www.elsevier.com/locate/theochem

Journal of Molecular Structure: THEOCHEM 809 (2007) 29–37

Intermediate ion stability and regioselectivity in propenepolymerization using neutral salicyladiminato nickel(II)

and palladium(II) complexes as catalysts

Yue Liu a,*,1, Michael G.B. Drew b, Ying Liu a,1

a School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221008, PR Chinab School of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK

Received 6 November 2006; received in revised form 5 January 2007; accepted 8 January 2007Available online 12 January 2007

Abstract

The mechanism of the Heck reaction has been studied with regard to transition metal catalysis of the addition of propene and theformation of unsaturated polymers. The reactivity of nickel and palladium complexes with five different bidentate ligands with O,Ndonor atoms has been investigated by computational methods involving density functional theory. Hence, it is possible to understandthe electronic and steric factors affecting the reaction and their relative importance in determining the products formed in regard of theircontrol of the regiochemistry of the products.

Our results show that whether the initial addition of propene is trans to O or to N of the bidentate ligand is of crucial importance tothe subsequent reactions. Thus when the propene is trans to O, 1,2-insertion is favoured, but when the propene is trans to N, then 2,1-insertion is favoured. This difference in the preferred insertion pathway can be related to the charge distribution engendered in the pro-pene moiety when the complex is formed. Indeed charge effects are important for catalytic activity but also for regioselectivity. Stericeffects are shown to be of lesser importance even when t-butyl is introduced into the bidentate ligand as a substituent.� 2007 Elsevier B.V. All rights reserved.

Keywords: Heck Reaction; Propene insertion; Nickel catalysts; Palladium catalysts; Density functional theory

1. Introduction

There has been a considerable amount of recentresearch, both experimental and theoretical, involved ininvestigating the Heck reaction [1,2] with reference to tran-sition metal catalysis in organic synthesis. This carbon–car-bon coupling reaction shown in Fig. 1 provides a directroute to synthetically important arylated and vinylatedolefin polymers.

The Heck reaction has been recently reviewed, [3–6] andwhile the mechanism is understood to some degree, there

0166-1280/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2007.01.002

* Corresponding author.E-mail addresses: yueliusd@163.com (Y. Liu), m.g.b.drew@reading.

ac.uk (M.G.B. Drew).1 Present address: College of Chemistry and Life Science, Shenyang

Normal University, Shenyang, PR China.

are several features that remain to be clarified. Theseinclude particularly control of the regiochemistry in theaddition to asymmetrical alkenes and hence the nature ofthe unsaturated polymers that can be formed. Someauthors emphasise the importance of steric effects [7–9]while others electronic effects [6].

Several experimental and theoretical studies have beencarried out which were specifically designed to understandthe electronic and steric factors affecting the reaction andtheir relative importance in determining the productsformed [10–18]. These studies involved varying the metalcatalyst (e.g. Ni or Pd), the type of ligand complexing themetal catalyst (e.g. bis-amides, salicylaldimine, triphenyl-phosphine), the substituents on the ligand (e.g. t-butyl,phenyl) and the solvent used.

The contrast between Ni and Pd is of interest because ofaim of finding a cheaper transition metal that is as efficient

NPd

O

R'

R

NPd

O

R'R

1N 1O

Compounds (D) R' = H, R = Ph (E) R' = t-Bu R = Ph

Fig. 3. Compounds studied by von Schenck et al. [11] relevant to thispresent work. Note that their nomenclature is changed to 1N and 1O to beconsistent with the present work.

R XR' R'

R

R'

R+ or + HX

Fig. 1. The Heck reaction.

30 Y. Liu et al. / Journal of Molecular Structure: THEOCHEM 809 (2007) 29–37

as the more expensive palladium catalyst [16]. Recent the-oretical work by von Schenk et al. [11] has used propeneas the inserting monomer with palladium(II) as the cata-lyst. A number of symmetrical and asymmetrical palladiumcomplexes were investigated and it was concluded that elec-tronic tuning of the catalytic center provided a powerfulmeans of controlling selectivity. Indeed it was concludedfrom their study that a systematic combination of electron-ic and steric effects could be employed to shift the selectiv-ity in the desired direction.

We have followed the method of these authors in thispresent work. In our previous work [12,14], we used DFTto study various aspects of propylene polymerisation usingpalladium(II) salicyladimine complexes as catalysts andconcluded that both 1,2- and 2,1-insertions are possible.Schenck et al. [11] concluded that the choosing a cationicor a neutral active complex will lead to 1,2- or 2,1-inser-tion, respectively, and therefore that the distribution ofcharge in the complex is crucial for the intermediate com-plexes on regioselectivity. A similar conclusion has beendrawn from calculations on methyl radical addition [19].In this work, we extend this study to investigate how thedistribution of partial charge in neutral ligands can effectthe resulting products. The importance of charge effectson activity for polymer catalysts has recently been empha-sized [20–22].

The system we considered previously [12] is shown inFig. 2 and involved R = Me with M = Pd, Ni.

The work of von Schenk et al. [11] involved exclusivelyM = Pd and they considered two symmetric ligands andthree asymmetric bidentate ligands complexing the metal.Donor atoms included P, N and O. They considered exclu-sively R = Ph and studied the insertion of propene into thePd–Ph bond. The two complexes most relevant to ourcurrent work D and E are shown in Fig. 3.

von Schenck et al. [11] used the B3LYP functional withthe 6-311G(d,p) basis set for all non-metal atoms and forPd the primitive double zeta basis set was recontracted to4s4p3d and an f-function of 1.472 was added. In ourwork, we used the smaller B3LYP/LANL2DZ methodol-

N

MO

R

R'

N

MO

R

R'

1N 1O

Fig. 2. Active catalyst structures of salicyladiminato metal complexes.Compounds A (R 0 = H, R = Ph); B (R 0 = t-But, R = Ph); C (R 0 = H,R = Me). 1N signifies the intermediate with R trans to N, and 1O with Rtrans to O.

ogy. In order to compare their work quantitatively withours, we repeated their work with ligands D and E at thislower basis set, using their published coordinates for start-ing models, and the results are reported here. We foundvery little difference in the energies and energy differences(e.g. those reported in Table 4), as well as moleculargeometries and electronic properties. It seems clear thatthe B3LYP/LANL2DZ methodology is sufficient for ourrequirements.

In this present paper we report new results on catalystsA and B, and compare the results found to those obtainedpreviously by us for C [12]. In addition, we have recalculat-ed the energies of the D and E systems calculated previous-ly [11] but at a lower basis set so that they can be compareddirectly with our results for A, B and C. Our comparison ofresults for A–E allow us to draw significant conclusionsconcerning electronic and steric control of the Heck reac-tion for propylene polymerisation. The calculations werecarried out for both palladium and nickel.

2. Results and discussion

As all five molecules A–E involve asymmetric bidentateligands with both nitrogen and oxygen donor atoms, wecan consider two different starting configurations 1N and1O for each molecule, thus there are 10 different startingmodels for both palladium and nickel.

Geometry optimisation was carried out on these com-plexes 1O and 1N for the five different systems (A)–(E)inclusive with the resulting relative energies between 1O

and 1N isomers given in Table 1. For compounds withR 0 = H, 1O is more stable than 1N by a margin of ca.35 kJ mol�1 for Pd and ca. 45 kJ mol�1 for Ni. Howeverwhen R 0 = t-butyl, then 1N is more stable than 1O presum-ably because in 1N the bulky phenyl and t-butyl groups are

Table 1Relative energy (kJ mol�1) of 1N compared to 1O after geometryoptimisation for compounds A–E shown in Figs. 2 and 3

E(1N) � E(1O) for Pd E(1N) � E(1O) for Ni

A 33.2 44.8B �13.3 �6.3C 35.8 46.6D 38.6 43.9E �10.8 �8.0

Y. Liu et al. / Journal of Molecular Structure: THEOCHEM 809 (2007) 29–37 31

significantly further apart. Energy differences are ca.�12 kJ mol�1 for Pd and ca. �7 kJ mol�1 for Ni.

It can be noted that the energy differences are signifi-cantly greater for nickel compared to palladium in all caseswith R 0 = H. The next stage in the reaction is the additionof propene to the metal to form a p-complex with two adja-cent carbon atoms bonded to the metal with the bond per-pendicular to the equatorial plane. These complexes arecalled 2N and 2O, with the nomenclature also dependentupon the atom trans to the R group.

There are two possible conformations for both 2O andfor 2N dependent upon whether the methyl group in pro-pylene and the R group bonded to the metal are on oppo-site sides (named trans) or on the same side (named cis) ofthe plane formed by the double bond of propylene and themetal atom. As the structures did not differ significantly inenergy having less than 4 kJ mol�1 difference, we decidedto restrict our calculations to just the lowest energy trans

conformations. 2N and 2O were energy minimised till con-vergence for molecules A–E inclusive for both Pd and Ni.

For all the complexes with both Pd and Ni, the energy of2N is significantly higher in energy than that of 2O and thisapplies for R 0 = t-butyl as well as R 0 = H. The results cantherefore be contrasted with those for 1N and 1O presentedin Table 1 where the conformational preference is depen-dent upon the different steric effects occurring whenR 0 = H or t-butyl. However for 2N and 2O with R 0 = t-bu-tyl for B and E, there is no significant difference in stericcrowding as in 2N the t-butyl group is adjacent to the pro-pylene in the equatorial plane while in 2O it is adjacent tothe R = phenyl group. The reason that the 2O complexeshave a lower energy than the 2N complexes is thereforelikely to be primarily electronic (Fig. 4).

In Table 2, enthalpy values are quoted for the reactions

1Nþ propene ¼ 2N

1Oþ propene ¼ 2O

N

MO

R

R'

N

MO

R

R'

2N 2O

Fig. 4. The structures of 2N and 2O.

Table 2Energy differences (kJ mol�1) for structures A–E inclusive

E(2N) � E(2O) E(2N) � E(1N) �E(propene)

E(2O) � E(1O) �E(propene)

A 38.6, 45.6 �97.7, �72.7 �103.1, �73.6B 37.5, 39.4 �35.0, �1.0 �85.7, �46.7C 26.8, 32.9 �114.1, �87.9 �105.1, �74.2D 41.2, 46.1 �98.4, �72.1 �101.0, �74.3E 34.9, 35.5 �36.2, �2.3 �81.9, �45.9

Values for M = Pd precede values for M = Ni.

and these are illustrated in Fig. 5 which clearly shows thatfor N the 1,2-insertion barriers to give 3N 0 are lower thanthe 2,1-insertion barriers to give 3N and for O, the reverseis true that the 2,1-insertion barriers to give 3O are lowerthan the 1,2-insertion barriers to give 3O.

In all cases, it can be noted that the coordinative abilityof Pd is greater than that of Ni, since E(2) � E(1) is morenegative for Pd than for Ni.

The energies for these two reactions follow a similar pat-tern with both M = Pd and Ni in that energies for complex-es A, C and D with R 0 = H can be distinguished from B

and E with R 0 = t-butyl.For A, C and D both reactions are strongly exothermic

with enthalpies of ca. �100 kJ mol�1 for Pd and �75kJ mol�1 for Ni. Despite having the extra fused phenyl ringthe reactions for A and B are approximately equivalent inenergy to those for D and E.

For molecule C, we argued previously [12] that becausethe difference in energy for 2N–2O at 26.8 kJ mol�1 forM = Pd is not so great as for 1N compared to 1O whichis 35.8 kJ mol�1, then the addition of propene will be morefavoured for 1N than for 1O by 9.0 kJ mol�1 despite thefact that 1N has a higher energy than 1O. It was concludedthat the propene molecule like the methyl group prefers tooccupy the trans-O position. Thus the higher energy species1N can be stabilized by a larger energy value than can 1O

by coordinating a propylene molecule. However for both A

and D the opposite is the case in that DE(1N–1O) is lessthan DE(2N–2O) and therefore the addition of propene ismore favoured for 1O than for 1N and 1O has a signifi-cantly lower energy than 1N this seems the likely reactionpath for these molecules. The same pattern is found forM = Ni, in that for C the addition of propene is morefavoured for 1N but for A and D addition to 1O isfavoured but by only a small amount (ca. 2 kJ mol�1).The difference between A, D and C is that A and D haveR = Ph and C has R = Me so that the electronic and stericproperties are very different. Indeed one major difference isthat the Mulliken partial charge on C11, the carbon atomfrom R that is bonded to the metal, has a value in the range�0.10 to 0.10 for R = Ph, but is ca. �0.75 for R = Methough when the CH3 group as a whole is consider the totalcharge on the four atoms becomes comparable to that forC11 in the phenyl ring.

The results with complexes B and E with R 0 = t-butylare strikingly different. Indeed for M = Pd, the enthalpiesof reaction, E(2) � E(1), are ca. �35 kJ mol�1 for 1N butca. �83 kJ mol�1 for 1O, the difference being due to thestability of 1N compared to 1O. For M = Ni, the resultsshow that the reactions for 1N are only just exothermicwith energies of reaction ca. �1.5 kJ mol�1 while for 1O

the energies are ca. �46 kJ mol�1. Thus although 1N ismore stable than 1O, for these complexes containing t-bu-tyl, 1O is much more likely by ca. 40 kJ mol�1 to react withpropene to form 2O. However the large difference in energybetween 1O and 1N suggests that 1O is unlikely to beformed particularly as there would be a negligible energy

1

2

3

3'

20

40

60

80

100

120

140

Ene

rgy

Strucures

N-AN-B

N-C N-D N-E

12

3

3'

0

20

40

60

80

100

120

140

structures

Ene

rgy

O-AO-B

O-D

O-C

O-E

12

33'

30405060708090100110120130

Structures

Ene

rgyN-A

N-BN-C

N-D

N-E

1

2

3

3'

-100102030405060708090

100

StructuresE

nerg

y

O-A

O-B

O-C

O-D

O-E

Fig. 5. The relative energies (kJ mol�1) of X1. X2, X3 and X3 0 complexes for complexes A–E inclusive. For clarity the energies are plotted relative to X2which is set at 0.00 kJ mol�1. (a) M = Pd, X = O, (b) M = Pd, X = N, (c) M = Ni, X = O, (d) M = Ni, X = N.

32 Y. Liu et al. / Journal of Molecular Structure: THEOCHEM 809 (2007) 29–37

barrier for the R = t-butyl to move from being trans to Oin 1O to trans to N in 1N.

The next stage in the reaction process from 2O or 2N isfor the propene to react with the R group. This can be donein two different ways either via 1,2-insertion, which givesthe transition state 3N 0 when starting from 2N, or the tran-sition state 3O 0 when starting from 2O. The alternative 2,1-insertion will give the transition state 3N when startingfrom 2N, or 3O when starting from 2O. The pathwaysfrom 2N are shown in Fig. 6 and from 2O in Fig. 7, bothfor molecule B.

The reaction path from 2O for M = Pd, molecule A isfirst considered in detail. Selected bond lengths in the com-plexes and transition states for complexes of Pd and Niwith A are given in Table 3.

In 2O the metal is bonded to both C22 and C23 as wellas C11 (distances 2.25, 2.32, 2.02 A, respectively). In theformation of the transition state 3O 0 the Pd–C22 bond isshortened (2.09 A), while Pd–C23 is lengthened consider-ably to 2.51 A, while Pd–C11 is also lengthened to2.14 A. However the most striking change is found inC11–C23 which decreases from 3.07 to 2.05 A. In the finalproduct 4N 0, the C11–C23 (1.55 A) bond is formed, as thePd–C23 (2.81 A) and Pd–C11 (2.38 A) bonds are broken.

By contrast in the alternative transition state 3O formedfrom 2O, now Pd1–C22 is lengthened to 2.42 A as is Pd1–C11 at 2.13 A while Pd1–C23 is shortened to 2.11 A andC11–C22 decreases from 3.13 to 2.00 A. In the final prod-uct 4N, shown in Fig. 8, the Pd1–C22 is definitely broken at3.00 , the Pd1–C11 bond is weakened at 2.38 while the

Fig. 6. Two possible insertion reactions, starting from 2N for compound B (R = t-Bu, R 0 = Ph).

Fig. 7. Two possible insertion reactions, starting from 2O for compound B (R = t-Bu, R 0 = Ph).

Y. Liu et al. / Journal of Molecular Structure: THEOCHEM 809 (2007) 29–37 33

C11–C22 bond is formed at 1.54. Similar changes in thedistances occur when starting from 2N giving transitionstates 3N or 3N 0 and products 4O and 4O 0.

This pattern of dimensions in the transition state indi-cating bond breaking and forming is maintained for the

five molecules A–E and for the two metals Pd and Ni withno significant differences even when R = t-butyl comparedto H. However whatever the nature of the products, theenergy differences shown in Table 4 for the various reactionpaths are significantly different. Following von Schenk

Table 3Selected bond lengths (A) for the Pd and Ni complexes with A

Reactant Transition states Reactant Transition states

2O 3O 0 3O 2N 3N 0 3N

Pd1–C11 2.02 2.14 2.13 2.03 2.12 2.12Pd1–C22 2.25 2.09 2.42 2.22 2.10 2.38Pd1–C23 2.32 2.51 2.11 2.28 2.47 2.12C11–C22 3.13 2.89 2.00 3.09 2.98 2.09C11–C23 3.07 2.05 2.94 2.97 2.14 3.03C22–C23 1.39 1.44 1.44 1.40 1.44 1.44Pd1–N2 2.03 2.08 2.08 2.12 2.05 2.06Pd1–O6 2.11 2.07 2.08 2.06 2.10 2.10Ni1–C11 1.92 2.02 2.01 1.92 1.99 1.98Ni1–C22 2.16 1.97 2.26 2.13 2.00 2.21Ni1–C23 2.25 2.33 2.00 2.18 2.25 2.04C11–C22 2.98 2.83 1.98 2.93 2.88 2.15C11–C23 2.25 2.02 2.90 2.87 2.14 3.01C22–C23 1.38 1.44 1.43 1.39 1.43 1.42Ni1–N2 1.88 1.91 1.91 1.95 1.90 1.90Ni1–O6 1.92 1.89 1.90 1.89 1.92 1.91Ni1–O6 1.92 1.89 1.90 1.89 1.92 1.91

The numbering system used is given in this Figure for 3O 0-A. Results forB, C, D and E are given in the Supplementary Publication.

Fig. 8. Three geometry optimised structures for 4N 0-A showing differentconformations, (a) 4N 0CH3-A, (b) 4N 0Ph-A, (c) 4N 0H-A.

2 Although we have used a different level of calculation from Schenket al. [11] which has resulted in different absolute energies, the energydifferences that we obtained which are shown in Table 4 are similaralthough for D they found that 1,2-insertion was favoured though by asmall amount.

34 Y. Liu et al. / Journal of Molecular Structure: THEOCHEM 809 (2007) 29–37

et al. [11] the regioselectivity in the Heck reaction can bedetermined by the relative energy difference between thetwo possible insertion pathways DDE = DE*(2,1) �DE*(1,2), the energies associated with the insertion of pro-pene into the Pd–C bond. The activation barriers DE*(2,1)and DE*(1,2) are calculated as the energy differencebetween the p-complex (2N or 2O) and the transition states(3N, 3N 0 or 3O, 3O 0). The selectivity is defined as the dif-ference in the barrier between the two possible insertionpathways i.e. 1,2- and 2,1-insertion. The barriers for isom-erization of related p-complexes DEiso are calculated basedon the reaction path involving the rotation of the ligand.This was found to be of lower energy than paths involvingeither a dissociative mechanism or an associative exchangemechanism.

As is apparent from Table 4, despite the values ofDE*(2,1) and DE*(1,2) being very different, DDE is positivefor 2N and negative for 2O for all the 10 calculations usingthe five molecules A–E and the two metals Pd and Ni. Thismeans that for 2N, 1,2-insertion is favoured to produce thetransition state 3N 0 and thence the product 4O 0, while for

2O, 2,1-insertion is favoured to produce the transition state3O and thence the product 4N.2

While the differences between the DDE value for 2N and2O are clear-cut, it is worth considering in detail why thesedifferences occur. While there are results [23] that indicatethe charge effects are important for catalytic activity, wefound that charge effects are also important for regioselec-tivity. To establish this fact, the partial atomic charges list-ed in Table 5 can be considered.

While results are only reported for molecules A and C inTable 5, the charges found for molecules B, D and E (in theSupplementary Publication) are very similar showing aconsistency in the Mulliken charges that allows conclusionsto be drawn. For example from Table 5, it can be seen thatfor 1,2-insertion which leads to the transition states 3N 0

and 3O 0, a positive partial charge is acquired by the middlecarbon atom C23 of the propylene molecule so that the

Table 4Activation energy of p-complex isomerization, the insertion of propene into the Pd–R bond and the thermodynamic driving force of the insertion step(kJ mol�1)

Compound* DE1a DE*(2,1)b DE*(1,2)c DDEd DED(2,1)e DED(1,2)f

A N 71.4, 46.5 60.1, 36.9 59.3, 36.1 0.8, 0.8 44.12, 19.85 49.49, 19.80O 109.9, 92.2 88.3, 68.6 94.0, 74.7 �5.7, �6.1 �4.65, 49.08 �8.57, 46.66

B N 36.5, 13.2 56.3, 38.9 50.4, 35.1 5.9, 3.8 45.18, 81.93 57.18, 86.09O 73.9, 52.6 90.8, 80.5 97.7, 85.2 �6.9, �4.7 8.00, 36.63 11.30, 32.74

C N 70.5, 47.9 79.0, 51.9 78.1, 49.7 0.9, 2.4 6.8, 38.0 7.7, 44.4O 97.3, 80.9 119.7, 93.8 123.2, 95.8 �3.5, �2.0 �48.3, �27.8 �43.7, �25.9

D (=6) N 70.4, 45.5 57.0(61.0), 36.9 56.4(62.3), 34.2 0.6(�1.3), 2.7 49.17(40.1) 54.01(44.7)65.37 66.97

(=5) O 111.6(105.8),91.6 92.4(96.1), 88.3 97.4(102.0),97.4 �5.0(�5.9),�9.1 �9.33(�13.8) �8.44(�13.4)0.83 �1.10

E (=8) N 37.7, 15.6 57.7(64.0), 38.0 52.0(54.8), 34.9 5.7(9.2), 3.1 45.88(25.5), 62.63 59.46(52.7), 74.30(=7) O 72.6(66.5), 51.1 93.9(92.0), 80.5 100.5(99.1), 87.4 �6.6(�7.1)�6.9 24.54(28.4) 12.14(14.6)

45.78 40.20

Energies for Pd precede those for Ni. Energies in brackets are abstracted from Ref. [11] and refer only to Pd.a DE1 is the barrier for p-complex isomerisation between 2O and 2N.b DE*(2,1) is the reaction activation energy forming 3X from 2X (X = N or O).c DE*(1,2) is the reaction activation energy forming 3X 0 from 2X (X = N or O).d DDE is DE*(2,1) � DE*(1,2).e DED(2,1) is the thermodynamic driving force of the insertion step passing through 3X, i.e. E(2X) � E(4X). Here, we used the lowest energy

conformation for 4X which included C–H–M agostic interactions.f DED(1,2) is the thermodynamic driving force of the insertion step passing through 3X 0 i.e. E(2X) � E(4X0). Here, we used the lowest energy

conformation for 4X0 which included C–H–M agostic interactions.* Numbers in brackets refer to the nomenclature used in Ref. [11].

Y. Liu et al. / Journal of Molecular Structure: THEOCHEM 809 (2007) 29–37 35

transition states can be considered as having a d+ carbonintermediate. By contrast for 2,1-insertion leading to thetransition states 3N and 3O this atom has a negative partialcharge leading to a d� carbon intermediate. Again we con-sider the example of the M = Pd, molecule A where in 2N

and 2O the Mulliken charges on C23 are 0.017, 0.018 whilein 3O 0, 3N 0, the charges are increased to 0.046, 0.030 and in3O, 3N reduced to �0.064, �0.100, respectively. Similarcharges are found for all the other systems B–E andM = Pd, Ni.

There is a major difference between the charge on C11for A, B, D, E with R = Ph where it is small, comparedto that for C with R = Me where it is ca �0.75, but thisseems to have little effect on the charge distribution else-where. No doubt this is because the three hydrogensattached to the methyl are positively charged such thatthe –CH3 group as a whole has a charge comparable to thatof the C11 bridgehead carbon atom in the phenyl ringwhen R=Ph. The total charge of the –CH3 group in 3N 0

of C is 0.02 for M = Pd and �0.05 for M = Ni very similarto that of C11 where R=Ph.3

We can conclude that any effect that increases the stabil-ity of this d+ carbon intermediate will favour 1,2-insertionwhile increasing the stability of the d� intermediate willfavour 2,1-insertion. This conclusion is consistent withthe result of Schenk et al. [11] that a cationic activecomplex will lead to 1,2-insertion and that a neutral active

3 It could be argued similarly that the highly negative charges on theatoms C22 and C24 (and indeed N2) given in Table 5 are similarlymisleading because they would be balanced to some extent by the positivecharges of the attached hydrogen atoms.

complex will lead to 2,1-insertion of propylene. HereSchenk’s result for cationic mechanism can be applied tothe mechanism for a neutral species, where the cationiccharacter can be considered as emanating from the positivecharge on the metal atom.

Thus in 1,2-insertion the metal atom of 2N or 2O

attacks the end carbon C22 of the double bond of propyl-ene, forming a carbocation transition state 3N 0 or 3O 0 withthe positive charge located on the middle carbon C23 of thepropylene. Transition state 3N 0 delocalizes the positivecharge on C23 more than 3O 0 (Table 3), making 3N 0 morestable than 3O 0, so that 1,2-insertion has a lower barriermoving from 2N to 3N 0 than from 2O to 3O 0. The carbonatom C11 of 2N or 2O attacks the terminal carbon C22 ofthe double bond of propylene, forming an intermediatecarboanion 3N or 3O on the middle carbon C23 of thepropylene.

One might expect that the results should fall into threegroups, namely C with R = Me in which C11 is a highly neg-ative methyl carbon atom, B and E which are stericallycrowded with R 0 = t-butyl, R = Ph and A and D withR 0 = H, R = Ph. However this does not appear to be thecase, presumably because the –CH3 group should be consid-ered as a whole with a charge that is comparable to that ofC11 where R = Ph, and the results for the five different struc-tures have more in common than might be expected. Exceptfor compound E which is crowded at the active site, transi-tion state 3O delocalizes the negative charge on C23 morethan 3N, making 3O more stable than 3N, so the 2,1-inser-tion has a lower barrier from 2O to 3O than from 2N to 3N.

The more negative (or less positive) the charge on thealkyl or the aryl carbon C11 then the more likely it is that

Table 5Mulliken partial charges for the complexes with M = Pd, Ni and ligands A

and C

2O 3O 0 3O 2N 3N 0 3N

For palladium

Molecule A

Pd1 0.15 0.19 0.16 0.19 0.19 0.18C22 �0.52 �0.57 �0.44 �0.52 �0.58 �0.45C23 0.02 0.05 �0.06 0.02 0.03 �0.10C24 �0.69 �0.69 �0.69 �0.67 �0.69 �0.67C11 0.09 0.04 0.05 0.12 0.09 0.10N2 �0.46 �0.45 �0.45 �0.45 �0.45 �0.46O6 �0.45 �0.44 �0.45 �0.43 �0.42 �0.41

Molecule C

Pd1 0.24 0.20 0.18 0.25 0.19 0.18C22 �0.53 �0.57 �0.38 �0.53 �0.58 �0.39C23 0.01 0.05 �0.11 0.01 0.02 �0.14C24 �0.68 �0.66 �0.69 �0.68 �0.66 �0.66C11 �0.77 �0.76 �0.75 �0.75 �0.72 �0.72N2 �0.46 �0.46 �0.46 �0.45 �0.45 �0.46O6 �0.45 �0.43 �0.44 �0.44 �0.44 �0.44

For nickel

Molecule A

Ni1 0.21 0.24 0.23 0.26 0.25 0.26C22 �0.52 �0.59 �0.45 �0.51 �0.58 �0.47C23 0.02 0.05 �0.09 0.02 0.03 �0.10C24 �0.70 �0.71 �0.70 �0.69 �0.70 �0.69C11 0.03 �0.01 0.00 0.07 0.04 0.17N2 �0.44 �0.45 �0.44 �0.46 �0.46 �0.45O6 �0.47 �0.45 �0.46 �0.43 �0.45 �0.43

Molecule C

Ni1 0.28 0.27 0.27 0.29 0.25 0.26C22 �0.52 �0.61 �0.39 �0.52 �0.61 �0.42C23 0.01 0.05 �0.13 0.01 0.03 �0.14C24 �0.70 �0.67 �0.70 �0.69 �0.67 �0.68C11 �0.79 �0.77 �0.76 �0.76 �0.75 �0.74N2 �0.43 �0.46 �0.46 �0.46 �0.44 �0.44O6 �0.48 �0.45 �0.46 �0.44 �0.45 �0.45

36 Y. Liu et al. / Journal of Molecular Structure: THEOCHEM 809 (2007) 29–37

this carbon attacks the terminal carbon C22 of propylene,and hence the more likely it is that the middle carbon ofpropylene C23 forms a carbanion. Indeed C11 is less posi-tively charged in 2O than in 2N which makes the 2,1-mech-anism more likely. Schenck et al. [11] also found thatincreased negative charge at the phenyl carbon C11 corre-lates with an increased 2,1-selectivity.

The next step in the polymerisation process involves thestructures of 4O, 4O 0, 4N and 4N 0. The figures shown inFigs. 6 and 7 are only one of several possible arrangements.Our calculations show that there are a variety of stableconformations for these species containing agostic interac-tions. For example for 4N 0, we have established three lowenergy conformations shown in Fig. 8 which differ accord-ing to which moiety group is in a cis position relative to themetal as defined by the M–C22–C23–X torsion angle. Thestructures are named 4N 0X, with X = CH3, Ph or H.

Relative energies of the three structures in Fig. 8 are0.00, 27.04, 40.58 kJ mol�1 for 4N 0Ph, 4N 0H, and 4N 0CH3,respectively. All contain a Pd(1)–C(22) bond of ca 2.04 Aand a C(23)–C(11) bond of ca. 1.55 A. In addition

4N 0Ph–A, which has the lowest energy, contains weakinteractions between Pd(1) and C(11) and C(12) of thephenyl ring at 2.38, 2.43 A. As there is an agostic interac-tion between Pd and C11–C23 in 4N 0Ph–A, the bondlength of C11–C23 is slightly longer at 1.551 A than inthe other two structures (1.528, 1.532 A). By contrast in4N 0CH3, there is a strong agostic interaction betweenPd(1) and a hydrogen of the methyl group at 2.04 A. ThePd(1). . .C(23) distance is 2.80 A. In 4N 0H, there is a strongagostic interaction between the metal and the cis hydrogenatom of 1.84 A and concomitantly the C(23)–H distance isincreased to 1.22 A. There is also an interaction betweenPd(1) and C(23) with a distance of 2.42 A.

Our calculations show that there are a variety of stableconformations for all the products 4N, 4O, 4N 0, 4O 0 andthe preferred structures is significantly dependent upon ste-ric effects and whether the metal is Ni or Pd. These resultswill be discussed in a subsequent paper together with thefurther reactivity of these species.

3. Conclusions

The mechanism of the Heck reaction has been studiedfor the addition of propene to transition metal complexesand the formation of unsaturated polymers. Ten differentmetal complexes have been considered, with two differentmetals Pd and Ni, and five different ligands. The resultsusing DFT show significant differences between the metalcomplexes. It is clear that whether the initial addition ofpropene is trans to O or to N of the bidentate ligand isof crucial importance to the subsequent course of the reac-tion. When R 0 is H, then 1O is more stable by 33–47 kJ mol�1 and when R 0 is the more sterically demandingt-butyl, then 1N is favoured but by a lesser margin (6–14 kJ mol�1). (Table 1) However for all complexes 2O isfavoured over 2N by 26–46 kJ mol�1 because steric effectsin these more crowded molecules are similar and the pref-erence for 2O is presumably electronic. It seems likely thenthat 2O is favoured for both R 0 = H and R 0 = t-propyl,though with the latter there is a finite chance of obtained2N.

The paths of subsequent reactions have been calculatedand it is clear that for 2O, 2,1-insertion is favoured to pro-duce the transition state 3O and thence the product 4N

rather than the transition state 3O 0 and the product 4N 0.In the unlikely event that 2N is formed, then 2,1-insertionis favoured to produce the transition state 3N 0 and thencethe product 4O 0 rather than the transition state 3N and theproduct 4O. We have shown that these preferences can berelated to the charge distribution in the intermediates.

Acknowledgements

We thank the Natural Science Foundation (No. B0313)of Heilongjiang Province of PR China and the ResearchFoundation of the China University of Mining andTechnology for financial support of this research.

Y. Liu et al. / Journal of Molecular Structure: THEOCHEM 809 (2007) 29–37 37

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.theochem.2007.01.002.

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