The single-site silica-supported group IV metal amido complex [Ti(NMe2)(4)] gives the tris(amido)-supported fragment [(equivalent to Si-O-)Ti(-NMe2)(3)], which transforms into a three-membered metallacycle (called a metallaaziridine) by an alpha-H transfer between two amido ligands. When the three-membered metallacycle reacts with 1-octene, it gives a five-membered metallacycle by insertion of the double bond into the M-C bond of the metallaziridine. These two metallacycles, key intermediates in the catalytic cycle of the hydroaminoalkylation of terminal olefins, were isolated and fully characterized following the surface organometallic chemistry (SOMC) concept and procedures. This paper shows that surface organometallic chemistry can be used to identify and fully characterize three- and five-membered metallacycles of Ti in the hydroaminoalkylation of olefins.

With this protocol, a well-defined singlesite silica-supported heterogeneous catalyst [(≡Si-O-)Hf(=NMe)(η1-NMe2)] is designed and prepared according to the methodology developed by surface organometallic chemistry (SOMC). In this framework, catalytic cycles can be determined by isolating crucial intermediates. All air-sensitive materials are handled under inert atmosphere (using gloveboxes or a Schlenk line) or high vacuum lines (HVLs, <10-5 mbar). The preparation of SiO2-700 (silica dehydroxylated at 700 °C) and subsequent applications (the grafting of complexes and catalytic runs) requires the use of HVLs and double-Schlenk techniques. Several well-known characterization methods are used, such as Fourier-transform infrared spectroscopy (FTIR), elemental microanalysis, solid-state nuclear magnetic resonance spectroscopy (SSNMR), and state-of-the-art dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS). FTIR and elemental microanalysis permit scientists to establish the grafting and its stoichiometry. 1H and 13C SSNMR allows the structural determination of the hydrocarbon ligands coordination sphere. DNP SENS is an emerging powerful technique in solid characterization for the detection of poorly sensitive nuclei (15N, in our case). SiO2-700 is treated with about one equivalent of the metal precursor compared to the amount of surface silanol (0.30 mmol·g-1) in pentane at room temperature. Then, volatiles are removed, and the powder samples are dried under dynamic high vacuum to afford the desired materials [(≡Si-O-)Hf(η2π-MeNCH2)(η1-NMe2)(η1-HNMe2)]. After a thermal treatment under high vacuum, the grafted complex is converted into metal imido silica complex [(≡Si-O-)Hf(=NMe)(η1-NMe2)]. [(≡Si-O-)Hf(=NMe)(η1-NMe2)] effectively promotes the metathesis of imines, using the combination of two imine substrates, N-(4-phenylbenzylidene)benzylamine, or N-(4-fluorobenzylidene)-4-fluoroaniline, with N-benzylidenetert-butylamine as substrates. A significantly lower conversion is observed with the blank runs; thus, the presence of the imido group in [(≡Si-O-)Hf(=NMe)(η1-NMe2)] is correlated to the catalytic performance.

Selective hydroamination of terminal alkynes with primary aryl amines is catalyzed by an unprecedented well-defined silica-supported tantalum complex [( Si-O-)Ta(eta(1)sigma-NEtMe)(2)(=(NBu)-Bu-t)]. A molecular-level characterization of the surface organometallic Ta species was done with the help of characterization techniques including in situ infrared, H-1 and C-13 solid-state NMR (including double and triple quanta sequencing), and X-ray absorption spectroscopies. These were complemented by the state-of-the-art DNP-SENS N-15 characterization. Several catalytic intermediates have been isolated, in particular the 4-membered metallacycle ring intermediate resulting from the anti-Markovnikov addition of the alkyne to the surface tantalum imido. A mechanism is proposed that is on the isolation of all intermediates. A density functional theory (DFT) calculation has confirmed all the elementary steps and intermediates that were fully characterized.

VO(OiPr)3 was grafted on highly dehydroxylated silica by a surface organometallic chemistry approach and its thermal evolution was analyzed with support of DFT calculations.

The broad challenges of energy and environment have become a main focus of research efforts to develop more active and selective catalytic systems for key chemical transformations. Surface organometallic chemistry (SOMC) is an established concept, associated with specific tools, for the design, preparation and characterization of well-defined single-site catalysts. The objective is to enter a catalytic cycle through a presumed catalytic intermediate prepared from organometallic or coordination compounds to generate well defined surface organometallic fragments (SOMFs) or surface coordination fragments (SCFs). These notions are the basis of the "catalysis by design" strategy ("structure-activity" relationship) in which a better understanding of the mechanistic aspects of the catalytic process led to the improvement of catalyst performances. In this review the application of SOMC strategy for the design and preparation of catalysts for industrially relevant processes that are crucial to the energy and environment is discussed. In particular, the focus will be on the conversion of energy-related feedstocks, such as methane and higher alkanes that are primary products of the oil and gas industry, and of their product of combustion, CO2, whose efficient capture and conversion is currently indicated as a top priority for the environment. Among the main topics related to energy and environment, catalytic oxidation is also considered as a key subject of this review.

The well-defined single-site silica-supported hafniaaziridine complex [(≡Si–O−)Hf(η2,π-MeNCH2)(η1-NMe2)(η1-HNMe2)] was prepared using surface organometallic chemistry. Upon thermal treatment under high vacuum, the grafted species was converted into the unprecedented hafnium imido bis-amido complex [(≡Si–O−)Hf(═NMe)(η1-NMe2)]. The surface complexes were characterized by elemental analysis and the following spectroscopic techniques: infrared, solid-state single and multiple quantum NMR, advanced DNP-SENS, and extended X-ray absorption fine structure. [(≡Si–O−)Hf(═NMe)(η1-NMe2)] catalyzed imine metathesis under mild conditions, and characterization of the reactivity showed that the imido exchange with N-(4-phenylbenzylidene)benzylamine yielded [(≡Si–O−)Hf (═NCH2Ar)(η1-NMe2)], demonstrating a kind of 2 + 2 mechanism involving the imine and the imido groups; the proposed reaction mechanism is also supported by DFT calculations.

‘Predictive catalysis’ or ‘catalysis by design’ has recently advanced heterogeneous catalysis by using the conceptual tool of surface fragments [1] such as ‘surface organometallic fragments’（SOMF）or ‘surface coordination fragments’（SCF） to achieve and understand a presumed catalytic cycle （see Fig. 1）.One or several fragments of the molecule

KAUST Catalysis Center, King Abdullah 23955-6900 Thuwal, Saudi-Arabia. E-mai cavallo@kaust.edu.sa Departments of Chemistry and Catalysis Munich, Lichtenbergstrasse 4, Ernst-Otto-Fis Department of Materials Science and Engin Engineering, Vidyasirimedhi Institute of Sc Payupnai, WangChan, Rayong, Thailand. E † Electronic supplementary information and details, additional IR spectra; comp diagram. See DOI: 10.1039/c8ra02419g ‡ These authors contributed equally. Cite this: RSC Adv., 2018, 8, 20801

A reusable zirconium-based catalyst for the cycloaddition of CO2 to propylene oxide (PO) was prepared by the surface organometallic chemistry (SOMC) methodology. Accordingly, well-defined amounts of the ZrCl4·(OEt2)2 precursor were grafted on the surface of silica dehydroxylated at 700°C (SiO2-700) and at 200°C (SiO2-200) in order to afford surface coordination compounds with different podality and chemical environment. The identity of the surface complexes was thoroughly investigated by FT-IR, elemental microanalysis and solid state NMR and applied as a recoverable and reusable heterogeneous catalyst for the title reaction using pure CO2 and flue gas samples from a cement factory. The observed catalytic activity for the isolated zirconium complexes is rationalized by means of systematic DFT calculations.

Well-defined silica-supported zirconium–imido complexes effectively catalyze imine/imine cross-metathesis and are thus considered as the first heterogeneous catalysts active for imine metathesis.

The ability of early transition metal halides to capture and convert CO2from diluted gas flows has been exploited in the cycloaddition of CO2to epoxides using industrial flue gas as an impure source of CO2.

Single-site, well-defined, silica-supported tantallaaziridine intermediates [≡Si-O-Ta(η(2) -NRCH2 )(NMe2 )2 ] [R=Me (2), Ph (3)] were prepared from silica-supported tetrakis(dimethylamido)tantalum [≡Si-O-Ta(NMe2 )4 ] (1) and fully characterized by FTIR spectroscopy, elemental analysis, and (1) H,(13) C HETCOR and DQ TQ solid-state (SS) NMR spectroscopy. The formation mechanism, by β-H abstraction, was investigated by SS NMR spectroscopy and supported by DFT calculations. The C-H activation of the dimethylamide ligand is favored for R=Ph. The results from catalytic testing in the hydroaminoalkylation of alkenes were consistent with the N-alkyl aryl amine substrates being more efficient than N-dialkyl amines.

Heterogeneous catalysis, a field important industrially and scientifically, is increasingly seeking and refining strategies to render itself more predictable. The main issue is due to the nature and the population of catalytically active sites. Their number is generally low to very low, their "acid strengths" or " redox properties" are not homogeneous, and the material may display related yet inactive sites on the same material. In many heterogeneous catalysts, the discovery of a structure-activity reationship is at best challenging. One possible solution is to generate single-site catalysts in which most, if not all, of the sites are structurally identical. Within this context and using the right tools, the catalyst structure can be designed and well-defined, to reach a molecular understanding. It is then feasible to understand the structure-activity relationship and to develop predictable heterogeneous catalysis. Single-site well-defined heterogeneous catalysts can be prepared using concepts and tools of surface organometallic chemistry (SOMC). This approach operates by reacting organometallic compounds with surfaces of highly divided oxides (or of metal nanoparticles). This strategy has a solid track record to reveal structure-activity relationship to the extent that it is becoming now quite predictable. Almost all elements of the periodical table have been grafted on surfaces of oxides (from simple oxides such as silica or alumina to more sophisticated materials regarding composition or porosity). Considering catalytic hydrocarbon transformations, heterogeneous catalysis outcome may now be predicted based on existing mechanistic proposals and the rules of molecular chemistry (organometallic, organic) associated with some concepts of surface sciences. A thorough characterization of the grafted metal centers must be carried out using tools spanning from molecular organometallic or surface chemistry. By selection of the metal, its ligand set, and the support taken as a X, L ligands in the Green formalism, the catalyst can be designed and generated by grafting the organometallic precursor containing the functional group(s) suitable to target a given transformation (surface organometallic fragments (SOMF)). The choice of these SOMF is based on the elementary steps known in molecular chemistry applied to the desired reaction. The coordination sphere necessary for any catalytic reaction involving paraffins, olefins, and alkynes also can thus be predicted. Only their most complete understanding can allow development of catalytic reactions with the highest possible selectivity, activity, and lifetime. This Account will examine the results of SOMC for hydrocarbon transformations on oxide surfaces bearing metals of group 4-6. The silica-supported catalysts are exhibiting remarkable performances for Ziegler-Natta polymerization and depolymerization, low temperature hydrogenolysis of alkanes and waxes, metathesis of alkanes and cycloalkanes, olefins metathesis, and related reactions. In the case of reactions involving molecules that do not contain carbon (water-gas shift, NH3 synthesis, etc.) this single site approach is also valid but will be considered in a later review.

An efficient and potentially scalable method is described for the synthesis of the silica-supported complexes [(equivalent to Si-O)WMe5] and [(equivalent to:Si-O)WMe2(equivalent to CH)] obtained by in situ alkylation of the surface-grafted tungsten chloride [(equivalent to Si-O)WCl5] (1). [(equivalent to Si-O)WCl5] can be readily prepared by the reaction of commercially available and stable tungsten hexachloride WCI6 with partially dehydroxylated silica at 700 degrees C (SiO2-700). Further reaction with ZnMe2 at room temperature rapidly forms a mixture of surfacealkylated tungsten complexes. They were fully characterized by microanalysis, FTIR, mass balance, and solid-state NMR (H-1, C-13, H-1-C-13 HETCOR, H-1-H-1 double quantum and triple quantum) and identified as [(equivalent to Si-O)WMes] and another product, [(equivalent to Si-O)WMe2(equivalent to CH)]. The latter might have been generated by partial decomposition of the tungsten methyl chloride compound, which is formed during the stepwise alkylation of [(equivalent to Si-O)WCl3]. DFT calculations were carried out to check the relative stability of the tungsten methyl chloride intermediates and the feasibility of the reaction and corroborate the experimental results. This tungsten complex and its derivative were found to be active catalysts for the metathesis of cydooctane.

Intermolecular catalytic hydroaminoalkylation of unactivated alkene occurs with silica-supported azazirconacyclopropane [ Si-O-Zr(HNMe2)(eta(2)-NMeCH2)(NMe2)]. Mechanistic studies were conducted using surface organometallic chemistry (SOMC) concepts to identify the key surface intermediates. The azametallacyclopentene intermediate { Si-O-Zr(HNMe2)[eta(2)-NMeCH2CH(Me)CH2](NMe2)} was isolated after treating with 1-propylene and characterized by FTIR spectroscopy, elemental analysis, H-1 C-13 HETCOR, DARR SS-NMR and DQ TQ SS-NMR. The regeneration of the catalyst was conducted by dimethylamine protonolysis to yield the pure amine.

The silica-supported azazirconacyclopropane ≡SiOZr(HNMe2)(η(2)-NMeCH2)(NMe2) (1) leads exclusively under hydrogenolysis conditions (H2, 150 °C) to the single-site monopodal monohydride silica-supported zirconium species ≡SiOZr(HNMe2)(NMe2)2H (2). Reactivity studies by contacting compound 2 with ethylene, hydrogen/ethylene, propene, or hydrogen/propene, at a temperature of 200 °C revealed alkene hydrogenation.

Recent discoveries highlighted the activity and the intriguing mechanistic features of NbCl5 as a molecular catalyst for the cycloaddition of CO2 and epoxides under ambient conditions. This has inspired the preparation of novel silica-supported Nb species by reacting a molecular niobium precursor, [NbCl5·OEt2], with silica dehydroxylated at 700 °C (SiO(2-700)) or at 200 °C (SiO(2-200)) to generate diverse surface complexes. The product of the reaction between SiO(2-700) and [NbCl5·OEt2] was identified as a monopodal supported surface species, [≡SiONbCl4·OEt2] (1a). The reactions of SiO(2-200) with the niobium precursor, according to two different protocols, generated surface complexes 2a and 3a, presenting significant, but different, populations of the monopodal surface complex along with bipodal [(≡SiO)2NbCl3·OEt2]. (93)Nb solid-state NMR spectra of 1a-3a and (31)P solid-state NMR on their PMe3 derivatives 1b-3b led to the unambiguous assignment of 1a as a single-site monopodal Nb species, while 2a and 3a were found to present two distinct surface-supported components, with 2a being mostly monopodal [≡SiONbCl4·OEt2] and 3a being mostly bipodal [(≡SiO)2NbCl3·OEt2]. A double-quantum/single-quantum (31)P NMR correlation experiment carried out on 2b supported the existence of vicinal Nb centers on the silica surface for this species. 1a-3a were active heterogeneous catalysts for the synthesis of propylene carbonate from CO2 and propylene oxide under mild catalytic conditions; the performance of 2a was found to significantly surpass that of 1a and 3a. With the support of a systematic DFT study carried out on model silica surfaces, the observed differences in catalytic efficiency were correlated with an unprecedented cooperative effect between two neighboring Nb centers on the surface of 2a. This is in an excellent agreement with our previous discoveries regarding the mechanism of NbCl5-catalyzed cycloaddition in the homogeneous phase.