2 Homometallic Alkoxides

Homometallic Alkoxides 115
R D Pr i ⊲soluble⊳. M D Sn; x D 4; y D 1 ⊲soluble⊳, 2 ⊲insoluble⊳;
R D Pr i . M D Sb; x D 3; y D 1 ⊲soluble⊳;RD Et, Pr i ⊳
Interestingly, when the reaction of Ge(OPri⊳4 with HOCH2CH2NH2 was carried out
in 1:2 molar ratio, the product was not Ge(OCH2CH2NH⊳2; instead,
Ge(OCH2CH2NH)(OCH2CH2NH2)(OPri ) was obtained, and if the reactants were in
1:3 molar ratio, Ge(OCH2CH2NH⊳(OCH2CH2NH2)2 was the main product.
In these cases, the increasing reactivity of the amino group in aminoalkoxides may
O
be due to intramolecular coordination of the type: M
C2H 4,
rendering the
N
H2 amino hydrogens more reactive.
Tin alkoxides form cyclic derivatives with alkanolamines in equimolar ratio and
the same is the case with germanium alkoxides but silicon alkoxides show reactivity
restricted to the hydroxyl group only. The poor reactivity of silicon alkoxides may be
attributed to the fact that silicon prefers to involve its d-orbitals by -bonding whereas
Sn and Ge prefer -bonding. 702 This may be reflected in the order of estimated M–O
bond energies of Si–O, Ge–O, and Sn–O, which are 112, 85, and 82 kcal mol 1 ,
respectively. 472,702,703
A comparative study of the reactions of M(OR)4 (M D Si, Ge, Sn) with alkanolamines
indicated the following order of reactivity: Sn > Ge × Si.
The non-involvement of the –NH2 group in the formation of intramolecular N !
Si bonding has been substantiated by infrared516,696,704 and proton magnetic resonance696,705
studies.
4.4.2 Dialkanolamines
Reactions of metal alkoxides with dialkanolamines afford a variety of structurally
interesting products depending on the stoichiometric ratios of the reactants as illustrated
by Eqs (2.207)–(2.217):
M⊲OPr i ⊳2 C 2⊲HOC2H4⊳2NH benzene
! MfOC2H4⊳NH⊲C2H4OH⊳g2 # C 2Pr
C
i OH
⊲2.207⊳
where M D Mg, Ca, Sr, Ba. 706,707
M⊲OPr i ⊳3 C ⊲HOC2H4⊳2NH benzene
! ⊲Pr i O⊳MfOC2H4⊳2NHgC2Pr i OH ⊲2.208⊳
where M D Al, 694 Ga, 185 Sb, 708 Fe, 283 Y, Gd, Er, Yb. 148,156
M⊲OPr i ⊳3 C 2⊲HOC2H4⊳2NH
benzene
! M[f⊲OC2H4⊳2NHgf⊲OC2H4⊳NH⊲C2H4OH⊳g] # C 3Pr
D
i OH ⊲2.209⊳