Synthesis, Crystal Structures, Magnetic Properties, and Theoretical Investigation of a New Series of Ni^II-Ln^III-W^V Heterotrimetallics: Understanding the SMM Behavior of Mixed Polynuclear Complexes

The polynuclear compounds containing anisotropic metal ions often exhibit efficient barriers for blocking of magnetization at fairly arbitrary geometries. However, at variance with mononuclear complexes, which usually become single-molecule magnets (SMM) under the sole requirement of a highly axial crystal field at the metal ion, the factors influencing the SMM behavior in polynuclear complexes, especially, with weakly axial magnetic ions, still remain largely unrevealed. As an attempt to clarify these conditions, we present here the synthesis, crystal structures, magnetic behavior, and ab initio calculations for a new series of Ni^II-Ln^III-W^V trimetallics, [(CN)₇W(CN)Ni(H₂O)(valpn)Ln(H₂O)₄]·H₂O (Ln = Y 1, Eu 2, Gd 3, Tb 4, Dy 5, Lu 6). The surprising finding is the absence of the magnetic blockage even for compounds involving strongly anisotropic Dy^III and Tb^III metal ions. This is well explained by ab initio calculations showing relatively large transversal components of the g-tensor in the ground exchange Kramers doublets of 1 and 4 and large intrinsic tunneling gaps in the ground exchange doublets of 3 and 5. In order to get more insight into this behavior, another series of earlier reported compounds with the same trinuclear [W^VNi^IILn^III] core structure, [(CN)₇W(CN)Ni(dmf)(valdmpn)Ln(dmf)₄]·H₂O (Ln = Gd^III 7, Tb^III 8a, Dy^III 9, Ho^III 10), [(CN)₇W(CN)Ni(H₂O)(valdmpn)Tb(dmf)_2.5(H₂O)_1.5]·H₂O·0.5dmf 8b, and [(CN)₇W(CN)Ni(H₂O)(valdmpn)Er(dmf)₃(H₂O)₁]·H₂O·0.5dmf 11, has been also investigated theoretically. In this series, only 8b exhibits SMM behavior which is confirmed by the present ab initio calculations. An important feature for the entire series is the strong ferromagnetic coupling between Ni(II) and W(V), which is due to an almost perfect trigonal dodecahedron geometry of the octacyano wolframate fragment. The reason why only 8b is an SMM is explained by positive zero-field splitting on the nickel site, precluding magnetization blocking in complexes with fewer axial Ln ions. Further analysis has shown that, in the absence of ZFS on Ni ion, all compounds in the two series (except those containing Y and Gd) would be SMMs. The same situation arises for perfectly axial ZFS on Ni(II) with the main anisotropy axis parallel to the main magnetic axis of Ln(III) ions. In all other cases the ZFS on Ni(II) will worsen the SMM properties. The general conclusion is that the design of efficient SMMs on the basis of such complexes should involve isotropic or weekly anisotropic metal ions, such as Mn(II), Fe(III), etc., along with strongly axial lanthanides.