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Organometallics 2009, 28, 3212–3217
Mechanism for Stereoblock Isotactic CO/Styrene Copolymerization Promoted 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, and Dipartimento 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 first intermediates in CO/p-methylstyrene copolymerization reactions catalyzed by Pd(II) complexes, bearing aryl R-diimine achiral ligands. The results of the computation are correlated with the microstructure of the resulting polyketones ranging from atactic to stereoblock isotactic, depending on the substituents on the aryls. In past years, we have been involved in CO/vinyl arene copolymerization reactions catalyzed by palladium complexes bearing nitrogen ligands.1 Recently, we found that achiral R-diimine Pd complexes of type [Pd(η1,η2-C8H12OMe)(Ar-NdC(R)-C(R)dN-Ar)]X (1) lead to the formation of CO/p-methylstyrene copolymers with a microstructure ranging from atactic to stereoblock isotactic, depending on the substituents on the aryls;2 this result was totally unexpected because isotactic polyketones are generally obtained with optically active C2 symmetric ligands.1b,3 In addition, it was observed that the isotacticity strongly increases using diimine ligands with methyl groups in the phenyl ortho-positions and in the backbone; this fact was ascribed to a locked perpendicular orientation of the aryl rings with respect to the metal coordination plane.2 Next, with the aim of developing catalytic systems able to improve the yields and useful to perform model studies, we modified the catalyst structure by replacing the bulky methoxycyclooctenyl fragment in complexes 1 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 catalysts for the production of atactic and isotactic copolymers, respectively.4 To understand how the steric arrangement of the catalyst is able to determine an enantioselective insertion of the olefin, we investigated the first intermediates of the copolymerization process starting from the methyl carbonyl complexes 4 and 5 * To whom correspondence should be addressed. E-mail: carla.carfagna@ uniurb.it (C.C.);
[email protected] (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 obtained with 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.
(Scheme 1) because they represent, in the copolymerization conditions, the real catalytic species.1,5 Insertion of p-methylstyrene in complexes 4 and 5 resulted in the quantitative formation of η3-allyl complexes 6 and 7 having one double bond of the p-methylstyrene ring involved in the Pd-coordination; complex 7 was structurally characterized also by X-ray diffraction (Figure 1).4a Bubbling carbon monoxide in CHCl3 solutions of 6 and 7 leads to the formation of compounds 8 and 9, respectively. Results from IR and NMR spectroscopy experiments on 8 and 9 and structural data retrieved from the Cambridge Structural Database (CSD)6 allowed us to assign to the growing chain the conformation sketched in Scheme 1. Moreover, we hypothesized that the insertion of the second styrene unit goes through an intermediate whose 3D structure is similar to 8 and 9, except for the CO, which is replaced by the olefin.4a,7 If the double bond of the latter is arranged perpendicularly to the metal coordination plane,8 four different isomers (I-IV, Scheme 2) are in principle possible. We also suggested that the relative steric hindrance between the aryl rings of the styrene and of the nitrogen ligand is crucial in determining the population of the individual isomers.4a Thus, when ortho-disubstituted aryl-diimine ligands are used, in the species II and IIIthe steric hindrance should be considerably larger than in I; as a consequence, I should be the preferred intermediate for the insertion reaction, thus justifying the observed high copolymer isotacticity. On the contrary, when no ortho-substituents are present on the aryl-diimine ligand, the three isomers I, II, and III should be energetically comparable, in agreement with the formation of atactic copolymers. Product IV should be anyway disfavored, because of the short contacts between the aryls of the two styrene units. This Article presents a theoretical work aiming to verify the above hypothesis, both on structural and on energetic grounds, to rationalize the formation of isotactic copolymer. With a view to shedding light on this issue, a DFT study was undertaken; first, simplified model structures of the intermediates 8 and 9 involved in the copolymerization, containing the growing chain, were devised. (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.
10.1021/om8011322 CCC: $40.75 2009 American Chemical Society Publication on Web 05/01/2009
Stereoblock Isotactic CO/Styrene Copolymerization
Organometallics, Vol. 28, No. 11, 2009 3213 Scheme 1
In particular, three cations were examined, 10, 11, and 12 (Scheme 3), with an increasing number of methyl groups in the nitrogen ligand: 10 without methyls, 11 with two methyls on the R-diimine backbone, and 12 with four additional methyls in ortho-positions on both phenyl rings. The increased steric hindrance in 11 and 12, relative to the reference system 10, should reduce their conformational freedom, affecting the coordination of the styrene units and the stereochemistry of the resulting intermediate complexes. Table 1 lists the most interesting geometrical parameters of the optimized structures of models 10-12. As a whole, the calculated geometries of the complex cations appear quite similar to the experimental ones retrieved from the CSD (version 5.38, November 2006) for analogous molecular fragments (Scheme 4a). The Pd-N bond trans to the acyl ligand is definitely longer than the corresponding experimental value. In all cases, the CdO group in R position to the metal ion is almost at a right angle with respect to the palladium mean coordination plane, as observed in the solid-state structure of the parent acetylcarbonyl-(1,10-phenanthrolinato)-palladium complex (TAX-
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 Resulting from the Coordination of the Second Styrene Unit
Scheme 3. Sketch of the DFT Studied Models Showing the Essential 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).
BAV,9 Scheme 4), while the mean angular value resulting from the CSD screening is smaller. The τ1 and τ2 values, which define the arrangement of the aryl groups with respect to the R-diimine moiety, change on going from 10 to 12, suggesting that the orientation of the aromatic rings relative to the metal coordination plane (Figure 2) is affected by the presence of bulky groups on the ligand backbone and/or in the phenyl ortho-positions as well as by the two others palladium ligands. Indeed, in model 10 with hydrogens on the R-diimine backbone, both phenyls are in a +syn-clinal10 conformation with respect to the NdC-CdN plane (τ1 and τ2 in Table 1) and almost perpendicular to each other (ca. 84°). The larger τ2 value (ca. 15°, with respect to τ1) suggests that the bulky acyl ligand exerts a significant steric hindrance on the facing phenyl ring. The introduction of methyl groups in the backbone of 11 increases the τ torsions (∆τmean ) 30°), and the angle between the aromatic rings reduces to 35°. Finally, the strong steric interactions between the methyl groups of the backbone and those in the phenyl ortho-positions in 12 constrain the phenyl rings to arrange almost perpendicular with respect to the palladium mean coordination plane and parallel to each other (they form an angle of 16°). The reliability of the optimized geometry of the growing chain has been checked by a comparison between the calculated geometry of various intermolecular contacts in models 11 and 12 (final section of data in Table 1) and the NMR evidence for the corresponding complexes 8 and 9 (Scheme 1).4a The latter suggested that protons H(2) and H(3) (Scheme 3 for atoms numbering) are in the shielding cone of the facing aromatic ring. Moreover, NOE experiment on complex 8 indicates that a relatively short distance separates H(1) and the opposite aromatic protons. Assuming that insertion of the successive styrene unit passes through 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.
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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
Pd-N(1) Pd-N(2) Pd-C(1) Pd-C(2) N(1)-Pd-N(2) N(1)-Pd-C(2) C(1)-Pd-C(2) C(1)-Pd-N(2) N(1)-Pd-C(1) N(2)-Pd-C(2) ∠PdN2C-PdCOe C(1)-O(1) C(2)-O(2) τ1f τ2f τ3f ∠Ar · · · H(2)-X(2)g H(2) · · · X(2) C(3)-H(2) · · · X(2) H(1) · · · H(4) O(2) · · · H(5)
2.197 2.335 1.950 2.060 75.60 94.04 87.08 103.3 178.9 169.6 87.2 1.137 1.189 36.3 50.9 -6.6 63.8 3.416 128.2 3.783 2.662
2.170 2.272 1.950 2.057 74.42 93.96 88.20 103.6 177.2 167.4 87.33 1.138 1.191 59.6 85.6 -7.1 83.5 3.083 132.0 2.957 3.223
2.175 2.257 1.950 2.060 74.67 94.26 88.65 102.7 176.0 167.7 86.6 1.138 1.193 88.3 90.3 -7.3 81.6 3.093 132.8
2.08 2.16
value in TAXBAVd 2.10 2.15 1.92 2.00 79.5 90.9 87.2 102.5 175.7 170.2 87.7 1.11 1.18
1.96 78.2 94.6
172.5 69.5 1.20
a For comparative purposes, data retrieved from the CSD have been also reported. b For the atoms labeling, refer to Scheme3. c The searched fragment is sketched in Scheme4a. Twelve Refcodes were retrieved in the CSD (13 fragments). d Acetyl-carbonyl-(1,10-phenanthrolinato)-palladium complex 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 low angular 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 to the 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 ring facing 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 the Second Styrene Unit Insertiona geometrical parametersb
11/I
11/II
11/III
12/I
12/II
12/III
mean value from CSDc
Pd-N(1) Pd-N(2) Pd-C(2) Pd-C(9) Pd-C(10) Pd-X(1)d N(1)-Pd-N(2) N(1)-Pd-C(2) N(2)-Pd-C(2) N(1)-Pd-X(1) N(2)-Pd-X(1) C(2)-Pd-X(1) Pd-X(1)-C(9) Pd-X(1)-C(10) ∠PdN2C-PdC2e ∠PdN2C-PdCOf C(9)-C(10) C(2)-O(2) τ1g τ2g τ3g ∠Ar · · · H(2)-X(2)h H(2) · · · X(2) C(3)-H(2) · · · X(2) H(1) · · · H(4) O(2) · · · H(5)
2.210 2.316 1.993 2.247 2.420 2.230 73.23 96.64 167.6 161.7 98.2 93.5 97.6 82.4 74.7 81.1 1.387 1.197 71.4 69.4 -6.1 87.4 2.86 137.2 3.153 3.198
2.209 2.340 2.009 2.248 2.565 2.311 73.10 95.59 168.7 175.8 104.8 86.4 76.2 103.8 73.3 71.1 1.382 1.200 112.7 79.0 -13.8 85.0 2.66 136.8 2.903 3.425
2.202 2.341 2.007 2.230 2.379 2.199 73.23 95.09 161.6 158.6 106.8 89.3 83.6 96.4 79.5 79.2 1.388 1.198 63.87 73.36 -8.11 82.7 3.10 132.4 2.882 3.426
2.210 2.315 1.998 2.255 2.457 2.254 73.34 96.44 169.3 164.3 97.6 93.1 81.2 98.8 78.7 83.2 1.385 1.198 85.1 80.3 -4.7 85.5 2.84 138.1
2.215 2.341 2.006 2.239 2.437 2.236 73.13 93.94 158.1 165.3 105.0 91.8 81.4 98.6 71.7 87.1 1.384 1.201 101.9 96.4 21.8 80.8 2.70 138.5
2.200 2.333 2.014 2.251 2.452 2.250 73.49 94.41 165.4 166.1 104.7 88.9 81.3 98.7 73.4 83.3 1.383 1.200 79.7 78.2 -7.7 82.1 2.96 135.5
2.10 2.14 2.03 2.19 2.19 2.09 80.9 99.5 169.2 170.2 99.8 80.4 90.0 90.0 75.5 1.38
a For comparative purposes, data retrieved from the CSD have been also reported. b For the atoms labeling, refer to Scheme3. c The searched fragment is sketched in Scheme4b. Eight refcodes (8 fragments) were retrieved in the CSD. d X(1) is the centroid between the carbon atoms C(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) and Pd-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.
and 3), the DFT analysis was carried out on these postulated intermediates, with exclusion of types IV, which were not considered due to the closeness of the phenyl ring of the olefin and the last inserted styrene unit. The optimized parameters defining the geometries are reported in Table 2, together with corresponding experimental values retrieved from the CSD for analogous palladium complexes featuring the molecular fragment sketched in Scheme 4b.
The Pd-N bond distances of these intermediates are longer in comparison both with the corresponding carbonyl complexes 10-12 and with the experimental values found for analogous species (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 phenyl group, a geometry optimization of the complex model 12/ety
Stereoblock Isotactic CO/Styrene Copolymerization
Organometallics, Vol. 28, No. 11, 2009 3215
Scheme 4. Left: Sketches of the Searched Fragments in the CSD (- - -, Any Bond; X ) Any Nonmetal); Right: Complex Cation of the TAXBAV Entry Retrieved in the CSD
Table 3. Relative Energies (kcal mol-1) and Concentrations of the Intermediates 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 Eb Ec Gd mole fraction xe
+0.45f +0.25 +0.32 -0.07 0.53
+3.83f +4.14 +4.00 +5.45 0.0
0.0 0.0 0.0 0.0 0.47
0.0 0.0 0.0 0.0 0.85
+2.11g +2.07 +2.12 +1.94 0.03
+0.64g +0.96 +0.82 +1.18 0.12
a Calculated from electronic energies. b Calculated from electronic energies corrected for ZPE. c Calculated from electronic energies including the thermal energies at 298 K. d Calculated from electronic energies including the Gibbs energy correction at 298 K. e Calculated from the differences in Gibbs free energy. f Energy value with respect to the 11/III isomer. g Energy value with respect to the 12/I isomer.
with ethylene replacing styrene was performed. In this case also, the metal ion shows a typical η2-coordination, and the Pd-C bond distances are nearly identical, 2.29 and 2.31 Å; the difference ∆q of the atomic Mullikan charges of the two olefin carbons is zero. In all of the studied intermediates, ∆q resulted in being around 0.2e;11 thus the styrene CdC bond appears scarcely polarized. Therefore, electronic factors appear not crucial 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 does not perturb appreciably the coordination sphere about the metal cation, except, as already pointed out, for the Pd-N bond distances, which are slightly lengthened. Only in the case of the 11/II form does the coordination of the second styrene unit produce a significant change, ∆τ1(11/II - 11) ) 53°, in the orientation of the phenyl ring of the nitrogen ligand, and this intermediate results the highest in energy, ∆G (11/II - 11/I) ) +5.52 kcal mol-1 (see Table 3, Figure 3). In 11/III the phenyl rings of styrene and of the ligand occupy two distinct regions (their centroids are 5.1 Å apart); at variance with our starting hypothesis, the energy of this intermediate is comparable to that of 11/I, ∆G (11/III - 11/I) ) +0.07 kcal mol-1. The ortho-substituted phenyl rings of 12, which are almost perpendicular to the NdC-CdN plane, do not undergo any substantial conformational rearrangement upon the coordination
of styrene (Table 2, Figure 4). The overall geometries of the 12/I and 12/III intermediates do not differ significantly from those corresponding to the 11 set, as provided by molecular superimpositions, while 11/II and 12/II differ a little bit for the position of the aromatic rings of both the growing chain and the styrene unit. In any case, forms II appear more crowded with respect to the other isomers given that the styrene ring is between the aryls of the azo-ligand and of the growing chain. As a consequence, the distances separating the centroid of the styrene ring and those of the nearby rings are considerably smaller than in forms I and III. The angle between the mean planes described by the N2-phenyls is, as expected, 20° smaller in forms 12/I and 12/III with respect to the corresponding 11 forms; in type II intermediates, the reverse holds. Also, in this set of intermediates, isomer 12/II is the highest in energy, isomer 12/I is the lowest, and 12/III is in between. The molar fractions of the individual isomers, evaluated from the differences in Gibbs free energy, are reported for comparison in Table 3. In addition, the rotation about τ1 appears actually prevented only in intermediate 12/III, as provided by the investigation of the potential energy profile about the torsion τ1, by systematic rigid scans. In fact, the profile of the potential energy surface about τ1 is almost flat when the model complexes 11 and 12 and the corresponding isomers 11/I, 11/III, and 12/I were considered; instead, in the case of 12/III a significant energy barrier (ca. 10 kcal mol-1) accompanies the rotation from 75° to 91°. This suggests that in intermediate 12/III a steric interaction between one methyl group on the aryl of nitrogen ligand and the phenyl of the coordinated styrene prevents a free rotation about τ1. In summary, isomers 11/I and 11/III are very similar: they are almost isoenergetic, and in both of them the torsion τ1 can vary with no significant energy cost; that is, they are both flexible. On the contrary, in the other set, 12/I is by far the most stable form, and it is largely more flexible than 12/III. The above results can help in rationalizing regioselectivity and stereoselectivity of the copolymerization reactions promoted by aryl R-diimine Pd(II) catalysts 2 and 3.13 Concerning regioselectivity, inspection of Scheme 2 shows that, while both forms I and III lead to insertion with a given regiochemistry, forms II and IV lead to insertion with the reverse regiochemistry. Now, DFT calculations on II and molecular modeling of IV advocate that their concentrations are in any case negligible and therefore predict that the insertion of the styrene is mainly restrained to just one kind of insertion, corresponding to highly regioregular copolymers. This guess is confirmed by the 13C NMR spectrum of the copolymers, where regioirregular head-to-head and tail-to-tail enchainments are not detected.14 Moreover, secondary insertion (Pd to CH) is actually observed in the study of the first steps of the copolymerization.4a
Figure 2. View of the complex cations 10 (left), 11 (center), and 12 (right) with the relevant atom labeling.
3216 Organometallics, Vol. 28, No. 11, 2009
Carfagna et al.
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.
Concerning stereoselectivity, by inspection of Scheme 2 it appears 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: ku
kl
u r R-Si a R-Re f l The ratio of molar fractions xl to xu depends on the equilibrium constant Keq ) xR-Re/xR-Si and on the rate constants kl and ku:
xl kl ) Keq xu ku Inserting the values of dyads molar fractions xu and xl (determined from the 13C NMR spectra of the prevalently atactic and isotactic copolymers obtained with catalysts 2 and 313) and the values of Keq calculated from the mole fractions of R-Re and R-Si reported in Table 3 results in values of kl/ku of an order of magnitude close to unity, 0.96 in the atactic case and 0.74 in the isotactic case. This suggests that the polymer tacticity is mainly controlled by Keq, that is, by the concentrations of the intermediates generated in the coordination of the second styrene unit. According to this, the concentrations of the l and u dyad are expected to be equal to the concentrations of intermediates R-Re I and R-Si III, respectively. Indeed, these expected values of dyad populations are very close to the values observed in the 13C NMR spectra of the copolymers, as reported in Table 4. The model can be further tested at the triad information level. Assuming Bernouillian statistics (commonly observed, within experimental error, for copolymers obtained with R-diimine Pd complexes15), the intensities of the triads can be calculated with the standard relationships:16
(ll) ) Pl2, (ul) ) 2PuPl, (uu) ) P2u where the conditional probabilities Pl and Pu are the molar fractions of R-Re and R-Si, respectively. The obtained results
Table 4. Peak Intensities for Stereosequences of Copolymers copolymer from catalyst 2 copolymer from catalyst 3 stereosequences dyads triads
a
l u ll ul/lu uu
observed
expecteda
observed
expectedb
0.52 0.48 0.27 0.51 0.22
0.53 0.47 0.28 0.50 0.22
0.84 0.16 0.70 0.27 0.03
0.88 0.12 0.78 0.21 0.01
Calculated from model 11. b Calculated from model 12.
are reported in Table 4 and are compared to the 13C NMR triad peak intensities of Figure 5. Considering the approximation of the postulated models, the agreement looks reasonably satisfactory, and it is an additional quantitative proof in favor of our previously proposed mechanism for the copolymerization. Of course, besides this, other factors could be invoked, to rationalize minor variations of the triad distributions, such as the nature of the counterion2b and the solvent utilized. In summary, the mechanism of CO/styrene copolymerization with aryl R-diimine Pd(II) catalysts supplemented by DFT calculations suggests that both regioselectivity and stereoselectivity are controlled by the intermediates resulting from the coordination of the styrene unit. This model was purposely called “ligand assisted chain-end control”,4a because both the ligand and the chain-end cooperate in selecting the enantioface and the direction of the incoming styrene unit.
Computational Details The Gaussian 03 (revision C.02)17 package was used. All of the studied species were fully optimized by using the density functional theory (DFT) method by means of Becke’s three-parameter hybrid method using the LYP correlation functional.18 The effective core potential of Hay and Wadt19 was used for the palladium atom. The
Stereoblock Isotactic CO/Styrene Copolymerization
Organometallics, Vol. 28, No. 11, 2009 3217 in principle can result from the insertion of the second styrene unit on the Pd-complexes, have been obtained by replacing CO with the olefin in the optimized species 11 and 12. The same procedure applies to 12/ety. We judged that the intermediate deriving from complex 10, due to the larger degree of freedom of the latter, should not add significant information and, as a consequence, has not been taken into account. Isomers of type IV were ruled out, due to closeness of the last inserted styrene unit to the phenyl of the ligand. Rigid potential energy surface scans about τ1 were performed on the R-diimine ligand of 11 and 12 (increment size, 2°; starting values, 60° (11) and 75° (12); number of steps, 9) and on their corresponding intermediate isomers of types I and III to check the torsional freedom of the phenyl ring bound to N(2). Only the organic moieties were considered, and the model chemistry was HF/6-31G*. OM8011322
Figure 5. Section of the 13C{1H} NMR spectrum (50.3 MHz, 308 K, (CF3)2CHOH/CDCl3 1/1 (v/v)) relative to the ipso-carbon resonances of CO/p-methylstyrene polyketones produced by the catalysts 2 above and 3 below. 6-31G* basis set20 was used for the remaining atomic species. The reliability of the found stationary points (minima on the potential energy surface) was assessed by evaluating the vibrational frequencies. Starting geometries for the model cations 10, 11, and 12 were based on already published IR, NMR, and X-ray diffraction evidence obtained for the corresponding complexes 8 and 9.4a The input geometries of the four isomers (I-IV, in Scheme 3), which
(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 reflect those in the unbound styrene moiety (-0.407e on C(9) and-0.138e on C(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 conditions (Pco ) 1 atm, T ) 17 °C); for details, see ESI of ref 4a. The copolymer tacticity was carefully measured using a 150 mg sample dissolved in a solution 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. Macromolecules 1968, 1, 533. (17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (18) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (19) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (20) Petersson, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham, M. A.; Shiirlay, W. A.; Mantzaris, J. J. Chem. Phys. 1998, 89, 2193.
