Conformationally flexible dimeric salphen complexes for bifunctional catalysis

Article abstract of DOI:10.1021/ja106484t

Appropriate modification of the salphen ligand allows an easy modular design of flexibly linked dimeric salphen species and their complexes, which can act as bifunctional catalysts. A series of chromium salphen systems including monomeric compound and dimers with different spacer lengths were tested for their catalytic performance in β-butyrolactone polymerization and CO ₂/propylene oxide copolymerization toward biodegradable materials. The results clearly show an enhancement in activity upon dimerization, thus underlining the role of bifunctional catalysis in the studied processes and extending the possible strategies for improvement of catalysts in these reactions.

Full text of DOI:10.1021/ja106484t

Published on Web 09/23/2010
Conformationally Flexible Dimeric Salphen Complexes for Bifunctional
Catalysis
Sergei I. Vagin, Robert Reichardt, Stephan Klaus, and Bernhard Rieger*
Wacker Lehrstuhl fu¨r Makromolekulare Chemie, Technische UniVersita¨t Mu¨nchen, Lichtenbergstrasse 4,
D-85748 Garching, Germany
Received July 22, 2010; E-mail: rieger@tum.de
Scheme 1. ROP of ꢀ-BL (1) and CO2/PO Copolymerization (2)
Abstract: Appropriate modification of the salphen ligand allows
an easy modular design of flexibly linked dimeric salphen
species and their complexes, which can act as bifunctional
catalysts. A series of chromium salphen systems including
monomeric compound and dimers with different spacer lengths
were tested for their catalytic performance in ꢀ-butyrolactone
polymerization and CO2/propylene oxide copolymerization
toward biodegradable materials. The results clearly show an
enhancement in activity upon dimerization, thus underlining
the role of bifunctional catalysis in the studied processes and
extending the possible strategies for improvement of catalysts
in these reactions.
salphens yields, in contrast to salens, a nearly statistical mixture of
products, unless the synthesis is metal-templated.9 This makes
further derivatization toward dimeric systems extremely complex.
For this reason, bridging two salphen units via the phenylenediamine
backbone seems to be more affordable. Up to now, only a few
dimeric salphen systems, all rigidly bridged via the backbone, have
been described in the literature.10 However, intramolecular interac-
Among metal-based catalysts, complexes with Schiff base ligands
tion of the two catalytic centers in these complexes is unlikely due
to their long metal-metal distance. Earlier we successfully
demonstrated a modular approach to dimeric phthalocyanines
flexibly interlinked by a bis(O,O′-resorcinato)hexamethylene
spacer.11 Here a similar approach has been adopted to prepare
dimeric salphen systems.
belong indisputably to a very important group because of their
multifunctionality and versatility.1 In this regard, salen and salphen
derivatives show appropriate activity in oxidation of alkenes and
other substrates,2 Diels-Alder, asymmetric hetero-Diels-Alder, and
Michael additions,3 stereoselective formation of cyanhydrines,4
stereo- and regioselective ring-opening of epoxides and their
copolymerization with CO2,5 and ring-opening polymerization
(ROP) of ꢀ-butyrolactone and lactide.6 The latter polymerization
and copolymerization reactions are of high interest, as they utilize
renewable resources like CO2 and lead to biodegradable materials
having a potential to compete with pure oil-based polymers like
polypropylene.
Scheme 2. Synthesis of Dimeric and Monomeric Salphen Ligandsa
One of the proposed mechanisms for the ROP of ꢀ-butyrolactone
(ꢀ-BL) as well as CO2/propylene oxide (PO) copolymerization using
CrIII(salphen) complexes implies a transition state, where the
growing chain at one metal center interacts with the activated
monomer at another metal center, giving high-molecular-weight
polymers.6a,5e This kind of bifunctional catalysis is, in general, a
widespread reaction mechanism that is currently gaining more and
more consideration.7 As shown in the case of salens, the activity
of catalyst in such reactions can be increased by utilization of
flexibly linked dinuclear species featuring a higher local concentra-
tion of catalytic centers.5d To the present, a great number of reports
dealing with preparation of di- and multinuclear salens has been
published.8 However, there is still a lack of knowledge on the
synthesis and catalytic properties of dimeric salphens. Here, we
report on a new synthetic approach to bimetallic salphen systems
for bifunctional catalysis which show the effect of dimerization in
the ROP of ꢀ-BL as well as for CO2/PO copolymerization reactions
(Scheme 1).
a Conditions: (i) NaH, resorcin in DMF; (ii) Pd/C, H2 in ethanol; (iii)
3,5-di(tert-butyl)salicyl aldehyde in ethanol; (iv) 1-bromohexane and Cs2CO3
in acetonitrile (4a) followed by metalation (4b); (v) 0.5 equiv of R,ω-
dibromoalkane and Cs2CO3 in acetonitrile followed by metalation (see
experimental details in Supporting Information).
The synthetic route presented in Scheme 2 is completely
chromatography-free, granting an advantage for high-scale
applications. The readily accessible intermediate 3 is a suitable
building unit for modular design of dimeric salphens, where the
length of the spacer and the nature of the metal can be easily
varied.
The salphen ligands are typically prepared under conditions
where the condensation of amine and aldehyde occurs in fast
equilibrium. That is why a stepwise condensation of two different
salicylaldehydes with phenylenediamine toward unsymmetrical
9
10.1021/ja106484t  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 14367–14369 14367

C O M M U N I C A T I O N S
Table 2. Copolymerization of Propylene Oxide with CO2 in the
Presence of Monomeric and Dimeric CrIII(salphen) Catalysts (60
°C, 40 bar CO2, 24 h)
The alkylation of 3 with excess of 1-bromohexane or with
0.5 equiv of R,ω-dibromoalkanes proceeds selectively at the
-OH group of the resorcin moiety. In the dimerization reactions,
which require a longer period of time for completion, partial
dehydrohalogenation of R,ω-dibromoalkanes and marginal hy-
drolysis of salphens lead to minor byproducts. Dimeric ligands
5a-c of good purity are readily obtained by reprecipitation from
acetonitrile due to a strong difference in solubility of bridged
and monomeric ligands in this solvent. Metalation of 5a-c with
CrCl2 according to a modified literature procedure gives the
corresponding CrIII complexes 6a-c in quantitative yields.12
These complexes were first tested for their catalytic performance
in the synthesis of poly(3-hydroxybutyrate) (PHB) via ROP of ꢀ-BL
(Scheme 1). Since the substitution pattern of the salphen ligand
plays a crucial role in the activity of the catalyst and changes the
characteristics of produced polymer,13a an analogous monomeric
Cr(III) complex, 4b, was synthesized for comparison. This high-
lighted the dimerization effect in the obtained polymerization results
(Table 1).
d
entry
cat.
[PO]/[Cr]a
PO/CO2b
cPC, %c
TOF, h-1
Mw, kg/mol
PD
8
9
10
11
4b
6c
4b
6c
2000
2000
20000
20000
1.7
2.3
1.3
2.4
<1
0
20
<2
67
49
7
70
40
9
2.0
1.8
2.0
3.3
82
46
a Molar ratio of propylene oxide to chromium. b Molar ratio of
propylene oxide to CO2 in the polymer (see Supporting Information for
details). c Weight part of cyclic propylene carbonate. d Turnover
frequency calculated for consumption of PO per Cr center; molar
TOFs for dimeric catalysts are twice higher.
and 9), though there is a drastic influence of dilution on the
polymerization results. The monomeric complex 4b loses its activity
in terms of much lower TOF and polymer molecular weight (entry
10), whereas in the case of catalysis with dimeric complex 6c, these
characteristics remain practically unaffected (entry 11). This is
consistent with our previous considerations on the mechanism of
the CO2/PO copolymerization.5e Additionally, the formation of
cyclic propylene carbonate (cPC) becomes noticeable only for
monomeric complex upon dilution (entry 10), which can be
attributed to an enhanced dissociation of the coordinated polymer
chain end from the catalyst, followed by back-biting.5e Investigation
of polymerization using dimeric catalysts or 4b in the presence of
cocatalysts is in progress.
In conclusion, a reliable and versatile synthetic approach toward
dinuclear salphen systems was developed. Using the corresponding
Cr(III) complexes, an increased activity upon dimerization was
demonstrated for such actually important reactions as polymeriza-
tion of ꢀ-butyrolactone and copolymerization of CO2 with PO,
which underlines the role of bimetallic processes in mechanisms
of both reactions.
Table 1. Results of Polymerization (100 °C, 1:1000 Cr/ꢀ-BL Ratio)
of ꢀ-BL with Different Complexes
b
entry cat. polymerization time, h yield of PHB, %a TOF, h-1
Mw, kg/mol PD
1
2
3
4
5
6
7
4b
4b
6a
6a
6a
6b
6c
5
24
5
15
24
5
6
30
35
86
>99
28
12
12
70
57
41
56
62
20
25
71
107
108
68
2.0
2.6
1.9
1.9
1.9
2.2
2.1
5
31
70
a Determined by integration of signals in 1H NMR spectrum.
b Turnover frequency calculated per Cr center; molar TOFs for dimeric
catalysts are twice higher.
The developed synthetic strategy can be easily adjusted for the
preparation of heteronuclear complexes and other multifunctional
systems, opening a wide area for different applications. This issue
is currently under investigation in our group.
Indeed, despite the slightly reduced activities of these new
complexes and the lower Mw of the resulting polymers as
compared to the best CrIII(salphen)-based catalysts published
earlier,6a the effect of dimerization on ROP of ꢀ-BL in the
systems studied herein is clear. The productivity of dimeric
complexes is practically independent of the length of the spacer
(entries 3, 6, and 7), giving ca. 5 times higher yield and higher
molecular weight of PHB in comparison with the monomeric
complex (entry 1).
Supporting Information Available: Detailed description of syn-
thesis and characterization of new products and polymers. This material
is available free of charge via the Internet at http://pubs.acs.org.
References
On the other side, polymerization with the complexes studied
herein leads to PHB of a relatively narrow polydispersity (PD) close
to 2, whereas PHBs of similar molecular weight obtained previously
with monomeric CrIII(salphens) display PD in the range of 4-8
and even higher (see also Figure S12 in Supporting Information).6a,13a
In addition, in 13C NMR spectra of PHBs prepared here, the ratio
of i- to s-signals in the carbonyl region, being a measure of the
polymer tacticity,13b is below 1.2. This is much lower than in the
case of the previously reported CrIII(salphen) catalysts, where it
reaches nearly 2.6a Investigations to get reasonable experimentally
supported explanations of these facts are in progress in our
laboratory.
Bifunctional salen catalysts were described recently for the
copolymerization of CO2 and PO to give aliphatic polycarbonate
(PPC) (Scheme 1).14 The similarity of the latter reaction to ROP
of ꢀ-BL, as well as a high value of PPC for applications and
utilization of carbon dioxide, encouraged us to examine the dimeric
and monomeric Cr(III) complexes 6c and 4b for their activity in
such a copolymerization.
(1) Gupta, K. C.; Sutar, A. K. Coord. Chem. ReV. 2008, 252, 1420–1450.
(2) (a) McGarrigle, E. M.; Gilheany, D. G. Chem. ReV. 2005, 105, 1563–1602.
(b) Gupta, K. C.; Sutar, A. K.; Lin, C. C. Coord. Chem. ReV. 2009, 253,
1926–1946.
(3) (a) Pellissier, H. Tetrahedron 2009, 65, 2839–2877. (b) Cort, A. D.;
Mandolini, L.; Schiaffino, L. Chem. Commun. 2005, 3867–3869. (c)
Castelli, V. A.; Cort, A. D.; Mandolini, L.; Pinto, V.; Schiaffino, L. J.
Org. Chem. 2007, 72, 5383–5386. (d) Madhavan, N.; Weck, M. AdV. Synth.
Catal. 2008, 350, 419–425.
(4) Achard, T. R. J.; Clutterbuck, L. A.; North, M. Synlett 2005, 1828–1847.
(5) (a) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps, A. L.; Billodeaux, D. R.
Acc. Chem. Res. 2004, 37, 836–844. (b) Darensbourg, D. J. Chem. ReV.
2007, 107, 2388–2410. (c) Coates, G. W.; Moore, D. R. Angew. Chem.,
Int. Ed. 2004, 43, 6618–6639. (d) Jacobsen, E. N. Acc. Chem. Res. 2000,
33, 421–431. (e) Luinstra, G. A.; Haas, G. R.; Molnar, F.; Bernhart, V.;
Eberhardt, R.; Rieger, B. Chem.sEur. J. 2005, 11, 6298–6314.
(6) (a) Zintl, M.; Molnar, F.; Urban, T.; Bernhart, V.; Preishuber-Pfluegl, P.;
Rieger, B. Angew. Chem., Int. Ed. 2008, 47, 3458–3460. (b) Dove, A. P.
Chem. Commun. 2008, 6446–6470.
(7) (a) Shibasaki, M.; Kanai, M.; Matsunaga, S.; Kumagai, N. Acc. Chem. Res.
2009, 42, 1117–1127. (b) Matsunaga, S.; Shibasaki, M. Bull. Chem. Soc.
Jpn. 2008, 81, 60–75. (c) Haak, R. M.; Wezenberg, S. J.; Kleij, A. W.
Chem. Commun. 2010, 46, 2713–2723.
(8) (a) Konsler, R. G.; Karl, J.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120,
10780–10781. (b) Widger, P. C. B.; Ahmed, S. M.; Hirahata, W.; Thomas,
R. M.; Lobkovsky, E. B.; Coates, G. W. Chem. Commun. 2010, 46, 2935–
2937. (c) Dioos, B. M. L.; Jacobs, P. A. Appl. Catal. A: General 2005,
282, 181–188. (d) Wezenberg, S. J.; Kleij, A. W. AdV. Synth. Catal. 2010,
352, 85–91. (e) Kleij, A. W. Eur. J. Inorg. Chem. 2009, 193–205.
As seen from Table 2, the activity of monomeric and dimeric
catalysts is comparable at a catalyst/PO ratio of 1/2000 (entries 8
9
14368 J. AM. CHEM. SOC. VOL. 132, NO. 41, 2010

C O M M U N I C A T I O N S
(9) Kleij, A. W.; Tooke, D. M.; Spek, A. L.; Reek, J. N. H. Eur. J. Inorg.
Chem. 2005, 4626–4634.
(13) (a) Reichardt, R.; Vagin, S.; Reithmeier, R.; Ott, A.; Rieger, B.
Macromolecules, submitted. (b) Bloembergen, S.; Holden, D. A.; Bluhm,
T. L.; Hamer, G. K.; Marchessault, R. H. Macromolecules 1989, 22, 1656–
1663.
(14) (a) Nakano, K.; Kamada, T.; Nozaki, K. Angew. Chem., Int. Ed. 2006, 45,
7274. (b) Ren, W.-M.; Zhang, X.; Liu, Y.; Li, J.-F.; Wang, H.; Lu, X.-B.
Macromolecules 2010, 43, 1396–1402. (c) Sujith, S.; Min, J. K.; Seong,
J. E.; Na, S. J.; Lee, B. Y. Angew. Chem., Int. Ed. 2008, 47, 7306.
(10) (a) Kuil, M.; Elsbeth Goudriaan, P.; van Leeuwen, P. W. N. M.; Reek,
J. N. H. Chem. Commun. 2006, 4679–4681. (b) Curreli, S.; Escudero-Adan,
E. C.; Benet-Buchholz, J.; Kleij, A. W. J. Org. Chem. 2007, 72, 7018–
7021. (c) Kleij, A. W. Chem.sEur. J. 2008, 14, 10520–10529.
(11) Lyubimtsev, A.; Vagin, S.; Syrbu, S.; Hanack, M. Eur. J. Org. Chem. 2007,
2000–2005.
(12) Darensbourg, D. J.; Mackiewicz, R. M.; Rodgers, J. L.; Fang, C. C.;
Billodeaux, D. R.; Reibenspies, J. H. Inorg. Chem. 2004, 43, 6024–6034.
JA106484T
9
J. AM. CHEM. SOC. VOL. 132, NO. 41, 2010 14369