2 Homometallic Alkoxides

Table 5.1 (continued)
Metal Oxo-alkoxides 393
⊲M–O⊳ 1 ⊲MOM⊳
Compound ⊲cm 1 ⊳ ⊲cm 1 ⊳ Reference
[Os2⊲O⊳2O2⊲OCMe2CH2NBu t ⊳2] 960 660 78
[U3⊲O⊳4⊲OBu t ⊳10] 931, 898 – 64
[UO2⊲OBu t ⊳2⊲Ph3PO⊳2] 861 (822) – 44
1 Values in parentheses are ⊲M– 18 O⊳ frequencies (cm 1 ).
be explored using 1Hand 13C, the oxo ligands using 17O, and the metal using a
range of metal nuclei ( 27Al, 119Sn, 207Pb, 89Y, 47,49Ti, 51V, 93Nb, and 183W). Fluxional
behaviour has been studied using variable-temperature (VT) studies and in many cases
the limiting low-temperature spectrum is in accordance with the known X-ray crystal
structure. Metals with I D 1
2 nuclei give useful spectra but the quadrupolar nuclei give
broad lines with obvious disadvantages. Although the 17O nucleus is quadrupolar the
chemical shift8 and line width are very sensitive to the oxo environment. Klemperer and
co-workers9–12 have demonstrated the value of 17O NMR in characterizing titanium
oxo-alkoxides. Some data on 17O chemical shifts are given in Table 5.2 relating specifically
to the oxo ligand. Unambiguous spectra were obtained by carrying out hydrolysis
with 17O-enriched water. Thus the MoDO terminal oxo ligands in MoO2⊲OR⊳2 and
MoO⊲OR⊳4 gave shifts in the 970–862 ppm range34 and the bridging oxo-ligands ( ,
3, 4) were identified in a number of titanium oxo-alkoxides. 9–12 The chemical shifts
for 4- and 5-O in several aluminium oxo-alkoxides have also been determined. 79
Heterometal oxo-alkoxides have also been studied using 17O NMR. In the cases of the
niobotungstate anions in ⊲Bu4N⊳3[Nb2W4O18⊲OMe⊳] and⊲Bu4N⊳3[Nb2W4O18⊲OBu t ⊳]
it was possible to identify to which metal atoms the different oxo ligands were
bonded. 80 Similar data have been reported for several more hexanuclear complexes
⊲NBu4⊳x [⊲MeO⊳MW5O18] x (M D Ti, Zr; x D 3. M D Nb, Ta; x D 2). 81 In these cases
the 6- 17O resonances were seen at 56 to 71 ppm (see Table 5.2). In the lead
oxo-alkoxides [Pb4⊲ 4-O⊳⊲OBu t ⊳6] and[Pb6⊲3-O⊳4⊲OBu t ⊳4] and the zirconium lead
heterometal oxo-alkoxide [Pb3Zr⊲ 4-O⊳⊲OBu t ⊳8] the relatively sharp 17O resonances
show the expected coupling to the 207Pb (22%) isotope. 22 The 17O resonances for
3-O(Ba, Ti) and 5-O⊲Ba2Ti3⊳ were in accordance with the X-ray crystal structure
of the hetero metal oxo-alkoxide [Ba4Ti13⊲ 5-O⊳6⊲ 3-O⊳12⊲OC2H4OMe⊳24]. 82
4.3 Mass Spectra
Mass spectral studies on metal oxo-alkoxides have sometimes given valuable analytical
data. As with metal alkoxides the parent molecular ion P D [Mx Oy⊲OR⊳z ] C is usually
of very low intensity and the strongest high mass fragment ion is often P Me, P R,
or P OR.
The appearance of P Me or P R is then a confirmation that the oxo-alkoxide has
retained the integrity of its Mx Oy core in spite of the disruptive effects of 70 eV electrons.
For example, although [Sc5O⊲OEt⊳13] gave a parent molecular ion 17 the pentanuclear
isopropoxo species [M5⊲ 5-O⊳⊲OPr i ⊳13] did not and were thus characterized
as (P OPr i )(MD Sc,Y)or(P 3OPr i )(MD Yb) 16a . Indium oxo-isopropoxide was