chemical reduction of triphenylphosphine (TPP) and its oxide (TPPO) in aprotic solvents such as acetonitrile, dimethylformamide and hexamethylphosphoramide ...
J. Electroanal. Chem., 88 (1978) 27--41 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
27
CATALYTIC PHENOMENA IN THE ELECTROCHEMICAL REDUCTION OF TRIPHENYLPHOSPHINE AND TRIPHENYLPHOSPHINE OXIDE IN NON-AQUEOUS SOLVENTS WITH TETRAALKYLAMMONIUM SUPPORTING ELECTROLYTES
J.M. SAVEANT and SU KHAC BINH
Laboratoire d'Electrochimie, Universitd de Paris VII, 2, place Jussieu, 75221 Paris-Cedex 05 (France) (Received l l t h June 1977; in revised form 2nd August 1977)
ABSTRACT Polarography, cyclic voltammetry and coulometry reveal that a catalytic reduction of the tetraalkylammonium cation, R4N +, of the supporting electrolyte is involved in the electrochemical reduction of triphenylphosphine (TPP) and its oxide (TPPO) in aprotic solvents such as acetonitrile, dimethylformamide and hexamethylphosphoramide. There is however progressive consumption of TPP and TPPO resulting in the final formation of phenyl substitution products (RP~)2 and ROP02). Comparison with the reduction of the BuP~)3 + cation allows to propose the following mechanism which involves a chemical type catalytic process: -"*P~)3(P~)30) + e ~-.
~P(~--(P~)30"-)
P ~ - - ( P ~ 3 ~ - ) + NR4 + Rla(~3(ROP(~3)
~Ri~03(ROP03) + NR 3 RP02(ROP~)2) + ~)"
~P(b3(P(~30) J
P(~3--(P(~30--) + R" 2R"
+ R"
~ P03(PO30) + ~)- (-+¢H) } R-- (-~RH) R--R RH + R(--H)
Redox type catalytic mechanisms are discussed and shown to be unlikely. Values of the alkylation rate constant are derived from the polarographie or the coulometric data or from cyclic voltammetry according to its magnitude which varies with the solvent. TPP anion radical appears as more readily alkylable than TPPO anion radical.
Electrochemical reduction of triphenylphosphine (TPP) and triphenylphosphine oxide (TPPO) in aprotic solvents such as DMF, as well as their reduction by alkali metals in ethers, have been the subject of several investigations. Agreement is however far from being general as concerns the product distribution and the mechanism of the electrochemical reduction and the possibility of an e.s.r. characterization reduction of the anion radicals. In ethers, the reaction of TPP and alkali metals yields essentially diphenylphosphide and the phenyl-metal [1--5]. Small amounts of biphenyl have also been reported among the reaction products [ 5]. The e.s.r, spectrum, obtained in
28 these conditions, initially thought to be that of the TPP anion radical [6], has been further assigned to the diphenylphosphide anion radical [5]. The anion radical of TPP thus appears as unstable in the presence of alkali cations even at temperature as low as--65°C [5]. In the case of TPPO the anion radical appears as more stable giving rise to an identifiable e.s.r, spectrum [7--9] although it also undergoes a cleavage reaction yielding apparently a larger a m o u n t of biphenyl than in the case of TPP. TPP gives a one-electron [10--12] reversible [11] reduction wave in DMF. The e.s.r, spectrum recorded u p o n electrochemical generation at --40°C has been claimed to be that of the TPP anion radical [12] whereas only a weak unassigned signal has been obtained at 3°C [11]. The products obtained upon preparative scale electrolysis, in DMF with Bu4NI as supporting electrolyte, differ from one author to the other. Wawzonek and Wagenknecht [ 10] have indicated the formation of benzene, diphenylphosphinic acid, triphenyl phosphine oxide and triphenyl-n-butylphosphonium iodide whereas Santhanam and Bard [ 11 ] have reported the formation of substantial amounts of biphenyl (identified by cyclic voltammetry and chromatography) besides diphenylphosphinic acid. In the case of TPPO [11,12] two waves have been observed the first of which is reversible. The reaction products were n o t identified. It is to be noted that coulometry of TPP has been reported to result in a steady-state current instead of going to completion which has been attributed to the proximity of the background discharge [ 11]. TPPO has been reported to give rise to a similar phenomenon for the explanation of which a catalytic type process has been invoked [ 11 ]. The purpose of the present paper is to analyze in detail the electrochemical reduction mechanism of TPP and TPPO in DMF and also in hexamethylphosphoroamide (HMPA). It will be shown that the reduction involves in b o t h cases an homogeneous catalytic process which consumes the tetraalkylammonium cation of the supporting electrolyte. The nature of the catalysis, redox or chemical [13] will be discussed on the basis of the recent understanding of the reduction mechanism of alkyltriphenylphosphonium cations in aprotic solvents [14] and of the observation that the catalyst progressively disappears during the reduction. Finally, the rate constants characterizing the catalytic process will be determined using polarography, coulometry in a rotated cell and linear sweep voltammetry. MECHANISM OF THE ELECTROCHEMICAL REDUCTION OF TPP AND TPPO. NATURE OF THE CATALYTIC PROCESS
Supporting electrolytes involving either the tetraethylammonium or the tetrab u t y l a m m o n i u m cations were used in the study of TPPO. For TPP, which is reduced more negatively it was only possible to use NBu4 ÷. DMF and HMPA were used in a purpose of comparison since a significant difference appears between the two solvents as to the rate of the catalytic process. Another reason for using HMPA is the possibility of carrying o u t cyclic voltammetry experiments at very low sweep rates {down to 0.008 V s-1) due to the high viscosity of this solvent. It was checked with an uncomplicated reversible couple (fluorenone/flurenone anion radical) that a diffusion controlled cyclic voltammogram can be obtained down to such low sweep rates (Fig. 1). It was thus ascer-
29
g
-1.5
o
i (pA)
;. (pA)
v=0.O005V s-I
.o.1
-0.5
-10
~
-5
li5
Ag/Ag+) 116
1.5 =
1~7
- E/V ( vs Ag/Ag+) 1.7 i
1.6 i
1
0.05 I 0.002.5
010 I 0.0100
015 I 0.0225
0.20 I 0.0400
t
0.25 V~ / V ~ $ t 0°0625 v/Vs-'
Fig. 1. F l u o r e n o n e 10 - 3 m o l l - 1 in H M P A with 0.1 M Bu4NBF 4. Test of control by diffusion with negligible interference o f natural c o n v e c t i o n d o w n to 0.008 V s- 1 . Cyclic v o l t a m m e t r y at 0.01 V s- 1 and 0.0005 V s- 1 , and p r o p o r t i o n a l i t y o f the peak current to square r o o t of the sweep rate.
tained that natural convection still plays a negligible role in these conditions which allows a c.v. study of slow catalytic processes. This would not have been possible otherwise since convection and slow catalysis give rise to a very similar behavior, i.e., the transformation of the peak-shaped reversible pattern into an S-shaped curve.
TPP A first indication of the existence of a catalytic process in TPP reduction is provided by the kinetic character of its polarographic wave in DMF. The ratio of the limiting currents of TPP and tetraphenylphosphonium cation (which is known to correspond to a diffusion controlled one-electron reduction [15]) increases with the drop-time 0 as follows: 0/s
1.5
1
2
3
5
iwpp/iPO4+
1.45
1.79
1.98
2.13
2.25
The effect of temperature also points to the same conclusion:
t/°C iwep/ifluorenon e
25 1.77
35 2.01
50 2.35
30 i/pA
v: o.o, Vs-'
i/pA
v:o.osVs"1
.o
-E/V(~sAg/A~)
- v(vs~/A¢) i/laA
i/laA
v: o.o,oVs-~
"0.5j
V'_
o. o o T V s -1
~
"0 -ElV(vs AglAg+)
IJ2 (ipI~)//~.AV" s"~ I
-EIV(vs AglAg÷) I
I
o;~
o. 2
i
o.?V'~l
s
Fig. 2. Triphenylphosphine 0.76 X 10 - 3 tool 1-1 in HMPA with 0.1 M Bu4NBF 4. Cyclic voltammetry at low sweep rates. Variation of the peak current with sweep rate.
Other evidence of the catalytic character of the reduction process in DMF as well as in HMPA is provided by the observation t h a t the c.v. wave becomes S-shaped and less and less reversible as the sweep rate decreases. At the same time the peak current becomes higher than predicted by proportionality to square root of the sweep rate. This is shown on Fig. 2 in the case of HMPA. That the supporting electrolyte interferes in the catalytic process is revealed by the fact that, e.g. in DMF, the polarographic wave is increased by 14% when the concentration of supporting electrolyte is doubled. Also, at a given sweep rate, the c.v. wave looses its reversibility and becomes progressively S-shaped upon increasing the supporting electrolyte concentration. Coulometry also shows the catalytic character of the TPP reduction. The apparent number of electrons na~ is indeed significantly larger than one: Solvent
Supporting electrolyte
Initial conc./M
nap
DMF DMF DMF HMPA
0.1 0.1 0.1 0.1
10 -3 2 × 10 -2 5 X 10 -2 1.7 × 1 0 - 3
6.5 7.8 8.7 7.7
M M M M
NBu4BF a NBuaBFa NBu4BF 4 NBu4BF4
31
However the catalytic process ends after the passage of a relatively small number of electrons per molecule showing that the catalyst, i.e. TTP, is progressively consumed in the reduction. Preparative scale electrolysis with identification of products allows a further insight in the nature of the catalytic process. In DMF, with 0.1 M NBu4BF4, electrolysis of 0.05 M TPP at the foot of the polarographic wave yielded large amounts of butyldiphenylphosphine (BuP(~2), tributylamine and benzene as shown by vapor-phase chromatography and thin-layer chromatography by comparison with authentic samples. Using the same methods it was shown that no biphenyl, diphenylphosphine, diphenylphosphinic acid, triphenylphosphine oxide were formed and that TPP has been completely consumed during electrolysis. Particular attention was devoted to the question of biphenyl formation [ 11 ]. Another electrolysis experiment was thus carried out in the same conditions as in ref. 11 (TPP conc.: 1.4 mmol 1-1, supporting electrolyte: 0.1 M NBu4I, ~ = --2.80 V vs. SCE). The solution obtained after the passage of 1 electron/ molecule, i.e. after uncomplete electrolysis was investigated by cyclic voltammetry. A reversible wave is indeed observed at a potential slightly m o r e positive (Fig. 3c) than the original TPP wave (Fig. 3a) corresponding approximately to
i/HA
~A
yY .2
-4
"O
-0
.
I /
~IIJA
-E/V(~SCE)
- E/V(~ 5CEJ
I
25
3
i+A
21+
#
T
,// "° ~
2a
- E/V(vs5CE)
-E/V(,,SCE)
3
215
13
Fig. 3. Cyclic v o l t a m m e t r y in D M F + 0.1 M Bu4NI of (a) TPP 1.4 × 10 - 3 tool 1-1 ; (b) biphenyl 10 _ 3 mol l--l; (c) an u n c o m p l e t e l y electrolyzed TPP solution, after c o n s u m p t i o n of 1 electron per molecule, starting TPP conc.: 1.4 × 1 0 - 3 tool 1-1 ; (d) the same solution as (c) with addition of 0.5 × 10 - 3 mol 1-1 biphenyl.
32 the location of the biphenyl wave (Fig. 3b). There is however a general positive shift of the potential scale u p o n electrolysis, probably due to a change in the junction potential. This is confirmed by addition of an equivalent a m o u n t of biphenyl in the electrolyzed solution which results in a new wave, positive to the first one (Fig. 3d). This shows that the wave observed after uncomplete electrolysis is that of TPP. After complete e l e c t r o , s i s no cathodic wave is observed. At any m o m e n t no wave corresponding to BuP03 is obtained. This c o m p o u n d would anyway be completely reduced at the potential where the electrolyses were carried out. The yield in BuP02 was found to be 60%. Taking into account the partial air oxidation of the phosphine into its oxide, as evaluated on an authentic sample, the actual yield was estimated as nearly quantitative. A first mechanism involving the nucleophilic attack of TPP anion radical on the supporting electrolyte cation, can thus be proposed to explain the catalytic p h e n o m e n o n as well as the consumption of TPP:
(1)
--*P¢3 + e ~ PO~P0h- + NBu4 + ~ BuP(~3 + NBua
(2)
Bu15~)3 ~ 0 - + BuP02
(3)
au'+
(4)
4
0" + P 0 3 - -+ P03 + 0 - (-~0 H) (DISP)
(5) (6)
Bu" + e -, B u - (-~BuH) (ECE)
(7)
Bu" + P 0 3 - -~ P 0 3 + B u - ( - ~ B u H ) (DISP)
(8)
O" + 1 e -~ O - (-+OH) (ECE)
(.,
I
2Bu" -+ Bu--Bu and or B u r + Bu(--H)
(9)
This is based on the results recently obtained in the electrochemical reduction of alkyltriphenylphosphonium cations [14] which have shown the existence of a competition between phenyl and alkyl cleavage in the initially formed neutral radical. Further electron transfer to ~" and Bu" m a y occur at the electrode (reactions 5 and 7) and in solution (reactions 6 and 8). The second route is more probable than the first since the first step (2) of the chemical process is slow. In these conditions 0" and Bu" are formed at relatively large distance from the electrode having thus no time to go back to the electrode to be further reduced (5, 7) instead of being reduced in solution (6, 8) (for a more quantitative analysis of this problem see refs. 16 and 17). An alternate path for the reduction of 0" into OH is H-atom abstraction from the solvent SH followed b y further reduction of the solvent radical: O'+SH
OH+s"
S'+e-* S-
s" +
+ S-
This would change neither the electron balance nor the product distribution.
33
Dimerization of ~)" has been shown to be of little importance as compared to further reduction in the reduction of tetraphenylphosphonium cation [15]• It was therefore neglected here• This is not a priori possible for Bu'. In that case, by analogy with other alkyl radicals [ 14], it can be predicted that dimerization and H-atom transfer disproportionation are likely to play a role in the present process. The competition between reactions (8) and (9) results in a dependence of the apparent number of electrons upon initial concentration. As shown in a detailed analysis of this type of systems [18] an increase in TPP concentration favors reaction (8) at the expense of reaction (9). According to which is the predominant reaction, two limiting mechanisms, involving the following reaction sequence, exist: (A): 1--6, 9 (reduction of Bu" negligible) (B): 1--8 (complete reduction of Bu', negligible dimerization and H-atom transfer disproportionation) For these two limiting situations the apparent n u m b e r of electrons are respectively [18]: hal; A = 2 + (k2/kl) , nat; B= 2 + 2 ( k 2 / k l )
It is noted that in any case n:~ A < na~ B. It is then expected that the apparent number of electrons increases upon raising the TPP concentration, which is actually observed experimentally. Additional evidence in favor of the proposed mechanism is provided by comparison of the value of the k2/k 1 ratio derived from the measured apparent number of electrons and that obtained from product distribution in the electrochemical reduction of the butyltriphenylphosphonium cation in the same medium. Electrolysis was carried out at --2 V vs. SCE in DMF + 0.1 M BuaNI with a BU~)03 c o n c e n t r a t i o n of 51.7 mmol 1-1. The yields in P03 a n d B u P 0 2 as determined by v.p.c, were found to be 63% and 14%, respectively. The losses in BuP(/)2 during work-up through an oxidation into the phosphine oxide were taken into account in the last estimation. The reduction reaction scheme of the phosphonium cation can be described as follows [14]: +
(10)
BuP¢)3 + e ~ But)O3
(3)
oll •
1--~I
k2
(~" + Bu~)~)a + H20-> OH + O H - + BuP03
(4) (5) (11)
BuPO3 + O H - -~ BuPO20 + OH
(12)
BuP¢~3-~ Bu" + P03 (~" +
e +
H20 -~ OH
+ OH-,
or
Bu" -+ (1/2)Bu--Bu and/or (1/2)Bull + Bu(--H) -Bu" + e + H20 -~ Bull + OH-, or
1-~
(9) (7)
Bu" + Bu~'Oa + H20 ~ Bull + O H - + Bu~03
(13)
BuPO3 + O H - -> BuPO20 + 0 H
(14)
34 + !
(2 -- ~ + c~I3)BuP93 + (2 --/3 + afl)e + (1 --/3 + ( ~ ) H 2 0 -~
(1 -- a)P03 + (~BuP02 + (1 --/3 + ( ~ ) B u P 0 2 0 + (1 + a --/3 + a ~ ) 0 H + (1 --a)(/3/2)l(Bu--Bu) + Bull + Bu(--H)I + (1 - - a ) ( 1 --13)Bull It appears immediately that the (k 1/k2) ratio is given by:
kl/k 2 = a / ( 1 - - a ) = (%
BuP(/)2)/(%PO3
) = 0.23
(15)
this estimation being n o t dependent upon initial concentration. It follows that = 0.18./3 can then be derived from the yields in either P03 or BuP02:/3 = 0.85. Competition between chemical evolution of Bu" (9) and further reduction {7,13) occurs here in quite different conditions than during the TPP catalyzed reduction of Bu4 N÷. In the reduction of BuP03, Bu" is formed at a very short distance from the electrode, certainly inside the diffusion layer, since formation of Bu1503 is at the electrode and since it is chemically very unstable. It follows that further reduction of Bu" is very likely to occur b y electrode electron transfer (7) rather than solution electron transfer (13). A decrease in the initial BuPO3 concentration would then favor reduction of Bu" at the expense of dimerization and H-atom transfer disproportionation (16}. On the contrary, in TPP catalyzed reduction of Bu4N +, due to the slowness of the catalytic process, Bu" is formed far from the electrode, practically in the bulk of the solution. Competition occurs then between reactions (8} and (9) which results in a decrease of initial TPP concentration favoring Bu" dimerization and H-atom transfer disproportionation at the expense of further reduction. It follows that the (kl/k2) ratio as determind b y eqn. (15) should lie between the values obtained from TPP catalysis according to mechanism (A) and (B), respectively. Also, the value of the (kl/k2) ratio from Bui~03 reduction should be closer to (kl/ke)A at small TPP concentration than at high TPP concentration. This is actually what is observed from the experimental values in DMF:
TPP conc. mmol 1-1
(kl/k2)A
(kl/k2)a
1 20 50
0.22 0.17 0.15
0.43 0.35 0.30
The above reaction scheme thus appears as a satisfactory representation of TPP reduction in the presence of Bu4N ÷ involving both a TPP catalysis of Bu4N÷ reduction and a progressive substitution of a phenyl substituent of TPP b y a butyl group. It is noted that in the framework of this scheme, catalysis appears as being of the chemical type rather than of the redox type [13]. It is therefore worth examining if a redox type catalysis reaction scheme would n o t be able to fit with the experimental observations (such redox catalyzed reductions of tetraalkylammoniums have already been reported in the case of naphthalene with Et4 N÷ [19], methylnaphthalene [20] and biphenyl [ 21] with Bu4N÷. Catalysis is however slow so that it cannot be detected by polarography
35
and appears only in preparative scale experiments):
(1) PO~- + NBu4 + i NBu'4 + P03
(16)
NBu'4 --> Bu" + NBu3 P~)~ + Bu" --> BuP0~-
(17) (18)
BuP0~- ~ BuP02 + O-(-+~)S)
(19)
BuP 3- --' P 8 + B u - ( ~ B u H )
(20)
2~ ".~Bu--Bu tm "'~BuH + Bu(--H)
(9)
If such a mechanism were to operate, it would imply that reaction (9) would not be completely dominant over reaction (18), otherwise there would not be formation of a n y BuP(~2. In these conditions, an increase in TPP concentration would result in a decrease of n~p [18] contrarily to what is observed experimentally. Decreasing the concentration the process would become purely catalytic and therefore n~'p would increase indefinitely. On raising the concentration n~p would tend toward 2 + 2(k1/k2) as in the above mechanism (B). In these conditions, there is no reason why the apparent number of electrons would be related to the ratio of ~" and Bu" cleavages in BuPO3 decomposition as shown above. Still another redox type catalysis mechanism may be considered in which Bu" reacts on TPP itself rather than on its anion radical: --~P03 + e -~- P ~
(1)
PO~- + NBu4 + 2 NBu~ + P@3
(16)
@.
NBu~-* Bu" + NBu3
(17)
P03 + Bu" -* BuP(~3
(21)
•
k1
BUD03--+ O" + SUP02 BuP(D3 (~ B u ' + P@a
((3) (4)
followed by reactions (5) to (9). It is not possible to discriminate between this redox mechanism and the chemical one on the basis of the available experimental data. However the catalytic redox mechanism seems unlikely since the butyl radicals would disappear through reaction (9), which is probably close to diffusion control, before having time to react on TPP according to reaction (21). There would not be thus any BuP~2 formed. It follows that the electron exchange from P ~ - to Bu4N ÷ appears as a slower reaction than the nucleophilic attack. This is in agreement with the observation that redox catalysis with compounds reducible in the vicinity of TPP, e.g. biphenyl, is not polarographically detectable in DMF.
36
TPPO iE, cath = In polarography or c.v. TPPO exhibits two reduction waves ~p,1
EC~th _-- --2.850 V vs. S C E in D M F p,2
--2.540,
+ 0.I M Bu4NI). T h e first w a v e appears as
reversible and the second irreversible. Since the first wave, which corresponds to the formation of the anion radical, is markedly positive to that of TPP, Et4N ÷ can be employed as well as Bu4N÷ as supporting electrolyte. Et4 N+ was used in acetonitrile (ACN) and Bu4N ÷ in DMF and HMPA as for TPP. The catalytic character of the first wave reduction does n o t appear significantly u p o n polarography in DMF + 0.1 M Bu4N÷. The c.v. waves tends to loose their reversibility and become S-shaped as the sweep rate decreases in DMF and HMPA. Accordingly the ip/V~vratio tends to increase over the value corresponding to reversibility at low sweep rates. However this occurs at such low sweep rates {0.02 V s-1 in DMF and 0.002 V s-1 in HMPA) that it cannot be excluded that natural convection be responsible, at least partially, of this phenomenon. The situation is clearer in ACN (Fig. 4) since the characteristic sweep rate is there on the order of 1 V S-1. At the coulometric scale the reduction is however clearly catalytic. In DMF
•
-1/2 t/2
(,p/~)I/~Av
s
-14 -12
V~/Vv2s -~'2
? •
--
('p/v~)/~A V
-t12 t/2
S
-2e
24
• 2O
•
%2 (;p/~)/~A
•
f
v?/V'/2s - w O,13
v'1% ~/2 c
.I
~o5
o;~
~ / v v 2 s -vz oi15
Fig. 4. Cyclic v o l t a m m e t r y o f TPPO. R a t i o o f p e a k c u r r e n t a n d square r o o t o f s w e e p rate as a f u n c t i o n o f s w e e p rate. (a) ACN + 0.1 M Et4NCIO4, (b) DMF + 0.1 M Bu4NI, (c) HMPA + 0.1 M Bu4NBF 4.
37
and HMPA + 0.1 M Bu4NBF4, with a starting concentration of TPPO of 10 -a mol 1-1, there is complete consumption of TPPO after passage of 7--8 electrons per molecule. In ACN the number of electrons is of the same order of magnitude both with Et4 N+ and Bu4N+ as supporting electrolytes. Analysis of products in ACN with Et4 N+, starting from a TPPO concentration of 0.05 mol 1-1 shows the formation of a substitution product E t - O - - P 0 2 by comparison with an authentic sample [22] in v.p.c. No biphenyl and no E t P 0 2 0 was detected. With Bu4N+, BuOP02 was similarly formed, yields were a b o u t 70--80% in b o t h cases. It follows that the reduction mechanism of TPPO appears as very similar to that of TPP:
--P0aO + e ~ PO3(~P0aO: + NR4 + ~ [)~)3OR + NR3
P03OR ~'--~ O"
+ Pt~2OR
PO3OR k2 R" + P(~30 (
r
O" + PO3 O - " PO30 + ~-(-~q)H) J
R" + P O 3 0 - ~ P ~ 3 0 + R-(-+RH) (
2R" -+
R--R RH + R(--H)
The k l / k 2 ratio is again on the order of 0.2. In the same solvent the butylation reaction b y NBu4 ÷ appears as significantly slower than in the case of TPP. It is noted that alkylation occurs on the oxygen rather than the phosphorus. This is in agreement with an electrophilic addition rather than a radical addition since, in TPPO radical anion, the negative charge is mostly on the oxygen and the spin density on the phosphorus. KINETICS
OF THE ELECTROCHEMICAL
REDUCTION
From the reaction scheme established above it results that the slow step which governs the kinetics of the overall reduction is the alkylation reaction of the anion radical b y the tetraalkylammonium cation. With TPP in DMF the rate of this reaction is such that polarography appears as a suitable technique for its determination. With TPP in HMPA the reaction is slower and cannot be kinetically characterized using polarography. We used for this purpose the coulometric current-time curves obtained on a mercury pool cathode with a rotated cell. The formal kinetics characterizing a catalytic process accompanied by consumption of catalyst of the type encountered here has been recently established for b o t h polarography and stationary state coulometry [18]. It will be used to analyze the data in the case of TPP in DMF and in HMPA. For these systems and for the other ones considered in this paper, the results obtained in linear sweep voltammerry will serve to give a crude estimation of the reaction rate, the formal kinetics for this technique having n o t been established yet.
38
T P P in D M F + N B u 4 ÷ ( p o l a r o g r a p h y )
The quantity to be determined is the ratio: t~ = (il)TPp/(id)TP P of the actually observed polarographic limiting current to the diffusion controlled limiting current, i.e. that would have been observed in the absence of catalytic process. ((il)wpp/(id)TPp = I(il)wPr/(id )F [I(id)F/(id )TPPJ, where (id)F is the diffusion controlled limiting current of fluorenone in the same medium. (id)F/(id)WP P = (ip)F/(ip)Wpp, where (ip)TPP and (ip)F are the c.v. peak currents at a sufficiently high sweep rate for the catalytic process to be negligible (v > 10 V s-l). For a 0.1 M concentration of Bu4N÷ it was found that: ( il)Tpp/( id)WPp = 1 . 7 8 We consider four possibilities corresponding to reaction schemes A and B and inside each of them the ECE mechanism and the solution electron transfer mechanism (DISP). The expressions of the current ratio ~ as a function of the kinetic dimensionless parameter k are given in Table 1 together with the values of p = k l / k 2 determined for a 10 -3 mol 1-1 TPP concentration. X was c o m p u t e d in each case from the ~ expression in which the proper value of p has been introduced. Finally the first order rate constant k is deduced from X taking into account the drop-time value: 0 = 2 s. The values given between brackets are those obtained for a 0.2 M Bu4N÷ concentration. As noted before the DISP mechanisms are likely to represent more correctly the actual process than the ECE reaction schemes. The actual value of the first order rate constant, for a 0.1 M Bu4N ÷ concentration, thus lies between 1.26 and 0.66 being most probably closer to the first figure than to the second as discussed above. If the Bu4N÷ concentration is raised this last trend is enhanced. This may explain the slight decrease of the second order rate constant which is observed upon doubling the Bu4N ÷ concentration. It follows that the most prob-
TABLE 1 Mechanism
ECE A
DISP A
Expression of the current ratio ~
p = kl/k 2
k = 37rkO/7
kls ~1
1 + Lp/(1 + p ) [ l l -- (tanhx/rx/x/~)l
0.22
3.28 (5.64)
1.22 (2.10)
0.22
3.39 (5.64)
1.26 (2.i0)
0.43
1.78 (2.83)
0.66 (1.05)
1.79
0.68 (1.11)
[p/(1 + p)] + [tanhV~/~v/X(1 + p)[ 2p+1 p + (1 + p ) t a n h X / ~ p + 1)k/(p + 1) %/(2 p + 1)~,/(p + 1) (1 + p)[2 - - (tanhV~/~d~) [
ECE B DISP B
p + (t~nhV%jX) 2(P + 1) p + (p + 2) ItanhX/2 ~/X/2 ~l
0.43
(3.00)
39
able value o f t h e s e c o n d o r d e r r a t e c o n s t a n t k ' is: k' = 10.5 m o 1 - 1 1 s- 1 T P P in H M P A
+ NBu4 + (coulometric
current-time
curves)
T h e r a t e c o n s t a n t was d e t e r m i n e d f r o m t h e i - - t curves using t h e results o f f o r m a l kinetics f o r t h e t w o r e a c t i o n s c h e m e s D I S P A a n d D I S P B [ 18]: i/i ° = e x p ( - - a t / 2 ) ] c o s h ( p t / 2 ) - - (/3/p) s i n h ( p t / 2 ) l
with, f o r D I S P A: a = p + k(2p +
l)/(p
+ 1),
/3 = p - - k ( 2 p + 1 ) / ( p + 1),
p = x//32 + 4 p k
f o r D I S P B: a = p + 2k,
/3 = p - - 2k,
U = x//32 + 4 p k ( p + 2 ) / ( p + 1)
(i ° is t h e initial (a) f r o m t h e p = i ° / F V c °, tration. (b) f r o m t h e
c u r r e n t . ) p was d e t e r m i n e d b y t w o m e t h o d s : equation: w h e r e V is t h e v o l u m e o f t h e s o l u t i o n a n d c o t h e s t a r t i n g c o n c e n i n t e g r a t i o n o f t h e (i/t~))--t curve:
t
p = n~p/f
(i/i°)d'r
o
T h e value f o u n d in b o t h cases was p r a c t i c a l l y t h e s a m e : p = 0 . 9 7 × 10 - 2 s-1. Figure 5 s h o w s t h e e x p e r i m e n t a l i/i ° c u r v e o b t a i n e d w i t h 0.1 M NBu4 + (full line). T h e t w o d a s h e d lines r e p r e s e n t t h e t h e o r e t i c a l curves c o r r e s p o n d i n g t o t h e
[/io
-O
1000
2090
%/s
Fig. 5. R e d u c t i o n o f 10 - 3 M - 1 T P P in H M P A . C o u l o m e t r i e c u r r e n t - t i m e c u r v e s . ( E x p e r i m e n t a l c u r v e , (o©©) t h e o r e t i c a l c u r v e f o r e a s e A w i t h k = 4.5 x 10 - 2 s - 1 , ( ~ ) t h e o r e t i c a l c u r v e f o r ease B w i t h k = 4 x 10 - 2 ~ - 1 .
40 TABLE 2 Compound
Solvent
Supporting electrolyte
TPP TPP TPPO TPPO TPPO
DMF HMPA ACN DMF HMPA
N.Bu4 ~Bu 4 iituU~
k'/mo1-1 l $--1
+
10 0.4 10 0.14 0.05
best fit in case A (k = 4.5 × 10 -~ s-1) and case B (k = 4 × 10 -~ s-l). The second order rate constant is then k' = 0.45 mo1-11 s-1. For a 0.01 M NBu4 ÷ concentration a slightly smaller value of k' = 0.3 mo1-1 1 s-1. However, the excess of Bua N÷ over TPP is now only 10 and this may result in second order effects, assuming that the NBu4 ÷ concentration in solution is n o t significantly affected by the course of TPP reduction. It is noted that the alkylation reaction is about t w e n t y times slower in HMPA than in DMF. This is probably related to the larger donicity of HMPA as compared to that of DMF (HMPA = 38.8, DMF = 30.9) [23] resulting in a larger solvation of the tetraalkylammonium which reduces its alkylating power.
Approximate evaluation o f the rate constant by linear sweep voltammetry It is found for the polarographic results concerning TPP in DMF + NBu4+ that the rate constants obtained neglecting the catalyst consumption reaction are of the same order of magnitude as those derived above from a more rigourous treatment (1 vs. 1.26 for DISPA and 0.5 vs. 0.66 for DISPB). It can be infered that the same is true for 1.s.v. An approximate evaluation of the rate constant can thus be obtained using the available treatment of formal kinetics for a purely catalytic process under a large excess of substrate [24,25]. ~ = RT/Fv is equal to 0.1 when ip/x/~is 1.10 times its value for diffusion. The determination of the v value corresponding to these conditions thus leads to the value of k and then of k' (Table 2). The values thus obtained for TPP are in good agreement with those derived from polarography and coulometry. On the other hand it is seen that TPPO anion radical is about ten times less reactive than TPP anion radical. It must however be borne in mind that the values of k' determined for TPPO in DMF and HMPA may be in excess owing to the possible interference of convection at the low sweep rates used.
EXPERIMENTAL Most of the compounds referred to in this work were of commercial origin. They were used without further purification, except for TPP, TPPO and biphenyl which were recrystallized from a water-acetonitrile mixture. The n-butyltriphenylphosphonium bromide was prepared reacting the n-butylbromide with TPP [14]. EtOP(/)2 was prepared in a manner similar to t h a t described by Aknes and
41
Aknes [22]: EtOH + (CHa)2N 0 + P02CI -> P02OEt + (CH3)2~)I~HC1The origin and purification of solvents and supporting electrolytes were the same as previously described [14,15]. Instrumentation and procedures for polarography, cyclic voltammetry, coulometry and electrolysis have been described elsewhere [14,15]. The work in HMPA was carried o u t using a glove box [14]. For kinetic determinations by coulometry, a rotated cell was used. The mercury pool with a surface of 13 cm 2, and the cell itself were rotated at a regular speed of a b o u t 120 r.p.m. The volume of solution was 2--5 cm 3. The reference electrode and the anodic compartment which was a clay cylinder (Haldenwanger, ABS), were held immobile. The rotated cell proved suitable for a smooth and reproducible recording of coulometric current-time curves. Reduction products from electrolyzed solutions were analyzed by v.p.c, on carbowax 20M or OV 17 columns. The thin layer chromatography experiments were performed on 60 F Merck Chromoplate. The eluent was 2% benzene-ether. Spots were then detected under exposure to u.v. light. c.v. was also used for titrating TPP and TPPO during electrolysis. ACKNOWLEDGEMENT The work was supported in part b y the C.N.R.S. (Equipe de Recherche Associ~s No. 309: Electrochimie Organique). REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
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