Electrochemical analysis of Friedel—Crafts acylation and alkylation of functional groups on highly ordered pyrolytic graphite

J. Electroanal. Chem., 112 (1984) 189-200 Else&r Sequoia S.A., Lausanne - Printed

189 in The Netherlands

ELECTROCHEMICAL ANALYSIS OF FRIEDEL-CRAFTS ALKYLATION OF FUNCTIONAL GROUPS ON HIGHLY PYROLYTIC GRAPHITE

WAYNE

E. BRITI’ON,

Chemistry Department, (Received

4th August

MAHER

EL-HASHASH,

MOHAMMED

University of Texas at Dallas, Richardson, 1983; in final form 23nd February

EL-CADY

ACYLATION ORDERED

and FAHAD

AND

ASSUBAIE

TX 75080 (U.S.A.)

1984)

ABSTRACT Friedel-Crafts alkylation and acylation of ferrocene via alcohols, lactones and anhydrides on the edges of highly ordered pyrolytic graphite (HOPG) is demonstrated. The alkylation and acylation reactions upon ferrocene induce a substituent effect on the oxidation potential of the immobilized ferrocene, which provides information about the nature of the chemical bond formed and, therefore, the nature of the functional groups present on the graphite surface. A reaction consistent with the sequential bonding of two ferrocenes to two different functional groups in known relative proximity on the graphite surface is described.

INTRODUCTION

Determining the nature of the functional groups present on the surface of various forms of carbon before and after chemical and physical treatments has been the subject of numerous investigations [l-6]. The properties of these functional groups are similar to these of carboxylic acids and anhydrides, lactones, phenols, quinones, esters and ethers as shown by chemical [l], electrochemical [2], infrared [3], mass spectrometric [4], internal reflectance infrared spectroscopy [5] and XPS [6] analytical methods. There is current interest in utilizing these functional groups in the preparation of chemical modified electrodes (CME) [7]. In addition to the intrinsic value of CME studies, much can be learned about graphite surface chemistry via electrochemical techniques. The numerous applications of graphite (e.g. as catalysts, [6e,8], structural fibers [9], batteries [lo], electrochromic devices [ll], dispersants and pigments [12], chromatographic supports [13], moderators in nuclear reactors [14], adsorbents for pollution control [la,lS], precious metal recovery systems [16], and ion selective electrodes [17], etc.) are influenced to varying degrees by the graphite surface properties, and appropriate graphite surface modifications could lead to improved performance. The anisotropic nature of highly ordered pyrolytic graphite (HOPG) [18] may afford an organized substrate for the synthesis of catalysts [8], electrochemical sensors [19] or discriminating electrodes for synthesis [lc]. Intercalation [20] which is involved in certain graphite catalysts [8h], batteries [lo] and 0022-0728/84/$03.00

0 1984 Elsevier Sequoia

S.A.

190

electrochromic phenomena [ 1 l] should be influenced by the chemical environment at the graphite edges, across which the intercalated species must move. Covalent bonds to graphite via esters [21], amides [lc,21,22], silyl esters [23], bis(amides) [24], hydrazones [23], cyanuric chlorides [25], and hydrocarbons [21,26] involve, with the exception of the last case, linkages through potentially hydrolyzable heteroatoms. Described below is the Friedel-Crafts * alkylation and acylation of HPOG with ferrocene, which has unique features compared with most other modification techniques employed on graphite. First, a bond is created directly between the surface functional group and the ferrocene which imposes a measurable substituent effect on the ferrocene oxidation potential, providing direct evidence for the type of bond formed and therefore the nature of the functional group on the graphite surface. Second, the Friedel-Crafts reaction forms a carbon-to-carbon bond less susceptible to attack and certainly of different reactivity than most other bonding methodologies employed (vide supra). It may also be possible to bind separate molecules onto graphite, in known relative proximity by opening anhydrides or lactones as a first step (see discussion below). The ability to bind two molecules on a surface in known relative proximity is an important step toward synthesis of a structured surface, of potential value in catalysis [27] or analysis. Shigehara and Anson [8i] have shown that two metal ions randomly arranged on an electrode surface give enhanced catalysis for oxygen reduction to water, but they point out that the binuclear cobalt porphyrins [Sj] with cobalt atoms in a specific relative geometry are superior catalysts for the same reaction [8i]. EXPERIMENTAL

Electrochemistry P.A.R.C. 173, 174, 175&176 instruments were used in conjunction with a Houston 2000 recorder to obtain voltammograms. IR compensation was employed in the cyclic voltammetry experiments. The three-electrode cell was fabricated from a Fisher-Porter vacuum stopcock with added side arms for sample introduction, nitrogen purge, an unfused Vycor separated Ag/AgNO, (0.1 M in acetonitrile) reference electrode and a carbon rod counter electrode separated with a glass frit (Fig. 1). The Teflon stopcock plug was slotted on the end, and a slice of HOPG was mounted in the slot flush with the end and sides of the plug, to expose a ca. 0.2 cm’ area of the graphite edges on the end. Contact to the graphite was made via gold foil inside the slot. The Teflon plug was then forced into the stopcock barrel comprising the cell. exposing one graphite edge to the solution. Baker HPLC grade acetonitrile and reagent grade sodium perchlorate were used as received. All cyclic voltammetry peaks are small, indicating low coverage. The peaks are * Friedel-Crafts alkylation of carbon has been carried out, but not discussed in detail, and electrochemical methods were not involved. See for example ref. 16.

191 Original

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counter

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Fig. 1. Stopcock modification to produce the electrochemical cell and graphite holder. Drawing is not to scale.

reproducible, however, and control experiments run without added ferrocenes failed to give waves. Control experiments using differential pulse voltammetry did reveal small waves for blank electrodes, but these could be differentiated from the waves for the ferrocene treated samples. H O P G treatment

All electrodes were treated in the following manner prior the chemical modification.

192

Electrodes were first cut to size with a scalpel, continuously extracted for 24 h with anhydrous methanol and then dried under vacuum. The electrodes were then heated at 5OO’C for 24 h, allowed to cool slowly to room temperature in the muffle furnace, and then immediately transferred to a desiccator. This procedure was followed to avoid hydrolysis of surface functional groups, such as anhydrides. Modification

of anhydrides

and lactones on HOPG

(a) Friedel-Crafts acylation of ferrocenes Electrodes were stirred for 24 h in a solution of 0.3 g of ferrocene and 0.35 g of anhydrous aluminum chloride (fresh bottle) in 20 ml of dry nitrobenzene in oil bath at 75 “C. Then the reaction mixture was cooled and poured over 50 g ice and 2 ml of concentrated hydrochloric acid. The electrodes were collected and washed several times with distilled water, then dried under vacuum and finally Soxhlet-extracted with dry benzene for 24 h and redried under vacuum. (b) Reduction of graphite surface by lithium aluminium hydride Electrodes were stirred 48 h in a refluxing solution of 1 g of lithium aluminum hydride in 50 ml of anhydrous ether or THF. Then excess lithium aluminum hydride was decomposed by slow addition of ethyl acetate. The suspension was poured over ice-dilute hydrochloric acid, then the electrodes were collected and washed with water several times and dried under vacuum. (c) Friedel-Crafts acylation and/or alkylation of ferrocene with alcohols or lactones on HOPG The same procedure as described under (a) above was followed. (d) Friedel-Crafts acylation of ferrocene with acid chlorides Step 1. Treatment of the surface carboxylic acid groups with thionyl chloride: Electrodes were stirred 24 h in a refluxing solution of 2 ml of thionyl chloride in 30 ml of dry benzene. After washing with benzene, the electrodes were dried under vacuum and used directly in the next step. Step 2. Friedel-Crafts acylation of ferrocene with acid chlorides. The same procedure as described under (a) above was followed. (e) Conversion of carboxyl groups to the amide groups containing ferrocene moiety, after acylation as described in (a) above Step 1. Electrodes were stirred overnight with a refluxing solution of 30 ml of anhydrous methanol containing 3 drops of concentrated H,SO,. Then the electrodes were collected, washed with anhydrous methanol several times and subjected directly to the second step. Step 2. Electrodes from the first step were stirred for 24 h in a refluxing solution of 1 ml of /?-ferrocenylethylamine in 4 ml of benzene. After washing with benzene the electrodes were extracted for 72 h with refluxing benzene and dried under vacuum.

193

Friedel-Crafts

acylation of ferrocene

with phthalic anhydride

A solution of 9.3 g (0.05 mol) of ferrocene in 100 ml of methylene chloride was added dropwise to a stirred solution of 7.4 g (0.05 mol) of phthalic anhydride and 10 g (0.075 mol) of anhydrous aluminum chloride (fresh bottle) in 100 ml of methylene chloride. The mixture was maintained at room temperature during addition and for 24 h thereafter, then was poured over ice-hydrochloric acid (200 g; 25 ml). The layers were separated and the aqueous layer was extracted repeatedly with ether, the combined organic phases were washed several times with water, then washed with 10% sodium bicarbonate solution. The alkaline extract was cooled acidified with diluted hydrochloric asid. The acidic precipitate was extracted with ether, the ethereal layer washed several times with water, then dried over sodium sulphate. Slow evaporation of the ether solution left 4.1 g crude acidic material (25% yield) and after crystallization from methanol 2 g (ca. 12% yield of 1-(2carboxybenzoyl)ferrocene, m.p. 251-252’C. Synthesis of 1-(2-P-ferrocenylethyIcarboxyamidebenzoyl)ferrocene

(1)

The corresponding acid [1.6 g (0.005 mol)] was ground with 2.1 g (0.01 mol) of phosphorous pentachloride in a mortar until the solid mixture melted. Then a solution of /3-ferrocenylethylamine [l.l g (0.005 mol)] in 100 ml of dry ether was added dropwise with stirring of mortar contents. The reaction mixture was left at room temperature for 2 h. Then water was added to decompose the excess PCl,. The precipitate obtained was extracted with ether, ethereal solution washed several times with water, then dried over sodium sulphate. Evaporation of the solvent left the amide as a red brown powder, 1.3 g, ca. 49% yield. Recrystallization from ethanol gave red brown crystals which decomposed at 277 ‘C. P-Ferrocenylethylamine

/?-Ferrocenylethylamine was prepared from N, N-dimethylaminomethylferrocene (Parish Chem. Co.) according to Hauser et al. [28,29]. RESULTS AND DISCUSSION

Modification

of anhydrides

and lactones on HOPG

Friedel-Crafts acylation of ferrocene with anhydrides depicted in eqns. (1) and (2) respectively.

ferrocene AM, + nitrobenzene

(Fc = ferrocenyl)

on graphite

is

0

ferrocene + AICI 3 nitrobenzene

or lactones

FC OH x

(1)

0

(2)

194 TABLE

1

Half-wave potentials for several ferrocenes, measured at a platinum anode in acetonitrile containing 0.1 M sodium perchlorate, vs. Ag/AgNO,

Ferrocene a

E”2 vs. Ag/AgNO,

Fc-CHO Fc-COCH,

0.37 0.32

Fc-CO

0.32

1 Fc-H

/V

COZH 0.32 and 0.12 0.07

a Fc = ferrocenyl moiety.

Table 1 lists the half-wave potentials for several substituted ferrocenes, which suggests that acylation according to eqns. 1 or 2 would shift the ferrocene oxidation potential positive by ca. 220 mV. Figure 2 confirms this expectation, which compares the cyclic voltammogram of the HOPG modified electrode with the solution analog (ferrocene plus phthalic anhydride). Typically, the cyclic voltammogram exhibits a large adsorption wave which is removed by potentiostating at 1.5 V for up to 40 min, revealing the more tenaciously bonded waves shown in Figure 2. Surface coverage is less than one monolayer using the geometric area of the electrode and cyclic voltammetry peak integrals; however consideration of surface roughness [30] would reduce the coverage value further. This is to be expected for two reasons: (1) the reaction shown in eqns. (1) and (2) would not normally afford 100% yield and (2)

1

I 0.2

0.4 +

0

vs Ag/AgNO,

Fig. 2. Comparison of electrochemistry of ferrocene in solution with that of ferrocene bound Friedel-Crafts modification of HOPG. (a) ca. 1 X lo-’ M l-(2-carboxybenzoyl)ferrocene in 0.1 M NaCIO, in acetonitrile. S = laA, u = 20 mV/s; (b) HOPG modified with ferrocene via the Friedel-Crafts reaction. S = 100 PA, u = 100 mV/s. The cross indicates the origin.

195

other functional groups expected on the surface do not react via the Friedel-Crafts reactions. The bonded waves are broad with wide peak-to-peak separations consistent with a range of activities [31] for ferrocene and/or surface heterogeneity [24,32,33] and slow electron transfer [33] respectively. The peak heights vary linearly with scan rate and are more symmetric than those of the solution analogs, consistent with a species immobilized on the electrode [34]. A peak about 200 mV negative of the acylated ferrocene wave is also observed (see Fig. 2). This is thought to arise via alkylation of surface aliphatic alcohols formed by opening of an aliphatic lactone (eqn. (2)). A more pronounced voltammetric wave at the same position is produced when the graphite is reduced with lithium aluminum hydride before running the Friedel-Crafts reaction (see below). This treatment would also be expected to produce aliphatic alcohols on the graphite surface. Attempt

to distinguish

between lactones and anhydrides

The procedure discussed above does not allow the differentiation between lactones and anhydrides on the graphite surface. However, lithium aluminum hydride reacts rapidly with anhydrides, but more slowly with lactones. the graphite surface was therefore reacted with lithium aluminum hydride followed by the Friedel-Crafts reaction. The resulting electrode gave the same two cyclic voltammetric peaks observed above the corresponding to acylated and alkylated ferrocenes (see Fig. 3). The acylated ferrocenes could result from the opening of the lactone ring under Friedel-Crafts conditions, while the alkylated ferrocenes are formed in a Friedel-Crafts alkylation with aliphatic alcohols formed by lithium aluminum hydride reduction of anhydrides or carboxylic acids. Some aliphatic alcohols may also be produced by opening the lactone ring (see discussion above).

0.4

02 vs Ag/AgN03

0

Fig. 3. Cyclic voltammogram of HOPG modified with ferrocene via Friedel-Crafts reaction after reducing the HOPG with lithium aluminum hydride. S = 33 pA, u = mV/s. The cross indicates the origin.

196

The voltammograms obtained when reduction with lithium aluminum hydride precedes the Friedel-Crafts reactions are better resolved with narrower, more reversible-looking waves. This is due possibly to the reduction of carboxylic acids and other groups on the surface, affording a chemically more homogeneous environment at the surface. Possible ion exchange phenomena or unique solvent structures associated with carboxyl groups would be absent on a reduced surface. These observations make the distinction between anhydrides and lactones tenuous. However, the fact that the acyl wave has not diminished relative to the aliphatic wave, which would be expected if anhydrides were reduced to alcohols, may suggest that anhydrides are not in significant concentration on the surface. Their complete absence would be surprising, however, since carboxylic acids are frequently implicated on graphite surfaces [l-6] and our conditions for graphite pretreatment would be expected to convert these into anhydrides as long as the regiochemical requirements for this reaction are met. The Friedel-Crafts acylation has also been carried out with acid chlorides produced by treating the surface carboxylic acid groups with thionyl chloride, then following with the Friedel-Crafts reaction (eqn. (3)): 0 SOCI, CO,H

h

FeCi COCI

3

?GEzGz

FC

(3) (Fc = ferrocenyl)

As expected, the acylferrocene oxidation wave appears in the cyclic voltammogram. Sequential HOPG

introduction

of two different

ferrocenes

in known

relative proximity

on

Reactions (1) and (2) indicate that a carboxylic acid and alcohol are formed respectively, in a fixed proximity to the acyl ferrocene. On the assumption that anhydrides are present on the surface, it was of interest to try to introduce a second ferrocene according to eqn. (4).

B

C-FC ?-OH 0

CH,OH H+

0

0

kc

C-Fc

$-OCHx

C-NHCH-JH,Fc

0

d

II

(4

(Fc = ferrocenyl)

The result of this reaction sequence is shown in the voltammogram in Fig. 4, which compares the surface modification with the solution analog, i.e., using

197

I

1 02

0.4

E

0

V vs Ag/AgN03 I

Fig. 4. HOPG modified sequentially with two different ferrocenes compared with solution analog. (a) ca. 1 x 10e3 M of 1-(2-P-ferrocenylethylcarboxamidebenzoyl)ferrocene, S = 10 pA, o = 50 mV/s, 0.1 M NaClO, in acetonitrile. (b) HOPG modified sequentially with two different ferrocenes, S = 100 PA, u = 100 mV/s, 0.1 M NaClO, in acetonitrile. The cross indicates the origin.

phthalic anhydride in the same reaction sequence to give compound 7;.

(Fc = ferrocenyl)

We have repeated the HOPG modification reaction described in eqns. (1) and (2) and analyzed the surface by differential pulse voltammetry *. Four separate HOPG samples were exposed to ferrocene and three controls were run (see experimental section). All ferrocene treated samples gave large ferrocene adsorption waves, and three out of the four samples give small waves of variable size which correspond with acylated ferrocene. Figure 5 presents the voltammogram of the best of the four runs made. This sample was not potentiostated at 1.,5 V and thus displays the large ferrocene adsorption wave, and the smaller wave at the more positive potential which we attribute to acylferrocene. While the chemistry represented in eqns. (l)-(4) is not proved by the voltammetry results in Figs. 2-5, it is consistent with it and could therefore represent a step

l

A referee recommended the graphite be examined by differential pulse voltammetry

198

E

I

V

“s

Ag/AgNO,

Fig. 5. Differential pulse voitammetric Modulation amplitude = 25 mV p-p.

analyses of Friedel-Crafts

modified HOPG. Scan rate = 5 mV/s,

toward the synthesis of a structured surface, of value as a catalyst, sensor, chromatography packing, etc. ACKNOWLEDGEMENTS

Our thanks are extended to A.W. Moore at Union Carbide Corporation for samples of HOPG, to Mr. Tim Mulone for technical assistance, and to Ain Shams University for a sabbatical leave for M.E-H. and M.E-C. This work was aided by Grant AT-748 from the Robert A. Welch Foundation. We also thank the Environmental Science Department at UTD for the loan of the P.A.R.C.-174 Polarographic Analyzer. REFERENCES (a) R.W. Coughhn and F.S. Ezra, Environ. Sci. Technol., 2 (1968) 291; (b) H.P. Boehm, E. Diehl, W. Heck and R. Sappok, Angew. Chem. Int. Ed. Eng., 3 (1969) 669; (c) B.F. Watkins, J.R. Behhng, E. Kariv and L.L. Miller, J. Am. Chem. Sot., 97 (1975) 3549; (d) Y. Minoura and M. Katano, J. Appl. Polym. Sci., 13 (1969) 2057; (e) C.M. Elliott and R.W. Murray, Anal. Chem., 48 (1976) 1247; (f) K. Ohkita, N. Nakayama and M. Shimomura, Carbon, 18 (1980) 277; (g) N. Tsubokawa, A. Funaki, Y. Hada and Y. Sone, J. Polym. Sci. Polym. Lett. Div., 20 (1982) 27. (a) B.D. Epstein, E. Dalle-Molle and J.S. Mattson, Carbon, 9 (1971) 609; (b) K.F. Blurton, Electrochim. Acta, 18 (1973) 869; (c) K. Kinoshita and J.A.S. Bett, Carbon, 11 (1973) 403; (d). K. Kinoshita and J.A.S. Bett, Carbon, 12 (1974) 525; (e) G. Bernard and J. Simonet, J. Electroanal. Chem., 112 (1980) 117.

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