Ni₅, Ni₈, and Ni₁₀ Clusters with 2,6-Diacetylpyridine-dioxime as a Ligand

In the present work, novel coordination possibilities for the system dapdoH2/NiII (dapdoH2 = 2,6-diacetylpyridine-dioxime) have been explored. Depending on the starting reagents and solution conditions, several clusters with nuclearities ranging from Ni5 to Ni10 were achieved and structurally characterized, namely, [Ni5(R-COO)2(dapdo)2(dapdoH)2(N(CN)2)2(MeOH)2] in which R-COO– = benzoate (1) or 3-chlorobenzoate (2), [Ni8(dapdo)4(NO3)4(OH)4(MeOH)4] (3), and [Ni10(dapdo)8(N(CN)2)2(MeO)(MeOH)](NO3) (4). For the first time, pentadentate coordination for the dapdo2– ligand has been established. All compounds show a combination of square-planar and octahedrally coordinated nickel atoms. According to the Ni2(sp)Ni3(Oh) (1 and 2), Ni4(sp)Ni4(Oh) (3), and Ni4(sp)Ni6(Oh) (4) environments, these systems magnetically behave as trimer, tetramer, and hexanuclear clusters, respectively. dc magnetic measurements in the 2–300 K range of temperature reveal antiferromagnetic coupling for all compounds, and the correlation of the superexchange interaction with the torsion angles involving the oximato bridges is experimentally confirmed

Ni₅, Ni₈, and Ni₁₀ Clusters with 2,6-Diacetylpyridine-dioxime as a Ligand

In the present work, novel coordination possibilities for the system dapdoH2/NiII (dapdoH2 = 2,6-diacetylpyridine-dioxime) have been explored. Depending on the starting reagents and solution conditions, several clusters with nuclearities ranging from Ni5 to Ni10 were achieved and structurally characterized, namely, [Ni5(R-COO)2(dapdo)2(dapdoH)2(N(CN)2)2(MeOH)2] in which R-COO– = benzoate (1) or 3-chlorobenzoate (2), [Ni8(dapdo)4(NO3)4(OH)4(MeOH)4] (3), and [Ni10(dapdo)8(N(CN)2)2(MeO)(MeOH)](NO3) (4). For the first time, pentadentate coordination for the dapdo2– ligand has been established. All compounds show a combination of square-planar and octahedrally coordinated nickel atoms. According to the Ni2(sp)Ni3(Oh) (1 and 2), Ni4(sp)Ni4(Oh) (3), and Ni4(sp)Ni6(Oh) (4) environments, these systems magnetically behave as trimer, tetramer, and hexanuclear clusters, respectively. dc magnetic measurements in the 2–300 K range of temperature reveal antiferromagnetic coupling for all compounds, and the correlation of the superexchange interaction with the torsion angles involving the oximato bridges is experimentally confirmed

Calorimetric Investigation of Triazole-Bridged Fe(II) Spin-Crossover One-Dimensional Materials: Measuring the Cooperativity

The relevance of abrupt magnetic and optical transitions exhibiting bistability in spin-crossover solids has been pointed out for their potential applications in optical or memory devices. In this respect, triazole-based one-dimensional coordination polymers are widely recognized as one of the most interesting systems. The measure of the interaction among spin-crossover centers at the origin of such cooperative behavior is of paramount importance and has so far been realized through modeling of spin-crossover curves derived mostly from magnetic measurements. Here, a new series of triazole-based one-dimensional coordination polymers of formula [Fe(Rtrz)3](A)2·xH2O with R = methoxyethyl and A = monovalent anion has been prepared that show complete and abrupt spin-crossover phenomenon as shown by magnetic measurements. The spin-crossover transition in these and related compounds is studied by differential scanning calorimetry, and the thermodynamic excess enthalpies and entropies associated with the phenomenon are derived systematically. Then the cooperative character of the spin-crossover in these materials is quantified by use of two widely used models, so-called Slichter and Drickamer and domain models. The same procedure is applied to spin-crossover curves of similar compounds available in the literature and for which calorimetric studies have been reported. The experimental thermodynamic figures, in particular the excess enthalpies, are shown to be clearly correlated to the output parameters of both models, thus providing a direct, experimental, quantitative measure of the cooperative character of the spin-crossover phenomenon

Influence of Selenocyanate Ligands on the Transition Temperature and Cooperativity of bapbpy-Based Fe(II) Spin-Crossover Compounds

Coordination of the ligand bapbpy (<b>1</b>, bapbpy<b> = </b><i>N,N</i>′-di­(pyrid-2-yl)-2,2′-bipyridine-6,6′-diamine), of one of its four dimethyl-substituted analogues <b>2</b>–<b>5</b> (R₂bapbpy = <i>N,N</i>′-di­(methylpyrid-2-yl)-2,2′-bipyridine-6,6′-diamine), or of one of its three bis­(iso)­quinoline analogues <b>6</b>–<b>8</b> (R₂bapbpy<i>= N,N</i>′-di­(quinolyl)-2,2′-bipyridine-6,6′-diamine), to Fe­(NCSe)₂, afforded eight new iron­(II) compounds of the type [Fe­(R₂bapbpy)­(NCSe)₂] (<b>9</b>–<b>16</b>). Three of these compounds (<b>11</b>, <b>13</b>, and <b>16</b>) were structurally characterized by single crystal X-ray diffraction, which showed similar molecular geometry and packing compared to their thiocyanate analogues. Magnetic susceptibility measurements were carried out for all iron compounds and revealed thermal spin-crossover (SCO) behavior for compounds <b>9</b>, <b>11</b>, <b>13</b>, <b>15</b>, and <b>16</b>. Compounds <b>11</b>, <b>13</b>, <b>15</b>, and <b>16</b> show an increased transition temperature compared to the thiocyanate analogues. [Fe­(bapbpy)­(NCSe)₂] (<b>9</b>) shows a gradual, one-step SCO, whereas its thiocyanate analogue [Fe­(bapbpy)­(NCS)₂] is known for its cooperative two-step SCO. To discuss the influence of S-to-Se substitution on the cooperativity of the SCO, heat capacity measurements were carried out for compounds <b>9</b>, <b>11</b>, <b>13</b>, <b>15</b>, and <b>16</b>, and fitted to the Sorai domain model. The number <i>n</i> of like-spin SCO centers per interacting domain, which is a quantitative measure of the cooperativity of the spin transition, was found to be high for compounds <b>11</b> and <b>15</b>, and low for compounds <b>9</b>, <b>11</b>, and <b>13</b>. Compound <b>15</b> is one of the few known SCO compounds that is more cooperative than its thiocyanate analogue. Altogether, X-ray diffraction, calorimetry, and magnetic data give a consistent structure–property relationship for this family of compounds: hydrogen-bonding networks made of intermolecular N–H···Se interactions are of paramount importance for the cooperativity of the SCO

Quantum Tunneling and Quantum Phase Interference in a [Mn^II₂Mn^III₂] Single-Molecule Magnet

[Mn4(hmp)6(H2O)2(NO3)2](NO3)2·2.5H2O (1) has been synthesized from the reaction of 2-hydroxymethylpyridine (Hhmp) with Mn(NO3)2·4H2O in the presence of tetraethylammonium hydroxide. 1 crystallizes in the triclinic P1̄ space group with two crystallographically independent centrosymmetrical [Mn4(hmp)6(H2O)2(NO3)2]2+ complexes in the packing structure. Four Mn ions are arranged in a double-cuboidal fashion where outer Mn2+ are heptacoordinated and inner Mn3+ are hexacoordinated. dc magnetic measurements show that both Mn2+···Mn3+ and Mn3+···Mn3+ interactions are ferromagnetic with Jwb/kB = +0.80(5) K, and Jbb/kB = +7.1(1) K, respectively, leading to an ST = 9 ground state. Combined ac and dc measurements reveal the single-molecule magnet (SMM) behavior of 1 with both thermally activated and ground-state tunneling regimes, including quantum phase interference. In the thermally activated regime, the characteristic relaxation time (τ) of the system follows an Arrhenius law with τ0 = 6.7 × 10-9 s and Δeff/kB = 20.9 K. Below 0.34 K, τ saturates indicating that the quantum tunneling of the magnetization becomes the dominant relaxation process as expected for SMMs. Down to 0.04 K, field dependence of the magnetization measured using the μ-SQUID technique shows the presence of very weak inter-SMM interactions (zJ‘/kB ≈ −1.5 × 10-3 K) and allows an estimation of D/kB at −0.35 K. Quantum phase interference has been used to confirm the D value and to estimate the transverse anisotropic parameter to E/kB = +0.083 K and the ground-state tunnel splitting ΔLZ = 3 × 10-7 K at Htrans = 0 Oe. These results rationalize the observed tunneling time (τQTM) and the effective energy barrier (Δeff)

Molecular [(Fe₃)–(Fe₃)] and [(Fe₄)–(Fe₄)] Coordination Cluster Pairs as Single or Composite Arrays

The synthesis of molecular cluster pairs is a challenge for coordination chemists due to the potential applications of these species in molecular spintronics or quantum computing. The ligand H4L, 1,3-bis-(3-oxo-3-(2-hydroxyphenyl)-propionyl)-2-methoxybenzene, has been successfully used to obtain a series of such complexes using the basic Fe­(III) trinuclear carboxylates as starting materials. Synthetic control has allowed the isolation of the two molecular cluster pairs that form the composite [Fe4O2(PhCO2)6(H2L)­(pz)]2[Fe3O­(PhCO2)5(py)­(H2L)]2 (1). The dimers of trinuclear units, [Fe3O­(PhCO2)5(H2O)­(H2L)]2 (2) and [Fe3O­(o-MePhCO2)5(H2L)­(py)]2 (3), and the dimers of tetranuclear units, [Fe4O2(PhCO2)6(H2L)­(pz)]2 (4) and [Fe4O2(o–MePhCO2)6(H2L)­(pz)]2 (5), are presented here. The magnetic properties of the reported aggregates show that they are pairs of semi-independent clusters weakly interacting magnetically as required for two-qubit quantum gates

Molecular [(Fe₃)–(Fe₃)] and [(Fe₄)–(Fe₄)] Coordination Cluster Pairs as Single or Composite Arrays

The synthesis of molecular cluster pairs is a challenge for coordination chemists due to the potential applications of these species in molecular spintronics or quantum computing. The ligand H₄L, 1,3-bis-(3-oxo-3-(2-hydroxyphenyl)-propionyl)-2-methoxybenzene, has been successfully used to obtain a series of such complexes using the basic Fe­(III) trinuclear carboxylates as starting materials. Synthetic control has allowed the isolation of the two molecular cluster pairs that form the composite [Fe₄O₂(PhCO₂)₆(H₂L)­(pz)]₂[Fe₃O­(PhCO₂)₅(py)­(H₂L)]₂ (<b>1</b>). The dimers of trinuclear units, [Fe₃O­(PhCO₂)₅(H₂O)­(H₂L)]₂ (<b>2</b>) and [Fe₃O­(<i>o</i>-MePhCO₂)₅(H₂L)­(py)]₂ (<b>3</b>), and the dimers of tetranuclear units, [Fe₄O₂(PhCO₂)₆(H₂L)­(pz)]₂ (<b>4</b>) and [Fe₄O₂(<i>o</i>–MePhCO₂)₆(H₂L)­(pz)]₂ (<b>5</b>), are presented here. The magnetic properties of the reported aggregates show that they are pairs of semi-independent clusters weakly interacting magnetically as required for two-qubit quantum gates

Liquid-Crystalline Zinc(II) and Iron(II) Alkyltriazoles One-Dimensional Coordination Polymers

Several series of unidimensional coordination polymers of formula [Zn­(C_<i>n</i>H_2<i>n</i>+1trz)₃]­(Cl)₂·<i>x</i>H₂O (<i>n</i> = 18, 16, 13, 11, 10, trz = 4-substituted-1,2,4-triazole), [Zn­(C₁₈H₃₇trz)₃]­(ptol)₂·<i>x</i>H₂O, [Fe­(C_<i>n</i>H_2<i>n</i>+1trz)₃]­(X)₂·<i>x</i>H₂O (<i>n</i> = 18, 16, 13, 10; X = Cl^– or ptol^–, where ptol^– = <i>p</i>-tolylsulfonate anion), and [Fe­(C₁₈H₃₇trz)₃]­(X)₂·<i>x</i>H₂O (X = C₈H₁₇PhSO₃^– and C₈H₁₇SO₃^–) are reported with their thermal, structural, and magnetic properties. Most of these materials exhibit thermotropic lamellar mesophases at temperatures as low as 410 K, as confirmed by textures observed by polarized optical microscopy. The corresponding phase diagrams deduced by differential scanning calorimetry are also reported. All iron-containing materials present a spin crossover phenomenon that occurs at temperatures ranging from 242 to 360 K, only slightly below the mesophase temperature domain, and remains complete and cooperative, even for the longer alkyl substituents. The use of stable diamagnetic Zn­(II) analogues proves to be very useful to characterize the comparatively less stable and less crystalline Fe­(II) analogues

Multifunctional Gels from Polymeric Spin-Crossover Metallo-Gelators

The gelation abilities toward organic solvents of a series of triazole-based coordination polymers of formula [M(Cntrz)3]A2 (M = Fe(II) or Zn(II); Cntrz =4-n-alkyl-1,2,4-triazole with n = 13, 16, 18; A = monovalent anions, abbreviated as MCnA) have been studied to form thermally responsive multifunctional metallogels, in particular for the iron polymers that present the spin-crossover phenomenon. Indeed thermo-reversible physical gels exhibiting thermally reversible magnetic and optical crossovers are formed in decane and toluene. The FeC18ptol/decane and FeC18ptol/toluene phase diagrams are described (ptol = p-toluene sulfonate anion), together with the rheological properties of the gels determined as a function of the solvent, the gelator concentration as well as temperature. Microscopic observations of the gel structure are correlated to the composition and rheological properties of the gels. Magnetic and thermal studies show that both the gel−liquid and spin-crossover phenomena can be adjusted through the composition of the gel mixture

Quantum Tunneling and Quantum Phase Interference in a [Mn^II₂Mn^III₂] Single-Molecule Magnet

[Mn4(hmp)6(H2O)2(NO3)2](NO3)2·2.5H2O (1) has been synthesized from the reaction of 2-hydroxymethylpyridine (Hhmp) with Mn(NO3)2·4H2O in the presence of tetraethylammonium hydroxide. 1 crystallizes in the triclinic P1̄ space group with two crystallographically independent centrosymmetrical [Mn4(hmp)6(H2O)2(NO3)2]2+ complexes in the packing structure. Four Mn ions are arranged in a double-cuboidal fashion where outer Mn2+ are heptacoordinated and inner Mn3+ are hexacoordinated. dc magnetic measurements show that both Mn2+···Mn3+ and Mn3+···Mn3+ interactions are ferromagnetic with Jwb/kB = +0.80(5) K, and Jbb/kB = +7.1(1) K, respectively, leading to an ST = 9 ground state. Combined ac and dc measurements reveal the single-molecule magnet (SMM) behavior of 1 with both thermally activated and ground-state tunneling regimes, including quantum phase interference. In the thermally activated regime, the characteristic relaxation time (τ) of the system follows an Arrhenius law with τ0 = 6.7 × 10-9 s and Δeff/kB = 20.9 K. Below 0.34 K, τ saturates indicating that the quantum tunneling of the magnetization becomes the dominant relaxation process as expected for SMMs. Down to 0.04 K, field dependence of the magnetization measured using the μ-SQUID technique shows the presence of very weak inter-SMM interactions (zJ‘/kB ≈ −1.5 × 10-3 K) and allows an estimation of D/kB at −0.35 K. Quantum phase interference has been used to confirm the D value and to estimate the transverse anisotropic parameter to E/kB = +0.083 K and the ground-state tunnel splitting ΔLZ = 3 × 10-7 K at Htrans = 0 Oe. These results rationalize the observed tunneling time (τQTM) and the effective energy barrier (Δeff)