Chemistry Reference
In-Depth Information
with similar covalent characteristics, as confirmed by Nyholm
et al
.
185
in 1970. This
suggestion of
Pr
i
OH
2
K
-OPr
i
2
Al
OPr
i
2
adduct formation received confirmation
in 1993 from the X-ray structural elucidation of this low-melting polymeric unit by
Gilje
et al
.
38
In fact, these workers have shown that the unsolvated KAl
OPr
i
4
is a
white solid (m.p. 380
Ž
C) insoluble in noncoordinating solvents, whereas the adduct
KAl
OPr
i
4
.
2Pr
i
OH (m.p. 31
Ž
C) is soluble in aromatic solvents. The most intense peak
in the mass spectrum of this material corresponds to [K
2
Al
2
OPr
i
7
HOPr
i
]
C
and a major
fragment contains three K and two Al atoms, which might correspond to a product of the
composition K
3
Al
2
OPr
i
9
, isolated
35
in a 3:2 molar reaction of KOPr
i
and Al
OPr
i
3
.
Compared to KAl
OPr
i
4
, which dissolves in isopropanol to give a homogeneous solu-
tion, NaOPr
i
yields only a thick suspension of white powder in isopropanol.
The coordination sphere of K in [
Pr
i
O
2
Al
-OPr
i
2
K
.
2Pr
i
OH] was found to be
similar to that of magnesium
37
in [f
Pr
i
O
2
Al
-OPr
i
2
g
2
Mg
.
2Pr
i
OH]
2
. This latter
product along with a series of volatile soluble products with the formula, MfM
0
OR
4
g
2
(M D Mg, Ca, Sr, Ba; M
0
D Al, Ga; R D Et, Pr
i
) were isolated (Eq. 3.25) by the disso-
lution of metal M in alcohol in the presence of 2 moles of M
0
OR
3
. This comparatively
facile dissolution of bivalent metal in alcohol has been ascribed
16
to the coordina-
tion of alcohol to the M
OR
3
, rendering the proton more labile and reactive. The
mass and NMR spectra of MgfAl
OBu
s
4
g
2
liquid, and the X-ray structure
206
of
Mg
-OPh
2
Al
OPh
2
,confirm a similar coordination model. Table 3.1 lists some
representative tetra-alkoxoaluminate derivatives.
Compared to the polymeric insoluble and nonvolatile nature of homo-alkoxides of
bivalent metals [M
OR
2
]
n
in general (where M D
i
Mg, Ca, Sr, Ba (Eq. 3.25),
(ii) Be, Zn, Cd, Hg(
II
), Sn(
II
) (Eq. 3.35), and (iii) Mn(
II
), Fe(
II
), Co(
II
), Ni(
II
),
Cu(
II
) (Eq. 3.40)), their tetraalkoxoaluminate derivatives MfAl
OR
4
g
2
are volatile and
soluble in organic solvents, in which they exhibit monomeric (and in a few cases
slightly associated) behaviour.
Synthesized by the reactions (Eqs 3.35 and 3.40) of their anhydrous chlorides
with 2 moles of KAl
OPr
i
4
, their framework MfAl
OPr
i
4
g
2
(M D Mg, Zn, Cd, Hg)
appears to be so stable that even in 1:1 molar reaction of MCl
2
with KAl
OPr
i
4
,
only half the amount of MfAl
OPr
i
4
g
2
is obtained leaving the remaining MCl
2
unreacted. However, a product ClBefAl
OPr
i
4
g could be obtained in such a 1:1
molar reaction; this was converted by reaction with KOPr
i
into volatile monomeric
Pr
i
O
BefAl
OPr
i
4
g which could be distilled at ¾157
Ž
C/0.3 mm pressure. However,
this monomeric product tended to dimerize on standing for a few days as shown
by its molecular weight and
1
H NMR spectrum. This change could be ascribed
to the tendency of beryllium to attain the four-coordination state by assuming
a structure of the type: [
Pr
i
O
2
Al
-OPr
i
2
Be
-OPr
i
2
Be
-OPr
i
2
Al
OPr
i
2
].
207
This observation immediately led to the synthesis of the first heterotrimetallic
alkoxides
7
[
Pr
i
O
2
Al
-OPr
i
2
Be
-OPr
i
2
Zr
OPr
i
3
]and[
Pr
i
O
2
Al
-OPr
i
2
Be
-
OPr
i
2
Nb
OPr
i
4
] by the reactions of
Pr
i
O
2
Al
-OPr
i
2
Be
OPr
i
with Zr
OPr
i
4
and
Nb
OPr
i
5
, respectively.
The structural features of derivatives (iii) of later 3d transition metals,
MfAl
OPr
i
4
g
2
97
,
101 - 104
were elucidated by their UV-Vis spectra and paramagnetic
characteristics, techniques commonly employed in the conventional coordination
derivatives dealing with such 3d
5
- 3d
9
systems and explaining the observations with
the help of ligand field theory. For example, the selective exchange ability of branched
