Octanuclear heterobimetallic {Ni4Ln4} assemblies possessing Ln4 square grid [2×2] motifs : synthesis, structure and magnetism

Octanuclear heterobimetallic complexes, [Ln4Ni4(H3L)4(µ3-OH)4(µ2-OH)4]4Cl·xH2O·yCHCl3 (Dy3+ , x = 30.6, y = 2 (1); Tb3+ , x = 28, y = 0 (2) ; Gd3+ , x = 25.3, y = 0 (3); Ho3+ , x = 30.6, y = 3 (4)) (H5L = N1, N3-bis(6-formyl-2-(hydroxymethyl) -4-methylphenol) diethylenetriamine) are reported. These are assembled by the cumulative coordination action of four doubly deprotonated compartmental ligands, [H3L] 2- , along with eight exogenous –OH ligands. Within the core of these complexes, four Ln3+ are distributed to the four corners of a perfect square grid while four Ni2+ are projected away from the plane of the Ln4 unit. Each of the four Ni2+ possesses distorted octahedral geometry while all the Ln3+ are crystallographically equivalent and are present in an elongated square antiprism geometry. The magnetic properties of compound 3 are dominated by an easy-plane single-ion anisotropy of the Ni2+ ions [DNi = 6.7(7) K] and dipolar interactions between Gd3+ centers. Detailed ac magnetometry reveals the presence of distinct temperature-dependent out-of-phase signals for compounds 1 and 2, indicative of slow magnetic relaxation. Magnetochemical analysis of complex 1 implies the 3d and the 4f metal ions are engaged in ferromagnetic interactions with SMM behavior, while dc magnetometry of compound 2 is suggestive of an antiferromagnetic Ni-Tb spin-exchange with slow magnetic relaxation due to a field-induced level crossing. Compound 4 exhibits an easy-plane single-ion anisotropy for the Ho3+ ions and weak interactions between spin centers

Experimental and theoretical electron density analysis of copper pyrazine nitrate quasi-low-dimensional quantum magnets

The accurate electron density distribution and magnetic properties of two metal-organic polymeric magnets, the quasi-one-dimensional (1D) Cu(pyz)(NO3)2 and the quasi-two-dimensional (2D) [Cu(pyz)2(NO3)]NO3·H2O, have been investigated by high-resolution single-crystal X-ray diffraction and Density Functional Theory calculations on the whole periodic systems and on selected fragments. Topological analyses, based on Quantum Theory of Atoms in Molecules, enabled the characterization of possible magnetic exchange pathways and the establishment of relationships between the electron (charge and spin) densities and the exchange-coupling constants. In both compounds, the experimentally observed anti-ferromagnetic coupling can be quantitatively explained by the Cu-Cu super-exchange pathway mediated by the pyrazine bridging ligands, via a σ-type interaction. From topological analyses of experimental charge-density data, we show for the first time that the pyrazine tilt angle does not play a role in determining the strength of the magnetic interaction. Taken in combination with molecular orbital analysis and spin density calculations, we find a synergistic relationship between spin delocalization and spin polarization mechanisms and that both determine the bulk magnetic behavior of these Cu(II)-pyz coordination polymers

Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S=1 quantum antiferromagnet

The magnetic ground state of the quasi-one-dimensional spin-1 antiferromagnetic chain is sensitive to the relative sizes of the single-ion anisotropy (D) and the intrachain (J) and interchain (J') exchange interactions. The ratios D/J and J'/J dictate the material's placement in one of three competing phases: a Haldane gapped phase, a quantum paramagnet and an XY-ordered state, with a quantum critical point at their junction. We have identified [Ni(HF)2(pyz)_2]SbF6, where pyz = pyrazine, as a rare candidate in which this behavior can be explored in detail. Combining neutron scattering (elastic and inelastic) in applied magnetic fields of up to 10~tesla and magnetization measurements in fields of up to 60~tesla with numerical modeling of experimental observables, we are able to obtain accurate values of all of the parameters of the Hamiltonian [D = 13.3(1)~K, J = 10.4(3)~K and J' = 1.4(2)~K], despite the polycrystalline nature of the sample. Density-functional theory calculations result in similar couplings (J = 9.2~K, J' = 1.8~K) and predict that the majority of the total spin population resides on the Ni(II) ion, while the remaining spin density is delocalized over both ligand types. The general procedures outlined in this paper permit phase boundaries and quantum-critical points to be explored in anisotropic systems for which single crystals are as yet unavailable

Enhancing easy-plane anisotropy in bespoke Ni(II) quantum magnets

We examine the crystal structures and magnetic properties of several S = 1 Ni(II) coordination compounds, molecules and polymers, that include the bridging ligands HF2-, AF62- (A = Ti, Zr) and pyrazine or non-bridging ligands F-, SiF62-, glycine, H2O, 1-vinylimidazole, 4-methylpyrazole and 3-hydroxypyridine. Pseudo-octahedral NiN4F2, NiN4O2 or NiN4OF cores consist of equatorial Ni-N bonds that are equal to or slightly longer than the axial Ni-Lax bonds. By design, the zero-field splitting (D) is large in these systems and, in the presence of substantial exchange interactions (J), can be difficult to discriminate from magnetometry measurements on powder samples. Thus, we relied on pulsed-field magnetization in those cases and employed electron-spin resonance (ESR) to confirm D when J 0) and range from ≈ 8-25 K. This work reveals a linear correlation between the ratio d(Ni-Lax)/d(Ni-Neq) and D although the ligand spectrochemical properties may also be important. We assert that this relationship allows us to predict the type of magnetocrystalline anisotropy in tailored Ni(II) quantum magnets

Antiferromagnetism in a family of S=1 square lattice coordination polymers NiX2(pyz)2 (X=Cl, Br, I, NCS; pyz=Pyrazine)

The crystal structures of NiX2(pyz)2 (X = Cl (1), Br (2), I (3), and NCS (4)) were determined by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN4X2 units that are bridged by pyz ligands. The 2D layered motifs displayed by 1–4 are relevant to bifluoride-bridged [Ni(HF2)(pyz)2]EF6 (E = P, Sb), which also possess the same 2D layers. In contrast, terminal X ligands occupy axial positions in 1–4 and cause a staggered packing of adjacent layers. Long-range antiferromagnetic (AFM) order occurs below 1.5 (Cl), 1.9 (Br and NCS), and 2.5 K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and g factor of 2, 3, and 4 were measured by electron-spin resonance with no evidence for zero–field splitting (ZFS) being observed. The magnetism of 1–4 spans the spectrum from quasi-two-dimensional (2D) to three-dimensional (3D) antiferromagnetism. Nearly identical results and thermodynamic features were obtained for 2 and 4 as shown by pulsed-field magnetization, magnetic susceptibility, as well as their Néel temperatures. Magnetization curves for 2 and 4 calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. Compound 3 is characterized as a 3D AFM with the interlayer interaction (J⊥) being slightly stronger than the intralayer interaction along Ni–pyz–Ni segments (Jpyz) within the two-dimensional [Ni(pyz)2]2+ square planes. Regardless of X, Jpyz is similar for the four compounds and is roughly 1 K

Quantum magnetism in coordination polymers

This thesis presents an experimental and theoretical examination of five polymeric quantum magnets. The first of these is Cu(pyrazine)(glycinate)ClO4, an exchange-coupled spin-dimer system that undergoes a powerful and continuous magnetocaloric effect (MCE) in a rapidly changing magnetic field H. The evolution of the sample temperature T with H must be accounted for in order to reconcile an apparent discrepancy between the results of magnetometry measurements performed in quasistatic and pulsed magnetic fields, and the MCE is likely to be an important consideration for pulsed-field experiments performed on similar insulating materials. Heat capacity measurements of Cu(pyrazine)(glycinate)ClO4 are perturbed by zero-point fluctuations for T > 400 mK, and these data further suggest that this system exhibits possible two-dimensional universal behaviour. The results of single crystal x-ray diffraction measurements of a second material [H2F]2[NiF2(3-fluoropyridine)4]3[SbF6]2 at 100 K indicate that the Ni2+ ions of this complex are arranged on the vertices of a two-dimensional kagome lattice, wherein the spin S = 1 ions are bridged via charge-assisted Ni-F· · · H-F-H· · · F-Ni linkages. However, a density-functional theory study indicates that a positional disorder of the H2F+ moieties within these bridges suppresses the intraplane spin-exchange interactions. Powder muon spin-rotation measurements imply that the system is paramagnetic for T > 19 mK, while polycrystalline electron spin-resonance (ESR), magnetization M(H), and heat capacity experiments together indicate that the unixial and rhombohedral single-ion anisotropy of the Ni2+ ions are approximately D/kb = 8.3(4) K and E/kb = 1.2(3) K respectively. Lastly, neutron powder diffraction measurements of three isotructural compounds [M(HF2)(pyrazine)2]SbF6 (M = Cu2+, Ni2+ or Co2+) reveal that each system is tetragonal (P4/nmm) and that the spin-exchange interactions facilitated by the pyrazine (Jpyz) and bifluoride (Jfhf) ligands are antiferromagnetic. The Cu2+ congener is a quasi-two-dimensional Heisenberg S = 1/2 antiferromagnet, which displays an ordered moment of 0.6(1)μb per ion that is reduced from its paramagnetic value by quantum fluctuations. For the S = 1 Ni2+ complex, powder M(H) measurements suggest that D has an easy-plane character while inelastic neutron scattering experiments determine D/kb = 13.3(3) K, Jfhf/kb = 10.4(3) K and Jpyz/kb = 1.4(2) K. The S = 3/2 Co2+ system adopts an Ising-like antiferromagnetic ground state below 7.1(1) K, and its magnetic properties are parameterized with an effective spin-1/2 Hamiltonian for T < 50 K

A single-ion magnet based on a heterometallic CoIII2DyIII complex

We report a CoIII2DyIII complex, which shows single-ion-magnet behaviour. AC susceptibility data of this compound reveals the presence of slow relaxation of the magnetization in zero-field below 15 K. The relaxation barrier is 88 K

Heterometallic trinuclear {CoIII2LnIII} (Ln = Gd, Tb, Ho and Er) complexes in a bent geometry. Field-induced single-ion magnetic behavior of the ErIIIand TbIIIanalogues

Through the use of a multi-site compartmental ligand, 2-methoxy-6-[{2-(2-hydroxyethylamino)ethylimino}methyl]phenol (LH3), the family of heterometallic, trinuclear complexes of the formula [CoIII2Ln(L)2(μ-O2CCH3)2(H2O)3]·NO3·xMeOH·yH2O has been expanded beyond Ln = DyIII to include GdIII (1), TbIII (2), HoIII (3) and ErIII (4) for 1, 3 and 4 (x = 1; y = 1) and for 2 (x = 0; y = 2). The metallic core of these complexes consists of a (CoIII–LnIII–CoIII) motif bridged in a bent geometry resulting in six-coordinated distorted CoIII octahedra and nine-coordinated LnIII monocapped square-antiprisms. The magnetic characterization of these compounds reveals the erbium and terbium analogues to display a field induced single-ion magnetic behavior similar to the dysprosium analogue but at lower temperatures. The energy barrier for the reversal of the magnetization of the CoIII2TbIII analogue is Ueff ≥ 15.6(4) K, while for the CoIII2ErIII analogue Ueff ≥ 9.9(8) K. The magnetic properties are discussed in terms of distortions of the 4f electron cloud