Valence Tautomeric Interconversion Research Articles

Valence tautomeric interconversion of bis-dioxolene cobalt complex with imino-pyridine functionalized by TEMPO moiety in solid solutions with isostructural nickel analogue: phase transitions and monocrystal destruction.

Valence tautomeric complexes (VT) are promising systems for creating molecular devices. From this viewpoint, valence tautomeric complexes with a hysteresis loop on the magnetic curve are of special interest as potential memory elements. The hysteresis loop is a consequence of retarded structural rearrangements which investigation is an actual problem. Recently, we have described a new VT transition taking place in a bis-dioxolene cobalt complex with imino-pyridine having a TEMPO substituent (A. A. Zolotukhin, et al., Inorg. Chem., 2017, 56, 14751-14754). Valence tautomeric transformation occurs with a hysteresis loop and is accompanied by a phase transition. The phase transition taking place during cooling is accompanied by crystal destruction. This fact makes it impossible to monitor the structural changes responsible for the hysteresis loop. The current research attempts to resolve this problem. A nickel compound of the same composition (TEMPO-imino-pyridine)Ni(3,6-DBSQ)2 was synthesized and characterized. It was established to be isostructural with the cobalt complex. It was used as an inert matrix for the dilution of the VT cobalt complex. The number of solid solutions with Co/Ni ratios of 1 : 1, 1 : 2, 1 : 4, and 1 : 8 was obtained. Variable temperature magnetic susceptibility measurements show that VT transformation with a hysteresis loop takes place in all solid solutions. The hysteresis loop is shifted to low temperatures primarily due to the shifting of its low-temperature boundary with dilution. The hysteresis width does not change significantly with dilution. DSC detected that transformations are accompanied by phase transitions at different temperatures at cooling and heating. The phase transition at the first cooling occurs at slightly lower temperatures compared with subsequent cycles. These temperatures correspond to the transition temperatures detected from the magnetic curves. The phase transition during the first cooling is accompanied by crystal destruction. Physical destruction takes place in the crystals of all solid solutions. X-ray diffraction powder patterns confirm that phase transition is accompanied by considerable reorganization of the crystal structure typical for the first order transitions. The unit cell volume of solid solutions is larger than that of pure complexes. Especially calculated crystal invariom indicated that the "lattice energy" in a solid solution is the lowest compared with that in "pure" nickel and cobalt complexes.

Valence tautomerism in a cobalt-dioxolene complex containing an imidazolic ancillary ligand.

This work reports the synthesis, structural, spectroscopic and magnetic investigation of two complexes, [Co(bmimapy)(3,5-DTBCat)]PF6·H2O (1) and [Co(bmimapy)(TCCat)]PF6·H2O (2), where bmimapy is an imidazolic tetradentate ancillary ligand and 3,5-DTBCat and TCCat are the 3,5-di-tert-butyl-catecholate and tetrachlorocatecholate anions, respectively. Their structures have been elucidated using single crystal X-ray diffraction, showing a pseudo-octahedral cobalt ion bound to a chelating dioxolene ligand and the ancillary bmimapy ligand in a folded conformation. Magnetometry displayed an entropy-driven, incomplete, Valence Tautomeric (VT) process for 1 in the 300-380 K temperature range, while 2 displayed a temperature independent, diamagnetic low-spin cobalt(iii)-catecholate charge distribution. This behaviour was interpreted on the basis of the cyclic voltammetric analysis, allowing the estimation of the free energy difference associated with the VT interconversion of +8 and +96 kJ mol-1 for 1 and 2, respectively. A DFT analysis of this free energy difference highlighted the ability of the methyl-imidazole pendant arm of bmimapy favouring the onset of the VT phenomenon. This work introduces the imidazolic bmimapy ligand to the scientific community working in the field of valence tautomerism, increasing the library of ancillary ligands to prepare temperature switchable molecular magnetic materials.

Photo- and Thermosensitive Molecular Crystals. Valence Tautomeric Interconversion as the Cause of the Photomechanical Effect in Crystals of Bis(dioxolene)cobalt Complex.

Especially grown crystals (elongated plates) of the complex (1,10-phen)Co(3,6-DBSQ)2 (1) reversibly elastically bend under laser excitation (532 or 808 nm) and/or heating in the temperature interval ∼242-265 K (1,10-phen = 1,10-phenanthroline, 3,6-DBSQ = anion-radical and 3,6-DBCat = dianion of 3,6-di-tert-butyl-o-benzoquinone). The abrupt interconversion of valence tautomers is observed in this temperature range: (phen)Co(3,6-DBCat)(3,6-DBSQ) ⇌ (phen)Co(3,6-DBSQ)2. Solving the problem of design of photo- (thermo-) actuators with preset parameters, the series of solid solutions of general formula (phen)x(bpy)1-xCo(3,6-DBSQ)2 was obtained and crystallized as thin elongated plates.Crystals of each solid solution demonstrate the maximal photomechanical response (bending) at a temperature close to its own valence tautomeric (VT) transition temperature. On the other hand, the VT transition temperature depends on the composition of the solid solution. So, by setting the composition of the solid solution, one can set the temperature of the photomechanical response. It is the first example of temperature-manageable photomechanical properties.

Novel bis-catecholato heterospin manganese complexes

A tris-dioxolene manganese complex was synthesized and characterized (dioxolene- mono- and dianions of 3,6-DBQ-pipe, which is 3,6-di-tert-butyl-o-benzoquinone having a N,N’-piperazine bicycle as substituent in 4,5-positions). According to the bonds lengths distribution in the chelate cycles of the ligands the complex was identified as a bis-o-semiquinone-catecholato derivative of Mn(IV). Variable temperature magnetic susceptibility measurements indicate one unpaired electron per complex molecule as a result of strong intramolecular antiferromagnetic exchange interactions. The bis-o-semiquinone-catecholato manganese complex interacts with pyridine substituted by a nitronyl-nitroxide moiety forming a six-coordinate compound. The X-ray structural diffraction study of the complex executed at 100 K indicates a trans-arrangement of the pyridine ligands and the two catecholato dianionic chelates. Variable temperature magnetic susceptibility measurements of the complex with nitronyl-nitroxide give a magnetic moment of 5.5 µB which cannot be achieved in the spin system S = 3/2 (Mn4+) and 2 × S = 1/2 (two nitronyl-nitroxides). A valence tautomeric interconversion is proposed when the high multiplicity states [Mn3+(catecholate)(semiquinone)] and [Mn2+(semiquinone)2] are partly populated.

Valence-Tautomeric Interconversion in a Bis(dioxolene)cobalt Complex with Iminopyridine Functionalized by a TEMPO Moiety. Phase Transition Coupled with Monocrystal Destruction.

Iminopyridine modified by TEMPO nitroxide was utilized for the synthesis of an octahedral bis(o-semiquinonato)cobalt complex. Variable-temperature magnetic susceptibility measurements detect a valence tautomeric transformation in the temperature range 200-300 K. A reproducible hysteresis loop of about 40 K width is observed on the magnetic moment temperature dependence in the transition region. Differential scanning calorimetry measurements confirm different temperatures of phase transitions accompanying a valence-tautomeric transformation upon heating and cooling. Attempts to study the structural changes associated with the valence-tautomeric transformation by single-crystal X-ray diffraction failed because of the crystal destruction taking place upon cooling from 220 K. The powder X-ray diffraction pattern indicated an essential change of the unit cell upon cooling from 240 K.

Hard X-ray-Induced Valence Tautomeric Interconversion in Cobalt-o-Dioxolene Complexes.

Valence tautomeric interconversion (VTI) is a reversible process occurring in metal complexes in which an intramolecular metal-ligand electron transfer is accompanied by a change of metal ion spin state, creating two switchable electronic states (redox isomers). Herein, we describe the low-temperature, 30-100 K, single-crystal study of the [Co(diox)2(4-CN-py)2]·benzene complex (1) (diox = 3,5-di-t-butylsemiquinonate (SQ•-) and/or 3,5-di-t-butylcatecholate (Cat2-) radical; 4-CN-py = 4-cyano-pyridine) using hard synchrotron X-ray radiation with different intensities. We demonstrate for the first time that hard X-rays can induce VTI, and that the interconversion molar fraction is dependent on both intensity and exposure time. This in turn shows that X-rays, as a probe, might be altering the very nature of many structures under investigation at low temperatures, and consequently their properties. Our findings add new perspectives to VTI studies and might be of significant interest to the entire community investigating photoresponsive complexes.

Hard X-ray-induced valence tautomeric interconversion in cobalt-o-dioxolene complexes

Hard X-ray-induced valence tautomeric interconversion in cobalt-<i>o</i>-dioxolene complexes

Toward Controlling the Solid State Valence Tautomeric Interconversion Character by Solvation

Crystals of [Co(diox)2(4-NO2-py)2] (1) and [Co(diox)2(4-CN-py)2] (2) where diox are the o-dioxolene 3,5-di-t-butylsemiquinonate (SQ•–) and/or 3,5-di-t-butylcatecholate (Cat2–) ions, 4-NO2-py is 4-nitro-pyridine, 4-CN-py is 4-cyano-pyridine, are among the few known crystals presenting both thermally induced and photoinduced ls-[M+3(SQ•–)(Cat2–)] ↔ hs-[M2+(SQ•–)2] valence tautomeric interconversion (VTI). In 2, the thermal-induced VTI is cooperative, characterizing an abrupt conversion, and in 1 it is noncooperative. In this work, crystals of [Co(diox)2(4-NO2-py)2]·benzene (1BZ), [Co(diox)2(4-NO2-py)2]·toluene (1TL), [Co(diox)2(4-CN-py)2]·benzene (2BZ), and [Co(diox)2(4-CN-py)2]·toluene (2TL) have been prepared and analyzed by single crystal X-ray diffraction in order to investigate how solvation modulates thermally induced VTI. Crystallographic data were also successfully used together with the two-state equilibrium equation to estimate ΔH° and ΔS° VTI thermodynamic parameters. The solvate crystals, like t...

Design and Synthesis of a Noninnocent Multitopic Catechol and Pyridine Mixed Ligand: Nanoscale Polymers and Valence Tautomerism.

The design and synthesis of a new redox-active ligand combining catechol and pyridine units have allowed the achievement of cobalt-based nanoscale coordination polymer particles in a single-step exhibiting a switchable valence tautomeric behavior and thermal hysteresis. The combination of polymerizing capabilities with redox-active responses in a unique ligand leads to the formation of nanoparticles exhibiting a gradual valence tautomeric interconversion in the 35-370 K temperature range. Using one single ligand to obtain these nanoparticles facilitates possible nanostructure formation methodologies.

Synthesis, crystal structure and valence tautomerism of a 4,4′-bipyridine-bridged one-dimensional chiral cobalt complex

A linear-chain cobalt coordination polymer, [Co(2,3-LH2)2(4,4′-bipy)]⋅2H2O⋅4,4′-bipy]n (1) (2,3-LH2=2,3-tetrahydroxy-9,10-dimethyl-9,10-dihydro- 9,10-ethanoanthracene, 4,4′-bipy=4,4′-bipyridine), has been synthesized and structurally characterized. Single-crystal X-ray analysis reveals that complex 1 is a chiral polymer assemblied from achiral components. The complex 1 crystallizes in the chiral space group P3221 and the central Co ion has a slightly distorted octahedral coordination environment. The temperature dependence of magnetic susceptibility indicates that the complex 1 undergoes valence tautomeric interconversion between low-spin ls-[CoIII(2,3-LH2Cat)(2,3-LH2SQ)] and high-spin hs-[CoII(2,3-LH2SQ)2] (2,3-LH2Cat=2,3-LH2catecholate, 2,3-LH2SQ=2,3-LH2semiquinone).

Photoswitchable stable charge-distributed states in a new cobalt complex exhibiting photo-induced valence tautomerism.

We report the synthesis and magnetic and photomagnetic behaviour of a novel valence tautomeric cobalt complex, [Co(3,5-dbbq)2(μ-bpym)] (1) (3,5-dbbq = 3,5-di-tert-butyl-1,2-benzoquinone and μ-bpym = 2,2'-bipyrimidine). The synthesis is performed by reacting Co2(CO)8 and μ-bpym in the presence of the ligand 3,5-dbbq in a mixed solvent under inert atmosphere. The magnetic behavior clearly shows the presence of electron transfer from the catecholate ligand to the cobalt center, producing valence tautomers of [Co(II)(SQ)2] with a transition temperature (T1/2) of 215 K. Photomagnetic studies, performed via both SQUID magnetometry and X-band electron paramagnetic resonance, show the clear presence of photoinduced valence tautomerism, at temperatures considerably higher than previous systems. A metastable charge distribution is observed, strengthening previous investigations on the character of mixed valence ligands. Entropy-driven valence tautomeric interconversion is observed, and drives the transition to the most stable charge distribution. The complex has the ability to coordinate and can be used as a photoswitchable building block, with the photomagnetic characterisation evidencing a metastable state lifetime of the photo-induced valence tautomeric process of ca. 2.9 × 10(4) s below 20 K. The observed yields are higher than ones in similar systems, showing that tiny changes in the molecular structures may have a huge impact.

Thermal and optical control of electronic states in a single layer of switchable paramagnetic molecules.

The control of the charge distribution in a monolayer of switchable cobalt-dioxolene complexes undergoing Valence Tautomerism (VT) has been achieved by means of thermal and optical stimuli. Thiol-derivatised VT molecules have been grafted on polycrystalline gold surface as monolayers from solution. X-ray photoelectron spectroscopy and time-of-flight secondary ions mass spectrometry evidenced the formation of a covalent bond between intact VT molecules and the surface and excluded the presence of physisorbed molecules. X-ray absorption spectroscopy revealed that the temperature- and light-induced conversion profiles of the monolayer closely reproduce the ones found for the crystalline phase. This study demonstrates that a wet chemistry based approach allows to transfer switchable paramagnetic molecules at the nanoscale, widening the playground to develop new hybrid molecular based architectures for novel technologies.

Electric Field Control of Valence Tautomeric Interconversion in Cobalt Dioxolene

We demonstrate that the critical temperature for valence tautomeric interconversion in cobalt dioxolene complexes can be significantly changed when a static electric field is applied to the molecule. This is achieved by effectively manipulating the redox potential of the metallic acceptor forming the molecule. Importantly, our accurate density functional theory calculations demonstrate that already a field of 0.1 V/nm, achievable in Stark spectroscopy experiments, can produce a change in the critical temperature for the interconversion of 20 K. Our results indicate a new way for switching on and off the magnetism in a magnetic molecule. This offers the unique chance of controlling magnetism at the atomic scale by electrical means.

X‐ray Absorption Spectroscopy as a Probe of Photo‐ and Thermally Induced Valence Tautomeric Transition in a 1:1 Cobalt–Dioxolene Complex

The role of the cobalt ion in the entropy- and optically-driven valence tautomeric (VT) interconversion exhibited by the [Co(Me(2)tpa)(DTBdiox)](PF(6))C(6)H(5)CH(3) complex (Me(2)tpa = bis (6-methyl-(2-pyridylmethyl))(2-pyridylmethyl)amine, DBdiox = 3,5-ditertbutyl-dioxolene) is established by means of X-ray absorption spectroscopy (XAS). Analysis of the pre-edge features at 6 and 300 K in the Co K-edge XAS spectra using a ligand field multiplet approach allows us to obtain detailed information on the electronic structures of the metal ion in the two redox isomers. The temperature dependence of the spectra confirms the occurrence of a thermally induced VT transition and suggests that nucleation and distortion of the phase boundaries take place during the process. Moreover, optically induced metastable state formation is monitored at low temperatures--with a high degree of reproducibility--without changing the position of measurement on the sample during the experiment. This result paves the way for the use of such a highly sensitive technique for the investigation of photoswitchable materials in non-crystalline and nanostructured environments.

Polymorph-Dependent Molecular Valence Tautomerism Synchronized with Crystal-Melt Phase Transitions

This paper describes a molecular valence tautomeric (VT) bistability synchronized with polymorphic crystal-melt phase transitions of a newly designed VT complex, [Co(C9Espy)2(3,6-DTBQ)2] (CoC9Espy, C9Espy = dinonyl-pyridine-3,5-dicarboxylate and 3,6-DTBQ = 3,6-di-tert-butyl-semiquinonate (3,6-DTBSQ) or catecholate (3,6-DTBCat)). Two polymorphic crystals which are the thermodynamically stable phase (K1) and the kinetically stable phase (K2), commonly containing a [low spin-CoIII(C9Espy)2(3,6-DTBSQ)(3,6-DTBCat)] (ls-[CoIII]) tautomer with a low-spin CoIII atom and mixed-valence ligands. These polymorphic crystals show a molecular VT interconversion that is synchronized with crystal-to-melt phase transitions at different temperatures, 368.2 and 362.6 K for the ls-[CoIII]⊂K1 and the ls-[CoIII]⊂K2 phases, respectively. Interestingly, the ls-[CoIII]⊂K2 obtained from a melt of a [high spin-CoII(C9Espy)2(3,6-DTBSQ)2] (hs-[CoII]) tautomer with a high-spin CoII atom with two 3,6-DTBSQ exhibits a double-melting phenomenon that includes, in part, the thermodynamically unfavorable hs-[CoII]-to-ls-[CoIII] VT interconversion. Eventually, three types of VT interconversions synchronized with macroscopic crystal-melt phase transitions appear either in equilibrium or nonequilibrium conditions: (1) the thermodynamically stable ls-[CoIII]⊂K1 to the hs-[CoII]⊂melt, (2) the metastable ls-[CoIII]⊂K2 to the hs-[CoII]⊂melt, and (3) the relaxing process of the metastable hs-[CoII]⊂melt to the thermodynamically stable ls-[CoIII]⊂K1. A new strategy for simultaneous control of molecular states and macroscopic phases using thermodynamically and kinetically formed polymorphic crystalline phases is presented.

Complete Direct and Reverse Optically Induced Valence Tautomeric Interconversion in a Cobalt–Dioxolene Complex

Complete Direct and Reverse Optically Induced Valence Tautomeric Interconversion in a Cobalt–Dioxolene Complex

Molecule-Based Valence Tautomeric Bistability Synchronized with a Macroscopic Crystal-Melt Phase Transition

Here, we show a synchronic bistability of valence tautomeric (VT) molecular interconversion and a macroscopic crystal-melt phase transition in long alkoxy-functionalized cobalt-dioxolene complexes. Studies have been carried out for a novel series of complexes, [Co(CnOpy) 2(3,6-DTBQ)2] (3,5-dialkoxy(C(n)H2(n+1)O-; n = 9, 12, and 17)pyridine (CnOpy) and 3,6-di-tert-butyl semiquinonate or catecholate ligands (3,6-DTBQ)). All complexes show the molecular VT interconversion with thermal hysteresis attributed to the synchronous crystal-melt phase transition. Thermodynamic analysis has revealed that the molecular VT interconversion is restricted over 50 K and crystal-melt phase transition is accelerated about 50 K by the synchronicity. The synchronicity is attributed to enthalpic and entropic effects of the alkoxy chains in the crystalline phase of the one tautomer and the melt phase of the other, respectively. Our results show efficient chemical and thermodynamic strategies to combine molecule-based and macroscopic bistabilities.

Thermodynamics of valence tautomeric interconversion in a tetrachlorodioxolene:cobalt 1:1 adduct

We report here the X-ray structure, solid state magnetic properties and temperature dependent solution electronic spectra of a 1:1 cobalt:tetrachlorodioxolene adduct. The study showed that in the solid state this system undergoes an intramolecular electron transfer above 300 K, passing from a Co(III)-Cat charge distribution to a Co(II)-SQ one (valence tautomeric interconversion). The occurrence of this process has also been evidenced in solution by the temperature dependent electronic spectra, showing the transition to take place around 250 K. This study also provided for the first time the thermodynamic parameters of the valence tautomeric process in a 1:1 adduct, confirming its entropy driven character.

Tuning the Charge Distribution and Photoswitchable Properties of Cobalt–Dioxolene Complexes by Using Molecular Techniques

A series of cobalt complexes [Co(Me(n)tpa)(diox)]PF(6)sol (diox=3,5-di-tert-butyl-1,2-dioxolene; sol=ethanol, toluene; tpa=tris(2-pyridylmethyl)amine) were prepared by using tripod-like Me(n)tpa (n=0, 1, 2, 3), derived from tpa by successive introduction of methyl groups into the 6-position of the pyridine moieties, as an ancillary ligand. The steric hindrance induced by this substitution modulates the redox properties of the metal acceptor, thus determining the charge distribution of the metal-dioxolene moiety at room temperature. All of these complexes were characterised by using diffractometric studies, electronic spectroscopic analysis, and magnetic susceptibility measurements. In the solid state, the [Co(Me(n)tpa)(diox)](+) ions (n=0, 1) can be described as diamagnetic cobalt(III)-catecholato derivatives, whereas a cobalt(II)-semiquinonato description seems appropriate for the paramagnetic [Co(Me(3)tpa)(diox)](+) complex. The complex [Co(Me(2)tpa)(diox)]PF(6)C(2)H(5)OH undergoes entropy-driven valence tautomeric interconversion at room temperature. Optically induced valence tautomerism was observed by irradiation of [Co(Me(n)tpa)(diox)]PF(6) complexes (n=0, 1, 2) at cryogenic temperatures. The different relaxation kinetics of the photoinduced metastable phases are related to the respective free-energy changes of the interconversion, as estimated by cyclic voltammetric experiments at room temperature, and to the different lattice interactions, as supported by structural data. These results show the importance of molecular techniques for controlling the relaxation properties of photoinduced metastable species. At the same time, this behaviour strongly suggests that this paradigm exhibits intrinsic limits because of the less controllable factors that affect the process.

Solvent effects on valence tautomerism: A comparison between the interconversion in solution and solid state

A detailed comparative study of the valence tautomeric interconversion of complexes 1– 3 in three different environments is reported. The three environments are solid state (both crystalline and amorphous materials), solution and embedded into a polymeric matrix. The VT behavior of the three complexes strongly differs from one to the other, though no systematization can be established. In solution, different solvatational parameters seem to affect the equilibrium. However, such solvate effect cannot be translated either into the solid state or polymeric matrix, where the equilibrium is controlled by the vibrational relaxation of the network.