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

Abstract

NbCl(5) x (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 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.

Reaction Mechanisms in the Direct Carboxylation of Alcohols for the Synthesis of Acyclic Carbonates

“…DFT calculations provided an energy profile for the reaction. An interesting additional piece of information is that the resulting DCU can be reconverted back into DCC [24,25], a finding that may support the practical application of such synthetic methodology for the synthesis of carbonates of alcohols bearing thermally unstable moieties that require low reaction temperatures. Conversely, the DCC based synthetic methodology is less suited with alcohols such as methanol or ethanol, that would be used at a [ 10 Mt/year scale.…”

Interaction of niobium and tantalum pentahalides with O-donors: coordination chemistry and activation reactions

“…Actually the detailed study of the systems NbCl 5 /2-butanone and NbCl 5 /9-heptadecanone clearly pointed out the generation of oxygen-depleted organic products, whose nature depends on the solvent. 29 Otherwise, products of formula NbOCl 3 L 2 were claimed to be formed by the reactions of NbX 5 (X = Cl, Br) with excess of oxygen molecules (L = Ph 3 PO, 28,30 Me 2 SO, 31 ketones, 18a ureas 20,32 ), but evidence for the contextual production of deoxygenated organics was given in few cases only. 28,31,32 In the course of our studies on the chemistry of MX 5 with limited amounts of oxygen compounds, we could isolate and fully characterize, by spectroscopic (IR, NMR) and analytical (elemental analysis, solution conductivity) techniques, a series of stable MX 5 L (M = Nb, Ta; X = F, Cl, Br) complexes.…”

N,N′-Dicyclohexyl-N-(phthaloylglycyl)urea

“…Orea Flores et al, 2006;Imhof, 2007;Pinheiro et al, 2012). While DCU has been found to be basic enough to coordinate to acid cations such as Nb 5+ or La 3+ (Aresta et al, 2010;Zhang et al, 2016), its derivatives obtained by functionalization of the amine groups cannot serve as ligands, because of the hindrance between urea substituents.…”