Structure and Bonding of Halonium Compounds

Abstract

The geometrical parameters and the bonding in [D···X···D]+ halonium compounds, where D is a Lewis base with N as the donor atom and X is Cl, Br, or I, have been investigated through a combined structural and computational study. Cambridge Structural Database (CSD) searches have revealed linear and symmetrical [D···X···D]+ frameworks with neutral donors. By means of density functional theory (DFT), molecular electrostatic potential (MEP), and energy decomposition analyses (EDA) calculations, we have studied the effect of various halogen atoms (X) on the [D···X···D]+ framework, the effect of different nitrogen-donor groups (D) attached to an iodonium cation (X = I), and the influence of the electron density alteration on the [D···I···D]+ halonium bond by variation of the R substituents at the N-donor upon the symmetry, strength, and nature of the interaction. The physical origin of the interaction arises from a subtle interplay between electrostatic and orbital contributions (σ-hole bond). Interaction energies as high as 45 kcal/mol suggest that halonium bonds can be exploited for the development of novel halonium transfer agents, in asymmetric halofunctionalization or as building blocks in supramolecular chemistry.

Halogen Bonds of Halogen(I) Ions─Where Are We and Where to Go?

“…Halogen(I) complexes do not behave as coordinative transition metal complexes, 43 for example, when exposed to solvents and counterions. 44 The bond has occasionally also been named a halonium bond 45 and the complexes superhalides. 21,22 We note that halogen(I) com-plexes should not be misconceived with λ 3 -halogane (λ 3iodane, λ 3 -bromane, λ 3 -chlorane) compounds, which sometimes are also termed halonium complexes yet encompass a halogen(III).…”

“…13,19 Still, a simple electrostatic model is unable to explain the stability of three-center halogenbond complexes. 45 These halogen bonds are characteristically short and strong, ΔG > 50 kJ/mol, with their bond strength and length depending on the involved halogen and Lewis bases. The bond distance is often characterized by the reduction of the contact distance of the interacting atoms in comparison to the sum of their van der Waals radii, which for three-center bonds is 60−70% (R XB = 0.6−0.7).…”

“…13,26 The bond lengths and the bond strengths decrease with the size of the involved halogen(I) decreasing. For [bis(pyridine)halogen- 19,45 Initially, even fluorine(I) complexes have been predicted to be symmetric, 70 a suggestion that was later experimentally and computationally refuted. 19,41 The bond energies strongly depend on the Lewis basicity of the electron donors involved.…”

“…82 This observation is also in line with the strength independence of the N−I bond lengths of [bis(pyridine)iodine(I)] + complexes, 62 56 The influence of substituents on the stability and the geometry of halogen(I) complexes has been extensively studied using solution NMR, X-ray diffraction, and computations. 45,62,63,80 In short, electron-donating substituents strengthen while electron-withdrawing ones weaken the 3c4e halogen bond. It is important to note that the halogen-bond strength shows a strong dependence on the electron density of the Lewis base, whereas the halogen-bond donor−acceptor distance and the NMR coordination shifts do not show a larger strength-dependent variation.…”

Halogen Bonding and/or Covalent Bond: Analogy of 3c–4e N···I···X (X = Cl, Br, I, and N) Interactions in Neutral, Cationic, and Anionic Complexes

“…The variety of applications of trihalides and halonium ions, from mediating electron transfer in solar cells to synthetic organic chemistry, − led to a large number of experimental and computational studies. − However, the nature of this bonding still remains a matter of debate. Besides weakly covalent 3c–4e bonding, it was sometimes referred to as a reversible or dynamic covalent bond or viewed as coordination bonding of I + with two Lewis bases.…”