Theoretical Insight into the Structural Nonplanarity in Aromatic Fused-Ring Metallabenzenes.

Metalla-aromatics have attracted considerable attention due to their fascinating structural and reactive properties as well as their potential as prospective functional materials. Metallabenzenes and their fused-ring counterparts are significant members of metalla-aromatics, while their crystal structures often display seemly counterintuitive nonplanar geometry. The geometric bending of metallabenzenes has been attributed to the unfavorable antibonding interactions in the σ-space orbitals rather than the general opinion regarding the π-space orbitals of an aromatic compound. However, the origin of the geometric bending in fused-ring metallabenzenes remains elusive. In this work, we elucidated that such a "σ-control mechanism" still holds for fused-ring metallabenzenes by performing systematic calculations for a plethora of metallabenzenes and fused-ring metallabenzenes. Furthermore, we found that a more bent geometry can be achieved for fused-ring metallabenzenes than their corresponding metallabenzenes by fusing the aromatic rings at the ortho-position of a metal center to induce extra repulsion. The more significant bending in fused-ring metallabenzenes also favors the aromaticity enhancement. These findings not only provide mechanistic insight into the unexpected geometric distortion in both metallabenzenes and fused-ring metallabenzenes but also pave the way to design and develop bent metalla-aromatics with enhanced metalla-aromaticity, which hold great potential as aromatic functional materials.