Molecular enneanuclear CuII phosphates containing planar hexanuclear and trinuclear sub-units: syntheses, structures, and magnetism

Highly symmetric enneanuclear copper(II) phosphates [Cu9(Pz)6(μ-OH)3(μ3-OH)(ArOPO3)4(DMF)3] (PzH =pyrazole, Ar = 2,6-(CHPh2)2-4-R-C6H2; R = Me, 2MeAr; Et, 2EtAr; iPr, 2iPrAr; and Ar = 2,6-iPr2C6H3, 2Dip) comprising nine copper(II) centers and pyrazole, hydroxide and DMF as ancillary ligands were synthesized by a reaction involving the arylphosphate monoester, 1, copper(I)chloride, pyrazole, and triethylamine in a 4 : 9 : 6 : 14 ratio. All four complexes were characterized by single crystal structural analysis. The complexes contain two distinct structural motifs within the multinuclear copper scaffold: a hexanuclear unit and a trinuclear unit. In the latter, the three Cu(II) centres are bridged by a µ3-OH. Each pair of Cu(II) centers in the trinuclear unit are bridged by a pyrazole ligand. The hexanuclear unit is made up of three dinuclear Cu(II) motifs where the two Cu(II) centres are bridged by an -OH and a pyrazole ligand. The three dinuclear units are connected to each other by phosphate ligands. The latter also aid the fusion of the trinuclear and the hexanuclear motifs. Magnetic studies reveal a strong antiferromagnetic exchange between the Cu(II) centres of the dinuclear units in the hexanuclear part and a strong spin frustration in the trinuclear part leading to a degenerate ground state

Influence of ligand field on magnetic anisotropy in a family of pentacoordinate Co complexes

International audienceA family of mononuclear penta-coordinated CoII complexes, [Co(L)Cl2]·CH3OH (1), [Co(L)Br2] (2) and [Co(L)(NCS)2] (3) (where L is 1-mesityl-N,N-bis(pyridin-2-ylmethyl)methanamine) were synthesized and characterized. In these complexes, the neutral non-planar ligand, L, binds to three coordination sites around the metal center while two others are bound by anionic halide/pseudo halide ligands. The coordination geometry of the complexes is dictated by the coordinated anionic ligands. Thus, the coordination geometry around the metal ion is distorted trigonal bipyramidal for complexes 1 and 3, while it is distorted square pyramidal for complex 2. Ab initio CASSCF/NEVPT2 calculations on the complexes reveal the presence of an easy plane magnetic anisotropy with the D and E/D values being, 13.3 and 0.14 cm-1 for 1; 36.1 and 0.24 cm-1 for 2 and ±8.6 and 0.32 cm-1 for 3. These values are in good agreement with the values that were extracted from the experimental DC data. AC magnetic measurements reveal the presence of a field-induced slow relaxation of magnetization. However, clear maxima in the out-of-phase susceptibility curves were not observed for 1 and 3. For complex 2, peak maxima were observed when the measurements were carried out under an applied field of 1400 Oe which allowed an analysis of the dynamics of the slow relaxation of magnetization. This revealed that the relaxation is mainly controlled by the Raman and direct processes with the values of the parameters found to be B = 0.77(15) s-1 K-6.35, n = 6.35(12) and A = 3.41(4) × 10-10 s-1 Oe-4 K-1 and m = 4 (fixed). The ab initio calculation which showed the multifunctional nature of the electronic states of the complexes justifies the absence of zero-field SIM behaviour of the complexes. The magnitude and sign of the D and E values and their relationship with the covalency of the metal-ligand bonds was analysed by the CASSCF/NEVPT2 as well as AILFT calculations

Slow Magnetic Relaxation in Dinuclear CoY Complexes

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Magnetocaloric effect and slow magnetic relaxation in peroxide-assisted tetranuclear lanthanide assemblies

The synthesis, structure and magnetism of tetranuclear complexes, Ln(4)(LH2)(2)(eta(1)-Piv)(2)(eta(2)-Piv)(2)(mu(3)-O-2)(2)(H2O)(2)center dot 2MeOH (1, Ln = Gd-III; 2,Ln = Tb-III; 3, Ln = Dy-III; and 4, Ln = Er-III) (LH4 = 6-((bis(2-hydroxyethyl)amino)-N '-(2-hydroxybenzylidene)picolinohydrazide) (piv = pivalate ion) are reported. Interestingly, the formation of these tetranuclear assemblies is assisted by two side-on coordinating peroxide ligands which bind in a mu(3)-eta(2):eta(2):eta(2) fashion. DC magnetic behaviour revealed the existence of a dominating but weak intramolecular antiferromagnetic interaction in the cases of 1, 2, and 4, while a small structural change in complex 3 turns this into a ferromagnetic interaction. We have investigated the magnetothermal behaviour of 1 which shows a magnetocaloric effect with the value of maximum entropy change of -Delta S-m = 33.60 J K-1 kg(-1) at 4 K (Delta H = 13-0 T). This is close to the calculated value of 36.45 J K-1 kg(-1). The ac magnetic susceptibility study confirms zero-field out-of-phase magnetic susceptibility signals only for 3 which get well resolved on the application of a 1 kOe dc magnetic field. The energy barrier for the Orbach process of spin reversal was found to be 23 K (tau(0) = 9 x 10(-7) s). We also report detailed theoretical studies to rationalize and understand the observed magnetic behaviour

Azide-Coordination in Homometallic Dinuclear Lanthanide(III) Complexes Containing Nonequivalent Lanthanide Metal Ions: Zero-Field SMM Behavior in the Dysprosium Analogue

International audienceA series of homometallic dinuclear lanthanide complexes containing nonequivalent lanthanide metal centers [Ln(LH)(LH)(CHOH)(N)]MeOHHO [, Ln = Dy, = 0, = 2; , Ln = Tb, = 1, = 1] have been synthesized [LH = 6-((bis(2-hydroxyethyl)amino)-'-(2-hydroxybenzylidene)picolinohydrazide] and characterized. The dinuclear assembly contains two different types of nine-coordinated lanthanide centers, because the nonequivalent binding of the azide co-ligand as well as the varying coordination of the deprotonated Schiff base ligand. Detailed magnetic studies have been performed on the complexes and . Complex and its diluted analogue () are zero-field SMMs with effective energy barriers () of magnetization reversal equal to 59(3) K and 66(3) K and relaxation times of τ = 10(4) × 10 s and 10(4) × 10 s, respectively. On the other hand, complex shows a field-induced SMM behavior. Combined and density functional theory calculations were performed to explain the experimental findings and to unravel the nature of the magnetic anisotropy, exchange-coupled spectra, and magnetic exchange interactions between the two lanthanide centers

Exploring Tuning of Structural and Magnetic Properties by Modification of Ancillary β‑Diketonate Co-ligands in a Family of Near-Linear Tetranuclear Dy^III Complexes

Three tetranuclear Dy^III complexes, [Dy₄­(LH)₂­(CH₃­OH)₄­(acac)₆] (<b>1</b>), [Dy₄­(LH)₂­(CH₃­OH)₄­(hmacac)₆]·​2CH₃OH (<b>2</b>), and [Dy₄­(LH)₂­(CH₃­OH)₄­(dpacac)₆]·​2CHCl₃·​2CH₃OH·​2H₂O (<b>3</b>), have been synthesized and characterized [LH₄ = (2<i>E</i>,​<i>N</i>′<i>E</i>)-<i>N</i>′-(2,3-dihy­droxy­benzyl­idene)-2-(hy­droxy­imino)­propane­hydra­zide; acacH = acetyl­acetone; hmacacH = 2,2,6,6-tetra­methyl-3,5-heptane­dione; dpacacH = dibenzoyl­methane]. The structural elucidation of these complexes reveals two types of Dy^III centers in terms of the number of ancillary β-diketonate co-ligands coordinated to the metal centers. Detailed magnetic studies have been carried out on <b>1</b>–<b>3</b> which reveal a slow relaxation of magnetization at low temperatures. The relaxation of complexes <b>2</b> and <b>3</b> is distributed in three temperature ranges: lower temperature process, transition range, and higher temperature process. In the higher temperature range, the best fitting of the data for <b>2</b> yields τ₀ = (6.3 ± 3.6) × 10^–6 s and <i>U</i>_eff = (23.8 ± 4.0) K, and for <b>3</b>, τ₀ = (9.4 ± 5.9) × 10^–6 s, <i>U</i>_eff = (29.0 ± 6.3) K

Structural Diversity in Supramolecular Organization of Anionic Phosphate Monoesters: Role of Cations

Syntheses and structures of anionic arylphosphate monoesters [ArOP­(O)2(OH)]− (Ar = 2,6-CHPh2-4-R-C6H2; R = Me/Et/iPr/tBu) with different counter cations are reported. The counter cations were varied systematically: imidazolium cation, 2-methyl imidazolium cation, N-methyl imidazolium cation, N,N′-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]­octan-1-ium cation, 4,4′-bipyridinium dication, and magnesium­(II) dication. The objective was to examine if the supramolecular structure of anionic arylphosphate monoesters could be modulated by varying the cation. It was found that an eight-membered P2O4H2-hydrogen-bonded dimeric motif involving intermolecular H-bonding between the [P­(O)­(OH)] unit of the anionic phosphate monoester along with the counter cation is formed with 2-methyl imidazolium cation, N-methyl imidazolium cation, N,N′-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]­octan-1-ium cation, and magnesium­(II) dication; both discrete and polymeric H-bonded structures are observed. In the case of imidazolium cations and 1,4-diazabicyclo[2.2.2]­octan-1-ium cation, the formation of one-dimensional polymers (single lane/double lane) was observed. On the other hand, two types of phosphate motifs, intermolecular H-bonded dimer and an open-form, were observed in the case of 4,4′-bipyridinium dication