The site of co-ordination in boron trihalide adducts of ureas and thioureas

Abstract

Boron trifluoride adducts with a wide variety of substituted ureas and thioureas have been prepared and their chemical and physical properties studied to determine whether nitrogen or oxygen (sulphur) is the donor atom. The chemical reactions indicate N-co-ordination and this is also consistent with the ultraviolet and infrared spectra. Nuclear magnetic resonance spectroscopy (¹H,¹¹B,¹⁹F) was of less value diagnostically but gave interesting information on intermolecular association and exchange effects in solution. The stable double complexes [BF₃,(PhNH)₂CO]₄NiBr₂,[BF₃,(PhNH)₂CS]₄NiBr₂, and [BF₃,(MeNH)₂CO]₂CdCl₂, and the unstable double complex [BF₃,(EtNH)₂CSMe]⁺l^– were also isolated.

Boron trichloride yields both covalent and ionic 1 : 1 adducts which differ in their solubility and spectroscopic properties. The covalent adducts of sym-dialkylureas disproportionated in solution to give alkylammonium tetrachloroborates and a new class of six-membered heterocyclic compounds of four-co-ordinate boron of which the first member is dichloro-(1,3,5-trimethylbiuretato)boron, [graphic omitted]HMe. Boron tribromide tended to give polymers rather than simple stable adducts.