Synthesis, structure, and catalytic activity of dinuclear aluminium bis(amidinate) and bis(guanidinate) complexes

Article abstract of DOI:10.1039/d0nj05007e

Eight dinuclear methyl aluminium(iii) bis(amidinate) and bis(guanidinate) complexes have been synthesized in good to very good yields and were fully characterized by means of 1H, 13C, and IR spectroscopy as well as elemental analysis. Five of them were successfully converted to the respective dinuclear aluminium iodide complexes and a dinuclear aluminium chloride bis(amidinate) complex was directly accessed by deprotonation of the ligand using ethyl aluminium dichloride. The molecular solid-state structures of eleven complexes were obtained from X-ray diffraction analysis. Furthermore, the catalytic activity of the dinuclear methyl aluminium complexes 3 and 4 has been probed in the ring-opening polymerization of ?-caprolactone and l-lactide and the results highlight the impact of the metal-metal separation and the ligands' backbone on activity and selectivity. This journal is

Full text of DOI:10.1039/d0nj05007e

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Synthesis, structure, and catalytic activity of
dinuclear aluminium bis(amidinate) and
bis(guanidinate) complexes†
Cite this: New J. Chem., 2021,
45, 972
Andreas Rosch,‡a Fabian Seifert,‡b Valentin Vass,a Helmar Gorlsb and
¨
¨
bc
Robert Kretschmer
*
Eight dinuclear methyl aluminium(III) bis(amidinate) and bis(guanidinate) complexes have been synthesized
1
in good to very good yields and were fully characterized by means of H, 13C, and IR spectroscopy as well
as elemental analysis. Five of them were successfully converted to the respective dinuclear aluminium
iodide complexes and a dinuclear aluminium chloride bis(amidinate) complex was directly accessed by
deprotonation of the ligand using ethyl aluminium dichloride. The molecular solid-state structures of
eleven complexes were obtained from X-ray diffraction analysis. Furthermore, the catalytic activity of the
dinuclear methyl aluminium complexes 3 and 4 has been probed in the ring-opening polymerization of
e-caprolactone and L-lactide and the results highlight the impact of the metal–metal separation and the
ligands’ backbone on activity and selectivity.
Received 12th October 2020,
Accepted 9th November 2020
DOI: 10.1039/d0nj05007e
rsc.li/njc
has only recently started to evolve, likely due to the higher
synthetic demands. In both cases, the two binding sites can be
Introduction
The previous decades have seen a remarkable development of connected through either one or two of the chelating nitrogen
new catalytic methods associated with increased reactivity and atoms, affording acyclic and macrocyclic ligands, respectively,
selectivity schemes, while using less toxic, abundant, and non- or through the ligands’ backbone, Fig. 1.6
endangered elements instead of scarce and correspondingly
The ligand framework affects the overall orientation of the
expensive noble metals.1 Among the various concepts, coopera- two metal sides and allows for or impedes cooperative effects as
tive catalysis is one of the most versatile and powerful illustrated in various applications of the related polynuclear
approaches, and dinuclear or bimetallic compounds often excel complexes in material science and catalysis.6a,7a–i However, exam-
the related single-site homogeneous catalysts.2 Reactivity and ples incorporating p-block elements remain rare,3,4e,j,l,m,n,o,p,y,7h,8
selectivity patterns can be controlled by designing tailor-made and only a few reports about dinuclear aluminium bis(amidinate)
ditopic ligands, which allow modifying the metal–metal separa- complexes exist in the literature.7g,h,8c In terms of catalysis,
tion, the relative orientation of both active sites and also the dinuclear aluminium bis(amidinate)s have been applied for the
steric constraints of the dinuclear catalyst. Ditopic ligands fixation of carbon dioxide into cyclic carbonates7h and for the
possessing two monoanionic N,N0-binding pockets have been ring-opening polymerization (ROP) of cyclic esters including
regularly utilized for the complexation of non-precious metals,3 e-caprolactone and L-lactide.7g,8c With respect to the latter,
and bis(amidine)s4 received considerable interest in the past. cooperative effects were not observed in case of the backbone-
The use of the more electron-rich bis(guanidine)s,5 however, bridged bis(amidinate)s,8c likely because of the opposing orienta-
tion of the two active sites, while in case of the nitrogen-bridged
relatives cooperativity was seen.7g However, only two aromatic
bridging groups have been studied and experiments in which
a
¨
Institute of Inorganic Chemistry, University of Regensburg, Universitatsstrasse 31,
93053 Regensburg, Germany
b Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena,
Humboldtstraße 8, 07743 Jena, Germany. E-mail: robert.kretschmer@uni-jena.de
c Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena,
Philosophenweg 7, 07743 Jena, Germany
† Electronic supplementary information (ESI) available: NMR and IR spectra,
cyclic voltammograms, and the crystallographic data. CCDC 2020619–2020629.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0nj05007e
Fig. 1 Various types of bis(amidine)s (R0 = alky, aryl) and bis(guanidine)s
(R0 = NR00R0 0 0).
‡ Both authors contributed equally to this work.
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Scheme 1 Synthesis of the dinuclear methyl aluminium bis(amidinate) 3 and bis(guanidinate) 4 complexes from the respective protio-ligands by
methane elimination. Dipp = 2,6-diisopropylphenyl.
the metal–metal separation is systematically altered have yet to be
performed. This is somewhat surprising as dinuclear alkyl
aluminium complexes based on various ditopic ligands have been
identified as powerful catalysts in the ring-opening polymerization
of polar monomers.7f,i,9 Hence, we set out to synthesize new
Fig. 2 Solid-state structure (hydrogen atoms are omitted for the sake of
dinuclear aluminium alkyl complexes, which possess more
flexible ethylene, propylene, butylene, and 1,3-xylylene bridging
clarity) with selected bond lengths [Å] and angles [1] of: (a) 3a: Al1–Al10
6.2911(7), Al1–C18 1.9580(14), Al1–C19 1.9507(14), Al1–N1 1.9441(10),
groups. Aiming to elucidate the impact of the ligands’ backbone
Al1–N2 1.915(1), N1–C13 1.3479(15), N2–C13 1.3379(15), N1–Al1–N2
without influencing the steric demand significantly, bis(guanidinate) 68.27(4), N1–C13–N2 107.47(9), C18–Al1–C19 117.18(6); (b) 3c: Al1–Al10
8.1793(6), Al1–C18 1.9599(14), Al1–C19 1.9589(13), Al1–N1 1.9358(9),
Al1–N2 1.9196(10), N1–C13 1.3449(14), N2–C13 1.3396(14), N1–Al1–N2
complexes have been synthesized and investigated as well and our
findings are reported in the following.
68.41(4), N1–C13–N2 107.68(9), C18–Al1–C19 117.56(6).
Results and discussion
Synthesis and structural characterization of dinuclear
for other methyl aluminium amidinate (1.937(6) and 1.958(3) Å),13
guanidinate (1.955(4) and 1.9726(19) Å),14 bis(amidinate)
(1.943(4) and 1.946(4) Å),4l and bis(guanidinate) complexes
aluminium complexes
The dinuclear aluminium bis(amidinate) and bis(guanidinate) (1.925(7) and 1.962(3) Å).15 The same holds true for the Al–N
complexes 3 and 4 were obtained from the related protio-ligands, bond lengths (1.915(1) to 1.9441(10) Å) and N–Al–N bite angles
which have been synthesized according to reported procedures,4ae,5d,10 (68.27(4) to 69.89(5)1). Although small differences in the C–N
using protocols established for dinuclear alkyl aluminium bis(b- bond lengths were observed between the bis(amidinate)s
diketiminate) complexes.11 Hence, toluene solutions of 1 or 2 were (1.3379(15) to 1.3479(15) Å) and bis(guanidinate)s (1.3421(18)
allowed to react with trimethyl aluminium at 100 1C overnight, to 1.3704(17) Å), the values indicate effective electron delocaliza-
Scheme 1, affording the respective complexes 3 and 4 in good to very tion throughout the N–C–N backbone in both cases. The
good yields as colourless, partially crystalline powders using a simple aluminium-aluminium separation in the solid state increases
work-up procedure. The complexes 3 and 4 have been characterized proportional to the number of carbon atoms in the linker moiety
1
by elemental analyses, H and 13C NMR as well as IR spectroscopy. ranging from 6.2911(7) Å (3a) to 8.2085(6) Å (4c).
Notably, 27Al NMR resonances could not been detected despite
Due to our interest in Group 13 analogues of bis(carbene)s16
extended numbers of scans; most likely, this is caused by the low we intended to convert the dinuclear methyl aluminium
symmetry of the aluminium centers, which causes considerable line- bis(amidinate) and bis(guanidinate) complexes 3 and 4 by iodiza-
broadening due to quadrupolar interactions.12 Common features tion using elemental iodide to the related dinuclear aluminium
observed in the respective 1H NMR spectra include one singlet for iodide complexes 5 and 6. While bis(amidinate) complexes 5 could
the aluminium methyl groups (d between ꢀ0.90 and ꢀ0.21 ppm) be obtained in acceptable to good yields, 6b was the only
as well as one septet and two doublets for iso-propyl groups of the bis(guanidinate) complex that we were able to isolate although
2,6-diisopropylphenyl substituent, a pattern consistent with confor- only in poor yield, Scheme 2. Possible side reactions of the strongly-
mational averaging on the NMR time scale at room temperature.
basic and nucleophilic ligand framework with electrophilic methyl
Furthermore, we were able to obtain single crystals suitable iodide formed during the reaction may account for this. Attempts
1
for an X-ray diffraction analysis of the species 3a, 3c, and 4a–c to follow this side reaction by H NMR were not successful and
and their molecular structures in the solid state are shown in gave complex spectra with broadened and overlapping resonances.
Fig. 2 and 3. The Al–C bond lengths with values in between
Besides the absence of the Al(CH3)2 resonances, the room-
1.9507(14) and 1.9692(16) Å are comparable to those reported temperature 1H NMR spectra resemble by and large those of the
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Fig. 3 Solid-state structure (hydrogen atoms are omitted for the sake of clarity) with selected bond lengths [Å] and angles [1] of: (a) 4a: Al1–Al10
6.3264(9), Al1–C18 1.9651(16), Al1–C19 1.9653(14), Al1–N1 1.9339(11), Al1–N2 1.9203(11), N1–C13 1.3543(16), N2–C13 1.3489(16), N3–C13 1.3484(16),
N1–Al1–N2 69.54(5), N1–C13–N2 108.8(1), N1–C13–N3 125.87 (11), N2–C13–N3 125.33(11), C18–Al1–C19 118.19(7); (b) 4b: Al1–Al2 6.5520(6),
Al1–C18 1.9691(16), Al1–C19 1.9553(17), Al1–N1 1.9271(13), Al1–N2 1.9196(13), N1–C13 1.3704(17), N2–C13 1.3421(18), N3–C13 1.3439(17), N1–Al1–N2
69.89(5), N1–C13–N2 108.64(11), N1–C13–N3 122.84(12), N2–C13–N3 128.50(13), C18–Al1–C19 117.70(7); (c) 4c: Al1–Al10 8.2085(6),
Al1–C18 1.9561(14), Al1–C19 1.9645(14), Al1–N1 1.929(1), Al1–N2 1.919(1), N1–C13 1.3549(15), N2–C13 1.3466(15), N3–C13 1.3441(15), N1–Al1–N2
69.69(4), N1–C13–N2 108.94(10), N1–C13–N3 124.93(10), N2–C13–N3 126.11(10), C18–Al1–C19 116.17(6).
Scheme 2 Synthesis of the dinuclear aluminium iodide complexes 5 and 6 from the respective alkyl aluminium bis(amidinate) and bis(guanidinate)
complexes. Dipp = 2,6-diisopropylphenyl.
dinuclear methyl aluminium complexes 3 and 4 and the simple related amidinate (Al–N 1.884(7) and 1.889(8) Å) and guanidinate
set of resonances associated with the 2,6-diisopropylphenyl complexes (Al–N 1.886(5) and 1.896(5) Å). In consequence, the
groups indicates a symmetric or averaged structure in solution. N–Al–N bite angles of 5a–5d (70.21(17) to 71.2(2)1) and 6b
In agreement with previous reports on dinuclear aluminium (72.21(7)1) are more obtuse. The Al–Al distances, finally, are only
iodide complexes,17 27Al NMR resonances could not been slightly longer than those of 3 and 4. In direct comparison of 5c
detected. In addition to the full characterization by elemental and 5d, twisting in case of the latter causes a significant shorter
1
analyses, H and 13C NMR as well as IR spectroscopy, we were Al–Al separation of 6.903(2) Å relative to 8.1977(14) Å for the former.
able to isolate single-crystals suitable for X-ray diffraction
analysis of the complexes 5a–d, and 6b. Their molecular plexes are also directly available from the protio-ligand. Indeed,
structures in the solid state are shown in Fig. 4 and 5a. reacting the bis(amidine) 1d with ethyl aluminium dichloride
We finally wondered if dinuclear aluminium halide com-
The aluminium–iodine bond lengths of the bis(amidinate) gives rise to the dinuclear aluminium chloride complex 7d by
complexes 5a–5d (2.4752(15) to 2.523(2) Å) are slightly longer elimination of ethane in almost quantitative yields, Scheme 3.
compared to their mononuclear relatives (2.438(3) and 2.481(3) Å).18 It readily crystallizes from toluene and its solid-state structure
In case of the bis(guanidinate) complex 6b the Al–I distances is given in Fig. 5b. The Al–Cl (2.0981(6) and 2.1143(5) Å) and
(2.5027(6) and 2.5076(6) Å) resemble values of the respective Al–N bond lengths (1.8667(13) and 1.9001(10) Å) as well as the
guanidinate complexes (2.5029(19) to 2.5169(16) Å),19 but N–Al–N bite angles (70.28(5) and 70.59(5)1) are similar to those
are slightly longer as compared to the only other yet reported for amidinate aluminium chloride complexes (Al–Cl
reported dinuclear bis(guanidinate) aluminium iodide complex 2.1018(14) to 2.1036(14) Å, Al–N 1.863(3) to 1.872(3) Å, N–Al–N
(2.4555(18) and 2.477(2) Å).15 By substituting the methyl for the 70.911).13a The Al–Al distance amounts to 7.4043(5) Å and is
iodo rests, the Al–N bond lengths of 5a–5d (1.868(4) to 1.896(3) Å) thus about 0.5 Å longer compared to 5d.
and 6b (1.8703(16) and 1.8757(2) Å) are shortened due to the
Attempts to obtain dinuclear aluminium(I) complexes by
increased polarization and match by and large reported values of reducing the dinuclear complexes 5c, 5d and 7d using either
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alkylene-bridged complexes 5a–c, no significant impact of the
linker group was observed while in case of the 1,3-xylylene-
bridged complex 5d no reduction event was observed above
ꢀ3.5 V. However, the impact of the backbone is well illustrated
by comparing 5b (ꢀ3.25 V) and 6b (ꢀ3.07 V). Notably, the
estimated value of potassium in THF amounts to ꢀ3.24 V,
which may explains the unsuccessful reduction experiments.
Ring-opening polymerization of e-caprolactone and L-lactide
The dinuclear methyl aluminium complexes 3 and 4 have been
tested as initiators for the ring-opening polymerization of
L-lactide (LLA) and e-caprolacton (eCL). While alcohols are
regularly used as activators of dinuclear alkylaluminium
complexes,6a,7f,i previous reports on mononuclear dialkyl alu-
minium amidinate complexes mention a rapid alcoholysis.13b
Hence, we first probed the stability of the four-membered
AlCN2-metallacycles towards protonolysis. Adding one or two
equivalents of benzyl alcohol to the complexes 3b and 4b,
respectively, Fig. 6, affords the related protio-ligands and
predominantly dimethyl aluminium benzyloxylate as evidenced
by the distinct 1H NMR shifts, Fig. S70–S75 (ESI†).20 In con-
sequence, and in agreement with previous reports on dialkyl
aluminium amidinate complexes,13b Me2Al(OR) (R = benzyl) is
formed and the complexes 3 and 4 serve only as alkyl aluminium
precursors. Most likely, this behaviour is due to the less stable four-
membered chelate ring in contrast to its five- and six-membered
relatives.
Fig. 4 Solid-state structure (hydrogen atoms are omitted for the sake of
clarity) with selected bond lengths [Å] and angles [1] of: (a) 5a: Al1–Al10
6.3599(16), Al1–I1 2.4902(12), Al1–I2 2.5005(8), Al1–N1 1.892(3),
Al1–N2 1.872(3), N1–C13 1.356(4), N2–C13 1.339(4), N1–Al1–N2 70.55(11),
N1–C13–N2 107.6(3), I1–Al1–I2 113.85(4); (b) 5b: Al1–Al2 6.4993(18),
Al1–I1 2.4800(12), Al1–I2 2.5037(9), Al1–N1 1.896(3), Al1–N2 1.880(3),
N1–C13 1.347(5), N2–C13 1.349(4), N1–Al1–N2 70.71(14), N1–C13–N2
108.3(3), I1–Al1–I2 112.07(4); (c) 5c: Al1–Al10 8.1977(14), Al1–I1 2.4925(8),
Al1–I2 2.4816(7), Al1–N1 1.894(2), Al1–N2 1.869(2), N1–C13 1.350(3),
N2–C13 1.346(3), N1–Al1–N2 70.60(9), N1–C13–N2 107.5(2), I1–Al1–I2
112.22(3); (d) 6b: Al1–Al10 6.5975(10), Al1–I1 2.5076(6), Al1–I2 2.5027(6),
Al1–N1 1.8703(16), Al1–N2 1.8757(17), N1–C13 1.369(2), N2–C13 1.356(2),
N3–C13 1.334(2), N1–Al1–N2 72.21(7), N1–C13–N2 108.19(15), N1–C13–N3
124.24(16), N2–C13–N3 127.56(17), I1–Al1–I2 110.16(2).
In previous reports, dinuclear aluminium alkyl amidinate
complexes have been successfully applied for the ROP of
e-caprolactone and L-lactide with and without the addition of
an activator under various conditions.7g Aiming to test the
catalytic performance of the complexes 3 and 4, we adopted
the experimental conditions and the results for e-caprolactone
and L-lactide are subsequently discussed in the following.
e-caprolactone was polymerized at 70 1C in toluene for eight
hours using a catalyst–monomer ratio of 1 : 200 and a monomer
concentration of 1.4 mol Lꢀ1, Table 1. Based on the obtained
material, conversions ranging from 75 to 90% are recognized
and higher values are associated with the bis(amidinate)s 3 in
direct comparison with their bis(guanidinate) relatives 4. The size-
exclusion chromatography (SEC) traces, Fig. S92–S99 (ESI†),
possess multimodal distributions showing predominantly
Fig. 5 Solid-state structure (hydrogen atoms are omitted for the sake of
clarity) with selected bond lengths [Å] and angles [1] of: (a) 5d: Al1–Al2 6.903(2),
Al1–I1 2.4752(15), Al1–I2 2.5209(14), Al1–N1 1.895(6), Al1–N2 1.875(4),
N1–C13 1.354(7), N2–C13 1.350(8), N1–Al1–N2 71.2(2), N1–C13–N2 108.4(5),
I1–Al1–I2 111.22(6); (b) 7d: Al1–Al2 7.4043(5), Al1–Cl1 2.0981(6),
Al1–Cl2 2.0987(6), Al1–N1 1.8860(12), Al1–N2 1.8673(13), N1–C13 1.3456(17),
N2–C13 1.3474(17), N1–Al1–N2 70.59(5), N1–C13–N2 107.28(12), Cl1–Al1–Cl2
112.79(3).
Scheme 3 Synthesis of the dinuclear aluminium bis(amidinate) chloride
complex 7d by ethane elimination. Dipp = 2,6-diisopropylphenyl.
magnesium powder, cobaltocene or potassium mirror
remained unsuccessful. As very low reduction potential have
been reported for dinuclear aluminium bis(b-diketiminate)
halide complexes,17 we performed cyclic voltammetry experi-
ments with the complexes 5a–d and 6b. Irreversible reduction
events with potentials (referenced to the Fc/Fc+ redox couple)
between ꢀ3.20 and ꢀ3.25 V, which are even lower than those
of [{CH(CMeNDipp)2}AlI2] (ꢀ3.16 V),17 were observed for the
bis(amidinate) complexes, Fig. S1–S6 (ESI†). Among the
Fig. 6 Excerpts of the 1H NMR spectra of (a) 3b and (b) 4b in C6D6 before
(blue) and after the addition of one (green) and two (red) equivalents of
benzyl alcohol, respectively.
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Table 1 ROP of e-caprolactone catalysed by the dinuclear complexes 3
and 4. Reaction conditions: toluene 2.1 mL, 8h, 70 1C, catalyst–monomer
ratio = 1 : 200, [catalyst] = 7 ꢁ 10ꢀ3 mol Lꢀ1, [monomer] = 1.4 mol Lꢀ1. SEC
data are relative to polystyrene standard in THF and refer to the fraction
with a molar mass 4 2000 Da
to be best within their complex class. The increased conversion
also affected the thus obtained poly-L-lactide (PLLA) and higher
molecular weights were hence observed at 90 1C. In most of the
cases, the experimental molecular weights are in good agree-
ment with the calculated values for one polymer chain growing
per metal centre. However, the molecular weights of the poly-
mers obtained using 3b, 3c or 4b at 80 1C as well as 3b, 4b or 4d
at 90 1C are 1.3 to 1.8 times higher than the calculated values
for one polymer chain per metal centre suggesting a concerted
chain propagation. Except the case of 3b, the increased conver-
sions at 90 1C come along with larger polydispersity indices
ranging from 1.35 to 2.31. More pronounced back-biting and
transesterification reactions are likely to account for the broader
molecular weight distributions. The experimental findings
illustrate well the crucial impact of both, the bridging group
and the ligands’ backbone. At 90 1C, the bis(guanidinate)s
4 gave rise to polymers with higher polydispersities as com-
pared to the related bis(amidinate)s 3. With respect to the
linker length, the propylene-bridged complexes 3b and 4b,
afforded products with the highest number average molar mass,
while the most narrow polydispersity indices among each com-
pound class are associated with the ethylene-bridged bis(amidinate)
3a (1.35) and the propylene-bridged bis(guanidinate) 4b (2.17).
Notably, an increase of the PDI with enlarging the linker length
has previously reported for bimetallic salen aluminium
complexes.7f However, the data reported in here do not allow
deducing a general trend.
Mn calca
Mn SEC
Ð SEC
(Mw/Mn)
Entry
Cat.
Conv. (%)
(g molꢀ1
)
(g molꢀ1
)
1
2
3
4
5
6
7
8
3a
3b
3c
3d
4a
4b
4c
4d
82
90
89
88
78
81
75
81
9.36 ꢁ 103
1.03 ꢁ 104
1.02 ꢁ 104
1.00 ꢁ 104
8.90 ꢁ 103
9.25 ꢁ 103
8.56 ꢁ 103
9.25 ꢁ 103
2.88 ꢁ 103
3.67 ꢁ 103
6.93 ꢁ 103
6.06 ꢁ 103
6.94 ꢁ 103
6.14 ꢁ 103
6.02 ꢁ 103
6.36 ꢁ 103
16.68
12.50
12.45
9.56
5.91
5.96
5.67
6.26
a
Calculated Mn = [monomer/Al] ꢁ (conversion/100) ꢁ MW eCL.
polymeric but also oligomeric products. A number average molar
mass Mn between 17.000 and 23.400 Da and rather large poly-
dispersities Ð ranging from 2.07 to 2.39 were obtained when
analysing the polymeric peak above 2000 Da. Overall, the
bis(amidinate)s 3 give rise to higher Mn values and except of
3d also to smaller polydispersity indices (PDIs). Notably, using 3c
affords the polymer with the lowest PDI and the highest molecular
weight Mn. When comparing 3 and 4, no distinct trend with
respect to the linker length can be recognized but within each
subset differences are observed. For the bis(amidinate)s 3, the
bridges containing more than two carbon atoms give rise to higher
conversions than 3a. Furthermore, the number average molar
mass of the polycaprolactone (PCL) obtained by using the 1,4-
butylene and 1,3-xylylene-bridged complexes 3c and 3d are signifi-
cantly higher compared to the catalysts containing smaller bridges.
In contrast, the impact of the linker on conversion and product
properties is only marginal in case of the bis(guanidinate)s 4. In
comparison with previous reports,7g both 3 and 4 afford products
with broader dispersities and smaller Mn values.
Experimental
General considerations
All preparations were performed under an inert atmosphere of
dinitrogen by means of standard Schlenk-line techniques,
while the samples for analytics were handled in a glovebox
(GS-Systemtechnik and MBraun). Traces of oxygen and moist-
ure were successively removed from the inert gas by passing it
over a BASF R 3-11 (CuO/MgSiO3) catalyst, through concen-
trated sulfuric acid, over coarsely granulated silica gel, and
finally P4O10. Dichloromethane, diethyl ether, and n-pentane
were freshly collected from a solvent purification system by
M. Braun (MB SPS-800). Benzene-d6 and toluene were used as
The ring-opening polymerization of L-lactide was performed
in toluene (monomer concentration of 2.0 mol Lꢀ1) at 80 and
90 1C using a catalyst–monomer ratio of 1 : 200, Table 2. After
25 hours, the reactions were quenched and the products
1
analysed by H NMR spectroscopy and SEC. Increased conver-
sions and yields were observed for the reactions at 90 1C as
compared to those at 80 1C and catalysts 3b and 4c were found
Table 2 ROP of L-lactide catalysed by the dinuclear complexes 3 and 4. Reaction conditions: toluene 1.5 mL, 25 h, catalyst–monomer ratio = 1 : 200,
[catalyst] = 1 ꢁ 10ꢀ2 mol Lꢀ1, [monomer] = 2.0 mol Lꢀ1. SEC data are relative to polystyrene standard in THF
Temp. Conv.a
Mn calcb
Mn SECb
Ð SEC
(Mw/Mn) Entry Cat. (1C)
Temp. Conv.a
Mn calcb
Mn SECb
Ð SEC
(Mw/Mn)
Entry Cat. (1C)
(%)
Yield (g molꢀ1
)
(g molꢀ1
)
(%)
Yield (g molꢀ1
)
(g molꢀ1
)
1
2
3
4
5
6
7
8
3a
3a
3b
3b
3c
3c
3d
3d
80
90
80
90
80
90
80
90
35
83
35
92
39
90
36
86
25
83
29
88
35
88
24
83
5.04 ꢁ 103 5.27 ꢁ 103 1.18
1.20 ꢁ 104 1.35 ꢁ 104 1.35
5.04 ꢁ 103 8.09 ꢁ 103 1.66
1.33 ꢁ 104 1.78 ꢁ 104 1.58
5.62 ꢁ 103 9.02 ꢁ 103 1.43
1.30 ꢁ 104 1.41 ꢁ 104 1.80
5.19 ꢁ 103 4.94 ꢁ 103 1.17
1.24 ꢁ 104 9.34 ꢁ 103 1.59
9
4a
4a
4b
4b
4c
4c
4d
4d
80
90
80
90
80
90
80
90
45
84
40
83
48
86
24
68
41
79
35
80
37
80
11
62
6.49 ꢁ 103 7.94 ꢁ 103 2.08
1.21 ꢁ 104 1.29 ꢁ 104 2.22
5.77 ꢁ 103 1.02 ꢁ 104 1.48
1.20 ꢁ 104 2.12 ꢁ 104 2.15
6.92 ꢁ 103 8.61 ꢁ 103 1.53
1.24 ꢁ 104 1.33 ꢁ 104 2.31
3.46 ꢁ 103 8.49 ꢁ 103 1.35
9.80 ꢁ 103 1.43 ꢁ 104 2.17
10
11
12
13
14
15
16
a
b
Determined by 1H NMR spectroscopy. Calculated Mn = [monomer/Al] ꢁ (conversion/100) ꢁ MW L-lactide.
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3
p.a. grade and were distilled from Na/benzophenone prior to use. (sept, JHH = 6.8 Hz, 4H, CH(CH3)2), 3.65 (t, JHH = 6.4 Hz, 4H,
CDCl3 was dried by distillation from calcium hydride. Pyrroli- CH2(CH2)CH2), 7.04–7.13 (m, 6H, C6H3); 13C{1H} NMR (101 MHz,
dine, lead(II) oxide, pivaloylchloride, PCl5, 1,2-ethylenediamine, CDCl3): d (ppm) = ꢀ9.7 (Al(CH3)2), 23.3 (CH(CH3)2), 26.2
1,3-diaminopropane, 1,4-diaminobutane, 1,3(aminomethyl)ben- (CH(CH3)2), 28.0 (CH(CH3)2), 28.7 (C(CH3)3), 34.1 (CH2(CH2)CH2),
zylamine, 2,6-diisopropylaniline, trimethylaluminium (2 M in 39.9 (C(CH3)3), 43.5 (CH2(CH2)CH2), 123.4 (m-C6H3), 125.0 (p-
toluene), iodine and ethylaluminium dichloride (1.8 M in toluene) C6H3), 140.2 (o-C6H3), 144.2 (i-C6H3), 180.2 (NC(C(CH3)3)N); 27Al
were purchased from Sigma-Aldrich. The bis(amidines) (1a, 1b, NMR (104 MHz, CDCl3): d (ppm) = no signal observed; IR (ATR):
1c)4ae as well as the bis(guanidines) (2a, 2b, 2c, 2d)4ae,5d,10 were n~ [cmꢀ1] = 2958 (m), 2927 (vw), 2873 (w), 1416 (s), 1335 (m), 1191
prepared according to published procedures. Further information (m), 775 (m), 729 (m), 669 (s); anal. calc. (found) for [C41H70N4Al2]:
are given in the ESI.†
C 73.43 (73.39), H 10.56 (10.55), N 8.16 (8.26).
3c: A solution of trimethylaluminium (2.00 mL, 4.0 mmol,
2.0 M in toluene) was added dropwise to a stirred solution of 1c
Characterization
The NMR spectra were recorded with Bruker Avance 300 and (1.15 g, 2.0 mmol) in toluene (20 mL) at rt, followed by stirring
400 spectrometers (T = 300 K) with d referenced to external at 100 1C overnight. The product crystallized upon slow cooling
tetramethylsilane (1H and 13C) and aluminium(III) nitrate (27Al). to rt over 1 h. After solvent filtration the white crystalline powder
1H and 13C NMR spectra were calibrated by using the solvent was washed with toluene (2 ꢁ 5 mL) to afford 3c (1.04 g, 1.5 mmol,
residual peak (CHCl3: d(1H) = 7.26) or (C6D6: d(1H) = 7.16) and the 75%) as a white crystalline powder. 1H NMR (300 MHz, CDCl3):
solvent peak (CDCl3: d(13C) = 77.16) or (C6D6: d(13C) = 128.06), d (ppm) = ꢀ0.69 (s, 12H, Al(CH3)2), 1.12–1.16 (m, 30 H, C(CH3)3,
respectively. 27Al NMR spectra were calibrated relative to external CH(CH3)2), 1.26–1.29 (m, 12H, CH(CH3)2), 1.66 (quint, 3JHH = 3.3 Hz,
Al(NO3)3ꢂ9H2O. Notably, the broad resonance at about 60 ppm is 4H, CH2(CH2)2CH2), 3.27 (sept, 3JHH = 6.9 Hz, 4H, CH(CH3)2), 3.53 (t,
a background signal associated with the probe. IR spectra were 3JHH = 6.0 Hz, 4H, CH2(CH2)2CH2), 7.03–7.20 (m, 6H, C6H3); 13C{1H}
recorded with a Bruker ALPHA spectrometer equipped with a NMR (101 MHz, CDCl3): d (ppm) = ꢀ9.7 (Al(CH3)2), 23.4 (CH(CH3)2),
diamond ATR unit. Elemental analysis was performed with a 26.2 (CH(CH3)2), 28.0 (C(CH3)3), 28.8 (CH(CH3)2), 30.2 (C(CH3)3), 39.8
Vario MICRO cube (Elementar Analysensysteme GmbH); the (CH2(CH2)2CH2), 46.6 (CH2(CH2)2CH2), 123.4 (m-C6H3), 125.0
presence of residual solvent molecules was verified by 1H NMR (p-C6H3), 140.3 (o-C6H3), 144.4 (i-C6H3), 179.6 (NC(C(CH3)3)N); 27Al
spectroscopy. NMR and IR spectra are given in the ESI,† Fig. S8–S70. NMR (104 MHz, CDCl3): d (ppm) = no signal observed; IR (ATR):
3a: A solution of trimethylaluminium (3.84 mL, 7.68 mmol, n~ [cmꢀ1] = 2958 (m), 2939 (w), 2872 (w), 1419 (s), 1351 (s), 1188 (m),
2.0 M in toluene) was added dropwise to a stirred solution of 1a 774 (m), 727 (m), 670 (s); anal. calc. (found) for [C42H72N4Al2]: C 73.17
(2.00 g, 3.66 mmol) in toluene (15 mL) at rt, followed by stirring (73.38), H 10.48 (10.25), N 8.33 (8.09).
at 100 1C overnight. After slowly cooling to rt colourless crystals
3d: A solution of trimethylaluminium (15.0 mL, 30.0 mmol,
grew from the solution, which were separated and further 2.0 M in toluene) was added dropwise to a stirred solution of 1d
washed with pentane (3 ꢁ 10 mL) to obtain 3a (2.22 g, (9.34 g, 15.0 mmol) in toluene (60 mL) at rt, followed by stirring
3.37 mmol, 92%) as a white crystalline solid. 1H NMR (400 at 100 1C overnight. All volatiles were removed en vacuo and the
MHz, C6D6): d (ppm) = ꢀ0.21 (s, 12H, Al(CH3)2), 1.05 (s, 18H, orange solid was recrystallized from Et2O to obtain 3d (10.3 g,
3
C(CH3)3,), 1.22–1.25 (m, 24H, CH(CH3)2), 3.41 (sept, JHH = 6.8 14.1 mmol, 94%) as a white crystalline solid. 1H NMR (400 MHz,
Hz, 4H, CH(CH3)2), 3.86 (s, 4H, CH2CH2), 7.00–7.10 (m, 6H, C6D6): d (ppm) = ꢀ0.74 (s, 12H, Al(CH3)2), 1.32 (d, 3JHH = 6.9 Hz,
3
C6H3); 13C{1H} NMR (101 MHz, C6D6): d (ppm) = ꢀ9.1 (Al(CH3)2), 12H, CH(CH3)2), 1.41 (s, 18H, C(CH3)3), 1.48 (d, JHH = 6.9 Hz,
23.4 (CH(CH3)2), 26.3 (CH(CH3)2), 28.3 (CH(CH3)2), 28.5 12H, CH(CH3)2), 3.49 (sept, 3JHH = 6.8 Hz, 4H, CH(CH3)2), 4.90 (s,
(C(CH3)3), 40.0 (C(CH3)3), 48.9 (CH2CH2), 123.8 (m-C6H3), 125.9 4H, CH2C6H4CH2), 7.23–7.60 (m, 10H, C6H3 + C6H4); 13C{1H}
(p-C6H3), 140.0 (o-C6H3), 144.3 (i-C6H3), 180.4 (NC(C(CH3)3)N); NMR (101 MHz, C6D6): d (ppm) = ꢀ9.4 (Al(CH3)2), 23.5
27Al NMR (104 MHz, C6D6): d (ppm) = no signal observed; IR (CH(CH3)2), 26.4 (CH(CH3)2), 28.2 (C(CH3)3), 28.8 (CH(CH3)2),
(ATR): n~ [cmꢀ1] = 2957 (m), 2928 (w), 2869 (w), 1412 (s), 1298 (s), 39.8 (C(CH3)3), 51.2 (CH2C6H4CH2), 123.8 (m-C6H3), 125.8
1186 (s), 773 (s), 720 (s), 673 (s); anal. calc. (found) for (p-C6H3), 127.7 (m-C6H4), 128.7 (o-C6H4), 129.4 (o-C6H4), 140.3
[C40H68N4Al2]: C 73.76 (73.47), H 10.32 (10.02), N 8.11 (8.35).
(o-C6H3), 140.8 (i-C6H4), 144.3 (i-C6H3), 179.8 (NC(C(CH3)3)N);
3b: A solution of trimethylaluminium (15.0 mL, 30.0 mmol, 27Al NMR (104 MHz, CDCl3): d (ppm) = no signal observed; IR
2.0 M in toluene) was added dropwise to a stirred solution of 1b (ATR): n~ [cmꢀ1] = 2961 (m), 2921 (w), 2867 (w), 1413 (s), 1321 (s),
(8.42 g, 15.0 mmol) in toluene (60 mL) at rt, followed by stirring 1184 (s), 669 (s); anal. calc. (found) for [C46H72N4Al2]: C 75.16
at 100 1C overnight. After slowly cooling to rt a crystalline solid (75.11), H 9.87 (9.65), N 7.62 (7.61).
precipitated, which was separated and further washed with
4a: A solution of trimethylaluminium (1.24 mL, 2.49 mmol,
hexane (3 ꢁ 10 mL). A second crop of crystals was obtained by 2.0 M in toluene) was added dropwise to a stirred solution of 2a
concentration of the mother solution followed by cooling to (680 mg, 1.18 mmol) in toluene (10 mL) at rt, followed by
ꢀ20 1C. Repeated washing with toluene gave 3b (9.05 g, 13.4 mmol, stirring at 105 1C overnight. Solvent concentration lead to
89%) as a white crystalline solid. 1H NMR (400 MHz, CDCl3): crystallization at rt overnight to obtain 4a (650 mg, 0.99 mmol,
3
1
d (ppm) = ꢀ0.66 (s, 12H, Al(CH3)2), 1.14 (d, JHH = 6.7 Hz, 12H, 80%) as a white crystalline solid. H NMR (400 MHz, CDCl3):
3
CH(CH3)2), 1.17(s, 18H, C(CH3)3,), 1.27 (d, JHH = 6.7 Hz, 12H, d (ppm) = ꢀ0.72 (s, 12H, Al(CH3)2), 1.13 (d, 3JHH = 6.8 Hz, 12H,
3
3
CH(CH3)2), 1.84 (quint, JHH = 6.3 Hz, 2H, CH2(CH2)CH2), 3.25 CH(CH3)2), 1.19 (d, JHH = 6.9 Hz, 12H, CH(CH3)2), 1.71–1.74
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(m, 8H, N(CH2)2(CH2)2), 3.09–3.14 (m, 8H, N(CH2)2(CH2)2), 3.33 at 105 1C overnight. All volatiles were removed under vacuum to
3
(sept, JHH = 6.8 Hz, 4H, CH(CH3)2), 3.44 (s, 4H, CH2CH2), obtain a yellow solid, which was further washed with pentane
7.04–7.10 (m, 6H, C6H3); 13C{1H} NMR (101 MHz, CDCl3): (4 ꢁ 5 mL). Recrystallization in DCM at rt overnight gave the
d (ppm) = ꢀ9.5 (Al(CH3)2), 23.0 (CH(CH3)2), 25.7 (N(CH2)2 product 4d (2.16 g, 2.83 mmol, 92%) as clear colorless crystals.
(CH2)2), 25.9 (CH(CH3)2), 27.7 (CH(CH3)2), 47.5 (CH2CH2), 48.6 1H NMR (400 MHz, CDCl3): d (ppm) = ꢀ0.90 (s, 12H, Al(CH3)2),
3
3
(N(CH2)2(CH2)2), 123.3 (m-C6H3), 124.5 (p-C6H3), 139.4 (o-C6H3), 1.15 (d, JHH = 6.8 Hz, 12H, CH(CH3)2), 1.20 (d, JHH = 6.9 Hz,
144.9 (i-C6H3), 162.5 (NC(N(CH2)2(CH2)2)3)N; 27Al NMR (104 MHz, 12H, CH(CH3)2), 1.62–1.65 (m, 8H, N(CH2)2(CH2)2), 3.04–3.07
3
CDCl3): d (ppm) = no signal observed; IR (ATR): n~ [cmꢀ1] = 2963 (m, 8H, N(CH2)2(CH2)2), 3.40 (sept, JHH = 6.8 Hz, 4H,
(m), 2924 (w), 2869 (vw), 1575 (w), 1461 (m), 1422 (s), 1335 (m), CH(CH3)2), 4.44 (s, 4H, CH2(C6H4)CH2), 7.03–7.11 (m, 6H,
1183 (s), 703 (s), 665 (vs); anal. calc. (found) for [C40H66N6Al2]: C6H3), 7.29–7.38 (m, 4H, CH2(C6H4)CH2); 13C{1H} NMR (101
C 70.14 (69.65), H 9.71 (9.23), N 12.27 (12.91).
MHz, CDCl3): d (ppm) = ꢀ10.3 (Al(CH3)2), 23.0 (CH(CH3)2), 25.6
4b: A solution of trimethylaluminium (2.10 mL, 4.19 mmol, (N(CH2)2(CH2)2), 25.9 (CH(CH3)2), 27.7 (CH(CH3)2), 48.5
2.0 M in toluene) was added dropwise to a stirred solution of 2b (N(CH2)2(CH2)2), 49.7 (CH2(C6H4)CH2) 123.3 (m-C6H3), 124.5
(1.17 g, 1.99 mmol) in toluene (10 mL) at rt, followed by stirring (p-C6H3), 125.5 (o-C6H4), 126.1 (o-C6H4), 128.8 (m-C6H4),
at 105 1C overnight. Solvent concentration followed by layering 139.5 (o-C6H3),141.9 (i-C6H4), 144.8 (i-C6H3), 162.8
with pentane lead to crystallization at rt overnight to obtain 4b (NC(N(CH2)2(CH2)2)3)N; 27Al NMR (104 MHz, CDCl3): d (ppm)
(1.21 g, 1.73 mmol, 87%) as clear colorless crystals. 1H NMR = no signal observed; IR (ATR): n~ [cmꢀ1]= 2960 (m), 2926 (m),
(400 MHz, CDCl3): d (ppm) = ꢀ0.72 (s, 12H, Al(CH3)2), 1.12 2889 (w), 2865 (w), 1519 (s), 1418 (s), 1332 (s), 1177 (m), 773 (m),
3
3
(d, JHH = 6.8 Hz, 12H, CH(CH3)2), 1.19 (d, JHH = 6.9 Hz, 12H, 709 (m), 666 (s); anal. calc. (found) for [C46H70N6Al2]: C 72.60
CH(CH3)2), 1.69–1.72 (m, 8H, N(CH2)2(CH2)2), 1.85 (qui, 2H, (72.30), H 9.27 (8.86), N 11.04 (10.72).
CH2(CH2)CH2, 3.08–3.11 (m, 8H, N(CH2)2(CH2)2), 3.30–3.37
5a: Iodine (2.31 g, 9.10 mmol) was added to a stirred
(m, 8H, CH(CH3)2) + CH2(CH2)CH2), 7.03–7.07 (m, 6H, C6H3); solution of 3a (1.20 g, 1.82 mmol) in toluene (10 mL) followed
13C{1H} NMR (101 MHz, CDCl3): d (ppm) = ꢀ9.6 (Al(CH3)2), 23.0 by stirring for eight days at rt. All volatiles were removed en vacuo
(CH(CH3)2), 25.7 (N(CH2)2(CH2)2), 25.9 (CH(CH3)2), 27.7 and the dark brown solid was extracted with DCM (2 ꢁ 10 mL).
(CH(CH3)2), 33.4 (CH2(CH2)CH2), 43.6 (CH2(CH2)CH2), 48.6 Solvent removal and recrystallization from DCM gave brownish
(N(CH2)2(CH2)2), 123.2 (m-C6H3), 124.5 (p-C6H3), 139.5 crystals which were further washed with pentane (5 ꢁ 10 mL).
(o-C6H3), 144.8 (i-C6H3), 162.7 (NC(N(CH2)2(CH2)2)3)N; 27Al A second crop of crystals was obtained by concentration of
NMR (104 MHz, CDCl3): d (ppm) = no signal observed; IR the mother solution, crystallization and washing with pentane
(ATR): n~ [cmꢀ1] = 2956 (m), 2925 (m), 2884 (w), 2864 (w), 1528 (7 ꢁ 10 mL). 5a (724 mg, 0.66 mmol, 36%) was obtained as a pale
1
(s), 1586 (s), 1417 (s), 1327 (m), 1178 (m), 775 (s), 716 (s), 669 (s); brown crystalline solid. H NMR (400 MHz, CDCl3): d (ppm) =
3
anal. calc. (found) for [C41H68N6Al2]: C 70.45 (70.48), H 9.81 1.25 (s, 18H, C(CH3)3), 1.28 (d, JHH = 6.8 Hz, 12H, CH(CH3)2),
(9.54), N 12.02 (11.93).
1.30 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 3.30 (sept, 3JHH = 6.7 Hz,
4c: A solution of trimethylaluminium (5.25 mL, 10.1 mmol, 4H, CH(CH3)2), 4.05 (s, 4H, CH2CH2), 7.11–7.23 (m, 6H, C6H3);
2.0 M in toluene) was added dropwise to a stirred solution of 2c 13C{1H} NMR (101 MHz, CDCl3): d (ppm) = 23.6 (CH(CH3)2), 28.1
(3.00 g, 5.00 mmol) in toluene (10 mL) at rt, followed by stirring (CH(CH3)2), 28.3 (C(CH3)3), 40.4 (C(CH3)3), 47.1 (CH2CH2), 124.2
at 105 1C overnight. Solvent concentration lead to crystal- (m-C6H3), 127.1 (p-C6H3), 136.4 (o-C6H3), 144.8 (i-C6H3), 185.7
lization at rt overnight to obtain 4c (2.97 g, 4.15 mmol, 83%) (NC(C(CH3)3)N); 27Al NMR (104 MHz, CDCl3): d (ppm) = no signal
as clear colorless crystals. 1H NMR (400 MHz, CDCl3): d (ppm) = observed. A small signal at ꢀ24.7 ppm was observed, which
ꢀ0.73 (s, 12H, Al(CH3)2), 1.12 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), account for the formation of [AlI4]ꢀ and might be caused by slow
3
1.18 (d, JHH = 6.9 Hz, 12H, CH(CH3)2), 1.62–1.67 (m, 8H, degradation; IR (ATR): n~ [cmꢀ1] = 2961 (m), 2927 (w), 2867 (w),
CH2(CH2)2CH2), 1.69–1.72 (m, 8H, N(CH2)2(CH2)2), 3.08–3.11 1435 (m), 1387 (s), 1182 (m), 770 (s), 694 (m), 542 (s), 433 (s);
(m, 8H, N(CH2)2(CH2)2), 3.23–3.26 (m, 4H, CH2(CH2)2CH2), 3.33 anal. calc. (found) for [C36H56N4Al2I2]: C 39.08 (39.10), H 5.10
3
(sept, JHH = 6.8 Hz, 4H, CH(CH3)2), 7.03–7.09 (m, 6H, C6H3); (5.15), N 5.06 (4.90).
13C{1H} NMR (101 MHz, CDCl3): d (ppm) = ꢀ9.5 (Al(CH3)2), 23.0
5b: Iodine (6.09 g, 24.0 mmol) was added to a stirred
(CH(CH3)2), 25.7 (N(CH2)2(CH2)2), 25.9 (CH(CH3)2), 27.7 solution of 3b (2.69 g, 4.0 mmol) in DCM (60 mL) followed by
(CH(CH3)2), 34.3 (CH2(CH2)2CH2), 46.2 (CH2(CH2)2CH2), 48.6 stirring for eight days at rt. All volatiles were removed en vacuo
(N(CH2)2(CH2)2), 123.4 (m-C6H3), 124.4 (p-C6H3), 139.5 (o-C6H3), and the dark brown solid was extracted with DCM (80 mL) and
144.8 (i-C6H3), 162.5 (NC(N(CH2)2(CH2)2)3)N; 27Al NMR (104 THF (20 mL). Solvent removal and recrystallization from DCM
MHz, CDCl3): d (ppm) = no signal observed; IR (ATR): gave brownish crystals which were further washed with toluene
n~ [cmꢀ1] = 2960 (m), 2926 (m), 2885 (w), 2867 (w), 1572 (m), (3 ꢁ 10 mL) to afford 5b (1.80 g, 1.6 mmol, 40%) as pale brown
1522 (m), 1444 (m), 1427 (m), 1347 (m), 1180 (m), 783 (s), 706 crystalline solid. 1H NMR (400 MHz, CDCl3): d (ppm) = 1.23
3
(s), 668 (s); anal. calc. (found) for [C42H70N6Al2]: C 70.75 (70.68), (s, 18H, C(CH3)3), 1.25 (d, JHH = 6.7 Hz, 12H, CH(CH3)2), 1.28
H 9.90 (9.43), N 11.79 (11.56).
(d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 2.20 (quint, 3JHH = 5.9 Hz, 2H,
3
4d: A solution of trimethylaluminium (3.24 mL, 6.47 mmol, CH2(CH2)CH2), 3.28 (sept, JHH = 6.7 Hz, 4H, CH(CH3)2), 3.90
3
2.0 M in toluene) was added dropwise to a stirred solution of 2d (t, JHH = 5.9 Hz, 4H, CH2(CH2)CH2), 7.09–7.22 (m, 6H, C6H3);
(2.00 g, 3.08 mmol) in toluene (10 mL) at rt, followed by stirring 13C{1H} NMR (101 MHz, CDCl3): d (ppm) = 23.6 (C(CH3)3), 28.2
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(CH(CH3)2), 28.3 (C(CH3)3), 31.5 (CH2(CH2)CH2), 40.4 (C(CH3)3), 2939 (vw), 2874 (w), 1627 (w), 1586 (w), 1463 (m), 1404 (s), 1333 (s),
43.7 (CH2(CH2)2CH2), 124.1 (m-C6H3), 127.0 (p-C6H3), 1198 (s), 771 (m), 690 (m); anal. calc. (found) for [C42H60N4Al2I4]:
136.6 (o-C6H3), 144.7 (i-C6H3), 186.1 (NC(C(CH3)3)N); C 42.66 (42.32), H 5.11 (5.38), N 4.4.76 (4.30).
27Al NMR (104 MHz, CDCl3): d (ppm) = no signal observed.
6b: Iodine (1.81 g, 7.15 mmol) was added to a stirred
A small signal at ꢀ24.8 ppm was observed, which account for solution of 4b (1.00 g, 1.43 mmol) in toluene (8 mL) followed
the formation of [AlI4]ꢀ and might be caused by slow degrada- by stirring for six days at rt. All volatiles were removed en vacuo
tion; IR (ATR): n~ [cmꢀ1] = 2963 (m), 2918 (w), 2869 (w), 1402 (s), and the dark brown mixture of products was washed with
1330 (s), 1188 (s), 771 (s), 732 (m), 690 (m), 539 (s), 437 (s); anal. pentane (6 ꢁ 5 mL). Recrystallization from a DCM solution
calc. (found) for [C42H72N4Al2ꢂ0.4 DCM]: C 38.91 (38.96), H 5.13 layered with pentane at ꢀ25 1C gave the product 6b as yellowish
(5.14), N 4.85 (4.73).
crystals in very low yields (74 mg, 0.11 mmol, 8%) after three days.
5c: Iodine (4.57 g, 18.0 mmol) was added to a stirred 1H NMR (400 MHz, CDCl3): d (ppm) = 1.21 (dd, 3JHH = 6.8 Hz, 14H,
solution of 3c (2.75 g, 4.0 mmol) in DCM (60 mL) followed by CH(CH3)2), 1.75–1.80 (m, 8H, N(CH2)2(CH2)2), 2.24 (qui, 2H,
stirring for four days at rt. All volatiles were removed en vacuo CH2(CH2)CH2), 3.00–3.29 (m, 8H, N(CH2)2(CH2)2), 3.35 (sept, 3JHH
=
3
and the dark brown solid was extracted with DCM (80 mL) and 6.9 Hz, 4H, CH(CH3)2), 3.58 (t, JHH = 6.9 Hz, 4H, CH2(CH2)CH2),
THF (20 mL). Solvent removal and recrystallization from DCM 7.08–7.19 (m, 6H, C6H3); 13C{1H} NMR (101 MHz, CDCl3): d (ppm) =
gave brownish crystals which were further washed with toluene 23.1 (CH(CH3)2), 25.6 (N(CH2)2(CH2)2), 27.4 (CH(CH3)2), 28.0
(3 ꢁ 10 mL) to afford 5c (3.16 g, 2.8 mmol, 70%) as pale brown (CH(CH3)2), 32.3 (CH2(CH2)CH2), 42.6 (CH2(CH2)CH2), 49.7
crystalline solid. 1H NMR (400 MHz, CDCl3): d (ppm) = 1.20 (N(CH2)2(CH2)2), 123.9 (m-C6H3), 126.4 (p-C6H3), 136.0 (o-C6H3),
3
(s, 18H, C(CH3)3), 1.26 (d, JHH = 6.7 Hz, 12H, CH(CH3)2), 1.31 145.4 (i-C6H3), 163.9 (NC(N(CH2)2(CH2)2)3)N; 27Al NMR (104 MHz,
(d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.95 (quint, 3JHH = 3.3 Hz, 4H, CDCl3): d (ppm) = no signal observed. A small signal at ꢀ24.9 ppm
3
CH2(CH2)2CH2), 3.32 (sept, JHH = 6.8 Hz, 4H, CH(CH3)2), 3.67 was observed, which account for the formation of [AlI4]ꢀ and might
(t, 3JHH = 6.0 Hz, 4H, CH2(CH2)2CH2), 7.09–7.21 (m, 6 H, C6H3); be caused by slow degradation; IR (ATR): n~ [cmꢀ1] = 2960 (m),
13C{1H} NMR (101 MHz, CDCl3): d (ppm) = 23.7 (C(CH3)3), 28.2 2924 (m), 2867 (w), 1546 (s), 1456 (m), 1348 (m), 1327 (m), 778 (m),
(CH(CH3)2), 28.2 (CH(CH3)2), 28.4 (CH(CH3)2), 29.4 732 (m), 638 (s), 428 (m); anal. calc. (found) for [C40H66N6Al2]:
(CH2(CH2)2CH2), 40.3 (C(CH3)3), 46.5 (CH2(CH2)2CH2), 124.1 even after repeated attempts no suitable elemental analysis
(m-C6H3), 126.9 (p-C6H3), 136.8 (o-C6H3), 145.0 (i-C6H3), 185.2 could be obtained.
(NC(C(CH3)3)N); 27Al NMR (104 MHz, CDCl3): d (ppm) = no
7d: A solution of ethylaluminium dichloride (5.6 mL,
signal observed. A small signal at ꢀ24.7 ppm was observed, 10.0 mmol, 1.8 M in toluene) was added dropwise to a stirred
which account for the formation of [AlI4]ꢀ and might be caused solution of 1d (3.11 g, 5.0 mmol) in toluene (40 mL) at rt,
by slow degradation; IR (ATR): n~ [cmꢀ1] = 2961 (m), 2929 (w), followed by stirring at 100 1C overnight. All volatiles were
2868 (w), 1406 (s), 1344 (s), 1186 (m), 773 (s), 728 (m), 435 (s); removed en vacuo and the pale yellow solid was recrystallized
anal. calc. (found) for [C42H72N4Al2ꢂ0.8 DCM]: C 38.76 (38.57), H from toluene to obtain 7d (4.02 g, 4.93 mmol, 98%) as a white
5.16 (5.35), N 4.66 (4.56).
crystalline solid. 1H NMR (400 MHz, CDCl3): d (ppm) = 1.23
3
5d: Iodine (8.12 g, 32.0 mmol) was added to a stirred (d, JHH = 6.7 Hz, 12H, CH(CH3)2), 1.29 (s, 18H, C(CH3)3), 1.32
solution of 3d (5.88 g, 8.0 mmol) in toluene (80 mL) followed (d, 3JHH = 6.7 Hz, 12H, CH(CH3)2), 3.26 (sept, 3JHH = 6.7 Hz, 4H,
by stirring for four days at rt. All volatiles were removed CH(CH3)2), 4.78 (s, 4H, CH2C6H4CH2), 7.09–7.57 (m, 10H,
en vacuo and the dark brown solid was extracted with DCM C6H3 + C6H4); 13C{1H} NMR (101 MHz, CDCl3): d (ppm) = 23.5
(2 ꢁ 40 mL). Solvent removal and recrystallization from DCM gave (C(CH3)3), 26.3 (CH(CH3)2), 28.4 (CH(CH3)2), 28.5 (CH(CH3)2),
brownish crystals which were further washed with pentane 40.0 (C(CH3)3), 50.5 (CH2C6H4CH2), 123.9 (m-C6H3), 126.7
(5 ꢁ 10 mL). A second crop of crystals was obtained by (p-C6H3), 128.0 (m-C6H4), 128.6 (o-C6H4), 129.8 (o-C6H4), 136.9
concentration of the mother solution, crystallization and (o-C6H3), 139.0 (i-C6H4), 144.6 (i-C6H3), 185.7 (NC(C(CH3)3)N);
washing with pentane (7 ꢁ 10 mL). 5d (6.17 g, 5.2 mmol, 27Al NMR (104 MHz, CDCl3): d (ppm) = 104.06; IR (ATR):
65%) was obtained as a pale brown crystalline solid. 1H NMR n~ [cmꢀ1] = 2962 (m), 2927 (w), 2869 (w), 1498 (w), 1416 (vs),
3
(400 MHz, CDCl3): d (ppm) = 1.26 (d, JHH = 6.7 Hz, 12H, 1312 (vs), 1181 (s), 770 (m), 671 (vs); anal. calc. (found) for
CH(CH3)2), 1.29 (s, 18H, C(CH3)3), 1.33 (d, 3JHH = 6.7 Hz, 12H, [C42H60N4Al2I4]: even after repeated attempts no suitable ele-
3
CH(CH3)2), 3.36 (sept, JHH = 6.7 Hz, 4H, CH(CH3)2), 4.83 mental analysis could be obtained.
(s, 4H, CH2C6H4CH2), 7.10–7.70 (m, 10H, C6H3 + C6H4);
13C{1H} NMR (101 MHz, CDCl3): d (ppm) = 23.7 (C(CH3)3),
Catalytic experiments
28.2 (CH(CH3)2), 28.3 (CH(CH3)2), 28.5 (CH(CH3)2), Ring-opening polymerization of e-caprolacton: In a glovebox, a
40.3 (C(CH3)3), 50.6 (CH2C6H4CH2), 124.1 (m-C6H3), 126.9 glass vial equipped with a stir bar was charged with the catalyst
(p-C6H3), 128.9 (m-C6H4), 126.6 (o-C6H4), 130.6 (o-C6H4), (15.2 ꢁ 10ꢀ6 mol, 1 equiv., 0.007 mol Lꢀ1), e-aprolacton (0.3 mL,
136.7 (o-C6H3), 138.0 (i-C6H4), 144.8 (i-C6H3), 185.0 30.3 ꢁ 10ꢀ3 mol, 200 equiv., 1.4 mol Lꢀ1) and 2.1 mL of toluene.
(NC(C(CH3)3)N); 27Al NMR (104 MHz, CDCl3): d (ppm) = no The vial was sealed with a crimp cap and taken out of the
signal observed. A small signal at ꢀ24.6 ppm was observed, glovebox, where the mixture was stirred at 70 1C for 8 h. Then,
which account for the formation of [AlI4]ꢀ and might be the reaction was quenched by the addition of 2 mL of a HCl
caused by slow degradation; IR (ATR): n~ [cmꢀ1] = 2957 (m), solution (5 mol Lꢀ1 in ethanol) and the product was
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NJC
Paper
115, 28; (g) Cooperative Catalysis: Designing Efficient Catalysts
for Synthesis, ed. R. Peters, Wiley, 2015; (h) N. P. Mankad,
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precipitated by adding 5 mL of ethanol. The product was
filtered off, washed with ethanol (2 ꢁ 5 mL) and dried in
vacuum to a constant weight. The SEC traces are shown in
Fig. S76–S91 (ESI†).
´
Lichtenfeld, G. Fernandez, S. Grimme and A. Studer, Chem. –
Eur. J., 2017, 23, 5864; ( j) D. R. Pye and N. P. Mankad, Chem.
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Ring-opening polymerization of L-lactide: in a glovebox, a
glass vial equipped with a stir bar was charged with the catalyst
15.2 ꢁ 10ꢀ6 mol, 1 equiv., 0.01 mol Lꢀ1
, L-lactid (437 mg,
30.3 ꢁ 10ꢀ3 mol, 200 equiv., 2 mol Lꢀ1) and 1.5 mL of toluene.
The vial was sealed with a crimp cap and taken out of the
glovebox, where the mixture was stirred at 80 1C or 90 1C for 25 h.
The reaction was quenched by the addition of 2 mL of a HCl
solution (5 mol Lꢀ1 in ethanol) and the product was precipitated
by adding 5 mL of ethanol. The product was filtered off, washed
with ethanol (2 ꢁ 5 mL) and dried in vacuum to a constant
weight. The SEC traces are shown in Fig. S92–S99 (ESI†).
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Conclusions
In summary, the synthesis and characterization of overall 14
dinuclear aluminium(III) bis(amidinate) and bis(guanidinate)
complexes is reported and the molecular structures in solid
state of eleven complexes was established by X-ray diffraction
analysis. The dinuclear methyl aluminium complexes 3 and 4
are efficient catalysts for the ring-opening polymerization of
e-caprolactone and L-lactide, but undergo protonolysis with alcohols,
which prohibits the use of acidic coinitiators. The bis(amidinate)s 3
were found to outperform their bis(guanidinate) relatives 4 with
respect to activity and selectivity as higher conversions and more
narrow polydispersity indices of both PCL and PLLA were observed
using the former. However, no general trend with respect to the linker
length has been observed.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work is dedicated to the memory of Tanja Hott. The
project was financially supported by the Elite Network of
Bavaria, the Friedrich Schiller University Jena, and the Deutsche
Forschungsgemeinschaft (DFG, KR4782/3-1). We are thankful to
Grit Festag for conducting the SEC measurements.
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