On the nature of nonplanarity in the [N]phenylenes

The [N]phenylenes display marked deformation from planarity in the crystall ine state. In order to probe the generality of this phenomenon, several der ivatives were synthesized and their single-crystal X-ray structures were ob tained. All new compounds displayed some degree of nonplanarity. Thus, for example, the parent triangular [4]phenylene (4b) has a median bend angle at the ring junction of 1.5 degrees and a range of 0.3 degrees to 3.5 degrees , whereas hexakis[triisopropylsilyl(ethynyl)] triangular [4]phenylene (4c) possesses the bulkiest appendages and the largest median bend angle and ran ge (3.8 degrees and 1.7 degrees-5.6 degrees, respectively). A detailed anal ysis of the bending and twisting angles at the ring junctions, however, rev ealed that the magnitude of deformations were independent of topology, mole cular size, and substituent type. In contrast to the phenylenes, a Cambridg e Structural Database (CSD) search of unsubstituted and non-peri-substitute d naphthalenes and anthracenes shows these molecules to be virtually planar in the solid state. A comparison of the single-point energies (HF/6-31G*) of the phenylenes with the acenes calculated for molecules possessing a fix ed bend angle at the ring fusion of 3 degrees, 6 degrees, 9 degrees, and 12 degrees reveals the former to be 26% to 45% easier to deform than the latt er. Based on these results, the nonplanarity seen for the phenylenes is mos t likely a consequence of crystal-packing forces deforming particularly fle xible molecules.