Synthesis and Characterization of the Dimethyl-Substituted Bisbenzimidazole Ligand and Its Manganese Complex

Macrocycles with unique properties provide new avenues for the design of novel catalysts and materials. Here,
we report, for the first time, the synthesis and characterization of the dimethyl-substituted bisbenzimidazole ligand
(Me2BBZ) and its manganese complex (Mn−Me2BBZ). The Me2BBZ ligand is similar to porphyrin and
phthalocyanine macrocycles in terms of its cavity size and metal-binding mode, but owing to electronic and
charge differences, it exhibits properties that make it distinct from its structural counterparts. For instance, the
optical spectra of bisbenzimidazoles lack transitions in the 500−900 nm region. Perhaps the most significant
feature of the Me2BBZ ligand, however, is its inherent nonplanarity. Geometric restraints within this nonplanar
ligand give rise to two atropisomers, which, when separated, could have potential in chiral catalysis and recognition.
In addition, here we show that this nonplanarity can help to promote unusual crystal-packing interactions. Within
the structure of the Mn−Me2BBZ complex, intermolecular π-stacking interactions of the phenyl and benzimidazole
groups lead to the formation of a distinct two-dimensional “staircase” lattice comprised of alternating Mn−Me2BBZ
atropisomers. The potential significance of this structural arrangement is revealed by temperature-dependent
magnetic studies that indicate weak antiferromagnetic coupling between the metal ions in the crystal. Fine-tuning
of these long-range electronic and magnetic interactions could be useful for the design of novel molecular materials.