Synthesis, characterization, and biological evaluation of neutral nitrido technetium(V) mixed ligand complexes containing dithiolates and aminodiphosphines. A novel system for linking technetium to biomolecules

A new biomolecule labeling method that utilizes the [(99m)Tc(N)(PNP)](2+) metal fragment is presented. Thus, a series of nitrido mixed-ligand M(V) complexes (M = (99m)Tc, (99g)Tc, Re), [M(N)(Ln)(PNP)], where Ln is the dianionic form of a dithiolate or substituted-dithiolate ligand and PNP is an aminodiphosphine, is described. (99m)Tc complexes can be prepared using either a two-step or a three-step procedure starting from generator-eluted pertechnetate through a prereduced mixture of [(99m)Tc(N)]-containing species, followed by sequential or contemporary addition of the relevant dithiolate and aminodiphosphine. The reactions of 2,3-dimercaptopropionic acid (H(2)L1) with [Tc(N)(PNP)](2+) were investigated in detail. It was found that this bidentate ligand coordinated the metal fragment through the [S(-),S(-)] donor atom pair, to yield neutral mixed-ligand complexes [(99m)Tc(N)(L1)(PNP)] in high specific activity. The additional carboxylic functional group was not involved in metal coordination, thus remaining available for conjugation to target-specific molecules. Dithiolates incorporating pendant functional group(s) gave rise to a 1:1 diastereoisomeric mixture of syn-[M(N)(Ln)(PNP)] and anti-[M(N)(Ln)(PNP)] derivatives, depending on the relative orientation of the dithiolate substituent(s) with respect to the terminal nitrido group, and no isomeric conversion was detected. (99m)Tc species had been proven to be identical with the (99g)Tc complexes prepared at the macroscopic level by comparison of the corresponding radiometric and UV/vis HPLC profiles. Challenge experiments with cysteine or glutathione indicated that these physiological agents had no effect on the stability of this class of mixed-ligand (99m)Tc-complexes. Biodistribution studies in rats of selected (99m)Tc-complexes showed a rapid clearance from the blood and tissues after 60 min pi.     

Rhodium(III) and iridium(III) complexes with 1,2-naphthoquinone-1-oximate as a bidentate ligand: synthesis,structure, and biological activity

The synthesis and characterization of three novel iridium(III) complexes and one rhodium(III) complex with 1-nitroso-2-naphthol (3) chelating as a 1,2-naphthoquinone-1-oximato ligand are described. The reaction of μ₂-halogenido-bridged dimers [(η⁵-C₅Me₅)IrX₂]₂ [X is Cl (1a), Br (1b), I (1c)] and [(η⁵-C₅Me₅)RhCl₂]₂ (2a) with 3 in CH₂Cl₂ yields the mononuclear complexes (η⁵-C₅Me₅)IrX(η²-C₁₀H₆N₂O) (4a, 4b, 4c) and (η⁵-C₅Me₅)RhCl(η²-C₁₀H₆N₂O) (5a). All compounds were characterized by their ¹H and ¹³C NMR, IR, and mass spectra, UV/vis spectra were recorded for 4a and 5a. The X-ray structure analyses revealed a pseudo-octahedral “piano-stool” configuration for the metals with bidentate coordination through oximato-N and naphthoquinone-O, forming a nearly planar five-membered metallacycle. The metal complexes 4a and 5a were evaluated in respect to their cytotoxicity and binding affinity toward double-stranded DNA. As determined in the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, both exerted a much stronger cytotoxic effect toward HeLa and HL60 cancer cell lines than did cisplatin. The remarkable cytotoxicity of the compounds tested may be attributed to necrosis, rather than to apoptosis, as it is evidenced by the caspase-3/7 activation assay. No clear evidence was found for interaction with double-stranded DNA. The melting experiments showed no significant differences between thermodynamic parameters of intact DNA and DNA incubated with 3, 4a, or 5a, although these derivatives altered DNA recognition by the BamHI restriction enzyme. Therefore, the screened iridium and rhodium complexes 4a and 5a may still be interesting as potential anticancer drugs owing to their high cytotoxicity toward cancer cell lines, whereas they do not modify DNA in a way similar to that of cisplatin.     

Mixed-ligand technetium(III) complexes with tetradendate/monodendate NS(3)/isocyanide coordination: a new nonpolar technetium chelate system for the design of neutral and lipophilic complexes stable in vivo.

Starting from the tripodal ligand 2,2',2' '-nitrilotris(ethanethiol) (NS(3)) and isocyanides (CNR) as co-ligands, neutral mixed-ligand technetium(III) complexes of the general formulation [Tc(NS(3))(CNR)] have been synthesized and characterized. The (99)Tc complexes can be( )()obtained by a two-step reduction/substitution procedure starting from [TcO(4)](-) via the phosphine-containing precursor complex [Tc(NS(3))(PMe(2)Ph)]. As shown by X-ray structural analyses, the complexes adopt a nearly ideal trigonal-bipyramidal geometry with the trigonal plane formed by the three thiolate sulfurs of the tripodal ligand. The central nitrogen atom of the chelate ligand and the monodendate isocyanides occupy the apical positions. The no-carrier-added preparation of the corresponding (99m)Tc complexes was performed by a one-step procedure starting from (99m)[TcO(4)](-) with stannous chloride as reducing agent. Biodistribution studies in the rat demonstrated for the nonpolar, lipophilic compounds a significant initial brain uptake. In vitro challenge experiments with glutathione clearly indicated that no transchelation reaction occurs. Furthermore, there were no indications for reoxidation of Tc(III) to Tc(V) species or pertechnetate. We propose this type of complexes as a useful tool in the design of lipophilic (99m)Tc or (186)Re/(188)Re radiopharmaceuticals.     

Synthesis and biological activities of linear and cyclic enkephalin analogues containing a psi (E,CH = CH) or psi (CH2CH2) isosteric replacement.

The peptide CO-NH function was replaced by a trans carbon-carbon double bond or by a CH2-CH2 isostere in enkephalin analogues of DADLE, DCDCE-NH2 or DPDPE. In DADLE the 2-3 and the 3-4 peptide bond was modified, whereas in the cyclic analogues the Gly3-Phe4 bond was replaced by the isosteres Gly psi (E,CH = CH)Phe [5-amino-2-(phenylmethyl)-3(E)-pentenoic acid] or Gly psi (CH2CH2)Phe [5-amino-2-(phenylmethyl)pentanoic acid]. In general, the modification results in a drop in potency which is the largest for the flexible CH2-CH2 replacement. The Gly3 psi (E,CH = CH)Phe4 DCDCE-NH2 analogue retains considerable potency. These results confirm the importance of the peptide function at the 2-3 and 3-4 position in enkephalin analogues for biological potency.     

Iron(III) complexes of tridentate N3 ligands as models for catechol dioxygenases: Stereoelectronic effects of pyrazole coordination

The iron(III) complexes of the tridentate N₃ ligands pyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L1), 3,5-dimethylpyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L2), 3-iso-propylpyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L3) and (1-methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethylamine (L4) have been isolated and studied as functional models for catechol dioxygenases. They have been characterized by elemental analysis and spectral and electrochemical methods. The X-ray crystal structure of the complex [Fe(L1)Cl₃] 1 has been successfully determined. The complex possesses a distorted octahedral coordination geometry in which the tridentate ligand facially engages iron(III) and the Cl⁻ ions occupy the remaining coordination sites. The Fe-N_pz bond distance (2.126(5) Å) is shorter than the Fe-N_py bond (2.199(5) Å). The systematic variation in the ligand donor substituent significantly influences the Lewis acidity of the iron(III) center and hence the interaction of the present complexes with a series of catechols. The catecholate adducts [Fe(L)(DBC)Cl], where H₂DBC = 3,5-di-tert-butylcatechol, have been generated in situ and their spectral and redox properties and dioxygenase activities have been studied in N,N-dimethylformamide solution. The adducts [Fe(L)(DBC)Cl] undergo cleavage of DBC²⁻ in the presence of dioxygen to afford major amounts of intradiol and smaller amounts extradiol cleavage products. In dichloromethane solution the [Fe(L)(DBC)Cl] adducts afford higher amounts of extradiol products (64.1-22.2%; extradiol-to-intradiol product selectivity E/I, 2.6:1-4.5:1) than in DMF (2.5-6.6%; E/I, 0.1:1-0.4:1). The results are in line with the recent understanding of the function of intra- and extradiol-cleaving catechol dioxygenases.     

A series of mononuclear Co(III) complexes using tridentate N,O-donor ligands: chemical properties and cytotoxicity activity

Continuing our interest in tridentate ligands to develop new prototypes of cobalt-based metallodrugs for combating cancer, modifications in the backbone of HL1, [(2-hydroxybenzyl)(2-(pyridil-2-yl)ethyl]amine) were proposed in order to modulate the redox potential of new Co(III) complexes. Three ligands with electron withdrawing groups were synthesized: HL2: [(2-hydroxy-5-nitrobenzyl)(2-(pyridil-2-yl)ethyl]amine); HL3: [(2-hydroxybenzyl)(2-(pyridil-2-yl)ethyl]imine) and HL4: [(2-hydroxy-5-nitrobenzyl)(2-(pyridil-2-yl)ethyl]imine). They were used to obtain the respective mononuclear complexes 2, 3 and 4, which are discussed compared to the previous reported complex 1 (obtained from HL1). The new complexes were characterized and studied by several techniques including X-ray crystallography, elemental and conductimetric analysis, IR, UV-vis and ¹H NMR spectroscopies, and electrochemistry. The substitutions of the group in the para position of the phenol (HL1 and HL2) and the imine instead of the amine (HL3 and HL4), promote anodic shifts in the complexes reduction potentials. The influence of these substitutions in the biological activities of the Co(III) complexes against the murine melanoma cell line (B16F10) was also evaluated. Little effect was observed on cellular viability decrease for all free ligands, however the coordination to Co(III) enhances their activities in the following range: 1 > 4 ≈ 2 > 3. The data suggest that no straight correlation can be addressed between the reduction potential of the Co(III) center and the cell viability.     

First example of thiostannates with lanthanide-containing counter cations: Solvothermal synthesis, crystal structures and properties of thiostannates with neodymium(III) and gadolinium(III) complexes of bidentate and tridentate amino ligands

Three new lanthanide thiostannates [Ln₂(en)₆(μ₂-OH)₂]Sn₂S₆ (Ln = Nd (1), Gd (2); en = ethylenediamine) and [Gd(dien)₃]₂[(Sn₂S₆)Cl₂] (3) (dien = diethylenetriamine) were first synthesized by treating LnCl₃ with SnCl₄ and S under mild solvothermal conditions. Compounds 1 and 2 are isostructural. They consist of a binuclear lanthanide(III) complex [Ln₂(en)₆(μ₂-OH)₂]⁴⁺ cation and a dimeric [Sn₂S₆]⁴⁻ anion. The anion is built up by two SnS₄ tetrahedra sharing a common edge. The Nd³⁺ and Gd³⁺ ions are in an eight-coordinated environment forming distorted bicapped trigonal prisms. Compound 3 is composed of two monouclear [Gd(dien)₃]³⁺ complex cations, a [Sn₂S₆]⁴⁻ anion, and two chlorine ions. The Gd³⁺ ion has a nine-coordinated environment forming a distorted tricapped trigonal prism. In compounds 1-3, extensive hydrogen bonds are formed leading to three-dimensional networks of anions and cations. The band gaps of 2.42 eV for 1 and 3.17 eV for 2 have been derived from optical absorption spectra. The new lanthanide compounds might be the precursors for ternary lanthanide thiostannates by the heat treatment under nitrogen atmosphere to get rid of organic components.     

CH...Metal(II) axial interaction in planar complexes (metal=Cu, Pd) and implications for possible environmental effects of alkyl groups at biological copper sites

Intramolecular M(II)...H-C interactions (M(II)=Cu(II), Pd(II)) involving a side chain alkyl group of planar d8 and d9 metal complexes of the N-alkyl (R) derivatives of N,N-bis(2-pyridylmethyl)amine with an N3Cl donor set were established by structural and spectroscopic methods. The methyl group from the branched alkyl group (R=2,2-dimethylpropyl and 2-methylbutyl) axially interacts with the metal ion with the M...C and M...H distances of 3.056(3)-3.352(9) and 2.317(1)-2.606(1) A, respectively, and the M-H-C angles of 122.4-162.3 degrees . The Cu(II) complexes showing the interaction have a higher redox potential as compared with those without it, and the (1)H NMR signals of the interacting methyl group in Pd(II) complexes shifted downfield relative to the ligand signals. Dependence of the downshift values on the dielectric constants of the solvents used indicated that the M(II)...H-C interaction is mainly electrostatic in nature and may be regarded as a weak hydrogen bond. Implications for possible environmental effects of the leucine alkyl group at the type 1 Cu site of fungal laccase are also discussed.     

DFT/TDDFT computational study of the structural,electronic and optical properties of rhodium (III) and iridium (III) complexes based on tris-picolinate bidentate ligands

The electronic structures and spectroscopic properties of two complexes [M(pic)₃] (M = Ir, Rh) containing picolinate as bidentate ligands have been calculated by means density functional theory (DFT) and time-dependent DFT/TD-DFT using three hybrid functionals B3LYP, PBE0 and mPW1PW91. The PBE0 and mPW1PW91 functionals, which have the same HF exchange fraction (25%), give similar results and do not differ drastically from B3LYP results. Calculated geometric parameters of the complexes are in good agreement with the available experimental data. The UV absorptions observed in acetonitrile were assigned on the basis of singlet state transitions. The most intense band observed in the UV-C region corresponds to ligand-to-ligand charge transfer states (LLCT) in both complexes. The theoretical spectrum of the rhodium complex is characterized by a large degree of mixing between metal-to-ligand-charge-transfer (MLCT), LLCT and metal centered (MC) states in the UV-A region. The presence of low-lying excited states with MC character affects the absorption spectrum under spin-orbit coupling (SOC) effects and play important roles in the photochemical properties.

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Graphical abstract Frontier molecular orbital diagram of mer-M(pic)₃ (M=Ir, Rh).

     

Solution equilibrium studies on metal complexes of 2,3-dihydroxy-phenylalanine-hydroxamic acid (Dopaha) and models: catecholate versus hydroxamate coordination in iron(III)-, aluminium(III)- and molybdenum(VI)-Dopaha complexes

Equilibrium results based on pH potentiometric, spectrophotometric and (1)H NMR measurements for the complexes of Fe(III), Al(III) and Mo(VI) with 2,3-dihydroxy-phenylalanine-hydroxamic acid (Dopaha) as well as for binary model systems Fe(III)-, Al(III)-, Mo(VI)-acetohydroxamic acid (Aha), -alpha-alaninehydroxamic acid (alpha-Alaha) and -1,2-dihydroxy-3,5-benzene-disulphonate (Tiron) and ternary model systems Fe(III)-, Al(III)-, Mo(VI)-Tiron-Aha, are summarized in this paper. The amine-type coordination mode is not detectable with these metal ions at all. Precipitation occurs at pH <5.5 with Fe(III) and Al(III) even at a Dopaha-to-metal ion ratio of 10:1. Hydroxamate-type coordination was demonstrated with both metals below the pH range of precipitation but, after dissolution, catecholate-type coordination was exclusively found. The hydroxamate-type coordination mode occurs only in the very acidic pH range for Mo(VI) complexes and the crossover from hydroxamate to catecholate binding occurs at pH >3. A ligand-bridged dinuclear species, [(MoO(2))(2)(Dopaha)(2)](2+), involving mixed-type (catecholate and hydroxamate) coordination modes is formed in the pH range 2.5-5.5. [MoO(2)A(2)H(2)], with catecholate-type coordination, forms above pH 3. On increasing the pH further, deprotonation of the coordinated Dopaha and hydrolytic processes result in the formation of catecholate-coordinated [MoO(3)AH] and [MoO(3)A]. MoO(4)(2-) and free Dopaha exist above pH 10.     

Enantiomeric NMR signal separation behavior and mechanism of samarium(III) and neodymium(III) complexes with (S,S)‐ethylenediamine‐N,N'‐disuccinate

Enantiomeric ¹H and ¹³C NMR signal separation behaviors of various α‐amino acids and DL‐tartarate were investigated by using the samarium(III) and neodymium(III) complexes with (S ,S )‐ethylenediamine‐N ,N' ‐disuccinate as chiral shift reagents. A relatively smaller concentration ratio of the lanthanide(III) complex to substrates was suitable for the neodymium(III) complex compared with the samarium(III) one, striking a balance between relatively greater signal separation and broadening. To clarify the difference in the signal separation behavior, the chemical shifts of β‐protons for fully bound D‐ and L‐alanine (δ_b(D) and δ_b(L)) and their adduct formation constants (K s) were obtained for both metal complexes. Preference for D‐alanine was similarly observed for both complexes, while it was revealed that the difference between the δ_b(D) and δ_b(L) values is the significant factor to determine the enantiomeric signal separation. The neodymium(III) and samarium(III) complexes can be used complementarily for higher and smaller concentration ranges of substrates, respectively, because the neodymium(III) complex gives the larger difference between the δ_b(D) and δ_b(L) values with greater signal broadening compared to the samarium(III) complex.     

Synthesis, crystal structures, in vitro biological evaluation of zinc(II) and bismuth(III) complexes of 2-acetylpyrazine N(4)-phenylthiosemicarbazone

Two metal complexes formulated as [Zn(L)(2)](2)·H(2)O (1) and [Bi(L)(NO(3))(2)(CH(3)OH)] (2), where HL=2-acetylpyrazine N(4)-phenylthiosemicarbazone, have been synthesized and characterized by elemental analysis, IR, MS, NMR and single-crystal X-ray diffraction studies. Biological studies, carried out in vitro against selected bacteria and the K562 leukemia cell lines, respectively, have shown that the free ligand and its two complexes may be endowed with important biological properties, especially HL with MIC=3.90 μg/mL against Pseudomonas aeruginosa, the zinc(II) complex 1 with IC(50)=1.0 μM against K562 leukemia cell lines, respectively. The compounds HL and 1 may exert their cytotoxicity activity via induced loss of mitochondria membrane potential (MMP).     

Synthesis and characterization of copper(I) and silver(I) complexes with heterocyclic bidentate ligands (N, X), X = S, Se

The synthesis and structural characterization of series of copper and silver homoleptic complexes [M(R-pyX)], M = Cu, Ag, X = S, Se; R = H, 3-CF₃, 5-CF₃ (not all combinations), is described. The copper compounds, as well as [Ag(pySe)] and [Ag(3-CF₃-pySe)], were synthesised by electrochemical oxidation of anodic metal in a cell containing an acetonitrile solution of the corresponding proligand. The other homoleptic silver complexes were obtained by direct reaction between AgNO₃ and the salt of the corresponding ligand in methanol. In addition, the reaction of the metal thiolate compounds with bis(diphenylphosphino)ethane (dppe) in acetone allowed the synthesis of heteroleptic compounds [M₂(R-pyX)₂(dppe)₃]. The compounds obtained have been characterized by microanalysis, IR spectroscopy and mass spectrometry and, in cases where the complexes were sufficiently soluble, by ¹H NMR spectroscopy. The proligands (3-CF₃pySe)₂ (1), (5-CF₃-pySe)₂ (2) and (5-CF₃-pySe-DMF) (3) and the complexes [Cu(3-CF₃-pyS)] (4), [Ag(3-CF₃-pyS)] (5) and [Cu₂(5-CF₃-pyS)₂(dppe)₃] (6) were obtained as crystalline products and were studied by X-ray diffraction methods.     

Low-spin, mononuclear, Fe(III) complexes with P,N donor hemilabile ligands: A combined experimental and theoretical study

Two new mononuclear Fe(III) complexes, [FeCl₃{PPh₂(p-C₆H₄NMe₂)-P}₃](1) (PPh₂(p-C₆H₄NMe₂): 4-(dimethylamino)phenyldiphenylphosphine) and [FeCl₃(PPh₂py-P)(PPh₂py-P,N)] (2) (PPh₂py: diphenyl(2-pyridyl)phosphine) were synthesized by reacting anhydrous FeCl₃ with respective ligand in acetonitrile solution under refluxing condition. Both the complexes were characterized by elemental analysis, FAB-Mass, FTIR, UV-Vis, ESR, Cyclic Voltammetry and magnetic measurement. The FAB mass spectra of complexes 1 and 2 show molecular ion peak at m/z 1078 [M]⁺ and m/z 687 [M−1]⁺, respectively, indicating mononuclear nature of the complexes. UV-Vis spectra of the complexes were consistent with low-spin, octahedral geometry. The variable temperature magnetic susceptibility measurement (73-323 K) of these complexes is also consistent with the paramagnetic nature of the complexes with a ground state spin S = ½. The Fe(III) centers of these two complexes remain low-spin, both at room temperature and liquid nitrogen temperature, was also indicated by the ESR analysis. Cyclic Voltammetry of both the complexes show an irreversible oxidation wave attributed to Fe³⁺ → Fe⁴⁺ + e⁻ along with the peak for ligand oxidation. Theoretical calculations (B3LYP) of the complexes show that for complex 1, a trans geometry of the two phosphorous atoms and for complex 2, a mer,cis structures are the most favored geometrical isomer. TDDFT calculations were performed to interpret the observed bands in the UV-Visible spectra.     

Synthesis and single crystal structures of first examples of tridentate ligands of (Te, N, S) type and their complexes with palladium(II), platinum(II) and ruthenium(II)

The First examples of (Te, N, S) type ligands, 2-CH₃SC₆H₄CHNCH₂CH₂TeC₆H₄-4-OCH₃ (L¹) and 2- CH₃SC₆H₄CHNHCH₂CH₂TeC₆H₄-4-OCH₃ (L²), and their metal complexes, [PdCl(L¹)]PF₆ · CHCl₃ · 0.5H₂O (4), [PtCl(L¹)]PF₆ (5), [PdCl(L²)]ClO₄.CHCl₃ (6), [PtCl(L²)]ClO₄ (7), and [Ru(p-cymene)(L²)](PF₆)₂ · CHCl₃ (8), have been synthesized and characterized. The single crystal structures of 4, 6 and 8 have revealed that both the ligands coordinate in them in a tridentate (Te, N, S) mode. The geometry around Pd in both the complexes has been found to be square planar, whereas for Ru in a half sandwich complex 8, it is found to be octahedral. Between two molecules of 4 there are intra and inter molecular weak Te?Cl [3.334(3) and 3.500(3) Å, respectively] interactions along with weak intermolecular Pd?Te [3.621(2) Å] interactions. The Pd-Te bond lengths are between 2.517(6) and 2.541(25) Å and the Ru-Te bond length is 2.630(6) Å. The crystal structure of [PdCl₂(4-MeO-C₆H₄- TeCH₂CH₂NH₂)] (9) is also determined. It is formed when KPF₆ is not added in the synthesis of 4 and Pd-complex of L¹ is recrystallized. Apart from Te?Cl secondary interactions, C-H?π interactions also exist in the crystal of 9.     

Donor-substituted allenylidene(carbonyl)(XR3)chromium complexes (X=P, As, Sb) - synthesis and properties

Photolysis of the allenylidene pentacarbonyl chromium complexes [(CO)₅CrCCC(R¹)R²] (R¹=NMe₂, NPh₂; R²=NMe₂, OMe, Ph) in THF in the presence of equimolar amounts of XR₃ (XR₃=various phosphanes, P(OMe)₃, AsPh₃, SbPh₃) affords cis-allenylidene tetracarbonyl XR₃ complexes, cis-[(CO)₄(XR₃)CrCCC(R¹)R²]. When in the photolysis of [(CO)₅CrCCC(NMe₂)Ph], the phosphanes PR₃ (R=C₆H₄F-p, C₆H₄Cl-p, OMe) are used in excess (three equivalents) two carbonyl ligands are displaced and the mer-tricarbonyl complexes mer-[(CO)₃(PR₃)₂CrCCC(NMe₂)Ph] are formed both PR₃ ligands being mutually trans. The structure of the new complexes is established by IR, NMR, and UV-Vis spectroscopy, that of cis-[(CO)₄(PPh₃)CrCCC(NMe₂)Ph] additionally by an X-ray structural analysis. As indicated by the spectroscopic data of the compounds, these complexes are best described as hybrids of allenylidene and zwitterionic alkynyl complexes with delocalization of the electron pair at nitrogen bonded to the C_γ atom of the allenylidene ligand towards the metal center. The relative contribution of the allenylidene and zwitterionic alkynyl resonance forms is influenced by XR₃. Increasing the donor properties of XR₃ favors the allenylidene resonance form.     

New pinch-porphyrin complexes with quantum mixed spin ground state S=3/2,5/2 of iron (III) and their catalytic activity as peroxidase

New complexes of the pinch-porphyrin family were obtained from the dimethylester of (proto-, meso-, and deutero-porphyrinato)iron(III) with the ligand [N,N'-bis-pyridin-2-ylmethyl-propane-1,3-diamine] 1-3 and with the ligand [N-pyridin-2-ylmethyl-N'-[3-[(pyridin-2-ylmethyl)-amino]-propyl]-propane-1,3-diamine] 4-6. The UV/VIS studies of 1-6 indicate an increase in the distortion of the ligand field excited state. The 1H NMR spectra of 1-6 at RT and over the range 223-328 K show iron(III)-complexes with quantum mixed spin state (qms) S=5/2, S=3/2. The chemical shifts of the meso protons are consistent with qms state S=3/2, S=5/2, where the S=3/2 spin state is lowest in energy. For methyl-heme the chemical shifts are also consistent with a qms state but now the S=5/2 ground state is lowest in energy. ESR spectra of 1-6 show two different species, B and C, of iron(III) with qms, S=5/2, S=3/2 consistent with the 1H NMR results. Species B with 70% of S=5/2 and species C with 72.5% of S=3/2. The catalytic activity as peroxidase of 1-6 was quantified by guaiacol test; their theoretical maximum rate constants were k(cat) approximately 10(2)-10(3) M(-1) s(-1). A quantitative empirical correlation is found: the higher the 32 spin contribution to the qms state and the higher proportion of this species into the samples, the higher the peroxidase activity. Such a correlation was also obtained for pinch-porphyrins already reported.     

Reactions of quadruply bonded dimolybdenum(II) and dirhenium(III) complexes with 2,6-bis(dicyclohexylphosphinomethyl)pyridine and the related behavior of the mixed P,S,P ligand bis(diphenylphosphinomethyl)sulfide

The first complexes that contain the 2,6-bis(dicyclohexylphosphinomethyl)pyridine ligand (PNP) have been isolated and characterized. The reactions of K₄Mo₂Cl₈, (n-Bu₄N)₂Re₂Cl₈ and PdBr₂(1,5-COD) afford Mo₂Cl₄(PNP)(HPCy₂) (1), ReCl₃(PNP) (2) and PdBr₂(PNP) (4), respectively, while from the reaction of PNP with cis-Re₂(μ-O₂CCH₃)₂Cl₄(H₂O)₂ the heteromacrocylic dication [Cy₂P{CH₂pyCH₂}₂PCy₂]²⁺ has been isolated as its mixed [Cl]⁻/[ReO₄]⁻ salt (3). The reaction of cis-Re₂(μ-O₂CCH₃)₂Cl₄(H₂O)₂ with bis(diphenylphosphinomethyl)sulfide (PSP) gives the mononuclear Re(V) complex ReO(OEt)Cl₂(PSP) (5) in which the S atom is not coordinated. The structures of 1-5 have been established by X-ray crystallography, that of 5 being the first for a complex of this ligand.     

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%.     

Copper (II) complexes with Ac-HXH-NHMe (X=Gly, Ala and Aib) peptide motifs: influence of increasing CH(3) groups at C(alpha) of residue X on the coordination in solution

The interaction of Cu(II) ion with small peptides has been an interesting subject to clarify the role of copper in detail. As various Cu(II)-oligopeptide complexes can also be good models for the active centers of metalloenzymes, complexes of tripeptide and tetrapeptides are frequently investigated instead of the complexes of large peptides. The histidine side-chains of various metalloproteins frequently take part in the copper(II) coordination. Accordingly, we studied the coordination of Cu(II) to the N and C terminal protected tripeptide ligands L(A) (Ac-HisGlyHis-NHMe), L(B) (Ac-HisAlaHis-NHMe) and L(C) (Ac-HisAibHis-NHMe) in aqueous solution potentiometrially in order to determine the effect of C(alpha) methyl groups at middle residue acid on the ligation of the backbone NH and also on histidine's N(im) of coordination. Species distribution curves indicates that in acidic pH, all three peptides behave as bidentate ligands and a macrochelate forms on the metal coordination with the two histidine imidazolyl N. This coordination remains unaffected with the +I effect of increasing CH(3) groups at C(alpha) of middle residue. In the pH range 4-8, the tridentate coordination from the peptide is seen in ligand L(A) and L(B) while it is absent in L(C) due to +I effect of two C(alpha) methyl groups at middle residue as they makes N-terminal NH deprotonation difficult in this pH range and it takes place along with C terminal NH and only 4N coordinated species formed at higher pH. These 4N (N(im), N(-), N(-), N(im)) coordinated species are formed by all the three ligands at higher pH values.