2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 267
Figure 4.34 Structure of [Th4⊲ -
OPr i ⊳6⊲OPr i ⊳10⊲py⊳2](ž D N; D O;
ž D Th; H atoms omitted).
(2.399 ˚A) is trans to a terminal isopropoxo group, which has a very short Th–O
(2.056 ˚A) bond. The coordinated isopropanol was trans to an iodide and appeared
to be hydrogen bonded to the iodide on the adjacent thorium ion. 147 Addition of
the strong Lewis base quinuclidine to the dimeric [Th2⊲OCHPr i 2 ⊳8] produced the
mononuclear adduct [Th⊲OCHPr i 2 ⊳4(quinuc)] with five-coordinated (TBP) thorium with
the quinuclidine occupying an axial site. 148 In [Th⊲OCHPr i 2 ⊳3I⊲C5H5N⊳2] the Th
is octahedrally coordinated with the alkoxo ligands in the facial configuration and
the pyridines in the cis configuration. 148 A most remarkable organometallic thorium
alkoxide is the hexanuclear complex [⊲C5H5⊳Th2⊲ -OPr i ⊳3⊲OPr i ⊳4]3 which has a ring
structure in which the pentahapto-cyclopentadienyls act as bridging ligands between
pairs of thoriums which in turn are linked facially by three -OPr i ligands. 149
2.8.2 Uranium
Only a few structures of homoleptic uranium alkoxides have been reported. The
hexamethoxide [U⊲OMe⊳6] is a mononuclear molecule with an octahedral UO6
framework. 150 The mixed valency [U(IV), U(V)] complex [U2⊲ -OBu t ⊳3⊲OBu t ⊳6] has
a face-sharing bi-octahedral structure. The U....U separation (3.549 ˚A) precludes
any metal–metal bonding and the fact that the U–O bond distances for one metal
are significantly shorter than for the other suggests that this molecule contains
isolated U(IV) andU(V) atoms. 151 The uranium pentaisopropoxide is also a dimer
[U2⊲ -OPr i ⊳2⊲OPr i ⊳8] with the edge-shared bi-octahedral structure (Fig. 4.2) (data in
Table 4.8, pp. 269–270). 151 The iodo compound [U2⊲ -OPr i ⊳2⊲OPr i ⊳2I4⊲Pr i OH⊳2] also
has the edge-shared bi-octahedral structure with the coordinated isopropanol ligands
trans to iodides and hydrogen bonded as in the analogous thorium compound. 152
The mononuclear bis-benzopinacolate complex with THF [U⊲OC2Ph4O⊳2⊲thf⊳2] hasa
distorted octahedral structure with the THF molecules cis to one another. 153 Structures
of [U⊲OCBu t 3 ⊳⊲BH4⊳3⊲thf⊳] and [U⊲OCBu t 3 ⊳3⊲BH4⊳] have been determined. 154 The