Schiff base tetranuclear Zn2Ln2 single-molecule magnets bridged by hydroxamic acid in association with near-infrared luminescence

Abstract

A series of Zn–Ln heteronuclear SMMs constructed by using a hexadentate compartment Schiff base Zn-precursor and lanthanoid ions were structurally and magnetically characterized, in which the two [Zn–Ln] moieties are bridged by a series of hydroxamic acids, resulting in double-decker tetranuclear complexes with the molecular formulae [ZnL¹Ln(C₂H₅O)(qua)]₂(CF₃SO₃)₂·2C₂H₅OH ((1) Ln = Dy; (7) Ln = Yb), [ZnL¹Ln(CH₃O)(bnz)]₂(CF₃SO₃)₂·2CH₃OH ((2) Ln = Dy), [ZnL¹Ln(CH₃O)(aca)]₂(CF₃SO₃)₂·2CH₃OH ((3) Ln = Dy; (8) Ln = Yb), [ZnL²Dy(CH₃O)(bnz)]₂(CF₃SO₃)₂·2CH₃OH (4), [ZnL²Dy(CH₃O)(aca)]₂(CF₃SO₃)₂·2CH₃OH (5), and [ZnL³Dy(CH₃O)(bnz)]₂(CF₃SO₃)₂·2CH₃OH (6) (HL¹ = N,N‘-bis(2-hydroxy-3-methoxybenzylidene)-1,2-phenylenediamine, HL² = N,N‘-bis(2-hydroxy-3-methoxybenzylidene)-propane-1,2-diamine, HL³ = N,N‘-bis(3-methoxysalicylidene)-1,3-propanediamine, qua = 2-quinolinecarboxylic acid, bnz = benzhydroxamic acid and aca = acetohydroxamic acid). Strikingly, the slow magnetic relaxation can be tuned by modifying the steric hindrance and/or electronic effect on the backbone of the Shiff base and the terminal substituents of hydroxamic acid, as well the magneto-structural correlations are studied. Furthermore, Yb congeners 7 and 8 were synthesized to explore dual-functional materials with both magnetic and fluorescence properties, and they displayed both slow magnetic relaxation and near-infrared (NIR) properties; the low temperature NIR spectroscopic data were correlated with the corresponding slow magnetic relaxation mechanism involving thermally activated ground states to the excited state.