title: Comparison of the electronic properties of diarylamido-based PNZ pincer ligands: redox activity at the ligand and donor ability toward the metal.
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.
authors.affiliation: Department of Chemistry, Texas A&M University , 3255 TAMU, College Station, Texas 77842, United States.
authors.forename: Jillian J
authors.initials: JJ
authors.lastname: Davidson
authors.forename: Jessica C
authors.initials: JC
authors.lastname: DeMott
authors.forename: Christos
authors.initials: C
authors.lastname: Douvris
authors.forename: Claudia M
authors.initials: CM
authors.lastname: Fafard
authors.forename: Nattamai
authors.initials: N
authors.lastname: Bhuvanesh
authors.forename: Chun-Hsing
authors.initials: CH
authors.lastname: Chen
authors.forename: David E
authors.initials: DE
authors.lastname: Herbert
authors.forename: Chun-I
authors.initials: CI
authors.lastname: Lee
authors.forename: Billy J
authors.initials: BJ
authors.lastname: McCulloch
authors.forename: Bruce M
authors.initials: BM
authors.lastname: Foxman
authors.forename: Oleg V
authors.initials: OV
authors.lastname: Ozerov