The Silica‐Like Extended Polymorphism of Cobalt( II ) Imidazolate Three‐Dimensional Frameworks: X‐ray Single‐Crystal Structures and Magnetic Properties

Five polymorphous frameworks of cobalt( II ) imidazolates ( 1 – 5 ) have been prepared by solvatothermal syntheses. Of these, compound 3 has already been synthesized in a gas‐phase reaction by Seel et al. in 1969 and structurally characterized by Sturm et al. in 1975. The new synthetic strategy affords four polymorphous frameworks of cobalt( II ) imidazolates ( 1, 2, 4, 5 ) of crystalline substances, of which the compound 4 ( a = b = 23.450(3), c = 12.460(3) Å, tetragonal, I 4 1 cd, Z = 16) is an isomorphous compound of [Zn(im) 2 ] ∞ , which was also synthesized in a gas‐phase reaction in 1980. The frameworks of compounds 1 and 2 are porous and isostructural; they have the same framework topology that represents a novel uninodal (6,4)‐net: 1 : a = 18.513(4), b = 24.368(5), c = 9.2940(19) Å, orthorhombic, Fdd 2, Z = 16; 2 : a = 17.635(4), b = 27.706(6), c = 9.0810(18) Å, orthorhombic, Fdd 2, Z = 16. The framework of compound 5 exhibits a topology of zeolitic structure with the unit‐cell parameters: a = 24.3406(8), b = 9.4526(3), c = 24.8470(8) Å, β = 91.977(1)°, monoclinic, P2 1 / n, Z = 4. All polymorphous frameworks of cobalt( II ) imidazolates reflect the structural features of silica (SiO 2 ) and also exhibit different magnetic behaviors, although the imidazolates transmit the antiferromagnetic coupling between the cobalt( II ) ions in all cases. However, the uncompensated antiferromagnetic couplings arise from spin‐canting are sensitive to the structures: compound 1 is an antiferromagnet with T N = 13.11 K; compounds 2 – 4 are weak ferromagnets (canted antiferromagnets): 2 shows a very weak ferromagnetism below 15 K, 3 exhibits a relatively strong ferromagnetism below 11.5 K and a coercive field ( H C ) of 1800 Oe at 1.8 K, and 4 displays the strongest ferromagnetism of the three cobalt imidazolates and demonstrates a T C of 15.5 K with a coercive field, H C , of 7300 Oe at 1.8 K. However, compound 5 seems to be a hidden canted antiferromagnet with a magnetic ordering temperature of 10.6 K.