Calibrating the Oxidative Capacity of Microdroplets.

Recently, redox chemical transformations have been reported to occur spontaneously in microdroplets. The origins of such novel reactivity are still debated, and any systematic correlation of the oxidative/reductive yield with the reactivity of the reactant is yet to be established. Towards this end, we report the simple, outer-sphere, one-electron oxidation of a series of ferrocene derivatives spanning a range of oxidation potentials from -0.1 V to +0.8 V vs. Ag/AgCl in acetonitrile microdroplets generated via nebulization and measured by mass spectrometry of the corresponding ferrocenium ions. The reaction environments and dynamics in the droplets are complex, and it is still unclear whether such reactivity correlates with any bulk thermodynamic values. Our key finding is that the ion yields decrease monotonically with the oxidation potential of the ferrocenes, which is a thermodynamic quantity. The ion yields emphatically do not obey the Nernstian ratio, revealing the redox processes in the droplets do not follow the assumptions of bulk steady-state electrochemistry. Furthermore, oxidative competition in the mixture of several ferrocenes suggest a finite oxidative capacity or oxidant concentration. These results demonstrate that even though ion generation could be an out-of-equilibrium and kinetically limited process, the oxidative yield in microdroplets does correlate with thermodynamics, suggesting a possible free energy relationship between the kinetics and thermodynamics of the process.