María A. Palacios

Family of Carboxylate- and Nitrate-diphenoxo Triply Bridged Dinuclear Ni(II)Ln(III) Complexes (Ln = Eu, Gd, Tb, Ho, Er, Y): Synthesis, Experimental and Theoretical Magneto-Structural Studies, and Single-Molecule Magnet Behavior

Seven acetate-diphenoxo triply bridged M(II)-Ln(III) complexes (M(II) = Ni(II) and Ln(III) = Gd, Tb, Ho, Er, and Y; M(II) = Zn(II) and Ln(III) = Ho(III) and Er(III)) of formula [M(μ-L)(μ-OAc)Ln(NO(3))(2)], one nitrate-diphenoxo triply bridged Ni(II)-Tb(III) complex, [Ni(μ-L)(μ-NO(3))Tb(NO(3))(2)]·2CH(3)OH, and two diphenoxo doubly bridged Ni(II)-Ln(III) complexes (Ln(III) = Eu, Gd) of formula [Ni(H(2)O)(μ-L)Ln(NO(3))(3)]·2CH(3)OH have been prepared in one pot reaction from the compartmental ligand N,N,N-trimethyl-N,N-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H(2)L). Moreover, Ni(II)-Ln(III) complexes bearing benzoate or 9-anthracenecarboxylate bridging groups of formula [Ni(μ-L)(μ-BzO)Dy(NO(3))(2)] and [Ni(μ-L)(μ-9-An)Dy(9-An)(NO(3))(2)]·3CH(3)CN have also been successfully synthesized. In acetate-diphenoxo triply bridged complexes, the acetate bridging group forces the structure to be folded with an average hinge angle in the M(μ-O(2))Ln bridging fragment of ~22°, whereas nitrate-diphenoxo doubly bridged complexes and diphenoxo-doubly bridged complexes exhibit more planar structures with hinge angles of ~13° and ~2°, respectively. All Ni(II)-Ln(III) complexes exhibit ferromagnetic interactions between Ni(II) and Ln(III) ions and, in the case of the Gd(III) complexes, the J(NiGd) coupling increases weakly but significantly with the planarity of the M-(O)(2)-Gd bridging fragment and with the increase of the Ni-O-Gd angle. Density functional theory (DFT) theoretical calculations on the Ni(II)Gd(III) complexes and model compounds support these magneto-structural correlations as well as the experimental J(NiGd) values, which were found to be ~1.38 and ~2.1 cm(-1) for the folded [Ni(μ-L)(μ-OAc)Gd(NO(3))(2)] and planar [Ni(H(2)O)(μ-L)Gd(NO(3))(3)]·2CH(3)OH complexes, respectively. The Ni(II)Dy(III) complexes exhibit slow relaxation of the magnetization with Δ/k(B) energy barriers under 1000 Oe applied magnetic fields of 9.2 and 10.1 K for [Ni(μ-L)(μ-BzO)Dy(NO(3))(2)] and [Ni(μ-L)(μ-9-An)Dy(9-An)(NO(3))(2)]·3CH(3)CN, respectively.

Bifunctional ZnIILnIII Dinuclear Complexes Combining Field Induced SMM Behavior and Luminescence: Enhanced NIR Lanthanide Emission by 9-Anthracene Carboxylate Bridging Ligands

There were new dinuclear Zn(II)-Ln(III) complexes of general formulas [Zn(μ-L)(μ-OAc)Ln(NO3)2] (Ln(III) = Tb (1), Dy (2), Er (3), and Yb (4)), [Zn(μ-L)(μ-NO3)Er(NO3)2] (5), [Zn(H2O)(μ-L)Nd(NO3)3]·2CH3OH (6), [Zn(μ-L)(μ-9-An)Ln(NO3)2]·2CH3CN (Ln(III) = Tb (7), Dy (8), Er (9), Yb(10)), [Zn(μ-L)(μ-9-An)Yb(9-An)(NO3)3]·3CH3CN (11), [Zn(μ-L)(μ-9-An)Nd(9-An)(NO3)3]·2CH3CN·3H2O (12), and [Zn(μ-L)(μ-9-An)Nd(CH3OH)2(NO3)]ClO4·2CH3OH (13) prepared from the reaction of the compartmental ligand N,N,N″-trimethyl-N,N″-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H2L), with ZnX2·nH2O (X = NO3(-) or OAc(-)) salts, Ln(NO3)3·nH2O, and, in some instances, 9-anthracenecarboxylate anion (9-An). In all these complexes, the Zn(II) ions invariably occupy the internal N3O2 site whereas the Ln(III) ions show preference for the O4 external site, giving rise to a Zn(μ-diphenoxo)Ln bridging fragment. Depending on the Zn(II) salt and solvent used in the reaction, a third bridge can connect the Zn(II) and Ln(III) metal ions, giving rise to triple-bridged diphenoxoacetate in complexes 1-4, diphenoxonitrate in complex 5, and diphenoxo(9-anthracenecarboxylate) in complexes 8-13. Dy(III) and Er(III) complexes 2, 8 and 3, 5, respectively, exhibit field induced single molecule magnet (SMM) behavior, with Ueff values ranging from 11.7 (3) to 41(2) K. Additionally, the solid-state photophysical properties of these complexes are presented showing that ligand L(2-) is able to sensitize Tb(III)- and Dy(III)-based luminescence in the visible region through an energy transfer process (antenna effect). The efficiency of this process is much lower when NIR emitters such as Er(III), Nd(III), and Yb(III) are considered. When the luminophore 9-anthracene carboxylate is incorporated into these complexes, the NIR luminescence is enhanced which proves the efficiency of this bridging ligand to act as antenna group. Complexes 2, 3, 5, and 8 can be considered as dual materials as they combine SMM behavior and luminescent properties.

Diphenoxo-Bridged NiIILnIII Dinuclear Complexes as Platforms for Heterotrimetallic (LnIIINiII)2RuIII Systems with a High-Magnetic-Moment Ground State: Synthesis, Structure, and Magnetic Properties

The first examples of pentanuclear heterotrimetallic [(LnNi)(2)Ru] [Ln(3+) = Gd (1) and Dy (2)] complexes were prepared and magnetostructurally characterized. They exhibit ferromagnetic interactions, leading to a high-magnetic-moment ground state.

Enhanced ferromagnetic interaction in metallacyclic complexes incorporating m-phenylenediamidato bridges.

The double stranded Cu(II)2-metallacyclic complex of formula [Cu2(mbpb)2].2H2O (1) and the triple stranded Ni(II)2-metallacyclic complexes of formula [Ni2(Hmbpb)3]PF6.21H2O (2), [Co(H2O)6][Ni2(mbpb)3)]THF.10H2O (3) and {Ag2(H2O)[Ni2(mbpb)3]}.11H2O (4) (where H2mbpb is the bisbidentate dinucleating bridging ligand 1,3-bis(pyridine-2-carboxamide) benzene) have been synthesised and characterised by single-crystal X-ray diffraction. Within the dinuclear molecules, metal ions are bridged by either fully or semideprotonated bisbidentate ligands, which are coordinated through the pyridine and amidato nitrogen donor atoms. In complex 4 the triple stranded dinuclear Ni2 units are connected to the Ag+ cations through O-amidato bridges and Ag-pi(benzene) interactions to afford a 1D bimetallic chain. Cu2 (1) and Ni2 (2-4) complexes exhibit ferromagnetic coupling between the metal ions through the bridging ligand with J(Cu-Cu) = 21.1 cm(-1) and J(Ni-Ni) in the range of 2.9-3.6 cm(-1), respectively. Amongst copper(II) dinuclear complexes bearing m-phenylenediamidato bridges, complex 1 exhibits the stronger ferromagnetic exchange coupling reported so far. DFT calculations firstly confirm that the spin polarisation mechanism is responsible for the ferromagnetic coupling, and secondly allows us to predict stronger ferromagnetic couplings in Cu(II)(2) complexes with larger tetrahedral distortions of the CuN4 coordination environment.

3d−3d−4f Chain Complexes Constructed Using the Dinuclear Metallacyclic Complex [Ni2(mbpb)3]2− [H2mbpb = 1,3-Bis(pyridine-2-carboxamide)benzene] as a Ligand: Synthesis, Structures, and Magnetic Properties

A series of one-dimensional Ni(2)Ln cationic complexes have been prepared by assembling the in situ generated dinuclear mesocate [Ni(2)(mbpb)(3)](2-) building block [H(2)mbpb is the ligand 1,3-bis(pyridine-2-carboxamide)benzene] with Ln(3+) metal ions (Ln(3+) = Gd, Tb, Dy). The crystal-field potentials for the two types of site symmetries found for these 3d-3d-4f complexes (LnO(7) and LnO(8)) are quite different, which has a direct influence on the depopulation of the Stark sublevels, the magnetic anisotropy, and the magnetic properties.

Analysis of Magnetic Anisotropy and the Role of Magnetic Dilution in Triggering Single-Molecule Magnet (SMM) Behavior in a Family of CoIIYIII Dinuclear Complexes with Easy-Plane Anisotropy

Three new closely related CoII YIII complexes of general formula [Co(μ-L)(μ-X)Y(NO3 )2 ] (X- =NO3- 1, benzoate 2, or 9-anthracenecarboxylato 3) have been prepared with the compartmental ligand N,N,N-trimethyl-N,N-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H2 L). In these complexes, CoII and YIII are triply bridged by two phenoxide groups belonging to the di-deprotonated ligand (L2- ) and one ancillary anion X- . The change of the ancillary bridging group connecting CoII and YIII ions induces small differences in the trigonally distorted CoN3 O3 coordination sphere with a concomitant tuning of the magnetic anisotropy and intermolecular interactions. Direct current magnetic, high-frequency and -field EPR (HFEPR), frequency domain Fourier transform THz electron paramagnetic resonance (FD-FT THz-EPR) measurements, and ab initio theoretical calculations demonstrate that CoII ions in compoundsu20051-3 have large and positive D values (≈50u2005cm-1 ), which decrease with increasing the distortion of the pseudo-octahedral CoII coordination sphere. Dynamic ac magnetic susceptibility measurements indicate that compoundu20051 exhibits field-induced single-molecule magnet (SMM) behavior, whereas compoundsu20052 and 3 only display this behavior when they are magnetically diluted with diamagnetic ZnII (Zn/Co=10:1). In view of this, it is always advisable to use magnetically diluted complexes, in which intermolecular interactions and quantum tunneling of magnetism (QTM) would be at least partly suppressed, so that hidden single-ion magnet (SIM) behavior could emerge. Field- and temperature-dependence of the relaxation times indicate the prevalence of the Raman process in all these complexes above approximately 3u2005K.

On the Magnetic Coupling and Spin Crossover Behavior in Complexes Containing the Head-to-Tail [FeII2(μ-SCN)2] Bridging Unit: A Magnetostructural Experimental and Theoretical Study

A new dinuclear complex [{Fe(tpc-OBn)(NCS)(μ-NCS)}2] (1) based on the tripodal tpc-OBn ligand (tpc-OBn = tris(2-pyridyl)benzyloxymethane), containing bridging μ-κN:κS-SCN and terminal κN-SCN thiocyanate ligands, has been prepared and characterized by single crystal X-ray diffraction, magnetic studies, and DFT theoretical calculations. This complex represents the first example of dinuclear FeII complex with double μ-κN:κS-SCN bridges in a head-to-tail configuration that exhibits ferromagnetic coupling between metal ions (JFeFe = +1.08 cm-1). Experimental and theoretical magnetostructural studies on this kind of infrequent FeII dinuclear complex containing a centrosymmetrically [Fe2(μ-SCN)2] bridging fragment show that the magnitude and sign of the magnetic coupling parameter, JFeFe, depend to a large extent on the Fe-N-C (α) angle, so that JFeFe decreases linearly when α decreases. The calculated crossover point below which the magnetic interactions change from ferromagnetic to antiferromagnetic is found at 162.3°. In addition, experimental results obtained in this work and those reported in the literature suggest that large Ntripodal-FeII distances and bent N-bound terminal κN-SCN ligands favor the high spin state of the FeII ions, while short Ntripodal-FeII distances and almost linear Fe-N-C angles favor a stronger ligand field, which enables the FeII ions to show spin crossover (SCO) behavior.

Varying structural motifs, unusual X-band electron paramagnetic spectra, DFT studies and superoxide dismutase enzymatic activity of copper(II) complexes with N′-[(E)-phenyl(pyridin-2-yl)methylidene]benzohydrazide

A diverse set of new copper(II) complexes with N′-[(E)-phenyl(pyridin-2-yl)methylidene]benzohydrazide have been designed and synthesized as possible models for antioxidant superoxide dismutase mimics. Five new coordination complexes, [Cu(L)(NO3)2(4,4-bipy)] (1), [Cu(L)(HL)]ClO4·1/2H2O (2), [Cu(L)(HL)][Cu(L)(HL)](ClO4)2·H2O (3), [Cu(L)(NO3)(μ-2-aminopyrazine)Cu(L)(NO3)2]·2H2O (4) and [Cu(L)(H2O)(ClO4)]n (5), have been synthesized. These copper(II) mono/bi and polynuclear complexes were characterized using various physico-chemical techniques, viz., microanalysis, FTIR, CV, UV-vis and EPR spectroscopy. The complexes were further characterized by single crystal X-ray methods. X-ray crystal structures reveal that complexes 1–3 are mononuclear, 4 is binuclear and 5 is a polynuclear species. In all of the complexes, H-bonding and π⋯π, CH⋯π and lone pair (lp⋯π) interactions are operative in forming supramolecular motifs. The copper ions in 1–4 are five coordinate distorted square pyramidal, while polynuclear complex 5 shows six coordinate geometry about a copper ion that is distorted pseudo octahedral. Variable temperature magnetic susceptibility measurements have been carried out for the mono and binuclear complexes and show an antiferromagnetic exchange interaction with coupling constants J = −0.3 to −3.8 cm−1. The X-band epr spectra for polycrystalline samples exhibit a copper(II) hyperfine structure and zero-field splittings which are appropriate for the triplet state (S = 1) of such inter dimer/dimer/poly pseudo dipolar zero-field interactions for mononuclear complexes. These results, from a spin–orbit admixture of the excited state, were unexpected and indicated exchange interactions in the excited state that were greater than those observed for the ground state. Cyclic voltammograms of the homo binuclear complexes (4 and 5) in DMSO gave reversible waves which correspond to Cu(II)–Cu(II)/Cu(II)–Cu(I) and Cu(II)–Cu(I)/Cu(I)–Cu(I) redox processes. Theoretical calculations by means of DFT at the B3LYP level were performed to support the experimental findings. In addition, all the complexes showed effective antioxidant superoxide anion free radical activity at biological pH. The trend of antioxidant superoxide dismutase activity varies in the order 4 > 5 > 2 > 1 > 3.