2804
HARTLEY C. ECICSTROM, DJONG-GIE OBI, AND LYLER. DAIVSON
Vol. 67
THF, BEHAVIOR OF OCTAHEDRAL COBALT COMPLEXES I N MIXTURES OF N-METHYLACETAhn/IIDE AND N,N-DIMETHYLACETAMIDE1i2 B Y E4RTLEY
c. ECKSTROM, DJONG-GIE OEI, A S D LYLER. DAWSON
Department of Chemistry, University of Kentucky, Lexington, Kentucky Received July 23, 1963 Spectrograms of solutions of cis- and trans- [ Co(en),( NO,)*]NO3 in N-methylacetamide (NM-4) and K,Kdimethylacetamide (DMA) did not change with time. Spectrograms of solutions of cis- and trans-[Co(en)&lz]Cl and [C~(en)~Br,]Br in DMA remained unchanged with time, but changed when KMA was a constituent of the solvent mixture. First-order reaction rate constants have been obtained for the reaction of the trans- [ Co(en)*ClzlC1 and of the trans-[C~(en)~Br~]Br in the solvent mixtures. The kinetics are consistent with the SNImechanism as the rate-determining step for the reaction [Co(en)&]X SLIA +.[Co(en)~X.ShfA] T2 2X-. The Arrhenius activation energy for this solvation reaction in N X 4 had a value of 25 kcal./mole for the chloro complex and 27 kcal./mole for the bromo complex.
+
During the past decade maiiy studies3 have been made in this Laboratory of the dielectric and conductometric characteristics of electrolytes in amide-type solvents of high dielectric constants. Recently, these studies have been extended to electromotive force measurements4 and reaction kinetics studies.6 It was the purpose of this work to study the behavior of cobalt complexes of the type [Co(en),Y,]+ in the solvents N-dimethy lacetamide S-methylacetamide (NMA) , S, (DMA), and mixtures of NhfA and DniIA and to obtain reaction kinetic data for the reaction of these cobalt complexes with the solvent. Experimental A. Preparation and Purification of Chemicals.-NMA and DMA were prepared and purified by methods developed in this Laboratory.' The NMA and DMB used in these studies had specific conductances at 40" of approximately 1 X 10-7 ohm-'
+
The cis isomers were not resolved into their optical isomers for this study. B. Apparatus and Procedures.-Densities were determined using conventional Reischauer specific gravity bottles. Dielectric constant measurements were made with a General Radio Type 821-A Twin-T impedance measuring circuit, with a General Radio Type 1001-A standard signal generator and a Hallicrafters Model S - 4 0 4 multiband receiver as a null detector. Cell calibrations and procedures followed were the same as described by Leader and Gormley.'* Conductance measurements of the solvent were made with a Jones bridge. Temperature control was maintained to =!zO.0lo b y constant temperature baths. The product of the reaction was determined by conductometric titration technique using a dip cell connected to a Serfass conductivity bridge hlodel RC Ml5, using silver nitmte solution in NMA which was 2.668 X fif and in DMB which was 5.406 X -11. Known amounts of the trans-chloro and trans-bromo compounds were dissolved in XMA and of the trans-bromo compound in DMA at least 48 hr. before titrating. Rate data were obtained by following the change in absorption of the solution of the coordination compounds in the different solvents using a Beckman DU or a Beckman DK spectrophotometer. The stop watch used to measure the time during the reaction was standardized through the NBS signals that are broadcast a t 10 Mc. The rate constants were calculated by the usual procedure13 except in some cases the method proposed by KingI4 was employed.
cm.-l and 2 X lo+ ohm-' cm.-', respectively. cis- and trans-dichlorobis( ethylenediamine)cobalt( 111) chloride were prepared according to Bailar's procedure.8 The crude trans[C~(en)~Clt]Cl was purified by washing with alcohol and ether before drying and expelling HC1 a t 110". The violet cis form was obtained by dissolving the green truns-[Co(en)&l~]Clin a minimum volume of water and evaporating the solution to dryness. Results trans-Dibromobis(ethylenediamine)cobaIt(III) bromide was Data for the solvents used in this study are summarprepared according to the method of Werner and c o - ~ o s k e r s . ~ ized in Tables I and 11. The results of spectrophotoThe crude green solid was powdered, dried a t 110', and washed metric studies are summarized in Table 111. with water, alcohol, and ether. To prepare the cis form the method recommended by Werners was modified as follows to TABLE I obtain a product free of the green trans isomer. The trans form SELECTED PHYSICAL PROPERTIES OF THE P C R E SOLVENlS was dissolved in a miaimurn amount of water, and instead of evaporating the solution to dryness, the precipitate formed N-hIethylN,N-Dimethylacetamide acetamide during partial evaporation was filtered from the hot solution. The dark grayish violet cis isomer was free from the green trans 58" at 14 mm. Boiling point ... form. 50" at 10 mm. cis- and trans-dinitrobis(ethylenediamine)cobalt(III) nitrate Density (g./ml.) 0 9502 (30') 0 9320 (30') were prepared using the procedure reported by Holtzclaw and 0 9231 (40") 0 9417 (40') co-workers.lO The potassium hexanitrocobaltate(II1) used in the Dielectric constant 175 3 (31") preparation of the cis isomer was prepared according to the 36 7 (35') 169 9 (38") method of Ray." 35 7 (40') 165 6 (40') Specific conductance 1 02 x 10-7 1 99 x 10-7 (1) Taken from a P1i.D. dissertation submitted by Djong-Gie Oei. (2) This work was supported in part by the U. S. Atomic Energy Com(400) (400) mission under Contract No. AT-(40-1)-2451. (3) L. R. Dawson, E. D. Wilhoit, R. R. Holmes, and P. G. Sears, J . A m . Chem. Soe., 79, 3004 (1957); L. R. Dawson and C. Berger, ibid., 79, 4269 (1957): W. D. Williams, J. A. Ellard, and L. R. Dawson, ibid., 79, 4652 (1957). (4) L. R. Uawsuii, K. c'. Sheridan, aiid IC. C. Ihkstroiii, J . I'hys. Cheni., 65, 1829 (1961). ( 5 ) L. R. Dawson, J. E. Berger, and H. C. Eckstrorn, i b i d . , 65, 986 ( l Y 6 1 ) . ( 6 ) Y is NOz, C1, or Br; en is ethylenediamine. (7) L. R. Dawson, P. G. Sears, and R. H. Graves, J . Am. Chem. Soe., 77, 1986 (1955); L. R. Dawson, J. E. Berger, and H, C , Eckstrom, J . I'hys. Chem., 6 5 , 986 (1961). (8) J. C. Bailar, Jr., Inuw. Syn., 2, 222 (1946). (8) A. Wcrner, et al., ,4nn., 386, 110 (1912). (10) H.F. Holtzclaw, Jr., et al., I n o r g . Syn., 2, 176 (1946).
Rate data gave typical first-order plots from which the reaction rate constants were computed. Averaged values for tlic rcuxtioii ratc coiistaiits arc given in Tables 1V a d V. 'l'lie values ol the Arrlieiiius activatioii eiiergy were coiiiputed fro111 the sets of data for the three temperatures as given in the tables. (11) P. C. Ray, J Chem. S o r , 89, 551 (1906). (12) G. R. Leadei and J. f.Gormiey, J. Bm. Chem. Soc., 73, 3731 (1951) (13) -4.A. Fiost and R G. Pearson, "Klnetms arid 1\Ieclianlsrrl," Jollri \\iley and Sons, Inc., New York, N. Y., 1958, P. 29. (14) E. L. King, J . Am. Chem. Soc., 74, 563 (1952).
Dec., 1963;
BEHAVIOR OF COBALT COMPLEXES IN AMIDEMIXTURES TABLEI1
DIELECTRIC CONSTANTS O F DMA AND hhXTURES O F hTMA DhlA
".."
y y l .
DMA
AND
1
I1
I11
IV 79
Weight 74 NMA
0
9
34
54
25 O 30 35" 40
38.8 37.8 36.7 35.7
..
..
..
45.3 44.0 43.1
63.1 61.3 60.1
85.6 82.7 80.8
123.0 119.0 117.2
TABLE 111 SPECTROPHOTOMETRIC DAT'A Maximum ---Ab8orbance------Compound
Solvent
cis- and trans-Nitro
Methanol Ethanol
NMA DMA trans-Chloro
Methanol Ethanol NM.4
DMA cis-Chloro
Methanol Ethanol
Nhld DMA trans-Bromo
Methanol Ethanol
NMA DMA cis-Bromo
Methanol Ethanol
NMA DM-4
----Wave 432 432 432 432 610 610 610 619;670 540 550 540 535 650 660 645 645; 670 555 555 555 555
NaxiMiniMinimum mum mum (after 24 hr. or more) length, mp7 405 405 405 405 520 520 520 615 700;4555 450 700;440 610;465 760;550 750;520
580 760; 655 720;495 495 710;490 720;490
432 432 432 432
405 405 405 405
...
...
...
...
520 710;440 610;670 615
... ...
..,
520 535
700;450 710;455
... ...
...
...
...
710;460 525 645; 670 760; 655
. .. ,..
... ...
525 555
700;455 720;490
TABLE IV FOR t~ans-[Co(en)~Cl~] C1 IN DIFFERENT RATECONSTAKW SOLVENTS Solvent
KMA YMA NMA 1V I11 I1 I Averogc ent runs.
Temp., OC.
Average kp, min. -1'
i
EA
45.1 (3.1 i 0 . 1 ) X 24.5 kcal, 39.5 (1.9 i 0 . 2 ) 35.6 ( 1 . 2 f 0.1) J 43.8 (2.39 f 0.03) 43.8 (2.5 =!= 0.3) 43.8 ( 1 . 7 f 0.1) 43.8 (1.3 f 0 . 2 ) rate constant from the weighted iiieon of tho differ-
TABLE V RATECONSTANT'S FOR Ir~ns-[Co(en)~Br~]Br IS DIFFERENT SOLVENTS Temp., solvent
oc.
XMA NMA NMA
39.5 35.5 31.5 38.5 35.5 35.5 31.5 39.0
IV
IC.!,
Average k,, min. -1"
(3.0 f 0.3) X 10-8) (1.65 zk 0.05) (0.99 f 0.09) (2.2 i 0.2) (1 .R8 0.05) (1.26 i 0.06) (0.76 f.0.03) ( 2 . 4 =k 0 . 2 )
/
27.3 kcal.
1
I11 I11 28.1 Ircn,l. I11 I1 I 39.0 ( 1 . 2 d~ 0 . 1 ) ' Average rate constant from the weighted mean of the different runs.
Discussion The spectrograms of solutions of cis- and trans[Co(en)z(NO&]NOI in NMA and 'DMA did not change wit'h time. The spectrograms of solutions of cis- and kans- [Co(en)~Cl2 IC1 and [Co(en)2Brz]Br remained unchanged also in DMA, but in IiMA and in
2805
mixtures of NMA and DMA there appeared definite changes in the spectrograms which gave evidence of the formation of the same product for the cis and trans isomers in each case. Positions of the maxima and minima for the different cases are given in Table 111; similar results for methanol and ethanol solutions are included for comparison. The initially bright green NRlA solution of trans[Co(en)&lS]Cl became pink upon standing, as did the pale purple solution of the cis isomer. The spectrograms of the two pink solutions seemed to be identical. The initially pale green N-CIA solution of the trans- [Co(enz)Brp]Br became pink upon standing, as did the initially gray-purple NAIA solution of the cis isomer, the spectrograms of the two pink solutions being almost identical. The rates of change of the two cis forms are very slow while those for the trans fornis are such that they lend themselves readily to kinetic studies. Solvation reaction studies of octahedral complexes have been limited generally to the reactions in water and in methanol, in which cases they followed firstorder kinetics. Solvation of complexes of the type [Co(en),X,]+ was found to proceed largely by a Ssl path where the rate-determining step is the release of one ligand from the octahedral complex to form a trigonal bypyrainid intermediate.I5 The rate data for the trans-chloro and trans-bromo coniplexes obtained from the spectrograms followed firstorder kinetics and the rate constants obtained by those computations are summarized in Tables IV and V. Several factors appear to influence the rate of solvation. The rate of solvation in methanol is slower than in mater, and Stranksi6 suggested that this was due presumably to the lower dielectric constant of methanol which renders charge separation less efficient. For comparison, values of the rate constants for solvation have been summarized in Table VI where it can be seen that the rate constant for aquation at 25' for trans-[Co(en)2C12]C1 is the same as that for solvation in NMA a t 39.5'. This is unexpected, since S M A has a much higher dielectric constant, and the reaction is taking place a t a higher temperature also. Extrapolation of the values for the rate constants for solvation of trans-[C~(eii)~Br~]Br in SAL4 gave 3 X niin.-* for the rate constant a t 25'. This is less than the corresponding rate constant for the aquation of this coinpound. One is forced to conclude from this that the relation between the rate constant and the dielectric constant is not a simple one. A study of the data given in Tables 11, I V , and V shows that the dielectric constant is a major factor in determining the rate constant, but no definite pattern is apparent from these studies. It is hoped that more information with these solvents will aid in determining the relation hetwccn the rate constant and the dielectric constant. Attempts to isolate the product of the reaction were unsuccessful so that the identification of the product was made by indirect arguments from conductometric titration data and from the spectrograms. The facts obtained from the spectrograms showed that DRTk was inert as a solvent so that in the mixed solvents the reaction with D N A can be excluded. The possibility (15) R 0. Pearson and F. Basolo, J d m Chem Soc , 78, 4878 (1956). (16) D. R. Stranks, in J Leais and I3 G l\ilkins, ' Modern Coordination Chemistry." Interscience Publishers, Inc , New York, N. Y., 1960, p 125.
2806
HANBo YUNAND HERBERTC. MOSER
of cis to trans isomerization in S M A can also be excluded, since it was found that the cis form dissolved in K-MA showed a spectrogram which changed with time, and furthermore the spectrograms of both the cis and trans forms of the chloro compound were almost identical after 24 hr. or more. The bromo compound in KMA behaved in a similar manner. Conductometric titrations of the chloro compound in NMA, after standing for 24 hr. or more a t 35O, with a AgNO, solution in NhL4 as a titrant revealed the presence of two equivalents of halide for every mole of the complex. Since only one equivalence point was observed during the titration, showing the availability of two chlorine atoms per molecule of trans- [Co(en),ClzlC1, one may conclude that one coordinated chlorine has been replaced by a molecule of NhIA. The titration of trans-[C~(en)~Br~ ]Br in NMA was performed in a similar manner. However, in this case two equivalence points were observed; the first corresponded to two bromine atoms per molecule of complex, while the second poiht indicated the presence of more than two bromine atoms per molecule. When the trans-bromo complex was titrated in DMA (in which no reaction takes place) two equivalence points were obtained; the first corresponded with the availability of one equivalent of bromine per mole of complex, and the second equivalence point indicated that another reaction is occurring in which a bromine atom from the coordination sphere is released. It appears logical that this second equivalence point is due to an incomplete replacement of the coordinated bromine atom by a nitrate ion. I n view of these results in DllIA the second equivalence point obtained in XMA may be interpreted in a similar way.
Vol. 67 TABLE VI
REACTIOX RATECONSTANTS FOR tran~-[Co(en)~X~]X IN WATER, AND NMA METHANOL, Soltent
Temp., OC.
truna-[Co(en)zClg]Cl trans-[Co(en)zBr~]Br ICr, min. -1 IC , inin.-l
Water‘ 25 1 9 x 10-3 8 . 4 x 10-3 Methanol’ 25 3 . 2 x 10-4 . . SbfA 39.5 1 9 x 10-3 30 X SRIA 35.5 1 2 x 10-8 17 x 10-3 a R. G. Pearson, R. E. Sleeker, and F. Rasolo, J . Am. Chenz. Sac., 78, 2673 (1956); F. Bmolo, R. W. JIatoush, and R. (4. Pearson, ibid., 78, 4883 (1965). F. Basolo and R. G. Pearson, “Mechanism of Inorganic Reactions,” John Wiley and Sons, Inc., Kew York, N. Y., 1958, p. 147.
It has been coiicluded from these observations that the following reaction is taking place in KMA and in mixtures of M I A and DMA
where X represents C1 or Br. Since the reaction obeys first-order kinetics, it has been assumed that 6he mechanism is Sxl. This implies that a mixture of the cis and trans products is formed; however, one of the isomers will be predominant. Based on the studies of Brasted and Hirayama17 of the cis-trans isomerization in alcohol solutions, it could be argued that the mixtures contain predominantly the trans isomers. Since the rate constants were determined by using differences in absorbance, the nature of the products is not of major importance. (17) R. C. Brasted and C . Hirayama,
J. Am. Chem. Soc., 80, 788 (1958).
REACTIONS OF ENERGETIC HYDROGEN ATOMS WITH SOLID FILMS OF PROPEKE AT 77OK.l BY HART B O Y U N AND HERBERT c. nIOSER Department of Chemistry, Kansas State University, Manhattan, Kansas Received J d y 24, 196s Tritium was used as a tracer to study reactions of hydrogen atoms with solid films of propene at 77°K. A tungsten filament centrally located in a spherical flask tyas used to atomize hydrogen gas a t pressures of 0.5 to 5 p. Products of reactions were separated by gas chromatography. A fraction of the hydrogen atoms reacted before reaching thermal equilibrium with the film. Products of combination reactions of three different Ca radicals (sec-propyl, n-propyl, and allyl) were obtained. sec-Propyl radicals were most abundant, and when helium gas was used to moderate the hydrogen atoms only sec-propyl radicals were produced. Fast, surface reactions of free radicals were indicated.
Introduction Hydrogen exchange occurred when solid hydrocarbons a t 77OK. were bombarded with tritium atoms thermally generated in the gaseous state a t lorn pressure. 2 , 3 The sizeable energy requirements for these reactions must be provided by the bombarding atoms. Therefore, reactions must occur before the atoms reach thermal equilibrium with the solid. We have been interested in the energy effects of hydrogen atom reactions with solid propene. (1) Presented in part a t the 145th National Meeting of the -4merican Chemical Society, New York, N. Y., Sept., 1963. Work performed under Contract AT(ll-1)584 with the U. S. Atomic Energy Commission. (2) R. D. Shores and H. C. Moser, J . Phys. Chem., 66, 570 (1961). ( 3 ) H. C. hfoser and R. D. Shores, zbzd., 66, 2272 (1962).
Hydrogen atoms can react with propene in a t least three different ways: hydrogen atom addition to form sec-propyl or n-propyl radicals,4 or hydrogen atom abstraction to form allyl radicak6 Previously hydrogen atoms have been observed to react with solid propene 77OK. by addition to form sec-propyl radicals only.6 Experimental The essential features of the reaction apparatus and techniques have been described previou~ly.~~3 Three sizes of reaction flasks (4) J. N. Bradley, H. W. Mel>-ille,and J . C. Robb, Proc. Roy. Boc. (London), AFS6, 339 (1956). ( 5 ) P. J. Boddy and J. C. Robb, ibid., A249, 518 (1959). (6) R. Klein and M. D. Scheer, J. Phys. Chem., 66, 324 (1961).
