A Dicyanomethyl Radical Conjugated with a Pyridylamino Group: Combining Radical-based Dynamic Covalent Chemistry and Coordination Chemistry.

In this work, we aimed to develop a dicyanomethyl radical that undergoes both reversible C-C bond formation/dissociation and metal-ligand coordination reactions to combine dynamic covalent chemistry (DCC) based on organic radicals with coordination chemistry. We have previously reported a dicyanomethyl radical conjugated with a triphenylamine (1•) that exhibits a monomer/dimer equilibrium between the σ-bonded dimer (12). We designed and synthesized a novel dicyanomethyl radical with a pyridyl group as a coordination point (2•) by replacing the phenyl group of 1• with a 3-pyridyl group. We showed that 2• is also in an equilibrium with the σ-bonded dimer (22) in solution and has suitable thermodynamic parameters for application in DCC. 22 coordinates to PdCl2 in a 2:2 ratio to selectively form a metallamacrocycle (22)2(PdCl2)2, and its structure was clarified by single crystal X-ray analysis. Variable-temperature NMR, ESR, and electronic absorption measurements revealed that (22)2(PdCl2)2 also undergoes the reversible C-C bond formation/dissociation reaction. Ligand-exchange experiment showed that 22 was liberated from (22)2(PdCl2)2 by the addition of another ligand with a higher affinity for Pd(II). This work demonstrated that DCC based on dicyanomethyl radicals works orthogonally to metal-ligand coordination reactions.