χMT Value Research Articles

A Molecular Crystal Shows Multiple Correlated Magnetic and Ferroelectric Switchings

Simultaneous control of the magnetic and electric properties of materials is crucial for their application in next-generation memory and sensor devices. Herein, we report a single-crystal Co(II) co...

Acidity-Driven Bidirectional Room-Temperature Spin-State Switch and Fluorescence Modulation of a Mononuclear Fe(II) Complex

Room-temperature switchable materials showing multiple responses toward external stimuli are highly desired. Herein, we report bidirectional spin-state switch and fluorescence modulation of an Fe(I...

A Hidden Coordination-Bond Torsional Deformation as a Sign of Possible Spin Transition in Nickel(II)-Bis(nitroxide) Compounds.

Complex formation of nickel(II) tetrafluoroborate and tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) in the presence of sodium cyanate gave a discrete molecule [Ni(phpyNO)2(X)2] (X = NCO). The Ni-O-N-Csp2 torsion angles were reduced on heating; 33.5(5)° and 36.2(4)° at 100 K vs. 25.7(10)° and 32.3(11)° at 400 K. The magnetic behavior was almost diamagnetic below ca. 100 K, and the χmT value reached 1.04 cm3 K mol−1 at 400 K. An analysis using the van’t Hoff equation indicates a possible spin transition at T1/2 >> 400 K. Density functional theory calculation shows that the singlet-quintet energy gap decreases as the structural change from 100 to 400 K. The geometry optimization results suggest that the diamagnetic state has the Ni-O-N-Csp2 torsion angles of 32.7° while the Stotal = 2 state has those of 11.9°. The latter could not be experimentally observed even at 400 K. After overviewing the results on the known X = Br, Cl, and NCS derivatives, the magnetic behavior is described in a common phase diagram. The Br and Cl compounds undergo the energy level crossing of the high-/low-spin states, but the NCS and NCO compounds do not in a conventional experimental temperature range. The spin transition mechanism in this series involves the exchange coupling switch between ferro- and antiferromagnetic interactions, corresponding to the high- and low-spin phases, respectively.

Multiresponsive Spin Crossover Driven by Rotation of Tetraphenylborate Anion in an Iron(III) Complex

Dynamic molecular materials, which involve mechanical motions in crystals, usually exhibit tunable properties related to conformational polymorphs. Herein, we describe a solvent-free molecular crys...

Synthsis, structures and magnetic properties of three new coordination polymers constructed by mixed-linker strategy

By the reaction of a rigid π-conjugated linker of H4L (5,5′-(1,3,6,8-tetraoxobenzo [lmn] [3,8] phenanthroline-2–7- diyl) bis-1,3-benzenedicarboxylic acid), nickel nitrate and N-donor auxiliary linkers under solvothermal conditions, three new Ni-based coordination polymers (CPs), namely {[Ni(L)0.5(1,4-bib)(H2O)2]·2·5H2O·DMA}n (1), {[Ni(H2L)(4,4′-bibp)(H2O)2]}n (2) and {[Ni(L)0.5(4,4′-bimb)(H2O)2]·3·5H2O·2DMA}n (3) {1,4-bib = 1,4-bis(1-imidazoly)benzene, 4,4′-bibp = 4,4′-bis(imidazolyl)biphenyl, 4,4′-bimb = 4,4′-bis(imidazol-1-ylmethyl) biphenyl} have been synthesized and characterized by EA, TG, IR, PXRD and magnetic properties. The structural analysis indicates both 1 and 2 are 2D layer structures, which are further extended to 3D supramolecular structures by H-bonding and intermolecular π…π interactions. While 3 exhibits a complicated 3D structure with the topology symbol of {64.82}. The difference of the structures in CPs 1–3 is mainly attributed to different auxiliary linkers used. Meanwhile, magnetic properties of CPs 1–3 show that the descent of χMT values is attributed to zero-field splitting (ZFS) effects of Ni2+ ions. In addition, on account of large Ni2+ Ni2+ separations within these frameworks, magnetic interactions between adjacent Ni2+ ions can be neglected.

An indication of spin-transition accompanied by an order-disorder structural transformation in [Ni(phpyNO)2(NCS)2] (phpyNO = tert-butyl 5-phenyl-2-pyridyl nitroxide).

Reaction of nickel(ii) thiocyanate and tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) afforded a 2p–3d–2p heterospin triad [Ni(phpyNO)2(NCS)2]. The compound crystallizes in the orthorhombic Pbcn space group. The whole molecule is crystallographically independent. The torsion angles around Ni–O–N–C2py are 26.8(4) and 27.3(4)° at 400 K, indicating appreciable orbital overlaps of the radical π* and nickel(ii) 3dx2−y2/3dz2 orbitals. In a low-temperature region, the torsion was enhanced, and the space group changed to monoclinic P21/c with a doubled asymmetric unit volume. The χmT value was practically null below ca. 140 K and, on heating to 400 K, gradually increased and reached 1.30 cm3 K mol−1. A van't Hoff analysis suggests a spin transition at T1/2 = 530(20) K. Density functional theory calculation reproduced ground Stotal = 0 with singlet-triplet gaps of 910 and 1263 K for the 140 K structure, and the gap was reduced to 297 K at 400 K. Consequently, the present compound can be considered as an incomplete spin-crossover material, as a result of T1/2 located above the experimental temperature window.

Trinuclear CoIILnIIICoII Complexes (Ln = La, Ce, Nd, Sm, Gd, Dy, Er, and Yb) with 2,6-Dipicolinoylbis(N,N-diethylthiourea): Synthesis, Structures, and Magnetism.

One-pot reactions of 2,6-dipicolinoylbis(N,N-diethylthiourea) (H2L) with Co(CH3COO)2·4H2O and LnCl3, where Ln = La, Ce, Nd, Sm, Gd, Dy, Er, and Yb, in warm methanol in the presence of Et3N, give stable trinuclear complexes of the composition [LnCo2(L)2(μ1,3-OOCCH3)2X] ("CoLnCo" complexes), where X- = κ2-CH3COO- or Cl-. X-ray structure determinations reveal symmetric trinuclear complexes containing two organic ligands (L2-), two terminal CoII ions, and one central LnIII ion. The organic ligands coordinate equatorially to the two CoII ions via two bidentate (O,S) N-acylthiourea moieties and tridentate to the central Ln ion via the (O,N,O) 2,6-dipicolinoyl moieties. Two acetate bridges established between each of the terminal Co and central Ln ions complete the square-pyramidal coordination spheres of CoII. All products possess an additional chlorido ligand axially coordinated to the lanthanide except the gadolinium(III) and lanthanum(III) complexes, where bidentate acetato ligands are coordinated. Fitting the χmT versus T data of the "CoLaCo" complex gives the axial and rhombic zero-field-splitting parameters D = 24.3(4) cm-1 and E = -1.0(2) cm-1, respectively, and anisotropic Landé values gx,y = 2.81(1) and gz = 2.00 as well as weak antiferromagnetic interactions between two high-spin CoII centers with J = -0.49(2) cm-1. The nature of the magnetic interactions between the LnIII ions and the CoII ions in the "CoLnCo" complexes is deduced by comparing their χMT values to the sum of χMT values of the analogous "CoLaCo" and related "ZnLnZn" complexes. The "CoDyCo" complex reveals an antiferromagnetic interaction, while the remaining "CoLnCo" complexes show ferromagnetic interactions.

Synthesis, nonlinear optical, magnetic and electrical properties of ultra-stable open-shell pancake bonding linked perylene diimide anion radicals π-oligomer

A stable open-shell bay-bromine substitute perylene diimide anion radical π-oligomer was facilely synthesized in N,N′-dimethylformamide with K2CO3. The unusual π-oligomer showed wide absorption of 400–850 nm and the spacing for adjacent perylene core was 3.25 Å. Z-scan measurements exhibited that π-oligomer had high nonlinear optical character with βeff = −1.85 × 10−13 m/W, γeff = 1.025 × 10−19 m2/W. χMT value of π-oligomer reached 13.376 emu·K·Oe−1·mol−1 at 300 K which was far more than the theoretical value for one isolated spin of radical (0.375 emu·K·Oe−1·mol−1). Further, Conductivity of the π-oligomer depended on the temperature.

Syntheses, structure and properties of dinuclear Co complexes with bis(catecholate) ligands – Effect of a quinoline ring in the terminal group

Two types of biscatechol, namely H4L1 (5,5′-(buta-1,3-diyne-1,4-diyl)bis(3-t-butylcatechol)) and H4L2 (5,5′-(ethyne-1,2-diyl)bis(3-t-butylcatechol)) were synthesized. In these ligands, two redox active catechol moieties are connected by one or two triple bonds. Also, tpa (tris(2-pyridylmethyl) amine), bpqa (bis(2-pyridylmethyl)(2-quinolylmethyl)amine) and pbqa ((2-pyridylmethyl)bis(2-quinolylmethyl)amine) were synthesized as terminal ligands of the tetracoordinated tripod type. In total, six dinuclear Co complexes were synthesized from these biscatechol and terminal ligands as follows: [Co2(L1)(tpa)2](BF4)2 (1), [Co2(L1)(bpqa)2](PF6)2 (2), [Co2(L1)(pbqa)2](PF6)2 (3), [Co2(L2)(tpa)2](BF4)2 (4), [Co2(L2)(bpqa)2](PF6)2 (5), [Co2(L2)(pbqa)2](PF6)2 (6). Of the six dinuclear Co complexes, complex 6, which was isolated as a mixed valent state CoII(HS)-[SQ-Cat]-CoIII(LS) compound, showed an absorption intensity at around 703 nm (MLCT bands) that increased with increasing temperature in acetonitrile solution. In addition, an investigation of the magnetic properties of the complex 6 with SQUID showed that the χMT value gradually increased as the temperature increased from 150 to 380 K. This suggests that a transition from CoIII(LS) (S = 0) to CoII(HS) (S = 3/2) accompanies the temperature rise. This means the steric hindrance and electronic effect of the quinolyl groups around the Co ion produce a coordination atmosphere weaker than that of pyridyl groups, with the result that the CoIII ions easily convert to CoII ions.

Modular construction, magnetism and photocatalytic properties of two new metal-organic frameworks based on a semi-rigid tetracarboxylate ligand

The solvothermal reactions of 5,5ˊ-(1,4-phenylenebis(methyleneoxy))diisophthalic acid (H4L) with cobalt(II) salts in presence of two N-donor ancillary ligands yielded two new metal–organic frameworks (MOFs) viz., [Co2(L)(bpz)(H2O)]n (bpz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole) (1) and [Co2(μ2-H2O)(H2L)2(tib)2·2CH3CN·2H2O]n (tib = 1,3,5-tris(1-imida-zolyl) benzene) (2). The single-crystal X-ray diffraction studies revealed that 1 displays 3,6-connected 3-periodic network with 2-fold interpenetration feature while 2 exhibits only 1D chain structure. The nature of ancillary ligands and the influence of N-donor ligands on coordination framework of these compounds have been discussed. The photocatalytic activities of these newly designed MOFs have been tested for the degradation of methyl violet (MV). The results of photocatalytic experiments demonstrated that both 1 and 2 displayed efficient photocatalytic performances to degrade MV under UV irradiation. The plausible mechanism through which both the MOFs displayed photocatalytic properties have been proposed through density of states (DOS) calculations. Also, the magnetic properties of both the MOFs have been analysed in detail which indicated χMT values of 6.02 and 6.54 cm3 K mol−1 for 1 and 2, respectively at 300 K which declined to 3.64 cm3 K mol−1 for 1 and 0.34 cm3 K mol−1 for 2 at 2 K.

Tetranuclear [FeII2FeIII2]2+ molecular switches: [FeII(bik)2(N–)2] spin-crossover complexes containing [FeIII(Tp)(CN)3]– metalloligands as N-donor

Three novel mixed valence cyanide-bridged {FeIII2FeII2} square complexes were obtained through the self-assembling of [FeIII(Tp)(CN)3]- or [FeIII(Tp*)(CN)3]− cyanido building blocks with the in situ formed [FeII(bik)2(S)2] complex (Tp = hydrotris (pyrazol-1-yl)borate, Tp* = hydrotris (3,5-dimethyl-pyrazol-1-yl)borate, bik = bis(1-methylimidazol-2-yl)ketone, S = solvent). The structures of these three complexes (2, 3 and 4) are reminiscent of that of our previously published square complex {[FeIII(Tp)(CN)3]2 [FeII(bik)2]2}·[FeIII(Tp)(CN)3]2·18H2O·4CH3OH (1). They consist of cyanide-bridged square dicationic complexes, ClO4− (2 and 3) or BF4− (4) counterions and solvate molecules. The FT-IR cyanide stretching vibrations observed at νCN ≈ 2145–60 cm−1 are typical of {FeIII–CN–FeII} moieties. The investigation of the magnetic properties of 2 reveals the occurrence of spin-crossover centered at T1/2 = 227 K. The χMT variation, ca. 7 cm3 mol−1 K, reflects the complete spin-state change occurring on both {FeII(bik)(–NC)2} moieties (–NC represents the cyanido building blocks). The Slichter–Drickamer model leads to a weak cooperativity factor, Γ = 1.6 kJ mol−1 (with Γ < 2RT1/2), which reflects the gradual spin-state change. This is in agreement with the molecular structure of 2, which does not present significant intermolecular interactions. The calculated enthalpy and entropy variations associated with the spin-state equilibrium are ΔH = 24 kJ mol−1 and ΔS = 105 J K−1 mol−1. In contrast, 3 and 4 show only partial spin-crossover in the accessible temperature range (2–400 K) as the T1/2 are shifted toward higher temperatures (ca. T1/2 > 400 K). Although no photomagnetic effect is observed for 3, compound 4 shows a moderate increase in the magnetization upon irradiation at low temperature. This phenomenon is ascribed to the light-induced excited spin-state trapping (LIESST) effect. Interestingly, the complex 2 also shows a remarkable LIESST effect, which is observed with different laser lights covering the visible and near-infrared range. The resulting χMT value obtained in the photoinduced state suggests the occurrence of a ferromagnetic interaction inside the {FeIII–CN–FeII} units. Trois nouveaux complexes carrés {FeIII2FeII2} à pont cyanure et à valence mixte ont été obtenus par auto-assemblage des précurseurs cyanurés [FeIII(Tp)(CN)3]− ou [FeIII(Tp*)(CN)3]− et du complexe cationique [FeII(bik)2(S)2] formé in situ (Tp = hydrotris (pyrazol-1-yl)borate, Tp* = hydrotris (3,5-dimethyl-pyrazol-1-yl)borate, bik = bis(1-methylimidazol-2-yl)cétone, S = solvant). Les structures de ces trois complexes (2, 3 et 4) sont similaires à celle du complexe {[FeIII(Tp)(CN)3]2[FeII(bik)2]2}[FeIII(Tp)(CN)3]2·18H2O·4CH3OH (1) préalablement publié. Elles sont constituées de complexes carrés dicationiques à pont cyanure, de contre-ions ClO4− (2 and 3) ou BF4− (4) et de molécules de solvant. Les vibrations d’élongation des cyanures, observée en spectroscopie IR à νCN ≈ 2145–60 cm−1, sont caractéristiques d'unités {FeIII–CN–FeII}. L’étude des propriétés magnétiques de 2 révèle un équilibre de spin centré à T1/2 = 227 K. La variation du produit χMT, ca. 7 cm3 mol−1 K, traduit une conversion de spin complète sur chacune des unités {FeII(bik)(–NC)2} du carré (–NC représente le complexe précurseur cyanuré). L'analyse des données par le modèle de Slichter–Drickamer conduit à un faible facteur de coopérativité, Γ = 1.6 kJ mol−1 K (with Γ < 2RT1/2), en accord avec un changement d’état de spin graduel. Ces données sont en accord avec la structure de 2 qui ne montre pas d'interactions intermoléculaires notables. Les valeurs des variations d'enthalpie et d'entropie associées à la conversion de spin sont ΔH = 24 kJ mol−1 et ΔS = 105 J K−1 mol−1. Au contraire de 2, les composés 3 and 4 présentent seulement une conversion de spin partielle dans le domaine de température exploré (2–400 K), avec des valeurs T1/2 déplacées vers les hautes températures (ca. T1/2 > 400 K). Tandis qu'on n'observe pas d'effet photomagnétique pour 3 et seulement un faible effet dans 4, le composé 2 présente une forte augmentation de son aimantation sous irradiation à basse température. Cet effet est dû au piégeage photo-induit d'un état excité de spin (effet « LIESST », Light-Induced Excited Spin-State Trapping). Il est observé avec différentes sources laser couvrant le spectre visible et le proche infrarouge. Les valeurs de χMT obtenues dans l’état photo-induit suggèrent la présence d'une interaction ferromagnétique au sein de la paire {FeIII–CN–FeII}.

Electrical and magnetic properties of a radical-based Co(II) coordination complex with C[sbnd]H⋯π and π⋯π supramolecular interactions

HAN– radical based supramolecular Co(II) complex, Co-HAN, has been synthesized with reaction of large π-conjugated hexaazatrinaphthylene (HAN), cobalt salt and ethylenediamine reductant through one-pot solvothermal method. In aggregating crystal network of Co-HAN, two kinds of Zig-Zag supramolecular channels are formed by π⋯π stacking and CH⋯π hydrogen-bond interactions, respectively. The supramolecular interactions, especially π⋯π stacking, help promote the charge transport, leading to an enhanced electrical conductivity (σ = 0.599 × 10−7 S·cm−1). In addition, electron paramagnetic resonance (EPR) measurements indicated of antiferromagnetic interactions between monoradical ligand and Co(II) ion. The temperature-dependent magnetic susceptibility and χmT value suggests that Co-HAN exhibits a normal paramagnetic behavior for an effective spin of 1/2 coming from the unquenched orbit magnetic moment of Co(II) ions.

Construction of a new double phenoxo bridged asymmetric manganese(III) Schiff base complex: Observation of ferromagnetic interaction within the dimer and antiferromagnetic interaction between dimers

A dimeric phenoxo bridged manganese(III) complex, [Mn2L2(OH2)(CH3OH)](ClO4)2 [H2L = N,N′-bis((2-hydroxy-1-phenyl)ethylidene)ethane-1,2-diamine] has been synthesized and characterized. The structure has been confirmed by single crystal X-ray crystallography. The χMT value at room temperature is slightly higher than that expected for two non-interacting species with S = 2 (g = 2), that corresponds to two high spin manganese(III) centers. That χMT value increases gradually with lowering temperature to reach a maximum and then an abrupt decrease of χMT value is observed at lower temperature. A ferromagnetic interaction (J = 1.44 cm−1) between the manganese(III) centers within the dimer is counter balanced by the presence of zero-field splitting (ZFS, D = −2.03 cm−1) at very low temperature and possibly also by antiferromagnetic interaction between dimers (zJ = -0.06 cm−1). In order to investigate the single-molecule magnet (SMM) nature of the complex, alternating current (ac) magnetic measurements were carried out from 2 to 7 K under a 3.5 Oe ac field between 1 and 1000 Hz.

Stepwise Synthesis, Hydrogen-Bonded Supramolecular Structure, and Magnetic Property of a Co–Mn Heterodinuclear Complex

A cobalt(III)–manganese(II) heterometallic dinuclear complex, [MnII{CoIII(µ-Himn)3}Cl2(CH3OH)], was prepared by a metalloligand approach. X-ray crystallographic analysis indicated that the metalloligand [CoIII(Himn)3] underwent mer/fac geometrical isomerization upon coordination to a Mn ion. Owing to the non-coordinating N–H bonds in the [CoIII(Himn)3] moiety, the heterodinuclear complex exhibited hydrogen bond interactions with the Cl− ligand of the neighboring complex to construct two-dimensional hydrogen-bond networks. The bond distances around the Mn center and the χMT value at 300 K indicate that the Mn center is in a divalent state. The temperature dependence of the χMT product and field dependence of the magnetization showed the isotropic nature of the MnII center.

Cobalt(II) Terpyridin-4′-yl Nitroxide Complex as an Exchange-Coupled Spin-Crossover Material

Spin-crossover (SCO) was studied in [Co(L)2](CF3SO3)2, where L stands for diamagnetic 2,2′:6′,2′′-terpyridine (tpy) and its paramagnetic derivative, 4′-{4-tert-butyl(N-oxy)aminophenyl}-substituted tpy (tpyphNO). The X-ray crystallographic analysis clarified the Co-N bond length change (Δd) in high- and low-temperature structures; Δdcentral = 0.12 and Δddistal = 0.05 Å between 90 and 400 K for L = tpy and Δdcentral = 0.11 and Δddistal = 0.06 Å between 90 and 300 K for L = tpyphNO. The low- and high-temperature structures can be assigned to approximate low- and high-spin states, respectively. The magnetic susceptibility measurements revealed that the χmT value of [Co(tpyphNO)2](CF3SO3)2 had a bias from that of [Co(tpy)2](CF3SO3)2 by the contribution of the two radical spins. The tpy compound showed a gradual SCO around 260 K and on cooling the χmT value displayed a plateau down to 2 K. On the other hand, the tpyphNO compound showed a relatively abrupt SCO at ca. 140 K together with a second decrease of the χmT value on further cooling below ca. 20 K. From the second decrease, Co-nitroxide exchange coupling was characterized as antiferromagnetic with 2JCo-rad/kB = −3.00(6) K in the spin-Hamiltonian H = −2JCo-rad(SCo·Srad1 + SCo·Srad2). The magnetic moment apparently switches double-stepwise as 1 μB ⇄ 3 μB ⇄ 5 μB by temperature stimulus.

Proton Relaxation Time in Water-soluble Metal Complex Nanoparticles

Abstract Water dispersible metal complex nanoparticles were synthesized using the polyethylene glycol (PEG) coating method. The χmT values at room temperature for the NPs 1–3 were calculated using the Evans method, and the proton relaxation times, T1 and T2, were measured employing MRI. Both relaxation times were observed to decrease with increasing χmT value.

Double-stranded helicates of Ni(II), Co(II), Fe(II) and Zn(II) with oligo-α-naphthyridylamino ligand: Synthesis, structure and properties

By using tetranaphthyridyl-triamine ligand, N2-(2-(1,8-naphthyridin-7-ylamino)-1,8-naphthyridin-7-yl)-N7-(1,8-naphthyridin-2-yl)-1,8-naphthyridine-2,7-diamine (H3tentra), double-stranded dinuclear helical compounds [M2(H3tentra)2(ClO4)4] [M = NiII (1), CoII (2), FeII (3), ZnII (4)] were synthesized and structurally characterized. The single X-ray crystal structure of iron(II) complex showed that both of H3tentra ligands wrap around the metal–metal axis giving rise to a double-stranded dinuclear helical compound with Fe–Fe separation of ca. 5.326 Å. Each ligand uses two nitrogen atoms from two different napthyridine rings, to coordinate one metal centre (Fe or Fe(A)) and therefore, the geometry about the metal ions was described as distorted tetrahedral, with both ligands acting as bisbidentate N4 donors. This distortion is shown by the angles between nitrogen and iron atoms, which range from 80.45(15) to 143.32(14)°. Distorted tetrahedral environment was also proposed for the other complexes on the basis of the magnetic measurements and X-ray results. Strong π–π interactions were observed for all synthesized double stranded helicates of H3tentra ligand. The measured χMT values at 300 K for complexes 2 and 3 found larger than the expected values for two high-spin Co(II) with S = 3/2 and for Fe(II) ions with S = 2 and g = 2, which attributed to the unquenched orbital coupling.

Mononuclear and One‐Dimensional Cobalt(II) Complexes with the 3,6‐Bis(picolylamino)‐1,2,4,5‐tetrazine Ligand

We describe herein the synthesis and structural characterization of a tetrazine‐based bis(picolylamine) ligand, namely, N3,N6‐bis(pyridin‐2‐ylmethyl)‐1,2,4,5‐tetrazine‐3,6‐diamine (L, 1), which was prepared by the direct reaction between 2 equiv. of picolylamine and 3,6‐dichloro‐1,2,4,5‐tetrazine. Its reaction with cobalt(II) hexafluoroacetylacetonate hydrate afforded the mononuclear complex [Co(hfac)2L]·CH3CN (2, Hhfac = hexafluoroacetylacetone) and the chain compound {[Co(hfac)2(µ‐L)][Co(hfac)2(CH3OH)2]}n (3). The crystal structures of 1–3 were determined by X‐ray structure analysis. The metal centre in 2 is trischelated by two hfac ligands and one picolylamine unit. The two crystallographically independent cobalt(II) ions in 3 (Co1 and Co2) are six‐coordinate. Two bidentate hfac groups in equatorial positions and two trans‐positioned pyridine nitrogen atoms build the coordination polyhedron around Co1 to form a neutral zigzag chain compound, which cocrystallizes with the mononuclear species trans‐[Co2(hfac)2(CH3OH)2]. The magnetic properties of 2 and 3 were investigated in the temperature range 1.9–300 K. The decrease of the χMT values of 2 and 3 upon cooling is mainly due to the depopulation of the higher energy Kramers doublets of the magnetically isolated six‐coordinate cobalt(II) ions. Alternating‐current (ac) magnetic susceptibility measurements of 2 and 3 revealed the occurrence of frequency‐dependent out‐of‐phase signals under applied direct‐current (dc) fields at very low temperatures, and this result is indicative of single‐ion magnet (SIM) behaviour.

Metal organic framework Cu9Cl2(cpa)6 as tunable molecular magnet

Chemical modifications of the magnetic metal organic framework (MOF) Cu9X2(cpa)6·42H2O (X = F, Cl, Br; cpa = anion of 2-carboxypentonicacid) have been investigated as a means of modifying, in a tunable manner, the magnetism of this 2-D material best described as a triangles-in-triangles (TIT) or triangulated-Kagomé-latttice (TKL). Since numerous theoretical studies have already attempted to describe the enigmatic ground state of this Heisenberg lattice, tunable chemical modifications should provide an excellent opportunity to expand this class of materials for studies concerning fundamental physics of frustrated spins, and applications such as adiabatic demagnetization refrigeration (ADR) that depend on the magnetocaloric effect (MCE). The chemical modification investigated is the intercalation of d- and f-orbital ions into the voids of the framework (channels of nearly 20 Å diameter). Magnetic measurements in the temperature range 1.8 – 300 K confirm signature features of TKL magnetism in intercalated samples persist, specifically: i) large negative Weiss constant (θCW); ii) absence of a phase transition down to 1.8 K; iii) minimum in χMT; iv) low temperature χMT values increasingly divergent at low fields indicating net ferromagnetic correlations; and, v) increasing field dependence of magnetization at low temperatures suggestive of intermediate plateaus, or ferrimagnetism, not saturation.

Three-Dimensional (3-D) Ferromagnetic Network of Mn12 Single-Molecule Magnets: Subtle Environmental Effects and Switching to Antiferromagnetic.

A new member of the Mn12 family of single-molecule magnets (SMMs) has been prepared and found to be the first of this family to give a 3-D ferromagnetic network. [Mn12O12(O2CC6H4-p-F)16(H2O)4] (2) was prepared by carboxylate substitution on the acetate derivative with p-F-benzoic acid and crystallizes as 2·8MeCN in space group I4̅2m with extensive formation of intermolecular C-H···F hydrogen-bonding. The latter leads to a combination of ferromagnetic (F) and antiferromagnetic (AF) interactions and an overall F network that gives a χMT value at low T that is abnormally high for an S = 10 ground state. 2·8MeCN undergoes solvent loss under vacuum to 2, with a decrease in unit-cell volume of 17%, primarily due to a 13% decrease in the c-axis. The χMT vs T plot for 2 indicates a switch to a net AF network. Exposure to air causes hydration to 2·3H2O, a concomitant increase in unit cell volume, and a switch back to a F network. The same conversion of 2·8MeCN to 2·3H2O can also be accomplished in one step rather than two steps, by leaving crystals of the former exposed to air at ambient temperature and pressure for 10 days, giving the same magnetic plots. Interestingly, the desolvation/solvation processes cause Jahn-Teller isomerism to occur, but the ratio of the faster-relaxing isomer to the normal slowly relaxing one does not change monotonically. Single-crystal micro-SQUID studies on 2·8MeCN show the expected magnetization hysteresis loops for a SMM and a small exchange-bias from the intermolecular interactions that is unexpectedly AF. Since the micro-SQUID study only identifies interactions along the easy-axis (z-axis) of the crystal, this is readily rationalized as due to the Jz components of the intermolecular interactions in 2·8MeCN being net AF. The combined results offer useful insights into the degree of sensitivity of the magnetic properties to small environmental perturbations.