Structure search for B7Mn2 clusters: Inverse sandwich geometry with a high-spin state

Abstract

Herein, we present a density functional theory (DFT) investigation of the B${}_7$Mn${}_2$ cluster, a boron-based system doped with two manganese atoms. The most stable structure adopts an inverse sandwich configuration, in which the B${}_7$ ring is symmetrically coordinated by two Mn atoms and exhibits a spin multiplicity of eight. Higher-energy isomers retain the B${}_7$ wheel-like motif, with Mn atoms positioned either above the ring or at peripheral sites. The Mn${}_2$–B${}_7$ complex exhibits moderate interaction energy, arising from a balance between favorable electrostatic and orbital contributions and significant Pauli repulsion. Strong $\pi$-type interactions between the Mn $d$-orbitals and the delocalized B${}_7$ ring lead to substantial charge transfer ($\sim 1.3e^{-}$), rendering the Mn centers electron-deficient. This behavior is consistent with their Lewis acidic character and a weak Mn–Mn bonding interaction. Nucleus-independent chemical shift (NICS) isosurface analysis reveals a pronounced antiaromatic character, with extended deshielding under a magnetic field applied along the $z$-axis. In contrast, fields oriented along the $x$- and $y$-directions produce more localized effects, highlighting the planar delocalization of the antiaromatic B${}_7$ framework.