Halogen-Bonded [N–I–N]⁻ Complexes with Symmetric or Asymmetric Three-Center–Four-Electron Bonds

Abstract

A series of LH[Z–I–Z] halogen(I) complexes, where Z = saccharinato or phthalimido anions and LH = pyridinium, pyrazinium, tetrabutyl (TBA)- or tetramethylammonium (TMA) cations, were prepared, structurally characterized, and discussed as complexes consisting of a [N–I–N]− anion with a three-center–four-electron (3c-4e) halogen bond, and a hydrogen-bonding (pyridinium or pyrazinium) or inert (TBA or TMA) cation. The symmetric [N–I–N]− anion, reminiscent of the triiodide [I–I–I]− anion, is found to be structurally equivalent to its cationic analogue [N–I–N]+ with N–I bond lengths of 2.26 Å. In contrast to the homoleptic [N–I–N]+ complexes, asymmetry of the N–I bond lengths (2.21 and 2.28 Å) was observed for those [N–I–N]− complexes which manifested a hydrogen bond to only one saccharinato moiety, thus being structurally analogous to the asymmetric heteroleptic [N–I–N]+ complexes. The results show that the 3c-4e [N–I–N] halogen bond, being either positively or negatively charged, can be asymmetrized by an external hydrogen bond ([N–I–N]−) or by using two different ligands (heteroleptic [N–I–N]+).

Solid‐state NMR Spectroscopy of Iodine(I) Complexes

“…À 320.6 (NMe 2 ), À 109.0 (py) À 328.7 (NMe 2 ), À 108.9 (py) [38] [Ag(DMAP) 2 ]PF 6 ( À 72.5 (À 70.1 # ) [45] [Ag(isoq) 2 ]PF 6 (4 a) 245.0 À 136.7 À 132.0 (À 106.1 # ) [45] [I(isoq) 2 ]PF 6 (4 b) 201.6 À 180.1 À 181.6 (À 181.9 # ) [45] distances and the NÀ IÀ N angles (Figure 4) are remarkably constant, fitting very well with the average values of a large set of similar iodine(I) complexes. [55] The very small NÀ IÀ N asymmetry observed in the X-ray structure the I-clamp (Figure 4) is not observed in the solution 15 N NMR spectra. [54] The three-center four-electron (3c-4e) [NÀ IÀ N] + halogen bond in these systems is very robust and retains its geometry regardless of the ligands (bases) used.…”

“…The AgÀ N and IÀ N bond distances and the NÀ IÀ N angles (Figure 4) are remarkably constant, fitting very well with the average values of a large set of similar iodine(I) complexes. [55] The very small NÀ IÀ N asymmetry observed in the X-ray structure the I-clamp (Figure 4) is not À 66.3 [38] [Ag(py) 2 ]PF 6 (1 a) 252.2 À 129.5 À 121.6 [38] [I(py) 2 ]PF 6 (1 b) 207.5 À 174.2 À 174.8 [38] 4-ethylpyridine (2) Liquid (m.p. À 91 °C) Liquid (m.p.…”

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

“…As such, these HaB complexes represent asymmetric 3c–4e systems with the central atom shifted one way or another (it should be noted in this respect that the symmetry of the N–I–N bonding in the halonium cation is also frequently distorted by crystal forces, noncovalent bonding to one side, etc. , ,, ).…”

“…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.…”

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

“…The epitome of halogen bonding is the linear halogen­(I) (also termed halonium) complexes, , which comprise a halenium ion (X + ; X = Cl, Br, and I) and a pair of stabilizing Lewis bases (L; commonly nitrogen-based aromatic ligands such as pyridine), [L–X–L] + . The stability of halogen­(I) complexes follows the trend: I > Br ≫ Cl, , which is reflected in the number of solid-state examples reported for each type, with Barluenga’s reagent, bis (pyridine)­iodine­(I) tetrafluoroborate, being the paradigm of iodine­(I) complexes due to its widespread use in a multitude of organic transformations as a mild iodinating and oxidizing reagent. − Halogen­(I) complexes, [L–X–L] + , feature a 3-center 4-electron ( 3c–4e ) bond, the symmetric nature of which has been confirmed computationally and in solution. , The negatively charged [O–I–O] − complexes are also known to have applications as organic reagents, and recently the ability to instigate asymmetry in the halogen bonding, via hydrogen bonding with one of the two saccharinato ligands, in the analogous [N–I–N] − complexes has been demonstrated . Interest in halogen­(I) chemistry has been steadily increasing in recent years, with a whole slew of recent advances being reported, including the first examples of unrestrained heteroleptic, − hierarchical, and nucleophilic interactions of iodine­(I) complexes, − as well as the resurgence of (isolable) non-chiral ,− and chiral carbonyl hypoiodites in the context of being halogen-bonded iodine­(I) complexes.…”