Exchange in Transient Covalent Assembly

Data archive for "Exchange in Transient Covalent Assembly". The hydration of carbodiimides is commonly used to drive nonequilibrium chemical systems, often by converting carboxylic acids into transient aqueous anhydrides. This fuel reaction has found broad applicability in functional systems, from single-molecule motors to responsive bulk materials. The reaction cycle is usually presented in terms of activation (anhydride formation) and deactivation (hydrolysis) steps. Here, we investigate the additional role of exchange within the activated state. Slowly hydrolyzing anhydride dimers and polymers, representative of transient covalent assemblies, were generated using carbodiimide and then treated with distinct carboxylic acids. The formation of new anhydrides was monitored using 13C NMR spectroscopy, allowing rate constants to be determined. The anhydrides exchange at an intermediate rate, with on the order of 10 exchange events per hydrolysis reaction under typical conditions. This process yields unique behavior. At long reaction times, the ratios of different anhydride assemblies do not trend toward the equilibrium distribution (fast exchange limit), nor are they determined solely by activation/deactivation rates (slow exchange limit). Instead, it is possible to favor distinct species because of the balance between exchange and deactivation. The implications of this result for the assembly of nonequilibrium species with covalent bonds is discussed.