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 bell-shaped heterometallic coordination clusters [(Mn9Mn2La2III)-Mn-III-La-II(mu(4)-O)(7)(mu(3)-O)(mu(3)-OH)(2)(piv)(10.8)(O2CC4H3O)(6.2)(NO3)(2)(OH2)(1.5)(MeCN)(0.5)]center dot 12CH(3)CN center dot 2H(2)O (1) and [Mn(9)(III)Mn(2)(II)Ln(2)(mu(4)-O)(7)(mu(3)-O)(mu(3)-OH)(2)(piv)(10.6)(O2CC4H3O)(6.4)(NO3)(2)(OH2)]center dot nCH(3)Cn center dot H2O (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 [(Mn2Mn4O2)-Mn-III-O-II(piv)(10)(4-Me-py)(2.5)(pivH)(1.5)] and Ln(NO3)(3)center dot 6H(2)O in the presence of 2-furan-carboxylic acid (C4H3OCOOH) in CH3CN. Compounds 1-5 are isomorphous, crystallizing in the triclinic space roup Pi with Z = 2. The Muni 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 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 Gd-2 unit in 5 is a necessary contribution to its SMM behavior.