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The reduction of tetraalkylammonium ions on metal electrodes
J. Electroanal. Chem., 195 (1985) 435-438 Elsevier Sequoia S.A., Lausanne -Printed in The Netherlands
435
Preliminary note THE REDUCTION OF TETRAALKYLAMMONIUM ELECTRODES
IONS ON METAL
CATHODIC CORROSION AND “TETRAALKYLAMMONIUM-METALS”
BSSIE KARIV-MILLER, Department
of Chemistry,
PHILLIP B. LAWIN and ZLATKO VAJTNER University of Minnesota, Minneapolis, MN 55455
(U.S.A.)
(Received 10th September 1985)
In contrast to the oxidative corrosion processes commonly observed with anodes, cathodic corrosion* of metals is an almost unexplored phenomenon. Most intriguing are the few reports involving the cathodic reduction of tetraalkylammonium cations on mercury [3] and graphite [4]. It was shown that, the cations and the cathode material participafe, but the nature of the processes or the products were not understood. It was proposed that amalgams are the products of mercury and that lamellar compounds are formed by graphite. In recent studies we have confirmed that reduction of simple tetraalkylammonium ions on mercury produces metastable solid products containing mercury and the tetraalkylammonium moiety [5,6]. Using dimethylpyrrolidinium (DMP) tetrafluoroborate in DMF the reduced product, a black solid, was sufficiently stable for electrochemical characterization [ 51. Additionally, it, has been shown that such tetraalkylammonium-mercury products are involved as catalysts in the preparative scale electroreduction [6] of numerous organic compounds and the nature of the organic product formed depends on the tetraalkylammonium catalyst [ 71. It seemed possible that it was not necessary to use a liquid electrode to observe such phenomena and it was interesting to find out if, like mercury, other metals would “corrode”. We report here that a number of metals do react cathodically in the presence of tetraalkylammonium salts, to produce solid products analogous to those formed by mercury and we believe this is logically identified as cathodic corrosion. Although the results are preliminary, these products seem to constitute a new class of tetraalkylammonium-metals. We first report visual observations, gravimetric and potential measurements which survey a number of metals to evaluate the occurrence of cathodic corrosion. The results are presented in Table 1. The experimental conditions were chosen following the detailed studies [5] on mercury. A diglyme solution (20 ml) containing 0.01 M (DMP)BF4 and 0.1 M tetrabutylammonium (TBA) tetrametal
*The reductive cleavage of alkyl halides and similar compounds on metal electrodes, to produce alkyl metals [ 11, could be considered cathodic corrosion, as could metal-amalgam
436 TABLE 1 Cathodic corrosion of various metals s Cathode Pb b Snb Sb c Bi d graphite d
E,IV
vs. SCE
Weight loss A w/mg
-2.1 -1.8 -1.9 -2.4 -2.0
14.0 13.0 4.0 7.3 -e
AM/mm01 cm-’ 2.7 4.4 4.1 6.8
Deposit color black dark rust orange rust dark blue black
a20 ml 0.01 M (DMP)BF, with 0.1 M (TBA)BF, as the electrolyte. Z = 20 mA, Q = 14.6 C. b25 cm’. c8 cma. d6 cma. eExtensive corrosion wss visible but, perhaps due to the porosity of graphite, reproducible values for weight loss were not obtained. 8s the supporting electrolyte was electrolyzed at constant current (20 mA). A divided cell was used, the amount of charge transferred was 14.6 C and the temperature was 0°C. The cathode potential was measured using an SCE and all potentials reported are vs. SCE. The potential values recorded during electrolysis were erratic, which is not surprising in view of the disruption of the metal surface. However, after the current was disconnected rather reproducible residual potentials (I&,& persisted and could be measured (Table 1). The fragile deposits which formed on the cathodes during electrolysis could be easily removed from the surface and had a powder like appearance. To determin whether the metal surface corrodes in this process, the cathodes were washed with water and acetone (to remove the powder deposits), dried and weighed. Comparing the weight of each electrode before and after electrolysis showed a considerable, reproducible weight loss for Pb, Sn, Sb, Bi and C (Table 1). In a different series of experiments, the reducing properties of the electrolysis products were tested by injecting 9-fluorenone or benzophenone into the cell immediately after disconnecting the current. The characteristic colors of the radical anions developed on the cathode surface and powder particles. 9-Fluorenone (red) was used for the dark.precipitates (Pb, Bi and C) and benzophenone (blue) for the reddish precipitates (Sn and Sb). fluoroborate
-1.6 0
1 5
10
TIME/min
15
437
When Ga and In were used, rather negative E res were measured, -2.3 V and -1.7 V respectively and surface reactions with 94luorenone were observed. However, only a small amount of white powder was visible on the surface of Ga, In appeared unchanged and the amount of gravimetric corrosion for either one was inconclusively small. Cr and Pt did not exhibit any indication of cathodic corrosion. It is hypothesized that corrosion depends in part on breaking apart the metal lattice. Experiments are in progress to determine whether it is possible to predict the occurrence of this cathodic corrosion process from physical properties of the metals, like the heat of vaporization and work function c One of the more stable systems* as judged by the decay of E, was DMPPb (Fig. 1) and it was selected for further study. Some results are briefly recapitulated here, to indicate that the cathodic corrosion products are tetraalkylammonium-metals. The constant current reduction of DMP on Pb was carried out under the conditions described above and the concentration of DMP was monitored by cyclic voltammetry on a mercury drop electrode [5]. As the black deposit formed on the cathode, concurrently the concentration of DMP in solution decreased. Upon interrupting the electrolysis after transfer of 0.25 mol electrons/ mol (for DMP) the amount of DMP remaining in the solution was equivalent to 75% of the original amount added. Admission of oxygen to the cell, at this experimental stage, caused an instantaneous decay of E, and an increase of the DMP concentration to 93% of its value before electrolysis. This experiment establishes that the deposit incorporated DMP as an intact entity, which could be released back into solution by oxidation. Thus, a DMP-metal composite, similar to that formed by Hg, can be obtained from Pb. It is a reducing agent and it can react in a catalytic sequence regenerating DMP. It is possible that the corrosion products formed using other tetraalkylammonium ions* and other metals are also tetraalkylammonium-metals. The work using Hg electrodes [6,7] has demonstrated that tetraalkylammonium-Hg provide a synthetically useful catalytic system, where the corrosion product acts to reduce organic compounds. We have now observed that using TBA salts and Pb or Sn electrodes, the reduction of fluorobenzene to benzene can be achieved [8]. Furthermore, we report that the reductive cyclization of 6-hepten-2one to cis-1 &dimethylcyclopentanol can be performed using DMPHg or DMP-C [9]. In all cases the corrosion product of the cathode seems to be intimately involved as a catalyst. In summary, we have shown for the first time that a number of solid metals will corrode cathodically in the presence of tetraalkylammonium ions. It is demonstrated that the corrosion produces tetraalkylammonium-metal composites and proposed that these are active electrocatalysts. *Corrosion was not limited to DMP electrolytes. In the presence of (TBA)BF, similar, but visually non-identical products were apparent on Pb, Sn, C and Bi. Positive reactions with Q-fluorenone and measurements of E - indicate that reductante were formed on these
438
ACKNOWLEDGEMENTS
The authors are grateful to the National Science Foundation for support of this research.
REFERENCES 1 (a) L.G. Feoktistov in M.M. Baizer and H. Lund (Eds.) Organic Electrochemistry, Marcel Dekker, New York, 1983, p. 276; (b) A.P. Tomilov, 6.0. Mairanovskii, M.Ya. Fioshin and V.A. Smirnov, Electrochemistry of Organic Compounds, Halsted Press, New York, 1972, p. 466. 2 2. Galus, Crit. Rev, Anal. Chem., (1975) 359. 3 (a) J.D. Littlehailes and B.J Woodhall, Discuss. Faraday Sot., 45 (1968) 187; (b) L. Horner in MM. Baizer and H. Lund (Eds.), Organic Electrochemistry, Marcel Dekker, New York, 1983, p. 397 and refs. therein. 4 (a) J. Simonet and H. Lund, J. Electroanal. Chem., 75 (1977) 719; (b) G. Bernard and J. Simonet, ibid., 112 (1980) 117; (c) J. Berthelot, M. Jubault and J. Simonet, J. Chem. Sot., Chem. Commun., (1982) 759. 5 (a) E. Kariv-Miller, C. Nanjundiah, J. Eaton and K.E. Swenson, J. Electroanal. Chem., 167 (1984) 141; (b) E. Kariv-Miller and R. Andruzzi, J. Electroanal. Chem., 187 (1985) 175. 6 (a) E. Kariv-Miller, K.E. Swenson and D. Zemach, J. Org. Chem., 48 (1983) 4210; (b) E. Kariv-Miller, K.E. Swenson, G.K. Lehman and R. Andruzzi, J. Org. Chem., 50 (1985) 557. 7 E. Kariv-Miller and Z. Vajtner, J. Org. Chem., 50 (1985) 1394. 8 Z. Vajtner, unpublished results, 9 T.J. Mahachi, unpublished results.
435
Preliminary note THE REDUCTION OF TETRAALKYLAMMONIUM ELECTRODES
IONS ON METAL
CATHODIC CORROSION AND “TETRAALKYLAMMONIUM-METALS”
BSSIE KARIV-MILLER, Department
of Chemistry,
PHILLIP B. LAWIN and ZLATKO VAJTNER University of Minnesota, Minneapolis, MN 55455
(U.S.A.)
(Received 10th September 1985)
In contrast to the oxidative corrosion processes commonly observed with anodes, cathodic corrosion* of metals is an almost unexplored phenomenon. Most intriguing are the few reports involving the cathodic reduction of tetraalkylammonium cations on mercury [3] and graphite [4]. It was shown that, the cations and the cathode material participafe, but the nature of the processes or the products were not understood. It was proposed that amalgams are the products of mercury and that lamellar compounds are formed by graphite. In recent studies we have confirmed that reduction of simple tetraalkylammonium ions on mercury produces metastable solid products containing mercury and the tetraalkylammonium moiety [5,6]. Using dimethylpyrrolidinium (DMP) tetrafluoroborate in DMF the reduced product, a black solid, was sufficiently stable for electrochemical characterization [ 51. Additionally, it, has been shown that such tetraalkylammonium-mercury products are involved as catalysts in the preparative scale electroreduction [6] of numerous organic compounds and the nature of the organic product formed depends on the tetraalkylammonium catalyst [ 71. It seemed possible that it was not necessary to use a liquid electrode to observe such phenomena and it was interesting to find out if, like mercury, other metals would “corrode”. We report here that a number of metals do react cathodically in the presence of tetraalkylammonium salts, to produce solid products analogous to those formed by mercury and we believe this is logically identified as cathodic corrosion. Although the results are preliminary, these products seem to constitute a new class of tetraalkylammonium-metals. We first report visual observations, gravimetric and potential measurements which survey a number of metals to evaluate the occurrence of cathodic corrosion. The results are presented in Table 1. The experimental conditions were chosen following the detailed studies [5] on mercury. A diglyme solution (20 ml) containing 0.01 M (DMP)BF4 and 0.1 M tetrabutylammonium (TBA) tetrametal
*The reductive cleavage of alkyl halides and similar compounds on metal electrodes, to produce alkyl metals [ 11, could be considered cathodic corrosion, as could metal-amalgam
436 TABLE 1 Cathodic corrosion of various metals s Cathode Pb b Snb Sb c Bi d graphite d
E,IV
vs. SCE
Weight loss A w/mg
-2.1 -1.8 -1.9 -2.4 -2.0
14.0 13.0 4.0 7.3 -e
AM/mm01 cm-’ 2.7 4.4 4.1 6.8
Deposit color black dark rust orange rust dark blue black
a20 ml 0.01 M (DMP)BF, with 0.1 M (TBA)BF, as the electrolyte. Z = 20 mA, Q = 14.6 C. b25 cm’. c8 cma. d6 cma. eExtensive corrosion wss visible but, perhaps due to the porosity of graphite, reproducible values for weight loss were not obtained. 8s the supporting electrolyte was electrolyzed at constant current (20 mA). A divided cell was used, the amount of charge transferred was 14.6 C and the temperature was 0°C. The cathode potential was measured using an SCE and all potentials reported are vs. SCE. The potential values recorded during electrolysis were erratic, which is not surprising in view of the disruption of the metal surface. However, after the current was disconnected rather reproducible residual potentials (I&,& persisted and could be measured (Table 1). The fragile deposits which formed on the cathodes during electrolysis could be easily removed from the surface and had a powder like appearance. To determin whether the metal surface corrodes in this process, the cathodes were washed with water and acetone (to remove the powder deposits), dried and weighed. Comparing the weight of each electrode before and after electrolysis showed a considerable, reproducible weight loss for Pb, Sn, Sb, Bi and C (Table 1). In a different series of experiments, the reducing properties of the electrolysis products were tested by injecting 9-fluorenone or benzophenone into the cell immediately after disconnecting the current. The characteristic colors of the radical anions developed on the cathode surface and powder particles. 9-Fluorenone (red) was used for the dark.precipitates (Pb, Bi and C) and benzophenone (blue) for the reddish precipitates (Sn and Sb). fluoroborate
-1.6 0
1 5
10
TIME/min
15
437
When Ga and In were used, rather negative E res were measured, -2.3 V and -1.7 V respectively and surface reactions with 94luorenone were observed. However, only a small amount of white powder was visible on the surface of Ga, In appeared unchanged and the amount of gravimetric corrosion for either one was inconclusively small. Cr and Pt did not exhibit any indication of cathodic corrosion. It is hypothesized that corrosion depends in part on breaking apart the metal lattice. Experiments are in progress to determine whether it is possible to predict the occurrence of this cathodic corrosion process from physical properties of the metals, like the heat of vaporization and work function c One of the more stable systems* as judged by the decay of E, was DMPPb (Fig. 1) and it was selected for further study. Some results are briefly recapitulated here, to indicate that the cathodic corrosion products are tetraalkylammonium-metals. The constant current reduction of DMP on Pb was carried out under the conditions described above and the concentration of DMP was monitored by cyclic voltammetry on a mercury drop electrode [5]. As the black deposit formed on the cathode, concurrently the concentration of DMP in solution decreased. Upon interrupting the electrolysis after transfer of 0.25 mol electrons/ mol (for DMP) the amount of DMP remaining in the solution was equivalent to 75% of the original amount added. Admission of oxygen to the cell, at this experimental stage, caused an instantaneous decay of E, and an increase of the DMP concentration to 93% of its value before electrolysis. This experiment establishes that the deposit incorporated DMP as an intact entity, which could be released back into solution by oxidation. Thus, a DMP-metal composite, similar to that formed by Hg, can be obtained from Pb. It is a reducing agent and it can react in a catalytic sequence regenerating DMP. It is possible that the corrosion products formed using other tetraalkylammonium ions* and other metals are also tetraalkylammonium-metals. The work using Hg electrodes [6,7] has demonstrated that tetraalkylammonium-Hg provide a synthetically useful catalytic system, where the corrosion product acts to reduce organic compounds. We have now observed that using TBA salts and Pb or Sn electrodes, the reduction of fluorobenzene to benzene can be achieved [8]. Furthermore, we report that the reductive cyclization of 6-hepten-2one to cis-1 &dimethylcyclopentanol can be performed using DMPHg or DMP-C [9]. In all cases the corrosion product of the cathode seems to be intimately involved as a catalyst. In summary, we have shown for the first time that a number of solid metals will corrode cathodically in the presence of tetraalkylammonium ions. It is demonstrated that the corrosion produces tetraalkylammonium-metal composites and proposed that these are active electrocatalysts. *Corrosion was not limited to DMP electrolytes. In the presence of (TBA)BF, similar, but visually non-identical products were apparent on Pb, Sn, C and Bi. Positive reactions with Q-fluorenone and measurements of E - indicate that reductante were formed on these
438
ACKNOWLEDGEMENTS
The authors are grateful to the National Science Foundation for support of this research.
REFERENCES 1 (a) L.G. Feoktistov in M.M. Baizer and H. Lund (Eds.) Organic Electrochemistry, Marcel Dekker, New York, 1983, p. 276; (b) A.P. Tomilov, 6.0. Mairanovskii, M.Ya. Fioshin and V.A. Smirnov, Electrochemistry of Organic Compounds, Halsted Press, New York, 1972, p. 466. 2 2. Galus, Crit. Rev, Anal. Chem., (1975) 359. 3 (a) J.D. Littlehailes and B.J Woodhall, Discuss. Faraday Sot., 45 (1968) 187; (b) L. Horner in MM. Baizer and H. Lund (Eds.), Organic Electrochemistry, Marcel Dekker, New York, 1983, p. 397 and refs. therein. 4 (a) J. Simonet and H. Lund, J. Electroanal. Chem., 75 (1977) 719; (b) G. Bernard and J. Simonet, ibid., 112 (1980) 117; (c) J. Berthelot, M. Jubault and J. Simonet, J. Chem. Sot., Chem. Commun., (1982) 759. 5 (a) E. Kariv-Miller, C. Nanjundiah, J. Eaton and K.E. Swenson, J. Electroanal. Chem., 167 (1984) 141; (b) E. Kariv-Miller and R. Andruzzi, J. Electroanal. Chem., 187 (1985) 175. 6 (a) E. Kariv-Miller, K.E. Swenson and D. Zemach, J. Org. Chem., 48 (1983) 4210; (b) E. Kariv-Miller, K.E. Swenson, G.K. Lehman and R. Andruzzi, J. Org. Chem., 50 (1985) 557. 7 E. Kariv-Miller and Z. Vajtner, J. Org. Chem., 50 (1985) 1394. 8 Z. Vajtner, unpublished results, 9 T.J. Mahachi, unpublished results.