Electrochemical behaviour of benzophenone-benzene-chromium

Electroanalytical Chemistry and Interracial Electrochemistry, 50 (1974) 359 372 ~() Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

ELECTROCHEMICAL CHROMIUM

BEHAVIOUR

OF

359

BENZOPHENONE-BENZENE-

S. VALCHER and G. CASALBORE

Laboratorio di Polarogra[~a ed Elettrochimica Preparativa del Consiglio Nazionale delle Ricerche, Via Monte Cen.qio 33, 35100 Padua (Italy) (Received 12th July 1973 )

INTRODUCTION

The electrochemical behaviour of the bisaromatic complexes of chromium has been investigated by several workers 1 -1 o. While the influence of the coordinated metal atom on the redox properties was investigated by changing the substituents on the benzene rings 2'3, no paper was found regarding the effect of coordination on the electrochemical behaviour of the organic ligands. The reducibility of the carbonylic group in benzophenone seemed to permit an investigation of such an effect in the case of benzophenone benzene~chromium. Therefore the electrochemical behaviour of benzophenone-benzenechromium(I) and (0) was investigated by polarography, linear sweep potential voltammetry, coulometry and macroelectrolysis followed by the analysis of the products. The electrodic process commonly observed in the polarographic studies of chromium diarenes, consists in the cathodic reduction of monovalent chromium when a salt of the complex (e.9. the iodide) is used and in the opposite anodic process with the zero-valent complex, that is: ArzCr++e

~Ar2Cr °

(1)

The cathodic wave implying the above process for the cation BzzCr + was studied by Furlani and Sartori 2 in aqueous and in methanol/benzene media. The conclusion was for the aqueous medium, that the cathodic wave for the process: BzzCr + + e ~

Bz2Cr °

(2)

with E~ = - 0 . 9 5 V (vs. SCE), can be described by the equation: E = E ' + (0.059/n) log(i d - i ) in accordance with the insolubility in aqueous medium of chromium(0) diarenes. The absorption prewave, which appears in the polarographic pattern of many bisarene complexes in aqueous solution, has been studied by Virtyulina et al. 9. The bisbenzene--chromium has been studied by Hsiung and Brown s in dimethylformamide (DMF). In this case the compound showed also a reversible cathodic wave caused by the electrodic process (2). Also for bisbenzene~zhromium diarenes, the possibility of the irreversible oxidation A r z C r + ~ C r 3+ + 2 A r + 2 e (with ligands removal) at the platinum electrode was made evident 11

360

S. VALCHER, G. CASALBORE

The electrochemical investigations of organic carbonyl compounds are very frequent in the literature. Explanations of the polarographic behaviour of these compounds were given in many papers 12 14. Whatever the proposed mechanism may be, the corresponding alcohol is generally identified as the product of carbonyl group reduction. When the reduction is a two-step process, in the first step the radical RR'C O H (or RR'42~O - in basic or aprotic media) is formed. At the electrode the dimeric pinacol is formed from this radical, with a rate depending on the employed depolarizer and on the experimental conditions ls'16. The electrochemical reduction of benzophenone is in accordance with the preceding general considerations on organic carbonyl compounds 17 19 In D M F at the dropping mercury electrode, the benzophenone shows two cathodic waves which correspond to the reduction of the carbonyl group. Halfwave potentials are as follows: - 1.72 and - 1.97 V (vs. SCE) using tetraethylammonium (TEA) iodide as the supporting electrolyte and - 1 . 2 2 and - 1 . 4 7 V (vs. mercury pool) 2°. Some papers 13'19'21 22 deal with the effect of substituents in carbonyl compounds on the E} of the waves which correspond to the reduction of the carbonyl group. As a general conclusion, the effect of the substituents is qualitatively and quantitatively determined by the value and sign of the group constants (Hammett's and Taft's 0). EXPERIMENTAL

The b e n z o p h e n o n e ~ e n z e n e - c h r o m i u m was prepared by the method used by Fisher and Brunner 24. First, the metallated (sodium) bisbenzene-chromium was prepared, and then this compound was treated with benzaldehyde. After extractive separation, coupled with oxidation and reduction, alternately, in water and in benzene, the reaction mixture was chromatographed using benzene as the eluent. A chromatographic column (600 mm length, ~ 22 mm) filled with "neuter" A120 a (Woelm, activity 1)was used. As a last step, 4.9 wt.~ of deaerated H 2 0 was added, in nitrogen atmosphere, to the A1203, which had been previously activated at 160°C for 1 h 30 min under vacuum. F r o m the benzene eluate the red band of benzophenone-benzene-chromium was isolated. The bisbenzene-chromium for this synthesis was prepared according to the method used by Fisher and Hafner 2s" 26 from anhydrous CrC13, obtained as described in ref. 27. The BpBzCr °, which results from the above preparation, is dissolved in benzene but cannot be crystallized from this solvent. The complex was therefore precipitated as iodide by adding a saturated solution of KI in water and by bubbling O2 (oxidation to BpBzCr+). The separated iodide was recrystallized from acetone. Standardized solutions of this complex as hydroxide have been obtained by stirring a known amount of iodide with moist Ag20 and by diluting the filtrate to a standard volume. By reduction of BpBzCr with LiA1H424 the benzhydrol-benzene~zhromium was obtained, and the products of its thermal decomposition were separated by chromatography on an A120 3 column and analyzed. The chromatographic column was of the same type as that used for the benzophenone-benzene-chromium purifica-

BENZOPHENONE

BENZENE~CHROMIUM ELECTROCHEMISTRY

361

tion. Standardized solutions of BpBzCr ° were prepared by reduction of the complex iodide and extraction in benzene. D M F , water and water.ethanol mixtures were employed as solvents for electrochemical experiments. D M F (RP, Carlo Erba) was kept over BaO for 24 h for dehydration and then distilled under reduced pressure in N 2 atmosphere. For the water-ethanol mixtures, ethanol RP Carlo Erba was used. Tetraethylammonium perchlorate (TEAC104) RS Carlo Erba was used as the supporting electrolyte. An amount of 0.04~ of gelatine was added to the solutions under investigation, because, in aqueous solution, the polarographic waves of the complex cation were distorted by maxima. The Britton-Robinson buffer was used in water and water.ethanol media. In the polarographic measurements, a dropping mercury electrode was used with a flow rate m = 1.88 mg s - 1 at h = 64 cm. The drop time was maintained constant by forced fall. A platinum spiral was used as the counter-electrode. The reference electrode (to which all the potentials reported in this paper are referred) was a saturated calomel electrode (SCE) connected to the cell by appropriate liquid junction. In the coulometric and electrolytic experiments the electrode c o m p a r t m e n t s were separated by a sintered glass disc (G4). The working electrode was a mercury pool and the reference electrode was, again in this case, an SCE. An Amel 462 multifunction apparatus (_+ 150 V potentiostat swing) was used for polarographic and coulometric measurements. The oscilloscopic polarograph Amel mod. 448 was used for the linear potential sweep voltammetric measurements on the DME. The presence of the carbonyl group was tested by the reaction with 2,4dinitrophenylhydrazine. For benzaldehyde the test based on the formation of indigo, was used. Benzilic acid was identified by the reaction with rhodamine B. The same test was also used for benzil after transposition into alkaline medium. These analytical tests are described in ref. 28. RESULTS

Polarography in D M F A 10 -3 M solution of b e n z o p h e n o n e ~ e n z e n e - c h r o m i u m ( 0 ) with 1 M TEAC104 as the supporting electrolyte shows three polarographic waves. The first anodic wave, with E ~ = - 0 . 5 9 V, is reversible, as shown by the slope of the plot E/log [ ( i d - i)/id] which is 0.059 V in accordance with a monoelectronic transfer. It is also diffusion controlled since the plot limiting current versus root of the height of the mercury reservoir is linear and proportional to the concentration of the depolarizer. The polarogram of the iodide of the complex is similar to that of the zero-valent complex, but in this case the wave with E~ = - 0.59 V is a cathodic one. It is therefore possible to ascribe this wave to the reversible process: +e

BpBzCr + ~

BpBzCr °

In the case of the iodide, two waves also appear with E ~ = - 0 . 2 6 V and +0.11 V, respectively, which are attributable (by comparison with the behaviour of a K I solution in the same solvent) to the I - anion itself.

362

S. VALCHER, G. CASALBORE

The polarograms of both the zero-valent complex and the mono-valent complex also show two cathodic waves of the same height as the first one with E~ = - 1.91 V and - 2 . 0 1 V, respectively. The entire polarogram of the iodide of the complex is shown in Fig. la. The temperature coefficients have been evaluated for the three cathodic waves of the iodide complex. The temperature coefficient of the first wave (E~= - 0 . 5 9 V) between 20 and 50°C was 1.8~ per degree in agreement with the value calculated theoretically (1.8~o) for the system under examination (forced fall of the drop). Because of insufficient separation of the last two waves, their temperature coefficients cannot be determined easily. Therefore the temperature coefficient of their sum was evaluated and resulted to be 2~o per degree, again close to the theoretical value for a diffusive process. For all waves the ratio limiting current/depolarizer concentration was a constant, independent of the concentration. Linear sweep potential measurement on the D M E performed on chromium(0) complex, under the same experimental conditions, showed peaks corresponding to every polarographic wave by potential sweep both towards negative and towards positive values. Only in the case of the first two polarographic cathodic waves is the difference between the potentials for the three couples of cathodic and anodic peaks in good agreement with reversible systems.

Coulometry and electrolysis in DMF The results of coulometric experiments, performed on BpBzCrI in D M F with TEAC104 as the supporting electrolyte, are in agreement with the conclusion that the three cathodic waves are coupled with single one-electron reduction processes. After the electrochemical reduction at - 0 . 9 V (plateau of the first wave) a polarogram was obtained where the initial cathodic wave was changed to an anodic one with E , equal to that of the oxidation wave of the zero-valent complex (Fig. lb). This is in accord with the hypothesis that the electrodic process corresponds to the reaction: +e

BpBzCr + ~- BpBzCr ° e

Despite the very small potential difference between the second and the third cathodic waves, some electrolyses were performed on the plateau of the second polarographic wave. As a result, the solution turned almost black and then became colourless in time. The polarogram performed after the electrolysis showed, at the potential of the preceding cathodic wave, an anodic wave which disappeared in time. These results are in accord with known stability of other radicals produced in the first monoelectronic reduction of the carbonyl group of aldehydes and ketones in aprotic solvents. On the other hand, the product which is detectable by polarography immediately after macro-electrolysis at the potential ( - 2 . 1 V) corresponding to the plateau of the third wave, is not the primary product of the electrodic reaction as evidenced by linear sweep voltammetry (1.s.v.) (E~, = - 1.9 V in reversal sweep).

BENZOPHENONE BENZENE-~CHROM1UMELECTROCHEMISTRY

J

b

363

E/V

112

2.4

J Fig. 1. Schematical polarograms. (a) In DMF of BpBzCrl 0.46x10 3 M; supporting electrolyte TEAC104 0.1 M. (b) Obtained after electrolysisat -0.9 V.

fj (3

f

~

1.'2

f f

/

L- E/V

2:4

Fig. 2. (a) Series of schematical polarograms obtained m time after electrolysis of 10 3 M BpBzCrI in DMF at -2.1 V with TEAC104 0.1 M as supporting electrolyte.(b) Schematical polarogram obtained after electrolysisat -2.1 V and immediatelysubsequent oxidation by air. In fact, after the electrolysis, it is possible to observe an anodic polarographic wave with E~= - 0 . 5 0 V, which decreases rapidly in time, giving rise to a wave (E~= - 0 . 8 2 V~) which can be attributed to bisbenzene chromium (Fig. 2a). If, immediately after the electrolysis, oxygen (air) is bubbled through the solution, the wave at - 0 . 5 0 V disappears and two cathodic waves are formed. The first wave (E~ = - 0 . 8 2 V) is much smaller than the preceding anodic one, while the second wave ( E ~ = - 0 . 5 9 V) demonstrates the formation of the initial compound. The limiting currents of these two waves are not well reproducible, but their sum is always equal to the limiting current of the initial anodic wave (Fig. 2b). The experimental results reported above can be explained as follows. R~.~, ~.

The dianion ( R / t : - u )

which is the primary product for a bielectronic transfer

on BpBzCr(0), is detectable by l.s.v. : but being quite unstable it is rapidly protonated, R-_' R-. and t h e a l c o h o l a t e ( R / C H - O ) [or perhaps the alcohol ( R / C H - O H ) I is the product with E ~ = - 0 . 5 0 electrolysis.

V which can be detected at the end of a macro-

364

S. VALCHER, G. CASALBORE

The alcoholate ion can be oxidized by 0 2 from the air to the starting ketone coordinated with Cr(I), The alcoholate is also unstable and slowly decomposes giving BzzCr (E~.= - 0 . 8 2 V) and other products. During the electrolysis at the plateau of the third wave the electronic exchange:

CP °

+2

Cr +

- 3

CP °

does not permit the product to appear until the chromium atom is completely reduced. In fact, at the beginning of the electrolysis the solution became red, while the colour of the zero-valent b e n z h y d r o l ~ e n z e n e - c h r o m i u m is clearer than the colour of BpBzCr°; in addition, the polarograms performed after an interruption of the electrolysis were identical with those performed after electrolysis at the plateau of the first wave. Polarographic experiments in water and water-ethanol The polarogram of the BpBzCrI in water with B.R. buffer showed a cathodic wave at - 0 . 7 V distorted by a prewave and by very strong maxima. On addition of 0.04~o gelatine these maxima disappeared and the resulting wave was more regular and was preceded by a little prewave. At more negative potentials, in acidic media, two further waves appeared about equal in height to that of the preceding wave; in more basic media, these two waves merged into one wave of double size. Because, as will be demonstrated by the results of the controlled potential electrolysis, the products of the electrodic reactions relative to the first, second and third wave are insoluble under the present experimental conditions, none of the usual mathematical analyses is allowed for the second and the third wave and a detailed study based on their shape is therefore impossible. The mathematical analysis of the first wave based on the equation E = a + b log (id - i) was not in agreement with the theoretical value of b (0.120 V experimental value, 0.059 V theoretical value) for a reversible one-electron process. Furthermore, under irreversible conditions, the prewave cannot be unmistakably correlated with an absorption process. A detailed treatment of this problem has not, as yet, been reported in the literature. The plot E ~ H for the observed waves is shown in Fig. 3. Analogous behaviour was also observed in aqueous alcoholic media (25 and 50~o) (Figs. 4 and 5). In these solutions, however, with increasing amount of ethanol, the point where the two waves merge is shifted towards more acidic pH. These polarographic patterns are similar to those obtained for other aldehydes and ketones 29'3° and particularly for benzophenone (Fig. 3) under the same experimental conditions. In some papers 12'31 the alcohol effect in such systems is explained by assuming that there is a remarkable variation of half-wave potentials of the ir-

BENZOPHENONE-BENZENE~CHROMIUM

E /V _

365

ELECTROCHEMISTRY

E~/V 1.s

1.5 0

Y oo

5

1o

PH

Fig. 3. Plot of E~ vs, pH for the waves of BpBzCrI in water with B,R. buffer ( phenone under the same conditions ( - ).

PH

) and of benzo-

Fig. 4, Plot of E~ vs. pH for the waves of BpBzCrI in water-ethanol 25~ (B.R. buffer).

E~/v r!.5

i o

o o

0

Q

s

O---O-~C

o

c.~0---.-0o¢

lo

pH

Fig. 5. Plot of E~ vs. pH for the waves of BpBzCrl in water-ethanol 50% (B.R, buffer).

reversible process caused by a double layer modification. On the basis of these considerations, our experimental results could indicate some degree of irreversibility for the second and third wave of the examined compound, but the different solubilities of the compounds concerned with the electrochemical process could also be responsible for the observed modifications. Coulometric and electrolytic experiments in water and water-ethanol Coulometric measurements have been performed in aqueous medium at pH 8.95 (B.R. buffer) on BpBzCrI. 25 ml of a 4 mM solution of BpBzCrI have been electrolyzed at a potential corresponding to the plateau of the first wave ( - 0.9 V). The electrolysis consumed one mole of electrons for a depolarizer mole.

366

s. VALCHER, O. CASALBORE

Furthermore, during the electrolysis a red-brown compound was precipitated; this compound was soluble with a red colour in a benzene layer on the electrolyzed solution. In the presence of a benzene layer, the solution resulting at the end of the electrolysis, was polarographically inactive; the oxidation by air decolourized the benzene layer (or the precipitate was dissolved in the absence of the benzene layer) and the aqueous solution once again turned orange-coloured. At the end of this experiment a polarogram equal to the initial one was obtained. The preceding coulometric results demonstrate that the process related to the first polarographic wave is a simple monoelectronic transfer which results in a stable product. The formation of a product which is insoluble in aqueous solution and soluble in benzene, and the colour of this product suggest the simple reaction mechanism +e

[BpBzCr] + (I) ~

BpBzCr(0)

+02

analogous to the one deduced for the first polarographic step in DMF. At the plateau of the second wave (at the p H of this experiment the two waves shown in acidic medium merge into one wave of double height) three moles of electrons per mole were consumed. By taking into account the electron exchanged in the first polarographic wave it is evident that the second wave is concerned with a two-electron process. Since the chromium under these conditions has an electronic shell of noble gas, this process cannot be ascribed to a further reduction of the zerovalent chromium to a negative chromium atom. On the contrary, by comparing the polarographic behaviour of the complex with that of other organic carbonyl compounds and by the nature of the electrolysis products (as will be described), it appears that this electrodic process is inherent to the reduction of the ketone group, in the complex, to an alcohol group. Furthermore, since the coulometric measurements were m accord with the ratio between the polarographic limiting currents, it is possible to exclude any slow chemical reaction subsequent to the electrodic reduction, yielding chemical species that, in our experimental conditions, can result in further reductive reactions. Exhaustive electrolysis performed at the plateau of the second wave ( + 1.7 V) under nitrogen atmosphere, produced a polarographically inactive solution. However, when the electrolyzed solution was reoxidized by air, the polarographic pattern of the starting compound was reproduced, but the limiting currents were smaller and a new cathodic wave with - 0 . 9 7 V was shown which can be attributed to the reduction of Bz2Cr + to Bz2Cr0. The presence of this compound was demonstrated by u.v. spectroscopic measurements. When oxidation is performed some time after electrolysis, the height of the wave of Bz2Cr + was greater and reached its maximum value if oxidation was carried out after a few hours. By the heights of the polarographic waves it was possible to verify that the amount of the bisbenzene~chromium obtained was equal to the amount of benzophenone-benzene-chromium initially reduced. F r o m these results it is evident that the bisbenzene~zhromium is a product of slow decomposition of the primary electroreduction product. Since the hydroxide of the complex behaved in the same manner, it can be concluded that the anion bonded to the complex did not influence

BENZOPHENONE BENZENE-CHROMIUM ELECTROCHEMISTRY

367

the electrochemical behaviour of the depolarizer. The polarographic pattern resulting from the retarded reoxidation of the electrolyzed solution also shows another, ill-defined wave with a scarcely reproducible half-wave potential of - - 1.1 V. This wave is diminished by extraction with benzene, and therefore the compound wl~ich produces this wave divides itself between water and benzene. In order to characterize the compounds which resulted together with Bz2Cr from the decomposition, some polarographic measurements were performed on several substances which could be considered possible decomposition products, since b e n z h y d r o l ~ e n z e n e - c h r o m i u m is the most probable primary product of the electroreduction. Therefore, benzaldehyde, benzoin, benzil and benzilic acid [q~-o~COOH qg"~'- O H ] were examined in aqueous medium at pH 8.95, and the same experiments were also performed in the presence of Bz2CrI. Only benzaldehyde showed a polarographic wave, with E ~ = - 1.4 V (the other compounds were polarographically inactive). In fact, as is known, benzil gives an inactive complex with borates of the B.R. buffer, benzoin was not sufficiently soluble in our experimental conditions and benzilic acid is not reducible under these conditions. Also, in water-ethanol (10~o) mixture, benzoin, benzil and benzilic acid did not show any polarographic wave. The complex known to be formed in alkaline solutions between benzoin and benzil was also examined, but it showed a polarographic wave with E~ = - 1.4 V under our experimental conditions. The preceding polarographic experiments demonstrate that benzaldehyde is the only compound whose derivation can be excluded from the decomposition of the reduction product. In our experiments this compound was reduced at a potential where no wave was shown in electrolyzed solutions. The benzene solution, derived by extraction with this solvent from the aqueous solution of BpBzCrI electrolyzed at pH 8.95 (at the plateau of the second wave) was examined. In this manner only benzilic acid was identified among the compounds previously considered. Benzhydrol-benzene-chromium was prepared as previously described in the experimental part of this paper, and its decomposition products, obtained after several hours, were separated by chromatography with benzene as eluent in a column equal to that used for the BpBzCr preparation. Two bands were visible in the column. The first contained a compound, identified by its chemical behaviour and u.v. spectra, as bisbenzene~zhromium. The second band was due to benzil. Therefore, the most reasonable hypothesis is that, during electrolysis at a potential corresponding to the plateau of the second wave, benzhydrol-benzenechromium is formed (i.e., the carbonylic group is reduced). The alcohol then decomposes and yields bisbenzene-chromium and benzoin which, in the presence of oxygen, is oxidized to benzil. The latter compound undergoes transposition to benzilic acid in the alkaline electrolyzed solution. To confirm this conclusion, aqueous and aqueousmthanolic solutions of

368

s. VALCHER, G. CASALBORE

Bz2Cr + were prepared, adding benzaldehyde, benzoin, benzil and benzilic acid respectively. BzzCr + was reduced by electrolysis and the solution was then oxidized by bubbling air. With the exception of benzaldehyde and benzilic acid, the polarographic pattern, obtained after the BpBzCrI electrolysis at the plateau of the second wave and subsequent reoxidation, was reproduced in this manner. Very similar results were obtained with benzoin, the product which is directly subsequent to benzhydrol benzene-chromium decomposition. The wave displayed in these solutions at -1.1 V did not seem to be directly attributable to any of the compounds cited, but rather to another oxidation product which was not identified. CONCLUSIONS

The electrochemical reduction of benzophenone-benzene-chromium(I) in water and D M F solutions follows a similar pattern, and in every case, the electrodic reduction of the carbonyl is followed by a cleavage of a carbon-carbon bond with the formation of bisbenzene-chromium and other organic products. The reaction schemes may be summarized as follows. In DMF: 0-

O-L

I

C6Hs-CO-C6H5

C6Hs-CO-C6H5

C6Hs-C-C6H5

C6H5 -C-C6H5

Cr+

Cr°

~r°

Cr °

t

C6H6

+e ~ -e

"

t

C6H6

+e ~.~ - e

+e"

t

C6H6

C6H6

fast H* (solv.) slow transformation 0I

C6Hs-C-C6H5 ro

02

t C6H6 I dec. H÷ (solv.)

C6H6

!r+

C6H6

o I

+ ~C6Hs-CO-CO-C6H5

r° + ½C6Hs-CO-C-C6H5

~ O2

t

t

C6H6

C6H6

and in alkaline aqueous or aqueous alcoholic solutions:

I

369

B E N Z O P H E N O N E B E N Z E N E - C H R O M I U M ELECTROCHEMISTRY C6H5 -CO-C6 H 5

~r°

(insol.)

C6H6 C6H5 -CO-C6 Hs Cr ÷

t

C6H6 OH I

C6Hs -CH-C6H5

C 6 I-t6

Cr°

Cr°

(insol.) ~

t

t

C6H6

C6H 6

OH

I (insol.) + ½C6H s-C -CO-C6H s I

H

C6H6 Cr +

t C6H6

+ 1/2 C6H s -CO-CO-C6 H 5-

I OHC6 H 5

\/ c /\

C6H5

COO-

+x (reducible at-I.1 V) OH

In water, the value of E~ ( - 0 . 7 V) for the first wave concerned with the reduction Cr(I)--*Cr(0) is 0.250 V more positive than that reported 2 for bisbenzenechromium. In D M F an analogous difference in the E~ value of the complexes (BzzCr, E~ = -0.80 V; BpBzCr, E~ = -0.59 V) is shown. This difference is in accord with the expected effect of the benzoyl group. Because of the insolubility of the intermediate products, no comparison is possible for water solutions between half-wave potentials concerning the carbonyl group of BpBzCr and those shown by other benzoyl derivatives such as benzophenone. For results obtained in D M F such a comparison is possible. It must be noted that half-wave potentials for reduction of the carbonyl group are shifted more towards the negative values in the case of BpBzCr ( - 1 . 9 1 V) than in that of benzophenone ( - 1.72 V). This fact may be connected to the decrease of the aromatic property of the benzene ring coordinated with the metal. Spectroscopic measurements demonstrate that the charge distribution on the carbonyl group is modified. The u.v. band

370

S. VALCHER, G. CASALBORE

TABLE 1 CHARACTERISTIC FREQUENCIES OF THE U.V. BANDS OF BENZOPHENONE BENZENE CHROMIUM AND OF SOME COMPARABLE C O M P O U N D S (n) Data from ref. 33; (p) from ref. 34. n ~ ~"

0<°0

(.)

Cr

©

© Cr

Benzenoid

ET

B 199

/

/

340

278

243

/

/

320

270

254

445

340

320

270

410

335

=(p)

275

/

(n)

(n)

shown by benzophenone at 340 nm and assigned to n--,~ transitions is strongly modified in its position and intensity by the ~ coordination with the chromium atom. Table 1 shows how this band is shifted in the case of BpBzCr towards the value observed for acetophenone. The assignment of the band is supported by the close correspondence between the other bands of the compounds examined. By considering the whole benzene--chromium~henyl group as a subsfituent in the series C6H5CO X it can be compared with other substituents for its effect on the reducibility of the carbonyl group and a value of the Taft constant can be attributed to it. The following substituents were compared: -CH3(acetophenone),

E /v

20

1

l.g

18

I

0.1

J

0.2

I

0,3

I

04

I ~

05

0.6

Fig. 6. Plot of E~ v s . Taft's ~ in DMF for the series C6HsCO~X: (1) acetophenone, (2) benzaldehyde, (3) benzophenone, (4) interpolated value of BpBzCrI.

BENZOPHENONE BENZENE CHROMIUM ELECTROCHEMISTRY

371

C 6 H s (benzophenone), for which the Taft's constants are 0, 0.49, and 0.60, respectively 32. The half-wave potentials in the same experimental conditions for the compounds considered here 2° are - 1.99 V, - 1.80 V and - 1.72 V, respectively. As can be seen in Fig. 6, the interpolated value for the -C6H5----~ C r + - - C 6 H 6 group ( E ~ = - 1.91 V in D M F ) is 0.19. Since a precise significance for a can be obtained only by the examination of a large number of cases the value reported here must be considered with the necessary caution. Nevertheless it can be outlined that the effect of the considered groups is, also in this case, more similar to that of a methyl group than to that of a phenyl group. Finally it must be noted that the ~ coordination with the chromium a t o m permits a fast oxidation of benzhydrol to benzophenone by air and that the presence of Bz2Cr permits the oxidation of benzoin. These facts suggest the possibility of using the bisarene complexes as redox catalyst compounds. -H(benzaldehyde),

ACKNOWLEDGEMENTS

This work has been initiated in the Centro di Polarografia del C.N.R. (c/o Istituto "G. Ciamician" dell'Universit& di Bologna) under the direction of Prof. E. Vianello. The authors are indebted to Professor E. O. Fischer and Dr. H. Brunner of the "Institut fiir Anorganische Chemie der Universit~t Mfinchen" where the practical details of the preparations were acquired. SUMMARY

An electrochemical study performed in aqueous and D M F been elucidated and the effect behaviour of the ketone has been

on b e n z o p h e n o n e - b e n z e n e - c h r o m i u m has been solutions. The mechanism of the reactions has of the coordinated c h r o m i u m a t o m on the outlined.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

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