A DFT and HF quantum chemical study of the tin nanocluster [(RSn)12O14(OH)6]2+ and its interactions with anions and neutral nucleophiles: confrontation with experimental data

Abstract

Calculations of traditional wave function and DFT based reactivity descriptors are reported for the nanocluster [(RSn)₁₂O₁₄(OH)₆]²⁺ (R = CH₃) in order to get insight into the factors determining the exact nature of its interactions with anions or neutral nucleophiles (F⁻, Cl⁻, OH⁻, H₂O, acetone, DMSO). Two levels of calculation (Hartree–Fock and DFT) were used in this study, giving the opportunity to compare the performance of both aproaches. As a whole the HF and DFT results, despite some numerical differences, are qualitatively and in most cases quantitatively similar, since no interchanges in the sequences are found. Calculated charge distributions indicate that the hexacoordinated tin atoms are expected to be harder, the pentacoordinated ones softer. This result is confirmed by the condensed local softness and experimental observations provided by the literature (X-ray, ¹¹⁹Sn NMR), thereby matching predictions made on the basis of the HSAB principle. Molecular electrostatic potential (MEP) calculations further confirm this selectivity, indicating that nucleophiles will approach preferentially the macrocation around the poles rather than at the equator of the cage. Calculated stabilization energies indicate that the charged (F⁻, Cl⁻ and OH⁻) and uncharged (DMSO, acetone and water) nucleophiles tested all interact preferentially with the cage pole region around the hexacoordinated tin atoms. This feature appears related to the electrostatic nature of the interaction for the charged nucleophiles and to the tendency, for all the nucleophiles, to form hydrogen bridges with the μ₂-OH moieties. The basicity of the three types of oxygen atoms [μ₂-OH, μ₃-O(I), μ₃-O(P)] present in [(RSn)₁₂O₁₄(OH)₆)]²⁺ has been addressed by protonation energy and MEP calculations, taking into account accessibility factors not represented by atomic charges. Summarizing, it turns out that the hardness/softness properties of tin atoms are modulated by their coordination; however, the sole consideration of these quantities is not sufficient to predict in all cases the strength and orientation of the interaction between various nucleophiles and the cluster, since electrostatic interactions and H-bonding play a major role.