Theoretical view on a linear end-on manganese–dioxygen complex bearing a calix[4]arene ligand

Abstract

By taking a Mn–dioxygen complex supported by a zwitterionic calix[4]arene ligand ([MnL(O₂)(H₂O)](PF₆)₂; H₄L = [5,11,17,23-tetrakis(trimethylammonium)-25,26,27,28-tetrahydroxycalix[4]arene], CSD refcode: CAGVAJ) as a model system, we investigated the geometric and electronic structures of [MnL(O₂)(H₂O)]²⁺ through density functional theory (DFT) calculations without and with the presence of additional waters. It was found that the ground-state structure of [MnL(O₂)(H₂O)]²⁺ possesses a typical bent end-on binding mode (Mn–O–O angle at around 130° and dioxygen bond length at ∼1.32 Å) within a sextet state (S = 5/2) irrespective of calculated methodologies. The experimently proposed linear end-on structure (Mn–O–O angle at ∼180°) and the side-on structure (Mn–O–O angle ∼75°) are shown to be transition-state structures in the process of dioxygen flipping from one bent-end-on structure to another. By the inclusion of water solvation effect via both the explicit water-cluster model ([MnL(O₂)(H₂O)]²⁺–nH₂O, n = 1, 4, 6, 8) and implicit CPCM solvation model, a good correlation was concluded: the more additional waters and the more contracted O_e square benefit the larger Mn–O–O angle, indicating a water solvation effect and a ligand size effect. Inspired by the relationship of linear end-on mode with bent end-on mode and the consistence of the projected dioxygen bond length of [MnL(O₂)(H₂O)]²⁺–8H₂O in the CPCM model along the Mn–O bond direction (d_O1–O2(p) = 1.240 Å) with the experimental dioxygen bond length (1.249 Å), we concluded that the linear Mn–O–O arrangement fitted by experimentalists should be a result of the flipping motion of dioxygen rotating about the Mn–O bond in solid-state structures of the [MnL(O₂)(H₂O)](PF₆)₂ complex based on analyses of the crystallographic data. Natural bond orbital (NBO) analyses show the sextet [MnL(O₂)(H₂O)]²⁺–nH₂O (n = 0, 1, 4, 6, 8) have a Mn(III)-superoxo nature, consistent with experimental observations. The differences in the bonding structures of the three binding modes were discussed by the second-order perturbation energies ΔE_i→j from the superoxo lone pair (LP) orbitals to Mn(III) 3d orbitals in β electron space. It can also give clues about the instability of the linear end-on binding mode and the increased Mn–O–O angle in the presence of water solvents.