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In-Depth Information
alkoxides involving tetragonally distorted octahedral copper. On the other hand, the
magnetic susceptibility measurements of copper dimethoxide over the wide range of
temperature (193 to 77
Ž
C) indicated a maximum around 13
Ž
C and exhibits anti-
ferromagnetic behaviour.
542
On the basis of the above observations, Adams
et al
.
542
concluded that copper(
II
) methoxide does not possess a distorted octahedral structure as
proposed by Brubaker and Wicholas;
221
instead it possesses a linear chain type polymer
involving methoxo bridges. A similar type of structure was earlier suggested for copper
halides,
601
which also showed magnetic susceptibility maxima similar to that observed
in copper(
II
) methoxide. As compared to dialkoxides, the chloride-alkoxides of copper
show higher magnetic moments. Thus monochloride monomethoxide and monochloride
monoethoxide have room temperature magnetic moments in the range 1.67 - 1.68 and
1.39 - 1.49 B.M., respectively.
542
The higher value of magnetic susceptibility observed
for the chloride-methoxide follows the Curie law with a moment
eff
D 2
.
30 B.M. per
copper atom. This differs from the value deduced from ESR measurements (
g
D 2
.
01).
These workers have interpreted the results in terms of the interaction of each unpaired
electron from pairs of ions to give a triplet ground state. The bromide methoxide
of copper on the other hand is almost diamagnetic,
600
and therefore it must involve
strong magnetic interactions between Cu
2C
ions (
i.e.
a singlet ground state). It may be
mentioned here that copper dibromide possesses stronger magnetic interactions than
the dichloride
601
and this behaviour is significantly enhanced on the replacement of a
bromide atom with alkoxide.
3.7
Mass Spectral Studies
Mass spectrometry has been a useful technique for elucidating the molecular complexity
of metal alkoxides, except for those cases which have extremely high molecular weights
or low volatility. There are three features of the mass spectrum that are important in
characterizing a metal alkoxide. First, the parent peak can give information regarding
the molecular weight of the compound. In cases where high-resolution mass spectral
data are obtained this can be an authentic piece of evidence for a specific molecular
formulation. However, care must be taken, to ensure that the observed peak in the mass
spectrum is not some fragment of larger species. Secondly, an analysis of the isotopic
distributions of the parent ion can give information as to the number of metal ions
present. Finally, the fragmentation pattern can provide valuable information as to the
type and number of ligands bound to the metal centres, particularly in cases of mixed
ligand-alkoxide derivatives. Thus mass spectrometry can potentially yield information
other than the molecular weight and/or molecular composition of the compound.
For alkali metal alkoxides the mass spectroscopic study is so far limited to lithium
tert
-
butoxide.
440
There were controversies
438
,
439
,
602
concerning the molecular complexity of
lithium
tert
-butoxide, but the appearance of fragment ions Li
6
OBu
t
5
OCMe
2
C
and
Li
6
OBu
t
5
C
in its mass spectrum (at 130
Ž
C) has finally established a hexa-
meric structure.
440
The parent molecular ion Li
6
OBu
t
5
OCMe
3
C
(an odd electron
species) on losing one methyl group would give rise to the even-electron species
Li
6
OBu
t
5
O
D
CMe
2
C
possessing an energetically favoured carbon - oxygen double
bond. Furthermore, mass spectroscopic studies by Chisholm
et al
.
28
in 1991 have also
confirmed that hexameric lithium
tert
-butoxide is extremely volatile, exhibiting not only
a molecular ion (M
C
480), but also fragments of the type [LiOBu
t
]
n
n
D 1 - 5). These
