Low pH constructed Co(ii) and Ni(ii) 1D coordination polymers based on C-substituted analogues of zoledronic acid: structural characterization, and spectroscopic and magnetic properties.

Three novel coordination compounds based on α,α-disubstituted analogues of zoledronic acid with a cyclopropane (cpp) or cyclobutane (cbt) ring on the C carbon, isomorphous [Co(HcppZol)(HO)]·HO (1a), [Ni(HcppZol)(HO)]·HO (1b) and [Co(HcbtZol)(HO)]·HO (2a), were synthesized under hydrothermal conditions at low pH. Single-crystal X-ray diffraction analysis revealed that all the compounds had a 1D double zig-zag chain architecture with an 8 + 8 ring motif formed by alternately arranged symmetrical (-O-P-O-) bridges linking equivalent octahedral metal centres. Both the ligand coordination mode and chain architecture displayed by 1a, 1b and 2a are unique among 1D [M(HL)(HO) ]·HO coordination polymers based on nitrogen-containing bisphosphonates reported so far. All the compounds exhibit similar decomposition pathways upon heating with thermal stabilities decreasing in the order 1b > 1a > 2a. The IR spectra revealed that lattice water release above 227, 178 and 97 °C, respectively, does not change the chain architecture leaving them intact up to 320, 280 and 240 °C. Magnetic behaviour investigations indicated that 1a, 2a and 1b exhibit weak alternating antiferromagnetic-ferromagnetic exchange interactions propagated between the magnetic centres through double (-O-P-O-) bridges. The boundary between antiferro- and ferromagnetic couplings for the Co-O⋯O-Co angle in 1a and 2a was estimated to be 80°. This value is also applicable for recently reported [M(HL)(HO)]·6HO (M = Co, Ni) complexes based on α,α-disubstituted analogues of zoledronic acid and can be used to the explain magnetic behaviour of 1b.