18-Crown-6 ether templated transition-metal dicyanamido complexes: Synthesis, structural characterization and DFT studies

Reaction of ethanolic solution of hydrated metal salts Mn(NO3)2/Fe(BF4)2 with sodium dicyanamide (dca) and 18-crown-6 ether (18C6) in water results in the isolation of 1:1 associates [Mn(dca)2(H2O)4]·18C6 (1) and [Fe(dca)2(H2O)4]·18C6 (2). Single crystal X-ray structure analysis revealed a high degree of isostructurality between the two 18C6 templated metal complexes (1) and (2) exhibiting second sphere coordination via intermolecular O (water)H⋯O (crown) hydrogen bonds. The octahedral Mn(II)/Fe(II) complexes are sandwiched between adjacent 18C6 molecules into one-dimensional polymeric chains along the [1 0 0] directions, which are further linked through intermolecular O (water)H⋯N (dca) hydrogen bonds, thus forming a two-dimensional supramolecular framework. The electronic structures of the complexes calculated at the DFT level showed a HOMO–LUMO energy separation of 2.45 eV in (1) and 0.83 eV in (2), which implied a higher kinetic stability of complex (1) than that of (2).

► The second sphere coordination between the crown ether (18C6) and [M(dca)2(H2O)4] units. ► One-dimensional polymeric chain with alternating – metal–crown–metal – sequence. ► Electronic properties evaluation by the DFT method. ► HOMO–LUMO energy gap and kinetic stability.