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

Abstract

The electron-rich squarate ion (C4O4 2–, SA 2–) possesses electronic delocalization over the entire molecule and good redox activity, and the functionalization of metal–organic complexes with the SA 2– group is desirable. In this work, a mixed-ligand method is used to construct novel uranyl squarate coordination polymers utilizing 4,4′-bipyridine (bpy), 4,4′-bipyridine-N,N′-dioxide (bpydo), 1,10-phenanthroline (phen), 4,4′-vinylenedipyridine (vidpy), and in situ formed oxalate (OA 2–) as ancillary ligands. Seven mixed-ligand uranyl compounds, [(UO2)­(OH)­(SA)]­(Hbpy) (1), [(UO2)­(H2O)­(SA)2]­(H2 bpy) (2), (UO2)­(H2O)­(SA)­(bpydo)·2H2O (3), (UO2)­(H2O)­(SA)­(phen)·H2O (4), (UO2)­(OH)­(SA)0.5(phen)·H2O (5), [(UO2)­(SA)­(OA)0.5]­(Hphen) (6), and [(UO2)­(SA)­(OA)0.5]­(Hvidpy) (7), with varying crystal structures were synthesized under hydrothermal conditions. Compound 1, together with bpy molecules filling in the interlayer space as template agents, has a two-dimensional (2D) network structure, while 2 gives a one-dimensional (1D) chain based on mononuclear uranium units. Compound 3 shows a neutral 2D network through the combined linkage of SA 2– and bpydo. Both 4 and 5 have a similar chain-like structure due to the capping effect of phen motifs, while phen molecules in 6 act as templating agents after protonation. Similar to 6, compound 7 has a “sandwich-like” structure in which the Hvidpy motifs locate in the voids of layers of 2D uranyl-squarate networks. The redox properties of typical mixed-ligand uranyl-squarate compounds, 1, 4, and 5 with high phase purity, are characterized using cyclic voltammetry. All three of these uranyl coordination compounds show anode peaks (E a) at 0.777, 0.804, and 0.760 V, respectively, which correspond to the oxidation process of SA 2– → SA. Meanwhile, cathodic peaks (E c) at −0.328, −0.315, and −0.323 V corresponding to the reduction process of U­(VI) → U­(V) are also observed. The results reveal that all three of these uranyl coordination compounds show good redox activity and, most importantly, the interplay between two different redox-active motifs of SA 2– organic linker and uranyl node. This work enriches the library of redox-active uranyl compounds and provides a feasible mixed-ligand method for regulating the synthesis of functional actinide compounds.

Unveiling Structural Diversity of Uranyl Compounds of Aprotic 4,4′-Bipyridine N,N′-Dioxide Bearing O-Donors

“…Each 7-fold coordinated uranyl cation in the polymeric uranyl chain has five oxygen atoms coordinated in its equatorial plane: four oxygen atoms from μ 2 -hydroxyl groups (U–O­(μ 2 -OH), 2.348(3), 2.351(3), 2.340(3), and 2.334(3) Å) and another one from a monocoordinated DPO ligand with a slightly longer U–O distance (U–O­(DPO), 2.441(3) Å) (Figure a). It is notable that, while a ditopic DPO ligand generally serves as a biconnected linker through two-end uranyl coordination, − , such a monodentate coordinated DPO is rarely reported, only being found in DPO ligands enclosed by cucurbituril macrocycles . All these monodentate DPO ligands are arranged alternately on both sides of the “spine” of the μ 2 -hydroxyl bridged uranyl chain, thus forming a “fishbone” shape (Figure b).…”

“…Compared to carboxylates and phosphonates, organic ligands with aprotic O-donors, such as amides, ethers, or N-oxides, are less pH sensitive and free of deprotonation before metal coordination (Scheme ). The attribute of this kind of ligand being inert to acids and bases has been utilized in metal complexation and separation. ,, Recently, 4,4′-bipyridine N , N ′-dioxide (DPO), an aprotic oxygen-donor ligand, has been employed as building structures to prepare UCPs with varying dimensionalities, ,− among which this bidentate ligand with a single O-donor at each end works together with other O-donors or N-donors from anions and auxiliary ligands added when coordinating to a uranyl node. The pioneering exploration of DPO-based uranyl compounds demonstrates the feasibility of such an aprotic ligand for the preparation of UCPs and inspires further research on UCPs based on this aprotic ligand.…”

Modular Assembly of Isostructural Mixed-Ligand Uranyl Coordination Polymers Based on a Patterning Strategy

“…During the past two decades, a large variety of uranium-based CPs with diverse structures have been reported, ,− where the common practice is to use a single ligand for the assembly of uranyl-based CPs. Meanwhile, a mixed-ligand assembly strategy has been widely applied to design and synthesize numerous CPs that are structurally diverse and controllable, , because various combinations of organic ligands broaden the variety of the resultant uranyl compounds, and also enables their better structural tunability and a greater likelihood of implanting the desirable functionality. In addition, the void positions around metal ions can be easily occupied by coordination with the mixed ligands instead of solvents or anions, which may improve the stability and performance diversity of the resulting CPs.…”