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

Previously, reactions of the "bow-tie'' Ni(eta(2)-TEMPO)(2) complex with an assortment of donor ligands have been characterized experimentally and computationally. The X-ray crystal structures afforded Ni(eta(2)-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(eta(2)-TEMPO)(kappa(1)-TEMPOH)(kappa(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 cis-Ni(eta(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(eta(2)-TEMPO)(2) and Ni(eta(2)-TEMPO)(eta(1)-TEMPO) structures has been performed. Our results show profound conformational and fluxional complexity for the trans-cis isomerization of the Ni(eta(2)-TEMPO)(2) precursor. Surprisingly, the proposed mechanism for trans-cis isomerization indicates that conformational distortion of one of the TEMPO ligands of transNi (eta(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. (C) 2015 Elsevier B.V. All rights reserved.