[C<sub>3</sub>H<sub>7</sub>N<sub>6</sub>]<sub>3</sub>[B<sub>3</sub>O<sub>5</sub>(OH)<sub>2</sub>] and [C<sub>3</sub>H<sub>8</sub>N<sub>6

The π-conjugated [C<sub>3</sub>H<sub>6+<i>x</i></sub>N<sub>6</sub>]<sup><i>x</i>+</sup> (<i>x</i> = 0–3) cations are good functional\ngroups, which\nare widely\nemployed in the preparations of nonlinear optical (NLO) and birefringent\nmaterials due to their high hyperpolarizability and optical anisotropy.\nIn this paper, the first melamine hydroxyborate [C<sub>3</sub>H<sub>7</sub>N<sub>6</sub>]<sub>3</sub>[B<sub>3</sub>O<sub>5</sub>(OH)<sub>2</sub>] (<b>MelBO-I</b>) was synthesized by the boric acid\nmelting method under the molar ratio of H<sub>3</sub>BO<sub>3</sub>:C<sub>3</sub>H<sub>6</sub>N<sub>6</sub> = 1:1. <b>MelBO-I</b> (<i>P</i>2<sub>1</sub>/<i>c</i>) exhibits a\ntwo-dimensional (2D) {[C<sub>3</sub>H<sub>7</sub>N<sub>6</sub>]<sub>3</sub>[B<sub>3</sub>O<sub>5</sub>(OH)<sub>2</sub>]}<sub>∞</sub> layer composed of [C<sub>3</sub>H<sub>7</sub>N<sub>6</sub>]<sup>+</sup> cations and [B<sub>3</sub>O<sub>5</sub>(OH)<sub>2</sub>]<sup>3–</sup> anions interconnected via hydrogen bonds. <b>MelBO-I</b> exhibits significant birefringence (Δ<i>n</i> =\n0.286@546 nm). Under the molar ratio of H<sub>3</sub>BO<sub>3</sub>/C<sub>3</sub>H<sub>6</sub>N<sub>6</sub> = 3:1, [C<sub>3</sub>H<sub>8</sub>N<sub>6</sub>]<sub>4</sub>[B<sub>12</sub>O<sub>19</sub>(OH)<sub>6</sub>] (<b>MelBO-II</b>) was isolated. In <b>MelBO-II</b> (<i>P</i>2<sub>1</sub>), highly polymerized [B<sub>12</sub>O<sub>19</sub>(OH)<sub>6</sub>]<sup>8–</sup> groups form a\n3D network through hydrogen bonding, featuring 1D tunnels of 8-membered\nand 16-membered rings filled by [C<sub>3</sub>H<sub>8</sub>N<sub>6</sub>]<sup>2+</sup> cations. <b>MelBO-II</b> is the first noncentrosymmetric\n(NCS) bifunctional melamine borate with a moderate SHG response (0.4\n× KDP) and large birefringence (Δ<i>n</i> = 0.285@546\nnm). The results indicate that incorporating [C<sub>3</sub>H<sub>6+<i>x</i></sub>N<sub>6</sub>]<sup><i>x</i>+</sup> (<i>x</i> = 0–3) cations into borate can effectively induce\nbirefringence. A high concentration of boric acid promotes the formation\nof large boric acid cluster anions and facilitates the transformation\nfrom the CS to NCS structure.