Novel zinc and magnesium alkyl and amido cations for ring-opening polymerization reactions

Article abstract of DOI:10.1021/om0497691

The reaction of [H(OEt₂)₂] [B(C₆F ₅)₄] with Zn[N(SiMe₃)₂]₂, MgBu₂, and {Mg[N(SiMe₃)₂]₂} ₂ in diethyl ether proceeds in very high yields to give the salts [(Et₂O)₃ ZnN(SiMe₃)₂] [B(C ₆F₅)₄] (2), [(Et₂O) ₃MgⁿBu][B(C₆F₅)₄] (3), and [(Et₂O)₃MgN(SiMe₃)₂] [B(C ₆F₅)₄] (4), respectively. The structures of 2 and 4 were determined by X-ray crystallography; the compounds contain distorted-tetrahedral metal centers and are isostructural. [(Et ₂O)₃ZnCH₂CH₃] [B(C₆F ₅)₄] (1) as well as 2-4 catalyze the ring-opening polymerization of epoxides and ε-caprolactone. Cyclohexene oxide is polymerized extremely rapidly to high molecular weight materials, while the polymerization of propene oxide gives low molecular weight materials via a cationic mechanism. The zinc species are more stable than their magnesium analogues and show higher productivities. ε-Caprolactone is polymerized efficiently to high molecular weight polymers.

Full text of DOI:10.1021/om0497691

3296
Organometallics 2004, 23, 3296-3302
Novel Zin c a n d Ma gn esiu m Alk yl a n d Am id o Ca tion s for
Rin g-Op en in g P olym er iza tion Rea ction s
Yann Sarazin, Mark Schormann, and Manfred Bochmann*
Wolfson Materials and Catalysis Centre, School of Chemistry and Pharmacy,
University of East Anglia, Norwich NR4 7TJ , U.K.
Received April 1, 2004
The reaction of [H(OEt2)2][B(C6F5)4] with Zn[N(SiMe3)2]2, MgBu2, and {Mg[N(SiMe3)2]2}2
in diethyl ether proceeds in very high yields to give the salts [(Et2O)3ZnN(SiMe3)2][B(C6F5)4]
(2), [(Et2O)3MgnBu][B(C6F5)4] (3), and [(Et2O)3MgN(SiMe3)2][B(C6F5)4] (4), respectively. The
structures of 2 and 4 were determined by X-ray crystallography; the compounds contain
distorted-tetrahedral metal centers and are isostructural. [(Et2O)3ZnCH2CH3][B(C6F5)4] (1)
as well as 2-4 catalyze the ring-opening polymerization of epoxides and ꢀ-caprolactone.
Cyclohexene oxide is polymerized extremely rapidly to high molecular weight materials,
while the polymerization of propene oxide gives low molecular weight materials via a cationic
mechanism. The zinc species are more stable than their magnesium analogues and show
higher productivities. ꢀ-Caprolactone is polymerized efficiently to high molecular weight
polymers.
In tr od u ction
as ꢀ-caprolactone and especially L-lactide for the produc-
tion of biodegradable polymers have become an impor-
tant target.1b Zinc and to a lesser extent magnesium
complexes with sterically hindered ligands have been
shown to be the most efficient initiators to date for the
controlled ring-opening polymerization of these mono-
mers.1b,c Coates and Chisholm independently reported
diketiminato and iminophenolato complexes of zinc that
are very efficient for the polymerization of lactides;9
Chisholm also found that tris(pyrazolato) borate com-
plexes of magnesium and especially calcium were highly
active.10 However, these complexes are inactive toward
epoxide polymerization.
The ring-opening polymerizations of polar monomers
such as epoxides and cyclic esters have attracted
considerable interest during the past few years.1 Poly-
(propene oxide) (PPO) is a key component for the
preparation of polyurethanes and is widely used as a
bulk commodity material.2 Aluminum-based complexes
have been developed for the living polymerization of
propene oxide,3 whereas cationic aluminum species
supported by the salen-type ligands afford high molec-
ular weight poly(propene oxide).4 Cyclohexene oxide and
carbon dioxide are successfully copolymerized by zinc
aryloxides,5 diketiminates,6 and carboxylates,7 although
these systems proved ineffective for epoxide homopo-
lymerization.8 The polymerizations of cyclic esters such
We showed recently that the reaction of zinc dialkyls
with B(C6F5)3 in toluene gives Zn(C6F5)2(toluene), while
in Et2O the cationic zinc alkyls [RZn(OEt2)3]+[B(C6F5)4]-
t
are formed (R ) Me, Et, Bu).11a Similarly, zinc cyclo-
* Corresponding author. E-mail: m.bochmann@uea.ac.uk.
(1) Darensbourg, D. J .; Holtcamp, M. W. Coord. Chem. Rev. 1996,
153, 155. (b) O’Keefe, B. J .; Hillmyer, M. A.; Tolman, W. B. J . Chem.
Soc., Dalton Trans. 2001, 2215. (c) Coates, G. W. J . Chem. Soc., Dalton
Trans. 2002, 467.
(2) Ultree, A. J . Encyclopedia of Polymer Science and Engineering;
J ohn Wiley and Sons: New York, 1986; Vol. 6, pp 733-755. (b) Bolick,
J . E.; J ensen, A. W. In Encyclopedia of Chemical Technology, 4th ed.;
Kirk-Othmer, Ed.; J ohn Wiley and Sons: New York, 1993; Vol. 10, pp
624-638.
pentadienyl complexes stabilized by chelating amines
form cationic species that are active for the polymeri-
zation of ꢀ-caprolactone and cyclohexene oxide.11b We
also reported the convenient synthesis of three-coordi-
nate zinc cations [DADZnR]+[X]- from ZnR2 and [HDAD]-
[X] (R ) Me, Et, N(SiMe3)2; X ) Me-B(C6F5)3, B(C6F5)4;
(3) Aida, T.; Inoue, S. Macromolecules 1981, 14, 1162. (b) Aida, T.;
Inoue, S. Macromolecules 1981, 14, 1166. (c) Aida, T.; Wada, K.; Inoue,
S. Macromolecules 1987, 20, 237. (d) Le Borgne, A.; Vincens, V.;
J ouglard, M.; Spassky, N. Makromol. Chem. Macromol. Symp. 1993,
73, 37. (e) Atwood, D. A.; J egier, J . A.; Rutherford, D. Inorg. Chem.
1996, 35, 63. (f) Emig, N.; Nguyen, H.; Krautscheid, H.; Re´au, R.;
Cazaux, J .-B.; Bertrand, G. Organometallics 1998, 17, 3599. (g) Braune,
W.; Okuda, J . Angew. Chem., Int. Ed. 2003, 42, 64.
(4) Mun˜oz-Hernandez, M.-A.; McKee, M. L.; Keizer, T. S.; Yearwood,
B. C.; Atwood, D. A. J . Chem. Soc., Dalton Trans. 2002, 410.
(5) (a) Darensbourg, D. J .; Holtcamp, M. W. Macromolecules 1995,
28, 577. (b) Darensbourg, D. J .; Holtcamp, M. W.; Struck, G. E.;
Zimmer, M. S.; Niezgoda, S. A.; Rainey, P.; Robertson, J . B.; Draper,
J . D.; Reibenspies, J . H. J . Am. Chem. Soc. 1999, 121, 107. (c)
Darensbourg, D. J .; Wildeson, J . R.; Yarbrough, J . C.; Reibenspies, J .
H. J . Am. Chem. Soc. 2000, 122, 12487.
(7) Super, M.; Berlucke, E.; Costello, C.; Beckman, E. J . Am. Chem.
Soc. 1997, 119, 368. (b) Darensbourg, D. J .; Wildeson, J . R.; Yarbrough,
J . C. Organometallics 2001, 20, 4413.
(8) The homopolymerization of cyclohexene oxide by zinc aryloxides
has been reported, see ref 5c.
(9) Chen, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. J .
Am. Chem. Soc. 1999, 121, 11583. (b) Chamberlain, B. M.; Cheng, M.;
Moore, D. R.; Ovitt, E. B. Lobkovsky, E. B.; Coates, G. W. J . Am. Chem.
Soc. 2001, 123, 3229. (c) Chisholm, M. H.; Galluci, J .; Zhen, H.;
Huffman, J . C. Inorg. Chem. 2001, 40, 5051. (d) Chisholm, M. H.;
Galluci, J .; Phomphrai, K. Inorg. Chem. 2002, 41, 2785.
(10) Chisholm, M. H.; Eilerts, N. W. Chem. Commun. 1996, 853.
(b) Chisholm, M. H.; Eilerts, N. W.; Huffman, J . C.; Iyer, S. S.; Pacold,
V.; Phomphrai, K. J . Am. Chem. Soc. 2000, 122, 11845. (c) Chisholm,
M. H.; Galluci, J .; Phomphrai, K. Chem. Commun. 2003, 48.
(11) (a) Walker, D. A.; Woodman, T. J .; Hughes, D. L.; Bochmann,
M. Organometallics 2001, 20, 3772. (b) Walker, D. A.; Woodman, T.
J .; Schormann, M.; Hughes, D. L.; Bochmann, M. Organometallics
2003, 22, 797.
(6) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J . Am. Chem. Soc.
1998, 120, 11018. (b) Cheng, M.; Moore, D. R.; Reczek, J . J .; Cham-
berlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J . Am. Chem. Soc. 2001,
123, 8738.
10.1021/om0497691 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 05/28/2004

Zn and Mg Cations for Ring-Opening Polymerization
Organometallics, Vol. 23, No. 13, 2004 3297
Sch em e 1
i
DAD ) (MeCdNC6H3 Pr2-2,6)2) suitable for the ring-
opening polymerization of epoxides and ꢀ-caprolactone.12
Previously, cationic zinc alkyls had been synthesized by
the addition of nitrogen macrocycles or crown ethers to
zinc alkyls in the presence of AlR3 or tetraphenylcyclo-
pentadiene13 or by the protolysis of ZnEt2 with [Bz3TAC-
H]+[PF6]- (TAC ) 1,3,5-tribenzyl-1,3,5-triazacyclohex-
ane).14
dichloromethane, but only moderately in toluene and
insoluble in light petroleum.
The new complexes were characterized by NMR
spectroscopy (1H, {1H}13C, 19F, and 11B) and elemental
analyses. In all cases, the 11B NMR of the borate anion
displays a single sharp peak at δ -13.6. Elemental
analyses showed good agreement with the theoretical
values. The synthesis of 3 from MgBusBun gave exclu-
sively the n-butyl complex, either by selective protona-
tion of s-Bu or via isomerization of the s-Bu groups.
Attempts to prepare aryloxy derivatives of compounds
1 and 2 either by direct reaction of J utzi’s acid with {Zn-
(OAr)2}2 (Ar ) 2,6-bis-But-4-methylphenyl) or via ad-
dition of an excess of ArOH to 2 proved unsuccessful.
In both cases, off-white sticky solids were obtained, and
neither washing with petrol nor attempts to recrystal-
lize in diethyl ether allowed the recovery of a clean
product.
Whereas single crystals of 2 suitable for X-ray dif-
fraction crystallography were readily grown from diethyl
ether/light petroleum at -26 °C, the isolation of crystals
of the magnesium derivatives 3 and 4 was complicated
by the extreme sensitivity and relative instability of
these compounds. The smallest crystals decomposed
almost immediately when removed from the Schlenk
tube under nitrogen, whereas the bigger ones degraded
in the diffractometer during the course of the acquisition
of the crystallographic data. Suitable crystals of 4 were
eventually obtained by recrystallization from an 8:1
mixture of diethyl ether and light petroleum after
several weeks at -26 °C, whereas suitable crystals of 3
could not be isolated. The difference in stability between
Zn compounds 1 and 2 and their Mg analogues 3 and 4
was further evidenced by the fact that the latter
decomposed in the inert atmosphere of the glovebox over
a period of a few months, while the former remained
unchanged under identical conditions.
On the other hand, very little is known about cationic
magnesium alkyls or amides. The reaction of MgR2 with
the sterically demanding tris(3-R′-pyrazolyl)hydrobo-
rato ligands (R′ ) Ph, tBu) yields [η3-HB(3-R′pz)3]-
MgR,9b,15 whereas magnesium alkyl cations are obtained
by reaction of dialkylmagnesium with the azacrown
ether (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetra-
decane).16 Here, we report the syntheses and the
structures of novel four-coordinate alkyl and amide zinc
and magnesium cations and their behavior in the ring-
opening polymerization of cyclohexene oxide, propene
oxide, and ꢀ-caprolactone.
Resu lts a n d Discu ssion
The reaction in diethyl ether of “Jutzi’s acid” [H(OEt2)2]-
[B(C6F5)4] with a slight excess of [MX2]n (M ) Zn, n )
1, X ) N(SiMe3)2; M ) Mg, X ) Bu, n ) 1; X )
N(SiMe3)2, n ) 2) affords the corresponding salts
[(Et2O)3ZnN(SiMe3)2][B(C6F5)4] (2), [(Et2O)3MgCH2CH2-
CH2CH3][B(C6F5)4] (3), and [(Et2O)3MgN(SiMe3)2][B(C6-
F5)4] (4), where three molecules of diethyl ether are
coordinated to the cationic metal center and where the
counteranion is the poorly coordinating pentafluorophe-
nyl borate anion. Following the procedure we reported
for the synthesis of [(Et2O)3ZnCH2CH3][B(C6F5)4] (1),11
compounds 2-4 were prepared in very good yields when
the reactions were carried out for 1 to 2 h in diethyl
ether at room temperature (Scheme 1). In the case of
the Mg species, the yields of isolated products were
typically well above 90%, and the reactions were com-
plete within 1 h. Analytically pure products 2-4 were
isolated as white crystalline materials from a diethyl
ether/light petroleum mixture (5:1) at -26 °C. All
compounds are extremely air and moisture sensitive.
They are highly soluble in diethyl ether, THF, and
The single-crystal X-ray structures of the salts 2 and
4 were determined (Figures 1 and 2). Selected bond
lengths and angles are collected in Table 1. They both
contain two anions and two cations per asymmetric unit.
The structure of the cation in 4 (4+) confirms the
presence of three molecules of diethyl ether coordinated
to the metal center and exhibits a distorted tetrahedral
arrangement, with N-Mg-O angles ranging from
107.93(12)° to 127.46(12)°. The Mg-N bond length in
4+ of 1.986(3) Å compares well with the literature values
for [(Et2O)Mg(Cl){N(SiMe3)2}]2 [1.970(3) Å] and Mg-
[N(SiMePh2)2]2 [1.963(6) and 1.969(6) Å],17 [{(Me3-
Si)2NMg[µ-OC(Me)-tBuCH2C(tBu)dO]}2] [1.9920(19) Å],18
(12) Hannant, M. D.; Schormann, M.; Bochmann, M. J . Chem. Soc.,
Dalton Trans. 2002, 4071.
(13) (a) Fabicon, R. M.; Pajerski, A. D.; Richey, H. G. J . Am. Chem.
Soc. 1991, 113, 6680. (b) Fabicon, R. M.; Parvez, M.; Richey, H. G.
Organometallics 1999, 18, 5163. (c) Tang, H.; Parvez, M.; Richey, H.
G. Organometallics 2000, 19, 4810.
(14) Haufe, M.; Ko¨hn, R. D.; Kociok- Ko¨hn, G.; Filippou, A. C. Inorg.
Chem. Commun. 1998, 1, 263.
(15) Haq, R.; Parkin, G. J . Am. Chem. Soc. 1992, 114, 748.
(16) Fabicon, R. M.; Pajerski, A. D.; Richey, H. G. J . Am. Chem.
Soc. 1993, 115, 9333.
(17) Bartlett, R. A.; Olmstead, M. M.; Power, P. P. Inorg. Chem.
1994, 33, 4800.

3298 Organometallics, Vol. 23, No. 13, 2004
Sarazin et al.
Ta ble 1. Selected In ter a tom ic Dista n ces (Å) a n d
An gles (d eg) for 2+ a n d 4+
to 2.550 Å, i.e., close to the sum of the van der Waals
radii. The presence of such H‚‚‚F contacts leads to the
formation of infinite chains in the crystal packing.23
[(Et2O)3ZnN(SiMe3)2]+ (2+)
[(Et2O)3MgN(SiMe3)2]+ (4+)
-
Whereas in many cases salts with B(C6F5)4 as anion
Zn(1)-N(4)
Zn(1)-O(1)
Zn(1)-O(2)
Zn(1)-O(3)
N(4)-Si(1)
N(4)-Si(2)
1.907(3)
2.092(2)
2.050(3)
2.056(3)
1.735(3)
1.728(3)
Mg(1)-N(4)
Mg(1)-O(3)
Mg(1)-O(1)
Mg(1)-O(2)
N(4)-Si(2)
N(4)-Si(1)
1.986(3)
2.045(2)
2.056(3)
2.060(3)
1.720(3)
1.724(3)
are extremely difficult to crystallize or remain oils, it
may be that these interactions assist in the facile
crystallization of compounds such as 2 and 4.
P olym er ization Stu dies. Complexes 1-4 were tested
for the ring-opening polymerization of various polar
monomers. In principle, polymerization can be envis-
aged to occur by two mechanisms: either the transfer
of the nucleophilic alkyl or amide ligand to the mono-
mer, with formation of a metal alkoxide-propagating
species (Scheme 2, path A), or ring opening and propa-
gation via a cationic (activated chain end, ACE) mech-
anism (path B).
N(4)-Zn(1)-O(2) 113.86(11) N(4)-Mg(1)-O(3) 127.46(12)
N(4)-Zn(1)-O(3) 133.52(11) N(4)-Mg(1)-O(1) 108.24(12)
N(4)-Zn(1)-O(1) 110.73(11) N(4)-Mg(1)-O(2) 107.93(12)
O(2)-Zn(1)-O(1) 112.23(11) O(3)-Mg(1)-O(2) 95.25(11)
O(3)-Zn(1)-O(1) 91.56(10)
O(2)-Zn(1)-O(3) 92.39(11)
Si(2)-N(4)-Zn(1) 116.7(2)
Si(1)-N(4)-Zn(1) 118.0(2)
O(1)-Mg(1)-O(2) 125.99(12)
O(3)-Mg(1)-O(1) 93.24(10)
Si(1)-N(4)-Mg(1) 121.67(14)
Si(2)-N(4)-Mg(1) 114.0(2)
Compounds 1-4 polymerize cyclohexene oxide (CHO)
under very mild conditions (Table 2). At room temper-
ature, the polymerization of CHO with 1 was extremely
rapid and highly exothermic. Even diluted in 5 volumes
of toluene, the polymerization of CHO (CHO:Zn ) 5000:
1) at an initial temperature of 0 °C was complete within
30 s (entry 1), with a productivity of 5.9 × 107 g polymer
(mol‚Zn)-1‚h-1. This corresponded to a 20-fold increase
in activity compared with that reported for [ZnMe-
(DAD)]+[B(C6F5)4]- under similar conditions, most likely
due to the facile displacement of diethyl ether ligands
by the epoxide.12 There was a considerable reaction
exotherm, with the temperature of the solution rising
quickly to >60 °C. However, this could be controlled by
lowering the temperature or by dilution in a large
volume of toluene. When the polymerizations were
carried out at very low temperatures (Table 2, entries
2, 3), the percentages of conversion and productivities
achieved were much lower, but the molecular weights
of the polymers increased significantly and at -78 °C
reached Mw ) 380 000 g‚mol-1 (entry 3). On the other
hand, no increase of temperature was observed when
the polymerization of 5.1 mL of CHO was performed in
200 mL of toluene on an ice-bath (Table 2, entries 4, 5).
The percentage of conversion of 5000 equiv of CHO
varied considerably from one complex to another.
F igu r e 1. X-ray structure of the cation (Et2O)3MgN-
+
(SiMe3)2 (4+) showing the atomic numbering scheme.
Thermal ellipsoids are drawn at the 50% probability level.
or [(Me3Si)2NMg(µ-Br)(OEt2)]2 [1.962(4) Å] and [(Me3-
Si)2NMg(µ-OEt)(THF)]2 [2.006(3) Å].19 The correspond-
ing Zn-N distance in the cation 2+ of 1.907(3) Å is
comparable to those in the neutral â-diketiminato
complex (BDI)ZnN(SiMe3) [1.896(2) Å]6b or [{CH(Ph2-
PNC6H2Me3-2,4,6)2}ZnN(SiMe3)2] [1.907(4) Å],20 but
longer than in Zn[N(SiMePh2)2]2 [1.849(5) and 1.850(3)
Å].21
Compounds 2 and 4 are isostructural, and the only
noticeable difference resides in the metal-nitrogen bond
length. Although the ionic radius of Mg2+ is smaller
than that of Zn2+ (0.71 and 0.74 Å, respectively22), the
magnesium compound shows the longer M-N bond
length, indicative of weaker bonding.
The crystal packing diagrams of 2 and 4 indicate the
presence of some close C-H‚‚‚F contacts, as illustrated
in Figure 2 for the structure of 2. Interactions can be
seen between some of the fluorine atoms in ortho and
meta positions of the pentafluorophenyl substituents of
the counteranion and some of the hydrogen atoms of
coordinated diethyl ether. Distances range from 2.463
Whereas 1 achieved 64% within 30 min (6.2 × 105
g
polymer (mol‚Zn)-1‚h-1, entry 5), 2, 3, and 4 under the
same conditions converted only 27%, 13%, and 4%,
respectively (entries 6, 7, and 9). Complete conversion
was reached with 1 after 2 h. However, the productivi-
ties decreased consistently with time, indicative of slow
deactivation processes. The polymers possessed high
molecular weights, which ranged from ca. 80 000 to
>100 000 g mol-1 for all complexes, whereas the poly-
dispersities were usually around 3.0. The poly(CHO)
was a very tough, nonstretchable material and soluble
in THF at room temperature.
From these results, the trend in activity is 1 > 2 > 3
> 4; that is, the zinc species were far more active than
their magnesium analogues. The order of activity was
rather surprising since Mg is the more electropositive
metal and the Mg2+ compounds were expected to be the
stronger Lewis acids. The first and second ionization
enthalpies of Mg are 737 and 1450 kJ mol-1, respec-
(18) Allan, J . F.; Henderson, K. W.; Kennedy, A. R. Chem. Commun.
1999, 1325.
(19) Yang, K.-C.; Chang, C.-C.; Huang, J .-Y.; Lin, C.-C.; Lee, G.-H.;
Wang, Y.; Chiang, M. Y. J . Organomet. Chem. 2002, 648, 176.
(20) Hill, M. S.; Hitchcock, P. B. J . Chem. Soc., Dalton Trans. 2002,
4694.
(21) Power, P. P.; Ruhlandt-Senge; Shoner, S. C. Inorg. Chem. 1991,
30, 5013.
(22) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(23) For information on C-H‚‚‚F-C interactions, see: Mountford,
A. J .; Hughes, D. L.; Lancaster, S. J . Chem. Commun. 2003, 2148, and
references therein.

Zn and Mg Cations for Ring-Opening Polymerization
Organometallics, Vol. 23, No. 13, 2004 3299
F igu r e 2. Crystal packing and anion-cation close contacts in 2.
Sch em e 2
Ta ble 2. Cycloh exen e Oxid e P olym er iza tion s
entry
initiatora
toluene [mL]
temp [°C]
time [min]
yield [g]
convb
prodc
Mw × 10-3
Mw/Mn
1
2
3
4
5
6
7
8
9
1
1
1
1
1
2
3
3
4
25
25
25
200
200
200
200
200
200
0
-35
-78
0.5
10
120
15
30
30
30
45
30
4.9
1.5
1.9
2.2
3.1
1.3
0.6
0.9
0.2
100
31
39
46
64
27
13
20
4
58,800
900
95
88
190
380
76
80
104
85
3.1
2.2
3.8
3.2
3.0
2.0
2.6
2.1
0
0
0
0
0
0
880
620
260
120
123
40
82
a
b
10 µmol of initiator, CHO 5.1 mL; CHO:initiator ratio 5000:1. Percentage conversion of the monomer (weight monomer/weight of
polymer recovered × 100). c kg polymer (mol metal)-1‚h-1
.
tively, while those of Zn are noticeably higher, 906 and
1734 kJ mol-1. The magnesium derivatives were there-
fore expected to bind the monomer more easily than
their zinc counterparts, which should result in an
enhanced polymerization ability, as has already been
reported for lactide polymerizations.9a,b,10a,b It appears
therefore that while the ligand environments around the
metal centers are essentially identical, the relative
instability of the Mg complexes 2 and 4 is responsible
for the inferior activity.
Due to its comparatively weaker ring strain, propene
oxide (PO) is significantly more difficult to polymerize
than CHO. In each case, 30 µmol of complexes 1-4 were
reacted at room temperature with 20 000 equiv of neat
PO (Table 3). Polymerization times were usually kept
below 24 h. Aliquots were taken directly from the
reaction mixture and immediately quenched with metha-
nol. The yields in Table 3 are related to the total amount
of monomer used for the polymerization. A high excess
of monomer was necessary to ensure that the solution
did not become too viscous since no other solvent was
used.
As described earlier for the polymerization of CHO,
the results here changed noticeably for each initiator.
Again, the highest conversions were obtained with Zn
complexes, with 58% conversion for 1 in 24 h (2.8 × 104
g PPO (mol‚Zn)-1‚h-1, entry 11), while 2, 3, and 4
converted, respectively, only 11%, 9%, and <1% over the
same period of time (entries 13, 14, and 16). The same
trend as previously noted could be observed for the
polymerization of PO: 1 . 2 > 3 . 4. As expected, the
productivities were low in comparison with those found
for CHO, but were better than those reported in the
literature for the polymerization of PO with aluminum
complexes.3f,g The molecular weights of the polymers
were low, typically Mw ≈ (1-3) × 103 g mol-1, and were

3300 Organometallics, Vol. 23, No. 13, 2004
Ta ble 3. P r op en e Oxid e P olym er iza tion sa
Sarazin et al.
entry
initiatorb
feed ratioc
time [h]
yield [g]
convd
prode
Mw
Mw/Mn
10
11
12
13
14
15
16
1
1
2
2
3
3
4
20 000:1
20 000:1
20 000:1
20 000:1
20 000:1
20 000:1
20 000:1
3
24
5
24
24
120
24
1.7
20.2
1.0
3.8
3.1
5
58
3
11
9
28
<1
19.4
28.0
7.0
5.3
4.4
1 000
1 600
oligomer
oligomer
1 200
1.8
2.9
1.8
3.6
0.47
traces
2.7
2 900
a
b
d
Performed in neat monomer at room temperature. 30 µmol of initiator. c Initial PO/initiator ratio. Percentage conversion of the
monomer (weight monomer/weight of polymer recovered × 100). e kg polymer‚(molmetal)-1‚h-1
.
Sch em e 3
Sch em e 4
comparable to those achieved with aluminum initiators.
Microstructure analyses of PPO samples produced
with 1-4 indicated that the polymers had a random
distribution of primary and secondary hydroxyl end
groups. On the basis of the signals in the area of the
CH2 groups, the polymers consisted of about equal
proportions of H-T diads and H-H, T-T defect diads;
that is, the distribution was random. GPC characteriza-
tion of PPO samples indicated the presence of a signifi-
cant amount of oligomeric species together with the low
molecular weight polymers. This was further confirmed
by GC-MS analysis, which allowed the qualitative and
quantitative determination of low molecular weight
byproducts. The analysis showed that some of the
samples contained up to 6.5% of volatile organic com-
pounds that are typically formed in the acid-catalyzed
polymerization of PO.
However, 2 led to the formation of oligomers only, and
the molecular weights never exceeded 1000 g mol-1
,
even after 24 h (entries 12, 13). 3 gave the polymers
with highest molecular weights, which reached almost
3000 g mol-1 (entry 15). Even though with 1, 2, and 3
the conversion increased linearly with time during the
first 24 h of the reaction, clearly such a linear relation-
ship between molecular weights and reaction time or
percentage of conversion could not be established.
As stated above, the polymerization of cyclic mono-
mers can be thought to occur via different mechanisms.
Whereas most Zn and Mg initiators for the controlled
ROP of epoxides proceed via a coordination mechanism,
where binding to an electrophilic metal center renders
the monomer more susceptible to intra- or intermolecu-
lar attack by a nucleophilic moiety,24 in the case of
cationic metal complexes a simple acidic mechanism had
to be considered. The coordination mechanism is char-
acterized by high regioregularity, where the polymer
chain consists mainly of head-to-tail (H-T) diad se-
quences (Scheme 3). By contrast, acidic polymerization
gives little regioselectivity, and the resulting polymers
usually exhibit high contents of head-to-head (H-H)
and tail-to-tail (T-T) defects.25
This, together with the NMR characterization of the
polymer microstructure, indicated that the epoxide
polymerizations initiated by cationic Zn and Mg alkyls
and amides follow a cationic (ACE) mechanism (Scheme
4). Following the reaction of 1 with 1-10 equiv of PO
1
by H NMR in CD2Cl2 or toluene-d8 indicated very fast
PO/Et2O exchange followed by the formation of oligo-
mers after ca. 15 min.
Complexes 1 and 2 were also tested for the polymer-
ization of ꢀ-caprolactone (CL) and L-lactide (Table 4).
(24) (a) Chisholm, M. H.; Crandall, J . C.; McCollum, D. G.; Pagel,
M. Macromolecules 1999, 32, 5744. (b) Antelmann, B.; Chisholm, M.
H.; Iyer, S. S.; Huffman, J . C.; Navarro-Llobet, D.; Pagel, M.; Simon-
sick, W. J . Macromolecules 2001, 34, 3159. (c) Chisholm, M. H.;
Navarro-Llobet, D. Macromolecules 2001, 34, 8851.
(25) Tsuruta, T.; Kawakami, Y. In Comprehensive Polymer Science,
1st ed.; Allen and Bevington, Eds.; Pergamon Press: New York, 1989;
Vol. 3, Chapter 31, pp 457-487. (b) Le Borgne, A.; Spassky, N.; Lin
J un, C.; Momtaz, A. Makromol. Chem. 1988, 189, 637.

Zn and Mg Cations for Ring-Opening Polymerization
Organometallics, Vol. 23, No. 13, 2004 3301
Ta ble 4. P olym er iza tion of E-Ca p r ola cton e w ith Com p lexes 1 a n d 2
entry
init (µmol)
feed ratioa
toluene [mL]
50
temp [°C]
time [h]
yield [g]
convb
prodc
Mw
Mw/Mn
17
18
1 (15)
2 (30)
1000
6000
22
50
120
20
1.6
3.0
93.5
14.5
53.3
300
55 000
2.3
a
b
Initial monomer/initiator ratio. Percentage conversion of the monomer (weight monomer/weight of polymer recovered × 100). c kg
polymer (mol metal)-1‚h-1
.
[bis(trimethylsilyl)amide]28 were prepared according to litera-
ture methods. Propene oxide, cyclohexene oxide, and ꢀ-capro-
lactone were dried for a minimum of 24 h over fresh calcium
hydride, then distilled under vacuum and stored over activated
4 Å molecular sieves. NMR spectra were recorded using a
Bruker Avance DPX-300 spectrometer. 1H NMR spectra (300.1
MHz) were referenced to the residual solvent proton of the
deuterated solvent used. 13C NMR spectra (75.5 MHz) were
referenced internally to the D-coupled 13C resonances of the
NMR solvent. Gel permeation chromatography (GPC) mea-
surements were performed on a Polymer Laboratories PL-
GPC-220 instrument equipped with a PLgel 5 Å Mixed-C
column, a refractive index detector, and a PD2040 light-
scattering detector. The GPC column was eluted with THF at
40 °C at 1 mL/min and was calibrated using eight monodis-
perse polystyrene standards in the range 580-483 000 Da.
[(Et2O)3Zn N(SiMe3)2][B(C6F5)4] (2). A solution of [H(OEt2)2]-
[B(C6F5)4] (0.97 g, 1.17 mmol) in diethyl ether (150 mL) was
added dropwise over 45 min at room temperature to a solution
of Zn[N(SiMe3)2]2 (0.710 mL, 1.75 mmol) in diethyl ether (20
mL). The resulting colorless solution was stirred at room
temperature for 1 h and was then concentrated under vacuum
to 50 mL. Light petroleum (10 mL) was added. Recrystalliza-
tion at -26 °C afforded 2 as colorless crystals, yield 0.95 g
(72.0%). Anal. Calcd for C42H48BF20NO3Si2Zn: C 44.75; H 4.29;
Whereas there was no reaction between L-lactide and
2, complex 1 initiates very rapid polymerization of CL
at room temperature (entry 17) and gives almost
complete conversion of 1000 equiv of CL in 2 h, with a
productivity of more than 5.3 × 104 g PCL (mol
Zn)-1‚h-1. 1 is therefore considerably more active at
room temperature than the diazadiene complex [ZnMe-
(DAD)][B(C6F5)4]- (ca. 1.5 × 104 g PCL (mol Zn)-1‚h-1
)
was at 60 °C.12 The bulk polymerization of CL performed
at 50 °C with complex 2 was extremely efficient (entry
18). High molecular weight polymer (Mw ) 55 000, Mw/
Mn ) 2.3) was obtained with an excellent productivity
of 3 × 105 g PCL (mol Zn)-1‚h-1. Due to the high
molecular weight, end-group analysis was not possible.
Con clu sion
Four-coordinate cationic zinc and magnesium amides
and alkyls stabilized by diethyl ether are readily acces-
sible from magnesium or zinc alkyls and amides and
[H(OEt2)2][B(C6F5)4] in almost quantitative yields, to our
knowledge the first structurally characterized examples
of such simple group 2 and 12 metal amido cations. In
the solid state the zinc and magnesium -N(SiMe3)2
compounds are isostructural. In contrast to neutral zinc
aryl oxides, these cationic species are highly active in
the polymerization of epoxides. Cyclohexene oxide is
rapidly polymerized to high molecular weight polymers,
whereas the polymerization of propene oxide proceeds
more slowly to give low molecular weight materials.
There is strong evidence that in these cases the Lewis
acidic metal centers act as initiators for an acidic
polymerization mechanism. Nevertheless, there was a
marked dependence of the polymerization activity on
the nature of the catalyst, with zinc compounds being
significantly more active than magnesium analogues.
ꢀ-Caprolactone is also polymerized very efficiently by
the zinc cations, and high molecular weight polyesters
were obtained.
1
N 1.24. Found: C 44.08; H 3.98; N 1.30. H NMR (300 MHz,
CD2Cl2, 25 °C): δ 3.95 (q, 12 H, J HH ) 7.1 Hz, CH3CH2O), 1.43
(t, 18 H, J HH ) 7.1 Hz, CH3CH2O), 0.16 (s, 18 H, SiMe3). 13C
{1H} NMR (75.5 MHz, CD2Cl2, 25 °C): δ 69.5 (CH3CH2O), 15.2
(CH3CH2O), 5.2 (SiMe3). 19F NMR (CD2Cl2, 282.4 MHz, 25
°C): δ -133.6 (br, 8 F, o-F), -164.1 (t, 4 F, J FF ) 19.8 Hz,
p-F), -167.9 (t, 8 F, J FF ) 19.8 Hz, m-F). 11B NMR (96.3 MHz,
CD2Cl2, 25 °C): δ -13.6.
[(Et2O)3MgBu n ][B(C6F 5)4] (3). [H(OEt2)2][B(C6F5)4] (0.65
g, 0.78 mmol) and MgBusBun (1.0 mL, 1.2 M, 1.2 mmol) were
reacted as described for 2. Volatiles were removed under
vacuum to leave a white sticky solid, which was washed with
3 × 20 mL of light petroleum to afford a white powder. Attempt
to recrystallize from a diethyl ether/light petroleum mixture
at -26 °C led to a white crystalline powder, yield 0.76 g
(99.1%). Anal. Calcd for C40H39BF20MgO3: C 48.88; H 4.00.
1
Found: C 48.13; H 3.35. H NMR (300 MHz, CD2Cl2, 25 °C):
δ 3.96 (q, 12 H, J HH ) 7.1 Hz, CH3CH2O), 1.40 (t, 18 H, J HH
)
7.1 Hz, CH3CH2O), 1.28 (m, 2 H, MgCH2CH2CH2CH3), 1.20
(m, 2 H, MgCH2CH2CH2CH3), 0.89 (t, 3 H, J HH ) 7.1 Hz,
Exp er im en ta l Section
MgCH2CH2CH2CH3), -0.44 (m, 2 H, MgCH2CH2CH2CH3). 13
C
Gen er a l P r oced u r es. All manipulations were performed
under nitrogen, using standard Schlenk techniques. Solvents
were predried over sodium wire (toluene, light petroleum,
THF, diethyl ether) or calcium hydride (dichloromethane) and
distilled under nitrogen from sodium (toluene), sodium-
potassium alloy (light petroleum, bp 40-60 °C), sodium-
benzophenone (THF, diethyl ether), or calcium hydride (dichlo-
romethane). Deuterated solvents were stored over activated
4 Å molecular sieves and degassed by several freeze-thaw
cycles. ZnEt2 (1.1 M solution in toluene) and MgBu2 (1.0
solution in heptane) were used as purchased (Aldrich). Com-
pounds [H(OEt2)2][B(C6F5)4],25 [(Et2O)3ZnCH2CH3] [B(C6F5)4]
(1),11 zinc bis[bis(trimethylsilyl)amide],27 and magnesium bis-
{1H} NMR (75.5 MHz, CD2Cl2, 25 °C): δ 67.1 (CH3CH2O), 31.8
(MgCH2CH2CH2CH3), 25.2 (MgCH2CH2CH2CH3), 14.1 (CH3-
CH2O), 13.9 (MgCH2CH2CH2CH3), 6.1 (MgCH2CH2CH2CH3).
19F NMR (CD2Cl2, 282.4 MHz, 25 °C): δ -133.5 (br, 8 F, o-F),
-164.2 (t, 4 F, J FF ) 19.8 Hz, p-F), -168.0 (t, 8 F, J FF ) 19.8
Hz, m-F). 11B NMR (96.3 MHz, CD2Cl2, 25 °C): δ -13.6.
[(Et2O)3MgN(SiMe3)2][B(C6F 5)4] (4). A procedure identical
to that of 2 was followed to react [H(OEt2)2][B(C6F5)4] (1.56 g,
1.88 mmol) with {Mg[N(SiMe3)2]2}2 (0.708 g, 1.02 mmol).
Recrystallization from diethyl ether/light petroleum afforded
colorless crystals, yield 1.90 g (93.0%). Single crystals suitable
for X-ray diffraction crystallography were obtained from
diethyl ether/light petroleum (8:1) after several weeks at -26
°C. Anal. Calcd for C42H48BF20NO3Si2Mg: C 46.45; H 4.45; N
1.29. Found: C 45.94; H 4.09; N 1.28. 1H NMR (300 MHz, CD2-
(26) J utzi, P.; Mu¨ller, C.; Stammler, A.; Stammler, H. G. Organo-
metallics 2000, 19, 1442.
(27) Bochmann, M.; Bwembya, G. Webb, K. J . Inorg. Synth. 1997,
31, 19.
(28) Westerhausen, M. Inorg. Chem. 1991, 30, 96.

3302 Organometallics, Vol. 23, No. 13, 2004
Sarazin et al.
Cl2, 25 °C): δ 4.03 (q, 12 H, J HH ) 7.0 Hz, CH3CH2O), 1.42 (t,
18 H, J HH ) 7.0 Hz, CH3CH2O), 0.08 (s, 18 H, SiMe3). 13C {1H}
NMR (75.5 MHz, CD2Cl2, 25 °C): δ 66.7 (CH3CH2O), 13.8 (CH3-
CH2O), 5.6 (SiMe3). 19F NMR (CD2Cl2, 282.4 MHz, 25 °C): δ
-133.5 (br, 8 F, o-F), -164.1 (t, 4 F, J FF ) 19.8 Hz, p-F), -168.0
(t, 8 F, J FF ) 19.8 Hz, m-F). 11B NMR (96.3 MHz, CD2Cl2, 25
°C): δ -13.6.
X-r a y Cr ysta llogr a p h y. Crystals coated with dry Nujol
were mounted on a glass fiber under a cold nitrogen steam.
Data were collected on a Rigaku R-Axis IIc image plate
diffractometer equipped with a rotating-anode X-ray source
(Mo KR radiation, λ ) 0.71073 Å) and graphite monochroma-
tor. Data were processed using the DENZO/SCALE-PACK
programs.29 The structures were determined by direct methods
in the SHELXS program30 and refined by full-matrix least-
squares methods, on F2 values, in SHELXL.31 The non-
hydrogen atoms were refined with anisotropic thermal pa-
rameters. Hydrogen atoms were included in idealized positions,
and their Uiso values were set to ride on the Ueq values of the
parent carbon atoms. Scattering factors for neutral atoms were
taken from ref 32.
final difference map, the highest peaks (to ca. 0.34 e A-3) were
close to C(23) and C(24).
P olym er iza tion P r oced u r es. Polymerizations of cyclo-
hexene oxide (CHO) were carried out on a Schlenk line in a
flame-dried round-bottom flask equipped with a magnetic
stirrer. In a typical procedure, the initiator was dissolved in
the appropriate amount of toluene, and temperature equilibra-
tion was ensured by stirring the solution for 15 min on an ice-
bath. CHO, kept at 0 °C, was then injected, and polymerization
times were measured from that point. Polymerizations were
terminated by addition of a solution of hydrochloric acid (10%
vol.) in methanol, and the polymers were precipitated in
methanol. After filtration, the polymers were washed with
methanol and then dried in vacuo to constant weight. When
it was necessary to take samples from the reaction, the
reaction mixture was stirred for the measured amount of time,
and an aliquot was removed with a gastight syringe, poured
in acidified methanol, and then treated as described above.
Polymerizations of neat propene oxide (PO) at room tem-
perature were performed in a flame-dried Schlenk tube on a
Schlenk line. The flask was charged in the glovebox with the
required amount of initiator and attached to the vacuum line.
The appropriate amount of PO was added via syringe. Vigorous
stirring ensured the initiator was immediately dissolved.
Polymerizations were stopped by injecting a solution of
hydrochloric acid (10 vol. %) in methanol. Volatiles were
removed in vacuo, and the polymers were dried in an oven at
60 °C to constant weight. When necessary, aliquots were taken
from the reaction mixture and injected in a vial containing
acidified methanol. The vial was evacuated, and the remaining
polymer was dried in the oven to determine the conversion.
Polymerizations of ꢀ-caprolactone (CL) were carried out
using the same procedure. The initiator was dissolved in the
desired amount of toluene, and after temperature equilibration
CL was added via syringe and the polymerization time
measured from that point. Polymerizations were stopped by
addition of a solution of acetic acid in methanol. The polymers
were precipitated in methanol, filtered, dissolved in THF,
reprecipitated in methanol, and dried in vacuo to constant
weight.
Cr ysta l Da ta for 2: C18H48ZnNO3Si2‚BC24F20, fw 1127.17;
crystal size 0.4 × 0.4 × 0.3 mm; triclinic, space group P1h; a )
16.039(3) Å, b ) 17.607(4) Å, c ) 18.853(4) Å; R ) 105.45(3)°,
â ) 104.25(3)°, γ ) 92.57(3)°; V ) 4939 (2) Å3; Z ) 4; Dcalc
)
1.516 g/cm3; µ ) 0.660 mm-1; F(000) ) 2296; -1.82° e θ e
25.44°; -19 e h e 17, -21 e k e 21, -22 e l e 22; 28 338
reflections collected, of which 16 976 were independent (Rint
) 0.0661) and 11 331 with I > 2σ(I) were observed; final R1 [I
> 2σ(I)] ) 0.0497, wR2 (all data) ) 0.1336, w ) [σ2(Fo2) +
(0.0614P)2]-1 with P ) (Fo + 2Fc2)/3; no. of data/restraints/
2
parameters 16 976/0/1261; goodness of fit, F2 ) 1.018. In the
final difference map, the highest peaks (to ca. 0.84 e A-3) were
close to Zn(2).
Cr ysta l Da ta for 4: C18H48MgNO3Si2‚BC24F20, fw 1086.11;
crystal size 0.45 × 0.45 × 0.10 mm; triclinic, space group P1h;
a ) 15.985(3) Å, b ) 17.603(4) Å, c ) 18.918(4) Å; R ) 105.32-
(3)°, â ) 104.00(3)°, γ ) 92.66(3)°; V ) 4947(2) Å3; Z ) 4; Dcalc
) 1.458 g/cm3; µ ) 0.198 mm-1; F(000) ) 2224; 1.16° e θ e
23.50°; -19 e h e 19, -21 e k e 21, -21 e l e 21; 22 489
reflections collected, of which 13 476 were independent (Rint
) 0.0922) and 8612 with I > 2σ(I) were observed; final R1 [I >
2σ(I)] ) 0.082, wR2 (all data) ) 0.1336, w ) [σ2(Fo2) +
Ack n ow led gm en t. This work was supported by the
European Commission, grant HPRN-CT2000-00004. We
thank Dr. J . Hoffmann (Bayer AG) for helpful discus-
sions and the analysis of PPO samples.
(0.0533P)2]-1 with P ) (Fo + 2Fc2)/3; no. of data/restraints/
2
parameters 13 476/0/1261; goodness of fit, F2 ) 0.941. In the
(29) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(30) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(31) Sheldrick, G. M. SHELXL, Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
(32) International Tables for X-ray Crystallography; Kluwer Aca-
demic Publishers: Dordrecht, 1992; Vol. C, pp 500, 219, and 193.
Su p p or tin g In for m a tion Ava ila ble: Full listings of
crystallographic details for compounds 2 and 4. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM0497691