3‑Bromopyridine As a Sixth Ligand in Sulfoxide-Based HoveydaComplexes: A Study on Catalytic PropertiesKarolina Zukowska,†,‡ Anna Szadkowska,† Bartosz Trzaskowski,*,† Aleksandra Pazio,† Łukasz Pączek,†,‡

Krzysztof Wozniak,† 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 anadditional 3-bromopyridine ligand was obtained and charac-terized. The initiators were compared in ring-closing meta-thesis reactions, showing a faster initiation of six-coordinatedanalogues. A theoretical study was performed utilizing thedensity functional theory to explain the observed reactivitydifferences. The results confirmed a different initiationmechanism for both compound classes, with a multistepsequence of initiation for pyridine-containing ones, explainingdifferences in catalytic behavior.

■ INTRODUCTION

In recent decades, olefin metathesis emerged as a valuable toolfor C−C double-bond formation.1 The great success of thismethodology is largely due to the introduction of catalystssuited for a variety of applications. Well-defined molybdenumand tungsten complexes of high activity and selectivity haveemerged.2 Also ruthenium complexes that, due to their lack ofsensitivity toward air and moisture, seem to be more “userfriendly” have undergone tremendous development.3 Tuningthe catalyst properties of these systems by structuralmodifications is a well-established research direction (Figure1).4

The first breakthrough in this area took place when N-heterocyclic carbenes (NHC) were introduced to the structureof phosphine-containing complexes,5 leading to the second-generation of ruthenium compounds (1 and 3).6 Anotherimportant advance came with the introduction of the chelatingbenzylidene ligands to create the Hoveyda-type system (5a).7

Modification of this type of ligand led to a huge diversificationof the properties, resulting in a variety of so-called latentcomplexes,4b e.g., 6,8 or more active species such as 5b.9

We became interested in complexes such as 2 and 4, whichare sometimes referred to as third-generation complexes.10

These ruthenium chelates are generally second-generationcomplexes with one or two pyridine or pyridine-derivedmonochelating ligands in the structure.11 Such complexes areof great importance, as they are known to initiate the reactionmore rapidly than the respective complexes lacking the pyridineligand.12 This fact is particularly important in polymerization-related applications. Pyridine-containing complexes have anenhanced initiation rate, which combined with a reducedpropagation rate allows obtaining polymers characterized by alow polydispersity index (PDI).

We became interested in the possibility of transferring thesebeneficial properties into Hoveyda-type systems by “doping”them with pyridine derivatives. Similar experiments wereperformed before, however, for another purpose. Addition ofpyridine to certain groups of catalysts was found to favor atrans−cis isomerization process.13 These experiments showedthat novel complexes are formed, but their catalytic behavior

Received: January 25, 2013Published: March 28, 2013

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

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was not investigated in detail. This made us wonder whetherHoveyda-type systems containing a pyridine ligand wouldbehave similarly to the third-generation complexes or wouldthey retain the characteristics of the parent Hoveydacompounds. Another interesting question is what kind ofmechanistic pathways would such initiators follow. This studyaims at answering these questions, which should improve ourunderstanding of the metathesis process.

■ RESULTS AND DISCUSSIONOur initial speculation was that six-coordinated Hoveyda-typecomplexes should be obtainable, and for evaluation of thishypothesis, we selected a series of sulfoxide chelates previouslystudied by us.8 During our former research we noted anunexpected coordination of a water molecule to the rutheniumcenter of some sulfoxide-containing compounds while growingX-ray-suitable crystals in reagent-grade solvents. This inspiredus to investigate whether compounds originating from this classwould 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).

For investigation of the newly formed system, diethylmalonate derivative 10 was chosen (Scheme 2). During ring-closing metathesis (RCM) a trisubstituted double bond isformed.

This substrate is known to be demanding, thus allowing foran easy differentiation of the metathesis-active complexes.Comparison of 7 and 9 started at a relatively high loading atlow temperature, which was gradually increased (Figure 2).The preliminary results revealed pronounced differences in

reactivity of the studied complexes, as the pyridine-containing 9initiates the reaction much faster than its precursor 7. At roomtemperature, the second-generation complex 7 was inactive, asdescribed before.8a Chelate 9, on the other hand, allowedobtaining the RCM product in a relatively low yield. Afterelevation of the temperature to 40 °C and simultaneousdecrease of complex loading, activity of both compounds couldbe noted. In these conditions, catalyst 9 was superior to 7. Topush the system even further, we continued to elevate thetemperature and decrease the catalyst loading. Results of thetests 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 temper-ature was strongly beneficial, allowing for the reduction of the

amount of the ruthenium complex used. In all investigatedconditions the pyridine-containing complex 9 initiated thereaction noticeably faster than the second-generation catalyst 7.In order to make sure that the aforementioned observations arenot just a specific effect, we decided to test a wider variety ofrelated initiators. Our choice was to examine rutheniumcomplexes with various ligand substitution patterns allowingfor diversification of both steric and electronic factors. To doso, the synthesis of additional compounds was performed(Scheme 3).The synthesized complexes were tested in conditions

previously found to be optimal. Results of structurediversification on the RCM reactivity of the obtained complexesare presented in Figure 4.The first observation is that irrespective of the character of

the substituent, the difference between the five- and six-coordinated complexes remains significant. Compoundsbearing a tert-butyl group stand out as the least stable, provingagain that too much steric hindrance close to the ruthenium-

Scheme 1. Synthesis of a Pyridine-Containing Catalyst 9

Scheme 2. Model RCM Reaction

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

Figure 3. Performance of 7 (solid line) and 9 (dashed line) at elevatedtemperatures. Conditions: toluene, C10 = 0.1 M. Lines are intended asvisual aid.

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heteroatom chelation destabilizes the initiator. The resultsobtained by the phenyl-substituted complexes were superior tothe p-nitrophenyl-containing catalysts utilized in the first part ofthe study. In this case, the electron-withdrawing character ofthe nitro group caused destabilization of the catalyst rather thanits activation. In all cases our finding that pyridine-containingcomplexes initiate faster remains valid.The last structural feature we selected to examine was the

influence of the NHC ligand. In order to monitor its electronicinfluence, we compared five- and six-coordinated IMes-(previously discussed 7 and 9) and SIMes-containingcompounds (14 and 17). The results are summarized inFigure 5.The results of these tests came as a surprise. The influence of

the NHC ligand is minor (less than 5%) when the second-generation compounds are considered. When it comes to thepyridine-containing initiators, there is a significant difference incatalytic performance. The IMes-containing complex 9 led to a50% conversion, whereas the SIMes-bearing compound 17 ledto a 35% conversion. This may seem unexpected, as in generalthe ruthenium chelates containing saturated NHC ligands aredescribed in the literature as better for the RCM of malonatederivatives.To fully explore the differences among two examined classes

of complexes, we turned our attention to structural studies.

Structures of complexes 9 and 15 have been determined usingsingle-crystal X-ray diffraction (Figure 6).The resulting geometries were compared in two pairs

containing a second- and third-generation complex with asimilar substitution pattern. The first pair bearing the p-nitrophenyl substituent differs in the coordination number ofthe ruthenium center. Compound 7 is five-coordinated, while 9contains six ligands around the ruthenium atom, including theadditional pyridine moiety. The geometry changes betweenthese 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 theRu1−C22 catalytic center) in 7 is directed exactly to the centerof the C4−C9 mesitylene ring, whereas in 9 it is deflected inthe direction of the C5 atom. Nevertheless, distances betweenC4−centroid−C9 and H22 are similar in both compounds andequal to 2.493 Å in 7 and 2.434 Å in 9.14

The second pair of compounds bearing the tert-butylsubstituent, 12 and 15, presents a six-coordinated rutheniumcenter, which differs only by the type of the sixth ligand: wateror pyridine molecule.15 The geometry changes between thesecompounds are apparently smaller, and the only effectsobserved in the ruthenium atom geometry are related to thesize of water and pyridine molecules. The nature of the H22and the second mesitylene ring interaction differs from the firstpair. In 15 the second mesitylene ring C4−C9 is shifted by ca.26° in comparison to the respective fragment of 12. Still theH22 atom is deflected in the direction of the C9 atom in bothcompounds. The distance between H22 and C4−centroid−C9equals 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 informationabout the studied complexes. Table 2 contains chemical shiftsof the benzylidene protons for all studied rutheniumcompounds, which is one of the basic facts derived from the1H NMR spectrum.The data for the second-generation compounds are

straightforward to analyze. Starting from the IMes-containingphenyl-substituted complex 13, the signal is shifted upfieldwhen an electron-withdrawing group is present (compound 7).

Scheme 3. Introduction of Structural Modifications

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

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

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

with a ca. 2.4 ppm difference. The chemical shift of thepyridine-containing compound 15 is unexpectedly high and ingood agreement with the single-crystal study. Contrary to thesolid-state structure of 9, where the p-nitrophenyl substituentundergoes the stacking-like interaction between the mesity-

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 P1 P21/c P21/n P1

crystal system triclinic monoclinic monoclinic triclinic

Bond Lengths (Å)Ru Center

Ru(1)−C(22) 1.834(4) 1.865(8) 1.850(2) 1.863(3)Ru(1)−C(1) 2.102(3) 2.114(8) 2.088(3) 2.093(3)Ru(1)−Cl(1) 2.3344(9) 2.364(2) 2.3883(7) 2.3904(8)Ru(1)−Cl(2) 2.3276(8) 2.405(2) 2.3768(7) 2.3931(8)Ru(1)−S(1) 2.332(1) 2.323(2) 2.378(1) 2.357(1)Ru(1)−N(4) 2.360(7) 2.363(2)Ru(1)−O(2) 2.304(2)

S AtomS(1)−O(1) 1.478(2) 1.472(6) 1.480(2) 1.477(2)S(1)−C(28) 1.768(4) 1.763(8) 1.781(2) 1.774(3)S(1)−C(29) 1.795(3) 1.785(9) 1.864(3) 1.868(3)Br(1)−C(42) 1.886(9) 1.892(3)

Valence Angles (deg)NHC

C(1)−N(1)−C(4) 126.7(3) 101.6(3) 129.4(2) 127.2(2)C(1)−N(2)−C(13) 127.9(3) 130.6(7) 127.9(2) 128.0(2)C(1)−Ru(1)−C(22) 99.5(1) 96.2(3) 94.1(1) 97.7(1)

Ru CenterCl(1)− Ru(1)−Cl(2) 156.95(4) 173.54(8) 167.98(2) 174.07(3)Ru(1)−C(22)−C(23) 124.9(3) 122.2(6) 120.8(2) 120.9(2)C(22)−C(23)−C(28) 118.5(3) 118.0(7) 118.3(2) 117.3(2)

Torsion Angles (deg)NHC

Ru(1)−C(1)−N(1)−C(4) 16.9(5) −9.1(13) −11.7(4) 16.5(4)Ru(1)−C(1)−N(2)−C(13) −16.2(4) 10.7(12) 0.8(4) −2.9(4)

BenzylideneC(22)−Ru(1)−S(1)−C(28) −2.52(16) 20.8(4) −25.22(11) −28.83(12)Ru(1)−C(22)−C(23)−C(28) −3.1(5) 15.8(10) −21.0(3) −22.7(3)C(22)−C(23)−C(28)−S(1) 0.2(4) 6.6(9) −6.8(3) −8.8(3)C(23)−C(28)−S(1)−Ru(1) 1.8(3) −19.3(6) 22.39(19) 26.43(19)S(1)−Ru(1)−C(22)−C(23) 3.4(3) −22.5(6) 28.27(19) 31.8(2)

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lene−3-bromopyridine−aromatic ring of the p-nitrophenylmoiety, the tert-butyl substituent in 15 is situated in front ofthe molecule and presents sufficient steric hindrance to preventfurther benzylidene bending. Thus, the H22 hydrogen atom ispushed into the C4−C9 mesitylene ring. This significantdeshielding can be observed by NMR technique and may leadto a decreased stability of the complex, which was observedduring the activity studies (Figure 4).Having explored the structure of the obtained compounds,

we decided to investigate the influence of the sixth ligand onthe mechanistic pathways of the initiators using computationalmethods. Although five-coordinated Hoveyda-type complexeshave been widely used for a long time, only recently has theirinitiation mechanism been proposed. On the basis ofexperimental work16 and computational investigation17 it wassuggested that an interchange mechanism involving asimultaneous alkoxy dissociation and an olefin binding is themost plausible option. The latest mechanistic study by Plenio etal.18 suggests, however, that the initiation reaction cansimultaneously follow two parallel pathways: a dissociativemechanism and an interchange mechanism. The preference forone of the two possible modes of precatalyst activation dependson the steric and electronic properties of the utilized rutheniumcomplexes and the olefins employed for the metathesisreaction. To the best of our knowledge, no mechanistic studiesof the six-coordinated ruthenium based compounds containinga pyridine ligand have been performed before.Considering the behavior of SIMes- and IMes-containing

complexes in the same conditions, we found an indication that

the second- and third-generation Hoveyda complexes followdifferent mechanistic pathways. The same conclusion may bereached when the previously discussed pronounced differencein the catalytic activity of all five- and six-coordinatedcomplexes is compared. The experimentally noticed differenceof reactivity between complexes 7 and 9, particularly at lowtemperatures, suggests that the presence of an additionalpyridine ligand has an important effect on the energetics of theinitiation step of the catalytic reaction. To suggest a feasiblereaction mechanism, we performed density functional theory(DFT) calculations of the initiation mechanism of obtainedcomplexes in order to show the impact of the additional ligandon the energetic profile of the reaction. As models for this studywe selected compounds 7, 9, and 17, bearing the p-nitrophenylgroup, for which detailed experiments were conducted.In the computational part of this study we used a well-

established protocol that was successfully used to describereaction paths and transition states of similar five-coordinatedHoveyda type complexes with good accuracy.19 To summarizethe most important points of the calculations, we have used anall-atom model for the catalyst and an ethylene molecule tomodel the substrate of olefin metathesis. For each system wehave performed full geometry optimizations of all theoreticallypossible and chemically feasible stationary points using B3LYP/lacvp**. To evaluate accurate free energies of all stationarypoints, we used M06/lacv3p++** single-point calculations, andto add solvent contribution, we used the Poisson−Boltzmannself-consistent polarizable continuum method. Details of thecomputational approach are given in the SupportingInformation.From a theoretical point of view, the pyridine-containing

Hoveyda complex 9 may undergo three different reactionpaths: associative, dissociative, or interchange. Due to the sterichindrance caused by the presence of the sixth ligand, there is,however, a limited space for an olefin attack, making it almostcompletely improbable. As a result, both associative andinterchange pathways are not likely, due the necessity offormation 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 third-generation sulfoxide Hoveyda derivatives is the dissociative

Table 2. 1H NMR Chemical Shifts of the BenzylideneProtons of Catalysts in the Study

compounds lackingthe pyridine ligand

pyridine-containingcompounds

NHC R complex δ (ppm) complexδ

(ppm)Δδ

(ppm)

SIMes p-C6H4NO2 14 16.70 17 17.16 0.46IMes p-C6H4NO2 7 16.77a 9 17.35 0.58IMes Ph 13 16.85a 16 17.42 0.57IMes t-Bu 12 16.99a 15 19.42 2.43

aData originating from ref 8a.

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

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mechanism. The results of the theoretical study of the initiationstep 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 theruthenium center and the 3-bromopyridine moiety stillcoordinated to the metal. INT1 is a five-coordinated systemwith a trigonal-bipyramid-like geometry. Interestingly, theinteraction energy between the 3-bromopyridine and theruthenium center is now stronger than in the original six-coordinated complex, meaning that the dissociation of 3-bromopyridine 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 interchangereaction, in which the olefin attack occurs simultaneously withthe pyridine derivative dissociation, or in a two-step associativemechanism, where the first step is the olefin associationfollowed by the 3-bromopyridine dissociation. Our computa-tional results show that the energy barrier of the one-stepreaction for 9 is 14.5 kcal/mol for the trans attack and 25.9kcal/mol for the cis attack (see the Supporting Information).On the other hand, the two-step associative mechanism has anenergy barrier of only 11.7 kcal/mol for 9, making it the mostprobable route from the energetic point of view andsimultaneously the limiting step for the entire initiationreaction.We also compared the initiation pathways for the six-

coordinated complexes containing various NHC ligands: IMesand SIMes (9 and 17, respectively). Our DFT analysis suggeststhat the initiation follows the same mechanism, which includessulfoxide dissociation followed by an olefin association. For theSIMes-bearing complex, the energy barrier for TS1 is lower, butfor TS2 the rate-limiting step is slightly higher. This results inan overall higher energy barrier for complex 17 that is in goodagreement with the decreased catalytic profile that has beenobserved during the preparative experiments.We can directly compare the evaluated energy barrier with

the energy barrier for 7, i.e., without the additional pyridineligand. In the case of these sulfoxide Hoveyda derivatives, thedifference between the energy barrier for the dissociative andinterchange paths is small and equal to 0.9 kcal/mol (Figure 8).

This value is in accordance with the recent experimental workof Plenio et al. for the similar five-coordinated Hoveyda−Grubbs-type complexes, which suggests that the initiationreaction may simultaneously follow two different mechanisticpathways.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

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

■ CONCLUSIONS

We report on the synthesis of ruthenium-based metathesiscatalysts containing simultaneously a sulfoxide moiety and a 3-bromopyridine ligand. The obtained complexes are active inolefin metathesis, leading to the formation of trisubstituteddouble bonds. The performed DFT study revealed energeticdifferences in the mechanistic pathways of the studiedcomplexes. In the course of the theoretical study it wasexplained that six-coordinated sulfoxide complexes initiatefaster due to a different initiation mechanism in operation,which has a lower energy barrier.

■ EXPERIMENTAL SECTIONGeneral Procedure for Pyridine-Containing Ruthenium

Complex Synthesis. A selected second-generation complex (0.2mmol) was placed in a Schlenk tube under argon. Next, 3-bromopyridine was added (3 mmol, 0.28 mL). The resulting mixturewas stirred for an hour at room temperature. After that time an excessof n-pentane was added, resulting in precipitation of the product. Theprecipitate was filtered off, washed with a portion of n-pentane, anddried 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 aninternal standard, were put in a Schlenk tube under Ar and dissolved in5 mL of anhydrous toluene. The mixture was heated to the desiredtemperature, and the examined ruthenium complex was added,marking this time as the reaction start. Samples (0.2 mL) weretaken and immediately quenched with 2 M ethyl vinyl ether solution inDCM (0.2 mL) after 5, 10, 15, and 30 min and 1 h and then everyhour until 8 h. GC analysis was performed shortly after samplecollection.

■ ASSOCIATED CONTENT

*S Supporting InformationDetails of experimental procedures, analytical data of theobtained compounds, and description of computationalmethods applied. This material is available free of charge viathe Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: klgrela@gmail.com.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

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

Figure 8. Free energies of the initiation of 7.

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