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

Halenium ions, X+, are particularly strong halogen-bond donors that interact with two Lewis bases simultaneously to form linear [D···X···D]+-type halonium complexes. Their three-center, four-electron halogen bond is both fundamentally interesting and technologically valuable as it tames the reactivity of halogen(I) ions, opening up new horizons in a variety of fields including synthetic organic and supramolecular chemistry. Understanding this bonding situation enables the development of improved halogen(I) transfer reactions and of advanced functional materials. Following a decade of investigations of basic principles, the range of applications is now rapidly widening. In this Perspective, we assess the status of the field and identify its key advances and the main bottlenecks. Clearing common misunderstandings that may hinder future progress, we aim to inspire and direct future research efforts.

The Impact of ortho-substituents on Bonding in Silver(I) and Halogen(I) Complexes of 2-Mono- and 2,6-Disubstituted Pyridines: An In-Depth Experimental and Theoretical Study

The coordination nature of 2-mono- and 2,6-disubstituted pyridines with electron-withdrawing halogen and electron-donating methyl groups for [N-X-N]+ (X=I, Br) complexations have been studied using 15 N NMR, X-ray crystallography, and Density Functional Theory (DFT) calculations. The 15 N NMR chemical shifts reveal iodine(I) and bromine(I) prefer to form complexes with 2-substituted pyridines and only 2,6-dimethylpyridine. The crystalline halogen(I) complexes of 2-substituted pyridines were characterized by using X-ray diffraction analysis, but 2,6-dihalopyridines were unable to form stable crystalline halogen(I) complexes due to the lower nucleophilicity of the pyridinic nitrogen. In contrast, the halogen(I) complexes of 2,6-dimethylpyridine, which has a more basic nitrogen, are characterized by X-crystallography, which complements the 15 N NMR studies. DFT calculations reveal that the bond energies for iodine(I) complexes vary between -291 and -351 kJ mol-1 and for bromine between -370 and -427 kJ mol-1 . The bond energies of halogen(I) complexes of 2-halopyridines with more nucleophilic nitrogen are 66-76 kJ mol-1 larger than those of analogous 2,6-dihalopyridines with less nucleophilic nitrogen. The experimental and DFT results show that the electronic influence of ortho-halogen substituents on pyridinic nitrogen leads to a completely different preference for the coordination bonding of halogen(I) ions, providing new insights into bonding in halogen(I) chemistry.

N-X⋅⋅⋅O-N Halogen Bonds in Complexes of N-Haloimides and Pyridine-N-oxides: A Large Data Set Study

N-X⋅⋅⋅- O-N+ halogen-bonded systems formed by 27 pyridine N-oxides (PyNOs) as halogen-bond (XB) acceptors and two N-halosuccinimides, two N-halophthalimides, and two N-halosaccharins as XB donors are studied in silico, in solution, and in the solid state. This large set of data (132 DFT optimized structures, 75 crystal structures, and 168 1 H NMR titrations) provides a unique view to structural and bonding properties. In the computational part, a simple electrostatic model (SiElMo) for predicting XB energies using only the properties of halogen donors and oxygen acceptors is developed. The SiElMo energies are in perfect accord with energies calculated from XB complexes optimized with two high-level DFT approaches. Data from in silico bond energies and single-crystal X-ray structures correlate; however, data from solution do not. The polydentate bonding characteristic of the PyNOs' oxygen atom in solution, as revealed by solid-state structures, is attributed to the lack of correlation between DFT/solid-state and solution data. XB strength is only slightly affected by the PyNO oxygen properties [(atomic charge (Q), ionization energy (Is,min ) and local negative minima (Vs,min )], as the σ-hole (Vs,max ) of the donor halogen is the key determinant leading to the sequence N-halosaccharin>N-halosuccinimide>N-halophthalimide on the XB strength.

Iodine(I) and Silver(I) Complexes Incorporating 3-Substituted Pyridines

Building upon the first report of a 3-acetaminopyridine-based iodine(I) complex (1b) and its unexpected reactivity toward ^tBuOMe, several new 3-substituted iodine(I) complexes (2b-5b) have been synthesized. The iodine(I) complexes were synthesized from their analogous silver(I) complexes (2a-5a) via a silver(I) to iodine(I) cation exchange reaction, incorporating functionally related substituents as 3-acetaminopyridine in 1b; 3-acetylpyridine (3-Acpy; 2), 3-aminopyridine (3-NH₂py; 3), and 3-dimethylaminopyridine (3-NMe₂py; 4), as well as the strongly electron-withdrawing 3-cyanopyridine (3-CNpy; 5), to probe the possible limitations of iodine(I) complex formation. The individual properties of these rare examples of iodine(I) complexes incorporating 3-substituted pyridines are also compared to each other and contrasted to their 4-substituted counterparts which are more prevalent in the literature. While the reactivity of 1b toward etheric solvents could not be reproduced in any of the functionally related analogues synthesized herein, the reactivity of 1b was further expanded to a second etheric solvent. Reaction of bis(3-acetaminopyridine)iodine(I) (1b) and ⁱPr₂O gave [3-acetamido-1-(3-iodo-2-methylpentan-2-yl)pyridin-1-ium]PF₆ (1d), which demonstrated potentially useful C-C and C-I bond formation under ambient conditions.

Solid-state NMR Spectroscopy of Iodine(I) Complexes

Solid-state NMR has been applied to a series of Barluenga-type iodine(I) [L-I-L]PF₆ (L=pyridine, 4-ethylpyridine, 4-dimethylaminopyridine, isoquinoline) complexes as their hexafluorophosphate salts, as well as their respective non-liquid ligands (L), their precursor silver(I) complexes, and the respective N-methylated pyridinium and quinolinium hexafluorophoshate salts. These results are compared and contrasted to the corresponding solution studies and single-crystal X-ray structures. As the first study of its kind on the solid-state NMR behavior of halogen(I) complexes, practical considerations are also discussed to encourage wider utilization of this technique in the future.