Slow magnetic relaxation in five-coordinate spin-crossover cobalt(II) complexes

Hui-Hui Cui a, Jing Wang a, Xue-Tai Chen *a and Zi-Ling Xue b
aState Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, China. E-mail:
bDepartment of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA

Received 20th June 2017 , Accepted 17th July 2017

First published on 17th July 2017

We present the first examples of the coexistence of field induced slow magnetic relaxation and spin-crossover observed in five-coordinate cobalt(II) complexes [Co(12-TMC)(CH3CN)](X)2 (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane, X = BF4, 1; PF6, 2). The direct-current (dc) magnetic data show a gradual and incomplete spin-crossover at high temperature. 1 and 2 display frequency- and temperature-dependent alternating-current (ac) magnetic susceptibility under an applied dc field of 2500 Oe, which originates from the S = 1/2 spin state of Co(II) ions.

A great number of mononuclear paramagnetic transition-metal and rare earth compounds are found to display slow relaxation of the magnetization at low temperatures,1,2 which constitute an important subclass of single-molecule magnets and termed as single-ion magnets (SIMs). The metal-ions found in 3d-ion SIMs include Cr(II),3 Mn(III,IV),4 Fe(I,II,III),5 Co(I,II),6 Ni(I,II)7 and Cu(II)8 ions, among which Co(II)-based SIMs are the largest family of d-ion SIMs due to its large magnetic anisotropy resulting from significant spin-orbital coupling. Slow magnetic relaxation is observed in high spin-state Co(II) complexes with the results of increasing examples of Co(II)-based SIMs with coordination numbers from 2 to 8 and varied coordination configurations.6

Spin-crossover complexes are another type of important magnetic materials, which have transition metal ions in a d4–d7 electronic configuration characterized by two low lying electronic states of different spin-multiplicities. So far, the majority of reported spin-crossover compounds contain Fe(II) or Fe(III) ions.9 Co(II) complexes can also exhibit spin-crossover between high-spin S = 3/2 and low-spin S = 1/2 states.10–13 Most of the Co(II)-based spin-crossover complexes are mononuclear six-coordinate species10,11 with few examples of four- and five-coordinate Co(II) complexes.10,12,13

It is of great interest to have two magnetic properties in the same material.5c,14 There is, to our knowledge, no example of slow magnetic relaxation in spin-crossover Co(II) complexes. Herein we report the first observations of the coexistence of spin-crossover and slow magnetic relaxation in five-coordinate Co(II) complexes.

Complexes [Co(12-TMC)(CH3CN)](X)2 (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane, X = BF4, 1; PF6, 2) were synthesized by the reactions of CoCl2, 12-TMC and salts of anions in good yields. The crystal structures of these complexes were determined by single-crystal X-ray crystallography at 155 K (Fig. 1 and Fig. S1, ESI). The cation of 1 or 2 has a five-coordinate Co(II) ion in a square pyramidal configuration formed by four nitrogen atoms of the macrocyclic 12-TMC ligand in equatorial positions and the nitrogen atom of CH3CN in an axial position. The distortion values of 0.234 and 0.036 relative to square pyramidal configurations were obtained by the continuous shape measurement by the SHAPE 2.1 program15 (Table S2, ESI). Both 1 and 2 have similar bond parameters. The average Co–Nequatorial and Co–Naxial bond lengths are 1.990 and 2.042 Å for 1 and 1.997 and 2.022 Å for 2, respectively. The shortest intermolecular Co–Co distance is 8.508 Å for 1 and 9.166 Å for 2.

image file: c7cc04785a-f1.tif
Fig. 1 Molecular structure of 1 (left). The H atoms and anions are omitted for clarity. Red, blue, and gray spheres represent Co, N, and C atoms, respectively. Variable-temperature dc susceptibility data under 1000 Oe applied dc field of 1 (right). Inset: Field dependence of the magnetization below 5 K for 1. Solid lines are fits to the data with the program PHI.17

Variable temperature direct-current susceptibilities were determined between 1.8 and 400 K under a field of 1000 Oe for polycrystalline samples of 1 and 2, whose phase purities were confirmed by powder X-ray diffraction (Fig. S3 and S4, ESI). The χMT versus T plots are shown in Fig. 1 and Fig. S7 (ESI). For 1, the χMT value is almost constant (ca. 0.47 cm3 K mol−1) in the temperature range of 13–250 K, corresponding to an isolated low-spin Co(II) center with g = 2.25. When the temperature is raised from 250 K, the χMT value smoothly increases, attaining a value of 1.26 cm3 K mol−1 at 400 K. The temperature dependence of χMT for 2 (Fig. S7, ESI) is very similar to 1. The χMT value (ca. 0.46 cm3 K mol−1) is also constant approximately in the temperature range of 10–210 K, corresponding to a low-spin Co(II) ion. When the temperature of 2 is raised, its χMT value increases more quickly than 1, reaching 1.74 cm3 K mol−1 at 400 K, a value smaller than those values (2.1–3.4 cm3 K mol−1) reported for the high-spin Co(II) complexes with a considerable orbital angular momentum contribution.16 It appears that a significant population of the S = 3/2 state is present in 1 and 2 at higher temperatures. This behaviour suggests the occurrence of an incomplete conversion of the low spin state to the high spin state. Such an incomplete spin-crossover has been found in several four- and six-coordinate Co(II) complexes.11,12 It is worth noting that few five-coordinate spin-crossover Co(II) complexes are known, and they are (PNP)CoX2 complexes, where PNP is a tridentate ligand.10a,131 and 2 are the first examples of five-coordinate Co(II) spin-crossover complexes containing a CoN5 motif.

The field-dependent magnetization data were collected in the range of 1–7 T below 5 K (Fig. 1 and Fig. S8, ESI). The magnetizations are 1.15 Nβ at 7 T for both 1 and 2, approximately reaching saturation, which are consistent with an S = 1/2 system. Simultaneous fits of magnetization data and the magnetic susceptibility from 1.8 to 250 K for 1 and 1.8 to 210 K for 2 yielded giso = 2.26 for 1, and giso = 2.25 for 2. In the fittings, only an isotropic g factor and spin Hamiltonian are considered in the program PHI,17 which includes some corrections for the temperature-dependent magnetism χTIP (1, 3.0 × 10−4 cm3 mol−1; 2, 2.2 × 10−4 cm3 mol−1) and intermolecular interactions at low temperatures zJ (1, −0.078 cm−1; 2, −0.042 cm−1). The g values derived from the fittings are found to be larger than the spin-only value of 2.00232, which indicates the spin–orbit coupling in these complexes. More detailed insights into the electronic structure were probed by EPR spectroscopy at 91 k. The g values are determined from the X-band EPR spectra of the solid powder samples, yielding gx = 2.38, gy = 2.34, gz = 2.06 for 1 and gx = 2.37, gy = 2.28, gz = 2.04 for 2 (Fig. S26 and S27, ESI). The average g values (1, 2.26; 2, 2.26) are in good agreement with the values determined by fitting of magnetic data.

Measurements of alternative-current magnetic susceptibility were performed at 1.8 K under different external dc fields from 0 to 5000 Oe. No out-of-phase ac susceptibility (χM′′) signal is observed in the absence of a dc field. However, the relaxation process is detected under an applied dc field to give both χM′ and χM′′ signals. The frequency-dependent ac susceptibility for both 1 and 2 changes as the external magnetic field is increased (Fig. 2 and Fig. S13, ESI). The characteristic frequency of this relaxation mode continuously decreases when applying higher fields. The amplitude of the mode (χ0χ) exhibits a maximum around 2500 Oe, which is used for the further magnetic dynamic studies as in the MnIV-SIM.4b

image file: c7cc04785a-f2.tif
Fig. 2 (a) Frequency dependence of out-of-phase (χM′′) ac susceptibility at 1.8 K under different applied static fields from 0 to 5000 Oe for 1. The solid lines are for eye guide. (b) Field dependence of the magnetic parameters collected from the χMvs. v for 1 (χ0χ: amplitude of the relaxation mode with χ0 and χ being the in-phase ac susceptibilities in the zero and infinite ac frequency limits, respectively4b).

It is found that there is slow dynamics of magnetization in 1 and 2, as revealed in ac susceptibility measurements under the particular applied dc field of 2500 Oe (Fig. 3 and Fig. S11, S12 and S14, S16, ESI). Thus, 1 and 2 behave as field-induced SIMs. Comparison of the positions of the maximums of χM′′ signals appeared at 1.8 K suggests that the magnetic relaxation is slower in 1 than in 2. The maximums of out-of-phase χM′′ from the frequency-dependent data were used to construct the Arrhenius plots in Fig. 3b and Fig. S15 (ESI). It is obvious that τ does not follow the Arrhenius behaviour, in accordance with the lack of magnetic/electronic states that can be thermally populated to provide a path for the multiphonon Orbach mechanism of relaxations in S = 1/2 systems.18 The graphs of ln(τ) vs. T−1 were fitted approximately with eqn (1), where the first term represents a direct process and the second term is for the Raman process. The fitting results are shown in Table 1.

τ−1 = AT + bTn(1)

image file: c7cc04785a-f3.tif
Fig. 3 (a) Frequency dependence of the out-of-phase ac susceptibility from 1.8 to 6.0 K under a 2500 Oe dc field for 1. The solid lines are for eye guide. (b) Relaxation time of the magnetization ln(τ) vs. T−1 plot under 2500 Oe for 1, and those diluted in a matrix of [Zn(12-TMC)(CH3CN)](X)2 under the same field. The solid lines fit by eqn (1). The data were collected from the maximum of χM′′ against frequency at different temperatures.
Table 1 Results of fitting of the natural logarithm relaxation time against the inverse temperature according to eqn (1)
  A b n
1 632.53 8.05 4.60
2 738.02 54.33 3.97
1′ 3.70 1.24 3.50
2′ 49.20 92.57 2.31

The obtained exponent values are much smaller than the expected value of 9 for the Raman process, indicating the presence of a Raman-like process involving both acoustic (lattice) and optical (molecular) vibrations.18 The fits reproduce the experimental data very well (Fig. S24 and S25, ESI). Thus, both direct and Raman-like processes are involved in spin–lattice relaxations of 1 and 2. The Cole–Cole plots from the ac magnetic susceptibility data and the fittings by the generalized Debye model19 are given in Fig. 4 and Fig. S17 with the fitting parameters in Tables S3 and S4 (ESI). The α values are in the ranges of 0.05–0.24 and 0.19–0.27 for 1 and 2, respectively, indicating a weak distribution of the relaxation times.

image file: c7cc04785a-f4.tif
Fig. 4 Frequency dependence of the out-of-phase ac susceptibility from 1.8 to 6.0 K under 2500 Oe dc field Cole–Cole plot obtained from the ac susceptibility data under 2500 Oe dc field in the temperature range of 1.8–5.0 K for 1. Solid lines represent the best fits to a generalized Debye model.19

In order to further probe the spin-crossover and the relaxation mechanism in 1 and 2, the diluted samples 1′ and 2′ in the diamagnetic matrix [Zn(12-TMC)(CH3CN)](X)2 (X = BF4, 3; X = PF6, 4) were prepared and magnetically characterized. Unfortunately, the ZnII analogs 3 and 4 and the diluted samples exhibit space groups different from 1 and 2 (Fig. S5 and S6, ESI). This kind of difference is not desired for spin-crossover and dynamic magnetic studies, as the subtle structural distortion imposed by crystal packing could change the spin-crossover and the dynamic process. But the study still gives some insights. Compared with 1 and 2, the dc magnetic data of diluted samples exhibit similar temperature-dependent magnetic behavior (Fig. S9 and S10, ESI), indicating the gradual spin-crossover behavior after dilution. The slow magnetic relaxation was also observed in 1′ and 2′ (Fig. S18, S19 and S21, S22, ESI), further suggesting that the magnetic dynamics is intrinsic to the individual [Co(12-TMC)(CH3CN)](X)2 molecules. The relaxation processes in 1′ and 2′ are much slower than those in the pure samples (Fig. 3b and Fig. S15, ESI). The graphs of ln(τ) vs. T−1 are fitted approximately with eqn (1) and the results are shown in Table 1 and Fig. S24, S25 (ESI) in comparison with undiluted samples. Obviously the direct process is suppressed and the Raman-like process is predominant after dilution. The Cole–Cole plots from the ac magnetic susceptibility data and the fittings by the generalized Debye model19 are given in Fig. S20, S23 and Tables S5, S6 (ESI). The α values are in the ranges of 0.06–0.19 and 0.08–0.10, respectively, indicating a single relaxation process.

The above experimental data clearly show that both 1 and 2 are new field-induced SIMs with the S = 1/2 ground state. So far slow magnetic relaxation has been found in few other S = 1/2 systems such as Ni(I)-,7a,b Mn(IV)-,4b and Cu(II)-SIMs.8 Unlike the two-coordinate Ni(I)-SIM with unquenched orbital contribution,7a the dc susceptibility reveals the absence of zero-field splitting in complexes 1 and 2. Static magnetic data and EPR spectra showed that spin–orbit coupling is present in the low spin state of 1 or 2 due to the large magnitude and significant anisotropy of g tensor. The analyses of the relaxation times indicate that both direct and Raman-like processes are operative in the spin–lattice relaxation in 1 or 2, which is due to the spin–orbit coupling.

In summary, the results described above present the first examples of the five-coordinate mononuclear Co(II) complex featuring both spin-crossover and field-induced slow magnetic relaxation. The ac susceptibility data of the diluted compounds further support that the slow magnetic relaxation is intrinsic attributes of [Co(12-TMC)(CH3CN)](X)2. Slow magnetic relaxation in 1 and 2 originates from the low spin state (S = 1/2) of Co(II) ions. This work suggests a new strategy to find more Co(II)-SIMs by examining the dynamic magnetics in spin-crossover Co(II) or low spin Co(II) complexes.

We thank the financial support from the National Basic Research Program of China (No. 2013CB922102), the Natural Science Grant of China (No. 21471078) and the US National Science Foundation (CHE-1633870 to ZLX).

Notes and references

  1. (a) G. A. Craig and M. Murrie, Chem. Soc. Rev., 2015, 44, 2135 RSC; (b) A. K. Bar, C. Pichon and J.-P. Sutter, Coord. Chem. Rev., 2016, 308, 346 CrossRef CAS; (c) J. M. Frost, K. L. M. Harriman and M. Murugesu, Chem. Sci., 2016, 7, 2470 RSC; (d) S. Gómez-Coca, D. Aravena, R. Morales and E. Ruiz, Coord. Chem. Rev., 2015, 379, 289 Search PubMed.
  2. (a) D. N. Woodruff, R. E. P. Winpenny and R. A. Layfield, Chem. Rev., 2013, 113, 5110 CrossRef CAS PubMed; (b) P. Zhang, L. Zhang and J.-K. Tang, Dalton Trans., 2015, 44, 3923 RSC; (c) P. Zhang, Y.-N. Guo and J.-K. Tang, Coord. Chem. Rev., 2013, 257, 1728 CrossRef CAS.
  3. A. Cornia, L. Rigamonti, S. Boccedi, R. Clérac, M. Rouzières and L. Sorace, Chem. Commun., 2014, 50, 15191 RSC.
  4. (a) G. A. Craig, J. J. Marbey, S. Hill, O. Roubeau, S. Parsons and M. Murrie, Inorg. Chem., 2015, 54, 13 CrossRef CAS PubMed; (b) M. Ding, G. E. Cutsail III, D. Aravena, M. Amoza, M. Rouzières, P. Dechambenoit, Y. Losovyj, M. Pink, E. Ruiz, R. Clèrac and J. M. Smith, Chem. Sci., 2016, 7, 6132 RSC.
  5. (a) J. M. Zadrozny, D. J. Xiao, M. Atanasov, G. J. Long, F. Grandjean, F. Neese and J. R. Long, Nat. Chem., 2013, 5, 577 CrossRef CAS PubMed; (b) D. E. Freedman, W. H. Harman, T. D. Harris, G. J. Long, C. J. Chang and J. R. Long, J. Am. Chem. Soc., 2010, 132, 1224 CrossRef CAS PubMed; (c) S. Mossin, B. L. Tran, D. Adhikari, M. Pink, F. W. Heinemann, J. Sutter, R. K. Szilagyi, K. Meyer and D. J. Mindiola, J. Am. Chem. Soc., 2012, 134, 13651–13661 CrossRef CAS PubMed.
  6. (a) Y. S. Meng, Z. Mo, B.-W. Wang, Y.-Q. Zhang, L. Deng and S. Gao, Chem. Sci., 2015, 6, 7156 RSC; (b) X.-N. Yao, J.-Z. Du, Y.-Q. Zhang, X.-B. Leng, M.-W. Yang, S.-D. Jiang, Z.-X. Wang, Z.-W. Ouyang, L. Deng, B.-W. Wang and S. Gao, J. Am. Chem. Soc., 2017, 139, 373 CrossRef CAS PubMed; (c) A. Eichhöfer, Y. Lan, V. Mereacre, T. Bodenstein and F. Weigend, Inorg. Chem., 2014, 53, 1962 CrossRef PubMed; (d) Y. Rechkemmer, F. D. Breitgoff, M. van der Meer, M. Atanasov, M. Hakl, M. Orlita, P. Neugebauer, F. Neese, B. Sarkar and J. van Slageren, Nat. Commun., 2016, 7, 10467 CrossRef CAS PubMed; (e) T. Jurca, A. Farghal, P.-H. Lin, I. Korobkov, M. Murugesu and D. S. Richeson, J. Am. Chem. Soc., 2011, 133, 15814 CrossRef CAS PubMed; (f) V. V. Novikov, A. A. Pavlov, Y. V. Nelyubina, M.-E. Boulon, O. A. Varzatskii, Y. Z. Voloshin and R. E. P. Winpenny, J. Am. Chem. Soc., 2015, 137, 9792 CrossRef CAS PubMed; (g) L. Chen, S.-Y. Chen, Y.-C. Sun, Y.-M. Guo, L. Yu, X.-T. Chen, Z. Wang, Z.-W. Ouyang, Y. Song and Z.-L. Xue, Dalton Trans., 2015, 44, 11482 RSC; (h) L. Chen, J. Wang, J.-M. Wei, W. Wernsdorfer, X.-T. Chen, Y.-Q. Zhang, Y. Song and Z.-L. Xue, J. Am. Chem. Soc., 2014, 136, 12213 CrossRef CAS PubMed.
  7. (a) R. C. Poulten, M. J. Page, A. G. Algarra, J. J. Le Roy, I. López, E. Carter, A. Llobet, S. A. Macgregor, M. F. Mahon, D. M. Murphy, M. Murugesu and M. K. Whittlesey, J. Am. Chem. Soc., 2013, 135, 13640 CrossRef CAS PubMed; (b) W. Lin, T. Bodenstein, V. Mereacre, K. Fink and A. Eichhöfer, Inorg. Chem., 2016, 55, 2091 CrossRef CAS PubMed; (c) K. E. R. Marriott, L. Bhaskaran, C. Wilson, M. Medarde, S. T. Ochsenbein, S. Hill and M. Murrie, Chem. Sci., 2015, 6, 6823 RSC.
  8. R. Boča, C. Rajnák, J. Titič and D. Valigura, Inorg. Chem., 2017, 56, 1478 CrossRef PubMed.
  9. (a) P. Gütlich, Y. Garcia and H. A. Goodwin, Chem. Soc. Rev., 2000, 29, 419 RSC; (b) M. A. Halcrow, Polyhedron, 2007, 26, 3523 CrossRef CAS; (c) M. Nihei, T. Shiga, Y. Maeda and H. Oshio, Coord. Chem. Rev., 2007, 251, 2606 CrossRef CAS.
  10. (a) H. A. Goodwin, Top. Curr. Chem., 2004, 234, 23 CrossRef CAS; (b) M. G. Cowan, J. Olguín, S. Narayanaswamy, J. L. Tallon and S. Brooker, J. Am. Chem. Soc., 2012, 134, 2892 CrossRef CAS PubMed.
  11. (a) R. Ishikawa, K. Matsumoto, K. Onishi, T. Kubo, A. Fuyuhiro, S. Hayami, K. Inoue, S. Kaizaki and S. Kawata, Chem. Lett., 2009, 38, 620 CrossRef CAS; (b) A. Galet, A. B. Gaspar, M. C. Muňoz and J. A. Real, Inorg. Chem., 2006, 45, 4413 CrossRef CAS PubMed.
  12. (a) D. M. Jenkins and J. C. Peters, J. Am. Chem. Soc., 2003, 125, 11162 CrossRef CAS PubMed; (b) D. M. Jenkins and J. C. Peters, J. Am. Chem. Soc., 2005, 127, 7148 CrossRef CAS PubMed.
  13. (a) D. W. Shaffer, I. Bhowmick, A. L. Rheingold, C. Tsay, B. N. Livesay, M. P. Shores and J. Y. Yang, Dalton Trans., 2016, 45, 17910 RSC; (b) L. Sacconi, Coord. Chem. Rev., 1972, 8, 351 CrossRef CAS.
  14. (a) A. Urtizberea and O. Roubeau, Chem. Sci., 2017, 8, 2290 RSC; (b) C. Mathonière, H.-J. Lin, D. Siretanu, R. Clérac and J. M. Smith, J. Am. Chem. Soc., 2013, 135, 19083 CrossRef PubMed; (c) X. Feng, C. Mathonière, I.-R. Jeon, M. Rouzières, A. Ozarowski, M. L. Aubrey, M. I. Gonzalez, R. Clérac and J. R. Long, J. Am. Chem. Soc., 2013, 135, 15880 CrossRef CAS PubMed.
  15. (a) S. Alvarez, P. Alemany, D. Casanova, J. Cirera, M. Llunell and D. Avnir, Coord. Chem. Rev., 2005, 249, 1693 CrossRef CAS; (b) S. Alvarez and M. Llunell, J. Chem. Soc., Dalton Trans., 2000, 3288 RSC.
  16. F. E. Mabbs and D. J. Machin, Magnetism and Transition Metal Complexes, Dover Pulications, Mineola, NY, 2008 Search PubMed.
  17. N. F. Chilton, R. P. Anderson, L. D. Turner, A. Soncini and K. S. Murray, J. Comput. Chem., 2013, 34, 1164 CrossRef CAS PubMed.
  18. K. N. Shrivastava, Phys. Status Solidi B, 1983, 117, 437 CrossRef CAS.
  19. (a) K. S. Cole and R. H. Cole, J. Chem. Phys., 1941, 9, 341 CrossRef CAS; (b) S. M. J. Aubin, Z. M. Sun, L. Pardi, J. Krzystek, K. Folting, L. C. Brunel, A. L. Rheingold, G. Christou and D. N. Hendrickson, Inorg. Chem., 1999, 38, 5329 CrossRef CAS.


Electronic supplementary information (ESI) available: Experimental, physical measurements, additional magnetic data, EPR spectra, and some figures including XRD patterns and crystal structure. CCDC 1546344 and 1546345. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cc04785a

This journal is © The Royal Society of Chemistry 2017