Methacrylate insertion into cationic diimine palladium(II)-alkyl complexes and the synthesis of poly(alkene-mocfc-alkene/carbon monoxide) copolymers

Article abstract of DOI:10.1021/om7006024

The reaction of cationic Pd(II) diimine complex [(N∧N)Pd(Me)(L)] [B(Ar_f)₄] ((N∧N) = 2,3-bis(2,6-di- isopropylphenylimino)butane, Ar_f -3,5-(CF₃) ₂C₆H₃, L = Et₂O) with m

Full text of DOI:10.1021/om7006024

Organometallics 2007, 26, 4711-4714
4711
Methacrylate Insertion into Cationic Diimine Palladium(II)-Alkyl
Complexes and the Synthesis of Poly(alkene-block-alkene/carbon
monoxide) Copolymers
Sachin Borkar, Hemant Yennawar, and Ayusman Sen*
Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
ReceiVed June 19, 2007
Scheme 1. Synthesis of MMA-Inserted Complex 2
Summary: The reaction of cationic Pd(II) diimine complex
[(N∧N)Pd(Me)(L)][B(Arf)4] ((N∧N) ) 2,3-bis(2,6-di-isopropyl-
phenylimino)butane, Arf ) 3,5-(CF3)2C6H3, L ) Et2O) with
methyl methacrylate results in 1,2-insertion regiochemistry that
is opposite of that obserVed with methyl acrylate. The enthalpy
of actiVation for MMA insertion is 4 kcal/mol higher than that
for the corresponding MA insertion. The MMA-inserted complex
can be employed for ethene or 1-hexene homopolymerization
and their alternating copolymerization with carbon monoxide.
Further, the quasi-liVing nature of the polymerizations allows
the synthesis of poly(hexene-b-hexene/CO) and poly(ethene-b-
ethene/CO) copolymers.
resultant radical. Another potential bonus of tuning the catalysts
to undergo 1,2-insertion of acrylate monomers is to be found
in the case of methacrylates, a class of monomers that has not
been shown to undergo metal-catalyzed addition polymerization.
When a methacrylate inserts in a 2,1 fashion, the ensuing alkyl
bears both an ester functionality and a methyl group on the
R-carbon, resulting in extreme steric congestion. This greatly
disfavors further monomer insertion but offers five â-hydrogens
resulting in facile elimination with an methacrylate-derived end
group.3a,b,4 On the other hand, incorporation of a methacrylate
with a 1,2 regioselectivity would potentially make it a good
monomer since the ensuing alkyl would have no substituents
on the R-carbon and would contain no â-hydrogens.
The catalytic copolymerization of polar vinyl monomers (e.g.,
acrylates and methacrylates) with unfunctionalized alkenes is
an area of great current interest,1 because of new desirable
properties that may emerge from such copolymers. Due to their
lower oxophilicity and relatively high functionality tolerance,
late transition metal-based systems have been the focus of much
of the recent work in the area. However, the interaction of the
oxygen functionality of the polar vinyl monomer with the metal
center and formation of cyclic chelates are the most commonly
cited problems that have stymied the development of suitable
catalysts. Additionally, for electronic reasons, acrylates have a
strong preference for 2,1-insertion into metal-carbon bonds.2
This insertion regioselectivity generates a metal-alkyl species
that is particularly prone to homolysis because of the enhanced
stability of the resultant alkyl radical, one that is essentially the
same as the propagating species in radical-initiated acrylate
polymerization.3
The cationic Pd(II) diimine ligand based system [(N∧N)Pd-
(Me)(L)][B(Arf)4], 1 ((N∧N) ) 2,3-bis(2,6-di-isopropylphe-
nylimino)butane, Arf ) 3,5-(CF3)2C6H3, L ) Et2O), reported
by Brookhart is among the very few systems that copolymerize
acrylates via insertion mechanism.5 The distinctive feature of
this system is the 2,1-insertion of acrylate and subsequent
rearrangement to form a six-membered cyclic chelate. The
rearrangement pushes the ester functionality away from the
R-carbon and prevents facile metal-carbon bond homolysis.
This unique feature of the system prompted us to examine the
corresponding insertion and polymerization of methacrylate. Our
hope was that the bulkier methacrylate monomer may undergo
1,2-insertion, thereby opening the possibility of copolymerizing
methacrylates by this system.
The reaction of complex 1 with 2 equiv of methyl methacry-
late (MMA) displaces ether and results in formation of 1,2-
inserted product [(N∧N)Pd(CH2-C(Me)2-C(O)OMe][B(Arf)4],
2 (Scheme 1). The 1H NMR spectrum (Figure 1) confirmed the
formation of complex 2. The methyl protons on the C-2 carbon
of inserted MMA appear as a singlet at 1.1 ppm, whereas the
Pd-CH2 protons appear at 1.32 ppm. The -C(O)OCH3 protons
In principle, it should be possible to override the electronic
preference for 2,1-insertion by increasing the steric crowding
around the metal through the use of bulkier monomers and/or
ligands. The ensuing alkyl will no longer have an ester
functionality on the R-carbon, making it less prone to undergo
metal-carbon bond homolysis due to lower stability of the
* Corresponding author. E-mail: asen@psu.edu.
(1) (a) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283-
316. (b) Boffa, L. S.; Novak, B. M. Chem. ReV. 2000, 100, 1479-1493.
(c) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100, 1169-
1203. (d) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170.
(2) (a) Philipp, D. M.; Muller, R. P.; Goddard, W. A.; McAdon, M.;
Mullin, M. J. Am. Chem. Soc. 2002, 124, 10198-10210. (b) Michalak, A.;
Ziegler, T. Top. Organomet. Chem. 2005, 12, 145-186. (c) Michalak, A.;
Ziegler, T. J. Am. Chem. Soc. 2001, 123, 12266-12278.
(3) (a) Sen, A.; Borkar, S. J. Organomet. Chem. 2007, 692, 3291-3299.
(b) Nagel, M.; Sen, A. Organometallics 2006, 25, 4722-4724. (c) Elia,
C.; Elyashiv-Barad, S.; Sen, A.; Lopez-Fernandez, R.; Albeniz, A. C.;
Espinet, P. Organometallics 2002, 21, 4249-4256. (d) Albe´niz, A. C.;
Espinet, P.; Lo´pez-Ferna´ndez, R. Organometallics 2003, 22, 4206-4212.
(e) Tian, G.; Boone, H. W.; Novak, B. M. Macromolecules 2001, 34, 7656-
7663. (f) Yang, P.; Chan, B. C. K.; Baird, M. C. Organometallics 2004,
23, 2752-2761.
(4) (a) Gibson, V. C.; Tomov, A. Chem. Commun. 2001, 1964-1965.
(b) Goodall, B. L.; Benedikt, G. M.; McIntosh, L. H.; Barnes, D. A.; Rhodes,
L. F. (BF Goodrich) U.S. Pat. 5,569,730, 1996.
(5) (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414-6415. (b) Mecking, S.; Johnson, L. K.; Wang, L.;
Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888-899.
10.1021/om7006024 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 08/16/2007

4712 Organometallics, Vol. 26, No. 19, 2007
Communications
Figure 1. 1H NMR (CD2Cl2) spectra of 1 (I) and MMA-inserted complex 2 (II).
Figure 3. Eyring plot for MMA insertion into the Pd-CH3 bond
of 1.
Figure 2. ORTEP diagram of the MMA-inserted complex 2. The
hydrogen and BArF4 anion are omitted for clarity.
-12.7 eu; the corresponding values for methyl acrylate (MA)
insertion are 12.1 kcal/mol and -14.1 eu as reported by
Brookhart and co-workers.5b
show a sharp singlet at 3.5 ppm and are slightly shifted upfield
from the corresponding monomer. The structural assignment
was further confirmed from 13C NMR and DEPT-135 NMR
spectra. The FT-IR analysis showed the carbonyl absorbance
at 1599 cm-1 for complex 2. Finally, the 1,2-insertion of MMA
in the Pd-CH3 bond was established by single-crystal X-ray
analysis. The X-ray quality single crystals were grown from
methylene chloride/pentane. The crystal structure of complex
2 is shown in Figure 2. The structure clearly shows 1,2-insertion
of MMA and the formation of a five-membered cyclic chelate
with Pd to O1 and C6 distances of 2.04(4) and 2.014(5) Å,
respectively. The methyl group from the starting complex
migrates to the C2 carbon of MMA, and the observed bond
angles for Pd1-C6-C3 and C4-C3-C5 carbons are 107.3-
(4)° and 109.5(5)°, respectively.
Unlike the copolymerization of MA with ethene reported by
Brookhart, attempts at the copolymerization of MMA with
ethene resulted in the formation of essentially branched poly-
ethene (PE). We ascribe the failure to incorporate MMA into
the polymer chain to the significantly higher enthalpy of
activation for MMA insertion compared to MA insertion.
Clearly, the same steric effect that forces the 1,2-insertion of
the bulkier MMA also severely attenuates its insertion rate.
The ability of the MMA-inserted complex, 2, to initiate alkene
homopolymerization and alkene/carbon monoxide copolymer-
ization was examined. Carbon monoxide (CO) was found to
insert into the Pd-CH2 bond of 2 as indicated by the shift in
1
CH2 protons from 1.32 to 2.42 ppm in the H NMR spectrum
The kinetics of MMA insertion was studied under pseudo-
first-order conditions and suggested that the insertion of MMA
is first order. Figure 3 shows the Eyring plot for MMA insertion,
and the obtained data gave ∆H# ) 16.4 kcal/mol and ∆S# )
(Scheme 2). However, the insertion step is reversible, as shown
by the re-formation of the starting complex 2 upon removal of
CO from the solution. As the CO concentration in the solution
1
is decreased, the H NMR spectrum shows the formation of

Communications
Organometallics, Vol. 26, No. 19, 2007 4713
1H NMR spectroscopy (CD2Cl2); the first block had resonances
at 0.9 and 1.0-1.6 ppm due to poly(1-hexene). The H NMR
Scheme 2. Schematic Presentation of CO Insertion in
Complex 2
1
spectrum of the final copolymer shows formation of additional
resonance at 2.4 ppm due to hexene/CO block. Because of the
relatively high molecular weight, it is difficult to observe the
CH and CH2 groups connecting the two blocks by NMR
spectroscopy (however, see below). To further test the formation
of a diblock copolymer versus a mixture of two single block
polymers, the material was washed in hexane (which dissolves
1
polyhexene). The H NMR spectrum (CD2Cl2) of the hexane-
insoluble residue showed slightly higher hexene/CO content (27
mol % versus 15 mol % before hexane wash), suggesting the
presence of some polyhexene in the original material. As
discussed above, this is consistent with the observation of some
termination of the initially synthesized polyhexene chains with
time. Similarly synthesized were diblock copolymers with
branched polyethene (PE) and poly(alt-ethene/CO) (PK) blocks.
In one example, a PE block was first synthesized by performing
the polymerization for short time (reaction conditions: 2, 5 mg;
ethene, 300 psi; CH2Cl2, 10 mL, RT, 1 h), and then the PK
block was synthesized by charging the same reactor with a
mixture of ethene and CO (ethene, 300 psi, CO, 50 psi, 0.5 h).
The obtained polymer was dissolved in a hexafluoroisopropyl
starting MMA-inserted complex. The addition of both CO and
an alkene to 2 results in the formation of the alternating
copolymer.
The homopolymerization of both ethene and 1-hexene initi-
ated by 2 was found to have some “living” characteristics.
Fractions withdrawn at different time intervals for hexene
homopolymerization (reaction conditions: 2, 10 mg; 1-hexene,
0.5 g; CH2Cl2, 5 mL, RT) showed an increase in Mn with time
(2 h, Mn ) 23 900; 8.5 h, Mn ) 32 100; 22 h, Mn ) 47 400).
That the Mn value does not continue to increase linearly with
time suggests that a fraction of the growing polymer chains
terminates at longer reaction times. Taking advantage of the
quasi-living nature of the polymerization, we successfully
synthesized polymers with poly(alkene) and poly(alt-alkene/
CO) blocks by sequential addition of monomers. For example,
a poly(1-hexene) block was synthesized by the reaction of 2
with 1-hexene (reaction conditions: 2, 5 mg; 1-hexene, 0.35 g;
CH2Cl2, 5 mL) at ambient temperature for 2 h (at this point, a
significant amount of 1-hexene remains). A small fraction of
the reaction mixture was syringed out, the reactor was charged
with 20 psi of CO, and the polymerization was continued for a
further 6 h. The polyhexene fraction withdrawn after the first
step had Mn ) 65 600 and Mw/Mn ) 2.48. The final diblock
copolymer had Mn ) 90 100 and Mw/Mn ) 1.99. The length of
the second block was kept short to avoid the solubility issues.
The formation of block copolymer was further confirmed by
1
alcohol and trace chloroform-d mixture and analyzed by H
NMR spectroscopy. The NMR spectrum shows resonances at
1.3 and 2.8 ppm due to PE and PK blocks, respectively (82
mol % PE, 18 mol % PK; Mn, 4000; Mw/Mn, 1.57). Similar to
1
the poly(hexene-b-hexene/CO) copolymer, the H NMR spec-
trum recorded after washing the PE-b-PK copolymer with
hexane (which dissolves the branched PE) shows higher PK
content (72 mol % PE and 28 mol % PK). The difference can
be attributed to some chain termination in the initially synthe-
1
sized PE. However, the presence of H NMR resonances for
both PE and PK chains in the hexane-insoluble fraction confirms
that this material is a real copolymer and not a mixture of two
homopolymers (PE and PK). In an experiment performed in a
Figure 4. 1H NMR (CD2Cl2) spectra of (I) polyethene (PE) and (II) polyethene-b-polyketone (PE-b-PK) copolymer.

4714 Organometallics, Vol. 26, No. 19, 2007
Communications
high-pressure NMR tube, a PE block was first formed (reaction
conditions: 2, 5 mg; ethene, 50 psi; CD2Cl2, 0.5 mL, RT, 1 h).
After the addition of CO (50 psi) to the preformed PE block a
new resonance attributable to the formation of a PK block was
observed at 2.6 ppm, and the intensity of this resonance
increased over time. In addition, the resonances at 1.6 and 2.3
ppm observed are the CH2 protons at the interface between PE
and PK blocks, respectively (Figure 4). The correlation between
the two sets of protons on the connecting carbons at the interface
was established by HCOSY analysis. The assignment of the
connecting CH2 groups was also confirmed by comparing with
the 1H NMR shifts of 3-hexanone. The protons labeled a and b
in Et-C(O)-CbH2-CaH2-Me resonate at 1.6 and 2.4 ppm,
respectively. The methoxy protons from the end group derived
from the initially inserted MMA were observed at 3.7 ppm.
Continuing the reaction for longer times results in polymer
precipitation.
The DSC of the PE-b-PK copolymer synthesized shows two
melt transitions at 105 and 220 °C, and these remain unaltered
after hexane wash. The first corresponds to that reported by
Brookhart for the branched PE formed by the diimine-Pd
system.5b The second is ascribable to the PK block and is
approximately 40 °C lower than that for pure PK.6 Pure PK is
very highly crystalline, and the lowered Tm in PE-b-PK is
presumably due to the disruption of the crystallinity because of
the presence of the branched PE block.
In conclusion, the reaction of a cationic Pd(II) diimine
complex with methyl methacrylate results in 1,2-insertion
regiochemistry that is opposite of that observed with methyl
acrylate. NMR, FT-IR, and single-crystal X-ray analysis con-
firms the 1,2-insertion of MMA and formation of a five-
membered cyclic chelate. The enthalpy of activation for MMA
insertion is 4 kcal/mol higher than that for the corresponding
MA insertion. The MMA-inserted complex, 2, can be employed
for ethene or 1-hexene homopolymerization and their alternating
copolymerization with carbon monoxide. Further, the quasi-
living nature of the polymerizations allows the synthesis of poly-
(hexene-b-hexene/CO) and poly(ethene-b-ethene/CO) copoly-
mers. The polymerization activity of 2 is comparable to that of
the starting Brookhart complex, 1. However, the stability of 2
is higher than 1 under ambient conditions and is, therefore, a
more attractive starting point.
Acknowledgment. This research was supported by a grant
from the U.S. Department of Energy, Office of Basic Energy
Sciences.
Supporting Information Available: Experimental procedures
and crystal structure data are available free of charge via the Internet
at http://pubs.acs.org.
(6) Catalytic Synthesis of Alkene-Carbon Monoxide Copolymers and
Cooligomers; Sen, A., Ed.; Kluwer Academic: Dordrecht, 2003.
OM7006024