Contribution of Spin and Anisotropy to Single Molecule Magnet Behavior in a Family of Bell-Shaped Mn11Ln2 Coordination Clusters

The synthesis, structures, and magnetic properties of a family of isostructural "bellshaped" heterometallic coordination clusters [MnIII9MnII2LaIII2(μ4-O)7(μ3-O)(μ3-OH)2(piv)10.8- (O2CC4H3O)6.2(NO3)2(OH2)1.5(MeCN)0.5] 3 12CH3CN 3 2H2O (1) and [MnIII9MnII2Ln2-(μ4-O)7(μ 3-O)(μ3OH)2(piv)10.6(O2CC4H3O)6.4(NO3)2(OH2)] 3nCH3CN3H2O (Ln = PrIII, n =8 (2); Ln =NdIII, n = 10 (3); Ln = EuIII, n = 17 (4); Ln = GdIII, n = 13 (5); piv = pivalate) are reported. The complexes were obtained from the reaction of [MnIII2MnII4O2(piv)10(4-Me-py)2.5(pivH)1.5] and Ln(NO3)3 3 6H2O in the presence of 2-furan-carboxylic acid (C4H3OCOOH) in CH3CN. Compounds 1-5 are isomorphous, crystallizing in the triclinic space group P1 with Z = 2. The MnIII and MnII centers together form the shell of the bell, while the two LnIII 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 GdIII ions leading to a large overall spin ground state of S = 11-12. While ac and dc magnetic measurements confirmed that Mn11Gd2 (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 Gd2 unit in 5 is a necessary contribution to its SMM behavior