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.