Bond Activation by Nitrogen Ligated Pt(II)/Pt(IV) Complexes

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  • March 20, 2019
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  • West, Nathan M.
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • Activation and functionalization of hydrocarbons by platinum has been studied using chelating nitrogen donor ligands to stabilize Pt(II) and Pt(IV) complexes. These bidentate and tridentate ligands allowed examination of oxidative addition of R-H (R = C, Si) bonds and reductive elimination of R′-H (R′ = C, H) bonds. In addition we have investigated functionalization of hydrocarbons by dehydrogenation of alkanes and by coupling of alkynes. Stoichiometric dehydrogenation of alkanes or simple ethers can be accomplished by stirring Me4Pt2(μ-SMe2)2 and nacnacH (nacnac = bis-N-aryl-β-diiminate) in the solvent to be dehydrogenated. This reaction yields Pt(II) alkene hydride complexes, (nacnac)Pt(H)(η2- alkene). The dimeric Pt reagent is protonated by nacnacH and ensuing methane elimination forms the (nacnac)PtMe fragment which then binds and cleaves a solvent C-H bond through oxidative addition to Pt. Methane elimination followed by β-H elimination from the activated solvent molecule yield the Pt alkene hydride complex, (nacnac)Pt(H)(η2-alkene). Linear alkanes undergo selective C-H activation of primary C-H bonds to form α-olefin complexes, but ethers add selectively through the secondary C-H α to oxygen; this is due to coordination of the ether oxygen to Pt preceeding C-H activation. The alkene hydride complex readily exchanges free and bound olefins. This facile ligand exchange has allowed us to explore the reactivity of the (nacnac)Pt(H) fragment. iv Triphenylsilane displaces pentene from (nacnac)Pt(H)(1-pentene) and oxidative addition of the Si-H bond forms a stable five-coordinate Pt(IV) silyldihydride complex (nacnac)Pt(H)2(SiPh3). This five-coordinate complex was characterized crystallographically and appears to be a square based pyramid with the vacant coordination site trans to the silyl ligand. If acetylene or phosphaalkyne is mixed with the five-coordinate species the triple bond inserts across the vacant site on the Pt and the central CH on the nacnac ligand. Alkynes easily displace pentene from (nacnac)Pt(H)(1-pentene) and rapidly undergo insertion into the Pt-H bond to initiate a reaction cascade. The identity of the final platinum product depends on the substituents on the alkyne reagent. Alkynes with propargylic protons bond favor allyl formation. Terminal silyl alkynes such as R3SiC≡CH (R3Si = Me3Si, Ph3Si, Ph2MeSi) insert into the Pt-H bond in a 2,1 fashion placing the silyl group and Pt on the same carbon, subsequent C-H activation of the silicon substituents, either methyl or phenyl, forms chelated vinyl silane products. Terminal alkynes with no propargylic hydrogens such as PhC≡CH and t-BuC≡CH insert into the Pt-H bond in a 1,2, rather than a 2,1, fashion, placing the substituent β to the metal and precluding C-H activation of the alkyne substituent. Instead, a second alkyne binds to the metal and inserts into the Pt-vinyl bond in a 1,2 fashion forming chelated η1-η2-butadienyl ligands with R groups at the 2 and 4 positions. Acid assisted reductive elimination of hydrogen from Tp'PtH3 (Tp' = hydridotris(3,5- dimethylpyrazolyl)borate) was examined. Loss of H2 is observed from Tp'Pt(H)3 upon protonation and addition of CO. No formation of hydrogen is observed if the reaction is conducted in the absence of CO. In contrast to most reductive eliminations from Pt(IV) which occur from five-coordinate intermediates, here reductive elimination occurs directly from the 6-coordinate [κ2 -(HTp')Pt(H)3(CO)][BAr'4] species.
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  • Templeton, Joseph
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  • October 19, 2010

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