title: 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.
authors: Department of Chemistry, National Institute of Technology Agartala, Pin 799046, Tripura, India.
authors: Subhadip
authors: S
authors: Roy
authors: Departamento de Química Aplicada, Facultad de Química, Universidad del País Vasco, UPV/EHU , Paseo Manuel Lardizabal, No. 3, 20018 Donostia-San Sebastián, Spain.
authors: Itziar
authors: I
authors: Oyarzabal
authors: Institut de Ciència Molecular, Universitat de València , 46980 Paterna, Valencia, Spain.
authors: Julia
authors: J
authors: Vallejo
authors: Institut de Ciència Molecular, Universitat de València , 46980 Paterna, Valencia, Spain.|Fundació General de la Universitat de València , Valencia, Spain.
authors: Joan
authors: J
authors: Cano
authors: Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada , Avenida de Fuentenueva s/n, 18071 Granada, Spain.
authors: Enrique
authors: E
authors: Colacio
authors: Departament de Quimica, Universitat de les Illes Balears , Crta. de Valldemossa km 7.5, 07122 Palma de Mallorca (Baleares), Spain.
authors: Antonio
authors: A
authors: Bauza
authors: Departament de Quimica, Universitat de les Illes Balears , Crta. de Valldemossa km 7.5, 07122 Palma de Mallorca (Baleares), Spain.
authors: Antonio
authors: A
authors: Frontera
authors: Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade de Lisboa , Av. Rovisco Pais, 1049-001 Lisbon, Portugal.
authors: Alexander M
authors: AM
authors: Kirillov
authors: School of Chemistry, The University of Reading , P.O. Box 224, Whiteknights, Reading RG66AD, U.K.
authors: Michael G B
authors: MG
authors: Drew
authors: Department of Chemistry, National Institute of Technology Patna , Ashok Rajpath, Patna 800005, Bihar, India.
authors: Subrata
authors: S
authors: Das