Homoleptic aminophenolates of Zn, Mg and Ca. Synthesis, structure, DFT studies and polymerization activity in ROP of lactides

Article abstract of DOI:10.1039/c3dt52868e

The reaction of MgBu₂, ZnEt₂ or Ca(O ⁱPr)₂ with 2 eq. of three-coordinating N-[methyl(2-hydroxy-3,5-dimethylphenyl)]-N-methyl-N-methyl-1,3-oxolaneamine (mpoa-H) or N-[methyl(2-hydroxy-3,5-di-tert-butylphenyl)]-N-methyl-N-methyl-1,3- oxolaneamine (tbpoa-H) gave neutral, monomeric [Mg(mpoa)₂], [Zn(mpoa)₂], [Zn(tbpoa)₂], and [Ca(tbpoa)₂] as white powders in 58-90% yields. The resulting aminophenolates were characterized in solution by NMR showing, in the case of [Zn(tbpoa)₂], interesting dynamics. [Zn(tbpoa)₂] and [Ca(tbpoa)₂] were characterized by X-ray crystallography to show the Zn atom to be pseudo-octahedrally coordinated and the Ca atom in six-coordination mode. The new homoleptic complexes were tested in the polymerization of lactide with an external alcohol to reveal stable behaviour (during the polymerization process) only in the case of [Zn(tbpoa)₂]. The high activity of the catalyst was correlated with a ligand flexibility that was further supported by theoretical studies.

Full text of DOI:10.1039/c3dt52868e

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Homoleptic aminophenolates of Zn, Mg and Ca.
Synthesis, structure, DFT studies and
polymerization activity in ROP of lactides†
Cite this: Dalton Trans., 2014, 43,
2424
Jakub Wojtaszak, Krzysztof Mierzwicki, Sławomir Szafert, Nurbey Gulia and
Jolanta Ejfler*
The reaction of MgBu2, ZnEt2 or Ca(OiPr)2 with 2 eq. of three-coordinating N-[methyl(2-hydroxy-3,5-
dimethylphenyl)]-N-methyl-N-methyl-1,3-oxolaneamine (mpoa-H) or N-[methyl(2-hydroxy-3,5-di-tert-
butylphenyl)]-N-methyl-N-methyl-1,3-oxolaneamine (tbpoa-H) gave neutral, monomeric [Mg(mpoa)2],
[Zn(mpoa)2], [Zn(tbpoa)2], and [Ca(tbpoa)2] as white powders in 58–90% yields. The resulting amino-
phenolates were characterized in solution by NMR showing, in the case of [Zn(tbpoa)2], interesting
dynamics. [Zn(tbpoa)2] and [Ca(tbpoa)2] were characterized by X-ray crystallography to show the Zn atom
to be pseudo-octahedrally coordinated and the Ca atom in six-coordination mode. The new homoleptic
complexes were tested in the polymerization of lactide with an external alcohol to reveal stable behaviour
(during the polymerization process) only in the case of [Zn(tbpoa)2]. The high activity of the catalyst was
correlated with a ligand flexibility that was further supported by theoretical studies.
Received 11th October 2013,
Accepted 19th November 2013
DOI: 10.1039/c3dt52868e
www.rsc.org/dalton
Nevertheless, since it is practically unviable to completely
remove catalyst residues from the polymer, which is important
Introduction
Over the past two decades biodegradable polymers have for biomedical applications and green packages, the most
attracted increasing attention as the subject of fundamental interesting remain environmentally friendly non-toxic
research and as products of the chemical industry.1 One of the catalysts.
most prominent examples of such molecules is polylactide
Among the galore of tested catalysts, the so-called well-
(PLA), which is presently developed as a commodity polymer defined heteroleptic catalysts of LnM-OR type possess a great
for packaging (bottles and thin films), fibres (tissue and advantage owing to their ability to facilitate ring-opening
clothes), as well as for biomedical applications as bioresorb- polymerization with control of both molecular weight and
able sutures, screws, orthopedic implants, drug delivery agents polymer microstructure. For a specific “single-site” LnM-OR
or scaffolds for tissue engineering.2 Due to its favorable catalyst, the relative rate of ROP correlates well with an M–O
material properties and the fact that it can be produced from bond polarity. For example, for a given ligand environment,
inexpensive renewable sources, PLA is qualified to be a viable the relative rate of ROP changes in the order Ca+2 > Mg+2
alternative to petrochemical-based plastics.3 Zn+2.5 An excellent study of a family of “single-site” divalent
>
Excellent reviews have recently appeared, describing the metal initiators supported by β-diketiminate, trispyrazolyl-
most effective method for the synthesis of PLAs, i.e. ring- borate or amino/imino-phenolate6 ligands, which included
opening polymerization (ROP) of lactides catalyzed by metal derivatives based on Zn, Mg, Ca, and Sn, has been reported.7
alkoxides.4 A wide variety of different kinds of complexes have Such initiators illustrate the anticipated trends in polymeriz-
been used for this purpose, comprising not only compounds ation rates, correlating well with the size of an initiating group
of biologically benign metals like lithium, sodium, mag- and the electronic properties of ancillary ligand substituents.8
nesium, zinc, calcium, and iron, but also more or even highly
For the above mentioned LnM-OR initiators, it is important
toxic ones like aluminum, tin, lead, bismuth, and lanthanides. to modify a metal coordination sphere by an ancillary ligand
with sufficient steric bulk to prevent bischelation, which
process is considered to represent a deactivation pathway by
the formation of inactive ML2 compounds.9 Therefore, homo-
Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław,
Poland. E-mail: jolanta.ejfler@chem.uni.wroc.pl; Tel: +48 (71) 375 73 10
†Electronic supplementary information (ESI) available: Additional tables and
leptic metal complexes have not been qualified as potentially
effective initiators for ROP of lactides. However, the efficacy of
figures of DFT calculations. CCDC 962645 and 969646. For ESI and crystallo-
ligands stabilizing heteroleptic LnM-OR (M = Ca, Mg, Zn)
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt52868e
2424 | Dalton Trans., 2014, 43, 2424–2436
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complexes is arguable due to high lability of such species. This million and referenced to the residual solvent signal. The
seems especially apparent for initiators derived in situ from weights and number-average molecular weights of PLAs were
organometallic LnM-R precursors. It also seems difficult to determined by gel permeation chromatography (GPC) using a
ensure the stability of such compounds towards an excess of HPLC-HP 1090 II with a DAD-UV/Vis and an RI detector HP
external alcohol in the case of iROP (immortal ROP).
1047A and polystyrene calibration. Microanalyses were con-
An increasing demand for highly active, non-toxic, colour- ducted with an ARL Model 3410+ICP spectrometer (Fisons
less, inexpensive, and stable complexes which can be easily Instruments) and a VarioEL III CHNS (in-house).
handled is forcing the development of an alternative to single-
Syntheses
site initiators. Among those, very attractive are catalytic
systems for the monomer activated pathway based on homo-
N-[Methyl(2-hydroxy-3,5-dimethylphenyl)]-N-methyl-N-methyl-
leptic compounds of ML2 type combined with an external 1,3-dioxolaneamine (mpoa-H). To a solution of 8.35 mL of 2,4-
alcohol.10 In this regard, the recent study on homoleptic mag- dimethylphenol (69.0 mmol) and 8.00 mL (70.3 mmol) of
nesium and zinc catalyst supported by bulky N,O-donor 2-methylaminomethyl-1,3-dioxolane in MeOH (100 mL)
ligands is very promising.11 As we have recently reported, the 10.46 mL (0.140 mol) of formaldehyde (37% solution in H2O)
aminophenolate ligands are able to form homoleptic zinc and was added. The solution was stirred and heated under reflux
magnesium monomers, which combine great potential as for 10 h until a crude product precipitated as a white solid. It
active catalysts in the ROP of lactide with an acceptable stabi- was collected by filtration, washed with cold methanol and
lity.12 Additionally, our studies indicate the dynamic behavior dried in vacuo to give mpoa-H in 72% yield (12.5 g,
of these coordinatively saturated complexes in solution, which 49.7 mmol). Anal. Calcd (found) for C14H21NO3 (%, 251.32):
can be crucial for both stability and catalytic activity in ML2/ 66.91 (66.73); H, 8.42 (8.51); N, 5.57 (5.65)%; ESI/MS: 252.2
ROH systems.
[M + 1]+; 1H NMR (300 MHz, C6D6, 300 K): δ = 10.54 (s, 1H,
A more in-depth mechanistic understanding of the acti- OH), 6.87 (s, 1H, ArH), 6.54 (s, 1H, ArH), 4.82 (t, JHH = 4.1 Hz,
vation process of aminophenolate complexes in lactide 1H, OCHO), 3.48 (s, 2H, N–CH2–Ar), 3.50–3.22 (m, 4H, 2CH2–
polymerization as well as the way in which the structural “per- O), 2.51 (d, JHH = 4.1 Hz, 2H, N–CH2–CH), 2.45 (s, 3H, N–CH3),
turbations” of the active centre and reaction conditions influ- 2.19 (s, 3H, CH3), 2.06 (s, 3H, CH3); 13C NMR (75 MHz, C6D6,
ence their catalytic activity constitutes the aim of the current 300 K): δ = 16.2, 20.7 (2C, CH3), 42.3 (N–CH3), 59.5 (N–CH2–
research. Herein we have described the synthesis and charac- CH), 62.4 (N–CH2–Ar), 64.8 (2C, CH2–O), 102.9 (1C, OCHO),
terization of magnesium, calcium, and zinc complexes sup- 121.5, 125.03, 127.1, 127.3, 131.2, 154.6 (6C, Ar).
ported by the aminophenolate ligands with hemilabile arms
[Mg(mpoa)2]. To a solution of mpoa-H (2.08 g, 8.28 mmol)
containing additional ether O-donor and their application as in hexanes (50 mL) MgBu2 (1.66 mL, 4.15 mmol) was added
initiators for lactide polymerization. The study extensively cor- dropwise at room temperature. The solution was stirred until a
relates the experimental outcomes with DFT calculations to white solid precipitated. It was filtered off, washed with
rationalize the results.
hexanes (20 mL) and dried in vacuo. Recrystallization from
toluene at −15 °C gave [Mg(mpoa)2] in 90% yield (1.96 g,
3.73 mmol). Anal. Calcd (found) for C28H40N2O6Mg (%,
524.93): C 64.07 (64.32), H 7.68 (7.25), N 5.34 (5.60); H NMR
(300 MHz, C6D6, 300 K): δ = 7.42 (br s, 2H, ArH), 7.03 (br s, 2H,
ArH), 5.69, 5.49 (2br s, 2H, OCHO), 4.02 (br s, 4H, N–CH2–Ar),
1
Experimental section
General materials, methods and procedures
All the reactions and operations were performed under an 3.89–3.60 (m, 8H, O–CH2), 3.40–3.28 (m, 4H, N–CH2–CH), 2.81
inert atmosphere of N2 using a glove-box (MBraun) or standard (s, 6H, N–CH3), 2.74 (s, 6H, CH3), 2.67 (s, 6H, CH3); 13C NMR
Schlenk techniques. Reagents were purified by standard (75 MHz, C6D6, 300 K): δ = 16.3 (2C, CH3), 20.9 (2C, CH3), 46.5
methods: THF, distilled from Na/benzophenone; toluene, dis- (2C, N–CH3), 59.6 (2C, N–CH2–CH), 62.5 (2C, N–CH2–Ar), 64.8
tilled from Na; CH2Cl2, distilled from P2O5; hexanes, distilled (2C, O–CH2), 65.1 (2C, O–CH2), 101.8, 102.9 (2C, OCHO),
from Na; methanol, distilled from Mg; C6D6, distilled from 119.6, 122.0, 126.7, 129.2, 131.9, 164.4 (12C, Ar).
CaH2. L-LA ((3S)-cis-3,6-dimethyl-1,4-dioxane-2,5-dione) (98%;
[Zn(mpoa)2]. A solution of ZnEt2 (2.00 mL, 2.00 mmol),
Aldrich) was sublimed and recrystallized from toluene prior to mpoa-H (1.01 g, 4.02 mmol), and hexanes (50 mL) were com-
use. Benzyl alcohol (Aldrich) was distilled prior to use. ZnEt2 bined in a procedure analogous to that for [Mg(mpoa)2].
(1.0 M solution in hexanes), MgBu2 (2.5 M solution in Recrystallization from toluene gave [Zn(mpoa)2] in 84% yield
hexanes), Ca(OiPr)2 (99.9+%), 2,4-dimethylphenol (98%), (1.68 mmol, 0.95 g). Anal. Calcd (found) for C28H40N2O6Zn (%,
2-methylaminomethyl-1,3-dioxolane (98%), and formaldehyde 566.01): C 59.42 (59.61), H 7.12 (7.08), N 4.95 (4.85)%; 1H NMR
(37% solution in H2O) were purchased from Aldrich and used (300 MHz, C6D6, 300 K): δ = 7.40 (s, 2H, ArH), 6.90 (s, 2H,
as received. The ligand tbpoa-H was prepared according to the ArH), 5.25 (t, JHH = 4.6 Hz, 2H, OCHO), 4.38 (br s, 4H, N–CH2–
literature.12a
Ar), 3.60–3.20 (m, 8H, CH2–O), 2.63 (d, JHH = 4.6 Hz, 4H,
1H and 13C NMR spectra were detected in the temperature N–CH2–CH), 2.58 (s, 6H, N–CH3), 2.45 (s, 12H, CH3), 2.30 (s,
range from 233 K to 333 K using Bruker ESP 300E or 500 MHz 12H, CH3); 13C NMR (125 MHz, C6D6, 300 K): δ = 16.2 (2C,
spectrometers. Chemical shifts are reported in parts per CH3), 20.8 (2C, CH3), 43.7 (2C, N–CH3), 59.5 (2C, N–CH2), 64.8
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(2C, N–CH2–Ar), 65.3 (4C, CH2–O), 102.9 (2C, OCHO), 119.4, collected using a KUMA KM4 CCD (ω scan technique) diffracto-
125.0, 129.7, 131.1, 132.2, 163.7 (12C, Ar). meter equipped with an Oxford Cryosystem-Cryostream
[Zn(tbpoa)2]. A solution of ZnEt2 (2.00 mL, 2.00 mmol), cooler.13 The space groups were determined from systematic
tbpoa-H12a (1.34 g, 4.00 mmol), and hexanes (50 mL) were absences and subsequent least-squares refinement. Lorentz
combined in a procedure analogous to that for [Mg(mpoa)2]. and polarization corrections were applied. The structures were
Recrystallization from CH2Cl2 at −15 °C gave [Zn(tbpoa)2] in solved by direct methods and refined by full-matrix-least
67% yield (0.99 g, 1.35 mmol). Anal. Calcd (found) for squares on F2 using SHELXTL Package.14 Scattering factors
C40H64N2O6Zn (%, 734.33): C 65.42 (64.96), H 8.78 (8.44), were taken from the literature.15 Non-hydrogen atoms were
N 3.81 (3.51); 1H NMR (300 MHz, C6D6, 300 K): δ = 7.56 (s, refined with anisotropic thermal parameters. Hydrogen atom
2H, ArH), 6.92 (s, 2H, ArH), 5.22, 5.13 (2 br s, 2H, OCHO), 4.53 positions were calculated and added to the structure factor
(d, JHH = 11.2 Hz, 2H, N–CH2–Ar), 4.23 (d, JHH = 11.2 Hz, 2H, calculations, but were not refined. All data (except structure
N–CH2–Ar), 3.41–3.09 (m, 8H, O–CH2), 2.53–2.25 (m, 4H, factors) have been deposited with the Cambridge Crystallo-
N–CH2–CH), 1.67 (s, 6H, N–CH3), 1.60 (s, 18H, C(CH3)3), 1.44 graphic Data Centre or as supplementary publications
(s, 18H, C(CH3)3); 13C NMR (125 MHz, C6D6, 300 K): δ = 30.2 CCDC-962646 and CCDC-962645. The solvated CH2Cl2 in
(4C, C(CH3)3), 32.3 (4C, C(CH3)3), 34.4 (4C, C(CH3)3), 35.6 (4C, [Zn(mpoa)2]·CH2Cl2 was distorted and it was refined with FVAR
C(CH3)), 42.7, 43.8 (2C, N–CH3), 59.3 (2C, N–CH2–CH), 64.9, of 0.69.
65.1 (2C, Ar–CH2–N), 66.1 (2C, CH2–O), 66.3 (2C, CH2–O),
102.9 (2C, OCHO), 120.4, 123.2, 125.9, 136.1, 138.2, 164.2
(12C, Ar).
Computational methods
Theoretical calculations of harmonic vibrational frequencies,
chemical shifts and energies for calcium, magnesium and zinc
[Ca(tbpoa)2]. To a solution of tbpoa-H (1.34 g, 4.00 mmol)
in toluene (50 mL) Ca(OiPr)2 (0.316 g, 2.00 mmol) was added
complexes were performed using TURBOMOLE 6.316 and
at room temperature. The solution was stirred and heated
Gaussian 09.17 During optimizations and frequency calcu-
under reflux for 48 hours. After cooling it was concentrated to
lations C2 point group symmetry constraints (for Ca-, Mg-, and
20 mL and a resulting white powder was filtered off, washed
with hexanes (20 mL) and dried in vacuo to give [Ca(tbpoa)2] in
SCF convergence criteria, and density fitting approach (resolu-
58% yield (0.82 g, 1.16 mmol). Anal. Calcd (found) for
few Zn-complexes), m4 grid (in TURBOMOLE notation), tight
tion-of-identity, RI) have been used. Geometries from crystal
C40H64N2O6Ca (%, 709.02): C 67.76 (67.91), H 9.10 (9.03),
1
structure investigations were taken as a starting point for the
N 3.95 (3.89); H NMR (300 MHz, C6D6, 300 K) δ: 7.60 (s, 2H,
full gas-phase optimization. Two terminal tBu groups were
ArH), 7.01 (s, 2H, ArH), 5.20 (br s, 2H, OCHO), 4.65 (br s, 4H,
N–CH2–Ar), 3.56–3.20 (m, 8H, O–CH2), 2.78–2.54 (m, 4H, N–
replaced by hydrogen atoms as shown in Fig. 1.
Since the X-ray data showed two slightly different isomers
for the zinc complex, both structures were considered in sub-
CH2–CH), 2.08 (s, 6H, N–CH3), 1.77 (s, 18H, C(CH3)3), 1.53 (s,
18H, C(CH3)3); 13C NMR (75 MHz, C6D6, 300 K): δ = 31.0 (4C,
sequent studies. In the first stage, geometry optimization and
C(CH3)3), 32.4 (4C, C(CH3)3), 35.1 (4C, C(CH3)3), 35.7 (4C,
C(CH3)), 42.7 (1C, N–CH3), 43.9 (1C, N–CH3), 62.8 (2C, N–CH2–
CH), 65.5 (2C, Ar–CH2–N), 66.5 (4C, CH2–O), 103.7 (2C,
TPSS-D3 means the meta-GGA functional with Grimme’s D3
OCHO), 124.0, 125.5, 128.0, 135.4, 139.2, 165.0 (12C, Ar).
vibrational analysis were performed at the TPSS-D3/def2-SVPD
level18–20 with respective default auxiliary basis sets,21 where
dispersion correction.22 Though for like-charged species (for
Representative procedure for solution polymerization
example two ligands in considered [ML2] complexes) the Cou-
lombic repulsion is usually dominant, as was recently shown
by Grimm et al., going from point charges model to real ions
one should also take into account London dispersion attrac-
tion. In some cases the dispersion not only overcomes electro-
static repulsion but also the entropy penalty of complex
In a typical experiment, the monomer L-LA and a solution of a
metal complex (M) in CH2Cl2 were placed in a Schlenk flask at
a fixed molar ratio. Then, after 10 minutes an external alcohol
in stoichiometric amount (M/ROH = 1/1) was added. The reac-
tion was stirred at the desired temperature for a prescribed
time. At certain time intervals, about 1 mL aliquots were
removed, precipitated with hexanes, and dried in vacuo. A con-
1
version was determined observing H NMR resonances of the
polymer and the monomer by dissolving the precipitates in
C6D6. After the reaction was completed, an excess of hexanes
was added to the reaction mixture. Filtration and vacuum
drying yielded a white polymer.
Details of X-ray data collection and reduction
[Zn(mpoa)2] and [Ca(tbpoa)2] (separately) were dissolved in
CH2Cl2 and placed in a freezer at −15 °C. After several days,
colourless, good quality crystals had formed. X-ray diffraction
Fig. 1 Schematic structure (with numbering of selected atoms;
H atoms omitted except for the CH protons of dioxolane rings) of [ML2]
data for [Zn(mpoa)2]·CH2Cl2 and [Ca(tbpoa)2]·C6H5CH3 were complexes (M = Ca, Mg, Zn).
2426 | Dalton Trans., 2014, 43, 2424–2436
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formation.22 Therefore the dispersion correction seems to be
important not only for quantitative but even for correct quali-
tative description of such molecules. All the vibration frequen-
cies in each studied complex were real. This proves that the
obtained structures are true minima. Such calculated mole-
cules were reoptimized at the TPSS-D3/def2-TZVPPD level20,23
(with respective auxiliary basis sets taken from TURBOMOLE
library). This functional was chosen because – as we found –
for the considered molecules it gives results which are quite
close to MP2 ones. The NMR results suggested that for the
zinc complex, depending on temperature, dioxolane arms
can be bound or non-bound. Therefore we optimized these
molecules with different dioxolane ring positions and found
additional isomers.
Isotropic 1H chemical shifts were computed (with the Gaus-
sian 09 package) as a difference between chemical shieldings
of reference hydrogen atom in tetramethylsilane (TMS, opti-
mized at the TPSS-D3/def2-TZVPPD level of theory) and proton
chemical shieldings in considered complexes. All these values
were obtained using the gauge independent atomic orbital
(GIAO) method24 for the gas-phase geometries. We employed
WP04 functional, proposed recently by Wiitala et al.25 and
6-31G(d,p) basis set. ChemCraft package has been used to
visualize some of the results.26
Scheme 1 Synthesis of homoleptic Zn, Mg, and Ca aminophenolates.
appear much poorer in these solvents. All compounds are
insoluble in aliphatic hydrocarbons.
Results and discussion
Synthesis and solid-state determination
The complexes were characterized by 1H and 13C NMR spec-
troscopy, which showed complicated dynamics in solution.
Monomeric, homoleptic magnesium, calcium or zinc alkox- Magnesium compound exhibited (both in 1H and 13C NMR
ides, like similar alkoxides of other metals, are rare. Their high spectra) multiple signals of the OCHO group of the dioxolane
tendency for bridging must be suppressed by the steric bulk of ring. In the proton spectrum there were two broad signals posi-
an alkoxo ligand and therefore proposed ligands contain tioned at δ: 5.69 and 5.49 ppm. Also the 13C spectrum showed
obstructed ortho and para positions of the phenol moiety as two signals of OCHO carbon, which were located at δ: 101.8
well as a hemilabile amino-arm with a coordinating dioxolane and 102.9 ppm. Moreover, the signal of the neighbouring
ring.
methylene N–CH2–CH protons appeared as a multiplet in the
Aminophenolates mpoa-H and N-[methyl(2-hydroxy-3,5-di- δ range: 3.40–3.28 ppm. The results suggest the coordination
tert-butylphenyl)]-N-methyl-N-methyl-1,3-dioxolaneamine (tbpoa-H) of just one dioxolane arm to the metal center in [Mg(mpoa)2]
were prepared according to modified Mannich condensation27 or the so-called “gorilla” effect (quick coordination and
using respective disubstituted phenol, paraformaldehyde, and decoordination of both dioxolane rings interchangeably).
2-methylaminomethyl-1,3-dioxolane as described in previous Interestingly, in [Zn(mpoa)2] the 1H NMR spectrum showed
literature12 and in the Experimental section. Next, both just one well resolved triplet of the OCHO group at δ: 5.25 ppm
ligands were used for complex syntheses. In the first thrust, a and the neighbouring methylene N–CH2–CH signal appeared
solution of mpoa-H was slowly treated with 2.5 M solution of as a doublet at δ: 2.63 ppm. Also the 13C NMR spectrum of
MgBu2 (0.5 eq.) in hexanes at room temperature to give a white [Zn(mpoa)2] exhibited a single peak of the OCHO group at δ:
solid of [Mg(mpoa)2] in 90% yield (after recrystallization from 102.9 ppm. This might be an effect of a slightly higher ionic
toluene) as shown in Scheme 1.
radius of Zn(II) compared to Mg(II), which enables a better
Analogous reactions of mpoa-H or tbpoa-H with ZnEt2 (1 M coordination of both dioxolane rings.28 An exchange of the
solution in hexanes) or tbpoa-H with Ca(OiPr)2 gave analyti- substituents in the phenolic part of the aminophenolate
t
cally pure [Zn(mpoa)2], [Zn(tbpoa)2], and [Ca(tbpoa)2] in ligand for larger the Bu group resulted in a “tighter” arrange-
84–58% yield as white solids. Only in the case of [Ca(tbpoa)2] a ment around the metal center, which affected its coordination
longer reaction time and an elevated temperature were mode. In [Zn(tbpoa)2], once again the 1H and 13C NMR spectra
applied. Compounds with a tbpoa ligand are well soluble in showed multiple signals for the OCHO group. The proton spec-
toluene, CH2Cl2 or THF, while the solubility of aminopheno- trum showed two broadened signals at δ: 5.22 and 5.13 ppm
lates with methyl substituents [Mg(mpoa)2] and [Zn(mpoa)2] and a multiplet of the adjacent methylene N–CH2–CH group
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groups coalesce at 333 K. This nice triplet of the methine
group can be observed, which proves the existence of pendant
dioxolane ring. It is worth mentioning that such bond-dan-
gling quick (on the NMR time scale) exchange of the coordi-
nation mode may have important implications on the catalytic
properties of this complex. Although some other isomers are
possible for [Zn(tbpoa)2] arising from different coordinations
of dioxolane rings or from the general geometry around the
central atom, there was no indication of those in the NMR or
X-ray data (similarly as for [Ca(tbpoa)2]).
t
In [Ca(tbpoa)2], despite the larger Bu substituent at the
phenolic moiety, the larger ionic radius of the calcium ion
enabled octahedral coordination. The 1H NMR spectra showed
a single broadened signal of the OCHO proton at δ: 5.20 ppm
and a multiplet in the δ range: 2.78–2.54 ppm indicating some
dynamics. A single signal was observed for the OCHO group in
13C NMR for [Ca(tbpoa)2] at δ: 103.7 ppm.
Fig. 2 The signals of methane OCHO (marked with bA, bB) and methyl-
ene N–CH2–Ar protons (marked with aA, aB) belonging to possible
isomers of [Zn(tbpoa)2] at room temperature (in d6-benzene). The signal
of methane OCHO proton of the internal standard (free tbpoa-H) was
marked with bC.
X-ray crystallography
Needle-shaped, colorless crystals of [Zn(tbpoa)2]·CH2Cl2 and
[Ca(tbpoa)2]·C6H5CH3 were grown by slow evaporation of
dichloromethane and toluene solution, respectively, and their
molecular structures were determined as outlined in Table 1
and summarized in the Experimental section.
in the δ range: 2.53–2.25 ppm. Interestingly the signal of the
methylene N–CH2–Ar protons in [Zn(tbpoa)2] appeared as a
pair of doublets, which might suggest that they became dia-
stereotopic as shown in Fig. 2. The effect is similar to that
observed for [Mg(mpoa)2] and suggests a similar “gorilla”
mechanism.
Interatomic distances and angles have been provided in
Table 2 and a view of each structure is given in Fig. 4 and 5.
The other explanation could be stereochemistry at the N
centers (the possible isomers for [Zn(tbpoa)2] are SS, RR and
R,S). This could be supported by the appearance of two signals
of the methyl N–CH3 groups in 13C NMR at δ: 42.7 and
43.8 ppm. This observation indicates that decoordination of
the dioxolane moiety causes stronger coordination of an N
arm of the aminophenolate ligand. In order to better under- Molecular weight
stand the nature of the phenomenon, a variable temperature
NMR experiment was performed in d8-toluene as shown in
Table 1 X-ray data for [Zn(tbpoa)2]·CH2Cl2 and [Ca(tbpoa)2]·C6H5CH3
[Ca(tbpoa)2]·
C6H5CH3
[Zn(tbpoa)2]·CH2Cl2
Molecular formula
C41Cl2H66N2O6Zn
819.23
C47H72CaN2O6
801.15
Temp. of collection (K)
Crystal system
Space group
100(2)
Monoclinic
C2/c
100(2)
Triclinic
ˉ
P1
Fig. 3. Noticeably, signals of all the methylene and methine a [Å]
18.403(4)
19.279(4)
25.718(5)
90
110.61(2)
90
8540(3)
8
1.274
10.293(3)
15.167(4)
16.176(4)
81.41(3)
74.00(3)
71.62(3)
2298.2(11)
2
b [Å]
c [Å]
α [°]
β [°]
γ [°]
V [Å3]
Z
dc [g cm−3
]
1.158
0.184
μ [mm−1
]
0.746
Crystal dimensions
[mm]
Θ Range [°]
0.33 × 0.20 × 0.03
0.24 × 0.18 × 0.15
2.61 ≤ Θ ≤ 25.04
−19,21; −22,22;
−30,30
43 167
7540
4984 [I > 2σ(I)]
489
0.0695
2.78 ≤ Θ ≤ 28.62
−13,13; −20,20;
−21,21
20 255
10 595
4007 [I > 2σ(I)]
508
0.0688
Range/indices (h,k,l)
No. of reflections
No. of unique data
No. of observed data
No. refined parameters
Rint
R indices [I > 2σ(I)]
R1 = 0.0765
wR2 = 0.1994
R1 = 0.1143
wR2 = 0.2274
1.048
R1 = 0.0688
wR2 = 0.1393
R1 = 0.1862
wR2 = 0.1799
0.900
R indices (all data)
Fig. 3 Variable temperature 1H NMR spectra for [Zn(tbpoa)2]. The signal Goodness of fit
Largest diff. peak hole
1.607, −0.716
0.812, −0.598
of methane OCHO proton of the internal standard (free tbpoa-H) was
marked with bC.
[e Å−3
]
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Table 2 Selected bond distances (Å) and angles (°) for [Zn(tbpoa)2]·CH2Cl2
and [Ca(tbpoa)2]·C6H5CH3
[Zn(tbpoa)2]·
CH2Cl2, A
[Zn(tbpoa)2]·
CH2Cl2, B
[Ca(tbpoa)2]·
C6H5CH3
Atoms
M–N(1)
M–N(2)a
M–O(1)
M–O(2)
M–O(4)a
M–O(5)a
2.092(4)
2.092(4)
2.106(4)
2.106(4)
2.608(3)
2.666(3)
2.189(3)
2.401(2)
2.210(2)
2.377(3)
1.923(4)
1.933(5)
(3.007(4))b
1.923(4)
(2.725(5))b
1.933(5)
(3.007(4))b
(2.725(5))b
Atoms
Angles
N(1)–M–N(2)a
N(1)–M–O(1)
N(1)–M–O(2)
N(1)–M–O(4)a
N(1)–M–O(5)a
N(2)a–M–O(1)
N(2)a–M–O(2)
N(2)a–M–O(4)a
N(2)a–M–O(5)a
O(1)–M–O(2)
O(1)–M–O(4)a
O(1)–M–O(5)a
O(2)–M–O(4)a
O(2)–M–O(5)a
O(4)a–M–O(5)a
147.4(2)
148.5(3)
96.25(17)
100.04(9)
78.70(9)
68.74(9)
171.73(9)
91.64(9)
165.81(10)
86.71(9)
80.16(9)
67.07(9)
105.68(9)
103.14(9)
98.77(10)
103.07(9)
144.36(9)
96.01(9)
96.20(16)
(71.03)b
98.99(16)
(82.01)b
98.99(16)
(82.01)b
96.20(16)
(71.03)b
(151.07)b
123.8(2)
(84.52)b
(84.52)b
(68.34)b
(151.07)b
(71.68)b
100.0(2)
(84.32)b
100.0(2)
(84.32)b
96.25(17)
(71.68)b
(159.58)b
117.1(4)
(81.87)b
(81.87)b
(80.60)b
(159.58)b
Fig. 5 The view of [Ca(tbpoa)2]·C6H5CH3 molecule (solvated toluene,
H-atoms and methyl group carbons from Bu substituents were omitted
for clarity).
t
metal is surrounded by two pairs of N,O atoms from two
aminophenolates.12b,29
a Atoms N(2), O(4), and O(5) are related to N(1), O(1), and (O2),
respectively, by symmetry operation: 1 − x, y, 1.5 − z. b Values in
parentheses are for the non-bonding interactions.
Nevertheless, a closer look at the positions of the cis-sited
dioxolane substituents shows that although the arms are dan-
gling they remain in close proximity to the metal center to
form a pseudooctahedral arrangement. Interestingly, the di-
oxolane O atoms being closer to the metal atoms differ signifi-
cantly in the M–O distance of 3.007(4) and 2.725(5) Å. The
pseudooctahedral arrangement is further supported by the
bond angles around the metal centers, as shown in Table 2.
Also here the O–M–O angles formed by O atoms from dioxo-
lane rings are markedly different for the two independent
molecules (68.3 vs. 80.6°). The observations – although made
for a solid state – support the NMR data, which showed impor-
tant dynamics of the ligands in [Zn(tbpoa)2]. The Zn(1)–N(1)
and Zn(2)–N(2) distances of 2.092(4) and 2.106(4) Å and Zn(1)–
O(1) and Zn(2)–O(2) distances of 1.923(4) and 1.933(5) Å are
similar to those found in Zn(tbpca)2 (L = N-[methyl(2-hydroxy-
3,5-di-tert-butylphenyl)]-N-methyl-N-cyclohexylamine; Zn–O(1) =
1.909(2) and Zn–N(1) = 2.130(2) Å), [Zn(L)]·H2O (L = 1-ethyl-
4,7-bis(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclo-
nonane; Zn–O = 1.963(1) and 1.934(1) Å; Zn–N = 2.113(1) and
2.277(1) Å)27 and [ZnL2] (L = N-(2-hydroxy-5-nitrobenzyl)-(R)-a-
methylbenzylamine; Zn–O = 1.935(2) and 1.933(2) Å; Zn–N =
2.0426(19) and 2.0458(19) Å).29
Fig. 4 The view of [Zn(tbpoa)2]·CH2Cl2 molecule (solvated CH2Cl2,
H-atoms and methyl group carbons from Bu substituents were omitted
for clarity; atoms with superscript are symmetry related, see Table 2
caption).
t
a
Although the solid structure of [Ca(tbpoa)2]·C6H5CH3 is
also a molecular monomer, it substantially differs from that of
The zinc and calcium solvates [Zn(tbpoa)2]·CH2Cl2 and [Ca- [Zn(tbpoa)2]·CH2Cl2. As has already been revealed by the NMR
ˉ
(tbpoa)2]·toluene crystallize in C2/c (monoclinic) and P1 (tri- data, both dioxolane rings in [Ca(tbpoa)2]·C6H5CH3 are co-
clinic) space groups, respectively. No internal or external ordinated to the metal center and remain in trans arrange-
hydrogen bonds were observed for both structures. There are ment. The Ca–O(2) and Ca–O(5) distances are 2.401(2) and
two independent molecules of [Zn(tbpoa)2]·CH2Cl2 in the unit 2.377(3) Å, respectively. The nitrogen atoms are cis one to the
cell, each lying at the two fold axis. In both, zinc(II) centers other and so are the phenoxo oxygens. The Ca–N(1) and Ca–
possess analogous four coordinated arrangement, where each N(2) distances are 2.608(3) and 2.666(3) Å and Ca–O(1) and
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Table 3 ROP of L-LA catalyzed by [Zn(tbpoa)2]
Ca [%]
Mn,calc b [g mol−1
]
Mn,obs c [g mol−1
]
Mw/Mn
d
Entry
ROH
[I]/[L-LA]/ROH
Time [min]
1
2
3
4
5
6
HCuCCH2OH
HCuCCH2OH
HCuCCH2OH
HCuCCH2OH
HCuCCH2OH
1/5/1
2
5
15
20
30
45
98
92
99
99
98
94
762
1382
2881
4337
7118
4335
908
1280
3333
4912
7612
4530
1.081
1.012
1.083
1.040
1.081
1.031
1/10/1
1/20/1
1/30/1
1/50/1
1/30/1
C
18H37OH
Reaction conditions: Vsolvent = 25 mL, toluene; T = 58 °C. a Obtained from 1H NMR. b Calculated from Mn,theo = [L-LA]0/[ROH]0 × C% × 144.13 +
MROH unless otherwise specified. c Determined by GPC calibrated versus polystyrene standards and corrected by a factor of 0.58 according to
literature recommendations.30 d Obtained from GPC.
Ca–O(4) are 2.189(3) and 2.210(2) Å. The octahedron around
the central atom is substantially distorted, which can be con-
cluded from the bond angles in Table 2.
Lactide polymerization
As has already been mentioned, a catalytic system based on a
homoleptic complex ML2/ROH combination for “the activated
monomer pathway” has been proposed for ROP of cyclic
esters, as an alternative to “single-site” LnM-OR catalysts. As
shown above, the aminophenolate ligands are able to form,
with M(II) ions, labile monomeric complexes whose dynamic
behaviour in solution can be crucial for catalytic activity in
Fig. 6 Polymerization of L-LA catalyzed by [Zn(tbpoa)2] in toluene at
ML2/ROH systems. The focus of our attention has now been
58 °C. The relationship between Mn (blue line) or PDI (red line) vs. the
initial molar ratio [L-LA]0/[ROH]0.
shifted towards verification of the reactivity of [M(tbpoa)2] (M =
Zn, Ca) and [M(mpoa)2] (M = Mg, Zn) as initiators in lactide
polymerization. After running several trials of the polymeriz-
Experimental results showed [Zn(tbpoa)2] to be an efficient
and the most interesting initiators for the polymerization of
L-LA. Representative results are collected in Table 3. The
choice of the propargyl alcohol was determined by the possi-
bility of conducting synthesis of end-functionalized oligomers,
which could later be applied as building blocks in molecular
engineering.
Based on the narrow PDI values (1.012–1.083) complex
[Zn(tbpoa)2] behaves in a controlled manner. The linear
relationship between the Mn and the conversion exhibited by
[Zn(tbpoa)2] implies the living character of the polymerization
process as shown in Fig. 6.
ation in THF, CH2Cl2, and toluene at 298–323 K, the latter
became the best choice for these systems. Nevertheless, poor
solubility of [M(mpoa)2] and low activity of [Ca(tbpoa)2]
excluded them from further experiments. Moreover, the
calcium complex in the presence of alcohols loses aminophe-
nolate ligands very easily and an insoluble mixture of calcium
compounds is formed. Ligand displacement is a prominent
feature of calcium complexes and the aminophenolate
[Ca(tbpoa)2] complex appeared to be no exception. Instead, the
most labile (according to the NMR data) compound
[Zn(tbpoa)2] appeared as efficient initiators for the polymeriz-
ation of L-LA.
The end-group analysis is demonstrated by the 1H NMR
spectrum of the PLA produced by initiators [Zn(tbpoa)2]/ROH
(ROH – propargyl alcohol, octadecanol) indicated that polymer
chains are terminated by hydroxyl and appropriate ester
groups as demonstrated in Fig. 7.
The polymerization conditions essentially determine the
activity of [Zn(tbpoa)2], which is in agreement with theoretical
calculations. At room temperature either the closed structure
or the structure with one pendant dioxolane arm is dominant.
The latter form after the coordination of lactide molecules
shows low polymerization activity. Instead, the change of con-
ditions to a fully open structure (both dioxolane arms
pendant) and the addition of lactide creates an active complex.
As proven by theoretical calculations, there is no big difference
which – of the many possible open conformations – is formed
Regardless of its low solubility, the magnesium complex
[Mg(mpoa)2] was reacted with 50 equiv. of L-LA and BzOH
(BzOH = benzyl alcohol) was added (1 eq.). It achieved high
conversion in 15 min and gave out polymers with moderate
Mw = 8700 and PDI = 1.1. The polymerization process is living
and polymer chains are terminated by aminophenolate and
hydroxyl groups. The polymerization results and NMR study
are consistent with the DFT study (see below), indicating a
possibility of equilibrium of five and six coordinated mag-
nesium species. Usually an octahedral environment of mag-
nesium atoms is preferred and therefore a coordination gap
(after decoordination of a hemilabile arm) is immediately sub-
stituted by lactide, which is the first step in the polymerization
process. These structural perturbations caused rending of one
aminophenolate ligand as a polymer end-group.
2430 | Dalton Trans., 2014, 43, 2424–2436
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Although 8′ has a structure which is close to the crystallo-
graphic one (or rather its mirror image), we found two similar
lower energy conformers: 8 and 8″, which differ mainly in
dioxolane rings arrangement (pseudo-rotation of this ring; see
ESI† for more detailed explanation).
Unlike Ca and Mg, which prefer octahedral arrangement,
zinc is usually tetrahedrally coordinated. The analysis of the
optimized geometrical parameters (Table 1S in ESI†) of
calcium and magnesium complexes show that Ca and Mg
cations are 6-coordinated and they are surrounded by six (four
O and two N) nearest neighbors in a distorted octahedral
arrangement. On the other hand, the optimized Zn initiator
appeared to have octahedral or rather pseudo-octahedral
coordination, but the analysis of M–N and M–O distances (see
Table 4 and 1S†), as well as ionic radii (0.74 Å, 4-coordinated
ion; 0.88 Å, 6-coordinated ion) led to the conclusion that its
structure more closely resembles a distorted tetrahedron. Cal-
culated and experimentally measured M–N and M–O (phenyl
oxygen) distances are the smallest in the Zn complex and the
largest in the Ca one. These results are consistent with ionic
radii of Ca (1.14 Å, 6-coordinated ion), Mg (0.86 Å, 6-co-
ordinated ion), and Zn (0.74 Å, 4-coordinated ion) cations.
Table 4 presents also ML-L and M-L interaction energies for
the ML2 complexes ad ML+ ions (M = Ca, Mg, Zn). The lowest
ΔE(ML-L) and ΔE(M-L) values have been found for calcium. In
the case of magnesium, ΔE(MgL-L) is over 20 kcal mol−1 larger
than for the Ca complex. The ΔE(Mg-L) value is much greater
than that for ΔE(Ca-L) and reaches 62 kcal mol−1. But the
difference between ΔE(MgL-L) and ΔE(ZnL-L) seems to be too
small to explain why in contrast to the zinc complex the mag-
nesium initiator loses one ligand during the polymerization
process. There may be different reasons for this: (i) in the Mg-
complex both ligands have conformations that are significantly
Fig. 7 1H NMR spectra of poly(L-lactide) with acetylene end-group.
after the dioxolane arms decoordinate, since they all easily
transform into each other. The most important thing is to
keep those reaction conditions that retain open forms. The
shutter of hemilabile arms at room temperature gave stable
and easy to store slipping catalysts.
DFT study
In order to gain a more detailed picture of the processes
taking place during the polymerization, theoretical calcu-
lations have been performed as described in the Experimental
section. First the interaction energies have been calculated:
ΔEðML-LÞ ¼ ꢀðEðML2Þ ꢀ EðMLþÞ ꢀ EðLꢀÞÞ
ΔEðM-LÞ ¼ ꢀðEðMLþÞ ꢀ EðM2þÞ ꢀ EðLꢀÞÞ
To calculate these values, in the first place we performed a
gas-phase geometrical optimization. The optimized structures distinct from those for Zn initiator (Fig. S1†), which may facili-
tate the incorporation of one ligand into the growing polymer
and (ii) one should remember that structures of these
are shown in Fig. 8 and the major conformers are marked 1
and 8′.
Fig. 8 Optimized structures of [Zn(tbpoa)2] complexes (H atoms omitted except for the CH protons of dioxolane rings) and their relative energies
(in kcal mol−1) calculated at the TPSS-D3/TZVPPD level of theory.
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Fig. 9 TPSS-D3/TZVPPD energy profile for the transition between 1
and 3. In () and [ ] parentheses are values calculated in dichloromethane
and toluene, respectively. All values in kcal mol−1
.
molecules in solution are not necessarily the same as in
crystal. As mentioned before the changes observed in NMR
signals for the Zn complex can be interpreted as a rotation of
dioxolane rings and hence the lack of coordination to the
metal cation. It can be seen from Fig. 9 that interconversion
between closed 1 and half-opened complex 2 though it is not
energetically favorable is possible.
Opening of the second dioxolane arm is also possible with
an even slightly smaller barrier. The influence of solvents on
calculated relative energies is not very significant (in all cases
obtained values were smaller). For the fully opened complex
we found additional lower energy conformer 3′ (Fig. 8). 3 and
3′ differ mainly in the distance of aromatic rings (see Fig. 2S
and Table 2S†). Interestingly, the energy difference between 3
and 3′ decreases going from gas phase to toluene (ε = 2.4) and
next to dichloromethane (ε = 8.9). In the case of the mag-
nesium complex such an opening of dioxolane arms is prob-
ably less favorable. To confirm this supposition we estimated
the contributions of individual fragments of the ligand (dioxo-
lane ring, amine part, and phenyl ring) to the total interaction
energy (see Fig. 3S in ESI† for more detailed explanation).
From Table 4, the contribution of dioxolane ring–metal inter-
action to the ligand–metal interaction energy is less than 7%
for Zn and 9–13% for Mg and Ca. As expected, the phenyl ring
oxygen–metal interaction is the largest part of the ΔE(M-L) and
it grows going from Ca to Mg and to the Zn complex. These
values are so large mainly due to the strong attractive Coulom-
bic interaction, and the observed trend is consistent with
decreasing M–OPh distances. A similar trend is observed for
the amine part–metal interaction, which is understandable,
taking into account the fact that zinc prefers coordination to
“softer” atoms like nitrogen.
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Fig. 10 TPSS-D3/TZVPPD energy profile for the transition between 1 and 8. In () and [ ] parentheses are values calculated in dichloromethane and
toluene, respectively. All values in kcal mol−1
.
In all considered complexes the ΔE(ML-L) is much smaller phenyl rings. The conformational flexibility of dioxolane and
than ΔE(M-L). It is worth noting that ΔE(M-L) − ΔE(ML-L) other five-membered ring molecules is a very well-known
difference is the smallest for CaL2 (∼160 kcal mol−1), medium fact.31–33 Therefore, in addition to the rotations of dioxolane
for MgL2 (∼201 kcal mol−1), and the largest for ZnL2 molecules ring around the N28–C33 (N72–C77) and C33–C36 (C77–C80)
(∼227–246 kcal mol−1). Because magnesium complex loses bonds it was important to consider pseudorotational
only one ligand during polymerization, a calculated larger motions.34,35 In this way, we obtained, for example, two
ΔE(M-L) − ΔE(ML-L) value for Mg in comparison to Ca is additional conformers of 1 and 8 (Fig. 8; see also Table 2S† for
consistent with experimental observations.
selected geometrical parameters of all Zn-complex confor-
Table 4 lists also partial NPA charges and metal cation mers). The energy differences between 1, 1′, and 1″ molecules
natural electron configurations for the ML2 complexes and are rather small and do not exceed 3 kcal mol−1. In the case of
ML+ ions. It is evident from this table that the most (negative) 8, 8′, and 8″ they are even smaller. Since the reported in litera-
charge is transferred to Zn and most of the additional electron ture values of the pseudorotation barrier for different oxolanes
density goes into the 3s and 4s orbitals for magnesium and are rather small, probably a multitude of conformers coexists
zinc, respectively. For the calcium complex a significant at room temperature.31,32,36,37 A similar difference in relative
portion of the electron density transferred to the metal ion energies was found for two other conformers: 3 and 3′.
resides in the 4s and 3d orbitals (Zn loses some electron However, as mentioned before, in contrast to 1, 1′, and 1″ or 8,
density in this orbital). In all cases, the presence of the metal 8′, and 8″, they differ in the mutual arrangement of aromatic
cation affects the electron density distribution of neighboring rings. In higher energy conformer 3 these rings are closer to
N and O atoms. As one can guess, the smallest change of this each other than in 3′.
distribution is observed for dioxolane oxygen atoms co-
ordinated to the zinc.
In Table 5 methine protons H45 and H81 (Fig. 1) chemical
shifts for all conformers which we found are reported.
Our calculations show that the lowest energy forms of the
1
For the Zn-complex, analysis of the experimental H NMR
spectrum indicates two signals of the methine protons of dioxo- zinc initiator (1, 1′, 1″, 5, 8, 8′, and 8″) have both dioxolane
lane rings, which appear at 5.22 and 5.13 ppm. But from arms closed. However, the comparison of experimental and
1
theoretical calculations we obtained two much more distant calculated H chemical shifts may suggest rather the presence
values: 5.26 ppm (conformer 1) and 5.76 ppm (conformer 8). of some half-opened or fully opened structures in solution. In
This may mean that instead of form 8 (or rather, as it was men- the opened 3, 3′, 11, and 12 forms the chemical shifts for H45
tioned before, its mirror image form) some other conformer(s) and H81 have the values of 5.21–5.42 ppm. For half-opened
is(are) present in solution. As mentioned before the solution molecules 2, 2′, 2″ signals in the similar range (5.18–5.38 ppm)
NMR studies indicate that bound and non-bound dioxolane have been observed. Although, for other conformers H45 and/
rings can readily exchange. Rotation of these groups may lead or H81 chemical shifts have larger values, nevertheless, their
to interconversion of 1 and 8. Fig. 10 shows such a possible presence in solution cannot be ruled out.
rotational path which passes through several minima.
It should be stressed that this is only one of the possible
rotational paths since this complex is quite flexible. During
the optimization we have obtained nineteen structures for the
Conclusions
Zn-complex (Fig. 8). The overall shape of these molecules is In summary, four Mg, Zn, and Ca complexes of aminopheno-
dependent on the combined effects of such factors as the late ligands with
a hemilabile (flexible) segment were
arrangement of dioxolane groups and distances between the described. The experimental data verified by theoretical
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Table 5 Chemical shift (ppm) relative to TMS for the Ca-, Mg-, and Zn-
complexes calculated at the WP04/6-31G(d,p)//TPSS-D3/TZVPPD level
of theory
δ(H45)
δ(H81)
[Ca(tbpoa)2]
[Mg(tbpoa)2]
[Zn(tbpoa)2]
5.70
6.45
5.70
6.45
1
5.26
5.26
5.40
5.21
5.18
5.18
5.21
5.23
5.54
5.86
6.53
6.57
5.94
5.76
6.44
5.46
5.27
5.38
5.42
5.26
5.39
5.40
5.26
5.38
5.23
5.21
5.23
5.27
5.31
5.25
5.81
5.70
5.76
6.44
5.31
5.23
5.24
5.42
1′
1″
2
2′
2″
3
3′
4
5
6
7
8
8′
8″
9
10
11
12
studies suggest that lability of dioxolane fragment is essential
to ensure a suitable structure of an active centre for zinc,
which is not the case for the calcium complex. Under opti-
mized conditions zinc bis(chelate)complex in the presence of
alcohol demonstrates efficient activities for living ROP of lac-
tides. The subtle changes observed in the calculated structure
of the complex allowed for optimization of the reaction con-
dition in order to improve the catalyst. Additionally these cal-
culations can elucidate the low stereocontrol in the ROP of
racemic lactide by complexes with prochiral ligands. Although
heteroleptic complexes are the most explored as initiators for
ROP of cyclic esters, yet, the combination of homoleptic ones
and external donors can provide an alternative way for catalysts
design based on kinetically labile complexes.
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Acknowledgements
The authors would like to thank the National Science Centre
in Poland (Grants N204 200640 and N204 136339) and the
Wroclaw Centre for Networking and Supercomputing (http://
www.wcss.wroc.pl; grant no. 162 and no. 011) for support of
this research.
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