Selective Synthesis and Thermodynamic Relations of Polymorphic Co(NCS)2‐4‐Dimethylaminopyridine Coordination Compounds

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Reaction of Co(NCS)2 with 4‐dimethylaminopyridine (DMAP) leads to the formation of four new compounds. The crystal structures of Co(NCS)2(DMAP)2(H2O)2·2H2O (1), Co(NCS)2(DMAP)2(MeOH)2 (2) and Co(NCS)2(DMAP)2(MeCN)2 (3) consist of discrete simple solvato complexes in which the Co cations are octahedrally coordinated by two terminally N‐bonded thiocyanate anions, two DMAP ligands as well as two solvato ligands. Co(NCS)2(DMAP)2 (4/II) also forms discrete complexes, but the Co cations are tetrahedral coordinated by two anionic ligands and two DMAP ligands. Upon heating, the hydrate 1 transforms into 4/II, whereas the methanol solvate 2 transforms into a new polymorphic modification of Co(NCS)2(DMAP)2 (4/I). For structure determination the corresponding Zn(NCS)2 compound was prepared that is isotypic to 4/I. Solvent mediated conversion experiments prove that 4/II represents the thermodynamic stable form at room‐temperature and density functional theory (DFT) calculations indicate that this form is also stable at 0 K. Upon heating both modifications show melting with the higher melting polymorph 4/I having the lower melting enthalpy. Temperature dependent X‐ray powder diffraction shows that 4/II transforms into 4/I upon heating. All experimental results indicate, that both modifications are related by enantiotropism.