Cyclic In-Plane Electron Delocalization (σ-Bishomoaromaticity) in 4N/5e Radical Anions and 4N/6e DianionsGeneration, Structures, Properties, Ion-Pairing, and Calculations

Abstract

Cyclic delocalization of five and six electrons, respectively, in the plane of four N centers was explored by one-/two-electron reduction of more or less rigid, parallel bisdiazenes (1-7), with NdN/NdN distances (d ππ ) ranging from ∼2.8 to 5.0 Å, interorbital angles (p,p; ω) from ∼l75°to 90°, and with more or less kinetic protection. For the "proximate" substrates 1-5 (d ππ ≈ 2.8-3.2 Å, ω ≈ 175-156°), short contact with alkali metals (Li, Na, K, Cs) generates turquoise to deeply green radical anions (λ max (DME) ≈ 700-900 nm). The character of these radical anions as cyclically in-plane delocalized bishomoconjugated 4N/5e species with a high concentration of spin density between the NdN units is established by extensive UV/vis, electrochemical, and EPR measurements (CW, pulsed) at temperatures down to 8 K, and by DFT calculations (B3LYP/6-31G*). After longer exposure to the metals, the three most persistent 4N/5e radical anions (M + 1 •-, M + 2 •-, M + 5 •-) are further reduced to the red dianions (2M + 1 2-, 2M + 2 2-, 2M + 5 2-, λ max (THF) ≈ 360-430 nm). Of the nine ion pair combinations with Li + , Na + , and K + all except one (2K + 1 2-) are thermally highly persistent. For the dianions the l H, l3 C, and 7 Li NMR analyses assisted by DFT calculations confirm the 4N/6e σ-bishomoaromatic electronic structure. The reduction potentials determined (CV) for dialkyl-diazenes (8-10) and bisdiazenes (1, 2, and 5) allow an estimate of the thermodynamic stabilization of the respective radical anions and of the gain in electron delocalization energy in the dianions (σ-bishomoaromaticity).

Secododecahedradienes - Syntheses, Reactivity, in-Plane Homoconjugated 3C/2e Cations, 4C/3e Radical Cations, andσ-Bishomoaromatic 4C/2e Dications?

“…Particularly, the intensity of m/2z 128 (48, À 4 Br) for 91a suggests for this specific substitution pattern a dominant fragmentation into two naphthalene parts, similar to that established for the parent pagodane skeleton (À [4 2]) [58]. Intensive doubly charged ions with m/2z 126 (C 20 H 12 ; À 2 Br, À 2 HBr) and m/2z 127 (C 20 Photoelectron Spectroscopy. The He(I) photoelectron spectra were recorded with a Leybold-Heraeus UPG 200.…”

“…Chemical shift and multiplicity of the methylene protons are readily recognizable manifestations of the substitution patterns. For 58a, the composition C 20 The MS fragmentation pattern of tetrabromodiene 58a (C 20 H 14 Br 4 ; Fig. 8) deserves special attention as demonstration of the stability of increasingly unsaturated, inreasingly strained cagelike C-skeletons generated by electron-impact ionization in the vapour phase [46].…”

“…filtered through a D4 glass filter, and the solid colorless residue washed thoroughly with warm CCl 4 and dried in vacuo (for analysis of the soln., see the seco-dienes 20a and 31 (below)). According to TLC, .0 13,17 .0 16,20 ]icosane-4anti,9anti-dicarboxylic Acid. A soln.…”

“…In the analogously conducted reaction of mono-ene 5a with Br 2 (Scheme 10), as notable distinction from diene 2a, the momentarily evolving HBr did not interfere: in the absence of any 1,2-dibromide (MS), the exclusively isolated product was the C 20 H 19 Br allylic bromide 60a (TLC, NMR, HR-MS), resulting from regiospecific deprotonation of HÀC (20) (rather than of the more acidic HÀC(14) which would yield 61a). Once more, the ester groups of 5b changed the picture; the isododecahedral 52d was formed together with 60b (ca.…”

“…Still, in weakly polar CH 2 Cl 2 , a concerted ene mechanism for the formation of 63a,b is not excluded [44]. The secoenes 5a,b behaved analogously (Scheme 11): after addition of PTAD, regiospecific abstraction of HÀC (20) led to the ene adducts 65a,b.…”