Stacking interactions of aromatic ligands in transition metal complexes

Abstract

In this review article we present all the recent research on stacking interactions of aromatic ligands that coordinate to transition metals through their π-electrons (η-coordination). These studies were mostly based on searching the crystal structures from the Cambridge Structural Database (CSD) and on quantum chemical calculations. Stacking interactions between coordinated and uncoordinated benzene reach the energy of −4.40 kcal/mol, while the strongest calculated staking between two coordinated benzenes has the energy of −4.01 kcal/mol; this is significantly stronger than stacking between two uncoordinated benzenes (−2.73 kcal/mol). It was determined that in crystal structures both coordinated benzene and coordinated cyclopentadienyl anion form stacking interactions that dominantly have large horizontal displacements (more than 4.5 Å). This dominance is caused by the relatively strong stacking interactions at large displacements between benzene or Cp ligands in sandwich compounds, while for half-sandwich compounds they are supported by additional interactions of the other (usually branched) ligands. Larger aromatic ligands, tropylium and cyclooctatetraenide, almost exclusively form stacking interactions with large horizontal displacements. Methyl substituted benzene and Cp ligands dominantly form stacking interactions in combination with C–H/π interactions. Moreover, there is an interplay of stacking and aromatic C–H/π interactions in the CSD crystal structures, both interactions being important energy contributors to the stability of supramolecular systems. Stacking interactions of η-coordinated aromatic ligands are important in materials science, crystal engineering and medicinal chemistry, primarily in the application of ruthenium-arene complexes, where they determine the strength of bonding of these complexes to the DNA.