Macrocyclic meta-carborane hexamer. Evaluation of aromatic characteristics as a cluster-based analog to phenyl-bridged macrocyclic structures

Abstract

Envisaging cluster-based aggregates of larger phenyl-bridged macrocyclic structures enables evaluation of resemblance and differences between planar- and spherical building blocks towards novel synthetic target architectures for molecular materials. Here, we explore the aromatic characteristics of the Hawthorne proposal for a rigid cyclic hexamer involving meta-carborane ((C₂B₁₀H₁₀)₆, 1). Such structure appears as a cluster-based analog to [6]cycloparaphenylene ([6]CPP), highlighting the resemblance between planar and spherical aromatic building blocks. Our results denote 1 six local spherical aromatic units, or spherical aromatic states, retain the behavior of the parent meta-[C₂B₁₀H₁₂] cluster, in line with the six local aromatic rings in [6]CPP featuring a similar diameter. The six adjacent spherical aromatic units overlap both shielding and deshielding regions, leading to an extension of such characteristics as a result of a cyclic array made from clusters. The magnetic anisotropy of 1 is ascribed mainly to bridged C–C bonds, with carborane cages as mostly non-anisotropic units owing to their spherical aromatic features, contrasting to [6]CPP given its planar aromatic characteristics. The [6]CPP²⁻ dianion exhibits a global aromatic character, which is also featured in 1²⁻, in addition to the retained spherical aromatic behavior of carboranes units. Thus, the resemblance between phenyl- and cluster-based macrocycles encourages further evaluation and explorative synthesis toward conceiving molecular materials involving cluster aggregates, retaining the characteristics of their isolated parents.