Carbon-Bromine Bond Formation through a Nickel-Centered Spin-Crossing Mechanism
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abstract
The mechanism of a Ni-centered C-Br bond-forming reaction has been investigated with density functional theory calculations. Unlike the typical behavior of heavier group 10 metals that make use of higher oxidation states, Ni undergoes a change in multiplicity to provide a low-energy path. The calculated pathway begins with the singlet state complex Ni(Ar)(Br)(pic) (Ar = 2-phenylpyridine, pic = 2-picoline) and remains on the singlet state reaction surface for the axial addition of Br2. The Br2 forms a three-center, four-electron bond with Ni that stabilizes the singlet state without increasing the oxidation state of the Ni center. The singlet state is then destabilized by the loss of the three-center, four-electron interaction as the Br-Br bond is broken and the second Br from the Br2 (the Br previously unbound to Ni) binds to the Ni center. At this point in the mechanism, the triplet state offers a lower energy pathway, as the triplet state is stabilized by a Br2-/NiIII interaction where one unpaired electron is localized on the Ni center and the second unpaired electron is localized in the Br2 * orbital. The reaction continues on the triplet reaction surface because the Br 2-/NiIII interaction can be maintained until the final reductive elimination takes place. The mechanism concludes with the formation of the reductively eliminated CAr-Br product and the addition of dppe to stabilize the Ni byproducts as a singlet-state Ni II(dppe)(Br)2 complex. 2011 American Chemical Society.
