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Magnetic anisotropy of copper(II) complxes from ESR studies in solution. Use of mixed ligand complexes
JOURNAL
OF MAGNETIC
RESONANCE
g,89-98
(1983)
Magnetic Anisotropy of Copper(I1) Complexes from ESR Studies in Solution. Use of Mixed Ligand Complexes* RANJAN
DAS AND B. S. PRABHANANDA
Chemical Physics Group, Tata Institute of Fundamental Research, Bombay 400 005. India Received January 18, 1983 ESR linewidths of the complex Cu(Qh and that of the mixed ligand complex Cu(P)(Q) in liquid solution, where P and Q are two different l&and groups, can be used to determine or confirm the anisotropy of Cu(Q)r, when the anisotropy parameters asscciated with Cu(P), are known. This method makes use of(i) the equations derived by Kivelson and coworkers (Wilson and Kivelson, J. Chem. Phys. 44, 154 (1966); n9r)in.s and Kivelson, .I. Chem. Phys. 44, 169 (1966)); (ii) equations which find justification in the model proposed by Kuska and coworkers (Ku&a, Rogers, and Drullinger, J. Phys. Chem. 71, 109 (1967); Ku&a, J. Am. Chem. Sot. 97,2289 (1975)) to explain the variation of metal hyperfine constant. Our procedure is shown to be promising by taking the example of Cu { (i-C3H70)2PSe2}2. This method could be useful when it is difficult to obtain good frozen solution/single crystal ESR spectra.
Anisotropic ESR parameters are often determined from frozen solution ESR spectra. In some unusual situations, such a procedure could be misleading. For example, one can obtain the mixed ligand complex Cu(dtc)(dSeP), where dtc = (C2H5)zNCSZ and dSeP = (i-C3H70)2PSe2, by the ligand exchange reaction (1) Cu(dtcb + Cu(dSeP)z = 2 Cu(dtc)(dSeP).
111
The ESR spectrum of a typical reaction mixture in chloroform at room temperature (Fig. la) can be understood as due to superposition of the spectra from the three species of reaction [ 11. On lowering the temperature, the concentration of the paramagnetic form of Cu(dSePh decreases (due to the formation of diamagnetic dimers). (This was seen even in solutions of Cu(dSeP)z.) There is also an associated decrease in Cu(dtc)(dSeP) relative to Cu(dtc)*, which can be seen by comparing Figs. la and b. The frozen solution spectrum (Fig. lc) can be interpreted as primarily resulting from only one species, the anisotropic parameters being similar to that of Cu(dtc), . Assignment of this spectrum to Cu(dtc)(dSeP), the dominant species at room temperature, would be incorrect. The inability to observe the frozen solution spectrum of Cu(dSePh in our previous work (2) can also be understood on the basis of such behavior. From this, we note that it is difficult to observe the frozen solution ESR spectra of Cu(I1) complexes which have dSeP as a l&and. However, Yordanov and Shopov (3) have reported a spectrum obtained from li A preliminary report of this work was presented at the Nuclear Physics and Solid State Physics Symposium, 1982, India. 89
0022-2364183 $3.00 Copyright Q 1983 by Academic Press. Inc. All rights of reproduction m any form reserved.
90
DAS AND PRABHANANDA
AA
BOG
3lSOG
FIG. 1. Temperature dependence of the ESR spectrum of a reaction mixture of reaction [I] in chloroform. (a) and (b) give only the high field lines corresponding to %I nuclear spin states 3 and x; and were obtained in liquid solutions at 24 and -2l”C, respectively. The spectrum (c) was obtained from frozen solutions at 77 K.
toluene solutions of Cu(dSePh at 77 K. In view of the observations mentioned above, it is difficult to say that this spectrum is really due to Cu(dSeP)z (as assumed by Yordanov and Shopov) and not due to some impurities present in the sample. Yordanov and Shopov have analyzed this spectrum under the assumption that the symmetry axes of g and A tensors coincide and find gil < gL. This has led to assigning d,z character to the unpaired electron on the metal (3). These results contradict the tensor components estimated from a study of ESR linewidths in chloroform liquid solutions at two temperatures and the assignment of dx+,z character to the unpaired electron on the metal in our earlier work (2), even though the room temperature ESR parameters given in both the papers are similar. (In our coordinate system the lobes of d,+,2 point towards near neighbors.) Our method (2,4) makes use of the expressions derived by Kivelson and coworkers (5, 6) to analyze the peak to peak derivative linewidths W of the ESR hyperfine transition, corresponding to nuclear spin state m of the metal ion:
ANISOTROPY
OF COPPER COMPLEXES
W = a’ + a + #rn + ym2 + 6m3.
91 PI
The quantity 6 is usually negligibly small, and (Y’is the concentrationdependent term. When we have nonnegligible spin-rotation interaction contribution to linewidths, (Y, /3, and y are independent parameters. On intuitive grounds we can assume axial sym.metry for the copper hyperfine tensor A, (i.e., A, = A,,) and the principal axes of g and A tensors to coincide. It is then possible to determine Ag and b along with the reorientation correlation time 7e for a given choice of Sg from an experimentally observed set of cy, 8, and y to a good approximation (2,4). & = gz - (&TX+ &J/2
[31
&! = (gy - &)/2
[41
b = 2[Az - (A, + A,)/2]/3.
[51
A unique value of Ag can be obtained by requiring that Ag values obtained from (Y, p and y at two temperatures are identical. This amounts to the assumption that the anisotropic ESR parameters do not vary significantly with temperature. In this method, Hubbard’s reciprocal relation (7) between the angular momentum correlation time 7/ and the reorientation correlation time T@is also assumed. Furthermore, it is assumed that the contribution to the concentration independent term (Y comes only from the spin-rotation interaction term and the modulation of g and A by the tumbling motion. Possible contributions to (Yfrom unresolved hyperfine structure (8) or unknown dynamic processes are assumed to be small. In comparison with the results of Yordanov and Shopov (3), our results (2) are supported by their similarity to those obtained by Attanasio et al. (9) in a study of bis(diethyldiselenophosphato)Cu(II) diluted in single crystals of analogous Ni(I1) complexes. Nevertheless, our method is open to criticism because of the crucial assumptions mentioned above and the approximate nature of Kivelson’s expressions. Furthermore, to get reliable estimates it is necessary to get LX,8, and y at two substantially difIerent temperatures at which the contributions of spin-rotation interaction and g modulation to (Yare comparable. This restriction limits the reliability of the results obtained by our procedure. Thus, there is a need for cross-checking the anisotropic ESR parameters by a different procedure. In this paper, we suggest the following method based on the properties of mixed ligand complexes of Cu(I1) which can be inferred from the results of our earlier work (I, 20). With the superscripts p, q, and pq referring to Cu(P),, Cu(Q)*, and Cu(P)(Q) we can, on the basis of the results of our earlier work (1, IO), which shows that the 3d spin density on the metal of Cu(P)(Q) is close to the average of that in Cu(P), and Cu(Qh, write the following: ,g +g:
= 2gow + c/3
[61
where the isotropic values go are obtained in liquid solution and C depends on the complex Cu(P)(Q). The results obtained in the case of the mixed ligand complex formed by N-benzoyl-N’diethylcarbamide (btc) and thiopicolineanilide (TPA) show that the dominant contribution to C can be attributed to g, and g,, components of the g tensor. (see Table I of (10)). Thus, it may not be unreasonable to write
92
DAS
AND
PRABHANANDA
g$ +gz = 2gp. Assuming equal contributions
[71
to C from g, and g,, we can write, Ag~+Agq=2Ag~-c/2 dgP + c3gq= 26gW.
PI
PI
The empirical relations [7]-[9] find an explanation in the following correlation observed experimentally. The isotropic 63Cu hyperline constants A0 measured in liquid solution are such that A! + A: = 2A$ + D. [lOI Table 1 lists C and D calculated from the results of our earlier works on a number of mixed ligand complexes (I, IO). In our earlier work (10) we had attributed a positive D to an increase in the metal 4s component and a negative D to a decrease in the metal 4s component of the unpaired electron orbital (compared to the average contribution on forming the mixed ligand complex). This conforms to an earlier suggestion by Kuska and coworkers (II, 12). The mixing of 4s component depends on the mixing of d,z orbital with d+,,z (II). A change in the perturbation from the z direction which decreases the energy of dz2 reduces the above mentioned mixing. Such a change, associated with mixed l&and complex formation, (when compared to the average value) makes D negative. It also decreases the energies of dxz and d,,* orbitals causing a reduction in g, and g,,. This makes C positive. Thus in systems similar to ours, a negative D is accompanied by a positive C in the above model. Furthermore, the magnitude of C can be expected to be substantial when D is large. Several systems (Table 1) show this trend. (It is not appropriate to look for quantitative correlation with this data in’ view of the larger errors in the estimates of C and D obtained from earlier work, where simulation of the complete spectrum had not been carried out.) Equations [8] and [9] follow from thg discussion that the effect of such a perturbation is mainly on g, and g,,. If Agp and 6gp are known, Agq against 6gq plots of Cu(Qh can be obtained either from the liquid solution linewidths of CUE or from the linewidths of Cu(P)(Q) (using [8] and [9]). The intersection of the two plots will determine a unique combination of Agq and 6gq. Equations [8] and [9] obviate the need for linewidth parameters at a second temperature. In this paper, we show that such a procedure is promising by obtaining the anisotropic ESR parameters of Cu(dSePlz and Cu(dtc)(dSeP). EXPERIMENTAL
The complexes Cu(dtc), and Cu(dSeP)* were prepared using cupric acetate (enriched to greater than 90% with 63Cu) as described in our earlier work (I). Reaction [l] was used to o&in the complex Cu(dtc)(dSeP). Degassed chloroform solutions sealed in evacuated sample tubes were used for obtaining the X-band ESR spectra. Dilution of the solutions were carried out by distilling the solvent from a side arm to the sample tube under vacuum. Variable temperature experiments were carried out using the experimental setup described elsewhere (13). Concentrations of the paramagnetic molecules were estimated with respect to a standard solution of Cu(dtc)*
ANISOTROPY
93
OF COPPER COMPLEXES TABLE
I
THE PARAMETERS C AND D OCXXJRRING [ 101 FOR TYPICAL MIXED LIGAND
Complex
C
Cu(btc)(TPA) Cu(acac)(TPA) Cu(TPA)(dSeP) Cu(dtc)(dSeP) Cu(dtpi)(dSeP) CWPM~P) Cu(btc)@SeP)
0.0558 0.0375 0.0246 0.0132 0.0060 0.0057 -0.0036
IN EQS. [6], COMPLEXES’
D G) -17.9 -16.8 -2.3 -1.0 -1.5 -1.5 4.0
[81, AND Reference (10) (10) (10) This
work
(1) (10) (10)
’ dtp = (C2Hs0)$S2; dtpi = (C6H&PS2; acac = (CH$ZO)$H.
using ESR. The relative concentrations of the three paramagnetic species were obtained by simulating a spectrum which matches with the experimentally observed spectrum. A computer program used in our earlier work (14) was used for this purpose and the simulations were carried out using a DEC-10 computer. The values of Ag for a given choice of Sg were obtained from (Y,/3, and y by an iterative procedure as described in our earlier works (2, 4). A computer programme written for our earlier work (2) was used in such calculations. RESULTS
Figures 2a and b give the experimentally observed ESR spectra at room temperature from a convenient reaction mixture of Cu(dtc)z and Cu(dSeP)z diluted to different extents. Figure 2c gives the spectrum obtained from a second reaction mixture in which the proportion of Cu(dtc), added was higher than the first. We can easily identify the lines due to Cu(dtc), and Cu(dSeP)z in these figures, since their charact.eristic features are known from earlier works. The remaining lines could be assigned to Cu(dtc)(dSeP) since they show the expected ligand hyperfine splitting from a single 31P. The fraction of “Se in natural abundance is p (= 7.5/100). Thus the fraction of Cu(dSeP)z which would show ligand hyperfme splittings of the ESR lines from a single “Se is 4p(l - P)~ (-23.7%). The fraction which would not show the sphttings from “Se is (1 - p)” (-73.2%). The corresponding fractions for Cu(dtc)(dSeP) are 2p(l - p) (- 13.9%) and (1 - p)’ (-85.6%). Some of the lines due to species with one “Se can also be identified in Fig. 2. The fraction of the species with more than one “Se is quite small and they make negligible contributions to spectral features. The parameters (Y,/3, and y associated with dilute solutions of Cu(dtc)2 and Cu(dSeP)2 redetermined by simulations in the present work are not very different from those reported in our earlier work (2). However, use of the above given values of isotopic composition requires us to use a smaller value of x (=0.2 G) in accounting for the widths of “Se hyperfine split lines, than reported earlier (=0.6 G). This correction does not alter the g-anisotropy estimates. Since the peak-to-peak height of the derivative lines is sensitive to width variations, matching of the relative heights of the ditFerent hyperfme transitions enables us to determine /3 and y with reasonable ac-
94
DAS AND PRABHANANDA 406
,
EXPERIMENTAL
b
FIG. 2. Experimentally observed and computer simulated ESR spectra of typical reaction mixtures in chloroform at room temperature. Total Cu(II) complex concentrations: (a) -1 X 10e4M, (b) - 1.0 X lo-’ M, (c) - 1.8 X 1O-3 M. Linewidth parameter a’: (a’) 0.06 G, (b’j 0.75 G, (I?) 1.1 G. [Cu(dtc)J:[Cu(dtc)(dSeP)]:[Cu(dSeP)z] used in the simulations: (a’) 35.5:52.9: 11.6 (b’J 37.1:5 1.6:11.3 (c’) 52.0~42.6~5.4.
ANISOTROPY
OF COPPER COMPLEXES
0,
SIMULATED
FIG.
0
Culdtc),
A
Cu(dtc)(d SIP)
2-Continued.
95
96
DAS AND PRABHANANDA TABLE 2 PARAMETERSUSED INTHE~IMULATIONS' Parameters
Cu(dtc)z
Cu(dtcMdSeP)
Cu(dSeP)l
a -B :, (%I)
2.0480 4.8 1.77 78.0 0.46
2.0362 2.6 0.712 77.35 0.42
2.0288 2.540 -0.372 75.7 0.533
Ao (“P) Ao (“Se)
-
9.75 34.60
11.3 38.0
go
’ All parameters except go are in gauss. For the “Se hyperline split lines x = 0.2, y, = 0.77, yz = 0.4 (see (2)). A0 (Y) is the hypertine splitting associated with the nucleus Y. The values of a’ and the relative concentrations of the three species are given in figure captions.
curacy (2). Matching the observed and simulated lines by superimposing one on the other will then help us determine (Y’+ LY.We have noticed that errors in linewidth parameters could be as much as 5% if we measure only one recording of the spectrum. This is mainly due to nonuniform magnetic field scan in our experiments. Figures 2a’-c’ have been simulated using the parameters given in Table 2. The values of (Y’ and the relative concentrations of the three species used in the simulations are given in the figure caption. There is good agreement between the observed and simulated spectra. The following features increase our confidence in the simulations and in the parameters given in Table 2. (i) Relative concentrations used in the simulations can be used to estimate the value of [Cu(dtc)(dSeP)]2/[Cu(dtc)2][Cu(dSeP)2J. The concentrations associated with Figs. 2a’-c’ show that this ratio is independent of the absolute concentrations of the species in the reaction mixture. This is expected from the reaction scheme [I]. (ii) The concentration dependent term (Y’is due to the dipoledipole and Heisenberg exchange type interactions (15) between the unpaired electron spins of the complexes in solutions. Since our earlier work has shown (2) that at room temperature in chloroform solutions, the reorientation correlation times r8 of Cu(dtch and Cu(dSeP)2 are comparable, we can say that the effective hydrodynamic radius of each of the three species is comparable. Thus we can expect the distance of “closest approach” and hence (Y’to have similar values for all the three species in a given solution. Our results are consistent with this expectation. (iii) We mentioned above that in addition to reaction [l] involving the paramagnetic species, we also have the formation of dimeric species (which do not give ESR) when the complexes involved have dSeP as a l&and. Thus on dilution, we should expect an increase in the monomeric Cu(dSeP)2 and an associated increase in Cu(dtc)(dSeP) relative to Cu(dtc)2. A comparison of the relative concentrations of the three species corresponding to Figs. 2a and b estimated from our simulations figures 2a’ and b’ indeed show such a behavior, We might also add here that viscosity of the solution is unlikely to change significantly in the concentration range of our experiments to affect (Y,j3, and y drastically.
ANISOTROPY
0.016
0.006
OF COPPER COMPLEXES
97
-
1
I 0,000
I 0.010
I OG?O
I 0.030
I 0,040
69 -
FIIG.3. Plots of Ag against bg for C~(dseP)~ obtained from ESR in liquid solutions in chloroform at room temperature. (a) Using linewidth parameters of Cu(dSeP)z. (b) and (c) Using linewidth parameters of Cu(dtc)(dSeP) with C = 0.0 and C = 0.0132, respectively, in Eq. [8].
Table 2 also gives the go of the three complexes estimated from the simulations with go = 2.0480 for Cu(dtc)Z (10). Changes in the fourth decimal place of go make the matching of the observed and simulated spectra poor. Thus we can say that C is accurate at least to the third decimal place. Single crystal studies (16) and frozen solution studies (17) have suggested that the g tensor of Cu(dtc)* is nearly axially symmetric and that the symmetry axes of g and A coincide. Linewidth studies in liquid solutions at different temperatures also lead to a similar conclusion with Ag = 0.078 and Sg = 0. Using the (Y, 8, and y of Cu(dtc)(dSeP), (Table 2), we get Ag for the mixed ligand complex for a given choice of 6g by the procedure outlined in our earlier works (2, 4). Using Eqs. [8] and [9] and using the above mentioned Ag for Cu(dtc),, we can get a set of Ag and Sg values for Cu(dSeP)* from the values associated with Cu(dtc)(dSeP). The broken line of Fig. 3 corresponds to such a set of Ag and 6g with C = 0 in Eq. [8]. The solid line of this figure was obtained with the experimentally determined value of C = 0.0132. The dotted line of Fig. 3 was obtained using values of (Y, /3, and y associated with the spectrum of Cu(dSeP)z. The values of Ag, Sg, and WT~(w = 2~ X microwave frequency) associated with the two possible points of intersection are given in Table 3 along with the value of C used in obtaining them. As mentioned above we expect the magnitudes of WT@ of the three species to be comparable. Such a result is obtained when the experimentally determined value of C is used. CONCLUSIONS
The values of Ag and 6g for Cu(dSeP)l obtained in the present work are close to the results of our earlier work (2). Furthermore, they are similar to the values reported by Attanasio et al. (9). This agreement of the results obtained by different methods
DAS AND PRABHANANDA TABLE 3 g ANI~~TROPY AND w,( OF THE COMPLEXES STUDIED IN THIS WORK Complex
Cu(dtc)* Cu(dtc)(dSeP) cuww*
Ag
h
w70
0.078 0.034 (0.033 -0.016 (-0.011
0.0 0.011 0.014 0.02 I 0.027
1.91 1.82” 1.92)b 1.99” 4.5)b
a Values obtained with C = 0.0132. b Values obtained with C = 0.0.
gives credence to our method and the validity of Eqs. [8] and [9]. The model which accounts for the A0 variation on the basis of changes in 4s mixing can be used to predict Eqs. [8] and [9]. Thus our results can be taken to support the explanation of A0 variation given by Kuska et al. (II, 12). It may be added that growing of single crystals with Cu(dtc)(dSeP) is a difficult task. Its frozen solution ESR spectrum could not be obtained (see the introduction). Nevertheless, by the application of our method we have obtained Ag and Sg for this complex along with that of C~(dseP)~. We also note that the g given by Yordanov and Shopov (3) will not correspond to Cu(dSeP), in liquid solutions. REFERENCES 1. G. KRISHNAMOORTHY,
B. S. PRABHANANLIA,
AND P. M. S~LOZHENKIN,
Proc. Indian Acad. Sci. Sect
A 87, 395 (1978). 2. B. S. PRABHANANDA, Mol. Phys. 38, 209 (1979). N. D. YORDANOV AND D. SHOPOV, Chem. Phys. Lett. 33, 162 (1975). B. S. PRABHANANDA, Indian J. Chem. A. 18,290 (1979). R. WILSON AND D. KIVEUON, J. Chem. Phys. 44, 154 (1966). P. W. ATKINS AND D. KIVELSON, J. Chem. Phys. 44, 169 (1966). 7. P. S. HUBBARD, Phys. Rev. 131, 1155 (1963). 8. B. S. PRABHANANDA, Indian J. Pare Appl. Phys. 18, 823 (1980). 9. D. A~TANASIO, C. P. KEUZERS, J. P. VAN DE BERG, AND E. DE BOER, Mol. Phys. 31, 10. G. KRISHNAM~~RTHY AND B. S. PRABHANANDA, J. Phys. Chem. 84,637 (1980). II. H. A. KUSKA, M. T. ROGERS, AND R. E. DRULLINGER, J. Phys. Chem. 71, 109 (1967).
3. 4. 5. 6.
501 (1976).
12. H. A. KUSKA, J. Am. Chem. Sot. 97,2289 (1975). 13. G. KRISHNAM~RTHY AND B. S. PRABHANANDA, J. Magn. Reson. 24,215 (1976). 14. Z. R. BARATOVA, E. V. SEMENOV, P. M. S~LOZHENKIN, AND B. S. PRABHANANDA, horg. 57 (1982). IS. M. P. EASTMAN, R. G. KOOSER, M. R. DAS, AND J. H. FREED, J. Chem. Phys. 51,269O 16. T. R. REDDY AND R. SRINIVASAN, J. Chem. Phys. 43, 1404 (1965). 17. H. R. GERSMANN AND J. D. SWALEN, J. Chem. Phys. 36, 3221 (1962).
Chem. 21, (1969).
OF MAGNETIC
RESONANCE
g,89-98
(1983)
Magnetic Anisotropy of Copper(I1) Complexes from ESR Studies in Solution. Use of Mixed Ligand Complexes* RANJAN
DAS AND B. S. PRABHANANDA
Chemical Physics Group, Tata Institute of Fundamental Research, Bombay 400 005. India Received January 18, 1983 ESR linewidths of the complex Cu(Qh and that of the mixed ligand complex Cu(P)(Q) in liquid solution, where P and Q are two different l&and groups, can be used to determine or confirm the anisotropy of Cu(Q)r, when the anisotropy parameters asscciated with Cu(P), are known. This method makes use of(i) the equations derived by Kivelson and coworkers (Wilson and Kivelson, J. Chem. Phys. 44, 154 (1966); n9r)in.s and Kivelson, .I. Chem. Phys. 44, 169 (1966)); (ii) equations which find justification in the model proposed by Kuska and coworkers (Ku&a, Rogers, and Drullinger, J. Phys. Chem. 71, 109 (1967); Ku&a, J. Am. Chem. Sot. 97,2289 (1975)) to explain the variation of metal hyperfine constant. Our procedure is shown to be promising by taking the example of Cu { (i-C3H70)2PSe2}2. This method could be useful when it is difficult to obtain good frozen solution/single crystal ESR spectra.
Anisotropic ESR parameters are often determined from frozen solution ESR spectra. In some unusual situations, such a procedure could be misleading. For example, one can obtain the mixed ligand complex Cu(dtc)(dSeP), where dtc = (C2H5)zNCSZ and dSeP = (i-C3H70)2PSe2, by the ligand exchange reaction (1) Cu(dtcb + Cu(dSeP)z = 2 Cu(dtc)(dSeP).
111
The ESR spectrum of a typical reaction mixture in chloroform at room temperature (Fig. la) can be understood as due to superposition of the spectra from the three species of reaction [ 11. On lowering the temperature, the concentration of the paramagnetic form of Cu(dSePh decreases (due to the formation of diamagnetic dimers). (This was seen even in solutions of Cu(dSeP)z.) There is also an associated decrease in Cu(dtc)(dSeP) relative to Cu(dtc)*, which can be seen by comparing Figs. la and b. The frozen solution spectrum (Fig. lc) can be interpreted as primarily resulting from only one species, the anisotropic parameters being similar to that of Cu(dtc), . Assignment of this spectrum to Cu(dtc)(dSeP), the dominant species at room temperature, would be incorrect. The inability to observe the frozen solution spectrum of Cu(dSePh in our previous work (2) can also be understood on the basis of such behavior. From this, we note that it is difficult to observe the frozen solution ESR spectra of Cu(I1) complexes which have dSeP as a l&and. However, Yordanov and Shopov (3) have reported a spectrum obtained from li A preliminary report of this work was presented at the Nuclear Physics and Solid State Physics Symposium, 1982, India. 89
0022-2364183 $3.00 Copyright Q 1983 by Academic Press. Inc. All rights of reproduction m any form reserved.
90
DAS AND PRABHANANDA
AA
BOG
3lSOG
FIG. 1. Temperature dependence of the ESR spectrum of a reaction mixture of reaction [I] in chloroform. (a) and (b) give only the high field lines corresponding to %I nuclear spin states 3 and x; and were obtained in liquid solutions at 24 and -2l”C, respectively. The spectrum (c) was obtained from frozen solutions at 77 K.
toluene solutions of Cu(dSePh at 77 K. In view of the observations mentioned above, it is difficult to say that this spectrum is really due to Cu(dSeP)z (as assumed by Yordanov and Shopov) and not due to some impurities present in the sample. Yordanov and Shopov have analyzed this spectrum under the assumption that the symmetry axes of g and A tensors coincide and find gil < gL. This has led to assigning d,z character to the unpaired electron on the metal (3). These results contradict the tensor components estimated from a study of ESR linewidths in chloroform liquid solutions at two temperatures and the assignment of dx+,z character to the unpaired electron on the metal in our earlier work (2), even though the room temperature ESR parameters given in both the papers are similar. (In our coordinate system the lobes of d,+,2 point towards near neighbors.) Our method (2,4) makes use of the expressions derived by Kivelson and coworkers (5, 6) to analyze the peak to peak derivative linewidths W of the ESR hyperfine transition, corresponding to nuclear spin state m of the metal ion:
ANISOTROPY
OF COPPER COMPLEXES
W = a’ + a + #rn + ym2 + 6m3.
91 PI
The quantity 6 is usually negligibly small, and (Y’is the concentrationdependent term. When we have nonnegligible spin-rotation interaction contribution to linewidths, (Y, /3, and y are independent parameters. On intuitive grounds we can assume axial sym.metry for the copper hyperfine tensor A, (i.e., A, = A,,) and the principal axes of g and A tensors to coincide. It is then possible to determine Ag and b along with the reorientation correlation time 7e for a given choice of Sg from an experimentally observed set of cy, 8, and y to a good approximation (2,4). & = gz - (&TX+ &J/2
[31
&! = (gy - &)/2
[41
b = 2[Az - (A, + A,)/2]/3.
[51
A unique value of Ag can be obtained by requiring that Ag values obtained from (Y, p and y at two temperatures are identical. This amounts to the assumption that the anisotropic ESR parameters do not vary significantly with temperature. In this method, Hubbard’s reciprocal relation (7) between the angular momentum correlation time 7/ and the reorientation correlation time T@is also assumed. Furthermore, it is assumed that the contribution to the concentration independent term (Y comes only from the spin-rotation interaction term and the modulation of g and A by the tumbling motion. Possible contributions to (Yfrom unresolved hyperfine structure (8) or unknown dynamic processes are assumed to be small. In comparison with the results of Yordanov and Shopov (3), our results (2) are supported by their similarity to those obtained by Attanasio et al. (9) in a study of bis(diethyldiselenophosphato)Cu(II) diluted in single crystals of analogous Ni(I1) complexes. Nevertheless, our method is open to criticism because of the crucial assumptions mentioned above and the approximate nature of Kivelson’s expressions. Furthermore, to get reliable estimates it is necessary to get LX,8, and y at two substantially difIerent temperatures at which the contributions of spin-rotation interaction and g modulation to (Yare comparable. This restriction limits the reliability of the results obtained by our procedure. Thus, there is a need for cross-checking the anisotropic ESR parameters by a different procedure. In this paper, we suggest the following method based on the properties of mixed ligand complexes of Cu(I1) which can be inferred from the results of our earlier work (I, 20). With the superscripts p, q, and pq referring to Cu(P),, Cu(Q)*, and Cu(P)(Q) we can, on the basis of the results of our earlier work (1, IO), which shows that the 3d spin density on the metal of Cu(P)(Q) is close to the average of that in Cu(P), and Cu(Qh, write the following: ,g +g:
= 2gow + c/3
[61
where the isotropic values go are obtained in liquid solution and C depends on the complex Cu(P)(Q). The results obtained in the case of the mixed ligand complex formed by N-benzoyl-N’diethylcarbamide (btc) and thiopicolineanilide (TPA) show that the dominant contribution to C can be attributed to g, and g,, components of the g tensor. (see Table I of (10)). Thus, it may not be unreasonable to write
92
DAS
AND
PRABHANANDA
g$ +gz = 2gp. Assuming equal contributions
[71
to C from g, and g,, we can write, Ag~+Agq=2Ag~-c/2 dgP + c3gq= 26gW.
PI
PI
The empirical relations [7]-[9] find an explanation in the following correlation observed experimentally. The isotropic 63Cu hyperline constants A0 measured in liquid solution are such that A! + A: = 2A$ + D. [lOI Table 1 lists C and D calculated from the results of our earlier works on a number of mixed ligand complexes (I, IO). In our earlier work (10) we had attributed a positive D to an increase in the metal 4s component and a negative D to a decrease in the metal 4s component of the unpaired electron orbital (compared to the average contribution on forming the mixed ligand complex). This conforms to an earlier suggestion by Kuska and coworkers (II, 12). The mixing of 4s component depends on the mixing of d,z orbital with d+,,z (II). A change in the perturbation from the z direction which decreases the energy of dz2 reduces the above mentioned mixing. Such a change, associated with mixed l&and complex formation, (when compared to the average value) makes D negative. It also decreases the energies of dxz and d,,* orbitals causing a reduction in g, and g,,. This makes C positive. Thus in systems similar to ours, a negative D is accompanied by a positive C in the above model. Furthermore, the magnitude of C can be expected to be substantial when D is large. Several systems (Table 1) show this trend. (It is not appropriate to look for quantitative correlation with this data in’ view of the larger errors in the estimates of C and D obtained from earlier work, where simulation of the complete spectrum had not been carried out.) Equations [8] and [9] follow from thg discussion that the effect of such a perturbation is mainly on g, and g,,. If Agp and 6gp are known, Agq against 6gq plots of Cu(Qh can be obtained either from the liquid solution linewidths of CUE or from the linewidths of Cu(P)(Q) (using [8] and [9]). The intersection of the two plots will determine a unique combination of Agq and 6gq. Equations [8] and [9] obviate the need for linewidth parameters at a second temperature. In this paper, we show that such a procedure is promising by obtaining the anisotropic ESR parameters of Cu(dSePlz and Cu(dtc)(dSeP). EXPERIMENTAL
The complexes Cu(dtc), and Cu(dSeP)* were prepared using cupric acetate (enriched to greater than 90% with 63Cu) as described in our earlier work (I). Reaction [l] was used to o&in the complex Cu(dtc)(dSeP). Degassed chloroform solutions sealed in evacuated sample tubes were used for obtaining the X-band ESR spectra. Dilution of the solutions were carried out by distilling the solvent from a side arm to the sample tube under vacuum. Variable temperature experiments were carried out using the experimental setup described elsewhere (13). Concentrations of the paramagnetic molecules were estimated with respect to a standard solution of Cu(dtc)*
ANISOTROPY
93
OF COPPER COMPLEXES TABLE
I
THE PARAMETERS C AND D OCXXJRRING [ 101 FOR TYPICAL MIXED LIGAND
Complex
C
Cu(btc)(TPA) Cu(acac)(TPA) Cu(TPA)(dSeP) Cu(dtc)(dSeP) Cu(dtpi)(dSeP) CWPM~P) Cu(btc)@SeP)
0.0558 0.0375 0.0246 0.0132 0.0060 0.0057 -0.0036
IN EQS. [6], COMPLEXES’
D G) -17.9 -16.8 -2.3 -1.0 -1.5 -1.5 4.0
[81, AND Reference (10) (10) (10) This
work
(1) (10) (10)
’ dtp = (C2Hs0)$S2; dtpi = (C6H&PS2; acac = (CH$ZO)$H.
using ESR. The relative concentrations of the three paramagnetic species were obtained by simulating a spectrum which matches with the experimentally observed spectrum. A computer program used in our earlier work (14) was used for this purpose and the simulations were carried out using a DEC-10 computer. The values of Ag for a given choice of Sg were obtained from (Y,/3, and y by an iterative procedure as described in our earlier works (2, 4). A computer programme written for our earlier work (2) was used in such calculations. RESULTS
Figures 2a and b give the experimentally observed ESR spectra at room temperature from a convenient reaction mixture of Cu(dtc)z and Cu(dSeP)z diluted to different extents. Figure 2c gives the spectrum obtained from a second reaction mixture in which the proportion of Cu(dtc), added was higher than the first. We can easily identify the lines due to Cu(dtc), and Cu(dSeP)z in these figures, since their charact.eristic features are known from earlier works. The remaining lines could be assigned to Cu(dtc)(dSeP) since they show the expected ligand hyperfine splitting from a single 31P. The fraction of “Se in natural abundance is p (= 7.5/100). Thus the fraction of Cu(dSeP)z which would show ligand hyperfme splittings of the ESR lines from a single “Se is 4p(l - P)~ (-23.7%). The fraction which would not show the sphttings from “Se is (1 - p)” (-73.2%). The corresponding fractions for Cu(dtc)(dSeP) are 2p(l - p) (- 13.9%) and (1 - p)’ (-85.6%). Some of the lines due to species with one “Se can also be identified in Fig. 2. The fraction of the species with more than one “Se is quite small and they make negligible contributions to spectral features. The parameters (Y,/3, and y associated with dilute solutions of Cu(dtc)2 and Cu(dSeP)2 redetermined by simulations in the present work are not very different from those reported in our earlier work (2). However, use of the above given values of isotopic composition requires us to use a smaller value of x (=0.2 G) in accounting for the widths of “Se hyperfine split lines, than reported earlier (=0.6 G). This correction does not alter the g-anisotropy estimates. Since the peak-to-peak height of the derivative lines is sensitive to width variations, matching of the relative heights of the ditFerent hyperfme transitions enables us to determine /3 and y with reasonable ac-
94
DAS AND PRABHANANDA 406
,
EXPERIMENTAL
b
FIG. 2. Experimentally observed and computer simulated ESR spectra of typical reaction mixtures in chloroform at room temperature. Total Cu(II) complex concentrations: (a) -1 X 10e4M, (b) - 1.0 X lo-’ M, (c) - 1.8 X 1O-3 M. Linewidth parameter a’: (a’) 0.06 G, (b’j 0.75 G, (I?) 1.1 G. [Cu(dtc)J:[Cu(dtc)(dSeP)]:[Cu(dSeP)z] used in the simulations: (a’) 35.5:52.9: 11.6 (b’J 37.1:5 1.6:11.3 (c’) 52.0~42.6~5.4.
ANISOTROPY
OF COPPER COMPLEXES
0,
SIMULATED
FIG.
0
Culdtc),
A
Cu(dtc)(d SIP)
2-Continued.
95
96
DAS AND PRABHANANDA TABLE 2 PARAMETERSUSED INTHE~IMULATIONS' Parameters
Cu(dtc)z
Cu(dtcMdSeP)
Cu(dSeP)l
a -B :, (%I)
2.0480 4.8 1.77 78.0 0.46
2.0362 2.6 0.712 77.35 0.42
2.0288 2.540 -0.372 75.7 0.533
Ao (“P) Ao (“Se)
-
9.75 34.60
11.3 38.0
go
’ All parameters except go are in gauss. For the “Se hyperline split lines x = 0.2, y, = 0.77, yz = 0.4 (see (2)). A0 (Y) is the hypertine splitting associated with the nucleus Y. The values of a’ and the relative concentrations of the three species are given in figure captions.
curacy (2). Matching the observed and simulated lines by superimposing one on the other will then help us determine (Y’+ LY.We have noticed that errors in linewidth parameters could be as much as 5% if we measure only one recording of the spectrum. This is mainly due to nonuniform magnetic field scan in our experiments. Figures 2a’-c’ have been simulated using the parameters given in Table 2. The values of (Y’ and the relative concentrations of the three species used in the simulations are given in the figure caption. There is good agreement between the observed and simulated spectra. The following features increase our confidence in the simulations and in the parameters given in Table 2. (i) Relative concentrations used in the simulations can be used to estimate the value of [Cu(dtc)(dSeP)]2/[Cu(dtc)2][Cu(dSeP)2J. The concentrations associated with Figs. 2a’-c’ show that this ratio is independent of the absolute concentrations of the species in the reaction mixture. This is expected from the reaction scheme [I]. (ii) The concentration dependent term (Y’is due to the dipoledipole and Heisenberg exchange type interactions (15) between the unpaired electron spins of the complexes in solutions. Since our earlier work has shown (2) that at room temperature in chloroform solutions, the reorientation correlation times r8 of Cu(dtch and Cu(dSeP)2 are comparable, we can say that the effective hydrodynamic radius of each of the three species is comparable. Thus we can expect the distance of “closest approach” and hence (Y’to have similar values for all the three species in a given solution. Our results are consistent with this expectation. (iii) We mentioned above that in addition to reaction [l] involving the paramagnetic species, we also have the formation of dimeric species (which do not give ESR) when the complexes involved have dSeP as a l&and. Thus on dilution, we should expect an increase in the monomeric Cu(dSeP)2 and an associated increase in Cu(dtc)(dSeP) relative to Cu(dtc)2. A comparison of the relative concentrations of the three species corresponding to Figs. 2a and b estimated from our simulations figures 2a’ and b’ indeed show such a behavior, We might also add here that viscosity of the solution is unlikely to change significantly in the concentration range of our experiments to affect (Y,j3, and y drastically.
ANISOTROPY
0.016
0.006
OF COPPER COMPLEXES
97
-
1
I 0,000
I 0.010
I OG?O
I 0.030
I 0,040
69 -
FIIG.3. Plots of Ag against bg for C~(dseP)~ obtained from ESR in liquid solutions in chloroform at room temperature. (a) Using linewidth parameters of Cu(dSeP)z. (b) and (c) Using linewidth parameters of Cu(dtc)(dSeP) with C = 0.0 and C = 0.0132, respectively, in Eq. [8].
Table 2 also gives the go of the three complexes estimated from the simulations with go = 2.0480 for Cu(dtc)Z (10). Changes in the fourth decimal place of go make the matching of the observed and simulated spectra poor. Thus we can say that C is accurate at least to the third decimal place. Single crystal studies (16) and frozen solution studies (17) have suggested that the g tensor of Cu(dtc)* is nearly axially symmetric and that the symmetry axes of g and A coincide. Linewidth studies in liquid solutions at different temperatures also lead to a similar conclusion with Ag = 0.078 and Sg = 0. Using the (Y, 8, and y of Cu(dtc)(dSeP), (Table 2), we get Ag for the mixed ligand complex for a given choice of 6g by the procedure outlined in our earlier works (2, 4). Using Eqs. [8] and [9] and using the above mentioned Ag for Cu(dtc),, we can get a set of Ag and Sg values for Cu(dSeP)* from the values associated with Cu(dtc)(dSeP). The broken line of Fig. 3 corresponds to such a set of Ag and 6g with C = 0 in Eq. [8]. The solid line of this figure was obtained with the experimentally determined value of C = 0.0132. The dotted line of Fig. 3 was obtained using values of (Y, /3, and y associated with the spectrum of Cu(dSeP)z. The values of Ag, Sg, and WT~(w = 2~ X microwave frequency) associated with the two possible points of intersection are given in Table 3 along with the value of C used in obtaining them. As mentioned above we expect the magnitudes of WT@ of the three species to be comparable. Such a result is obtained when the experimentally determined value of C is used. CONCLUSIONS
The values of Ag and 6g for Cu(dSeP)l obtained in the present work are close to the results of our earlier work (2). Furthermore, they are similar to the values reported by Attanasio et al. (9). This agreement of the results obtained by different methods
DAS AND PRABHANANDA TABLE 3 g ANI~~TROPY AND w,( OF THE COMPLEXES STUDIED IN THIS WORK Complex
Cu(dtc)* Cu(dtc)(dSeP) cuww*
Ag
h
w70
0.078 0.034 (0.033 -0.016 (-0.011
0.0 0.011 0.014 0.02 I 0.027
1.91 1.82” 1.92)b 1.99” 4.5)b
a Values obtained with C = 0.0132. b Values obtained with C = 0.0.
gives credence to our method and the validity of Eqs. [8] and [9]. The model which accounts for the A0 variation on the basis of changes in 4s mixing can be used to predict Eqs. [8] and [9]. Thus our results can be taken to support the explanation of A0 variation given by Kuska et al. (II, 12). It may be added that growing of single crystals with Cu(dtc)(dSeP) is a difficult task. Its frozen solution ESR spectrum could not be obtained (see the introduction). Nevertheless, by the application of our method we have obtained Ag and Sg for this complex along with that of C~(dseP)~. We also note that the g given by Yordanov and Shopov (3) will not correspond to Cu(dSeP), in liquid solutions. REFERENCES 1. G. KRISHNAMOORTHY,
B. S. PRABHANANLIA,
AND P. M. S~LOZHENKIN,
Proc. Indian Acad. Sci. Sect
A 87, 395 (1978). 2. B. S. PRABHANANDA, Mol. Phys. 38, 209 (1979). N. D. YORDANOV AND D. SHOPOV, Chem. Phys. Lett. 33, 162 (1975). B. S. PRABHANANDA, Indian J. Chem. A. 18,290 (1979). R. WILSON AND D. KIVEUON, J. Chem. Phys. 44, 154 (1966). P. W. ATKINS AND D. KIVELSON, J. Chem. Phys. 44, 169 (1966). 7. P. S. HUBBARD, Phys. Rev. 131, 1155 (1963). 8. B. S. PRABHANANDA, Indian J. Pare Appl. Phys. 18, 823 (1980). 9. D. A~TANASIO, C. P. KEUZERS, J. P. VAN DE BERG, AND E. DE BOER, Mol. Phys. 31, 10. G. KRISHNAM~~RTHY AND B. S. PRABHANANDA, J. Phys. Chem. 84,637 (1980). II. H. A. KUSKA, M. T. ROGERS, AND R. E. DRULLINGER, J. Phys. Chem. 71, 109 (1967).
3. 4. 5. 6.
501 (1976).
12. H. A. KUSKA, J. Am. Chem. Sot. 97,2289 (1975). 13. G. KRISHNAM~RTHY AND B. S. PRABHANANDA, J. Magn. Reson. 24,215 (1976). 14. Z. R. BARATOVA, E. V. SEMENOV, P. M. S~LOZHENKIN, AND B. S. PRABHANANDA, horg. 57 (1982). IS. M. P. EASTMAN, R. G. KOOSER, M. R. DAS, AND J. H. FREED, J. Chem. Phys. 51,269O 16. T. R. REDDY AND R. SRINIVASAN, J. Chem. Phys. 43, 1404 (1965). 17. H. R. GERSMANN AND J. D. SWALEN, J. Chem. Phys. 36, 3221 (1962).
Chem. 21, (1969).