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Comparison of the relative efficiency of peroxomolybdenum complexes as oxidants of the alcoholic function
Journal of Molecular Catalysis, 83 (1993) 95-105 Elsevier Science Publishers B.V., Amsterdam
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Comparison of the relative efficiency of peroxomolybdenum complexes as oxidants of the alcoholic function Sandro Campestrini*, Fulvio Di Furia*, Pietro Rossi and Alessandro Torboli Centro CNR di Studio sui Meccanismi di Reazioni Organiche, Dipartimento di Chimica Organica, Universitb di Padova, Via Marzolo 1,35131 Padua (Italy)
Giovanni Valle Centro CNR di Studio sui Biopolimeri, Dipartimento di Chimica Organica, Universith di Padova, Via Marzolo I,35131 Padua (Italy) (Received October 26,1992; accepted March 9,1993)
Abstract The oxidative ability toward the alcoholic function of a series of peroxomolybdenum complexes has been evaluated by measuring the oxidation rates and the nature of the products in the oxidation of two model alcohols, i.e. cyclohexanol and I-octanol. Included in the series are two complexes, i.e. [MoO(O~)~(C~H&OO)](Bu),N+ and [2M00(02)2(MeOH),(MoO(O~)O)~ (MeO),]‘[ (Bu), N’12 which have been synthesized for the fiit time and characterized by Xray analysis. On the basis of the reactivity data and of the structural features of the oxidants, the requisites that allow the peroxomolybdenum complexes to act as effective and selective oxidants of the alcoholic function have been selected. The necessary requisite appears to be the anionic nature of the oxidant. An important role is also played by the availability of a free or releasable coordination site on the complexes. Such a site should be the second apical position of the species which in all cases displays pentagonal pyramid or bipyramid geometry. Some preliminary mechanistic considerations based on the results obtained are presented envisaging the coordination of the substrate as an alkoxo derivative prior to the oxidation step. Key words: alcoholic function; oxidation; peroxomolybdenum complexes
Introduction The versatility, of the transition metal peroxocomplexes, MO ( 02)mLn, where M=Ti(IV), V(V), Mo(V1) and W(VI), and L can be either a neutral or a negatively charged ligand, as oxidants of organic substrates, is well documented [ 11. Among the various transformations, an important one is the oxidation of the alcoholic function carried out by MO(VI) peroxocomplexes [ 2lo]. The first example of such reactivity has been reported by Mares et al. [ 21 *Corresponding authors.
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0 1993 - Elsevier Science Publishers B.V. All rights reserved.
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x=H
x=p-OCH, (Bu).,N+
x=p-NH, X=lTl-Cl
x=p-NO,
Scheme 1.
who showed that complex 1 in polar media oxidizes secondary alcohols to ketones either under stoichiometric or catalytic (H,O,) conditions (Scheme 1) . Subsequently, we prepared the analogous complexes 2 and 3 [ 71. Taking advantage of their much higher lipophilicity, as compared with that of 1, we were able to run our oxidation experiments in dichloroethane where 1 is almost insoluble. Under such conditions, 2 and 3 proved to be considerably more reactive than 1,also oxidizing primary alcohols to aldehydes without any overoxidation to carboxylic acids [ 7,9,10]. We noticed that 3 is far more effective than 2 [ 71. At the same time, we also explored the synthetic scope of 3 in the oxidation of a number of multifunctional molecules containing the OH group [8] thus establishing that indeed 3 is not only fairly active but also rather selective. Other procedures for the oxidation of alcohols, based on the use of peroxomolybdenum derivatives, have been reported [ 4-61. A common feature of
S. Campestrini
et al. 1 J. Mol. Catal. 83 (1993) 95-105
97
these methods is the in situ formation of the oxidant by addition of hydrogen peroxide to a suitable molybdenum precursor. It is noteworthy that the peroxomolybdenum oxidants thus formed are anionic species. However, a comparison of the behavior of such systems with that of 2 and 3 reveals some important differences. In particular, anionic peroxomolybdenum species formed in situ show lower selectivity. As an example, primary alcohols are mainly oxidized to carboxylic acids [ 4-61. Finally, it should be mentioned that no information is available on the reactivity of neutral peroxomolybdenum complexes, such as 4 and 5, as oxidants of the alcoholic function. All these pieces of information, taken together, appear rather puzzling and therefore need to be rationalized. Unfortunately, the oxidation of alcohols has not been studied in great detail from a mechanistic point of view. Recently we reported on the oxidation of diols by 3 [ 111 providing evidence of an associative mechanism where the substrate can enter the coordination sphere of the peroxocomplex acting both as a monodentate or as a bidentate ligand. In the former situation, where the analogy with the simple oxidation of alcohols is rather strict, it has been suggested that the diol replaces the neutral tooth of the bidentate ligand of 3 (Scheme 2) with one of its two OH groups.
- kl k-1
+ +OdH H,O
Scheme 2.
In this paper we present results that provide more information on the general aspects of the simple oxidation of alcohols by a series of peroxomolybdenum derivatives. Namely, we investigated the relationships existing between the nature of the peroxomolybdenum oxidizing species and their oxidative behavior, in an effort to establish which features render some of these species effective and selective oxidants of alcohols. Our data point to a major role played by the anionic nature of the peroxomolybdenum complexes.
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Experimental
Materials Complexes 2, 3, 4 and 5 were obtained and purified ( [Oact.] > 95% ), iodometric titre) according to published procedures [ 7,12,13]. Cyclohexanol and 1-octanol were commercially available, high purity products (Aldrich), used as received. 1,2-Dichloroethane (DCE) was purified by distillation over P&k.
Preparation of complexes 6 NazMo0,*2H20 (3.36 g, 14 mmol) was dissolved in 10 ml of water, the acidity of the solution was adjusted at pH=2 with dilute HzS04, and 7 ml of H202 (36% w/v) were added (solution A). 15 mmol of the variously substituted benzoic acids were dissolved in 10 ml of an aqueous solution of tetrabutylammonium hydroxide 1.5 M (solution B). In an ice-cold bath, under vigorous magnetic stirring, solution B was added dropwise to solution A, maintaining the acidity of the media at pH = 2 with dilute H,SO,. The amorphous yellow precipitate was filtered off and washed first with water and then with E&O. The product was crystallized from DCE and dried under vacuum (0.1 mmHg ), in the presence of Pz05. Suitable crystals for X-ray analysis were obtained for complex 6a by slow crystallization in acetonitrile at - 18” C. The crystal system was obtained with an automatic Philips PW 1100 with standard software. Cell parameters: a= 1709.6 (2) pm, b= 1616.0 (2) pm, c= 1003.9 (2) pm, a~90.0” /3= 106.8”, y=90.0”; 2=4; space group P2,,,. The structure was solved with the Patterson method and refined to R=0.045 by using 3488 reflections with $5 70(F) (see Fig. 1).
Fig. 1. X-ray structure of complex 6a. Only non-hydrogen atoms are shown.
S. Campestrini et al. /J. Mol. Catal. 83 (1993) 95-105 Selected bond lengths (in pm) MO-O, 193.6(5) MO-Oa 191.4(5) MO-Os 190.6(5) MO-Ol 193.7(4) MO-O5 165.4(4) Selected bond angles (in degrees) 01-MO-O2 44.5(2) 03-MO-O1 44.4(3) OS-MO-O, 105.6(3) O,-MO-O, 102.5(3)
99
MO-O6 MO-OT G-01 G-06
208.8(3) 254.4(3) 124.5(5) 128.2(5)
07-MO-O1 O,-MO-O, O?-MO-O6 OS-MO-O,
76.6(2) 76.6(2) 153.7(2) 98.2(2)
Preparation of complex 7 In an ice-cold bath, under vigorous magnetic stirring, 10 ml of 1.5 M tetrabutylammonium hydroxide were added dropwise to solution A (see above ) , maintaining the acidity of the media at pH = 2 with dilute HzS04. The amorphous yellow precipitate was filtered off, and washed with the minimum amount of cold water (cu. 20 ml) and then with ether. After several attempts with various solvents, suitable crystals for X-ray analysis were obtained only by slow crystallixation at - 18 oC in methanol. Cell parameters: a = 2097.5 (2 ) pm, b=1085.6(2) pm, c=1241.5(2) pm, a!=90.0”, 8=105.05”, y=90.0”. Space group P2,,,. The structure was solved by the Patterson method and refined to R = 0.062 by using 3106 reflections with F> 70(F) (see Fig. 2). Procedures In a typical run, 5 ml of DCE containing 6.0 mmol of substrate were added to a DCE solution (10 ml) containing the oxidant (0.3 mmol) and the internal GC standard (n-hexadecane for cyclohexanone and n-tetradecane for octanal)
Fig. 2. X-ray structure of complex 7. Only non-hydrogen atoms are shown.
100 Selected bond lengths (in pm) Mot-O, 202.7 (8) MO,-O2 237.2(8) MO,-Os 193.9(8) MO,-O., 193.0(l) Mol-O6 198.4(7) MO*-O6 194.0(l) 153.0(2) 0,-O, Selected bond angles (in degrees) 03-Mol-O4 46.6(4) 105.4(5) 04-Mol-O7 05-Mol-O6 43.2(4) 169.9(4) 07-Mol-O2
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MO,-O7 Mo2-0, MO,-0, MO*--O,, Moz-O9 08-09
145.0(l) 170.1(8) 211.4(7) 193.5(9) 198.1(g) 193.9 (7) 144.0(l)
O,-MoI-OI 09-MO*-OS 08-Moz-O,,,
43.2(4) 81.2(3)
OS-OS
102.9(4)
in a glass reactor maintained at the appropriate temperature. Aliquots of the reaction mixture were withdrawn at various times, quenched with P ( Ph)s in excess, and the amount of the product determined by GC analysis on a Carbowax 20 M 10% (1.8 m column) on Chromosorb WAW-DMCS. Kinetic measurements were carried out under pseudo-first-order conditions by using an excess of the substrate over the oxidant, and monitoring the appearance of the product by GC. The pseudo-first-order rate constants were obtained from plots of In ( [Product] o3- [Product] t) us. time, linear up to 80% reaction. Results and discussion Together with the complexes mentioned in the Introduction, we have prepared, for comparison purposes, the novel complexes 6 and 7. Complexes 6 are representative of anionic peroxomolybdenum species containing a monodentate ligand whereas complex 7 is an acceptable model for anionic complexes not containing a carboxylato ligand. Clearly, a mononuclear complex without any organic ligand would have been preferable in order to keep the analogy among the various oxidants as close as possible. Unfortunately, any attempt to obtain crystals suitable for X-ray analysis, other than recrystallization from methanol which gives the tetrameric species 7, failed. Details on the procedures adopted for the synthesis and isolation of the complexes 6 and 7 are given in the Experimental section. Of all the peroxomolybdenum species employed in this study, the solid state structure, as determined by X-ray diffraction, is known either from previous work [7,13,14], complexes l-5, or from our own determination carried out within the framework of the present investigation. Details on the structures of the novel complexes 6 and 7 are reported in the Experimental section. We have also determined, under identical experimental conditions, both the nature and the yields of the products of the oxidation of two model alcohols, Le. cyclohexanol and 1-octanol by the peroxomolybdenum complexes. The corresponding oxidation rates and pseudofirst-order kinetic constants have also been obtained. All these data are pre-
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sented in Table 1. It may be noticed, in passing, that the reactivities of the primary and secondary alcohols are as already mentioned [ 4,6] rather similar; the reactivity ratio is about 4, in favor of the secondary alcohol. Even though careful examination of the structural features of the complexes prepared for the first time, e.g. 6 and 7, is of general interest, we shall discuss this topic elsewhere, limiting ourselves here to underlining those aspects which should be more closely related to the reactivity of the species, as provided by the data of Table 1. The usual caution in correlating solid state features with the behavior in solution must be exercised even though the solvent in which the reactivity has been measured is unlikely to produce severe modifications of the species owing to its low polarity and coordinating ability. Both neutral and anionic complexes and mono or diperoxo derivatives have been tested (see Scheme 1) . These differences among the molybdenum derivatives appear to have little effect on the geometry of the species. In all cases a pentagonal pyramid or bipyramid arrangement is observed. The bipyramid TABLE 1 Oxidation of 1-octanol and cyclohexanol by peroxomolybdenum complexes, in DCE, at 60°C” Substrate
Oxidant
cyclohexanol cyclohexanol cyclohexanol cyclohexanol cyclohexanol cyclohexanol cyclohexanol cyclohexanol cyclohexanol 1-0ctan01 1-0ctanol
1-0ctan01
1-octanol 1-octanol 1-octanol 1-octanol 1-0ctanol I-octanol
Product
Yield (% )b
klx 10’ (s-l)’
5
n.r.d
-
-
4 2 3
t?b 6c 6d 6e 4
cyclohexanone cyclohexanone cyclohexanone cyclohexanone cyclohexanone cyclohexanone cyclohexanone cyclohexanone n.r.d
35” 95 100 100 100 100 95 100 _e
3 0.2 46 50 57 82 37 23 -
2 3 6a 6b 6c 6d 6e 7
octanal octanal octanal o&anal o&anal o&anal octanal o&anal
90 100 100 100 85 95 98 100
0.3 11 12 14 20 9 6 17
Ba
‘Concentrations: substrate, 0.4 M; oxidant, 0.02 M. bBased on the oxidant. “From plots of ln( [Product],[Product],) us. time. dNo product formation after 24 h. *Extensive decomposition of the oxidant occurs.
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geometry occurs when the complexes are coordinatively saturated* e.g. l-3,6 and also 7. Other common features are the presence of an oxo-oxygen in the apical position and the preference of the peroxo oxygens and of the carboxylato oxygens, when present, for the equatorial ones. These observations suggest that the structural differences which might be responsible for the different oxidative behavior revealed by the data of Table 1 are those involving the second apical position. This would be in line with the results, mentioned in the Introduction, for the oxidation of diols. A first and rather obvious differentiation among the complexes can be therefore made by comparing those whose second apical position is free, with those in which it is occupied by a ligand. Subsequently, it may be useful to take into account the strength of the bond between the molybdenum atom and the ligand in order to have an indication of the feasibility of the substitution processes involving the apical ligand. Owing to the higher basicity and, very likely, nucleophilicity of nitrogen as compared with oxygen it may be expected that the neutral tooth of the bidentate picolinate-N-oxido ligand should be more easily removed than that of the picolinato ligand. Accordingly, the data of Table 1 indicate that 3 is 200 times more active than 2 in the oxidation of alcohols. In contrast, this line of reasoning could lead to the conclusion that complexes not containing any apical ligand at all should be superior oxidants of alcohols. This is only partly confirmed by the results of Table 1. In fact, it is observed that complexes 6, containing the monodentate carboxylato ligands are effective and selective oxidants of the two model alcohols but it may also be noticed that their reactivity is not greater than that of 3, whose apical position is occupied by the oxygen of the picolinate-N-oxido ligand. In contrast, inspection of the structural date reveals that the apical position in 6 is not completely free. In particular, the distance between the molybdenum atom and the carbonyl oxygen of the carboxylato group is 254 pm which indicates that some hindrance to the entering of a ligand may also occur in 6. However, the data of Table 1 reveal that the presence of a free or easily removable coordination site on the peroxocomplexes cannot be the only requisite that makes them effective and selective oxidants of the alcoholic function. In fact, if this were the crucial feature for such reactivity, complexes 4 and 5 should be, by far, the most efficient as their second apical position is absolutely free. In contrast, both the yields and the reaction rates observed when 4 and 5 are the oxidants of the two model alcohols indicate that they behave very poorly. Therefore, together with an accessible coordination site, there must be some other factor which is essential for the oxidation of alcohols. Our data rule out that such a factor is the presence of a carboxylato ligand. *The structure of 4, as such, which cannot he obtained as a crystalline material, is not available. However, that of the complex containing the water molecule has been determined [ 141. Such a water molecule present in the apical position is easily removed simply by heating under vacuum. A similar situation is found for complex 6, which has been used in this work after removal of the water molecule [ 131.
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Again, the behavior of complex 5 which is completely ineffective from an oxidative point of view militates against any unique role played by such ligands. The suggestion that the reactivity toward alcohols is due to the presence of a carboxylato ligand on the metal is also strongly in question, in view of the efficiency of complex 7, where only methoxo groups and methanol molecules are bound to the metal. Moreover, the presence of a bidentate ligand, as has been suggested [ 21, cannot be the key feature. Complexes 6, that contain only monodentate ligands, as already mentioned, are effective oxidants. Therefore we are left with only one possibility. The necessary requisite of peroxomolybdenum complexes as oxidants of alcohols, which is shared by all the species found to be active, is their anionic nature. Such a requisite is even more important than the ease of coordination of the substrate to the metal. Further evidence favoring such a conclusion is given by the oxidative behavior observed within the series of substituted carboxylato complexes 6. Such behavior is graphically shown in Fig. 3. Besides the observation that the oxidation rates may be nicely correlated via the Hammett equation with the Q values, the finding that the Hammett plot provides a p value of - 0.38 confirms that an increase of the electrodonating character of the ligand accelerates the oxidation rates. Again, one has to conclude that an increase of the anionic character of the oxidant results in an increase of its efficiency. To summarize the results presented so far: the most important feature of peroxomolybdenum complexes as oxidant of alcohols is their anionic character. The availability of a coordination site on molybdenum does play a role in determining the efficiency of the oxidation but it is a less stringent requisite.
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
(I
Fig. 3. Hammett plot for the oxidation of 1-octanol by complexes 6, in DCE, at 60°C.
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These conclusions have, of course, mechanistic significance which cannot be ignored in proposing a detailed mechanism for the oxidation of alcohols. On the other hand, in order to gain a better understanding of such a mechanism, more data are required and it is along this line that we are currently carrying on our investigation. It is, at any rate, tempting to suggest that the role of the anion is to ionize the alcohol substrates to the corresponding alkoxo derivatives which, inter aliu, should be better ligands than the parent compounds. This is equivalent to saying that in the oxidation of alcohols, a nucleophilic catalysis operates, the peroxocomplex being both the oxidant and the catalyst. Even though a direct test of this hypothesis should be possible, it must be recalled that such experiments are hampered by the extensive decomposition of the complexes in basic media. At any rate literature data provide clear evidence that, in protic media, under basic conditions, the oxidation of alcohols by peroxomolybdenum complexes formed in situ proceeds much faster than in neutral or acidic ones [ 31. The last point to be made here, on the basis of the data so far available, concerns the differing selectivities of the various oxidizing procedures. In particular, one may ask why, with the complexes employed here, the oxidation of primary alcohols stops at the level of aldehydes whereas with the other systems mentioned in the Introduction [ 4-61, the major products are carboxylic acids. A tentative rationale takes into account the possibility that also aldehydes, in order to be oxidized, have to coordinate to molybdenum, unless free-radical autoxidative processes occur. As it is very likely that carbonyl compounds behave as weaker ligands than the alkoxo ones, it may be observed that in all the anionic complexes examined here, some hindrance to the coordination to the second apical position does exist. This is not only true for complexes 2 and 3 but also for complex 6, owing to the proximity of the carbonyl oxygen of the carboxylato group and also for complex 7 due to its unique structure. Consequently, for these oxidants the coordination of the aldehyde is hampered, whereas in the system where the peroxomolybdenum species, whose structure is unknown, is formed in situ, such hindrance could be relatively unimportant thus allowing the coordination and hence the overoxidation of the aldehydes. Alternatively, one might envisage that, in the latter systems, owing to the presence of carbon-centered free radicals, the oxidation of the aldehydes to carboxylic acid is greatly facilitated.
Acknowledgments This research was carried out within the framework of Progetto Finalizzato Chimica Fine II of CNR. Financial support by MIJRST is also gratefully acknowledged.
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References 1
V. Conte and F. Di Furia, in G. Strukul (Ed.), Catalytic Oxidations with H,O,, Kluwer, Rotterdam, 1992, chap. 7. 2 SE. Jacobson, D.A. Muccigrosso and F. Mares, J. Org. Chem., 44 (1979) 921. 3 V. Come, F. Di Furia and G. Modena, J. Org. Chem., 53 (1988) 1665. 4 0. Bortolini, V. Conte, F. Di Furia and G. Modena, J. Org. Chem., 51 (1986) 2661. 5 B.M. Trost and Y. Masuyama, Tetrahedron Lett., 25 (1984) 173. 6 Y. Masuyama, M. Takahasi, Y. Kurusu, Tetrahedron I&t., 25 (1984) 4417. 7 0. Bortolini, S. Campestrini, F. Di Furia, G. Modena and G. Valle, J. Org. Chem., 52 (1987) 5467. 8 0. Bortolini, S. Campestrini, F. Di Furia and G. Modena, J. Org. Chem., 55 (1990) 3658. 9 S. Campestrini, F. Di Furia, G. Modena and F. Novello, in L.I. Simandi (Ed.), Dioxygen Activation and Homogeneous Catalytic Oxidation, Elsevier, Amsterdam, 1991, p. 375. 10 S. Campestrini, V. Conte, F. Di Furia and F. Novello, in M. Graziani and C.N.R. Rao (Eds.), Advances in Catalyst Design, World Scientific Singapore, 1991, p. 333. 11 S. Campestrini, F. Di Furia, G. Modena and F. Novello, J. Mol. Catal., 78 (1993) 159. 12 H. Mimoun, I. Seree De Roth and L. Sajus, Bull. Sot. Chim. France, (1969) 1481. 13 SE. Jacobson, R. Tang and F. Mares, Inorg. Chem., 17 (1978) 3055. 14 J.M. Le Carpentier, A. Mitschler, P. Weiss, Acta Crystallogr. B, 28 (1972) 1288.
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Comparison of the relative efficiency of peroxomolybdenum complexes as oxidants of the alcoholic function Sandro Campestrini*, Fulvio Di Furia*, Pietro Rossi and Alessandro Torboli Centro CNR di Studio sui Meccanismi di Reazioni Organiche, Dipartimento di Chimica Organica, Universitb di Padova, Via Marzolo 1,35131 Padua (Italy)
Giovanni Valle Centro CNR di Studio sui Biopolimeri, Dipartimento di Chimica Organica, Universith di Padova, Via Marzolo I,35131 Padua (Italy) (Received October 26,1992; accepted March 9,1993)
Abstract The oxidative ability toward the alcoholic function of a series of peroxomolybdenum complexes has been evaluated by measuring the oxidation rates and the nature of the products in the oxidation of two model alcohols, i.e. cyclohexanol and I-octanol. Included in the series are two complexes, i.e. [MoO(O~)~(C~H&OO)](Bu),N+ and [2M00(02)2(MeOH),(MoO(O~)O)~ (MeO),]‘[ (Bu), N’12 which have been synthesized for the fiit time and characterized by Xray analysis. On the basis of the reactivity data and of the structural features of the oxidants, the requisites that allow the peroxomolybdenum complexes to act as effective and selective oxidants of the alcoholic function have been selected. The necessary requisite appears to be the anionic nature of the oxidant. An important role is also played by the availability of a free or releasable coordination site on the complexes. Such a site should be the second apical position of the species which in all cases displays pentagonal pyramid or bipyramid geometry. Some preliminary mechanistic considerations based on the results obtained are presented envisaging the coordination of the substrate as an alkoxo derivative prior to the oxidation step. Key words: alcoholic function; oxidation; peroxomolybdenum complexes
Introduction The versatility, of the transition metal peroxocomplexes, MO ( 02)mLn, where M=Ti(IV), V(V), Mo(V1) and W(VI), and L can be either a neutral or a negatively charged ligand, as oxidants of organic substrates, is well documented [ 11. Among the various transformations, an important one is the oxidation of the alcoholic function carried out by MO(VI) peroxocomplexes [ 2lo]. The first example of such reactivity has been reported by Mares et al. [ 21 *Corresponding authors.
0304-5102/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved.
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x=H
x=p-OCH, (Bu).,N+
x=p-NH, X=lTl-Cl
x=p-NO,
Scheme 1.
who showed that complex 1 in polar media oxidizes secondary alcohols to ketones either under stoichiometric or catalytic (H,O,) conditions (Scheme 1) . Subsequently, we prepared the analogous complexes 2 and 3 [ 71. Taking advantage of their much higher lipophilicity, as compared with that of 1, we were able to run our oxidation experiments in dichloroethane where 1 is almost insoluble. Under such conditions, 2 and 3 proved to be considerably more reactive than 1,also oxidizing primary alcohols to aldehydes without any overoxidation to carboxylic acids [ 7,9,10]. We noticed that 3 is far more effective than 2 [ 71. At the same time, we also explored the synthetic scope of 3 in the oxidation of a number of multifunctional molecules containing the OH group [8] thus establishing that indeed 3 is not only fairly active but also rather selective. Other procedures for the oxidation of alcohols, based on the use of peroxomolybdenum derivatives, have been reported [ 4-61. A common feature of
S. Campestrini
et al. 1 J. Mol. Catal. 83 (1993) 95-105
97
these methods is the in situ formation of the oxidant by addition of hydrogen peroxide to a suitable molybdenum precursor. It is noteworthy that the peroxomolybdenum oxidants thus formed are anionic species. However, a comparison of the behavior of such systems with that of 2 and 3 reveals some important differences. In particular, anionic peroxomolybdenum species formed in situ show lower selectivity. As an example, primary alcohols are mainly oxidized to carboxylic acids [ 4-61. Finally, it should be mentioned that no information is available on the reactivity of neutral peroxomolybdenum complexes, such as 4 and 5, as oxidants of the alcoholic function. All these pieces of information, taken together, appear rather puzzling and therefore need to be rationalized. Unfortunately, the oxidation of alcohols has not been studied in great detail from a mechanistic point of view. Recently we reported on the oxidation of diols by 3 [ 111 providing evidence of an associative mechanism where the substrate can enter the coordination sphere of the peroxocomplex acting both as a monodentate or as a bidentate ligand. In the former situation, where the analogy with the simple oxidation of alcohols is rather strict, it has been suggested that the diol replaces the neutral tooth of the bidentate ligand of 3 (Scheme 2) with one of its two OH groups.
- kl k-1
+ +OdH H,O
Scheme 2.
In this paper we present results that provide more information on the general aspects of the simple oxidation of alcohols by a series of peroxomolybdenum derivatives. Namely, we investigated the relationships existing between the nature of the peroxomolybdenum oxidizing species and their oxidative behavior, in an effort to establish which features render some of these species effective and selective oxidants of alcohols. Our data point to a major role played by the anionic nature of the peroxomolybdenum complexes.
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Experimental
Materials Complexes 2, 3, 4 and 5 were obtained and purified ( [Oact.] > 95% ), iodometric titre) according to published procedures [ 7,12,13]. Cyclohexanol and 1-octanol were commercially available, high purity products (Aldrich), used as received. 1,2-Dichloroethane (DCE) was purified by distillation over P&k.
Preparation of complexes 6 NazMo0,*2H20 (3.36 g, 14 mmol) was dissolved in 10 ml of water, the acidity of the solution was adjusted at pH=2 with dilute HzS04, and 7 ml of H202 (36% w/v) were added (solution A). 15 mmol of the variously substituted benzoic acids were dissolved in 10 ml of an aqueous solution of tetrabutylammonium hydroxide 1.5 M (solution B). In an ice-cold bath, under vigorous magnetic stirring, solution B was added dropwise to solution A, maintaining the acidity of the media at pH = 2 with dilute H,SO,. The amorphous yellow precipitate was filtered off and washed first with water and then with E&O. The product was crystallized from DCE and dried under vacuum (0.1 mmHg ), in the presence of Pz05. Suitable crystals for X-ray analysis were obtained for complex 6a by slow crystallization in acetonitrile at - 18” C. The crystal system was obtained with an automatic Philips PW 1100 with standard software. Cell parameters: a= 1709.6 (2) pm, b= 1616.0 (2) pm, c= 1003.9 (2) pm, a~90.0” /3= 106.8”, y=90.0”; 2=4; space group P2,,,. The structure was solved with the Patterson method and refined to R=0.045 by using 3488 reflections with $5 70(F) (see Fig. 1).
Fig. 1. X-ray structure of complex 6a. Only non-hydrogen atoms are shown.
S. Campestrini et al. /J. Mol. Catal. 83 (1993) 95-105 Selected bond lengths (in pm) MO-O, 193.6(5) MO-Oa 191.4(5) MO-Os 190.6(5) MO-Ol 193.7(4) MO-O5 165.4(4) Selected bond angles (in degrees) 01-MO-O2 44.5(2) 03-MO-O1 44.4(3) OS-MO-O, 105.6(3) O,-MO-O, 102.5(3)
99
MO-O6 MO-OT G-01 G-06
208.8(3) 254.4(3) 124.5(5) 128.2(5)
07-MO-O1 O,-MO-O, O?-MO-O6 OS-MO-O,
76.6(2) 76.6(2) 153.7(2) 98.2(2)
Preparation of complex 7 In an ice-cold bath, under vigorous magnetic stirring, 10 ml of 1.5 M tetrabutylammonium hydroxide were added dropwise to solution A (see above ) , maintaining the acidity of the media at pH = 2 with dilute HzS04. The amorphous yellow precipitate was filtered off, and washed with the minimum amount of cold water (cu. 20 ml) and then with ether. After several attempts with various solvents, suitable crystals for X-ray analysis were obtained only by slow crystallixation at - 18 oC in methanol. Cell parameters: a = 2097.5 (2 ) pm, b=1085.6(2) pm, c=1241.5(2) pm, a!=90.0”, 8=105.05”, y=90.0”. Space group P2,,,. The structure was solved by the Patterson method and refined to R = 0.062 by using 3106 reflections with F> 70(F) (see Fig. 2). Procedures In a typical run, 5 ml of DCE containing 6.0 mmol of substrate were added to a DCE solution (10 ml) containing the oxidant (0.3 mmol) and the internal GC standard (n-hexadecane for cyclohexanone and n-tetradecane for octanal)
Fig. 2. X-ray structure of complex 7. Only non-hydrogen atoms are shown.
100 Selected bond lengths (in pm) Mot-O, 202.7 (8) MO,-O2 237.2(8) MO,-Os 193.9(8) MO,-O., 193.0(l) Mol-O6 198.4(7) MO*-O6 194.0(l) 153.0(2) 0,-O, Selected bond angles (in degrees) 03-Mol-O4 46.6(4) 105.4(5) 04-Mol-O7 05-Mol-O6 43.2(4) 169.9(4) 07-Mol-O2
S. Campestrini et al. /J. Mol. Catal. 83 (1993) 95-105
MO,-O7 Mo2-0, MO,-0, MO*--O,, Moz-O9 08-09
145.0(l) 170.1(8) 211.4(7) 193.5(9) 198.1(g) 193.9 (7) 144.0(l)
O,-MoI-OI 09-MO*-OS 08-Moz-O,,,
43.2(4) 81.2(3)
OS-OS
102.9(4)
in a glass reactor maintained at the appropriate temperature. Aliquots of the reaction mixture were withdrawn at various times, quenched with P ( Ph)s in excess, and the amount of the product determined by GC analysis on a Carbowax 20 M 10% (1.8 m column) on Chromosorb WAW-DMCS. Kinetic measurements were carried out under pseudo-first-order conditions by using an excess of the substrate over the oxidant, and monitoring the appearance of the product by GC. The pseudo-first-order rate constants were obtained from plots of In ( [Product] o3- [Product] t) us. time, linear up to 80% reaction. Results and discussion Together with the complexes mentioned in the Introduction, we have prepared, for comparison purposes, the novel complexes 6 and 7. Complexes 6 are representative of anionic peroxomolybdenum species containing a monodentate ligand whereas complex 7 is an acceptable model for anionic complexes not containing a carboxylato ligand. Clearly, a mononuclear complex without any organic ligand would have been preferable in order to keep the analogy among the various oxidants as close as possible. Unfortunately, any attempt to obtain crystals suitable for X-ray analysis, other than recrystallization from methanol which gives the tetrameric species 7, failed. Details on the procedures adopted for the synthesis and isolation of the complexes 6 and 7 are given in the Experimental section. Of all the peroxomolybdenum species employed in this study, the solid state structure, as determined by X-ray diffraction, is known either from previous work [7,13,14], complexes l-5, or from our own determination carried out within the framework of the present investigation. Details on the structures of the novel complexes 6 and 7 are reported in the Experimental section. We have also determined, under identical experimental conditions, both the nature and the yields of the products of the oxidation of two model alcohols, Le. cyclohexanol and 1-octanol by the peroxomolybdenum complexes. The corresponding oxidation rates and pseudofirst-order kinetic constants have also been obtained. All these data are pre-
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sented in Table 1. It may be noticed, in passing, that the reactivities of the primary and secondary alcohols are as already mentioned [ 4,6] rather similar; the reactivity ratio is about 4, in favor of the secondary alcohol. Even though careful examination of the structural features of the complexes prepared for the first time, e.g. 6 and 7, is of general interest, we shall discuss this topic elsewhere, limiting ourselves here to underlining those aspects which should be more closely related to the reactivity of the species, as provided by the data of Table 1. The usual caution in correlating solid state features with the behavior in solution must be exercised even though the solvent in which the reactivity has been measured is unlikely to produce severe modifications of the species owing to its low polarity and coordinating ability. Both neutral and anionic complexes and mono or diperoxo derivatives have been tested (see Scheme 1) . These differences among the molybdenum derivatives appear to have little effect on the geometry of the species. In all cases a pentagonal pyramid or bipyramid arrangement is observed. The bipyramid TABLE 1 Oxidation of 1-octanol and cyclohexanol by peroxomolybdenum complexes, in DCE, at 60°C” Substrate
Oxidant
cyclohexanol cyclohexanol cyclohexanol cyclohexanol cyclohexanol cyclohexanol cyclohexanol cyclohexanol cyclohexanol 1-0ctan01 1-0ctanol
1-0ctan01
1-octanol 1-octanol 1-octanol 1-octanol 1-0ctanol I-octanol
Product
Yield (% )b
klx 10’ (s-l)’
5
n.r.d
-
-
4 2 3
t?b 6c 6d 6e 4
cyclohexanone cyclohexanone cyclohexanone cyclohexanone cyclohexanone cyclohexanone cyclohexanone cyclohexanone n.r.d
35” 95 100 100 100 100 95 100 _e
3 0.2 46 50 57 82 37 23 -
2 3 6a 6b 6c 6d 6e 7
octanal octanal octanal o&anal o&anal o&anal octanal o&anal
90 100 100 100 85 95 98 100
0.3 11 12 14 20 9 6 17
Ba
‘Concentrations: substrate, 0.4 M; oxidant, 0.02 M. bBased on the oxidant. “From plots of ln( [Product],[Product],) us. time. dNo product formation after 24 h. *Extensive decomposition of the oxidant occurs.
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geometry occurs when the complexes are coordinatively saturated* e.g. l-3,6 and also 7. Other common features are the presence of an oxo-oxygen in the apical position and the preference of the peroxo oxygens and of the carboxylato oxygens, when present, for the equatorial ones. These observations suggest that the structural differences which might be responsible for the different oxidative behavior revealed by the data of Table 1 are those involving the second apical position. This would be in line with the results, mentioned in the Introduction, for the oxidation of diols. A first and rather obvious differentiation among the complexes can be therefore made by comparing those whose second apical position is free, with those in which it is occupied by a ligand. Subsequently, it may be useful to take into account the strength of the bond between the molybdenum atom and the ligand in order to have an indication of the feasibility of the substitution processes involving the apical ligand. Owing to the higher basicity and, very likely, nucleophilicity of nitrogen as compared with oxygen it may be expected that the neutral tooth of the bidentate picolinate-N-oxido ligand should be more easily removed than that of the picolinato ligand. Accordingly, the data of Table 1 indicate that 3 is 200 times more active than 2 in the oxidation of alcohols. In contrast, this line of reasoning could lead to the conclusion that complexes not containing any apical ligand at all should be superior oxidants of alcohols. This is only partly confirmed by the results of Table 1. In fact, it is observed that complexes 6, containing the monodentate carboxylato ligands are effective and selective oxidants of the two model alcohols but it may also be noticed that their reactivity is not greater than that of 3, whose apical position is occupied by the oxygen of the picolinate-N-oxido ligand. In contrast, inspection of the structural date reveals that the apical position in 6 is not completely free. In particular, the distance between the molybdenum atom and the carbonyl oxygen of the carboxylato group is 254 pm which indicates that some hindrance to the entering of a ligand may also occur in 6. However, the data of Table 1 reveal that the presence of a free or easily removable coordination site on the peroxocomplexes cannot be the only requisite that makes them effective and selective oxidants of the alcoholic function. In fact, if this were the crucial feature for such reactivity, complexes 4 and 5 should be, by far, the most efficient as their second apical position is absolutely free. In contrast, both the yields and the reaction rates observed when 4 and 5 are the oxidants of the two model alcohols indicate that they behave very poorly. Therefore, together with an accessible coordination site, there must be some other factor which is essential for the oxidation of alcohols. Our data rule out that such a factor is the presence of a carboxylato ligand. *The structure of 4, as such, which cannot he obtained as a crystalline material, is not available. However, that of the complex containing the water molecule has been determined [ 141. Such a water molecule present in the apical position is easily removed simply by heating under vacuum. A similar situation is found for complex 6, which has been used in this work after removal of the water molecule [ 131.
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Again, the behavior of complex 5 which is completely ineffective from an oxidative point of view militates against any unique role played by such ligands. The suggestion that the reactivity toward alcohols is due to the presence of a carboxylato ligand on the metal is also strongly in question, in view of the efficiency of complex 7, where only methoxo groups and methanol molecules are bound to the metal. Moreover, the presence of a bidentate ligand, as has been suggested [ 21, cannot be the key feature. Complexes 6, that contain only monodentate ligands, as already mentioned, are effective oxidants. Therefore we are left with only one possibility. The necessary requisite of peroxomolybdenum complexes as oxidants of alcohols, which is shared by all the species found to be active, is their anionic nature. Such a requisite is even more important than the ease of coordination of the substrate to the metal. Further evidence favoring such a conclusion is given by the oxidative behavior observed within the series of substituted carboxylato complexes 6. Such behavior is graphically shown in Fig. 3. Besides the observation that the oxidation rates may be nicely correlated via the Hammett equation with the Q values, the finding that the Hammett plot provides a p value of - 0.38 confirms that an increase of the electrodonating character of the ligand accelerates the oxidation rates. Again, one has to conclude that an increase of the anionic character of the oxidant results in an increase of its efficiency. To summarize the results presented so far: the most important feature of peroxomolybdenum complexes as oxidant of alcohols is their anionic character. The availability of a coordination site on molybdenum does play a role in determining the efficiency of the oxidation but it is a less stringent requisite.
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
(I
Fig. 3. Hammett plot for the oxidation of 1-octanol by complexes 6, in DCE, at 60°C.
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These conclusions have, of course, mechanistic significance which cannot be ignored in proposing a detailed mechanism for the oxidation of alcohols. On the other hand, in order to gain a better understanding of such a mechanism, more data are required and it is along this line that we are currently carrying on our investigation. It is, at any rate, tempting to suggest that the role of the anion is to ionize the alcohol substrates to the corresponding alkoxo derivatives which, inter aliu, should be better ligands than the parent compounds. This is equivalent to saying that in the oxidation of alcohols, a nucleophilic catalysis operates, the peroxocomplex being both the oxidant and the catalyst. Even though a direct test of this hypothesis should be possible, it must be recalled that such experiments are hampered by the extensive decomposition of the complexes in basic media. At any rate literature data provide clear evidence that, in protic media, under basic conditions, the oxidation of alcohols by peroxomolybdenum complexes formed in situ proceeds much faster than in neutral or acidic ones [ 31. The last point to be made here, on the basis of the data so far available, concerns the differing selectivities of the various oxidizing procedures. In particular, one may ask why, with the complexes employed here, the oxidation of primary alcohols stops at the level of aldehydes whereas with the other systems mentioned in the Introduction [ 4-61, the major products are carboxylic acids. A tentative rationale takes into account the possibility that also aldehydes, in order to be oxidized, have to coordinate to molybdenum, unless free-radical autoxidative processes occur. As it is very likely that carbonyl compounds behave as weaker ligands than the alkoxo ones, it may be observed that in all the anionic complexes examined here, some hindrance to the coordination to the second apical position does exist. This is not only true for complexes 2 and 3 but also for complex 6, owing to the proximity of the carbonyl oxygen of the carboxylato group and also for complex 7 due to its unique structure. Consequently, for these oxidants the coordination of the aldehyde is hampered, whereas in the system where the peroxomolybdenum species, whose structure is unknown, is formed in situ, such hindrance could be relatively unimportant thus allowing the coordination and hence the overoxidation of the aldehydes. Alternatively, one might envisage that, in the latter systems, owing to the presence of carbon-centered free radicals, the oxidation of the aldehydes to carboxylic acid is greatly facilitated.
Acknowledgments This research was carried out within the framework of Progetto Finalizzato Chimica Fine II of CNR. Financial support by MIJRST is also gratefully acknowledged.
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105
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