Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “complexity”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 433 records · Page 24Linked to original sources
PubMed2005

First example of a 2:1 cocrystal of mixed Cu(I)/Cu(II) complexes and a novel ferromagnetic bis(mu-hydroxo)dicopper(II) complex with a bis(pyrazol-1-yl)methane bidentate ligand.

A unique 2:1 cocrystal of mixed Cu(I)/Cu(II) complexes [Cu(I)(H2CPz2)(MeCN)2](ClO4) (1) and [Cu(II)(H2CPz2)2(ClO4)2] (4), a novel ferromagnetic ClO(4-)-bridged bis(mu-hydroxo)dicopper(II) complex, [Cu2(H2CPz2)2(OH)2(ClO4)](ClO4)(CH3CN)(0.5) (5), and a bischelated copper(I) complex, [Cu(H2CPz2)2](ClO4) (2), prepared from a one-pot reaction of [Cu(MeCN)4](ClO4) and H2CPz2, are described. The structures of these complexes have been determined by X-ray crystallographic methods. The Cu(I)-N(acetonitrile) bond distances in complex 1 are nonequivalent (1.907(8) and 2.034(9) A), leading to the dissociation of one MeCN to form a Y-shaped complex, [Cu(I)(H2CPz2)(MeCN)](ClO4) (3), which is oxidized readily in air to form complex 5 with a butterfly Cu2O2 core.

Journal Article↗
PubMed2004

The activation of tertiary carboxamides in metal complexes: an experimental and theoretical study on the methanolysis of acylated bispicolylamine copper(II) complexes.

It is a well-established concept that the C-N bond cleavage of carboxamide functions is facilitated by the coordination of a metal ion to the carbonyl oxygen atom. In contrast, the alternative C-N bond activation by coordination of a neutral tertiary carboxamide nitrogen atom has not been studied. We present the first results on the effect of nitrogen pyramidalization in N-coordinated metal complexes on the methanolysis of tertiary carboxamide groups. An analysis of the reactions products obtained from the methanol cleavage of [(N-Acyl-bpa)Cu]2+ (bpa = N,N-bispicolylamine) complexes is presented together with experimental and high-level theoretically calculated structures. The strong effect of different anions on the amide pyramidalization and subsequent C-N-bond cleavage is evaluated. We show that dichloro complexes [(N-Acyl-bpa)CuCl2] have much less activated amide groups than the corresponding triflate species. They should therefore be less reactive. However, [(N-Acyl-bpa)CuCl2] complexes dissociate in solution to give cationic monochloro complexes [(N-Acyl-bpa)Cu(S)Cl]+ (S = solvent molecule). Theoretical calculations show that the amide pyramidalization in the monochloro complexes is equal to that in the corresponding CF3SO3- salts. Consequently, chloro and triflato complexes are cleaved with similar rates and efficiencies. Parallels to and differences in the reactivity of purely organic distorted amides are discussed.

Journal Article↗
PubMed2005

NMR and EPR studies of low-spin Fe(III) complexes of meso-tetra-(2,6-disubstituted phenyl)porphyrinates complexed to imidazoles and pyridines of widely differing basicities.

A series of bis-axially ligated complexes of iron(III) tetramesitylporphyrin, TMPFe(III), tetra-(2,6-dibromophenyl)porphyrin, (2,6-Br2)4TPPFe(III), tetra-(2,6-dichlorophenyl)porphyrin, (2,6-Cl2)4TPPFe(III), tetra-(2,6-difluorophenyl)porphyrin, (2,6-F2)4TPPFe(III), and tetra-(2,6-dimethoxyphenyl)porphyrin, (2,6-(OMe)2)4TPPFe(III), where the axial ligands are 1-methylimidazole, 2-methylimidazole, and a series of nine substituted pyridines ranging in basicity from 4-(dimethylamino)pyridine (pK(a)(PyH(+)) = 9.70) to 3- and 4-cyanopyridine (pKa(PyH+) = 1.45 and 1.1, respectively), have been prepared and characterized by EPR and 1H NMR spectroscopy. The EPR spectra, recorded at 4.2 K, show "large g(max)", rhombic, or axial signals, depending on the iron porphyrinate and axial ligand, with the g(max) value decreasing as the basicity of the pyridine decreases, thus indicating a change in electron configuration from (d(xy))2(d(xz),d(yz)3 to (d(xz),d(yz))4(d(xy))1 through each series at this low temperature. Over the temperature range of the NMR investigations (183-313 K), most of the high-basicity pyridine complexes of all five iron(III) porphyrinates exhibit simple Curie temperature dependence of their pyrrole-H paramagnetic shifts and beta-pyrrole spin densities, rho(C) approximately 0.015-0.017, that are indicative of the S = 1/2 (d(xy))(2)(d(xz),d(yz))(3) electron configuration, while the temperature dependences of the pyrrole-H resonances of the lower-basicity pyridine complexes (pK(a)(PyH(+)) < 6.00) show significant deviations from simple Curie behavior which could be fit to an expanded version of the Curie law using a temperature-dependent fitting program developed in this laboratory that includes consideration of a thermally accessible excited state. In most cases, the ground state of the lower-basicity pyridine complexes is an S = 1/2 state with a mixed (d(xy))2(d(xz),d(yz))3/(d(xz),d(yz))4(d(xy))1 electron configuration, indicating that these two are so close in energy that they cannot be separated by analysis of the NMR shifts; however, for the TMPFe(III) complexes with 3- and 4-CNPy, the ground states were found to be fairly pure (d(xz),d(yz))4(d(xy))1 electron configurations. In all but one case of the intermediate- to low-basicity pyridine complexes of the five iron(III) porphyrinates, the excited state is found to be S = 3/2, with a (d(xz),d(yz))3(d(xy))1(d(z)2)1 electron configuration, lying some 120-680 cm(-1) higher in energy, depending on the particular porphyrinate and axial ligand. Full analysis of the paramagnetic shifts to allow separation of the contact and pseudocontact contributions could be achieved only for the [TMPFe(L)2]+ series of complexes.

Electron Spin Resonance Spectroscopy↗
PubMed2006

Effects of steric constraint on Chromium(III) complexes of tetraazamacrocycles. 3. Insights into the temperature-dependent radiationless deactivation of the 2Eg (Oh) excited state of trans-[Cr(N4)(CN)2]+ complexes.

Macrocyclic complexes of the type trans-[Cr(N4)(CN)2]+, where N4 = cyclam, 1,11-C3-cyclam, and 1,4-C2-cyclam demonstrate significant variation in their room-temperature excited-state behavior; namely, the lifetimes of the 2Eg (Oh) excited states are 335, 23, and 0.24 micros, respectively. The lifetimes of these complexes have been measured in acidified H2O/dimethyl sulfoxide over the temperature range between -30 and +95 degrees C. Arrhenius activation parameters were calculated from these data. There was very little variation in the values of the Arrhenius preexponential factor between these three complexes, whereas the value of Ea is 40.6 kJ/mol for the cyclam complex, 35.5 kJ/mol for the 1,11-C3-cyclam complex, and 22.3 kJ/mol for the 1,4-C2-cyclam complex. Thus, differences in the room-temperature excited-state lifetimes can be rationalized based on the competition between thermally independent nonradiative relaxation and a thermally activated channel. To test whether a photodissociation mechanism involving Cr-macrocyclic N bond cleavage is a plausible explanation for the thermally activated relaxation pathway, samples of the cyclam complex were photolyzed in acidified D(2)O. A marked increase in the lifetime after photolysis demonstrated the occurrence of photodeuteration and thus a likely photodissociation of a macrocyclic N.

Journal Article↗
PubMed1997

New Adducts of Dirhodium(II) Formamidinate Complexes with Polycyano Acceptor Molecules. X-ray Crystal Structure of the Tricyanomethanide Complex Rh(2)(form)(4)[C(CN)(3)] (form = N,N'-Di-p-tolylformamidinate).

The dirhodium(II) formamidinate complexes Rh(2)(form)(2)(O(2)CCF(3))(2)(H(2)O)(2) (I) and Rh(2)(form)(4) (II, form = N,N'-di-p-tolylformamidinate) react with the polycyano acceptor molecules tetracyanoethylene (TCNE), tetracyano-p-quinodimethane (TCNQ), 2,5-dimethyl-N,N'-dicyano-p-quinonediimine (2,5-DMDCNQI), and N,N'-naphthocyano-p-quinonediimine (NCNQI) giving species whose nature is critically dependent on the redox potentials of the two parent complexes. Complex I reacts via axial coordination with negligible charge transfer (CT) from the dimetal unit to the ligand. With TCNE, it gives the labile monoaxial adduct Rh(2)(form)(2)(O(2)CCF(3))(2)(TCNE) (1), which easily loses the cyano ligand restoring the parent complex. TCNQ, 2,5-DMDCNQI, and NCNQI react with I giving polymeric materials of composition {[Rh(2)(form)(2)(O(2)CCF(3))(2)](2)TCNQ)}(n)() (2) and [Rh(2)(form)(2)(O(2)CCF(3))(2)X](n)() (X = 2,5-DMDCNQI (3), NCNQI (4)). The reaction of II with TCNE, TCNQ, and 2,5-DMDCNQI proceeds via a single electron transfer from the dimetal unit to the cyano ligand to form the CT species [Rh(2)(form)(4)X] (X = TCNE (5), TCNQ (6), 2,5-DMDCNQI (7)). Electrochemical and EPR measurements suggest a different extent of coordination between the polycyano fragment and the dirhodium unit, depending upon the polarity of the solvents. Attempts to crystallize complex 5 from acetonitrile unexpectedly led to the formation of the tricyanomethanide complex Rh(2)(form)(4)[C(CN)(3)] (5A), arising from the unprecedented transformation of the tetracyanoethylenide ion into the tricyanomethanide anion. The complex crystallizes in the tetragonal P4/ncc space group with a = 14.169(6) Å, c = 29.20(2) Å, V = 5863(5) Å(3), and Z = 4. The molecule consists of a dirhodium unit symmetrically bridged by four formamidinate ligands and one tricyanomethanide anion N-coordinated at the axial position of Rh(2).

Journal Article↗
PubMed1997

Synthesis and Magnetic Properties of Six New Trinuclear Oxo-Centered Manganese Complexes of General Formula [Mn(3)O(X-benzoato)(6)L(3)] (X = 2-F, 2-Cl, 2-Br, 3-F, 3-Cl, 3-Br; L = Pyridine or Water) and Crystal Structures of the 2-F, 3-Cl, and 3-Br Complexes.

The reaction of N-n-Bu(4)MnO(4) or NaMnO(4) with appropriate reagents in ethanol-pyridine leads to the high-yield formation of new mixed-valence trinuclear oxo-centered Mn(III,III,II) complexes of general formulation [Mn(3)O(X-benzoato)(6)L(3)] (1, X = 2-fluoro, L = pyridine; 2, X = 2-chloro, L = pyridine; 3, X = 2-bromo, L = pyridine; 4, X = 3-fluoro, L = 2 pyridine + 1 H(2)O; 5, X = 3-chloro, L = 2 pyridine + 1 H(2)O; 6, X = 3-bromo, L = 2 pyridine + 1 H(2)O). The crystal structures of 1, 5, and 6 were determined. Complex 1 crystallizes in the monoclinic system, space group C2/c with a = 15.774(2) Å, b = 17.269(2) Å, c = 21.411(2) Å, beta = 91.11(1) degrees, and Z = 4. Complex 5 crystallizes in the monoclinic system, space group P2(1)/n with a = 15.172(2) Å, b = 17.603(2) Å, c = 21.996(3) Å, beta = 106.300(10), and Z = 4. Complex 6 crystallizes in the monoclinic system, space group P2(1)/n with a = 15.533(3) Å, b = 17.884(2) Å, c = 21.997(4) Å, beta = 106.95(1) degrees, and Z = 4. The three complexes are neutral and possess an oxo-centered Mn(3)O unit with peripheral ligands provided by bridging carboxylate and terminal pyridine or H(2)O groups. Each manganese ion is distorted octahedral, and consideration of overall charge necessitates a mixed-valence Mn(II)Mn(III)(2) description. In 1, the presence of a C(2) axis through the central O atom and one of the manganese atoms (Mn(II)) and the absence of imposed symmetry elements in 5 and 6 (they have the two Mn(III) with a terminal pyridine group and the Mn(II) with a H(2)O terminal molecule) suggest a trapped-valence situation in all three cases. The Mn(II) is assigned on the basis of its longer metal-ligand distances. Variable-temperature magnetic susceptibility studies were performed on 1-6 in the temperature range 2-300 K. Satisfactory fits to the observed susceptibility data were obtained by assuming isotropic magnetic exchange interactions and using the appropriate spin Hamiltonian and susceptibility equation. The derived J and J exchange parameters are all relatively small in magnitude, |J| < 10 cm(-)(1). J characterizes the Mn(II).Mn(III) interactions and J the Mn(III).Mn(III) interaction. Magnetization measurements at 2 K up to 50 kG indicate the variability of the ground state: S = (3)/(2) for 2 and 3; S = (1)/(2) for 1, 4, and 5; and S = (3)/(2), (1)/(2) for 6. X-band EPR spectra measured from 4 K to room temperature on polycrystalline samples of 1-6 show highly significant differences when the ground state is (3)/(2) or (1)/(2). For S = (3)/(2) complexes (2 and 3), there is a transition centered at g approximately 4, which decreases in intensity with increasing temperature. For S = (1)/(2) complexes, this g approximately 4 band does not appear but instead there are broad bands centered at g approximately 2. These results are discussed in terms of spin frustration within the Mn(3)O core, which produces different spin ground states and susceptibility values.

Journal Article↗
PubMed1998

Reactivity of the Verdoheme Analogues, 5-Oxaporphyrin Complexes of Cobalt(II) and Zinc(II), with Nucleophiles: Opening of the Planar Macrocycle by Alkoxide Addition To Form Helical Complexes.

The 5-oxaporphyrin macrocycle is a modified porphyrin (with an oxygen atom replacing a meso-methine unit) that is produced during heme degradation in biological and chemical systems. We have undertaken an investigation into the chemical reactivity of metal complexes of this macrocycle. Here we report that the diamagnetic zinc(II) and paramagnetic cobalt(II) verdoheme analogues, [Zn(II)(OEOP)](O(2)CCH(3)) and [Co(II)(OEOP)](PF(6)) (OEOP is the monoanion of octaethyl-5-oxaporphyrin), undergo ring opening when treated with alkoxide ions. The zinc(II) complex Zn(II)(OEBOMe), where OEBOMe is the dianion of octaethylmethoxybiliverdin, is sufficiently stable to be isolated in crystalline form, but the cobalt(II) analogue, Co(II)(OEBOMe), is less stable and has been characterized primarily by (1)H NMR spectroscopy in solution. The reactions are accompanied by a color change from blue-green for the verdoheme complexes to yellow-green for the ring-opened compounds. The ring-opened product Zn(II)(OEBOMe) has been subjected to a crystallographic study which reveals that the complex contains a four-coordinate zinc(II) ion whose geometry is significantly controlled by the helical tetrapyrrole ligand so that tetrahedral coordination is not achieved. The structure of this complex is compared to that of a closely related free ligand and those of copper and cobalt complexes of tetrapyrrole ligands that also form helical complexes.

Journal Article↗
PubMed1998

Formation and Deprotonation Kinetics of the Sitting-Atop Complex of Copper(II) Ion with 5,10,15,20-Tetraphenylporphyrin Relevant to the Porphyrin Metalation Mechanism. Structure of Copper(II)-Pyridine Complexes in Acetonitrile As Determined by EXAFS Spectroscopy.

The formation of a sitting-atop (SAT) complex of Cu(II) ion with 5,10,15,20-tetraphenylporphyrin (H(2)tpp) in acetonitrile has been observed, and the kinetic parameters for the formation were determined as follows: k(S0) = (3.6 +/- 0.1) x 10(5) mol(-)(1) dm(3) s(-)(1) at 25.0 degrees C, DeltaH(S0)() = 56 +/- 5 kJ mol(-)(1), and DeltaS(S0)() = 46 +/- 19 J mol(-)(1) K(-)(1). The (1)H NMR spectrum of the SAT complex (Cu(H(2)tpp)(2+)) indicated that two pyrrolenine nitrogens coordinate to the Cu(II) ion and that two protons bound to the pyrrole nitrogens remain. The protons were abstracted by the addition of pyridine (py) as the Brønsted base to give the Cu(tpp) metalloporphyrin. In the presence of py, the product for the reaction of the Cu(II) ion with H(2)tpp was Cu(tpp) instead of the SAT complex. The observed conditional rates for the formation of Cu(H(2)tpp)(2+) and Cu(tpp) were interpreted by the contribution of Cu(2+), Cu(py)(2+), and Cu(py)(2)(2+) species, and the second-order rate constants of the SAT complex formation were k(S1) = (3.5 +/- 0.3) x 10(4) mol(-)(1) dm(3) s(-)(1) for Cu(py)(2+) and k(S2) = 90 +/- 2 mol(-)(1) dm(3) s(-)(1) for Cu(py)(2)(2+). Deprotonation rates were measured by following the reaction between the SAT complex and py as a function of the py concentration, and the second-order rate constant was determined to be (2.3 +/- 0.1) x 10(2) mol(-)(1) dm(3) s(-)(1). The present kinetic results have indicated that the SAT complex exists during the course of the metalation process and that the SAT complex formation is a rate-determining step.

Journal Article↗
PubMed2000

Synthesis and characterization of mixed-ligand oxorhenium complexes with the SNN type of ligand. Isolation of a novel ReO[SN][S][S] complex.

A new series of mixed-ligand oxorhenium complexes 4-9, with ligands 1-3 (L1H2) containing the SNN donor set and monodentate thiols as coligands (L2H), is reported. All complexes were synthesized using ReOCl3(PPh3)2 as precursor. They were isolated as crystalline products and characterized by elemental analysis and IR and NMR spectroscopy. The ligands 1 and 2 (general formula RCH2CH2NHCH2CH2SH, where R = N(C2H5)2 in 1 and pyrrolidin-1-yl in 2) act as tridentate SNN chelates to the ReO3+ core, leaving one open coordination site cis to the oxo group. The fourth coordination site is occupied by a monodentate aromatic thiol which acts as a coligand. Thus, three new "3 + 1" [SNN][S] oxorhenium complexes 4-6 (general formula ReO[RCH2CH2NCH2CH2S][SX], where R = N(C2H5)2 and X = phenyl in 4, R = N(C2H5)2 and X = p-methylphenyl in 5, and R = pyrrolidinlyl and X = p-methylphenyl in 6) were prepared in high yield. Complex 4 adopts an almost perfect square pyramidal geometry (tau = 0.07), while 6 forms a distorted square pyramidal geometry (tau = 0.24). In both complexes 4 and 6, the basal plane is formed by the SNN donor set of the tridentate ligand and the S of the monodentate thiol. On the other hand, the ligand 3, [(CH3)2CH]2NCH2CH2NHCH2CH2SH, acts as a bidentate ligand, probably due to steric hindrance, and it coordinates to the ReO3+ core through the SN atoms, leaving two open coordination sites cis to the oxo group. These two vacant positions are occupied by two molecules of the monodentate thiol coligand, producing a novel type of "2 + 1 + 1" [SN][S][S] oxorhenium mixed-ligand complexes 7-9 (general formula ReO[[(CH3)2CH]2NCH2CH2NHCH2CH2S][SX][SX], where X = phenyl in 7, p-methylphenyl in 8, and benzyl in 9). The coordination sphere about rhenium in 7 and 8 consists of the SN donor set of ligand 3, two sulfurs of the two monodentate thiols, and the doubly bonded oxygen atom in a trigonally distorted square pyramidal geometry (tau = 0.44 and 0.45 for 7 and 8, respectively). Detailed NMR assignments were determined for complexes 5 and 8.

Journal Article↗
PubMed2001

Electronic structure of bis(o-iminobenzosemiquinonato)metal complexes (Cu, Ni, Pd). The art of establishing physical oxidation states in transition-metal complexes containing radical ligands.

The ligand 2-anilino-4,6-di-tert-butylphenol and its 2-(3,5-dichloroanilino)-4,6-di-tert-butylphenol analogue react in CH(3)CN or CH(3)OH solutions with divalent transition metal ions in the presence of air and triethylamine. Depending on the metal:ligand ratio (1:1, 1:2, or 1:3) and the presence (or absence) of the cyclic amine 1,4-dimethyl-1,4,7-triazacyclononane (dmtacn), the following complexes have been isolated as crystalline solids: [Co(III)(L(ISQ))(3)] (1); [Cu(II)(dmtacn)(L(ISQ))]PF(6) (2); [Cu(II)(L(ISQ))(2)] (3); [Ni(II)(L(ISQ))(2)] (4a); [Ni(II)((Cl)L(ISQ))(2)] (4b); [Pd(II)(L(ISQ))(2)] (5). (L(ISQ))(-) represents the monoanionic o-iminobenzosemiquinonate radical (S(rad) = (1)/(2)). Compounds 1-5 have been characterized by single-crystal X-ray crystallography at 100(2) K. For all complexes it is unambiguously established that the O,N-coordinated o-iminobenzosemiquinonato(1-) ligand is present. Complexes 3, 4b, and 5 are square planar molecules which possess an S(t) = (1)/(2), 0, and 0 ground state, respectively, as was established by (1)H NMR and EPR spectroscopies and variable-temperature magnetic susceptibility measurements. Complex 2 possesses an S(t) = 1 ground state which is attained via strong intramolecular ferromagnetic coupling (J = +195 cm(-1)) between the d(x)2-(y)2 magnetic orbital of the Cu(II) ion and the pi-orbital of the ligand radical. Complex 1 contains three mutually orthogonal (L(ISQ))(-*) ligands and has an S(t) = (3)/(2) ground state. It is shown that the electronic structure of 4a and 5 is adequately described as singlet diradical containing a divalent, diamagnetic d(8) configurated central metal ion and two strongly antiferromagnetically coupled (L(ISQ))(-) radical ligands. It is concluded that the same electronic structure prevails in the classic bis(o-diiminobenzosemiquinonato)- and bis(o-benzosemiquinonato)metal complexes of Ni(II), Pd(II), and Pt(II). The electrochemistry of all complexes has been investigated in detail. For 3, 4a, and 5 a series of reversible one-electron-transfer waves leads to the formation of the anions and cations [M(L)(2)](2-),(1-),(1+),(2+) which have been characterized spectroelectrochemically. All redox processes are shown to be ligand-based.

Journal Article↗
PubMed2004

14-electron disilene palladium complex having strong pi-complex character.

The first 14-electron disilene palladium complex eta2-[tetrakis(tert-butyldimethylsilyl)disilene](tricyclohexylphosphine)palladium (4) was synthesized. In the solid state, complex 4 has one tricyclohexylphosphine ligand bound unsymmetrically to the palladium center in regard to the eta2-disilene moiety. The elongation of the Si-Si bond length from that of the corresponding free disilene (3.2%) and the bent back angles of the disilene moiety (4.41 degrees and 9.65 degrees ) for 4 were much smaller than those for the corresponding 16-electron eta2-disilene complex (Me3P)2Pd[tetrakis(tert-butyldimethylsilyl)disilene] (4.6% and 27.2 degrees , respectively). Complex 4 is regarded as the complex having the strongest pi-complex character among the known disilene complexes. Highly symmetric NMR spectra were observed for 4; central Si1 and Si2 nuclei were equivalent and appeared as a doublet with 2J(29Si-31P) of 19 Hz, indicating facile flipping of the phosphine ligand in the Pd-Si-Si plane.

Journal Article↗
PubMed2004

Hydrogenation of carbon dioxide and aryl isocyanates by a tetranuclear tetrahydrido yttrium complex. Isolation, structures, and CO2 insertion reactions of methylene diolate and mu3-oxo yttrium complexes.

The reaction of carbon dioxide with a tetranuclear tetrahydrido yttrium complex [(C5Me4SiMe3)Y(mu-H)]4(L) (L = Me3SiCC(H)C(H)CSiMe3) (1) rapidly afforded the corresponding bis(methylene diolate) complex [(C5Me4SiMe3)Y]4(mu-O2CH2)2(L) (2), while the reactions of an aryl isocyanate with 1 led to selective formation of the mu3-oxo complex [(C5Me4SiMe3)Y]4(mu-O)( mu-H)2(L) (5) or [(C5Me4SiMe3)Y]4(mu-O)2(L) (7), depending on the substrate ratio. Both the methylene diolate and the oxo complexes can undergo CO2 insertion reactions to give the corresponding carbonate complexes. These reactions not only yield a new series of polynuclear yttrium complexes having novel structures but also shed new light on the mechanistic aspects of the heterogeneous hydrogenation of COmicron2. The high reactivity of the polynuclear mu3-oxo yttrium complexes 5 and 7 could also make them novel molecular models for study of metal oxide-supported catalysts.

Journal Article↗
PubMed2005

Mechanism of the mild functionalization of arenes by diboron reagents catalyzed by iridium complexes. Intermediacy and chemistry of bipyridine-ligated iridium trisboryl complexes.

This paper describes mechanistic studies on the functionalization of arenes with the diboron reagent B(2)pin(2) (bis-pinacolato diborane(4)) catalyzed by the combination of 4,4'-di-tert-butylbipyridine (dtbpy) and olefin-ligated iridium halide or olefin-ligated iridium alkoxide complexes. This work identifies the catalyst resting state as [Ir(dtbpy)(COE)(Bpin)(3)] (COE = cyclooctene, Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl). [Ir(dtbpy)(COE)(Bpin)(3)] was prepared by independent synthesis in high yield from [Ir(COD)(OMe)](2), dtbpy, COE, and HBpin. This complex is formed in low yield from [Ir(COD)(OMe)](2), dtbpy, COE, and B(2)pin(2). Kinetic studies show that this complex reacts with arenes after reversible dissociation of COE. An alternative mechanism in which the arene reacts with the Ir(I) complex [Ir(dtbpy)Bpin] after dissociation of COE and reductive elimination of B(2)pin(2) does not occur to a measurable extent. The reaction of [Ir(dtbpy)(COE)(Bpin)(3)] with arenes and the catalytic reaction of B(2)pin(2) with arenes catalyzed by [Ir(COD)(OMe)](2) and dtbpy occur faster with electron-poor arenes than with electron-rich arenes. However, both the stoichiometric and catalytic reactions also occur faster with the electron-rich heteroarenes thiophene and furan than with arenes, perhaps because eta(2)-heteroarene complexes are more stable than the eta(2)-arene complexes and the eta(2)-heteroarene or arene complexes are intermediates that precede oxidative addition. Kinetic studies on the catalytic reaction show that [Ir(dtbpy)(COE)(Bpin)(3)] enters the catalytic cycle by dissociation of COE, and a comparison of the kinetic isotope effects of the catalytic and stoichiometric reactions shows that the reactive intermediate [Ir(dtbpy)(Bpin)(3)] cleaves the arene C-H bond. The barriers for ligand exchange and C-H activation allow an experimental assessment of several conclusions drawn from computational work. Most generally, our results corroborate the conclusion that C-H bond cleavage is turnover-limiting, but the experimental barrier for this bond cleavage is much lower than the calculated barrier.

Benzene Derivatives↗
PubMed2007

Size distribution, complexing capacity, and stability of phosphate-metal-humic complexes.

Size distribution, maximum complexing ability, and stability constants for phosphate-metal-humic (PO43--M-HA) complexes involving two trivalent (Fe and Al) and five divalent metal (M) bridges (Zn, Cu, Mn, Ca, and Mg) were investigated at the pH values 4, 6, and 8. Results highlighted the existing competition between metal-humic acid (M-HA) aggregation and the formation of PO43--M-HA complexes. However, the fact that only a very low fraction of complexed metal is involved in PO43- fixation seems to be related to the existence of specific electronic and/or steric requirements in the binding site in the metal-humic complex. The importance of the ionic form of phosphate (H2PO4- or HPO42-) and the involvement of phenolic and especially carboxylic groups in the phosphate binding are discussed. Finally, the order of stability obtained for PO43--M-HA complexes was similar to that of M-HA complexes. This result suggests that PO43--M-HA might play a significant role in the dynamics of phosphorus in certain soil types.

Binding Sites↗
PubMedDate not supplied

Metal complexes of poly(alpha-amino acids). A potentiometric and circular dichroism investigation of Cu(II) complexes of poly(L-lysine), poly(L-ornithine), and poly(L-diaminobutyric acid).

The conformational properties of cupric complexes of poly(L-lysine), poly(L-ornithine), and poly(L-diaminobutyric acid) have been investigated by potentiometric, visible and UV absorption, and circular dichroism (CD) techniques. The three polymers form two kinds of complexes stable at pH less than 8.5 (type I complexes) and at pH less than 8.5 (type II complexes). It has been found that in the low pH complexes of poly(L-diaminobutyric acid) at least one deprotonated amido nitrogen is coordinated to cupric ions. Type II complexes involve always amide nitrogens in the coordination sphere of Cu(II). Evidence is presented that the structure of such complexes is not compatible with the alpha-helical conformation of the peptide backbone.

Aminobutyrates↗
PubMed2003

Correlations between complexation modes and redox activities of Ni(II)-GSH complexes.

The formation of Ni(II) complexes of GSH in conditions of 4-fold GSH excess over Ni(II) was studied by potentiometric titrations, UV-vis and CD spectroscopies, and magnetic susceptibility measurements. The following set of complexes was obtained in the pH range of 6-12: NiHL, Ni(2)L(2)(2)(-), NiHL(2)(3)(-), NiL(2)(4)(-), and NiH(-)(1)L(2)(5)(-). The first of these is an octahedral species, coordinated through the donors of the Glu moiety of GSH, while the remaining ones are largely square-planar, with participation of the thiol in Ni(II) coordination. Magnetic moments indicate the presence of a spin equilibrium for Ni(2)L(2)(2)(-), NiHL(2)(3)(-), and NiL(2)(4)(-) complexes. Phosphate ions apparently decompose the Ni(2)L(2)(2)(-) complex, converting it into a monomeric, high spin, ternary species. Among the molecular forms of GSH, HL(2)(-) is the one most susceptible to air oxidation, due to a presence of ionic interactions between its protonated amine and deprotonated thiol moieties. The complexation of Ni(II) accelerates air oxidation of GSH in alkaline solutions by a factor of 4, but this effect is absent at neutral pH. The damage to plasmid DNA by H(2)O(2) is facilitated by Ni(II) ions and inhibited by excess of GSH. However, the analysis of the concentration profile of this process indicates that octahedral Ni(II) complexes with GSH are involved in the formation of double strand breaks. Finally, numerical simulations of intracellular Ni(II) distribution, made possible by the determination of stability constants of Ni(II) complexes of GSH, indicate that histidine and ATP, rather than GSH, may act as ligands for Ni(II) in vivo. Altogether, our results suggest that the direct impact of GSH on Ni(II) toxicity may be of a limited character.

Air↗
PubMed2005

Sec17p and HOPS, in distinct SNARE complexes, mediate SNARE complex disruption or assembly for fusion.

SNARE functions during membrane docking and fusion are regulated by Sec1/Munc18 (SM) chaperones and Rab/Ypt GTPase effectors. These functions for yeast vacuole fusion are combined in the six-subunit HOPS complex. HOPS facilitates Ypt7p nucleotide exchange, is a Ypt7p effector, and contains an SM protein. We have dissected the associations and requirements for HOPS, Ypt7p, and Sec17/18p during SNARE complex assembly. Vacuole SNARE complexes bind either Sec17p or the HOPS complex, but not both. Sec17p and its co-chaperone Sec18p disassemble SNARE complexes. Ypt7p regulates the reassembly of unpaired SNAREs with each other and with HOPS, forming HOPS.SNARE complexes prior to fusion. After HOPS.SNARE assembly, lipid rearrangements are still required for vacuole content mixing. Thus, Sec17p and HOPS have mutually exclusive interactions with vacuole SNAREs to mediate disruption of SNARE complexes or their assembly for docking and fusion. Sec17p may displace HOPS from SNAREs to permit subsequent rounds of fusion.

Carrier Proteins↗
PubMed2006

Zinc complexes of anionic NPPN and NP(S)PN ligands and rearrangement to the isomeric NPNP and NP(S)NP ligands in mercury complexes.

The lithium (imido)diphosphineimide Li(Et2O)[DippNPhP-P((n)Bu)PhNDipp] (1) (Dipp = 2,6-(i)Pr2C6H3) undergoes simple metathesis reactions with equimolar amounts of zinc halides, ZnCl2 and (t)BuZnBr, to give the respective N,N'-chelated complexes {Zn(micro-Cl)[DippNPhP-P((n)Bu)PhNDipp]}2 (2) and (t)BuZn[NDippPhP-P((n)Bu)PhNDipp] (3). In contrast, the reaction of two equivalents of complex 1 with HgCl2 affords the rearranged bis(imidodiphosphinoamine) complex, Hg[PhP([double bond, length as m-dash]NDipp)(micro-NDipp)P((n)Bu)Ph]2 (4), where the ligand acts as a P-centered anion. The (imido)diphosphineimide backbone of remains intact on oxidation with elemental sulfur to afford the lithium (imido)diphosphineimine sulfide complex, Li(Et2O)[DippNPhP(S)-P((n)Bu)PhNDipp] (6). Reactions of 6 with group 12 metal halides show similar behaviour to those of complex 1. The N,N' chelated metathesis products RZn[DippNPhP(S)-P((n)Bu)PhNDipp] (7, R = Cl; 8, R = (t)Bu) are obtained on reaction with ZnCl2 and (t)BuZnBr, respectively. Isomerization of the ligand backbone occurs on reaction of 6 with HgCl2 to form the homoleptic P,S-chelated mercury complex Hg[Ph(S)P(=NDipp)(micro-NDipp)P((n)Bu)Ph]2 (9). Complexes 2, 3, 4, 6, 8 and 9 have been characterized by X-ray crystallography.

Journal Article↗