Deuterated N2Py2 Ligands: Building More Robust Non-Heme Iron Oxidation Catalysts

Abstract

Fe(N2Py2)/H 2 O 2 /AcOH catalytic systems provide powerful tools for efficient C-H and C=C bond oxidations (N2Py2 = bis-alkylamine-bis-pyridine ligand). Yet, the stability of these catalysts under the oxidizing conditions still remains a problem. The generally accepted catalyst decomposition pathway of Fe(N2Py2) complexes is through oxidative dimerization to form inactive oxo-bridged Fe 2 (μ-O)(N2Py2) 2 dimers. Detailed ESI-MS analysis has now shown a catalyst decomposition pathway of ligand oxidation via C-H oxidation on the 2-pyridinylmethylene sites, followed by dissociation of the oxidized ligand from the iron center. By deuterating the 2-pyridinylmethylene sites of a series of N2Py2 ligands with variations on both alkylamine and pyridine fragments, providing access to the corresponding Fe(N2Py2-D 4 ) complexes, longer catalysts lifetimes are achieved in catalytic oxidation reactions with all complexes. As a consequence, improved substrate conversions and product yields were consistently observed in both aliphatic C-H oxidations and alkene epoxidations. Kinetic and catalytic studies revealed that deuteration does not change the intrinsic reactivity and product selectivity of Fe(N2Py2) complexes. In addition, different Fe(N2Py2-D 4 ) complexes provide different improvements in catalytic performances and lifetimes, responding to the differences in ligand rigidity and robustness of the corresponding nondeuterated N2Py2 ligands. Accordingly, these improvements are more pronounced for ligands with a more flexible bis-alkylamine backbone. These observations provide insights into the development of more robust ligands for homogeneous oxidation catalysis.