Three-Step Spin State Transition and Hysteretic Proton Transfer in the Crystal of an Iron(II) Hydrazone Complex.

A proton-electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.

Observation of Proton Transfer Coupled Spin Transition and Trapping of Photoinduced Metastable Proton Transfer State in an Fe(II) Complex.

An important technique to realize novel electron- and/or proton-based functionalities is to use a proton-electron coupling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However, although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit proton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II) complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respectively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunction with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at low temperatures in the Fe(II) complex-a phenomenon known as the light-induced excited-spin-state trapping effect-the light-induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light irradiation, is useful for modulating proton-based functionalities in molecular devices.

Changing Single-Molecule Magnet Properties of a Windmill-Like Distorted Terbium(III) α-Butoxy-Substituted Phthalocyaninato Double-Decker Complex by Protonation/Deprotonation.

Synthesis, structures, and magnetic properties of α-butoxy-substituted phthalocyaninato double-decker complexes Tb(α-obPc)2 (1-) (α-obPc: dianion of 1,4,8,11,15,18,22,25-octa(n-butoxy)phthalocyaninato) with protonated (1H), deprotonated (1[HDBU]), and diprotonated forms (1H2+) are discussed. X-ray analysis was used to confirm the position of the proton in 1H, and it was revealed that the protonation induced asymmetric distortion in 1H. In contrast, 1[HDBU] was distorted in a highly symmetric windmill-like fashion. 1H is arranged in a slipped column array in the crystal packing, whereas 1[HDBU] is arranged in a one-dimensional fashion, in which the magnetic easy axes of 1[HDBU] lie along the same line. From direct-current (dc) magnetic measurements, ferromagnetic Tb-Tb interactions occur in both 1H and 1[HDBU], and magnetic hysteresis was observed. However, the area of the magnetic hysteresis in 1[HDBU] is larger than that in 1H, meaning that magnetic relaxation time (τ) is longer in 1[HDBU]. In addition, the results of alternating-current magnetic measurements in a zero dc magnetic field indicate that τ of 1[HDBU] is longer as compared to 1H. In other words, protonation/deprotonation affects not only the molecular structures and crystal packing but also the single-molecule magnet properties.

α-Substituted Bis(octabutoxyphthalocyaninato)Terbium(III) Double-Decker Complexes: Preparation and Study of Protonation by NMR and DFT.

Synthesis of the anionic, α-substituted, bis(phthalocyaninato)Tb(III) complex [Tb(α-obPc)2](-) ([1](-)) (obPc = α-octabutoxyphthalocyaninato) leads to the isolation of its protonated form [1H](0). This complex was characterized by X-ray diffraction (XRD), mass spectroscopy (MS), infrared (IR) and ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy. Crystal structure analysis did not allow localization of the additional proton, which is probably attached to the meso-N atom or isoindole-N atom of the phthalocyaninato ligand. [1H](0) can easily be deprotonated or protonated, giving the corresponding anionic and cationic complexes. The three compounds [1H](0), [1](-), and [1HH](+) were studied by a combination of paramagnetic NMR experiments ((1)H, (13)C, variable-temperature measurements, two-dimensional nuclear magnetic resonance and DFT calculations (done on Y(III) analogues with octamethoxyphthalocyaninato ligands), for the purpose of elucidating the positions of the acidic protons and for understanding the structural changes of the coordination environment of the Tb ion induced by protonation.

Macroscopic Polarization Change via Electron Transfer in a Valence Tautomeric Cobalt Complex.

Polarization change induced by directional electron transfer attracts considerable attention owing to its fast switching rate and potential light control. Here, we investigate electronic pyroelectricity in the crystal of a mononuclear complex, [Co(phendiox)(rac-cth)](ClO4)·0.5EtOH (1·0.5EtOH, H2phendiox = 9, 10-dihydroxyphenanthrene, rac-cth = racemic 5, 5, 7, 12, 12, 14-hexamethyl-1, 4, 8, 11-tetraazacyclotetradecane), which undergoes a two-step valence tautomerism (VT). Correspondingly, pyroelectric current exhibits double peaks in the same temperature domain with the polarization change consistent with the change in dipole moments during the VT process. Time-resolved Infrared (IR) spectroscopy shows that the photo-induced metastable state can be generated within 150 ps at 190 K. Such state can be trapped for tens of minutes at 7 K, showing that photo-induced polarization change can be realized in this system. These results directly demonstrate that a change in the molecular dipole moments induced by intramolecular electron transfer can introduce a macroscopic polarization change in VT compounds.