Structural aspects of the mechanical and thermal dissociation of the central bond in 2,2′-bis(2,3,4-triarylchromenyl)s

Abstract

2,2′-Bis(2,3,4-triphenylchromenyl)(1a) has been shown to undergo reversible homolytic cleavage of the C(2)–C(2′) bond to give green-coloured chromenyl radicals when subjected to mechanical force or heating in the solid state. 2,4-Bis-(p-chlorophenyl)(1b), 2,4-bis-(p-tolyl)(1c), 2,4-bis-(p-methoxyphenyl)(1d), and 2,4-bis-(p-bromophenyl)(1e) derivatives have been synthesized, and substituent effects on the radical dissociation investigated. Upon pressing 1b exhibits the highest degree of dissociation, while thermal dissociation is facilitated by electron-releasing methyl (1c) and methoxy (1d) groups. Dissociation enthalpies (ΔH) for 1a–e in dichloromethane solutions are evaluated to be 12–15 kcal mol^–1. The small values of ΔH are considered to be mainly due to overcrowding around the C(2)–C(2′) bond. MMP2 calculation predicts that meso-1a is more stable than the (±)-isomer and the lowest-energy conformation of meso-1a is a centrosymmetric anti form. In the optimized geometry of meso-1a C(2)–C(2′) the bond length is calculated to be 1.584 Å, which is rather longer than a normal C_sp³–C_sp² bond. AM1 calculations give similar results and suggest that through-bond interactions between the lone-pairs on the oxygen atoms and/or 2- and 2′-aryl groups are not significant compared with the steric repulsion.