Nuclear Magnetic Resonance Studies of Metal Ami no poIyca rboxyla te Complexes Lability of Individual Metal Ligand Bonds in (Ethyle ne d initri Io)tet ra a ceta te Co m pIexes ROBERT J. DAY and CHARLES N. REILLEY Department of Chemistry, Universify o f North Carolina, Chapel Hill, N. C. Application of nuclear magnetic resonance (NMR) to the study of metal complexes in aqueovs solutions is discussed. The presenc:e or absence of various splitting patterns is used to determine qualitatively the lability of individual metal-ligand bonds in (ethylenedinitri1o)tetrciacetate (EDTA) complexes. If all metal-EDTA bonds have a short lifetime, simple NMR spectra are obtained. If some bonds, such as metal-nitrogen, have a long lifetime, while othw bonds such as metal-oxygen (acetate) have lifetimes that are still short, spectra of intermediate complexity are obtained. When all metal-EDTA bonds have a long lifetime, the NMR spectra may b e complex. Some examples of these situations are presented and discussed, including the effect of metal-ion isotopes with nuclear spin.
T
nuclear magnetic resonance (KMR) spectra of the ligand protons of a metal chelate can be valuable in determining the lability of bonding in the chelate. For example, if all ligand-to-metal bonds have a short lifetime, the spectrum of the protons of the ligand will be relatively simple. This arises because all protons equivalent in the absence of metal ion will be equivalent in the complex becau,e of internal averaging; the chemical shifts will, however, be different because the average magnetic: environments in the two situations :Ire not identical. On the other hand, when one or more of the ligand-to-metal bonds have lifetimes sufficiently long so that these bonds may be considered permanent so far as KMR is concerned, 1,hen certain splittings may not be averaged out, and, hence, more conipler spectra are obtained. For example, the two protons on a carbon adjacent to an asymmetric quaternary nitrogen ittom (such as the nitrogen being bound to the metal) are expected to be noneqtdvalent just as in the case of two proton ion a carbon atom zdjacent to an asymmetric carbon atom. HE PROTON
Thus, consider the metal-nitrogen bond in the following structure where R is not a hydrogen and where R1, R2, and RCHz are all I,,'
R-CHz--N-,M---
I
I ' f
Rz different. The nitrogen atom is, therefore, asymmetric. When the metalnitrogen bond has a long lifetime, the two RCHz protons made nonequivalent by the asymmetry of the nitrogen atom are not able to average out through inversion of the nitrogen atom, and these protons split each other to yield an AB-type splitting pattern. Analogously, when a similar tertiary amine is placed in acid solution, the acid hydrogen bonds with the nitrogen so that the situation is similar to the above, with the acid hydrogen taking the place of the metal ion; AB splitting patterns for the RCHz protons have been obtained when the lifetime of the nitrogen-hydrogen bond is long. A B splitting will also occur even when RCH2 is identical to R, or Rz (4, 6). Another useful effect which can be exploited arises when the metal has abundant isotopes with nuclear spin of one half. In these cases, the metal ion can couple to the RCHz protons in the ligand via the ill-N-C-H bonds and provide further splitting. The above-mentioned effects are useful in that they provide a knowledge of the lability of individual metalligand bonds, a feature which is difficult to obtain from other techniques. The purpose of this paper is to demonstrate the existence of these effects and to interpret their meaning for some selected metal-EDTA complexes. EXPERIMENTAL
The proton nuclear magnetic resonance spectra were recorded using a Varian -460 high resolution XMR instrunent. The values of the chemical shifts reported are relative to sodium 3(trimethylsily1)-1-propanesulfonate,ab-
breviated TMS*. Chemicals were of the highest commercially available purity and were used without further purification; to eliminate any effect of the sodium ion, disodium EDTA was first converted to the acid form. The KMR spectra were recorded using aqueous solutions of 0.5 to lM, this concentration being chosen to provide adequate signal amplitudes for protons which are multiply split or which yield broad resonances. RESULTS AND DISCUSSION
EDTA contains both oxygen (acetate) and nitrogen ligand atoms, and, according to the lability of the various metal-ligand bonds, several situations may arise: The lifetimes of both the metaloxygen and metal-nitrogen bonds are short. The lifetime of the metal-oxygen bond is short while that of the metalnitrogen bond is long. The lifetime of the metal-oxygen bond is long while that of the metalnitrogen bond is short. In view of the structure of metal-EDTA complexes, it is unlikely that the metal-nitrogen bond could be broken without prior rupture of the metal-oxygen bonds on its two associated acetate groups. Investigations thus far have tended to confirm this conclusion in that they have not provided an example of this type. The lifetimes of both the metaloxygen and metal-nitrogen bonds are long. I n the first situation, the proton NMR spectra of complexes of diamagnetic metal ions (and having negligible abundances of isotopes with spin one half) should be simple and exhibit two sharp peaks corresponding to the ethylenic and to the acetate protons, as has been reported for a number of EDTA complexes ( 2 ) . Further, the complex should exist in various stages of unwrapping of the EDTA molecule, with the more complete stages of unwrapping being the less prevalent. Although the rates of unwrapping and wrapping may VOL. 36, NO. 6, MAY 1964
1073
proton splitting persists despite the short lifetime of the metal-nitrogen bonds and rapid inversion of the nitrogen because the nitrogen-metal bond always reforms to the same metal ion; hence, all nuclei are still in the same spin states 85 present prior to bond rupture, and the coupling is not relaxed. An example of this type of system is given in Figure 1, whioh is the spectrum of lead-EDTA, Lead-207, which occurs in a natural abundance of 2175, couples to the two types of protons on EDTA
Pb EDTA
1
I 40
I
35
,
3.0
Sva ~ ~ S : p p m
Figure 1.
NMR spectrum of lead-
EDTA Upper: Natural isotopic abundance of lead (21% lead-207) Lower: 93% enriched in lead-207 (inferior resolution ascribed to use of microcell)
be fast, the lifetime of a given EDTA molecule on a given metal ion may be long, Thus, any possible multiplets caused by proton-proton splitting are collapsed because the corresponding to a given type-i,e., ethylmade equivalent eic or by internal exchange (caused by rapid inversion ,f the nitrogen), If the metal ion has an appreciable &bundance of isotopes with spin of one half, then even if the lifetime of the individual metal-EDTA bonds is short, these isotopes can still couple to the EDTA protons so long as the chelate does not exchange rapidly with free metal ion or free EDTA. This metsl-
the ioublets are now the major peak; and only small peaks remain in the unsplit position (Figure 1). The leadproton coupling constants are 21.5 C.P.S. for the acetate protons, and 19.0 C.Pms* for the protons* I n the second situation, in which the metal-nitrogen bond has a long lifetime, while the metal-oxygen bond lifetime is short, the acetate and ethylenic protons exhibit different NMR patterns, Figure 2 indicates the two most probable conformations for the ethylenic carbons. These two conformations can rapidly interconvert and, thus, average out any differences of the two protons on a given ethylenic carbon. Also the symmetry of the chelate makes the two carbons equivalent so that all four ethylenio protons are equivalent even if the metalnitrogen bond has a long fifetime. Only one resonance peak is expected under these conditions (assuming the metal in question has zero spin) for the
ethylenic protons. However, the two protons on a given acetate are not equivalent. Figure 3 shows the diagram of B chelate with a fixed metal-nitrogen bond and an acetate group which is free to rotate about the acetate-carbon-tonitrogen bond designated by the m o w . Also shown are three rotational staggered configurations. These three positions are not equivalent just as in the casea where the carbon is bound to an asymmetrio carbon atom (8). In effect, the quaternary nitrogen etom is asymmetric as far as one acetate is concerned. The nonequivalences of the two acetate protons, Haand Hb, which leads to an BB splitting pattern, is caused by the inherent dissymmetry of the situation and/or by the different amounts of time the proton spends in the various rotational conformations (1). AD example of a metal-EDTA complex where the long lifetime of the metal-nitrogen bond causes an AB splitting pattern of the acetate protons is cadmium-EDTA. The NMR mectrum is given in Figure 4. The spectrim here is further complicated because of the nuclear spin of the cadmium isotopes 111 and 113, each of which has a spin of one half, and which together have a natural abundance of 25%. I n the NMR spectrum of cadmium-EDTA, the pattern of the ethylenic protons is quite simple and is analogous to that found in the lead-EDTA case. A single sharp ethylenic proton peak occurs for EDTA bound to those cadmium ions with zero nuclear spin and a doublet for the EDTA moleoules bound to cadmium111 and -113. The acetate proton spectrum is more complex. An AB pattern occurs for those EDTA molecules bound to the cadmium ions having no nuclear spin and an ABX spectrum occurs for those EDTA molecules bound to cadmium-Ill and -113, The entire .4BX spectrum is not visible, for some of these peeks are Of rather low intensity and are lost in the
1
GHd
no
H&Hb
KMON
NLfi
Figure 2. Conformations of the ethylenic group of EDTA for a complex where the metal-nitrogen bond has a long lifetime
I074
ANALYTICAL CHEMISTRY
Ac@M
A@"
Ac@ Hb
Ha Figure 3.
Hb
co;:
-02c
Configurations for a freely rotating acetate group
Ha
Cd EDTA
ISOTOPES 106 l O 8 , l l O 112: 114, I t s :
111, 113 28.0
I = I/
L 'r
.'..
35
8
Figure 4.
vs
M S:
1'.
ppm
NkrR spectrum of cadmium-EDTA
Pattern for acetate protops to the left of the dotted line; ethylenic protons to the right
I
Figure 5. Acetate portion of the NMR spectrum of cadmium EDTA
-
Top: AB pattern due to cadmium with spin zero Bottom: ABX pattern due to cadmium with spin one half
noise level. I n Figure 5, the top assignment is for the AB pattern for cadmium with no nuclear spin and the bottom assignment is for the visible part of the ABX patt,ern for cadmium with nuclear spin of one half. Analysis of the spectra shows that JAx (the metal to proton coupling constants for one of the acetate protons) is very nearly equal to JBx (the metal to proton coupling constant for the other acetate proton). Although the two coupling constants are not exactly equivalent, they are very close, suggesting rapid rotation about the acetate-nitrogen bond. This rapid rotational averaging is possible only if the metal-oxygen bond is labile, a feature which is also suggested by the effect of added ammonia which sharpens the spectra but does not alter the chemical shif ;s. The ammonia increases the rate of making and breaking of the acetate-m3tal bonds which causes a sharpening of the spectra, The coupling constants are as follows: the proton-proton coupling constant Ja is 17.1 c.P.s., J A X a.id J B X are each about 14 c.P.s., the difference in the latter two being ,%pproximately 1 C.P.S. The metal to ethylenic proton coupling constant is 12 C.P.S. The relative chemical shift of the acetate protons, ( 6 A - 6B), is nearly 10 C.P.S. or 0.17 p.p.m. The values of J A x and JBX mere calculated by the method cited by Pople, Schneider, and Bernstein (3); some of the parameters needed were first okltained from the AB pattern, thus siniplifying the calculation. For the fourth situation, in which both the nitrogen-to-metal bonds and the acetate-to-metal bonds each have relatively long lifetimes, rather complex spectra are expected because the acetate groups will not be equivalent. In the case of an octahedral Icomplex where all
ligand atoms are bound, dl forms are present. I n a given optical form, two of the acetate groups are in the ethylenic nitrogen-to-metal plane and are identical, and the other two acetate groups are out of this plane and are identical. The two methylene protons on a given acetate group are in different magnetic environments and ,yield an A B pattern. Therefore, the two types of acetates may exhibit two different AB patterns. Also the rigidity of the structure may affect the ethylenic protons so that they are no longer equivalent. Therefore, in addition to the splitting of the acetate protons, one may get a complex splitting type pattern for the ethylenic protons. An example of this is cobalt(II1)-EDTA. Although a complete numerical analysis of this NMR spectrum was not made, a number of conclusions may be drawn from the spectrum given in Figure 6. The acetates exhibit two different A B patterns as indicated in the assignments given below the spectra. The areas of each AB pattern are equal in agreement with the above-mentioned point that two acetate types exist-i.e., two inplane and two out-of-plane in respect to the bonding of their oxygens relative to the ethylene nitrogens. A tentative assignment of the A B patterns can be made on the basis of x-ray crystal studies (6) and on preliminary studies on complexes of the type K2[Co(III) (EDTA)X], where only three of the acetate groups are coordinated, the fourth hanging free, having been replaced by X . The AB pattern a t higher field (lower 6) is probably due t o the in-plane acetates, while the pattern at lower field is assigned to the out-of-plane acetate groups. The use of techniques such as multiple
resonance, use of a variable temperature probe, deuterium substitution, analysis of carbon-13 satellites, and " 6 or 0x7 resonance should also prove useful in these studies. Also, the ratio of ligand to metal ion can be important since an excess of either may cause exchange broadening. Thus, in the study of the structure of a complex, a stoichiometric ratio is normally best, while nonstoichiometric ratios may be used for kinetic studies based on line broadening.
Figure 6. (Ill)-EDTA
NMR spectrum of cobalt
Two AB patterns for the acetate groups indicated below spectrum Remaining peaks due to the highly split ethylenic protons
VOL. 36, NO. 6, MAY 1964
1075
From an analytical point of view, it is obvious that KMR measurements of metal chelates permit quantitative estimates of certain metal ion mixturesLe., Cd-Pb mixtures-and of isotope content-Le., Pb“7 in lead. This procedure also forms the basis for isotope exchange studies-i.e., Pb”7Y Pb+2 --* PbY Pb”7+2. The kinetic study of chelate exchange reactions (PbaiY* PbY is PbY* PbmiY) by normal mixing methods is also possible by employing deuterated chelates, Y *; the S M R technique has the advantage of continuous measurement of the course
+
+
+
+
of reaction, thus avoiding the usual timeconsuming separations and their possible effect on the amount of exchange and the use of radioactive isotopes. One of the chief disadvantages of the NMR technique is the concentration level which must be employed. LITERATURE CITED
(1) Gutowsky, H. S., J . Chem. Phys. 37, 2196 (1962). \ - I
(2) Kula, R. J., Sawyer, D. T., Chan, 8. I., Finley, C. M., J . Am. Chem. SOC. 85,2930 (1963). (3) Pople, J. A,, Schneider, W. G., Bernstein, H. J., “High Resolution Nuclear
Magnetic Resonance,” McGraw-Hill, New York, 1959. (4)Saunders, bi., Yamada, M., J . A m . Chem. SOC.85, 1882 (1963). ( 5 ) Singer, J., Sudrneier, J. L., Reilley, C. N., unpublished work. (6) Weakliem, H. A., Hoard, J. L., J. Am. Chem. SOC.81, 549 (1959). RECEIVED for review December 26, 1963. .4ccepted March 9, 1964. Research SUPported in part by National Institutes of Health Grant RG-8349. One of the authors (R. J. D.) gratefully acknowledges the help of a Kational Science Foundation Coo erative Graduate Fellowship. Presentel at the Southeastern Regional ACS Meeting, Xovember 15, 1963.
Colorimetric Detection and Spectrophotometric Determination of Vanadium Using a Specific Reaction P. L. SARMA Department of Chemistry, University of North Dakota, Grand Forks, N. D.
b In the presence of nonreducing acids, small quantities of vanadium(V) give a yellow color, large quantities an orange color. It i s a specific test for vanadium(V). The color was most stable and intense with concentrated sulfuric acid than with many other acids. Using a spot technique and one drop of a sodium metavanadate solution, a minimum of 10 p.p.m. of vanadium(V) was detected with a few drops of concentrated sulfuric acid. Absorption by a mixture of concentrated sulfuric acid and sodium metavonadate solutions followed Beer’s law between 450 and 540 mp. An optimum concentration range for estimating vanadium(V) by this method will depend upon the wavelength and the amount of acid used.
D
OF VANADIUM by redox titrimetry (6) involves a prior separation of vanadium from other oxidizable or reducible materials. Likewise, detection and determination of vanadium using hydrogen peroxide (14)) diphenylamine (7), phosphotungstic acid ( I @ , strychnine (4, benzohydroxamic acid (18), 2,6-pyridinedicarboxylic acid (9), benzoylphenylhydroxylamine ( I I ) , and many other colorimetric reagents use nonspecific reactions of vanadium. Simple vanadate ions have a tendency to undergo condensation reactions, especially in the presence of a nonreducing acid, forming polyvanadic acids (IO). Such a condensation reaction is generally accompanied by deepening of color. The mono-, di-, and tetravanadic acids are colorless; ETERMINATION
1076
ANALYTICAL CHEMISTRY
but the pentavanadic acid is orangeyellow, and the octsvanadic acid is brown-red. Also, the vanadates show a tendency to form complexes with acids forming heteropoly acids. Both these types of reactions are characteristic of vanadium@), but the composition and stability of the products are dependent upon the nature and the amount of acid inducing these reactions. Therefore, an investigation was begun in 1962 utilizing these specific reactions in the detection and determination of vanadium. Independently, Mittal and Mehrotra (8) worked on a method of estimating vanadium(V) with acetic, succinic, malonic, benzoic, and phthalic acids. However, the results of the present investigation showed that, because of smaller pK,‘s, the color intensities and stabilities were better with some inorganic acids than with most, or perhaps any, organic acid. EXPERIMENTAL
Apparatus. Beckman Model D B Spectrophotometer, Beckman Laboratory Potentiometric Recorder, and 0.998-cm. silica cells. Procedure. About 2.4 rams of sodium metavanadate, r\la#03, were dissolved in about one liter of water, filtered through a highly retentive quantitative filter paper, and the vanadium concentration was determined by titrating it against reagent grade ferrous ethylenediammonium sulfate, Fe[C2H4(NH&! (S0&.4H20, using sodium diphenylamne sulfonate as the indicator (2). This solution contained 0.975 mg. of vanadium per milliliter. -411 quantitative measurements were made with dilutions from this solution. At lower concentrations of vanadium, sodium metavanadate solutions pro-
duced a yellow color with sulfuric, hydrochloric, nitric, phosphoric, iodic, perchloric, periodic, molybdic, formic, acetic, trichloroacetic, ethylenediamine tetraacetic, sulfamic, sulfanilic, and sulfosalicylic acids; but the color faded away partially or completely through hydrolysis. Citric, oxalic, and tartaric acids also gave a yellow color; but on standing they reduced vanadium, producing the blue color of vanadium(1V). Ascorbic acid produced a transient greenish-blue color a t the moment of mixing. Thus, it provided a sensitive and specific test ( I S ) for the detection of vanadium@). An increase in the amount of a nonreducing acid decreased hydrolysis and increased the color intensity. Because the water content is small, concentrated sulfuric and glacial acetic acids were investigated as reagents for the detection and determination of vanadium(V). With a few hundred micrograms of vanadium(V) in one milliliter of a sodium metavanadate solution, small quantities of glacial acetic acid produced only a light yellow color and larger quantities gave an orange color. However, a large excess of this acid precipitated vanadium if the solution contained milligram quantities of vanadium per milliliter. For example, 5.0 ml. of a solution containing 5 mg. of vanadium(V) per milliliter produced an orange color with 10 ml. of glacial acetic acid. It changed to a yellow colored solution almost immediately, to a yellow colored turbidity after about 20 minutes, and to an orange colored precipitate in about 45 minutes. An increase in the amount of glacial acetic acid caused a more rapid precipitation of the heteropoly acids. No amount of sulfuric acid produced any turbidity or precipitate even on prolonged standing. A.R. grade ammonium metavanadate
