Synthesis, structural studies, and oxidation catalysis of the manganese(II), iron(II), and copper(II) complexes of a 2-pyridylmethyl pendant armed side-bridged cyclam

Abstract

The first 2-pyridylmethyl pendant armed structurally reinforced cyclam ligand has been synthesized and successfully complexed to Mn²⁺, Fe²⁺, and Cu²⁺ cations. X-ray crystal structures were obtained for the diprotonated ligand and its Cu²⁺ complex demonstrating pentadentate binding of the ligand with trans-II configuration of the side-bridged cyclam ring, leaving a potential labile binding site cis to the pyridine donor for interaction of the complex with oxidants and/or substrates. The electronic properties of these complexes were determined by means of solid state magnetic moment, with a low value of μ = 3.10 μ_B for the Fe²⁺ complex suggesting it has a trigonal bipyramidal coordination geometry, matching the crystal structure of the Cu²⁺ complex, while the μ = 5.52 μ_B value for the Mn²⁺ complex suggests it is high spin octahedral. Cyclic voltammetry in acetonitrile revealed reversible redox processes in all three complexes, suggesting catalytic reactivity involving electron transfer processes are possible for these complexes. Screening for oxidation catalysis using hydrogen peroxide as the terminal oxidant identified the Fe²⁺ complex as the oxidation catalysts most worthy of continued development.

Keywords: 2-pyridylmethyl; oxidation catalysis; pendant-arm macrocycle; side-bridged cyclam; structurally reinforced cyclam.