Chemistry Reference
In-Depth Information
A few structures of tetranuclear tungsten oxo-alkoxides have been reported. In
the remarkable molecule [W
4
-O
2
-OPr
i
3
OPr
i
9
] where W has the average
oxidation state of four there is one triple bonded (WW, 2.404 A) ditungsten
W
2
OPr
i
5
unit linked by two
-oxo ligands to one edge of a confacial bi-
octahedral W
2
3
-OPr
i
OPr
i
unit (W-W, 2.684 A). With respect to oxygen the
tungstens are three, four, and six-coordinated.
35
Another unusual structure was found
for [W
4
-O
-OPr
i
2
OPr
i
7
Cl] which has an apical two-coordinated tungsten
W
OPr
i
Cl capping a triangle of four-coordinated basal metals linked by metal - metal
bonds. The
-oxo ligand bridges two of the basal tungsten atoms which are each
linked to the third basal tungsten by
-OPr
i
ligands. The oxygen coordination around
the basal metals is approximately square planar.
72
In the tetranuclear
4
-carbido oxo-alkoxides [W
4
4
-C
-O
-OR
4
OR
8
](RD
CH
2
Bu
t
,Pr
i
) the four metals adopt the “butterfly” configuration. In the neopentyloxide
the
-oxo ligand bridges the backbone tungsten atoms whereas in the isopropoxide
it links one backbone W to a wingtip W.
38
A zig-zag array of four W atoms occurs
in [f
CO
dmpe
2
W
.
W
O
2
g
2
-O
] (dmpe D Me
2
PC
2
H
4
PMe
2
) in which the exterior
five-coordinated zero valent tungstens are metal - metal bonded (W-W, 2.648 A) to
three-coordinated pentavalent inner metals, which in turn are linked by the
-oxo
ligand. The diamagnetism is presumed to arise from spin pairing of oxo-bridged interior
d
1
-d
1
metal centres.
171
The two centrosymmetrical hexanuclear oxo-alkoxides [W
6
-O
4
-CR
2
OBu
t
10
]
(R D Et, Pr
n
) reported
164
,
172
have the same general structure involving two trinuclear
subunits joined by a pair of unsymmetrical
-oxo bridges (W-O, 1.755 A, 2.119 A).
Each isosceles W
3
triangle has a
-oxo and a
-carbyne ligand bridging the longer sides
and each metal has two terminal
tert
-butoxo groups. The only octanuclear tungsten oxo-
alkoxide reported to date is the dimeric W
4
unit in [W
8
-O
4
-OPr
i
4
OPr
i
12
].
173
This molecule is centrosymmetric and the two W
4
units are connected by a
W
-OPr
i
2
W double bridge. Each W
4
unit consists of a W
3
triangle bridged by two
-
oxo and one
-OPr
i
group, and linked to the fourth tungsten by three tungsten - tungsten
bonds.
5.7 Manganese Sub-group Metal-containing Oxo-alkoxides (Including
Heterometallic Compounds) (Table 5.7)
5.7.1
Manganese
The use of the tridentate alkoxo ligand MeC
CH
2
O
3
gave rise to the decanuclear mixed-
valence manganese oxo-alkoxo-chloro anion [Mn
10
6
-O
2
f
OCH
2
3
CMeg
6
Cl
8
]
2
.
88
The Mn
10
O
20
Cl
8
structure (Fig. 5.20) can be considered as the fusion of two octa-
hedral Mn
6
O
13
Cl
6
units by eliminating two
cis
manganese moieties. The six alkoxo
ligands symmetrically cap the eighteen surface bridging oxo-sites and the mixed valence
is accounted for by eight Mn(
III
) and two Mn(
II
)atoms.
The remarkable mixed-ligand mixed valence neutral tridecanuclear molecule
[Mn
13
5
-O
6
3
-O
2
3
-OEt
6
-O
2
CPh
12
] has a supercubane structure with a
central Mn(
IV
) atom octahedrally surrounded by six
5
-oxo ligands which are
symmetrically bonded to six Mn(
III
) and six Mn(
II
) atoms whose octahedral coordination
is completed by the twelve bidentate bridging benzoate ligands.
89
