Hydrogen-Bond-Directed Assembly of Polyanionic Cluster [Mo₂O₅(IO₃)₄]^2- for Nonlinear Optical Crystal Design

Iodates are promising candidates for second-order nonlinear optical (NLO) materials due to the stereochemically active lone pair of I⁵⁺ in their functional units [(IO₃)^-/(IO₄)^3-], which confers large second-harmonic generation (SHG) coefficients and high birefringence. However, their natural tendency to crystallize in centrosymmetric structures, driven by electrostatic stabilization, severely restricts the design and development of high-performance nonlinear optical crystals. Although incorporating d⁰ transition metals or other NLO-active units has been studied, the rational design of hybrid iodates through polyanionic cluster modulation with organic cations remains underexplored. Electrostatic potential analysis revealed the [Mo₂O₅(IO₃)₄]^2- cluster as an optimal building block with multiple hydrogen-bonding sites. By employing distinct planar π-conjugated cations to modulate the polyanionic building blocks, two organic-inorganic hybrid molybdenyl iodates were successfully synthesized: centrosymmetric (C₅H₆NO)₂Mo₂O₅(IO₃)₄·2H₂O (1) and noncentrosymmetric (C₂H₄N₃)₂Mo₂O₅(IO₃)₄·4H₂O (2). 2 exhibits a strong SHG response of 4.2 times that of KH₂PO₄ (KDP) and high birefringence of 0.298 at 546 nm, highlighting its potential for UV linear and nonlinear optical applications.