1,3-Bis-(3-Nitrophenyl)Urea can form Crystalline Solid Complexes with Simple Lewis Bases

If you have a compound that is at least as effective an electron pair donor as a di-aliphatic ether which will not crystallize at all or will not produce good quality crystals,  it may be possible to crystallize it as a molecular complex with a special hydrogen bond acceptor.

The late Margaret C. Etter, [Acct. Chem. Res. 1990, 23, 120-126]  may have found this special substance and reported it in a paper that organic chemists and process chemists in prticular are unlikely to have read because the lead author was a crystallographer and solid-state chemist.  1,3-Bis-(3-nitrophenyl)urea can produce solid stoichiometric compositions from substances with poor Lewis basicity; as as poor as aliphatic ethers.  

The compound itself can easily be synthesized from inexpensive commercial 3-nitroaniline. The α form of 1,3-Bis-(3-nitrophenyl)urea is sufficiently poorly soluble to smoothly crystallize as yellow prisms from acetic acid, benzene, chloroform, methylene chloride, 95% ethanol, ethanol or ethylene glycol. The compound precipitates when formed in benzene.

How could one use this urea compound to isolate a material that exists as an oil and will not crystallize? One would mix it with an approximately stoichiometric amount of the Bis(3-nitrophenyl) urea. On a small scale, this could be done by grinding them together in a mortar. By rubbing the materials together strongly one would provide the best opportunity to form a co-crystal.  Then one could dilute the comingled mass with an antisolvent and filter the complex, washing it with more of the antisolvent. Impurities that did not form the complex would be washed away leaving behind a combination of excess N, N’-bis-(3-nitrophenyl)urea and its complex with the electron-pair donor. To this one could then add 300,000 MW polyethylene glycol and either grind together or heat them with stirring in an antisolvent. According to   Acct. Chem. Res. 1990, 23, 120-126, this particular substituted urea forms a strong non-stoichiometric complex with polyethylene glycol. This should liberate the first complexant which would be taken into the antisolvent. The N,N’-bis(3-nitrophenyl)urea /polyethylene glycol complex and residual excess polyethylene glycol could be filtered off as insoluble solids. Only the components of the original non-crystalline oil mixture that formed the complex should be retained in the antisolvent along with residual polymeric polyethyleneglycol.

Lewis Base/ Bis-N,N’-(3-Nitrophenyl)Urea Complex

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Polyethylene glycol

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Lewis Base

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Polyethylene glycol/ n. Bis-N,N’-(3-Nitrophenyl)Urea Complex

How this would work with a multifunctional molecule would need to be investigated.