Effects of bimetallic doping on small cyclic and tubular boron clusters: B7M2 and B14M2 structures with M = Fe, Co

Abstract

Using density functional theory with the TPSSh functional and the 6-311+G(d) basis set, we extensively searched for the global minima of two metallic atoms doped boron clusters B₆M₂, B₇M₂, B₁₂M₂ and B₁₄M₂ with transition metal element M being Co and Fe. Structural identifications reveal that B₇Co₂, B₇Fe₂ and B₇CoFe clusters have global minima in a B-cyclic motif, in which a perfectly planar B₇ is coordinated with two metallic atoms placed along the C₇ axis. The B₆ cluster is too small to form a cycle with the presence of two metals. Similarly, the B₁₂ cluster is not large enough to stabilize the metallic dimer within a double ring 2 × B₆ tube. The doped B₁₄M₂ clusters including B₁₄Co₂, B₁₄Fe₂ and B₁₄CoFe have a double ring 2 × B₇ tubular shape in which one metal atom is encapsulated by the B₁₄ tube and the other is located at an exposed position. Dissociation energies demonstrate that while bimetallic cyclic cluster B₇M₂ prefers a fragmentation channel that generates the B₇ global minimum plus metallic dimer, the tubular structure B₁₄M₂ tends to dissociate giving a bimetallic cyclic structure B₇M₂ and a B@B₆ cluster. The enhanced stability of the bimetallic doped boron clusters considered can be understood from the stabilizing interactions between the anti-bonding MOs of metal–metal dimers and the levels of a disk aromatic configuration (for bimetallic cyclic structures), or the eigenstates of the B₁₄ tubular form (in case of bimetallic tubular structure).