Small Ring Molecules Comprising 3-6 Boron Atoms: An Account on Synthesis, Structure, and Orbital Engineering

ConspectusUnlike carbon, boron does not usually form ring compounds due to its electron-deficiency-driven affinity toward polyhedral geometries. The polyhedral boranes having closo-, nido-, arachno-, or hypho-shapes can be structurally and electronically correlated using various electron counting rules developed by Wade, Mingos, and one of us. However, in the last few decades, boron chemistry progressed significantly toward ring systems. In this regard, three of our research groups have made significant contributions to the development of boron ring molecules through different synthetic approaches. While the Ghosh group generally starts from transition metal (TM) stabilized boron species, the Himmel group typically starts from electron-deficient TM-free boron ring compounds. On the other hand, the Jemmis group studies boron rings and their analogous structures computationally and develops electron counting rules to describe them. Over the past few years, through different synthetic approaches, several boron ring molecules have been prepared by our research groups and others. Recently, the Ghosh group has reported the synthesis of an almost planar B₆-ring that is stabilized by a TM template. Similarly, the B₃-, B₄-, and B₅-rings have also been stabilized in the coordination spheres of early and late TMs. The recent work of Himmel has uncovered some remarkable diversity in the structures and bonding of B₃ and B₄ rings, along with their redox reactions. The well-known hydrocarbon analogues of these borane rings, i.e., two-dimensional aromatic compounds [C₃H₃]⁺, [C₅H₅]^-, [C₆H₆], etc., are governed by Hückel's (4n + 2) π-electron rule. However, planar or nearly planar borane rings are not seriously thought of as achievable targets. One of the reasons for this is the influence of the Rudolph diagram in the thought process of chemists that the nido- and arachno-structures generated from closo-polyhedral boranes must also be three-dimensional (3D) fragments. However, this is not the only possibility. Flat arachno- and nido-boranes reminiscent of their organic counterparts follow from an equivalent of the Rudolph diagram. Therefore, this Account is very much necessary for the boron community, in particular, to design and synthesize 3-6 membered boron rings or beyond. This Account aims to highlight significant ongoing experimental and theoretical results in this area from our groups, in addition to relevant works from other groups wherever appropriate. This will also bring into focus various ways in which the flat B-systems can be stabilized, such as the utilization of TM or main group caps, utilization of various Lewis bases, edge-condensation of small rings, control over the electron count, and orbital engineering.