Reactivity of the Indole Ring in Palladium(II) Complexes of 2N1O-Donor Ligands: Cyclopalladation and π-Cation Radical Formation

The Pd(II) complexes of new 2N1O-donor ligands containing a pendent indole, 3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-iepp), 1-methyl-3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-miepp), 3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]methylindole (Htbu-impp), and 3-(N-2-pyridylmethyl-N-4-hydroxybenzylamino)ethylindole (Hp-iepp) (H denotes a dissociable proton), were synthesized, and the structures of
[Pd(tbu-iepp)Cl] (1a), [Pd(tbu-iepp-c)Cl] (1b), [Pd(tbu-miepp)Cl] (3), and [Pd(p-iepp-c)Cl] (4) (tbu-iepp-c
and p-iepp-c denote tbu-iepp and p-iepp bound to Pd(II) through a carbon atom, respectively) were
determined by X-ray analysis. Complexes 1a prepared in CH2Cl2/CH3CN and 3 prepared in CH3CN have
a pyridine nitrogen, an amine nitrogen, a phenolate oxygen, and a chloride ion in the coordination plane.
Complex 1b prepared in CH3CN has the same composition as 1a and was revealed to have the C2 atom
of the indole ring bound to Pd(II) with the Pd(II)−C2 distance of 1.973(2) Å. The same Pd(II)−indole C2
bonding was revealed for 4. Interconversion between 1a and 1b was observed for their solutions, the
equilibrium being dependent on the solvent used. Reaction of 1b and 4 with 1 equiv of Ce(IV) in DMF gave
the corresponding one-electron-oxidized species, which exhibited an ESR signal at g = 2.004 and an
absorption peak at ∼550 nm, indicating the formation of the Pd(II)−indole π-cation radical species. The
half-life, t1/2, of the indole radical species at room temperature was calculated to be 20 s (kobs = 3.5 × 10-2
s-1) for 1b. The cyclic voltammogram for 1b in DMF gave two irreversible oxidation peaks at Epa = 0.68
and 0.80 V (vs Ag/AgCl), which were ascribed to the oxidation processes of the coordinated indole and
phenolate moieties, respectively.