(3) Corsini, A., Fernando, Q., Freiser, H.,ANAL.CHEM.35, 1424 (1963). (4) Freiser, H.1 Charles, R. G.1 Johnston1 w. D., J.Am. Chem. SOc. 74,1383 (1952). (5) Irving, H., Butler, E . J., Ring, hf. F., J. Chem. SOC.1949,p. 1489. (6) Irving, H., Mellor, D. H., Zbid., 1962, p. 5237.
(7) Irving, H., Rossotti, H. S., Zbid., 1954, p. 2910. (8) Johnston, W. D., Freiser, H., Anal. Chim. A , . ~11, 201 (1954). (9) K e a b , D*, Freiser~ H . ~Talanta 13, in Press (1966). (10) Merritt, L. L., Walker, J. K., IND.
ENG.CHEM.,ANAL.ED. 16, 387 (1944). (11) Sekido, E., Fernando, Q., Freiser, H., ANAL.CHEM.36, 1768 (1964). RECEIVEDfor review June 20, 1966. Accepted July 29, 1966, The financial assistance of the U. S. Atomic Energy Commission is acknowledged.
Solution Equilibria and Structures of MoIy bde num(VI) Che Iates (E th y le ned initri lo)tetra acetic Acid RICHARD J. KULA Department o f Chemistry, The University o f Wisconsin, Madison, Wis.
b Several of the equilibria in the (ethylenedinitri1o)tetraacetic Mo(VI) acid system have been investigated using proton nuclear magnetic resonance techniques and pH titrations. Between pH 9 and 5, two complexes are formed, one with a one-to-one metal-ligand ratio and the other with a two-to-one metal-ligand ratio. The formation constants of both chelates have been determined and compared with the formation constant of the one-to-one Mo(VI)-methyliminodiacetote chelate. In acid solutions the two-to-one chelate polymerizes by
0 forming Mo/ \Mo linkages between individual chelate species. From the nuclear magnetic resonance spectra, structures of the complexes are proposed; and from the aqueous solution equilibria, the possibility of a direct titrimetric determination of Mo(VI) using (ethylenedinitri1o)tetraacetic acid is considered.
A
the existence of an (ethylenedinitri1o)tetraacetic acid (EDTA) chelate having two Mo(V1) ions has been proved, no quantitative information concerning the stability of this species is available (,9,8, 11-13, 16). One report suggests that a one-to-one Mo(V1)-EDTA complex also forms, but no work seems to have been initiated to characterize this complex (5). I n the present work, the aqueous solution equilibria of the Mo(V1)EDTA system are examined in detail, and structural and bonding features of the solution species are considered. As in the preceding paper concerning the complexes of Mo(V1) and methyliminodiacetic acid (MIDA), the data were obtained mainly by proton nuclear magnetic resonance (NMR) and by pH titrations (6). I n addition, certain features of the aqueous infrared spectra are considered. LTHOUGH
12
I
10
t:.
I
two equivalents of acid are consumed for each equivalent of Mo between pH 11 and 5, and one equivalent of acid is consumed for each Mo below pH 5. By analogy with the RIIDA system, the overall stoichiometry between pH 11 and 5 can be viewed as the reaction 2Hf MOO^-^ e MOO, H20, followed by coordination of MOO, with EDTA. With two equivalents of M o and four equivalents of H + per EDTA, the resultant product is the two-to-one ( M O O J ~ E D T A - chelate. ~ In the MIDA system, the consumption of one equivalent of H + per Mo below pH 5 leads to a dimeric chelate. A similar reaction can be envisaged for the EDTA system, but on the basis of structures proposed for the EDTA-4 chelate (2, 8 ) , the product would be polymeric rather than dimeric. Consistent with this interpretation is the observation that solutions of pH less than 4 become cloudy after standing for about an hour. I n the vicinity of pH 2, a solution which is 0.5M in (MoO&EDTA-~ completely solidifies. This solid dissolves only very slowly if a NaOH solution is added. NMR. Spectra of solutions with varying metal-ligand ratios indicate that ligand exchange is relatively slow on the N M R time scale, and t h a t the predominant Mo(V1) chelate contains two Mo ions per EDTA. Most of the remaining work was carried out on solutions with this metal-ligand ratio. The spectra between pH 10 and 5 consist of resonances other than those of free EDTA (7) and of the two-to-one Mo-EDTA complex ( 2 ) , as illustrated in Figure 2. The main features of these additional resonances are a four-line A B multiplet whose chemical shift is independent of pH; a single sharp resonance whose chemical shift varies with pH and whose intensity is equal to the total intensity of the A B multiplet;
+
moles H' moles EDTA
Figure 1.
pH titration curves
- - - 0.2M NarEDTA4
-25'C; 0.4M Na2Ma04: 0.2M NarEDTA titrated with 5 M H N 0 3 EXPERIMENTAL
The experimental techniques were identical to those outlined previously (6), a Varian A-60 N M R spectrometer being employed using water as a solvent and tetramethylammonium chloride as an internal reference. Crystalline Na4(MOO~)ZEDTA.~HZ was O synthesized according to the method of Pecsok and Sawyer (8). Solution infrared spectra were obtained on a Perkin-Elmer Model 421 instrument, equipped with sample cells made of BaFz and having 0.025-mm. spacing. The solutions for the infrared studies were 0.05M in the Mo-EDTA chelate, and 99.8% D 2 0 was the solvent. RESULTS
pH Titrations. The p H titration curves for a solution of NalEDTA and for a two-to-one solution of Ner MoOrNalEDTA are shown in Figure 1. The corresponding titration curve for a solution of NazMoO4 was presented previously and showed acid consump tion only below pH 6.2 (6). The titration curve for the t w o - t w n e solution of Mo and EDTA is similar to that for the one-to-one Mo-MIDA system where
+
VOL 38, NO. 11, OCTOBER 1966
1581
,
20
W.S.
,
n
shift downfield slightly and at the same time decrease nearly linearly in intensity with added acid. Simultaneously, several extremely broad resonances (linewidths greater than 10 c.P.s.) appear; these resonances are centered a t -43 C.P.S. from ThlA and do not shift with pH. In the 20-minute interval between sample preparation and recording of the spectra no appreciable precipitate had formed in the NMR tubes, and the pH had not changed significantly. The down-field shift of the resonances presumably results from protonation of the (MoO&EDTA-' chelate, and the new resonances are those of the partially polymerized chelate. The extreme breadth of these resonances-Le., short relaxation time-is consistent with a large correlation time, as typically found for polymeric species (9). In Table I the NMR parameters of all the species have been summarized. Equilibria. The following equilibria were established and evaluated as in the preceding paper:
Frrr EDTA, Ac
L
rrc
I'
Ac $reo
H+
+ Y-'
s HY-3
(1)
+ HY-a + H + + MOOaY-' + HzO MOO4-* + Y-' + 2H+ s MOOsY-' + HzO H + + MoOaY-' + MOO SHY-^ 2M004-2 + Y-' + 4H+ + (kf003)2Y-' + 2Hz0 M004-*
Figure 2. NMR spectrum of u two-to-one Mo-EDTA (pH 7.8) and line assignments
and several smaller resonances, part of a highly split multiplet whose chemical shifts vary with pH. Because the intensities of these smaller resonances are so low, and because they are partially obscured by larger resonances, no definite chemical shifts can be determined, but they seem to lie mainly between the acetate and ethylenic resonances of free EDTA. For solutions with Mo-EDTA ratios less than two, the latter multiplet collapses to a single resonance below pH 6 (intensity equal to the intensity of the other singlet) and the chemical shift is no longer pH dependent. The species giving rise to these additional resonances is concluded to be the one-to-one MOOsY-' complex from considerations of the total EDTA in the system, the H+ consumed in its formation, and its NMR spectrum. A partial resonance assignment for the one-to-one complex is shown in the bottom portion of Figure 2. The
solution
chemical shift results and the relative concentrations determined from the NMR spectra are shown in Figure 3. As the pH is decreased below 4, the resonances for the two-bone complex 401
-----
Lower: pH dependence of relative concentrations of EDTA species
20
1582
ANALYTICAL
CHEMISTRY
I
I
4
1
J6'
-
0-
-20
-
-
1.00 1.00
75
L- 2
PH
2
3
I
I
-**. -**.
3
/* /*
ti . I
:I
.25 -
.so
I.
III ...
II .. II
r
0
Mo-EDTA ratio = 211
I
-
Figure 3. Upper: pH dependence of chemical shifts for various EDTA species Mo-EDTA Ratio = 211 Ac denotes ocetote protons Et denotes ethylenic protons MozEDTA EDTA MoEDTA 0 0 0 0 Polymer
I
//'
..
e
4
'
I
I
I
I
(2) (3)
(4) (5)
Q
Figure 4.
Structures of Mo-EDTA Complexes I. II.
+ & 1 0 0 4 - ~ + 2H+
hI003Y-~
+
(Moody4 (MoO~)~Y-~
F=
(&TOO~)ZY-~ HzO
(6)
Each equilibrium constant was calculated a t a minimum of five different pH values. The log of the average value of each of these determinations is given in Table I1 along with the range of calculated values. No attempt has been made to calculate the constants for the processes occurring below pH 4 because of their nonequilib r i m nature and the undefined nature of the species. However, the equilibria occurring are probably:
crystalline material in a KBr matrix (12, 13). Of particular interest here is the band around 1600 cm.-l which is assigned to the antisymmetrical carboxylate stretching mode. In both solution and solid this band is split, showing absorption maxima at 1600 and a t 1630 cm.-l DISCUSSION
is interesting that this ethylenic resonance lies downfield from the center of the one-to-one A B multiplet in exact analogy with the two-to-one complex. Similar explanations may be applicable to both complexes. The structure previously suggested for (MoO&Y-~ (2, 8) is represented by I1 in Figure 4. The multiplet assigned to the acetate protons is nearly identical to that from the acetate protons of MoOJfIDA-2 and can be interpreted similarly. Because the coupling constants between the geminal acetate protons for the one-to-one and two-toone Mo-EDTA complexes and for the Mo-MIDA complex are approximately equal, and because the chemical shift differences between the geminal acetate protons for the three complexes are also approximately equal, the bonding in all three cases is undoubtedly similar. The extreme downfield shift of the ethylenic resonance in (31003)2Y-4 results from the anisotropic deshielding of these protons by the Mo=O bonds of Moo3. Similar shifts have been observed for resonances of protons lying close to C=O and C=S groupings ( 2 7 ) . X configuration consistent with this effect and also with the downfield shift of the ethylenic resonance of ~ I O O ~ H Yis - that ~ in which the Mo ions lie trans to one another with respect to the XIo-?T-CH2CHz-N-h40 grouping, and in which deshielding of results protons on a given -CH2from the interactions with the nearest Moo3 group. To observe a single ethylenic resonance for the ~ ~ o O , H Y - ~ complex, the -CHzprotons adjacent to the protonated nitrogen must be deshielded by the H C (through bonds) to the same extent that the 0th-r -CHzprotons are deshielded by the Moo3 (through space). An x-ray structural determination has just been completed on the crystalline Na4(51003)2Ycomplex by J. L. Hoard and coworkers at Cornel1 University (3). The x-ray results confirm the octahedral Mo geom-
Structural Considerations. As in the Mo-MIDA case, the Mo(V1) in the chelated forms is assumed to be octahedrally coordinated in aqueous and - ~ the Hf (&f003)2Y-4 e H ( M O O ~ ) ~ Y - ~ solution. For the ~ I O O ~ Y ~ ~ O O I H Yspecies, -~ the NXIR spectra and are consistent with structure I in Figure 4,where the end of the ligand not (72 1) (03hf0-1'-A1003)-4 f coordinated to Mo behaves much like 2nH+ $ an iminodiacetate group. That is, the chemical shifts of the free acetate and 0 the ethylenic resonances shift downfield 03MO-Y (--MOO2 /, \Mo02-Y-), with decreasing pH, an effect indicative h~003-'2"+4'. of nitrogen protonation ('7). The collapse of the ethylenic multiplet to a From the pH titration data there seems single resonance below pH 6 is preto be very nearly one H+ consumed for sumably the result of the chemical each Mo which would imply that n is shifts of the protons in the two CH2 quite large. groups becoming accidentally equal. It By incorporating Equation 7 into the calculations, and by assuming that H2hIo04 = Mo03.H20, the Hf dependence of Equations 3 , 5 , and 6 can be Table 1. NMR Parameters eliminated. The resulting expressions Species VEthylenic' VAcetgte' JGeminal' are :
+
+
+ Mood-'
Hzhf.004 $2"
+ Y-' 2h1003 + Y-4 * (MO03)2Y-* h1003
&TOO8
?bfOO3Y-'
(7)
where Ks
=
Ks
. K,,
K9
=
K5
+32.6 +0.4
(8)
HY-a MOOaY -'
(9)
M o O a H Y -3
-36.5
(h'f003)2Y-' H(M00a)iY -3 M o Y polymer
-40.0 -43.3
f MOO3Y-'
(M003)zY-'
Y-4
(10) K72,
Infrared. The infrared spectrum of the (M003)2Y-' chelate in DtO is nearly identical with the spectrum of the
+1.3 -25.4 -26.0* -5.2d -26. ObmC; -43.0d -29.8b -33.5b sC;
VGeminsl"
... ...
... ...
16.5c
16.iG
16.5c
16.ic
16.5"
18.IC
(-43)O
Chemical shifts in C.P.S. from internal TMA. Denotes center of multiplet. Acetate protons adjacent to coordinated carboxylates. Acetate protons adjacent to uncoordinated carboxylates. * Estimated. f S in coupling constant in C.P.S. between geminal acetate protons. 0 Ctemical shift difference in C.P.S. between geminal acetate protons. VOL 38, NO. 1 1 , OCTOBER 1966
1583
Table II.
Equilibrium
log K 9.8 f0.1 10.1 8.8 f 0.3 18.5 f 0 . 3 7 . 5 f 0.2 35.1 + 0.3 17.2 f 0.2 -7.8 10.7 f 0 . 3 19.5 f 0 . 3 8 . 8 f 0.5 0.26
1
2 3
4 5 6 7 8 9
10 11
0
ANALYTICAL CHEMISTRY
Reference This work
NMR Potentiometric NMR NMR NhtR NMR NMR Potentiometric Calcd. from 3 and 7 Calcd. from 5 and 7 Calcd. from 8 and 9 NMR
etry with cis carboxylate bonding at each hlo, and the trans hfo geometry in the two-to-one chelate. Because the infrared spectra of Na4( M O O ~ ) ~inYthe crystalline form and in solution are nearly identical, the structure of the chelate probably does not change significantly on going from crystal to solution. From the work in KBr, it was concluded that because the antisymmetric carboxylate stretching band was split, either some of the carboxylates in the chelate were not coordinated or there was more than one type of Mo-carboxylate bonding in the chelate (12, 13). On the other hand, neither the solution NMR studies nor the x-ray results give any evidence for different types of carboxylates or for any free carboxylates. Therefore, it seems likely that while splitting of the infrared antisymmetric carboxylate stretching band may sometimes give information about the number of carboxylates coordinated to a metal ion ( I ) , it does not always give completely unambiguous information. Equilibria. The formation constants in Table I1 indicate that the Mo-EDTA chelates are quite stable and the stabilities are comparable in magnitude with the ,MIDA chelate as expected from similarities in structures and bonding. Although the resultsmay be fortuitous, the factor of two between K Efor the one-to-one hfo-EDTA complex and the formation constant for the Mo-MIDA complex reflects the statistical expectations for these species. Both the formation constant, Klo,for addition of a second &foO3to hfo03Y-4 and the protonation constant, K4, for are lower by H+ addition t o about two orders of magnitude than the corresponding constants K s and K 1 for addition of the first hl003 and H + to Y -4. Such behavior may reflect either a steric or an inductive influence from the MOO, group in hlo03Y-4. Several polarographic investigations of the Mo(V1)-EDTA system have been carried out, but these have been concerned mainly with solutions whose pH is less than 7. While some of the polarographic data may be consistent with the formation of polymeric species, this possibility has not been suggested.
1584
This corresponds to the equilibrium,
Equilibrium Constants Method of detn.
(15)
This work This work This work This work This work
(MoO&Y-~
+ H2Y-2 e ~ ( M o O ~ ) H Y (11) -~
The equilibrium constant, K 1 , , is equal
(14)
... ...
This 'work
In view of the discrepencies which exist among the data from individual workers-e.g., half-wave reduction potentials for hlo(V1)-EDTA solutions with comparable compositions vary considerably (4, 8, 10, 16), and there is some question as to whether the reduction product is hfo(V) or Mo(II1) (4, 8, 16)-polarographic interpretations of the solution equilibria must be weighed rather lightly. I n terms of analytical applications, the EDTA complex of hfo may be contrasted to that of MIDA. At the concentrations employed in the hfo-MIDA investigation, complete conversion to the one-to-one chelate was never achieved: a t pH 5 where the Mo-MIDA concentration maximized there was always about 4% of the total hlo which was not complexed, and therefore MIDA did not seem to be a suitable titrant for the quantitative determination of Mo (VI). With EDTA, on the other hand, the two-to-one complex is sufficiently stable that (R;IoO~)~Y-~ formation is complete between pH 6 and 4, and the competing formation of paramolybdate is inconsequential. It would appear then that EDTA might be employed advantageously for the direct titrimetric determination of Mo(V1) , although attempts in this direction have been frustrated by lack of an appropriate indicator and by poorly defined end points (11). The present results suggest that these difficulties may arise from several factors: in acidic solution Mo (VI) is polymeric and may react slowly; the Mo(V1) chelates are kinetically rather inert, and structurally related hlo-indicator complexes may also react slowly; and both one-to-one and two-to one Mo-EDTA complexes may form. The third factor was demonstrated to be a pertinent consideration by simulating typical titration conditions [?rlo(VI) solution buffered at pH 5 with acetate, titration with H2Y-21 and following the titration by NMR. Up to an EDTA concentration corresponding to the two-to-one Mo-EDTA complexi.e., EDTA/Mo 1/2-only the two-toone complex is formed. At higher EDTA concentrations-i.e., past the equivalence point-the appearance of the one-to-one complex is observed.
