2 Homometallic Alkoxides

shown in Eq. (2.286):
O
Mo2(OEt)6
Homometallic Alkoxides 135
CO2 + ROH HOCOR
(ROCO)2Mo2(OR)4 (2.286)
Nickel and cobalt methoxides, M(OMe)2 (M D Ni, Co), are reported to be unreactive
towards carbon dioxide. However, it has been demonstrated that Cu⊲OMe⊳2 in the
presence of pyridine reacts with two moles of CO2 to yield the corresponding carbonate
(Eq. 2.287): 934
py/25°C
Cu(OMe) 2 + 2CO2 Cu(OCOMe) 2(py) (2.287)
Further the dimeric copper(II) acetylacetonate-methoxide was also found to take up
two moles of carbon dioxide in the presence of pyridine (Eq. 2.288): 934
(acac)Cu
Me
O
Cu(acac)
O
Me
py/CO2
O
O
O
2 (acac)Cu(OCOMe)(py) (2.288)
Tsuda et al. 935 also found that copper(I) t-butoxide in the presence of t-butyl
isocyanide or certain other ligands ⊲PEt3, PPh3⊳ reacted reversibly with carbon dioxide
to give the t-butyl carbonate complex (Eq. 2.289):
(Bu
benzene/20°C
benzene/reflux
tO)Cu(CNBut ) 2CNBut CO2 (ButOCO)Cu(CNBut + +
O
) 3
⊲2.289⊳
Titanium and zirconium tetra-alkoxides undergo insertion reactions of both organic
isocyanates936,937 and carbodiimides938 into M–O bonds, as shown by Eqs (2.290) and
(2.291):
M⊲OR⊳4 C xR 0 NCO ! M⊲OR⊳4 x fNR 0 C⊲O⊳ORgx ⊲2.290⊳
⊲M D Ti; 936,937 R D Et, Pr i , Bu t ;R 0 D Me, Et, Ph,˛-naphthyl.
M D Zr; 938 R D Pr i ;R 0 D Ph⊳
Ti(OPr i )4
2R′N C NR′ (Pri + O2)Ti{N-C NR′}2 (2.291)
where R 0 D p-tolyl. 938
Both types of reaction are reversible in the case of Ti⊲OR⊳4. 937
With niobium 939 and tantalum 940 penta-alkoxides, the degree of insertion of phenyl
isocyanate may be controlled, resulting in the formation of mono-, di-, tri-, tetra-, and
R′
OPr i