Oxalic amidines-protonation studies and activity in lactide polymerisation

Article abstract of DOI:10.1002/ejic.201200838

The oxalic amidine N,N′-dimethyl-N,N′-diphenyl-1,2-bis[4- tolylimino]ethane-1,2-diamine was used to synthesise a chloridozinc complex and two monoprotonated species either with chloride or triflate counterions. The compounds have been structurally characterised by X-ray crystallography. Proton affinities have been determined by quantum chemical calculations to investigate why only the iminium protonated species is selectively formed upon mild protonation. To better understand the coordination properties of the ligand, different possible coordination modes of the ligand to zinc chloride have been investigated by DFT calculations. The chloridozinc complex was applied as a catalyst in the melt polymerisation of rac-lactide and exhibited high activity combined with excellent agreement between the obtained and calculated molecular weights.

Full text of DOI:10.1002/ejic.201200838

FULL PAPER
DOI:10.1002/ejic.201200838
Oxalic Amidines – Protonation Studies and Activity in
Lactide Polymerisation
Ines dos Santos Vieira,[a] Christina Dietz,[a] Fabian Mohr,[b]
Rainer Beckert,[c] and Sonja Herres-Pawlis*[a,d]
Dedicated to Professor Ernst Anders on the occasion of his 70th birthday
Keywords: Polymers / Polymerization / Ring-opening polymerization / Zinc / N ligands / Sustainable chemistry / Density
functional calculations
The oxalic amidine N,NЈ-dimethyl-N,NЈ-diphenyl-1,2-bis[4-
tolylimino]ethane-1,2-diamine was used to synthesise
understand the coordination properties of the ligand, dif-
ferent possible coordination modes of the ligand to zinc
chloride have been investigated by DFT calculations. The
chloridozinc complex was applied as a catalyst in the melt
polymerisation of rac-lactide and exhibited high activity
combined with excellent agreement between the obtained
and calculated molecular weights.
a
chloridozinc complex and two monoprotonated species either
with chloride or triflate counterions. The compounds have
been structurally characterised by X-ray crystallography.
Proton affinities have been determined by quantum chemical
calculations to investigate why only the iminium protonated
species is selectively formed upon mild protonation. To better
with different metals and ligand classes are active initiators
in the ROP of lactide, but many of them lack industrial
applicability because they contain toxic heavy metals or are
not stable under industrial conditions.[3] Until now, most
large-scale PLA production processes use tin compounds as
initiators; however, these are undesirable for widespread use
because accumulation effects are suspected.[4] To obtain
biocompatible initiator systems, zinc complexes with N-do-
nor ligands are a viable possibility because they are mostly
colourless, inexpensive and nontoxic. Most zinc complexes
investigated as ROP initiators make use of anionic N-donor
ligands such as β-diketiminates,[5] trispyrazolylborates,[6]
aminophenolates[7] or phenolate Schiff bases.[8] In general,
such systems show sensitivity towards air and moisture,
which is detrimental for industrial use.
Introduction
Polylactide (PLA) is in great demand as a sustainable
and biodegradable alternative to petrochemical-based com-
modity plastics. It can be produced from inexpensive renew-
able raw materials like corn or sugar beets. Its mechanical
properties are similar to those of poly(ethylene
terephthalate) (PET) and polypropylene (PP). Therefore,
PLA is a viable alternative to petrochemical based plastics
in many fields of application, for example, packaging or
fibres and it helps to minimise the problem of waste dis-
posal as it can be recycled or composted after use.[1]
Most commonly, PLA is produced by a ring-opening
polymerisation (ROP) of the cyclic diester lactide by a
metal-based initiator system.[2] A large variety of complexes
The use of neutral N-donor ligands in zinc-based ROP
initiators is still rather uncommon as anionic ligand systems
have dominated this field to date.[9] Recently, promising
classes of neutral ligands for lactide polymerisation includ-
ing guanidines,[10] carbenes,[11] phosphinimines,[12] trispyr-
azolylmethanes,[13] substituted amines[14] or pyridines[15]
have emerged. For the stabilisation of active catalysts,
strong donors are needed.[2] Peralkylated oxalic amidines
appear to be a feasible neutral ligand class for ROP-active
zinc complexes because of their strong donor properties and
high nucleophilicity.
[a] Fakultät Chemie, Anorganische Chemie, Technische
Universität Dortmund,
44221 Dortmund, Germany
[b] Fachbereich C, Anorganische Chemie, Bergische Universität
Wuppertal,
42097 Wuppertal, Germany
[c] Institut für Organische Chemie und Makromolekulare Chemie,
Friedrich-Schiller-Universität,
Jena, 07743 Jena, Germany
[d] Department Chemie, Ludwig-Maximilians-Universität
München,
81377 München, Germany
Fax: +49-89-218077867
E-mail: Sonja.Herres-Pawlis@cup.uni-muenchen.de
Homepage: http://www.cup.lmu.de/ac/herres-pawlis/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200838.
Oxalic amidines are specific derivatives of oxalic acid
that can be used as multifunctional ligands in organometal-
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lic and coordination chemistry because of their four nitro- slow diffusion of pentane into the solution. The monopro-
gen donor atoms.[16–18] These bifunctional amidines can act tonated compounds 3 and 4 (Figure 1) were isolated after
as diamide or diamidinate ligands after deprotonation, and reaction of MgCl2 or Zn(OTf)2, respectively, with the ligand
their electronic and steric effects can be easily modified by in absolute THF under inert gas conditions. Slow diffusion
variation of the substituents on the nitrogen atoms. More- of diethyl ether produced single crystals suited for structure
over, they can adopt several possible coordination modes in elucidation by X-ray crystallography. Acidic impurities in
their metal complexes.
the salts are probably responsible for the protonation of the
For example, binuclear oxalamidinate complexes of pal- ligand. Interestingly, under the given conditions only the
ladium were described to be highly selective precatalysts in monoprotonated species could be obtained. The fact that
the copper-free Sonogashira reaction.[19] Similarly, spectral even under these relatively “dry” conditions protonation
properties such as UV/Vis absorption maxima are affected could be observed gives a first clue about the basicity of the
by using oxalamidines as coligands in polypyridyl rutheni- oxalic amidine ligand. The reaction of 1 with trifluorometh-
um(II) complexes.[20] Recently, 4H-imidazoles which can be anesulfonic acid or aqueous HCl in MeCN did not lead to
regarded as cyclic versions of oxalamidines were success- growth of single crystals, but the formation of the products
fully used as ligands for palladium.[21] These deeply col- 3 and 4 could be traced in the IR spectra after evaporation
oured metallacycles display catalytic activity and, in ad- of the solvent.
dition, are fully reversible two-step redox systems.
The zinc complex 2 crystallises in the monoclinic space
In this contribution, we report the synthesis and charac- group P21/n with four molecules in the unit cell. The ligand
terisation of a chloridozinc complex with the oxalic amidine coordinates to the zinc centre through the 1,4-diazadiene
ligand N,NЈ-dimethyl-N,NЈ-diphenyl-1,2-bis[4-tolylimino]- moiety, which leads to a pseudo-C2-symmetric arrange-
ethane-1,2-diamine[22] and its application in the bulk poly- ment. In the complex, zinc is fourfold coordinated by the
merisation of d,l-lactide at 150 °C. Additionally, DFT two N-imine atoms of the oxalic amidine ligand and two
analyses of the complex provide further information on the chlorido ligands (Figure 2). Selected bond lengths and
binding properties of the oxalic amidine ligand. Proton af- angles are given in Table 1. The zinc atom exhibits a dis-
finity calculations in combination with experimentally ob- torted tetrahedral coordination geometry because of the li-
tained structures of the protonated ligand illuminate the gand bite angle of 80.7(1)°. The angle between the ZnCl2
protonation behaviour of the oxalic amidine. This proton- and the ZnN2 plane is close to the angle for an ideal tetra-
ation study helps to understand the coordination properties hedron with a value of 86.7(1)°.
of the neutral ligand. DFT and Hartree–Fock (HF) calcula-
tions have proven to be a viable method for the determi-
nation of the proton affinities of N-donor ligands.[23]
Results and Discussion
Syntheses and Molecular Structures
N,NЈ-Dimethyl-N,NЈ-diphenyl-1,2-bis[4-tolylimino]-
ethane-1,2-diamine (1) was synthesised according to litera-
ture procedures.[22] Reacting the ligand with zinc chloride in
absolute tetrahydrofuran (THF) resulted in a clear orange
solution. Single crystals of 2 (Figure 1) were obtained by
Figure 2. Molecular structure of 2.
Table 1. Selected bond lengths [Å] and angles [°] of 2 (experimen-
tally in the solid state and calculated by B3LYP-DFT).
2 (X-ray)
2 (DFT)
Zn–Nimine
Zn–Cl
C–Nimine
2.089(2)/2.074(2)
2.191(1)/2.184(1)
1.289(3)/1.290(3)
80.7(1)
2.094/2.086
2.225/2.217
1.296/1.296
79.7
N–Zn–N
Є(ZnCl2,ZnN2)
Є(CN2,CN2)
86.7(1)
30.9(2)
89.9
18.5
The Zn–Nimine bond lengths are 2.089(2) and 2.074(2) Å,
which are similar to the Zn–Nimine bond lengths in zinc
guanidine complexes that are active lactide polymerisation
catalysts.[10] The zinc coordination only leads to a very
Figure 1. Ligand 1, chloridozinc complex 2 and protonated com-
pounds 3 and 4.
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slight increase in the C–Nimine bond lengths to 1.289(3) and a chloride anion crystallises in the monoclinic space group
1.290(3) Å [compared to the nonprotonated C–Nimine of 3 P21/c with four molecules in the unit cell. The triflato com-
and 4 of 1.275(2) Å], which indicates that the double bonds pound 4 crystallises in the monoclinic space group P21/n
are basically maintained. The angle between the CN2 planes with eight molecules in the unit cell. The asymmetric unit
exhibits a rather small value of 30.9(2)°, which can be as- contains two molecules, of which only one is discussed here
cribed to the rigidity caused by the coordination of the N as both molecules are very similar.
1
atoms to the zinc. In the H and 13C NMR spectra only
The geometries at the imine double bonds are both (Z)
one series of signals for half of the ligand is observed, which at the protonated and (E) at the nonprotonated imine sites.
shows that the symmetric coordination geometry remains In both protonated compounds, a hydrogen bond between
in solution. In the literature, chloridozinc complexes with the proton at the imine N atom and the corresponding
different bis(arylimino)acenaphthenes (BIANs) are de- anion is present, which exhibits a N–H···O distance of
scribed that exhibit similar geometries with Zn–Nimine bond 2.774(2) Å to the triflate (nearest O atom) and a N–H···Cl
lengths ranging from 2.071 to 2.128 Å and C–Nimine bond distance of 2.991(2) Å. Both N–H···X distances are rela-
lengths between 1.260 and 1.295 Å. Also, the bite angles of tively short but in the normal range for hydrogen bonds at
the bisimino ligands (79.7–81.7°) lie in the same range as imines.[25] In both cases, the C–Nimine bond including the
that of the oxalic amidine ligand.[24]
protonated Nimine still has partial double bond character
The protonated compounds 3 and 4 show very similar with bond lengths of 1.313(2) and 1.316(2) Å for 3 and 4,
geometries in the solid phase (Figure 3). Compound 3 with respectively, but in comparison to the non-protonated
Nimine side [C–Nimine = 1.275(2) and 1.276(2) Å, respec-
tively] the bonds are elongated. In the reported crystal
structures of differently substituted nonprotonated oxalic
amidine ligands, the C–Nimine bond lengths range between
1.281 and 1.294 Å, which is in between the values for the
nonprotonated and the protonated site in complexes 3 and
4.[22,26]
The angles between the two CN2 planes are much larger
in 3 [77.3(2)°] and 4 [63.3(2)°] than in the zinc complex 2
[30.9(2)°]. The angles between the plane of the phenyl rings
of the p-tolyl units and the adjacent CN2 plane lie between
47.6(2) and 51.1(1)° in 4 and 62.4(2) to 67.0(1)° in 3, which
means that the π-system of the phenyl rings and the imine
moiety is discontinuous; this is caused by steric repulsion
of the phenyl rings (Table 2). This repulsion effect is also
observed in other phenyl-containing oxalic amidine li-
gands.[22,26]
Table 2. Selected bond lengths [Å] and angles [°] of 3 and 4 (experi-
mentally in the solid state and calculated by B3LYP-DFT).
3 (X-ray) 4 (X-ray) HF
B3LYP
1.316(2) 1.315 1.314
1.276(2) 1.253 1.285
C–Nimine (prot.)
C–Nimine (non-prot.)
Є(CN2,CN2)
Є(CN2,Ph) (prot. imine)
Є(CN2,Ph) (non-prot. imine)
1.313(2)
1.275(2)
77.3(2)
62.4(2)
67.0(1)
63.3(2)
51.1(1)
47.6(2)
67.5
88.3
65.1
60.5
84.8
59.7
Quantum Chemical Calculations
Geometry optimisations of the chloridozinc complex 2
were performed by using B3LYP density functional theory
(DFT) with the economical 6-31G(d) basis set implemented
in the Gaussian09 suite of programs.[27] In previous studies
of zinc complexes with other N-donor ligands, this combi-
nation of functional and basis set has appropriately de-
scribed this type of complex.[10b,10c,10e] For the protonated
ligand structures, HF and B3LYP geometry optimisations
with the same basis set have been carried out. The different
methods showed good agreement with each other and the
Figure 3. Molecular structures of 3 and 4. The asymmetric unit of
4 contains two molecules, of which only one is displayed here.
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solid-state structure. The starting point for all calculations determined by single point calculations using the 6-
was the solid-state structure determined by X-ray diffrac- 311+G(d,p) basis set and by considering solvent effects with
tion analysis. For the protonated ligands the calculations the conductor-like polarisable continuum model (CPCM)
were performed without the anions.
in acetonitrile (ε = 36.64). The proton affinities for different
The geometry optimisations of all protonated species possible sites of proton attack (imine or amine nitrogen
ended up in the same geometry with the same self-consis- atom, Figure 4) taking into account mono- and diproton-
tent field (SCF) energy, which suggests that the space ation were calculated starting from the crystal structure of
requirement of the anion and packing effects lead to the 3 and altering the protonation sites. The results are summa-
slightly different geometries in the crystal structures.
rised in Table 3. Comparative calculations with the two dif-
The calculated geometries fit well with the solid-state ferent molecules in the crystal structure of 4 led to almost
structures. However, for 2 the angle between the CN2 planes the same results with differences in the proton affinity (PA)
of the 1,4-diazadiene moiety is even smaller in the calcu- values not exceeding 4 kJ/mol. The protonation at the other
lated structure. In the protonated compounds the alignment one of the two Nimine sites in 1pi or of the two Namine sites
of the phenyl rings deviates slightly from the solid-state in 1pa led to similar PAs (at the mostϮ8 kJ/mol).
structures: this is illustrated by the angle between the imine
Table 3. Calculated proton affinities PA [kJmol–1] for different sites
of proton attack.
CN2 plane and the adjacent plane of the p-tolyl ring, which
is wider in the calculated structures.
Base
Conjugate acid
PA
Proton Affinity Studies
1
1pi
1124
1072
1017
995
Structures 3 and 4 of the protonated ligand were taken 1pi
1dpi
1pa
1
1pa
as starting structures for the proton affinity calculations.
Quantum chemistry geometry optimisation calculations of
1dpa
the ligand and different mono- and diprotonated forms
were performed at the Hartree–Fock level using the 6-
31G(d) basis set, which has already been used for proton
affinity studies of similar systems by Maksic et al.[23] Zero
point vibrational energy (ZPVE) contributions were consid-
ered by vibrational analyses at the same level by application
of the common scale factor 0.89.[28] Electronic energies were
A comparison of the determined PAs reveals that mono-
protonation of the imine function has the highest proton
affinity, which makes the imine function the most basic po-
sition in 1. With a value of 1124 kJmol–1, the proton affin-
ity is comparable to those of other organic bases such as
some guanidines.[23b] Protonation of the second imine func-
tionality is much less favoured with a PA of 1072 kJmol–1,
which explains why only monoprotonation occurs under the
given mild conditions. As assumed, the basicity of the
amine is much lower than that of the imine functionality as
illustrated by the more than 100 kJmol–1 lower PA. Dipro-
tonation at the amine nitrogen atoms is even less favoured.
Zinc Coordination Studies
The crystal structure of 2 shows that the ligand coordi-
nates through the more basic imine nitrogen atoms. Even
with two or more equivalents of zinc chloride, the same
complex is formed. As coordination of the amine moieties
to the zinc should also be possible, but is not observed, we
theoretically generated different possible coordination
modes for the ligand to zinc chloride (depicted in Figure 5).
They represent the coordination of either two imine N
atoms, one imine and one amine N atom or two amine N
atoms. All geometries were geometry optimised by DFT
calculations using the B3LYP density functional and the 6-
31G(d) basis set. For the coordination modes 2ia (coordina-
tion by one imine and one amine nitrogen atom) and 2aa
(coordination by both amine nitrogen atoms), two different
geometries were calculated [2ia(a), 2ia(b), 2aa(a), 2aa(b)],
which differ in the arrangement of the phenyl and methyl
substituents at the coordinating amine nitrogen atom (see
Supporting Information, Figure S1).
As expected, the imine coordination that is also experi-
mentally obtained has the lowest energy (Table 4), which is
therefore set as zero. The 2ia coordination shows energies
Figure 4. Calculated protonated compounds with different proton-
ation sites for the determination of proton affinities.
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Table 4. Energies and key geometric parameters of the calculated structures 2, 2ia(a), 2ia(b), 2aa(a) and 2aa(b).
Compound
Ee [Hartree]l
ΔEel [kJmol–1]
Zn–Cl [Å]
Zn–N [Å]
ZnCl2/ZnN2 [°]
CN2/CN2 [°]
2
–4079.9324
–4079.9144
–4079.9115
–4079.8891
–4079.8903
0
2.225/2.217
2.208/2.228
2.204/2.223
2.233/2.210
2.219/2.218
2.094/2.086
2.035/2.306
2.035/2.341
2.144/2.192
2.148/2.217
89.9
86.3
78.5
86.7
81.8
18.5
38.3
26.6
37.6
57.7
2ia(a)
2ia(b)
2aa(a)
2aa(b)
+47
+55
+114
+110
polymerisation conditions were oriented to studies already
reported for robust zinc guanidine complexes.[10] The poly-
merisations were performed in the melt of rac-lactide at
150 °C with a monomer to initiator ratio [M]/[I] of 500:1.
The rac-lactide was used as purchased without purification.
The polymerisation results after 24 h were promising
and, therefore, a kinetic study was performed. After dif-
ferent reaction times of 45 min to 24 h, the conversion was
determined by 1H NMR spectroscopy, and after dissolution
in CH2Cl2 and precipitation in EtOH the molecular mass
distribution was analysed by gel permeation chromatog-
raphy (GPC). The polymerisation results are summarised in
Table 5. The plot of ln[LA]0/[LA]t (Figure 6) shows a linear
progression up to a polymerisation time of about 7 h and
80% conversion, which indicates 1st order kinetics for the
chain growth and thus a controlled polymerisation behav-
iour. Afterwards the polymerisation proceeds more slowly
because of the high viscosity of the mixture.
Table 5. Results of the polymerisation with 2 at 150 °C. [M]/[I] =
500:1.
t [min]
Conv. [%] Mth [gmol–1] Mn [gmol–1] PD
Figure 5. Calculated coordination modes of 1 to zinc chloride.
45
10.6
23.6
39.3
60.4
70.6
76.7
79.9
84.4
88.3
7000
4000
12000
28000
41000
n.d.
45000
n.d.
1.33
1.38
1.29
1.34
n.d.
1.40
n.d.
n.d.
1.27
105
195
360
480
600
720
960
1440
17000
28000
44000
51000
55000
56000
61000
64000
of +47 kJmol–1 and +55 kJmol–1, which are considerably
higher than those of the coordination through both imine
functionalities. Therefore, the coordination of two ZnCl2
molecules in a transoid arrangement is unlikely because the
1,4-diazadiene coordination is energetically highly fav-
oured. Coordination through the two amine functions is
even less favourable with +110 and +114 kJmol–1. This ex-
plains why no complexation by the amine functions, for ex-
ample, in a binuclear manner could be observed even in
high excess of the zinc salt.
The coordination of the amine is weaker than that of the
imine, which is seen in the Zn–N bond lengths. The Zn–
Namine distances in 2aa are 2.144 to 2.217 Å. In the 2ia geo-
metries, the Zn–Namine bonds are longer (2.306 to 2.341 Å),
and the coordination of the Nimine atoms is even closer than
in 2 with Zn–Nimine bond lengths of 2.035 Å in both cases.
n.d.
51000
Polymerisation Activity
Zinc complexes with neutral N-donor ligands such as
guanidines showed very good activity in the polymerisation
of rac-lactide.[10] As the oxalic amidine ligand 1 also showed
to be highly basic, the catalytic activity of the synthesised
chloridozinc complex in lactide polymerisation was exam-
Figure 6. First-order plot of ln([LA]0/[LA]t) vs. time at 150 °C for
ined. Owing to the stability of complex 2 under air, the the lactide polymerisation with 2.
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The molecular weight distributions are relatively narrow
To determine the end group and possibly acquire insights
with polydispersities (PD) of around 1.3–1.4, which indi- into the polymerisation mechanism with this new catalyst
cates a controlled polymerisation process. In Figure 7, the class, high resolution ESI mass spectra were collected with
number average molecular weight Mn is plotted vs. the con- low molecular weight PLA, which was obtained by poly-
version. This plot shows a linear behaviour over the whole merisation for 10 min with 2 at 150 °C. After the polymeri-
range, which proves that the molecular mass is controllable. sation, the polymer was dissolved in CH2Cl2 and precipi-
Additionally, the experimental molecular weights are close tated in water. The negative mode spectrum shows peaks
to the theoretical values Mth calculated from the [M]/[I] ra- with masses between 1457.43 and 377.11 u with intervals of
tio and the conversion.
72.02 u. This reveals that transesterification processes take
place during the polymerisation, because otherwise inter-
vals of 144.04 u (the molecular mass of the monomer)
would be observed. The subtraction of further 72.02 u units
ends with a mass of 17.00 u, which suggests that one end
group is an OH group. This was expected after the precipi-
tation in water. The positive mode spectrum also shows a
series of signals with a 72.02 u interval with the lowest m/z
value at 540.19 u, which could not be assigned to a possible
end group. According to the isotope pattern no zinc or
chlorine is contained in the polymer.
The outstanding feature of the new catalyst is its robust-
ness in combination with an excellent match between the
observed and theoretical molecular masses. Other chlorido-
zinc complexes with guanidines that were investigated un-
der the same conditions are in general less active in ROP
and need longer polymerisation times to achieve high con-
versions.[10c] Cationic zinc bis(chelate) complexes with
guanidine quinoline ligands are also robust and in compari-
son more active, but the molecular masses are considerably
higher than the theoretical molecular weights.[10b,10e] Simi-
larly robust tridentate Schiff base complexes of titanium are
much more active in the melt polymerisation with unpuri-
Figure 7. Plot of number average molecular weight Mn vs. conver-
sion for the lactide polymerisation with 2.
To evaluate the thermal stability of 2, differential scan- fied lactide (90% yield within 0.5 h), but the obtained mo-
ning calorimetry (DSC) measurements were performed and lecular masses are considerably lower than the theoretical
showed that no phase transition or decomposition takes values.[29] Dagorne et al. recently developed a robust zirco-
place up to a temperature of 250 °C. Furthermore, poly- nium N-heterocyclic carbene complex that polymerises un-
merisations were carried out at 180 °C. Within 24 h a high purified lactide and combines a good activity at 130 °C
monomer conversion of 97% was reached but the molecular (86% conversion within 20 h) with high stereoselectivity
mass is lower (Mn = 34000 gmol–1) and the molecular mass and a narrow molecular mass distribution (PD = 1.03) in
distribution broader (PD = 1.80) than at 150 °C, which in- the resulting polymer.[30] Davidson et al. reported Zr and
dicates that more side reactions occur at the higher tem- Hf complexes with tetradentate amine tris(phenolate) li-
perature.
gands to be active initiators in the solvent free ROP of lact-
Additionally, the [M]/[I] ratio was increased to 1000:1 be- ide.[31] Both catalysts polymerise lactide much faster than 2
cause lower catalyst loadings are targeted for industrial pur- (Zr complex: 78% yield within 0.1 h; Hf complex: 95%
poses. With the higher [M]/[I] ratio, the polymerisation pro- within 0.5 h) and additionally give PLA with a high degree
ceeds more slowly: within 48 h a conversion of 92.2% was of heterotacticity (Zr complex: Pr 0.96; Hf complex: Pr
obtained. The resulting Mn of the polymer amounts to 0.88). For industrial purposes these systems are not suited
59000 gmol–1, which is considerably lower than the theoret- because the robustness of these complexes is obviously not
ical value (Mth = 133000 gmol–1). The PD of 1.65 is also high enough for the polymerisation with unpurified lactide
higher than that for the [M]/[I] ratio of 500:1. This shows as purified lactide (recrystallised from toluene and sublimed
that the catalyst is not particularly suited for high monomer twice) was used and the utilised metals are expensive. Also,
loadings.
an example of the use of an air-stable Ti–alkoxide metal–
A cross-check experiment proved that the ligand 1 itself organic framework (MOF) is described that polymerises
shows a very poor activity. After a reaction time of 24 h at lactide in the melt.[32] Within 2 h a conversion of 90% was
150 °C, a lactide conversion of only 7% was observed. The achieved with a good match between theoretical and experi-
catalytic activity of zinc chloride alone has been investi- mental molecular mass of PLA. In comparison to 2, the
gated before.[10b] In this case under the same reaction condi- MOF compound polymerises faster but it is not described
tions the polymer yield was 42% and Mn amounted to whether purified lactide was used and the authors used a
14000 gmol–1.
smaller [M]/[I] ratio (300:1).
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Conclusions
Experimental Section
Materials and Methods: All manipulations were performed under
nitrogen (99.996%) dried with P4O10 granules using Schlenk tech-
niques. Solvents were purified according to literature procedures
and also kept under nitrogen.[33] Zinc(II) chloride (99.99%, Acros),
magnesium(II) chloride (99.9%, abcr), zinc(II) trifluoromethansul-
fonate (98%, Sigma Aldrich) and d,l-lactide (3,6-dimethyl-1,4-di-
oxane-2,5-dione, Purac) were used as purchased.
The oxalic amidine N,NЈ-dimethyl-N,NЈ-diphenyl-1,2-
bis[4-tolylimino]ethane-1,2-diamine was treated with ZnCl2,
MgCl2 and Zn(CF3SO3)2, which led to the formation of a
chloridozinc complex and two monoprotonated com-
pounds that were structurally characterised by X-ray dif-
fraction.
The protonated compounds were used for proton affinity
studies of the ligand by means of quantum chemical calcu-
lations. The calculated proton affinities explain the site of
proton attack and the fact that only monoprotonation is
Physical Measurements: Spectra were recorded with the following
spectrometers: 1H NMR [internal standard CDCl3 (δ = 7.26)]:
Bruker DRX 400 (400.1 MHz), Bruker DRX 500 (500.1 MHz),
Varian Inova 500 (499.8 MHz). 13C NMR [internal standard
observed under the given conditions. The imine site has a CDCl3 (δ = 77.16)]: Bruker DRX 300 (75.5 MHz), Bruker DRX
proton affinity more than 100 kJmol–1 higher than that of
400 (100.6 MHz), Bruker DRX 500 (125.8 MHz), Varian Inova 500
(100.6 MHz). Elemental analysis: Leco Instrument CHNS-932.
the amine site. Investigations of different conceivable coor-
Mass spectrometry: The electrospray mass spectra were collected
dination modes of the ligand to zinc chloride by DFT cal-
with a TSQ Thermoquest Finnigan Instrument with acetonitrile as
culations revealed that the coordination mode observed in
mobile phase. The high-resolution ESI-MS were performed with a
the crystal structure is the energetic minimum and that zinc
Thermo Finnigan LTQ FT Ultra Fourier Transform mass spec-
coordination through the amine nitrogen atoms is much less
trometer with an IonMax ion source with an ESI head and CHCl3
favoured. Hence, the donor difference between imine and
as the mobile phase. The spray capillary voltage was 4 kV, the heat-
amine donors within this oxalic amidine has been quantita-
tively evaluated.
ing capillary temperature 250 °C, the nitrogen sheath gas flux 25
and the sweep gas flux 5 units. The flux injection analysis ESI (FIA/
The chloridozinc complex was applied in the melt poly- ESI) was performed with a Surveyor MS pump at a flux rate of
100 μL/min with 20:80 water/acetonitrile as mobile phase. 1–10 μL
sample solutions were injected with the use of inline filters. Infrared
spectroscopy: Spectra were collected with a Bruker IFS 28 Fourier
spectrometer. Differential Scanning Calorimetry: Measurements
were performed with a Perkin–Elmer Pyris1 instrument between 20
and 250 °C.
merisation of rac-lactide and shows very good activity with
a well-controlled polymerisation process and good agree-
ment between the theoretical and experimental molecular
weights. As both the ligand and the chloridozinc complex
are storable under air, the presented catalyst may also be
suited for industrial application. The combination of high
polymerisation control and robustness was rather unexpec-
ted and demonstrates that strong neutral donors stabilising
a Lewis acid such as zinc yield excellent ROP catalysts. This
first example of an oxalic amidine as ligand in the polymeri-
sation of lactide opens up a new neutral N-donor ligand
class that is very valuable for lactide polymerisation chemis-
try.
Crystal Structure Analyses: The crystal data for compounds 2 and 3
were collected with an Oxford Diffraction XcaliburS diffractometer
using the programs CRYSALIS (Oxford, 2008) and CRYSALIS
RED (Oxford, 2008). The data for compound 4 were collected with
an Xcalibur Eos Gemini Ultra diffractometer using the programs
CRYSALIS (Oxford, 2008) and CRYSALIS RED (Oxford, 2008).
All structures were solved using Direct Methods (SHELXS-90),[34]
and structural refinement was done with SHELXL-97 (Table 6).[35]
Table 6. Crystallographic data for the compounds 2, 3 and 4.
2
3
4
Empirical formula
Formula weight [gmol–1]
Temperature [K]
Crystal system
Space group
a [Å]
C30H30Cl2N4Zn
582.85
173(2)
monoclinic
P21/n
11.3642(7)
16.8265(9)
15.1925(7)
91.109(4)
2904.6(3)
4
1.333
1.055
0.48ϫ0.23ϫ0.18
2.16–25.50
–12/13, Ϯ20, Ϯ18
16577
C30H31ClN4
483.04
173(2)
monoclinic
P21/c
11.2483(9)
8.5364(5)
28.0417(18)
103.025(6)
2623.3(3)
4
1.223
0.171
0.14ϫ0.08ϫ0.07
2.10–25.50
–11/13, –7/10, Ϯ33
10022
C31H31F3N4O3S
596.66
173(2)
monoclinic
P21/n
18.5097(5)
15.2581(3)
21.5596(5)
94.131(2)
6037.1(3)
8
1.305
0.163
0.14ϫ0.11ϫ0.03
3.01–29.52
–19/23, –15/20, Ϯ28
36985
b [Å]
c [Å]
β [°]
Volume [Å3]
Z
Dcalc [gcm–3]
μ(Mo-Kα) [mm–1]
Crystal size [mm]
θ range [°]
h, k, l
Reflections collected
Independent reflections
Parameters
5333
338
4845
292
14467
765
R1 [IՆ2σ(I)]
wR2 (all data)
Largest diff. peak, hole [eÅ–3]
0.0314
0.0663
0.366 /–0.379
0.0418
0.0842
0.229 /–0.261
0.0498
0.1220
1.704 /–0.701
Eur. J. Inorg. Chem. 2013, 99–108
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FULL PAPER
CCDC-887839 (for 2), -887840 (for 3) and -873845 (for 4) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
510 (m) cm–1. MS (ESI): m/z (%) = 447 (100) [M – ZnCl2 + H]+,
223 (26) [M – ZnCl2 – C15H15N2]+.
Protonated Compounds 3 and 4: A solution of the ligand 1 (1 mmol,
0.447 g) in dry THF was added to a suspension of MgCl2 (1 mmol,
0.095 g) or Zn(CF3SO3)2 (0.5 mmol, 0.182 g) in dry THF. The re-
sulting suspension was stirred for 10 min and then heated to reflux
for 30 min. The resulting clear solution was allowed to cool to
room temperature. Single crystals were obtained by diffusion of
diethyl ether. The crystals were washed with diethyl ether and dried
in vacuo. Data for 3: Yield 0.20 g (0.42 mmol, 42%). C30H31N4Cl
(483.05): calcd. C 74.6, H 6.5, N 11.6; found C 74.3, H 6.9, N 11.5.
1H NMR (300 K, CDCl3, 400.13 MHz): δ = 1.99 (s, 6 H, CH3–
tolyl), 2.32 (s, 6 H, N–CH3), 5.85 (m, 4 H, CH), 6.48 (m, 2 H, CH),
6.73 (m, 6 H, CH), 7.16 (m, 2 H, CH), 7.49 (m, 2 H, CH), 7.93
(m, 2 H, CH), 14.72 (br. s, 1 H, N–H) ppm. IR (KBr, 25 °C, 32
Computational Details: Quantum chemical calculations were per-
formed with the Gaussian 09 program suite.[27] The geometries of
the complexes 2, 2aa and 2ia were optimised using the B3LYP[36]
hybrid DFT functional and the 6-31G(d) basis set implemented in
Gaussian on all atoms. Tight conversion criteria were applied.
Proton affinities were calculated according to the following pro-
cedure: A geometry optimisation of different protonated species of
the ligand 1 was performed by using HF calculations with the 6-
31G(d) basis set. Zero point vibrational energies (ZPVE) were de-
termined by subsequent frequency analysis at the same level and
application of the common scale factor 0.89. Electronic energies
were determined by single point DFT calculations of the optimised
geometries using the functional B3LYP[36] and the basis set 6-
311+G(d,p) and considering solvent effects with the conductor-like
polarisable continuum model (CPCM) in acetonitrile (ε = 36.64).
scans): ν = 3023 (w), 2918 (w), 2859 (vw), 2333 (m), 1629 (vs), 1583
˜
(s), 1509 (m), 1493 (m), 1439 (w), 1392 (m), 1306 (w), 1219 (m),
1152 (w), 1103 (m), 1073 (w), 1022 (w), 909 (w), 819 (w), 804 (m),
784 (w), 766 (w), 699 (m), 577 (w), 549 (m), 517 (w) cm–1. MS
The proton affinities (PA) were determined according Equa- (ESI): m/z (%) = 447 (100) [M – Cl]+. Data for 4: Yield 0.28 g
tions (1), (2) and (3), where B and BH+ denote the respective base
and its conjugate acid.
(0.47 mmol, 47%). C31H31F3N4O3S (596.66): calcd. C 62.4, H 5.2,
N 9.4; found C 62.3, H 5.5, N 9.3. 1H NMR (300 K, CDCl3,
400.13 MHz): δ = 1.95 (s, 6 H, CH3–tolyl), 2.29 (s, 6 H, N–CH3),
5.79 (m, 4 H, CH), 6.44 (m, 2 H, CH), 6.69 (m, 6 H, CH), 7.13
(m, 2 H, CH), 7.46 (m, 2 H, CH), 7.60 (m, 2 H, CH) ppm. IR
PA(Bα) = (ΔEel)α + (ΔZPVE)α
(ΔEel)α = E(B) – E(BαH+)
(1)
(2)
(KBr, 25 °C, 32 scans): ν = 3025 (m), 2921 (m), 1620 (vs), 1592
˜
(vs), 1495 (m), 1458 (m), 1437 (w), 1383 (m), 1285 (vs), 1243 (m),
1223 (s), 1155 (s), 1108 (m), 1029 (vs), 920 (w), 876 (vw), 814 (m),
(ΔZPVE)α = ZPVE(B) – ZPVE(BαH+)
(3) 779 (w), 757 (w), 696 (m), 636 (m), 573 (w), 5419 (w), 517 (m) cm–1.
MS (ESI): m/z (%) = 447 (100) [M – Cl]+.
Gel Permeation Chromatography: The average molecular weights
and the weight distributions of the obtained polylactide samples
were determined by gel permeation chromatography (GPC) in THF
as mobile phase at a flow rate of 1 mL/min. The utilised GPCmax
VE-2001 from Viscotek is a combination of an HPLC pump, a
SDV column (PSS) with a porosity of 500 Å and a refractive index
detector (VE-3580). Universal calibration was applied to evaluate
the chromatographic results. Kuhn–Mark–Houwink (KMH) pa-
General Procedure for D,L-Lactide Polymerisation: d,l-Lactide (3,6-
dimethyl-1,4-dioxane-2,5-dione, 3.603 g, 25 mmol, used as pur-
chased) and the initiator 2 (I/M ratio 1/500) were weighed into a
50 mL flask, which was flushed with argon and closed with a glass
stopper. The reaction vessel was then heated to 150 °C. After the
reaction time, the polymer melt was allowed to cool to room tem-
perature and was dissolved in dichloromethane (20 mL). The PLA
was precipitated in ice-cooled ethanol (300 mL) and dried under
vacuum at 50 °C. For the polymerisation kinetic measurements, a
homogeneous blend of d,l-lactide and the catalyst complex (I/M
ratio 1/500) was prepared. Portions of 2 g of the blend were
weighed into 50 mL flasks, which were flushed with argon and
closed with a glass stopper. The reaction vessels were heated to
150 °C. After different reaction times, the flasks were cooled with
ice water. The polymers were dissolved in dichloromethane
(10 mL). The PLA was precipitated in ice-cooled ethanol (150 mL)
and dried under vacuum at 50 °C.
rameters for the polystyrene standards (KPS = 0.011 mL/g, aPS
0.725) were taken from the literature.[37] Previous GPC measure-
ments utilising online viscosimetry detection revealed the KMH
parameters for polylactide (KPLA = 0.053 mL/g, aPLA = 0.610).[10a]
=
Preparation of Compounds
Chloridozinc Complex 2: A solution of the ligand 1 (1 mmol,
0.447 g) in dry THF was added to a suspension of zinc chloride
(1 mmol, 0.136 g; 2 mmol, 0.272 g or 4 mmol, 0.545 g) in dry THF.
The resulting clear solution was stirred for 10 min. Single crystals
were obtained by diffusion of diethyl ether. The orange crystals
were washed with diethyl ether and dried in vacuo to yield 0.50 g
of 2 (0.86 mmol, 86%) for the reaction with 1 equiv. ZnCl2.
C30H30N4Cl2Zn (582.87): calcd. C 61.8, H 5.2, N 9.6; found C 61.5,
Supporting Information (see footnote on the first page of this arti-
cle): Calculated geometries for the coordination of 1 to zinc chlor-
ide and Cartesian coordinates of the computed structures.
1
H 5.4, N 9.6. H NMR (300 K, CDCl3, 500.14 MHz): δ = 2.14 (s,
6 H, CH3–tolyl), 2.34 (s, 6 H, N-CH3), 6.75 (br. s, 4 H, CH), 7.18
3
Acknowledgments
(m, 4 H, CH), 7.25 (m, 6 H, CH), 7.45 (t, J = 7.9 Hz, 4 H, CH)
ppm. 13C{1H} NMR (300 K, CDCl3, 125.77 MHz): δ = 21.2 (CH3–
tolyl), 37.4 (CH3–N), 119.7 (CH), 122.8 (CH), 125.0 (CH), 129.7 Financial support by the Fonds der Chemischen Industrie (fellow-
(CH), 130.2 (CH) 138.5 (Ar–C), 140.5 (Ar–C), 143.0 (Ar–C), 156.7 ship to S. H.-P.), the Bundesministerium für Bildung und For-
(C=N) ppm. IR (KBr, 25 °C, 32 scans): ν = 3025 (vw), 2924 (w),
2866 (vw), 2371 (vw), 1611 (s), 1579 (vs), 1493 (s), 1451 (vw), 1427
(w), 1376 (m), 1351 (w), 1311 (vw), 1201 (vw), 1179 (vw), 1151 (m),
schung (BMBF) (MoSGrid, 01IG09006), the Deutsche For-
schungsgemeinschaft (DFG) (HE 5480/3-1) and the MERCATOR
Stiftung is gratefully acknowledged. S. H.-P. thanks Prof. K. Jurk-
˜
1111 (m), 1086 (w), 1065 (vw), 1031 (w), 1014 (vw), 900 (m), 864 schat for his valuable support. The authors thank Purac Biochem
(w), 820 (m), 766 (s), 732 (m), 697 (m), 648 (w), 625 (w), 570 (w),
for lactide samples.
Eur. J. Inorg. Chem. 2013, 99–108
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