A plier-shaped binary molecular wheel B7Mg4+ cluster: hybrid in-plane heptacoordination, double π/σ aromaticity, and electronic transmutation

Abstract

Quantum chemical calculations in combination with unbiased global searches reveal a unique binary B₇Mg₄⁺ cluster, in which a vertically oriented Mg₂ dimer is incorporated into a ringent (or Pac-Man shaped) B₇ disk, forming a hybrid Mg₂B₇ molecular wheel with pseudo-seven-fold in-plane coordination. The hybrid wheel assumes a plier shape. The remaining Mg₂ dimer floats on the hybrid wheel via electrostatic interaction, with relatively minor covalent Mg–B bonding. The B₇Mg₄⁺ cluster can be described as a charge transfer [Mg₂]²⁺[Mg₂B₇]⁻ complex. Chemical bonding analysis indicates double 6π/6σ aromaticity in B₇Mg₄⁺ cluster. The unique plier shape of the cluster is elucidated using the concept of electronic transmutation, in which the vertical Mg₂ dimer closely mimics the chemistry of an atomic B⁻ or C site.