- Email: [email protected]
The reaction of [Pt(dien)Br]+ with a series of pyridine derivatives
J. inorg, nucl.Chem. 1967,Vol. 29, pp. 2401 to 241}9. Pergamon Press Ltd. Printed in Northern Ireland
THE REACTION OF [Pt(dien)Br] + WITH A SERIES OF P Y R I D I N E DERIVATIVES W. R. RIMM, D. O. JOHNSTON, C. H. OESTREICH, D. G. LAMBERT and MARK i . JONES Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37203 (First received 23 December 1966; in revised form 10 March 1967)
Abstract--The rate of substitution of bromide in [Pt(dien)Br]+ by a series of pyridine derivatives was found to be governed primarily by steric properties of the entering group. There was no general relationship between this rate and the basicity of the entering group. A more complete analysis of the proposed mechanism of such reactions is given and it is shown that the two term rate law, commonly used to describe such reactions, may be only an approximation. INTRODUCTION REACTIONS of the type: [Pt(dien)X] + + Y = [Pt(dien)Y] 9+ + X-,
(1)
(where dien = diethylenetriamine) have been examined extensively. In particular, the reactions of the [Pt(dien)Br] + ion have been studied in detail because of the directness with which they bear upon the mechanism of substitution reactions in square planar complexes, tx-a~ For these systems the experimental rate law is +
+
Rate = ~l[Pt(dien)Br] +/~2[Pt(Dien)Br][Y].
(2)
According to the interpretation of Equation (2) given by previous workers, k x is the rate constant for hydrolysis of the eomplex and ~z is the rate constant for reaction of the entering group (Y) with the complex. I f this interpretation is correct/~1 must be constant and independent of the nature of the entering group; /~1 is the same when the entering group is chloride or hydroxide, but is considerably lower when the entering group is pyridine. The present work is an extension of the earlier studies. It is shown that when various substituted pyridines are the entering group both/~x and /~2 vary by several orders of magnitude, and a tentative explanation of these observations is presented. EXPERIMENTAL Preparation of [Pt(dien)Br]Br The [Pt(dien)Brl Br used for this work was prepared by the method of DIN and BAILARc5~with some modifications. Five grams of KaPtCl~ (City Chemical Company) was dissolved with 3-8 gofreerystallized diethylenetriamine trihydrochloride in 100 ml of water. The mixture was refluxed gently on a steam bath for 5 hr in the dark. After filtering, the volume was reduced to 65 ml by evaporation at room temperature and 1 ml of HBr and 100 ml of warm saturated NH4Br were added. The mixture was stirred vigorously and the salt which precipitated was filtered, washed with 95 per cent ethanol, and then ether. The product was dried 1 hr at 45°C and was observed to melt with decomposition at ~1~F. BASOLO, H. B. GRAY and R. G. PEARSON,d. Am. chem. Soc. 82, 4200 (1960). I~ F. BASOLO,J. CHATr, H. B. GRAY, R. G. PEARSONand B. L. SHAW,d. Am. chem. Soe. 83,2207 (1961). t3~ H. B. GRAY, d. Am. chem. Soc. 84, 1548 (1962). t~ H. B. GRAY, Ph.D. Thesis, Northwestern University, Evanston, Ill. (1961). c5~BADAR-UD-DIN and J. C. BAILAR, JR., J. inorff, nucL Chem. 22, 241 (1961). 18
2401
2402
W.R. ~
et aL
299°C. Yield was 18 per cent. (A second crop of crystals could have been obtained from the mother liquor). Anal Cale. for [Pt(dien)Br]Br: C, 10"49; H, 2'86; N, 9.17; Br, 34-89. Found: C, 10.31; H, 2"83; N, 9.29; Br, 34.79.
Pyridine bases The pyridine bases used were the purest available commercially. Where these were not of sufficient purity they were purified by sublimation or fractional distillation until their m.p. or b.p. was in accord with the literature values.
Determination of reaction rates The reaction examined in this study was the replacement of B r - in the complexion ion [Pt(dien)Br] + by substituted pyridines. These reactions are of the type: [Pt(dien)Br]+ + Y ~
[Pt(dien)Y] + + Br-.
The reactions were followed by measuring changes in electrical conductivity of the aqueous solution under investigation. The apparatus and conductance cells have been described previously, c.3 The typical procedure for a run was as follows. Three solutions of the ligand at concentrations varying from 0-06 M to 0.2 M and a solution of 0.008 M [Pt(dien)Br]Br were thermostated for at least 10 hr in a 25°C water, bath before each run. To start a kinetic run, equal aliquots of both ligand and complex were mixed, the cell rinsed at least three times and the cell finally filled. Normally the first reading could be made within 4 min. In this manner, the initial concentration of the complex was 0.0004 M and the concentration of the ligands between 0.003 M and 0.100 M as shown in Table 1. TABLE 1.--OBF.~WD RATE CONSTANTS OF SOME stJBsrrrtrr~D pC~DnqES w r r a 0.0004 M [Pt(dien)Br]Br itq WATER at 25"0°C [Pt(dien)Br] + + Y ~ Compound Pyridine
2-Picoline
3-Picoline
4-Picoline
2,3-Lutidine
2,4-Lutidine
2,6-Lutidine
3,4-Lutidine
[Pt(dien)Y] 2+ + B r Concentration (M) 0.00372 0.0124 0.0620 0.00372 0"0124 0.0372 0-0620 0"010 0'050 0-100 0-010
kobB (min ~1) 1.04 2.56 1.30 1.51 2.53 4.46 6"60 2"55 1.31 2"55 6"56 1.40
x x x × x x x × × × ×
10-a 10-8 10-2 10-4 10-4 10-' 10-4
× × / × x × × x × <~1 ×
10 -2 I0 -~ 10 -4 10 -4 I0-' 10 -5 10 -4 I0 -4 10 -4
0"050 0.I00 O'OlO 0"050 0.I00 0.00372 0"124 0-0372 0"0620
2"31 1.40 2"50 2"81 9"89 1"56 3-01 4"59
0"010 0"050 0.100 0"010 0"050
.~4 48 7"29 1.33
x x x x
c~ T. H. LARKINS, JR., Ph.D. Thesis, Vanderbilt University, Nashville, Tenn. (1963).
10 -8 I0 -z
10-2 I0 -s
10-s 10-5 10-5 10-a 10-2
The reaction of [Pt(dien)Br] + with a series o f pyridine derivatives
2403
Table 1. Cont. Compound
3,5-Lutidine
2,4,6-Collidine
2-Ethyl pyridine
4-Ethyl pyridine
2-Pyridone
3-Pyridol
Picolinic acid
Nicotinic acid
Aniline
2-Amino pyridine
3-Amino pyridine
4-Amino pyddine
2-Amino, 4-methyl pyridine
2-Amino, 6-methyl pyridine
2-Chloro pyridine
3-Chloro pyridine
2-Propanol pyridine
Concentration ( M ) 0.100 0"010 0"050 0.100 0.010 0-050 0.100 0"010 0"050 0.100 0.010 0"0375 0"050 0"010 0"050 0.100 0'010 0"050 0-100 0.010 0"050 0.100 0"010 0-0400 0.050 0.010 0"050 0.100 0.010 0-050 0-100 0.010 0.050 0.100 0.010 0.050 0.100 0"010 0.050 0"100 0"010 0"050 0.100 0"010 0"025 0-050 0.00925 0.04125 0.0925 0"0075 0-010 0-050
kob.(Min -x) 2"08 7"70 1"48 2"35 .~1 ,~4 ,~8 1.22 2"27 3.61 7.73 1-19 1"35 1"65 4-65 8-50 1"54 5"82 1'11 2"92 9"06 1"61 1"29 3-98 4.60 5"47 1.16 1"90 5.98 7.21 8.60 7.70 1.52 2.49 6.27 1.61 2'85 5"91 6.71 7"73 9"43 1-14 1.40 5'40 1"13 2"10 2"60 7"35 1.50 1"15 1"26 2"94
× × × X × × × × × × × × × x × × × x x x X x X × × × × X × x × × × × × × × × × X X X × × × × x × × × × X
10 -2 10 -2 10 -2 10 -2 10 -5 10 -5 10-s
10 -4 10 -4 10 -4 10 -s 10 -2 10 -2 10 -s 10. 5 10 -~ 10 -2 10.2 10 -2 10.4 10 .4 10 -2 10-2 10 .2 10. 2 10. 2 10 -2 10 -2 10. 2 10 -s 10. 2 10 -s 10 -~ 10. 2 10. 2 10 -2 10. 2 10 .2 10 .2 10 -s 10 -2 10 -~ 10. 4 10. 4 10 .2 10 .2 10 .2 10.2 10. 2 10.4 10 .4 10 .4
2404
W. R. ~
et aL
All these reactions went to completion as judged by the total conductance changes observed and by comparison of the pyridine data with that reported earlier."} The reactions were carried out with an excess of ligand and the conductance data analysed to give pseudo-first-order rate constants (/COBB)as evaluated graphically from the equation: k = 2.303 log
Ro
(3)
where R0 is the resistance at zero time, R~o is the resistance at completion of the reaction, and R~ is the resistance at time t. Since R0 and R~ are constants during any run, log [R,[R~ -- Roo] is plottedvs, t. A typical plot for [Pt(dien)Br]Br is shown in Fig. 1. 0,7,
i
"
i
"' : .010 M. :)-Amino Pyridine [] : .050M. 2-Amino Pyridine
0.6
0~
0,5
0 .._1
0.4
I 50
0.3
0
1 I00
MINUTES
FIG. 1.--Determination of kobs for 2-amino-pyridine at 25 °. In all cases, the variation of the pseudo-first order rate constants (kobs) with initial substituted pyridine concentration was consistent with the equation: kob, = ~'x + ~'~(Y).
(4)
A typical plot of kobs vS. ligand concentration is shown in Fig. 2. The slope of the line is kj and the intercept is ~x. In obtaining kobs by the conductance method, some deviations from strictly first order plots were observed. Other workers have observed similar deviations,t~) The initial fast reaction which occurs at low pyridine concentrations (see Fig. 3) was attributed to addition of an insufficient amount of ligand to produce pseudo-first-order kinetics. At high concentrations of ligand a tailing off in the latter part of the reaction (Fig. 3) which is attributed to the diminished amount of complex remaining at that point. These points were taken into consideration in evaluating kobs. RESULTS I n all cases kobs was d e t e r m i n e d a t three o r m o r e different c o n c e n t r a t i o n s o f the substituted pyridine, Y ; these results are given in T a b l e 1. T h e r a t e c o n s t a n t s ~1 a n d ~2 in E q u a t i o n (4) were c o m p u t e d f r o m the d a t a in T a b l e 1 a n d a r e collected in (T) R. G. PEARSON. Private communication.
The reaction of [Pt(dien)Br]+ with a series of pyridine derivatives
2405
3-OH py.
8
T z
2-NH2 py, y.
x 3-COzH py.
2 - C I py. PY"
0.00
I
O, lO
0.05 [YI,M
FIG. l--Determination of k~ and ~=. 1313
i
I
0:°00372 In ~ °0620
0.9
M. Pyridinll M. Pyridine
0.7
-J 0,5
I
L
I
tO0
200
500
MINUTES
FIa. 3.--Determination of kob, for pyridine at 25°. Table 2. The accuracy of the/~x values reported in Table 2 is estimated as 4-20 per cent. A correlation between the basicity of the ligand and the second-order rate constant /~, was sought. Figure 4 shows very vividly that there is no simple correlation between the pKa of the ligand and/~2. This unexpected result can be attributed to the over-riding importance of the sterie interactions exhibited by the substituted pyridines. DISCUSSION Figure 3 shows very vividly that the relationship between the basicity of the ligand, the pKa, and the second order rate constant, k,, is lacking here due to the over-riding importance of the steric interactions exhibited by the substituted pyridines. Equation (2) might be written in the form: rate ---- (/~1[S] q-/~[Y])[complex],
(5)
where/~z would be the rate constant for reaction with the solvent and/~2 would be the
W . R. P,IMM et aL
2406
TABLE 2.--SLrRWY OF FIRST ORDER RATE CONSTANTS AND BIMOLECULAR RATE CONSTANTS FOR REACTIONS OF [Pt(dien)Br]Br wrrri VAmOUS s t m s m t r l X D P Y R m n ~ s IN WATER AT 25-0°C Compound
pK~*
Pyridine 2-Picoline 3-Picoline 4-Picoline 2,3-Lutidine 2,4-Lutidine 2,6-Lutidine 3,4-Lutidine 3,5-Lutidine 2,4,6-Collidine 2-Ethyl pyridine 4-Ethyl pyridine 2-Pyridol 3-Pyridol Picolinic acid Nicotinic acid Aniline 2 - A m i n o pyridine 3 - A m i n o pyridine 4 - A m i n o pyridine 2 - A m i n o , 4-methyl pyridine 2 - A m i n o , 6-methyl pyridine 2-Chloro pyridine 3-Chloro pyridine 2-Propanol pyridine
5.29 5.95 5"85 6.11 6-56 6-80 6"72 6"61 6-34 7"63
kl ( m i n - 0 4.0 1.7 2"0 4"8 1.2 8.0 ,(<9 5"8 6"0 ,~<9 9"5 7.1 9.0 5-0 1.4 4.7 4.0 5"7 5.7 5.7 5-7 9.0 1.6 1.2 8.4
4"86 5.52 4.77 4.58 6.86 6.09 9.17
× × × × × x × × × × × x x x × × × × × × × × × × ×
10-' 10 -~ 10-' 10 -8 10-' 10 -5 10 -5 10 -8 10 -8 10 -s 10 -5 10 -3 10 -6 10 -4 10 -4 10 -4 10 -s 10 -s 10 - s 10 -3 10 -a 10 -s 10 -4 10 -s 10 -s
~= ( m i n - O
2-03 7.90 2"53 1-83 1.55 3.80 ,<5.00 1.50 1"75 ,~5.00 2.66 1.28 7.60 1.06 1.47 8"26 1.50 2.90 1.92 2.47 2.03 5.00 3.88 1.48 4.20
X × x x × × x × × × × x × x x × × × × × × × × x ×
10 -x 10 4 10 -1 10 -I 10 -a 10 - s 10-' 10 -1 10 -1 10 -~ 10 - s 10 -1 I0 -~ 10 -1 10 -= 10 -2 10 -] 10 -= 10 -1 10 -1 10 -s 10 -4 10 -= 10 -1 10 -8
* T h e pKa values were obtained f r o m : H. H . JAFrEE a n d G. O. DOAK, d. Am. chem. Soc. 77, 4441 (1955), a n d Ref. (5) therein.
I
t
An;fine <) 0 A o
S.O
t t : :
2-S.bstituted Pyrldines 3-Substituted Pyridines 4-Substiluted Pyridlaes Multiply Substituted Pyridine|,
pKa
O o
o
o AD
6,0
go x 4.0
f 2,50
I 2,00
I 1.50 LOG
I 1,00
kz
FIG. 4.---Graphical presentation o f pKa vs. log k2 data. T h e lack o f correlation illustrated here e m p h a s i z e s t h e i m p o r t a n c e o f steric factors.
The reaction of [Pt(dien)Br]+ with a series of pyridinederivatives
2407
rate constant for reaction with the entering group. This interpretation leads to the anticipation that/~1 will be the same for all the ligands, if the leaving group is lost in the rate-determining step. GRAYtmhas shown that ~1 is the same for the reactions of CI-I-, NO2-, SCN-, Na-, and SC(NH~)~ with [Pt(dien)Br]+, bu t n o t for pyridine. The slower rate observedtl) was explained by the fact that the more important first-order reaction intermediate reacts much more rapidly with bromide ion than with pyridine. This was shown experimentally by reacting [Pt(dien)Br]NOa, which furnishes no free bromide ion initially, with pyridine. The complex reacts rapidly at first but as bromide is liberated the reaction rate is reduced until only a slow first-order rate, which is equal to the kl value reported for the [Pt(dien)Br]Brpyridine reactions, remains. The initial rapid reaction rate is almost equal to the first-order rate observed for reactions of [Pt(dien)Br]Br with ligands of the type CI-, I-, NO2-, SCN-, Nz- and SC(NH~)2.TM Therefore, the other ligands must compete with bromide much more effectively than pyridine does for the principal intermediate. The same might be said for the substituted pyridines studied in this work, but with the wide variation of kl observed it is felt that the possibility of concurrent alternative reaction paths (of lesser importance than the primary associative path) needs to be considered. A sequence of steps which is capable of explaining the variation in k~is the following one:
Pl
--
X k
,
I
II
S
_
-
k- 2 S m
y
=
,
y
]I
Y v
S HI
~
+S Y
2408
W. R. RaMM et al.
By applying the steady-state assumption to species II and III, one obtains:
[II] =
kl[I](k2 + k4[Y]) (k-a[X] + ka[Y])(k_2 + k4[Y]) + k~k4[Y]'
[III]=
klk~[I] (k_l[X] + ka[Y])(k_~ + k4[Y]) + ks k4[Y]
and:
Now the rate of product formation will be: Rate = ka[II][Y] + ka[III][Y], and using the steady state concentrations of II and III one obtains:
Rate =
(kxk2k3 + klk2k4 + klk3k4[Y])[I] k-lk-~[X] q- k_2k a @ k-lka[X] -q- kzk4[Y ] q- k~k 4 [YI (a) (b)
If terms (a), and (b) in the denominator may be neglected we may write:
Rate = {(klk-2ka -q- klkzk4) + kxk3k4[Y]}[I] (kzk4 + k-~ka + k _ l k 4 [ X - ] ) Since we have previously used:
kob8=/~1 + ~[Y], we can see that in terms of the reaction sequence proposed:
klk_2k a -@ kxk2k 4 l~1 = k ~ 4 + k-~ka + k_tk4[X] '
and: fc~ =
klksk4 k_~ka + k_xk4[X] + k2k4'
"
The reaction of [Pt(dien)Br]+ with a series of pyridine derivatives
2409
If k_2k3 >~ k_lk4[X ] + kek4: /~,=ki+
\k_,l~=A+Bk'-~.
For different ligands,/~x will be a constant only if k J k 3 is constant. This allows some understanding of a possible explanation of the fact that the kx values do vary. From this same development:
s o / ~ is a measure of k4, i.e. the rate of the reaction in which the ligand replaces the solvent. The mass of evidence collected to date favours these reactions proceeding primarily along the path of direct substitution; the experimental variation ofkt values, however, would appear to indicate that a contribution from a dissociative mechanism may be an alternative path of only relatively lesser importance than the associative mechanism. Such an assumption is not in accord with recent evidence obtained on the hindered system [Pd(Et~dien)C1] + which indicates that a dissociative pathway must be several orders of magnitude slower than an associative one for this ion. tS) Acknowledgement--We wish to acknowledge with thanks the financial support for this work provided by the U.S. Atomic Energy Commission. ~8~W. H. BADDLEVand F. BASOLO,J. Am. chem. Soc. 86, 2075 (1964).
THE REACTION OF [Pt(dien)Br] + WITH A SERIES OF P Y R I D I N E DERIVATIVES W. R. RIMM, D. O. JOHNSTON, C. H. OESTREICH, D. G. LAMBERT and MARK i . JONES Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37203 (First received 23 December 1966; in revised form 10 March 1967)
Abstract--The rate of substitution of bromide in [Pt(dien)Br]+ by a series of pyridine derivatives was found to be governed primarily by steric properties of the entering group. There was no general relationship between this rate and the basicity of the entering group. A more complete analysis of the proposed mechanism of such reactions is given and it is shown that the two term rate law, commonly used to describe such reactions, may be only an approximation. INTRODUCTION REACTIONS of the type: [Pt(dien)X] + + Y = [Pt(dien)Y] 9+ + X-,
(1)
(where dien = diethylenetriamine) have been examined extensively. In particular, the reactions of the [Pt(dien)Br] + ion have been studied in detail because of the directness with which they bear upon the mechanism of substitution reactions in square planar complexes, tx-a~ For these systems the experimental rate law is +
+
Rate = ~l[Pt(dien)Br] +/~2[Pt(Dien)Br][Y].
(2)
According to the interpretation of Equation (2) given by previous workers, k x is the rate constant for hydrolysis of the eomplex and ~z is the rate constant for reaction of the entering group (Y) with the complex. I f this interpretation is correct/~1 must be constant and independent of the nature of the entering group; /~1 is the same when the entering group is chloride or hydroxide, but is considerably lower when the entering group is pyridine. The present work is an extension of the earlier studies. It is shown that when various substituted pyridines are the entering group both/~x and /~2 vary by several orders of magnitude, and a tentative explanation of these observations is presented. EXPERIMENTAL Preparation of [Pt(dien)Br]Br The [Pt(dien)Brl Br used for this work was prepared by the method of DIN and BAILARc5~with some modifications. Five grams of KaPtCl~ (City Chemical Company) was dissolved with 3-8 gofreerystallized diethylenetriamine trihydrochloride in 100 ml of water. The mixture was refluxed gently on a steam bath for 5 hr in the dark. After filtering, the volume was reduced to 65 ml by evaporation at room temperature and 1 ml of HBr and 100 ml of warm saturated NH4Br were added. The mixture was stirred vigorously and the salt which precipitated was filtered, washed with 95 per cent ethanol, and then ether. The product was dried 1 hr at 45°C and was observed to melt with decomposition at ~1~F. BASOLO, H. B. GRAY and R. G. PEARSON,d. Am. chem. Soc. 82, 4200 (1960). I~ F. BASOLO,J. CHATr, H. B. GRAY, R. G. PEARSONand B. L. SHAW,d. Am. chem. Soe. 83,2207 (1961). t3~ H. B. GRAY, d. Am. chem. Soc. 84, 1548 (1962). t~ H. B. GRAY, Ph.D. Thesis, Northwestern University, Evanston, Ill. (1961). c5~BADAR-UD-DIN and J. C. BAILAR, JR., J. inorff, nucL Chem. 22, 241 (1961). 18
2401
2402
W.R. ~
et aL
299°C. Yield was 18 per cent. (A second crop of crystals could have been obtained from the mother liquor). Anal Cale. for [Pt(dien)Br]Br: C, 10"49; H, 2'86; N, 9.17; Br, 34-89. Found: C, 10.31; H, 2"83; N, 9.29; Br, 34.79.
Pyridine bases The pyridine bases used were the purest available commercially. Where these were not of sufficient purity they were purified by sublimation or fractional distillation until their m.p. or b.p. was in accord with the literature values.
Determination of reaction rates The reaction examined in this study was the replacement of B r - in the complexion ion [Pt(dien)Br] + by substituted pyridines. These reactions are of the type: [Pt(dien)Br]+ + Y ~
[Pt(dien)Y] + + Br-.
The reactions were followed by measuring changes in electrical conductivity of the aqueous solution under investigation. The apparatus and conductance cells have been described previously, c.3 The typical procedure for a run was as follows. Three solutions of the ligand at concentrations varying from 0-06 M to 0.2 M and a solution of 0.008 M [Pt(dien)Br]Br were thermostated for at least 10 hr in a 25°C water, bath before each run. To start a kinetic run, equal aliquots of both ligand and complex were mixed, the cell rinsed at least three times and the cell finally filled. Normally the first reading could be made within 4 min. In this manner, the initial concentration of the complex was 0.0004 M and the concentration of the ligands between 0.003 M and 0.100 M as shown in Table 1. TABLE 1.--OBF.~WD RATE CONSTANTS OF SOME stJBsrrrtrr~D pC~DnqES w r r a 0.0004 M [Pt(dien)Br]Br itq WATER at 25"0°C [Pt(dien)Br] + + Y ~ Compound Pyridine
2-Picoline
3-Picoline
4-Picoline
2,3-Lutidine
2,4-Lutidine
2,6-Lutidine
3,4-Lutidine
[Pt(dien)Y] 2+ + B r Concentration (M) 0.00372 0.0124 0.0620 0.00372 0"0124 0.0372 0-0620 0"010 0'050 0-100 0-010
kobB (min ~1) 1.04 2.56 1.30 1.51 2.53 4.46 6"60 2"55 1.31 2"55 6"56 1.40
x x x × x x x × × × ×
10-a 10-8 10-2 10-4 10-4 10-' 10-4
× × / × x × × x × <~1 ×
10 -2 I0 -~ 10 -4 10 -4 I0-' 10 -5 10 -4 I0 -4 10 -4
0"050 0.I00 O'OlO 0"050 0.I00 0.00372 0"124 0-0372 0"0620
2"31 1.40 2"50 2"81 9"89 1"56 3-01 4"59
0"010 0"050 0.100 0"010 0"050
.~4 48 7"29 1.33
x x x x
c~ T. H. LARKINS, JR., Ph.D. Thesis, Vanderbilt University, Nashville, Tenn. (1963).
10 -8 I0 -z
10-2 I0 -s
10-s 10-5 10-5 10-a 10-2
The reaction of [Pt(dien)Br] + with a series o f pyridine derivatives
2403
Table 1. Cont. Compound
3,5-Lutidine
2,4,6-Collidine
2-Ethyl pyridine
4-Ethyl pyridine
2-Pyridone
3-Pyridol
Picolinic acid
Nicotinic acid
Aniline
2-Amino pyridine
3-Amino pyridine
4-Amino pyddine
2-Amino, 4-methyl pyridine
2-Amino, 6-methyl pyridine
2-Chloro pyridine
3-Chloro pyridine
2-Propanol pyridine
Concentration ( M ) 0.100 0"010 0"050 0.100 0.010 0-050 0.100 0"010 0"050 0.100 0.010 0"0375 0"050 0"010 0"050 0.100 0'010 0"050 0-100 0.010 0"050 0.100 0"010 0-0400 0.050 0.010 0"050 0.100 0.010 0-050 0-100 0.010 0.050 0.100 0.010 0.050 0.100 0"010 0.050 0"100 0"010 0"050 0.100 0"010 0"025 0-050 0.00925 0.04125 0.0925 0"0075 0-010 0-050
kob.(Min -x) 2"08 7"70 1"48 2"35 .~1 ,~4 ,~8 1.22 2"27 3.61 7.73 1-19 1"35 1"65 4-65 8-50 1"54 5"82 1'11 2"92 9"06 1"61 1"29 3-98 4.60 5"47 1.16 1"90 5.98 7.21 8.60 7.70 1.52 2.49 6.27 1.61 2'85 5"91 6.71 7"73 9"43 1-14 1.40 5'40 1"13 2"10 2"60 7"35 1.50 1"15 1"26 2"94
× × × X × × × × × × × × × x × × × x x x X x X × × × × X × x × × × × × × × × × X X X × × × × x × × × × X
10 -2 10 -2 10 -2 10 -2 10 -5 10 -5 10-s
10 -4 10 -4 10 -4 10 -s 10 -2 10 -2 10 -s 10. 5 10 -~ 10 -2 10.2 10 -2 10.4 10 .4 10 -2 10-2 10 .2 10. 2 10. 2 10 -2 10 -2 10. 2 10 -s 10. 2 10 -s 10 -~ 10. 2 10. 2 10 -2 10. 2 10 .2 10 .2 10 -s 10 -2 10 -~ 10. 4 10. 4 10 .2 10 .2 10 .2 10.2 10. 2 10.4 10 .4 10 .4
2404
W. R. ~
et aL
All these reactions went to completion as judged by the total conductance changes observed and by comparison of the pyridine data with that reported earlier."} The reactions were carried out with an excess of ligand and the conductance data analysed to give pseudo-first-order rate constants (/COBB)as evaluated graphically from the equation: k = 2.303 log
Ro
(3)
where R0 is the resistance at zero time, R~o is the resistance at completion of the reaction, and R~ is the resistance at time t. Since R0 and R~ are constants during any run, log [R,[R~ -- Roo] is plottedvs, t. A typical plot for [Pt(dien)Br]Br is shown in Fig. 1. 0,7,
i
"
i
"' : .010 M. :)-Amino Pyridine [] : .050M. 2-Amino Pyridine
0.6
0~
0,5
0 .._1
0.4
I 50
0.3
0
1 I00
MINUTES
FIG. 1.--Determination of kobs for 2-amino-pyridine at 25 °. In all cases, the variation of the pseudo-first order rate constants (kobs) with initial substituted pyridine concentration was consistent with the equation: kob, = ~'x + ~'~(Y).
(4)
A typical plot of kobs vS. ligand concentration is shown in Fig. 2. The slope of the line is kj and the intercept is ~x. In obtaining kobs by the conductance method, some deviations from strictly first order plots were observed. Other workers have observed similar deviations,t~) The initial fast reaction which occurs at low pyridine concentrations (see Fig. 3) was attributed to addition of an insufficient amount of ligand to produce pseudo-first-order kinetics. At high concentrations of ligand a tailing off in the latter part of the reaction (Fig. 3) which is attributed to the diminished amount of complex remaining at that point. These points were taken into consideration in evaluating kobs. RESULTS I n all cases kobs was d e t e r m i n e d a t three o r m o r e different c o n c e n t r a t i o n s o f the substituted pyridine, Y ; these results are given in T a b l e 1. T h e r a t e c o n s t a n t s ~1 a n d ~2 in E q u a t i o n (4) were c o m p u t e d f r o m the d a t a in T a b l e 1 a n d a r e collected in (T) R. G. PEARSON. Private communication.
The reaction of [Pt(dien)Br]+ with a series of pyridine derivatives
2405
3-OH py.
8
T z
2-NH2 py, y.
x 3-COzH py.
2 - C I py. PY"
0.00
I
O, lO
0.05 [YI,M
FIG. l--Determination of k~ and ~=. 1313
i
I
0:°00372 In ~ °0620
0.9
M. Pyridinll M. Pyridine
0.7
-J 0,5
I
L
I
tO0
200
500
MINUTES
FIa. 3.--Determination of kob, for pyridine at 25°. Table 2. The accuracy of the/~x values reported in Table 2 is estimated as 4-20 per cent. A correlation between the basicity of the ligand and the second-order rate constant /~, was sought. Figure 4 shows very vividly that there is no simple correlation between the pKa of the ligand and/~2. This unexpected result can be attributed to the over-riding importance of the sterie interactions exhibited by the substituted pyridines. DISCUSSION Figure 3 shows very vividly that the relationship between the basicity of the ligand, the pKa, and the second order rate constant, k,, is lacking here due to the over-riding importance of the steric interactions exhibited by the substituted pyridines. Equation (2) might be written in the form: rate ---- (/~1[S] q-/~[Y])[complex],
(5)
where/~z would be the rate constant for reaction with the solvent and/~2 would be the
W . R. P,IMM et aL
2406
TABLE 2.--SLrRWY OF FIRST ORDER RATE CONSTANTS AND BIMOLECULAR RATE CONSTANTS FOR REACTIONS OF [Pt(dien)Br]Br wrrri VAmOUS s t m s m t r l X D P Y R m n ~ s IN WATER AT 25-0°C Compound
pK~*
Pyridine 2-Picoline 3-Picoline 4-Picoline 2,3-Lutidine 2,4-Lutidine 2,6-Lutidine 3,4-Lutidine 3,5-Lutidine 2,4,6-Collidine 2-Ethyl pyridine 4-Ethyl pyridine 2-Pyridol 3-Pyridol Picolinic acid Nicotinic acid Aniline 2 - A m i n o pyridine 3 - A m i n o pyridine 4 - A m i n o pyridine 2 - A m i n o , 4-methyl pyridine 2 - A m i n o , 6-methyl pyridine 2-Chloro pyridine 3-Chloro pyridine 2-Propanol pyridine
5.29 5.95 5"85 6.11 6-56 6-80 6"72 6"61 6-34 7"63
kl ( m i n - 0 4.0 1.7 2"0 4"8 1.2 8.0 ,(<9 5"8 6"0 ,~<9 9"5 7.1 9.0 5-0 1.4 4.7 4.0 5"7 5.7 5.7 5-7 9.0 1.6 1.2 8.4
4"86 5.52 4.77 4.58 6.86 6.09 9.17
× × × × × x × × × × × x x x × × × × × × × × × × ×
10-' 10 -~ 10-' 10 -8 10-' 10 -5 10 -5 10 -8 10 -8 10 -s 10 -5 10 -3 10 -6 10 -4 10 -4 10 -4 10 -s 10 -s 10 - s 10 -3 10 -a 10 -s 10 -4 10 -s 10 -s
~= ( m i n - O
2-03 7.90 2"53 1-83 1.55 3.80 ,<5.00 1.50 1"75 ,~5.00 2.66 1.28 7.60 1.06 1.47 8"26 1.50 2.90 1.92 2.47 2.03 5.00 3.88 1.48 4.20
X × x x × × x × × × × x × x x × × × × × × × × x ×
10 -x 10 4 10 -1 10 -I 10 -a 10 - s 10-' 10 -1 10 -1 10 -~ 10 - s 10 -1 I0 -~ 10 -1 10 -= 10 -2 10 -] 10 -= 10 -1 10 -1 10 -s 10 -4 10 -= 10 -1 10 -8
* T h e pKa values were obtained f r o m : H. H . JAFrEE a n d G. O. DOAK, d. Am. chem. Soc. 77, 4441 (1955), a n d Ref. (5) therein.
I
t
An;fine <) 0 A o
S.O
t t : :
2-S.bstituted Pyrldines 3-Substituted Pyridines 4-Substiluted Pyridlaes Multiply Substituted Pyridine|,
pKa
O o
o
o AD
6,0
go x 4.0
f 2,50
I 2,00
I 1.50 LOG
I 1,00
kz
FIG. 4.---Graphical presentation o f pKa vs. log k2 data. T h e lack o f correlation illustrated here e m p h a s i z e s t h e i m p o r t a n c e o f steric factors.
The reaction of [Pt(dien)Br]+ with a series of pyridinederivatives
2407
rate constant for reaction with the entering group. This interpretation leads to the anticipation that/~1 will be the same for all the ligands, if the leaving group is lost in the rate-determining step. GRAYtmhas shown that ~1 is the same for the reactions of CI-I-, NO2-, SCN-, Na-, and SC(NH~)~ with [Pt(dien)Br]+, bu t n o t for pyridine. The slower rate observedtl) was explained by the fact that the more important first-order reaction intermediate reacts much more rapidly with bromide ion than with pyridine. This was shown experimentally by reacting [Pt(dien)Br]NOa, which furnishes no free bromide ion initially, with pyridine. The complex reacts rapidly at first but as bromide is liberated the reaction rate is reduced until only a slow first-order rate, which is equal to the kl value reported for the [Pt(dien)Br]Brpyridine reactions, remains. The initial rapid reaction rate is almost equal to the first-order rate observed for reactions of [Pt(dien)Br]Br with ligands of the type CI-, I-, NO2-, SCN-, Nz- and SC(NH~)2.TM Therefore, the other ligands must compete with bromide much more effectively than pyridine does for the principal intermediate. The same might be said for the substituted pyridines studied in this work, but with the wide variation of kl observed it is felt that the possibility of concurrent alternative reaction paths (of lesser importance than the primary associative path) needs to be considered. A sequence of steps which is capable of explaining the variation in k~is the following one:
Pl
--
X k
,
I
II
S
_
-
k- 2 S m
y
=
,
y
]I
Y v
S HI
~
+S Y
2408
W. R. RaMM et al.
By applying the steady-state assumption to species II and III, one obtains:
[II] =
kl[I](k2 + k4[Y]) (k-a[X] + ka[Y])(k_2 + k4[Y]) + k~k4[Y]'
[III]=
klk~[I] (k_l[X] + ka[Y])(k_~ + k4[Y]) + ks k4[Y]
and:
Now the rate of product formation will be: Rate = ka[II][Y] + ka[III][Y], and using the steady state concentrations of II and III one obtains:
Rate =
(kxk2k3 + klk2k4 + klk3k4[Y])[I] k-lk-~[X] q- k_2k a @ k-lka[X] -q- kzk4[Y ] q- k~k 4 [YI (a) (b)
If terms (a), and (b) in the denominator may be neglected we may write:
Rate = {(klk-2ka -q- klkzk4) + kxk3k4[Y]}[I] (kzk4 + k-~ka + k _ l k 4 [ X - ] ) Since we have previously used:
kob8=/~1 + ~[Y], we can see that in terms of the reaction sequence proposed:
klk_2k a -@ kxk2k 4 l~1 = k ~ 4 + k-~ka + k_tk4[X] '
and: fc~ =
klksk4 k_~ka + k_xk4[X] + k2k4'
"
The reaction of [Pt(dien)Br]+ with a series of pyridine derivatives
2409
If k_2k3 >~ k_lk4[X ] + kek4: /~,=ki+
\k_,l~=A+Bk'-~.
For different ligands,/~x will be a constant only if k J k 3 is constant. This allows some understanding of a possible explanation of the fact that the kx values do vary. From this same development:
s o / ~ is a measure of k4, i.e. the rate of the reaction in which the ligand replaces the solvent. The mass of evidence collected to date favours these reactions proceeding primarily along the path of direct substitution; the experimental variation ofkt values, however, would appear to indicate that a contribution from a dissociative mechanism may be an alternative path of only relatively lesser importance than the associative mechanism. Such an assumption is not in accord with recent evidence obtained on the hindered system [Pd(Et~dien)C1] + which indicates that a dissociative pathway must be several orders of magnitude slower than an associative one for this ion. tS) Acknowledgement--We wish to acknowledge with thanks the financial support for this work provided by the U.S. Atomic Energy Commission. ~8~W. H. BADDLEVand F. BASOLO,J. Am. chem. Soc. 86, 2075 (1964).