2 Homometallic Alkoxides

Homometallic Alkoxides 121
about 1.5 in boiling benzene, which suggests the presence of dimeric species. The
structure: (ˇ-dik)(OPr i )BuSn( -OPr i ⊳2SnBu(OPr i )(ˇ-dik) has been suggested for the
dimeric form, in which tin tends to attain the hexacoordinate state through isopropoxide
bridging.
The synthesis of a number of tin(II) ˇ-diketonates has been carried out 761 by the
reactions of tin(II) dimethoxide with the ˇ-diketones:
Sn⊲OMe⊳2 C 2⊲R 0 COCH2COR 00 ⊳ ! Sn⊲OCR 0 DCHCOR 00 ⊳2 C 2MeOH " ⊲2.228⊳
where R 0 , R 00 D Me; R 0 D Me, R 00 D CF3; R 0 , R 00 D CF3.
Whitley 762 showed that Ta2(OEt)10 reacted with excess benzoylacetone (bzac)
to give Ta(OEt)2(bzac)3 which may contain eight-coordinated tantalum. Mehrotra
and co-workers 763–767 have also investigated the reactions of niobium and tantalum
pentaethoxides with acetylacetone, benzoylacetone, dibenzoylmethane, methyl and
ethyl acetoacetates and ethylbenzoylacetate and only observed partial replacement in
these cases. It has been shown that the above reactions proceeded quite smoothly
up to the formation of the bis-derivative, and slowed down considerably beyond that
stage, but could be forced to trisubstitution for some ˇ-diketone or ketoester molecules;
similar results have been reported 768,769 in the reactions of U(OMe)5 with ˇ-diketones:
M⊲OEt⊳5 C xacacH ! ⊲EtO⊳⊲5 x⊳M⊲acac⊳x C xEtOH ⊲2.229⊳
M⊲OEt⊳5 C xCH3COCH2COOR ! ⊲EtO⊳⊲5 x⊳M⊲OCCH3CHCOOR⊳x C xEtOH
⊲2.230⊳
where M D Nb, Ta, U; R D Me, Et; x D 1–3.
These derivatives readily interchange their ethoxo groups with higher acohols. The
non-replaceability of the last two ethoxo groups by ˇ-diketonate ligands may be due
either to steric factors or to the inability of niobium and tantalum to increase their
coordination number beyond eight in these derivatives. These derivatives are generally
monomeric in boiling benzene and could be distilled unchanged in vacuo. Holloway563 has studied the variable temperature proton magnetic resonance of Ta(OR)4(acac)
derivatives which have been shown to be fluxional molecules.
4.6 Reactions with Carboxylic Acids and Acid Anhydrides
The reactions of metal alkoxides with carboxylic acids and acid anhydrides 628 can be
represented by Eqs (2.231) and (2.232):
M⊲OR⊳x C yR 0 COOH ! M⊲OR⊳x y⊲OOCR 0 ⊳y C yROH ⊲2.231⊳
M⊲OR⊳x C y⊲R 0 CO⊳2O ! M⊲OR⊳x y⊲OOCR 0 ⊳y C yR 0 COOR ⊲2.232⊳
The reaction of silicon ethoxide with acetic anhydride was investigated as early as
1866 by Friedel and Crafts 770 who reported the preparation of triethoxysilicon monoacetate.
The reaction has been reinvestigated by Post and Hofrichter 771 as well as by
Narain and Mehrotra. 772
In view of the difficulties 628 in the preparation of aluminium tricarboxylates from
aqueous systems, the first attempt to prepare a metal carboxylate from its alkoxide
appears to have been made in 1932 by McBain and McLatchie 773 from the reaction