The trans–cis isomerization of Ni(η2-TEMPO)2: Interconnections and conformational complexity

•The trans–cis isomerization of Ni(TEMPO)2 has been modeled with Density Functional Theory.•The Ni(TEMPO)2 complex exhibits conformational complexity and fluxionality.•New visualization techniques are needed for conformational dynamics of Ni(TEMPO)2.

Previously, reactions of the “bow-tie” Ni(η2-TEMPO)2 complex with an assortment of donor ligands have been characterized experimentally and computationally. The X-ray crystal structures afforded Ni(η2-TEMPO)2 with trans-disposed TEMPO ligands, which was validated theoretically. Experimentally, proton transfer from the C–H bond of a variety of alkyne substrates (R) mostly produced cis-disposed ligands of the form Ni(η2-TEMPO)(κ1-TEMPOH)(κ1-R). While computations validated that the experimentally observed cis-disposed products were thermodynamically favored, in all cases, the proposed mechanisms of alkyne addition to cis-Ni(η2-TEMPO)2 were kinetically disfavored. Therefore, a trans–cis isomerization must occur along the addition pathway. In order to better understand this phenomenon, an exhaustive theoretical conformational search of cis-/trans-Ni(η2-TEMPO)2 and Ni(η2-TEMPO)(η1-TEMPO) structures has been performed. Our results show profound conformational and fluxional complexity for the trans–cis isomerization of the Ni(η2-TEMPO)2 precursor. Surprisingly, the proposed mechanism for trans–cis isomerization indicates that conformational distortion of one of the TEMPO ligands of trans-Ni(η2-TEMPO)2 before ring-opening provides a transition state free energy of activation stabilization of nearly 4.0 kcal mol−1 versus the most “straightforward” isomerization mechanism.

Graphical abstractThe trans–cis isomerization of Ni(TEMPO)2 exhibits fascinating conformational complexity and fluxionality of the coordinated TEMPO ligand. The mechanism of Ni(TEMPO)2 isomerization has been explored with Density Functional Theory. It is observed that conformational distortion of a TEMPO ring leads to lower ring-opening/ring-closing free energies of activation.Figure optionsDownload full-size imageDownload as PowerPoint slide