Contribution of spin and anisotropy to single molecule magnet behavior in a family of bell-shaped Mn_(11)Ln_2 coordination clusters

摘要

The synthesis, structures, and magnetic properties of a family of isostructural "bell-shaped" heterometallic coordination clusters [Mn_9~(III)Mn_2~(II)La_2~(III)(μ_4-O)_7(μ_3-O) (μ_3-OH)_2(piv)_(10.8)(O_2CC _4H_3O)_(6.2)(NO_3)_2(OH _2)_(1.5)(MeCN)_(0.5)]?12CH_3CN? 2H_2O (1) and [Mn_9~(III)Mn_2~(II)Ln_2(μ_4-O)_7(μ _3-O)(μ_3-OH)_2(piv)_(10.6)(O _2CC_4H_3O)_(6.4)(NO_3) _2(OH_2)]?nCH_3CN?H_2O (Ln = Pr~(III), n = 8 (2); Ln = Nd~(III), n = 10 (3); Ln = Eu ~(III), n = 17 (4); Ln = Gd~(III), n = 13 (5); piv = pivalate) are reported. The complexes were obtained from the reaction of [Mn _2~(III)Mn_4~(II)O_2(piv) _(10)(4-Me-py)_(2.5)(pivH)_(1.5)] and Ln(NO _3)_3?6H_2O in the presence of 2-furan-carboxylic acid (C_4H_3OCOOH) in CH_3CN. Compounds 1-5 are isomorphous, crystallizing in the triclinic space group P1 with Z = 2. The Mn~(III) and Mn~(II) centers together form the shell of the bell, while the two Ln~(III) centers can be regarded as the bell's clapper. The magnetic properties of 1-4 reveal dominant antiferromagnetic interactions between the magnetic centers leading to small spin ground states; while those of 5 indicate similar antiferromagnetic interactions between the manganese ions but with unusually strong ferromagnetic interactions between the Gd~(III) ions leading to a large overall spin ground state of S = 11-12. While ac and dc magnetic measurements confirmed that Mn_(11)Gd_2 (5) is a single-molecule magnet (SMM) showing hysteresis loops at low temperatures, compounds 1-4 do not show any slow relaxation of the magnetization, indicating that the S = 7 spin of the ferromagnetic Gd_2 unit in 5 is a necessary contribution to its SMM behavior.