Ag(I) cationization mass spectrometry : substituent effects in cation-pi interactions

Abstract

Cation-π interaction has been recently recognized as a new type of non-covalent interaction important in biological recognition, and the design of functional materials in the nanoscale or sub-nanoscale. The binding Ag+ ion to alkylbenzenes (RBz), substituted naphthalenes (X-Nap, where X = H, Me, Et, i-Pr, OH, OMe, OEt, CN, NH2), substituted indoles (X-Indole, where X = H, Me, OH, OMe, NO2, CN) and substituted phenols (X-PhOH, where X = H, Me, Et, i-Pr, t-Bu, OMe, OEt, NO2) are chosen as model cation aromatic-π systems to study substituent effects on cation-π binding, as well as the factors governing the strength of cation-π interactions. The Ag+ binding affinities of these aromatic model ligands (L) at 0K were measured by vigorous application of the mass spectrometric kinetic method, which is based on the competitive dissociation of the Ag+ bound heterodimers complexes, [ L1 + Ag +L2]+, to their respective monomer complexes. The Ag+ bound heterodimers were generated by electrospray ionization (ESI), and their dissociations were conducted under low-energy (Ar as collision gas) and / or high-energy (He as collision gas) collision-induced dissociation (CID) conditions. The Ag+ binding affinities of alkylbenzenes (157 - 216 kJmol-1), substituted naphthalene (177 - 211 kJmol-1), substituted indoles (201 - 225 kJmol-1), and substituted phenols (158 - 185 kJmol-1) were found to be in good agreement (experimental uncertainty of +- 10 - 14 kJ mol-1) with theoretical ab initio affinities estimated at the CCSD(T)/[HW(f), 6-31G+(d)] level. Our experimental and theoretical studies show that Ag+ cation-π binding is the most stable and preferred binding mode for alkyl, hydroxyl and alkoxy substituted benzenes, naphthalenes, indoles and phenols, indicating that non-covalent cation-π interaction is indeed strong enough to compete against Ag+ binding to oxygen-donor binding sites. On the other hand, Ag+ cation-π binding is not as strong as Ag+ binding to heteroatom nitrogen I oxygen binding sites in -CN, -NH2 and -NO2 substituted benzenes, naphthalenes, indoles and phenols. Our results indicate that Ag+-π interaction is mainly electrostatic in nature, though charge-transfer covalent interaction is noticeably present. The relative stability of the cation-π and non-π binding to O/N heteroatom sites is mainly determined by the interplay of electrostatic ion-quadrupole, ion-dipole, ion-induced dipole interactions, as well as the extent of charge-transfer (covalent) interaction present in the Ag+-1igand complexes. Ag+ binding to substituted naphthalenes and substituted indoles are measured to be -9 - 12 % and -11 - 33 % stronger than of substituted benzenes, respectively, indicating tat both the quadrupole moment and polarizability of fused aromatic ring systems could greatly enhance cation-π binding energy. Substituted phenols show comparable Ag+ affinities as substituted benzenes, i.e., the phenolic-OH group does not have any significant effect on Ag+ cation-π binding affinities. The increasing trend of Ag+ affinities within a series of substituted benzenes, naphthalenes, indoles and phenols is shown to be mainly due to the increasing molecular polarizability of the substituted aromatic systems, which could enhance the Ag+ affinities via both ion-induced dipole and charge-transfer interactions. Ag+ cation-π binding affinities are comparable to tat of Li+, but greater than Na+ affinities, even though the ionic radius of Ag+ is larger than that of Nat. The greater Ag+ affinity is attributed to the noticeable presence of charge-transfer (covalent) interactions, i.e., π-->Ag+ (σ-donor interaction) and Ag+-->π* (4d10 back donation), present in Ag+-ligand interactions. In fact, the order of transitional metal cation-π affinities: Co+>Fe+> Cr+ ~ Ag+ is rationalized in terms of the variation in charge-transfer interaction for different transition metal cations.