An unprecedented Zn10O4 heteroadamantane cage containing anilido-pyridinate ligand and its activity for ring opening polymerization of l-lactide and ?-caprolactone

Article abstract of DOI:10.1039/c7dt01925d

(NPyC₂OMe)₈Zn₁₀Et₄(μ⁴-O)₄ (1) has been synthesized in situ by hydrolysis of ZnEt₂ in a one pot process and it was proved by applying X-ray structure analysis that the organozinc anilido-pyridinate moiety encapsulated the zinc oxide cluster, which demonstrated one self-assembled coordinate mode Zn₁₀O₄ . Moreover, the catalytic activity was investigated and good activity for l-lactide and ?-caprolactone ring opening polymerization was observed with both living and controllable properties.

Full text of DOI:10.1039/c7dt01925d

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An unprecedented Zn10O4 heteroadamantane
cage containing anilido-pyridinate ligand and
its activity for ring opening polymerization of
L-lactide and ε-caprolactone†
b
Cite this: DOI: 10.1039/c7dt01925d
Received 27th May 2017,
Accepted 6th July 2017
DOI: 10.1039/c7dt01925d
rsc.li/dalton
Ming-Tsz Chen *a and Chi-Tien Chen
*
(NPyC2OMe)8Zn10Et4(μ4-O)4 (1) has been synthesized in situ by
hydrolysis of ZnEt2 in a one pot process and it was proved by
applying X-ray structure analysis that the organozinc anilido-pyri-
dinate moiety encapsulated the zinc oxide cluster, which demon-
strated one self-assembled coordinate mode “Zn10O4”. Moreover,
the catalytic activity was investigated and good activity for
L-lactide and ε-caprolactone ring opening polymerization was
observed with both living and controllable properties.
Low-oxidation metal oxides can serve as valuable models in
various areas, including surface substrate interactions in
heterogeneous catalysis, metal oxide particles in porous solids,
deposition, precipitation, and condensation reactions that
lead to MxOy frameworks.1–5 In particular, zinc oxide com-
plexes have been well studied; possessing steric substituents
on zinc, the coordination number of oxide ions can vary from
two to four. Based on this early work, several structural motifs
were reported, including Zn2O,6–8 Zn3O,9 Zn4O,10–12 Zn4O2,13
Zn6O2 14,15 and Zn7O2 16,17 (Fig. 1).
Besides, Steiner et al. found that zinc oxide clusters intro-
duced two high nuclearity zinc oxide centres with types of
“Zn9O3” and “Zn12O6” which was generated by phosphazenate
ligands,18 as well as versatile zinc oxide clusters created by
Williams’ group.19 In this communication, we report the un-
precedented heteroadamantane cage zinc complex exhibiting
self-assembly coordinatate behavior of “Zn10O4” and its appli-
cation to perform the ring opening polymerization reaction
(ROP) of ε-caprolactone and L-lactide.
Fig. 1 Various molecular Zn–O assemblies.
oxazolinate/-pyrazolinate26 systems, motivated by interest in
ligand precursors bearing similar chelating systems and iso-
electronic feature related to amidonate ligands. Therefore, we
now aimed at he target of anilido-pyridnated ligands bearing
different functional groups, which was different from Kempe
et al.’s system.27 Herein, the ligand precursor HNPyC2
OMe
bearing the aliphatic pendant functional group was prepared
from amination of 2-bromopyridine with 2-methoxyethylamine
in the presence of CuI, L-proline and K3PO4 in DMSO at 110 °C
for 17 hours with moderate yield (84%),‡ following our pre-
viously established procedures.20–24
Previous studies on the compatibility of our Zn complexes
synthesis protocol were carried out employing HNPyC2
During the past few years, we systematically studied the
catalytic actvities for ROP containing anilido-oxazolinate,20–23
anilido-pyrazolate,24 anilido-aldimine25 and sulfonamido-
OMe
as
the ligand. It was surprisingly and reproducibly to isolate the
self-assembly molecule through metal–ligand-directed inter-
action to produce the specific structure 1 (Yield, 83% ),‡ even
in the presence of equivalent water as a distinct hydrate or
using dry toluene or hexane as the solvent, and all results were
in line with complex 1, which was fully characterized by NMR
spectroscopy and elemental analyses. The synthesis and pro-
posed structure are summarized in Scheme 1.§
aDepartment of Applied Chemistry, Providence University, Taichung 43301, Taiwan,
Republic of China. E-mail: mschen@pu.edu.tw
bDepartment of Chemistry, National Chung Hsing University, Taichung 402, Taiwan,
Republic of China. E-mail: ctchen@dragon.nchu.edu.tw
†Electronic supplementary information (ESI) available. CCDC 1545500. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c7dt01925d
A suitable crystal of 1 for structural determination was
obtained from hexane/THF solutions and the molecular struc-
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2.047(4) and 2.060(4) Å, respectively; the μ-oxo groups hϕad
Zn(2)–O(5) and Zn(4)–O(6) bond distances of 1.949(3) and
1.949(3) Å. Both Zn(3) and Zn(5) occupied vertex points of the
adamantyl ring and were coordinated with two nitrogen atoms
of the anilido groups with Zn(3)–N(2), Zn(3)–N(8); Zn(5)–N(4),
and Zn(5)–N(6) bond distances of 1.966(4), 2.061(3), 2.058(3),
and 1.972(4) Å, respectively. They were also coordinated with
the same μ-oxo groups with Zn(3)–O(5), Zn(3)–O(6), Zn(5)–O(5),
and Zn(5)–O(6) bond distances of 2.028(3), 1.918(3), 1.916(3),
and 2.035(3) Å, respectively.
The terminal coordination sites of the zincs were occupied
by the pyridine, μ-oxo and ethyl groups to create trigonal
planar and tetrahedral geometry at the zinc(1) and zinc(6),
respectively. There was one nitrogen atom of the pyridine
groups with Zn(1)–N(1) and Zn(6)–N(4) bond distances of
2.072(4) and 2.325(4) Å, respectively, one μ-oxo group with
Zn(1)–O(5) and Zn(6)–O(6) bond distances of 1.989(3) and
1.986(3) Å, respectively, and one carbon atom of the ethyl
group with Zn(1)–C(9) and Zn(6)–C(27) bond distances of
1.978(4) and 1.969(5) Å, respectively. However, for Zn(6), there
was one additional anilido group to coordinate with the metal
centre with a Zn(6)–N(5) bond distance of 2.068(4) Å. Overall,
not only was complex 1 a centrosymmetric complex that
induced self-assembly by the ligand and was simliar to
wurtize,28 but the bond angles and lengths also were similar to
those in zinc pyridyl systems.29–31
Scheme 1
ture is depicted in Fig. 2. The zinc centres of 1 possessed both
distorted tetrahedral and trigonal planar geometry modes. In
the adamantyl motif, each of the zinc atoms was four-co-
ordinated; Zn(2) and Zn(4) adopted the same coordinated be-
havior, which was bonded to the nitrogen atoms of the pyridyl
groups with Zn(2)–N(7) and Zn(4)–N(3) bond distances of
To our best knowledge, β-diketiminate complexes as cata-
lysts have shown excellent activities and a controllable manner
in ring opening polymerization,32–37 but with a few exceptions
for anilido-pyridinate systems. Moreover, zinc oxide complexes
bearing alkyl groups were not investigated in ROP studies;38,39
therefore, this is the first case study to use a zinc oxide cluster
as a catalyst in ROP. Polymerization of L-lactide (LA) and
ε-caprolactone (CL) employing 1 as the catalyst in the presence
of benzyl alcohol was investigated under a dry nitrogen atmo-
sphere. Representative results are listed in Table 1. The
optimum solvent/temperature combination was found to be
toluene at 50 °C in the presence of benzyl alcohol after several
Fig. 2 X-ray crystal structure of complex 1. Selected bond lengths (Å)
and bond angles (°): Zn(1)–C(9), 1.978(4); Zn(1)–O(5), 1.989(3); Zn(2)–
O(5), 1.949(3); Zn(3)–O(5), 2.028(3); Zn(3)–O(6), 1.918(3); Zn(4)–O(6),
1.949(3); Zn(1)–O(5)–Zn(3), 85.62(11); Zn(1)–O(5)–Zn(2), 111.69(13);
Zn(3)–O(6)–Zn(6), 126.13(15); Zn(4)–O(6)–Zn(3), 108.98(13). Hydrogen
atoms are omitted for clarity.
Table 1 Polymerization of L-lactide and ε-caprolactone using compound 1 as catalyst at 50 °Ca
Conv.d (%)
Yielde (%)
Mw/Mn
b
Entry
Monomer
{[M]0 : [ZnEt]0} : [BnOH]
Time (h)
Mn b (obsd)
Mn c (calcd)
1
2
3
4
5
6
50 : 1
50 : 1
100 : 1
200 : 1
300 : 1
100 : 2
0.17
0.67
0.75
1.25
2
—
—
35
87
92
84
97
80
—
77
83
76
87
72
—
14 700 (8500)
24 200 (14 000)
30 600 (18 400)
55 500 (32 200)
13 300 (7700)
6400
13 400
24 300
42 100
5900
1.10
1.12
1.08
1.13
1.08
0.67
7
8
9
10
50 : 1
100 : 1
150 : 1
200 : 1
1
2
2.25
3
13 000 (7300)
21 300 (11 900)
38 000 (21 300)
64 400 (36 100)
5700
11 200
16 500
33 700
98
97
96
98
80
83
81
84
1.16
1.18
1.14
1.15
a For L-lactide in 10 mL of toluene, [ZnEt]0 = 0.05 M, [BnOH] = 0.05 M and for ε-caprolactone in 15 mL of toluene, [ZnEt]0 = 0.125 M, [BnOH] =
0.125 M. b Obtained from GPC analysis and calibrated by polystyrene standard. Values in parentheses are the values obtained from GPC times
0.58 for L-lactide, and 0.56 for ε-caprolactone. c Calculated from [M(lactide or ε-caprolactone) × [M]0/[ZnEt]0 × conversion yield/([BnOH](eq)] +
M(BnOH)). d Obtained from 1H NMR analysis. e Isolated yield.
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trials of running the polymerization with CH2Cl2, THF and
toluene using 1 as the catalyst.
Experimental results showed that compound 1 demon-
strated comparable activities with anilido-oxazolinate zinc
systems within a similar period (50 equiv. monomer loading),
whereas
1 exhibited good control polydispersity index
(PDI).20,23 Following this optimized protocol, we then
increased the monomer/catalyst ratio from 50 to 300 equiv. to
obtain the corresponding polylactides in good yield with
controllable PDIs (1.08–1.13). However, using the high ratios
of monomer at 50 °C (entries 4–5), demonstrated by 1 might
result from the interaction between the pendant functionality,
the metal centre and the rigidity of adamantyl core group,
which hindered the coordination of benzyl alcohol or
monomer to the metal centre, leading to a decrease in propa-
gation compared to the anilido-oxazolinate zinc systems.20,23
The living and immortal characters were examined and the
linear relationship between the number-average molecular
weight (Mn) and the monomer-to-initiator ratio ([M]0/[I]0) demon-
strated in Fig. 3 (Table 1, entries 2–5) implies the “living” charac-
ter of the polymerization process. The “immortal” character was
examined using two equiv. ratios (of [M]0/[ZnEt]0) of benzyl
alcohol as the chain transfer agent (Table 1, entry 6). The end
group analysis was demonstrated by the 1H NMR spectrum of
the polymer produced from L-lactide and 1 ([M]0/[BnOH] = 50).
Peaks were assignable to the corresponding protons in the pro-
posed structure with the capped benzyl alkoxyl group, as shown
in Fig. 4. This implied the complex 1/BnOH system might
undergo an ‘activated monomer’ pathway, in which monomer or
BnOH were activated by 1 and BnOH acted as a nucleophile.40,41
Complex 1 was also investigated to explore its catalytic be-
havior in the ROP of CL. Selected representative experimental
results indicated that complex 1 has the better controlled
manner in ROP of CL in Table 1 (controllable Mn in entries
7–10). These reactions gave PCLs with reasonable number-
average molecular weight (Mn) and narrow PDI values
(1.15–1.18). The plot of Mn vs. ([M]0/[I]0) demonstrated by
those data initiated by 1 exhibited a linear relationship, indi-
cating the “living” character of the polymerization process, as
shown in Fig. S1 and S2† (Table 1, entries 7–10).¶ The catalytic
mode of CL was observed and was similar to that for LA.
Fig. 4 1H NMR spectrum of PLLA-50 catalyzed by 1 in toluene at 50 °C.
Conclusions
In summary, a self-assembled anilido-pyridinate zinc cage
complex bearing Zn10O4 coordinate behavior was investigated
and applied to the ring opening polymerization of L-lactide
and ε-caprolactone, in high yields with controllable behaviors
within a short reaction time. In addition, more efforts in the
expansion of anilido-pyridinate containing different moieties
systems are currently ongoing.
Acknowledgements
We are grateful to the Ministry of Science and Technology of
Taiwan for financial support (grant number MOST 106-2113-
M-126-001-MY2 and MOST 105-2119-M-005-006).
Notes and references
‡HNPyC2OMe and compound 1 experimental and spectral data are listed in ESI.†
§Crystallographic data of compound 1 is summarized in Table S1 of ESI,† and
crystal data of 1 has been deposited with the Cambridge Crystallographic data-
base as CCDC numbers 1545500.†
¶Fig. S1 and S2 are depicted in ESI.†
1 A. Erxleben, Coord. Chem. Rev., 2003, 246, 203–228.
2 M. Melnik, K. Györyová, J. Skoršepa and C. E. Holloway,
J. Coord. Chem., 1995, 35, 179–279.
3 H. W. Roesky, I. Haiduc and N. S. Hosmane, Chem. Rev.,
2003, 103, 2579–2596.
4 V. Chandrasekhar, S. Nagendran and V. Baskar, Coord.
Chem. Rev., 2002, 235, 1–52.
5 D. Prochowicz, K. Sokołowski and J. Lewiński, Coord. Chem.
Rev., 2014, 270–271, 112–126.
6 F. Meyer and P. Rutsch, Chem. Commun., 1998, 1037–1038.
7 M. Ruf and H. Vahrenkamp, Inorg. Chem., 1996, 35, 6571–
6578.
8 M. Ruf and H. Vahrenkamp, J. Chem. Soc., Dalton Trans.,
1995, 1915–1916.
Fig. 3 Polymerization of L-lactide catalyzed by 1 in toluene at 50 °C.
9 A. Murso and D. Stalke, Dalton Trans., 2004, 2563–2569.
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Communication
Dalton Transactions
10 J. L. C. Rowsell, E. C. Spencer, J. Eckert, J. A. K. Howard 27 A. Noor, S. Schwarz and R. Kempe, Organometallics, 2015,
and O. M. Yaghi, Science, 2005, 309, 1350–1354. 34, 2122–2125.
11 H. Koyama and Y. Saito, Bull. Chem. Soc. Jpn., 1954, 2, 112– 28 W. Bury, E. Krajewska, M. Dutkiewicz, K. Sokolowski,
114.
I. Justyniak, Z. Kaszkur, K. J. Kurzydlowski, T. Plocinski
and J. Lewinski, Chem. Commun., 2011, 47, 5467–
5469.
12 L. Hiltunen, M. Leskelä, M. Mäkelä and L. Niinistö, Acta
Chem. Scand., 1987, 548–555.
13 G. Zhu, J. M. Tanski and G. Parkin, J. Chem. Crystallogr., 29 M. Westerhausen, A. N. Kneifel and A. Kalisch, Angew.
2002, 32, 469–475. Chem., Int. Ed., 2005, 44, 96–98.
14 M. A. Malik, P. O’Brien, M. Motevalli and A. C. Jones, Inorg. 30 S. Javed and D. M. Hoffman, Dalton Trans., 2010, 39,
Chem., 1997, 36, 5076–5081. 11439–11444.
15 R. E. Marsha and A. L. Spekb, Acta Crystallogr., Sect. B: 31 M. Westerhausen, T. Bollwein, N. Makropoulos,
Struct. Sci., 2001, 57, 800–805.
T. M. Rotter, T. Habereder, M. Suter and H. Nöth,
16 N. Lalioti, S. P. Perlepes, E. Manessi-Zoupa,
Eur. J. Inorg. Chem., 2001, 2001, 851–857.
C. P. Raptopoulou, A. Terzis, A. E. Aliev and 32 D. R. Moore, M. Cheng, E. B. Lobkovsky and G. W. Coates,
I. P. Gerothanassis, Chem. Commun., 1998, 1513–1514. Angew. Chem., Int. Ed., 2002, 41, 2599–2602.
17 D. J. Darensbourg, J. R. Wildeson and J. C. Yarbrough, 33 H. Hao, C. Cui, H. W. Roesky, G. Bai, H.-G. Schmidt and
Inorg. Chem., 2002, 41, 973–980. M. Noltemeyer, Chem. Commun., 2001, 1118–1119.
18 G. T. Lawson, C. Jacob and A. Steiner, Eur. J. Inorg. Chem., 34 M. Cheng, A. B. Attygalle, E. B. Lobkovsky and
1999, 1881–1887. G. W. Coates, J. Am. Chem. Soc., 1999, 121, 11583–11584.
19 S. D. Pike, E. R. White, M. S. P. Shaffer and C. K. Williams, 35 L. R. Rieth, D. R. Moore, E. B. Lobkovsky and G. W. Coates,
Nat. Commun., 2016, 7, 13008. J. Am. Chem. Soc., 2002, 124, 15239–15248.
20 C. T. Chen, C. Y. Chan, C. A. Huang, M. T. Chen and 36 H.-Y. Chen, B.-H. Huang and C.-C. Lin, Macromolecules,
K. F. Peng, Dalton Trans., 2007, 4073–4078. 2005, 38, 5400–5405.
21 C. T. Chen, H. J. Weng, M. T. Chen, C. A. Huang and 37 J. Wu, T.-L. Yu, C.-T. Chen and C.-C. Lin, Coord. Chem. Rev.,
K. F. Peng, Eur. J. Inorg. Chem., 2009, 2129–2135. 2006, 250, 602–626.
22 M. T. Chen, P. J. Chang, C. A. Huang, K. F. Peng and 38 R. Boomishankar, P. I. Richards and A. Steiner, Angew.
C. T. Chen, Dalton Trans., 2009, 9068–9074. Chem., Int. Ed., 2006, 45, 4632–4634.
23 M. T. Chen and C. T. Chen, Dalton Trans., 2011, 40, 12886– 39 P. I. Richards, R. Boomishankar and A. Steiner,
12894.
J. Organomet. Chem., 2007, 692, 2773–2779.
40 C.-A. Huang, C.-L. Ho and C.-T. Chen, Dalton Trans., 2008,
3502–3510.
41 C. Romain, J. A. Garden, G. Trott, A. Buchard, A. J. P. White
and C. K. Williams, Chem. – Eur. J., 2017, 23, 7367–
7376.
24 K.-F. Peng and C.-T. Chen, Dalton Trans., 2009, 9800–9806.
25 C.-T. Chen, M.-C. Wang and T.-L. Huang, Molecules, 2015,
20, 5313.
26 C.-T. Chen, C.-H. Liao, K.-F. Peng, M.-T. Chen and
T.-L. Huang, J. Organomet. Chem., 2014, 753, 9–19.
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