Cyclic in-plane electron delocalization (sigma-bishomoaromaticity) in 4N/5e radical anions and 4N/6e dianions - Generation, structures, properties, ion-fairing, and calculations

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 l ess rigid, parallel bisdiazenes (1-7), with N=N/N=N distances (d(pi pi)) ra nging from similar to2.8 to 5.0 Angstrom, interorbital angles (p,p; omega) from similar to 175 degrees to 90 degrees, and with more or less kinetic pr otection. For the "proximate" substrates 1-5 (d(pi pi) approximate to 2.8-3 .2 Angstrom, omega approximate to 175-156 degrees), short contact with alka li metals (Li, Na, K, Cs) generates turquoise to deeply green radical anion s (lambda (max)(DME) approximate to 700-900 nm). The character of these rad ical anions as cyclically in-plane delocalized bishomoconjugated 4N/5e spec ies with a high concentration of spin density between the N=N units is esta blished by extensive UV/vis, electrochemical, and EPR measurements (CW, pul sed) at temperatures down to 8 K, and by DFT calculations (B3LYP/6-31G*). A fter longer exposure to the metals, the three most persistent 4N/5e radical anions (M(+)1(radical anion-), M(+)2(.-), M(+)5(radical anion-)) are furth er reduced to the red dianions (2M(+)1(2-) 3M(+)2(2-), 2M(+)5(2-), lambda ( max)(THF) approximate to 360-430 nm). Of the nine ion pair combinations wit h Li+, Na+, and K+ all except one (2K(+)1(2-)) are thermally highly persist ent. For the dianions the H-1,C-13, and Li-7 NMR analyses assisted by DFT c alculations confirm the 4N/6e sigma -bishomoaromatic electronic structure; The reduction potentials determined (CV) for dialkyl-diazenes (8-10) and bi sdiazenes (1, 2, and 5) allow an estimate of the thermodynamic stabilizatio n of the respective radical anions and of the gain in electron delocalizati on energy in the dianions (sigma -bishomoaromaticity).