Synthesis, structures and magnetic properties of Fe(II) and Co(II) thiocyanato coordination compounds: on the importance of the diamagnetic counterparts for structure determination.

Reaction of Fe(NCS)2 and Co(NCS)2 with 2-methylpyrazine in different molar ratios and solvents at room temperature leads to the formation of five new coordination compounds of composition M(NCS)2(2-methylpyrazine)2(H2O)2 (M = Fe (1-Fe) , Co (1-Co)), Co(NCS)2(2-methylpyrazine)2(CH3OH)2 (2-Co) and Co(NCS)2(2-methylpyrazine)4·2-methylpyrazine solvate (3-Co). In all of these compounds, discrete complexes are found in which the metal cations are octahedrally coordinated by two terminal N-bonded thiocyanato anions and four N- or O-donor co-ligands. On heating compounds 1-3 in a thermobalance new coordination polymers of composition M(NCS)2(2-methylpyrazine)2 (M = Co (4-Co) , Fe (4-Fe)) are obtained in the first step, which transform into M(NCS)2(2-methylpyrazine) (M = Co (5-Co) , Fe (5-Fe)) in the second. Because of the low chalcophilicity of these cations, compounds 4 and 5 are not accessible from solution. Further investigations prove that 4-Co and 4-Fe obtained by thermal decomposition are of low crystallinity, might be isotypic and might consist of metal cations, in which the anionic ligands are only terminal N-bonded. In contrast, 5-Co and 5-Fe are of good crystallinity but their structure cannot be solved from X-ray powder data. However, a compound of the same composition (5-Cd) based on the more chalcophilic cadmium can easily be crystallized from solution and characterized by single crystal X-ray diffraction. This compound is isotypic to 5-Co and 5-Fe and therefore their structures were determined by Rietveld refinements. In their crystal structures the metal atoms are linked by μ-1,3-bridging thiocyanato anions into a 2D network. Magnetic measurements reveal that compounds 1-4 show only Curie-Weiss paramagnetism and that for 5-Fe antiferromagnetic ordering is observed. In contrast, 5-Co shows metamagnetic behavior with a very large critical field. Finally, it was shown that 4-Co and 4-Fe are hygroscopic and transform within minutes into the hydrates 1-Co and 1-Fe.