Jamie L. Manson

Strong H...F hydrogen bonds as synthons in polymeric quantum magnets: structural, magnetic, and theoretical characterization of [Cu(HF2)(pyrazine)2]SbF6, [Cu2F(HF)(HF2)(pyrazine)4](SbF6)2, and [CuAg(H3F4)(pyrazine)5](SbF6)2.

Three Cu(2+)-containing coordination polymers were synthesized and characterized by experimental (X-ray diffraction, magnetic susceptibility, pulsed-field magnetization, heat capacity, and muon-spin relaxation) and electronic structure studies (quantum Monte Carlo simulations and density functional theory calculations). [Cu(HF(2))(pyz)(2)]SbF(6) (pyz = pyrazine) (1a), [Cu(2)F(HF)(HF(2))(pyz)(4)](SbF(6))(2) (1b), and [CuAg(H(3)F(4))(pyz)(5)](SbF(6))(2) (2) crystallize in either tetragonal or orthorhombic space groups; their structures consist of 2D square layers of [M(pyz)(2)](n+) that are linked in the third dimension by either HF(2)(-) (1a and 1b) or H(3)F(4)(-) (2). The resulting 3D frameworks contain charge-balancing SbF(6)(-) anions in every void. Compound 1b is a defective polymorph of 1a, with the difference being that 50% of the HF(2)(-) links are broken in the former, which leads to a cooperative Jahn-Teller distortion and d(x(2))(-y(2)) orbital ordering. Magnetic data for 1a and 1b reveal broad maxima in chi at 12.5 and 2.6 K and long-range magnetic order below 4.3 and 1.7 K, respectively, while 2 displays negligible spin interactions owing to long and disrupted superexchange pathways. The isothermal magnetization, M(B), for 1a and 1b measured at 0.5 K reveals contrasting behaviors: 1a exhibits a concave shape as B increases to a saturation field, B(c), of 37.6 T, whereas 1b presents an unusual two-step saturation in which M(B) is convex until it reaches a step near 10.8 T and then becomes concave until saturation is reached at 15.8 T. The step occurs at two-thirds of M(sat), suggesting the presence of a ferrimagnetic structure. Compound 2 shows unusual hysteresis in M(B) at low temperature, although chi vs T does not reveal the presence of a magnetic phase transition. Quantum Monte Carlo simulations based on an anisotropic cubic lattice were applied to the magnetic data of 1a to afford g = 2.14, J = -13.4 K (Cu-pyz-Cu), and J(perpendicular) = -0.20 K (Cu-F...H...F-Cu), while chi vs T for 1b could be well reproduced by a spin-1/2 Heisenberg uniform chain model for g = 2.127(1), J(1) = -3.81(1), and zJ(2) = -0.48(1) K, where J(1) and J(2) are the intra- and interchain exchange couplings, respectively, which considers the number of magnetic nearest-neighbors (z). The M(B) data for 1b could not be satisfactorily explained by the chain model, suggesting a more complex magnetic structure in the ordered state and the need for additional terms in the spin Hamiltonian. The observed variation in magnetic behaviors is driven by differences in the H...F hydrogen-bonding motifs.

Crystal structures and magnetic properties of Mn[N(CN)2]2L {L=2,5-dimethylpyrazine and aminopyrazine}

Abstract In this paper, the crystal structures and preliminary magnetic properties of two new molecule-based magnets, Mn[N(CN)2]2(H2O)2(2,5-me2pyz)2 (1) and Mn[N(CN)2]2(H2O)(NH2-pyz)1.5 (2) (pyz=pyrazine) are reported. The structure of 1 consists of 2D Mn[N(CN)2]2(H2O)2 layers which are hydrogen bonded together by the 2,5-me2pyz ligands while that of 2 forms a novel 1D ladder structure with μ1,5-[N(CN)2]− rungs. Magnetically, 1 exhibits a spin canted phase transition at 1.78 K while 2 remains paramagnetic down to 1.7 K.

Crystal structure and magnetic properties of [Fe{N(CN)2}2(MeOH)2] a 2-D layered network consisting of hydrogen-bonded 1-D chains

A novel 2-D layered network structure [Fe{N(CN) 2 } 2 (MeOH) 2 ] was synthesized and characterized by X-ray crystallography, vibrational, and magnetic susceptibility. The neutral 2-D stair-like framework consists of hydrogen-bonded infinite 1-D {Fe[N(CN) 2 ] 2 } ribbons that pack in a staggered arrangement where nearest-neighboring chains are slipped a/2 relative to one another. Two methanol molecules are coordinated to the Fe II center via the oxygen atoms in a trans configuration resulting in a compressed FeN 4 O 2 octahedron. Hydrogen-bond interactions occur via N(2)‥H(1)-O(1) where N(2) is the amide nitrogen atom of a nearby ribbon. The magnetic susceptibility was interpreted according to an S=2 expression which includes the Weiss constant and zero-field splitting giving g=2.04, θ=–2.0 K, and D=–1.7 K. Intrachain exchange interactions were determined from a fit to an S=2 antiferromagnetic chain model leading to g=2.04 and J/k B =–0.23 K. Further interchain interaction via the hydrogen bond was determined by incorporation of a molecular-field correction term yielding J′/k B =–0.02 K indicating very weak antiferromagnetic coupling between chains.

Experimentally determining the exchange parameters of quasi-two-dimensional Heisenberg magnets

Though long-range magnetic order cannot occur at temperatures T > 0 in a perfect two-dimensional (2D) Heisenberg magnet, real quasi-2D materials will invariably possess nonzero inter-plane coupling J? driving the system to order at elevated temperatures. This process can be studied using quantum Monte Carlo calculations. However, it is difficult to test the results of these calculations experimentally since for highly anisotropic materials in which the in-plane coupling is comparable with attainable magnetic fields J? is necessarily very small and inaccessible directly. In addition, because of the large anisotropy, the Neel temperatures are low and difficult to determine from thermodynamic measurements. Here, we present an elegant method of assessing the calculations via two independent experimental probes: pulsed-field magnetization in fields of up to 85T, and muon-spin rotation.

Cluster glass state and photoinduced effects on the freezing dynamics in KxCo[Fe(Cn)6]y⋅ZH2O (x∼0.16, y∼0.72, z∼4.4)

The magnetic properties and photoinduced magnetization of a Prussian blue analog, KxCo[Fe(Cn)6]y⋅ZH2O (x∼0.16, y∼0.72, z∼4.4) were systematically studied. The frequency dependence of the linear ac susceptibility, the irreversibility in the field-cooled/zero-field-cooled magnetization (MFC/MZFC), and the relaxation of MZFC suggest a cluster glass behavior. Illumination with red light leads to an increase in the magnetic irreversibility. While MFC is significantly increased after illumination, MZFC at low temperatures is decreased. The observed photoinduced magnetic effects are explained within a cluster glass model. The photoinduced increase in the concentration of spins leads to a shift of dynamics toward longer length and time scales, resulting in freezing of spin clusters at a higher temperature.

Pressure‐Induced Sequential Orbital Reorientation in a Magnetic Framework Material

Pressure has been applied to manipulate the structure-property relationship of the copper(II)-based coordination network CuF{sub 2}(H{sub 2}O){sub 2}(pyz) (pyz=pyrazine). The elongated Jahn-Teller axis was found to switch sequentially from the NCuN ( 3.1 GPa; see picture). This orbital reordering leads to a drastic change in the magnetic properties, whereby the magnetic structure changes from two-dimensional to one-dimensional above 0.9 GPa.

Mechanical properties of a metal–organic framework containing hydrogen-bonded bifluoride linkers

We report the mechanical properties of a framework structure, [Cu2F(HF)(HF2)(pyz)4][(SbF6)2]n (pyz = pyrazine), in which [Cu(pyz)2](2+) layers are pillared by HF2(-) anions containing the exceptionally strong F-H···F hydrogen bonds. Nanoindentation studies on single-crystals clearly demonstrate that such bonds are extremely robust and mechanically comparable with coordination bonds in this system.

MnII(N3)2(pyrazine). A 2-D layered structure consisting of ferromagnetically coupled 1-D {Mn(µ-1,1-N3)2}n chains

Mn(N3)2(pyz) (pyz = pyrazine) consists of ferromagnetically coupled linear chains of {Mn(N3)2}n comprised of µ-1,1-azido bridges together with µ-pyz ligands to afford 2-D planar layers.

Long-range magnetic order in the quasi-1D metalloporphyrin family of molecule-based magnets

Abstract The unusual magnetic properties of various members of the quasi-1D manganese-porphyrin family are reviewed. Typical members of the family consist of spin S = 2 Mn III porphyrin ions alternating with trans-μ 2 -bonded spin s = 1/2 acceptors [C 2 (CN) 4 ] − or [C 4 (CN) 6 ] − . All members exhibit 1D behavior at high temperatures as evidenced by their similarities to predictions for mixed quantumclassical ferrimagnetic chains. Below approximately 50 K, deviations from one-dimensionality are observed due to the effects of interchain interactions. The competition between interchain exchange, single-ion anisotropy, and dipolar interactions are considered when examining the remarkable differences in the low temperature magnetic states of the various compounds.

Photoinduced magnetism in a cluster glass: Co–Fe Prussian blue

Abstract Static and dynamic magnetic properties and photoinduced magnetism of a Co–Fe Prussian blue analog, K 1−2 x Co 1+ x [Fe(CN) 6 ]· y H 2 O ( x ≈0.2, y ≈4.9), are studied in detail. The magnetic behavior of the material is indicative of a cluster glass state. Our model of ‘spin doping’ of cluster glass is consistent with the observed photoinduced magnetic effects, including the previously unaccounted for increase in coercivity.