Stabilization of
ions and radicals often determines reaction kinetics
and thermodynamics, but experimental determination of the stabilization
magnitude remains difficult, especially when the species is short-lived.
Herein, a competitive kinetic approach to quantify the stabilization
of a halide ion toward oxidation imparted by specific stabilizing
groups relative to a solvated halide ion is reported. This approach
provides the increase in the formal reduction potential, Δ
E
°′(Χ
•/–
), where
X = Br and I, that results from the noncovalent interaction with stabilizing
groups. The [Ir(dF-(CF
3
)-ppy)
2
(tmam)]
3+
photocatalyst features a dicationic ligand tmam [4,4′-bis[(trimethylamino)methyl]-2,2′-bipyridine]
2+
that is shown by
1
H NMR spectroscopy to associate
a single halide ion,
K
eq
= 7 × 10
4
M
–1
(Br
–
) and
K
eq
= 1 × 10
4
M
–1
(I
–
). Light excitation of the photocatalyst in
halide-containing acetonitrile solutions results in competitive quenching
by the stabilized halide and the more easily oxidized diffusing halide
ion. Marcus theory is used to relate the rate constants to the electron-transfer
driving forces for oxidation of the stabilized and unstabilized halide,
the difference of which provides the increase in reduction potentials
of Δ
E
°′(Br
•/–
) = 150 ± 24 meV and Δ
E
°′(I
•/–
) = 67 ± 13 meV. The data reveal that
K
eq
is a poor indicator of these reduction potential
shifts. Furthermore, the historic and widely used assumption that
Coulombic interactions alone are responsible for stabilization must
be reconsidered, at least for polarizable halogens.