Mechanism for Stereoblock Isotactic CO/Styrene CopolymerizationPromoted by Aryl r-Diimine Pd(II) Catalysts: A DFT Study

Carla Carfagna,*,† Giuseppe Gatti,† Paola Paoli,*,‡ and Patrizia Rossi‡

Istituto di Scienze Chimiche, UniVersita di Urbino, Piazza Rinascimento 6, 61029 Urbino, Italy, andDipartimento di Energetica, UniVersita di Firenze, Via S. Marta 3, 50139 Firenze, Italy

ReceiVed December 2, 2008

The density functional theory has been utilized to study a series of model compounds of the firstintermediates in CO/p-methylstyrene copolymerization reactions catalyzed by Pd(II) complexes, bearingaryl R-diimine achiral ligands. The results of the computation are correlated with the microstructure ofthe resulting polyketones ranging from atactic to stereoblock isotactic, depending on the substituents onthe aryls.

In past years, we have been involved in CO/vinyl arenecopolymerization reactions catalyzed by palladium complexesbearing nitrogen ligands.1

Recently, we found that achiral R-diimine Pd complexes oftype [Pd(η1,η2-C8H12OMe)(Ar-NdC(R)-C(R)dN-Ar)]X (1)lead to the formation of CO/p-methylstyrene copolymers witha microstructure ranging from atactic to stereoblock isotactic,depending on the substituents on the aryls;2 this result was totallyunexpected because isotactic polyketones are generally obtainedwith optically active C2 symmetric ligands.1b,3 In addition, itwas observed that the isotacticity strongly increases usingdiimine ligands with methyl groups in the phenyl ortho-positionsand in the backbone; this fact was ascribed to a lockedperpendicular orientation of the aryl rings with respect to themetal coordination plane.2 Next, with the aim of developingcatalytic systems able to improve the yields and useful toperform model studies, we modified the catalyst structure byreplacing the bulky methoxycyclooctenyl fragment in complexes1 with a methyl group and an acetonitrile molecule. Thus,two complexes of the type [Pd(Me)(NCMe)(Ar-NdC(Me)-C(Me)dN-Ar)]PF6, Ar ) p-OMe-C6H5 (2) and Ar ) 2,6-(Me)2C6H3 (3), were synthesized as typical examples of catalystsfor the production of atactic and isotactic copolymers, respec-tively.4 To understand how the steric arrangement of the catalystis able to determine an enantioselective insertion of the olefin,we investigated the first intermediates of the copolymerizationprocess starting from the methyl carbonyl complexes 4 and 5

(Scheme 1) because they represent, in the copolymerizationconditions, the real catalytic species.1,5

Insertion of p-methylstyrene in complexes 4 and 5 resultedin the quantitative formation of η3-allyl complexes 6 and 7having one double bond of the p-methylstyrene ring involvedin the Pd-coordination; complex 7 was structurally characterizedalso by X-ray diffraction (Figure 1).4a

Bubbling carbon monoxide in CHCl3 solutions of 6 and 7leads to the formation of compounds 8 and 9, respectively.Results from IR and NMR spectroscopy experiments on 8 and9 and structural data retrieved from the Cambridge StructuralDatabase (CSD)6 allowed us to assign to the growing chain theconformation sketched in Scheme 1. Moreover, we hypothesizedthat the insertion of the second styrene unit goes through anintermediate whose 3D structure is similar to 8 and 9, exceptfor the CO, which is replaced by the olefin.4a,7 If the doublebond of the latter is arranged perpendicularly to the metalcoordination plane,8 four different isomers (I-IV, Scheme 2)are in principle possible.

We also suggested that the relative steric hindrance betweenthe aryl rings of the styrene and of the nitrogen ligand is crucialin determining the population of the individual isomers.4a Thus,when ortho-disubstituted aryl-diimine ligands are used, in thespecies II and IIIthe steric hindrance should be considerablylarger than in I; as a consequence, I should be the preferredintermediate for the insertion reaction, thus justifying theobserved high copolymer isotacticity. On the contrary, whenno ortho-substituents are present on the aryl-diimine ligand, thethree isomers I, II, and III should be energetically comparable,in agreement with the formation of atactic copolymers. ProductIV should be anyway disfavored, because of the short contactsbetween the aryls of the two styrene units. This Article presentsa theoretical work aiming to verify the above hypothesis, bothon structural and on energetic grounds, to rationalize theformation of isotactic copolymer. With a view to shedding lighton this issue, a DFT study was undertaken; first, simplifiedmodel structures of the intermediates 8 and 9 involved in thecopolymerization, containing the growing chain, were devised.

* To whom correspondence should be addressed. E-mail: carla.carfagna@uniurb.it (C.C.); paolapaoli@unifi.it (P.P.).

† Universita di Urbino.‡ Universita di Firenze.(1) (a) Carfagna, C.; Gatti, G.; Martini, D.; Pettinari, C. Organometallics

2001, 20, 2175. (b) Binotti, B.; Carfagna, C.; Gatti, G.; Martini, D.; Mosca,L.; Pettinari, C. Organometallics 2003, 22, 1115.

(2) (a) Binotti, B.; Carfagna, C.; Zuccaccia, C.; Macchioni, A. Chem.Commun. 2005, 92. (b) Binotti, B.; Cardaci, G.; Carfagna, C.; Zuccaccia,C.; Macchioni, A. Chem.-Eur. J. 2007, 13, 1570.

(3) (a) Brookhart, M.; Wagner, M. I.; Balavoine, G. G. A.; Haddou,H. A. J. Am. Chem. Soc. 1994, 116, 3641. (b) Bartolini, S.; Carfagna, C.;Musco, A. Macromol. Rapid Commun. 1995, 16, 9. (c) Reetz, M. T.;Haderlein, G.; Angermund, K. J. Am. Chem. Soc. 2000, 122, 996.

(4) (a) Carfagna, C.; Gatti, G.; Mosca, L.; Passeri, A.; Paoli, P.; Guerri,A. Chem. Commun. 2007, 4540. (b) Atactic copolymers were also obtainedwith analogous Pd complexes bearing meta-substituted aryl-BIAN ligands.(c) Scarel, A.; Axet, M. R.; Amoroso, F.; Ragaini, F.; Elsevier, C. J.;Holuigue, A.; Carfagna, C.; Mosca, L.; Milani, B. Organometallics 2008,27, 1486.

(5) Carfagna, C.; Formica, M.; Gatti, G.; Musco, A.; Pierleoni, A.J. Chem. Soc., Chem. Commun. 1998, 1113.

(6) Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380.(7) Drent, E.; Budzelaar, P. H. M. Chem. ReV. 1996, 96, 663.(8) Fusto, M.; Giordano, F.; Orabona, I.; Ruffo, F. Organometallics 1997,

16, 5981.

Organometallics 2009, 28, 3212–32173212

10.1021/om8011322 CCC: $40.75 2009 American Chemical SocietyPublication on Web 05/01/2009

In particular, three cations were examined, 10, 11, and 12(Scheme 3), with an increasing number of methyl groups inthe nitrogen ligand: 10 without methyls, 11 with two methylson the R-diimine backbone, and 12 with four additional methylsin ortho-positions on both phenyl rings. The increased sterichindrance in 11 and 12, relative to the reference system 10,should reduce their conformational freedom, affecting thecoordination of the styrene units and the stereochemistry of theresulting intermediate complexes.

Table 1 lists the most interesting geometrical parameters ofthe optimized structures of models 10-12. As a whole, thecalculated geometries of the complex cations appear quitesimilar to the experimental ones retrieved from the CSD (version5.38, November 2006) for analogous molecular fragments(Scheme 4a).

The Pd-N bond trans to the acyl ligand is definitely longerthan the corresponding experimental value. In all cases, the CdOgroup in R position to the metal ion is almost at a right anglewith respect to the palladium mean coordination plane, asobserved in the solid-state structure of the parent acetyl-carbonyl-(1,10-phenanthrolinato)-palladium complex (TAX-

BAV,9 Scheme 4), while the mean angular value resulting fromthe CSD screening is smaller.

The τ1 and τ2 values, which define the arrangement of thearyl groups with respect to the R-diimine moiety, change ongoing from 10 to 12, suggesting that the orientation of thearomatic rings relative to the metal coordination plane (Figure2) is affected by the presence of bulky groups on the ligandbackbone and/or in the phenyl ortho-positions as well as bythe two others palladium ligands. Indeed, in model 10 withhydrogens on the R-diimine backbone, both phenyls are in a+syn-clinal10 conformation with respect to the NdC-CdNplane (τ1 and τ2 in Table 1) and almost perpendicular to eachother (ca. 84°). The larger τ2 value (ca. 15°, with respect to τ1)suggests that the bulky acyl ligand exerts a significant sterichindrance on the facing phenyl ring. The introduction of methylgroups in the backbone of 11 increases the τ torsions (∆τmean

) 30°), and the angle between the aromatic rings reduces to35°. Finally, the strong steric interactions between the methylgroups of the backbone and those in the phenyl ortho-positionsin 12 constrain the phenyl rings to arrange almost perpendicularwith respect to the palladium mean coordination plane andparallel to each other (they form an angle of 16°).

The reliability of the optimized geometry of the growing chainhas been checked by a comparison between the calculatedgeometry of various intermolecular contacts in models 11 and12 (final section of data in Table 1) and the NMR evidence forthe corresponding complexes 8 and 9 (Scheme 1).4a The lattersuggested that protons H(2) and H(3) (Scheme 3 for atomsnumbering) are in the shielding cone of the facing aromatic ring.Moreover, NOE experiment on complex 8 indicates that arelatively short distance separates H(1) and the opposite aromaticprotons.

Assuming that insertion of the successive styrene unit passesthrough the intermediates 11/I-IV and 12/I-IV (Schemes 2

(9) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996,118, 4746.

(10) Klyne, W.; Prelog, V. Experentia 1960, 16, 521.

Scheme 1

Figure 1. X-ray structure of the complex cation 7 from ref 4a.Only the heteroatoms have been labeled.

Scheme 2. Sketches of the Possible Intermediates Resultingfrom the Coordination of the Second Styrene Unit

Scheme 3. Sketch of the DFT Studied Models Showing theEssential Atoms Labeling Used in Tables 1 and 2a

a 10: R ) R′ ) H and L ) CO. 11: R ) Me and R′ ) H, L ) CO.12: R ) R′ ) Me and L ) CO. 11/I-IV L ) styrene; 12/I-IV L )styrene. When R′ ) H, the hydrogen atoms are labeled H(4) and H(5).

Stereoblock Isotactic CO/Styrene Copolymerization Organometallics, Vol. 28, No. 11, 2009 3213

and 3), the DFT analysis was carried out on these postulatedintermediates, with exclusion of types IV, which were notconsidered due to the closeness of the phenyl ring of the olefinand the last inserted styrene unit. The optimized parametersdefining the geometries are reported in Table 2, together withcorresponding experimental values retrieved from the CSD foranalogous palladium complexes featuring the molecular frag-ment sketched in Scheme 4b.

The Pd-N bond distances of these intermediates are longerin comparison both with the corresponding carbonyl complexes10-12 and with the experimental values found for analogousspecies (especially the Pd-N bond trans to the acyl bond).Concerning the coordination of the olefin, the two Pd-C(styrene)bonds are of different length: Pd-C(9) is shorter than Pd-C(10);they differ 0.2 Å on average. To assess the effect of the phenylgroup, a geometry optimization of the complex model 12/ety

Table 1. Most Significant Geometrical Parameters Defining the Geometry of the Model Cations 10, 11, and 12a

geometrical parametersb 10 11 12 mean value from CSDc value in TAXBAVd

Pd-N(1) 2.197 2.170 2.175 2.08 2.10Pd-N(2) 2.335 2.272 2.257 2.16 2.15Pd-C(1) 1.950 1.950 1.950 1.92Pd-C(2) 2.060 2.057 2.060 1.96 2.00N(1)-Pd-N(2) 75.60 74.42 74.67 78.2 79.5N(1)-Pd-C(2) 94.04 93.96 94.26 94.6 90.9C(1)-Pd-C(2) 87.08 88.20 88.65 87.2C(1)-Pd-N(2) 103.3 103.6 102.7 102.5N(1)-Pd-C(1) 178.9 177.2 176.0 175.7N(2)-Pd-C(2) 169.6 167.4 167.7 172.5 170.2∠ PdN2C-PdCOe 87.2 87.33 86.6 69.5 87.7C(1)-O(1) 1.137 1.138 1.138 1.11C(2)-O(2) 1.189 1.191 1.193 1.20 1.18τ1

f 36.3 59.6 88.3τ2

f 50.9 85.6 90.3τ3

f -6.6 -7.1 -7.3∠ Ar · · · H(2)-X(2)g 63.8 83.5 81.6H(2) · · · X(2) 3.416 3.083 3.093C(3)-H(2) · · · X(2) 128.2 132.0 132.8H(1) · · · H(4) 3.783 2.957O(2) · · · H(5) 2.662 3.223

a For comparative purposes, data retrieved from the CSD have been also reported. b For the atoms labeling, refer to Scheme3. c The searchedfragment is sketched in Scheme4a. Twelve Refcodes were retrieved in the CSD (13 fragments). d Acetyl-carbonyl-(1,10-phenanthrolinato)-palladiumcomplex from ref 9 (Scheme4). e Angle formed by the planes Pd-N(1)-N(2)-C(2) and Pd-C(2)-O(2). Only the refcode LUBRIJ shows a very lowangular value (32°), which was not included for the mean value calculation. f τ1, τ2, and τ3 define the orientation of the aromatic rings with respect tothe N(2)-C(6), N(1)-C(5), and C(7)-C(8) bonds, respectively. g Angle between the vector X(2)-H(2) and the mean plane defined by the aryl ringfacing it. X(2) is the centroid of the aromatic ring.

Table 2. Most Significant Geometrical Parameters Defining the Geometry of the Intermediates (11/I-III and 12/I-III) Deriving from theSecond Styrene Unit Insertiona

geometrical parametersb 11/I 11/II 11/III 12/I 12/II 12/III mean value from CSDc

Pd-N(1) 2.210 2.209 2.202 2.210 2.215 2.200 2.10Pd-N(2) 2.316 2.340 2.341 2.315 2.341 2.333 2.14Pd-C(2) 1.993 2.009 2.007 1.998 2.006 2.014 2.03Pd-C(9) 2.247 2.248 2.230 2.255 2.239 2.251 2.19Pd-C(10) 2.420 2.565 2.379 2.457 2.437 2.452 2.19Pd-X(1)d 2.230 2.311 2.199 2.254 2.236 2.250 2.09N(1)-Pd-N(2) 73.23 73.10 73.23 73.34 73.13 73.49 80.9N(1)-Pd-C(2) 96.64 95.59 95.09 96.44 93.94 94.41 99.5N(2)-Pd-C(2) 167.6 168.7 161.6 169.3 158.1 165.4 169.2N(1)-Pd-X(1) 161.7 175.8 158.6 164.3 165.3 166.1 170.2N(2)-Pd-X(1) 98.2 104.8 106.8 97.6 105.0 104.7 99.8C(2)-Pd-X(1) 93.5 86.4 89.3 93.1 91.8 88.9 80.4Pd-X(1)-C(9) 97.6 76.2 83.6 81.2 81.4 81.3 90.0Pd-X(1)-C(10) 82.4 103.8 96.4 98.8 98.6 98.7 90.0∠ PdN2C-PdC2

e 74.7 73.3 79.5 78.7 71.7 73.4 75.5∠ PdN2C-PdCOf 81.1 71.1 79.2 83.2 87.1 83.3C(9)-C(10) 1.387 1.382 1.388 1.385 1.384 1.383 1.38C(2)-O(2) 1.197 1.200 1.198 1.198 1.201 1.200τ1

g 71.4 112.7 63.87 85.1 101.9 79.7τ2

g 69.4 79.0 73.36 80.3 96.4 78.2τ3

g -6.1 -13.8 -8.11 -4.7 21.8 -7.7∠ Ar · · · H(2)-X(2)h 87.4 85.0 82.7 85.5 80.8 82.1H(2) · · · X(2) 2.86 2.66 3.10 2.84 2.70 2.96C(3)-H(2) · · · X(2) 137.2 136.8 132.4 138.1 138.5 135.5H(1) · · · H(4) 3.153 2.903 2.882O(2) · · · H(5) 3.198 3.425 3.426

a For comparative purposes, data retrieved from the CSD have been also reported. b For the atoms labeling, refer to Scheme3. c The searchedfragment is sketched in Scheme4b. Eight refcodes (8 fragments) were retrieved in the CSD. d X(1) is the centroid between the carbon atomsC(9)dC(10). e Angle formed by the planes Pd-N(1)-N(2)-C(2) and Pd-C(9)-C(10). f Angle formed by the planes Pd-N(1)-N(2)-C(2) andPd-C(2)-O(2). g τ1, τ2, and τ3 define the orientation of the aromatic rings with respect to the N(2)-C(6), N(1)-C(5), and C(7)-C(8) bonds,respectively. h Angle between the vector X(2)-H(2) and the mean plane defined by the aryl ring facing it. X(2) is the centroid of the aromatic ring.

3214 Organometallics, Vol. 28, No. 11, 2009 Carfagna et al.

with ethylene replacing styrene was performed. In this case also,the metal ion shows a typical η2-coordination, and the Pd-Cbond distances are nearly identical, 2.29 and 2.31 Å; thedifference ∆q of the atomic Mullikan charges of the two olefincarbons is zero. In all of the studied intermediates, ∆q resultedin being around 0.2e;11 thus the styrene CdC bond appearsscarcely polarized. Therefore, electronic factors appear notcrucial in determining the regiochemistry of styrene insertion,which instead should be ascribed to steric effects.12

As a general remark, the introduction of the styrene unit doesnot perturb appreciably the coordination sphere about the metalcation, except, as already pointed out, for the Pd-N bonddistances, which are slightly lengthened. Only in the case ofthe 11/II form does the coordination of the second styrene unitproduce a significant change, ∆τ1(11/II - 11) ) 53°, in theorientation of the phenyl ring of the nitrogen ligand, and thisintermediate results the highest in energy, ∆G (11/II - 11/I))+5.52 kcal mol-1 (see Table 3, Figure 3). In 11/III the phenylrings of styrene and of the ligand occupy two distinct regions(their centroids are 5.1 Å apart); at variance with our startinghypothesis, the energy of this intermediate is comparable to thatof 11/I, ∆G (11/III - 11/I) ) +0.07 kcal mol-1.

The ortho-substituted phenyl rings of 12, which are almostperpendicular to the NdC-CdN plane, do not undergo anysubstantial conformational rearrangement upon the coordination

of styrene (Table 2, Figure 4). The overall geometries of the12/I and 12/III intermediates do not differ significantly fromthose corresponding to the 11 set, as provided by molecularsuperimpositions, while 11/II and 12/II differ a little bit forthe position of the aromatic rings of both the growing chainand the styrene unit. In any case, forms II appear more crowdedwith respect to the other isomers given that the styrene ring isbetween the aryls of the azo-ligand and of the growing chain.As a consequence, the distances separating the centroid of thestyrene ring and those of the nearby rings are considerablysmaller than in forms I and III. The angle between the meanplanes described by the N2-phenyls is, as expected, 20° smallerin forms 12/I and 12/III with respect to the corresponding 11forms; in type II intermediates, the reverse holds. Also, in thisset of intermediates, isomer 12/II is the highest in energy, isomer12/I is the lowest, and 12/III is in between. The molar fractionsof the individual isomers, evaluated from the differences inGibbs free energy, are reported for comparison in Table 3.

In addition, the rotation about τ1 appears actually preventedonly in intermediate 12/III, as provided by the investigation ofthe potential energy profile about the torsion τ1, by systematicrigid scans. In fact, the profile of the potential energy surfaceabout τ1 is almost flat when the model complexes 11 and 12and the corresponding isomers 11/I, 11/III, and 12/I wereconsidered; instead, in the case of 12/III a significant energybarrier (ca. 10 kcal mol-1) accompanies the rotation from 75°to 91°. This suggests that in intermediate 12/III a stericinteraction between one methyl group on the aryl of nitrogenligand and the phenyl of the coordinated styrene prevents a freerotation about τ1.

In summary, isomers 11/I and 11/III are very similar: theyare almost isoenergetic, and in both of them the torsion τ1 canvary with no significant energy cost; that is, they are bothflexible. On the contrary, in the other set, 12/I is by far themost stable form, and it is largely more flexible than 12/III.

The above results can help in rationalizing regioselectivityand stereoselectivity of the copolymerization reactions promotedby aryl R-diimine Pd(II) catalysts 2 and 3.13

Concerning regioselectivity, inspection of Scheme 2 showsthat, while both forms I and III lead to insertion with a givenregiochemistry, forms II and IV lead to insertion with thereverse regiochemistry. Now, DFT calculations on II andmolecular modeling of IV advocate that their concentrationsare in any case negligible and therefore predict that the insertionof the styrene is mainly restrained to just one kind of insertion,corresponding to highly regioregular copolymers. This guessis confirmed by the 13C NMR spectrum of the copolymers,where regioirregular head-to-head and tail-to-tail enchainmentsare not detected.14 Moreover, secondary insertion (Pd to CH)is actually observed in the study of the first steps of thecopolymerization.4a

Scheme 4. Left: Sketches of the Searched Fragments in theCSD (- - -, Any Bond; X ) Any Nonmetal); Right: Complex

Cation of the TAXBAV Entry Retrieved in the CSD

Figure 2. View of the complex cations 10 (left), 11 (center), and 12 (right) with the relevant atom labeling.

Table 3. Relative Energies (kcal mol-1) and Concentrations of theIntermediates 11/I-III and 12/I-III Derived from the Coordination

of the Second Styrene Unit

configuration: R-Re R-Si R-Re R-Si

conformation: 11/I 11/II 11/III 12/I 12/II 12/III

Ea +0.45f +3.83f 0.0 0.0 +2.11g +0.64g

Eb +0.25 +4.14 0.0 0.0 +2.07 +0.96Ec +0.32 +4.00 0.0 0.0 +2.12 +0.82Gd -0.07 +5.45 0.0 0.0 +1.94 +1.18mole fraction xe 0.53 0.0 0.47 0.85 0.03 0.12

a Calculated from electronic energies. b Calculated from electronicenergies corrected for ZPE. c Calculated from electronic energiesincluding the thermal energies at 298 K. d Calculated from electronicenergies including the Gibbs energy correction at 298 K. e Calculatedfrom the differences in Gibbs free energy. f Energy value with respect tothe 11/III isomer. g Energy value with respect to the 12/I isomer.

Stereoblock Isotactic CO/Styrene Copolymerization Organometallics, Vol. 28, No. 11, 2009 3215

Concerning stereoselectivity, by inspection of Scheme 2 itappears that the R-Re form I generates the isotactic (l) dyad,while the R-Si form III generates the syndiotactic dyad (u),as summarized in the following:

u rku

R-Si a R-Re fkl

lThe ratio of molar fractions xl to xu depends on the equilibriumconstant Keq ) xR-Re/xR-Si and on the rate constants kl and ku:

xl

xu) Keq

kl

ku

Inserting the values of dyads molar fractions xu and xl (determinedfrom the 13C NMR spectra of the prevalently atactic and isotacticcopolymers obtained with catalysts 2 and 313) and the values ofKeq calculated from the mole fractions of R-Re and R-Si reportedin Table 3 results in values of kl/ku of an order of magnitude closeto unity, 0.96 in the atactic case and 0.74 in the isotactic case.This suggests that the polymer tacticity is mainly controlled byKeq, that is, by the concentrations of the intermediates generatedin the coordination of the second styrene unit. According to this,the concentrations of the l and u dyad are expected to be equal tothe concentrations of intermediates R-Re I and R-Si III,respectively. Indeed, these expected values of dyad populationsare very close to the values observed in the 13C NMR spectra ofthe copolymers, as reported in Table 4. The model can be furthertested at the triad information level. Assuming Bernouillian statistics(commonly observed, within experimental error, for copolymersobtained with R-diimine Pd complexes15), the intensities of thetriads can be calculated with the standard relationships:16

(ll) ) Pl2, (ul) ) 2PuPl, (uu) ) Pu

2

where the conditional probabilities Pl and Pu are the molarfractions of R-Re and R-Si, respectively. The obtained results

are reported in Table 4 and are compared to the 13C NMR triadpeak intensities of Figure 5.

Considering the approximation of the postulated models, theagreement looks reasonably satisfactory, and it is an additionalquantitative proof in favor of our previously proposed mecha-nism for the copolymerization. Of course, besides this, otherfactors could be invoked, to rationalize minor variations of thetriad distributions, such as the nature of the counterion2b andthe solvent utilized. In summary, the mechanism of CO/styrenecopolymerization with aryl R-diimine Pd(II) catalysts supple-mented by DFT calculations suggests that both regioselectivityand stereoselectivity are controlled by the intermediates resultingfrom the coordination of the styrene unit. This model waspurposely called “ligand assisted chain-end control”,4a becauseboth the ligand and the chain-end cooperate in selecting theenantioface and the direction of the incoming styrene unit.

Computational Details

The Gaussian 03 (revision C.02)17 package was used. All of thestudied species were fully optimized by using the density functionaltheory (DFT) method by means of Becke’s three-parameter hybridmethod using the LYP correlation functional.18 The effective corepotential of Hay and Wadt19 was used for the palladium atom. The

Figure 3. View of the intermediates 11/I (left), 11/II (center), and 11/III (right) with the relevant atom labeling.

Figure 4. View of the intermediates 12/I (left), 12/II (center), and 12/III (right) with the relevant atom labeling.

Table 4. Peak Intensities for Stereosequences of Copolymers

copolymer from catalyst 2 copolymer from catalyst 3

stereosequences observed expecteda observed expectedb

dyads l 0.52 0.53 0.84 0.88u 0.48 0.47 0.16 0.12

triads ll 0.27 0.28 0.70 0.78ul/lu 0.51 0.50 0.27 0.21uu 0.22 0.22 0.03 0.01

a Calculated from model 11. b Calculated from model 12.

3216 Organometallics, Vol. 28, No. 11, 2009 Carfagna et al.

6-31G* basis set20 was used for the remaining atomic species. Thereliability of the found stationary points (minima on the potentialenergy surface) was assessed by evaluating the vibrational frequen-cies. Starting geometries for the model cations 10, 11, and 12 werebased on already published IR, NMR, and X-ray diffractionevidence obtained for the corresponding complexes 8 and 9.4a Theinput geometries of the four isomers (I-IV, in Scheme 3), which

in principle can result from the insertion of the second styrene uniton the Pd-complexes, have been obtained by replacing CO withthe olefin in the optimized species 11 and 12. The same procedureapplies to 12/ety. We judged that the intermediate deriving fromcomplex 10, due to the larger degree of freedom of the latter, shouldnot add significant information and, as a consequence, has not beentaken into account. Isomers of type IV were ruled out, due tocloseness of the last inserted styrene unit to the phenyl of the ligand.Rigid potential energy surface scans about τ1 were performed onthe R-diimine ligand of 11 and 12 (increment size, 2°; startingvalues, 60° (11) and 75° (12); number of steps, 9) and on theircorresponding intermediate isomers of types I and III to check thetorsional freedom of the phenyl ring bound to N(2). Only the organicmoieties were considered, and the model chemistry was HF/6-31G*.

OM8011322

(11) The mean Mullikan atomic charges on the styrene carbon atoms,which are-0.38e and-0.14e on C(9) and C(10), respectively, closely reflectthose in the unbound styrene moiety (-0.407e on C(9) and-0.138e onC(10)). For a discussion of the interplay between steric and electronic effects,see also: (a) Carfagna, C.; Gatti, G.; Mosca, L.; Paoli, P.; Guerri, A. HelV.Chim. Acta 2006, 89, 1660.

(12) Consiglio, G. Chimia 2001, 55, 809.(13) The copolymerization reactions were performed under mild condi-

tions (Pco ) 1 atm, T ) 17 °C); for details, see ESI of ref 4a. The copolymertacticity was carefully measured using a 150 mg sample dissolved in asolution of 1,1,1,3,3,3-hexafluoro-2-propanol/CDCl3 1/1 (v/v) at 308 K.

(14) Aeby, A.; Gsponer, A.; Consiglio, G. J. Am. Chem. Soc. 1998, 120,11000.

(15) Unpublished results of our laboratory.(16) Frisch, H. L.; Mallows, C. L.; Heatley, F.; Bovey, F. A. Macro-

molecules 1968, 1, 533.(17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,

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Figure 5. Section of the 13C{1H} NMR spectrum (50.3 MHz, 308K, (CF3)2CHOH/CDCl3 1/1 (v/v)) relative to the ipso-carbonresonances of CO/p-methylstyrene polyketones produced by thecatalysts 2 above and 3 below.

Stereoblock Isotactic CO/Styrene Copolymerization Organometallics, Vol. 28, No. 11, 2009 3217