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η 6 ‐Arene‐Zirconium‐PNP‐Pincer Complexes: Mechanism of Their Hydrogenolytic Formation and Their Reactivity as Zirconium(II) Synthons
Author(s) -
Plundrich Gudrun T.,
Wadepohl Hubert,
Clot Eric,
Gade Lutz H.
Publication year - 2016
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201601213
Subject(s) - chemistry , hydrogenolysis , ligand (biochemistry) , medicinal chemistry , metathesis , toluene , reactivity (psychology) , pincer ligand , zirconium , intramolecular force , pincer movement , agostic interaction , stereochemistry , inorganic chemistry , organic chemistry , polymerization , metal , medicine , biochemistry , polymer , receptor , alternative medicine , pathology , catalysis
The cyclometalated monobenzyl complexes [(Cbzdiphos R ‐CH)ZrBnX] 1 i Pr Cl and 1 Ph I reacted with dihydrogen (10 bar) to yield the η 6 ‐toluene complexes [(Cbzdiphos R )Zr(η 6 ‐tol)X] 2 i Pr Cl and 2 Ph I (cbzdiphos=1,8‐bis(phosphino)‐3,6‐di‐ tert ‐butyl‐9 H ‐carbazole). The arene complexes were also found to be directly accessible from the triiodide [(Cbzdiphos Ph )ZrI 3 ] through an in situ reaction with a dibenzylmagnesium reagent and subsequent hydrogenolysis, as exemplified for the η 6 ‐mesitylene complex [(Cbzdiphos Ph )Zr(η 6 ‐mes)I] ( 3 Ph I ). The tolyl‐ring in 2 i Pr Cl 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 Ph I was reacted with 2,6‐diisopropylphenyl isocyanide giving the deep blue, diamagnetic Zr II ‐diisocyanide complex [(Cbzdiphos Ph )Zr(CNDipp) 2 I] ( 4 Ph I ). DFT modeling of 4 Ph I 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 Ph I with trimethylsilylazide (N 3 TMS) and 2 i Pr Cl with 1‐azidoadamantane (N 3 Ad) resulted in the formation of the imido complexes [(Cbzdiphos R )Zr=NR′(X)] 5 i Pr Cl‐NAd and 5 Ph I‐NTMS , respectively. Reaction of 2 i Pr Cl with azobenzene led to N−N bond scission giving 6 i Pr Cl , in which one of the NPh‐fragments is coupled with the carbazole nitrogen to form a central η 2 ‐bonded hydrazide(−1), whereas the other NPh‐fragment binds to zirconium acting as an imido‐ligand. Finally, addition of pyridine to 2 i Pr Cl yielded the dark purple complex [(Cbzdiphos i Pr )Zr(bpy)Cl] ( 7 i Pr Cl ) through a combination of CH‐activation and C−C‐coupling. The structural data and UV/Vis spectroscopic properties of 7 i Pr Cl indicate that the bpy (bipyridine) may be regarded as a (dianionic) diamido‐type ligand.