Ethylene cross-bridged tetraazamacrocycles have found particular success as ligands in supporting Mn and Fe complexes used in catalytic oxidation of organic substrates. Several ligand derivatives have been made where the two unbridged nitrogen atoms are alkylated with different substituents, including methyl, benzyl, and ethyl groups. However, more extremely sterically bulky groups have not yet been utilized. The purposes of the proposed sterically bulky substituents are three-fold: (1) To prevent dimerization, allowing the study of monomeric complexes. Previous Mn and Fe work indicates that lack of steric bulk on the non-bridged nitrogens may allow dimers to form, which will alter the chemistry. Both dimers and monomers should be studied, thus the need for steric bulk. (2) In similar systems, bulky tBu groups lengthen and weaken M-N bonds and cause macrocycle twisting to keep the tBu groups far apart. Modification of the electronic properties of the complexes caused by these sterically induced complex deformations may help realize the specific properties needed for catalysis. (3) To encourage dissociation of one or more macrocyclic nitrogen due to steric bulk. Two and three-coordinate configurations have been critical to some catalysts' reactivity, so using steric bulk to encourage them to form (due to bond elongation weakening) is a logical structural modification to test. These structural changes may lead to electronic and reactivity changes which should be explored. In this project, we have successfully synthesized an isopropyl substituted ethylene cross-bridged cyclen and its transition metal complexes. Synthetic details of the ligand synthesis and selected properties of the resulting metal complexes will be presented.

Sterically hindered cross-bridged tetraazamacrocyles

2019 Spring National Meeting

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