New. J. Chem., 1994, 18, 115-122

Surface organometallic chemistry: a molecular approach to surface

Susannah L. SCOTT, Jean-Marie BASSET*, Gerald P. NICCOLAI, Catherine C. SANTINI, Jean-Pierre CANDY, Christine LECUYER, Frangoise QUIGNARD and Agnes CHOPLIN Laboratoire de chimie organometallique de surface, EP CNRS 48, Ecole Superieure de Chimie Industrielle de Lyon, 43, bd du 11-Novembre-1918, 69616 Villeurbanne, France.

Received June 21, 1993, accepted July 20, 1993.

Abstract. - The study of the reactions of organometallic complexes with the surfaces of inorganic oxides, zeolites and metals constitutes a new aspect of the coordination chemistry of organometallic compounds. We will demonstrate that the basic rules of organometallic chemistry are often valid when applied to surfaces, using as an example the chemistry of bis(allyl)rhodium grafted onto silica. In addition, concepts can be developed which are new both in molecular chemistry and in surface science (unusual oxidation state, coordination number and electron count). These new concepts allow the design of a new generation of catalysts with unexpected activity and/or selectivity. Two examples will be presented: the low temperature activation of the C- Hand C-C bonds of by a hydride supported on silica, and the regioselective of citral to geraniol with a metallic surface of Rh covered with alkyltin fragments.

Introduction surlace 1. This approach is called Surface Organometallic Chemistry (SOMe). Site isolation and limited mobility of catalytically active molecules prohibit bimolecular Modem organometallic chemistry has had an enOlmous decomposition reactions, and catalyst recovery becomes impact on homogeneous catalysis in the last two a simple matter of phase separation. An additional and decades. Soluble metal complexes bind organic substrate considerable advantage is that unsaturated intermediates molecules, which then undergo a large variety of are readily fonned and stabilized by the surlace, which transfonnations in the coordination sphere of the metal. acts as a large and rigid ligand to trap them. Although The strategy for developing new homogeneous catalysts the supported organometallic complexes superficially is based on knowledge of the series of elementary steps resemble heterogeneous catalysts, their relatively unifOlm which constitute the catalytic mechanism. This govems structure, reactivity and distribution on the support the choice of the central metal atom, oxidation state, material make them essentially homogeneous in nature. ancillary ligands and reaction conditions. Mechanistic The similarities between molecular species and supported studies carried out with well-defined organometallic organometallic complexes are illustrated by the example complexes have identified key intennediates in many of allylrhodium(III) complexes, discussed below. catalytic processes. However, the efficiency of homoge­ The potential benefit to catalysis is now beginning to neous catalysts is often limited by instability caused by be realized. Catalyst active sites can be custom-designed irreversible ligand dissociation and side reactions such with the ligand environment, oxidation state and other as bimolecular reactions of unsaturated (highly reactive) chemical properties desired for a given reaction. Any intermediates. Separation and recovery of the catalyst, organometallic material can be used (transition metal, which sometimes contains an expensive metal such as Rh main group, rare ) with a large variety of supports or Ir, is usually not efficient (note that biphasic systems (inorganic oxides, zeolites, metal surfaces). The activity are an important exception). of these SOM catalysts is often very high, since the A recent development in homogeneous catalysis involves concentration of active sites is controlled by the level of immobilizing the organometallic complex directly on a organometallic loading. The SOM catalysts may possess

NEW JOURNAL OF CHEMISTRY, VOL. 18, N° 1-1994. - 1144-0546/94/01/$ 4.00/© CNRS-Gauthier-Villars S. L. Scott et al.

enhanced selectivity usually associated with traditional and is consistent with molecular modelling 4 and homogeneous catalysts. theoretical calculations 6 on the surface complex. Two such systems have been thoroughly characterized in our laboratory 2. Tetra(neopentyl)zirconium, when 3 supported on silica, has been converted into a Reaction of (==SiO)(=SiOH)Rh(rJ -C3Hsh hydride which catalyses the low-temperature C-C with CO bond hydrogenolysis of alkanes, activates C-H bonds The reaction of the molecular complex [Rh(7]3­ (even of methane) and performs the stoichiometric C H hClh with CO induces the 7]3 ---. 7]1 shift of functionalisation of alkanes to aldehydes, alcohols and 3 s longer alkanes. The reaction of tetra(n-butyl)tin with the allyl ligands followed by reductive elimination of 1,5-hexadiene, eq. 2 7,8. highly dispersed Rh metal supported on silica gives a new generation hydrogenation catalyst which selectively activates C = 0 bonds in the presence of C = C bonds. It has been demonstrated that the high chemoselectivity is directly related to the presence of organotin fragments -- [Rh(COhClh+ ~ (2) grafted onto the surface of the metallic particles. The surface interaction between (== SiO)(== SiOH) Rh(7]3 -C3Hsh and CO on silica(400) is exactly analo­ Rules of surface organometallic chemistry gous to the homogeneous reaction. Quantitative formation Grafting of Rh(rl-C3Hsh onto silica of 1,5-hexadiene was observed when (== SiO)(== SiOH) Rh(7]3_C3Hsh was exposed to 0.1 torr CO 7. Removal of surface water from silica (Aerosil, 300 m2/g) by treatment in vacuum (10-4 tOlT) at 200°C creates a surface covered with exposed hydroxyl groups. Rh(7]3 -C3Hsh reacts via the gas phase with these ~ hydroxyl groups, which results in the formation of a -- [(=Si-O)Rh(COhh + (3) grafted organometallic complex and propene, eq. 1 3. The same reaction was observed for bis(allyl)rhodium(III) == Si - OH + Rh(7]3_C3Hsh grafted onto alumina and titania. However, the reaction was found to depend strongly on ---. == Si - 0 - Rh(7]3_C H C H 3 sh + 3 6 (I) the CO pressure and the proton content (extent of dehy­ droxylation) of the silica surface. At higher CO pressure This reaction has been studied in detail by IR (2":10 ton), insertion of CO into a Rh-C bond precedes spectroscopy 4. Sublimation of Rh(7]3 -C H )3 onto silica, 3 s reductive elimination of 1,6-heptadien-4-one, eq. 4. which originally contains no v(CH) bands, results 1 in bands due to physisorbed (3063, 3004 cm- ) and 1 chemisorbed (3049, 2990 cm- ) species. Evacuation of the sample at room temperature removes physisorbed If'Rh"'3.l...... 1j [r<~]If ~ Rh - co oc, Rhco } r:c ~ Rh(7]3-C3Hs)3 from the surface. During the grafting 0' 'OH 0/ 'OH CO rf 'OH () (4) I I II -- I I + reaction, the intensity of the v(OH) band at 3750 cm- I, .Si Si .Si Si. Si Si attributed to free silanol groups, decreases. At the same "J 'r5 \" "J '0 \' "J V \" ~ time, a new type of hydroxyl group, characterized by When the proton content of the surface is higher, a sharp band at 3636 cm- I, appears. This band was attributed to a silanol which is strongly perturbed by the as in silica(200), the major gaseous product is grafted organometallic fragment. The following structure, proprene instead of 1,5-hexadiene. Insertion of CO which provides a stable 18 electron configuration to the into the remaining Rh - C bond gives an acyl complex 1 grafted species, has been proposed: (v(CO) = 1 692 cm- ). Reductive elimination of the acyl ligand with a siloxy ligand leads to the formation ~FU1?\\ of a silyl ester, strongly bound to the surface. The monomeric dicarbonylrhodium(l) product ultimately d' 'OH I I dimerizes on silica, as shown by the evolution of the IR .Si Si ,,'/ 'if \', spectIllm in the v(CO) region. The two bands at 2012 and 2087 cm-1 which characterize the monomeric species This structure is a monomeric form of the well­ are replaced (over 30 min at room temperature) by three characterized molecular analogue, [Rh(7]3_C3HshOHh s, bands at 2109, 2092 and 2035 cm-1, characteristic of

116 NEW JOURNAL OF CHEMlSTRY. VOL. 18 NO 1-1994 A molecular approach to sUlface catalysis

A surface organometallic catalyst for the low temperature C-H and (5) C-C bond activation of alkanes

Grafting of Zr(CH2C(CH3h)4 onto silica Sublimation of Zr(CHzC(CH3)3)4 onto silica partially dehydroxylated at 500°C results in the electrophilic cleavage of a Zr-C bond by surface protons, with formation of a grafted species described as == 8i-0-Zr(CHzC(CH3 hh, eq.9. a nonplanar dicarbonylrhodium dimer. The dimer is strongly bound to the surface, and so is formulated as == 8i - OH + Zr(CHzC(CH3 h)4 [(== 8iO)Rh(CO)2lz. ---> == 8i-0-Zr(CH2C(CH3 hh + C(CH3 )4 (9) Reaction of (== SiO)(=SiOH)Rh(rl-C3Hsh with PMe3 Thus the intensity of the v(OH) vibration at 3750 cm-1 attributed to surface silanols disappears during the Coordination of PMe3 to the bis(allyl)rhodium(III) reaction, and v(C-H), 8(CHz) and 8(CH3) bands appear complex on silica(400) results in formation of 1,5­ which are stable to prolonged exposure to dynamic hexadiene, as in the reaction with CO. On silica(200), the vacuum. The surface species has also been characterized exclusive gas phase product is propene. The second allyl by 13C CP-MAS NMR (33.9 ppm, CH2C(~H3h) and ligand remains bound to Rh, as shown by the appearance elemental analysis, which shows that, on average, 3 alkyl I of a band at 1612 cm- in the JR, characteristic of the groups remain bound to the surface after grafting 10. v(C =C) stretch of a (T-allyl group 9. This ligand can be removed from the surface as allyl alcohol by reaction with H20. Formation of a grafted Zr hydride

The reaction of == 8i-0-Zr(CH2C(CH3 hh with dry Hz (450 torr, 150°C) leads to hydrogenolysis of the Zr-C bonds with formation of mixture of Zr hydrides, eq. 10 II.

(== 8iO)ZrR3 + Hz ---> (== SiO)ZrRz(H) + RH

---> ---> (== 8iO)n Zr(H)x + 3RH (10)

The grafted Zr hydride complexes are highly oxophilic, reacting with siloxane bridges to give silicon hydrides, eq. 11. The v(Si-H) vibrations appear at 2253 and 1 Reaction of the grafted allyltriphosphinorhodium species 2 191 cm- , and do not shift in the presence of Dz. The I with H2 causes the disappearance of the band at v(Zr-H) bands are observed in the region of 1635 cm- . l 1612 cm- and the appearance of a new band at They disappear in the presence of Dz (90 torr, 25°C), 1938 cm- I, attributed to v(Rh - H). The latter is however, the v(M - D) bands are obscured by the strong 1 displaced to I 390 cm- in the presence of D2, consistent absorption of silica below 1200 cm-I IZ with exchange to form Rh-D. The exchange is completely reversible when Dz is replaced by Hz. These reactions are summalized in eq. 8. (== 8iO)nZr(H)x + 2 == 8i - °-8i == ---> (== 8iOhZrH + 2 == 8i - H (11)

(8) The reaction of the grafted metal hydride with CH3J (400 torr, 25°C) liberates C~.

NEW JOURNAL OF CHEMISTRY, VOL. 18 NO 1-1994 ll7 S. L. Scoll et at.

C-H bond activation by a supported Zr hydride == Si-0-Zr(CH2C(CH3hh + 3/202 The grafted hydride (== SiOhZrH is highly electrophilic, having formally only 8 electrons (neglecting the p1r­ ----+ == Si-0-Zr(OCH2C(CH3hh (13) d1r interaction between surface oxygens and Zr), as well as being sterically unhindered. These properties are undoubtedly responsible for the ability of the grafted == Si-0-Zr(OCH2C(CH3hh + 3HCl hydride to react with unactivated C - H bonds in CH4 (150°C) and cyclooctane (25°C). The reaction has been ----+ == Si-0-ZrCI3 + 3HOCH2C(CH3hh (14) proposed to proceed by a 4-centre O"-bond metathesis mechanism, eq. 12 13 The mechanism of oxygen insertion into the Zr-C bond remains unclear.

To aldehydes (12) The grafted metal alkyls inselt CO into the M-C bond to form acyl species. At room temperature, the first step is the coordination of CO to Zr, accompanied by a color change from white to yellow and the appearance of two new bands at 2027 and 1938 cm-1 in the IR This mechanism has been demonstrated in homogeneous spectrum 16. No CO insertion occurs at this temperature. chemistry for activation by dO transition metal At 100°C, insertion of CO into the alkylzirconium bond complexes of Sc and Y 14. 15. results in the appearance of a u(C = 0) band in the IR The selectivity of stoichiometric activation of propane by spectrum at 1652 cm-I (cyclooctyl) and 1643 cm-I (n­ the Zr hydride species has been studied 16. When the Zr propyl). Treatment with HCl gives the product aldehyde, hydride was converted to propylzirconium under propane, eg. 15-17. then oxidized with dry O2 , the resulting propoxide ligands were extracted with HCl/ether. The ratio (== SiOhZrCH2CH2CH3 + CO n-propanol/2-propanol was determined quantitatively ----+ (== SiOhZr(CH2CH2CH3)(CO)2 (15) by GC to be > 37. Therefore, activation occurs predominantly at the primary carbon. In a separate experiment, the propylzirconium species was hydrolysed (== SiOhZr(CH2CH2CH3)(COh with D20 and the resulting propane-dl was analysed by mass spectroscopy. The isotopic pattern for the ----+ (== SiOhZr(C(0)CH2CH2CH3) + CO (16) primary fragment peaks indicated that the ratio CH3CH2CH2D/CH3CHDCH3 was approx. 49. Catalytic exchange of propane C-H bonds with D2 (== SiOhZr(C(0)CH2CH2CH3) + HCl revealed that propane activation is extremely rapid at room temperature (70 turnovers/min) 16. The kinetics ----+ (== SiOhZrCI + CH3CH2CH2CHO (17) of the exchange reaction were biphasic, corresponding to primary and secondary C - H exchange with a rate constant ratio k 1o/k2o =6.4. Taking into account the number of primary and secondary C - H bonds, this To longer-chain alkanes corresponds to a total kinetic selectivity for primary C-H The grafted metal alkyls will inselt ethylene into activation of 19. At room temperature, hydrogenolysis of the M -C bond, effectively extending the alkyl chain, propane is not significant (see below). 13 eg. 18 .

== Si-0-Zr(CH2C(CH3hh + n CH2CH2 Stoichiometric alkane functionalisation ----+== Si-0-Zr(CH2)n CH2C(CH3hh (18) To alcohols The grafted metal alkyls react with dry O2 to give This is a route to polymers. Indeed, the polymerisation alkoxy ligands, which can be hydrolysed with HCI to activity of organozirconium catalysts is enhanced by the corresponding alcohol, eg. 13-14 10. immobilization on an alumina surface 17, 18.

118 NEW JOURNAL OF CHEMISTRY, VOL. 18 N° 1-1994 A molecular approach to surface catalysis

Catalytic low temperature C-C bond what occurs in the polymerisation of propene catalysed hydrogenolysis of alkanes: a unique by supported zirconium complexes 20,21. example of catalysis by a-bond metathesis and f3-methyl transfer

In the presence of H2 , the grafted Zr hydride cracks into smaller fragments. Thus, 40 ton neopentane was converted into isobutane and methane by the catalyst (= SiOhZrH in the presence of 230 ton H2 at 50°C 19. The mechanism is proposed to involve the formation of a Zr - CH2 C(CH3 )3 species by a-bond metathesis, as in eg. 12, followed by ,B-methyl transfer to give isobutene and a Zr - CH3 species. The latter is converted to methane by H2 , with regeneration of the Zr - H catalyst, Scheme 1.

Scheme II. - Analogy between alkane hydrogenolysis and olefin polymerisation.

In support of this hypothesis, a small amount of 2­ methylbutane was observed during the hydrogenolysis of neopentane 19, arising from the anti-Markovnikov insertion of isobutene into the Zr-CH3 bond, eg. 19. .~me(hyllr;msfer ( )=

Scheme I. - Catalytic hydrogenolysis of neopentane by silica-supported zirconium hydride.

The primary product isobutene was not identified because Recently, ,B-methyl transfer has been identified as a chain the catalyst converts it rapidly to isobutane under H2 . Ultimately, isobutane undergoes further hydrogenolysis to termination step in propene polymerization catalyzed by propane and methane. Likewise, propane becomes ethane homogeneous group IV metal complexes 22,23. In this and methane. Ethane is not cleaved, presumably because respect, the mechanisms of alkane hydrogenolysis and the ethylzirconium intermediate lacks a ,B-methyl group. olefin polymerisation are consistent with each other. Key parameters which control the direction of the reaction are temperature and H2 pressure. Low temperatures Relationship between elementary steps of C-C favor C-C bond fOlmation whereas high H2 pressure bond cleavage in alkane hydrogenolysis and C-C favors C-C bond cleavage. It is well-known that H2 is a molecular-weight regulator in catalysed olefin bond formation in olefin polymerisation polymerisation 24. ,B-alkyl transfer conesponds to the microscopic reverse The reason why ,6-methyl transfer rather than ,B- of olefin insertion into a metal-alkyl bond, presumed to elimination occurs with supported and homogeneous Zr be a crucial step in polymerization of olefins catalysed catalysts is unclear. Steric effects in the ground state by early transition metals, which are often dO systems. have been proposed to be important 22. The importance For example, in the hydrogenolysis of isobutane, propene of electronic effects has also been suggested 25. Thus the and a methylzirconium species are the proposed primary highly electrophilic nature of the Zr center is thought products, Scheme II. The reverse reaction is precisely to favor ,B-methyl transfer. Although direct evidence

NEW JOURNAL OF CHEMISTRY, VOL. 18 N° 1-1994 119 S. L. Scott et at. is lacking, an agostic C-C interaction with the early temperature is raised to 100°C, and the surface complex transition metal center can be envisaged. is stable up to 200°e.

== Si - OH + Sn(n -C4H g )4

----> == Si - 0 - Sn(n -C4 H g h + n - C4 H lO (20)

This reaction can be minimized by avoiding an excess of SnBu4 (relative to exposed Rh atoms) and by removing unreacted SnBu4 with a cold trap. Thus by JR, the band 1 A surface organometallic catalyst attributed to free silanol, v(OH) = 3748 cm- , is fully for the selective hydrogenation restored after heating the sample to 100°e. The 13C CP/MAS NMR spectra shows no signal attribuable to of conjugated C =0 bonds == Si-O-Sn(n-C4 Hg h if Sn/Rhs < 1.5. Reaction of Sn(n-Bu)4 with metallic The organometallic fragment described as (SnBu2) Rh dispersed on silica Rh/Si02 has been characterized by several techniques 31. Electron microscopy revealed an increase in average From an applied perspective, metaJs are probably the particle diameter to 2.2 nm, consistent with the fOimation most important surfaces in SOMe. The controlled of 1-2 monolayers of alkyltin on the Rh surface. However, reaction of organometallic compounds with the surfaces adsorption measurements showed that 1/3 of the surface of metals can lead to bimetallic particles whose Rh atoms are still accessible for binding of CO. XPS and properties are significantly different from those of Mossbauer spectroscopy identified the oxidation state of the monometallic materials 26-28. Our investigations of the organometallic fragment, corresponding to 75% Sn(II transition metal/main group metal combinations have or IV) and 25% Sn(O). By EXAFS, Sn has an average proved particularly fruitful. of 2 light neighbours (C or 0) at 0.217 nm and 2 heavy The possibility of coordinating an organometallic neighbours (Sn or Rh) at 0.268 urn. fragment to the smface of a metal particle is highly Molecular modelling of the grafted surface organo­ relevant to the field of heterogeneous catalysis. Metals, metallic fragment demonstrated that it is impossible whether supported or in the bulk, present at their surfaces to fit enough SnBu2 entities around the Rh palticles a variety of crystallographic planes, edges and corners. to satisfy the ratio Sn/Rhs =1 without unreasonable The relative ratios of these sites depends on the particle steric interference. An alternate model was proposed, size. Even particles which are fairly uniform in size are Scheme III, which is consistent with the spectroscopic heterogeneous in terms of surface composition. Selective data and results in a sterically accessible structure: grafting of an organometallic fragment onto a specific (Bu3Sn)2SnRh/Si02· surface site (e.g. low coordination sites) leads to selective poisoning of that site. The nature of the ligands on the surface organometallic fragment also influences the propelties of the modified metal particle. Small particles of Rh (1.4 nm mean diameter), dispersed on silica (Aerosil, 300 m2/g), were reduced under flowing H2 at 673 K to ensure complete reduction of the surface to the metallic state. Rh surface atoms are covered with dissociated hydrogen under these conditions. The reaction with Sn(n-Bu)4 from the physisorbed state evolves BuH in a stepwise hydrogenolysis reaction, leading eventually to a Sn/Rh 29. Depending on the experimental conditions, the reaction can be intercepted at an Scheme III. - Proposed structure for SnBu4-modified Rh. internlediate stage. At 100°C, a species with the empirical formula (SnBu2)Rh/Si02 is present on the surface. Organometallic modification of Rh has proved promising A possible side reaction, the protolytic cleavage for the molecular control of chemo-, regio- and of a Sn - C bond by surface silanols, has been enantioselectivity in Rh-catalysed reactions. One example studied independently 30. Chemisorption begins when the is discussed below.

120 NEW JOURNAL OF CHEMISTRY. VOL. 18 N° 1-1994 A molecular approach to surface catalysis

Chemoselective hydrogenation of citral Conclusion

Hydrogenation of citral, an 0:, ,B-unsaturated aldehyde, on a conventional supported Rh catalyst leads to a variety In the long term, the systematic study of the of products, because Rh is fairly unselective for the site reactivity of organometallic compounds with surfaces of hydrogenation. may have an impact on surface catalysis rivalling that of organometallic chemistry on homogeneous ~OH catalysis. Compared to the direct study of heterogeneous systems, the SOMC approach offers several advantages, H'_~,,~~o including the preparation of well-defined active sites, the ~O __ possibility of observing elementary reaction steps and the RhiSiO, development of a fundamental basis for the synthesis of cilml ~O tailor-made catalysts. The impact of SOMC on the methodology of In the presence of the grafted butyltin fragment, the heterogeneous catalysis is still limited by the difficulty yield of unsaturated alcohol, geraniol, is 96% at 100% of applying the technology of organometallic chemistry conversion 32. This increase in selectivity is attlibuted (e.g., preparation of the catalyst in a controlled to (1) an electronic effect, in which coordination and atmosphere) to large-scale operations. However, this is activation of the aldehydic function of citral is enhanced merely a matter of approach, and should not be an by the presence of butyltin moieties, and (2) a steric insurmontable obstacle. There are already examples of effect, in which limited access to the metal surface favors catalysts prepared by the SOMC technique which are hydrogenation at the less hindered double bond of citral, being used in industry. For example, the dehydrogenation i.e., the aldehyde function. of 2-butanol to methyl ethyl ketone is now performed in Japan with a Raney nickel catalyst covered with alkyltin fragments. In five years of use, no sign of aging or loss ~O (R'Sn)~:nRhls~, ~OH of selectivity has become apparent.

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