Kinetics of oxidation of thiosulfate ion with [MnIV2(µ-O)2(µ -MeCO2)(H2O)2(bipy)2] 3+ (bipy = 2,2′-bipyridine) and its hydrolytic derivatives
Abstract
The complex
[Mn
IV
2
(µ-O
2
)
2
(
µ-MeCO
2
)(H
2
O)
2
(bipy)
2
]
3+
1 (bipy = 2,2′-bipyridine) has
been found to co-exist in rapid equilibrium with
[Mn
2
O
2
(H
2
O)
4
(bipy)
2
]
4+
2 and
[Mn
2
O
2
(MeCO
2
)(H
2
O)
4
(bipy)]
3+
3 in aqueous buffer containing an excess of
bipy and MeCO
2
-
in the range pH
4.0–5.5. Equilibrium constants (mol dm
-3
) for
1 ⇌ 2 +
MeCO
2
-
and
1 ⇌ 3 +
bipy are 10
-2
and 10
-3
, respectively.
Water molecules co-ordinated to the manganese(IV) centres are
stable at least up to pH 5.5. Reduction of the binuclear
manganese(IV) species by thiosulfate ion proceeded
via
[(bipy)
2
Mn
III
O
2
Mn
IV
(bipy)
2
]
3+
4, in the presence of an excess of bipy. An
excess of thiosulfate reduced complex 4 further and ultimately produced
Mn
II
. Rate constants for reduction of 1–3 to 4, as
determined under second-order conditions, increase with increasing
number of water molecules at the manganese centre. A plausible mechanism
is proposed and the relevance of the results to photosystem II
discussed.
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