Synthesis, Structural Characterization and Ligand-Enhanced Photo-Induced Color-Changing Behavior of Two Hydrogen-Bonded Ho(III)-Squarate Supramolecular Compounds

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 1,2-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–HO 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, 1,2,3-trismonodentate and 1,2-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.

Mixed-Ligand Uranyl Squarate Coordination Polymers: Structure Regulation and Redox Activity

“…Squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione, namely as H 2 C 4 O 4 or H 2 SA ) is a small-molecule cyclic compound belonging to the family of oxocarbons H 2 C n O n ( n = 3–6 for deltic, squaric, croconic, and rhodizonic acids, respectively; see Scheme a). , Because of the characteristic electron delocalization for the deprotonanted form of squaric acid (Scheme b), the corresponding squarate (C 4 O 4 2– , SA 2– ) ion has the attributes of structural rigidity, good coplanarity, and high symmetry. Furthermore, this electron-rich system can undergo oxidation reaction and has good redox activity. ,− To date, SA 2– has been widely used as a versatile ligand with various coordination modes to construct various CPs with novel extended connection, including one-dimensional (1D) chains, − two-dimensional (2D) networks, , and three-dimensional (3D) frameworks. , Meanwhile, it also acts as hydrogen bonding acceptor or a π–π stacking constructor for the stabilization of extended 3D supramolecular networks. ,− However, up to now, compared to coordination polymers based on transition-metal-squarate compounds, only a limited number of actinide-based squarate coordination compounds have been reported, and there is still broad room for further development of actinide-based compounds. Cahill et al studied hydrothermal synthesis of uranyl squarates and the effects of pH and hydrolysis of uranyl cations on reaction products of uranyl nitrate and squaric acid. , Forbes et al probed the effects of uranyl oligomerization, squaraic acid coordination, and intermolecular interactions on the formation of structural building units using a combination of solid and solution spectroscopy .…”

A Semiconducting Bi2O2(C4O4) Coordination Polymer Showing a Photoelectric Response

“…Despite its interesting properties, squarate has been scarcely used as a ligand for homoleptic CPs. Some of these works ,− report squarate-based CPs exhibiting interesting performances in separation and heterogeneous catalysis, with, however, a lack of sufficient information about their optical properties.…”