Dipyridophenazine as electronic tunable ligands for the palladium-catalyzed synthesis of polyketones

Article abstract of DOI:10.1021/om700524v

A series of Pd - methyl complexes, 1a - 6a, with 11-R-dipyrido[3,2-a: 2′,3′-c]phenazine ligands (dppz-R) being electronically modified with an R sustituent, efficiently catalyze CO/ styrene copolymerization under mild conditions. Increased catalytic activity is observed on increasing the electron-donating character of the remote R substituent in the order R = NO ₂ < COOH < Cl < H < CH₃ < NH₂.

Full text of DOI:10.1021/om700524v

Copyright 2008
Volume 27, Number 6, March 24, 2008
American Chemical Society
Communications
Dipyridophenazine as Electronic Tunable Ligands for the
Palladium-Catalyzed Synthesis of Polyketones
Diego Villagra,† Rosa López,‡ Sergio A. Moya,‡ Carmen Claver,*,§ and Amaia Bastero*,§,
Departamento de Química, Facultad de Ciencias, UniVersidad de Tarapacá, casilla 7 D, Arica, Chile,
Facultad de Química y Biología, UniVersidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile,
and Departament de Química Física i Inorgànica, Facultat de Química, UniVersitat RoVira i Virgili,
C/Marcel.li Domingo s/n, 43007 Tarragona, Spain
ReceiVed May 28, 2007
of polyketones is still low catalyst activity and metallic
palladium deposition during the process of polymer synthesis.2
This catalyst decomposition is of particular importance for
palladium catalysts bearing nitrogen ligands,1a,3 which are the
most active ones for CO/styrene copolymerization.4 The de-
composition of such catalysts mostly takes place after ꢀ-H
elimination (a possible chain-end termination reaction), since
the palladium-hydride species formed are very unstable and
CO may substitute the coordination of the N ligand, precipitating
black palladium. Therefore, investigations aimed at finding more
stable catalysts for this process are still of interest. Catalysts
with enhanced stability have been obtained by the coordination
of two chelating nitrogen ligands to the palladium precursor.
Although dissociation of one ligand must occur to generate two
Summary: A series of Pd-methyl complexes, 1a-6a, with 11-
R-dipyrido[3,2-a:2′,3′-c]phenazine ligands (dppz-R) being elec-
tronically modified with an R sustituent, efficiently catalyze CO/
styrene copolymerization under mild conditions. Increased
catalytic actiVity is obserVed on increasing the electron-donating
character of the remote R substituent in the order R ) NO2 <
COOH < Cl < H < CH3 < NH2.
Over the last few years several families of palladium
precatalysts for alternating CO/olefin copolymerization have
been developed, with the common feature being at least one
chelating ligand.1 Despite intense research on this reaction over
the past decade, the major drawback to the industrial production
* To whom correspondence should be addressed. E-mail: abastero@
dow.com (A.B.); carmen.claver@urv.cat (C.C.).
(2) (a) Catalytic Synthesis of Alkene - Carbon Monoxide Copolymers
and Cooligomers; Sen, A., Ed.; Kluwer Academic: Dordrecht, The
Netherlands, 2003. (b) Consiglio, G. In Late Transition Metal Polymerization
Catalysis; Rieger, B., Ed.; Wiley-VCH: Weinheim, Germany, 2003; pp 279–
305.
† Departamento de Química, Facultad de Ciencias, Universidad de
Tarapacá, Casilla 7 D, Arica, Chile.
‡ Departamento de Química de los Materiales. Facultad de Química y
Biología, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago,
Chile.
(3) Recent examples of palladium-methyl catalysts with nitrogen
ligands: (a) Durand, J.; Zangrando, E.; Stener, M.; Fronzoni, G.; Carfagna,
C.; Binotti, B.; Kamer, P. C. J.; Mueller, C.; Caporali, M.; van Leeuwen,
P. W. N. M.; Vogt, D.; Milani, B. Chem. Eur. J. 2006, 12, 7639. (b) Schaetz,
A.; Scarel, A.; Zangrando, E.; Mosca, L.; Carfagna, C.; Gissibl, A.; Milani,
B.; Reiser, O. Organometallics 2006, 25, 4065. (c) Scarel, A.; Durand, J.;
Franchi, D.; Zangrando, E.; Mestroni, G.; Carfagna, C.; Mosca, L.; Seraglia,
R.; Consiglio, G.; Milani, B. Chem. Eur. J. 2005, 11, 6014.
§ Departament de Química Física i Inorgànica, Facultat de Química,
Universitat Rovira i Virgili, C/Marcel.li Domingo s/n, 43007 Tarragona,
Spain.
Present address: Dow Chemical Ibérica, Autovia Tarragona Salou,
43006 Tarragona, Spain.
(1) Recent reviews on palladium copolymerization catalysts: (a) Durand,
J.; Milani, B. Coord. Chem. ReV. 2006, 250, 542. (b) Bianchini, C.; Meli,
A. Coord. Chem. ReV. 2002, 225, 35. (c) García Suárez, E. J.; Godard, C.;
Ruiz, A.; Claver, C. Eur. J. Inorg. Chem. 2007, 2582.
(4) Unexpected 1,2-styrene insertion explains the polymer production
of a P-P-based palladium catalyst: Nozaki, K.; Komaki, H.; Kawashima,
Y.; Hiyama, T.; Matsubara, T. J. Am. Chem. Soc. 2001, 123, 534.
10.1021/om700524v CCC: $40.75
2008 American Chemical Society
Publication on Web 03/04/2008

1020 Organometallics, Vol. 27, No. 6, 2008
Communications
Table 1. CO/tert-Butylstyrene Copolymerization with 1a–6aa
Scheme 1. Dipyridophenazine Ligands (dppz-R)
productivityb
(g of PK/g of Pd) Mw (Mw/Mn) Tg (°C)
c
d
entry precursor
R
1
2
3
4
5
6
1a
2a
3a
4a
5a
6a
NH2
CH3
H
Cl
COOH
NO2
2669
2153
1332
722
240
259
1.0 × 105 (1.7)
4.3 × 104 (2.0)
3.6 × 104 (1.9)
3.9 × 104 (1.8)
4.0 × 104 (2.1)
4.6 × 104 (1.9)
154
153
152
151
150
142
a Reaction conditions: 5 µmol catalyst; TBS/Pd ) 3270; 8 mL of
CH2Cl2; 1 bar of CO; room temperature; 24 h. b Calculated from
weighed polyketone (PK). c Measured at 25 °C against polystyrene
standards. d Determined by DSC.
vacant positions at palladium for the polymerization reaction,
the presence of free ligand in the reaction medium greatly
increases the stability of the catalytic systems.5
Our approach to increase catalyst stability has been the use
of planar-around-metal ligands whose electronics could be easily
tuned.6 11-R-dipyrido[3,2-a:2′,3′-c]phenazine ligands (R ) NH2
(1), Me (2), H (3), Cl (4), COOH (5), NO2 (6); referred to here
as dppz-R),7 which can be considered as π-extended phenan-
throlines,8 were chosen (Scheme 1). Due to the π-delocalization
present in the phenazine backbone, we envisaged that the
electronic properties of R could be transferred to the metal and
affect its reactivity. Electronic effects have been observed for
substituted diphosphine ligands in the related palladium-
catalyzed CO/ethylene polymerization. The rate-enhancing effect
of the electron-donating substituents has been related to the
decreased strength of the interaction of the ꢀ-ketone moiety in
the growing polymer chain with the metal center, which can
favor the opening of the five- or six-membered oxametallacycle
involved in the progress of the reaction.9
The ligands presented in this work have been previously used
for the Ru-catalyzed water-gas shift reaction.10 It must be here
noted that during preparation of this work, Milani et al. reported
the novel use of unsubstituted dppz as a ligand for this
reaction.11 We prepared isolated single-site polymerization
catalysts via a two-step procedure (Scheme 2).12 Reaction of
ligands 1–6 with [PdClMe(cod)] (cod ) 1,5-cycloctadiene) in
refluxing dichloromethane gave insoluble products, which were
filtered and washed to remove free cyclooctadiene.13 Further
reaction of the insoluble products with NaBArF (BArF )
[B(3,5-(CF3)2C6H3)4]-) to abstract chlorine afforded the cationic
palladium-methyl complexes 1a–6a as slightly soluble dark
red-brown solids.14
Figure 1. Influence of the electronic density of the R substituent
(Hammett constants) on productivity (g of PK (g of Pd)-1 h-1).
Analysis by 1H and 13C NMR of the most soluble complex,
2a (R ) CH3), shows twice the number of signals for protons
H2,2′, H3,3′, and H4,4′ (Scheme 3), indicating the presence of
two coordination isomers in a ca. 1:1 ratio, owing to the
unsymmetrical nature of the ligand.15 For remote atoms (10,
1
12, and 13) only one signal is observed for each atom by H
NMR, while the signals of both isomers are observed by 13C
(see the Supporting Information for experimental details and
characterization data). Although the cis/trans regioisomeric ratio
of cationic palladium-methyl complexes should not influence
the catalytic activity, it could, however, affect the stereoregu-
larity of the reaction due to the different arrangements around
the metal.
(5) Selected examples of [Pd(N-N)][X2] catalysts: (a) Scarel, A.; Milani,
B.; Zangrando, E.; Stener, M.; Furlan, S.; Fronzoni, G.; Mestroni, G.;
Gladiali, S.; Carfagna, C.; Mosca, L. Organometallics 2004, 23, 1974. (b)
Milani, B.; Scarel, A.; Mestroni, G.; Gladiali, S.; Taras, R.; Carfagna, C.;
Mosca, L. Organometallics 2002, 21, 1323. (c) Milani, B.; Corso, G.;
Mestroni, G.; Carfagna, C.; Formica, M.; Seraglia, R. Organometallics 2000,
19, 3435.
Table 1 summarizes the results of the experiments with
complexes 1a–6a, which were studied as CO/tert-butylstyrene
(TBS) copolymerization precursors under mild conditions (1
bar of CO, room temperature, 24 h).16 Under the studied
conditions high productivities are obtained for complexes 1a–3a,
comparable to those of the most active palladium-methyl
systems in the literature.3 Notably we observed no decomposi-
tion to black palladium at the end of the polymerization runs.
(6) Electronic effects can be crucial in this process: (a) Reference 3a.
(b) Bastero, A.; Ruiz, A.; Claver, C.; Castillón, S.; Daura, E.; Bo, C.;
Zangrando, E. Chem. Eur. J. 2004, 10, 3747. (c) Gambs, C.; Chaloupka,
S.; Consiglio, G.; Togni, A. Angew. Chem., Int. Ed. 2000, 39, 2486.
(7) (a) Amouyal, E.; Homsi, A.; Chambron, J. C.; Sauvage, J. P. J. Chem.
Soc., Dalton Trans. 1990, 1841. (b) López, R.; Loeb, B.; Bossie, T.; Meyer,
T. J. Tetrahedron Lett. 1996, 37, 5437. (c) Arancibia, A.; Concepción, J.;
Daire, N.; Leiva, G.; Leiva, A. M.; Loeb, B.; del Rio, R.; Díaz, R.; Francois,
A.; Valdivia, M. J. Coord. Chem. 2001, 54, 323.
(8) Phenanthroline (phen) and phen derivatives have shown superior
performance as ligands in this reaction.3a,5
(9) See ref 2a, in particular Chapter 4, p 110 by Mul, W. P.; van der
Made, A. W.; Smaardijk, A. A., Drent, E.
(14) Complexes 1a-6a could be characterized by 1H NMR, but their
solubility was not sufficient to obtain good 13C spectra, except for 2a.
(15) Such mixtures of geometric isomers have reported for similar
palladium-methyl complexes: (a) Leone, A.; Gischig, S.; Elsevier, C. J.;
Consiglio, G. J. Organomet. Chem. 2007, 692, 2056. (b) Reference 3b. (c)
Soro, B.; Stoccoro, S.; Cinellu, M. A.; Minghetti, G.; Zucca, A.; Bastero,
A.; Claver, C. J. Organomet. Chem. 2004, 689, 1521.
(10) Aguirre, P.; López, R.; Villagra, D.; Azocar-Guzman, I.; Pardey,
A. J.; Moya, S. A. Appl. Organomet. Chem. 2003, 17, 36–41.
(11) Bergman, S. D.; Goldberg, I.; Carfagna, C.; Mosca, L.; Kol, M.;
Milani, B. Organometallics 2006, 25, 6014.
(12) Brookhart, M.; Rix, F. C.; DeSimone, J. M. J. Am. Chem. Soc.
1992, 114, 5894.
(16) tert-Butylstyrene instead of styrene is used to afford soluble alt-
CO-tert-butylstyrene polyketones: Brookhart, M.; Rix, F. C.; DeSimone,
J. M. J. Am. Chem. Soc. 1992, 114, 5895–5897.
(13) NMR spectra were not informative, due to the low solubility of
the products.

Communications
Organometallics, Vol. 27, No. 6, 2008 1021
Scheme 2. Preparation of Palladium-Methyl Precursors 1a–6a
Scheme 3. Numbering Scheme of Ligands in the Cationic
Palladium Complexes
effect on the relative rates of the chain transfer reaction vs
the chain propagation reaction, probably due to the remote
position of the substituents in the ligand. The highest
molecular weight was obtained with the most active catalyst
1a (Table 1, entry 1).
Narrow polydispersities of ca. 1.7–2 indicate single active
species (see Figure S1 in the Supporting Information). Dif-
ferential scanning calorimetry (DSC) measurements allowed us
to determine the Tg value (ca. 152 °C), which is slightly lower
for the polyketone obtained with catalyst 6a (142 °C). No
melting transitions were observed, as expected for amorphous
polymers. The stereoregularity of the polymers was determined
by 13C NMR. Highly syndiotactic materials (97% of the uu triad)
were obtained with all precursors, as expected for planar
ligands.1a
When productivities are related to the electronic nature of
the R substituent in the ligand, an increase in productivity is
observed with increasing electron-donating character. As a way
to evaluate the electronic effect of substituents on the reaction,
the Hammet parameter (σp) for the R substituents are plotted
against the productivities of the different complexes in Figure
1.17 A correlation appears, the catalysts bearing electron-rich
substituents (1a and 2a with NH2 and CH3, respectively) being
the most productive, in contrast to those having electron-
deficient substituents which are an order of magnitude less active
(4a, 5a, and 6a with Cl, COOH, and NO2). The complex 3a,
with unmodified dppz, has an intermediate performance (entry
3) and notably behaves better than the related catalyst with
phenanthroline as the chelating ligand,3a as recently reported
for [Pd(dppz)2][X]2 complexes.11 Therefore, it seems that the
extended aromatic system indeed exerts a positive influence on
the catalytic system stabilizing it.18
In conclusion, we have reported the synthesis of cationic
palladium-methyl complexes containing dppz-R ligands, which
are highly active catalysts for CO/tert-butylstyrene copoly-
merization under mild conditions of pressure and temperature.
Electronic modification of the dppz-R ligand at a position remote
from the coordinated nitrogen atoms tunes the productivity of
the palladium precursors. The most productive catalysts are those
containing electron-rich substituents.
Acknowledgment. We thank the CYTED V9 Iberoameri-
cano de Cooperación Project, the MCYT (No. CTQ2004-
04412/BQU, Consolider Ingenio 2010 CSD2006-0003), and
the Generalitat de Catalunya (No. 2005SGR007777 and
Distinction for Research Promotion, 2003, to C.C.). A.B.
thanks the Juan de la Cierva program (MCYT) for a research
contract. S.A.M. and R.L. thank the Fondecyt-Chile (Project
No. 1085135). Dr. Juan Guerrero is acknowledged for his
collaboration in data interpretation (Mecesup 0007 RMN-
USACH).
Molecular weights were determined by gel permeation
chromatography against polystyrene standards. No clear rela-
tionship was found between the electronics of the dppz-R ligand
and the molecular weight of the polymer (e.g., for 2a (R )
CH3) Mw ) 4.3 × 104 and for 6a (R ) NO2) Mw ) 4.6 × 104).
This indicates that there is no influence of the ligand electronic
Supporting Information Available: Text and a figure giving
experimental details and characterization data for the complexes.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(17) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165–195.
(18) Although activity measurements should be performed to state this
point, the absence of black palladium at the end of the reaction seems to
indicate the higher stability of these catalysts.
OM700524V