Kinetics and mechanism of CO substitution of [η-CpFe(CO)3]PF6 with PPh3 in the presence of substituted pyridine N-oxide

The kinetics of substitution reactions of [η-CpFe(CO)₃]PF₆ with PPh₃ in the presence of R-PyOs have been studied. For all the R-PyOs (R = 4-OMe, 4-Me, 3,4-(CH)₄, 4-Ph, 3-Me, 2,3-(CH)₄, 2,6-Me₂, 2-Me), the reactions yeild the same product [η⁵-CpFe(CO)₂PPh₃]PF₆, according to a second-order rate law that is first order in concentrations of [η⁵-CpFe(CO)₃]PF₆ and of R-PyO but zero order in PPh₃ concentration. These results, along with the dependence of the reaction rate on the nature of R-PyO, are consistent with an associative mechanism. Activation parameters further support the bimmolecular nature of the reactions: ΔH^≠ = 13.4 ± 0.4 kcal mol⁻¹, ΔS^≠ = −19.1 ± 1.3 cal k⁻¹ mol⁻¹ for 4-PhPyO; ΔH^≠ = 12.3 ± 0.3 kcal mol⁻¹, ΔS^≠ = 24.7 ±1.0 cal K⁻¹ mol⁻¹ for 2-MePyO. For the various substituted pyridine N-oxides studied in this paper, the rates of reaction increase with the increasing electron-donating abilities of the substituents on the pyridine ring or N-oxide basicities, but decrease with increasing ¹⁷O chemical shifts of the N-oxides. Electronic and steric factors contributing to the reactivity of pyridine N-oxides have been quantitatively assessed.     

Stereognostic coordination chemistry 4 the design and synthesis of a selective uranyl ion complexant

A new approach to ligand design for the sequestration of metal-oxo cations has been called stereognostic coordination chemistry, in that the ligand incorporates a traditional Lewis base coordination to the metal center and a hydrogen bond donor to interact with the oxo group. This paper reports the synthesis of ligands that are more rigid and sterically predisposed to bind the targeted UO₂²⁺ cation. These are the tripod ligands tris-N,N′,N′′-[2-(2-carboxy-phenoxy)ethyl]-1,4,7-triazacyclononane bis-hydrochloride (ETAC · 2HCl) and tris-N,N′,N′′-[2-(2-carboxy-4-decyl-phenoxy)ethyl]-1,4,7-triazacyclononane tris-hydrochloride (DETAC · 3HCl), which chelate uranyl with a tris-carboxylate coordination sphere and provide a hydrogen bond donor through a protonated amine on the triazacyclononane macrocycle to interact with one uranyl oxo atom. Structural models predict that upon uranyl binding the hydrogen bond donor must point directly towards the oxo atom, enforcing a stereognostic interaction. Both ETAC and DETAC chelate the uranyl ion; DETAC is a powerful extractant and will quantitatively extract uranyl into an organic phase at pH 1.9 and above. The extraction coefficient is estimated to be 10¹⁴ in neutral aqueous conditions. Vibrational spectra of ¹⁸O labeled UO₂²⁺ have been used to probe the stereognostic coordination to uranyl utilizing hydrogen bonding.     

Formation of dioxygen complexes of transition metal chelates in oxygen saturated aprotic solvents. Cyclic voltammetric evidence at a micro glassy carbon disk electrode

Cyclic voltammetry at a micro electrode of Co(II) salen, Fe(II) salen, electrode generated Fe(II)(acac)₂, Fe(II) (salicylaldehyde)₂, Fe(II) (salicylaldoxime)₂, Fe(II) (bipy)₃, Fe(II) (bipy)₂, Co(II) (bipy)₃, Co(II) (benzacac)₂, and electrode generated Co(acac)₂ in oxygen saturated aprotic solvents show positive shift of the O₂ sigmoidal wave, as well as enhancement of the limiting current in the case of the first five compounds. In the case of Co(II) (bipy)₃ the slope of the sigmoidal wave due to O₂ becomes more positive, while for the other two Co(II) complexes there is no change except a small decrease in the wave height. The data are used to correlate and predict the O₂ binding properties of the chelates in solution. The data for the diketone complexes of Co(II) indicate absence of any direct association, which is in line with the interpretation offered in the literature on the mechanism of their catalytic role in the O₂ oxidation of substrates. The mechanism of the autoxidation of dimethylformamide in the presence of Fe(III) (bipy)₃ and Cu(II) (bipy)₂ is elucidated by the observation that these higher valent compounds are reduced to their next lower oxidation state by DMF.     

Chemistry in environmentally benign media Part 1. Synthesis and characterization of 1,2-bis[bis(hydroxymethyl)phosphino]ethane (‘HMPE’). X-ray structure of [Pt{(HOH2C)2PCH2CH2P(CH2OH)2}2](Cl)2

The water-soluble bisphosphine, 1,2-bis(bis(hydroxymethyl)phosphino)ethane (1), was synthesized in near quantitative yield by the reaction of bisphosphine, H₂PCH₂CH₂PH₂, with an aqueous formaldehyde in the presence of K₂PtCl₄. The reaction of this water-soluble bisphosphine 1 with cis-Pt(COD)Cl₂ affords the mononuclear bischelate complex, [Pt{(HOH₂C)₂PCH₂CH₂P(CH₂OH)₂}₂](Cl)₂ (2), in near quantitative yield. The new ligand and complex have been characterized spectroscopically and the structure of the metal complex, 2, was determined by X-ray crystallography. The Pt(II) complex 2 crystallizes in the orthorhombic space group Pbca(a=14.623(1), B=16.216(2), C=9.319(4) Å) with Z=4. The final R value is 0.024.     

Properties of monovalent nickel complexes with tetraaza-macrocyclic ligands in aqueous solutions

The effects of N-alkylation on the redox potential of the couples NiLⁱ²⁺/NiLⁱ⁺, L = tetraaza-14-membered-macrocyclic ligands, and on the properties of the monovalent nickel complexes in aqueous solutions are reported for 14 complexes. The spectra and lifetimes of the NiLⁱ⁺ complexes are reported. The self-exchange rates for the couples NiLⁱ²⁺/NiLⁱ⁺ were determined. Two of the ligands were synthesized for the first time for this study. Cyclic voltammetry and pulse radiolysis were used. The results point out that: (i) N-alkylation always shifts the redox potential to a less cathodic one; this effect stems to a large degree from the decrease in the solvation energy of the complex caused by the N-alkylation of the ligand. (ii). The lifetime of the monovalent complexes is not linearly related to the redox potential of the NiLⁱ²⁺/NiLⁱ⁺ couples. (iii) The NiLⁱ⁺ complexes exist in several isomeric forms; the rate of the isomerization depends on the structure of the ligand. (iv) Different isomers of NiLⁱ⁺ may be formed when the complex NiLⁱ²⁺ is reduced by different reagents; therefore, the pulse-radiolytically formed NiLⁱ⁺ complexes might have different properties than those formed electrochemically.     

Olefin versus sulfur coordination of benzo[b]thiophene (BT) in Cp(CO)2Re(η:η-μ2-BT)Cr(CO)3

Reactions of Cr(CO)₃(η⁶-BT), in which the Cr is π-coordinated to the benzene ring of benzo[b]thiophene (BT), with Cp′(CO)₂Re(THF), where Cp′ = η⁵-C₅H₅ or η⁵-C₅Me₅, give the products Cp′(CO)₂Re(η²:η⁶-μ₂-BT)Cr(CO)₃ in which the Cr remains coordinated to the benzene ring and Re is bound to the C(2)=C(3) double bond. An X-ray diffraction study of Cp(CO)₂Re(η²:η⁶-μ₂-BT)Cr(CO)₃ (3) provides details of the geometry. This structure contrasts with that of the Cp′(CO)₂Re(BT) complexes that exist as mixtures of isomers in which the BT is coordinated to the Re through either the double bond (2,3-η²) or the sulfur (η¹(S)). Thus, the electron-withdrawing Cr(CO)₃ group in 3 stabilizes the 2,3-η² mode of BT coordination to the Cp′(CO)₂Re fragment. Implications of these results for catalytic hydrodesulfurization of BT are discussed. Crystal data for 3: triclinic, space group

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.     

The kinetics and energetics of formation of a molecular hydrogen complex involving a dinuclear ruthenium(II) centre

The reversible equilibrium conversion under H₂ of [RuCl(dppb) (μ-Cl)]₂ (1) to generate (η²-H₂) (dppb) (μ-Cl)₃RuCl(dppb) in CH₂Cl₂ (dppb = Ph₂P(CH₂)₄PPh₂) has been studied at 0–25 °C by UV-Vis and ³¹P{¹H} NMR spectroscopy, and by stoppe kinetics; the equilibrium constant and corresponding thermodynamic parameters, and the forward and reverse rate constants at 25 °C have been determined. A measured ΔH° value of 0 kJ mol⁻¹ allows for an estimation of an exothermicity of 60 kJ mol⁻¹ for binding an η²-H₂ at an Ru(II) centre; a ΔS° value of 60 J mol⁻¹ K⁻¹ indicates that in solution 1 contain s coordinated CH₂Cl₂. The kinetic and thermodynamic data are compared to those obtained from a previously studied hydrogenation of styrene catalyzed by 1. Preliminary findings on related systems containing Ph₂P(CH₂)₃PPh₂ and (C₆H₁₁)₂P(C₆H₁₁)₂ are also noted.     

Formation of a metallocycle by dimerization of acrylonitrile. Crystal structure of the hexakis{(1,4-dicyanobutane-1,4-diyl)-nickel(II)} complex

A novel hexanickel(II) complex [Ni₆(NCCHCH₂CH₂CHCN)₆] (2) with 1,4-dicyanobutane-1,4-diyl (L) which was produced by the metal-induced dimerization of acrylonitrile (AN) has been isolated and the structure has been determined crystallographically. Complex 2 is triclinic, space group

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. Each nickel atom is coordinated by two carbon atoms of L and two nitrogen atoms of the cyano group of two other L, providing a square-plenar geometry. The six nickel atoms are bridged by the cyano group and carbon atom to form the slightly distorted octahedral Ni₆ core.     

A method for long-term measurement of respiration in intertidal fishes during simulated intertidal conditions

Aquatic and aerial respiration of the amphibious fishes Lipophrys pholis and Periophthalmus barbarus were examined using a newly designed flow-through respirometer system. The system allowed long-term measurements of oxygen consumption and carbon dioxide release during periods of aquatic and aerial respiration. The M o ₂ of L. pholis , measured at 15° C, was 2·1 μmol O₂ g^–1 h^–1 during aquatic and 1·99 μmol O₂ g^–1 h^–1 during aerial exposure. The corresponding values of the M co₂ were 1.67 and 1.59 μmol O₂ g^–1 h^–1 respectively, giving an aquatic respiratory exchange ratio (RER) of 0·80 and an aerial RER of 0·79. The M o₂ of P. barbarus , measured at 28°C, was 4·05 μmol O₂ g^–1 h^–1 during aquatic and 3·44 μmol O₂ g^–1 h^–1 during aerial exposure. The corresponding values of the Mco₂ were 3·29 μmol CO₂ g^–1 h^–1 and 2·63 μmol CO₂ g^–1 h^–1 respectively, giving an aquatic RER of 0·81 and an aerial RER of 0·77. While exposed to air for at least 10 h, both species showed no decrease in metabolic rate or carbon dioxide release. The RER of these fishes equalled their respiratory quotient. After re-immersion an increased oxygen consumption, due to the payment of an oxygen debt, could not be detected.