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

5.2 Experimental procedures
5.2.1 Kinetic experiments
Chapter 5
All reagents and chemicals were of analytical grade. [NbCl5]2 and acacH were used
as received from Sigma Aldrich and the solvent (MeOH) was dried and distilled prior
to use. Kinetic measurements were performed on Varian Cary 50 Conc UV/Vis
(slower reactions) or Applied Photophysics Stopped-flow (fast reactions)
spectrophotometers. Temperature control of the reaction solutions was maintained to
within ± 0.1 ˚C by means of a circulating water bat h system.
The kinetics involve time resolved scanning of a sample over a range of preset
wavelengths to determine absorption changes in this range. The sample in this case
would be a mixture of a metal complex solution and a ligand solution with known
concentrations. The software program Scientist 1 from Micromath was used to fit, by
mathematical calculations, the data to selected kinetic equations for single step
reactions as discussed below in Section 5.3 and in Chapter 3.
Stability tests of the reagent solutions were the first examinations performed in the
kinetic study. The solutions were scanned for a longer period of time than the studied
reactions occur, to establish that the reagents themselves do not undergo changes
such as decomposition, polymerisation or coordination while in solution, since these
additional reactions would significantly complicate the nature of the kinetics.
Solutions of [NbCl5]2 and acacH in methanol were monitored for ± 3 days, signifying
no noteworthy decomposition or interaction between the solvent and starting
reagents.
Reaction solutions were prepared to comply with pseudo first-order conditions. The
metal solution was prepared throughout to maintain a final concentration of 2.5 x 10 -5
M ([NbCl5]2 in MeOH), whereas the ligand solution was prepared as a stock solution
of 5 x 10 -3 M final concentration and dilution was carried out across a series of 5
solutions with minimum final concentration of 5 x 10 -4 M (acacH in MeOH). Fresh
solutions were prepared for separate days of kinetic experiments. Even though
stability of solutions were confirmed for ca. 24 hour periods, it was considered
unwise to utilize solutions older than that time.
1 MicroMath Scientist for Windows. Version 2.01. Copyright © 1986 – 1995, MicroMath Inc.
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