Chemistry Reference
In-Depth Information
The product [KZr
OBu
t
5
]
n
was actually isolated
41
,
182
in 1:1 molar reaction of
KOBu
t
and Zr
OBu
t
4
and characterized
182
by X-ray crystallography.
In view of the larger size of the central metal atoms, the stability of [KU
2
OBu
t
9
]
,
217
[NaCe
2
OBu
t
9
]
,
218
and [NaTh
2
OBu
t
9
]
219
can be easily understood.
In contrast to the stability of KZr
2
OPr
i
9
,
41
the comparative instability of simi-
larly synthesized KTi
2
OPr
i
9
was explained on the basis of the difficulty of the
smaller titanium (0.64 A) atom to accommodate six OPr
i
groups around itself like
zirconium (0.80 A). However, Veith
164
has been able to characterize X-ray crystallo-
graphically the identity of products such as KTi
2
OPr
i
9
(low temperature technique)
and of BaTi
3
OPr
i
14
as fTi
OPr
i
5
gBafTi
2
OPr
i
9
g;
140
the latter finding is of special
interest as it might finally throw light on the nature of products like K
2
Zr
3
OPr
i
14
41
and may lead to the isolation of similar derivatives, MTi
3
OPr
i
14
, of other divalent
metals like Ca, Sr, Zn, Cd, Ni, Co, and Cu.
The stability of monomeric [ClCufZr
2
OPr
i
9
g]
126
in contrast to the dimeric
[CdfZr
2
OPr
i
9
-Cl
g]
2
135
again illustrates the effect of the size of central metal atom.
Further, the monomeric nature of isostructural CdfM
2
OPr
i
9
gI derivatives (M D Zr,
Hf, Ti, Sn(
IV
)) can also be ascribed to the large size of I
160 - 163
compared to that
of Cl.
135
Although [ClSnfZr
2
OPr
i
9
g]
2
is also dimeric like [ClCdfZr
2
OPr
i
9
g]
2
,the
fZr
2
OPr
i
9
g
ligand binds Sn(
II
) in a bidentate manner
141
,
142
in place of the usual
tetradentate ligating mode
188
in the cadmium analogue; this difference has been
ascribed
141
to the presence of a lone pair of electrons on tin(
II
). The replacement
of Cl in [ClSnM
2
OPr
i
9
]
2
by C
5
H
5
(cyclopentadienyl) by interaction with NaC
5
H
5
led
142
to monomeric derivatives [
C
5
H
5
SnM
2
OPr
i
9
], as confirmed by
119
Sn NMR
spectra and X-ray crystallography.
An eight-coordinated barium derivative, [BafZr
2
OPr
i
9
g
2
]
131
has been crystallo-
graphically characterized; the structure consists of a “bow-tie” or “spiro” Zr
2
BaZr
2
unit wherein barium is eight-coordinated by two face-shared bi-octahedral fZr
2
OPr
i
9
g
units.
3.4.2.2 Tri- and bidentate modes of coordination
The small lithium ion (0.78 A) interacts with only three isopropoxo groups
131
of a
fZr
2
OPr
i
9
g
unit in contrast to the four utilized by its larger congeners like Na
C
(0.98 A) and K
C
(1.33 A). Lithium appears to be too small to span the distance between
isopropoxo groups on both zirconium centres (see Chapter 4). The fourth coordination
site on lithium is occupied by an isopropanol molecule, which is hydrogen bonded to
an oxygen of a terminal isopropoxo group on zirconium.
The structure of [SnfZr
2
OPr
i
9
-I
g
2
]
161
also shows a fZr
2
OPr
i
9
g
unit inter-
acting with Sn(
II
) in a tridentate fashion, which appears to result from the presence of
the stereochemically active lone pair of electrons on tin(
II
).
The electronic spectra of [CofZr
2
OPr
i
9
g
2
]
115
and [CufZr
2
OPr
i
9
g
2
]
122
show the
central transition metals Co and Cu to be hexacoordinate, indicating a tridentate
bonding mode of the fZr
2
OPr
i
9
g
ligand.
Although the size of lead(
II
) is larger (1.32 A) than that of tin(
II
) (0.93 A),
fZr
2
OPr
i
9
g
appears to bind the former in a bidentate (
2
) manner, as revealed
by the X-ray structure of [PbfZr
2
OPr
i
9
g
-OPr
i
]
2
with a “serpentine” rather than
“close” pattern
181
as exhibited by the [Sn
-OPr
i
3
Zr
OPr
i
3
] derivative in its crystal
structure. In fact, Caulton
et al
.
82
have shown that the reactions of Zr
OBu
t
4
and
M
OBu
t
2
(M D Sn, Pb) yielded
