2 Homometallic Alkoxides

240 Alkoxo and Aryloxo Derivatives of Metals
characterized. 48 The structure consists of a triangle of Ba atoms capped above and
below the plane by two 3-OSiPh3 ligands and equatorially bridged by three -OSiPh3
ligands. One barium is four-coordinated by two 3-OSiPh3 and two -OSiPh3 ligands
whilst the others are five-coordinated, one having the terminal OSiPh3 ligand and the
other having the THF molecule. In view of the unusual coordination of the fourcoordinated
barium the possibility of agostic C....Ba interactions was investigated
but the closest approach to a phenyl carbon was 3.31 ˚A. The first barium alkoxide to
be structurally characterized was the mononuclear [BafOC2H4N⊲C2H4OH⊳2g2]EtOH
which is chelated by two quadridentate (O3N) ligands making the barium eightcoordinated.
49 The lattice ethanol molecules are involved in extensive hydrogen bonding.
Bulky tertiary alkoxides and siloxides give rise to dimeric species. Thus the
triphenylcarbinolate has the formula [Ba2⊲ -OCPh3⊳3⊲OCPh3⊳⊲thf⊳3]. 50 The molecular
structure is unsymmetrical with one five-coordinated barium having one terminal
OCPh3 ligand and one THF whilst the other barium has two terminal THF molecules.
The dimer is sustained by three -OCPh3 face-sharing ligands making each barium
five-coordinated (square pyramidal). With the very bulky tris-tert-butylsiloxide another
unsymmetrical dimer [Ba2⊲ -OSiBu t 3⊳3⊲OSiBu t 3⊳⊲thf⊳] is formed but in this molecule
the barium atoms are four-coordinated. 50 With the less sterically demanding hexafluoroisopropoxo
ligand (hfip) the pentanuclear complex [Ba5⊲ 5-OH⊳⊲ 3-hfip⊳4⊲ -
hfip⊳4⊲hfip⊳⊲thf⊳4⊲H2O⊳].thf was obtained51 (Fig. 4.15) having a distorted square pyramidal
array of barium ions encapsulating the 5-OH ligand. The apical barium has a
coordination number of 6 as have three of the basal bariums with the fourth having sixcoordination
owing to the additional molecule of coordinated water. Agostic CF....Ba
interactions are also featured. The mononuclear [Ba⊲OSiPh3⊳2(15-crown-5)(thf)] 46 is
isostructural with the strontium analogue. 46
Finally it is interesting to note that the tetranuclear complex [Ba4⊲ 3-
OBut⊳4⊲OBu t ⊳4⊲ButOH⊳8] has a structure based on the cubane unit Ba4⊲ 3-OBu t ⊳4. 52
Each barium ion is six-coordinated by virtue of terminal bonding to one tert-butoxo
ligand and two tert-butanol molecules with intramolecular hydrogen bonding.
Figure 4.15 Ba5( 5-O)( 3-O)4)( -O)4O6
core structure of [Ba5( 5-OH)( 3-hfip)4( -
hfip)4(hfip)(thf)4(H2O)], ⊲hfip D OCH⊲CF3⊳2;
C and H atoms omitted).