Zinc(II)-regulation of hydrazone switch isomerization kinetics.

The extra H-bond in a bipyridyl-functionalized hydrazone rotary switch slows down its Z→E isomerization rate by 2 orders of magnitude (k = (3.5 ± 0.2) × 10(-6) s(-1)). The coordination of Zn(2+) with the bipyridyl subgroup simultaneously 'unlocks' this H-bond and accelerates the isomerization rate by at least 6 orders of magnitude (k > 6.9 s(-1)). This coordination-regulated kinetic control could open the way to molecular timers that can be used in guiding temporal events.