Triple-Stranded Helicates of Zinc(II) and Cadmium(II) Involving a New Redox-Active Multiring Nitrogenous Heterocyclic Ligand: Synthesis, Structure, and Electrochemical and Photophysical Properties

The protonated form [H-2(L)](CF3SO3)(2) (1) of a new redox-active bis-bidentate nitrogenous heterocyclic ligand, viz., 3,3'-dipyridin-2-yl[1,1']bi[imidazo[1,5-a]pyridinyl] (L), and its zinc(II) and cadmium(II) complexes (2 and 3) have been synthesized and characterized by single-crystal X-ray diffraction analysis. In the solid state, both 2 and 3 have triple-stranded helical structures involving ligands that experience twisting and bending to the extent needed by the stereoelectronic demand of the central metal ion. The metal centers in the zinc(II) complex [Zn-2(L)(3)](ClO4)(4) (2) are equivalent, each having a distorted octahedral geometry, flattened along the C-3 axis with a Zn1 center dot center dot center dot Zn1# separation of 4.8655(13) angstrom. The cadmium complex [Cd-2(L)(3)(H2O)](ClO4)(4) (3), on the other hand, has a rare type of helical structure, showing coordination asymmetry around the metal centers with a drastically reduced Cd1 center dot center dot center dot Cd2 separation of 4.070 angstrom. The coordination environment around Cd1 is a distorted pentagonal bipyramid involving a N6O donor set with the oxygen atom coming from a coordinated water, leaving the remaining metal center Cd2 with a distorted octahedral geometry. The structures of 2 and 3 also involve anion-pi- and CH-pi-type noncovalent interactions that play dominant roles in shaping the extended structures of these molecules in the solid state. In solution, these compounds exhibit strong fluxional behavior, making the individual ligand strands indistinguishable from one another, as revealed from their H-1 NMR spectra, which also provide indications about these molecules retaining their helical structures in solution. Electrochemically, these compounds are quite interesting, undergoing ligand-based oxidations in two successive one-electron steps at E-1/2 of ca. 0.65 and 0.90 V versus a Ag/AgCl (3 M NaCl) reference. These molecules are all efficient emitters in the red and blue regions because of ligand-based pi*-pi fluorescent emissions, tuned appropriately by the attached Lewis acid centers.