Mononuclear and Dinuclear Complexes of Isoeilatin

This work describes the synthesis and characterization of mononuclear and dinuclear Ru(II) and Os(II) complexes based on the symmetrical bridging ligand isoeilatin (1). The crystal structure of 1·[HCl]₂ consists of layers of tightly π-stacked molecules of the biprotonated isoeilatin. The mononuclear complexes [Ru(bpy)₂(ieil)]²⁺ (2²⁺) and [Os(bpy)₂(ieil)]²⁺ (3²⁺) form discrete dimers in solution held together by face-selective π-stacking interactions via the isoeilatin ligand. Coordination of a second metal fragment does not hinder the π-stacking completely, as demonstrated by the concentration dependence of the ¹H NMR spectra of the dinuclear complexes [{Ru(bpy)₂}₂{μ-ieil}]⁴⁺ (4⁴⁺), [{Os(bpy)₂}₂{μ-ieil}]⁴⁺ (5⁴⁺), and [{Ru(bpy)₂}{μ-ieil}{Os(bpy)₂}]⁴⁺ (6⁴⁺) and supported by the solid-state structure of meso-4·[Cl]₄. The bridging isoeilatin ligand conserves its planarity even upon coordination of a second metal fragment, as demonstrated in the solid-state structures of meso-4·[Cl]₄, meso-4·[PF₆]₄, and meso-5·[PF₆]₄. All of the dinuclear complexes exhibit a preference (3/2−3/1) for the formation of the heterochiral as opposed to the homochiral diastereoisomer. Absorption spectra of the mononuclear complexes feature a low-lying d_π(M) → π*(ieil) MLCT band around 600 nm that shifts to beyond 700 nm upon coordination of a second metal fragment. Cyclic and square-wave voltammetry measurements of the complexes exhibit two isoeilatin-based reduction waves that are substantially anodically shifted compared to [M(bpy)₃]²⁺ (M = Ru, Os). Luminescence spectra, quantum yields, and lifetime measurements at room temperature and at 77 K demonstrate that the complexes exhibit ³MLCT emission that occurs in the IR region between 950 and 1300 nm. Both the electrochemical and photophysical data are consistent with the low-lying π* orbital of the isoeilatin ligand. The dinuclear complexes exhibit two reversible, well-resolved, metal-centered oxidation waves, despite the chemical equivalence of the two metal centers, indicating a significant metal−metal interaction mediated by the bridging isoeilatin ligand.