Abstract
Oxidation of [(Pip)
n
CuX]
4 (
n
=
1 or 2, Pip
=
piperidine, X
=
Cl, Br or I) by dioxygen to form homogeneous oxidative coupling catalysts [(Pip)
n
CuX]
4O
2, represents one half of the catalytical cycle. The mechanism can be summarized as a pre-equilibrium
K between [(Pip)
n
CuX]
4 and O
2, which is followed by rate determining step
k
2. The overall reaction is a second-order process, first order in each [[(Pip)
n
CuX]
4] and [O
2], with rate constant
k
on (
k
on
=
Kk
2).
This paper reports the mechanism of formation of oxidative coupling catalysts [(Pip)
n
CuX]
4O
2,
n
=
1 or 2 and X
=
Cl, Br or I, which represent half of the catalytical cycle, Scheme
1. The mechanism has been described as a pre-equilibrium between [(Pip)
n
CuX]
4 and O
2.
K values are very sensitive to how strong the hydrogen-bonding between copper (I) tetranuclear and incoming dioxygen is, such association is also sensitive to the variation of X. The pronounced pre-equilibrium is the reason behind the oxidation of [(Pip)
n
CuI]
4, which is not the case for pyridine type of ligands. The pre-equilibrium followed by rate determining step
k
2, which is responsible to the formation of the oxidative coupling catalysts [(Pip)
n
CuX]
4O
2. The overall reaction is a second-order process, first order in each [[(Pip)
n
CuX]
4] and [O
2], with rate constant
k
on (
k
on
=
Kk
2) and exothermic Δ
H
≠ varying from −3 to −12 kcal
mol
−1 and Δ
S
≠ varying from −87 to −65 cal
deg
−1
mol
−1.
k
on were found to be very sensitive to
n value 1 or 2 and to the type of X (Cl, Br or I).