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.

Ligand survey results in identification of PNP pincer complexes of iridium as long-lived and chemoselective catalysts for dehydrogenative borylation of terminal alkynes

“…8-H was prepared via the intermediate S1 . The synthesis of S1 relied on the same selective dilithiation of bis(2-bromo-4-methylphenyl)amine we previously used in the synthesis of S2 , 49 , 50 followed by quenching with water. Pure S1 was isolated in 86% yield by column chromatography.…”

“…Nonetheless, we were encouraged by the results because the PNP framework offers facile opportunities for optimization of the ligand via substituent variation. We selected previously reported PNP ligands 15-H , 50 16-H , 60 and 17-H 50 , 61 for further testing in DHBTA. The syntheses of the corresponding Ir-COE complexes are depicted in Scheme 4 .…”

Concerted aryl-sulfur reductive elimination from PNP pincer-supported Co(iii) and subsequent Co(i)/Co(iii) comproportionation

“…2 , which may help stabilize a low-spin configuration. Amido/bis(phosphine) PNP pincer ligands can be oxidized at the ligand and thus are potentially redox non-innocent, 54 but we see no evidence of it in this work.…”

Synthesis and rhodium complexes of macrocyclic PNP and PONOP pincer ligands

“…The ν(CO) bands of rhodium(I) carbonyl derivatives are diagnostic reporter groups for the donor properties of pincer ligands. 20,21 Comparison of the carbonyl bands of 9a/b with those of acyclic congeners 9a/b′, 15,22 recorded under the same conditions, suggests PNP-14 and PONOP-14 are marginally weaker net donors than PNP-tBu and PONOP-tBu, respectively (…”