Two Polymorphic Forms of a Six-Coordinate Mononuclear Cobalt(II) Complex with Easy-Plane Anisotropy: Structural Features, Theoretical Calculations, and Field-Induced Slow Relaxation of the Magnetization

Abstract

A mononuclear cobalt(II) complex [Co(3,5-dnb)2(py)2(H2O)2] {3,5-Hdnb = 3,5-dinitrobenzoic acid; py = pyridine} was isolated in two polymorphs, in space groups C2/c (1) and P21/c (2). Single-crystal X-ray diffraction analyses reveal that 1 and 2 are not isostructural in spite of having equal formulas and ligand connectivity. In both structures, the Co(II) centers adopt octahedral {CoN2O4} geometries filled by pairs of mutually trans terminal 3,5-dnb, py, and water ligands. However, the structures of 1 and 2 disclose distinct packing patterns driven by strong intermolecular O-H···O hydrogen bonds, leading to their 0D→2D (1) or 0D→1D (2) extension. The resulting two-dimensional layers and one-dimensional chains were topologically classified as the sql and 2C1 underlying nets, respectively. By means of DFT theoretical calculations, the energy variations between the polymorphs were estimated, and the binding energies associated with the noncovalent interactions observed in the crystal structures were also evaluated. The study of the direct-current magnetic properties, as well as ab initio calculations, reveal that both 1 and 2 present a strong easy-plane magnetic anisotropy (D > 0), which is larger for the latter polymorph (D is found to exhibit values between +58 and 117 cm(-1) depending on the method). Alternating current dynamic susceptibility measurements show that these polymorphs exhibit field-induced slow relaxation of the magnetization with Ueff values of 19.5 and 21.1 cm(-1) for 1 and 2, respectively. The analysis of the whole magnetic data allows the conclusion that the magnetization relaxation in these polymorphs mainly takes place through a virtual excited state (Raman process). It is worth noting that despite the notable difference between the supramolecular networks of 1 and 2, they exhibit almost identical magnetization dynamics. This fact suggests that the relaxation process is intramolecular in nature and that the virtual state involved in the two-phonon Raman process lies at a similar energy in polymorphs 1 and 2 (∼20 cm(-1)). Interestingly, this value is recurrent in Co(II) single-ion magnets, even for those displaying different coordination number and geometry.

Reversible solvatomagnetic switching in a single-ion magnet from an entatic state

“…However, since about a dozen examples of Co( ii ) SIMs with large and positive values of D can already be found in the literature, explaining the magnetic switch between 1 and 2 taking into account only the D sign is naïve. 17,18,23,49,54,55 On the other hand, the magnetic relaxation through a high energy barrier induced by an axial zfs could be assisted by extra energy investment from phonons emitted by the solid network from a vibrational relaxation. 54 Hence, the synthesis and investigation of novel cobalt( ii ) complexes are essential to understanding this singular magnetic behaviour, although an improvement of the current theoretical models or the development of new ones that answer the open questions will be a landmark in the field.…”

Single-ion anisotropy and exchange coupling in cobalt(ii)-radical complexes: insights from magnetic and ab initio studies

“…52,[100][101][102][103][104] In this approach the 4 T 1g ground state, which arises from the 4 F free ion term in octahedral symmetry, is treated as an effective orbital moment of L eff ¼ 1, spin S ¼ 3/2 with explicit SOC using the T, P isomorphism. 73,105,106 A combined orbital reduction parameter, a, is also included, which incorporates the isomorphism coefficient (À3/2) and reduction of the orbital angular momentum (L eff # 1) due to the mixing of higher energy states into the ground state and metal-ligand bond covalency (a varies from À3/2 for a weak ligand eld to À1 for a strong ligand eld). 52 The spin-orbit coupling constant, l SO , is expected to take values in the range À90 to À180 for six-coordinate Co(II) complexes.…”

“…Low symmetry is described by effective crystal eld parameters for axial (B 0 2 ) and rhombic (B 2 2 ) distortions from octahedral symmetry. 73,104,105 According to the Griffith model, the magnetic susceptibility and magnetization data and EPR effective g-values for 1 were simultaneously t to a phenomenological Hamiltonian (eqn (1), where I is the identity matrix) in the program PHI. 104 The overall best t was achieved with the following parameters: a ¼ À1.113(8), B 0 2 ¼ 161(5) cm À1 and |B 2 2 | ¼ 111(5) cm À1 , resulting in a residual of 5.23 Â 10 À5 .…”

Field Induced Slow Magnetic Relaxation in a Non Kramers Tb(III) Based Single Chain Magnet

“…At 2000 Oe dc field the frequency dependency of out of phase ac susceptibility ( Figure S10) and the temperature dependency of in phase (χ M ) ( Figure 5b) and out of phase (χ M ") ( Figure 5a) ac susceptibility clarifies the SMM behaviour of complex 1. Although the values of χ M " at higher temperature goes negative (Figure 5a) [54,61] and lower than the expected range also indicates the small fractions of the molecules are participating the slow relaxation of magnetization. And may be for the same reason the χ M values are quite smaller as compared to χ M value in the AC susceptibility plots (Figure 5a,b).…”

“…The stretching frequencies observed at ca. 1514 cm −1 and 1344 cm −1 reflect the asymmetric and symmetric stretching modes of the nitro group [54].…”