The relative stabilities of two different isomers of d0 MoO2Cl2L2 species are examined for two di... more The relative stabilities of two different isomers of d0 MoO2Cl2L2 species are examined for two different complexes with the help of the integrated molecular orbital molecular mechanics method IMOMM. The experimentally reported preference of the MoO2Cl2(N,N,N‘,N‘-tetramethylethylenediamine) complex for the all-cis arrangement and that of the MoO2Cl2(N,N‘-di-tert-butyl-1,4-diaza-1,3-butadiene) complex for the cis,trans,cis arrangement are properly reproduced. The inversion of relative stabilities is shown to be associated to the balance between electronic and steric contributions, the former favoring the cis,trans,cis arrangement and the latter stabilizing the all-cis form.
The origin of the barrier for the uncatalyzed epoxidation of alkenes by iodosylbenzene is examine... more The origin of the barrier for the uncatalyzed epoxidation of alkenes by iodosylbenzene is examined from a computational point of view. The reaction of a monomeric unit with ethylene presents a very low barrier, in disagreement with experimental data indicating the requirement of a catalyst. The polymeric structure of iodosylbenzene was then analyzed and the importance of the presence of a terminal hydration water in its linear structure confirmed. The reaction of a model dimer with ethylene is shown to have a high barrier, in good agreement with experiment. Implications of this result with respect to the nature of the catalytic mechanism are briefly discussed.
The referred elongated dihydrogen compounds cannot be described by simply interpolating dihydroge... more The referred elongated dihydrogen compounds cannot be described by simply interpolating dihydrogen and dihydrido models. According to the results reported here, it is more appropriate to describe them as complexes containing two hydrogen atoms moving freely ...
The relative stabilities of two different isomers of d0 MoO2Cl2L2 species are examined for two di... more The relative stabilities of two different isomers of d0 MoO2Cl2L2 species are examined for two different complexes with the help of the integrated molecular orbital molecular mechanics method IMOMM. The experimentally reported preference of the MoO2Cl2(N,N,N‘,N‘-tetramethylethylenediamine) complex for the all-cis arrangement and that of the MoO2Cl2(N,N‘-di-tert-butyl-1,4-diaza-1,3-butadiene) complex for the cis,trans,cis arrangement are properly reproduced. The inversion of relative stabilities is shown to be associated to the balance between electronic and steric contributions, the former favoring the cis,trans,cis arrangement and the latter stabilizing the all-cis form.
Reaction of the hexahydride complex OsH(6)(P(i)Pr(3))(2) (1) with pyridine-2-thiol leads to the t... more Reaction of the hexahydride complex OsH(6)(P(i)Pr(3))(2) (1) with pyridine-2-thiol leads to the trihydride derivative OsH(3){kappa-N,kappa-S-(2-Spy)}(P(i)Pr(3))(2) (2). The structure of 2 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid with the phosphine ligands occupying axial positions. The equatorial plane contains the pyridine-2-thiolato group, attached through a bite angle of 65.7(1) degrees, and the three hydride ligands. The theoretical structure determination of the model complex OsH(3){kappa-N,kappa-S-(2-Spy)}(PH(3))(2) (2a) reveals that the hydride ligands form a triangle with sides of 1.623, 1.714, and 2.873 Å, respectively. A topological analysis of the electron density of 2a indicates that there is no significant electron density connecting the hydrogen atoms of the OsH(3) unit. In solution, the hydride ligands of 2 undergo two different thermally activated site exchange processes, which involve the central hydride with each hydride ligand situated close to the donor atoms of the chelate group. The activation barriers of both processes are similar. Theoretical calculations suggest that the transition states have a cis-hydride-dihydrogen nature. In addition to the thermally activated exchange processes, complex 2 shows quantum exchange coupling between the central hydride and the one situated close to the sulfur atom of the pyridine-2-thiolato group. The reactions of 1 with L-valine and 2-hydroxypyridine afford OsH(3){kappa-N,kappa-O-OC(O)CH[CH(CH(3))(2)]NH(2)}(P(i)Pr(3))(2) (3) and OsH(3){kappa-N,kappa-O-(2-Opy)}(P(i)Pr(3))(2) (4) respectively, which according to their spectroscopic data have a similar structure to that of 2. In solution, the hydride ligands of 3 and 4 also undergo two different thermally activated site exchange processes. However, they do not show quantum exchange coupling. The tetranuclear complexes [(P(i)Pr(3))(2)H(3)Os(&mgr;-biim)M(TFB)](2) [M = Rh (5), Ir (6); H(2)biim = 2, 2'-biimidazole; TFB = tetrafluorobenzobarrelene] have been prepared by reaction of OsH(3)(Hbiim)(P(i)Pr(3))(2) with the dimers [M(&mgr;-OMe)(TFB)](2) (M = Rh, Ir). In solution the hydride ligands of these complexes, which form two chemically equivalent unsymmetrical OsH(3) units, undergo two thermally activated site exchanges and show two different quantum exchange coupling processes.
IMOMM (Becke3LYP:MM3) calculations are performed on different proposed intermediates in the react... more IMOMM (Becke3LYP:MM3) calculations are performed on different proposed intermediates in the reaction pathway of biomimetic models of catalase and peroxidase enzymes. These calculations allow the identification of their ground state, structural features and relative energies. The relative energies are shown to depend heavily on the nature of the substrates involved in the reaction, and they are analyzed in detail for the case of the oxidation of alcohol to aldehyde by hydrogen peroxide. All the considered intermediates are found to be within a reasonable energy span.
The relative stabilities of two different isomers of d0 MoO2Cl2L2 species are examined for two di... more The relative stabilities of two different isomers of d0 MoO2Cl2L2 species are examined for two different complexes with the help of the integrated molecular orbital molecular mechanics method IMOMM. The experimentally reported preference of the MoO2Cl2(N,N,N‘,N‘-tetramethylethylenediamine) complex for the all-cis arrangement and that of the MoO2Cl2(N,N‘-di-tert-butyl-1,4-diaza-1,3-butadiene) complex for the cis,trans,cis arrangement are properly reproduced. The inversion of relative stabilities is shown to be associated to the balance between electronic and steric contributions, the former favoring the cis,trans,cis arrangement and the latter stabilizing the all-cis form.
The origin of the barrier for the uncatalyzed epoxidation of alkenes by iodosylbenzene is examine... more The origin of the barrier for the uncatalyzed epoxidation of alkenes by iodosylbenzene is examined from a computational point of view. The reaction of a monomeric unit with ethylene presents a very low barrier, in disagreement with experimental data indicating the requirement of a catalyst. The polymeric structure of iodosylbenzene was then analyzed and the importance of the presence of a terminal hydration water in its linear structure confirmed. The reaction of a model dimer with ethylene is shown to have a high barrier, in good agreement with experiment. Implications of this result with respect to the nature of the catalytic mechanism are briefly discussed.
The referred elongated dihydrogen compounds cannot be described by simply interpolating dihydroge... more The referred elongated dihydrogen compounds cannot be described by simply interpolating dihydrogen and dihydrido models. According to the results reported here, it is more appropriate to describe them as complexes containing two hydrogen atoms moving freely ...
The relative stabilities of two different isomers of d0 MoO2Cl2L2 species are examined for two di... more The relative stabilities of two different isomers of d0 MoO2Cl2L2 species are examined for two different complexes with the help of the integrated molecular orbital molecular mechanics method IMOMM. The experimentally reported preference of the MoO2Cl2(N,N,N‘,N‘-tetramethylethylenediamine) complex for the all-cis arrangement and that of the MoO2Cl2(N,N‘-di-tert-butyl-1,4-diaza-1,3-butadiene) complex for the cis,trans,cis arrangement are properly reproduced. The inversion of relative stabilities is shown to be associated to the balance between electronic and steric contributions, the former favoring the cis,trans,cis arrangement and the latter stabilizing the all-cis form.
Reaction of the hexahydride complex OsH(6)(P(i)Pr(3))(2) (1) with pyridine-2-thiol leads to the t... more Reaction of the hexahydride complex OsH(6)(P(i)Pr(3))(2) (1) with pyridine-2-thiol leads to the trihydride derivative OsH(3){kappa-N,kappa-S-(2-Spy)}(P(i)Pr(3))(2) (2). The structure of 2 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid with the phosphine ligands occupying axial positions. The equatorial plane contains the pyridine-2-thiolato group, attached through a bite angle of 65.7(1) degrees, and the three hydride ligands. The theoretical structure determination of the model complex OsH(3){kappa-N,kappa-S-(2-Spy)}(PH(3))(2) (2a) reveals that the hydride ligands form a triangle with sides of 1.623, 1.714, and 2.873 Å, respectively. A topological analysis of the electron density of 2a indicates that there is no significant electron density connecting the hydrogen atoms of the OsH(3) unit. In solution, the hydride ligands of 2 undergo two different thermally activated site exchange processes, which involve the central hydride with each hydride ligand situated close to the donor atoms of the chelate group. The activation barriers of both processes are similar. Theoretical calculations suggest that the transition states have a cis-hydride-dihydrogen nature. In addition to the thermally activated exchange processes, complex 2 shows quantum exchange coupling between the central hydride and the one situated close to the sulfur atom of the pyridine-2-thiolato group. The reactions of 1 with L-valine and 2-hydroxypyridine afford OsH(3){kappa-N,kappa-O-OC(O)CH[CH(CH(3))(2)]NH(2)}(P(i)Pr(3))(2) (3) and OsH(3){kappa-N,kappa-O-(2-Opy)}(P(i)Pr(3))(2) (4) respectively, which according to their spectroscopic data have a similar structure to that of 2. In solution, the hydride ligands of 3 and 4 also undergo two different thermally activated site exchange processes. However, they do not show quantum exchange coupling. The tetranuclear complexes [(P(i)Pr(3))(2)H(3)Os(&mgr;-biim)M(TFB)](2) [M = Rh (5), Ir (6); H(2)biim = 2, 2'-biimidazole; TFB = tetrafluorobenzobarrelene] have been prepared by reaction of OsH(3)(Hbiim)(P(i)Pr(3))(2) with the dimers [M(&mgr;-OMe)(TFB)](2) (M = Rh, Ir). In solution the hydride ligands of these complexes, which form two chemically equivalent unsymmetrical OsH(3) units, undergo two thermally activated site exchanges and show two different quantum exchange coupling processes.
IMOMM (Becke3LYP:MM3) calculations are performed on different proposed intermediates in the react... more IMOMM (Becke3LYP:MM3) calculations are performed on different proposed intermediates in the reaction pathway of biomimetic models of catalase and peroxidase enzymes. These calculations allow the identification of their ground state, structural features and relative energies. The relative energies are shown to depend heavily on the nature of the substrates involved in the reaction, and they are analyzed in detail for the case of the oxidation of alcohol to aldehyde by hydrogen peroxide. All the considered intermediates are found to be within a reasonable energy span.
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