Supramolecular motifs and solvatomorphism within the compounds [M(bpy)_3]_2[NbO(C_2O_4)_3]Cl•nH_2O (M=Fe^2+, Co^2+, Ni^2+, Cu^2+ and Zn^2+; n=11, 12). Syntheses, structures and magnetic properties

Solvatomorphism has been found between two series of complexes of the composition [M(bpy)_3]_2[NbO(C_2O_4)_3]Cl•nH_2O [M = Fe^2+(1, 2), Co^2+(3, 4), Ni^2+(5, 6), Cu^2+(7) and Zn^2+(8, 9); bpy=2,2'-bipyridine)], crystallizing in the monoclinic space group P2_1/c[3, 5, 8 (n=11)] or in the orthorhombic space group P2_12_12_1[2, 4, 6, 7 (n= 12)]. All the structures contain two symmetry independent [M(bpy)_3]^2+ cations, one [NbO(C_2O_4)_3]^3- anion, one Cl– anion, and crystal water molecules. The cations possess a trigonally distorted octahedral geometry, with an additional tetragonal distortion in 7. Analysis of crystal packing reveals a specific type of supramolecular contact comprising four bipyridine ligands from two neighbouring [M(bpy)_3]^2+ cations – quadruple aryl embrace (QAE) contact. The contact is realized by the alignment of two molecular two-fold rotation axes, preserving the parallel orientation of the molecular three-fold rotation axes. The resulting two-dimensional honeycomb lattices of [M(bpy)_3]^2+ cations are placed between the hydrogen bonding layers made of [NbO(C_2O_4)_3]^3– and Cl– anions and the majority of crystal water molecules. The temperature-dependent magnetic susceptibility measurements (1.8–300 K) show a significant orbital angular momentum contribution for 3 and 4 (high-spin Co^2+), the influence of zero-field splitting for 5 and 6 (Ni^2+) and a substantially paramagnetic Curie behaviour for the Cu^2+ compound (7).