η 6 ‐Arene‐Zirconium‐PNP‐Pincer Complexes: Mechanism of Their Hydrogenolytic Formation and Their Reactivity as Zirconium(II) Synthons

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.