Effect Of Metal Coordination Research Articles

Fluorinated dienes in transition-metal chemistry – the rich chemistry of electron-poor ligands

The coordination chemistry of alkenes is among the fundamentals of organometallic chemistry. However, despite the beneficial effects of fluorination to organic molecules, their fluorinated counterparts are rarely found in transition-metal complexes and most examples are limited to simple monoalkenes like tetrafluoroethene. The paucity of fluorodiene complexes is contrasted by their structural diversity. Seemingly similar dienes containing conjugated, isolated or cumulated double bonds exhibit largely different ligand properties. While the most prominent group, the conjugated fluorodienes, is very flexible in hapticity, their analogues bearing isolated double bonds behave more like monoalkenes. Fluorinated cumulenes in contrast are thermally labile, their chemistry is dominated by the stabilising effect of metal coordination. This review outlines the synthesis, structure and reactivity of transition-metal complexes derived from fluorinated dienes.

Effect of metal coordination on the interaction of substituted phenanthroline and pyridine ligands with quadruplex DNA

A series of new mono-carboxamido phenanthrolines and bis-carboxamido pyridines have been prepared and their coordination chemistry to platinum(II) and palladium(II) investigated. Both the ligands and metal complexes have been designed with the appropriate geometric requirements to favour pi-pi interactions with guanine quartets, which are the core motif in quadruplex structures formed by folding single-stranded telomeric DNA. The interactions of these compounds with quadruplex-DNA have been investigated. The results show that some of these compounds induce the stabilisation of quadruplex-DNA (e.g. DeltaTm at 1 microM of 20 degrees C for one of the platinum(II) complexes) while there is no measurable interaction with duplex DNA at similar concentrations. In addition, the important role played by the metal centre in increasing the interaction of these planar molecules with quadruplex-DNA is highlighted.

Potent in vitro anti-Trypanosoma cruzi activity of pyridine-2-thiol N-oxide metal complexes having an inhibitory effect on parasite-specific fumarate reductase

In the search for new therapeutic tools against Chagas disease (American trypanosomiasis) palladium and platinum complexes of the bioactive ligand pyridine-2-thiol N-oxide were exhaustively characterized and evaluated in vitro. Both complexes showed high in vitro growth inhibition activity (IC(50) values in the nanomolar range) against Trypanosoma cruzi, the causative agent of the disease. They were 39-115 times more active than the antitrypanosomal drug Nifurtimox. The palladium complex showed an approximately threefold enhancement of the activity compared with the parent compound. In addition, owing to their low unspecific cytotoxicity on mammalian cells, the complexes showed a highly selective antiparasite activity. To get an insight into the mechanism of action of these compounds, DNA, redox metabolism (intraparasite free-radical production) and two parasite-specific enzymes absent in the host, namely, trypanothione reductase and NADH-fumarate reductase, were evaluated as potential parasite targets. Additionally, the effect of metal coordination on the free radical scavenger capacity previously reported for the free ligand was studied. All the data strongly suggest that trypanocidal action of the complexes could mainly rely on the inhibition of the parasite-specific enzyme NADH-fumarate reductase.

Diaquabis(2,5-dihydroxybenzoato-κO)bis(1,10-phenanthroline-κ2N,N′)strontium(II) bis(1,10-phenanthroline) tetrahydrate

The title compound, [Sr(C(7)H(5)O(4))(2)(C(12)H(8)N(2))(2)(H(2)O)(2)].2C(12)H(8)N(2).4H(2)O, consists of an Sr(II) complex, uncoordinated phenanthroline (phen) molecules and solvent water molecules. The Sr(II) ion is located on a twofold axis and is coordinated by two phen ligands, two dihydroxybenzoate anions and two water molecules in a distorted tetragonal antiprismatic geometry. Partially overlapped arrangements exist between parallel coordinated and parallel uncoordinated phen rings; the face-to-face separations between the former (coordinated) and the latter (uncoordinated) rings are 3.436 (13) and 3.550 (14) A, respectively. This difference suggests the effect of metal coordination on pi-pi stacking between phen rings.

Influence of Metal Coordination on the Mismatch Tolerance of Ligand-Modified PNA Duplexes

Recent studies on metal incorporation in ligand-modified nucleic acids have focused on the effect of metal coordination on the stability of metal-containing duplexes or triplexes and on the metal binding selectivity but did not address the effect of the sequence of the nucleic acid in which the ligands are incorporated. We have introduced 8-hydroxyquinoline Q in 10-mer PNA strands with various sequences and have investigated the properties of the duplexes formed from these strands upon binding of Cu(2+). Variable-temperature UV-vis spectroscopy shows that, in the presence of Cu(2+), duplexes are formed even from ligand-modified Q-PNA strands that have a large number of mismatches. Spectrophotometric titrations demonstrate that at any temperature, one Cu(2+) ion binds a pair of Q-PNA strands that each contain one 8-hydroxyquinoline, but below the melting temperature, the PNA duplex exerts a supramolecular chelate effect, which prevents the transformation in the presence of excess Cu(2+) of the 1:2 Cu(2+):Q-PNA complexes into 1:1 complexes. EPR spectroscopy gives further support for the existence in the duplexes of [CuQ(2)] moieties that are similar to the corresponding square planar synthetic complex formed between Cu(2+) and 8-hydroxyquinoline. As PNA duplexes show a preferred handedness due to the chiral induction effect of a C-terminal l-lysine, which is transmitted through stacking interactions within the duplex, only if the metal-containing duplex has complementary strands, does it show a chiral excess measured by CD spectroscopy. The strong effect of the metal-ligand moiety is suggestive of an increased correlation length in PNA duplexes that contain such moieties. These results indicate that strong metal-ligand alternative base pairs significantly diminish the importance of Watson-Crick base pairing for the formation of a stable PNA duplex and lead to high mismatch tolerance, a principle that can be used in the construction of hybrid inorganic-nucleic acid nanostructures.

Metal coordination to the formal P=N bond of an iminophosphorane and charge-density evidence against hypervalent phosphorus(V).

The iminophosphorane Ph(2)P(CH(2)Py)(NSiMe(3)) (1) was treated with deprotonating alkali metal reagents to give [(Et(2)O)Li[Ph(2)P(CHPy)(NSiMe(3))]] (2), [[Ph(2)P(CH(2)Py)(NSiMe(3))]Li[Ph(2)P(CHPy)(NSiMe(3))]] (3) and [[Ph(2)P(CH(2)Py)(NSiMe(3))]Na[Ph(2)P(CHPy)(NSiMe(3))]] (4). We report their coordination behaviour in solid-state structures and NMR spectroscopic features in solution. Furthermore, we furnish experimental evidence against hypervalency of the phosphorus atom in iminophosphoranes from experimental charge-density studies and subsequent topological analysis. The topological properties, correlated to the results from NMR spectroscopic investigations, illustrate that the formal P=N double bond is better written as a polar P(+)--N(-) single bond. Additionally, the effects of metal coordination on the bonding parameters of the iminophosphorane and the related anion are discussed.

Metal coordination and imine-amine hydrogen bonding as the source of strongly shifted adenine pKa values.

The X-ray crystal structure of a Pt(II) complex of composition trans-[(NH(3))(2)Pt(1,9-DimeA) (1,9-DimeAH)](ClO(4))(3) (2) with 1,9-DimeA = 1,9-dimethyladenine and 1,9-DimeAH(+) = 1,9-dimethyladeninium) is presented. Complex 2 forms upon deprotonation of one of the exocyclic amino groups of the adeninium ligands in trans-[(NH(3))(2)Pt(1,9-DimeAH)(2)](ClO(4))(4) (1), where the two nucleobases are in a head-tail arrangement. The low pK(a1) of 1 (4.1 +/- 0.2) is a consequence of a combination of the effects of metal coordination to N7 of the purine base and efficient stabilization of the deprotonated species. This feature is supported by the results of the structure determination of 2, which displays a head-head orientation of the two bases and intramolecular H-bonding between the imine group of 1,9-DimeA and the amino group of 1,9-DimeAH. In the fully deprotonated species trans-[(NH(3))(2)Pt(1,9-DimeA)(2)](ClO(4))(2) (3), the two nucleobases are again in a head-tail arrangement. The findings are of relevance with regard to the concept of "shifted pK(a) values" of nucleobases. This concept is applied to rationalize acid-base catalysis reactions involving nucleobases of DNA and RNA which occur in the near-physiological pH range.

Cycloaddition of nitrones to free and coordinated (E)-cinnamonitrile: effect of metal coordination and microwave irradiation on the selectivity of the reaction

[2 + 3] Cycloaddition of N-methyl-C-phenylnitrone to transition metal coordinated (E)-cinnamonitrile occurs exclusively at the nitrile CN bond, leading to Δ4-1,2,4-oxadiazoline complexes, from which the heterocyclic ligand can be released and isolated in high yield. In contrast, the reaction of the nitrone with free cinnamonitrile involves the CC bond only, yielding a diastereomeric mixture of isoxazolidine-4-carbonitriles. Microwave irradiation enhances the reaction rates of both transformations considerably, without changing their regioselectivity with respect to the thermal reactions. The two nitrile ligands in complexes of the type [MCl2(cinnamonitrile)2] (M = Pt or Pd) are significantly different in reactivity. Thus, short-time microwave irradiation allows for the selective synthesis of the mono-cycloaddition product [PtCl2(cinnamonitrile)(oxadiazoline)], even in the presence of an excess of nitrone. Using longer irratiation times, this complex can be further transformed into the bis-cycloaddition product [PtCl2(oxadiazoline)2]. The latter compound is also produced when thermal heating is applied, however, the formation of the mono-cycloaddition product fails to be selective under thermal conditions.

DFT study of electronic structure of saccharin, thiosaccharin, and their respective ions: Effects of metal coordination on thiosaccharinate electronic structure

AbstractA rationalization of the conspicuous different abilities of saccharin and thiosaccharin to form coordination compounds with the first series of transition metal elements and to interact with heavy metal cations is sought. Their electronic molecular structures as well as those of their respective ions are compared performing natural bond orbitals (NBO) analyses of the four species. Upon deprotonation, the negative charge at the N atom in saccharin is almost constant while it decreases in thiosaccharin and the negative charge at the monocoordinated chalcogen atom is notably more increased in the latter than in the former. Apparently, the negative charge reorganization makes difficult the coordination of thiosaccharin with the first series of transition metal elements and favors its experimentally observed interaction with heavy metal cations such as cadmium, lead, mercury, silver, and thallium. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

Influence of metal ions binding on free radical concentration in humic acids. A quantitative electron paramagnetic resonance study

The influence of metal ions, e.g. Co(II), Cu(II), Mn(II), Ni(II), Fe(II), on free radical concentration in humic acids isolated from soil, peat and compost was investigated by electron paramagnetic resonance (EPR). The results show that metal ions with unfilled d-shell exhibit antiferromagnetic interactions with semiquinone radicals. Moreover, coordinated metals shift the quinone–semiquinone–hydroquinone equilibrium in the macromolecular matrix of humic acids. A strong decrease of semiquinone radical concentration in humic acid–metal complexes is observed. This effect is caused by interactions of metal ions with oxygen-containing stable radicals occurring in the aromatic systems of humic acids. Furthermore, the effect of metal coordination on free radical concentration in humic acids–metal complexes depends on the humic acid origin. FTIR spectroscopy was also used as an additional tool for studies of the metal ions interactions with carboxylic groups.

Effect of Metal Coordination on the Charge Distribution over the Cation Binding Sites of Zeolites. A Combined Experimental and Theoretical Study

Simplified cluster models representing cation binding sites in a zeolite of ferrierite structure have been optimized with a series of divalent (MgII, MnII, CoII, NiII) and monovalent (NaI, CuI, AgI) metal cations. It has been found that the coordination of metal cations significantly perturbs the charge distribution over the cation binding sites. The perturbation is of electronic origin and arises from the strong electric field concentrated on the metal cation and from framework to metal charge-transfer effects. Its extent is specific to the type of cation and can be expressed with an empirical formula, which contains the formal charge of the cation and bond distance parameters. A sound correlation has been found between the extent of perturbation and the measured infrared vibrational wavenumbers of the shifted T−O−T antisymmetric skeletal vibrations, appearing in the 1020−780 cm-1 region of the FTIR spectra of ferrierite exchanged with metal cations.

Steric effect and effect of metal coordination on the reactivity of nitric oxide with cysteine-containing proteins under anaerobic conditions

A comparison is made between the reactivity of nitric oxide (NO) with cysteine, bovine serum albumin (BSA) and metallothionein-1 (MT1) at pH 7 under strictly anaerobic conditions. The rate of reaction of NO with these amino acid/proteins was found to be of the order: cysteine>BSA≫MT1, in clear disparity with the size of the reactants. The difference in the reaction rates is attributed to steric effects due to the high molecular size in the case of BSA and to effects of metal coordination proper as well as to steric effects associated with the closed dual shell-like structure resulting from the tight coordination of the thiolate groups with Zn 2+ in MT1. The mechanisms of the reaction of NO with cysteine, BSA and MT and its possible implication for the rate of the respective reactions are discussed.

X-ray structures of trans-dichloro(4-methylpyrimidine) ( η1-phenyl)bis(pyridine)rhodium(III) hydrate and trans-dichloro(phenyl)tris(triphenylstibine)rhodium(III) ethylacetate solvate. Molecular orbital analysis of trans-dichloro (4-methylpyrimidine)( η1-phenyl)bis(pyridine)rhodium(III)

The crystal and molecular structure of trans-dichloro(4-methylpyrimidine)( η 1 -phenyl)cis-bis(pyridine)rhodium(III) water solvate, 1x 0.17H 2 O, and trans-dichloro( η 1 -phenyl)tris(triphenylstibine)rhodium(III) ethylacetate solvate, 2x CH 3 CO 2 CH 2 CH 3 have been studied via X-ray diffraction from a single crystal at room temperature. The final refinement converged to R1 conventional index of 0.0350 and 0.0361 for the structural analysis of 1x 0.17H 2 O [space group R(-3) and 2x CH 3 CO 2 CH 2 CH 3 P(-1) , respectively. The 4-methylpyrimidine ligand (Pym) is only weakly bound to Rh in 1, as shown by the long Rh-N distance (2.251(4) Å), compared to the Rh N(pyridine) lengths average, 2.066(4) . N-C bond distances involving the N donor average 1.329(6) and 1.345(6) for Pym and pyridine (Py) ligands, respectively. The C N C bond angle on the donor is 114.1(5)° for Pym and average 117.4(4) ° for Py. The structure of the complex molecule of 2x CH 3 CO 2 CH 2 CH 3 has some differences when compared to that of the corresponding acetone solvate previously studied in this laboratory (Cini, R., Giorgi, G. and Pasquini, L., Inor`. Chim. Acta, 1992, 196, 7). The two structures differ mainly by the orientation of the phenyl donor with respect to the Cl-Rh-Cl axis (which is more eclipsed for the ethylacetate solvate) and by the conformation of the SbPh 3 ligands. Density functional calculations at the B3LYP/LANL2DZ level with full geometry optimization were carried out on the free Pym molecule and on some Sc(N 1 Pym) 3 +and Sc(N 2 Pym) 3 + model molecules. The effect of metal coordination consists mainly in enlarging the (Sc)N-C bond distances up to 0.150Å, whereas the C-N(Sc)-C bond angle decreases of 1.9°. Significant changes on other bond lengths and angles relevant to ring atoms of Pym occur upon metal coordination to the nitrogen atom. The metal coordination to N(2) is less favorable than to N(1) of 7.5 kcal for 1:1 species of Sc 3 + Extended Hückel calculations showed that HOMO consists mostly of metal-d orbitals with some character of chloride and phenyl and pyrimidine ligands, whereas LUMO is composed of phenyl, pyridine and pyrimidine orbitals. The method well reproduces the Rh-N and Rh-C bonding distances and gives Rh-C dissociation energy 2.38 and 5.48 times that for the Rh-N(Py) and Rh-N(Pym) bonds, respectively.

Effects of metal coordination on the reactivity of 1,3,4-triphenyl-1,2-dihydrophosphete

Effects of metal coordination on the reactivity of 1,3,4-triphenyl-1,2-dihydrophosphete

Conformational analysis of 14- and 16-membered unsaturated oxa, thia, and selena macrocyclic ligands

Crystallographic results retrieved from the Cambridge Structural Database (CSD) have been used to perform systematic conformational analyses of the free and metal-coordinated ligands 1,4,8,11-tetraoxa/thia/selenacyclotetradecane, 1,5,9,13-tetraoxa/thia/selenacyclohexadecane, and their derivatives. Conformational classifications established using symmetry-modified Jarvis-Patrick cluster analysis have been displayed in torsional space by principal component analysis (PCA) plots. Relative molecular mechanics energies of free macrocycles in the observed conformations are compared with the cluster populations and the effect of metal coordination investigated. While the conformations of the free macrocycles in the solid state are those with low molecular mechanics energies, substantial reorganization is required to produce tetradentate chelating conformers, particularly with S donors. With the latter, several such conformers occur, while one predominates with O donors for both macrocycles, in agreement with molecular mechanics predictions. Factors influencing the conformations adopted are discussed.

Structures of Z-(nitrostilbene)chromium tricarbonyl complexes: The effect of metal coordination on the nonplanarity of the stilbene system

The structures of the nitrostilbene complexes Z-(4-R−C6H4−CH=CH−C6H4-4′-NO2)Cr(CO)3 (R=Me2N,1; H,2; F3C,3), are reported. The copounds were prepared as part of a study of the synthesis of organometallic complexes with potential nonlinear optics applications. For Z-(4-Me2N−C6H4−CH=CH−C6H4-4′-NO2)Cr(CO)3,1, triclinic, $$P\bar 1$$ ,a=7.214 (3) A,b=7.340 (3) A,c=17.238 (9)A, α=88.23 (5)°, β=88.76 (4)°, γ=87.66 (3) °,Z=2. For Z-(C6H5−CH=CH−C6H4-4′-NO2)Cr(CO)3,2, monoclinic, P21/C,a=12.255 (3) A,b=9.415 (1) A,c=13.579 (4) A, β=105.60 (2)°,Z=4. For Z-(4-F3C−C6H4−CH=CH−C6H4-4′-NO2)Cr(CO)3,3, triclinic, $$P\bar 1$$ ,a=7.094 (2) A,b=8.057 (5) A,c=17.295 (9) A, α=87.95 (5)° β=85.94 (4)°, γ=64.24 (5)°,Z=2. The structural data show that the Z isomers exhibit bond lengths and angles similar to those observed in the E isomers, but the stilbene fragment is considerably more nonplanar. As a consequence, these molecules are unlikely to behave as NLO materials.

Vibrational and electronic spectroscopic study of lanthanides and effect of sodium on the aromatic system of benzoic acid

AbstractRaman spectra of benzoic acid, sodium benzoate and complexes of lanthanides with benzoic acid, of general formula Ln(Obz)3 (Ln = lanthanum, neodymium, terbium, dysprosium or lutetium; OBz = benzoic acid radical) were recorded. The Raman bands of lanthanide benzoates were interpreted on the basis of the Raman and IR spectra. Analysis of benzene ring vibrations, active in the Raman and IR spectra, and electronic transitions in the UV and visible spectra provided information on the effect of metal coordination on the aromatic system of benzoic acid.

Effect of lanthanides on the aromatic system of benzoic acid

The stucture of lanthanide complexes with benzoic acid was investigated by IR and UV absorption spectra. To determine the effect of metal coordination on the aromatic system of benzoic acid, IR spectra of Ln(OBz) 3 (Ln is a lanthanide except promethium; BzO is benzoic acid radical) were compared with ligand and sodium benzoate spectra. Also, changes in frequency and relative intensity of the ▪ bands in the 1600–1400 cm −1 region, were analyzed in terms of the atomic number of lanthanides. It is shown that lanthanides disturb the aromatic system of the benzoate ligand less than sodium. This effect is discussed in terms of the bonds formed.

The synthesis and characterization of mixed isocyanide complexes of cyclopentadienyliron

The alkylation of η 5-C 5H 5Fe(CO)(CN)(CNR) complexes with primary, secondary, and tertiary alkyl iodides, R′I (R ≠ R′) produces the mixed isocyanide species [η 5-C 5H 5Fe(CO)(CNR)(CNR′)]I. Infrared and NMR spectra of the new complexes are dischased. The effect of metal coordination on the 13C spectra of cationic and neutral isocyanide complexes is considered. The use of the chiral alkyl iodides, secbutyliodide and α-phenylethyliodide, results in configurationally non-labile diastereomers which, in the case of compounds of the latter, can be detected by 1H and 13C NMR spectra.

The Effect of Metal-Coordination on the Reactivity of Amino Acids and their Related Compounds

The Effect of Metal-Coordination on the Reactivity of Amino Acids and their Related Compounds