Thermodynamically stable and metastable coordination polymers synthesized from solution and the solid state

Abstract

The reaction of Fe(NCS)₂ with 4-picoline in solution leads to two new discrete complexes, Fe(NCS)₂(4-picoline)₄·solvate (1-Fe) and Fe(NCS)₂(4-picoline)₂(H₂O)₂ (2-Fe–H₂O). Upon heating, both compounds transform into two isomers with the composition [Fe(NCS)₂(4-picoline)₂]_n (2-Fe/I and 2-Fe/II) that consist of chains and that are related by monotropy, with 2-Fe/I as the thermodynamically stable form. Upon further heating, a transformation into a more condensed chain compound of unusual structure with the composition [Fe(NCS)₂(4-picoline)]_n (3-Fe) is observed. The crystal structures of 2-Fe/I and 2-Fe/II were determined by Rietveld refinements, as it was found that 2-Fe/I is isotypic to the corresponding Cd compound 2-Cd/I, whereas 2-Fe/II is isotypic to [Cu(NCS)₂(4-cyanopyridine)₂]_n which was reported recently. To retrieve structural information on 3-Fe the corresponding Cd compound (3-Cd) was synthesized and was shown to be isotypic to 3-Fe. Finally, upon heating the already known compounds [Cd(NCS)₂(4-picoline)₄]·0.67·4-picoline·0.33·H₂O solvate (1-Cd) and [Cd(NCS)₂(4-picoline)₂]_n (2-Cd/I) a new thermodynamically metastable modification was found (2-Cd/III) that is isotypic to [Cd(NCS)₂(4-cyanopyridine)₂]_n and behaves monotropically to 2-Cd/I with the latter as the thermodynamically stable phase.