A two-step valence tautomeric transition in a dinuclear cobalt complex

A dinuclear cobalt complex with cobalt centers bridged by a bis(dioxolene) ligand exhibits a rare two-step valence tautomeric transition.     

Side-effect of ancillary ligand on electron transfer and photodynamics of a dinuclear valence tautomeric complex

A cobalt complex [[Co(dpqa)]2(dhbq)](PF6)3 ((PF6)3, dhbq = deprotonated 2,5-dihydroxy-1,4-benzoquinone, dpqa = di(2-pyridylmethyl)-N-(quinolin-2-ylmethyl)amine) was prepared and studied by X-ray diffraction, electrochemistry, ESR, thermally and photo-induced magnetic measurements; the results show that the ancillary ligand finely tuned structural factors as well as intermolecular interactions that affect the VT behavior.     

Polytypic phase transition in alkyl chain-functionalized valence tautomeric complexes

A novel series of valence tautomeric (VT) complexes, [Co(Cnbpy)(3,5-DTBQ)2] (Cnbpy = 4,4'-dialkyl-2,2'-bipyridine (alkyl = (C(n)H(2n+1)) where n = 0 (CoC0bpy), 1 (CoC1bpy), 5 (CoC5bpy), 9 (CoC9bpy) and 13 (CoC13bpy); 3,5-DTBQ = 3,5-di-tert-butylsemiquinonate or catecholate), including two previously reported complexes (CoC0bpy and CoC1bpy), were systematically prepared and their properties examined. Introduction of alkyl groups into VT chromophores leads to unexpected variations in the VT process, depending on the lengths of the alkyl chains. An even more interesting observation is that significant polytypic phase transitions with crystallographic symmetry changes accompanying the abrupt VT conversion are induced for the first time in CoC9bpy and CoC13bpy.     

Isotopic effects may induce cooperativity in valence tautomeric transition

The replacing of CH(2)Cl(2) solvent molecules with their deuterated analogues in a Co complex undergoing valence-tautomeric interconversion drastically modifies its magnetic properties giving rise to a thermal hysteresis.     

Temperature-induced solid-state valence tautomeric interconversion in two cobalt-Schiff base diquinone complexes

The mixed-ligand complexes [Co(III)(tpy)(Cat-N-SQ)]Y and [Ni(II)(tpy)(Cat-N-BQ)]PF(6) (tpy = 2,2':6',2' '-terpyridine; Cat-N-BQ, Cat-N-SQ = mononegative and radical dinegative Schiff base diquinone ligand; Y = PF(6), BPh(4)) were prepared. Structural and spectroscopic data support the different charge distribution of the two compounds. The temperature-dependent electronic and spectral properties of solutions containing the [Co(III)(tpy)(Cat-N-SQ)](+) suggest that this compound undergoes a thermally driven valence tautomeric interconversion to [Co(II)(tpy)(Cat-N-BQ)](+) complex, the metal ion being in high-spin configuration. The comparison of the electrochemical properties of the cobalt and nickel derivatives supports the observed behavior. The same interconversion process was found to occur also in the solid state with a significant higher T(c) value than in solution. It was found that the previously reported [Co(III)(Cat-N-BQ)(Cat-N-SQ)] shows a similar behavior. The large difference between the interconversion T(c) in the solid state and in solution is suggested to come from the entropy changes associated with the modifications of vibronic interactions.     

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.     

Pressure- and temperature-induced valence tautomeric interconversion in a o-dioxolene adduct of a cobalt-tetraazamacrocycle complex

An electronic switch at the molecular level has been realized by using a class of ionic compounds of the formula [Co(L)(diox)]Y (L = tetraazamacrocyclic ligand, Y = mononegative anion). Such compounds undergo temperature- and pressure-induced intramolecular one-electron transfer equilibria. The transition temperature of interconversion varies with the nature of the counterions Y (Y = PF6, BPh4, I). Surprisingly the effect of the anion on the transition temperature is not only governed by its volume but also by its coulombic interaction.     

Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state

A detailed investigation of a valence tautomeric (VT) transition for the new complex [Co(III)(3,5-DBCat)(3,5-DBSQ)(py)?]/[Co(II)(3,5-DBSQ)?(py)?] (1) is reported, where 3,5-DBCatH? is 3,5-di-tert-butyl-catechol, 3,5-DBSQH is 3,5-di-tert-butyl-semiquinone and py is pyridine. Complex 1 exists as a mixture of the two valence tautomers, with the relative proportion of each depending on the external conditions. Three differently solvated forms of the complex have been synthesized and variable temperature structural and magnetic investigations of one of these, 1·0.5py, reveals that this compound undergoes a thermally-induced VT transition from the [Co(III)(3,5-DBCat)(3,5-DBSQ)(py)?] tautomer at temperatures below 150 K to a 1 : 1 mixture of the two tautomers at temperatures above 300 K. The VT transition may also be photo-induced at 9 K, affording a similar mixture of the two tautomers. In both cases the incomplete transition is attributed to the presence of π-π stacking interactions between the pyridine molecules of solvation and one of the two crystallographically independent complex molecules, which inhibits the expansion of this molecule that would accompany a VT transition. Studies on alternatively solvated forms 1·2MeCN and 1·1.67hexane also suggest a significant dependence of the VT transition on solvation-induced packing effects and/or intermolecular interactions.     

Dinuclear cobalt bis(dioxolene) complex exhibiting two sequential thermally induced valence tautomeric transitions

4,6-Di-2'-pyridylpyrimidine is employed as a bis(diimine) ligand bridging two cobalt bis(dioxolene) centers. The thermally induced valence tautomeric transitions of these two metal centers are coupled through the ligand. The result is that sequential switching from high-spin Co(II) to low-spin Co(III) of one center, followed by the onset of switching of the other center at lower temperature, is observed in a solid amorphous thin film by IR absorption spectroscopy.     

Coordination-tuned single-molecule-magnet behavior of TbIII-CuII dinuclear systems

Tb (III)-Cu (II)-based single-molecule magnet (SMM) and non-SMM were synthesized to investigate the relationship between magnetic anisotropy and the symmetry of the ligand field by the reaction of [TbCu( o-vanilate) 2(NO 3) 3] with methoxypropylamine (MeOC 3H 6NH 2, 1) or ethoxyethylamine (EtOC 2H 4NH 2, 2). In both complexes, Tb (III) ions have a bicapped square-antiprism coordination geometry. When the Tb (III) ion is in a less symmetrical ligand field, it has an easy-axis anisotropy and shows SMM behavior, whereas when it is in a more symmetrical environment, it has an easy-plane anisotropy and exhibits non-SMM behavior.     

Reversible bridge-mediated excited-state symmetry breaking in stilbene-linked DNA dumbbells

The excited-state behavior of synthetic DNA dumbbells possessing stilbenedicarboxamide (Sa) linkers separated by short A-tracts or alternating A-T base-pair sequences has been investigated by means of fluorescence and transient absorption spectroscopy. Electronic excitation of the Sa chromophores results in conversion of a locally excited state to a charge-separated state in which one Sa is reduced and the other is oxidized. This symmetry-breaking process occurs exclusively via a multistep mechanism-hole injection followed by hole transport and hole trapping-even at short distances. Rate constants for charge separation are strongly distance-dependent at short distances but become less so at longer distances. Disruption of the A-tract by inversion of a single A-T base pair results in a pronounced decrease in both the rate constant and efficiency of charge separation. Hole trapping by Sa is highly reversible, resulting in rapid charge recombination that occurs via the reverse of the charge separation process: hole detrapping, hole transport, and charge return to regenerate the locally excited Sa singlet state. These results differ in several significant respects from those previously reported for guanine or stilbenediether as hole traps. Neither charge separation nor charge recombination occur via a single-step superexchange mechanism, and hole trapping is slower and detrapping faster when Sa serves as the electron donor. Both the occurrence of symmetry breaking and reversible hole trapping by a shallow trap in a DNA-based system are without precedent.     

Broken symmetry density functional study of a mixed‐valence unsymmetrical dinuclear iron complex

Density functional theory (DFT) calculations with different exchange‐correlation functionals were performed for a mixed valence Fe(II)/Fe(III) binuclear complex with μ‐methoxo and two μ‐carboxylate bridging ligands, (1) with geometry optimizations being performed for all possible spin multiplicities (M_S = 2, 4, 6, 8, and 10). Within the exchange‐correlation functionals studied, only the hybrid GGA functionals B3P and B3LYP and also the pure GGA functional RPBE, predicts the geometry with high spin (S = 9/2) to be more stable than the geometry with low spin state (S = 1/2) by 20 kcal/mol, in agreement with the experimental findings. These functionals also predict the same stability order for the different spin states, being M_S = 10>8>6>2>4. The meta‐GGA functionals TPSS and TPSSh and also the pure GGA functionals BLYP and BP86 predict different stability orders. The computed average EPR g‐tensor, g_av, of 2.03, at the B3LYP level, is in good agreement with the experimental findings. Heisenberg exchange coupling constants, J, were calculated within the broken‐symmetry formalism, at the B3LYP level, showing that the two iron centers are antiferromagnetic coupling, with a very weak coupling constant of about ?7 cm^?1, in good agreement with the experimental value. Additionally, the effect of using different multiplicities of the reference geometries on the computed J value is discussed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

New route to the mixed valence semiquinone-catecholate based mononuclear FeIII and catecholate based dinuclear MnIII complexes: first experimental evidence of valence tautomerism in an iron complex

The semiquinone-catecholate based mixed valence complex, [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] x DMF (1), and catecholate based (H2bispictn)[Mn2III(Cl4Cat)4(DMF)2] (2) (bispicen = N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine, bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, Cl4Cat = tetrachlorocatecholate dianion, and Cl4SQ = tetrachlorosemiquinone radical anion) were synthesized directly utilizing a facile route. Both the complexes have been characterized by single crystal X-ray diffraction study. The electronic structures have been elucidated by UV-vis-NIR absorption spectroscopy, cyclic voltammetry, EPR, and magnetic properties. The structural as well as spectroscopic features support the mixed valence tetrachlorosemiquinone-tetrachlorocatecholate charge distribution in 1. The ligand based mixed valence state was further confirmed by the presence of an intervalence charge transfer (IVCT) band in the 1900 nm region both in solution and in the solid. The intramolecular electron transfer, a phenomenon known as valence tautomerism (VT), has been followed by electronic absorption spectroscopy. For 1, the isomeric form [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] is favored at low temperature, while at an elevated temperature, the [FeII(bispicen)(Cl4SQ)2] redox isomer dominates. Infrared as well as UV-vis-NIR spectral characterization for 2 suggest that the MnIII(Cat)2- moiety is admixed with its mixed valence semiquinone-catecholate isomer MnII(SQ)(Cat)-, and the electronic absorption spectrum is dominated by the mixed charged species. The origin of the intervalence charge transfer band in the 1900 nm range is associated with the mixed valence form, MnII(Cl4Cat)(Cl4SQ)-. The observation of VT in complex 1 is the first example where a mixed valence semiquinone-catecholate iron(III) complex undergoes intramolecular electron transfer similar to manganese and cobalt complexes.     

Experimental and DFT study of the tautomeric behavior of cobalt-containing secondary phosphine oxides

New cobalt-containing secondary phosphine oxides [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4){mu,eta-PhC[triple chemical bond]CP(==O)(H)(R)}] (8 a: R=tBu; 8 b: R=Ph) were prepared by reaction of secondary phosphine oxides PhC[triple chemical bond]CP- (==O)(H)(R) (6 a: R=tBu; 6 b: R=Ph) with dppm-bridged dicobalt complex [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(6)] (2). The molecular structures of 8 a and 8 b were determined by single-crystal X-ray diffraction. Although palladium-catalyzed Heck reactions employing 8 b as ligand gave satisfying results, 8 a performed poorly in the same reaction. Judging from these results, a tautomeric equilibrium between 8 b and its isomeric form [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4){mu,eta-PhC[triple chemical bond]CP(OH)(Ph)}] 8 b' indeed takes place, but it is unlikely between 8 a and [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4){mu,eta-PhC[triple chemical bond]CP(OH)(tBu)}] (8 a'). The DFT studies demonstrated that reasonable activation energies for the tautomeric conversions can be achieved only via a bimolecular pathway. Since a tBu group is much larger than a Ph group, the conversion is presumably only feasible in the case of 8 bright harpoon over left harpoon8 b', but not in the case of 8 aright harpoon over left harpoon8 a'. Another cobalt-containing phosphine, namely, [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4){mu,eta-PhC[triple chemical bond]CP(NEt(2))(tBu)}] (7 a), and its oxidation product [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4){mu,eta-PhC[triple chemical bond]CP(==O)(NEt(2))(tBu)}] 7 a' were prepared from the reaction of PhC[triple chemical bond]CP(NEt(2))(tBu) (5 a) with 2. The molecular structures of 7 a and 7 a' were determined by single-crystal X-ray diffraction. The phosphorus atom is surrounded by substituents in a tetrahedral environment. A P--N single bond (1.676(3) A) is observed in the molecular structure of 7 a. Heck reactions employing 7 a/Pd(OAc)(2) as catalyst system exhibited efficiency comparable to that of 8 a/Pd(OAc)(2).     

Quantum chemical modeling of valence tautomeric adducts of Co<Superscript>II</Superscript> bischelates with pyrene-4,5-diimines

Quantum chemical study of mixed-ligand (1: 1) adducts of tetracoordinated cobalt complexes (diketonates, bis-aminovinylketonates, and bis-salicylaldiminates) with pyrene-4,5-diimines using density functional theory (DFT B3LYP*/6-311++G(d,p)) revealed compounds possessing valence tautomeric properties. The mechanisms of intramolecular electron transfer accompanied by changing magnetic properties of the complexes under consideration were studied. It was shown that the variation of substituents at the nitrogen atoms of the diimine ligand and the donor groups in the cobalt bischelate, as well as the annulation of five- and six-membered rings to the azomethine fragment exerts a considerable influence on the stability of the formed adducts and the energy difference between their low- and high-spin isomers.     

Electronic and photophysical properties of a novel phenol bound dinuclear ruthenium complex: evidence for a luminescent mixed valence state

A novel dinuclear ruthenium(II) complex bridged by dianionic bridge 3-(2-phenol)-5-(pyridin-2-yl)-1,2,4-triazole in which the ruthenium metal atoms are bound through N,N coordination to the pyridine and triazole and O,N coordination to the triazole and phenolate is described. The electrochemical, spectroscopic and photophysical behaviour of the dimer is compared with its associated N,N- and O,N-coordinated mononuclear complexes. The mixed valence complex was prepared electrochemically and a weak inter-valence charge transfer transition is observed which from Hush theory provides an electronic coupling matrix element of 666 cm(-1), suggesting the complex is weakly coupled and valence trapped. In its native state the dinuclear compound is essentially non-emissive but upon the oxidation of the O,N moiety luminescence from the complex is reversibly switched on at 0.3 V and reversibly switched off by application of 1.3 or 0 V. To our knowledge this is the first report of a luminescent mixed valence ruthenium complex.     

Single-molecule magnet behavior for an antiferromagnetically superexchange-coupled dinuclear dysprosium(III) complex

A family of five dinuclear lanthanide complexes has been synthesized with general formula [Ln(III)(2)(valdien)(2)(NO(3))(2)] where (H(2)valdien = N1,N3-bis(3-methoxysalicylidene)diethylenetriamine) and Ln(III) = Eu(III)1, Gd(III)2, Tb(III)3, Dy(III)4, and Ho(III)5. The magnetic investigations reveal that 4 exhibits single-molecule magnet (SMM) behavior with an anisotropic barrier U(eff) = 76 K. The step-like features in the hysteresis loops observed for 4 reveal an antiferromagnetic exchange coupling between the two dysprosium ions. Ab initio calculations confirm the weak antiferromagnetic interaction with an exchange constant J(Dy-Dy) = -0.21 cm(-1). The observed steps in the hysteresis loops correspond to a weakly coupled system similar to exchange-biased SMMs. The Dy(2) complex is an ideal candidate for the elucidation of slow relaxation of the magnetization mechanism seen in lanthanide systems.