DOI: 10.1002/chem.201403792
Communication
& Single-Molecule Magnets
Enhancement of Spin Relaxation in an FeDy2Fe Coordination Cluster by Magnetic Fields Guo Peng,[a] Valeriu Mereacre,*[b] George E. Kostakis,[a] Juliusz A. Wolny,[c] Volker Schnemann,[c] and Annie K. Powell*[b] Dedicated to Prof. Marius Andruh on the occasion of his 60th birthday
Abstract: Two [FeLn2Fe(m3-OH)2(teg)2(N3)2(C6H5COO)4] compounds (where Ln = YIII and DyIII ; teg = triethylene glycol anion) have been synthesized and studied using SQUID and Mçssbauer spectroscopy. The magnetic measurements on both compounds indicate dominant antiferromagnetic interactions between the metal centers. Analysis of the 57Fe Mçssbauer spectra complement the ac magnetic susceptibility measurements, which show how a static magnetic field can quench the slow relaxation of magnetization generated by the anisotropic DyIII ions.
Since it was recognized that paramagnetic molecules can show magnetic bistability either as single-ion systems or, more commonly, as cooperatively coupled coordination cluster entities, such single-molecule magnets (SMMs) have been an intensively researched area of interest. Nonetheless, many challenges for optimizing the desired behavior of such molecules need to be addressed. One of the most important of these in the first line of enquiry is how to direct, or at least understand, the parameters that need to be tuned to achieve control over the characteristics of the energy barrier that gives the system its inherent magnetic bistability. This arises as a result of a slow relaxation of magnetization owing to an energy barrier (Ueff) to spin inversion, but the quantum nature of such molecular-based systems also allows for tunneling processes that can render the height of the barrier an essentially useless parameter. This has been seen in its most extreme form for the many cases of systems based on various lanthanide ions, which possess both high spin and large magnetic anisotropies.[1] Although energy barriers for relaxation of magnetization
of up to 800 K have been achieved,[2] which should be sufficient to have such SMMs operating at useful temperatures, quantum tunneling of the magnetization (QTM) means that the system is short-circuited in terms of showing the desired useful hysteresis effects associated with magnetic behavior. One particularly useful method for testing magnetic relaxation is using alternating current (ac) susceptibility measurements with an oscillating ac field (usually 3–5 Oe) at different frequencies. When QTM is present, a small external dc field to fully or partly suppress the QTM is applied. The last procedure will shift the maxima in c’ and c“ towards higher temperatures thereby increasing the relaxation time significantly. Herein we show how application of an external field may give rise to an increase in the rate of the relaxation process and how this effect can also be microscopically monitored using 57Fe Mçssbauer spectroscopy. Two cluster molecules are reported: [FeLn2Fe(m3-OH)2(teg)2(N3)2(C6H5COO)4] where H2teg = triethylene glycol, Ln = Y (1), Dy (2). These compounds have a similar core motif to previously reported Fe2Ln2 coordination clusters.[3] Compound 2 crystallizes in triclinic P1¯ space group. Compound 1 was found to be isostructural with 2 by comparing the powder XRD patterns (Supporting Information, Figure S6). The central core of compound 2 presents an Fe2Dy2 planar butterfly topology with the Dy in the body (Figure 1). The core
[a] Dr. G. Peng, Dr. G. E. Kostakis Institute of Nanotechnology, Karlsruhe Institute of Technology Postfach 3640, 76021 Karlsruhe (Germany) [b] Dr. V. Mereacre, Prof. Dr. A. K. Powell Institute of Inorganic Chemistry, Karlsruhe Institute of Technology Engesserstrasse 15, 76128 Karlsruhe (Germany) E-mail: [email protected] [email protected] [c] Dr. J. A. Wolny, Prof.Dr. V. Schnemann Institute of Physics, University of Kaiserslautern Erwin Schrçdingerstrasse 56, 67653 Kaiserslautern (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403792. Chem. Eur. J. 2014, 20, 12381 – 12384
Figure 1. Molecular structure of compound [FeDy2Fe(m3-OH)2(teg)2(N3)2(C6H5COO)4] (2).[8] H atoms on carbon atoms are omitted for clarity.
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Communication is held together by two m3-OH groups above and below the plane. The teg ligands coordinate to Dy in a tetradentate chelated mode and bridge to two adjacent Fe centers with two deprotonated alkoxide arms. Benzoate and azide groups complete the coordination sphere of Dy and Fe centers. The Dy centers are eight-coordinate with trigonal dodecahedral coordination geometry, whereas the Fe centers are six-coordinate possessing slightly distorted octahedral coordination geometry. The dc magnetic susceptibilities of compounds 1 and 2 were measured in the temperature range 1.8–300 K under an applied field of 1000 Oe (Figure 2). The dc data listed in the
through a quantum tunneling mechanism, the ac susceptibility measurements were performed under small dc fields. However, ac magnetic measurements as a function of the frequency show absence of quantum tunneling. Moreover, the applied fields induce faster relaxation of the magnetization (Supporting Information, Figure S5). Since the weak ac signal in ac magnetic susceptibility measurements as function of the temperature is due the slow relaxation of magnetization generated by the anisotropic DyIII ions, we conclude that application of a static magnetic field is fastening their relaxation of magnetization. Herein, we will show how this process affects the hyperfine parameters of the iron nuclei and interaction between iron and dysprosium ions. The Mçssbauer spectrum of 1 obtained at 3 K and zero applied external fields displays an asymmetric doublet with an isomer shift d = 0.57 mm s1 and a quadrupole splitting DEQ = 0.39 mm s1 and line width T = 0.80 mm s1 (Figure 3 a). These
Figure 2. The cT versus T plots for compounds 1 and 2 at 1000 Oe.
Supporting Information, Table S1 show that the cT values of 1 and 2 at room temperature are close to the expected values for two uncoupled Ln and two isolated Fe ions. The cT values of compound 1 decrease slowly with decreasing temperature until 1.8 K, indicating that the interactions between the two separated Fe centers are very weak. This conclusion is further confirmed by M versus H plots of 1, in which the magnetization reaches 9.91 NmB at 2 K and 70 kOe. This value is consistent with that expected for two isolated Fe ions with S = 5/2. In the case of compound 2, the cT products decrease with cooling, which is largely due to the thermal depopulation of excited j mJ > states of the DyIII ion. The field dependence of the magnetization of 2 increases slowly with increasing applied field without saturation even at 70 kOe. This behavior suggests the presence of magnetic anisotropy and/or low lying-excited states. The M versus H/T curves are non-superposed, which further highlights the presence of these effects. We then performed ac susceptibility measurements to probe the dynamic properties of these two compounds. As expected, complex 1 did not show any out-of-phase (c’’) signal. The temperature dependence of the in-phase and out-of-phase ac susceptibilities of compound 2 show frequency dependence, but with no maxima above 1.8 K even at 1500 Hz (Supporting Information, Figure S3), which is indicative of very fast relaxation of the magnetization. To check whether this can be attributed to the presence of a fast relaxation of the magnetization Chem. Eur. J. 2014, 20, 12381 – 12384
www.chemeurj.org
Figure 3. 57Fe Mçssbauer spectra of 1 at 3 K in zero field and applied external magnetic fields.
parameters are typical for octahedral coordinated ferric highspin FeIII ions with an O/N ligand sphere. The observed linewidth asymmetry is typical for magnetically isolated single ferric ions[4] and is the result of paramagnetic relaxation times that are only slightly above the typical time window of Mçssbauer spectroscopy, which is about 107 s. The application of an external field of 4 T leads to a broad magnetic sextet, which has been analyzed by means of the hyperfine field distribution with a maximum hyperfine field Bhf = 42.3 T (Figure 3 b). At a field of 6 T, again a broad magnetic sextet is obtained, now with a maximal hyperfine field of Bhf = 46.2 T (Figure 3 c). At small external fields, magnetically isolated ferric high-spin FeIII ions display in general sharp magnetic sextets at liquid helium temperatures and thus have spin relaxation times that are longer than the time window of Mçssbauer spectroscopy (ca. 107 s). However, compound 1 shows no magnetic sextet even in the small earth magnetic field at T = 3 K. This can be explained by the presence of spin–spin relaxation between the two ferric high-spin FeIII ions, which have a distance of 5.48 .
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication As the YIII ions are diamagnetic, these do not influence the spin–spin relaxation properties of 1. It is known that the application of large external fields slows down the spin–spin relaxation process. This is reflected in the observation of magnetically split patterns at external fields of 4 and 6T (Figure 3 b, c). The fact that Bhf increases with increasing applied external fields supports the conclusion that spin-spin relaxation is the dominant relaxation process in 1. Surprisingly, compound 2 shows a well-defined magnetic sextet at T = 3 K and zero external fields (Figure 4 b), whereas
Figure 4. 57Fe Mçssbauer spectra of 2 at 30 K and 3 K in zero field and applied external magnetic field. d = 0.56 mm s1, DEQ = 0.35 mm s1, T = 0.85 mm s1.
the Mçssbauer pattern of 2 obtained at T = 30 K (Figure 4 a) resembles that observed for 1 at T = 3 K (Figure 3 a). Given the 107 s time window of Mçssbauer spectroscopy, the ferric high-spin FeIII ions in 2 must relax with a relaxation rate much smaller than 107 s1. Obviously the presence of paramagnetic DyIII ions cause a significant decrease in the iron spin–spin relaxation rate from much more than 107 s1 in the YIII complex 1 to much less than 107 s1 in the DyIII complex 2. The magnetic hyperfine field of 2 obtained at 3 K is Bhf = 53.9 T, which corresponds very well with the hyperfine field of about 55 T observed for magnetically isolated ferric high-spin FeIII ions with saturated spin expectation values < S > x,y,z = 5/2.[4, 5] The application of an external field of 4 T leads to a magnetically broadened sextet with a maximum hyperfine field of Bhf = 46.1 T. The fact that Bhf is decreasing with increasing external field is caused by the dominating negative Fermi contact field of ferric high-spin FeIII ions. Surprisingly, the magnetically broadened Mçssbauer signatures of 1 (Figure 3 b) and of 2 (Figure 4 c) obtained at 3 K and 4 T are strikingly similar. All of these experimental observations described above can be explained by two scenarios: 1) In 1, spin–spin relaxation is the dominant relaxation process. At zero external field, the spin–spin relaxation rate is Chem. Eur. J. 2014, 20, 12381 – 12384
www.chemeurj.org
larger than 107 s1, and with increasing external field spin– spin relaxation is slowed down. The paramagnetic DyIII ions in 2 influence the magnetic moments of the FeIII sites in such a way that the relaxation rate of the FeIII sites is smaller than 107 s1. This process has been recently reported as a magnetic pinning effect between the DyIII and the FeIII ions.[6, 7] If the relaxation time is longer than the typical time window of Mçssbauer spectroscopy (107 s), a magnetic pattern should be observable, which corresponds to the magnetic hyperfine fields that are caused by the spin expectation values of the three Kramers doublets of the FeIII S = 5/2 spin system. An application of a large external field leads in general to the occupation of an isolated ground state which causes a well-defined sextet with a hyperfine field of up to 55 T.[4] However, Figure 4 c does not show a sharp, but a broadened sextet which is very similar to the spectrum of 1 obtained under the same experimental conditions (Figure 3 b). Since the 4 T Mçssbauer spectrum of 1 is doubtless influenced by relaxation effects, it is tempting to conclude that also the 4 T spectrum of 2 reflects FeIII sites that display a relaxation rate slightly faster than 107 s1. This implies that in compound 2 the application of an external magnetic field at 3 K leads to an enhancement of the spin relaxation rate from ! 107 s1 without an external field (Figure 4 b) to slightly above 107 s1 at an external field of 4 T. 2) In the alternative scenario, the DyIII ions also pin the spins of the FeIII ions. At 3 K and zero applied fields, the magnetic moments of the DyIII ions would be randomly oriented, and as the spins of the FeIII ions are pinned to the magnetic moments of the DyIII ions, their spin expectation values and therefore also the magnetic hyperfine fields of the FeIII sites are randomly oriented. The application of a 4 T external field would induce an incomplete orientation of the DyIII moments with respect to the 4 T external field. A perfect alignment of the moments of the DyIII and the FeIII ions with the external field would lead to a sharp magnetic sextet with line widths of less than 0.4 mms1 and an intensity ratio of 3:4:1:1:4:3. However, an incomplete alignment of the DyIII ions would lead to an incomplete alignment of the FeIII spins and therefore to the observed distribution in hyperfine field, as evident from Figure 4 c. Although scenario (1) has never been reported to date, it is also consistent with ac magnetic susceptibility measurements in static magnetic fields (Supporting Information, Figure S5) for 2; with increasing dc field, the curves maxima are moving to higher frequencies, showing in this way a speeding up of relaxation of the DyIII magnetic moments. To explore whether this unprecedented effect is a consequence of different electronic structures of 1 and 2, density functional calculations have been performed with the PBE/ CEP-31 g functional/basis set according to a procedure described previously.[7b] The analysis of the so obtained electronic structures of both complexes (see the Supporting Information) indicates that the framework of the orbitals involving d-orbitals of iron atoms of 1 and 2 is the same.
12383
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication In summary, we have outlined the role that 57Fe Mçssbauer spectroscopic characterization can play in understanding lanthanide anisotropy in molecular clusters. It has been demonstrated that this technique has much to offer as an improved analytical method for determining forms of magnetic behavior in lanthanides which are not observed with commonly used techniques. This study has shown that 57Fe Mçssbauer spectroscopy is a very sensitive method for diagnosing very fine microscopic changes of iron ions electronic structure affected by coordination environments and anisotropic weakly interacting neighbors, and gives us microscopic information about metal–metal communication and relaxation dynamics on specific centers (in the present case, Fe and Dy ions) and the influence of applied external magnetic fields on all these hyperfine effects.
Acknowledgements This work was supported by the DFG-funded transregional collaborative research center SFB/TRR 88 “3MET”. Keywords: cooperative effects · heterometallic complexes · iron · lanthanides · Moessbauer spectroscopy
Chem. Eur. J. 2014, 20, 12381 – 12384
www.chemeurj.org
[1] a) D. N. Woodruff, R. E. P. Winpenny, R. A. Layfield, Chem. Rev. 2013, 113, 5110; b) P. Zhang, Y. N. Guo, J. Tang, Coord. Chem. Rev. 2013, 257, 1728; c) F. Habib, M. Murugesu, Chem. Soc., Rev. 2013, 42, 3278; d) J. D. Rinehart, J. R. Long, Chem. Sci. 2011, 2, 2078; e) C. R. Ganivet, B. Ballesteros, G. de La Torre, J. M. Clemente-Juan, E. Coronado, T. Torres, Chem. Eur. J. 2013, 19, 1457. [2] R. J. Blagg, L. Ungur, F. Tuna, J. Speak, P. Comar, D. Collison, W. Wernsdorfer, E. J. L. McInnes, L. F. Chibotaru, . E. P. Winpenny, Nat. Chem. 2013, 5, 673, and references therein. [3] G. Peng, G. E. Kostakis, Y. Lan, A. K. Powell, Dalton Trans. 2013, 42, 46. [4] a) Y. Maeda, N. Tsutsumi, Y. Takashima, Inorg. Chem. 1984, 23, 2440; b) M. D. Timken, D. N. Hendrickson, E. Sinn, Inorg. Chem. 1985, 24, 3947. [5] N. N. Greenwood, T. C. Gibbs, Mçssbauer Spectroscopy Chapman and Hall Ltd., London, 1971. [6] a) V. Mereacre, F. Klçwer, Y. Lan, R. Clrac, J. A. Wolny, V. Schnemann, C. E. Anson, A. K. Powell, Beilstein J. Nanotechnol. 2013, 4, 807; b) M. N. Akhtar, V. Mereacre, G. Novitchi, J. P. Tuchagues, C. E. Anson, A. K. Powell, Chem. Eur. J. 2009, 15, 7278. [7] a) V. Mereacre, A. Baniodeh, C. E. Anson, A. K. Powell, J. Am. Chem. Soc. 2011, 133, 15335; b) A. Baniodeh, V. Mereacre, N. Magnani, Y. Lan, J. A. Wolny, V. Schnemann, C. E. Anson, A. K. Powell, Chem. Commun. 2013, 49, 9666. [8] CCDC 1006048 (2) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Received: June 3, 2014
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication
& Single-Molecule Magnets
Enhancement of Spin Relaxation in an FeDy2Fe Coordination Cluster by Magnetic Fields Guo Peng,[a] Valeriu Mereacre,*[b] George E. Kostakis,[a] Juliusz A. Wolny,[c] Volker Schnemann,[c] and Annie K. Powell*[b] Dedicated to Prof. Marius Andruh on the occasion of his 60th birthday
Abstract: Two [FeLn2Fe(m3-OH)2(teg)2(N3)2(C6H5COO)4] compounds (where Ln = YIII and DyIII ; teg = triethylene glycol anion) have been synthesized and studied using SQUID and Mçssbauer spectroscopy. The magnetic measurements on both compounds indicate dominant antiferromagnetic interactions between the metal centers. Analysis of the 57Fe Mçssbauer spectra complement the ac magnetic susceptibility measurements, which show how a static magnetic field can quench the slow relaxation of magnetization generated by the anisotropic DyIII ions.
Since it was recognized that paramagnetic molecules can show magnetic bistability either as single-ion systems or, more commonly, as cooperatively coupled coordination cluster entities, such single-molecule magnets (SMMs) have been an intensively researched area of interest. Nonetheless, many challenges for optimizing the desired behavior of such molecules need to be addressed. One of the most important of these in the first line of enquiry is how to direct, or at least understand, the parameters that need to be tuned to achieve control over the characteristics of the energy barrier that gives the system its inherent magnetic bistability. This arises as a result of a slow relaxation of magnetization owing to an energy barrier (Ueff) to spin inversion, but the quantum nature of such molecular-based systems also allows for tunneling processes that can render the height of the barrier an essentially useless parameter. This has been seen in its most extreme form for the many cases of systems based on various lanthanide ions, which possess both high spin and large magnetic anisotropies.[1] Although energy barriers for relaxation of magnetization
of up to 800 K have been achieved,[2] which should be sufficient to have such SMMs operating at useful temperatures, quantum tunneling of the magnetization (QTM) means that the system is short-circuited in terms of showing the desired useful hysteresis effects associated with magnetic behavior. One particularly useful method for testing magnetic relaxation is using alternating current (ac) susceptibility measurements with an oscillating ac field (usually 3–5 Oe) at different frequencies. When QTM is present, a small external dc field to fully or partly suppress the QTM is applied. The last procedure will shift the maxima in c’ and c“ towards higher temperatures thereby increasing the relaxation time significantly. Herein we show how application of an external field may give rise to an increase in the rate of the relaxation process and how this effect can also be microscopically monitored using 57Fe Mçssbauer spectroscopy. Two cluster molecules are reported: [FeLn2Fe(m3-OH)2(teg)2(N3)2(C6H5COO)4] where H2teg = triethylene glycol, Ln = Y (1), Dy (2). These compounds have a similar core motif to previously reported Fe2Ln2 coordination clusters.[3] Compound 2 crystallizes in triclinic P1¯ space group. Compound 1 was found to be isostructural with 2 by comparing the powder XRD patterns (Supporting Information, Figure S6). The central core of compound 2 presents an Fe2Dy2 planar butterfly topology with the Dy in the body (Figure 1). The core
[a] Dr. G. Peng, Dr. G. E. Kostakis Institute of Nanotechnology, Karlsruhe Institute of Technology Postfach 3640, 76021 Karlsruhe (Germany) [b] Dr. V. Mereacre, Prof. Dr. A. K. Powell Institute of Inorganic Chemistry, Karlsruhe Institute of Technology Engesserstrasse 15, 76128 Karlsruhe (Germany) E-mail: [email protected] [email protected] [c] Dr. J. A. Wolny, Prof.Dr. V. Schnemann Institute of Physics, University of Kaiserslautern Erwin Schrçdingerstrasse 56, 67653 Kaiserslautern (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403792. Chem. Eur. J. 2014, 20, 12381 – 12384
Figure 1. Molecular structure of compound [FeDy2Fe(m3-OH)2(teg)2(N3)2(C6H5COO)4] (2).[8] H atoms on carbon atoms are omitted for clarity.
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication is held together by two m3-OH groups above and below the plane. The teg ligands coordinate to Dy in a tetradentate chelated mode and bridge to two adjacent Fe centers with two deprotonated alkoxide arms. Benzoate and azide groups complete the coordination sphere of Dy and Fe centers. The Dy centers are eight-coordinate with trigonal dodecahedral coordination geometry, whereas the Fe centers are six-coordinate possessing slightly distorted octahedral coordination geometry. The dc magnetic susceptibilities of compounds 1 and 2 were measured in the temperature range 1.8–300 K under an applied field of 1000 Oe (Figure 2). The dc data listed in the
through a quantum tunneling mechanism, the ac susceptibility measurements were performed under small dc fields. However, ac magnetic measurements as a function of the frequency show absence of quantum tunneling. Moreover, the applied fields induce faster relaxation of the magnetization (Supporting Information, Figure S5). Since the weak ac signal in ac magnetic susceptibility measurements as function of the temperature is due the slow relaxation of magnetization generated by the anisotropic DyIII ions, we conclude that application of a static magnetic field is fastening their relaxation of magnetization. Herein, we will show how this process affects the hyperfine parameters of the iron nuclei and interaction between iron and dysprosium ions. The Mçssbauer spectrum of 1 obtained at 3 K and zero applied external fields displays an asymmetric doublet with an isomer shift d = 0.57 mm s1 and a quadrupole splitting DEQ = 0.39 mm s1 and line width T = 0.80 mm s1 (Figure 3 a). These
Figure 2. The cT versus T plots for compounds 1 and 2 at 1000 Oe.
Supporting Information, Table S1 show that the cT values of 1 and 2 at room temperature are close to the expected values for two uncoupled Ln and two isolated Fe ions. The cT values of compound 1 decrease slowly with decreasing temperature until 1.8 K, indicating that the interactions between the two separated Fe centers are very weak. This conclusion is further confirmed by M versus H plots of 1, in which the magnetization reaches 9.91 NmB at 2 K and 70 kOe. This value is consistent with that expected for two isolated Fe ions with S = 5/2. In the case of compound 2, the cT products decrease with cooling, which is largely due to the thermal depopulation of excited j mJ > states of the DyIII ion. The field dependence of the magnetization of 2 increases slowly with increasing applied field without saturation even at 70 kOe. This behavior suggests the presence of magnetic anisotropy and/or low lying-excited states. The M versus H/T curves are non-superposed, which further highlights the presence of these effects. We then performed ac susceptibility measurements to probe the dynamic properties of these two compounds. As expected, complex 1 did not show any out-of-phase (c’’) signal. The temperature dependence of the in-phase and out-of-phase ac susceptibilities of compound 2 show frequency dependence, but with no maxima above 1.8 K even at 1500 Hz (Supporting Information, Figure S3), which is indicative of very fast relaxation of the magnetization. To check whether this can be attributed to the presence of a fast relaxation of the magnetization Chem. Eur. J. 2014, 20, 12381 – 12384
www.chemeurj.org
Figure 3. 57Fe Mçssbauer spectra of 1 at 3 K in zero field and applied external magnetic fields.
parameters are typical for octahedral coordinated ferric highspin FeIII ions with an O/N ligand sphere. The observed linewidth asymmetry is typical for magnetically isolated single ferric ions[4] and is the result of paramagnetic relaxation times that are only slightly above the typical time window of Mçssbauer spectroscopy, which is about 107 s. The application of an external field of 4 T leads to a broad magnetic sextet, which has been analyzed by means of the hyperfine field distribution with a maximum hyperfine field Bhf = 42.3 T (Figure 3 b). At a field of 6 T, again a broad magnetic sextet is obtained, now with a maximal hyperfine field of Bhf = 46.2 T (Figure 3 c). At small external fields, magnetically isolated ferric high-spin FeIII ions display in general sharp magnetic sextets at liquid helium temperatures and thus have spin relaxation times that are longer than the time window of Mçssbauer spectroscopy (ca. 107 s). However, compound 1 shows no magnetic sextet even in the small earth magnetic field at T = 3 K. This can be explained by the presence of spin–spin relaxation between the two ferric high-spin FeIII ions, which have a distance of 5.48 .
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication As the YIII ions are diamagnetic, these do not influence the spin–spin relaxation properties of 1. It is known that the application of large external fields slows down the spin–spin relaxation process. This is reflected in the observation of magnetically split patterns at external fields of 4 and 6T (Figure 3 b, c). The fact that Bhf increases with increasing applied external fields supports the conclusion that spin-spin relaxation is the dominant relaxation process in 1. Surprisingly, compound 2 shows a well-defined magnetic sextet at T = 3 K and zero external fields (Figure 4 b), whereas
Figure 4. 57Fe Mçssbauer spectra of 2 at 30 K and 3 K in zero field and applied external magnetic field. d = 0.56 mm s1, DEQ = 0.35 mm s1, T = 0.85 mm s1.
the Mçssbauer pattern of 2 obtained at T = 30 K (Figure 4 a) resembles that observed for 1 at T = 3 K (Figure 3 a). Given the 107 s time window of Mçssbauer spectroscopy, the ferric high-spin FeIII ions in 2 must relax with a relaxation rate much smaller than 107 s1. Obviously the presence of paramagnetic DyIII ions cause a significant decrease in the iron spin–spin relaxation rate from much more than 107 s1 in the YIII complex 1 to much less than 107 s1 in the DyIII complex 2. The magnetic hyperfine field of 2 obtained at 3 K is Bhf = 53.9 T, which corresponds very well with the hyperfine field of about 55 T observed for magnetically isolated ferric high-spin FeIII ions with saturated spin expectation values < S > x,y,z = 5/2.[4, 5] The application of an external field of 4 T leads to a magnetically broadened sextet with a maximum hyperfine field of Bhf = 46.1 T. The fact that Bhf is decreasing with increasing external field is caused by the dominating negative Fermi contact field of ferric high-spin FeIII ions. Surprisingly, the magnetically broadened Mçssbauer signatures of 1 (Figure 3 b) and of 2 (Figure 4 c) obtained at 3 K and 4 T are strikingly similar. All of these experimental observations described above can be explained by two scenarios: 1) In 1, spin–spin relaxation is the dominant relaxation process. At zero external field, the spin–spin relaxation rate is Chem. Eur. J. 2014, 20, 12381 – 12384
www.chemeurj.org
larger than 107 s1, and with increasing external field spin– spin relaxation is slowed down. The paramagnetic DyIII ions in 2 influence the magnetic moments of the FeIII sites in such a way that the relaxation rate of the FeIII sites is smaller than 107 s1. This process has been recently reported as a magnetic pinning effect between the DyIII and the FeIII ions.[6, 7] If the relaxation time is longer than the typical time window of Mçssbauer spectroscopy (107 s), a magnetic pattern should be observable, which corresponds to the magnetic hyperfine fields that are caused by the spin expectation values of the three Kramers doublets of the FeIII S = 5/2 spin system. An application of a large external field leads in general to the occupation of an isolated ground state which causes a well-defined sextet with a hyperfine field of up to 55 T.[4] However, Figure 4 c does not show a sharp, but a broadened sextet which is very similar to the spectrum of 1 obtained under the same experimental conditions (Figure 3 b). Since the 4 T Mçssbauer spectrum of 1 is doubtless influenced by relaxation effects, it is tempting to conclude that also the 4 T spectrum of 2 reflects FeIII sites that display a relaxation rate slightly faster than 107 s1. This implies that in compound 2 the application of an external magnetic field at 3 K leads to an enhancement of the spin relaxation rate from ! 107 s1 without an external field (Figure 4 b) to slightly above 107 s1 at an external field of 4 T. 2) In the alternative scenario, the DyIII ions also pin the spins of the FeIII ions. At 3 K and zero applied fields, the magnetic moments of the DyIII ions would be randomly oriented, and as the spins of the FeIII ions are pinned to the magnetic moments of the DyIII ions, their spin expectation values and therefore also the magnetic hyperfine fields of the FeIII sites are randomly oriented. The application of a 4 T external field would induce an incomplete orientation of the DyIII moments with respect to the 4 T external field. A perfect alignment of the moments of the DyIII and the FeIII ions with the external field would lead to a sharp magnetic sextet with line widths of less than 0.4 mms1 and an intensity ratio of 3:4:1:1:4:3. However, an incomplete alignment of the DyIII ions would lead to an incomplete alignment of the FeIII spins and therefore to the observed distribution in hyperfine field, as evident from Figure 4 c. Although scenario (1) has never been reported to date, it is also consistent with ac magnetic susceptibility measurements in static magnetic fields (Supporting Information, Figure S5) for 2; with increasing dc field, the curves maxima are moving to higher frequencies, showing in this way a speeding up of relaxation of the DyIII magnetic moments. To explore whether this unprecedented effect is a consequence of different electronic structures of 1 and 2, density functional calculations have been performed with the PBE/ CEP-31 g functional/basis set according to a procedure described previously.[7b] The analysis of the so obtained electronic structures of both complexes (see the Supporting Information) indicates that the framework of the orbitals involving d-orbitals of iron atoms of 1 and 2 is the same.
12383
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication In summary, we have outlined the role that 57Fe Mçssbauer spectroscopic characterization can play in understanding lanthanide anisotropy in molecular clusters. It has been demonstrated that this technique has much to offer as an improved analytical method for determining forms of magnetic behavior in lanthanides which are not observed with commonly used techniques. This study has shown that 57Fe Mçssbauer spectroscopy is a very sensitive method for diagnosing very fine microscopic changes of iron ions electronic structure affected by coordination environments and anisotropic weakly interacting neighbors, and gives us microscopic information about metal–metal communication and relaxation dynamics on specific centers (in the present case, Fe and Dy ions) and the influence of applied external magnetic fields on all these hyperfine effects.
Acknowledgements This work was supported by the DFG-funded transregional collaborative research center SFB/TRR 88 “3MET”. Keywords: cooperative effects · heterometallic complexes · iron · lanthanides · Moessbauer spectroscopy
Chem. Eur. J. 2014, 20, 12381 – 12384
www.chemeurj.org
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Received: June 3, 2014
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