J. Phys. Chem. B 1997, 101, 3919-3926
3919
NMR Spectroscopic Studies on Interactions of Tripositive Ions of Cobalt(III) and Chromium(III) Complexes in Micellar Solutions Yuri Mizuno and Masayasu Iida* Department of Chemistry, Faculty of Science, Nara Women’s UniVersity, Kitauoya-nishi-machi, Nara 630, Japan ReceiVed: May 28, 1996; In Final Form: NoVember 1, 1996X
Interactions between micelles and tripositive ions of metal complexes such as [M(en)3]3+ (M ) Co or Cr, en ) ethylenediamine), [M(chxn)3]3+ (chxn ) (R,R)-1,2-cyclohexanediamine), and [M(phen)3]3+ (phen ) 1,10-phenanthroline), were studied using 59Co NMR spectroscopies and paramagnetic Cr(III)-induced 13C NMR relaxations of the surfactants. The magnitudes of the changes in 59Co chemical shifts and relaxation rates for [Co(en)3]3+ () en complex) and [Co(chxn)3]3+ () chxn complex) were dependent on the type of the head group: The carboxylate and alaninate surfactants caused larger changes in the 59Co NMR parameters than the sulfonate and sulfate ones did. The 59Co longitudinal and transverse relaxation rates were measured for the typical systems; these results gave effective 59Co electric field gradients (efgs) and effective correlation times for the three complex ions in the micellar solutions. The effective 59Co efgs obtained for the en and chxn complexes were larger in the carboxylate micelles than in the sulfonate and sulfate micelles. This result is in accord with the larger changes in the 59Co chemical shifts. The paramagnetic Cr(III)-induced 13C relaxation studies for each carbon of the surfactants suggest that [Cr(chxn)3]3+ is located at the micellewater interface as well as the en complex. The correlation times and their activation energies also gave useful information on the selective interactions of the metal complex ions in the micellar solutions.
Introduction Specific interaction of metal complex ions in micellar systems is of fundamental importance since extensive functionalities of metal complex ions are enhanced in micelles.1 NMR spectroscopy is a very powerful tool for studying microscopic behavior of molecules in complicated solutions.2,3 Therefore, we can expect to get useful information concerning an extent of binding and environment of metal complex ions in micelles using NMR. The mode of the interactions of cobalt(III) complexes with amphiphilic systems is monitored by 59Co NMR chemical shifts which are sensitive to the extent of solvation of the cobalt(III) complex ion:4 One of the present authors (M.I.) with co-workers has found that if cobalt(III) complex cation is present in the hydrophobic region of anionic amphiphiles the 59Co signal tends to shift downfield relative to the “free” hydrated state.5 This trend is consistent with the relationship between the donor numbers of solvents and the 59Co chemical shifts for [Co(en)3]3+; that is, less solvation of the cobalt(III) complex ion will make the 59Co NMR resonance downfield shift.4 In the present study, we deduce the location of solubilization and the extent of the interactions in micelles for [M(en)3]3+, [M(chxn)3]3+ , and [M(phen)3]3+, where M denotes Co or Cr. We focus on the utility of the 59Co NMR parameters in micellar systems to see selective interactions between the metal complex ions and the head group. The phen complex is the most hydrophobic among them and is revealed to be dissolved in the hydrophobic cores of micelles.6,7 The en complex is much more hydrophilic and it may interact with micelles at the aqueous side. On the other hand, the chxn complex which has both hydrophilic (having amino protons in the C3 axis) and hydrophobic sites (having cyclohexane ring in the ligands) is not known in which side (outside or inside) of micelles it is located. The chxn and en complexes have similar common features except for the bulkiness and hydrophobicity of the X
Abstract published in AdVance ACS Abstracts, April 15, 1997.
S1089-5647(96)01541-6 CCC: $14.00
ligands. They have analogous hydrophilic (diamine) sites, their association constants with the sulfate ion are comparable each other,8 and their 59Co chemical shifts are changed similarly due to the sulfate and oxalate ions.9,10 The location of the complex ions in micelles was deduced by 13C paramagnetic relaxations of the surfactants in the presence of the corresponding chromium(III) complexes. This method is available for the chromium(III) complexes if the relaxation time is simply proportional to the distance of the chromium(III) center from the observed carbon nuclei.11-13 Since cobalt(III) and chromium(III) complexes having the same ligands show very similar physical properties in solution,8 it is reasonable to deduce the positions of the cobalt(III) complexes from those of the corresponding chromium(III) complexes. The paramagnetically induced 13C relaxation studies were thus carried out for the sulfonate-surfactant systems in the presence of [Cr(chxn)3]3+, [Cr(OH2)6]3+, [Cr(en)3]3+, [Cr(phen)3]3+, and [Cr(acac)3], whose locations in micelles have been known except for [Cr(chxn)3]3+. The interaction of the cobalt(III) complex ions in micelles was also studied by 59Co NMR relaxation time measurements. Although the dominant process of 59Co NMR relaxation is generally quadrupolar in origin, other mechanisms are often effective on the relaxations of the symmetric ions.14 For the amine complexes, there are significant contributions to transverse relaxation rate (R2) from scalar coupling to 14N(R2SC),15 and for [Co(NH3)6]3+ the temperature dependence suggests the presence of the spin-rotation mechanism (R2SR).16 If the quadrupolar relaxation is dominant, the longitudinal (T1) and transverse (T2) relaxation rates are to a first approximation expressed as17,18
T1-1(ω0) )
2π2 2 χ (0.2J(ω0) + 0.8J(2ω0) 49
© 1997 American Chemical Society
(1)
3920 J. Phys. Chem. B, Vol. 101, No. 20, 1997
T2-1(ω0) )
Mizuno and Iida
2π2 2 χ (0.3J(0) + 0.5J(ω0) + 0.2J(2ω0) (2) 49
where ω0 is the Larmor frequency (ω0 ) 2πν0 where ν0 is the observation frequency), χ is the quadrupole coupling constant in frequency units, and J(ω0) is the reduced spectral density function which describes the frequency dependence of the molecular motions modulating the quadrupolar interaction. The 59Co (I ) 7/ ) NMR relaxation is nearly exponential provided 2 that the effective spectral density is only weakly frequency dependent (i.e., under nearly extreme narrowing conditions).17 When isotropic rotational motions dominate the relaxation, the reduced spectral densities have the following Lorentzian form:
J(ω0) )
τC 1 + (ω0τC)2
(3)
where τC is the correlation time for isotropic rotation of the cobalt(III) complex ion and follows the Arrhenius relationship. Equations 1 and 3 say that T1-1 increases with an increase of temperature below τC ) 0.62ω0-1 (where τC is around 2 ns, in the present case). In order to check the dominant process of the relaxation, it seems useful to measure the temperature dependences of both relaxation times (T1 and T2), since ln(T1-1) and ln(T2-1) increases with a decrease in temperature in the above condition while ln(T2SC-1) and ln(T2SR-1) increases with an increase in temperature as follows.15,16,19,20 For the complex ions studied here, the T2SC values may be effective on T2 and can be expressed as
( ) 1 T2SC
Co
2 ) (2πJ)2T1N 3
(4)
where J is the cobalt-nitrogen coupling constant in hertz and T1N the nitrogen longitudinal relaxation time in the case where it relaxes rapidly compared with any chemical exchange. The temperature dependences of τC and T1N-1 should follow the Arrhenius relationship. If the contribution of T2SC to T2 is significant, the Arrhenius plot of T2 will be curved as has been reported for [Co(en)3]3+ in aqueous solution.21 In colloidal systems, however, the longitudinal relaxation times (T1N or T1) are appreciably smaller than those in aqueous solution and therefore the contribution of T2SC-1 to T2-1 should also be much smaller. The relaxation for the ions interacting with micelles is usually under nearly extreme narrowing condition.2 In this case, 59Co NMR relaxation processes can be described by comparing the longitudinal (T1) and transverse relaxation times (T2). From these relaxation times, we tried to estimate the correlation time (τC) and the quadrupolar coupling constant (χ), which provide useful information on the selective interactions of the complex ions in micelles. Experimental Section The complexes of [Co(en)3]Cl3‚3H2O,22 [Cr(en)3]Cl3‚3H2O,23 [Co(chxn)3]Cl3‚5H2O,24 [Cr(chxn)3]Cl3‚3H2O,25 [Co(phen)3]Cl3‚7H2O,26 [Cr(phen)3]Cl3‚6H2O,27 and [Cr(acac)3]28 were prepared by literature methods. The purification was confirmed by the absorption spectra and by SP-Sephadex column chromatography. The sodium 1-alkanesulfonates (NaAlkaneSO3) and 1-dodecyl sulfate (SDS) were guaranteed reagents of Tokyo Kasei Ltd and of Nacalai Chemicals for quantitative analysis, respectively. Potassium dodecanoate (KDodec), sodium Ndodecanoylalaninate (NaDDA), and sodium N-octanoylalaninate (NaOA) were prepared as previously reported.5 The 59Co and
13C
NMR spectra were measured with a JEOL GX-270 FT NMR spectrometer operating at 64.6 and 67.9 MHz, respectively. The concentration of the cobalt(III) complex ion was always adjusted to 5 × 10-3 mol dm-3. In the paramagnetic chromium(III)-induced relaxation studies, the surfactants used are the sodium salts of 1-hexanesulfonate, 1-octanesulfonate, and 1-decanesulfonate. 13C NMR were measured for 0.3 or 0.8 mol dm-3 (only for 1-hexanesulfonate) of the surfactants in the presence (3-10 mmol dm-3) and absence of the chromium(III) complexes. The longitudinal relaxation times (T1) were determined by the usual inversion-recovery method, using the pulse sequence of (-180° pulse-t-90° pulse-T-)n. For measurements of T1, 8-10 different pulse intervals (t) were used with a waiting time (T) of more than 8T1. The 59Co 90° pulse widths were typically 13-15 µs. When the relaxation time is too small, there is a possibility that 90° pulse widths and predelay (dead) times are effective on the relaxation times. We therefore changed these parameters for the system in the measurements (e.g., the optimum 90°-pulse widths were changed from 13-14 to 22-30 µs in combination with a change in probe-tuning dials and then the predelay times were also changed from 60 (usual condition) to 5 µs). Conclusively, the effects of these parameter changes on the relaxation times were within experimental errors. The relaxation rates (R1) were obtained as the reciprocal of T1. The transverse relaxation rates (R2) were also obtained from the linewidths (∆ν1/2) using the relationship, R2 ) π∆ν1/2. In order to check the relaxation mechanism, we measured temperature dependences (20-45 °C) of the relaxation rates (R1 and R2) for the typical systems. In the paramagnetic-induced relaxations, the inverse relaxation times (T1M-1) were simply obtained as T1M-1 ) T1-1 - T10-1, where T1 and T10 are the relaxation times in the presence and absence of the chromium(III) complexes, respectively. The solvent for the 13C NMR measurement was 5% D2O-95% H2O for the deuterium lock and sample solutions for the 13C relaxation measurements were bubbled with argon gas for 10 min before each NMR measurement in order to remove dissolved oxygen gas. The temperature of the sample solution was controlled at 27 ( 0.5 °C for the NMR measurements except for the temperature-dependence studies. The kinematic viscosities (ν) for micellar solutions were measured with a Cannon-Fenske capillary viscometer at 27 ( 0.1 °C. The shear viscosity (η) was obtained from the relationship ν ) η/F, where the density (F) was measured with an Ostwald-type pycnometer. Results and Discussion 59Co
NMR Chemical Shifts in Micellar Solutions. The chemical shift dependence on various surfactant concentration is shown in Figure 1 where broken lines (parts a and b) indicate that precipitation occurs. For these cases the complex ions are dissolved above the cmcs. (The cmc values are listed in Table 1.)29-31 For the phen complex, there is a steady downfield change in the chemical shift below the cmc. However, the chemical shift remains almost constant above cmc (Figure 1c). The exception is for the case of the 1-hexanesulfonate where the cmc is not so clear as for the other surfactants.29 Figure 1, a and c, clearly shows that the interactions with micelles cause an upfield shift for the hydrophilic ion, [Co(en)3]3+, while they cause a downfield shift for the hydrophobic ion, [Co(phen)3]3+. On the other hand, for [Co(chxn)3]3+, an intermediate situation was obtained. The micelles having either a sulfonate or a sulfate head group (referred to as “sulfonate/ sulfate micelles” hereafter) caused a downfield shift for 59Co resonance from the free hydrated complex ion in a fashion 59Co
Tripositive Ions of Co(III) and Cr(III)
J. Phys. Chem. B, Vol. 101, No. 20, 1997 3921 TABLE 1: Critical Micelle Concentrations of Surfactants in Aqueous Media
Figure 1. 59Co chemical shifts (δ) of the cobalt(III) complex ions as functions of the surfactant concentrations. The lines are drawn as an aid to the eye. See the Experimental Section for the abbreviations of the surfactants. (a) [Co(en)3]3+, (b) [Co(chxn)3]3+, (c) [Co(phen)3]3+.
compound
temp/°C
cmc/mol dm-3
ref
NaHexaneSO3 NaOctaneSO3 NaDecaneSO3 SDS NaDDA KDodec
25 40 40 40 40 25
0.5-1.0 0.16 0.040 0.0086 0.012 0.026
29 30 30 30 31 31
similar to the phen complex case, while the micelles having carboxylate or alaninate head group (referring to as the “carboxylate micelles” hereafter) have almost the same effect on the 59Co chemical shift as that for the en complex. This trend suggests that the variation in the 59Co chemical shift reflects specific changes in the solubilization environment (extent of the hydration or of the ion pairing) about [Co(chxn)3]3+ in the different micelles. In the carboxylate-derived micelles, both [Co(en)3]3+ and [Co(chxn)3]3+ would interact with the carboxylate group by the hydrogen bonds using their amine protons as well as by the electrostatic force; this strong interaction will fix the position of these complex ions in the hydrophilic region of the micelles. The 59Co NMR signal for the cobalt(III) complex cations tends to shift downfield if the hydrophobic interaction acts additively with the electrostatic interaction.5 Thus it is possible that the chxn complex ion is present in the hydrophobic cores of the “sulfonate/sulfate micelles”. Alternatively, hydrogen bonds of this complex with water molecules are weakened by ion pairing interactions, as has been reported from chemical shift changes for the probe ions, e.g., 23Na ion by sulfonate/ sulfate micelles,32,33 39K ion by SO42- or CrO42-,34 and [59Co(en)3]3+ by I-.9 In these cases, the anions have structurebreaking character so that the hydration around the cations can be reduced by the ion pairing with these anions. This case is also probable in the interaction between [Co(chxn)3]3+ and the sulfonate/sulfate head group, since the chxn complex is somewhat hydrophobic and the sulfonate/sulfate head group is weakly hydrated. Therefore, it is not easy to deduce whether [Co(chxn)3]3+ is present in the hydrophilic region or in the hydrocarbon cores of the sulfonate/sulfate micelles from the 59Co chemical shifts. In order to see the location of [Co(chxn)3]3+ in micelles, paramagnetic Cr(III)-induced 13C relaxations for the surfactants will be discussed in the next section. Anyway, the 59Co chemical shifts suggest that the changes in hydration around [Co(en)3]3+ and [Co(chxn)3]3+ occurs to a less extent in the sulfonate/sulfate micellar systems than in the carboxylate system. This trend is stronger for the chxn complex which has more hydrophobic ligands. The appreciable differences in the interactions of the chxn complex between the carboxylate and sulfonate/sulfate micellar systems that appeared in the 59Co chemical shifts can be attributed to the double-faced (hydrophilic and hydrophobic) feature of this complex and to the smaller hydration of the sulfonate/sulfate head group than that of the carboxylate one. Figure 1a-c shows some other trends concerning the 59Co chemical shift changes as follows. (1) The magnitude of the chemical shift change is larger in the dodecanoate (KDodec) system than in the alaninate (NaDDA) system, probably because of the higher charge density of the head group due to its smaller size. (2) Except for some cases (the above case (1) and a few cases of the sulfonate/sulfate systems for the en and chxn complexes), the chemical shift changes increase with an increase in the alkyl chain length of the surfactant. The second trend follows a general tendency that the counterion binding is stronger for larger micelles. For [Co(phen)3]3+, which is strongly localized in the hydrocarbon core, the surfactants having
3922 J. Phys. Chem. B, Vol. 101, No. 20, 1997
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longer alkyl chains seem to facilitate the interaction with this complex. This complex follows the above two trends more faithfully than the other two complexes. This may be because specific interactions, such as direct bonding contacts with the head groups of the micelles, are less likely to occur. 13C Relaxations Induced by Paramagnetic Chromium(III) Complexes. As the chemical shift results cannot determine the position of [Co(chxn)3]3+ in the sulfonate/sulfate micelles, we measured the 13C relaxation times of the sulfonate systems in the presence of the chromium(III) complexes. Although detergents we used are 1-decanesulfonate and 1-octanesulfonate, we here show the results only for the octyl derivatives, since their spectra are simpler than those of the longer chained analogues while the results are very similar. An addition of chromium(III) complexes appreciably decreases the relaxation times. The geometrical relationship between the paramagnetically induced 13C NMR relaxation times (T1M) and the timeaveraged distance (〈d〉) of 13C nuclei from the center of the paramagnetic ions can be expressed as relative values of the ith carbon to those of the jth carbon as follows.11,12,35
〈di-6〉fi(ω0, τC)
(T1M)i-1 ) (T1M)j-1
〈dj-6〉fj(ω0, τC)
(5)
where fi(ω0,τC) is the spectral density for the i-carbon as a function of both the frequency of the spin transition (ω0) and the correlation time (τC) for the motion that modulates the interaction. It is necessary to consider the physical meaning of the dipolar correlation time, τC. In the absence of chemical exchange phenomena, the dipolar correlation rate is expressed as
τC-1 ) τs(e)-1 + τR-1
(6)
where τs(e)-1 () 2 × 109 s-1 for the hexaaqua ion35) is the electronic relaxation rate for Cr(III) and τR-1 is the appropriate rotational correlation rate. We cannot exactly estimate the τs(e) values for each chromium(III) complex and they may be significantly different from that for the hexaaqua ion; however, these values may be comparable in a comparison between analogous complex ions. Furthermore, the rotational correlation times (τR) for the segmental motions of the alkyl chains of micelles are around 10-9 s and thus the differences in τR between the positions of the carbon atoms (i) may also contribute to the τC-1 and then to the fi(ω0,τC). Previously, Cabane measured12 13C relaxation rates of SDS micelles in the presence of Co2+ (τs(e)-1 ) 1012 s-1) and Mn2+ (τs(e)-1 ) 108 - 109 s-1) which are typically extreme two cases of the paramagnetic ions concerning the τs(e)-1 values. In this study it was shown that the 13C relaxation rates of SDS are governed by the distances (di) between the measuring nuclei and paramagnetic ions and that the plots of relative 13C relaxation rates vs the carbon number show similar profiles between the two kinds of the paramagnetic ions in spite of using the two extreme cases. This is because the di enters the relaxation equation to the sixth power while the τR values for the respective carbon atoms in the surfactant change at most by a factor of 5.12 Therefore, it is reasonable that a comparison of T1M-1 values between the chromium(III) complexes (the differences of τs(e)-1 between the chromium(III) complexes should be less than those between Co2+ and Mn2+) can be used to estimate their rough positions in the micelles. Figure 2a-c shows the 13C relaxation times for each atom of the 1-octanesulfonate in micelles depending on the chromium(III) complex concentrations. Figure 3 is a similar plot for the 1-octanesulfonate in a comparison between the chromium(III)
Figure 2. 13C relaxation rates for each atom of the sodium octanesulfonate in its micellar solutions (0.3 mol dm-3) depending on the concentrations of the chromium(III) complexes (M ) mol dm-3). The signals for the C4 and C5 atoms are overlapped. (a) [Cr(en)3]3+, (b) [Cr(chxn)3]3+, (c) [Cr(phen)3]3+.
complexes. It is noticeable that the relaxation rates show weak dependences on the carbon positions, since the ion location may be considerably distributed in the micelles; here we know the highest probability position of the chromium(III) complexes. The profiles of T1M-1 vs carbon-number plot can be classified into two categories: one group is for the hexaaqua, en, and chxn complexes and the other is for the phen and acac complexes. This result suggests that the position of the chxn
Tripositive Ions of Co(III) and Cr(III)
J. Phys. Chem. B, Vol. 101, No. 20, 1997 3923
Figure 3. 13C relaxation rates for each atom of the sodium octanesulfonate in its micellar solution (0.3 mol dm-3) depending on the kinds of the chromium(III) complexes. All the complex concentrations are 5 mmol dm-3.
complex in the sulfonate micelle is similar to that of the hydrophilic complex ions. Figure 2c shows that, with increasing [Cr(phen)3]3+ concentrations, the T1M-1 values become the largest at the C4-C6 carbons of the surfactant. This position may be optimum by considering both molecular size of [Cr(phen)3]3+ and the electrostatic attraction between the chromium(III) center and the headgroup. From this analogy, the profiles of Figure 2b ([Cr(chxn)3]3+) similar to those of Figure 2a ([Cr(en)3]3+) suggest that [Cr(chxn)3]3+ is also present at the waterheadgroup interface. Therefore, we conclude that the downfield shift of 59Co spectra of [Co(chxn)3]3+ in the sulfonate/sulfate micelles will be due to the dehydration of the hydrophobic chxn complex by the interaction with the sulfonate/sulfate micelles. 59Co NMR Relaxation Studies. Figure 4 a-c shows the R1 values for the respective cobalt(III) complex ions as a function of surfactant concentrations. Most of the systems measured here are the same as those used for the chemical shift studies. (As the reproducibility of the relaxation times measured for [Co(chxn)3]3+ in 0.4 mol dm-3 of KDodec was not good because precipitation occurs near this concentration, this data was omitted from Figure 4b.) A characteristic of the present results is that the R1 values for [Co(chxn)3]3+ and [Co(en)3]3+ associated with the carboxylate micelles are significantly larger than those from the sulfonate/ sulfate micelles. There is a rough correlation between the chemical shift and the relaxation rate for the three complexes; i.e., the system having the larger relaxation rate also shows the higher field shift of the 59Co resonance. However, this relationship does not hold in the NaDDA micelle compared to the KDodec micelle: The 59Co relaxation rate is larger in the NaDDA micelle than in the KDodec system, although 59Co chemical shift change is smaller. The reason for the chxn complex will be discussed later. In order to obtain more details of the relaxation mechanism, the influence of temperature on the relaxation times (T1 and T2) was measured for the typical systems (Figure 1s). These typical systems were composed of 0.4 or 0.6 mol dm-3(only for the KDodec in the chxn complex system) surfactant where the relaxation rates are almost independent of the surfactant concentrations. It is characteristic that the plot is curved only for the en complex in aqueous solution.21 Furthermore, in the aqueous solutions of the chxn and phen complexes, the temperature dependence of R2 is clearly smaller than those of R1 and the absolute R2 values are always larger than the R1
Figure 4. 59Co longitudinal relaxation rates (R1) as functions of the surfactant concentrations. The lines are drawn as an aid to the eye. The broken lines mean the region of the precipitation. (a) [Co(en)3]3+, (b) [Co(chxn)3]3+, (c) [Co(phen)3]3+.
values. These results suggest that in these two systems the contribution of the 14N-59Co scalar coupling (R2SC) to the R2 values is significant. On the other hand, in the micellar systems,
3924 J. Phys. Chem. B, Vol. 101, No. 20, 1997
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TABLE 2: 59Co NMR Longitdudinal (R1) and Transverse (R2) Relaxation Rates at 27 °C for the Cobalt(III) Complex Ions in the Micelles complex [Co(en)3]3+
[Co(chxn)3]3+
[Co(phen)3]3+
surfactanta
τeff/ns
χeff/MHz
1.0 ( 0.2 1.2 ( 0.2 1.8 ( 0.3 1.4 ( 0.3 1.3 ( 0.3 0.8 ( 0.2 1.0 ( 0.2 1.4 ( 0.3 2.1 ( 0.3 1.5 ( 0.3 1.0 ( 0.2 1.3 ( 0.2 1.5 ( 0.3 1.7 ( 0.3 1.6 ( 0.3
1.5 ( 0.1 1.7 ( 0.1 1.7 ( 0.05 2.5 ( 0.1 2.5 ( 0.1 2.1 ( 0.1 2.2 ( 0.1 2.3 ( 0.1 3.2 ( 0.05 3.3 ( 0.1 4.0 ( 0.2 4.1 ( 0.2 4.4 ( 0.1 4.4 ( 0.1 4.1 ( 0.1
R1/s-1 R2/s-1
NaOctaneSO3 NaDecaneSO3 SDS NaDDA KDodec NaOctaneSO3 NaDecaneSO3 SDS NaDDA KDodecb NaOctaneSO3 NaDecaneSO3 SDS NaDDA KDodec
614 826 790 1704 1686 1059 1287 1464 2703 3096 4167 4608 5376 5465 4762
801 1178 1403 2639 2435 1272 1649 2258 5466 4909 5498 6676 8718 9424 7854
a The concentration of the micelles is 0.4 mol dm-3, except for the cases labeled b. b KDodec ) 0.6 mol dm-3.
the slope for the 59Co R1 value is always comparable to or smaller than that for the corresponding 59Co R2 value; we can consider that the relaxations are mainly governed by the quadrupolar mechanism. A comparison of the aqueous and micellar systems is consistent with the explanation for eq 4, i.e., although a contribution of R2SC to the R2 values is significant for the three complexes in the aqueous solutions (it is the largest for the en complex), the magnitude is slight and it becomes negligible in the micellar solutions. Only in the case of the en complex in the octanesulfonate system is the slope for the R2 value slightly smaller than that for the R1 value. In this system the contribution of the scalar coupling (R2SC) to the R2 may still be somewhat effective. The R2 values at 27 °C are listed in Table 2 together with the R1 values. We can in principle discuss these values on the basis of eqs 1-3. In the isotropic micellar system where the quadrupolar interaction is partially averaged by a fast and slightly anisotropic motion, the simplest model can be expressed on the basis of a two-step spectral density function as17,18,36
J(ω0) ) (1 - A)τf +
AτS 1 + (ω0τS)2
(7)
where A()S2) is a squared order parameter, τf is a correlation time characterizing the fast motion, and τS is the correlation time for tumbling of the micelles. Equations 1, 2, and 7 should be applicable for the description of the dynamic behavior of cobalt(III) complex ions in spherical, i.e., semidilute solutions of micelles (below 0.5-0.6 mol dm-3 for usual surfactant solutions).36,37 In eq 7, A takes values between 0 (no immobilization) and 1 (complete immobilization) and in the micellar system this value may be closer to 1. Furthermore, if τf is assumed to be much smaller than τS, the first term on the right-hand side of eq 7 is negligible in spherical micellar systems and then we can write the following equation for the spectral density.
J(ω0) )
AτS 1 + (ω0τS)2
(8)
We use here the residual quadrupole coupling constant remaining after the rapid averaging process defined as
χeff ) Aχ
(9)
Furthermore, as has been previously pointed out,18,21 the correlation time (τS) possibly includes contributions (τf) arising
TABLE 3: Viscosities of the Micellar Solutions surfactant
η/mPa s
NaOctaneSO3, 0.4 mol dm-3 NaDecaneSO3, 0.4 mol dm-3 SDS, 0.4 mol dm-3 NaDDA , 0.4 mol dm-3 KDodec, 0.4 mol dm-3 KDodec, 0.6 mol dm-3
1.00 1.37 1.77 1.74 1.44 2.12
from ions in rotationally nonrestrictive environments on the micellar surfaces. We therefore use effective correlation times τeff in place of τS. The R1 and R2 values in Table 2 give the two parameters τeff and χeff using eqs 1, 2, 8, and 9 for the respective systems; the results are listed in Table 2. As the τeff value means the rotational correlation time, this value is in principle proportional to the viscosity according to the StokesEinstein-Debye equation. The rotational motions of [CoL3]3+ (L ) en, phen, and 2,2′-bipyridine) have been previously studied on the basis of the following Stokes-Einstein-Debye equation38,39
τ ) CVη/kT
(10)
where V is the volume of the metal complex ions, η the shear viscosity of the solution, k the Boltzmann constant, T the absolute temperature, and C an experimentally determined dimensionless parameter which is concerned with the shape of the rotating molecule and the hydrodynamic boundary condition. It has been revealed that the rotational motions of the complex ions are nearly isotropic and the C value in eq 10 is 0.83 (for the phen complex) - 1.0 (for the en complex).38,39 That is, the metal complex ions used here would almost follow the StokesEinstein-Debye equation. Therefore, τeff is also proportional to the shear viscosity (η) unless a specific interaction between the metal complex ion and the micelle is present. The viscosities for the respective micellar solutions were thus measured and the results are given in Table 3. Except for a few systems, a rough correlation between τeff and η is seen for each complex as described in Figure 5. As stated above, if the contribution of the scalar coupling (R2SC) to the R2 is still present in the micellar solutions, the R2 values obtained are larger than those dominated by the quadrupolar relaxations and therefore the estimated τeff value becomes larger than the true values. This effect may be the largest for the en complex in the octanesulfonate and so the τeff value in this system (Figure 5a) may be too large. Therefore, in Figure 5a, we cannot clearly say whether the SDS system is out of the linear relationship. For the chxn complex (Figure 5b), the appreciably larger deviation from the linear τeff vs η correlation is seen in the NaDDA system. This result suggests a specific interaction between the chxn complex and the NaDDA micelles. As the chxn complex is bulky, steric hindrance will be effective in the interaction with the bulky alaninate head group. This selectivity is analogous to the chiral discrimination of [Co(en)3]3+ enantiomers in the phenylalaninate system compared to the other less bulky head groups.40 The correlation times for the phen complex also have a rough linear relationship with the corresponding viscosities of the various micellar solutions. (Figure 5c) As the τeff values may include estimation errors considerably, the presence of the specific interaction is not clear except for the chxn complex ion in NaDDA system. When the relaxation is governed by the quadrupolar mechanism, the temperature dependence of the relaxation rates (R1 and R2), is considered to come from the Arrhenius relationship of the correlation times, since the χeff value is almost independent of temperature. This temperature dependence will also give useful information concerning the extent of the interactions
Tripositive Ions of Co(III) and Cr(III)
J. Phys. Chem. B, Vol. 101, No. 20, 1997 3925 TABLE 4: Activation Energies (Ea) and the Correlation Times (τeff, 27 °C) system
Ea/kJ mol-1
τeff/ns
3+
(a) [Co(en)3] 17.8 ( 0.5 22.5 ( 1.5 21.1 ( 1.1 35.1 ( 2.8 31.6 ( 1.0 29.4 ( 1.8
0.05a 1.0 1.2 1.8 1.4 1.3
(b) [Co(chxn)3]3+ aqueous 23.3 ( 0.4 NaOctaneSO3 (0.4 M) 25.1 ( 1.0 NaDecaneSO3 (0.4 M) 26.6 ( 0.4 SDS (0.4 M) 31.6 ( 0.5 NaDDA (0.4 M) 34.5 ( 1.2 KDodec(0.6 M) 28.7 ( 0.8
0.08a 0.8 1.0 1.4 2.1 1.5
(c) [Co(phen)3]3+ aqueous 20.8 ( 0.3 NaOctaneSO3 (0.4 M) 25.5 ( 1.1 NaDecaneSO3 (0.4 M) 26.5 ( 0.6 SDS (0.4 M) 31.2 ( 2.1 NaDDA (0.4 M) 35.0 ( 3.1 KDodec (0.4 M) 30.0 ( 1.1
0.1a 1.0 1.3 1.5 1.7 1.6
aqueous NaOctaneSO3 (0.4 M) NaDecaneSO3 (0.4M) SDS (0.4 M) NaDDA (0.4 M) KDodec(0.4 M)
a These values were estimated on the basis of the Stokes-EinsteinDebye equation using the limiting partial molar volumes (refs 8, 38, and 39).
Figure 5. Relationship between τeff obtained from the R2/R1 ratios and viscosity. (a) [Co(en)3]3+, (b) [Co(chxn)3]3+, (c) [Co(phen)3]3+. The concentrations of the surfactant are 0.4 mol dm-3 except for (b) KDodec (0.6 mol dm-3).
by providing activation energies for the correlation times. In the estimation of the activation energies, systematic errors included in the relaxation rates are expected to be compensated by each other. The activation energies in the aqueous solution (without surfactants) can be obtained only from the R1 values since the complicated mechanisms involving scalar couplings influence the R2 values in this system. On the other hand, we used the R2 values for the calculations of the activation energies in the micellar solutions. The contribution of scalar couplings to the R2 may be negligible in the micellar systems, while the
activation energies obtained from the R1 values have less accuracy because of a larger contribution of the denominator in eq 3 or 7 (the relationship of 2πν0τeff ) 1 holds for τeff ) 2.46 ns).41 We therefore determined the activation energies for the τeff values from the temperature dependences of R2 values. All the results are listed in Table 4 together with the τeff values at 27 °C for comparison. Although we can see a rough correlation between the activation energies and the τeff values, the activation energies will give more definite results concerning the micellar effects; i.e., the extent of the interaction for the en complex is in the order NaOctaneSO3 ≈ NaDecaneSO3 < KDodec < NaDDA < SDS, that for the chxn complex is in the order NaOctaneSO3 < NaDecaneSO3 < KDodec < SDS < NaDDA, and that for the phen complex is also the same. The order in the interaction obtained is almost dependent on the kind of micelles; especially, it is significant that the activation energy in 0.6 mol dm-3 KDodec is the same as that in 0.4 mol dm-3 KDodec and is smaller than that in the SDS micelle in spite of the larger viscosity. One possible explanation for these results is that the activation energies are mainly governed by the respective micellar sizes, which will be related to the reciprocal of the cmc values listed in Table 1. However, it is remarkable that the motion of the chxn is slightly more restricted by the NaDDA micelle than by the SDS micelle in spite of the smaller viscosity. This result is consistent with the specific interaction predicted from the τeff vs η plot in Figure 5b. For the phen complex, the orders in the R1 and R2 values (Figure 4c) were the same as that in the activation energies. Furthermore, all the dynamic data (R1, R2, τeff, and the activation energies) predict that this complex is more motionally restricted in the NaDDA system than in the SDS and KDodec systems, although the interaction of the phen complex with the NaDDA head group is considered to be smaller than that in the other two systems on the basis of the 59Co chemical shift data. This complex is dissolved in the hydrocarbon cores of micelles and the relaxation rate is mainly dependent on the motional restriction by the interactions in the micellar cores. The χeff values in the carboxylate micelles are clearly larger than those in the sulfonate/sulfate micelles for the en and chxn complexes. This result is consistent with the magnitude of the change in the 59Co chemical shift. When comparing between
3926 J. Phys. Chem. B, Vol. 101, No. 20, 1997 the NaDDA and KDodec system for the chxn complex, we can also see that the smaller chemical shift change in the NaDDA system (Figure 1b) is consistent with the smaller χeff (Table 2). It is reasonable to consider that the relatively large χeff values for [Co(en)3]3+ and [Co(chxn)3]3+ in the carboxylate micelles are due to the strong electrostatic interaction and correlates with the chemical shifts, while the τeff values are governed by the motional restriction or by the exchange between the mobile and immobilized states. Recent studies have revealed significant effects of the sulfate ion on the 59Co quadrupole coupling constants in [Co(en)3]3+ and [Co(chxn)3]3+.9,10 In the present systems, χeff is affected by the interaction with the micellar surface having larger anionic charge density than the sulfate ion. We should notice that χeff also includes the order parameter and the larger χeff value often indicates a smaller rotational degree of freedom by the interaction with the micelles. We did not significantly detect this effect here. For the phen complex, the differences in the χeff values between the two types of the surfactants are not significant; however, the χeff values in the C12-alkyl chained micelles are slightly larger than the other shorter-alkyl chained systems. We can understand that the larger relaxation rates in the carboxylate micelles than those in the sulfonate/sulfate micelles for the en and chxn complexes arise from the larger χeff values in the former systems. The relaxation rates also support the above results that both the en and chxn complexes may directly interact with the headgroups of micelles at the aqueous sites while the phen complex is dissolved in the hydrophobic cores of micelles. Conclusion The interactions of [Co(phen)3]3+, [Co(chxn)3]3+, and [Co(en)3]3+ in micelles of various amphiphiles were studied by 59Co NMR spectroscopy. The influences observed were attributed to the differences in the hydrophobic properties of the complex ions. Distinct differences between the phen complex and the en and chxn complexes were observed. The results are consistent with localization of the phen complex to the interior of the micelles while the en and chxn complexes interact with the head groups in the intrafacial region of the micelle. The 59Co chemical shifts and effective electric field gradient of the en and chxn complexes both suggest there are stronger interactions between the carboxylate head groups and the cobalt(III) complex ions than between the sulfonate/sulfate head groups and the cobalt(III) complex ions. The correlation times of these complexes are not strongly sensitive to the differences between the head groups except for [Co(chxn)3]3+ in NaDDA. However, differences are observed and the effect is more clearly revealed by the activation energies of the correlation times. These are governed by the micellar sizes and by the specific interactions between the metal complex ions and the micelles. Acknowledgment. The authors are grateful to Dr. Alan S. Tracey (Simon Fraser University, Canada) for useful discussions. This work was partially supported by a Grant-in Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (No. 07640752). Supporting Information Available: Arrhenius plots for the longitudinal (R1) and transverse (R2) relaxation rates measured at 64.6 MHz and 27 °C (4 pages). Ordering information is given any current masthead page.
59Co
References and Notes (1) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975; p 250.
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