Synthesis, structural characterization and computational studies of layered metal phosphonates: [M(HO3P–C5H4N–PO3H)2(H2O)2]n [MII = Co, Zn, Cd]

Abstract

Three novel isomorphous coordination polymers with a general formula [M(HO₃P–C₅H₄N–PO₃H)₂(H₂O)₂]_n [M^II = Co (1), Zn (2) and Cd (3)] have been synthesized and characterized by powder and single-crystal X-ray diffraction, vibrational spectroscopy (IR and Raman) and simultaneous thermal analysis (TG-DTA-MS). Crystal structure analyses of compounds 1–3 have revealed that both phosphonate groups of (pyridin-1-ium-3,5-diyl)diphosphonate ligands serve to extend the metal cations into a strongly undulated grid structure parallel to the (101) plane. The adjacent monolayers are held together via multiple hydrogen bonds and offset face-to-face π–π interactions. The interlayer interaction energy values, estimated using the CRYSTAL code, have revealed that the stabilization energies of compounds 1–3 are quite similar, where 1 is least and 3 most advantageous. Computational studies have been further related to the results of thermal analyses. In the case of compound 1, which has unpaired electrons at the metal centre, the investigation has been supplemented by magnetic measurements. The magnetic properties of 1 (1.7–300 K) have been analyzed assuming that any possible exchange interactions are very weak. The obtained best-fit parameters are: Δ (axial splitting of ⁴T_1g term) = −478 cm⁻¹, λ (spin–orbit coupling) = −103 cm⁻¹, α (orbital reduction factor) = 1.38, and zJ′ (mean-field exchange parameter) = −0.11 cm⁻¹. The negative value of Δ implies that the ground state is the orbital doublet, which cannot be treated within the spin-Hamiltonian approach.