Catalytic-Site-Mediated Chain-End Control in the Polymerization of rac-Lactide with Copper Iminopyrrolide Complexes

Article abstract of DOI:10.1021/acs.organomet.8b00196

Reaction of copper(II) methoxide with N-R-2-iminopyrroles (LH) and pyridylmethanol (R′OH) provided the dinuclear complexes {LCu(μ-OR′)}₂ (R = naphtyl, CHPh₂, 2,6-xylyl, 2,6-diisopropylphenyl (diip), or p-bromobenzyl). All complexes c

Full text of DOI:10.1021/acs.organomet.8b00196

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Catalytic-Site-Mediated Chain-End Control in the Polymerization of
rac-Lactide with Copper Iminopyrrolide Complexes
Pargol Daneshmand, Jose L. Jimenez-Santiago, Maxime Aragon--Alberti, and Frank Schaper*
́
́
Centre in Green Chemistry and Catalysis, Department of Chemistry, Universite
Quebec H3T 3J7, Canada
́ ́ ́
de Montreal, C. P. 6128 Succ. Centre-Ville, Montreal,
S
* Supporting Information
ABSTRACT: Reaction of copper(II) methoxide with N-R-2-iminopyrroles (LH) and pyridylmethanol (R′OH) provided the
dinuclear complexes {LCu(μ-OR′)}2 (R = naphtyl, CHPh2, 2,6-xylyl, 2,6-diisopropylphenyl (diip), or p-bromobenzyl). All
complexes crystallized as dinuclear compounds with a square-pyramidal coordination geometry around copper and either imine
or pyridine (for R = diip) in the apical position. The naphthyl-substituted complex was inactive in rac-lactide polymerization at
room temperature in benzene. All other complexes showed good activity with apparent rate constants of kobs = 0.16(1)−1.89(8)
h−1 at 2 mM catalyst concentration. All complexes showed a preference for slight isotactic monomer enchainment with Pm =
0.60−0.68. Stereoerror analysis indicate that the chain-end determines stereocontrol. A dependance of stereocontrol on the steric
bulk of the ligand, on the initial monomer concentration and on the symmetry of the catalytic site support that the chiral
information on the chain-end is mediated via the catalytic site (catalytic-site-mediated chain-end control).
varying success on providing catalyst systems which allow the
control of stereochemistry and reactivity in lactide polymer-
ization.12−38 Unlike the industrially employed polymerization
of L-lactide, which can only provide isotactic PLLA, polymer-
ization of racemic lactide can give rise to atactic polymer in the
absence of stereocontrol. Stereocontrolled polymerization
provides isotactic or heterotactic polymer with different degrees
of stereoselectivity (Scheme 1), of which only isotactic PLA is
of current industrial interest. Typically, high degrees of
heterotacticity are comparatively easily achieved, while highly
active, isoselective polymerization still poses a catalytic
challenge.39−48 Coordination−insertion polymerization cata-
lyzed by a discrete metal alkoxide species is the most employed
mechanism.
INTRODUCTION
■
Polylactic acid (PLA) is the most important biodegradable
polyester and typically obtained by ring-opening polymer-
ization of lactide, the dimeric anhydride of lactic acid (Scheme
1).1−11 Given its increasing economic importance and the
currently unselective polymerization catalysis employed by
industry, a large number of academic studies have focused with
Scheme 1
For various reasons, such as the general biocompatibility and
the ease of complex characterization by 1H NMR, most studies
focused on d0- or d10-metal systems. Mid- to late dn-transition
metal complexes, on the other hand, have been sparingly
studied. Very few studies investigated the performance of Cr,49
Mn,50−53 or Co systems.51,54,55 Next to iron,52,56−81 copper
complexes received the highest attention.82−104 We have
reported that copper diiminopyrrolide complex 1 polymerizes
rac-lactide with an isoselectivity of Pm = 0.7 at room
Received: April 3, 2018
© XXXX American Chemical Society
A
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temperature (Scheme 2).94 Isoselective stereocontrol was
unprecedented for copper complexes. The copper complexes
obtained by (catalytic-site-mediated) chain-end control.
Copper diiminopyrrolides, such as 1, follow a similar
mechanism, in which catalytic-site epimerization is assisted by
coordination of the pendant imine (Scheme 4). A catalytic-site-
Scheme 2
Scheme 4
mediated chain-end control combines advantages of the more
typical catalytic-site control and chain-end control mechanisms:
(a) Chain-end control and catalytic-site control are often both
present in lactide polymerization and in isotactic catalysts often
with opposing stereoselectivities. (b) Chain-end control
provides longer isotactic blocks with the same degree of
control, since only one r-dyad is introduced per stereoerror. (c)
Chain-end control mechanisms are less influenced by fast
chain-transfer reactions and thus more suitable for immortal
polymerizations. (d) Since monomer selection is governed by
the catalytic-site, stereocontrol can thus be more directly
influenced than in pure chain-end control. (e) No chiral ligands
are required as long as a chiral catalytic site is formed. Despite
the advantages of this mechanism, only the very few cases
above have been reported, and the stereocontrol mechanism
was often more postulated than proven. The same,
unfortunately, was also true for diiminopyrrolide complex 1.
In the following we will offer further evidence to confirm the
existence of this rather unusual stereocontrol mechanism.
remain dinuclear throughout polymerization and successful
stereocontrol depends on the presence of a bridging
pyridylmethoxide ligand in the active species (Scheme 2).85
The stereocontrol mechanism proposed for lactide polymer-
ization with copper iminopyrrolide complexes was catalytic-site-
mediated chain-end control.105,106 In this mechanism the chiral
information is derived from the polymer chain end, which in
turn determines the configuration of the flexible catalytic site.
Monomer selectivity is then either enhanced or even
determined by the configuration of the catalytic site. The
mechanism is based on initial observations by Carpentier and
Okuda that increased flexibility of the ligand/catalytic site led to
increased heterotactic stereocontrol (Scheme 3).105,106 Okuda
proposed that heterotactic stereocontrol involves catalytic-site
inversion after each insertion step.105 Davidson proposed that a
similar mechanism is in place for C3-symmetric germanium and
zirconium complexes.107,108 Jones obtained isotactic PLA using
zirconium complexes with either chiral or achiral ligands.109,110
With chiral ligands, the complexes are locked into either Δ- or
Λ- configuration and stereocontrol follows catalytic-site control.
Achiral ligands allow Δ/Λ-isomerization, and epimerization of
the catalytic site after a misinsertion led to stereoblock PLA
RESULTS AND DISCUSSION
■
Impact of Sterically Bulky Ligands on Stereocontrol.
Given the strong implication of the catalytic site in the
a
Scheme 3. Catalyst Systems Showing Catalytic-Site-Mediated Chain-End Control
a
RR and SS denote R,R- and S,S-lactide.
B
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Figure 1. Crystal structures of 2−5. Thermal ellipsoids are drawn at 50% probability (at 30% for 2). Hydrogen atoms, a second independent
molecule (4), and the minor component of N-aryl disorder (4) omitted for clarity.
Table 1. Bond Distances [Å] and Bond Angles [deg] in Iminopyrrolide Copper Complexes
a
ab
,
c
2
3
4
5
9
6
7
Cu−Npyrrole
Cu−Nimine
Cu−Npyridine
Cu−Oshort
Cu−Olong
Cu−Cu
1.98(2)
2.37(2)
1.953(19)
1.942(16)
1.976(15)
3.030(7)
139.6(17)
0.3
1.936(2)
2.314(2)
2.049(2)
1.931(1)
1.974(1)
3.018(1)
136.4(1)
0.7
1.938(7),1.930(8)
2.421(7), 2.374(8)
2.011(8), 2.000(8)
1.929(6), 1.925(7)
1.942(6), 1.953(6)
3.025(3), 2.996(3)
1.952(4)
2.076(4)
2.202(4)
1.962(3)
1.965(3)
3.0098(12)
128.2(3)
0.4
1.934(8)
2.295(8)
2.034(8)
1.937(6)
1.967(6)
3.025(3)
1.931(1), 1.935(1)
2.317(2), 2.398(1)
2.022(1), 2.026(1)
1.927(1), 1.931(1)
1.951(1), 1.951(1)
3.0, 3.1
1.960(2)
2.047(2)
2.173(2)
1.974(2)
1.986(2)
2.992(1)
128.9(2)
0.6, 0.6
d
d
C6−Nimine−Cu
τ
139, 138
134
137.2(1), 134.8(1)
0.4, 0.4
0.5, 0.5
0.5
group in apical position
imine
imine
imine, imine
pyridine
imine
imine
pyridine
a
b
c
d
Two independent molecules in the asymmetric unit. Taken from ref 85. Taken from ref 84. Averaged value of the observed disorder.
a
Table 2. rac-Lactide Polymerizations with 2−5 and 9
b
c
d
e
catalyst
final conversion (time)
kobs [h−1
]
Mn
Mn (calcd)
Mw/Mn
# chains
Pm
f
2
3
4
5
4−8% (24−32 h)
83% (27 h)
99% (21 h)
99% (3 h)
95% (2 h)
99% (24 h)
99% (32 h)
99% (23 h)
0.16(1)
1.89(8)
1.74(8)
1.79(3)
1.29(1)
1.07(4)
0.55(1)
12.2 kDa
8.3 kDa
12.0 kDa
14.3 kDa
14.3 kDa
13.7 kDa
14.3 kDa
14.3 kDa
14.3 kDa
1.6
1.7
1.8
1.3
2.2
2.1
1.6
1.0
1.7
0.9
1.3
0.8
2.1
1.3
0.57
0.68
0.65
0.65
0.63
0.60
0.60
15.9 kDa
10.4 kDa
16.9 kDa
7.0 kDa
5 + 1 Ph3COH
g
6
h
7
9
11.1 kDa
a
Conditions: C6D6, RT, [lactide] = 200 mM, [L2Cu2(OR)2] = 2 mM. The time of final conversion should not be considered a measure of activity
b
but indicates after what time the reaction was quenched. Mn and Mw determined by size exclusion chromatography vs polystyrene standards, with a
c
Mark−Houwink correction factor of 0.58. Mn expected if one alkoxide per catalyst dimer initiates polymerization, calculated from [lactide]/[cat]·
d
e
conversion·Mlactide + MROH Number of chains per catalyst dimer, calculated from the ratio of expected and obtained polymer molecular weight. Pm
determined from decoupled 1H NMR by Pm = 1 − 2·I1/(I1 + I2), with I1 = 5.20−5.25 ppm (rmr, mmr/rmm), I2 = 5.13−5.20 ppm (mmr/rmm, mmm,
f
g
h
mrm). Two experiments. Taken from ref 85. Taken from ref 84.
proposed stereocontrol mechanism, a notable influence of the
steric bulk provided by the ligand on selectivity would have
been expected. Unfortunately, variations of the N-substituent in
1 either provided complexes with very similar stereocontrol or,
for sterically demanding N-CHPh2, N-naphthyl, or N-xylyl
substituents, the complexes were synthetically not accessible
(Scheme 2).84 Influence of ligand steric bulk was thus
investigated for the respective monoiminopyrrolide complexes,
in the expectation that the removal of one imino-substituent
would allow the incorporation of a wider range of N-
substituents.
Synthesis of ligands L2−L5 followed either literature
protocols or procedures successfully employed for similar
ligands (see the Experimental Section). Corresponding
monoiminopyrrolide complexes 2−4 were accessible using
the same synthetic protocols employed for 1 (Scheme 4). In
addition, complex 5, containing a N-2,6-diisopropyphenyl
(diip) substituent was prepared using the same methodology.
Monoiminopyrrolide complexes 6 and 7 with methylbenzyl and
benzyl N-iminosubstituents have been prepared previously.85,94
All complexes were characterized by X-ray diffraction studies.
Previously obtained diimino- and monoiminopyrrolide com-
plexes LCu(OR) typically form dinuclear copper complexes
with a square-pyramidal coordination geometry around copper.
The pyrrolide nitrogen and the bridging alkoxides were always
found in the equatorial plane, while either the pyridyl or the
imino group occupied the axial position.84,85,94 Complexes 2−5
follow the same structural pattern (Figure 1): Despite τ values
up to 0.7,111 the coordination geometry around copper is best
described as square-pyramidal with two short Cu−N distances,
two short Cu−O distances, and one elongated Cu−N distance
to the ligand in the apical position (Table 1). Complexes 2−4,
as well as 6,85 display the imino group in the apical position.
The N-dipp complex, 5, coordinates the pyridyl group in the
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apical position instead of the imine (Figure 1, Table 1). While it
is tempting to ascribe this to the increased steric bulk of the
diip substituent, coordination of the pyridyl group in the apical
position was also favored upon reducing the steric bulk from N-
CH(Me)Ph in 6 to N-CH2Ph in 7.84,85 In general, bond
distances and angles do not show any clear dependence on the
bulk of the N-imino substituent. In concurrence with earlier
findings, iminopyrrolide complexes 2−5 thus show an invariant
structural motif (square-pyramidal coordination with the
anionic ligands in equatorial positions) combined with a
flexibility of the remaining coordination geometry.
Figure 3. 13C{1H}-NMR of PLA obtained with 4. Left: carbonyl
region, right: methine region. Tetrad assignments according to refs
113 and 114.
Lactide Polymerization. Complexes 2−5 were tested for
the polymerization of rac-lactide at room temperature in C6D6
solution (Table 2, Figures S1 and S2). Complex 2 was barely
active at all and reached less than 10% conversion even after 24
h, while 3 showed moderate activity, 6−8 times slower than less
sterically demanding complexes 6 or 7, reported previously.84,85
Slight curvatures in the semilogarithmic conversion−time plot
(Figure 2) and 83% final conversion for 3 indicate that this
polydispersities of 1.6−1.8 are surprisingly broad for such
well-controlled reactions. Broadened polydispersities are most
likely associated with the (reversible) formation of an inactive
species. Addition of 1 equiv of trityl alcohol has been shown to
enforce fast chain-transfer between inactive and active species
without generating additional polymer chains.85 Addition of
Ph3COH to polymerizations with 5 (Figures S4 and S5)
consequently reduced polydispersities to 1.3 (Table 2).
Complexes 3−5 all produce isotactically enriched PLA.
Replacing H or Me in the N−CH(R)Ph substituents of 6 and 7
by phenyl did not increase isotacticity and 3 displayed an even
lower Pm value (Table 2). N-Aryl substituted 4 and 5, however,
showed a notable increase in stereocontrol to Pm = 0.68 and
0.65, respectively. Monoiminopyrrolide complexes thus show a
clearer and more pronounced dependence of stereocontrol on
ligand bulk than diiminopyrrolide complexes (Scheme 2),
which confirms the participation of the ligand environment on
monomer selection.
Impact of Site Epimerization on Stereocontrol. In the
proposed stereocontrol mechanism, a misinsertion is followed
by fast catalytic-site inversion and continued isotactic polymer-
ization (Scheme 3) and would thus produce an isolated mrm-
tetrad. Contrary to typical chain-end control, the insertion rate,
or more precisely the insertion/isomerization rate ratio, can
have an influence on stereocontrol. If insertion occurs before
isomerization, then an rmr-tetrad might be produced, either
because the ligand environment is solely responsible for
monomer selection or because the chain-end/catalytic-site
mismatch decreases stereocontrol. In other words, if epimeriza-
tion is slow relative to insertion, then the catalyst partially
behaves as being either under catalytic-site control or under no
stereocontrol, both of which provide lower total isotacticities
than pure chain-end control (given the same selectivity,
catalytic-site control provides up to 10% less isotactic tetrads
than chain-end control in lactide polymerization).
Since insertion rate is dependent on monomer concen-
tration, while epimerization is not, one would thus expect that
stereocontrol increases with conversion since lower lactide
concentrations favor epimerization. Isotacticities indeed in-
crease during the polymerization in lactide polymerization with
1,85 as well as in polymerizations with 3−5 (Figures S2 and S5),
but the small amount of the changes make it impossible to
delineate this effect from chain-end effects at lower chain-
lengths. We thus conducted polymerizations with 5 at constant
catalyst concentration but varying monomer concentrations
(Table S2, Figure S4). Complex 5 was chosen since it was most
likely to show slow epimerization and one equiv of Ph3COH
was added to avoid any influence of reversible or irreversible
catalyst decomposition on stereocontrol. At higher lactide
concentrations which favor insertion over epimerization,
stereocontrol was indeed reduced for 0.67 to 0.63 (Table 3,
Figure 2. Semilogarithmic conversion−time plot for rac-lactide
polymerizations with 2 (circles), 3 (squares), 4 (triangles), and 5
(diamonds).
might be partially due to complex decomposition. Complexes 4
and 5, however, showed even higher activities than those of 6
or 7 (Figure 2, Table 2). Variation of the ortho-substituent on
the N-aryl between methyl and isopropyl did not notably
influence activity.
PLA produced with 3 and 5 shows the molecular weight
expected for 1 polymer chain per catalysts dimer (Table 2).
The lower polymer molecular weight obtained with complex 4
corresponds to 1.7 polymer chains per dimer. The MALDI-MS
spectrum of PLA obtained with 4 shows however the presence
of cyclic oligomers (Figure S3), which led to the increased
number of polymer chains. Stereoerror analysis by 13C NMR of
PLA produced with 4 yielded a ratio of mrm/mmr/rmm/rmr of
15%:11%:11%:5%, which is the exact ratio expected for chain-
end control with a Pm value of 0.69 (Figure 3, Table S1).112
Monoiminopyrrolide complexes 3−5 thus follow the same
mechanism as determined for diiminopyrrolide 1 in which only
one pyridylmethoxide substituent initiates chain growth and the
active species is dincuclear.85,94
As generally observed for iminopyrrolide copper complexes,
polymer molecular weight control is relatively poor and
D
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structure of 9 shows coordination of the imine in the apical
position (Figure 4), again indicating that the different
Table 3. rac-Lactide Polymerization with 4 at Different
Lactide Concentrations
a
b
[4]
[lactide]
kobs [h−1
]
conversion
Pm
2.0 mM
2.0 mM
2.0 mM
2.0 mM
0.10 M
0.20 M
0.20 M
0.40 M
1.13(2)
1.79(3)
1.74(8)
1.34(5)
98%
95%
99%
97%
0.67
0.65
0.65
0.63
c
a
b
Conditions: C6D6, in the presence of 2 mM Ph3COH. Pm value
averaged for a polymerization degree >40 lactide units. Typically at
polymerization degrees above 40 units the influence of the chain-end
became negligible and Pm values remained constant to 1% (cf. Figure
c
S5). No Ph3COH added.
Figure S5). The observed trend thus supports the necessary
involvement of catalytic-site epimerization in the mechanism,
although the differences were barely larger than the typical
error ( 2% for Pm in repeated experiments).
On the basis of the provided mechanism, polymerization of
enantiopure lactide should be faster than that of rac-lactide,
since all catalyst would be present in the same isomer enabling
isotactic insertion with the full monomer concentration. In rac-
lactide polymerization, the catalyst is present 50% in SS-
selective form and 50% in RR-selective form, and half of the
monomer can only be incorporated by misinsertion (which is
by necessity slower). Polymerizations of 5 with L-lactide
instead of rac-lactide showed very similar kinetics with identical
induction periods of 11 and 12 min, respectively. As expected,
the apparent rate constant for L-lactide polymerization was
50% higher (2.85(3) h−1 for L-lactide compared to that for rac-
lactide, 1.74(8) h−1). This is a somewhat larger difference than
expected and translates to an isotacticity of Pm = 0.82 (see the
Supporting Information). While there are possible mechanistic
explanations for this, to differentiate between Pm = 0.82 and the
observed Pm = 0.66, rate constants would have to be accurate
within 10%. The difference is thus most likely due to the
experimental error in working with two different batches of
lactide.
Impact of Catalytic-Site Symmetry on Stereocontrol.
The proposed stereocontrol mechanism relies on the catalytic
site to transfer (and amplify) the chiral information on the
chain-end. Accordingly, in the presence of a symmetric catalytic
site, no stereocontrol is expected. We have previously reported
that 8 carrying a para-bromobenzyl N-substituent unexpectedly
coordinated both iminogroups to the copper center which
resulted in a Cs-symmetric catalytic site (Scheme 5).84 The
difference in coordination mode was attributed to increased π-
stacking interactions in this complex. To distinguish between
effects of catalytic site symmetry and effects due to changes in
the substitution pattern, such as increased π-stacking
interactions, respective monoiminopyrrolide 9 was prepared
(Scheme 5). Contrary to its benzyl analogue 7, the crystal
Figure 4. Crystal structure of 9. Hydrogen atoms and the minor part
of the disorder omitted for clarity. Thermal ellipsoids shown at the
50% probability level.
coordination isomers observed in solid state are likely very
close in energy. Bond lengths and angles are similar to those of
2−4 and 6 (Table 1).
While 9 can provide the same interactions as 8, it cannot
form a Cs-symmetric complex and thus allows us to delineate
between the two influences. rac-Lactide polymerization with 9
followed clean first-order kinetics (Figures S8 and S9) with a
rate constant approximately half of that of the respective
benzyl-substituted complex 7. Polymer molecular weight data
corresponded to one pyridylmethoxide per catalyst dimer
initiating polymerization but was slightly depressed from that
value (Table 2). MALDI-MS analysis again indicated the
presence of intramolecular transesterification reactions (Figure
S10).
More importantly, monoiminopyrrolide complex 9 showed
isotactic stereocontrol identical to that of its N-benzyl analogue
7 (Pm = 0.60 in both cases, Table 2), thus indicating a negligible
influence of the para-bromosubstituent on stereocontrol. In
rac-lactide polymerizations with 8, however, atactic PLA (Pm =
0.53) was obtained, with Cs-symmetric 8 being the only
pyridylmethoxide containing complex investigated which did
not produce isotactically enriched PLA.84,85,94 This strongly
supports that the chirality of the catalytic site is responsible for
monomer selection, even though stereoerror analysis and other
data clearly indicated that the chiral information is provided by
the chain-end.
Scheme 5
CONCLUSION
Investigations into monoiminopyrrolide complexes showed that
their lactide polymerization behavior models that of their
■
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3
pyrrole), 6.25 (dd, JHH = 4, 3 Hz, 1H, 4-pyrrole), 4.65 (s, 2H, CH2).
13C{1H} NMR (CDCl3, 75 MHz): δ 152.8 (NC), 138.8 (ipso-Ph),
131.7 (m-Ph), 130.0 (2-pyrrole), 129.7 (o-Ph), 122.1 (5-pyrrole),
121.0 (p-Ph), 114.8 (3-pyrrole), 110.1 (4-pyrrole), 63.8 (CH2). ESI-
HRMS (m/z): M + H+ (C12H12BrN2) calcd 263.0178; found:
263.0189.
respective diiminopyrrolide analogues. The observed impact of
ligand congestion on stereocontrol, the dependence of
stereocontrol on monomer concentration, and most impor-
tantly, the absence of stereocontrol if the catalytic site becomes
symmetrical strongly support the presence of a catalytic-site-
mediated chain-end control mechanism in this type of
complexes.
Despite the advantages inherent with this stereocontrol
mechanism, a successful application will have to rely on
establishing the same mechanism in a different catalytic system.
Although iminopyrrolide complexes provided the only isotactic
copper complexes reported so far, their inherent weaknesses
(poor molecular weight control, sluggish response to steric
bulk, and limited synthetic variability) where again underlined
in this study and make it unlikely that this class of complexes
can be improved much further.
(L2)2Cu2(μ-O,κN-OCH2Py)2, 2. Cu(OMe)2 (57 mg, 0.45 mmol)
was suspended in toluene (3 mL). 2-Pyridylmethanol (87 μL, 0.90
mmol) was added to the blue suspension, which was stirred for 45
min. A freshly prepared brown solution of L2 (100 mg, 0.45 mmol) in
toluene (2 mL) was added dropwise, resulting in a dark green solution.
The reaction was stirred 48 h at RT, filtered to remove trace
impurities, concentrated to 1/3 of the volume, diluted with hexane (18
mL), and kept at −30 °C for 4 h, resulting in 31 mg (18%) of green X-
ray-quality crystals. Samples for elemental analysis were obtained by
diffusion of hexane into dichloromethane (1:3).
UV−vis (toluene, 2.3 × 10−6 M) λmax, nm (ε, mol−1 cm2): 355
(25 200), 522 (sh), 603 (252), 666 (200). Anal. Calcd for
C42H34Cu2N6O2: C, 64.52; H, 4.38; N, 10.75; Found: C, 65.09; H,
4.99; N, 11.33. (Recrystallized twice, final result shown.)
(L3)2Cu2(μ-O,κN-OCH2Py)2, 3. Analogous to 2, from Cu(OMe)2
(48 mg, 0.38 mmol) in toluene (3 mL), 2-pyridylmethanol (73 μL,
0.76 mmol), and L3 (100 mg, 0.38 mmol) in toluene (2 mL) afforded
32 mg (20%) of green X-ray-quality crystals. Samples for elemental
analysis were obtained by diffusion of hexane into dichloromethane
(1:3).
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out using
Schlenk or glovebox techniques under nitrogen atmosphere. Cu-
(OMe)2,115 L4,116 and L5117 were prepared according to literature.
1H-Pyrrole-2-dicarbaldehyde was prepared according to literature and
recrystallized from hexane at −80 °C.118 Solvents were dried by
passage through activated aluminum oxide (MBraun SPS), deoxy-
genated by repeated extraction with nitrogen, and stored over
molecular sieves. C6D6 was dried over molecular sieves. rac-Lactide
(98%) was purchased from Sigma−Aldrich, purified by 3×
recrystallization from dry ethyl acetate and kept at −30 ◦C. All
other chemicals were purchased from common commercial suppliers
UV−vis (toluene, 2 × 10−6 M) λmax, nm (ε, mol−1 cm2): 364
(4700), 509 (550), 603 (500), 671 (470). Anal. Calcd for
C48H42Cu2N6O2·1/4CH2Cl2: C, 65.62; H, 4.85; N, 9.52; Found: C,
66.03; H, 4.96; N, 9.90.
(L4)2Cu2(μ-O,κN-OCH2Py)2, 4. Analogous to 2, from Cu(OMe)2
(63 mg, 0.50 mmol) in toluene (3 mL), 2-pyridylmethanol (96 μL, 1.0
mmol), and L4 (100 mg, 0.50 mmol) in toluene (2 mL) afforded 41
mg (23%) of green X-ray-quality crystals. Samples for elemental
analysis were obtained by diffusion of hexane into dichloromethane
(1:3).
and used without further purification. H and 13C NMR spectra were
1
acquired on Bruker Avance 300 and 400 spectrometers. Chemical
shifts were referenced to the residual signals of the deuterated solvents
1
1
(CDCl3: H: δ 7.26 ppm, 13C: δ 77.16; C6D6: H: δ 7.16 ppm, 13C: δ
128.38 ppm). Elemental analyses were performed by the Laboratoire
́ ́ ́ ́
d’analyse elementaire (Universite de Montreal). All UV−vis measure-
UV−vis (toluene, 2.5 × 10−6 M) λmax, nm (ε, mol−1 cm2): 350
(21 400), 388 (sh), 542 (780), 607 (800), 664 (675). Anal. Calcd for
C38H38Cu2N6O2·1/4CH2Cl2: C, 60.52; H, 5.11; N, 11.07; Found: C,
60.72; H, 5.15; N, 11.36.
ments were conducted in anhydrous and degassed toluene at room
temperature in a sealed quartz cell on a Cary 500i UV−vis−NIR
spectrophotometer.
2-((Naphthyl)aldimino)pyrrole, L2. 1H-Pyrrole-2-carbaldehyde
(1.0 g, 11 mmol) was dissolved in dry toluene (25 mL). MgSO4 (5 g),
a catalytic amount of Amberlyst 15, and 1-naphtylamine (1.5 g, 11
mmol) were added. The reaction mixture was stirred overnight at
room temperature. The brown suspension was filtered, and the solvent
was removed under vacuum. The residue was treated with hexane (20
mL), resulting in a brown oil. The oil was separated by decantation
and dried under vacuum (1.8 g, 78%). The 1H NMR spectra is
identical to an alternative preparation published earlier.119
1H NMR (CDCl3, 400 MHz,): δ 8.35−8.27 (m, 2H, (NC)H and
Ar), 7.85 (d, JHH = 7 Hz, 1H, Ar), 7.70 (d, JHH = 8 Hz, 1H, Ar), 7.55−
7.42 (m, 3H, Ar), 7.27−7.25 (m, 1H, Ar), 7.05 (d, 3JHH = 7 Hz, 1H, 5-
pyrrole), 7.03 (br s, 1H, NH), 6.76−6.72 (m, 1H, 3-pyrrole), 6.38−
6.31 (m, 1H, 4-pyrrole).
(L5)2Cu2(μ-O,κN-OCH2Py)2, 5. Analogous to 2, from Cu(OMe)2
(49 mg, 0.39 mmol) in toluene (3 mL), 2-pyridylmethanol (75 μL,
0.78 mmol), and L5 (100 mg, 0.39 mmol) in toluene (2 mL) afforded
40 mg (24%) of green X-ray-quality crystals. Samples for elemental
analysis were obtained by diffusion of hexane into dichloromethane
(1:3).
UV−vis (toluene, 3.6 × 10−5 M) λmax, nm (ε, mol−1 cm2): 355
(18 500), 388 (sh), 472 (1400), 673 (320). Anal. Calcd for
C46H54Cu2N6O2: C, 65.00; H, 6.40; N, 9.89; Found: C, 64.60; H,
6.10; N, 9.78.
(L9)2Cu2(μ-O,κN-OCH2Py)2, 9. Analogous to 2, from Cu(OMe)2
(50 mg, 0.40 mmol) in toluene (3 mL), 2-pyridylmethanol (77 μL,
0.80 mmol), and L9 (100 mg, 0.40 mmol) in toluene (2 mL).
Decantation and washing with hexane (3 × 10 mL) afforded 31 mg
(18%) of green X-ray quality crystals.
2-((Benzylhydryl)aldimino)pyrrole, L3. Analogous to L2, from
1H-pyrrole-2-carbaldehyde (1.0 g, 11 mmol) and benzhydrylamine
UV−vis (toluene, 1.2 × 10−5 M) [λmax, nm (ε, mol−1 cm2)]: 351
(18 600), 371 (sh), 490 (1200), 603 (800), 664 (720). Anal. Calcd for
C36H32Br2Cu2N6O2: C, 49.84; H, 3.72; N, 9.69; Found: C, 50.06; H,
3.73; N, 9.43.
1
(1.9 g, 11 mmol) to yield a brown oil (2.2 g, 81%). The H NMR
spectra is identical to an alternative preparation published earlier.120
1H NMR (CDCl3, 400 MHz,): δ 8.43 (br s, 1H, NH), 7.97 (s, 1H,
(NC)H), 7.33−7.16 (m, 10H, Ph), 6.98−6.93 (m, 1H, 5-pyrrole),
6.56 (d, 3JHH = 4 Hz, 1H, 3-pyrrole), 6.21 (dd, 3JHH = 4, 3 Hz, 1H, 4-
pyrrole), 5.62 (s, 1H, CH).
rac-Lactide Polymerization. All manipulations took place in a
glovebox under nitrogen atmosphere. Stock solutions of the catalysts
and BnOH were prepared in dry C6D6 and stored at −30 °C to avoid
concentration changes. The desired amount of rac-lactide was placed
into a J. Young tube together with C6D6. A stock solution of benzyl
alcohol was added, where required, followed by a stock solution of the
catalyst (ca. 20 mM in C6D6) to give final concentrations of 2.0 mM
catalyst dimer and of 0.20 M lactide. After complete dissolution was
assured by shaking, the reaction was followed by 1H NMR. The
reaction was quenched by addition of 5−10 equiv of a CDCl3 solution
2-((4-Bromobenzyl)aldimino)pyrrole, L9. Analogous to L2,
from 1H-pyrrole-2-carbaldehyde (1.0 g, 11 mmol) and p-bromoben-
zylamine (2.9 g, 16 mmol) to give 2.5 g (91%) of a 1:3 mixture of p-
bromobenzylamine and L9. Purification attempts were unsuccessful
and the mixture was used without purification in further synthesis.
1H NMR (CDCl3, 300 MHz): δ 8.15 (dd, JHH = 2, 1 Hz, 1H,
((N)C)H), 7.49−7.41 (m, 2H, Ar), 7.19−7.14 (m, 2H, Ar), 6.90−
3
4
6.86 (m, 1H, 5-pyrrole), 6.53 (dd, JHH = 4, JHH = 1 Hz, 1H, 3-
F
DOI: 10.1021/acs.organomet.8b00196
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 4. Details of X-ray Diffraction Experiments
2
3
4
5
9
formula
C42H34Cu2N6O2
781.83; 1.201
150; 3618
C48H42Cu2N6O2
861.95; 1.428
150; 892
C38H38Cu2N6O2
737.82; 1.410
150; 764
C46H54Cu2N6O2
850.03; 1.303
150; 446
C36H32Br2Cu2N6O2
867.57; 1.696
150; 868
Mw (g/mol); dcalcd (g/cm3)
T (K); F(000)
crystal system
space group
trigonal
monoclinic
P21/c
triclinic
triclinic
monoclinic
P21/c
R3
P1
̅
P1
̅
̅
unit cell
a (Å)
35.261(5)
35.261(5)
9.0334(17)
90
8.8407(3)
16.3755(5)
14.1657(4)
90
7.9918(7)
12.0774(12)
18.2565(17)
95.832(5)
95.745(5)
94.455(5)
1737.3(3); 2
6.83; multiscan
3.2−61.3; 0.96
67959; 0.126
7954; 0.186
0.126
10.2561(6)
10.8248(6)
11.7883(7)
65.984(3)
83.830(3)
65.374(3)
1083.67(11); 1
5.52; multiscan
3.6−59.9; 0.99
27019; 0.052
4963; 0.079
0.082
11.7994(7)
20.5590(13)
7.0097(4)
90
b (Å)
c (Å)
α (deg)
β (deg)
90
102.2090(10)
90
92.497(4)
90
γ (deg)
V (Å3); Z
120
9727(3); 9
5.51; multiscan
2.2−33.6; 0.99
9578; 0.183
1320; 0.251
0.114
2004.40(11); 2
5.98; multiscan
3.6−60.7; 0.98
28616; 0.026
4539; 0.046
0.039
1698.83(18); 2
8.90; multiscan
3.3−55.9; 0.99
34561; 0.059
3889; 0.099
0.103
μ (mm−1); abs. corr.
θ range (deg); completeness
collected reflections; Rσ
unique reflections; Rint
R1(F) (I > 2σ(I))
wR(F2) (all data)
GoF (F2)
0.299
0.107
0.351
0.233
0.259
1.07
1.11
1.08
1.06
1.08
residual electron density
0.48, −0.28
0.32, −0.95
1.07, −0.82
1.41, −0.77
1.04, −0.69
of acetic acid (10 mM, drops). The volatiles were evaporated and solid
polymer samples were stored at −80 °C for further analysis.
Accession Codes
CCDC 1581210−1581214 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Conversion was determined from 1H NMR by comparison to
remaining lactide. Pm values were determined from homodecoupled
1H NMR spectra and calculated from Pm = 1 − 2·I1/(I1 + I2), with I1 =
5.15−5.21 ppm (rmr, mmr/rmm), I2 = 5.21−5.25 ppm (mmr/rmm,
mmm, mrm). The integration of the left multiplet and right multiplet
(I1 and I2) required only one very reproducible dividing point of the
integration, which was always taken as the minimum between the two
multiplets. Pm values obtained this way were typically consistent to
1% over the course of one experiment and 3% between different
experiments under identical conditions. Molecular weight analyses
were performed on a Waters 1525 gel permeation chromatograph
equipped with three Phenomenex columns and a refractive index
detector at 35 °C. THF was used as the eluent at a flow rate of 1.0 mL·
min−1 and polystyrene standards (Sigma−Aldrich, 1.5 mg·mL−1,
prepared and filtered (0.2 mm) directly prior to injection) were used
for calibration. Obtained molecular weights were corrected by a
Mark−Houwink factor of 0.58.121
AUTHOR INFORMATION
Corresponding Author
*E-mail: Frank.Schaper@umontreal.ca.
■
ORCID
Frank Schaper: 0000-0002-3621-8920
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
X-ray Diffraction. Single crystals were obtained directly from
isolation of the products as described above. Diffraction data were
collected on a Bruker Venture METALJET diffractometer (Ga Kα
radiation) using the APEX2 software package.122 Data reduction was
performed with SAINT,123 and absorption corrections were carried
out with SADABS.124 Structures were solved by dual-space methods
(SHELXT).125 All non-hydrogen atoms were refined anisotropic using
full-matrix least-squares on F2 and hydrogen atoms refined with fixed
isotropic U using a riding model (SHELXL2014).126 Complexes 2, 4,
and 9 were found to be twinned. Complex 2 diffracted very weakly,
and a general thermal parameter restraint (RIGU) was necessary in
refinement due to the resulting bad data quality. No better crystal
could be obtained. Further experimental details can be found in Table
4 and in the Supporting Information.
■
Funding was supplied by the NSERC discovery program
(RGPIN-2016-04953) and the Centre for Green Chemistry
́
and Catalysis (FQRNT). We thank Francine Belanger for
support with X-ray diffraction, Elena Nadezhina for elemental
analysis, and Marie-Christine Tang, Karine Gilbert and Dr.
Alexandra Furtos for support with MALDI-MS. Polymerization
with L-lactide was proposed by a reviewer.
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■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00196.
̈
̊
Additional tables and figures (PDF)
G
DOI: 10.1021/acs.organomet.8b00196
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DOI: 10.1021/acs.organomet.8b00196
Organometallics XXXX, XXX, XXX−XXX