( Peter Letkeman Brandon Univerw Brandon. Manitoba, Canada R7A 6A9
I(
An NMR Protonation Study of Metal Diethylenetriaminepentaacetic Acid Complexes An integrated senior chemistty experiment
Most chemistry majors in their junior or senior year study the properties of aminocarhoxylic acids. This may come about in numerous ways; that is, their preparations or structures, their dipolar behavior (ampholytes), their metal complexing abilities (EDTA), their ionization constants, solubilities, iso-electric points, etc. It follows that an experiment involving an aminocarhoxylic acid could be used for the integrated laboratory course where most branches of chemistry would he represented adequately. This paper is limited to a protonation study of an aminocarboxylic acid and its metal complexes; however, the reader will soon realize that it can he expanded to cover topics from instrumental techniques to a study of the kinetics of metal-ligand bond formation. Diethylenetriaminepentaacetic acid was chosen because of its symmetry (as will he shown later) and its commercial availability in a pure form a t a relatively low cost. Theory
The possibility that ions carrying bath a positive charge and a negative charge could exist in solutions of amphoteric electrolytes was suggested by G. Bredig in 1894 (I). These ions became known as zwitterions; that is, hermaphrodite ions, ampholytes, or dual ions. In the early 1900's scientists (L. H. Adams and N. Bjerrum) thought only a small fraction of the molecules could exist as dual ions. Today we know that virtually all of the aminocarhoxylic acid molecules exist as dually charged ions in aqueous solutions (2).The zwitterion is formed when the proton from the carhoxyl group is transferred to the basic nitrogen atom in the amino group. After World War I1 Schwarzenhach (3) introduced a series of new chelatlng agents known generally as polyaminopolycarhoxylic acids. The most familiar is ethylenediaminetetraacetic (EDTA). The next higher analog to EDTA is diethylenetriaminepentaacetic acid (DPTA). It became an intriguing question as to what charged species would exist in aqueous solutions for these complexing agents. In a brief communication Frost (4) indicated a few possible structures for aminopolycarboxylic acids. Martell (51,Langer ( 6 ) ,Sawyer (71, and Sievers ( 8 )all used infrared spectroscopy to investigate the protonation of EDTA and higher analogs such as DTPA and TTHA. A controversy arose between Langer ( 6 )and Sawyer (7) as to the sequence of protonation on the nitrogen and carhoxyl sites. This disagreement was finally resolved when Sawyer and his co-workers (9,10) used nuclear magnetic resonance spectroscopy to follow the protonation of EDTA and DTPA. They as well as Sudmeier and Reilley ( I I ) proved conclusively that the nitrogen sites were protonated preferentially before the carhoxyl groups when the anions of EDTA or DTPA were titrated with acid. The same can be said for triethylenetetraaminehexaacetic acid (TTHA) and tetraethylenepentaamineheptaacetic acid (TPHA) which have been studied and reported by the author (13, 14). More recently an excellent review article appeared by Kostromina (15) summarizing the various nmr protonation studies of complexones. Fortunately the literature (10-12) is consistent in labelling the non-labile protons of the anion of DTPA as follows 348 1 Journal of Chemical Education
I
Ib)
Icl
(1)N-CHi-CHI-N-CH,-CH?-
-
0,C-CH.
(11
I
I
CHI-COIla1
1.")
where one expects singlets for (a) and (dl protons in the ratio of 4:l and an AzBz pattern for the spin-coupled (h) and (c) protons as shown in Figure 1. If the experiment is performed in aqueous solutions then the resonance signals of the ionizable protons will merge with the water proton signal due to rapid exchange and the pmr spectra will indicate changes in the chemical environment surrounding the uon-labile methylene protons. Rigorous mathematical methods of predicting or calculating the extent of protonation are given by Reilley e t al. (11) and Letkeman et al. (13) respectively. However, since this paper is designed for an undergraduate laboratory experiment, the writer will develop a more simplified approach. Id1
"""CHt
\
CH&OO-
( 77
'1 7
p m lo)
IblN-C-C-N-C-C-N / I i -0oc-CH, H H
ZYCOO'
IUId
1clIU
DTPA = 0.20 M Temp = 38-C pH = 4.00
o 0 00
I
b
d I
I
3 50
400
6, p p m
vs D S S .
Figure 1. Chemical shins of DTPA versus pH.
c
protons I
3 00
I
In a compound, such a s DTPA, which contains a number of different basic groups capable of accepting protons, microscopic equilibria exist among the various forms which have all possible combinations of protonated and unprotonated sites. As one adds acid t o t h e fully basic form of DTPA, each site (carhoxyls and nitrogens) is protonated for a certain average fraction of the time a t any given pH. T h e pmr spectra indicate t h a t the methvlenic protons (type a , h, c, and d) a r e deshielded by protonation of a nearby basic site a n d by a n amount which depends upon t h e nature of t h e basic site, its proximity to t h e particular methylene protons, and the fraction of time protonated (10-14). Since there are three basic nitroeen sites. two of which are eauivalent bv svmmetrv. a stud'nt might ask readily which sites are proionated a t &e, two. a n d three eouivalents of acid per eauivalent of the basic DTPA anion. his experiment i; designed t o answer t h a t auestion and t o studv metal comnlex formation. '1'0 a first apprnxtmation one can assume that the chemical shtft for a Droton will be affecred t h r most for any chance in its chemical environment occurring only one bbnd length awav. It follows then t h a t t h e position of a proton signal can he &en ( a t any p H ) by t h e equation ( 1 5 ) -
+
6 = NABA NBBB
where NA, NB, 8 ~ ' a n 6~ d are, respectively, t h e mole fractions ( N ) a n d t h e chemical shifts (6) of t h e protonated (A) and non-protonated (B) forms. T h e basic 6 ~ ' scan he taken a t p H = 12 when t h e anion would he virtually non-protonated, a n d 6 ~ ' as t p H = 2 when t h e molecule is almost completely protonated a s seen from t h e ~ u b l i s h e ddissociations constants (16), t h a t is,pK, = 1 . 8 2 , d = ~ ~2.65, p K ~ = ~ 4 . 2 8 , p K 4 = 8 . 5 5 , and pK6 = 10.45. T h e fraction V) of time t h a t a particular site is protonated can he defined a s 6 - SR f. =&A
- 6s
where i refers t o t h e various donor sites. for examnle, two different nitrogen atoms and two different carboxyigrkps. In this DaDer onlv t h e nitrogen donor sites will be considered ilnre lr'does herome sorneu,har n m p l i r a t e d t o bring in the carhorylate protonation a s well ( 1 I, 1.71.It is well knou,n bv now thkt the-nitrogen sites are more basic than t h e carhoxyl sites especially a t pH's above 4. In following t h e chemical shifts of a particular signal one finds a point of inflection where 6 = 1 / 2 ( 6 ~ 6 ~or) where t h e concentrations of t h e protonated and non-protonated forms are equal. Here i t is possible t o calculate t h e dissociation constants, because if in general
mine the pH of the various DTPA solutions. The pH is adjusted simply by the addition of potassium hydroxide or nitric acid as required because DTPA serves as its own buffering agent. Potassium hydroxide is preferred to sodium hydroxide because the latter has a higher tendency to form complexes with DTPA (3).Similarly, the metal-ligand solutions are prepared by mixing equal amounts of the acid and metal nitrate with the addition of enough KOH to prevent any precipitation of the protonated complex at low pH. Again a cancentration o f 0.20 M in ligand and metal is sufficient for good nmr spectra. Since less than a milliliter is needed for a typic nmr run, students are advised to work with small auantities. The Droton nuclear magnetic resonance spectra given in this paper were recorded with a Varian A56160A spectrometer at 38 f I T though any available nmr spectrometer will suffice. The values of chemical shifts were measured with respect to t-butyl alcohol (TBA) but reported with respect to DSS (3-trimethylsilylpropanesulfonicacid, sodium salt). The advantage of using TBA is that its signal is independent of pH and ionic strength (11).6 also has a signal E~aseto theregion under investigation at 1.233 oom .. downfield from DSS and turns out be an inexoens l r c reference chemical for aqueuw proronnricm studies by nmr. A m e l x n m r by volume of'I'Hz\ \\a wii;irnr to ywld n g w d reference signal. Results and Discussion The proton nmr spectra of 0.20 M DTPA varying from pH 2 to 14 are given in Figure 1and in Table 1.The (a) and (dl protons exhibit single peaks whereas the (b) (c) protons yield an A2Bs type spectrum since due to svmmetrv considerations they are non-equivalent ~~~~~~~
~~~
~
~
~
+
in Figure 1. When acid is added to a basic solution (pH = 12.0) of DTPA, the protonation of the DTPA anion can be followed by observing the chemical shift of the (a) and (dl protons. As the first equivalent of acid isadded, one notices that the (dl protons shift downfield faster than the iai orotons: however. when a second eauivalent of acid is added lhr d , pn.r
