Study of Imido-Exchange Reaction Mechanism

In the second example we present a study where theoretical helped unravel an unexpected result in the laboratory a reaction expected to lead to the thermodynamically most stable species (uranium oxo), instead generated a uranium imido species. As mentioned earlier, the uranyl ion possesses U-O bonds with high thermodynamic stability and extreme kinetic inertness. As a result, the majority of uranyl ion reaction chemistry involves substitution of equatorially coordinated ligands while leaving the U-O bond unaffected. Its isoelectronic bis(imido) [U(NR)2] + analog, while possessing many of the bonding features found in UO2+ [46] exhibits reactivity quite distinct from UO.  

In contrast to this mechanism, the calculated [2 -h 2] C = O cycloaddition bond pathway (Path 2) involves the formation of the higher energy A(9-bound ureato intermediates 15 

Given the evidence from DFT calculations and the results from I lN-labeling studies, it appears the mechanism for the formation of 9 involves the [2 - - 2] cycloaddition of the aryl isocyanate C = N bond across the U = N imido moiety. These results are quite surprising given the thermodynamic and kinetic stability of U = O bonds. The further reaction at elevated temperature gives a mixed oxo-imido complex 8 which is consistent with the relative energies determined by DFT calculations. 

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