2 Homometallic Alkoxides

Table 2.20 (Continued)
Homometallic Alkoxides 79
Compound Solvent υ 1 ⊲ˇ-CH⊳ υ 1 ⊲˛-CH⊳ Reference
Mo4Cl3(OPr i )9 C7D8 1.97, 1.70, 1.58, 6.09, 5.91, 5.79, 1053
1.52, 1.44, 1.38, 4.52, 4.45 (septets)
1.15, 0.90, 0.89
(doublets)
W4(OPr i )12 C6D6 1.2–1.6 (br, overlapping 6.1 (br), 5.0 (sharp), a
resonance) 4.7 (br), 4.2 (br)
[W(OMe)6] Toluene – 4.51 (s) 5
[Al(OPr i )3]4 CCl4 1.28 ( 2); 489
1.47 ( 2) doublets;
1.12 (d)
[Al(OBu t )3]2 CCl4 1.23 (s), 1.5 ( 2) 489
[Sn(OBu t )3(OAc)(Py)] C5D5N 1.55 (s) – 644
Se(OCHMe2)4 C7D8 1.42 (d) 5.41 (septet) 146a
Te(OMe)4 C7D8 – 3.83 (s) 146a
Te(OCH2CH3)4 C7D8 1.26 (t) 4.09 (q) 146a
Te(OCH2Bu t )4 C7D8 – 3.72 (s) 146a
Te(OCHMe2)4 C7D8 1.22 (d) 4.62 (septet) 146a
1 Chemical shifts relative to internal TMS (υ D 0).
a M.H. Chisholm, D.L. Clark, K. Folting, J.C. Huffman, and M. Hampden-Smith, J. Am. Chem.
Soc., 109, 7750 (1987).
2 Relative to Me3SiOSiMe3 (υ D 0).
based on the observation that the downfield chemical shifts relative to trimethyl silicone
(TMS) follow the trend: 3–OR > 2–OR > terminal.
Apart from homometallic alkoxides involving only one type of metal atom, 1 H
NMR spectra have also proved of great significance in elucidation of structures of
heterometallic alkoxides (Section 3) involving more than one different metal atom.
This aspect of investigations will be presented in Chapter 3, whilst in this section
a brief account is given of the NMR (mainly 1 Hand 13 C) spectra of homo- and
heteroleptic metal alkoxides arranged group-wise.
3.4.2 Alkoxides of Group 1 Metals
The 1 H NMR spectrum of lithium tert-butoxide in carbon tetrachloride solution, 438
showed a singlet at υ 1.53 due to methyl protons of the tert-butoxo group, indicative
of either equivalent OBu t groups or fast exchange between different environments of
the associated molecules. The corresponding peak in tert-butyl alcohol appears at υ
1.23 and this shifting of the Bu t signal to lower fieldinLiOBu t led Bains to assume
a higher covalency of the Li–O bond in alkoxide than of the O–H bond in alcohol.
Variable temperature 1 H NMR studies on tert-butoxides of heavier alkali metals (K,
Rb, Cs) by Weiss et al. 443 have shown that the Bu t group rotates freely above 20 Ž C
but below this temperature the rotation is restricted. Variable temperature 1 Hand 13 C
NMR studies 28 have shown that the derivatives [LiOBu t ]n and [LiOCMe2Ph]6 exhibit
a single peak for the methyl groups at room temperature, which do not change on
cooling to 53 Ž C.
In benzene-d6 and toluene-d8 the complexes [MOBu t .Bu t OH]1 (M D K, Rb, Cs)
exhibit only a single 1 H NMR peak due to t-butoxo ligand environment 29 at room