Eilatin as a Bridging Ligand in Ruthenium(II) Complexes: Synthesis, Crystal Structures, Absorption Spectra, and Electrochemical Properties

The potential of the heptacyclic aromatic alkaloid eilatin (1), that features two nonequivalent binding sites, to serve
as a bridging ligand is reported. The nonequivalency of the binding sites allowed the selective synthesis of both
mono- and dinuclear complexes. The mononuclear Ru(II) complexes [Ru(dmbpy)2(eilatin)]2+ (2) and [Ru(tmbpy)2(eilatin)]2+ (3) in which eilatin selectively binds “head-on” were synthesized and employed as building blocks in the
synthesis of the dinuclear complexes [{Ru(dmbpy)2}2(μ-eilatin)]4+ (4) and [{Ru(tmbpy)2}2(μ-eilatin)]4+ (5). Complete
structure elucidation of the complexes in solution was accomplished by 1D and 2D NMR techniques. The X-ray
structures of the mononuclear complex 3 and of the two dinuclear complexes 4 and 5 were solved, and absorption
spectra and electrochemical properties of the complexes were explored. Both dinuclear complexes formed as
racemic mixtures in a 3:1 diastereoisomeric ratio, the major isomer being the heterochiral one (ΔΛ/ΛΔ) as revealed
by crystallography. The mononuclear complexes feature an exceptionally low energy MLCT band around 600 nm
that shifted to over 700 nm upon the binding of the second Ru(II) center. The mononuclear complexes show one
reversible oxidation and several reversible reduction waves, the first two reductions being substantially anodically
shifted in comparison with [Ru(bpy)3]2+, attributed to the reduction of eilatin, and consistent with its low lying π*
orbital. The dinuclear complexes follow the same reduction trend, exhibiting several reversible reduction waves,
and two reversible well-resolved metal centered oxidations due to the nonequivalent binding sites and to a significant
metal−metal interaction mediated by the bridging eilatin.