J . Phys. Chem. 1992, 96, 2376-2381
2376
Intramolecular Relaxation Dynamics and Infrared Spectra of Diaza Macrocyclic Complexes with Na+ and Ag+ in Propylene Carbonate Paul Firman, Licesio J. Rodriguez, Sergio Petrucci,* Weber Research Institute and Department of Chemistry, Polytechnic University, Long Island Campus, Route 110, Farmingdale. New York 1 1 735
and Edward M. Eyring Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: September 12, 1991: In Final Form: January 2, 1992)
Ultrasonic relaxation spectra and infrared spectra (N-H stretch) are reported for the macrocycles 1,lO-diaza-18-crown-6 (Kryptofix 22), monoaza-18-crown-6, and 18-crown-6(18C6) and their complexes with Na+ and Ag+ in the solvent propylene carbonate (PC) at 25 OC. The relaxation spectra are attributed to intramolecular conversions on the microsecond to nanosecond time scale between various conformers of the macrocyclic ligands. When Na+ is present, a shift of the relaxation frequencies for Kryptofix 22 and the appearance of a new relaxation process a t f C 1 MHz with monoaza-18C6 are recorded. Infrared spectra suggest the existence of more than one configuration of the ligand, this being evident both with and without Na+ in PC. For Ag+ complexes of diaza-18C6 in PC, the formation constant is too high for the two observed relaxation processes to be consistent with a previously envisaged three species mechanism involving the complete rotation of the nitrogens, one of them carrying Ag'. Rather the three species are visualized as conformers of the same endo-endo species. The IR spectra seem to confirm this view.
Introduction Several years ago, we investigated the molecular relaxation dynamics of complexes of l,lO-diaza-l8C6 (Kryptofix 22) (I) with Na+ and with Ag+ in the solvent acetonitrile and compared the observed relaxation processes with those obtained with the macrocycle 18C6, (II).2 The results were interpreted in terms of an intramolecular rearrangement of Kryptofix 22 involving possible stepwise rotation of the nitrogens of the ring with the metal cation always in contact with the Kryptofix 22 ligand. The lateral chains containing the ethereal groups were also thought to be involved in the molecular rearrangements with interaction with the metal cation in the case of Na+. The assignment of the two observed relaxation processes to the rapid equilibrium between three complexed species M'C
+(
MT)
KZ
((M'C))
M+...C
KI
M+C
(M'C)
ti
Thl LO ro
__
((M+C))
K3
(2)
with M+...C symbolizing a solvent-separated entity and M+C, (M'C), and ((M+C)) denoting three distinct contact species. The last species, ((M+C)), would correspond to the complex form with ( 1 ) Rodriguez, L. J.; Eyring, E. M.; Petrucci, S. J . Phys. Chem. 1989, 93, 5087. (2) Rodriguez, L. J.; Eyring, E. M.; Petrucci, S. J . Phys. Chem. 1989, 93, 5916. ( 3 ) Cox, B. G.;Firman, P.; Horst, H.; Schneider, H. Polyhedron 1983, 2, 343. (4) Kulstadt, S.; Malmsten, A. J . Inorg. Nucl. Chem. 1980, 42, 573. ( 5 ) Eigen, M.; Winkler, R. In Neurosciences: Second Study Program; Schmitt, F. 0.. Ed.; Rockefeller University Press: New York, 1970; pp 685-696.
Y o 1
::
O]
q Y
O3
L o 3
c,"
O3
L o >
H
(n)
(1)
(m)
the metal ion, M+, imbedded in the cavity of the ligand, C. In the above mechanism, KO= [M+...C]/[M+][Cl, K, = [M+C]/ [M+...C], K2 = [(M+C)]/[M+C], and K3 = [((M+C))]/[(M+C)], resulting in the overall complexation constant KZ given by K, =
seemed to be confirmed by infrared spectra related to the N-H stretch region of the Kryptofix 22 ligand. Three bands were present, suggesting the presence of three molecular configurations of Kryptofix 22 and of its complexes with both Na+ and Ag+ in acetonitrile. The overall formation constant for complexation of Na+ ion by Kryptofix 22 in acetonitrile is log K, = 4.4S or 4.30.4 Influenced by the IR spectra in acetonitrile, one may write the Eigen-Winkler schemeS of complexation applied to these systems with three contact species M+ + C
CHART I
[((M'C))]
+ [(M'C)] + [M+C] + [M+...C]
K, = Ko[1
[M+l [CI
+ K, + KlK2 + KIK2K31
(3)
From classical theorie~,~.' KO = (4*Ld3/3000) exp(eFlcd2kT). Combination of co = c, = (the permittivity of acetonitrile a t frequencies high with respect to the dipolar rotation of the solvent dipoles, corresponding to the solvent irrotationally bound around the ions), a value for fi = 2 X esu cm as a reasonable parameter, and a distance d = 2 X cm or d = 3 X lo-* cm leads to KO= 2.4 X lo3 and KO = 12.2, respectively. Thus KO can change from 10 to lo3 depending on the separation distance, d, in the solvent separated species, [M+...C]. Assuming KO N IO2 (as an average order of magnitude), the quantity [ l + K , + KIK2+ K,K2K3] = lo2from eq 3 remains to be evaluated. This condition can be easily met with K,, K2, K3 I5 ; namely, the three species M+C, (M+C), and ((M+C)) have concentrations within 1 order of magnitude of each other, in the sequence [((M+C))] > [(M+C)] > [M+C]. Under this condition, the multiple equilibrium (2) approximates equilibrium 1 with the three contact species at reasonable concentration ratios. Two ultrasonic relaxation processes may be observable, and the infrared envelope corresponding to the N-H stretch of the ligand Kryptofix 22 (6) Fuoss, R. M. J . Am. Chem. Soc. 1958.80, 5059. (7) Chen, C. C.; Petrucci, S. J . Phys. Chem. 1982,86,2601. Maaser, H. E.: Delsienore. M.: Newstein. M.: Petrucci. S. J . Phvs. Chem. 1984. 88. 5100. (8) Firman, P.; Marchetti, A.; Xu,M.; Eyring, E, M.; Petrucci, S. J . Phys. Chem. 1991, 95. 7055
-
0022-365419212096-2376$03.00/0 0 1992 American Chemical Society
Relaxation Dynamics of Diaza Macrocyclic Complexes should show three molecular configurations. In acetonitrile, we indeed observed' two relaxation processes for Na+ complexed to Kryptofix 22. The infrared spectrum indicated three molecular configurations, suggesting that the distribution of three species M+C, (M'C), and ((M'C)) is such that comparable relative amounts are present with small or relatively insignificant amounts of M+, C, and M+...C also present. For Ag+ complexed with Kryptofm 22, two ultrasonic relaxation processes were also observed. Although the I R spectrum also showed three bands in the N-H stretch region, there were dramatic shifts to lower frequencies on the order of 70-90 cm-'. This indicated specific interactions, as would be expected for the two nitrogen atoms of the ligand. The relative amplitudes of the bands indicated a favoring of two of the three species over the third. The formation constant of Ag+ with Kryptofix 22 is3 log K = 7.94, which stretches to the limit the possibility of interpreting the two observed ultrasonic relaxation processes by the same scheme 1, unless we assume that the three species responsible for the observed process are subspecies of ( ( W C ) ) . Thus the pristine meaning of the equilibria in scheme 1 is abandoned. Even assuming KO= lo4, we are left with the quantity [ 1 + Kl + KlK2 + K1K2K3]N lo4. This would require the values of K , , K,, and K3 to each be larger than 10 to satisfy the above condition, a requirement at the limit of the evidence shown by the amplitude of the IR bands' of Ag+ Kryptofix 22 in acetonitrile. Thus it would be interesting to see whether the results in the solvent acetonitrile are general in nature and whether by changing the structure of the ligand one could gain more insight into the molecular details of the observed processes. We reasoned that by using a solvent of comparable donor number to acetonitrile (DN = 14.1)9 but of larger permittivity than acetonitrile (e25 = 36), namely by using propylene carbonate (PC) (DN = 15.1, e25 = 64),9 we could check the generality of the above findings for the complexation of Na+ and Ag+ with Kryptofix 22. Further, by using ligands such as monoaza-l8C6 (111) and 18C6 (II), namely by reducing the number of nitrogen atoms in the ring to one and zero, respectively, we should uncover the molecular details of the dynamics of the observed processes. The formation constant of Ag+ + Kryptofix 22 in PC has been reportedlo to be log K = 15.57, namely more than 7 orders of magnitude larger than that in acetonitrile. Hence by taking KO lo4 as the upper limit, one would expect to have from eq 3 [ 1 + Kl + K1K2+ KlK2K3] = Kz/Ko = 3.7 X 10". This would carry as a consequence values of Kl, K2, and K3 each on the order of lo3 or larger, namely a preponderance of the inclusive complex ((M+C)) with respect to the other species, Le. [ ( ( M T ) ) ] r 1O3[(M+C)] and [M+C)] 103[M+C]. Consequently, no ultrasonic relaxation should be detected in the accessible 0.5600-MHzfrequency range and a single band in the N-H stretch region of the IR spectrum would be expected. As quoted below (ref 15), ultrasonic relaxation spectra, to be detectable, need at least two species at equilibrium, with a relative concentration of 0.1-1% at the least. Infrared bands can be detected from the total spectral envelope for a relative concentration of at least a few percent. Thus a combined ultrasonic-infrared absorption study of Ag+ Kryptofix 22 in PC should be illuminating.
+
=
=
+
Experimental Section The equipment and procedures for the ultrasonic and infrared measurements have been described elsewhere." Sealed infrared cells (Perkin-Elmer) with 0.025- and 0.10-mm spacers and BaF2 windows were used. The IR cells were recalibrated for window (9) Gutmann, V. Coordination Chemistry in Nonaqueous Solvents; Springer-Verlag: Vienna, 1968. (10) Ref. 3; a more recent review reports log K = 16.33: Cox, B. G.; Schneider, H. Pure Appl. Chem. 1989, 61, 17 1. (11) Eggers, F.;Funck, T.; Richmann, K. H.; Schneider, H.; Eyring, E.; Petrucci, S.J. Phys. Chem. 1987,91, 1961. Saar, D.; Petrucci, S . J . Phys. Chem. 1986, 90, 3326. Chen, C.; Wallace, W.; Eyring, E. M.;Petrucci, S . J. Phys. Chem. 1984,88, 2541. ( 1 2) See,for instance: Farber, H.; Petrucci, S . In The Chemical Physics of Solvation; Dogonadze, R., et al., Eds.; Elsevier: New York, 1986; Vol. B and references contained therein.
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2377
1
,ooo
Kryptofix 22 0.13M in Pc; t=25OC
/ -
1
I
I
I
I
I
I
I
I
2
5
10
20
50
100
200
500
I(MHz)+ 0.04
Kryptofix 22 0.10M in PC; t=25"C
r 0
5
c
0.02
B
n d
0.004 34
IO t i(cm.')
Figure 1. (A) Ultrasonic relaxation spectrum expressed as p , the excess sound absorption per wavelength, VSJ the frequency, for Kryptofix 22 (0.13 M) in F T at 25 OC. (B) Infrared spectrum expressed in absorbance A vs wavenumber, B, for Kryptofix (0.10 M) in PC at 25 OC. The spectrum refers to the N-H stretch region.
spacing before each run by the fringe method. Propylene carbonate (Aldrich) was dried over P205for a night, decanted, and distilled in vacuo in an all-glass apparatus with no grease on the joints. NaC10, (Aldrich), NaB(Ph),, and AgC104 were dried in vacuo a t 100 and 50 OC and room temperature, respectively, to constant weight. 1,1Miaza-l8C6 and monoaza-18C6 (Aldrich) were used as received. 18C6 (Aldrich) was recrystallized from distilled acetonitrile, and the acetonitrile adduct formed (kept in sealed bottles in dessicators up to the time of use) was desolvated in vacuo a t room temperature at the time of use. Solutions were prepared by weight of the solute directly in volumetric flasks. (The salts were first subjected to the vacuum-drying procedure, the macrocycle was added followed by redrying in vacuo at room temperature to constancy in weight). The solvent propylene carbonate was then added to produce the required volume of the solution in the volumetric flask. Exposure of the solvent and the prepared solution to the atmosphere during preparation of the solutions and filling of the ultrasonic and infrared cells was limited to about 30 s overall.
Results and Discussion l,lO.Diaza-l8CCQ.In Figure 1A the excess ultrasonic absorption per wavelength, 1,is plotted vs the frequency,f, for a representative concentration of the macrocycle Kryptofix 22 in propylene carbonate at 25 OC. The quantity p = (a - Bf)(u/f) and B = ( ( C X / ~ ) ) ~ namely , ~ ~ , , the value of a/f at frequencies above the observed relaxation processes. The symbol a denotes the total attenuation coefficient of sound, and u is the sound velocity. The solid line in Figure 1A corresponds to the sum of two Debye relaxation processes
characterized by the relaxation frequenciesf, andfIl with 1 1 and
2378 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
t
loo0l
1
lool
looo
Kryptofix 22 0.235M+NaB$. 0.235M in PC; k25.C
J
0
0
r
x
x
Firman et al. Monoaza-1BCS 0.22111 in PC; t = z s * ~
1
X
a F 10
r
1
2
5
10
20
50
100
200
500
f(MHz),025
f(MHz) -+
Monoaza-18C6 0.09M in PC
.0250 Kryptofix 22 0.095M+NaCtO4 0.095M in PC; t=25"C
I,,,,
t,,,,=0.01056 cm
I
=0.00468Cm
P 6
3400
3346
tv(cm-1)
3293
3260
Figure 3. (A, top) Ultrasonic relaxation spectrum expressed as p vsffor monoaza-18C6 (0.22 M) in PC at 25 O C . (B, bottom) Infrared spectrum expressed as A vs J for monoaza-18C6 (0.09 M) in PC. te ("')
Figure 2. (A) Ultrasonic relaxation spectrum expressed as u/f vs/for Kryptofix 22 (0.235 M) + NaB(Ph), (0.235 M) in PC at 25 OC. (B) Infrared spectrum expressed as A vs B for Kryptofix 22 (0.095 M) + NaCIO, (0.095 M) in PC. pIIthe maximum sound absorption values per wavelength of the two normal modes of the relaxation processes I and 11. Figure 1B is a representative infrared spectrum plotted as absorbance vs wavenumber, I (cm-I), in the 3400-3200-~m-~ region corresponding to the N-H stretching group frequency for Kryptofix 22 in PC at 25 OC. The spectral envelope can be interpreted by the sum of three Gaussian-Lorentzian product functions of the typei3
with j' = 1, 2, and 3 and uj = (A11/2)j/ 1.46 at AjI2.Tables I and I1 in the supplementary material present the ultrasonic parameters pI,/I, pII,fiI, B, the ultrasonic velocity I(, and the infrared parameters AIo,vj, and ( A V ~ , ~respectively. )~, The presence of two ultrasonic relaxations suggests the presence of three species in rapid equilibrium. Scrutiny of Table I reveals that pI and pIIare linear with the concentration, C, and that the values offi andfiI are independent of C a s a first-order process of the type C, e C, e C3requires. Further, the presence of three bands in the infrared spectrum reinforces the notion of the N-H moiety being assoCiated with three distinct configurations as the above process requires. Linear regression, assigning 50% statistical weight to the intercept, gives for the pI vs C and pIIvs C data pI = -0.68 X + l l . 0 C r2 = 0.99, plI = -0.61 X
10"
+ 12.0C
r2 = 0.96
with C expressed in units of mol/cm3. Figure 2A is a representative ultrasonic absorption relaxation spectrum of the system NaB(Ph), + Kryptofix 22 in a molar ratio (13) For a discussion of this function, see: Maaser, H.; Xu, M.; Hemmas, P.; Petrucci, S. J. Phys. Chem. 1987, 91, 3047. Inoue, N.; Xu, M.; Petrucci, S. J. Phys. Chem. 1987, 91, 4268.
R = 1 in PC at 25 OC in the form of p vsf. The spectrum shows a faster relaxation process at frequencies around 105 MHz with the onset of a lower frequency relaxation process centered below our accessible range, namely below -1 MHz. An extremely tentative fit would place this second process at -0.4 MHz. Similar results were obtained for NaClO,+ + Kryptofix systems in the solvent acetonitrile. Replacement of the anion B(Ph), with C104- does not affect the results in the concentration range investigated. Figure 2B is an infrared spectrum, corresponding to the N-H stretch region, of a representativeNaC10, plus Kryptofix 22 system in PC. The spectrum can be reproduced by the sum of three Gaussian-Lorentzian functions (eq 5 ) of altered amplitudes with respect to the one found with no Na+ present. Tables I and I1 present the parameters for the ultrasonic and infrared spectra, respectively, that were used to interpret the componding data. Linear regression, giving 50% statistical weight to the intercept, gives for the pI vs C (mol/cm3) data 13 = 0.96 p1 = 2.4 X lo-' 7.8C
+
Linear correlation between p I and C and independence within experimental error of thefi values with C suggest that also for Na+, interacting with Kryptofix 22, a first-order process occurs. Presence of a second relaxation suggests that process 1 might also explain the equilibrium between the three molecular forms indicated by the infrared spectra. The same conclusions were reached for this system (NaC10, + Kryptofm 22) in acetonitrile, supporting the generality of the conclusions1 for the Na+ + Kryptofix 22 combination. Monoaza-18CQ. We consider next the results obtained with monoaza-l8C6 (111). Figure 3A is a representative ultrasonic spectrum expressed as p vsffor monoaza-18C6 in PC at 25 OC. The spectrum can be interpreted by a single Debye relaxation centered at -70 MHz. The infrared spectrum of the same system at a representative concentration in the N-H stretch region is shown in Figure 3B. The spectral envelope can be interpreted by the sum of three. Gaussian-Lorentzian functions, eq 5 . A representative ultrasonic spectrum expressed as p vsffor NaC10, + monoaza-18C6 in PC at 25 OC is shown in Figure 4A. The spectrum can be interpreted by a Debye relaxation process also centered at 70 MHz with the onset of another process centered at frequencies below our accessible range. Figure 4B is a rep-
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2319
Relaxation Dynamics of Diaza Macrocyclic Complexes
Monoaza-18C6 O.I5M+AgC&,
Monoaza-18C6 O.ISM+NaCb, 0.15M in PC t=25'C "
0.15M In PC; k25.C
I
i
\
\
I
\
I
I
1
\.I
2
5
10
I
I
100
200
1
20 50 f(MHz) +
1
500
2
I
I
I
I
5
10
20
50
\
I
100
500
f(MHz) -+
,050 Monoaza-l8C6 0.22M+NaB@. 0.22,M in PC 1,,,,=0.0106 cm
t
0
f
2
.028
n 4
,005
I
3
DO
c f(cm")
Figure 4. (A) Ultrasonic spectrum expressed as p vsffor monoaza-18C6 (0.15 M) NaC10, (0.15 M) in PC at 25 "C. (B) Infrared spectrum expressed as A vs B for monoaza-18C6 (0.22 M) + NaB(Ph)4 (0.223M) in PC.
+
+
resentative infrared spectrum of NaB(Ph), monoaza-18C6 for the N-H stretch region. As for the monoaza-18C6 alone, three Gaussian-Lorentzian bands of enhanced absorbance can be used to interpret the spectrum. Seeking to characterize the nature of the observed relaxation process, we have run a single experiment with AgC104 monoaza-18C6 at molar ratio R = 1 in PC. Figure 5A is the ultrasonic relaxation spectrum for this system. Again the spectrum can be interpreted by the sum of two relaxation processes, one also centered at 70 MHz and the other centered at 6 MHz. Figure 5B is the IR spectral envelope for the N-H stretch in this system. Two Gaussian-Lorentzian bands are almost adequate to describe the spectrum although a third one (extremely small) may be present at the lower end of the spectrum. Tables I and I1 present the results of the analysis of the above ultrasonic and IR data. It appears that the relaxation centered at 70 MHz is a property of the ligand monoaza-18C6, whereas the one for Ag+ at 6 MHz and the one for Na+ at f, < 1 MHz are attributable to the presence of the cations, and their positions are dependent on the nature of the cation. A possible interpretation of the spectra would assign the upper relaxation process to the rotation of the nitrogen of the ring without or with the cation for the free macrocycle and complexed macrocycle, respectively. Comparatively little has been reported concerning formation constants of macrocycles with metal cations in PC. Boss and P ~ p o v reported l~ that for Na+ 18C6 in PC pK = 5.16 f 0.13 and for Na+ + l,lO-diaza-l8C6 in PC pK = 4.62 k 0.03. Other laboratories have reported that for Ag+ + 18C6 in PC K = 1.1 X lo714b and for Ag+ l,lO-diaza-l8C6 K = 3.7 X or K = 2.1 X which average to R = 1 X loi6. Hence one would expect KAg+for monoaza-18C6 to be between lo7 and
+
+
+
(14) (a) Boss, R. D.; Popov, A. I. Inorg. Chem. 1986, 25, 1747. (b) Arnaud-Neu, F.; Loufoiulou, E. L.; Schwing-Weil, M. J. J . Chem. Soc., Dalton Trans. 1986, 2629. (c) Buschmann, H. J. J . Solution Chem. 1988, 17,277. Nakamura, T.; Yumoto, Y.; Izutsu, K. Bull. Chem. Soc. Jpn. 1982, 55, 1850.
+i (cm" )
Figure 5. (A) Ultrasonic spectrum expressed as p vsffor monoaza-18C6 (0.15 M) AgCIO, (0.15 M) in PC at 25 O C . (B) Infrared spectrum expressed as A vs B for monoaza- 18C6 (0.15 M) + AgC104 (0.15 M) in
+
PC .
+
As noted above, the K reported14afor Na+ 18C6 in PC is 1.4 X lo5 and that of Na+ + l,lO-diaza-l8C6 is 4.2 X lo4. We would expect the formation constant of Na+ + monoaza-18C6 to be somewhat smaller than K = 1.4 X lo5, judging from the lower affinity of Na+ for nitrogen compared to that for oxygen. One can arrive at the same approximate value of K for Na+ + monoaza-18C6 in PC from what is known'" about formation constants in acetonitrile. For both Ag+ and Na+ monoaza-18C6 in PC at molar ratio R = 1, the extent of free ligand and free cation ought to be negligible at the concentration used in this work. Hence, most of the cations should always be in contact with the nitrogen and possibly rotating with it. The slower relaxation process could be ascribed to the final desolvation of the metal cation by the ethereal chain of the macrocycle monoaza- 18C6 after inward partial rotation of the nitrogen. The ratedetermining step of the slow relaxation process would then be the desolvation of the cation (since it is cation dependent) and not the rearrangement of the ethereal chain. The rate-determining step of all the intramolecular processes (or pseudointramolecular processes if solvent molecules are exchanged with the bulk) is also desolvation of the cation going in and out of the "inclusive" position with the cation always attached to the nitrogen. It is known that Ag+ exchanges solvent molecules from its first coordination shell faster than Na+ does, as appears to be the case here, if the above interpretation is correct. The infrared spectra of monoaza-18C6 and of its complexes with Na+ and Ag+ are more difficult to interpret. Existence of three bands, presumably of three molecular configurations, can be harmonized with the ultrasonic spectrum of monoaza-l8C6 alone (Figure 3) either by postulating a closed, coupled isomeric process C I F! C, C,, with only one visible relaxation process, or by assuming that one of the two processes is outside the range accessible to our ultrasonic instrumentation. Similarly, the smallness and possibly lack of a third band, for the IR spectrum of Ag+ + monoaza-l8C6 in PC, can be made
+
2380 The Journal of Physical Chemistry, Vo1. 96, No. 5, 1992
consistent with the ultrasonic spectrum by assuming that the concentration of one of the three molecular configurations possibly present, inclusive, exclusive desolvated, and exclusive solvated, is extremely small for Ag+ complexes. It is known that ultrasonic methods are more sensitive detectors than other methods of minor amounts of species in dynamic equi1ibri~m.l~ For monoaza- 18C6 alone and for monoaza- 18C6 + NaC10, at R = 1 in PC, constancy within experimental error of the relaxation frequencies with concentration and linearity of the recorded p1 point to a first-order or pseudo-first-order process interpretable by intramolecular mechanisms. Linear regression of pIvs C giving 50% statistical weight to the intercept for monoaza-18C6 gives p, = 8.0 X 16.7C and r2 = 0.99 with C in units of mol/cm3. Similarly, linear regression of pIvs C giving 50% statistical weight to the intercept for NaC10, + monoaza8.4C and rz = 18C6 at R = 1 in PC gives pI = -3.0 X 0.99 with C expressed in mol/cm3. It is now feasible to attempt a comparative interpretation of the data for Kryptofix 22 and monoaza-l8C6 in PC. The two ligands alone show two and one ultrasonic relaxations, respectively. In the simplifying assumption that each relaxation corresponds to the rotation of the nitrogen, it is to be expected that two and one relaxations, respectively, are present for Kryptofix 22 and monoaza-18C6. That the explanation may be an oversimplification is suggested by the two IR spectra showing three bands for each spectrum. EigenS warned that kinetic schemes such as the Eigen-Winkler process, eq 2, may be oversimplified representations of more complicated mechanisms. Addition of Na+ to Kryptofix 22 seems to speed up somewhat the faster relaxation process with the slower one becoming far slower. The same conclusion is reached for Na+ monoaza-18C6 with no apparent effect on the faster process compared to the pure ligand but a much slower and new slow relaxation process. It would appear that for both ligands the possible ultimate entrance of Na+ into the cavity forming an inclusive complex is relatively slow, perhaps because of Na+ desolvation. 18C6. Figure 6A is a representative ultrasonic spectrum of 18C6 in PC at 25 OC. A single Debye ultrasonic relaxation process can describe the data. Parts B and C of figure 6 are representative ultrasonic spectra for the system NaB(Ph), 18C6 at R = 2 and for AgC10, 18C6 at R = 2 in PC, respectively. A single Debye relaxation process can describe these data. Table I and Table I1 present the ultrasonic parameters for 18C6 in PC, for 18C6 NaB(Ph), in PC, and for 18C6 AgClO, in PC at 25 OC. The data for NaB(Ph), 18C6 in PC confirm, within experimental error, previous data for the same system.I6 From the data of Table I for 18C6 in PC a t 25 OC, one can deduce the relaxation frequency& Linear regression, giving 50% statistical weight to the origin, yields 1.1 = 3.7 X 11.9C and P = 0.995 with C expressed in mol/cm3. From the data of Table I for NaB(Ph), + 18C6 in PC, and from the previously reported data,I6it is evident that there is constancy offi with concentration within experimental error. Linear regression of all the available data, giving 50% statistical weight to the origin, results in the function p = 1.68 X 1 9 . X and r = 0.97 with C expressed in mol/cm3. From the constancy offI with C and from the linearity of pIvs C, there is strong support for the assumption of a first-order or pseudofirst-order process. The position of the relaxation frequency for 18C6 alone,fi z 115 MHz, vsfi z 80 MHz for NaB(Ph), 18C6 andfi zi 90 MHz for AgC10, 18C6 in PC indicates that cations affect somewhat the relaxation process related to isomeric conformational changes of a macrocycle such as 18C6 in PC. In acetonitrile, 18C6 shows a relaxation process at -90 MHz. Addition of NaC10, at molar ratio R = [NaC104]/[18C6] = 5 causesZthe relaxation frequency to move to an average value of 100 MHz, the difference from 90 MHz not being very significant.
Firman et al.
'---I
I
18C6 0.20M in PC; 1=25OC
1
2
1
1
5
10
,
I
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l
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500
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+
18C6 0,15M+NaB@, 0.30M in PC; 1 = 2 5 T
+
1
2
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20 50 100 200 l(MHz)-+
500
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18C6 O.lOM+AgCLO, 0.20M in PC; t=25'C
I '""I 9
10
+
+
+
+
+
+
+
+
+
+
(15) Lamb, J. In Physical Acoustics; Mason, P., Ed.; Academic Press: New York, 1964; Vol. 11, Part A. (16) Rodriguez, L. J.; Eyring, E. M.; Petrucci, S.J . Phys. Chem. 1990, 94, 2 150. (17) Rodriguez, L. J.; Eyring, E. M.; Petrucci, S. J . Phys. Chem. 1989, 93, 6351.
1
I
I
l
l
2
5
10
20
1
1
1
50 100 200
500
l(MHz)+
Figure 6. (A) Ultrasonic spectrum expressed as p vsffor 18C6 (0.20 M) in PC at 25 "C. (B) Ultrasonic spectrum expressed a s p vsffor 18C6 (0.15 M) + NaB(Ph)4 (0.30 M) in PC at 25 "C. (C) Ultrasonic spectrum expressed as p vsffor 18C6 (0.10 M) + AgC104 (0.20 M) in PC at 25 "C.
The stability constant of Na+ with 18C6 in PC, to our knowledge, is not known. In CH3CN it is large (K= 5.2, X lO4).'" Assuming the same order of magnitude for K in PC, the mechanism of complexation, eq 2, can be written according to the scheme M+...C
is
M+C is (M+C)
(6)
where M+...C is a solvent separated species, M+C a contact species, and (M+C) is an inclusive complex. In both PC and CH3CN, Na+ 18C6 shows only one ultrasonic relaxation process in the 1-500-MHz region at 25 OC. In DMF,I8 ethanol,18 and methanol,I9 a second relaxation process at lower frequencies also appears. Observation of only one relaxation process in PC and CH3CN may be taken to indicate close coupling of the two processes depicted in scheme 6. This in turn may be attributed to the lower donor number of PC and CH3CN not requiring an additional step for the complete desolvation and wrapping of the cation by the ligand 18C6. An alternative explanation may be valid if the second relaxation process occurs at frequencies below our accessible range. Under this latter hypothesis, the observed relaxation process would correspond to the first step of scheme 6. This step involves the initial desolvation of the cation and the initial rearrangement of the macrocycle 18C6 in forming the contact species M+C. Obviously, the ratedetermining event for this process would be either the removal of the first solvent molecule from the first coordination sphere of the cation or the rearrangement of 18C6, depending on the relative sizes of the energy barriers for the two events. If the rate-determining event were the rearrangement of the ligand, then
+
(18) Chen, C.; Wallace, W.; Eyring, E. M.; Petrucci, S. J . Phys. Chem. 1984, 88, 2541.
(19) Wallace, W.; Chen, C.; Eyring, E. M.; Petrucci, S. J . Phys. Chem. 1985, 89, 1357.
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2381
Relaxation Dynamics of Diaza Macrocyclic Complexes
IOoo
t 1
I
I
I
2
5
10
I
*
I
I
I
50
100
200
d
500
t
IfMHz)+ .060 Kryptofix 22 O.lOM+AgCtO,O.lOM in PC; t=25*C .!..,,=0.00461
MHz) for the ligand Kryptofix 22 alone, reported above. The infrared spectrum of Kryptofix 22 (0.10 M) AgClO, (0.10 M) in PC at 25 OC is shown in Figure 7B. The spectral envelope can be interpreted by two Gaussian-Lorentzian bands. The parameters used for the fits of the ultrasonic and infrared data are given in Tables I and 11. The above results are indeed surprising. As stated in the introduction, a reportedlo K, = 3.7 X 1015is not compatible with the experimental results interpreted by scheme 2 or even by scheme 1 if one assumes that M+C and (M+C) involve, respectively, the so-called exo-exo and exc-endo orientations of the two nitrogen atoms of the ring. Scheme 1 is still applicable, however, if one assumes the existence of more than one endo-endo or inclusive configuration of the ((M+C)) complex and identifies the three species in scheme 1 as three forms of ((M+C)). Stated in other words, the high affinity of Ag+ for the nitrogen lone electron pairs may force a contact with both ligand nitrogen atoms as the stable configuration. Intramolecular movements of ((M'C)) with Ag+ partially moving out of the ring plane, without leaving the two nitrogen atoms, with concomitant distortion of the ring may be possible and would explain the presence of two intramolecular relaxation processes for ((Ag+C)). That the above is not an entirely speculative rationalization is evident from the IR spectra of Kryptofix 22 Ag' in both acetonitrile1 and PC. In both c a m , the N-H stretch envelope is totally displaced to lower wavenumbers by large amounts (-60 cm-' in PC). There is no residual band left at the original position for the Kryptofix 22. This means that the weakening of the N-H bond occurs for both nitrogen atoms when Ag+ is present and that Ag+ is always (within IW3 s) bound to the nitrogen atoms, a notion incompatible with (M+C) and M+C interpreted as endc-exo and exo-exo species, where one nitrogen has to remain free from cation bonding. Notice that this shift in the I R spectrum is true for both acetonitrile' and PC solvents, despite the large difference in the formation constants (7 orders of magnitude). Notice also that for the Na+ there is not a significant shift in the frequency of the N-H stretch for the Kryptofix in both acetonitrile' and PC. This is attributed to weak interactions of the nitrogen atoms with the Na+ cation. Thus the new evidence found for the Ag+ Kryptofix complexes in PC and the previous evidence' in CH$N suggest that for Ag+ salts the ligand nitrogens cannot undergo a full rotation (to the exo-endo and exo-exo configurations) thereby suggesting a modification of the interpretation for Ag+ of our previous results in acetonitrile.' The fact that we are able to interpret the IR spectrum of Ag+ + Kryptofix by two bands instead of three and yet we still find two ultrasonic relaxation processes (requiring three species in equilibrium, hence presumably three IR bands) has no apparent explanation except for the more limited sensitivity of IR spectra than the sensitivity of the ultrasonic method to small amounts of a species at eq~i1ibrium.l~
+
Kryptofix 22 O.lM+AgCtQ, 0.1M in PC; t=25*C
cm
+
c 5 (cm-')
Figure 7. (A) Ultrasonic spectrum expressed as p vsffor Kryptofix 22 (0.10 M) + AgC104 (0.10 M) in PC at 25 OC. (B) Infrared spectrum expressed as A vs I for Kryptofix 22 (0.10 M) + AgC104 (0.10 M) in
PC.
the relaxation frequency for the same cation and ligand should be independent of the solvent. By way of contrast, it has been reported18g20that Na+ + 18C6 in DMF shows an upper relaxation frequencyfi N 60 MHz. In ethanol (EtOH) the same solute systemL8gavefi N 50 MHz, and in methanol (MeOH),fi r 45 MHz compared to the present work where in PCfi = 80 MHz and in CH3CN2fi = 90 MHz. Hence there is a solvent dependence of the value of the relaxation frequency observed with Na+ + 18C6 in various solvents. In aprotic solvents such as acetonitrile (DN = 14.1,fi = 90 MHz)? PC (DN = 15.1,fi = 80 MHz), and DMF (DN 26.6,fi = 60 MHZ),'~.~O there appears to be a rough inverse correlation between the donor number, DN, of the solvent and the value of the relaxation frequency, fi. Furthermore, the hypothesis that the observed relaxation frequency would correspond to the first step of scheme 6 would go along with the explanation given above for the existence of a lower relaxation process centered at about 6 MHz for Ag+ and below 1 MHz for Na+, respectively (interacting with monoaza 18C6 in PC). Lacking further evidence, however, we cannot take a definite stand on either of the two hypotheses above, namely whether the observable relaxation frequency of Na+ 18C6 in PC is due to the two processes in scheme 6 being closely coupled or whether the observed process is due to step I of scheme 6. Diaza-18C6 (Kryptofix 22) + AgC104. We come now to the data for the system l,lO-diaza-l8C6 AgClO, in PC at 25 O C . Figure 7A is the ultrasonic relaxation spectrum plotted as p vs f for Kryptofix 22 (0.1 M) AgC104 (0.1 M) in PC at 25 OC. The spectrum can be interpreted by the sum of two relaxation processes centered at 70 MHz and 14 MHz, respectively, not very different from the averaged values (fI = 85 MHz and fil = 15
=
+
+
+
(20) Maynard, K. J.; Irish, Chem. 198488, 729.
D.E.;Eyring, E. M.; Petrucci, S. J . Phys.
+
Conclusions For Ag+ interacting with Kryptofix 22 in both acetonitrile' and PC, new evidence suggests that the main conformation is an endo-endo inclusive complex that is still able to undergo intramolecular movements. Infrared spectra of the N-H stretch region indicate the complete linkage of Ag+ with both ligand nitrogen atoms at all times. Acknowledgment. We acknowledge generous financial support from the National Science Foundation (Grant CHE-88-22333). Registry No. I, 23978-55-4; I.Na+, 74775-56-7; I.Ag+, 31096-77-2; 11, 17455-13-9; IbNa', 31270-12-9; II.Ag+, 33775-63-2; 111, 33941-15-0; III.Na+, 88676-98-6; III.Ag+, 3 1 1 78-54-8.
Supplementary Material Available: Table I listing ultrasonic relaxation parameters and sound velocities for Kryptofix 22, monoaza-18C6 and 18C6, and their complexes with Na+ and Ag+ and Table I1 listing infrared parameters for the Gaussian-Lorentzian bands (4 pages). Ordering information is given on any current masthead page.
