6 CRITERIOS DE EVALUACIÓN DE LOS DIFERENTES MATERIALES DE LA PRESA

The ligand binding energies and the strain energies, as defined in the computational details section, are reported in Table 2.3. In general, the [CuII_{(Clip-Phen)] complexes have the} lowest ligand binding energies and the [CuI_{(Clip-Phen)Cl] complexes the highest, following the} net charges of the complexes. This fits with the elongated Cu-N bond lengths observed in the previous section. Furthermore, the complexes 2dft-6dft (Figure 2.1), characterized by short bridges, have higher complex ligand binding energies than complex 1dft. Complex 2dft exhibits the highest ligand binding energy with one exception: the ligand binding energy is lower for 2Bdft than for 5Bdft. A reasonable assumption to explain this higher energy obtained for 5Bdft is the steric hindrance of the chloride atom by the oxygen from the acetamide group of the bridge. Table 2.3 Ligand binding energies and strain energies (on ligand coordination) of complexes 1dft- 7dft. Complex Ligand binding energy (kJ mol−1₎ Strain energy (kJ mol−1₎ Complex Ligand binding energy (kJ mol−1₎ Strain energy (kJ mol−1₎ 1Adft -759 0 1Cdft -1892 0 2Adft -656 110 2Cdft -1818 95 3Adft -692 76 3Cdft -1867 58 4Adft -662 76 4Cdft -1838 56 5Adft -727 69 5Cdft -1920 53 6Adft -711 49 6Cdft -1892 35 7Adft -730 26 7Cdft -1922 11 1Bdft -232 0 1Ddft -707 0 2Bdft -194 48 2Ddft -631 74 3Bdft -204 38 3Ddft -673 43 4Bdft -215 36 4Ddft -645 42 5Bdft -180 35 5Ddft -696 45 6Bdft -214 21 6Ddft -686 24 7Bdft -214 5 7Ddft -709 3

In order to investigate the geometrical constraints imposed by the bridge of the Clip- Phen-based complexes, the strain energy (see computational details) was also calculated (Table 2.3 and Figure 2.3). The CuI _{structures 2A}dft_-7Adft _{show the highest strain energies, which can be}

understood because the difference between the geometries of these complexes and the tetrahedral environment of [CuI_(phen)

2] (1Adft) is substantial. Upon binding of the chloride, the

Chapter 2

geometry of 1Bdft changes drastically to a trigonal bipyramidal environment, comparable to the environment of complexes 2Bdft-7Bdft. As a result, the strain energies of the CuI_Cl– _complexes 2Bdft-7Bdft are much lower. The small variations in strain energy between the CuII _{and Cu}II_Cl– structures are ascribed to the minor structural differences between 1Cdft and 1Ddft. As expected, complex 2dft, holding the shortest bridge, has the highest strain energy, which can be explained by steric repulsions between the protons H1 and H2 of the phenanthrolines. Strain energies are lower for the complexes with a three-methylene bridge, i.e. complexes 3dft-5dft (Figure 2.1). No significant differences are observed between these three complexes, indicating that the functionalization of the bridge has no influence on the magnitude of the strain energy. A further increase of the bridge length leads to computed structures closely related to the one of complex 1dft. Thus, complexes 6dft and 7dft show markedly lower strain energies than complexes 2dft-5dft. In particular, complex 7dft displays a very low strain energy, which clearly indicates that the long bridge does not induce significant geometrical constraints. The coordination characteristics of 7dft are therefore very close to those of [CuI/II_(phen)

2] complexes.

Figure 2.3 Strain energies of complexes 2dft-7dft. The letters A, B, C and D symbolize the structures holding a CuI_{, Cu}I_{–Cl, Cu}II _{or Cu}II_{–Cl moiety, respectively.}

The effect of the bridge on the redox properties was estimated for these complexes, by calculation of the inner-sphere reorganization energy (see computational details). The redox properties of the complexes are important, because the compounds have to go through a full redox cycle during their cleavage activity. Table 2.4 displays the λred, λox and λi values of complexes 1dft-7dftin vacuo. Cu(phen)2 has a λred of 19 kJ mol−1 and a λox of 25 kJ mol−1. The sum of these components, i.e. λi, amounts to 44 kJ mol−1. Complexes 2dft-4dft, 6dft and 7dft have comparable inner-sphere reorganization energies; only minor differences are noticed. Complexes

3-Clip-Phen complexes studied by DFT 3dft and 4dft have a slightly lower inner-sphere reorganization energy, which can be expected, since the structural differences between the reduced and oxidized structures are less significant than for complex 1dft. Complex 2dft has a higher λi compared to compound 1dft, possibly due to the distorted CuI _{structure. Only the λ}

i value of complex 5dft is markedly higher than those of the other complexes. In general, the complexes bearing a chlorido ligand (1BDdft-7BDdft) have higher λi values compared to the compounds without chlorido ligand (1ACdft-7ACdft). The highly distorted CuI_Cl– _{structures of all the complexes are reflected by the large differences observed for} the λox values for the chloride-containing complexes. Consequently, it is not possible to draw any general conclusions from the differences in the λi values of the complexes with a chlorido ligand, other than that distortion by additional ligands can affect the redox properties rather drastically. Cyclic voltammetry experiments performed by Pitié et al.[22] _{indicated that the redox properties of} the complexes 2dft-7dft are analogous. The small differences noticed for the complexes without chlorido ligand reflect these experimental findings.

Table 2.4 Inner-sphere reorganization energies (kJ mol−1_{) of complexes 1}dft_-7dft_.

Complex λred λox λi Complex

+ Cl λred λox λi 1dft 19 25 44 1dft + Cl 26 50 76 2dft 22 23 46 2dft + Cl 37 40 77 3dft 23 18 41 3dft + Cl 19 60 79 4dft 21 17 38 4dft + Cl 22 34 55 5dft 25 30 55 5dft+ Cl 60 78 138 6dft 19 27 46 6dft + Cl 35 48 83 7dft 25 24 48 7dft + Cl 25 63 88