An Investigation of Superexchange in Dinuclear Mixed-Valence Ruthenium Complexes

The kinetics and thermodynamics of intramolecular electron transfer (IET) can be subjected to redox control in a bistable [2]rotaxane comprised of a dumbbell component containing an electron-rich 1,5-dioxynaphthalene (DNP) unit and an electron-poor phenylenebridged bipyridinium (P-BIPY 2þ ) unit and a cyclobis (paraquatp-phenylene) (CBPQT 4þ ) ring component. The [2]rotaxane exists in the ground-state co-conformation (GSCC) wherein the CBPQT 4þ ring encircles the DNP unit. Reduction of the CBPQT 4þ leads to the CBPQT 2ð•þÞ diradical dication while the P-BIPY 2þ unit is reduced to its P-BIPY •þ radical cation. A radical-state co-conformation (RSCC) results from movement of the CBPQT 2ð•þÞ ring along the dumbbell to surround the P-BIPY •þ unit. This shuttling event induces IET to occur between the pyridinium redox centers of the P-BIPY •þ unit, a property which is absent between these redox centers in the free dumbbell and in the 1∶1 complex formed between the CBPQT 2ð•þÞ ring and the radical cation of methyl-phenylene-viologen (MPV •þ ). Using electron paramagnetic resonance (EPR) spectroscopy, the process of IET was investigated by monitoring the line broadening at varying temperatures and determining the rate constant (k ET ¼ 1.33 × 10 7 s −1 ) and activation energy (ΔG ‡ ¼ 1.01 kcal mol −1 ) for electron transfer. These values were compared to the corresponding values predicted, using the optical absorption spectra and Marcus-Hush theory.T he simplest organic intervalence compounds consist (1) of two redox centers (X) which are connected by a covalent bridge (b) wherein the two centers possess oxidation states that range from being equal ( nþ0.5 X − b − X nþ0.5 ) to differing by as much as one unit ( n X − b − X nþ1 ). According to the Robin-Day classification (2), there are three main types of organic compounds, which can be defined by the degree of the electronic coupling element, H, that exists between the two interacting redox centers. Compounds that exhibit low electronic coupling between each X redox center have localized charges and exhibit no intramolecular electron transfer (IET) are classified as Class I systems ( n X − b − X nþ1 , H ¼ 0), whereas those that possess a strong electronic coupling between X redox centers and undergo complete delocalization of charge and fast IET are considered Class III systems ( nþ0.5 X − b − X nþ0.5 , H ≫ 0). In between these two classifications lie Class II systems, where the electronic coupling between X redox centers is moderate, leading to a mixture of charge between each unit as well as a measurable IET event.In the 1960s, Marcus (3-6) developed a theory for electron transfer in bridged transition metal-coordinated complexes by identifying the reorganizational energy, λ, required for solvent polarization and subsequent electron transfer between two metal centers. Shortly thereafter, Hush (7-9) proposed a theoretical method that could be applied to Class II intervalence compounds by focusing on the low-energy intervalence charge transfer (IT) bands in optical abso...

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“…•þ and the neutral P-BIPY 0 , i.e, ΔG r ≈ ΔG disp (33)(34)(35), the electronic coupling between the two redox centers can be obtained using Eq. 3…”

Section: Resultsmentioning

confidence: 99%

Mechanically induced intramolecular electron transfer in a mixed-valence molecular shuttle

Barnes

Fahrenbach

Dyar

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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“…[11] We will only consider systems for which they were able to get data allowing estimation of H by three different methods. These are B dicyd (we abbreviate it H 4 ), and three derivatives: 2,5-dimethyl-(Me 2 H 2 ), 2,5-dichloro-(Cl 2 H 2 ) and tetrachloro-substituted (Cl 4 ) dicyd.…”

Section: Introductionmentioning

confidence: 99%

“…The M groups include (NH 3 ) 5 Ru, (NH 3 ) 4 pyrRu, and (NH 3 ) 3 bpyRu. The compound ± solvent combinations for which complete data have been published [11] appear in Table 1. It will be noted from the R Dn Ä 1/2 (obsd)/Dn Ä 1/2 (HTL) column that most of the observed band widths are significantly smaller than the Dn Ä 1\2 (htl) values obtained from Equation (8).…”

Section: Introductionmentioning

confidence: 99%

“…These optical methods of estimating H were compared with the thermodynamic estimate of electronic coupling obtained from the difference in formal potentials, DE8, for ET on each side of the intervalence oxidation state (determined by cyclic voltammetry), and the singlet, triplet gap for the diradical oxidation state. [11] The parameter thus obtained, DG' r , was equated to H 2 /n Ä max (the stabilization of the adiabatic energy minimum compared with the diabatic minimum) and the energy minimum stabilizations estimated by the three methods were compared. It was decided that H CNS gave reasonable predictions throughout the range of coupling studied because H CNS 2 /n Ä max correlates reasonably with DG' r .…”

Section: Introductionmentioning

confidence: 99%

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“Almost Delocalized” Intervalence Compounds

Nelsen

2000

Chem. Eur. J.

289

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Interest is high in +M-bridge-M systems that have very low barriers to intramolecular electron transfer giving M-bridge-M+ ("almost delocalized" systems). Hush showed how to evaluate the electronic coupling across the bridge (H) from the M-to-M charge-transfer optical absorption band, but did not point out that his classical model causes the extinction coefficient to suddenly drop to zero at a photon energy of 2 H. Ignoring this band cutoff leads to a low estimation of H.

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“…7). [16,40] From our study (Supporting information), it is clear that the delocalization term (GD or mesomery between A and C) will not contribute much to the large difference found (0.73 V), while the so-called entropic factor (GE) is also too weak for making a significant contribution.…”

Section: Electronic Structure Of 2[pf6]mentioning

confidence: 76%

[Fp*Fc][PF6]: A remarkable non-symmetric dinuclear cation in a very stable mixed-valent state

Argouarch

Grelaud

Roisnel

et al. 2017

Journal of Organometallic Chemistry

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Abstract.We report herein dicarbonyl(pentamethylcyclopentadienyl)(ferroceniumyl)iron hexafluorophosphate, which contains a dimetallic cation with a remarkably stable mixed-valent (MV) structure and unexpected redox features. The electronic structure of this compound is discussed in the light of the existing Hush model, which suggests an unusually strong electronic coupling between the two iron centers, despite the non-symmetric environment of the metallic centers. End-to-end charge delocalization is not the main contributor to the extra-large stability of this MV complex, which originates from electronic effects other than the energetic difference between the two MV redox isomers (G 0 ). This open-shell derivative has also been briefly tested as a catalyst in the reductive etherification of aldehydes by hydrosilanes.

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An Investigation of Superexchange in Dinuclear Mixed-Valence Ruthenium Complexes

Cited by 160 publications

References 25 publications

Mechanically induced intramolecular electron transfer in a mixed-valence molecular shuttle

Mechanically induced intramolecular electron transfer in a mixed-valence molecular shuttle

“Almost Delocalized” Intervalence Compounds

[Fp*Fc][PF6]: A remarkable non-symmetric dinuclear cation in a very stable mixed-valent state

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