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Interpreting Oxidative Addition of Ph−X (X = CH3, F, Cl, and Br) to Monoligated Pd(0) Catalysts Using Molecular Electrostatic Potential Bai Amutha Anjali†,‡ and Cherumuttathu H. Suresh*,†,‡ †

Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum 695019, India ‡ Academy of Scientific & Innovative Research (AcSIR), New Delhi 110020, India S Supporting Information *

ABSTRACT: A B3LYP density functional theory study on the oxidative addition of halogenobenzenes and toluene to monoligated zerovalent palladium catalysts (Pd−L) has been carried out using the “L” ligands such as phosphines, N-heterocyclic carbenes, alkynes, and alkenes. The electron deficiency of the undercoordinated Pd in Pd−L is quantified in terms of the molecular electrostatic potential at the metal center (VPd), which showed significant variation with respect to the nature of the L ligand. Further, a strong linear correlation between ΔVPd and the activation barrier (Eact) of the reaction is established. The correlation plots between ΔVPd and Eact suggest that a priori prediction on the ability of the palladium complex to undergo oxidative addition is possible from VPd analysis. In general, as the electron-donating nature of ligand increases, the suitability of Pd(0) catalyst to undergo oxidative addition increases. VPd measures the electron-rich/-deficient nature of the metal center and provides a quantitative measure of the reactivity of the catalyst. By tuning the VPd value, efficient catalysts can be designed.

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unreactive,7 and thus efforts have been made to make them more reactive.7,12,34,39 Mechanistic studies in this area are pioneered by the works of Hartwig,14,35,39 Amatore,40 and Norrby.16,17,32,41 Various studies identified either the presence of coordinatively unsaturated 14-electron complex with two donor ligands (PdL2) or the one-ligand-dissociated 12-electron complex (PdL) as the major species in solution based on their ligand size.42 As per the typical mechanisms summarized in Scheme 1, the active form of the catalyst PdL2 either interacts with Ar−X directly and follows an associative pathway to give the oxidation product or dissociates to PdL and reacts with Ar− X to form a σ adduct. In the σ adduct, the C−X bond is activated and eventually breaks to form the oxidative addition product. Kozuch et al. have shown that the tricoordinated anionic Pd intermediate (Pd0L2Cl−) formed by introducing anions such as chlorides is more susceptible to an oxidative addition of aryl halides than the regular Pd0L2 catalyst.43,44 Senn and Ziegler have investigated the oxidative addition of phenyl halides to bidentate phosphines.22 A density functional theory study conducted by Ahlquist and Norrby on oxidative addition of aryl chlorides to monoligated Pd(0) has gained much attention recently. 16 Hartwig et al. studied the mechanism for the oxidative addition of haloarenes to trialkylphosphine Pd(0) complexes and evaluated the steric properties of ligands.14 Numerous studies show that the use of hindered phosphines as ligands for palladium complexes has

INTRODUCTION Palladium catalysts have emerged as an invaluable tool for organometallic synthesis, mainly in cross-coupling reactions such as Heck,1 Negishi,2 Suzuki−Miyaura,3 Stille,4 Sonogashira,5 and so forth, to form carbon−carbon and carbon-hetero atom bonds.6−11 In general, the initial step of the catalytic cycle comprises an oxidative addition of electrophilic aryl halide to zerovalent palladium center (Pd(0)), generating a Pd(II) aryl halo complex, which is often observed as the rate-limiting step.12,13 Hence, activation energy (Eact) for the oxidative addition becomes a key thermodynamic parameter to be tuned for the successful design of efficient catalysts, and several experimental14,15 and computational16−22 studies have been devoted to such attempts. Many of the developed Pd(0) catalysts are found to be most effective with two electrondonating phosphines15,23,24 and N-heterocyclic carbenes (NHCs),25−29 while a few studies have been reported for Pd(0) complexes of alkenes30 and alkynes.31,32 In the catalyst design strategy, the choice of ligands in Pd(0) systems can have a considerable impact on the reaction pathway, as the finetuning of Eact is often achieved by a balanced mix of steric and electronic effect exerted by the ligand on the metal center, which effectively determines the kinetic aspects of the reaction.33 Oxidative addition of aryl halides (Ar−X) to Pd(0) complexes has gained considerable interest in both experimental34,35 and theoretical20,36−38 studies owing to their wide range of applications in modern organic synthesis. Aryl bromides, iodides, and triflates are the general substrates used in this category, whereas chlorides are found to be generally © 2017 American Chemical Society

Received: June 7, 2017 Accepted: July 19, 2017 Published: August 3, 2017 4196

DOI: 10.1021/acsomega.7b00745 ACS Omega 2017, 2, 4196−4206

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In organometallic complexes, metal center plays the pivotal role in executing a reaction, and a single parameter that explains the reactivity of the metal center is yet to be established with respect to a correlation with the activation barrier of the reaction. Further, the question of how the selectivity of a ligand influences the performance of the metal center in the ratedetermining step of a reaction remains unanswered. For the first time, we provide a mechanistic interpretation of the oxidative addition of aryl halides to Pd(0) solely based on the MESP at the Pd center, which undergoes delicate changes with respect to changes in the ligand environment. MESP is an electronic property, and the quantification of it on the metal center provides an easy measure of the activation barrier of the oxidative addition. Thus, a novel use of MESP is unraveled in this work, which will also establish this quantity as an excellent electronic parameter for the direct quantification of the chemical reactivity. We have carried out the oxidative addition of aryl bromide, aryl chloride, aryl fluoride, and toluene to Pd(0) centers ligated with phosphines, NHCs, alkenes, and alkynes. As there is an increasing tendency to replace traditional phosphines with NHCs in organic synthesis, this work will also enable the comparison of the suitability of phosphine and NHC ligands. Several groups have tried to make Pd−phosphines and Pd− NHCs more bulky to ease the oxidative addition; however, the molecular design strategies were not based on a quantitative measure for the electron-rich/-deficient character of the metal center. MESP at Pd(0) gives a convenient measure of the ability of Pd(0) to undergo oxidative addition, and its calculation is available with many computational software. Our findings will pave the way for developing a rational design strategy for making efficient ligands in oxidative addition.

Scheme 1. Associative and Dissociative Pathways of Oxidative Addition

significantly improved the catalytic activity.45 In the case of Pd−phosphine complexes, Hartwig and Paul,46 Mitchell and Baird,47 Harvey et al.,20 and Brown and Jutand15 have demonstrated that even small changes to PR3 ligands can influence the mechanism of the reaction substantially, showing equilibria among PdL4, PdL3, and PdL2, as well as a preference for associative displacement pathways for the oxidative addition step. The present study focuses only on the reactivity of the monoligated PdL complexes (L = phosphines, NHCs, alkynes, and alkenes) in the dissociative pathway and uses molecular electrostatic potential (MESP) analysis as a tool to understand the subtle variations in the energetics of the oxidative addition of Ph−X (X = F, Cl, Br, and CH3) to Pd(0). MESP analysis has been established as an effective tool to forecast various aspects of the reactivity of chemical systems, including biomolecules.48,49,50 It has made a significant impact in predicting the stabilities and reactivities of diverse organometallic catalysts including first-generation Grubbs olefin metathesis catalysts;51 pincer catalysts;52 metal hydrides of Mo, W, Mn, Re, Fe, and Ru, which are applicable as watersplitting catalysts;53 and so forth. Recently, we have predicted the reduction potential (E0) values of mononuclear cobalt catalysts with the help of MESP at the cobalt nucleus54 and also showed that E0 of organometallic cobalt complexes can be finetuned using MESP at the metal center. MESP is also used to characterize and quantify the electron-donating ability of two electron ligands. Suresh and Koga55 characterized the lone-pair region of various substituted phosphine ligands and quantified their electron-donating power with the aid of MESP. MESP analysis has helped theoreticians and experimentalists in understanding the stereoelectronic profile of phosphine ligands, which led to the rational design of superior ligands.56 Quantification of steric and electronic effects of NHC ligands toward metal coordination has also been carried out using topographical analysis of MESP in which a linear correlation between Tolman electronic parameter and electrostatic potential at carbene carbon (VC) was established.57 Structure and reactivity of substituted arene−Cr(CO)3 complexes are also well-explained using MESP topography analysis.58 These studies of MESP have extensively helped in understanding the chemical reactivity.

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RESULTS AND DISCUSSION Ligands and MESP Features. The selected sets of ligands are depicted in Figure 1. The abbreviation ImNX2Y2 is used for naming the NHC ligands, where ImN represents the imidazole core unit while X and Y represent the N and C substituents, respectively.

Figure 1. Schematic representation of ligands selected for the study.

In Figure 2, MESP isosurface at −20.0 kcal/mol (−0.0319 au) is plotted for a representative set of optimized ligands along with their MESP minimum [a (3, +3) critical point (CP)] Vmin in kilocalorie per mole. This figure also depicts MESP value in au at the nucleus of phosphorus (VP) for phosphines, at the carbene carbon (VC) for NHC, at the alkyne carbon (VC), and at the alkene carbon (VC). The MESP value at the nucleus is 4197


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Table 2. Vmin, ΔVmin, and ΔVC of NHC Ligands (in kcal/ mol) L

Vmin

ΔVmin

ΔVC

ImNMe2H2 ImNH2H2 ImNMe2(COOMe)2 ImNMe2F2 ImNMe2Cl2 ImNH2F2 ImNMe2(CF3)2 ImN(CF3)2H2 ImNMe2(CN)2 ImNMe2(NO2)2

−82.1 −80.8 −72.1 −70.7 −70.1 −68.5 −63.0 −57.1 −55.7 −51.2

−1.3 0.0 8.7 10.1 10.7 12.3 17.8 23.7 25.1 29.6

−8.1 0.0 2.9 6.2 5.5 16.6 13.7 31.8 23.9 29.5

Table 3. Vmin, ΔVmin, and ΔVC of Alkynes (in kcal/mol) L

Vmin

ΔVmin

ΔVC

C2(NH2)2 C2(NMe2)2 C2Me2 C2(SiMe3)2 C2Et2 C2Ph2 C2H2 C2Br2 C2Cl2 C2F2

−30.4 −27.2 −25.0 −24.8 −24.6 −19.2 −18.1 −7.9 −6.9 nil

−12.3 −9.1 −6.9 −6.7 −6.5 −1.1 0.0 10.2 11.2 nil

−1.6 −6.1 −16.1 −30.6 −18.4 −6.0 0.0 31.8 37.8 71.7

Table 4. Vmin, ΔVmin, and ΔVC of Alkenes (in kcal/mol) Figure 2. Representation of MESP isosurface at −20.0 kcal/mol. Vmin in kcal/mol and VC and VP in au.

very high compared with the Vmin. Hence, in Tables 1−4, the relative values of VP and VC with respect to their respective unsubstituted systems PH3, ImNH2H2, C2H2, and CH2CH2 are reported along with their Vmin and ΔVmin values (usually, the term “unsubstituted” indicates that the substituent is H). ΔVmin Table 1. Vmin, ΔVmin, and ΔVP of Phosphine Ligands (in kcal/mol) L

Vmin

ΔVmin

ΔVP

PCy3 PtBu3 PiPr3 PMe3 PEt3 P(SiMe3)3 PHMePh PPh3 PH3 P(Ph−F)3 P(thiophene)3 P(Ph−Cl)3 P(SMe)3 P(Ph−CF3)3 PH2CF3 PCl2Ph PCl2Me PCl3 P(CF3)3 PF3

−43.1 −42.4 −40.5 −39.9 −39.7 −35.5 −32.1 −30.7 −25.5 −23.8 −23.8 −21.5 −17.7 −14.0 −9.3 −6.8 −6.3 nil nil nil

−17.6 −16.9 −15.0 −14.3 −14.1 −10.0 −6.5 −5.1 0.0 1.8 1.8 4.1 7.8 11.5 16.2 18.7 19.2 nil nil nil

−12.0 −8.7 −10.6 −4.4 −9.2 −33.9 0.2 5.4 0.0 14.4 20.9 16.6 33.9 25.2 23.8 60.5 61.3 92.5 59.5 129.7

L

Vmin

ΔVmin

ΔVC

CH2CMe2 CH2CEt2 CH2CH2 CH2C(SiMe3)2 CH2CPh2 CH2CCl2 CH2CBr2 CH2CF2 CH2C(CF3)2 CH2C(CN)2

−20.3 −19.5 −18.9 −17.8 −15.2 −7.5 −7.1 −0.8 nil nil

−1.4 −0.6 0.0 1.10 3.7 11.4 11.8 18.1 nil nil

−4.9 −6.6 0.0 −13.0 3.9 49.0 46.7 64.3 44.5 64.7

is the difference between the Vmin of the ligand and that of the unsubstituted reference ligand. The notations used for the relative values for phosphorus and carbon nuclei are ΔVP and ΔVC, respectively. Among the phosphines, PCy3 shows the most negative Vmin (−43.1 kcal/mol) followed by PtBu3 (−42.4 kcal/mol). Vmin of PiPr3, PMe3, and PEt3 (∼−40.0 kcal/mol) lies very close to PCy3, whereas a significant reduction in the negative character of Vmin is observed for PPh3 (−30.7 kcal/mol). The ligands PCl3, PCF3, and PF3 do not have Vmin, indicating the high electron-withdrawing nature of their P substituents (Table 1). Similarly, the ligands PH2CF3, PCl2Ph, and PCl2Me show substantial decrease in the negative character of Vmin, suggesting the electron-deficient nature of these ligands.62 For a particular ligand, the negative sign of ΔVmin indicates the electrondonating nature and the positive sign indicates the electronwithdrawing nature. A trend very similar to Vmin is observed for ΔVP, ΔVN, and ΔVC for most of the ligands (Tables 1−4). Nearly, a twofold increase in the negative character of Vmin is observed for NHC compared with phosphines, which suggests that the lone pair of NHC is more electron-rich and more 4198


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electron-donating toward coordination bonds than phosphine. A detailed elucidation of MESP analysis for several phosphines and NHCs is stated elsewhere.55,63 Very recently, Suresh, Gadre, and co-workers have shown that MESP Vmin characterizes lone pairs in molecules.64,65 In the case of alkyne ligands, amino-substituted C2(NH2)2 and C2(NMe2)2 show the most negative Vmin, whereas the severe withdrawing effect of F substituents in C2F2 leads to the disappearance of the negative MESP region for the CC triple bond. Similarly, alkylsubstituted alkenes show the most electron-rich double bonds (Vmin ≈ −20 kcal/mol), whereas CF3- and CN-substituted cases do not exhibit Vmin. The ΔVC follows a trend similar to that of ΔVmin for electron-withdrawing substituents, whereas alkyl- and silyl-substituted systems appear more electron-rich than amino-substituted systems in ΔVC than ΔVmin. The discrepancy may be due to the difference in the throughspace and through-bond interactions of the alkyl and amino groups; the former is mainly through bond active via inductive effect, whereas the through-space effect of the lone pair on the amino group may strongly influence the absolute value of Vmin for the CC π bond. Dissociation of Pd−L from PdL2. As stated in the Introduction, in the monoligated pathway, the substrate is oxidatively added to a 12-electron active catalyst (PdL), which is formed by the dissociation of the 14-electron bisligated complex (PdL2). The MESP values at the Pd nucleus of PdL2 and PdL are designated as VPd1 and VPd2, respectively. The VPd1 and VPd2 values of phosphine-, NHC-, alkyne-, and alkeneligated complexes are given in Tables S1−S4 of Supporting Information. The negative character of these quantities decreases with increasing electron-withdrawing power of the ligand. The relative values of VPd1 and VPd2 with respect to the unsubstituted systems (ΔVPd1 and ΔVPd2) are useful to make a quick comparison of the electron-donating/-withdrawing power of the ligands. Tables 5−8 depict these MESP parameters along with the Pd−P distance d1 for PdL2, Pd−P distance d2 for PdL,

Table 6. Pd−C Distances (Å), Relative MESP Values (kcal/ mol), and NHC Dissociation Energy (kcal/mol) of Pd(0) Catalysts

L

d1

d2

ΔVPd1

ΔVPd2

Edis

2.341 2.365 2.342 2.321 2.329 2.351 2.316 2.328 2.302 2.328 2.320 2.327 2.308 2.325 2.295 2.301 2.285 2.288 2.286 2.269

2.241 2.259 2.239 2.219 2.227 2.260 2.215 2.230 2.206 2.228 2.220 2.227 2.206 2.223 2.192 2.192 2.186 2.181 2.177 2.154

−16.0 −15.7 −14.1 −11.1 −12.9 −16.5 −4.4 −5.9 0.0 5.1 −0.1 7.6 9.1 17.5 23.0 25.7 34.4 47.5 47.6 53.0

−9.5 −10.0 −8.5 −5.9 −7.4 −11.8 −1.4 −1.7 0.0 5.7 3.0 7.3 10.0 14.5 16.3 23.2 24.2 36.3 35.8 37.2

31.8 30.4 30.9 31.1 30.6 27.4 29.9 29.8 26.8 30.1 29.0 29.8 26.8 29.0 25.5 25.2 26.6 21.3 22.3 22.3

d1

d2

ΔVPd1

ΔVPd2

Edis


2.053 2.031 2.051 2.050 2.051 2.027 2.050 2.031 2.044 2.044

1.971 1.951 1.971 1.971 1.971 1.949 1.971 1.951 1.962 1.963

−0.9 0.0 10.2 11.7 11.1 14.4 22.5 20.3 32.1 38.3

−1.7 0.0 5.3 6.0 5.5 8.7 12.9 14.3 19.3 24.1

41.3 40.9 41.3 41.5 41.4 41.0 41.5 36.2 41.1 40.8

Table 7. Pd−C Distances (Å), Relative MESP Values (kcal/ mol), and Alkyne Dissociation Energy (kcal/mol) of Pd(0) Catalysts L

d1

d2

ΔVPd1

ΔVPd2

Edis

C2(NH2)2 C2(NMe2)2 C2Me2 C2(SiMe3)2 C2Et2 C2Ph2 C2H2 C2Br2 C2Cl2 C2 F 2

2.085 2.089 2.128 2.167 2.132 2.114 2.114 2.070 2.064 2.033

2.077 2.092 2.096 2.138 2.099 2.083 2.083 2.046 2.041 2.020

−7.3 −13.4 −16.2 −20.2 −15.9 −2.1 0.0 35.5 39.3 62.2

−9.1 −13.7 −9.4 −11.9 −8.9 1.9 0.0 23.8 25.2 35.8

32.9 33.6 28.2 29.3 28.1 28.6 30.2 30.8 31.0 34.9

Table 8. Pd−C Distances (Å), Relative MESP Values (kcal/ mol), and Alkene Dissociation Energy (kcal/mol) of Pd(0) Catalysts

Table 5. Pd−P Distances (Å), Relative MESP Values (kcal/ mol), and Phosphine Dissociation Energy (kcal/mol) of Pd(0) Catalysts

PCy3 PtBu3 PiPr3 PMe3 PEt3 P(SiMe3)3 PHMePh PPh3 PH3 P(Ph−F)3 P(thiophene)3 P(Ph−Cl)3 P(SMe)3 P(Ph−CF3)3 PH2CF3 PCl2Ph PCl2Me P(CF3)3 PCF3 PF3

L

L

d1

d2

ΔVPd1

ΔVPd2

Edis


2.240 2.238 2.223 2.241 2.247 2.195 2.186 2.210 2.196 2.218

2.155 2.158 2.140 2.163 2.166 2.101 2.096 2.099 2.113 2.130

−9.6 −9.0 0.0 1.3 −1.8 28.7 30.6 23.8 48.8 57.6

−4.0 −3.5 0.0 2.2 3.0 24.9 26.3 22.8 34.6 41.6

21.5 21.9 25.2 21.4 20.4 17.7 20.2 16.1 18.6 17.3

and dissociation energy of L from PdL2 (Edis) for phosphines, NHCs, alkynes, and alkenes, respectively. In both PdL2 and PdL complexes, the Pd−P bond length is the highest for the PtBu3-ligated complex and the lowest for the PF3-ligated complex, which suggest that the Pd−P distance increases with electron-donating and bulky ligands. When the ligand is more electron-donating, ΔVPd values become more negative. The PCy3, PtBu3, and P(SiMe3)3 complexes show more negative ΔVPd2 and ΔVPd1 values. The electron richness of the metal center is directly proportional to its tendency to undergo oxidative addition. The Edis is more or less the same for Pd(0) complexes coordinated with alkyl-/phenyl-substituted phosphines, whereas it decreases with increasing electronwithdrawing effect by other ligands. A contradictory correlation aspect can be immediately noted between d1 or d2 distances and Edis. The Pd−P bond shortening leads to a decrease in the bond 4199


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strength. In the case of alkyl-/phenyl-substituted phosphines, the steric effect may retard the closer approach of the ligand to the metal, whereas the electron-rich nature of the ligand compensates it by providing more sharing of electrons in the Pd−P bond. In the case of electron-withdrawing ligands, the Pd−P bond is inherently weak because of the diminished electron density, whereas the reduced steric effect decreases the bond length. In addition, the back-bonding effect from metal to ligand may also influence the bond strength. From Table 6, it is evident that the Pd−C distances of Pd coordinated to NHCs fall on a narrow range of 2.03−2.05 Å for d1 and 1.95−1.97 Å for d2. The most negative ΔVPd1 and ΔVPd2 values are observed when the coordinating ligand is ImNMe2H2, indicating the high electron-donating character of the CH3 substituent at the N atom of NHC. Except for ImN(CF3)2H2, all the dissociation energy fall in the narrow range of 40.8−41.5 kcal/mol. In the case of alkyne complexes, the Pd−C distance in both Pd−L2 and Pd−L is higher when C2(SiMe3)2 is employed as a ligand. This is due to the steric influence of bulky SiMe3 substituent. The Pd−C1 and Pd−C2 distances of all the PdL complexes are the same, except that of Pd(C2(NMe2)2); in this case, Pd−C1 is 2.133 Å and Pd−C2 is 2.051 Å. The average of the two Pd−C distances is given in Table 7. Although the MESP parameters clearly distinguish the electron-rich ligands from the electron-deficient ones, the Edis values do not show a correlation pattern with these parameters or a pattern with the Pd−P bond distance data. This indicates that in addition to the σ-donating electronic effect from ligands, binding of the ligand to the metal is influenced by the steric effect and back-bonding effect arising from the interaction of filled metal d orbitals and π* orbital of alkyne. Among the PdL2 complexes of alkenes, the Cl, Br, F, CF3, and CN systems show two different Pd−C bond lengths while the rest of the systems show the same bond length for all the four Pd−C bonds. Two examples, viz., Pd(CH2CMe2)2 and Pd(CH2CCl2)2, are shown in Figure 3 to illustrate this

Figure 4. Illustration of the energy profile describing the oxidative addition of Ph−X to Pd(PtBu3).

distances in P1 for aryl halides are found to be 1.98 and 2.51 Å, respectively, whereas those of toluene are found to be 2.02 and 2.49 Å, respectively. The Eact of oxidative addition of substrates follows the order PhF ≈ Ph−Me ≫ PhCl > PhBr. Compared with the strong C−C and C−F bonds, the weaker C−Cl and C−Br bonds cleave with significantly less energy. The Eact data for all the Pd(PR3) complexes are provided in Table 9. In general, electron-rich phosphines such as alkyl substituted show lower Eact than those with electron-withdrawing substituents. Energy-profile diagram for the oxidative addition of Ph−X to the active catalyst Pd(ImNMe2H2) is given in Figure 5. The Edis of NHC in Pd(ImNMe 2 H 2 ) 2 is 41.3 kcal/mol. The monoligated complex forms an adduct I2 with Ph−X, which subsequently passes through a transition state TS2 to form the product P2. The adduct formation of substrate with Pd(NHC) leads to more energy lowering than that with Pd(PR3), whereas Eact data given in Table 10 show that NHC ligation is more favorable for the reaction than PR3 ligation. The Eact for the substrate addition follows the order Ph−F ≈ Ph−Me ≫ Ph−Cl > Ph−Br. The maximum Eact is observed for ImNMe2NO2, the least donating NHC ligand to Pd. Energy-profile diagram for the oxidative addition of Ph−X to the active catalyst Pd(C2(NMe2)2) is given in Figure 6. The Edis of Pd(C2(NMe2)2)2 is 33.6 kcal/mol. For Ph−Br, Ph−Cl, Ph− F, and Ph−Me, the adduct I3 formation takes place at relative energies 19.4, 20.4, 18.0, and 19.8 kcal/mol, respectively. With respect to I3, the oxidative addition is significantly exothermic for Ph−Cl and Ph−Br, whereas the reaction is highly endothermic for Ph−Me and Ph−F. All Eact values for adding Ph−X to Pd−alkynes are given in Table 11. The presence of the F substituent in the alkyne ligand gives the highest Eact in all the four cases of additions, viz., 12.7, 17.6, 37.8, and 46.8 for Ph−Br, Ph−Cl, Ph−F, and Ph−Me, respectively. Among all, the alkyne with NMe2 substituent gives the least Eact, viz., 5.4, 11.1, 31.5, and 38.8 kcal/mol for Ph−Br, Ph−Cl, Ph−F, and Ph−Me, respectively, which is even less than that of phosphine complexes except for the case of Ph−Br. These results support the findings of Ahlquist et al. that alkynes are excellent ligands for Pd(0) complexes for oxidative addition reactions.32

Figure 3. Optimized structures of Pd(CH 2CMe2) 2 and Pd(CH2CCl2)2. Distances are given in Å.

geometric feature. For those showing different Pd−C bond lengths, the average bond distance is given in Table 8. Overall, the dissociation energies of alkenes (16.1−25.2 kcal/mol) are found to be significantly smaller than other sets of ligands. Oxidative Addition of Ph−Br, Ph−Cl, Ph−F, and Ph− Me. A typical energy-profile diagram for the oxidative addition of Ph−X to Pd(PtBu3) is given in Figure 4. Pd(PtBu3) in the reaction is generated by dissociating PtBu3 from Pd(PtBu3)2. The adduct of Ph−X and Pd(PtBu3) (I1) subsequently passes through a transition state TS1 to form a tricoordinated product P1. In cases of Ph−Br and Ph−Cl, I1 is formed as a result of η2type coordination of Pd to one of the ortho CC bonds of the arene ring, whereas in cases of Ph−F and Ph−Me, the η2-type coordination of Pd occurs on one of the meta CC bonds (Figure S1 of Supporting Information). The Pd−C and Pd−P 4200


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Table 9. Phosphine Ligands and the Corresponding Energy Barriers on Addition of Ph−X to Pd(phosphine) L

Eact on adding Ph−Br

Eact on adding Ph−Cl

Eact on adding Ph−F

Eact on adding Ph−Me

PCy3 PtBu3 PiPr3 PMe3 PEt3 P(SiMe3)3 PHMePh PPh3 PH3 P(Ph−F)3 P(thiophene)3 P(Ph−Cl)3 P(SMe)3 P(Ph−CF3)3 PH2CF3 PCl2Ph PCl2Me P(CF3)3 PCF3 PF3

4.7 4.6 4.6 5.5 5.0 4.7 5.3 5.2 6.6 5.5 5.5 5.8 6.6 6.4 8.1 7.9 8.2 10.8 11.3 11.9

11.6 11.4 11.8 12.6 12.3 12.0 12.5 12.4 13.8 12.8 12.7 13.1 13.8 13.2 15.3 15.3 15.7 18.3 18.9 19.2

39.9 39.2 40.1 41.2 40.7 40.2 41.0 40.7 43.4 41.1 40.9 41.5 42.1 43.2 45.1 43.6 45.3 48.2 48.3 48.8

41.8 42.2 42.2 43.3 43.0 39.6 43.7 43.5 44.1 44.5 43.5 44.4 45.1 45.1 46.2 46.5 48.4 47.2 41.8 42.2

Figure 6. Illustration of the energy profile describing the oxidative addition of Ph−X to Pd(C2(NMe2)2).

Figure 5. Illustration of the energy profile describing the oxidative addition of Ph−X to Pd(ImNMe2H2).

Table 10. NHC Ligands and the Corresponding Energy Barriers on Addition of Ph−X to Pd(NHC) L






4.6 4.7 4.9 5.0 5.0 5.2 5.3 5.1 5.7 6.0

11.4 11.7 12.1 12.0 11.9 12.3 12.4 12.5 12.9 13.3

39.0 39.1 39.6 39.9 39.9 39.9 40.5 41.8 41.7 43.1

38.6 39.8 39.5 39.5 39.4 40.7 40.1 41.1 40.7 41.3

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Table 11. Alkyne Ligands and Their Corresponding Energy Barriers on Addition of Ph−X to Pd(Alkyne) L





C2(NH2)2 C2(NMe2)2 C2Me2 C2(SiMe3)2 C2Et2 C2Ph2 C2H2 C2Br2 C2Cl2 C2 F 2

5.9 5.4 5.6 4.9 5.6 7.1 7.2 10.9 11.2 12.7

11.9 11.1 12.8 12.0 12.9 13.6 14.2 15.9 16.1 17.6

32.6 31.5 33.8 35.0 34.1 34.9 35.8 36.8 36.9 37.8

39.1 38.8 39.4 39.6 39.8 41.2 40.9 45.4 45.6 46.8

and 40.8 kcal/mol for Ph−Br, Ph−Cl, Ph−F, and Ph−Me, respectively. Alkenes with electron-donating ligands appear as excellent ligands for palladium. The presence of substituents CF3 and CN makes the ligand electron-deficient, leading to high Eact values. The relative energies of all adduct systems (I1−I4), transition states (TS1−TS4), and the product complexes (P1−P4) are provided in Tables S5−S8 of the Supporting Information. Correlation Plot of ΔVPd2 versus Eact. Figure 8 depicts ΔVPd2 versus Eact correlation plots corresponding to phosphines, NHCs, alkynes, and alkenes. All cases show excellent linear correlations, which strongly suggest that MESP at the palladium nucleus serves as an effective electronic parameter for predicting Eact. From the linear equations in the graph, the unknown Eact of a ligand can be calculated by knowing the ΔVPd value. The Eact increases with an increase in the ΔVPd2 value (from negative to positive), meaning that improving the electron density at the Pd nucleus by appropriate ligation can improve the efficiency of oxidative addition. It is evident from the correlation that all the ligands behave in a similar fashion with respect to the substrates because the slope of the graph is fairly close for most of them. Among all the substrates, Ph−Me is the most difficult to cleave by oxidative addition followed by Ph−F. In Pd(phosphine), Pd(NHC), and Pd(alkene), complexes, Ph−Me and Ph−F, show a similar reactivity toward oxidative addition, whereas in Pd(alkyne), the reactivity of Ph− F is significantly higher than that of Ph−Me. The Pd(alkyne) complex formed with electron-rich ligands emerged as the most promising systems for activating Ph−F bonds under oxidative addition conditions.

Energy-profile diagram for the oxidative addition of Ph−X to a representative Pd−alkene complex, Pd(CH2CEt2), is given in Figure 7. The Edis is 21.9 kcal/mol for this complex. Association

Figure 7. Illustration of the energy profile describing the oxidative addition of Ph−X to Pd(CH2CEt2).

of Ph−X to the monoligated complex leads to the formation of the adduct I4. The adduct formation stabilizes the complex by 8−11 kcal/mol. The oxidative addition of Ph−F and Ph−Me is highly endothermic and passes through high-energy transition states (TS4), whereas moderate values of Eact, viz., 10.1 and 13.7 kcal/mol, are observed for the cleavage of Ph−Cl and Ph−Br bonds, respectively. Table 12 depicts the Eact values for adding the four substrates to the Pd(alkene) complex. The least Eact is obtained for the Pd(CH2CEt2) complex, viz., 3.7, 10.2, 37.6,

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CONCLUSIONS In summary, we have investigated the C−Br, C−Cl, C−F, and C−C bond breaking via oxidative addition on monoligated

Table 12. Alkene Ligands and Their Corresponding Energy Barriers on Addition of Ph−X to Pd(Alkene) L






4.7 3.7 5.8 5.6 5.3 9.1 9.4 8.7 12.1 12.5

11.1 10.2 12.3 12.0 11.9 15.6 15.9 15.3 18.4 18.8

39.2 37.6 40.2 39.4 39.4 43.5 43.8 43.4 46.4 47.0

41.6 40.8 43.0 43.1 42.6 44.9 45.1 45.1 46.9 48.0

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Figure 8. Correlation between activation barrier (Eact) and relative MESP at the Pd nucleus of Pd−L (ΔVPd2). (a) L = phosphine, (b) L = NHC, (c) L = alkyne, and (d) L = alkene.

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COMPUTATIONAL METHODS All the calculations have been carried out using Gaussian 09 suite of programs.59 The B3LYP density functional with the 631+G(d,p) basis set for all atoms except Pd and LANL2DZ basis set for Pd is used for optimizing the molecular geometries. This method abbreviated as B3LYP/BS1 is also used for vibrational frequency calculation and MESP calculation. A minimum energy structure shows zero imaginary frequency, and a transition state is characterized with one imaginary frequency along the bond-breaking/-forming direction of the oxidative addition. The energy profiles of the computed mechanisms are derived from the total energy of the complexes. The MESP, V(r) at a point r in space is calculated by the following equation

palladium catalysts. MESP-derived parameters Vmin and VPd emerge as good measures to characterize the electron-rich/poor character of the ligands and the complexes. The steric effect influences the absolute value of Vmin to some extent, whereas VPd reflects mostly the overall electronic effect of the ligand environment at the nucleus. Both MESP parameters undergo subtle variations with respect to change in the ligand environment. With respect to a reference complextypically the complex coordinated with the unsubstituted ligandthe observed change in VPd (ΔVPd) gives a measure of electron donation to or electron withdrawal from the metal center. A linear correlation is established between MESP at the Pd center and Eact. Ligands of electron-donating nature favor oxidative addition reaction. Thus, use of ligands showing strong electronrich character can be proposed as a common strategy for designing efficient catalysts susceptible for oxidative addition. Alkenes and alkynes have shown excellent ligand property to oxidative addition by Pd(0). Although the high energy barrier observed for adding Ph−F and Ph−Me suggests a nonfeasible reaction, the ΔVPd versus Eact correlations aid us to develop feasible oxidative additions by tuning VPd via appropriate ligands. In summary, the MESP-based electronic parameter VPd emerges as an easy tool for fine-tuning the reactivity of the Pd(0) catalysts.

N

V (r) =

∑ A

ZA − |r − R A |

∫

ρ(r′)d3r′ r − r′

(1)

where ZA is the charge on the nucleus A, which is located at the position RA, ρ(r′) is the electron density, and N is the total number of nuclei in the molecule. The MESP at a nucleus A (VA) is computed by removing the nuclear contribution because of ZA from eq 1. The topographical analysis of V(r) is based on locating and characterizing the CPs. These are points in space at which first-order partial derivatives of the V(r) vanish. A CP is represented as an ordered pair consisting 4203


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of rank and signature, which is grouped into (3, +3), (3, −3), (3, +1), and (3, −1). A MESP minimum (Vmin) corresponds to a (3, +3) CP, which represents the potential binding sites for electrophiles, and (3, −3) stands for a maximum, whereas (3, +1) and (3, −1) denote saddle points. We have taken the Vmin at lone-pair region of P in the case of phosphines, lone-pair region of N in the case of NHCs, and π regions in cases of alkynes and alkenes. The Vmin values are located numerically by computing MESP on a three-dimensional grid using Gaussian 09. Selection of 50 ligands from four categories leads to the study of 250 complexes (including transition states) for elucidating the mechanism of the oxidative addition of one-substituted benzene. Hence, a total of 750 complexes are analyzed in this study. To understand the effect of solvation (solvent = THF) and dispersion, the reaction of aryl bromide with a selected set of 15 phosphine-coordinated complexes is also described at the B3LYP-D3/BS1 level60 in conjunction with the self-consistent reaction field approach with the “solvation model density” method61 as implemented in Gaussian 09 in the Supporting Information.59 We have also tested the SDD basis set for Pd along with dispersion and solvation (B3LYP-D3/BS2) corrections for the test systems. The B3LYP-D3/BS1 and B3LYP-D3/BS2 results for the tested systems show close similarity to the B3LYP/BS1 results, and hence, only the data on the latter are discussed.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00745. Coordinates and energies of optimized geometries, tables containing MESP and energy values, figures of optimized geometries and dispersion-corrected correlation plot (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-471-2515472 (C.H.S). ORCID

Cherumuttathu H. Suresh: 0000-0001-7237-6638 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the project CSC0129 by the Council of Scientific and Industrial research (CSIR), India. B.A.A. thanks UGC, India, for a research fellowship.

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