A structural and functional model of galactose oxidase: control of the one-electron oxidized active form through two differentiated phenolic arms in a tripodal ligand

Diversity of oxidation state in copper complexes with phenolate ligands

The phenoxyl radical binding copper complexes have been widely developed and their detailed geometric and electronic structures have been clarified. While many one-electron oxidized CuII-phenolate complexes have been reported previously, recent studies of the Cu-phenolate complexes proceed toward elucidation of the complexes with other oxidation states, such as the phenoxyl radical binding CuI complexes and CuIV-phenolate complexes in the formal oxidation state. This Perspective focuses on new aspects of the properties and reactivities of various Cu-phenolate and Cu-phenoxyl radical complexes with emphasis on the relationship between geometric and electronic structures.

Altering the Localization of an Unpaired Spin in a Formal Ni(V) Species

The participation of both ligand and the metal center in the redox events has been recognized as one of the ways to attain the formal high valent complexes for the late 3d metals, such as Ni and Cu. Such an approach has been employed successfully to stabilize a Ni(III) bisphenoxyl diradical species in which there exist an equilibrium between the ligand and the Ni localized resultant spin. The present work, however, broadens the scope of the previously reported three oxidized equivalent species by conveying the approaches that tend to affect the reported equilibrium in CH3 CN at 233 K. Various spectroscopic characterization revealed that employing exogenous N-donor ligands like 1-methyl imidazole and pyridine favors the formation of the Ni centered localized spin though axial binding. In contrast, due to its steric hinderance, quinoline favors an exclusive ligand localized radical species. DFT studies shed light on the novel intermediates' complex electronic structure. Further, the three oxidized equivalent species with the Ni centered spin was examined for its hydrogen atom abstraction ability stressing their key role in alike reactions.

Magnetostructural Properties of Some Doubly-Bridged Phenoxido Copper(II) Complexes

Three new tripod tetradentate phenolate-amines (H2L1, H2L4 and H2L9), together with seven more already related published ligands, were synthesized, and characterized. With these ligands, two new dinuclear doubly-bridged-phenoxido copper(II) complexes (3, 4), and six more complexes (1, 2, 5–8), a new trinuclear complex (9) with an alternative doubly-bridged-phenoxido and –methoxido, as well as the 1D polymer (10) were synthesized, and their molecular structures were characterized by spectroscopic methods and X-ray single crystal crystallography. The Cu(II) centers in these complexes exhibit distorted square-pyramidal arrangement in 1–4, mixed square pyramidal and square planar in 5, 6, and 9, and distorted octahedral (5+1) arrangements in 7 and 8. The temperature dependence magnetic susceptibility study over the temperature range 2–300 K revealed moderate–relatively strong antiferromagnetic coupling (AF) (|J| = 289–145 cm−1) in complexes 1–6, weak-moderate AF (|J| = 59 cm−1) in the trinuclear complex 9, but weak AF interactions (|J| = 3.6 & 4.6 cm−1) were obtained in 7 and 8. No correlation was found between the exchange coupling J and the geometrical structural parameters of the four-membered Cu2O2 rings.

Dinuclear doubly bridged phenoxido copper(II) complexes as efficient anticancer agents

Two cationic [Cu₂(L^1-2)₂](ClO₄)₂ (1, 2), and four neutral doubly bridged-phenoxido-copper(II) complexes [Cu₂(L^3-4)₂] (3, 4) and [Cu₂(L^5-6)₂(H₂O)]‧2H₂O (5, 6) as well as 1D polymeric catena-[Cu(L⁷)] (7), where HL^1-2 and H₂L^3-7 represent tripodal tetradentate pyridyl or aliphatic-amino groups based 2,4-disubstituted phenolates, were synthesized and thoroughly characterized by various spectroscopic methods and single crystal X-ray analysis. The molecular structures of the complexes exhibited diverse geometrical environments around the central Cu(II) atoms. The in vitro antiproliferative activity of the isolated complexes and selected parent free ligands were screened against some human cancer cell lines (A2780, A2780R, PC-3, 22Rv1, MCF-7). The most promising cytotoxicity against cancer cells were obtained for 1-6, while complex 6 was found as the best performing as compared to the reference drug cisplatin. The cytotoxicity study of complex 6 was therefore extended to wider variety of cancer cell lines (HOS, A549, PANC-1, CaCo2, HeLa) and results revealed its significant cytotoxicity on all investigated human cancer cells. The cell uptake study showed that cytotoxicity of 6 (3 μM concentration and 24 h of incubation) against A2780 cells was almost independent from the intracellular levels of copper. The effect of complexes 4, 6 and 7 on cell cycle of A2780 cells indicates that the mechanism of action in these complexes is not only different from that of cisplatin but also different among them. Complex 7 was able to induce apoptosis in A2780 cells, while complexes 4 and 6 did not and on the other hand, they showed considerable effect on autophagy induction and there are some clues that these complexes were able to induce cuproptosis in A2780 cells.

A Copper(I) Complex with Two Unpaired Electrons, Synthesised by Oxidation of a Copper(II) Complex with Two Redox-Active Ligands

Two homoleptic copper(II) complexes [Cu(L1)2 ] and [Cu(L2)2 ] with anionic redox-active ligands were synthesised, one with urea azine (L1) and the other with thio-urea azine (L2) ligands. One-electron oxidation of the complexes initiates an unprecedented redox-induced electron transfer process, leading to monocationic copper(I) complexes [Cu(L1)2 ]+ and [Cu(L2)2 ]+ with two oxidised ligands. While [Cu(L1)2 ]+ is best described as a CuI complex with two neutral radical ligands that couple antiferromagnetically, [Cu(L2)2 ]+ is a CuI complex with two clearly different ligand units in the solid state and with a magnetic susceptibility close to a diamagnetic compound. Further one-electron oxidation of the complex with L1 ligands results in a dication [Cu(L1)2 ]2+ , best described as a CuI complex with a twofold oxidised, monocationic ligand and a neutral radical ligand. The stability in at least three redox states, the accumulation of spin density at the ligands and the facile ligand-metal electron transfer make these complexes highly attractive for a variety of applications; here the catalytic aerobic oxidation of alcohols to aldehydes is tested.

Formation of Ni(II)-phenoxyl radical complexes by O2: a mechanistic insight into the reaction of Ni(II)-phenol complexes with O2

A reaction of Ni(ClO4)2·6H2O with a tripodal ligand having two di(tert-butyl)phenol moieties, H2tbuL, and 1 equivalent of triethylamine in CH2Cl2/CH3OH (1 : 1, v/v) under N2 gave a NiII-(phenol)(phenolate) complex, [Ni(HtbuL)(CH3OH)2]ClO4. The formation of the NiII-phenoxyl radical complex by O2 was observed in the reaction of this complex in the solid state. On the other hand, the NiII-phenoxyl radical complex [Ni(Me2NL)(CH3OH)2]ClO4 was obtained by the reaction of H2Me2NL having a p-(dimethylamino)phenol moiety with Ni(ClO4)2·6H2O in a similar procedure under O2, through the oxidation of the NiII-(phenol)(phenolate) complex. However, a direct redox reaction of the NiII ion could not be detected in the phenoxyl radical formation. The results of the reaction kinetics, XAS and X-ray structure analyses suggested that the O2 oxidation from the NiII-(phenol)(phenolate) complex to the NiII-phenoxyl radical complex occurs via the proton transfer-electron transfer (PT-ET) type mechanism of the phenol moiety weakly coordinated to the nickel ion.

Distorted copper(ii) radicals with sterically hindered salens: electronic structure and aerobic oxidation of alcohols

The sterically hindered salen ligands featuring biphenyl and tetramethyl putrescine linkers were synthesized and chelated to copper. The resulting complexes CuL^bp,tBu, CuL^bp,OMe, CuL^pu,tBu and CuL^pu,OMe were structurally characterized, showing a significanty tetrahedrally distorted metal center. The complexes show two reversible oxidation waves in the range 0.2 to 0.8 V vs. Fc⁺/Fc. A further reduction wave is detected in the range -1.4 to -1.7 V vs. Fc⁺/Fc. It is reversible for CuL^bp,tBu and CuL^bp,OMe and assigned to the Cu^II/Cu^I redox couple. One-electron oxidation of CuL^bp,OMe, CuL^pu,tBu and CuL^pu,OMe was performed chemically and electrochemically. It is accompanied by a quenching of the EPR resonances. Phenoxyl radical formation was established by X-Ray diffraction on the cations [CuL^bp,OMe]⁺ and [CuL^pu,OMe]⁺, whereby the coordination sphere is elongated upon oxidation with quinoidal distributions of bond distances. The cations exhibit a NIR band of moderate intensity in their optical spectrum, supporting their classification as class II mixed-valent radical species according to the Robin Day classification. The proposed electronic structures are supported by DFT calculations. The cations [CuL^bp,OMe]⁺, [CuL^pu,tBu]⁺ and [CuL^pu,OMe]⁺ were active towards aerobic oxidation of the unactivated alcohol 2-phenylethanol, with TON numbers up to 58 within 3 h.

Formation of the CuII -Phenoxyl Radical by Reaction of O2 with a CuII -Phenolate Complex via the CuI -Phenoxyl Radical

Reaction of Cu(ClO₄ )₂ ⋅6 H₂ O with a tripodal 2N2O ligand, H₂ Me₂ NL, having a p-(dimethylamino)phenol moiety, in CH₂ Cl₂ /MeOH (1:1 v/v) under basic conditions under an inert gas atmosphere gave [Cu(Me₂ NL)(H₂ O)] (1). The same reaction carried out under aerobic conditions gave [Cu(Me₂ NL)(MeOH)]ClO₄ (2), which could be obtained also from the isolated complex 1 by reaction with O₂ in CH₂ Cl₂ /MeOH. The X-ray crystal structures of 1 and 2 revealed similar square-pyramidal structures, but 2 showed the (dimethylamino)phenoxyl radical features. Complex 1 exhibits characteristic Cu^II EPR signals of the d x 2 - y 2 ground state in CH₂ Cl₂ /MeOH at 77 K, whereas 2 is EPR-silent. The EPR and X-ray absorption fine structure (XAFS) results suggest that 2 is assigned to the Cu^II -(dimethylamino)phenoxyl radical. However, complex 1 showed different features in the absence of MeOH. The EPR spectrum of the CH₂ Cl₂ solution of 1 exhibits distortion from the d x 2 - y 2 ground state and a temperature-dependent equilibrium between the Cu^II -(dimethylamino)phenolate and the Cu^I -(dimethylamino)phenoxyl radical. From these results, Cu^II -phenoxyl radical complex 2 is concluded to be formed by the reaction of 1 with O₂ via the Cu^I -phenoxyl radical species.

Electronic Structure and Reactivity Studies of a Nonsymmetric One-Electron Oxidized CuII Bis-phenoxide Complex

The tetradentate mixed imino/amino phenoxide ligand (N-(3,5-di-tert-butylsalicylidene)-N'-(2-hydroxyl-3,5-di-tert-butylbenzyl))-trans-1,2-cyclohexanediamine (salalen) was complexed with CuII, and the resulting Cu complex (2) was characterized by a number of experimental techniques and theoretical calculations. Two quasi-reversible redox processes for 2, as observed by cyclic voltammetry, demonstrated the potential stability of oxidized forms, and also the increased electron-donating ability of the salalen ligand in comparison to the salen analogue. The electronic structure of the one-electron oxidized [2]+ was then studied in detail, and Cu K-edge X-ray Absorption Spectroscopy (XAS) measurements confirmed a CuII-phenoxyl radical complex in solution. Subsequent resonance Raman (rR) and variable temperature 1H NMR studies, coupled with theoretical calculations, showed that [2• ]+ is a triplet (S = 1) CuII-phenoxyl radical species, with localization of the radical on the more electron-rich aminophenoxide. Attempted isolation of X-ray quality crystals of [2• ]+ afforded [2H]+, with a protonated phenol bonded to CuII, and an additional H-bonding interaction with the SbF6 - counterion. Stoichiometric reaction of dilute solutions of [2• ]+ with benzyl alcohol showed that the complex reacted in a similar manner as the oxidized CuII-salen analogue, and does not exhibit a substrate-binding pre-equilibrium as observed for the oxidized bisaminophenoxide CuII-salan derivative.

Seven-coordinate lanthanide complexes with a tripodal redox active ligand: structural, electrochemical and spectroscopic investigations

One-electron oxidation of the lanthanide complexes affords phenoxyl radical species. Radical formation is accompanied by a quenching of the metal-based luminescence.

Activation of dioxygen by copper metalloproteins and insights from model complexes

Nature uses dioxygen as a key oxidant in the transformation of biomolecules. Among the enzymes that are utilized for these reactions are copper-containing metalloenzymes, which are responsible for important biological functions such as the regulation of neurotransmitters, dioxygen transport, and cellular respiration. Enzymatic and model system studies work in tandem in order to gain an understanding of the fundamental reductive activation of dioxygen by copper complexes. This review covers the most recent advancements in the structures, spectroscopy, and reaction mechanisms for dioxygen-activating copper proteins and relevant synthetic models thereof. An emphasis has also been placed on cofactor biogenesis, a fundamentally important process whereby biomolecules are post-translationally modified by the pro-enzyme active site to generate cofactors which are essential for the catalytic enzymatic reaction. Significant questions remaining in copper-ion-mediated O2-activation in copper proteins are addressed.

Roles of phenol groups and auxiliary ligand of copper(ii) complexes with tetradentate ligands in the aerobic oxidation of benzyl alcohol

Mimics structurally assembled to form the metal center of GOase. The phenol group(s) and substituent (R) and the auxiliary ligand (L) of the mimics significantly affect catalysis during the aerobic oxidation of benzyl alcohol.

Copper(ii) mediated phenol ring nitration by nitrogen dioxide

Addition of nitrogen dioxide in the THF solutions of Cu(ii) complexes of N₂O₂type ligands, L¹H₂and L²H₂resulted in the nitration at the 4-position of coordinated equatorial phenolate ring of the ligand frameworks. Spectroscopic evidence suggests that the reaction proceeds through a phenoxyl radical complex formation.

Synthesis and characterization of copper(ii) complexes with multidentate ligands as catalysts for the direct hydroxylation of benzene to phenol

Like iron(iii) complexes, copper(ii) complexes catalyse the direct hydroxylation of benzene to phenol with H₂O₂ as the oxidant and their catalytic efficiency correlates with the reduction potentials of the copper(ii) complexes.

Inter-ligand azo (NN) unit formation and stabilization of a Co(ii)-diradical complex via metal-to-ligand dπ–pπ* back donation: synthesis, characterization, and theoretical study

Ligand H₂Rich^(AP)N3 provided a diradical-containing Co(ii) complex via an inter-ligand azo (NN) bond formation.

Copper Catalysts for Aerobic Oxidation of Alcohols

Inspired by reactions catalyzed by galactose oxidase, a copper-containing enzyme, extensive studies were carried out on copper-based catalysts for alcohol oxidation using O2 as the terminal oxidant. Significant advances have been made towards the development of homogeneous and heterogeneous copper catalysts. These advances over the past decades are reviewed.

Copper(II)-mediated transformation of a tridentate non-innocent ligand into a tetradentate salen-type innocent ligand

A non-innocent ligand, H4L(CH2NH2), was synthesized by introducing a -CH2NH2 group at the ortho carbon atom to the aniline moiety of 2-anilino-4,6-di-tert-butylphenol. The new ligand was characterized by IR and NMR spectroscopy and mass spectrometry techniques. Upon treatment with CuCl2⋅2H2O, this non-innocent ligand provided a mononuclear four-coordinate salen-type Cu(II) complex by complete modification of the ligand backbone. The complex was characterized by IR spectroscopy, mass spectrometry, X-ray single-crystal diffraction, electron paramagnetic resonance (EPR) spectroscopy, and UV/Vis/near-IR spectroscopy techniques. X-ray crystallographic analysis showed an asymmetric environment around the Cu(II) center with a small (≈12°) twist between the two biting planes. Analysis of the X-band EPR spectrum also supported the asymmetric environment and also indicated the presence of an unpaired electron on the dx2-y2 orbital. The UV/Vis/near-IR spectrum showed strong absorption bands for metal-to-ligand charge transfer and ligand-to-metal charge transfer along with a Cu(II) -centered d-d transition. Mechanistic investigation of the formation of complex 1 indicated that modification of the ligand backbone proceeded through ligand-centered amine to imine oxidation as well as through C-N bond-breaking processes. During these processes, 3,5-di-tert-butyl-1,2-benzoquinone and 2-aminobenzylidene were produced. Ammonia, generated in situ through hydrolysis of the imine to the aldehyde, reacted with 3,5-di-tert-butyl-1,2-benzoquinone to form the corresponding 3,5-di-tert-butyl-1,2-iminobenzoquinone moiety, which upon two-electron reduction in the reaction medium formed 3,5-di-tert-butyl-1,2-aminophenol. This aminophenol underwent condensation with the H2L5 ligand that was formed by self-condensation of two molecules of 2-aminobenzaldehyde and provided the modified ligand backbone.

Effect of ligand substituent coordination on the geometry and the electronic structure of Cu(II)-diradical complexes

Two organic moieties, known as ligands, having -OMe and -SePh as the ortho substituent attached to the aniline moiety of the parent 2-anilino-4,6-di-tert-butylphenol ligand, were synthesized. The ligands reacted with CuCl2·2H2O in a 2:1 ratio in CH3CN in the presence of Et3N and provided the corresponding mononuclear Cu(II)-diradical complexes 1 (-OMe) and 2 (-SePh). Complex 1 was square planar, while complex 2 was in distorted square planar geometry due to the secondary coordination between the Se atom and the central Cu(II) center. Both complexes were comprised of multi-paramagnetic centers and exhibited an St = 1/2 ground state as established by variable-temperature magnetic susceptibility measurements. X-band electron paramagnetic resonance measurements indicated the presence of an unpaired electron at the Cu(II) center in complex 1 and at the ligand center (π-radical) in complex 2. The extent of the secondary interaction was found to be dependent on the "softness" of the donor atom belonging to the ortho substituent.

Synthesis, X-ray crystallography, spectroscopic characterization and spectroscopic/electrochemical evidence of formation of phenoxy free radical in active center analogs of galactose oxidase - [Cu(Salgly)H₂O] and [Cu(Salphenylalanine)H₂O]

The formation of phenolate free radical is the factor of high turnover for catalytic activity of galactose oxidase (GO) compared to that by inorganic complexes. A new active center analog of GO, [Cu(II)(Salphenylalanine)H(2)O] have been synthesized and its single crystal X-ray analysis was done. In aqueous surfactant micellar solution chemical oxidation as well as electrochemical oxidation of structural models of galactose oxidase - [Cu(II)Salgly·H(2)O] and [Cu(II)(Salphenylalanine)·H(2)O], have been found to generate free radical originating at the phenolate group. Formation of the free radical have been proved by electron paramagnetic resonance spectroscopy, electronic spectroscopy and electrochemistry.

Ligand contributions to the electronic structures of the oxidized cobalt(II) salen complexes

Square planar cobalt(II) complexes of salen ligands N,N'-bis(3-tert-butyl-5R-salicylidene)-1,2-cyclohexanediamine), where R = OMe (1) and tert-butyl (2), were prepared. 1 and 2 were electrochemically reversibly oxidized into cations [1-H(2)O](+) and [2-H(2)O](+) in CH(2)Cl(2). The chemically generated [1-H(2)O](SbF(6))·0.68 H(2)O·0.82CH(2)Cl(2) and [2-H(2)O](SbF(6))·0.3H(2)O·0.85CH(2)Cl(2) were characterized by X-ray diffraction and NIR spectroscopy. Both complexes are paramagnetic species containing a square pyramidal cobalt ion coordinated at the apical position by an exogenous water molecule. They exhibit remarkable NIR bands at 1220 (7370 M(-1) cm(-1)) and 1060 nm (5560 M(-1) cm(-1)), respectively, assigned to a CT transition. DFT calculations and magnetic measurements confirm the paramagnetic (S = 1) ground spin state of the cations. They show that more than 70% of the total spin density in [1-H(2)O](+) and [2-H(2)O](+) is localized on the metal, the remaining spin density being distributed over the aromatic rings (30% phenoxyl character). In the presence of N-methylimidazole 1 and 2 are irreversibly oxidized by air into the genuine octahedral cobalt(III) bis(phenolate) complexes [1-im(2)](+) and [2-im(2)](+), the former being structurally characterized. Neither [1-im(2)](+) nor [2-im(2)](+) exhibits a NIR feature in its electronic spectrum. 1 and 2 were electrochemically two-electron oxidized into [1](2+) and [2](2+). The cations were identified as Co(III)-phenoxyl species by their characteristic absorption band at ca. 400 nm in the UV-vis spectrum. Coordination of the phenoxyl radical to the cobalt(III) metal ion is evidenced by the EPR signal centered at g = 2.00.

Bis(acetonitrile)bis(acetylacetonato)ruthenium(III) mediated chemical transformations of coordinated 2-methylthioanilide

Chemical reaction of [Ru(III)(acac)(2)(CH(3)CN)(2)]ClO(4) (1) with 2-methylthioaniline, HL(1) in ethanol under basic conditions yielded three new complexes Ru(II)(acac)(2)(L(1b)) (1b), (L(1b) = 4-imino-3-(methylsulfanyl)cyclohexa-2,5-dien-1-one), Ru(III)(acac)(2)(L(1c)) (1c), (HL(1c) = N-(2-methylthiophenyl)formamide) and (acac)(2)Ru(II)(μ-L(1d))Ru(II)(acac)(2) (1d), (L(1d) = 1,4-bis(2-methylthioaniline)-1,4-diazabutadiene) via the intermediate Ru(III)(acac)(2)(L(1a)) (1a, L(1a) = (L(1))(-) = 2-methylthioanilide). The reaction proceeded through temperature induced valence tautomerisation between the Ru(III) center and its 2-methylthioanilide counterpart in 1a with concomitant reduction of ruthenium from +III to +II oxidation state and oxidation of ligand L(1a), resulting in aromatic ring hydroxylation, N-formylation and C-C bond formation reactions. All the complexes have been characterised by their single-crystal X-ray structure determination and various spectroscopic and electrochemical techniques. The identity of complex 1a has been confirmed by X-ray crystal structure determination of complex 2, a phenyl analogue of complex 1a. The complexes (1a-d) showed intense charge transfer (MLCT/LMCT) transition in the long wavelength region. The paramagnetic compounds, 1a and 1c, along with the diamagnetic compound 1b showed two one-electron responses in the ranges, -0.4 to -1.0 V and 0.3 to 1.1 V. The diamagnetic complex 1d displayed two successive one-electron reversible reductions (-1.31 and -1.55 V) and two one-electron reversible oxidation processes (-0.036 and 0.51 V). The redox processes are characterized by EPR spectroscopy and spectroelectrochemistry. The compound 1c has been found to exhibit solvatochromism and concentration dependent aggregation in solution.

Electrochemical and spectroscopic effects of mixed substituents in bis(phenolate)-copper(II) galactose oxidase model complexes

Nonsymmetric substitution of salen (1(R(1),R(2))) and reduced salen (2(R(1),R(2))) Cu(II)-phenoxyl complexes with a combination of -(t)Bu, -S(i)Pr, and -OMe substituents leads to dramatic differences in their redox and spectroscopic properties, providing insight into the influence of the cysteine-modified tyrosine cofactor in the enzyme galactose oxidase (GO). Using a modified Marcus-Hush analysis, the oxidized copper complexes are characterized as Class II mixed-valent due to the electronic differentiation between the two substituted phenolates. Sulfur K-edge X-ray absorption spectroscopy (XAS) assesses the degree of radical delocalization onto the single sulfur atom of nonsymmetric [1((t)Bu,SMe)](+) at 7%, consistent with other spectroscopic and electrochemical results that suggest preferential oxidation of the -SMe bearing phenolate. Estimates of the thermodynamic free-energy difference between the two localized states (ΔG(o)) and reorganizational energies (λ(R(1)R(2))) of [1(R(1),R(2))](+) and [2(R(1),R(2))](+) lead to accurate predictions of the spectroscopically observed IVCT transition energies. Application of the modified Marcus-Hush analysis to GO using parameters determined for [2(R(1),R(2))](+) predicts a ν(max) of ∼13600 cm(-1), well within the energy range of the broad Vis-NIR band displayed by the enzyme.

Synthesis, spectroscopic characterization and reactivity studies of oxovanadium(IV) complexes with bulky N,N'-polymethylenebis(3,5-(t)Bu(2)salicylaldimine) ligands

A series of new sterically hindered N,N'-polymethylenebis(3,5-(t)Bu(2)salicylaldimine) ligands (H(2)L(x)) VO(IV) complexes, [VO{(2-O-3,5-(t)Bu(2)C(6)H(2))CHN-R-NCH-(3,5-(t)Bu(2)-C(6)H(2)O-2)] (X), where R=-(CH(2))(3)- (3), -(CH(2))(4)- (4), -(CH(2))(5)- (5), -(CH(2))(6)- (6) and -CH(2)C(CH(3))(2)CH(2)- (7) and early reported -(CH(2))(2)- (1) and -CH(2)CH(CH(3))- (2), has been synthesized and characterized by spectroscopic (IR, UV/vis, (1)H NMR, EPR), electrochemical and magnetic susceptibility measurements. Complexes 1-7 are described a trigonal distorted pyramids. All seven compounds give nearly the same parallel hyperfine coupling constant (A(z)) regardless that the geometry of VO(IV) changes from square pyramidal to trigonal distorted pyramids. Chemical oxidation of 1-7 by one equiv Ce(IV) leads to the formation of stable [VO(V)L(x)](+) complexes. Cyclic voltammograms of 2-6 in DMSO along with a quasi-reversible VO(IV)/VO(V) redox couple also showed irreversible phenolate/phenoxyl responses. Each 1 and 7 shows only one reversible VO(IV) centered oxidation waves. Chemical oxidation of H(2)L(x) forms the stable [H(2)L(x)](+) radical species.

A stereodynamic tripodal ligand with three different coordinating arms: synthesis and zinc(II), copper(I) complexation study

A tetradentate tripodal ligand containing a chiral center and three different coordinating arms was designed and synthesized. Its complexation properties with Zn(II) and Cu(I) were studied by NMR and optical spectroscopy. NMR experiments demonstrated the formation of two diastereomers, indicating the stabilization of the central tertiary amine configuration by metal coordination. The inversion of pyramidalization of the central tertiary amine of the ligand was found to be highly dependent upon metal ion, solvent, and temperature. Dynamic NMR measurements were used to estimate the energy of activation required for nitrogen atom inversion. Finally, absorption and circular dichroism measurements confirmed the expectation that metal complexes of the ligand gave rise to circular dichroism but that such spectra were not characterized by exciton-coupling, in contrast to previously described ligands containing two identical arms with strong chromophores.