Redox-Active Bridging Ligands Based on Indigo Diimine (“Nindigo”) Derivatives

Reactions of indigo with a variety of substituted anilines produce the corresponding indigo diimines (“Nindigos”) in good yields. Nindigo coordination complexes are subsequently prepared by reactions of the Nindigo ligands with Pd(hfac)2. In most cases, binuclear complexes are obtained in which the deprotonated Nindigo bridges two Pd(hfac) moieties in the expected bis-bidentate binding mode. When the Nindigo possesses bulky substituents on the imine (mesityl, 2,6-dimethylphenyl, 2,6-diisopropylphenyl, etc.), mononuclear Pf(hfac) complexes are obtained in which the Nindigo core has isomerized from a trans- to a cis-alkene; in these structures, the palladium is bound to the cis-Nindigo ligand at the two indole nitrogen atoms; the remaining proton is bound between the imine nitrogen atoms. The palladium complexes possess intense electronic absorption bands [near 920 nm for the binuclear complexes and 820 nm for the mononuclear cis-Nindigo complexes; extinction coefficients are (1.0–2.0) × 104 M–1 cm–1] that are ligand-centered (π–π*) transitions. Cyclic voltammetry investigations reveal multiple redox events that are also ligand-centered in origin. All of the palladium complexes can be reversibly oxidized in two sequential one-electron steps; the binuclear complexes are reduced in a two-electron process whose reversibility depends on the Nindigo ligand substituent; the mononuclear palladium species show two one-electron reductions, only the first of which is quasi-reversible.