'%d
NMR Study of Cadmium(I1) Halide
Complexes
electron expansion term and therefore a low-field shift. Although the effect of electron redistribution in the thiocyanate anion may actually be rather complex it is likely that the large upfield variations following isothiocyanate bonding are mainly due to an increase of excitation energies. Such an increase arises from the bonding of the nitrogen lone pair with the lithium atom. Acknowledgment. The infrar:d measurements were conducted by DaniBle Paoli to whom grateful appreciation is expressed. References and Notes
J. Phys. Chem. 1978.82:2423-2426. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 08/25/15. For personal use only.
(1) M. S. Greenberg, R. L. Bodner, and A. I. Popov, J . Phys. Chem., 77. 2449 (19731. (2) Y. 'M. Cahen, P.'R. Handy, E. T. Roach, and A. I. Popov, J . Phys. Chem., 79, 80 (1975).
The Journal of Physical Chemistry, Vol. 82, No. 22, 1978 2423
G. E. Maciel, J. K. k n & , L. F. Lafferty, P. A. Mueller, and K. M a k e r , Inorg. Chem., 5, 554 (1966). W. J. Dewitte, L. Liv, J. L. Dye, and A. I. Popov, J . Solution Chem., 6. 337 (1977). d. J. Martin, J. P. Gouesnard,J. Dorie, C. Rabiller, and M. L. Martin, J. Am. Chem. Soc., 99, 1381 (1977). M. Chabanel, C. MBnard, and G. GuihBneuf, C. R . Acad. S d . , Ser. C , 272, 253 (1971); D. Menard and M. Chabanel, J. Phys. Chem., 75, 1081 (1975). A. Rykowski and H. L. Vanderplas, Rec. J . Roy. Neth. Soc., 94, 204 (1975). D. Paoli, M. Lucon, and M. Chabanel, Spectrochim. Acta, in press. 0. W. Howarth, R. E. Richards, and L. M. Venanzi, J . Chem. SOC., 3335 (1964). A. H. Norbury, Adv. Coord. Chem., 17, 246 (1975). D. Paoli and M.Chabanel, C. R . Acad. Sci., Ser. C , 264, 95 (1977). R. G. Pearson, J . Chem. Educ., 45, 643 (1968). J. L. Burmeister,R. L. Massel, and R. J. Phelan, Chem. Commun., 679 (1970). C . MBnard. 8. Woitkowiak. and M. Chabanel, C. R . Acad. Sci., Ser. C , 278, 553 (19j4).
Cadmium-113 Nuclear Magnetic Resonance Study of Cadmium(I1) Halide Complexes in Water and Dimethyl Sulfoxide Torbjorn Drakenberg,' Nils-Olof Bjork,' and Roberto Portanovat Division of Physical Chemistry, and Division of Inorganic Chemistry, Chemical Center, University of Lund, Lund, Sweden, and Istituto di Impianti Nuclear!, Universiti di Palerrno, Palermo, Italy (Received March 9, 1978; Revised Manuscript Received June 12, 1978)
'H NMR of the MezSO protons, 13CNMR of the Me2S0 carbons, and l13Cd NMR have been used to study the formation of various cadmium(I1) halide complexes in ivIezSO solutions. The same complexes have also been studied in water solutions by l13Cd NMR. The l13Cd chemical shifts for the various complexes have been evaluated from both solvents. The differences in the chemical shifts of the individual complexes between water and Me2S0 solutions are in agreement with the fact that the change from octahedral to tetrahedral complexes take place at different complexation steps in the two solvents.
Introduction X-ray diffraction measurements carried out for aqueous solutions of cadmium(I1) perchlorate' have shown that the hydrated Cd2+ion is coordinated to six water molecules with an octahedral configuration. On the other hand, it has also been shown that the tetraiodo complex, CdId2-, is a regular tetrahedron in aqueous solution.' In water, therefore, a change of coordination certainly occurs, and the thermodynamic functions AC,",AHj",and ASj" for the cadmium(I1)-halide complex formation in aqueous solution2 are consistent with a change in coordination taking place with the formation of the third complex, so that the initially octahedral hydrated Cd2+ion is converted to a tetrahedral halide complex. More recently, X-ray diffraction investigations on solutions of cadmium(I1) perchlorate in MezSO have shown that the solvated metal ion is coordinated to six Me$O molecules3 and that the tetraiodo complex is tetrahedrally ~ o o r d i n a t e d .The ~ changes in AGjo, AHjo,and ASj" for cadmium halide complex formation have also been interpreted as due to a change in coordination from a purely Octahedral configuration to a tetrahedral 0118,~s~ similar to that taking place in aqueous solution. In Me2S0,however, this change seems to occur at an earlier step than in ~ater.~,~ *Division of Physical Chemistry. 'Division of Inorganic Chemistry. t Instituto di Impianti Nucleari. 0022-3654/78/2082-2423$01 .OO/O
In an attempt to obtain more information about the conformational changes in the cadmium(I1) halide complexes, we decided to study these systems with various NMR methods and in the present work some results from the NMR studies on these systems will be presented. Information about the sensitivity of 'H, 13C, and l13Cd chemical shifts to variations of the cadmium(I1) species in solution is reported, and the advantage of l13Cd NMR compared to 'H and I3C is shown. Experimental Section (a) Measurements. The l13Cd NMR spectra were recorded on a modified Varian XL-100-15 spectrometer operating in the Fourier transform (FT)mode at 22.2 MHz and using 20-mm sample tubes. An external proton lock was used, and typical FT parameters were as follows: spectral width 8000 Hz, acquisition time 0.5 s, flip angle 20" (pulse width 30 p s ) and lo00 to 1OOo00 transients. The 13C NMR spectra were run on a Jeol FX 60 FT spectrometer at 15 MHz using a 10-mm sample tube containing acetone-d, as lock signal surrounding a 5-mm tube containing the sample. Typical FT parameters were spectral width 500 Hz, acquisition time 16 s, and 60" flip angle (9 11s). The 'H NMR spectra were run at 100 MHz on a Jeol MH 100 spectrometer. All spectra were recorded at 30 f 2 "C. (b) Chemicals. The hexasolvate Cd(MezSO)6(C104)z was prepared and analyzed as described beforea7Ammonium chloride (BDH), ammonium bromide (B&A), and am@ 1978 American Chemical Society
2424
The Journal of Physical Chemistry, Vol. 82, No, 22, 1978
4
4
A
I !
I/
400 .
'*/
\
\
Flgure 1. Observed line width of the 'I3Cd NMR signal as a function of the CI-:Cd2' ratio. The total cadmium concentrationwas held constant at 0.6 M.
J. Phys. Chem. 1978.82:2423-2426. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 08/25/15. For personal use only.
T. Drakenberg, N.-0. Bjork, and R. Portanova
monium iodide (Baker Analyzed) were dried at 120 "C; ammonium thiocyanate (Merck) and ammonium perchlorate (Baker Analyzed) were dried at 110 "C and used without further purification. Dimethyl sulfoxide was purified as described previ~usly.~ Solutions of Cd(I1) in Me$O were prepared by dissolving known quantities of Cd(Me2S0)6(C104)2, NH4C104,and the ammonium salt of the halide being studied in Me2S0. The water solutions were prepared by mixing adequate amounts of the two solutions containing (i) 0.1 M Cd(C10J2 and 3 M NaC104 and (ii) 0.1 M Cd(C10& and 3 M NaX. In some cases 1 M instead of 3 M of NH4C104and NaX were used.
Results In a first attempt to study the change in solvent coordination around the Cd2+ion in MezSO solution, proton NMR was used since it had been observed that the proton NMR signal from MezSO bound to AP+ was shifted ca. 0.5 ppm downfield from the position of the free Me2S0 signal? In that case, the exchange between free and metal bound Me2S0 was sufficiently slow to allow the observation of separate signals from these two sites. When the metal was Cd2+, however, the exchange was so fast that only one resonance could be observed. The effect of the addition of halide ions on the MezSO resonance, which is assumed to reduce the number of bound MezSO molecules, is too small (ca. 1 Hz) to allow any conclusion to be drawn regarding the change in coordination number. Since it is well known that 13CNMR is normally more sensitive than proton NMR to changes in the binding of the molecules studied, the 13Cshifts of the Me2S0 carbons were also measured. However, it is found that the change in the 13C shift of the methyl carbon in Me2S0 is of the same order of magnitude as for the proton signal. We therefore turned our interest to l13Cd NMR. We initially found that the l13Cd resonance for a solution of 0.3 M Cd(C104)2in pure Me2S0 is ca. 27 ppm upfield from that of a solution of 0.1 M Cd(C104)2in water. Kostelnik and Bother-Byg have shown that the Cd2' ion chemical shift is solvent dependent, and found an upfield shift when the solvent was changed from pure water to mixtures of water with MezSO, DMF, or acetone. When halide ions are added to these solutions, a marked downfield shift is observed both in aqueous and in MezSO solutions, in agreement with previous observati~ns.~J~ For the MezSO solution there is also a broadening of the signal. For a 0.6 M cadmium perchlorate solution a maximum broadening was observed after the addition of 0.3 M NaC1. In Figure 1 the l13Cd line width as a function of the ClCd ratio is shown. Maxima are found close to the C1:Cd ratios of 0.5 and 1.5 and also a shoulder at 2.5. This is the type of curve to be expected if the broadening is due to a slow exchange among the various c~mplexes.~'At a C1:Cd ratio of 0.5,
1 200 -
0
I
2 4 XICd Figure 2. 'I3Cd chemical shifts, in Me2S0 solutions, as a function of X-:Cd2+ ratio where X- is a halide ion: Cd2+ = 0.6 M, (0) CI-, (X) Br-, and (a)I-. Shifts in ppm downfield to 0.1 M Cd(CI0,) in water. The curves are calculated by means of eq 2 and the data given in Table 11.
Cd2+and CdCl+ are the only species present in appreciable amounts? and the rate of exchange can be calculated from the difference between their chemical shifts (58 ppm, see below) by means of T
= 2 A v l j 2 / ~6v2
(1)
Here T is the mean lifetime of the site, Avlj2 is the broadening due to exchange and 6 v is the chemical shift difference between the two sites, which has been determined as described below. A mean lifetime of ca. 2 X s is found for these species, which means a free energy of activation for the exchange reaction of 52.3 kJ/mol. In a similar way the free energy of activation for the exchange between CdCP and CdClz can be estimated to be-46.0 kJ/mol. This latter value is, however, much less reliable than that for the exchange between Cd2+and CdCP because no solution can be prepared combining only the species CdCP and CdClZa5 We have also observed that the broadening depends upon the cadmium(I1) concentration, being approximately doubled when the concentration is reduced from 0.6 to 0.3 M, whereas the observed shifts are not significantly affected by the total cadmium concentration at a constant halide to cadmium ratio. The broadening of the l13Cd signal for the bromide and iodide systems in MezSO is even larger than for the chloride. This shows that the exchange is even slower than for the chloride system, for if the rate were independent of the ligand, the broadening should decrease in the order C1> Br > I, as the chemical shift differences decrease in this order. The exchange rate mainly depends upon solvent, however, as there is no observable broadening in water solutions. A detailed discussion of the exchange processes is, however, outside the scope of this work and will not be further pursued. The observed l13Cd chemical shifts vs. the ligand-tocadmium ratio for the MezSO and water solutions, respectively, are shown in Figures 2 and 3. As discussed by Maciel et a1.l0 in their study of the chemical shift of 67Zn caused by the formation of chloride and bromide com-
'%d
The Journal of Physical Chemistry, Vol. 82, No. 22, 1978 2425
NMR Study of Cadmium(I1) Halide Complexes
PP
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f
I
1
2
3
4
5
L
6
7
8
XiCd
Figure 3. '%d chemical shifts, in water solutions, as a function of X-:Cd2+ ratio, where X- is a halide ion. Shifts measured in ppm downfield from 0.1 M Cd(C104)2in water: Cd2+ = 0.1 M, (0)CI-, ( 0 )Br-, (*) I-, and (0) SCN-. The curves are calculated by means of eq 2 and the data given in Table 11.
TABLE I: Stability Constants of Cadmium(I1) Halide Complexes in Water and Me,SO Solutions wateru Cl-
Br-
Me,SOb I-
logK, 1.59 1.76 2.08 log K, 0.64 0.59 0.70 log K, 0.18 0.98 2.14 log K , 0.38 1.60 3 M medium. Reference 14. erence 5.
C1-
Br-
I-
3.23 1.98 2.57 1.76
2.92 1.91 2.75 1.68
2.18 1.40 2.93 1.17
1M medium. Ref-
plexes, the chemical shift of the central atom in the individual complexes can be calculated if the equilibrium constants are known. For the cadmium halide complexes, the equilibrium constants are known in both waterl1-l4 and Me2S0,5and are collected in Table I. The constants determined, admittedly, do not refer to exactly the same media as used for the present measurements, since useful NMR spectra demand much higher cadmium concentrations than those used in the determination of the equilibrium constants. However, on the assumption that the constants are not changed appreciably by this change of medium, it is possible to calculate the concentration of each species in each solution. Hence, the various shifts 6i can be calculated from eq 2 by means of a least-squares procedure.
In eq 2 pi is the part of the cadmium present as the species CdXi and cYi the shift of l13Cd in this species. The resulting shifts are given in Table 11.
Discussion Available experimental results on l13Cd chemical shifts are limited,9J5J6J8-23 and as far as we have found in the literature there is only one paper dealing with the l13Cd chemical shifts of individual c0mp1exes.l~ Interest has primarily been directed toward cadmium(I1) complexes, in which the ligands are bound to cadmium via a sulfur atom. From the data it is, however, not possible to draw any conclusions regarding the effect of coordination number or geometry on the l13Cd shift. Most of the available data on l13Cd shifts in complexes are reported as observed shifts in a given solution, but with no attempt to evaluate the shifts of the various complexes p r e ~ e n t . ~ ~ ' ~ As discussed by Maciel et al.,1° the chemical shifts, calculated from observed chemical shifts and known complex constants, can never be better determined than the accuracy of the complex constants used. For the systems studied in the present work, the equilibrium constants are well known, but only for cadmium concentrations lower than those used in this work. The 0.1 M concentration used for the water solutions is not very far from the concentrations for which equilibrium constants have been determined, and the literature data are probably also valid for our solutions, with the possible exception for the Cd-C1 system, for which no complex constant for CdC1:- is given. In solutions with the highest C1- concentrations used in the present work, some of the fourth complex is probably formed, making the determination of the '13Cd shift for CdC13- uncertain. The value given should therefore be regarded as an upper limit. For the water solutions, where the observed shifts are accurate to about 0.1 ppm, the reliability of the calculated shifts for the individual complexes is completely determined by the accuracy of the equilibrium constants. To obtain an estimate of the effects of errors in the equilibrium constants, we ran two series of measurements for the Cd-I system with 1 and 3 M ionic strength, respectively. For both of these ionic media the equilibrium constants are known, and the calculated shifts when these constants were used are given in Table 111, together with the error square sum obtained in the calculations using eq 2. We then made another calculation using the constants for 3 M ionic strength for the shifts from 1 M ionic strength and vice versa. The results from these calculations are also given in Table 111. We first observe that all the shifts from the two series, when correct equilibrium constants are used, show satisfactory agreement. Secondly, the effect of using the incorrect set of equilibrium constants is to drastically change the calculated shift of the second complex (Cd12), and furthermore the error square sum is increased significantly above the value obtained when the correct set of equilibrium constants was used. These data thus indicate that the equilibrium constants are valid for our solutions, and that the shifts, except that for Cd12,are well determined. The reason for the ill-defined value of the shift for CdIz is that its population is always very low, a maximum of 5 and 10% of the total cadmium concen-
TABLE 11: I13CdChemical Shifts of the Cadmium(I1) Halide Complexes in Water and Me,SO Solutionsu ligand
solvent
Cdtot, M
&Cd2+
Clb
H*O
0.1
-1 - 27
c1
Me,SO
0.1-0.6
BrC HZO 0.1 -3.6 Br 0.6 - 27 Ib 0.1 -1 I Me,SO 0.6 - 27 SCNb HZO 0.1 -1 Shifts in ppm downfield from external 0.1 M Cd(ClO,), in water.
::to
CdL'
CdL
CdL,-
CdL, '-
63 31 62
98 204 83 373 47 286 85
282 398 186 28 5 122 124 101
442 399 351 71 44 174
11 46 -14 40
1 M ionic strength.
3 M ionic strength.
2426
The Journal of Physical Chemistry, Vol. 82,No. 22, 1978
TABLE 111: Calculated I 1 T dChemical S h i f t s for t h e C d - I System for Two D i f f e r e n t I o n i c Strengths a n d Two Sets of Equilibrium Constantsa i o n i c strength
i o n i c strength
3Mb
lMC
3 MC
-3.6 43.8 46.2 129.2 71.2 1.4
-1.3 46.4 46.9 121.7 70.6 0.94
-3.6 44.7 86.7 121.2 68.6 3.6
1 Mb
-1.1 43.1 8, 31.2 6 3 119.5 73.9 2SSd 5.0 S h i f t s in ppm d o w n f i e l d from e x t e r n a l 0.1 M Cd(ClO,), in water. l o g K , = 2.08, l o g K , = 0.70, l o g K,= 2.14, l o g K, = 1.60. l o g K,= 1.88, l o g K,= 0.78, l o g K, = 1.69, l o g K , = 1.28. T h e error square 6 0
6,
J. Phys. Chem. 1978.82:2423-2426. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 08/25/15. For personal use only.
sum.
trations for the 3 and 1 M ionic strength solutions, respectively. For the other ligands, all complexes are present to at least 30%, for some ligand concentration making the calculated shifts quite reliable. For the MezSO solutions, the situation is quite different, as high concentrations ((Cd), = 0.6 M) had to be used and the lines were, for the intermediate ligand concentrations, very broad. It is therefore difficult to estimate the accuracy in the calculated shifts. Let us assume that the equilibrium constants used are reasonably good, then the accuracy in the calculated shifts is determined by the accuracy of the observed shifts, which for some cases might be as poor as f5 ppm. Some test calculations indicate that it is only the shifts of the second complexes that are not well determined by more than f10 ppm. The error in the shifts of the second complexes might be as large as f50 ppm. The values of the chemical shifts derived for the different Cd(I1) species in water and Me2S0, reported in Table I, show that the effect of the first ligand on the l13Cd shift is almost solvent independent, and secondly that the effect of the second ligand is very different in the two solvents. The pronounced downfield shifts for the second complexes in MezSO indicate that with the formation of these complexes there is a change in the coordination number for the MezSO solutions, whereas no such changes take place with the formation of the second complex in water. For the water solutions, the pronounced downfield shifts occur for the third complexes. These observations thus indicate that the change in coordination takes place with the formation of the second complex in MezSO and with the third complex in water solution. This conclusion is further strengthened by other observations that the l13Cd NMR signal is shifted downfield for tetrahedral compared to octahedral ~omp1exes.l~ Our conclusion that a change in coordination number takes place with the formation of the second halide complex in Me2S0 and the third one in water is in perfect agreement with the conclusions drawn from equilibrium
T. Drakenberg, N.-0. Bjork, and R. Portanova
and calorimetric rnea~urements.~-~ The main conclusion that can be drawn from the results reported here deals with the role that metal NMR can play in studies of solution chemistry. We believe that the present work illustrates clearly the potential utility of lI3Cd NMR to study variations in coordination number and geometry of different cadmium(I1) species in solution. Before the full strength of this technique can be used, for '13Cd and other metal nuclei much more data of the kind presented in this work, must be collected. We hope, and believe, that in the near future it will be possible to use metal NMR data, at least on empirical grounds, in a more direct way to discuss the coordination geometry of various metal complexes in solution. Acknowledgment. The authors thank Professor S. Ahrland for his interest in this work and for helpful discussions. Dr. R. E. Carter is thanked for valuable linguistic criticism. This project was supported by the Swedish Natural Science Research Council.
References and Notes (1) H. Ohtaki, M. Maeba, and S. Ito, Bull. Chem. SOC. Jpn., 47, 2217 (1974). (2) S. Ahrland, Struct. Bonding, 15, 167 (1973). (3) M. Sandstrom, G. Johansson, S. Ahrland, and I. Person, to be published. (4) G. Johansson, personal communication. (5) S. Ahrland and N. 0. Bjork, Acta Chem. Scand., Ser. A , 30, 249 (1976). (6) S. Ahrland and N. 0.Bjork, Acta Chem. Scand., Ser. A , 30, 257 (1976). (7) S. Ahrland and N. 0. Bjork, Acta Chem. Scand., Ser. A , 28, 623 (1974). (8) A. J. Brown, D. A. Couch, 0. W. Howarth, and P. Moor, J . Magn. Reson., 21, 503 (1976). (9) R. J. Kostelnik and A. A. Bothner-By, J. Magn. Reson., 14, 141 (1974). (10) G. E. Maciel, L. Simeral, and J. J. H. Ackerman, J . fhys. Chem., 81, 263 (1977). (11) C. P. Vanderzee and H. J. Dawson, J . Am. Chem. Soc., 75, 5659 (1953). (12) P. Kivalo and P. Ekari, Suom. Kemi. 6 , 30, 116 (1957). (13) P. Gerding, Acta Chem. Scand., 22, 1283 (1968). (14) P. Gerding, Acta Chem. Scand., 20, 79 (1966). (15) R. A. Haeberkorn, L. Que, Jr., W. 0. Gillum, R. H. Holm, C. L. Liu, and R. C. Lord, Inorg. Chem., 15, 2408 (1976). 16) A. Catdin, P. D. Ellis, J. D. Odom, and J. W. Howard, Jr., J. Am. Chem. Soc., 97, 1672 (1975). 17) After a suggestion from one of the referees that the variation in line width might be caused by viscosity variations we measured TI for two solutions: 0.6 M Cd(C10& and 0.6 M Cd(ClO,& 1.2 M NH,CI in Me,SO. The T,values found were ca. 25 and ca. 15 s,respectively, showing that there is no line broadening caused by short relaxation times and no effect from variations in viscosity should be expected. 18) P. D. Ellis, Third European Experimental NMR Conference, Elslnor, 1977. (19) I.M. Armitage, R. T. Pajer, A. J. M. Schoot Uiterkamp, J. F. Chlebowski, and J. E. Coleman, J. Am. Chem. SOC.,98, 571 (1976). (20) J. L. Sudmeier and S. J. Bell, J . Am. Chem. Soc., 99, 4499 (1977). (21) J. F. Chlebowski, I. M. Armitage, and J. E. Coleman, J . Bloi. Chem., 252, 7053 (1977). (22) B. Birgersson, R. E. Carter, and T. Drakenberg, J . Magn. Reson., 28, 299 (1977). (23) I. M. Armitage, A. J. M. Schoot Uiterkamp, J. F. Chlebowski, and J. E. Coleman, J . Magn. Reson., 29, 375 (1978).
+
