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

Abstract

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.

The Reasons for Ligand‐Dependent Quantum Yields and Absorption Spectrum of Four Polypyridylruthenium(II) Complexes with a Tetrazolate‐Based Ligand: TDDFT Study

“…The calculated results agree with the experimental observations [10] that the MLCT component is more remarkable in the lower energy region. The above calculation indicates that spin-selection rules are not strictly obeyed for heavy metal complexes and the singlet-to-triplet transitions are facilitated by the enhanced spin-orbit coupling in heavy atom.…”

“…[10] Obviously, our calculated absorption spectra can well reproduce the experimental features in terms of band positions, intensities and separations. Tables 3-6 show that the lowest energy transitions follow the order RuL1 (485 nm) Ͼ RuL2 (468 nm) Ͼ RuL2Me (459 nm) Ͼ RuL1Me (427 nm), which is consistent with the trend of the energy gap because of the HOMOǞLUMO transition (the coefficient in the configuration interaction wave functional is more than 0.70), which contributes mainly to the S 0 ǞS 1 electronic absorption.…”

“…The experimentally observed strongest absorptions around 280 nm are mainly assigned as ligand-centered bpybased π Ǟ π* absorptions. [10] In our calculations, these absorption peaks are blueshifted to a higher energy region, and all the bands arise from a combination of several transitions, among which only those having the major contribution are listed in Tables 3-6. The observed absorption band at 289 nm for RuL1 is mainly described by a series of very close transitions (271, 270, and 266 nm) and they are assigned to {[d(Ru) + π(L1 + bpy)] Ǟ [π*(bpy)]} with character of MLLCT and ILCT.…”

“…The main geometry structural parameters are summarized in Table 1 together with the X-ray crystal structures data of RuL2. [10] As a result of the more spatially extended 4d orbital and the large ligand field splitting, in Ru II the three d orbitals are doubly occupied by six d electrons; therefore, Ru II tends to form low-spin pseudo-octahedral coordination. This low-spin arrangement can be obtained from the higher-energy e g *-like orbital, which will forbid electron transition from the low-energy t 2g orbital to the e g *-like orbital and therefore maintain the low-spin arrangement.…”

“…[where L is represented by the deprotonated form of 2-(1H-tetrazol-5-yl)pyridine (L1) or 2-(1H-tetrazol-5-yl)pyrazine (L2)], as well as their methylated complexes RuL1Me and RuL2Me, [10] the emission energy, quantum yields, and lifetime of RuL2Me are all much larger than those of RuL2 and RuL1. It is still not very clear how the structures of the ligands influence the phosphorescence efficiency after modification.…”