THE EFFECT OF SODIUM SALTS ON THE CATALYTIC ACTIVITY OF Ni(Ii) COMPLEXES IN OXIDATION PROCESSES L. A. Mosolova,

L. I. Matienko,

and Z. K. Maizus

UDC 546.33-38:541.49:546.74:542.943

Nickel(II) compounds catalyze the oxidation of ethylbenzene (EB), giving ~-phenylethyl hydroperoxide (PhEH) as the principal oxidation product [i, 2]. The degree of EB conversion achieved in this process is relatively low, reaction being inhibited by modification of the catalyst under the action of phenol (Ph), one of the reaction products [2J. The attempt has been made in the present work to increase the catalytic efficiency of nickel acetylacetonate [Ni(acac)2], using sodium stearate (NaSt) as a promoter. The fact that fixed-valence metallic salts can affect the catalytic activity of transition-metal compounds has been established in [3]. EXPERIMENTAL Ethylbenzene was oxidized by 02 at 120~ The content of EB and its oxidation products was determined by GLC in a glass column chromatograph equipped with a flame-ionization detector, ~-cyanoethylmethylsilicone supported on Gas-chrom G serving as the mobile phase. The PhEH content of the oxidation products was also determined icdometrically. The electronic spectra were obtained with a Specord UV-VIS spectrometer, working at 20~ DISCUSSION OF RESULTS It can be seen from the kinetic curves for buildu p of the products from EB oxidation on the Ni(acac)2 catalyst (Fig. i) that NaSt markedly increased both the PhEH yield and the length of the induction period for Ph formation. Since the Ph concentration in a mixture of NaSt and Ph in O2-free EB was not affected by heating the mixture to 120~ for 4 h, it could be concluded that the inhibition of Ph formation was not due to interaction of the latter with the NaSt. The buildup of methylphenylcarbinol (MPhC) and acetophenone (AP) was less affected by the presence of NaSt. Measured as the sum of the rates of accumulation of all of the reaction products, the initial rate of EB oxidation increased with increasing concentration of t~e added NaSt (Fig. 2). Increasing the NaSt concentration als0 markedly increased the degree of EB Conversion, comparison being with the Ni(acac) 2 -catalyzed oxidation in systems free of NaSt (curve i) where reaction essehtially ceased at 7-8% conversion. Comparison of the amount of EB consumed (ARH) with the amount of PhEH, MPhC, AP, and Ph formed showed the four latter compounds to account for 85-90% of the hydrocarbon reacted (Table i). The formation of styrene and its decomposition products would account for the fact that the balance was less than perfect. The added NaSt also affected the selectivity of the EB oxidation. The ratio of the PhEH concentration to the sum of the concentrations of all of the reaction products was 1~gher in systems containing small amounts of NaSt than in systems containing Ni(acac) 2 alone (Fig. 3). The selectivity remained high, even at a high degree of EB conversion, in systems containing N~St, the situation there being different from that met in systems containing Ni(acac)= alone where the selectivity fell off as oxidation proceeds. It can be assumed that the promoting action of the NaSt is due to its ability to complex with the Ni(acac) 2, thereby reducing the concentration of the Ni(acac)2--phenol complexes which are responsible for inhibiting the EB oxidation. The formation of complexes of KSt with MnSt2 has been demonstrated in [4]. It is also known that adducts can be formed between Co, Ni, Cu N'N'-ethylenebis(salicylideneiminates) and fixed-valance metal salts [5, 6]. The formation of complexes of Ni(acac)2 with NaSt has been confirmed by the study of electronic spectra. The UV spectrum of Ni(acac)2 shows a band at 300 nm which is associated with acetylInstitute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1760-1765, August, 1978. Original article submitted April 7, 1977.

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9 1979 Plenum Publishing Corporation

*

o,z u

Y

0

0

~

10

0,~

2O -

30

~0

o

80 3'

~qe

o,z s

70

15

Time, h Fig. 1

zo

2,~

0

r

l

~

I0

I

15

tO

Time, h Fig. 2

Fig. i. The oxidation of ethylbenzene, catalyzed by Ni(acac)= (3,10 -3 mole/liter) (1-4), by Ni(acac) 2 + NaSt (9.8.i0 -~ mole/liter) (1'-4'), and by Ni(acac)2 + NaSt (1.62.10 -= mole/liter) (1"-4"): i, i', i") Kinetic curves for hydroperoxide buildup; 2, 2', 2") for phenol buildup; 3, 3 v, 3") for acetophenone buildup; 4, 4', 4") for methylphenylcarbinol buildup. Figo 2. Kinetic curves for the buildup of all products in EB oxidation in systems containing 3,10 -3 mole/liter Ni(acac) 2 and NaSt at the following concentrations, mole/liter: l) without addition; 2) 4.9.10-~; 3) 9.8"i0-~; 4) 3.26.10-3; 5) 8.15~ 6) 1.62"10 -2 . acetonate absorption in the metal complex (Fig. 4, curve i). Addition of the NaSt markedly reduced the absorption maximum (curve 2), the ligand being displaced from the Ni coordination sphere in forming the NaSt complex. The processes in question here are similar to those responsible for displacement of the acetylacetonate ligand from the Co coordination sphere during complexing of tert-butyl hydroperoxide with Co(acac) 2 [7]. Spectrum 2 of Fig. 4 was obtained in EB, NaSt being only slightly soluble in paraffin hydrocarbons. The addition of phenol to solutions of Ni(acac)= in decane also altered the Ni(acac)= spectrum, the alterations being similar to those resulting from the introduction of NaSt into the system (curve 3). The band appearing here at 270 nm was that arising from the uncoordinated ligand. It is clear that the complexing of Ni(acac) 2 with NaSt or phenol results in the displacement of a single ligand from the Ni coordination sphere, the band at 300 nm failing to disappear completely as it does when CH3COOH is added to a solution of Ni(acac) 2 in n-decane (curve 4). Acetic acid acts in a similar manner when taken in excess with respect to the Ni(acac)2. The alteration in the Ni(acac) 2 spectrum resulting from the addition of NaSt to the system cannot be due to reaction between the Ni(acac)= and NaSt with the formation of N a ( a c a c ) and a mixed-ligand Ni(acac)St complex whose catalytic activity is higher than that of Ni(acac)= itself. We have shown that ethylbenzene will undergo oxidation in systems containing Ni(acac)2 and HSt at half the concentration of the latter, just as it does in systems conatining Ni(acac) 2 alone, and this despite the fact that the conditions are such as to favor formation of a Ni(acac)St complex. Actually the observed alteration in the Ni(acac)2 spectrum resulting from the addition of NaSt could be due to NaSt penetration into the Ni(acac)2 coordination sphere with formation of a [Ni(acac)NaSt][acac] complex. The kinetic data suggest the formation of Ni(acac)2 complexes with NaSt during the EB oxidation. Determined from the curves of Fig. 2, Wox, the initial rate of EB oxidation, at first increases with rising NaSt concentration and then becomes constant (Fig. 5). Such

1539

TABLE i. Comparison of the Amount of Ethylbenzene Consumed, A[RH], with the Total Amount of Products Formed in the Oxidation of Ethylbenzene at 120~ in Systems Containing [Ni(acac) 2] = 3-i0 -3 mole/liter and [NeSt] = 9.8.10 -4 mole/ liter.

Time, h

t 0,22 0,22

A ['RH], mole/liter

E products, mole/liter

I

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314 Io,4o

710115

J

0,50'[0,43 0,70 0,75 0,44 0,49 0,59 0,69

0,34

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80! 60 ~

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-

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40 0

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8

I 12

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20

Time, h Fig. 3. Variation of the selectivity of the oxidation of ethylbenzene to hydroperoxide with the degree of conversion, in systems containing (mole/ liter): i) 3.10 -3 Ni(acac) 2; 2) 3.10 -3 Ni(acac) 2 and 9.8-i0 -~ NaSt; 3) 3-10 -3 Ni(aCac) 2 and 3.26. 10 -8 NaSt; 4) 3-10 -3 Ni(acac) 2 and 1.62.10 -2 NaSt; 5) 3-10 -3 Ni(acac) 2 and 3.10 -a DMFA. relations would be consistent with the supposition of Ni(acac) 2 complexing with the NaSt. The value of tf, the length of the phenol induction period, is similarly dependent on the NaSt concentration, increasing as this concentration rises. Modification of the catalyst in the Ni(acac) a-catalyzed EB oxidation leads to the appearance of Ni(acac) 2-phenol complexes on which phenol is formed through hydroperoxide breakdown [2]. It is possible that any added NaSt will complex with the Ni(acac) 2, reducing the concentration of the Ni(acac) 2--phenol complex and the phenol concentration in the system. The addition of NaSt reduces the rate of the Ni(acac) 2-catalyzed breakdown of the PhEH. This suggests the formation of Ni(acac) 2--NaSt complexes which promote hydroperoxide decomposition, but at a rate lower than that met with Ni(acac) 2 itself. The role played by donor--transitional-metal compound complexes in the Ni(acac) 2-catalyzed ethylbenzene oxidation was confirmed in experiments with D~iFA, a compound which readily coordinates with other metal compounds [8]. Oxidation of EB in the presence of Ni(acac) 2 and DMFA is similar in every respect to oxidation in Ni(acac) 2 systems containing NaSt (cf. Figs. 3 and 5). Since the Ni(acac) 2--NaSt complexes are more effective catalysts for the EB oxidation than theNi(acac) a--DMFA complexes, the initial oxidation rates and tf values were higher for the NeSt systems. The rate of hydroperoxide breakdown under the combined action of Ni(acac)= and NaSt being lower than the rate of breakdown in systems containing Ni(acac) 2 alone, it would be reasonable to suppose that the increase in initial oxidation rate resulting from NaSt addition was due to participation of the Ni(acac) 2--NaSt couple in chain initiation for the RH-02 reaction. The fact is, however, that inhibitor-method measurements show [2] Wo, the rate of radical formation over the initial period of oxidation at 120~ to be 1.6-10 -6 mole/liter. sec ([NaSt] = 9.8.10 -~ mole/liter), a value some three times greater than that obtained for EB oxidation in systems containing Ni(acac) 2 alone (Wo = 6"10 -7 mole/liter.sec [2]).

1540

[DIV~A]-103, rnole/h~er 10

4

Z0

30

40 I m

.J F

0

I

,

25O

.|

3OO

~,nM

~ [NaSt] 9103, mole Biter

Fig. 4

Fig. 5

Fig. 4. Electronic spectra of: i) a solution containing only Ni(acac)2 (5.10 -5 mole/liter); 2, 3, 4) Ni(acac) 2 solutions (5.10 -5 mole/liter) containing (mole/liter): 2) 1"10 -3 NaSt (solvent, ethylbenzene); 3) 5.5" 10 -4 phenol (solvent, n-decane; spectrum relative to that of phenol at the same concentration); 4) 7"10 -4 CH3COOH (solvent, n-decane). Fig. 5. Variation of the initial rate of ethylbenzene oxidation (i, i'), and tf, the time required for forming 0.01 mole/liter of phenol in the oxidation process (2, 2'), in systems containing Ni(acac) 2 and NaSt (unprimed numbers), or Ni(acac) 2 and DMFA (primed numbers), with [NaSt] (i and 2), or [DMFA] (i' and 2'). Free-radical generation through the reaction of hydrocarbons with the 02 of transitionmetal coordination compounds has been postulated in many instances of autooxidation catalyzed by polyvalent metal compounds [9-11]. The fact that the Ni(acac) 2--NaSt complex coordinates more readily with 02 than does Ni(acac) 2 itself may explain the high rate of chain initiation in the oxidation of EB in systems containing both Ni(acac) 2 and NaSt. For example, it is known that dipolar ligands such as pyridine, DMFA, and DMSO add to Co(II) bis(salicylidene)ethylenediamine, thereby markedly promoting the complexing of the latter with 02 [12]. Cumyl hydroperoxide radicals behave in a similar fashion in the Rh(PPh3)3COCI coordination sphere [13]. It is clear that the inclusion of electron-donor molecules in the transition-metal coordination sphere reduces the oxidation--reduction poten~ tial of the complex. Thus it can be shown that there is a correlation between the ease of oxygen adduct formation and the ease of oxidation of Co(II) to Co(III) in cobalt chelates with electron-donor ligands [14]. In considering the activities of chelates in oxidation-reduction reactions, it is, however, necessary to take account not only of the oxidation-reduction potentials themselves but also of the complex structure, a factor which can adversely affect substrate coordination. It is possible that the reduction in Ni(acac)2 activity in hydroperoxide decomposition resulting from NaSt introduction traces back to t~e formation of an NaSt complex which sets up steric hindrances to hydroperoxide reactions. CONCLUSIONS i. Addition of sodium stearate to a system in which ethylbenzene is being catalyzed under the action of a nickel acetylacetonate catalyst will increase the initial reaction rate, the selectivity of the reaction, and the degree of ethylbenzene conversion. 2. UV spectroscopy has been used to demonstrate the formation of a Ni(acac]2--NaSt complex which catalyzes the ethylbenzene oxidation. 3. Addition of NaSt reduces the rate of the Ni(acac)2-catalyzed ~-phenylethyl hydroperoxide radical breakdown. The resulting increase in the rate of ethylbenzene oxidation is due to an increase in the rate of chain initiation~ LITERATURE CITED i. 2. 3.

L. I. Matienko and Z. K. Maizus, Izv. Akad. Nauk SSSR, Ser. Khim., 1971, 1207. L. I. Matienko and Z. K. Maizus, Kinet. Katal., 15, 317 (1974). D. I. Metelitsa, Usp. Khim., 41, 1737 (1972).

1541

4. 5. 6. 7. 8. 9. i0. ii. 12. 13. 14.

V . M . Gol'dberg and L. K. Obukhova, Neftekhimiya, 2, 294 (1964). C. Floriani and F. Calderazzo, J. Chem. Soc. Chem. Communs., 1973, 387. M. D. Hobday and T. D. Smith, J. Chem. Soc. A, 1971, 1453, 3424. R. B. Svitych, A. L. Buchachenko, O. P. Yablonskii, N. N. Rzhevskaya, V. A. Belyaev, and A. A. Petukhov, Kinet. Katal., 17, 73 (1976). V. A. Alekseevskii, A. G. Muftakhov, and G. A. Eliseeva, Zh. Neorg. Khim., 20, 2392 (1975). N. Uri, Nature, 117, 1177 (1956). Yu. D. Norikov, E. A. Blumberg, and L. V. Salukvadze, in: Problems of Kinetics and Catalysis [in Russian], Nauka (1975), pp. 16, 150. E. V. Abel, J. M. Pratt, R. Whelan, and P. J. Wilkinson, J. Am. Chem. Soc., 96, 7119 (1974). E. Bayer and P. Shretsman, in: Structure and Bonding [Russian translation], Mir (1969), p. 304. L. D. Tyutchenkova, V. G. Vinogradova, and Z. K. Maizus, Izv. Akad. Nauk SSSR, Ser. Khim., 1978, 773. M. J. Carter, D. P. Rillema, and F. Basolo, J. Am. Chem. Soc., 9-6, 392 (1974).

MIXED HYDRIDE COMPLEXES OF RHODIUM WITH TRIPHENYLPHOSPHINE AND d-~-METHYLBENZYLAMINE AS CATALYSTS FOR HOMOGENEOUS ASYMMETRIC HYDROGENATION REACTIONS L. M. Koroleva, E. V. Borisov, V. K. Latov, and V. M. Belikov

UDC 541.49:546.97:541.128:542.941

We have shown in our earlier work [i] that asymmetrical catalysts for homogeneous olefin hydrogenation can be obtained by substituting one of the triarylphosphine ligands in the coordination sphere of tris(triarylphosphine) complexes of Rh(1) by a chiral amine molecule. In the case of d-~-methylbenzylamine (MBA) substitution, an increase in the mole ratio reduces the initial rate of reaction (vo), and increases the degree of asymmetric synthesis of phenylalanine (p), in the hydrogenation of ~-acetaminoeinnamic acid (AAA) and its methyl ester (MEAA). It is clear that these changes result from the formation of mixed complexes under increasing excess MBA. It therefore seemed of interest to study the formation of such complexes in the reactions of MBA with RhCI(PPh3) 3 and its hydride form, H2RhCI(PPh3)3. Proton magnetic resonance methods, electronic spectroscopy in the visible region of the spectrum, and circular dichroism (CD) were used here to follow the interaction of MBA with RhCI(PPh3)3 in Ar and H=. EXPERIMENTAL We synthesized RhCI(PPh3)3 by the procedure of [2]: amine was a Schuchardt product, [~]D 25 + 30.3 ~ .

mp 154-157~

The d-~-methylbenzyl-

The methyl ester of ~-acetaminocinnamic acid (MFTLA) was prepared by the method of [3], esterifying the azlactone of ~-acetaminocinnamic acid (AAA) with methanol in the presence of NaOH; mp 120-121.5~ The AAA was synthesized, in turn, by the method of [4], hydrolyzing the azlactone of AAA with an aqueous solution of acetone; mp 192-193~ The electronic spectra were obtained with a Specord UV-VIS spectrometer, working in 1:2 benzene-methanol solutions with the concentration of the RhCI(PPh3)3 fixed at 1.25.10 -4 M, and the concentration of the MBA varied from 0 to 6.25,10 -4 M. The mixed complex solutions followed the Lambert--Beer law over the entire concentration range. The CD spectra were obtained with an R.J. Mark III Dichrograph, working again in 1:2 benzene-methanol solutions with the RhCI(PPh3)3 concentration fixed at 4.10 -4 M. The PMR spectra were obtained with a Hitachi--Perkin--Elmer R-20 spectrograph (60 MHz), working in chlorobenzene solutions with the Institute of Heteroorganic Compounds, Academy of Sciences of the USSR, Moscow. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1765-1770, August, 1978. Original article submitted April 26, 1977.

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9 1979 Plenum Publishing Corporation