Homogeneous hydrogenation of aromatic hydrocarbons with Rh(acac)-(P(OPh)3)2 catalyst

Journal

ofMolecular

Catalysis,

18 (1983)

193

193 - 195

Letter

Homogeneous (P(OPh),),

D. PIETA, Institute

Hydrogenation

of Aromatic

Hydrocarbons

with Rh(acac)-

Catalyst

A. M. TRZECIAK of Chemistry,

and J. J. ZIbLKOWSKI

University

of WrocFaw,

50-383

Wroctaw

(Poland)

Homogeneous, metal complex-catalyzed hydrogenation reactions of aromatic hydrocarbons have been a subject of particular interest for many years mainly because the mechanism is still not fully defined [ 11. One of the crucial and practical questions connected with hydrogenation of arenes is the metal complex catalyst design. In this paper we describe the catalytic properties of rhodium(I) acetylacetonato-bis-triphenylphosphite in the hydrogenation of arenes.

Results and discussion Rhodium(I) acetylacetonato-bis-triphenylphosphite complex has been recently prepared by the reaction of Rh(acac)(CO)* with P(OPh), [Z]. 31P NMR studies of this complex have shown that in the system containing complex and free triphenylphosphite practically no phosphite ligand exchange is observed [ 21. This is in contrast to the complex Rh(acac)(CO)(PPh,) which exhibits very fast exchange of phosphine [3]. The relatively high stability of the coordination sphere of the rhodium(I) acetylacetonato-bistriphenylphosphite complex, and its reactivity towards carbon monoxide and hydrogen indicates this complex as a potential catalyst for hydrogenation reactions. In our preliminary studies on hydroformylation reactions of olefins, we have found that when benzene was used as a solvent some amounts of cyclohexane were observed in the reaction products [4]. This fact stimulated us to more detailed studies of the hydrogenation of aromatic hydrocarbons. The results presented in Table 1 show that rhodium(I) acetylacetonato-bistriphenylphosphite is a quite active and very selective hydrogenation catalyst. In each experiment, hydrogenated substrate was observed as practically the only reaction product. Catalytic activity of the rhodium complex significantly decreases when excess free triphenylphosphite is added to the reaction mixture. The catalyst is not sensitive to air but its activity increases when stored in aqueous atmoOElsevier

Sequoia/Printed

in the Netherlands

194

TABLE

1

Hydrogenation triphenylphosphite

of arenes [ Rh]

Hydrocarbon

and olefins catalyzed at 80 “C and 10 atm

by rhodium(I) acetylacetonato-bisHz initial pressure

Catalyst/ hydrocarbon ratio

0.4 0.7 1.5 0.7 5.9 5.8 6.2 6.3 6.2 6.0

1 1 1 1 1 1 1 1 1 1

toluene

9.1

1 : 1030

6.5

methylcyclohexane

pyridine

9.1

1 : 680

6.0

piperidine

nitrobenzene

9.1

1 : 1000

cyclohexene

7.2 7.2

1 : 1250 1 : 1250

0.33 0.33

cyclohexane cyclohexane

(32.8) (8.1)

9.1

1 : 1100 1 : 600

5.5

cyclohexane

(95.6)

1 : 1100 1 :600

4.5

1 : 1000 1 : 615

6.5

benzene

cyclohexene benzene

+

1,3-cyclohexadiene + toluene

9.1

hexene-1 toluene

9.1

+

: 28000 : : : : : : : : :

16000 7500 16000 1900 1900 1800 1800 1800 1850

Reaction duration

Cycles/ Rh atom/h

mm01 Rh catalyst (X 103)

Product (% yield)a

(h)

10 11 11 10 13 4 4 4 4 4

15.0

cyclohexane cyclohexane cyclohexane cyclohexane cyclohexane cyclohexane cyclohexane cyclohexane cyclohexane cyclohexane

aniline

105 294 182 29rJ 28 21 9c 25d 59e 21f

(3.7) (18.4) (25.3) (1.8) (18.9) (4.3) (2.1) (5.6) (12.8) (4.5) (2.3)

3.5 11.3s

(10.0)

(50.0)

33.3s 1333 331c 191 _

aSelectivity practically 100%. bWith addition of 0.035 mmol triphenylphosphite. CWith addition of 0.0129 mmol triphenylphosphite. dCatalyst stored in argon atmosphere. eCatalyst stored in aqueous atmosphere. fWith addition of 0.0107 mmol triphenylphosphite;cataIyst sHydrogenation products determined by ‘H NMR.

cyclohexane (100) methylcyclohexane

(2.5)

hexane (94.6) methylcyclohexane

(< 1)

stored

127 3.3 145

in aqueous

-

atmosphere.

sphere (Table 1, d, e, f). From the results presented in Table 1, we conclude that the rhodium complex under investigation looks promising as a structurally well-determined catalyst for mechanistic studies in model reactions. Further investigations are in progress aimed at elucidating the reaction mechanism as well as the formation of catalytically active intermediates, and explaining the role of free phosphite ligand in this catalytic system.

195

Experimental Materials

Rh(acac)(P(OPh)& was obtained according to the synthesis procedure reported recently [2]. Analytical grade chemicals were generally used without additional purification. Procedure

for the hydrogenation

reaction

Hydrogenation reaction procedures were carried out in a steel reactor of total volume 20 ml equipped with a manometer thermostat, magnetic stirrer and a gas inlet/outlet system. The reactor was flushed before use with hydrogen and then loaded with catalyst (in some experiments also with triphenylphosphite) placed in a small Teflon vessel. The liquid substrates (hydrocarbons) were injected into the reactor using the syringe technique. Then the reactor was filled with hydrogen to the require pressure and heated to the desired temperature. The reaction was not considered to have begun until the temperature had reached the desired level, and all reaction times were measured from this point. The reaction course was followed by recording HZ-pressure changes and by the reaction products as determined by GLC and NMR methods. Physical measurements

Gas chromatography was performed on a Chromatograf 504 instrument equipped with a flame ionization detector. A 2 m column filled with 10% tricresol phosphate on Chromosorb was used. NMR spectra were recorded at ambient temperature on a Tesla 80 MHz instrument.

References 1 E. L. Muetterties and J. R. Blecke, Act. Chem. Res., 12 (1979) 327 and references therein. 2 A. M. Trzeciak and J. J. ZiSkowski, Znorg. Chim. Acta Lett., 64 (1982) L267. 3 A. M. Trzeciak, M. Jon and J. J. ZiSkowski, React. Kinet. Catal. Lett., (in press). 4 D. Pieta, A. M. Trzeciak and J. J. Zi61_kowski, unpublished results.