Steric Maps to Evaluate the Role of Steric Hindrance ...

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Procedia Computer Science 18 (2013) 845 – 854

2013 Internationall Conference on Computational Science

Steric maps to evaluate the role of steric hindrance on thhe IPr NHC ligand Albert Poatera,*, Laura Faliveneb, César A. Urbina-Blancoc, Simone Manzinnic, Steven P. Noolanc, Luigi Cavallob,d a

Institut de Química Computacional, Departament de Química, Q Universitat de Girona, Campus de Montilivi, E-17071 Girrona, Catalonia, Spainn. E-mail: [email protected] b Dipartimento di Chimica e Biologia, Università U di Salerno, Via Ponte don Melillo, 84084, Fisciano, Italy.. c EaStCHEM School of Chemistry, University of o St Andrews, North Haugh, St Andrews, Fife, KY16 9ST, United Kingdom. K d King Abdullah University of Science and Technologgy (KAUST), Physical Sciences and Engineering, Kaust Catalysis Ceenter, Thuwal 23955-6900, Saudi Arabia.

Abstract

Density functional theory (DFT) calculationss were used to predict and rationalize the effect of thee modification of the structure of the prototype 1,3-biss(2,6-diisopropylphenyl)imidazol-2-ylidene) (IPr) N-heterocyclic N carbene (NHC) ligand. The modification coonsists in the substitution of the methyl groups of orrtho isopropyl substituent with phenyl groups, and here we w plan to describe how such significant changes efffect the metal environment and therefore the related catalyttic behaviour by simple steric maps. Bearing in mind that there is a significant structural difference between IPrr and IPr* ligands, that translated in different reactiviity for several olefin metathesis reactions, here by means of o DFT we characterize where the NHC ligand plays a more active role and where it is a simple spectator, or at a least its modification does not significantly changee its catalytic role/performance. Furthermore, this communnication endeavours to modify further the skeleton of the IPr NHC ligand. The optimization of these bulky new systems go to the limits of the DFT computational meethod. Keywords: NHC ligand; olefins metathesis; ruthenium catalysts; c DFT calculations; IPr

1. Introduction Society is facing tremendous challenges in thhe 21st century to maintain earth and its living beings in a suustainable way. With a growing population and dwindling resourcces, the need to provide clean water, food and energy, as weell as affordable health care for everybody in an environmentallyy benign and sustainable way, will occupy the efforts of many m scientists. Seeking to contribute to solutions for the Worldd’s well-being from many different angles, we regard thee synthesis and characterization of more efficient catalysts for oleefin metathesis. It is fundamental to reduce the waste for ennvironment, but

1877-0509 © 2013 The Authors. Published by Elsevier B.V. Selection and peer review under responsibility of the organizers of the 2013 International Conference on Computational Science doi:10.1016/j.procs.2013.05.249

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Albert Poater et al. / Procedia Computer Science 18 (2013) 845 – 854

also for the reduction of the economic cost of transforming organic substrates. Finding new catalysts is a hard but exciting task, often driven by trial and error. In this respect, computational techniques can be used to screen novel catalyst architectures more rapidly to obtain insights that could help in the design and experimental synthesis of novel and improved catalysts. Bearing in mind the great work done in metathesis and mainly in Ru-based catalysts with N-heterocyclic carbenes (NHC, Figure 1),[1] here we wish to use Density Functional Theory (DFT) to characterize how adding steric hindrance to a particular NHC such as IPr ( IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) affects its reactivity.[2,3] DFT-based prediction of the reactivity of new catalysts is not a new approach, and systematic methods are searched for a priori computational prediction to see whether a catalyst will be feasible experimentally, productive and fruitful in industry.[4]

Figure 1. Commonly used NHC in ruthenium olefin metathesis.

Taking into consideration that the latest big leap in the evolution of ruthenium based olefin metathesis catalysts was made by exchanging one of the phosphines in the first generation catalyst for a N-heterocyclic carbene ligand,[5,6] here we search how present computational tools might help improve the design of NHC ligands. The so-called second generation catalysts (those bearing a NHC ligand) are much more active in olefin metathesis than their first generation counterparts, and although many variations of the basic layout have been disclosed over the past 10 years,[7,8] a single universal catalyst superior to the rest of the field in all metathesis transformations has not been achieved, instead, a range of catalysts able to fulfil particular requests have been found. Here we want to examine if it is reasonable to continue exploring new NHC ligands with ever increasing complexity with respect to the common IPr, its saturated backbone relative SIPr and the 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIMes) congener,[8,9] and rationalize the reactivity differences observed between IPr-, IPr*- and IPr*Tol-bearing olefin metathesis catalysts.[10]

1

2

3

Figure 2. Ruthenium based olefin metathesis catalysts with IPr (1), IPr* (2), and IPr*tol (3) NHC ligands.

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2. Computational Details All DFT static calculations were performed at the GGA level with the Gaussian09 set of programs,[11] using the BP86 functional of Becke and Perdew.[12] The electronic configuration of the molecular systems was described with the standard split-valence basis set with a polarization function of Ahlrichs and co-workers for H, C, N, P, and Cl (SVP keyword in Gaussian).[ 13 ] For Ru we used the small-core, quasi-relativistic Stuttgart/Dresden effective core potential, with an associated valence basis set contracted (standard SDD keywords in gaussian09).[11] The geometry optimizations were performed without symmetry constraints, and the characterization of the located stationary points was performed by analytical frequency calculations. The reported energies have been optimized via single point calculations on the BP86 geometries with triple zeta valence plus polarization (TZVP keyword in Gaussian) using the M06 functional,[14] however estimating solvent effects with the polarizable continuous solvation model PCM using CH2Cl2 as solvent.[15] For all energies given throughout the text, zero point energies and thermal corrections calculated at the BP86 level were added to the M06 in solvent energies to approximate free energies in solvent. Bearing in mind that Mayer Bond Order (MBO) theory gives insight into the strength of the bonds,[16] MBOs between two atoms A and B have been calculated through the eq 1,[17] where S is the atomic orbital overlap matrix and P is the density matrix. The sums run over the basis set functions belonging to a given atom A or B. (1) The electrophilicity of the complexes was evaluated thanks to the Parr electrophilicity index shown in eq 2.[18]

ω=

μ2 2η

,

(2)

where ȝ and Ș are the chemical potential and the molecular hardness, respectively. In the framework of DFT,[19] ȝ and Ș for a N-electron system with total electronic energy E are defined as the first and second derivatives of the energy with respect to N at a fixed external potential.[20] In numerical applications, ȝ and Ș are calculated with the finite difference formulas of eq 3, which are based on Koopmans’ approximation,[21] 1 1 (3) μ ≅ (ε L + ε H ) and η ≅ (ε L − ε H ) , 2 2 where εǾ and εL are the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. Over the last years, conceptual DFT has been used to explain the reactivity pattern, and in particular the regioselectivity in chemical reactions.[22]

3. Results and Discussion DFT calculations have initially been performed on the geometry of complexes 1-3 in Figure 3 and the optimized structure of complex 1 is overall in excellent agreement with the X-ray structure (rmsd=0.033 Å on distances and 0.8° on angles).[23,24] The effect of the coordination of IPr and IPr* in the Ru catalysts was first evaluated by analysing the strength of the RuNHC bond. As expected, the Ru-IPr bond is somewhat shorter than the Ru-IPr* and the Ru- IPr*Tol bonds for the complexes containing PPh3, while the difference is minimal in the later complexes. To have a better understanding of the different stability of the Ru-NHC and Ru-P bonds in 1-3, we performed a Mayer Bond Order (MBO) analysis.[16,17] The analysis showed that the Ru-P bond trans to the NHC ligand, displays MBO values of 0.550, 0.587, and 0.581 for 1-3 respectively; while the Ru-NHC MBOs are 0.817, 0.777, and 0.776. These results are in line with the evidence that the modifications on the IPr ligand weaken the Ru-NHC bond significantly (comparing systems 2 and 3 with 1), thus enhancing interaction of the other labile ligand with the metal. Focusing on the MBOs of the Ru-indenylidene bond, 1.472, 1.456, and 1.457 for 1-3 respectively indicates that replacing the IPr ligand by the sterically demanding IPr* and IPr*Tol ligands impacts the strength of the Ru-NHC and Ru-P bonds, but has minor influence on the Ru-alkylidene bond. The weakened Ru-IPr* and Ru-IPr*Tol bonds in 2 and 3 can be ascribed to steric repulsion between the bulky ortho-CHPh2 groups of these NHC ligands and PPh3.

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1

2

3

Figure 3. 1-3 catalysts.

The energy cost for the dissociation of PPh3 was evaluated next (Figure 4), spanning from 14.9 for 1 to 12.0 for 2 and 19.4 kcal/mol for 3. Although the differences are not big, they are significant. Surprisingly, the energies follow the trend 3 (IPr*Tol) > 1 (IPr) > 2 (IPr*). The value for 2 is uncertain because it was expected that the substitution of IPr by IPr* could improve the activation significantly, however, the dissociation is disfavored by 2.9 kcal/mol. In contrast, and as expected, dissociation of PPh3 from 3 requires 4.5 kcal/mol more than for 1. Although the barrier for dissociation could be helpful for the representation of the thermodynamic of the labile ligand dissociation, the large size of the modified IPr ligands, specially the IPr*Tol, excludes the search for these barriers. However, here conceptual DFT supposes a great tool to overcome such problems and it is a cheap tool to compare the reactivity between similar catalysts from an electronic point of view, and could avoid the study of all the species involved in the olefin metathesis pathway for future comparisons with other NHC ligands. Thus, although this modification of the IPr ligand deals principally with the steric influence of the IPr* ligand, we also examined simple electronic properties such as chemical hardness and electrophilicity for complexes 1-3.[1821] The chemical hardness values are 0.0123, 0.0126, and 0.0148 a.u., which means that complexes containing the modified IPr ligands are less hard, and thus tend to be less reactive, specially complex 3 with the IPr*Tol ligand. On the other hand, the electrophilicity values are 0.719, 0.701, and 0.670 a.u., and these values confirm that the affinity to react with a nucleophile is similar for all complexes, following the trend of the bulkier the NHC ligand the less reactive the system is, mainly due to the reduction of the free space around the Ru center. Thus, these low differences confirm that the diverse catalytic performance of these complexes should be mainly related to different steric properties. Moving to the olefin metathesis reaction itself, for the sake of clarity we used ethylene as the olefin substrate in the calculations.[25] Coordination of ethylene to the 14e- substrate, see Figure 4, is an endothermic step for all complexes except for 3, where there is a release of 1.8 kcal/mol.

Figure 4. Energy profile of olefin metathesis of complexes 1-3 with ethylene (energies are in kcal/mol).


The following step is the metallacycle formaation, and again, the complexes behave similarly with an energy e release, although small differences can be found. For all species the metallacycle is more stable than the coordinatioon intermediate CI1, but for system 1, with IPr, the metallacycle is more stable than the CI species, by 2.8 kcal/mol. Then, in the presence of the bulky IPr*Tol in 3, the metallacycle is 3.1 kcal/mol less stable than the CI intermediate, whereas forr the less bulky 1 kcal/mol. However, these energy differences are scarce and no trend is IPr* in system 2 this difference reduces to just 1.0 detected. The main goal of the geometrical modificationn of the IPr ligand was to understand how the modified steriic properties of IPr* effected the reactivity. To analyze the steriic influence of the ligand we used topographic steric maps. The points in space defining the steric map were located with thhe SambVca package developed by us.[26] This program annalyzes the first coordination sphere around the metal, which is thhe place where catalysis occurs. It is normally used to calcuulate the buried volume of a given ligand, which is a number that t quantifies the amount of the first coordination spherre of the metal occupied by this ligand.[27] A modified versioon of SambVca allows the user to perform a more detailled analysis by evaluating the %VBur in the single quadrants around the Ru center, as well as the steric map of the NHC in thhe Ru-systems. Splitting the total %VBur into quadrant contributioons quantifies any asymmetry in the way the ligand wraps arround the metal and allows to understand how changing the ligandd from IPr to IPr* modifies the shape of the reactive pockeet.[28] We have already introduced topographic steric maps to chharacterize Ru-complexes relevant to olefin metathesis.[266,28] The steric maps were calculated for the free ligands first, without w symmetry constraints, and then for the ligands in com mplexes 1-3 as well as in the next 14e species, as well as for the metallacycle m intermediates.

Figure 5. Stteric maps vs geographic physical maps.

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For an easy comprehension, these steric maps allow comparative analyses between different complexes. Indeed, steric maps could be considered as the classical geographic physical maps (see Figure 5), that depict the physical features like various landforms and water bodies present on the Earth’s surface. Different colors, lines, tints, shading and spot elevations are used to show the elevation and to differentiate lowlands from the mountains in physical maps. We used the same philosophy to build the steric maps of the complexes. The Ru center is at level zero, and the ligands are placed at the same plane or below the metal. In this framework, brown areas indicate zones where the ligand protrudes like a mountain towards the reacting groups, thus limiting the space at their disposal, whereas blue areas indicate empty zones where the ligand retracts like a lake from the reacting groups. The map of the free NHC ligand for IPr* reported in Figure 6 shows clearly that is much more sterically demanding than IPr, whereas for IPr*Tol this comparison is not so clear. The buried %VBur is 50.0% for IPr* and and 51.0% for IPr*Tol, whereas a value of only 42.2% is found for IPr. Bearing in mind that the topographic maps characterize the ligand surface offered to the substrate, it is clear that the modified IPr ligands have an intrinsic strong steric hindrance that after binding to the metal precludes coordination of big substrates.

(a)

(b)

(c)

Figure 6. Topographic steric maps of the free (a) IPr, (b) IPr* and (c) IPr*Tol NHC ligands. The isocontour curves of the steric maps are in Å. The xz plane is the mean plane of the NHC ring, whereas the yz plane is the plane orthogonal to the mean plane of the NHC ring, and passing through the carbene C atom of the NHC ring. The carbene C atom of the NHC ring is at the origin.

Focusing on the steric maps in Figure 6 in more detail,(1994) the IPr map shows that the two quadrants on the right are slightly more hindered, but this asymmetry is negligible compared to the asymmetry in the IPr* map, where the distribution of the steric bulk around the metal is remarkably different and the hindrance is much more localized into two quadrants (top right and bottom left quadrants %VBur~70%, top left and bottom right quadrants %VBur~29%). Thus, although the %VBur values are similar for IPr* and IPr*Tol the steric maps are key to further understand that the simple para substitution of the phenyl groups in IPr*Tol with respect to IPr* allows more reactive surface for a substrate around the metal with IPr*Tol. Figure 7 reports the steric maps for the three ligands in catalysts 1-3. The maps are very different from those of the free NHCs. In particular the IPr* ligand is able to rearrange in less sterically demanding conformations with the aromatic groups on the NHC somewhat bent to adopt a final position further away from the metal. The %VBur 29.1, 28.6 and 28.7 for complexes 1-3, respectively, thus with no significant differences. To further evaluate differences between IPr and IPr* we calculated the steric maps for the 14e- species, see again Figure 8. In these systems the modified IPr* and IPr*Tol ligands create higher steric constraints than IPr. In particular, they again adopt a quite asymmetric folding, creating a groove into which the olefin has to coordinate. Differently, the IPr ligand shapes a flat surface to host the incoming olefin. Overall, this analysis indicates that modified IPr ligands can be seen as rather flexible ligands that are able to exert steric pressure to push away the incoming substrate. At the same time, they are not too rigid, so that they can retract away to make space for other ligands. This analysis highlights the intrinsic dynamic behavior of the modified IPr ligands, indicating that the interaction between the complex and the substrate requires a conformation rearrangement of the complex, more consistent with an induce fit model rather than with a key and lock model.

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1

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3

Figure 7. Topographic steric maps of catalysts 1-3. The isocontour curves of the steric maps are in Å. The xz plane is the mean plane of the NHC ring, whereas the yz plane is the plane orthogonal to the mean plane of the NHC ring, and passing through the carbene C atom of the NHC ring. The Ru atom is at the origin.

1

2

3

Figure 8. Topographic steric maps of the 14e- species for complexes 1-3. The isocontour curves of the steric maps are in Å. The xz plane is the mean plane of the NHC ring, whereas the yz plane is the plane orthogonal to the mean plane of the NHC ring, and passing through the carbene C atom of the NHC ring. The Ru atom is at the origin.

1

2

3

Figure 9. Topographic steric maps of the CI species for complexes 1-3. The isocontour curves of the steric maps are in Å. The xz plane is the mean plane of the NHC ring, whereas the yz plane is the plane orthogonal to the mean plane of the NHC ring, and passing through the carbene C atom of the NHC ring. The Ru atom is at the origin.

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1

2

3

Figure 10. Topographic steric maps of the Metallacycle species for complexes 1-3. The isocontour curves of the steric maps are in Å. The xz plane is the mean plane of the NHC ring, whereas the yz plane is the plane orthogonal to the mean plane of the NHC ring, and passing through the carbene C atom of the NHC ring. The Ru atom is at the origin.

Experimentally, it was observed that complexes bearing the IPr* ligand are only more active than complexes bearing the IPr ligand when low hindered substrates are used, and that complex 1 was the most active of all.10 However, when bulkier substrates were used, the IPr bearing complexes showed always superior activity. This can be easily explained by analysis of the steric maps and of the energy profiles we calculated. Even though the energy barrier of the phosphine dissociation is similar and even higher for IPr*, from the steric maps the bulkier IPr* ligand favors dissociation of bulky labile ligands such as PPh3, and still can coordinate well small substrates, thus resulting in good catalytic performances. Consistently, coordination of bulky substrates is more difficult for the IPr* bearing complexes, thus making the reaction proceed significantly slower than with their IPr counterparts. In Figures 8-10 the steric maps of the 14e-, CI and Metallacycle species show that surprisingly the %VBur values for IPr*Tol are lower than for IPr*. For the 14e- species are 33.0, 35.3 and 34.4 for complexes 1-3, respectively; for the CI 30.9, 32.4, and 31.3, whereas for the Metallacycle intermediate 32.5, 33.5, and 32.9. Thus, the results of IPr*Tol look promising because they seem to collect the advantages of the higher steric pressure of IPr* with respect to IPr. The para substitution of the phenyl groups allows to increase the free surface around the metal because the modification renders the IPr*Tol less flexible than IPr* as can be observed in the steric maps of the free ligand species in Figure 7. However, bearing in mind previous studies,29 we must point out that the differences between IPr* with respect to IPr*Tol are subtle, and the a priori expected higher sterical hindrance of IPr*Tol is not translated into new relevant insights, suggesting that the modification of the NHC must be focused in the part nearer to the metal core.

Conclusions Screening of new catalysts in silico is an area that could assist experimental tests in targeting preferred structural types. In this contribution, the catalytic performance of ruthenium IPr, IPr*, and IPr*Tol complexes were tested in such a manner for an olefin metathesis reaction. The calculations allowed the rationalization of the experimental findings and thus validate the computational mode, as well as to predict a new NHC ligand such as IPr*Tol. These results also highlight the potential of computational methods to assist in the development of novel catalyst types in silico thereby providing a significant resource-saving tool to rational catalyst design.


Acknowledgements The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° CP-FP 211468-2 EUMET. A.P. and L.C. thank BSC (QCM-2010-2-0020), and the HPC team of Enea for using the ENEA-GRID and the HPC facilities CRESCO in Portici (Italy) for access to remarkable computational resources. A.P. thanks the Spanish MICINN for a Ramón y Cajal contract (RYC-2009-05226), European Commission for a Career Integration Grant (CIG09-GA-2011-293900), and Generalitat de Catalunya (2011BE100793). SPN is a Royal Society Wolfson Research Merit Award holder.

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