The
mononuclear square-planar Rh{κ2-X,N-(Xpy)}(η2-coe)(IPr) (X = O,
NH, NMe, S) complexes have been synthesized from the dinuclear precursor
[Rh(μ-Cl)(IPr)(η2-coe)]2 and the
corresponding 2-heteroatom-pyridinate salts. The Rh-NHC-pyridinato
derivatives are highly efficient catalysts for gem-specific alkyne dimerization. Particularly, the chelating N,O-pyridonato
complex displays turnover frequency levels of up 17 000 h–1 at room temperature. Mechanistic investigations and
density functional theory calculations suggest a pyridonato-based
metal–ligand cooperative proton transfer as responsible for
the enhancement of catalytic activity. The initial deprotonation of
a Rh-π-alkyne complex by the oxo-functionality of a κ1-N-pyridonato moiety has been established to be the rate-limiting
step, whereas the preferential protonation of the terminal position
of a π-coordinated alkyne accounts for the exclusive observation
of head-to-tail enynes. The catalytic cycle is closed by a very fast
alkenyl–alkynyl reductive elimination.
Using a low loading of the iridium(III) complexes [Ir(CF3SO3)(κ2-NSiiPr)2] (1) (NSiiPr = (4-methylpyridin-2-iloxy)diisopropylsilyl and [{Ir(κ2-NSiMe)2}2(µ-CF3SO3)2] (2) (NSiMe = (4-methylpyridin-2-iloxy)dimethylsilyl) in presence of Et3N, it has been possible to achieve the...
The unsaturated hydride complex RhClH{κ 2 -N,C-(C 11 H 8 N)}(IPr) {IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-carbene} (2) has been prepared via C− H activation of 2-phenylpyridine and fully characterized by spectroscopic methods and X-ray diffraction analysis. Complex 2 efficiently catalyzes the isomerization of terminal and internal olefins under mild conditions to give preferentially the E regioisomers. Complex 2 also catalyzes the hydroarylation of terminal olefins with 2-phenylpyridine to yield selectively mono-ortho-alkylated derivatives. Tandem isomerization−alkylation processes were observed for internal olefins. In contrast to olefins, double alkenylation is operative for internal alkynes. The marked complementary reactivity of olefins and alkynes allows for a tandem alkylation/alkenylation of 2-phenylpyridine to yield substituted styrenes. These heterobiaryl compounds exhibit axial chirality. The rotational barrier has been experimentally calculated and corroborated by density functional theory (DFT) calculations. A catalytic cycle for hydroarylation reactions has been proposed based on the identification of key reaction intermediates and H/D exchange experiments. The reaction seems to proceed by initial C−H activation of 2-phenylpyridine, subsequent insertion of alkene or alkyne, and reductive elimination steps. According to experimental results, DFT calculations have shown a higher energy barrier for bis-alkylation processes than for bis-alkenylation ones that display a feasible activation energy. Moreover, it has been found that reductive elimination is the rate-limiting step for alkene hydroarylation, whereas migratory insertion is the rate-limiting step for alkyne hydroarylation processes.
Complex [(CNC)MesRh(PMe2Ph)]PF6 (1) has been found to be an effective catalyst for solventless formic acid (FA) dehydrogenation, affording exclusively H2 and CO2 as decomposition products. The effect of the addition...
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