2 Homometallic Alkoxides

Table 2.21 1 H NMR spectra of Al4(OPr i )12
Methyl protons chemical shift (υ)
Homometallic Alkoxides 91
Solvent Terminal Bridging Reference
Benzene 1.27 1.37 1.65 596
1.30 1.39 1.66 439
Cyclohexane 1.16 1.34 1.54 578a
Carbon tetrachloride 1.10 1.28 1.47 596
1.24 1 1.30 1 1.48 1 596
1.12 1.28 1.47 489
Chloroform-d 1.13 1.32 1.50 578
Carbon disulphide 1.09 1.26 1.43 578a
1 At 100 MHz; all other data at 60 MHz.
peaks of equal intensity ratio. According to Shiner et al., 489 the spectrum ought to have
shown three sets of peaks for methine protons.
1 H NMR studies by Oliver and Worrall 573 have convincingly demonstrated that the
asymmetric central aluminium in tetrameric Al4⊲OCH2R⊳12 (R D C6H5, 4-ClC6H4)
causes nonequivalence of the methylene protons with consequent appearance of an AB
quartet (JAB D 11 Hz). In the spectra of both the compounds the bridging CH2 groups
appear as well-defined quartets (υAB D 22.5 and 21.3 Hz, respectively) at 60 MHz,
but terminal methylenes gave unresolved singlets. However, at 220 MHz AB quartets
(υAB D 0.5 and 6 Hz converted to 60 MHz equivalents) have been observed even for
terminal methylene protons. The larger υAB values for the bridging CH2 groups could
be ascribed to their closer proximity to the asymmetric aluminium centre.
Ayres et al. 582 measured the 1 H NMR spectrum of trimeric aluminium tribenzyloxide
in carbon tetrachloride solution at 40 Ž C which showed a singlet due to terminal aryloxo
groups and a four line multiplet with intensity ratio 2:1 downfield relative to the TMS
due to bridging benzyloxo groups. No solvent or internal reference was suitable for
the measurement of temperature-dependent 1 H NMR spectra in the wide range of
temperature. However, benzene was used up to 70 Ž C and biphenyl in the temperature
range 70–170 Ž C. The spectra showed a broad peak below 70 Ž C, probably due to a
mixture of trimeric and dimeric species; the dimeric species appeared to dominate in the
range 85–140 Ž C. It was assumed that monomeric species may also exist above 170 Ž C,
but the 1 H NMR measurement was rather impracticable. Huckerby et al. 491 observed
a singlet and a quartet for aluminium tribenzyloxide, but the intensity ratio was found
to be 1:1 rather than 2:1 as observed by Ayres et al. 582 in their 1 H NMR spectrum.
On this basis Huckerby et al. 491 assumed the presence of equal numbers of terminal
and bridging benzyloxide groups which is a requirement of the tetrameric species.
Shiner et al. 489 examined the 1 H NMR spectra of super-cooled trimeric liquid
aluminium isopropoxide in various solvents, which showed the presence of a single
isopropoxide species with a chemical shift close to that observed for the terminal
isopropoxo groups of the tetramer. At low temperatures the trimer signals broadened
and, in some cases, split into two (approximately in 1:2 intensity ratio), consistent with
the requirements of a cyclic trimer (2-X).
Kleinschmidt 572 proposed an alternative structure for the trimer (2-XI) involving a
central five-coordinated aluminium bridged to two four-coordinated aluminium atoms.