Theoretical Investigation of the Mechanism of Primary Amines Reacting with Hexamolybdate: An Insight into the Organoimido Functionalization and Related Reactions of Polyoxometalates

The functionalization of polyoxometalates (POMs), especially with an amino group to yield organonitrogenous derivatives of POMs, is an efficient approach to the enrichment of their structures and the diversification of their properties for various applications. The mechanism for the formation of organonitrogenous-derivatized hexamolybdates was explored by investigating the monofunctionalization of the [Mo6O19]2- ion with methylamine using the density functional theory (DFT) method. The calculations show that the direct imidoylization of hexamolybdate with methylamine is both kinetically and thermodynamically unfavorable. However, this imidoylization was found to take place readily in the presence of dimethylcarbodiimide (DMC), for which the free-energy barrier was calculated to be +32.5 kcal?mol-1 in acetonitrile. Moreover, various factors controlling the efficiency of the imidoylization were examined. The calculations show that [W5MoO19]2- has a relatively lower reactivity than [Mo6O19]2-, and that the imidoylization of [W6O19]2- is an unfavorable process. With respect to the effect of carbodiimides, it is found that the catalytic activity is directly proportional to the electron-withdrawing effects of the substituents. As to the reactivity of R?NH2, the computation results indicate that the free-energy barriers of the substitution reactions are linearly correlated with the basicity constants (pKb) of the amino groups. It is noteworthy that the introduction of the proton dramatically decreases the free-energy barrier of the imidoylization of [Mo6O19]2- catalyzed by DMC to 24.3 kcal?mol-1 in acetonitrile.