Valence Tautomerism in One-Dimensional Coordination Polymers

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Simultaneous manipulation of iron(II) spin crossover and LIESST behaviour using pressure, temperature and light

Spin crossover (SCO) and light-induced excited spin state trapping (LIESST) effects were studied using high pressure X-ray diffraction at cryogenic temperatures on a single crystal of the {[FeII(pyrazole)4]2[NbIV(CN)8]·4H2O}n (FeNb) coordination polymer. The studied compound does not show SCO or LIESST at ambient pressure, but these effects can be enforced by a mechanical stimulus. The obtained results demonstrate the manipulation of the spin state via the appropriate combination of multiple stimuli simultaneously.

Molecular Twist-Induced Single-Crystal Isomerization and Valence Tautomeric Transitions in a Cobalt-Dioxolene Complex

A mononuclear valence tautomeric (VT) complex, [Co(pycz)2(Sq)(Cat)] (1-trans), where pycz = 9-(pyridin-4-yl)-9H-carbazole, Sq⋅- = 3,5-di-tert-butyl-semiquinonato, and Cat2- = 3,5-di-tert-butyl-catecholato, is synthesized in the trans configuration, which undergoes one-step valence tautomeric transition above room temperature. Remarkably, 1-trans can transform into its isomeric structure, [Co(pycz)2(Sq)(Sq)] (1-cis), at temperature above 350 K in a single-crystal-to-single-crystal way by in situ molecular twist, and the resulting 1-cis exhibits a pronounced two-step VT transition during magnetic measurements that is rare for mononuclear VT complexes. Such drastic solid-state structural transformation is reported in VT compounds for the first time, which is actuated by a crystal surface's melting-recrystallization induced phase transition process. DFT calculations offer an underlying mechanism suggesting a concerted bond rotation during the structural transformation. The results demonstrate an unconventional approach that realizes structural transformation of VT complexes and the control of VT performance.

Dual-radical-based molecular anisotropy and synergy effect of semi-conductivity and valence tautomerization in a photoswitchable coordination polymer

Organic radicals are widely used as linkers or ligands to synthesize molecular magnetic materials. However, studies regarding the molecular anisotropies of radical-based magnetic materials and their multifunctionalities are rare. Herein, a photoisomerizable diarylethene ligand was used to form {[Co^III(3,5-DTSQ^·-)(3,5-DTCat^2-)]₂(6F-DAE-py₂)}·3CH₃CN·H₂O (o-1·3CH₃CN·H₂O, 6F-DAE-py₂ = 1,2-bis(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene), a valence-tautomeric (VT) coordination polymer. We directly observed dual radicals for a single crystal using high-field/-frequency (∼13.3 T and ∼360 GHz) electron paramagnetic resonance (EPR) spectroscopy along the c-axis, which was further confirmed by angle-dependent Q-band EPR spectroscopy. Moreover, a conductive anomaly close to the VT transition temperature was observed only when probes were attached at the ab plane of the single crystal, indicative of synergy between valence tautomerism and conductivity. Structural anisotropy studies and density functional theory (DFT) calculations revealed that this synergy is due to electron transfer associated with valence tautomerism. This study presents the first example of dual-radical-based molecular anisotropy and charge-transfer-induced conductive anisotropy in a photoswitchable coordination polymer.

Electronic structure of cobalt valence tautomeric molecules in different environments

Future molecular microelectronics require the electronic conductivity of the device to be tunable without impairing the voltage control of the molecular electronic properties. This work reports the influence of an interface between a semiconducting polyaniline polymer or a polar poly-D-lysine molecular film and one of two valence tautomeric complexes, i.e., [Co^III(SQ)(Cat)(4-CN-py)₂] ↔ [Co^II(SQ)₂(4-CN-py)₂] and [Co^III(SQ)(Cat)(3-tpp)₂] ↔ [Co^II(SQ)₂(3-tpp)₂]. The electronic transitions and orbitals are identified using X-ray photoemission, X-ray absorption, inverse photoemission, and optical absorption spectroscopy measurements that are guided by density functional theory. Except for slightly modified binding energies and shifted orbital levels, the choice of the underlying substrate layer has little effect on the electronic structure. A prominent unoccupied ligand-to-metal charge transfer state exists in [Co^III(SQ)(Cat)(3-tpp)₂] ↔ [Co^II(SQ)₂(3-tpp)₂] that is virtually insensitive to the interface between the polymer and tautomeric complexes in the Co^II high-spin state.

One-Step versus Two-Step Valence Tautomeric Transitions in Tetraoxolene-Bridged Dinuclear Cobalt Compounds

The syntheses of valence tautomeric compounds with multistep transitions using new redox-active ligands are the long-term goal of the field of bistable materials. The redox-active tetraoxolene ligand, 2,7-di-tert-butylpyrene-4,5,9,10-tetraone (pyrene^Q-Q), is now developed to synthesize a pair of dinuclear compounds {[CoL₂]₂(pyrene^Sq-Sq)}[Co(CO)₄]₂·xCH₂Cl₂·2C₆H₅CH₃ (1, x = 2, L = 1,10-phenanthroline, phen; 2, x = 1.5, L = 2,2'-bipyridine, bpy). Variable-temperature magnetic susceptibilities and single-crystal X-ray diffraction measurements indicate a partial one-step valence tautomeric transition for 1 and a rare two-step valence tautomeric transition for 2, respectively. DFT calculation results are consistent with the experimental data, revealing the correlation between thermodynamic parameters and the one-step/two-step valence tautomeric behaviors.

Tunable Synchronicity of Molecular Valence Tautomerism with Macroscopic Solid-Liquid Transition by Molecular Lattice Engineering

The combination of a cobalt-dioxolene core that exhibits valence tautomerism (VT) with pyridine-3,5-dicarboxylic acid functionalized with chains bearing two, four, or six oxyethylene units led to new complexes ConEGEspy (n = 2, 4, and 6). These complexes commonly form violet crystals of the low-spin (ls)-[Co^III (nEGEspy)₂ (3,6-DTBSQ)(3,6-DTBCat)] (ls-[Co^III ], 3,6-DTBSQ = 3,6-di-tert-butyl semiquinonato, 3,6-DTBCat = 3,6-di-tert-butyl catecholato). Interestingly, violet crystals of Co2EGEspy in the ls-[Co^III ] transitioned into a green liquid, accompanied by an almost complete VT shift (94 %) to the high-spin (hs)-[Co^II (nEGEspy)₂ (3,6-DTBSQ)₂ ] (hs-[Co^II ]) upon melting. In contrast, violet crystals of Co4EGEspy and Co6EGEspy in the ls-[Co^III ] exhibited partial VT (33 %) and only a 9.3 % VT shift after melting, respectively. These data demonstrate the tunability of the synchronicity of the molecular VT and macroscopic solid-liquid transitions by optimizing the tethered chains, thus establishing a new strategy for coupling bistable molecules with the macroscopic world.

Photo-induced variation of magnetism in coordination polymers with ligand-based electron transfer

Photo-induced variation of magnetism from ligand-based electron transfer has been extensively studied because of its potential applications in magneto-optical memory devices, light-responsive switches, and high-density information storage materials. In this review, we discussed the progress in the photo-induced variation of magnetism in coordination polymers with ligand-to-metal charge transfer (LMCT), ligand-to-ligand charge transfer (LLCT) and internal ligand charge transfer (ILCT), which provides fundamentals for the rational design of multi-functional materials. We also discussed the design and synthetic strategy of such molecule-based materials and gave views on the current challenges and growth trends in this field.

Influence of the Increasing Number of Organic Radicals on the Structural, Magnetic, and Electrochemical Properties of the Copper(II)-Dioxothiadiazole Family of Complexes

The preparation, structures, and electrochemical and magnetic properties supported by density functional theory (DFT) calculations of three new copper(II) compounds with [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide (td) and its radical anion (td·-) are reported: {[CuIICl(td)](μ-Cl)2[CuIICl(td)]} (1), which incorporates only neutral td ligands; [CuIICl(td·-)(td)]·2MeCN (2), which comprises one neutral td and one radical td·-; and PPN[CuIICl(td·-)2]·2DMA (3), where CuII ions are coordinated by two radical anions td·- (DMA, dimethylacetamide; PPN+, the bis(triphenylphosphine)iminium cation). All three compounds show interesting paramagnetic behavior with low-temperature features indicating significant antiferromagnetic coupling. The magnetic properties of 1 are dominated by CuII···CuII interactions (JCuCu) mediated through the Cl- bridges, while the magnetic properties of 2 and 3 are governed mainly by the td·-···td·- (Jtdtd) and CuII-td·- (JCutd) exchange interactions. The structure of 2 features only two major magnetic coupling pathways enabling the fitting of experimental data with Jtdtd = -36.0(5) cm-1 and JCutd = -12.6(2) cm-1 only. Compound 3 exhibits a complex network of magnetic contacts. Attempt to approximate its magnetic behavior using only a local magnetic contacts model resulted in Jtdtd = -5.6(1) cm-1 and two JCutd constants, -12.4(2) and -22.6(4) cm-1. The experimental fitting is critically compared with the results of broken symmetry density functional theory (BS DFT) calculations for inter- and intramolecular contacts. More consistent results were obtained with the M06 functional as opposed to popular B3LYP, which encountered problems reproducing some of the experimental intermolecular exchange interactions. Electrochemical measurements of 2 and 3 in MeCN showed three reversible nearly overlapping redox peaks appearing in a narrow potential range of -600 to -100 mV vs Fc/Fc+. Small differences between the redox events suggest that such compounds may be good candidates for new switchable materials, where the electron transfer between the metal and the ligand center is triggered by temperature, pressure, or light (valence tautomerism).

Chameleonic layered metal-organic frameworks with variable charge-ordered states triggered by temperature and guest molecules

Molecular materials whose electronic states are multiply varied depending on external stimuli are among the most promising targets for the development of multiply accessible molecular switches. Here, we report a honeycomb layer composed of tetraoxolene-bridged iron (Fe) subunits whose charge-ordered states are multiply variable via thermal treatments and solvation/desolvation with the crystallinity intact. The compound is (NPr4)2[Fe2(Cl2An)3] (1-d; NPr4 + = tetra-n-propylammonium; Cl2An2- = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinonate), which possesses three charge-ordered states: a low-temperature (LT) phase [(Fe3+)2(Cl2An2-)(Cl2An˙3-)2]2-; an intermediate (IM) phase [(Fe2.5+)2(Cl2An2-)(Cl2An2.5-)2]2-; and a high-temperature (HT) phase [(Fe2+)2(Cl2An2-)3]2- that varies according to temperature. In addition, the LT phase of 1-d is reversibly changeable to another IM phase in its solvated compound 1 via a solvation/desolvation process at room temperature. This example demonstrates a new multiple-switching system based on electron transfer and host-guest chemistry in a charge-flexible metal-organic framework.

Thermally-induced hysteretic valence tautomeric conversions in the solid state via two-step labile electron transfers in manganese-nitronyl nitroxide 2D-frameworks

Near room temperature hysteretic thermo-induced valence tautomerism was discovered in a layered 2D-coordination polymer of manganese(ii) with nitronyl nitroxide radicals separated by ClO4- anions (1). This opens a novel approach towards switchable materials with hysteresis and under ambient conditions with prospects for applications and for investigating solid-state intramolecular electron transfers. Herein, two new compounds with similar layered structures where the anions (X) are BF4- (2) or PF6- (3) are presented. Their magnetic behaviors also reveal hysteretic thermo-induced valence tautomeric conversions but in two steps and evidencing a strong effect of the anion. This occurs near room temperature (278-220 K) for 2 and higher for 3 (380-330 K). Their single crystal structures at different temperatures show that this involves two successive thermally-triggered electron transfers with switching between three redox tautomers formulated as {[MnII2-yMnIIIy(NITIm)3-y(NITRed)y]X}n, where y is temperature dependent. Upon cooling from the high-temperature redox-tautomer (y = 0) to the intermediate one (y = 1), half of the manganese(ii) centers are oxidized to manganese(iii) and 1/3 of the nitronyl nitroxide radicals (NITIm-) are reduced to the aminoxyl form (NITRed2-). On further cooling, the second half of the manganese(ii) centers are oxidized and another 1/3 of the radicals are reduced to reach the low-temperature redox-tautomer (y = 2). Upon reheating, reverse electron transfers occur. This is complementarily supported by X-ray powder measurements, differential scanning calorimetry, and electron paramagnetic resonance and Raman spectroscopies. These multi-stable compounds in which manganese ions exchange reversibly their electron with the nitronyl nitroxide radical are outstanding rare examples of two-step valence tautomerism in the solid state promoted by the polymeric structure.

Rational Design of Electronically Labile Dinuclear Fe and Co complexes with 1,10-Phenanthroline-5,6-Diimine: A DFT study

A series of coordination compounds of redox-active 1,10-phenanthroline-5,6-diimine with Co^II bis-diketonates and Fe^II dihydrobis(pyrazolyl)borates has been computationally designed by means of density functional theory (DFT UB3LYP*/6-311++G(d,p)) calculations of their electronic structure, energy characteristics, and magnetic properties. Four types of complexes differing by the nature and position of the terminal metal-centered fragments have been considered. The performed systematic calculations have revealed the systems capable of undergoing thermally initiated spin-state switching rearrangements, including those governed by the synchronized mechanisms of spin crossover and valence tautomerism. The predicted magnetic characteristics allow one to consider the dinuclear cobalt complexes and heterometallic Co/Fe compounds with 1,10-phenanthroline-5,6-diimine as building blocks for molecular and quantum electronics devices. © 2019 Wiley Periodicals, Inc.

Unconventional Method for Fabricating Valence Tautomeric Materials: Integrating Redox Center within a Metal-Organic Framework

Due to the structural advantages displayed by Metal-Organic Frameworks (MOFs), integrating Valence Tautomerism (VT) systems within MOFs could be an effective strategy in order to break through the constraints of the traditional ones. Herein, we report the first successful integration of a VT system into a MOF termed VT-MOF-1. The structural characteristics of VT-MOF-1, such as dinuclear cobalt-catechol clusters and solvent-accessible pores, are both innovative and novel, potentially yielding new vitality within VT field. In addition, VT-MOF-1 exhibits specific behaviors responsive to temperature and different solvent molecules as n-butanol, tert-butanol, and isopropyl alcohol. The entropy values and configurations of the solvent molecules might be responsible for the tunable sensing behaviors.

Valence tautomerism and spin crossover in pyridinophane–cobalt–dioxolene complexes: an experimental and computational study

Variation of alkyl substituents on pyridinophane ancillary ligands governs temperature-dependent valence tautomeric or spin crossover equilibria in a family of cobalt–dioxolene complexes.

A Photomagnetic Sponge: High-Temperature Light-Induced Ferrimagnet Controlled by Water Sorption

"Converting" light energy to magnetization is the attribute of molecule-based compounds called photomagnets and is inaccessible for conventional magnetic solids. The design and synthesis of such compounds, however, is a formidable challenge, and only a few examples are known, all with rather low magnetic ordering temperatures well below the boiling point of liquid nitrogen. Herein, a cyanide-bridged coordination polymer, {[Mn^II(imidazole)]₂[W^IV(CN)₈]} _n, exhibiting the highest light-induced magnetic ordering temperature ever observed and a magnetic hysteresis loop up to 90 K is reported. The photomagnetic effect results from the blue light excitation (450 nm) of the constituent octacyanotungstate(IV) moiety, which then couples magnetically with manganese(II), resulting in light-induced ferrimagnetic ordering. The reported coordination framework shows also outstanding water sorption properties that are strongly correlated with the photomagnetic functionality. The photoswitching observed in the anhydrous state is completely quenched by the reversible capture of water, with the fully hydrated phase becoming practically non-photomagnetic.

Cobalt complexes with redox-active anthraquinone-type ligands

Three anthraquinone-type multidentate ligands, HL1-3 (HL = 2-R-1H-anthra[1,2-d]imidazole-6,11-dione; HL1: R = (2-pyridyl), HL2; R = (4,6-dimethyl-2-pyridyl), HL3; R = (6-methoxy-2-pyridyl)), were prepared, and their complexation behaviour was investigated. Three bis-chelate cobalt complexes with the formula [CoII(L1-3)2]·n(solv.) (1, 2, and 3 for HL1, HL2, and HL3, respectively), in which the ligands adopted tridentate binding modes, were synthesized and structurally characterized by single-crystal X-ray analyses. Electrochemical studies of 1-3 in CH2Cl2 reveal three reversible redox waves, assigned to ligand and cobalt-centred processes. Additional complexes were obtained in which HL1 adopted a bidentate binding mode, stabilising the mono-chelate [CoII(HL1)(NO3)2(DMF)2] (4) species and tris-chelate [CoIII(L1)3] (5) complex in which the cobalt ion was in its 3+ state. The electrochemical properties of complex 5 were investigated in DMF, and the Co(ii)/Co(iii) redox couple was found to be negatively shifted compared to that of complex 1, while the ligand-based processes became irreversible. Tridentate chelation is found to stabilise the anthraquinone ligands and unlocks their redox multi-stability.

Structural investigation of one- and two-dimensional coordination polymers based on cobalt-bis(dioxolene) units and 1-hydroxy-1,2,4,5-tetrakis(pyridin-4-yl)cyclohexane

The combination of cobalt, 3,5-di-tert-butyldioxolene (3,5-dbdiox) and 1-hydroxy-1,2,4,5-tetrakis(pyridin-4-yl)cyclohexane (tpch) yields two coordination polymers with different connectivities, i.e. a one-dimensional zigzag chain and a two-dimensional sheet. Poly[[bis(3,5-di-tert-butylbenzene-1,2-diolato)bis(1,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-yl-3-olato)[μ₄-1-hydroxy-1,2,4,5-tetrakis(pyridin-4-yl)cyclohexane]cobalt(III)]-ethanol-water 1/7/5], {[Co₂(C₁₄H₂₀O₂)₄(C₂₆H₂₄N₄O)]·7C₂H₅OH·5H₂O}_n or {[Co₂(3,5-dbdiox)₄(tpch)}·7EtOH·5H₂O}_n, is the second structurally characterized example of a two-dimensional coordination polymer based on linked {Co(3,5-dbdiox)₂} units. Variable-temperature single-crystal X-ray diffraction studies suggest that catena-poly[[[(3,5-di-tert-butylbenzene-1,2-diolato)(1,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-yl-3-olato)cobalt(III)]-μ-1-hydroxy-1,2,4,5-tetrakis(pyridin-4-yl)cyclohexane]-ethanol-water (1/1/5)], {[Co(C₁₄H₂₀O₂)₂(C₂₆H₂₄N₄O)]·C₂H₅OH·5H₂O}_n or {[Co(3,5-dbdiox)₂(tpch)]·EtOH·5H₂O}_n, undergoes a temperature-induced valence tautomeric interconversion.

Steric control in the metal–ligand electron transfer of iminopyridine–ytterbocene complexes

A systematic study of reactions between Cp*₂Yb(THF) and iminopyridine ligands featuring similar electron accepting properties but variable denticity and steric demand, has provided a new example of steric control on the redox chemistry of ytterbocenes.