On the influence of the titanium source on the composition and structure of novel titanoniobates

Abstract

Systematic variation of the titanium source and the reaction temperature applied during hydrothermal synthesis led to crystallization of four new titanoniobates: {[Ni(cyclam)]₄[Ti₂Nb₈O₂₈]}_n·∼28nH₂O (I), K[Ni(cyclam)]₃[TiNb₉O₂₈]·xH₂O; x = 18 (II), x = 14 (III) and x ∼ 10 (IV). These are the first titanoniobates with Ni²⁺-centered amine complexes acting as counter cations and additionally, this is the first report of transition metal complexes expanded by monotitanoniobates. While I is obtained using Ti(OⁱPr)₄, II–IV are formed using K₂TiO(C₂O₄)·2H₂O as the educt. The presence of oxalate anions seems to influence the coordination environments of the Ni²⁺ cations that are octahedrally coordinated in I, and in a square-planar environment in II–IV. The titanium source also affects the degree of substitution of Nb^V by Ti^IV. Temperature-dependent syntheses demonstrate that the formation of I does not depend on the reaction temperature, while the formation of II–IV is clearly affected by this parameter. Regarding the arrangement of crystal water molecules, patterns of different dimensionalities ranging from 0D to 3D are formed which can be classified as water clusters. Each compound exhibits a pronounced plateau in the thermogravimetric curves after the removal of crystal H₂O molecules. Rehydration experiments after water removal proved that except for compound III, the re-integration of water was successful.