Spontaneous Assembly of a Polymeric Helicate of Sodium with LVO2Units Forming the Strand: Photoinduced Transformation into a Mixed-Valence Product

The anionic cis-dioxovanadium(V) complex species LVO(2)(-) of a tridentate ONS ligand (H(2)L) can bind sodium ion in a bis-monodentate fashion like a bridging carboxylate group. The product [LVO(2)Na(H(2)O)(2)](infinity) (1) is a water soluble polymeric compound in which the complementary units are held together by the simultaneous use of hydrogen bonding and Coulombic interactions. Crystallographic characterization reveals that 1 is a single stranded helicate with LVO(2)(-) units forming the strand which surrounds the labile sodium ions that occupy the positions on the axis. In solution of protic solvents, viz. water and methanol, 1 is quite stable as indicated by electrical conductivity and (1)H NMR measurements. In aprotic solvents, viz. CH(3)CN, DMF, or DMSO, however, the extended hydrogen bonded network in 1 breaks apart and the helical structure collapses when irradiated with visible light. The product is a mixed-oxidation vanadium(IV/V) species obtained by photoinduced reduction as confirmed by EPR, time dependent (1)H NMR, and electronic spectroscopy. Compound 1 is a rare example of a nonnatural helix where hydrogen bonding interactions play a crucial role in stabilizing the single stranded polymeric structure such as that frequently observed in the biological world.