Computational Study on the Role of Zn(II) Z‑Type Ligands in Facilitating Diaryl Reductive Elimination from Pt(II)

Electron-accepting, or Z-type, ligands offer new potential
for
affecting the reactivity of transition-metal complexes. A better understanding
of the precise mechanisms by which Z-type ligands affect reactivity
will aid in taking advantage of their unique properties. It was previously
shown that a ZnArF2 additive (where ArF = C6F5) can bind to phenPt(II)Ar2 (where phen = phenanthroline and Ar = p-C6H4tBu) as a Z-type ligand and enable
quantitative reductive elimination at 60°C [Liberman-Martin, A. L.Chem. Commun.2016, 52(43), 7039−7042]. In the present study, density functional theory (DFT) calculations
were used to gain insight into the role of the ZnArF2 Z-type ligand in facilitating reductive elimination. The
computed Gibbs activation energies with and without ZnArF2 are 41.4 and 44.5 kcal mol–1, respectively.
Therefore, simply the presence of the Z-type ligand lowers the barrier
by 3.1 kcal mol–1, which is insufficient to account
for the experimentally observed effect of ZnArF2. An alternative mechanism is therefore identified. First, the phenanthroline
is sequestered by excess ZnArF2 followed by
the transfer of one of the ArF groups to the Pt(II) center
to form a zwitterionic [ZnArF]+[ArFPtAr2]− complex. An estimate of the
overall Gibbs activation energy for this process is 25.1 kcal mol–1, and subsequent reductive elimination occurs with
a Gibbs activation energy of 10.1 kcal mol–1. These
values are consistent with experimental observations of the reaction.
An analysis of the interaction energies of each of the ligands in
the reductive elimination model systems generated by the mechanistic
study suggests that the effective charge transfer to the Z-type ligand
in the zwitterionic complex helps lower the reductive elimination
barrier, but the ability to obtain a three-coordinate complex (e.g.,
in the absence of phenanthroline) is more important.