2 Homometallic Alkoxides

Homometallic Alkoxides 87
septet was assumed to be due to monomeric Ta(OPr i )5 and the low-field septets due to
resolved terminal and bridging isopropoxo groups of dimeric Ta2(OPr i )10. The added
alcohol rapidly exchanged with the isopropoxy protons of the monomer but not with
the protons of the dimer, although the intensities of the latter were diminished. The
existence of a monomer/dimer behaviour in tantalum isopropoxide was supported by
the appearance of the spectrum in benzene solution which gave three doublets in
the range υ 1.23–1.28. The low-field doublets were in the intensity ratio 4:1 (due to
dimeric species) and their decrease in intensity with increase in temperature or decrease
in concentration with the simultaneous increase in intensity of the upfield doublet
(Figs 2.5 and 2.6) accorded with the conversion of dimer to monomer species. The
measurements of equilibrium constants at various temperatures indicate the equilibrium
constants were twice the values in benzene than in carbon tetrachloride.
The 1 H NMR spectrum of niobium pentaisopropoxide at room temperature indicates
a broad resonance and an average doublet which at low temperatures was resolved into
peaks of the dimer (at υ 1.25 and 4.90) and monomer (at υ 1.19 and 4.68). Although
the monomer/dimer equilibrium for niobium pentaisopropoxide also existed in benzene
solution, the value of the equilibrium constant at a given temperature was about 100
times greater for niobium than for tantalum isopropoxide, thereby demonstrating the
greater degree of dissociation of niobium isopropoxide into monomeric species.
The 1 H NMR spectra of niobium and tantalum tert-butoxides 562 gave single peaks at
υ 1.37 and 1.36, respectively which did not change with temperature or concentration.
The addition of tert-butyl alcohol gave a single coalesced signal indicating rapid alcohol
interchange, a behaviour which is in contrast to that observed in case of titanium tetratert-butoxide.
548 This abnormal behaviour was not clearly understood on steric grounds,
which should prevent the exchange between tert-butoxo groups and tert-butyl alcohol
much more in niobium or tantalum penta-alkoxides than in titanium tetra-alkoxide.
Riess and Hubert-Pfalzgraf 564,565 measured the temperature and concentration dependence
of the 1 H NMR spectra of niobium and tantalum pentamethoxides in nonpolar
solvents like toluene, carbon disulphide, and octane at ambient temperature and at
low temperatures ( 11 to 74 Ž C) and at different concentrations and confirmed that
at low temperatures and higher concentrations methyl protons having peak intensity
ratio 2:2:1 are obtained. This is consistent with the X-ray structure of [(MeO)4Nb( -
OMe)2Nb(OMe)4]. 566 The coalescence of these peaks occurs at higher temperature
(10 Ž C) and low concentration (0.01 M).
On the basis of variable temperature 1 H NMR studies Holloway 563 reasserted that
in dimeric niobium and tantalum alkoxides, the terminal–terminal alkoxide exchange
occurs at a faster rate than terminal–bridging exchange. 562
Oxovanadium alkoxides, VO(OR)3 (R D Me, Et, Pr i , Bu s , Bu t , CH2CH2F,
CH2CH2Cl, CH2CCl3), have been investigated by 51 V NMR studies, 567 which indicate
that the limiting (low concentration) υ⊲ 51 V⊳ values depend on the bulk of R (highest 51 V
shielding for Bu t ). Shielding decreases with increasing concentration (more pronounced
for smaller R groups), owing to the formation of oligomers involving alkoxo bridging.
Similar observations were also reported by Lachowicz and Thiele 568 in 1977 on
51 V NMR data for oxovanadium alkoxides. For example, the chemical shift for
oxovanadium trimethoxide changes more than 40 ppm when the concentration increase
from 1 mM to more than 50 mM.