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, β′′-(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.

(HNEt 3 ) 2 [Co(pdms) 2 ] was synthesized following reported procedures. [2] Preparation of βʺ-(BO) 4  Physical characterization: Single-crystal crystallographic data were collected at 120 K on a Rigaku Saturn70 CCD Diffractometer (Rigaku, Tokyo, Japan) with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) produced by a VariMax microfocus X-ray rotating anode source. Data processing was performed using the Crystal-Clear crystallographic software package. [3] The structures were solved by using direct methods included in SIR-92, [4] and refinement was carried out using SHELXL-2013. [5] The non-H atoms were refined anisotropically using weighted full-matrix least squares, and H atoms attached to the C atoms were positioned using idealized geometries and refined using a riding model. Elemental analysis was performed at the Research and Analytical Centre for Giant Molecules, Tohoku University.
UV-Vis spectra were acquired in the solid-state as KBr pellets on a Shimadzu UV-3100pc (Shimadzu, Kyoto, Japan). Reflectance IR spectra were acquired on a JASCO IRT-5000 microscope and an FT-IR-6200YMS infrared spectrometer (JASCO, Tokyo, Japan) in the ab plane of a single crystal. IR spectra were acquired in the solid state as KBr pellets on a FT-IR-6200YMS infrared spectrometer (JASCO, Tokyo, Japan).

Raman spectroscopy was performed on single crystals using a Micro Laser Raman
Spectrometer LabRam H-800 at an excitation of 532 nm. EPR: The EPR spectrum was acquired at room temperature by using a JEOL JES-FA100.
Magnetic susceptibility measurements were conducted on polycrystalline samples using a Quantum Design SQUID magnetometer MPMS-7L. Diamagnetic corrections were estimated from Pascal's constants (-960 × 10 -6 cm 3 mol -1 ). [6] Ac measurements were performed in the frequency range of 0.1-1000 Hz. Temperature dependences of σ and MR of single crystals were determined with a four-probe method using a Quantum Design PPMS 6000 equipped with horizontal rotator probe.
Electronic structure calculations: The band structure was calculated by using periodic boundary conditions (PBC) Kohn-Sham density functional theory (DFT) calculations with the VASP [7] package employing the Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional. [8] PAW pseudopotentials and plane-wave basis sets with cutoff energies of 800 eV were used. A 4 × 4 × 2 Monkhorst-Pack k-mesh was employed for SMM and metallic layers, and a 4 × 4 × 1 Monkhorst-Pack k-mesh was employed only for the metallic layer. In addition, the band structure was calculated using an extended Hückel method in the software package developed by T. Mori et al. [9] via a the tightbinding band approximation in which transfer integrals were evaluated with t = ES, where E is the energy of the HOMO (= -10 eV) and S is the overlap integrals between the HOMOs of the different molecules. The Hückel parameters used are listed in Table   S1.                 The magnetic susceptibilities for the data between 10-300 K were fitted by using equation (S1): (S1) = (1 -Δ) + 1 -( 2) ( + ) where Δ is the fraction of magnetic impurity ( = 1.55 × 10 -5 emu mol -1 ), η is Δ = 0.0026, Δ the temperature-independent paramagnetism, is Bohr magneton, N A is Avogadro's number and zJ is the intermolecular interaction parameter between two Co(pdms) 2 units (z = 2). χ (η) = 0.0032 cm 3 mol -1 under ZFC, and χ (η) = 0.0028 cm 3 mol -1 under FC conditions