Slow magnetic relaxation of mononuclear complexes based on uncommon Kramers lanthanide ions CeIII, SmIII and YbIII.

Based on the uncommon Kramers ions CeIII, SmIII and YbIII, complexes [Ce(dppbO2)2Cl3] (1, dppbO2 = 1,2-bis(diphenylphosphino)benzene dioxide), [Sm(dppbO2)2Cl3] (2) and [Yb(dppbO2)2Cl2]Cl (3) toward single-ion magnets were obtained and fully characterized. Complexes 1 and 2 are isostructural seven-coordinate with distorted geometries between a capped trigonal prism and a capped octahedron, while 3 is six-coordinate with an octahedral geometry. Dynamic magnetic property measurements reveal their field induced slow magnetic relaxation behaviours. Fits to the temperature dependent relaxation time (τ) result in effective barriers of 38(2), 11.7(2) and 20.2(6) cm-1 for 1, 2 and 3, respectively, as well as relatively low Raman exponents (3.4(2) for 1 and 2.1(1) for 3). Ab initio calculations were then performed and they indicated that the first excited Kramers doublets (KDs) for 1, 2 and 3 lie at ca. 291, 63.5 and 137 cm-1, respectively, which are much higher than the fitting results. Combining the significant transverse magnetic moments between the ground state KDs and the first excited KDs, we thus attribute the dominant relaxation pathway of the three compounds to a Raman process, while quantum tunnelling of magnetization and direct relaxation processes may coexist. In this case, the obtained effective barriers in a high temperature region for 1 and 3 are actually the energies of the vibrational modes, which lead to second-order Raman processes with exponential temperature dependence.

Stereoisomeric coordination polymers based on facial and meridional six-coordinate dysprosium(III ).

Due to the small differences in the chemical properties of facial (fac) and meridional (mer) stereoisomers, selective synthesis of one of the isomers is challenging, especially for lanthanide complexes. By using a flexible bidentate phosphine oxide ligand, we managed to isolate three stereoisomeric 2D and 3D coordination polymers, in which six-coordinate Dy(III) ions possess fac- or mer-Cl3O3 coordination environments. Structural studies indicate that the stereochemistry differences result from their various supramolecular interactions (e.g., hydrogen bonding and π⋯π stacking). Magnetic property measurements reveal the different static and dynamic magnetic behaviours of the three stereoisomers. Ab initio CASSCF calculations were then performed which indicated that their distinct magnetic behaviours arise from their fac/mer configurations. Compared to fac-Dy(III), mer-Dy(III) possesses more axial ground-state KDs and higher first excited KDs.

Phosphine Oxide Ligand Based Tetrahedral CoII Complexes with Field-induced Slow Magnetic Relaxation Behavior Modified by Terminal Ligands.

Two isostructural mononuclear CoII complexes, [Co(xantpo)(NCE)2 ] (E=S (1) and O (2); xantpo=9,9-dimethyl-4,5-bis(diphenylphosphoryl) xanthene), supported by a bidentate phosphine oxide ligand are reported. The cobalt complexes exhibit characteristic tetrahedral structures coordinated with two oxygen and two nitrogen atoms. Magnetic property measurements show their similar static magnetic behaviours but very different dynamic magnetic behaviours. Both complexes show field-induced slow magnetic relaxation behaviours, but the relaxation of 2 is much slower than that of 1. Fittings to the magnetic data and ab initio CASSCF calculations reveal significant changes in the zero field splitting (ZFS) parameters (D and E), which can be attributed to the small geometrical changes of the Co ions and the different ligand field strength of the two terminal ligands.

Salt metathesis routes to homoleptic near-linear Mg(ii) and Ca(ii) bulky bis(silyl)amide complexes.

Herein we investigate salt metathesis reactions of bulky bis(silyl)amide potassium salts [K{N(SiMe2tBu)2}]n, [K{N(SiMe2tBu)(SiiPr3)}]n, [K{N(SiiPr3)2}]n and [K{N(SiPh3)2}]n (1) with AeI2 (Ae = Mg, Ca, Sr) in an effort to synthesise rare homoleptic two-coordinate complexes. Recrystallization of 1 from THF provided structural authentication of the solvated adduct [K{N(SiPh3)2}(THF)3] (1-THF). For Ae = Mg we were able to identify products for three of the four bis(silyl)amides investigated: [Mg{N(SiMe2tBu)2}2(THF)] (2), [Mg{N(SiMe2tBu)(SiiPr3)}2] (3) and [Mg{N(SiPh3)2}2] (4). Complexes 2-4 were isolated in good yields, and on one occasion during recrystallization of 2 a crystal of the heteroleptic complex [Mg{N(SiMe2tBu)2}(I)(THF)2] (5) was identified by XRD. We found that these methodologies were not widely applicable for larger Ae = Ca and Sr, as large quantities of the parent bis(silyl)amines formed in reaction mixtures; however, we were able to isolate the first near-linear Ca(ii) amide [Ca{N(SiiPr3)2}2] (6) from these studies. No Ae-containing products could be isolated from KC8/crown ether reductions of 3, 4 and 6. Complexes 1-4 and 6 were characterised by single crystal XRD, multinuclear NMR and FTIR spectroscopy and elemental analysis, whereas complex 5 was identified by single crystal XRD only.

Chromium chains as polydentate fluoride ligands for actinides and group IV metals.

The reactions of {Cr6} horseshoe chains {[nPr2NH2]3[Cr6F11(O2CtBu)10]}2, 1 and precursors of actinides and group IV metals led to a series of ring complexes [nPr2NH2][Cr7TiF6O2(O2CtBu)16], 2, [nPr2NH2][Cr6Ti2F5O3(O2CtBu)16], 3, [Cr6ThF7(O2CtBu)15 (Me2SO)], 4, [(nPr2NH2)2(Cr6Th2F12(O2CtBu)16)], 5 and [nPr2NH2][Cr6U2O2F8(O2CtBu)16(Me2SO)], 6. X-ray structure studies indicate that the {Cr6} chains maintain their structures in these complexes, acting as polydentate fluoride ligands. Their static magnetic properties were measured and fitted by isotropic exchange Hamiltonian. In accordance with 1, the magnetic exchanges between CrIII are antiferromagnetic, while the exchange interactions can be modified by the tetravalent metals. For compound 6, ferromagnetic exchanges JCr-U and JU-U are obtained. EPR spectra of compounds 2-5 were measured at Q band and were simulated. The spectrum of 2 has the same profile as {Cr7Cd} and {Cr7Zn} rings with a ground state S = 3/2. 3, 4 and 5 give similar EPR spectra with S = 0 ground states.

Reversible crystal-to-crystal transformation from a trinuclear cluster to a 1D chain and the corresponding spin crossover (SCO) behaviour change.

Reversible crystal-to-crystal transformation between a linear trinuclear Fe(ii) complex [Fe3(NH2-trz)6(SCN)5(H2O)] (SCN)·4H2O (1, NH2-trz = 4-amino-1,2,4-triazole) and a 1D chain [Fe3(NH2-trz)6(SCN)5]n(SCN)n (1a) and the SCO behaviour change have been studied by X-ray single-crystal diffraction, magnetic measurements and DSC. Complex 1a exhibits one more SCO step at a low temperature.

Field and dilution effects on the magnetic relaxation behaviours of a 1D dysprosium(iii)-carboxylate chain built from chiral ligands.

A one-dimensional dysprosium(iii)-carboxylate chain in which the Dy(III) ions sit in a pseudo D(2d)-symmetry environment is synthesized and shows different slow magnetic relaxation behaviours depending on the field and dilution effects. Besides, the chiral ligand introduces the additional functions of the Cotton effect and polarization for this compound.

Polymeric Perturbation to the Magnetic Relaxations of the C2v-Symmetric [Er(Cp)2(OBu)2](-) Anion.

To test the coordination symmetry effect on the magnetization-reversal barrier trend of Er(III)-based single-ion magnets, the C2v-symmetric organolanthanide anion [Er(Cp)2(O(t)Bu)2](-) has been incorporated with different countercations, resulting in two structures, namely, the discrete [K2(Cp)(18-C-6)2][Er(Cp)2(O(t)Bu)2] (1) and the polymeric [ErK2(Cp)3(O(t)Bu)2(THF)2]n (2), where 18-C-6 = 18-crown-6 ether and Cp = cyclopentadienide. Surprisingly, the polymeric 2 exhibits much stronger field-induced magnetization relaxing behavior compared to the monomeric 1. Such disparate dynamic magnetism is attributable to the subtle coordination environmental perturbations of the central Er(III) ions.

Disklike hepta- and tridecanuclear cobalt clusters. Synthesis, structures, magnetic properties, and DFT calculations.

The synthesis, structure and magnetic properties are reported of two disklike mixed-valence cobalt clusters [Co(III)Co(II)6(thmp)2(acac)6(ada)3] (1) and [Co(III)2Co(II)11(thmp)4(Me3CCOO)4(acac)6(OH)4(H2O)4](Me3CCOO)2·H2O (2). Heptanuclear complex 1 was prepared by solvothermal reaction of cobalt(II) acetylacetonate (Co(acac)2), 1,1,1-tris(hydroxymethyl)-propane (H3thmp), and adamantane-1-carboxylic acid (Hada), whereas by substituting Hada with Me3CCO2H, tridecanuclear complex 2 was obtained with an unexpected [Co(III)2Co(II)11] core. The core structures of 1 and 2 are related to each other: that of 1 arranges as a centered hexagon of a central Co(III) ion surrounded by a [Co(II)6] hexagon, while that of 2 can be described as a larger oligomer based on two vertex-sharing [Co(III)Co(II)6] clusters. Variable-temperature direct-current magnetic susceptibility measurements demonstrated overall ferromagnetic coupling between the Co(II) ions within both clusters. The magnetic exchange (J) and magnetic anisotropy (D) values were quantified with appropriate spin-Hamiltonian models and were also supported by density functional theory calculations. The presence of frequency-dependent out-of-phase (χM″) alternating current susceptibility signals at temperatures below 3 K suggested that 2 might be a single-molecule magnet.

Switching of the magnetocaloric effect of Mn(II) glycolate by water molecules.

The transformation of Mn(II) glycolates (glc) between the three-dimensional coordination polymer [Mn(glc)2]n (1) and discrete mononuclear phase [Mn(glc)2 (H2O)2] (2) can be reversibly switched by water molecules, which dramatically change the magnetocaloric effect (MCE) of Mn(II) glycolates from the maximum of 6.9 J kg(-1) K(-1) in 1 to 60.3 J kg(-1) K(-1) in 2. This case example reveals that the effect of magnetic coupling on MCE plays a dominant role over that of other factors such as magnetic density for 3d-type magnetic refrigerants.

Cu(II)-Gd(III) cryogenic magnetic refrigerants and Cu8Dy9 single-molecule magnet generated by in situ reactions of picolinaldehyde and acetylpyridine: experimental and theoretical study.

A series of heterometallic [Ln(III)(x)Cu(II)(y)] complexes, [Gd2Cu2]n (1), [Gd4Cu8] (2), [Ln9Cu8] (Ln=Gd, 3·Gd; Ln=Dy, 3·Dy), were successfully synthesized by a one-pot route at room temperature with three kinds of in situ carbonyl-related reactions: Cannizzaro reaction, aldol reaction, and oxidation. This strategy led to dysprosium analogues that behaved as single-molecule magnets (SMMs) and gadolinium analogues that showed significant magnetocaloric effect (MCE). In this study a numerical DFT approach is proposed by using pseudopotentials to calculate the exchange coupling constants in three polynuclear [Gd(x)Cu(y)] complexes; with these values exact diagonalization or quantum Monte Carlo simulations have been performed to calculate the variation of the magnetic entropy involved in the MCE. For the [Dy9Cu8] complexes, local magnetic properties of the Dy(III) centers have been determined by using the CASSCF+RASSI method.

Anion-templated assembly and magnetocaloric properties of a nanoscale {Gd38} cage versus a {Gd48} barrel.

The comprehensive study reported herein provides compelling evidence that anion templates are the main driving force in the formation of two novel nanoscale lanthanide hydroxide clusters, {Gd38(ClO4)6} (1) and {Gd48Cl2(NO3)} (2), characterized by single-crystal X-ray crystallography, infrared spectroscopy, and magnetic measurements. {Gd38(ClO4)6}, encapsulating six ClO4(-) ions, features a cage core composed of twelve vertex-sharing {Gd4} tetrahedrons and one Gd⋅⋅⋅Gd pillar. When Cl(-) and NO3(-) were incorporated in the reaction instead of ClO4(-), {Gd48Cl2(NO3)} is obtained with a barrel shape constituted by twelve vertex-sharing {Gd4} tetrahedrons and six {Gd5} pyramids. What is more, the cage-like {Gd38} can be dynamically converted into the barrel-shaped {Gd48} upon Cl(-) and NO3(-) stimulus. To our knowledge, it is the first time that the linear M-O-M' fashion and the unique µ8-ClO4(-) mode have been crystallized in pure lanthanide complex, and complex 2 represents the largest gadolinium cluster. Both of the complexes display large magnetocaloric effect in units of J kg(-1) K(-1) and mJ cm(-3) K(-1) on account of the weak antiferromagnetic exchange, the high N(Gd)/M(W) ratio (magnetic density), and the relatively compact crystal lattice (mass density).

Unprecedented ferromagnetic dipolar interaction in a dinuclear holmium(III) complex: a combined experimental and theoretical study.

A ferromagnetic intramolecular dipolar interaction is observed and verified by both experimental studies and ab initio calculations on a dinuclear holmium(III) complex [Ho2(H2cht)2Cl4(H2O)(MeCN)]·MeCN.

Gadolinium(III)-hydroxy ladders trapped in succinate frameworks with optimized magnetocaloric effect.

Two kinds of inorganic gadolinium(III)-hydroxy "ladders", [2×n] and [3×n], were successfully trapped in succinate (suc) coordination polymers, [Gd2(OH)2(suc)2(H2O)]n·2nH2O (1) and [Gd6(OH)8(suc)5(H2O)2 ]n·4n H2O (2), respectively. Such coordination polymers could be regarded as alternating inorganic-organic hybrid materials with relatively high density. Magnetic and heat capacity studies reveal a large cryogenic magnetocaloric effect (MCE) in both compounds, namely (ΔH=70 kG) 42.8 J kg(-1) K(-1) for complex 1 and 48.0 J kg(-1) K(-1) for complex 2. The effect of the high density is evident, which gives very large volumetric MCEs up to 120 and 144 mJ cm(-3) K(-1) for complexes 1 and 2, respectively.

Wheel-shaped nanoscale 3d-4f {Co(II)16Ln(III)24} clusters (Ln = Dy and Gd).

Two unprecedented wheel-shaped nanoscale clusters {Co(II)16Ln(III)24} (Ln = Dy and Gd), with a diameter and a thickness of 3.0 nm and 2.0 nm, respectively, were obtained from the self-assembly of Co(NO3)2, Ln(NO3)3 and a pyridyl-functionalized ß-diketone ligand. Notably, the gadolinium species exhibited a relatively large magnetocaloric effect.

Fluorescent single-ion magnets: molecular hybrid (HNEt3)[DyxYb1-x(bpyda)2] (x = 0.135-1).

Four single-phase isostructural mononuclear complexes (HNEt3)[DyxYb1-x(bpyda)2]·3H2O (x = 1 (1), 0 (2), 0.367 (3), 0.135 (4), bpyda = 2,2'-bipyridine-6,6'-dicarboxylate) show characteristics of controllable slow relaxation and photoluminescence. The molecular hybrids 3 and 4 exhibit the Orbach process for the DyIII component and a mixture of direct and Raman processes for the YbIII component. The presence of paramagnetic YbIII enhances the relaxation time of the DyIII component, originating from the suppression of the direct process/QTM. The correlation between photoluminescence, static susceptibilities and dynamic magnetic relaxation in the hybrid species was also analysed by comparing the energy gap between the ground and first excited states to the energy barrier.

Lanthanide oxide clusters: from tetrahedral [Dy4(µ4-O)](10+) to supertetrahedral [Ln20(µ4-O)11]38+ (Ln = Tb, Dy, Ho, Er).

Supertetrahedral clusters: A family of lanthanide oxide supertetrahedral T3{Ln20} clusters (Ln = Tb, Dy, Ho, Er; see figure) were obtained from the solvothermal reaction of lanthanide(III) salts with polytriazolate ligands that could be methylated and oxidized in situ.

Two 3d-4f nanomagnets formed via a two-step in situ reaction of picolinaldehyde.

Two 3d-4f nanomagnets, [Dy(III)2Cu(II)7] (1) and [Gd(III)6Cu(II)12] (2), are synthesized under a two-step in situ reaction of picolinaldehyde. Not only the final adduct (hemiacetal), but also the intermediate (gem-diol) are "visualized" by X-ray single crystal diffraction. The Dy complex behaves as a single-molecule magnet, while the Gd complex exhibits a significant magnetocaloric effect. Theoretical calculations are employed to obtain the orientation of the magnetic moments and the magnetic exchange.

High symmetry superoctahedron cluster [Mn(III)18O14]26+ from the use of N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine.

The use of flexible N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine has provided one new high symmetry superoctahedron with unprecedented aesthetically-pleasing Mn(III)18 clusters, which exhibits out-of-phase signals indicative of the slow magnetic relaxation.