Crystal structure of a hybrid salt–cocrystal and its resolution by preferential crystallization: ((±)trans-N,N′-dibenzyldiaminocyclohexane)(2,3-dichlorophenylacetic acid)4

Abstract

trans-N,N′-Dibenzyldiaminocyclohexane (B) crystallizes with 2,3-dichlorophenylacetic acid (AH); the crystal structure, resolved by using crystal X-ray diffraction, revealed an odd stoichiometry composed of H₂B²⁺, two A⁻ and two AH forming an unexpected hybrid salt–cocrystal. As this compound is a stable conglomerate (i.e. every single crystal contains the enantiomerically pure cation H₂B²⁺, RR or SS), several preferential crystallization attempts (AS3PC) were performed in methanol and in THF and gave unexpected final enantiomeric excesses greater than 20% for the entrainment in methanol. These results suggest that the crystal growth mechanism preferentially involves building units composed of [H₂B²⁺; 2A⁻ and 2AH] or reconstruction of some crystal interfaces rather than a layer by layer construction.