LMCT Band Research Articles

Photoinduced Phase Transition of the Coordinationally Unsaturated d9 Metal Centers of Bis(N,N‐diethylethylenediamine)copper(II) Perchlorate within the Thermal Hysteresis of the Spin‐Exchange Interaction

AbstractDiffraction evidence is presented that the structure of one‐dimensional arrays of strained, coordinationally unsaturated d9 pseudo‐Jahn–Teller centers of bis(N,N‐diethylethylenediamine)copper(II) perchlorate is affected by excitation of the LMCT bands within the hysteresis region of a low‐temperature ferromagnetic‐to‐antiferromagnetic phase transition. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis of Mononuclear Non-Heme Iron(III)-Hydroperoxo Complex as an Oxidative Catalyst

Mononuclear FeII complex [FeII(ppba)]2+ (ppba = (6-pivalamido-2-pyridylmethyl) (2-pyridylmethyl)tert-butylamine) (1) has been synthesized and structurally characterized. The addition of H2O2 (10eq) to an acetone solution of 1 at -78°C generated FeIIIPPBA-OOH species, accompanied by a color change from pale yellow to reddish purple, which showed an intense absorption band corresponding to LMCT band (HOO- to Fe) at 528 nm (ε = 1120 M-1cm-1). The ESR spectrum for FeIIIPPBA-OOH species (g// = 4.22, g⊥ = 7.70 in acetone at 77 K) was typical of a high-spin FeIII species, suggesting that the FeIIIPPBA-OOH species is maintained as a mononuclear complex in the solution state. The resonance Raman spectra, when H2 16O2 and H2 18O2 were used, exhibited intense bands characteristic to the O-O stretching vibrations at 880 cm-1 ν(16O-16O) and 824 cm-1 ν(18O-18O), respectively. These findings suggest that FeIIIPPBA-OOH species is [FeIII(ppba)(OOH)]2+ (2). In order to understand the catalytic reactivity of complex 2, oxidative reactions for some organic substrates by complex 2 were carried out in the presence of H2O2; thioanisole was oxidized to the corresponding sulfoxide and sulfone, respectively, and furthermore, cyclohexene was converted to cyclohexene oxide, cyclohexenol and cyclohexenone.

Binuclear manganese(III) complexes with carboxylate bridges: Synthesis, structure, electrochemistry and magnetic properties

A series of μ-oxo, μ-carboxylato binuclear manganese(III) complexes derived from bipy, phen and different carboxylate ligands have been prepared and characterized by IR and UV–vis spectroscopies, cyclic voltammetry, magnetic susceptibility and the X-ray crystallography. The electronic spectra of all the complexes show very similar features consisting of two absorptions in the visible region, one high intensity LMCT band and two intra-ligand transitions in the UV region. The cyclic voltammograms of the complexes exhibit four steps in most cases, each consisting of a one-electron transfer reaction. These probably correspond to Mn 2 III ↔ Mn IV Mn III ↔ Mn 2 IV oxidation and Mn 2 III ↔ Mn III Mn II ↔ Mn 2 II reduction processes. The structure of complex 2 has been determined by the X-ray crystallography and consists of a binuclear [Mn 2(bipy) 2(μ- O)(μ-OOCR) 2(NO 3)(H 2O)] + cation (R = CH 2Cl) with a ClO 4 - anion. Low temperature magnetic susceptibility measurements show that there is a weak ferromagnetic interaction in 1 (R = Ph) ( J = +0.43 cm −1) and a weak antiferromagnetic interaction between the metal centers in 2 (R = CH 2Cl) ( J = −0.15 cm −1).

A time-dependent density functional theory investigation on the nature of the electronic transitions involved in the nonlinear optical response of [Ru(CF3CO2)3T] (T = 4′-(C6H4-p-NBu2)-2,2′:6′,2″-terpyridine)

We report a theoretical study based on density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on the nature and role of the absorption bands involved in the nonlinear optical response of the complexes [Ru(CF3CO2)3T] (T = T1, T2; T1 = 4'-(C6H4-p-NBu2)-2,2':6',2''-terpyridine, T2 = 4'-(C6H4-p-NMe2)-2,2':6',2''-terpyridine). Geometry optimizations, performed without any symmetry constraints, confirm a twisting of the -C6H4-p-NBu2 moiety with respect to the plane of the chelated terpyridine. Despite this lack of strong pi interaction, TDDFT excited states calculations of the electronic spectrum in solution provide evidence of a relevant role of the NBu2 donor group in the low-energy LMCT band at 911 nm. Calculations also show that the two bands at higher energy (508 and 455 nm) are not attributable only to LMCT and ILCT transitions but to a mixing of ILCT/MLCT and ILCT/pi-pi* transitions, respectively. The 911 nm LMCT band, appearing at lower wavelength of the second harmonic (670 nm) of the EFISH experiment, controls the negative value of the second-order NLO response. This is confirmed by our calculations of the static component beta0(zzz) of the quadratic hyperpolarizability tensor, showing a large positive value. In addition we have found that the increase of the dipole moment upon excitation occurs, in all the characterized transitions, along the dipole moment axis, thus explaining why the EFISH and solvatochromic experimental values of the quadratic hyperpolarizability agree as sign and value.

Photoinduced phase transition of coordinationally unsaturated d9 metal centers within the thermal hysteresis of the spin exchange interaction

Diffraction evidence is presented that the structure of one-dimensional arrays of strained, coordinationally unsaturated d9 pseudo-Jahn-Teller centers is affected by excitation of the LMCT bands within the hysteresis region of the low-temperature ferromagnetic-to-antiferromagnetic phase transition.

N2 Hydrogenation Promoted by a Side-On Bound Hafnocene Dinitrogen Complex

Reduction of the hafnocene diiodide (η5-C5Me4H)2HfI2 with excess sodium amalgam under 1 atm of dinitrogen yielded the hafnocene dinitrogen complex [(η5-C5Me4H)2Hf]2(μ2,η2:η2-N2). X-ray diffraction has established a strongly activated, side-on bound, bridging N2 ligand with an N−N bond length of 1.423(11) Å. Exposure of the dinitrogen compound to 1 atm of dihydrogen at 23 °C resulted in rapid hydrogenation of the N2 ligand, affording the hydrido hafnocene diazenido complex [(η5-C5Me4H)2HfH]2(μ2,η2:η2-N2H2). Confirmation of N2 hydrogenation has been provided by NMR and IR spectroscopic studies and X-ray diffraction. Pseudo-first-order observed rate constants for H2 addition have been measured by monitoring the disappearance of a principally LMCT band centered at 886 nm [(η5-C5Me4H)2Hf]2(μ2,η2:η2-N2) as a function of time. The hafnocene complex hydrogenates dinitrogen approximately 4 times faster than the corresponding zirconocene compound. Divergent chemistry from zirconium is observed upon thermolysis of [(η5-C5Me4H)2HfH]2(μ2,η2:η2-N2H2), which results in cyclopentadienyl methyl group cyclometalation rather than N−N bond scission.

Hemes férricos pentacoordenados e hexacoordenados dos monômeros d nativo e reconstituído da hemoglobina extracelular de Glossoscolex paulistus: estudos espectroscópicos em meio ácido

UV-Vis and fluorescence spectroscopic studies of the native and reconstituted d monomers of Glossoscolex paulistus were performed in acid medium. The coexistence of distinct species shows the complexity of the equilibria. Besides the hexacoordinate low spin hemichrome, with bands at 535 and 565 nm, a pentacoordinate high spin hemichrome is identified by the blue-shifted low intensity Soret band (371 nm) and the LMCT band (643 nm). The pentacoordinate hemichrome must be related to the partial unfolding of the polypeptide.

Axial Ligand Substituted Nonheme FeIVO Complexes: Observation of Near-UV LMCT Bands and FeO Raman Vibrations

Axial ligand substitution of a mononuclear nonheme oxoiron(IV) complex, [FeIV(O)(TMC)(NCCH3)]2+ (1) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), leads to the formation of new FeIV=O species with relatively intense electronic absorption features in the near-UV region. The presence of these near-UV features allowed us to make the first observation of Fe=O vibrations of S = 1 mononuclear nonheme oxoiron(IV) complexes by resonance Raman spectroscopy. We have also demonstrated that the reactivity of nonheme oxoiron(IV) intermediates is markedly influenced by the axial ligands.

Biophysical and biochemical investigation on the binding of a manganese–cyanonitrosyl complex with DNA

The interaction of a cyanonitrosyl–manganate complex with DNA has been studied by u.v.–vis spectroscopy, circular dichroism, fluorescence and gel electrophoresis techniques. As the DNA 260 nm band remained insensitive to interaction, the interaction ratio was determined by monitoring the CN−→ Mn LMCT band observed at 222 nm, and the interaction ratio was found to be in the complex (D): DNA (P)=1:0.30 from the spectrophotometric titration. The above-mentioned physical measurements indicate that the binding mode is not intercalative and the cyanonitrosyl system is a groove binder.

A family of mixed-ligand oxovanadium(V) complexes incorporating tridentate ONO donor hydrazone ligands derived from acetylhydrazide and 2-hydroxybenzaldehyde/2-hydroxyacetophenone

In the title family the tridentate ONO donor ligands are the fully deprotonated forms of acetylhydrazones of 2-hydroxybenzaldehyde (H2L1) and 2-hydroxyacetophenone (H2L2) (general abbreviation H2L), while bidentate mononegative OO donor ligands are the deprotonated salicylaldehyde (Hsal), vanillin (Hvan) and monodeprotonated 1,2-ethanediol (H2ed) (general abbreviation HB). The reaction of VIVO(acac)2 with H2L and Hsal or Hvan in equimolar ratio in MeOH afforded the complexes of the type [VVO(L)(B)], (1)–(4). The reaction of VIVO(acac)2 with H2L1 (in an equimolar ratio) and an excess of H2ed in MeOH yielded the complex [VVO(L1)(Hed)], (5) but the similar reaction with H2L2 ligand failed to produce such a type of complex. Complexes have been characterized by elemental analyses and by i.r., n.m.r. and u.v.-vis. spectroscopies. All the complexes are diamagnetic and display only LMCT bands. 1H-n.m.r. spectral data indicate that complexes (1)–(4) exist in two isomeric forms [(1A), (1B); (2A), (2B); (3A), (3B) and (4A), (4B)] in different ratios in CDCI3 solution. Complexes (1)–(4) display a quasi-reversible one electron reduction peak in the −0.06 to +0.05 V versuss.c.e. region in CH2CI2 solution and (5) displays an irreversible reduction peak at −0.46 V versuss.c.e. in DMF solution. The trend in the redox potential values has been correlated with the basicity of both the primary and auxiliary ligands.

Photolysis of CH 3Re(O 2) 2O induced by ligand-to-metal charge transfer and by peroxide intraligand excitation

The electronic spectrum of the diperoxo complex MeRe VII(O 2) 2O shows a peroxide IL (intraligand) absorption at λ max ∼ 260 nm in addition to the well-known peroxide to Re(VII) LMCT band at λ max = 358 nm. Upon IL excitation, the diperoxo group undergoes a dismutation. Accordingly, MeRe(O 2) 2O photolyzes to MeRe VIIO 3 and O 2. LMCT excitation at λ irr = 405 nm is assumed to generate the radical pair MeRe VI ( O 2 ) O + / O 2 - in the primary photochemical step. Back electron transfer and reaction with water leads finally to the monoperoxo complex MeRe VII(O 2)O 2 and H 2O 2.

Synthesis, spectroscopy and redox properties of mononuclear manganese(II) and manganese(IV) complexes with N-(aryl)-pyridine-2-aldimine (L) and its amide derivatives. X-ray structural characterization of [Mn(MeL)2(NCS)2] (MeL = N-(4-methylphenyl)-pyridine-2-aldimine)

A series of mononuclear MnII and MnIV complexes of general formulae [MnL2(NCS)2] (1a–1d) and [Mn(L′)2(NCS)2] (2a–2c) have been prepared where L are Schiff bases obtained by the condensation of pyridine-2-aldehyde with para-alkyl-substituted aniline, and L′ are the corresponding amide ligands. The room temperature magnetic susceptibility data of (1a–1d) indicate that MnII is in a high spin state. The cyclic voltammograms of (1a–1d) exhibit a one-electron quasi-reversible MnII→MnIII oxidation. A linear correlation has been found when E0[MnIII/MnII] is plotted against Hammett σp parameters. X-ray crystallographic data of (1b) shows that the central MnII ion adopts a distorted octahedral geometry with six different Mn–N distances. Upon oxidation of MnII complexes (1b–1d) by H2O2, the corresponding MnIV complexes (2a–2c) were obtained, and the Schiff base ligands were oxidized to the corresponding amides. The lowest energy LMCT bands of these MnIV complexes correlate linearly with Hammett σp parameters. The redox behavior of the MnIV complexes has been investigated by cyclic voltammetry. E.p.r. spectra of the MnII and MnIV complexes are also reported.

Group 8 and 10 metal acetylide naphthalimide dyads

[M]–C C–naphthalimide derivatives {[M] = CpNi(PPh 3), CpFe(dppe), CpRu(dppe) and Cp*Ru(dppe)} have been prepared and their X-ray structures determined. The structures show significant non-linearity of the acetylide link, with concomitant ν C C at low energy and high intensity. Other properties confirm the strong-donor/strong-naphthalimide acceptor nature of the compounds. All the compounds exhibit an intense MLCT band in the visible spectrum that resolves into two bands in non-polar solvents. Spectroelectrochemistry of the CpFe species shows that the MLCT band disappears upon oxidation, and the appearance of a LMCT band at lower energy.

A linear correlation between energy of LMCT band and oxygenation reaction rate of a series of catecholatoiron(III) complexes: initial oxygen binding during intradiol catechol oxygenation

The oxygen reactivity of catecholatoiron(III) complexes has been examined using a series of catecholate ligands as the substrate. All the complexes examined here, [Fe III(TPA)(R-Cat)]BPh 4 ( 1– 9) (TPA: tris(pyridin-2-ylmethyl)amine; R-Cat: substituted catecholate ligand, R = 3,5- t Bu 2 ( 1), 3,6- t Bu 2 ( 2), 3,5-Me 2 ( 3), 3,6-Me 2 ( 4), 4- t Bu ( 5), 4-Me ( 6), H ( 7), 4-Cl ( 8) and 3-Cl ( 9)), exclusively afforded the intradiol cleaving products of the catecholate ligands upon exposure to O 2. It was revealed that 1– 7 can be categorized into two classes based on their electrochemical properties; i.e., the complexes having the dialkyl-substituted (group A) and the mono- or non-substituted (group B) catecholate ligands. In spite of their classification, these two groups show a linear correlation between the logarithm of the reaction rate constant with O 2 and the energy of the catecholate-to-iron(III) LMCT band, although 2 shows a large negative deviation from the correlation line. Based on this LMCT-energy dependent reactivity of 1 and 3– 9 as well as the very low reactivity of 2, we have discussed on the mechanisms of the reaction of [Fe III(TPA)(R-Cat)]BPh 4 with O 2.

Molecular and electronic structure of square-planar nickel(ii), nickel(iii) and nickel(iii)π-cation radical complexes with a tetradentate o-phenylenedioxamidate redox-active ligand

The molecular and electronic structures of the electron transfer series of four-coordinate square-planar nickel complexes with the ligand o-phenylenebis(N'-methyloxamidate), [NiL]z (z = 2-, 1-, 0), have been evaluated by DFT and TDDFT calculations, and most of their experimentally available structural and spectroscopic properties (X. Ottenwaelder et al., Dalton Trans., 2005, DOI: 10.1039/b502478a) have been reasonably reproduced at the B3LYP level of theory. The anionic species [NiL]2- and [NiL]- are genuine low-spin nickel II and nickel III complexes with diamagnetic singlet (S = 0) and paramagnetic doublet (S = 1/2) states, respectively. The nickel III complex presents shorter Ni-N(amidate) bond distances (1.85-1.90 A) than the parent nickel II complex (1.88-1.93 A) and characteristic LMCT bands in the NIR region (lambda max = 794 and 829 nm) while the analogous MLCT bands for the nickel(II) complex are in the UV region (lambda max = 346 and 349 nm). The neutral species [NiL] is a nickel III o-benzosemiquinonediimine pi-cation radical complex with a diamagnetic singlet (S = 0) and a paramagnetic triplet (S = 1) states fairly close in energy but fundamentally different in orbital configuration. The singlet metal-radical ground state results from the antiferromagnetic coupling between the 3d(yz) orbital of the Ni III ion (S(M) = 1/2) and the pi(b) orbital of the benzosemiquinone-type radical ligand (S(L) = 1/2), which have a large overlap and thus strong covalent bonding. The triplet metal-radical excited state involves the ferromagnetic coupling between the Ni III 3d(zx) orbital and the benzosemiquinone-type pi(b) orbital, which are orthogonal to each other. The singlet and triplet states of the nickel III pi-cation radical complex possess characteristic quinoid-type short-long-short alternating sequence of C-C bonds in the benzene ring, as well as intense MLCT transitions in the VIS (lambda max = 664 nm) and NIR (lambda max = 884 nm) regions, respectively.

Synthesis, structure, spectroscopy and redox chemistry of square-planar nickel(ii) complexes with tetradentate o-phenylenedioxamidates and related ligands

A series of four-coordinate square-planar nickel(II) complexes of o-phenylenebis(N'-methyloxamidate)(L1) and related o-phenylene(N'-methyloxamidate)oxamate (L2) and o-phenylenebis(oxamate)(L3) tetradentate ligands have been synthesized and characterized structurally, spectroscopically and electrochemically. The parent nickel(II)-L1 complex presents an intense MLCT band in the UV region (lambda max = 357 nm) and a distinctive 1 s --> 4p CT satellite in the Ni K-edge XANES spectrum (E = 8339.2 eV). These features together with the short Ni-N(amidate) bond lengths (1.85-1.93 A) as revealed by the analysis of the Ni K-edge EXAFS spectrum and confirmed by single-crystal X-ray diffraction are typical of square-planar low spin (S = 0) Ni(II) ions. The dianionic nickel(II) complexes, [Ni(II)L(i)](2-)(i = -3), experience two redox processes in acetonitrile at 25 degrees C. The first redox process, at moderately low potentials (E1 = 0.12-0.52 V vs. SCE), is a reversible one-electron metal-centered oxidation to the corresponding monoanionic nickel(III) complexes, [Ni(III)L(i)]-. The second redox process, at relatively high potentials (E2 = 0.86-1.04 V vs. SCE), is a quasireversible to irreversible one-electron oxidation largely centered on the o-benzenediamidate fragment of the non-innocent ligand, yielding the corresponding neutral nickel(iii) complexes with a o-benzosemiquinonediimine pi-cation radical ligand, [Ni(III)(L(i))*+]. The singly and doubly oxidized species of the parent nickel(II)-L1 complex have been prepared by chemical oxidation and characterized spectroscopically in acetonitrile at -40 degrees C. The stable singly oxidized nickel(III)-L1 species presents an intense LMCT band in the NIR region (lambda max = 910 nm) and a rhombic X-band EPR spectrum (g1 = 2.193, g2 = 2.080 and g3 = 2.006) characteristic of square-planar low spin (S = 1/2) Ni(III) ions. The unstable double oxidized nickel(III)-L1 pi-cation radical species exhibits a rather intense visible band (lambda max = 645 nm) that is tentatively assigned as a MLCT transition from the Ni(III)-benzosemiquinone type ground state to the Ni(IV) excited state.

Synthesis, Characterization, and Thermal Stability of New Mononuclear Hydrogenperoxocopper(II) Complexes with N3O-Type Tripodal Ligands Bearing Hydrogen-Bonding Interaction Sites

Abstract In order to understand the effect of an oxygen-containing ligand on the physico-chemical properties and reactivities of hydrogenperoxocopper complexes, new copper(II) complexes with the N3O-type tripodal ligand bearing pivalamido groups, N,N-bis(6-pivalamido-2-pyridylmethyl)glycine (Hbpga), and N,N-bis(6-pivalamido-2-pyridylmethyl)-β-alanine (Hbpaa), have been designed and synthesized. Copper(II) complexes without any external ligand and those with a monodentate ligand, such as azido and chloro, have been prepared and characterized with the aid of electronic absorption and ESR spectroscopic, cyclic voltammetric, and X-ray structure analytical methods. The redox potential values of the Cu(II) complexes, when they were compared with the Cu(II) complex of bis(6-pivalamido-2-pyridylmethyl)(2-pyridylmethyl)amine (bppa), reported previously, shifted toward the negative side upon the introduction of a carboxylate group in the place of one pyridine of bppa. Reactions of [Cu(bpga)]ClO4 (1a) and [Cu(bpaa)]PF6 (2a) with hydrogen peroxide in the presence of triethylamine in both MeCN and MeOH solutions gave mononuclear copper(II) complexes with hydrogenperoxide(1−), Cu–bpga–OOH (1d) and Cu–bpaa–OOH (2d) systems, respectively. The intense absorption bands, assignable to LMCT (HOO− → Cu(II)) and d–d bands, and ESR and resonance Raman spectra have revealed that they form trigonal bipyramidal copper complexes with OOH− in an end-on fashion. The thermal stabilities of 1d and 2d have also been studied by following the reduction rate of the LMCT bands at 283 K. Those of copper(II) complexes with hydrogenperoxide(1−) have been reduced in the order 1d > 2d >> [Cu(bppa)(OOH)]+ (3d), all of which are rather stable compared with that of Cu(II)–tpa–OOH (tpa = tris(2-pyridylmethyl)amine). These findings indicate that the hydrogenperoxocopper(II) complexes are activated by introducing carboxylate coordination, although they are stabilized by hydrogen-bonding interactions.

Synthesis, structure and solution chemistry of mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes incorporating tridentate ONO donor hydrazone ligands

In the title family, the ONO donor ligands are the acetylhydrazones of salicylaldehyde (H 2L 1) and 2-hydroxyacetophenone (H 2L 2) (general abbreviation, H 2L). The reaction of bis(acetylacetonato)oxovanadium(IV) with a mixture of tridentate H 2L and a bidentate NN donor [e.g., 2,2′-bipyridine(bpy) or 1,10-phenanthroline(phen), hereafter B] ligands in equimolar ratio afforded the tetravalent complexes of the type [V IVO(L)(B)]; complexes ( 1)–( 4) whereas, if B is replaced by 8-hydroxyquinoline(Hhq) (which is a bidentate ON donor ligand), the above reaction mixture yielded the pentavalent complexes of the type [V VO(L)(hq)]; complexes ( 5) and ( 6). Aerial oxygen is most likely the oxidant (for the oxidation of V IV → V V) in the synthesis of pentavalent complexes ( 5) and ( 6). [V IVO(L)(B)] complexes are one electron paramagnetic and display axial EPR spectra, while the [V VO(L)(hq)] complexes are diamagnetic. The X-ray structure of [V VO(L 2)(hq)] ( 6) indicates that H 2L 2 ligand is bonded with the vanadium meridionally in a tridentate dinegative fashion through its phenolic-O, enolic-O and imine-N atoms. The general bond length order is: oxo < phenolato < enolato. The V–O (enolato) bond is longer than V–O (phenolato) bond by ∼0.07 Å and is identical with V–O (carboxylate) bond. 1H NMR spectrum of ( 6) in CDCl 3 solution indicates that the binding nature in the solid state is also retained in solution. Complexes ( 1)–( 4) display two ligand-field transitions in the visible region near 820 and 480 nm in DMF solution and exhibit irreversible oxidation peak near +0.60 V versus SCE in DMSO solution, while complexes ( 5) and ( 6) exhibit only LMCT band near 535 nm and display quasi-reversible one electron reduction peak near −0.10 V versus SCE in CH 2Cl 2 solution. The VO 3+–VO 2+ E 1/2 values shift considerably to more negative values when neutral NN donor is replaced by anionic ON donor species and it also provides better VO 3+ binding via phenolato oxygen. For a given bidentate ligand, E 1/2 increases in the order: (L 2) 2− < (L 1) 2−.

Metal-induced folding of a designed metalloprotein

The metal-induced assembly of a designed peptide-based rubredoxin model is described. The C16C19-GGY peptide has the sequence Ac-K(IEALEGK) 2(CEACEGK)(IEALEGK)GGY-amide in which the presence of the Cys–X–X–Cys metal binding domain of rubredoxin was used to place cysteine residues at the hydrophobic “a” and “d” positions upon formation of a homodimeric α-helical coiled-coil. Circular dichroism spectroscopy shows that the apopeptide exists as a random coil and assembles into a coiled-coil in the presence of Cd 2+. Metal binding is monitored by the appearance of a new LMCT band at 238 nm. UV–Vis titrations and SDS–PAGE experiments are used to show that this designed metalloprotein exists as a metal-bridged coiled-coil dimer.

Synthesis, Spectral and Electrochemical Studies of Mixed-Ligand Oxovanadium(IV) and Oxovanadium(V) Complexes Incorporating the Tridentate ONO Donor Schiff Base Derived from Acetylacetone and Benzoylhydrazine

Mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes with a tridentate dinegative ONO donor Schiff base ligand [viz., 4-(1-hydroxybenzylidenehydrazono)-2-penten-2-ol (H2L)] and bidentate NN [viz., 2,2′-bipyridine (bipy) and 1,10-phenanthroline (phen): complexes (1) and (2), respectively] or OO−[viz., ethylene glycol (H2gol), salicylaldehyde (Hsal) and vanillin (Hvan): complexes (3)–(5), respectively] donor ligands have been prepared and characterized by elemental analyses and by i.r., e.p.r., n.m.r., and u.v.–vis. spectroscopies. The complexes with NN donor ligands are one electron paramagnetic, displaying axial e.p.r. spectra and exhibiting two ligand-field transitions in the visible region, whereas the complexes with OO− donor ligands are diamagnetic and display only LMCT bands. 1H n.m.r. spectral data indicate that the pentavalent complexes (4) and (5) exist in two isomeric forms [(4A), (4B) and (5A), (5B) in 1:1 and 4:1 ratios, respectively]. The vanadyl(IV) complexes display an irreversible oxidation peak near +0.67 V while among the vanadyl(V) complexes, (3), displays an irreversible reduction peak near −0.20 V and (4) and (5) display a quasi-reversible one electron reduction peak near +0.25 V versus s.c.e. The trend in redox potential values and the selective stabilization of VO2+ and VO3+ motifs have been explained on the basis of the basicity of the bidentate auxiliary ligands.