As longtime readers know, I have a minor obsession with molecular machines. A recent JACS paper discloses how researchers in Japan have combined molecular machinery with hypervalent activation to develop a “declutchable” molecular gear system. Di-triptycyl-difluorosilane 1 is so sterically hindered that the two triptycyl groups act like intermeshed gears, bumping into each other and pushing each other around like possessed, interlocked merry-go-rounds. In the presence of a fluoride source, however, a trigonal bipyramidal, hypervalent silicate 2 forms in which the two “gear groups” occupy axial positions. The two collinear groups are then free to rotate independently of one another–they are essentially “declutched” through the action of a fluoride source (specifically, KF and 18-crown-6). Interestingly (but not surprisingly, considering sterics), 2 contradicts the usual rule that electronegative atoms occupy the apical positions in hypervalent compounds.
If, like me, you need a reminder of what a clutch is, here you go.