2 Homometallic Alkoxides

88 Alkoxo and Aryloxo Derivatives of Metals
Interestingly, the 51 V chemical shift of oxovanadium trialkoxides of bulky and chiral
alcohols, VO⊲OR 0 ⊳3 (where R 0 D 1-admantanyl, endo-borneyl, and exo-norborneyl) 569
did not change in toluene, hexanes, and chloroform solutions from 1.0 to 500 mM; this
is possibly due to the tendency of the bulky alkoxo ligands to inhibit association. The
1 H NMR chemical shifts for the protons on the carbon ˛ to the oxygen shift from 1.3
to 1.5 ppm upon vanadylation. Protons on remote sites show a shift ranging from 0 to
0.4 ppm. The 13 C NMR spectra exhibit the carbon ˛ to the oxygen shift from 19 to
25 ppm downfield. All the other carbons shift less than 2 ppm. The broadening of the
signal for the ˛-carbon indicated the possibility of the occurrence of exchange process
at ambient temperature. Furthermore, the hydrolysis of the oxovanadium trialkoxides
have also been investigated in organic solvents using 51 V NMR spectroscopy. 569
3.4.7 Alkoxides of Group 6 Metals
A number of chromyl(VI) alkoxides have been investigated by NMR spectroscopy 233
and the observed data have been interpreted either in terms of a static dimeric structure
in solution or due to rapid exchange (on NMR time scale) between terminal and
bridging alkoxo ligands.
Mo2(OBu t )6 shows only a methyl signal in the 1 H NMR spectrum at 1.42 ppm
downfield from hexamethyldisiloxane. 360 Variable temperature 1 Hand 13 CNMRspectroscopic
studies 361 on Mo2(OR)6 (R D PhMe2C, Me3C, Me3CCH2) tend to support
the well-established formulation containing Mo Mo bonds for these alkoxides.
The diamagnetic tetrameric ethoxide Mo4(OEt)12 obtained by the reaction of
Mo2(NMe2)6 with ethanol (½6 equiv.) shows a very complex 1 H NMR spectrum
due to overlapping of several ethyl resonances. 361 In benzene-d6 the 13 Cf 1 Hg spectrum
at 40 Ž C shows eight methylene carbon signals in the intensity ratio 1:1:1:1:2:2:2:2,
consistent with the tetrameric, Mo4(OEt)12 structure.
The diamagnetic Mo2(OPr i )8 gives sharp NMR spectra 371 at room temperature. The
low-temperature limiting spectra ( 1 Hand 13 C) are consistent with the adoption of
a structure having a pair of bridging alkoxo ligands: [Pr i O3Mo( -OPr i )]2 (see also
Chisholm et al. 372 ).
The 1 H NMR spectrum of the diamagnetic W4(OMe)16 shows rigid behaviour on
the NMR time scale by depicting eight singlets, 367 consistent with the maintenance
of the centrosymmetric structure found in the solid state. It is noteworthy that the
tetrameric titanium ethoxide Ti4(OEt)16 dissociates in solution. The 1 H NMR spectrum
of W4(OEt)16 shows many overlapping methylene and methyl resonances.
Hexaalkoxides M(OR)6 (R D Me, Et, Pr n ,Pr i ,CH2CHDCH2) are only known for
tungsten (M D W) and 1 Hand 13 C NMR spectra of these compounds 570 in the temperature
range C90 to 90 Ž C exhibit in all cases only one kind of alkyl group.
Room temperature 1 H NMR spectra (in C6D6) forW2(OMe)10 and Re2(OMe)10
exhibit three sharp singlets in a 2:2:1 ratio consistent with an edge-shared bioctahedral
geometry. 429
3.4.8 Alkoxides of Group 13 Metals
The observations of Mehrotra 293 regarding the existence of aluminium alkoxides in
different oligomeric forms such as dimers, trimers, and tetramers depending upon the