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The role of molecular oxygen in the iron(iii)-promoted oxidative dehydrogenation of amines
A mechanistic study is presented of the oxidative dehydrogenation of the iron( iii ) complex [Fe III L 3 ] 3+ , 1 , (L 3 = 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanonane) in ethanol in the presence of molecular oxygen. The product of the reaction was identified by NMR spectros...
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| Published in: | Dalton transactions : an international journal of inorganic chemistry 2015-03, Vol.44 (12), p.551-5519 |
|---|---|
| Main Authors: | , , |
| Format: | Article |
| Language: | English |
| Subjects: | |
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| Online Access: | Get full text |
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| Summary: | A mechanistic study is presented of the oxidative dehydrogenation of the iron(
iii
) complex [Fe
III
L
3
]
3+
,
1
, (L
3
= 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanonane) in ethanol in the presence of molecular oxygen. The product of the reaction was identified by NMR spectroscopy and X-ray crystallography as the identical monoimine complex [Fe
II
L
4
]
2+
,
2
, (L
4
= 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanon-1-ene) also formed under an inert nitrogen atmosphere. Molecular oxygen is an active player in the oxidative dehydrogenation of iron(
iii
) complex
1
. Reduced oxygen species,
e.g.
, superoxide, (O
2
&z.rad;
−
) and peroxide (O
2
2−
), are formed and undergo single electron transfer reactions with ligand-based radical intermediates. The experimental rate law can be described by the third order rate equation, −d[(Fe
III
L
3
)
3+
]/dt =
k
OD
[(Fe
III
L
3
)
3+
][EtO
−
][O
2
], with
k
OD
= 3.80 ± 0.09 × 10
7
M
−2
s
−1
(60 °C,
μ
= 0.01 M). The reduction O
2
→ O
2
&z.rad;
−
represents the rate determining step, with superoxide becoming further reduced to peroxide as shown by a coupled heme catalase assay. In an independent study, with H
2
O
2
, replacing O
2
as the oxidant, the experimental rate law depended on [H
2
O
2
]: −d[(Fe
III
L
3
)
3+
]/dt =
k
H
2
O
2
[(Fe
III
L
3
)
3+
][H
2
O
2
]), with
k
H
2
O
2
= 6.25 ± 0.02 × 10
−3
M
−1
s
−1
. In contrast to the reaction performed under N
2
, no kinetic isotope effect (KIE) or general base catalysis was found for the reaction of iron(
iii
) complex
1
with O
2
. Under N
2
, two consecutive one-electron oxidation steps of the ligand coupled to proton removal produced the iron(
ii
)-monoimine complex [Fe
II
L
4
]
2+
and the iron(
ii
)-amine complex [Fe
II
L
3
]
2+
in a 1 : 1 ratio (disproportionation), with the amine deprotonation being the rate determining step. Notably, the reaction is almost one order of magnitude faster in the presence of O
2
, with
k
EtO
−
= 3.02 ± 0.09 × 10
5
M
−1
s
−1
(O
2
) compared to
k
EtO
−
= 4.92 ± 0.01 × 10
4
M
−1
s
−1
(N
2
), documenting the role of molecular oxygen in the dehydrogenation reaction.
A mechanistic study is presented of the oxidative dehydrogenation of the iron(
iii
) complex [Fe
III
L
3
]
3+
,
1
, (L
3
= 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanonane) in ethanol in the presence of molecular oxygen. |
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| ISSN: | 1477-9226 1477-9234 |
| DOI: | 10.1039/c4dt03606a |