The "Magic Linker": Highly Effective Gelation from Sterically Awkward Packing.

Abstract

Bis(urea)s based on the 4,4′-methylenebis(2,6-diethylphenylene) (4,4′-MDEP) spacer are highly effective low molecular weight gelators, and the first single crystal structure of a bis(urea) based on this spacer is reported. The structure is a conformational isomorph with eight crystallographically independent molecules (Z′ = 8) arranged in four tennis-ball type dimers with the 2,6-diethylphenylene units adopting five different conformations in the ratio 4:5:3:2:2. The awkward shape and conformational promiscuity arising from the orientations of the ethyl groups in this system is linked to its gelation behavior. A total of seven 4,4′-MDEP derivatives have been prepared, and six are versatile gelators, confirming the particularly effective nature of the MDEP spacer. Only the nitrophenyl derivative does not form gels, likely because of intramolecular CH···O hydrogen bonding arising from the electron-withdrawing nature of the nitro substituent and hence inhibition of the urea α-tape hydrogen-bonded motif.