A solution and solid state study of niobium complexes University of ...

Chapter 5
of our results to that listed in Table 5.1 indicates the presence of three-coordinated
methoxido ligands under our conditions.
Table 5.1: Assignment of 93 Nb NMR signals to [NbCl5-x(OMe)x] species. 4
Solvent x = 0 x = 1 x = 3 x = 4 x = 5
Benzene 2.6 -497 -810 -1010 -1160
Toluene 2 -495 -810 - -1150
Methanol -0.5 -480 -820 -1015 -1155
Ethanol - -470 -830 -1020 -1180
Isopropanol - -470 -830 -1025 -1135
Acetonitrile - -495 / -560 -850 - -1160
x = number of coordinated methoxido species
Further experiments by Lee et al. 4 and others 3,5 indicated that there are no bridged
(polymeric) niobium species in solution at metal concentrations below 0.185 M.
Under our conditions in the kinetic studies, the final metal concentration after mixing
is 5 x 10 -5 M, much lower than these conditions. It is therefore postulated here that
the starting material in our kinetic studies is in fact a mono-nuclear niobium complex,
[NbCl2(OMe)3(MeOH)]. The presence of a coordinated methanol solvento species
in the niobium coordination sphere is the most obvious choice since the
concentration of methanol under our conditions is in the region of 25 M and since
charge balance is maintained in this manner. This bridge splitting by coordinating
solvents have also been illustrated for platinum(II) chloride olefin complexes. 6
Another aspect that is important to note here is that there is seemingly only one
reaction observed under the conditions of the NMR experiment when excess of
acacH is added to [NbCl2(OMe)3(MeOH)]. Later paragraphs would show that there
are in fact two reactions in this process which are only observed under conditions
where the concentrations of the starting materials are much lower.
6 M. R. Plutino, S. Otto, A. Roodt, L. I. Elding, Inorg. Chem., 38, 1233, 1999.
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