Dithiodiolate Ligands: Group 4 complexes and application in lactide polymerization

Article abstract of DOI:10.1021/ic100390x

Dithiodiolate ligands were synthesized by reacting 1,2-ethanedithiol or 1,2-benzenedithiol with 2,2-bis(trifluoromethyl)oxirane, led selectively to mononuclear octahedral group 4 complexes of the type [{OSSO}M(OR)₂], which features C₂

Full text of DOI:10.1021/ic100390x

Inorg. Chem. 2010, 49, 3977–3979 3977
DOI: 10.1021/ic100390x
Dithiodiolate Ligands: Group 4 Complexes and Application
in Lactide Polymerization
Ekaterina Sergeeva, Jacob Kopilov, Israel Goldberg, and Moshe Kol*
School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv,
Tel Aviv 69978, Israel
Received February 26, 2010
Dithiodiolate ligands were synthesized by reacting 1,2-ethane-
dithiol or 1,2-benzenedithiol with 2,2-bis(trifluoromethyl)oxirane,
led selectively to mononuclear octahedral group 4 complexes of the
type [{OSSO}M(OR)2], which features C2 symmetry and fluxional
behavior, and were highly active in the ring-opening polymerization
of rac- and L-lactide.
to other metals, group 4 metal complexes are less explored for
cyclic ester ROP.7-13 Typical catalysts are composed of
chelating phenoxo-type spectator ligands and labile alkoxo
groups.14,15 Of those, zirconium (and hafnium) complexes
are typically more active and exhibit superior stereocontrol in
comparison to the analogous titanium species.12,13
Chelating ligands featuring phenoxo-type anionic ligands
have dominated the coordination chemistry of oxophilic
main-group and transition metals and have led to numerous
successful catalysts16 while the related nonaromatic alkoxo-
based ligands have been explored to a lesser extent.17 For
example, complexes of {OSSO}-type dithiodiphenolate
ligands have led to the isospecific polymerization of styrene
(when bound to titanium)18 and to heteroselective poly-
merization of rac-lactide (when bound to scandium),19
whereas {OSSO}-type dithiodiolate ligands have never been
reported, to our knowledge. Herein, we describe the synthesis
of the first dithiodiolate ligand precursors, their coordination
chemistry around group 4 metals, and the application of
Poly(lactic acid) (PLA) is often referred to as “bioplastic”
or “greenplastic” because it is produced from renewable
resources and because of its ability to biodegrade.1 Having
mechanical properties similar to those of polystyrene.2 it may
replace less environmentally friendly plastics in packaging
applications, while its biocompatibility enables its applica-
tion in biomedical products.3,4 The preferred method for
producing PLA is ring-opening polymerization (ROP) of the
dilactone of lactic acid;lactide;because it enables control
of the molecular weight and tacticity, which affect the
plastic’s mechanical properties and its tendency to degrade.5
these complexes in the ROP of rac- and L-lactide.
Polymerization of L-lactide leads to isotactic PLA, whereas
The ligands were designed to include either an aliphatic or
an aromatic two-carbon bridge between the sulfur donors
and two trifluoromethyl groups on the R-carbon atoms to
sterically and electronically diminish the bridging tendency of
polymerization of rac-lactide can lead to atactic, heterotactic,
stereoblock, or stereocomplex PLA as determined by the
catalyst employed.6 Therefore, in the past decade, consider-
able efforts were made to design well-defined metal-based
catalysts as active polymerization initiators. In comparison
(12) Chmura, A. J.; Davidson, M. G.; Frankis, C. J.; Jones, M. D.; Lunn,
M. D. Chem. Commun. 2008, 1293.
(13) Zelikoff, A. L.; Kopilov, J.; Goldberg, I.; Coates, G. W.; Kol, M.
Chem. Commun. 2009, 6804.
(14) For titanium complexes of a bis(phenoxide) spectator ligand and
amido labile groups, see: Takashima, Y.; Nakayama, Y.; Hirao, T.; Yasuda,
H.; Harada, H. J. Organomet. Chem. 2004, 689, 612.
(15) For group 4 complexes featuring spectator chelating tosylamido
ligands, see: Schwarz, A. D.; Thompson, A. L.; Mountford, P. Inorg. Chem.
2009, 48, 10442.
*To whom correspondence should be addressed. E-mail: moshekol@
post.tau.ac.il.
(1) Williams, C. K.; Hillmyer, M. A. Polym. Rev. 2008, 48, 1.
(2) Dorgan, J. R.; Lehermeier, H.; Mang, M. J. Polym. Environ. 2000, 8, 1.
(3) Auras, R.; Harte, B.; Selke, S. Macromol. Biosci. 2004, 4, 835.
(4) Albertsson, A.-C.; Varma, I. K. Biomacromolecules 2003, 4, 1466.
(5) For a recent review, see: Platel, R. H.; Hodgson, L. M.; Williams,
C. K. Polym. Rev. 2008, 48, 11.
(6) For a recent review, see: Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165.
(7) (a) Kim, Y.; Verkade, J. G. Organometallics 2002, 21, 2395. (b) Kim, Y.;
Jnaneshwara, G. K.; Verkade, J. G. Inorg. Chem. 2003, 42, 1437.
(8) Russell, S. K.; Gamble, C. L.; Gibbins, K. J.; Juhl, K. C. S.; Mitchel,
W. S., III; Tumas, A. J.; Hofmeister, G. E. Macromolecules 2005, 38, 10336.
(9) Gendler, S.; Segal, S.; Goldberg, I.; Goldschmidt, Z.; Kol, M. Inorg.
Chem. 2006, 45, 4783.
(10) Gregson, C. K. A.; Blackmore, I. J.; Gibson, V. C.; Long, N. J.;
Marshall, E. L.; White, A. J. P. Dalton Trans. 2006, 3134.
(11) Chmura, A. J.; Davidson, M. G.; Jones, M. D.; Lunn, M. D.; Mahon,
M. F.; Johnson, A. F.; Khunkamchoo, P.; Roberts, S. L.; Wong, S. S. F.
Macromolecules 2006, 39, 7250.
(16) For selected examples, see: (a) Makio, H.; Fujita, T. Acc. Chem. Res.
2009, 42, 1532. (b) Tshuva, E. Y.; Goldberg, I.; Kol, M. J. Am. Chem. Soc. 2000,
122, 10706. (c) Yoon, T. P.; Jacobsen, E. N. Science 2003, 299, 1691. (d) Saito, B.;
Egami, H.; Katsuki, T. J. Am. Chem. Soc. 2007, 129, 1978.
(17) See, however: Carpentier, J.-F. Dalton Trans. 2010, 39, 37.
€
€
(18) (a) Capacchione, C.; Proto, A.; Ebeling, H.; Mulhaupt, R.; Moller,
K.; Spaniol, T. P.; Okuda, J. J. Am. Chem. Soc. 2003, 125, 4964. (b) Beckerle,
K.; Manivannan, R.; Lian, B.; Meppelder, G.-J. M.; Raabe, G.; Spaniol, T. P.;
Ebeling, H.; Pelascini, F.; M€ulhaupt, R.; Okuda, J. Angew. Chem., Int. Ed. 2007,
46, 4790.
(19) Ma, H.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2006, 45,
7818.
r
2010 American Chemical Society
Published on Web 03/31/2010
pubs.acs.org/IC

3978 Inorganic Chemistry, Vol. 49, No. 9, 2010
Sergeeva et al.
Scheme 1. Dithiodiolate Ligands and Their Group 4 Metal Complexes
the oxygen donors.20 The two ligand precursors Lig1H2 and
Lig2H2 were conveniently prepared in a one-step synthesis by
reacting 2,2-bis(trifluoromethyl)oxirane with 1,2-ethane-
dithiol and 1,2-benzenedithiol, respectively (see the Support-
ing Information). Each ligand was then reacted with Ti(O-i-
Pr)4, Zr(O-t-Bu)4, and Hf(O-t-Bu)4 to give well-defined
mononuclear complexes of the type [{OSSO}M(OR)2] in
good yield (see Scheme 1).21 The complexes were easily
isolated as white solids by the removal of volatiles under
reduced pressure and pentane washings. All of the complexes
(and in particular complex 4) showed signs of decomposition
within a few days. Therefore, spectroscopic characterization
and polymerization reactions were performed shortly after
their preparation.
Figure 1. ORTEP representation of complex 1 with 50% probability
ellipsoids. Hydrogen atoms were omitted for clarity. Selected bond
lengths (A) and angles (deg) for Lig Ti(O-i-Pr)2: Ti1-O2, 1.938(3);
Ti1-O3, 1.921(3); Ti1-O4, 1.792(3); Ti1-O5, 1.751(3); Ti1-S6,
2.762(1); Ti1-S7, 2.740(1); O2-Ti1-O3, 150.0(1); S6-Ti-S7, 78.6(4);
O4-Ti-O5 106.0(1).
1
˚
[{OSSO}Ti(O-i-Pr)2] and 17.0 kcal mol-1 for the complex of
the softer metal [{OSSO}Zr(O-t-Bu)2].23 For the dithiodi-
phenolate complexes [{OSSO}Ti(O-i-Pr)2] (featuring
a
5-5-5 chelate array), a barrier for interconversion that
Symmetric tetradentate ligands may wrap in three geome-
tries around octahedral metal centers designated as mer-mer
(trans), fac-fac (cis-R), or fac-mer (cis-β).22 Because the
sulfur donors become stereogenic upon binding to the metal,
the possible symmetries of the isomers of these {OSSO}
complexes are C2, Cs, or C1. Single peaks of the methylene
depended on the ligand bulk was found, with the lowest being
13.4 kcal mol-1 24,25
.
Single crystals of 1 were grown from cold pentane, and its
X-ray structure was solved. The structure features a mono-
nuclear complex of octahedral geometry around the titanium
center; i.e., the sulfur donors are bound to the metal,
completing an array of 5-5-5-membered chelating rings.26
Lig1 binds in a fac-fac mode around the titanium metal
center (Figure 1), which is consistent with the species ob-
served in the 1H NMR spectra at low temperature. Following
this wrapping mode, the diolate oxygen atoms are located
mutually trans, and the monodentate ligands are mutually cis
and are trans to the sulfur donors. The chelating ligand bond
1
groups in the H NMR spectra of all complexes at room
temperature were consistent with C2v-symmetric species. This
indicates a dynamic process of low barrier that interconverts
isomers of lower symmetry. These isomers could be observed
by variable-temperature NMR. For example, the titanium
complex 1 dissolved in toluene-d8 showed coalescence of a
broad signal of the -SCH2C(CF3)2- group and a single peak
of the -SCH2CH2S- group at -45 °C, which split to two
sets of two doublets at -65 °C. This corresponds to a
C2-symmetric complex having a low barrier to enantiomer
interconversion of 11.7 kcal mol-1. Slightly higher barriers for
interconversion were found for zirconium (2) and hafnium (3)
complexes of Lig1 of 13.4 and 13.8 kcal mol-1, respectively
(see the Supporting Information). The complexes of Lig2
(4-6) were even more fluxional, with the titanium and
zirconium complexes showing only broadening of the peaks
upon cooling to -65 °C (estimated ΔG‡ < 10.0 kcal mol-1). It
is notable that similar low barriers for interconversion are
observed for the two ligands around the three metals. In
comparison, the barrier for interconversion showed a strong
dependence on the metal for the dithiodiphenolate ligands
(featuring a 6-5-6 chelate array): 11.5 kcal mol-1 for
24
24
˚
˚
lengths of S-Ti (2.74-2.76 A) and O-Ti (1.92-1.94 A)
are typical, indicating unstrained binding. A narrow
O3-Ti-O2 angle of 150.0° (relative to 163.8° for the related
{ONNO} ligand)25 results from the “pulling-backwards” of
the ligand by the long S-Ti bonds, thus leaving a relatively
open space for the labile groups.
Preliminary activity studies of these complexes included
polymerization of 300 equiv of rac-lactide in the melt and in a
toluene solution at 75 °C (see the Supporting Information).
(23) Cohen, A.; Yeori, A.; Goldberg, I.; Kol, M. Inorg. Chem. 2007, 46,
8114.
(24) Capacchione, C.; Manivannan, R.; Barone, M.; Beckerle, K.;
Centore, R.; Oliva, L.; Proto, A.; Tuzi, A.; Spaniol, T. P.; Okuda, J.
Organometallics 2005, 24, 2971.
(25) Titanium complexes of {ONNO} ligands featuring the same diolate
arms appeared rigid at room temperature. See:Lavanant, L.; Chou, T.-Y.;
Chi, Y.; Lehmann, C. W.; Toupet, L.; Carpentier, J.-F. Organometallics
2004, 23, 5450.
(20) Marquet, N.; Kirillov, E.; Roisnel, T.; Razzavi, A.; Carpentier, J.-F.
Organometallics 2009, 28, 606.
(26) In contrast, X-ray structures of thiodiolate {OSO} complexes
showed that the sulfur group was not coordinated to the metal. See:
Lavanant, L.; Silvestru, A.; Faucheux, A.; Toupet, L.; Jordan, R.;
Carpentier, J.-F. Organometallics 2005, 24, 5604.
(21) Attempts to prepare complexes of the type [{OSSO}M(O-i-Pr)2] for
M=Zr and Hf resulted in ill-defined mixtures.
(22) Sergeeva, E.; Kopilov, J.; Goldberg, I.; Kol, M. Inorg. Chem. 2009,
48, 8075.

Communication
Inorganic Chemistry, Vol. 49, No. 9, 2010 3979
All complexes were able to catalyze the ROP of rac-lactide;
however, the activity and stereoselectivity varied depending
on the metal, whereas the nature of the bridge between the
two sulfur donors (i.e., Lig1 vs Lig2) was considerably less
significant. The two titanium complexes 1 and 4 showed the
lowest activities and led to practically atactic PLA for
polymerizations carried out in a neat monomer at 144 °C.
Still, consumption of ca. 150 equiv of rac-lactide in the melt
within 17 min (by 4) corresponds to one of the highest
activities reported to date for titanium complexes.10,12,13
Higher activities and a clear inclination toward heterotacti-
city were found for the zirconium and hafnium complexes 2,
3, 5, and 6 of the two ligands. For polymerizations in the melt,
the hafnium complexes were somewhat more active than the
zirconium complexes. In particular, an almost quantitative
consumption of 300 equiv of rac-lactide within 1 min (and
3000 equiv within 5 min) by complex 6 corresponds to the
highest activity of any group 4 metal catalyst reported to
date, to our knowledge.12,13,15 The PLA obtained featured a
degree of heterotacticity (Pr) of approximately 70%. Run-
ning the polymerizations in toluene solutions at lower tem-
peratures resulted in higher degrees of heterotacticities, with
the highest being 89% achieved by complex 5 at 75 °C.
Molecular weight distributions in the range of PDI =
1.17-1.78 and lower than expected molecular weights indi-
cate that transesterification processes are taking place.
As expected, catalysts that exhibit a mild tendency for
heterotactic enchainment in the melt showed a relatively high
activity toward
plexes of Lig2 fully consumed 300 equiv of
15-25 min (see the Supporting Information).
L
-lactide as well.27 For example, all com-
-lactide within
L
In conclusion, the first dithiodiolate ligands were synthe-
sized via a straightforward one-step reaction. The ligands led
to C2-symmetric complexes around octahedral centers of
group 4 metals that exhibited a high degree of fluxionality.
These complexes were highly active in the ROP of lactide,
showing a tendency for heterotactically enriched PLA in the
polymerization of rac-lactide. The study of the coordination
chemistry of these ligands around other metals and further
possible catalytic applications are underway.
Acknowledgment. We thank the Israel Science Founda-
tion and the United States-Israel Binational Science
Foundation for financial support.
Supporting Information Available: Experimental details for
synthesis of the ligands and complexes and crystallographic data
of complex 1 in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
(27) Although polymerization demonstrates a relatively high activity
toward
of a mild kinetic preference for consecutive polymerization of
-lactide to give heterotactically enriched poly(lactide).
L
-lactide, it is significantly slower than that of rac-lactide because
D
- and
L