Family of Lanthanide–Manganese Heterometallic Metallacrowns: Syntheses, Structures, Magnetic Exchange, and Magnetocaloric Effects

The intermetallic magnetic interaction is a key role for exploring magneto-structure relationship. Herein, a series of Mn-based clusters featuring metallacrowns with the formulas [(MnMn4III)-Mn-II(L-1)(4)(HL2)(2)(DMF)(6)]middot4MeOHmiddot3H(2)O (1), [GdMn4III(L-1)(4)(OAc)(4)]middotH(4)TEAmiddot2MeOHmiddotH(2)O (2), [Gd4Mn4III(L-1)(4)(HL2)(2)(H2L1)(4)(OAc)(4)(mu(3)-OH)(2)(DMF)(2)(H2O)(2)]middot4DMFmiddotH(2)Omiddot16MeOHmiddotH(3)TEA (3), and [(Gd6Mn4Mn2IV)-Mn-III(L-1)(10)(HL1)(2)(H2L1)(4)(DMF)(4)(H2O)(4)(MeOH)(4)]middot4DMFmiddot2MeOHmiddot6H(2)O (4) (H3L1 = salicylhydroxamic acid, H2L2 = salicylic acid, MeOH = methanol, DMF = N,N-dimethylformamide, and H(3)TEA = triethylamine) were prepared using salicylhydroxamic acid as organic ligands. The structures of compounds 1-2, 3, and 4 featured 12-metallacrown-4, 16-metallacrown-6, and 28-metallacrown-10, respectively. Magnetic calculations showed that the obvious antiferromagnetic couplings of compounds 1-2 were mainly derived from Mn(III)middotmiddotmiddotMn(III). For compound 3, the weak ferromagnetic couplings of Gd(III)middotmiddotmiddotMn(III) and Mn(III)middotmiddotmiddotMn(III) bridged through [M-O-N-M] manner were smaller than the antiferromagnetic couplings of Gd(III)middotmiddotmiddotGd(III) and Gd(III)middotmiddotmiddotMn(III) bridged by O atoms, resulting in the overall antiferromagnetic behavior. In compound 4, the weak ferromagnetic couplings of Gd(III)middotmiddotmiddotMn(III)/Mn(III)middotmiddotmiddotMn(III)/Gd(III)middotmiddotmiddotMn(IV)/Mn(III)middotmiddotmiddotMn(IV) bridged by the way of [M-O-N-M] were dominant. The abovementioned results revealed that the arrangement of metal ions displaying [M-N-O-M] is more likely to produce weak ferromagnetic coupling between metal ions, which is beneficial to the magnetocaloric effect (MCE). Meanwhile, compounds 3-4 showed considerable MCEs with a magnetic entropy change (-delta S-m) of 13.86 J kg(-1) K-1 at 3.5 K, 7 T and 16.05 J kg(-1) K-1 at 3 K, 7 T, respectively.