Transition metals, complexes with pyridines

Syntheses, reactions, and biological activity of pyridine oximes 03KGS963. Transition metal complexes with 2,2 -bipyridines, pyridine-phosphines, and pyridine A-oxides as ligands 04IZV1733. 

Other common transition metal corrosion products typically monitored at various sites within the plant include iron, copper, nickel, zinc, and chromium. More than 80% of BWR plants analyze for iron, nickel, copper, and zinc in reactor water, and nearly all of the BWR plants determine these metals in feed water. In addition, zinc is also an additive used in many plants to control the shutdown radiation dose rate. Nickel and chromium are corrosion products in BWR plants fi-om stainless-steel piping. The best selectivity and sensitivity for achieving low to submicrogram/Liter detection limits for transition metals can be obtained by separating transition metal complexes using pyridine-2, 6-dicarboxylic acid (PDCA) or oxalic acid as chelators in the eluent, followed by postcolumn derivatization with 4-(2-pyridylazo)resorcinol (PAR) and absorbance detection at 520 nm (see Section 8.2.1.2). This approach was successfully used to determine trace concentrations of iron, copper, nickel, and zinc in BWR and PWR matrices [197]. Figure 10.113 compares the chromatograms from the... 

A unique method to generate the pyridine ring employed a transition metal-mediated 6-endo-dig cyclization of A-propargylamine derivative 120. The reaction proceeds in 5-12 h with yields of 22-74%. Gold (HI) salts are required to catalyze the reaction, but copper salts are sufficient with reactive ketones. A proposed reaction mechanism involves activation of the alkyne by transition metal complexation. This lowers the activation energy for the enamine addition to the alkyne that generates 121. The transition metal also behaves as a Lewis acid and facilitates formation of 120 from 118 and 119. Subsequent aromatization of 121 affords pyridine 122. 

Pyridine 1-oxide, like pyridine, can act as a ligand in transition metal complexes, but unfortunately good stability constants are not known. However, Shupack and Orchin have found that the C===C stretching frequency of the ethylene ligand in trans-ethylene pyridine 1-oxide dichloroplatinum(II) varies linearly with the pA and hence with the C7-value (ct+ or a, respectively) of substituents in the pyridine oxide. The data for the above reaction series are included in Table V. 

Transition metal complex-mediated hydroacylation of pyridine derivatives with aldehydes 99SL1. 

Three approaches have been tested, as already described above for inorganic supports. The first attempts concern the direct reaction of transition metal carbonyls with unmodified organic polymers like poly-2-vinyl-pyridine.61 62 However, this kind of anchoring is restricted to only a few complexes. Various polymers have been functionalized with donor groups 63-72 ligand displacement reactions using these afforded the corresponding immobilized complexes. Finally, tests with modified complexes and unmodified polymers are scarce because of the low stability of these complexes under the conditions of reactions. 

The coordination shell comprises three fac-S and three fac-N-donor atoms in [Fe(6mp)3] [FeClJCl, where 6mp = 6-thiopurine, (189). Both iron(II) and iron(III) complexes are included in a review of transition metal complexes of thiosemicarbazones. " 5,5 -Dimethyl-l,2,3-cyclohex-anetrione-l,2-dioxime-3-thiosemi-carbazone, dcdt (190), acts as an A, A, 5 -donor to Fe +, giving a bis-ligand complex (contrast [Fe(7V,7V -dcdt)3] with Fe " "). The Schiff bases from pyridine 2-carboxaldehyde and thiosemicarbazide or 4-phenyl thiosemicarbazide also act as A, VV, 5 -donors, both to Fe " " and to Fe " ". The antibacterial activity of these complexes was assessed, in... 

The free electron pair(s) in the concave pyridines 3 (Table 1), 13 (s. Scheme 3) and 29 (s. Scheme 5) and especially in the concave 1,10-phenanthrolines 11 (s. Scheme 2) and 21 (Structures 3) are not only able to bind a proton, they may also be used to coordinate a metal ion. For concave 1,10-phenanthrolines 11 and 21, transition metal complexes 87 (Structure 11) have already been generated [18, 55]. They form readily in acetonitrile solution with binding constants of 10 10 and larger. Of great importance is the nature of the chains X in the concave 1,10-phenanthrolines 21 (Structures 3). Pure aliphatic chains lead to smaller association constants than polyether chains. 

Acetylacetone usually forms transition metal complexes by coordination through bidentate oxygens. Since platinum forms unusually strong bonds to carbon, acetylacetonato complexes of platinum(II) are frequently C-bonded. When Pt(acac)2 is treated with 1 mole of pyridine, a bidentate oxygen-bonded acetylacetonate ligand is converted to a y-carbon-bonded ligand. 

Pyridines are also well known as ligands in transition metal complexes, and if the equilibrium constants for the formation of such complexes can be related to base strength, it is expected that such constants would follow the Hammett equation. The problem has been reviewed,140 and a parameter S, formulated which is a measure of the contribution of the additional stabilization produced by bond formation to the stabilization constants of complexes expressed in terms of a.141 The Hammett equation has also been applied to pyridine 1 1 complexation with Zn(II), Cd(II), and Hg(II) a,/3,y,<5-tetraphenylporphins,142 143 the a values being taken as measures of cation polarizing ability. Variation of the enthalpy of complexation for adducts of bis(2,4-pentanediono)-Cu(II) with pyridines plotted against a, however, exhibited a curved relationship.144... 

Ammonium dithiocarbamates can be prepared through reaction (11). With HC1 or H2S04 at lower temperatures the free acids are formed. Only one transition metal complex (37) is known, with a pseudo-octahedral coordination of Ni four sulfur atoms of the dithiocarbamate in the equatorial plane and two pyridine solvent molecules at the apical positions. Sulfides can be eliminated from monosubstituted dithiocarbamates to give isothiocyanates.53... 

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