Polyamide-anchored palladium(0): Catalysts for the hydrogenation of olefins and nitro compounds

Reactive Polymers, 3 (1985) 191-198 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

191

POLYAMIDE-ANCHORED PALLADIUM(0): CATALYSTS FOR THE HYDROGENATION OF OLEFINS AND NITRO COMPOUNDS YU-PEI WANG and D.C. NECKERS

Department of Chemistry, BowlingGreen State University, BowlingGreen, OH 43403 (U.S.A.) (Received May 8, 1984; accepted in revised form December 31, 1984)

The reaction of a polyamide prepared from 2,2'-bipyridine-4,4'-dicarboxylic acid and 2,6-diaminopyridine with palladium dichloride was carried out in tetrahydrofuran, resulting in formation of the polyamide Pd(II) complex [1 4]. This was reduced in alkaline methanol under hydrogen to give the polyamide Pd(O) complex, which is" a very efficient catalyst for the reduction of olefins and nitro compounds.

INTRODUCTION

It has long been recognized that the incorporation of metallocene units into a polymeric backbone may give rise to certain bulk properties or a combination of pertinent chemical and physical features not found in the non-polymeric complexes. A large amount of work has been carried out on the catalytic properties of complexes of transition metal ions and synthetic low and high molecular polymers [4 9]. Polymer-supported metallic catalysts are able to catalyze chemical reactions both with a high activity and selectivity. They are, therefore, of interest from both a practical and a theoretical point of view. Previous studies [10,11] have been concerned with the site isolation hypothesis, size and polar selectivities, and regioselec6vity and stereoselectivity of polymer-based metal catalysts in contrast to their monomeric analogs. 0167-6989/85/$03.30

In general, these phenomena are related to the configuration and conformation of one macromolecular chain; i.e., a random, helical coil. Metals in the polymer matrix behave as a second phase in which their concentration is higher than that in the bulk of solution. This study concentrates on polymeric complexes derived from Pd(II) and a new polyamide derived from 2,6-diaminopyridine. The polymer complex was prepared according to Scheme 1. As noted above, polyamide palladium(II) was reduced by a different method than previously reported [4-9]. In the present study N a O H is used as a reducing catalyst for polyamide palladium(II) in a H 2 atmosphere. It is evident that base is necessary for polyamide Pd(II) to be reduced to Pd(0). Hydrogenations were carried out under ambient conditions using the polyamide Pd(0). In fact, all hydrogenation processes were heterogeneous and occurred on the surface of the macromolecular complex.

~5 1985 Elsevier Science Publishers B.V.

192

Polyconden sot ion 0

COOH

c,-~

COOH

K M,,%

SOCI 2

KOH

>

~

0

~-c,

IH-C

C-NH"-~N .~--NH

2r 6-Diominopyridine > DMF

Complexotlon

II II N - ~ H

NH

NaOH/MeOH~. "N~ H2

EtOH • P~CI2

?

o,

(~ H'-J~N,,,Y"NH'

•... pdO"

"Pd CI~

Scheme 1.

RESULTS AND D I S C U S S I O N

IL Catalytic activity of polyamide Pd(O)

I. The polymer complex formation

II. 1. Hydrogenation of olefins

In general, macromolecular complex formation represents a complicated process. The polymer matrix absorbs solvent and swells. This is accompanied by a change in polymer structure. The swollen polymer then absorbs the metal ions, and complexation takes place. One can consider that the individual functional groups (ligand units) bind metal ions and form "inner coordination nodes". Then the spatial arrangement of the complexing groups over the macromolecular ligand chain takes various orientation with respect to the main chain. As a result, the total polymer complex as a whole is formed. Macromolecular complex formation also represents a relaxation process, which is related to the probability of transition of the system from one state of equilibrium to another. In the present study, the polyamide does not dissolve in tetrahydrofuran (THF); in contrast, PdCI 2 is soluble in THF, so the complexation process itself is heterogeneous throughout.

It is known that Pd(0) catalysts are usually very active toward reducing carbon-carbon TABLE 1 Hydrogenation of olefins at 25°C and 1 atm of hydrogen, 10 ml methanol with 150 mg polyamide Pd(0) catalyst Run Reactant

1 2 3 4 5 6 7 8 9 10

Rate of Conversion hydrogenation (%) (ml/min)

Styrene 13 Methyl methacrylate 7.8 Allyl alcohol 6.3 2-Methyl-3-buten-2-ol 6.0 1-Hexene 6.0 1-Octene 5.7 Carvone a 4.5 Cyclopentene 2.4 Methyl vinyl ketone b 2.4 Cyclohexene 1.8

100 100 100 98 97 96 98 72 98 66

a In the case of carvone, the rate means that of the reduction of the C=C bond in isopropenyl group. There is no reduction of the carbonyl group. b In the case of methyl vinyl ketone, there is no reduction of the carbonyl group.

193 multiple bonds. A number of different olefins have been reduced using polymer Pd(0) catalysts with excellent results [9]. The polyamide Pd(0) complex is more efficient and more stereoselective than the non-polymer bound Pd(0) complex. The present investigation has determined the scope and versatility, in synthetic applications, of polymeric palladium complexes. The results listed in Table 1 indicate that: (i) Linear olefins are reduced more rapidly than cyclic olefins. (ii) Functional groups which form conjugated systems or increase the electron density around the C=C bond lead to more rapid reactions, e.g., styrene and methyl methacrylate. (iii) In general, carbonyl groups are not reduced with palladium catalysts. (iv) As mentioned above, polymer catalysts display peculiar features, such as regioselectivity and stereoselectivity, which are also observed in the present investigation. We will discuss this in the following section by comparing the polymeric catalyst with homogeneous catalyst.

TABLE 2 Hydrogenation of nitro compounds at 25°C and 1 atm of hydrogen, 10 ml methanol with 150 mg polymer Pd(0)

11.2. Hydrogenation of nitro compounds

The results with the monomeric palladium complex as catalyst are summarized in Table 3, and compared with those obtained with polyamide Pd(0) complex. The polymer complex is more efficient and stereoselective than the corresponding monomer complex. Though several mechanistic studies of palladium hydrogenation have been reported [14,17,22], reactions catalyzed by a polymeric Pd(0) complex are limited to a few examples. In polymeric catalysts, "coordinatively unsaturated" metal species, to which the substrate may be easily coordinated without displacement of ligands, are possible because the ligands are held away from one another. The present results provide data which support some of the hypotheses [10,14,15,17] regarding site isolation of metals on a catalytic

Under the present conditions the reduction of the nitro group is not simply a regular catalytic hydrogenation. Actual experimental studies directed toward the determination of the mechanism of these reductions are limited [12,13]. As indicated by the data in Table 2, a series of nitro compounds with various functional groups can be reduced to corresponding amino compounds in good yields with polyamide Pd(0) as the catalyst, and the rate of reduction of nitro functions is enhanced by electron-donating groups. In the series of the s u b s t i t u t e d n i t r o t o l u e n e s , m- a n d pnitrotoluene are reduced faster than o-nitrotoluene. We found a similar order of reactivity in the series of both the nitrophenols and nitroacetophenones. Aliphatic nitro corn-

Rate of Conversion hydrogenation (%) (ml/min) Nitrobenzene 4.0 100 o-Nitrotoluene 5.0 100 m-Nitrotoluene 5.1 100 p-Nitrotoluene 5.3 100 o-Nitrophenol 3.8 96 m-Nitrophenol 4.0 98 p-Nitrophenol 4.1 98 o-Nitroacetophenone 2.9 90 m-Nitroacetophenone 4.1 91 o-Nitrobenzaldehyde 4.1 91 p-Nitroaniline 3.1 92 Nitrosobenzene 3.9 75 l-Nitro-l-hexene 1.5 53 2-Nitrobutane 1.2 70

Run Reactant

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

pounds, on the other hand, are reduced with more difficulty using our catalyst.

III. Reductions catalyzed by the monomeric complex

194 TABLE 3 Rate of hydrogenation with polyamide Pd(0) and rnonomeric Pd(0)[bipy] at 25°C and 1 atm of hydrogen, 10 ml methanol Run Reactant

Rate of hydrogenation (ml/min) Polyamide Pd(0)

1 2 3 3 5 6 7 8 9

Styrene 13 Methyl methacrylate 7.8 Allyl alcohol 6.3 1-Hexene 6.0 Nitrobenzene 4.0 o-Nitrotoluene 5.0 o-Nitrophenol 3.8 o-Nitroacetophenone 2.9 m-Nitroacetophenone 4.1

Pd(0)[bipy] 2.5 2.2 2.2 2.1 1.8 2.2 1.8 1.7 2.0

polymeric surface. In contrast, non-polymersupported catalysts aggregate and form a catalytically inactive state. The polymer matrix plays an important role in the catalytic processes because the motion of the macromolecular segments provides the useful possibility of conveniently varying the contact time between catalyst and substrate, thus allowing sequential reactions to be performed. In the case of the polymer catalyst, the heterogeneous mechanism seems to operate with substrate molecules becoming adsorbed onto the internal surfaces of the supports and then undergroing reaction in this adsorbed state. The processes of the physical adsorption of a sorbate in the pores of a sorbent and the swelling of the latter occur simultaneously, and it is impossible to separate them. Therefore, the reaction involves simultaneous diffusion and chemical transformation of a substrate in the solid catalyst. We visualize that the surface of a polymer matrix can be divided arbitrarily into area sections beyond which a catalyst may not migrate due to matrix restrictions. The area sections are assumed to be randomly occupied and the metals which are supported on the polymers behave as a second phase. This, we

suggest, is how metal catalysts are "site isolated" on polymeric supports. From a comparison of the data for styrene and 1-hexene with both polymeric and nonpolymeric palladium catalysts, it was obvious that the rate of hydrogenation of styrene with the polymeric catalyst is six times faster than that with the non-polymeric catalyst under identical conditions. Comparing the rate of styrene and 1-hexene, in the case of the polymeric catalyst, the former is twice as rapid as the latter. In contrast, such differences were not observed with the corresponding homogeneous catalyst. We may summarize the data, therefore, by saying that the polymer matrix increases catalytic activity, stereoselectivity and regioselectivity. 1 V. Effect of reaction conditions on the rate of hydrogenation IV. 1. Concentration of base The rate of hydrogenation depends on not only the character of the catalyst but also on reaction conditions. To probe the requirements of the catalyst system, two control ex-

65

,

4-

.

,

c

~3 I

g2

0

0.5

1.0

1,5

Concentrot;on (M)

Fig. 1. Effect of base concentration on the reduction of styrene with polyamide palladium(II) at 25°C and 1 atm of hydrogen.

195

perimental factors lead to the following observations: (1) In the absence of any palladium complex and only with NaOH as the base, no product formation was detected. (ii) No reaction was catalized by the Pd(II) complex in the absence of base. It is evident that base is necessary for polyamide Pd(II) to be reduced to Pd(0); in other words, alkali is a reducing catalyst for polyamide palladium(II) in a H 2 atmosphere. To choose an ideal concentration in which palladium(II) is reduced completely, both reduction of Pd(II) and hydrogenation of olefins with Pd(0) are carried out in the same apparatus. As illustrated in Fig. 1, the optimum concentration of NaOH is about 0.25 M. In order to prove the effect of the concentration of base on the rate of hydrogenation, polyamide Pd(II) was reduced first, then isolated. Afterward, a number of hydrogenations were carried out with various concentrations of alkali and with the same polyamide Pd(0). From Fig. 2, it can be seen that base is unnecessary for the hydrogenation of olefins and nitro compounds; on the contrary, hydrogenation must be carried out in a neutral environment with polymer Pd(0) complex in the present experiment in order to achieve the

highest rate. Basic conditions, however, are required to reduce Pd(II) to Pd(0), where Pd(0) is more stable [18].

IV.2. Effect of solvents on the rate of hydrogenation All the reactions described previously were carried out in methanol, but the effect of solvents on the course of the reactions has been studied in a limited number of cases [19-211. The interaction of a solvent with a polymer is a complicated process. Macromolecules will absorb significant quantities of solvents, swell in volume and change their physical state and conformation. The processes of diffusion, absorption and swelling of polymeric matrices occur simultaneously. The diffusion mechanism with polymeric catalysts is quite different from the known simple bimolecular processes of homogeneous reactions. The nature of the solvent influences the long-range flexibility of polymers, and the swollen polymer segments change their physical state. This contributes to the rate of the catalytic reaction. As described previously [9], the solvent plays two major roles: one is swelling polymer segments, the other is to serve as a medium for chemical transformation of a substrate. The results in Table 4 indicate that a linear aliphatic alcohol is more favorable as a reac-

15

-£

TABLE 4

-'~o

Effect of solvents on the rate of hydrogenation at 250(_` and 1 atm of hydrogen

£

Run

Solvent

Rate of hydrogenation (ml/min)

3 4 5 6 7 8 9

Methanol Ethanol 2-Propanol Butanol 1-Pentanol Benzylalcohol a-Phenethyl alcohol Cyclohexanol 2-Methyl-cyclohexanol

3.3 2.5 2.0 1.7 1.4 1.1 0.8 0.4 0.35

2 £s

0.5

1.0 Concentrotlon

!.5 (M)

Fig. 2. Effect of base concentration on the hydrogenation of olefins and nitro compounds at 25°C and 1 atm: O: styrene; ~ : methyl methacrylate; A: 1-hexene; + : o-nitrotoluene; O: nitrobenzene; In: p-nitroaniline.

196

tion medium than are either aromatic and cyclic aliphatic alcohols. In a series of aliphatic alcohols, the rate of hydrogenation decreases on increasing the molecular weight because molecules move with difficulty in these solvents. On the other hand, larger molecules are more subject to diffusional restrictions within the matrix, particularly in the vicinity of many of the active sites.

V. Lifetime and stability of the polyamide Pd complex In the present experiment, the total amount of catalyst was reduced by 1-2% (depending on the separatory procedures) and the amount of palladium was reduced by 0.5% after five consecutive uses. There was no significant difference in activity after a number of reactions for the same kind of reactions. The reaction solution itself was not an active catalyst for hydrogenation after removal of the catalyst, so leaching was not sufficient in and of itself to account for the observed decreased activity on re-use of the catalyst. We found that centrifugation was a very efficient method for recovery of the catalyst. If it was separated by filter, some catalyst powder passed through a filter paper or clogged in the pores of a fritted glass so as to cause catalyst loss. Bipyridine (or the bipyridine unit of the polymers) is an efficient chelating ligand and forms strong complexes with many different metals. Thus, we prepared a (polyamide palladium) solution for atomic absorption analysis, in order to confirm the validity of the statement above. The procedure was as follows: to boiling concentrated HC1 acid weighed polyamide palladium was added, and the complex in the solvent was boiled for ten minutes. The metallic ions were leached from the ligands and dissolved in the acid solvent, but the polymer matrix was insoluble in the solvent. It was centrifuged from the resulting

solution which contained residual Pd ions. The polyamide matrix can be re-used for incorporating new metals after separation from the acidic solution. This indicates that, once formed, a polymer complex with metal ions becomes a part of the macromolecular structure. In general, polymer catalysts are very stable. In fact, the polyamide palladium was used for over 30 successive catalytic hydrogenation reactions with some catalytic activity in the present experiment.

EXPERIMENTAL All elemental analyses were performed by Galbraith Microanalyses Laboratory. Infrared spectra were obtained on a Nicolet 20 DX FTIR, nuclear magnetic resonance spectra on a Varian Associates CFT-20 N M R spectrometer, GLC analysis on a Hewlett-Packard 5710A gas chromatograph. All solvents and organic compounds were distilled before use. Differential scanning calorimetry (DSC) was performed on a Perkin-Elmer DSC-2C 3600 Data Station Thermoanalyzer. 2,2'-Bipyridine-4,4'-dicarboxylic acid and its acid chloride were prepared by the method of Bos [23]. Pd[bipy]C12 was prepared by procedure of Livingstone [24].

L Polycondensation of 2,2'-bipyridine-4,4'-diacid chloride with 2,6-diaminopyridine 2,2'-Bipyridine-4,4'-diacid chloride, 1.4 g (5 mmol), was added to 10 ml of dry dimethylformamide (DMF) and 6 ml of N-methylpyrrolidone in a three-neck, 50 ml round-bottom flask fitted with a condenser, nitrogen inlet and funnel with stopper; then it was stirred under a slow flow of dry N 2. The mixture which contained 0.55 g (5 mmol) of 2,6-diaminopyridine, 1 g (10 mmol) of triethylamine and 5 ml of DMF, was introduced to

197 the funnel; afterward the mixture was dropped from the funnel to the flask over a period of 30 minutes. The reaction mixture was held for 1 h at 0°C, 1 h at 5°C, and 5 h at room temperature. The final mixture was poured into methyl butyl ether to precipitate a lightbrown solid, which was collected by filtration, extracted with acetone overnight and dried in vacuo. The viscosity of a diluted polymer solution in dimethylsulfoxide (DMSO) was 0.17 at 35°C (inherent viscosity). A doublestep thermal decomposition (DSC) occurs in the range 508-640 K. In other words, the temperature of use of the polymer should be below 508 K.

II. Complexation of polyamide with palladium dichloride Palladium dichloride (0.55 g) was dissolved in 5 ml of T H F in a 50 ml round-bottom flask; then polyamide (0.5 g) suspended in T H F (10 ml) was added to the flask. The mixture was heated and refluxed for 1 h; afterward it was held for 10 h at 30°C, and for 24 h at room temperature. At the end of the reaction, the color of the solution changed from red brown to light yellow. The resulting insoluble complex, whose color was light brown, was filtered and thoroughly washed with THF, benzene, and ethyl acetate, and finally dried in vacuo. Elemental analysis for palladium of the polymer indicates a Pd content of 19.67%. The thermal decomposition of polyamide Pd consists of four stages: the first one starts from 565 K, the second from 610 K, the third from 625 K and the fourth from 640 K. An endothermic peak in the range 397-448 K might be attributed to partial breaking of the coordination of the bond N - P d . From a comparison of the DSC curves for both polymer and polymer Pd, it seems that the metal incorporation in the polymer chains results in an increased thermal stability of the material.

IlL Reduction of polyamide palladium(II) complex Polyamide palladium dichloride (1 g) was added to 20 ml of 0.25 M NaOH methanolic solution; then it was hydrogenated for 3 h at 25°C. The black polymer was filtered through fritted glass and washed thoroughly with methanol, ethanol, and ethyl acetate.

IV. Example of hydrogenation of a nitro compound Nitrobenzene (1.23 g, 10 mmol) and 150 mg of polymer palladium(0) complex (0.28 mmol Pd) were added to 10 ml of methanol in a 100 ml round-bottom flask. The flask was attached to a 1 atm hydrogenation apparatus, the system purged with hydrogen, and the mixture stirred under 1 atm of hydrogen. The hydrogen consumed was measured as a function of time. The reaction was completed to form the corresponding aniline, after 30 mmol hydrogen uptake. The catalyst was separated by centrifugation, and the solvent removed by distillation at reduced pressure. The product was identified by NMR, IR and GLC spectra. The yield and the products ratio were calculated from the relative peak areas in the GLC spectra, which indicated 100% conversion.

ACKNOWLEDGMENT This work has been supported by the National Science Foundation (DMR 8103100); the authors are grateful for this support. The authors also acknowledge, with gratitude, many helpful discussions with Keda Zhang.

REFERENCES 1 K. Kurita and R.L. Williams, Synthesis and characterization of soluble polyamides from various bipyridyl diamines, Amer. Chem. Soc., Div. Org. Coat. Plast. Chem., Pap., 33(1) (1973) 177-183.

198 2 R. Chapurlat and E. Kuntz, Complex forming polymers, U.S. Patent 3810888, to Rhone-Poulence S.A., 1974; Chem. Abstr., 83 (1975) 59801d. 3 Hitachi, Ltd., Jpn. Kokai Tokkyo Koho, Ruthenium-polymer coordination compound, Japanese Patent 8,230,727, 1982; Chem. Abstr., 97 (1982) 93042f. Hitachi, Ltd. Jpn. Kokai Tokkyo Koho, Japanese Patent 8,230,728, 1982; Chem. Abstr., 97 (1982) 93030a. 4 R.J. Card and D.C. Neckers, Preparation of polymer-bound bipyridine and some of its transition metal complexes, J. Amer. Chem. Soc., 99 (1977) 7733. 5 R.J. Card and D.C. Neckers, (Polystyrylbipyridine) (tetracarbonyl) tungsten. An active, reusable heterogeneous catalyst for metathesis of internal olefins, J. Amer. Chem. Soc., 100 (1978) 6635. 6 R.J. Card and D.C. Neckers, [Poly(styryl)bipyridine]palladium(0)-catalyzed isomerization of quadricyclene, J. Org. Chem., 43 (1978) 2958. 7 R.J. Card and D.C. Neckers, Poly(styryl)bipyridine: Synthesis and formation of transition-metal complexes and some of their physical, chemical, and catalytic properties, Inorg. Chem., 9 (1978) 2345. 8 R.J. Card and D.C. Neckers, [Poly(styryl)bipyridine]palladium complexes as heterogeneous catalysts for hydrogenation of alkenes and alkynes, J. Org. Chem., 44 (1979) 1095. 9 Keda Zhang and D.C. Neckers, Diaminobipyridine-TDI polyureas: Synthesis, metal complexes, and catalytic activity, J. Polym. Sci., Polym. Chem. Ed., 21 (1983) 3115. 10 C.U. Pittman, Jr., Catalysis by polymer-supported transition metal complexes, in: P. Hodge and D.C. Sherrington (Eds.), Polymer-Supported Reactions in Organic Synthesis, Wiley-Interscience, New York, 1980, pp. 249-259. 11 C. Carlini and G. Sbrena, Catalysis by transition metal derivative bound to structurally ordered polymers, in: C.E. Carraher, Jr., J.E. Sheats and C.U. Pittman, Jr. (Eds.), Advances in Organometallic and Inorganic Polymer Science, Marcel Dekker, Inc., New York, 1982, p. 323. 12 G. Brieger and T. Nestrick, Catalytic transfer hydrogenation, Chem. Rev., 74 (1974) 567.

13 D.C. Bailey and S.H. Langer, Immobilized transition-metal carbonyls and related catalysts, Chem. Rev., 81 (1981) 110. 14 R.H. Grubbs, C. Gibbons, L.C. Kroll, W.D. Bonds and C.H. Brubaker, Jr., Activation of homogeneous catalysts, J. Amer. Chem. Soc., 95 (1973) 2373. 15 W.D. Bonds, C.H. Brubaker, Jr., E.S. Chandrasekaran, C. Gibbons, R.H. Grubbs and L.C. Kroll, Polystyrene attached titanocene species, J. Amer. Chem. Soc., 97 (1975) 2128. 16 E.S. Chandrasekaran, R.H. Grubbs and C.H. Brubaker, Jr., Polymer-supported organometallic compounds of titanium, zirconium and hafnium as hydrogenation catalysts, J. Organomet. Chem., 120 (1976) 49. 17 R.H. Grubbs, C.P. Lau, R. Cukien and C.H. Brubaker, Jr., Polymer attached metallocenes. Evidence for site osalation, J. Amer. Chem. Soc., 99 (1977) 4517. 18 J. Tsuyi (Ed.), Organic Synthesis with Palladium Compounds, Springer Verlag, Berlin, 1980, Chap. II, p. 2. 19 R.H. Grubbs and L.C. Kroll, Catalytic reduction of olefins with a polymer-supported rhodium(I) catalyst, J. Amer. Chem. Soc., 93 (1971) 3062. 20 B.M. Trost and R.W. Warner, Macrocyclization via an isomerization at high concentrations promoted by palladium templates, J. Amer. Chem. Soc., 104 (1982) 6112. 21 P.N. Rylander, Solvents in catalytic hydrogenation, H. Jones (Ed.), Catalysis in Organic Syntheses, Academic Press, New York, 1980, p. 155. 22 R.J. Card and D.C. Neckers, (Polystyrylbipyridine)palladium(0): Heterogeneous catalyst for the mild hydrogenation of dienes to monoemes, Isr. J. Chem., 17 (1978) 269. 23 K.D. Bos, J.G. Kraaijkamp and J.G. Nattes, Improved synthesis of 4,4'-disubstituted-2,2'-bipyridines, Synth. Commun., 9(6) (1979) 497. 24 S.E. Livingstone and B. Wheelahan, Complexes of 2,2'-bipyridine and 1.10-phenanthraline with bivalent Pd and Pt having covalencies greater than four, Austr. J. Chem., 17(2) (1964) 219.