Crystallographic characterisation of novel Zn(II) silsesquioxane complexes and their application as initiators for the production of polylactide

Article abstract of DOI:10.1016/j.poly.2009.05.024

Herein, the solid state structures of the products from the reaction of the silsesquioxane triol (iso-C₄H₉)₇Si₇O₁₂(OH)₃ (1) with two equivalents of ZnMe₂ in both THF and toluene are reported. In both cases tetrametallic Zn(II) complexes were isolated, with toluene [(iso-C₄H₉)₇Si₇O₁₂]₂Zn₄Me₂ (2) was prepared while performing the reaction in THF the analogous complex [(iso-C₄H₉)₇Si₇O₁₂]₂Zn₄Me₂(THF)₂ (3) was formed. Both species have also been characterised via ¹H, ¹³C{¹H} and ²⁹Si{¹H} NMR spectroscopy, which confirm the solid state structures are maintained in solution. Both 2 and 3 show modest activities for the polymerisation of rac-lactide and a heterogeneous catalyst has also been prepared.

Full text of DOI:10.1016/j.poly.2009.05.024

Polyhedron 29 (2010) 312–316
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Crystallographic characterisation of novel Zn(II) silsesquioxane complexes
and their application as initiators for the production of polylactide
a
a
b
Matthew D. Jones a, , Callum G. Keir , Andrew L. Johnson , Mary F. Mahon
*
a Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK
b Bath Chemical Crystallography Unit, Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 19 May 2009
Herein, the solid state structures of the products from the reaction of the silsesquioxane triol (iso-
C4H9)7Si7O12(OH)3 (1) with two equivalents of ZnMe2 in both THF and toluene are reported. In both cases
tetrametallic Zn(II) complexes were isolated, with toluene [(iso-C4H9)7Si7O12]2Zn4Me2 (2) was prepared
while performing the reaction in THF the analogous complex [(iso-C4H9)7Si7O12]2Zn4Me2(THF)2 (3) was
formed. Both species have also been characterised via 1H, 13C{1H} and 29Si{1H} NMR spectroscopy, which
confirm the solid state structures are maintained in solution. Both 2 and 3 show modest activities for the
polymerisation of rac-lactide and a heterogeneous catalyst has also been prepared.
Keywords:
Silsesquioxane
Zinc
Crystallography
Polylactide
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
pared by reacting one equivalent of triol 1 with two equivalents of
ZnMe2 in toluene (2) or THF (3) as the solvent (Scheme 1). The re-
It is known that silsesquioxanes can be used as models for silica
surfaces and major advances in this area and general silsesquiox-
ane coordination chemistry have been achieved [1–5]. For exam-
ple, Johnson and Crocker have prepared titanium silsesquioxanes
as models for heterogeneous epoxidation catalysts [6–8]. We have
recently utilised Ti(IV) and Al(III) silsesquioxane complexes as
models for heterogeneous initiators for polylactide production
[9]. This paper details the preparation and characterisation of
two silsesquioxane complexes based on Zn(II) and the single crys-
tal X-ray structure of the parent trisilanol. To the best of our
knowledge (from the Cambridge structural data base) only one
crystallographically characterised Zn(II) silsesquioxane has been
reported [10]. In this example, a Zn–Me complex of the silsesquiox-
anedisilanol (c-C5H9)7Si7O9(OSiMePh2)(OH)2 was described [11].
Hitherto, there are no literature reports of crystallographically
characterised examples of the reaction of the silsesquioxanetrisila-
nol family with a Zn(alkyl)2 [10]. However, an analogous tetranu-
clear magnesium silsesquioxane complex has been prepared by
the reaction of the triol with methylmagnesium chloride [12].
Other examples of silsesquioxane metal complexes include alu-
minium [13–17], vanadium [18], tin [19], gallium [15], molybde-
num [20] and group 4 metals [21–24].
sults of which will be compared to a heterogeneous system.
2. Results and discussion
The molecular structure of the parent silsesquioxane (iso-
C4H9)7Si7O12(OH)3 (1) was determined and is shown in Fig. 1. This
was previously reported by Feher, however, no discussion of metric
data was presented due to the quality of the data set [25].
The bond lengths and angles are in agreement with previously
reported triol silsesquioxanes in the literature [26,27]. Recently,
Unno and co-workers have reported the structure of 1, which is
analogous (in respect to metric data including H-bonding) to that
reported herein [28]. One equivalent of 1 was treated with two
equivalents of ZnMe2 in toluene, recrystallisation in hexane
yielded crystals of 2 upon standing at room temperature (see
Fig. 2). Complex 2 is a tetranuclear zinc compound with two chem-
ically distinct zinc centres {Zn(1) and Zn(2)} and 2 is soluble in
common organic solvents such as CH2Cl2, THF and toluene. There
is a crystallographic centre of inversion in the molecule and 2 crys-
tallises in the triclinic space group P1. The four zinc centres form an
interlayer between two trianionic silsesquioxane units. There are
three planar four membered rings (two zinc and two oxygen cen-
tres) present. The central ring {Zn(2), Zn(2A), O(13), O(13A)} is al-
most perpendicular to the two outer rings, with the angle between
the planes formed from Zn2A–O13–Zn2–O13A and Zn2–O2–Zn1–
O5A being 87°. Zn(1) is bound to two oxygen moieties {O(2) and
O(5A)} and the methyl group C(1), with the metric data for Zn(1)
being in agreement with the only previously crystallographically
characterised Zn–silsesquioxane complex in the literature (see
There is continued interest in the coordination chemistry of sils-
esquioxanes and the use of these as soluble models for homoge-
neous catalysts. Therefore, in this paper we report the full
characterisation of two novel Zn(II) silsesquioxane complexes pre-
* Corresponding author. Tel.: +44 (0)1225 384908; fax: +44 (0)1225 386231.
E-mail address: mj205@bath.ac.uk (M.D. Jones).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.05.024

M.D. Jones et al. / Polyhedron 29 (2010) 312–316
313
Scheme 1. Zn(II) silsesquioxane complexes prepared in this study.
Table 1) [11]. Zn(1) is in an extremely distorted trigonal planar
geometry, as exemplified by the O(2)–Zn(1)–C(1) 139.70(18)°,
O(5A)–Zn(1)–C(1) 135.60(18)° and O(2)–Zn(1)–5(A) 84.64(11)° an-
gles. The Zn(1)–C(1) distance of 1.922(5) Å, is in agreement with
the literature precedent [29,30]. Zn(2) is bound to four oxygen cen-
tres and has a distorted tetrahedral geometry, which is exemplified
by the angles O(13)–Zn(2)–O(13A) 87.71(11)° and O(13)–Zn(2)–
O(2) 120.70(10)°. The central metallic core in 2 is different to that
produced from the reaction of (c-C5H9)7Si7O9(OSiMePh2)(OH)2
with Zn(Me)2 where one of the oxygen moieties is bound to three
Zn-centres and all Zn(II) centres remain coordinated to a methyl
group, which is not the case in this example [11]. The metallic core
of 2 is analogous to that of Mg(II)–(c-C5H9)7Si7O9(OH)3 complex
[12]. In solution the 29Si{1H} NMR spectrum (C6D6) shows five res-
onances in a 2:1:1:2:1 ratio which is consistent with the solid state
structure being maintained in solution. In the 1H NMR spectrum a
singlet at À0.12 ppm was detected, characteristic for the Zn–Me
resonance, this was further confirmed via a sharp singlet at
À15.7 ppm in the 13C{1H} NMR spectrum [11,30].
When the same reaction was performed in THF complex 3
(Fig. 3) was isolated, which is highly soluble in THF, toluene and
hexane. Crystals suitable for X-ray diffraction were obtained by
cooling (À20 °C) a saturated solution of 3 in hexane. Complex 3 also
crystallises in the triclinic space group P1. Again a tetranuclear
arrangement of Zn(II) is observed with three Zn2O2 rings. These
rings are more puckered than in 2, this is exemplified by the angle
between the Zn2–O5A–O2 and O2–Zn1–O5A planes of 161° cf. 177°
for the analogous angle in 2. Zn(1) has a highly distorted tetrahedral
geometry which is illustrated by the following O(2)–Zn(1)–C(1)
131.17(14)° and O(2)–Zn(1)–O(1) 97.76(10)°. The Osil–Zn(1) dis-
tances are significantly longer in 3 than 2 (Osil–Zn av. 2.018 Å cf.
1.957 Å for 2) due to the coordinated THF molecule. Zn(2) sits in a
distorted tetrahedral environment, analogous to 2. Again the solu-
tion 1H and 13C{1H} NMR spectra clearly show resonances for the
Zn–Me moiety (À0.18 ppm in the 1H NMR and À15.6 ppm in the
13C{1H} NMR). The 29Si{1H} NMR has five resonances in
a
2:1:1:2:1 ratio indicating that the structure is maintained in solu-
tion. The metric data for 2 and 3 are summarised in Table 1.
Fig. 1. The molecular structure of 1. Selected bond lengths (Å) and angles (°) are
Si(1)–O(1) 1.6293(16), Si(1)–O(2) 1.6205(18), Si(1)–O(5) 1.6143(17), Si(4)–O(5)
1.6133(17), Si(4)–O(7) 1.6190(18), Si(4)–O(8) 1.6190(18), O(2)–Si(1)–O(5)
110.34(9), O(2)–Si(1)–O(1) 108.92(10). Hydrogen atoms not involved in hydrogen
bonding have been removed for clarity.
Fig. 2. The molecular structure of 2. The iso-butyl groups and hydrogen atoms have
been omitted for clarity. Labels with suffix A relate to those in the asymmetric unit
by the Àx, À y + 2, Àz symmetry operation.

314
M.D. Jones et al. / Polyhedron 29 (2010) 312–316
of the 1H NMR spectra were all atactic. A blank run with pure
SiO2 failed to produce polymer under analogous conditions.
Table 1
Selected bond lengths (Å) and angles (°) for complexes 2 and 3.
2
3
Zn(1)–O(1)
Zn(1)–C(1)
Zn(1)–O(2)
Zn(1)–O(5A)
Zn(2)–O(2)
Zn(2)–O(13)
Zn(2)–O(13A)
Zn(2)–O(5A)
O(1)–Zn(1)–C(1)
O(2)–Zn(1)–C(1)
O(2)–Zn(1)–O(5A)
O(1)–Zn(1)–O(2)
O(13)–Zn(2)–O(2)
O(13)–Zn(2)–O(5A)
2.127(3)
1.951(4)
2.015(2)
2.021(2)
1.949(2)
1.946(2)
1.984(2)
1.961(2)
107.42(14)
131.17(14)
83.06(8)
97.76(10)
123.43(8)
129.48(9)
3. Conclusions
1.923(5)
1.954(3)
1.959(3)
1.956(3)
1.959(3)
1.951(3)
1.946(3)
In conclusion two novel Zn–silsesquioxane complexes are re-
ported. Both complexes have a tetrameric zinc layer with two cap-
ping silsesquioxanes. The complexes have been used as
homogeneous initiators for the ring opening polymerisation of
rac-lactide and a heterogeneous catalyst has been prepared for
comparison.
139.71(18)
84.65(11)
4. Experimental
120.69(10)
119.41(12)
4.1. General procedures
1H, 13C{1H}and 29Si{1H} NMR spectra were recorded on a Bruker
500 MHz spectrometer, and referenced to residual solvent peaks of
C6D6 which was dried prior to use. Coupling constants are given in
Hertz. Elemental analysis was performed by Mr. A.K. Carver at the
Department of Chemistry, University of Bath. In all cases the yields
quoted are for those of the crystalline material. Both 2 and 3
decomposed before they melted.
0
4.2. X-ray crystallography
All data were collected on a Nonius Kappa CCD area detector
diffractometer using Mo Ka radiation (k = 0.71073 Å) at a temper-
ature of 150(2) K, and all structures were solved by direct methods
and refined on all F2 data using the SHELXL-97 suite of programs [41].
All hydrogen atoms, were included in idealised positions and re-
fined using the riding model. For both 2 and 3 the ADPs for the car-
bons of the iso-butyl groups are slightly less isotropic than
desirable, where appropriate these have been modelled. For 3 the
iso-butyl groups adoring Si(2), Si(3) and Si(5) were modelled over
two sites, in all cases a 50:50 model provided the best convergence.
In the case of 2 it was slightly more complex with the carbons of
the iso-butyl groups of the following silicon sites being disordered
Si(1) {55:45}, Si(2) {65:35}, Si(3) {60:40}, Si(5) {50:50}, Si(6)
{80:20} with the numbers in brackets representing the occupancy
that afforded the best convergence. For the pure iso-butyl trisilanol
1 the iso-butyl group from Si(6) was disordered in a 65:35 occu-
pancy. Multi-scan absorption corrections were applied where
appropriate.
Fig. 3. The molecular structure of 3. The iso-butyl groups, hydrogen atoms and the
carbons of the THF moiety have been omitted for clarity. Labels with suffix A relate
to those in the asymmetric unit by the Àx, Ày + 1, Àz + 1 symmetry operation.
Table 2
Polymerisation data.
b
Initiator
Yielda
Mw
PDIb
Prc
4.3. Synthesis and characterisation of 1–3
2
3
15
10
10
2400
1900
1800
1.08
1.12
1.08
0.5
0.5
0.5
Heterogeneous
During the course of this work the crystal structure of the free
silsesquioxane was determined: Crystal data for 1: C28H66O12Si7,
M = 791.44, 0.20 Â 0.20 Â 0.20 mm3, monoclinic, space group P21/
a
Isolated yield.
b
Determined from GPC measurements (THF at 1 ml per minute, relative to
polystyrene standards).
n,
a = 14.102(1) Å,
b = 21.335(2) Å,
V = 4368.79(6) Å3,
Z = 4,
a radiation, 2hmax = 55.0°, 77 415 reflections col-
c = 14.748(1) Å,
Calculated from analysis of the methine region of the 1H homonuclear decou-
c
b = 100.072(1)°,
F000 = 1712, Mo K
Dc = 1.203 g cmÀ3
,
pled spectrum.
lected, 10 008 unique (Rint = 0.0396). Final GooF = 1.130,
R1 = 0.0467, wR2 = 0.1187, R indices based on 8672 reflections with
Zn(II) complexes have found great utility as initiators for the
production polylactide (PLA) [31–36]. Heterogeneous catalysts
are finding utility in the area of ring opening polymerisation
(ROP) initiators for PLA [9,37–40]. A heterogeneous initiator was
prepared by treating a silica surface with ZnMe2, in an analogous
fashion to the soluble silsesquioxane complexes 2 and 3. Com-
plexes 2, 3 and the heterogeneous system were trialled for the
ROP of rac-lactide in toluene (iso-propyl alcohol was added to gen-
erate the Zn–OiPr in situ, see Section 4). Modest conversions were
obtained after prolonged reaction time (Table 2). The polymers
produced had low PDIs and from analysis of the methine region
I > 2
= 0.268 mmÀ1
r 0 restraints,
(I) (refinement on F2), 468 parameters,
l
.
4.3.1. Preparation of [(iso-C4H9)7Si7O12]2Zn4Me2 (2)
1 (1.57 g, 2 mmol) was dissolved in toluene (20 ml). To this
ZnMe2 (2 ml, 2 M solution in toluene, 4 mmol) was added, there
was vigorous effervescence. The mixture was stirred at room tem-
perature for 1 h. After which time the toluene was removed in va-
cuo and the white solid was recrystallised in the minimum amount
of hexane. This was left at room temperature and after 24 h crys-

M.D. Jones et al. / Polyhedron 29 (2010) 312–316
315
tals suitable for X-ray diffraction were obtained, yield 0.95 g 50%.
1H (C6D6) À0.12 (6H, s, Zn–CH3), 0.77–0.99 (28H, m, SiCH2),
1.02–1.34 (84H, m, CH3), 2.02–2.30 (14H, m, CH). 13C{1H} (C6D6)
À15.7 (Zn–CH3), 22.9, 23.2 (CH2), 24.4 (CH2), 24.4, 24.5, 24.6
(CH), 24.7 (CH2), 24.8 (CH), 24.9 (CH2), 25.9, 26.1, 26.2, 26.2,
26.4, 26.6, 26.7 (CH3). 29Si{1H} (C6D6) À57.7, À62.5, À64.9, À65.3,
À70.3 (2:1:1:2:1 ratio). Elemental analysis: Anal. Calc. for
C58H132Zn4O24Si14: C, 37.28; H, 7.12. Found: C, 37.3; H, 6.94%. Crys-
tal data for 2: C29H66O12Si7Zn2, M = 934.19, 0.20 Â 0.15 Â
0.10 mm3, triclinic, space group P1, a = 13.7070(5) Å, b =
clear decoupled 1H NMR spectra, the equations used to calculate Pr
and Pm are given by Coates et al. [32]. Gel permeation chromatog-
raphy (GPC) analyses were performed on a Polymer Laboratories
PL-GPC 50 integrated system using
a PLgel 5 lm MIXED-D
300 Â 7.5 mm column at 35 °C, THF solvent (flow rate, 1.0 ml/
min). The polydispersity index (PDI) was determined from Mw/
Mn, where Mn is the number average molecular weight and Mw
the weight average molecular weight. The polymers were refer-
enced to 11 narrow molecular weight polystyrene standards with
a range of Mw 615–568 000 Da. For PLA the Mn values obtained
from GPC are typically higher than expected due to the hydrody-
namic volume difference between PLA and the polystyrene stan-
dards [42].
14.1560(7) Å, c = 14.3980(8) Å,
c
a = 71.362(2)°, b = 68.977(2)°,
= 62.959(3)°, V = 2281.18(19) Å3, Z = 2, Dc = 1.360 g cmÀ3, F000
=
988, 2hmax = 50.3°, 21 134 reflections collected, 7987 unique
(Rint = 0.0664). Final GooF = 1.042, R1 = 0.0485, wR2 = 0.0994, R indi-
ces based on 5347 reflections with I > 2
r
(I) (refinement on F2), 640
.
Supplementary data
parameters, 0 restraints,
l
= 1.285 mmÀ1
CCDC 711114–711116 contain the supplementary crystallo-
graphic data. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk.
4.3.2. Preparation of [(iso-C4H9)7Si7O12]2Zn4Me2THF2 (3)
1 (1.57 g, 2 mmol) was dissolved in THF (20 ml). To this ZnMe2
(2 ml, 2 M solution in toluene, 4 mmol) was added, there was vig-
orous effervescence. The mixture was stirred at room temperature
for 1 h. After which time the THF was removed in vacuo and the
white solid was recrystallised in the minimum amount of hexane.
This was left at À20 °C and after several weeks crystals suitable for
X-ray diffraction were obtained, yield 0.85 g, 43%. 1H (C6D6) À0.18
(6H, s, Zn–CH3), 0.76–0.98 (28H, m, SiCH2), 1.05–1.40 (84H, m,
CH3), 1.53 (8H, br m, CH2 THF), 2.03–2.30 (14H, m, CH), 3.82 (8H,
br m, CH2 THF). 13C{1H} (C6D6) À15.6 (Zn–CH3), 23.0, 23.5 (CH2),
24.5, 24.5, 24.6 (CH), 24.6 (CH2), 24.7 (CH2), 24.8 (CH), 25.1
(CH2), 25.5 (CH2 THF), 26.0, 26.2, 26.2, 26.3, 26.5, 26.7, 26.8
(CH3), 68.8 (O–CH2 THF). 29Si{1H} (C6D6) À58.1, À62.9, À64.9,
À65.1, À70.4 (2:1:1:2:1 ratio). Elemental analysis: Anal. Calc. for
C66H148Zn4O26Si14: C, 39.39; H, 7.41. Found: C, 38.9; H, 7.01%. Crys-
tal data for 3: C33H74O13Si7Zn2, M = 1006.29, colourless block,
Acknowledgments
We gratefully acknowledge the EPSRC (M.D. Jones, C.G. Keir) for
funding. In addition, Dr. John Lowe is thanked for 29Si NMR
measurements.
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4.3.3. Preparation of SiO2–Zn–Me
Sixty angstrom silica (2.0 g) was dried at 130 °C under a dy-
namic vacuum for 24 h. To this hexane (20 ml) was added and
ZnMe2 (2 M in toluene, 3 ml, 6 mmol) was slowly added. Vigorous
effervescence was observed and the solution was left for 2 h. The
solution was filtered and the white solid washed with toluene
(2 Â 25 ml) and hexane (2 Â 25 ml) and dried in vacuo. The white
solid was stored in a glove-box and used as required.
4.4. Polymerisation procedure
The monomer rac-lactide (0.72 g, 5 mmol) was added to a
Schlenk flask to which toluene {20 ml containing 2 equivalents of
iso-propyl alcohol (IPA)} was added. The initiator 2 (47 mg) or 3
(50 mg) was then added so that the monomer:initiator:IPA was
100:1:2. In the case of the heterogeneous catalyst 50 mg of mate-
rial was used. The mixture was heated at 110 °C for 72 h, after this
time the flask was cooled and the solvent removed in vacuo. The
residue was washed with MeOH (3 Â 50 ml) and the residue was
dried. 1H NMR spectroscopy (CDCl3) and GPC (THF) were used to
determine tacticity and molecular weights (Mn and Mw) of the
polymers produced; Pr (the probability of heterotactic linkages)
were determined by analysis of the methine region of the homonu-
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