Two Self‐Catenated Nickel(II) Hybrid Vanadates with Honeycomb‐Like 3D Inorganic Frameworks Stabilized by Crossed Organic Bpe Pillars: Thermal, Spectroscopic and Magnetic Properties

Abstract

AbstractThe three‐dimensional hybrid compounds [{Ni6(H2O)10(Bpy)6}V18O51]·1.5H2O (1, Bpy = 4,4′‐bipyridine) and [{Ni(H2O)2(Bpe)}V4O11]·0.5H2O (2, Bpe = 1,2‐bis(4‐pyridyl)ethene) have been synthesized under mild hydrothermal conditions. The 3D crystal structures of both compounds are closely related, with an inorganic honeycomb‐like framework stabilized by the organic ligands. The latter are oriented in a crossed way into the channel of the inorganic backbone, and directly attached to the nickel(II) metal centers. Increasing of the ligand length causes widening of the inorganic skeleton with retention of the main structural organization. Both compounds possess self‐catenated nets, a 24‐nodes net (3‐c)2(4‐c)3(5‐c)7 for 1 and a binodal net (4‐c)(7‐c) for 2. These rigid architectures exhibit very interesting thermal properties. The co‐crystallized water molecules can be removed without changes in the crystal structures. The initial phases show a low thermal expansion of the cell volume, with a negative thermal expansion of the b parameter for compound 1. Moreover, at high temperatures structural irreversible transformations are observed in both compounds, due to the removal of coordinated water molecules. These transformations have been followed by thermodiffractometry and IR spectra. The UV/Vis spectra show the characteristic bands of the NiII d8‐high spin cation in slightly distorted octahedral environment. The magnetic behaviour is strongly dependent of the connectivity of the metal centres through the VO5 polyhedra. Compound 1 exhibits a typical Zero Field Splitting for six nickel(II) cations, while the shortest distances between the nickel(II) ions in 2 allow a weak antiferromagnetic 1D coupling. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)