Mono(arylhydrazino)acenaphthenones as a platform for the design of NIR chromophores based on Pd(ii)-BIAN complexes

A new synthetic procedure for mono(arylhydrazino)acenaphthenones Ph-mhan (1) and 2-tol-mhan (2), based on the reaction of acenaphthenequinone with an arylhydrazonium salt, has been developed. These compounds were used further to obtain a series of heteroleptic cationic palladium(ii) complexes containing bis(arylimino)acenaphthene (Ar-bian) and mono(arylhydrazino)acenaphthenone (Ar-mhan) ligands: [Pd(tmp-bian)(Ph-mhan)](CF3SO3)(2) (3), [Pd(tmp-bian)(2-tol-mhan)](CF3SO3)(2), (4), and [Pd(dpp-bian)(Ph-mhan)](CF3SO3)(2) (5). The resulting compounds were characterized by X-ray diffraction analysis, 1H NMR, IR and UV spectroscopy, cyclic voltammetry, and quantum chemical calculations. All complexes show a reversible reduction at -0.31 to -0.41 V (vs. Ag/AgCl) localized on the Ar-bian ligand, followed by an irreversible reduction at -1.27 to -1.46 V (vs. Ag/AgCl) of the Ar-mhan moiety. Electronic absorption spectra of 3-5 reveal a characteristic band in the near-IR region (similar to 700 nm), corresponding to the intramolecular ligand-to-ligand charge transfer (LL ' CT) from Ar-mhan (pi-donor) to Ar-bian (pi-acceptor).