Metal–Ligand
Synergistic Effects in the Complex
Ni(η<sup>2</sup>‑TEMPO)<sub>2</sub>: Synthesis, Structures,
and Reactivity
- Publication date
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Abstract
In
the current investigation, reactions of the “bow-tie”
Ni(η<sup>2</sup>-TEMPO)<sub>2</sub> complex with an assortment
of donor ligands have been characterized experimentally and computationally.
While the Ni(η<sup>2</sup>-TEMPO)<sub>2</sub> complex has <i>trans</i>-disposed TEMPO ligands, proton transfer from the C–H
bond of alkyne substrates (phenylacetylene, acetylene, trimethylsilyl
acetylene, and 1,4-diethynylbenzene) produce <i>cis</i>-disposed
ligands of the form Ni(η<sup>2</sup>-TEMPO)(κ<sup>1</sup>-TEMPOH)(κ<sup>1</sup>-R). In the case of 1,4-diethynylbenzene,
a two-stage reaction occurs. The initial product Ni(η<sup>2</sup>-TEMPO)(κ<sup>1</sup>-TEMPOH)[κ<sup>1</sup>-<i>C</i>C(C<sub>6</sub>H<sub>4</sub>)CCH] is formed first but can react further
with another equivalent of Ni(η<sup>2</sup>-TEMPO)<sub>2</sub> to form the bridged complex Ni(η<sup>2</sup>-TEMPO)(κ<sup>1</sup>-TEMPOH)[κ<sup>1</sup>-κ<sup>1</sup>-<i>C</i>C(C<sub>6</sub>H<sub>4</sub>)C<i>C</i>]Ni(η<sup>2</sup>-TEMPO)(κ<sup>1</sup>-TEMPOH). The corresponding reaction with
acetylene, which could conceivably also yield a bridging complex,
does not occur. Via density functional theory (DFT), addition mechanisms
are proposed in order to rationalize thermodynamic and kinetic selectivity.
Computations have also been used to probe the relative thermodynamic
stabilities of the <i>cis</i> and <i>trans</i> addition products and are in accord with experimental results. Based
upon the computational results and the geometry of the experimentally
observed product, a <i>trans</i>–<i>cis</i> isomerization must occur
