Please cite this article as: N.J. DeYonker, C.E. Webster, The trans-cis Isomerization of Ni(η 2 -TEMPO) 2 : Interconnections and Conformational Complexity, Inorganica Chimica Acta (2015), doi: http://dx.
ABSTRACTPreviously, reactions of the "bow-tie" Ni(η 2 -TEMPO) 2 complex with an assortment of donor ligands have been characterized experimentally and computationally. The X-ray crystal structures afforded Ni(η 2 -TEMPO) 2 with trans-disposed TEMPO ligands, which was validated theoretically. Surprisingly, proton transfer from the C-H bond of a variety of alkyne substrates (R) mostly produced cis-disposed ligands of the form Ni(η 2 -TEMPO)(κ 1 -TEMPOH)(κ 1 -R).While computations validated that the experimentally observed cis-disposed products were thermodynamically favored, in all cases, the proposed mechanisms of alkyne addition to cisNi(η 2 -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(η 2 -TEMPO) 2 and Ni(η 2 -TEMPO)(η 1 -TEMPO) structures has been performed. Our results show profound conformational and fluxional complexity for the trans-cis isomerization of the Ni(η 2 -TEMPO) 2 precursor. Surprisingly, the proposed mechanism for trans-cis isomerization indicates that conformational distortion of one of the TEMPO ligands of trans-Ni(η 2 -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.
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KEYWORDSDensity Functional Theory, TEMPO ligand, reaction mechanisms, conformational distortion
HIGHLIGHTSThe trans-cis isomerization of Ni(TEMPO) 2 has been modeled with Density Functional Theory.The cis and trans isomer of Ni(TEMPO) 2 exhibit fascinating conformational complexity and fluxionality of the coordinated TEMPO ligand.New visualization techniques are needed for the presentation of the conformational dynamics of Ni(TEMPO) 2 .