Ferromagnetic Polarization: The Quantum Picture of Switching On/Off Single-Molecule Magnetism

Magnetic behavior and ground spin states for coordination {L·[MII(Hal)2]3}3- assemblies (Hal = Cl or I) of radical trianion hexacyanohexaazatriphenylenes (L) with three coordinated high-spin FeII (S = 2) or CoII (S = 3/2) centers

A series of trianion assemblies of hexaazatriphenylenehexacarbonitrile {HAT(CN)6} and hexaazatrinaphthylenehexacarbonitrile {HATNA(CN)6} with three Fe(II) or Co(II) ions: {cryptand(K+)}3·{HATNA(CN)6·(FeIII2)3}3-·2C6H4Cl2 (1), {cryptand(K+)}3·{HATNA(CN)6·(CoIII2)3}3-·2C6H4Cl2 (2), and (CV+)3·{HAT(CN)6·(CoIICl2)3}3-·0.5(CVCl)·2.5C6H4Cl2 (3) are synthesized (CVCl = crystal violet). Salt 1 has a χMT value of 9.80 emu K mol-1 at 300 K, indicating a contribution of three high-spin FeII (S = 2) and one S = 1/2 of HATNA(CN)6˙3-. The χMT value increases with cooling up to 12.92 emu K mol-1 at 28 K, providing a positive Weiss temperature of +20 K. Such behavior is described using a strong antiferromagnetic coupling between S = 2 and S = 1/2 with J1 = -82.1 cm-1 and a weaker FeII-FeII antiferromagnetic coupling with J2 = -7.0 cm-1. As a result, the spins of three Fe(II) ions (S = 2) align parallel to each other forming a high-spin S = 11/2 system. Density functional theory (DFT) calculations support a high-spin state of CoII (S = 3/2) for 2 and 3. However, the χMT value of 2 and 3 is 2.25 emu K mol-1 at 300 K, which is smaller than 6 emu K mol-1 calculated for the system with three independent S = 3/2 and one S = 1/2 spins. In contrast to 1, the χMT values decrease with cooling to 0.13-0.36 emu K mol-1 at 1.9 K, indicating that spins of cobalt atoms align antiparallel to each other. Data fitting using PHI software for the model consisting of three high-spin Co(II) ions and an S = 1/2 radical ligand shows very large CoII-L˙3- coupling for 2 and 3 with J1 values of -442 and -349 cm-1. The CoII-CoII coupling via the ligand (J2) is also large, being -100 and -84 cm-1, respectively, which is more than 10 times larger than that of 1. One of the reasons for the J2 increase may be the shortening of the Co-N(L) bonds in 3 and 2 to 2.02(2) and 1.993(12) Å. DFT calculations support the population of the quartet state for the Co3 system, whereas the high-spin decet (S = 9/2) state is positioned higher by 680 cm-1 and is not populated at 300 K. This is explained by the large CoII-CoII coupling. Thus, a balance between J1 and J2 couplings provides parallel or antiparallel alignment of the FeII and CoII spins, leading to high- or low-spin ground states of {L·[MII(Hal)2]3}3-.

Guest-Induced Switching of a Molecule-Based Magnet in a 3d-4f Heterometallic Cluster-Based Chain Structure

With the motivation of controlling a magnetic switch by external stimuli, we report here an infinite chain structure formed from the secondary building units of Cu₃Tb₂ clusters through the linkage of nitrate ions. It behaves as a molecule-based magnet with the highest energy barrier among isolated Tb/Cu-based single-molecule magnets and single-chain magnets, which is close to a dimer of a Cu₃Tb₂ cluster unit from a magnetic point as revealed by its correlation length of 2.23 Cu₃Tb₂ units. This kind of molecule-based magnet in a chain structure is rare. The removal of its guest ethanol molecules leads to the complete disappearance of slow magnetic relaxation behavior. Interestingly, the capture and removal of guest ethanol molecules are reversible, mediating a rare ON/OFF switching of a 3d-4f heterometallic molecule-based magnet, which was interpreted by the theoretical calculations based on the structural difference upon desolvation.

Crystal structures of three anionic lanthanide-aluminium [3.3.1] metallacryptate complexes

The three isomorphous [3.3.1] metallacryptate complexes bis-(pyridinium) di-aqua-dipyridine-hexa-kis-[μ3-salicyl-hydroximato(3-)]bis-[μ2-salicyl-hydroxim-ato(1-)]hexa-aluminiumgadolinium-pyridine-water (1/7.396/1), (C5H6N)2[GdAl6(C7H6NO3)2(C7H4NO3)7(C5H5N)1.855(H2O)2]·7.396C5H5N·H2O or [Hpy]2[GdAl6(H2shi)2(shi)7(py)1.855(H2O)2]·7.396py·H2O, 1, bis-(pyridinium) di-aqua-dipyridine-hexa-kis-[μ3-salicyl-hydroximato(3-)]bis-[μ2-salicyl-hydroxim-ato(1-)]hexa-aluminiumdysprosium-pyridine-water (1/7.429/1), (C5H6N)2[DyAl6(C7H6NO3)2(C7H4NO3)7(C5H5N)1.855(H2O)2]·7.429C5H5N·H2O or [Hpy]2[DyAl6(H2shi)2(shi)7(py)1.891(H2O)2]·7.429py·H2O, 2, and bis-(pyrid-in-ium) di-aqua-dipyridine-hexa-kis-[μ3-salicyl-hydroximato(3-)]bis-[μ2-salicyl-hydrox-imato(1-)]hexa-aluminiumytterbium-pyridine-water (1/7.386/1), (C5H6N)2[YbAl6(C7H6NO3)2(C7H4NO3)7(C5H5N)1.855(H2O)2]·7.429C5H5N·H2O or [Hpy]2[YbAl6(H2shi)2(shi)7(py)1.818(H2O)2]·7.386py·H2O, 3, where Hpy+ is pyridinium, shi3- is salicyl-hydroximate, and py is pyridine, consist of an aluminium-based metallacryptand that captures an Ln III ion in the central cavity. The metallacryptand portions are comprised of an Al-N-O repeat unit; thus, they can be considered three-dimensional metallacrowns. The encapsulated Ln III ions are nine-coordinate with a spherical capped-square-anti-prism geometry, while the six AlIII ions are all octa-hedral. Four of the AlIII ions are chiral centers with 2 Δ and 2 Λ stereoconfigurations. The remaining two AlIII ions have trans chelate rings from two different shi3- ligands. For 1-3, a section of the main mol-ecule is disordered induced by the presence or absence of a pyridine ligand coordinated to one of the AlIII ions. In the absence of the pyridine moiety, an H2shi- ligand moves into the space otherwise occupied by the pyridine and the phenol oxygen atom coordinates to the AlIII ion. The movement of the H2shi- ligand induces movement for the Ln III ion, for another AlIII ion that also binds the same H2shi- ligand, and for one of the shi3- ligands coordinated to the latter AlIII ion. For 1-3 the occupancy ratio of the metallacryptand portions refined to 0.8550 (13):0.1450 (13), to 0.8909 (13):0.1091 (13), and to 0.8181 (14):0.1819 (14), respectively.

Crystal structures of two heterotrimetallic dysprosium-manganese-sodium 12-metallacrown-4 complexes with the bridging ligands 3-hy-droxy-benzoate and 4-hy-droxy-benzoate

The syntheses and crystal structures for the compounds tetra-μ-aqua--tetra-kis-{2-[aza-nid-yl-ene(oxido)meth-yl]phenolato}tetra-kis-(μ2-3-hy-droxy-benzoato)dys-pro-s-ium(III)-tetra-manganese(III)sodium(I) N,N-di-methyl-acetamide deca-solvate, [DyMn4Na(C7H5O3)4(C7H4NO2)4(H2O)4]·10C4H9NO or [DyIIINa(4-OHben)4{12-MCMn(III)N(shi)-4}(H2O)4]·10DMA, 1, and tetra-μ-aqua--tetra-kis-{2-[aza-nid-yl-ene(oxido)meth-yl]phenolato}tetra-kis-(μ2-3-hy-droxy-benzoato)dys-pros-ium(III)tetra-manganese(III)sodium(I) N,N-di-methylformamide tetra-solvate, [DyMn4Na(C7H5O3)4(C7H4NO2)4(H2O)4]·4C3H7NO or [DyIIINa(3-OHben)4{12-MCMn(III)N(shi)-4}(H2O)4]·4DMF, 2, and where MC is metallacrown, shi3- is salicyl-hydroximate, 3-OHben is 3-hy-droxy-benzoate, DMA is N,N-di-methyl-acetamide, 4-OHben is 4-hy-droxy-benzoate, and DMF is N,N-di-methyl-formamide, are reported. For both 1 and 2, the macrocyclic metallacrown consists of an [MnIII-N-O] ring repeat unit, and the domed metallacrown captures two ions in the central cavity: a DyIII ion on the convex side of the metallacrown and an Na+ ion the concave side. The MnIII ions are six-coordinate with an elongated tetra-gonally distorted octa-hedral geometry. Both the DyIII and Na+ ions are eight-coordinate. The DyIII ions possess a square-anti-prismatic geometry, while the Na+ ions have a distorted biaugmented trigonal-prismatic geometry. Four 3-hy-droxy-benzoate or 4-hy-droxy-benzoate ligands bridge each MnIII ion to the central DyIII ion. For 1, whole-mol-ecule disorder is observed for the main mol-ecule, excluding only the DyIII and Na+ ions, and the occupancy ratio refined to 0.8018 (14):0.1982 (14). Three DMA mol-ecules were refined as disordered with two in general positions by an approximate 180° rotation and the third disordered twice by general disorder as well as by an exact 180° rotation about a twofold axis that bis-ects it. The occupancy ratios refined to 0.496 (8):0.504 (8), 0.608 (9):0.392 (9), and 2×0.275 (7):2×0.225 (7), respectively. For 2, segments of the metallacrown are disordered including the DyIII ion, one of the Mn ions, two of the Mn-bound 4-hy-droxy-benzoate ligands, the Mn-bridging salicyl-hydroximate ligand, and portions of the remaining three shi3- ligands. The occupancy ratio for the metallacrown disorder refined to 0.849 (9):0.151 (9). Two DMF solvent mol-ecules are also disordered, each over two orientations. The disorder ratios refined to 0.64 (3):0.36 (3) and to 0.51 (2):0.49 (2), respectively. For 2, the crystal under investigation was refined as a non-merohedric twin by a 90° rotation around the real a axis [twin ratio 0.9182 (8):0.0818 (8)].

Octacyanidometallates for multifunctional molecule-based materials

Octacyanidometallates have been successfully employed in the design of heterometallic coordination systems offering a spectacular range of desired physical properties with great potential for technological applications. The [M(CN)8]n- ions comprise a series of complexes of heavy transition metals in high oxidation states, including NbIV, MoIV/V, WIV/V, and ReV. Since the discovery of the pioneering bimetallic {MnII4[MIV(CN)8]2} and {MnII9[MV(CN)8]6} (M = Mo, W) molecules in 2000, octacyanidometallates were fruitfully explored as precursors for the construction of diverse d-d or d-f coordination clusters and frameworks which could be obtained in the crystalline form under mild synthetic conditions. The primary interest in [M(CN)8]n--based networks was focused on their application as molecule-based magnets exhibiting long-range magnetic ordering resulting from the efficient intermetallic exchange coupling mediated by cyanido bridges. However, in the last few years, octacyanidometallate-based materials proved to offer varied and remarkable functionalities, becoming efficient building blocks for the construction of molecular nanomagnets, magnetic coolers, spin transition materials, photomagnets, solvato-magnetic materials, including molecular magnetic sponges, luminescent magnets, chiral magnets and photomagnets, SHG-active magnetic materials, pyro- and ferroelectrics, ionic conductors as well as electrochemical containers. Some of these materials can be processed into the nanoscale opening the route towards the development of magnetic, optical and electronic devices. In this review, we summarise all important achievements in the field of octacyanidometallate-based functional materials, with the particular attention to the most recent advances, and present a thorough discussion on non-trivial structural and electronic features of [M(CN)8]n- ions, which are purposefully explored to introduce desired physical properties and their combinations towards advanced multifunctional materials.

Field-Induced Single-Ion Magnetic Behavior in Two Mononuclear Cobalt(II) Complexes

The employment of a new rigid N-tridentate ligand, bis(1-chloroimidazo[1,5-a]pyridin-3-yl)pyridine (bcpp), in the construction of cobalt(II) single-ion magnets is reported. Two cobalt(II) complexes, [Co(bcpp)Cl₂ ] (1) and [Co(bcpp)Br₂ ] (2), have been prepared and characterized. Single-crystal XRD analyses reveal that complexes 1 and 2 are isostructural. They are pentacoordinated mononuclear cobalt(II) compounds with expected trigonal bipyramidal geometry. Both analysis of the magnetic data and ab initio calculations reveal easy-plane magnetic anisotropy (D>0) for 1 and 2. Detailed alternating current magnetic susceptibility measurements reveal the occurrence of slow magnetic relaxation behavior for the cobalt(II) centers of 1 and 2; thus indicating that both complexes are field-induced single-ion magnets.

Synthesis, crystal structures and magnetic properties of a series of pentanuclear heterometallic [CuII3LnIII2] (Ln = Ho, Dy, and Gd) complexes containing mixed organic ligands

Three novel heterometallic compounds [CuII3LnIII2] [Ln = Ho (1), Dy (2) and Gd (3)] containing mixed organic ligands were obtained. 1 and 2 show slow magnetic relaxation behaviour.

A new family of {Co4Ln8} metallacrowns with a butterfly-shaped structure

A series of cobalt–lanthanide metallacrowns (MCs) {CoIII4Ln₈} (Ln = Dy³⁺1, Ho³⁺2 and Tm³⁺3) based on pyrazinehydroxamic acid (H₂pyzha) and pyrazinic acid (Hpyzic) ligands have been synthesized.

Regulation of magnetic relaxation behavior by replacing 3d transition metal ions in [M2Dy2] complexes containing two different organic chelating ligands

Two [MIII2DyIII2] complexes (M = Fe for 1 and Co for 2) with mixed organic ligands were obtained. Complex 2 exhibits single molecule magnet behavior with U_eff = 64.0(9) K.

Magnetic on-off switching in redox non-innocent ligand bridged binuclear cobalt complexes

The redox non-innocent ligand 2,2'-bipyrimidine (bpym) reacted with two molar equivalents of LCo(tol) (L = CH(MeC[double bond, length as m-dash]NAr)₂, Ar = 2,6-iPr₂C₆H₃) to afford the binuclear Co^II complex (LCo)₂(μ-bpym) (1), in which the bpym moiety accepts two electrons from the metal centers and in the dianion state. Complex 1 readily underwent one-electron oxidation when treated with [Cp₂Fe]⁺[BPh₄]^- affording the cation salt [(LCo)₂(μ-bpym)][BPh₄] (2) with the bpym moiety in the radical anion state. Meanwhile, complex 2 could be converted back to 1 with elemental potassium. The magnetic studies revealed that complex 2 is a single molecular magnet (SMM) under a small external field, while 1 has no SMM activity. This represents a new example of a SMM exhibiting a fully reversible 'on/off' switching ability in anodic and neutral states.

Metal-ion induced ferromagnetic polarization in a mixed-spin system

Three new 3d-3d heterometallic complexes, [CuM^II(hmb)₆(OH)]ClO₄·H₂O (M = Zn (1), Ni (2) and Co (3)) (Hhmb = 2-hydroxy-3-methoxy-benzaldehyde), have been synthesized. Structural analysis reveals that the three complexes are isostructural. Three Cu^II ions are in a perfect trigonal geometrical frustration and form a tetrahedral frustrated system with the introduced fourth ion Zn^II/Ni^II/Co^II. Magnetic studies indicate the antiferromagnetic coupling between metal ions in all 1, 2 and 3. Due to the geometrical frustration, spin-frustrated magnetism is also a typical feature of 1-3 and the magnetostructural correlation is far more complicated than their structures. The detailed magnetic investigation found that the ground state spin of 2 cannot be simply determined by the antiferromagnetically linear arrangement of spins, because the strong antiferromagnetic coupling between Cu^II and Ni^II ions quenches the spin frustration and ferromagnetically polarizes the spins of the [Cu] unit. Contrarily, the antiferromagnetic coupling between Cu^II and Co^II ions is not strong enough in 3, so there is no similar behaviour compared to 2.

Manganese clusters of aromatic oximes: synthesis, structure and magnetic properties

With the aim of tuning the structures by using oxime ligands with different non-coordinating groups, three aromatic oxime ligands were designed by fusing oxime groups ([double bond, length as m-dash]N-OH) onto different non-coordinating groups. Their reactions with the corresponding Mn(ii) salts gave five manganese clusters [Mn(μ₃-O)(L¹)₃(DMF)(H₂O)₃Cl]·2DMF·CH₃OH (1), [Mn(μ₃-O)(L²)₃(OAc)(CH₃OH)₂] (2), [Mn(μ₃-O)₂(L²)₆(H₂O)(py)₇](ClO₄)₂·py·0.5CH₃OH·2H₂O (3), [MnO₄(L²)₈(DMF)₄]·DMF·6CH₃CN (4), and [MnMnO₄(L³)₁₂]·3DMF·6H₂O (5), in which H₂L¹, H₂L² and HL³ represent indane-1,2,3-trione-1,2-dioxime, acenaphthenequinone dioxime, and 9,10-phenanthrenedione-9-oxime, respectively. Their structures were determined and studied in detail. 1 and 2 show planar triangular trinuclear Mn structures. 3 has a hexanuclear Mn skeleton formed from two Mn triangular units through inter-trinuclear mutual coordination. 4 and 5 present octanuclear skeletons constructed from planar triangular Mn₃O and tetrahedral Mn₄O secondary building units, respectively, with different symmetries and different oxidation states of the manganese ions. Their structural studies reveal a significant contribution of the parent rings for fusing oxime groups, different non-coordinating groups and anions to the formation of different cluster skeletons. Their magnetic properties were investigated and simulated, which revealed the presence of dominant antiferromagnetic interactions between the metal ions in these compounds.

Two unprecedented decanuclear heterometallic [MnII2MnIII6LnIII2] (Ln = Dy, Tb) complexes displaying relaxation of magnetization

The synthesis and magnetic properties of two novel [Mn₈Ln₂] complexes containing the gem-diol form of di-2-pyridyl ketone and triethanolamine were investigated.