2 Homometallic Alkoxides

122 Alkoxo and Aryloxo Derivatives of Metals
of aluminium sec-butoxide with palmitic acid. However, the end product obtained
was only dipalmitate mono-butoxide leading Gray and Alexander 636 to doubt the very
existence of tricarboxylates of aluminium. In view of his experience in the reactions of
aluminium alkoxides with ramified alcohols, 293 Mehrotra was able to demonstrate 637
that the reaction can be slowly driven to completion by continuous fractionation of
isopropyl alcohol formed during the third stage of reaction azeotropically with benzene:
Al⊲OPr i ⊳3 C 2RCOOH benzene
!
fast
1
2 [RCOO⊳2Al⊲ -OPri⊳2Al⊲OOCR⊳2 ⏐
C RCOOH⏐
�slow C 2Pr i OH "
Al⊲OOCR⊳3 C Pr i OH "
⊲2.233⊳
Rai and Mehrotra 774 later showed that the reaction of Al(OBu t ⊳3 with carboxylic
acids yields the tricarboxylates more readily, as in this case the intermediate product
Al(OBu t )(OOCR)2 is monomeric and continues to react in a facile manner with the
third mole of carboxylic acids. In fact the reactions of metal alkoxides with carboxylic
acids are facilitated thermodynamically since the carboxylate moiety tends to bind the
metal in a bidentate mode, thus:
R′C
In view of Mehrotra’s unique success, 637 the reactions of alkoxides of a number
of other metals and metalloids, Ti, 775–779 Zr, 780–783 Si, 628,770–772,784–786 Ge, 787 Ga, 185
Ln, 148,156,788 Fe(III), 283,641,642 V, 789 Nb, 790 Ta, 790 and Sb 791 have been studied extensively.
In most cases, however, homoleptic carboxylates could not be obtained, as the
intermediate heteroleptic alkoxide carboxylates M⊲OR⊳x y⊲OOCR 0 ⊳y tend to decompose
with increasing value of y yielding oxo-carboxylates and alkyl esters by intramolecular
coupled with intermolecular reactions as illustrated below in a few typical cases.
The reactions of titanium alkoxides with carboxylic acids can be represented by
Eqs (2.234)–(2.239):
O
O
M
Ti⊲OR⊳4 C R 0 COOH ! Ti⊲OR⊳3⊲OOCR 0 ⊳ C ROH ⊲2.234⊳
Ti⊲OR⊳4 C 2R 0 COOH ! Ti⊲OR⊳2⊲OOCR 0 ⊳2 C 2ROH ⊲2.235⊳
Ti⊲OR⊳4 C 3R 0 COOH ! Ti⊲OR⊳⊲OOCR 0 ⊳3 C 3ROH
⏐
�
⊲2.236⊳
ODTi⊲OOCR 0 ⊳2 C R 0 COOR ⊲2.237⊳
Ti⊲OR⊳⊲OOCR 0 ⊳3 C Ti⊲OR⊳2⊲OOCR 0 ⊳2 ! Ti2O⊲OR⊳2⊲OOCR 0 ⊳2 C R 0 COOR
⊲2.238⊳
Ti2O⊲OR⊳2⊲OOCR 0 ⊳4 C 2R 0 COOH ! Ti2O⊲OOCR 0 ⊳6 C 2ROH ⊲2.239⊳
where R D C2H5, ⊲CH3⊳2CH; R 0 D CH3, C4H n 9 ,C17H35, C21H43.
The products up to titanium dialkoxide dicarboxylates are quite stable. Higher
carboxylate products tend to decompose in a variety of ways depending on the relative