2 Homometallic Alkoxides

Bu t O
Bu t O
Bu t O
Th
O
O
Bu
O
t
But H Bu t
Th
OBu t
OBu t
OBu t
2 Py
Bu t O
Bu t O
Bu
O
t
Th
Py
Scheme 2.4
Bu
O
t
O
But Homometallic Alkoxides 83
OBu
Th
t Py
O
Bu t
OBu t
2 Py
But But Bu
O
Th
tO But Py
O
O
Py
It seems clear that octahedral coordination is the preferred coordination environment
around thorium for the t-butoxo ligand.
1 H NMR spectral studies 407 of Th2(OCHPr i 2 )8 in noncoordinating solvents (C6D6 or
C7D7) reveal the presence of two different alkoxide environments (broadened methine
resonances at υ 2.0 and 3.72 and a smaller sharp set of resonances at υ 1.75 and
3.38; the two overlapping sets of diastereotopic methyl group resonances are seen in
the region υ 1.0–1.2) consistent with dimer/monomer equilibrium in solution at room
temperature (Eq. 2.181a):
Th2⊲OCHPr i 2 ⊳8
⇀
↽ 2Th⊲OCHPr i 2 ⊳4 ⊲2.181a⊳
A detailed 1 H NMR investigation of the above system has been used to determine
both kinetic and thermodynamic parameters for the equilibrium. Thermodynamic
parameters for the equilibrium process are 1H Ž D 17 kcal mol 1 , 1G Ž D 5 kcal mol 1 ,
1S Ž D 40 cal deg 1 mol 1 .
Karraker et al. 553 recorded the 1 H NMR spectrum of dimeric uranium pentaethoxide
in CFCl3 and CDCl3 at 65 Ž C and observed a large signal at υ 2.26 and a small one at
υ 1.06 due to methyl protons and a large very broad signal at υ 20.2 and a smaller very
broad signal at υ 16.1 due to the methylene protons. Below 65 Ž C, the spectrum is
very complex and on the basis of the quoted results it was assumed that above 65 Ž C,
the product is dimeric whereas below this temperature it is oligomeric. The 13 CNMR
data are consistent with these structural characteristics. 554
The 1 H NMR spectrum of uranium pentaisopropoxide at 13 Ž C shows a doublet at
υ 1.26, a much larger broad signal at υ 1.78 and a very broad signal, which is just
detectable above the unavoidable noise, at υ 15. These results have been interpreted
in terms of the dimer/monomer behaviour of the isoproxide. At 65 Ž C, the high-field
signal splits into two equal signals and the dimeric species predominates below this
temperature. 553
In benzene-d6 the 1 H NMR spectrum 330 of U2(OBu t )8(HOBu t ) shows only one
broad Bu t signal at υ 1.5 and a broad signal at υ 13.0 due to the OH proton; the
intensity ratio of these two signals is ¾80:1. Variable temperature 1 H NMR studies in
toluene-d8 in the range C25 to 90 Ž C have been incapable of “freezing out” a static
structure.
Although 1 H NMR peaks in benzene-d6 at 22 Ž C of X-ray crystallographically characterized
complex U2I4(OPr i )4(HOPr i )2 are quite broad and paramagnetically shifted 109 as
is typical of U(IV) systems, 555,556 the above formulation is consistent with the observed
1 H NMR data: υ 32(br, CHMe2), 5 (vv br, CHMe2).