Polarographic and voltammetric investigations in N-methylpyrrolidinone(2) and N-methylthiopyrrolidinone(2)

J. Electroanal. Chem., 7 5

(1977) 739--746 © Elsevier S e q u o i a S.A., L a u s a n n e -- P r i n t e d in T h e N e t h e r l a n d s

739

POLAROGRAPHIC A N D VOLTAMMETRIC INVESTIGATIONS IN N-METHYLPYRROLIDINONE(2) A N D N-METHYLTHIOPYRROLIDINONE(2) *

G. G R I T Z N E R , P. R E C H B E R G E R a n d V. G U T M A N N

Institute of Inorganic Chemistry, Technical University, Getreidemarkt 9, A-1060 Vienna (Austria) (Received 2 0 t h May 1 9 7 6 )

ABSTRACT P o l a r o g r a p h i c a n d v o l t a m m e t r i c m e t h o d s were e m p l o y e d t o s t u d y t h e i n f l u e n c e o f Nm e t h y l p y r r o l i d i n o n e ( 2 ) (NMP) a n d N - m e t h y l t h i o p y r r o l i d i n o n e ( 2 ) ( N M T P ) t o w a r d s a series of cations. In NMP reversible e l e c t r o d e r e a c t i o n s were o b s e r v e d for Na + , K + , T1 +, Z n 2+ , Cd 2+, Cu 2+, Ag + a n d irreversible r e d u c t i o n s for Ba 2+, M n 2+, Co 2+ a n d Ni 2+. 0.1 tool 1 - 1 t e t r a e t h y l a m m o n i u m p e r c h l o r a t e s o l u t i o n s served as s u p p o r t i n g electrolytes. Li + was n o t e l e c t r o a c t i v e in t h e s u p p o r t i n g e l e c t r o l y t e m e n t i o n e d , b u t yielded an irreversible c a t h o d i c wave in t e t r a - n - b u t y l a m m o n i u m p e r c h l o r a t e . In NMTP, Li + , Na + , T1+, Z n 2+ , Cd 2+, Cu + a n d Ag + gave reversible c a t h o d i c waves o n t h e DME, while M n 2+, Co 2+ a n d Ni 2+ were r e d u c e d in a n irreversible e l e c t r o d e process. B i s b i p h e n y l c h r o m i u m iodide serving as a r e f e r e n c e s y s t e m t h r o u g h o u t this s t u d y s h o w e d reversible b e h a v i o u r in b o t h solvents. A c o m p a r i s o n o f Ell 2 for given ions in b o t h s o l v e n t s s h o w e d a s h i f t o f a b o u t 0.5 V t o m o r e positive values in t h e case of a t y p i c a l l y h a r d c a t i o n s u c h as Na + w h e r e a s soft c a t i o n s such as Ag + a n d Cu + s h i f t e d b y m o r e t h a n 0.8 V t o m o r e negative values. T h e e f f e c t s o f t h e s e t w o s o l v e n t s o n t h e c a t i o n s s t u d i e d is discussed in t e r m s of d o n o r a c c e p t o r i n t e r a c t i o n s b e t w e e n t h e c a t i o n a n d t h e s o l v e n t m o l e c u l e s w i t h special r e s p e c t t o t h e c h a n g e s caused b y replacing t h e o x y g e n a t o m in NMP b y a s u l p h u r a t o m .

INTRODUCTION

For reversible polarographic electrode processes the E 1/2 Yalues can be correlated to the AG vatues. In cases where the reversible reduction of a cation leads to the formation of an amalgam or of the metal, the variation of halfwave potentials in different solvents is a measure of the donor acceptor interactions of the solvent molecules and cations [ 1--7]. N-Methylpyrrolidinone(2) (NMP) will exhibit donor properties via the oxygen atom whereas N-methylthiopyrrolidinone(2) (NMTP} will act as a donor via the sulphur atom. NMP therefore is considered to be a " h a r d " solvent and NMTP a " s o f t " solvent. This study is primarily concerned with the behaviour of these solvents towards * In h o n o u r o.f Dr. G.C. B a r k e r ' s 6 0 t h b i r t h d a y .

740

a variety of cations ranging from typically hard cations such as Na ÷ to typically soft ones such as Ag ÷. In order to compare half-wave potentials in the two solvents all potential data are referred to the bisbiphenylchromium(I)/bisbiphenylchromium(0) system [4--7]. EXPERIMENTAL

The apparatus and experimental procedures have been described previously [ 5--7]. Signal generators, potentiostat, coulometer and current voltage converters were made at our Institute and are described elsewhere [8]. Measurements of the dielectric constant were made on a Dipolmeter DMO 1 (Wiss. Techn. Werkstiitten, Weilheim/F.R.G.), NMTP was prepared from NMP and P4S10 in xylene [9--11]. NMTP was purified by triple vacuum distillation after xylene had been removed under reduced pressure. Commercially available NMP was purified according to published procedures [ 12] and dried over molecular sieves prior to a triple vacuum distillation. Physical properties of both solvents are listed in Table 1. AgC104, LiC104, KC104, NaC104, Ba(C104)2 were obtained in anhydrous form according to methods mentioned elsewhere [6,7]. Only dihydrates were obtainable of the perchlorates of Z n 2+, C o 2+, Ni 2÷, Mn 2+, Cu 2÷ and Cd 2+ employing procedures already published [6,7]. The preparation of bisbiphenylchromium iodide [BBCr(I)I] has been described [13]. Tetraethylammonium perchlorate served as supporting electrolyte. All measurements were made at 25 ± 0.02°C. All potentials unless stated otherwise are referred to BBCr(I) as a reference system. The potential range on the dropping mercury electrode (DME) extends in NMTP from --1.6 to + 0.4 V (vs. BBCr) and to +1.0 V on the rotating

TABLE 1 Physical properties of N - m e t h y l p y r r o l i d i n o n e ( 2 ) and N - m e t h y l t h i o p y r r o l i d i n o n e ( 2 )

Melting point Boiling p o i n t Density d 25 Dynamic viscosity 725 Refractive index n 25 Dielectric constant Dipole m o m e n t p

N-Methylpyrrolidinone(2)

N-Methylthiopyrrolidinone(2)

--24.4 ° C [ 14 ] 202°C (760 Tort) [14] 1.0279 g cm - 3 [14] 1.666 cP a [14] 1.468 [14] 32.0 [14] 4.09 b D [14]

+19.3 ° C 1 4 4 - - 1 4 5 ° C (15 Torr) [10] 1.0839 g c m - 3 4.25 cP 1.5837 47.5 4.86 c D

a l c P = 10 - 3 k g m - 1 s - 1 . b At 30°C in benzene [14]. c At 25°C calculated according to Onsager [15].

741 p l a t i n u m e l e c t r o d e . A p o t e n t i a l range f r o m - - 2 . 1 t o +1.1 V is accessible on the D M E in NMP w h i c h can be e x t e n d e d t o +2.1 V b y m e a n s o f a R P E . RESULTS

N-Me thylthiopy rrolidinone(2) T h e p o l a r o g r a p h i c a n d v o l t a m m e t r i c d a t a o b t a i n e d in N M T P are s u m m a rized in T a b l e 2. T h e limiting c u r r e n t s were in all cases c o n t r o l l e d b y diffusion. Li ÷ , T1÷ a n d N a ÷ gave reversible o n e - e l e c t r o n r e d u c t i o n s t o t h e m e t a l amalgam. KC104, a n d Ba(C104) 2 w e r e insoluble in N M T P . Since t h e r e d u c t i o n o f BBCr ÷ in N M T P was f o u n d t o be reversible, t h e B B C r ( I ) / B B C r ( 0 ) c o u p l e can be e m p l o y e d as a r e f e r e n c e s y s t e m in N M T P . Zn 2+ and Cd 2÷ w e r e reversibly r e d u c e d in t w o - e l e c t r o n steps. A n o d i c stripping p r o v e d a m a l g a m f o r m a t i o n in t h e case o f Cd 2÷, n o a n o d i c p e a k was o b s e r v e d f o r Zn 2÷. Cyclic v o l t a m m e t r y o n t h e d r o p p i n g m e r c u r y elect r o d e s h o w e d c o r r e s p o n d i n g c a t h o d i c - a n o d i c p e a k s f o r b o t h Zn 2÷ a n d Cd 2+. T h e t w o - e l e c t r o n r e d u c t i o n o f Mn 2÷ was f o u n d t o b e irreversible, a b s o r p t i o n p h e n o m e n a usually c o n n e c t e d w i t h t h e r e d u c t i o n o f Mn 2÷ in n o n - a q u e o u s solvents w e r e absent. T h e r e d u c t i o n o f Co 2÷ o c c u r r e d in an irreversible t w o - e l e c t r o n step, additions o f 4% w a t e r did n o t e f f e c t t h e p o l a r o g r a p h i c wave. H i g h e r c o n c e n t r a tions s h i f t e d t h e h a l f wave p o t e n t i a l t o m o r e negative values. T h e El/2 value at 24% w a t e r was f o u n d t o be - - 0 . 1 3 V. A t c o n c e n t r a t i o n s higher t h a n 20% w a t e r t h e c o l o u r o f t h e s o l u t i o n c h a n g e d f r o m green t o p i n k i n d i c a t i n g a change f r o m t e t r a h e d r a l c o o r d i n a t i o n t o o c t a h e d r a l c o o r d i n a t i o n [ 11 ]. T h e c a t h o d i c p o l a r o g r a p h i c wave o f Ni 2÷ o n t h e D M E was n o t s e p a r a t e d f r o m t h e d i s s o l u t i o n o f m e r c u r y a n d h a d a m a x i m u m o f t h e first kind. A n irreversible wave was o b s e r v e d o n t h e RPE. T h e r e d u c t i o n o f Cu 2÷ o c c u r r e d on t h e R P E in t w o irreversible o n e e l e c t r o n steps via Cu ÷ t o t h e m e t a l . O n l y the r e d u c t i o n o f Cu ÷ c o u l d b e o b s e r v e d o n t h e D M E since t h e r e d u c t i o n o f Cu 2÷ t o Cu ÷ o c c u r s at p o t e n t i a l s m o r e positive t h a n t h e d i s s o l u t i o n o f m e r c u r y in N M T P . T h e c o r r e s p o n d i n g o n e e l e c t r o n p r o c e s s was reversible. T h e r e d u c t i o n o f Ag + c o u l d b e o b s e r v e d in N M T P o n t h e D M E in t h e f o r m o f a reversible o n e e l e c t r o n process.

N-Me th y lpy rrolidinone( 2 ) T h e p o l a r o g r a p h i c a n d v o l t a m m e t r i c d a t a are listed in T a b l e 3. N o p o l a r o graphic w a v e was o b s e r v e d f o r Li ÷ using t e t r a e t h y l a m m o n i u m p e r c h l o r a t e as s u p p o r t i n g e l e c t r o l y t e . A n irreversible w a v e was o b t a i n e d e m p l o y i n g t e t r a nb u t y l a m m o n i u m p e r c h l o r a t e as s u p p o r t i n g e l e c t r o l y t e . Reversible one-elect r o n r e d u c t i o n s w e r e f o u n d f o r Na÷, K ÷ a n d T1÷ o n t h e DME, Ba 2+ was red u c e d in an irreversible t w o - e l e c t r o n process.

--0.065 +0.37

--1.025 --0.945 +-0.0 +0.15 --0.245 +0.068 --0.365 --0.05

55 55

60 60 58 55 30--35 35--40 55--60 30--40 +0.14 a +0.32 a --0.29 a 0.0 a +0.265 b

E1/2/V

E1/2/V

T" T/mV c

RPE

DME

a E l ~ 2 for 2 × 10 - 4 mol 1-1 s o l u t i o n s . b E~BC r o b t a i n e d f r o m p o t e n t i o m e t r i c m e a s u r e m e n t s . c T o m e h test.

Cu(I)/(0) Ag(I)/(0)

Cu(II)/(1)

Li(I)/(0) Na(I)/(0) BBCr(I)/(0) Tl(I)/(0) Zn(II)/(0) Cd(II)/(0) Mn(II)/(0) Co(II)](0) Ni(II)/(O)

Depol.

All p o t e n t i a l s vs. b i s b i p h e n y l c h r o m i u m iodide

Polarographie and v o l t a m m e t r i c data in NMTP

TABLE 2

90 80

T" T / m V c

1.42 1.57 0.78 0.67 0.78 0.74 0.81 1.78 2.99 1.57

1.78

106 D / c m 2 s--1

rev.

rev. rev. rev. rev. rev. rev. irrev. irrev.

Kalousek test

yes

no

no

yes

no

yes

yes

yes

Anodic stripping a n o d i c peak

irrev. irrev. irrev. irrev. rev. rev.

(Hg) (Hg) (Pt) (Pt)

irr. (Hg)

rev. (Hg)

Cycl. volt.

b~

+0.75 +0.53 d +0.87 d +1.16 e

f Tome~ test.

a 0.1 mol 1-1 t e t r a - n - b u t y l a m m o n i u m p e r c h l o r a t e . b 1 X 10 - 3 m o l l - 1 , t = 6 s . c Ill d e f i n e d d o u b l e wave. d 2 × 10 - 4 mol 1 - 1 . e EBBC r* obtained from potentiometric measurements.

Cu(I)/(0) Ag(I)/(0)

Cu(ID/(0)

.75 60 60 40 58 55 30 35 220 180 80

E1/2/V

E1/2/V

T. T/mV f

RPE

DME

Li(I)/(0) --1.72 a Na(I)/(0) --1.47 K(I)/(0) --1.387 Ba(II)/(0) - - 1 . 3 9 BBCr(I)/(0) -+0 Tl(I)/(0) +0.227 Zn(II)/(0) --0.26 C d ( I I ) / ( 0 ) +0.13 Mn(II)/(0) --0.4 Co(II)/(0) --0.13 b Ni(II)/(0) - - 0 . 1 2 b Cu(II)/(0) 0.74 c

Depol.

All p o t e n t i a l s vs. b i s b i p h e n y l c h r o m i u m iodide.

P o l a r o g r a p h i c and v o l t a m m e t r i c d a t a in NMP

TABLE 3

65

T. T/mV f

1.74 1.63 1.74 2.94

2.04 4.26 4.55 1.78 2.4 4.76 1.55

106 D / c m 2 s--1

irrev. rev. rev. irrev. rev. rev. rev. rev. irrev. irrev. irrev. rev.

Kalousek test

yes

no

no

no

no

no

yes

no

yes yes

no

Anodic stripping anodic peak

rev. (Pt)

rev. (Pt)

rev. (Hg)

Cycl. volt.

5o

744

Ag ÷ yielded a reduction wave only on the RPE. Cyclic v o l t a m m e t r y on a Pt electrode showed voltammograms typical for the reduct i on to the metal. The reduction of Cu 2÷ occurred in two steps. The waves of the r e d u c t i o n Cu 2÷ to Cu ÷ and Cu ÷ to Cu ° were very close t oget her and could n o t be separated in classical polarography, a bet t er separation was obtained in cyclic v o ltamme t r y. At water cont ent s of 10% and higher only one r e d u c t i o n wave occurred. A two-electron r educt i on overlapped by absorption was observed in the case of Zn 2+. According to the Kalousek-technique the r e d u c t i o n was reversible. No anodic peak was obtainable in anodic stripping experiments, i--t curves on single drops proved the occurrence of adsorption. Two polarographic waves were observed during the polarographic reduction of Cd 2+. The wave at more positive potentials proved to be an adsorption prewave. The second wave was reversible according to the Kalousek-test. i--t curves showed that this reduction was overlapped by adsorption phenom ena. Anodic stripping did n o t result in the f o r m a t i o n of an anodic peak. Flat irreversible waves were obtained for bot h Co 2÷ and Ni 2÷. Maxima of the first kind accompanied bot h waves. The El~ 2 value of the r e d u c t i o n wave of Co 2÷ depended on the drop time. The bisbiphenylchromium ion was f o u n d to undergo a reversible one-electron reduction [16]. DISCUSSION

Polarographic half wave potentials for reversible electrode processes agree with the standard r e dox potential for the electrode process within a few millivolts, as long as no complex f o r m a t i o n with cations or anions present in solution occurs. The b i s b i p h e n y l c h r o m i u m ( I ) / b i s b i p h e n y l c h r o m i u m ( 0 ) couple served as a reference system and E*BBcr has been proposed [5] as a symbol for r e d o x potentials n o t corrected for the ratio of activity coefficients and diffusion coefficients. EBBcr * is only a few mV different f r o m the standard

Ag+/Ag Cu~Cu(Hg) ,l, i , ,l, , j \\\+~, 0 +~5 \

nYn(Hg) I

I

~/

~

No+/NcIHg) K+IK(,HgJ1 I

. . . .

I

0.0 I

,

-0,5

I I

,

I

-1,0 /

/

//.5

//

IV]

I

I ,I

A g+/.4g~tg) n+/rff~- ,

,

,

,

,

I Zff+/Zn{Hg) u/cu~g)

/

//

P

//

I I

/

/

//

I

,

I

-/.5

i"

i+/Li(Hg) No+/Na(Hg)

Fig. 1. E~BCr and aEBBCr* values in DMF and DMTF (E1/2(BBCr) for Zn 2+ and Cd 2+ ).

745

redox potential with respect to BBCr. In most cases in this study the metal forms an amalgam. In such a case the redox potential includes amalgam formation and the subscript a is added to the E*Bc r values. Employing E*Bc rdata the series of redox potentials for each NMP and NMTP has been arranged, which are shown in Fig. 1. It can easily be seen that a typically hard cation such as Na ÷ undergoes much stronger interactions with the hard solvent NMP than with NMTP. Unfortunately the perchlorates of K ÷, Rb ÷ and Cs÷ are n o t soluble in NMTP, thus preventing a comparison. The " s o f t " silver cation is much more strongly solvated in NMTP than in NMP, which is also true for Cu ÷. T h e aE~BBCr)-value of T1÷ (generally considered to be soft) is not affected to an appreciable degree in going from NMP to NMTP. The comparison of the redox potentials o f Z n 2+ and C d 2+ is somewhat affected by the occurrence of adsorption phenomena overlapping the polarographic reduction, but it seems t h a t for these two cations the differences of the half-wave potentials in the two solvents * is not strictly permisare modest. A conversion of the E 1 ]2(BBCr) into EBBCr sible in the cases of Z n 2+ a n d C d 2+ for the reasons mentioned above. The E 1/2 values have, however, been included in Fig. 1 to facilitate a comparison. Cations undergoing irreversible electrode processes had to be eliminated from the comparison. The data confirm the expectations that NMP interacts more strongly with hard cations than with soft cations. Accordingly the oxidizing properties of hard cations are more strongly decreased in this solvent than those of soft cations. On the other hand NMTP interacts more strongly with soft cations, than with hard cations. Thus the oxidizing properties of soft cations are more strongly decreased than those of hard cations. These results further support the concept of donor acceptor interactions between the solvent and cations. No such effects would be expected on purely electrostatic considerations. The acceptor properties, a measure for the interaction of the solvent with anions, of NMP are modest (acceptor number: 13.3 [17]). The acceptor properties of NMTP are expected to be similar. The solubility of salts will therefore in each of the solvents be dominated by the interaction of the respective solvent with the cations. It can safely be predicted that silver salts including silver halides and salts of Cu ÷ will be quite soluble in NMTP. Similar results leading to the preparation of silver halide solvates have been observed in N,N-dimethylthioformamide [7,18]. ACKNOWLEDGEMENT

This study has been supported by the Fonds zur FSrderung der wissenschaftlichen Forschung in (Ssterreich, Project 3004.

746 REFERENCES 1 V. Gutmann, Topics Current Chem., 27 (1972) 103. 2 V. Gutmann, Coordination Chemistry in Non-Aqueous Solutions, Springer, WienNew York, 1968. 3 V. G u t m a n n and R. Schmid, Monatsh. Chem., 100 (1969) 2113. 4 G. Gritzner, P. Rechberger and V. Gutmann, Monatsh. Chem.,107 (1976) 809. 5 G. Gritzner, Monatsh. Chem., in press. 6 O. Duschek, V. Gutmann, P. Rechberger, Monatsh. Chem., 105 (1974) 62. 7 V. Gutmann, K. Danksagmfiller and O. Duschek, Z. Phys. Chem. N. F., 92 (1974) 199 8 P. Rechberger, Dissertation, TU Wien, 1976. 9 O. Riester and F. Bauer, DBP 927 043; C.A., 52 (1958) P939a. 10 H. Eilingsfeld, M. Seefelder and H. Weidinger, Ber., 96 (1963) 2671. 11 S.K. Madan and M. Sulich, Inorg. Chem., 5 (1965) 1662. 12 M. Breant, M. Bazouin, C. Buisson, M. Dupin and J.M. Rebattu, Bull. Soc. Chim., (1968) 5065. 13 V. G u t m a n n and G. Peychal-Heiling, Monatsh. Chem., 100 (1969) 813. 14 J.A. Riddick and W.B. Burger, Techniques of Chemistry, Vol. II, Organic Solvents, Wiley-Interscience, New York, 1970, p. 454. 15 G. Geiseler, Ausgew~hlte physikalische Methoden in der organischen Chemie, Vol. II, Akademie Verlag, Berlin, 1963, p. 307. 16 V. Gutmann, G. Gritzner and K. Danksagmfiller, Inorg. Chim. Acta, 17 (1976) 81. 17 U. Mayer, V. G u t m a n n and W. Gerger, Monatsh. Chem., 106 (1975) 1235. 18 K. Danksagmiiller, G. Gritzner and V. Gutmann, Inorg. Chim. Acta, 18 (1976) 2 6 9