Guiding Principles for the Rational Design of Hybrid Materials: Use of DFT Methodology for Evaluating Non‐Covalent Interactions in a Uranyl Tetrahalide Model System

Abstract

Together with the synthesis and experimental characterization of 14 hybrid materials containing [UO2X4]2− (X=Cl− and Br−) and organic cations, we report on novel methods for determining correlation trends in their formation enthalpy (ΔHf) and observed vibrational signatures. ΔHf values were analyzed through isothermal acid calorimetry and a Density Functional Theory+Thermodynamics (DFT+T) approach with results showing good agreement between theory and experiment. Three factors (packing efficiency, cation protonation enthalpy, and hydrogen bonding energy [ ]) were assessed as descriptors for trends in ΔHf. Results demonstrated a strong correlation between and ΔHf, highlighting the importance of hydrogen bonding networks in determining the relative stability of solid‐state hybrid materials. Lastly, we investigate how hydrogen bonding networks affect the vibrational characteristics of uranyl solid‐state materials using experimental Raman and IR spectroscopy and theoretical bond orders and find that hydrogen bonding can red‐shift U≡O stretching modes. Overall, the tightly integrated experimental and theoretical studies presented here bridge the trends in macroscopic thermodynamic energies and spectroscopic features with molecular‐level details of the geometry and electronic structure. This modeling framework forms a basis for exploring 3D hydrogen bonding as a tunable design feature in the pursuit of supramolecular materials by rational design.

Synthesis, Characterization, and Density Functional Theory Investigation of the Solid-State [UO2Cl4(H2O)]2– Complex

“…In contrast, the analogous (C 4 H 12 N 2 )-[UO 2 Cl 4 ] showed extensive hydrogen bonding through axial oxygen (BO = 0.161). 9 Within the literature, this phenomenon is typically attributed to the reduced Lewis basicity of axial oxygen due to U�O bond strengthening 22,48,49 and agrees with observations described in Section 3.2. The equatorial water molecule engages in strong H-bonding (Cl equiv •••H− O water 2.167 Å, BO = 0.128) with neighboring uranyl units, forming supramolecular chains of the [UO 2 Cl 4 (H 2 O)] 2− unit extending in the [100] direction (Figure 3d).…”

“…The hydrogen bonding energy ( E H,norm total ) (equations used in the E H,norm total calculation are provided in the Supporting Information, SI eqs 2 and eq 3) and the number of hydrogen ( N H ) bonding interactions of (C 4 H 12 N 2 ) 2 [UO 2 Cl 4 (H 2 O)]Cl 2(s) averaged per formula unit were −2404.69 kJ/mol and 39, respectively. Both E H,norm total and N H are substantially different than related (C 4 H 12 N 2 )[UO 2 Cl 4 ] ( E H,norm total = −1394.49 kJ/mol N H = 20) . A primary reason for the more negative E H,norm total for (C 4 H 12 N 2 ) 2 [UO 2 Cl 4 (H 2 O)]Cl 2(s) is the presence of uncoordinated chloride ions in the lattice, which can act as additional hydrogen bond acceptors.…”

“…Cl 4 ] (s) material (ΔH f = −152.41 ± 1.28 kJ/mol). 9 In previous work, we found that protonation enthalpy of the organic base and E H total positively correlates with ΔH f uranyl hybrid materials, 9 Cl 4 ] (s), it is somewhat surprising that that the aquatetrachloroU(VI) complex is not observed more often during crystallization in these systems. In this particular case, a higher ratio of the organic base is necessary to create pure yields (Supporting Information; Table S1) of (C 4 H 12 N 2 ) 2 [UO 2 Cl 4 (H 2 O)]Cl 2(s) , and that is consistent with the need for additional counterions in the solid phase.…”

“…hydrogen bond acceptor) type hydrogen bonds, which is in agreement with our previous observations. 9 The Cl equiv •••H−N bond distances range from 2.114 Å (BO = 0.153) to 2.683 Å (BO = 0.037), whereas the Cl equiv •••H−C interactions are much weaker than Cl equiv •••H−N and have BOs ranging from 0.067 to 0.020 (Figure 3b). Participation of the uranyl oxo group in hydrogen bonding is minimal, where only type O axial •••H−C bonding is observed (2.258 Å and BO = 0.067) (Figure 3c).…”

“…1,4 For example, the commonality of the [UO 2 Cl 4 ] 2− anion enabled a systematic evaluation of noncovalent interactions in solid-state materials that led to insights regarding the crystallization of U(VI) coordination compounds. [1][2][3][4]8,9 In addition, the [UO 2 Cl 4 ] 2− complex has been crystallized with a range of alkali and organic cations and used to determine the optical, 1 vibrational, 3,5,6 and thermodynamic 2,9,10 properties of these phases to provide insights into the nature of bonding within the 5f system.…”