Porous lanthanide–copper coordination frameworks exhibiting reversible single-crystal-to-single-crystal transformation based on variable coordination number and geometry

Abstract

Two serial heterometallic Ln–Cu coordination polymers [Ln₂Cu(NIPH)₄(H₂O)₄]·6H₂O (Ln = Gd (1a), Nd (2a), Sm (3a), Eu (4a), Tb (5a), Dy (6a), Ho (7a) and Er (8a)) and [Ln₂Cu(NIPH)₄(H₂O)₂] (Ln = Gd (1b), Nd (2b), Sm (3b), Eu (4b), Tb(5b), Dy (6b), Ho (7b) and Er (8b) (NIPH = 5-nitroisophthalate) have been successfully synthesized and exhibit reversible single-crystal-to-single-crystal transformation behavior based on a dehydration and rehydration process involving coordinated and uncoordinated water molecules. The coordination polymers 1a–8a and 1b–8b belong to two sets of isomorphous coordination frameworks respectively; their 3D frameworks are constructed by different heterometallic polynuclear chains. For the serial crystals 1a–8a, the Ln³⁺ and Cu²⁺ ions are 8-coordinate and 4-coordinate respectively. While for the other serial crystals, 1b–8b, the Ln³⁺ and Cu²⁺ ions are 9-coordinate and 6-coordinate respectively. La³⁺, Ce³⁺ and Pr³⁺ were also employed to construct Ln–Cu coordination frameworks [LaCu(NIPH)₂(HNIPH)(H₂O)₂]·2H₂O (La-1), [La₂Cu(NIPH)₄(H₂O)₂] (La-2), [Ce₂Cu(NIPH)₄(H₂O)₆] (Ce-1) and [Pr₂Cu(NIPH)₄(H₂O)₄]·6H₂O (Pr-a), which do not display the crystal-to-crystal transformation characteristic.