β-Hydrogen Abstraction of an Ethyl Group Provides Entry to Titanium and Zirconium Ethylene Complexes

Abstract

Treatment of trichloride complexes [(PNP)-MCl 3 ] (M = Ti or Zr) with 3 equiv of ethyl-Grignard reagent cleanly results in transmetalation followed by β-hydrogen abstraction to form the ethylene-ethyl species [(PNP)M(η 2 -H 2 CCH 2 )(CH 2 CH 3 )]. In the case of zirconium, we can replace the ethylene ligand with an imido or 2,2′-bipyridine (bpy) to form [(PNP)ZrN[1-adamantyl](CH 2 CH 3 )] or [(PNP)Zr(bpy)(CH 2 CH 3 )], respectively. The ethylene-methyl derivative [(PNP)Zr(η 2 -H 2 CCH 2 )(CH 3 )] can also be prepared cleanly from the precursor [(PNP)Zr(CH 3 ) 2 (OTf)] and ethyl-Grignard reagent. Using the isotopologue [(PNP)Zr(CD 3 ) 2 (OTf)] we found that α-abstraction is not a pathway to ethylene formation.

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

“…[49][50][51][52][53][54][55][56][57][58] Early studies also include the synthesis of arene-phosphine-M 0 complexes (M = Zr,H f) through metal-ligand co-condensation reported by Clokea nd Green. [59,60] Polydentate amidophosphine-hybridl igandsw ere found to be capableo fs tabilizing Zr II species, [20,[61][62][63][64] which provided the pointo fe ntry for the reductive activation of N 2 referred to above. [20][21][22][23][24] The generation of Ti II compoundsh as been achieved both by direct reduction as wella st hrough dialkyl or hydridoalkyl precursors that underwent reductivee limination.…”

“…Mindiola et al recently characterizedt he natureo ft heir (PNP)Ti(h 2 -ethylene) species as amaskedT i II reagent, [61][62][63] by reaction with two-electron oxidants, analogous to earlier work on [(h 5 -Cp*) 2 Ti(h 2 -C 2 H 4 )] by Bergman and Andersen. [96][97][98][99][100] Given that complexes 2 iPr Cl and 2 Ph I were precursors for Zr II species discussed in the previous section, it was of interestwhether reactions with potentialZ r =Nb ond-formingt wo-electron oxidants, would lead to the corresponding Zr IV species.…”

“…The relativelys hort ZrÀN(2) distances [1.8298(13) ( 5 Ph I-NTMS); 1.8185(17) ( 5 iPr Cl-NAd)] together with the approximately linear ZrÀN(2)-R' angle [171.17(9)8 (5 Ph I-NTMS); 173.69(14)8 (5 iPr Cl-NAd)] are consistentw ith significant ZrÀN multiple bond character,a sh as been observedp reviously for relatedz irconium imido complexes. [63,[101][102][103][104][105][106] Interestingly,w hen 2 iPr Cl was reacted with azobenzene (Scheme 4, bottom), NÀNb ond scissiona nd ac oupling of one of the NPh-fragmentsw ith the carbazole nitrogen occurred giving ac entral h 2 -bonded hydrazide(À1). Theo ther NPh-fragment binds to zirconium acting as an imido-ligand.…”

“…Mindiola et al recently reported the synthesis of the bpy complexes [(PNP)Ti(CH 2 tBu)(bpy)] [107] and [(PNP)Zr(Et)(bpy)] [63] by reacting[ (PNP)Ti(h 2 -CH 2 =CH 2 )CH 2 tBu] and [(PNP)Zr(h 2 -CH 2 = CH 2 )Et] with 2,2'-bipyridine, respectively.I nt he case of the titanium complex, an antiferromagnetic coupling of the Ti III d 1 radical to the bpy radicala nion (bpyC À )w as proposed. In the zirconium complex, however,t he metric parameters of the 2,2'-bipyridine ligand suggest ad ianionic ligand.…”

“…[9,19,56,[76][77][78][79][80][81][82][83][84][85][86] An otable alternative, which has been reported recently by Mindiola and co-workers, is based on the generation of isolable M IV compounds containing side-on coordinated ethylene (as an ethene-1,2-diyl) generatedf rom ethyl precursors by intramolecular a-or b-H-abstraction;T i-species reactivity has been demonstratedw ith aT i II synthon. [61][62][63] There is ag eneral interest in the developmento fs toichiometric and catalytic Group 4m etal chemistry that does not merely involve redox-neutral transformations,a so bserved in most cases studied to date. [9,87,88] This requires ancillary ligands that stabilizen ot only av ariety of coordination geometries but also redox states lower than + +IV.H ere we present such ac ase, based on the recently reported monobenzylated zirconium complexes A,containing ac yclometalated PNP-pincer.…”