Crystal structure, ultraviolet spectrum, and electronic structure of a strongly twisted push-pull ethylene, approaching an amidinium-vinylogous dithioate zwitterion

Abstract

The crystal structure of 1,3-dibenzyl-2-(4,4-dimethyl-2,5-bisthioxocyclohexylidene)imidazolidine (2) shows a twist of 80.8(5)° about the inter-ring bond, which has a length of 1.482(6)Å ;. The near orthogonality of the donor and acceptor parts of this formal push-pull ethylene makes the structure approach that of a zwitterion, as evidenced by bond lengths indicating strong electron delocalization. The acceptor part approaches a vinylogous dithioate structure, the donor part an amidinium system. The u.v. spectrum shows an n→π^∗ and a π→π^∗ transition, at 511 and 417.5 nm, respectively; according to CNDO/S calculations these are located entirely in the [S-C-C-C-S]^- part. Two bands at shorter wavelength are ascribed to transitions from combinations of the lone-pair orbitals on the sulphur atoms to a π^∗ orbital in the [N-C-N]⁺ part; this is facilitated by the near perpendicularity of the two parts of the molecule.