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

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.