Charge-localized p-phenylenedihydrazine radical cations: ESR and optical studies of intramolecular electron transfer rates

1,4-Bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)benzene-1,4-diyl (2) its 2,5-dimethyl and 2,3,5,6-tetramethyl derivatives (3 and 4), their radical cations, and bis-radical dications are studied. Crystal structures including those of 2^+BPh_4^-, 3^(2+)(BPh_4^-)_2, 4^+BPh_4^-, and 4^(2+)(BPh_4^-)_2 establish that ring methylation causes more N-lone pair, aryl π twist without changing the NAr,NAr distance significantly and that both 2^+ and 4^+ have the charge localized in one hydrazine unit. NMR measurements show that 3^+ has about 6% of its spin at the four aryl CH and CMe carbons, while 4^+ has about 1.5% of its spin at the four CMe carbons. The average distance between the unpaired electrons of 3^(2+) and 4^(2+) was obtained from the dipolar splittings of their thermally excited triplet states and, as expected, is significantly smaller for 3^(2+) (5.25 Å) than for 4^(2+) (5.63 Å). Rate constants for electron transfer between the hydrazine units of 3^+ and 4^+ in CH_2Cl_2 and CH_3CN were determined by dynamic ESR. The intervalence radical cations show charge transfer bands corresponding to vertical electron transfer between the ground state and the highly vibrationally excited electron-shifted state, allowing calculation of the parameters controlling electron transfer. Electron transfer parameters obtained from the CT bands using adiabatic energy surfaces which approximate the CT band shapes observed produce rate constants within experimental error of those extrapolated to room temperature from the ESR data for both 3^+ and 4^+ in both solvents, without using tunneling corrections. The effects of mixing of the electronic wave functions of the reduced and oxidized hydrazine units of 2^+ on d_(NN), the C(t-Bu)N,NA(Ar) twist angle, and the aryl nitrogen lone pair, aryl π twist angle which are observed by X-ray are close to those predicted from the position of the minima on the ET coordinate X of the adiabatic energy surface calculated from the CT band

Adiabatic electron transfer: Comparison of modified theory with experiment

The radical cations of properly designed bishydrazines allow comparison of observed and calculated electron transfer rate constants. These compounds have rate constants small enough to be measured by dynamic electron spin resonance spectroscopy and show charge transfer bands corresponding to vertical excitation from the energy well for the charge occurring upon one hydrazine unit to that for the electron-transferred species. Analysis of the data for all six compounds studied indicates that the shape of the adiabatic surface on which electron transfer occurs can be obtained from the charge transfer band accurately enough to successfully predict the electron transfer rate constant and that explicit tunneling corrections are not required for these compounds

Solvent effects on charge transfer bands of nitrogen-centered intervalence compounds

Electron transfer parameters are extracted from the optical spectra of intervalence bis(hydrazine) radical cations. Compounds with 2-tert-butyl-3-phenyl-2,3-diazabicyclo[2.2.2]octyl-containing charge-bearing units that are doubly linked by 4-σ-bond and by 6-σ-bond saturated bridges are compared with ones having tert-butylisopropyl- and diphenyl-substituted charge bearing units and others having the aromatic units functioning as the bridge. Solvent effect studies show that the optical transition energy (E_(op)) does not behave as dielectric continuum theory predicts but that solvent reorganization energy may be usefully separated from the vibrational reorganization energy by including linear terms in both the Pekar factor (γ) and the Gutmann donor number (DN) in correlating the solvent effect. Solvation of the bridge for these compounds is too large to ignore, which makes dielectric continuum theory fail to properly predict solvent effects on either E_(op) or the free energy for comproportionation

Crystallographic characterization of the geometry changes upon electron loss from 2-tert-butyl-3-aryl-2,3-diazabicyclo 2.2.2 octanes

Crystal structures of 2-tert-butyl-3-(2,3,5,6-tetramethylphenyl)-2,3-diazabicyclo[2.2.2]-octane radical cation nitrate (HyDU+NO_3-) [Hy = (2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl]; 2-tert-butyl-3-(1-naphthyl)-2,3-diazabicyclo[2.2.2]octane radical cation hexafluoroantiminate (Hy^1NA+SbF_6-); 2-tert-butyl-3-(2-naphthyl)-2,3-diazabicyclo-[2.2.2]octane radical cation hexafluoroantiminate (Hy^2NA+SbF_6-); 1,5-bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)naphthalene dication bis(tetraphenylborate) (Hy_2^(15)NA^(2+)(Ph_4B^-)_2); and 2,7-bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)naphthalene dication bis(hexafluoroantiminate) (Hy_2^(27)NA^(2+)(SbF_6^-)_2·CH_3CN) are reported, and the geometries about the oxidized Hy units compared with literature data for neutral Hy-substituted analogues and the geometry changes upon electron loss for these compounds, which have a lone pair, lone pair twist angle in the neutral form (θ(0)) in the range 122−130°, are compared with those for tetraalkylhydrazines that have θ(0) values near 180, 90, and 0°

Charge localization in a dihydrazine analogue of tetramethyl-p-phenylenediamine radical cation

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σ,π Interaction in Halogen-Substituted Biadamantylidene Radical Cations

The order of E°‘ and vIP for 4-eq-halogenated-biadamantylidene is F > Cl Br, and the 5-F-substituted compound is harder to ozidize than the 4-eq-F-substituted one. The former result is most consistent with a detectable resonance contribution through the σ-framework, and the latter with σ-hyperconjugative destablilization proceeding through two pathways being more than double the same effect through one pathway (the Whiffen effect). AM1 calculations predict these results. The facial selectivity for epoxidation and diazetidine formation from 4-eq-halogenated 3 (4(X)) is in the order Cl > F > Br, and the 5-fluoro compound (8) is less selective than 4(F) for both reactions. Steric as well as electronic factors might well contribute to these results, neither of which was expected from consideration of σ,π interaction. Cation radical catalyzed chain dioxetane formation from 4(F) and 3(Cl) is significantly more face selective than epoxidation or diazetidine formation, as expected on electronic grounds; σ,π interaction should be larger in the radical cation