Reatividade de ligantes na química dos cianoferratos

Autor: Iha, Neyde Yukie Murakami
Jazyk: portugalština
Rok vydání: 1981
Předmět:
Druh dokumentu: Tese de Doutorado
Popis: As reações entre o íon pentacianoferrato(II) e os isômeros de acetilpiridina (py(CO)CH3) foram estudadas com o intuito de estabelecer o mecanismo da reação de ataque nucleofílico ao grupo nitrosilo coordenado. Estas reações de adição foram acompanhadas pela técnica stopped flow ll a 25,OºC, em condições de pseudo primeira ordem, em meio alcalino com força iônica 0,10 M ajustada com LiC104. Durante o processo, constatou-se que a formação dos produtos é precedida pelo aparecimento de intermediário azul com pequenos tempos de vida. A formação deste intermediário envolve o ataque da acetilpiridina na forma de carbânion, ao grupo nitrosilo coordenado, em competição com a adição do íon hidroxila. A lei da velocidade para o mecanismo proposto nesta etapa, tem a expressão abaixo para a constante de velocidade observada kobsl= KkL [py(CO)CH3] [OH-] + kOH [OH-] + k-L Os valores de KkL e k-L são respectivamente 1,91 ± 0,05 X 102M-2s-1 e 0,90± 0,02 X 10-1 s-1
The reactions between the pentacyano(nitrosyl) ferrate(II) ion and the o-, m-, p-isomers of acetylpyridine (py(CO)CH3) were investigated by stopped flow technique, in order to evaluated the mechanisms of nucleophilic attack on the coordinated nitric oxide ligand. The kinetics of these addition reactions were followed spectrophotometrically, under pseudo first order conditions, at a temperature of 25°C, and µ= 0,100 (lithium perchlorate and lithium hydroxide). A deep blue compound can be observed with a half-1ife of a few seconds. The formation of such an intermediate was described in terms of the attack of the carbanion of acetylpyridine to the coordinate nitric oxide ligand, in competition with the addition of the hydroxide ion. The experimental rate 1aw for the reaction can be expressed by a pseudo first order equation, consistent with our proposed mechanism; where kobsl= K.kL [py(CO)CH3] [OH- + kOH [OH-] + k-LThe rate constants kL and k-L refer to the nucleophilic attack step and the reversal reaction, respectively. The equilibrium constant K represents the formation of the carbanion, while kOH is associated with the parallel addition of the hydroxide ion to NO+. The values determined for KkL and k-L were, respectively, 196 M-2s-1, and 0.090 s-l for p-py(CO)CH3 ; 370 M-2s-1 and 0.080 s-l for m-py(CO)CH 3; and 14,2M-2s-1 and 0.046s-1 for o-py(CO)CH3. The kOH value obtained under the experimental conditions is 0,302 M-1s-1 The decay of the intermediate yields the free carbonyloximes in solution. It may be associated with an intrinsic lability of the nitroso species, or even by an internal isomerization leading to a sterically hindered configuration of the ligand. The observed rate law was found to depend on the acetylpyridine and the hydroxide ion concentrations in the following way [see figure PDF file] where KT = K.kL/k-L, and. kd is the rate constant of dissociation of coordinated 1igand. The experimental values of KT and kd were, respectively, 2.1x103 M-2 and 0.082s-1 for p-py(CO)CH3; 390 M-2 and 0.050 s-1 for m-py (CO) CH3;and 295 M-2 and 0.032 s-1 for o-py(CO)CH3. Independent experiments using the intermediate species isolated as solids, allowed to confirm the consistency of our two step mechanism. From the pure carbonyl-oxime ligands obtained in this work we have synthesized the substituted complexes of pentacyanoferrate(II) coordinated by pyridine group. An extensive characterization of the ligands and of the complexes was carried out in this work, including the analysis of the electronic spectra in terms of Gaussians and log-normal functions, and the assignments of the vibrational spectra. Also examined were the pH effects on the charge transfer spectra, as well as the evaluation of the basicity constants of the coordinated cyanide and the heterocyclic ligands. We have also characterized the intermediates in terms of their electronic and vibrational spectra. Based on an electrochemical study, using mostly cyclic voltammetry, we have obtained the thermodynamic parameters of the several complexes. A linear correlation between the formal potentials and the energies of the charge transfer transitions was found in this work. The Fe (CN)5py (CO) CHNOH3- complexes were found to be photosensitive, turning slowly to a deep blue color under sun or ordinary light. The photolysis under controlled conditions, with a mercury lamp, in the visible - near uv region, produced neat spectral changes, with isosbestic points indicating a well behaved photochemical process. The product, in contrast to pentacyanoferrate(II) species, was found to be inert toward dimethyl sulfoxide. The same behavior has been observed for tetracyanoferrate(II) complexes with bidentate ligands. In analogy, the blue product was formulated as [see figure PDF file] Quantum yields were determined at several wavelengths however, it was not possible to distinguish, between the following photochemical processes, leading to the tetracyano species : [see figure PDF file] The second process taking place on the oxime isomer, which may be eventually present in very small amount, is expected to be highly effective because of the neighboring effect enhancing the formation of the 5 member ring. To clarify this point, we have investigated the photochemistry of the Fe(CN)5(en)3- complex. The results indicated that the formation of the Fe(CN)4en2- complex takes place with a quantum yield of 0,087± 0,005 at 366nm, which is nearly 10 times the values obtained for carbonyl-oxime complexes. In this case, the photochemical process can be analysed quantitatively by determining the amount of the starting complex, or by isolating the tetracyano species by ionic exchange chromatography. Both ways gave comparable numbers, indicating that the measured quantum yields refer to the process [see figure PDF file] We therefore concluded that the labilization of the CN ligands in the carbonyl-oxime complexes should take place mostly according to the first route.
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