[C3H7N6]3[B3O5(OH)2] and [C3H8N6]4[B12O19(OH)6]: Two Melamine Borates with Large Birefringence

The π-conjugated [C₃H_6+xN₆]^x+ (x = 0-3) cations are good functional groups, which are widely employed in the preparations of nonlinear optical (NLO) and birefringent materials due to their high hyperpolarizability and optical anisotropy. In this paper, the first melamine hydroxyborate [C₃H₇N₆]₃[B₃O₅(OH)₂] (MelBO-I) was synthesized by the boric acid melting method under the molar ratio of H₃BO₃:C₃H₆N₆ = 1:1. MelBO-I (P2₁/c) exhibits a two-dimensional (2D) {[C₃H₇N₆]₃[B₃O₅(OH)₂]}_∞ layer composed of [C₃H₇N₆]⁺ cations and [B₃O₅(OH)₂]^3- anions interconnected via hydrogen bonds. MelBO-I exhibits significant birefringence (Δn = 0.286@546 nm). Under the molar ratio of H₃BO₃/C₃H₆N₆ = 3:1, [C₃H₈N₆]₄[B₁₂O₁₉(OH)₆] (MelBO-II) was isolated. In MelBO-II (P2₁), highly polymerized [B₁₂O₁₉(OH)₆]^8- groups form a 3D network through hydrogen bonding, featuring 1D tunnels of 8-membered and 16-membered rings filled by [C₃H₈N₆]²⁺ cations. MelBO-II is the first noncentrosymmetric (NCS) bifunctional melamine borate with a moderate SHG response (0.4 × KDP) and large birefringence (Δn = 0.285@546 nm). The results indicate that incorporating [C₃H_6+xN₆]^x+ (x = 0-3) cations into borate can effectively induce birefringence. A high concentration of boric acid promotes the formation of large boric acid cluster anions and facilitates the transformation from the CS to NCS structure.