Single-crystal-to-single-crystal structural transformation in a three-dimensional bimetallic (4f−3d) supramolecular porous framework

Abstract

A three-dimensional (3D) bimetallic (4f−3d) supramolecular framework, {[Nd(pyno)₂(H₂O)₄][Fe(CN)₆]·H₂O}_n (1) (pyno = pyridine-N-oxide), has been synthesized and structurally characterized. Structure determination reveals that one-dimensional (1D) cyano-bridged chains of Nd^III and Fe^III form an alternative basket-like arrangement and are connected by (O−H···N) H-bonding interactions through coordinated water molecules and pendent CN groups forming a two-dimensional (2D) sheet-like structure. 2D sheets are further connected by another set of similar (O−H···N) interactions resulting in a 3D supramolecular framework with 1D water-filled channels. Controlled heating of the as-synthesized crystal at 85 °C causes a color change from yellow to a light brown compound {[Nd(pyno)₂(H₂O)₄][Fe(CN)₆]}_n (1′) and structure determination shows significant contraction of the overall framework with selective removal of the guest water molecules. 1′ exhibits a similar framework topology as 1. The dehydrated crystal (1′) regenerates the virgin as-synthesized framework (1a) with structural expansion upon exposure to the water vapor. Adsorption studies reveal that 1′ can selectively uptake H₂O but not MeOH, EtOH, and CO₂ consistent with the channel size. Removal of the guest as well as coordinated water molecules gives another phase {[Nd(pyno)₂][Fe(CN)₆]}_n (2), which shows sorption of H₂O and MeOH vapors but not CO₂. Temperature-dependent magnetic measurement (300−2 K) suggests the as-synthesized framework is antiferromagnetically coupled at low temperature.