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
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Abstract
The protonated form [H<sub>2</sub>(L)](CF<sub>3</sub>SO<sub>3</sub>)<sub>2</sub> (<b>1</b>) of a new redox-active
bis-bidentate
nitrogenous heterocyclic ligand, viz., 3,3′-dipyridin-2-yl[1,1′]bi[imidazo[1,5-<i>a</i>]pyridinyl] (L), and its zinc(II) and cadmium(II) complexes
(<b>2</b> and <b>3</b>) have been synthesized and characterized
by single-crystal X-ray diffraction analysis. In the solid state,
both <b>2</b> and <b>3</b> 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<sub>2</sub>(L)<sub>3</sub>](ClO<sub>4</sub>)<sub>4</sub> (<b>2</b>) are
equivalent, each having a distorted octahedral geometry, flattened
along the <i>C</i><sub>3</sub> axis with a Zn1···Zn1#
separation of 4.8655(13) Å. The cadmium complex [Cd<sub>2</sub>(L)<sub>3</sub>(H<sub>2</sub>O)](ClO<sub>4</sub>)<sub>4</sub> (<b>3</b>), on the other hand, has a rare type of helical structure,
showing coordination asymmetry around the metal centers with a drastically
reduced Cd1···Cd2 separation of 4.070 Å. The coordination
environment around Cd1 is a distorted pentagonal bipyramid involving
a N<sub>6</sub>O 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 <b>2</b> and <b>3</b> also
involve anion−π- and CH−π-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 <sup>1</sup>H 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 <i>E</i><sub>1/2</sub> 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 π*−π fluorescent emissions, tuned
appropriately by the attached Lewis acid centers