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 NdIII and FeIII 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.