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.

Quantitative Electromerism of a Well-defined Mononuclear Copper Complex Through Reversible Intramolecular Metal-Ligand Electron Transfer.

Copper amine oxidases are enzymes that exhibit in their active site a mononuclear copper complex and a 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor; in the oxidative half of the catalytic cycle, the enzymes regulate their activity by a temperature-dependent electron transfer equilibrium between the CuII complex with the reduced, aminoquinol form of the cofactor and the reactive CuI complex with the corresponding oxidized, semiquinone form of the cofactor. Here, we report the first mononuclear copper complex with redox-active ligands showing quantitative, reversible electromerism between a CuII eletromer with reduced, neutral ligand and a CuI electromer with an oxidized, radical monocationic ligand. The CuII form, being exclusively present at low temperature, exhibits a lower enthalpy (like the enzymes), but the CuI complex exhibits a higher entropy and is exclusively present at room temperature in CH2 Cl2 solution. Further analysis, based on six different copper complexes, discloses a large solvent effect on electromerism.

A New Type of Valence Tautomerism in Cobalt Dioxolene Complexes - Temperature-Induced Transition from a Cobalt(III) Catecholate to a Low-Spin Cobalt(II) Semiquinonate State.

The synthesis and characterization of the monocationic cobalt(III) catecholate complex [Co(L-N4 t Bu2 )(Cl2 cat)]+ (L-N4 t Bu2 =N,N'-Di-tert.-butyl-2,11-diaza[3.3](2,6)pyridinophane, Cl2 cat2- =4,5-dichlorocatecholate) are presented. The complex exhibits valence tautomeric properties in solution; but, in contrast to the usually observed conversion from a cobalt(III) catecholate to a high-spin cobalt(II) semiquinonate state, valence tautomerism of [Co(L-N4 t Bu2 )(Cl2 cat)]+ leads to the formation of a low-spin cobalt(II) semiquinonate complex upon raising the temperature. This new type of valence tautomerism for a cobalt dioxolene complex has been unambiguously established by a detailed spectroscopic investigation using variable-temperature NMR, IR and UV-Vis-NIR spectroscopy. Determination of the enthalpies and entropies characterizing the valence tautomeric equilibria in various solutions shows that the influence of the solvent is almost exclusively entropic.

Ligand Modifications Produce Two-Step Magnetic Switching in a Cobalt(dioxolene) Complex.

Mononuclear monodioxolene valence tautomeric (VT) cobalt complexes typically exist in their low spin (l.s.) CoIII (cat2- ) and high spin (h.s.) CoII (sq⋅- ) forms (cat2- =catecholato, and sq⋅- =seminquinonato forms of 3,5-di-t Bu-1,2-dioxolene), which reversibly interconvert via temperature-dependent intramolecular electron transfer. Typically, the remaining four coordination sites on cobalt are supported by a tetradentate ligand whose properties influence the temperature at which VT occurs. We report that replacing one chelating pyridyl arm of tris(2-pyridylmethyl)amine (tpa) with a weaker field ortho-anisole moiety facilitates access to a third magnetic state, and examine a series of related complexes. Variable temperature crystallographic, magnetic, calorimetric, and spectroscopic studies support that this third state is consistent with l.s. CoII (sq⋅- ). Thus, our ligand modifications not only provide access to the VT transition from l.s. CoIII (cat2- ) to l.s. CoII (sq⋅- ), but at higher temperatures, the complex undergoes spin crossover from l.s. CoII (sq⋅- ) to h.s. CoII (sq⋅- ), representing the first example of two-step magnetic switching in a mononuclear monodioxolene cobalt complex. We hypothesize that ligand dynamicity may facilitate access to the rarely observed l.s. CoII (sq⋅- ) intermediate state, suggesting a new design criterion in the development of stimulus-responsive multi-state molecular switches.

Macroscopic Polarization Change of Mononuclear Valence Tautomeric Cobalt Complexes Through the Use of Enantiopure Ligand.

The crystallization of a complex having electron transfer properties in a polar space group can induce the polarization switching of a crystal in a specific direction, which is attractive for the development of sensors, memory devices, and capacitors. Unfortunately, the probability of crystallization in a polar space group is usually low. Noticing that enantiopure compounds crystallize in Sohncke space groups, this paper reports a strategy for the molecular design of non-ferroelectric polarization switching crystals based on the use of intramolecular electron transfer and chirality. In addition, this paper describes the synthesis of a mononuclear valence tautomeric (VT) cobalt complex bearing an enantiopure ligand. The introduction of enantiomer enables the crystallization of the complex in the polar space group (P21 ). The polarization of the crystals along the b-axis direction is not canceled out and the VT transition is accompanied by a change in the macroscopic polarization of the polar crystal. Polarization switching via electron transfer is realized at around room temperature.

Combining Open-Shell Verdazyl Environment and Co(II) Spin-Crossover: Spinmerism in Cobalt Oxoverdazyl Compound.

The spin states of a Co(II) oxoverdazyl compound are investigated by means of wavefunction-based calculations. Within a ca. 233 K energy window, the ground state and excited states display a structure-sensitive admixture of low-spin SM =1/2 in a dominant high-spin SM =3/2 Co(II) ion as indicated by the localized molecular orbitals. The puzzling spin zoology that results from the coupling between open-shell radical ligands and a spin-crossover metal ion gives rise to this unusual scenario, which extends the views in molecular magnetism. In agreement with experimental observation, the low-energy spectroscopy is very sensitive to deformations of the coordination sphere, and a growing admixture of Co(II) low-spin is evidenced from the calculations. In analogy with mesomerism that accounts for charge delocalization, entanglement combines different local spin states to generate a given total spin multiplicity, a spinmerism phenomenon.

Valence Tautomerism of p-Block Element Compounds - An Eligible Phenomenon for Main Group Catalysis?

Valence tautomerism has had a remarkable impact on several branches of transition metal chemistry. By switching between different valence tautomeric states, physicochemical properties and reactivities can be triggered reversibly. Is this phenomenon transferrable into the p-block - or is it already happening there? This Perspective collects observations of p-block element-ligand systems that might be assignable to valence tautomerism. Further, it discusses occurrences in p-block element compounds that exhibit the related effect of redox-induced electron transfer. As disclosed, the concept of valence tautomerism with p-block elements is at a very early stage. However, given the substantial disparity in the properties of those elements in different redox states, it might offer a valid extension for future developments in main group catalysis.

Tracking Topological and Electronic Effects on the Folding and Stability of Guanine-Deficient RNA G-Quadruplexes, Engineered with a New Computational Tool for De Novo Quadruplex Folding.

The initial aim of this work was to elucidate the mutual influence of different single-stranded segments (loops and caps) on the thermodynamic stability of RNA G-quadruplexes. To this end, we used a new NAB-GQ-builder software program, to construct dozens of two-tetrad G-quadruplex topologies, based on a designed library of sequences. Then, to probe the sequence-morphology-stability relationships of the designed topologies, we performed molecular dynamics simulations. Their results provide guidance for the design of G-quadruplexes with balanced structures, and in turn programmable physicochemical properties for applications as biomaterials. Moreover, by comparative examinations of the single-stranded segments of three oncogene promoter G-quadruplexes, we assess their druggability potential for future therapeutic strategies. Finally, on the basis of a thorough analysis at the quantum mechanical level of theory on a series of guanine assemblies, we demonstrate how a valence tautomerism, triggered by a coordination of cations, initiates the process of G-quadruplex folding, and we propose a sequential folding mechanism, otherwise dictated by the cancellation of the dipole moments on guanines.

Lanthanoid Complexes as Molecular Materials: The Redox Approach.

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.

Tailoring Valence Tautomerism by Using Redox Potentials: Studies on Ferrocene-Based Triarylmethylium Dyes with Electron-Poor Fluorenylium and Thioxanthylium Acceptors.

Three new electrochromic ferrocenyl triarylmethylium dyes with fluorenylium (1 a+ , 1 b+ ) or thioxanthylium (1 c+ ) residues were selected in order to keep the intrinsic differences of redox potentials for ferrocene oxidation and triarylmethylium reduction small and to trigger valence tautomerism (VT). UV/Vis/NIR and quantitative EPR spectroscopy identified paramagnetic diradical isomers 1 a..+ -1 c..+ alongside diamagnetic forms 1 a+ -1 c+ , which renders these complexes magnetochemical switches. The diradical forms 1 a..+ -1 c..+ as well as the one-electron-reduced triarylmethyl forms of the complexes were found to dimerize in solution. For radical 1 a. , dimerization occurs on the timescale of cyclic voltammetry; this allowed us to determine the kinetics and equilibrium constant for this process by digital simulation. Mößbauer spectroscopy indicated that 1 a+ and 1 b+ retain VT even in the solid state. UV/Vis/NIR spectro-electrochemistry revealed the poly-electrochromic behaviour of these complexes by establishing the distinctly different electronic absorption profiles of the corresponding oxidized and reduced forms.

Large-Amplitude Thermal Vibration-Coupled Valence Tautomeric Transition Observed in a Conductive One-Dimensional Rhodium-Dioxolene Complex.

The exploration of dynamic molecular crystals is a fascinating theme for materials scientists owing to their fundamental science and potential application to molecular devices. Herein, a one-dimensional (1D) rhodium-dioxolene complex is reported that exhibits drastic changes in properties with the phase transition. X-ray photoelectron spectroscopy (XPS) revealed that the room-temperature (RT) phase is in a mixed-valence state, and therefore, the drastic changes originate from the mixed-valence state appearing in the RT phase. Another notable feature is that the mean square displacements of the rhodium atoms along the 1D chain dramatically increased in the RT phase, indicating a large-amplitude vibration of the Rh-Rh bonds. From these results, a possible mechanism for the appearance of the mixed-valence state in the RT phase was proposed based on the thermal electron transfer from the 1D d-band to the semiquinonato π* orbital coupled with the large-amplitude vibration of the Rh-Rh bonds.

Metal-to-Ligand Charge Transfer Induced Valence Tautomeric Forms of Non-Innocent 2,2'-Azobis(benzothiazole) in Ruthenium Frameworks.

The impact of metal-to-ligand charge transfer towards the redox noninnocence of 2,2'-azobis(benzothiazole) (abbt) has been highlighted on coordination to {RuII (acac)2 } (acac=2,4-pentanedionato). It led to the authentication of a series of mononuclear and dinuclear complexes incorporating variable oxidation states of abbt (abbt0/.-/2- ). Mononuclear 1 was identified as [RuIII (abbt.- )], a MLCT excited state of [RuII (abbt)]. Dinuclear 2 was however recognized as two discrete redox isomers: (i) radical bridged mixed-valent meso-[Ru2.5 (µ-abbt.- )Ru2.5 ] (2a) and (ii) dianionic ligand bridged isovalent meso-[RuIII (µ-abbt2- )RuIII ] (2b), demonstrating unprecedented structural confirmation of valence tautomerism in azo-based ligand systems. A crystal structure of [2]ClO4 validated the formation of [RuIII (µ-abbt.- ) RuIII ]ClO4 . Analysis of electronic structural forms of 1 and 2 in accessible redox states via spectroelectrochemistry and DFT revealed their electron reservoir feature.

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 CoII bis-diketonates and FeII 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.

Valence tautomerism in synthetic models of cytochrome P450.

CytP450s have a cysteine-bound heme cofactor that, in its as-isolated resting (oxidized) form, can be conclusively described as a ferric thiolate species. Unlike the native enzyme, most synthetic thiolate-bound ferric porphyrins are unstable in air unless the axial thiolate ligand is sterically protected. Spectroscopic investigations on a series of synthetic mimics of cytP450 indicate that a thiolate-bound ferric porphyrin coexists in organic solutions at room temperature (RT) with a thiyl-radical bound ferrous porphyrin, i.e., its valence tautomer. The ferric thiolate state is favored by greater enthalpy and is air stable. The ferrous thiyl state is favored by entropy, populates at RT, and degrades in air. These ground states can be reversibly interchanged at RT by the addition or removal of water to the apolar medium. It is concluded that hydrogen bonding and local electrostatics protect the resting oxidized cytP450 active site from degradation in air by stabilizing the ferric thiolate ground state in contrast to its synthetic analogs.

Thermally Induced Valence Tautomeric Transition in a Two-Dimensional Fe-Tetraoxolene Honeycomb Network.

A novel tetraoxolene-bridged Fe two-dimensional honeycomb layered compound, (NPr4 )2 [Fe2 (Cl2 An)3 ] ⋅2 (acetone)⋅H2 O (1), where Cl2 Ann- =2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonate and NPr4+ =tetrapropylammonium cation, has been synthesized. 1 revealed a thermally induced valence tautomeric transition at T1/2 =236 K (cooling)/237 K (heating) between Fem+ (m=2 or 3) and Cl2 Ann- (n=2 or 3) that induced valence modulations between [FeIIHS FeIIIHS (Cl2 An2- )2 (Cl2 An.3- )]2- at T>T1/2 and [FeIIIHS FeIIIHS (Cl2 An2- )(Cl2 An.3- )2 ]2- at T

Deactivation of a Cobalt Catalyst for Water Reduction through Valence Tautomerism.

The activity of the water reduction catalyst [CoIII (L1 )(pyr)2 ]PF6 (1), where (L1 )2- is a bis-amido pyridine ligand and pyr is pyrrolidine, is investigated. Catalyst 1 has an overpotential of 0.54 V and a high observed TOF of 23 min-1 , albeit for a relatively short time. Considering the significant activity of 1 and aiming to improve catalyst design, a detailed structural and electronic study is performed to understand the mechanisms of deactivation. Experimental and theoretical evidence support that the metal-reduced [CoI (L1 )]- is in tautomeric equilibrium with the ligand-reduced [CoII (L1. )]- species. While [CoI (L1 )]- favors formation of a CoIII -H- relevant for catalysis, the [CoII (L1. )]- species leads to ligand protonation, structural distortions, and, ultimatley, catalyst deactivation.

Electronic Structure of Manganese Complexes of the Redox-Non-innocent Tetrazene Ligand and Evidence for the Metal-Azide/Imido Cycloaddition Intermediate.

The first synthetic manganese tetrazene complexes are described as a redox pair comprising anionic [Mn(N4 Ad2 )2 ](-) (1) and neutral Mn(N4 Ad2 )2 (2) complexes (N4 Ad2 =[Ad-N-N=N-N-Ad](2-) ). Compound 1 is obtained in two forms as lithium salts, one as a cationic Li2 Mn cluster, and one as a Mn-Li 1D ionic polymer. Compound 1 is electronically described as a Mn(III) center with two [N4 Ad2 ](2-) ligands. The one-electron oxidized 2 is crystalized in two morphologies, one as pure 2 and one as an acetonitrile adduct. Despite similar composition, the behavior of 2 differs in the two morphologies. Compound 2-MeCN is relatively air and temperature stable. Crystalline 2, on the other hand, exhibits a compositional, dynamic disorder wherein the tetrazene metallacycle ring-opens into a metal imide/azide complex detectable by X-ray crystallography and FTIR spectroscopy. Electronic structure of 2 was examined by EPR and XPS spectroscopies and DFT calculations, which indicate 2 is best described as a Mn(III) ion with an anion radical delocalized across the two ligands through spin-polarization effects.

Influence of Intermolecular Interactions on Valence Tautomeric Behaviors in Two Polymorphic Dinuclear Cobalt Complexes.

Two polymorphic structures have been well determined in a valence tautomeric (VT) dinuclear cobalt complex. These polymorphs showed distinct thermal- and photomagnetic behavior, and are thus ideal for studying the "pure" intermolecular factors to VT transitions. In polymorph 1A, the VT cations are arranged head-to-waist with their neighbors and exhibit weak π⋅⋅⋅π interactions, resulting in a gradual and incomplete thermal VT transition. In contrast, the cations in polymorph 1B are arranged head-to-tail and exhibit relatively strong π⋅⋅⋅π interactions, leading to an abrupt and complete thermal VT transition with adjustable hysteresis loop at around room temperature. The VT process for both polymorphs can be induced by light, but the light-excited state of 1B⋅2H2 O has a higher thermal relaxation temperature than that of 1A⋅3H2 O.

Copper Complexes of New Redox-Active 4,5-Bisguanidino-Substituted Benzodioxole Ligands: Control of the Electronic Structure by Counter-Ligands, Solvent, and Temperature.

Herein, we analyze the possibility of controlling the electronic structure of mononuclear copper complexes featuring new redox-active 4,5-bisguanidino-substituted benzodioxole ligands. The nature of the guanidino groups, the anionic counter-ligands, the applied solvent (polarity), and the temperature are the parameters that decide if a CuII complex with neutral ligand unit or a CuI complex with radical monocationic ligand unit is the adequate description. Under special conditions, a temperature-dependent equilibrium of the two valence tautomeric forms (CuII /neutral ligand and CuI /radical monocationic ligand) is achieved. Removal of a ligand-centered electron from a paramagnetic CuII complex with a neutral ligand unit leads to a diamagnetic CuI complex with a dicationic ligand unit through a redox-induced electron-transfer (RIET) process.

Synthesis of Barbaralones and Bullvalenes Made Easy by Gold Catalysis.

The gold(I)-catalyzed oxidative cyclization of 7-ethynyl-1,3,5-cycloheptatrienes gives 1-substituted barbaralones in a general manner, which simplifies the access to other fluxional molecules. As an example, we report the shortest syntheses of bullvalene, phenylbullvalene, and disubstituted bullvalenes, and a readily accessible route to complex cage-type structures by further gold(I)-catalyzed reactions.