Synthesis, Structural Studies, and Oxidation Catalysis of the Late-First-Row-Transition-Metal Complexes of a 2‑Pyridylmethyl Pendant-Armed Ethylene Cross-Bridged Cyclam

The first 2-pyridylmethyl pendant-armed
ethylene cross-bridged cyclam ligand has been synthesized and successfully
complexed to Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ cations. X-ray crystal
structures were obtained for all six complexes and demonstrate pentadentate
binding of the ligand with the requisite cis-V configuration of the
cross-bridged cyclam ring in all cases, 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 the
complexes were evaluated using solid-state magnetic moment determination
and acetonitrile solution electronic spectroscopy, which both agree
with the crystal structure determination of high-spin divalent metal
complexes in all cases. Cyclic voltammetry in acetonitrile revealed
reversible redox processes in all but the Ni2+ complex,
suggesting that catalytic reactivity involving electron-transfer processes
is possible for complexes of this ligand. Kinetic studies of the dissociation
of the ligand from the copper­(II) complex under strongly acidic conditions
and elevated temperatures revealed that the pyridine pendant arm actually
destabilizes the complex compared to the parent cross-bridged cyclam
complex. Screening for oxidation catalysis using hydrogen peroxide
as the terminal oxidant for the most biologically relevant Mn2+, Fe2+, and Cu2+ complexes identified
the Mn2+ complex as a potential mild oxidation catalyst
worthy of continued development.