1282
Organometallics 2009, 28, 1282–1285
Activated Zinc Complexes Supported by a Neutral,
Phosphinimine-Containing Ligand: Synthesis and Efficacy for the
Polymerization of Lactide
Craig A. Wheaton, Benjamin J. Ireland, and Paul G. Hayes*
Department of Chemistry and Biochemistry, UniVersity of Lethbridge, 4401 UniVersity DriVe,
Lethbridge, AB, T1K 3M4, Canada
ReceiVed October 27, 2008
Summary: A general approach for the synthesis of cationic zinc
complexes supported by neutral ancillary ligands has been
deVeloped and exploited to prepare [LH][B(C6F5)4] (L )
4-(2,6-iPr2-C6H3NPPh2)dbf), 1a, and [LH][SO3CF3], 1b) using
[HNMe2Ph][B(C6F5)4] and triflic acid, respectiVely. Reactions
of 1a and 1b with diethylzinc afforded the corresponding
complexes [LZnEt][B(C6F5)4], 2a, and [LZnEt(OSO2CF3)], 2b.
Both zinc complexes haVe exhibited notable actiVities for the
catalytic polymerization of L-lactide.
Figure 1. General structure of ligand L, to which a chiral group
will be added in future generations of catalyst design.
Polylactide has received growing attention in recent years as
an environmentally friendly, potentially carbon neutral alterna-
tive to conventional polyolefins. Consequently, the development
of new single-site metal catalysts for the ring-opening polym-
erization of lactide has seen tremendous growth over the past
decade.1 Several important families of single-site zinc catalysts
have been developed that exhibit high polymerization activity;2
however, these studies have predominantly employed neutral
catalyst species supported by anionic ancillary ligands. A
handful of recent studies have considered the use of neutral
ligands,3 though very few cationic species have been success-
fully applied to lactide polymerization.2e,4 This is surprising
given that borane-activated ion-pairs have realized significant
success in the field of olefin polymerization catalysis.5 For these
reasons, we were motivated to prepare sterically and electroni-
cally unsaturated zinc complexes of the form [LZnR]+ as
homogeneous catalysts for lactide polymerization. Such acti-
vated species might be expected to show enhanced activity due
to more facile coordination of lactide to the metal center.6 To
the best of our knowledge, the results reported herein constitute
the first single-site cationic zinc species supported by neutral
ligands to be used for the polymerization of lactide.
A novel neutral ligand (L) has been designed whereby a single
phosphinimine substituent is attached at the 4 position of
dibenzofuran (dbf). This ligand is intended to coordinate in a
bidentate manner through the phosphinimine nitrogen and the
oxygen of the dbf backbone to afford six-membered metalla-
cycles. The modular phosphinimine functionality, which allows
for a high degree of steric and electronic tunability, is also
chemically robust and serves as an excellent electron donor.7
In addition, the 31P NMR handle is particularly useful, as the
phosphinimine is highly sensitive to the chemical environment,
exhibiting large downfield shifts upon coordination to a Lewis
acid. As the dbf backbone can be easily substituted at the 6
position,8 we envision developing enantioselective catalysts by
installation of chiral functionalities at this site (Figure 1).
* Corresponding author. E-mail: p.hayes@uleth.ca.
(1) For general reviews see: (a) O’Keefe, B. J.; Hillmyer, M. A.;
Tolman, W. B. J. Chem. Soc., Dalton Trans. 2001, 2215–2224. (b) Wu,
J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C. Coord. Chem. ReV. 2006, 250,
602–626. (c) Platel, R. H.; Hodgson, L. M.; Williams, C. K. Polym. ReV.
2008, 48, 11–63.
(2) (a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W.
J. Am. Chem. Soc. 1999, 121, 11583–11584. (b) Chisholm, M. H.; Eilerts,
N. W.; Huffman, J. C.; Iyer, S. S.; Pacold, M.; Phomphrai, K. J. Am. Chem.
Soc. 2000, 122, 11845–11854. (c) Williams, C. K.; Breyfogle, L. E.; Choi,
S. K.; Nam, W.; Young, V. G., Jr.; Hillmyer, M. A.; Tolman, W. B. J. Am.
Chem. Soc. 2003, 125, 11350–11359. (d) Hill, M. S.; Hitchcock, P. B.
J. Chem. Soc., Dalton. Trans. 2002, 4694–4702. (e) Lian, B.; Thomas, C. M.;
Casagrande, O. L., Jr.; Lehmann, C. W.; Roisnel, T.; Carpentier, J.-F. Inorg.
Chem. 2007, 46, 328–340. (f) Alonso-Moreno, C.; Garce´s, A.; Sa`nchez-
Barba, L.-F.; Fajardo, M.; Ferna´ndez-Baeza, J.; Otero, A.; Lara-Sa´nchez,
A.; Antin˜olo, A.; Broomfield, L.; Lo´pez-Solera, M. I.; Rodrı´guez, A. M.
Organometallics 2008, 27, 1310–1321. (g) Chen, H.-Y.; Tang, H.-Y.; Lin,
C.-C. Macromolecules 2006, 39, 3745–3752. (h) Zhang, C.; Wang, Z.-X.
J. Organomet. Chem. 2008, 693, 3151–3158.
(3) (a) Jensen, T. R.; Breyfogle, L. E.; Hillmyer, M. A.; Tolman, W. B.
Chem. Commun. 2004, 2504–2505. (b) Boerner, J.; Herres-Pawlis, S.;
Fluorke, U.; Huber, K. Eur. J. Inorg. Chem. 2007, 5645–5651. (c) Jeong,
J. H.; An, Y. H.; Kang, Y. K.; Nguyen, Q. T.; Lee, H.; Novak, B. M.
Polyhedron 2007, 27, 319–324.
(4) (a) Sarazin, Y.; Schormann, M.; Bochmann, M. Organometallics
2004, 23, 3296–3302. (b) Samantaray, M. K.; Katiyar, V.; Roy, D.; Pang,
K.; Nanavati, H.; Stephen, R.; Sunoj, R. B.; Ghosh, P. Eur. J. Inorg. Chem.
2006, 2975–2984. (c) Dagorne, S.; Le Bideau, F.; Welter, R.; Bellemin-
Laponnaz, S.; Maisse-Francoise, A. Chem.-Eur. J. 2007, 13, 3202–3217.
(5) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391–1434.
The general ligand framework is easily synthesized by
reaction of the phosphine precursor, generated according to a
modified literature procedure,9 with an appropriate aryl-azide
(6) The ring-opening polymerization of lactide by metal catalysts is
known to occur by a coordination-insertion mechanism. For details, see:
(a) Kricheldorf, H. R.; Berl, M.; Scharnagl, N. Macromolecules 1988, 21,
286–293. (b) Dubois, P.; Jacobs, C.; Jerome, R.; Teyssie, P. Macromolecules
1991, 24, 2266–2270.
(7) (a) Courtenay, S.; Walsh, D.; Hawkeswood, S.; Wei, P.; Das, A. K.;
Stephan, D. W. Inorg. Chem. 2007, 46, 3623–3631. (b) Cavell, R. G.;
Kamalesh Babu, R. P.; Aparna, K. J. J. Organomet. Chem. 2001, 617, 158–
169. (c) Welch, G. C.; Piers, W. E.; Parvez, M.; McDonald, R. Organo-
metallics 2004, 23, 1811–1818. (d) Zhu, D.; Budzelaar, P. H. M.
Organometallics 2008, 27, 2699–2705.
(8) Haenel, M. W.; Fieseler, H.; Jakubik, D.; Gabor, B.; Goddard, R.;
Kruger, C. Tetrahedron Lett. 1993, 34, 2107–2110.
(9) Haenel, M. W.; Jakubik, D.; Rothenberger, E.; Schroth, G. Chem.
Ber. 1991, 124, 1705–1710.
10.1021/om801034k CCC: $40.75
2009 American Chemical Society
Publication on Web 02/16/2009