Reversible water intercalation accompanied by coordination and color changes in a layered metal-organic framework

Abstract

A new two-dimensional 3d-4f mixed-metal mixed dicarboxylate (homocyclic and heterocyclic) of the formula [Gd₂(H₂O)₂Ni(H₂O)₂(1,2-bdc) ₂(2,5-pydc)₂]·8H₂O (1; 1,2-H₂bdc = 1,2-benzenedicarboxylic acid and 2,5-H₂pydc = 2,5-pyridinedicarboxylic acid) has been prepared by employing the hydrothermal method. The structure has infinite one-dimensional -Gd-O-Gd- chains formed by the edge-shared GdO₉ polyhedral units, resulting exclusively from the connectivity between the Gd³⁺ ions and the 1,2-bdc units. The chains are connected by the [Ni(H₂O)₂(2,5-pydc)₂]^2- metalloligand, forming the two-dimensional layer arrangements. The stacking of the layers creates hydrophilic and hydrophobic spaces in the interlamellar region. A one-dimensional water ladder structure, formed by the extraframework water molecules, occupies the hydrophilic region while the benzene ring of 1,2-bdc occupies the hydrophobic region. To the best of our knowledge, the present compound represents the first example of a 3d-4f mixed-metal carboxylate in which two different aromatic dicarboxylate anions act as the linkers. The stabilization energies of the water clusters have been evaluated using density functional theory calculations. The water molecules in 1 are fully reversible accompanied by a change in color (greenish blue to brown) and coordination around Ni²⁺ ions (octahedral to distorted tetrahedral).