V. V. Gorbatov, N. V. Yablokova, Yu. A. Aleksandrov, and V. I. Ivanov, Zh. Obshch. Khim., 53, 1752 (1983). V. M. Zhulin, O. B. Rudakov, G. A. Stashina, eL al., Izv. Akad. Nauk SSSR, Ser. Khim., 1979 (1985). V. M. Zhulin, O. B. Rudakov, A. V. Ganyushkin, and V. A. Yablokov, Izv. Akad. Nauk SSSR, Ser. Khim., 2206 (1986). N. P. Sluchevskaya, N. V. Yablokova, V. A. Yablokov, and Yu. A. Aleksandrov, Zh. Obshch. Khim., 48, 1136 (1978). V. M. Zhulin, G. A. Stashina, A. V. Ganyushkin, et al., Izv. Akad. Nauk SSSR, Ser. Khim., 1659 (1983). V. A. Yablokov, N. V. Yablokova, A. P. T a r a b a r i n a , and M. N. Shemuranova, Zh. Obshch. Khim., 41, 1565 (1971). V. A. Yablokov, A. V. Ganyushkin, M. Ya. Botnikov, and V. M. Zhulin, Izv. Akad. Nauk SSSR, Set. Khim., 950 (1980). N. S. Isaacs, Liquid Phase High Pressure Chemistry, Wiley-Interscience, New York (1981), p.172; V. M. Z h u l i n , P h y s i c a l C h e m i s t r y . C u r r e n t Problems [ i n R u s s i a n ] , Khimiya, Moscow (1984), p. 144. M. G. Gonikberg, Chemical Equilibrium and Rates of Reactions at High Pressures [in Russian], Khimiya, Moscow (1969), p. 207. V. A. Yablokov, S. Ya. Khorshev, A. P. Tarabarina, and A. N. Sunin, Zh. Obshch. Khim., 43, 607 (1973). S. Ya. Khorshev, S. E. Skobeleva, A. N. Egorochkin, et al., Zh. Obshch. Khim., 4__77,1357 (1977). B. S. El'yanov and E. M. Vasylvitskaya, Rev. Phys. Chem. Jpn., 5__00,169 (1980).
POLAROGRAPHIC STUDY OF Me(V) IN METHANOL IN THE PRESENCE OF PHOSPHATIDYL CHOLINE S. I. Kulakovskaya and L. P. Didenko
In a nitrogen-fixing system containing a complex of Mo(III) with Mg(II) (catalyst) and an amalgam of Na (reducing agent) in alkaline methanol solution, the rate of formation of hydrazine increases by tens of times when phosphatidyl choline (PC, where RICO and R2CO are residues of oleic and stearic acids) or dipalmitoyl phosphatidyl choline (DPC, where RICO R2CO are palmitic acid residues) are added to this system [i]
R1COOCH.~ I R2COOCH O i
0 We found that in oxidation of Mo(III) in methanol solutions of MeOLi or MeONa in concentrations not exceeding 0.i M, particles of Me(V) are formed and their electrochemical properties are identical to the properties of particles obtained on addition of a methanol solution of MoCI s to these solutions. Since Me(V) is more stable in these solutions than Mo(III), the study was conducted in solutions containing Me(V). We studied the electrolytic reduction of Me(V) on a mercury drop electrode (MDE) in methanol solutions of MeOLi and MeONa with additives of PC, DPC, and glycerophosphatidyl choline (GPC, whose molecule contains no fatty acid residues), and palmitic acid (PA) to determine the cause of the strong effect of the phospholipids. In addition, it was interesting to follow the effect of addition of PC and DPC on electrolytic reduction of Me(V) in guanidine buffer solution (pK a 13.6). This would permit predicting the effect of addition of these phospholipids on the electrochemical behavior of the Mo(III) catalyst in a nitrogen-fixing system containing Mo(III), Ti(III), and guanidine hydrochloride [2-4]. Branch of the Institute of Chemical Physics, Academy of Sciences of the USSR, Chernogolovka. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 14961501, July, 1987. Original article submitted January 14, 1986.
Plenum Publishing Corporation
EXPERIMENTAL The study was conducted with methods of classic and alternating current polarography on MDE in methanol solutions of MeOLi and MeONa using a PU-I univeral polarograph equipped with a device for force rupture of the mercury drops. MDE characteristics: m = 4.18 mg/sec, 9 = 2.5 sec, and m = 0.48 mg/sec, T = 25 sec in 0.07 M MeOLi. The alternating current curves were recorded in 2.1 sec at a frequency of 25 z and an alternating voltage amplitude of 20 mV. The capacitance component of the current was separated with a phase-sensitive detector in which the reference signal was phase-shifted 90 ~ with respect to the alternating voltage on the cell. To separate the capacitance component, the alternating current curves were made with a phase shift of the sinusoidal alternating voltage reference pulses of 90 ~ An aqueous normal calomel electrode separated from the cell by a transition bridge filled with the background electrolyte was used as the reference electrode. The potentials were given with a precision of • mV. The alternating current curves were converted to capacitance curves for C with the E curve of LiCI in MeOH . All measurements were made in an atmosphere of Ar (high purity). All solutions were prepared under Ar in MeOH dehydrated according to  and distilled twice under argon. The solutions of MeOLi and MeONa were prepared by dissolving the corresponding metals in MeOH and 1 M MoCI 5 was prepared by dissolving a weighed portion in MeOH. The solutions were subsequently used for preparing a 0.05 M solution of MoCls, and aliquots were added to the solutions studied. RESULTS AND DISCUSSION The presence of a bipolar "head" and long hydrocarbon "tails" in PC favor their adsorption both with positive and with negative surface charges and consequently affects the course of electrode reactions. Adsorption of PC and DPC was studied in a wide range of MDE potentials (from -0.i to -2.0 V). The capacitance curves of PC are shown in Fig. i; they are identical to the curves for DPC, also obtained in 0.07 M MeOLi. The capacitance curves obtained here are similar to the curves obtained for DPC [7, 8]. Adsorption of PC and DPC is observed from -0.i to -2.0 V. There are two adsorption regions: the molecules of PC and DPC which lie flat on the surface of the MDE are adsorbed in the region of -0.i to -0.9 V. A reorientation peak then follows, after which the region of adsorption of molecules of PC and DPC in the vertical position from -1.5 to -2.0 V is located. However, we did not observe the sharp decrease in the capacitance with high negative MDE potentials when CpC > 5"10 -4 M due to the formation of a condensed adsorption layer, since the methanol solutions of PC and DPC studied here, in contrast to [7, 8], contained no water, which causes formation of micelles. The electrolytic reduction of Mo(V) on MDE in methanol alkaline and guanidine buffer solutions in the absence of PC and DPC was studied previously . The electrochemical process in these systems takes place irreversibly with the participation of two electrons. The saturation currents I d are diffusion currents. A wave (WA) of reduction of Mo(V) +~e Mo(Ill) with El/2 = --1.61 V and a one-electron wave of oxidation Mo(V) -~ Mo(VI) with EI/2 = -43.36 V are observed in alkaline methanol solutions. In solutions of 5.10 -s M PC or 7.5.10 -2 M DPC containing 10 -3 M Mo(V) and 0.07 M MeOLi, a new cathode wave (WB) with El/2 = --1.4 V appears in the polarograms with more positive potentials. When the concentration of phospholipid C L is increased to 8.10 -3 M, WB is monotonically shifted to the cathode region (Fig. 2). El/2 of WA changes weakly. With an increase in CL, the saturation current of WA decreases, and the I d of WB increases, which indicates an increase in the number of electroactive particles formed in the reaction of M o ( V ) w i t h PC or DPC. With a 7.5.10 -4 M concentration of the lipid, only WB is observed, while WA disappears. With C L of 7.5.10 -4 M, I d for WB is a diffusion wave [proportional to CMo(V ) in the range of concentrations from 5.10 -~ to 1.5.10 -3 M and the square root of the height of the mercury column]. We note that substitution of Li by Na in the background electrolyte does not alter the overall character of the polarograms and El/2 in the presence of these ions almost coincide. The capacitance curves show that the Mo(V) reduction wave lies in the region of molecules of PC and DPC vertically adsorbed on the MDE. It is suggested in [7, 8] that binding of the background Li + cations with the phospholipid gives the lipid layer an additional positive charge. The particles of Mo(V) bear a negative charge in alkaline methanol solutions [i0]. For this reason, binding of Mo(V) with the polar head of the phospholipid in the pores of the positively charged adsorption layer will facilitate discharge of Mo(V) on the negatively charged surface of the MDE. The appearance of WB at -1.4 V confirms this hypothesis. According to [ii], facilitation of the reduction of complexes of metals with an adsorbed ligand, catalyst, can be due both the appearance of a "bridge effect" and to the significant effect of the electric field.
ZO~ f 3 I I L I -65 -$0 -$5 -gOE~V Fig. i. Capacitance curves of PC in 0.07 M MeOLi with Cpc , mole/liter: i) O, 2) 2.5. 10 -4 , 3) 10 -3 , 4) 4.10 -3 , 5) 8.10 -a The dependences of Ez/2 on log C L are shown in Fig. 2. With an increase in CL, the position of Ez/2 of WB is shifted to the cathode region, and I d increases. The anode Wwith E~/= = -0.36 V is shifted toward less negative potentials, which indicates the formation of a complex of Mo(V) with the phospholipid. It is possible to determine the difference in the coordination numbers of the complex primarily present in the solution (n) and discharged on the electrode (k) with the equation in  from the slope of the dependence of Ez/2 on log C L (214 mV) with the value of ~n~ = 0.5 found
lg CL =
I t was f o u n d t h a t (n - k) = 2. As a c o n s e q u e n c e , two m o l e c u l e s o f p h o s p h o l i p i d t h e c o o r d i n a t i o n s p h e r e o f Mo(V) d u r i n g i t s r e d u c t i o n .
go o u t o f
The f o r m a t i o n o f t h e M o ( V ) - p h o s p h o l i p i d complex can be j u d g e d by t h e c h a n g e i n t h e s h a p e o f t h e c a p a c i t a n c e c u r v e o f PC o r DPC: a d d i t i o n o f 10 -3 M Mo(V) b r o a d e n s t h e r e g i o n o f a d s o r p t i o n of t h e p h o s p h o l i p i d in t h e v e r t i c a l p o s i t i o n , while t h e r e o r i e n t a t i o n peak i s s h i f t e d to anode p o t e n t i a l s (Fig. 3). The a p p e a r a n c e o f t h e M o ( V ) - p h o s p h o l i p i d complex i n t h e s o l u t i o n i s c l e a r l y e v i d e n t on t h e c a p a c i t a n c e c u r v e s made on a d d i t i o n o f i n c r e a s i n g c o n c e n t r a t i o n s o f p h o s p h o l i p i d t o a s o l u t i o n w i t h a c o n s t a n t c o n c e n t r a t i o n o f Mo(V) (10 -3 M) ( F i g . 4 ) ; t h e i n c r e a s e i n t h e c a p a c i t a n c e peak i n d i c a t e s an i n c r e a s e i n t h e c o n c e n t r a t i o n o f t h e Mo(V)phospholipid complex. The i n c r e a s e i n t h e y i e l d o f h y d r a z i n e by s e v e r a l t i m e s on a d d i t i o n o f M g ( I I ) t o t h e nitrogen-fixing s y s t e m ( c o n t a i n i n g no p h o s p h o l i p i d ) was a t t r i b u t e d i n  t o t h e f o r m a t i o n o f a M o ( I I I ) - M g ( I I ) c o m p l e x , w h i c h was c o n f i r m e d by t h e a p p e a r a n c e o f a new c a t h o d e WC w i t h E1/2 = - 1 . 2 8 V on a d d i t i o n o f M g ( I I ) t o m e t h a n o l s o l u t i o n s o f MeOLi o r NaOH w i t h 10 -3 M Mo(V). WC a t t a i n e d t h e maximum v a l u e w i t h CMg(II)/CMo(V ) = 10, and WA t o t a l l y d i s a p p e a r e d . We f o u n d that addition of 7.5.10 -s M PC or DPC to methanol solutions of 0.07 M MeOLi with 10 -3 M Mo(V) and 10 -2 M MgCI 2 results in the appearance of two overlapping cathode waves with Ei/2 = -1.28 V (WC) and at -1.4 (WB). With C L ~ 7.5.10 -4 M, only wave B is observed, WC disappears, the previously absent Mo(V) oxidation wave with EI/2 = -0.36 V also appears, and the dependences of El/2 on log C L for these waves is the same as for solutions containing no MgCI 2. This means that the phospholipid forms a more stable complex with Mo(V) than Mo(V) does with Mg(II) (see Fig. 2). Hydrolysis of the phospholipid with the formation of fatty acids and glycerophosphatidyl choline can take place in methanol alkaline solutions. Addition of PC from 5.10 -4 to 2.5. 10 -2 M to the solutions studied did not affect the polarograms of Mo(V), i.e., no complex of Mo(V) with PA is formed, and second, adsorption of PC did not affect discharge of Mo(V). The capacitance curves show that PC is desorbed from the surface of the MDE with E = -i.i V, and PC consequently cannot affect reduction of Mo(V), which takes place with -1.61 V. On addition of glycerophosphatidyl choline (GPC) to methanol alkaline solutions with 10 -3 M Mo(V), a new W with EI/2 = -1.55 V appears only with CGp C = 10 -3 M, while 7.5.10 -s M PC or DPC is sufficient for the appearance of WB. An increase in CGp C results in a shift in
, U ~0,5 i
I I -J,5 -J,o Fig. 2
-ho Fig. 3
-go E. V
Fig. 2. Dependence of EI/2 of WB on log C L in MeOH in the presence of 0.07 M MeOLi and 10 -3 M Mo(V): i) PC; 2) DPC; 3) GPC. Black points: solutions with addition of 10 -2 M MgCI2; 4) PC in 0.07 M MeONa; 5) PC in guanidine buffer (1.5"i0 -l M). TMD E = 2.5 sec, m = 4.18 mg/sec. Fig. 3. Capacitance curves in 0.07 M MeOLi in the presence of: i) background, 2) 8.10 -3 M PC; 3) 8.10 -~ M PC, and 10 -3 M Mo(V). EI/2 to the cathode region and an increase in I d. As a consequence, a complex of Mo with GPC is formed in this case, too. Similar changes in the polarograms of Mo(V) are observed in solutions containing 10 -2 M MgCI2, for which an anode W with El/2 = -0.3 V appears with CGp C 10 -2 M, and the dependence of EI/2 on log CGp C coincides with the dependence obtained in the absence of MgCI2; the slope is 210 mV, which corresponds to emergence of two molecules of GPC from the coordination sphere of the discharged particle of Mo(V) when ~n~ = 0.5. A comparison of the data obtained shows that the polar head of the phospholipid participates in complexing of Mo(V) with a phospholipid. It is necessary to note than on addition of PC to the solution, the saturation current initially increases and becomes constant after I0 min. This is either due to the slow reaction of the phospholipid with Mo(V) in the volume of the solution or to destruction of the micelles which exist in a concentrated solution of the additive and are destroyed on dilution in the background solution. A decrease in I d in time is observed in methanol alkaline solutions in the absence of PC, due to a decrease in the number of e!ectroactive particles of Mo(V), perhaps due to their association . The effect of the phospholipid on the saturation current indicates the formation of a stable Mo(V)-phospholipid complex in this medium~ We note that the capacitance curves of the phospholipids and Mo(V)-phospholipid do not change for 1 h, which additionally confirms their stability. It is known [2-4] that Mo(III) is a catalyst of reduction of molecular nitrogen in the Ti(llI)-Mo(III)-guanidine hydrochloride system. A cathode wave of reduction of Mo(V) +~e Mo(lll) with El/2 = --1.28 V and an anode wave of oxidation of Mo(V) -~ Mo(VI) with El/2 = -0.4 V are observed in guanidine buffer solution in the absence of PC . On addition of 5.10 -s to 10 -4 M PC to a solution of 10 -3 M Mo(V) in 0.07 M MeOLi, the position of the polarographic waves of Mo(V) does not change. With a further increase in Cpc, the reduction wave is shifted to the cathode region and i d increases. This means that a Mo(V)-PC complex is formed in guanidine buffer solution, similar to what is observed in methanol alkaline solutions. We
Fig. 4. Capacitance curves for solutions in 0.07 M MeOLi, I0 -a M Mo(V), and CDPC: i) 2.5.10-4; 2) 5.i0-4; 3) 1.5oi0-~; 4) 2.10-3; 5) 4.3.10 -3 M.
JO 20 iO-
find from the value of the slope of the graph of the dependence of EI/2 on log Cpc (see Fig. 2) with ~n~ = 0.5 that two molecules of PC leave the coordination sphere of Mo(V) during its reduction. Addition of MgCI2 with Cpc e i0 -3 M does not affect the position of I and the E curves. Similar to methanol alkaline solutions, EI/2 of the anode waves are weakly dependent on log Cpc (see Fig. 2). According to the data reported, it can be concluded that there is no major difference in the effect of phosphatidyl choline on discharge of Mo(V) in methanol alkaline and guanidine buffer solutions. We would like to thank A. E. Shilov for his constant attention to the study and discussion of the results obtained. CONCLUSIONS I. Adsorption of phosphatidyl choline on a mercury drop electrode and its effect on reduction of Mo(V) in methanol alkaline and guanidine buffer solutions were studied by the methods of classic and alternating current polarography and it was shown that Mo(V) forms a complex with the polar part of the phosphatidyl choline molecule. 2. It was hypothesized that the complex of molybdenum with the phospholipid increases the yield of hydrazine in the reaction of reduction of molecular nitrogen. LITERATURE CITED 1. 2. 3. 4. 5. 6. 7. 8. 9. I0. ii. 12. 13.
L. P. Didenko, A. K. Shilova, and A. E. Shilov, Dokl. Akad. Nauk SSSR, 254, 643 (1980). S. I. Kulakovskaya, V. N. Tsarev, O. N. Efimov, and A. E. Shilov, Kinet. Katal., 18, 1045 (1977). S. I. Kulakovskaya, L. P. Vasilieva, and O. N. Efimov, React. Kinet. Catal. Lett., 14, 181 (1980). G. V. Nikolaeva, S. I. Kulakovskaya, O. N. Efimov, and A. E. Shilov, Kinet. Katal., 26, 473 (1985). S. Minc and J. Jastrzebska, J. Electrochem. Soc., 107, 135 (1960). A. Weissberger, E. Proskauer, J. Riddick, and E. Toops, Organic Solvents, Wiley, New York (1955). E. Muller and H. Dorfler, J. Electroanal. Chem., 116, 459 (1980). L. Pospisil, K. Kuta, E. Muller, and H. Dorfler, J. Electroanal. Chem., 106, 359 (1980). S. I. Kulakovskaya and O. N. Efimov, Elektrokhimiya, 16, 593 (1980). A. F. Zueva, O. N. Efimov, and V. V. Strelets, Elektrokhimiya, 15, 232 (1979). Ya. I. Tur'yan, Chemical Reactions in Polarography [in Russian], Khimiya, Moscow (1980), p. 175. H. Matsyda and G. Ayabe, Bull. Chem. Soc. Jpn., 29, 134 (1956). L. P. Didenko, A. E. Shilov, and A. K. Shilova, Kinet. Katal., 20, 1488 (1979).