Chemistry Reference
In-Depth Information
A contribution from the second resonance form should be indicated structurally by
the presence of relatively long M-O-Ar bonds and a concomitant shortening of the
M-O(“aldehyde”) bond length. For all metals, the M-O distance to aryloxide/alkoxide
ligands is considerably shorter than that found for simple donor keto groups. Analysis
of the data in Table 6.6 shows that for early d-block metals (
e.g.
Ti, Zr) the distance
to the aryloxide oxygen is comparable to that found for simple, terminal aryloxides of
these metals. The M-O (aldehyde) distances are 0.2 - 0.3 A longer. In contrast it can be
seen for the later transition metals that the two parameters are much more comparable.
This only partly reflects the decreased amount of aryloxide oxygen
-donation to these
metals and there appears to be a significant contribution from the alternative resonance
picture for these molecules.
In an interesting reaction, treatment of [AlMe(OC
6
H
2
Bu
t
2
-2,6-Me-4)
2
] with acetyl
chloride was found to yield the complex [AlMe(OC
6
H
2
Bu
t
2
-2,6-Me-4)(OC
6
H
2
Bu
t
-2-
Me-4-CMeO-6)] in which one of the aryloxide ligands was acylated.
255
The structural
parameters for this latter compound were discussed in terms of the above two resonance
forms. The structural parameters for both
˛
-and
ˇ
-hydroxy carbonyl ligands bound to
aluminium have been discussed.
256
6.1.3
Metal Salicylates
In nearly all derivatives, this ligand is di-anionic with both the aryloxide and carboxy-
late oxygen atoms bound to the metal centre (Table 6.7). In one structurally determined
example, the carboxylic acid group remains protonated and chelates to the metal
through the carbonyl function.
257
6.1.4
Metal Biphenolates and Binaphtholates
The straightforwardly resolved 2,2
0
-dihydroxy-1,1
0
-binaphthyl (binol) ligand is one of
the most important chiral auxiliaries in chemistry, and its application to organic synthe-
sis has been reviewed.
46
,
258
The unsubstituted binol has been used in a vast number
of asymmetric catalytic applications, although in some of these applications the exact
nature (molecularity) of the active species is uncertain.
259
In order to help control
solubility and increase the chiral impact and steric size of the ligand, various strategies
have been devised to introduce substituents at the 3,3
0
-positions.
260
,
261
These bulkier
ligands have also been applied to asymmetric syntheses and have been instrumental
in allowing the isolation and characterization of discrete molecular species. These bis-
aryloxide ligands can adopt a variety of bonding modes (Tables 6.8 and 6.9). Simple
chelation to a single metal centre leads to the formation of a seven-membered ring
with M-O-Ar angles in the 110 - 130
Ž
range. Alternatively, the ligands can bind in a
terminal fashion to two different, nonconnected metal centres. In this case the M-O-Ar
angles are less constrained and therefore tend to be larger than for chelating examples.
There are also examples of complexes where one of the aryloxides of a bi-phenolate
is bridging two metals while the other is terminally bound to one of the metal centres.
Much less common are situations in which both oxygen atoms of bi-phenolate or
bi-naphtholate ligands adopt bridging modes. It is also possible for only one of the
aryloxide oxygen atoms to be deprotonated.
262
