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

5.2.2 93 Nb NMR
Chapter 5
All 93 Nb NMR spectra were obtained in CD3OD on a Bruker 600 MHz nuclear
magnetic resonance spectrometer using a TBI 600 MHz S3 5 mm broadband probe.
Although 93 Nb is quadrupolar (I = 9/2; Q = -0.22 x 10 -28 m 2 ) the broadness of the
lines are compensated for by a wide niobium(V) chemical shift range (δ = -1500 –
500) and good sensitivity ( 1 H = 1; 93 Nb = 0.48). 2 The specific resonance frequency
for 93 Nb NMR is 146.93 MHz.
As discussed in Section 2.7.2, niobium(V) chloride forms various niobium(V)-chloro-
methoxo species when dissolved in methanol. By controlling the reaction conditions,
it is possible to isolate a single niobium(V)-chlorido-methoxido species. 3,4,5 The
formation of different species is dependent on the [NbCl5]2 concentration, storage
time and temperature. Experimental work done by Karaliota et al. 3 also indicated that
at low niobium concentrations, 0.185 M and lower, no bridging methoxides can be
observed and the niobium(V)-chlorido-methoxido species exists as a monomer in
solution.
Figure 5.1: [NbCl2(OMe)3]2 that exists as a dimer in solutions at 0.185 M and above. 3
In order to accurately predict the mechanism of any reaction, the correct molecular
structure of the starting materials have to be known. Subsequently, a 0.1 M sample
of [NbCl5]2 in deuterated MeOH was prepared for NMR studies, similar to that
conducted by Lee et al. 4
The results from our NMR study are presented in Figure 5.2 and will be discussed in
the following paragraphs. Only one niobium signal was observed at -822 ppm, that
2 J. Mason, Multinuclear NMR, Plenum Press, New York, 493, 1987.
3 A. Karaliota, M, Kamariotaki, D, Hatzipanayioti, Transition Met. Chem., 22, 411, 1997.
4 G. R. Lee, J. Crayston, Dalton Trans., 3073, 1991.
5 M. Schönherr, L. Kolditz, Z. Chem., 10, 72, 1970.
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