of radioactivity-e.g., in biological and health physics problems. Fluoride ion must be removed OF effectively compIexed prior to the extraction. Chloride ion concentration should not exceed 0.5-V. Recoveries through the standard procedure averaged 98.8% berkelium-249 with a relative sta.ndard deviation of 0.5% (n = 6). ANALYTICAL APPLICATIONS
The TTA method simplifies the determination of berkelium nuclides produced in transuranium element processes and the preliminary isolation of berkelium prior t o study by other methods. A most practical advantage is the ease of plate preparation for alpha measurements. One simply evaporates an aliquot of the TTA-xylene solution on inexpensive stainless steel plates. After ignition, the essentially solid free plates produced are excellent for alpha spectrometry. On the author’s particular counting arrangement (silicon diode detector = 3 sq. cm.), a resolution of about 40 k.e.v. is possible. Because such resolution closely approaches that possible with electrodeposited plates, the need for an additional electrodeposition step is obviated for most work. In the method (8) currently used for berkelium, the direct evaporation of the high boiling solvents, di(2-ethylhexy1)orthophosphoric acid or tri-
caprylamine, is precluded because of the solids problem. Moreover, the use of hydroch’oric acid necessitates expensive platinum or tantalum plates.
in the experimental work and of TV. R. Laing for some of t’he analyses. He is also indebted to R. D. Baybarz for sharing his berkelium tracer.
GENERAL PURIFICATION WORK
LITERATURE CITED
The TTA method is promising for the general purification and isolation of berkelium nuclides in preparative and industrial work. It is readily adaptable to remote control and continuous countercurrent processing a t room temperature; TTA can be easily recovered for re-use. It provides higher separation factors from other actinide elements, fission products, and associated elements than any other single solvent developed t o date. Although no studies have been done on its radiation resistance to alpha particles, TTA possesses relatively high stability to gamma radiation. Zittel ( I S ) observed no detectable effect a t an accumulated dose of 1 X loEr. For the difficult separation of berkelium(ITi) and cerium(IV), a promising avenue lies in the application of the, TTA method in multistage systemse.g., liquid-liquid extraction or extraction chromatography. Although these elements can be separated in other systems ( I , 4, 6 ) , none of these methods provides the concomitant high selectivity possible with TTA.
(1) Higgins, G. H.,. “The Radioc%mistry of the Transcurium Elements, NASNS-3031 (1960). (2) Katz, J. J., Seaborg, G. T., “Th: Chemistry of the Actinide Elements, p. 437, Wiley, New York, 1957. (3) ?vIagiiusson, L. E., Anderson, M. L., J . Am. Chem. Sac. 76, 6207 (1954). (4) itfoore, F. L., ANAL. CHEW33, 748 (1961). (5) Ibid., 36, 2188 (1964). (6) Moore, F. L., “Metals Analysis with
ACKNOWLEDGMENT
The author gratefully acknowledges the capable assistance of G. I. Gault
TTA,” Symposium on Solvent Extraction in the Analysis of Metals, ASTM Spec. Tech. Publ. No. 238 (1958). (7) Moore, F. L., Fairman, W. D., Ganchoff, J. G., Surak, J. G., ANAL.CHEM.
31, 1148 (1959). (8) Noore, F. L., Mullins, W. T., B i d , , 37, 687 (1965). (9) Peppard, D. F., nloline, 8. W., Mason, G. W., J . Inarg. Nucl. Chem. 4, 344 (1957). (10) Poskanzer, A. hl., Foreman, B. M., Jr., Ibfd., 16, 323 (1961). (11) Smith, G. W., Moore, F. L., ANAL. CHEV.29, 448 (1957). (12) Stokely, J. R., Moore, F. L., Ibid., 36,1203 (1964). (13) Zittel, H. E., Oak Ridge National Laboratory, Oak Ridge, Tenn., personal communication (1964).
RECEIVEDfor review August 3, 1966. Accepted October 14, 1966. Research s onsored by the U. S. Atomic Energy 8ommission under contract with the Union Carbide Corp.
Base-Catalyzed Hydrogen-Deu terium Exchange in Bivalent Metal-EDTA Chelates J. 6. TERRlLL and C. N. RElLLEY Department of Chemistry, University of North Carolina, Chapel Hill, The hydrogen-deuterium exchange in alkaline heavy-water solutions at 95’ C. of bivalent metal-(ethylenedinitrile)-tetraacetates (denoted EDTA) was followed with nuclear magnetic resonance (NMR) spectrometry by observing decreases in spectral intensities. The exchange process, studied in 0 . l M to 0.8M OD-, was first order in complex and first order in OD-. The order of reactivity of the metal chelates, C U + ~ Nif2 Co+2 Zn+2 Pb+2 Cd+2 Mg+2 Ca+Z Sr+2 Ba+2,correlates well with the strength of the metal-ligand bonds. Only the acetate methylene protons exchanged at a measurable rate. In similar studies with unassociated model compounds, a much slower OD- hydrogen-deuterium exchange was observed for acetate and
>
>
1876 *
>
>
>
>
>
>
ANALYTICAL CHEMISTRY
>
N. C. 275 14
glycinate; in contrast, the exchange rates for imidodiacetate (IDA), ethylenediamine N,N’ diacetate (EDDA), and EDTA were faster and were zero order in OD-, suggesting an intramolecular base-catalysis process.
-
F
-
of bonds between a metal ion and a ligand molecule results in significant ohanges in the chemical properties of the organic compared to those of the unassociated ligand molecule. Extensive investigations of the chemical properties of the met,allocene (ferrocene) and the metal acetylacetonate complexes have often illustrated the differential chemical behavior of the chelate to that of the free ligand (1, 7, 18, 83). As most of these studies were done in nonaqueous media, a large variety of reactions were feasible, ORMATION
In aqueous media, notable studies have been made of the enhancement of the rate of hydrolysis in the metal peptide (9)and amino acid ester (8) complexes over that of the free ligand. The enhancement in reactivity of the glycinate methylene protons in glycinato-bis(ethglenediamine)cobalt(III) denoted [ C ~ ( e n ) ~ g l y +over ~ ] that of the unreactive unassociated glycine was shown by the successful condensation of the cobalt complex with acetaldehyde to give theonine (8% optical yield) (bI,92). Subsequently, Williams and Busch observed the hydrogen-deuterium exchange of the acetate methylene proton in basic heavy-water solutions of three cobalt(II1) chelates: [ C ~ ( e n ) ~ g l y ] + ~ , [Co(en)2(al)]+z, and [CO(EDTA)]+~, where a1 is alanine (SO). The influence of molecular stereochemistry on hydro-
gen-deuterium exchange processes in acidic heavy-water solutions was further studied in our laboratory using [Co(EDTA)] -1, trans-l,2-cyclohexanediamine - N,N' -tetraacetatocobalt(III), and dll-1,2-propylenediamine-N,iL"tetraacetatocobalt(II1) as model compounds (97). IP this latter work, high yields of asymmetric products could be obtained by utilizing the stereochemistry of the chelate molecule. Furthermore. identification of given geminal acetate methylene protons with their corresponding NMR A B patterns was made possible by the differential isotopic exchange rates. Interestingly, different reaction rates for the same ligand are observed when different metal ions are used; for example, Bostic, Fernando, and Freiser (6) pointed out that the rate of iodination of 8-quinol-5-sulfonic acid decreased in the order C U + > ~ Nie2 > Gof2 > Fe+2 > Zn+2 > Mn+2. A similar metal ion effect was exhibited in the hydrolysis of C-terminal peptide residues ($6, $9) (namely, C O + > ~ > Zn+2 > M n f 2 > Fe+2), and in a study of the inhibition of various tetracycline metal chelates on the reduction rate of alanine dehydrogenase ( C U +> ~ C O +> ~ Xi+* > Afg+2 > Ca+Z) which is thought to proceed via a mixed complex intermediate ( l a ) . A study of the change in the reactivity of the labile acetate methylene protons in a series of bivalent metal-EDTA chelates would hopefully provide further insight into the metal-ligand bonding of this analytically useful reagent. For this reason, base-catalysed hydrogendeuterium exchange, which is a measure of the lability of these protons, was investigated. The reaction, which is easily followed by NMR techniques, is also significant in that the deuteriumlabeled EDTA product is a useful reagent in itself.
Table I.
System
~ / ( o D - )x 104
M(0D-)
...
0.078 0.158 0.288 0.392 0.532 0.675
1.44 1.15 1.44 1.15 1.44 1.44
Zero order
MgY
0.069 0.140 0.227 0.288 0.410 0.515
2.88 5.05 10.0 6.75 8.10
Precipitate
4.16 3.60 4.40 2.3 2.0
CaY
0.083 0.169 0.276 0.350 0.503 0.670
0.635 0.95 1.58 2.04 2.95 3.9
0.765 0 565 0.570 0.57 0.585 0.58
SrY
0.048 0.126 0.227 0.295 0.436 0.535
0.039 0.16 0.334 0.384 0.60 0.69
[ 0,08151 0.127 0.147 0.130 0,137 0.130
BaY
0.090 0.182 0.300 0.415 0.543 0.716
Very slow
CdY
0.027 0.047 0,088 0,115 0.151 0.198
0.11 0.11 0.34 0.53 0.67 0.83;
PbYc
0.046 0.095 0.145 0.197 0.304 0.41
0.92 3.81 6.1 7.1 11.3 15.6
ZnY
0.0013 0.0035 0.0062 OIO09O 0,024 0.062
0.46 0.71 1.10 1.57 2.70 3.12
COY
0.033 0.066 0.125 0.181 0.293 0.417
4.2 7.6 11 14 18 22.3
N iY
0.046 0.095 0.145 0.197 0.304 0.41
10 13.9 20.0 24 30.5 32.3
K4Y
(1:5)
EXPERIMENTAL
The proton magnetic resonance spectra were recorded with a Varian A-60 high resolution NMR spectrometer a t ambient probe temperature (32.3 0.3" C,). The spectrometer was tuned with acetaldehyde to a resolution of 0.6 c.p.s. or greater before each spectral recording. Moreover, the instrument was rechecked with acetaldehyde after the recording of kinetic data. The hydrogen-deuterium exchange process was monitored by spectral area measurements made directly from peak heights in the case of singlets whose peak width remained constant. In other cases (unresolved multiplets), areas were measured using a Keuffel and Esser fixedarm polar planimeter (Model 4326). The precision was 2% in both cases. The collected isotopic exchange data were then analyzed by conventional kinetic methods. Chemical shifts of the diamagnetic metal chelates agreed with those previ-
Hydrogen-Deuterium Exchange Rates in Basic D2O Solution of Bivalent Chelates at 95" C.
(1:5)
*
(1:5)
CUY*
(1:s)
0,0095 0.034 0.103 0.176 0.28 0.55
.,.
7.45 18.3 20.2 24.5 29.1
k *b
70
1.55
90
4.3
92
0.65
92
0.145
90
0.050
90
4.65
90
... .,,
I
0.095 0.11 0.18 0.24 0.35
11.1
Deuterium content,
0.052 0,037 0.043 0.044 0.049 4.1
P.31 3.8 4.6 4.4 4.2
(19) 40.0 42.0 36.0 37.0 38.0
41.0
93
350 200 180 180 110 50
230
90
127 116 88 74 63 56
139
90
138 122 100 78
170
88
785 325 178 115 88 52
680
88
(?;;I
a Reaction rate observed except where MY was 1:5. In this case, kobs was multiplied by five. Accuracy of k o p is 3%. The average k* value incorporates a correction for the small amounts of light wateras indicated-in the heav water runs. Ratio in parentheses fyor a system denotes ratio of (M:Y) used experimentally. CuY decomposes in strongly basic solution.
VOL. 38, NO. 13, DECEMBER 1966
0
1877
ously reported (9). In the case of paramagnetic chelates, the metal to ligand ratio was made 1 to 5 so that the kinetics of the hydrogen-deuterium exchange process could be followed by monitoring the resonances of the free ligand which was only slightly broadened a t the concentrations used. The kinetic data were subsequently corrected for deuterium exchange occurring on the uncoordinated excess ligand, but in most cases this correction was less than 5%. Moreover, in utiliaing the change of resonance intensity of the excess free ligand to monitor the deuterium exchange in the chelate, it was assumed that intermolecular exchange between the coniplexed ligand and free excess ligand is fast relative to the rate of hydrogen-deuterium exchange occurring in the chelate. Solutions of the various metal chelates were prepared from metal salts of the highest available commercial purity and from reagent grade EDTA obtained commercially from Fisher Chemical CO. All reagents were used without further purification. The dry reagents, standardized potentiometrically using a mercury indicator electrode us. primary standard calcium carbonate (24), were weighed out directly in the desired quantity, then heavy waterobtained in 99.'i70 purity from the Colunibia Chemical Go.-was added for solution. Finally, ACE grade potassium hydroxide was added to adjust the pE4 to 11. At this pH the solution contains only bIP-2, U-4, but negligible excess deuieroxide. Individual S M R samples were prepared from this stock solution by addition of known amounts of standard ootassium hydroxide (in D@). In the case of the ZnY?. COY-2. and GUY-2, the decrease in t6e hydrogendeuterium exchange rate (Tables I and IB) suggested the formation of mixed hydroxyl complexes. A mixed CdEDTA complex was evidenced by a 2C.P.S. decrease in the AVABof the methylene protons. The stability constants of these mixed hydroxyl complexes were determined potentiometrically with a Leeds and Northrup expanded seat pH meter, using a Thomas high pH combination electrode. This electrode had excellent response (ctQ.05 pH unit) when calibrated with a series of potassium hydroxide standard solutions (0.001 to 0.5-TI). If no mixed hydroxide complex was formed, the deuteroxide concentration was directly computed from the amount of base added. All samples were heated in sealed KMR tubes placed in a conventional constant temperature bath regulated to 95"i 0.1" c. As the isotopic nature of the solvent and the contribution of secondary iaotope effects niay have a significant role in the transition state (H), the exchange rate was determined as a fmction of the isotopic ratio of the solvent for CaEDTA and Pb-EDTA. I n both cases, the reaction rate was linearly dependent (precision 10%) on the percentage of heavy water (11). 4 s the isotopic composition of the MY-2 heavy-water stock solution varied from 85 to 93% f
e
ANALYTICAL CHEMISTRY
Table II.
Isotopic Exchange Rates in Basic
95" System CaY
CdY
PbY
ZnY
COY
(1:5)*
NiY (1:5)
CUYC (1:5)
HzO Solutions of Bivalent Chelates at
c.
M(0H-) 0.075 0.140 0.286 0.434 0.588 0.700 0.027 0,047 0.088 0.155 0.151 0.198 0,005 0.0515 0.107 0,214 0.318 0.432 0.0030 0.0070 0,0130 0.020 0.037 0.094 0,033 0.066 0.125 0.181 0.293 0.417 0.100
0,150 0.210 0.318 0,432 0.545 0 .0010 0.0125 0.055 0.125 0.185
lkl(OH-)I 0.110 0.167 0.412 0.53 0.725 0.875 0.0245 0.485 0,0735 0.096 0.142 0.160 0.0278 0.435 0.836 1.83 3.06 4.05 0.139 0.348 0.404 0.78 1.07 1.22 0.945 1.67 2.3 3.03 3.15 4.9 2.5 5.5 8.00 10.2
16.7 24.4 125 146 311 430 590
x
lo*
0.142 0.120 0.144 0.123 0.124 0.125 0.90 1.00 1.20 0.83 0.93 0.81
(86;:)
IC* 0.13
0.95
8.7
7.8 8.6 9.4 9.3 46 50 31 39 29 13 28.5 25.3 18.5 16.9 12.8 11.8 25 36 38 32 38 45 180 117 56 34 32
45
28
35
150
Reaction rate observed, except where MY was 1:5. In this case, k&a was multiplied by five. Accuracy of kaba is 3%. Ratio in parentheses for a system denotes ratio of (M:Y) used experimentally. c CuY decomposes in strongly basic solution.
deuterium, the kinetic data listed has been corrected to that for pure solvent; also, only the initial portion (below tllz) of the raw data was considered to avoid the effect of a back reaction (at longer periods) caused by the small increase in quantity of light water. No attempt was made to keep the ionic strength of the various metal-chelate solutions constant. Octadeutero-EDTA (Ds-EDTA) was prepared by refluxing a 0.5M tetrapotassium EDTA heavy-water solution for 15 hours and stripping the solvent off twice with subsequent additions of equal volumes of 99.7% DzO to enrich the isotopic ratio. After the second enrichment had been removed and heavy water added again (to effect a 0.5M solution), the solution was refluxed an additional 15 hours. The ligand, as the tetraacid, was precipitated by adjusting the p H to 2.0 with concentrated hydrochloric acid, yielding the tetraacid solid. The isotopic
yield, by NMR, was 90% (deuterium) and material yield was 80%. RESULTS
The NMR spectra of the bivalent metal-EDTA chelates have been discussed previously (9, l 7 ) , and the lower field resonances were assigned to the acetate methylene protons and the higher field resonances to the ethylenic protons. Singlets are observed for both kinds of protons in most cases; an A B multiplet is, however, observed for the acetate methylene protons of cadmiumEDTA, and further spIitting is produced by ligand-metal spin interaction in the leadzo7 and cadmium1119 11s (9). The NMR techniques used to study the kinetics of hydrogen-deuterium exchange of the various heavy-water solutions have been discussed under Experimental. The ethylenic protons were
not observed to undergo hydrogendeuterium a t any measurable rate. The hydrogen-deuterium exchange process followed the generalized rate expression:
AH
9-L
500 20
+- 25 in which all species present are considered. r is &IT/YT where MT is total metal concentration in chelated form and YTis total EDTA in all forms. The first term is for hydrogen-deuterium exchange occurring in the metal chelate (MY); the second is for the same process occurring in the mixed hydroxyl metal-EDTA complex [M(OD)Y]; the last term on the right is for any hydrogen-deuterium exchange taking place in the free ligand. The subscript H in SfYx-’, h f ( o D ) Y ~ - ~ Y , H - ~ denotes the hydrogen isotopic form of the ligand a t the start of the heavy-water experiments. The value of m was observed to be 1.0 (as shown by a constant kl/ [OD-]ratio in each case). The value of n was much more difficult to determine experimentally as the solutions partially decomposed at extreme basicities, but it is felt that n is unity. On this basis, ICl > kz in all cases. Interestingly, the value of p was zero. The primary isotope effect, which was studied by comparing the exchange rates of the deuterated chelate in light water, ka, with that of the proterated chelate in heavy water, kn, gave an average valued for k ~ l ofk 4.7. ~ This value shows the rate-determining step of the exchange reaction to be the rupture of the methylene C-H (or C-D) bond to yield an anion intermediate. The deuteroxide ion acts as the proton acceptor in this step. The anion intermediate rapidly abstracts a deuterium from the heavywater solvent to give the deuterated product (3, 19). The contribution of secondary isotope effects to the isotope exchange process could not be analyzed quantitatively because of spectral overlap of partially deuterated (H D) intermediate with
\/
C the starting material (H
H).
How-
\/
C ever, the contribution of the secondary isotope effect in relation to that of the primary isotope effect is negligibly small (13, 19). DISCUSSION
The hydrogen-deuterium exchange rates of the alkaline earths were observed to follow the order Mg+2 > Ca+2 > Sr+2 > Ba+2 (Tables I and 11). As the
40 0
15
e20 300 10
el 5
200 .5
e10
Figure 1. Correlation of gross heat of ligation for the gaseous metal ions, the second ionization potential, and log stability constant with the hydrogen-deuterium exchange rate (1 .OM ODin DzO, 95” C.) Heat of hydration is equal (within 3%) to the gross heat of ligation for the metalEDTA chelates. 0 AHg-i = gross heat of ligation in kcal./mole X log Kaiy is the logarithm of stability of the metal chelates A 12 is the second ionization potential gaseous metal ions
ions of this group have inert gas structures, the bonding between the central metal ion and the ligand is predominantly ionic (14, $1). Hence, the order of the rates in this group is attributed to an increasing size-to-charge ratio. On the other hand, the order of rates for the first transition-metal series C U + ~> Ni+2 > > Zn+2is rationalized on the basis of covalent bonding and ligand field considerations (6, 10). In this work, it will be demonstrated that the inductive effect at the acetate methylene carbon of the EDTA attributed to the associated metal ion will be dependent on the strength of the metalligand bonds. The type of bonding, ionic or covalent, is relevant in that covalent metal-ligand bonds are usually stronger. Thus, ions with a tendency to bind more tightly to the ligand will produce a greater lability of the acetate methylene protons; hence, a correlation of the hydrogen-deuterium exchange rate should follow the same order as metal-ligand bonding strengths. Related parameters used as a measure of the
bonding strength of a metal ion are the heats of hydration of the gaseous metal ion (water as a ligand), second ionization potentials (an electron as the ligand), and the gross heat of ligation (39). The gross heat of ligation, which is defined as the enthalpy of reacting a gaseous metal ion with an aqueous ligand, is equal to the sum of the heat of hydration of the metal ion and the observed heat of ligation. As the gross heat of ligation for the metal-EDTA chelates (except lead) considered here is within three per cent of the heat of hydration, only gross heat of ligation (32) is shown in Figure 1. On the other hand, the enthalpy contribution to the stability constant of the metal chelate is a relative measure of the metal-ligand bonding strength, namely a comparison vs. water. Hence, a correlation of stability constant with the reaction rate is not anticipated. I n Figure 1, the linearity between the gross heat of ligation and the reaction rate for the two metal-ion series suggests that the nature of the metal chelate in respect to coordinated water molecules VOL. 38,
NO. 13, DECEMBER 1966
1879
is largely the same. That is, if a metal chelate had coordinated waters (attached to the metal ion) to which considerable bonding energy would be given, then the inductive effect of the metal ion on the hydrogen-deuterium exchange process would not be so great as in the case where the only ligands attached to the metal ion are those of EDTA. It is felt that the cadmium-EDTA system il-
Table 111. Hydrogen-Deuterium Exchange Rates of Mixed Hydroxyl Complexes and Hydroxy Formation Constants, K&r(oa)Y for Mixed Hydroxyl EDTA Complexes. Exchange rate (sec.-l) X System 106 in D2Ob 0.5M [Cd(OD)(EDTA)I--8 4.5 0.5111 [Zn(OD)(EDTA)]-3 20 0.5111 [Co(OD)(EDTA)]--8 40 0 . 5 M [Cu(OD)(EDTA)]-3 80c System Kx,rcoH)ud [Cd(OH)(EDTA)]--3 2.8 [Zn(OH)(EDTA)]-3 1008-f [Co(OH)(EDTA)I-s 7.8 [Cu(OH)(EDTA)]--8 1 . 5 X lo2, a I n 0.5M chelate with no supporting electrolyte b S'alues have been extrapolated t o pure D20 and 1.OJ1 base with an estimated accuracy of 10%. e Solution decomposition became ap-
Value of Kula (16) is 100. R. A. Care and L. A. K. Stavely, J . Chem. Soc. 1956. p. 4571) for [Cu(OH)(EDTA)l-a 3. X 102: for [Zn (OH)(EDTAjj-3 1.2 X lo2; for [Ni(OH) (EDTA)]-3 63; however, a hydroxyl complex could not. be detected in the Xi case either potentlometrically or on the basis of contact-shift measurements. a f
lustrates the former while the alkaline earth and first-row transition metalEDTA chelates belong to the latter. No explanation is offered for the deviation found in the case of lead-EDTA. Mixed Hydroxyl Complexes. Cadmium, copper, zinc, and cobalt form M(0H)Y complexes a t the base concentrations employed in the isotopic exchange studies. I n each of these cases the rate of hydrogen-deuterium exchange of the [M(OD)Y]3 complex was less than that of the (MY)+ chelate. This result is attributed to a decrease of positive charge on the metal with either a decrease in the extent of coordination of EDTA-Le., one acetate becoming nonbound-or a general decrease in the various metal-ligand bond strengths. The structure of [Zn(OH) (EDTA)]-3 has been discussed, on the basis of KMR investigations, by Kula (16) who suggested that one acetate became uncoordinated in going to the hydroxyl complex; the hydrogendeuterium exchange rate for Zn-EDTg is about fivefold greater than the rate of [Zn(OH)(EDTA)]-3. This is to be contrasted with [Cd(OH)(EDTA)]-a, which gives a methylene A B pattern much like that observed for Cd-EDTA and has about the same hydrogendeuterium exchange rate as Cd-EDTA. The foregoing suggest that all four acetates remain bonded in [Cd(OH) (EDTA) with the hydroxyl coordinated in a facial position, such as in [i?In(OH)(EDTA)]-3 (14). The exchange rates of the mixed hydroxyl complexes are listed in Table 111 along with their hydroxy formation constants. Unassociated Model Ligands. Because chelation changes the chemical reactivity of the ligand, it was of interest to study the hydrogen-deuterium exchange rate of the unassociated
Isotopic Exchange Rate of Unassociated Ligands in Heavy Watera at 95" C. Rate order M (OD -) k (set.-1) k l ( O D -1 in base System (Table I ) 1.55 x 10-5 ... Zero KdEDTA 0.07-0.7 0 . 2 9 x 10-6 *.. Zero NAzIDA 0.07-0.7 0 . 4 8 X 10-6 Zero K2EDDA 0.122 0.031 x 10-6 0 . 2 5 ' x ' 10-6 5 . O M glycinate 0.21 0,063 0.30 First Table IV.
Q . 5 M glycinate
0.31 0.40 0,047 0.122 0.21 0.31 0.40 0.53
0.093 0.170 0.019 0.028 0.039 0.052 0.122 0.155
x
10-6
0.29 0.42 0 . 4 2 X 10-6 0.23 0.19 0.17 0.30 0.29
0.8144 acetateb
a
Rate constants have been extrapolated t o pure heavy-water solution. 1.23 X 10-5 (set.-') is reported by Bok and Geib (4).
* A rate constant of 1880 *
ANALYTICAL CHEMISTRY
First
EDTA. I n contrast to the metal chelates, the rate of isotopic exchange for Y-4 was independent of the amount of OD- added (0.1 to 0.8X OD-) and was faster than the rate for the barium chelate (1.0JI OD-). These results suggested an intramolecular mechanism in which the proton abstracting sites of the unassociated ligands are the nitrogens and the carboxylates present on the same molecule. The possibility of a mechanism involving the a-nitrogen was ruled out as the hydrogen-deuterium exchange rate for glycinate was slower than for acetate, and both were first order in added deuteroxide and independent of the ligand concentration (4). To weigh the contribution of the remote nitrogen os. the carboxylates in the exchange mechanism, the rates for imidodiacetate (IDA) and ethylenediamine-N,N '-diacetate (EDDA) were determined (Table IV) This suggests that the more remote nitrogen is more effective (70%) than the carboxylates in the proton abstraction. I n the transition intermediate (below), the proton is labilized t o exchange by the remote nitrogen. I
where Ac = acetate. The intermediate involving carboxylate, (Ac,), also forms a six-membered ring intermediate. On the other hand, in the chelate structures, such groups are bound to the metal ion, and the only proton abstracting agent is the deuteroxide ion. The hydrogen-deuterium exchange rate for monoprotonated unassociated EDTA, HY-3, was immeasurably slow. This observation suggests that all methylene proton labiliaing groups are constrained in the monoprotonated species. A hydrogen-bonded structure has been proposed for HY-3. Additional studies, beyond the scope of this paper, are being made to characterize the structure and bonding of monoprotonated aminopolycarboxylic acid molecules and for species such as As the order of effect of different metal ions on the deuterium exchange rates investigated is the same as that observed for various biochemical studies and as predicted by the gross heats of ligation, it is asserted that metal ions will have a significant role in the rate of other chemical reactions of the organic ligand, I n this respect, conventional metal-ion masking techniques would be useful in custom synthesis. It is anticipated that the basic features of this work can be extended to efficient prep-
aration of other deuterium-labeled aminopolycarboxylic acid chelates which should make feasible the spectral elucidation of pending contact-shift studies of Ni+Z and C O +chelates ~ ($0). Intermolecular exchange rate studies, particularly MYH YD + MYn YH, and deuterium labeling to denature various a-methylene protons in natural amino acids are currently under investigation.
+
+
LITERATURE CITED
(1) Ababori, S., Ofani, T. T., Marshall,
R., Winitz, R., Greenstein, J. P.
Arch. Biochem. Biophys. 83, 1 (1954).
(2) Bamann, E., Hass, J. G., Trapmann, H., ilrch. Pharm. 294, 509 (1961). (3) Bell, R. . P., “Acid-Base Catalysis,” Oxford University Press, London, 1941. (4) Bok, L. D. C., Geib, K. H., 2. Physzk. Chem. A183, 353 (1939). (5) Bostic, C., Fernando, Q., Freiser, H., Inorg. Chem. 2, 232 (1963). (6) Carlson, R. L., Brown, T. L., Ibid., 5 , 268 (1965). (7) Collman, J. P., “Advances in Chemistry Series, No. 37,” “Reactions of Coordinated. Ligands and Homogeneous Catalysis,” in press.
(8) Collman, J. P., Buckingham, D. A., J . Am. Chem. Xoc. 85, 3039 (1963). (9) Day, R. J., Reilley, C. N., ANAL. CHEY.36, 1073 (1964). (10) Figgis, B. N., “Introduction to Ligand Fields,” Interscience, New York, 1Q66
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(21) Murahmi, AI., Itatani, H., Tahahasi, K., Kang, J. W., Suzuki, K., bIemoirs of Institute of Scientific and Industrial Research, Osaka University XX, 95 (1963). (22) Murahmi, M., Tahahasi, K., Bull. Chem. SOC.Japan 32, 308 (1959). (23) Rausch, RI., Vogel, AI., Rosenburg, H., J. Org. Chem. 2 2 , 900 (1957). (24) Schmidt, R. W., Reilley, C. N., AXAL.CHEM.29, 264 (1957). ( 2 5 ) Simpson, R. T., Riordan, J. F., Tallee, B. L. Biochemistry 2 , 616 (1963). (26) Swain, C. G., Thornton, E. R., J . Am. Chem. SOC.83, 3890 (1961). (27) Terrjll, J. B., Reilley, C. N., Inorg. Chem., in press. (28) Arthur H. Thomas Compan “Specifications Data for High pH 8;mbination Electrode.” (29) Vallee, B. L., Coombs, T. L., Hoch, F. L., J . Biol. Chem. 235, 609 (1960). (30) Williams, D. H., Busch, D. H., J . Am. Chem. SOC.87,4944 (1965). (31) Williams, R. H. P., J . Chem. SOC. 1952, p. 3770. (32) Wright, D. L., Holloway, J. H., Reilley, C. N., ANAL. CHEM.37, 884 ~
(196.5). ---, \
RECEIVED for review August 22, 1966. Accepted October 17, 1966. One of the authors (J. B. T.) acknowledges the financial assistance of National Institutes of Health Grant GM-12598-02 in support of this project.
Twin-Electrode Thin-Layer Electrochemistry Kinetics of Second-Order Disproportionation of Uranium(V) by Decay of Steady-State Current BRUCE McDUFFlEl and CHARLES N. REILLEY Department of Chemistry, University of North Carolina, Chapel Hill,
b The method of steady-state current decay with a twin working-electrode thin-layer cell, applied earlier to a kinetic study of a first-order following reaction, is extended here to the case of a second-order coupled reaction, using the disproportionation of uranium(V) as the model system. A general rate equation for decay of the steady-state current is derived for the second-order case and compared with experimental data on the uranium system. Rate data at several different acidities and initial U(Vl) concentrations are in reasonable agreement with previous work and with the proposed second-order mechanism. Limitations of both the chemical system and the steady-state decay method on the range of determinable secondorder rate constants are discussed.
I
previous study (10),the method of steady-state current decay in a twin-electrode thin-layer electrochemical cell was used to determine the rate constant of a pseudo-first-order chemical reaction that followed a charge-transfer N A
N. C.
step. The present work extends the method to the case of a second-order coupled reaction, using the disproportionation of uranium(V) in acidic perchlorate media as a model system. The case of a one-electron redox system coupled with irreversible disproportionation of the reduced species, R , is represented by the scheme:
e 0
,
s
R
t- - - - - - -
I
2R
UO
+ ( 2 - u)Z
The disproportionation proceeds with a second-order rate constant, k’, regenerating the oxidized species, 0, a t a rate equal to a / 2 times the rate of disproportionation and forming also the nonelectroactive species, 2,a reduced state of R. A similar case is that of the dimerization of R , where 0 is not regenerated. Such systems, second-order in a single component, are difficult to handle by transient electrochemical techniques (1,7,8, l 7 ) , since the data must be treated with
reference to nonlinear differential equations, analytical solutions for which have not yet been obtained. The present steady-state method, where applicable, offers a simpler relationship of the experimental parameters to the rate constant. CONDITION OF STEADY-STATE CURRENT
The twin-electrode thin-layer cell, described previously (1, do), has a cylindrical layer of solution of thickness, 1, in the axial or x-direction between two planar, working electrodes, each with circular area, A . With a redox system present in solution, if limiting potentials are applied to the working electrodes and if mass transfer in the a-direction is rapid compared to the rate of any coupled chemical reaction, a quasisteady-state current is generated through the cell. The concentration profiles of 0 and R between the electrodes are then linear, as shown in Figure 1. The concentration of R varies from 1 On leave from State University of New York at Binghamton, Binghamton, N. y.
VOL. 38, NO. 13, DECEMBER 1966
* 1881
