Sizeable Effect of Lattice Solvent on Field Induced Slow Magnetic Relaxation in Seven Coordinated Co^II Complexes

Abstract

We have demonstrated the effect of a solvent at the second coordination sphere on slow relaxation of magnetization for hepta-coordinated cobalt­(II) complexes with the formulas [Co­(H4L)­(DMF)­(H2O)]­(NO3)2·(DMF) (1), [Co­(H4L)­(MeOH)­(H2O)]­(NO3)2·(MeOH) (2), and [Co­(H4L)­(DEF)­(H2O)]­(NO3)2 (3) (H4L = 2,2′-(pyridine-2,6-diylbis­(ethan-1-yl-1-ylidene))­bis­(N-phenylhydrazinecarboxamide). Structural analysis reveals that the presence of lattice solvent molecule in 1 and 2 dramatically changes the crystal packing and noncovalent interactions as compared to 3 where no solvent molecule is present in the crystal lattice. The dc and ac magnetic susceptibility measurements reveal the presence of easy-plane magnetic anisotropy for all the complexes, and field induced slow relaxation behavior has been observed above 2 K for 1 and 2 in contrast to 3 due to the availability of the solvent molecules in the crystal lattice. The ab initio calculations further support the sign of D and the negligible effect of the first co-ordination sphere, as almost similar D values were obtained for all the complexes. The field and temperature dependence of relaxation time confirm that quantum tunnelling of magnetization (QTM) plays a major role in slow magnetic relaxation, and thermal dependence like an optical or acoustic Raman pathway is also important. To further analyze the effect of dipole–dipole interaction on slow magnetic relaxation, a dilution experiment has been performed.

Combined Experimental and Theoretical Investigation of the Origin of Magnetic Anisotropy in Pentagonal Bipyramidal Isothiocyanato Co(II), Ni(II), and Fe(III) Complexes with Quaternary-Ammonium-Functionalized 2,6-Diacetylpyridine Bisacylhydrazone

“…Perfect match with D = 30.01 cm –1 , E = 0.00 cm –1 , g = 2.32, and zJ = −0.03 cm –1 was achieved. This result is in agreement with Co­(II) in a similar PBPY-7 geometry with pentadentate equatorial ligands. − ,− …”

“…This result is in agreement with Co(II) in similar PBPY-7 geometry with the pentadentate equatorial ligands. [27][28][29][30][31][32][33][34][35][39][40][41][42][43][44][45][46] Complex 2 contains two Co(II) ions, with one of them having the same PBPY-7 geometry as in previously described complex 1, while the other is in tetrahedral (T-4) coordination. Due to magnetic anisotropy, the experimentally found χ*T value of 5.35 emuK/molOe (Figure 2b) is higher than theoretical spin-only value of 3.75 emuK/molOe, expected for two Co(II) ions with S = 3/2 and g=2 at 300 K. Simultaneous fitting of experimental M(H) and χ*T data was performed assuming possible exchange interactions between two paramagnetic Co(II) within the monomer unit.…”

“…PBPY-7 Co(II) complexes are relatively common and show magnetic anisotropy with D values in the range of 13-68 cm -1 . [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] On the other hand, Ni(II) complexes in PBPY-7 environment are scarce. 47 The rare examples of PBPY-7 Ni(II) complexes, with magnetic properties investigated, involve mononuclear high-spin Ni(II) complexes 32,33,35,40,43,46 as well as two pentanuclear [Ni 3 W 2 ] compounds containing the PBPY-7 Ni(II) moieties associated with [W(CN) 8 ] 3− .…”

The Structural and Magnetic Properties of FeII and CoII Complexes with 2-(furan-2-yl)-5-pyridin-2-yl-1,3,4-oxadiazole

“…This is reflected in the energy levels of the ligand-field terms and, as expected, in ligand-field multiplets showing the zero-field splitting pattern (Figures 7 and 8). The importance and the impact of the second coordination sphere on the magnetism was also recently studied in other Co(II) complexes [50][51][52]. In other words, the perchlorate induced hydrogen bonding alters the axial ligand field of aqua ligands, and, therefore, we decided to model this phenomenon in an alternative way.…”

Alignment of Axial Anisotropy in a 1D Coordination Polymer shows Improved Field Induced Single Molecule Magnet Behavior over a Mononuclear Seven Coordinated FeII Complex

“…Usually, the proper alignment of the anisotropic axis (preferably easy‐axis anisotropy) and strong intramolecular exchange coupling between the metal centers are the most suitable condition for such single chain magnet. Our group is also extensively involved in the investigation of magnetic properties of transition metal based hepta coordinated complexes . In our previous work, we reported that the weak π donor ligands in the axial position enhance the magnetic anisotropy in Fe(II) seven coordinated complexes .…”