2 Homometallic Alkoxides

Homometallic Alkoxides 27
The heteroleptic chloride tert-butoxide of rhenium Re3(OBut )6Cl3 was prepared by
Wilkinson et al. 172 by the reaction in THF of Re3( -Cl)3Cl6(thf)3 with NaOBut in 1:6
molar ratio:
THF
t
Re3⊲ -Cl⊳3Cl6⊲thf⊳3 C 6NaOBu ! Re3⊲OBu t ⊳6Cl3 C 6NaCl # ⊲2.56⊳
In an attempt to prepare Re3(OPri )6Cl3, Hoffman et al. 173,174 reacted Re3( -
Cl)3Cl6(thf)3 with NaOPri in 1:6 molar ratio in THF and isolated the X-ray
crystallographically characterized174 green complex Re3( -OPri )3(OPri )6. 1
3PriOH in a
very low (18%) yield. Naturally the unreacted Re3( -Cl)3Cl6(thf)3 had to be removed
from the reaction medium.
By contrast, an isopropyl alcohol-free product Re3( -OPri )3(OPri )6 has been prepared,
but again in a low (31%) yield according to Eq. (2.57):
THF
i
Re3⊲ -Cl⊳3Cl6⊲thf⊳3 C 9NaOPr ! Re3⊲ -OPr i ⊳3⊲OPr i ⊳6 C 9NaCl # ⊲2.57⊳
The yield of Re3( -OPr i )3(OPr i )6 (Eq. 2.57) could be improved (i.e. from 31%
to 53%) considerably by the addition of a few drops of acetone to the solvent of
crystallization. 174
The synthesis of simple generally insoluble alkoxides, M(OR)2 (M D Mn,Fe,Co,Ni,
Cu, Zn; R D Me, Et, Pr i ), was found not to be feasible owing to the difficulty of separating
them from the insoluble alkali metal (Na, K) chlorides (cf. preparation through
LiOR in Section 2.5.3). However, a soluble copper(II) fluoroalkoxide, Cu(ORf)2(py)2
where Rf D CH⊲CF3⊳2 or C(CF3)3, 175 has been synthesized as shown in Eq. (2.58):
CuBr2 C 2NaORf C 2py ! Cu⊲ORf⊳2⊲py⊳2 C 2NaBr # ⊲2.58⊳
where Rf D CH⊲CF3⊳2 or C(CF3)3.
In the absence of ancillary ligands such as C5H5, CO,PR3, andR2PCH2CH2PR2
(R D alkyl or aryl) there are relatively few stable platinum group metal (Ru, Rh, Pd;
Os, Ir, Pt) alkoxides 9,10,21 because these metals prefer softer donor ligands (relative
to the hard (oxygen) donor alkoxo groups) and the alkoxide ligands when bonded to
platinum group metals are labile to thermal decomposition (Section 2.1), typically by
a ˇ-hydrogen elimination pathway. However, with the use of some special (fluorinated
and/or donor-functionalized) type of alkoxo ligands, 21 the synthesis of hydrocarbonsoluble
and monomeric alkoxides of later transition metals including palladium(II) and
platinum(II) can be achieved (Eqs 2.59–2.62):
cis-⊲R3P⊳2MCl2 C 2NaOCH⊲CF3⊳2 ! cis-⊲R3P⊳2MfOCH⊲CH3⊳2g2 C 2NaCl #
⊲2.59⊳
⊲M D Ni, R D Et; 176 M D Pt, R D Ph 176 ⊳
(dppe)PtCl2 C 2NaOCH3
C6H6/MeOH
! (dppe)Pt⊲OCH3⊳2 C 2NaCl #
slight excess
177
(2.60)
2KOH
M M{OC(CF3)2CH2PPh2}2
2+ + 2 HOC(CF3) 2CH2PPh2 + 2H + (2.61)
where M D Co,Ni,Pd,Pt. 10,24