Protonation and olation of 2,2′-bipyridyl and 1,10-phenanthroline in γ-titanium phosphate dihydrate

Abstract

2,2′-bipyridyl (bpy) and 1,10-phenanthroline (phen) intercalate topotactically into Ti(H₂PO₄)(PO₄)·2H₂O (γ-titanium phosphate, γ-TiP), the aromatic rings undergoing hydration owing to the strongly acidic character of the > P(OH)₂ groups. For protonated phen, a ‘slotted’ assembly in the interlayer phosphate surface is given, via NH⁺⋯ HO —P hydrogen bonding (from indirect spectroscopic results and TG /DSC). Instead, protonated bpy adopts a slanted orientation within the layers, as does 2,9-dimethyl-1,10-phenanthroline (dmphen) which remains unprotonated. These differing assemblies lead to differing accessibilities of intercalated amine to metal ion (dmphen > bpy > phen) as confirmed by Cu²⁺ uptake diffusion rates. Cation exchange of Co²⁺, Ni²⁺ and Cu²⁺-exchanged composites do not give coordination pillaring, even after dehydration of the materials. Ag⁺ exchanges with Ti(H₂PO₄)(PO₄)(phen)_0.44·2.5H₂O and Ti(H₂PO₄)(PO₄)(dmphen)_0.23·2.2H₂O to give highly expanded composites (d₀₀₂= 20–21.3 Å). The former has high thermal stability, decomposing only at > 450 °C, and may be considered a pillared material. Low Ag⁺-loaded forms give time-dependent staging, ascribed to intralayer Ag⁺ diffusion. Ag⁺-phen·γ-TiP materials show enhanced selectivities for Cu²⁺(Cu²⁺≫ Ni²⁺= Co²⁺) from binary, ternary and quaternary mixtures and the anhydrous Ni²⁺-exchanged material has a square-planar coordination. Intralayer coordination models are suggested.