Investigation of the preparation and reactivity of metal-organic frameworks of cerium and pyridine-2,4,6-tricarboxylate.

Juliana Fonseca De Lima,Richard I Walton,Guy J Clarkson,Siân A Franklin,Osvaldo A Serra,Marc Walker,Fernanda V S Moreno,Matthew C Waddington,Jader Da Silva Barbosa,Bruno A T Menezes

doi:10.1039/d1dt03514b

Abstract

The synthesis of three coordination polymers of cerium(III) and the ligand pyridine-2,4,6-tricarboxylate (PTC) is reported. Two of the materials crystallise under hydrothermal conditions at 180 °C, with [Ce(PTC)(H2O)2]·1.5H2O, (1), being formed on extended periods of reaction time, 3 days or longer, and Ce(PTC)(H2O)3, (2), crystallising after 1 day. Both phases contain Ce(III) but are prepared using the Ce(IV) salt Ce(SO4)2·4H2O as reagent. Under solvothermal conditions (mixed water-N,N-dimethylformamide (DMF)), the phase [Ce(PTC)(H2O)(DMF)]·H2O (3) is crystallised. The structures of the three materials are resolved by single crystal X-ray diffraction, with the phase purity of the samples determined by powder X-ray diffraction and thermogravimetric analysis. (1) is constructed from helical chains cross-linked by the PTC linkers to give a three-dimensional structure that contains clusters of water molecules in channels that are hydrogen-bonded to each other and to additional waters that are coordinated to cerium. (2) also contains nine-coordinate cerium but these are linked to give a dense framework, in which water is directly coordinated to cerium. (3) contains corner-shared nine-coordinate cerium centres, linked to give a framework in which Ce-coordinated DMF fills space. Upon heating the material (1) in air all water is irreversibly lost to give a poorly crystalline anhydrous phase Ce(PTC), as deduced from X-ray thermodiffractometry and thermogravimetric analysis. The material (1), however, is hydrothermally stable, and is also stable under oxidising conditions, where immersion in 30% H2O2 gives no loss in crystallinity. Oxidation of around 50% of surface Ce to the +4 oxidation state is thus possible, as evidenced by X-ray photoelectron spectroscopy, which is accompanied by a colour change from yellow to orange. Photocatalytic activity of (1) is screened and the material shows effective degradation of methyl orange.

Highlights

Metal–organic frameworks of cerium have attracted significant interest in the past few years.[1]. Part of this focus lies in the fact that cerium is one of the few lanthanide elements that adopts two stable oxidation states that can interconvert readily
A variety of carboxylate linkers have been used for the preparation of cerium MOFs, each with inherent structural directing connectivity, depending on the number of connection points, and with the possibility of structural flexibility, depending on the organic backbone of the ligand.1a Carboxylates provide versatile ligands for construction of MOFs based on lanthanide cations because the variety of potential bonding modes of the carboxylate function allows the high coordination number of the cation to be satisfied,[5] as well as the strong electrostatic contribution to bonding leading to thermally, and potentially hydrothermally, robust materials
Application in photocatalysis were predicted by computational simulation for Ce(IV)-UiO-66, which showed that only the Ce analogue of material could allow ligand-to-metal charge transfer to separate photogenerated charges.[8]

Summary

Introduction

Metal–organic frameworks of cerium have attracted significant interest in the past few years.[1]. The structure of (1) is constructed from 9-coordinate Ce centres created from coordination of the PTC ligand and water molecules, Fig. 1a.

Results

Conclusion