[3+2] Cycloadditions and Retrocycloadditions of Niobium Imido Complexes: An Experimental and Computational Mechanistic Study

We demonstrate reactivity between a beta-diketiminate-supportedniobium(III) imido complex and alkyl azides to form niobatetrazene complexes(BDI)Nb(NtBu)(RNNNNR) (BDI =N,N-bis(2,6-diisopropylphenyl)-3,5-dimeth-yl-beta-diketiminate; R = cyclohexyl (1), benzyl (2)). Intriguingly, niobatetrazenecomplexes1and2can be interconverted via addition of an appropriate alkyl azide,likely through a series of concerted [3 + 2] cycloaddition and retrocycloadditionreactions in which pi-loaded bis(imido) intermediates are formed. The bis(imido)intermediates were trapped upon addition of alkyl isocyanides to yieldfive-coordinate bis(imido) complexes (BDI)Nb(NtBu)(NCy)(CNR) (R =tert-butyl (4a), cyclohexyl (4b)). Two computationalmethods???density functional theory and density functional tight binding (DFTB)???were employed to calculate the lowest energypathway across the potential energy surface for this multistep transformation. Reaction path calculations for individual cycloadditionor retrocycloaddition processes along the multistep reaction pathway showed that these transformations occur via a concerted, yethighly asynchronous mechanism, in which the two bond-breaking or -making events do not occur simultaneously. The use of theDFTB method in this work highlights its advantages and utility for studying transition metal systems

Automated summary

In this study, the reactivity between a beta-diketiminate-supported niobium(III) imido complex and alkyl azides was demonstrated, resulting in the formation of niobatetrazene complexes. The reaction mechanism was also determined using computational methods.