Quantifying the Donor−Acceptor Properties of Phosphine and N

Jul 17, 2009 - rove the catalytic performance of their corresponding phosphine analogues, and therefore, understanding their differences could have no...
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Organometallics 2009, 28, 4283–4287 DOI: 10.1021/om900180m

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Quantifying the Donor-Acceptor Properties of Phosphine and N-Heterocyclic Carbene Ligands in Grubbs’ Catalysts Using a Modified EDA Procedure Based on Orbital Deletion Nadya S. Antonova, Jorge J. Carb o,* and Josep M. Poblet* Departament de Quı´mica Fı´sica i Inorg anica, Universitat Rovira i Virgili, 43007 Tarragona, Spain Received March 9, 2009

The σ-donation and π-acceptor properties of phosphine and N-heterocyclic carbene (NHC) ligands in first- and the second-generation Grubbs’ catalysts have been theoretically evaluated by means of a modified version of the energy decomposition analysis (EDA). According to the calculations, the NHC ligand is significantly more tightly bonded than the phosphine ligand as a consequence of the larger energetic contributions of the σ-donation, π-back-donation, and synergic effects. Despite its larger σ-donor strength, the NHC ligand is a slightly poorer charge σ-donor and better π-acceptor than phophine, explaining the recently observed overall poorer charge donation of the NHC ligand.

During the past few years N-heterocyclic carbenes (NHC) have become an alternative to the ubiquitous phosphine ligands in transition metal chemistry and homogeneous catalysis.1 Initially, NHC ligands were regarded as predominantly σ-donors, while their π-acceptor strength was thought to be rather weak, although the carbene carbon atom has a formally empty pπ orbital.2 Nowadays, it is accepted that NHC ligands can act as π-donors in electron-deficient metal complexes,3 and there is increasing evidence that these ligands can also be regarded as π-acceptors,4,5 the extent of the latter property being a matter of debate.5b Very recently, Cavallo and co-workers have reviewed the computational research to understand the nature of the TM-NHC bond.6 Scheme 1 shows the most important interactions between NHC ligands and transition metals. Frequently, the NHC carbene ligands complement and improve the catalytic performance of their corresponding phosphine analogues, and therefore, understanding their differences could have notable implications on catalyst design. An outstanding example is the Grubbs’ catalyst (PCy3)2Cl2RudCHPh and its

Scheme 1

second-generation derivatives modified with N-heterocyclic carbenes (H2IMes)(PCy3)Cl2RudCHPh (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazoline-2-ylidene), which catalyze effectively a variety of olefin metathesis reactions.7 The replacement of phosphine ligand (PCy3) in the former complex by a N-heterocyclic carbene (H2IMes) improves the catalytic activity and stability at high temperatures.7,8 Initially, it was thought that the stronger σ-donation from NHC ligands would accelerate the initiation step, i.e., the dissociation of the tricyclohexylphosphine (PCy3) ligand. However, Grubbs and co-workers showed that NHC ligands do not labilize the trans-PCy3 and that phosphine dissociation is faster in the first- than in the second-generation Grubbs’ catalysts.9 Improved catalytic efficiency was therefore rationalized by a shift in the rate-determining step, which is phosphine dissociation in the second-generation catalysts, whereas the rate-determining step in the first-generation catalyst occurs later in the catalytic cycle.10,11

*Corresponding authors. E-mail: [email protected]. Phone: +34 977 55 81 81. Fax: +34 977 55 95 63. (1) N-Heterocyclic Carbenes in Transition Metal Catalysis; Glorius, F., Ed.; Springer: Berlin, 2007. (2) (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39–92. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309. (3) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516–3526. (4) In vitro evidence of π-back-donation: (a) Sanderson, M. D.; Kamplain, J. W.; Bielawski, C. W. J. Am. Chem. Soc. 2006, 128, 16514– 16515. (b) Mercs, L; Labat, G.; Neels, A.; Ehlers, A.; Albrecht, M. Organometallics 2006, 25, 5648–5656. (c) Khramov, D. M.; Lynch, V. M.; Bielawski, C. W. Organometallics 2007, 26, 6042–6049. (5) In silico evidence of π-back-donation: (a) Hu, X.; Castro-Rodriguez, I.; Olsen, K.; Meyer, K. Organometallics 2004, 23, 755–764. (b) Nemcsok, D.; Wichmann, K.; Frenking, G. Organometallics 2004, 23, 3640–3646. (c) Jacobsen, H.; Correa, A.; Costabile, C.; Cavallo, L. J. Organomet. Chem. 2006, 691, 4350–4358. (d) Tonner, R.; Heydenrych, G.; Frenking, G. Chem. Asian J. 2007, 2, 1555–1567. (e) Radius, U.; Bickelhaupt, M. Organometallics 2008, 27, 3410–3414. (6) Jacobsen, H.; Correa, A.; Poater, C.; Cavallo, L. Coord. Chem. Rev. 2009, 253, 687–703.

(7) (a) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100–110. (b) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953–956. (8) (a) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674–2678. (b) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Hermann, W. A. Angew. Chem., Int. Ed. 1998, 37, 2490–2493. (9) (a) Sanford, M. S.; Ulmann, M.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 749–750. (b) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 6543–6554. (10) (a) Straub, B. F. Adv. Synth. Catal. 2007, 349, 204–214. (b) Straub, B. F. Angew. Chem., Int. Ed. 2005, 44, 5974–5978. (11) (a) Torker, S.; Merki, D.; Chen, P. J. Am. Chem. Soc. 2008, 130, 4808–4814. (b) Adlhart, C.; Chen, P. J. Am. Chem. Soc. 2004, 126, 3496–3510.

r 2009 American Chemical Society

Published on Web 07/17/2009

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Organometallics, Vol. 28, No. 15, 2009 Scheme 2

Antonova et al. Table 1. Calculated and Experimental Phosphine Dissociation Energies and Atomic Chargesa 1a ΔEdiss (expt)b qRu (Mull.) qRu (MDC) ΔqRu (Mull.)c ΔqRu (MDC)c

18.5 (19.9) 0.61 0.69

2a

1b

2b

0.56 0.77

23.4 (23.0) 0.93 0.96 +0.32 +0.27

0.83 0.83 +0.27 +0.06

Energies in kcal 3 mol-1 and atomic charge in au (Mulliken and multipole derived charges including quadrupole contributions, MDC). b Values taken from ref 9 for (L)(PCy3)Cl2RudCHPh (L=PCy3 and H2IMes) complexes. c ΔqRu is defined as the variation of atomic charge on going from phosphine (a) to NHC carbene (b) complexes for penta(1) and tetra-coordinated (2) species. a

Recently, Kennepolh and co-workers have explained the slower phosphine dissociation in the initiation step observed for the second-generation Grubbs’ catalyst relative to the first one from the different donor ability of the phosphine and NHC auxiliary ligands.12 The authors concluded that NHC ligands are poorer charge donors than the phosphine ligands, leading to a more electron-deficient metal center. This is in apparent contrast with the traditional view of N-heterocyclic carbenes as better σ-donors than phosphines. However, we should note that although the overall ligand donation can be determined experimentally, its decomposition into different factors and their individual contributions is more difficult. Here, we compare the donor-acceptor properties of phosphine and N-heterocyclic carbene ligands in first- and second-generation Grubbs’ catalyst by means of DFT calculations,13 energy decomposition analysis (EDA),14 and a modified version of EDA based on an orbital deletion procedure. The latter methodology allows separation and quantification of ligand donor-acceptor properties in real systems without symmetry. The theoretical study was performed on (L)(PCy3)Cl2RudCH2 (L=PCy3 1a, H2IMes 1b) and (L)Cl2Rud CH2 (L=PCy3 2a, H2IMes 2b) complexes (see Scheme 2). Table 1 collects the calculated phosphine dissociation energies and selected atomic charges. The computed values for dissociation energies are very close (∼1 kcal 3 mol-1) to those experimentally determined by means of NMR spectroscopy.9 More importantly, they reproduce the trend that dissociation is faster in first- than in the second-generation Grubbs’ catalysts. Recently, Harvey and co-workers15 and Truhlar and co-workers16 reported that some popular functionalities fail to predict the trend of the phosphine binding energies among these complexes. However, others authors11b,17 have also shown that the BP86/TZP level of theory is able to predict the right trend of bond dissociation energy on real ligands. The larger dissociation energy for the NHC complex with respect to the phosphine ones was attributed to a further (12) Getty, K.; Delgado-Jaime, M. U.; Kennepohl, P. J. Am. Chem. Soc. 2007, 129, 15774–15776. (13) DFT calculations were carried out with the ADF program by using a triple-ζ-quality basis set augmented with one set of polarization functions (TZP) for all atoms. The core electrons were kept frozen (C and N, 1s; Cl and P, 1s2p; Ru, 1s3d), applying to them the zero-order regular approximation (ZORA) to account for scalar relativistic effects. See the Supporting Information for further details. (14) (a) Morokuma, K. J. Chem. Phys. 1971, 55, 1236. (b) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1755–1759. (c) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1558–1565. (15) Tsipis, A. C.; Orpen, A. G.; Harvey, J. N. Dalton Trans. 2005, 2849–2858. (16) Zhao, Y.; Truhlar, D. G. Org. Lett. 2007, 9, 1967–1970. (17) Cavallo, L. J. Am. Chem. Soc. 2002, 124, 8965–8973.

destabilization of the 14e unsaturated complex (H2IMes)Cl2RudCHPh with respect to (PCy3)Cl2RudCHPh.12 The authors based their conclusions on the experimental ionization energies and calculated DFT atomic charges of the fourcoordinate phosphonium alkylidene complexes [(L)Cl2Rud CHPCy3]þ (L=PCy3, H2IMes). Our calculated ruthenium atomic charges for complexes 2a and 2b follow the same trends as experimentally characterized model phosphonium complexes, showing that the metal center has less electron density (more positive) for 2b, where L = H2IMes. The same trend is observed for the pentacoordinated species 1a and 1b (see Table 1). Thus it seems clear that, in the outstanding Grubbs’ catalysts species, the NHC ligands are poorer overall charge donors than phosphine ligands, leading to more electron-deficient metal centers. However, the σ-donor and π-acceptor contributions to the overall ligand interaction remain to be assessed, aiming to separate and quantify them. The energy decomposition analysis method divides the interaction between fragments into three terms: (i) the classical electrostatic interaction ΔEelstat between the unperturbed charges of the fragments, (ii) the Pauli term ΔEPauli, which accounts for repulsion between electrons of the same spin, and (iii) the orbital term ΔEorb that accounts for charge transfer and polarization effects.18 Since the original proposition by Morokuma,14a this energy decomposition scheme has been used in many metal-ligand systems,19 including the comparison between donor/acceptor contributions of phosphine and NHC ligands to group 6 (TM(CO)5, TM = Cr, Mo, W)5d and group 11 (Cl-Au)20 metal fragments. Table 2 summarizes the EDA results for the bonding interactions between the metal fragment (PR3)Cl2(CH2)Ru and the phosphine and N-heterocyclic carbene ligands. The metal-ligand interaction energies (ΔEint) in 1a and 1b were computed to be rather high, the latter being 14.7 kcal 3 mol-1 larger in energy. Therefore, we can claim that the NHC ligand is significantly more tightly bonded than the phosphine. The ratios between the attractive interactions (ΔEelstat/ΔEorb) are nearly identical for phosphine- and carbene-Ru bonds in 1a and 1b, respectively, suggesting that the two bonds display very similar electrostatic and covalent character. Interestingly, the difference in the interaction energy ΔEint on going from 1a to 1b (-14.7 kcal 3 mol-1) compares better with the difference in orbital energies (-19.2 kcal 3 mol-1) than with the (18) Bickelhaupt, F. M.; Nibbering, N. M. M.; van Wezenbeek, E. M.; Baerends, E. J. J. Phys. Chem. 1992, 96, 4864–4873. (19) For a review, see for example: Frenking, G.; Wichmann, K.; Fr€ ohlich, N.; Loschen, C.; Lein, M.; Frunzke, J.; Ray on, V. M. Coord. Chem. Rev. 2003, 238-239, 55–82. (20) Pyykk€ o, P.; Runeberg, N. Chem. Asian J. 2006, 1, 623–628.

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Table 2. Energy Decomposition Analysis for (PCy3)Cl2(CH2)RuL (1a and 1b) and (PH3)Cl2(CH2)Ru-L (1b0 ) Complexesa L

ΔEint ΔEPauli ΔEelstat ΔEorb ΔEσ(a0 ) ΔEπ(a00 )

PCy3 (1a)

H2IMes (1b)

Im (1b0 Cs)

-42.8 147.0 -123.1 (65%)b -66.5 (35%)b

-57.5 194.4 -166.1 (66%)b -85.7 (34%)b

-58.8 189.1 -170.5 (69%)b -77.4 (31%)b -64.7 (84%)c -12.7 (16%)c

a Energies in kcal 3 mol-1. b Percentage of total attractive interactions ΔEelest. þ ΔEorb. c Percentage of total orbital interactions ΔEorb.

differences associated with Pauli and electrostatic energies (þ47.4 and -43.0 kcal 3 mol-1, respectively). For molecules containing symmetry elements the ΔEorbital term can be easily divided into the contributions associated with each irreducible representation. Often the σ- and π-interactions fall in different symmetry representations, allowing their separation. For example, in the case of the NHC ligand, if we use Cs symmetry with the mirror plane containing the five-membered ring atoms the a0 contributions to the orbital interaction energy can be associated with σ-bonding, whereas a00 contributions can be related to the π-bonding (see Scheme 1). To obtain a more detailed picture of the contributions from different molecular bonding mechanisms (donation and back-donation), it is possible to extend the EDA analysis by successively excluding virtual orbitals from the variational space.5c,5d,21 Here, we will further extend the use of this orbital deletion procedure to analyze real size systems where symmetry restrictions cannot be used.22 Figure 1 schematically describes the methodology employed for the analysis of Ru-NHC and Ru-phophine bonding in complexes 1a and 1b. To account for σ-donation, referred to as ΔEσ(LfRu), we repeat the EDA analysis while consecutively deleting all the virtual molecular orbitals except those metal orbitals (dz2- and 5s-type) with the appropriate shape for σ-donation (Figure 1a). Repeating the same procedure but keeping the appropriate virtual orbitals of the ligand fragment provides data for the π-back-donation, referred to as ΔEπ(RufL). For NHC we keep the out-of-plane pπ-type orbital of carbene carbon, while for the PCy3 ligand two π*-type phosphorus orbitals perpendicular to the bond axis (see Figure 1b). This gives the energetic contribution to the bonding from the interaction between the occupied orbitals of one fragment and the selected unoccupied orbitals of the other fragment, which can be used to estimate the corresponding charge transfer from these orbital interactions (Δqσ(LfRu) and Δqπ(RufL)).21 We have also measured the synergic effect (ΔEsyn.) obtained from the extra stabilization gained by allowing for σ-donation and π-back-donation interactions taking place at the same time.21 Table 3 collects the results. To gain further insight into the σ- and π-partition of the orbital interaction energy, we have additionally considered the (21) Heinz-Bernhard, K.; Jacobsen, H.; Ziegler, T.; Boorman, P. M. Organometallics 1993, 12, 76–80. (22) Note also that in the case of model phophine ligands under Cs symmetry part of the π-interactions fall in the same symmetry representation as the σ ones.

Figure 1. Schematic representation of the modified EDA procedure based on orbital deletion. Evaluation of σ-donation (a) from the phosphine and NHC ligands (L) to (PCy3)Cl2RudCH2 metal fragment (TM) and of π-back-donation (b) from TM to L. Table 3. EDA Partition Energies (kcal 3 mol-1) and Mulliken Charge Transfer (au) for (PCy3)Cl2(CH2)Ru-L (1a and 1b) and (PH3)Cl2(CH2)Ru-L (1b0 ) Complexesa L

ΔEorb ΔEσ(LfRu)a ΔEπ(RufL) ΔEsyn ΔEpol(Ru) ΔEpol(L) Δqσ(LfRu)b Δqπ(RufL)b

PCy3 (1a)

H2IMes (1b)

Im (1b’ Cs)

-66.5 -31.9 (93%)a -2.6 (7%)a -2.1 -8.2 -22.0 0.36 0.05

-85.7 -35.9 (84%)a -7.1 (16%)a -6.2 -12.0 -27.2 0.33 0.10

-77.4 -39.1 (86%)a -6.6 (14%)a -6.7 -9.8 -20.7 0.34 0.07

a Percentage of total interaction ΔEσ(LfRu) þ ΔEπ(RufL). b Δqσ(LfRu) and Δqπ(RufL) are defined as the increase of electrons of ruthenium and ligand fragments, respectively, upon fragment interaction.

model complex (Im)(PH3)Cl2RudCH2 (Im=1,3-imidazoline2-ylidene, 1b0 ), which has a symmetry plane that contains the five-membered ring atoms, the RudC bond, and the P atom. The Ru-carbene bond in complex 1b0 exhibits a qualitatively similar interaction energy and ΔEelstat/ΔEorb ratio to those computed for 1b (see Table 2). The breakdown of the orbital term results in a σ-contribution (ΔEσ(a0 )) equal to -64.7 kcal 3 mol-1 and a π-contribution (ΔEπ(a00 )) equal to -12.4 kcal 3 mol-1. Although the σ-contribution dominates in the orbital interaction, the π-contribution is still noticeable (16% of the total ΔEorb), lying within the limits (about 15-30%) of previous calculations on TM-Im bonds for a number of model metal fragments.5c,5d The orbital deletion procedure described above estimates the L f Ru σ-donation as -39.1 kcal 3 mol-1 and the TM f L π-back-donation as -6.6 kcal 3 mol-1. Note that these values are significantly lower than those calculated for ΔEσ(a0 ) and ΔEπ(a00 ). The ΔEorb term accounts for the mixing between occupied orbitals of one fragment and unoccupied orbitals of the other fragment (σ-donation and π-backdonation), as well as the mixing of occupied and virtual orbitals within the same fragment (previously called

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intrafragment polarization).23 Unfortunately, the EDA method does not allow evaluating the latter effect separately. Nevertheless, we have performed additional calculations to estimate the polarization effects (ΔEpol.) of both fragments by allowing the orbital involved in donation/back-donation and all the remaining virtual orbitals of one fragment to interact at the same time (Table 3). These additional calculations should include the stabilization energy coming from polarization and the previously discussed effects (σ-donation, π-back-donation, and synergic effects). To ensure that most of the new stabilization does not come from orbital mixing between occupied and unoccupied orbital interfragments, we checked that the variational Mulliken charge transfer upon introduction of new virtual orbitals does not vary significantly. It also interesting to note that the sum of ΔEσ(LfRu), ΔEπ(RufL), and ΔEsyn terms for 1b0 is -52.2 kcal 3 mol-1, closer to the interaction energy ΔEint (-58.8 kcal 3 mol-1) than the orbital term ΔEorb (-77.4 kcal 3 mol-1). Thus, the values from the EDA orbital deletion procedure can be considered more reliable for quantifying the bonding mechanisms, at least in bonds with mostly dative character. Nevertheless, the σ-donation and π-back-donation percentages of the total interaction remain nearly identical to the percentages of the ΔEσ(a0 ) and ΔEπ(a00 ) contribution to the total ΔEorb, and therefore, the same conclusions about the nature of the metal-carbene bonding appear from both methodologies. Moreover, this also indicates that, as expected,6 the ΔEπ(a00 ) interaction should mostly correspond to the M f L π-back-donation, whereas L f Ru π-donation is significantly lower. Finally, we calculated σ-donation and π-acceptor contributions of the real size phosphine and N-heterocyclic carbene ligands in complexes 1a and 1b, respectively. Calculations on these complexes cannot make use of the symmetry partition, and therefore, the bonding analysis has to be restricted to the use of the EDA procedure based on the orbital deletion scheme. We estimated the strength of the L f Ru σ-donation by carrying out EDA calculations where all vacant orbitals were deleted except the dz2 Rubased orbitals (LUMO and LUMOþ1) and the 5s Ru-based orbitals (LUMOþ3) of the metal fragment (Figure S1). The analysis shows that the σ-donation of PCy3 and IMes ligands contribute notably to the overall interaction: -31.9 and -35.9 kcal 3 mol-1, respectively (Table 3). The major contribution to ΔEσ(LfRu) comes from the interaction with the dz2 Ru-based LUMO orbital (-28.9 and -30.6 kcal 3 mol-1 for PCy3- and IMes-Ru bonds, respectively), whereas the interaction with the 5s Ru-based LUMOþ3 orbital has a minor contribution (-0.3 and -1.0 kcal 3 mol-1, respectively), the 5s orbital being 3.3 eV higher in energy than 4dz2. Thus, the overall energy contribution from σ-donation to the metal-ligand bonding is larger (∼4 kcal 3 mol-1) for the NHC ligand than for the phophine. On the other hand, the calculated nonsynergistic Mulliken charge transfer is slightly smaller (∼0.03 au)24 for the NHC ligand, which seems in contradiction with the (23) (a) Mitoraj, M. P.; Michalak, A.; Ziegler, T. J. Chem. Theory Comput. 2009, 5, 962–975. (b) Rodríguez-García, C.; Gonzalez-Blanco, O.; Oliva, A.; Ortu~ no, R. M.; Branchadell, V. Eur. J. Inorg. Chem. 2000, 1073– 1078. (24) The same trends of calculated Mulliken charge transfers were observed for multipole derived charges including quadrupole contributions (MDC): Δq(LfRu)=0.29 and 0.22 au for 1a and 1b, respectively; Δq(RufL)=0.04 and 0.07 au for 1a and 1b, respectively.

Antonova et al.

Figure 2. Representation of selected frontier molecular orbitals: out-of-plane pπ-type orbital LUMOþ4 for the NHC fragment in 1b (a) and phosphorus π*-type LUMOþ10 for the phosphine fragment in 1a (b).

traditional view of NHC ligands as better σ-donors than phosphines. It seems clear that in this case energy contribution to the bonding and charge rearrangement do not correlate. Nevertheless, it is necessary to consider also the π-acceptor mechanism to have an overall understanding of Ru-L bonding. To evaluate the Ru f NHC π-back-donation in 1b, we consider the mixing between occupied metal fragment orbitals and the virtual molecular orbitals of the ligand fragment LUMO þ 4 and LUMO þ 8 (see Figure 2a and S2, repectively). These orbitals formally correspond to the empty out-of-plane pπ-type orbital of the carbene carbon. The calculated interaction is noticeable for the LUMOþ 4 (-5.5 kcal 3 mol-1) and residual for LUMO þ 8 (-1.6 kcal 3 mol-1), totaling -7.1 kcal 3 mol-1. In the former molecular orbital the contribution of carbene p orbitals is over 50%, whereas in the latter it is only about 15%. For the phophine fragment, the phosphorus π*-type orbitals (Figure 1b) are spread in four molecular orbitals, LUMOþ 1, LUMOþ 5, LUMOþ 7, and LUMO þ 10, with contributions of phosphorus p atomic orbitals ranging from 10% to 26% (see Figure 2b and S3). In these conditions, the π-acceptor interaction energy amounts to -2.6 kcal 3 mol-1, which is 3-fold lower than that of the carbene ligand. The molecular orbitals formally corresponding to the out-of-plane pπ orbital of the NHC ligand lie deeper in energy (-0.3 and þ0.7 eV) than those formally corresponding to phosphorus π*-type orbitals of the PCy3 ligand (from þ0.1 to þ1.2 eV), justifying the minor interaction extent of the latter orbital set. In this case, the calculated Mulliken charge transfers associated with the orbital deletion scheme follow the same trend as the π-back-bonding energies, experiencing a 2-fold increase for the NHC ligand (Table 3).24 Thus, comparing the calculated interaction energies and the corresponding charge rearrangements for the two types of ligands (Table 3), we observe that σ, π, and synergic interactions favor the Ru-L bonding in NHC complexes and that the differences in charge transfer are more pronounced in π-back-donation than in σ-donation. Consequently, the NHC ligand in 1b results as an overall poorer charge donor than the PCy3 ligand in 1a. This agrees with experimental findings12 and follows the trend of previously computed atomic ruthenium charges on complexes 1a and 1b. Note also that the trend is less pronounced in the orbital deletion scheme than in the Ru atomic charge analysis, where we considered overall metal-ligand interactions. In summary, our calculations show that the replacement of the phosphine ligand in a first-generation Grubbs’ catalyst with a NHC ligand in a second-generation Grubbs’ catalyst reduces the electron donation at the metal center, in

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agreement with previous findings. A detailed analysis of the Ru-L bond reveals that the overall charge donation of the ligands is a consequence of the balance between two distinct mechanisms: σ-donation and π-back-donation. The NHC ligand is significantly more tightly bonded than the phosphine ligand due to the larger σ-donation, π-back-donation, and synergic effects. Despite its larger σ-donor strength, the NHC ligand is a slightly poorer charge σ-donor and also better π-acceptor, resulting in a further overall decrease of the electron density at the metal center. Thus, the differences between Ru-phosphine and -NHC bonding in these catalysts are dominated by the further π-acidity of the NHC ligand. Moreover, the understanding of the bonding could have important implications on the design of more active catalysts. A less π-acid NHC ligand would increase its overall charge donor ability, and consequently, it would stabilize

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the unsaturated (H2IMes)Cl2RudCHPh intermediate 2. In turn, this would accelerate the phosphine dissociation, which is the rate-determining step in the olefin metathesis catalyzed by a second-generation Grubbs complex.

Acknowledgment. Financial support for this work was provided by the Spanish Ministery of Science and Innovation (CTQ2008-06549-C02-01/BQU) and by the DURSI of the Generalitat de Catalunya (2005SGR-00104). Supporting Information Available: Computational details, the 3D representation of frontier molecular orbitals for phosphine and NHC ligand fragments of complexes 1a and 1b, and the Cartesian coordinates of all computed structures. This material is available free of charge via the Internet at http://pubs.acs.org.