Stacking interactions between ruthenium p-cymene complexes. Combined crystallographic and density functional study

Abstract

The Cambridge Structural Database search for stacking interactions between p-cymene (1-methyl-4-isopropylbenzene) ligands of transition metal (mostly ruthenium) complexes revealed three preferred interaction geometries, all with antiparallel orientation. The most frequent one involves both stacking of aromatic rings and C-H/π interactions of methyl substituents with aromatic rings, while the second most frequent has stacking of aromatic rings and C-H/π interactions of methyl groups of isopropyl substituents with aromatic rings. The results of CSD search are in agreement with DFT calculations of interaction energies, since all the preferred CSD geometries correspond to minima on potential energy curves. The strongest calculated interaction between p-cymene ligands of model complexes [Ru(p cym)Cl¬¬2(NH3)] corresponds to the most frequent geometry found in crystal structures, and it has the B97-D2/def2-TZVP interaction energy of -7.56 kcal/mol. This is significantly stronger than interaction between benzene ligands of [Ru(benzene)Cl¬¬2(NH3)] complexes (-3.93 kcal/mol), revealing that substituents increase interaction strength substantially. All interaction geometries and their relative strengths are in agreement with electrostatic potentials of the monomer complex.