Nonplanar radical cations

H321). Tliese radical cations exist in a nonplanar conformation. With fra s-3,4-dimethyl substituents, the barrier to ring flip was estimated to be >5 kcal/mol. In connection with reactions of thiirane with the thiirane radical cation, ab initio calculations were reported on the 1,2-dithietane radical cation (93JA12510). 

More recently, the radical cation of the parent system has been characterized by ESR/ENDOR spectroscopy [340], These experiments revealed medium-sized hfcs for both the bridgehead (—11.4 G) and the equatorial protons (+11.4G) and large positive hfcs (+ 77 G) for the axial protons (which in 75 are replaced by the trimethylene bridge). The large difference between the couplings of the axial and equatorial protons indicates a nonplanar, puckered geometry of the radical cation. No evidence for interconversion is apparent up to 160 K this finding requires an inversion barrier of at least 12 kJ mol-1. 

Schemes. Nonplanar bicyclo[2.1.0]-pentene radical cation.

The possibility of preparing radical-cation salts of these nonplanar aromatic compounds is an interesting prospect. Many of these compounds contain pyrene and perylene substructures, both of which form well-defined salts when the procedures outlined and reviewed in the chapter by Enkel-mann (8) are used. However, the morphologies of crystals of radical-cation salts derived from the nonplanar compounds are difficult to envisage. Consequently, one cannot be certain that the resulting salts will have useful conducting properties. 

Barrier to deformation is relatively small nonplanar radicals can be made, unlike nonplanar cations. 

When multiple peripheral substituents, as in [Ni(2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenyl-porphyrin)], [Ni(5,10,15,20-tetra(/-butyl)porphyrin)], [Ni(2,3,7,812,13,17,18-tetracyclohexenyl-5,10,15,20-tetraphenylporphyrin)], and [Ni(OETPP)], cause the macrocycles to become nonplanar,283,284 the HOMOs of the porphyrins are destabilized and the molecules become easier to oxidize.283,285 In accord with the general discussion above, these compounds are oxidized to tt cation radicals and remain so even at low temperatures in CH2C12. However, upon addition of... 

The functionalization of zinc porphyrin complexes has been studied with respect to the variation in properties. The structure and photophysics of octafluorotetraphenylporphyrin zinc complexes were studied.762 Octabromoporphyrin zinc complexes have been synthesized and the effects on the 11 NMR and redox potential of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin were observed.763 The chiral nonplanar porphyrin zinc 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin was synthesized and characterized.764 X-ray structures for cation radical zinc 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin and the iodinated product that results from reaction with iodine and silver(I) have been reported.765 Molecular mechanics calculations, X-ray structures, and resonance Raman spectroscopy compared the distortion due to zinc and other metal incorporation into meso dialkyl-substituted porphyrins. Zinc disfavors ruffling over doming with the total amount of nonplanar distortion reduced relative to smaller metals.766 Resonance Raman spectroscopy has also been used to study the lowest-energy triplet state of zinc tetraphenylporphyrin.767... 

Sometimes transformation of aromatic componnds into ion-radicals leads to stereochemically unusual forms. Octamethylnaphthalene is a nonplanar molecule twisted around the bond that is common for the two six-membered rings. The nitrosonium oxidation results in the formation of the cation-radical with the centrosymmetric flatten chairlike geometry (Rosokha and Kochi 2006). According to the authors, such a skeletal transformation improves the overall planarity of octamethylnaphthalene. For example, the mean deviation of the carbon atoms in the naphthalene core for the flatten chairlike cation-radical (0.007 nm) is less than half of the corresponding value for the neutral twisted parent (0.016 nm). Within this flatten carcass of the anion-radical, the spin density can be delocalized more effectively. 

Ph2C=NEt " ), dibenzo[fl,e]cyclooctene (54, a nonplanar cation radical)," ... 

One way to eliminate the effect of stabilization of the cyclopentenyl produrts is to make use of reactions sucJi as those noted above for the cyclopropenyl series. The G2 energy (Jianges are Not surprisingly, the radical is somewhat stabilized. The anion is greatly stabilized, and the cation is more destabilized than the cyclopropenyl anion. This should be expected since the cation is planar, forcing the //-orbitals to interact with eacJi other, whereas cyclopropenyl anion is nonplanar, thus minimizing //-electron interactions. 

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