Thermal Decomposition Reactions as a Tool for the Synthesis of New Thermodynamic Metastable Modifications: Synthesis, Structures, and Properties of (Formato)nickel(II) Coordination Polymers Based on 4,4′-Bipyridine

The reaction of nickel formate with 4,4'-bipyridine (bipy) in aqueous solution at room temperature leads to the formation of the hydrated compound [Ni(HCO2)(2)(H2O)(bipy)center dot 4H(2)O](n) (1) reported recently. On heating, compound 1 decomposes into the new anhydrous compound of composition [Ni(HCO2)(2)-(bipy)](n) (2II), which decomposes on further heating. Interestingly, if the anhydrous compound is prepared from solution, a new modification 2I is obtained. Investigations on the stability of both forms show that modification 2I presents the thermodynamically most stable form between room and decomposition temperature, whereas modification 2II, which can only be prepared by thermal decomposition, is metastable. In the crystal structure of 2I, the Ni2+ cations are coor-dinated by four mu(2)-anti,anti bridging formato anions and two bridging mu(2)-bipy ligands in a slightly distorted octahedral geometry. The formato anions bridge the metal cations in zigzag chains, which are further connected by mu(2)-anti,anti formato anions and bipy ligands to give a three-dimensional coordination network. IR spectroscopic investigations on the metastable form 2II also indicate that all formato anions act as bridging ligands. Magnetic measurements of the hydrated and anhydrous compounds reveal different behavior with a ferromagnetic ordering for compound 2I and an antiferromagnetic ordering for compound 1. For form 2II, only Curie-Weiss paramagnetism was found.