Step-by-Step Electrocrystallization Processes to Make Multiblock Magnetic Molecular Heterostructures.

Assembling conductive or magnetic heterostructures by bulk inorganic materials is important for making functional electronic or spintronic devices, such as semiconductive p-doped and n-doped silicon for P-N junction diodes, alternating ferromagnetic and nonmagnetic conductive layers used in giant magnetoresistance (GMR). Nonetheless, there have been few demonstrations of conductive or magnetic heterostructures made by discrete molecules. It is of fundamental interest to prepare and investigate heterostructures based on molecular conductors or molecular magnets, such as single-molecule magnets (SMMs). Herein, we demonstrate the fabrication of a series of molecular heterostructures composed of (TTF)2M(pdms)2 (TTF = tetrathiafulvalene, M = Co(II), Zn(II), Ni(II), H2pdms = 1,2-bis(methanesulfonamido)benzene) multiple building blocks through a well-controlled step-by-step electrocrystallization growth process, where the Co(pdms)2, Ni(pdms)2, and Zn(pdms)2 anionic complex is a SMM, paramagnetic, and diamagnetic molecule, respectively. Magnetic and SMM properties of the heterostructures were characterized and compared to the parentage (TTF)2Co(pdms)2 complex. This study presents the first methodology for creating molecule-based magnetic heterostructural systems by electrocrystallization.

4f-π Molecular Hybrid Exhibiting Rich Conductive Phases and Slow Relaxation of Magnetization.

Cooperation between single-molecule magnets and electrical conductivity holds promise for preparing high-density magnetic devices; however, there are only a few reports so far. Here we report a 4f-π-based molecular hybrid, k-(ET)5Dy(NCS)7(KCl)0.5 (1) (ET = bis(ethylenedithio)tetrathiafulvalene, NCS- = thiocyanate), which undergoes slow relaxation of the magnetization and electrical conductivity. Unlike common ET-based conductive salts, K+ ions were intercalated into ET layers and coordinated with ET radicals. We found that the ET charges were sensitive to temperature, resulting in rich conductive phases at 75-300 K. In particular, the upturn in conductivity with a clear hysteresis loop was explained by the formation of partially oxidized states with charges close to 0.5+, which accounts for a metallic state. From the results of electronic structure calculations, the hole concentration increased to 125 K, which is consistent with a partially oxidized state upon cooling. The weak antiferromagnetic interactions accompanied by a dual magnetic relaxation process below 4 K are closely associated with the weak 4f-π interactions.

Emergence of Metallic Conduction and Cobalt(II)-Based Single-Molecule Magnetism in the Same Temperature Range.

Single-molecule magnets exhibit magnetic bistabililties at the molecular level, making them promising for molecule-based spintronics due to high magnetic densities. The incorporation of SMM behavior and electrical conductivity in one compound is rare because these two physical properties often do not operate in the same temperature range, which further hinders their use in practical applications. Here we present an organic-inorganic molecular hybrid, ß″-(BEDO-TTF)3[Co(pdms)2]·(MeCN)(H2O)2 (BO3) (BEDO-TTF = bis(ethylenedioxy)tetrathiafulvalene and H2pdms = 1,2-bis(methanesulfonamido)benzene), which manifests both metallic conduction (electrical conductivity up to 1000 S cm-1 at 12 K under 2.0 gigapascal pressure) and SMM behavior in the temperature range 12-26 K for the first time.

An Organic-Inorganic Hybrid Exhibiting Electrical Conduction and Single-Ion Magnetism.

The first three-dimensional (3D) conductive single-ion magnet (SIM), (TTF)2 [Co(pdms)2 ] (TTF=tetrathiafulvalene and H2 pdms=1,2-bis(methanesulfonamido)benzene), was electrochemically synthesised and investigated structurally, physically, and theoretically. The similar oxidation potentials of neutral TTF and the molecular precursor [HNEt3 ]2 [M(pdms)2 ] (M=Co, Zn) allow for multiple charge transfers (CTs) between the SIM donor [M(pdms)2 ]n- and the TTF.+ acceptor, as well as an intradonor CT from the pdms ligand to Co ion upon electrocrystallisation. Usually TTF functions as a donor, whereas in our system TTF is both a donor and an accepter because of the similar oxidation potentials. Furthermore, the [M(pdms)2 ]n- donor and TTF.+ acceptor are not segregated but strongly interact with each other, contrary to reported layered donor-acceptor electrical conductors. The strong intermolecular and intramolecular interactions, combined with CT, allow for relatively high electrical conductivity even down to very low temperatures. Furthermore, SIM behaviour with slow magnetic relaxation and opening of hysteresis loops was observed. (TTF)2 [Co(pdms)2 ] (2-Co) is an excellent building block for preparing new conductive SIMs.

An electrically conductive metallocycle: densely packed molecular hexagons with π-stacked radicals.

Electrical conduction among metallocycles has been unexplored because of the difficulty in creating electronic transport pathways. In this work, we present an electrocrystallization strategy for synthesizing an intrinsically electron-conductive metallocycle, [Ni6(NDI-Hpz)6(dma)12(NO3)6]·5DMA·nH2O (PMC-hexagon) (NDI-Hpz = N,N'-di(1H-pyrazol-4-yl)-1,4,5,8-naphthalenetetracarboxdiimide). The hexagonal metallocycle units are assembled into a densely packed ABCABC… sequence (like the fcc geometry) to construct one-dimensional (1D) helical π-stacked columns and 1D pore channels, which were maintained under the liberation of H2O molecules. The NDI cores were partially reduced to form radicals as charge carriers, resulting in a room-temperature conductivity of (1.2-2.1) × 10-4 S cm-1 (pressed pellet), which is superior to that of most NDI-based conductors including metal-organic frameworks and organic crystals. These findings open up the use of metallocycles as building blocks for fabricating conductive porous molecular materials.

Melamine-induced synthesis of a structurally perfect kagomé antiferromagnet.

We report here a structurally perfect kagomé lattice {[Cu3(bpy)6](SiF6)3(melamine)8}n (1), where bpy is 4,4'-bipyridine and [SiF6]2- is a hexafluorosilicate anion. In comparison to general 1D linear, 2D layered and 3D cubic metal-organic frameworks, by using Cu2+ nodes and bpy ligands, a perfect kagomé lattice was synthesized by introducing C3 symmetrical melamine molecules. Magnetic susceptibility and low-temperature heat capacity measurements indicated weak antiferromagnetic interactions between the spins and no long-range magnetic ordering to 0.7 K. Using C3 symmetrical melamine molecules can be considered as a challenging synthetic strategy to afford new topological materials.

Heterospin frustration in a metal-fullerene-bonded semiconductive antiferromagnet.

Lithium-ion-encapsulated fullerenes (Li+@C60) are 3D superatoms with rich oxidative states. Here we show a conductive and magnetically frustrated metal-fullerene-bonded framework {[Cu4(Li@C60)(L)(py)4](NTf2)(hexane)}n (1) (L = 1,2,4,5-tetrakis(methanesulfonamido)benzene, py = pyridine, NTf2- = bis(trifluoromethane)sulfonamide anion) prepared from redox-active dinuclear metal complex Cu2(L)(py)4 and lithium-ion-encapsulated fullerene salt (Li+@C60)(NTf2-). Electron donor Cu2(L)(py)2 bonds to acceptor Li+@C60 via eight Cu‒C bonds. Cu-C bond formation stems from spontaneous charge transfer (CT) between Cu2(L)(py)4 and (Li+@C60)(NTf2-) by removing the two-terminal py molecules, yielding triplet ground state [Cu2(L)(py)2]+(Li+@C60•-), evidenced by absorption and electron paramagnetic resonance (EPR) spectra, magnetic properties and quantum chemical calculations. Moreover, Li+@C60•- radicals (S = ½) and Cu2+ ions (S = ½) interact antiferromagnetically in triangular spin lattices in the absence of long-range magnetic ordering to 1.8 K. The low-temperature heat capacity indicated that compound 1 is a potential candidate for an S = ½ quantum spin liquid (QSL).

Simultaneous manifestation of metallic conductivity and single-molecule magnetism in a layered molecule-based compound.

Single-molecule magnets (SMMs) show superparamagnetic behaviour below blocking temperature at the molecular scale, so they exhibit large magnetic density compared to the conventional magnets. Combining SMMs and molecular conductors in one compound will bring about new physical phenomena, however, the synergetic effects between them still remain unexplored. Here we present a layered molecule-based compound, ß''-(BEDO-TTF)4 [Co(pdms)2]·3H2O (BO4), (BEDO-TTF (BO) and H2pdms are bis(ethylenedioxy)tetrathiafulvalene and 1,2-bis(methanesulfonamido)benzene, respectively), which was synthesized by using an electrochemical approach and studied by using crystal X-ray diffraction. This compound simultaneously exhibited metallic conductivity and SMM behaviour up to 11 K for the first time. The highest electrical conductivity was 400-650 S cm-1 at 6.5 K, which is the highest among those reported so far for conducting SMM materials. Furthermore, antiferromagnetic ordering occurred below 6.5 K, along with a decrease in conductivity, and the angle-independent negative magnetoresistance suggested an effective electron correlation between the conducting BO and Co(pdms)2 SMM layers (d-π interactions). The strong magnetic anisotropy and two-dimensional conducting plane play key roles in the low-temperature antiferromagnetic semiconducting state. BO4 is the first compound exhibiting antiferromagnetic ordering among SMMs mediated by π-electrons, demonstrating the synergetic effects between SMMs and molecular conductors.

Exploration of SMM behavior of Ln2 complexes derived from thianaphthene-2-carboxylic acid.

Two lanthanide-based dinuclear complexes [LnIII(L)3(DMF)(H2O)]2 (Ln = DyIII (1)/TbIII (2)) derived from thianaphthene-2-carboxylic acid have been synthesized and characterized in detail. Single-crystal XRD analyses confirm that the centro-symmetric discrete dimeric structures contain eight-coordinated lanthanide centers with biaugmented trigonal prism geometry in the complexes. Magnetic studies reveal the presence of antiferromagnetic interaction as well as thermal depopulation of Stark sublevels with a decrease in temperature for both complexes, whereas prominent field-induced single-molecule magnet behavior was observed for 1. Also, an isostructural yttrium-based complex (3) and a magnetically dilute sample of complex 1 (1') have been prepared for magnetic dilution study. 1' shows a combination of Orbach and Raman relaxation processes with effective energy barrier ΔE = 19.508 cm-1 and relaxation time τo = 9.710 × 10-10 s.

Conducting single-molecule magnet materials.

Multifunctional molecular materials exhibiting electrical conductivity and single-molecule magnet (SMM) behaviour are particularly attractive for electronic devices and related applications owing to the interaction between electronic conduction and magnetization of unimolecular units. The preparation of such materials remains a challenge that has been pursued by a bi-component approach of combination of SMM cationic (or anionic) units with conducting networks made of partially oxidized (or reduced) donor (or acceptor) molecules. The present status of the research concerning the preparation of molecular materials exhibiting SMM behaviour and electrical conductivity is reviewed, describing the few molecular compounds where both SMM properties and electrical conductivity have been observed. The evolution of this research field through the years is discussed. The first reported compounds are semiconductors in spite being able to present relatively high electrical conductivity, and the SMM behaviour is observed at low temperatures where the electrical conductivity of the materials is similar to that of an insulator. During the recent years, a breakthrough has been achieved with the coexistence of high electrical conductivity and SMM behaviour in a molecular compound at the same temperature range, but so far without evidence of a synergy between these properties. The combination of high electrical conductivity with SMM behaviour requires not only SMM units but also the regular and as far as possible uniform packing of partially oxidized molecules, which are able to provide a conducting network.

Anion-driven structures and SMM behavior of dinuclear terbium and ytterbium complexes.

The work in this present investigation reports the syntheses, crystal structures and magnetic properties of five dinuclear lanthanide complexes having compositions [Tb2(HL)4(NO3)6] (1), [Tb2(HL)4Cl6]·2EtOH (2), [Yb2(HL)4(NO3)6] (3), [Yb2(HL)4Cl6]·2EtOH (4) and [Y2(HL)4(NO3)6] (5) with HL = 8-hydroxyquinaldine. It is evident from the crystal structures that the coordination number of trivalent lanthanide ions in compounds 1, 3 and 5 is nine, whereas that for compounds 2 and 4 is six. A dynamic magnetic study shows that both compounds 1 and 3 exhibit single-molecule-magnet (SMM) behavior while compounds 2 and 4 do not have any SMM property.