On the pi-electron distribution in biphenylene analogues

The bonding situation in a series of biphenylene analogues - benzo[b]biphen ylene and its dication, 4,10-dibromobenzo[b]biphenylene, naphtho[2,3-b]biph enylene and its dianion benzo[a]biphenylene, (biphenylene)tricarbonylchromi um, benzo[3,4]cyclobuta[1,2-c]thiophene. benzo[3,4]cyclobuta[1,2-c]thiophen e 2-oxide, benzo[3,4]cyclobuta[1,2-c]thiophene 2,2-dioxide, 4,10-diazabenzo [b]biphenylene, biphenylene-2,3-dione, benzo[3,4]cyclobuta[1,2-b]anthracene -6,11-dione, and 3,4-dihydro-2H-benzo[3,4]cyclobuta[1,2]cycloheptene - wher e one of the two benzo rings of biphenylene is replaced by a different pi - system (B) was investigated on the basis of the NMR parameters of these sys tems. From the vicinal H-1,H-1 spin-spin coupling constants, the electronic structure of the remaining benzo ring (A) is derived via the Q-value metho d. It is found that increasing tendency of B to tolerate exocyclic double b onds at the central four-membered ring of these systems favors increased pi -electron delocalization in the A ring. The analysis of the chemical shift s supports this conclusion. NICS (nucleus-independent chemical shift) value s as well as CC bond lengths derived from ab initio calculations are in exc ellent agreement with the experimental data. The charged systems benzo[b]bi phenylene dication and naphtho[2,3-b]biphenylene dianion (7(2)) are also st udied by C-13 NMR measurements. The charge distribution found closely resem bles the predictions of the simple HMO model and reveals that 7(2) can be r egarded as a benzo[3,4]cyclobuta[1,2-b]-substituted anthracene dianion. It is shown that the orientation of the tricarbonylchromium group in complexes of benzenoid aromatics can be derived from the vicinal 1H,1H coupling cons tants.