2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 261
Figure 4.30 Structure of [CefOC⊲CF3⊳2CH2C⊲Me⊳DNC2H4NDC⊲Me⊳CH2C⊲CF3⊳2Og2]
(H and F atoms omitted).
complex [CefOC⊲CF3⊳2CH2C⊲Me⊳DNC2H4NDC(Me)CH 2C⊲CF3⊳2Og2] (Fig. 4.30)
was obtained using the tetradentate functionalized diolate ligand obtained by template
condensation of ethylene diamine and 5,5,5-trifluoro-4-hydroxy-4-trifluoromethyl-2pentanone.
125 The structure involves a distorted square antiprism with the two diolate
ligands occupying meridional configurations. The triphenylsiloxo ligand gave rise to
the mononuclear [Ce(OSiPh 3⊳3⊲thf⊳3] 126 isostructural with the yttrium and lanthanum
analogues (Table 4.7). The unsolvated compound [Ce2⊲ -OSiPh3⊳2⊲OSiPh3⊳4] hasthe
familiar dimeric structure with a distorted tetrahedral structure. There appears to be
interaction between the ipso-carbon atom of one phenyl group of the bridging OSiPh3
group with a cerium atom (Ce ....C D 2.982⊲9⊳ ˚A). 117
2.7.3 Praseodymium
Some structures of praseodymium(III) complexes with fluorinated tertiary alkoxo
ligands have been reported. The tris-trifluoro-tert-butoxide adopts the trinuclear
structure [Pr3⊲ 3-tftb⊳2⊲ -tftb⊳3(tftb) 2](tftb D OCMe2⊲CF3)) 127 with the three metals
in an isosceles triangle capped above and below by two 3-OCMe2⊲CF3⊳ ligands. The
-ligands span the edges of the triangle, and one Pr atom, which has two terminal
alkoxo groups, is in a distorted octahedral coordination while the other two, with one
terminal ligand apiece, are five-coordinated. However, the five-coordinated metals are
close enough to fluorine atoms of CF3 groups (Pr....F, 2.756, 2.774 ˚A) to suggest
distorted six- and seven-coordination (Fig. 4.31). This trinuclear structure is clearly
related to the other [M3⊲ 3-OR⊳2⊲ -OR⊳3(OR) 4L2] structures (Fig. 4.5) but without
the neutral donor ligands L. With the even more acidic hftb group the ammonia
produced in a side reaction was captured by the praseodymiums to give the dimeric
ammine [Pr2⊲ -hftb⊳2(hftb) 4⊲NH3⊳4]. The centrosymmetric molecule has the typical
edge-shared octahedral structure (cf. Fig. 4.20) but with one hftb and one ammonia in
the Pr2O2 plane and the other hftb at right angles to the Pr2O2 plane and trans to the
second ammonia on each praseodymium. 128 This is a different arrangement from that
found in [Sc2⊲ -hfip⊳2(hfip) 4 ⊲NH3⊳4]. 103