Organometallics 1995, 14, 2288-2296
2288
Oxidative Insertion as Frontside s N 2 Substitution: A Theoretical Study of the Model Reaction System Pd CH&1
+
F. Matthias Bickelhaupt,*B’ Tom Ziegler,” and Paul von R a p 6 Schleyert Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N I N 4 Received December 7, 1994@
+
The potential energy surface of the model reaction system Pd CH3C1 has been explored using density functional theory based on the local density approximation (LDA) and its nonlocal extension NL-SCF. Oxidative insertion (OxIn) of Pd into the C-C1 bond has the lowest activation barrier (AE*= -1.5 kcaVmol relative to the separated reactants; NL-SCF) and leads to exothermic production of CH3PdC1 (hE, = -7.7 kcaYmo1). The “straight” sN2 substitution is not competitive as it leads to the highly endothermic formation of PdCHS+ C1- (AE,= 145.2 kcallmol). However, in combination with a concerted rearrangement of the C1- leaving group from C to Pd, the substitution process (S~2/Cl-ra)leads to the exothermic formation of CH3PdCl via a still high but much lower energy barrier (AE*= 29.6 kcaYmo1). Furthermore, radical mechanisms proceeding via single electron transfer (SET) have been considered. Solvent effects, estimated using a simple electrostatic continuum model, tend to favor the straight sN2 substitution because of the charge separation in the products, but oxidative insertion remains dominant. In order to explain the intrinsic preference of the Pd atom to react via oxidative insertion, a detailed analysis of the bonding mechanism between Pd and CH3C1 has been carried out. It is argued that oxidative insertion in organometallic chemistry corresponds to frontside S Nsubstitution ~ in organic chemistry, in spite of obvious differences. Finally, possible effects of ligands are discussed.
+
1. Introduction
metal ions or atoms is studied in the absence of ligands or solvent molecules, the effects of which may be added stepwise at later stages. Experimentally, this has been achieved using mass spectrometric3r4(metal ions) or spectroscopic5 (metal atoms) techniques. Theoretical method^^,^ play a key role in this approach. They enable the study of model reaction systems, which are indispensable for the achievement of a real understanding
Oxidative addition and reductive elimination (the reverse process) represent a fundamental class of organometallic reactions which occur in nearly all homogeneous, catalytic processes (eq 11.l Therefore, these
Mb
t R-X
Oxidative Addition - 4
R-ML-X
Reductive Elimination
processes are of major significance for synthesis and industrial processes and have been the subject of many e~perimentall-~ and theoreticaV studies. There are basically two different approaches to the investigation of oxidative addition. In the first approach, particular transition metal complexes are studied e~perimentallyl-~~ as well as theoretically,6 using more or less realistic model systems in the latter case. In the second and more recent approach, the intrinsic reactivity of the Present address: Computer Chemie Centrum, Universitat Erlangen-Nurnberg, Nagelsbachstr. 25,D-91052Erlangen, Germany. Abstract published in Advance ACS Abstracts, April 1, 1995. (1)(a) Collman, J . P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (b) Elschenbroich, Ch.; Salzer, A. Organometallics. A Concise Introduction, 2nd ed.; VCH: Weinheim, Germany 1992. (2)(a) Janowicz, A. H.; Bergman, R. G. J . Am. Chem. SOC.1982, 104,352.(b) Janowicz, A. H.; Bergman, R. G. J . Am. Chem. SOC.1983, 105,3929.(c) Jones, W. D.;Feher, F. J . J . Am. Chem. Soc. 1982,104, 4240. (d) Sakakura, T.;Sodeyama, T.; Sasaki, K.; Wada, K.; Tanaka, M. J . Am. Chem. SOC.1990, 112, 7221. (e) Casalnuovo, A. L.; Calabrese, J. C.; Milstein, D.J . Am. Chem. SOC.1988,110,6738. (0 Wright, M.W.; Smalley, T. L.; Welker, M. E.; Rheingold, A. L. J . Am. Chem. SOC.1994,116,6777.(g) Grushin, V.V.; Alper, H. Chem. Reu. 1994,94,1047. (h) Ellis, P.R.; Pearson, J. M.; Haynes, A,; Adams, H.; Bailey, N. A,; Maitlis, P. M. Organometallics 1994,13,3215. +
@
(3)(a) Armentrout, P. B.; Beauchamp, J . L. Acc. Chem. Res. 1989, 22,315.(b) Eller, K.;Schwarz, H. Chem. Rev. 1991,91,1121.(c) van den Berg, K. J.; Ingemann, S.; Nibbering, N. M. M.; Gregor, I. K. Rapid Commun. Mass Spectrom. 1993,7,769.(d) Wesendrup, R.; Schroder, D.;Schwarz, H. Angew. Chem. 1994, 105, 1232. (e) Chen, Y.-M.; Clemmer, D.E.; Armentrout, P. B. J . Am. Chem. SOC.1994,116,7815. (4)(a)Jones, R. W.; Staley, R. H. J . Am. Chem.SOC.1980,102,3794. (b)Jones. R. W.: Stalev. R. H. J . Phvs. Chem. 1982.86.1669.(c) Weil. D.A,;Wiikins, C.L. J . L . Chem.S&. 1985,107,7316. ’(d)Chowdhury; A. K.; Wilkins, C. L. J . Am. Chem. Soc. 1987,109,5336. (5)(a) Mitchell, S. A,; Hackett, P. A. J . Chem.Phys. 1990,93,7822. (b) Ritter, D.;Weisshaar, J . C. J . Am. Chem. SOC.1990,112,6425.(c) Chertihin, G. V.; Andrews, L. J . Am. Chem. SOC.1994,116,8322. (6)(a) Ziegler, T.;Tschinke, V.; Fan, L.; Becke, A. D.J . Am. Chem. SOC.1989,111,9177. (b) Bickelhaupt, F. M.; Baerends, E. J.; Ravenek, W. Inorg. Chem. 1990,29,350. (c) Ziegler, T. Chem. Rev. 1991,91, 651. (d) Koga, N.; Morokuma, K. Chem. Rev. 1991,91,823. (e) Low, J.J.; Goddard, W. A., 111. J.Am. Chem. SOC.1984,106,6928.(f)Ibid. 1984,106,8321. (g)Ibid.1986,108,6115.(h) Irikura, K. K.; Goddard, W. A,, 111.J . Am. Chem.SOC.1994,116,8733.(i) Perry, J. K.; Goddard, W. A,, 111. J . Am. Chem. SOC.1994,116,5013. (7) (a) Siegbahn, P. E. M.; Blomberg, M. R. A. J . Am. Chem. SOC. 1992, 114, 10548. (b) Siegbahn, P. E. M.; Blomberg, M. R. A.; Svensson, M. J . Am. Chem. SOC.1993,115,1952. ( c ) Siegbahn, P. E. M. Organometallics 1994,13,2833. (d) Carter, E. A.; Goddard, W. A,, 111. J . Phys. Chem. 1988,92,5679. (e) Perry, J. K.; Ohanessian, G.; Goddard, W. A,, 111. Organometallics 1994,13,1870. (8)(a) Blomberg, M.R. A,; Siegbahn, P. E. M.; Svensson, M. J . Am. Chem. SOC.1992,114, 6095. (b) Siegbahn, P. E. M.; Blomberg, M. R. A.; Svensson, M. J . Am. Chem. Soc. 1993,115,4191. (c) Svensson, M.; Blomberg, M. R. A,; Siegbahn, P. E. M. J . Am. Chem. SOC.1991, 113,7076. (d) Blomberg, M.R. A.; Siegbahn, P. E. M.; Svensson, M. Inorg. Chem. 1993,32,4218. (e) Siegbahn, P. E. M.; Blomberg, M. R. A,; Svensson, M. J . Phys. Chem. 1993,97,2564. (0 Siegbahn, P. E. M. J.Am. Chem. SOC.1994,116,7722.
0 1995 American Chemical Society
Oxidative Insertion as Frontside S N Substitution ~
Organometallics, Vol.14,No. 5, 1995 2289
of the reaction mechanism, but which often cannot be realized experimentally. In the past, theoretical investigations were mainly focused on the oxidative addition reactions of organotransition metal systems to H-H,Gb-f C-H,Ga,c,d,f,g,7b,c,e,8a-c and C-C6d,f9g,7abonds but also to F-F,GCN-H,8d and 0-H8e bonds. In the present study,'we have carried out a high-level density-functional theoretical (DFT)9 investigation on the intrinsic reactivity of palladium-dlO toward chloromethane. Thus, we follow the second approach (vide supra) using the model system Pd CH3C1. The calculations were carried out with the ADF program.lOJ1 First, the oxidative insertion (cis oxidative addition) of Pd to the C-C1 bond (OxIn, eq 2) is considered.
+
acetic acid process in which the rate-determining step is oxidative addition of a Rh(1) complex to CH31.14 The question is addressed if nucleophilic substitution or a radical mechanism can be competitive in the formation of the apparent oxidative insertion product, CH3PdC1, as indicated in eqs 3 and 4. Another main objective is to get insight into mechanistic and electronic differences and analogies between our organometallic (Pd CH3C1) and related organic reaction systems involving main group bases (B CH3Cl). For this purpose, we have performed an advanced analysisll of the electronic structure of Pd CH&1 in selected stationary points on the potential energy surface. This analysis enables us to interpret our quantitative results in familiar terms from MO t h e 0 ~ 7 . l ~
+
+
+
2. Methods
Pd + CH3Cl
SET
PdCI'
+ CH,'
-
CHQdCl
(4)
Furthermore, the competing backside nucleophilic substitution on carbon ( s N 2 , eq 3) and a radical mechanism proceeding via a single electron transfer (SET) and C1 abstraction (eq 4) are investigated. Solvent effects have been estimated using a simple electrostatic continuum model.12 One objective is to arrive a t a better understanding of the important class of oxidative addition reactions in which a metal center inserts into the polar, electrophilic carbon-halogen bond.13 It is noted, that our model reactions are closely related to the famous Monsanto (9) (a) Dreizler, R. M.; Gross, E. K. U. Density Functional Theory, An Approach to the Quantum Many-Body Problem; Springer-Verlag: Berlin, 1990. (b) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (c) Slater, J. C. Quantum Theory of Molecules and Solids; McGraw-Hill: New York, 1974; Vol. 4. (10) (a) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973,2, 41. (b)Baerends, E. J.; Ros, P. Chem. Phys. 1976,8,412. (c)Baerends, E. J.; Ros, P. Znt. J . Quantum Chem., Quantum Chem. Symp. 1978, S12, 169. (d) Ravenek, W. In Algorithms and Applications on Vector and Parallel Computers; Riele, H. H. J., Dekker, Th. J., van de Vorst, H. A, Eds.;Elsevier: Amsterdam, 1987. (e)Boerrigter, P. M.; te Velde, G.; Baerends, E. J. Znt. J. Quantum Chem. 1988,33,87. (D te Velde, G.; Baerends, E. J. J . Comp. Phys. 1992, 99, 84. (g) Snijders, J. G.; Baerends, E. J.; Vernooijs, P. At. Nucl. Data Tables 1982,26,483.(h) Krijn, J.;Baerends, E. J. Fit-Functionsin the HFS-Method, Internal Report (in Dutch), Vrije Universiteit Amsterdam, The Netherlands, 1984. (i) Versluis, L.;Ziegler, T. J . Chem. Phys. 1988, 88, 322. (i) Fan, L.;Versluis, L.; Ziegler, T.; Baerends, E. J.; Ravenek, W. Znt. J . Quantum Chem., Quantum Chem. Symp. 1988, S22,173. (k) Fan, L.; Ziegler, T. J . Chem. Phys. 1990, 92, 3645. (1) Banerjee, A.; Adams, N.; Simons, J.; Shepard, R. J . Phys. Chem. IS=, 89, 52. (m) Baker, J.J . Comput. Chem. 1986, 7, 385. (n)Vosko, S.H.; Wilk, L.; Nusair, M. Can. J . Phys. 1980,58,1200. ( 0 ) Becke, A.D. J . Chem. Phys. 1986, 84,4524. (p)Becke, A.D. Phys. Rev. A 1988,38,3098. (9)Perdew, J. P. Phys. Rev. B 1986, 33, 8822. Erratum: Ibid. 1986, 34, 7406. (r) Fan, L.;Ziegler, T. J . Chem. Phys. 1991, 94, 6057. (11) (a) Bickelhaupt, F. M.; Nibbering, N. M. M.; van Wezenbeek, E. M.; Baerends, E. J. J . Phys. Chem. 1992, 96,4864. (b) Ziegler, T.; Rauk, A. Znorg. Chem. 1979,18, 1558. (c) Ziegler, T.;Rauk, A. Znorg. Chem. 1979, 18, 1755. (d) Ziegler, T.;Rauk, A. Theoret. Chim. Acta 1977. . 46. 1. -. 7
- - 7
(12) (a) Born, M. 2.Phys. 1920,1,45. (b) Onsager, L.J. Am. Chem. Soc. 1936,58, 1486. (c) Wong, M. W.; Frisch, M. J.; Wiberg, K. B. J . Am. Chem. SOC. 1991,113,4776. (13) Reference la, Chapter 5.
A. General Procedure. All calculations were performed using the Amsterdam-Density-Functional (ADF) program,l0 developed by Baerends et aZ.loa-cand vedorized by Ravenek.lod The numerical integration was performed using the procedure developed by te Velde et uZ.loe,f The MOs were expanded in an uncontracted set of Slater type orbitals (STOs).lOg For H and C the basis is of double-5 quality, augmented with a 2p and a 3d polarization function, respectively. For C1 the basis is of triple-5 quality, augmented with two 3d polarization functions. For Pd the basis is of double-l; quality for the 4s shell and of triple-l; quality for the 4p, 4d, 5s, and 5p shells. The core shells of carbon (Is),chlorine (ls2s2p), and palladium (ls2s2p3s3p3d) were treated by the frozen-core approximation.loa An auxiliary set of s, p, d, f, and g STOs was used to fit the molecular density and to represent the Coulomb and exchange potentials accurately in each SCF cycle.loh Geometries and frequencies were calculated at the LDA level. Equilibrium structures were optimized using analytical gradient techniques.10i FrequencieslOj were calculated by numerical differentiation of the analytical energy gradients. Transition state structureslokwere optimized using the algorithm developed by Simonslol in the implementation due to Baker.lorn Energies were evaluated using the local density approximation (LDA) as well as density functionals including nonlocal corrections (NL). At the LDA level exchange is described by Slaters X a potentialgeand correlation is treated in the VoskoWilk-Nusair (VWN) parameterization.lon At the NL level nonlocal corrections for the exchange due to Beckeloo,*and for correlation due to Perdew'Oq are added self-consistently.lor B. Solvent Effects. Solvent effects have been estimated using a simple electrostatic continuum model12 based on the expressions derived by OnsagerIZaand Born.lZb The solute is considered as a point dipole, p , andor point charge, Q, which is located in the center of a spherical cavity with radius uo. This cavity is surrounded by the solvent which is represented as a dielectric continuum with relative dielectric constant cr. The solvation energy, AEsolv,is the sum of a dipole (A&,I~,+) and a charge (AEsolv,~) term (eq 5). These terms are given in
(14) (a) Forster, D. J . Am. Chem. Soc. 1976, 98, 846. (b) Forster, D. Adv. Organomet. Chem. 1979, 17, 255. (c) Reference la, Chapter 12. (d) Forster, D. J . Am. Chem. Soc. 1975,97,951. (e) Hickey, C. E.; Maitlis, P. M. J . Chem. Soc., Chem. Commun. 1984, 1609. (15) (a) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; Wiley-Interscience: New York, 1985. (b) Rauk, A. Orbital Interaction Theory of Organic Chemistry; WileyInterscience: New York, 1994. (c) Fleming, I. Grenzorbitale und Reaktionen organischer Verbindungen;VCH Verlagsgcsellschafk Weinheim, Germany 1990.
Bickelhaupt et al.
2290 Organometallics, Vol. 14, No. 5, 1995
Table 1. Energies (in kcaymol) of Stationary Points Relative to the Reactants Pd CH&l (LDA, NL-SCFand NL-SCFSsolv)
+
AEsolv,B = -l/
2(
1 - - --
(7)
4)E :o1:
SI units in eqs 6 and 7. Overall charges, Q, and dipole moments, p, calculated at the NL-SCF level with respect t o the center of electronic charge were used.lZc The effective cavity radius a0 for a molecule or ion was calculatedlZcas the sum of the greatest internuclear distance and the van der Waals radii16J7 of the two atoms involved. The expressions were evaluated for two values of the relative dielectric constant corresponding to diethyl ether (er = 4.34 at 20 "C) and water (er = 78.54 at 25 'C).18 C. Bonding Energy Analysis. The bonding mechanism between Pd and CH&1 was analyzed in selected stationary points using a n extended transition state (ETS) method.ll The overall bond energy AE is made up of two major components is the amount of energy (eq 8). The preparation energy Uprep
required to deform the separated fragments from their equilibrium structure to the geometry which they acquire in the overall molecule. The interaction energy AEbt corresponds to the actual energy change when the prepared fragments are combined to form the overall molecule. The interaction energy is further split up in two physically meaningful terms (eq 9).11
mi, = melst + upauli + moi= AEO
+A
E ~ ~(9)
The term AEelst corresponds to the classical electrostatic interaction between the unperturbed charge distributions of the prepared fragments and is usually attractive. The Pauli repulsion A E p a d i comprises the 4-electron destabilizing interactions between occupied orbitals and is responsible for the steric repulsion. For neutral fragments, it is useful to combine AEebt and AEpadi in the steric interaction AEo (eq 9). The orbital interaction AEoi accounts for charge transfer (interaction between occupied orbitals on one moiety with unoccupied orbitals of the other, including the HOMO-LUMO interactions) and polarization (empty/occupied orbital mixing on one fragment).
3. Results and Discussion
The results are summarized in Figure 1(geometries), Table 1 (energies), and Table 2 (analysis). In the following, we will discuss the competition between the various reaction mechanisms (subsection A) and the possible influences of solvation (subsection B). Furthermore, a detailed analysis of the electronic structure of the reaction system Pd CH3C1 is presented (subsection C) and analogies with related organic systems involving main group bases (subsection D) as well as expectations for ligand effects (subsection E) are discussed. Energies were evaluated at the LDA and NLSCF level using LDA geometries. At the LDA level, molecular complexes are stronger bound and transition state energies are lower than at the NL-SCF level (Table 11, in agreement with the general tendency of LDA methods to lead to overbinding.6c This overbinding is (partly) compensated by the introduction of gradient
+
~~
(16) CRC Handbook of Chemistry and Physics, 63rd ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1982; p D-195.
(17)Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: Oxford, U.K., 1990. (18)Reference 16, pp E-51, E-52.
NL-SCF
NL-SCF+SOIV~ diethyl ether water
System'
IDA
tkuckuu Pd + CHjCl
0.0
0.0
0.0
0.0
-23.0 -16.2
-9.9 -3.7
-9.9 -3.1
-9.8 -3.7
-16.1 22.9 -16.1
-1.5 29.6 1.7
-1.4 28.9 1.7
-1.4 28.5
35.2 50.6
29.3 46.9
28.9 46.9
28.7 46.9
149.0 -25.3
145.2 -7.7
50.6 -7.8
23.8 -1.9
[CH,CI. Pdl [Pd, CHjCI]
PdCI' + CH3' PdCHj' + CI'
I .7
ctuiuas
PdCHp' + CICH jPdCl
See Figure 1 for structures. Solvent effects were calculated using the Onsager-Born model (section 2.B). Diethyl ether: cr = 4.34. Water: er = 78.54.
corrections in the NL-SCF scheme. The discussion is therefore based on the nonlocal results! k Reaction Mechanisms. Oxidative Addition. First, the oxidative insertion of Pd to CH3C1 is considered (eq 2, OxIn). The corresponding reaction energy profile is depicted in Figure 2. The reactants Pd CH3C1 can combine in two different ways to give the reactant complexes [CH3C1, Pdl, in which Pd coordinates to C1, and [Pd, CH3Cl1, in which Pd coordinates t o the CH3 backside (Figure 1). The most stable reactant complex is [CHsCl, Pdl with a complexation energy hE = -9.9 kcaymol (Table 1, NL-SCF). In this C, symmetric complex, the Pd-C1 bond is oriented staggered with res ect to CH3 (Figure 1). The Pd-C1 bond length is 2.258 while.the Pd-C1-C angle amounts to 112.05'. The C-Cl bond is only slightly extended from 1.750 (in CH3C1) to 1.781 The oxidative addition reaction proceeds from [CH3C1, Pdl via a strong bending of C1 toward a C-H bond, leading to the C, symmetric transition state TS(Ox1n) which is 8.4 kcal/mol higher in energy (Figure 2) and is characterized by one imaginary frequency of i 161.7 cm-l (Figure 1). The overall activation energy, A@ = 1.7 kcaymol (Table 11, is rather low. In TS(Ox1n) the C-C1 and Pd-C1 bonds are elongated to 1.896 and 2.361 A,respectively (Figure 1). The Pd-C bond is being formed and comes to 2.227 A while the Pd-C-C1 angle amounts to 69.33'. Palladium is involved in an agostic interaction with a C-H bond, as indicated by a relatively short Pd-H bond of 1.857 A and an elongated C-H bond of 1.154 A. The oxidative addition is completed by further insertion of Pd into the C-C1 bond and results in the product CH3PdC1 which is at -7.7 kcdmol with respect to the reactants (Table 1). During this final stage of the reaction the C-C1 bond further elongates to 2.374 A,whereas the Pd-C and Pd-C1 bonds are shortened to 2.008 and 2.192 A, respectively. s N 2 Substitution. The reaction energy profile for nucleophilic substitution is shown in Figure 3. The reactant complex [Pd, CH3C11 is relatively weakly bound by AE = -3.7 kcdmol and can be formed either directly from the separated reactants or by rearrangement of the more stable complex [CHsCl, Pdl. During this
+
x
A.
A
Organometallics, Vol. 14,No. 5,1995 2291
Oxidative Insertion as Frontside Shr2 Substitution
c3v
CH3C1
C, [CH3Cl, Pd]
cs [Pd, CH3C11
1,105
750 ,103
1.145
C, (i 104.4 cm-')
C, (i 102.3 cm-')
C, (i 161.7 cm-')
TS(Pd-ra)
TS(SN2/Cl-ra)
TS(0xIn)
1.105 (1.1041
PdCH3'
cs
(PdCHi)
CH3PdC1
CH3' Figure 1. Optimized geometries (LDA; in A and degrees) for stationary points of the Pd
J
Figure 2. Reaction energy profile (NL-SCF; in kcaymol) for the oxidative insertion (OxIn) of Pd CH3Cl.
+
rearrangement, Pd moves from C1 to the CH3 backside via the C1 symmetric transition state TS(Pd-ra) (i 104.4 cm-l) in which Pd interacts weakly with C1 and a C-H 1: dpd-ci = 2.688 A,dpd-c = 2.370 A,d p d - H bond = 1.870 1. In the C, symmetric reactant complex [Pd, CH3C11, palladium is involved in a n agostic interaction with a C-H bond a t the backside of the methyl group (dpd-c = 2.381 8, dpd-H = 1.766 8,dC-H = 1.154 8). The C-C1 bond is slightly extended to 1.783 8. The "straight" sN2 substitution proceeds via attack of Pd on carbon and expulsion of the C1- leaving group
?!
10.0
5 O.O-;
+ CH&l reaction system.
-v [ [Pd, CH3CI] TS(Pd-ra)
-10.0
-
[CH3CI, Pd]
CH3PdCl
2292 Organometallics, Vol. 14, No. 5, 1995
Bickelhaupt et al.
This reaction is highly endothermic, i.e. AE = +145.2 kcal/mol, due t o the charge separation in the products C1-. The reverse process, backside sN2 PdCH3+ attack by C1- on PdCH3+, proceeds barrierless (Figure 3). The high endothermicity of the sN2 process of eq 10 prevents this route to be a competitive alternative for the formation of CH3PdC1, e.g. by recombination of PdCH3+ and C1- via palladium (eq 11).
+
+
Cl-
-
PdCH3'
(11)
CH3PdC1
+
*
-
(12)
SET
PdCH;
c
+
+
CI'
-
CH3PdC1 (13)
endothermic by 46.9 kcal/mol (Table 1)and is thus 17.6 kcal/mol less favorable, and not competitive, with respect to Cl' abstraction (eq 4), reflecting the difference in Pd-C and Pd-C1 bond strengths. Again, recombination of the intermediates, PdCH3' Cl', results in the formation of CH3PdC1 (eq 13). Comparison of OxIn/S&SET. The intrinsic reactivity of Pd toward CH3C1 is clearly in favor of oxidative
+
10.0A
h
F
1
0.0
1
[CHlCl, Pd]
Figure 4. Reaction energy profile (NL-SCF;in kcaymol) for radical pathways of Pd CH3Cl.
+
+
-
#
-
process advances via transition state TS(s~2/Cl-ra)(i 102.3 cm-l), which is 33.3 kcaYmol above [Pd, CH3ClI (Figure 3). The overall barrier is AE* = 29.6 kcaYmo1. In the C, symmetric TS(S~2/cl-ra),the C1- ion binds electrostatically to PdCH3+ in an v2 fashion, i.e. via two bridging H-atoms (Figure 1: dC1-H = 2.363 A, ~ C - H= 1.110 A). The Pd-C bond length equals 1.989 A and is thus somewhat shorter than in the final product (CH3PdC1: dPd-C = 2.192 A). The activation barrier (29.6 kcaYmo1) is still too high for the S~2/Cl-raprocess (eq 12) to be competitive with oxidative insertion (1.7 kcall mol, eq 2). This result shows however than an ion-pair mechanism dramatically reduces the extremely high energy found for the straight dissociation process (145.2 kcal/mol). Radical Mechanisms. Next, we discuss briefly two conceivable radical mechanisms which proceed via single electron transfer (SET), focusing on the reactive intermediates. The reaction energy profiles are displayed in Figure 4. Starting from the most stable reactant complex, [CH3C1, Pdl, this leads apparently to C1 atom abstraction (actually C1- abstraction by Pd+*) and the formation of PdCP CHI (eq 4, Figure 1: dPd-C1 = 2.154 A), which is 29.3 kcaYmol endothermic (Table 1). Recombination of the radical intermediates via palladium results in the exothermic formations of CH3PdCl (eq 4). Alternatively, SET can occur in the less stable complex [Pd, CH3C11, inducing methyl radical Cl' transfer to palladium and formation of PdCH3' (eq 13, Figure 1: dPd-C = 1.970 A). This process is CH3C1
20.0-
-10.0
CH3PdCl
+
=
2
c
Pd
1
30.0A
W
S~2ICl-ra
CH3Cl
\
0
There appears to be an alternative sN2 pathway in which the C1- leaving group undergoes concerted rearrangement to palladium during the Pd-C bond formation (eq 12). This one-step substitutiodrearrangement Pd
I
40.0-
insertion (eq 2, OxIn). Nucleophilic substitution may only become a viable pathway for oxidative addition (e.g. through its S~2ICl-raion-pair mechanism, eq 12) after adjustment of additional reaction parameters (ligands, solvent). The SET mechanism proceeding via C1 atom abstraction by palladium and recombination of the intermediates (eq 4) has a comparable barrier as the SN2/Cl-ra process and is thus neither competitive with respect to oxidative insertion, for our model system. B. Solvent Effects. Solvent effects were estimated for diethyl ether (er = 4.34) and water (cr = 78.541, i.e. for a weakly and a strongly polar solvent, using a simple electrostatic continuum model (Table 1,NL-SCF solv; see also section 2.B). These simple calculations can give a good impression of the effects that can be expected, although a full understanding of the phenomenon of solvation requires the rather expensive incorporation of discrete solvent molecules in the quantum chemical c a l c ~ l a t i o n .In ~ ~general, complexation, activation and reaction energies change only very slightly (0-0.6 kcal/ mol) upon introduction of solvent effects, as long as only neutral species are involved. Oxidative insertion (eq 2), for example, remains the dominating process in both diethyl ether and water, with an unchanged activation energy (AE*= 1.7 kcallmol). An exception is seen for the activation energy of the S~2/Cl-ramechanism (eq 12) which is reduced by ca 1kcal/mol to 28.9 and 28.7 kcal/mol for ether and water, respectively. This has to be ascribed to the rather large dipole moment @NL-SCF = 7.108 D) in the strongly polarized TS (S~2ICl-ra). In contrast, the reaction endothermicity for the straight sN2 substitution (eq 10)is dramatically reduced from 145.2 kcallmol in the gas phase to 50.6 kcal/mol in diethyl ether and 23.8 kcallmol in water (Table 1). This enormous change is mainly ('99.9%) caused by the Born term (eq 71, thus by the strong solvation of the separated charges in PdCH3+ C1-. In diethyl ether, the (solvent caged) ion-pair S~2/Cl-ramechanism (eq 12) is still the faster substitution process with a lower activation energy than the straight sN2 substitution
+
+
(19)Bickelhaupt, F.M.; Baerends, E.J.; Nibbering, N. M. M. To be published.
Organometallics, Vol. 14, No. 5, 1995 2293
Oxidative Insertion as Frontside S N Substitution ~
---
5p
3el
--
5al
-
4al
-
%H*
%I*
5s 4 d i t it it it t+
yz
xy $-y2
z2 Y
xz,
3al -Hle1 it it
4dA
2al la1
Yf - x
Pd
A
-Hit
-8,
CH3C1
Figure 5. Valence MO scheme of Pd and CH&1 (for CH&1 the orbital counting begins with the lowest valence orbital). Table 2. Analysis of the Bonding Mechanism between Pd and CH&l in Selected Stationary Points [CH,CI, Pdl
[Pd. CH3Cl1
TS(0xIn)
CHjPdCI
a
AI?
26.6
-36.5
AEoi
25.8 51.4 -3 1.O~ -57.2
84.8 -132.2
-9.9 0.0
-5.2 1.5
-5.8 7.5
-47.4 39.7
-9.9
-3.7
1.7
-1.1
-
~~~~
Figure 6. Realistic 3D representation of the 4al-LUMO of CH3C1. Note that there is no "normal" backside lobe! (Table 1). In water, however, the reaction energy of the straight sN2 substitution (A& = 23.8 kcal/mol) falls below the activation energy of the S~2/Cl-raprocess (hE*= 28.5 kcal/mol). This shows that solvation favors the straight sN2 substitution (eq 10) and that the two substitution processes may become competitive with respect to each other in strongly polar solvents. However, an accurate assessment of the solvent dependence of this competition requires more quantitative methods for the evaluation of solvent effects,lg e.g. a search for and optimization of a possible transition state of the straight sN2 substitution in the presence of a (small) number of solvent molecules. Summarizing, the preference of palladium to insert into the C-C1 bond is preserved under (electrostatic) solvation, although sN2 substitution becomes less unfavorable. We conclude that still other reaction parameters, e.g. ligands, must be invoked to shift the reactivity toward substitution. C. Electronic Structure and Bonding. We have analyzed the electronic structure of and bonding between Pd and CH3C1 for the reactant complexes [CH3C1, Pd] and [Pd, CH3C13, the transition state TS(OxIn), and the product CH3PdC1 (Figures 5 and 6, Table 2). As mentioned before, there is no TS structure corresponding to the straight sN2 substitution, as shown in eq 3. For the sake of clarity, we use C3" symmetry labels for the CH3C1 orbitals, even though this fragment has the lower Cs point group symmetry in the four analyzed systems. Separated Reactants. First, we inspect the valence electronic structures of the separated reactants, which
AE 1 Overlam (Pd I
amb
(MAI2el.,)
0.21 0.08 0.05 0.19
0.09 0.06 0.17 0.06
0.21 0.09 0.1 1
0.17 0.18 0.14 0.15
P(4dA) P(5s)
5.73 0.23
5.74 0.12
5.57 0.09
5.23 0.14
P(2el.y) P(4al) P(5al)
1.89 0.08
1.99 0.01 0.08
1.93 0.18 0.02
1.85 0.61 0.01
QVd) Q(CH3) QKl)
0.03 -0.09
0.15
0.38 -0.23
0.64 -0.22 -0.42
(MAI4a1) ( M AI Sad (5s I2e1.~)
Orbital P
0.10
w
0.00
d-
0.06
-0.21 0.06
-0.15
a AEo = steric interaction, AE,i = orbital interaction, Uprep = preparation energy, required to deform the separated fragments to their geometry in t h e overall molecule (section 2.C). Overlaps between Pd and CH3CI orbitals; ( 4 d ~ + p )= (4dxzIfp) (4dzzlfp) (4dx2-y21fp). Gross Mulliken populations of Pd and CH3C1 orbitals; P ( 4 d ~ t= ) P(4d,) P(4dzz) P(4dxz,z). Sums of gross Mulliken charges of the constituting atoms.
+
+
+
+
are schematically displayed in Figure 5. Palladium has a closed 4d1° shell containing the five degenerate HOMOs of the metal. The 5s LUMO is only 1.3 eV higher in energy, followed by the 5p orbitals at 4.1 eV above the HOMOs. The 4d orbitals can act as electron donors in all reactions, whereas the 5p orbitals play essentially no role. Next, the substrate (CH3C1) is considered for which the orbital counting begins with the lowest valence orbital. Thus, the two lowest valence MO levels, la1 and 2a1, are essentially the bonding and antibonding combinations of carbon 2s and chlorine 3s 1 . le1 and 3al levels are OCH and orbitals: 2sc f 3 ~ ~ The accl bonding, respectively, while the 2el HOMOs are the
-
Bickelhaupt et al.
2294 Organometallics, Vol. 14,No. 5,1995
chlorine lone pairs, LPcl. The 4al LUMO has strong oCcI* antibonding character, while the unoccupied 5al and 3el orbitals are mainly OCH*antibonding. The 4al LUMO represents the well-known oCcI* orbital which is also involved in the donor/acceptor interaction between nucleophile and substrate in organic sN2 reactions .15b,c 3 4 Interestingly, we find that the CH3C14al LUMO has essentially no backside lobe as shown by its 3D reprewith the 4al LUMO are very poor (P(4al) = 0.01 el.). sentation in Figure 6. This is in line with the relatively The back-donation into 5s is also strongly reduced, poor (4d~14al)overlap of 0.06 in [Pd, CH3ClI (Table 2). because of the poor overlap of 0.06 with the chlorine lone pair, 2e1-~,which is only slightly depopulated: P(2e1,) This result differs from the general view in which the = 1.99 el. (Table 2, Figure 5). HOMO of an approaching nucleophile can overlap with We conclude that the electronic structure of the metal the pronounced backside lobe of the ocx* of a substrate CHSX,which is the onset to sN2 s u b s t i t ~ t i o n . ~ ~ ~ ~ ~is more suitable for frontside than for backside complexation to the C-C1 bond of the substrate. Frontier Orbitals in the PdCH3Cl Interaction. Oxidative Insertion. The orbital interaction (-57.2 Next, we consider which are the most important frontier kcaVmo1) and more so the steric interaction (51.4 kcaV orbitals in the bonding between Pd and CH3C1 (Table mol) increase strongly when going from the reactant 2). The orbital interactions (AEoi) in the C, symmetric complex [CH3Cl, Pd] to the transition state TS(OxIn), CH&l] species occur mainly (ca. 90%) in A’ [Pd resulting in an reduced interaction of m i n t = -5.8 kcaV symmetry. The frontier orbitals appear to be 4d,,, 4dz2, mol between palladium and chloromethane (Table 2). and 5s for Pd and 2e1-~,4a1, and 5al for CH3C1 (Figure The deformations of the substrate increase the prepara5). Other orbitals play no important role due to poor tion energy to AEprep= 7.5 kcaVmo1, leading to an overlap or energy mismatchingwith orbitals of the other overall energy difference AE = 1.7 kcaVmol relative to fragment. Note, that in all calculations Pd keeps its the reactants. The increase in orbital interaction is due orientation as in Figure 5, whereas CH3C1 may be to the favorable overlap of interaction 5 between 4 d ~ rotated around the z-axis by an arbitrary amount. This means, that the palladium 4d,,, 4dz2, and 4dX2-$ (all in A’ symmetry)interchange their function in the PdCH3C1 interaction from case to case in the actual calculation and must be treated as one set, i.e. 4 d ~ as , far as overlaps and populations are involved. Therefore, we use the sum of their overlaps with a specific CH3C1 5 6 7 orbital q, (4d&), and the sum of their populations, P ( 4 d ~ )in , the discussion (see footnotes to Table 2). and 4al ((4d~14al)= 0.10),which leads to effective Reactant Complexes. The bonding orbital interacpopulation of the C-C1 antibonding 4al (P(4aSl)= 0.18 tion (AEoi = -36.5 kcaVmo1) in [CH3C1, Pd] is caused el., Table 2). The resulting C-Cl bond elongation by a synergic combination of “back-donation” 1 from further amplifies interaction 5 because of the lowering of the 4al orbital energy. Similarly, the favorable ~ the chlorine overlap and interaction 6 between 4 d and lone pair ((4d~12el-J = 0.21) causes the relatively strong steric repulsion. Note, that the (4d~14al)overlap in TS(OxIn), 0.10, is nearly two times larger than in the backside complex [Pd, CH3C13, 0.06, whereas the C-C1 bond length is approximately equal, i.e. 2.3 f 0.1 1 2 A. This illustrates again the suitability of the metal electronic structure for frontside and not for backside metal 4 d to ~ substrate 4al LUMO (occl*)and “donation” reaction with the C-C1 bond. 2 from a substrate lone pair (LPc1) to metal 5s (Figure Mainly due to 5, the orbital interaction (-132.2 kcaV 5, Table 2). This is also reflected by the populations of mol) increases very strongly, when the oxidative inserthe orbitals involved (Table 2: P ( 4 d ~= ) 5.73, P(4al) = tion completes and CH3PdC1 is formed. This is reflected 0.08 el.; P(2e1,) = 1.89, P(5s) = 0.23 el.). The bonding by the strong depopulation and population of 4 d and ~ donationhack-donation interaction is counteracted by 4a1, respectively (Table 2: P(4dk) = 5.23 el., P(4al) = the steric interaction ( A E O = 26.6 kcaVmo1) of 4 d ~with . 0.61 el.). The reason is a further increase of the overlap LPcl, leading to an overall interaction AE = AEht = -9.9 to (4dx14al) = 0.18 as well as the extra lowering of the kcaVmol (the preparation energy is zero, because CH34al energy, caused by further C-C1 elongation. The 4al C1 is hardly deformed). undergoes a slight but significant change, as 1s lobe If we examine the backside complex [Pd, CH&l], the develops on H, out-of-phase with the frontside lobe on overall interaction is reduced to AE = -3.7 kcaVmo1, C (not illustrated). This modifies the nature of 5 in the mainly because of a weaker orbital interaction ( A E O i = sense that it also provides the agostic interaction which -31.0 kcaVmol) which is now provided by donation 3 elongates the C-H bond (Figure 1). The steric interacinto the higher energy 5al (P(5al) = 0.08 el.). This is tion (84.8 kcaVmol) also increases considerably, but less the agostic interaction leading to C-H bond lengthening so then AEoi. It is provided by a somewhat weaker (Figure 1). Note, that overlap and interaction 4 of 4 d ~ interaction 6 ((4d~12el-J = 0.17) and by Pauli repulsion
+
a
Organometallics, Vol. 14,No. 5, 1995 2295
Oxidative Insertion as Frontside s N 2 Substitution
Scheme 1
B
Frontside Attack (Main Group Base)
Backside Attack
7 between palladium 4d and the substrate 3a1, i.e. the occl orbital whose energy is increased during C-C1 bond breaking. Due to the larger increase of bonding interaction 5 than of repulsive 6 and 7 , the net interaction (mint= -47.4 kcavmol) is strong enough to make the overall energy change exothermic (AE= -7.7 kcdmol), in spite of the unfavorable preparation energy (AE,, = 39.7 kcavmol) connected with the heavy deformation of the substrate. During the insertion process the charge &(Pd) on palladium increases continuously from 0.03 in [CH&l, Pdl via 0.38 in TS(Ox1n) to 0.64 el. in CH3PdC1 (Table 2). The fragment charges of CH3 and C1 in CH3PdC1 are -0.22 and -0.42 el, respectively. This is in line with the oxidation of the metal. The formal charges are, of course, more pronounced and the final product CH3PdC1 can be conceived as Pd(II), coordinated by CH3- and C1-. Summarizing, the electronic structure analysis ascribes the intrinsic preference of Pd for frontside attack on the C-C1 bond to the better 4d/occl* overlap in 5 than in 4. D. Analogies with Related Organic Reactions. It is interesting to compare our results for Pd CH3Cl with the related organic system B CH3C1, where B represents a main group base. In general, the backside sN2 substitution dominates in the organic reaction system,20which is explained on the basis of overlap arguments.20a Principally, a main group base B has a p- or sp"-type HOMO, which favorably overlaps and interacts with the backside of the o*cc1 orbital of the substrate (Scheme 1). In the case of frontside attack, however, the interaction is very poor due to the wellknown cancellation of overlap, as the HOMO lobe approaches on a nodal surface of o*cc1 (Scheme 1).Yet, our results show that this frontside attack becomes favorable for a metal base M, because the d-type HOMO is ideally suitable for frontside interaction with the o*ccl of the substrate (Scheme 1). In this sense, oxidative insertion can be conceived as frontside sN2 substitution which is favored by metal bases. We believe that the recognition of this analogy can be a helpful concept for the understanding and also the designing of organometallic and organic reactions. We stress that this concept should not be overestimated. Of course, there are obvious differences between the reactions of M CH3C1 and B CH3C1, the most important one being the strong binding of the leaving group to the base in the final stage of the metal reaction. This difference is due to the special bonding capabilities of M, in particular the large flexibility to change the oxidation state.
+
+
+
+
(20) (a) March, J. Advanced Organic Chemistry; Wiley-Interscience: New York, 1992. (b) Deng, L.; Branchadell, V.; Ziegler, T. J. Am. Chem. Soc. 1994,116,10645.
Frontside Attack (Metal Base)
Nevertheless, one should also be aware not to overrate CH3C1 too can the differences. The reaction of B proceed via a product complex [CH3B+, C1-1, in which the expelled leaving group is bound either to B or CH3. The interaction here is only weaker and more electrostatic in nature than in the metal case. E. Expectations for Ligand Effects. In this investigation, we have tried to understand the intrinsic reactivity of palladium toward CH3C1. This is the necessary basis for the next step, namely the development of insight into the working of ligand^.^,^ First, we take a look at the changes in the electronic structure of the Pd-dlo center upon introduction of one C1- ligand in PdC1-. The Pd-C1 bond is provided by a donor/ acceptor interaction between chlorine 3p2and palladium 5s. The introduction of an overall negative charge causes all 4d orbital energies to rise by ca. 5 eV. This charge effect generally increases the reactivity, as it improves backside interaction 4 as well as frontside interaction 5. However, the 4d22rises the most and ends up as the HOMO, due to the Pauli repulsion with C1 3p2. This effect strengthens backside interaction 4 and shifts the reactivity of the metal system from frontside to backside sN2 substitution (eq 14). Furthermore, CH3PdCl is directly formed; no C1- rearrangement (eq 12) is necessary and no charge separation (eq 10) occurs in the products.
+
CIPd-
+
CH$l
anion assisted sN2
* ClPdCH, +
C1-
Finally, it is pointed out that ligands can also influence the reactivity of a metal center by changing the character or appearance of the frontier d orbitals. This is clearly demonstrated by the preference of the square planar d8 system PtC142- to coordinate end-on, but not side-on, to F2 or 12, in spite of the fact that the 5d, orbitals (5d,,, 5dy2)are higher in energy than the 5d, orbital ( 5 d , ~ ) . We ~ ~ have shown previously that the reason is the special nodal structure of the metauigand hybrid 5d, orbitals, which results in poor overlap with ~ F - F * or 01-1" in the side-on coordination mode.6b 4. Conclusions
The intrinsic reactivity of palladium strongly favors oxidative insertion (eq 2) over backside sN2 substitution (eq 3) in the model system Pd CH3C1, as has been shown by our NL-SCF density-functional investigation. The reason is that the metal 4d orbitals are more suitable for frontside (4dX2) than backside (4d9) overlap with the occl* LUMO of the substrate. We have argued that it can be a useful concept to conceive oxidative insertion as frontside sN2 substitution, in spite of
+
2296 Organometallics, Vol. 14, No. 5, 1995 obvious differences. Thus, frontside sN2 attack is favored by (bare) metal bases due to the presence of valence d orbitals, whereas for main group bases only backside sN2 substitution is feasible (Scheme 1). The backside sN2 substitution of Pd CH3C1 can proceed in two ways: (i)The “straight” sN2 process (eq 10) is highly endothermic due to charge separation in the products PdCH3’ C1-. (ii)In the SNWC1-ra process (eq 121, substitution occurs with concerted rearrangement of the C1- leaving group and leads to the exothermic formation of CH3PdC1 via a still high but much lower energy barrier. A similar activation barrier has been found for a radical mechanism (eq 4) proceeding via single electron transfer (SET) and C1- abstraction by Pd+*,followed by recombination of the intermediates CH3* PdCP to form CH3PdC1. Solvent effects, estimated using a simple electrostatic continuum model, tend to favor the straight sN2 substitution due to the charge separation in the products. However, oxidative insertion remains dominant and other reaction parameters, namely ligands, must be invoked to shift the reactivity toward substitution.
+
+
+
Bickelhaupt et al.
Ligands can effect the reactivity of a metal center in two different ways: (i) by changing the (relative)energy of the d orbitals; (ii)by changing their character. The combined action of both mechanisms explains, for example, the working of the square planar d8 system [Rh(C0)2121-, i.e. the catalytic species of the well-known Monsanto acetic acid process. In the rate-determining step, [Rh(C0)2121- reacts specifically via backside sN2 substitution with CH31. This reactivity is further enhanced by prior coordination with I-, under formation of the pentacoordinate d8 complex [Rh(C0)&12-.13J4d,e It will be the subject of our future efforts to quantitatively analyze the nature of ligand effects on the reactivity of different metal centers.
Acknowledgment. This investigation was supported by the Deutsche Forschungsgemeinschaft (DFG), the Netherlands Organization for Scientific Research (NCFNWO),and the Natural Sciences and Engineering Research Council of Canada (NSERC). F.M.B. gratefully acknowledges a postdoctoral DFG fellowship. OM940931H
