Example of Highly Stereoregulated Ruthenium Amidine Complex Formation: Synthesis, Crystal Structures, and Spectral and Redox Properties of the Complexes [Ru^II(trpy){NC₅H₄CHNN(C₆H₅)C(CH₃)NH}](ClO₄)₂ (<b>1</b>) and [Ru^II(trpy)(NC₅H₄CHNNHC₆H₅)Cl]ClO₄ (<b>2</b>) (trpy = 2,2‘:6‘,2‘ ‘-Terpyridine)

The reaction of Ru(trpy)Cl3 (trpy = 2,2‘:6‘,2‘ ‘-terpyridine) with the pyridine-based imine function NpC5H4−CHNi-NH−C6H5 (L), incorporating an NH spacer between the imine nitrogen (Ni) and the pendant phenyl ring, in ethanol medium followed by chromatographic work up on a neutral alumina column using CH3CN/CH2Cl2 (1:4) as eluent, results in complexes of the types [Ru(trpy)(L‘)](ClO4)2 (1) and [Ru(trpy)(L)Cl]ClO4 (2). Although the identity of the free ligand (L) has been retained in complex 2, the preformed imine-based potentially bidentate ligand (L) has been selectively transformed into a new class of unusual imine−amidine-based tridentate ligand, NpC5H4CHNiN(C6H5)C(CH3)NaH (L‘), in 1. The single-crystal X-ray structures of the free ligand (L) and both complexes 1 and 2 have been determined. In 2, the sixth coordination site, that is, the Cl- function, is cis to the pyridine nitrogen (Np) of L which in turn places the NH spacer away from the RuCl bond, whereas, in 1, the corresponding sixth position, that is, the RuNa (amidine) bond, is trans to the pyridine nitrogen (Np) of L‘. The trans configuration of Na with respect to the Np of L‘ in 1 provides the basis for the selective L → L‘ transformation in 1. The complexes exhibit strong Ru(II) → π* (trpy) MLCT transitions in the visible region and intraligand transitions in the UV region. The lowest energy MLCT band at 510 nm for 2 has been substantially blue-shifted to 478 nm in the case of 1. The reversible Ru(III)−Ru(II) couples for 1 and 2 have been observed at 0.80 and 0.59 V versus SCE, respectively. The complexes are weakly luminescent at 77 K, exhibiting emissions at λmax, 598 nm [quantum yield (Φ) = 0.43 × 10-2 ] and 574 nm (Φ = 0.28 × 10-2 ) for 1 and 2, respectively.