A series of s-block (Ca, Sr and Ba) metal–organic frameworks: synthesis and structure–property correlation

A series of metal–organic frameworks based on alkaline earth metal ions (Ca, Sr and Ba) and 1,3,5-benzenetribenzoic acid (BTB) have been synthesized and characterized. These are [H2N(CH3)2][Ca7(BTB)5(H2O)8(DMF)4]·4H2O (1), [H2N(CH3)2]2[Sr5(H2O)6(BTB)4] (2) and [H2N(CH3)2][Ba(H2O)(BTB)] (3). All the structures are three-dimensional in nature with different secondary building units. Compound 1 contains one-dimensional Ca–O–Ca zigzag chains while compound 2 contains Sr5O28 pentameric clusters and compound 3 contains one-dimensional Ba–O–Ba chains. Both compounds 1 and 3 form (3,6)-net connectivity while compound 2 forms (3,12)-net connectivity. Optical band gap energy measurements show that compound 1 (2.65 eV) has low band gap energy compared to 2 (3.22 eV) and 3 (3.32 eV). This variation in band gap energy may be due to the difference in structural arrangement. Compound 3 crystallizes in a non-centrosymmetric space group (Pna21), which belongs to the polar point group C2v. This compound displays a strong SHG response and good ferroelectric and piezoelectric properties.