Stacking Interactions between Indenyl Ligands of Transition Metal Complexes: Crystallographic and Density Functional Study

The analysis of crystal structures deposited in the Cambridge Structural Database showed that indenyl ligands of transition metal complexes prefer to form stacking interactions with one of the three geometries: two of them (types 1 and 2) at small horizontal displacements and one (type 3) at large horizontal displacements. Density functional theory calculations on several model molecules showed that types 1 and 2 are minima at potential energy surfaces, with substantial interaction energies that surpass -8.0 kcal/mol. Type 3 has a small energy contribution (around -2.0 kcal/mol) to the stability of supramolecular structures; however, it is combined with simultaneous stronger stacking or aromatic C-H/pi interactions. Stacking of indenyl ligands is significantly stronger than the stacking of corresponding cyclopentadienyl ligands (-3.0 kcal/mol), due to the larger size of the indenyl ligand. The strength of stacking interactions depends on the electrostatic potential surface of indenyl ligands, depending on the nature and number of the other ligands of the transition metal.