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Electrochemically catalyzed cleavage of carbon-carbon bond The intermediate formation of metal—Carbene complexes
J. Electroanal. Chem., 95 (1979) 103--108 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
103
Preliminary note ELECTROCHEMICALLY CATALYZED CLEAVAGE OF CARBON--CARBON BOND THE INTERMEDIATE FORMATION OF METAL--CARBENE COMPLEXES
V.V. STRELETS, V.M. RUDAKOV and O.N. EFIMOV
Institute of Chemical Physics, U.S.S.R. Academy of Sciences, Chernogolovka, Moscow 142432 (U.S.S.R.) (Received 4th September, 1978)
INTRODUCTION
Recently it has been shown [1--5] that the electrocatalytic reduction of acetylene in aqueous solutions in the presence of various Mo(III) complexes results in the formation of ethane and ethylene. At the same time the reaction of hydrogen evolution catalyzed by the same Mo(III) complexes proceeds in a parallel way on the electrode. Therefore, there is always a temptation to explain the formation of acetylene reduction products by the hydride transfer in the H - . . . M o ( I I I ) . . . C2H: complex as done in the papers [2,4]. Due to the fast competing hydrogen evolution the C2H4 and C2H6 current efficiency does not exceed some per cent in most of the C2H2 reduction experiments. To decrease the contribution of the competing evolution of H2 we have decided to study the electrocatalytic reduction of acetylene in the Mo(III)--citric acid (H3Cit) system in the aprotic solvent--dimethylformamide (DMF) medium in the presence of the mere traces of water. As supporting electrolyte tetrabutylammonium tetrafluoroborate (Bu4NBF4) was employed which "hydrophobized" to a large degree the near electrode layer where in our opinion [5] the reduction of the C2H2 coordinated with the Mo(III) complex takes place. We hoped that the DMF + Bu4NBF4 used as electrolyte might in the first place slow down the hydrogen evolution rate in the system and in the second influence the distribution of acetylene reduction products due to low proton content in the reaction layer. EXPERIMENTAL
The Mo(III)--H3Cit complex solutions were prepared by mixing of the MoCls and H3Cit. H20 solutions in DMF + 0.05M Bu4NBF4 after which the bright-green solution of the Mo(V)--H3Cit complex (Xmax = 455 and 752 nm) obtained was electrochemically reduced at a stirred Hg-pool cathode with a potential of -1.50V (SCE) until the yellow-brown solution of the Mo(III)--H3Cit complex was produced*. After that, acetylene (50--250 Torr) was introduced into the *In accord with data of polarography and coulometry the limiting diffusion current of the Mo(V)--H3Cit + 2e ~ Mo(III)--H3Cit reduction is reached at E < - 1 . 4 V (SCE). In the presence of excess of H3Cit at E ~< 1.6 V(SCE) a sharp current increase is observed due to catalytic hydrogen evolution.
104 sealed cell and its further reduction and chromatographic analysis of gaseous reduction products were carried out according to a technique reported earlier [3,5]. In most cases the procedure was much simplified by replacing a mercury cathode by sodium amalgam (ca. 1 wt.%) which markedly shortened the experiment. In this case acetylene was introduced simultaneously with the preliminarily evacuated Mo(V)--H3Cit in DMF + 0.05M Bu4NBF4 solution (total volume 3.75 ml) being mixed with the amalgam (0.5--1.0 cm 3 ). After this the reaction vessel was shaken up vigorously for 20--60 min. The analysis of vapour phase from the reaction vessel was carried out after the proton content (H20 and H3Cit) in the system was exhausted (ceasing of hydrogen bubbles formation at the amalgam surface). It is worth mentioning that the amalgam remained rather active after the reaction stopped. The DMF purification was carried out according to a well-known technique [6]. Analytical grade reagents were employed in all experiments without further purification. The preparation of the solutions and all of the electrochemical measurements were done under argon. With H3Cit. H 2 0 being used the water content in the reaction mixture was comparable to that of H3Cit. Polarographic measurements were carried o u t with a polarograph OH-102 (Hungary) three-electrode apparatus. An Ag/AgI/O.1M Bu4NI was used as reference electrode (E = +0.73V, SCE). Controlled potential electrolysis and coulometric measurements were performed with a P-5848 potentiostat. During the electrolysis the temperature was maintained at 20 + 2°C. The acetylene and ethylene reduction products were analysed by gas chromatography and massspectrometry. Before mass-spectrometric analysis the sample was cooled in liquid nitrogen to prevent water, DMF, and decomposition products of DMF produced in the reaction by amalgam reduction from getting into the ionizing chamber of the mass-spectrometer. RESULTS AND DISCUSSION Application of potential to the reduced m o l y b d e n u m solution under C~H2 (50--250 Torr) results in the reduction of acetylene and production of ethylene, ethane, methane, traces of propylene and hydrogen. Although we have failed to suppress the hydrogen evolution entirely even under such relatively " h y d r o p h o b i c " conditions as they seemed to be (it is caused by the nearness of the potentials of catalytic hydrogen evolution in given system to those of C2H2 electroreduction) nevertheless, in a number of cases the current efficiency of the C2H2 reduction products (ethane and ethylene) increased considerably compared to aqueous solutions. Thus, at E = - 1 . 5 V ( S C E ) a total C2H4 and C2H6 current efficiency is 47%; it is only ca. 10% at E = - 1 . 7 V (SCE). Fig. 1 illustrates kinetic curves of ethane and ethylene accumulation at electrocatalytic acetylene reduction in time*. It is evident that the rate of C2H4 and C2H6 formation at E = const remains constant which in our opinion confirms the heterogeneous character of the reaction, i.e., C2H2 reduction takes place in the thin reaction layer near the electrode surface. The different influence of the electrode potential on the rates of the 2e,2H + (ethylene p r o d u c t i o n ) and 4e,4H + (ethane production) reactions of *Note that C2H2 reduction does not occur in the absence of Mo(III) catalyst or in the presence of Mo(III) but without potential applied.
105
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~2.£
~ ..-1.7 V 1.0
~r 60
120
180
Time/rain
Fig. 1. Ethylene and ethane formation during electrocatalytic acetylene reduction in DMF (0.05M Bu4NBF 4) at mercury pool. Experimental conditions: 0.08mM MoC15 ; 40 mM H3Cit.H20; catholyte volume 10 ml; complex prereduced at -1.50 V (SCE);Pc2H 2 = 75 Torr; potential reapplied as indicated.
C2H2 reduction might be said to be the most interesting feature of Fig. 1. The rate of growth of C2H6 formation with the increase of the cathodic potential corresponds to the data for electrocatalytic acetylene reduction in aqueous medium [4,5]. It is quite reasonable to explain this fact in terms of a slow step of electron transfer (slow discharge) to the acetylene molecule coordinated with Mo(III)-H3Cit [5]. Unlike aqueous solutions where the rate of C2H4 formation grows with the increase of the cathodic potential too, in DMF + Bu4NBF4 the potential change does not influence the rate of C2H4 production. It is very likely that now the slow step is n o t the electron transfer b u t some chemical step, for example, the step of protonation of the 2e-reduction product which may be linked to the increased kinetical C--H acidity of alkenes compared to alkanes [7], which is especially marked under the conditions of a " h y d r o p h o b i c " reaction layer. Thus, changing the applied potential it is easy to vary the C2H6 : C2H4 : H2 p r o d u c t ratio. At first sight it was rather unexpected that methane was discovered in the C2H2 reduction products. Compared to ethane and ethylene, methane formation occurred in much smaller amounts and depended mainly on the applied potential. The CH4 current efficiency at E = - 1 . 5 to - 1 . 7 V (SCE) was 0.5--1.5%. It is n o t e w o r t h y that the CH4 yield grows with the increase of the applied potential as in the case of C~H6. In view of the fact that the experiments on electrocatalytic C2H2 reduction in electrochemical cell require a lot of time and complex apparatus and do n o t allow much variation of parameters during the electrolysis, we have decided to car-
106
ry o u t the further experiments using sodium amalgam as "electrode". The possibility of such approach has been already discussed [8], the experimental procedure has been considerably improved and the duration of each experiment has been much reduced. Of course, along with the C2H2 reduction by amalgam the hydrogen evolution proceeds parallel. However, even in this case it is possible to obtain some useful information on the mechanism of the C2H2 reduction. Under these conditions acetylene reduces catalytically n o t only to ethylene and ethane b u t also to methane. Like the hydrogen evolution the C2H2 reduction ceases after exhaustion of the proton content (H3Cit and H20) in the reaction mixture what is indicated by the amalgam remaining active after the reaction has come to the end. The CzH4, C2H6 and CH4 yields depend to a large degree both on shaking frequency of the vessel and amalgam volume (speaking in favour of the heterogeneous character of the reaction) and medium acidity (HzO H3Cit), acetylene pressure, Mo(III) concentration and etc. An increase in PC2H: (50--250 Tort) only slightly influences the hydrocarbon yields which might be connected with the intermediate complexation of C2H2 with Mo(III)-H3Cit and, thus, taking into account the high solubility of acetylene in DMF a rapid approach to the steady state conditions may be expected. Finally, the H3Cit concentration increase at the beginning leads to the increased yields of all the products of acetylene reduction (up to a five-fold H3Cit excess) b u t then the hydrocarbon yields decrease. Such an extreme relation can be explained in terms of a dual role of H3Cit being not only a ligand b u t also a proton d o n o r and in the end the hydrogen content in the reduction products at a ratio [H3Cit] : [Mo] > 5 becomes predominant. Fig. 2 gives the dependence of the Mo(III) concentration on the hydrocarbon yields at the optimal five-fold H3Cit excess under amalgam C2Hz reduction. On the basis of the results obtained it is rather difficult to make a conclusion on the kinetics and mechanism of the catalytic reaction. But it is clear that an acetylene coordinated with the Mo(III)--H3Cit is subjected to reduction. Thus, the primary function of the Mo(III)--H3Cit catalyst is to bind C2H2 as a substrate. One can suppose that the coordination t y p e of such unsaturated substrates must influence the character of reduction products considerably. In this connection we have studied the possibility of catalytic ethylene reduction in the same system. Apart from the expected C2H6 reduction products, methane formation .go L
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2
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I
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Fig. 2. E t h y l e n e (o), ethane ( , ) and m e t h a n e (~) yields as f u n c t i o n of Mo(III)-catalyst concentration by amalgam C2H ~ r e d u c t i o n in D M F (0.05M Bu4NBF 4 ) at five-fold H3Cit excess. E x p e r i m e n t a l conditions: amalgam v o l u m e 1 cm 3 ; reaction m i x t u r e 3.75 m l ; P c 2 H 2 = 100 Tort.
107
was also observed, and with an increase in the Mo(III) concentration the C2H6and CH4 yields grew linearly. Thus as the reaction mixture of volume 3.75 ml, [MoCls ] = 10 -2 M, five-fold H3Cit excess, [Na(Hg)] = 1 cm 3 and PC:H, = 70 Torr, the C2H6 and CH4 yields are equal accordingly to 1.1 and 1.7 pmoles. Both in cases of C2H2 and C2H4 reduction by the amalgam the reaction of hydrogen evolution proceeds parallel to h y d r o c a r b o n formation and as a result the total hydrocarbon current efficiency calculated from the amalgam decomposed does not exceed 2--3%. It is known [6,9] that in the course of electroreduction in the DMF medium at high cathodic potentials the solvent itself can decompose and/or reduce to produce methane. In some experiments w i t h o u t substrates (C2H2 and C2H4 ) we also observed CH4 formation from DMF and/or its decomposition products b u t the total CH4 yield that did not exceed 5--10%. Nevertheless to confirm the methane formation from the reductive cleavage of the carbon--carbon bond reaction we subjected the gaseous C2D4 reduction products to mass-spectrometry. The appearance of the corresponding CH2D2 peaks 18 and 17 masses in mass-spectrum finally confirms the methane formation from C2-hydrocarbons. Undoubtedly C2H4 and C2H6 formation proceeds in a parallel way at least at small amounts of C2H2 reduction* b u t the mechanism of CH, formation requires further investigation. It is difficult to conclude with certainty whether the C~and C2-hydrocarbons formation should be assigned to one reaction centre or whether there is an alternative mechanism that is more plausible. More attractive as in case of electrocatalytic acetylene reduction in aqueous solutions [5], is the attempt to explain the C1- and C2-hydrocarbons formation with the same dimeric Mo(III)--H3Cit species. In this case according to the modern mechanism of dinitrogen fixation [10] the electrocatalytic C2H2 and C2H4 reduction can be imagined by following sequence of reactions: +
Mo L
Mo
C2H2 ~
///~'Je,c.t.r,ed,'d//7
M6-"CH=-CI'L'-Mo~
M6,~CH-CH%Mo ~ 3H 4 Mo i Mo ÷ C2H6 (I)
I-/777777-~
~
/////////I///
'
(2)
'
7777777-77
(A)
Mo
Mo
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77777777/-
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(3)~ 2e, H+ MOOCH2'H2C"~Mo~4H+ Mo
II/111111iiii
M,o+2CFI2
/-)TTT77T
(B)
Reaction (1) takes into account the enhancement of the basic properties of carbon in the intermediate complex (A). Formation of ethylene from C2H2 (and ethane from C2H4 ) occurs if the substrate coordinates with the dimeric Mo(III)-H3Cit species as a ~-complex: CH-:CH MO
( or"
Mo
~~~~~~~/~lit
CH2=CH2) 20, 2N +
~
Mo
Mo + C2H4
( or
C2H 6 )
///////////if
*Otherwise the kinetic curve of C2H6 accumulation (Fig. 1) should have been deflected at larger time to greater C~H~ yields.
108 To take into account the heterogeneous character of the catalytic reaction in this system makes clear the cause of CH4 formation. Unlike aqueous solutions in DMF the medium acidity is considerably lower with the result that the life time of intermediate complex (A) must increase. Moreover, the BuaN + ions accumulate in the double layer around the amalgam and establish here a layer of low proton content [8]. The protonation (reaction 2) may, thus, be slowed down sufficiently so the further reduction of intermediate complex (A) to "carbene" complex (B) (reaction 3) can compete with it. Insufficient reaction layer acidity appears to lead to the rate determining step in the case of C2H2 reduction to ethylene becoming the protonation step with the result that rate of the ethylene production ceases to depend on the applied potential (Fig. 1). In this case it is clear why the C2H4 and C2H6 yields were equal (Fig. 2) by amalgam C2H2 reduction (E~qa(Hg) = - 2 . 1 4 V(SCE) [8] while the CzH4 yield exceeded C2H6 yield by direct electrolysis at mercury pool in the less negative potential range (Fig. 1). Indeed, the increasing of the negative potential (or transition to amalgam)led to the increasing of the rate of C2H6 production with the result that the CzH4 and C2H6 production rates at amalgam potential were equal. The increase of the CH4 yield at amalgam C2H2 reduction can be explained similarly. The hypothesis of intermediate "carbene" (B) formation, in our opinion, is quite logical just as similar structures have been supposed in another homogeneous and heterogeneous reactions so, for example, olefine metathesis at Mo- and W-containing catalysts (see e.g. [11] ). The reactivity of such complexes is, of course, most interesting feature and is an aim of our further investigation. In this connection it is worth while mentioning the additional minor products of electrocatalytic acetylene reduction. We have detected the trace of propylene that, in our opinion, could not be explained without intermediate formation such as --Mo=CHz "carbenes". In conclusion one other feature needs to be noted. The C:H: reduction with ethylene and ethane formation has been considered as an example of a multielectron transfer reaction catalyzed by transition-metal complexes [10]. In this case the reductive carbon--carbon breaking catalysis is an example of a six-electron redox processes like the dinitrogen reduction to ammonia. The work on the Mo(III)--H3Cit catalysis with acetylene as the substrate is progressing further. ACKN OWLED GEMENTS
We are grateful to Prof. A.E. Shilov for helpful discussions. REFERENCES 1 M. I c h i k a w a a n d S. M e s i t s u k a , J. A m . C h e m . S o c . , 9 5 ( 1 9 7 3 ) 3 4 1 1 . 2 D . A . L e d w i t h a n d F . A . S c h u l t z , J. A m . C h e m . Soe., 9 7 ( 1 9 7 5 ) 6 5 9 1 . 3 0 . N . E f i m o v , A . F . Z u e v a , G . N . P e t r o v a a n d A.E. Shilov, K o o r d . K h i m . , 2 ( 1 9 7 6 ) 6 5 1 . 4 F . A . S e h u l t z , D . A . L e d w i t h a n d L . O . L e a z e n b e e , A C S S y r u p . Series, N o , 38, E l e c t r o c h e m i c a l S t u d i e s o f B i o l o g i c a l S y s t e m s . N e w Y o r k , 1 9 7 7 , p. 78. 5 G.N. P e t r o v a , A . F . Z u e v a , O.N. E f i m o v a n d V.V. S t r e l e t s , Zh. Fiz. K h i m . , 5 2 ( 1 9 7 8 ) in press. 6 C.K. M a n n , in E l e c t r o a n a l y t i c a l C h e m i s t r y , Vol. 3, A.J. B a r d (Ed.), M a r c e l D e k k e r , N e w Y o r k , 1 9 6 9 . 7 K.P. B u t i n , I.P. B e l e t s k a j a , A . N . K a s h i n a n d O . A . R e u t o v , J. O r g a n o m e t a l . C h e m . , 10 ( 1 9 6 7 ) 1 9 7 . 8 H. L u r i d , in O r g a n i c E l e c t r o c h e m i s t r y , M.M. B a i z e r (Ed.), M a r c e l D e k k e r , N e w Y o r k , 1 9 7 3 , p. 8 0 5 . 9 L.V. K a a b a k , A.P. T o m i l o v , S.L. V a r s h a v s k i i a n d M . L K a b a c h n i k , Zh. Org. K h i m . , 3 ( 1 9 6 7 ) 3. 1 0 A.E. Shilov, in B i o l o g i c a l A s p e c t s o f I n o r g a n i c C h e m i s t r y , D. D o l p h i n (Ed.), Wiley, N e w Y o r k , 1 9 7 7 , p. 1 9 8 . 11 E . A . L o m b a r d o , M. H o u a U e a n d W . K . Hall, J. C a t a l y s i s , 51 ( 1 9 7 8 ) 2 5 6 .
103
Preliminary note ELECTROCHEMICALLY CATALYZED CLEAVAGE OF CARBON--CARBON BOND THE INTERMEDIATE FORMATION OF METAL--CARBENE COMPLEXES
V.V. STRELETS, V.M. RUDAKOV and O.N. EFIMOV
Institute of Chemical Physics, U.S.S.R. Academy of Sciences, Chernogolovka, Moscow 142432 (U.S.S.R.) (Received 4th September, 1978)
INTRODUCTION
Recently it has been shown [1--5] that the electrocatalytic reduction of acetylene in aqueous solutions in the presence of various Mo(III) complexes results in the formation of ethane and ethylene. At the same time the reaction of hydrogen evolution catalyzed by the same Mo(III) complexes proceeds in a parallel way on the electrode. Therefore, there is always a temptation to explain the formation of acetylene reduction products by the hydride transfer in the H - . . . M o ( I I I ) . . . C2H: complex as done in the papers [2,4]. Due to the fast competing hydrogen evolution the C2H4 and C2H6 current efficiency does not exceed some per cent in most of the C2H2 reduction experiments. To decrease the contribution of the competing evolution of H2 we have decided to study the electrocatalytic reduction of acetylene in the Mo(III)--citric acid (H3Cit) system in the aprotic solvent--dimethylformamide (DMF) medium in the presence of the mere traces of water. As supporting electrolyte tetrabutylammonium tetrafluoroborate (Bu4NBF4) was employed which "hydrophobized" to a large degree the near electrode layer where in our opinion [5] the reduction of the C2H2 coordinated with the Mo(III) complex takes place. We hoped that the DMF + Bu4NBF4 used as electrolyte might in the first place slow down the hydrogen evolution rate in the system and in the second influence the distribution of acetylene reduction products due to low proton content in the reaction layer. EXPERIMENTAL
The Mo(III)--H3Cit complex solutions were prepared by mixing of the MoCls and H3Cit. H20 solutions in DMF + 0.05M Bu4NBF4 after which the bright-green solution of the Mo(V)--H3Cit complex (Xmax = 455 and 752 nm) obtained was electrochemically reduced at a stirred Hg-pool cathode with a potential of -1.50V (SCE) until the yellow-brown solution of the Mo(III)--H3Cit complex was produced*. After that, acetylene (50--250 Torr) was introduced into the *In accord with data of polarography and coulometry the limiting diffusion current of the Mo(V)--H3Cit + 2e ~ Mo(III)--H3Cit reduction is reached at E < - 1 . 4 V (SCE). In the presence of excess of H3Cit at E ~< 1.6 V(SCE) a sharp current increase is observed due to catalytic hydrogen evolution.
104 sealed cell and its further reduction and chromatographic analysis of gaseous reduction products were carried out according to a technique reported earlier [3,5]. In most cases the procedure was much simplified by replacing a mercury cathode by sodium amalgam (ca. 1 wt.%) which markedly shortened the experiment. In this case acetylene was introduced simultaneously with the preliminarily evacuated Mo(V)--H3Cit in DMF + 0.05M Bu4NBF4 solution (total volume 3.75 ml) being mixed with the amalgam (0.5--1.0 cm 3 ). After this the reaction vessel was shaken up vigorously for 20--60 min. The analysis of vapour phase from the reaction vessel was carried out after the proton content (H20 and H3Cit) in the system was exhausted (ceasing of hydrogen bubbles formation at the amalgam surface). It is worth mentioning that the amalgam remained rather active after the reaction stopped. The DMF purification was carried out according to a well-known technique [6]. Analytical grade reagents were employed in all experiments without further purification. The preparation of the solutions and all of the electrochemical measurements were done under argon. With H3Cit. H 2 0 being used the water content in the reaction mixture was comparable to that of H3Cit. Polarographic measurements were carried o u t with a polarograph OH-102 (Hungary) three-electrode apparatus. An Ag/AgI/O.1M Bu4NI was used as reference electrode (E = +0.73V, SCE). Controlled potential electrolysis and coulometric measurements were performed with a P-5848 potentiostat. During the electrolysis the temperature was maintained at 20 + 2°C. The acetylene and ethylene reduction products were analysed by gas chromatography and massspectrometry. Before mass-spectrometric analysis the sample was cooled in liquid nitrogen to prevent water, DMF, and decomposition products of DMF produced in the reaction by amalgam reduction from getting into the ionizing chamber of the mass-spectrometer. RESULTS AND DISCUSSION Application of potential to the reduced m o l y b d e n u m solution under C~H2 (50--250 Torr) results in the reduction of acetylene and production of ethylene, ethane, methane, traces of propylene and hydrogen. Although we have failed to suppress the hydrogen evolution entirely even under such relatively " h y d r o p h o b i c " conditions as they seemed to be (it is caused by the nearness of the potentials of catalytic hydrogen evolution in given system to those of C2H2 electroreduction) nevertheless, in a number of cases the current efficiency of the C2H2 reduction products (ethane and ethylene) increased considerably compared to aqueous solutions. Thus, at E = - 1 . 5 V ( S C E ) a total C2H4 and C2H6 current efficiency is 47%; it is only ca. 10% at E = - 1 . 7 V (SCE). Fig. 1 illustrates kinetic curves of ethane and ethylene accumulation at electrocatalytic acetylene reduction in time*. It is evident that the rate of C2H4 and C2H6 formation at E = const remains constant which in our opinion confirms the heterogeneous character of the reaction, i.e., C2H2 reduction takes place in the thin reaction layer near the electrode surface. The different influence of the electrode potential on the rates of the 2e,2H + (ethylene p r o d u c t i o n ) and 4e,4H + (ethane production) reactions of *Note that C2H2 reduction does not occur in the absence of Mo(III) catalyst or in the presence of Mo(III) but without potential applied.
105
£ L 3.C o
L)
~2.£
~ ..-1.7 V 1.0
~r 60
120
180
Time/rain
Fig. 1. Ethylene and ethane formation during electrocatalytic acetylene reduction in DMF (0.05M Bu4NBF 4) at mercury pool. Experimental conditions: 0.08mM MoC15 ; 40 mM H3Cit.H20; catholyte volume 10 ml; complex prereduced at -1.50 V (SCE);Pc2H 2 = 75 Torr; potential reapplied as indicated.
C2H2 reduction might be said to be the most interesting feature of Fig. 1. The rate of growth of C2H6 formation with the increase of the cathodic potential corresponds to the data for electrocatalytic acetylene reduction in aqueous medium [4,5]. It is quite reasonable to explain this fact in terms of a slow step of electron transfer (slow discharge) to the acetylene molecule coordinated with Mo(III)-H3Cit [5]. Unlike aqueous solutions where the rate of C2H4 formation grows with the increase of the cathodic potential too, in DMF + Bu4NBF4 the potential change does not influence the rate of C2H4 production. It is very likely that now the slow step is n o t the electron transfer b u t some chemical step, for example, the step of protonation of the 2e-reduction product which may be linked to the increased kinetical C--H acidity of alkenes compared to alkanes [7], which is especially marked under the conditions of a " h y d r o p h o b i c " reaction layer. Thus, changing the applied potential it is easy to vary the C2H6 : C2H4 : H2 p r o d u c t ratio. At first sight it was rather unexpected that methane was discovered in the C2H2 reduction products. Compared to ethane and ethylene, methane formation occurred in much smaller amounts and depended mainly on the applied potential. The CH4 current efficiency at E = - 1 . 5 to - 1 . 7 V (SCE) was 0.5--1.5%. It is n o t e w o r t h y that the CH4 yield grows with the increase of the applied potential as in the case of C~H6. In view of the fact that the experiments on electrocatalytic C2H2 reduction in electrochemical cell require a lot of time and complex apparatus and do n o t allow much variation of parameters during the electrolysis, we have decided to car-
106
ry o u t the further experiments using sodium amalgam as "electrode". The possibility of such approach has been already discussed [8], the experimental procedure has been considerably improved and the duration of each experiment has been much reduced. Of course, along with the C2H2 reduction by amalgam the hydrogen evolution proceeds parallel. However, even in this case it is possible to obtain some useful information on the mechanism of the C2H2 reduction. Under these conditions acetylene reduces catalytically n o t only to ethylene and ethane b u t also to methane. Like the hydrogen evolution the C2H2 reduction ceases after exhaustion of the proton content (H3Cit and H20) in the reaction mixture what is indicated by the amalgam remaining active after the reaction has come to the end. The CzH4, C2H6 and CH4 yields depend to a large degree both on shaking frequency of the vessel and amalgam volume (speaking in favour of the heterogeneous character of the reaction) and medium acidity (HzO H3Cit), acetylene pressure, Mo(III) concentration and etc. An increase in PC2H: (50--250 Tort) only slightly influences the hydrocarbon yields which might be connected with the intermediate complexation of C2H2 with Mo(III)-H3Cit and, thus, taking into account the high solubility of acetylene in DMF a rapid approach to the steady state conditions may be expected. Finally, the H3Cit concentration increase at the beginning leads to the increased yields of all the products of acetylene reduction (up to a five-fold H3Cit excess) b u t then the hydrocarbon yields decrease. Such an extreme relation can be explained in terms of a dual role of H3Cit being not only a ligand b u t also a proton d o n o r and in the end the hydrogen content in the reduction products at a ratio [H3Cit] : [Mo] > 5 becomes predominant. Fig. 2 gives the dependence of the Mo(III) concentration on the hydrocarbon yields at the optimal five-fold H3Cit excess under amalgam C2Hz reduction. On the basis of the results obtained it is rather difficult to make a conclusion on the kinetics and mechanism of the catalytic reaction. But it is clear that an acetylene coordinated with the Mo(III)--H3Cit is subjected to reduction. Thus, the primary function of the Mo(III)--H3Cit catalyst is to bind C2H2 as a substrate. One can suppose that the coordination t y p e of such unsaturated substrates must influence the character of reduction products considerably. In this connection we have studied the possibility of catalytic ethylene reduction in the same system. Apart from the expected C2H6 reduction products, methane formation .go L
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g,
J~ 3.0
C2 H 6
t
0 b 2.0 L u 1,0
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E=-
0
i
I
I
2
4
6
L
I
8
10
1o3 [Mo(m)]/M
Fig. 2. E t h y l e n e (o), ethane ( , ) and m e t h a n e (~) yields as f u n c t i o n of Mo(III)-catalyst concentration by amalgam C2H ~ r e d u c t i o n in D M F (0.05M Bu4NBF 4 ) at five-fold H3Cit excess. E x p e r i m e n t a l conditions: amalgam v o l u m e 1 cm 3 ; reaction m i x t u r e 3.75 m l ; P c 2 H 2 = 100 Tort.
107
was also observed, and with an increase in the Mo(III) concentration the C2H6and CH4 yields grew linearly. Thus as the reaction mixture of volume 3.75 ml, [MoCls ] = 10 -2 M, five-fold H3Cit excess, [Na(Hg)] = 1 cm 3 and PC:H, = 70 Torr, the C2H6 and CH4 yields are equal accordingly to 1.1 and 1.7 pmoles. Both in cases of C2H2 and C2H4 reduction by the amalgam the reaction of hydrogen evolution proceeds parallel to h y d r o c a r b o n formation and as a result the total hydrocarbon current efficiency calculated from the amalgam decomposed does not exceed 2--3%. It is known [6,9] that in the course of electroreduction in the DMF medium at high cathodic potentials the solvent itself can decompose and/or reduce to produce methane. In some experiments w i t h o u t substrates (C2H2 and C2H4 ) we also observed CH4 formation from DMF and/or its decomposition products b u t the total CH4 yield that did not exceed 5--10%. Nevertheless to confirm the methane formation from the reductive cleavage of the carbon--carbon bond reaction we subjected the gaseous C2D4 reduction products to mass-spectrometry. The appearance of the corresponding CH2D2 peaks 18 and 17 masses in mass-spectrum finally confirms the methane formation from C2-hydrocarbons. Undoubtedly C2H4 and C2H6 formation proceeds in a parallel way at least at small amounts of C2H2 reduction* b u t the mechanism of CH, formation requires further investigation. It is difficult to conclude with certainty whether the C~and C2-hydrocarbons formation should be assigned to one reaction centre or whether there is an alternative mechanism that is more plausible. More attractive as in case of electrocatalytic acetylene reduction in aqueous solutions [5], is the attempt to explain the C1- and C2-hydrocarbons formation with the same dimeric Mo(III)--H3Cit species. In this case according to the modern mechanism of dinitrogen fixation [10] the electrocatalytic C2H2 and C2H4 reduction can be imagined by following sequence of reactions: +
Mo L
Mo
C2H2 ~
///~'Je,c.t.r,ed,'d//7
M6-"CH=-CI'L'-Mo~
M6,~CH-CH%Mo ~ 3H 4 Mo i Mo ÷ C2H6 (I)
I-/777777-~
~
/////////I///
'
(2)
'
7777777-77
(A)
Mo
Mo
~C2H4~ Mo~'CH2=CH2"Mo~
77777777/-
///H/i/I/i///
(3)~ 2e, H+ MOOCH2'H2C"~Mo~4H+ Mo
II/111111iiii
M,o+2CFI2
/-)TTT77T
(B)
Reaction (1) takes into account the enhancement of the basic properties of carbon in the intermediate complex (A). Formation of ethylene from C2H2 (and ethane from C2H4 ) occurs if the substrate coordinates with the dimeric Mo(III)-H3Cit species as a ~-complex: CH-:CH MO
( or"
Mo
~~~~~~~/~lit
CH2=CH2) 20, 2N +
~
Mo
Mo + C2H4
( or
C2H 6 )
///////////if
*Otherwise the kinetic curve of C2H6 accumulation (Fig. 1) should have been deflected at larger time to greater C~H~ yields.
108 To take into account the heterogeneous character of the catalytic reaction in this system makes clear the cause of CH4 formation. Unlike aqueous solutions in DMF the medium acidity is considerably lower with the result that the life time of intermediate complex (A) must increase. Moreover, the BuaN + ions accumulate in the double layer around the amalgam and establish here a layer of low proton content [8]. The protonation (reaction 2) may, thus, be slowed down sufficiently so the further reduction of intermediate complex (A) to "carbene" complex (B) (reaction 3) can compete with it. Insufficient reaction layer acidity appears to lead to the rate determining step in the case of C2H2 reduction to ethylene becoming the protonation step with the result that rate of the ethylene production ceases to depend on the applied potential (Fig. 1). In this case it is clear why the C2H4 and C2H6 yields were equal (Fig. 2) by amalgam C2H2 reduction (E~qa(Hg) = - 2 . 1 4 V(SCE) [8] while the CzH4 yield exceeded C2H6 yield by direct electrolysis at mercury pool in the less negative potential range (Fig. 1). Indeed, the increasing of the negative potential (or transition to amalgam)led to the increasing of the rate of C2H6 production with the result that the CzH4 and C2H6 production rates at amalgam potential were equal. The increase of the CH4 yield at amalgam C2H2 reduction can be explained similarly. The hypothesis of intermediate "carbene" (B) formation, in our opinion, is quite logical just as similar structures have been supposed in another homogeneous and heterogeneous reactions so, for example, olefine metathesis at Mo- and W-containing catalysts (see e.g. [11] ). The reactivity of such complexes is, of course, most interesting feature and is an aim of our further investigation. In this connection it is worth while mentioning the additional minor products of electrocatalytic acetylene reduction. We have detected the trace of propylene that, in our opinion, could not be explained without intermediate formation such as --Mo=CHz "carbenes". In conclusion one other feature needs to be noted. The C:H: reduction with ethylene and ethane formation has been considered as an example of a multielectron transfer reaction catalyzed by transition-metal complexes [10]. In this case the reductive carbon--carbon breaking catalysis is an example of a six-electron redox processes like the dinitrogen reduction to ammonia. The work on the Mo(III)--H3Cit catalysis with acetylene as the substrate is progressing further. ACKN OWLED GEMENTS
We are grateful to Prof. A.E. Shilov for helpful discussions. REFERENCES 1 M. I c h i k a w a a n d S. M e s i t s u k a , J. A m . C h e m . S o c . , 9 5 ( 1 9 7 3 ) 3 4 1 1 . 2 D . A . L e d w i t h a n d F . A . S c h u l t z , J. A m . C h e m . Soe., 9 7 ( 1 9 7 5 ) 6 5 9 1 . 3 0 . N . E f i m o v , A . F . Z u e v a , G . N . P e t r o v a a n d A.E. Shilov, K o o r d . K h i m . , 2 ( 1 9 7 6 ) 6 5 1 . 4 F . A . S e h u l t z , D . A . L e d w i t h a n d L . O . L e a z e n b e e , A C S S y r u p . Series, N o , 38, E l e c t r o c h e m i c a l S t u d i e s o f B i o l o g i c a l S y s t e m s . N e w Y o r k , 1 9 7 7 , p. 78. 5 G.N. P e t r o v a , A . F . Z u e v a , O.N. E f i m o v a n d V.V. S t r e l e t s , Zh. Fiz. K h i m . , 5 2 ( 1 9 7 8 ) in press. 6 C.K. M a n n , in E l e c t r o a n a l y t i c a l C h e m i s t r y , Vol. 3, A.J. B a r d (Ed.), M a r c e l D e k k e r , N e w Y o r k , 1 9 6 9 . 7 K.P. B u t i n , I.P. B e l e t s k a j a , A . N . K a s h i n a n d O . A . R e u t o v , J. O r g a n o m e t a l . C h e m . , 10 ( 1 9 6 7 ) 1 9 7 . 8 H. L u r i d , in O r g a n i c E l e c t r o c h e m i s t r y , M.M. B a i z e r (Ed.), M a r c e l D e k k e r , N e w Y o r k , 1 9 7 3 , p. 8 0 5 . 9 L.V. K a a b a k , A.P. T o m i l o v , S.L. V a r s h a v s k i i a n d M . L K a b a c h n i k , Zh. Org. K h i m . , 3 ( 1 9 6 7 ) 3. 1 0 A.E. Shilov, in B i o l o g i c a l A s p e c t s o f I n o r g a n i c C h e m i s t r y , D. D o l p h i n (Ed.), Wiley, N e w Y o r k , 1 9 7 7 , p. 1 9 8 . 11 E . A . L o m b a r d o , M. H o u a U e a n d W . K . Hall, J. C a t a l y s i s , 51 ( 1 9 7 8 ) 2 5 6 .