Preparation and oxidation of some quinone adducts of transition metal complexes

Deactivation of Co-Schiff base catalysts in the oxidation of para-substituted lignin models for the production of benzoquinones

Those features which enhance the reactivity of Co-Schiff base oxidation catalysts can also contribute to their demise.

The semiquinone radical anion of 1,10-phenanthroline-5,6-dione: synthesis and rare earth coordination chemistry

Reduction of 1,10-phenanthroline-5,6-dione (pd) with CoCpR2 resulted in the first molecular compounds of the pd˙- semi-quinone radical anion, [CoCpR2]+[pd]˙- (R = H, (1); R = Me4, (2)). Furthermore compounds 1 and 2 were reacted with [Y(hfac)3(thf)2] (hfac = 1,1,1-5,5,5-hexafluoroacetylacetonate) to synthesise the rare earth-transition metal heterometallic compounds, [CoCpR2]+[Y(hfac)3(N,N'-pd)]˙- (R = H, (3); R = Me4, (4)).

The modular synthesis of rare earth-transition metal heterobimetallic complexes utilizing a redox-active ligand

We report a robust and modular synthetic route to heterometallic rare earth-transition metal complexes. We have used the redox-active bridging ligand 1,10-phenathroline-5,6-dione (pd), which has selective N,N' or O,O' binding sites as the template for this synthetic route. The coordination complexes [Ln(hfac)₃(N,N'-pd)] (Ln = Y [1], Gd [2]; hfac = hexafluoroacetylacetonate) were synthesised in high yield. These complexes have been fully characterised using a range of spectroscopic techniques. Solid state molecular structures of 1 and 2 have been determined by X-ray crystallography and display different pd binding modes in coordinating and non-coordinating solvents. Complexes 1 and 2 are unusually highly coloured in coordinating solvents, for example the vis-NIR spectrum of 1 in acetonitrile displays an electronic transition centred at 587 nm with an extinction coefficient consistent with significant charge transfer. The reaction between 1 and 2 and VCp₂ or VCp^t₂ (Cp^t = tetramethylcyclopentadienyl) resulted in the isolation of the heterobimetallic complexes, [Ln(hfac)₃(N,N'-O,O'-pd)VCp₂] (Ln = Y [3], Gd [4]) or [Ln(hfac)₃(N,N'-O,O'-pd)VCp^t₂] (Ln = Y [5], Gd [6]). The solid state molecular structures of 3, 5 and 6 have been determined by X-ray crystallography. The spectroscopic data on 3-6 are consistent with oxidation of V(ii) to V(iv) and reduction of pd to pd^2- in the heterobimetallic complexes. The spin-Hamiltonian parameters from low temperature X-band EPR spectroscopy of 3 and 5 describe a ²A₁ ground state, with a V(iv) centre. DFT calculations on 3 are in good agreement with experimental data and confirm the SOMO as the d_x²-y² orbital localised on vanadium.

Synthesis and structural characterization of iron complexes bearing N-aryl-phenanthren-o-iminoquinone ligands

Treatments of N-aryl-phenanthren-o-iminoquinone (aryl = 2,6-Me₂C₆H₃ (^MeL); 2,6-ⁱPr₂C₆H₃ (^iPrL)) with iron powder in THF at 75 °C generate complexes [η²L]₂Fe[η¹LH] (1a, L = ^MeL; 1b, L = ^iPrL) in moderate yields. The X-ray crystallography analysis reveals that the molecule of 1b consists of a Fe(iii) center coordinated by three phenanthren-o-iminosemiquinone ligands, two of which are in an η² fashion while the remaining one is in an η¹ fashion. The analysis of the bond parameters of ligands indicates that the η²-fashioned ligands are radical anions and the η¹-fashioned one is in an aminephenolato form. Reactions of ^MeL and ^iPrL with FeCl₂ in THF produce Fe(iii) complexes [L]₂FeCl (2a, L = ^MeL; 2b, L = ^iPrL) with the two ligands in the radical anionic form. However, similar reactions of PIQ ligands with FeCl₂ in CH₂Cl₂ yield ion-pair complexes {[L]₂FeCl}⁺[FeCl₄]^- (3a, L = ^MeL; 3b, L = ^iPrL), in which the iron center chelated by two neutral ligands can be formulated as Fe(ii). Reduction of 2b with sodium provides a salt-type complex [^iPrL^2-]₂Fe(ii)Na₂ (4), in which a high spin Fe(ii) atom is ligated by two amidophenolate ligands, and the sodium atoms attached to the oxygen atoms of ligands are η³-coordinated by the aryl ring in amido moieties.

Quantum chemical modeling of magnetically bistable metal coordination compounds. Synchronization of spin crossover, valence tautomerism and charge transfer induced spin transition mechanisms

Heterometallic complexes of 1,10-phenanthroline-5,6-dione exhibiting unprecedented dynamic behaviour due to synchronized thermally induced intramolecular rearrangements were computationally studied.

Crystal structure of aqua-{μ-N-[3-(di-methyl-amino)-prop-yl]-N'-2-(oxidophen-yl)oxamidato}(1,10-phen-anthroline-5,6-dione)dicopper(II) perchlorate hemihydrate

The N-[3-(di­methyl­amino)­prop­yl]-N′-(2-hy­droxy­phen­yl)oxamide trianion bridges two Cu^II cations to form the binuclear complex, in which the Cu^II cations have distorted square-planar and square-pyramidal coordination geometries.

The title compound, [Cu₂(C₁₃H₁₆N₃O₃)(C₁₂H₆N₂O₂)(H₂O)]ClO₄·0.5H₂O, consists of a cis-oxamide-bridged binuclear Cu^II complex cation, a perchlorate anion and half a solvent water mol­ecule. One Cu^II cation is N,N′,N",O-chelated by an N-[3-(di­methyl­amino)­prop­yl]-N′-(2-hy­droxy­phen­yl)oxamide trianion in a distorted square-planar geometry, whereas the other Cu^II cation is O,O′-chelated by the oxamide moiety of the anion and N,N′-chelated by a 1,10-phenanthroline-5,6-dione mol­ecule, and a water mol­ecule further coordinates the second Cu^II cation, completing a distorted square-pyramidal coordination geometry. In the crystal, classical O—H⋯O hydrogen bonds, weak C—H⋯O hydrogen-bonding inter­actions and π–π stacking inter­actions link the complex cations, anions and solvent water mol­ecules into a three-dimensional supra­molecular architecture. In the crystal, the di­methyl­amino­propyl unit of the oxamide anion is disordered over two positions with an occupancy ratio of 0.561 (11):0.439 (11); the solvent water mol­ecule is also disordered over two positions, the occupancy ratio being 0.207 (10):0.293 (10).

Synthesis, Crystal Structure and Photoluminescence of [Zn(phon)(H2O)2(OH)]ClO4·2H2O (Phon=1,10-Phenathrolinedione)

A new coordination polymer [Zn(phon)(H2O)2(OH)]·(ClO4)·2H2O (phon=1,10-phenathrolinedione) has been synthesised and its photoluminescence is reported. Its X-ray structure shows it to have octahedral coordination and be stabilised by π—π stacking and hydrogen bonding. 1,10-Phenanthroline and its derivatives play important roles in supramolecular assemblies.

Cyclometallated Iridium and Platinum Complexes with Noninnocent Ligands

The electronic properties of the cyclometalated (CwedgeN) complexes of iridium and platinum metals with a catechol ligand have been studied experimentally and computationally. The synthesis and characterization of (p-tolylpyridine)Ir(3,5-di-tert-butylcatechol) (abbreviated Ir-sq) and (2,4-diflorophenylpyridine)Pt(3,5-di-tert-butylcatechol) (abbreviated Pt-sq) are reported along with their structural, spectral, and electrochemical properties. Reaction of the 3,5-di-tert-butylcatechol (DTBCat) ligand with the prepared cyclometalated metal complex was carried out in air in the presence of a base. The resulting complexes are air stable and are paramagnetic with the unpaired electron residing mainly on the catechol ligand. The bond lengths obtained from X-ray structure analysis and the theoretical results suggest the semiquinone form of the catechol ligand. Low-energy, intense (approximately 10(3) M-1 cm-1) transitions are observed in the visible to near-infrared region (600-700 nm) of the absorption spectra of the metal complexes. Electrochemically, the complexes exhibit a reversible reduction of the semiquinone form to the catechol form of the ligand and an irreversible oxidation to the unstable quinone form of the ligand. The noninnocent catechol ligand plays a significant role in the electronic properties of the metal complexes. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations on the two open-shell molecules provide the ground-state and excited-state energies of the molecular orbitals involved in the observed low-energy transitions. The spin density in the two complexes resides mainly on the catechol ligand. The intense transition arises from excitation of the beta electron from a HOMO-n (n = 1 or 2 here) to the LUMO, rather than from the excitation of the unpaired alpha electron.

Synthesis and X-ray crystal structure of [Ag(phendio)2]ClO4 (phendio = 1,10-phenanthroline-5,6-dione) and its effects on fungal and mammalian cells

The Cu(II) and Ag(I) complexes, [Cu(phendio)3](ClO4)2 x 4H2O and [Ag(phendio)2]ClO4 (phendio = 1,10-phenanthroline-5,6-dione), are prepared in good yield by reacting phendio with the appropriate metal perchlorate salt. The X-ray crystal structure of the Ag(I) complex shows it to have a pseudo tetrahedral structure. 'Metal-free' phendio and the Cu(II) and Ag(I) phendio complexes strongly inhibit the growth of the fungal pathogen Candida albicans, and are more active than their 1,10-phenanthroline analogues. The simple Ag(I) salts, AgCH3CO2, AgNO3 and AgClO4 x H2O display superior anti-fungal properties compared to analogous simple Cu(II) and Mn(II) salts, suggesting that the nature of the metal ion strongly influences activity. Exposing C. albicans to 'metal-free' phendio, simple Ag(I) salts and [Ag(phendio)2]ClO4 causes extensive, non-specific DNA cleavage. 'Metal-free' phendio and [Ag(phendio)2]ClO4 induce gross distortions in fungal cell morphology and there is evidence for disruption of cell division. Both drugs also exhibit high anti-cancer activity when tested against cultured mammalian cells.

Sensitive Oxidation State Ambivalence in Unsymmetrical Three-Center (M/Q/M) Systems [(acac)2Ru(μ-Q)Ru(acac)2]n, Q = 1,10-Phenanthroline-5,6-dione or 1,10-Phenanthroline-5,6-diimine (n = +, 0, −, 2−)

The new redox systems [(acac)2 Ru(mu-Q1)Ru(acac)2](n) (1(n)) and [(acac)2 Ru(mu-Q2)Ru(acac)2](n) (2(n)) with Q1 = 1,10-phenanthroline-5,6-dione and Q2 = 1,10-phenanthroline-5,6-diimine were studied for n = +, 0, -, and 2- using UV-Vis-NIR spectroelectrochemistry and, in part, EPR and susceptometry. The ligands can bind the first metal (left) through the phenanthroline nitrogen atoms and the second metal (right) at the o-quinonoid chelate site. The neutral compounds are already different: Compound 1 is formulated as a Ru(II)(mu-Q1)*- Ru(III) species with partially coupled semiquinone and ruthenium(III) centers. In contrast, a Ru(III)(mu-Q2)2- Ru(III) structure is assigned to 2, which shows a weak antiferromagnetic spin-spin interaction (J = -1.14 cm(-1)) and displays an intense half-field signal in the EPR spectrum. The one-electron reduced forms are also differently formulated as Ru(II)(mu-Q1)2- Ru(III) for 1(-) with a Ru(III)-typical EPR response and as Ru(II)(mu-Q2)*- Ru(II) for 2(-) with a radical-type EPR signal at g = 2.0020. In contrast, both 1(2-) and 2(2-) can only be described as Ru(II)(mu-Q)2- Ru(II) species. The monooxidized forms 1(+) and 2(+) show very similar spectroscopy, including a Ru(III)-type EPR signal. Although no unambiguous assignment was possible here for the alternatives Ru(II)(mu-Q)0Ru(III), Ru(III)(mu-Q)2- Ru(IV) or Ru(III)(mu-Q)*- Ru(III), the last description is favored. The reasons for identical or different oxidation state combinations are discussed.

Electronic Structure and Spectra of Linkage Isomers of Bis(bipyridine)(1,2-dihydroxy-9,10-anthraquinonato)ruthenium(II) and Their Redox Series

Linkage isomers of bis(bipyridine)(1,2-dihydroxy-9,10-anthraquinonato)ruthenium(II), 1,2- and 1,9-coordinated complexes, and several of their oxidation products have been prepared chemically and/or electrochemically. For the 1,2-coordinated complex, the one- and two-electron oxidized species have been characterized, and for the 1,9-coordinated complex, the one-electron oxidized species has been characterized. The rich redox activity of these complexes leads to ambiguity in assessing the electronic structure. This paper reports EPR spectra of odd-electron species and detailed analyses of electronic spectra and structure of the complexes, based on INDO molecular orbital calculations. Results of calculations on the related 1-hydroxyanthraquinone complex and the free ligands,1,2-dihydroxy-9,10-anthraquinone (alizarin) and 1-hydroxyanthraquinone, are also briefly discussed.

Kinetics and mechanism of the reaction between chromium(III) and 3,4-dihydroxyphenylpropionic (dihydrocaffeic) acid in weak acidic aqueous solutions

The reaction of 3,4-dihydroxyphenylpropionic acid (dihydrocaffeic acid, hydcafH3) with chromium(III) in weak acidic aqueous solutions has been shown to take place through various oxygen-bonded intermediates. The formation of the oxygen-bonded complexes upon substitution of water molecules of the chromium(III) coordination sphere takes place in at least three stages, the first of which has an observed rate constant k1(obs)=k1K0'[hydcafH3]/[H+] where K0' corresponds to the Cr(H2O)6(3+) complex dissociation equilibrium. The second and third stages are ligand concentration independent and are thus attributed to isomerisation and chelation processes. The corresponding activation parameters are DeltaH2(not equal)=78+/-3 kJmol(-1), DeltaS2(not equal)=-49+/-9 JK(-1)mol(-1), DeltaH3(not equal)=60+/-9 kJmol(-1) and DeltaS3(not equal)=-112+/-39 JK(-1)mol(-1). The kinetic results support associative mechanisms and the nature of the electronic spectra a catecholic-type of coordination at the pH and concentration range studied and reported in this paper. The associatively activated substitution processes are accompanied by proton release causing a pH decrease. At lower acid concentration oxidation of the ligand takes place with concomitant high increase in the UV and VIS absorbance.

Mono- and Dinuclear Ruthenium Carbonyl Complexes with Redox-Active Dioxolene Ligands: Electrochemical and Spectroscopic Studies and the Properties of the Mixed-Valence Complexes

The mononuclear complex [Ru(PPh(3))(2)(CO)(2)(L(1))] (1; H(2)L(1) = 7,8-dihydroxy-6-methoxycoumarin) and the dinuclear complexes [[Ru(PPh(3))(2)(CO)(2)](2)(L(2))][PF(6)] [[2][PF(6)]; H(3)L(2) = 9-phenyl-2,3,7-trihydroxy-6-fluorone] and [[Ru(PBu(3))(2)(CO)(2)](2)(L(3))] (3; H(4)L(3) = 1,2,3,5,6,7-hexahydroxyanthracene-9,10-dione) have been prepared; all complexes contain one or two trans,cis-[Ru(PR(3))(2)(CO)(2)] units, each connected to a chelating dioxolene-type ligand. In all cases the dioxolene ligands exhibit reversible redox activity, and accordingly the complexes were studied by electrochemistry and UV/vis/NIR, IR, and EPR spectroscopy in their accessible oxidation states. Oxidation of 1 to [1](+) generates a ligand-centered semiquinone radical with some metal character as shown by the IR and EPR spectra. Dinuclear complexes [2](+) and 3 show two reversible ligand-centered couples (one associated with each dioxolene terminus) which are separated by 690 and 440 mV, respectively. This indicates that the mixed-valence species [2](2+) has greater degree of electronic delocalization between the ligand termini than does [3](+), an observation which was supported by IR, EPR, and UV/vis/NIR spectroelectrochemistry. Both [2](2+) and [3](+) have a solution EPR spectrum consistent with full delocalization of the unpaired electron between the ligand termini on the EPR time scale (a quintet arising from equal coupling to all four (31)P nuclei); [3](+) is localized on the faster IR time scale (four CO vibrations rather than two, indicative of inequivalent [Ru(CO)(2)] units) whereas [2](2+) is fully delocalized (two CO vibrations). UV/vis/NIR spectroelectrochemistry revealed the presence of a narrow, low-energy (2695 nm) transition for [3](+) associated with the catecholate --> semiquinone intervalence transition. The narrowness and solvent-independence of this transition (characteristic of class III mixed-valence character) coupled with evidence for inequivalent [Ru(CO)(2)] termini in the mixed-valence state (characteristic of class II character) place this complex at the class II-III borderline, in contrast to [2](2+) which is clearly class III.