Role of the cation formal charge in cation-π interaction. A survey involving the [2.2.2]paracyclophane host from relativistic DFT calculations

The role of the metal formal charge in the cation-π interactions has been evaluated with relativistic DFT methods involving a versatile π-cryptating structure, namely [2.2.2]paracyclophane. Our study focuses on experimentally characterized [([2.2.2]pCp)M]ⁿ⁺ systems with M = Ag⁺ and Sn²⁺ and their Cd²⁺ and In⁺ counterparts, which exhibit 5s⁰5p⁰ and 5s²5p⁰ electron configurations. The acceptor capabilities increase when the metal charges go from 1+ to 2+, resulting in a large stabilization of the interaction. For the studied 5s⁰5p⁰ cations Ag⁺ and Cd²⁺, the most stable conformation namely [(η²:η²:η²-[2.2.2]pCp)M]ⁿ⁺, the electrostatic contribution is more favorable by -9.3 kcal mol^-1, whereas the ΔE^Orb contribution increases by -151.6 kcal mol^-1 towards a more favourable situation in the 2+ counterpart. Similarly in the 5s²5p⁰ cationic group, the isoelectronic Sn²⁺ and In⁺ systems depict variation of the electrostatic and orbital terms, with a considerable decrease of the stabilizing ΔE^Orb contribution, and in a lesser amount the ΔE^Elstat term. Thus, the variation of the interaction energy between the M⁺ and M²⁺ isoelectronic counterparts can be ascribed mainly to the variation of the ΔE^Orb term, leading to a more covalent character of the interaction retaining a similar bonding scheme.