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}](ClO ₄)₂ (1) and [Ru^II(trpy)(NC₅H₄-CH=N-NH-C₆H₅)Cl]ClO₄ (2) (trpy = 2,2':6',2' '-Terpyridine)

Abstract

The reaction of Ru(trpy)Cl₃ (trpy = 2,2':6',2''-terpyridine) with the pyridine-based imine function N_pC₅H₄-CH=N_i-NH-C₆H₅ (L), incorporating an NH spacer between the imine nitrogen (N_i) 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, N_pC₅H₄-CH=N_iN(C₆H₅)C(CH₃)=N _aH (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 (N_p) of L which in turn places the NH spacer away from the RuCl bond, whereas, in 1, the corresponding sixth position, that is, the RuNa (amidine) bond, is trans to the pyridine nitrogen (N_p) of L'. The trans configuration of N_a with respect to the N_p 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 κ, exhibiting emissions at λ _max, 598 nm [quantum yield (F) = 0.43 × 10^-2 ] and 574 nm (F = 0.28 × 10^-2 ) for 1 and 2, respectively.