Publications by authors named "Peter Rapta"

54 Publications

Triapine Analogues and Their Copper(II) Complexes: Synthesis, Characterization, Solution Speciation, Redox Activity, Cytotoxicity, and mR2 RNR Inhibition.

Inorg Chem 2021 Jul 19. Epub 2021 Jul 19.

Institute of Inorganic Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria.

Three new thiosemicarbazones (TSCs) - as triapine analogues bearing a redox-active phenolic moiety at the terminal nitrogen atom were prepared. Reactions of - with CuCl·2HO in anoxic methanol afforded three copper(II) complexes, namely, (), [ (), and (), in good yields. Solution speciation studies revealed that the metal-free ligands are stable as - at pH 7.4, while being air-sensitive in the basic pH range. In dimethyl sulfoxide they exist as a mixture of and isomers. A mechanism of the isomerization with an inversion at the nitrogen atom of the Schiff base imine bond is proposed. The monocationic complexes are the most abundant species in aqueous solutions at pH 7.4. Electrochemical and spectroelectrochemical studies of , , and confirmed their redox activity in both the cathodic and the anodic region of potentials. The one-electron reduction was identified as metal-centered by electron paramagnetic resonance spectroelectrochemistry. An electrochemical oxidation pointed out the ligand-centered oxidation, while chemical oxidations of and as well as and afforded several two-electron and four-electron oxidation products, which were isolated and comprehensively characterized. Complexes and showed an antiproliferative activity in Colo205 and Colo320 cancer cell lines with half-maximal inhibitory concentration values in the low micromolar concentration range, while with the most closely related ligand to triapine displayed the best selectivity for cancer cells versus normal fibroblast cells (MRC-5). and in the presence of 1,4-dithiothreitol are as potent inhibitors of mR2 ribonucleotide reductase as triapine.
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http://dx.doi.org/10.1021/acs.inorgchem.1c01275DOI Listing
July 2021

Coumarin-Based Triapine Derivatives and Their Copper(II) Complexes: Synthesis, Cytotoxicity and mR2 RNR Inhibition Activity.

Biomolecules 2021 Jun 9;11(6). Epub 2021 Jun 9.

Institute of Inorganic Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria.

A series of thiosemicarbazone-coumarin hybrids ( and ) has been synthesised in 12 steps and used for the preparation of mono- and dinuclear copper(II) complexes, namely (), (), () and (), isolated in hydrated or solvated forms. Both the organic hybrids and their copper(II) and dicopper(II) complexes were comprehensively characterised by analytical and spectroscopic techniques, i.e., elemental analysis, ESI mass spectrometry, 1D and 2D NMR, IR and UV-vis spectroscopies, cyclic voltammetry (CV) and spectroelectrochemistry (SEC). Re-crystallisation of from methanol afforded single crystals of copper(II) complex with monoanionic ligand , which could be studied by single crystal X-ray diffraction (SC-XRD). The prepared copper(II) complexes and their metal-free ligands revealed antiproliferative activity against highly resistant cancer cell lines, including triple negative breast cancer cells MDA-MB-231, sensitive COLO-205 and multidrug resistant COLO-320 colorectal adenocarcinoma cell lines, as well as in healthy human lung fibroblasts MRC-5 and compared to those for triapine and doxorubicin. In addition, their ability to reduce the tyrosyl radical in mouse R2 protein of ribonucleotide reductase has been ascertained by EPR spectroscopy and the results were compared with those for triapine.
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http://dx.doi.org/10.3390/biom11060862DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8230303PMC
June 2021

Interfering with Metabolic Profile of Triple-Negative Breast Cancers Using Rationally Designed Metformin Prodrugs.

Angew Chem Int Ed Engl 2021 06 6;60(24):13405-13413. Epub 2021 May 6.

Department of Chemistry, National University of Singapore, 3 Science Drive 2, 117543, Singapore, Singapore.

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer, characterized by an aberrant metabolic phenotype with high metastatic capacity, resulting in poor patient prognoses and low survival rates. We designed a series of novel Au cyclometalated prodrugs of energy-disrupting Type II antidiabetic drugs namely, metformin and phenformin. Prodrug activation and release of the metformin ligand was achieved by tuning the cyclometalated Au fragment. The lead complex 3met was 6000-fold more cytotoxic compared to uncoordinated metformin and significantly reduced tumor burden in mice with aggressive breast cancers with lymphocytic infiltration into tumor tissues. These effects was ascribed to 3met interfering with energy production in TNBCs and inhibiting associated pro-survival responses to induce deadly metabolic catastrophe.
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http://dx.doi.org/10.1002/anie.202102266DOI Listing
June 2021

Spectroelectrochemical Properties and Catalytic Activity in Cyclohexane Oxidation of the Hybrid Zr/Hf-Phthalocyaninate-Capped Nickel(II) and Iron(II) tris-Pyridineoximates and Their Precursors.

Molecules 2021 Jan 11;26(2). Epub 2021 Jan 11.

Institute of Inorganic Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria.

The in situ spectroelectrochemical cyclic voltammetric studies of the antimony-monocapped nickel(II) and iron(II) tris-pyridineoximates with a labile triethylantimony cross-linking group and Zr(IV)/Hf(IV) phthalocyaninate complexes were performed in order to understand the nature of the redox events in the molecules of heterodinuclear zirconium(IV) and hafnium(IV) phthalocyaninate-capped derivatives. Electronic structures of their 1e-oxidized and 1e-electron-reduced forms were experimentally studied by electron paramagnetic resonance (EPR) spectroscopy and UV-vis-near-IR spectroelectrochemical experiments and supported by density functional theory (DFT) calculations. The investigated hybrid molecular systems that combine a transition metal (pseudo)clathrochelate and a Zr/Hf-phthalocyaninate moiety exhibit quite rich redox activity both in the cathodic and in the anodic region. These binuclear compounds and their precursors were tested as potential catalysts in oxidation reactions of cyclohexane and the results are discussed.
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http://dx.doi.org/10.3390/molecules26020336DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7827310PMC
January 2021

Triapine Derivatives Act as Copper Delivery Vehicles to Induce Deadly Metal Overload in Cancer Cells.

Biomolecules 2020 09 19;10(9). Epub 2020 Sep 19.

Institute of Inorganic Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria.

Thiosemicarbazones continue to attract the interest of researchers as potential anticancer drugs. For example, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone, or triapine, is the most well-known representative of this class of compounds that has entered multiple phase I and II clinical trials. Two new triapine derivatives and were prepared by condensation reactions of 2-pyridinamidrazone and S-methylisothiosemicarbazidium chloride with 3--(-butyloxycarbonyl) amino-pyridine-2-carboxaldehyde, followed by a Boc-deprotection procedure. Subsequent reaction of and with CuCl·2HO in 1:1 molar ratio in methanol produced the complexes () and (). The reaction of with Fe(NO)∙9HO in 2:1 molar ratio in the presence of triethylamine afforded the complex (), in which the isothiosemicarbazone acts as a tridentate monoanionic ligand. The crystal structures of , and metal complexes and were determined by single crystal X-ray diffraction. The UV-Vis and EPR spectroelectrochemical measurements revealed that complexes and underwent irreversible reduction of Cu(II) with subsequent ligand release, while showed an almost reversible electrochemical reduction in dimethyl sulfoxide (DMSO). Aqueous solution behaviour of and as well as of and its complex , was monitored as well. Complexes - were tested against ovarian carcinoma cells, as well as noncancerous embryonic kidney cells, in comparison to respective free ligands, triapine and cisplatin. While the free ligands and were devoid of antiproliferative activity, their respective metal complexes showed remarkable antiproliferative activity in a micromolar concentration range. The activity was not related to the inhibition of ribonucleotide reductase (RNR) R2 protein, but rather to cancer cell homeostasis disturbance-leading to the disruption of cancer cell signalling.
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http://dx.doi.org/10.3390/biom10091336DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7564244PMC
September 2020

Insight into the Anticancer Activity of Copper(II) 5-Methylenetrimethylammonium-Thiosemicarbazonates and Their Interaction with Organic Cation Transporters.

Biomolecules 2020 08 20;10(9). Epub 2020 Aug 20.

Institute of Inorganic Chemistry, Faculty of Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria.

A series of four water-soluble salicylaldehyde thiosemicarbazones with a positively charged trimethylammonium moiety ([HL]Cl, R = H, Me, Et, Ph) and four copper(II) complexes [Cu(HL)Cl]Cl (-) were synthesised with the aim to study (i) their antiproliferative activity in cancer cells and, (ii) for the first time for thiosemicarbazones, the interaction with membrane transport proteins, specifically organic cation transporters OCT1-3. The compounds were comprehensively characterised by analytical, spectroscopic and X-ray diffraction methods. The highest cytotoxic effect was observed in the neuroblastoma cell line SH-5YSY after 24 h exposure and follows the rank order: > > > > >>[HL]Cl. The copper(II) complexes showed marked interaction with OCT1-3, comparable to that of well-known OCT inhibitors (decynium 22, prazosin and corticosterone) in the cell-based radiotracer uptake assays. The work paves the way for the development of more potent and selective anticancer drugs and/or OCT inhibitors.
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http://dx.doi.org/10.3390/biom10091213DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7565988PMC
August 2020

Nickel(II), Copper(II) and Palladium(II) Complexes with Bis-Semicarbazide Hexaazamacrocycles: Redox-Noninnocent Behavior and Catalytic Activity in Oxidation and C-C Coupling Reactions.

Inorg Chem 2020 Aug 10;59(15):10650-10664. Epub 2020 Jul 10.

University of Vienna, Institute of Inorganic Chemistry, Währinger Strasse 42, A-1090 Vienna, Austria.

Nickel(II), copper(II), and palladium(II) complexes , where M = Ni (), Cu (), Pd (), and , where M = Ni (), Cu (), Pd (), have been prepared by reactions of NiCl·6HO, Cu(OAc)·HO, and PdCl(MeCN) with 14-membered bis-semicarbazide hexaazamacrocycles and in dimethylformamide (DMF). The compounds were characterized by elemental analysis, ESI mass spectrometry, IR, UV-vis, and 1D (H, C) and 2D (H-H COSY, H-H TOCSY, H-H NOESY, H-C HSQC, H-C HMBC) NMR spectra (, , , and ), and X-ray diffraction (, -). The complexes with MN coordination environment have = 0, 1/2, 0 ground states for Ni, Cu, and Pd, respectively. The electrochemical behavior of - was investigated in detail. The electronic structures of 1e-oxidized species were studied by EPR, UV-vis-NIR spectroelectrochemistry, and DFT calculations, indicating the redox-noninnocent behavior of the ligands. Compounds - were tested in the oxidation of styrene and C-C coupling (Henry and Knoevenagel condensations). Compounds and selectively catalyze the microwave-assisted oxidation of neat styrene to benzaldehyde (up to 88% yield), whereas the and catalytic systems afforded up to 99% β-nitroethanol yield with an appreciable diastereoselectivity toward the formation of the isomer.
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http://dx.doi.org/10.1021/acs.inorgchem.0c01119DOI Listing
August 2020

Tuning Redox Properties and Self-Assembly of Thienoacene-Extended Tetrathiafulvalenes.

Chempluschem 2019 09 14;84(9):1279-1287. Epub 2019 Jan 14.

Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Copenhagen Ø, Denmark.

Turning on and off associations between molecules by a reversible change in their redox states is a convenient way of controlling self-assembly and hence for advancing supramolecular chemistry. Here we present systematic studies on a selection of extended tetrathiafulvalenes with thienoacene spacers. By cyclic and differential pulse voltammetry and in situ EPR/UV-Vis-NIR spectroelectrochemistry, in combination with computations, we have elucidated how the number and orientations of thiophene rings in the spacer between the two dithiafulvene rings influence both the donor strength and association properties. The radical cations and their associates were found to cover a remarkable large region of the UV-Vis-NIR spectrum, but the appearance of the absorption spectrum of the radical cations as well as of the unassociated dications also depended strongly on the exact molecular structure.
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http://dx.doi.org/10.1002/cplu.201800626DOI Listing
September 2019

Nickel(II) Complexes with Redox Noninnocent Octaazamacrocycles as Catalysts in Oxidation Reactions.

Inorg Chem 2019 Aug 2;58(16):11133-11145. Epub 2019 Aug 2.

Institute of Inorganic Chemistry , University of Vienna , Währinger Strasse 42 , A-1090 Vienna , Austria.

Nickel(II) complexes with 15-membered (-) and 14-membered () octaazamacrocyclic ligands derived from 1,2- and 1,3-diketones and -methylisothiocarbohydrazide were prepared by template synthesis. The compounds were characterized by elemental analysis, electrospray ionization mass spectrometry, IR, UV-vis, H NMR spectroscopies, and X-ray diffraction. The complexes contain a low-spin nickel(II) ion in a square-planar coordination environment. The electrochemical behavior of - was investigated in detail, and the electronic structure of 1e-oxidized and 1e-reduced species was studied by electron paramagnetic resonance, UV-vis-near-IR spectroelectrochemistry, and density functional theory calculations indicating redox noninnocent behavior of the ligands. Compounds - were tested in the microwave-assisted solvent-free oxidation of cyclohexane by -butyl hydroperoxide to produce the industrially significant mixture of cyclohexanol and cyclohexanone (i.e., A/K oil). The results showed that the catalytic activity was affected by several factors, namely, reaction time and temperature or amount and type of catalyst. The best values for A/K oil yield (23%, turnover number of 1.1 × 10) were obtained with compound after 2 h of microwave irradiation at 100 °C.
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http://dx.doi.org/10.1021/acs.inorgchem.9b01700DOI Listing
August 2019

Novel latonduine derived proligands and their copper(ii) complexes show cytotoxicity in the nanomolar range in human colon adenocarcinoma cells and in vitro cancer selectivity.

Dalton Trans 2019 Jul;48(28):10464-10478

Institute of Inorganic Chemistry of the University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria.

Four Schiff bases derived from 7-hydrazin-yl-5,8-dihydroindolo[2,3-d][2]benzazepin-(6H)-one and its bromo-substituted analogue (HL1-HL4) and four copper(ii) complexes 1-4 have been synthesised and fully characterised by standard spectroscopic methods (1H and 13C NMR, UV-vis), ESI mass spectrometry, single crystal X-ray diffraction and spectroelectrochemistry. In addition, two previously reported complexes with paullone ligands 5 and 6 were prepared and studied for comparison reasons. The CuII ion in 1-4 is five-coordinate and adopts a square-pyramidal or slightly distorted square-pyramidal coordination geometry. The ligands HL1-4 act as tridentate, the other two coordination places are occupied by two chlorido co-ligands. The organic ligands in 2 and 3 are bound tighter to copper(ii) when compared to related paullone ligands in 5 and 6. The new compounds show very strong cytotoxic activity against human colon adenocarcinoma doxorubicin-sensitive Colo 205 and multidrug resistant Colo 320 cancer cell lines with IC50 values in the low micromolar to nanomolar concentration range.
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http://dx.doi.org/10.1039/c9dt01238aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6635014PMC
July 2019

Dinuclear manganese(iii) complexes with bioinspired coordination and variable linkers showing weak exchange effects: a synthetic, structural, spectroscopic and computation study.

Dalton Trans 2019 May;48(18):5909-5922

Inorganic Polymers Department, "Petru Poni" Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A, Iasi 700487, Romania.

Three dimanganese(iii) complexes have been synthesised and fully characterised by standard spectroscopic methods and spectroelectrochemistry. Each MnIII ion is chelated by a salen type ligand (H2L), but there is variation in the bridging group: LMn(OOCCH[double bond, length as m-dash]CHCOO)MnL, LMn(OOCC6H4COO)MnL, and LMn(OOCC6H4C6H4COO)MnL. X-ray diffraction revealed an axial compression of each six-coordinate high-spin d4 MnIII ion, which is a Jahn-Teller-active ion. Temperature dependent magnetic susceptibility and variable temperature-variable field (VTVH) magnetisation measurements, as well as high-frequency and -field EPR (HFEPR) spectroscopy were used to accurately describe the magnetic properties of the complexes, not only the single-ion spin Hamiltonian parameters: g-values and zero-field splitting (ZFS) parameters D and E, but also the exchange interaction constant J between the two ions, which has been seldom determined for a di-MnIII complex, particularly when there is more than a single bridging atom. Quantum chemical calculations reproduced well the electronic and geometric structure of these unusual complexes, and, in particular, their electronic absorption spectra along with the spin Hamiltonian and exchange parameters.
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http://dx.doi.org/10.1039/c8dt04596hDOI Listing
May 2019

New Water-Soluble Copper(II) Complexes with Morpholine-Thiosemicarbazone Hybrids: Insights into the Anticancer and Antibacterial Mode of Action.

J Med Chem 2019 01 18;62(2):512-530. Epub 2018 Dec 18.

Institute of Inorganic Chemistry , University of Vienna , Währinger Strasse 42 , A-1090 Vienna , Austria.

Six morpholine-(iso)thiosemicarbazone hybrids HL-HL and their Cu(II) complexes with good-to-moderate solubility and stability in water were synthesized and characterized. Cu(II) complexes [Cu(L)Cl] (1-6) formed weak dimeric associates in the solid state, which did not remain intact in solution as evidenced by ESI-MS. The lead proligands and Cu(II) complexes displayed higher antiproliferative activity in cancer cells than triapine. In addition, complexes 2-5 were found to specifically inhibit the growth of Gram-positive bacteria Staphylococcus aureus with MIC values at 2-5 μg/mL. Insights into the processes controlling intracellular accumulation and mechanism of action were investigated for 2 and 5, including the role of ribonucleotide reductase (RNR) inhibition, endoplasmic reticulum stress induction, and regulation of other cancer signaling pathways. Their ability to moderately inhibit R2 RNR protein in the presence of dithiothreitol is likely related to Fe chelating properties of the proligands liberated upon reduction.
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http://dx.doi.org/10.1021/acs.jmedchem.8b01031DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6348444PMC
January 2019

NO Releasing and Anticancer Properties of Octahedral Ruthenium-Nitrosyl Complexes with Equatorial 1 H-Indazole Ligands.

Inorg Chem 2018 Sep 14;57(17):10702-10717. Epub 2018 Aug 14.

Institute of Inorganic Chemistry, Faculty of Chemistry , University of Vienna , Währinger Strasse 42 , A-1090 Vienna , Austria.

With the aim of enhancing the biological activity of ruthenium-nitrosyl complexes, new compounds with four equatorially bound indazole ligands, namely, trans-[RuCl(Hind)(NO)]Cl·HO ([3]Cl·HO) and trans-[RuOH(Hind)(NO)]Cl·HO ([4]Cl·HO), have been prepared from trans-[Ru(NO)(Hind)] ([2]). When the pH-dependent solution behavior of [3]Cl·HO and [4]Cl·HO was studied, two new complexes with two deprotonated indazole ligands were isolated, namely [RuCl(ind)(Hind)(NO)] ([5]) and [RuOH(ind)(Hind)(NO)] ([6]). All prepared compounds were comprehensively characterized by spectroscopic (IR, UV-vis, H NMR) techniques. Compound [2], as well as [3]Cl·2(CH)CO, [4]Cl·2(CH)CO, and [5]·0.8CHCl, the latter three obtained by recrystallization of the first isolated compounds (hydrates or anhydrous species) from acetone and dichloromethane, respectively, were studied by X-ray diffraction methods. The photoinduced release of NO in [3]Cl and [4]Cl was investigated by cyclic voltammetry and resulting paramagnetic NO species were detected by EPR spectroscopy. The quantum yields of NO release were calculated and found to be low (3-6%), which could be explained by NO dissociation and recombination dynamics, assessed by femtosecond pump-probe spectroscopy. The geometry and electronic parameters of Ru species formed upon NO release were identified by DFT calculations. The complexes [3]Cl and [4]Cl showed considerable antiproliferative activity in human cancer cell lines with IC values in low micromolar or submicromolar concentration range and are suitable for further development as potential anticancer drugs. p53-dependence of Ru-NO complexes [3]Cl and [4]Cl was studied and p53-independent mode of action was confirmed. The effects of NO release on the cytotoxicity of the complexes with or without light irradiation were investigated using NO scavenger carboxy-PTIO.
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http://dx.doi.org/10.1021/acs.inorgchem.8b01341DOI Listing
September 2018

cis-Tetrachlorido-bis(indazole)osmium(iv) and its osmium(iii) analogues: paving the way towards the cis-isomer of the ruthenium anticancer drugs KP1019 and/or NKP1339.

Dalton Trans 2017 Sep;46(35):11925-11941

University of Vienna, Faculty of Chemistry, Institute of Inorganic Chemistry, Währinger Str. 42, A-1090 Vienna, Austria.

The relationship between cis-trans isomerism and anticancer activity has been mainly addressed for square-planar metal complexes, in particular, for platinum(ii), e.g., cis- and trans-[PtCl(NH)], and a number of related compounds, of which, however, only cis-counterparts are in clinical use today. For octahedral metal complexes, this effect of geometrical isomerism on anticancer activity has not been investigated systematically, mainly because the relevant isomers are still unavailable. An example of such an octahedral complex is trans-[RuCl(Hind)], which is in clinical trials now as its indazolium (KP1019) or sodium salt (NKP1339), but the corresponding cis-isomers remain inaccessible. We report the synthesis of Na[cis-OsCl(κN2-1H-ind)]·(Na[1]) suggesting a route to the cis-isomer of NKP1339. The procedure involves heating (Hind)[OsCl(κN1-2H-ind)] in a high boiling point organic solvent resulting in an Anderson rearrangement with the formation of cis-[OsCl(κN2-1H-ind)] ([1]) in high yield. The transformation is accompanied by an indazole coordination mode switch from κN1 to κN2 and stabilization of the 1H-indazole tautomer. Fully reversible spectroelectrochemical reduction of [1] in acetonitrile at 0.46 V vs. NHE is accompanied by a change in electronic absorption bands indicating the formation of cis-[OsCl(κN2-1H-ind)] ([1]). Chemical reduction of [1] in methanol with NaBH followed by addition of nBuNCl afforded the osmium(iii) complex nBuN[cis-OsCl(κN2-1H-ind)] (nBuN[1]). A metathesis reaction of nBuN[1] with an ion exchange resin led to the isolation of the water-soluble salt Na[1]. The X-ray diffraction crystal structure of [1]·MeCO was determined and compared with that of trans-[OsCl(κN2-1H-ind)]·2MeSO (2·2MeSO), also prepared in this work. EPR spectroscopy was performed on the Os complexes and the results were analyzed by ligand-field and quantum chemical theories. We furthermore assayed effects of [1] and Na[1] on cell viability and proliferation in comparison with trans-[OsCl(κN1-2H-ind)] [3] and cisplatin and found a strong reduction of cell viability at concentrations between 30 and 300 μM in different cancer cell lines (HT29, H446, 4T1 and HEK293). HT-29 cells are less sensitive to cisplatin than 4T1 cells, but more sensitive to [1] and Na[1], as shown by decreased proliferation and viability as well as an increased late apoptotic/necrotic cell population.
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http://dx.doi.org/10.1039/c7dt02194aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5605806PMC
September 2017

A five-coordinate manganese(iii) complex of a salen type ligand with a positive axial anisotropy parameter D.

Dalton Trans 2017 Sep;46(35):11817-11829

Inorganic Polymers Department, "Petru Poni" Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A, Iasi 700487, Romania.

A new high-spin d roughly trigonal-bipyramidal (TBP) manganese(iii) complex with a salen type ligand (HL), namely MnL(NCS)·0.4HO, has been synthesised and characterised by elemental analysis, ESI mass spectrometry, IR and UV-vis spectroscopy, and spectroelectrochemistry. X-ray diffraction analysis revealed an axial compression of the approximate TBP. Temperature dependent magnetic susceptibility and variable-temperature variable-field (VTVH) magnetisation measurements, as well as high-frequency and -field EPR (HFEPR) spectroscopy, were used to accurately describe the magnetic properties of this complex and, in particular, determine the spin Hamiltonian parameters: g-values and the zero-field splitting (ZFS) parameters D and E. The HFEPR spectra allowed the extraction of fourth order ZFS parameters. Quantum chemical calculations reproduced well the electronic and geometric structures of this unusual complex and, in particular, its electronic absorption spectrum along with the spin Hamiltonian parameters.
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http://dx.doi.org/10.1039/c7dt01809fDOI Listing
September 2017

Copper(ii) thiosemicarbazone complexes induce marked ROS accumulation and promote nrf2-mediated antioxidant response in highly resistant breast cancer cells.

Dalton Trans 2017 Mar;46(12):3833-3847

Institute of Inorganic Chemistry of the University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria.

A series of water-soluble sodium salts of 3-formyl-4-hydroxybenzenesulfonic acid thiosemicarbazones (or sodium 5-sulfonate-salicylaldehyde thiosemicarbazones) containing different substituents at the terminal nitrogen atom (H, Me, Et, Ph) and their copper(ii) complexes have been prepared and characterised by elemental analysis, spectroscopic techniques (IR, UV-vis, H NMR), ESI mass spectrometry, X-ray crystallography and cyclic voltammetry. The proligands and their copper(ii) complexes exhibit moderate water solubility and good stability in aqueous environment, determined by investigating their proton dissociation and complex formation equilibria. The copper(ii) complexes showed moderate anticancer activity in established human cancer cell lines, while the proligands were devoid of cytotoxicity. The anticancer activity of the copper(ii) complexes correlates with their ability to induce ROS accumulation in cells, consistent with their redox potentials within the biological window, triggering the activation of antioxidation defense mechanisms in response to the ROS insult. These studies pave the way for the investigation of ROS-inducing copper(ii) complexes as prospective antiproliferative agents in cancer chemotherapy.
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http://dx.doi.org/10.1039/c7dt00283aDOI Listing
March 2017

Protonation and electronic structure of 2,6-dichlorophenolindophenolate during reduction. A theoretical study including explicit solvent.

J Mol Model 2016 Oct 29;22(10):251. Epub 2016 Sep 29.

Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, SK-812 37, Bratislava, Slovak Republic.

Protonation in the two-electron/two-proton reduction processes of 2,6-dichlorophenolindophenolate (DCIP) is investigated combining density functional theory (DFT) and molecular dynamics (MD) methods. DCIP (anion), DCIP (radical anion), and DCIP (dianion) are considered, including the electronic structure analysis from the prospective of quantum theory of atoms and molecules (QTAIM). It is shown that oxygen on the indophenolate moiety and nitrogen are the first and/or the second proton acceptor sites and their energetic order depends on the total charge of the system. MD simulations of differently charged species interacting with the solvent molecules have been performed for methanol, water, and oxonium cation (HO). Methanol and water molecules are found to form only hydrogen bonds with the solute irrespective of its charge. The calculated pK values show that the imino group of DCIPH is a weaker acid than water. While in the case of DCIP (and DCIP) plus oxonium cation, proton transfer from the solvent to the solute was evidenced for both aforementioned acceptor sites. In addition, MD simulations of bulks containing 15 and 43 molecules of water around the DCIP molecule have been performed, revealing the formation of 2-4 hydrogen bonds. Graphical Abstract 2,6-Dichlorophenolindophenolate interacts with solvent molecules (water, oxonium cation and methanol). Hydrogen transfer and electronic structure are studied by DFT and molecular dynamics methods.
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http://dx.doi.org/10.1007/s00894-016-3109-7DOI Listing
October 2016

Vanadium(V) Complexes with Substituted 1,5-bis(2-hydroxybenzaldehyde)carbohydrazones and Their Use As Catalyst Precursors in Oxidation of Cyclohexane.

Inorg Chem 2016 Sep 26;55(18):9187-203. Epub 2016 Aug 26.

Institute of Inorganic Chemistry, University of Vienna , Währinger Strasse 42, A-1090 Vienna, Austria.

Six dinuclear vanadium(V) complexes have been synthesized: NH4[(VO2)2((H)LH)] (NH4[1]), NH4[(VO2)2((t-Bu)LH)] (NH4[2]), NH4[(VO2)2((Cl)LH)] (NH4[3]), [(VO2)(VO)((H)LH)(CH3O)] (4), [(VO2)(VO)((t-Bu)LH)(C2H5O)] (5), and [(VO2)(VO)((Cl)LH)(CH3O)(CH3OH/H2O)] (6) (where (H)LH4 = 1,5-bis(2-hydroxybenzaldehyde)carbohydrazone, (t-Bu)LH4 = 1,5-bis(3,5-di-tert-butyl-2-hydroxybenzaldehyde)carbohydrazone, and (Cl)LH4 = 1,5-bis(3,5-dichloro-2-hydroxybenzaldehyde)carbohydrazone). The structures of NH4[1] and 4-6 have been determined by X-ray diffraction (XRD) analysis. In all complexes, the triply deprotonated ligand accommodates two V ions, using two different binding sites ONN and ONO separated by a diazine unit -N-N-. In two pockets of NH4[1], two identical VO2(+) entities are present, whereas, in those of 4-6, two different VO2(+) and VO(3+) are bound. The highest oxidation state of V ions was corroborated by X-ray data, indicating the presence of alkoxido ligand bound to VO(3+) in 4-6, charge density measurements on 4, magnetic susceptibility, NMR spectroscopy, spectroelectrochemistry, and density functional theory (DFT) calculations. All four complexes characterized by XRD form dimeric associates in the solid state, which, however, do not remain intact in solution. Compounds NH4[1], NH4[2], and 4-6 were applied as alternative selective homogeneous catalysts for the industrially significant oxidation of cyclohexane to cyclohexanol and cyclohexanone. The peroxidative (with tert-butyl hydroperoxide, TBHP) oxidation of cyclohexane was performed under solvent-free and additive-free conditions and under low-power microwave (MW) irradiation. Cyclohexanol and cyclohexanone were the only products obtained (high selectivity), after 1.5 h of MW irradiation. Theoretical calculations suggest a key mechanistic role played by the carbohydrazone ligand, which can undergo reduction, instead of the metal itself, to form an active reduced form of the catalyst.
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http://dx.doi.org/10.1021/acs.inorgchem.6b01011DOI Listing
September 2016

Effect of venlafaxine on scavenging free radicals in vitro.

Neuro Endocrinol Lett 2016 ;37(1):59-64

Unlabelled: O BJECTIVE: Venlafaxine (VLF) was examined as a potential donor of H atom(s) to scavenge hydroxyl and peroxy-type radicals generated under aerobic conditions by catalytic oxidation of ascorbate with Cu²⁺ ions. Kinetics of the electron-donor property of VLF was investigated by standard ABTS and DPPH assays. Electron paramagnetic resonance measurements were applied to prove/disprove the VLF ability to scavenge superoxide anion radical.

Results: Results indicated that the drug venlafaxine was slightly capable of donating ·H, this way VLF scavenged the in situ generated hydroxyl radicals. Under the experimental conditions VLF was not able to inhibit/retard the propagation of the peroxy-type radicals. Regarding to the drug electron donating property, VLF did not show any ABTS·⁺ or DPPH· radical quenching property. Venlafaxine was not effective in scavenging O2·⁻.

Conclusion: Results of ABTS and DPPH assay showed a negligible redox activity of venlafaxine to both DPPH· and ABTS·⁺. Venlafaxine was not capable of scavenging the superoxide anion radical generated in KO₂/DMSO system, which indicates that VLF is not an efficient electron/proton donor molecule.
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July 2016

On the association of neutral and cationic tris(tetrathiafulvaleno)dodecadehydro[18]annulenes.

Org Biomol Chem 2016 Jan;14(2):425-429

Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.

Here, we report the first X-ray crystal structure of a tetrathiafulvalene-fused dehydroannulene with peripheral ethylthio substituents. In addition, we have subjected this compound to electrochemical and UV-Vis-NIR/ESR spectroelectrochemical studies to elucidate the degree to which the oxidised species associate.
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http://dx.doi.org/10.1039/c5ob02087eDOI Listing
January 2016

Interactions between tetrathiafulvalene units in dimeric structures - the influence of cyclic cores.

Beilstein J Org Chem 2015 2;11:930-48. Epub 2015 Jun 2.

Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.

A selection of cyclic and acyclic acetylenic scaffolds bearing two tetrathiafulvalene (TTF) units was prepared by different metal-catalyzed coupling reactions. The bridge separating the two TTF units was systematically changed from linearly conjugated ethyne, butadiyne and tetraethynylethene (trans-substituted) units to a cross-conjugated tetraethynylethene unit, placed in either acyclic or cyclic arrangements. The cyclic structures correspond to so-called radiaannulenes having both endo- and exocyclic double bonds. Interactions between two redox-active TTF units in these molecules were investigated by cyclic voltammetry, UV-vis-NIR and EPR absorption spectroscopical methods of the electrochemically generated oxidized species. The electron-accepting properties of the acetylenic cores were also investigated electrochemically.
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http://dx.doi.org/10.3762/bjoc.11.104DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4464444PMC
June 2015

Charge and Spin States in Schiff Base Metal Complexes with a Disiloxane Unit Exhibiting a Strong Noninnocent Ligand Character: Synthesis, Structure, Spectroelectrochemistry, and Theoretical Calculations.

Inorg Chem 2015 Jun 1;54(12):5691-706. Epub 2015 Jun 1.

⊥Faculty of Chemistry, Institute of Inorganic Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria.

Mononuclear nickel(II), copper(II), and manganese(III) complexes with a noninnocent tetradentate Schiff base ligand containing a disiloxane unit were prepared in situ by reaction of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with 1,3-bis(3-aminopropyl)tetramethyldisiloxane followed by addition of the appropriate metal(II) salt. The ligand H2L resulting from these reactions is a 2:1 condensation product of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with 1,3-bis(3-aminopropyl)tetramethyldisiloxane. The resulting metal complexes, NiL·0.5CH2Cl2, CuL·1.5H2O, and MnL(OAc)·0.15H2O, were characterized by elemental analysis, spectroscopic methods (IR, UV-vis, X-band EPR, HFEPR, (1)H NMR), ESI mass spectrometry, and single crystal X-ray diffraction. Taking into account the well-known strong stabilizing effects of tert-butyl groups in positions 3 and 5 of the aromatic ring on phenoxyl radicals, we studied the one-electron and two-electron oxidation of the compounds using both experimental (chiefly spectroelectrochemistry) and computational (DFT) techniques. The calculated spin-density distribution and localized orbitals analysis revealed the oxidation locus and the effect of the electrochemical electron transfer on the molecular structure of the complexes, while time-dependent DFT calculations helped to explain the absorption spectra of the electrochemically generated species. Hyperfine coupling constants, g-tensors, and zero-field splitting parameters have been calculated at the DFT level of theory. Finally, the CASSCF approach has been employed to theoretically explore the zero-field splitting of the S = 2 MnL(OAc) complex for comparison purposes with the DFT and experimental HFEPR results. It is found that the D parameter sign strongly depends on the metal coordination geometry.
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http://dx.doi.org/10.1021/acs.inorgchem.5b00229DOI Listing
June 2015

Osmium(III) analogues of KP1019: electrochemical and chemical synthesis, spectroscopic characterization, X-ray crystallography, hydrolytic stability, and antiproliferative activity.

Inorg Chem 2014 Oct 7;53(20):11130-9. Epub 2014 Oct 7.

Faculty of Chemistry, Institute of Inorganic Chemistry, University of Vienna , Währinger Strasse 42, A-1090 Vienna, Austria.

A one-electron reduction of osmium(IV) complexes trans-[Os(IV)Cl4(Hazole)2], where Hazole = 1H-pyrazole ([1](0)), 2H-indazole ([2](0)), 1H-imidazole ([3](0)), and 1H-benzimidazole ([4](0)), afforded a series of eight new complexes as osmium analogues of KP1019, a lead anticancer drug in clinical trials, with the general formula (cation)[trans-Os(III)Cl4(Hazole)2], where cation = H2pz(+) (H2pz[1]), H2ind(+) (H2ind[2]), H2im(+) (H2im[3]), Ph4P(+) (Ph4P[3]), nBu4N(+) (nBu4N[3]), H2bzim(+) (H2bzim[4]), Ph4P(+) (Ph4P[4]), and nBu4N(+) (nBu4N[4]). All complexes were characterized by elemental analysis, (1)H NMR spectroscopy, electrospray ionization mass spectrometry, UV-vis spectroscopy, cyclic voltammetry, while H2pz[1], H2ind[2], and nBu4[3], in addition, by X-ray diffraction. The reduced species [1](-) and [4](-) are stable in aqueous media in the absence of air oxygen and do not react with small biomolecules such as amino acids and the nucleotide 5'-dGMP. Cell culture experiments in five different human cancer cell lines (HeLa, A549, FemX, MDA-MB-453, and LS-174) and one noncancerous cell line (MRC-5) were performed, and the results were discussed and compared to those for KP1019 and cisplatin. Benzannulation in complexes with similar structure enhances antitumor activity by several orders of magnitude, implicating different mechanisms of action of the tested compounds. In particular, complexes H2ind[2] and H2bzim[4] exhibited significant antiproliferative activity in vitro when compared to H2pz[1] and H2im[3].
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http://dx.doi.org/10.1021/ic501710kDOI Listing
October 2014

Effect of the Schiff base complex diaqua-(N-salicylidene-l-glutamato)copper(II) monohydrate on human tumor cells.

Eur J Pharmacol 2013 Dec 7;721(1-3):178-84. Epub 2013 Oct 7.

Institute of Medical Chemistry, Biochemistry and Clinical Biochemistry, Faculty of Medicine, Comenius University, Sasinkova 2, 813 72 Bratislava, Slovak Republic.

The aim of our study was to estimate cytostatic/cytotoxic activity of the copper(II) Schiff base complex of the composition [Cu(N-salicylidene-l-glutamato)(H2O)2]·H2O, further Cu(SG-L)H2O, against human colon carcinoma cell line HT-29, as well as to determine type of cell death and to find out the molecular mechanism of apoptosis induced by this complex. Two highest concentrations (50, 100 µmol/l) of the complex showed a strong cytotoxic activity against human colon carcinoma cells HT-29 after 72 h of influence. Other concentrations had a cytostatic activity. Unchelated copper(II) ions and free ligands had no effect on the cell growth. Cu(SG-L)H2O preferentially reduced cancer cell viability compared to healthy cells (NIH-3T3). Cu(SG-L)H2O induced apoptosis of cells HT-29 at all concentrations used (1-100 µmol/l) after 48 h of influence. Apoptosis was carried out by the mitochondrial pathway with active caspases 3 and 9. By the spin-trapping technique combined with electron paramagnetic resonance we found that our complex is photochemically stable in aqueous systems and does not exhibit radical-scavenging activity when 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) cation radical was used as an oxidant. The complex exhibits a strong prooxidant property in the initial stages of thermal decomposition of K2S2O8 in water solutions leading to the massive production of (·)OH radicals. Therefore, this complex could strongly participate in anticancer action via a free radical mechanism.
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http://dx.doi.org/10.1016/j.ejphar.2013.09.038DOI Listing
December 2013

A highly cytotoxic modified paullone ligand bearing a TEMPO free-radical unit and its copper(II) complex as potential hR2 RNR inhibitors.

Chem Commun (Camb) 2013 Nov;49(85):10007-9

Institute of Inorganic Chemistry of the University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria.

A new paullone-TEMPO conjugate and its copper(II) complex inhibit RNR activity and show high antiproliferative activity in human cancer cell lines.
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http://dx.doi.org/10.1039/c3cc45743eDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4047831PMC
November 2013

Synthesis and optical properties of various thienyl derivatives of pyrene.

J Fluoresc 2014 Jan 6;24(1):153-60. Epub 2013 Aug 6.

Institute of Organic and Macromolecular Chemistry, Friedrich-Schiller University Jena, Humboldstraße 10, 07743, Jena, Germany,

A series of various thienyl derivatives of pyrene were synthesized by Stille cross-coupling procedure. Their structures were characterized by (1)H NMR, (13)C NMR and elemental analysis. The spectroscopic characteristics were investigated by UV-vis absorption and fluorescence spectra. Based on quantum chemical calculations, the energy levels of investigated molecules with respect to the pyrene molecule were also discussed.
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http://dx.doi.org/10.1007/s10895-013-1281-zDOI Listing
January 2014

Combined spectroelectrochemical and theoretical study of electron-rich dendritic 2,5-diaminothiophene derivatives: N,N,N',N'-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine.

J Phys Chem A 2013 Aug 22;117(30):6702-11. Epub 2013 Jul 22.

Center of Spectroelectrochemistry, Department of Electrochemistry and Conducting Polymers, Leibniz Institute for Solid State and Materials Research , D-01069 Dresden, Germany.

The in situ spectroelectrochemical and electron spin resonance (ESR) behavior of the recently prepared N,N,N',N'-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine 11 is presented. The results are compared to the ones of the parent 2,5-bis-diphenylamino-thiophene 41 as well as to the corresponding high-molar third dendrimer generation 8 containing the same thiophene-2,5-diamine core. The dendritic compound 11 can be reversibly oxidized in three separated steps to yield the corresponding stable monocation 11(•+), dication 11(2+), and tetracation 11(4+). A well resolved ESR spectrum of the corresponding cation radical 11(•+) with dominating splittings from two nitrogen atoms and two hydrogen atoms was observed at the first oxidation peak similar to 41(•+). The shape of the SOMOs orbitals very well correlates with the proposed distribution of the unpaired electron mainly on the thiophene center and neighboring nitrogen atoms. The spin delocalization on the central thiophene moiety in the monocations for all three model compounds 41(•+), 11(•+), and 8(•+) was confirmed. The computed single occupied molecular orbital (SOMO) for trication 11(•3+) is completely different compared to the SOMO of the corresponding monocation 11(•+), and it confirms a largely delocalized unpaired spin density. Dominating diamagnetic product was determined at the third oxidation peak, confirming the formation of a tetracation by a two electron oxidation of ESR silent dication. The positive charge is fully delocalized over the lateral parts of the molecule leading to the high stability of tetracation 11(4+). The estimated theoretical limit energy of the lowest optical transition S0 → S1 is 2.90 eV, and it can be achieved for the 3D dendrimer generation.
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http://dx.doi.org/10.1021/jp405212eDOI Listing
August 2013

Marked stabilization of redox states and enhanced catalytic activity in galactose oxidase models based on transition metal S-methylisothiosemicarbazonates with -SR group in ortho position to the phenolic oxygen.

Inorg Chem 2013 Jul 12;52(13):7524-40. Epub 2013 Jun 12.

Institute of Inorganic Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria.

Reactions of 5-tert-butyl-2-hydroxy-3-methylsulfanylbenzaldehyde S-methylisothiosemicarbazone and 5-tert-butyl-2-hydroxy-3-phenylsulfanylbenzaldehyde S-methylisothiosemicarbazone with pentane-2,4-dione (Hacac) and triethyl orthoformate in the presence of M(acac)2 as template source at 107 °C afforded metal complexes of the type M(II)L(1) and M(II)L(2), where M = Ni and Cu, with a new Schiff base ligand with thiomethyl (H2L(1)) and/or thiophenyl (H2L(2)) group in the ortho position of the phenolic moiety. Demetalation of NiL(1) in CHCl3 with HCl(g) afforded H2L(1). The latter reacts with Zn(OAc)2·2H2O with formation of ZnL(1). The effect of -SR groups and metal ion identity on stabilization of phenoxyl radicals generated electrochemically was studied in detail. A marked stabilization of phenoxyl radical was observed in one-electron-oxidized complexes [ML(2)](+) (M = Ni, Cu) at room temperature, as demonstrated by cyclic voltammetry, EPR spectroscopy, and UV-vis-NIR measurements. In solution, the oxidized CuL(2) and NiL(2) display intense low-energy NIR transitions consistent with their classification as metal-delocalized phenoxyl radical species. While the CuL(2) complex shows reversible reduction, reduction of NiL(2), CuL(1), and NiL(1) is irreversible. EPR measurements in conjunction with density functional theory calculations provided insights into the extent of electron delocalization as well as spin density in different redox states. The experimental room temperature spectroelectrochemical data can be reliably interpreted with the (3)[CuL(2)](+) and (2)[NiL(2)](+) oxidation ground states. The catalytic activity of synthesized complexes in the selective oxidations of alcohols has been studied as well. The remarkable efficiency is evident from the high yields of carbonyl products when employing both the CuL(2)/air/TEMPO and the CuL(2)/TBHP/MW(microwave-assisted) oxidation systems.
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http://dx.doi.org/10.1021/ic4004966DOI Listing
July 2013

Molecular structure, UV/vis spectra, and cyclic voltammograms of Mn(II), Co(II), and Zn(II) 5,10,15,20-tetraphenyl-21-oxaporphyrins.

Inorg Chem 2013 Feb 14;52(3):1515-24. Epub 2013 Jan 14.

Department of Chemistry and Food Chemistry, TU Dresden, 01062 Dresden, Germany.

The 5,10,15,20-tetraphenyl-21-oxaporphyrin complexes of Mn(II), Co(II), and Zn(II) have been crystallized and studied by X-ray diffraction, NMR and UV/vis spectroscopy, and mass spectrometry as well as cyclic voltammetry. The X-ray structure of the earlier described Cu(II) complex is also reported. All complex structures possess a five-coordinate, approximately square-pyramidal geometry with a slight deviation of the heteroaromatic moieties from planarity. The packing structures are characterized by parallel strands of complex molecules interacting by weak hydrogen bonds. In the case of Zn(II) an octahedral complex has also been isolated using a side-chain hydroxy functionalized oxaporphyrin ligand; the structure was verified by NMR and EXAFS spectroscopy. Cyclic voltammetry studies reveal that the reduction of the complex bound Mn(II), Co(II), and Zn(II) ions is a ligand-centered process whereas the first oxidation step depends on the metal ion present.
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http://dx.doi.org/10.1021/ic302268hDOI Listing
February 2013

Stable radical trianions from reversibly formed sigma-dimers of selenadiazoloquinolones studied by in situ EPR/UV-vis spectroelectrochemistry and quantum chemical calculations.

J Phys Chem A 2012 Oct 27;116(40):9919-27. Epub 2012 Sep 27.

Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovak Republic.

The redox behavior of the series of 7-substituted 6-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-h]quinolines and 8-substituted 9-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-f]quinolines with R(7), R(8) = H, COOC(2)H(5), COOCH(3), COOH, COCH(3), and CN has been studied by in situ EPR and EPR/UV-vis spectroelectrochemistry in dimethylsulfoxide. All selenadiazoloquinolones undergo a one-electron reduction process to form the corresponding radical anions. Their stability strongly depends on substitution at the nitrogen atom of the 4-pyridone ring. The primary generated radical anions from N-ethyl-substituted quinolones are stable, whereas for the quinolones with imino hydrogen, the initial radical anions rapidly dimerize to produce unusually stable sigma-dimer (σ-dimer) dianions. These are reversibly oxidized to the initial compounds at potentials considerably less negative than the original reduction process in the back voltammetric scan. The dimer dianion can be further reduced to the stable paramagnetic dimer radical trianion in the region of the second reversible reduction step. The proposed complex reaction mechanism was confirmed by in situ EPR/UV-vis cyclovoltammetric experiments. The site of the dimerization in the σ-dimer and the mapping of the unpaired spin density both for radical anions and σ-dimer radical trianions with unusual unpaired spin distribution have been assigned by means of density functional theory calculations.
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http://dx.doi.org/10.1021/jp307270bDOI Listing
October 2012