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

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 (H 2 O) 2 ( 2 -Fe-H 2 O ). 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·H 2 O 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. Different Fe( ii ) and Cd( ii ) thiocyanate coordination compounds with unusual topology including isomeric modifications were synthesized with 4-picoline and investigated in regard to their thermodynamic relations.