Electrochemistry, Chemical Reactivity, and Time-Resolved Infrared Spectroscopy of Donor–Acceptor Systems [(Q^x)Pt(pap^y)] (Q = Substituted o‑Quinone or o‑Iminoquinone; pap = Phenylazopyridine)

The donor–acceptor complex [(^O,NQ^2–)­Pt­(pap⁰)] (1; pap = phenylazopyridine, ^O,NQ⁰ = 4,6-di-tert-butyl-N-phenyl-o-iminobenzoquinone), which displays strong π-bonding interactions and shows strong absorption in the near-IR region, has been investigated with respect to its redox-induced reactivity and electrochemical and excited-state properties. The one-electron-oxidized product [(^O,NQ^•–)­Pt­(pap⁰)]­(BF₄) ([1]­BF₄) was chemically isolated. Single-crystal X-ray diffraction studies establish the iminosemiquinone form of ^O,NQ in [1]⁺. Simulation of the cyclic voltammograms of 1 recorded in the presence of PPh₃ elucidates the mechanism and delivers relevant thermodynamic and kinetic parameters for the redox-induced reaction with PPh₃. The thermodynamically stable product of this reaction, complex [(^O,NQ^•–) Pt­(PPh₃)₂]­(PF₆) ([2]­PF₆), was isolated and characterized by X-ray crystallography, electrochemistry, and electron paramagnetic resonance spectroscopy. Picosecond time-resolved infrared spectroscopic studies on complex 1b (one of the positional isomers of 1) and its analogue [(^O,OQ^2–)­Pt­(pap⁰)] (3; ^O,OQ = 3,5-di-tert-butyl-o-benzoquinone) provided insight into the excited-state dynamics and revealed that the nature of the lowest excited state in the amidophenolate complex 1b is primarily diimine-ligand-based, while it is predominantly an interligand charge-transfer state in the case of 3. Density functional theory calculations on [1]ⁿ⁺ provided further insight into the nature of the frontier orbitals of various redox forms and vibrational mode assignments. We discuss the mechanistic details of the newly established redox-induced reactivity of 1 with electron donors and propose a mechanism for this process.