RESUMO

The synthesis of cyclometalated osmium complexes is usually more complicated than of other transition metals such as Ni, Pd, Pt, Rh, where cyclometalation reactions readily occur via direct activation of C-H bonds. It differs also from their ruthenium analogs. Cyclometalation for osmium usually occurs under more severe conditions, in polar solvents, using specific precursors, stronger acids, or bases. Such requirements expand reaction mechanisms to electrophilic activation, transmetalation, and oxidative addition, often involving C-H bond activations. Osmacycles exhibit specific applications in homogeneous catalysis, photophysics, bioelectrocatalysis and are studied as anticancer agents. This review describes major synthetic pathways to osmacycles and related compounds and discusses their practical applications.

RESUMO

A zwitterionic-based chemical, the 3,3'-(octadecylamino)dipropionic acid, was quantum-theoretically designed to be applied as a corrosion inhibitor for protecting oxidized iron surfaces against the attack of very corrosive gasolines. Its performance, as well as those of worldwide-employed nitrogen-free carboxylic-diacid-based corrosion inhibitors, were experimentally evaluated and compared. Through Density-Functional-Theory calculations of the molecular interactions of the corrosion inhibitors with an iron-oxide cluster model, along with the experimental corrosion-inhibiting evaluations, it is revealed that the zwitterionic-based chemical substantially overcomes the performance of nitrogen-free chemicals. It is shown by the theoretical results that the two carboxylic heads of either, the zwitterionic-based or the nitrogen-free corrosion inhibitors, reinforce the octahedral coordination around the exposed Fe3+ atom of the iron oxide. Furthermore, when the zwitterionic-based chemical is bonded to the Fe3+ atom, a two-rings chelate is formed, in contrast to the one-ring chelate formed by the nitrogen-free corrosion inhibitors. Finally, it is theoretically predicted that oleic solvents improve the performance of the zwitterionic-based corrosion inhibitor because preclude the steric hindrance of nitrogen.

Assuntos

RESUMO

Three cyclometalated and one coordination compounds [Os(C-N)x(bpy)3-x](m) (x/m=0/2+ (4); 1/1+ (3); 2/1+ (2); 3/0 (1); (-)C-N=2-phenylpyridinato, bpy=2,2'-bipyridine) with drastically different reduction potentials have been used for analyzing the second-order rate constants for one-electron, metal-based osmium(II) to osmium(III) oxidation of the complexes by compound I (k2) and compound II (k3) of horseradish peroxidase. Previously unknown k2 and k3 have been determined by digital simulation of cyclic voltammograms measured in phosphate buffer of pH7.6 and 21 ± 1°C. Osmium(II) species derived from osmium(III) complexes 1 and 2 were generated electrochemically in situ. Under the conditions used the reduction potentials for the Os(III/II) feature equal -0.90, -0.095, 0.23 and 0.85V versus NHE (normal hydrogen electrode) for 1-4, respectively. The rate constants k2 equal ~5 × 10(7), 6 × 10(8), 2 × 10(6) and 1 × 10(5)M(-1)s(-1) and the rate constants k3 equal ~9 × 10(6), 4× 10(7), 1 ×10(6) and 1 × 10(5)M(-1)s(-1) for complexes 1-4, respectively. Both rate constants k2 and k3 first increase with increasing the reaction driving force on going from 4 to 2 but then both decrease on going to complex 1 though the reaction driving force is the highest in this case. The system described has been explored theoretically using docking Monte Carlo simulations.

Assuntos

RESUMO

Electrochemical studies of [Os(C-N)(x)(N-N)(3-x)](m+) (1) consisting of known Os(II) species with x = 0 (a) and 1 (b) and crystallographically characterized new Os(III) bis- and tris-metalacycles with x = 2 (c) and 3 (d) (N-N = 2,2'-bipyridine, (-)C-N = 2-phenylpyridinato) revealed a Nernstian behavior in MeCN. A stepwise replacement of neutral N-N ligands by three anionic C-N donors covers a 2 V potential range from -1 to +1 V vs. Ag/AgCl for the Os(III)/Os(II) feature.

RESUMO

Mild electrophilic C(sp2)-H cyclometalation of 2-phenylpyridine and N,N-dimethylbenzylamine by the chloro-bridged osmium(II) dimer [OsCl(micro-Cl)(eta6-C6H6)]2 in acetonitrile affords cyclometalated pseudotetrahedral OsII complexes [Os(C approximately N)(eta6-C6H6)(NCMe)]PF6 (C approximately N=o-C6H4py-kappa C,N (2) and o-C6H4CH2NMe2-kappa C,N (5), respectively) in good to excellent yields. The cyclometalation reactions are super sensitive to the nature of an external base. Sodium hydroxide is essential for cyclometalation of 2-phenylpyridine, but NaOH retards metalation of N,N-dimethylbenzylamine, the tertiary amine being self-sufficient as a base. Further reactions of compounds 2 and 5 with 1,10-phenanthroline or 2,2'-bipyridine (N approximately N) lead to the substitution of the eta6-bound benzene to produce octahedral species [Os(C approximately N)(N approximately N)(NCMe)2]PF6 or [Os(C approximately N)(N approximately N)2]PF6 in MeCN or MeOH as solvent, respectively. The cis configuration of the MeCN ligands in [Os(C approximately N)(phen)(NCMe)2]PF6 has been confirmed by an X-ray crystallographic study. Electrochemical investigation of the octahedral osma(II)cycles by cyclic voltammetry showed a pseudoreversible MIII/II redox feature at (-50)-(+109) and 190-300 mV versus Ag/AgCl in water and MeCN, respectively. As a possible application of the compounds, a rapid electron exchange between the reduced active site of glucose oxidase enzyme from Aspergillus niger and the electrochemically generated OsIII species has been demonstrated. The corresponding second-order rate constants cover the range (0.7-4.8)x10(6) M(-1) s(-1) at 25 degrees C and pH 7.

Assuntos