Eilatin Complexes of Ruthenium and Osmium. Synthesis, Electrochemical Behavior, and Near-IR Luminescence

The synthesis and characterization of new Ru(II) and Os(II) complexes of the ligand eilatin (1) are described. The new complexes [Ru(bpy)(eil)₂]²⁺ (2), [Ru(eil)₃]²⁺ (3), and [Os(eil)₃]²⁺ (4) (bpy = 2,2‘-bipyridine; eil = eilatin) were synthesized and characterized by NMR, fast atom bombardment mass spectrometry, and elemental analysis. In the series of complexes [Ru(bpy)_x(eil)_y]²⁺ (x + y = 3), the effect of sequential substitution of eil for bpy on the electrochemical and photophysical properties was examined. The absorption spectra of the complexes exhibit several bpy- and eil-associated π−π* and metal-to-ligand charge-transfer (MLCT) transitions in the visible region (400−600 nm), whose energy and relative intensity depend on the number of ligands bound to the metal center (x and y). On going from [Ru(bpy)₂(eil)]²⁺ (5) to 2 to 3, the d_π(Ru) → π*(eil) MLCT transition undergoes a red shift from 583 to 591 to 599 nm, respectively. Electrochemical measurements performed in dimethyl sulfoxide reveal several ligand-based reduction processes, where each eil ligand can accept up to two electrons at potentials that are significantly anodically shifted (by ca. 1 V) with respect to the bpy ligands. The complexes exhibit near-IR emission (900−1100 nm) of typical ³MLCT character, both at room temperature and at 77 K. Along the series 5, 2, and 3, upon substitution of eil for bpy, the emission maxima undergo a blue shift and the quantum yields and lifetimes increase. The radiative and nonradiative processes that contribute to deactivation of the excited level are discussed in detail.