193084-64-9Relevant articles and documents
Nickelation of PCP- and POCOP-type pincer ligands: Kinetics and mechanism
Vabre, Boris,Lambert, Melinda L.,Petit, Alban,Ess, Daniel H.,Zargarian, Davit
, p. 6041 - 6053 (2012)
This report describes the results of a combined experimental and computational investigation on the kinetics and mechanism of the C-H metalation step involved in the formation of PCP- and POCOP-type complexes of nickel. The kinetics of the C-H nickelation reaction was probed through competition studies involving two ligands reacting with a substoicheometric quantity of {(i-PrCN)NiBr2}n. These experiments have confirmed that metalation is more facile for aromatic ligands 1,3-(i-Pr2PE) 2C6H4 vs their aliphatic counterparts 1,3-(i-Pr2PECH2)2CH2 (sp2 C-H > sp3 C-H; E = O, CH2), ligands bearing phosphine moieties vs those with phosphinite moieties (PCP > POCOP), ligands bearing P substituents i-Pr2P vs t-Bu2P and Ph2P, and POC sp2OP ligands 1,3-(i-Pr2PO)2C 6RnH4-n bearing electron-donating vs electron-withdrawing substituents (p-OMe p-Me > m-CO 2Me > p-CO2Me > m,m-Cl2). Among the latter, there is a 6-fold difference in C-H metalation rate between ligands bearing p-OMe and p-COOMe, whereas the most readily metalating ligand, 1,3-(i-Pr2PCH2)2C6H4, is metalated ca. 270 times more readily relative to the least reactive ligand, 1,3-(i-Pr2POCH2)2CH2. Density functional calculations indicate that PCP- or POCOP-type pincer ligands react with NiBr2 to generate nonmetalated intermediates that form the corresponding pincer complexes via a two-step mechanism involving an ionic dissociation of the bromide to give a tight ion pair intermediate, followed by bromide-assisted deprotonation of the C-H bond. The type of structure adopted by the nonmetalated intermediates (mono- or dinuclear; tetrahedral, cis or trans square planar) and the energy barriers for the metalation transition states depend on the steric properties of the PR2 moiety. The presence of a base that can neutralize the HBr generated in the metalation step is crucial for rendering the metalation process exergonic. One rationale for the more facile metalation of PCP ligands in comparison to their POCOP counterparts is the greater donor character of phosphine moieties, which allows a more effective stabilization of the coordination and metalation transition states wherein the strongly donor halide ligand is displaced by a much weaker C-H bond donor. The aromatic ligands metalate more readily than their aliphatic analogues for multiple reasons, including the higher ground state energy of the nonmetalated intermediates formed with aromatic ligands, the stronger C sp2-Ni bond formed via metalation, and the more stabilized anionic charge on the C atom being metalated.
Practical Gas Cylinder-Free Preparations of Important Transition Metal-Based Precatalysts Requiring Gaseous Reagents
Ahrens, Alexander,Donslund, Bjarke S.,Gausas, Laurynas,Kristensen, Steffan K.,Skrydstrup, Troels,Sun, Hongwei
supporting information, p. 2300 - 2307 (2021/09/28)
A simple and safe setup for the synthesis of a selection of important transition metal-based precatalysts is reported, all requiring low-molecular weight gaseous reagents for their preparation. Hydrogen, carbon monoxide, ethylene, and acetylene are each liberated in a controlled manner from a corresponding easy-to-handle precursor in a closed two-chamber reactor. Gas cylinders and elaborate setups/techniques connected to handling toxic and/or flammable gases as reported in the literature can thus be avoided. The corresponding precatalysts are of high relevance in the active research fields of C-H bond activation, dehydrogenation, hydrogenation, and coupling reactions. The selection of complexes shown is meant to serve as examples for the usefulness and broadness of the presented methods, allowing precatalysts requiring gaseous reagents to become available for the research community.
One-Pot Synthesis of 1,3-Bis(phosphinomethyl)arene PCP/PNP Pincer Ligands and Their Nickel Complexes
Shih, Wei-Chun,Ozerov, Oleg V.
, p. 4591 - 4597 (2015/10/06)
A one-pot synthesis of arene-based PCP/PNP ligands has been developed. The reaction of 1,3-bis(bromomethyl)benzene or 2,6-bis(bromomethyl)pyridine with various chlorophosphines in acetonitrile afforded bis-phosphonium salts. These salts can then be reduced by magnesium powder to yield PCP or PNP ligands. In comparison to traditional synthetic methods for making PCP/PNP ligands involving the use of secondary phosphines, this new alternative method allows for the use of chlorophosphines, which are cheaper, safer to handle, and have a broader range of commercially available derivatives. This is especially true for the chlorophosphines with less bulky alkyl groups. Moreover, the one-pot procedure can be extended to allow for the direct synthesis of PCP/PNP nickel complexes. By using nickel powder as the reductant, the resulting nickel halide was found to directly undergo metalation with the PCP or PNP ligand to generate nickel complexes in high yields.
