2 Homometallic Alkoxides

102 Alkoxo and Aryloxo Derivatives of Metals
observations are in accord with those of Schleyer and co-workers 603 and Hartwell and
Brown, 440 who had earlier demonstrated the hexameric form of LiOBu t in the vapour
phase. The derivative [LiOCMe2Ph]6 also exhibits a molecular ion in the vapour phase. 28
Although the derivative Ca[OC⊲Bu t ⊳⊲CH2OPr i ⊳2]2 is composed of dimeric units in
the solid state, 53 mass spectroscopic studies reveal that it is monomeric in the gas
phase.
The suggested tetrameric form 155,158 of some lanthanide isopropoxides [Ln⊲OPr i ⊳3]4
(Ln D Nd, Tb, Er, Lu) has been confirmed by their mass spectroscopic studies. 55
Although mass spectra 245,349–352 of none of the X-ray crystallographically characterized
345,349–352 mononuclear tertiary alkoxides (including fluorinated analogues) gave
parent molecular ions, they were interesting in showing the presence of some important
di- and or trinuclear species.
Mass spectral studies by Mazdiyasni et al. 604,605 of some tetravalent metal(loid)tert-amyloxides
and hexafluoroisopropoxides such as M⊲OAm t ⊳4 (M D Ti, Zr, Hf) and
M[OCH⊲CF3⊳2]4 (M D Ti, Zr, Hf, Si, Ge) have revealed the monomeric nature of
these compounds in the gaseous state, by the detection of parent molecular ions. The
appearance of weak parent molecular ion peaks in the mass spectra of tert-butoxides
appear to indicate: (i) the general instability of the molecular ion peaks resulting from
the successive losses of CH3 and C2H5 groups, and (ii) the thermal instability of these
products as is indicated by the presence of strong peaks in the spectra characteristic
of OH and H2O fragments at 17 and 18, respectively. The first peak observed for
metal hexafluoroisopropoxides is due to the loss of one fluorine atom from the parent
species M[OCH⊲CF3⊳2]4. A similar behaviour was observed earlier in fluorinated ˇdiketonates.
606 The m/z values observed for higher mass numbers appear to indicate the
general instability of the molecular ions as a result of the successive loss of CF3, CF2,
and C2F3 groups and the next observed peak is due to the loss of one OCH⊲CF3⊳2 group.
The recent mass spectral findings of Turevskaya et al. 607 on tetra-alkoxides
of zirconium and hafnium are usually in good agreement with their molecular
complexity. 113,298,432,548 However, the existence of a high intensity of M3O⊲OR⊳9 C
or M4O⊲OR⊳13 C ions in the mass spectra of [M⊲OR⊳4]n tends to indicate the complex
nature of such species. The X-ray crystal structure determination of [M⊲OR⊳4]n is
called for to solve such an ambiguity.
The fragmentation pattern for tantalum penta-alkoxides Ta⊲OR⊳5 (R D Me, Et, Pr i ,
CH2CH2OMe), in their mass spectra 81 is similar to that for alkoxides of zirconium
and hafnium, 607 niobium, 608 molybdenum, 609 and tungsten. 610 For example, at first the
loss of OR groups occurs, followed by elimination of the molecules of unsaturated
hydrocarbons (usually, with the same number of carbon atoms as R). However, the
decomposition of Ta⊲OMe⊳5 differs from that of other homologues by the loss of
HCHO molecules and H atoms. Elimination of ethers, R2O, which is accompanied
by the formation of the metal oxoalkoxide ions of the above mentioned alkoxides is
typical. In the absence of X-ray crystallographic data, considerable effort had been
directed earlier to throw light on the molecular complexities of aluminium trialkoxides
in different (solid, liquid, or vapour) states.
The tetrameric behaviour of aluminium triisopropoxide was confirmed from mass
spectral studies of [Al⊲OCHMe2⊳3]4 and [Al⊲OCDMe2⊳3]4 by Fieggen et al. 611 The
highest mass fragment was ascribed to Al4⊲OCHMe2⊳11⊲OCHMe⊳ C , in addition to
very intense peaks due to Al4⊲OCHMe2⊳11 C ,andAl4O⊲OCHMe2⊳9 C . Besides these