The first example of an unsaturated transition metal complex that is supported exclusively by the intriguing ?2-TEMPO ligand [Ni(?2-TEMPO)2] was previously reported from one of our laboratories. In the current investigation, reactions of the �bow-tie� Ni(?2-TEMPO)2 complexwith an assortment of donor ligands have been characterized experimentally and computationally. While the Ni(?2-TEMPO)2 complex has trans-disposed TEMPO ligands, C-H bond activation of various substrates (acetylene, phenylacetylene, 1,4-diacetylenebenzene, and trimethylsilyl acetylene) produce cis-disposed ligands of the form Ni(?2-TEMPO)(?1O-TEMPO-H)(?1-R). Via Density Functional Theory, reaction mechanisms are proposed in order to determine whether the trans-cis isomerization occurs before or after substrate addition. Based on multiple mechanistic possibilities, the pre- and post-addition free energies of activation could be thermodynamically and kinetically competitive. Computations have also been used to validate the thermodynamic stabilities of various bond-activation products. For example, in the case of 1,4-diacetylenebenzene, a second Ni(?2-TEMPO)2 can subsequently activate the dangling C-H bond of ?1-bound ligand (to form a bridged bimetallic complex); whereas for acetylene, Ni(?2-TEMPO)2 cannot. Based upon computational results, the observed fluxional behavior of the complexes obtained from addition of tert-butyl isocyanide or pyridine ligands correspond to asymmetric ?1/?2 exchange in the two TEMPO ligands.

Metal – ligand synergistic effects in Ni(η<sup>2</sup>-TEMPO)<sub>2</sub>, structures and reactivity

245th National Meeting (2013)

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Metal – ligand synergistic effects in Ni(η2-TEMPO)2, structures and reactivity

Metal – ligand synergistic effects in Ni(η²-TEMPO)₂, structures and reactivity