Design, synthesis, and evaluation of bromo-retrochalcone derivatives as protein tyrosine phosphatase 1B inhibitors

A series of bromo-retrochalcones was designed, synthesized and evaluated as PTP1B inhibitors based on licochalcone A and E. Compounds 6, 12, 13, 14, 25, 36, 37, 39, and 41 showed potent inhibitory effects against PTP1B, and compound 37, the most potent among the series, had an IC₅₀ value of 1.9 μM, about two-fold better than that of the positive control, ursolic acid.     

Efficient synthesis and biological evaluation of epiceanothic acid and related compounds

Epiceanothic acid (1) is a naturally occurring, but very rare pentacyclic triterpene with a unique pentacyclic triterpene (PT) structure. An efficient synthesis of 1 starting from betulin (3) has been accomplished in 12-steps with a total yield of 10% in our study. Compound 1 and selected synthetic intermediates were further evaluated as anti-HIV-1 agents, inhibitors of glycogen phosphorylase (GP), and cytotoxic agents. Compound 1 exhibited moderate HIV-1 inhibition. Most importantly, compound 5, with an opened A-ring, showed significant GP inhibitory activity with an IC₅₀ of 0.21 μM, suggesting a potential for development as an anti-diabetic agent. On the other hand, compound 12, with a closed A-ring, showed potent cytotoxicity against A549 and MCF-7 human tumor cell lines, with IC₅₀ values of 0.89 and 0.33 μM, respectively. These results suggest that the A-ring of PTs is an important pharmacophore that could be modified to involve different biological activities.     

Probing the active site of rat porphobilinogen synthase using newly developed inhibitors

The structurally related tetrapyrrolic pigments are a group of natural products that participate in many of the fundamental biosynthetic and catabolic processes of living organisms. Porphobilinogen synthase catalyzes a rate-limiting step for the biosyntheses of tetrapyrrolic natural products. In the present study, a variety of new substrate analogs and reaction intermediate analogs were synthesized, which were used as probes for studying the active site of rat porphobilinogen synthase. The compounds 1, 3, 6, 9, 14, 16, and 28 were found to be competitive inhibitors of rat porphobilinogen synthase with inhibition constants ranging from 0.96 to 73.04 mM. Compounds 7, 10, 12, 13, 15, 17, 18, and 26 were found to be irreversible enzyme inhibitors. For irreversible inhibitors, loose-binding inhibitors were found to give stronger inactivation. The amino group and carboxyl group of the analogs were found to be important for their binding to the enzyme. This study increased our understanding of the active site of porphobilinogen synthase.     

Molecular modeling based approach, synthesis and in vitro assay to new indole inhibitors of hepatitis C NS3/4A serine protease

In an attempt to identify potential HCV NS3 protease inhibitors lead compounds, a series of novel indoles (10a-g) was designed. Molecular modeling study, including fitting to a 3D-pharmacophore model of the designed molecules (10a-g), with HCV NS3 protease hypothesis using catalyst program was fulfilled. Also, the molecular docking into the NS3 active site was examined using Discovery Studio 2.5 software. Several compounds showed significant high simulation docking score and fit values. The designed compounds with high docking score and fit values were synthesized and biologically evaluated in vitro using an NS3 protease binding assay. It appears that most of the tested compounds reveal promising inhibitory activity against NS3 protease. Of these, compounds 10a and 10b demonstrated potent HCV NS3 protease inhibitors with IC₅₀ values of 9 and 12 ??g/mL, respectively. The experimental serine protease inhibitor activities of compounds 10a-g were consistent with their molecular modeling results. Inhibitors from this class have promising characteristics for further development as anti-HCV agents.     

1,3-Dipolar cycloaddition to the FeSC fragment 20. Preparation and properties of carbonyliron complexes of di-thiooxamide. Reactivity of the mononuclear (di-thiooxamide)Fe(CO)3 towards dimethyl acetylenedicarboxylate

Reaction of Fe₂(CO)₉ at room temperature in THF with the di-thiooxamides (L), SC{N(R,R′)}C{(R,R′)N}S [R=Me, R′-R′=(CH₂)₂ (a); R=H, R′=ⁱPr (b); R=H, R′=iPr (c), R=H, R′=benzyl (d); R=H, R′=H (e)], results for ligands a-d initially in the formation of the mononuclear σ-S, σ-S′ chelate complexes Fe(CO)₃(L) (7a-d), which could be isolated in case of 7a and 7d. Under the reaction conditions, complexes 7a-d react further with [Fe(CO)₄] fragments to give three types of Fe₂(CO)₆(L) complexes (8a-d) in high yields, depending on the di-thiooxamide ligand used together with traces of the known complex S₂Fe₃(CO)₉ (14). The molecular structures of these complexes have been established by the single crystal X-ray diffraction determinations of 8a, 8b and 8d. In the reaction with ligand e the corresponding complex 7e was not detected and the well-known complexes 14 and S₂Fe₃(CO)₉ (15) were isolated in low yield. In situ prepared 7a reacts in a slow reaction with 1 equiv. of dimethyl acetylene dicarboxylate in a 1,3-dipolar cycloaddition reaction to give the stable initial ferra [2.2.1] bicyclic complex 10a in 60% yield. In complex 10a an additional Fe(CO)₄ fragment is coordinated to the sulfido sulfur atom of the cycloadded FeSC fragment. When a toluene solution of 10a is heated to 50 °C it loses two terminal CO ligands to give the binuclear FeFe bonded complex 11a in almost quantitative yield. The molecular structures of 10a and 11a have been confirmed by single crystal X-ray diffraction. Reaction of 7d at room temperature with 2 equiv. of dimethyl acetylene dicarboxylate results in the mononuclear complex 12d in 5% yield. The molecular structure of 12b has been established by single crystal X-ray diffraction and comprises a tetra dentate ligand with two ferra-sulpha cyclobutene, and a ferra-disulpha cyclopentene moiety. When the reaction is performed at 60 °C a low yield of 2,3,4,5-thiophene tetramethyl tertracarboxylate is obtained besides complex 12d.     

New pyridyl modified phosphines: Synthesis and late transition-metal coordination studies

Using a phosphorus based Mannich condensation reaction the new pyridylphosphines {5-Ph₂PCH₂N(H)}C₅H₃(2-Cl)N (1-Cl) and {2-Ph₂PCH₂N(H)}C₅H₃(5-Br)N (1-Br) have been synthesised in good yields (60% and 88%, respectively) from Ph₂PCH₂OH and the appropriate aminopyridine. The ligands 1-Cl and 1-Br display variable coordination modes depending on the choice of late transition-metal complex used. Hence P-monodentate coordination has been observed for the mononuclear complexes AuCl(1-Cl) (2), AuCl(1-Br) (3), RuCl₂(p-cymene)(1-Cl) (4), RuCl₂(p-cymene)(1-Br) (5), RhCl₂(Cp^∗)(1-Cl) (6), RhCl₂(Cp^∗)(1-Br) (7), IrCl₂(Cp^∗)(1-Cl) (8), IrCl₂(Cp^∗)(1′-Cl) (8′), IrCl₂(Cp^∗)(1-Br) (9), cis-/trans-PdCl₂(1-Cl)₂ (10), cis-/trans-PdCl₂(1-Br)₂ (11), cis-PtCl₂(1-Cl)₂ (12) and cis-PtCl₂(1-Br)₂ (13). Reaction of Pd(Me)Cl(cod) (cod = cycloocta-1,5-diene) with either 1 equiv. of 1-Br or the known pyridylphosphines 1′-Cl, 1-OH or 1-H gave the P/N-chelate complexes Pd(Me)Cl(1-Br-1-H) (14)-(17). All new compounds have been fully characterised by spectroscopic and analytical methods. Furthermore the structures of 4, 5, 10 and 16 · (CH₃)₂SO have been elucidated by single crystal X-ray crystallography. A crystal structure of the dinuclear metallocycle trans,trans-[PdCl₂{μ-P/N-{Ph₂PCH₂N(H)}C₅H₄N}]₂ · CHCl₃, 18 · CHCl₃, has also been determined. Here 1-H bridges, using both P and pyridyl N donors, two dichloropalladium centres affording a 12-membered ring with the PdCl₂ units adopting a head-to-tail arrangement.     

Construction of carbon chains on ruthenium and osmium carbonyl clusters

We have used the elimination of AuX(PR₃) (X = halide, R = Ph, tol) that occurs in reactions of alkynylgold(I)-phosphine complexes with M₃(μ-H)₃ (μ₃-CBr) (CO)₉ (M = Ru, Os) to prepare the complexes M₃(μ-H)₃(μ₃-CCCR)(CO)₉ [M = Ru, R = Ph 2, CCSiMe₃3, Fc 4, CCFc 6-Ru, CC[Ru(PPh₃)₂Cp] 8; M = Os, R = CCFc 6-Os, CCCCFc 7], Fc′{(μ₃-CCC)Ru₃(μ-H)₃(CO)₉}₂5, and bis-cluster-capped carbon chain complexes {M₃(μ-H)₃(CO)₉}₂{μ₃:μ₃-C(CC)_nC} (M = Ru, n = 2 9, 3 10-Ru; M = Os, n = 3 10-Os) and {(L)(OC)₈(μ-H)₃M₃}C(CC)_nC{Co₃(μ-dppm)(CO)₇} (n = 1, M = Ru, L = CO 11, PPh₃12-Ru/P; n = 2, L = CO 12-Ru, PPh₃13; M = Os, L = CO 12-Os) in good to excellent yields. X-ray structural determinations of 2-5, 6-Ru, 6-Os, 7, 9, 11, 12-Ru, 12-Os and 12-Ru/P are reported.     

Biological evaluation of 3'-O-alkylated analogs of salacinol, the role of hydrophobic alkyl group at 3' position in the side chain on the α-glucosidase inhibitory activity

Four analogs with 3′-O-alkyl groups (9a: CH₃, 9b: C₂H₅, 9c: C₁₃H₂₇ or 9d: CH₂Ph) instead of the 3′-O-sulfate anion in salacinol (1), a naturally occurring potent α-glucosidase inhibitor, were synthesized by the coupling reaction of 1,4-dideoxy-1,4-epithio-d-arabinitols (18a and 18b) with appropriate epoxides (10a-10d). These analogs showed equal or considerably higher inhibitory activity against rat small intestinal α-glucosidases than the original sulfate (1), and one of them (9d) was found more potent than currently used α-glucosidase inhibitors as antidiabetics. Thus, introduction of a hydrophobic moiety at the C3′ position of this new class of inhibitor was found beneficial for onset of stronger inhibition against these enzymes.     

Synthesis, transport and antiviral activity of Ala-Ser and Val-Ser prodrugs of cidofovir

We report the synthesis and biological evaluation of Ala-(Val-)l-Ser-CO₂R prodrugs of 1, where a dipeptide promoiety is conjugated to the P(OH)₂ group of cidofovir (1) via esterification by the Ser side chain hydroxyl group and an ethyl group (4 and 5) or alone (6 and 7). In a murine model, oral administration of 4 or 5 did not significantly increase total cidofovir species in the plasma compared to 1 or 2, but 7 resulted in a 15-fold increase in a rat model and had an in vitro EC₅₀ value against human cytomegalovirus comparable to 1. Neither 6 nor 7 exhibited toxicity up to 100 μM in KB or HFF cells.     

Cholinergic and neuroprotective drugs for the treatment of Alzheimer and neuronal vascular diseases. II. Synthesis, biological assessment, and molecular modelling of new tacrine analogues from highly substituted 2-aminopyridine-3-carbonitriles

The synthesis, biological assessment, and molecular modelling of new tacrine analogues 11-22 is described. Compounds 11-22 have been obtained by Friedländer-type reaction of 2-aminopyridine-3-carbonitriles 1-10 with cyclohexanone or 1-benzyl-4-piperidone. The biological evaluation showed that some of these molecules were good AChE inhibitors, in the nanomolar range, and quite selective regarding the inhibition of BuChE, the most potent being 5-amino-2-(dimethylamino)-6,7,8,9-tetrahydrobenzo[1,8-b]-naphthyridine-3-carbonitrile (11) [IC₅₀ (EeAChE: 14 nM); IC₅₀ (eqBuChE: 5.2 ??M]. Kinetic studies on the easily available and potent anticholinesterasic compound 5-amino-2-(methoxy)-6,7,8,9-tetrahydrobenzo[1,8-b]-naphthyridine-3-carbonitrile (16) [IC₅₀ (EeAChE: 64 nM); IC₅₀ (eqBuChE: 9.6 ??M] showed that this compound is a mixed-type inhibitor (K_i = 69.2 nM) of EeAChE. Molecular modelling on inhibitor 16 confirms that this compound, as expected and similarly to tacrine, binds at the catalytic active site of EeAChE. The neuroprotective profile of molecules 11-22 has been investigated in SH-SY5Y neuroblastoma cells stressed with a mixture of oligomycin-A/rotenone. Compound 16 was also able to rescue by 50% cell death induced by okadaic acid in SH-SY5Y cells. From these results we conclude that the neuroprotective profile of these molecules is moderate, the most potent being compounds 12 and 17 which reduced cell death by 29%. Compound 16 does not affect ACh- nor K⁺-induced calcium signals in bovine chromaffin cells. Consequently, tacrine analogues 11-22 can be considered attractive therapeutic molecules on two key pharmacological targets playing key roles in the progression of Alzheimer, that is, cholinergic dysfunction and oxidative stress, as well as in neuronal cerebrovascular diseases.     

Synthesis and biological activity of hydroxy substituted phenyl-benzo[d]thiazole analogues for antityrosinase activity in B16 cells

In this study, we synthesized hydroxy and/or alkoxy substituted phenyl-benzo[d]thiazole derivatives using substituted benzaldehydes and 2-aminothiophenol in MeOH. The structures of these compounds were established by ¹H and ¹³C NMR and mass spectral analyzes. All synthesized compounds were evaluated for their mushroom tyrosinase inhibition activity. Out the 12 generated compounds, 2a and 2d exhibited much higher tyrosinase inhibition activity (45.36-73.07% and 49.94-94.17% at 0.01-20 μM, respectively) than kojic acid (9.29-50.80% at 1.25-20 μM), a positive control.The cytotoxicity of 2a and 2d was evaluated using B16 cells and the compounds were found to be nontoxic. Compounds 2a and 2d were also demonstrated to be potent mushroom tyrosinase inhibitors, displaying IC₅₀ values of 1.14 ± 0.48 and 0.01 ± 0.0002 μM, respectively, compared with kojic acid, which has an IC₅₀ value of 18.45 ± 0.17 μM. We also predicted the tertiary structure of tyrosinase, simulated the docking with compounds 2a and 2d and confirmed that the compounds strongly interact with mushroom tyrosinase residues. Kinetic plots showed that 2a and 2d are competitive tyrosinase inhibitors. Substitutions with a hydroxy group at R³ or both R³ and R¹ of the phenyl ring indicated that these groups play a major role in the high binding affinity to tyrosinase. We further found that compounds 2a and 2d inhibit melanin production and tyrosinase activity in B16 cells. These results may assist in the development of new potent tyrosinase inhibitors against hyperpigmentation.     

Dibenzocyclooctane lignans from the stems of Schisandra bicolor

A new dibenzocyclooctane lignan, schisanbicolorin A, together with fifteen known lignans, were isolated from the stems of Schisandra bicolor Cheng. Their structures were identified as (aS,6R,7S)-5,6,7,8-tetrahydro-2,3,13-trimethoxy-6,7-dimethyldibenzo[3,4]cycloocta [1,2-f][1,3]benzodioxol-1-ol (1), neglschisandrin C (2), angeloylgomisin R (3), schisantherin D (4), gomisin F (5), schisantherin B (6), tigloylgomisin Q (7), gomisin G (8), interiotherin B (9), schisandrin (10), angeloylgomisin H (11), benzoylgomisin H (12), gomisin H (13), angeloyl−(+)−gomisin K₃ (14), deoxyschizandrin (15), and (+)-gomisin K₃ (16), respectively, based on spectroscopic analysis and by comparison of their spectral data with those reported previously in the literature.     

Palladium(II), platinum(II), ruthenium(II) and mercury(II) complexes of potentially tridentate Schiff base ligands of (E, N, O) type (E = S, Se, Te): Synthesis, crystal structures and applications in Heck and Suzuki coupling reactions

Schiff bases of 2-hydroxybenzophenone (HBP) (C₆H₅)(2-HOC₆H₄)CN(CH₂)_nEAr (L1/L2: E = S, Ar = Ph, n = 2/3; L3/L4: E = Se, Ar = Ph, n = 2/3; L5/L6: E = Te, Ar = 4-MeOC₆H₄, n = 2/3) and their complexes [PdCl(L-H)] (L = L1−L6; 1, 2, 3, 5, 7, 11), [PtCl(L3-H/L5-H)] (4/8), [PtCl₂(L4/L6)₂] (6/12), [(p-cymene)RuCl(L5/L6)]Cl (9/13) and [HgBr₂(L5/L6)₂] (10/14) have been synthesized and characterized by proton, carbon-13, selenium-77 and tellurium-125 NMR, IR and mass spectra. Single crystal structures of L1, 1, 3, 4, 5 and 7 were solved. The Pd-E bond distances (Å): 2.2563(6) (E = S), 2.3575(6)−2.392(2) (E = Se); 2.5117(5)−2.5198(5) (E = Te) are near the lower end of the bond length range known for them. The Pt-Se bond length, 2.3470(8) Å, is also closer to the short values reported so far. The Heck and Suzuki reaction were carried out using complexes 1, 3, 5 and 7 as catalysts under aerobic condition. The percentage yields for trans product in Heck reaction were found upto 85%.     

Syntheses, structures, and mesomorphic properties of two series of oxovanadium(IV) salen and salpn complexes with 4-substituted long alkoxy chains

Two series of oxovanadium(IV) salen and salpn complexes containing 4-substituted alkoxy chains of aromatic rings, [VO((4-C_nH_2n+1O)₂salen)] (n = 3 (1), 4 (2), 6 (3), 8 (4), 10 (5), 12 (6), 14 (7), 16 (8), 18 (9), and 20 (10) and salen = N,N′-ethylenebis(salicylideneiminato)), and [VO((4-C_nH_2n+1O)₂salpn)] (n = 8 (11), 10 (12), 12 (13), 14 (14), 16 (15), and 18 (16) and salpn = N,N′-propylenebis(salicylideneiminato)), have been prepared and their mesomorphic properties have been investigated. The crystal structures of 1-9 except for 7 by an X-ray crystallographic analysis have been revealed. Complexes 4-9 in the solid state have been confirmed as novel bilayer crystal structures composed of only the VO(IV) complex without linear chains via the VO units. The VO(IV) complexes with longer alkoxy chains of 8-10 transferred from the bilayer crystal to the bilayer metallomesogens (liquid crystals). Based on the X-ray analyses and the precise extinction rules for 8-10 with the bilayer metallomesogens, complexes 8-10 were identified as the liquid crystalline (M(Pa2₁)) phase derived from 80 layer groups. On the other hand, the 4-alkoxysalpn complexes of 14-16 showed the unusual rectangular columnar mesophase (Col_r) with the linear chain via the VO units supported by the existence of the VO stretching band characteristic of weak linear chain formation via the VO units in the liquid crystal.     

Sargachromanols as inhibitors of Na+/K+ ATPase and isocitrate lyase

Sargachromanols A-P (1-16), 16 meroterpenoids of the chromene class isolated from the brown alga Sargassum siliquastrum, were evaluated for their inhibitory activities toward Na⁺/K⁺ ATPase from porcine cerebral cortex and isocitrate lyase (ICL) from Candida albicans. These studies led to the identification of compounds 4, 6, 8, and 12 as potent Na⁺/K⁺ ATPase inhibitors. Compounds 12, 13, and 16 exhibited moderate ICL inhibitory activity. Compound 12 also showed weak antibacterial activity. The preliminary structure-activity relationship of these compounds is described to elucidate the essential structural requirements.     

Synthesis and antitumor activity of new d-seco and d-homo androstane derivatives

Starting from 3β-hydroxy-17-oxo-16,17-secoandrost-5-ene-16-nitrile (1), the new 16,17-secoandrostane derivatives 4-9 were synthesized. On the other hand, 3β-hydroxy-17-oxa-d-homoandrost-5-ene-16-one (10) yielded the new d-homo derivatives 12, 13 and 15. In vitro antiproliferative activity of selected compounds against three tumor cell lines (human breast adenocarcinoma ER+, MCF-7, human breast adenocarcinoma ER−, MDA-MB-231, prostate cancer AR−, PC-3, and normal fetal lung fibroblasts, MRC-5) was evaluated. Compounds 3 and 12 showed strong antiproliferative activity against PC-3 cells, the IC₅₀ values being 2 μM and 0.55 μM, respectively. Compounds 6 (10 μM) and 14 (9 μM) showed moderate activity against MDA-MB-231 cells. The synthesized compounds 1-3, 5-8, 10 and 12-15 were not toxic to normal fetal lung fibroblasts cells, MRC-5.     

Study of the coordination behaviour of (3,5-diphenyl-1H-pyrazol-1-yl)ethanol against Pd(II), Zn(II) and Cu(II)

A new easily synthetic route with a 96% yield of ligand 2-(3,5-diphenyl-1H-pyrazol-1-yl)ethanol (L) is obtained. The reactivity of L against Pd(II), Zn(II) and Cu(II) leads to [PdCl₂(L)₂] (1), [ZnCl₂(L)] (2) and [CuCl(L′)]₂ (3) (L′ is the ligand L without alcoholic proton), respectively. According to the different geometries imposed by the metallic centre and the capability of L to present various coordination links, it has been obtained complexes with square planar (1 and 3) or tetrahedral (2) geometry and different nuclearity: monomeric (1 and 2) or dimeric (3). Complete characterisation by analytical and spectroscopic methods, resolution of L and 1-3 by single-crystal X-ray diffraction and magnetic studies for complex 3 are presented.     

Antiproliferative and apoptotic sesquiterpene lactones from Carpesium faberi

Four new (1-4) and 13 known (5-17) sesquiterpene lactones along with two known diterpenes (18, 19) were isolated from the whole plant of Carpesium faberi. The new structures were elucidated by means of spectroscopic techniques and some chemical transformations to be pseudoguaian-1α(H)-8α,12-olide-4β-O-β-d-glucopyranoside (1), 4β,10α-dihydroxy-5α(H)-1,11(13)-guaidien-8α,12-olide (2), 4β,10β-dihydroxy-5α(H)-1, 11(13)-guaidien-8β,12-olide (3), and (4S)-acetyloxyl-11(13)-carabren-8β,12-olide (4). All isolates were tested against MCF-7 human breast cancer cells using the MTT assay. Among them, the sesquiterpene lactones (except tomentosin 17) possessing an α-methylene-γ-lactone moiety were found to have in vitro antiproliferative activities, with IC₅₀ values of 3.0-38.8 μg/mL. The effects of four selected sesquiterpene lactones (guaianolide 2, carabranolide 4, pseudoguaianolide 9, eudesmanolide 13) on the cell cycle were examined using flow cytometry (FCM).     

Iridium assisted S-H and C-H activation of benzaldehyde thiosemicarbazones. Synthesis, structure and electrochemical properties of the resulting complexes

Reaction of five 4-R-benzaldehyde thiosemicarbazones (R = OCH₃, CH₃, H, Cl and NO₂) with [Ir(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃) affords complexes of three different types, viz. 1-R, 2-R and 3-R. In the 1-R complexes the thiosemicarbazone is coordinated to iridium as a monoanionic bidentate N,S-donor forming a four-membered chelate ring. Two triphenylphosphines, a hydride and a chloride are also coordinated to the metal center. The 2-R complexes are very similar in composition and stereochemistry to the corresponding 1-R complexes, except that a second hydride is bound to iridium instead of the chloride. In the 3-R complexes, the thiosemicarbazones are coordinated to iridium as dianionic tridentate C,N,S-donors forming two adjacent five-membered chelate rings. Two triphenylphosphines and a hydride are also coordinated to the metal center. Structures of the 1-NO₂, 2-NO₂ and 3-NO₂ complexes have been determined by X-ray crystallography. Reaction of the same 4-R-benzaldehyde thiosemicarbazones with [Ir(PPh₃)₃Cl] in refluxing toluene in the presence of NEt₃ affords complexes of two types, viz. 3-R and 4-R. The 4-R complexes are very similar in composition and stereochemistry to the corresponding 3-R complexes, except that a chloride is bound to iridium instead of the hydride. Structure of the 4-CH₃ complex has been determined by X-ray crystallography. In all the complexes the two PPh₃ ligands are trans. All the complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on the complexes shows an Ir(III)-Ir(IV) oxidation on the positive side of SCE followed by an oxidation of the coordinated thiosemicarbazone. A reduction of the coordinated thiosemicarbazone is also observed on the negative side of SCE.     

6-Halogenopyridin-2-olato complexes with the diruthenium(2+) core: Equilibria between head-to-head and head-to-tail structures

The dinuclear bis(6-X-pyridin-2-olato) ruthenium complexes [Ru₂(μ-XpyO)₂(CO)₄(PPh₃)₂] (X = Cl (4B) and Br (5B)), [Ru₂(μ-XpyO)₂(CO)₄(CH₃CN)₂] (X = Cl (6B), Br (7B) and F (8B)) and [Ru₂(μ-ClpyO)₂(CO)₄(PhCN)₂] (9B) were prepared from the corresponding tetranuclear coordination dimers [Ru₂(μ-XpyO)₂(CO)₄]₂ (1: X = Cl; 2: X = Br) and [Ru₂(μ-FpyO)₂(CO)₆]₂ (3) by treatment with an excess of triphenylphosphane, acetonitrile and benzonitrile, respectively. In the solid state, complexes 4B-9B all have a head-to-tail arrangement of the two pyridonate ligands, as evidenced by X-ray crystal structure analyses of 4B, 6B and 9B, in contrast to the head-to-head arrangement in the precursors 1-3. A temperature- and solvent-dependent equilibrium between the yellow head-to-tail complexes and the red head-to-head complexes 4A-7A and 9A, bearing an axial ligand only at the O,O-substituted ruthenium atom, exists in solution and was studied by NMR spectroscopy. Full ¹H and ¹³C NMR assignments were made in each case. Treatment of 1 and 2 with the N-heterocyclic carbene (NHC) 1-butyl-3-methylimidazolin-2-ylidene provided the complexes [Ru₂(μ-XpyO)₂(CO)₄(NHC)], X = Cl (11A) or Br (12A). An XRD analysis revealed the head-to-head arrangement of the pyridonate ligands and axial coordination of the carbene ligand at the O,O-substituted ruthenium atom. The conversion of 11A and 12A into the corresponding head-to-tail complexes was not possible.