DFT and QTAIM Study of Intramolecular and Intermolecular Fe–Hδ−···Hδ+–O Dihydrogen Bonds

Abstract

The geometric structures of three recently discovered iron complexes with one intramolecular and two intermolecular Fe–Hδ−···Hδ+–O dihydrogen bonds are optimized using the density functional theory. Then the topology of electron densities and charge distributions in optimized structures were analyzed using Bader’s quantum theory of atoms in molecules method. The Hδ−···Hδ+ distances in three optimized structures are 1.415, 1.549 and 1.608 A, respectively, much shorter than the H···H distances of 1.7–2.2 A in most M–H···H–X dihydrogen bonds discovered so far. The corresponding electron densities at the bond critical points between those Hδ−···Hδ+ pairs are 0.047, 0.031, and 0.029, respectively. Such short Hδ−···Hδ+ distances and high electron density between Hδ− and Hδ+ indicate that the Fe–Hδ−···Hδ+–O bonds have unusually strong interactions between hydridic Feδ+–Hδ− hydrogen and protonic Oδ−–Hδ+ hydrogen. Selected molecular orbitals that show the H···H interactions are also analyzed. Quantum theory of atoms in molecules analysis indicates that the intramolecular Fe–Hδ−···Hδ+–O dihydrogen bonds with a Hδ−···Hδ+ distance of 1.415 A is much stronger than those two intermolecular dihydrogen bonds.