Densely Packed CO2 Aids Charge, Spin, and Lattice Ordering Partially Fluctuated in a Porous Metal-Organic Framework Magnet

Partial charge fluctuations in the charge-ordered state of a material, often triggered by structural disorders and/or defects, can significantly alter its physical characteristics, such as magnetic long-range ordering. However, it is difficult to post-chemically fix such accidental partial fluctuations to reconstruct a uniform charge-ordered state. Herein, we report CO2 -aided charge ordering demonstrated in a CO2 -post-captured layered magnet, [{Ru2 (o-ClPhCO2 )4 }2 {TCNQ(OMe)2 }] ⋅ CO2 (1⊃CO2 ; o-ClPhCO2 - =ortho-chlorobenzoate; TNCQ(OMe)2 =2,5-dimethoxy-7,7,8,8-tetracyanoquinodimethane). Pristine porous layered magnet 1 had a partially charge-fluctuated ordered state, which provided ferrimagnetic ordering at TC =65 K. Upon loading CO2 , 1 adsorbed one mole of CO2 , forming 1⊃CO2 , and raising TC to 100 K. This was because of the vanishing charge fluctuations without significantly changing the framework structure. This research illustrates the post-accessible host-guest chemistry delicately combined with charge, spin, and lattice ordering in a spongy magnet. Furthermore, it highlights how this innovative approach opens up new possibilities for technology and nanoscale magnetism manipulation.

Donor–acceptor systems in metal–organic frameworks: design, construction, and properties

In this highlight, the development of donor acceptor (D–A) MOF was briefly reviewed and summarized in the aspects of design, construction, and properties. Also, an outlook about the research and potential application of D–A MOF has been presented.

Magnet Creation by Guest Insertion into a Paramagnetic Charge-Flexible Layered Metal-Organic Framework

Changing nonmagnetic materials to spontaneous magnets is an alchemy-inspiring concept in materials science; however, it is not impossible. Here, we demonstrate chemical modification from a nonmagnet to a bulk magnet of either a ferrimagnet or antiferromagnet, depending on the adsorbed guest molecule, in an electronic-state-flexible layered metal-organic framework, [{Ru₂(2,4-F₂PhCO₂)₄}₂TCNQ(EtO)₂] (1; 2,4-F₂PhCO₂^- = 2,4-difluorobenzoate; TCNQ(EtO)₂ = 2,5-diethoxy-7,7,8,8-tetracyanoquinodimethane). The guest-free paramagnet 1 undergoes a thermally driven intralattice electron transfer involving a structural transition at 380 K. This charge modification can also be implemented by guest accommodations at room temperature; 1 adsorbs several organic molecules, such as benzene (PhH), p-xylene (PX), 1,2-dichloroethane (DCE), dichloromethane (DCM), and carbon disulfide (CS₂), forming 1-solv with intact crystallinity. This induces an intralattice electron transfer to produce a ferrimagnetically ordered magnetic layer. According to the interlayer environment tuned by the corresponding guest molecule, the magnetic phase is consequently altered to a ferrimagnet for the guests PhH, PX, DCE, and DCM or an antiferromagnet for CS₂. This is the first demonstration of the postsynthesis of bulk magnets using guest-molecule accommodations.

Magnetic Correlation Engineering in Spin-Sandwiched Layered Magnetic Frameworks

The insertion of "sandwiched spins" between magnetic layers could efficiently affect the interlayer magnetic correlations, but doing so increases the complexity in the interlayer spin alignment because of competition between the inserted spin-layer interaction J_NNI and the interlayer through-space interaction J_NNNI if the magnitude of J_NNI is of the same order as J_NNNI with reciprocal signs of the respective interactions. Herein, systematic tuning of the magnetic phase variations by J_NNI and J_NNNI in two kinds of metal-variable isostructural series of supramolecular pillared layer magnets [MCp*₂ ][{Ru₂ ^II,II (2,3,5,6-F₄ CO₂ )₄ }₂ (TCNQ)]⋅2 DCE (M=Co, Fe, Cr; 2,3,5,6-F₄ PhCO₂ ^- =2,3,5,6-tetrafluorobenzoate; TCNQ=7,7,8,8-tetracyano-p-quinodimethane; DCE=1,2-dichloroethane) and their DCE-free series, in which [MCp*₂ ]⁺ (Cp*=η⁵ -C₅ Me₅ ) species with S=0, 1/2, and 3/2 for M=Co, Fe, Cr, respectively, are sandwiched between ferrimagnetic layers of [{Ru₂ }₂ (TCNQ)]^- , is demonstrated. The results showed that the flexible magnetic natures of these magnets are changeable in dependence on J_NNI and J_NNNI , as well as on interlayer inserted spins M.

Metal-Organic Framework Magnets

Metal-organic frameworks represent the ultimate chemical platform on which to develop a new generation of designer magnets. In contrast to the inorganic solids that have dominated permanent magnet technology for decades, metal-organic frameworks offer numerous advantages, most notably the nearly infinite chemical space through which to synthesize predesigned and tunable structures with controllable properties. Moreover, the presence of a rigid, crystalline structure based on organic linkers enables the potential for permanent porosity and postsynthetic chemical modification of the inorganic and organic components. Despite these attributes, the realization of metal-organic magnets with high ordering temperatures represents a formidable challenge, owing largely to the typically weak magnetic exchange coupling mediated through organic linkers. Nevertheless, recent years have seen a number of exciting advances involving frameworks based on a wide range of metal ions and organic linkers. This review provides a survey of structurally characterized metal-organic frameworks that have been shown to exhibit magnetic order. Section 1 outlines the need for new magnets and the potential role of metal-organic frameworks toward that end, and it briefly introduces the classes of magnets and the experimental methods used to characterize them. Section 2 describes early milestones and key advances in metal-organic magnet research that laid the foundation for structurally characterized metal-organic framework magnets. Sections 3 and 4 then outline the literature of metal-organic framework magnets based on diamagnetic and radical organic linkers, respectively. Finally, Section 5 concludes with some potential strategies for increasing the ordering temperatures of metal-organic framework magnets while maintaining structural integrity and additional function.

Layered ferrimagnets constructed from charge-transferred paddlewheel [Ru2] units and TCNQ derivatives: the importance of interlayer translational distance in determining magnetic ground state

A donor (D)/acceptor (A) assembly reaction using the paddlewheel-type diruthenium(ii,ii) complex [Ru2(3,5-F2PhCO2)4(THF)2] (3,5-F2PhCO2- = 3,5-difluorobenzoate; abbreviated hereafter as [Ru2]) as D and 7,7,8,8-tetracyano-p-quinodimethane derivatives (TCNQRx; 2,5-R-substituted TCNQ) as A in a DCM/TCE or DCE solvent system (DCM = dichloromethane, TCE = 1,1,2,2-tetrachloroethane, DCE = 1,2-dichloroethane) led to the formation of D2A-type two-dimensional layered compounds [{Ru2(3,5-F2PhCO2)4}2{TCNQRx}]·nsolv (Rx = H2 (1), Rx = Me2, (2), and Rx = (OEt)2 (3)). All the compounds had similar two-dimensional fishnet-type structures, where the two [Ru2] units were fully coordinated with the four cyano groups of TCNQRx. The compounds 1-3 were categorized as a one-electron transferred ionic state (1e-I) with D+-A--D formulation (D+[triple bond, length as m-dash][RuII,III2]+; A-[triple bond, length as m-dash]TCNQRx˙-). This subunit state formally derived a ferrimagnetically ordered state composed of up-spins with S = 1 for [RuII,II2] and S = 3/2 for [RuII,III2]+ and a down-spin of S = 1/2 for TCNQRx˙-, with antiferromagnetic superexchange interactions in each layer. Eventually, 1-3 became three-dimensional ferrimagnets with Curie temperatures TC = 93, 93, and 92 K, respectively, owing to the presence of interlayer ferromagnetic interactions. The interlayer translational distances (l2) for the three compounds were 10.56, 10.54, and 10.84 Å, respectively, which agreed with the empirical prediction based on the threshold value of l2 > 10.3 Å for a valid ferromagnetic interaction.

trans-Heteroleptic carboxylate-bridged paddlewheel diruthenium(ii, ii) complexes with 2,6-bis(trifluoromethyl)benzoate ligands

Carboxylate-ligand substitution reactions of paddlewheel-type diruthenium(ii, iii) complexes ([Ru(RCO2)4](+)) with 2,6-bis(trifluoromethyl)benzoate (2,6-(CF3)2PhCO2(-)) involving a selective reduction to [Ru] provide a series of trans-substituted paddlewheel-type diruthenium(ii, ii) complexes, [Ru(2,6-(CF3)2PhCO2)2(RCO2)2(THF)2] (R = CH3, ; C2H5, ; C3H7, ; C4H9, ; C(CH3)3, ; 2,3,5,6-F4Ph, ). Crystal structures of were determined, and their electronic states were investigated by cyclic voltammetry, density functional theory (DFT) and magnetic measurements. This is the first example of trans-heteroleptic carboxylate-bridged [Ru] complexes.

Construction of an Artificial Ferrimagnetic Lattice by Lithium Ion Insertion into a Neutral Donor/Acceptor Metal-Organic Framework

Construction of a molecular system in which the magnetic lattice exhibits long-range order is one of the fundamental goals in materials science. In this study, we demonstrate the artificial construction of a ferrimagnetic lattice by doping electrons into acceptor sites of a neutral donor/acceptor metal-organic framework (D/A-MOF). This doping was achieved by the insertion of Li-ions into the D/A-MOF, which was used as the cathode of a Li-ion battery cell. The neutral D/A-MOF is a layered system composed of a carboxylate-bridged paddlewheel-type diruthenium(II,II) complex as the donor and a TCNQ derivative as the acceptor. The ground state of the neutral form was a magnetically disordered paramagnetic state. Upon discharge of the cell, spontaneous magnetization was induced; the transition temperature was variable. The stability of the magnetically ordered lattice depended on the equilibrium electric potential of the D/A-MOF cathode, which reflected the electron-filling level.

Tuning of Stepwise Neutral-Ionic Transitions by Acceptor Site Doping in Alternating Donor/Acceptor Chains

The stepwise neutral-ionic (N-I) phase transition found in the alternating donor/acceptor (DA) chain [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)]·2(p-xylene) (0; 2,3,5,6-F4PhCO2(-) = 2,3,5,6-tetrafluorobenzoate; DMDCNQI = 2,5-dimethyl-N,N'-dicyanoquinonediimine) was tuned by partly substituting the acceptor DMDCNQI with 2,5-dimethoxy-N,N'-dicyanoquinonediimine (DMeODCNQI), which displays a poorer electron affinity in an isostructural series. The site-doped series comprised [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)1-x(DMeODCNQI)x]·2(p-xylene) for doping rates (x) = 0.05 (0.05-MeO), 0.10 (0.10-MeO), 0.15 (0.15-MeO), and 0.20 (0.20-MeO). The neutral chain [Ru2(2,3,5,6-F4PhCO2)4(DMeODCNQI)]·4(p-xylene) (1), which only contained DMeODCNQI, was also characterized. All site-doped compounds were isostructural to 0 except 1 despite their identical DA chain motif. Except at an x value of 0.20, they displayed a two-step N-I transition involving an intermediate phase. This transition occurred at high temperatures in 0 but shifted to lower temperatures in a parallel manner with increasing doping rate. Simultaneously, each transition broadened with increasing doping rate, leading to a convergence of two transitions at an x value approximating 0.2. Donor/acceptor-site-doping techniques present somewhat different impacts in terms of interchain Coulomb effects.

Electron-Transferred Donor/Acceptor Ferrimagnet with T(C) = 91 K in a Layered Assembly of Paddlewheel [Ru2] Units and TCNQ

The donor (D)/acceptor (A) assembly reaction of the paddlewheel-type diruthenium(II,II) complex [Ru2(2,4,6-F3PhCO2)4(THF)2] (2,4,6-F3PhCO2(-) = 2,4,6-trifluorobenzoate; abbreviated hereafter as [Ru2]) with 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) in a p-xylene/CH2Cl2 solvent system led to the formation of a two-dimensional layered compound, [{Ru2(2,4,6-F3PhCO2)4}2(TCNQ)]·2(p-xylene)·2CH2Cl2 (1). As expected from this D/A combination, 1 has a one-electron-transfer ionic state with the D(0.5+)2A(-) formulation. This state formally derives a heterospin state composed of S = 1 for [Ru(II,II)2], S = 3/2 for [Ru(II,III)2](+), and S = ½ for TCNQ(•-), possibly causing intralayer ferrimagnetic spin ordering. Most of these types of compounds have an antiferromagnetic ground state because of the coupling of ferrimagnetically ordered layers in dipole antiferromagnetic interactions. However, 1 became a three-dimensional ferrimagnet with T(C) = 91 K because of the presence of interlayer ferromagnetic interactions.

The effect of chlorine and fluorine substitutions on tuning the ionization potential of benzoate-bridged paddlewheel diruthenium(ii, ii) complexes

A series of paddlewheel diruthenium(ii, ii) complexes with various chlorine-substituted benzoate ligands (Cl-series) was synthesized as tetrahydrofuran (THF) adducts [Ru2(ClxPhCO2)4(THF)2]; where ClxPhCO2(-) = o-chlorobenzoate, ; m-chlorobenzoate, ; p-chlorobenzoate, ; 2,3-dichlorobenzoate, ; 2,4-dichlorobenzoate, ; 2,5-dichlorobenzoate, ; 2,6-dichlorobenzoate, ; 3,4-dichlorobenzoate, ; 3,5-dichlorobenzoate, ; 2,3,4-trichlorobenzoate, ; 2,3,5-trichlorobenzoate, ; 2,4,5-trichlorobenzoate, ; 3,4,5-trichlorobenzoate, ; 2,3,4,5-tetrachlorobenzoate, . This Cl-series and the previously synthesized F-series together with four new fluorine-substituted derivatives, [Ru2(FxPhCO2)4(THF)2] (where FxPhCO2(-) = 2,3-difluorobenzoate, ; 2,4-difluorobenzoate, ; 2,5-difluorobenzoate, ; 2,3,5-trifluorobenzoate, ), were experimentally characterized with respect to solid-state structure, magnetic properties and electrochemistry. By tuning the substituents of the benzoate ligands using chlorine or fluorine atoms, the redox potential (E1/2) for [Ru2(II,II)]/[Ru2(II,III)](+) varied over a wide range of potentials from -40 mV to 360 mV (vs. Ag/Ag(+) in THF). This was dependent on (i) the number of ortho-substituents, i.e. non-, mono- and di-o-substituted groups, with quasi-Hammett parameters for ortho-Cl and -F substitutions (σo = -0.272 and -0.217, respectively) and (ii) the general Hammett constants, σm and σp, for each group. The HOMO energy level calculated on the basis of the atomic coordinates of the solid-state structure was strongly affected by Cl- and F-substitutions as well as the redox potential in solution, which emphasizes the steric contribution of ortho-substituents in the energy level giving a deviation of EHOMO < 0.3 eV and <0.55 eV for the Cl- and F-series, respectively.

A charge-disproportionate ordered state with δ = 0.75 in a chemically sensitive donor/acceptor Dδ+2A2δ− layered framework

A novel charge-disproportionation state with δ = 0.75 was observed in an electron-donor (D)/-acceptor (A) D^δ+₂A^2δ− layered framework by chemically tuning the electron-donating affinity of D.

Crystal-to-crystal transformation of fishnet-like layered compounds: a self-locking structure with position-variable intercalated molecules

Fishnet-like layered compounds crystallized with anthracene (ANT) molecules between their layers and crystallization solvents at their hexagonal columnar pores; these complexes undergo a solvent-release crystal-to-crystal transformation involving the migration of a portion of the ANT molecules from between the layers to the hexagonal pores.

Magnet design by integration of layer and chain magnetic systems in a π-stacked pillared layer framework

The control of inter-lattice magnetic interactions is a crucial issue when long-range ordered magnets that are based on low-dimensional magnetic frameworks are designed. A "pillared layer framework (PLF)" model could be an efficient system for this purpose. In this report, A magnet based on a π-stacked PLF with a phase transition temperature of 82 K, which can be increased to 107 K by applying a pressure of 12.5 kbar, is rationally constructed. Two types of low-dimensional magnetic framework systems, an electron donor/acceptor magnetic layer and a charge transfer [FeCp*2](+)TCNQ(.-) columnar magnet ([FeCp*2](+) = decamethylferrocenium; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane), are integrated to fabricate the magnet. This synthetic strategy employing a combination of layers and chains is widely useful not only for magnet design, but also for the creation of multifunctional materials with pores and anisotropic frameworks.

Magnetic and Electronic Properties of Three New Hetero-Bimetallic Coordination Frameworks [Ru2(O2CR)4][Au(CN)2] (R = Benzoic Acid, Furan-2-carboxylate, or Thiophen-2-carboxylate)

The crystal structures and magnetic and electronic properties of three new hetero-bimetallic coordination frameworks, [Ru2(O2CR)4][Au(CN)2] (R = phenyl (1(Ph)), 2-furan (1(Furan)), and 2-thiophene (1(Thio)), have been characterised. Through variation of the functionalised carboxylate group, both the one-dimensional chain structure and properties of the frameworks have been systematically modulated. The magnetic behaviour for each framework has been modelled with a zero-field splitting model and the presence of weak inter-dimer coupling was assessed.

Selective NO trapping in the pores of chain-type complex assemblies based on electronically activated paddlewheel-type [Ru2(II,II)]/[Rh2(II,II)] dimers

The design of porous materials that undergo selective adsorption of a specific molecule is a critical issue in research on porous coordination polymers or metal-organic frameworks. For the purpose of the selective capture of molecules possessing an electron-acceptor character such as nitric oxide (NO), one-dimensional chain compounds possessing a high donor character have been synthesized using 4-chloroanisate-bridged paddlewheel-type dimetal(II, II) complexes with M = Ru and Rh and phenazine (phz) as the chain linker: [M2(4-Cl-2-OMePhCO2)4(phz)]·n(CH2Cl2) (M = Ru, 1; Rh, 2). These compounds are isostructural and are composed of chains with a [-{M2}-phz-] repeating unit and CH2Cl2 occupying the void space between the chains. Compounds 1 and 2 change to a new phase (1-dry and 2-dry) upon evacuating the crystallization solvent (CH2Cl2) and almost lose their pores in the drying process: no void space in 1-dry and 31.8 Å(3), corresponding to 2.9% of the cell volume, in 2-dry. Nevertheless, the compounds show a unique gas accommodation ability. Accompanied by a structural transformation (i.e., the first gate-opening) at low pressures of <10 kPa, both compounds show a typical physisorption isotherm for O2 (90 K) and CO2 (195 K), with the adsorption amount of ca. 2-4 gas molecules per [M2] unit. In addition, the adsorption isotherm for NO (121 K) involves the first gate-opening followed by a second gate-opening anomaly at NO pressures of ≈52 kPa for 1-dry and ≈21 kPa for 2-dry. At the first gate-opening, the absorbed amount of NO is ca. 4 molecules per [M2] unit, and then it reaches 8.4 and 6.3 for 1-dry and 2-dry, respectively, at 95 kPa. Only the isotherm for NO exhibits hysteresis in the desorption process, and some of the NO molecules are trapped in pores even after evacuating at 121 K, although it recovers to the original dried sample on heating to room temperature. The adsorbed NO molecules accrue a significant electron donation from the host framework even in the [Rh2] derivative, indicating that such simple porous compounds with electron-donor characteristics are useful for the selective adsorption of NO.

Axial-site modifications of paddlewheel diruthenium(II, II) complexes supported by hydrogen bonding

The reactions of paddlewheel-type diruthenium(II, II) complexes, [Ru2(II,II)(x-FPhCO2)4(THF)2] (x-FPhCO2(-) = x-fluorobenzoate with x- = o-, m-, p-), with 2,6-diaminopyridine (dapy) and 7-azaindole (azain) afford axially capped discrete compounds, [Ru2(II,II)(x-FPhCO2)4(dapy)2] (x = o-, 1; m-, 2; p-, 3) and [Ru2(II,II)(o-FPhCO2)4(azain)2] (4), respectively. In these compounds, intramolecular hydrogen bonds are observed between NH2 groups for 1-3 or imine NH groups for 4 and oxygen atoms of carboxylate groups. In addition, hydrogen bonds of NH2···F are also observed for 1 and 4 with an o-positioned F atom on benzoate. This coordination mode, i.e., a dual bonding mode with σ-bonding and hydrogen bonding, should assist ligand coordination to the axial position of the [Ru2] unit. The Ru-N bond distance in 1-4 is shorter than that observed in related compounds reported previously. In a similar fashion, reactions with planar M(II) dithiobiuret (dtb) complexes, [M(II)(dtb)2] (M(II) = Pd(II) and Pt(II)), were carried out. One-dimensional alternating chains, [{Ru2(II,II)(o-FPhCO2)4}{M(II)(dtb)2}] (M(II) = Pd(II), 5; Pt(II), 6), were obtained, in which the hydrogen-bonding modes of NH2···O and NH2···F are present, as expected. DFT calculations for the [M(II)(dtb)2] unit revealed that the LUMO of [M(II)(dtb)2] lies at -2.159 and -1.781 eV for M = Pd and Pt, respectively, which is much higher than HOMO energy at -4.184 eV calculated for [Ru2(II,II)(o-FPhCO2)(THF)2], proving that the respective units are essentially electronically isolated in the chains.

First row transition metal(II) thiocyanate complexes, and formation of 1-, 2-, and 3-dimensional extended network structures of M(NCS)2(solvent)2 (M = Cr, Mn, Co) composition

The reaction of first row transition M(II) ions with KSCN in various solvents form tetrahedral (NMe4)2[M(II)(NCS)4] (M = Fe, Co), octahedral trans-M(II)(NCS)2(Sol)4 (M = Fe, V, Ni; Sol = MeCN, THF), and K4[M(II)(NCS)6] (M = V, Ni). The reaction of M(NCS)2(OCMe2)2 (M = Cr, Mn) in MeCN and [Co(NCMe)6](BF4)2 and KSCN in acetone and after diffusion of diethyl ether form M(NCS)2(Sol)2 that structurally differ as they form one-dimensional (1-D) (M = Co; Sol = THF), two-dimensional (2-D) (M = Mn; Sol = MeCN), and three-dimensional (3-D) (M = Cr; Sol = MeCN) extended structures. 1-D Co(NCS)2(THF)2 has trans-THFs, while the acetonitriles have a cis geometry for 2- and 3-D M(NCS)2(NCMe)2 (M = Cr, Mn). 2-D Mn(NCS)2(NCMe)2 is best described as Mn(II)(μ(N,N)-NCS)(μ(N,S)-NCS)(NCMe)2 [= Mn2(μ(N,N)-NCS)2(μ(N,S)-NCS)2(NCMe)4] with the latter μ(N,S)-NCS providing the 2-D connectivity. In addition, the reaction of Fe(NCS)2(OCMe2)2 and 7,7,8,8-tetracyanoquino-p-dimethane (TCNQ) forms 2-D structured Fe(II)(NCS)2TCNQ. The magnetic behavior of 1-D Co(NCS)2(THF)2 can be modeled by a 1-D Fisher expression (H = -2JS(i)·S(j)) with g = 2.4 and J/kB = 0.68 K (0.47 cm(-1)) and exhibit weak ferromagnetic coupling. Cr(NCS)2(NCMe)2 and Fe(II)(NCS)2TCNQ magnetically order as antiferromagnets with Tc's of 37 and 29 K, respectively, while Mn(NCS)2(NCMe)2 exhibits strong antiferromagnetic coupling. M(NCS)2(THF)4 and K4[M(NCS)6] (M = V, Ni) are paramagnets with weak coupling between the octahedral metal centers.

Control of charge transfer in donor/acceptor metal-organic frameworks

Charge transfer (CT) of D(0)A(0) ↔ 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(m)A(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 ([Ru(2)(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 D(2)A type: (ii-1) the 1e(-) transferred D(2)A-MOFs provide mixed-valence systems of D(+)D(0)A(-) 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(+)(2)A(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 [Ru(2)(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.

Magnetically ordered molecule-based materials

Magnets composed of molecular components that provide both electron spins and spin-coupling pathways can stabilize bulk magnetic ordering. This was first reported for the ionic, zero-dimensional (0-D) electron transfer salt [Fe(C(5)Me(5))(2)](+)[TCNE]˙(-) (TCNE = tetracyanoethylene), which orders as a ferromagnet at T(c) = 4.8 K. Later V[TCNE](x) (x ∼ 2) was characterized to order above room temperature at 400 K (127 °C). Subsequently, numerous examples of organic- and molecule-based magnets have been characterized. In this critical review, after a discussion of the important aspects of magnetism pertaining to molecule-based magnets, including the determination of the magnetic ordering temperature (T(c)) these magnetically ordered materials are reviewed from a perspective of the structural dimensionality (208 references).

Molecular magnetic materials based on 4d and 5d transition metals

The study of paramagnetic compounds based on 4d and 5d transition metals is an emerging research topic in the field of molecular magnetism. An essential driving force for the interest in this area is the fact that heavier metal ions introduce important attributes to the physical properties of paramagnetic compounds. Among the attractive characteristics of heavier elements vis-à-vis magnetism are the diffuse nature of their d orbitals, their strong magnetic anisotropy owing to enhanced spin-orbit coupling, and their diverse structural and redox properties. This critical review is intended to introduce readers to the topic and to report recent progress in this area. It is not fully comprehensive in scope although we strived to include all relevant topics and a large subset of references in the area. Herein we provide a survey of the history and current status of research that has been conducted on the topic of second and third row transition metal molecular magnetism. The article is organized according to the nature of the precursor building blocks with special topics being highlighted as illustrations of the special role of heavier transition metal ions in the field. This paper is addressed to readers who are interested in molecular magnetism and the application of coordination chemistry principles to materials synthesis (231 references).