Synthesis and characterization of the dimethyl-substituted bisbenzimidazole ligand and its manganese complex

Macrocycles with unique properties provide new avenues for the design of no vel catalysts and materials. Here, we report, for the first time, the synth esis and characterization of the dimethyl-substituted bisbenzimidazole liga nd (Me(2)BBZ) and its manganese complex (Mn-Me(2)BBZ). The Me(2)BBZ ligand is similar to porphyrin and phthalocyanine macrocycles in terms of its cavi ty size and metal-binding mode, but owing to electronic and charge differen ces, it exhibits properties that make it distinct from its structural count erparts. For instance, the optical spectra of bisbenzimidazoles lack transi tions in the 500-900 nm region. Perhaps the most significant feature of the Me(2)BBZ ligand, however, is its inherent nonplanarity, Geometric restrain ts within this nonplanar ligand give rise to two atropisomers, which, when separated, could have potential in chiral catalysis and recognition In addi tion, here we show that this nonplanarity can help to promote unusual cryst al-packing interactions. Within the structure of the Mn-Me(2)BBZ complex, i ntermolecular pi-stacking interactions of the phenyl and benzimidazole grou ps lead to the formation of a distinct two-dimensional "staircase" lattice comprised of alternating Mn-Me(2)BBZ atropisomers. The potential significan ce of this structural arrangement is revealed by temperature-dependent magn etic studies that indicate weak antiferromagnetic coupling between the meta l ions in the crystal. Fine-tuning of these long-range electronic and magne tic interactions could be useful for the design of novel molecular material s.