Synthesis, Crystal Structures, Magnetic Properties, and Theoretical Investigation of a New Series of NiII-LnIII-WV 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 NiII-LnIII-WV 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 DyIII and TbIII 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 [WVNiIILnIII] core structure, [(CN)7W(CN)Ni(dmf)(valdmpn)Ln(dmf)4]·H2O (Ln = GdIII 7, TbIII 8a, DyIII 9, HoIII 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.

One- and two-photon induced emission in heterobimetallic Zn(II)-Sm(III) and Zn(II)-Tb(III) complexes with a side-off compartmental ligand.

Heterobimetallic [Zn(II)Ln(III)] complexes have been obtained using a compartmental Schiff-base ligand, H(2)valdmpn, resulting from the 2:1 condensation between o-vanillin and 2,2-dimethyl-propilenediamine: [Zn(H(2)O)(valdmpn)Sm(O(2)NO)(3)] 1, [Zn(H(2)O)(valdmpn)Tb(O(2)NO)(3)] 2a, [Zn(H(2)O)(valdmpn)Tb(O(2)NO)(3)]·H(2)O 2b, and [Zn(H(2)O)(valdmpn)Gd(O(2)NO)(3)]·H(2)O 3. The crystal structures of 1, 2b, and 3 have been solved. Compounds 1 and 2a crystallize in a non-centrosymmetric space group (P2(1)2(1)2(1)), being isomorphous. Crystals 2b and 3 are also isomorphous (space group P1[combining macron]). The complex entities in the four crystals are similar and their structures consist of binuclear species with the pentacoordinated zinc(II) ion hosted into the N(2)O(2) compartment and the lanthanide(III) ion in the large, open compartment, with a coordination number of 10. The photophysical properties of the four compounds have been investigated. Strong visible excited (excitation tails extend up to 420-430 nm) one photon antenna sensitization was obtained with the samarium(III) and terbium(III) derivatives. Following femtosecond Ti:Sapphire laser at λ(ex) = 775 nm, both second-harmonic generation at λ(em) = 775/2 nm and two-photon induced emission in the VIS range were obtained, extending thus the excitation range of these complexes from the VIS to the NIR spectral range. The two-photon induced emission and second harmonic generation effect for a samarium(III) complex are reported for the first time.

Preparation, crystal structures, and magnetic features for a series of dinuclear [Ni(II)Ln(III)] Schiff-base complexes: evidence for slow relaxation of the magnetization for the Dy(III) derivative.

A series of dinuclear [Ni(II)Ln(III)] Schiff-base complexes (using a Schiff-base dicompartmental ligand derived from o-vanillin [H(2)valpn = 1,3-propanediylbis(2-iminomethylene-6-methoxy-phenol)]) with Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and a hydroxo-bridged tetranuclear [Ni(II)Yb(III)] are reported. The crystal structures have been solved for 10 dinuclear complexes revealing four arrangements for the dinuclear units, which are modulated by the coordinated solvent molecules and the nitrato-anion interactions. The magnetic behaviors have been investigated, and the nature of the Ni(II)-Ln(III) exchange interaction has been emphasized by comparison with the behavior of the related [Zn(II)Ln(III)] derivatives. This allowed for establishing that the interaction within these compounds is antiferromagnetic with the 4f ions of the beginning of the Ln series and turns ferromagnetic from Gd(III) toward the end of the series. AC susceptibility investigations clearly show the occurrence of slow relaxation processes of the magnetization close to 2 K for the dinuclear [Ni(II)Dy(III)] complex.

Study of the luminescent and magnetic properties of a series of heterodinuclear [Zn(II)Ln(III)] complexes.

Herein, we report the synthesis, structural investigation, and magnetic and photophysical properties of a series of 13 [Zn(II)Ln(III)] heterodinuclear complexes, which have been obtained employing a Schiff-base compartmental ligand derived from o-vanillin [H(2)valpn = 1,3-propanediylbis(2-iminomethylene-6-methoxy-phenol)]. The complexes have been synthesized starting from the [Zn(valpn)(H(2)O)] mononuclear compound and the corresponding lanthanide nitrates. The crystallographic investigation indicated two structural types: the first one, [Zn(H(2)O)(valpn)Ln(III)(O(2)NO)(3)], contains 10-coordinated Ln(III) ions, while in the second one, [Zn(ONO(2))(valpn)Ln(III)(H(2)O)(O(2)NO)(2)]·2H(2)O, the rare earth ions are nine-coordinated. The Zn(II) ions always display a square-pyramidal geometry. The first structural type encompasses the larger Ln ions (4f(0)-4f(9)), while the second is found for the smaller ions (4f(8)-4f(11)). The dysprosium derivative crystallizes in both forms. Luminescence studies for the heterodinuclear compounds containing Nd(III), Sm(III), Tb(III), Dy(III), and Yb(III) revealed that the [Zn(valpn)(H(2)O)] moiety acts as an antenna. The magnetic properties for the paramagnetic [Zn(II)Ln(III)] complexes have been investigated.

Temperature switch of LMCT role: from quenching to sensitization of europium emission in a Zn(II)-Eu(III) binuclear complex.

The synthesis, structural investigation, and photophysical properties of a new heterobinuclear complex, [Zn(H(2)O)(valpn)Eu(NO(3))(3)], are reported [H(2)valpn = 1,3-propanediylbis(2-iminomethylene-6-metoxy-phenol)]. In the absence of the antenna-type sensitization of europium emission at room temperature, the strongest metal-centered emission was obtained following excitation into the (7)F(0)-(5)D(2) transition at 535 nm. In contrast, at 80 K, the strongest emission of europium was obtained by exciting into the maximum of a high-intensity, low-lying ligand-to-metal charge-transfer band (LMCT) located at approximately 425 nm. The overall temperature-induced changes of the photophysical properties of europium were assigned to the relative location of the LMCT and (3)pi pi* ligand states to the europium excited levels. The results may explain the lack of the antenna effects reported for some of the europium complexes with this type of ligand.

Octanuclear [Ni(II)4Ln(III)4] complexes. Synthesis, crystal structures and magnetocaloric properties.

Two original heterooctanuclear [Ni(II)4Ln(III)4] complexes (Ln(III) = Sm(III), Gd(III)) have been obtained starting from the [Ni(II)(valpn)(H2O)2] mononuclear precursor [H2valpn = 1,3-propanediylbis(2-iminomethylene-6-methoxy-phenol)] and the corresponding lanthanide nitrates, in the presence of azide anions, through slow capture of atmospheric CO2. Three weak and competitive exchange interactions, J(GdGd), J(GdNi), J(NiNi), make the ground state of this magnetic system degenerate at cryogenic temperature and zero field. This, along with the high spin of Gd(III), lead to a significant magnetocaloric effect spread in the temperature range 1 to 20 K (ΔSm[0-7 T, 3.5 K] = 19 J kg(-1) K(-1)).

Dimers and chains of {3d-4f} single molecule magnets constructed from heterobimetallic tectons.

A tetranuclear complex and a 1-D coordination polymer with a ladder-like topology have been obtained by connecting [Ni(II)Dy(III)] nodes with dicarboxylato ligands: [Ni2(valpn)2Dy2(III)(pdca)2(NO3)(H2O)6](NO3)·4H2O 1, and (∞)¹[Ni2(H2O)2(valpn)2Dy2(tfa)3]·4CH3CN 2 (valpn²â» = the dianion of the Schiff base resulting from reacting o-vanillin with 1,3-propanediamine; pdca²â» = the dianion of 2,6-pyridinedicarboxylic acid; tfa²â» = the dianion of the terephthalic acid). The magnetic measurements show a ferromagnetic interaction between Ni(II) and Dy(III), and that both compounds behave like SMM with strong tunnelling. The barrier of 2 (17.4 K) is higher than that of 1 (13.6 K).