- POCOP-Type Pincer Complexes of Nickel: Synthesis, Characterization, and Ligand Exchange Reactivities of New Cationic Acetonitrile Adducts
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This report describes the synthesis, characterization, and ligand exchange studies of a family of cationic acetonitrile adducts of nickel featuring resorcinol-based, pincer-type POCOP ligands. The compounds [(R-POCOPR′)Ni(NCMe)][OSO2CF3] (R-POCOPR′ = 2,6-(R′2PO)2(RnC6H3-n); R′ = i-Pr: R = H (1), p-Me (2), p-OMe (3), p-CO2Me (4), p-Br (5), m,m-t-Bu2 (6), m-OMe (7), m-CO2Me (8); R′ = t-Bu: R = H (9), p-CO2Me (10)) were prepared in 80-93% yields by reacting the corresponding charge-neutral bromo derivatives with Ag(OSO2CF3) in acetonitrile. The impact of the R- and R′-substituents on electronics and structures of 1-10 have been probed by NMR, UV-vis, and IR spectra, X-ray crystallography, and cyclic voltammetry measurements. The observed ν(C≡N) values were found to increase with the increasing electron-withdrawing nature of R, i.e., in the order 7 2C-substituted cations showed the highest oxidation potentials. Moreover, all cationic adducts showed greater oxidation potentials compared with their corresponding charge-neutral bromo precursors. Equilibrium studies conducted with selected [(R-POCOPR′)Ni(NCMe)][OSO2CF3] and (R-POCOPR′)NiBr (R′ = i-Pr) have confirmed facile MeCN/Br exchange between these derivatives and show that the cationic adducts are stabilized with MeO-POCOP, whereas the charge-neutral bromo species are stabilized with MeO2C-POCOP. The potential implications of these findings for the catalytic reactivities of the title cationic complexes have been discussed. (Chemical Equation Presented).
- Lapointe, Sbastien,Vabre, Boris,Zargarian, Davit
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- High-Turnover Aromatic C-H Borylation Catalyzed by POCOP-Type Pincer Complexes of Iridium
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The catalytic C-H borylation of arenes with HBpin (pin = pinacolate) using POCOP-type pincer complexes of Ir has been demonstrated, with turnover numbers exceeding 10 000 in some cases. The selectivity of C-H activation was based on steric preferences and largely mirrored that found in other Ir borylation catalysts. Catalysis in the (POCOP)Ir system depends on the presence of stoichiometric quantities of sacrificial olefin, which is hydrogenated to consume the H2 equivalents generated in the borylation of C-H bonds with HBpin. Smaller olefins such as ethylene or 1-hexene were more advantageous to catalysis than sterically encumbered tert-butylethylene (TBE). Olefin hydroboration is a competing side reaction. The synthesis and isolation of multiple complexes potentially relevant to catalysis permitted examination of several key elementary reactions. These experiments indicate that the C-H activation step in catalysis ostensibly involves oxidative addition of an aromatic C-H bond to the three-coordinate (POCOP)Ir species. The olefin is mechanistically critical to gain access to this 14-electron, monovalent Ir intermediate. C-H activation at Ir(I) here is in contrast to the olefin-free catalysis with state-of-the-art Ir complexes supported by neutral bidentate ligands, where the C-H activating step is understood to involve trivalent Ir-boryl intermediates.
- Press, Loren P.,Kosanovich, Alex J.,McCulloch, Billy J.,Ozerov, Oleg V.
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- Impact of backbone substituents on POCOP-Ni pincer complexes: A structural, spectroscopic, and electrochemical study
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When treated at room temperature and in the presence of NEt3 with {(i-PrCN)NiBr2}n, the pincer-type ligands R-POC HOPR′ undergo direct C-H nickellation to give the pincer complexes (R-POCOPR′)NiBr in 45-92% yields (R-POCOP = κP,κC,κP-{R n-2,6-(R′2PO)2C6H 3-n}; Rn = 4-OMe, 4-Me, 4-CO2Me, 3-OMe, 3-CO2Me, 3,5-t-Bu2; R′ = i-Pr, t-Bu). These complexes have been characterized by multinuclear NMR and UV-vis spectroscopy as well as single-crystal X-ray diffraction studies to delineate the impact of R and R′ on Ni-ligand interactions. The solid-state structural data have revealed slightly shorter Ni-Br bonds in the complexes bearing a 4-CO 2Me substituent, shorter Ni-P bonds in the complex bearing t-Bu substituents at the 3- and 5-positions, and longer Ni-P bonds in complexes featuring OP(t-Bu)2 donor moieties. The UV-vis spectra indicate that a 4-CO2Me substituent causes a red-shift in the frequency of the MLCT bands (330-365 nm), whereas the ligand field transitions appearing in the 380-420 nm region are influenced primarily by the P-substituents. Cyclic voltammetry measurements have shown that the oxidation potentials of the title complexes are affected by P- and ring-substituents, oxidation being somewhat easier with t-Bu2PO (vs i-Pr2PO), OMe and Me (vs CO 2Me), and t-Bu (vs Cl). Moreover, oxidation potentials are affected more by the aromatic substituents at the 4-position vs those at the 3- and 5-positions.
- Vabre, Boris,Spasyuk, Denis M.,Zargarian, Davit
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p. 8561 - 8570
(2013/02/23)
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