pH-dependent concatenation of an inorganic complex results in solid state helix formation.

Growth of the library of tetraaza macrocyclic pyridinophane ligands is a result of the potential to treat neurodegenerative diseases by binding unregulated redox active metal-ions, scavenging radicals, and reducing oxidative stress. As part of this work, the copper complex of OH PyN 3 Cu (3,6,9,15-tetraazabicyclo[9.3.1]penta-deca-1(15),11,13-trien-13-ol) was previously identified as a discrete molecule in the solid state when isolated at lower pH values. However, here we report that OH PyN 3 Cu forms a helical structure upon crystallization around pH 6.5. Several properties of the ligand and complex were evaluated to understand the driving forces that led to the concatenation and formation of this solid-state helix. DFT studies along with a comparison of keto/enol tautomerization stability and bond lengths were used to determine the keto-character of the C=O within each subunit. This pH dependent keto-enol tautomerization is responsible for the solid state intermolecular C=O···Cu bonds observed in this metallohelix (Cu1 H ) when produced around pH 6.5. Perchlorate templating that occurs through hydrogen bonding between perchlorate counter ions and each Cu1 H unit is the primary driving factor for the twist that leads to the helix structure. Cu1 H does not exhibit the typical factors that stabilize the formation of helices, such as intra-strand hydrogen bonding or π-stacking. The helix structure further highlights the diversity of inorganic metallohelices and demonstrates the importance of tautomerization and pH that occurs with the pyridinophane ligand used in this study. To our knowledge and although these phenomenon have been observed individually, this is the first example of a pH dependent keto-enol tautomerization in an azamacrocycle being the driving force for the formation of a metallohelix solid state structure and is a particularly unique observation for pyridinophane complexes.