A comparison of the structures of N-chloroacetylindigo (N-chloroacetyl-2,2′-bi-indolinylidene-3,3′-dione) and N,N′-bis(chloroacetyl)indigo [N,N′-bis(chloroacetyl)-2,2′-bi-indolinylidene-3,3′-dione] in the solid and solution state

Abstract

N,N′-Bis(chloroacetyl)indigo [N,N′-bis(chloroacetyl)-2,2′-bi-indolinylidene-3,3′-dione], (3) and N-chloroacetylindigo (N-chloroacetyl-2,2′-bi-indolinylidene-3,3′-dione), (4) were synthesized, their solid state molecular structures characterized by X-ray crystallography and their solution state structures in various solvents probed by visible and NMR spectroscopy. In the solid state, 3 adopts a bent structure that is twisted about its central CC bond by 20°, the structure of 4 is more planar with a twist angle of only 8°. Despite the difference in CC twist angles both molecules have the same CC bond length. NMR studies indicate that the N-acyl side chains of these molecules suffer restricted flexibility, hindered rotation of the CO–CH₂Cl bonds was clearly observable. For compound 4 the barrier for this rotation was observed to be lower in non-polar, non-hydrogen bonding solvents. Molecular modelling studies suggest that the barrier to CO–CH₂Cl rotation decreases as the central CC bond becomes less twisted. The small hypsochromic shift observed between 3 and 4 is interpreted in terms of the large anticipated hypsochromic effect due to the inductively withdrawing N-acyl substituent being attenuated by a bathochromic effect due to sterically induced twisting of the central CC bond.