On the Interplay between Charge‐Shift Bonding and Halogen Bonding

Abstract

The nature of halogen-bond interactions has been analysed from the perspective of the astatine element, which is potentially the strongest halogen-bond donor. Relativistic quantum calculations on complexes formed between halide anions and a series of Y 3 CÀ X (Y=F to X, X=I, At) halogen-bond donors disclosed unexpected trends, e. g., At 3 CÀ At revealing a weaker donating ability than I 3 CÀ I despite a stronger polarizability. All the observed peculiarities have their origin in a specific component of CÀ Y bonds: the charge-shift bonding.Descriptors of the Quantum Chemical Topology show that the halogen-bond strength can be quantitatively anticipated from the magnitude of charge-shift bonding operating in Y 3 CÀ X. The charge-shift mechanism weakens the ability of the halogen atom X to engage in halogen bonds. This outcome provides rationales for outlier halogen-bond complexes, which are at variance with the consensus that the halogen-bond strength scales with the polarizability of the halogen atom.

Is There a Single Ideal Parameter for Halogen‐Bonding‐Based Lewis Acidity?

“…Next, we extended our set of halogen-bond donors to see whether M05-2X/D3/TZVP(D) would also reproduce the CCSD(T) CBSd ata for am ore diverse array of compounds ( Figure 5). As mosta pplicationso fh alogen bonding are based on iodine-bearing carbon backbones, [10e] we focused on halo- Overview of the test set molecules 1-18,i ncludingf ive halomethanes 1-5,s ix more sp 3 -hybridised XB donors featuring moreo rl ess electron-withdrawing oxygen and sulfur substituents( 6-9, 14 and 15), four aromatic systems, three of them polyfluorinated (11-13)a nd the very electron-deficient pentacyanoiodobenzene (10), and three alkyne-bonded iodine compounds (16)(17)(18). gen-bond donors featuring CÀIb onds and strived to select representative small-molecule examples for all variationso fh ybridisation with different electron-withdrawing substituents.…”

“…[16] In the contexto fo ur studies to utilise halogen bondingi n organocatalysis, we had performed orientating computations on simple halogen-bond donors (halogen-basedL ewis acids) and had found an unexpectedt rend in the series of complexes CX 3 I···Cl À and CX 3 I···NH 3 (X = F, Cl, Br,I ). [17] For instance,t he tri- fluoromethyl group is surely more electronegative than the triiodomethyl group, and thusC F 3 I( 2)s hould feature ad eeper s-hole than CI 4 (CI 3 I, 5)a nd should consequently also form stronger complexest ob oth Lewis bases ( Figure 2). The opposite is the case, however,a sh as recentlyb een re-confirmed by high-level calculations anda ne nergy decomposition analysis.…”

Towards a Stronger Halogen Bond Involving Astatine: Unexpected Adduct with Bu3PO Stabilized by Hydrogen Bonding

“…Indeed, the molecular structure(s)c an readily be determined from the (experimental) stoichiometry of acompound, as well as many other spectroscopic signatures at ad esired level of accuracy.I nt his work, the twocomponent relativistic density functionalt heory (DFT) approach is used, as in previous successful works. [11,22] According to the former benchmark study focused on astatine species, the B3LYP [23] hybrid functional appears as as afe cost-effective choice, [24] whichi sa lso true for describing astatine-mediated halogen bonds. [11] The calculated value of log K BAtI is 4.37 at the B3LYP/AVDZ [13] level of theory,i ne xcellent agreement with the experimental one.…”

Quantum chemical topology at the spin–orbit configuration interaction level: Application to astatine compounds

“…[8,9,[11][12][13][14][15]) or (b) determine the electron density in the real space, its topology (QTAIM), [16] as well as that of the electron localization function (ELF) [17,18] at relativistic DFT levels with or without the SOC (see refs. [7,[19][20][21][22][23][24][25][26][27][28] for case studies).…”