Synthesis and Characterization of Cobalt-Cage Complexes with Pendant Phenol Groups

The synthesis and characterization of four compounds formed from the reductive amination of m- and o-hydroxybenzaldehyde to (1,8-diamino-3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane)cobalt(III) are presented. These compounds are the mono- and dialkylated derivatives of the starting cobalt complex. The X-ray crystal structures of each compound, as its fully protonated chloride salt, have been obtained and are reported. The pK(a)'s of the compounds were determined by nonlinear least-squares analysis of data from potentiometric titrations and UV/vis spectrophotometry. The K(a)'s of the phenol groups of the compounds are reduced by roughly 1 order of magnitude compared to their analogous organic phenols. In the solid state, the compounds with one pendant phenol group (2, 4) are yellow and adopt the ob(3) conformation of their ethylene diamine rings in their crystal structures, while the compounds with two pendant phenol groups (3, 5) are orange and have the lel(3) conformation in their crystal structures. Solid-state reflectance UV/vis spectroscopy confirms these structural differences. In water, all four compounds form orange solutions and adopt the lel(3) conformation, as indicated by comparison of UV/vis spectroscopy and cyclic voltammetry properties with the literature.