Pd halide and hydride complexes of a new PNP pincer ligand with a central diarylamido moiety can be prepared via N-H cleavage in a neutral amine/ diphosphine PNP ligand. The solid-state structure of (PNP)PdCl shows a meridional PNP ligand about an approximately square-planar Pd center. (PNP)PdH hydrodehalogenates alkyl and certain aryl halides, while (PNP)PdX (X ) Cl, I, H, OAc) complexes catalyze Heck coupling of ethyl acrylate with aryl halides.Utilization of tridentate PCP pincer ligands (A), pioneered by Shaw in the 1970s, 1 has been increasing in recent years, owing to their potential for supporting unusual chemical transformations on transition-metal centers. 2 These complexes have been found to be efficient catalysts for alkane dehydrogenation 3 and Heck coupling, 4-6 in addition to providing a platform for seminal mechanistic studies of C-C, 7 C-N, 8 and C-O 9 bond activation. 2 The realm of anionic PNP ligands has been mostly limited to the family of Fryzuk's ligands (B). 10,11 An anionic amido-PNP ligand may be viewed not only as a component of an organometallic chemist's "pincer toolbox" complementary to the PCP ligands but also as a chelate version of the ubiquitous mer,transCl(R 3 P) 2 motif. A combination of soft phosphines and hard π-base amido on the same metal is often difficult to achieve without linking these into a chelate, and such a hybrid environment has been shown to lead to unusual structures and reactivity in complexes of B. 10 However, B suffers from the high oxophilicity of Si in its backbone, leading to high sensitivity to OH and other O-containing groups. 10d,11 Cyclometalation of the Si-CH 2 -P group has also been described. 10f PNP ligands featuring aliphatic linkers (C) between N and P have been reported, 12-15 yet, for these, few metal complexes utilizing a C-type ligand in its anionic form are known. 14 In addition, pincers B and C are more flexible than A and therefore do not have as strong a preference, if any, for the meridional geometry.We were attracted to the ligand construction wherein o-arylene units link the amido and phosphine sites, because it offers increased rigidity and is devoid of -hydrogens and moisture-sensitive functionalities. At the time of writing, reports were published presenting synthesis of the ligand D and its Ni 16a and Rh 16b complexes, as well as similar reasoning behind the Czerw, M.; Zhu, K.; Singh, B.; Kanzelberger, M.; Darji, N.; Achord, P. D.; Renkema, K. B.; Goldman, A. S. Oevers, S.; Montag, M.; Vigalok, A.; Rozenberg, H.; Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2001, 123, 9064. (b) Sundermann, A.; Uzan, O.; Milstein, D.; Martin, J. M. L. J. Am. Chem. Soc. 2000, 122, 7095. (c) van der Boom, M. E.; Kraatz, H.-B.; Hassner, L.; Ben-David, Y.; Milstein, D. Organometallics 1999, 18, 3873. (d) van der Boom, M. E.; Shyh-Yeon, L.; Ben-David, Y.; Gozin, M.; Milstein, D. Haddad, T. S. Coord. Chem. Rev. 1990, 99, 137. (c) Fryzuk, M. D.; Montgomery, C. D. Coord. Chem. Rev. 1989, 95, 1. (d) Fryzuk, M. D.; MacNeil, P. A. (a) Steffey, B. D.; Miedan...

Reactions of chelating pincer-type PNP ligands based on the bis(ortho-phosphinoaryl)amine substructure and containing either an N-H (PN(H)P, 1) or N-Me (PN(Me)P, 2) central moiety with group 10 complexes have been explored. Reactions with MCl 2 (MCl 2 ) NiCl 2 , (COD)PdCl 2 , (COD)PtCl 2 , COD ) 1,5-cyclooctadiene) proceed readily with the loss of either HCl or MeCl and the formation of (PNP)MCl ( 7) where PNP is an anionic, meridional amido-PNP ligand. Alkylation of (PNP)MeCl with MeMgCl gives (PNP)MMe ( 9), and reaction of (PNP)MCl with excess NaBH 4 provides (PNP)MH ( 8). (PNP)MH ( 8) compounds react with CDCl 3 to regenerate (PNP)MCl ( 7). The transformations 7 f 8 f 7 f 9 are sluggish for M ) Pt compared with M ) Ni or Pd. Solid-state structures of (PNP)PdH (8b-Pd) and (PNP)-PdMe (9b-Pd) were determined. The environment about Pd in either structure is approximately square planar with a meridional amido-PNP ligand. Reactions of 1 and 2 with L n M 0 (L n ) (COD) 2 , (PPh 3 ) 4 , (PBu t 3 ) 2 ) proceed in some cases via N-H or N-C oxidative addition to give either (PNP)MH ( 8) or (PNP)MMe ( 9). The N-H oxidative addition reactions are more facile. Both the N-H and N-Me oxidative addition reactions are kinetically inhibited by liberated phosphines from the L n M 0 starting material. Thermolysis of (PNP)MMe (9, M ) Ni, Pd, Pt) in the presence of excess PPh 3 does not lead to N-C reductive elimination, thus indicating irreversibility of the N-C oxidative addition.

A (PNP)Ir fragment undergoes facile, room-temperature oxidative addition of C−H bonds in arenes and haloarenes in preference to aromatic carbon−halogen bonds. This preference, however, is determined to be kinetic in nature. Oxidative addition of C−Cl and C−Br is preferred thermodynamically. The products of the C−Cl or C−Br oxidative addition are separated from the C−H oxidative addition products by a high activation barrier and are only accessible at >100 °C. Of the C−H oxidative addition products of chlorobenzene, the isomer with the o-ClC6H4 ligand has the lowest energy.

A Ni complex of a diarylamido-based PNP ligand is an efficient and robust catalyst for coupling of acetonitrile with aldehydes.

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.

The (PNP)PdOTf complex is a suitable synthetic equivalent of the [(PNP)Pd](+) fragment in reactions with various HX substrates. The [(PNP)Pd](+) fragment either simply binds HX molecules as L-type ligands (X = NH(2), PCy(2), imidazolyl) or heterolytically splits the H-X bond to produce [(PN(H)P)Pd-X](+) (X = H, CCR, SR). DFT calculations analyze the relative energetics of the two outcomes and agree with the experimental data. Calculations also allow to assess the unobserved Pd(IV) isomer [(PNP)Pd(H)(2)](+) and validate its unfavourability with respect to the Pd(II) isomer [(PN(H)P)PdH](+).

The (PNP)Ir fragment displays a thermodynamic preference for the oxidative addition of aromatic vs benzylic C−H bonds. However, in the case of the mesitylene activation products, the benzylic isomer is kinetically accessible and can be trapped by an external donor ligand. The preference for the benzylic isomer in the six-coordinate Ir(III) adduct of mesitylene activation is ascribed to steric factors.

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