Palladium Vacataporphyrin Reveals Conformational Rearrangements Involving Hückel and Möbius Macrocyclic Topologies

Wormholes in chemical space connecting torus knot and torus link pi-electron density topologies

Möbius aromaticities can be considered as deriving from cyclic delocalized pi-electron densities rho(r)(pi) which have the topological form of either a two-component torus link or a single-component torus knot. These two topological forms are distinguished by their (non-zero) linking number L(k), which describes how many times the two components of a torus link cross each other or the single component of a torus knot crosses with itself. The special case of Hückel or benzenoid aromaticity is associated with a pi-electron density that takes the form of a two-component torus link for which the linking number is zero. A class of molecule has been identified which here is termed a Janus aromatic, and which bears the characteristics of both a two-component torus link and a single-component torus knot in the topology of the pi-electron density. This is achieved by the formation of one (or more) wormholes or throats in the pi-electron density connecting the two torus forms, which can impart a Janus-like dual personality to the aromaticity of the system. The impact of such wormholes on the overall pi-delocalized aromaticity of such molecules is approximately estimated using a NICS(rcp) index, and subdivides into two types; those where the forms of aromaticity associated with a torus link and a torus knot cooperate and those where they oppose.

A computational investigation of the structure of polythiocyanogen

Although polythiocyanogen (SCN)(x), has been known for a long time, its solid state structure continues to be uncertain and controversial. Reported here are density functional calculations for linear polymers comprising repeating 1,2,4-dithiazole or 1,2,4-thiadiazole rings, an S-S bridged isomer, and a variety of cyclic phyrin-like systems resulting from cyclization of these linear polymers. The computed energies and comparison of GIAO-derived relative magnetic shieldings for the (13)C and (15)N nuclei with experimental values reported by Woollins and co-workers confirms their assignment of the polymer as based on the 1,2,4-dithiazole ring. It is proposed here that the cyclic forms of this polymer are a better match with experiment than the purely linear forms; the (33)S spectra are likely to be the best discriminator for these diverse structures. A number of these cyclic forms are modelled on the conformations of analogous phyrins such as the recently reported dodecaphyrin, and have lemniscular figure-eight or helical topologies which are recognized here as being double or higher twisted Möbius pi-conjugated rings. The aromatic character of several forms is dissected using ELF(pi) (electron localization) analyses. Based on the propensity of the phyrin rings to form both mono and dinuclear metal complexes, and a specific analogy to a uranylpentaphyrin, it is proposed that a UO(2) complex of e.g. a (SCN)(2) pentamer might be isolable.