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 (−CHNX) 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 −PiPr2 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.