Modeling the catalyst resting state in aryl tin(IV) polymerizations of lactide and estimating the relative rates of transamidation, transesterification and chain transfer

Article abstract of DOI:10.1039/b306700a

The preparation and characterization (IR, ¹H, ¹³C{¹H}, ¹¹⁹Sn NMR spectroscopy, elemental analysis and single crystal X-ray structure determination) are reported for Ph₃SnOCMe₂C(O)OEt (1) and Ph₂Sn[OCMe ₂C(O)NMe₂]₂ (2). In the solid state, compound 1 contains four-coordinate tin with evidence for incipient bond formation to the ester oxygen: Sn...O = 2.648(2) A. Compound 2 contains six-coordinate tin in a pseudo-octahedral geometry. The OCMe₂C(O) NMe₂ groups form cis-chelates with short, ca. 2.03 A, and long, ca. 2.26 A, Sn-O bonds to alkoxide and amide oxygen atoms, respectively. In solution, compound 1 remains four-coordinate but compound 2 exists as an equilibrium mixture of six-coordinate and five-coordinate species as judged by NMR spectroscopy. At -50°C in toluene-d₈, the six-coordinate isomer is favored and the NMR data are consistent with the structure observed in the solid state. At +50°C, the NMR data are consistent with a five-coordinate species in which reversible chelation of η²- and η¹-OCMe₂C(O)NMe₂ is fast on the NMR time scale. The molecular structure of 2 and its dynamic solution behavior is proposed to resemble that of Ph₂Sn[OCHMeC(O)NMe₂] ₂ formed in the polymerization of L-lactide by Ph₂ Sn(NMe₂)₂. The high formation tendency of this compound is proposed to be responsible for the preferential formation of cyclic lactide oligomers (LA/₂)_n by intrachain transesterification, in contrast to polymerizations employing Ph ₂Sn(OPrⁱ)₂, which produce long chains of H-(LA/₂)_n-OPrⁱ where LA = [OCHMeC(O)OCHMeC(O)]. The kinetics of the reactions between Ph₃SnX and each of Me ₂CHC(O)OMe, Me(MeO)CHC(O)OEt and Ph₃SnOCHMeC(O)OEt, have been determined from NMR studies in benzene-d₆ where X = NMe ₂ or OPrⁱ. Similarly, the reaction between Ph ₃SnOBu^t and (p-tolyl)₃SnOPrⁱ has been followed. The former reactions represent transamidation and transesterification, and the latter models chain transfer. These findings, when compared to the earlier studies of the ring-opening of lactide and its subsequent ring-opening polymerization, indicate that the rate follows the order: chain transfer > ring-opening > ring-opening polymerization > transesterification, although the latter is influenced by the ester end-group.

Full text of DOI:10.1039/b306700a

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P A P E R
Modeling the catalyst resting state in aryl tin(IV) polymerizations
of lactide and estimating the relative rates of transamidation,
transesterification and chain transfery
Malcolm H. Chisholm,* Ewan E. Delbridgez and Judith C. Gallucci
Department of Chemistry, The Ohio State University, 100 W 18th Avenue, Columbus,
OH 43210-1185, U.S.A.. E-mail: chisholm@chemistry.ohio-state.edu;
Fax: þ1 614 292 0368; Tel: þ1 614 292 7216
Received (in Montpellier, France) 12th June 2003, Accepted 19th September 2003
First published as an Advance Article on the web 21st November 2003
1
The preparation and characterization (IR, H, 13C{1H}, 119Sn NMR spectroscopy, elemental analysis
and single crystal X-ray structure determination) are reported for Ph3SnOCMe2C(O)OEt (1) and
Ph2Sn[OCMe2C(O)NMe2]2 (2). In the solid state, compound 1 contains four-coordinate tin with evidence for
˚
incipient bond formation to the ester oxygen: Snꢀ ꢀ ꢀO ¼ 2.648(2) A. Compound 2 contains six-coordinate tin in
˚
a pseudo-octahedral geometry. The OCMe2C(O)NMe2 groups form cis-chelates with short, ca. 2.03 A, and
˚
long, ca. 2.26 A, Sn–O bonds to alkoxide and amide oxygen atoms, respectively. In solution, compound 1
remains four-coordinate but compound 2 exists as an equilibrium mixture of six-coordinate and five-coordinate
species as judged by NMR spectroscopy. At ꢁ50 ꢂC in toluene-d8 , the six-coordinate isomer is favored and the
NMR data are consistent with the structure observed in the solid state. At þ50 ꢂC, the NMR data are consistent
with a five-coordinate species in which reversible chelation of Z2- and Z1-OCMe2C(O)NMe2 is fast on the NMR
time scale. The molecular structure of 2 and its dynamic solution behavior is proposed to resemble that of
Ph2Sn[OCHMeC(O)NMe2]2 formed in the polymerization of L-lactide by Ph2Sn(NMe2)2 . The high formation
tendency of this compound is proposed to be responsible for the preferential formation of cyclic lactide
oligomers (LA/2)n by intrachain transesterification, in contrast to polymerizations employing Ph2Sn(OPri)2 ,
which produce long chains of H–(LA/2)n–OPri where LA ¼ [OCHMeC(O)OCHMeC(O)]. The kinetics of the
reactions between Ph3SnX and each of Me2CHC(O)OMe, Me(MeO)CHC(O)OEt and Ph3SnOCHMeC(O)OEt,
have been determined from NMR studies in benzene-d6 where X ¼ NMe2 or OPri. Similarly, the reaction
between Ph3SnOBut and (p-tolyl)3SnOPri has been followed. The former reactions represent transamidation
and transesterification, and the latter models chain transfer. These findings, when compared to the earlier
studies of the ring-opening of lactide and its subsequent ring-opening polymerization, indicate that the rate
follows the order: chain transfer > ring-opening > ring-opening polymerization > transesterification,
although the latter is influenced by the ester end-group.
Coates2i has recently shown that stereochemical control in the
Introduction
polymerization of meso-lactide can generate syndiotactic-PLA
and rac-LA can be converted to heterotactic-PLA by what
appears to be end-group control of the ring-opening event at
sterically demanding zinc metal centers. Control of polymer
microstructure as well as molecular weight and molecular weight
distribution, together with the preparation of co-polymers
and polymer blends involving LA, represent important
challenges in what is emerging as a significant new field in
polymer science.
In the ring-opening polymerization of LA by a coordination
metal complex, the active species contains a metal–alkoxide
bond. This metal–oxygen bond is kinetically labile to other
reactions, namely chain transfer and transesterification. An
earlier claim9 that rac-salen Al–OR complexes could poly-
merize rac-LA to the stereoplex polymer [poly(L-LA) þ poly-
(D-LA)] was refuted2i,j based on an analysis of the mistakes
in the polymer ,which revealed that the polymer was a blocky
polymer of (L-LA)n–(D-LA)m where n ꢄ m ꢄ 10 or 11. Chain
transfer in conjunction with ring-opening polymerization
would give such a polymer. The addition of an alcohol would
also lead to chain transfer and limit molecular weight by
increasing the number of growing chains. The process of trans-
esterification leads to loss of stereochemistry in the resulting
polymer and also leads to a broadening of the molecular
weight distribution with time. It is, however, a process by
Biodegradable and biocompatible polymers formed from inex-
pensive renewable resources are attracting considerable current
attention in both academic and industrial circles.1–3 For exam-
ple, polylactides (PLAs) formed by the ring-opening polymeri-
zation (ROP) of lactide (LA), find numerous applications
ranging from environmentally friendly bulk packaging
materials4 to control-released drug delivery agents1,5, arti-
ficial sutures6 and polymer matrices for tissue engineering.1,7
Cargill–Dow has entered a joint venture for the production
of PLA on the order of 3 ꢃ 108 1b per annum based on an
enzymatic process for the formation of LA from corn. Ring-
opening polymerization can be brought about in a melt with
catalysts such as tin(II) octanoate, or in a more controlled
manner in solution by a variety of well-defined coordination
catalyst precursors3 or organic bases in the presence of alcohol
initiators.8 An excellent recent review documents work
employing coordination catalysts.3 Work by Ovitt and
y Electronic supplementary information (ESI) available: kinetic data
of reactions
suppdata/nj/b3/b306700a/
C and E in Scheme 2. See http://www.rsc.org/
z Current address: Chemistry Department, University of North
Dakota, P.O. Box 9024, Grand Forks, ND 58203-9024, USA. E-mail:
edelbridge@chem.und.edu
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 6 , 1 4 5 – 1 5 2
145

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Table 1 Crystallographic data for 1 and 2
which LA can be introduced into other polyesters, thus
providing a potential route to new co-polymers and blends.
In our studies of ring-opening polymerization of lactide by
aryl tin(IV) complexes, we observed both chain transfer and
transesterification.10 Furthermore, when Sn–NMe2 groups
were used as initiators, we observed a marked preference for
the formation of cyclic oligomers of PLA, (LA/2)n , along with
long chains of H(LA/2)nNMe2 [where LA ¼ OCHMeC(O)-
OCHMeC(O)], when compared with Sn–OPri initiators. Since
these tin(IV) catalyst systems are relatively slow, we were
determined to examine the origin of these effects and to
try to understand the fundamental reactions involved. We
describe here our findings, which were prompted by these
considerations.
1
2
Formula
FW
Crystal system
C
481.14
24H26O3Sn
C24H34N2O4Sn
533.22
Monoclinic
0.71073
P21/c
Triclinic
0.71073
¯
P1
˚
Wavelength/A
Space group
˚
a/A
9.433(1)
10.255(1)
12.180(2)
81.66(1)
87.44(1)
73.95(1)
1120.3(2)
2
11.554(1)
11.075(1)
19.704(2)
90
˚
b/A
˚
c/A
a/ꢂ
b/ꢂ
g/ꢂ
98.947(4)
90
3
˚
U/A
2490.8(4)
4
200(2)
Z
T/K
200(2)
30902
5108
Results and discussion
Total reflections
Independent reflections
Rint
44002
5704
Syntheses
0.030
4503
0.030
4678
Obs. reflections [I > 2s(I)]
R1 [I > 2s(I)]a
wR2 [I > 2s(I)]b
The synthesis of aryl tin alkoxides and dimethylamides has
been described previously.10 The details of the new tin alkoxy-
ester synthesis are described in the experimental. The forma-
tion of Ph2Sn[OCMe2C(O)NMe2]2 , compound 2, in the
reaction between Ph2Sn(NMe2)2 and HOCMe2C(O)OEt is
not immediately obvious. However, it can be reasonably well
understood in terms of the reaction sequence displayed in
Scheme 1. Following the initial alcoholysis there is a rapid
intramolecular amidation reaction leading to formation of
Ph2Sn(OEt)[OCMe2C(O)NMe2)], which then undergoes ligand
(alkoxide) exchange to form the stable bis chelate complex 2.
An attempt was made to prepare the ester analog of 2,
namely Ph2Sn[OCMe2C(O)OEt]2 , from the reaction between
Ph2SnCl2 and two equivalents of LiOCMe2C(O)OEt. How-
ever, the product of this reaction was very insoluble in organic
solvents, which suggests it may be polymeric with m-OR
bridges. That the compound is not similar to 2 serves to
demonstrate the significant difference in the chelating ability
of the OCMe2COX ligands where X ¼ NMe2 and OEt,
though ethyl lactate has been seen to chelate zinc(II) in the
solid state.2g
0.024
0.0527
0.0349
0.0926
a
b
R1 ¼ S||Fo| ꢁ |Fc||/S|Fo|. wR2 ¼ [Sw(F2o ꢁ F2c )2/Sw(Fo2)2]1/2
.
Sn(IV) center as seen, for example, in the tropinato complex
Ph3Sn(trop)11 (shown in Fig. 3). In contrast, compound 2 most
definitely contains six-coordinate Sn(IV). The Sn–O bonds fall
˚
into two groups, short bonds with Sn–O ca. 2.03 A to the alk-
˚
oxide oxygen atoms and long bonds with Sn–O ca. 2.26 A to
the amide oxygen atoms. Again, a comparison can be made
11
with the tropanato complex Me2Sn(trop)2 (shown in Fig.
3). As indicated below the comparison is pertinent not only
from a structural standpoint but also in terms of the solution
NMR spectroscopy of these molecules.
NMR spectral properties of compounds 1 and 2
Compound 1 displays NMR behavior in benzene-d6 typical of
a four-coordinate Ph3SnOR compound. In particular, the
119Sn resonance is at d ꢁ143, which contrasts with five-coordi-
nate Sn(IV) signals that are found around d ꢁ200. That of
Ph3Sn(trop) is reported at d ꢁ182,11 for example. In contrast,
compound 2 shows interesting temperature-dependent NMR
spectra (Fig. 4 and 5), which is uncommon for R2SnX2 com-
pounds. At low temperatures the 1H NMR spectra are consis-
tent with the structure seen in the solid state (Fig. 5). Notably
Solid state and molecular structures
A summary of crystal data for compounds Ph3SnOCMe2-
C(O)OEt (1) and Ph2Sn[OCMe2C(O)NMe2]2 (2) is given in
Table 1 and ORTEP drawings of the molecular structures
are displayed in Figs. 1 and 2, respectively. Selected bond
lengths and angles are given in Tables 2 and 3. Compound 1
can be considered to contain a four-coordinate Sn(IV) center
with evidence of incipient Snꢀ ꢀ ꢀO bond formation
˚
(Snꢀ ꢀ ꢀO ¼ 2.65 A) to the ketonic carbon of the ester. How-
ever, the molecule clearly does not contain a five-coordinate
Scheme 1 Pathway to the formation of Ph2Sn[OCMe2C(O)NMe2]2 (2).
Fig. 1 ORTEP plot of 1 showing 50% probability ellipsoids.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
146
N e w . J . C h e m . , 2 0 0 4 , 6 , 1 4 5 – 1 5 2

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ꢂ
˚
Table 3 Selected bond lengths (A) and angles ( ) for 2
Sn–O(1)
Sn–O(2)
Sn–O(3)
2.267(2)
2.037(2)
2.258(2)
74.02(8)
77.18(8)
86.59(9)
162.09(10)
91.61(11)
85.29(8)
154.38(9)
92.76(11)
Sn–O(4)
Sn–C(13)
Sn–C(19)
2.024(2)
2.149(3)
2.146(3)
O(1)–Sn–O(2)
O(1)–Sn–O(3)
O(1)–Sn–O(4)
O(1)–Sn–C(13)
O(1)–Sn–C(19)
O(2)–Sn–O(3)
O(2)–Sn–O(4)
O(2)–Sn–C(13)
O(2)–Sn–C(19)
O(3)–Sn–O(4)
O(3)–Sn–C(13)
O(3)–Sn–C(19)
O(4)–Sn–C(13)
O(4)–Sn–C(19)
C(13)–Sn–C(19)
102.97(13)
73.99(8)
89.97(9)
163.84(12)
101.96(11)
93.99(13)
103.30(12)
Furthermore, Ph2Sn[OCHMeC(O)NMe2]2is formed in the re-
action between Ph3SnNMe2 and L-lactide by disproportionation
of Ph3SnOCHMeC(O)NMe2 into Ph4Sn and Ph2Sn[OCH-
MeC(O)NMe2]2 .10 The compound Ph3SnOCHMeC(O)-
OCHMeC(O)NMe2 also is kinetically labile to the formation
of Ph3SnOCHMeC(O)NMe2 and long chain Ph3SnOCH-
MeC(O)–(LA/2)n–OCHMeC(O)NMe2
by
intermolecular
transesterification. It is the thermodynamic preference for che-
lation of the OCHMeC(O)NMe2 ligand to the Sn(IV) center, as
seen in the structure of the model compound 2, and in its
solution behavior that is dominating the behavior of the
Ph2Sn(NMe2)2 catalyst systems in the ROP of L-lactide.
Fig. 2 ORTEP plot of 2 showing 40% probability displacement
ellipsoids. Hydrogen atoms omitted for clarity. Only one orientation
of the disordered phenyl ring [at C(19)] is shown.
Transesterification and transamidation reactions
there are two NMe singlets, consistent with restricted rotation
about the C–N partial double bond due to the resonance struc-
ture shown in Fig. 6. The CMe2 methyl groups are diastereo-
topic as a result of the virtual C2 symmetry of the molecule.
At low temperatures (ꢁ50 ꢂC and below) there is just one
119Sn resonance at d ꢁ383, which predominates, and which
may be compared with the Ph2Sn(trop)2 compound for which
a peak at d ꢁ359 is reported.11
A series of bimolecular reactions was investigated as outlined
in Scheme 2. These reactions attempt to model transesterifica-
tion, transamidation and chain transfer reactions, which are
prevalent in the ROP of lactide. Both the steric and electronic
effects of the substrates have been considered and how they
influence the rates of product formation. It is apparent from
Scheme 2 that the transamidation reactions A–C are not equi-
librium reactions but are quantitative ( > 95% conversion)
since a tin–amide bond is activated and forms the thermodyna-
mically more stable tin–alkoxide bond. Conversely, in the
transesterification reactions D and E a tin–alkoxide bond is
activated to form another tin–alkoxide, bond resulting in an
Upon raising the temperature the 1H signals associated with
the two CMe2 and two NMe2 resonances broaden, coalesce
and above 50 ꢂC appear as sharp signals (Fig. 5). This is con-
sistent with dechelation to give a five- or four-coordinate
Sn(IV) centers. A five-coordinate monochelate of the structural
type shown in Fig. 7 would have a mirror plane of symmetry
and with rapid and reversible chelation would lead to a single
CMe2 proton signal. With dechelation, rotation about
the C–N bond of the amide group would allow N-methyl
site exchange, as is well-known for organic amides such
as DMF.
equilibrium being established where at t all four species,
1
[A], [B], [C] and [D], are in equal concentration (see Experi-
mental). It is also important to note that none of the trans-
esterification reactions occur at room temperature whilst the
transamidation reactions do. Kinetic data were ascertained
from 1H NMR data collected on samples made up in
benzene-d6 (see Experimental and Electronic supplementary
information for details).
The variable temperature 119Sn NMR spectra (Fig. 4) are
particularly informative and support the view that at room
temperature and above the predominant species present in
solution is a five-coordinate Sn(IV) complex with d ꢁ276
(Fig. 4). Paricularly pertinent to the discussion that follows
is the fact that similar NMR behavior was observed for
Ph2Sn[OCHMeC(O)NMe2]2 , which is the compound formed
in the reaction between Ph2Sn(NMe2)2 and L-lactide (1 equiv.);
it is also the resting state of the tin catalyst in the ring-opening
polymerization (ROP) reaction between Ph2Sn(NMe2)2 and
excess L-lactide when the L-lactide has been consumed.10
Transamidation reactions. Two types of reactions have been
carried out. The first involves treatment of a free ester,
Me2CHC(O)OMe, or ether-ester, MeOCHMeC(O)OEt, with
Ph3SnNMe2 and the second involves reaction of a tin(IV) ester,
Ph3SnOCHMeC(O)OEt, with Ph3SnNMe2 . It was found that
the Ph3Sn[OCHMeC(O)OEt] compound reacts more slowly
ꢂ
˚
Table 2 Selected bond lengths (A) and angles ( ) for 1
Sn–O(1)
Sn–C(7)
Sn–C(13)
Sn–C(19)
Snꢀ ꢀ ꢀO(2)
1.996(1)
2.126(2)
2.141(2)
2.148(2)
2.648(2)
O(1)–Sn–C(7)
O(1)–Sn–C(13)
O(1)–Sn–C(19)
C(7)–Sn–C(13)
C(7)–Sn–C(19)
C(13)–Sn–C(19)
113.25(7)
118.73(7)
93.86(6)
114.44(7)
108.47(7)
105.05(7)
Fig. 3 Molecular structures of Ph3Sn(trop) and Me2Sn(trop)2 .
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 6 , 1 4 5 – 1 5 2
147

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one methyl with a methoxy group on the ester leads to a faster
rate of reaction. This may be expected if the methoxy group is
considered inductively electron withdrawing, as this would
increase the electrophilicity of the carbonyl carbon and thus
its susceptibility to nucleophilic attack by NMe2 . We also
investigated the effect of adding an inert organic amide such
as DMF (5 equiv.) to the reaction mixture [reactions A(i)
and (ii)] and found that it suppressed the reaction rate
Fig. 4 119Sn NMR of Ph2Sn[OCMe2C(O)NMe2]2 (2) performed at
high and low temperatures in toluene-d8 .
[k ¼ 7.8(2) ꢃ 10ꢁ4 to 5.2(2) ꢃ 10ꢁ4 Mꢁ1
s
ꢁ1]. This can be
attributed to DMF competing with Me2CHC(O)OMe for
coordination to the Ph3SnNMe2 prior to nucleophilic attack
by NMe2 on the ester. Coordination is presumably a fast equi-
librium process, forming an adduct that can then undergo
reaction as shown below:
with Ph3SnNMe2 [reaction C; k ¼ 4.7(2) ꢃ 10ꢁ4 Mꢁ1 sꢁ1] than
the free esters [reactions A and B; k ¼ 7.8(2) ꢃ 10ꢁ4 to
1.4(2) ꢃ 10ꢁ2 Mꢁ1
s
ꢁ1]. This unexpected result is in contrast
with that found for transesterification reactions where reaction
D proceeds more rapidly than reaction E. Based on electronic
arguments alone reaction C would be expected to proceed
more rapidly than reaction A(i), which would be consistent
with the relative rates seen below for transesterification reac-
tions D and E. A plausible explanation for this apparent con-
tradiction in relative rates in these transamidation and
transesterification reactions may lie in the fact that steric hin-
drance of Ph3Sn maybe more dominant in reaction C than in
reaction D. Reaction C is performed at room temperature
where the ketonic chelation to tin is more dominant than in
reaction D, which is carried out at 60 ꢂC. Under these lower
temperature conditions the bulky Ph3Sn group would more
effectively hinder access to the ketonic carbon, resulting in sup-
pression of reaction rate for the room temperature reaction. It
is interesting to compare reactions rates for reactions A(i) and
B [k ¼ 7.8(2) ꢃ 10ꢁ4 vs. 1.4(2) ꢃ 10ꢁ2 Mꢁ1 sꢁ1] where replacing
Transesterification reactions. Transesterification reactions
are much less facile compared with transamidation reactions,
with none proceeding significantly at room temperature
[RT ¼ 26(1) ꢂC]. It is interesting to compare reactions D
[Ph3SnOCHMeC(O)OEt þ Ph3SnOPri; k ¼ 2.8(2) ꢃ 10ꢁ3 Mꢁ1
s
Mꢁ1 ꢁ1] in Scheme 2 where the rates of reaction are in the
reverse order of that found in the corresponding transamidation
but are consistent with the electron-withdrawing effect of the
Ph3Sn group (see above for discussion).
s
ꢁ1] and E [MeOCHMeC(O)OEt þ Ph3SnOPri; k ¼ 4.0 ꢃ 10ꢁ5
Fig. 5 Variable temperature 1H NMR spectra of 2 measured in toluene-d8 showing the methyl region. * denotes a small impurity and the pentet
is due to toluene-d8 .
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
N e w . J . C h e m . , 2 0 0 4 , 6 , 1 4 5 – 1 5 2
148
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4

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chosen instead of more complicated OR ligands that more
closely resemble a lactide chain since simple triaryl tin(IV)
alkoxides are not susceptible to transesterification reactions
or indeed aryl group migration, as seen for Ph3Sn[OCHMe-
C(O)]nX compounds. At room temperature the reaction of
Ph3SnOBut and (o-MeC6H4)3SnOPri in benzene-d6 proceeded
Fig. 6 Resonance structure of the amide.
rapidly towards equilibrium [k ¼ 3.0(2) ꢃ 10ꢁ3 Mꢁ1
s
ꢁ1; see
reaction I in Scheme 2]. It is important to note that every
attempt was made to remove even trace amounts of free alco-
hol from these reagents since free alcohol was found to drasti-
cally increase the rate of the alkoxide transfer. Moisture was
rigorously excluded from the reagents and solvents and the
reagents were heated under vacuum for an extended period
to remove trace alcohol prior to their use in this reaction.
As observed for the transamidation reactions the electronic
properties of the ester affect its reactivity towards transesterifi-
cation with Ph3SnOPri such that no reaction is observed in
reaction F [Me2CHC(O)OMe þ Ph3SnOPri] at 60 ꢂC. The elec-
tron-withdrawing properties of the MeO group in the ester in
reaction E enhances reactivity such that transesterification can
occur, albeit slowly.
It is also clear that steric factors play a significant role in
transesterification reactions and this is illustrated in the com-
parison between reactions D and G where the –OCHMeC–
backbone has been replaced by –OCMe2C–. The substitution
of H for a second methyl group has provided sufficient protec-
tion such that no discernible reaction was observed at 60 ꢂC in
reaction G. This is not surprising since we have previously
observed no transesterification for Ph3SnOCMe2C(O)OCH-
MeC(O)NMe2 .
We have also explored the steric effect of the nucleophile
(OR) and its ability to undergo transesterification with the
ester group in Ph3SnOCHMeC(O)OCHMeC(O)NMe2 (reac-
tion H). It was found that reaction of Ph3SnOPri with
Ph3SnOCHMeC(O)OCHMeC(O)NMe2 yielded two major
products, Ph3SnOCHMeC(O)X where X ¼ NMe2 or OPri;
the OPri product results from transesterification with Ph3SnO-
Pri. However, the corresponding reaction with Ph3SnOBut
containing the bulkier OBut nucleophile only yielded one
major product, namely Ph3SnOCHMeC(O)NMe2 . This pro-
duct, which is present in both reactions in H, results
from Ph3SnOCHMeC(O)OCHMeC(O)NMe2 molecules re-
acting together in an intermolecular self-transesterification
reaction to form Ph3SnOCHMeC(O)NMe2 and Ph3Sn-
[OCHMeC(O)]1.5NMe2 . Further transesterification leads to
Ph3SnOCHMeC(O)NMe2 and long chains of Ph3Sn–(LA/2)n–
OCHMeC(O)NMe2 , together with some cycles of PLA.
To summarize, these transesterification reactions are sensi-
tive to not only the electronic and steric environments around
the ester group but also those of the nucleophile. The trans-
amidation reactions are also sensitive to the electronic
environment of the ester group.
Kinetic summary
From this and our previous two studies,10 it is now possible to
estimate the relative rates of the various processes in the ROP
of lactide, namely ring-opening, propagation, transesterifica-
tion and chain transfer. This allows us to order or rank the
various rates of these processes (Table 4) such that the rate
of chain transfer (kct) > transesterification (ktrans) > ring-
opening (kro) > propagation (kprop). Note that the order is
slightly different if one considers the metal-containing trans-
esterification reaction with Ph3SnOCHMeC(O)OEt rather than
the organic ester MeOCHMeC(O)OEt, where for the organic
ester the order would switch to kct > kro > kprop > ktrans . This
indicates that transesterification near a metal center is much
more facile than somewhere along the polymer chain.
Conclusions
This study of kinetically slow reactions of Sn(IV) compounds
has given considerable insight into the intricacies of the poly-
merization of lactide by Ph2SnX2 and Ph3SnX compounds
and its competing side reactions. We have quantified the rates
of chain transfer, transesterification and transamidation reac-
tions and compared some of these with previously determined
ring-opening and propagation rates and found that the rates
of such reactions follow kct > kro > kprop > ktrans or kct
>
ktrans > kro > kprop , depending on which transesterification
reaction is employed. This is because transesterification
occurs more rapidly near a metal center than in the absence
of one (such as in the middle of a polymer chain). It should be
noted that whilst we feel this overall order of reactivity to be
accurate, these rate constants are derived from different reac-
tions. In spite of this, and some similarities in some of the
rate constant values, it is still possible to draw meaningful con-
clusions about the relative rates of these processes, which
are prevalent in ROP lactide reactions. Overall it was found
that transamidation reactions are in general faster than
transesterification reactions but both are slower than chain
transfer reactions.
Chain transfer reactions
Chain transfer reactions involving the propagating lactide
chain are important as polymer molecular weight can be con-
veniently modified via addition of a transfer agent such as an
alcohol. As such, we wanted to quantify the ability of simple
alkoxides, which mimic a propagating polylactide chain, to
exchange between triaryl tin(IV) centers. Simple alkoxides were
This study has also investigated some solid state and solu-
tion properties of two key tin(IV) compounds. Both the coordi-
nation number and structure of Ph3SnOCMe2C(O)OEt (1) and
Ph2Sn[OCMe2C(O)NMe2]2 (2) have been determined and the
ketonic oxygen exhibits much stronger coordination to tin in
Ph2Sn[OCMe2C(O)NMe2]2 than in Ph3SnOCMe2C(O)OEt.
This is an important finding since previously we have found
that the ability of the ketonic group to coordinate to the metal
center influences many properties of polylactide formation,
including molecular weight distribution and the ratio of
polymer cycles to chains.
Fig. 7 Five-coordinate representation of 2.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 6 , 1 4 5 – 1 5 2
149

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Scheme 2 Transamidation, transesterification and chain transfer reactions between tin(IV) compounds and organic moieties [T ¼ 26(1) ꢂC].
Experimental
Table 4 Comparison of rates of various processes in L-lactide
polymerization by aryl tin(IV) compounds
General methods and materials
Caution: organotin(IV) compounds are highly toxic and require
appropriate handling!
A
B
T/ꢂC k/Mꢁ1 sꢁ1
The manipulation of air-sensitive compounds involved
standard Schlenk line and dry box techniques. All solvents
were distilled under nitrogen from alkali metals (sodium or
Ph3SnOPri
L-Lactide
61
61
60
60
26
kro ¼ 1.3(2) ꢃ 10ꢁ4 a
Ph2Sn(OPri)2 L-Lactideb
kprop ¼ 2.6(2) ꢃ 10ꢁ5 sꢁ1 c d
ktrans ¼ 4.0(2) ꢃ 10ꢁ5 a
ktrans ¼ 2.8(2) ꢃ 10ꢁ3 a
kct ¼ 3.0(2) ꢃ 10ꢁ3 a
Ph3SnOPri
Ph3SnOPri
Ph3SnOBut
MeOCHMeC(O)OEt
Ph3SnOCHMeC(O)OEt
(o-MeC6H4)3SnOPri
˚
sodium/potassium alloy) and stored over 4 A molecular sieves.
Benzene-d6 , toluene-d8 , ethyl L-(ꢁ)-lactate and ethyl (L)-(ꢁ)-2-
methoxypropionate were purchased from Acros Scientific
while ethyl 2-hydroxyisobutyrate was purchased from Aldrich
and diphenyltin(IV) dichloride was purchased from Alfa Aesar.
a
b
c
d
This work.
1:100 [Sn]:[L-lacitide]. Pseudo 1st order (sꢁ1).
Previous
work.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
150
N e w . J . C h e m . , 2 0 0 4 , 6 , 1 4 5 – 1 5 2

View Article Online
Tetrahydrofuran-d8 was purchased from Cambridge isotopes.
Ethyl (L)-(ꢁ)-2-methoxypropionate was purified by column
chromatography (diethyl ether–pentane) prior to drying and
degassing. All esters were degassed and dried over activated
molecular sieves. Benzene-d6 , tetrahydrofuran-d8 and toluene-
d8 were dried over sodium and vacuum transferred to a
Schlenk flask containing activated molecular sieves. Ph3SnX,
(o-MeC6H4)3SnOPri and Ph2SnX2 [where X ¼ NMe2 ,
OR or OCHMeC(O)OCHMeC(O)NMe2] were synthesized
as described previously.10
slowly. The colorless reaction was stirred for 30 min after
which time the hexane was removed, giving 0.42 g (90%) of
an opaque semi-solid. Anal. calcd for C23H24O3Sn: C, 59.14;
H, 5.18; found: C, 58.56; H, 4.56. IR (liquid): n/cmꢁ1 3065
m, 3050 m, 2980 m, 1710 br vs, 1481 m, 1447 w, 1439 s,
1375 m, 1335 w, 1302 m, 1235 br s, 1150 br s, 1076 s, 1057
m, 1023 m, 997 m, 941 w, 859 w, 799 vw, 729 s, 698 s, 659
m, 540 w, 492 w, 450 w.1H NMR (500 MHz, benzene-d6):
d 0.68 (t, OCH2Me, 3H), 1.40 (d, SnOCHMe, 3H), 3.64 (q,
OCH2Me, 2H), 4.64 [q, SnOCHMe, 3H, 119/117Sn satellites
1H NMR spectra were obtained from either Bruker DPX-
400 or DRX-500 NMR spectrometers using either benzene-
d6 , toluene-d8 or tetrahydrofuran-d8 . Spectra were referenced
JSnH
H, 3H), 7.91 ([dd, o-H, 6H, JHH 4.6 and 1.9 Hz, 119/117Sn satel-
lites JSnH
117Sn) 64, (119Sn) 48 Hz].13C{1H} NMR (126 MHz,
(
117Sn) 54, (119Sn) 40 Hz], 7.23 (t, m-H, 6H), 7.17 (d, p-
(
1
internally to the residual protio impurities for H (benzene-d6
benzene-d6): d 13.81 (s, OCH2Me), 22.99 (s, SnOCHMe), 61.76
(s, OCH2Me), 68.99 (s, SnOCH2Me, 119/117Sn satellites JSnC 34
Hz), 128.72 (s, m-C, 119/117Sn satellites JSnC 60 Hz), 129.49
(s, p-C 119/117Sn satellites JSnC 13 Hz), 137.31 (s, o-C, 119/117Sn
satellites JSnC 57 Hz), 142.52 (s, ipso-C), 181.13 [s, SnOCH-
(Me)C(O)]. 119Sn NMR (187 MHz, benzene-d6): d ꢁ129 (s).
ESI-HR-MS: m/z calcd for C23H24O3Sn (MNaþ): 491.0650;
found: 491.0623 (5.5 ppm).
d 7.15; toluene-d8 d 2.09; tetrahydrofuran-d8 d 1.73) and 13C
(benzene-d6 d 128) or externally to Me4Sn for 119Sn (d 0.0).
Infrared data were obtained from a Perkin–Elmer Spectrum
GX spectrophotometer with samples sandwiched between
potassium bromide or sodium chloride plates as Nujol mulls
(solids) or as neat liquids/semi-solids. Mass spectra were
obtained from a Mircomass QTOF mass spectrometer. Ele-
mental analyses were performed by Atlantic Microlab, Inc.,
Norcross, GA on samples sealed under an inert atmosphere
in glass ampoules.
Ph3SnOCMe2C(O)OEt. To a cooled hexane solution (0 ꢂC
10 mL) of dimethylamidotriphenyltin(IV) (0.39 g, 1.00 mmol)
ethyl 2-hydroxyisobutyrate (150 mL, 1.10 mmol) was added
slowly. The colorless reaction was stirred for 30 min after
which time the hexane was removed, giving 0.46 g (96%) of a
white solid. Some of this white solid was dissolved in pentane
from which colorless crystals suitable for X-ray analysis
formed. Anal. calcd for C24H26O3Sn: C, 59.91; H, 5.45; found:
C, 59.10; H, 5.42. IR (Nujol): n/cmꢁ1 3067 m, 3040 m, 2980 m,
1710 br vs, 1481 m, 1447 w, 1439 s, 1375 m, 1335 w, 1302 m,
1235 br s, 1150 br s, 1076 s, 1057 m, 1023 m, 997 m, 941 w, 859
w, 799 vw, 729 s, 698 s, 659 m, 540 w, 492 w, 450 w.1H NMR
(500 MHz, benzene-d6): d 0.68 (t, OCH2Me, 3H), 1.40 (d,
SnOCHMe, 3H), 3.64 (q, OCH2Me, 2H), 4.64 [q, SnOCHMe,
Reaction kinetics
All kinetic data were obtained from NMR scale reactions.
Standard solutions of Ar3SnX (X ¼ NMe2 , OPri, OBut;
Ar ¼ Ph, o-MeC6H4) and the appropriate ester [or tin(IV) con-
taining compound] were made in benzene-d6 and stored in the
dry box. Appropriate aliquots 1:1 of both reagents were trans-
ferred to a J. Young1 NMR tube. The total volume was
made up to 800 mL with benzene-d6 to ensure a constant
initial Ar3SnX concentration (0.0388 M). The reaction tem-
peratures were regulated via a thermostatically controlled oil
bath. Room temperature reactions were performed in air at
25 ꢂC.
3H 119/117Sn satellites JSnH
(t, m-H, 6H), 7.17 (d, p-H, 3H), 7.91 [dd, o-H, 6H JHH 4.6
and 1.9 Hz, 119/117Sn satellites JSnH 117Sn) 64, (119Sn) 48
(
117Sn) 54, (119Sn) 40 Hz], 7.23
(
An overall second-order process, first-order in both Ph3SnX
[A] and ester or Sn(IV) containing compound [B], was assumed.
Two different rate laws were used depending on whether the
process was an equilibrium or not, that is A þ B ! C þ D or
A þ B $ C þ D. In either case the disappearance of both [A]
and/or [B] was determined based on the formation of [C]
and [D]. For non-equilibria cases plotting ln([B]/[A]) vs.
time(s) produced a straight line {initial concentrations of A
were 0.0388 M (for Ph3SnNMe2) and of B were 0.0310
[Ph3SnOCHMeC(O)OEt], 0.0335 [MeOCHMeC(O)OEt] and
0.0327 M [Me2CHC(O)OMe]}. The rate constants for these
reactions were determined from the gradient of the graph via
m ¼ k([Bo] ꢁ [Ao]). In the equilibria cases, the derived rate
law of King12 was used:
Hz].13C{1H} NMR (126 MHz, benzene-d6) d: 13.81 (s,
OCH2Me), 22.99 (s, SnOCHMe), 61.76 (s, OCH2Me), 68.99
(s, SnOCH2Me, 119/117Sn satellites JSnC 34 Hz), 128.72 (s,
m-C, 119/117Sn satellites JSnC 60 Hz), 129.49 (s, p-C 119/117Sn
satellites JSnC 13 Hz), 137.31 (s, o-C, 119/117Sn satellites JSnC
57 Hz), 142.52 (s, ipso-C), 181.13 [s, SnOCH(Me)C(O)].119Sn
NMR (187 MHz, benzene-d6): d ꢁ143 (s).
Ph2Sn[OCMe2C(O)NMe2]2 , 2. To a pentane solution (20
mL) of bis(dimethylamido)diphenyltin(IV) (1.07 g, 3.00 mmol)
ethyl 2-hydroxyisobutyrate (0.40 g, 3.00 mmol) was added
slowly, leading to the immediate precipitation of a sticky white
solid. After stirring the solution for 30 min the pentane was
separated from the precipitate and the solution allowed to sit
at room temperature overnight after which time colorless crys-
tals (suitable for X-ray analysis) formed. The crystals were
separated from the mother liquor and washed with cold pen-
tane and dried under vacuum to give 0.08 g (13%) of the title
complex. Anal. calcd for C18H22NO2Sn: C, 54.06; H, 6.43; N
5.25; found: C, 52.60; H, 6.36; N, 4.78. IR (Nujol): n/cmꢁ1
1594 s, 1574 s, 1504 m, 1257 m, 1282 s, 1265 s, 1075 m, 1057
w, 1024 vw, 996 m, 869 w, 801 w, 730 s, 702 s, 641 m, 569
m, 525 m. See Fig. 5 for a stacked plot of 1H NMR data.
Selected variable temperature data for 1H NMR (500 MHz,
toluene-d8) at 323 K: d 1.54 (s, OCMe2 , 6H), 2.45 (s, NMe2 ,
6H), 7.15 (d, p-H, 2H), 7.25 (t, m-H, 4H), 8.11 [dd, o-H, 6H,
½Aꢅꢁ½Aꢅeq
¼
½Aꢅꢁ½Aꢅeq½1ꢁð1=KÞꢅþ½Aꢅeq þ½Bꢅeq þð1=KÞð½Cꢅeq þ½Dꢅeq
ꢁkf½Aꢅeq þ½Bꢅeq þ1=Kð½Cꢅeq þ½Dꢅeqgtþconst:
where K is the equilibrium constant (which is determined from
the equilibrium concentrations of A, B, C and D) and [X]eq is
the concentration of X at equilibrium. In this case the disap-
pearance of [A] was determined from 1H NMR data and
plotted via the above equation to produce a straight line from
which the value of k was determined.
JHH 7.9 and 1.6 Hz, 119/117Sn satellites JSnH
(
(
117Sn) 53,
Syntheses
119Sn) 36 Hz]; at 223 K: d 1.49 (s, OCMe2 , 3H), 1.77 (s,
Ph3SnOCHMeC(O)OEt, 1. To a cooled hexane solution
(0 ꢂC 10 mL) of dimethylamidotriphenyltin(IV) (0.39 g, 1.00
mmol) ethyl L-(ꢁ)-lactate (125 mL, 1.10 mmol) was added
OCMe2 , 3H), 1.88 (s, NMe2 , 3H), 2.11 (s, NMe2 , 3H), 7.26
(t, p-H, 2H), 7.41 (t, m-H, 4H), 8.45 [d, o-H, 6H, JHH 7.0
Hz, 119/117Sn satellites JSnH 117Sn) 72, (119Sn) 57 Hz].119Sn
(
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 6 , 1 4 5 – 1 5 2
151

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NMR (187 MHz, toluene-d8) at 323 K: d ꢁ276 (s); at 223 K:
d ꢁ383 (s).
agreement factors of R1(F) ¼ 0.045 and wR2(F2) ¼ 0.098.
For the subset of data with I > 2s(I), the R1(F) value is
0.024 for 4503 reflections for 1 and 0.035 for 4678 reflections
for 2. The final difference electron density map contains ꢁm3axi-
Reaction between Ph2SnCl2 and 2Li[OCMe2C(O)OEt] to
give Ph2Sn[OCMe2C(O)OEt]2. To a benzene solution (80
mL) of ethyl 2-hydroxyisobutyrate (2.58 g, 19.6 mmol) solid
lithium dimethylamide (1.00 g, 19.6 mmol) was added in the
dry box. Gas evolution was observed and the clear colorless
reaction mixture warmed; it was stirred for 30 min, after which
time diphenyltin(IV) dichloride (3.35 g, 9.79 mmol) was added.
Immediately a white precipitate formed and the reaction mix-
ture was stirred at room temperature overnight. The benzene
was separated from the white precipitate and the benzene
was removed to give a trace amount of a white solid. The ori-
ginal precipitate was extracted with THF (50 mL) and sepa-
rated from the remaining white solid. The THF was removed
under vacuum to give (0.3 g) of a white solidm which reluc-
tantly redissolved in any organic solvents. IR (Nujol): n/
cmꢁ1 1678 s, 1590 m, 1307 m, 1256 m, 1209 w, 1174 m, 1156
m, 1094 w, 1081 w, 1047 w, 1016 m, 1002 m, 910 w, 889 w,
801 m, 733 m, 698 m, 667 s, 540 m. 1H NMR (500 MHz,
THF-d8): d 1.05 (t, OCH2Me, 6H), 1.45 (s, SnOCMe2 , 12H,
JSnH 117/119Sn 12 Hz), 3.64 (q, OCH2Me, 2H), 4.03 [q,
C(O)OCH2Me, 4H], 7.23 (m, p-H and m-H, 6H), 7.85 [dd,
˚
mum and minimum peak heights of 0.77 and ꢁ0.56 e A for
ꢁ3
˚
1 and 0.71 and ꢁ0.91 e A for 2. Neutral atom scattering
factors were used and include terms of anomalous dispersion.16
A summary of the X-ray data is given in Table 1.x
Acknowledgement
The authors wish to acknowledge the financial support from
the Department of Energy, Office of Basic Sciences, Chemistry
Division.
References
1
2
(a) R. Langer, Science, 2001, 293, 58; (b) A. Lendlein and
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(
(a) T. M. Ovitt and G. W. Coates, J. Am. Chem. Soc., 1999, 121,
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Chem. Soc., 2002, 124, 1316.
43, (119Sn) 28 Hz].119Sn NMR (187 MHz, THF-d8): d ꢁ227
(s), ꢁ255 (s), ꢁ319 (br s).
X-Ray crystallography
The crystal of 1 was a colorless plate while 2 was a colorless
chunk. Examination of the diffraction patterns on a Nonius
Kappa CCD diffractometer indicated a triclinic crystal system
for 1 and a monoclinic crystal system for 2. All data collection
was performed at 200 K using an Oxford Cryosystems Cryo-
stream Cooler. For 1 the data collection strategy was set up
to measure a hemisphere of reciprocal space with a redundancy
factor of 3.0, meaning that 90% of the reflections were mea-
sured at least 3.0 times. For 2 the data collection strategy
was set up to measure a quadrant of reciprocal space with a
redundancy factor of 3.7, meaning that 90% of the reflections
were measured at least 3.7 times. A combination of phi and
omega scans with a frame width of 1.0ꢂ was used. Data inte-
gration was done with Denzo13 whilst scaling and merging of
the data was performed with Scalepack.13 Merging the data
and averaging the symmetry equivalent reflections resulted in
a Rint value of 0.030 for both data sets.
3
4
5
6
7
8
B. J. O’Keefe, M. A. Hillmyer and W. B. Tolman, J. Chem. Soc.,
Dalton Trans, 2001, 2215.
Ecochem is a polylactide based packaging material developed by
DuPont ConAgra.
Leupron Depot is a product of Takeda Chemical Industries, Ltd.,
Japan for drug delivery purposes.
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Hedrick, J. Am. Chem. Soc., 2002, 124, 914; (b) G. W. Nyce, T.
Glauser, E. F. Conner, A. Mock, R. M. Waymouth and J. L.
Hedrick, J. Am. Chem. Soc., 2003, 125, 3046.
The structures were solved by the Patterson method in
SHELXS-8614 in space groups P1 and P21/c for 1 and 2,
¯
respectively. For 2 one of the phenyl rings is rotationally dis-
ordered about the Sn–C(19) bond. The disorder is modelled
with two orientations for this ring. The C(19) and C(22) atoms
are common to both orientations, while the other four carbon
atoms of the ring are disordered over two sites each. Only the
C(19) atom of this phenyl group is refined anisotropically; the
other carbon atoms are kept isotropic. The occupancy factors
for the two orientations refined to 0.529(8) and 0.471(8). Full-
matrix least-squares refinements based on F2 were performed
in SHELXL-93.15
For each methyl group, the hydrogen atoms were added
at calculated positions using a riding model with U(H) ¼
1.5ꢀU(eq) (bonded C atom) for 1 and 2. The torsion angle,
which defines the orientation of the methyl group about the
C–C or N–C bond, was refined. The other hydrogen atoms
were included in the model at calculated positions using a rid-
ing model with U(H) ¼ 1.2ꢀU(eq) (bonded C atom). For 1 the
final refinement cycle was based on 5108 intensities and 256
variables and resulted in agreement factors of R1(F) ¼ 0.031
and wR2(F2) ¼ 0.055. For 2 the final refinement cycle was
based on 5704 intensities and 280 variables and resulted in
9
C. P. Radano, G. L. Baker and M. R. Smith, J. Am. Chem. Soc.,
2000, 122, 1552.
10 (a) M. H. Chisholm and E. E. Delbridge, Chem. Commun., 2001,
1308; (b) M. H. Chisholm and E. E. Delbridge, New J. Chem.,
2003, 27, 1167.
11 C. Camacho-Camacho, R. Contreras, H. No¨th, M. Bechmann, A.
Sebald, W. Milius and B. Wrackmeyer, Magn. Reson. Chem.,
2002, 40, 31.
12 E. L. King, Int. J. Chem. Kinet., 1982, 14, 1285.
13 DENZO: Z. Otwinowski and W. Minor, in Macromolecular
Crystallography (vol. 276 of the Methods in Enzymology series),
eds. C. W. Carter, Jr. and R. M. Sweet, Academic Press, 1997,
Part A, p. 307.
14 SHELXS-86: G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990,
46, 467.
15 SHELXL-93: G. M. Sheldrick, Universit¯at Go¨ttingen, Germany,
1993.
16 International Tables for Crystallography, Kluwer Academic
Publishers., Dordrecht, 1992, vol. C.
x CCDC reference numbers 210 941 and 210 942. See http://
www.rsc.org/suppdata/nj/b3/b306700a/ for crystallographic data in
.cif or other electronic format.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
152
N e w . J . C h e m . , 2 0 0 4 , 6 , 1 4 5 – 1 5 2