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PubMed2004

Activation and cleavage of dinitrogen by three-coordinate metal complexes involving Mo(III) and Nb(II/III).

Density functional calculations have been employed to rationalize why the heteronuclear N(2)-bridged Mo(III)Nb(III) dimer, [Ar((t)Bu)N](3)Mo(mu-N(2))Nb[N((i)Pr)Ar](3)(Ar = 3,5-C(6)H(3)Me(2)), does not undergo cleavage of the dinitrogen bridge in contrast to the analogous Mo(III)Mo(III) complex which, although having a less activated N-N bond, undergoes spontaneous dinitrogen cleavage at room temperature. The calculations reveal that although the overall reaction is exothermic for both systems, the actual cleavage step is endothermic by 144 kJ mol(-1) for the Mo(III)Nb(III) complex whereas the Mo(III)Mo(III) system is exothermic by 94 kJ mol(-1). The reluctance of the Mo(III)Nb(III) system to undergo N(2) cleavage is attributed to its d(3)d(2) metal configuration which is one electron short of the d(3)d(3) configuration necessary to reductively cleave the dinitrogen bridge. This is confirmed by additional calculations on the related d(3)d(3) Mo(III)Nb(II) and Nb(II)Nb(II) systems for which the cleavage step is calculated to be substantially exothermic, accounting for why in the presence of the reductant KC(8), the [Ar((t)Bu)N](3)Mo(mu-N(2))Nb[N((i)Pr)Ar](3) complex was observed to undergo spontaneous cleavage of the dinitrogen bridge. On the basis of these results, it can be concluded that the level of activation of the N-N bond does not necessarily correlate with the ease of cleavage of the dinitrogen bridge.

Journal Article↗
PubMed2005

Structure and photoluminescence property of two-dimensional coordination polymer complexes involving Cu(I)(6)X(6)(X = Cl, Br, I) hexagon prism cluster supported by a tripodal tripyridine ligand with 1,3,5-triethylbenzene spacer.

Treatment of cuprous halide (Cu(I)X, X = Cl, Br, and I) and a tripodal tripyridine ligand (L) consisting of a 1,3,5-triethylbenzene spacer gave a unique two-dimensional (2D) polymer sheet structure involving a rare Cu(I)(6)X(6) hexagon prism cluster unit, which exhibits intense fluorescence around 448-476 nm.

Journal Article↗
PubMed2004

Complex unitary vectors for the direction of propagation and for the polarization of electromagnetic waves in absorbing isotropic media.

We introduce a formalism based on complex unitary vectors for the direction of propagation and for the polarization in order to describe in detail the propagation of inhomogeneous plane waves in absorbing isotropic media. We obtain analytic expressions for the displacement vector, the electric field, the magnetic field, and the Poynting vector, and we study their geometry in terms of the geometrical interpretation of the complex directions of propagation inside the material. We introduce a complex coordinate system based on complex unitary vectors, where the description of the polarization states of the field vectors and the Poynting vector becomes simpler. The physical meaning and the interpretation of the mathematical operations involving these complex unitary vectors is provided.

Journal Article↗
PubMed2002

Synthesis and characterization of three-coordinate and related beta-diketiminate derivatives of manganese, iron, and cobalt.

Treatment of M[N(SiMe(3))(2)](2) (M = Mn, Fe, Co) with various bulky beta-diketimines afforded a variety of new three-coordinate complexes which were characterized by UV-vis, (1)H NMR and IR spectroscopy, magnetic measurements, and X-ray crystallography. Reaction of the beta-diketimine H(Dipp)NC(Me)CHC(Me)N(Dipp) (Dipp(2)N(wedge)NH; Dipp = C(6)H(3)-2,6-Pr(i)(2)) with M[N(SiMe(3))(2)](2) (M = Mn or Co) gave Dipp(2)N(wedge)NMN(SiMe(3))(2) (M = Mn, 1; Co, 3) while the reaction of Fe[N(SiMe(3))(2)](2) with Ar(2)N(wedge)NH (Ar = Dipp, C(6)F(5), Mes, C(6)H(3)-2,6-Me(2), or C(6)H(3)-2,6-Cl(2)) afforded the series of iron complexes Ar(2)N(wedge)NFe[N(SiMe(3))(2)] (Ar = Dipp, 2a; C(6)F(5), 2b; Mes, 2c; C(6)H(3)-2,6-Me(2), 2d; C(6)H(3)-2,6-Cl(2), 2e). This represents a new synthetic route to beta-diketiminate complexes of these metals. The four-coordinate bis-beta-diketiminate complex Fe[N(wedge)N(C(6)F(5))(2)](2), 4, was also isolated as a byproduct from the synthesis of 2b. Direct reaction of the Dipp(2)N(wedge)NLi with CoCl(2) gave the "ate" salt Dipp(2)N(wedge)NCoCl(2)Li(THF)(2), 5, in which the lithium chloride has formed a complex with Dipp(2)N(wedge)NCoCl through chloride bridging. The Fe(III) species Dipp(2)N(wedge)NFeCl(2), 6, was obtained cleanly from the reaction of FeCl(3) with Dipp(2)N(wedge)NLi. Magnetic measurements showed that all the complexes have a high spin configuration. The different substituents in the series of iron complexes 2a-e allowed assignment of their paramagnetically shifted (1)H NMR spectra. The X-ray crystal structures 1-2d and 3 showed that they have a distorted three-coordinate planar configuration at the metals whereas complexes 4-6 have highly distorted four-coordinate geometries.

Journal Article↗
PubMed2005

Theoretical investigations of uranyl-ligand bonding: four- and five-coordinate uranyl cyanide, isocyanide, carbonyl, and hydroxide complexes.

The coordination and bonding of equatorial hydroxide, carbonyl, cyanide (CN-), and isocyanide (NC-) ligands with uranyl dication, [UO2]2+, has been studied using density functional theory with relativistic effective core potentials. Good agreement is seen between experimental and calculated geometries of [UO2(OH)4]2-. Newly predicted ground-state structures of [UO2(OH)5]3-, [UO2(CO)4]2+, [UO2(CO)5]2+, [UO2(CN)4]2-, [UO2(CN)5]3-, [UO2(NC)4]2-, and [UO2(NC)5]3- are reported. Four-coordinate uranyl isocyanide complexes are the predicted gas-phase species while five-coordinate uranyl cyanide complexes are energetically favorable in aqueous solution. Small energy differences between cyanide and isocyanide complexes indicate the energetic feasibility of mixed cyanide and isocyanide complexes. A D2d uranyl tetrahydroxide is the dominant gas-phase and aqueous species, but formation of uranyl carbonyl complexes is seen to be exothermic in the gas-phase and endothermic in aqueous solution.

Journal Article↗
PubMed2003

Synthetic analogues of the histidine-chlorophyll complex: a NMR study to mimic structural features of the photosynthetic reaction center and the light-harvesting complex.

Mg(II)-porphyrin-ligand and (bacterio)chlorophyl-ligand coordination interactions have been studied by solution and solid-state MAS NMR spectroscopy. (1)H, (13)C and (15)N coordination shifts due to ring currents, electronic perturbations and structural effects are resolved for imidazole (Im) and 1-methylimidazole (1-MeIm) coordinated axially to Mg(II)-OEP and (B)Chl a. As a consequence of a single axial coordination of Im or 1-MeIm to the Mg(II) ion, 0.9-5.2 ppm (1)H, 0.2-5.5 ppm (13)C and 2.1-27.2 ppm (15)N coordination shifts were measured for selectively labeled [1,3-(15)N]-Im, [1,3-(15)N,2-(13)C]-Im and [1,3-(15)N,1,2-(13)C]-1-MeIm. The coordination shifts depend on the distance of the nuclei to the porphyrin plane and the perturbation of the electronic structure. The signal intensities in the (1)H NMR spectrum reveal a five-coordinated complex, and the isotropic chemical shift analysis shows a close analogy with the electronic structure of the BChl a-histidine in natural light harvesting 2 complexes. The line broadening of the ligand responses support the complementary IR data and provide evidence for a dynamic coordination bond in the complex.

Bacteriochlorophyll A↗
PubMed2001

The cerebellum and parietal cortex play a specific role in coordination: a PET study.

The synthesis of complex, coordinated movements from simple actions is an important aspect of motor control. Lesion studies have revealed specific brain areas, particularly the cerebellum, to be essential for a variety of coordinated movements, and lend support to the view that the cerebellum is engaged in the integration of simple movements into compound ones. A PET study was therefore conducted to show which brain areas were active specifically during the coordinated execution of an arm and finger movement to visual targets. A two-by-two factorial design was employed, in which subjects either made arm or finger movements alone, made coordinated arm-finger movements, or made no movements. Voxels were identified where activity was significantly greater during the execution of coordinated movements than when movements were made alone and in which this increased activity could not be accounted for simply by the additive effects of the activations for each movement in isolation. The behavioral results showed that subjects coordinated arm and finger movements well during coordination scans. Coordination-specific activations were found in left anterior lobe and bilaterally in the paramedian lobules of the cerebellum. These are known to receive forelimb-specific spinocerebellar proprioceptive inputs that may be related to multijoint movements. The same areas also receive corticocerebellar afference from motor areas that may convey efference copy information to the cerebellum. Coordination-specific activations were also seen in areas of the posterior parietal cortex. The results provide direct evidence in healthy human subjects of specific cerebellar engagement during the coordination of movement, over and above the control of constituent movements.

Adult↗
PubMed2005

Designing tridentate ligands for ruthenium(II) complexes with prolonged room temperature luminescence lifetimes.

Coordination complexes have been used extensively as the photoactive component of artificial photosynthetic devices. While polynuclear arrays increase the probability of light absorption, the incorporation of the stereogenic Ru(2,2'-bipyridine)(3)(2+) motif gives rise to diastereomeric mixtures whereas the achiral Ru(2,2':6',2"-terpyridine)(2)(2+) motif creates stereopure polynuclear complexes. Thus, polynuclear arrays composed of ruthenium(II) complexes of tridentate ligands are the targets of choice for light-harvesting devices. As Ru(II) complexes of tridentate ligands have short excited state lifetimes at room temperature (r. t.), considerable effort has been focused on trying to increase their r. t. luminescence lifetime for practical applications. This tutorial review will report on the sophisticated synthetic strategies currently in use to enhance the room temperature photophysical properties of Ru(II) complexes of tridentate ligands.

Ligands↗
PubMed2006

Reduction of vanadium(V) by L-ascorbic acid at low and neutral pH: kinetic, mechanistic, and spectroscopic characterization.

L-Ascorbic acid interacts with vanadium(V) over the pH range of 0.4-7.0 to form three different coordination complexes. Both inner- and outer-sphere electron-transfer pathways are proposed to form vanadium(IV) complexes with L-ascorbate or dehydroascorbate, respectively. Effects of the pH on the coordination of L-ascorbic acid to the vanadium(V) center were observed and are presumably related to the speciation of the vanadium(V) ion. Three vanadium(IV) complexes were observed using ambient-temperature electron paramagnetic resonance spectroscopy. Two of these complexes are proposed to be vanadium(IV) L-ascorbate complexes, and one is consistent with a vanadium(IV) dehydroascorbic acid complex proposed earlier. These reduction reactions will occur under physiological conditions and could be important to the reduction of vanadium(V)-containing coordination complexes used as insulin-enhancing agents for treatment of diabetes.

Ascorbic Acid↗
PubMed2005

Direct probe of iron vibrations elucidates NO activation of heme proteins.

We use nuclear resonance vibrational spectroscopy (NRVS) to identify the Fe-NO stretching frequency in the NO adduct of myoglobin (MbNO) and in the related six-coordinate porphyrin Fe(TPP)(1-MeIm)(NO). Frequency shifts observed in MbNO Raman spectra upon isotopic substitution of Fe or the nitrosyl nitrogen confirm and extend the NRVS results. In contrast with previous assignments, the Fe-NO frequency of these six-coordinate complexes lies 70-100 cm-1 lower than in the analogous five-coordinate nitrosyl complexes, indicating a significant weakening of the Fe-NO bond in the presence of a trans imidazole ligand. This result supports proposed mechanisms for NO activation of heme proteins and underscores the value of NRVS as a direct probe of metal reactivity in complex biomolecules.

Hemeproteins↗
PubMed2004

Structural characterization and electronic properties determination by high-field and high-frequency EPR of a series of five-coordinated Mn(II) complexes.

The isolation, structural characterization, and electronic properties of a series of high-spin mononuclear five-coordinated Mn(II) complexes, [Mn(terpy)(X)(2)] (terpy = 2, 2':6',2' '-terpyridine; X = I(-) (1), Br(-) (2), Cl(-) (3), or SCN(-) (4)), are reported. The X-ray structures of the complexes reveal that the manganese ion lies in the center of a distorted trigonal bipyramid for complexes 1, 2, and 4, while complex 3 is better described as a distorted square pyramid. The electronic properties of 1-4 were investigated by high-field and high-frequency EPR spectroscopy (HF-EPR) performed between 5 and 30 K. The powder HF-EPR spectra have been recorded in high-field-limit conditions (95-285 GHz) (D << gbetaB). The spectra are thus simplified, allowing an easy interpretation of the experimental data and an accurate determination of the spin Hamiltonian parameters. The magnitude of D varies between 0.26 and 1.00 cm(-)(1) with the nature of the anionic ligand. Thanks to low-temperature EPR experiments, the sign of D was unambiguously determined. D is positive for the iodo and bromo complexes and negative for the chloro and thiocyano ones. A structural correlation is proposed. Each complex is characterized by a significant rhombicity with E/D values between 0.17 and 0.29, reflecting the distorted geometry observed around the manganese. Finally, we compared the spin Hamiltonian parameters of our five-coordinated complexes and those previously reported for other analogous series of dihalo four- and six-coordinated complexes. The effect of the coordination number and of the geometry of the Mn(II) complexes on the spin Hamiltonian parameters is discussed.

Crystallography, X-Ray↗
PubMed1999

Carboxylate binding modes in zinc proteins: a theoretical study.

The relative energies of different coordination modes (bidentate, monodentate, syn, and anti) of a carboxylate group bound to a zinc ion have been studied by the density functional method B3LYP with large basis sets on realistic models of the active site of several zinc proteins. In positively charged four-coordinate complexes, the mono- and bidentate coordination modes have almost the same energy (within 10 kJ/mol). However, if there are negatively charged ligands other than the carboxylate group, the monodentate binding mode is favored. In general, the energy difference between monodentate and bidentate coordination is small, 4-24 kJ/mol, and it is determined more by hydrogen-bond interactions with other ligands or second-sphere groups than by the zinc-carboxylate interaction. Similarly, the activation energy for the conversion between the two coordination modes is small, approximately 6 kJ/mol, indicating a very flat Zn-O potential surface. The energy difference between syn and anti binding modes of the monodentate carboxylate group is larger, 70-100 kJ/mol, but this figure again strongly depends on interactions with second-sphere molecules. Our results also indicate that the pK(a) of the zinc-bound water ligand in carboxypeptidase and thermolysin is 8-9.

Carboxylic Acids↗
PubMed1999

Synthesis and Characterization of Indium Compounds with Phosphinothiol Ligands. The Crystal and Molecular Structures of [In{2-(Ph(2)P)C(6)H(4)S}(3)], [In{2-(Ph(2)P)-6-(Me(3)Si)C(6)H(3)S}(2){2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)S}], and [NMe(4)][In{PhP(C(6)H(4)S-2)(2)}(2)].CH(3)CN.

The electrochemical oxidation of anodic indium metal in an acetonitrile solution of phosphinothiol ligands affords [In{2-(Ph(2)P)C(6)H(4)S}(3)] (1), [In{2-(Ph(2)P)-6-(Me(3)Si)C(6)H(3)S)}(2){2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)S}] (2), [In{2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)S}(3)] (3), and [NMe(4)][In{PhP(C(6)H(4)S-2)(2)}(2)].CH(3)CN (4) complexes exhibiting distorted six-coordinate geometries based on {InS(3)P(3)}, {InS(3)P(2)O}, and {InS(4)P(2)} cores, respectively. In all cases, the In-P bond distances are anomalously long, presumably as a consequence of steric crowding. The anion of 4 provides an unusual example of an In(III)-thiolate coordination complex ion. Crystal data: 1, C(54)H(42)InP(3)S(3), monoclinic, P2(1)/c, a = 16.6579(1) Å, b = 12.6628(2) Å, c = 22.5520(3) Å, beta = 96.42(1) degrees, V = 4727.15(1) Å(3), Z = 4, 6176 reflections, R = 0.0579; 2, C(63)H(66)InOP(3)S(3)Si(3), monoclinic, P2(1)/c, a = 11.53090(10) Å, b = 26.2505(3) Å, c = 20.4206(2) Å, beta = 94.0870(10) degrees, V = 6165.43(11) Å(3), Z = 4, 10 699 reflections, R = 0.0621; 4, C(42)H(41)InN(2)P(2)S(4), monoclinic, P2(1), a = 13.0052(2) Å, b = 11.2240(2) Å, c = 14.3070(3) Å, beta = 93.190(1) degrees, V = 2085.16(7) Å(3), Z = 2, 6651 reflections, R = 0.0352.

Journal Article↗
PubMed1991

The chemical basis for the ferriprotoporphyrin IX-chloroquine complex induced lipid peroxidation.

Ferriprotoporphyrin IX (FPIX) forms a coordination complex with chloroquine, an anti-malarial drug. The FPIX-chloroquine complex strongly promotes the peroxidative cleavage of phospholipid membrane. Iron in the complex is essential for the complex to induce lipid peroxidation. In this paper a more detailed mechanism of the complex promoted lipid peroxidation was investigated. Apotransferrin exhibited no apparent inhibition of the complex evoked lipid peroxidation, indicating no mobilization of iron from the complex. No significant inhibitory effect by superoxide dismutase, catalase and sodium benzoate on the complex induced lipid peroxidative reaction, suggesting little involvement of superoxide anion, hydrogen peroxide and hydroxyl radical in the reaction. Quinine and mefroquine, blood shizontocidal drugs as well as chloroquine, formed a complex with FPIX and each complex more rapidly induced lipid peroxidation than FPIX alone. Primaquine, which is not as effective as quinine or mefroquine on an intraerythrocytic malaria parasite, neither coordinated to FPIX nor promoted lipid peroxidation. The complex formation between FPIX and chloroquine, quinine or mefroquine could play a key role in their anti-malarial actions.

Animals↗
PubMed2005

Novel CdCl2 and HgCl2 complexes with 3-monosubstituted and 3,3-disubstituted 1-furoylthioureas: IR and Raman spectra.

Two series of coordination complexes of CdCl(2) and HgCl(2) with 3-monosubstituted and 3,3-disubstituted 1-furoylthioureas were prepared and characterized. These complexes were obtained with a medium to high yield from ethanolic solutions of both ligand and salt. The formed complex results from the salt-ligand interaction with participation of both the salt anion and cation. Information on the coordination chemistry of these complexes was derived from thermal stability data, and IR, Raman and (13)C CPMAS NMR spectra. On coordination the electronic structure of these ligands changes as a whole, affecting practically all their vibrational pattern, however, within that complex pattern some vibrations provide valuable information on the nature of the studied complexes. These thiourea derivatives behave as neutral ligands, which coordinate the metal ion through the sulfur atom of the thiocarbonyl group. This fact is supported by the observed frequency shift, to lower values, in the nu(CS) vibration on the coordination and the appearance of a low frequency Raman line which was assigned to the metal-sulfur stretching, nu(M-S), in the formed complex. The frequency of the nu(CO) vibration always increases on complex formation, which discards the participation of the carbonyl group in the coordination process. The complexation takes place preserving the free ligand conformation, established from intra-molecular interactions, particularly in 3-monosubstituted ligands. Such features of the studied ligands and their complexes are also supported by (13)C CPMAS NMR spectra. This spectroscopic information correlates with the reported behavior of the ligands in ion selective electrodes.

Cadmium Chloride↗
PubMed1984

Phosphorus-31 nuclear relaxation rate studies of the nucleotides on phosphoenolpyruvate carboxykinase.

The interactions of nucleotide substrates with the enzyme phosphoenolpyruvate carboxykinase and its Mn2+ complex were investigated by several methods. Direct binding shows the formation of stoichiometric complexes. The presence of Mn2+ increases the affinity of the enzyme for nucleotide. A higher affinity for GTP (Kd less than 2 microM) than for GDP (Kd = 15 microM) was determined. Solvent proton relaxation rate studies indicate no substantial difference in titration curves for free nucleotide or for Mg-nucleotide to the enzyme-Mn complex. The effect of Mn2+ on the 31P relaxation rates of IDP and of ITP in the binary Mn-nucleotide complex indicates the formation of direct coordination complexes. The distances of the alpha- and beta-31P of IDP to Mn2+ are identical (3.5 A). The Mn2+ distance to the beta- and gamma-31P of ITP is also identical (3.7 A) and is 0.2 A further from the alpha-phosphorus. In the presence of P-enolpyruvate carboxykinase, the effect of Mn2+ on the 31P relaxation rates was measured at 40.5 MHz and at 121.5 MHz. The dipolar correlation time was calculated to be 0.6-5.4 ns, depending upon assumptions made. The Mn2+ to phosphorus distances indicate the nucleotide substrates form a second sphere complex to the bound Mn2+. From 1/T2 measurements, electron delocalization from Mn2+ to the phosphorus atoms is indicated; this effect occurs although direct coordination does not take place. The exchange rate of GTP from the enzyme-Mn complex (koff = 4 X 10(4) s-1) is rapid compared to kcat with a lower energy of activation (9.2 kcal/mol) than for catalytic turnover.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗