Crystallographic and Quantum Chemical Study of NH/π Interactions of Metal Ammine Complexes with Aromatic Rings in the Second Coordination Sphere

Abstract

Interactions of coordinated ammonia and C6-aromatic rings were studied by analyzing crystal structures in the Cambridge Structural Database (CSD) and by quantum chemical calculations. Interactions between coordinated ammonia and C6-aromatic rings in the crystal structures from the CSD can have one, two, or three ammine ligands interacting with the aromatic ring, while the most frequent are the interactions with one interacting ammine ligand. The distance between coordinated ammonia and the aromatic ring is significantly influenced by charges of the species; the shortest distances were observed for interactions with oppositely charged species. The interaction energies were calculated in model systems between the benzene molecule and coordinated ammonia in metal ammine complexes. In the calculations, we studied the influence of the number of interacting ammine ligands, the complex charge, the metal atom size and the coordination number on the strength of the interactions. The results show that the interaction strength increases with an increasing number of interacting ammine ligands, with an increasing complex charge and with a decreasing metal atom size and the coordination number in the ammine complex. For example, the interaction between the [Co(NH3)6]3+ complex and benzene is the weakest in the case of one interacting ammine ligand (−30.82 kcal/mol), stronger in the case of two interacting ammine ligands (−33.61 kcal/mol), and the strongest in the case of three interacting ligands (−34.16 kcal/mol). For metal complexes with charges +1, +2, and +3, the calculated interaction energies with one interacting ammine ligand can be as strong as −10.51, −15.75, and −30.82 kcal/mol. Complexes [Ni(NH3)6]2+ and [Cd(NH3)6]2+ differ by the metal atom size. The interaction is stronger in the case of the smaller [Ni(NH3)6]2+ complex (−15.75 kcal/mol) compared to the larger [Cd(NH3)6]2+ complex (−12.94 kcal/mol). In addition, complexes with coordination number four form stronger interactions than complexes with coordination number six. In the cases with two interacting ammine ligands, the interacting energy in the [Pt(NH3)4]2+/benzene system is −22.09 kcal/mol, while in the [Cd(NH3)6]2+/benzene system, it is −14.93 kcal/mol.