Two Alkaline\nMetal Borates: from Layer to Layer-Pillared\nFramework

Two borates, Na<sub>2</sub>K[B<sub>5</sub>O<sub>8</sub>(OH)<sub>2</sub>]·H<sub>2</sub>O (<b>1</b>) and Na<sub>4</sub>K<sub>2</sub>[{B<sub>5</sub>O<sub>8</sub>}<sub>2</sub>{B<sub>3</sub>O<sub>5</sub>}{BO<sub>2</sub>(OH)<sub>2</sub>}]·2H<sub>2</sub>O (<b>2</b>) have been designed and made under solvothermal\nconditions.\nCompound <b>1</b> exhibits a 2D fluctuant layer based on the\n[B<sub>5</sub>O<sub>10</sub>(OH)<sub>2</sub>]<sup>7–</sup> clusters,\ncontaining two types of 9-membered ring (MR) channels and showing\na four-connected sql topology net. By modifying the reactants and\nreaction temperature, compound <b>2</b> was obtained from compound <b>1</b>. The addition of boron-containing components not only increased\ntheir concentration but also enriched the B–O fragment while\nhigher temperatures promoted greater condensation of the anionic clusters.\nCompound <b>2</b> crystallizes in the acentric space group <i>Pc</i> and is composed of two types of [B<sub>5</sub>O<sub>11</sub>]<sup>7–</sup>clusters, [B<sub>3</sub>O<sub>7</sub>]<sup>5–</sup> clusters and [BO<sub>2</sub>(OH)<sub>2</sub>]<sup>3–</sup> tetrahedra. The two kinds of [B<sub>5</sub>O<sub>11</sub>]<sup>7–</sup> clusters form a 2D layer similar to <b>1</b>, which is further\nconnected by the chain built from the [B<sub>3</sub>O<sub>7</sub>]<sup>5–</sup> clusters and decorated by [BO<sub>2</sub>(OH)<sub>2</sub>]<sup>3–</sup> groups to produce a 3D layer-pillared\nframework. Notably, compound <b>2</b> is the first layer-pillared\nborate containing three types of B–O clusters and six types\nof channels. Second harmonic generation (SHG) measurements reveal\nthat compound <b>2</b> displays a moderate SHG response of about\n1.1 times that of KH<sub>2</sub>PO<sub>4</sub> (KDP).