Polypyridyl Ruthenium(II) Complexes with Tetrazolate-Based Chelating Ligands. Synthesis, Reactivity, and Electrochemical and Photophysical Properties

In this contribution, we report the synthesis, the chemical and photophysical characterization, and the study of the
reactivity toward electrophiles of two mononuclear complexes of the type [Ru(bpy)2L]+ (bpy is 2,2‘-bipyridyl), in
which L is represented by the deprotonated form of 2-(1,H-tetrazol-5-yl)pyridine (L1) or 2-(1,H-tetrazol-5-yl)pyrazine
(L2). The 1H and 13C NMR experiments that were performed on complexes RuL1 and RuL2 allowed us to establish
that the tetrazolate moiety is bonded to the metal center via the N-1 nitrogen, while the coplanar arrangement
adopted by the coordinated ligand upon coordination and the consequent interannular conjugation effect accounts
for the unexpectedly low field resonance of the tetrazole carbon. The 13C NMR spectroscopy is also of fundamental
importance to determine the chemo- and regioselectivity of the addition of a methyl group to RuL1 and RuL2,
which takes place at the N-3 nitrogen of the five-membered ring. All these features were confirmed by the X-ray
diffraction structures of RuL1 and of the methylated compounds RuL1Me and RuL2Me. Relative to these latter
complexes, the presence of a methyl moiety does not cause any distortion from coplanarity of the coordinated
tetrazolates. The redox properties of the complexes were investigated by cyclic voltammetry and indicated a quite
different behavior between the pyrazinyl−tetrazolate and the pyridyl−tetrazolate complexes as the consequence of
the higher electron-withdrawing character of the pyrazine ring. The study of the photophysical properties of the
complexes also shows a significant diversity between the luminescent RuL1 and the rather poorly emissive RuL2.
Interestingly, the methylated compounds RuL1Me and RuL2Me display radiative excited-state decays with longer
lifetimes than their precursors; this feature indicates that methylation is a useful reaction for the tuning of the light
emission performances of similar tetrazolate complexes. The synthesis and the characterization of a novel dinuclear
complex of type [(bpy)2Ru-L3-Ru(bpy)2]2+, Ru(L3)Ru, where L3 is the bis-anion derived from bis-2,3-(1,H-tetrazol-5-yl)pyrazine, is also reported.