3-Bromopyridine As a Sixth Ligand in Sulfoxide-Based Hoveyda

Article pubs.acs.org/Organometallics

3‑Bromopyridine As a Sixth Ligand in Sulfoxide-Based Hoveyda Complexes: A Study on Catalytic Properties Karolina Ż ukowska,†,‡ Anna Szadkowska,† Bartosz Trzaskowski,*,† Aleksandra Pazio,† Łukasz Pączek,†,‡ Krzysztof Woźniak,† and Karol Grela*,† †

Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland

‡

S Supporting Information *

ABSTRACT: A novel series of sulfoxide complexes with an additional 3-bromopyridine ligand was obtained and characterized. The initiators were compared in ring-closing metathesis reactions, showing a faster initiation of six-coordinated analogues. A theoretical study was performed utilizing the density functional theory to explain the observed reactivity differences. The results confirmed a different initiation mechanism for both compound classes, with a multistep sequence of initiation for pyridine-containing ones, explaining differences in catalytic behavior.

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INTRODUCTION In recent decades, olefin metathesis emerged as a valuable tool for C−C double-bond formation.1 The great success of this methodology is largely due to the introduction of catalysts suited for a variety of applications. Well-defined molybdenum and tungsten complexes of high activity and selectivity have emerged.2 Also ruthenium complexes that, due to their lack of sensitivity toward air and moisture, seem to be more “user friendly” have undergone tremendous development.3 Tuning the catalyst properties of these systems by structural modifications is a well-established research direction (Figure 1).4 The first breakthrough in this area took place when Nheterocyclic carbenes (NHC) were introduced to the structure of phosphine-containing complexes,5 leading to the secondgeneration of ruthenium compounds (1 and 3).6 Another important advance came with the introduction of the chelating benzylidene ligands to create the Hoveyda-type system (5a).7 Modification of this type of ligand led to a huge diversification of the properties, resulting in a variety of so-called latent complexes,4b e.g., 6,8 or more active species such as 5b.9 We became interested in complexes such as 2 and 4, which are sometimes referred to as third-generation complexes.10 These ruthenium chelates are generally second-generation complexes with one or two pyridine or pyridine-derived monochelating ligands in the structure.11 Such complexes are of great importance, as they are known to initiate the reaction more rapidly than the respective complexes lacking the pyridine ligand.12 This fact is particularly important in polymerizationrelated applications. Pyridine-containing complexes have an enhanced initiation rate, which combined with a reduced propagation rate allows obtaining polymers characterized by a low polydispersity index (PDI). © 2013 American Chemical Society

Figure 1. Selected ruthenium-based catalysts for olefin metathesis.

We became interested in the possibility of transferring these beneficial properties into Hoveyda-type systems by “doping” them with pyridine derivatives. Similar experiments were performed before, however, for another purpose. Addition of pyridine to certain groups of catalysts was found to favor a trans−cis isomerization process.13 These experiments showed that novel complexes are formed, but their catalytic behavior Received: January 25, 2013 Published: March 28, 2013 2192

dx.doi.org/10.1021/om400064c | Organometallics 2013, 32, 2192−2198

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was not investigated in detail. This made us wonder whether Hoveyda-type systems containing a pyridine ligand would behave similarly to the third-generation complexes or would they retain the characteristics of the parent Hoveyda compounds. Another interesting question is what kind of mechanistic pathways would such initiators follow. This study aims at answering these questions, which should improve our understanding of the metathesis process.

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RESULTS AND DISCUSSION Our initial speculation was that six-coordinated Hoveyda-type complexes should be obtainable, and for evaluation of this hypothesis, we selected a series of sulfoxide chelates previously studied by us.8 During our former research we noted an unexpected coordination of a water molecule to the ruthenium center of some sulfoxide-containing compounds while growing X-ray-suitable crystals in reagent-grade solvents. This inspired us to investigate whether compounds originating from this class would lead to pyridine-containing chelates. To our satisfaction, we found that upon addition of 3-bromopyridine to complex 7, a new compound was formed quantitatively (Scheme 1).

Figure 2. Results of RCM catalyzed by complexes 7 (solid line) and 9 (dashed line). Conditions: toluene, C10 = 0.1 M. Lines are intended as visual aid.

Scheme 1. Synthesis of a Pyridine-Containing Catalyst 9

For investigation of the newly formed system, diethyl malonate derivative 10 was chosen (Scheme 2). During ringclosing metathesis (RCM) a trisubstituted double bond is formed. Scheme 2. Model RCM Reaction Figure 3. Performance of 7 (solid line) and 9 (dashed line) at elevated temperatures. Conditions: toluene, C10 = 0.1 M. Lines are intended as visual aid.

amount of the ruthenium complex used. In all investigated conditions the pyridine-containing complex 9 initiated the reaction noticeably faster than the second-generation catalyst 7. In order to make sure that the aforementioned observations are not just a specific effect, we decided to test a wider variety of related initiators. Our choice was to examine ruthenium complexes with various ligand substitution patterns allowing for diversification of both steric and electronic factors. To do so, the synthesis of additional compounds was performed (Scheme 3). The synthesized complexes were tested in conditions previously found to be optimal. Results of structure diversification on the RCM reactivity of the obtained complexes are presented in Figure 4. The first observation is that irrespective of the character of the substituent, the difference between the five- and sixcoordinated complexes remains significant. Compounds bearing a tert-butyl group stand out as the least stable, proving again that too much steric hindrance close to the ruthenium-

This substrate is known to be demanding, thus allowing for an easy differentiation of the metathesis-active complexes. Comparison of 7 and 9 started at a relatively high loading at low temperature, which was gradually increased (Figure 2). The preliminary results revealed pronounced differences in reactivity of the studied complexes, as the pyridine-containing 9 initiates the reaction much faster than its precursor 7. At room temperature, the second-generation complex 7 was inactive, as described before.8a Chelate 9, on the other hand, allowed obtaining the RCM product in a relatively low yield. After elevation of the temperature to 40 °C and simultaneous decrease of complex loading, activity of both compounds could be noted. In these conditions, catalyst 9 was superior to 7. To push the system even further, we continued to elevate the temperature and decrease the catalyst loading. Results of the tests are presented in Figure 3. Having performed the RCM of 10 at 50, 60, and 80 °C with various catalyst loadings, we found that elevation of temperature was strongly beneficial, allowing for the reduction of the 2193


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Scheme 3. Introduction of Structural Modifications

Figure 5. Influence of the NHC ligand in malonate RCM. Secondgeneration complexes, solid line; pyridine-containing compounds, dashed line. Conditions: 0.5 mol % catalyst, toluene, 80 °C, C10 = 0.1 M. Lines are intended as visual aid.

Structures of complexes 9 and 15 have been determined using single-crystal X-ray diffraction (Figure 6). The resulting geometries were compared in two pairs containing a second- and third-generation complex with a similar substitution pattern. The first pair bearing the pnitrophenyl substituent differs in the coordination number of the ruthenium center. Compound 7 is five-coordinated, while 9 contains six ligands around the ruthenium atom, including the additional pyridine moiety. The geometry changes between these chelates take place mainly on the Cl1−C22−Cl2 plane, without significant alteration along the S1−Ru1−C1 plane (Table 1). The H22 atom (hydrogen atom belonging to the Ru1−C22 catalytic center) in 7 is directed exactly to the center of the C4−C9 mesitylene ring, whereas in 9 it is deflected in the direction of the C5 atom. Nevertheless, distances between C4−centroid−C9 and H22 are similar in both compounds and equal to 2.493 Å in 7 and 2.434 Å in 9.14 The second pair of compounds bearing the tert-butyl substituent, 12 and 15, presents a six-coordinated ruthenium center, which differs only by the type of the sixth ligand: water or pyridine molecule.15 The geometry changes between these compounds are apparently smaller, and the only effects observed in the ruthenium atom geometry are related to the size of water and pyridine molecules. The nature of the H22 and the second mesitylene ring interaction differs from the first pair. In 15 the second mesitylene ring C4−C9 is shifted by ca. 26° in comparison to the respective fragment of 12. Still the H22 atom is deflected in the direction of the C9 atom in both compounds. The distance between H22 and C4−centroid−C9 equals 2.700 Å in 12 and 2.465 Å in 15. Structural studies led us to conduct an examination of the 1H NMR spectra, which are an additional source of information about the studied complexes. Table 2 contains chemical shifts of the benzylidene protons for all studied ruthenium compounds, which is one of the basic facts derived from the 1 H NMR spectrum. The data for the second-generation compounds are straightforward to analyze. Starting from the IMes-containing phenyl-substituted complex 13, the signal is shifted upfield when an electron-withdrawing group is present (compound 7).

Figure 4. Influence of the sulfur substituent. Second-generation complexes, solid line; pyridine-containing compounds, dashed line. Conditions: 0.5 mol % catalyst, toluene, 80 °C, C10 = 0.1 M. Lines are intended as visual aid.

heteroatom chelation destabilizes the initiator. The results obtained by the phenyl-substituted complexes were superior to the p-nitrophenyl-containing catalysts utilized in the first part of the study. In this case, the electron-withdrawing character of the nitro group caused destabilization of the catalyst rather than its activation. In all cases our finding that pyridine-containing complexes initiate faster remains valid. The last structural feature we selected to examine was the influence of the NHC ligand. In order to monitor its electronic influence, we compared five- and six-coordinated IMes(previously discussed 7 and 9) and SIMes-containing compounds (14 and 17). The results are summarized in Figure 5. The results of these tests came as a surprise. The influence of the NHC ligand is minor (less than 5%) when the secondgeneration compounds are considered. When it comes to the pyridine-containing initiators, there is a significant difference in catalytic performance. The IMes-containing complex 9 led to a 50% conversion, whereas the SIMes-bearing compound 17 led to a 35% conversion. This may seem unexpected, as in general the ruthenium chelates containing saturated NHC ligands are described in the literature as better for the RCM of malonate derivatives. To fully explore the differences among two examined classes of complexes, we turned our attention to structural studies. 2194


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Figure 6. Atomic displacement parameters and the labeling of atoms for molecules 9 and 15. Thermal ellipsoids are at the 30% level of probability. Hydrogen atoms are omitted for clarity.

Table 1. Geometry Comparison for the Investigated Compounds 7

9

12

15

space group

P1̅

P21/c

P21/n

P1̅

crystal system

triclinic

monoclinic

monoclinic

triclinic

1.865(8) 2.114(8) 2.364(2) 2.405(2) 2.323(2) 2.360(7)

1.850(2) 2.088(3) 2.3883(7) 2.3768(7) 2.378(1)

Ru(1)−C(22) Ru(1)−C(1) Ru(1)−Cl(1) Ru(1)−Cl(2) Ru(1)−S(1) Ru(1)−N(4) Ru(1)−O(2)

Bond Lengths (Å) Ru Center 1.834(4) 2.102(3) 2.3344(9) 2.3276(8) 2.332(1)

2.304(2) S Atom 1.478(2) 1.768(4) 1.795(3)

S(1)−O(1) S(1)−C(28) S(1)−C(29) Br(1)−C(42)

C(1)−N(1)−C(4) C(1)−N(2)−C(13) C(1)−Ru(1)−C(22) Cl(1)− Ru(1)−Cl(2) Ru(1)−C(22)−C(23) C(22)−C(23)−C(28)

Ru(1)−C(1)−N(1)−C(4) Ru(1)−C(1)−N(2)−C(13) C(22)−Ru(1)−S(1)−C(28) Ru(1)−C(22)−C(23)−C(28) C(22)−C(23)−C(28)−S(1) C(23)−C(28)−S(1)−Ru(1) S(1)−Ru(1)−C(22)−C(23)

1.863(3) 2.093(3) 2.3904(8) 2.3931(8) 2.357(1) 2.363(2)

Valence Angles (deg) NHC 126.7(3) 127.9(3) 99.5(1) Ru Center 156.95(4) 124.9(3) 118.5(3) Torsion Angles (deg) NHC 16.9(5) −16.2(4) Benzylidene −2.52(16) −3.1(5) 0.2(4) 1.8(3) 3.4(3)

1.472(6) 1.763(8) 1.785(9) 1.886(9)

1.480(2) 1.781(2) 1.864(3)

1.477(2) 1.774(3) 1.868(3) 1.892(3)

101.6(3) 130.6(7) 96.2(3)

129.4(2) 127.9(2) 94.1(1)

127.2(2) 128.0(2) 97.7(1)

173.54(8) 122.2(6) 118.0(7)

167.98(2) 120.8(2) 118.3(2)

174.07(3) 120.9(2) 117.3(2)

−9.1(13) 10.7(12)

−11.7(4) 0.8(4)

16.5(4) −2.9(4)

20.8(4) 15.8(10) 6.6(9) −19.3(6) −22.5(6)

−25.22(11) −21.0(3) −6.8(3) 22.39(19) 28.27(19)

−28.83(12) −22.7(3) −8.8(3) 26.43(19) 31.8(2)

with a ca. 2.4 ppm difference. The chemical shift of the pyridine-containing compound 15 is unexpectedly high and in good agreement with the single-crystal study. Contrary to the solid-state structure of 9, where the p-nitrophenyl substituent undergoes the stacking-like interaction between the mesity-

When the substituent has an electron-donating influence, the benzylidene signal can be observed downfield. The addition of pyridine causes generally a 0.5−0.6 ppm shift of the signal in the downfield direction. In the case of the tert-butyl-substituted complexes (12 vs 15), this change is much more pronounced, 2195


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Table 2. 1H NMR Chemical Shifts of the Benzylidene Protons of Catalysts in the Study compounds lacking the pyridine ligand

a

the second- and third-generation Hoveyda complexes follow different mechanistic pathways. The same conclusion may be reached when the previously discussed pronounced difference in the catalytic activity of all five- and six-coordinated complexes is compared. The experimentally noticed difference of reactivity between complexes 7 and 9, particularly at low temperatures, suggests that the presence of an additional pyridine ligand has an important effect on the energetics of the initiation step of the catalytic reaction. To suggest a feasible reaction mechanism, we performed density functional theory (DFT) calculations of the initiation mechanism of obtained complexes in order to show the impact of the additional ligand on the energetic profile of the reaction. As models for this study we selected compounds 7, 9, and 17, bearing the p-nitrophenyl group, for which detailed experiments were conducted. In the computational part of this study we used a wellestablished protocol that was successfully used to describe reaction paths and transition states of similar five-coordinated Hoveyda type complexes with good accuracy.19 To summarize the most important points of the calculations, we have used an all-atom model for the catalyst and an ethylene molecule to model the substrate of olefin metathesis. For each system we have performed full geometry optimizations of all theoretically possible and chemically feasible stationary points using B3LYP/ lacvp**. To evaluate accurate free energies of all stationary points, we used M06/lacv3p++** single-point calculations, and to add solvent contribution, we used the Poisson−Boltzmann self-consistent polarizable continuum method. Details of the computational approach are given in the Supporting Information. From a theoretical point of view, the pyridine-containing Hoveyda complex 9 may undergo three different reaction paths: associative, dissociative, or interchange. Due to the steric hindrance caused by the presence of the sixth ligand, there is, however, a limited space for an olefin attack, making it almost completely improbable. As a result, both associative and interchange pathways are not likely, due the necessity of formation of a labile seven-coordinated ruthenium complex. We were able to estimate the energy barrier of the transition states for these unlikely events to be around 40−50 kcal/mol. Thus, the only possible option for the initiation step of thirdgeneration sulfoxide Hoveyda derivatives is the dissociative

pyridinecontaining compounds

NHC

R

complex

δ (ppm)

complex

δ (ppm)

Δδ (ppm)

SIMes IMes IMes IMes

p-C6H4NO2 p-C6H4NO2 Ph t-Bu

14 7 13 12

16.70 16.77a 16.85a 16.99a

17 9 16 15

17.16 17.35 17.42 19.42

0.46 0.58 0.57 2.43

Data originating from ref 8a.

lene−3-bromopyridine−aromatic ring of the p-nitrophenyl moiety, the tert-butyl substituent in 15 is situated in front of the molecule and presents sufficient steric hindrance to prevent further benzylidene bending. Thus, the H22 hydrogen atom is pushed into the C4−C9 mesitylene ring. This significant deshielding can be observed by NMR technique and may lead to a decreased stability of the complex, which was observed during the activity studies (Figure 4). Having explored the structure of the obtained compounds, we decided to investigate the influence of the sixth ligand on the mechanistic pathways of the initiators using computational methods. Although five-coordinated Hoveyda-type complexes have been widely used for a long time, only recently has their initiation mechanism been proposed. On the basis of experimental work16 and computational investigation17 it was suggested that an interchange mechanism involving a simultaneous alkoxy dissociation and an olefin binding is the most plausible option. The latest mechanistic study by Plenio et al.18 suggests, however, that the initiation reaction can simultaneously follow two parallel pathways: a dissociative mechanism and an interchange mechanism. The preference for one of the two possible modes of precatalyst activation depends on the steric and electronic properties of the utilized ruthenium complexes and the olefins employed for the metathesis reaction. To the best of our knowledge, no mechanistic studies of the six-coordinated ruthenium based compounds containing a pyridine ligand have been performed before. Considering the behavior of SIMes- and IMes-containing complexes in the same conditions, we found an indication that

Figure 7. Free energies for the initiation step of the six-coordinated sulfoxide Hoveyda derivatives 9 and 17. 2196


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initiation of the pyridine-containing complex. A different initiation mechanism of the studied six-coordinated complexes rationalizes starting the catalytic cycle at lower temperature, which is in perfect agreement with the experimental data presented in this work (see Figure 2).

mechanism. The results of the theoretical study of the initiation step for compounds 9 and 17 are presented in Figure 7. The first stage of this mechanism has an energy barrier of only 10.9 kcal/mol for 9 and leads to an intermediate (INT1) that has the p-nitrophenyl group rotated away from the ruthenium center and the 3-bromopyridine moiety still coordinated to the metal. INT1 is a five-coordinated system with a trigonal-bipyramid-like geometry. Interestingly, the interaction energy between the 3-bromopyridine and the ruthenium center is now stronger than in the original sixcoordinated complex, meaning that the dissociation of 3bromopyridine may occur only if forced by an olefin attack. The dissociation of the 3-bromopyridine ligand may be accomplished in two manners: either in a one-step interchange reaction, in which the olefin attack occurs simultaneously with the pyridine derivative dissociation, or in a two-step associative mechanism, where the first step is the olefin association followed by the 3-bromopyridine dissociation. Our computational results show that the energy barrier of the one-step reaction for 9 is 14.5 kcal/mol for the trans attack and 25.9 kcal/mol for the cis attack (see the Supporting Information). On the other hand, the two-step associative mechanism has an energy barrier of only 11.7 kcal/mol for 9, making it the most probable route from the energetic point of view and simultaneously the limiting step for the entire initiation reaction. We also compared the initiation pathways for the sixcoordinated complexes containing various NHC ligands: IMes and SIMes (9 and 17, respectively). Our DFT analysis suggests that the initiation follows the same mechanism, which includes sulfoxide dissociation followed by an olefin association. For the SIMes-bearing complex, the energy barrier for TS1 is lower, but for TS2 the rate-limiting step is slightly higher. This results in an overall higher energy barrier for complex 17 that is in good agreement with the decreased catalytic profile that has been observed during the preparative experiments. We can directly compare the evaluated energy barrier with the energy barrier for 7, i.e., without the additional pyridine ligand. In the case of these sulfoxide Hoveyda derivatives, the difference between the energy barrier for the dissociative and interchange paths is small and equal to 0.9 kcal/mol (Figure 8).

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CONCLUSIONS

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

We report on the synthesis of ruthenium-based metathesis catalysts containing simultaneously a sulfoxide moiety and a 3bromopyridine ligand. The obtained complexes are active in olefin metathesis, leading to the formation of trisubstituted double bonds. The performed DFT study revealed energetic differences in the mechanistic pathways of the studied complexes. In the course of the theoretical study it was explained that six-coordinated sulfoxide complexes initiate faster due to a different initiation mechanism in operation, which has a lower energy barrier.

General Procedure for Pyridine-Containing Ruthenium Complex Synthesis. A selected second-generation complex (0.2 mmol) was placed in a Schlenk tube under argon. Next, 3bromopyridine was added (3 mmol, 0.28 mL). The resulting mixture was stirred for an hour at room temperature. After that time an excess of n-pentane was added, resulting in precipitation of the product. The precipitate was filtered off, washed with a portion of n-pentane, and dried to afford dark green crystals. General Procedure for Metathesis Reactions. Compound 10 (0.5 mmol, 127 mg) and n-dodecane (0.15 mmol, 26 mg), as an internal standard, were put in a Schlenk tube under Ar and dissolved in 5 mL of anhydrous toluene. The mixture was heated to the desired temperature, and the examined ruthenium complex was added, marking this time as the reaction start. Samples (0.2 mL) were taken and immediately quenched with 2 M ethyl vinyl ether solution in DCM (0.2 mL) after 5, 10, 15, and 30 min and 1 h and then every hour until 8 h. GC analysis was performed shortly after sample collection.

S Supporting Information *

Details of experimental procedures, analytical data of the obtained compounds, and description of computational methods applied. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ̇ K.Z. and K.G. acknowledge the “TEAM” project operated within the Foundation for Polish Science Team program cofinanced by the EU European Regional Development Fund, Operational Program Innovative Economy 2007−2013. B.T. would like to acknowledge financial support from the Foundation for Polish Science “Powroty/Homing” program. All calculations were performed at the Interdisciplinary Center for Mathematical and Computational Modeling of University of Warsaw.

Figure 8. Free energies of the initiation of 7.

This value is in accordance with the recent experimental work of Plenio et al. for the similar five-coordinated Hoveyda− Grubbs-type complexes, which suggests that the initiation reaction may simultaneously follow two different mechanistic pathways.17 The energy barrier for 7 (17.4 kcal/mol) is more than 5 kcal/ mol higher than for 9 (11.7 kcal/mol), which results in faster 2197


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(h) Nuñez-Zarur, F.; Solans-Monfort, X.; Rodríguez-Santiago, L.; Sodupe, M. Organometallics 2012, 31, 4203. (i) Nuñez-Zarur, F.; Poater, J.; Rodríguez-Santiago, L.; Solans-Monfort, X.; Solà, M.; Sodupe, M. Comput. Theor. Chem. 2012, 996, 57. (j) Minenkov, Y.; Occhipinti, G.; Heyndrickx, W.; Jensen, V. R. Eur. J. Inorg. Chem. 2012, 9, 1507.

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3-Bromopyridine As a Sixth Ligand in Sulfoxide-Based Hoveyda

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