Hitoshi Miyasaka

A porous one-dimensional coordination chain compound, [Ru₂(p-F-PhCO₂)₄(phz)] (1′; p-F-PhCO₂ ⁻ = para-fluorobenzoate; phz = phenazine) derived by drying its nitrobenzene-solvated compound, specifically adsorbs CO₂ at 195 K in a stepwise sorption manner.

Charge transfer (CT) of D⁰A⁰ ↔ D^δ+A^δ− not only involves an electron transfer from D to A, but also generates a new spin set of S = 1/2 spins with an exchange interaction. Therefore, the control of CT in multidimensional frameworks could be an efficient way to design electronically/magnetically functional materials. The use of redox-active metal complexes as D and/or A building blocks expands the variety of such D/A frameworks with the formulation of D_mA_n (m, n ≥ 1), permitting the design of donor/acceptor metal–organic frameworks (D/A-MOFs). This Account summarizes our ongoing research on the design of D/A-MOFs and on the systematic control of CT in such D/A-MOFs toward the discovery of unique electronic/magnetic materials exhibiting nontrivial phenomena. For this purpose, the D/A combinations of carboxylate-bridged paddlewheel-type diruthenium(II,II) complexes ([Ru₂^II,II]) that act as one-electron (1e^–) donors and polycyanoorganic acceptors such as 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and N,N′-dicyanoquinodiimine (DCNQI) have been chosen. Even in the covalently bonded motif, the CT in this system is systematically dependent on the intrinsic ionization potential (I_D) and electron affinity (E_A) of the D and A units, respectively, which is controllable by chemical modification of the D/A units. As we consider the energy difference between the HOMO of D and the LUMO of A (ΔE_H-L(DA)) instead of hν_CT ∝ |I_D – E_A|, the neutral (N) and ionic (I) states can be defined as follows: (i) the D/A materials with ΔE_H-L(DA) > 0 (i.e., the LUMO level of A is higher than the HOMO level of D) should be neutral, and (ii) complexes adopted when ΔE_H-L(DA) < 0 are, meanwhile, ionic. Materials located near ΔE_H-L(DA) ≈ 0, that is, at the boundary between the N and I phases, are candidates for the N–I transition driven by external stimuli such as temperature, pressure, and photoirradiation. Even in the ionic state, two distinct states could be isolated for the D₂A type: (ii-1) the 1e^– transferred D₂A-MOFs provide mixed-valence systems of D⁺D⁰A^– possibly involving intervalence CT, which produce magnetic correlations via radical A^– units, and (ii-2) when the 2e^– reduced form of A (e.g., TCNQ^2–) is energetically favored beyond the on-site Coulomb repulsion on A, the oxidation state of D⁺₂A^2– is produced, for which magnetic measurements reveal a paramagnetic state attributed to the isolated D⁺ units. The interspatial Coulombic interaction is another factor in determining the charge distribution in materials, which is related to the spatial Coulombic stability of D/A packing and possibly yields a mixture of N and I domains when it is more advantageous to get Coulombic gain than in the uniform N or I phase. Such a phase could be observed at the boundary between N and I phases involving the N–I transition. These charge-distributed states/phases are systematically demonstrated in a D/A-MOF system made by the combination of [Ru₂^II,II] and TCNQ/DCNQI; however, we immediately recognize the charge distribution of D/A-MOF only by understanding the nature of the starting D/A units. The present D/A-MOF system should be an intriguing platform to look for new functionalities with synergistic correlations among charge, spin, and lattice.

A Mn^III ₂Fe^II chain with a [–Mn^III–(O_Ph)₂–Mn^III–ON–Fe^II–NO–] repeat unit was synthesized via the assembling reaction of Mn^III salen-type dimers and Fe^II pyridyloximate complexes, where the –(O_Ph)– and –ON– bridges represent a biphenolate bridge and an oximate bridge, respectively. The bulky counter anions, BPh₄ ⁻, which surround the chain to form a zeolite-like brick wall, successfully isolate the chains from a magnetic point of view. This compound is isostructural with Mn₂Ni–BPh₄, which is an SCM with S_T = 3 (H. Miyasaka, A. Saitoh, M. Yamashita and R. Clérac, Dalton Trans., 2008, 2422). Because the Fe^II unit is diamagnetic, the Mn^III dimer, which has the potential to be a single-molecule magnet (SMM), is nearly magnetically isolated, although a weak antiferromagnetic interaction with J/k_B ≈ −0.1 K is perturbed between the Mn^III dimers with S_T = 4 via the Fe^II unit. Ac susceptibility data shows that two types of relaxation of the magnetization are present, which are attributed to SMM and chain relaxations.

The assembly of linear edges and triangle nodes in a 3 : 2 molar ratio possibly forms a honeycomb topology: this was achieved by the use of paddlewheel-type dimetal complexes and 2,4,6-tris(4-pyridyl)-1,3,5-triazine, in which the framework with a very large mesh of 39 × 29 Å diameters was stabilized by π-stacked coronene molecules.

The reaction of Mn^III salen-type complexes with di- and tetraanionic α-Keggin-type polyoxometalates (POMs) was performed, and three types of Coulombic aggregations containing Mn^III out-of-plane dimeric units (abbreviated as [Mn₂]²⁺) that are potentially single-molecule magnets (SMMs) with an S_T = 4 ground state were synthesized: [Mn₂(5-MeOsaltmen)₂(acetone)₂][SW₁₂O₄₀] (1), [Mn₂(salen)₂(H₂O)₂]₂[SiW₁₂O₄₀] (2), and [Mn(5-Brsaltmen)(H₂O)(acetone)]₂[{Mn₂(5-Brsaltmen)₂}(SiW₁₂O₄₀)] (3), where 5-Rsaltmen^2– = N,N′-(1,1,2,2-tetramethylethylene)bis(5-R-salicylideneiminate) with R = MeO (methoxy), Br (bromo) and salen^2– = N,N′-ethylenebis(salicylideneiminate). Compound 1 with a dianionic POM, [SW₁₂O₄₀]^2–, is composed of a 1:1 aggregating set of [Mn₂]²⁺/POM, and 2, with a tetraanionic POM, [SiW₁₂O₄₀]^4–, is a 2:1 set. Compound 3 with [SiW₁₂O₄₀]^4– forms a unique 1D coordinating chain with a [−{Mn₂}–POM−]^2– repeating unit, for which a hydrogen-bonded dimeric unit ([Mn(5-Brsaltmen)(H₂O)(acetone)]₂²⁺) is present as a countercation. Independent of the formula ratio of [Mn₂]²⁺/POM, Mn^III dimers and POM units in 1–3 form respective segregated columns along a direction of the unit cell, which make an alternate packing to separate evenly identical species in a crystal. The nearest intermolecular Mn···Mn distance is found in the order 2 < 3 < 1. The segregation of the [Mn₂]²⁺ dimer resulted in interdimer distances long enough to effectively reduce the intermolecular magnetic interaction, in particular in 1 and 3. Consequently, an intrinsic property, SMM behavior, of Mn^III dimers has been characterized in this system, even though the interdimer interactions are still crucial in the case of 2, where a long-range magnetic order competitively affects slow relaxation of the magnetization at low ac frequencies.

A series of isostructural cyano‐bridged Mn^III(h.s.)–M^III(l.s.) alternating chains, [Mn^III(5‐TMAMsalen)M^III(CN)₆]⋅4H₂O (5‐TMAMsalen²⁻=N,N′‐ethylenebis(5‐trimethylammoniomethylsalicylideneiminate), Mn^III(h.s.)=high‐spin Mn^III, M^III(l.s.)=low‐spin Co^III, MnCo; Fe^III, MnFe; Mn^III, MnMn; Cr^III, MnCr) was synthesized by assembling [Mn^III(5‐TMAMsalen)]³⁺ and [M^III(CN)₆]³⁻. The chains present in the four compounds, which crystallize in the monoclinic space group C2/c, are composed of an [‐Mn^III‐NC‐M^III‐CN‐] repeating motif, for which the ‐NC‐M^III‐CN‐ motif is provided by the [M^III(CN)₆]³⁻ moiety adopting a trans bridging mode between [Mn^III(5‐TMAMsalen)]³⁺ cations. The Mn^III and M^III ions occupy special crystallographic positions: a C₂ axis and an inversion center, respectively, forming a highly symmetrical chain with only one kind of cyano bridge. The Jahn–Teller axis of the Mn^III(h.s.) ion is perpendicular to the N₂O₂ plane formed by the 5‐TMAMsalen tetradentate ligand. These Jahn–Teller axes are all perfectly aligned along the unique chain direction without a bending angle, although the chains are corrugated with an Mn‐N_axis‐C angle of about 144°. In the crystal structures, the chains are well separated with the nearest inter‐chain M⋅⋅⋅M distance being relatively large at 9 Å due to steric hindrance of the bulky trimethylammoniomethyl groups of the 5‐TMAMsalen ligand. The magnetic properties of these compounds have been thoroughly studied. MnFe and MnMn display intra‐chain ferromagnetic interactions, whereas MnCr is characterized by an antiferromagnetic exchange that induces a ferrimagnetic spin arrangement along the chain. Detailed analyses of both static and dynamic magnetic properties have demonstrated without ambiguity the single‐chain magnet (SCM) behavior of these three systems, whereas MnCo is merely paramagnetic with S_Mn=2 and D/k_B=−5.3 K (D being a zero‐field splitting parameter). At low temperatures, the MnM compounds with M=Fe, Mn, and Cr display remarkably large M versus H hysteresis loops for applied magnetic fields along the easy magnetic direction that corresponds to the chain direction. The temperature dependence of the associated relaxation time for this series of compounds systematically exhibits a crossover between two Arrhenius laws corresponding to infinite‐chain and finite‐chain regimes for the SCM behavior. These isostructural hetero‐spin SCMs offer a unique series of alternating [‐Mn‐NC‐M‐CN‐] chains, enabling physicists to test theoretical SCM models between the Ising and Heisenberg limits.