Radical Ions in the Pentalene Series [1]. Part II. Dibenzo [b,f]pentalene and its 5, 10‐dimethyl derivative

Abstract

Proton hyperfine data have been determined for the radical anions and cations of dibenzo [b,f]pentalene (III) and its 5, 10‐dimethyl derivative (IV). The assignment of the coupling constants to pairs of equivalent protons follows from a simple MO model, the use of which enables one to reproduce satisfactorily the experimental values. The proton hyperfine data, aHμ , for the radical anion III⊖. correlate fairly well with the π‐charge populations ϵμ derived from 1H‐NMR. spectra for the carbon centres μ in the dianion III2⊖. The analogous correlation is less good in the case of the radical cation III⊕. and the dication III2⊕., presumably due to the rough approximations involved in the evaluation of the numbers ϵμ for the latter species. The coupling constants aH5,10 for III⊖. and III⊕. are very close to the corresponding values aH4,6 for the radical ions of 1,3,5‐tri‐t‐butylpentalene (II), in accord with the prediction of the MO model. A similarity is also found between the proton hyperfine data for III⊖. and those for the radical anions of structurally related 1,4‐diphenyl‐1,3‐butadiene (V). On the other hand, there are striking changes in the coupling constants of the analogous protons on passing from III⊖. to the radical anions of dibenzo [a,e] cyclooctene (VI) and [16] annulene (VII), as a consequence of raising the symmetry from C2h to D2h and D4h, respectively.

Molecular Structure and Electronic State of the Dibenzo[a,e]pentalene Anion Radical

“…Dibenzo[ a , e ]pentalene 1 (Scheme ), denoted as dibenzopentalene hereafter) has been known since its first synthesis in 1912;1 however, it still has received considerable attention in the past few years and its convenient synthesis has recently been reported 2. 3 The redox reactions of 1 are of considerable interest, leading to the corresponding aromatic dication and dianion having 14 π‐ and 18 π‐electron systems, respectively. The oxidation of 1 and its dimethyl derivative provided the corresponding cation radical3 and dication,4 which were identified by ESR and NMR spectroscopy, respectively.…”

“…3 The redox reactions of 1 are of considerable interest, leading to the corresponding aromatic dication and dianion having 14 π‐ and 18 π‐electron systems, respectively. The oxidation of 1 and its dimethyl derivative provided the corresponding cation radical3 and dication,4 which were identified by ESR and NMR spectroscopy, respectively. On the other hand, the reduction of 1 and its dimethyl derivative provided the corresponding anion radical3 and dianion,4 which were also identified by ESR and NMR spectroscopy, respectively.…”

“…The oxidation of 1 and its dimethyl derivative provided the corresponding cation radical3 and dication,4 which were identified by ESR and NMR spectroscopy, respectively. On the other hand, the reduction of 1 and its dimethyl derivative provided the corresponding anion radical3 and dianion,4 which were also identified by ESR and NMR spectroscopy, respectively. We have recently reported the synthesis and structure of dilithium 5,10‐disilyldibenzopentalenide 2 (Scheme ), which is a dianion equivalent 5–7.…”

Synthesis and Reactions of Dibenzo[a,e]pentalenes

“…Dications and dianions are formed through the cation radical and the anion radical, respectively. The synthesis of cation radicals 41a and 41b of dibenzopentalenes were accomplished by the reactions of dibenzopentalenes 1e and 1g with aluminum trichloride, the formation of which was evidenced by ESR spectroscopy (Scheme 32) [58]. Respective anion radicals 42a and 42b of 1e and 1g were synthesized by the reactions of 1e and 1g with potassium (Scheme 33) [58].…”

“…The synthesis of cation radicals 41a and 41b of dibenzopentalenes were accomplished by the reactions of dibenzopentalenes 1e and 1g with aluminum trichloride, the formation of which was evidenced by ESR spectroscopy (Scheme 32) [58]. Respective anion radicals 42a and 42b of 1e and 1g were synthesized by the reactions of 1e and 1g with potassium (Scheme 33) [58]. Subsequent challenging tasks are to characterize molecular structures of cation radicals, anion radicals and dications of dibenzopentalenes.…”