Abstract
Two coordination polymers (CPs) with chemical formulas, [Ho2(C4O4)2(C2O4)(H2O)8]·4H2O (1) and [Ho(C4O4)1.5(H2O)3] (2), (C4O42− = dianion of squaric acid, C2O42− = oxalate), have been synthesized and their structures were determined by single-crystal X-ray diffractometer (XRD). In compound 1, the coordination environment of Ho(III) ion is eight-coordinate bonded to eight oxygen atoms from two squarate, one oxalate ligands and four water molecules. The squarates and oxalates both act as bridging ligands with μ1,2-bis-monodentate and bis-chelating coordination modes, respectively, connecting the Ho(III) ions to form a one-dimensional (1D) ladder-like framework. Adjacent ladders are interlinked via O–H⋅⋅⋅O hydrogen bonding interaction to form a hydrogen-bonded two-dimensional (2D) layered framework and then arranged orderly in an AAA manner to construct its three-dimensional (3D) supramolecular architecture. In compound 2, the coordination geometry of Ho(III) is square-antiprismatic eight coordinate bonded to eight oxygen atoms from five squarate ligands and three water molecules. The squarates act as bridging ligands with two coordination modes, μ1,2,3-trismonodentate and μ1,2-bis-monodentate, connecting the Ho(III) ions to form a 2D bi-layered framework. Adjacent 2D frameworks are then parallel stacked in an AAA manner to construct its 3D supramolecular architecture. Hydrogen bonding interactions between the squarate ligands and coordinated water molecules in 1 and 2 both play important roles on the construction of their 3D supramolecular assembly. Compounds 1 and 2 both show remarkable ligand-enhanced photo-induced color-changing behavior, with their pink crystals immediately turning to yellow crystals under UV light illumination.
Highlights
Lanthanide metal-organic frameworks (LnMOFs) have received much attention, for their fascinating structural variety, and for their potential applications of MOFs like porosity for gas storage [1–9], their specific characteristics arising from 4f electrons for luminescence [10–22].The inherent character of lanthanide ions with high affinity for oxygen atoms and high coordination numbers, result in the formation of a number of MOFs with flexible coordination geometry and various structural dimensionality from multi-carboxylate ligands [13–29]
With our continuous effort on the study of metal-squarate coordination polymers (CPs) [58–63], we report here the synthesis, structural characterization, thermal stability and light-induced color-changing behavior of two Ho(III)-squarate hydrogen-bonded supramolecular networks, [Ho2 (C2 O4 )(C4 O4 )[2] (H2 O)8 ]·4H2 O (1) and [Ho(C4 O4 )1.5 (H2 O)3 ] (2), (C4 O4 2− = dianion of squaric acid, C2 O4 2− = dianion of oxalic acid), in which the squarate acts as the bridging ligands with μ1,2 -coordination mode (Scheme 1b) for 1 and combined μ1,2 -plus μ1,2,3 -coordination modes (Scheme 1b,c) for 2 to build up 1D ladder-like CP and 2D bi-layered MOF, respectively
In method 1, the oxalate ligand was obtained via the in-situ synthesis from the squarate ligand, indicating a slow release of oxalate from the squarate could be helpful for the formation of compound 1
Summary
Lanthanide metal-organic frameworks (LnMOFs) have received much attention, for their fascinating structural variety, and for their potential applications of MOFs like porosity for gas storage [1–9], their specific characteristics arising from 4f electrons for luminescence [10–22].The inherent character of lanthanide ions with high affinity for oxygen atoms and high coordination numbers, result in the formation of a number of MOFs with flexible coordination geometry and various structural dimensionality from multi-carboxylate ligands [13–29]. With our continuous effort on the study of metal-squarate coordination polymers (CPs) [58–63], we report here the synthesis, structural characterization, thermal stability and light-induced color-changing behavior of two Ho(III)-squarate hydrogen-bonded supramolecular networks, [Ho2 (C2 O4 )(C4 O4 ) (H2 O)8 ]·4H2 O (1) and [Ho(C4 O4 )1.5 (H2 O)3 ] (2), (C4 O4 2− = dianion of squaric acid, C2 O4 2− = dianion of oxalic acid), in which the squarate acts as the bridging ligands with μ1,2 -coordination mode (Scheme 1b) for 1 and combined μ1,2 -plus μ1,2,3 -coordination modes (Scheme 1b,c) for 2 to build up 1D ladder-like CP and 2D bi-layered MOF, respectively.
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