Polymorphism in a Cobalt-Based Single-Ion Magnet Tuning Its Barrier to Magnetization Relaxation

Abstract

A large barrier to magnetization reversal, a signature of a good single-molecule magnet (SMM), strongly depends on the structural environment of a paramagnetic metal ion. In a crystalline state, where SMM properties are usually measured, this environment is influenced by crystal packing, which may be different for the same chemical compound, as in polymorphs. Here we show that polymorphism can dramatically change the magnetic behavior of an SMM even with a very rigid coordination geometry. For a cobalt(II) clathrochelate, it results in an increase of the effective barrier from 109 to 180 cm, the latter value being the largest one reported to date for cobalt-based SMMs. Our finding thus highlights the importance of identifying possible polymorphic phases in search of new, even more efficient SMMs.

Novel mononuclear and 1D-polymeric derivatives of lanthanides and (η6-benzoic acid)tricarbonylchromium: synthesis, structure and magnetism

“…28,29 The theoretical explanation of the appearance of two maxima was carried out previously 30 . Polymorphism can dramatically change the magnetic behavior of SMM 31 and may also result in the appearance of the second maxima. Comparable intensities of the χ" signal for both maxima indicate that the quantitative ratio of phases must be near 1:1 if the polymorphism takes place.…”

A Pseudo‐Octahedral Cobalt(II) Complex with Bispyrazolylpyridine Ligands Acting as a Zero‐Field Single‐Molecule Magnet with Easy Axis Anisotropy

“…[6][7][8] Complexes that contain only one metal centerm ay represent the smallestc hemically tuneableS MMs for spin-based devices, [9] and significant progress has been made in this regard by using lanthanides [3,10] and, more recently,3 dm etal ions. However,f ew examples of zero-field SMMs based on mononuclear complexes of iron(I) [12] iron(III), [13] and cobalt(II) have been reported, [14][15][16][17][18][19][20][21][22][23][24][25] which all feature half-integer spin states (S = 3/2 or 5/2). However,f ew examples of zero-field SMMs based on mononuclear complexes of iron(I) [12] iron(III), [13] and cobalt(II) have been reported, [14][15][16][17][18][19][20][21][22][23][24][25] which all feature half-integer spin states (S = 3/2 or 5/2).…”

“…[10,11] In the latter case, relaxation by quantum tunneling is usuallyv ery fast in zero field, so that application of astatic field is required to observeS MM behaviorb ya lternating-current (ac) susceptibility measurements. In particular, individual cobalt(II) ions were found to exhibit zero-field SMM behavior when embedded in tetrahedral, [14-17, 20, 22, 24] trigonal prismatic, [18,19,23] or linear [21] coordination environments or in as olid-state lattice. In particular, individual cobalt(II) ions were found to exhibit zero-field SMM behavior when embedded in tetrahedral, [14-17, 20, 22, 24] trigonal prismatic, [18,19,23] or linear [21] coordination environments or in as olid-state lattice.…”

Template Synthesis and X-Ray Structure of the First Cobalt(II) Glyoximate Clathrochelate with Terminal Formyl Groups

“…Such low-spin state of the cobalt(II) clathrochelates can be stabilized by the crystal packing effects. [11][12][13] In the crystal CoGm 3 (B4-C 6 H 4 CHO) 2 ·0.5CH 2 Cl 2 , the observed π...π stacking between N=C-C=N chelate fragments of its neighboring macrobicyclic molecules, forming a clathrochelate dimer (Figure 3), cause the shortening of the corresponding coordination Co-N bonds, while the third ribbed chelate fragment of these interacting macrobicyclic molecules of CoGm 3 (B4-C 6 H 4 CHO) 2 , showing the greatest Co-N distances, is not included in such bonding. The reagents used, anhydrous CoCl 2 , 4-formylphenylboronic acid, sorbents and organic solvents were obtained commercially (Sigma-Aldrich).…”

“…The measurements were done using the residual signal of DMSO-d 6 ( 1 H 2.50 ppm). 13 C NMR spectrum could not be obtained due to paramagnetic broadening and poor solubility of the complex. IR spectrum of the solid sample (KBr tablet) in the range of 400-4000 cm -1 was recorded with a Perkin Elmer FT-IR Spectrum BX II spectrometer.…”