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]+).