis of the corresponding surface alkyls. Well-defined Al−H’s have been made by grafting aluminum alkyls on silica and alumina surfaces. Al(iBu)3 was grafted on high surface area silica (SBA-15500) and produced a complex mixture of dimeric surface species that transferred some or all of their alkyl groups onto the surface Si atoms along with some Si− H.115,116 This mixture of surface species was found to react with H2 at elevated temperatures (100−400 °C) to give surface Al−H species with νAl−H at 1940, 1644, and 1610 cm−1 in the IR spectrum.117 The peak at 1940 cm−1 corresponds to a terminal Al−H, while those at lower frequencies correspond to bridging Al−H−Al species. Grafting Al(iBu)3 on Al2O3−500118 generated a mixture of fourand five-coordinate surface Al species, which upon treatment with H2 (0.73 bar) at 400 °C yield the corresponding surface aluminum hydrides with νAl−H at 1940 cm−1. By using J-HMBC filtering, they obtained the 27Al NMR corresponding only to the Al atoms in close proximity to hydride ligands. Three different Al−H signals could be identified in the 27Al NMR spectrum, corresponding to four-, five-, and six-coordinate surface species. These all overlapped into one signal in the 1H NMR that showed unusual, field-dependent line shapes due to unresolved J, dipolar, and quadrupolar coupling to 27Al. As was discussed above, surface aluminum hydrides are also made by hydride transfer from hydridic metal complexes on the surface to the surface Al atoms. This often occurs by opening strained Al−O rings on the surface and gives the metal a new Al−O ligand. These are usually ill-defined, since the formation of strained rings on the surface is uncontrolled during the alumina dehydroxylation. Rather poorly defined surface boron hydrides have been claimed upon impregnation of silica with B(OH)3, followed by exposure of B sites to methanol to make B−OMe species. Heating this at temperatures up to 500 °C under vacuum causes β-hydride elimination from the alkoxide ligand and generation of a new species.119 Heating the methanol-treated species under vacuum results in loss of the IR bands associated with the methoxy ligands and growth of new bands at 2621, 2601, and 2580 cm−1. If the surface is first treated with CD3OD instead of CH3OH, the IR bands shift to 1957 and 1937 cm−1. These

Scheme 15. Grafting Boron Hydrides on SiO2 and Al2O3 Surfaces

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one to achieve high Si−H loadings, estimated around 5 Si−H per nm−2,127 which is very close to the silanol content of a fully hydrated silica.138 Note that this kind of silane precursor can also be used on the surface of other oxides, like alumina, zirconia, or titania,139,140 forming Si−H coated surfaces. This process can be used for the preparation of pH-resistant static phases for liquid chromatography.139,140 Alternatively, the precursor can be directly hydrolyzed to yield polyhydrosiloxane,126,141,142 a stoichiometric compound of the formula HSiO3/2 in which each tetrahedral silicon atom is bound to one hydrogen atom, forming of a xerogel of high specific surface area (about 200 m2 g−1). Such silicon hydrides are widely used for hydrosilylation reactions.128,129 In such reactions, the surface of a hydridemodified silica or polyhydrosiloxane reacts with an alkene, forming a Si−C bond, and thus a surface-grafted alkylsilyl compound. However, given the stability of the Si−H bonds, the reaction requires catalytic activation, using conventional homogeneous catalysts for hydrosylilation reactions, like Speier’s catalyst (H2PtIICl6),143 and is usually performed either in toluene or in the neat alkene. This procedure allows introduction of a large variety of alkylsilyl groups on the surface of silica with various sizes (up to C18 linear alkenes) and functionalities (styrene, vinyl acetate, acrylamide).128 Regarding the formation of alkylsilyl grafted on silica, the silanization−hydrosilylation sequence is preferred over the direct grafting of alkylsilyl compounds. The main advantage is the higher density that can be achieved via this route, including for relatively long alkyl chains.129 This is particularly useful for the preparation of stationary phases for liquid chromatography, were a high surface concentration of bonded alkyls improves pH resistance, stability, and performance with respect to certain applications.144−146 Additionally, surface silicon hydrides have been shown to interact with metals. They can act as relatively strong reducing reagents for metal salts, including silver, palladium, platinum, and mercury, which can be reduced to their metallic counterparts. Manganese oxide, iron, and copper can also be reduced, albeit only to a partially reduced state. These properties were employed to prepare silver, gold, and platinum nanoparticles supported on silica by reduction at room temperature of grafted hydrides of metallic precursors in aqueous solution (AgNO3, HAuCl4, and H2PtCl6, respectively).147,148 The Si−H band disappears upon contact with the precursor solution while particles are formed, which have been characterized by XRD and electronic microscopy. Finally, Si−H bonds can oxidatively add to a homogeneous metal complex to make new Si−M and H−M bonds (vide infra). However, with silicon hydrides prepared by such a sol−gel method, the high silane density does not allow formation of a well-defined grafted complex but, rather, metal particles formed after reduction of the metal by the hydride itself.88 Finally, it is also possible to obtain hydride-terminated flat silicon surfaces by treating a superficially oxidized silicon wafer with HF or NH4F.149 Different terminations can be obtained: dihydrides with HF and monohydrides with NH4F. Subsequent hydrosilylation allows for introduction of alkyl groups at the surface of the silicon wafers. Note that this procedure can also be applied to germanium wafers with similar results.149 Germanium hydrides do not react with hydroxyl groups on oxide surfaces.150 To our knowledge, no examples of Ge−H on nonmetallic surfaces have been reported. However, alkyl−GeH3 can be physisorbed on these oxide surfaces. For instance, addition of AdGeH3 (Ad = adamantyl) to SiO2 dehydroxylated

Trichlorosilane (HSiCl3), methyldichlorosilane (HSiCl2CH3), or dimethylchlorosilane [HSiCl(CH3)2] have been used for that purpose (Scheme 16).130−132 The adsorption is usually performed in the gas phase and proceeds by exchange with the hydroxyl groups of silica. The number of ligands exchanged with hydroxyl groups depends on the structure of the silane, i.e., the number and type of ligands (Scheme 16b), and the hydriderelated stretching vibration is displaced accordingly.133 The amount of grafted Si−H bonds was found to be very dependent on the reaction temperature,132 ranging from 0.14 Si−H per nm−2 at 373 K to 1.0 Si−H per nm−2 at 1073 K. Alkylamidosilylating agents, in particular HN(HSiR2)2, can also be employed and allow for the introduction of Si−H bonds at room temperature without the use of a catalyst (Scheme 16c).134 In fact, an amido ligand is a better leaving group than alkoxy groups in triethoxysilane, which makes the exchange reaction faster. The Si−H coverage achieved was estimated to be around 1.8 Si−H per nm−2.134 Two other methods exist to prepare silicon hydrides on silica that involve the direct formation of the Si−H bond. The surface of silica can be reacted with inorganic hydrides such as LiAlH4. The surface has to be prepared using SOCl2 to exchange silanol groups with chloride that are in turn exchanged with the hydrides. This method was used by Pesek and Sandoval,135 but due to the precursor’s high water sensitivity, the introduction of aluminum salt decomposition products in the final compounds, and the comparatively low Si−H density achieved (1.2 Si−H per nm−2),127 this method was replaced by the triethoxysilane method discussed below. The thermal decomposition of grafted methoxy groups under vacuum was also employed to generate surface hydrides. The method requires high temperatures (above 600 °C), and the reaction time has to be controlled because the hydride can decompose under such conditions. Morterra et al.136 proposed that thermal treatment induced the homolytic splitting of Si−O bonds and the formation of dangling bonds, which can react with decomposition products of the alkoxy groups. Finally, as in the case of alumina, Si−H groups can be observed in the vicinity of single-site organometallic compounds supported on silica after reductive treatment, ultimately leading to a silicon hydride.18,32,137 This is also the origin of the extra oxygens in the coordination sphere of metal hydride complexes made by this method (vide supra). 2.3.2.2. Silicon Hydrides Prepared by Sol−Gel Chemistry. Another approach to introduce surface silanes involves sol−gel chemistry. In particular, triethoxysilane, HSi(OCH2CH3)3, can be hydrolyzed on the surface of silica in dioxane at about 100 °C, using an aqueous acidic catalyst (HCl).127 This procedure allows Scheme 16. Formation of Surface Silicon Hydrides

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Scheme 17. Tin Hydrides on Silica Surfaces

at various temperatures leads to a new species with νGe−H at 2055 cm−1 that is removed by heating under vacuum.151 Molecular Sn hydrides react with oxide surfaces by release of H2 concomitant with formation of a new Sn−O bond to the surface.150 In the case of Sn monohydrides, this means that only alkyl ligands remain on the grafted Sn atom. Sn dihydrides also graft to form surface species. For example, Bu2SnH2 reacts with SiO2−200 to only form (SiO)2SnBu2 (Scheme 17a). However, use of SiO2−500 slows the grafting of the second hydride ligand so that (SiO)SnBu2H forms after heating at 100 °C for 8 h, for which the νSn−H vibration can be observed at 1854 cm−1 in the IR spectrum.152 This species ultimately yields the dialkyl product (SiO)2SnBu2. Sn−H can be tethered onto the surface by attaching a triethoxysilane linker to a molecular Sn hydride. Compounds 3-, 4-, and 5-Sn were grafted onto SiO2, Al2O3, TiO2, ZrO2, or C (Scheme 17b,c).153,154 They were then characterized by elemental analysis and titration of the hydride components with iodomethane. The materials had up to 0.8 mmol Sn−H g−1 in the case of SiO2, whereas ZrO2 and C had very low loadings. Around 50% of the Sn sites retained the Sn−H functionality. No spectroscopic characterization was reported. 2.3.3. Main Group Hydride Reactivity. Main group hydrides are in general not as reactive as the early-transitionmetal hydrides. They do not exchange with D2 under the typical H2 treatment conditions (150 °C, 1 bar), nor are they consumed under the same reaction conditions as their transition-metal counterparts. Aluminum hydrides have been shown to both hydrogenate and polymerize ethylene at 100 and 400 °C, respectively.155 All the boron hydrides presented here are active in hydroboration of alkenes.120 For complexes 2-, 3-, and 4-B, the terminal B−H bonds substitute before the bridging B−H−M bonds. The supported metal borohydride complexes 4-B are coordinatively unsaturated and will coordinate H2, CO, and other ligands.122,123 They have been used as catalysts in a variety of reactions including ethylene polymerization (Ti, Zr, Hf)121 and polymerization of rac-β-butyrolactone (La, Nd).125

Surface silanes prepared by grafting of HN(HSi(CH3)2)2 can be used to graft metallic complexes such as Ru(COD)(COT) (vide infra). This allows forming well-defined single-sitesupported ruthenium complexes.87 Sn hydrides in particular are different than most of the other hydrides examined in this review. Most Sn−H reactions involve radical chain reactions, similar to that of Sn−H in solution. For example, 5-Sn and 7-Sn react with haloalkanes in the presence of initiators by a radical chain mechanism to form alkyl radicals and Sn−X (X = Br or I). These alkyl radicals are involved in useful organic transformations, such as hydrodehalogenation and radical cyclizations. The Sn−X reacts with DIBAL or with NaBH4 to reproduce the Sn−H. 2.4. Putative Supported Metal Hydrides

Numerous supported metal alkyl surface complexes of transition metals and actinides have been treated under H2 or used as hydrogenation precatalysts. In many instances, the putative metal hydride could not be observed, despite indirect evidence. For example, Cp2ThMe2 and Cp2UMe2 grafted on various supports are highly active alkene hydrogenation catalysts.1 When supported on dehydroxylated alumina, it has a low number of active sites (40% active sites during hydrogenation (eq 2). Reacting 1-Th with H2 at 100 °C causes reduction of NMR signals associated with Th−Me and Mg−Me groups, but none could be assigned to a new hydride species.156

Similarly, Cp2Zr(CH3)2, Cp*2Zr(CH3)2, and Cp*Zr(CH3)31 supported on a variety of surfaces (such as SiO2, Al2O3 and sulfated alumina or zirconia) are very active alkene and arene hydrogenation catalysts but contain a low number of active sites. 8481


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Scheme 18. Tethered Late-Transition-Metal Complexes That Catalyze Hydrogenations

Presumably, the mechanism of hydrogenation also involves hydride intermediates similar to those proposed above for Th. Indeed exposure of the catalyst to H2 causes consumption of alkyl ligands with the presumption of a hydride product. However, no evidence for a hydride ligand has been observed. Grafting the same Zr precursors onto sulfated alumina generates more active hydrogenation catalyst with >90% active sites (eq 3), but no hydride could be detected.157 The difficulty in observation may be caused by the reactivity of these complexes. The complexes Ti(CH2SiMe3)4, Zr(CH2Ph)4, Hf(CH2Ph)4, [Nb(CSiMe3)(CH2SiMe3)2]2, [Ta(CSiMe3)(CH2SiMe3)2]2, and Mo2(CH2SiMe3)6 also hydrogenate arenes when grafted on silica. However, characterization of the putative hydride intermediates was never reported.13

Scheme 19. Metal Hydride Intermediates Formed during Hydrogenation and Dehydrogenation

Transient hydrides have also been proposed as intermediates during the hydrogenation and dehydrogenation of propene and propane catalyzed by several metal silicates (Cr,163 Fe,164 Co,165 Zn;166 Scheme 19). These materials consist of isolated metal atoms on the surface of silica and/or alumina with empty coordination sites. The proposed mechanism for propene hydrogenation is thought to occur through cleavage of the H−H bond across a M−O bond to form new M−H and OH species. During propane dehydrogenation, β-hydride elimination from a metal propyl species is postulated. This would produce one new M−H bond and 1 equiv of alkene. However, due to the high temperatures of these reactions and the low surface concentration of the hydride intermediates, these species have never been observed.

Chromium hydrides are thought to be formed during ethylene polymerization using Phillips catalysts. When the Phillips catalyst, CrO3@SiO2, is treated with silane (SiH4), the polymer produced is of much lower molecular weight with a greater degree of chain branching. This is likely caused by the intermediacy of a Cr−H species that is formed by β-H elimination and can reinsert to form the branches (eq 2 and 3).158 The hydrogenation catalyst [(SiO)Pd(R)(4,4′-ditBuBiPy)] [R = −CH3, −OSi(OtBu)3 and 4,4′-tBuBiPy = 4,4′-di-tert-butyl2,2′-bipyridyl] grafted on SBA-15120 likely proceeds via a transient Pd−H complex during the semihydrogenation of alkynes.159 Other hydrogenation catalysts on hybrid organosilica materials possessing dispersed imidazolium groups have also been described for ruthenium, [(Si(CH2)3(1-C(NCHCHNMes))RuCl2(PMe3)x],160 palladium, [(Si(C6H4−CH2)(1-C(NCHCHN-Mes))PdCl(η3-C3H5)],161 and iridium, [(Si(CH 2 ) 3 (1-C(NCHCHN-Mes))IrCl(SiOSi)(SiOSiMe3)] (Scheme 18).162 In the case of ruthenium, hydrogenation of CO2 in the presence of pyrrolidine likely proceeds via a Ru−H intermediate to yield 1-formylpyrrolidine (Scheme 18a).160 For palladium (Scheme 18b)161 and iridium (Scheme 18c),162 M−H intermediates likely facilitate the semihydrogenation of alkynes and hydrogenation of alkenes, respectively.

2.5. Concluding Remarks

Surface metal hydrides can be prepared on a broad range of oxide surfaces and are involved as intermediates in numerous reactions. Isolated and characterized species are mainly limited to early-transition metals (Ti, Zr, Hf, Ta, and W). No group 3 or lanthanide surface hydrides have been reported so far. This is likely due to the difficulty in preparation of appropriate peralkyl complexes of these elements. It could also be related to the fact that these elements are mainly trivalent and make strong M−O bonds, hence the difficulty in generating low-coordinate metal hydride. Similarly, while V, Nb, Cr, Mo, Mn, and Re surface hydrides have not been reported so far, one may expect that they should be accessible, provided appropriate molecular precursors are available. For later transition metals (groups 8−11), it seems that their weak M−O bonds lead to unstable hydrides. This promotes the formation of nanoparticles, unless additional stabilizing ligands (Si, phosphine, etc.) are coordinated to the metal center.

3. HYDRIDES IN ZEOLITES Surface hydrides can also be formed inside the porous network of zeolites. Zeolites are aluminosilicates with well-defined 8482


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Figure 1. (a) Molecular model for the gallium extraframework species Ga+, Ga(O)+, and Ga2O22+ in the mordenite zeolite. Reproduced in part from refs 78 (copyright 2010 American Chemical Society) and 188 (copyright 2009 Royal Society of Chemistry) with permission. (b) Formation of hydrides upon hydrogenation of the gallium species in zeolites.

microporous structures. Their framework is composed of fourcoordinate silicon or aluminum [SiO4] or [AlO4−] subunits, with various Si/Al ratios. In order to compensate for the negative charge on the aluminum subunits, the structures accommodate cations, typically Na+ or H+, the latter being responsible for the strong Brønsted acidity of zeolites. These cations can be exchanged by other cations in extraframework positions, generating Lewis acidic sites in the zeolite. In some cases, such sites are able to dissociate dihydrogen or alkanes, sometimes forming metal hydrides. While framework aluminum atoms in proton- or metal-exchanged zeolites react sparingly with dihydrogen, DFT calculations suggest that extraframework aluminum species can.167−169 Such species are formed during partial dealumination of zeolites and consist of mono- or oligomeric octahedral aluminum complexes bearing water or hydroxyl ligands. Due to their being less covalent than framework aluminum atoms, they are more Lewis acidic.168 Co- and Mo-exchanged zeolites display a high activity for C−H activation, but little is known about their potential to form hydrides upon exposure to H2.170−172 The case of silverexchanged A- and Y-zeolites was addressed in a number of experimental studies.173−177 Upon H2 adsorption on such materials, two 1H NMR signal appear at 3.9 and −1.8 ppm, attributed to OH and AgH groups, respectively.173,174 However, the relative intensities of these two signals, close to 3:1, is not consistent with a pure heterolytic splitting of H2 but, rather, with a succession of one homolytic and one heterolytic dissociation of H2. This process forms partially reduced silver clusters according to eqs 4−6. Here, H+ ions form OH groups with framework oxygen atoms. This is consistent with calculations, as isolated Ag+ seems to be unable to dissociate H2.167 Ag-exchanged zeolites are also active for methane activation, which proceeds by the formation of methoxy (OSCH3) and silver-clustered hydride species (Ag3H).175,176 2Ag + + H 2 = 2Ag 0 + 2H+

(4)

2Ag 0 + Ag + = Ag 3+

(5)

Ag 3+ + H 2 = Ag 3H + H+

(6)

The dissociation energies of dihydrogen in mordenite (MOR) exchanged with various cations were calculated. 167 The dissociation was strongly unfavorable for large, monovalent cations like Na+, Cu+, and Ag+, and dissociation on Cu2+ is nearly thermoneutral. In contrast, dissociation of hydrogen on Al3+, Zn2+, and Ga3+ was found to be very exothermic. This effect was attributed to the relatively low electronegativity of these cations. Ga- and Zn-exchanged zeolites are known to heterolytically dissociate dihydrogen experimentally,178,179 and both they and their bulk oxide counterparts show high activities in alkane dehydrogenation (vide infra). The following section addresses the formation and intermediacy of hydrides in the reactivity of Ga- and Zn-exchanged zeolites in more detail. 3.1. Ga-Exchanged Zeolites

Gallium ions can be exchanged as charge-compensating ions in the micropores of various aluminum-containing zeolites.180 The pores contain either mononuclear species [Ga + , 181,182 Ga(O)+ 182−185] or binuclear species (Ga2O2+)186−188 (Figure 1a). Such materials can dissociate H2 to form Ga−H hydrides characterized by IR stretching bands in the range 2040−2070 cm−1.179,185,189 These frequencies are somewhat higher than those observed on Ga2O3 (2020 cm−1), probably because of the different electronic and geometric environments of gallium in zeolites. Different surface hydrides, such as gallium dihydride [GaH2+],190−192 hydrido-hydroxy gallium [GaH(OH)+],182,193 or [H−Ga(O)(OH)Ga]2+,186,188,194 have been proposed as the products of reaction with H2 (Figure 1b). The mononuclear gallium hydrides [GaH2+] and [GaH(OH)+] are kinetically very stable, with activation barriers for the desorption of H2 higher than 300 kJ mol−1.195 Thus, decomposition of gallium dihydride is very slow, even at temperatures above 800 K.195−197 The decomposition of hydrido-hydroxy gallium [GaH(OH)+] into water and Ga+ (accompanied by reduction of Ga3+ to Ga+) is faster than the direct loss of H2. In contrast, desorption of H2 from the dimeric species is faster (activation barrier of 168 kJ mol−1). This effect is attributed to the presence of lessbasic bridging oxygen atoms that lower the reactivity toward H2. 8483


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Scheme 20. Formation of Ga Hydrides in Ga-Exchanged Zeolites

and the nature of the zeolite (Si/Al ratio), different zinc species are produced. When zinc is introduced in aqueous medium, either by equilibrium adsorption or incipient wetness impregnation, isolated zinc species are obtained in conventional exchange positions, with complete charge compensation.206−208 Zn2+ ions can balance the charge of two adjacent [AlO4]− units (Scheme 21a) or they can form [Zn(OH)]+ on isolated [AlO4]− positions (Scheme 21c). The latter is unstable at elevated temperatures, and the formation of binuclear [Zn−O−Zn2+] species by the condensation of two nearby ZnOH+ units may occur upon heating (Scheme 21c). However, the existence of the binuclear species is questioned, since no Zn−Zn scattering paths are observed in the EXAFS spectra of these materials,209,210 and furthermore, DFT calculations found their formation to be unfavorable.211 The deposition of zinc ions using a gas-phase precursor allows a stoichiometric exchange of the protons of H-zeolites with Zn2+.178,212 In this case, the charge of some of the zinc ions is only partially compensated by the nearby [AlO4]− tetrahedron, one being located farther from the Zn2+ (Scheme 21b). This produces a strong local electric field, thereby increasing the Lewis acidity of the zinc ion. Such species can be identified by the strong red shift of the H−H or C−H stretching frequencies of molecular hydrogen or methane adsorbed at low temperature.178,213,214 Zn-exchanged zeolites dissociate dihydrogen at room temperature. This gives rise to two well-defined IR bands at 1936 and 3612 cm−1 corresponding to Zn−H and O−H stretching, respectively.178,214 The Zn−H band in Zn-exchanged zeolites is blue-shifted with respect to the Zn−H band on ZnO (1710 cm−1). This is attributed to a different coordination environment of zinc ions in these materials. A number of computational studies have been performed and structures proposed for the products of dihydrogen dissociation. The observed IR frequencies cannot be reproduced computationally by adsorption of hydrogen on either ZnOH+ or isolated zinc

In this case, desorption of water is less favorable than desorption of H2.188 The gallium hydrides in Ga-modified zeolites are intermediates in H/D exchange reactions195,198−200 and alkane dehydrogenation and aromatization. Ga-modified zeolites have indeed been used for decades in industrial processes, like the CYCLAR process, for converting light alkane mixtures (propane, butane) into aromatics and H2.201 The rate-determining step in this reaction is the initial activation of the C−H bond. The use of Gacontaining materials for these reactions has been reviewed recently.78,202 Several mechanisms were proposed, all of them involving gallium hydride intermediates. The desorption of chemically adsorbed dihydrogen from gallium hydride species is a key step in most of the proposed mechanisms. Since desorption of H2 from isolated gallyl [GaO+] species is slow, it must not be the active site of these reactions. A three-step mechanism was proposed, in which gallium dihydride was the active site (Scheme 20a)190−192,200,203 involving (i) rupture of the C−H bond to form new Ga−alkyl and hydroxyl species, (ii) desorption of dihydrogen from a Ga-hydride and the proton of the hydroxyl, and (iii) β-elimination from the Ga−alkyl to yield alkene and regenerate the dihydride site. Other authors proposed, instead, a one-step, concerted mechanism featuring a six-centered transition state.192,197,204,205 This mechanism is energetically less favorable than the three-step mechanism for small alkanes (ethane) but more favorable for larger alkanes like isobutane. This is said to be due to larger steric hindrance around Ga−alkyl intermediates. Some authors consider the dinuclear Ga species to be the active site, since desorption of H2 from these sites is faster (Scheme 20b).186,188 3.2. Zn-Exchanged Zeolites

Zinc-exchanged zeolites are similar to their Ga-exchanged counterparts in many ways. They readily dissociate H2 to form zinc hydrides. They also have particularly high reactivity in alkane dehydrogenation. Depending on the preparation method 8484


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Scheme 21. Hydrogen Activation by Zn-Exchanged Zeolites

Scheme 22. (a) Heterolytic and (b) Homolytic H2 Activation Mechanisms on Metal Oxide Surfaces Sites

aquo species (the dissociation is endothermic in both cases).215 Isolated Zn2+ sites can explain the IR spectrum, but the adsorption energy strongly depends on the coordination environment of Zn and the positions of the surrounding Al atoms. The dissociation energy is higher when the two Al atoms are separated by more than one Si atom, causing the Zn ion to be more Lewis acidic.215−218 Thus, the formation of hydride is favored in samples prepared using gas-phase deposition of Zn2+, due to its propensity to generate such sites.178 The structure of the zeolite also plays a role.216,219 DFT calculations showed that ZnOZn2+ sites also dissociate H2 and generate hydrides,215,219,220 but with smaller adsorption energies than isolated sites. However, the calculated νZn−H of hydrogen adsorbed on such sites (1835 cm−1) does not match the experimental frequency.215 Finally, isolated Zn−H species formed upon H2 adsorption may evolve toward reduced Zn0 species. The result of the whole process is equivalent to a homolytic dissociation of H2 on the zeolite, with zinc hydride as an intermediate, as shown in eq 7.221

hydride is further transferred to an adjacent O atom. The presence of the M−H stretching bands in the IR spectrum is diagnostic of heterolytic dissociation on oxide surfaces (see Table 5).235 Observation of the isotopic shift by adsorption of D2 provides additional confirmation of these species. Reducible supports, such as transition-metal oxides (TiO2, CeO2, etc.), tend to favor homolytic dissociation.236,237 Nonreducible oxides, such as SiO2,133 Al2O3,238 and MgO,239 favor the heterolytic dissociation of H2 and form stable surface hydrides. Despite this general trend, the formation of surface hydrides has been observed on several reducible oxides, such as ZnO,240,241Ga2O3,242,243 or Cr2O3.244 The formation of hydrides is in fact often controlled by specific properties of the material, including electronic and structural surface defects. The rate of H2/D2 exchange may be used to probe the formation of transient surface hydrides. MgO, ZnO, and Ga2O3 catalyze this reaction at room temperature. Al2O3 requires temperatures of 100−200 °C, while with SiO2 even higher temperatures (above 400 °C) are required, but at such high temperatures Brønsted acid catalysis may also occur.230,231 Note that this reactivity cannot be directly linked to the Lewis acidic or basic properties of the different oxides. Additionally, there does not seem to be a relationship between the rate of the H2/D2 exchange and the mechanism of H2 splitting (homolytic vs heterolytic), as Ga2O3 and ZnO exhibit higher rates in comparison to TiO2, MnO, Fe2O3, or CuO, but much lower ones than Cr2O3, Co3O4, or NiO.230 The following section covers the formation of hydrides on a variety of bulk oxides formed upon adsorption of dihydrogen, with a focus on the reported structures. The involvement of surface hydrides on bulk oxides in hydro- or dehydrogenation reactions of C−O and C−C bonds is then discussed.

Zn−H+ + Si−O−Al−(OH)−Si →Zn 0 + (Si−(OH)−Al−(OH)−Si)+

(7)

Besides H2 activation, these materials have high reactivity for C−H bond activations in which Zn hydrides play a role. This includes H/D exchange,199 activation of methane,222,223 and alkane dehydrogenation.224,225 The proposed mechanisms are very similar to the ones proposed for Ga-exchanged zeolites. Here again, a three-step mechanism is the more often reported,219,226,227 where formation of Zn−alkyl species228 is followed by a β-H transfer to form Zn-hydride and alkene.229 However, the concerted one-step mechanism has also been proposed for Zn-exchanged zeolites.215 The more polarized isolated zinc ions are more active in the initial C−H activation. Zn−O−Zn2+ structures activate the C−H bonds of alkanes, but the alkyl species formed cannot easily eliminate alkene to produce the hydride intermediate (activation barrier of 190 kJ mol−1). Thus, this is probably not the active site.211,215

4.1. Zinc Oxide

The adsorption of hydrogen on zinc oxide has been studied extensively.240,241,245−259 When H2 is absorbed on ZnO, IR spectroscopy shows two new species. The first hydride (referred to as type I) is formed reversibly under H2 and shows two new IR bands at 3495 and 1710 cm−1, attributed to O−H and Zn−H stretches. The second hydride (referred to as type II) is formed irreversibly with weak bands at 1450 cm−1 attributed to Zn−H.240,246,250,251,253−255 Both of these species show signals in 1H NMR around 0 ppm.249,259 Increasing H2 coverage induces a red shift of the Zn−H frequencies from 1710 to 1690 cm−1 due to lateral and through-solid interactions.253,257,260 Thus, hydrides are adsorbed either as patches on neighboring, isolated Zn2+ ions on extended surfaces257 or on patches of Zn2+ ions as found at defects like oxygen vacancies or on the reconstructed polar facets (Figure 2).247,253,261 Both experiment and theory suggest that oxygen vacancies may play an important role in this reaction.247,251,262−269 DFT calculations showed that H2 adsorption is strongest on oxygen

4. SURFACE HYDRIDES ON BULK OXIDES Many bulk oxide surfaces react with dihydrogen.230,231 Oxide surfaces can split H2 by either a heterolytic or homolytic mechanism, depending on the nature of the oxide (Scheme 22). Heterolytic H−H bond cleavage (Scheme 22a) occurs on Lewis acidic Mδ+−Oδ‑ sites to form new surface hydroxyl and surface hydride species Mδ+−Hδ‑. In contrast, homolytic dissociation (Scheme 22b) involves proton coupled electron transfer (PCET), resulting in the formation of two new OH groups and reduced metal centers (either isolated or delocalized in the conduction band of the material).232−234 Note also that the latter process may also occur via heteroltyic splitting if the metal 8485


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Figure 3. Calculated structures for the irreversible and reversible adsorption modes detected by IR spectroscopy on MgO on a reversecorner location (green, magnesium; red, oxygen; white, hydrogen). Adapted with permission from ref 283. Copyright 2003 American Chemical Society.

Figure 2. Top view of the structure of surface hydrides at an oxygen vacancy on the polar oxygen-terminated (0001̅) facet of zinc oxide (blue, zinc; red, oxygen; and white, hydrogen). Adapted with permission from ref 246. Copyright 2003 Springer.

Hydrogen desorption is possible when the hydride and hydroxyl are close (reversible mode) but is much more difficult when farther apart (irreversible mode). Calculations show that the adsorption of H2 in both modes is not activated. The calculated frequencies match the experimental vibrational frequencies observed.239,283 Alternatively, oxygen vacancies on the terraces have been proposed as potential sites for hydride formation.284 A specific feature of the proton/hydride pair on the surface of MgO is that, upon UV irradiation, the hydride decomposes into a neutral hydrogen atom that evolves to the gas phase and a trapped electron in the vicinity of the proton, a so-called color center that can be observed and examined by EPR spectroscopy.285 Similar features are observed on CaO.286 Thus, H2 adsorption accompanied by UV irradiation on MgO produces a single OH stretching band at 3698 cm−1. This can be attributed to a homolytic splitting of H2, yielding a color center.287−289 The splitting is also homolytic in the case of MgO ultrathin film deposited on a gold substrate. In this case, the electrons from dihydrogen are transferred to the metal substrate by tunneling through the film.290

vacancies on the polar oxygen-terminated (0001̅) facet of ZnO particles.263,264,266 Figure 2 shows the most favorable computed structures for the zinc hydrides at oxygen vacancies on the polar facets of ZnO. The computed properties of these species are similar to those observed by experiment.263 The type II hydride is thought to be bridging between either two or three Zn atoms in the center of the oxygen vacancy. The high symmetry of this position explains its low IR activity (calculated at 1507 cm−1) and thus the weakness of the band. The optimized structure for the type I hydride is a terminal Zn hydride. It has a νZn−H of 1745 cm−1, very similar to the experimental value. In both cases, the hydridic and protonic moieties remain close to each other. Electron transfer from the hydride species to the anionic vacancy is also thought to occur.266 Note that the polar surfaces of ZnO are unstable and undergo reconstruction.261,270,271 A possible (2 × 2) reconstruction involves the departure of one O atom out of four from the (0001̅) facets. This generates Zn2+ arrangements similar to those at oxygen vacancies, as shown on Figure 2. Hydrides on the other, nonpolar facets are less stable from DFT calculations,262,263,272 although some calculated geometries found on the (0101̅) facets also explain well the main Zn−H stretching frequency at 1710 cm−1 262 or the specific Raman feature observed at 1600 cm−1.273 This type of hydride on pristine, extended surfaces may be present on ZnO powders where nonpolar facets (101̅0) and (112̅0) are most abundant.256,257

4.3. Group 13 (Al2O3, Ga2O3, In2O3)

4.3.1. Al2O3. Surface aluminum hydrides have been observed on γ- and η-alumina during H2/D2 equilibration reactions.238,291,292 At room temperature, H2 dissociatively adsorbs on γ-alumina that has been pretreated at 500 °C under vacuum. Al−H stretching bands are observed at 1900 and 1866 cm−1. H2 adsorption is thought to occur at a small fraction of defect sites. The concentration of these sites was estimated by DFT calculations and site titration to be ca. 0.1 per nm−2 (much less than the surface OH density of ca. 4.0 OH per nm−2). 291,293−295 DFT calculations showed that these active sites consist of either three-coordinate aluminum centers (AlIII) or four-coordinate aluminum centers (AlIV) that are located on the (110) facet of γ-alumina particles. Reaction of H2 with these sites gives either four-coordinate (AlIV−H) or five-coordinate (AlV− H) centers along with nearby hydroxyl groups (Figure 4a,b). The formation of surface aluminum hydrides has been proposed on other transition aluminas (δ and η)238,292 and on the most thermodynamically stable allotropic variety, α-alumina.296 Note that at low temperatures (20 K) alumina does not dissociate H2 but only forms H2−Al adducts, whose relative frequencies parallels what is observed for CO adsorption.297 4.3.2. Ga2O3. Gallium oxide also forms stable surface hydrides, characterized by νGa−H at 2020 cm−1.242,298 Depending on the polymorph used, two other bands are also observed that are attributed to octahedral (2003 cm−1) or tetrahedral (1980 cm−1) gallium hydrides.243,298 These assignments are

4.2. Group 2 Metal Oxides

The dissociation of H2 on MgO, CaO, and SrO is also heterolytic, resulting in the formation of surface hydrides.274 MgO has been the subject of numerous studies regarding hydrogen adsorption. The adsorption of hydrogen on MgO produces two different species, as detected by IR spectroscopy. The first has bands at 3454 and 1325 cm−1, attributed to the hydroxyl O−H and Mg−H, respectively, whereas the second mode is characterized by bands at 3712 and 1125 cm−1.239,275,276 The formation of the first species is reversible while the second is irreversible. Most of the structural proposals involve structural defects, such as corner and edges on the surface of MgO crystallites. Well-defined structures have been proposed.277−283 The most favorable are formed at defect sites where di- and tricoordinated hydrides are possible, referred to as “reverse corners” or “reverse edges” (Figure 3).283 This structure is common to both the reversible and irreversible adsorption sites. The main difference between reversible and irreversible adsorption appears to be the distance between the hydride and the hydroxyl groups. 8486


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4.4. Group 14 (SiO2, GeO2, SnO2)

4.4.1. SiO2. Silica reacts with molecular hydrogen at relatively high temperatures (>400 °C).231 Heterolysis of H2 produces new O−H and Si−H groups. This reaction is observed during reductive treatments of silica, with νSi−H at 2300−2100 cm−1. The dissociation of hydrogen probably occurs at silica defects, like dangling bonds or strained Si−O bonds.308−310 This effect is often used to enhance the properties of silica for electronic311,312 and optical applications.313 4.4.2. GeO2. Akin to silica, germanium oxide is used as a material in microelectronic applications. Dangling bonds on the surface are sometimes quenched by treatment with H2. However, this is much less efficient than for silica.314,315 DFT calculations suggested, indeed, that homolytic dissociation, accompanied by formation of hydroxyl groups and reduction of the Ge centers, is favored over heterolytic dissociation and the formation of hydrides for GeO2.316 4.4.3. SnO2. No νSn−H is observed after adsorption of dihydrogen on tin oxide. Instead, the IR absorbance of the entire spectrum increases, up to full extinction if high amounts are adsorbed.317,318 This is accompanied by changes in conductivity.319,320 Both phenomena are consistent with homolytic dissociation of dihydrogen on SnO2, with transfer of electrons to the conduction band.321,322 The adsorption is fully reversible, as is the change in conductivity.320 For that reason, tin oxide is commonly used as a material for gas sensors.323

Figure 4. DFT-calculated structure of the (a) (AlIV-H,OH) and (b) (AlV-H,OH) pairs. Al−H distance: ca. 1.60 Å (pink, aluminum; red, oxygen; white, hydrogen). Adapted with permission from ref 291. Copyright 2006 American Chemical Society.

Scheme 23. H2 Activation on ZrO2 Surface

supported by DFT calculations.299 Similar to other oxides, surface defects, such as oxygen vacancies, may play an important role in the formation of hydrides. Increasing the activation temperature of the material increases the concentration of defects, which is paralleled by an increase in the concentration of hydrides formed by hydrogen adsorption.300−302 DFT calculations showed that while hydrogen adsorption is endothermic on defect-free Ga2O3 surfaces, it is exothermic at oxygen vacancies on the (100) facet of β-Ga2O3.299,302,303 Gallium oxide can sometimes adsorb H2 in a homolytic fashion, accompanied by the reduction of GaIII to GaI. In this case, the adsorption of H2 is irreversible. It may though desorb in the form of a water molecule accompanied by the formation of an oxygen vacancy.304 4.3.3. In2O3. On In2O3, homolytic adsorption of H2 is thermodynamically favored over heterolytic dissociation. However, calculations show that the latter is kinetically favored. Thus, the formation of indium hydride is likely an intermediate step in the homolytic adsorption on In2O3305,306 and probably involved in the hydrogenation of CO2 on In-doped ZrO2.307

4.5. Lanthanide, Group 3, and Transition-Metal Oxides

As stated above, it is considered that reducible oxides react homolytically with H2. In some cases, homolytic adsorption is accompanied by loss of water, the formation of an oxygen vacancy, and transfer of electrons to the metal centers. The reaction of these oxides with H2 is often the first step in production of the metal. 4.5.1. CeO2. Adsorption of H2 on ceria produces various OH bands, with no observed Ce−H band (expected at ca. 1500 cm−1).324,325 Instead, a feature around 2125 cm−1 appears, attributed to the presence of Ce3+.324,326,327 This band is also observed in CeO2 samples that have been heated under vacuum

Figure 5. (a) Proposed reaction pathway for the dissociation of H2 on ceria,330,359 RuO2,357 PdO,358 and In2O3.306 (b) Elimination of H2 from hydroxo species through hydride intermediate and oxidation of the metal sites on analogous U complexes. 8487


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Table 4. A Collection of Energetic Parameters for the Oxides Mentioned in This Review electronic structure entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 a

oxide CaO MgO SrO γ-Al2O3 CeO2 ZrO2 TiO2-rutile SiO2 β-Ga2O3 ZnO Cr2O3 In2O3 GeO2 SnO2 WO3 PdO RhO2

M−H bond:362χH−χM

reducibility:aΔrH° of reduction (kJ mol−1)

1.2 0.9 1.3 0.6 1.1 0.9 0.7 0.3 0.4 0.6 0.5 0.4 0.2 0.2 −0.2 0.0 −0.1

393 360 350 317 302367 270 230 214 118372 109 98 66375 48 47 21 −127380 −160381

band gap (eV) 7.1 7.8 5.2 8.7 3.1 5.0 3.1 9.0 4.9 3.5 3.4 3.0 5.5 3.6 2.6 1.5 0.0

ref 363 364 365 366 368 369 370 371 373 374 370 376 377 378 379 380 382

Unless stated otherwise, the data are taken from the NIST Chemistry Webbook.383

below 175 K. The second adsorbed species is attributed to a regular heterolytic dissociation, with a sharp band at 1562 cm−1 (Zr−H stretching vibration) and a band at 3668 cm−1 attributed to an O−H stretching vibration. Above room temperature, reductive homolytic dissociation occurs, characterized by two new νOH at 3778 and 3668 cm−1.341,346−348 However, these bands cannot be distinguished from the OH stretching vibrations obtained after hydration of the zirconia.349 4.5.4. WO3. The adsorption of hydrogen on tungsten oxide forms tungsten bronzes of the generic formula HxWO3, with diffusion of hydrogen into the bulk.350,351 This results in a color change, due to the injection of electrons in the conduction band. The most favorable adsorption (or insertion) mode for hydrogen atoms is as a hydroxyl.352,353 Thus, the formation of hydrides on these materials is unlikely. 4.5.5. Others. Despite their high reducibility, H2 adsorption on several transition-metal oxides (such as cobalt, manganese, and chromium oxide) causes new M−H stretching vibrations to appear in the IR spectrum (Table 5).244,354,355 However, on α-Cr2O3, in addition to the new M−H and related O−H stretching bands, the absorbance of O−H bands normally associated with water adsorption increases.244 This could be due to homolytic splitting of hydrogen, suggesting that both processes occur simultaneously to some extent. Additionally, the formation of metastable (H+, H−) heterolytic pairs on the (110) surface of ruthenium oxide was observed by combining STM observation and TPR.356,357 Similar reactions have been discussed for PdO(101) thin films.358 Surface hydrides have thus also been shown on reducible oxides, and in some cases, heterolytic splitting appears to be in competition with homolytic splitting. Recent DFT calculations proposed a different scenario to explain these observations. In the case of ceria, even though the product of the homolytic splitting of H2 is more stable than the product of heterolytic splitting, the barrier to heterolytic splitting is lower than that of the homolytic pathway (Figure 5a).330,359 Thus, the dissociation of H2 on this surface is proposed to proceed via a sequential heterolytic splitting and a H migration−reduction mechanism. In that sense, cerium hydride may be considered an intermediate in the formation of the reduced oxide. This mechanism has also

for several hours. Its intensity increases when H2 is adsorbed at higher temperatures. The feature at 2125 cm−1 is accompanied by UV−visible bands and XPS signals that are also attributed to Ce3+.327,328 All these data are consistent with overall homolytic dissociation of H2, through the formation of Ce hydride intermediates and the heterolytic dissociation of H2 on Ce−O bond (vide infra).329−331 4.5.2. TiO2. Contradictory results have been reported for the adsorption of hydrogen on titania. Similar to ceria, adsorption of hydrogen on titania produces an IR band around 2000 cm−1 that cannot be attributed to νTi−H (expected around 1600 cm−1). The band does not shift when hydrogen is replaced with deuterium. This band is rather associated with the formation of Ti3+.332 No IR signal attributable to a Ti−H bond can be observed.333 This hypothesis is supported by several computational studies.236,334−336 However, both low-energy ion scattering (LEIS)337 and electronically stimulated desorption (ESD)338 indicated the presence of titanium hydride species, although indirectly. Titanium hydrides were only observed by LEIS on nonstoichiometric titania. For a stoichiometric sample, an XPS signal characteristic of Ti3+ species was observed. This suggests that oxygen vacancies could play a role determining the mechanism of hydrogen adsorption and the stability of putative hydride intermediates. However, the ability of hydrogen to easily diffuse into bulk titania complicates these studies. Protons in the bulk oxide are associated with reduced Ti3+ centers and can be formulated as interstitial hydroxyls.339,340 The computed barrier for diffusion of a proton from the surface into the bulk is 140 kJ mol−1 lower than the desorption of H2.334,335 4.5.3. ZrO2. Zirconium oxide is a poorly reducible oxide. However, adsorption of dihydrogen at low temperature (up to 373 K) forms two kinds of hydride species.341,342 The first one, stable at only low temperature, is the result of a homolytic oxidative dissociation on two zirconium atoms in the vicinity of an oxygen vacancy343−345 and shows a broad band at 1540 cm−1 not associated with other O−H bands. These species are the result of electron transfer from reduced Zr3+ ions or oxygen vacancies to the hydrogen atoms, which become formally hydride ions (Scheme 23).343 This adsorption mode can be observed below 373 K, but it is only thermodynamically stable 8488


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Table 5. M−H Stretching Vibrations Observed for Various Surface Hydrides on Bulk Oxides entry

species

compound

IR frequency (cm−1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Mg−H Ca−H Sr−H Al−H Zn−H

MgO CaO SrO Al2O3 ZnO Zn-HZSM5 Ga2O3 Ga-HZSM5 SiO2 polyhydrosiloxane ZrO2 α-Cr2O3 MgO-CoO MnO-Cr2O3

1325 (rev) /1125 (irr) 930 850 1902−1866 1710 1936 2020 2093, 2069, 2050 2260−2180 2260 1565 (term.)/ 1360 (bridg) 1714−1697 1669, 1585, 1542 1506

Ga−H Si−H Zr−H Cr−H Co−H Mn−H

been suggested for RuO2,357 ReO3,352 and PdO.360 Note that very similar behaviors have been observed on molecular compounds, although in the reverse direction. 3 6 1 [(1,3-(Me3Si)2C5H3)4U2(μ-OH)2] complexes, for instance, decompose between 30 and 107 °C and form molecular hydrogen and [(1,3-(Me3Si)2C5H3)4U2(μ-O)2], where uranium ions are oxidized. A uranium hydride was shown to be an intermediate in this process (Figure 5b). We showed here that, in general, H2 adsorption on nonreducible oxides occurs by heterolytic dissociation, while adsorption on reducible oxides leads to an overall hemolytic process. However, the relationship is not straightforward, since several reducible oxides, such as Cr2O3, ZnO, and Ga2O3, have been shown to form surface hydrides. In order to gain insight into the fundamental reasons why certain oxides form surface hydrides upon adsorption of H2, we gathered the following parameters for bulk oxides into Table 4: M−H bond strength, reducibility of the oxide, and the band gap. The strength of the M−H bond can be assessed by the difference in electronegativities according to Pauling’s definition (the higher the difference, the stronger the bond). The reducibility of the oxide is assessed by the standard enthalpy of reduction of the oxide by one hydrogen molecule, according to eq 8. 1 x MxOy + H 2 = M + H 2O y y

ref 275 274 274 238 240 213 242 179 133 142 341 244 355 244

H2 by forming hydroxyls and transfer of electrons to the conduction band. The turning point for the band gap is between 3.1 eV (CeO2 and TiO2) and 3.4 eV (Cr2O3), with the exception of SnO2 and GeO2. The calculated M−H bond strengths in Table 4 do not seem to correlate with the dissociation mechanism. The ability of an oxide to form stable hydrides seems to be related to electronic parameters, but we could not find a straightforward correlation. Other parameters, such as the local coordination environments of the surface ions and the ease of forming vacancies, have been shown to play a role. However, these parameters are difficult to quantify. 4.6. Hydrides on Bulk Oxide Surfaces as Intermediates in Hydrogenation or Dehydrogenation Reactions

As for most of the other types of hydrides discussed so far, surface hydrides on bulk oxides are involved in the mechanisms of hydrogenation and dehydrogenation of C−C and C−O multiple bonds. 4.6.1. C−C and CC Bonds. The earliest evidence for the direct interaction of a surface hydride with an alkene was given by Dent and Kokes.241,384−386 They showed that Zn−H bonds on the surface of ZnO deform when contacted with a reaction mixture of ethylene and H2. The proposed mechanism involves hydride transfer to a gas-phase alkene molecule to form a Zn−alkyl species. A proton from a hydroxyl group is then transferred to the alkyl species to form the alkane (Scheme 24). This reaction can be carried out at room temperature in the case of ZnO. Only the reversible hydride species (type I) is involved in this reaction. Moreover, performing the reaction with deuterium yields only the bideuterated compound ethane-d2 (CH2D−CH2D) while using an equimolar mixture of H2 and D2 mainly yields ethane-d0 or ethane-d2.387,388 Both of these results indicate that both hydrogen addition steps (hydride or proton) are irreversible under the reaction conditions and both come from the same molecule of H2. This latter feature is consistent with the proximity between the H2 fragments mentioned above for the adsorption of H2 on ZnO. MgO,389−391 other alkaline-

(8)

The band gap between the valence band and the conduction band gives information about the electronic structure of the oxide. The data in Table 4 are sorted according to their reducibility. The formation of hydrides is the more favorable pathway for most of the oxides with a high, positive standard enthalpy of reduction. This is in line with the general consideration that nonreducible oxides favor heterolytic dissociation and easily reducible oxides rather favor homolytic dissociation. The turning point seems to be between 66 kJ mol−1 (In2O3) and 98 kJ mol−1 (Cr2O3). However, TiO2 and CeO2 are clearly outside of this range, with very high reduction enthalpies (230 and 302 kJ mol−1, respectively). Nonetheless, these two materials have quite small band gaps compared to the other materials having high enthalpies of reduction. The band gap also seems to be a relevant parameter, with large band gap materials favoring heterolytic dissociation and small band gap materials favoring homolytic dissociation. This is consistent with the adsorption of

Scheme 24. ZnO-Catalyzed Hydrogenation of Ethylene

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earth oxides,392−394 Ga2O3,395 and Al2O3396,397 all hydrogenate alkenes by a similar mechanism involving a surface hydride. Ga2O3 and alkaline-earth oxides (MgO, BaO, SrO) catalyze the hydrogenation of ethylene at 100 and 200 °C, respectively. Yet, the activity was shown in both cases to strongly depend on the activation temperatures, all giving volcano-type trends.398 Increasing activation temperatures removes adsorbed water or CO2 molecules, causing an increase in the number of surface defect sites. However, at higher temperatures, the surface defect concentration decreases, possibly due to surface reconstructions. These volcano trends match the evolution of the maximum hydrogen uptake. 395 Additionally, the hydrogenation of 1,3-butadiene can be promoted by MgO at room temperature, essentially yielding trans-but-2-ene through 1,4-addition (Scheme 25). In this case again, the added hydrogen molecule retains its molecular identity.391 This latter feature may also be observed by the para-hydrogen induced polarization (PHIP) method. Indeed PHIP effects have been observed during the hydrogenation of 1,3-butadiene on CaO and ZrO2 but also for hydrogenation of propene on ZnO, Cr2O3, and CeO2,399 indicating that H2 retains its identity in the hydrogenated molecule, consistent with the heterolytic splitting of H2 on M,O sites. Other materials like alumina and ceria catalyze hydrogenation of C−C multiple bonds, although the intervention of a hydride may be questioned. Al2O3 requires higher reaction temperatures to catalyze hydrogenation of ethylene (between 200 and 400 °C),397 probably due to the lower amount of suitable defect sites for the formation of the hydride. At such high temperatures, other mechanisms involving radicals or Brønsted acidicty may occur. On cerium oxide, the most favorable calculated reaction pathway for the semihydrogenation of propyne on ceria involves the addition of a hydride-like species to the alkyne by a concerted mechanism.237 In that sense, hydrides are involved as a transient intermediate species, just as they are for the dissociation of H2, despite their being less thermodynamically stable than other adsorption modes. Metal oxides without metal particles also catalyze the dehydrogenation of alkanes (the microscopic reverse of hydrogenation of alkenes). The principle of microreversibility implies that the mechanism of this reaction should be the microscopic reverse of the mechanism of the hydrogenation and as such will involve the intermediacy of surface hydrides. Dehydrogenation of alkanes is an endothermic reaction. As a consequence, this reaction requires much higher temperatures than hydrogenation reactions (500−600 °C). ZnO shows lowto-medium activity and MgO and Al 2 O 3 are almost inactive.304,400−402 Ga2O3, on the contrary, is one of the most active materials for the dehydrogenation of alkanes.304 The intermediacy of a surface gallium hydride was shown

Scheme 26. Ga2O3-Catalyzed Dehydrogenation of Alkanes

unambiguously,403 by the appearance of a Ga−H stretching band in the IR spectrum of Ga2O3 previously heated in the presence of ethane. The order of appearance and disappearance of the C−H and Ga−H bands led to the proposition that dehydrogenation occurs by the reverse mechanism of hydrogenation. This involves an initial C−H activation by a surface Ga−O pair, forming a gallium alkyl and a hydroxyl group (Scheme 26). An alternative mechanism has been proposed, in which the initial C−H bond activation occurs by formation of a gallium hydride along with an alkoxide.404 On a defect-free Ga2O3(100) crystal facet, a mechanism involving alkyl radicals and reduction of a gallium center is more favorable than the heterolytic pathway.405 In this case, no gallium hydride is formed. However, we recall here that involvement of structural defects seems to greatly enhance the formation of the hydride. Thus, a mechanism involving a hydride species might be disfavored on a defect-free surface. Finally, surface hydride species may also participate in the dehydrogenation of alkanes on chromium406 and vanadium oxide.407,408 4.6.2. C−O and CO Bonds. Hydrogen that is dissociatively adsorbed on ZnO can reduce carbon monoxide, carbon dioxide, and carbonyl groups.409,410 The effect of contacting CO with a hydrogen-covered ZnO surface has been studied.241,247,251,252,411 A red shift of the Zn−H stretching band from 1720 to 1691 and 1672 cm−1 is observed as the CO pressure is increased. This affect was attributed to a strong interaction between adsorbed CO and surface hydrides. Further increasing the CO partial pressure led to a decrease of the intensity of the Zn−H bands accompanied by the formation of surface formyl or formate species. The reduction of CO double or triple bonds was proposed to be a key step in the formation of methanol from CO or CO2. Several computational studies calculated reaction pathways in which CO2 or CO is hydrogenated to methanol through stepwise reduction. This occurs on an oxygen vacancy by successive incorporation of H− and H+ from heterolytically dissociated H2.264,265,412,413 There is a correlation between CO hydrogenation activity and the number of O vacancies (from EPR spectroscopy)268 that favor the formation of surface hydrides. ZnO is one of the components of the industrial catalyst for methanol synthesis from CO2 (Cu/ZnO/Al2O3), and the role of the zinc oxide in this reaction is still debated. Currently, there are three main hypotheses as to the role of ZnO in this catalyst: (1) electronic and morphologic effects between copper and ZnO,414−416 (2) decoration of the copper particles with Zn atoms,417,418 and (3) hydrogen spillover from the metal particles to the oxide.249,419 Some authors suggest that the active sites are located on zinc oxide. More specifically, the role of hydrogen adsorbed on oxygen-deficient ZnO, probably as hydrides, has been raised by several others.420−422 The direct involvement of a hydride was also shown by IR spectroscopy in the photocatalytic reduction of CO2 on gallium oxide.423 Hydrides were also proposed to be directly involved in the reduction of CO to formaldehyde on MgO based on IR424,425 and UV−visible

Scheme 25. Isotopic Labeling during the MgO-Catalyzed Hydrogenation of 1,3-Butadiene

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Scheme 27. ZnO-Catalyzed Conversion of CO to Methanol

spectroscopies.426 Scheme 27 shows a proposed mechanism for the hydrogenation of CO to methanol. The active site is thought to be around an oxygen vacancy, required for both the adsorption and activation of CO and for the heterolytic dissociation of H2. Successive incorporation of H2 leads to formaldehyde and then to methanol.412 The intermediacy of surface hydrides is also suspected for the dehydrogenation of alcohols to aldehydes or ketones on ZnO,427,428 MgO,429 and other alkaline-earth oxides.430 However, it remains mostly speculative, since no direct evidence was reported. There is a correlation between the rates of dehydrogenation of ethanol and the H2/D2 exchange rate for Mg/Al mixed oxides.431 Hydride species on MgO have been proposed as possible intermediates in the dehydrogenation of alcohols to ketones and related condensation reactions of ethanol such as the Guerbet and Lebedev reactions.429,430,432 Lewis acid sites are thought to be necessary for the α-hydrogen abstraction step, forming the ketone and a surface hydride (Scheme 28). However, this proposal was mainly based on the knowledge that hydrides can exist on such materials rather than on experimental evidence. Other authors have proposed mechanisms that do not involve a surface hydride, especially for the abstraction of the α-hydrogen atom.431,433 γ-alumina is a poor catalyst for the dehydrogenation of alcohols.434,435 The intermediacy of metal hydrides during α-H abstraction is possible, just as for the other oxides mentioned. Here again, the very low concentration of the required sites might explain the low activity measured. Additionally, in the case of alcohol dehydration, γ-alumina efficiently catalyzes the competitive dehydration reactions, producing water that strongly binds to the defect sites, reducing their surface concentration.436−438 However, alumina samples prepared by a sol−gel route display higher activity for the dehydrogenation of ethanol.439 Such samples might expose a higher density of surface defects, thereby increasing the number of suitable sites for the formation of a hydride.

5. CONCLUSIONS AND OUTLOOK In this review, we have shown that surface metal hydrides are involved in key reaction and chemical processes. In many instances, they are stable species that can be isolated and spectroscopically characterized. They are often generated by reaction of H2 on M,O sites of metal oxide and zeolite surfaces or from surface organometallic species. They display characteristic IR signatures, which can be confirmed by reversible H/D exchange experiments, in the case of main group and earlytransition-metal hydrides. However, such IR signatures are often absent for late-transition metals because of the low dipole moment and thereby intensity of such M−H bonds. They have also been identified by specific 1H NMR signatures, ranging from −40 to 25 ppm in a few cases. Other techniques, such as 2 H NMR, EPR, and neutron diffraction, could also be useful in the future. Both NMR and neutron techniques have been used to characterize homogeneous and heterogeneous hydride complexes to great success.6,440 Different types of isolated hydrides have been considered throughout this review: hydride complexes grafted on the surface of a support, hydrides generated from well-defined surface species, or hydrides formed upon dissociation of molecular hydrogen on metal sites at the surface of metal-exchanged zeolites or bulk oxides. Most hydrides are from the main group element and early-transition metals and show similar hydridic properties and reactivity. For instance, they react with proton sources and are key reactants or reaction intermediates in hydroor dehydrogenation processes involving hydrocarbons (alkanes, alkenes, alkynes) or oxygenates (CO2, CO, ketones, alcohols). In very few instances, late-transition-metal hydrides are found. As discussed above, surface metal hydrides can be formed from various routes. Surface organometallic chemistry has mainly led to the formation of early-transition-metal hydrides, mostly group 4, Ta, and W, as well as main group elements (Si, Al, Ga, Sn). There are fewer surface hydrides of late-transition metals (groups 8−11), mainly because they suffer from weak M−O bonds and thus easily generate metal nanoparticles. Many earlier-transition metals should be in principle accessible. One limitation is probably related to the availability of the appropriate molecular precursors (typically metal alkyls). In addition, the generation of hydrides via SOMC has been so far mainly limited to one approach, i.e., the treatment of supported perhydrocarbyl species by H2. As mentioned previously, late-transition metals are less oxophilic and more easily reduced in comparison to their early-metal counterparts. As a result, decomposition of late-

Scheme 28. MgO-Catalyzed Dehydrogenation of Alcohols

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prepared on silica. This is illustrated by the fact that tungsten hydrides can be generated on alumina-containing supports but not on silica. This difference likely arises from the Lewis acidity of the latter supports, which may prevent the lability of metal cations. However, one cannot draw general conclusions, since only a few supports have been investigated. There may be fundamental limitations that prevent the formation of stable hydrides on all metal oxides. We have seen that metal oxides react with H2 via heterolytic or homolytic processes. Activation of H2 by a heterolytic process involves formation of new M−H and O−H bonds. This reaction can either occur across directly bonded M−O sites (as in 1,2 additions)447 or across unbound M,O sites (like in frustated Lewis pairs, FLP).448 Both of these processes are related to σ-bond metathesis in that formation of the new hydride does not involve oxidation or reduction of the metal site. Homolytic activation of H2, which leads to the formation of O−H bonds in parallel with reduction of the metal centers, may occur directly without any intermediates, but it could also occur by a mechanism involving heterolytic H2 activation. In the latter case, heterolytic dissociation of H2 on M,O sites would occur followed by rapid hydride migration to the adjacent O, thus yielding O−H and a reduced metal site (Scheme 29). The factors that affect the rate and the thermodynamics of both heterolytic and homolytic H2 splitting are not wellunderstood. Likely, they involve many factors, such as the difference in electron density across M,O sites (Lewis acidity and basicity of M and O sites), the distribution of spin density on the M sites, the M−H bond strength, and the reducibility and the electronic structure of the oxides (band gap, position of band edges). For instance, oxide supports that have more spin density on the metal atoms should kinetically favor homolytic H2 splitting rather than heterolytic splitting. Making the analogy to homogeneous chemistry, these oxide supports can be thought of as highly noninnocent ligands.449 Such processes will be favored on oxides for which the metal orbitals have a large contribution to the valence band molecular orbitals.450 Investigations on the relationship between the electronic structure of oxides and the stability of their hydrides is necessary. Other factors may also contribute to hydride formation and stability on metal oxide surfaces, such as whether the hydride acts as an isolated entity or is part of a larger ensemble. This will depend on factors such as diffusion of H atoms through the

transition-metal hydrides on oxide supports occurs more readily and generally at lower temperatures than supported earlytransition-metal hydrides. This decomposition typically occurs by reductive elimination to the surface, resulting in surface O−H bonds, and subsequent formation of metal aggregates. However, the examples presented show that hydrides of late-transition metals can be stabilized through incorporation of additional ligands. Both monodentate and polydentate ligands can be introduced to the molecular precursor or to a surface complex prior to hydride formation. These ancillary ligands provide a stronger interaction with the metal center than the oxide surface, which ultimately suppresses aggregation. While this methodology has helped in the development and characterization of late-transition-metal hydrides on oxide surfaces, it does not always prevent leaching of a molecular complex from the surface. While not discussed here, supported nanoparticles of latetransition metals can also lead to the formation of hydrides at the surface of the oxide support, which may be essential for the activity and stability of such catalysts. In fact, hydrogen atom transfer from the particle to the surface of the support is a wellknown process in heterogeneous catalysts, coined spillover.441 It is likely involved in many catalytic reactions, when the support is not innocent (e.g., hydrogenation of COx). The generation of hydrides has also mainly been limited to a small number of oxide supports. In addition, one would expect a new generation of metal hydrides to appear with the development of functional materials (MOF, COF, PMO’s, etc.). Anchoring metal sites directly at the surface of an organic moiety isolated within a solid matrix could in principle combine the advantages of site isolation with the variety of organic ligands.5 Whether looking at hydrides generated via SOMC or from zeolite/bulk oxides, they are typically more stable than their molecular analogues, but their stability is highly dependent on the support. Many hydrides decompose quickly in solution at low temperatures.442,443 This is due to the common decomposition of hydrides via bimolecular mechanisms, yielding metal−metal bonded species and H2444 or bridging hydrides (eq 9 and 10).445,446 In contrast, surface hydrides benefit from site isolation, which prevents bimolecular reactions. Thus, some of the hydrides we have shown here can be reliably heated up to much higher temperatures (up to 250 °C) for prolonged periods of time without significant decomposition. This explains their use as catalyst precursors for high-temperature alkaneconversion processes (dehydrogenation, hydrogenolysis, metathesis, nonoxidative coupling, vide supra).

Scheme 29. Possible Mechanisms for Homolytic Hydrogen Dissociation on Metal Oxide Surfaces: (a) Direct vs (b) Indirect, via Heterolytic Splitting

The stability of the surface hydrides depends on many parameters, for example, (1) the ease to transfer the hydride ligand to the surface, (2) the reductive elimination of hydride and another ligand attached to the metal, e.g., OS−M−H, which generates surface O−H and metal particles, (3) the reducibility of metal oxides, which will lead to the formation of oxygen vacancy, bronze, color center, etc. All these factors modify the stability of the oxide support and the surface hydrides. Thus, the identity and properties of the support will determine the stability of the surface hydrides. With surface hydrides prepared via SOMC, alumina or silica− alumina supported hydrides are usually more stable than those 8492


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support or contributions of H atoms to the band structure. The presence of defect sites or vacancies could also contribute to the stability and mechanism of hydride formation. While surface metal hydrides have been discussed for already more than 4 decades, a better understanding of their structures, stabilities, and reactivities is still required in view of their importance in many chemical processes. This should probably involve more quantitative thermodynamic data and a better understanding of the electronic structure of oxides. For instance, although there is a huge amount of thermodynamic data for welldefined molecular complexes,451−453 there is hardly any data for the corresponding surface species. Work in the area of chemisorption and microcalorimetry is needed to bridge the gap between molecular and surface entities. Metal hydrides are also involved as transient high-energy intermediates, like in hightemperature dehydrogenation processes. It is thus very difficult to have direct evidence for such species. In these cases, PHIP can help to evidence the formation of such (transient) hydrides.399 Similarly, examining the band structure of metal oxides with advanced spectroscopic techniques will be an important step toward the understanding of these highly reactive entities. Improving our knowledge of the surface structure of such species on a broader range of materials is clearly necessary, from both an experimental and a theoretical point of view.

laboratory of Prof. Serge H. P. Schreiner before beginning graduate work at Indiana University. He earned his Ph.D. in 2015 under the joint supervision of Profs. Kenneth G. Caulton and Daniel J. Mindiola. He then moved to ETH Zürich, where he is currently a postdoctoral fellow in the laboratory of Prof. Copéret.

ACKNOWLEDGMENTS We are grateful to the Marie-Curie Fellowship program, Swiss Competence Center for Energy ResearchHeat & Energy Storage, SNF, and ETHZ for financial support. REFERENCES (1) Marks, T. J. Surface-bound metal hydrocarbyls. Organometallic connections between heterogeneous and homogeneous catalysis. Acc. Chem. Res. 1992, 25, 57−65. (2) Copéret, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, J.M. Homogeneous and Heterogeneous Catalysis: Bridging the Gap through Surface Organometallic Chemistry. Angew. Chem., Int. Ed. 2003, 42, 156−181. (3) Lamb, H. H.; Gates, B. C.; Knözinger, H. Molecular Organometallic Chemistry on Surfaces: Reactivity of Metal Carbonyls on Metal Oxides. Angew. Chem., Int. Ed. Engl. 1988, 27, 1127−1144. (4) Serna, P.; Gates, B. C. Molecular Metal Catalysts on Supports: Organometallic Chemistry Meets Surface Science. Acc. Chem. Res. 2014, 47, 2612−2620. (5) Copéret, C.; Comas-Vives, A.; Conley, M. P.; Estes, D. P.; Fedorov, A.; Mougel, V.; Nagae, H.; Núñez-Zarur, F.; Zhizhko, P. A. Surface Organometallic and Coordination Chemistry toward SingleSite Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities. Chem. Rev. 2016, 116, 323. (6) Paál, Z.; Menon, P. G. Hydrogen Effects in Catalysis: Fundamentals and Practical Applications; Marcel Dekker, Inc.: New York, 1988. (7) Rascon, F.; Wischert, R.; Coperet, C. Molecular nature of support effects in single-site heterogeneous catalysts: silica vs. alumina. Chem. Sci. 2011, 2, 1449−1456. (8) Anwander, R. SOMC@PMS. Surface Organometallic Chemistry at Periodic Mesoporous Silica†. Chem. Mater. 2001, 13, 4419−4438. (9) Zakharov, V. A.; Dudchenko, V. K.; Paukshtis, E. A.; Karakchiev, L. G.; Yermakov, Y. I. Formation of zirconium hydrides in supported organozirconium catalysts and their role in ethylene polymerization. J. Mol. Catal. 1977, 2, 421−435. (10) Dudchenko, V. K.; Zakharov, V. A.; Echevskaya, L. G.; Bukatov, G. D.; Ermakov, Y. I. Catalysts Obtained by Reaction of TransitionElement Organometallic Compounds with Oxide Supports. The Effect of Hydrogen Treatment on the Acitivity of Ethylene Polymerization Catalysts Obtained by Reaction of Allyl Zirconium Complexes with Silica Gel. Kinet. Catal. 1978, 19, 6. (11) Zakharov, V. A.; Yermakov, Y. I. Supported Organometallic Catalysts for Olefin Polymerization. Catal. Rev.: Sci. Eng. 1979, 19, 67− 103. (12) Zakharov, V. A.; Ryndin, A. Y. Surface hydride complexes of group IV transition metals as active sites for polymerization and hydrogenation reactions. J. Mol. Catal. 1989, 56, 183−193. (13) Profilet, R. D.; Rothwell, A. P.; Rothwell, I. P. Surface-supported group 5 metal organometallic compounds for catalytic arene hydrogenation. J. Chem. Soc., Chem. Commun. 1993, 42−44. (14) King, S. A.; Schwartz, J. Chemistry of (silica)zirconium dihydride. Inorg. Chem. 1991, 30, 3771−3774. (15) Schwartz, J.; Ward, M. D. Silica-supported zirconium hydrides as isomerization or hydrogenation catalysts for long-chain olefins. J. Mol. Catal. 1980, 8, 465−469. (16) Quignard, F.; Choplin, A.; Basset, J.-M. Alkane activation by a highly electrophilic zirconium hydride complex supported on silica. J. Chem. Soc., Chem. Commun. 1991, 1589−1590. (17) Quignard, F.; Lecuyer, C.; Choplin, A.; Basset, J.-M. From synthesis to chemical reactivity of supported d0 complexes. Part 1. An in

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Christophe Copéret was trained in chemistry and chemical engineering at CPE Lyon and then undertook a Ph.D. in chemistry at Purdue University under the supervision of Prof. Negishi (1996). After a postdoctoral stay in the group of Prof. Sharpless, he joined C2P2 (at the time LCOMS) as a CNRS researcher in 1998 and was promoted CNRS research director in 2008. Since 2010, he has been professor at the Department of Chemistry and Applied Biosciences at ETH Zürich. His research interest lies at the interface of molecular, material, and surface chemistry with applications in catalysis, energy, imaging, and microelectronics, and his work relies on the combination of advanced spectroscopic methods like surface-enhanced NMR spectroscopy and computational chemistry. Deven P. Estes received a B.S. in chemistry from the University of Oklahoma in 2009. In 2014 he earned his Ph.D. from Columbia University, where he was a Department of Energy Predoctoral Fellow in the laboratory of Prof. Jack Norton. In 2014, he moved to ETH Zürich, and he is currently a Marie Curie Postdoctoral Fellow in the Laboratory of Prof. Copéret. Kim Larmier earned his B.S. and M.S. degrees in chemistry at École Normale Supérieure in Paris, France, in 2010. He received his Ph.D. in materials chemistry and physics from Paris VI University (UPMC) in 2015, where he studied the reactivity of alcohols on the surface of oxides by combined computational and experimental approaches under the joint supervision of Prof. Hélène Pernot and Dr. Eric Marceau. He is currently a postdoctoral fellow in the laboratory of Prof. Copéret at ETH Zürich. Keith Searles received a B.S. in chemistry from Randolph-Macon College in Ashland, VA, in 2009 under the supervision of Prof. Rebecca R. H. Michelson. He then spent 1 year as a research assistant in the 8493


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