η6-Arene-Zirconium-PNP-Pincer Complexes: Mechanism of Their Hydrogenolytic Formation and Their Reactivity as Zirconium(II) Synthons

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• 作者：Gudrun T. Plundrich ; Prof. Hubert Wadepohl ; Dr. Eric Clot and Prof. Lutz H. Gade
• 刊名：Chemistry - A European Journal
• 出版年：2016
• 出版时间：June 27, 2016
• 年：2016
• 卷：22
• 期：27
• 页码：9283-9292
• 全文大小：3410K
• ISSN：1521-3765

文摘

The cyclometalated monobenzyl complexes [(Cbzdiphos^R-CH)ZrBnX] 1 ^iPrCl and 1 ^PhI reacted with dihydrogen (10 bar) to yield the η⁶-toluene complexes [(Cbzdiphos^R)Zr(η⁶-tol)X] 2 ^iPrCl and 2 ^PhI (cbzdiphos=1,8-bis(phosphino)-3,6-di-tert-butyl-9H-carbazole). The arene complexes were also found to be directly accessible from the triiodide [(Cbzdiphos^Ph)ZrI₃] through an in situ reaction with a dibenzylmagnesium reagent and subsequent hydrogenolysis, as exemplified for the η⁶-mesitylene complex [(Cbzdiphos^Ph)Zr(η⁶-mes)I] (3 ^PhI). The tolyl-ring in 2 ^iPrCl adopts a puckered arrangement (fold angle 23.3°) indicating significant arene-1,4-diido character. Deuterium labeling experiments were consistent with an intramolecular reaction sequence after the initial hydrogenolysis of a Zr−C bond by a σ-bond metathesis. A DFT study of the reaction sequence indicates that hydrogenolysis by σ-bond metathesis first occurs at the cyclometalated ancillary ligand giving a hydrido-benzyl intermediate, which subsequently reductively eliminates toluene that then coordinates to the Zr atom as the reduced arene ligand. Complex 2 ^PhI was reacted with 2,6-diisopropylphenyl isocyanide giving the deep blue, diamagnetic Zr^II-diisocyanide complex [(Cbzdiphos^Ph)Zr(CNDipp)₂I] (4 ^PhI). DFT modeling of 4 ^PhI demonstrated that the HOMO of the complex is primarily located as a “lone pair on zirconium”, with some degree of back-bonding into the C≡N π* bond, and the complex is thus most appropriately described as a zirconium(II) species. Reaction of 2 ^PhI with trimethylsilylazide (N₃TMS) and 2 ^iPrCl with 1-azidoadamantane (N₃Ad) resulted in the formation of the imido complexes [(Cbzdiphos^R)Zr=NR′(X)] 5 ^iPrCl-NAd and 5 ^PhI-NTMS, respectively. Reaction of 2 ^iPrCl with azobenzene led to N−N bond scission giving 6 ^iPrCl, in which one of the NPh-fragments is coupled with the carbazole nitrogen to form a central η²-bonded hydrazide(−1), whereas the other NPh-fragment binds to zirconium acting as an imido-ligand. Finally, addition of pyridine to 2 ^iPrCl yielded the dark purple complex [(Cbzdiphos^iPr)Zr(bpy)Cl] (7 ^iPrCl) through a combination of CH-activation and C−C-coupling. The structural data and UV/Vis spectroscopic properties of 7 ^iPrCl indicate that the bpy (bipyridine) may be regarded as a (dianionic) diamido-type ligand.