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]+).