Two pairs of geometrical isomers of heteroleptic Ru(II)-polypyridyl complexes incorporating asymmetric bidentate ligands, 2-Picolylamine (2-PA) and 2-Picolylimine (2-PI), in combination with phenyl-terpyridine (ptpy), were synthesized and systematically investigated. The complexes, formulated as Ru(ptpy)(2-PA)ClPF6 (1AM, 2AM) and Ru(ptpy)(2-PI)ClPF6 (1IM, 2IM), differ only in the spatial orientation of the pyridyl nitrogen of the bidentate ligand relative to the chloride (Npy-cis-Cl vs Npy-trans-Cl). Structural comparisons reveal strengthened Ru-N(imine) bonding in the 2-PI analogues driven by enhanced π-accepting ability. Despite their closely related coordination environments, the complexes display sharply contrasting thermal and photochemical behavior that is strongly dependent on the coordinating nature of the solvents. The Npy-cis-Cl isomer 1AM undergoes clean amine-to-imine conversion with H2 evolution under thermal or photoactivation in both coordinating and noncoordinating solvents. In contrast, the Npy-trans-Cl isomers 2AM preferentially undergo Ru-N bond cleavage in coordinating solvents, whereas noncoordinating media promote amine-to-imine transformation accompanied by isomerization to 1AM. Among all the species, 1IM exhibits exceptional thermal and photochemical stability toward both coordinating and non-coordinating solvents. All the complexes were moderate singlet oxygen generators, highlighting their potential in photooxidative and photochemotherapeutic applications. Overall, this work establishes coordinating ability of solvents to Ru(II) as a decisive factor governing divergent stimuli-responsive reactivity pathways in Ru(II)-polypyridyl-based geometrical isomers.
Chatterjee et al. (Tue,) studied this question.
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