New procedures for the preparation of
[Mo3S4(H2O)9]4+,
[Mo4S4(H2O)12]5+
and
[Mo7S8(H2O)18]8+
and their Se analogues: redox and substitution studies on the double
cube
[Mo7S8(H2O)18]8+
Abstract
Alternative improved methods for the preparation of
[Mo
3
S
4
(H
2
O)
9
]
4+
,
[Mo
4
S
4
(H
2
O)
12
]
5+
and
[Mo
7
S
8
(H
2
O)
18
]
8+
are described from polymeric
{Mo
3
S
7
Br
4
}
x
(obtained by heating together the elements), via water
soluble [Mo
3
S
7
Br
6
]
2-
.
Abstraction of neutral sulfur with phosphines and aquation of terminal
ligands in dilute acids gives
[Mo
3
S
4
(H
2
O)
9
]
4+
in yields of up to 85%. The single cube
[Mo
4
S
4
(H
2
O)
12
]
5+
,
and corner-shared double cube
[Mo
7
S
8
(H
2
O)
18
]
8+
,
are obtained by treating
[Mo
3
S
4
(H
2
O)
9
]
4+
with different reductants. The best procedure for
[Mo
7
S
8
(H
2
O)
18
]
8+
is with hypophosphorous acid (H
3
PO
2
), when yields
of up to 20% are obtained. Yields of
[Mo
4
S
4
(H
2
O)
12
]
5+
are variable and in the range 10–65% depending on the reductant
and procedure employed. The reactions provide examples of
Mo
III
and Mo
IV
–sulfido reassembly from
[Mo
3
S
4
(H
2
O)
9
]
4+
following reduction. Surprisingly, the more direct approach for the
preparation of
[Mo
7
S
8
(H
2
O)
18
]
8+
,
involving addition of
[Mo
3
S
4
(H
2
O)
9
]
4+
to
[Mo
4
S
4
(H
2
O)
12
]
4+
,
gives only ≈3% yield, most likely due to the inertness of
[Mo
4
S
4
(H
2
O)
12
]
4+
.
The procedures described are effective also for
[Mo
3
Se
7
Br
6
]
2-
, and in
exploratory studies the double cube
[Mo
7
Se
8
(H
2
O)
18
]
8+
has been prepared for the first time. The kinetics of substitution of
H
2
O on
[Mo
7
S
8
(H
2
O)
18
]
8+
by NCS
-
have been investigated, and two stages
identified. One of these is a [NCS
-
]-dependent
equilibration, with rate constants (25 °C) for formation
k
f
= 0.173 M
-1
s
-1
and aquation
k
aq
= 0.20 × 10
-3
s
-1
at
[H
+
] = 1.96 M, I = 2.00
M (Lipts),
pts
-
= p-toluenesulfonate. The
other is a [NCS
-
]-independent step assigned as
isomerisation of the S-bonded thiocyanato product (k
≈1.5 × 10
-3
s
-1
). With [Co(dipic)
2
]
-
(dipic = pyridine-2,6-dicarboxylate) as oxidant,
second-order kinetics are observed with the rate constant 0.31
M
-1
s
-1
at 25 °C
independent of [H
+
] in the range 0.87–2.00 M,
I = 2.00 M (Lipts). The stoichiometry indicates
higher than expected consumption (≈7 equivalents) of
[Co(dipic)
2
]
-
. A mechanism involving
transient formation of
[Mo
7
S
8
(H
2
O)
18
]
9+
,
which fragments to
[Mo
3
S
4
(H
2
O)
9
]
4+
and
[Mo
4
S
4
(H
2
O)
12
]
5+
,
is proposed. In separate experiments the latter was found to be oxidised
by [Co(dipic)
2
]
-
yielding first
[Mo
4
S
4
(H
2
O)
12
]
6+
,
which itself fragments with formation of
[Mo
3
S
4
(H
2
O)
9
]
4+
,
the Mo
V
2
dimer
[Mo
2
O
2
S
2
(H
2
O)
6
]
2+
, and other products.
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