4',4'""-Bis(methylthio)-4",4""-bis(n-propylthio)septipyridine (1), a new heptadentate ligand, reacted with Co(II) acetate at ambient temperature yielding a bimetallic complex of composition [LCo11] 2 [PFe] 4. The complex crystallized in the triclinic space group PI (No. 2) with unit cell dimensions a = 15.561(4) A, b = 16.879(4) A, c = 20.632(5) A, a = 80.02(2)°, 0 = 81.88(2)°, and 7 = 74.92(2)°, with V= 5127(2) A3 and Z = 2. The X-ray data showed that the two ligand strands were intertwined about each other and around the two Co(II) cations in a double-helical fashion. Each ligand strand acted as two discrete terpyridine units, and in one strand, pyridine ring one was uncoordinated, while, in the other strand, ring four was uncoordinated. Both Co(II) ions have distorted octahedral geometry (N6, N6), and the Co-Co distance was 4.606 A. The cyclic voltammogram of the complex showed a metal-based, two-electron oxidation as well as three ligand based reduction waves. Bi-, tri-, and tetrametallic doublehelical complexes resulted from reaction of the ligand 1 and copper ions of both oxidation states under different reaction conditions. Their compositions were established by a combination of combustion analytical data, FAB mass spectral data, and electrochemical data. Redox-induced transformations among this group of copper helicates were established spectroelectrochemically. An intramolecular diastereotopic effect in the 'H NMR spectrum of the diamagnetic, tetrametallic complex showed its chiral nature and its stability in solution. An absorption at 1420 nm in the near infrared spectrum of the mixed-valence, trimetallic complex is attributed to an intervalence electrontransfer transition.
This paper describes approaches to reduce susceptibility of perfluorosulfonic acid membranes such as Nafion® to degradation through chemical attack by oxygen-based radicals and physical degradation as a result of mechanical stress. These approaches include the incorporation of a continuous reinforcing layer and modification of the polymer to stabilize it against attack by peroxide radicals.
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