Probing electronic structures of redox-active ruthenium-quinonoids appended with polycyclic aromatic hydrocarbon (PAH) backbone

doi:10.1016/j.ica.2018.08.047

Inorganica Chimica Acta

Volume 483, 1 November 2018, Pages 343-351

https://doi.org/10.1016/j.ica.2018.08.047 Get rights and content

Highlights

•
Ruthenium-quinonoids appended with polycyclic aromatic hydrocarbon (PAH) backbone.
•
Intermolecular π-π interactions between the extended π-system of quinonoids.
•
Variation of redox potentials based on the donor centers of quinonoids.
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Varying electronic forms at the metal-quinonoid interface as a function of donors.
•
Revelation of intermediate description instead of any precise electronic form.

Abstract

The newly designed electrically neutral complexes [Ru(acac)₂(Q)] (1–3) involving redox-active polycyclic aromatic hydrocarbon (PAH) derived quinonoids (Q): Q₁^(O,O) (1) and Q₂^(O,NH) (2), Q₂^(NH,NH) (3) (acac = acetylacetonate) were prepared from the metal precursor [Ru^II(acac)₂(CH₃CN)₂] and preformed pyrene-4,5-dione (Q₁) and partially deprotonated pyrene-4,5-diamine (H₄Q₂), respectively. The structural characterization of 1–3 established their molecular identities including intermolecular π-π stacking interactions between the extended π-system of pyrene in the adjacent molecules and the hydrogen bonded 1D-polymeric form of 3. The redox sensitive C-O and C-N bond distances of Q in 1, 2 and 3 revealed the dominating ground state electronic forms of [(acac)₂Ru^III-Q₁^(O,O)•−] (S = 0), [(acac)₂Ru^III-Q₂^(O,NH)•−] (S = 0) and [(acac)₂Ru^II-Q₂^(NH,NH)o] (S = 0), respectively, where strong antiferromagnetic coupling between Ru^III(t_2g⁵) and Q^•− resulted in S = 0 state in 1 or 2. Complexes 1–3 exhibited reversible single oxidation and reduction within the potential window of ± 1.5 V versus SCE in CH₃CN, which progressively shifted to the negative potential on moving from 1 to 2 to 3, primarily due to the difference in electronegativity between O and N donors of Q. The collective consideration of experimental (EPR, electronic spectra) and theoretical (DFT, TD-DFT) results of 1ⁿ-3ⁿ (n = +1, 0, −1) revealed (i) extensive mixing of metal–ligand orbitals due to the inherent covalency factor and (ii) Q^•− and Ru^II based oxidations of 1/2 and 3, respectively, led to the {Ru^III-Q^o} electronic form at the metal–ligand interface of the oxidized state (1⁺-3⁺), while the reduced state (1 ⁻ −3 ⁻) could best be described by the resonating form of {Ru^II-Q^•−}↔{Ru^III-Q²⁻}.

Graphical abstract

Ruthenium-quinonoids with extended π-system in 1–3 exhibiting varying electronic structural forms including resonating description in accessible redox states as a function of donor centers.

Introduction

Accessibility of varying redox states of biochemically relevant quinonoid moieties [1] (Q⁰, Q^•−, Q²⁻ [2]) and ruthenium ion (Ru^II, Ru^III, Ru^IV) as well as extensive delocalization of charge at the Ru-Q interface due to the closeness of their frontier orbitals [3] and the covalency factor [4] have led to unpredictable electronic structural forms (Scheme 1) [5].

Intensive studies using a wide variety of quinonoid frameworks in combination with ruthenium metal fragments having co-ligands of different electronic and steric features have revealed the following significant points. (i) Quite often intermediate description (i.e. the resonating form) fits rather better than any precise electronic form (Ru^II-Q^o or Ru^II-Q^•− or Ru^II-Q²⁻) [6], (ii) emergence of complex phenomenon such as valence tautomerism [7] or redox induced electron transfer (RIET) [8] and (iii) difficulty in sketching a general narrative even out of the analogous systems [9]. Furthermore, potential application of ruthenium-quinonoid systems in catalysis has been addressed [10]. This indeed has prompted the continuing efforts in evaluating newer classes of ruthenium-quinonoid based molecular set up [11].

In this context, the present article deals for the first time with a group of ruthenium complexes (1–3) involving polycyclic aromatic hydrocarbon (PAH, pyrene) derived cis-quinonoids [12] comprising of O,O (quinone, 1), O,NH (iminoquinone, 2) and NH,NH (diminoquinone, 3) donors.

Besides structural elucidation, electronic structural aspects of 1ⁿ-3ⁿ have been assessed by a combined experimental and theoretical approach. This establishes the intrinsic sensitivity of the valence and spin situations both at the native and accessible reversible redox states of 1ⁿ-3ⁿ (n = +1, 0, −1) as a function of the nature of the donor centers in the quinonoids.

Section snippets

Materials

The precursor complex Ru^II(acac)₂(CH₃CN)₂ [13] and the ligands [14], [15] were prepared according to reported procedures. Pyrene was purchased from Sigma-Aldrich. All other chemicals and reagents were reagent grade and were used as received. For spectroscopic and electrochemical studies HPLC grade solvents were used.

Physical measurements

¹H NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer. The electrical conductivity was checked by using an autoranging conductivity meter (Toshcon Industries,

Synthesis and general characterization

The electrically neutral [Ru(acac)₂(Q₁^(O,O))] (1) (acac⁻=acetylacetonate) was prepared by reacting the metal precursor Ru^II(acac)₂(CH₃CN)₂ with the preformed pyrene-4,5-dione (Q₁) in refluxing ethanol under dinitrogen atmosphere followed by purification on a neutral alumina column. On the other hand, the reaction of Ru^II(acac)₂(CH₃CN)₂ and the preformed pyrene-4,5-diamine (H₄Q₂) in refluxing ethanol and in the presence of NEt₃ base under dinitrogen atmosphere resulted in the simultaneous

Conclusion

The present combined experimental and theoretical approach in probing the delicate electronic forms of a set of structurally characterized redox active ruthenium-quinonoids encircling polycyclic aromatic hydrocarbon (PAH = pyrene) backbone with varying donor centers in 1–3 reveals the following important points:

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The extended π-system of the polycyclic aromatic hydrocarbon at the backbone of Q in the crystals of 1–3 furnishes π-π interactions between the adjacent molecules.
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Redox potential of the

Acknowledgements

Financial support received from the Science and Engineering Research Board (SERB, Department of Science and Technology), Council of Scientific and Industrial Research (CSIR) (fellowship to M.C and P.G.), University Grant Commission (fellowship to A.H.), New Delhi (India) is gratefully acknowledged.

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Research paperProbing electronic structures of redox-active ruthenium-quinonoids appended with polycyclic aromatic hydrocarbon (PAH) backbone

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Physical measurements

Synthesis and general characterization

Conclusion

Acknowledgements

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Research paper
Probing electronic structures of redox-active ruthenium-quinonoids appended with polycyclic aromatic hydrocarbon (PAH) backbone