Preparation, structure and reactivity of the di-µ-disulfido
NbIV2 aqua ion
[Nb2(µ-S2)2(H2
O)8]4+, the structure of
[Nb2(µ-S2)2(NCS)8
]4-, and properties of the related
(µ-S)2 aqua ion
Abstract
Using Nb
IV
S
2
Cl
2
as starting
compound, and reaction routes involving aquation of Cl
-
(or NCS
-
) in 4 M Hpts
(Hpts = p-toluenesulfonic acid,
CH
3
C
6
H
4
SO
3
H), an aqua ion
product was obtained and purified by cation-exchange chromatography.
Yellow crystals of
[Nb
2
(µ-S
2
)
2
(H
2
O)
8
](pts)
4
·4H
2
O 1, and orange crystals
of the thiocyanato complex
Cs
4
[Nb
2
(µ-S
2
)
2
(NCS)
8
]·2H
2
O 2 were isolated, and the structures of
both determined by X-ray diffraction. The compounds have short
Nb–Nb distances (2.85–2.90 Å) consistent with
metal–metal bonding, two η
2
:η
2
disulfido bridges (S–S distance 1.99 Å), and 2 has N-bonded
thiocyanate. Compound 2 was also shown to be diamagnetic. Conventional
time range equilibration studies on the substitution of
NCS
-
(>10-fold excess) for H
2
O on
[Nb
2
(µ-S
2
)
2
(H
2
O)
8
]
4+
indicate
[H
+
]
-1
-dependent rate laws,
k
f
= k
1
k
2
[H
+
]
-1
for
formation, and
k
aq
= k
-1
+ k
-2
[H
+
]
-1
for aquation. At 25 °C
k
1
= 0.88 M
-1
s
-1
and
k
-1
= 2.4 ×
10
-3
s
-1
,
I = 2.00 M (LiClO
4
). The slow reaction
of the low electron count di-µ-disulfido ion
[Nb
2
(µ-S
2
)
2
(H
2
O)
8
]
4+
is an unusual feature. Neutral S-abstraction
reactions from
[Nb
2
(µ-S
2
)
2
(H
2
O)
8
]
4+
with PPh
3
or the water-soluble
phenylsulfonate derivative
(3-SO
3
C
6
H
4
)
3
P
3-
(PR
3
3-
) have stoichiometries of 2
moles of phosphine per dimer, and give a green product assigned as
[Nb
2
(µ-S)
2
(H
2
O)
8
]
4+
. Stopped-flow studies on the substitution of
NCS
-
for H
2
O on
[Nb(µ-S)
2
(H
2
O)
8
]
4+
give a
second-order formation rate constant k = 37
M
-1
s
-1
in 2.0 M Hpts solutions. On
varying [H
+
] in the range 1.0–2.0 M,
[Nb
2
(µ-S)
2
(H
2
O)
8
]
4+
gives a reversible green to yellow colour change with an
[H
+
]
-1
dependent rate constant
≈4.8 × 10
-4
s
-1
. An irreversible step is incident at
[H
+
] < 0.5 M. Comparisons are made of the
inhibitory effect of η
2
:η
2
µ-disulfido
bridges on substitution of H
2
O on
[Nb
2
(µ-S
2
)
2
(H
2
O)
8
]
4+
as compared with
[Nb
2
(µ-S)
2
(H
2
O)
8
]
4+
, and reference is made to a similar trend for
[Mo
3
(µ
3
-S)(µ–S
2
)
3
(H
2
O)
6
]
4+
and
[Mo
3
(µ
3
-S)(µ-S)
3
(H
2
O)
9
]
4+
. Attempts to prepare the
Mo
V
2
analogue
[Mo
2
(µ-S
2
)
2
(H
2
O)
8
]
6+
gave
[Mo
2
O
2
(µ-S)
2
(H
2
O)
6
]
2+
.
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