No abstract

This paper presents the synthesis and characterization of a series of pincer ligands and their Ni, Pd, Pt, and Rh complexes. The ligands under examination are based on a diarylamine which is modified either by two phosphino (-PR2) substituents in the ortho-positions (PNP ligands) or by a combination of a phosphino and an iminyl (-CH═NX) substituent (PNN ligands). The ligands can be broken down into three groups: (a) C2v-symmetric PNP ligands with identical side -PR2 donors, (b) Cs-symmetric PNP' ligands with different -PR2 side donors, and (c) PNN ligands containing a -P(i)Pr2 side donor. All of the ligands under study readily formed square-planar complexes of the types (PNZ)PdCl, (PNZ)Pd(OAc), and (PNZ)RhCO, where PNZ is the corresponding anionic tridentate pincer ligand. For select PNP ligands, (PNP)NiCl and (PNP)PtCl were also studied. The (PNZ)MCl complexes (M = Ni, Pd, Pt) underwent quasireversible oxidation in cyclic voltammetry experiments. Based on the close similarity of formal potentials for Ni, Pd, and Pt analogs, and based on the previous literature evidence, these oxidation events are ascribed primarily to the PNZ ligand, and the E1/2 values can be used to compare the ease of oxidation of different ligands. A (PNP)PdCl complex containing methoxy substituents para- to the central nitrogen underwent two quasireversible oxidations. Two mono-oxidized complexes were isolated and structurally characterized in comparison to their neutral analog, revealing minimal changes in the bond distances and angles. Several other neutral complexes were also structurally characterized. The carbonyl stretching frequency in (PNZ)RhCO complexes was used to gauge the donating ability of the various pincer ligands toward the metal. Comparison of E1/2 values for (PNZ)PdCl and νCO values for (PNZ)RhCO revealed that the two are not consistently correlated across all the studied ligands and can be tuned to different degrees through judicious ligand alteration.

The Pd(I)-Pd(I) dimer [((F)PNP)Pd-](2) reacts with O(2) upon exposure to light to produce either the superoxide ((F)PNP)PdO(2) or the peroxide [((F)PNP)PdO-](2), which exist in equilibrium with free O(2). Both complexes contain square-planar Pd(II) centers. The unpaired electron density in ((F)PNP)PdO(2) is localized on the superoxide 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](+).