Kristall- und Molekülstruktur von „Urindigo“ (4,4,4′,4′-Tetramethyl-2,2′-bipyrrolidinyliden-3,3′-dion) und die Strukturverwandtschaft zum Indigo

Abstract

Abstract The crystal structure of 4,4,4′,4′-tetramethyl-2,2′-bipyrrolidinyliden-3,3′-dione (3) („urindigo“ = primordial indigo) was determined by X-ray diffraction. The molecule has the trans-configuration as indigo itself. Within the parent group trans-(OC)(HN)C=C(CO)(NH) the distances and angles of 3 are very similar to that in indigo: C=C (136.6 pm), C–N (140.9 pm), C–C (145.6 pm), C=O (122.6 pm). The intramolecular N–H···O bonds in 3 and indigo are similar whereas the intermolecular hydrogen bonds in the former are substantially weaker. The latter observation finds its interesting parallel in the markedly smaller difference between the Vis absorptions of the gaseous and the solid primordial indigo in comparison to that of indigo. The present results support the early view by Lüttke and Klessinger that the trans-(OC)(HN)C=C(CO)(NH) fragment is the parent chromophor of indigo dyes.