Anagostic Axial Interactions Inhibit Cross-Coupling Catalytic Activity in Square Planar Pyridinophane Nickel Complexes

Abstract

Herein, we report for the first time the use of the nitrogen-based bidentate molecule [2.2]pyridinophane (N2) as a ligand for metal complexes. Additionally, its improved synthesis allows for electronic modification of the pyridine rings to access the new para-dimethylamino-[2.2]pyridinophane ligand (p-NMe2N2). These ligands bind nickel in an analogous fashion to other pyridinophane ligands, completing the series of tetra-, tri-, and bidentate pyridinophane-nickel complexes. The new compounds exhibit geometrically enforced C−H anagostic interactions between the ethylene bridge protons and the nickel center that are not present in other pyridinophane systems. These ethylene bridge groups also act as an unusual form of steric encumbrance, enforcing square planar geometries in ligand fields that would otherwise adopt tetrahedral structures. In addition, these anagostic interactions inhibit the catalytic performance in Csp3–Csp3 Kumada cross coupling reactions relative to other common bidentate N-ligand platforms, possibly by preventing the formation of the 5-coordinate oxidative addition intermediates.