Mg(II)-induced second-harmonic generation based on bis-monodentate coordination mode of thiobarbiturate

Utilizing a SHG inactive ligand, 2-thiobarbituric acid (H3L), three metal complexes formulated as M(H2L)2(H2O)2·2DMF (M = Mg 1 and Ni 2) and Co(H2L)2(H2O)2 (3) were synthesized and structurally characterized by single-crystal X-ray diffractions. Complexes 1 and 2 exhibit a similar uninodal 3D acentric diamondoid framework. Complex 1 displays SHG response using 1064 nm radiation, whereas complex 2 is SHG inactive under the same condition. Complex 3 is also SHG inactive, which exhibits a centro-symmetrical chain-like structure built from Co(II) ions and double strands of H2L−-bridge with the inversion center occupied by the metal(II) ion. In the three complexes, the negative charge of H2L− ligand is delocalized over the OC–CH–CO group and H2L− can be considered as a β-diketo derivative. H2L− shows a bis-monodentate coordination fashion and acts as a u2-bridge in the three complexes, which is different from the classical chelating fashion of β-diketo ligand. In complexes 1 and 2, the two β-diketo oxygen atoms of H2L− are coordinated to two metal centers. Whereas in complex 3, H2L− ligand links two Co(II) centers via one oxygen and one sulphur atoms. The three complexes exhibit different UV-vis absorption, photoluminescence properties and thermal stabilities.