2 Homometallic Alkoxides

Metal Aryloxides 561
The thermal stability of the dialkyls [(ArO)2MR2] is a function of all three variables:
metal, aryloxide, and alkyl. With 2,6-di-tert-butylphenoxide the bis-benzyls for all
three metals undergo elimination of toluene and formation of cyclometallated products
generated by activation of tert-butyl CH bonds (see Section 5.1).
The migratory insertion of organic isocyanides (RN C) into the metal–alkyl bonds
of [(ArO)xMR4 x] species has been extensively investigated. 362 The initial product of
these reactions is typically the corresponding 2 -iminoacyl derivatives. The structural
and spectroscopic properties of these derivatives has been reviewed and discussed in
terms of the bonding of the iminoacyl function. 363 In the case of titanium the mono-
2 -iminoacyls [(ArO)2Ti( 2 -R 0 NCR)(R)] react with pyridine to generate the 2 -imines
[(ArO)2Ti( 2 -R 0 NCR2)(py)]. 364,365 The structural parameters for these compounds fit
an aza-titanacyclopropane bonding picture. Further reaction with nitrogen heterocycles
leads to elimination of imine (eneamine tautomers) and generation of low valent titanium
aryloxides. 366 For simple pyridine, radical-induced coupling of py groups leads
to a dimeric derivative of Ti(III). In the case of 4-phenylpyridine, a five-coordinate,
mononuclear compound [(ArO)2Ti(py-4Ph)3] is produced. Structural studies rule out
formulation as a d 2 –Ti(II)-containing molecule as the equatorial pyridine (tbp) has
clearly undergone one electron reduction. 367 The octahedral species [(ArO)2Ti(bipy)2]
has a cis arrangement of aryloxide groups. Here again it is probable that significant
amounts of electron density are residing on the bipy ligands. This uncertainty in formal
metal oxidation state is absent in the compound trans-[Ti(OAr)2(dmpe)2] wherethe
supporting donor ligands are redox innocent. 368
A series of bis- 2 -iminoacyl compounds [(ArO)2Zr( 2 -R 0 NCR)2] has been isolated by
insertion of two equivalents of isocyanide with the corresponding di-alkyls (Table 6.22).
Subsequent thermolysis leads to an intramolecular coupling with formation of enediamido
derivatives. 369,370 Related titanium compounds are generated directly without
isolation of the intermediate bis- 2 -iminoacyl. A non-planar conformation for the diaza-metallacyclopent-3-ene
ring has been shown both in the solid state and solution.
Variable-temperature NMR studies have allowed measurement of the barriers to flipping
of these folded rings. The formation of the CDC double bond by coupling at zirconium
metal centres has been kinetically studied and shown to be a first order, intramolecular
event. The use of differing aryl-substituted isocyanides showed the coupling reaction
to be accelerated by electron-withdrawing substituents. 371
The reaction of mixed alkyl, aryloxides of the group 4 metals with CO has not yielded
simple insertion products. However, final products are clearly the result of subsequent
reactivity of 2 -acyl intermediates. The compound [(ArO)2Ti( 2 -R 0 NCR)(R)]
undergoes reaction with CO to produce the non-planar oxa-aza-metallacycle
[⊲ArO⊳2TifOC⊲R⊳DC⊲R⊳N⊲R 0 ⊳g]. 362 Carbonylation of [(ArO)2MR2] in the presence of
pyridine remarkably leads to pyridine-di-methoxide complexes. The threo and erythro
isomeric mixture can be rationalized on the basis of nucleophilic attack of acyl groups
on coordinated pyridine ligands. 372,373
The insertion chemistry of the di-alkyls [Cp(ArO)Zr(CH2Ph)2] with organic isocyanides
also leads to mono- and bis- 2 -iminoacyl derivatives. Their spectroscopic
and structural parameters are very similar to the corresponding bis-aryloxides. 374
The reaction of the bis-benzyls [(ArO)2M(CH2Ph)2] with the Lewis acidic [B(C6F5)3]
leads to the zwitterionic species [(ArO)2M C (CH2Ph)f( 6 -C6H5CH2)B (C6F5)3g]. 375
Structural studies show the M–OAr distances to be among the shortest for Ti and