Molecular electronics has been a subject of growing interest in recent years 1 . As was shown, a monoelectronic transistor can be constructed on the basis of a single p-phenylenevinylene oligomer that can have a varied oxidation state and is composed of five benzene rings connected by four double bonds 2 . Great opportunities are also associated with metal cluster complexes used as a basis for various nanodevices and molecular motors 3 . Metal cluster compounds are perspective elements of synthetic metals including superconductors. Some of them (e.g., Chevrel phases 4 5 6) are characterized by high parameters of critical magnetic fields of superconductivity reaching 60 T. Structurally, the phases are characterized by metal bonds being formed between triangular faces of the neighboring octahedral units.

Recent research reported 12-rhenium cluster complexes [Re₁₂CS₁₇(CN)₆ ⁿ⁻ (n = 6, 8) with a unique crystal structure and a reversible redox two-electron transformation {[Re₁₂CS₁₇(CN)₆ ⁶⁻↔[Re₁₂CS₁₇(CN)₆ ⁸⁻} 7 . The complexes are dimers composed of two Re₆ octahedrons connected by rhenium atoms through the interstitial carbon with a trigonal-prismatic coordination, {Re₃(μ₆-C)Re₃}, and by three bridging sulfur atoms μ₂-S with corner coordination of rhenium atoms, {Re(μ₂-S)Re}. According to X-ray data, the change in the oxidation state of the complex is accompanied by substantial changes in internuclear distances (Reⁱⁿ – Reⁱⁿ ~ 0.3 Å) between two metal subunits {Re₆}, whereas inside {Re₆} subunits interatomic distances Re–Re do not change more than for 0.01 Å (Fig. 1). In other words, the two subunits {Re₆} are closer to each other in [Re₁₂CS₁₇(CN)₆ ⁶⁻ than in [Re₁₂CS₁₇(CN)₆ ⁸⁻.

As is well known, a nanoscale device can be constructed from a molecule only if the molecule exhibits such electronic effects as rectification 8 , amplification 9 or switching 10 11. Here, we study electronic effects in complexes [Re₁₂CS₁₇(CN)₆ ⁿ⁻ (n = 6, 8) by NMR spectroscopy and quantum chemical method of electron localization function (ELF) 12 13 and consider the effects as an evidence of molecular switching.

¹³C NMR spectra were measured from the polycrystalline samples K₈Re₁₂CS₁₇(CN)₆ 20H₂O (I) and K₆Re₁₂CS₁₇(CN)₆ (II) synthesized by precipitating [Re₁₂CS₁₇(CN)₆ ^8−/6− complexes from solutions 7 . ¹³C NMR spectra of (I) and (II) enriched by ¹³C were measured by Avance-400 spectrometer of BRUKER Bio-Spin in an ampoule for a rotor with the outer diameter 4 mm, the sample being rotated at the magic angle (MAS) at room temperature. The rotation rate was 7, 11 and 15 kHz. The duration of the stimulating 90° pulse was 5 μs, the accumulated amount was 200. The relaxation delay was 10 s. Tetramethylsilane (tms) was used as an external standard for the scale of chemical shifts.

Quantum-chemical study of the electronic structure of model systems [Re₁₂CS₁₇(CN)₆ ⁿ⁻ (n = 6, 8) was carried out by a spin-restricted DFT method (code ADF2006 14 ) with model Hamiltonians of density functionals given by the sum of a local density functional LDA (VWN 15 ) and a gradient exchange functional GGA (Becke 16 and Perdew 17 ). The all-electron basis set of the TZP type, as included in the ADF2006 program, was used for all atoms. Geometry optimization was performed for D _3h point group; relativistic effects and spin–orbital interaction were taken into account with the zero-order relativistic approximation method ZORA 18 . ¹³C NMR chemical shifts were calculated by the method GIAO in view of scalar relativistic effects and spin–orbital interaction 19 20 21. Interatomic interactions were studied by a quantum-chemical topological method “Electron localization function” (ELF) 13 . In this method, we analyze the function.

D _h(r) is the density of the Thomas–Fermi kinetic energy for the homogeneous electron gas, which acts as a normalizing multiplier; D(r) is interpreted as excess density of the local kinetic energy of electrons (fermions) resulting from repulsion according to the Pauli principle relative to the density of the local kinetic energy of bosons 22 . The ELF is assumed to approximately 1 in the regions of space that are typical of the maximum localization of electron pairs with antiparallel spins (colored blue in our drawings). The ELF is 0.5 in the regions where the electron density is close to that of the homogeneous electron gas (green) and ~0 in the regions with delocalized electrons (red).

It was found out that both structure and ¹³C NMR spectra change dramatically depending on the oxidation state (n) of the complex [Re₁₂CS₁₇(CN)₆ ⁿ⁻ (Fig. 2). According to the obtained data, the reduction/oxidation of the complex [Re₁₂CS₁₇(CN)₆ ⁿ⁻ is accompanied by the change of ¹³C NMR signal from μ₆-C for 415 ppm (or ~37G) that substantially exceeds chemical shifts due to the change of the distances between carbon–transition metal 23 . Besides, there is obviously a fundamental difference between the electrically conductive properties of the complexes since the chemical shift is caused by induced electric currents to arise in the complexes in an external magnetic field 24 .

Quantum-chemical DFT calculations show a good agreement with the data of X-ray-structural analysis 7 25 and NMR data (Table 1). The fact suggests a high reliability of the model of electronic structure of complexes [Re₁₂CS₁₇(CN)₆ ⁿ⁻ (n = 6, 8) received at the DFT level.

^aCalculated absolute isotropic chemical shielding σ_tms = 177.4 ppm, and δ = σ_tms − σ_substance

The energy gaps between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO gap) are 1.05 eV (n = 6) and 1.31 eV (n = 8), respectively 25 . The difference between the energy gaps and distance between the two subunits {Re₆} also suggests different resistance of the molecular complexes 26 . Note that according to Kozlova et al. 25 , HOMO orbital is characterized by some bonding interaction between Re atoms from different subunits {Re₆} in the complex [Re₁₂CS₁₇(CN)₆ ⁶⁻ and by antibonding interaction in [Re₁₂CS₁₇(CN)₆ ⁸⁻.

The study of the electronic density by the method of electron localization function (ELF) revealed weak exchange interactions (ELF ~ 0.3) between μ₆-C and μ₂-S in the complex [Re₁₂CS₁₇(CN)₆ ⁶⁻, the interaction almost disappears in the complex [Re₁₂CS₁₇(CN)₆ ⁸⁻ 25 . The interaction can affect the structural properties, since even weak localization of electron pairs with antiparallel spins between two positively charged Re atoms should result in some reduction of the distances between subunits {Re₆}. Therefore, the internuclear distances Reⁱⁿ – Reⁱⁿ can be changed if we somehow succeed to weaken or strengthen this interaction.

We placed hypothetical point chargesQin the plane {(μ₆-C) − (μ₂-S)₃} at the distance ~5 Å from μ₂-S atoms of [Re₁₂CS₁₇(CN)₆]⁸⁻complex (Fig. 3) and calculated ELF distribution maps. Various chargesQand their various positions around [Re₁₂CS₁₇(CN)₆]⁸⁻complex were tried. The most demonstrative impact of the external electric field was seen forQ = ±25e. The modeled point chargeQ = ±25e creates on the μ₂-S atom an electrostatic potential ~10⁻⁹ V which is 10–100 times higher than typical an electrostatic potentials created by the atoms inside the complexes. As can be seen, the electrostatic field of a point charge (especially, a negative charge) reinforces the exchange interaction between μ₆-C and μ₂-S atoms. As mentioned earlier, [Re₁₂CS₁₇(CN)₆]⁶⁻is characterized by increased exchange interaction between μ₆-C and (μ₂-S)₃. Therefore, the magnification of exchange interaction between μ₆-C and (μ₂-S)₃in the complex [Re₁₂CS₁₇(CN)₆]⁸⁻should result in its ionization and reduction in the distances between {Re₆} subunits. In fact, the increased localization of electronic lone pairs observed at the sulfur atoms in the positions μ₃-S^up(Q = −25e) and μ₂-S (Q = +25e) can signify the beginning of the oxidation process ([Re₁₂CS₁₇(CN)₆]⁸⁻→[Re₁₂CS₁₇(CN)₆]⁶⁻) (Fig. 4).

Thereby, the oxidation state and the distances between {Re₆} subunits in the [Re₁₂CS₁₇(CN)₆]⁸⁻complexes and, therefore, the electron properties of the system are shown to be controlled by an external electric field. We believe that the ON condition with higher conductivity corresponds to the complex [Re₁₂CS₁₇(CN)₆]⁶⁻, since NMR spectroscopy reveals the strongest paramagnetic currents induced by external magnetic field. The state can be achieved by applying external electric field in the plane containing three atoms μ₂-S and the atom μ₆-C of the complex [Re₁₂CS₁₇(CN)₆]⁸⁻. Similarly, OFF state should result from turning off the field. We believe that the system is an example of a monomolecular switch.