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.

This work describes how reactions of oxidative addition of N-C and C-H bonds are in competition in the PNP-ligated Rh and Ir complexes. Iridium appears to have a higher preference than Rh for the C-H activation over the N-C activation, and the Ir C-H activated complexes are more kinetically stable than their Rh analogues. A new generation of a diarylamido-based PNP pincer is presented, a "tied" PNP ligand 1c based on the iminodibenzyl substructure. This ligand is more definitively prearranged for binding to a metal center in a meridional, anionic PNP fashion. As a result, its N-C cleavage reactions (that lead to complexes of anionic PNP) are faster than for the "untied" ligands 1a,b. Structural evidence indicates that the "tied" anionic PNP pincer ligand is bulkier than the "untied" ligands when bearing the same substituents on the donor atoms because of the influence of the conformation of the pincer backbone. The "tied" pincer ligand also allows for the observation of the products of activation of the C-H bonds of the central N-CH 3 group, which are not detected with the "untied" ligands. The N-C oxidative addition reaction with "untied" ligands proceeds in the solid state as well as in solution. A remarkable result is reported where the solid-state N-C oxidative addition reaction displays superior selectivity to the solution reaction. The mechanistic studies are augmented by the investigations of the isotopically labeled ( 2 H and 13 C) ligands.

Several new N-methylated diarylamine-based PNP pincer ligands have been prepared. The synthesis of these ligands is modular and allows incorporation of a variety of substituents that change the solubility and the stereoelectronic properties of the ligand as well as allow for the introduction of a sensitive 19 F NMR spectroscopic probe. The reactions of PN(Me)P ligands with PdX 2 (X ) Cl, OAc) initially proceed with formation of an adduct, (PN(Me)P)-PdX 2 , that may exist in either the neutral or the ionic forms. These adducts are unreactive in the case of PPh 2 -bearing ligands, but with the more donating PPr i 2 -bearing ligands, the adduct evolves into square planar (PNP)PdX with irreversible loss of MeX. Thus, the feasibility of cleavage of an unstrained N-C bond by Pd II is demonstrated. The N-C cleavage is accelerated by decreasing the solvent polarity. The mechanism may involve either N-C oxidative addition or a nucleophilic attack (external or internal) of Xon the Me group of the N-bound PN(Me)P ligand.

Palladium(II) complexes of the PCHP pincer ligand were prepared from PdCl 2 , a "proto-pincer" P 2 -CH 2 , and base. The resulting (PCHP)PdCl was converted to (PCHP)PdOTf. (PCHP)PdCl did not react with NaN(SiMe 3 ) 2 but produced (PCHP)PdR (R ) Me, Et, Ph) upon reaction with the corresponding organolithium, Grignard, or organozinc reagent. Trityl cation abstracted a hydride from (PCHP)PdCl to yield cationic Pd carbene complex (P 2 Cd)PdCl. Analysis of the structural data points to minimal π-interaction between the carbene center and palladium.

A Ru center ligated by a pincer bis(o-phosphinoaryl)amine (PNP) ligand in (PNP)RuH(H2) is sufficiently electron rich to break C−O and C−C bonds, resulting in the ultimate decarbonylation of hydrocarbonate and acetone, respectively, to give (PNP)RuH(CO). The decarbonylation of acetone is accompanied by hydrogenolysis of the C−C bonds to produce methane.