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

The donor–acceptor complex [(O,NQ2–)­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,OQ2–)­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.