Cover Picture: Chem. Eur. J. 21/2002

Sesto, Rico E. Del; Miller, Joel S.; Lafuente, Pilar; Novoa, Juan J.

doi:10.1002/1521-3765(20021104)8:21<4821::aid-chem4821>3.0.co;2-y

Cited by 53 publications

(202 citation statements)

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Section: Early Examples Of Radical Dimersmentioning

confidence: 99%

“…Some of the most common radical π-dimeric species are illustrated in Fig. 1, including the associated radical anions of singly reduced tetracyanoethylene (TCNE) [7], and the neutral radical pairs of phenalenyl [8], as well as the radical cation π-dimers of singly oxidized tetrathiafulvalene (TTF) [9,10] and singly reduced bipyridium (BIPY 2+ ) units [11,12].The π-association of phenalenyl has received much attention [8,13] as a model for the radical dimerization of large planar π-systems. The persistent (i.e., stable) neutral radical is readily generated during the oxidation of phenylene.…”

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confidence: 99%

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Molecular recognition and switching via radical dimerization

Spruell

2010

Pure and Applied Chemistry

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This article highlights the emerging use of the interactions of radical π-dimers to drive both molecular recognition and switching processes within supramolecular systems and mechanically interlocked molecular architectures. The enhanced stability experienced by dimers of radical cation species when encapsulated, as compared to when they are free in solution, is driving their useful incorporation into functional systems. The redox stimulation used in the production of radical cation species provides the ideal trigger for molecular switching events. Moreover, the nature and strength of the radical dimerization events introduces a completely novel recognition motif within supramolecular and mechanically interlocked molecular systems, complementing well-established techniques and enabling new research opportunities to blossom.atom-centered radical dimerization and, thereafter, I will highlight the recent advances that have led to the use of this unusual class of bonding as a practical and fully orthogonal molecular recognition motif. EARLY EXAMPLES OF RADICAL DIMERSThe π-dimerization of planar radical species was first observed in the middle of the last century [1][2][3][4][5][6] in both the solid state and in concentrated solutions at low temperatures. Under these conditions, equilibria are established between two paramagnetic free radical species and a diamagnetic dimer, most often with a particular equilibrium favoring the dissociated, non-dimer state. These types of dimeric associations have been observed for a whole range of different types of planar radical species, varying in both the size of the planar systems and the charges of the dimer components. Both radical anions and cations, as well as neutral free radicals, have been observed to associate with one another in this manner. Some of the most common radical π-dimeric species are illustrated in Fig. 1, including the associated radical anions of singly reduced tetracyanoethylene (TCNE) [7], and the neutral radical pairs of phenalenyl [8], as well as the radical cation π-dimers of singly oxidized tetrathiafulvalene (TTF) [9,10] and singly reduced bipyridium (BIPY 2+ ) units [11,12].The π-association of phenalenyl has received much attention [8,13] as a model for the radical dimerization of large planar π-systems. The persistent (i.e., stable) neutral radical is readily generated during the oxidation of phenylene. Its stability can be attributed to the delocalization of the radical throughout the fused polycyclic structure, its character residing predominately on 6 of the 12 peripheral carbons [14,15]. When substituted with bulky side groups to prevent σ-binding (the radical-radical coupling to form a traditional covalent sigma (σ) bond), π-dimer formation is observed in concentrated deoxygenated solutions. Moreover, this π-dimer is comprised of two perfectly overlapping delocalized singly occupied molecular orbitals (SOMOs) on each phenalenyl unit in such a way that the two formal free radical electrons pair to form what has been described as a two-electron...

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Section: Early Examples Of Radical Dimersmentioning

confidence: 99%

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confidence: 99%

Molecular recognition and switching via radical dimerization

Spruell

2010

Pure and Applied Chemistry

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“…[2][3][4][5][6][7][8][9] One example of crystal where this type of bond is found is the K 2 TCNE 2 glyme 2 crystal, 10 where one finds K 2 TCNE 2 aggregates that clearly show the presence of -TCNE 2 2À dimers (Fig. 1).…”

Section: Introductionmentioning

confidence: 99%

The origin of the two‐electron/four‐centers CC bond in π‐TCNE₂²⁻ dimers: Electrostatic or dispersion?

2006

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Abstract:The structure and stability of the -TCNE 2 2À dimers in K 2 TCNE 2 aggregates is revisited trying to find if the origin of their two-electron/four-centers CÀ ÀC bond are the electrostatic K þ -TCNE À interactions or the dispersion interactions between the anions. The study is done at the HF, B3LYP, CASSCF (2,2), and MCQDPT/CASSCF (2,2) levels using the 6-31þG(d) basis set. Our results show that the only minima of this aggregate that preserves the -TCNE 2 2À structure has the two K þ atoms placed in equatorial positions in between the two TCNE À planes. When the K þ atoms are placed along the D 2h axis of the anions the structure is not a minimum. The main energetic component responsible for the stability of these aggregates comes from the cation-anion interactions. However, a proper accounting of the dispersion component (as done in the MCQDPT/CASSCF (2,2) calculations) is needed to make the closedshell singlet more stable than the open-shell singlet. Thus, the bond results from the combination of the electrostatic and dispersion components, being the first the dominant one. The optimum geometry of the closed-shell singlet is very similar to the experimental one found in crystals.

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“…D 2 h π–[TCNE] 2 2− forms at low temperature, but is not spectroscopically evident at room temperature due to being present at very low concentration (<10 n m (vide infra)). MeTHF was identified as a suitable solvent for these studies due to it forming a glass below 137 K (−136 °C), its known density as a function of temperature, and it dissolves around 0.1 mmol [NMe 4 ] 2 [TCNE] 2 , [Mepy] 2 [TCNE] 2 , and [NEt 4 ] 2 [TCNE] 2 , which is appropriate for spectroscopic studies. Hence, equilibrium (2) can be experimentally studied via UV/Vis and EPR above 137 K in MeTHF .…”

Section: Resultsmentioning

confidence: 99%

Cation Dependence of the Dimerization Enthalpy for A₂[tetracyanoethylene]₂ (A=NMe₄, Mepy, NEt₄) Possessing a Long, Multicenter Bond

Graham

Fedin

Miller

2017

Chemistry A European J

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[TCNE] (TCNE=tetracyanoethylene) has been isolated as D π-[TCNE] possessing a long, 2.9 Å multicenter 2-electron-4-center (2e /4c) C-C bond, and as C π-[TCNE] possessing a longer, 3.04 Å multicenter 2e /6c (4 C+2 N atoms) bond. Temperature-dependent UV/Vis spectroscopic measurements in 2-methyltetrahydrofuran (MeTHF) has led to the determination of the dimerization, 2[TCNE] ⇌π-[TCNE] , equilibrium constants, K (T), [[TCNE] ]/[[TCNE] ] , enthalpy, ΔH, and entropy, ΔS, of dimerization for [Mepy] [TCNE] (Mepy=N-methylpyridinium, H CNC H ) possessing D π-[TCNE] and [NMe ] [TCNE] possessing C π-[TCNE] conformations in the solid state; however, both form D π-[TCNE] in MeTHF solution. Based on ΔH=-3.6±0.1 kcal mol (-15.2 kJ mol ), and ΔS=-11±1 eu (-47 J mol K ) and ΔH=-2.4±0.2 kcal mol (-10.2 kJ mol ), and ΔS=-8±1 eu (-32 J mol K ) in MeTHF for [NMe ] [TCNE] and [Mepy] [TCNE] , respectively, the calculated K (298 K) are 1.6 and 1.3 m , respectively. The observed K (145 K) are 3 and 2 orders of magnitude greater for [NMe ] [TCNE] and [Mepy] [TCNE] , respectively. The K (130 K) is 4470, 257, ≈0.8, and ≪0.1 m for [NMe ] [TCNE] , [Mepy] [TCNE] , [NEt ] [TCNE] , and [N(nBu) ] [TCNE] , respectively, decreasing with increasing cation size. At standard conditions and below ambient temperature the equilibrium favors the dimer for the NMe and Mepy cations. From the decreasing enthalpy, NMe >Mepy , along with the decrease in dimer formation K (T) as NMe >Mepy >NEt >N(nBu) , the dimer bond energy decreases with increasing cation size in MeTHF. This is attributed to a decrease in the [A] ⋅⋅⋅[TCNE] attractive interactions with increasing cation size. Solid state UV/Vis spectroscopic determinations of [NMe ] [TCNE] are reported and compared to D π-[TCNE] conformers. The feasibility and limitations of temperature-dependent electron paramagnetic resonance (EPR) measurements for the determination of K (T) are also discussed.

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Cover Picture: Chem. Eur. J. 21/2002

Cited by 53 publications

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Molecular recognition and switching via radical dimerization

Molecular recognition and switching via radical dimerization

The origin of the two‐electron/four‐centers CC bond in π‐TCNE₂²⁻ dimers: Electrostatic or dispersion?

Cation Dependence of the Dimerization Enthalpy for A₂[tetracyanoethylene]₂ (A=NMe₄, Mepy, NEt₄) Possessing a Long, Multicenter Bond

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Cover Picture: Chem. Eur. J. 21/2002

Cited by 53 publications

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Molecular recognition and switching via radical dimerization

Molecular recognition and switching via radical dimerization

The origin of the two‐electron/four‐centers CC bond in π‐TCNE22− dimers: Electrostatic or dispersion?

Cation Dependence of the Dimerization Enthalpy for A2[tetracyanoethylene]2 (A=NMe4, Mepy, NEt4) Possessing a Long, Multicenter Bond

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The origin of the two‐electron/four‐centers CC bond in π‐TCNE₂²⁻ dimers: Electrostatic or dispersion?

Cation Dependence of the Dimerization Enthalpy for A₂[tetracyanoethylene]₂ (A=NMe₄, Mepy, NEt₄) Possessing a Long, Multicenter Bond