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


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.

Publication Title

Inorganica Chimica Acta