Structures and chemical bonding of boron-based B12O and B11Au clusters. A counterexample in boronyl chemistry

Boron oxide clusters have structural diversity and unique chemical bonding, and recent literature has shown that boronyl complexes dominate boron-rich oxide clusters. A counterexample in boronyl chemistry is presented in this work. Using global structural searches, electronic structure calculations, and chemical bonding analyses, we shall report on the computational design of two boron-based quasi-planar or planar clusters: B12O and B11Au. Contrary to expectation, the B12O cluster has a circular quasi-planar shape with a peripheral B-O-B bridge, which resembles bare B-12 cluster. It does not contain a boronyl ligand. The isomeric boronyl complex turns out to be 10.32 kcal mol(-1) higher in energy at the single-point CCSD(T) level. In contrast, B11Au cluster behaves normally with an elongated B-11 moiety and a terminal Au ligand. Chemical bonding analyses reveal three-fold pi/sigma aromaticity in circular B12O cluster, including global 6 pi aromaticity, as well as spatially isolated inner 2 sigma aromaticity and outer 10 sigma aromaticity. The three-fold 6 pi/2 sigma/10 sigma aromaticity underlies the stability of B12O cluster. This bonding picture is unknown for bare B-12 cluster and its derivatives. The elongated B11Au cluster has conflicting pi/sigma aromaticity (with 6 pi versus 8 sigma electron-counting). The B12O cluster is actually isoelectronic with bare B-12 cluster in terms of delocalized pi/sigma bonding, which inherits the structural and electronic robustness of the latter.

Automated summary

This study computationally designed two boron-based quasi-planar or planar clusters, and found that contrary to expectations, the B12O cluster does not contain a boronyl ligand. Chemical bonding analysis reveals that the B12O cluster has three-fold pi/sigma aromaticity, which underlies its stability.