Boron and carbon: Antagonistic or complementary? Proposal for a simple prototype of a molecular clutch or molecular switch

Abstract

Boron and carbon, either in elemental form or when combined, are structurally very different. They are indeed complementary, and the weaknesses of one can be complemented by the strengths of the other, and vice versa. The structural complementarity can be readily observed in the shape of [XnHn]y– (X = C or B) compounds. One visualization of this complementarity can be found by comparing the most popular carbon and boron organometallic sandwich molecules, [Fe(C5H5)2] and [3,3'-Co(1,2-C2B9H11)2]–. Both obey the 18e– rule, and in both the metal is η5 coordinated by two pentagonal faces. However, for [Fe(C5H5)2], the first ring of atoms outside the pentagonal face is coplanar with the coordinating face, whereas for [3,3'-Co(1,2-C2B9H11)2]– the substituents are out of the coordinating face featuring a canopy shading the metal. Taking advantage of this feature, [3,3'-Co(1,2-C2B9H11)2]– can be a well-performing molecular clutch electrochemically driven. When it is engaged, the beams of the upper [7,8-C2B9H11]2– ligand in [3,3'-Co(1,2-C2B9H11)2]– mesh the beams of the lower [7,8-C2B9H11]2–. This occurs when the molecular friction disk, the Co, is as Co3+. When Co3+ is reduced to Co2+, its radius is elongated, and both sets of beams are unmeshed allowing for a more free rotation, or molecular clutch disengagement.