Porphyrins fused to N-heterocyclic carbenes (NHCs): Modulation of the electronic and catalytic properties of NHCs by the central metal of the porphyrin

Article abstract of DOI:10.1002/chem.201301483

We report herein a detailed study of the use of porphyrins fused to imidazolium salts as precursors of N-heterocyclic carbene ligands 1 M. Rhodium(I) complexes 6 M-9 M were prepared by using 1 M ligands with different metal cations in the inner core of the porphyrin (M=Ni^II, Zn ^II, Mn^III, Al^III, 2H). The electronic properties of the corresponding N-heterocyclic carbene ligands were investigated by monitoring the spectroscopic changes occurring in the cod and CO ancillary ligands of [(1 M)Rh(cod)Cl] and [(1 M)Rh(CO)₂Cl] complexes (cod=1,5-cyclooctadiene). Porphyrin-NHC ligands 1 M with a trivalent metal cation such as Mn^III and Al^III are overall poorer electron donors than porphyrin-NHC ligands with no metal cation or incorporating a divalent metal cation such as Ni^II and Zn^II. Imidazolium salts 3 M (M=Ni, Zn, Mn, 2H) have also been used as NHC precursors to catalyze the ring-opening polymerization of L-lactide. The results clearly show that the inner metal of the porphyrin has an important effect on the reactivity of the outer carbene. Porphyrin-NHC hybrids: The electron-donating properties of N-heterocyclic carbenes (NHCs) fused to porphyrins can be finely tuned by varying the nature of the inner metal in the macrocycle (see figure). It also dramatically modulates their catalytic properties, as illustrated for the ring-opening polymerization of L-lactide. Copyright

Full text of DOI:10.1002/chem.201301483

DOI: 10.1002/chem.201301483
Porphyrins Fused to N-Heterocyclic Carbenes (NHCs): Modulation of the
Electronic and Catalytic Properties of NHCs by the Central Metal of the
Porphyrin
Jean-FranÅois Lefebvre,[a] Mamadou Lo,[a] Jean-Paul Gisselbrecht,[b]
Olivier Coulembier,[c] Sꢀbastien Clꢀment,[a] and Sꢀbastien Richeter*[a]
Abstract: We report herein a detailed
study of the use of porphyrins fused to
imidazolium salts as precursors of N-
heterocyclic carbene ligands 1M. Rho-
dium(I) complexes 6M–9M were pre-
pared by using 1M ligands with differ-
ent metal cations in the inner core of
the porphyrin (M=NiII, ZnII, MnIII,
AlIII, 2H). The electronic properties of
the corresponding N-heterocyclic car-
bene ligands were investigated by mon-
itoring the spectroscopic changes oc-
curring in the cod and CO ancillary
ligands of [(1M)Rh(cod)Cl] and
[(1M)Rh(CO)2Cl] complexes (cod=
1,5-cyclooctadiene). Porphyrin–NHC li-
gands 1M with a trivalent metal cation
such as MnIII and AlIII are overall
poorer electron donors than porphy-
rin–NHC ligands with no metal cation
AHCTUNGTRENNUNG
or incorporating
a
divalent metal
cation such as NiII and ZnII. Imidazoli-
um salts 3M (M=Ni, Zn, Mn, 2H)
have also been used as NHC precur-
sors to catalyze the ring-opening poly-
merization of l-lactide. The results
clearly show that the inner metal of the
porphyrin has an important effect on
the reactivity of the outer carbene.
Keywords: carbenes · organocataly-
sis · porphyrinoids · ring-opening
polymerization · UV/Vis spectros-
copy
Introduction
electronic properties of a given NHC is not trivial, and often
requires the synthesis of new NHC precursors from scratch.
This explains why switchable NHCs, which allow the reversi-
ble and controlled switching of their electronic properties,
have attracted so much attention: Redox-active,[4] pH-
active,[5] and photoswitchable[6] NHCs are representative ex-
amples of such switchable NHCs. The coordination of
a metal complex to a distant coordination site attached to
the NHC can also dramatically modulate the electronic and
steric properties of the NHC moiety. For example, Ganter
and co-workers showed that a cationic pentamethylcyclo-
pentadienyl–ruthenium complex ([RuCp*]+) can be anch-
ored to a benzimidazol-2-ylidene ligand to significantly de-
crease the overall electron-donating ability of the NHC
ligand compared with the parent benzimidazol-2-ylidene
ligand.[7] Although this approach cannot be described as
a switchable process, it represents an interesting procedure
for modulating the electronic properties of NHC ligands.
We have recently developed the chemistry of NHCs fused
to porphyrins (1M) with the aim of modulating the electron-
ic and catalytic properties of the peripheral carbene unit
(Figure 1).[5b] For example, we have shown that NHC ligands
annulated to free-base porphyrins can be reversibly switched
between electron-poor and -rich states upon protonation
and deprotonation of the inner nitrogen atoms of the por-
phyrin. The porphyrin-NHC ligands thus appear to be inter-
esting candidates for catalytic applications as they should
allow the activity of the catalyst to be tuned through simple
chemical transformations of the porphyrin, for example, by
metalation or protonation of the inner core. Indeed, other
Over the last 20 years, N-heterocyclic carbenes (NHCs)
have become ubiquitous ligands in organometallic and coor-
dination chemistry.[1] NHCs are able to catalyze a wide
range of chemical transformations, either as ligands of tran-
sition-metal catalysts or directly as organocatalysts.[2] Under-
standing and tuning the electronic and steric properties of
NHCs allows the design, selection, and use of tailor-made
NHCs for a given catalytic application. Thus, there is a grow-
ing interest in developing new NHCs, the electronic proper-
ties of which can be finely varied.[3] The modulation of the
[a] Dr. J.-F. Lefebvre, Dr. M. Lo, Dr. S. Clꢀment, Dr. S. Richeter
Institut Charles Gerhardt de Montpellier, UMR 5253
Universitꢀ Montpellier 2, CC 1701
Place E. Bataillon, 34095 Montpellier Cedex 05 (France)
Fax : (+33)4-6714-3852
E-mail: sricheter@univ-montp2.fr
[b] Dr. J.-P. Gisselbrecht
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide
Institut de Chimie, UMR 7177 CNRS Universitꢀ de Strasbourg
4 rue Blaise Pascal, 67000 Strasbourg (France)
[c] Dr. O. Coulembier
Laboratory of Polymeric and Composite Materials (LPCM)
Center of Innovation and Research in Materials and Polymers
ACHTUNGTRENNUNG(CIRMAP), University of Mons (UMONS)
Place du Parc 20, 7000 Mons (Belgium)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301483 and contains
1H NMR and UV/Vis spectra of the compounds described in the
manuscript.
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FULL PAPER
Results and Discussion
Synthesis of imidazolium salts fused to porphyrins: Imidazo-
lium salts fused to the p system of a porphyrin macrocycle
(3M) are the key precursors of the hybrid porphyrin-NHC
ligands 1M (Scheme 1).[12] To exclude steric effects in the
following study, only imidazolium salts with N-methyl
Figure 1. Structures of porphyrin-NHC ligand 1M, a porphyrin fused to
the imidazole ring 2M, and a porphyrin fused to imidazolium salts 3M
(Ar=4-tBuPh).
research groups have synthesized porphyrins incorporating
peripheral metal complexes and tuned their catalytic activity
by changing the inner metal of the porphyrin. For example,
the groups of Osuka-Shinokubo,[8] Klein Gebbink,[9] Ima-
hori,[10] and Liu[11] have independently shown that peripher-
ally anchored palladium complexes exhibit different catalyt-
ic activities in Heck reactions depending on the nature of
the metal in the porphyrin (Figure 2). Herein we show that
NHCs fused to the p system of porphyrins (1M) are able to
catalyze the ring-opening polymerization (ROP) of l-lactide
(l-LA) and that the inner metal of the porphyrin plays a cru-
cial role in the catalytic process.
Scheme 1. Preparation of imidazolium salts 3M.
groups were synthesized. The porphyrins fused to an imida-
zole ring, 2Ni and 2NiMe, were obtained by the stepwise
functionalization of a NiII meso-tetraarylporphyrin complex
(meso-aryl=4-tBuPh).[13] Free-base imidazoles 2H2 and
2H2Me were obtained by treating the corresponding NiII
complexes 2Ni and 2NiMe with a mixture of H2SO4 and tri-
fluoroacetic acid (TFA). Interestingly, 2H2 and 2H2Me ap-
peared to be air-sensitive both in solution and in the solid
state, and were slowly oxidized to free-base secochlorins
4H2 and 4H2Me (Scheme 2).[14] Because singlet oxygen
(1O2) generated by free-base macrocycles under ambient
light is the dominant oxidant, 2H2 and 2H2Me should be
manipulated under argon in the absence of light to prevent
this ring-opening reaction. Metalation of the corresponding
free-base 2H2 with ZnACTHNUTRGNEUNG
(OAc)2·2H2O afforded ZnII imidazole
Figure 2. Examples of peripherally metalated porphyrin complexes for
catalytic applications.[8–11]
Scheme 2. Formation of free-base secochlorins 4H2 and 4H2Me.
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S. Richeter et al.
2Zn in 86% yield (Scheme 1). Alkylation of imidazoles
2Ni, 2H2, and 2Zn with iodomethane afforded the corre-
sponding imidazolium salts 3Ni, 3H2,and 3Zn in yields of
were obtained in yields of 71–94% from imidazolium salts
5M (M=Ni, Zn, H2) as NHC precursors (Scheme 4). For
these complexes, EDX spectroscopy confirmed that chloride
was coordinated to the rhodium center (Rh/Clꢀ1:1; no I
1
80–83% (Scheme 1). The H NMR spectra of 3M (M=Ni,
2H and Zn) recorded in CDCl3 revealed downfield signals
at d=11.01, 10.77, and 10.17 ppm, respectively, due to hy-
drogen bonds between the electron-deficient imidazolium
rings and iodide.[13a] The corresponding imidazolium salts
5M (M=Ni, Zn, H2) with tetrafluoroborate as counter
anion were then obtained in yields of 90–94% by treating
imidazolium salts 3M with AgBF4 in acetone (Scheme 3).
The imidazolium protons of 5Ni, 5H2, and 5Zn were ob-
detected). The formation of the peripheral [RhXACTHNGUTRENNU(G cod)] com-
plexes (X=I or Cl) in all the complexes 6M and 7M was
confirmed by 1) the absence of a signal in the 1H NMR
spectrum arising from the imidazolium proton due to the
À
formation of the NHC Rh bond, 2) the four characteristic
signals of the coordinated cod ligands in the H NMR spec-
1
tra of the 7M complexes with M=Ni, Zn, and H2 (see Fig-
ure S2 in the Supporting Information), and 3) the molecular
mass peaks observed by MALDI-TOF mass spectrometry
1
served by H NMR spectroscopy in CDCl3 at higher fields
1
than the corresponding imidazolium salts 3M, at d=9.27,
9.26, and 9.04 ppm (see Figure S1 in the Supporting Infor-
mation).
for these complexes. H NMR spectroscopy can be a useful
technique for measuring electron densities in Rh–olefin
compounds.[16] Olefinic protons, particularly those trans to
the NHC ligand, are shifted upfield in the presence of elec-
tron-donating groups in the 4,5-positions of the NHC. Here,
olefinic protons trans to the NHC ligand were observed at
5.00, 5.00, and 4.96 ppm for 7Ni, 7Zn, and 7H2, respective-
ly, which indicates that the electron-donating abilities of all
the NHC ligands are very similar according to 1H NMR
spectroscopy.
A more common method for evaluating the electron-do-
nating properties of NHC ligands involves the synthesis of
cis-[(NHC)MCl(CO)2] (M=Rh, Ir) complexes and deter-
mining their CO stretching vibrations, n(CO), by IR spec-
troscopy: lower n(CO) frequencies are observed with stron-
ger electron-donating NHC ligands.[17,18] Complexes 8M and
9M (M=Ni, Zn, H2) were synthesized by bubbling carbon
monoxide through a solution of complexes 6M and 7M, re-
spectively, in CH2Cl2 to quantitatively substitute the cod
ligand by two CO ligands (Scheme 5). Two CO stretching
Scheme 3. Preparation of imidazolium salts 5M.
Electronic properties of porphyrin-NHC 1M ligands: Sever-
al distinctly different spectroscopic and analytical methods
can be used to quantify the electron-donating properties of
the NHC ligands. Notably, the RhI and IrI complexes
[(NHC)MCl
G
(cod=1,5-cyclooctadiene)
and
[(NHC)MCl(CO)2] (M=Rh, Ir) are routinely used to evalu-
ate the electron-donating ability of NHCs by monitoring the
1H NMR and IR spectroscopic changes that occur in the cod
and CO ancillary ligands, respectively.[15] RhI complexes
[(1M)RhI
yields of 63–92% by treating imidazolium salts 3M with
[{RhCl(cod)}2] (0.5 equiv) and tBuOK in THF (Scheme 4).
ACHTUNGTRENNUNG(cod)] (6M; M=Ni, Zn, H2) were obtained in
ACHTUNGTRENNUNG
Scheme 5. Preparation of RhI complexes 8M and 9M.
frequencies were observed for all complexes 8M and 9M by
IR spectroscopy and confirmed their cis geometry (Table 1).
Similar CO stretching vibrations were recorded in CH2Cl2
with nav(CO)=2037, 2037.5, and 2038 cmÀ1 for 8H2, 8Zn,
and 8Ni, respectively (all n(CO) frequencies are given in
Table 1). Higher CO stretching vibrations were recorded in
CH2Cl2 for 9M complexes with chloride coordinated to the
rhodium center, all with similar nav(CO) values of 2040.5,
2041, and 2041.5 cmÀ1 for 9Zn, 9H2, and 9Ni, respectively.
These IR data are consistent with previously reported
n(CO) data for [(NHC)RhX(CO)2] complexes with X=Cl
and I.[15,19] They are also indicative of small differences be-
Scheme 4. Preparation of RhI complexes 6M and 7M.
Energy-dispersive X-ray (EDX) spectroscopy showed that
iodide is coordinated to the rhodium center (Rh/Iꢀ1:1; no
Cl detected) in these complexes.[5b] Because the data report-
ed on the electronic properties of NHC–Rh complexes were
obtained with chloride coordinated to the rhodium center,
the RhI complexes [(1M)RhCl
ACTHNUTRGNE(UNG cod)] (7M; M=Ni, Zn, H2)
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Porphyrins Fused to NHCs
FULL PAPER
Table 1. Electrochemical and spectroscopic data for the porphyrin-NHC
complexes.
M
Eox1/Eox2
d
G
d
R
n
G
nACHTUNGTRENNUNG
for 3M[a]
A
A
A
]
A
]
[V vs. Fc+/Fc]
Zn
Ni
H2
Mn
AlMe
AlCl
H42+[d]
0.36/0.66
0.64/0.64
0.57/0.75
5.22
5.23
5.16
5.00
5.00
4.96
2073, 2002
2074, 2002
2073, 2001
2080, 2001
2081, 2002
2080, 2002
2084, 2004
2083, 2003
2084, 2005
2089, 2010
2080, 2010
[a] Measured by cyclic voltammetry in CH2Cl2 containing 0.1m Bu4NPF6
(see ref. [13a]). [b] Signal of CH=CH trans to the NHC observed in the
1H NMR spectrum (300 MHz) recorded in CDCl3 at 258C. [c] Measured
by FTIR spectroscopy in CH2Cl2, average values nav(CO) are given in the
text. [d] See ref. [5b]).
Scheme 6. Preparation of RhI complexes 9AlCl and 9Mn.
tween the electron-donating abilities of NHC ligands 1M
with M=H2 and divalent metal cations such as NiII or ZnII.
Porphyrin-NHC ligands 1M with M=Ni, Zn, and H2 can be
seen as intermediate electron-donors in the range of imida-
zol-2-ylidene and benzimidazol-2-ylidene ligands.[5b,15,19]
To detect subtle differences between NHC ligands with
similar electron-donating properties, the IrI/II or RhI/II redox
potentials may serve as a more sensitive measurement.[18]
This approach was not straightforward in our case because
porphyrins are redox-active molecules with oxidation poten-
tials in the range of those observed for RhI/II. To overcome
this problem, the electron-donating abilities of the porphy-
rin p system of the corresponding NHC precursors 3M were
evaluated by cyclic voltammetry measurements of their elec-
trochemical oxidation processes. The first oxidation poten-
tials of the porphyrins (EOX1 vs. Fc+/Fc) are shifted anodi-
cally in the order 3Zn<3H2<3Ni (Table 1). Based on
these results, the electron-donating ability of the porphyrin
p system fused to the imidazolium unit depends, as expect-
ed, on the nature of the metal in the porphyrin and increas-
es in the order Ni<2H<Zn.[10] This order is in agreement
with the small differences observed by IR spectroscopy for
9M, but electrochemical measurements of the NHC precur-
sors 3M may be seen as a more informative technique in
our case compared with IR spectroscopy.
À
stitute CH3 by chloride (Scheme 6). The stretching vibra-
tions of the resulting 9AlCl were observed at nav(CO)=
2044.5 cmÀ1 (Table 1). These stretching frequencies are in
good agreement with previously reported data for
[(NHC)RhCl(CO)2] complexes.[15] Here, the slightly higher
n(CO) frequencies observed for 9AlCl (Dn(CO)=4 cmÀ1
between the ZnII and AlIII complexes, Table 1) show that the
corresponding 1AlCl ligand is a weaker NHC electron-
donor than 1M ligands with M=Ni, Zn, or H2. These data
show that the electronic properties of the outer NHC can be
finely modulated by 1) the nature of the inner metal of the
porphyrin and its valence, and 2) the nature of the axial li-
gands, which are more or less able to compensate the posi-
tive charge of the inner metal. It is also interesting to note
that the stretching frequencies of 9AlCl (nav(CO)=
2044.5 cmÀ1) are intermediate between those of 9Zn
(nav(CO)=2040.5 cmÀ1), which incorporates the most elec-
tron-donating zinc porphyrin, and those of 9H42+
(nav(CO)=2049.5 cmÀ1), which incorporates the least elec-
tron-donating protonated porphyrin (Table 1).[5b]
Another trivalent metal cation such as MnIII was also in-
cluded in the inner core of the porphyrin to confirm that the
donating ability of the peripheral NHC is weaker in the
presence of a trivalent metal cation in the porphyrin. The
synthesis of MnIII complexes performed with 2H2 as the
starting material proved to be tedious. However, imidazole
2Mn was obtained in a yield of 91% by metalation of
2H2Me with MnCl2·4H2O in air to oxidize the initially
formed MnII porphyrin (Scheme 1). The MALDI-TOF mass
spectrum of 2Mn is in agreement with the metalation of the
inner core of the porphyrin by MnIII and showed the molec-
ular ion peak at m/z=945.4 [MÀCl]+. The chloride anion
was evidenced by EDX spectroscopy (Mn/Clꢀ1:1). Magnet-
ic measurements showed that the cT value at 300 K is equal
to 3.04 emuKmolÀ1, which corresponds to the value calcu-
lated for a magnetically isolated high-spin MnIII ion with S=
2 and g=2.0. Imidazolium salt 3Mn was obtained by alky-
lating the corresponding imidazole 2Mn with excess iodo-
methane (Scheme 1). The molecular mass ion peak expected
Because similar n(CO) frequencies were measured for
RhI complexes 9M with divalent metal cations NiII and ZnII,
trivalent metal cations such as AlIII and MnIII were also in-
cluded in the inner core of the porphyrin to study the influ-
ence of the valence of the metal on the electronic properties
of the peripheral NHC ligand. AlIII complexes 9AlX (X=
Me, Cl) were prepared by using 9H2 as the starting materi-
al. We first synthesized 9H2 by bubbling CO through a solu-
tion of 7H2 in CH2Cl2 (Scheme 5). Then AlMe3 (2m solu-
tion in n-heptane) was added to the freshly prepared solu-
tion of 9H2 (Scheme 6). At this stage, the fifth coordination
À
site of the AlIII is coordinated by CH3 in 9AlMe and CO
stretching frequencies were observed at nav(CO)=2043 cmÀ1
(Table 1).[20] Then a vial of a solution of 9AlMe in CH2Cl2
was placed under an atmosphere of HCl for 30 min to sub-
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S. Richeter et al.
for the imidazolium cation 3Mn was observed by ESI-TOF
MS (m/z=480.2 [MÀ2I]2+). Iodide was also detected by
ESI-TOF MS (m/z=127.0 [I]À). EDX spectroscopy showed
a Mn/I ratio of around 1:2, and therefore we concluded that
chloride was exchanged by iodide as the axial ligand. We
presume that chloride reacts with CH3I to form iodide as
the axial ligand and CH3Cl gas. The results of a UV/Vis
spectroscopic analysis are also in agreement with the metal-
ation of the inner core of the porphyrins 2Mn and 3Mn by
MnIII (see Figures S3 and S4 in the Supporting Informa-
tion).[21] Imidazolium salt 3Mn was then treated with a satu-
rated aqueous solution of NaCl. The crude dry residue was
Scheme 7. Hydrolytic cleavage of 1Ni.
and Figure S5 in the Supporting Information). The polymer-
ization reactions were therefore conducted in a glovebox at
ambient temperature by using an alcohol initiator, such as
benzyl alcohol (BnOH), with porphyrin-NHCs 1M as cata-
subsequently treated with [{RhClACTHNUTRGNENUG(cod)}2] (0.5 equiv) and
tBuOK in THF (Scheme 6) to give the RhI complex 7Mn in
a yield of 58%. The classical absorption bands of MnIII–por-
phyrin complexes were observed by UV/Vis spectroscopy in
ethanol, with one intense Soret band at l=474 nm and two
Q bands at l=572 and 609 nm (Figure 3).[21] EDX spectros-
lysts generated in situ ([3M] /
LA were evaluated in THF for a [l-LA] /
[tBuOK]0 =1). The ROPs of l-
0 A[BnOH]0/A[1M]0
G
CHTUNGTRENNUNG
ratio of 100:1:1 ([l-LA]0 ꢀ2m) and followed by size exclu-
sion chromatography (SEC; Table 2). The number-averaged
Table 2. Properties of poly(l-lactide)s obtained from Benz and 1M car-
benes.[a]
[c]
[c]
Cat.[b]
t
Conv.
[%]
Mn
[gmolÀ1
]
ꢀM
[h]
U
Benz[d]
1Zn
0.33
99
14500
2.40
0.33
0.58
83
93
15500
17000
1.17
1.12
1H2
1Ni
0.83
86
16000
1.20
0.83
3.33
23
93
4100
17000
1.20
1.60
1Mn
0.33
1.00
3.17
6.00
6
10
18
27
1000
1900
3350
4200
1.03
1.04
1.14
1.16
Figure 3. UV/Vis spectrum of RhI complex 7Mn in ethanol.
[a] The reactions were performed in THF with BnOH as initiator. l-
LA=2m, [l-LA]0/A[BnOH]0/A[carbene]0 =100:1:1. [b] The NHC was gener-
C
CHTUNGTRENNUNG
ated in situ by deprotonating the imidazolium salts 3M with tBuOK.
[c] Determined by SEC in CHCl3 at 358C by using polystyrenes stand-
ards. [d] Benz=N,N-dimethylbenzimidazol-2-ylidene generated in situ by
deprotonating N,N-dimethylbenzimidazolium iodide.
copy showed a Mn/Rh/Cl ratio of approximately 1:1:2, and
iodide was detected in only trace amounts. Carbon monox-
ide was then bubbled through a solution of 7Mn to afford
9Mn with nav(CO)=2044 cmÀ1 (Table 1). These stretching
frequencies are in excellent agreement with those obtained
for 9AlCl, and also show that porphyrin-NHC ligands incor-
porating trivalent metal cations are weaker electron-donors
than those with divalent metal cations.
molecular weights (Mn) obtained for each catalyst vary line-
arly with conversion (Figure 4). The linearity of this plot
demonstrates the controlled nature of each polymerization
reaction and indicates that little chain transfer occurs. More-
over, differential scanning calorimetry (DSC) analyses con-
firmed that no racemization occurred in the course of the
ROP of l-LA.[23] In comparison with the polymerization re-
action catalyzed by N,N-dimethylbenzimidazol-2-ylidene
(Benz), 1M catalysts are less active. Nevertheless, the reac-
tions appeared to be more controlled in the studied poly-
merization because narrower ꢀM values were observed for
1M compared with Benz (ca. 2.4, Table 2). It has been
shown that benzimidazol-2-ylidene with small nitrogen sub-
stituents, such as Benz, can dimerize to form enetetra-
Ring-opening polymerization of l-lactide: The imidazolium
salts 3M were used as NHC precursors to catalyze the ROP
of l-lactide (l-LA). Recent studies have shown that the
NHC structure (i.e., its steric and electronic properties) can
dramatically modulate the polymerization rates, polydispers-
ity (ꢀM =Mw/Mn), and molecular weights of the obtained
polymers.[22] As expected, free carbenes 1M prepared by de-
protonating 3M with tBuOK are too unstable to be isolated.
For example, 1Ni is readily hydrolyzed in humid air to the
ring-opened amine formamide derivative 10Ni (Scheme 7
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Porphyrins Fused to NHCs
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Figure 5. Plot of ln([l-LA]0/ACHTNUTRGNE[NUG l-LA]t) versus time for [l-LA]0 =2m and
~
*
^
a targeted DP of 100: ) 1Zn, ) 1H2, ) 1Ni, and &) 1Mn.
Figure 4. Plot of Mn (estimated by SEC) versus l-LA conversion:
~
*
^
) 1Zn, ) 1H2, ) 1Ni, and &) 1Mn.
performed with meso-tetrakisACHTNUTRGNE(UNG 4-tBuPh)porphyrin derivatives
(i.e., 1M (M=Zn, H2, Ni) without the fused NHC ligand)
showed that they are unable to catalyze the ROP of l-LA
alone without the NHC in the reaction mixture. Moreover,
we also verified that the Lewis acidic core of the porphyrin
does not activate the l-LA: In the polymerization reaction,
upon combining the less active 1Mn catalyst with 1 equiv of
mines,[24] and Wanzlick showed that these enetetramines
have the reactivity of nucleophilic carbenes.[25] Such “Wan-
zlick” carbenes were used by Hedrick and co-workers as nu-
cleophilic catalysts for the ROP of lactide.[22b] Their results
showed that controlled polymerizations are difficult to ach-
ieve and that the obtained polymers had a broad PDI. Simi-
lar reactivity was observed in this work with Benz as cata-
lyst. In contrast, the much smaller values of PDI observed
for 1M suggest that porphyrin-NHC ligands are not prone
to dimerize due to the presence of the porphyrin backbone
and the two sterically demanding meso 4-tBuPh groups
close to the NHC moiety.
the free base or the Zn complex of meso-tetrakisACTHNUTRGENUG(N 4-tBuPh)-
porphyrin, the observed rate remained unchanged. This
shows that the Lewis acidic core of the porphyrin is not able
to co-catalyze the polymerization reaction.
Conclusion
Despite identical steric properties and small differences
between the electron-donating capacities of 1M, the results
of this work show striking differences in their reactivity, in-
creasing in the order 1Mn !1Ni<1H2<1Zn. Although
83% conversion was achieved within 20 min with 1Zn as
catalyst, only 6% conversion was observed with 1Mn under
identical conditions. This clearly shows the important effect
of the inner metal of the porphyrin on the outer carbene re-
activity (Table 2): the more electron-rich the NHCs are (i.e.,
the porphyrin p system is electron-rich), the faster the ROP
reactions. Polymers with Mn =16000–17000 gmolÀ1 and
ꢀM =1.1–1.2 were obtained with the most active catalysts
1Zn and 1H2. Interestingly, the polymerizations of l-LA in-
itiated by BnOH/1M (M=Zn, Ni, H2) mixtures are also
living processes, the semi-logarithmic dependence of ln([l-
We have described herein the electronic and catalytic prop-
erties of NHC ligands fused to porphyrins. In this study, the
porphyrin moiety was modified to tune the properties of the
NHC ligands. We have largely investigated how the inner
metal of the porphyrin is able to modulate the properties of
the outer NHC. We have shown that fine modulation of the
electronic properties of the outer NHC ligand can be ach-
ieved by varying the nature of the inner metal of the por-
phyrin. Porphyrin-NHC ligands 1M with divalent metal cat-
ions such as NiII and ZnII are intermediate electron donors
and display similar electronic properties to the NHC ligand
fused to the corresponding free-base porphyrin. Porphyrin-
NHC ligands 1M with trivalent metal cations, such as AlIII
and MnIII, are weaker NHC electron donors than those with
divalent metal cations. These data show that the valence,
and as a consequence the charge of the porphyrin backbone,
is responsible for the electronic properties of the outer
NHC ligand. To study if the inner metal of the porphyrin
plays a significant role in the catalytic properties of the
outer NHC ligands, we used these porphyrin-NHC ligands
to organocatalyze the ring-opening polymerization of l-lac-
tide. Surprisingly, huge differences in reactivity were ob-
served between the 1M catalysts despite the small differen-
ces in their electron-donating capacities. Such differences in
reactivity may thus only be partly related to the small varia-
LA] /
toacceleration was observed only when 1Ni was used, but
this did not influence the final properties of the poly(l-LA),
0 ACHTUNGTRENNUNG[l-LA]t) on time being linear (Figure 5). A marked au-
AHCTUNGTRENNUNG
with Mn =17000 gmolÀ1 and ꢀM =1.6 (Table 2). This might
be due to a change in the proportion between aggregated
and nonaggregated species,[26] the aggregation of porphyrin
derivatives being a commonly observed phenomenon.[27]
During the course of the ROP of l-LA, the electronic spec-
tra of the reaction mixtures did not undergo substantial
changes, which confirms that the porphyrin remained intact
and that demetalation did not occur. Control experiments
Chem. Eur. J. 2013, 19, 15652 – 15660
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15657

S. Richeter et al.
tion mixture was stirred in the dark for 1 h. After evaporation of solvent,
the imidazolium salt 5Ni was purified by column chromatography on
silica gel (eluent: from CH2Cl2 to CH2Cl2/EtOH 95:5). Evaporation of
the solvent afforded the imidazolium salt 5Ni as a purple solid in a yield
of 92% (74 mg). 1H NMR (200 MHz, CDCl3, 258C): d=9.27 (s, 1H; CH
iminium), 8.75 (s, 2H; pyrrole), 8.72 (d, J=5.0 Hz, 2H; pyrrole), 8.68 (d,
J=5.0 Hz, 2H; pyrrole), 8.06 (d, J=8.2 Hz, 4H; Ar meso), 7.95 (d, J=
8.2 Hz, 4H; Ar meso), 7.80 (d, J=8.2 Hz, 4H; Ar meso), 7.73 (d, J=
8.2 Hz, 4H; Ar meso), 3.16 (s, 6H; N-CH3), 1.56 (s, 18H; tBu), 1.54 ppm
(s, 18H; tBu); UV/Vis (CH2Cl2): lmax (e)=426 (222000), 537 (16000),
575 nm (sh 4000 molÀ1 dm3 cmÀ1); MS (ESI-TOF+): m/z calcd for
C63H65N6Ni+: 963.46 [M]+; found: 963.3; MS (ESI-TOFÀ): m/z calcd for
BF4À: 87.0 [M]À; found: 87.0.
tions in the electron-donating properties of 1M, and other
phenomena should also be taken into consideration. Indeed,
in our systems (and probably also in other reported systems
dealing with the use of a porphyrin backbone to tune the
catalytic activity of a peripheral catalytic site), the aggrega-
tion of the porphyrin catalyst (e.g., for 1Ni) or the coordina-
tion of additional innocent or non-innocent ligands on the
inner metal of the porphyrin, could also have a significant
effect on the properties of the peripheral catalytic site. Fur-
ther developments are now in progress to use these highly
tunable NHC ligands for other catalytic applications with or
without a metal anchored to the periphery of the porphyrin.
Imidazolium salt 5Zn: The imidazolium salt 5Zn was obtained in a yield
of 94% from 3Zn following the same procedure as described for 5Ni.
1H NMR (200 MHz, CDCl3, 258C): d=9.04 (s, 1H; CH iminium), 8.93 (s,
2H; pyrrole), 8.92 (d, J=4.8 Hz 2H; pyrrole), 8.80 (d, J=4.8 Hz 2H;
pyrrole), 8.27 (d, J=7.8 Hz, 4H; Ar meso), 8.12 (d, J=7.8 Hz, 4H; Ar
meso), 7.84 (d, J=7.8 Hz, 4H; Ar meso), 7.77 (d, J=7.8 Hz, 4H; Ar
meso), 3.20 (s, 6H; N-CH3), 1.62 (s, 18H; tBu), 1.59 ppm (s, 18H; tBu);
UV/Vis (CH2Cl2)=lmax (e): 428 (540000), 556 (25000), 600 nm
(8000 molÀ1 dm3 cmÀ1); MS (ESI-TOF+): m/z calcd for C63H65N6Zn+:
969.46 [M]+; found: 969.5; MS (ESI-TOFÀ): m/z calcd for BF4À: 87.0
[M]À; found: 87.0.
Experimental Section
General: Reactions were performed under argon using oven-dried glass-
ware. Dry THF was obtained by distillation over CaH2 and then Na/ben-
zophenone. Preparative separations were performed by silica gel flash
column chromatography (Baeckeroot-Labo 60M). Chemicals were ob-
tained from Alfa-Aesar, Sigma–Aldrich, and Acros and used as received.
Reactions were monitored by thin-layer chromatography using Merckꢁ
TLC silica gel 60 F254 plates. NMR spectra were recorded in CDCl3 on
either a Bruker 300 MHz or 200 MHz Fourier-transform spectrometer.
Chemical shifts are reported in ppm referenced to the CHCl3 solvent re-
Imidazolium salt 5H2: The imidazolium salt 5H2 was obtained in a yield
of 90% from 3H2 following the same procedure as described for 5Ni.
1H NMR (200 MHz, CDCl3, 258C): d=9.26 (s, 1H; CH iminium), 8.95 (s,
4H; pyrrole), 8.75 (s, 2H; pyrrole), 8.39 (d, 3J
ACTHNUTRGNEUNG(H,H)=7.8 Hz, 4H; Ar
1
1
meso), 8.20 (d, J=7.8 Hz, 4H; Ar meso), 7.93 (d, J=7.8 Hz, 4H; Ar
meso), 7.82 (d, J=7.8 Hz, 4H; Ar meso), 3.16 (s, 6H; N-CH3), 1.62 (s,
18H; tBu), 1.61 (s, 18H; tBu), À2.97 ppm (s, 2H; NH); UV/Vis
sidual peak at 7.26 ppm for H. Abbreviations for H NMR spectra are as
follows: s, singlet; d, doublet; t, triplet; q, quadruplet; m, multiplet. UV/
Vis spectra were recorded on a Perkin–Elmer Lambda 35 spectropho-
tometer in quartz cells. IR spectra were recorded in CH2Cl2 solutions in
a CaF2 cell with a Nicolet Avatar 320 FT-IR spectrometer. ESI mass
(CH2Cl2): lmax (e)=428 (353000), 528 (13000), 567 (8000), 597 (5000),
+
657 nm (7000 molÀ1 dm3 cmÀ1); MS (ESI-TOF+): m/z calcd for C63H67N6
:
spectra were recorded on
a
Q-Tof Waters 2001 MS spectrometer.
907.54 [M]+; found: 907.5; MS (ESI-TOFÀ): m/z calcd for BF4À: 87.0
MALDI-TOF mass spectra were recorded on a Bruker Ultraflex III MS
spectrometer with anthracene-1,8,9-triol as matrix. The electrochemical
studies and the syntheses of the imidazole derivatives 2M and the imida-
zolium salts 3M are described in ref. [13a].
[M]À; found: 87.0.
RhI complex 7Ni: A 25 mL flask was charged with imidazolium salt 5Ni
ACTHNUTRGNEUNG
(35 mg, 3.33ꢂ10À5 mol), [{RhCl(cod)}2] (10 mg, 2.02ꢂ10À5 mol), and
tBuOK (4,1 mg, 3.65ꢂ10À5 mol) in THF (10 mL). The reaction mixture
was stirred under an atmosphere of argon for 2 h at ambient tempera-
ture. Completion of the reaction was verified by TLC analysis. THF was
removed under reduced pressure leaving a crude product that was puri-
fied by silica gel column chromatography with dichloromethane as
eluent. Evaporation of the solvent afforded rhodium(I) complex 7Ni in
a yield of 94% (37.7 mg) as a purple solid. 1H NMR (300 MHz, CDCl3,
258C): d=8.69 (s, 2H; pyrrole), 8.65 (s, 4H; pyrrole), 8.31–8.19 (brd,
2H; Ar meso), 7.97 (d, J=7.8 Hz, 4H; Ar meso), 7.90–7.77 (brd, 4H; Ar
meso), 7.70 (d, J=7.8 Hz, 6H; Ar meso), 5.00 (brs, 2H; CH cod), 3.44 (s,
6H; N-CH3), 3.35 (brs, 2H; CH cod), 2.46–2.28 (brm, 4H; CH2 cod),
2.07–1.87 (brm, 4H; CH2 cod), 1.58 (m, 18H; tBu), 1.55 ppm (m, 18H;
tBu); UV/Vis (CH2Cl2): lmax (e)=426 (296000), 538 (22000), 572 nm (sh
Syntheses of the imidazole and imidazolium salts
Imidazole 2Mn: Nickel(II) N-methylimidazole 2NiMe (380 mg, 4.00ꢂ
10À4 mol) was dissolved in a TFA/H2SO4 mixture (20 mL/5 mL) and the
solution was stirred at room temperature for 30 min. It was then poured
onto ice, diluted with chloroform (200 mL), neutralized with saturated
K2CO3, washed with water, dried (Na2SO4), and evaporated. Crystalliza-
tion from CH2Cl2/MeOH afforded the free-base imidazole 2H2Me in
a yield of 98% (350 mg). A solution of 2H2Me (350 mg, 3.92ꢂ10À4 mol)
in
a
CHCl3/MeOH mixture (80 mL/20 mL) was prepared and
MnCl2 4H2O (800 mg, 4.04ꢂ10À3 mol) was added. The mixture was
heated at 508C for 12 h. After evaporation of the solvents, the crude
product was purified by column chromatography on alumina (eluent:
CH2Cl2 to CH2Cl2/MeOH 95:5), and crystallization from CH2Cl2/n-pen-
tane afforded 2Mn in a yield of 91% (348 mg). UV/Vis (EtOH): lmax
(e)=383 (62000), 403 (64000), 472 (100000), 566 (12000), 603 nm
(9000 molÀ1 dm3 cmÀ1); MS (ESI-TOF+): m/z calcd for C62H62N6Mn+:
945.44 [MÀCl]+; found: 945.5.
*
7000 molÀ1 dm3 cmÀ1);
MS
(MALDI-TOF+):
m/z
calcd
for
C71H76N6ClNiRh
*
+: 1208.42 [M]
*
+; found: 1208.4.
RhI complex 7Zn: Rhodium(I) complex 7Zn was obtained in a yield of
71% from 5Zn following the same procedure as described for 7Ni.
1H NMR (300 MHz, CDCl3, 258C): d=8.93 (s, 2H; pyrrole), 8.92 (d, J=
4.8 Hz, 2H; pyrrole), 8.86 (d, J=4.8 Hz, 2H; pyrrole), 8.58 (dd, J=7.5,
2.3 Hz, 2H; Ar meso), 8.21–8.10 (m, 4H; Ar meso), 7.99 (dd, J=7.5,
2.3 Hz, 2H; Ar meso), 7.93 (dd, J=7.5, 2.3 Hz, 2H; Ar meso), 7.83–7.69
(m, 6H; Ar meso), 5.00 (brs, 2H; CH cod), 3.63 (s, 6H; N-CH3), 3.50
(brs, 2H; CH cod), 2.52–2.31 (brm, 4H; CH2 cod), 2.11–1.92 (brm, 4H;
CH2 cod), 1.65 (s, 18H; tBu), 1.62 ppm (s, 18H; tBu); UV/Vis (CH2Cl2):
lmax (e)=428 (561000), 555 (31000), 596 nm (9000 molÀ1 dm3 cmÀ1); MS
Imidazolium salt 3Mn: ManganeseACTHNUGRTNEUNG(III) N-methylimidazole 2Mn
(132 mg, 1.35ꢂ10À4 mol) was dissolved in acetone (10 mL). Iodomethane
(5 mL) was added and the solution was stirred at 408C under argon for
48 h. The completion of the alkylation was verified by silica gel TLC and
the solvent was evaporated. Then the solvent was evaporated and crystal-
lization from CH2Cl2/n-pentane afforded 3Mn in
a yield of 82%
(133 mg). UV/Vis (EtOH)=lmax (e): 387 (51000), 404 (51000), 426
+
+
(MALDI-TOF+): m/z calcd for C71H76N6ClRhZn
*
:
1214.42 [M]
*
;
(43000), 473 (89000), 574 (12000), 614 nm (10000 molÀ1 dm3 cmÀ1); MS
(ESI-TOF+): m/z calcd for C63H65N6Mn2+
:
480.23 [MÀ2I]2+
; found:
found: 1214.5.
480.2; MS (ESI-TOFÀ): m/z calcd for IÀ: 126.90 [MÀ]; found: 127.0.
RhI complex 7H2: Rhodium(I) complex 7H2 was obtained in a yield of
92% from 5H2 following the same procedure as described for
7Ni.1H NMR (300 MHz, CDCl3, 258C): d=8.90 (d, J=5.1 Hz, 2H; pyr-
role), 8.86 (d, J=5.1 Hz, 2H; pyrrole), 8.72 (s, 2H; pyrrole), 8.65 (brd,
Imidazolium salt 5Ni: Imidazolium salt 3Ni (84 mg, 7.69ꢂ10À5 mol) was
dissolved in acetone (20 mL). Then a solution of silver tetrafluoroborate
(16 mg, 8.22ꢂ10À5 mol) in acetone (1 mL) was slowly added and the reac-
15658
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15652 – 15660

Porphyrins Fused to NHCs
FULL PAPER
J=7.8 Hz, 2H; Ar meso), 8.33–8.05 (brm, 4H; Ar meso), 8.11 (brd, J=
7.8 Hz, 2H; Ar meso), 7.98 (brd, J=7.8 Hz, 2H; Ar meso), 7.72–7.89
(brm, 6H; Ar meso), 4.96 (brs, 2H; CH cod), 3.46 (s, 6H; N-CH3), 3.43
(brs, 2H; CH cod), 2.50–2.27 (brm, 4H; CH2 cod), 2.08–1.89 (brm, 4H;
CH2 cod), 1.64 (s, 18H; tBu), 1.61 (s, 18H; tBu), À2.92 ppm (s, 2H; NH);
UV/Vis (CH2Cl2): lmax (e)=430 (322000), 528 (16000), 564 (11000), 599
(5000), 661 nm (4000 molÀ1 dm3 cmÀ1); MS (MALDI-TOF+): m/z calcd
for C71H79N6ClRh+: 1153.51 [M+H]+; found: 1153.5.
guard column and three PL gel Mixed-B 10 mm columns). Polystyrene
standards were used for calibration. Differential scanning calorimetry
(DSC) measurements were carried out with a DSC Q200 apparatus from
T.A. Instruments under a flow of nitrogen (heating and cooling rate
58CminÀ1).
General polymerization procedure illustrated with 3Ni as catalyst: In a glo-
vebox, a dried vial equipped with a stirrer was charged with l-lactide
(0.13 g, 9.0ꢂ10À4 mol), 3Ni (10 mg, 9.17ꢂ10À6 mol), and tBuOK (1 mg,
9.17ꢂ10À6 mol) in dry THF (0.3 mL). Benzyl alcohol (1 mg, 9.0ꢂ
10À6 mol) was added to the solution. The sealed vial was then maintained
under agitation and several withdrawals were realized to follow the ki-
netics by SEC.
RhI complex 7Mn: Imidazolium salt 3Mn (65 mg, 5.34ꢂ10À5 mol) was
dissolved in CH2Cl2 (15 mL) and a saturated aqueous solution of NaCl
(50 mL) was added. The mixture was vigorously stirred for 24 h. Then
the organic layer was separated, washed with water, and dried with
Na2SO4. After drying under vacuum, the crude product was dissolved in
THF (10 mL). Then [{RhClACHTNUTGRNEUNG
(cod)}2] (16 mg, 3.24ꢂ10À5 mol) and tBuOK
(7.2 mg, 6.42ꢂ10À5 mol) was added and the reaction mixture was stirred
under an atmosphere of argon for 2 h at ambient temperature. THF was
removed under reduced pressure to leave the crude product that was pu-
rified by silica gel column chromatography (eluent: from CH2Cl2 to
CH2Cl2/acetone 9:1). Evaporation of the solvent and crystallization from
CH2Cl2/n-pentane afforded 7Mn in a yield of 58% (38 mg). UV/Vis
(EtOH): lmax (e)=387 (54000), 405 (57000), 474 (92000), 572 (12000)
609 nm (10000 molÀ1 dm3 cmÀ1); MS (MALDI-TOF+): m/z calcd for
C71H76N6MnClRh+: 1205.43 [MÀCl]+; found: 1205.5.
Acknowledgements
The authors are grateful to the CNRS and the Agence Nationale de la
Recherche (ANR) for financial support (research project ANR-09-JCJC-
0089-01. We also greatly acknowledge Dr. Jꢀrꢃme Long for magnetic
measurements. O.C. is grateful to the Belgian National Fund for Scientif-
ic Research (FRS-FNRS).
General procedure for the preparation of carbonyl RhI complexes 8M and
9M: Rhodium(I) complex 6M or 7M (20 mg) was dissolved and stirred
in CH2Cl2 (20 mL). Then carbon monoxide was bubbled through the so-
lution for 1 h and the corresponding carbonyl RhI complex 8M or 9M
was obtained. Aliquots of the reaction mixture were sampled for IR spec-
troscopy analyses. Complex 9AlMe was synthesized by adding 2m AlMe3
in n-heptane (1.2 equiv, 21 mL) to a solution of 9H2 in CH2Cl2. A vial of
9AlMe was placed under an atmosphere of HCl for 30 min to generate
9AlCl. RhI complexes 8M and 9M were stable for hours in solution but
decomposed upon evaporation of dichloromethane. The CO stretching
frequencies measured by IR spectroscopy are presented in Table 1.
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Ring-opened amine formamide derivative 10Ni: A solution of imidazoli-
um salt 3Ni (60 mg, 5.50ꢂ10À5 mol) and tBuOK (20 mg, 1.78ꢂ10À4 mol)
in non-distilled THF (20 mL) was stirred for 24 h at room temperature
under air. The completion of the reaction was verified by silica gel TLC
analysis (one brown spot). After solvent evaporation, the compound was
purified by silica gel column chromatography (eluent: from CH2Cl2/n-
pentane 1/1 to CH2Cl2). After solvent evaporation, 10Ni was dried under
vacuum for one night and obtained in a yield of 96% (52 mg). 1H NMR
(200 MHz, CDCl3, 258C): d=8.70 (s, 2H; pyrrole), 8.69 (d, J=5.0 Hz,
1H; pyrrole), 8.64 (d, J=5.0 Hz, 1H; pyrrole), 8.61 (d, J=5.0 Hz, 1H;
pyrrole), 8.52 (d, J=5.0 Hz, 1H; pyrrole), 8.30 (s, 1H; CHO), 7.95–7.61
(m, 16H; Ar meso), 4.25 (q, J=5.5 Hz, 1H; NH-CH3), 2.81 (d, J=
5.5 Hz, 3H; NH-CH3), 2.78 (s, 3H; CH3), 1.57 (s, 18H; tBu), 1.56 (s, 9H;
tBu), 1.57 ppm (s, 9H; tBu); IR (CH2Cl2): n=1682 cmÀ1 (C=O); UV/Vis
(CH2Cl2):
lmax
(e)=420
(150000),
542
(11000),
597 nm
(6000 molÀ1 dm3 cmÀ1); MS (ESI-TOF+): m/z calcd for C63H67N6NiO+:
981.47 [M+H]+; found: 981.3.
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Polymerization reactions
Materials: l-Lactide (GALACTIC, Belgium) was recrystallized from
dried toluene and stored in a glovebox under dry nitrogen before use.
Benzyl alcohol (Sigma–Aldrich, 97%) was dried over calcium hydride
for 48 h, distilled under reduced pressure, and stored in a glovebox under
dry nitrogen before use. Imidazolium salts 3M and tBuOK (Sigma–Al-
drich, 98%) were dried overnight or for 48 h under vacuum at 1008C and
stored in a glovebox under dry nitrogen before use. THF solvent was
dried by using a MBraun Solvent Purification System (model MB-SPS
800) equipped with alumina drying columns.
Characterization: Size exclusion chromatography (SEC) was performed
in chloroform at 358C by using a Polymer Laboratories liquid chromato-
graph equipped with a PL-DG802 degasser, an isocratic LC 1120 HPLC
pump (flow rate=1 mLminÀ1),
a triple detector (refractive index
(ERMA 7517), capillary viscometry, and light scattering RALS (Viscotek
T-60) (Polymer Laboratories GPC-RI/CV/RALS)), an automatic injector
(Polymer Laboratories GPC-RI/UV), and four columns (a PL gel 10 mm
Chem. Eur. J. 2013, 19, 15652 – 15660
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: April 18, 2013
Revised: July 30, 2013
Published online: October 7, 2013
15660
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15652 – 15660