Weak Antiferromagnetic Coupling Research Articles

A Neutral Fe4tBuL4 All Ferric Grid Complex: Structural and Variable Temperature Magnetic Properties

A neutral all ferric [2×2] grid complex Fe4tBuL4 1 was synthesized and characterized in the solid‐state by X‐ray crystallography, variable temperature magnetic susceptibility and Mössbauer spectroscopy. The absence of any coordinate bond or other structural changes between 90(2) and 320(2) K indicated no changes to the electronic structure of 1 occurred in the solid‐state over this temperature range. Variable temperature magnetic susceptibility data were fitted to an isotropic spin Hamiltonian model and suggested weak antiferromagnetic exchange coupling between the high‐spin ferric ions (J = –16.1 cm‐1). Mössbauer spectroscopy parameters (δ = 0.416(9) mm s–1 & ΔEQ = 0.87(7) mm s‐1), powder EPR spectra, and broken symmetry density functional theory calculations supported the high‐spin ferric oxidation state assignment.

Structure, DFT studies, magnetism and biological activity of bis[(µ-azido)-chloro-(1,10-phenanthroline)-copper(II)] complex

The dinuclear complex Bis[(µ-azido)-chloro-(1,10-phenanthroline)-copper(II)] (1) was synthesized, and characterized by X-ray. Complex (1) crystallizes in the monoclinic system, it consists of centrosymmetric [CuCl(phen)µ-N3]2 dimers bridged by azide groups. The phenanthroline ligand, chloride ion, and ƞ-N of equatorial bridging azide ligand are coplanar and the square pyramidal copper ion is displaced slightly out of this basal plane, toward the axial bridging azide. The presence of H-bonds, CH-π and π-π interactions form layers of type ABAB within the supramolecular structure of (1). The magnetic susceptibility of (1) versus temperature data showed a weak antiferromagnetic coupling between Cu(II) ions. The best fitting parameters were for g = 2.01 ± 0.01 and J = −0.55 ± 0.01 cm−1. Thermal analysis of (1) exhibited an exothermic peak due to decomposition of azide ligands. The calculated HOMO–LUMO gap from DFT is 0.03098 a.u. (0.08430 eV).Besides, complex (1) exhibited potent chemotherapeutic potential with strong activity after 48 h against MDA-MB-32 (breast adenocarcinoma), HT-29 (colon adenocarcinoma), A549 (lung adenocarcinoma), SF (astrocytoma) and B16F10 (melanoma) cell lines, with IC50 value 0.78 μg/ml (1.21 µM) which is several folds higher than cisplatin 4.88 μg/ml (16.3 µM). (1) also has a better therapeutic index and toxicological profile compared to cisplatin, as evidenced by the median lethal dose (LD50).

Coexistence of Metal Phthalocyanine and Fullerene C60 Anions in Mixed {cryptand[2.2.2](Na+)}x(MPc)–x‐1·(C60) – Salts (M = CoI, VIVO, x = 2 and 1.5)

Salts containing metal phthalocyanine and fullerene anions: {cryptand[2.2.2](Na+)}1.5{CoI(Pc2–)}–0.5(C60·–)·2C6H4Cl2 (1) and {cryptand[2.2.2](Na+)}2{VIVO(Pc·3–)}–(C60–)·C6H4Cl2 (2) were obtained. The {CoI(Pc2–)}–anions are formed by metal centered reduction whereas the Pc2– macrocycles are reduced at the formation of {VIVO(Pc·3–)}–. Fullerenes form zigzag chains in 1 in which they are positionally disordered showing different degree of dimerization [loose packed or π‐stacked pairs from the monomers and singly bonded (C60–)2 dimers]. Singly bonded (C60–)2 dimers in 2 also coexist with pairs of π‐stacked C60·–monomers. The (MPc)– anions are isolated in the salts alternating with the {cryptand(Na+)} cations. Magnetic measurements by EPR technique show the presence of spins in 1 both on C60·–(S = 1/2) and the CoI atoms having S = 1 spin state. These spins are weakly antiferromagnetically coupled in the fullerene chains (Θ = –6 K) but show nearly paramagnetic behaviour for isolated {CoI(Pc2–)}– anions. Both S = 1/2 spins in 2 are mainly localized on VIV and delocalized over Pc·3– since only about 10 % of C60·– from total amount of fullerene is preserved. Spins in 2 show only weak antiferromagnetic coupling with Θ = –2 K within one {VIVO(Pc·3–)}– unit.

Structural and Magnetic Properties of Three One-Dimensional Nitrato-, Azido- and Phenoxido-Bridged Copper(Ii) Coordination Polymers

Three coordination polymers, 1 ∞ [Cu 3 L 2 (NO 3 )](NO 3 )·2MeOH·2H 2 O ( 1 ), 1 ∞ [Cu 3 L 2 (N 3 )](CH 3 COO) ( 2 ) and 1 ∞ [Cu 3 L 2 (H 2 O)](ClO 4 ) 2 ( 3 ), based on the Schiff base H 2 L (H 2 L = N,N’-bis[(2-hydroxybenzilideneamino)-propyl]-piperazine) have been synthesized and characterized. X-ray single-crystal analysis shows that compounds 1 - 3 consist of trinuclear complex entities, {Cu 3 L 2 } 2+ , connected via different bridges, nitrato ( 1 ), azido ( 2 ) and phenoxido ( 3 ), depending on the nature of the counterion. The cryomagnetic measurements reveal weak ferromagnetic interactions between the copper(II) ions bridged by the nitrato ligands in 1 ( J = +1.026 cm -1 , H = - J S 1 S 2 ) and a weak antiferromagnetic coupling between the copper(II) ions connected by azido groups in 2 ( J = -0.295 cm -1 ). The exchange interaction between the copper(II) ions connected by two phenoxido groups is antiferromagnetic, J = -12.780 cm -1 .

Assessment of density functional approximations for calculation of exchange coupling constants in thiocyanato and cyanato double bridged binuclear Ni(II) complexes

In the present work, we examine the magnetic properties of 8 ?endto- end? thiocyanato, and 3 ?end-to-end? cyanato double bridged Ni(II) binuclear complexes. Thiocyanato complexes are weakly ferromagnetic. Cyanato bridged complexes exhibit weak antiferromagnetic coupling. Therefore, it is a challenge for computational chemistry to calculate the exchange coupling constant in these systems accurately. 17 different density functional approximations with different flavours are used to find the method of choice to study magnetic properties in binuclear Ni(II) complexes within the broken-symmetry approach. It is found that M06-2X and PWPB95 performed the best compared to the experimental values for the entire set of examined complexes. Furthermore, the magneto-structural correlation rationalizes the results.

Accuracy of density functional theory methods for the calculation of magnetic exchange couplings in binuclear iron(III) complexes

We assess the performance of popular hybrid density functional approximations for the prediction of magnetic exchange couplings (J) in FeIII complexes. To this end, eleven dinuclear oxo-bridged iron(III) complexes with accurately determined J values were chosen to cover cases of weak and strong antiferromagnetic couplings ranging from −6 cm-1 to −132 cm-1. Seven representative exchange-correlation functionals ranging from the hybrids PBEh and B3LYP to meta hybrids (M06, M062X), range separated functionals (HSE, LC-ωPBE), and LSDA with 30% of HF exchange were used for this assessment. These functionals have been suggested as good performers for this particular property in transition metal complexes and hence this assessment aims to narrow the interval of confidence for the particular case of Fe complexes. In addition, we examined the performance of an alternative method based on differential local spin rotations to calculate magnetic exchange coupling parameters in these complexes. Our results show that, although some of these functionals perform on par, the M06 and HSE functionals yield magnetic exchange couplings in closer agreement with experimental data, with mean absolute percentage errors of 8.6 and 9.2%, respectively.

DFT analysis, spectroscopic study and biological activity of a newly synthesized benzoylhydrazone binuclear Cu(II) complex

The current work reports a joint experimental and theoretical study of a novel Cu(II) complex [CuL(μ-CH3COO)]2, based on 2-acetylpyridine-benzoylhydrazone ligand (HL). The two Cu(II) atoms are five-coordinated, consisting of three NNO-donor atoms from the hidrazone ligand connected by acetate bridges. In addition to the structural analysis, the complete characterization includes magnetic susceptibility, elemental analysis, FT-IR and UV–Vis. Two different approaches of Density Functional Theory (DFT) with localized basis set and plane waves were performed. The theoretical calculations were used to optimize the complex geometry of the new structure allowing a better understanding of its spectroscopic properties with insights of most important interactions. Thereunto, we made a comparison between the following functionals: B3LYP, PBE1PBE, B3PW91, ω-B97XD, M06 (hybrids), CAM-B3LYP (long range hybrid), and B97-d (pure), using localized basis set, and for plane waves we used the PW91 functional. Considering the expected antifungal and antibacterial activity of hydrazone derivative ligand, the antimicrobial activity of HL and the complex [CuL(μ-CH3COO)]2 was tested against pathogenic Gram-positive and Gram-negative bacteria and fungi. The synthesized new structure of hydrazone complexed with copper (II) shows antimicrobial activity, and magnetic susceptibility results indicate weak antiferromagnetic coupling between the copper atoms.

Syntheses, crystal structures and magnetic properties of redox active cyanide-bridged trinuclear Ru-M2 (M = Co, Ni, Cu) complexes

Three redox active cyanide-bridged trinuclear complexes trans-(DMAP)4Ru(μ-CN)2- [M(PY5Me2)]2[PF6]4 (M = Co (1), Ni (2), Cu (3)) {DMAP = 4-dimethylamino-pyridine, PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine} were designed, synthesized and fully characterized by single-crystal X-ray diffraction, elemental analysis, IR spectrum, cyclic voltammetry and magnetic measurement. Due to the Jahn-Teller effect of CuII ion, the structure of compound 3 shows peculiar disordered configuration. Cyclic voltammograms show that compounds 1 and 2 possess redox-active Co and Ni centers except for the Ru center. Magnetic measurements show compound 1 behaves as paramagnetism with strong magnetic anisotropy of CoII ion and compound 2 has a very weak anti-ferromagnetic coupling constant, but compound 3 is only simple paramagnetism.

Magnetic Molecular Conductors Based on Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and the Tris(chlorocyananilato)ferrate(III) Complex.

Electrocrystallization of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) organic donor in the presence of the [Fe(ClCNAn)3]3- tris(chlorocyananilato)ferrate(III) paramagnetic anion in different stoichiometric ratios and solvent mixtures afforded two different hybrid systems formulated as [BEDT-TTF]4[Fe(ClCNAn)3]·3H2O (1) and [BEDT-TTF]5[Fe(ClCNAn)3]2·2CH3CN (2) (An = anilato). Compounds 1 and 2 present unusual structures without the typical segregated organic and inorganic layers, where layers of 1 are formed by Λ and Δ enantiomers of the anionic paramagnetic complex together with mixed-valence BEDT-TTF tetramers, while layers of 2 are formed by Λ and Δ enantiomers of the paramagnetic complex together with dicationic BEDT-TTF dimers and monomers. Compounds 1 and 2 show semiconducting behaviors with room-temperature conductivities of ca. 6 × 10-3 S cm-1 (ambient pressure) and 1 × 10-3 S cm-1 (under applied pressure of 12.1 GPa), respectively, due to strong dimerization between the donors. Magnetic measurements performed on compound 1 indicate weak antiferromagnetic coupling between high-spin FeIII (SFe = 5/2) and mixed-valence radical cation diyads (BEDT-TTF)2+ (Srad = 1/2) mediated by the anilate ligands, together with an important Pauli paramagnetism typical for conducting systems.

Partial Oxidation-Induced Electrical Conductivity and Paramagnetism in a Ni(II) Tetraaza[14]annulene-Linked Metal Organic Framework.

We report the synthesis and characterization of a two-dimensional (2D) conjugated Ni(II) tetraaza[14]annulene-linked metal organic framework (NiTAA-MOF) where NiTAA is a macrocyclic MN4 (M = metal, N = nitrogen) compound. The structure of NiTAA-MOF was elucidated by Fourier-transform infrared, X-ray photoemission, and X-ray diffraction spectroscopies, in combination with density functional theory (DFT) calculations. When chemically oxidized by iodine, the insulating bulk NiTAA-MOF (σ < 10-10 S/cm) exhibits an electrical conductivity of 0.01 S/cm at 300 K, demonstrating the vital role of ligand oxidation in the electrical conductivity of 2D MOFs. Magnetization measurements show that iodine-doped NiTAA-MOF is paramagnetic with weak antiferromagnetic coupling due to the presence of organic radicals of oxidized ligands and high-spin Ni(II) sites of the missing-linker defects. In addition to providing further insights into the origin of the induced electrical conductivity in 2D MOFs, both pristine and iodine-doped NiTAA-MOF synthesized in this work could find potential applications in areas such as catalase mimics, catalysis, energy storage, and dynamic nuclear polarization-nuclear magnetic resonance (DNP-NMR).

Dysprosium Chlorocyanoanilate-Based 2D-Layered Coordination Polymers.

A series of two-dimensional (2D)-layered coordination polymers (CPs) based on the heterosubstituted anilate ligand ClCNAn2- derived from 3-chloro-6-cyano-2,5-dihydroxybenzoquinone and DyIII are reported. By changes in the synthetic methods (layering technique, solvothermal or conventional one-pot reactions) and conditions (solvent, concentration, etc.), different types of 2D extended networks could be prepared and structurally characterized. Compounds 1 and 1', two polymorphs with the formula [Dy2(ClCNAn)3(DMSO)6]n·(H2O)x [x = 7 (1), 0 (1')], were prepared by a conventional one-pot reaction and recrystallized at different concentrations. Compound 2, formulated as [Dy2(ClCNAn)3(DMF)6]n, was prepared by a layering technique, while compound 3, formulated as {(Me2NH2)2[Dy2(ClCNAn)4(H2O)2]·(DMF)2·(H2O)5}n, was obtained by a solvothermal method. Compounds 1 and 2 are neutral 2D CPs of the ClCNAn2- ligand and DyIII ions, while 3 presents 2D anionic layers of [Dy2(ClCNAn)4(H2O)2]2- alternating with cationic layers of Me2NH2+ ions. These compounds show very diverse networks, with compound 1 forming 2D (8,3) and (4,3) topology with eight- and four-membered rings with square cavities, 1' and 2, respectively, a 2D (6,3) topology with six-membered rings (a rectangular cavity for 1' and a regular hexagonal cavity for 2), and 3 a 2D (4,4) topology with distorted square cavities. In this respect, 1 and 1' represent the first examples of polymorphism in the family of anilate-based CPs. Thermal analysis measurements (differential scanning calorimetry and thermogravimetry) show an exothermic polymorphic transformation from the kinetically stable 1' phase to the thermodynamically stable phase 1. The magnetic behavior of 1-3 very likely indicates depopulation of the mJ levels, while the presence of weak antiferromagnetic coupling between the DyIII centers mediated by the anilate bridge cannot be excluded.

Crystal Structures, Photoluminescence, and Magnetism of Two Novel Transition-Metal Complex Cocrystals with Three-Dimensional H-Bonding Organic Framework or Alternating Noncovalent Anionic and Cationic Layers.

Cocrystallization may alter material physicochemical properties; thus, the strategy of forming a cocrystal is generally used to improve the material performance for practical applications. In this study, two transition-metal complex cocrystals [Zn(bpy)3]H0.5BDC·H1.5BDC·0.5bpy·3H2O (1) and [Cu2(BDC)(bpy)4]BDC·bpy·2H2O (2) have been achieved using a hydrothermal reaction, where bpy and H2BDC represent 2,2′-bipyridine and benzene-1,3-dicarboxylic acid, respectively. Cocrystals were characterized by microanalysis, infrared spectroscopy, and UV–visible spectroscopy. Cocrystal 1 contains five components and crystallizes in a monoclinic space group P21/n. The H0.5BDC1.5–, H1.5BDC0.5–, and H2O molecules construct three-dimensional H-bonding organic framework; the [Zn(bpy)3]2+ coordination cations and uncoordinated bpy molecules reside in channels, where two coordinated bpy ligands in [Zn(bpy)3]2+ and one uncoordinated bpy adopt sandwich-type alignment via π···π stacking interactions. Cocrystal 2 with four components crystallizes in a triclinic space group P-1 to form alternating layers; the binuclear [Cu2(bpy)4(BDC)]2+ cations and uncoordinated bpy molecules build the cationic layers, and the BDC2– species with disordered lattice water molecules form the anionic layers. Cocrystal 1 shows intense photoluminescence at an ambient condition with a quantum yield of 14.96% and decay time of 0.48 ns, attributed to the π* → π electron transition within phenyl/pyridyl rings, and 2 exhibits magnetic behavior of an almost isolated spin system with rather weak antiferromagnetic coupling in the [Cu2(bpy)4(BDC)]2+ cation.

Tailored Tunnel Magnetoresistance Response in Three Ultrathin Chromium Trihalides.

Materials that demonstrate large magnetoresistance have attracted significant interest for many decades. Extremely large tunnel magnetoresistance (TMR) has been reported by several groups across ultrathin CrI3 by exploiting the weak antiferromagnetic coupling between adjacent layers. Here, we report a comparative study of TMR in all three chromium trihalides (CrX3, X = Cl, Br, or I) in the two-dimensional limit. As the materials exhibit different transition temperatures and interlayer magnetic ordering in the ground state, tunneling measurements allow for an easy determination of the field-temperature phase diagram for the three systems. By changing sample thickness and biasing conditions, we then demonstrate how to maximize and further tailor the TMR response at different temperatures for each material. In particular, near the magnetic transition temperature, TMR is nonsaturating up to the highest fields measured for all three compounds owing to the large, field-induced exchange coupling.

Structure, DFT Calculations, and Magnetic Characterization of Coordination Polymers of Bridged Dicyanamido-Metal(II) Complexes

Three coordination polymers of metal(II)-dicyanamido (dca) complexes with 4-methoxypyridine-N-oxide (4-MOP-NO); namely, catena-[Co(µ1,5-dca)2(4-MOP-NO)2] (1), catena-[Mn(µ1,5-dca)2(4-MOP-NO)2] (2), catena-[Cd(µ1,5-dca)2(4-MOP-NO)2] (3), and the mononuclear [Cu(κ1dca)2(4-MOP-NO)2] (4), were synthesized in this research. The complexes were analyzed by single crystal X-ray diffraction as well as spectroscopic methods (UV/vis, IR). The polymeric 1-D chains in complexes 1–3 were achieved by the doubly µ1,5-bridging dca ligands and the O-donor atoms of two axial 4-MOP-NO molecules in trans configuration around the distorted M(II) octahedral. On the other hand, the two “trans-axial” pyridine-N-oxide molecules in complexes 2 and 3 display opposite orientation (s-trans). The DFT (density functional theory) computational studies on the complexes 1–3 were consistent with the experimentally observed crystal structures. Compounds 1 and 2 display weak antiferromagnetic coupling between metal ions (J = −10.8 for 1 and −0.35 for 2).

Exploring the coordination abilities of 1,5-diisopropyl-3-(4′-carboxyphenyl)-6-oxoverdazyl

The reactivity of 1,5-diisopropyl-3-(4′-carboxyphenyl)-6-oxoverdazyl H I i -Pr toward divalent metal ions is dominated by the coordination of the carboxylate moiety onto the metal ions. The formula of the compound obtained by the reaction with the earth-alkaline magnesium (II) is [Mg( I i -Pr ) 2 ·2H 2 O] n ·0.35H 2 O ( I i -Pr –Mg) whereas the reaction with 3d divalent metal ions yields compounds of general formula [M( I i -Pr ) 2 (CH 3 OH) 4 ] (M = Mn ( I i -Pr –Mn), Co ( I i -Pr –Co), Ni ( I i -Pr –Ni), Zn ( I i -Pr –Zn)). The single-crystal X-ray diffraction analysis of I i -Pr –Mg reveals that the magnesium (II) adduct forms a one-dimensional (1D) coordination polymer with a terminal radical in a bidentate κ(O,O′) coordination mode and a bridging one in a μ − 1κ(O); 1κ (O′) syn – anti mode. The coordination sphere of the magnesium (II) is completed by two water molecules that established H-bonds with the carboxylate moiety of the terminal radicals that further reinforce the 1D coordination polymer. Adjacent chains interact through π-stacking of head-to-tail radicals. The single-crystal X-ray diffraction analysis of I i -Pr –Mn and I i -Pr –Ni reveals that the two compounds are isostructural. They are composed of mononuclear complexes where the divalent metal is surrounded by two radicals coordinated by monodentate carboxylate and four methanol molecules in the equatorial plane. Two methanol ligands act as H-bond donors toward the second oxygen atom of the carboxylate groups. The complexes interact through π-stacking of head-to-tail radicals and H-bonds between the two remaining methanol ligands and the oxygen atom of the oxoverdazyl moieties. In all of the compounds, the exchange interaction is dominated by the weak antiferromagnetic coupling related to the π-stacking of the radicals.

Sulfonate‐Ligated Coordination Polymers Incorporating Paramagnetic Transition Metals

The functionalized linker SNDC (4,8‐disulfonyl‐2,6‐naphthalenedicarboxylate) has been incorporated into 2D extended structures featuring FeII or CoII. These materials are isostructural and exhibit coordination via both the carboxylate and sulfonate groups of the SNDC linker. The variable temperature magnetic behavior of the FeII and CoII materials has also been measured. A simplified model of these systems implies weak (|J| &lt; 5 cm–1) antiferromagnetic coupling, as is expected for a superexchange mechanism operating through the carboxylate paddlewheels. The combination of functionalized linkers, such as SNDC, with paramagnetic metal centers is a promising pathway towards functionalized materials, and the coordination polymers reported join a small class of known materials featuring SNDC.

Trinuclear and Mononuclear Lanthanoid Complexes Containing 2‐Methyl‐8‐quinolinolate: Synthesis, Structures, and Magnetic Properties

Two series of new lanthanoid complexes containing 2‐methyl‐8‐quinolinolate (MQ–) have been prepared at elevated temperatures. Lighter elements yield trinuclear complexes [Ln3(MQ)9(HMQ)]·5H2O·3MeOH (1Ln, where Ln = Pr, Eu, Gd, Tb, Dy and Ho) by solvothermal reactions, and heavier lanthanoids yield the mononuclear complexes [Ln(MQ)3(HMQ)]·HMQ·MeOH (2Ln, where Ln = Dy, Ho, Er and Yb), with overlap between the two series found in the cases of Dy and Ho. Magnetic studies reveal that 1Tb, 1Dy, and 2Er show distinct slow magnetic relaxation under external DC fields. The magnetic data for 1Gd yielded the J value for weak antiferromagnetic coupling and the magnetocaloric –ΔSm value of 20 J kg–1 K–1 at 2 K and ΔH = 5 T.

Evolution of interlayer and intralayer magnetism in three atomically thin chromium trihalides

We conduct a comprehensive study of three different magnetic semiconductors, CrI3, CrBr3, and CrCl3, by incorporating both few-layer and bilayer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, and magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest-neighbor exchange interactions, we are able to further determine a simple spin Hamiltonian describing all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in more isotropic CrBr3, with transition temperature still close to that of the bulk.

Thiosemiquinoid Radical-Bridged CrIII2 Complexes with Strong Magnetic Exchange Coupling.

Semiquinoid radical bridging ligands are capable of mediating exceptionally strong magnetic coupling between spin centers, a requirement for the design of high-temperature magnetic materials. We demonstrate the ability of sulfur donors to provide much stronger coupling relative to their oxygen congeners in a series of dinuclear complexes. Employing a series of chalcogen donor-based bis(bidentate) benzoquinoid bridging ligands, the series of complexes [(TPyA)2Cr2(RL4-)]2+ (OLH4 = 1,2,4,5-tetrahydroxybenzene, OSLH4 = 1,2-dithio-4,5-dihydroxybenzene, SLH4 = 1,2,4,5-tetrathiobenzene, TPyA = tris(2-pyridylmethyl)amine) was synthesized. Variable-temperature dc magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange coupling between CrIII centers in these complexes, with exchange constants of J = -2.83(3) (OL4-), -2.28(5) (OSL4-), and -1.80(2) (SL4-) cm-1. Guided by cyclic voltammetry and spectroelectrochemical measurements, chemical one-electron oxidation of these complexes gives the radical-bridged species [(TPyA)2Cr2(RL3-•)]3+. Variable-temperature dc susceptibility measurements in these complexes reveal the presence of strong antiferromagnetic metal-semiquinoid radical coupling, with exchange constants of J = -352(10) (OL3-•), - 401(8) (OSL3-•), and -487(8) (SL3-•) cm-1. These results provide the first measurement of magnetic coupling between metal ions and a thiosemiquinoid radical, and they demonstrate the value of moving from O to S donors in radical-bridged metal ions in the design of magnetic molecules and materials.

Using Redox-Active π Bridging Ligand as a Control Switch of Intramolecular Magnetic Interactions.

Intramolecular magnetic interactions in the dinuclear complexes [(tpy)Ni(tphz)Ni(tpy)] n+ ( n = 4, 3, and 2; tpy, terpyridine; tphz, tetrapyridophenazine) were tailored by changing the oxidation state of the pyrazine-based bridging ligand. While its neutral form mediates a weak antiferromagnetic (AF) coupling between the two S = 1 Ni(II), its reduced form, tphz•-, promotes a remarkably large ferromagnetic exchange of +214(5) K with Ni(II) spins. Reducing twice the bridging ligand affords weak Ni-Ni interactions, in marked contrast to the Co(II) analogue. Those experimental results, supported by a careful examination of the involved orbitals, provide a clear understanding of the factors which govern strength and sign of the magnetic exchange through an aromatic bridging ligand, a prerequisite for the rational design of strongly coupled molecular systems and high TC molecule-based magnets.