Abstract
It is widely known that the skeletal structure of clusters reflects the number of skeletal bonding electron pairs involved, which is called the polyhedral skeletal electron pair theory (PSEPT) or Wade and Mingos rules. While recent computational studies propose that the increase of skeletal electrons of polyhedral clusters leads to the flat structure beyond the PSEPT, little experimental evidence has been demonstrated. Herein, we report the synthesis of a C2B4R4 carborane 2 featuring a flat ribbon-like structure. The C2B4 core of 2 bearing 16 skeletal electrons in the singlet-ground state defies both the [4n + 2] Hückel’s rule and Baird’s rule. Nevertheless, the delocalization of those electrons simultaneously induces two independent π- and two independent σ-aromatic ring currents, rendering quadruple aromaticity.
Highlights
It is widely known that the skeletal structure of clusters reflects the number of skeletal bonding electron pairs involved, which is called the polyhedral skeletal electron pair theory (PSEPT) or Wade and Mingos rules
The cage framework of carborane clusters can be rationalized by the polyhedral skeletal electron pair theory (PSEPT), known as Wade–Mingos’ rules, which provides the relationship with the skeletal bonding electron pairs (SEPs)[16,17,18]
The B1–B1’ distance (1.601(4) Å), which falls into the range of typical B–B double bond distances observed in base-stabilized diborenes[27], is slightly shorter than those in the aromatic carboranes (1.625–1.636 Å)[26,28], The B1–C1 (B1’–C1’) (1.421(3) Å) and B2–C1 (B2’–C1’) (1.501(3) Å) distances are significantly longer than the reported B–C double bond (1.351 Å), but comparable to the typical B–C unsaturated bonds seen in the aromatic carboranes (1.486–1.502 Å)26,29. 2 represents a rare example of a molecule involving planar tetracoordinate boron centers[30,31,32]
Summary
It is widely known that the skeletal structure of clusters reflects the number of skeletal bonding electron pairs involved, which is called the polyhedral skeletal electron pair theory (PSEPT) or Wade and Mingos rules. The group of Wang and Boldyrev reported that despite the singlet ground state, the B19− cluster with 12 π electrons exhibits aromaticity[9] This can be rationalized by the fact that the B19− cluster possesses two concentric π-aromatic networks, involving a 10 π-electron system and a 2 π-electron system, both of which satisfy the [4n + 2] Hückel’s rule, independently. The discovery of molecules of type C2B4H6RR’ (R, R’ = H, Me, C3H7) by Onak, Williams and Weiss in 1962, is considered to be the beginning of the chemistry of carboranes, a class of boron clusters with the general formula of [(CH)a(BH)mHb]c, mostly having polyhedral structures[10]. The n-vertex carboranes with n, (n + 1), (n + 2) and (n + 3) SEPs prefer to adopt hypercloso-, closo-, nido- and arachno-structures, respectively
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