Hydrogen bonding induced polymorphism in the scandium(III) complex with ε-caprolactam

Abstract

Abstract Two polymorphs of [Sc(cpl)6][Cr(NCS)6] (cpl=ε- C6H11NO), trigonal and monoclinic, form purple elongated narrow plates and brownish-purple prisms and are formed concomitantly irrespectively of the crystallization conditions. In the trigonal polymorph both cation and anion possess C 3i site symmetry while in the monoclinic form cation and anion lie on inversion centre and 2-fold axis respectively. The nature of the polymorphism traces back to a redistribution of inter- and intramolecular hydrogen bonds that causes different conformation of the complex cations, different hydrogen bonding and different molecular packings. The [Sc(cpl)6]3+ cations in the structure of the trigonal polymorph form intermolecular N(H)…S, and in the monoclinic form both N(H)…S inter- and N(H)…O intramolecular hydrogen bonds with NCS groups of [Cr(NCS)6]3− and cpl ligands. This aggregation leads to chains, where the cations and the anions alternate, in the trigonal modification and to layers, in which each ion is surrounded by four counterions, in the monoclinic form. Both polymorphs possess thermochromic properties, and a reversible color change from light purple to dark green takes place at 470–475 K.