A flexible tris-phosphonate for the design of copper and cobalt coordination polymers: unusual cage array topology and magnetic properties

Abstract

The reaction of the flexible tris-phosphonate precursor benzene-1,3,5-tris(methylenephosphonic acid) (H₆btmp) with CuCl₂·2H₂O in H₂O under hydrothermal conditions afforded the complex [Cu₃(btmp)(H₂O)_3.6]·H₂O (1·H₂O). A similar reaction between 2,4,6-trimethylbenzene-1,3,5-tris(methylenephosphonic acid) (Me₃-H₆btmp) and Cu(NO₃)₂·3H₂O or Co(NO₃)₂·6H₂O gave the complex [Cu₄(Me₃-H₂btmp)₂(H₂O)₄] (2) or [Co₆(Me₃-btmp)₂(H₂O)₄] (3), respectively. The cis,trans,trans-conformation of the flexible tris-phosphonate ligands resulted in the formation of 3D networks 1 and 3. Interestingly, the cis,cis,cis-conformation of the Me₃-btmp ligand in 2 results in the formation of M₄L₂-type copper-based metalloprismatic cages, which can serve as building units for subsequent assemblies with phosphonate (P–O) linkages to afford a 2D array of nanocages. The magnetic study indicated that complexes 1·H₂O, 2 and 3 all exhibit antiferromagnetic behaviors. Interestingly, results from alternating current (ac) magnetic susceptibility measurements revealed that complex 3 exhibits slow magnetic relaxation behavior at low temperatures.