Five new rubidium borates with 0D clusters, 1D chains, 2D layers and 3D frameworks

Five new rubidium borates, namely Na_1/3Rb_5/3[B₄O₅(OH)₄] (1), Rb₄[B₁₂O₁₈(OH)₄] (2), Rb₂Ba[B₁₂O₁₈(OH)₄] (3), Rb₂[B₅O₈(OH)] (4) and NaRb_0.5[B₆O₁₀]·0.5H₃O (5) have been synthesized under solvothermal conditions. Compound 1 features a 0D structure composed of [B₄O₅(OH)₄]²⁻ clusters. After changing the metal salt, new cyclic [B₁₂O₂₀(OH)₄]⁸⁻ clusters in 2 and 3 are formed, resulting in 1D chains. Such a chain structure is first discovered in dodecaborates. By tuning the reaction temperature and pH of the reaction media, compound 4 with a 4-connected 2D oxoboron layer constructed from partially protonated [B₅O₁₀(OH)]⁶⁻ and compound 5 with a 6-connected 3D framework constructed from [B₆O₁₃]⁸⁻ clusters are successfully isolated. Since compounds 1 and 4 crystallize in the non-centrosymmetric (NCS) space group, second harmonic generation (SHG) measurements and dipole moment calculations have been carried out. The SHG response of 4 is 2.0 times that of KH₂PO₄ (KDP), which is higher than those of related analogues.