Ko Yoneda

Bidirectional Chemo‐Switching of Spin State in a Microporous Framework

The ins and outs of spin: Using the microporous coordination polymer {Fe(pz)[Pt(CN)(4)]} (1, pz=pyrazine), incorporating spin-crossover subunits, two-directional magnetic chemo-switching is achieved at room temperature. In situ magnetic measurements following guest vapor injection show that most guest molecules transform 1 from the low-spin (LS) state to the high-spin (HS) state, whereas CS(2) uniquely causes the reverse HS-to-LS transition.

Selective sorption of oxygen and nitric oxide by an electron-donating flexible porous coordination polymer

Porous coordination polymers are materials formed from metal ions that are bridged together by organic linkers and that can combine two seemingly contradictory properties—crystallinity and flexibility. Porous coordination polymers can therefore create highly regular yet dynamic nanoporous domains that are particularly promising for sorption applications. Here, we describe the effective selective sorption of dioxygen and nitric oxide by a structurally and electronically dynamic porous coordination polymer built from zinc centres and tetracyanoquinodimethane (TCNQ) as a linker. In contrast to a variety of other gas molecules (C2H2, Ar, CO2, N2 and CO), O2 and NO are accommodated in its pores. This unprecedented preference arises from the concerted effect of the charge-transfer interaction between TCNQ and these guests, and the switchable gate opening and closing of the pores of the framework. This system provides further insight into the efficient recognition of small gas molecules. Porous coordination polymers can form materials that are both crystalline and flexible, creating regular yet dynamic channels that are promising for guest sorption. Guest selectivity is difficult to achieve, however, and typically relies on size- or shape-recognition. A framework has now been assembled that combines charge-transfer interactions and structural flexibility to only accommodate O2 and NO.

Oxalate-Bridged Bimetallic Complexes {NH(prol)3}[MCr(ox)3] (M = MnII, FeII, CoII; NH(prol)3+ = Tri(3-hydroxypropyl)ammonium) Exhibiting Coexistent Ferromagnetism and Proton Conduction

The oxalate-bridged bimetallic complexes {NH(prol)(3)}[M(II)Cr(III)(ox)(3)] (M(II) = Mn(II), Fe(II), Co(II)) with hydrophilic tri(3-hydroxypropyl)ammonium (NH(prol)(3)(+)) were prepared by a new synthetic procedure, and the effects of the NH(prol)(3)(+) ion upon the structure, magnetism, and electrical conduction were studied. An X-ray crystallographic study of the MnCr dihydrate, {NH(prol)(3)}[MnCr(ox)(3)].2H(2)O, was performed. Crystal data: hexagonal, P6(3), a = b = 9.3808(14) A, c = 15.8006(14) A, Z = 2. The structure comprises oxalate-bridged bimetallic layers interleaved by NH(prol)(3)(+) ions. The ions assume a tripodal configuration and are hydrogen bonded to the bimetallic layers together with water molecules, giving rise to a short interlayer separation (7.90 A) and unsymmetrical faces to the bimetallic layer. Cryomagnetic studies demonstrate ferromagnetic ordering with transition temperature of 5.5 K for the MnCr complex, 9.0 K for the FeCr complex, and 10.0 K for the CoCr complex. The interlayer magnetic interaction is negligibly weak in all of the complexes despite the short interlayer separation. A slow magnetization is observed in all the complexes. This is explained by spin canting associated with the unsymmetrical feature of the bimetallic layer. The complexes show proton conduction of 1.2 x 10(-10) to 4.4 x 10(-10) S cm(-1) under 40% relative humidity (RH) and approximately 1 x 10(-4) S cm(-1) under 75% RH. On the basis of water adsorption/desorption profiles, the conduction under 40% RH is mediated through the hydrogen-bonded network formed by the bimetallic layer, NH(prol)(3)(+) ions, and water molecules (two per MCr). Under 75% RH, additional water molecules (three per MCr) are concerned with the high proton conduction. This is the first example of a metal complex system exhibiting coexistent ferromagnetism and proton conduction.

Precise control and consecutive modulation of spin transition temperature using chemical migration in porous coordination polymers.

Precise control of spin transition temperature (T(c)) is one of the most important challenges in molecular magnetism. A Hofmann-type porous coordination polymer {Fe(pz)[Pt(II)(CN)(4)]} (1; pz = pyrazine) exhibited cooperative spin transition near room temperature (T(c)(up) = 304 K and T(c)(down) = 284 K) and its iodine adduct {Fe(pz)[Pt(II/IV)(CN)(4)(I)]} (1-I), prepared by oxidative addition of iodine to the open metal sites of Pt(II), raised the T(c) by 100 K. DSC and microscopic Raman spectra of a solid mixture of 1-I and 1 revealed that iodine migrated from 1-I to 1 through the grain boundary after heating above 398 K. We have succeeded in precisely controlling the iodine content of {Fe(pz)[Pt(CN)(4)(I)(n)]} (1-In; n = 0.0-1.0), which resulted in consecutive modulation of T(c) in the range 300-400 K while maintaining the hysteresis width. Furthermore, it was demonstrated that iodine migration in the solid mixture was triggered by the spin transition of 1-I. The magnetically bistable porous framework decorating guest interactive open-metal-site in the pore surface makes it possible to modulate T(c) ad arbitrium through unique postsynthetic method using iodine migration.

A pentanuclear iron catalyst designed for water oxidation

Although the oxidation of water is efficiently catalysed by the oxygen-evolving complex in photosystem II (refs 1 and 2), it remains one of the main bottlenecks when aiming for synthetic chemical fuel production powered by sunlight or electricity. Consequently, the development of active and stable water oxidation catalysts is crucial, with heterogeneous systems considered more suitable for practical use and their homogeneous counterparts more suitable for targeted, molecular-level design guided by mechanistic understanding. Research into the mechanism of water oxidation has resulted in a range of synthetic molecular catalysts, yet there remains much interest in systems that use abundant, inexpensive and environmentally benign metals such as iron (the most abundant transition metal in the Earth’s crust and found in natural and synthetic oxidation catalysts). Water oxidation catalysts based on mononuclear iron complexes have been explored, but they often deactivate rapidly and exhibit relatively low activities. Here we report a pentanuclear iron complex that efficiently and robustly catalyses water oxidation with a turnover frequency of 1,900 per second, which is about three orders of magnitude larger than that of other iron-based catalysts. Electrochemical analysis confirms the redox flexibility of the system, characterized by six different oxidation states between FeII5 and FeIII5; the FeIII5 state is active for oxidizing water. Quantum chemistry calculations indicate that the presence of adjacent active sites facilitates O–O bond formation with a reaction barrier of less than ten kilocalories per mole. Although the need for a high overpotential and the inability to operate in water-rich solutions limit the practicality of the present system, our findings clearly indicate that efficient water oxidation catalysts based on iron complexes can be created by ensuring that the system has redox flexibility and contains adjacent water-activation sites.

Direct two-step spin-crossover through [HS–HS]⋯[LS–LS] at the plateau in dinuclear diiron(II) complex [{Fe(NCBH3)(4phpy)}2(μ-bpypz)2]

The variable temperature magnetic susceptibility, X-ray crystallography, and IR and Raman spectra of a new dinuclear complex [{Fe(II)(NCBH(3))(4-phpy)}(2) mu-bpypz)(2)] demonstrated the first two-step spin-crossover associated with a 1 : 1 mixture of high-spin pair [HS-HS] and low-spin pair [LS-LS] at the plateau.

A Switchable Molecular Rotator: Neutron Spectroscopy Study on a Polymeric Spin-Crossover Compound

A quasielastic neutron scattering and solid-state (2)H NMR spectroscopy study of the polymeric spin-crossover compound {Fe(pyrazine)[Pt(CN)(4)]} shows that the switching of the rotation of a molecular fragment--the pyrazine ligand--occurs in association with the change of spin state. The rotation switching was examined on a wide time scale (10(-13)-10(-3) s) by both techniques, which clearly demonstrated the combination between molecular rotation and spin-crossover transition under external stimuli (temperature and chemical). The pyrazine rings are seen to perform a 4-fold jump motion about the coordinating nitrogen axis in the high-spin state. In the low-spin state, however, the motion is suppressed, while when the system incorporates benzene guest molecules, the movements of the system are even more restricted.

Guest-responsive porous magnetic frameworks using polycyanometallates

Three types of cyanide-bridged porous magnetic frameworks based on polycyanometallates exhibited reversible magnetic conversions associated with different guest-responsive structural transformation, which highlights the available strategy using a polycyanometallate for constructing the guest-responsive magnetic frameworks.

Porous protein crystals as reaction vessels for controlling magnetic properties of nanoparticles

Magnetic bimetallic CoPt nanoparticles are synthesized in the solvent channels of hen egg white lysozyme crystals by the reduction of Co(2+) and Pt(2+) ions pre-organized on the interior surface of the solvent channels. By using different lysozyme crystal systems, the magnetic properties of CoPt nanoparticles can be controlled.

Proton Conduction Study on Water Confined in Channel or Layer Networks of LaIIIMIII(ox)3·10H2O (M = Cr, Co, Ru, La)

Proton conduction of the La(III)M(III) compounds, LaM(ox)3·10H2O (abbreviated to LaM; M = Cr, Co, Ru, La; ox(2-) = oxalate) is studied in view of their networks. LaCr and LaCo have a ladder structure, and the ladders are woven to form a channel network. LaRu and LaLa have a honeycomb sheet structure, and the sheets are combined to form a layer network. The occurrence of these structures is explained by the rigidness versus flexibility of [M(ox)3](3-) in the framework with large La(III). The channel networks of LaCr and LaCo show a remarkably high proton conductivity, in the range from 1 × 10(-6) to 1 × 10(-5) S cm(-1) over 40-95% relative humidity (RH) at 298 K, whereas the layer networks of LaCr and LaCo show a lower proton conductivity, ∼3 × 10(-8) S cm(-1) (40-95% RH, 298 K). Activation energy measurements demonstrate that the channels filled with water molecules serve as efficient pathways for proton transport. LaCo was gradually converted to La(III)Co(II)(ox)2.5·4H2O, which had no channel structure and exhibited a low proton conductivity of less than 1 × 10(-10) S cm(-1). The conduction-network correlation of LaCo(ox)2.5·4H2O is reported.