Template-Free Synthesis of a Macrocyclic Bis(pyridine-dienamine) Proligand and Metal Complexes of Its Bis(pyridine-diimine) and Bis(pyridine-dienamido) Forms

We describe the template-free synthesis of the bis(pyridine-dienamine) proligand [4,5-(m-xylylenediamine)NH–C═(CH)(9-butyl-octahydroacridine)]2 (2′), a variant of Burrows’s macrocyclic bis(pyridine-diimine) (bis-PDI) ligand [2,6-(m-xylylenediamine)N═C(py)]2 (A), using octahydroacridine as the ligand backbone. The octahydroacridine backbone favors macrocyclization by constraining the PDI units in the (s-cis)2 conformation. The template-free synthesis of 2′ enables facile access to a wide array of bis-PDI and bis(pyridine-dienamido) (bis-PDE) metal complexes. Five-coordinate binuclear bis-PDI (2)M2Cl4 complexes {2 = [4,5-(m-xylylenediamine)N═C(9-butyl-octahydroacridine)]2; M = Zn, Co, or Fe} and a four-coordinate bis-PDI [(2)Pd2Br2][B(3,5-(CF3)2-Ph)4]2 complex were synthesized and characterized. (2)Zn2Cl4 undergoes macrocyclic ring inversion on the nuclear magnetic resonance (NMR) time scale with a free energy barrier ΔG⧧ of 15.5(3) kcal/mol at 295 K. In contrast, (2)Fe2Cl4 and (2)Co2Cl4 undergo slow ring i...