Thermochromic Uranyl Isothiocyanates: Influencing Charge Transfer Bands with Supramolecular Structure

Abstract

The synthesis and structural characterization of seven new [UO(NCS)]- and [UO(NCS)Cl]-containing materials charge balanced by 4-phenylpyridinium or 4,4'-bipyridinium cations are reported. Assembly of these materials occurs via a diverse set of noncovalent interactions, with the most prevalent involving the terminal sulfur atoms, which can both accept hydrogen bonds and/or form S···S and S···O interactions. The electrostatic potential of the [UO(NCS)] and [UO(NCS)Cl] anions was calculated and mapped on the 0.001 au isodensity surface to rationalize the observed assembly modes and to provide an electrostatic basis to elucidate the role of the S atoms as both donors and acceptors of noncovalent interactions. Compounds 1-7 display a range of colors (red to yellow) as well as pronounced thermochromism. A computational treatment (time-dependent density functional theory, TDDFT) of the absorbance properties supports the temperature dependence on the ratio of inter- to intramolecular ligand to metal charge transfer (LMCT) bands as obtained from UV-vis diffuse reflectance analysis. Finally, the luminescence profiles of these materials feature additional peaks atypical for most uranyl-containing materials, and a combined spectroscopic (Raman, IR, and fluorescence) and computational (harmonic frequency calculations) effort assigns these as originating from vibronic coupling between the ν U═O symmetric stretch and bending modes of the isothiocyanate ligands.

Non-covalent interactions of uranyl complexes: a theoretical study

“…10 Much weaker hydrogen bonding via C-H … Oyl-U interactions, 11 sometimes via charge assisted hydrogen bonding, 12 have been reported and can be used to selectively separate or sense the uranyl ion selectively from complex mixtures. 13 14 as ell as "…H-OH hydrogen bonds that facilitate formation of infinite chains. 15 Carter et al de o strated that ura l o ge s participation in halogen bonding interactions with iodine determines the 3D crystal structure and spectroscopy in a range of complexes.…”

Restricted Speciation and Supramolecular Assembly in the 5f Block

“…Figure adapted with permission from ref. (R. G. Surbella III, L. C. Ducati, J. Autschbach, N. P. Deifel, C. L. Cahill, Inorg. Chem .…”

“…Homoleptic [UO 2 (NCS) 5 ] 3À anionsm ay be formed preferentially over the above mentioned [UO 2 (NCS) 4 (H 2 O)] 2À through judicious choice of organic cation, and we have prepared a series of eight compounds that continue our efforts to establish ah ierarchy of assembly criteria.Beyond structural interests, however,w en ote deep-red crystal colors, pronounced thermochromism, and additional vibronic features in their luminescence spectra (Figure 9). [24] These properties are not commonly observed among uranylc ontaining materials, anda re au nique characteristic of the uranyl-isothiocyanate system. [12c, 25] Each phenomenar equired as eparate computationala nalysis as a clear crystallographic explanation for the thermochromism and luminescence was insufficient to explain the origin of these observations.…”

Uranyl Coordination Compounds with Alkaline Earth Metals and Crotonate Ligands

“…Most commonly uranium atoms exist in solutions in the oxidation state +6 in the form of uranyl ions UO 2 2+ . Such uranyl ions originate many coordination compounds with diverse organic and inorganic ligands (See for example, refs …”