Palladium(II) complexes of a redox-active o-aminophenolate-based ONSN ligand. Proof-of-concept of hemilability in reactivity with PPh3 providing ONNP and ONSP coordination

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208 016, Uttar Pradesh, India and

Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata,

Mohanpur-741 246, West Bengal, India

E-mail : rnm@iitk.ac.in, rnm@iiserkol.ac.in

Using a potentially tetradentate redox-active o-aminophenol-based ligand, H₂L = 2-(2-ethylthio)pyridine-anilino4,6-di-tert-butylphenol, we recently reported synthesis and structural characterization of three O,N,S,N-coordinated squareplanar Pd^II complexes, green [Pd^II{(L^AP) ^2–}] 1 (S = 0), red [Pd^II{(L^ISQ) ^•–}][PF₆ ]·CH₂Cl₂ 2 (S = 1/2), and dark green [Pd^II{(L^IBQ) ⁰}][BF₄ ]₂ ·2CH₂Cl₂ 3 (S = 0), where the coordinated ligand is present as dianionic (L^AP) ^2–, monoanionic oiminobenzosemiquinonate(1–) π-radical (S_rad = 1/2) (L^ISQ) ^{• –} , and neutral o-iminobenzoquinone (L^IBQ) ⁰ redox level. Reaction of complexes 1 and 2 with PPh3 afforded isolation of two crystalline complexes, dark green [Pd^II{(L^Ap) ^2–}(PPh₃ )] 4 and red [Pd^II{(L^ISQ) ^•–}(PPh₃ )][PF₆ ]·CH₂Cl₂ 5. X-Ray structure determination of complex 5 at 100(2) K revealed Pd^IION₂P coordination environment (A. Ali, S. K. Barman and R. Mukherjee, Inorg. Chem., 2015, 54, 5182–5194). The present work reports X-ray structure determination of complex 4, which revealed Pd^IIONSP coordination environment. Notably, the ethylpyridine arm remains non-coordinated. When examined by cyclic voltammetry (CV), complex 4 exhibits two quasireversible oxidative responses at E_1/2 = 0.08 (peak-to-peak separation, ΔE_p = 140 mV) and 0.65 V (ΔE_p = 150 mV) vs SCE (saturated calomel electrode). Both the redox processes are supposedly ligand-based. Absorption spectral property of complex 4 has also been investigated. Density Functional Theory (DFT) calculations at B3LYPlevel of theory adequately describe the electronic structure of complex 4, containing a spin-paired d⁸ Pd^II ion. TimeDependent (TD)-DFT calculation shed light on the origin of observed spectral absorptions for complex 4.