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

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 CoII single-ion magnets, even for those displaying
different coordination number and geometry.