On the π‐Electron Distribution in Biphenylene Analogues

The bonding situation in a series of biphenylene analogues – benzob]biphenylene and its dication, 4,10‐dibromobenzob]biphenylene, naphtho2,3‐b]biphenylene and its dianion, benzoa]biphenylene, (biphenylene)tricarbonylchromium, benzo3,4]cyclobuta1,2‐c]thiophene, benzo3,4]cyclobuta1,2‐c]thiophene 2‐oxide, benzo3,4]cyclobuta1,2‐c]thiophene 2,2‐dioxide, 4,10‐diazabenzob]biphenylene, biphenylene‐2,3‐dione, benzo3,4]cyclobuta1,2‐b]anthracene‐6,11‐dione, and 3,4‐dihydro‐2H‐benzo3,4]cyclobuta1,2]cycloheptene – where one of the two benzo rings of biphenylene is replaced by a different π‐system (B) was investigated on the basis of the NMR parameters of these systems. From the vicinal ¹H,¹H spin‐spin coupling constants, the electronic structure of the remaining benzo ring (A) is derived via the Q‐value method. It is found that increasing tendency of B to tolerate exocyclic double bonds at the central four‐membered ring of these systems favors increased π‐electron delocalization in the A ring. The analysis of the chemical shifts supports this conclusion. NICS (nucleus‐independent chemical shift) values as well as C,C bond lengths derived from ab initio calculations are in excellent agreement with the experimental data. The charged systems benzob]biphenylene dication and naphtho2,3‐b]biphenylene dianion ( 7 ²⁻) are also studied by ¹³C NMR measurements. The charge distribution found closely resembles the predictions of the simple HMO model and reveals that 7 ²⁻ can be regarded as a benzo3,4]cyclobuta1,2‐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 constants.