The solid state structure and reactivity of NbCl5·(N,N′-dicyclohexylurea) in solution: evidence for co-ordinated urea dehydration to the relevant carbodiimide

Abstract

NbCl₅·(N,N′-dicyclohexylurea) 1a owns a distorted octahedral structure due to intramolecular NH⋯Cl bonding. The unit cell contains four units which are intermolecularly NH⋯Cl and NH⋯N bonded. An extended intramolecular network of H-bonding (N–H⋯Cl, CH⋯Cl, CH⋯N) causes the 3D self assembling of the units. Upon addition of base, the HCl release from 1a is observed with the transfer to Nb of the O-atom of the carbonylic function of the starting urea which is converted into the relevant carbodiimide CyNCNCy 4. The latter is quantitatively released by adding an excess of NEt₃ at 308 K (py and DBU are less efficient) with formation of the known NbOCl₃(NEt₃)₂, isolated in quantitative yield. Increasing the temperature leads to a loss in selectivity as the formed DCC undergoes further reactions. At 350 K, the isocyanate CyNCO has been isolated in 60% yield besides a mixture of Nb-complexes. DFT calculations have been coupled to IR and NMR experiments for characterizing possible reaction intermediates and the behaviour of 1a. Several other MCl_x species (ScCl₃,YCl₃,LaCl₃,TiCl₄, TaCl₅, AlCl₃, SnCl₄) have been shown to be able to co-ordinate DCU but not all of them promote the conversion of urea into DCC.