Cation-pi interactions in Ag(I)-substituted aromatic complexes : an Ab initio molecular orbital study

Abstract

Non-covalent cation-π interaction (a cation binding to the π-surface of aromatic rings) is recently recognized to be important in molecular recognition in biological systems and design of novel functional materials. In the present study, silver(I) cation binding to substituted aromatics with four different classes of aromatic compounds /platforms, i.e., benzene, naphthalene, indole and phenol, were used as model systems to investigate the binding nature, effects of substituents and aromatic platforms on cation-π interaction. Ab initio theoretical calculations at the CCSD(T)/[HW(f), 6-31+G(d)]//MP2/[HW, 3-21G(d)] level of theory was used to determine the Ag+ binding sites and affinities (energies) of the four classes of aromatics with alkyl, alkoxy, halogen, cyano, nitro and amino substituents. Molecular properties of the aromatic ligands, including quadrupole moment, polarizability and dipole moment, were also calculated to elucidate / rationalize the binding geometries and binding strength of different cation-π systems. The theoretical Ag+ affinities of substituted benzenes (136-217 kJmol-1), substituted naphthalenes (175 - 204 kJmol-1), substituted indoles (199 - 214 kJmol-1), and substituted phenols (160 - 185 kJmol-1) were found to be in good agreement (within the experimental uncertainty of +- 10 - 14 kJ mol-1) with the experimental values detennined by the mass spectrometric kinetic method. Two types of stable binding geometries, the cation-π binding and non-π binding to O/N heteroatom of the substituents, are generally located. Natural Population Analysis (NPA) revealed that the binding nature of the Ag+ bound substituted aromatic complexes is mainly electrostatic in nature, but charge-transfer (covalency) is noticeably present. The cation- π binding mode is the most stable binding mode for alkyl-, alkoxy- and halogen-substituted aromatics. Substituted aromatics generally showed enhanced Ag+ affinities which are mainly due to the increase in molecular polarizability and ion-induced dipole interactions. On the other hand, non-π Ag+ binding to O/N heteroatom sites of the substituent are energetically more favorable for CN-, NO2-and NH2- substituted aromatics. The strength of the non-π Ag+ binding modes is derived mainly from the strong Ag+ binding to the dipole moment of CN- and NO2-substituted aromatics, and the charge-transfer interaction with the lone pair electrons of the -NH2 substituent. In addition, the position of para (p-), meta (m-) and ortho (o-) substitution also affects the non-π binding strength by different extent of polarization and electron donating / withdrawing effects. The aromatic platform can have significant effects on the strength of cation-π interaction, which are clearly shown by the decreasing trend of Ag+ affinities of substituted indoles > substituted naphthalenes > substituted phenols - substituted benzenes with the same substituent attached. The significance of charge-transfer interaction is reflected by the stronger Ag+ -cation π binding than tat of Na+ and K+, even though the ionic radius of Ag+(1.26 A) is in-between Na+(0.95 A) and K+(1.33 A). On the other band, the order of binding affinities: Co+ > Fe+ > Cr+ ~ Ag+ is governed by the different extents of charge-transfer interactions for the transition metal ions.