2 Homometallic Alkoxides

10 −1 /xM
12
10
−200 −100 0
[O, R = Me; x, R = Et; and ∆, R = Bu n ]
8
6
4
2
(b)
(a)
100
T (K)
Homometallic Alkoxides 99
200 300
Figure 2.8 Variation of magnetic susceptibility of [Fe⊲OR⊳3]3 with temperature.
It has been observed that the benzene solutions of d 5 iron trimethoxide and triethoxide
do not affect the magnetic moment at the room temperature and the value remains
around 4.40 B.M. which is independent of the concentration of the solute. Thus the
anomalous magnetic behaviour exhibited by iron(III) alkoxides is intramolecular in its
origin. In a detailed study on iron(III) alkoxides Adams et al. 599 interpreted the anomalous
temperature dependence of the magnetic susceptibility (Fig. 2.8b) of the trimeric
methoxide, ethoxide, and n-butoxide in terms of the spin–spin coupling between three
iron atoms building the Fe3 core of a model involving corners sharing FeO4 tetrahedra
to give the symmetry point group D3h. The paramagnetic behaviour exhibited by iron
trialkoxides is consistent with a spin quantum number of S D 5/2 foreachironatom
(d 5 high spin) and the magnetic behaviour corresponds to the J value in the spin–spin
Hamiltonian (D 2JS i Si being 15 K).
Similar temperature-dependent magnetic behaviour has also been observed in the
case of iron(III) halide alkoxides. 220 For example, iron monochloride dimethoxide
shows magnetic moments of 3.67 B.M. and 2.63 B.M. at 19 Ž Cand 176 Ž Crespectively.
Similarly, the magnetic moments per iron atom observed for Fe4Cl3⊲OMe⊳9
and Fe4Br3⊲OMe⊳9 at room temperature are 4.16 and 4.28 B.M., respectively, which
are less than the expected value of 5.90 B.M. for the spin only value for high-spin
iron(III) compounds. The magnitude of the variations of susceptibility with temperature
is consistent with the magnitude of the interaction J D 15 K and 13 K for chloride
and bromide alkoxides, respectively, which are comparable with the value J D 15 K
observed for trialkoxides, 599 and thus exhibits antiferromagnetism. A critical examination
of the above values of J indicates that the value is lower for the bromide alkoxide
than for the chloride alkoxide. It is well known that bromide bridges provide a more
efficient superexchange mechanism than chloride and in the above halide-alkoxides,
if the polymerization occurs through halide bridges, on this basis the J value should
be more for bromide alkoxide than for chloride alkoxide. However, the reverse order
was observed for the above derivatives, thereby indicating that the association occurs
through methoxo bridges with the halide groups occupying terminal positions.
5
4
3
2
1
m eff (B.M.)