Spectral Analysis of the Uranyl Squarate and Croconate System: Evaluating Differences between the Solution and Solid-State Phases

Abstract

The design of hybrid materials relies on an understanding of the structural building units present in the reaction, but identifying these building units in solution can be hampered by difficulties in the interpretation of the characteristic spectroscopic signals. The importance of speciation and intermolecular interactions in identifying building units was explored in the uranyl squarate and croconate system with the isolation and characterization of six compounds: ( USq1 ) [C 4 H 12 N 2 ][(UO 2 )(C 4 O 4 ) 2 (H 2 O)]·(H 2 O), ( USq2 ) [C 5 H 6 N][(UO 2 )(C 4 O 4 )(μ 2 -OH)]·2H 2 O, ( USq3 ) [C 2 H 10 N 2 ] 2 [(UO 2 ) 6 (C 4 O 4 ) 3 (μ 3 -O) 2 (μ 2 -OH) 6 ], ( UCr1 ) [C 4 H 12 N 2 ] 2 [(UO 2 )(C 5 O 5 ) 3 (H 2 O)]·3H 2 O, ( UCr2 ) [C 5 H 6 N] 4 [(UO 2 ) 4 (C 5 O 5 ) 4 (μ 2 -OH) 4 (H 2 O) 4 ], and ( UCr3 ) [C 2 H 10 N 2 ] 5 [(UO 2 ) 6 (C 5 O 5 ) 6 (μ 3 -O) 2 (μ 2 -OH) 6 (H 2 O) 4 ]. These compounds are built upon the traditional monomeric, dimeric, and trimeric uranyl oligomers that occur upon hydrolysis of the metal center. A combined solid and solution spectroscopic approach was used to understand the impact of uranyl oligomerization, ligand coordination, and intermolecular interactions. The v 1 (UO 2 2+ ) Raman shift and the UV–visible spectrum for each solid uranyl-squarate and uranyl-croconate compound reveals important information regarding the impact of π–π interactions on the chemical properties of uranyl hybrid materials.