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

Abstract

The cyclometalated monobenzyl complexes [(Cbzdiphos(R) -CH)ZrBnX] 1 (iPr) 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 (iPr) Cl and 2 (Ph) I (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 η(6) -mesitylene complex [(Cbzdiphos(Ph) )Zr(η(6) -mes)I] (3 (Ph) I). The tolyl-ring in 2 (iPr) 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 (N3 TMS) and 2 (iPr) Cl with 1-azidoadamantane (N3 Ad) resulted in the formation of the imido complexes [(Cbzdiphos(R) )Zr=NR'(X)] 5 (iPr) Cl-NAd and 5 (Ph) I-NTMS, respectively. Reaction of 2 (iPr) Cl with azobenzene led to N-N bond scission giving 6 (iPr) 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 (iPr) Cl yielded the dark purple complex [(Cbzdiphos(iPr) )Zr(bpy)Cl] (7 (iPr) Cl) through a combination of CH-activation and C-C-coupling. The structural data and UV/Vis spectroscopic properties of 7 (iPr) Cl indicate that the bpy (bipyridine) may be regarded as a (dianionic) diamido-type ligand.

η1‐Arene Complexes as Intermediates in the Preparation of Molecular Phosphorescent Iridium(III) Complexes

“…[6] In addition, hole-transporting carbazole groups have been introduced into the core of ligands coordinated to ap hosphorescent transition-metal emitter to reduce the energyb arrierf or hole injection and to decrease triplet-triplet annihilation. Four approachesh ave been developed to achieve this:1 )The direct use of carbazoles as amide ligands to form homoleptic low-coordinate transition-metal complexes; [7] 2) carbazolide groups used as central anionic linkersi nC ,N,C, [8] P, N,P, [9] and N,N,N pincers [10] based on bis(NHC), bis(phosphine), and bis(imino)l igands;3 )chelate-supported metalated carbazole groups formingf ive- [11] and six-membered [12] heterometallacycles; 4) carbazole groups used as peripherala rylamine substituentso fc oordinated bidentate ligandsi nh omo-a nd heteroleptic complexes. [13] However,a sf ar as we know,c omplexes containing ac arbazole groupc oordinated as an h 1 -arene are unprecedented.…”

Phosphines and N‐Heterocycles Joining Forces: an Emerging Structural Motif in PNP‐Pincer Chemistry

“…The groups of Gade, Nishibayashi and Hou have shown that N ‐heterocyclic PNP pincer complexes of the group 4 metals may be employed for the activation of substituted pyridines, and dinitrogen,, respectively. Using the carbazole‐based CbzC ( R PNP) ligand, the zirconium cyclohexadiene‐1,4‐diido complexes 28 (X = Cl, I) have been isolated (see Scheme ) via cyclometalated intermediates, which will be discussed in more detail below (Scheme ) , , . Although the zircona‐norbornadiene structure depicted in Scheme was found to be slightly more stable than the corresponding zirconium(II) η 6 ‐arene isomer, it was shown that complexes 28 (R = i Pr with X = Cl, R = Ph with X = I) are valuable zirconium(II) precursors for the in situ generation of CbzC ( R PNP)Zr II X building blocks.…”

“…Although the zircona‐norbornadiene structure depicted in Scheme was found to be slightly more stable than the corresponding zirconium(II) η 6 ‐arene isomer, it was shown that complexes 28 (R = i Pr with X = Cl, R = Ph with X = I) are valuable zirconium(II) precursors for the in situ generation of CbzC ( R PNP)Zr II X building blocks. The rare zirconium(II) isocyanide derivative 29 , for example, was prepared starting from 28 (see Scheme ) . Complex 28 was also shown to react cleanly with 4‐substituted pyridines to afford the corresponding bipyridine derivatives 30 via dehydrogenative C–C coupling.…”

“…In the synthesis of the CbzC ( R PNP)‐coordinated zircona‐norbornadiene 28 (see Scheme ), ligand‐cooperativity involving a cyclometalated intermediate has been exploited by Gade and co‐workers. Treatment of the zirconium trihalides 65 with equimolar amounts of Bn 2 Mg(thf) 2 led to the cyclometalated mono‐benzyl complexes 66 comprising a dianionic κ 4 ‐ P,C,N,P ‐bound ligand.…”

“…Treatment of the zirconium trihalides 65 with equimolar amounts of Bn 2 Mg(thf) 2 led to the cyclometalated mono‐benzyl complexes 66 comprising a dianionic κ 4 ‐ P,C,N,P ‐bound ligand. This complex was hydrogenated in the next step to regenerate the pincer ligand via addition of H 2 across the activated Zr–C bond . The hydride in the ensuing hydrido benzyl intermediate then shifted to the benzylic carbon to afford the final product (see Scheme ).…”