Two isomorphous Co(ii) coordination polymers based on new α,α-disubstituted derivatives of zoledronic acid: synthesis, structures and properties

Abstract

Two novel α,α-disubstituted derivatives of zoledronic acid, namely 1-hydroxy-2-(1H-imidazol-1-yl)-2-methylpropylidene-1,1-diphosphonic acid (H₄L1) and 1-hydroxy-2-[1-(1H-imidazol-1-yl)cyclopropyl]ethylidene-1,1-diphosphonic acid (H₄L2) were synthesized and structurally characterized by single-crystal X-ray diffraction. The reaction of cobalt acetate with H₄L1 and H₄L2 carried out under hydrothermal conditions afforded two isomorphous Co₃(HL1)₂(H₂O)₆·6H₂O (1a) and Co₃(HL2)₂(H₂O)₆·6H₂O (2a) complexes. Both compounds are characterized by means of X-ray crystallography, IR and NIR–Vis–UV spectroscopic methods. Furthermore, their magnetic properties and thermal stabilities are reported. The crystals of 1a and 2a feature infinite 1D polymeric chains built from alternately arranged dinuclear [Co2(HL1/HL2)(H₂O)₂]₂ units and {Co1O₆} octahedra running along the [10] crystallographic direction. In both compounds, crystallographically distinct Co1 and Co2 atoms are six-coordinated. As is reflected in T values (T – index of tetragonality), the coordination environment of Co1 generates a slightly elongated octahedron (T = 0.94), whereas a slightly compressed octahedron (T = 1.06 for 1a and 1.05 for 2a) is formed around Co2. An assumption that the d–d type absorption is mainly attributed to the inversion related Co2 centers, whose population is two times higher than that of Co1, afforded a good correlation between calculated transition energies and experimental NIR–Vis–UV spectra. The magnetic susceptibility measurements analyzed in terms of a spin-3/2 Heisenberg trimer chain revealed that Co1⋯Co2 interactions within the trimer are antiferromagnetic whereas Co2⋯Co2 intertrimer interactions are ferromagnetic.