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 PdII complexes, green [PdII{(LAP) 2–}] 1 (S = 0), red [PdII{(LISQ) •–}][PF6 ]·CH2Cl2 2 (S = 1/2), and dark green [PdII{(LIBQ) 0}][BF4 ]2 ·2CH2Cl2 3 (S = 0), where the coordinated ligand is present as dianionic (LAP) 2–, monoanionic oiminobenzosemiquinonate(1–) π-radical (Srad = 1/2) (LISQ) • – , and neutral o-iminobenzoquinone (LIBQ) 0 redox level. Reaction of complexes 1 and 2 with PPh3 afforded isolation of two crystalline complexes, dark green [PdII{(LAp) 2–}(PPh3 )] 4 and red [PdII{(LISQ) •–}(PPh3 )][PF6 ]·CH2Cl2 5. X-Ray structure determination of complex 5 at 100(2) K revealed PdIION2P 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 PdIIONSP coordination environment. Notably, the ethylpyridine arm remains non-coordinated. When examined by cyclic voltammetry (CV), complex 4 exhibits two quasireversible oxidative responses at E1/2 = 0.08 (peak-to-peak separation, ΔEp = 140 mV) and 0.65 V (ΔEp = 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 PdII ion. TimeDependent (TD)-DFT calculation shed light on the origin of observed spectral absorptions for complex 4