Interpenetrated Uranyl-Organic Frameworks with bor and pts Topology: Structure, Spectroscopy, and Computation

Two novel three-dimensional interpenetrated uranyl-organic frameworks, (NH₄)₄[(UO₂)₄(L¹)₃]·6H₂O (1) and [(UO₂)₂(H₂O)₂L²]·2H₂O (2), where L¹ = tetrakis(3-carboxyphenyl)silicon and L² = tetrakis(4-carboxyphenyl)silicon, were synthesized by a combination of two isomeric tetrahedral silicon-centered ligands with 3-connected triangular [(UO₂)(COO)₃]^- and 4-connected dinuclear [(UO₂)₂(COO)₄] units, respectively. Structural analyses indicate that 1 possesses a 2-fold interpenetrating anion bor network, while 2 exhibits a 3-fold interpenetrated 4,4-connected neutral network with pts topology. Both compounds were characterized by thermogravimetric analysis and IR, UV-vis, and photoluminescence spectroscopy. A relativistic density functional theory (DFT) investigation on 10 model compounds of 1 and 2 shows good agreement of the structural parameters, stretching vibrational frequencies, and absorption with experimental results; the time-dependent DFT calculations unravel that low-energy absorption bands originate from ligand-to-uranium charge transfer.