Toward Molecular Magnets of Organic Origin via Anion-π Interaction Involving m-Aminyl Diradical: A Theoretical Study.

Abstract

Here we study a set of novel magnetic organic molecular species with different halide ions (fluoride, chloride, bromide) absorbed ∼2 Å above or below the center of an aromatic π-ring in an m-aminyl diradical. Focus is on the nature of anion-π interaction and its impact on magnetic properties, specifically on magnetic anisotropy and on intramolecular magnetic exchange coupling. In the development of single molecule magnets, magnetic anisotropy is considered to be the most influential factor. A new insight regarding the magnetic anisotropy that determines the barrier height for relaxation of magnetization of m-aminyl diradical-derived anionic complexes is obtained from calculations of the axial zero-field-splitting (ZFS) parameter D. The noncovalent anion-π interaction strongly influences magnetic anisotropy in m-aminyl-halide diradical complexes. In particular, the change of D values from positive (for the m-aminyl diradical, m-aminyl diradical/fluoride, and m-aminyl diradical/chloride complexes) to negative D-values in m-aminyl diradical complexes containing bromide signals a change from oblate to prolate type of spin-density distribution. Furthermore, the noncovalent halide-π interactions lead to large values of intramolecular magnetic exchange coupling coefficients J exhibiting a ferromagnetic sign. The magnitude of J steadily increases going from anionic complexes containing fluoride to chloride and then to bromide. Relations are sought between the magnetic exchange coupling coefficients J and aromaticity, namely structural HOMA (harmonic oscillator model of aromaticity) and magnetic NICS (nucleus independent chemical shift) aromaticity indices, in particular, the NICSzz(+1) component. Finally, possible numerical checks on the conditions relating to validity of the well-known Yamaguchi's formula for calculating the exchange coupling coefficient J in diradical systems are discussed.