III.3.1. systems [Sc(CH
3COO
-)
m]
(3-m)Studies performed before, showed that the DOTA chelator is the most suitable ligand for Sc(III) ion, since they form complexes, thermodynamically stable and kinetically inert [24].
The objective of this study is to understand the role of the acetate group from the DOTA chelator and also its contribution to the stability of the complexes formed between the Sc (III) and the chelating agent DOTA. DFT calculations are performed on systems [Sc(CH3COO)m]3-
m (m=1-5).
Two basis set were used in the calculation in the following order:BS1 and BS2.
The total binding energy was obtained according to the expression:
ΔE=E[Sc(CH3COO)m]3-m-E(Sc3+)-mE(CH3COO)- (m=1-5) (1.b)
The main results obtained with the two basis set as the total complexation energies, corrected with zero-point energies of the [Sc(CH3COO)m]3-m clusters, the (Sc-O) distance and the Mulliken charge for the Sc(III) ion are summarized in table 1. The optimized geometries of the [Sc(CH3COO)m](3-m) (m=1-5) are shown in Fig.1.
The results obtained showed that as the number of the acetate pendant arm increase until to 4, the binding energy of the [Sc(CH3COO)m](3-m) clusters, also the Mulliken charge decrease however the Sc-O distance increases. Beyond 4 acetate arm, the energy of complexation increases and the complex becomes less stable, which means that the presence of four carboxylic arms is ideal to form the most stable complex with Sc (III) ion. This is the case with the DOTA chelating agent.
The stability of the complexes formed is also related directly to the charge effect; the explanation for the above is given by the fact that more charge on an atom decreases stability and less charge on an atom increases stability. For a given stoichiometry, a structure with the highest reasonable symmetry was tried first, followed by desymmetrized structures, to obtain finally complexes with optimized the symmetries Cs, C2, C3 for m=1-3 respectively. However, C1 symmetry was obtained for both [Sc(CH3COO)m](3-m) (m=4-5).
[Sc(CH3COO)m](3-m)
(1.a)
Sc3++ m(CH3COO)- ΔE
154
Both basis sets used in the calculations, lead to satisfactory structures with same symmetries.
However, the BS2 basis set which gives the lowest energies in all calculations compared to the BS1 basis set, although the computation time is extended in the first case.
Table 1: Acetate binding energy, Sc-O distance and Mulliken charge for [Sc(CH3COO)m](3-m) cluster
complex symmetry BS1 BS2
ΔE (KJ/mol) R (Sc-O), Å Mulliken charge ΔE (KJ/mol) R (Sc-O), Å Mulliken charge Sc(CH3COO)1+2
Cs -2753.57 1.903 2.26 -2818.11 1.904 2.22
Sc(CH3COO)2+1 C2 -4198.48 2.02529x2 2.02534x2
1.94 -4320.11 2.026x2
2.027x2
1.95
Sc(CH3COO)3 C3 -4949.67 2.139x3 2.135x3
1.59 -5128.52 2.13635x3
2.13607x3
1.61
Sc(CH3COO)4-1 C1 -5118.92 1.962 1.997 2.161 2.295 1.949 4.188 3.552 4.167
1.61 -5333.50 1.953
2.302 2.157 1.962 2.010 4.185 3.472 4.193
1.62
Sc(CH3COO)5-2 C1 -5004.49 2.017 2.035 2.068 2.033 2.076 4.219 4.292 4.247 4.246 4.300
1.57 -5299.61 2.081
2.039 2.034 2.074 2.021 4.306 4.256 4.220 4.302 4.248
1.58
155 Figure 1: Structures of the [Sc(CH3COO-)m](3-m) cluster
III.3.2. systems [Sc(H
2O)
(n-m)(CH
3COO
-)
m]
(3-m)From the ab initio studies done before on [Sc(H2O)n]3+(n=1–10) clusters and systems [Sc(H2O)(n-m)(NH3)m]3+, we have found that replacing a water molecule with ammonia cluster does not really contribute to the stability of the Sc(III) complexes formed, in other words ammonia should bond less strongly than water.
In order to more understand the affinity of both acetate and water molecules to the Sc(III) ion, the idea here, is to realize some exchange reaction in the optimized Sc(H2O)83+
cluster, varying the number of acetate group in the octascandium (III) complex from 1 to 5 molecules, as is shown in the reactions:
(2.a)
Sc(CH3COO)1+2 (Cssym) Sc(CH3COO)2+1(C2 sym) Sc(CH3COO)3 (C3 sym)
Sc(CH3COO)4-1 (C1sym) Sc(CH3COO)5-2 (C1 sym)
[Sc(H
2O)
(n-m)(CH3COO)
m]3-m+m(H
2O) [Sc(H2O)n]3++m(CH3COO)-
ΔE
[Sc(H
2O)n’(CH3COO)m’]3-m‘
(2.b) Sc3++ n‘(H ΔE’
2O) +m‘(CH3COO)-
156
The total binding energy corresponding to these reactions are obtained according to the expressions:
ΔE=E[Sc(H2O)(n-m)(CH3COO)m]3-m +m E(H2O)- E[Sc(H2O)n]3+-m E(CH3COO)- (3.a) ΔE‘=E [Sc(H2O)n’(CH3COO)m’] 3+-E(Sc3+)-n‘E(H2O)-m‘E(CH3COO)- (3.b)
The principally results obtained such as the binding energy of the Sc(III) complexes, average Sc-O, distance and Mulliken charge are given in table 2. Theoretical structures of
[Sc(H2O)(n-m)(CH3COO)m](3-m) complexes are also shown in Fig 2.
The results obtained using both BS1 or BS2 level sets, showed that as the number of water molecules exchanged by acetate group increase, the binding energy of the
[Sc(H2O)(n-m)(CH3COO)m](3-m) cluster and the Mulliken charge of the Sc(III) ion decrease, which confirms that the acetate group contributes more effectively to the stability of the Sc(III) complexes formed compared to the water molecule. This was not really the case when water molecules are replaced by ammonia.
The decrease in the Mulliken charge can be explained by a charge transfer from Sc (III) ion to the neighboring atoms corresponding respectively to the water molecules or acetate groups related to the scandium.
When one or two water molecules were replaced with one or two acetate groups respectively, the complexes formed are located in the first coordination sphere. However when three water molecules were replaced with three acetate groups, one water molecule in the [Sc-(H2O)5- (CH3COO)3] complex migrate from the first to the second coordination sphere and connected to two water molecules and an acetate group from the first shell. Also when four water molecules were exchanged with acetate groups, one acetate group and one water molecule migrate from the first to the second coordination sphere of Sc(III) ion, and are connected to two water molecules and an acetate group from the first sphere.
157
Table 2: Acetate binding energy, Sc-O distance (a) from water molecule, (b) from acetate group and Mulliken charge for [Sc(H2O)(n-m)(CH3COO-)m] (3-m) cluster
complex BS1 BS2
ΔE (KJ/mol)
ΔE’
(KJ/mol)
R(Sc-O),Å Mulliken charge
ΔE (KJ/mol)
ΔE’
(KJ/mol)
R(Sc-O),Å Mulliken charge
(a) (b) (a) (b)
[Sc-(H2O)7(CH3COO)1]2+ -1266.12 -4247.27 2.256 2.211 2.317
2.482 2.268 2.355 2.383 1.934
2.63 -890.61 -3871.76 2.379 2.292 2.397
2.381 2.380 2.397 2.379 2.381
2.51
[Sc-(H2O)6( CH3COO)2] 1+ -1469.54 -4450.69 2.134 2.031 2.330 2.093 2.263
2.338 2.171 4.130
1.36 -1709.72 -4690.87 2.455 2.131 2.263 2.020 2.396
2.305 2.262
1.63
[Sc-(H2O)5( CH3COO)3] -2314.83 -5295.98 2.243 2.116 2.380 2.094 2.230 2.072 2.224
4.585
1.35 -2625.08 -5606.23 2.172 2.063 2.268 2.100 2.307 2.125 2.340
4.340
1.48
[Sc-(H2O)4( CH3COO)4] -1 -2470.91 -5452.06 2.124 1.996 2.212 2.056
2.235 2.117 4.075 3.807 5.054
1.49 -2803.23 -5784.38 2.126 1.992 2.238 2.058 2.210 2.119 4.096 3.814 5.039
1.58
158
Figure 2: Structures of the [Sc(H2O)(n-m)(CH3COO-)m] (3-m) clusters (C1 symmetry)