A “nucleophilic” iodine in a halogen-bonded iodonium complex manifests an unprecedented I+···Ag+ interaction

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.

Carbonyl hypoiodites from pivalic and trimesic acid and their silver(I) intermediates

The first tris(O-I-N) carbonyl hypoiodites have been synthesised based on trimesic acid and pyridine or 4-methylpyridine, with their structures definitively confirmed by single crystal X-ray diffraction (SCXRD). The more soluble carbonyl hypoiodites based on pivalic acid have also been studied via NMR, SCXRD, and computational analyses, enabling the study of the direct silver(I) precursor and intermediates of the resulting carbonyl hypoiodites generated using a range of substituted pyridines.

Iodine(i) complexes incorporating sterically bulky 2-substituted pyridines

The silver(i) and iodine(i) complexes of the 2-substituted pyridines 2-(diphenylmethyl)pyridine (1) and 2-(1,1-diphenylethyl)pyridine (2), along with their potential protonated side products, were synthesised to investigate the steric limitations of iodine(i) complex formation. The complexes were characterised by ¹H and ¹H–¹⁵N HMBC NMR, X-ray crystallography, and DFT calculations. The solid-state structures for the silver(i) and iodine(i) complexes were extensively compared to the literature and analysed by DFT to examine the influence of the sterically bulky pyridines and their anions.

The silver(i) and iodine(i) complexes of two sterically bulky 2-substituted pyridines, along with their potential protonated side products, were synthesised to investigate the steric limitations of iodine(i) ion formation.

Dimeric iodine(i) and silver(i) cages from tripodal N-donor ligands via the [N–Ag–N]+ to [N–I–N]+ cation exchange reaction

Complexation of tripodal ligands with silver(I) salts generated M3L2 cage complexes that encapsulated anions within their cavities. Subsequent [N–Ag–N]+ to [N–I–N]+ cation exchange with I2 resulted in the corresponding halogen-bonded iodine(I) cages.

Ligand Exchange Among Iodine(I) Complexes

A detailed investigation of ligand exchange between iodine(I) ions in [N···I···N]+ halogen-bonded complexes is presented. Ligand exchange reactions were conducted to successfully confirm whether iodine(I) complex formation, via the classical...

The Covalent AuI-AuI Bond in (AuF)n (n = 2∼4): A Perspective to Understand the Closed-Shell AuI···AuI Interaction

The nature of closed-shell Au^I···Au^I attraction is still a conundrum in theoretical chemistry. However, for Au₂F₂ with a zigzag conformation, the d¹⁰-d¹⁰ closed-shell interaction between the AuF monomers is demonstrated as a coordinate covalent bond. Chemical bonding analysis reveals that the strong Au^I···Au^I attraction is caused by the participation of the extraordinary active 5d orbital of Au. Based on our study, one of the 5d orbitals of the Au atom is activated to hybridize with its 6s and 6p orbitals to form hybridized dsp² orbitals, where each Au atom is both an electron donor (Lewis base) and acceptor (Lewis Acid) in dimerization. Actually, the closed-shell Au^I···Au^I interaction in the zigzag conformation of Au₂X₂ (X = F, Cl, Br, I, or NH₂) is covalent. Our results provide a rather simple but clear-cut example, where mysterious Au^I···Au^I attractions can be possibly explained by the covalent bond theory.

Iodine(I) and Silver(I) Complexes of Benzoimidazole and Pyridylcarbazole Derivatives

The synthesis of iodine(I) complexes with either benzoimidazole or carbazole-derived sp² N-containing Lewis bases is described, as well as their corresponding silver(I) complexes. The addition of elemental iodine to the linear two-coordinate Ag(I) complexes produces iodine(I) complexes with a three-center four-electron (3c-4e) [N-I-N]⁺ bond. The ¹ H and ¹ H-¹⁵ N HMBC NMR studies unambiguously confirm the formation of the complexes in all cases via the [N-Ag-N]⁺ →[N-I-N]⁺ cation exchange, with the ¹⁵ N NMR chemical shift change between 94 to 111 ppm when compared to the free ligand. The single crystal X-ray crystallographic studies on eight I⁺ complexes revealed highly symmetrical [N-I-N]⁺ bonds with I-N bond distances of 2.21-2.26 Å and N-I-N angles of 177-180°, whilst some of the corresponding Ag⁺ complexes showed a clear deviation from linearity with N-Ag-N angles of ca. 150° and Ag-N bond distances of 2.09-2.18 Å.

"Anti-electrostatic" Halogen Bonding between Ions of Like Charge

Halogen bonding occurs between molecules featuring Lewis acidic halogen substituents and Lewis bases. It is often rationalized as a predominantly electrostatic interaction and thus interactions between ions of like charge (e. g., of anionic halogen bond donors with halides) seem counter-intuitive. Herein, we provide an overview on such complexes. First, theoretical studies are described and their findings are compared. Next, experimental evidences are presented in the form of crystal structure database analyses, recent examples of strong "anti-electrostatic" halogen bonding in crystals, and the observation of such interactions also in solution. We then compare these complexes to select examples of "counter-intuitive" adducts formed by other interactions, like hydrogen bonding. Finally, we comment on key differences between charge-transfer and electrostatic polarization.

On the nature of recurrent Au⋯π motifs in tris(2,2'-bipyridine)M(II) (M = Fe, Co and Ni) dicyanoaurate(I) salts: X-ray analysis and theoretical rationalization

This manuscript reports the synthesis, X-ray characterization and DFT study of three new [M(bipy)₃]₂[Au(CN)₂]₃(X) (M = Fe, Co, and Ni; bipy = 2,2'-bipyridine; X = anion) ionic compounds. These salts are composed of [M(bipy)₃]²⁺ dications and [Au(CN)₂]^- anions in a 2 : 3 ratio. The positive charge is compensated by X = Cl^- anions in compounds 1 (M = Fe) and 2 (M = Co) and X = OH^- in 3 (M = Ni). The three tridentate bipyridine ligands define the coordination of the M²⁺ cation, resulting in a nearly octahedral coordination sphere. The linear dicyanoaurate(I) anions are completely surrounded by a cradle of aromatic rings with Au-ring centroid distances below the sum of van der Waals radii, evidencing the existence of a specific Au⋯π attraction. This interaction has been analyzed in terms of the role of the Au-atom (Lewis acid or Lewis base) using DFT calculations combined with the quantum theory of atoms in molecules (QTAIM), noncovalent interaction plot index (NCIplot) and natural bond orbital (NBO) computational tools. The NBO suggests that the Au⋯π interaction is an example of a coinage bond in spite of the anionic nature of the acceptor and the cationic nature of the donor.

Macrocyclic complexes based on [N⋯I⋯N]+ halogen bonds

New 1-2 nm macrocyclic iodine(I) complexes prepared VIA a simple ligand exchange reaction manifest rigid 0.5-1 nm cavities that bind the hexafluorophosphate anion in the gas phase. The size of the cavities and the electrostatic interactions with the iodine(I) cations influence the anion binding properties of these macrocyclic complexes.

Dihypoiodites stabilised by 4-ethylpyridine through O-I-N halogen bonds

Four bis(O-I-N) compounds have been synthesised from various dihypoiodites and 4-ethylpyridine. The compounds were characterised in both the solution and solid states by NMR spectroscopy (¹H, ¹⁵N), X-ray diffraction, and computational calculations.

Carbonyl Hypoiodites as Extremely Strong Halogen Bond Donors

Neutral halogen-bonded O-I-N complexes were prepared from in situ formed carbonyl hypoiodites and aromatic organic bases. The carbonyl hypoiodites have a strongly polarized iodine atom with larger σ-holes than any known uncharged halogen bond donor. Modulating the Lewis basicity of the selected pyridine derivatives and carboxylates leads to halogen-bonded complexes where the classical O-I⋅⋅⋅N halogen bond transforms more into a halogen-bonded COO- ⋅⋅⋅I-N+ ion-pair (salt) with an asymmetric O-I-N moiety. X-ray analyses, NMR studies, and calculations reveal the halogen bonding geometries of the carbonyl hypoiodite-based O-I-N complexes, confirming that in the solid-state the iodine atom is much closer to the N-atom of the pyridine derivatives than its original position at the carboxylate O-atom.

Unprecedented [d9]Cu[d10]Au coinage bonding interactions in {Cu(NH3)4[Au(CN)2]}+[Au(CN)2]- salt

The X-ray structure of the {Cu(NH₃)₄[Au(CN)₂]}⁺[Au(CN)₂]^- salt is reported showing an unprecedented [d⁹]Cu[d¹⁰]Au coinage bond. The physical nature of the interaction has been studied using DFT calculations, including the quantum theory of atoms-in-molecules, the noncovalent interaction plot and the natural bond orbital analysis, revealing the nucleophilic role of the [d¹⁰]Au metal and the electrophilic role of [d⁹]Cu metal.

Iodonium complexes of the tertiary amines quinuclidine and 1-ethylpiperidine

Iodonium complexes incorporating tertiary amines have been synthesised to study and explore why such species comprised of alkyl amines are relatively rare. The complexes were characterised in solution (1H and 15N NMR spectroscopy) and the solid state (SCXRD), and analysed computationally.

Utility of Three-Coordinate Silver Complexes Toward the Formation of Iodonium Ions

The work herein describes the synthesis of five three-coordinate silver(I) complexes comprising a bidentate ligand L1, either bpy (2,2'-bipyridyl) or bpyMe₂ (4,4'-dimethyl-2,2'-dipyridyl), and a monodentate ligand L2, either mtz (1-methyl-1H-1,2,3-triazole), 4-Etpy (4-ethylpyridine), or 4-DMAP (N,N-dimethylpyridin-4-amine). Upon reaction of the three-coordinate silver(I) complexes with 0.5 equiv of I₂, the reactions quantitatively produce a 1:1 pair of complexes of a four-coordinate silver(I) complex [Ag(L1)₂]PF₆ and a two-coordinate iodonium complex [I(L2)₂]PF₆. The combination of [Ag(bpyMe₂)₂]PF₆ and [I(4-DMAP)₂]PF₆ gave rise to an I⁺···Ag⁺ interaction where the I⁺ acts as a nucleophile, only the second example of which, that was observed in both the solution (NMR) and solid (X-ray) states.

On the importance of RH3C⋯N tetrel bonding interactions in the solid state of a dinuclear zinc complex with a tetradentate Schiff base ligand

The tetrel bonding and π-stacking interactions in a new dinuclear zinc complex using a tetradentate N₂O₂ donor Schiff base have been analysed energetically using DFT calculations and several computational tools.

Nucleophilic iodonium interactions (NIIs) in 2-coordinate iodine(i) and silver(i) complexes

A range of Nucleophilic Iodonium Interaction (NII) complexes were synthesised by partial cation exchange, leading to the observation of the first NII-complex involving a 2-coordinate silver(i) complex.