Structural diversity and properties of M(ii) 4-carboxyl phenoxyacetate complexes with 0D-, 1D-, 2D- and 3D M-cpoa framework

Abstract

Nine new metal complexes with an asymmetric H₂cpoa ligand, [Co(Hcpoa)₂(bpe)₂(H₂O)₂] (1), [Co(cpoa)(bpe)_1.5(H₂O)]·H₂O (2), [Co(Hcpoa)₂(4,4′-bpy)₂] (3), [Co(4,4′-bpy)(H₂O)₄]·[Co(cpoa)₂(H₂O)₂]·2H₂O (4), [Ni(cpoa)(4,4′-bpy)(H₂O)₂] (5), [Ni(cpoa)(bpe)_1.5(H₂O)]·H₂O (6), [Zn₂(cpoa)₂(bpe)(H₂O)] (7), [Zn(cpoa)(bpe)_0.5]·0.5(i-C₃H₇OH) (8) and [Zn(cpoa)(bpe)_0.5]·0.5(n-C₃H₇OH) (9) (H₂cpoa = 4-carboxyphenoxy acetic acid, bpe = 1,2-bis(4-pyridyl)ethane, 4,4′-bpy = 4,4′-bipyridine) were synthesized under hydro/solvothermal conditions. X-ray single-crystal structural analyses reveal that they have metal–organic frameworks ranging from 0D (1), 1D (3, 4), 2D (5, 7, 8) to 3D structure (2, 6, 9). The reaction of the CoCl₂/H₂cpoa/bpe (1 : 1 : 1) system results in the formation of complex 1 at the starting pH of ca. 8, and complex 2 at 12. The reaction of CoCl₂/H₂cpoa/4,4′-bpy (1 : 1 : 1) forms complex 3 with a 1D chain structure, while a molar ratio of 1 : 1 : 0.5 leads to the formation of complex 4 with two 1D metal–organic frameworks of rarely reported homometal complex ion pairs. By changing the added organic solvent, the 2D layer framework of complex 8 and the microporous 3D framework of complex 9 were obtained under the same reaction conditions. The guest n-C₃H₇OH molecules occupy 1D channels in 9. Thermogravimetric and powder X-ray diffraction analyses show that the microporous 3D framework of 9 is maintained after the removal of the guest molecule. And a reversible dealcoholization/realcoholization process can be observed. The magnetic measurements show antiferromagnetic interactions between the metal ions in 2, 4 and 5, and antiferromagnetic interaction in the range of 300–18 K and ferromagnetic interaction between Ni(II) ions below 18 K in 6.