Two alkali metal borates built by {Li@B₁₄} and {B₁₅} cluster units

Two alkali metal borates, K₇[{Li@B₁₂O₁₈(OH)₄}{BO(OH)₃}₂] (1) and LiNa₂K₂[B₁₅O₂₅] (2), were successfully synthesized <i>via</i> mid-temperature hydrothermal methods. 1 features a Li-centered [{Li@B₁₂O₁₈(OH)₄}{BO(OH)₃}₂]^7- ({Li@B₁₄}) cluster, forming a 3D supramolecular framework through hydrogen bonds. Notably, the {B₁₄} cluster is reported for the first time, containing a [12:6Δ + 6T]-configured [B₁₂O₁₈(OH)₆]^6- cluster terminally decorated with two BO(OH)₃ tetrahedra. By varying the reactant concentrations and temperature based on 1, 2 was obtained, achieving the expansion of dimensionality. 2 exhibits a 3D open-framework templated by three kinds of distinct alkali metal ions (Li⁺, Na⁺, K⁺), and represents a rare example of a B₁₅O₂₉-based framework obtained under hydrothermal conditions. The {B₁₅} cluster is formed by a [12:4(3:2Δ + T)]-type [B₁₂O₂₄]^12- cluster linked to a B₃O₇ ring. Both compounds exhibit ultraviolet (UV) cut-off edges and moderate birefringence, indicating promising potential for UV optical applications.