Methylammonium Tetrel Halide Perovskite Ion Pairs and Their Dimers: The Interplay between the Hydrogen-, Pnictogen- and Tetrel-Bonding Interactions

Abstract

The structural stability of the extensively studied organic–inorganic hybrid methylammonium tetrel halide perovskite semiconductors, MATtX3 (MA = CH3NH3+; Tt = Ge, Sn, Pb; X = Cl, Br, I), arises as a result of non-covalent interactions between an organic cation (CH3NH3+) and an inorganic anion (TtX3−). However, the basic understanding of the underlying chemical bonding interactions in these systems that link the ionic moieties together in complex configurations is still limited. In this study, ion pair models constituting the organic and inorganic ions were regarded as the repeating units of periodic crystal systems and density functional theory simulations were performed to elucidate the nature of the non-covalent interactions between them. It is demonstrated that not only the charge-assisted N–H···X and C–H···X hydrogen bonds but also the C–N···X pnictogen bonds interact to stabilize the ion pairs and to define their geometries in the gas phase. Similar interactions are also responsible for the formation of crystalline MATtX3 in the low-temperature phase, some of which have been delineated in previous studies. In contrast, the Tt···X tetrel bonding interactions, which are hidden as coordinate bonds in the crystals, play a vital role in holding the inorganic anionic moieties (TtX3−) together. We have demonstrated that each Tt in each [CH3NH3+•TtX3−] ion pair has the capacity to donate three tetrel (σ-hole) bonds to the halides of three nearest neighbor TtX3− units, thus causing the emergence of an infinite array of 3D TtX64− octahedra in the crystalline phase. The TtX44− octahedra are corner-shared to form cage-like inorganic frameworks that host the organic cation, leading to the formation of functional tetrel halide perovskite materials that have outstanding optoelectronic properties in the solid state. We harnessed the results using the quantum theory of atoms in molecules, natural bond orbital, molecular electrostatic surface potential and independent gradient models to validate these conclusions.

Definition of the Halogen Bond (IUPAC Recommendations 2013): A Revisit

“…This accords with the second feature in ref . k The UV–vis absorption bands of the HaBD chromophore may experience a shift to shorter wavelengths; − the binding energies of the peaks associated with X with the X-ray photoelectron spectrum (XPS) of the R−X···Y complex shift to lower energies relative to unbonded X, in accordance with the eighth and tenth features, respectively, in ref . l A bond path and a bond critical point between HaBD X and HaBA Y may be found when an electron density topology analysis based on the quantum theory of atoms in molecules (QTAIM) , is carried out, together with the appearance of other charge density-based signatures such as those associated with the sign and magnitude of the Laplacian of the charge density and the total energy density at bond critical points . This may be in accordance with the sixth feature in ref . m Isosurface volumes (colored greenish, blue, or blue-green between HaBD X and HaBA Y, representative of attractive interactions ,,, ) may be seen if a noncovalent index analysis (NCI) based on reduced density gradients is performed; − similar isosurface features may also emerge when an independent gradient model (IGM), , an IGM based on Hirshfeld partition of molecular density (IGMH), or an interaction region indicator (IRI) analysis is performed. , Feature l may fail in some chemical systems, especially in the weak bonding regime, ,,− but the isosurface analysis is expected to recover inter- and/or intramolecular interaction between interacting atomic basins. ,, n At least some transfer of charge density from the frontier HaBA orbital to the frontier HaBD orbital may occur (viz. from a (filled) lone-pair type orbital ( n ) to an empty σ*-/π*-type anti-bonding orbital); when the transfer of charge density between the orbitals is significant, the formation of a dative (covalent) interaction is likely .…”

“…An analysis of the geometric feature alone is insufficient to determine whether the I···F close contact is a HaB. However, as we will show below, and have demonstrated recently elsewhere, the I···F close contact could be regarded as HaB given that the interaction between the anion and the cation in the crystal is likely to play an important role in polarizing the electrostatic field of the anion; the induction of a positive potential on the surface of the I in C 6 ­H 5 ­F 5 ­I – along the C–I bond extension may occur, which shall interact attractively with the lateral portion of F in a neighboring C 6 H 5 F 5 I – . This speculative scenario for the occurrence of halogen bonding in the crystal is very different from that found in the crystals of tri­fluoro­meth­yl-tetra­fluoro-iodine (CF 7 I) and penta­fluoro­phen­yl-tetra­kis­(fluoro)­iodine (C 6 F 9 I) ( vide infra ).…”

“…81 A positive σ-hole on covalently bonded halogen X may be introduced in an anion in the presence of cation that enables it to engage attractively with a HaBA site Y, developing a halogen bond between them, as observed in the case of a tetrel derivative in an anion. 119 Note 17. An attractive noncovalent interaction between an electrophilic (or nucleophilic) region on a halogen atom in one molecular entity and an electrophilic (or nucleophilic) region in the same or a neighboring interacting molecular entity may not be considered to be a halogen bond if both carry the same charge polarity (see Notes 2, 6, and 7 and the definition).…”

“…The highly negative σ-hole may become less negative because of electron-withdrawing substituents at the peripheral sites of a halogenated molecule . A positive σ-hole on covalently bonded halogen X may be introduced in an anion in the presence of cation that enables it to engage attractively with a HaBA site Y, developing a halogen bond between them, as observed in the case of a tetrel derivative in an anion …”

Halogen Bond via an Electrophilic π-Hole on Halogen in Molecules: Does It Exist?

“…The characteristics of the π-hole can be understood based on positive or negative signs of the electrostatic potential. A region on a molecular entity can have either V S,max > 0 (or V S,min > 0) if the π-hole is electrophilic (electron density deficient) [52,122,123], or V S,max < 0 (or V S,min < 0) if nucleophilic (electron density rich) [47,55,56]. When such a situation is encountered, the former and latter characteristics signify the presence of positive and negative π-holes, respectively, and appear on the outer extensions around the covalently bonded atom (or molecular plane) orthogonal to the R-A bond axis [55].…”