Chelate-Enforced Phosphine Coordination Enables α-Abstraction to Give Zirconium Alkylidenes

Abstract

The rigid PNP pincer ligand (PNP ) deprotonated anion of bis(ortho-diisopropylphoshinoaryl)amine) is shown to stabilize the (ZrdCHR) 2+ fragment. (PNP)Li(THF) (2-THF) contains P-Li bonds, as evinced by the observation of the Li-P coupling in the solution 31 P NMR spectrum and by the X-ray structural determination in the solid state. 2-THF reacts with ZrCl 4 (Et 2 O) 2 to give (PNP)ZrCl 3 (3). (PNP)ZrCl 3 (3) can be alkylated with RCH 2 MgCl to give (PNP)Zr(CH 2 R) 3 (4a-c). (PNP)ZrMe 3 (4a) is thermally stable, and its solid-state structure is characterized by severe distortions from the octahedral geometry. The (PNP)Zr(CH 2 R) 3 (R ) phenyl (4b) or p-tolyl (4c)) compounds undergo R-abstraction at ambient temperature to give isolable Zr alkyl/alkylidenes (PNP)Zr(dCHR)(CH 2 R) (5b,c). The reaction follows a first-order rate law (t 1/2 at 298 K ≈ 2.3 h). The activation parameters were determined from the VT NMR studies (4b f 5b): ∆H q ) 19(1) kcal/mol; ∆S q ) -14(3) cal/(mol K); ∆G q 298 ) 23( 2) kcal/mol. The importance of the enforcement of the phosphine coordination by the rigid PNP ligand is discussed.

Phosphine- and thiophosphorane-amine ligands: Lithiation and coordination to Rh(I)

“…6 24.3 a in kcal mol À1 . b Full geometry optimization of the anion actually leads to its dissociation into phosphide or thiophosphinite anion and imine.…”

“…The [PNP] ligand developed by Liang et al [3] and Ozerov and co-workers [4] may appear similar since it also features aromatic rings linked by heteroatoms but the nitrogen atom is central and the phosphorus donors are located at the periphery [5]. This ligand proved to be very versatile since its coordination to electron deficient metal centres such as group 4, 5 [6][7][8] and actinides [9] or to electron rich metals such as group 9 and 10 [4,10] was described. When di(isopropyl) groups are present as substituents on phosphorus, insertions of rhodium(I) and iridium(I) in NC or NH bonds were observed [10a].…”

Gallium and indium compounds supported by a [PNP] pincer ligand

“…1 [20]. The first such ligand to be described, namely [PNP Ph ], features phenyl substituents on phosphorus and was reported independently by two research groups in 2003 [21,22] [25,26] as illustrated in Scheme 1, i.e. sequential treatment of (2-Br, 4-Me-C 6 H 3 ) 2 NH with (i) Bu n Li, (ii) Ph 2 PCl and (iii) HCl.…”

“…[21]. [21,26,29,30], and there are no structurally characterized gallium and indium derivatives listed in the Cambridge Structural Database [31]. Therefore, it is noteworthy that the gallium and indium halide complexes, [28].…”

“…2 Since the methyl group of the tolylene linker provides a good 1 H NMR spectroscopic probe, we were interested in the [MePNP Ph ] hybrid that features both a tolylene linker and phenyl substituents on phosphorus. Indeed, the bis(phosphino)amine [MePNP Ph ]H may be synthesized by appropriate modification of the methods used for [PNP Ph ]H [22] and [MePNP Pr i ]H [ 25,26] as illustrated in Scheme 1, i.e. sequential treatment of (2-Br, 4-Me-C 6 H 3 ) 2 NH with (i) Bu n Li, (ii) Ph 2 PCl and (iii) HCl.…”