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On the stability of the RuCl2(triphenylphosphine)2(amine) complexes: Ligand substituent effects of cyclic and acyclic amines
Polyhedron 81 (2014) 661–667
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On the stability of the RuCl2(triphenylphosphine)2(amine) complexes: Ligand substituent effects of cyclic and acyclic amines Rommel B. Viana a,⇑, Aguinaldo R. de Souza b, Benedito S. Lima-Neto a, Albérico B.F. da Silva a a b
Instituto de Química de São Carlos, Universidade de São Paulo, São Carlos, SP, Brazil Faculdade de Ciências de Bauru, Departamento de Química, Universidade Estadual Paulista Júlio de Mesquita Filho, Bauru, SP, Brazil
a r t i c l e
i n f o
Article history: Received 3 April 2014 Accepted 9 July 2014 Available online 24 July 2014 Keywords: Ruthenium complexes NBO Reactivity Charge distribution Vibrational frequencies
a b s t r a c t In this investigation we applied the Density Functional Theory to understand the substituent effects of cyclic and acyclic amines on the stability of RuCl2(PPh3)2(amine) complexes. In order to evaluate the relative stability of each complex, we analyzed five conformations considering different positions of the chlorine atoms and the triphenylphosphine ligand: a cis–cis (1), a trans–trans (2), a trans–cis (3), a cis–cis (4) and a cis–trans configuration (5). In addition, eight different amine ligands were considered: two acyclic (ammonia and trimethylamine), two cyclic aliphatic (piperidine and pyrrolidine) and four aromatic amine ligands (pyridine, pyrazine, pyrimidine and pyridazine). All the structures presented a square pyramid geometry, and in all systems the stereoisomer 3 is the most stable arrangement among the five isomers. Among the complexes with cyclic aliphatic amine ligands, the energy gap between arrangements 2 and 3 are the most sensitive to the substituent change. Furthermore, when it is considered the replacement by an aromatic amine, there are a large decrease in the energy difference between the arrangements 2 and 3. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, ring opening metathesis polymerization (ROMP) has been applied to the synthesis of several polymers with pharmacological applications [1,2], as well to the development of different medical materials [3–7]. RuCl2(PPh3)2(amine) complexes demonstrated to be a versatile precursor to the synthesis of new polynorbornenes [8–17]. Nevertheless, it is noteworthy to comment that there is any crystallographic data of these ruthenium complexes from our knowledge, mainly due to the difficulty of obtaining a single-crystal from these compounds. In addition, notwithstanding quantum chemical calculations has been emphasized as an important tool to understand different chemical properties in a molecular level [18–29], until now there is no computational analysis on the relative stability of these complexes. An important issue, which will be addressed here, is the conformational aspects of the RuCl2(PPh3)2(amine) complexes with the 31 1 P{ H} NMR spectroscopy in CDCl3 at 25 °C [8,12–17]. Matos et al. [8] detected two sharp peaks at 62.7 and 45.0 ppm indicating that the two PPh3 ligands were not trans-positioned, however after eleven hours the authors observed only a single peak which showed
⇑ Corresponding author. E-mail address: [email protected] (R.B. Viana). http://dx.doi.org/10.1016/j.poly.2014.07.028 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
that the two triphenylphosphines were equivalents. Cavalcante Jr. et al. [13] reported an opposite situation, where initially the authors observed a singlet and after 8.5 h was detected another singlet at 45 ppm indicating a cis-positioned of the PPh3 ligands, which was also seen in the case of other piperidine derivative complexes [13,17]. This cis-isomer is also emphasized due to the two Ru–Cl stretch modes in the infrared spectrum. In addition, these studies [8–17] also suggested that this unimolecular rearrangement may be a geometric change from a trigonal bipyramidal into a square pyramidal geometry, which were based in the FT-IR, the 31P{1H} NMR and the ROMP results. Finally, two important questions can be pointed out looking at these results. First, if there stereoisomers, which would isomer be the most stable one? The second question is: what is the geometry of the RuCl2(PPh3)2(amine) complexes, a trigonal bipyramidal or a square pyramidal geometry? With the aim to shed a light on the relative stability of the RuCl2(PPh3)2(amine) complexes, the purpose of this study is to apply the Density Functional Theory (DFT) to assess the most stable conformations. One of the primary goals of this work will be the characterization the PPh3 cis- and trans-positioned stereoisomers. As can be seen in Fig. 1, it will be considered the five possible isomers involving the different positions of the chlorine atoms and the triphenylphosphine molecules: a cis-cis (1), trans–trans (2), trans–cis (3), cis-cis (4) and a cis–trans (5) isomer. In this study we will also evaluate the effect of eight different amine ligands
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Fig. 1. The five isomers of the RuCl2(PPh3)2(amine) complexes, considering the different positions of the Cl atoms and the PPh3 molecules: the cis-cis (1), trans–trans (2), trans–cis (3), cis-cis (4) and the cis–trans (5).
Fig. 2. The eight different amines considered for analysis of the substituent effect on the RuCl2(PPh3)2(amine) complexes.
on the stability of these complexes (see Fig. 2): ammonia (n), trimethylamine (nt), piperidine (pip), pyrrolidine (pcpy), pyridine (pyr), pyrazine (pyzn), pyrimidine (rpy) and pyridazine (pydz). In this investigation will provide a comprehensive study that will confirm the most stable arrangement of the RuCl2(PPh3)2(amine) complexes and the energy gap between the different isomers, as well as the electronic properties of each ruthenium complex.
2. Methodology All the calculations were performed with the Gaussian 09 program [30]. Stationary points on the potential energy surface were fully optimized followed by evaluating harmonic vibration frequencies to characterize their nature as minima, and the absence of imaginary frequencies was the indicative that all optimized
structures were a true minimum. For the optimization procedure we applied the B3LYP functional [31,32], and the basis set employed for the ruthenium atom was the LANL2DZ one [33], while for the other atoms was used the 6-31G(d,p) basis sets [34,35]. In the analysis of the electronic energy was also employed the B3LYP function, nevertheless we used the LANL2TZ(f) [36,37] basis set for ruthenium and the 6-311++G(2df,2pd) basis sets [34,35] for the other atoms. For the atomic charge distribution we applied the Natural Population Analysis (NPA) [38], and to understand the bond order we used the Wiberg method [39]. The Natural Bond Orbital calculations was performed with the NBO6 program [40]. All the calculations were performed in the gas phase. Furthermore, it is important to explain that in the optimization procedure of each arrangement 5 was necessary to freeze some of the redundant internal coordinate definitions, which was required because in some situations during the optimization of 5
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the final structure converges for the isomers 2 or 4. Due to impositions in the optimization to obtain the arrangements 5, we can see small variations in their angles on the coordination sphere. 3. Results and discussions Considering RuCl2(PPh3)2(amine) structures, an important aspect among these complexes is the geometry. The several attempts to obtain a structure with a trigonal bipyramidal geometry always lead to one of the five isomers studied here with a square pyramidal arrangement, independently of the amine ligand. At this point, it is important to remember that all of the species studied here are 16-electron complexes, that would be expected to have octahedral geometries with a vacant coordination sphere, which corresponds to the observed square pyramidal geometry. This is in good agreement with the results of Hoffman and Caulton [41]. Furthermore, if we consider the Cl–Ru–Cl angle of the isomers 3 (157°–164°) in the coordination sphere, or even the Cl-Ru-N angle of the isomers 4 (152°–167°), these small angles can not be compared with the angles of a trigonal bipyramidal structure, where it would be expected angles ranging from 98° to 105° [41]. The same is also valid if we analyze the Cl–Ru–Cl angle of the isomers 2 (158°–178°), the Cl–Ru–P angles of the isomers 1 (150°–174°), or the Cl–Ru–N angles of the isomers 5 (162–166°). Closer inspection of the difference between the Cl–Ru–P angles, in the stereoisomers 1 are predicted high values for pyrrolidine, piperidine and trimethylamine ligand, following this respective order (see Table ST1, in Supplementary materials). In addition, analyzing the arrangement 2 with the NH3 ligand is observed a value of 178° for Cl–Ru–Cl angle, while with the pyridine ligand this value decreases to 158°. Nevertheless, if we compare the difference between the maximum and minimum value of the Cl–Ru–Cl angle, we have angles of 20° for the complexes 2, while among the arrangements 3 this value is 7°. On the other hand, examining the deformation in the square pyramidal base by the difference between the angles in the coordination sphere, it is observed that the stereoisomers 3 are the most susceptible to the nature of the amine ligand. A particular situation are the arrangements 5 where small changes are observed comparing the difference between the P–Ru–P angles and the Cl–Ru–N ones by the changes of the amine ligand. Moreover, we chose to not employ a continuum solvent effect because in the spectroscopy analysis of these complexes were used solvents with a small dielectric constants, as chloroform [8–17], and the effect of a continuum solvent like chloroform leads to very small changes in the electronic properties [42,43] and also show a very small influence on the stability of the isomers [44]. Concerning the structural changes in the ruthenium-ligand bond distances, due to the amine substitution ligand, it is interesting to see that the Wiberg bond order values are lower than one, indicating their r nature for the Ru–Cl (0.38–0.67), Ru–P (0.43–0.83) and Ru–N (0.19–0.64) bonds (see Tables ST2–ST7, in Supplementary materials). The highest variations of the ruthenium-chlorine bond lengths are seen for the complexes 1, while the smallest changes are observed for the structures 2. Taking a glance in the ruthenium-phosphorous bonds, it can be noticed that the largest variation are observed for the arrangements 3, whereas the ruthenium-nitrogen bond distances of the complexes 4 show the highest susceptibility to the amine substitution. Nevertheless, in some cases the Wiberg bond order presents values lower than 0.3 showing an increase in their ionic character. A particular situation is the trimethylamine complexes, where using the Natural Resonance Theory [45,46] it was confirmed a high ionic nature on their Ru–N bonds when we compared the values obtained from the other structures. In Fig. 3 is shown the relative energy profile among the five isomers with ammonia and the trimethylamine ligand. The n4 isomer
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resides 5.29 kcal mol 1 lower in energy than the n3 one, while in n1 and n2 was observed an energy difference above 10 kcal mol 1 when compared with the most stable isomer. Meanwhile, when we replace ammonia by trimethylamine these energy gaps greatly increases, where the difference between nt1 and nt3 is almost 23 kcal mol 1, and the gap between nt4 and nt3 is 15.4 kcal mol 1. When we compared the effect on the arrangements 2, due to the replacement of the trimethylamine ligand instead of ammonia, it is observed a decrease of 1.3 kcal mol 1 in energy relative to the most stable isomer. In the case of the trimethylamine ligand, it can be noted the absence of the stereoisomer 5. This happens because after several attempts to obtain the nt5 geometry, the result of the vibrational frequencies were two imaginary frequencies for this stereoisomer indicating that is not a stationary point. Fig. 4 presents the relative stability among the isomers with the pyrrolidine and piperidine ligands. Among the complexes with cyclic aliphatic amine ligands, the energy gap between arrangements 2 and 3 are the most sensitive to the substituent change, where the energy difference between pcpy2 and pcpy3 is 5 kcal mol 1 smaller than the difference between pip2 and pip3. Nevertheless, the energy profile among the other arrangements seems to be not affected by the replacement of pyrrolidine for piperidine, whereas these gaps between both ligands do not reach a value higher than 1 kcal mol 1. Regards to the comparison with the available 31P{1H} NMR experimental data of the RuCl2(PPh3)2(piperidine) complex [8,13–17], these results are in accordance with our computational chemical calculations. Comparing the energy gap between the PPh3 cis- and trans-positioned stereoisomers, the energy difference range from 7 to 15 kcal mol 1, while the highest value predicted for the gap between the trans- and the cis-arrangement is 7 kcal mol 1. These findings may be an explanation for the long time needed in the structural rearrangement detected by the experimental 31P{1H} NMR analysis [8,13,14,17]. Moreover, Silva Sa and Lima-Neto [14] demonstrated that the presence of free PPh3 in the solution, or even the free amine ligand, can favor the production of a PPh3 trans-positioned arrangement, or can inhibit the formation of a PPh3 cis-positioned stereoisomer. Fig. 5 demonstrates the trends on the relative stability among the ruthenium complexes when it is considered an aromatic amine ligand like pyridine and pyrazine, while Fig. 6 presents two other different aromatic amines as pyrimidine and pyridazine. A first glance in Fig. 5 reveals that the energy gap among the different stereoisomers shows only a slight difference when we take into account the replacement of pyridine by pyrazine, however these energy gaps are smaller than those seen in the acyclic amine ligands (Fig. 3), or even by the cyclic aliphatic amine ligands (Fig. 4). Another interesting feature from the values shown in Fig. 5 is that an aromatic amine substituent greatly decreases the difference in the relative energy of the arrangements 2 and 3. The energy gap between pyr2 and pyr3 is 4.1 kcal mol 1, whereas pyzn2 resides only 3.7 kcal mol 1 in energy above pyzn3. On the other hand, looking at Fig. 6 we can note that the change for a pyridazine ligand favors the rearrange of the stereoisomer 3 into the arrangement 2, where the gap between pydz2 and pydz3 is 1.8 kcal mol 1. Furthermore, the energy difference between pydz3 and pydz4 is 5.8 kcal mol 1. It is important to realize that, comparing the isomers with pyrimidine and those with pyridine, the variations do not lead to high values in the energetic analysis among the five arrangements where the difference in the energy values does not reach 2 kcal mol 1. In order to examine the impact of each arrangement on the bond nature in the coordination sphere, we also applied the three-center-four-electron r hyperbond search in the stereoisomers with NBO6 program. In this context, the hypervalent moiety refers to a bond beyond the reduced 12-electron valence space. In the isomers 1 and 4 were seen the P:–Ru–:Cl hyperbond, while in
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Fig. 3. The energetic profile (in kcal mol
Fig. 4. The energetic profile (in kcal mol
1
) on the relative stability among the isomers with ammonia and trimethylamine.
1
) on the relative stability among the isomers with pyrrolidine and the piperidine ligands.
the arrangements 2 and 5 were predicted the P:–Ru–:P hyperbond. A different situation is observed in the case of the stereoisomers 3, where we noticed the P:–Ru–:N hyperbond. Based on these results, we can see that the hyperbond found in each arrangement is basically associated with the trans effect. An important issue in the spectroscopy characterization of the unimolecular arrangement nature of these complexes is the ruthenium-chlorine stretching modes, which the predicted results can be seen in Table 1. Matos and Lima-Neto [10] commented the difficulty to characterize the position of the chlorine atoms by an infrared analysis, because the ruthenium-chlorine stretching frequencies are weak bands in the RuCl2(PPh3)2(amine) complexes. There are two important modes when we have in mind the RuCl2 modes, the symmetric [msym(RuCl2)] and asymmetric [mas(RuCl2)]
stretching. The authors [10] mentioned that in the case of the chlorine atoms with a trans- configuration is detected only one band, which is due to the very small intensity of the msym(RuCl2) mode (see Table ST8), while in the case of the chlorine atoms with a cis- configuration can be observed the two vibrational modes. In the stereoisomers 2 and 3 the mas(RuCl2) were predicted in 294– 308 cm 1, which is a very similar value when compared with the experimental result of Silva Sa and Lima-Neto [12], at 301 cm 1 [RuCl2(benzyldiphenylphosphine)2(piperidine) complex], and Carvalho Jr. et al. [13], at 320 cm 1 [RuCl2(PPh3)2(3,5-dimethylpiperidine) complex]. In the arrangements 1, 4 and 5 the separation between the two RuCl2 modes ranges from 12 to 44 cm 1, where we noted that this value is more susceptible to the arrangement of the complex than to the substitution of the amine ligand. Matos
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Fig. 5. The energetic profile (in kcal mol
Fig. 6. The energetic profile (in kcal mol
Table 1 The frequency (in cm Amine ligand
NH3 N(NCH3)3 Pyrrolidine Piperidine Pyridine Pyrazine Pyrimidine Pyridazine
1
1
) on the relative stability among the isomers with pyridine and the pyrazine ligands.
) on the relative stability among the isomers with pyrimidine and the pyridazine ligands.
1
) of the symmetric [msym(RuCl2)] and asymmetric RuCl2 [mas(RuCl2)] stretching modes for the RuCl2(PPh3)2(amine) complexes. 1
2
3
4
5
msym
mas
msym
mas
msym
mas
msym
mas
msym
mas
297.91 293.68 291.53 292.97 304.68 307.61 307.77 313.45
263.98 262.58 256.43 252.82 265.87 268.13 268.26 281.35
256.76 255.80 250.15 251.71 252.01 254.24 253.90 249.71
297.84 298.12 294.66 293.55 303.91 307.98 306.31 304.87
262.38 264.61 266.82 256.62 260.90 258.94 260.55 259.36
293.58 301.44 296.70 294.55 301.81 303.08 302.46 301.23
300.51 305.85 295.08 295.38 300.45 301.43 301.76 297.87
267.47 280.25 272.46 275.88 280.17 281.99 280.77 285.52
283.85 – 281.38 279.90 287.67 290.61 291.89 287.30
312.77 – 319.11 318.91 319.94 322.71 320.85 331.66
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and Lima-Neto [8] detected the separation between RuCl2 mode in 20 cm 1 which is in good agreement with the values predicted here. An interesting aspect in the arrangements 5 is that the mas(RuCl2) mode is localized with smaller values than the msym(RuCl2) one, while in the stereoisomers 4 was predicted the lowest values for the separation between the two RuCl2 modes. In addition, in the stereoisomers 1, 4 and 5 the relation between the intensities of the asymmetric and symmetric RuCl2 modes varies from 0.4 to 2.2 (see Table ST8, in Supplementary materials). Considering the importance of the reactivity index parameters, the calculations were performed based on the methodology of Pearson [47] and Parr et al. [48] (see Tables ST9–ST12, in Supplementary materials). The chemical hardness can give an important indicative of the resistance of compounds to change its electronic configuration, and according to this the arrangements 1 and 2 are the most affected by the amine substitution effect. Another reactivity parameter is the electronic chemical potential which shows the escaping tendency of electrons in the complex, and it is also associated with the electronic charge rearrangement associated to any chemical process. In this aspect, the stereoisomer 2 presents the higher electron-donor nature among the five isomers, independently of the amine ligand nature. Moreover, according to the electrophilicity (see ST12, in Supplementary materials) we can see that the values do not suffer much changes among the stereoisomers 3, 4 and 5, while in the arrangements 1 and 2 it was seen a decreasing in the electrophilicity index when we replace the acyclic amine by the cyclic aliphatic amine demonstrating an increase in the nucleophile nature of these stereoisomers, which become higher for the aromatic amine ligands. Comparing the reactivity parameter results reported here (Tables ST9–ST12, in Supplementary materials) with the ruthenium structures studied by Schott et al. [49], can be seen that the RuCl2(PPh3)2(amine) complexes present high values of hardness, nevertheless the electrophilicities are very small. Now we turn our attention to the atomic charge distribution in the coordination sphere (see Tables ST13–ST16, in Supplementary materials). The phosphorous and chlorine charges are not very sensitive to the changes in the amine ligand, independently of the arrangement. Observing the complexes with the phosphorous atoms in a trans- configuration, the phosphorous charge values are basically the same to both atoms, whereas in cis- the difference in the phosphorous charges are more visible for the arrangements 3 and 4 than for 1. Regarding to the ruthenium charge, the complexes 4 are the most susceptible to the nature of the amine ligand and, consequently, show the highest variations in rutheniumnitrogen bond orders. In addition, examining the ruthenium charge on the arrangements 4, the highest value was observed for trimethylamine, and the lowest one for ammonia. Notwithstanding the arrangements 1 and 2 are the most sensitive in the case of the nitrogen charge due to the substitution on the amine ligand, they are also the complexes with the lowest susceptibility on the ruthenium-nitrogen bond orders, demonstrating that this bond order are more sensitive to the behavior of the ruthenium charge than to the nitrogen one. There are some important results which comes out from the dipole moment among the ruthenium complexes (see Table ST17, in Supplementary materials). The highest susceptibility on the dipole moment value is seen among the complexes with the arrangement 2, followed by the arrangement 3. Moreover, the lowest value is predicted for the isomer 2 with the pyrazine ligand, 0.45, whereas the highest values is noticed among the structures with the arrangement 1. Meanwhile, a different result is obtained from the dipole moments of the stereoisomers 4, where this is the arrangement less affected by the amine substitution. Further insights can be obtained from the composition of the highest occupied molecular orbitals (HOMO) (see Table ST18, in
Supplementary materials). The HOMOs exhibit a mainly contribution from the Ru d-population and the p-electron population of chlorine (pCl). In the structures with the arrangement 1, in almost of all the cases their HOMO composition are represented by the eg-like d-orbitals of ruthenium (dz2 and dx2–y2), with a contribution ranging from 48% to 56%. The exception are the pyr1 and pydz1, where are observed a partial contribution from the t2g-like d-electron population of Ru (dxy, dxz and dyz). In the complexes 1, a decrease in the fraction of pCl was seen from cyclic aliphatic amine ligand and the trimethylamine one. Among the ruthenium complexes with the arrangements 2, 3 and 5, the d-population is mainly represented by the eg d orbitals, and only in some cases are noticed a mixed Ru t2g character, like in n3, n5 and pip5. In stereoisomers 2 and 3, the highest contribution from the Ru d-population to HOMO are observed by the complexes with the cyclic aliphatic amine ligands. Nevertheless, the structures with the major Ru d-population mixed, presenting a eg and t2g character, are those with arrangement 4, where the Ru d contribution to HOMO are not affect by the amine substituent.
4. Conclusion This study shed a light on the conformational aspects on the stability of the RuCl2(triphenylphosphine)2(amine) complexes by the substitution of the amine ligand. In this investigation all the structures presented a square pyramid geometry, and in all systems the stereoisomer 3 is the most stable arrangement among the five isomers. In addition, the r hyperbond search in the stereoisomers lead into hyperbonds that were consequence of the trans effect. An analysis of the Wiberg bond order shows values lower than one indicating their r nature in the coordination sphere, where the bond orders of the Ru–Cl bonds vary from 0.38–0.67, and the Ru–P and Ru–N ones vary from 0.43–0.83 and 0.19–0.64, respectively. In same cases, as in the trimethylamine complexes, the bond order is lower than 0.3 indicating an increase in their ionic character. In the stereoisomers 2 and 3, the mas(RuCl2) were predicted in 294–308 cm 1, whereas in the other arrangements the separation between the two RuCl2 modes ranges from 12 to 44 cm 1. Therefore we noticed a higher susceptibility to the different arrangements than for the substitution of the amine ligand. In the atomic charge distribution, in the coordination sphere, it was noted that the phosphorous and chlorine charges are not very sensitive to the changes in the amine ligand, or even affected by the arrangement. In regards to the ruthenium charge, the structures 4 are the most affected by the nature of the amine ligand and we found also the highest variations in ruthenium–nitrogen bond orders.
Acknowledgments The authors are grateful for the financial support given by the Brazilian agencies FAPESP, CAPES and CNPq. We would like to thank NCC/GridUNESP and CENAPAD/SP for the provision of computational facilities. RB Viana acknowledges FAPESP for the research fellowship (12/19175-2). We would like also to thank Valdemiro Pereira de Carvalho Júnior (UNESP), José Luiz Silva Sá (UESPI) and Camila Palombo Ferraz (USP/IQSC) for their important comments.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2014.07.028.
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On the stability of the RuCl2(triphenylphosphine)2(amine) complexes: Ligand substituent effects of cyclic and acyclic amines Rommel B. Viana a,⇑, Aguinaldo R. de Souza b, Benedito S. Lima-Neto a, Albérico B.F. da Silva a a b
Instituto de Química de São Carlos, Universidade de São Paulo, São Carlos, SP, Brazil Faculdade de Ciências de Bauru, Departamento de Química, Universidade Estadual Paulista Júlio de Mesquita Filho, Bauru, SP, Brazil
a r t i c l e
i n f o
Article history: Received 3 April 2014 Accepted 9 July 2014 Available online 24 July 2014 Keywords: Ruthenium complexes NBO Reactivity Charge distribution Vibrational frequencies
a b s t r a c t In this investigation we applied the Density Functional Theory to understand the substituent effects of cyclic and acyclic amines on the stability of RuCl2(PPh3)2(amine) complexes. In order to evaluate the relative stability of each complex, we analyzed five conformations considering different positions of the chlorine atoms and the triphenylphosphine ligand: a cis–cis (1), a trans–trans (2), a trans–cis (3), a cis–cis (4) and a cis–trans configuration (5). In addition, eight different amine ligands were considered: two acyclic (ammonia and trimethylamine), two cyclic aliphatic (piperidine and pyrrolidine) and four aromatic amine ligands (pyridine, pyrazine, pyrimidine and pyridazine). All the structures presented a square pyramid geometry, and in all systems the stereoisomer 3 is the most stable arrangement among the five isomers. Among the complexes with cyclic aliphatic amine ligands, the energy gap between arrangements 2 and 3 are the most sensitive to the substituent change. Furthermore, when it is considered the replacement by an aromatic amine, there are a large decrease in the energy difference between the arrangements 2 and 3. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, ring opening metathesis polymerization (ROMP) has been applied to the synthesis of several polymers with pharmacological applications [1,2], as well to the development of different medical materials [3–7]. RuCl2(PPh3)2(amine) complexes demonstrated to be a versatile precursor to the synthesis of new polynorbornenes [8–17]. Nevertheless, it is noteworthy to comment that there is any crystallographic data of these ruthenium complexes from our knowledge, mainly due to the difficulty of obtaining a single-crystal from these compounds. In addition, notwithstanding quantum chemical calculations has been emphasized as an important tool to understand different chemical properties in a molecular level [18–29], until now there is no computational analysis on the relative stability of these complexes. An important issue, which will be addressed here, is the conformational aspects of the RuCl2(PPh3)2(amine) complexes with the 31 1 P{ H} NMR spectroscopy in CDCl3 at 25 °C [8,12–17]. Matos et al. [8] detected two sharp peaks at 62.7 and 45.0 ppm indicating that the two PPh3 ligands were not trans-positioned, however after eleven hours the authors observed only a single peak which showed
⇑ Corresponding author. E-mail address: [email protected] (R.B. Viana). http://dx.doi.org/10.1016/j.poly.2014.07.028 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
that the two triphenylphosphines were equivalents. Cavalcante Jr. et al. [13] reported an opposite situation, where initially the authors observed a singlet and after 8.5 h was detected another singlet at 45 ppm indicating a cis-positioned of the PPh3 ligands, which was also seen in the case of other piperidine derivative complexes [13,17]. This cis-isomer is also emphasized due to the two Ru–Cl stretch modes in the infrared spectrum. In addition, these studies [8–17] also suggested that this unimolecular rearrangement may be a geometric change from a trigonal bipyramidal into a square pyramidal geometry, which were based in the FT-IR, the 31P{1H} NMR and the ROMP results. Finally, two important questions can be pointed out looking at these results. First, if there stereoisomers, which would isomer be the most stable one? The second question is: what is the geometry of the RuCl2(PPh3)2(amine) complexes, a trigonal bipyramidal or a square pyramidal geometry? With the aim to shed a light on the relative stability of the RuCl2(PPh3)2(amine) complexes, the purpose of this study is to apply the Density Functional Theory (DFT) to assess the most stable conformations. One of the primary goals of this work will be the characterization the PPh3 cis- and trans-positioned stereoisomers. As can be seen in Fig. 1, it will be considered the five possible isomers involving the different positions of the chlorine atoms and the triphenylphosphine molecules: a cis-cis (1), trans–trans (2), trans–cis (3), cis-cis (4) and a cis–trans (5) isomer. In this study we will also evaluate the effect of eight different amine ligands
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Fig. 1. The five isomers of the RuCl2(PPh3)2(amine) complexes, considering the different positions of the Cl atoms and the PPh3 molecules: the cis-cis (1), trans–trans (2), trans–cis (3), cis-cis (4) and the cis–trans (5).
Fig. 2. The eight different amines considered for analysis of the substituent effect on the RuCl2(PPh3)2(amine) complexes.
on the stability of these complexes (see Fig. 2): ammonia (n), trimethylamine (nt), piperidine (pip), pyrrolidine (pcpy), pyridine (pyr), pyrazine (pyzn), pyrimidine (rpy) and pyridazine (pydz). In this investigation will provide a comprehensive study that will confirm the most stable arrangement of the RuCl2(PPh3)2(amine) complexes and the energy gap between the different isomers, as well as the electronic properties of each ruthenium complex.
2. Methodology All the calculations were performed with the Gaussian 09 program [30]. Stationary points on the potential energy surface were fully optimized followed by evaluating harmonic vibration frequencies to characterize their nature as minima, and the absence of imaginary frequencies was the indicative that all optimized
structures were a true minimum. For the optimization procedure we applied the B3LYP functional [31,32], and the basis set employed for the ruthenium atom was the LANL2DZ one [33], while for the other atoms was used the 6-31G(d,p) basis sets [34,35]. In the analysis of the electronic energy was also employed the B3LYP function, nevertheless we used the LANL2TZ(f) [36,37] basis set for ruthenium and the 6-311++G(2df,2pd) basis sets [34,35] for the other atoms. For the atomic charge distribution we applied the Natural Population Analysis (NPA) [38], and to understand the bond order we used the Wiberg method [39]. The Natural Bond Orbital calculations was performed with the NBO6 program [40]. All the calculations were performed in the gas phase. Furthermore, it is important to explain that in the optimization procedure of each arrangement 5 was necessary to freeze some of the redundant internal coordinate definitions, which was required because in some situations during the optimization of 5
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the final structure converges for the isomers 2 or 4. Due to impositions in the optimization to obtain the arrangements 5, we can see small variations in their angles on the coordination sphere. 3. Results and discussions Considering RuCl2(PPh3)2(amine) structures, an important aspect among these complexes is the geometry. The several attempts to obtain a structure with a trigonal bipyramidal geometry always lead to one of the five isomers studied here with a square pyramidal arrangement, independently of the amine ligand. At this point, it is important to remember that all of the species studied here are 16-electron complexes, that would be expected to have octahedral geometries with a vacant coordination sphere, which corresponds to the observed square pyramidal geometry. This is in good agreement with the results of Hoffman and Caulton [41]. Furthermore, if we consider the Cl–Ru–Cl angle of the isomers 3 (157°–164°) in the coordination sphere, or even the Cl-Ru-N angle of the isomers 4 (152°–167°), these small angles can not be compared with the angles of a trigonal bipyramidal structure, where it would be expected angles ranging from 98° to 105° [41]. The same is also valid if we analyze the Cl–Ru–Cl angle of the isomers 2 (158°–178°), the Cl–Ru–P angles of the isomers 1 (150°–174°), or the Cl–Ru–N angles of the isomers 5 (162–166°). Closer inspection of the difference between the Cl–Ru–P angles, in the stereoisomers 1 are predicted high values for pyrrolidine, piperidine and trimethylamine ligand, following this respective order (see Table ST1, in Supplementary materials). In addition, analyzing the arrangement 2 with the NH3 ligand is observed a value of 178° for Cl–Ru–Cl angle, while with the pyridine ligand this value decreases to 158°. Nevertheless, if we compare the difference between the maximum and minimum value of the Cl–Ru–Cl angle, we have angles of 20° for the complexes 2, while among the arrangements 3 this value is 7°. On the other hand, examining the deformation in the square pyramidal base by the difference between the angles in the coordination sphere, it is observed that the stereoisomers 3 are the most susceptible to the nature of the amine ligand. A particular situation are the arrangements 5 where small changes are observed comparing the difference between the P–Ru–P angles and the Cl–Ru–N ones by the changes of the amine ligand. Moreover, we chose to not employ a continuum solvent effect because in the spectroscopy analysis of these complexes were used solvents with a small dielectric constants, as chloroform [8–17], and the effect of a continuum solvent like chloroform leads to very small changes in the electronic properties [42,43] and also show a very small influence on the stability of the isomers [44]. Concerning the structural changes in the ruthenium-ligand bond distances, due to the amine substitution ligand, it is interesting to see that the Wiberg bond order values are lower than one, indicating their r nature for the Ru–Cl (0.38–0.67), Ru–P (0.43–0.83) and Ru–N (0.19–0.64) bonds (see Tables ST2–ST7, in Supplementary materials). The highest variations of the ruthenium-chlorine bond lengths are seen for the complexes 1, while the smallest changes are observed for the structures 2. Taking a glance in the ruthenium-phosphorous bonds, it can be noticed that the largest variation are observed for the arrangements 3, whereas the ruthenium-nitrogen bond distances of the complexes 4 show the highest susceptibility to the amine substitution. Nevertheless, in some cases the Wiberg bond order presents values lower than 0.3 showing an increase in their ionic character. A particular situation is the trimethylamine complexes, where using the Natural Resonance Theory [45,46] it was confirmed a high ionic nature on their Ru–N bonds when we compared the values obtained from the other structures. In Fig. 3 is shown the relative energy profile among the five isomers with ammonia and the trimethylamine ligand. The n4 isomer
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resides 5.29 kcal mol 1 lower in energy than the n3 one, while in n1 and n2 was observed an energy difference above 10 kcal mol 1 when compared with the most stable isomer. Meanwhile, when we replace ammonia by trimethylamine these energy gaps greatly increases, where the difference between nt1 and nt3 is almost 23 kcal mol 1, and the gap between nt4 and nt3 is 15.4 kcal mol 1. When we compared the effect on the arrangements 2, due to the replacement of the trimethylamine ligand instead of ammonia, it is observed a decrease of 1.3 kcal mol 1 in energy relative to the most stable isomer. In the case of the trimethylamine ligand, it can be noted the absence of the stereoisomer 5. This happens because after several attempts to obtain the nt5 geometry, the result of the vibrational frequencies were two imaginary frequencies for this stereoisomer indicating that is not a stationary point. Fig. 4 presents the relative stability among the isomers with the pyrrolidine and piperidine ligands. Among the complexes with cyclic aliphatic amine ligands, the energy gap between arrangements 2 and 3 are the most sensitive to the substituent change, where the energy difference between pcpy2 and pcpy3 is 5 kcal mol 1 smaller than the difference between pip2 and pip3. Nevertheless, the energy profile among the other arrangements seems to be not affected by the replacement of pyrrolidine for piperidine, whereas these gaps between both ligands do not reach a value higher than 1 kcal mol 1. Regards to the comparison with the available 31P{1H} NMR experimental data of the RuCl2(PPh3)2(piperidine) complex [8,13–17], these results are in accordance with our computational chemical calculations. Comparing the energy gap between the PPh3 cis- and trans-positioned stereoisomers, the energy difference range from 7 to 15 kcal mol 1, while the highest value predicted for the gap between the trans- and the cis-arrangement is 7 kcal mol 1. These findings may be an explanation for the long time needed in the structural rearrangement detected by the experimental 31P{1H} NMR analysis [8,13,14,17]. Moreover, Silva Sa and Lima-Neto [14] demonstrated that the presence of free PPh3 in the solution, or even the free amine ligand, can favor the production of a PPh3 trans-positioned arrangement, or can inhibit the formation of a PPh3 cis-positioned stereoisomer. Fig. 5 demonstrates the trends on the relative stability among the ruthenium complexes when it is considered an aromatic amine ligand like pyridine and pyrazine, while Fig. 6 presents two other different aromatic amines as pyrimidine and pyridazine. A first glance in Fig. 5 reveals that the energy gap among the different stereoisomers shows only a slight difference when we take into account the replacement of pyridine by pyrazine, however these energy gaps are smaller than those seen in the acyclic amine ligands (Fig. 3), or even by the cyclic aliphatic amine ligands (Fig. 4). Another interesting feature from the values shown in Fig. 5 is that an aromatic amine substituent greatly decreases the difference in the relative energy of the arrangements 2 and 3. The energy gap between pyr2 and pyr3 is 4.1 kcal mol 1, whereas pyzn2 resides only 3.7 kcal mol 1 in energy above pyzn3. On the other hand, looking at Fig. 6 we can note that the change for a pyridazine ligand favors the rearrange of the stereoisomer 3 into the arrangement 2, where the gap between pydz2 and pydz3 is 1.8 kcal mol 1. Furthermore, the energy difference between pydz3 and pydz4 is 5.8 kcal mol 1. It is important to realize that, comparing the isomers with pyrimidine and those with pyridine, the variations do not lead to high values in the energetic analysis among the five arrangements where the difference in the energy values does not reach 2 kcal mol 1. In order to examine the impact of each arrangement on the bond nature in the coordination sphere, we also applied the three-center-four-electron r hyperbond search in the stereoisomers with NBO6 program. In this context, the hypervalent moiety refers to a bond beyond the reduced 12-electron valence space. In the isomers 1 and 4 were seen the P:–Ru–:Cl hyperbond, while in
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Fig. 3. The energetic profile (in kcal mol
Fig. 4. The energetic profile (in kcal mol
1
) on the relative stability among the isomers with ammonia and trimethylamine.
1
) on the relative stability among the isomers with pyrrolidine and the piperidine ligands.
the arrangements 2 and 5 were predicted the P:–Ru–:P hyperbond. A different situation is observed in the case of the stereoisomers 3, where we noticed the P:–Ru–:N hyperbond. Based on these results, we can see that the hyperbond found in each arrangement is basically associated with the trans effect. An important issue in the spectroscopy characterization of the unimolecular arrangement nature of these complexes is the ruthenium-chlorine stretching modes, which the predicted results can be seen in Table 1. Matos and Lima-Neto [10] commented the difficulty to characterize the position of the chlorine atoms by an infrared analysis, because the ruthenium-chlorine stretching frequencies are weak bands in the RuCl2(PPh3)2(amine) complexes. There are two important modes when we have in mind the RuCl2 modes, the symmetric [msym(RuCl2)] and asymmetric [mas(RuCl2)]
stretching. The authors [10] mentioned that in the case of the chlorine atoms with a trans- configuration is detected only one band, which is due to the very small intensity of the msym(RuCl2) mode (see Table ST8), while in the case of the chlorine atoms with a cis- configuration can be observed the two vibrational modes. In the stereoisomers 2 and 3 the mas(RuCl2) were predicted in 294– 308 cm 1, which is a very similar value when compared with the experimental result of Silva Sa and Lima-Neto [12], at 301 cm 1 [RuCl2(benzyldiphenylphosphine)2(piperidine) complex], and Carvalho Jr. et al. [13], at 320 cm 1 [RuCl2(PPh3)2(3,5-dimethylpiperidine) complex]. In the arrangements 1, 4 and 5 the separation between the two RuCl2 modes ranges from 12 to 44 cm 1, where we noted that this value is more susceptible to the arrangement of the complex than to the substitution of the amine ligand. Matos
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Fig. 5. The energetic profile (in kcal mol
Fig. 6. The energetic profile (in kcal mol
Table 1 The frequency (in cm Amine ligand
NH3 N(NCH3)3 Pyrrolidine Piperidine Pyridine Pyrazine Pyrimidine Pyridazine
1
1
) on the relative stability among the isomers with pyridine and the pyrazine ligands.
) on the relative stability among the isomers with pyrimidine and the pyridazine ligands.
1
) of the symmetric [msym(RuCl2)] and asymmetric RuCl2 [mas(RuCl2)] stretching modes for the RuCl2(PPh3)2(amine) complexes. 1
2
3
4
5
msym
mas
msym
mas
msym
mas
msym
mas
msym
mas
297.91 293.68 291.53 292.97 304.68 307.61 307.77 313.45
263.98 262.58 256.43 252.82 265.87 268.13 268.26 281.35
256.76 255.80 250.15 251.71 252.01 254.24 253.90 249.71
297.84 298.12 294.66 293.55 303.91 307.98 306.31 304.87
262.38 264.61 266.82 256.62 260.90 258.94 260.55 259.36
293.58 301.44 296.70 294.55 301.81 303.08 302.46 301.23
300.51 305.85 295.08 295.38 300.45 301.43 301.76 297.87
267.47 280.25 272.46 275.88 280.17 281.99 280.77 285.52
283.85 – 281.38 279.90 287.67 290.61 291.89 287.30
312.77 – 319.11 318.91 319.94 322.71 320.85 331.66
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and Lima-Neto [8] detected the separation between RuCl2 mode in 20 cm 1 which is in good agreement with the values predicted here. An interesting aspect in the arrangements 5 is that the mas(RuCl2) mode is localized with smaller values than the msym(RuCl2) one, while in the stereoisomers 4 was predicted the lowest values for the separation between the two RuCl2 modes. In addition, in the stereoisomers 1, 4 and 5 the relation between the intensities of the asymmetric and symmetric RuCl2 modes varies from 0.4 to 2.2 (see Table ST8, in Supplementary materials). Considering the importance of the reactivity index parameters, the calculations were performed based on the methodology of Pearson [47] and Parr et al. [48] (see Tables ST9–ST12, in Supplementary materials). The chemical hardness can give an important indicative of the resistance of compounds to change its electronic configuration, and according to this the arrangements 1 and 2 are the most affected by the amine substitution effect. Another reactivity parameter is the electronic chemical potential which shows the escaping tendency of electrons in the complex, and it is also associated with the electronic charge rearrangement associated to any chemical process. In this aspect, the stereoisomer 2 presents the higher electron-donor nature among the five isomers, independently of the amine ligand nature. Moreover, according to the electrophilicity (see ST12, in Supplementary materials) we can see that the values do not suffer much changes among the stereoisomers 3, 4 and 5, while in the arrangements 1 and 2 it was seen a decreasing in the electrophilicity index when we replace the acyclic amine by the cyclic aliphatic amine demonstrating an increase in the nucleophile nature of these stereoisomers, which become higher for the aromatic amine ligands. Comparing the reactivity parameter results reported here (Tables ST9–ST12, in Supplementary materials) with the ruthenium structures studied by Schott et al. [49], can be seen that the RuCl2(PPh3)2(amine) complexes present high values of hardness, nevertheless the electrophilicities are very small. Now we turn our attention to the atomic charge distribution in the coordination sphere (see Tables ST13–ST16, in Supplementary materials). The phosphorous and chlorine charges are not very sensitive to the changes in the amine ligand, independently of the arrangement. Observing the complexes with the phosphorous atoms in a trans- configuration, the phosphorous charge values are basically the same to both atoms, whereas in cis- the difference in the phosphorous charges are more visible for the arrangements 3 and 4 than for 1. Regarding to the ruthenium charge, the complexes 4 are the most susceptible to the nature of the amine ligand and, consequently, show the highest variations in rutheniumnitrogen bond orders. In addition, examining the ruthenium charge on the arrangements 4, the highest value was observed for trimethylamine, and the lowest one for ammonia. Notwithstanding the arrangements 1 and 2 are the most sensitive in the case of the nitrogen charge due to the substitution on the amine ligand, they are also the complexes with the lowest susceptibility on the ruthenium-nitrogen bond orders, demonstrating that this bond order are more sensitive to the behavior of the ruthenium charge than to the nitrogen one. There are some important results which comes out from the dipole moment among the ruthenium complexes (see Table ST17, in Supplementary materials). The highest susceptibility on the dipole moment value is seen among the complexes with the arrangement 2, followed by the arrangement 3. Moreover, the lowest value is predicted for the isomer 2 with the pyrazine ligand, 0.45, whereas the highest values is noticed among the structures with the arrangement 1. Meanwhile, a different result is obtained from the dipole moments of the stereoisomers 4, where this is the arrangement less affected by the amine substitution. Further insights can be obtained from the composition of the highest occupied molecular orbitals (HOMO) (see Table ST18, in
Supplementary materials). The HOMOs exhibit a mainly contribution from the Ru d-population and the p-electron population of chlorine (pCl). In the structures with the arrangement 1, in almost of all the cases their HOMO composition are represented by the eg-like d-orbitals of ruthenium (dz2 and dx2–y2), with a contribution ranging from 48% to 56%. The exception are the pyr1 and pydz1, where are observed a partial contribution from the t2g-like d-electron population of Ru (dxy, dxz and dyz). In the complexes 1, a decrease in the fraction of pCl was seen from cyclic aliphatic amine ligand and the trimethylamine one. Among the ruthenium complexes with the arrangements 2, 3 and 5, the d-population is mainly represented by the eg d orbitals, and only in some cases are noticed a mixed Ru t2g character, like in n3, n5 and pip5. In stereoisomers 2 and 3, the highest contribution from the Ru d-population to HOMO are observed by the complexes with the cyclic aliphatic amine ligands. Nevertheless, the structures with the major Ru d-population mixed, presenting a eg and t2g character, are those with arrangement 4, where the Ru d contribution to HOMO are not affect by the amine substituent.
4. Conclusion This study shed a light on the conformational aspects on the stability of the RuCl2(triphenylphosphine)2(amine) complexes by the substitution of the amine ligand. In this investigation all the structures presented a square pyramid geometry, and in all systems the stereoisomer 3 is the most stable arrangement among the five isomers. In addition, the r hyperbond search in the stereoisomers lead into hyperbonds that were consequence of the trans effect. An analysis of the Wiberg bond order shows values lower than one indicating their r nature in the coordination sphere, where the bond orders of the Ru–Cl bonds vary from 0.38–0.67, and the Ru–P and Ru–N ones vary from 0.43–0.83 and 0.19–0.64, respectively. In same cases, as in the trimethylamine complexes, the bond order is lower than 0.3 indicating an increase in their ionic character. In the stereoisomers 2 and 3, the mas(RuCl2) were predicted in 294–308 cm 1, whereas in the other arrangements the separation between the two RuCl2 modes ranges from 12 to 44 cm 1. Therefore we noticed a higher susceptibility to the different arrangements than for the substitution of the amine ligand. In the atomic charge distribution, in the coordination sphere, it was noted that the phosphorous and chlorine charges are not very sensitive to the changes in the amine ligand, or even affected by the arrangement. In regards to the ruthenium charge, the structures 4 are the most affected by the nature of the amine ligand and we found also the highest variations in ruthenium–nitrogen bond orders.
Acknowledgments The authors are grateful for the financial support given by the Brazilian agencies FAPESP, CAPES and CNPq. We would like to thank NCC/GridUNESP and CENAPAD/SP for the provision of computational facilities. RB Viana acknowledges FAPESP for the research fellowship (12/19175-2). We would like also to thank Valdemiro Pereira de Carvalho Júnior (UNESP), José Luiz Silva Sá (UESPI) and Camila Palombo Ferraz (USP/IQSC) for their important comments.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2014.07.028.
R.B. Viana et al. / Polyhedron 81 (2014) 661–667
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