A Molecule that gives Crystalline Solid Complexes with Simple Lewis Bases

This particular urea molecule seems likely to present interesting chemistry. From what I had seen up until my retirement about eight years ago chemists had not paid much attention to it. Of course a thorough up today literature search would be needed as well as a careful reading of Margaret Etter’s paper.

What particularly struck me was how this adducts were made by grinding together the components and that an adduct even formed with ethyl ether. What would happen with molecules with more than one Lewis base functions the question I had when I first blogged about the subject.

The origin blog article is reproduced below. 

Bis-N,N’-(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 an aliphatic ether and that compound will not crystallize at all or will not produce good quality crystals, there is a possibility that it will crystallize as a molecular complex with a hydrogen bond acceptor. Is anything known about choosing such a complexing agent wisely?

 An answer is proposed in a paper by the late Margaret C. Etter,[Acct. Chem. Res. 1990, 23, 120-126].  This is not a paper that organic chemists or process chemists are likely to read.  The lead author was a crystallographer and solid-state chemist.  Bis-N,N’-(3-nitrophenyl)urea can produce solid stoichiometric compositions from substances as non-basic as aliphatic ethers.  It in turn can easily be synthesized from inexpensive commercial 3-nitroaniline. The α form of 1,3-Bis(3-nitrophenyl)urea is sufficiently poorly soluble to be crystallized in yellow prisms from acetic acid, benzene, chloroform, dichloromethane, 95% ethanol, ethanol,  or ethylene glycol. The compound precipitated when formed in benzene.

How could one use the urea compound to isolate a material that exists as an oil and will not crystallize? One could mix it with an approximately stoichiometric amount of the Bis(3-nitrophenyl) urea. One could mix the urea with your oil on a small scale by grinding them together in a mortar. By rubbing the materials together strongly one would form the co-crystals if formation was possible. Then dilute the solid with an antisolvent and filter the complex washing the complex with the antisolvent. Impurities that did not form the complex would be washed through leaving excess urea and urea-complex. To this add 300,000 MW polyethylene glycol and either grind together or heat in an antisolvent. According to   Acct. Chem. Res. 1990, 23, 120-126, the urea forms a non-stoichiometric complex with polyethylene glycol. This should liberate the first complexant which should be taken into the antisolvent. The urea and polyethylene glycol complex of the urea would be removed as insoluble solids. Only the components of the non-crystalline oil that formed the complex should be retained in the antisolvent.

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

+

Polyethylene glycol

↓

Lewis Base

+

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

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