Surface organometallic chemistry: Reductive carbonylation of silica-supported MCl3·3H2O (M=Rh, Ir)

Journal of Molecular Catalysis, 74 (1992) 391-400 M2929

391

Surface organometallic chemistry: reductive carbonylation of silica-supported MC1 3 · 3H 20 (M=Rh, Ir) R. Psaro, D. Roberto, R. Ugo, C. Dossi and A. Fusi Centro C. N. R. and Dipartimento di Chimica lnorganica e Metallorganica, Via Venezian 21, 20133 Milan (Italy)

Abstract The reductive carbonylation of silica-supported MCI3·3H20 (M=Rh, Ir) was studied at atmospheric pressure under CO and under a mixture of CO and H 20. It was observed that these salts may be converted respectively into [Ir(CO)3Clln and [Rh(CO)2CI)z under CO. At 70°C, in the presence of CO and H 20, Ir.(CO)12 is selectively obtained. Under the same experimental conditions [Rh(CO)2CI]2 sublimes, while at 25°C the liberated HCI inhibits further reduction to rhodium carbonyl clusters. However, Rh 6(CO)16 might easily be obtained, working at 25°C with CO and H20, using [Rh(CO)2CI)z as the starting material instead of RhCI 3. The inhibiting effect on the nucleation process of HCI and the role of water are discussed in the comparison with the reductive carbonylation of a chlorine-free rhodium salt, [Rh(02CCH3)2]2'

Introduction

In the last few years many aspects of the surface organometallic chemistry of metal carbonyl complexes supported on inorganic oxides and zeolites have been studied [1]. Some transition metal ions ionically linked to the NaY zeolite may undergo in situ a reductive carbonylation to generate, inside the zeolite cage, mononuclear monovalent carbonyl compounds M1(CO)n (e.g., Rh I(CO)2 [2], Ir(CO)3 [3]). By further treatment with CO and HzO or CO and Hz the encaged M1(CO)n species migrate progressively through the porous network of the zeolite and are reduced until they form the so-called 'ship-in-a-bottle' polynuclear metal carbonyl clusters (e.g., Rh 4(CO)lZ [4], Rh 6(CO)16 [5-8], Ir 4(CO)lz [4], Ir6(CO) 16 [8-10]). With conventional supports such as silica and alumina, a similar surface chemistry is observed. Rh 6(CO)16 is obtained in high yield from the anchored species [RhI(CO)z(O-S)(HO-S)) (S = SiE, AI <) by treatment with CO and H 20 at 25°C and atmospheric pressure [11]. With highly basic oxides, such as MgO, surface-bound high-nuclearity Os and Ru anionic clusters are generated in high yield by reductive carbonylation of surface species obtained by aqueous impregnation with HzOsCl6 and RuCI 3, respectively [12]. The surface aggregation processes to form polynuclear structures from mononuclear surface metal species require a rather unexpectedly high surface mobility of metal fragments toward

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392

nucleation, probably via redox condensations, as occurring in homogeneous phase [13}. The present work deals with the study of the surface organometallic chemistry of reductive carbonylation, under mild conditions, ofsilica-supported MCI3·3H20 (M = Rh, Ir). Experimental Silica (Aerosil 200, Degussa) was used as the support. It was treated under vacuum (10- 2 torr) at 25°C for 3 h prior to use. In a typical experiment, the metal salt (IrCI 3· 3H 20, RhCI3· 3H 20, [Rh(02CCH3)2h), degassed water and the support were stirred overnight under argon. The resulting slurry was dried at 80°C under vacuum (10- 2 torr), affording a powder that was stored under argon. [Rh(CO)2Clh was supported similarly, using pentane instead of water. In all samples the metal loading was about 1% by weight of metal. The physisorbed sample was put in a glass cell equipped with Teflon@ stopcocks, which can work under vacuum or controlled atmosphere, and treated with CO or with a mixture of CO and H 20 at atmospheric pressure. Depending on the nature of the metal the carbonylation was carried out at temperatures between 25-150 DC. The surface reactions were monitored by FT-IR using a Nicolet MX-1 spectrophotometer. Samples were taken from the glass cell under argon and studied as nujol mull. Physisorbed metal carbonyl species were quantitatively extracted from the surface with a suitable organic solvent and identified by FT-IR. Results and discussion

Surface reactivity of frCl3 • 3H2 0 supported on Si02 with CO or with CO+H20 When silica-supported IrCl 3 is heated at 50°C under CO for a few days, an iridium carbonyl species is formed on the surface (Fig. l(A)). Its infrared spectrum in the v(CO) region is characterized by two strong bands at 2124 and 2094 cm- I, which are different from those reported by Tanaka et al. [14} for Ir(CO)2 species on the silica surface (2080 and 2008 cm- I ) . The doublet at higher frequencies may be attributed to a surface 'IrII(CO) 2' species. In fact, this spectrum is quite similar to that of solid-state [IrI 3(CO)2h as nujol mull [15}. However, while [IrI 3(CO)2h is soluble in organic solvents such as hexane and benzene, the surface species could not be extracted either with hexane or with dichloromethane. This would indicate that the surface species is probably interacting with the surface. This interaction would prevent it from being extracted from the surface by non-donor solvents. By further treatment under CO at 70°C a new broad band centered at 2100 cm- I appears (Fig. l(B)). When the temperature is raised up to 100-

393

i \.

/\

--------J 'J ··~D

·~c

2200

2100 2000 1900 Wavenumbers/ em-1

Fig. 1. Infrared spectra in the v(CO) region of physisorbed IrCI 3/Si0 2 • Reactivity under CO: (A) after 6 days at 50°C; (B) after a further 7 days at 70 DC. Reactivity under CO + H 20: (C) after 48 h at 70°C; (D) after extraction with THF.

150°C, the relevant sublimation of a brown material is observed on the cold part of the cell. The infrared spectrum of this sublimate, taken as nujol mull, shows a strong and very broad band at 2081 em - 1, in agreement with that reported for [Ir(CO)3CI]n [16]. In the homogeneous phase, IrCI 3· 3H 20 is very reactive toward CO, being converted into [Ir(CO)2CI2]- by refluxing in ethanol containing 10% water [17].

On the silica surface we could not detect the formation of the anion [Ir(CO)2CI2]-, probably owing to the instability of anionic complexes on weakly acidic surfaces. In fact, when a yellow methanol solution of [Ir(CO)2CI2]- [H 30] + is impregnated onto a silica wafer, the support immediately becomes blue. After evacuation of the solvent the colour changes to brown; the infrared spectrum shows only the broad band, centered at 2103 cm - 1. Further treatment under CO at 100°C leads to sublimation of [Ir(CO)3Cl ]nWhen physisorbed IrCI 3· 3H 20 is treated at 50°C with a mixture of CO + H 20 (12 wt.% H 20/Si02), the initial formation of the 'uI l1(CO)2' surface

394 IU(CO)12

co + H20

~HF Ir4(CO)l2/Si02

IrCI3/Si02 CO

[Ir(CO)3CI]n sublimed

Scheme 1.

species is still observed. At 70°C physisorbed Ir4(CO)12 is obtained (Fig. I(C)). It can easily be extracted with tetrahydrofuran (Fig. 1(D)). These results suggest that the process of aggregation of metal ions to a polynuclear structure by reductive carbonylation depends on the surface water content (Scheme 1). The hydrochloric acid formed during the reduction of iridium trichloride inhibits further reduction to Ir4(CO) 12. This inhibiting effect of HCI was previously reported to occur in the homogeneous synthesis of Ir 4(CO)12 [17], where it is necessary to buffer the solution with disodium citrate in order to convert [Ir(CO)2CI2]- into Ir4(CO)12. When a large amount of water is present on the silica surface, buffering is not necessary to obtain Ir 4(CO) 12. It is known that under these conditions a water layer is present on the surface [18], where the reductive carbonylation probably occurs. Probably the surface acidity due to HCI is strongly reduced at 70°C in this layer, since further nucleophilic attack by water on CO ligands occurs, together with a nucleation process.

Surface reactivity of RhCl3 • 3H2 0 supported on Si0 2 with CO or with CO+H20 When physisorbed RhCI3· 3H 20 is treated with CO at room temperature for 24 h the infrared spectrum in the v(CO) region shows bands at 2104, 2089, 2033 and 2015 cm- 1 (Fig. 2 (A)). These bands are quite broad, but with the exception of the band at lower frequency, they resemble those characteristic of physisorbed [Rh(CO)2Clb (2104, 2089, 2033 cm- 1; Fig. 2(B)). As expected, the surface species shown in Fig. 2(A) can be completely extracted by simple treatment with dichloromethane, affording pure [Rh(CO)2Clb (Scheme 2). This conversion promoted by the silica surface has been previously reported, at 50°C under CO on pressed wafer, by Keyes and Watters [19]. 'Rhlll(CO)xCI3' species, characterized by a CO band at 2140 cm -1, had been observed by some of us after exposure of silica-supported RhCl 3 to CO at 50°C for 1 h [20].

395

,:\

.~

~c

\

.'---~\,

r.

I:

iv

2200

2100

B

I

I

'

A

2000

1900

Wavenumbers/ cm-1 Fig. 2. Infrared spectra in the veCO) region of silica-supported rhodium complexes: (A) physisorbed RhCl3 after 24 h at 25°C under CO; (B) physisorbed [Rh(CO)2Clh; (C) physisorbed RhC13 after 24 h at 25°C under CO + H 20; (D) as in (C) after extraction with CH 2Cl z.

[Rh(CO)2CI]z/SiOz + Rh(CO)zCI(OHz)/SiOz RhC13/SiOz 70°C, CO

sublimed

co +

H20

--------1t>

Rh6(CO)16/SiOz

CO ---------1t>

Rh6(CO)16/SiOz

Scheme 2.

When physisorbed RhCl 3 is exposed to a mixture of CO + H 20 (15% by weight H 20/Si02 ) , after 24 h at room temperature the infrared spectrum shows only two bands of equal intensity at 2089 and 2015 cm- l (Fig. 2(C)).

396

An extraction with dichloromethane affords pure [Rh(CO)2Clb (Fig. 2(0)). The new two-band spectrum (Fig. 2(C)) is markedly different from that of physisorbed [Rh(CO)2Clb, which shows three bands, but it is very similar to those reported for molecular cis Rh(CO)2CIL compounds [21]. Consequently, we infer the formation on the surface of monomeric Rh(CO)2CI(OH2) species, supported by the behaviour of physisorbed [Rh(CO)2CI]2o This latter species is stable under CO at room temperature, for a prolonged time. However, after exposure to a mixture of CO + H 20, relevant changes occur in its infrared spectrum in the v(CO) region. The higher-frequency band at 2104 em- 1 disappears, while the two bands at 2089 and 2033 cm - 1 are shifted to 2084 and 2012 em-I, respectively. This shift may be ascribed to the splitting of the chloride bridge by coordination of water, yielding the surfaee monomeric species Rh(CO)2CI(OH2). Obviously, when the conversion of Si0 2supported RhCl a to [Rh(CO)2Clb is performed in the presence of CO only, a mixture of dimeric and monomeric species are formed on the silica surface (compare Figs. 2(A) and 2(C)). Physisorbed RhCl a treated with CO + H 20 between 25 and 70°C does not lead to the formation of rhodium clusters (viz., Rh 4(CO)I2 or Rh 6(CO)I6)'

o

\~c

B

'--

2200

2100

2000

~~_A

1900

1800

Wavenumbers/ cm-l Fig. 3. Infrared spectra in the v(CO) region of: (A) physisorbed [Rh(CO)2C1blSi02; (B) after 18 h at 25°C under CO + H 20; (C) after a further 6 days at 25 °C under CO + H 20; (D) after extraction with CH 2C12.

397

At 70°C the complete sublimation of [Rh(CO)2Clh occurs. Most probably the large amount of hydrochloric acid liberated during the surface formation of [Rh(CO)2Clh inhibits at relatively low temperatures the further reductive aggregation to rhodium clusters, as it was reported to occur in homogeneous synthesis [22]. However, when physisorbed [Rh(CO)2Clh is used as starting material, instead of physisorbed RhCI 3 , rhodium carbonyl clusters might be obtained (Scheme 2). In this case no HCI is present initially. When physisorbed [Rh(CO)2Clh is treated at room temperature with CO + H 20 (12 wt.% H 20/Si02) a reduction to physisorbed Rh 6(CO)16 occurs slowly in 7 days. Rh 6(CO)16 could be extracted easily using dichloromethane (Fig. 3). In order to confirm the positive effect of water, physisorbed [Rh(CO)2Clh was reacted in the presence of a large amount of water (89 wt.% H 20/Si02), previously saturated with carbon monoxide. The reaction is quite fast at room temperature, affording a mixture of physisorbed Rh 6(CO)16 and Rh 4(CO)12 even after 24 h. These results clearly indicate the role played by water in decreasing the inhibiting effect of hydrochloric acid.

- - - - - - - - - - - - - - - -_ _ A

2100

2000 1900 1800 Wavenumbers/ em-1

1700

Fig. 4. Infrared spectra in the v(CO) region of physisorbed [Rh(OzCCH 3)zh/Si02 : (A) after 24 h at 25°C under CO; (B) after a further 48 h at 100°C under CO; (C) after extraction with CHzCl z·

398

Surface reactivity of [Rh(02CCH3)212 supported on Si0 2 with CO The study of the surface reactivity of a chlorine-free Rh salt precursor, which does not liberate HCI during the reductive carbonylation on the silica surface, was an obvious extension of this investigation. No reaction occurs when physisorbed [Rh(02CCHa)21z is exposed to CO at room temperature (Fig. 4(A)). The infrared spectrum shows bands at 1583 and 1420 em - I , corresponding to the acetate ion coordinated to rhodium [23). After 24 h at 50°C, very weak absorptions appear in the v(CO) region. At 70°C physisorbed Rh 6(CO)16 is obtained. This aggregation reaction is faster at 100°C, although 4 days are necessary to obtain high yields of the hexanuclear cluster. Acetic acid is formed during the reaction, as can be seen by the appearance of a band at 1718 cm - 1 in the infrared spectrum (Fig. 4(B)). When the reaction is stopped before completion, for example after 2 days under CO at 100°C, extraction with dichloromethane affords Rh 6(CO)16, acetic acid, and traces of [Rh(CO)2(02CCHa) lz characterized by weak CO bands at 2100,2076 and 2027 cm- I [24) (Fig. 4(C)). The direct conversion of [Rh(02CCHa)21z to Rh 6(CO)16 on the silica surface is not surprising, because the conditions used are quite similar to those reported in the homogeneous synthesis [25). It is remarkable that the water content on the surface of hydrated silica is now enough for the reaction to occur. The evolved acetic acid, weak bridging ligand, does not strongly inhibit the process of reductive aggregation of Rh I species. We believe that the key step of the reductive carbonylation is the slow formation of physisorbed [Rh(CO)2(02CCHa) b an intermediate that is rapidly converted into physisorbed Rhe(CO)16.

Conclusion We have shown that the amount of surface water has a significant role in the reductive carbonylation of silica-supported MCla (M=Rh, Ir) under mild conditions. At low water contents the process stops at [Ir(CO)aCI)n and [Rh(CO)2Clb, which can sublime at 100-150 °C and 70°C, respectively. Their rather easy sublimation indicates the absence of strong interactions with the silanol groups of the surface. The role played by the hydrated silica surface is that of modifying the coordination sphere of the metal halide salts. The first step is the formation of 'MIII(CO)2' species, followed by a nucleophilic attack of surface water on CO ligands, affording physisorbed 'MI(CO)n' species. The hydrochloric acid thus formed prevents further reduction to zero-valent carbonyl clusters. In the presence of large amounts of water, the process will eventually end with metal carbonyl clusters. In fact, Ir4 (CO)12 is selectively obtained from physisorbed IrCl a by treatment at 70°C with a mixture of CO and

399

H 20. Under the same experimental conditions, the surface reactions of RhCI s stops only at Rh\ because the intermediate species [Rh(CO)2Clh completely sublimes. However, when the dimeric species is used as starting material, reduction to Rh 6(CO) 16 occurs at room temperature under CO + H 20. In this latter case, the presence of a large excess of water favours the nucleation process. The inhibiting effect on the nucleation process of HCI and the role of water have been confirmed by comparison with the reductive carbonylation of a chlorine-free rhodium salt. It is possible that similar surface effects are acting in the reduction processes to form metal particles and influence in particular their properties and sizes. Aclmowledgements This work was supported by the Ministry of University and Scientific and Technological Research. One of us (D.R.) thanks the Natural Sciences and Engineering Research Council of Canada for a NATO postdoctoral fellowship. We thank Prof. S. Martinengo for helpful discussions. References 1 (a) J. M. Basset, J. P. Candy, A. Choplin, M. Leconte and A. Theolier, in R. Ugo (ed.), Aspects of Homoqeneous Catalysis, Vol. 7, Kluwer, Dordrecht, 1990, p. 85; (b) P. A. Jacobs, in B. C. Gates, L. Guczi and H. Knozinger (eds.), Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986, Chap. 8, p. 357 . .2 M. Primet, J. Chern. Soc., Faraday Trans. 1, 74 (1978) 2579. 3 P. Gelin, G. Coudurier, Y. Ben Taarit and C. Naccache, J. Catal., 70 (1981) 32. 4 P. Gelin, F. Lefebvre, B. Elleuch, C. Naccache and Y. Ben Taarit, in G. D. Stucky and F. G. Dwyer (eds.), Intrazeolite Chernistry, ACS Symp. Ser., 218 (1983) 455. 5 F. Mantovani, N. Palladino and A. Zanobi, J. Mol. Catal., 3 (1977) 265. 6 G. Bergeret, P. Gallezot, P. Gelin, Y. Ben Taarit, F. Lefebvre, C. Naccache and R. D. Shannon, J. Catal., 104 (1987) 279. 7 L. Rao, A. Fukuoka, N. Kosugii, H. Kuroda and M. Ichikawa, J. Phys. Chern., 94 (1990) 5317. 8 M. Ichikawa, L. Rao, T. Ito and A. Fukuoka, Faraday Discuss. Chern. Soc., 87 (1989) 232. 9 G. Bergeret, P. Gallezot and F. Lefebvre, Stud. Surf. Sci. Catal., 28 (1986) 401. 10 S. Kawi and B. C. Gates, J. Chern. Soc., Chern. Commun., (1991) 994. 11 A. K. Smith, F. Hugues, A. Theolier, J. M. Basset, R. Ugo, G. M. Zanderighi, J. L. Bilhou, V. Bilhou-Bougnol and W. F. Graydon, lnorg. Chem., 18 (1979) 3104. 12 H. H. Lamb, A. S. Fung, P. A. Tooley, J. Puga, T. R. Krause, M. J. Kelly and B. C. Gates, J. Am. Chern. Soc., 111 (1989) 8367. 13 P. Chini, G. Longoni and G. Albano, Adv. Organomet. Chern., 14 (1976) 285. 14 K. Tanaka, K. L. Watters and R. F. Howe, J. Catal., 75 (1982) 23. 15 L. Malatesta, L. Naldini and F. Cariati, J. Chern. Soc., (1964) 961. 16 (a) A. H. Reis, Jr., V. S. Hagley and S. W. Peterson, J. Am. Chern. Soc., 99 (1977) 4184; (b) F. Canziani, M. Tuissi and M. Angoletta, J. Organometall. Chem., 256 (1983) 169. 17 R. Della Pergola, L. Garlaschelli and S. Martinengo, J. Organometall. Chern., 331 (1987) 271.

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