The transition metals in groups 3–5 of the periodic table tend to engage in redox-neutral bond making and breaking processes, such as s-bond metathesis. One method to expand the reactivity of these complexes is to employ a p-donor ligand—such as an oxo, imido, or nitrido—that forms more than one bonding interaction with the high valent metal center. The electrons contained in the metal–ligand p-bond(s) can undergo cycloadditions with substrates containing additional p bonds. We recently reported a niobium(III) imido complex supported by a β-diketiminate ligand. In this work, we show that this complex reacts with alkyl azides to form the niobatetrazene complexes (BDI)Nb(N^tBu)(RNNNNR) (BDI = N,N- bis(2,6-diisopropylphenyl)-3,5-dimethyl-b-diketiminate; R= cyclohexyl, benzyl). Intriguingly, these niobatetrazene complexes can be interconverted via addition of the appropriate alkyl azide, likely through a series of concerted [3+2] cycloaddition and retrocycloaddition reactions in which bis(imido) intermediates are formed. To provide support for this proposed reaction mechanism, density functional theory (DFT) and density functional tight binding (DFTB) computational methods were employed. DFTB uses the xTB program and is an approximation to DFT that allows for significant savings in computational time, while retaining some basic quantum mechanical characteristics of DFT. The lowest energy pathway across the potential energy surface of this reaction scheme was calculated; our results were compatible with our experimental insights using both methods. Intrinsic reaction coordinate calculations showed that some of the steps in this reaction pathway follow a concerted asynchronous mechanism, in which intermediates are not formed, but bond-breaking events do not occur simultaneously. The use of xTB methods in this work highlight its advantages and potential for use in transition metal systems such as this one.

Jade Fostvedt

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