The solid state structure and reactivity of NbCl(5)center dot(N,N '-dicyclohexylurea) in solution: evidence for co-ordinated urea dehydration to the relevant carbodiimide

NbCl(5)center dot(N,N '-dicyclohexylurea) 1a owns a distorted octahedral structure due to intramolecular NH center dot center dot center dot Cl bonding. The unit cell contains four units which are intermolecularly NH center dot center dot center dot Cl and NH center dot center dot center dot N bonded. An extended intramolecular network of H-bonding (N-H center dot center dot center dot Cl, CH center dot center dot center dot Cl, CH center dot center dot center dot 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 CyN=C=NCy 4. The latter is quantitatively released by adding an excess of NEt(3) at 308 K (py and DBU are less efficient) with formation of the known NbOCl(3)(NEt(3))(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 CyN=C=O 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 (ScCl(3), YCl(3), LaCl(3), TiCl(4), TaCl(5), AlCl(3), SnCl(4)) have been shown to be able to co-ordinate DCU but not all of them promote the conversion of urea into DCC.