New one-dimensional vanadyl iodates:: Hydrothermal preparation, structures, and NLO properties of A[VO2(IO3)2] (A = K, Rb) and A[(VO)2(IO3)3O2] (A = NH4, Rb, Cs)

Under mild hydrothermal conditions V2O5 reacts with AIO(4) (A = NH4, K, Rb, Cs) and I2O5 to yield A[VO2(IO3)(2)] (K (1), Rb (2)) and A[(VO)(2)(IO3)(3)O-2] (A = NH4 (3), Rb (4), Cs (5)) in the form of yellow to dark orange crystals. While 1 and 2 are not isostructural, both contain one-dimensional (1)(infinity)[VO2(IO3)(2)](1-) chains separated by K+ or Rb+ cations. These chains are constructed from five-coordinate V(V) atoms that are bound by two terminal oxo atoms to create a cis-VO2+ moiety and by one monodentate and two bridging iodate anions to create a distorted square pyramidal geometry. Compounds 3-5 are isostructural and contain orthorhombically distorted, octahedral V(V) with a single terminal oxo ligand. In addition, each vanadium center forms bonds with two bridging oxo atoms that are trans to one another and three bridging iodate anions in a mer arrangement. The bridging of the vanadium atoms by the oxo and iodate anions creates zigzagging one-dimensional (1[)(infinity)(VO)(2)(IO3)(3)O-2](1-) chains that are separated from one another by NH4+, Rb+, or Cs+ cations that reside in asymmetric environments. Compounds 3-5 crystallize in the polar space group Ima2. The polarity in these structures is imparted by the alignment of the stereochemically active lone pairs of electrons of the iodate anions along the c-axis. Second-harmonic generation measurements on 5 show a large response of 500 x alpha-quartz.