Intramolecular metal ligand aromatic cation-pi interactions in crystal structures of transition metal complexes

Abstract

Cation-pi interactions between ligands coordinated to a metal cation and aromatic groups have been predicted by theoretical calculations, and have been found in crystal structures of metalloproteins from the Protein Data Bank (PDB), We proposed metal ligand aromatic cation-pi (MLAC pi) interaction as intramolecular interactions in transition metal complexes. Searching crystal structures of transition metal complexes from the Cambridge Structural Database (CSD) reveals that there are quite a number of metal complexes, where an aromatic ring interacts with a hydrogen atom from a ligand in the same complex. The hydrogen atom is separated from the metal center by a few bonds and gains positive charge from the metal atom. In the complexes there are various metal atoms and various ligands. In the most of the complexes, the aromatic ring is separated from the metal center by two or four bonds, and interacts with the hydrogen atom of another ligand. In complexes where the aromatic ring is separated from the metal center by four or five bonds, there are examples of aromatic groups interacting with the hydrogen atom in the same ligand. By quantum chemical computations it was evaluated for the model systems of cationic cobalt(III) complexes with a charge of +1, that the energy of this intramolecular MLAC pi interaction is about 4 kcal/mol.