Heterospin single-molecule magnet behaviors after irradiation of polymorphous 2:2 cyclic diazo-cobalt(II) complexes

Abstract

Three kinds of crystals (crystals, α, β, and γ) of 2:2 cyclic cobalt complexes formulating as [Co(Br-hfpip)2(D2py2)]2 X: X=2CH2Cl2 (1dα), 4CH2Cl2 (1dβ), and 4CH2Cl2·4EtOH (1dγ) and Br-hfpip=1,1,1,5,5,5-hexafluoro-4-(4-bromophenylimino)-2-pentanonate, were obtained and were determined to be polymorphous by X-ray crystallography, having the space groups of P1¯ for 1dα and 1dβ, and P21/n for 1dγ. In three polymorphs, the local coordination structures of cobalt complex units in the molecule were the same; compressed octahedral. The molecular structure for 1dα was different from those for 1dβ and 1dγ. The dihedral angles between the X–Y plane and the pyridine ring were 43° and 44° for 1dα and 70–80° for 1dβ and 1dγ. In the crystal packings, 1dα had a short contact between the bromo substituent in hfpip ligand and the carbon at the β position of the pyridine ring in D2py2, while 1dβ and 1dγ had no short contact between the magnetically significant atoms. The powder X-ray diffraction (PXRD) patterns for the crushed samples of three polymorphs indicated that the crystal structures were changed by loosing crystal solvent and 1dα and 1dβ were microcrystals (1dα′ and 1dβ′), while 1dγ became amorphous (1dγ′). In dc and ac magnetic measurements after irradiation of crushed samples (1dα′, 1dβ′, and 1dγ′), the photoproducts, 1cα′, 1cβ′, and 1cγ′ exhibited slow magnetic relaxation and magnetic hysteresis. 1cα′ showed a single-molecule magnet (SMM) behavior with the effective activation barrier, Ueff/kB=98K and coercive force, Hc=2.9kOe at 1.9K affected by the intermolecular antiferromagnetic interaction. 1cβ′ showed the magnetic behavior with Ueff/kB=134K and Hc=13kOe at 1.9K typical of heterospin SMM. In 1cγ′, weak χmol″ signals with frequency dependency and the temperature dependent hysteresis loop; Hc=1.1kOe at 1.9K, were observed.