Reductive carbonylation of nitrobenzene catalyzed by heteropolyanion-modified palladium

Journal of Molecular Catalysis, 72 (1992) 37-46 M2822

37

Reductive carbonylation of nitrobenzene catalyzed by heteropolyanion-modified palladium Y. Izumi*, Y. Satoh, H. Kondoh and K. Urabe Department Chikusa-kw,

of Applied Chemistry, Nagoya 464 (Japan.

School of Engineering,

Nagoya

University,

Furo-cho,

(Received July 8, 1991; accepted November 20, 1991)

Abstract heteropolyanioncontainingMO or V as addendaatoms shows an appreciablepromotingeffect on the catalysis of PdClz in the reductive carbonylation of

The Keggin-type

nitrobenzene to form methyliV-phenylcarbamate in the presence of methanol in a moderately polar solvent such as l,Z-dhnethoxyethane at 130-170 “C under an initial CO pressure of 11-41 atm. The catalyst was recovered and reused to give a total Pd turnover of 168. Partially reduced heteropolyanion was effective for modifying Pd(II), presumably as a basic macroligand.

Introduction The Keggin-type heteropolyacids containing molybdenum or vanadium exhibit efficient cocatalytic redox behavior in the PdS04-catalyzed Wacker reaction [ 11, where heteropolyacids act as oxidants for Pd(0) in the redox cycle. Since the deposition of metallic Pd is barely observed in the course of the reaction under appropriate conditions, Pd(0) must be stabilized by the heteropolyacid to form a soluble species, probably through coordination with heteropolyanion [ 11. Heteropolyanion, therefore, appears to behave as a ligand of Pd(II) and Pd(O), like chloride ion in the conventional Wacker catalyst system. Such information about the affinity of a heteropolyanion for Pd leads us to believe that heteropolyanions may also exert certain electronic and steric modifying effects on several transition metals as a large polyoxometalate ligand. As expected, it was pointed out that heteropolyanion showed certain modifying effects on palladium and rhodium catalysts in the semihydrogenation of acetylenic compounds [ 2 ] and substrate-selective hydrogenation of alkenes [ 31. Recently several types of heteropoly compounds which incorporate different’ kinds of transition metals have been applied as catalysts for various types of oxidation and reduction reactions; these compounds include Mn(II)- or Ru(III)-substituted heteropolyanion [ 4, 5 1, cationic Rh(I) complexes with a heteropoly anion as a counter-anion [S], and heteropolyanion-supported Ir(I) complexes [ 7 I. *Author to whom correspondence should be addressed.

0304-5102/92/$5.00

0 1992 - Elsevier Sequoia. All rights reserved

38

Fig. 1. The Keggin structure of PMo~~O~~~-anion.

We have recently reported [8] a novel efficient catalyst system which consists of PdCla combined with a Keggin-type heteropolyanion containing MO or V as addenda atoms; Pig. 1 shows the structure of a typical PMo120403anion, in which the central tetrahedron of PO4 is surrounded by twelve octahedra of MOO,. This PdCla-heteropolyanion system catalyzes the following carbonylation reaction to give methyl IV-phenylcarbamate in high yield. PhNOa + 3C0 + CH,OH *

PhNHC0&H3 + 2COa

The reductive carbonylation of nitrobenzene is an important reaction for the manufacture of polyurethane products, since this route is expected to replace the existing phosgene process. Several efficient catalyst systems already proposed relevant to the reductive carbonylation of nitroaromatic compounds include PdC12 coupled with Lewis acids [9] or bulky bidentate tertiary amines such as 1 JO-phenanthrolme [lo], and Ru and Rh complexes 1111. The present paper deals with the catalytic features of the PdClaheteropolyanion system in the reductive carbonylation of nitrobenzene in detail, since even such a completely inorganic system can exhibit good catalytic performance almost comparable to systems containing organic ligands. In addition, the role of heteropolyanion as a macroligand (cu.1 run diameter) to modify Pd will be discussed based on some experimental observations on the interaction between heteropolyanion and palladium.

Experimental

Materials All organic and inorganic reagents other than heteropolyacids were commercial products of highest purity (> 99%). Nitrobenzene and solvents were dried and distilled prior to use. The Keggin-type heteropolyacids H4SrW12040and H4PVMo11040were HaPMo120.10,H3~12040, Hd=fGh, of commercially available purity. The other mix-coordinated heteropolyacids

39

and Li2HSPV12036 were prepared according to the literature ([ 12) and [ 13 J, respectively). Neutral lithium salts of heteropolyacids were obtained by reacting the parent acids with stoichiometric amounts of lithium carbonate. Heteropolyacids and their salts Were dehydrated at 150-200 “C for 3 h in vacua before use. Reaction procedure The reductive carbonylation of nitrobenzene was conducted in a Pyrex glass tube (50 ml) inserted in a stainless steel autoclave (60 ml). In a typical experiment, PdCla (0.1 mmol), anhydrous heteropolyacid or its salt (0.05 mmol), nitrobenzene (10 mmol), methanol (20 mmol) and solvent (40 mmol) were charged. The autoclave was fh-t pressurized with CO to 10 atm, and then the gas was purged; this procedure was repeated four times to eliminate oxygen from the reaction system. The autoclave was finally pressurized with CO to 41 atm, and set in an oil bath held at 170 “C. The reaction mixture was agitated for 3 h; the maximum pressure during the reaction was 51-52 atm. After the reaction, the autoclave was promptly quenched with cold water. Product analysis The reaction product, methyl N-phenylcarbamate, was determined by GLC (SE 30, 3 m cohmm), and a sample isolated was confirmed by ‘H NMR and IR measurements(S 3.80 (s, 3H, OCH,), 6.50-7.90 (br, m, 6H, PhNHC=O); 1750 cm-’ (>C=O)). Aniline was the major byproduct, the other being a trace amount of N,N’-diphenylurea.

Results

and discussion

Catalytic features of heteropol~animwnodi~d PdC12 The catalytic efficiency of heteropolyanion-modified PdC12in the reductive carbonylation of nitrobenzene in the presence of methanol is shown in Table 1 compared with the effects of other modi6ers (promoters or ligands). Only a trace amount of methyl N-phenylcarbamate was obtained with PdCla alone, and heteropolyacid itself showed no catalytic activity in the absence of PdC12.This observation means that Pd was essential for inducing the reaction. PdCla worked as an efficient catal@ only when coupled with a heteropoly compound, ammonia or an organic amine. In particular, Moor V-containing heteropolyacids or their Li salts exhibited a remarkable promoting effect, affording a total Pd turnover of 84-99. In addition, the PdC12-H3PMo,z040catalyst recovered from the product and solvent by distillation in vcccuo could be reused, and almost the same conversion and urethane selectivity were obtained; at this stage the total Pd turnover reached 168. The use of hydrous H,PMo1204,. 2OH,O in place of the anhydrous compound lowered the urethane selectivity, but not drastically (from 97% to 94%).

40 TABLE 1 Reductive carbonylationof nitrobenzenecatalyzedby PdClz-modifiersystemsin dimethoxyethane solvent” Catalyst system

PhN02 conv. (%)

PdCl, H3PMo1z040

trace 0

PdClrH3PMo,2040 PdC12-Li3PMo,z040 PdCl,-H,SiMolzOlo PdCl,-&Siio,2040 PdClz-H3pw,204o PdC1ZH4SlW,,z040 PdClzH4PVMo,,040

84 74 40 19 2 trace >99

PdCl,-FeCl, PdClZH2Mo04 PdQ--CNH&Mo,Oz4

trace 6 9

Urethane select. (%)

97 99 96 99

92 91 82

PdCl,-NHsb PdCl,pyzidineb PdCl,-piperszineb PdClz-pyrazineb PdCl,l ,10-phenanthrolineb PdCl,l,7-phenanthrolineb

46 42 11 0 32 14

>99 95 98

1PdCph~1z1F’F&= W@henMIPF& Gd

10 99

87 92

>99 99

‘PhN02 0.01 mol, PdC12/modifier/solvent@ME)/MeOH/PhN02= l/0.5/400/266/100 (molarratio), 170 “C, CO 41 ah, 3 h. bPd/motier = l/3 (molar ratio). Cohen= l,lO-phenanthroline. dMeOH solvent, Pd/MeOH/PhN02=l/4938/250 (molar ratio), 150 “C.

Considering that a neutral Li salt, Li3PMo12040, which is reasonably soluble in 1,2-dimethoxyethane solvent, exhibited a high promoting effect almost comparable to the parent free acid, and that both molybdic acid as a simple molybdate compound and ammonium paramolybdate as a tiopolg molybdate compound showed quite weak effects, not the strongly acidic nature of heteropolyacid but rather the heteropoly structure of MOoxometallate must be responsible for the activation of Pd. Indeed, it was confirmed through IR measurements that H3PMo12040 retained its Keggin structure in the course of the reaction, though changed into a reduced form; the absorption band of the edge-sharing MO-O-MO bond (790 cm-‘) was splitting, but those of the central P-O bond (1080 cm-‘) and the terminal Mo=O bond (965 cm-‘), which are both characteristic of the Keggin structure*, did not change after the reaction. *The absorption band of the P-O bond of a simple phosphate anion (P043-) appears at 1010 cm-l, and those of the Mo=O bonds of a simpIe molybdate anion (MoO,~-) and an isopolymolybdate anion (Mo,O~~‘-) at 820-900 and 84&920 cm-‘, respectively.

41

1 , 10-Phenanthroline being designated as a suitable ligand for Pd(.II) [lo] was not as effective as H3PMo1a04,,, Li3PMor204,, and H4PVMo11040 under the present reaction conditions, whereas it gave higher turnover of Pd (248) in methanol solvent: In contrast to MO- or V-containing heteropolyacid, heteropolytungstic acid was quite ineffectual as a modifier. The change in promoting effect on Pd with mix-coordination of heteropolyacid is depicted in Figs. 2 and 3 with respect to Mo-W and Mo-V binary systems, together with the literature data [ 141 of the maximum redox potential of mix-coordinated heteropolyacids. In the Mo-W system, catalytic activity increased monotonically with increasing number of MO atoms in the Keggin unit. For the Mo-V system, the highest activity was obtained when a single V atom was substituted for one of twelve MO atoms. As a qualitative understanding the promoting ability of heteropoly acid appears to be related with its maximum redox potential, that is, the more reducible heteropolyacid exerts a more effective modification on Pd. In addition to PdCla, other Pd(II) compounds containing chlorine were effective for the present reductive carbonylation, and neither chlorine-free Pd(II) compounds nor metallic Pd were active (Table 2). These facts indicate that the active Pd species may be Pd(II) with chloride ligands. Indeed, an inactive catalyst system of Pd(N03)2-H3PMo12040 became active when nBu_,NCl was added as a chloride donor, though not so active as the PdC12-H3PMo12040 system. Other transition metal chlorides such as RhCla and RuC13 could be also activated by heteropolyanion, but were much less active than PdCla.

l(

H3PMOxW12-~040~ =-

H3+~P",~~l2-x~40~

*-

Fig. 2. Effect of mix-coordination of Mo-W

heteropolyacid system on the catalytic activity of heteropolyanion-modd PdCl& PhN02 0.01 mol, PdClz/modifler/solvent(DME)/MeOH/PhNOz= 1/0.5/400/200/100(molar ratio), 170 “C, CO 41 atm, 3 h. F’ig. 3. Effect of mix-coordination of M-V heteropolyacid system on the catalytic activity of heteropolyanion-modd PdCl,; PhN02 0.01 mol, PdCl,/modifier/solvent(DME)/MeOH/PhNOz= 1/0.5/400/200/100(molar ratio), 150 “C, CO 41 atm, 3 h, 12-vanadophosphoric acid: WM'V12033.

42 TABLE 2 H,PMo,,Ol,,-modified transition metal catalysis in reductive carbonylation of nitrobenaene* Transition metal compound

PhNOa conv. W)

PdCl, Pd(OAc)a PdSO* Pd(NWz Pd(NH&Clab Pd(PPhs),Cl, PdlC Pd black RhClg

a4 none none none 21 5 none none 12 3

Urethane select. (%) 97

94 >99

99 99

‘Dimethoxyethane solvent, PhNOa= 0.01 mol, PdClz/HaPMo120&PhN02/MeOH/DME = l/0.5/ 100/200/400(molar ratio), 170 “C, CO 41 atm, 3 h. b150 “C, 3 h.

TABLE 3 Solvent effect in reductive carbonylation of nitrobenxene catalyzed by PdCla-H3PMo1a04,,* Solvent

PhNOa conv. (46)

Urethane select. (%)

acetone 2-butanone 2-pentanone methyl isopropyl ketone 2-hexanone cyclohexanone tetrahydrofuran dimethoxyethane diisopropyl ether methyl n-caproate dimethylsulfoxide dimethylfommmide n-octane 1,2dichloroethane MeOHb

78 86 71 74 75 57 70 50 34 70

90

6 4 58 18

75 89 89 86 30 90 95 93 92 20 97 96

‘PhNOa = 0.01 mol, PdC1z/HaPMoIz040/PhN0.JMeOH/solvent = 1/0.5/100/200/400 (molar ratio), 150 “C, CO 41 atm, 3 h. b170 “C, 3 h.

Moderately polar solvents that can dissolve heteropoly compounds are suitable for the present heteropolyanion-modified PdC12 catalyst system, i.e. ethers and chlorinated hydrocarbons (Table 3). Ketones were inadequate as solvents because they reacted with methanol to form ketals in acidic reaction

43

media. Strongly polar solvent such as DMSO and DMF were unsuitable for the present catalyst system, suggesting that polar solvent may prevent the necessary interaction between Pd and heteropolyanion. Methanol, therefore, was not very suitable as a solvent for the present catalyst system, although its use is the most desirable from the practical viewpoint. The change in the conversion of nitrobenzene was measured for the PdC12-H3PMo12040 catalyst system at 130-l 70 “C under an initial CO pressure of 11-41 atm, on varying the concentrations of nitrobenzene (0.4-3.2 M) and PdClz (0.005-0.01 in terms of Pd/PhNOa molar ratio at constant Pd/ H3PMo1z04,,molar ratio (2.0)). The reaction rate, calculated from the PhN02 conversion data based on the empirical zero-order dependence of the conversion on reaction time, increased in proportion to both CO pressure and Pd concentration, but showed 0.43-order dependence on the concentration of nitrobenzene. The apparent activation energy was estimated as 38 kJ mol-’ from the temperature dependence of the rate (Fig. 4); this value is somewhat lower thanthat reported for the Pd(H)-1 , 10-phenanthroline complex catalyst system (60 kJ mol- ‘) [lob]. Several reaction mechanisms have been proposed for the Pd-catalyzed reductive carbonylation of nitrobenzene, including possible intermediates such as nitrene [lla, 151, clusters [15] and metallacycles [lob, lla, 161. It was recently reported that a stable metallacyclic complex (Pd(ophen)(C,H,)(C,NO,)) was first isolated as a possible intermediate of the reductive carbonylation of nitrobenzene from the reaction of nitrobenzene with CO using the Pd(II)-1 ,lO-phenanthroline system [ 171. The reaction mechanism for the Pd(II)-heteropolyanion catalyst system is still unclear. However, since heteropolyanions with higher redox potentials caused greater modifications in the Pd catalysis (Figs. 2 and 3), it seems necessary to examine whether the carbonylation reaction proceeds via a Pd(0) + Pd(I1) redox cycle similar to that involved in the Wacker reaction using heteropolyacid as an oxidant for Pd(0) in the presence of oxygen [ 11. If the redox cycle occurs, nitrobenzene must be the oxidant for the reduced heteropolyanion formed from the reaction with Pd(0) because oxygen is absent in this carbonylation reaction. In this context, it was confirmed from a separate experiment that nitrobenzene was not reduced and remained unchanged when reacted with a reduced heteropolyacid (HsPMov~Movr9040) in the absence of PdCla at 170 “C under a CO pressure of 41 atm; i.e. nitrobenzene could not act as an oxidant for the reduced heteropolyanion. This fact, together with the fact that the Pd(0)-H3PMo12040 system could not catalyze the reaction (Table 2), suggests that a redox mechanism is unlikely for the carbonylation of nitrobenzene catalyzed by the Pd(II)-heteropolyanion system. Role of the h&eropol~aniun The change in catalytic activity of the heteropolyanion-modified PdCla system on varying the molar ratio of heteropolyanion to PdClz is illustrated in Fig. 5. A neutral heteropoly salt of L&PMo~~O~~was used as an anion

5.0

6.0 :\,_ 2.2

2.3 l/T

2.4 x

lo3

2.5

1 2.10

K

Fig. 4. Effect of temperature on the rate of carbonylation of nitrobenzene catalyzed by PdC1,-H,PMo120,0; PhN02 0.01 mol, CO 41 atm, 3 h, PdClz/H~PMo,z04,/solvent(DME)/MeOH/ PhN02 = 1/0.5/400/200/100 (molar ratio). Fig. 5. Effect of Li,PMo,,O,,/PdClz ratio on the catalytic activity of PdCl,-Li,PMo,,O,,,; PhN02 0.01 mol, 150 “C, CO 41 atm, 3 h, PdClJ&PMo,~O,,Jsolvent(DME)/MeOH/F%NO~= l/0.5/400/ 200/100 (molar ratio).

source to eliminate the effect of acidity change. The maximum activity was attained at a ratio of unity. This fact suggests that a 1:l molecular interaction may exist between the heteropolyanion and PdC12, although no definite complexes of Pd containing heteropolyanion could be isolated, and the liberation of LiCl was not observed due to its moderate solubility in 1,2dimethoxyethane. However, several heteropolyanion complexes with Rh(I) and Ix(I) are reported to be isolable [6, 181. Another concern is the electronic state of the heteropolyanion during modification of Pd. It was observed that heteropolyanion was reduced, showing ‘heteropoly blue’, in the course of the reaction. In order to clarify the situation, the dependence of activity on the extent of reduction of heteropolyanion was examined using heteropolymolybdic acids in different degrees of reduction as starting modiiers, obtained by reacting H~PMo~~O~~with L-ascorbic acid (Pig. 6). The catalytic activity of Pd increased with the extent of the reduction of the heteropolyanion until each anion was reduced with three electrons. This observation indicates that the heteropolymolybdate anion effective for modifying Pd was PMo~,Mo~~~_~O~~(~+~)- (na3). Such a reduced, highly basic heteropolyanion could interact with Pd effectively. Thus the efficient modifying effects caused by heteropolyacids with high redox potentials (Pigs. 1 and 2) reflect their great reducibility.

Conclusion

Heteropoly compounds with high redox potentials effectively modify PdClz to promote the reductive carbonylation of nitrobenzene, affording N-

45

a

60I

0

I

2

CL-Ascorbic

I

I

4

I

I

6

acidl/CH3PMo120401

Effect of H3PMor204e reduction with ascorbic acid on the catalytic activity of PdC1~HaPMo120,,; PhNOa 0.01 mol, 170 “C, CO 41 atm, 3 h, PdClz/HsPMo,zO,o/sohnt(DME)/ MeOH/PhNOa= 1/0.5/400/200/100 (molar ratio).

Fig. 6.

TABLE 4 PdCla-catalyzed reductive carbonylation of nitrobenzene to N-phenylisocyanatea Catalyst system

PdCla PdCIa-FeCla PdCla-HaPMo,eO,, PdCla-H,PVMo,,O,e PdCla-HaMoO, PdClz-VOS04 PdCk@H&Mo& lPd~J%),JC~z-H,PMo,zo,, IPd@henMIPFd2b

PhNOa conv.

(w

Isocyanate select. W)

trace trace 4 11 1 6 7 19 trace

87 74 0 0 0 95 0

‘PhN02= 0.01 mol; solvent: dimethoxyethane;PdCl,/modifier/PhN02/solv. = 1/l/100/400(molar ratio); 170 “C, CO 41 atm, 3 h. bphen= 1,lo-phenanthroline.

phenylcarbamate in the presence of methanol_ The modification appears to be strongly associated with the interaction of the reduced heteropolyanion with Pd(II) as an inorganic basic macroligand. An additional experimental result shown in Table 4 suggests that heteropolyanion may be useful not only for urethane formation, but also for the reductive carbonylation of nitroaromatic compounds into the corresponding isocyanates in the absence of alcohol.

46

Acknowledgements

This research was supported by a Grant-in-Aid for Scientifk Research from the Ministry of Education, Science and Culture, Japan.

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