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Solvent effects on the redox potentials of tetraethylammonium hexacyanomanganate(III) and hexacyanoferrate(III)
J. Electroanal. Chem., 90 (1978) 203--210 © Elsevier Sequoia S.A., Lausanne - - Printed in The Netherlands
203
SOLVENT EFFECTS ON THE REDOX POTENTIALS OF TETRAETHYLAMMONIUM HEXACYANOMANGANATE(III) AND HEXACYANOFERRATE(III)
G. GRITZNER, K. DANKSAGMULLER and V. GUTMANN
Institute of Inorganic Chemistry, Technical University Vienna, Getreidemarkt 9, A-1060 Vienna (Austria) (Received 6th October 1977)
ABSTRACT Tetraethylammonium hexacyanomanganate(III) was studied in formamide, N-methylformamide, methanol, propylenecarbonate, N,N-dimethylthioformamide, N-methylthiopyrrolidinone(2), butyrolactone, acetonitrile, dimethylsulfoxide, N,N-dimethylformamide, N-methylpyrrolidinone(2)~ nitromethane and tetramethylenesulfone employing polarographic and voltammetric techniques. Reversible or nearly reversible behaviour for the reaction Mn(CN)63--/Mn(CN) 2 - was observed in most solvents on the stationary platinum electrode. The reaction Mn(CN)3--/Mn(CN) 4 - was studied on both the dropping mercury electrode and the stationary platinum electrode. Besides the reaction Mn(CN)3--/Mn(CN)64-- several anodic waves due to successive reactions of mercury with the cyano-ligand of the complex occurred at the dropping mercury electrode. No redox reaction for (et4N)3Mn(CN)6 was found in nitromethane. The polarographic behaviour of tetraethylammonium hexacyanoferrate(III) was studied in formamide, N-methylformamide, N-methylpyrrolidinone(2) and butyrolactone. The variation of E 1/2 and 1 (Epa + Ep c ) values versus bisphenylchromium(I)/ bisbiphenylchromium(0) as reference redox system of the processes Mn(CN)~--/Mn(CN)~--, Mn(CN)63--/Mn(CN)64-- and Fe(CN)3--/Fe(CN)64-- with the nature of the solvent is discussed within the donor-acceptor concept. Correlations between the El~2 and 1 (Epa + Epc) values and the acceptor properties of the solvent have been observed. The preparation of tetraethyl- and tetrabutylammonium hexacyanomanganate(III) is described.
INTRODUCTION
Solvent effects on the redox potentials of anions so far have been studied only on tetraethylammonium hexacyanoferrate(III} and tetrabutylammonium hexacyanoferrate(III} [1,2]. Tetraethylammonium hexacyanomanganate(III) was chosen as a further model system to enlarge the understanding of solvent effects on anions. In order to study solvent effects on redox properties of (et4N)sMn(CN)s by polarographic and voltammetric methods it was necessary to find reversible redox processes in which both forms of the redox couple remain hexacoordinated by the cyanide ligands. In cases where the number of ligands changes upon either reduction or oxidation the effect of ligand and the solvent cannot generally be separated. As in previous studies all potentials are referred to bisbiphenylchromium(I)/bisbiphenylchromium(0) [BBCr(I}/BBCr(0)] as reference redox system [1--3].
204 EXPERIMENTAL The purification of solvents and the apparatus for polarographic and voltammetric measurements have been described previously [1--4]. The u.v.-visible spectrum was measured on a Cary 17 D spectrophotometer. All procedures were carried out under dry nitrogen. The preparation of tetraethylammonium hexacyanoferrate(III) [2] and of bisbiphenylchromium iodide [5] has already been reported. Potassium hexacyanomanganate(III) prepared according to a published method [6] was reacted with the stoichiometric amount of tetraethylammonium perchlorate in absolute methanol. The mixture containing undissolved potassium hexacyanomanganate(III) was stirred overnight under the exclusion of moisture. Upon filtration of the precipitated KC104, the solution was concentrated and cooled to --10°C. The crystallized tetraethylammonium hexacyanomanganate(III) was filtered and then redissolved in ethanol. Traces of KC104 were removed by filtration and the tetraethylammonium hexacyanomanganate(III) was Crystallized from ethanol and dried at 50 ° and 10 - a Torr. [(C2Hs)4N]aMn(CN)6, MW: 601.82. Calculated: C, 59.87, H, 10.05, N, 20.94; found: C, 59.10; H, 10.12, N, 20.32. Tetrabutylammonium hexacyanomanganate(III) was prepared by metathesis of tetrabutylammonium perchlorate and potassium hexacyanomanganate(III) in absolute ethanol. The filtered solution was cooled to --10°C and diethylether was added. A syrup containing crystals formed upon reaching room temperature. Several cooling steps to --10°C followed by warming to room temperature increased the crystalline fraction. The crystals were filtered and dried at 50°C a n d l 0 - 3 Torr. [(C4H9)4N]sMn(CN)6, MW: 938.47 analyzed as follows: calculated: C, 69.11%, H, 11.60%; N, 13.43%; found: C, 68.87%, H, 11.31%, N, 13.09%. 0.1 M solutions of tetraethylammonium perchlorate served as supporting electrolyte in all solvents but methanol where a saturated solution of tetraethylammonium perchlorate was used. All measurements were made at 25 ° + 0.02°C. RESULTS One cathodic wave was observed in all solvents but methanol during the studies of tetraethylammonium hexacyanomanganate(III) on the dropping mercury electrode. The limiting current was controlled by diffusion. Two cathodic waves appeared in methanol. Besides the cathodic wave, anodic waves were also observed on the dropping mercury electrode in several solvents. The half-wave potentials and the E3/4 --E1/4 values are listed in Table 1. The E3/4 --E1/4 values indicate a reversible or nearly reversible one-electron process for the cathodic waves. Employing a Kalousek commutator, reversible behaviour was found at frequencies above 20 Hz. At lower frequencies the anodic part rapidly decreased, completely disappearing at frequencies of 5 Hz. Since the reduced form of this process is therefore not very stable in solution, a comparison of the heights of the (et4N)sFe(CN)6 wave, where a reversible one-electron reduction has been established, and of the (et4N)3Mn(CN)6 wave at the same concentration was made. Based on such a comparison the number of electrons participating in this process was found to be one. These findings were confirmed
205
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206 by data obtained on the stationary platinum electrode, which are discussed later. The cathodic wave corresponds to a reduction of (et4N)3 Mn(CN)6 to (et4N)4Mn(CN)6. A corresponding pair of cathodic and anodic peaks due to the reaction Mn(CN)~-/Mn(CN) 2 - and a cathodic peak due to the reaction Mn(CN)~-/ Mn(CN)~- were observed on the stationary platinum electrode at slow scan rates (10--50 mV s- I ) in all the solvents studied but N-methylformamide. In N-methylformamide corresponding cathodic and anodic peaks appeared for both reactions. The peak currents in all cases were controlled by diffusion. 1 (Epa + Epe ) values as well as (Epa --Epc) values are listed in Table 1. The (Epa --Epc ) values are uncorrected for the ohmic drop between the working electrode and the reference electrode. The peak heights of the cathodic peaks for both of the above mentioned reactions were the same. Since coulometric determinations yielded one as the number of electrons for the reaction assigned to the process Mn(CN)3-/Mn(CN) 2 - , the comparison of peak heights confirmed the one-electron reaction previously assigned to the reaction Mn(CN)3-/Mn(CN)4,. Large scale electrolysis in DMSO led to the preparat i o n of (et4N)2Mn(CN)e in solution. The u.v.-visible spectrum o f (etaN)sMn(CN)e in methanol is given in Table 2 together with the previously reported spectrum of K3Mn(CN)s in 1.5 M KCN solution [ 7 ]. The anodic waves observed in several solvents were in m a n y cases accompanied with maxima and adsorption phenomena. No corresponding process could be observed on the stationary platinum electrode. On the mercury electrode successive reaction of the electrode material with the cyano-ligands forming mercury cyanides can be held responsible for the anodic waves observed. Since they could not be related to an oxidation process in which the number of ligands and the form of coordination remains the same these waves were not studied in detail. The number of waves and the E1/2(BBCr ) are given below: in acetonitrile three anodic waves with E1/2(BBCr ) of +0.77 V, +0.66 V and +0.30 V were observed. The E1/2(BBCr ) for three anodic waves in butyrolactone were +1.03 V, +0.65 V, and +0.43 V. No anodic waves were recorded in N-methylthiopyrrolidinone(2),
TABLE 2 U.V.-visible spectra and molar absorption coefficient (e) of (et4N)3Mn(CN)6 in methanol and of K3Mn(CN)6 in 1.5 M solutions of KCN in water sh = shoulder (et4N)3Mn(CN)6 in methanol
KsMn(CN) 6 in 1.5 M aqueous KCN solution
v/cm--1
e
v/cm-1
23800 sh 31130 37300 sh 40000 sh 41770 50500 sh
204 sh 1830 3080 sh 4059 sh 4277 8520
21000 30800 37200 40200 sh 42200 sh >50000
e 4 2980 1680 3540 sh 4110 sh
207 TABLE 3 Polarographic and voltammetric data of tetraethylammonium hexacyanoferrate(III) in various solvents All potentials vs. BBCr(I)/BBCr(0 ) as reference redox system. Supporting electrolyte 0.1 M tetraethylammonium perchlorate Solvent
EI/2/V
(E3/4 -- Ell4)/mV
Butyrolactone (BL) Formamide (FA) N-Methylformamide (NMF) N-Methylthiopyrrolidinone(2) (NMTP)
--0.246 +0.780 +0.62 s
65 57 58
---0.260
60
two anodic waves (El/2¢BBCr), +0.42 V and +0.17 V) occurred in N-methylpyrrolidinone(2). In propylenecarbonate two flat waves (El/2¢ssc~, +1.14 V and +0.74 V) and one wave perturbed by a maximum of the first kind (El12{BBC~, +0.41 V) were observed. Poorly defined anodic waves with maxima occurred in dimethylsulfoxide and dimethylformamide. In methanol one extremely flat anodic wave and two cathodic waves were observed. The wave at Elt2~BBC~ -- 0.23 V corresponds to the process Mn(CN)~-/ Mn(CN)~-. A further wave with a maximum at E1/2CBBCr~ -- 1.54 V was observed. This wave was very close to the end of the potential range at the dropping mercury electrode and thus could not be evaluated in detail. In acetonitrile decomposition of (et4N)sMn(CN)6 was observed after some time yielding a dark green solution. A similar solution was obtained in N-methylpyrrolidinone(2) upon heating. The data for polarographic reduction of (et4N)sFe(CN)6 in butyrolactone, formamide, N-methylformamide and N-methylthiopyrrolidinone(2) are summarized in Table 3. A reversible or nearly reversible electrode process was found for the reaction Fe(CN)~-/Fe(CN)~-. DISCUSSION
The redox reactions of Mn(CN)~- lead to Mn(CN)62- and Mn(CN)~-, respectively, the potassium salts of which are known. K2Mn(CN)6 has been obtained by oxidation of KsMn(CN)6 with NOC1 in dimethylformamide [10,11] and found to be sensitive to hydrolysis and light. It decomposes slowly in many solvents. K4Mn(CN)6 • 3 H20 is very sensitive to oxidation and hydrolysis and known only as the trihydrate [7,12--15], just as the K4Fe(CN)6 • 3 H20. All of the hexacyanomanganates Mn(CN)~-, Mn(CN)~- and Mn(CN)~- are stable in water only in the presence of excess cyanide ions. The redox potentials of both the oxidation of (et4N)sMn(CN)6 to (et4N)2Mn(CN)6 and the reduction of (et4N)sMn(CN)6 to (et4N)4Mn(CN)6 vary considerably with the nature of the solvent. It was of interest to investigate whether such shifts in redox potentials can be correlated with solvent parameters. Plots of the El/2¢BBC~ and the ½ (Epa + Epe)CBscr~values versus the dielectric constant, or the dipole moment showed no correlations, whereas linear relationships were
208
found with the solvent acceptor number (Figs. 1 and 2). These results can be explained on basis of the donor-acceptor concept [8] for solvent-solute interactions between the hexacyanomanganate ions as donors and the solvent molecules as acceptors. The acceptor n u m b e r has been recently introduced as a measure of the solvent acceptor properties [9]. The donor properties of the hexacyanomanganate ions are exhibited via the nitrogen atoms of the cyano ligands. A decrease in formal oxidation state of the manganese ion will lead to an increase in donor properties of the complex since more negative charge will be located on the nitrogen atoms. The donor properties of the hexacyanomanganate ions therefore increase in the order Mn(CN) 2 - , Mn(CN)6S - and Mn(CN)64-. Thus in both of the redox couples studied, the reduced form is more strongly effected by the interaction with the solvent than the oxidized form. Hence a shift in redox potential towards positive values is observed the extent of which is a function of the solvent acceptor number. There are a few deviations from the relationship presented in Figs. 1 and 2, the most apparent ones occurring in methanol. No explanation for such behaviour in methanol can be offered at this time. The oxidation of Mn(CN)~- in
MeOH
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E,~.,~,~/ v ¢2 (Epo.Epo) I v Fig. 1. Relationship between the half-wave potentials of the reduction (et4N)3Mn(CN)6/ (et4N)4Mn(CN)6 versus bisbiphenylchromium and the acceptor number of the solvents studied. (FA) Formamide, (NMF) N-methylformamide, (MeOH) methanol, (PC) propylene carbonate, (DMTF) dimethylthioformamide, (DMF) dimethylformamide, (DMSO) dimethylsulfoxide, (BL) butyrolactone, (NMTP) N-methylthiopyrrolidinone(2), (NMP) N-methylpyrrolidinone(2), (CH3CN) acetonitrile. Fig. 2. Relationship between the ½ (Epa + Epc ) values versus bisbiphenylchromium of the reaction (et4N)3Mn(CN)6/(et4N)2Mn(CN)6 and the acceptor number of the solvents studied. (FA) Formamide, (NMF) N-methylformamide, (MeOH) methanol, (PC) propylene carbonate, (DMTF) dimethylthioformamide, (DMF) dimethylformamide, (DMSO) dimethylsulfoxide, (BL) butyrolactone, (NMTP) N-methylthiopyrrolidinone(2), (NMP) N-methylpyrrolidinone(2), (CH3CN) acetonitrile.
209
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Fig. 3. Relationship between the half-wave potentials versus bisbiphenylchromium of the reductions of (et4N)3Mn(CN)6 and (et4N)3Fe(CN)6 in several solvents. (FA) Formamide, (NMF) N-methylformamide, (MeOH) methanol, (PC) propylene carbonate, (DMTF) dimethylthioformamide, (DMF) dimethylformamide, (DMSO) dimethylsulfoxide, (BL) butyrolactone, (NMTP) N-methylthiopyrrolidinone(2), (NMP) N-methylpyrrolidinone(2), (CH3CN)acetonitrile.
propylene carbonate is irreversible and this is the most likely reason for the deviation of the ½ (Epa + Epc)(SBC~) value in this solvent (Fig. 2). Modest deviations from the plot acceptor number versus E1/2(Bscr) and ! (Ep + Ea),however, are to be expected. From studies on the redox po2 tentials of the system Fe(CN)~-/Fe(CN)~- it is known that the redox potential of this system is strongly influenced by the addition of other electrolytes [1,2]. The E1/2(sscr) values of the system Fe(CN}~-/Fe(CN)~- were found to differ considerably in solutions containing et4NC104 and bu4NC104 as supporting electrolytes [ 1 ]. For the very similar hexacyanomanganates the solvent-anion interaction therefore will also be overlapped by cation-anion interactions. As the acceptor properties and the dielectric constant of the solvent decrease cationanion interaction will increase. Such cation-anion interactions will also affect the redox potentials and can be held responsible for deviations from the relationship between the acceptor number and the Ez/~(sscr) and ~ (E~ -- E~) values, respectively. Figure 3 shows a plot of the Ez/2
Thanks are due to the Fonds zur FSrderung der wissenschaftlichen Forschung in ()sterreich for generous financial support (project 3004}.
210 REFERENCES 1 2 3 4 5
6 7 8 9 10 11 12 13 14 15
G. V. G. G.
Gritzner, K. Danksagmiiller and V. Gutmann, J. Electroanal. Chem., 72 (1976) 177. Gutmann, G. Gritzner and K. Danksagmiiller, Inorg. Chim. Acta, 17 (1976) 81. Gritzner, Inorg. Chim. Acta, 24 (1977) 5. Gritzner, Monatsh. Chem., 107 (1976) 1056. V. G u t m a n n and G. Peychal-Hefling, Monatsh. Chem., 100 (1969) 813. J.A. Lower and W.C. Fernelius, Inorg. Synthesis, 2 (1946) 213. J.J. Alex ander and H.B. Gray, J. Amer. Chem. Soc., 90 (1968) 4260. V. Gutmann, FAectrochim. Acta, 21 (1976) 661; V. C-utmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York, to appear early 1978. U. Mayer, W. Gerger and V. Gutmann, Monatsh. Chem., 106 (1975) 1235. J.R. Fowler and J. IGeinberg, Inorg. Chem., 9 (1970) 1006. G. Trageser and H.H. Eysel, Z. Anorg. Allgem. Chem., 420 (1976) 273. B.N. Figgis, Trans. Faraday Soc., 57 (1961) 204. O.T. Christensen, J. Prakt. Chem., [2] 31 (1885) 171. A. Descamps, Ann. Chim. Phys., [5] 24 (1881) 178. J.H. Eaton and R. Fittig, Liebigs Ann. Chem., 145 (1868) 157.
203
SOLVENT EFFECTS ON THE REDOX POTENTIALS OF TETRAETHYLAMMONIUM HEXACYANOMANGANATE(III) AND HEXACYANOFERRATE(III)
G. GRITZNER, K. DANKSAGMULLER and V. GUTMANN
Institute of Inorganic Chemistry, Technical University Vienna, Getreidemarkt 9, A-1060 Vienna (Austria) (Received 6th October 1977)
ABSTRACT Tetraethylammonium hexacyanomanganate(III) was studied in formamide, N-methylformamide, methanol, propylenecarbonate, N,N-dimethylthioformamide, N-methylthiopyrrolidinone(2), butyrolactone, acetonitrile, dimethylsulfoxide, N,N-dimethylformamide, N-methylpyrrolidinone(2)~ nitromethane and tetramethylenesulfone employing polarographic and voltammetric techniques. Reversible or nearly reversible behaviour for the reaction Mn(CN)63--/Mn(CN) 2 - was observed in most solvents on the stationary platinum electrode. The reaction Mn(CN)3--/Mn(CN) 4 - was studied on both the dropping mercury electrode and the stationary platinum electrode. Besides the reaction Mn(CN)3--/Mn(CN)64-- several anodic waves due to successive reactions of mercury with the cyano-ligand of the complex occurred at the dropping mercury electrode. No redox reaction for (et4N)3Mn(CN)6 was found in nitromethane. The polarographic behaviour of tetraethylammonium hexacyanoferrate(III) was studied in formamide, N-methylformamide, N-methylpyrrolidinone(2) and butyrolactone. The variation of E 1/2 and 1 (Epa + Ep c ) values versus bisphenylchromium(I)/ bisbiphenylchromium(0) as reference redox system of the processes Mn(CN)~--/Mn(CN)~--, Mn(CN)63--/Mn(CN)64-- and Fe(CN)3--/Fe(CN)64-- with the nature of the solvent is discussed within the donor-acceptor concept. Correlations between the El~2 and 1 (Epa + Epc) values and the acceptor properties of the solvent have been observed. The preparation of tetraethyl- and tetrabutylammonium hexacyanomanganate(III) is described.
INTRODUCTION
Solvent effects on the redox potentials of anions so far have been studied only on tetraethylammonium hexacyanoferrate(III} and tetrabutylammonium hexacyanoferrate(III} [1,2]. Tetraethylammonium hexacyanomanganate(III) was chosen as a further model system to enlarge the understanding of solvent effects on anions. In order to study solvent effects on redox properties of (et4N)sMn(CN)s by polarographic and voltammetric methods it was necessary to find reversible redox processes in which both forms of the redox couple remain hexacoordinated by the cyanide ligands. In cases where the number of ligands changes upon either reduction or oxidation the effect of ligand and the solvent cannot generally be separated. As in previous studies all potentials are referred to bisbiphenylchromium(I)/bisbiphenylchromium(0) [BBCr(I}/BBCr(0)] as reference redox system [1--3].
204 EXPERIMENTAL The purification of solvents and the apparatus for polarographic and voltammetric measurements have been described previously [1--4]. The u.v.-visible spectrum was measured on a Cary 17 D spectrophotometer. All procedures were carried out under dry nitrogen. The preparation of tetraethylammonium hexacyanoferrate(III) [2] and of bisbiphenylchromium iodide [5] has already been reported. Potassium hexacyanomanganate(III) prepared according to a published method [6] was reacted with the stoichiometric amount of tetraethylammonium perchlorate in absolute methanol. The mixture containing undissolved potassium hexacyanomanganate(III) was stirred overnight under the exclusion of moisture. Upon filtration of the precipitated KC104, the solution was concentrated and cooled to --10°C. The crystallized tetraethylammonium hexacyanomanganate(III) was filtered and then redissolved in ethanol. Traces of KC104 were removed by filtration and the tetraethylammonium hexacyanomanganate(III) was Crystallized from ethanol and dried at 50 ° and 10 - a Torr. [(C2Hs)4N]aMn(CN)6, MW: 601.82. Calculated: C, 59.87, H, 10.05, N, 20.94; found: C, 59.10; H, 10.12, N, 20.32. Tetrabutylammonium hexacyanomanganate(III) was prepared by metathesis of tetrabutylammonium perchlorate and potassium hexacyanomanganate(III) in absolute ethanol. The filtered solution was cooled to --10°C and diethylether was added. A syrup containing crystals formed upon reaching room temperature. Several cooling steps to --10°C followed by warming to room temperature increased the crystalline fraction. The crystals were filtered and dried at 50°C a n d l 0 - 3 Torr. [(C4H9)4N]sMn(CN)6, MW: 938.47 analyzed as follows: calculated: C, 69.11%, H, 11.60%; N, 13.43%; found: C, 68.87%, H, 11.31%, N, 13.09%. 0.1 M solutions of tetraethylammonium perchlorate served as supporting electrolyte in all solvents but methanol where a saturated solution of tetraethylammonium perchlorate was used. All measurements were made at 25 ° + 0.02°C. RESULTS One cathodic wave was observed in all solvents but methanol during the studies of tetraethylammonium hexacyanomanganate(III) on the dropping mercury electrode. The limiting current was controlled by diffusion. Two cathodic waves appeared in methanol. Besides the cathodic wave, anodic waves were also observed on the dropping mercury electrode in several solvents. The half-wave potentials and the E3/4 --E1/4 values are listed in Table 1. The E3/4 --E1/4 values indicate a reversible or nearly reversible one-electron process for the cathodic waves. Employing a Kalousek commutator, reversible behaviour was found at frequencies above 20 Hz. At lower frequencies the anodic part rapidly decreased, completely disappearing at frequencies of 5 Hz. Since the reduced form of this process is therefore not very stable in solution, a comparison of the heights of the (et4N)sFe(CN)6 wave, where a reversible one-electron reduction has been established, and of the (et4N)3Mn(CN)6 wave at the same concentration was made. Based on such a comparison the number of electrons participating in this process was found to be one. These findings were confirmed
205
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206 by data obtained on the stationary platinum electrode, which are discussed later. The cathodic wave corresponds to a reduction of (et4N)3 Mn(CN)6 to (et4N)4Mn(CN)6. A corresponding pair of cathodic and anodic peaks due to the reaction Mn(CN)~-/Mn(CN) 2 - and a cathodic peak due to the reaction Mn(CN)~-/ Mn(CN)~- were observed on the stationary platinum electrode at slow scan rates (10--50 mV s- I ) in all the solvents studied but N-methylformamide. In N-methylformamide corresponding cathodic and anodic peaks appeared for both reactions. The peak currents in all cases were controlled by diffusion. 1 (Epa + Epe ) values as well as (Epa --Epc) values are listed in Table 1. The (Epa --Epc ) values are uncorrected for the ohmic drop between the working electrode and the reference electrode. The peak heights of the cathodic peaks for both of the above mentioned reactions were the same. Since coulometric determinations yielded one as the number of electrons for the reaction assigned to the process Mn(CN)3-/Mn(CN) 2 - , the comparison of peak heights confirmed the one-electron reaction previously assigned to the reaction Mn(CN)3-/Mn(CN)4,. Large scale electrolysis in DMSO led to the preparat i o n of (et4N)2Mn(CN)e in solution. The u.v.-visible spectrum o f (etaN)sMn(CN)e in methanol is given in Table 2 together with the previously reported spectrum of K3Mn(CN)s in 1.5 M KCN solution [ 7 ]. The anodic waves observed in several solvents were in m a n y cases accompanied with maxima and adsorption phenomena. No corresponding process could be observed on the stationary platinum electrode. On the mercury electrode successive reaction of the electrode material with the cyano-ligands forming mercury cyanides can be held responsible for the anodic waves observed. Since they could not be related to an oxidation process in which the number of ligands and the form of coordination remains the same these waves were not studied in detail. The number of waves and the E1/2(BBCr ) are given below: in acetonitrile three anodic waves with E1/2(BBCr ) of +0.77 V, +0.66 V and +0.30 V were observed. The E1/2(BBCr ) for three anodic waves in butyrolactone were +1.03 V, +0.65 V, and +0.43 V. No anodic waves were recorded in N-methylthiopyrrolidinone(2),
TABLE 2 U.V.-visible spectra and molar absorption coefficient (e) of (et4N)3Mn(CN)6 in methanol and of K3Mn(CN)6 in 1.5 M solutions of KCN in water sh = shoulder (et4N)3Mn(CN)6 in methanol
KsMn(CN) 6 in 1.5 M aqueous KCN solution
v/cm--1
e
v/cm-1
23800 sh 31130 37300 sh 40000 sh 41770 50500 sh
204 sh 1830 3080 sh 4059 sh 4277 8520
21000 30800 37200 40200 sh 42200 sh >50000
e 4 2980 1680 3540 sh 4110 sh
207 TABLE 3 Polarographic and voltammetric data of tetraethylammonium hexacyanoferrate(III) in various solvents All potentials vs. BBCr(I)/BBCr(0 ) as reference redox system. Supporting electrolyte 0.1 M tetraethylammonium perchlorate Solvent
EI/2/V
(E3/4 -- Ell4)/mV
Butyrolactone (BL) Formamide (FA) N-Methylformamide (NMF) N-Methylthiopyrrolidinone(2) (NMTP)
--0.246 +0.780 +0.62 s
65 57 58
---0.260
60
two anodic waves (El/2¢BBCr), +0.42 V and +0.17 V) occurred in N-methylpyrrolidinone(2). In propylenecarbonate two flat waves (El/2¢ssc~, +1.14 V and +0.74 V) and one wave perturbed by a maximum of the first kind (El12{BBC~, +0.41 V) were observed. Poorly defined anodic waves with maxima occurred in dimethylsulfoxide and dimethylformamide. In methanol one extremely flat anodic wave and two cathodic waves were observed. The wave at Elt2~BBC~ -- 0.23 V corresponds to the process Mn(CN)~-/ Mn(CN)~-. A further wave with a maximum at E1/2CBBCr~ -- 1.54 V was observed. This wave was very close to the end of the potential range at the dropping mercury electrode and thus could not be evaluated in detail. In acetonitrile decomposition of (et4N)sMn(CN)6 was observed after some time yielding a dark green solution. A similar solution was obtained in N-methylpyrrolidinone(2) upon heating. The data for polarographic reduction of (et4N)sFe(CN)6 in butyrolactone, formamide, N-methylformamide and N-methylthiopyrrolidinone(2) are summarized in Table 3. A reversible or nearly reversible electrode process was found for the reaction Fe(CN)~-/Fe(CN)~-. DISCUSSION
The redox reactions of Mn(CN)~- lead to Mn(CN)62- and Mn(CN)~-, respectively, the potassium salts of which are known. K2Mn(CN)6 has been obtained by oxidation of KsMn(CN)6 with NOC1 in dimethylformamide [10,11] and found to be sensitive to hydrolysis and light. It decomposes slowly in many solvents. K4Mn(CN)6 • 3 H20 is very sensitive to oxidation and hydrolysis and known only as the trihydrate [7,12--15], just as the K4Fe(CN)6 • 3 H20. All of the hexacyanomanganates Mn(CN)~-, Mn(CN)~- and Mn(CN)~- are stable in water only in the presence of excess cyanide ions. The redox potentials of both the oxidation of (et4N)sMn(CN)6 to (et4N)2Mn(CN)6 and the reduction of (et4N)sMn(CN)6 to (et4N)4Mn(CN)6 vary considerably with the nature of the solvent. It was of interest to investigate whether such shifts in redox potentials can be correlated with solvent parameters. Plots of the El/2¢BBC~ and the ½ (Epa + Epe)CBscr~values versus the dielectric constant, or the dipole moment showed no correlations, whereas linear relationships were
208
found with the solvent acceptor number (Figs. 1 and 2). These results can be explained on basis of the donor-acceptor concept [8] for solvent-solute interactions between the hexacyanomanganate ions as donors and the solvent molecules as acceptors. The acceptor n u m b e r has been recently introduced as a measure of the solvent acceptor properties [9]. The donor properties of the hexacyanomanganate ions are exhibited via the nitrogen atoms of the cyano ligands. A decrease in formal oxidation state of the manganese ion will lead to an increase in donor properties of the complex since more negative charge will be located on the nitrogen atoms. The donor properties of the hexacyanomanganate ions therefore increase in the order Mn(CN) 2 - , Mn(CN)6S - and Mn(CN)64-. Thus in both of the redox couples studied, the reduced form is more strongly effected by the interaction with the solvent than the oxidized form. Hence a shift in redox potential towards positive values is observed the extent of which is a function of the solvent acceptor number. There are a few deviations from the relationship presented in Figs. 1 and 2, the most apparent ones occurring in methanol. No explanation for such behaviour in methanol can be offered at this time. The oxidation of Mn(CN)~- in
MeOH
40 ~A
30
2
0
40
l
\
F•MeOH
3o
\ ,el
~ 2o
\c,.3c M
E
PC- "XoDM F
'~
20 PC DMTF~DMF
NMPo~ i
o
-02 -o.,~ -o.s -o.8 -7.0
~
NMP
10 ~.6
I/
1.2
1.o
o.e
o.6
E,~.,~,~/ v ¢2 (Epo.Epo) I v Fig. 1. Relationship between the half-wave potentials of the reduction (et4N)3Mn(CN)6/ (et4N)4Mn(CN)6 versus bisbiphenylchromium and the acceptor number of the solvents studied. (FA) Formamide, (NMF) N-methylformamide, (MeOH) methanol, (PC) propylene carbonate, (DMTF) dimethylthioformamide, (DMF) dimethylformamide, (DMSO) dimethylsulfoxide, (BL) butyrolactone, (NMTP) N-methylthiopyrrolidinone(2), (NMP) N-methylpyrrolidinone(2), (CH3CN) acetonitrile. Fig. 2. Relationship between the ½ (Epa + Epc ) values versus bisbiphenylchromium of the reaction (et4N)3Mn(CN)6/(et4N)2Mn(CN)6 and the acceptor number of the solvents studied. (FA) Formamide, (NMF) N-methylformamide, (MeOH) methanol, (PC) propylene carbonate, (DMTF) dimethylthioformamide, (DMF) dimethylformamide, (DMSO) dimethylsulfoxide, (BL) butyrolactone, (NMTP) N-methylthiopyrrolidinone(2), (NMP) N-methylpyrrolidinone(2), (CH3CN) acetonitrile.
209
DMSO /
l- 1,0
SLb,O"NMF NMTP".~'DMF pr .~ 0 CH~CN
0,6
i ~-o2 - 0/ .
-Off "~F,4
' i
o,e
o,~
i
o,4
o2
i
o,o -o2
-o,~
E,/2(ooc,)[~'o(c~),3"/F,IC~)~-1/V
Fig. 3. Relationship between the half-wave potentials versus bisbiphenylchromium of the reductions of (et4N)3Mn(CN)6 and (et4N)3Fe(CN)6 in several solvents. (FA) Formamide, (NMF) N-methylformamide, (MeOH) methanol, (PC) propylene carbonate, (DMTF) dimethylthioformamide, (DMF) dimethylformamide, (DMSO) dimethylsulfoxide, (BL) butyrolactone, (NMTP) N-methylthiopyrrolidinone(2), (NMP) N-methylpyrrolidinone(2), (CH3CN)acetonitrile.
propylene carbonate is irreversible and this is the most likely reason for the deviation of the ½ (Epa + Epc)(SBC~) value in this solvent (Fig. 2). Modest deviations from the plot acceptor number versus E1/2(Bscr) and ! (Ep + Ea)
Thanks are due to the Fonds zur FSrderung der wissenschaftlichen Forschung in ()sterreich for generous financial support (project 3004}.
210 REFERENCES 1 2 3 4 5
6 7 8 9 10 11 12 13 14 15
G. V. G. G.
Gritzner, K. Danksagmiiller and V. Gutmann, J. Electroanal. Chem., 72 (1976) 177. Gutmann, G. Gritzner and K. Danksagmiiller, Inorg. Chim. Acta, 17 (1976) 81. Gritzner, Inorg. Chim. Acta, 24 (1977) 5. Gritzner, Monatsh. Chem., 107 (1976) 1056. V. G u t m a n n and G. Peychal-Hefling, Monatsh. Chem., 100 (1969) 813. J.A. Lower and W.C. Fernelius, Inorg. Synthesis, 2 (1946) 213. J.J. Alex ander and H.B. Gray, J. Amer. Chem. Soc., 90 (1968) 4260. V. Gutmann, FAectrochim. Acta, 21 (1976) 661; V. C-utmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York, to appear early 1978. U. Mayer, W. Gerger and V. Gutmann, Monatsh. Chem., 106 (1975) 1235. J.R. Fowler and J. IGeinberg, Inorg. Chem., 9 (1970) 1006. G. Trageser and H.H. Eysel, Z. Anorg. Allgem. Chem., 420 (1976) 273. B.N. Figgis, Trans. Faraday Soc., 57 (1961) 204. O.T. Christensen, J. Prakt. Chem., [2] 31 (1885) 171. A. Descamps, Ann. Chim. Phys., [5] 24 (1881) 178. J.H. Eaton and R. Fittig, Liebigs Ann. Chem., 145 (1868) 157.