Lewis acidity of benzene in half-sandwich ruthenium arene complex. A computational study

Abstract

Abstract [Ru(benzene)(en)Cl]+ (en = ethylenediamine) is a very active anticancer drug like cisplatin. It has been shown that upon coordination to metal in half sandwich ruthenium complex the benzene ring becomes electron deficient. A library of Lewis bases: Tetrahydrofuran (THF), Acetonitrile (MeCN), Diethyl ether (Et2O), Water (H2O), Trimethylamine (Me3N), Ammonia (NH3), Pyridine (Py), Benzene (Bz), Furan, Adenine, Carbon monoxide (CO), Thiophene and Phosphine (PH3) are considered to study the Lewis acid character of the metal bound benzene. The binding energies of the bases are calculated at M06-2X/LanL2DZ(Ru), 6-31+G∗(C, H, N, O, P, S, Cl) level using Density Functional Theory (DFT) and the values are compared with free benzene. A conformational sampling analysis is carried out to get the most stable supermolecul. It is found that the binding energy is significantly higher in case of coordinated benzene in [Ru(benzene)(en)Cl]+ (e.g., interaction energy of Py is −5.56 Kcal mol−1 more in [Ru(benzene)(en)Cl]+ than that of free benzene). An Energy Decomposition Analysis shows that for the coordinated benzene the stability of the acid-base adduct comes from the combination of electrostatic and dispersion term. From the Quantum Theory of Atoms in Molecules (QTAIM) a cage critical point (CCP) is found in between the donor atom of base and centroid of the benzene ring. The values of electron density of the bond critical point (BCP) in the supermolecule are higher in coordinated benzene than that of the free one. Moreover, a change in the electron density and its Laplacian of the ring critical point (RCP) and bond critical points of the benzene ring in metal bound and free state have been observed. Nucleus Independent Chemical Shift (NICS) calculations also show change in the values of NICS(0) and NICS(1) upon interaction of benzene with Lewis base. An interesting experimental observation of the formation of substituted cyclohexadienyl complex of ruthenium(II) from the reaction of [Ru(benzene)Cl2]2 with nucleophilic reagents hydride, hydroxide and cyanide have been justified from DFT calculations. Thermochemical parameters; enthalpy (ΔH) and free energy (ΔG) of the reactions suggest the adduct formation follows the order: H− > OH− > CN−. This is because of the electron deficient nature of coordinated benzene and hence the nucleophilic reagent undergoes addition reaction. Global and local reactivity parameters such as chemical potential, hardness, electrophilicity index and Fukui function are calculated to understand the electronic properties.