1442
Organometallics 1995, 14, 1442-1449
Stepwise Mechanism for Oxidative Addition of Iodine to Organotellurium(I1)Compounds As Observed by Stopped-Flow Spectroscopy Michael R. Detty,*lt Alan E. Friedman,* and Martin McMillang OfficeImaging Research and Technical Development, Clinical Diagnostics Research Laboratories, and Analytical Technology Division, Eastman Kodak Company, Rochester, New York 14650 Received November 7, 1994@ The reaction of iodine with a series of diorgano tellurides was monitored by stopped-flow spectroscopy. For dihexyl telluride (11, diphenyl telluride (2), di-p-anisyl telluride (31, and phenyl 2-((dimethylamino)methyl)phenyl telluride (4), an initial “fast” reaction was first order (E),the “fast” in substrate and second order in iodine. For 2,6-di-tert-butyltelluropyran-4-one reaction was second order overall and first order in iodine. The “fast” reaction is actually two reactions: the addition of iodine to the tellurium atom to form a n +RZTe-Iz complex followed by the addition of a second iodine to form presumably a n q1-R2Te-14 complex. The first reaction is faster than the second leading to a rapid preequilibrium and apparent inverted Arrhenius behavior in the temperature dependence of the rate constants. The “fast” reaction(s) is followed by a “slow” reaction, which is first order overall and independent of iodine concentration. The rate constant for the “slow” reaction increases with increasing solvent polarity, which is consistent with a dissociative process leading to ionic intermediates.
Introduction and Background Among the oldest reactions of diorganotellurium(I1) and diorganoselenium(I1) compounds are the addition While the reactions of halogens to these material~.l-~ additions of bromine and chlorine to both diorganotellurium(I1) and diorganoselenium(I1) compounds give products of oxidative addition with nearly linear halogen-chalcogen-halogen the additions of iodine to these materials give different products, depending upon the chalcogen atom. In the 1-chalcogena-4-selenane and 1,kdiselenane systems, McCullough and C h a ~ ’ -observed ~ that the addition of iodine did not give products of oxidative addition. Instead, association complexes with nearly linear chalcogen-1-1 arrays were characterized by X-ray crystallographic analysis (structure I for the iodine association complex with 1,4-diselenane). Dissociation constants show that the selenide-iodine comM) are stronger than correspondplexes (K,% 5 x M).l0 ing sulfide-iodine complexes (K,% 5 x The addition of iodine to diorganotellurium(I1) compounds gives different products. Iodine oxidative-ad+ Office Imaging Research and Technical Development. Current address: Department of Medicinal Chemistry, School of Pharmacy, University at Buffalo, Buffalo, N Y 14620. Clinical Diagnostics Research Laboratories. P Analytical Technology Division. Abstract published in Advance ACS Abstracts, February 1, 1995. (1)Lederer, K. Ber. Bunsen-Ges. Phys. Chem. 1916,49,2002. (2)Lederer, K. Ber. Bunsen-Ges. Phys. Chem. 1916,49,1076. (3)Lederer, K.Justus Liebigs Ann. Chem. 1916,391,326. (4)McCullough, J. D.; Hamburger, G. J . Am. Chem. SOC.1941,63, 803. (5)Christofferson, G.D.; McCullough, J.D. Acta Crystallogr. 1958, 11, 249. (6)McCullough, J. D.;Marsh, R. E. Acta Crystallogr. 1950,3 , 41. (7)McCullough, J. D.; Chao, G. Y.; Zuccaro, D. E. Acta Crystallogr. 1959,12,815. ( 8 ) Maddox, H.; McCullough, J. D. Inorg. Chem. 1966,5 , 522. (9)McCullough, J. D. Inorg. Chem. 1964,3,1425. (10)McCullough, J. D.; Brunner, A. Inorg. Chem. 1967,6, 1251.
*
1
I
I1
1
dition products from diary1 tellurides have been characterized by X-ray crystallographyll and have been shown to have the general structure 11. The tellurium(IV)adduct I1 is a trigonal bipyramid with the C-Te bonds and a Te atom lone pair of electrons occupying the equatorial sites and with a nearly linear, transdiaxial array of the iodide ligands to the Te atom. Tellurium analogues of association complexes I have not been characterized, although one might postulate their formation along the path t o 11. Iodine association complexes with diorganochalcogen species can involve more than one molecule of iodine. Sulfur-iodine complexes with two iodine molecules are known. Both PhsP=S--I4 (III) and ethylenethiourea-14 (Wcomplexes have been isolated and characterized by
Ph3P=s-11-1*
@
[NhS-I1-I2
I I3
I
111
I4
YH
I I3
I
IV
I4
X-ray crystallographic analysis.12J3 Materials that are (11)Chao, G.Y.;McCullough, J. D. Acta Crystallogr. 1962,15,887. (12)(a) Schweikert, W.W.; Meyers, E. A. J.Phys. Chem. 1968,72, 1561.(b) Bransford, J.W.; Meyers, E. A. Cryst. S t r u t . Commun. 1978, 7,697. Schwotzer, W. J. Am. Chem. SOC.1984,106, (13)Herbstein, F.H.; 2367.
0276-733319512314-1442$09.00/0 0 1995 American Chemical Society
Oxidative Addition of I to Organotellurium(II) Compounds
Organometallics, Vol. 14, No. 3, 1995 1443
Scheme 1 1.400
(CH3CH2CH2CH2CH2CHz)zTe 1
I
12
acetone or cc1,
I
1.200
I
1.000
(CH-,CH2CH2CHzCH2CH2)zTe 6
I
0.800 0.600
2,R=H 3, R = OMe
7,R=H 8, R = OMe
0.400
0.200
I
0.000 I 250
9
4
9
9
I2
acetone
terr-Bu
,err-Bu
Or cc14
tert-Bu 10
5
analyzed as covalent R2Te-14 complexes with intact 1-1 bonds from spectroscopic analysis have been prepared by the addition of excess iodine to tellura~yclopentane~~ and telluracyclohexane (tellurane).15 However, these materials have not been characterized by X-ray crystallographic analysis. Similar complexes with seleniumcontaining molecules have not been described although it is not clear that they have been so~ght.~-lO One might expect the oxidative addition of iodine across organochalcogen species to follow a reaction course similar t o that observed for oxidative addition of bromine in which an initial fast reaction, which is first order in both bromine and substrate, is followed by one or more slower first-order reactions leading to the final product(s).16 In this paper, we describe a multistep process for the oxidative addition of iodine, which is characterized by a second-order dependence in iodine for the initial fast reaction. Furthermore, the initial reaction(s) in some substrates shows pronounced inverted Arrhenius behavior.
Results and Discussion Product Studies for the Oxidative Addition of Iodine to Diorganochalcogen(I1)Compounds. The diorganotellurium(I1) compounds 1-5 react with iodine in either acetone or carbon tetrachloride to give oxidative-addition products 6- 10,respectively, as shown in Scheme 1. The products 6-10 were spectroscopically identical with the final products produced by oxidative addition of iodine in the stopped-flowspectrophotometer in experiments described below. Absorption and infrared spectra for compound 9 are nearly identical with those of compound 11, whose structure has been established by X-ray crystallography.16 Crystals of 9 were highly twinned and were not
+ I
11
12
suitable for X-ray crystallographic analysis. For both
300
350
400
450
500
h, nm
Figure 1. Transient spectra for the fast reaction for oxidative addition of iodine (5 x M) to di-p-anisyl telluride 3 (5 x M) at 295.1Kin carbon tetrachloride. Transient spectra were determined at 0.012,0.0375,0.0875, 0.1375, 0.2125, 0.325, 0.4125, 0.587, 0.800, and 2.687 s (traces are sequential from the arrow). 9 and 11, lH NMR spectra were highly broadened due to exchange processes. From the spectral similarities, we have assumed compound 9 to have a structure similar to that of 11. The structures of compounds 6-8 and 10 should be similar to that reported for 12, which is trigonal bi~yramidal.~ Stopped-Flow Spectroscopic Study of the Oxidative Addition of Iodine. The oxidation of organochalcogen compounds 1-5 with iodine in several solvents was monitored via stopped-flow spectroscopy. As was observed with bromination reactions of similar substrates,16 several kinetically distinct steps were observed in the oxidation of 1-5 with iodine. A fast reaction was followed by sequentially slower reactions to give the final products. Unlike the “fast” reactions of bromine with diary1 tellurides, which were too fast for accurate kinetic measurements with our system, the “fast” reactions with iodine were observable. Typical spectral changes for the “fast” reaction are illustrated in Figure 1 for iodine (5 x M) and 3 (5 x M) in carbon tetrachloride between 0.012 and 2.687 s. Kinetic traces of this process for equimolar concentrations of iodine and diorgano telluride follow second-order behavior, as illustrated in the second-order curve fitting of the kinetic trace at 310 nm of Figure 2 for the fast reaction between M) and diphenyl telluride 2 ( 5 x 10-5 iodine ( 5 x M) at 281.8 K.17J8 The “fast” reaction is followed by one or more “slow” reactions. Typical spectral changes for the “slow”reacM) tion are illustrated in Figure 3 for iodine (5 x and 3 (5 x loT5M) in carbon tetrachloride between 25.0 and 1000 s. Kinetic traces for the “slow”process follow first-order behavior, as illustrated in Figure 4 for the M) and telluroslow reaction between iodine (5 x M) a t 288.8 K as monitored a t pyranone 5 (5 x 310 nm. (14) Srivastava, T. N.; Srivastava, R. C.; Singh, M. Indian J.Chem. 1979,17A, 615. (15)Morgan, G . T.; Burstall, F. H. J. Chem. SOC.1931, 180. (16)Detty, M. R.; Friedman, A. E.; McMillan, M. Organometallics 1994,13,3338. (17)The dead time of our stopped-flow instrument has been measured to be 1.2 ms: Tonomura, B.; Nakatani, H.; Ohnishi, M.; Yamaguchi-Ito, J.; Hiromi, K. Anal. Bwchem. 1978, 84, 370-383.
1444 Organometallics, Vol.14,No. 3, 1995 0.900
Detty et al.
t
I
0.800
8 m
C
e 8
0.700
Table 1. Effects of Concentration on the Observed Rate Constants in the “Fast” Reactions of Iodine Oxidations of Diorgano Tellurides and Calculated Third-Order Rate Constants in Carbon Tetrachloride“
0.600 0.500
9
0.400 0.300
0.200
1
I 0.200
0.600
0.400
1.000
0.800
t, s
Figure 2. Kinetic trace obtained at 310 nm for the fast reaction between iodine (5 x M) and diphenyl telluride 2 at 281.8 K with second-order curve fitting.
2 5 x 5 x 10-5 5x 2.5 x
5 x
3 5 x 5 x 5 x
5 x
5 5 x 5 x
5 x
13 5 5 5 5
x x x x
5 x 10-4 3x 5 x
1.5 x 5 x 5 x 5 x 5 x
5x 5 x
292.8 293.2 293.2 292.6
310 310 310 310
(1.05 f 0.06) x 10’ 4.2 x lo9 (1.0 f 0.1) x 103 4.0 x 109 (3.58 f 0.06) x lo2 3.98 x lo9 (7.9 f 0.3) x lo2 3.2 x lo9
297.3 310 (2.6 f 0.1) x 10’ 297.3 310 (2.4 f 0.1) x lo2 297.3 310 (2.6 f 0.3) x lo3
1.0 x lolo 1.1 x loLo 1.0 x 1Olo
293.2 280 (2.5 f0.3) x 10-I 5.0 x lo3 292.8 280 3.1 f 0 . 7 6.2 x 103d 283.7 300 283.7 300 293.0 300 293.0 300
(6.8 f 0.3) x 10’ (5.0 f 0.1) x lo2 ( 1 . 8 f 0 . 6 ) x103‘ (2.5 f 0.5) x
1.4 x 1.0 x 3.6 x 5.0 x
Second-order curve fitting at 5 x M iodine, pseudo-first-order curve fitting at higher iodine concentrations. First-order curve fitting in s-l. Second-order curve fitting. Second-order rate constant in L mo1-I S-1.
260
280
300
320 340
360 380
400
420
440
460
k,nm
Figure 3. Transient spectra for the final slow reaction for oxidative addition of iodine (5 x lov5 M) to di-p-anisyl telluride 3 ( 5 x M) at 298.0 K in carbon tetrachloride. Transient spectra were determined at 25.0,50.0,100, 150, 200, 250, 375, 625, 875, and 1000 s (traces are sequential from the arrow). The spectral window is smaller than in Figure 1 to shorten the acquisition time of the spectral region of interest (20 mid10 nm). 1.220 1.200
1.180 Q,
1.180 Q
g
9m
1.140
observed rate constants a t 5 x M iodine for 2 and 3, which followed pseudo-first-order behavior, suggested that the reaction was second order in iodine. For both 2 and 3, a 10-fold increase in iodine concentration gave a 100-fold increase in observed rate (Table 1). The reactions of di-n-hexyl telluride (1)or diary1telluride 4 with iodine at concentrations rl x M in carbon tetrachloride were too fast for meaningfid measurement. Under pseudo-first-order conditions in iodine (5 x M) a decrease in substrate concentration from 5 x low5M in 2 to 2.5 x M in 2 had little effect on the observed rate constant (hobs, Table 1). This is consistent with a first-order dependence on substrate in the kinetics of the reaction. The “fast” reaction of 5 with iodine followed secondorder kinetics with a first-order dependence in iodine concentration. Second-orderrate constants of 5.0 x lo3 and 6.2 x lo3 L mol-l s-l at 5 x and 5 x M iodine, respectively, were calculated a t 293 K (Table 1). The higher order kinetic behavior with respect to iodine concentration has not been previously described for reactions of diorgano chalcogenides with iodine. For comparison purposes, the reactions of diphenyl selenide, diphenyl sulfide, and dihexyl sulfide (13)with iodine were followed by stopped-flow spectroscopy. Surprisingly, no spectral changes were observed in the reactions
1.120
1.100 1.080
1.060 0
200
400
600
800
1000
t, s Figure 4. Kinetic trace obtained at 310 nm for the slow M) and telluropyranone reaction between iodine ( 5 x 5 (5 x M) at 288.8 K with first-order curve fitting.
The “Fast”Reaction. A. ConcentrationStudies. The effects of concentration on the initial ”fast”reaction are compiled in Table 1. Although the kinetic traces for these systems follow second-order behavior at 5 x lop5M concentrations in both substrate and iodine, the
(18) Concerns have been raised with respect to the measurements of rate constants of 23.5 x lo2 8-l under pseudo-first-order conditions (T 5 2 ms). With a “dead time” of 1.2 ms17 and the assumption that mixing is instantaneous, a value of T of 20.4 ms (k 5 1.7 x l o 3 s-1) and an absorbance change of 1.0 during the course of reaction would leave an absorbance change of at least 0.12 (after 3 half-lives) for the collection of kinetic data. Of the 17 pseudo-first-order observed rates listed in Tables 1 and 2, 15 are less than 1.7 x los s-l, one is comparable in magnitude, and one is 50% larger (k = 2.6 x 103 9-1). Some of the “dead time” of the instrument is undoubtedly consumed by a finite mixing time and, for the reactions studied here, by the time involved in establishing a preequilibrium. Consequently, a pseudofirst-order observed rate of 2.6 x lo3 s-l is not without significance. Similar concerns were expressed with respect to second-order observed rates of 1.4 x lo4 and 1.7 x lo4 s-l contained in Table 2. Although the initial value of the half-life, t,is described by l/kobs[A],where [AI is arbitrarily 1 M in the curve fitting, t is continually increasing as reagent is consumed. After 3 half-lives, T is a factor of 8 larger. Thus, initial half-lives of 70 and 60 ps, respectively, have increased to 0.57 and 0.47 ms after 3 half-lives. Again, an absorbance change of 1.0 during the course of reaction would leave an absorbance change of at least 0.12 for the collection of kinetic data after 3 half-lives.
Oxidative Addition of I to Organotellurium(II) Compounds
Organometallics, Vol. 14, No. 3, 1995 1445
Table 2. Observed Second-Order or Pseudo-Fimt-Order Rate Constants and Calculated Third-Order Rate Constants for the "Fast" Reactions in the Oxidative Addition of Iodine to Diorgano Tellurides 1-6 in CC4, EtOAc, or CH3CNa
1
CC4
5x 5 x
2
3
4
5
CC4 CC4 CC4 CC4 CC14 EtOAc CH3CN
5 x 5 x 5 x 5 x
5 x
CC14 CC4 CCl4 CC4 CC4 CC4 EtOAc CH3CN
5x
CC4 CC4 CC4 CC4 CC4 EtOAc EtOAc EtOAc EtOAc CH3CN
5 x 5 x 5x 5x 5x 5x 5 x
CC4 CC14 CC4 CC4 13 CC4 CC4 (I
5 x 5 x
5 x 5 x 5 x 5 x
5 x 5 x 5 x
5 x 5 x 5 x
5 x 5 x 5 x 5 x
292.8 290 2 2 x 104 297.9 290 (1.4f0.2) x 104 310 310 310 310 310 360 310
(1.79 f 0.06) x 10' (1.43 f 0.02) x 10' (1.06f0.07) x 10' 7.1 f 0.4 6.2f0.3 (1.7 f 0.3) x lo4 2104
282.7 287.3 293.0 297.3 302.2 307.8 312.9 293.1
300 300 300 300 300 300 300 310
(4.5 f 0.2) x (3.8 f 0.1) x (3.1 f 0.2) x (2.6 f 0.1) x (2.16 f 0.07) (1.72 i0.04) 2104 2104
282.6 287.6 292.4 297.5 301.8 286.9 295.6 302.9 312.8 293.1
290 290 290 290 -290 300 300 300 300 310
(10 f 1) x lo3 (8 f 2) x lo3 (6 f 1) x lo3 (4.2 & 0.2) x lo3 (3.0 ?C 0.5) x lo3 (6.2 f 0.1) x lo2 (4.1 f 0.2) x lo2 (3.9 f 0.7) x lo2 (3.12 f 0.03) x lo2 r104
4.0 x lo'* 3.2 x lo1* 2.4 x lo1* 1.7 x lo'* 1.2 x lo'* 2.5 x 10" 1.6 x 10" 1.6 x 10'' 1.25 x 10"
288.8 292.8 298.0 308.5
280 280 280 280
2.3 f 0 . 6 3.1 f O . 7 5.2 f 0 . 2 6.2 f 0 . 7 5
4.6 x 6.2 x 1.0 x 1.2 x
7.18 x lo9 5.72 x lo9 4.24 x lo9 2.8 x 109 2.5 x 109 6.8 x l O I 3
10' 10' 10' 10' x 10'
1.8 x 1Olo 1.5 x 1Olo 1.2 x 1OO ' 1.0 x 1Olo 8.64 x lo9 x loL 6.88 x lo9
283.7 300 (6.8 f 0.3) x 10' 5 x loW5295.3 300 (7.9 f 0.2) x 10'
4 I
"1
5.5 x 1OIz
281.8 288.0 292.8 307.3 311.1 293.0 293.1
5x
30
103" 103" 104" 104"
1.4 x lo6" 1.6 x lo6"
Second-order rate constant in L mol-' s-I.
of either diphenyl selenide or diphenyl sulfide (5 x M) with iodine (5 x M) a t 293.0 K. However, a "fast" reaction was observed for the reaction of iodine with 13 and could be followed at 5 x and 5 x lov4 M combinations of both reagents (Table 1). The rate behavior for the reaction of 13 with iodine was second order overall and first order in each of the reagents. No spectral changes were observed over a time frame of several hours following the initial second-order reaction with iodine. B. Inverted Arrhenius Behavior for the "Fast" Reaction of Diorgano Tellurides with Iodine. Observed rate constants a t different temperatures M in under second-order conditions for 1-5 (5 x both substrate and iodine) are compiled in Table 2. Solvents examined in this study were carbon tetrachloride, ethyl acetate, and acetonitrile. For comparison purposes, the temperature dependence of the reaction of iodine with dihexyl sulfide 13 was also followed. The rate of reaction increased with increasing solvent polarity in 1-3. Rate increases of several orders of magnitude were observed at a common temperature from carbon tetrachloride to ethyl acetate to acetonitrile in these systems. For telluride 4, the observed rate in ethyl acetate was actually somewhat slower than in carbon tetrachloride. However, the rate of reaction in acetonitrile was faster than in either carbon tetrachloride or ethyl acetate.
o ! 3.2
3:3
3.4
3.5
3.6
lOOOrr,K' Figure 5. Inverted Arrhenius behavior for In k as a function of 10OOlT for 2-4 (5 x M in both substrate and iodine): 2, E , = -6.2 f 0.2 kcal mol-'; 3,E, = -6.6 f 0.4kcal mol-l; 4,E, = -9.6 f 0.2 kcal mol-l. These data can be contrasted with the normal Arrhenius behavior observed for 13 (for two points) and for 5 (5 x M in substrate and 5 x M in iodine): 5,E , = 9 f. 2 kcal mol-'. Inverted Arrhenius behavior was observed in carbon M iodine as shown tetrachloride for 2-4 with 5 x in Figure 5 . For dihexyl telluride (l),the rate of reaction was too fast t o measure a t temperatures between 282.5 and 292.8 K. However, at 297.9 K the observed rate of reaction had slowed such that a rate constant could be measured, which suggested that the apparent rate of this reaction was slowing with increasing temperature as well. The initial reactions of iodine with 1-4 in acetonitrile were too fast for meaningful measurements of rate constants. In contrast to these results, the reactions of iodine M iodine and di-n-hexyl sulfide (13, with 5 at 5 x over a small temperature range) show normal Arrhenius behavior (Figure 5, Table 2). In carbon tetrachloride, the rate of reaction in the "fast" reactions is a function of the electronic environment of the tellurium atom. The fastest rates are observed in the two most electron-rich systems: dihexyl telluride (1)and the (dimethy1amino)methyLsubstituted derivative 4, which can form a chelate between the amine and the tellurium atom. The slowest rate is observed for telluropyranone 5, which has the most electron-withdrawing organic ligands of the compounds of this study. Di-p-anisyl telluride (3),which is more electron rich than 2, is faster than diphenyl telluride (2).
Mechanistic Implications of the "Fast"Reaction. On the basis of oxidative-addition reactions of bromine with diorgano chalcogenides,16the initial "fast" reaction between iodine and diorgano telluride is most likely the formation of an +association complex. The secondorder dependency in iodine suggests that the association complex may involve more than one molecule of iodine. The structure of such complexes might resemble that of complexes I11 and W . The initial association of iodine with diorgano telluride might be viewed as the formation of an 7l-R~Te-12 complex (reaction l), which then reacts with a
Detty et al.
1446 Organometallics, Vol. 14, No. 3, 1995 Scheme 2 R2Te
+
kl
I,
e R2TeBitaI--I
(1)
k-1 k2
R~T~,,,,I-I + I2
k.2
i
f.
6’
6-
e R2Te-I~~~~@J (2)
-
f
k.3
second iodine molecule t o give an R2Te-14 complex (reaction 2). In analogy t o structures I11 and IV,the R2Te-14 complex may be an 7’-complex of 14, although an association complex of two 9l-12 ligands cannot be excluded in the absence of structural proof. The R2Te-14 complex presumably goes on to products via the “slow” reaction as shown in reaction 3 (vide infra). If one assumes that reactions 1 and 2 describe the “fast” reaction and that K 3 in reaction 3 is much smaller than the products kl[I2l and k2[123, then one can describe the formation of R2Te-14 species as an equilibrium process. The R2Te-I4 complex reacts to give products as shown in reaction 3 of Scheme 2 on a much longer time scale such that k-3 can be ignored in establishing the initial equilibrium concentrations of R2Te-12 and R2Te-I4 from reactions 1 and 2, respectively. The rate expressions for the “fast” reactions as described by reactions 1 and 2 of Scheme 2 can be combined to generate an overall rate expression for formation of an iodine-diorgano telluride association complex (R2Te-14) as described by eq 4. In this analysis, d[productsl
dt
k1 k2 = K --[R2Tel[1212
3k-, k-,
(4)
the final rate expression in eq 4 shows an apparent second-order dependence in iodine for the process, which is consistent with the 100-foldincrease in observed rates of reaction for 2 and 4 when the iodine concentration is increased by a factor of 10. The ratios of kllk-1 and kd k-2, the equilibrium constants for the two steps, cannot be measured directly in our studies. However, from dissociation studies of iodine-sulfide and iodine-selenide complexes,1° one can approximate the k1/k-1 ratio as lo4 if one assumes that dissociations of R2Te-I2 complexes are similar to dissociations of R2Se-12 complexes.1° Inverted Arrhenius behavior typically is associated with intermediate complex formation (a preequilibrium) prior to the rate-determining transition state.lg For the condition k1 > 122 in reactions 1 and 2 of Scheme 2, a preequilibrium for R2Te-12 could be established for reaction 1. If the proportional decrease in K1 is greater than the increase in k2 with increasing temperature, (19)For illustrative examples: (a) Turro, N. J.; Hrovat, D. A.; Gould, I. A.; Padwa, A.; Dent, W.; Rosenthal, R. J. Angew. Chem., Int. Ed. (b) Maharaj, U.;Winnik, M. A. J . Am. Chem. Soc. Engl. 1983,22,625. 1981,103,2328.(c) Turro, N. J.; Lehr, G. F.; Butcher, J. A.; Moss, R. A.; Guo, W. J . Am. Chem. Soc. 1982,104, 1754. (d) Gorman, A. A.; Gould, I. R.; Hamblett, I. J . Am. Chem. Soc. 1982,104,7098. (e) Could, I. R.; Turro, N. J.;Butcher, J.;Doubleday, C.; Hacker, N. P.; Lehr, G. F.; Moss, R. A,; Cox, D. P.; Guo, W.; Munjal, R. C.; Perez, L. A.; Feydorynski, M. Tetrahedron 1985,41,1587.(0 Nicovich, J. M.; van Dijk, C. A,; Kreutter, K. D.; Wine, P. H. J . Phys. Chem. 1991,95,9890. (g) Chen, Y.;Tschuikow-Roux, E. J . Phys. Chem. 1993,97,3742.
then inverted Arrhenius behavior would be observed in the apparent rate of formation of R2Te-14. The “fast” reaction for the addition of iodine to telluropyranone 5 follows second-order kinetics overall and is first order in iodine. The tellurium atom of telluropyranone 5 is electron deficient relative to the other diorgano tellurides of this study and may follow a different mechanism. An R2Te-I4 species need not be invoked in the chemistry of this substrate, or its rate constant for formation (and equilibrium concentration) is sufficiently small that it is unimportant in the kinetics scheme. One precedent for R2Te-14 complexes is the isolation of sulfur-+iodine complexes Ph3P=S-I4 (111) and ethylenethiourea-14 (IV).12J3 In the tetraiodo-sulfur complexes I11 and IV,the S--I4 array is L-shaped with the linear S-11-12 array and linear 12-13-14 array at nearly right angles. The S-I1 distance in the S-I4 complexes is shorter (roughly 2.50 A in IV) than the S-I1 distances found in hS-12 complexes (roughly 2.72.8 A).13,20 Furthermore, the 11-12 distances in the S-I4 complexes are longer than the 11-12 distances found in the R2S-I2 complexes. The 12-13-14 array, which is not symmetrical, is the same length as symmetrical Is- found in ionic complexes, and the short S-I1 bonds suggest that partial separation of charge to sulfonium and triiodide species has occurred in these covalent complexes.’3 Although R2Te-I4 complexes are not plentiful in the literature,14J5 the larger, more polarizable tellurium atom might be more prone to formation of tetraiodo complexes such as shown in reaction 2 of Scheme 2 than the sulfur atom of similar organosulfur compounds. The formation of this complex from R2Te-I2 complexes plus I 2 or from R2Te plus 1 4 might involve changes in the bonding of Te to I1 from the former or bond formation in the latter to give telluronium character as well as formation of the “triiodide” interactions of 12, 13, and 14. The development of partial positive character at tellurium would be consistent with the observations that Mfast) is accelerated with increased electron donation from the organic groups attached to tellurium and with increased solvent polarity. (On the basis of data described below for the presence of ((slow”reactions and for the addition of excess iodide as well as the dissociation constant of triiodide,21we do not believe that the “fast” reactions are forming a telluronium species and triiodide as the second “fast” reaction.) In ethyl acetate, which is more polar than carbon tetrachloride, the observed rates of reaction are typically accelerated relative to the observed rates in carbon tetrachloride (Table 2). For 4, which could be studied over a fairly wide temperature range, inverted Arrhenius behavior was found in ethyl acetate (Table 2). Unfortunately, the spectral changes associated with the reaction of 5 with iodine in ethyl acetate were not well suited for kinetic analysis. Effects of Added Iodide in Sequential StoppedFlow Experiments. The reaction of iodide with iodine to give triiodide has been studied in several solvents. (20)(a) Chao, G. Y.; McCullough, J. D. Acta Crystullogr. 1960,13, 727.(b) Ahlsen, E.L.; Stremme, K. 0.Acta Chem. Scand., Ser. A 1974, (d) Hartl, 28,175.(c) Wmming, Chr. Acta Chem. Scand. 1960,14,2145. H.; Steidl, S. Z . NatuTforsch., B 1977,32B,6. (21)Turner, D. H.; Flynn, G. W.; Sutin, N.; Beitz, J. V. J . A m . Chem. Soc. 1972,94,1575.
Organometallics, Vol. 14,No. 3, 1995 1447
Oxidative Addition of I to Organotellurium(II) Compounds 1.800
I
1
1.600 1.400
8 c m e
1.200
Table 3. Effects of Concentration on the Observed Rate Constants in the “Slow” Reactions of Iodine Oxidations of Diorgano Tellurides 1-3 and 5 in CCl, R2Te [RzTe], M [I,], M T,K Jobs, nm k o ~ , ( ~ l 0 W ) S, K 1 1
5 x loT5 5 x 5x 5x 5x 1x
292.8 293.0 293.0
290 330 290
(3.4f0.6) x lo-, (3.8 f 0.5) x lo-, (3.6 f 0.3) x lo-*
2
5x 5 x 10-5
5x 5 x 10-4
291.5 292.6
310 310
(3.83 f 0.06) x (3.7 f 0.1) x 10-3
3
5 x 10-5 1x 5 x 5x
5 5 5 5
x 10-4 x x x
281.6 281.6 295.1 297.3
380 380 380 380
(9.5 f o . 3 ) x 10-4 (1.09 f 0.06) x (7.2 f 0.3) x (1.25 f 0.09) x lo-,
5
5 x 10-5 5 x 10-5
5 x 10-5 5 x 10-4
307.5 307.5
280 280
(5.1 f o . 2 ) x 10-3 (4.5 0.7) x 10-3
1,000
v) 0
0.600
L1 U
0.600 0.400 0.200 0.000
250
300
350
400
450
500
h, nm
Figure 6. Spectra generated by sequential stopped-flow spectroscopy. The bottom trace is starting di-p-anisyl telluride (3,5 x M in cc14). The top trace was generated by premixing 250-pL aliquots of 2 x M iodine and 2 x M di-p-anisyl telluride (3)at 298 K followed by injection of 500 pL of 1 x M tetra-nbutylammonium iodide 0.500 s later. Sampling was at 0.5 s. The middle trace was taken 1000 s after the time of the top trace at 298 K. Scheme 3
+ I-
R2TellilI-I
=R,Te
+I~-
(5)
tI The second-order rate constant for the reaction of iodide and iodine has been measured as 6.2 x lo9 L mol-l s-l in water at 298 K, while the first-order rate constant associated with dissociation of triiodide has been measured as 8.5 x lo6 s-l at 298 K.21 The resulting M at 298 K.21 In dissociation constant, K,,is 1.4 x carbon tetrachloride, K, for KIs is only slightly smaller M at 303 K.22 M a t 288 K and 1.5 x a t 1.1 x The dissociation constants associated with triiodide are similar in magnitude to dissociation constants measured for association complexes of diorgano chalcogenides and iodine.1° Added iodide should compete with diorgano tellurides for iodine as shown in Scheme 3. Ideally, added iodide would compete more effectively for iodine from the R2Te-I4 complex than from the R2Te-12 complex, which would shift the equilibrium from the “fast”sequence of reactions and allow observation of the RzTe-12 complex. We were able to add iodide to the reaction sequence following the “fast” reaction of iodine with 3 through the use of sequential stopped-flow techniques. In this M iodine and 2.0 process, 250-pL aliquots of 2.0 x x lop4M 3 in carbon tetrachloride were premixed and M tetra-n-butylammoa 500-pL aliquot of 1.0 x nium iodide in carbon tetrachloride was added 0.500 s later in the stopped-flow spectrophotometer (5 x M each in total RzTe, iodine, and iodide). The spectral results of the sequential stopped-flow experiment are shown in Figure 6 along with the spectrum of starting diary1 telluride 3. Similar behavior was observed with 2 under identical conditions. (22) Watts, H. Aust. J. Chem. 1961, 14, 15. (23) Detty, M. R.; Hassett, J. W.; Murray, B. J.; Reynolds, G. A. Tetrahedron 1985, 41, 4853.
In the presence of added iodide, the spectrum of the product mixture is quite different from that observed M3 in Figure 1 after 0.5 s for the reaction of 5 x and 5 x M iodine. In particular, the absorbances are much larger and the band structure of the complex in the presence of iodide is quite similar to that of the starting telluride 3. The spectral features of the product mixture in the presence of iodide are little changed aRer 1000 s (Figure 6). It is intriguing t o consider the spectrum of the complex of Figure 6 as being primarily due to the ArzTe-12 complex. Interestingly, the spectral features of Figure 6 can be generated by the addition of iodide to 8, the final product from iodine oxidation of 3, which suggests that the entire oxidative-addition sequence is reversible in the presence of added iodide. Furthermore, the “slow” reaction of Figure 3 at 310 nm and the “slow”reaction to 1000 s in the presence of iodide in Figure 6 at 310 nm have identical first-order rate constants (7.2 x s-l at 295.1 K in the absence of iodide, 7.4 x s-l at 295.8 K in the presence of iodide). While the presence of added iodide has shifted the equilibrium concentrations of the various diorganotellurium species, the “slow” rate constant, which is the final process observed in these studies, is the same in both systems. “Slow” Reactions. A. Concentration Studies. A “slow” reaction (or reactions) was observed for 1-5 in carbon tetrachloride. As shown in Table 3, increasing concentrations of iodine have little effect on the observed first-order rate constants for the %low” step, which suggests that the rate expression for the “slow”step is not dependent on iodine concentration at the concentrations of this study. A discrete “slow” step was not always observed in ethyl acetate or acetonitrile. The “slow” reactions are the rate-limiting processes for the generation of the oxidative-addition products 6-10 in the compounds of this study. Furthermore, in the reactions of Scheme 2, k3 in reaction 3 leading to products can be defined as kslow. B. Temperature and Substrate Dependence for the “Slow” Reactions. The “slow”reactions followed normal Arrhenius behavior as shown in the rate data of Table 4 for reactions in carbon tetrachloride, ethyl acetate, and acetonitrile. For 2-5, the observed rates of reaction at 293 K are surprisingly constant, varying by less than a factor of 4. For the “fast” reaction, relative rates for these same substrates varied by lo5lo6. If a common “slow”process is described in all three solvents, then one may assume several characteristics of this process: (1)the reaction is little influenced by
1448 Organometallics, Vol. 14, No. 3, 1995
Detty et al.
Table 4. Observed First-Order Rate Constants for the bbSlow”Reactions in the Oxidative Addition of Iodiie to Diorgano Tellurides 1-5 in CCL,EtOAc, and CHJCN Lobs,
[Iz], M
RzTe solvent
T,K nm
k O b 8 ( d O W ) ,s-’
E,, kcal mol-’
1
CC4
5 x
292.8 290 (3.4f0.6) x
2
CC4 CC4 CC4 EtOAc EtOAc EtOAc EtOAc
5 x
283.6 291.5 300.5 287.9 293.0 301.9 310.6
310 310 310 360 360 360 360
(1.6f0.1) x (3.8 f 0.4) x (8.8 f 0.5) x (4.9 f 0.2) x (7.8 f 0.3) x (1.30 5 0.02) x lo-’ (2.21 f 0.04) x lo-’
17.1 f 0 . 6
282.7 287.3 295.1 305.0 287.9 293.0 301.7 310.6 284.5 293.2 304.6 309.8
380 380 380 380 300 300 300 300 360 360 360 360
(3.9 f 0.3) x (5.3 f 0.1) x (7.2 f 0.3) x (1.1 f 0 . 2 ) x (2.23f 0.03) x 10-1 (3.3 i 0.6) x lo-’ (5.8 f 0.1) x lo-’ 1.11 f 0 . 0 3 1.245 2~ 0.02 1.54f0.03 1.8 f 0.1 2.0 f 0.3
7.7 f 0.2
282.6 293.0 286.9 295.6 302.9 312.8
380 380 370 370 370 370
(1.113 f 0.003) x (1.635 f 0.003) x (1.3 f 0.3) x lo-’ (2.1 f O . l ) x lo-’ (2.7 f 0.4) x lo-’ (4.1 f 0.2) x lo-’
3
4
5
5x 5 x 5 x 5 x 5x 5x
CCL CC4 CC4 CC4 EtOAc EtOAc EtOAc EtOAc CH3CN CH3CN CH3CN CH3CN
5 5 5 5 5 5
CC4 CC4 EtOAc EtOAc EtOAc EtOAc
5 x 5 x 5 x
CC4 CC4 CC4
5x 5x
x x x x
x
x 5x 5 x
5 x 5 x 5x 5 x
5 x 5x 5x
5 x
292.8 280 (2.6 f 0 . 9 ) x 298.0 280 (3.6 f 0.3) x 307.5 280 (4.5 f 0.7) x
Scheme 4 kslow +
11.6 f 0.6
12.6 f 0.4
3.1 f 0.3
7.7 f 0.4
6.6 f 0.1
RzTe-1-1
e R2Te-I
+ I-
(7)
RzTe--I-!
-=R2Te-I+
+ I,
(8)
kSlW
tI
, (9)
the concentration of iodine, (2)increasing solvent polarity facilitates reaction, and (3) the reaction is little influenced by the electronic character of the organo groups attached to tellurium in the substrate. Mechanistic Implications of the “Slow” Reaction. The characteristics of the “slow” reaction are consistent with a first-order, dissociative process to generate ionic intermediates as shown in Scheme 4. Telluronium salt 9 is an example of such intermediates. Dissociation to generate telluronium intermediates could occur from either the R2Te-I2 or R2Te-14 complexes. If dissociation were to occur from the latter species, triiodide would be largely dissociated a t the concentrations of this study. The final oxidative-addition products 6-8 and 10 are formed by iodide addition to the tellurium atom of the telluronium intermediate. In telluronium salt 9, the attack of iodide is blocked by the (dimethy1amino)methyl substituent.
Summary and Conclusions The oxidative addition of iodine to diorgano tellurides is a complex process. An initial “fast” series of reactions
can form either 1:l or 2:l iodine-diorgano telluride complexes. In the case of substrate 5, the “fast”reaction with iodine is second order overall and first order in both substrate and iodine. The other diorgano tellurides of this study (1-4) show a second-order dependence on iodine and presumably form 2:l iodine-diorgano telluride complexes. The formation of the 1:l complex follows normal Arrhenius behavior. Formation of the 2:1 complexes displays inverted Arrhenius behavior, which can be rationalized in terms of a rapid preequilibrium to form the 1:1 complex from diorgano telluride and iodine followed by reaction with a second iodine molecule. The initial association complexes with iodine can dissociate to telluronium intermediates, which generate the final tellurium(N) species by addition of iodide t o the tellurium atom of the telluronium salt. The dissociative process follows first-order rate behavior and is much slower than the initial reactions. The rates of reaction for both the “fast”and “slow”processes increase with increasing solvent polarity, which is consistent with partial charge separation in both processes.
Experimental Section Melting points were determined on a Thomas-Hoover melting point apparatus and are uncorrected. ’H NMR and I3C NMR spectra were recorded on a General Electric QE-300 spectrometer or on a Varian Gemini-200 spectrometer. UVvisible-near-infrared spectra were recorded on a Perkin-Elmer Lambda 9 spectrophotometer. Infrared spectra were recorded on a Beckman IR 4250 instrument. Microanalyses were performed on a Perkin-Elmer 240 C, H, and N analyzer. Dichloromethane, ethyl acetate, acetonitrile, tetrahydrofuran (THF), and carbon tetrachloride were obtained as anhydrous solvents from Aldrich Chemical Co. and were used as received. Diphenyl selenide and diphenyl telluride (2) were obtained from various commercial sources and were distilled prior to use. Diary1 telluride 3 was prepared according to refs 1and 2. Telluropyranone 5 was prepared according t o ref 21. Preparation of Di-n-hexylTelluride (1). Sodium borohydride (1.9 g, 0.050 mol) was added in several portions every 15 min to a refluxing slurry of tellurium powder (2.55 g, 0.0200 mol) in 50 mL of 0.3 M sodium ethoxide in ethanol. After the tellurium was consumed and a chalky white mixture was obtained, hexyl bromide (6.6 g, 0.040 mol) in 20 mL of ethanol was added. The reaction mixture was stirred for 3 h at ambient temperature and was then poured into water. The products were extracted with hexanes (3 x 50 mL). The combined organic extracts were washed with brine, filtered through Celite, dried over magnesium sulfate, and concentrated. The residue was purified via short-path distillation over the range 75-100 “C (0.1 Torr) t o give 5.85 g (98%) of a colorless oil: ‘H NMR (CDC13) 6 2.58 (t, 4 H, J = 7.6 Hz), 1.70 (m, 4 H), 1.26 (m, 12 H),0.85 (t, 6 H, J = 6.7 Hz); I3C NMR (CDCl3) 6 30.89,30.37,29.82,21.17,12.63,1.27;IR(film, NaC1) 2955,2919,2850,1455,1211,1150 cm-l; UV-vis (CHZC12)A, nm (E) 352 (900); FDMS, m l z 300 (C12H26~~~Te). Preparation of Phenyl %((Dimethylamino)methyl)phenyl Telluride (4). n-Butyllithium (10 mL of 2.5 M solution in hexanes) was added dropwise via syringe to dimethylbenzylamine (2.70 g, 0.020 mol) in 50 mL of ether at ambient temperature under a n argon atmosphere. After 5 h, a 0.5 M solution of phenyltellurenyl bromide in THF was added dropwise via syringe. After the addition of 38.5 mL, the color of the reaction mixture darkened with the characteristic orange-red color of the phenyltellurenyl bromide. The reaction mixture was poured into 100 mL of ether. The combined organics were washed with brine (3 x 50 mL), dried over magnesium sulfate, and concentrated. The residual oil was
Organometallics, Vol. 14,No. 3, 1995 1449
Oxidative Addition of I to Organotellurium(II, Compounds mostly the desired product by lH NMR. The residue was purified by chromatography on silica gel eluted with 1:9:90 methanol-ethyl acetate-dichloromethane to give 3.14 g (46.5%) of the amine as an off-white solid, mp 51-54 "C: lH NMR (CDCl3) 6 7.88 (d x d, 2 H, J = 1, 8 Hz) 7.4-6.8 (multiplets, 7 H), 3.51 (s, 2 H), 2.26 ( 8 , 6 H); IR (KBr) 3057, 2976, 2944, 2874,2850,2780,1458,1449,1440,1430,1026,1017,845,747, 731,693 cm-'; UV-vis (CH2C12)A,, nm (E) 283 (10 300), 231 Anal. (12 4001,226 (12 000); FDMS, m l z 341 (C15H1~Nl~~Te). Calcd for C15H17NTe: C, 53.16; H, 5.06; N, 4.13. Found: C, 52.83; H, 4.94; N, 4.03. The phenyltellurenyl bromide solution was prepared by the addition of 4.80 g (30.0 mmol) of bromine to 12.30 g (30.0 mmol) of diphenyl ditelluride in 120 mL of anhydrous THF. The resulting solution was stirred at ambient temperature for 0.5 h prior to use and was stored under an inert atmosphere in the dark at ambient temperature.
General Procedure for the Oxidative Addition of Iodine on a Preparative Scale. Tellurium-containing substrate (2.0 mmol) was dissolved in 10 mL of acetone. Iodine (0.51 g, 2.0 mmol) was added, and the resulting solution was stirred at ambient temperature for 0.5 h and was then chilled. The crystalline product was collected by filtration. The crystals were washed with small portions of cold acetone and dried. For 6:98%, red oil; lH NMR (CDC13) 6 3.60 (t, 4 H, J = 7.9 Hz), 2.14 (m, 4 H), 1.45 (m, 4 H), 1.37 (m, 8 H), 0.91 (t, 6 H, J = 6.9 Hz); IR (film, NaC1) 2950,2918,2848,1450,1400,1215, 1160,705 cm-'; UV-vis (CH2C12) I,,, nm (E) 355 (2100). Anal. Calcd for ClzH26IzTe: C, 26.12; H, 4.75. Found: C, 26.53; H, 5.05. For 7: 95%, mp 232-235 "C (lit.3mp 237-238 "C); lH NMR (CDC13) 6 8.14 (m, 4 H), 7.58 (m, 2 H), 7.45 (m, 4 H); IR (KBr) 3053,3047,1572,1472,1431,1326,1155,1050,993,910,833, 729, 680, 457, 444 cm-$ UV-vis (CH2C12) A,, nm (E) 372 (13 loo), 286 (26 400). Anal. Calcd for C12HloI2Te: C, 26.91; H, 1.88. Found: C, 27.00; H, 1.95. For 8: 73%, mp 167.5-169 "C (lit.2 mp 166-167 "C); 'H NMR (CDC13) 6 8.03 (AA'BB, 4 H, J("doub1et") = 9 Hz), 6.95
(AA'BB', 4 H), 3.87 (s, 6 H); IR (KBr) 1581, 1569, 1489, 1296, 1256,1177,1053,1021,818,511 cm-l; UV-vis (CHzClz)Am,, nm ( E ) 380 (sh, 13 OOO), 340 (20 1001, 288 (26 800). Anal. Calcd for C14H141202Te: C, 28.23; H, 2.37. Found: C, 28.15; H, 2.35. For 9: 73%, mp 178-179 "C; IR (KBr) 2859, 2830, 2785, 1715 (acetone of crystallization), 1479,1461,1452,1438,1431, 1425,1159,1053,1023,996,972,843,823,756,730,688,453, 432,423 cm-l; UV-vis (CH2ClZ) I,,, nm ( E ) 381 (12 3001,340 (10 000), 279 (31 500). Anal. Calcd for C15H17IzNTe'/2-acetone: C, 31.87; H, 3.22; N, 2.25. Found: C, 31.99; H, 3.27; N, 2.18. For 10 46%, mp 114-130 "C dec; IH NMR (CDC13) 6 6.95 (s, 2 H), 1.35 (s, 18 H); IR (KBr)1970, 1625, 1585, 1280, 1190, 1155 cm-l. Anal. Calcd for C13H20120Te: C, 27.2; H, 3.5; Te, 22.2. Found: C, 27.0; H, 3.5; Te, 22.5. Stopped-Flow Experiments. All stopped-flow experiments were performed on a Sequential DX17 M V stopped-flow spectrometer (Applied Photophysics, Leatherhead, U.K.). All experiments incorporated the instrument in stopped-flow mode only. The sample handling unit was fitted with two drive syringes that are mounted inside a thermostated-bath compartment, which allowed for variable-temperature experimentation. The optical-detectioncell was set up in the 10-mm path length. First- and second-order curve fitting and rate constants used a Marquardt algorithmz4 based on the routine CurfkZ5 Absorption spectra at indicated time points were calculated through software provided by Applied Photophysics. This consisted of slicing the appropriate time points across a series of kinetic traces (at different wavelengths) and then splining the points of a specific time group. Stock solutions of substrates and iodine at appropriate concentrations described in the text were utilized in the stopped-flow experiments. OM940846+ (24) Marquardt, D. W. J. SOC.Ind. Appl. Math. l S s 3 , 1 1 , 431. (251 Curfit is found in: Bevington, P. R. Data Reduction and Error
Analysis for the Physical Sciences; McGraw-Hill: New York, 1969.
