Example of highly stereoregulated ruthenium amidine complex formation:: Synthesis, crystal structures, and spectral and redox properties of the complexes [RuII(trpy) {NC5H4-CH=N-N(C6H5)C(CH3)=NH}](ClO4)2 (1) and [RuII(trpy)(NC5H4-CH=N-NH-C6H5)Cl]ClO4 (2) (trpy=2,2′:6′,2"-terpyridine)

The reaction of Ru(trpy)Cl-3 (trpy = 2,2':6',2"-terpyridine) with the pyridine-based imine function NpC5H4-CH=NiNH-C6H5 (L), incorporating an NH spacer between the imine nitrogen (N) 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= N-i-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 (N-p) 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-N-a (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) --> pi* (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 lambda(max), 598 nm [quantum yield (Phi) = 0.43 x 10(-2)] and 574 nm (Phi = 0.28 x 10(-2)) for 1 and 2, respectively.