Multinuclear nuclear magnetic resonance study of three aqueous

dentate Ln(DOTA)~ complexes are the best shift reagents, as ...... (12) Lee, Byungkook, Ph.D. Thesis, Cornell University, 1967. (13) Hoard, J. L.;Lee,...
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1418

Anal. Chem. 1981, 53, 1418-1425

Multinuclear Nuclear Magnetic Resonance Study of Three Aqueous Lanthanide Shift Reagents: Complexes with EDTA and Axially Symmetric Macrocyclic Polyamino Polyacetate Ligands Charles C. Bryden’ and Charles N. Reilley” Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 275 14

Jean F. Desreux Analytical and Radiochemistry, University of Liege, Sarf Tilman, 8-4000, Liege, Belgium

Lanthanide complexes with the macrocycles 1,4,7,1O-tetraazacyclododecane-N, N’, N”, N”‘4etraacetate (DOTA) and 1,4,7-trlazacyclononane-N,N’, ”’-triacetate (NOTA) and with ethylenediamlnetetraacetate(EDTA) were prepared in D20 solution; ten paramagnetic lanthanide ions and one or two diamagnetic lanthanide Ions were complexed with each ligand. The shift reagent properties of these three types of complexes were studied with positively charged (Na’), neutral (D20), and negatively charged (Cl-) substrates, using 23Na, 2H, ”0, and %I NMR. The axially symmetric octadentate Ln(D0TA)- complexes are the best shlft reagents, as the Paramagnetic ’H, 23Na, and 35CIshifts measured in these solutlons were purely dipolar. However, the best fit of the 23Na shlfts to the theoretical dipolar shifts was obtalned for Na+ in LnNOTA solutlons, indicating that the axially symmetric hexadentate LnNOTA complexes are best for posltlvely charged substrates. Proton NMR spectra of the organic substrates 1-butanol, n-butylammonium cation, and n-valerate anion in Tm( D0TA)- solutlon all showed paramagnetic shifts.

Lanthanide aquo ions have frequently been used as shift reagents for water-soluble molecules (1-6); more recently, lanthanide EDTA complexes have been used for the same purpose (7-9). The LnEDTA complexes were introduced to alleviate the problems of low solubility (7) and hydroxide precipitation (8)encountered with the aquolanthanides. The applicability and utility of aqueous shift reagents could be further increased if problems such as limited pH range, presence of higher than 1:l complexes, contact shifts, and nonaxial symmetry could be overcome. As is well-known, the use of the aquolanthanides is restricted due to precipitation of hydroxides above about pH 6. The LnEDTA complexes are useful at higher pHs, although hydroxide coordination to the metal (10) can produce a change in the chemical shift of the substrate above pH 9 (8),limiting the use of LnEDTA shift reagents to pHs below about 9. Another problem that is frequently confronted when aqueous (or nonaqueous) shift reagents are used is the formation of higher than 1:l adducts with substrate molecules, making the calculation of the chemical shift of the bound substrate more difficult. The aquolanthanides are known to form mixtures of 1:l and 2:l adducts in solution (5, 8 ) , and this possibiIity must be considered for LnEDTA complexes as well, since EDTA is only six-coordinate, with 3 A 1water molecules occupying the remaining coordination sites on the ‘Present address: Research Center, Hercules Inc., Wilmington,

DE 19899.

lanthanide ion (11-14). More than one substrate molecule might displace these water molecules and bind to the LnEDTA complex (depending on the charge and size of the substrate), producing higher than 1:l adducts. The possible contribution of contact shifts to the paramagnetic shift of the substrate is a third problem which complicates the application of shift reagents, both aqueous and nonaqueous. The paramagnetic shift is a combination of a contact (through-bonds) shift and a dipolar (throughspace) shift, expressed by the equation 6 = F(S,)

+ GCD

where the measured shift in parts per million is 8, the first term on the right-hand side represents the contact shift as the product of a ligand-dependent parameter F and a lanthanide ion dependent parameter {Sz), and the second term on the right-hand side represents the dipolar shift as the product of a ligand-dependent parameter G and a lanthanide ion dependent parameter CD (15). The lanthanide-dependent parameters { S z ) and CD have been calculated for all the lanthanides (26-19) and will be referred to respectively as theoretical contact shifts and theoretical dipolar shifts. It is the dipolar shift that is usually of interest, since it contains structural information. A number of methods for separating contact and dipolar shifts have been developed (20),but a shift reagent which produces purely dipolar shifts would be the best solution to this problem. Even when the paramagnetic shift of a particular nucleus in a substrate molecule is known to be purely dipolar, the shift reagent-substrate adduct must have axial symmetry (at least a threefold symmetry axis) if geometric information about the substrate is to be easily obtained (20). With the above requirements in mind, we decided to investigate the shift reagent properties of the lanthanide complexes of two ligands, one a derivative of a tetraaza macrocycle and the other a derivative of a triaza macrocycle, and compare them to the LnEDTA shift reagents. We prepared 11 or 12 lanthanide complexes with each of the ligands 1,4,7,10-tetraazacyclododecane-N,N”N”,N’”-tetraacetate (DOTA), 1,4”-triazacyclononane-N,N‘,N‘‘-triacetate(NOTA), and ethylenediaminetetraacetate (EDTA), the structures of which are shown in Figure 1. The complexes with NOTA have a C3 (threefold) symmetry axis (Figure 4),while the complexes with DQTA have a C4 symmetry axis (Figure 2); both complexes thus have axial symmetry. The LnDOTA complexes are eight-coordinate and are unlikely to form higher than 1:l adducts with substrate molecules. Potentiometric titrations show that LnDOTA complexes do not coordinate OH- even above pH 13 (22). These features suggest that LnNOTA and LnDOTA complexes might be superior to LnEDTA as

0003-2700/81/0353-1418$01.25/00 1981 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981 -ooc^\

A

/-coo-

Table 1. NMR Experimental Conditions DOTA

ohsd nucleus freq, ohsd MHz 'Wa 26.47 DOC-,

/-cocT

-OOC~N-N~COO-

1419

EDTA

Flgure 1. Structures of the three ligands,

pulse" width.

acq

time. s

no.of transients

halfheightb line width. Hz

0.20

600

10-30

"C

25.16

0.80

500

2-3

2H

15.36

1.00

50

2-5

1 7 0

13.57

0.05

10000

60-90

9.81

0.10

4000

20-50

35c1

The 90' pulse width is in parentheses. Typical line widths in paramagnetic solutions (except for solutions of Gd complexes, where the line widths are usually much lamer).

Flgure 2. Model of the lanthanum DOTA complex, showing the C, symmeby axis and illustrating the d i m r angle fw the case of a sodium

ion (the positon of the sodium ion is arbdrary). The obne of the four coordinated oxygen8 is above the lanthanum ion.

LOIDOTAI.

Flgure 3. Space-filling model of the LaDOTA complex.

aqueous shift reagents (especially the LnDOTA complexes). To determine the relative utility of the three types of complexes as aqueous shift reagents with respect to positively charged, neutral, and negatively charged substrates, we measured the 23Na shifts of Na+, the ,H and shifts of deuterated water, and the 35Cl shifts of C1- in solutions of all 35 complexes. We were particularly interested in determining which (if any) of these substrates would show significant paramagnetic shifts, and if so, what was the contact contribution to the shifts. Also, for those species where the contact contribution was negligihle (Na+, at least, would have no contact shift contribution), it would he important to compare the shifts to the theoretical dipolar shifts (CDvalues). A close correspondence of the measured shifts to the theoretical dipolar shifts would be evidence that the crystal field coefficients were constant along the lanthanide series, that the shift reagent-substrate stability constant did not vary along the series, and that the shift reagentsubstrate adducts were isostructural. EXPERIMENTAL SECTION Chemicals and Solutions. The ligand DOTA was prepared a~ deserihed previously (221, and the ligand NOTA was prepared as the trisodium salt according to ref 23. The tetrasodium salt of EDTA was purchased from Sigma Chemical Co., and the disodium salt of EDTA was purchased from Aldrich Chemical Co. The 0.1 M LnDOTA solutions were prepared in D,O as in ref 22;

the 0.1 M LnEDTA and LnNOTA solutions were prepared in D,O from anhydrous lanthanide chlorides and adjusted to pH 7.58.0 with NaOD (not corrected for deuterium isotope effect). The EuEDTA, GdEDTA, and TbEDTA complexes had lower solubilities, and their solutionswere prepared at 0.03,0.038, and 0.05 M concentrations, respectively. All solutions contained 0.15 M dioxane (1.25%)as an internal heteronuclear reference. LuDOTA, LuEDTA, LuNOTA, LaEDTA, and LaNOTA were prepared for use as diamagnetic references. The Na+ concentration was 0.35-0.63 M in LnDOTA solutions, 0.36-0.44 M in LnEDTA solutions, and 0.13-0.79 M in LnNOTA solutions; the CI- concentration was 0.30.39M in LnDOTA solutions, 0.30.31 M in LnEDTA solutions, and 0.784.79 M in LnNOTA solutions, NMB Measurements. AU spectra were obtained on a Varian XL-100/12 NMR spectrometer modified for multinuclear observation as described previously (24). The spectra were recorded at an ambient probe temperature of 28 + 2 "C, using 2-3 mL of solution in IO-mm NMR tubes, spinning for 13Cand 2Hspectra, nonspinning for 35CI,23Na,and 170spectra. The 13Cspectra of the dioxane internal reference were obtained with low power, 200-Hz bandwidth 'H decoupling to minimize heating of the solutions. The 13Cspectra of dioxane, and the z3Naspectra of Na+ were obtained twice, once with internal deuterium lock and spectra of Cl-, the I7O spectra once with "V external lock; the of water, and the 'H spectra of water were all obtained by using 'F external lock. Other experimental conditions are listed in Table I. The shift due to the bulk paramagnetic susceptibility of each solution was accounted for by subtracting the '3c shift of dioxane shift in parts in parts per million from the 23Na,2H, I7O, or per million. This is the same procedure that is routinely followed with 'H spectra using Me& (homonuclear internal reference), except that in our case, the observe nucleus is different from the reference nucleus (heteronuclear internal reference). The paramagnetic shift for a given nucleus was then obtained by subtracting the shift measured in the diamagnetic LuDOTA, LuEDTA, or LuNOTA solution. All shifts for a given nucleus measured in LuDOTA, LuEDTA, LuNOTA, LaEDTA, and LaNOTA solutions, and in a 0.4 M NaCl in D20 solution were virtually identical, indicating a very small diamagnetic shift in every case. For the 100-MHz 'H spectra of n-butylammonium ion, l-hutanol, and n-valerate ion, 0.05 mmol of each substrate was dissolved in 0.5 mL of D@ and in 0.5 mL of 0.1 M TmDOTA solution in D20. The pH of the solutions containing the n-butylammonium and n-valerate ions were adjusted with NaOD or DCI to the values indicated in Figures 9 and 11(not corrected for deuterium isotope effect). Dioxane was already present in the three solutions containing TmDOTA but was added to the other three solutions and was assigned a shift of 3.53 ppm M. Me,Si for all six solutions, thus correcting for the hulk paramagnetic susceptibility of the TmDOTA solutions. In the discussion which follows, positive shifts are to higher frequency (lower field).

ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981

1420

THEORY Equation 1is the expression for the measured paramagnetic shift, which is the average of the shifts of the free and hound substrate. In the following sections, we will be interested in the paramagnetic shift of the bound substrate

IC, 4“iS

Here A” and Y are both in hertz (Le., the bound shift is not expressed in parts per million). Fb and Gbare related to F and G as follows: LolEDTAT

(3) (4) where n is the coordination number of the substrate in the shift reagenthubstrate adduct, CsRs is the concentration of the shift reagenthubstrate adduct in moles per liter of solution, and Cs is the total concentration of substrate (free + hound) in moles per liter of solution. Geometric information is contained in the ligand-dependent parameter Gb Gb =

(

Zg2 ( r Z ) A , 0 ( 3 y s 8Z - 1) 60k2p

0 0

Flgure 4. Models of the lanthanum NOTA and lanthanum EDTA corn plexes. The plane of the cwdinated oxygens is below the lanthanum ion in both complexes.

method A, and F for method A,) were calculated hy using the linear least-squares technique, with shifts measured in solution for ten paramagnetic lanthanide complexes. Coupling constants and spin densities were calculated by using the following equations (20):

+

~

(?)AzZ(sinZ 8 cos 24) r3

)

(5)

where (?)A$ and (?)A2, are two crystal field coefficients, and r, 8, and 4 are the spherical coordinates of the observed nucleus with respect to the lanthanide ion a t the origin of the coordinate system (18). When the shift reagent-substrate complex is axially symmetric or has effective axial symmetry because of motional averaging, eq 5 simplifies to

Equation 6 can he used to find r and 8 for several nuclei in (and hence the solution structure 00 the substrate molecule and is clearly preferable to eq 5 for this purpose. As has been pointed out by Sherry, Yang, and Morgan (2.51,a linear correlation between the observed shifts and the theoretical CD values is not proof of axial symmetry, as such a correlation can he obtained with G represented by either eq 5 or eq 6. For the same reason, constancy of the shift ratios of two nuclei for several lanthanides is not proof of axial symmetry. Consequently, experimental proof of effective axial symmetry is difficult to obtain, and it is therefore desirable to have a shift reagent which is axially symmetric and which can he expected to form axially symmetric adducts with substrate molecules. Where contact shifts might be expected, we used the procedures designated in ref 20 as “method A,” and “method AT to separate the contact and dipolar shifts. In method A,, which was used when contact shifts were dominant, eq 1was rearranged to

(7) and 6/CD was plotted against (S,)/CD. In method A,, which was used when dipolar shifta were dominant, eq 1was rearranged to

and 6/(S,)was plotted against CD/(S,). The slope (F for method A, and G for method A,) and the intercept (G for

where A / h is the electron-nucleus coupling constant in megahertz, y is the magnetogyric ratio of the nucleus, and p is the electron spin density a t the nucleus. Values of lU(0)12/ IPH(0)(2 are tabulated in ref 26. When the substrate is the solvent (D,O), C,, can be replaced by CsR, the concentration of the shift reagent in moles per liter of solution. Theoretically one could ohserve dipolar paramagnetic shifts, even when the shift reagent does not form a complex with the substrate, through what we call the “excluded volume effect”. This effect can occur in complexes where the lanthanide ion is closer to one periphery of the complex. Under such circumstances, the substrate would be able to approach the lanthanide ion more closely in one region of the molecule, and the dipolar shift may no longer average to zero as a result of random motion. Figures 2 and 4 show that the effect can occur for all three complexes, with the substrate approaching the lanthanide ion more closely in the region above each complex. T o determine whether the effect was important for the LnDOTA, LnEDTA, and LnNOTA shift reagents, we plotted the I3C shifts of dioxane vs. the paramagnetic susceptibilities of the lanthanide aquo ions. As dioxane does not complex with these shift reagents, the dioxane shifts will correlate well with the magnetic susceptibilities provided the excluded volume effect is negligible (and provided the ligands do not significantly alter the paramagnetic susceptibilities of the aquo ions). The correlation coefficient was 0.99 or better in all three cases, demonstrating that the excluded volume effect was indeed negligible.

RESULTS The TI,“0,W a , and ,H shifts for each set of lanthanide complexes are plotted in Figures 5-7, along with scaled theoretical dipolar (19)or contact (17)shifts ( d i d line in figures). The theoretical shifta were scaled by using the zero-intercept least-squares slope of the line fitting the observed shifts to the unsealed theoretical shifts. A table of the observed shifta is given in Table 11. In Figure 8, the theoretical shifts (solid line) have been scaled to fit previously measured I7O and ‘H shifta of H,O in lanthanide aquo complexes (16,27); the shifta have been extrapolated down to 0.1 M for comparison to the shifts in Figures 5-7.

ANALYTICAL CHEMISTRY, VOL. 53,NO. 9, AUGUST 1981 L n D O T A SOLUTIONS

1421

L n AQUO SOLUTIONS

;-Yfi E " 0

-

-2

-2

4

-4

E -20

In

-

6

w

-

d

L o b R m F m S m E u G d TbDyHoErTmYbLu

,-

4,

I 23No Shift of

No'

In

-6

-30 -40

-40

-8

* I

La Cd Pr m Fm Sm Eu W TbDy 1% Er TmYbLu

LOCeRMFmSmEuGdTbDyHoErTmYbLu

La Ce PrNd hSm EuGd Tb Dy MEr Tm Yb t u

Flgure 5. Measured vs. theoretical dipolar NMR solutions.

shifts in 0.1 M LnDOTA In

-0.4

u

-o.8 -1.2L o Ce RNdPm Sm Eu WTbOy Ho E l TmYb Lu

Figure 8. Measured vs. theoretical contact ("0) or dipolar ('H) NMR shifts in 0.1 M Ln aquo complexes. The 'H shifts, and the I7O shifts labeled (a) are from ref 27; the I7O shifts labeled (b) are from ref 16. ~

Figure 6. Measured vs. theoretical contact (35CI,I7O)or dipolar (23Na, 2H) NMR shifts in 0.1 M LriEDTA solutions. LnNOTA SOLUTIONS

E -2

-8

-10

2H S h i f t o f W a t e r

LaCdPrNdPmSmEuGdTbDyHoGTmYbLu In

-2 Lo Ce R Nd FmSm Eu Gd TbDyno Er TmYbLu

Flgure 7. Measured vs. theoretical contact (%I, I7O)or dipolar (23Na, 2H) NMR shifts in 0.1 M LnNOTA solutions. I n the plot of 35CIshifts, 0 = observed shift and A := shift after calculated dipolar shift was subtracted.

Table 111gives the results when separation method AI or A2was applied to each set of 170,2H, lH, and %C1shifts, except where the relative error in shift measurement was large (2H shifts for water in LnEDTA and LnNOTA solutions, %C1shifts for chloride ion in LnEDTA solutions). The 1 7 0 shift of water in the Sm aquo complex (reported in ref 16) was excluded, as the measured and theoretical contact shifts of Sm are very small and thus have large relative uncertainties; the ratio of the two ( 8 / ( S z ) )used in the separation has an even larger relative error. In Table .[V, the Sm l70shift was again excluded. In Tables I11 and IV, N is the number of lanthanides

~~

Table 11. Paramagnetic Shifts in Parts per Million for GI-, Na+, and D,O in 0.1 M Solutions of LnDOTA, LnNOTA, and L ~ E D T AComplexes 23Na 1 shifts I7Oshifts 2H shifts 35Clshifts Ln LnDOTA La 0.34 -0.18 -0.14 -0.02 Pr 0.13 Nd 0.54 -0.08 0.09 0.13 0.04 0.29 -1.44 Eu 0.28 0.06 -0.76 Gd -1.13 -1.11 -1.52 -3.96 -7.06 Tb -1.68 -2.01 -7.41 -4.80 DY -1.01 Ho -1.02 -4.66 -2.54 1.25 Er 0.82 0.59 -0.23 2.43 3.56 Tm 3.38 1.54 0.83 1.04 0.85 0.47 Yb Lu 0.00 0.00 0.00 0.00 LnNOTA -0.07 0.01 -0.25 0.03 La -0.08 0.99 -0.02 0.49 Pr 1.65 Nd -0.05 0.16 0.23 -3.74 0.08 Eu -0.13 -1.70 0.27 -8.60 0.06 Gd -5.03 -10.25 -0.18 3.16 -9.04 Tb -10.21 -0.77 3.68 -10.22 DY Ho 2.16 -6.22 -0.33 -6.04 Er -0.11 --1.97 -0.79 -4.40 Tm -1.74 -0.14 0.23 -2.26 Yb 0.14 0.06 0.57 -0.84 Lu 0.00 0.00 0.00 0.00 LnEDTA 0.01 La 0.03 -0.22 0.05 Pr 0.03 0.84 0.55 0.05 Nd 1.58 0.02 0.32 0.11 -1.25 0.05 Eu a -0.07 -0.51 Gd -3.86 0.08 0.21 -0.57 Tb -4.77 0.00 2.78 -0.83 -0.56 -9.11 2.73 -1.14 DY Ho -6.44 -0.16 2.24 -0.23 Er -5.49 -0.08 0.03 0.11 Tm -2.85 -0.84 -0.05 -0.22 0.00 -0.18 Yb -0.87 0.14 0.00 0.00 0.00 Lu 0.00 a 0.03 M. 0.0375 M. 0.05 M. used in the separation and r is the correlation coefficient. The shifts measured in EuEDTA, GdEDTA, and TbEDTA solutions were linearly extrapolated to 0.1 M for comparison

1422

ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981

Table 111. Separation of Contact and Dipolar Shiftsa

Table IV. Separation of Contact and Dipolar “0 Shifts for Light and Heavy Lanthanide Complexesa

35c1

complex LnDOTA

I7O shifts N

F G

LnEDTA

r N F

G

LnNOTA

r N F

G

Ln aquae

r N F

G r

Ln aquof

N F G

r

loc

0.116 (25%) 0.0489’ (15%) 0.921’ 9b 0.375 (2.9%) -0.00787 (140%) 0.997 9b 0.348 (3.0%) 0.00289 (360%) 0.997 8b

1.50

(2.2%) -0.0352 (48%) 0.9985

2H shifts 1oc

-0.00197 (320%) 0.0228 (7.1%) 0.980

shifts

loc

0.0269 (82%) 0.0470 (12%) 0.947

loc

0.174 (9.8%) 0.0433 (10%) 0.961 10C,d

-0.001 57

(570%) -0.0119 (19%) -0.898

lob

1.35 (4.0%) -0.0900 (57%) 0.994

Separaa Relative standard deviations in parentheses. tion method A,. Se aration method A,. ‘H shifts. e Shifts from ref 27. Shifts from ref 16.

P

to the shifts for the other lanthanides. The validity of this extrapolation was verified by measuring shifts in 0.025,0.05, and 0.10 M HoEDTA solutions.

DISCUSSION Oxygen-17 Shifts. The 1 7 0 shifts of water in LnDOTA solution tend to follow the scaled theoretical dipolar shifts (Figure 5), although there appears to be a small contact shift contribution. The substantial dipolar contribution to the water 1 7 0 shifts in LnDOTA solution means that at least one water molecule must have a preferred orientation with respect to the LnDOTA complex and must approach the lanthanide ion fairly closely. If there were no preferred orientation, the 3 cos20 - 1term in eq 6 would average to zero, and there would be no dipolar shift. A molecular model (Figure 2) shows that the distance of closest approach is for a water molecule approaching the lanthanide ion along the C4symmetry axis from above, so that it is equidistant from the four coordinated carboxylate oxygens. A space-filling model (Figure 3) shows that only one water molecule at a time can closely approach the lanthanide ion from above. As there is a contact contribution to the water 1 7 0 shifts in LnDOTA solution, a water molecule must be coordinated to the lanthanide ion a t least some of the time. However, the water molecule above the complex cannot approach the lanthanide ion as closely as the coordinated water does in lanthanide aquo complexes as long as all four carboxylate oxygens are rigidly coordinated. Thus the complex must be sufficiently flexible that a water molecule can coordinate to the complex a large fraction of the time. The 1 7 0 shifts of water in solutions of LnEDTA, LnNOTA, and Ln aquo complexes all closely follow the scaled theoretical contact shifts (Figures 6-8); thus water must be coordinating to the metal in each of these three types of complexes. Given

heavy

complex LnDOTA

light lanthanides

lanthanides

6c 0.0923 (17%) 0.0527 (6.3%) 0.992 LnEDTA 6b 0.319 (3.3%) -0.00853 (46%) 0.998 LnNOTA 6b 0.278 (7.5%) 0.0139 (55%) r 0.989 Ln aquod N 2f 6b F 1.46 1.43 (1.9%) -0.0137 (74%) G -0.100 r 0.9993 Lnaquoe N 4b3g 6b F 1.23 (6.6%) 1.37 (2.1%) G -0.316 (37%) -0.0235 (46%) r 0.996 0.9991 a Light lanthanides are Pr, Nd, and Eu, except as noted. Heavy lanthanides are Tb, Dy, Ho, Er, Tm, and Yb. Separation method A,. Separation method A,. Shifts from ref 27. e Shifts from ref 16. f No Eu shift available. g Includes Ce. N F G r N F G r N F G

3c 0.131 (5.7%) 0.00481 (70%) 0.817 3b 0.405 (2.6%) 0.0425 (42%) 0.9997 3b 0.350 (2.9%) -0.00463 (370%) 0.9996

the large 170dipolar shifts in LnDOTA solutions (e.g., a -4.9 ppm dipolar shift for DyDOTA using separation method A2), it is surprising at first to see little dipolar contribution to the 170shifts in Ln aquo, LnEDTA, and LnNOTA solutions. However, rapid ligand exchange averages the dipolar 1 7 0 shifts of Ln aquo complexes to zero, while water dipolar angles in LnEDTA and LnNOTA solutions are near the magic angle of 54.7’, resulting in small dipolar shifts. The close correspondence of the 1 7 0 shifts of water to the theoretical contact shifts in solutions of Ln aquo and LnEDTA complexes means that the electron-nucleus coupling constant does not vary along the lanthanide series; this coupling constant can be calculated given the water coordination number n. Although there is controversy as to whether n changes along the lanthanide series in Ln aquo and LnEDTA complexes (14), there is general agreement that n is constant in both cases for the heavy lanthanides (Tb-Yb). If n for the heavy lanthanides is taken to be eight for the Ln aquo solutions (28) and three for the LnEDTA solutions (14),then the coupling constants can be calculated for these two systems using eq 3,4, and 9 (with water as the substrate, CSR-s = CSR= 0.1 M, while CS = 55.5 M). For the Ln aquo complexes, using a average F of 1.40 (mean of 1.43 and 1.37; see Table 111),and taking the temperature to be 300 K, A / h = 0.75 MHz. If n is taken to be nine (12,14),A / h = 0.67 MHz. For LnEDTA, using F = 0.319, and a temperature of 300 K, A / h = 0.46 MHz. We see that the coupling constant decreases at least 0.21 MHz, or 31% when the lanthanide ion is chelated with EDTA. Thus the coupling constant can change significantly when the complex is varied, in contrast to the result when the lanthanide ion is varied. If the coupling constant did not vary from complex to complex, we could reverse the above process for the LnDOTA and LnNOTA complexes and calculate n for the heavy lanthanides from the known contact shifts in these solutions. Unfortunately, the coupling constant does vary; however, some useful estimates can still be made if we assume that the coupling constants in LnDOTA and LnNOTA complexes vary no more than 33% from the LnEDTA complexes (Le., A / h = 0.46 f 0.15 MHz). Using F = 0.278 for the 1 7 0 shifts of

ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981

water in LnNOTA solutions (Table 1111,we can estimate a water coordination number of 3 f 1for the heavy lanthanides. For LnDOTA solutions, using F = 0.0923, we estimate a water coordination number of' 1 f 0.3 for the heavy lanthanides. Sodium-23Shifts. The 23Nashifts of Na+ in LnDOTA solutions follow the theoretical dipolar shifts, which means that the sodium ion m w t have a preferred orientation with respect to the LnDOTA complex. If there were no preferred orientation of the sodium ion, it would experience all possible dipolar shifts equally, with a resultant average (observed) dipolar shift of zero. The sign of the 23Nashifts is the same as the water 2H shifts and the dipolar part of the water 170 shifts; since the dipolar angles for water (00 and 0,) are less than 54.7", the dipolar angle 0Na must also be less than 54.7". The sodium ion must prefer one or both of the cones (defined by 0" 5 0 I 54.7') above lor below the LnDOTA complex. The lower portion of the LnDOTA complex is hydrophobic, and there is no reason for charged or polar species to associate closely with this region of the complex. There are four carboxylate oxygens in the vicinity of the upper cone, and as we expect to find the positively charged sodium ion near the negatively charged carboxylate oxygens, the sodium ion undoubtedly prefers the upper cone. We also expect the carboxylate groups to attract the sodium ion away from the C4 symmetry axis, so that it13 preferred position will be in a cone about the axis, making BNe substantially greater than zero. We emphasize that the geometric terms such as 0~~ and r N a refer only to the weighted dipolar average position of the sodium ion (or other substrate). Given that 0Na is large in LnDOTA solutions, we can easily explain the change in sign of the 23Nashifts in LnEDTA and LnNOTA solutions. The 23Na shifts in LnEDTA and LnNOTA solutions also follow the theoretical dipolar shifts, but with a change in sign relative to the 23Na shifts in LnDOTA solution. It is clear from the sign change that the dipolar angle ONa, already large in LnDOTA solution, is greater than 54.7" in LnEDTA and LnNOTA solutions. X-ray structures and molecular models show that it is reasonable for to be larger in LnISDTA and LnNOTA solutions. For LaEDTA and DyEDTA, X-ray crystal structures show that the four carboxylate oxygens form a plane below the lanthanide ion ( I I , 1 3 ) . A model of LaNOTA (Figure 4)shows that the three carboxylate oxygens in this complex also form a plane below the lanthanide ion. However, a model of LaDOTA clearly demonstrates that the four carboxylate ions form a plane above the lanthanide ion (Figure 2). Since the sodium ion will be attracted to the carboxylate oxygens in each complex, the sodium ion will have a larger dipolar angle in LnEDTA and LnNOTA solutions than in LnDOTA solutions. Consequently it is not surprising to find that the 23NaNMR results show that ONa > 647O in LnEDTA and LnNOTA solutions and 0 N a < 54.7" in LnDOTA solutions. (It could be argued that the crystal field coefficients change sign in LnEDTA and LnNOTA complexes, but the 35Clresults show that this does not happen.) Deuterium Shifts. From Figure 5, it can be seen that the 'H shifts of water in Ln1)OTA solutions closely follow the theoretical dipolar shifts. We conclude that the crystal field coefficient, (r2)Az0,is conofant along the lanthanide series and that the deuterium dipolar angle 0D and the Ln-D distance rD do not change appreciably, despite the decrease in radius from La to Lu. The sign of the 2H shifts is the same as for the dipolar portion of the l7O shifts, as expected. The 'H shifts in LnDOTA solutions are twice as large as the proton shifts in Ln aquo solutions, which also follow the theoretical dipolar shifts, although not as closely (Figure 8). The shifts in LnDOTA solutions are even larger if the comparison is adjusted to reflect the different water coordination

1423

numbers in the two complexes. As discussed earlier, only one water molecule can closely approach the lanthanide ion in LnDOTA complexes; if we take the coordination number of water in Ln aquo complexes to be 9, the ratio of water coordination numbers in the two complexes is 91, and the shift per water molecule in LnDOTA solutions is 2 X 9 = 18 times the shift per water molecule in Ln aquo solutions. Large dipolar shifts are expected for LnDOTA solutions, as the closest water molecule is located on the C4 symmetry axis, and the dipolar angle 0D for water deuterons must therefore be small (3 cos2 0 - 1 large). On the other hand, small dipolar shifts are expected for Ln aquo solutions as a result of the averaging process discussed in the section on " 0 chemical shifts. From the crystal structures of Pr(H20):+, Nd(HzO)t+, and Yb(HzO)2+(29,30),12 protons should have 0H < 54.7", and their instantaneous dipolar shift will be opposite to the six protons of the equatorial water molecules, which have 0~ near 90'. Rapid exchange will lead to a small average (observed) shift. (If there is an outer sphere contribution (311, there will be a similar averaging process.) Since large shifts are expected for LnDOTA solutions, and small shifts per water molecule are expected for Ln aquo solutions, the LnDOTA/Ln aquo shift ratio of 18 per water molecule is not surprising. The 2H shifts of water in LnDOTA solutions are not only larger but also opposite in sign to the proton shifts in Ln aquo solutions, which could be the result of a change in sign of (r2)Az0.However, the difference in sign may simply be due to geometry. The six hydrogens of the equatorial water molecules (eH near 90") in the aquo complex have dipolar shifta opposite in sign to the hydrogens of water in LnDOTA solutions (0, small). If the other 12 hydrogens in the Ln aquo complex have BH > 45", the six equatorial hydrogens will dominate the average (observed) shift, which will then be opposite in sign to the shifts in LnDOTA solutions. In contrast to the large 2H shifts observed in LnDOTA solutions, the 2H shifts in LnEDTA and LnNOTA solutions are very small (Figures 6 and 7). These results are similar to the 170results for the two complexes, where the dipolar contributions to the shifts are also very small. Again, the best explanation for the small shifts is that the dipolar angle 0H is near 54.7" for water hydrogens in LnEDTA and LnNOTA solutions. Chlorine-35 Shifts. The 36Cl shifts of chloride ion in LnDOTA solutions closely follow the scaled theoretical dipolar shifts (Figure 5), which means that C1-, like Na+ and D20, must have a preferred orientation with respect to the LnDOTA complex. The close correspondence to the theoretical dipolar shifts also shows that ( r2)AZo,the chloride dipolar angle e,,, and the Ln-Cl distance rcl are constant along the lanthanide series in LnDOTA solutions. The sign of the Q21 shifts in LnDOTA solutions is the same as the 23Nashifts, the 2H shifts, and the dipolar portion of the 170shifts in LnDOTA solutions, so the C1- ion must have a preferred orientation near the Na+ ion above the LnDOTA complex where 0" I 0 I 54.7". The chloride ion will be repelled by the four negatively charged carboxylate oxygens and attracted by the positively charged lanthanide ion, so the chloride ion should be located on the C4axis equidistant from the carboxylate oxygen, i.e., 0 ~ 1should be near zero. Given a 0a near O", we can account for the fact that the %C1 shifts have the same magnitude as the 23Nashifts in LnDOTA solution. The C1- ion should be farther away from the negatively charged LnDOTA complex than the Na+ ion, and on this basis should have a smaller dipolar shift. However, we previously concluded that Na+ has a large dipolar angle, and this decreases the 23Nashifts relative to the 36Clshifts to the point where the 35Clshifts are comparable to (and for some lanthanides larger than) the 23Nashifts in LnDOTA solutions.

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ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981

The %C1shifts in LnDOTA solutions, unlike the 170shifts, show little or no evidence of a contact contribution. The C1-Ln interaction, like the D20-Ln interaction, could have some small covalent character, which would then give rise to a contact contribution to the 35Clshifts. Indeed, there is a substantial contact contribution to the %C1shifts in LnNOTA solutions (see below), and there is a contact contribution to the 170shifts in all three complexes; however, there is little or no contact contribution to the %C1shifts in LnDOTA solutions. We conclude that the chloride ion is farther away from the lanthanide ion in LnDOTA solutions than in LnNOTA solutions, so that the C1-LnDOTA interaction is entirely ionic. These results are consistent with the 170results, where a much smaller contact contribution was observed for water in LnDOTA solutions than in LnEDTA or LnNOTA solutions. It must be that the region above the LnDOTA complex is quite crowded, preventing the chloride ion or water molecule from approaching the lanthanide ion as closely as in Ln aquo, LnEDTA, or LnNOTA complexes. The chloride ion, because of its negative charge and larger radius, cannot approach the negatively charged LnDOTA complex as closely as the water molecule and experiences a negligible contact shift. In LnNOTA solution, the 35Clshifts follow a pattern which is clearly a sum of contact and dipolar shifts. When the dipolar shifts obtained by using separation method A2 (G = 0.0433) are subtracted from the measured shifts, the residual shifts fit the scaled theoretical contact shifts quite closely (Figure 7); the good fit demonstrates that the 35C1-electron coupling constant does not vary much along the lanthanide series. Chlorine-35 contact shifts have been observed previously for chloride ion in aqueous LnC13 solutions (32). A contact contribution to the LnNOTA 35Clshifts means that the chloride ion is coordinated to the lanthanide ion in LnNOTA complexes a t least some of the time. Although a number of studies show no detectable inner sphere chloride in Ln aquo complexes (28, 33), NMR contact shifts can be observed even when only a very small amount of inner sphere complex is present. If we assume that 170of the LnNOTA complexes have one chloride ion in the inner sphere (i.e., CSR.S = 1%of 0.1 M, or 0,001 M), then using eq 3, 4, and 10, we can estimate the spin density p a at the 35Clnucleus required to produce the observed contact shifts. The chloride ion concentration is 0.78 M, and F is 0.174 (Table 111);using CSR.~ = 0.001 M, and n = 1,we obtain pc1 = -1.6 X This spin density is similar to that obtained for directly coordinated nuclei in other systems; for example, po = -1.15 X lod for 170 of water in LnEDTA solutions (taking n = 3), and PN = -2.6 X for 14N of pyridine in L n ( d ~ madducts )~ (20). Thus the observed %C1contact shifts can be accounted for by a very small amount of chloride ion in the inner coordination sphere. The signs of the dipolar contribution to the 35Clshifts are the same in LnNOTA and in LnDOTA solutions, but the signs are opposite for the 23Nashifts in the two solutions. If we tried to account for the opposite signs of the "a shifts on the grounds that (r2)Az0was of opposite sign in the two complexes, then the signs of the dipolar %C1shifts should be opposite also. Since in fact the dipolar 35Clshifts have the same sign, it is clear that a change in sign of ( r Z ) A Zcannot 0 be the reason for the opposite signs of the 23Nashifts. The best explanation is that ON, < 54.7' in LnDOTA solutions, and > 54.7O in LnNOTA solutions, as stated in the section on 23Na shifts, and that Ocl < 54.7O for both LnDOTA and LnNOTA solutions. Aqueous Shift Reagent Utility. As all of our studies were on inorganic substrates, we felt it would be useful to include some organic molecules, thus verifying the utility of the LnDOTA complexes for organic substrates. To determine the

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-A

5

4

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i

3

7

P3M

Flgure 9. The 'H spectra of 0.1 M n-butylammonium ion in D,O, with dioxane reference: (A) in 0.1 M TrnDOTA, pH 9.0 (the HDO resonance is at 6.13 pprn. The small resonances due to excess DOTA ligand are under the a-proton and dioxane resonances); (B) no addition, pH 9.1.

1

5

4

L

2-

3

2

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. A -

I

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Figure 10. The 'H spectra of 0.1 M 1-butanol D20, with dioxane reference: (A) in 0.1 M T m W T A (the HDO resonance is at 6.19 ppm. One of the resonances due to free DOTA is under the left portion of the a-proton multiplet); (E) no additions. 6 v, ,D a CH,Ch,Ch,C!i,COO

5

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& .

4

..

A

2

3

_

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0

PPM

Flgure 11. The 'H spectra of 0.1 M n-valerate in D,O, with dioxane reference: (A) in 0.1 M TrnDOTA, pH 11.0 (the HDO resonance is at 6.26 ppm); (B) no additions, pH 11.9.

extent to which LnDOTA complexes would induce paramagnetic shifts in organic substrates, we measured the 'H shifts in TrnDOTA solution of three molecules containing the n-butyl group. Figures 9-11 show the results for the positively charged n-butylammonium ion (pK, = 10.6), the neutral 1butanol, and the negatively charged n-valerate ion (valeric acid pK, = 4.8). Paramagnetic shifts were observed for all three

ANALYTICAL CHEMISTRY, VOL.

charge types, with the two charged molecules showing the largest shifts. The 'H shift of H b O was also measured and, as expected, was the same as the *H shift of D20. For the n-butylammonium ion (Figure 9), the shifts of the a and P protons are to high frequency (the same direction as the water protons), while the shift of the 6 protons is to lower frequency. This result shows that the association of the nbutylammonium ion with the TmDOTA complex is similar to that of the Na+ ion. The positively charged alkyl ammonium ion is located near the negatively charged carboxylate oxygens of the TmDOTA complex, so that the protons closest to the nitrogen (the a and j3 protons) are in the same region as the Na+ ion (6 < 54.7") and have paramagnetic shifts of the same sign as the 23Nashifts of the Na+ ion in TmDOTA solution. The end of the alkyl chain extends down into the equatorial shift cone, so that the 6 protons have 0 > 54.7" and experience a paramagnetic shift opposite in sign to the a and P protons and to the water protons. The shifts of the 1-butanol protons (Figure 10) are small, and in the opposite direction to the water proton shift, with the 6 protons experiencing the largest shift. Clearly, the 1-butanol molecule does not compete with water for the region above the TmDOTA complex, but instead hydrogen bonds to the carbonyl oxygens, with the alkyl chain extending down into the region where 6 > 54.7". The 1-butanol molecule is thus farther away from the TmDOTA complex than the water molecule at 6 near O", which accounts for the smaller shifts, as well as the opposite sign of the shifts. The 6 protons, having OH nearest 90°, have the largest shift. The opposite direction of the 1-butanol and water shifts suggests an application of LnDOTA complexes for solutions prepared in H20, namely, shifting the large H 2 0 peak away from the alkyl resonances for better qualitative and quantitative analysis. In contrast to the 1-butanol results, the paramagnetic shifts of the n-valerate protons (Figure 11)are in the same direction as the water protons, with the protons nearest the carboxylate group having the largest shifts. This result shows that the association of the n-valerate ion with the TmDOTA complex is similar to that of the Cl- ion. The valerate carboxylate goup is located above the TmDOTA complex, so that the alkyl chain is in the axial shift cone; the valerate protons thus have paramagnetic shifts of the same sign as the C1- ion and the water protons. In this case, the a protons, which have OH nearest 0" and are closest to the carboxyl group of the valerate ion, have the largest shift. In all three spectra, the P and y resonances are partially or completely resolved, but our proposed analytical application of the LnDOTA and LnNOTA complexes has nothing to do with the resolution of overlapping resonances. The application is to the determination of the structure of molecules in solution, and for this purpose we would prefer to start with a spectrum having all multiplets resolved (e.g., obtain the 'H spectrum at 360 MHz) and then add an axially symmetric shift reagent to obtain purely dipolar paramagnetic shifts. The dipolar shifts of the substrate (if rigid) can then be fitted to the dipolar surface, and r and 6 can be calculated for each proton (and carbon if '3c spectra are obtained) in the substrate molecule. As outlined in the introduction, our major objective was to find an aqueous shift reagent whose adducts with substrate

53, NO. 9, AUGUST 1981

1425

molecules were axially symmetric and which induced only dipolar shifts in the substrate. Other desirable features were 1:l stoichiometry and a wide pH range. The LnDOTA complexes satisfy these requirements. They are axially symmetric, induce only dipolar shifts in Na+, C1-, and the 2H nuclei of D20,and induce largely dipolar shifts in the "0 nuclei of D20. In contrast to the Ln aquo, LnEDTA, and LnNOTA complexes, where hydroxide coordination is a problem in the basic pH region, LnDOTA complexes are usable to pH 13 or higher. Additionally, the eight-coordinate LnDOTA complexes are unlikely to form higher than 1:l complexes with substrate molecules.

ACKNOWLEDGMENT The authors thank Dennis S. Everhart and Ronald F. Evilia for providing the trisodium salt of NOTA. LITERATURE CITED (1) Barry, C. 9.; North, A. C. T.; Gbsel, J. A.; Williams, Robert J. P.; Xavier. Antonio V. Nature (London) 1971. 232. 236-245. Desreux. Jean F.: Reillev. Charles N. J . Am: Chem. SOC.1976. 98. 2105-2109. Elgavlsh, Gabriel A.; Reuben, Jacques J . Am. Chem. SOC.1977, 99, 5590-5591 - - - . - - - .. Sherry, A. Dean; Pascual, E. J . Am. Chem. SOC. 1977, 99, 5871-5876. Fazakerley, G. Victor; Linder, Peter W.; Reid, David G. f u r . J . 810chem. 1977, 81, 507-514. Izumi, Kunihiko Agric. Blol. Chem. 1960, 4 4 , 1623-1631. Dobson, Christopher M.; Williams, Robert J. P.; Xavier, Antonio V. J. Chem. Soc., Dalton Trans. 1974, 1762-1764. Elgavish, Gabriel A.; Reuben, Jacques J. Am. Chem. SOC. 1976, 98. 4755-4759. Geraldes, C. F. G. C. J . Magn. Reson. 1979, 36, 89-98. SouthwoodJones, Rosalind V.; Merbach, Andre E. Inorg. Chlm. Acta 1978, 30, 77-82. Nassimbeni, Luigi R.; Wright, M. Robert W.; van Niekerk, Jill C.; McCallum, Pamela A. Acta Crystallogr., Sect B 1979, 835, 1341-1345. Lee, Byungkook, Ph.D. Thesis, Cornell University, 1967. Hoard, J. L.: Lee, Byungkook; Lind, M. D. J . Am. Chem. SOC. 1965, 8 7 , 1612-1613. Horrocks, William Dew., Jr.; Sudnick, Daniel J . Am. Chem. SOC. 1979, 101. 334-340. Reilley, Charles N.; Good, Bennie W.; Desreux, Jean F. Anal. Chem. 1975, 47, 2110-2116. Lewb, W. Burton; Jackson, Jasper A.; Lemons, Joe Fred; Taube, Henry J . Chem. Phys. 1962, 36, 694-701. Golding, R . M.; Halton. Margaret P. Aust. J . Chem. 1972, 2 5 , 2577-2581. Bleaney, B. J . Magn. Reson. 1972, 6 , 91-100. Golding, R. M.; qykko, P. Mol. Phys. 1973, 26, 1389-1396. Reilley, Charles N.; Good, Bennie W.; Allendoerfer, Robert D. Anal. Chem. 1976, 48, 1446-1458. Desreux, Jean F.; University of Liege, 1979, personal communicatlon. Desreux, Jean F. Inorg. Chem. 1980, 19, 1319-1324. Everhart. Dennis S.; Evilia, Ronald F., unpublished work, University of New Orleans. Good, Ben& W., Ph.D. Thesis, University of North Carolina, 1978. Sherry, A. Dean; Yang, Ping P.; Mwgan, Leon 0. J . Am. Chem. Soc. 1980, 102, 5755-5759. McGarvey, B. R.; Kurbnd, R. J. "NMR of Paramagnetic Molecules", La Mar, Gerd N., Horrocks, William Dew., Holm, R. H., Eds., Academic Press, New York, 1973; p 559. Reuben, Jacques: Flat, Daniel J. Chem. Phys. 1969, 51, 4909-4917. Habenschuss, Anton; Spedding, Frank H. J . Chem. Phys. 1960, 73, 442-450. Albertsson, Jhgen; EMing, Inga Acta Clystalbgr., Sect. B 1977, 833, 1460-1469. Helmholz, L. J. Am. Chem. SOC.1939, 61, 1544-1550. Zaev, E. E.; Khalilov, L. M. Zh. Strukt. Khlm. 1980, 2 1 , 53-58. Barbaht-Rey. Francoise Helv. Phys. Acta 1969, 42, 515-538. Choppin, G. R. Pure Appl. Chem. 1971, 27, 23-41.

RECEIVED for review January 14,1981. Accepted May 11,1981, We gratefully acknowledge research support from the National Science Foundation and the F.R.N.S. of Belgium.