T . E. ROGERS, J. H. SWINEHART, AND H. TAUBE
134
The Exchange of Methanol between Solvated Cations and Solvent. I1
by Terrence E. Rogers, James H. Swinehart, and Henry Taube’ George Herbert Jones Laboratory of the University of Chicago, Chicago 57, Illinoie, and the Department of Chemistry, Stanford University, Stanford, California (Received J u n e 85, 1964)
The exchange of methanol between solvated C O + or ~ Ni+2 and methanol as solvent was studied by the isotopic dilution method. The enthalpy of activation, A H * , for the exchange process is 11.5 f 0.7 kcal./mole for C O + and ~ 13.1 f 0.7 kcal./mole for S i f 2 . Qualitative observations have been made on methanol-water solvation equilibria for Co f 2 solutions in CH30H containing some HzO. The “kinetic solvation number” is shown to be 6. Preliminary data for the PrC13-methanol system indicate that solvation of Pr+3 in CH30H can be studied by the isotopic dilution method. I n this case it appears that HzO in small amounts decreases the lability of the solvation methanol. Analysis of the isotopic dilution data for the perchlorate salts of C O + ~ , or Fe+3shows that in certain cases the holdback of methanol for each mole of solute may exceed that ascribable to first coordination sphere interactions.
Introduction
Nevertheless, comparison of the activation energies measured by this direct method with those obtained by This paper describes a study of the exchange of methods capable of responding to rapid exchange rates methanol between solvated C O + ~Ni+2, , or Fe+3 and is of interest. In principle, too, the isotopic dilution the solvent. Some preliminary data on the Pr+3method can yield important information about the methanol system are included. Since the preparation composition of the solvated c ~ n i p l e x . ~ -Its ~ full poof the first paper in this series,2 the nuclear magnetic tential in this respect has not yet been realized for resonance line broadening method for determining the methanol solutions and awaits refinement of the exrate of solvent exchange has been developed so that it perimental method. is now possible by studying the line width as a function of temperature to determine which of two mechanisms Experimental causcs the broadening of the n.m.r. line. Luz and RleiA detailed description of the procedure, including boom3 have used the proton n.ni.r. of mixed methanolmethods of analysis, preparation of solutions and rewater solutions of C O +and ~ Ni+2 to determine the rate agents, and evaluation of errors, is found in a previous of methanol exchange for various mixed complexes of paPerez However, the preparations of the solutions of these ions and have determined the solvation number the particular salts studied in the present work are deof C O + ~ .Recently, a complete study of the rate of scribed. water exchange between the solvated ;\In+2, Fef2, Cof2, Ni+*,and Cuf2, and the solvent water a ~ p e a r e d . ~ (1) Department of Chemistry, Stanford University, Stanford, Calif. All reprint requests should be addressed to this author. Eigen and his co-workers5 have recently summarized (2) J. H. Swinehart, T. E. Rogers, and H. Taube, J . Chem. Phys., the results of their studies on the rates of solvent re38, 398 (1963). placement on many cations in aqueous solution; these (3) Z. Luz and S.Meiboom, ibid., 40, 1058, 1066, 2686 (1964). rates are very close to those measured by the n.1ii.r. (4) T. J. Swift and 11. E. Connick, ibid., 37, 307 (1962). niethod. (5) > I . Eigen and L. de Maeyer in “Technique of Organic Chemistry.” A. U’eissberger, Ed., Vol. VIII, P a r t 11, Interscience PublishDirect coinparison of the rates obtained by the n.1n.r. ers, Inc., New York, N. Y . , 1961, Chapter XVIII. or by relaxation techniques with those we have deter(6) H. W.Crandall, J . Chem. P h y s . , 17, 602 (1949). mined is not possible, because the isotopic dilution tech(7) J. 1’. Hunt and H. Taube, ibid.. 18, 757 (1950). nique for the labile ions is applicable only a t low tem(8) J. 1’. Hunt and H. Taube, ibid., 19, 602 (1951). peratures where the rates of exchange are quite slow. (9) H. W.Baldwin and H. Taube, ibid., 33, 206 (1960). The Journal of Phyeical Chemistry
EXCHANGE O F AIETHANOL
BETWEEK
SOLVATED CATIONS AND SOLVENTS
CoClz and Co(C104)z Solutions. Anhydrous CoClz was prepared from reagent grade hydrated CoClz by heating the salt at 135' for several days. Anhydrous methanol was added to the anhydrous salt. A small amount of anhydrous HC1 gas was added to repress acid dissociation. I n several runs this stock CoClz solution was used. Cobalt perchlorate solutions were prepared by adding the necessary amount of anhydrous AgC104 to give a slight excess of silver ion. In both the CoC12 and Co(C104)2 solutions the cobaltous ion was deterniined as [ C O ( C ~ H ~ K ) ~ ] ( C XThe S ) ~precision .~~ of the analysis was =t2%. The acidity was determined to be 0.001 m. The water content could not be determined by the Karl Fischer reagent, but on the basis of the experience with other solutions, even for the perchlorates it is less than 2 moles of water/niole of cation. NiClz and Ni(C104)Z Solutions. The nickel solutions were prepared in the same way as the cobaltous solutions. Nickel was determined as the dimethylglyoxime by the standard procedure. The mater content of the solutions is less than 2 nioles of water/niole of cation. FeC13 and Fe(C104)3Solutions. Dry chlorine gas was passed through a heated Pyrex tube containing coils of high-purity iron wire. Ferric chloride condensed on the cooler parts of the tube." The FeC13 was collected and stored in sealed Pyrex ampoules. Anhydrous methanol, freshly prepared by distillation from niagnesiuni, was used to prepare solutions of the salt. A small amount of anhydrous HCI was bubbled into the solution to prevent acid dissociation of the solvated ferric ion. A ferric perchlorate solution was prepared by adding the necessary amount of anhydrous -4gC104 to give a slight excess of silver ion. The following analytical procedures were used for both FeC13 and Fe(C104)s-n~ethanolsolutions. The ferrous content was estimated by direct titration to a ferroin end point using ceric ammonium sulfate solution which had itself been standardized against pure iron wire. Total iron was determined by reducing weighed aliquots of solution on a Jones reductor and then titrating the solution with Ctl(1V). The acidity of the solution was estimated by direct titration of a methanol solution diluted with water with standard base to pH 1 using cresol red as an indicator. The water content was estiniated to be less than 2 moles of waterlmole of cation. PrC13 Solution. Hydrated PrC13 was heated a t 180' for 5 hr. in a stream of HCl. Freshly distilled inethanol was added. Water as determined by the Karl Fischer reagent was found to be present to the extent of 1 mole/ mole of salt in a 1 m Praseodymium chloride was analyzed by evaporating the solution and then heating the residue a t 230-250O for several hours
135
in the absence of air to minimize the decomposition of PrC13to PrOCl. The residue was assunied to be PrC13.
Results and Discussion Definitions of Symbols. The symbols used to describe the results are the sanie as those of the previous paper.2 For convenience, they are repeated here. R , represents the mass spectrometer value of [mass mass 33)] for the sample of solvent 34/(mass 32 methanol taken a t time t. R , represents the mass spectrometer value of [mass 34/(niass 32 mass 33)] for the sample of solvent niethanol taken at infinite time. Re and R, represent the mass spectrometer values of mass 33)] for oxygen-18 enriched [mass 34/(niass 32 methanol and methanol of normal isotopic composition. R,' represents [mass 34/(niass 32 mass 33)] as calculated using b', R,, and Re for the solvent methanol at infinite time assuming random wixing. Ro represents the extrapolated mass spectrometer mass 33)] value a t zero time of [mass 34/(mass 32 for the solvent methanol. Ro is determined from a plot of ( R , - R,) US. t. a represents the ratio of the number of moles of salt to the number of moles of methanol of nornial isotopic coiiiposition in the solution before mixing with the oxygen-18 enriched methanol. b represents the ratio of the number of moles of oxygen-18 enriched methanol to the number of moles of methanol of normal isotopic composition as calculated from R,, Re,and Rn. b' is b , but calculated from the weights of the component solutions in the mixture, a,R,, and R,. q represents the "kinetic solvation number" calculated froin Ro, a, b, Re,and R,. Cobaltous Solutions. Table I suniniarizes the isotopic exchange results for CoClz-niet hanol solutions a t -82.5'. The data indicate that the exchange is complete by the time of sampling, 0.5 min. If the chloride anion is replaced by perchlorate, the rate of exchange of methanol between the solvated cation and the solvent is observable. Thus the cobaltous species present in solution when chloride is the anion exchanges methanol more rapidly than the conipletely niethanolated cobaltous species. The work of Katzin's on
+
+
+
+
+
(10) A . I. Vonel. "Textbook of Cluantitative Inormnic Analvsis. Theory and Pr&ice," Longmans, &reen and ( ' 0 , Inc., New York. N. Y . . 1951, p . 463. (11) B. R. Tarr, Inorg. S y n . , 3 , 191 (1957). (12) J. Mitchell, Jr., and D. 11.Smith, "Aquametry, Application of Karl Fischer Reagent to Quantit,ative Analyses Involving Water," Interscience Publishers, Inc., New York, iY Y.. 1948, 11. 231. for the analysis of water in inorganic d t s . (13) L. I. Katzin, J . Chem. P h y s . , 3 5 , 467 (1961).
Volume 69, ivztmber 1
J a n u a r y 1965
136
T. E. ROGERS, J. H. SWINEHART, ATD H. T A ~ J B E
(iiij The addition of excess water to the oxygen-18 enriched methanol before mixing with the Co(C1O4j2 solution has no effect on the solvation number of isotopic exchange rate (run 1-1). It should be noted that Run t, no. 5 R, R-0 Rt min. when the experiment is done in this may, the excess water is not introduced into Cof? solution until the 0 8 1 0 2.521 0 02000 0 04644 0 04655 0 04565 2 8 solutions are cooled to the temperature of the experi0 04570 4 5 nien t s. 0 04570 10 0 The observations cited indicate that there is a slow 0 5 2 0 2591 0 02060 0 04544 0 04705 re-equilibration of water in the solution at -97” 0 04705 1 8 and that as tinie progresses a water-containing Co(I1) 0 04700 3 0 0 04675 10 0 species which exchanges methanol rapidly transforms to a Co(I1) species containing less water and evchanging a u = 0.03988, Re = 0.14765. methanol slowly. The labilization of methanol by water in mixed methanol-water complexes of Co +* has also been observed by Luz and lIeiboom.3 the absorption spectrum of the CoClz-methanol The presence initially of a water-containing species system indicates that an octahedral form of Co+* conwhich exchanges methanol rapidly is indicated by taining chloride predominates a t the low temperature several items of evidence. First, at rooni tcniperaof our experiments. In view of the results obtained ture there is definitely a preference for water over for Co(C104jz(vide infra) we can conclude that addimethanol in the first coordination sphere of Cr+3 tion of chloride to the coordination sphere of octaand E u + ~l 4., l 5 Bjerrum and Jdrgensen have shown hedral cobaltous increases the rate of exchange of the that there is a preference for water over ethanol in the bound methanol; it may do this by increasing the rate first coordination sphere of the cobaltous ion,l6,l7 of the octahedral-tetrahedral transforniatiori for Co +z. and there is no reason to believe that the situation will The quality of the exchange data obtained for Co(C104)Z in CHSOH is siniilar to that for l I g ( C l O 4 ) ~ ~ be different in the case of niethanol-water solutions of cobaltous ion. Second, the coordinated water must and the exchange curves are not reported in detail. be unifornily distributed over the cobaltous ions iniTable I1 sunimarizes the results for cobaltous pertially a t -97’ in order to explain the difference bechlorate solutions. tween (iiaj, where the addition of ahout l mole of In appraising the data shown in Table 11, it must be water,’mole of C O + ~results in rapid exchange of all appreciated that the amount of adventitious water is the bound methanol and (i) where noritial exchange is not constant from experiment to experiment; thus the observed. results tend to be soniewhat erratic. However, even The evidence for the re-equilibration of water in with the limitations on precision which this imposes, the solution is the fact pointed out in (ij that the certain conclusions are suggested by the data recorded longer the cooling time the larger the solvation number in Table I1 observed, and the difference in the behavior noted (i) Coniparison of runs 3-9 at -97’ shows that for under (iiaj and (iibj. I t is possible for this re-equilsolutions with a constant amount of mater (less than ibration to occur in two mays. The water can be 2 moles of water,’niole of Co+2) the “kinetic solvation redistributed on cooling in such a manner so as to number” usually increases with the cooling time. yield a slowly exchanging cobaltous species Tvith little (ii) The addition of excess water to the Co(ClO4)2 or no water and a rapidly exchanging species enriched solution before mixing with the oxygen-18 enriched in water, or water may be transferred to the solvent methanol yields results which depend on the cooling during the redistribution process. Experiment 14 time. (iia) If the amount of water added is about 1 indicates that external HzO only slowly enters the comole/niole of C o f 2 and the cooling tinie is short, the ordination sphere at low temperatures. Sirice direct exchange is complete within the time of the second transfer of HzO from one C o t 2 complex to another is sampling, 1 . 5 to 2.0 niin., and the “kinetic solvation number” is sinal1 compared to the value generally ob(14) E. L. King, private communication served (runs 10 and 11). (iib) If less than 1 niok of (15) E. V. Snyre, D. G. 1Iiller. and 9. Freed, J . Chem. Phys., 2 6 , water/Co+:’ is added and the cooling time is short or 109 (1957). about 1 mole of water is added and the cooling time is (16) J. Bjerrum and C. K. Jdrgensen, Acta Chem. S c a n d . , 7, 951 longer, nornial kinetics and nearly nornial solvation (1953). numbers arc’ observed (runs 12 and 13). (17) C. K. J@rgensen,ibid.. 8 , 175 (1954). Table I : Summary of Data for the Isotope Exchange Reaction of CoC19 Solutions ( a t -82.5’)“
T h e Jo7irnaZ o,f Physical Chemistry
EXCHAXGE O F I\IETHAKOL
137
BETWEEN SOLVATED CATIONS AND SOLVENTS
Table I1 : Summar! of Isotopic Exchange Data for Co( C10a)2-llethanolSolutions
Run no.
Tenip.,
“C.
3 4
- (37
D
- $17
-
- 97
6 1
8 9 10 11 1% 13 14 15 16 17 1X 19 20 21
-97
- 97 - 97 -97 - 97 -97 - 07 - 97 - ‘37 - 97 - 97 - 97 -52 -82 -82 -101
5 3 5 .i
Cooling time, hr.
1 5 1 % 1 5 3 3 4 0 4 5 7 3 2 0 2 0 2 0 5 8 2 0 1-4 1-4 1-4 1-4 1-4 1-4 1-4
tll2,
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
02421 02912 01675 02999 02442 03988 02763 02587 03988 02649 02853 02991 03988 03988 03988 02988 03988 03988 02989
R,
b
a
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1420 1490 1786 1995 1352 2497 2176 1967 2491 1935 2244 2308 251i 1951 2122 2663 2786 2479 2469
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
02115 02041 02070 02092 02085 02031 02077 02083 02045 02102 02060 02093 02072 02040 02070 02102 02017 02068 02021
RO
Re
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
14765 14765 14979 14765 14765 14765 14979 14979 14765 14979 14979 14979 14765 14765 14765 14765 14765 14765 14979
0 0 0 0 0 0 0 O >O 0 0 0 0
03790 03936 04206 04590 03798 05019 04589 04279 04700 04190 04575 04829 05624 05117 05293 05345 05620 05470 04935
min.
P
2 3 3 6 5 4 3 12 >9 6 5 8 5
9 2 7 3 5 7 6 6*
10 9 11 10 12 10 12
3a 40 60 oa
4a oa
00
Ob
6 1 8 8 6 6 2 8 1 O’,Q
(11 (11 (10 (>18 (>a0 (>lo 0 0 0 25
6)c8h 2)d
0)e 6)j O)f 0)f 8P 75. 85“ 5f.Q
0 H20/Co-? e.;tiniated as
