Spontaneous Assembly of a Polymeric Helicate of Sodium with LVO₂ Units Forming the Strand: Photoinduced Transformation into a Mixed-Valence Product

The anionic cis-dioxovanadium(V) complex species LVO2- of a tridentate ONS ligand (H2L) can bind sodium ion in a bis-monodentate fashion like a bridging carboxylate group. The product [LVO2Na(H2O)2]∞ (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 LVO2- 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 1H NMR measurements. In aprotic solvents, viz. CH3CN, 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 1H 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.