Palladium(II) Complex of a Redox-Active Amidophenolate-Based <i>O</i>,<i>N</i>,<i>S</i>,<i>N</i> Ligand: Its Monocation and Dication and Reactivity with PPh₃

A new
potentially tetradentate redox-active o-aminophenol-based
ligand, H2L = 2-(2-ethylthio)­pyridine-anilino-4,6-di-tert-butylphenol, reacts with PdII(O2CCH3)2 in CH3OH in the presence
of air and Et3N affording isolation of a green crystalline
solid of composition [Pd­(L)] 1. When examined by cyclic
voltammetry (CV), 1 exhibits two quasireversible oxidative
responses at E1/2 = 0.16 (peak-to-peak
separation, ΔEp = 100 mV) and 0.89
V (ΔEp = 90 mV) vs SCE (saturated
calomel electrode). Chemical oxidation of 1 by [FeIII(η5-C5H5)2]­[PF6] and AgBF4 in CH2Cl2 led to the isolation of two crystalline solids, red [Pd­(L)]­[PF6]·CH2Cl2 2 and dark
green [Pd­(L)]­[BF4]2·2CH2Cl2 3, respectively. Single-crystal X-ray crystallography
at 100(2) K unambiguously established that the O,N,S,N-coordinated ligand
is present in the square-planar complexes [PdII{(LAP)2–}] 1, [PdII{(LISQ)•–}]­[PF6]·CH2Cl2 2, and [PdII{(LIBQ)0}]­[BF4]2·2CH2Cl2 3, as dianionic (LAP)2–, monoanionic o-iminobenzosemiquinonate­(1−)
π-radical (Srad = 1/2) (LISQ)•–, and neutral o-iminobenzoquinone (LIBQ)0 redox
level. Reaction of 1 and 2 with PPh3 afforded isolation of two crystalline complexes: dark green
[PdII(L)­(PPh3)] 4 and red [PdII{(LISQ)•–}­(PPh3)]­[PF6]·CH2Cl2 5. X-ray structure determination of 5 at 100(2) K revealed
PdIION2P coordination environment. The square-planar
complexes 1–5 possess an S = 0, 1/2, 0, 0, and 1/2 ground-state, respectively, as was established by 1H NMR and EPR spectroscopy, and room-temperature magnetic
moment data. All redox processes are thus shown to be ligand-based.
Absorption spectral measurements were done for all complexes. DFT
calculations at B3LYP-level of theory adequately describe the electronic
structures of 1–3, and 5, containing a spin-paired d8 PdII ion. Time-dependent-DFT
calculations on 1−3 and 5 shed light on the origin of UV−vis−NIR spectral absorptions.