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

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.

N,N′-diethyl and N-ethyl,N′-methyl glyoxal-bridged cyclams: synthesis, characterization, and bleaching activities of the corresponding Mn(II) complexes

“…Glyoxal-bridged cyclam-based ligands (L-R,R 0 ) have attracted much interest [1][2][3][4][5][6][7][8][9] in the last decades, due to their ability to coordinate mid-late transition metals (Scheme 1).…”

“…These macrocycles can be easily N-functionalized with various groups, allowing the preparation of a wide range of ligands suitable for important applications such as for catalysis [4,8], and the development of metal-based imaging and therapeutic agents in medicine [10].…”

Synthesis and structural studies of two pyridine-armed reinforced cyclen chelators and their transition metal complexes

“…[10] [12] Analogous conditions were used for the formation of 1 . Using dichloromethane gave the desired product in relatively high yields (~80%), see Scheme 1.…”

“…[8] [9] Recently we have been interested in the use of this complex type as oxidation catalysts. [10] [11] [12] A recent papers by Que, et al, has shown that addition of a pyridyl pendant arm to a tetraazamacrocycle can result in new and improved oxidation catalysis. [13] The literature contains numerous other pyridyl pendant armed tetraazamacrocycles, indicating a general interest in the community and known synthetic techniques for adding pendant pyridyl donors.…”

“…A cross-bridged cyclen ligand with two pyridyl pendant arms (CB-Py 2 Cyclen, Figure 1) has been reported [30], but its hexadentate nature coordinatively saturates its complexed metal ions, while we wished to leave at least one coordination site available for interaction with oxidant and/or substrate. Our first communications regarding complexes incorporating a single pyridyl labile coordinating group have described reinforced cyclam derivatives with modified steric and electronic properties to cause improved properties as oxidation catalysts, [10] [12] see Figure 1. Herein, we report the synthesis and characterization of analogous smaller cyclen derivatives as potential future oxidation catalysts.…”