Experimental and Theoretical Study of the Kinetics of Dissociation in cis-[ RuCl 2 ( PP ) ( N-N ) ] Type Complexes

As reações de substituição [RuCl 2 (P-P)(N-N)] + L → [RuCl(L)(P-P)(N-N)] + Cl-, onde P-P = 1,4-bis(difenilfosfino)butano e N-N = 2,2 ́-bipiridina, 4,4 ́-dimetóxi-2,2 ́-bipiridina, 4,4 ́-dimetil-2,2 ́-bipiridina e 4,4 ́-dicloro-2,2 ́-bipiridina, L = piridina (py) ou 4-metilpiridina (4-pic), foram estudadas sob condições de pseudo-primeira ordem. As reações ocorrem por um mecanismo dissociativo e as constantes de velocidade nas reações de substituição aumentam com o aumento do pK a dos ligantes N-heterocíclicos e com a diminuição dos potenciais de oxidação do centro metálico. Quanto mais alta é a porcentagem de participação dos orbitais atômicos d do metal na formação do HOMO, conforme calculado pelo método DFT, mais fácil é a dissociação do cloreto da esfera de coordenação do complexo. Nos espectros de P{H} RMN da série de complexos de fórmula geral [RuCl(L)(P-P)(N-N)]PF 6 , há dois dubletos com Δσ < 1, o que é consistente com produtos formados pela dissociação do cloreto trans ao átomo de fósforo nos precursores.


Introduction
A phenomenon in which one ligand labilizes another trans to itself is known as the "trans effect". 1 According to the classical theory, this effect is strong when the metal is coordinated to p-acids, which are able to form bonds by back donation, accepting electron density from the metal center.In this case, the trans effect probably takes place because the p-acids generate a pathway for the removal of electron density from the vicinity of the trans ligands, thereby stabilizing the transition state and facilitating nucleophilic attack on the metal center.
Quantum chemical calculations, especially by the DFT method, have turned out to be a valuable instrument for describing the molecular properties of diverse transition metal coordination compounds.][9][10][11] This paper describes the use of this method as an aid to the investigation of the possible influence of the diphosphine and X-bipy ligands on the rate of dissociation of one chloride from the precursors, leading to the corresponding [RuCl(L)(P-P)(N-N)] + complexes.Differential pulse voltammetry was used to track the kinetics of the reactions.The DFT calculations were then employed with the objective of supporting the interpretation of the experimental rate constants found for the reactions.
Although the kinetic measurements and DFT calculations were conducted only with the ligands pyridine and 4-methylpyridine, any other pyridine derivative could be used with the same precursors to give similar results, since the controlling process is dissociative.In addition to the kinetic experiments and the theoretical calculations, the X-ray structure of [RuCl(4-vinylpyridine)(P-P)(bipy)]PF 6 is reported here.
The goals of this research were to determine the experimental kinetic data for the chloride dissociation in the cis-[RuCl 2 (P-P)(N-N)] series and to analyze how these data correlate with the results of DFT calculations.

Instrumentation
The NMR experiments were performed at 293 K on a Bruker 9.4 T spectrometer.The 31 P{ 1 H} NMR spectra were recorded in dichloromethane solutions at 161.98 MHz with H 3 PO 4 (85%) as external reference.Differential pulse voltammetry experiments (used for the kinetics measurements) were carried out with a Bioanalytical Systems BAS-100B/W electrochemical analyzer in dichloromethane containing 0.10 mol L -1 Bu 4 NClO 4 (tbap) (Fluka Purum) as electrolyte.The working and auxiliary electrodes were stationary Pt foils; a Lugging capillary probe was used and the reference electrode was Ag/AgCl.Under these conditions, ferrocene is oxidized at 0.43 V (Fc + /Fc).Cyclic voltammetry was used to measure the oxidation potentials of the complexes; the experimental conditions were the same as those employed for pulse voltammetry.
Substitution reactions were performed under pseudofirst order conditions (excess of the entering ligand L).The kinetic data were analyzed in terms of the system 17,18 where S (solvent) is CH 2 Cl 2 , and the equation: Values of k obs were obtained from the ln(C ∞ -C t ) vs. time plots.The solutions were prepared by dissolving the precursor (1.0 × 10 -3 mol L -1 ) in dichloromethane containing 0.1 mol L -1 tbap and then adding the N-heterocyclic ligand L (concentration of 2.0; 5.0; 10.0; 15.0; 20.0 and 40.0 × 10 -3 mol L -1 ).
Thermodynamic parameters were determined by inserting data in the Arrhenius and Eyring equations. 17,18or this purpose, the experiments were carried out at 5 °C steps in the temperature range of 5 to 25 °C.

X-ray crystallography
Crystals of [RuCl(4-vnpy)(P-P)(bipy)]PF 6 were grown by slow evaporation of a dichloromethane/diethyl ether solution.The crystals were mounted on an Enraf-Nonius Kappa-CCD diffractometer with graphite-monochromated Mo-K a (l = 0.71073 Å) radiation.The final unit-cell parameters were based on all reflections.Data were collected with the COLLECT program; 19 integration and scaling of the reflections were performed with the HKL DENZO-SCALEPACK software package. 20Absorption correction was carried out by the Gaussian method. 21The structure was determined by direct methods with SHELXS-97. 22The model was refined by full-matrix least squares on F 2 by means of SHELXL-97. 23All hydrogen atoms were stereochemically positioned and refined with a riding model.The ORTEP view shown in Figure 1 was prepared with ORTEP-3 for Windows. 24Hydrogen atoms on the aromatic rings were refined isotropically, each one with a thermal parameter 20% greater than the equivalent isotropic displacement parameter of the atom to which it was bonded.The data collection and experimental details are summarized in Table 1, and the selected bond distances and angles are given in Table 2.

Theoretical calculations
6][27][28] A PC with an Intel Dual Core processor (3.0 GHz), 4 GB of RAM memory and 160 GB of hard disk space was used for the these calculations.
The proximity of the doublets in the 31 P{ 1 H} NMR spectra of the complexes in the series [RuCl(L)(P-P)(N-N)]PF 6 (N-N = 2,2´-bipyridine and L = 4-methylpyridine, 4-phenylpyridine, pyridine and vinylpyridine), Δσ < 1, is consistent with both phosphorus atoms being positioned trans to the nitrogen atoms.This suggests that the chloride dissociated from the precursors, in this series, is always the one that is trans to a phosphorus atom.This is confirmed by the X-ray structures of the complexes. 7,29,300][31] In this work, we report the structure of [RuCl(4-vnpy)(P-P)(bipy)]PF 6 (Figure 1).
The X-ray structural analysis of the complex [RuCl(4vnpy)(P-P)(bipy)]PF 6 shows that the chloride is trans to one of the 2,2´-bipyridine nitrogens (N1) and the 4-vinylpyridine Scheme 1. Kinetics of dissociation of a chloride from cis-[RuCl 2 (P-P)(N-N)]PF 6 complexes (L = pyridine or 4-metylpyridine).  2 are in the range expected for ruthenium diphosphine complexes. 7,12-14,29-32Similar behavior was observed for the complexes containing 4-methylpyridine and 4-phenylpyridine, as previously reported. 7,29,30In all three complexes, the dissociated chloride was always the one trans to a phosphorus atom in the precursor cis-[RuCl 2 (P-P) (bipy)], as expected, given the stronger trans effect of the phosphorus atom and in accordance with the X-ray structures.
The electrochemical data for the cis-[RuCl 2 (P-P)(N-N)] complexes, obtained by cyclic voltammetry, are shown in Table 3.
Typical differential pulse voltammograms recorded during the kinetic experiments can be seen in Figure 2, which shows the consumption of the precursor (E ap ca.700 mV) and appearance of the product of the reaction (E ap ca.1.200 mV) over time.
The plot of k obs vs.
[py] (Figure 3) gives the dissociation constant of one chloride from the coordination sphere of the ruthenium center.The values for all four complexes tested in this experiment are shown in Table 4.    Figures 4A and 4B show the inverse linear relationship between the k diss values obtained for the cis-[RuCl 2 (P-P)(N-N)] complexes and their oxidation and half-wave potentials [calculated from (E 1 + E 2 )/2], while Figures 5A and  5B show the linear relationship between the k diss of the complexes and the percentage of contribution of the Ru d-orbitals to the electron density in the HOMO.
As can be seen in Figures 4 and 5, there is a good correlation between the values plotted for the substitution of the chloride by the pyridine ligand, suggesting that the higher the electron density of the metal center, the easier is the dissociation of the chloride from the coordination sphere of the complex.As can be observed from the plots in Figure 5, although the difference in the percentages of Ru(d) contribution to the HOMOs of the complexes is quite small, k diss increases linearly as the percentage of Ru(d) in the HOMO increases.It is worth pointing out that there is a better correlation between k diss and the electron density  contribution from the Ru d-orbitals to the HOMO than between k diss and the electron density in the metal center as a whole (not shown).][9][10][11] Moreover, as can be seen in Tables 3 and 5, the higher the percentage of the electron density of the Ru d-orbital in the HOMO, as calculated by the DFT method, the lower are the oxidation potentials of the metal center in the complexes.
Figure 6 shows a graphic representation of the HOMO and LUMO of the cis-[RuCl 2 (P-P)(bipy)] complex under study and Table 5 lists the electron density contributions of selected atoms to the HOMO of the complexes.
As can be seen in Table 5, in all four complexes the chloride trans to the phosphorus atom (Cl1) contributes more electron density to the HOMO of the compound than the chloride (Cl2) trans to the nitrogen atom.It is likely that this is a consequence of the stronger trans effect exerted by the phosphorus atoms than by the nitrogen atoms.Because of this, the Ru-Cl bond distances in the cis-[RuCl 2 (P-P)(N-N)] complexes for the chloride trans to the phosphorus atoms are always longer than those for chlorides trans to the nitrogen atoms of X-bipy ligands. 31It can also be seen from Table 5 that the HOMO electron densities on the two chlorides of each complex are different, allowing them to be dissociated selectively.Accordingly, the chloride trans to the phosphorus atom should be more labile, which was confirmed experimentally.Thus, in the reactions of the cis-[RuCl 2 (P-P)(N-N)] complexes with N-heterocyclic ligands, the substituted chloride is always the one that is trans to the phosphorus atom, as seen in the X-ray structures of the [RuCl(L)(P-P)(N-N)]PF 6 (L= 4-pic, 4-Phpy, py, 4-vnpy) complexes. 7,29,30The data in Table 5 show that the Ru d-orbital contributes more effectively to the HOMO of the complexes in the order: MeO-bipy > Me-bipy > H-bipy > Cl-bipy.This is the same order as that of the dissociation rates of the substitution reactions of the cis-[RuCl 2 (P-P)(N-N)] complexes with the N-heterocyclic ligands and also the same order as that of increasing halfwave oxidation potentials of the metal center (Tables 3  and 4).All these data lead to the conclusion that the rate of substitution reactions in the cis-[RuCl 2 (P-P)(N-N)] complexes increases with the electron density from Ru(d) located in the HOMO of the complex, and this electron density is expressed experimentally by the oxidation potential of the metal center in the compound.
The rate constant for the dissociation of the chloride from the coordination sphere of the metal center in [RuCl(NH 3 ) 5 ] + is 4.4 s -1 at 20 °C, higher than for the analogous dissociation in the cis-[RuCl 2 (P-P)(N-N)] species. 32This can be explained by the fact that in [RuCl(NH 3 ) 5 ] + there are only σ-donor ligands, making the  ruthenium softer, the Ru-Cl bond weaker and the chloride easier to be dissociated from the metal center.The k diss values reported here are also lower than those obtained for the trans-[Ru(NH 3 ) 4 (PPh 3 )(H 2 O)] 2+ complexes, for which the value 3.9 L mol -1 s -1 was found. 33he thermodynamic parameters for the reactions, calculated with the Arrhenius and Eyring equations, 17,18 are given in Tables 6 and 7.
Again, it is seen that the rate constants for chloride substitution in the cis-[RuCl 2 (P-P)(N-N)] compounds increase in the same order as that for decreasing activation energies and enthalpies, suggesting that, indeed, the breaking of the Ru-Cl bond in the complexes is the rate-determining step of these reactions.It should be mentioned that ΔS # for a dissociation mechanism should be positive; however, in the present reactions these values were negative.Certainly, the reason for this is that in these processes charged species were produced during the substitution reactions from the uncharged cis-[RuCl 2 (P-P)(N-N)] precursors.

Conclusions
The trans effect has long been recognized as a strong driving force for ligand substitution reactions in coordination complexes.It can exert a great influence upon the metal-to-ligand bonding and the lability of ligands within a complex.This effect was observed clearly in the dissociation reactions of cis-[RuCl 2 (P-P)(N-N)] complexes, in that only the chloride trans to the phosphorus atom of the 1,4-bis(diphenylphosphino)butane ligand was dissociated, even in the presence of an excess of the entering ligands, forming the cis-[RuCl(L)(P-P)(N-N)]PF 6 (L = N-heterocyclic monodentate ligand) products.The easier dissociation of the chloride trans to the phosphorus atom is consistent with the results of DFT calculations, which show that the contribution of the electron density of the chloride to the HOMO is higher when the Cl − ligand is trans to the phosphorus atom than when it is trans to the nitrogen atom in the cis-[RuCl 2 (P-P)(N-N)] complexes.Thus, the DFT calculations can be a useful tool, aiding the understanding of the reactivity of coordination compounds.
The ΔH # values of the dissociation reactions of the cis-[RuCl 2 (P-P)(N-N)] complexes are close to 40.0 kJ mol -1 and show a tendency to decrease with the increase in the pK a of the N-N ligands.The negative ΔS # values of these reactions are probably due to the charge generated in the [RuCl(L)(P-P)(N-N)] + products.a kJ mol -1 ; b J mol -1 K -1 .

Table 5 .
Percentages of contribution of selected atoms to the HOMO of cis-[RuCl 2 (P-P)(N-N)] complexes