A Series of Non-Oxido VIV Complexes of Dibasic ONS Donor Ligands: Solution Stability, Chemical Transformations, Protein Interactions, and Antiproliferative Activity.

A series of mononuclear non-oxido vanadium(IV) complexes, [VIV(L1-4)2] (1-4), featuring tridentate bi-negative ONS chelating S-alkyl/aryl-substituted dithiocarbazate ligands H2L1-4, are reported. All the synthesized non-oxido VIV compounds are characterized by elemental analysis, spectroscopy (IR, UV-vis, and EPR), ESI-MS, as well as electrochemical techniques (cyclic voltammetry). Single-crystal X-ray diffraction studies of 1-3 reveal that the mononuclear non-oxido VIV complexes show distorted octahedral (1 and 2) or trigonal prismatic (3) arrangement around the non-oxido VIV center. EPR and DFT data indicate the coexistence of mer and fac isomers in solution, and ESI-MS results suggest a partial oxidation of [VIV(L1-4)2] to [VV(L1-4)2]+ and [VVO2(L1-4)]-; therefore, all these three complexes are plausible active species. Complexes 1-4 interact with bovine serum albumin (BSA) with a moderate binding affinity, and docking calculations reveal non-covalent interactions with different regions of BSA, particularly with Tyr, Lys, Arg, and Thr residues. In vitro cytotoxic activity of all complexes is assayed against the HT-29 (colon cancer) and HeLa (cervical cancer) cells and compared with the NIH-3T3 (mouse embryonic fibroblast) normal cell line by MTT assay and DAPI staining. The results suggest that complexes 1-4 are cytotoxic in nature and induce cell death in the cancer cell lines by apoptosis and that a mixture of VIV, VV, and VVO2 species could be responsible for the biological activity.

Tuning the Biological Activity of Camphorimine Complexes through Metal Selection.

The cytotoxic activity of four sets of camphorimine complexes based on the Cu(I), Cu(II), Ag(I), and Au(I) metal sites were assessed against the cisplatin-sensitive A2780 and OVCAR3 ovarian cancer cells. The results showed that the gold complexes were ca. one order of magnitude more active than the silver complexes, which in turn were ca. one order of magnitude more active than the copper complexes. An important finding was that the cytotoxic activity of the Ag(I) and Au(I) camphorimine complexes was higher than that of cisplatin. Another relevant aspect was that the camphorimine complexes did not interact significantly with DNA, in contrast with cisplatin. The cytotoxic activity of the camphorimine complexes displayed a direct relationship with the cellular uptake by OVCAR3 cells, as ascertained by PIXE (particle-induced X-ray emission). The levels of ROS (reactive oxygen species) formation exhibited an inverse relationship with the reduction potentials for the complexes with the same metal, as assessed by cyclic voltammetry. In order to gain insight into the toxicity of the complexes, their cytotoxicity toward nontumoral cells (HDF and V79 fibroblasts) was evaluated. The in vivo cytotoxicity of complex 5 using the nematode Caenorhabditis elegans was also assessed. The silver camphorimine complexes displayed the highest selectivity coefficients (activity vs. toxicity).

Synthesis and Biological Activity of Antimicrobial Agents.

New antimicrobial agents are urgent and necessary to overcome the acquired resistance of microorganisms to existing antibiotics and antifungals [...].

Synthesis and Characterization of Camphorimine Au(I) Complexes with a Remarkably High Antibacterial Activity towards B. contaminans and P. aeruginosa.

Fourteen new camphorimine Au(I) complexes were synthesized and characterized by spectroscopic (NMR, FTIR) and elemental analysis. The structural arrangement of three selected examples were computed by Density Functional Theory (DFT) showing that the complexes essentially keep the {AuI-CN} unit. The Minimum Inhibition Concentrations (MIC) were assessed for all complexes showing that they are active towards the Gram-negative strains E. coli ATCC25922, P. aeruginosa 477, and B. contaminans IST408 and the Gram-positive strain S. aureus Newman. The complexes display very high activity towards P. aeruginosa 477 and B. contaminans IST408 with selectivity towards B. contaminans. An inverse correlation between the MIC values and the gold content was found for B. contaminans and P. aeruginosa. However, plots of MIC values and Au content for P. aeruginosa 477 and B. contaminans IST408 follow distinct trends. No clear relationship could be established between the MIC values and the redox potentials of the complexes measured by cyclic voltammetry. The MIC values are essentially independent of the redox potentials either cathodic or anodic. The complexes K3[{Au(CN)2}3(A4L)] (8, Y = m-OHC6H4) and K3[{Au(CN)2}3(B2L)]·3H2O (14, Z = p-C6H4) display the lower MIC values for the two strains. In normal fibroblast cells, the IC50 values for the complexes are ca. one order of magnitude lower than their MIC values, although higher than that of the precursor KAu(CN)2.

Bioactive Coatings with Ag-Camphorimine Complexes to Prevent Surface Colonization by the Pathogenic Yeast Candida albicans.

Currently there is a gap between the rate of new antifungal development and the emergence of resistance among Candida clinical strains, particularly threatened by the extreme adhesiveness of C. albicans to indwelling medical devices. Two silver camphorimine complexes, [Ag(OH){OC10H14N(C6H4)2NC10H14O}] (compound P) and [{Ag(OC10H14NC6H4CH3-p)}2(µ-O)] (compound Q), are herein demonstrated as having high inhibiting activity towards the growth of Candida albicans and Candida glabrata clinical strains resistant to azoles, the frontline antifungals used in clinical practice. Compounds P and Q were also explored as bioactive coatings to prevent colonization by C. albicans and colonize the surface of indwelling medical devices, resulting in persistent infections. Functionalization of stainless steel with polycaprolactone (PCL) matrix embedded with compounds P or Q was reported for the first time to inhibit the colonization of C. albicans by 82% and 75%, respectively. The coating of PCL loaded with Q or P did not cause cytotoxic effects in mammalian cells, demonstrating the biocompatibility of the explored approach. The identification and further exploration of new approaches for surface engineering based on new molecules that can sensitize resistant strains, as herein demonstrated for complexes P and Q, is a significant step forward to improve the successful treatment of candidiasis.

Key Parameters on the Antibacterial Activity of Silver Camphor Complexes.

Nine new complexes with camphor imine or camphor sulfonimine ligands were synthesized and analytically and spectroscopically characterized, aiming to identify the key parameters that drive the antibacterial activity of the complexes with metal cores and imine substituents with distinct electronic and steric characteristics. The antimicrobial activity of all complexes was evaluated by determining their minimum inhibitory concentrations (MIC) against the Gram-negative Escherichia coli ATCC25922, Pseudomonas aeruginosa 477, and Burkholderia contaminans IST408, and the Gram-positive Staphylococcus aureus Newman. Camphor imine complexes based on the hydroxyl silver center ({Ag(OH)}) typically perform better than those based on the nitrate silver center ({Ag(NO3)}), while ligands prone to establish hydrogen bonding facilitate interactions with the bacterial cell surface structures. A different trend is observed for the silver camphor sulfonimine complexes that are almost non-sensitive to the nature of the metal cores {Ag(OH)} or {Ag(NO3)} and display low sensitivity to the Y substituent. The antibacterial activities of the Ag(I) camphor sulfonimine complexes are higher than those of the camphor imine analogues. All the complexes display higher activity towards Gram-negative strains than towards the Gram-positive strain.

New VIV, VIVO, VVO, and VVO2 Systems: Exploring their Interconversion in Solution, Protein Interactions, and Cytotoxicity.

The synthesis and characterization of one oxidoethoxidovanadium(V) [VVO(L1)(OEt)] (1) and two nonoxidovanadium(IV) complexes, [VIV(L2-3)2] (2 and 3), with aroylhydrazone ligands incorporating naphthalene moieties, are reported. The synthesized oxido and nonoxido vanadium complexes are characterized by various physicochemical techniques, and their molecular structures are solved by single crystal X-ray diffraction (SC-XRD). This revealed that in 1 the geometry around the vanadium atom corresponds to a distorted square pyramid, with a O4N coordination sphere, whereas that of the two nonoxido VIV complexes 2 and 3 corresponds to a distorted trigonal prismatic arrangement with a O4N2 coordination sphere around each "bare" vanadium center. In aqueous solution, the VVO moiety of 1 undergoes a change to VVO2 species, yielding [VVO2(L1)]- (1'), while the nonoxido VIV-compounds 2 and 3 are partly converted into their corresponding VIVO complexes, [VIVO(L2-3)(H2O)] (2' and 3'). Interaction of these VVO2, VIVO, and VIV systems with two model proteins, ubiquitin (Ub) and lysozyme (Lyz), is investigated through docking approaches, which suggest the potential binding sites: the interaction is covalent for species 2' and 3', with the binding to Glu16, Glu18, and Asp21 for Ub, and His15 for Lyz, and it is noncovalent for species 1', 2, and 3, with the surface residues of the proteins. The ligand precursors and complexes are also evaluated for their in vitro antiproliferative activity against ovarian (A2780) and prostate (PC3) human cancer cells and in normal fibroblasts (V79) to check the selectivity of the compounds for cancer cells.

Trinuclear vanadium(iv) and vanadium(v) complexes derived from 2,4,6-triacetylphloroglucinol and study of their peroxidase mimicking activity.

Novel dibasic Schiff bases with three tridentate sites were obtained from the condensation of the triketone 2,4,6-triacetylphloroglucinol (H3ptk) with four different hydrazides, benzoyl hydrazide (bhz), furoyl hydrazide (fah), isonicotinoyl hydrazide (inh) and nicotinoyl hydrazide (nah): H6ptk(bhz)3I, H6ptk(fah)3II, H6ptk(inh)3III and H6ptk(nah)3IV. These ligand precursors I-IV, each being an ONO donor, are tricompartmental building blocks able to form trinuclear complexes having C3 symmetry. The reaction of I-IV with [VIVO(acac)2] leads to the formation of [{VIVO(H2O)}3(ptk(bhz)3)] 1, [{VIVO(H2O)}3(ptk(fah)3)] 2, [{VIVO(H2O)}3(ptk(inh)3)] 3, and [{VIVO(H2O)}3(ptk(nah)3)] 4. In methanol/aqueous solutions of M2CO3 (M+ = Na+, K+ and Cs+), these complexes are slowly converted into dioxidovanadium(v) compounds, namely, M3[(VVO2)3{ptk(bhz)3}]·6H2O [M+ = K+5, Na+9, Cs+13], M3[(VVO2)3{ptk(fah)3}]·6H2O [M+ = K+6, Na+10, Cs+14], M3[(VVO2)3{ptk(inh)3}]·6H2O [M+ = K+7, Na+11, Cs+15] and M3[(VVO2)3{ptk(nah)3}]·6H2O [M+ = K+8, Na+12, Cs+16]. All ligand precursors and complexes are characterized by various techniques such as FT-IR, UV/Visible, EPR, NMR (1H, 13C and 51V), elemental analysis, thermal studies, cyclic voltammetry (CV) and single-crystal X-ray analysis. X-ray diffraction studies of complexes K2.7[{(VVO2)3ptk(fah)3}]·11.5H2O·MeOH 6a, Cs3[{(VVO2)3ptk(bhz)3}]·7H2O 13a and Cs3[{(VVO2)3ptk(nah)3}]·7.3H2O 16a reveal their distorted square pyramidal geometry by coordinating through phenolate oxygen (of ptk), azomethine nitrogen and enolate oxygen (of hydrazide) atoms. The reactivity of complexes 5-16 and their catalytic potential were screened towards their peroxidase mimetic activity in the oxidation of dopamine to aminochrome driven by H2O2 as an oxidant. The conversion of dopamine to aminochrome with different catalysts was monitored by HPLC showing high activity under mild conditions with good conversions within 1 h. Kinetic studies using compounds 13-16 as catalyst precursors reveal that the reaction follows a Michaelis-Menten-like kinetics.

Ag(I) camphor complexes: antimicrobial activity by design.

Eleven new complexes of general formula [Ag(NO3)(L-Y)2] corresponding to Ag(I) camphorimine complexes [Ag(NO3)(OC10H14NY)2] (Y=NMe2 (1); OH (2); C6H5 (3); 4-MeC6H4, (4); 3,5-(CH3)2C6H3 (5); 3-OHC6H4, (6); 3-ClC6H4 (7); 4-ClC6H4 (8); 4-FC6H4 (9); 4-CF3C6H4 (10)) and the camphor sulfonylimine complex [Ag(NO3)(O2SNC10H14NY)2] (Y=NH2) were synthesized/characterized and their structural properties and antibacterial activity studied to gain insights into the structure-antimicrobial activity relationships. Five of the complexes were selected as representative examples and structures were optimized by Density Functional Theory calculations. The results show that the imine substituents (Y) at the camphor ligands drive the structure of the complexes from distorted octahedral to trigonal prismatic or linear ionic while the effect of the sulfonylimine ring does not appreciably affects the geometry of the complex. The lipophilicity and polarity which are important parameters concerning the biological activity of the complexes are also high dependent of the characteristics of the camphor ligands. The redox properties of the complexes studied by cyclic voltammetry showed that their reduction potentials are essentially independent of their electronic and steric properties. The antibacterial activity of all the complexes, against Gram-positive (S. aureus Newman) and Gram-negative (Escherichia coli ATCC25922, Pseudomonas aeruginosa 477, Burkholderia contaminans IST408) strains was evaluated through calculation of MIC values. Results show that complexes with camphor imine ligands (1-10) that combine high lipophilicity with low dipolar moment (3-5) exhibit enhanced antibacterial activity. The ability to establish hydrogen bonding emerged as an important contribution to the antibacterial activity of the camphor sulphonylimine complex 11 (Y=NH2).

Search for cytotoxic compounds against ovarian cancer cells: Synthesis, characterization and assessment of the activity of new camphor carboxylate and camphor carboxamide silver complexes.

Five silver camphor complexes of formulae [Ag2(L)(L')2] (1,3,5) or [Ag(L)2(L')] (2,4) were synthesized from silver nitrate and the suitable camphor carboxylate (L1) or camphor carboxamides (L3, L4). The complexes were characterized by elemental analysis and spectroscopic techniques (NMR, FTIR, XPS). Computational calculations support coordination of the carboxylate group to silver, in the case of complex 2 and combined mixed keto/carboxylate in the case of complex 1. The stability of the complexes highly relies on the tetrahedral geometry of the lithium ion that binds to four oxygen atoms of the camphor carboxylate ligands. The redox properties of complexes 1 and 4 studied by cyclic voltammetry confirm the facile reduction of the metal sites that depending on the experimental conditions may lead to formation of silver nanoparticles as confirmed by XPS and TEM. Complexes 1, 2 and 4 were tested for cytotoxic activities against A2780 (IC50, 11-14 µM) and A2780 cisplatin resistant (A2780cisR) (IC50, 4-7 µM) cells using the MTT assay. The result showed that the complexes have anticancer activity higher than cisplatin. Complex 1 was also probed for cytotoxicity against the non-tumoral human embryonic kidney (HEK 293, IC50, 62.2 ±â€¯16 µM) cells showing low toxicity in agreement with the silver camphor carboxylate complexes having a considerable selectivity for the ovarian cancer cells A2780 and cisplatin resistant A2780cisR which is a key point under pharmacological uses.

Synthesis of alkynyl-substituted camphor derivatives and their use in the preparation of paclitaxel-related compounds.

Compounds containing two alkyne groups in close vicinity at the rigid skeleton of camphorsulfonamide show unique reactivities when treated with electrophiles or catalytic amounts of platinum(II). The formed product structures depend not only on the reagents used but also on the substituents attached to the triple bonds. Cycloisomerisations with perfect atom economy lead to polycyclic heterocycles that resemble to some extent the AB ring system of paclitaxel. Herein, we present practical synthetic methods for the selective synthesis of precursor dialkynes bearing different substituents (alkyl, aryl) at the triple bonds, based on ketals or an imine as protecting groups. We show for isomeric dialkynes that the reaction cascade induced by Pt(II) includes ring annulation, sulphur reduction, and ring enlargement. One isomeric dialkyne additionally allows for the isolation of a pentacyclic compound lacking the ring enlargement step, which we have proposed as a potential intermediate in the catalytic cycle.

Ag(I) camphorimine complexes with antimicrobial activity towards clinically important bacteria and species of the Candida genus.

The present work follows a previous report describing the antibacterial activity of silver camphorimine complexes of general formula [Ag(NO3)L]. The synthesis and demonstration of the antifungal and antibacterial activity of three novel [Ag(NO3)L] complexes (named 1, 2 and 3) is herein demonstrated. This work also shows for the first time that the previously studied complexes (named 4 to 8) also exert antifungal activity. The antibacterial activity of complexes was evaluated against Staphylococcus aureus, Pseudomonas aeruginosa, Burkholderia contaminans and Escherichia coli strains, while antifungal activity was tested against the Candida species C. albicans, C. glabrata, C. parapsilosis and C. tropicalis. The antimicrobial activity of the complexes ranged from very high (complex 4) to moderate (complex 6) or low (complex 8), depending on the structural and electronic characteristics of the camphorimine ligands. Notably, the highest antibacterial and anti-Candida activities do not coincide in the same complex and in some cases they were even opposite, as is the case of complex 4 which exhibits a high anti-bacterial and low antifungal activity. These distinct results suggest that the complexes may have different mechanisms against prokaryotic and eukaryotic cells. The antifungal activity of the Ag(I) camphorimine complexes (in particular of complex 1) was found to be very high (MIC = 2 µg/mL) against C. parapsilosis, being also registered a prominent activity against C. tropicalis and C. glabrata. None of the tested compounds inhibited C. albicans growth, being this attributed to the ability of these yeast cells to mediate the formation of less toxic Ag nanoparticles, as confirmed by Scanning Electron Microscopy images. The high antibacterial and anti-Candida activities of the here studied camphorimine complexes, especially of complexes 1 and 7, suggests a potential therapeutic application for these compounds.

Synthesis of Ag(I) camphor sulphonylimine complexes and assessment of their cytotoxic properties against cisplatin-resistant A2780cisR and A2780 cell lines.

Camphorsulphonylimine complexes [Ag(NO3)(IL)2] (IL=C12H19N3SO2, 1) and [(AgNO3)2(IIL)] (IIL=C22H23N3SO2, 2) were synthesized and characterized by elemental analysis, spectroscopy (IR, NMR) and cyclic voltammetry. [Ag(NO3)(IL)2] crystalizes in the monoclinic C2 space group with a triangular geometry assuming a chalice-type shape. The anti-proliferative properties of the new complexes 1 and 2 and those of the previously reported [Ag(NO3)(IIIL)] (IIIL=C16H18N3SO2, 3) were assessed against the human ovarian cancer cells (cisplatin-sensitive A2780, cisplatin-resistant A2780cisR) and the non-tumoral human HEK 293 cell line, using the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay. The NR (3-amino-7-dimethylamino-2-methylphenazine hydrochloride) assay was alternatively used to assess the cytotoxicity on the A2780 cells. Results from the MTT assay (48h exposure) show that the complexes display IC50 values lower (by at least one order of magnitude) than cisplatin, while the cytotoxicity of AgNO3 is of the same order of cisplatin. The camphorsulphonylimine ligands display irrelevant (IL, IIIL) or no cytotoxicity (IIL). The highest cytotoxicity (lower IC50) was found for [(AgNO3)2(IIL)]. The binding ability of the complexes to calf thymus-deoxyribonucleic acid (CT-DNA) was studied by fluorescence. Constants (Ksv, Ka) and the number (n) of binding centres to DNA were calculated showing that DNA intercalation possibly occurs in the cases of complexes 2 and 3, while a more complicated process operates for 1. As expected from the cytotoxicity, [(AgNO3)2(IIL)] displays the highest binding affinity (Ka=1.61×105 M-1). No binding to DNA was detected for AgNO3 or IIL under the experimental conditions used. The binding trend to CT-DNA found by fluorescence was corroborated by cyclic voltammetry.

Versatile Reactivity and Theoretical Evaluation of Mono- and Dinuclear Oxidovanadium(V) Compounds of Aroylazines: Electrogeneration of Mixed-Valence Divanadium(IV,V) Complexes.

The substituted hydrazones H2L(1-4) (L(1-4) = dibasic tridentate ONO(2-) donor ligands) obtained by the condensation of 2-hydroxy-1-naphthaldehyde and 2-aminobenzoylhydrazine (H2hnal-abhz) (H2L(1)) , 2-hydroxy-1-naphthaldehyde and 2-hydroxybenzoylhydrazine (H2hnal-hbhz) (H2L(2)), 2-hydroxy-1-acetonaphthone and benzoylhydrazine (H2han-bhz) (H2L(3)), or 2-hydroxy-1-acetonaphthone and 2-aminobenzoylhydrazine (H2han-abhz) (H2L(4)) are prepared and characterized. Reaction of ammonium vanadate with the appropriate H2L(1-4) results in the formation of oxidoethoxidovanadium(V) [V(V)O(OEt)(L(1-4))] (1-4) complexes. All compounds are characterized in the solid state and in solution by spectroscopic techniques (IR, UV-vis, (1)H, (13)C, and (51)V NMR, and electrospray ionization mass spectrometry). Single-crystal X-ray diffraction analysis of 1, 3, and 4 confirms the coordination of the corresponding ligands in the dianionic (ONO(2-)) enolate tautomeric form. In solution, the structurally characterized [V(V)O(OEt)(L)] compounds transform into the monooxido-bridged divanadium(V,V) [(V(V)OL)2-µ-O] complexes, with the processes being studied by IR and (1)H, (13)C, and (51)V NMR. The density functional theory (DFT) calculated Gibbs free energy of reaction 2[V(V)O(OEt)(L(4))] + H2O ⇆ [(V(V)OL(4))2-µ-O] + 2EtOH is only 2-3 kcal mol(-1), indicating that the dinuclear complexes may form in a significant amount. The electrochemical behavior of the complexes is investigated by cyclic voltammetry, with the V(V)-V(IV) E1/2(red) values being in the range 0.27-0.44 V (vs SCE). Upon controlled potential electrolysis, the corresponding (L)(O)V(IV)-O-V(V)(O)(L) mixed-valence species are obtained upon partial reduction of the [(V(V)OL)2-µ-O] complexes formed in solution, and some spectroscopic characteristics of these dinuclear mixed-valence complexes are investigated using DFT calculations and by electron paramagnetic resonance (EPR), with the formation of V(IV)-O-V(V) species being confirmed by the observation of a 15-line pattern in the EPR spectra at room temperature.

Strecker degradation of amino acids promoted by a camphor-derived sulfonamide.

A camphor-derived sulfonimine with a conjugated carbonyl group, oxoimine 1 (O2SNC10H13O), reacts with amino acids (glycine, L-alanine, L-phenylalanine, L-leucine) to form a compound O2SNC10H13NC10H14NSO2 (2) which was characterized by spectroscopic means (MS and NMR) and supported by DFT calculations. The product, a single diastereoisomer, contains two oxoimine units connected by a -N= bridge, and thus has a structural analogy to the colored product Ruhemann´s purple obtained by the ninhydrin reaction with amino acids. A plausible reaction mechanism that involves zwitterions, a Strecker degradation of an intermediate imine and water-catalyzed tautomerizations was developed by means of DFT calculations on potential transition states.

Antibacterial activity of silver camphorimine coordination polymers.

Five new silver camphorimine complexes of general formula [Ag(NO3)(Y)L] were synthesized and fully characterized using spectroscopic and analytical techniques. The structure of [Ag(NO3)(OC10H14NC6H4NC10H14O)] () was analyzed using single crystal X-ray diffraction, showing that it arranges as a coordination polymer formed by sequential Ag(NO3) units bridged by the bi-camphor ligand (). The antimicrobial properties of the new complexes were screened using the disk diffusion method and their Minimal Inhibitory Concentrations (MIC) were assessed against selected bacterial strains of the Gram-positive Staphylococcus aureus and the Gram-negative Escherichia coli, Pseudomonas aeruginosa, and Burkholderia contaminans. The lowest MICs were observed for , with estimated values of 72, 20, 32 and 19 µg mL(-1) for S. aureus, E. coli, B. contaminans, and P. aeruginosa, respectively. In the case of S. aureus, similar MIC values were obtained for silver nitrate and compound . All five compounds were bactericidal when used in concentrations equal or above the MIC value, as found by enumerating the total colony forming units (CFUs) after incubation in their presence.

Chemistry of Monomeric and Dinuclear Non-Oxido Vanadium(IV) and Oxidovanadium(V) Aroylazine Complexes: Exploring Solution Behavior.

A series of mononuclear non-oxido vanadium(IV) [V(IV)(L(1-4))2] (1-4), oxidoethoxido vanadium(V) [V(V)O(L(1-4))(OEt)] (5-8), and dinuclear µ-oxidodioxidodivanadium(V) [V(V)2O3(L(1))2] (9) complexes with tridentate aroylazine ligands are reported [H2L(1) = 2-furoylazine of 2-hydroxy-1-acetonaphthone, H2L(2) = 2-thiophenoylazine of 2-hydroxy-1-acetonaphthone, H2L(3) = 1-naphthoylazine of 2-hydroxy-1-acetonaphthone, H2L(4) = 3-hydroxy-2-naphthoylazine of 2-hydroxy-1-acetonaphthone]. The complexes are characterized by elemental analysis, by various spectroscopic techniques, and by single-crystal X-ray diffraction (for 2, 3, 5, 6, 8, and 9). The non-oxido V(IV) complexes (1-4) are quite stable in open air as well as in solution, and DFT calculations allow predicting EPR and UV-vis spectra and the electronic structure. The solution behavior of the [V(V)O(L(1-4))(OEt)] compounds (5-8) is studied confirming the formation of at least two different types of V(V) species in solution, monomeric corresponding to 5-8, and µ-oxidodioxidodivanadium [V(V)2O3(L(1-4))2] compounds. The µ-oxidodioxidodivanadium compound [V(V)2O3(L(1))2] (9), generated from the corresponding mononuclear complex [V(V)O(L(1))(OEt)] (5), is characterized in solution and in the solid state. The single-crystal X-ray diffraction analyses of the non-oxido vanadium(IV) compounds (2 and 3) show a N2O4 binding set and a trigonal prismatic geometry, and those of the V(V)O complexes 5, 6, and 8 and the µ-oxidodioxidodivanadium(V) (9) reveal that the metal center is in a distorted square pyramidal geometry with O4N binding sets. For the µ-oxidodioxidodivanadium species in equilibrium with 5-8 in CH2Cl2, no mixed-valence complexes are detected by chronocoulometric and EPR studies. However, upon progressive transfer of two electrons, two distinct monomeric V(IV)O species are detected and characterized by EPR spectroscopy and DFT calculations.

Amino acid derived CuII compounds as catalysts for asymmetric oxidative coupling of 2-naphthol.

We report the synthesis and characterization of several novel aminopyridine - L-amino acid derived Cu(II)-complexes. The ligands are prepared by a one-pot reductive alkylation of the L-amino acid scaffold and the respective aminopyridine Cu(II)-complexes were obtained by reaction with CuCl2 or Cu(acetato)2. All compounds were characterized by spectroscopic techniques, as well as ESI-MS. Two of the Cu(II)-complexes were characterized by single-crystal X-ray diffraction, one of them, [Cu(II)(L)(CH3COO)] (HL = (S)-3-phenyl-2-(pyridin-2-ylmethylamino)propanoic acid), being the first ever reported aminopyridine-class Cu(II) complex bearing a tridentate N,N,O donor set and a monodentate acetato ligand. The complexes are tested as catalysts in the oxidative coupling of 2-naphthol in organic solvent-water mixtures using dioxygen as the terminal oxidant. The effect of variables such as ligand denticity and substituents, as well as solvent, temperature and oxidant intake, on the overall performance is studied. In general, moderate to low conversions of 2-naphthol to 1,1'-bi-2-naphthol (BINOL) are obtained. The catalysts also showed moderate to low enantioselectivity. Some aspects of the reaction mechanism were elucidated by spectroscopy, electrochemical and theoretical studies. It was found that basic additives are important for activity, but these also increase the formation of secondary oxidation products. The addition of peroxide scavengers such as KI resulted in an increase of conversion, the yield of BINOL and enantioselectivity.

Electronic communication in linear oligo(azobenzene) radical anions.

The radical anions of five bis(azobenzene) and one tris(azobenzene) compounds were studied by optical and electron paramagnetic resonance (EPR) spectroscopies in polar aprotic solvents. The radicals with planar or almost-planar bridges are charge-delocalized mixed-valence species. Localization of charge occurs only on radicals with highly twisted biphenyl bridges. The electronic coupling between the azobenzene charge-bearing units, calculated as half the energy of the intervalence band for the charge-delocalized and by the Hush equation for the charge-localized radicals, decreases with the distance and torsion angle between azobenzene units. These radicals have smaller electronic couplings between charge-bearing units than other mixed-valence organic radicals with similar bridges. However, the application of a three-stage model to the tris(azobenzene) radical anion intervalence band yields an electronic coupling between consecutive azobenzenes that is higher than in any of the bis(azobenzene) radicals studied.

Dynamics of intramolecular electron transfer in dinitrodibenzodioxin radical anions.

The activation energy for intramolecular electron transfer in radical anions of 2,7-dinitrodibenzodioxin and 2,8-dinitrodibenzodioxin, obtained by simulation of their temperature-dependent EPR spectra, are well predicted by the values calculated by the two-state Marcus-Hush model from the optical charge-transfer bands using quartic-adjusted energy surfaces. The electronic coupling is higher in the 2,8-dinitrodibenzodioxin (H(ab) = 485 cm(-1)) than in the 2,7-dinitrodibenzodioxin radical anion (H(ab) = 250 cm(-1)), but for each solvent the reorganization energy, taken as the maximum of the optical band, is only slightly higher in the latter. These values are consistent with the fact that the reaction is faster in the 2,8-dinitrodibenzodioxin radical anion isomer, as determined by EPR spectroscopy. The pre-exponential factors obtained combining the EPR-derived rate constants and the activation energies calculated from the optical bands fit well the theoretical (modified) nonadiabatic values in the less viscous solvents. However, for the more viscous solvents, the trend of the pre-exponential values with solvent can only be explained if dynamical solvent effects increasingly influence their value. The influence of solvent dynamics in the 2,8-dinitrodibenzodioxin radical anion starts in the less viscous solvents DMF and DMSO, but in the 2,7-dinitrodibenzodioxin isomer this is only fully evident for the more viscous PhCN and HMPA. The influence of solvent dynamics is higher in the radical with the lowest activation barrier.