An insight into the molecular structures, theoretical calculation and catalytic activities of novel heterotrinuclear [CuII2CeIII] and heterohexanuclear [CuII4YIII2] bis(salamo)-based complexes

Abstract

Two heteropolynuclear complexes, namely [Cu₂(L)Ce(NO₃)₃] and [{Cu₂(L)Y(NO₃)₂(μ-AcO)}{Cu₂(L)Y(NO₃)₂(μ-NO₃)(CH₃OH)}]·4CH₃OH, were synthesized through the reaction of a newly synthesized bis(salamo)-based ligand (H₄L), Cu(OAc)₂·H₂O and Ln(NO₃)₃·6H₂O (Ln = Ce and Y), respectively. Single crystal structure analyses showed that the [Cu^II₂Ce^III] and [Cu^II₄Y^III₂] complexes contained similar trinuclear [Cu^II₂Ln^III] structures. The difference was that the [Cu^II₄Y^III₂] complex consisted of two crystallographically independent and chemically different trinuclear complexes with coordination and free solvent molecules. Among the two heteropolynuclear metal complexes, only the [Cu^II₂Ce^III] complex showed catecholase activity and benzoxazinone synthase activity under normal temperature and aerobic conditions. For catecholase activity, the calculated K_cat of the [Cu^II₂Ce^III] complex was 346.5 h⁻¹, and the K_cat of p-benzoxazinone synthase activity was 9.73 h⁻¹. Compared with the [Cu^II₄Y^III₂] complex, we speculated that the main reason for the high catalytic function of the [Cu^II₂Ce^III] complex is the presence of three nitrate groups coordinated with the central Ce^III atom. Evidence from mass spectrometry and cyclic voltammetry curves indicated that there may be heterometallic cooperation, in which Ce^III binds to the substrate, while the metallic copper transformed between Cu^II and Cu^I through redox reactions. In addition, the catalytic mechanism was inferred through electrospray mass spectrometry and cyclic voltammetry, and fluorescence properties were studied and Hirshfeld surface analyses were also conducted.