Coordination polymer changing its magnetic properties and colour by thermal decomposition: synthesis, structure and properties of new thiocyanato iron(II) coordination polymers based on 4,4'-bipyridine as ligand.

Thermal decomposition of the ligand-rich 1:2 precursor solvate (1:2 = ratio between metal and ligand) [{Fe(mu2-bipy-N,N')(NCS)2(H2O)2} x bipy]n (1I) (bipy = 4,4-bipyridine) reported recently leads to the stepwise formation of two new intermediates: a ligand-rich solvate-free 1:2 compound of composition [Fe(NCS)2(bipy)2]n (2) and a ligand-deficient 1:1 compound of composition [Fe(NCS)2(bipy)]n (3). The thermal decomposition is accompanied by a colour change from red (1I), via orange (2) into yellow (3). In the crystal structure of 2 the iron(II) cations are coordinated by four N atoms of bridging bipy ligands, which connect the metal centers into layers, and two terminal N-bonded thiocyanato anions within slightly distorted octahedra. In the crystal structure of 3 the iron(II) cations are coordinated by two N atoms of bridging bipy ligands, which are connected by the metal centers into chains, and two N atoms as well as two S atoms of mu-1,3 bridging thiocyanato anions within a slightly distorted octahedral geometry. The thiocyanato anions connect these chains into layers. Due to their connection mode of the metal centers, only for the latter compound can cooperative magnetic phenomena be expected. Magnetic measurements revealed two different magnetic properties: The 1:2 compounds 1I and 2 show Curie-Weiss paramagnetism and the 1:1 compound 3 shows an antiferromagnetic ordering at T(N) = 5.5 K. The reversibility of water reintercalation in the solvent-free compound 2 was investigated and followed by UV-Vis spectroscopy. Herein a new polymorphic metastable modification 1II of the pristine precursor compound 1I was obtained and characterized by single crystal X-ray determination.