Boron Anchors Expandable Ferrocene Dimer

A diborylated ferrocene dimer (shown) has been synthesized and found to undergo reversible conformational changes promoted by redox chemistry or by complexation with nucleophiles. The ability to expand or contract the structural framework of the molecule at will could make it a useful building block in molecular machines, conclude Krishnan Venkatasubbaiah and Frieder J?kle of Rutgers University, Newark, N.J., and their colleagues (Angew. Chem. Int. Ed. 2005, 44, 5428). The molecule has strong boron-iron electronic interactions, and it can undergo reduction at the diboryl ring or oxidation at a ferrocene unit. The diboryl ring sits slightly tilted in the plane between the ferrocene molecules, but when the neutral compound is oxidized or reduced, the ring flattens in the plane, pushing the iron atoms farther apart. A greater expansion is observed when the neutral molecule is complexed with two equivalents of a substituted pyridine. The pyridine nitrogen atoms coordinate to the boron centers, which causes the ring to tilt even more and, coupled with steric effects, to push the iron atoms even farther apart.