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

The π-conjugated [C3H6+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 [C3H6+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.