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EPR of mixed valence complexes: Hexaaminecobalt(III) chlorocuprates(I, II)
J. inorg, nucl. Chem., 1975, Vol. 37, pp. 70%711, Pergamon Press. Printed in Great Britain
EPR OF MIXED VALENCE COMPLEXES'HEXAAMINECOBALT(III) CHLOROCUPRATES(I, II) L. SHIA*, R. SCARINGEt and G. KOKOSZKA:~ State University of New York, College of Arts and Science, Plattsburgh, New York 12901 (Received 5 April 1974) Abstract--Eleven compounds in the mixed valence series of hexaaminecobalt(III) chlorocuprate(I, II), which may be formulated as [Co(NH3)6][CuCI,]~[CuCI,],_~ have been investigated by electron paramagnetic resonance at room temperature and liquid nitrogen temperature. The data show a variation of linewidth with concentration that is consistent with the structural data at room temperature. At 77°K a somewhat different variation is observed which suggests that the preferred crystal structure is a function of temperature. The interpretation indicates that exchange narrowing is a major factor in determining the linewidth but it is a function of several parameters.
INTRODUCTION
COMPLEXES incorporating copper in its two common valence states and the chloride anion form in a wide variety of coordination geometries. One of the more interesting members of this class of complexes incorporates the trigonal bipyramidal copper(II) ion and tetragonal copper(I) ion in the mixed valence complex M(NH3)6[CuCI~]x[CuCh]~_x where M may be Co(III), Cr(III), Rh(III), or Ru(III)[1, 2]. Experimental investigations on this series have included magnetic susceptibility [3, 4], EPR [5, 6], electronic spectra [2, 3, 7-9], and i.r. [2]. The X-ray structure of the Cr(llI) complex has been recently redetermined[10]. Day and coworkers have discussed the spectra, structural variations and semiconducting properties of the cobatic chlorocuprates(I, II) as a function of copper(II) concentration [7-9, 11]. They find [11] that the Co(NH3)6CuC15 phase (space group Fd3c) persists until the ratio of copper(I) to total copper (hereafter referred to as R) is at least 0.5. When R is in the 0.8-1.00 range the crystals are isotopic with the Co[(NH3)6],Cu~Cltv phase (space group Fd3m). In the 0.5-0.8 range the samples appear inhomogeneous. Day[8] indicates that the copper(II) species is always CuCI53- across the whole range. We have investigated the EPR spectra of 10 samples of this mixed valence complex at both room temperature and liquid nitrogen temperature in order to provide new experimental information on these complexes.
the full concentration range. Total copper was determined iodometrically after oxidizing copper(l) in nitric acid. The per cent copper(I) was determined by dissolving in an excess of acidified ferric ammonium sulfate and back titrating with standard dichromate. Electrolytic iron wire was used to standardize the dichromate solution. The analytic results are tabulated in Table 1. The ratio of per cent total copper(II) minus per cent copper(1) to per cent total copper is the fractional copper(II) concentration. The data listed in Table 1 are the experimental results without any additional corrections. In some cases small corrections[6] are needed near the high copper(I) end of the scale to account for the change in chemical composition but these are not significant for purposes of this paper. The samples were further characterized by an X-ray diffractometer study. The unit cell dimension[12] was determined from 12 strong reflections and produced an average value of 21.9-+ 0.2 ~, in good agreement with the values obtained by Day [81. The liquid nitrogen temperature data were obtained with a 9GHz spectrometer and a field modulation of 1000Hz. The modulation amplitude was always under 1 G and the field was calibrated with the well characterized material MnF~ whose peakto-peak linewidth was taken as 280 G. The lineshapes for samples A, B, C and E were the anisotropic line characteristic of axially asymmetric complexes. The g values and linewidths were obtained using the method of Searle et al.[13]. For samples F-K the lines were symmetric with only one g value. The uncertainty in Table 1. Analytical and linewidth (298K) data % Total % Copper(I) copper % Copper(I) Total
EXPERIMENTAL
the hexaaminecobalt(IfI) chlorocuprates were prepated by Mori's method[l]. A total of 10 complexes were investigated over *A portion of this work was submitted to the State University of New York in partial fulfillment of the requirements of the degree of Master of Arts in Chemistry. Present Address, Department of Chemistry, Tufts University, Medford, Mass. 02155. *Present address: Department of Chemistry, University of North Carolina, Chapel Hill. ~To whom correspondence should be addressed.
G H I j K 709
15.5 16.1 17.0 16.1 16.5 17.3 16.4 18.0 19.6 18.5 19.5
0.0 0.7 2-3 2.5 3.7 6.5 7.7 11-4 14.4 17.0 19.0
0.00 0.04 0.14 0.16 0.22 0.38 0.47 0.63 0.73 0.92 0.97
X-Band data
P-Band data
80.8 79.1 74.4 74.2 68.8 66.5 66.3 70.1 67.5 66.0 84.8
78.3 79.7 77.1 76.4 71-9 66.5 66.3 72.1 71.3 70.8
710
L. SHIA et al. Table 2. g Values andlinewidths at77K gi
g~
AHI
AHI~
A B C D E
2.199 2.199 2.195 2.201 2.198 g
2.081 2.086 2.110 2.095 2.0% AH
46 56 67 72 73
62 65 70 72 76
F G H I K
2.178 2.176 2.173 2.175 2.180
75 79 77 78 92
the linewidths (Table 2) is estimated to be 5 per cent in samples E-K and 10 per cent in the first four samples. The room temperature EPR data listed in Table 1 are the averages of at least 10 field sweeps at each frequency for each sample in both the upfield and downfielddirections. These results are obtained at X-band (9 GHz) and P-band (14 GHz). Additional data obtained at 25 GHz are in good agreement with these results. Some of the X-band data were independently confirmed on a Varian spectrometer at Clark University. The most reproducible method for determining linewidths was to measure between outermost halfheights in the derivative presentation. This width is just 4.159 times the peak-to-peak width for a Lorentzian lineshape. Considerable care was taken to use small sample volumes to avoid artificial broadening due to the "sample size effect." Except for the bottom three samples listed in Table 1 all lineshapes were Lorentzian. In the lower samples the lineshape deviated from Lorentzian geometry and the linewidth reported in the peak-to-peak linewidth for the slightly asymmetrical shape. The shapes were very similar to those reported by Mori[5] showing the spectrum obtained from sample J. The high field halfline was rather broader than the low field hairline. Assuminga linewidthindependent of angular dependence, this is the expected behavior for a case of gi >g~l. RESULTS The high temperature data show that in the range where the ratio of copper(I) to total copper increase from 0 to about 0.5 there is a decrease in the linewidth. The three points H, I and J show a second separate decrease while the last sample, K is rather broader. The break at about 0.5 in the linewidth correlates with the room temperature structural work[ll]. At this temperature the linewidth in the EPR spectrum is probably due to several factors including dipolar, spin lattice, and antisymmetric exchange contributions to line broadening as well as exchange narrowing. Furthermore, there is some possibility of fluxional averaging of the g factors but this is considered less likely in view of the antisymmetry of the lineshape of the last three samples. Since there is not a major difference between the low and high temperature linewidth values over most of the range we believe that spin-spin interactions must be playing a major role determining the linewidth at high temperatures. The dipolar variation with concentration produces a monotonically decreasing linewidth which is proportional to the factional concentration [14]. While this is consistent
with the general trend such a functional relationship does not account in detail for the data from the first seven samples. This suggests that other factors which influence the linewidth must also have a concentration dependence. One of these could be the concentration dependence of the exchange field. The low temperature data may be divided into three categories. The first five samples show the characteristic anisotropic behavior with linewidth that increase with decreasing concentration while samples F-J show an isotropic spectrum with linewidths which are independent of concentration. The last sample, K, is in a category by itself. This is due, we believe, to the rather dilute magnetic concentration. It is noteworthy that the break in behavior occurred at a value of R below 0.38. This result is lower than the room temperature result. The explanation of the break in the low temperature results may be associated with crystallographic changes. The incomplete averaging of the g factors in the first five samples indicates exchange pathways which do not fully exchange average the various sites. This is confirmed in a single crystal EPR spectrum obtained for the full Cu(II) end member of this series in which three lines were observed along the [100] direction. The isotropic behavior of samples F-I with an average of g of 2.175 and an average linewidth of 77 G indicates a crystal structure with exchange pathways which allow a more complete exchange averaging over a shorter concentration range at low temperature. This suggests that the Fd3c structure may be the dominant form over a smaller concentration range at 77°K. At low temperatures we believe that the exchange interactions are again dominating the linewidth. In this case, however, the linewidth increase with decreasing copper(II) for one group and then appear independent of concentration in the second group. These results: (1) the general decrease in linewidth with decreasing copper(II) concentration at high temperatures, (2) the increase in average linewidth in the first five samples with decreasing copper(II) concentration at low temperature in the second four samples are reminiscent of the linewidth behavior of semidilute Europium complexes studied by Somokhvalov and coworkers[15]. These results suggest that the exchange field may be a rather complex function sensitive to changes in both concentration and lattice parameters and, possibly by the latter mechanism, temperature as well. Finally we note that the weighted average of the anisotropic g values is in good agreement with the isotropic values. This result suggests that the paramagnetic species is the same for all the samples and, in accordance with Day's suggestion[8], we associate this with the trigonal bypyramidal (CuCls) -3 complex. We conclude (1) that the high temperature EPR results show a concentration dependence which cannot be accounted for by only the concentration dependence of the dipolar interaction, (2) that the break in the high temperature EPR behavior reflects the structural changes noted in earlier studies, (3) that at low temperatures again two main groups of spectra are observed but the break is
EpR of mixed valence complexes below R = 0.38 and (4) that the most probable explanation lies in a concentration dependence of the exchange field which is also dependent both on temperature and lattice geometry. Acknowledgements--Acknowledgement is made to the donors of the Petroleum Research Fund administered by the American Chemical society for support of this work. Equipment purchases from funds made available by the Research corporation and the State University of New York Research foundation are also ticknowFedged. REFERENCES
1. M. Mori. Bull. chem. Soc. Japan 33, 985 (1960), 34, 1249 ~1961). 2. G. C. Allen and N. S. Hugh, lnorg. Chem. 6, 4 (1967). 3. W. E. Hatfield and T. S. Piper, lnorg. Chem. 3, 841 (1%4),
711
4. M. Mori. Bull. chem. Soc. Japan. 34, 454 (1961). 5. M. Mori and S. Fujiwara, Bull. chem. Soc. Japan. 41, 1636 (1963). 6. R. Scaringe and G. Kokoszka, J. chem. Phys. 60, 40 (19741. 7. D. Culpin, P. Day, P. R. Edwards and R. J. P. Williams, J. chem. Soc. 1155 (1968). 8. P. Day, J. chem. Soc. A, 1045 (1967). 9. P. Day and D. W. Smith J. chem. Soc. A, 1045 (1967). 10. K. N. Raymond, D. U. Meek and J, A. lbers, lnor,e. Chem. 7, 1111 (1968). 11. P. Murry-Rust, P. Day and C. K. Prout, Chem. Comm. 277 (1966). 12. P. Day. Diffractometer data. Private communication. 13. Jr W. Searl, R. C. Smith and S. J. Wyard, Proc. Phys. Soc. 78, 1174 (1961). 14. C. Kittel and E. Abrahams, Phys. Rev. 90, 238 (1953). 15. A. Samokhvalov, V. Babushkin, V. Bamburov, N~ Lobachevskaya, Soviet Phys. solid St. 13, 2530 (1972~.
EPR OF MIXED VALENCE COMPLEXES'HEXAAMINECOBALT(III) CHLOROCUPRATES(I, II) L. SHIA*, R. SCARINGEt and G. KOKOSZKA:~ State University of New York, College of Arts and Science, Plattsburgh, New York 12901 (Received 5 April 1974) Abstract--Eleven compounds in the mixed valence series of hexaaminecobalt(III) chlorocuprate(I, II), which may be formulated as [Co(NH3)6][CuCI,]~[CuCI,],_~ have been investigated by electron paramagnetic resonance at room temperature and liquid nitrogen temperature. The data show a variation of linewidth with concentration that is consistent with the structural data at room temperature. At 77°K a somewhat different variation is observed which suggests that the preferred crystal structure is a function of temperature. The interpretation indicates that exchange narrowing is a major factor in determining the linewidth but it is a function of several parameters.
INTRODUCTION
COMPLEXES incorporating copper in its two common valence states and the chloride anion form in a wide variety of coordination geometries. One of the more interesting members of this class of complexes incorporates the trigonal bipyramidal copper(II) ion and tetragonal copper(I) ion in the mixed valence complex M(NH3)6[CuCI~]x[CuCh]~_x where M may be Co(III), Cr(III), Rh(III), or Ru(III)[1, 2]. Experimental investigations on this series have included magnetic susceptibility [3, 4], EPR [5, 6], electronic spectra [2, 3, 7-9], and i.r. [2]. The X-ray structure of the Cr(llI) complex has been recently redetermined[10]. Day and coworkers have discussed the spectra, structural variations and semiconducting properties of the cobatic chlorocuprates(I, II) as a function of copper(II) concentration [7-9, 11]. They find [11] that the Co(NH3)6CuC15 phase (space group Fd3c) persists until the ratio of copper(I) to total copper (hereafter referred to as R) is at least 0.5. When R is in the 0.8-1.00 range the crystals are isotopic with the Co[(NH3)6],Cu~Cltv phase (space group Fd3m). In the 0.5-0.8 range the samples appear inhomogeneous. Day[8] indicates that the copper(II) species is always CuCI53- across the whole range. We have investigated the EPR spectra of 10 samples of this mixed valence complex at both room temperature and liquid nitrogen temperature in order to provide new experimental information on these complexes.
the full concentration range. Total copper was determined iodometrically after oxidizing copper(l) in nitric acid. The per cent copper(I) was determined by dissolving in an excess of acidified ferric ammonium sulfate and back titrating with standard dichromate. Electrolytic iron wire was used to standardize the dichromate solution. The analytic results are tabulated in Table 1. The ratio of per cent total copper(II) minus per cent copper(1) to per cent total copper is the fractional copper(II) concentration. The data listed in Table 1 are the experimental results without any additional corrections. In some cases small corrections[6] are needed near the high copper(I) end of the scale to account for the change in chemical composition but these are not significant for purposes of this paper. The samples were further characterized by an X-ray diffractometer study. The unit cell dimension[12] was determined from 12 strong reflections and produced an average value of 21.9-+ 0.2 ~, in good agreement with the values obtained by Day [81. The liquid nitrogen temperature data were obtained with a 9GHz spectrometer and a field modulation of 1000Hz. The modulation amplitude was always under 1 G and the field was calibrated with the well characterized material MnF~ whose peakto-peak linewidth was taken as 280 G. The lineshapes for samples A, B, C and E were the anisotropic line characteristic of axially asymmetric complexes. The g values and linewidths were obtained using the method of Searle et al.[13]. For samples F-K the lines were symmetric with only one g value. The uncertainty in Table 1. Analytical and linewidth (298K) data % Total % Copper(I) copper % Copper(I) Total
EXPERIMENTAL
the hexaaminecobalt(IfI) chlorocuprates were prepated by Mori's method[l]. A total of 10 complexes were investigated over *A portion of this work was submitted to the State University of New York in partial fulfillment of the requirements of the degree of Master of Arts in Chemistry. Present Address, Department of Chemistry, Tufts University, Medford, Mass. 02155. *Present address: Department of Chemistry, University of North Carolina, Chapel Hill. ~To whom correspondence should be addressed.
G H I j K 709
15.5 16.1 17.0 16.1 16.5 17.3 16.4 18.0 19.6 18.5 19.5
0.0 0.7 2-3 2.5 3.7 6.5 7.7 11-4 14.4 17.0 19.0
0.00 0.04 0.14 0.16 0.22 0.38 0.47 0.63 0.73 0.92 0.97
X-Band data
P-Band data
80.8 79.1 74.4 74.2 68.8 66.5 66.3 70.1 67.5 66.0 84.8
78.3 79.7 77.1 76.4 71-9 66.5 66.3 72.1 71.3 70.8
710
L. SHIA et al. Table 2. g Values andlinewidths at77K gi
g~
AHI
AHI~
A B C D E
2.199 2.199 2.195 2.201 2.198 g
2.081 2.086 2.110 2.095 2.0% AH
46 56 67 72 73
62 65 70 72 76
F G H I K
2.178 2.176 2.173 2.175 2.180
75 79 77 78 92
the linewidths (Table 2) is estimated to be 5 per cent in samples E-K and 10 per cent in the first four samples. The room temperature EPR data listed in Table 1 are the averages of at least 10 field sweeps at each frequency for each sample in both the upfield and downfielddirections. These results are obtained at X-band (9 GHz) and P-band (14 GHz). Additional data obtained at 25 GHz are in good agreement with these results. Some of the X-band data were independently confirmed on a Varian spectrometer at Clark University. The most reproducible method for determining linewidths was to measure between outermost halfheights in the derivative presentation. This width is just 4.159 times the peak-to-peak width for a Lorentzian lineshape. Considerable care was taken to use small sample volumes to avoid artificial broadening due to the "sample size effect." Except for the bottom three samples listed in Table 1 all lineshapes were Lorentzian. In the lower samples the lineshape deviated from Lorentzian geometry and the linewidth reported in the peak-to-peak linewidth for the slightly asymmetrical shape. The shapes were very similar to those reported by Mori[5] showing the spectrum obtained from sample J. The high field halfline was rather broader than the low field hairline. Assuminga linewidthindependent of angular dependence, this is the expected behavior for a case of gi >g~l. RESULTS The high temperature data show that in the range where the ratio of copper(I) to total copper increase from 0 to about 0.5 there is a decrease in the linewidth. The three points H, I and J show a second separate decrease while the last sample, K is rather broader. The break at about 0.5 in the linewidth correlates with the room temperature structural work[ll]. At this temperature the linewidth in the EPR spectrum is probably due to several factors including dipolar, spin lattice, and antisymmetric exchange contributions to line broadening as well as exchange narrowing. Furthermore, there is some possibility of fluxional averaging of the g factors but this is considered less likely in view of the antisymmetry of the lineshape of the last three samples. Since there is not a major difference between the low and high temperature linewidth values over most of the range we believe that spin-spin interactions must be playing a major role determining the linewidth at high temperatures. The dipolar variation with concentration produces a monotonically decreasing linewidth which is proportional to the factional concentration [14]. While this is consistent
with the general trend such a functional relationship does not account in detail for the data from the first seven samples. This suggests that other factors which influence the linewidth must also have a concentration dependence. One of these could be the concentration dependence of the exchange field. The low temperature data may be divided into three categories. The first five samples show the characteristic anisotropic behavior with linewidth that increase with decreasing concentration while samples F-J show an isotropic spectrum with linewidths which are independent of concentration. The last sample, K, is in a category by itself. This is due, we believe, to the rather dilute magnetic concentration. It is noteworthy that the break in behavior occurred at a value of R below 0.38. This result is lower than the room temperature result. The explanation of the break in the low temperature results may be associated with crystallographic changes. The incomplete averaging of the g factors in the first five samples indicates exchange pathways which do not fully exchange average the various sites. This is confirmed in a single crystal EPR spectrum obtained for the full Cu(II) end member of this series in which three lines were observed along the [100] direction. The isotropic behavior of samples F-I with an average of g of 2.175 and an average linewidth of 77 G indicates a crystal structure with exchange pathways which allow a more complete exchange averaging over a shorter concentration range at low temperature. This suggests that the Fd3c structure may be the dominant form over a smaller concentration range at 77°K. At low temperatures we believe that the exchange interactions are again dominating the linewidth. In this case, however, the linewidth increase with decreasing copper(II) for one group and then appear independent of concentration in the second group. These results: (1) the general decrease in linewidth with decreasing copper(II) concentration at high temperatures, (2) the increase in average linewidth in the first five samples with decreasing copper(II) concentration at low temperature in the second four samples are reminiscent of the linewidth behavior of semidilute Europium complexes studied by Somokhvalov and coworkers[15]. These results suggest that the exchange field may be a rather complex function sensitive to changes in both concentration and lattice parameters and, possibly by the latter mechanism, temperature as well. Finally we note that the weighted average of the anisotropic g values is in good agreement with the isotropic values. This result suggests that the paramagnetic species is the same for all the samples and, in accordance with Day's suggestion[8], we associate this with the trigonal bypyramidal (CuCls) -3 complex. We conclude (1) that the high temperature EPR results show a concentration dependence which cannot be accounted for by only the concentration dependence of the dipolar interaction, (2) that the break in the high temperature EPR behavior reflects the structural changes noted in earlier studies, (3) that at low temperatures again two main groups of spectra are observed but the break is
EpR of mixed valence complexes below R = 0.38 and (4) that the most probable explanation lies in a concentration dependence of the exchange field which is also dependent both on temperature and lattice geometry. Acknowledgements--Acknowledgement is made to the donors of the Petroleum Research Fund administered by the American Chemical society for support of this work. Equipment purchases from funds made available by the Research corporation and the State University of New York Research foundation are also ticknowFedged. REFERENCES
1. M. Mori. Bull. chem. Soc. Japan 33, 985 (1960), 34, 1249 ~1961). 2. G. C. Allen and N. S. Hugh, lnorg. Chem. 6, 4 (1967). 3. W. E. Hatfield and T. S. Piper, lnorg. Chem. 3, 841 (1%4),
711
4. M. Mori. Bull. chem. Soc. Japan. 34, 454 (1961). 5. M. Mori and S. Fujiwara, Bull. chem. Soc. Japan. 41, 1636 (1963). 6. R. Scaringe and G. Kokoszka, J. chem. Phys. 60, 40 (19741. 7. D. Culpin, P. Day, P. R. Edwards and R. J. P. Williams, J. chem. Soc. 1155 (1968). 8. P. Day, J. chem. Soc. A, 1045 (1967). 9. P. Day and D. W. Smith J. chem. Soc. A, 1045 (1967). 10. K. N. Raymond, D. U. Meek and J, A. lbers, lnor,e. Chem. 7, 1111 (1968). 11. P. Murry-Rust, P. Day and C. K. Prout, Chem. Comm. 277 (1966). 12. P. Day. Diffractometer data. Private communication. 13. Jr W. Searl, R. C. Smith and S. J. Wyard, Proc. Phys. Soc. 78, 1174 (1961). 14. C. Kittel and E. Abrahams, Phys. Rev. 90, 238 (1953). 15. A. Samokhvalov, V. Babushkin, V. Bamburov, N~ Lobachevskaya, Soviet Phys. solid St. 13, 2530 (1972~.