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

Chapter 2
Table 2.7: Kinetic and thermodynamic parameters on the displacement reactions of complex
1 and 2 with Hacac. 86
Experiment kapp x 10 3
1
2
(s -1 )
T (K)
0.3 ± 0.2 293
0.6 ± 0.2 298
1.2 ± 0.2 303
2.1 ± 0.2 308
3.1 ± 0.2 313
0.1 ± 0.2 293
0.2 ± 0.2 298
0.5 ± 0.2 303
0.9 ± 0.2 308
1.5 ± 0.2 313
Ea
(kJ.mol -1 )
39
∆H ‡
(kJ.mol -1 )
∆S ‡
(J.mol -1 .K -1 )
90.3 ± 0.1 87.8 ± 0.1 -11 ± 4
105.5 ± 0.1 102.9 ± 0.1 31 ± 4
The kapp values were fit to the Arrhenius plot, kapp = Aexp(-Ea/RT), and the Ea values
for the two processes were calculated (Table 2.7). The activation parameters, ∆H ‡
and ∆S ‡ , were determined from the Eyring plot. The differences in the activation
energy of the two processes were attributed to the more stable bond between the
dbm ligand and the niobium atom in complex 2, through a delocalized π system due
to the phenyl rings of the ligand, than in complex 1 with the dpm ligand
In 2003 Aresta et al. 86 published an article on CO2 insertion into Nb-OR bonds of
various Nb(OR)5 (R = Me, Et, allyl) complexes. Their study also included the
absorption kinetics of CO2 insertion into Nb(OEt)5. As mentioned before, the niobium
penta-alkoxo complexes are dimeric in the solid state and in solution. They
conducted a 1 H NMR investigation and found that [Nb(OEt)5]2 undergoes a
dissociation equilibrium in solution. The dissociation equilibrium (Keq) was calculated
as 2.07 x 10 -4 .
Figure 2.18: Reaction of niobium penta ethoxide with CO2.
86 M. Aresta, A. Dibenedetto, C. Pastore, Inorg. Chem., 42, 10, 3256, 2003.