Na₂(H₂en)[B₅O₈(OH)]₂[B₃O₄(OH)]₂ and Na₃(HCOO)[B₅O₈(OH)]: Two Borates Co-Templated by Inorganic Cations and Organic Compounds

The cooperative effect of inorganic and organic co-templates led to two layered borates: centrosymmetric (CS) Na₂(H₂en)[B₅O₈(OH)]₂[B₃O₄(OH)]₂ (1; en = ethylenediamine) and noncentrosymmetric (NCS) Na₃(HCOO)[B₅O₈(OH)] (2). Compound 1 features coupled layers extended by two different oxoboron clusters, which are rare in borates. The Na⁺ cations are located in the channel and hold the coupled layers in tight inorganic B–O–Na layers, while H₂en²⁺ cations reside in channels between inorganic layers. Compound 2 is the first NCS borate directed by inorganic and organic species simultaneously and features acentric [B₅O₈(OH)]_n^2n– layers. It exhibits a good second harmonic generation efficiency of about 1.9 times that of potassium dihydrogenphosphate and is phase-matchable, and the UV cutoff edge below 200 nm makes 2 a potential UV nonlinear optical material. Investigation of the fluorescence properties indicated that the new borates are potential blue fluorescent materials. In addition, structure elucidation revealed that host–guest symmetry matching plays a key role in the formation of CS 1 and NCS 2.