Abstract
The first 2-pyridylmethyl pendant-armed ethylene cross-bridged cyclam ligand has been synthesized and successfully complexed to Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+) 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 Ni(2+) 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 Mn(2+), Fe(2+), and Cu(2+) complexes identified the Mn(2+) complex as a potential mild oxidation catalyst worthy of continued development.
Highlights
We offer our initial contribution to this field with the synthesis and characterization of a 2-pyridylmethyl N-pendant arm ethylene cross-bridged cyclam (PyMeEBC, Scheme 1) and its late first row transition metal (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) complexes
A new pyridylmethyl N-pendant arm cross-bridged cyclam ligand, PyMeEBC, has been synthesized with a key synthetic step using non-polar chloroform as the solvent to reduce the selfreactivity of picolyl chloride in the presence of the cyclam-glyoxal nucleophile
Mn(PyMeEBC)Cl+ demonstrates powerful, yet selective oxidation reactivity that may lead to improved applications where the Mn(Me2EBC)Cl2 catalyst may be too reactive, such as in laundry bleaching
Summary
Oxidation catalysis by cross-bridged cyclam complexes of manganese and iron has been studied for nearly a decade and a half. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] The manganese complex of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane [15] (Me2EBC, Scheme 1) in particular, has a diverse and rich oxidation chemistry utilizing oxidation mechanisms ranging from hydrogen atom abstraction, electron transfer, concerted oxygen transfer, to the oxygen rebound mechanism. [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] This compound, which we propose to call “the Busch catalyst”, was initially targeted as a potential oxidation catalyst because the rigid cross-bridged ligand could strongly bind the oxygen-reactive manganese ion and prevent it from being deactivated in the form of MnO2. [1] [2] [3] [4] Additional critical ligand properties are thought to be the two available cis labile coordination sites for oxidant and substrate interaction, the methyl groups sterically preventing dimerization which might deactivate the catalyst, and the saturated and all-tertiary nitrogen nature of the ligand, which minimizes the possibility of ligand oxidation and catalyst destruction. [1] [2] [3] [4] Mn(Me2EBC)Cl2 is a patented bleach catalyst heavily invested in by the laundry detergent industry because of its ability to activate O2/H2O2 in water and remove stain molecules from cloth. [16] [17] [18] [19] [20] it has not been fully implemented in consumer products. The reaction was stirred at room temperature for 2 days during which the pink MnCl2 beads dissolved and the white precipitate powder product precipitated This product was obtained by filtration on a glass frit, washed with ether, and allowed to dry open to the atmosphere of the glovebox for four days.
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