Langmuir 1999, 15, 7621-7625
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A Catalytic Probe of the Surface of Colloidal Palladium Particles Using Heck Coupling Reactions Joe¨l Le Bars,† Ullrich Specht, John S. Bradley,*,† and Donna G. Blackmond*,† Max-Planck-Institut fu¨ r Kohlenforschung, D-45470 Mu¨ lheim an der Ruhr, Germany Received February 10, 1999. In Final Form: May 24, 1999
A series of well-defined homopolymer-stabilized Pd colloids with varying metal particle size was used to study the Heck coupling reaction between aryl halides and olefins. Correlation of initial reaction rates and particle sizes determined by transmission electron microscopy demonstrates that the Heck reaction is a sensitive probe of metal surface structure. The presence of the colloid-stabilizing homopolymer affords a much more stable catalyst than that obtained using the colloid-precursor metal complexes alone. Extremely high total turnover numbers (TON, moles of substrate/moles of Pd; TON ) 100 000) and turnover frequencies (TOF ) TON/h; TOF > 80 000) are easily attainable with poly(vinylpyrrolidone)-stabilized colloidal palladium in the coupling of p-bromobenzaldehyde with butyl acrylate.
Introduction The concept of structure sensitivity, based on the observation that some reactions demand either low coordination number metal atoms or a specific arrangement of metal sites, is one of the fundamental phenomena in heterogeneous catalysis.1 Reactions have been grouped as structure-insensitive, such as simple hydrogenation reactions, or structure-sensitive, including carbon-carbon bond-breaking reactions. The observation of structure sensitivity in a heterogeneously catalyzed reaction classically requires the preparation of a series of catalysts which differ only in particle size. This is notoriously difficult to accomplish since heterogeneous catalysts of differing particle size are typically prepared by using different precursors, supports, or preparation conditions, producing catalysts which are intrinsically different from one another in ways other than simply their particle size. Colloid preparation techniques, however, can provide catalysts in which the size or structure of the metal particles may be varied in the absence of other perturbations2 and thus are excellent materials for investigations of catalytic site requirements. While studies of this phenomenon have largely focused on heterogeneous catalyzed reactions of simple gaseous molecules, the concept of structure sensitivity applies equally to catalytic reactions involving liquid-phase organic molecules. We report in this paper studies of the Heck coupling reaction between aryl halides and olefins using a series of well-defined homopolymer-stabilized Pd colloids with varying metal particle size. The Heck reaction3,4 is an important synthetic tool in the production of intermediates for pharmaceuticals and fine chemicals (eq 1). While current research has focused primarily on soluble palladium metal complexes with coordinating phosphine ligands,5 several reports of stabilized colloidal * Corresponding authors:
[email protected].
[email protected]. † Present address: Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom. (1) Boudart, M. Adv. Catal. 1969, 20, 153. (2) Bradley, J. S. In Clusters and Colloids; Schmid, G., Ed.; VCH: Weinheim, 1994; p 459. (3) (a) Heck, R. F.; Nolley, J. P., Jr. J. Org. Chem. 1972, 37, 2320. (b) Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971, 44, 581.
Pd catalysts6 and heterogeneous catalysts7 have also appeared in the literature. We confirm that a quantitative relationship exists between the reaction rate and the number of lowcoordination number or “defect” Pd surface atoms, as was found for heterogeneous Pd catalysts for this reaction over a narrower range of defect site concentration.7a Indeed, correlation of initial reaction rates and particle sizes determined by transmission electron microscopy demonstrates that the Heck coupling reaction itself may be used as a sensitive probe of metal surface structure. We show in addition that the presence of the colloid-stabilizing homopolymer affords a much more stable catalyst than that obtained using the colloid-precursor metal complexes alone. In addition, we report that extremely high total turnover numbers and turnover frequencies are easily (4) Recent reviews: (a) Bra¨se, S.; de Meijere, A. In Metal-Catalyzed Cross-Coupling Reactions; Sland, P. J., Diederich, F., Eds.; WileyVCH: Weinheim, 1998; Chapter 3, p 99. (b) Cabri, W.; Candiani, I.; Bedeschi, A.; Penco, S. J. Org. Chem. 1992, 57, 1481. (c) Riermeier, T. H.; Zapf, A.; Beller, M. In Top. Catal. 1997, 4, 301. (d) Heck, R. F. In Comprehensive Organic Synthesis; Trost, B. M.; Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 4, p 833. (e) Herrmann, W. A. In Applied Homogeneous Catalysis with Organometallic Compounds; Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim, 1996; p 712. (5) (a) Ziegler, C. B.; Heck, R. F. J. Org. Chem. 1978, 43, 2941. (b) Ben-David, Y.; Portnoy, M.; Gozin, M.; Milstein, D. Organometallics 1992, 11, 1995. (c) Portnoy, M.; Milstein, D. Organometallics 1993, 12, 1655. (d) Herrmann, W. A.; Brossmer, C.; O ¨ fele, K.; Reisinger, C.-P.; Priermeier, T.; Beller, M.; Fischer, H. Angew. Chem. 1995, 107, 1989, Angew. Chem., Int. Ed. Engl. 1995, 34, 1844. (e) Herrmann, W. A.; Brossmer, C.; O ¨ fele, K.; Beller, M.; Fischer, H. J. Organomet. Chem. 1995, 491, C1. (f) Herrmann, W. A.; Brossmer, C.; Reisinger, C.-P.; Riermeier, T. H.; O ¨ fele, K.; Beller, M. Chem. Eur. J. 1997, 3, 1357. (6) (a) Beller, M.; Fischer, H.; Ku¨hlein, K.; Reisinger, C.-P.; Herrmann, W. A. J. Organomet. Chem. 1996, 520, 257. (b) Reetz, M. T.; Lohmer, G. Chem. Commun. 1996, 1921. (c) Klingelho¨fer, S.; Heitz, W.; Greiner, A.; Oestreich, S.; Fo¨rster, S.; Antonietti, M. J. Am. Chem. Soc. 1997, 119, 10116. (7) (a) Augustine, R. A.; O’Leary, S. T. J. Mol. Catal. A 1995, 95, 277. (b) Eisenstadt, A. In Catalysis of Organic Reactions; Herkes, F., Ed.; Marcel Dekker: New York, 1998; Vol. 36, p 36. (c) Mehnert, C. P.; Ying, J. Y. Chem. Commun. 1997, 2215. (d) Mehnert, C. P.; Weaver, D. W.; Ying, J. Y. J. Am. Chem. Soc. 1998, 120, 12289.
10.1021/la990144v CCC: $18.00 © 1999 American Chemical Society Published on Web 08/26/1999
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Figure 1. Transmission election micrographs for the four catalysts given in Table 1: (a) Pd17 (1.7 nm); (b) Pd23 (2.3 nm); (c) Pd28 (2.8 nm); (d) Pd37 (3.7 nm). Scale bars ) 25 nm. The average diameter of the particles is in brackets.
attainable with poly(vinylpyrrolidone) (PVP)-stabilized colloidal palladium in the coupling of p-bromobenzaldehyde with butyl acrylate. Experimental Section A series of colloidal catalysts of mean Pd particle sizes from ca. 1.7 to 3.7 nm were prepared by variations on literature methods.8 Solutions of Pd(dba)2 (dba ) dibenzylideneacetone) and PVP (MW ) 30 000) in dichloromethane were treated with hydrogen at 1-4 bar for 1-3 h to give the Pd catalysts shown in Table 1. Each catalyst was isolated by evaporation of the solvent and stored under argon as a dry solid. Catalyst particle size distributions were determined by transmission electron microscopy (TEM) (Hitachi HF 2000, 200 keV). Mean Pd particle sizes were determined as the average diameter of a minimum of 300 particles. The Heck coupling of p-bromobenzaldehyde with butyl acrylate was carried out at 140 °C in dimethylacetamide solvent with sodium acetate as base. Typical reaction conditions employed 100 mmol of the aryl halide, 140 mmol of the olefin and base, and 25 mmol of Pd (0.025 mol %) as the colloidal catalyst in a total reaction volume of 500 mL. Because it was noted that catalysts exhibited an initial transient period before attaining peak activity, the catalysts were pretreated in a solution containing the solvent, the olefin, and the base for 1 h at reaction temperature prior to (8) (a) Bradley, J. S.; Hill, E. W.; Behal, S.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1992, 4, 1234. (b) Bradley, J. S.; Via, G. H.; Bonneviot, L.; Hill, E. W. Chem. Mater. 1996, 8, 1895.
Table 1. Characteristics of the Polymer-Stabilized Palladium Colloids (2.2 wt % Pd)a catalyst reduction reduction desigpressure time nation (bar H2) (h) Pd37 Pd28 Pd23 Pd17
1.5 1.0 1.0 4.0
1.15 1.0 2.0 3.0
particle sizeb (nm) 3.7 ( 0.3 2.8 ( 0.6 2.3 ( 0.6 1.7 ( 0.5
disper- defect heat sionc sitesc flowd (%) (%) (W) 33.0 42.1 51.5 60.2
7.9 13.4 20.2 31.1
6.6 11.8 14.2 22.3
a Preparation of polymer-stabilized Pd colloids by hydrogenation (time and H2 pressure given in table) of solutions containing: 0.5 mmol of Pd(dba)2, 1.77 g of poly(vinylpyrrolidone) K30, 100 mL of absolute CH2Cl2, stirring at 2000 rpm and 20 °C. b Pd particle sizes, determined from TEM as the average diameter of 300 particles. c Using the statistics for closed face cubic centered cuboctahedra (9). d At 10% conversion of p-bromobenzaldehyde in the coupling with butyl acrylate at 140 °C, 25 µmol of Pd.
injection of the aryl halide to commence the reaction. Product formation was monitored by GLC over the course of the reaction. Over 95% yield to n-butyl (E)-4-formylcinnamate was obtained in all reactions. Reaction rates were measured by reaction calorimetry (Mettler RC1). In a system in which a single reaction is occurring, the measured heat flow q(t) is proportional to the rate of reaction dn/dt as given in eq 2, with the heat of reaction, ∆Hrxn as the proportionality constant.
q(t) ) ∆Hrxn
dn dt
(2)
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Figure 2. Histograms of Pd particle size calculated from micrographs in Figure 1 for the four catalysts given in Table 1. 300 particles were counted in each case: (a) Pd17; (b) Pd23; (c) Pd28; (d) Pd37. Agreement between GLC (gas-liquid chromatography) analysis as a function of reaction time and conversions from the heat-flow data demonstrates the validity of the reaction calorimetric method for measuring the rate of the coupling reaction. The heat of reaction ∆Hrxn was found experimentally from integration of the heat flow curves to be 36.8 ( 0.5 kcal/mol. This value was used to convert the heat flow measured in units of watts to product formation rates (moles/hour). This overall rate may be further converted to a turnover frequency (TOF) by dividing by the moles of active Pd.
Results and Discussion It is known that the zero-valent Pd complex Pd(dba)2 is easily hydrogenated to colloidal Pd in solutions containing polymers such as PVP. By varying the hydrogen pressure and reduction time, we were able to prepare a series of colloidal dispersions with different particle sizes and narrow particle size distributions. Table 1 presents information about the preparation and characteristics of the four catalysts employed in this study. Surprisingly, higher pressure and longer reduction time favored production of smaller metal crystallites. The more forcing conditions may result in the rapid nucleation of a greater number of small crystallites compared to a slower sustained growth of fewer particles under milder reduction conditions. Figure 1 shows TEM images and Figure 2 shows the corresponding particle size histograms for the catalysts. The micrographs and the histograms reveal uniform particle shapes and narrow size distribution for all of the samples. Additional analyses by high-resolution TEM revealed diffraction planes of the Pd particles, indicating that they correspond to crystalline materials.
While the precise morphology of the particles cannot be defined, it is clear that they are quite equidimensional, thus allowing us to represent the particles as regular polyhedra for the following analysis of surface site statistics. To allow an estimate of the particle size dependence of the relative abundance of various possible surface sites in this series of catalysts, we have adopted the statistics for closed face-centered cubic (fcc) cuboctahedra over this size range.9 Figure 3 shows the different types of sites that comprise the surface of an idealized cuboctahedral particle. High-coordination number “terrace sites” exist on the faces of a crystallite and low-coordination number “defect” sites are situated at the edges and vertices. The relative abundance of each type of site varies with Pd cluster size as simple geometric functions of the length of the cuboctahedron edge. Figure 3 illustrates that the greatest change in defect site concentration with particle size is exhibited over the particle size range of the samples given in Figure 1. The Heck coupling of p-bromobenzaldehyde with butyl acrylate (eq 1, R1 ) CHO; R2 ) Br; base ) NaOAc) was carried out for each of these colloidal catalysts, using reaction calorimetry as an in situ method of monitoring reaction rate. Figure 4 shows a representative heat flow curve for the Heck coupling reaction using a Pd catalyst of 2.8 nm particle size. By carrying out reactions over a range of catalyst and substrate concentrations, we have been able to achieve turnover numbers of 100 000 with a yield of 99% and turnover frequencies of over 80 000/h (9) van Hardeveld; Hartog Surf. Sci. 1969, 15, 189.
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Figure 3. Calculation of the fraction of high coordination (“terrace”) and low coordination (“defect”) sites present on a regular cuboctahedron particle as a function of crystallite size.
Figure 4. Raw data giving reaction rate vs time in terms of the reaction heat flow in the Heck coupling of p-brombenzaldehyde with butylacrylate (eq 1) for catalyst Pd28 (0.10 mol of p-bromobenzaldehyde, 0.14 mol of butyl acrylate, 0.14 mol of sodium acetate in DMA solvent, 500 mL total reaction volume, 25 µmol of Pd catalyst).
with these catalysts (TON ) moles substrate per mole of total Pd; and TOF )TON/h). These are comparable to or higher than those reported in previous studies of colloidal catalysts.6 They are several orders of magnitude higher than those reported recently for supported metal catalysts using similar activated aryl halide substrates7d and approach some of the best homogeneous catalysts4 developed for Heck coupling reactions of activated aryl halides. For comparison of catalysts of different particle sizes, initial rates as TOFs were calculated from heat flow curves at 10% conversion of the aryl bromide substrate. Figure
Le Bars et al.
5 shows these rates as a function of Pd particle size for the series of colloids shown in Table 1. The number of active Pd atoms used as the basis for the TOF calculation has been calculated in two different ways. In Figure 5a, the TOF was calculated on the basis of the total number of surface Pd atoms present in each catalyst, assuming cuboctahedral morphology. Figure 5b compares the TOF for each catalyst by taking into account only the number of defect surface Pd atoms, calculated on the same geometric basis. If the active surface atoms have been properly identified, the TOF would be expected to appear as a horizontal line in Figure 5, since the rate normalized to the active sites should have no dependence on Pd particle size. The initial TOF changed by more than a factor of 2 over the range of Pd crystallite sizes used when the rate was based on total surface Pd (Figure 5a). This indicates that all of the surface Pd atoms do not possess equal reactivity in this reaction. When only the defect Pd sites were considered to be active sites for the calculation of TOF (Figure 5b), the dependence of rate on crystallite size disappeared, confirming the suggestion made previously7a that the low-coordination number Pd defect sites are the active centers for catalysis in Heck coupling reactions. The trends noted here would also hold for less regular particles than the idealized cuboctahedral geometries used here. Augustine and co-workers7a were the first to note such structure sensitivity in Heck coupling reactions in their studies of a series of heterogeneous catalysts for the coupling of acyl chlorides with enol ethers. In that case an experimental correlation was made between the reaction rate and the number of “alkene saturation sites”, which, the authors’ previous work suggested, correlate with corner and adatom abundances. Our conclusion is the same as theirs: sites of low metal-metal coordination are the active centers for carbon-carbon coupling reactions. That the use of heterogeneous catalysts in such a systematic study of the effects of surface structure may be problematic was, however, also revealed by Augustine’s work: the defect site concentration for the series of heterogeneous catalysts employed bore no apparent relationship to that calculated for an idealized crystallite shape. In fact, in some cases catalysts of lower dispersion (Pd surface atoms/total Pd atoms) exhibited higher defect site concentrations. Thus in the absence of an independent measurement of surface structure, elucidation of the structure sensitivity of the reaction for that series of heterogeneous catalysts would not have been possible. One of the benefits of using colloidal preparation methods is that a series of closely related metal catalysts may be prepared for systematic studies of the effects of particle size and surface structure in complex catalytic reactions. The correlation to an idealized statistical distribution of surface sites which we were able to obtain emphasizes this advantage of colloidal systems for fundamental studies of surface structure and properties as they relate to heterogeneous catalytic systems. Figure 6 shows that when the Pd(dba)2 precursor complex was used in the absence of the polymer as a catalyst in the reaction, significantly longer reaction times were required to reach high conversion, compared to the preprepared Pd/PVP catalyst with small Pd crystallites or to an in situ preparation of the Pd complex and the PVP polymer added together to the reaction mixture. This result highlights the efficiency of stabilization of the small metal crystallites by the polymer and suggests that the reaction does not occur via solution-phase catalysis by leached Pd
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Figure 5. Normalized site-specific initial reaction rates in the Heck coupling reaction (see eq 1, 0.10 mol of p-bromobenzaldehyde, 0.14 mol of butyl acrylate, 0.14 mol of sodium acetate in DMA solvent, 500 mL total reaction volume, 25 µmol of Pd catalyst), for the four catalysts given in Table 1. Rates are given at 10% conversion of the aryl bromide and are normalized to that of the catalyst with the largest crystallite size: (a) rate calculated on the basis of total surface Pd atoms; (b) rate calculated on the basis of total number of low coordination “defect” surface Pd atoms.
Conclusions
Figure 6. Conversion of the aryl bromide substrate as a function of time the catalyst prepared in situ from Pd(dba)2 + PVP and for Pd(dba)2 in the absence of stabilizing polymer. Preprepared 1.7 nm colloidal catalysts shown for comparison. Total Pd is 25 µmol in all three experiments.
species. This supports the suggestion of Beller et al.6a that the significant involvement of unstabilized colloidal Pd formed adventitiously is unlikely in Heck reactions employing homogeneous catalysts.
These results confirm that the Pd-metal catalyzed Heck coupling of aryl halides with olefins is a structure-sensitive reaction and that the Pd sites responsible for high activity are those with low metal-metal coordination number. The activity of these colloidal systems is comparable to that exhibited in similar reactions using the most efficient homogeneous Pd catalysts with coordinating ligands,5f even after the colloidal catalysts have stabilized following an initial period of high activity. Moreover, this work shows that colloidal preparation techniques can provide a series of well-defined catalysts of differing particle sizes (and thus surface statistics) for systematic study of structure sensitivity in the Heck coupling as well as other reactions. An understanding of the site requirement for the Heck reaction provides an important prerequisite toward developing appropriate catalyst modifications for the practical use of colloidal or hetereogeneous Pd catalysts, especially in the more demanding coupling reactions of deactivated aryl halide substrates. Acknowledgment. Funding from the Max-Planck Gesellschaft is gratefully acknowledged. D.G.B. thanks Novartis Pharma Corporation for an unrestricted research grant. The authors thank B. Spliethoff for assistance with electron microscopy and W. Ko¨nen for assistance with distillation of reagents and GC analysis. D.G.B. thanks Professor Dr. Andreas Pfaltz for helpful discussions. LA990144V
