Theoretical Study for the Site Exchange Mechanism of Anionic 5-Coordinate Pt(II) Complexes with Halide, [PtX(hfac)2]- (X = Cl, Br, I, hfac = hexafluoroacetylacetonate)



1 Introduction

The exchange rates of aqua ligand in water have been reported for a variety of metal complexes[1]. In the case of mixed complexes these are affected by the supporting ligands, the so-called cis and trans effects. The order of the trans effect of halide ligands is well known to be I > Br > Cl > F, but few reports about the cis effect have been presented[2]. For example, however, the water exchange rate constant in [Pt(H2O)4]2+ was reported to be of the order of 10-4 s-1 using 195Pt nmr[3] and 17O nmr[4] spectroscopy, while that in [PtX(H2O)3]+ (X = halide), which contains two exchange routes, is unknown. One question is to what degrees halide ligands activate the water exchange reaction in the cis and trans positions, and the other question is what are the orders of the cis and trans effects of halide ligands, respectively? The answers of these problems are important to understand the biochemistry of Pt(II). Since it is difficult to determine separately the water exchange rates of the trans position to X ligand and the cis position, we chose the [PtX(hfac)2]- (X = Cl, Br, I; hfac = hexafluoroacetylacetonate) system as a model, which has a 5-coordinate distorted square pyramidal structure[5]. This structure is maintained in solution and the fast dynamic site exchange process has been observed in the temperature variable 1H and 19F NMR spectra. One of the oxygen donors occupies an apical position in the apically wounded hfac chelate; Oap has four possibilities to assault the positions in the square plane, X, Ocis-1, Ocis-2 and Otrans. Considering the coalesce mode of the 1H and 19F signals with temperature variation, it was found that two independent site exchange paths occur, in which Oap assaults the positions of Ocis-2 and Otrans, respectively. The line shape analyses were performed qualitatively, and the orders of cis and trans effects were determined as follows; cis effect; Cl > Br > I, and trans effect; I > Br > Cl. It is very interesting that the order of the cis effect is the reverse of that of the trans effect. In this paper the ab initio calculations to determine the optimal structures of [PtX(hfac)2]- and the trigonal bipyramidal transition state energies of each path are reported.

2 Calculations

It is well recognized that the B3LYP hybrid functional based on DFT (density functional theory), is useful for the quantum chemical calculations of transition metal complexes[6]. Therefore, geometry optimizations were performed with Gaussian 98 program package[7] at the B3LYP/3-21G level of theory, except Pt and X (X = Cl, Br, and I), for which LanL2DZ basis sets were adopted. The calculations for the transition state structure optimization were performed by using the STQN method which automatically generates a starting structure.

3 Results and Discussion

3. 1 Geometry optimization for possible structures of the initial states

We optimized three types of structures associated with the possible initial states for [PtX(hfac)2]-. They are shown in Figure 1, I-III, respectively. Their total electronic energies (in a.u) and the relative energies (in kcal/mol) are summarized in Table 1. Molecular Gibbs free energies were also calculated using the Gaussian 98 program package and the calculated values are summarized in Table 1. Comparison of the calculated electronic energies (and also calculated molecular Gibbs free energies) makes clear that the energy of structure III is much higher than that of structure I or II for all complexes concerned. This means that structure III is probably a local minimum state and we can omit this structure hereafter from the discussions. The electronic energies of structure I are slightly lower than those of structure II (X = Cl; 0.2 kcal/mol, X = Br; 0.4 kcal/mol, and X = I; 0.4 kcal/mol).

Figure 1. B3LYP optimized structures of [PtX(hfac)2]- (X = Cl).

Table 1. Calculated energies of [PtX(hfac)2]- isomers (I - III)
XStructureElectronic energyFree energy
a.u.DE (kcal mol-1)a.u.DG (kcal mol-1)

Table 2. Calculated bond lengths (A) and bond angles (degree) in [PtX(hfac)2]-
Calc (B3LYP)Exp[5]
X = Cl (I)X = Cl (II)X = Br (I)X = Br (II)X = I (I)X = I (II)X = Br (II)X = I (I)
Pt-Otrans -Ocis-1 -Ccis-1178.9177.2178.5177.7178.0177.4176.78179.74

Table 3. Calculated energies of transition states for ligand exchange reactions of [PtX(hfac)2]-
XElectronic energyFree energy
a.u.DE (kcal mol-1)a.u.DG (kcal mol-1)
Cl (I = initial state)-2005.3396030-2005.2685840
Cl (TS A, 162.3i cm-1)-2005.31930912.7-2005.24849112.6
Cl (TS B, 121.4i cm-1)-2005.32235910.8-2005.25055611.3
Cl (TS C, 113.3i cm-1)-2005.31137617.7-2005.23965318.2
Br (I = initial state)-2003.5620260-2003.4938350
Br (TS A, 165.2i cm-1)-2003.54117413.1-2003.47198413.7
Br (TS B, 102.6i cm-1)-2003.5471999.3-2003.4786139.6
Br (TS C, 104.3i cm-1)-2003.53566316.5-2003.46564517.7
I (I = initial state)-2001.7865740-2001.7193180
I (TS A, 169.8i cm-1)-2001.76530613.3-2001.69695014.0
I (TS B, 87.9i cm-1)-2001.7747297.4-2001.7065388.0
I (TS C, 95.3i cm-1)-2001.76312914.7-2001.69381216.0

There are too many optimized structure parameters for each complex. Therefore, we have summarized only some selected structural parameters in Table 2 with the corresponding experimental ones (X-ray crystal structure data). From Table 2, one can see that the agreement between the calculated values and the experimental ones is satisfactory. The calculated dihedral angles, Pt-Otrans-Ocis-1-Ccis-1 between the two planes containing Pt-Otrans-Ocis-1 and Otrans-Ocis-1-Ccis-1 for all complexes, are near 180°, which indicates that one hfac is almost coplanar with the coordination square plane. However the dihedral angle, Pt-Oap-Ocis-2-Ccis-2 was calculated to be smaller than 180°, which indicates that the plane of the other hfac and the plane containing Pt-Oap-Ocis-2 are bent down. From the X-ray structure it can be seen that, while this hfac plane bends away from the halogen ligand in the Br complex (Structure II), it bends toward the halogen in the I complex (Structure I). However, the following discussions associated with the effect of the halide ligand on the reaction mechanism are based on structure I as an initial state.

3. 2 Geometry optimization for transition state

The most interesting problem in theoretical works about the reaction mechanism is the calculation of the electronic structure and the geometry of the transition states appearing in the course of the reaction path. The coordination site exchange is attained through a pseudo-rotation path with minimal structural change between the square pyramidal and trigonal bipyramidal structures (see Scheme 1). There are two possible rotations, around the X-Pt-O2 axis or the O1-Pt-O3 axis. Because there are two possible directions for each rotation, four cases in total, which correspond to the substitution of O4 with O1, O2, O3 or X, respectively, need to be taken into consideration (Scheme 1, path A~D). But path D can be omitted since both O3 and O4 belong to one hfac ligand and it would be very difficult for them to take trans positions relative to each other. Because the energy of the structure with an apical X (Structure III) is greater than that of the structure with a basal X (Structure I) as mentioned above, path C corresponding to the basal-apical exchange of the X ligand need not to be considered either. In path A, apical O4 assaults the cis-2 position (O3) in the same hfac and in path B, O4 assaults the trans position (O2) in the other hfac. Accordingly paths A and B are models for the substitution reaction of the square planar [PtXO3] type complex by an O-donor ligand in the X-cis and X-trans positions, respectively. The optimized structures at the transition state in paths A, B and C (TS A, B and C) for Cl complex are shown in Figure 2. From the frequency analysis it was confirmed that these transition states correspond to the ligand exchange at the cis and trans positions, respectively.

Figure 2. B3LYP optimized structures of transition states for ligand exchange reactions of [PtX(hfac)2]- (X = Cl).

The energies of the trigonal bipyramidal transition states (TS A, B and C) are shown in Table 3. The order of the relative electronic energies in TS B is Cl > Br > I in accordance with the order of the trans effect. However, though the energy difference is not so large, the order of the energies at the transition state (TS A) is reversed, I > Br > Cl, which suggests that this is due to the cis effect. Because TS C for all halide complexes has a higher energy than the corresponding TS A or TS B, the basal-apical exchange of the X ligand should be an unfavorable energy path compared with those of oxygen donors in the hfac chelates.

4 Conclusion

From the DFT calculation based on the B3LYP/3-21G level calculations, the optimal structures of [PtX(hfac)2]- fit well with the crystal structures. The trigonal bipyramidal transition state energies of three possible paths, two of which actually seem to function, show that the order of the trans effect of halide ligands is Cl < Br < I, and that of the cis effect is Cl > Br > I, which is in accordance with the experimental results.


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