Conformational study on sterically hindered bis(arylimino)-isoindolines

Abstract

Single crystal X-ray crystallographic analyses of three bis(arylimino)isoindoline ligands (bai's) with terminal thiazolyl (bti) and pyridyl groups (bpi) and differing degrees of sterical encumbrance are reported and discussed in the context of conformation, conjugation, and coordination properties. 4-Mebti 2 forms yellow plates from CH 2 Cl 2/pentane at -20 °C in P21/c with Z = 4, 6-Mebpi 3 crystallizes as yellow blocks from CH 2 Cl 2/n-hexane at -20 °C in P21/c with Z = 8 (two independant molecules), and 4-tBubti 4 is obtained from CHCl 3/n-hexane at ambient temperature as yellow plates in P21 with Z = 2. The latter compound forms pseudomerohedrically twinned crystals and was treated accordingly in the structure solution and refinement. The resulting molecular structures provide insight into the stepwise increase of sterical hindrance from 2 over 3 to 4 which results in a rotational displacement of one of the terminal heterocycles from the major plane. The ease of this rotation process proves a low degree of conjugation in bis(arylimino)isoindolines and stands in contrast with the formation of a helically distorted structure for a related, though strongly conjugated, t-butyl terminated tripyrrin 1.