From mononuclear to two-dimensional cobalt(ii) complexes based on a mixed benzimidazole–dicarboxylate strategy: syntheses, structures, and magnetic properties

Abstract

Based on a benzimidazole derivative and three different dicarboxylates, three new cobalt(II) complexes, namely [Co(1,3-BDC)₂(MBIm)₂] (1), [Co(SCA)₂(MBIm)₂]_n (2), and {[Co(1,4-BDC)₃(MBIm)₂]·2DMF}_n (3), (1,3-BDC = benzene-1,3-dicarboxylic acid, SCA = succinic acid, 1,4-BDC = benzene-1,4-dicarboxylic acid, MBIm = 5,6-dimethylbenzimidazole), have been hydrothermally synthesized and characterized structurally and magnetically. Single crystal X-ray diffraction revealed that 1 is a mononuclear neutral complex possessing a distorted tetrahedral geometry, 2 is a one-dimensional (1D) chain coordination polymer with the Co²⁺ centers arranged in T_d symmetry, and 3 is a geometrically frustrated two-dimensional (2D) framework constructed from linear trimeric Co₃ units with a mixed T_d–O_h–T_d geometry. Magnetic investigation revealed the easy-axis magnetic anisotropy of the Co²⁺ ions in 1 and 2, and field-induced slow magnetic relaxation behaviors were observed at very low temperatures with an effective energy barrier of U_eff = 51.8 K and 11.3 K for 1 and 2, respectively. Notably, complex 2 is the first example of a 1D SIM featuring four-coordinate metal centers. For 3, no long-range magnetic ordering was established even at 1.8 K, which is mainly attributed to spin frustration. These studies illustrate that a mixed benzimidazole–dicarboxylate strategy is an effective strategy for the design and construction of functional molecular magnetic materials with distinct magnetic properties and structural diversity.