Hybrid scorpionate/cyclopentadienyl magnesium and zinc complexes: Synthesis, coordination chemistry, and ring-opening polymerization studies on cyclic esters

Article abstract of DOI:10.1021/ic902399r

The reaction of the hybrid scorplonate/cyclopentadlenyl lithium salt [Li(bpzcp)(THF)] [bpzcp = 2,2-bis(3,5-dimethylpyrazol-1 -yl)-1,1- dlphenylethylcyclopentadlenyl] with 1 equiv of RMgCI proceeds cleanly to give very high yields of the corresponding monoalkyl k²-NNn ⁵-C₅H₄ magnesium complexes [Mg(R)(k ²n⁵-bpzcp)] (R = Me 1, Et 2, ⁿBu 3, ^tBu 4, CH²SiMe₃ 5, CH₂Ph 6). Hydrolysis of the hybrid lithium salt [Li(bpzcp)(THF)] with NH ₄CI/H₂O in ether cleanly affords the two previously described regiolsomers: (bpzcpH) 1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1- diphenylethyl]-1,3-cyclopentadlene (a) and 2-[2,2-bis(3,5-dlmethylpyrazol-1-yl)- 1,1-dlphenylethyl]-1,3-cyclopentadlene (b). Subsequent reaction of the bpzcpH hybrid ligand with ZnR₂ quantitatively yields the monoalkyl k ²-NN-n¹(π)-C₅H₄ zinc complexes [Zn(R){k²n¹(π)-bpzcp}] (R = Me 7, Et 8, 'Bu 9, CH ₂SiMe₃10). Additionally, magnesium alkyls 1, 2, 4, and 5 can act as excellent cyclopentadlenyl and alkyl transfers to the zinc metal center and yield zinc alkyls 7-10 in good yields. The single-crystal X-ray structures of the derivatives 4,5,7, and 10 confirm a 4-coordlnative structure with the metal center in a distorted tetrahedral geometry. Interestingly, whereas alkyl magnesium derivatives 4 and 5 present a n⁵ coordination mode for the cyclopentadlenyl fragment, zinc derivatives 7 and 10 feature a peripheral n¹(π) arrangement in the solid state. Furthermore, the reaction of the hybrid lithium salt [Li(bpzcp)(THF)] with 1 equiv of ZnCI ₂ in tetrahydroturan (THF) affords very high yields of the chloride complex [ZnCI{k² -n¹ (π)-bpzcp}] (11). Compound 11 was used as a convenient starting material for the synthesis of the aromatic amide zinc compound [Zn(NH-4-MeC₆H₄){k²-n ¹(π)-bpzcp}] (12), by reaction with the corresponding aromatic primary amide lithium salt. Alternatively, aliphatic amide and alkoxide derivatives were only accessible by protonolysls of the bis(amide) complexes [M{N(SiMe₃)₂}₂] (M = Mg, Zn) and the mixed ligand complex [EtZnOAr)] with the hybrid ligand bpzcpH to afford [Zn(R){k ²-n¹)-bpzcp}] (R = N(SiMe₃)₂13, R = 2,4,6Me₃C₆H₂O14) and [Mg{N(SiMe ₃)₂}(k²-n⁵bpzcp)] (15). Finally, alkyl and alkoxide-containlng complexes 1 -10 and 14 can act as highly effective single-component living initiators for the ring-opening polymerization of ε-caprolactone and lactldes over a wide range of temperatures. ε-Caprolactone is polymerized within minutes to give high molecular weight polymers with medium-broad polydlspersitles (M_n > 10⁵, M_w/M_n = 1.45). Lactide afforded poly(lactide) materials with medium molecular weights and polydilspersitles as narrow as M _w/M_n = 1.02. Additionally, polymerization of L-lactide occurred without racemlzation in the propagation process and offered highly crystalline, isotactlc poly(L-lactldes) with very high melting temperatures (T_m = 165 °C). Microstructural analysis of poly(rac-lactide) by¹H NMR spectroscopy revealed that propagations occur without appreciable levels of stereoselectivity. Polymer end group analysis showed that the polymerization process is initiated by alkyl transfer to the monomer.

Full text of DOI:10.1021/ic902399r

Inorg. Chem. 2010, 49, 2859–2871 2859
DOI: 10.1021/ic902399r
Hybrid Scorpionate/Cyclopentadienyl Magnesium and Zinc Complexes:
Synthesis, Coordination Chemistry, and Ring-Opening Polymerization
Studies on Cyclic Esters
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Andres Garces, Luis F. Sanchez-Barba,* Carlos Alonso-Moreno, Mariano Fajardo, Juan Fernandez-Baeza,
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Antonio Otero,* Agustın Lara-Sanchez, Isabel Lopez-Solera, and Ana Marıa Rodrıguez
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Departamento de Quımica Inorganica y Analıtica, Universidad Rey Juan Carlos, Mostoles-28933-Madrid,
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Spain, and Departamento de Quımica Inorganica, Organica y Bioquımica, Universidad de Castilla-La Mancha,
Campus Universitario, 13071-Ciudad Real, Spain
Received December 4, 2009
The reaction of the hybrid scorpionate/cyclopentadienyl lithium salt [Li(bpzcp)(THF)] [bpzcp = 2,2-bis(3,5-dimethyl-
pyrazol-1-yl)-1,1-diphenylethylcyclopentadienyl] with 1 equiv of RMgCl proceeds cleanly to give very high yields of the
corresponding monoalkyl κ2-NN-η5-C5H4 magnesium complexes [Mg(R)(κ2-η5-bpzcp)] (R = Me 1, Et 2, nBu 3, tBu 4,
CH2SiMe3 5, CH2Ph 6). Hydrolysis of the hybrid lithium salt [Li(bpzcp)(THF)] with NH4Cl/H2O in ether cleanly affords the
two previously described regioisomers: (bpzcpH) 1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopen-
tadiene (a) and 2-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene (b). Subsequent reaction
of the bpzcpH hybrid ligand with ZnR2 quantitatively yields the monoalkyl κ2-NN-η1(π)-C5H4 zinc complexes [Zn(R)-
{κ2-η1(π)-bpzcp}] (R = Me 7, Et 8, tBu 9, CH2SiMe3 10). Additionally, magnesium alkyls 1, 2, 4, and 5 can act as
excellent cyclopentadienyl and alkyl transfers to the zinc metal center and yield zinc alkyls 7-10 in good yields. The
single-crystal X-raystructuresofthe derivatives4, 5, 7, and 10confirma 4-coordinativestructure with the metal centerin a
distorted tetrahedral geometry. Interestingly, whereas alkyl magnesium derivatives 4 and 5 present a η5 coordination
mode for the cyclopentadienyl fragment, zinc derivatives 7 and 10 feature a peripheral η1(π) arrangement in the solid
state. Furthermore, the reaction of the hybrid lithium salt [Li(bpzcp)(THF)] with 1 equiv of ZnCl2 in tetrahydrofuran (THF)
affords very high yields of the chloride complex [ZnCl{κ2-η1(π)-bpzcp}] (11). Compound 11 was used as a convenient
starting material for the synthesis of the aromatic amide zinc compound [Zn(NH-4-MeC6H4){κ2-η1(π)-bpzcp}] (12), by
reactionwith the correspondingaromatic primary amidelithiumsalt. Alternatively, aliphaticamideand alkoxide derivatives
were only accessible by protonolysis of the bis(amide) complexes [M{N(SiMe3)2}2] (M = Mg, Zn) and the mixed ligand
complex [EtZnOAr)] with the hybrid ligand bpzcpH to afford [Zn(R){κ2-η1(π)-bpzcp}] (R = N(SiMe3)2 13, R = 2,4,6-
Me3C6H2O 14) and [Mg{N(SiMe3)2}(κ2-η5-bpzcp)] (15). Finally, alkyl and alkoxide-containing complexes1-10 and 14
can act as highly effective single-component living initiators for the ring-opening polymerization of ε-caprolactone and
lactides over a wide range of temperatures. ε-Caprolactone is polymerized within minutes to give high molecular weight
polymers with medium-broad polydispersities (Mn > 105, Mw/Mn = 1.45). Lactide afforded poly(lactide) materials with
medium molecular weights and polydispersities as narrow as Mw/Mn = 1.02. Additionally, polymerization of L-lactide
occurred without racemization in the propagation process and offered highly crystalline, isotactic poly(L-lactides) with very
high melting temperatures (Tm = 165 °C). Microstructural analysis of poly(rac-lactide) by 1H NMR spectroscopy revealed
that propagations occur without appreciable levels of stereoselectivity. Polymer end group analysis showed that the
polymerization process is initiated by alkyl transfer to the monomer.
Introduction
and improvement of new catalytic processes is crucial in the
search for attractive alternatives to the bioresistant poly-
olefins.1 Lactide, the cyclic dimer of lactic acid, is an in-
expensive and annually renewable natural feedstock2 that is a
byproduct of biomass fermentation (e.g., beets and corn).
In the present era of depleting fossil-based feedstocks
and increasing environmental awareness, the exploration
*To whom correspondence should be addressed. E-mail: luisfernando.
sanchezbarba@urjc.es (L.F.S.-B.),antonio.otero@uclm.es (A.O.).
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ACS Symposium Series 921; American Chemical Society: Washington, DC,
2006; Vol. 9, pp 116-129. (b) Goodstein, D. Out of Gas: The End of the Age of
Oil; W. W. Norton & Company: New York, 2004.
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7989. (b) Biela, T.; Kowalski, A.; Libiszowski, J.; Duda, A.; Penczek, S.
Macromol. Symp. 2006, 47–55.
r
2010 American Chemical Society
Published on Web 02/10/2010
pubs.acs.org/IC

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Garces et al.
2860 Inorganic Chemistry, Vol. 49, No. 6, 2010
Ring-opening polymerization (ROP) of lactide2a,3,4 yields
poly(lactides) (PLAs), an important emerging bulk com-
modity material5,6 given its utility in environmentally friendly
and recyclable thermoplastics, which have the added bene-
fit of biodegradability.7,8 Additionally, the biocompatible
nature9 of lactide systems with living tissue and the non-
toxicity of the bioassimilable PLAs has attracted significant
attention in biomedical and pharmaceutical applications, for
example, in resorbable surgical sutures,9,10 drug delivery
vehicles,10 and artificial tissue matrixes.11 These medicinal
applications have promoted the use of biocompatible metals
in the design of new catalysts, such as magnesium12 and
zinc13,14 systems. Furthermore, zinc and magnesium-based
catalysts have been extensively employed in ROP and are
among the most efficient initiators used to date for the well-
controlled polymerization of cyclic esters such as ε-capro-
lactone and lactides.3,4,15-17
In this context, our research group has recently reported
amidinate-based heteroscorpionate ligands,18 related to the
bis(pyrazol-1-yl)methane system,19 with different levels of
steric congestion as convenient ancillary ligands for the
synthesis of well-defined alkyl magnesium20 and alkyl21-
amide22 zinc complexes of the type [M(R)(κ3-NNN)] (M =
Mg, Zn; R = alkyl, amide). Moreover, heteroscorpionate
alkyl magnesium complexes proved to be highly effective
single-component living initiators20 for the well-controlled
ROP of ε-caprolactone and lactides over a wide range of
temperatures. Additionally, the more sterically hindered
alkyl zinc initiators21 offered good control of the polymer
microstructures and promoted the formation of a hetero-
tactic bias in the polymerization of rac-lactide. Furthermore,
heteroscorpionate amide zinc22 derivatives also behaved as
efficient single-component initiators for the ROP of ε-capro-
lactone at room temperature.
The possibility of designing alternative magnesium and
zinc initiators for efficient ROP stimulated our interest in a
new scorpionate/cyclopentadienyl hybrid ligand of the type
NNCp, derived from the bis(3,5-dimethylpyrazol-1-yl)-
methane, to compare their synthetic accessibility, structural
arrangements, and catalytic behavior in the ROP of these
polar monomers with the previously reported amidinate-
based magnesium20 and zinc21,22 analogues. This attrac-
tive scaffold, which was previously prepared in our group
in the form of the lithium salt,23,24b has proven to act as an
excellent ancillary ligand for the synthesis of discrete tita-
nium24 and zirconium24 complexes and in efficient scan-
dium and yttrium initiators for the ROP of cyclic esters25a
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Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2861
Chart 1. Possible Coordination Modes of the Cyclopentadienyl Ligand
in Metallocenes
Scheme 1. Synthesis of Scorpionate/Cyclopentadienyl Magnesium
Alkyls [Mg(R)(κ2-η5-bpzcp)] (1-6)
and styrene polymerization.25b Related cyclopentadienyl/
scorpionate hybrid ligands derived from hydrobis(3,5-di-
methylpyrazolyl)borate have been also reported.26
In the field of cyclopentadienyl-containing metal com-
plexes it is usual to find different metal-cyclopentadienyl
coordination modes. Thus, the most representative example,
a η5 coordination mode for the cyclopentadienyl fragment,
appears in metallocenes, which exhibit a ferrocene-type
structure (1), and in bent-metallocenes. Furthermore, cyclo-
pentadienyl derivatives of the main-group elements present a
rich variety of coordination modes, including a η1(π) coordi-
nation one which involves metal coordination through one
sp2-hybridized carbon atom of the ring by means of the
π cloud. This is found in the complexes exhibiting η5/η1(π)
slipped-sandwich geometries (2). Additionally, it is possible to
find a rigid κ1(σ) -Cp coordination mode with two localized
CdC double bonds in the ring, giving rise to η5/κ1(σ)
complexes where the two cyclopentadienyl rings are not
parallel (3) (Chart 1).27 In particular, cyclopentadienyl deri-
vatives of magnesium and zinc may adopt a variety of
structures. For instance, whereas magnesium complexes
predominately present a η5-cyclopentadienyl coordination
mode,28a-c with the exception of very few cases,28d interest-
ingly, zinc derivatives offer a rich structural diversity in the
hapticity count from η1 or κ1 to η5, as recently reported by
Carmona et al.27a in zincocenes with substituted cyclopenta-
dienyl rings.
The study of the different structural arrangements for the two
metals, their reactivity against nucleophilic reagents, and the
use of these materials as single-component living initiators
for the ROP of ε-caprolactone and L-/rac-lactide under well-
controlled conditions are also discussed in detail.
Results and Discussion
Synthesis and Characterization of Scorpionate/Cyclo-
pentadienyl Monoalkyl Magnesium and Zinc Complexes.
A toluene solution of the hybrid scorpionate/cyclo-
pentadienyl lithium salt [Li(bpzcp)(THF)]23,24b [bpzcp =
2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethylcyclo-
pentadienyl] was treated with a series of commercially
available Grignard reagents RMgCl in Et2O or n-hexane
solutions in a 1:1 molar ratio at -70 °C. These reactions
gave the corresponding alkyl magnesium complexes
[Mg(R)(κ2-η5-bpzcp)] (R = Me 1, Et 2, tBu 4, CH2SiMe3
5, CH2Ph 6) as yellow or orange solids in good yields (ca.
80%) after the appropriate workup (see Scheme 1a).
Furthermore, hydrolysis of the aforementioned hybrid
lithium salt [Li(bpzcp)(THF)]23,24b with NH4Cl/H2O in
diethyl ether21,22 cleanly afforded the two previously
described scorpionate/cyclopentadiene regioisomers25a
(bpzcpH) 1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenyl-
ethyl]-1,3-cyclopentadiene (a) and 2-[2,2-bis(3,5-dimethyl-
pyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene (b) in
very good yields (Scheme 1b). Additionally, derivative 3
could be only prepared by reacting bpzcpH with the bisalkyl
Mg(nBu)2 (Scheme 1c).
The work described here concerns a new approach in the
chemistry of these biocompatible metals and, from this view-
point, our initial study was aimed at preparing novel organo-
magnesium and zinc complexes of the type [M(R)(NNCp)].
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(25) (a) Otero, A.; Fernandez-Baeza, J.; Antinolo, A.; Lara-Sanchez, A.;
Alternatively, subsequent reaction of the bpzcpH
ligand with ZnR2 (1 equiv vs Zn) at low temperature
(-70 °C) in toluene cleanly afforded the corresponding
zinc complexes [Zn(R){κ2-η1(π)-bpzcp}] (R = Me 7, Et 8,
tBu 9, CH2SiMe310) in very good yields (>75%) as
brown microcrystalline powders (Scheme 2a). Further-
more, inspired by Carmona’s work,27a we successfully
proved that the use of alkyl magnesium complexes 1, 2, 4,
and 5 enabled excellent cyclopentadienyl and alkyl trans-
fers to the zinc metal center and allowed the formation of
derivatives 7-10 in good isolated yields (Scheme 2b).
Compounds 1-10 are extremely air- and moisture-sensi-
tive, are highly soluble in tetrahydrofuran (THF), to-
luene, and diethyl ether, but sparingly soluble in n-hexane -
apart from derivatives with R = nBu, tBu, and CH2SiMe3.
All compounds decompose in dichloromethane. Qualitative
ꢀ
Martınez-Caballero, E.; Tejeda, J.; Sanchez-Barba, L. F.; Alonso-Moreno,
´
C.; Lopez-Solera, I. Organometallics 2008, 27, 976–983. (b) Otero, A.;
Fernandez-Baeza, J.; Lara-Sanchez, A.; Antinolo, A.; Tejeda, J.; Martínez-
Caballero, E.; Marquez-Segovia, I.; Lopez-Solera, I.; Sanchez-Barba, L. F.;
Alonso Moreno, C. Inorg. Chem. 2008, 47, 4996–5005.
(26) (a) Kunz, K.; Bolte, M.; Lerner, H.-W.; Wagner, M. Organometallics
2009, 28, 3079–3087. (b) Kunz, K.; Vitze, H.; Bolte, M.; Lerner, H. W.; Wagner,
M. Organometallics 2007, 26, 4663–4672. (c) Lopes, I.; Lin, G. Y.; Domingos,
A.; McDonald, R.; Marques, N.; Takats, J. J. Am. Chem. Soc. 1999, 121, 8110–
8111.
´
(27) (a) Fernandez, R.; Grirrane, A.; Resa, I.; Rodrıguez, A.; Carmona,
E.; Alvarez, E.; Gutierrez-Puebla, E.; Monge, A.; Lopez del Amo, J. M.;
Limbach, H.-H.; Lledos, A.; Masera, F.; del Rıo, D. Chem.;Eur. J. 2009,
´
15, 924–935. (b) Walker, D. A.; Woodmann, T. J.; Schormann, M.; Hughes, D. L.;
Bochmann, M. Organometallics 2003, 22, 797–803.
(28) (a) Hanusa, T. P. Organometallics 2002, 21, 2559–2571. (b) Jutzi, P.;
Burford, N. Chem. Rev. 1999, 99, 969–990. (c) El-Kaderi, H. M.; Xia, A.; Heeg,
M. J.; Winter, C. H. Organometallics 2004, 23, 3488–3495. (d) Xia, A.; Knox,
J. E.; Heeg, M. J.; Schlegel, H. B.; Winter, C. H. Organometallics 2003, 22, 4060.
ꢀ
ꢀ
ꢀ
~
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ

ꢀ
Garces et al.
2862 Inorganic Chemistry, Vol. 49, No. 6, 2010
Scheme 2. Synthesis of Scorpionate/Cyclopentadienyl Zinc Alkyls
[Zn(R){κ2-η1(π)-bpzcp}] (7-10)
Table 1. Selected Interatomic Distances (A) and Angles (deg) for 4 and 5
4
5
Bond Distances (A)
Mg(1)-N(1)
Mg(1)-N(3)
Mg(1)-C(13)
Mg(1)-C(21)
Mg(1)-C(22)
Mg(1)-C(23)
Mg(1)-C(24)
Mg(1)-C(25)
C(1)-C(2)
C(1)-C(21)
C(21)-C(22)
C(21)-C(25)
C(22)-C(23)
C(23)-C(24)
C(24)-C(25)
2.208(4)
2.223(4)
2.179(4)
2.442(4)
2.507(4)
2.543(4)
2.489(4)
2.423(4)
1.587(6)
1.536(5)
1.404(6)
1.421(5)
1.396(5)
1.406(6)
1.408(6)
2.20(1)
2.22(1)
2.15(1)
2.44(1)
2.48(1)
2.54(1)
2.53(1)
2.46(1)
1.61(1)
1.54(1)
1.44(2)
1.42(1)
1.40(1)
1.40(1)
1.36(1)
Bond Angles (deg)
tests on complexes 1-6 showed the absence of halide
through the formation of a [MgCl(κ2-η5-bpzcp)] derivative.
N(1)-Mg(1)-N(3)
N(1)-Mg(1)-C(13)
N(3)-Mg(1)-C(13)
Cent(1)-Mg(1)-N(1)
Cent(1)-Mg(1)-N(3)
Cent(1)-Mg(1)-C(13)
C(2)-C(1)-C(21)
83.6(1)
113.5(1)
115.5(2)
106.4
102.9
126.4
111.5(3)
82.6(4)
110.5(4)
107.2(4)
105.5
106.0
133.3
115.3(8)
1
The H and 13C-{1H} NMR spectra of 1-10 in ben-
zene-d6 at room temperature display a singlet for the H4,
Me3, and Me5 pyrazole protons, indicating that both
pyrazoles are equivalent, along with two multiplets for
the cyclopentadienyl protons (Hc and Hd) and one reso-
nance for the alkyl ligands. The pattern corresponding to
the cyclopentadienyl fragment remains unchanged down
to -80 °C, and this clearly suggests that a very fast hapto-
tropic exchange27b takes place in solution for the alkyl zinc
derivatives (7-10), even at the lowest temperature investi-
gated, since it is well established that these systems present a
η1(π)-C5H4-coordination mode to the zinc metal center in the
solid state. This situation has been observed previously by
Carmona et al. in beryllocenes and zincocenes with sub-
stituted and unsubstituted cyclopentadienyl rings.27a Such
systems are well-known to exhibit dynamic behavior both in
solution29 and in the solid state.30 All of these data indi-
cate that “in solution” in the zinc complexes the metal atom
adopts a tetrahedral disposition, as a result of this hapto-
tropic exchange,27b a symmetry plane exists and contains the
metal center, the bridging carbon of the pyrazole rings, and
the Ca and Cb atoms from the cyclopentadienyl fragment
Table 2. Selected Interatomic Distances (A) and Angles (deg) for 7 and 10
10
7
Bond Distances [A]
Zn(1)-N(1)
Zn(1)-N(3)
Zn(1)-C(14)
Zn(1)-C(13)
Zn(1)-C(15)
Zn(1)-C(16)
Zn(1)-C(17)
Zn(1)-C(30)
C(11)-C(12)
C(13)-C(14)
C(13)-C(17)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
2.099(7)
2.094(6)
2.137(8)
2.74(1)
2.81(1)
3.51(1)
3.48(1)
1.966(8)
1.60(1)
1.47(1)
1.36(1)
1.44(1)
1.36(1)
1.42(1)
2.104(6)
2.171(6)
2.166(7)
2.787(2)
2.659(1)
3.015(1)
3.414(2)
1.985(7)
1.61(1)
1.43(1)
1.38(1)
1.42(1)
1.38(1)
1.41(1)
1
Bond Angles (deg)
(Schemes 1 and 2). The phase-sensitive H NOESY-1D
spectra were also obtained to confirm the assignments of
the signals for Hc and Hd from the cyclopentadienyl group
and Me3 and Me5 from the pyrazole rings.
Complexes 4, 5, 7, and 10 ꢀ 0.5C7H8 were characteri-
zed by single-crystal X-ray diffraction. Selected bond
lengths and angles are collected in Tables 1 and 2. Crystal-
lographic details are reported in Table 5, and the mole-
cular structures are illustrated in Figures 1-4, respectively.
All complexes present a monomeric structure in the solid
N(1)-Zn(1)-N(3)
83.9(2)
118.6(3)
118.7(3)
88.4(3)
108.4(3)
126.8(4)
115.4(6)
97.3(5)
101.8(6)
105.2
88.6(2)
114.6(3)
112.7(3)
100.1(2)
87.7(3)
139.3(3)
113.4(5)
99.6(4)
93.5(5)
96.46
N(1)-Zn(1)-C(30)
N(3)-Zn(1)-C(30)
N(1)-Zn(1)-C(14)
N(3)-Zn(1)-C(14)
C(14)-Zn(1)-C(30)
C(11)-C(12)-C(13)
Zn(1)-C(14)-C(13)
Zn(1)-C(14)-C(15)
Zn(1)-C(14)-plane of ring
Zn(1)-C(30)-Si(1)
118.2(4)
€
(29) (a) Wang, H.; Kehr, G.; Frohlich, R.; Erker, G. Angew. Chem. 2007,
state, and the metal centers exhibit a distorted tetrahedral
geometry, in which the pyrazolic nitrogens N(1) and N(3)
occupy two positions and the alkyl group and the cyclo-
pentadienyl fragment the other two positions.
The molecular structures of 4 and 5 are presented in
Figures 1 and 2, respectively, and show a distortion of
the tetrahedral geometry of the metal center because of
the scorpionate ligand, which acts in a κ2-NN-η5-C5H4
coordination mode with the Mg(1) atom 0.883(2) A and
119, 4992–4995. Angew. Chem., Int. Ed. 2007, 46, 4905-4908.(b) Carmona, E.;
Fernandez, R. Eur. J. Inorg. Chem. 2005, 3197–3206. (c) Conejo, M. M.;
ꢀ
ꢀ
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Fernandez, R.; del Río, D.; Carmona, E.; Monge, A.; Ruiz, C.; Marquez, A. M.;
Sanz, J. F. Chem.;Eur. J. 2003, 9, 4452–4461. (d) Fischer, B.; Wijkens, P.;
Boersma, J.; van Koten, G.; Smeets, W. J. J.; Spek, A. L.; Budzelaar, P. H. M.
J. Organomet. Chem. 1989, 376, 223–233.
(30) (a) Lopez del Amo, J. M.; Buntkowsky, G.; Limbach, H. H.; Resa, I.;
Fernandez, R.; Carmona, E. J. Phys. Chem. A 2008, 112, 3557–3565.
(b) Hung, I.; Macdonald, C. L. B.; Schurko, R. W. Chem.;Eur. J. 2004, 10,
5923–5935.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2863
Figure 1. ORTEP view of [Mg(tBu)(κ2-η5-bpzcp)] (4). Hydrogen atoms
have been omitted for clarity. Thermal ellipsoids are drawn at the 30%
probability level.
Figure 3. ORTEP view of [Zn(CH3){κ2-η1(π)-bpzcp}] (7). Hydrogen
atoms have been omitted for clarity. Thermal ellipsoids are drawn at the
30% probability level.
Figure 2. ORTEP viewof[Mg(CH2SiMe3)(κ2-η5-bpzcp)] (5). Hydrogen
atoms have been omitted for clarity. Thermal ellipsoids are drawn at the
30% probability level.
Figure 4. ORTEP view of [Zn(CH2SiMe3){κ2-η1(π)-bpzcp}] (10 ꢀ
0.5C7H8). Hydrogen atoms have been omitted for clarity. Thermal
ellipsoids are drawn at the 30% probability level.
The most notable feature in the X-ray molecular struc-
tures of complexes 7 and 10 (Figures 3 and 4, respectively)
is undoubtedly the peripheral η1(π) coordination mode of
the cyclopentadienyl ring. In this situation, the cyclopen-
tadienyl fragment maintains partial aromatic character,
with C-C bond lengths ranging from 1.36(1) A to 1.47(1)
A for complex 7, and from 1.38(1) A to 1.43(1) A for
complex 10, clearly in the range between simple and
double bonds. For instance, the difference between the
CβCβ and CRCβ bonds [R and β refer to the positions with
respect to C(14), that is, the four coordinated carbon of
the η1(π) ring] is 0.06 A in 7 and 0.03 A in 10. The
calculated difference in similar zincocenes with one of
the two C5Me4(SiMe3) moieties in a η1(π)-coordination
mode is also 0.07 A,31a while in [B(C5Me5)2]þ it has
double that value, that is, 0.14 A, in agreement with a
localized structure.31b
-0.996(6) A out of the plane defined by N(1)-N(3)-
C(13) for 4 and 5, respectively. The N(1)-Mg and N(3)-
Mg bond lengths are well-balanced at 2.208(4) A and
2.223(4) A for complex 4, and 2.20(1) A and 2.22(1) A for
complex 5 (Table 1). The four-membered Mg-C(1)-
C(2)-C(21) fragment and the alkyl group lie around a
pseudo-mirror plane that shows the symmetry in the
molecule.
The solid-state structures also reveal that the C5H4
fragment is coordinated in a pentahapto fashion to the
magnesium atom, a situation that has been previously
observed in several complexes23-25 synthesized in our
group and supported with this hybrid ligand. Delocaliza-
tion is also evidenced in the C5H4 ring, with the C-C
bond lengths ranging on average from 1.396(5) A to
1.421(5) A for derivative 4, and from 1.36(1) A to
1.44(2) A for derivative 5 (Table 1). The C5H4 ring is
unsymmetrically bonded to the Mg atom, with Mg-C
bond lengths ranging from 2.423(4) to 2.543(4) A for 4,
and from 2.44(1) to 2.54(1) A for 5 (Table 1). Finally, the
alkyl moiety is directly bonded to the Mg center with a
Mg-C(13) bond length of 2.179(4) A for 4 and 2.15(1) A
for 5 (Table 1); the bond in 5 is shorter than that in 4 as a
result of steric hindrance because of the tert-butyl group.
ꢀ
(31) (a) Fernandez, R.; Resa, I.; del Rıo, D.; Carmona, E. Organome-
´
tallics 2003, 22, 381–383. (b) Voigt, A.; Filipponi, S.; Macdonald, C. L. B.;
Gorden, J. D.; Cowley, A. H. Chem. Commun. 2000, 911–912. (c) Burkey, D. J.;
Hanusa, T. P. J. Organomet. Chem. 1996, 512, 165–173. (d) Data taken from a
ꢀ
computer search at the CCDC.(e) Cordero, B.; Gomez, V.; Platero-Prats, A. E.;
ꢀ
ꢀ
ꢀ
Reves, M.; Echeverría, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans.
2008, 2832–2838.

ꢀ
Garces et al.
2864 Inorganic Chemistry, Vol. 49, No. 6, 2010
Additionally, the Zn(1)-C(14) bond length of 2.137(8)
A for 7 and 2.166(7) A for 10, as well as the angle of about
105.2° for 7 and 96.46° for 10 formed by the Zn(1)-C(14)
bond with the plane of this ring, are typical for this type of
η1(π)-coordination mode.27,31c,32 As a comparison, the
C(14)-Zn distance of 2.166(7) A in 10, although shorter
than the corresponding average distance in C(21-25)-
Mg [2.490(3) A] in 5, evidence the higher C-M bond
strength in the former and is well above the average for
Zn-C covalent bonds31d [ca. 2.005(3) A] and 0.23 A
longer than the 1.930(2) A distance found for the Zn-C
σ-bond in dimethylzinc.33
Scheme 3. Synthesis of the Scorpionate/Cyclopentadienyl Zinc
Chloride [ZnCl{κ2-η1(π)-bpzcp}] (11) and Amide Complex [Zn(NH-
4-MeC6H4){κ2-η1(π)-bpzcp}] (12)
In a similar way to the magnesium crystal structures,
the alkyl moiety is directly bonded to the Zn center with a
Zn-C(30) bond length of 1.966(8) A in complex 7 and
1.985(7) A in complex 10 (Table 2).
Finally, comparison between the trimethylsilylmethyl
magnesium and zinc derivatives 5 and 10 reveals that the
M-C alkyl bond length in complex 10 [1.985(7) A] is
shorter than that in complex 5 [2.15(1) A]. This situation
is probably the result of the higher magnesium covalent
radius (1.41 A) in comparison with the zinc one (1.22 A);31e
however, electronic effects derived from the η1(π)-coordina-
tion mode of the C5H4 ring in the zinc compound 10 can
neither be discarded.
Synthesis and Characterization of Scorpionate/Cyclo-
pentadienyl Zinc and Magnesium Chloride, Amide, and
Alkoxide Complexes. Given the interest in amide34 and
alkoxide34b,35 complexes for the ROP of cyclic esters, and
considering our previous experience in the preparation of
analogous heteroscorpionate zinc chloride22 derivatives
as convenient starting materials for the preparation of
alkoxide and amide complexes, we decided to react the
scorpionate/cyclopentadienyl hybrid ligand in the form
of the lithium salt23,24b with ZnCl2 (1 equiv vs Zn) at low
temperature to afford cleanly the corresponding chloride
complex [ZnCl{κ2-η1(π)-bpzcp}] (11) in very good yield
(>75%) as a white microcrystalline powder (Scheme 3a).
The heteroscorpinate chloride 11 is extremely air- and
moisture-sensitive, is soluble in THF but sparingly solu-
ble in toluene and diethyl ether, and insoluble in n-hexane.
Subsequent reaction of the monochloride zinc species
11 with an aromatic (primary)amide lithium salt in to-
luene at room temperature afforded the corresponding
scorpionate/cyclopentadienyl amide complex of the type
[Zn(NH-4-MeC6H4){κ2-η1(π)-bpzcp}] (12) (NH-4-MeC6H4=
4-methylphenylamide) (Scheme 3b). Attempts to synthesize
aliphatic amide derivatives employing the zinc chloride
Scheme 4. Synthesis of the Scorpionate/Cyclopentadienyl Zinc
Amide and Alkoxide Complexes [Zn(R){κ2-η1(π)-bpzcp}] (13-14) and
the Magnesium Amide [Mg{N(SiMe3)2}(κ2-η5-bpzcp)] (15)
(32) (a) Darensbourg, D. J.; Wildeson, J. R.; Yarbrough, J. C. Organo-
metallics 2001, 20, 4413–4417. (b) Blom, R.; Boersma, J.; Budzelaar, P. H. M.;
Fischer, B.; Haalan, A.; Volden, H. V.; Weidlein, J. Acta Chem. Scand. Ser. A
1986, 40, 113–120. (c) Fischer, B.; Wijkens, P.; Boersma, J.; van Koten, G.;
Smeets, W. J. J.; Spek, A. L.; Budzelaar, P. H. M. J. Organomet. Chem. 1989,
376, 223–233.
precursor 11 failed and produced intractable mixtures,
as previously observed for the preparation of amidinate-
heteroscorpionate amide zinc complexes.22
(33) Almenningen, A.; Helgaker, T. U.; Haaland, A.; Samdal, S. Acta
Chem. Scand. 1982, A36, 159.
In an effort to prepare aliphatic amide and alkoxide
species analogous to 12, we employed an alternative route
that involved a protonolysis reaction of the correspond-
ing bis-amides [M{N(SiMe3)2}2] (M = Mg, Zn) and the
alkoxo-ethyl zinc mixed-ligand complex [EtZn(2,4,6-
Me3C6H2O)] (2,4,6-Me3C6H2O = mesityloxy) with the
hybrid ligand bpzcpH (Scheme 4). Thus, the reaction of
bpzcpH with the amide and alkoxide starting materials
(34) (a) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K. Inorg. Chem.
2005, 44, 8004–8010. (b) Chisholm, M. H.; Gallucci, J.; Phomphrai, K. Inorg.
Chem. 2002, 41, 2785–2794. (c) Cheng, M.; Moore, D. R.; Reczek, J. J.;
Chamberlain, B.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001,
123, 8738–8749.
(35) (a) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K. Inorg. Chem.
2004, 43, 6717–6725. (b) Rieth, L. R.; Moore, D. R.; Lobkovsky, E. B.; Coates,
G. W. J. Am. Chem. Soc. 2002, 124, 15239–15248. (c) Chisholm, M. H.;
Gallucci, J. C.; Zhen, H.; Huffman, J. C. Inorg. Chem. 2001, 40, 5051–5054.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2865
Table 3. Polymerization of ε-Caprolactone Catalyzed by Complexes 1, 4, 5, 7, 9, and 10a
conv (%)b
prodc
Mn (Da)d
Mw/Mn
d
entry
initiator
temp (°C)
time (h)
yield (g)
1
2
3
4
5
6
7
8
1
4
4
5
20
20
70
20
0
20
20
80
80
80
20
20
18
18
2
1 (min)
2.90
3.70
4.90
4.71
1.24
7.41
4.98
2.60
3.20
3.30
traces
1.19
56
72
95
92
24
65
97
51
62
64
2
2
27
3 139
7
2 223
3 320
0.6
0.7
2
30 200
39 500
53 200
51 800
12 800
151 000
36 900
27 200
33 700
34 600
1.38
1.39
1.49
1.19
1.08
1.45
1.41
1.53
1.49
1.33
5
2
5e
10 (min)
1 (min)
48
48
18
[Mg(CH2SiMe3)(tbpamd)]f
7
9
10
9
10
11
12
10
[Zn(CH2SiMe3)(tbpamd)]f
48
15
24
1
18 600
1.09
a Polymerization conditions: 90 μmol of initiator, 20 mL of toluene as solvent, [ε-CL]0/[initiator]0 = 500. b Percentage conversion of the monomer
[(weight of polymer recovered/weight of monomer) ꢀ 100]. c kg polymer ꢀ [mol of metal (Mg/Zn)]-1 h-1
.
d Determined by GPC relative to polystyrene
3
standards in THF. Experimental Mn was calculated considering Mark-Houwink’s corrections48 for Mn [Mn(obsd) = 0.56 ꢀ Mn(GPC)]. e 20 μmol of
3
initiator, 30 mL of toluene as solvent, [ε-CL]0/[initiator]0 = 5000. f These data have been included for comparison in ROP with the alkyl magnesium20
and zinc21 analogues.
(1 equiv vs M) at low temperature (-70 °C) in toluene
cleanly afforded the corresponding monoamide/alkoxide
scorpionate/cyclopentadienyl complexes [Zn(R){κ2-η1(π)-
bpzcp}] (R = N(SiMe3)2 13, R = 2,4,6-Me3C6H2O 14)
and [Mg{N(SiMe3)2}(κ2-η5-bpzcp)] (15) in very good
yields (>80%) as brown and white microcrystalline
powders, respectively.
A variety of polymerization conditions were explored.
The magnesium alkyl derivatives 1 and 4 effectively
polymerize CL at room temperature (entries 1 and 2)
and 4 gives rise to 72% of conversion of 500 equiv of CL
after 18 h. The polymerization is well controlled and gives
a high molecular weight polymer with a medium broad
polydispersity (Mn = 39 500, Mw/Mn = 1.39). Not
unexpectedly, an increase in the temperature up to 70 °C
led to complete conversion of the monomer in 2 h without
a notable increase in the polydispersity index (Mw/Mn =
1.49, entry 3), showing a higher productivity [27 ꢀ 103 g
The 1H and 13C-{1H} NMR spectra of 11-15 in benzene-
d6 at room temperature display a single set of resonances for
the Me5 and Me3 groups from both pyrazole rings, indicat-
ing their equivalence, and for the bridging CH, along with
two multiplets for the cyclopentadienyl protons (Hc and
PCL (mol Mg)-1
h
-1]. Surprisingly, magnesium alkyl 5
3
1
Hd). Additionally, H NOESY-1D experiments were also
initiates very rapid polymerization at room temperature,
accompanied by a marked increase in the viscosity of the
solution, and gives almost complete conversion in 1 min
with a productivity of more than 3 139 ꢀ 103 g PCL (mol
performed to confirm the assignment of the signals for the
Me3 or 5 groups of the pyrazole rings as well as for Hc and Hd
in the cyclopentadienyl fragment. These data suggest a
tetrahedral disposition in solution for the metal center. In
this arrangement a plane of symmetry exists for both the
pyrazole rings and the C5H4 fragment, and this contains
both the metal center and the chloride, amide, or alkoxide
ligand (Schemes 3 and 4). As discussed above, zinc com-
plexes 11-14 show an unchanged pattern corresponding to
the cyclopentadienyl fragment down to -80 °C. This find-
ing is due to the fast haptotropic exchange27b in solution. In
a similar way to the zinc alkyls 7-10, in the solid state a
η1(π)-C5H4 arrangement is assumed for complexes 11-14,
although unfortunately evidence for this proposal could not
be obtained by X-ray analysis.
Polymerization Studies on Scorpionate/Cyclopentadie-
nyl Magnesium and Zinc Alkyl, Amide, and Alkoxide
Complexes with ε-Caprolactone and Lactides. Complexes
1, 4, 5, 7, 9, 10, 13 and 14 were assessed in the ROP of the
polar monomers ε-caprolactone (CL) and L-/rac-lactide
(L-LA, rac-LA) to probe their potential as initiators for
cyclic esters. These reactivity studies also allowed a
comparison of the Mg-alkyl relative to Zn-alkyl moieties
as initiating groups in these hybrid scorpionate/cyclopen-
tadienyl alkyl systems.
Mg)-1 h-1 (entry 4). This activity is retained at 0 °C
3
(entry 5) and after 2 h 24% of the monomer was converted
with a narrow molecular weight distribution, which sug-
gests living behavior (Mw/Mn = 1.08, entry 5). Alkyl
magnesium 5 shows an extraordinarily high activity,
which is comparable to the previously described amidi-
nate-based alkyl magnesium initiator [Mg(CH2SiMe3)-
(tbpamd)]20 (entry 7).
As a comparison, alkyl zinc initiators 7, 9, and 10
showed significantly lower activity than the analogous
heteroscorpionate amidinate-based alkyl zinc derivatives
recently reported by our group21 (Table 3, entry 12). For
instance, 9 and 10 polymerize CL effectively at 80 °C
(entries 9 and 10) and in the case of 10 64% of the
monomer was transformed after 18 h to produce medium-
high molecular weight materials with a medium-broad
polydispersity index (entry 10, Mn = 34 600, Mw/Mn =
1.33). The productivity decreases markedly on cooling,
and in the reaction at 20 °C, the catalytic activity of 10 is
drastically reduced and only traces of product are formed
(entry 11).
In these tests the polymer molecular weights were limited
by the monomer/initiator ratio of 500:1. An increase in this
ratio by a factor of 10 gave polymers with significantly
higher molecular weight (Mn > 105) and broader molecular
weight distribution (entry 6, Mw/Mn = 1.45), possibly
because of the occurrence of backbiting as well as tran-
sesterification as side reactions, resulting in the formation of
ε-Caprolactone Polymerization Promoted by Scorpionate/
Cyclopentadienyl Alkyl Magnesium and Zinc Complexes.
Initiators 1, 4, 5, 7, 9, and 10 act as efficient single-
component catalysts for the polymerization of ε-capro-
lactone (CL) to give high molecular weight polymers;
the results of these experiments are collected in Table 3.

ꢀ
Garces et al.
2866 Inorganic Chemistry, Vol. 49, No. 6, 2010
Table 4. Polymerization of
L
-Lactide and rac-Lactide Catalyzed by Complexes 5, 10, 13, and 14a
monomer temp (°C) time (h) yield (g) conv (%)b Mn(theor.) (Da)c Mn(Da)d Mw/Mn Tm(°C)e [R]D22 (deg)f
d
entry
initiator
1
2
3
4
5
6
7
8
5
5
5
5
5
5
5
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
90
90
90
90
0.5
1
1.5
2
2.5
5
0.5
96
0.59
1.06
1.50
2.05
2.51
1.81
2.09
1.20
23
41
58
79
97
70
81
93
6 600
5 800
1.03
1.02
1.02
1.03
1.05
1.15
1.38
1.19
161
160
165
165
164
165
155
160
-146
-147
-148
-145
-146
-149
-155
-146
11 800
16 700
22 700
27 900
20 100
23 300
13 400
11 400
16 500
22 500
27 600
21 100
22 200
12 600
90
65(THF)
bulk
70
[Mg(CH2SiMe3)- L-LA
(pbpamd)]g
9
10
13
14
5
5
5
L-LA
L-LA
L-LA
rac-LA
rac-LA
rac-LA
90
90
90
90
90
bulk
30
72
18
4
8
2
2.09
81
23 300
21 000
1.18
160
-153
-146
10
11
12
13
14
2.40
0.82
1.52
1.06
92
32
59
41
26 500
9 200
17 000
11 800
21 500
8 800
15 500
10 900
1.19
1.07
1.12
1.41
165
126
127
135
a Polymerization conditions: 90 μmol of initiator; 70 mL of toluene as solvent, [L-lactide]0/[initiator]0 = 200 and [rac-LA]0/[initiator]0 = 200.
b Percentage conversion of the monomer [(weight of polymer recovered/weight of monomer) ꢀ 100]. c Theoretical Mn = (monomer/initiator) ꢀ
(% conversion) ꢀ (Mw of lactide). d Determined by GPC relative to polystyrene standards in THF. Experimental Mn was calculated considering
Mark-Houwink’s corrections48 for Mn [Mn(obsd) = 0.56 ꢀ Mn(GPC)]. e PLA melting temperature. f Optical rotation data of poly(L-lactide) obtained.
[R]D of L-lactide and poly(L-lactide) are -285° and -144°, respectively.17d g These data have been included for comparison in ROP with the alkyl
22
magnesium analogues,20 90 μmol of initiator; [L-lactide]0/[initiator]0 = 100.
macrocycles with a wide range of molecular weight distribu-
tions. The lower Mn values (as compared to those expected
for a well-controlled polymerization) observed in all cases
(entries 1-6) are also indicative of transfer reactions. In
accordance with the aforementioned data, it is worth noting
that the polymerization processes initiated by alkyl zinc
initiators 7, 9, and 10 are significantly slower than those
initiated by their alkyl magnesium counterparts 1, 4, and 5.
This marked difference between the two types of deriva-
tives, which under the present polymerization conditions
decreases in the order [Mg(R)(K2-η5-bpzcp)] . [Zn(R){K2-
η1(π)-bpzcp}], is presumably a consequence of the M-C
bond strength; this trend is consistent with the decrease in
the lability of the M-C bond36and may rationalize the delay
in the initiation step observed on employing these zinc
initiators. Additionally, the nature of the alkyl group in
both families of initiators affects the catalytic activity, which
decreases in the order CH2SiMe3 . tBu >Me, andis alsoin
agreement with the decrease in the lability of the M-C
bond.36 This behavior has also been observed in analogous
amidinate-based magnesium20 and zinc alkyl21/amide22 de-
rivatives.
Very few alkyl magnesium and zinc initiators38 have been
shown to polymerize ε-caprolactone to medium mole-
cular weight polymers. By contrast, catalysts 5 and 10, as
well as analogous alkyl magnesium20 and alkyl21/amide22
zinc initiators described by our group, have proved to be
efficient initiators for the production of medium-high
molecular weight polymers with medium-broad mole-
cular weight distributions, even at a catalyst loading of
0.08%, which yielded Mn values as high as 151 kg mol-1
3
(entry 6).
L- and rac-Lactide Polymerization Initiated by Scorpionate/
Cyclopentadienyl Alkyl Complexes. Derivatives 5, 10, 13,
and 14 were also examined for the production of PLAs
(Table 4). Complex 5 proved to be an active catalyst for
the polymerization of L-lactide at 90 °C in toluene without
co-catalyst or activator (Table 4, entries 1-5). In all cases,
the PLAs produced have molecular weights in close
agreement with calculated values for one polymer chain
per metal center [Mn(calcd)PLA200 = 28 800] (Table 4).
The gel permeation chromatography (GPC) data for the
resulting polyesters show a monomodal weight distribu-
tion, with polydispersities ranging from 1.02 to 1.05. At
this point, it is worth mentioning that polymerization of
the optically active (S,S)-lactide (L-lactide) afforded 58%
conversion of 200 equiv in 1.5 h, with a very narrow
molecular weight distribution (Mw/Mn = 1.02, entry 3).
On extending the reaction time up to 2.5 h, 97% of the
monomer was transformed without an appreciable in-
crease in the molecular weight distribution (Mw/Mn =
1.05, entry 5). The polymerization occurs without obser-
vable epimerization reactions at the chiral centers, as
evidenced by the homonuclear decoupled 1H NMR spec-
trum, which has only a single resonance at δ 5.16 ppm in
the methine region, affording highly crystalline, isotactic
polymers with a Tm in the range of 160-165 °C.39 The
low level of stereochemical imperfections is also revealed
in the poly(L-lactide) with Mn > 25 000, for which the
The 1H NMR spectrum of poly(ε-caprolactone) oligo-
mer, derived from the reaction of 5 with 25 equiv of ε-CL
exhibits characteristic resonances that are consistent with
the presence of one -CH2SiMe3 end group per CH2OH
hydroxyl chain end. This provides evidence confirming
that the polymerization follows a nucleophilic route and
is initiated by the transfer of an alkyl ligand to the
monomer, with cleavage of the acyl-oxygen bond and
formation of a metal alkoxide-propagating species.37
(36) Martinho, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629–688.
ꢀ
(37) (a) Sanchez-Barba, L. F.; Hughes, D. L.; Humphrey, S. M.;
ꢀ
Bochmann, M. Organometallics 2006, 25, 1012–1020. (b) Sanchez-Barba,
L. F.; Hughes, D. L.; Humphrey, S. M.; Bochmann, M. Organometallics 2005, 24,
ꢀ
5329–5334. (c) Sanchez-Barba, L. F.; Hughes, D. L.; Humphrey, S. M.;
Bochmann, M. Organometallics 2005, 24, 3792–3799.
(38) (a) Sarazin, Y.; Howard, R. H.; Hughes, D. L.; Humphrey, S. M.;
Bochmann, M. Dalton Trans. 2006, 340–350. (b) Sarazin, Y.; Schormann, M.;
Bochmann, M. Organometallics 2004, 23, 3296–3302. (c) Walker, D. A.;
Woodman, T. J.; Schormann, M.; Hughes, D. L.; Bochmann, M. Organometallics
2003, 22, 797–803.
(39) (a) Radano, C. P.; Baker, G. L.; Smith, M. R. J. Am. Chem. Soc.
2000, 122, 1552–1553. (b) Zhong, Z.; Dijkstra, J. P.; Feijen, J. J. Am. Chem. Soc.
2003, 125, 11291–11298.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2867
Table 5. Crystal Data and Summary of Data Collection and Refinement Details for 4, 5, 7 and 10 ꢀ 0.5C7H8
3
4
7
10 ꢀ 0.5C7H8
empirical formula
formula weight
temperature, K
wavelength, A
crystal system
space group
a, A
b, A
c, A
R, deg
β, deg
C33H40MgN4Si
545.09
180(2)
0.71073
triclinic
P1
9.06(1)
13.58(2)
13.95(2)
114.12(2)
96.94(3)
90.90(1)
1552(3)
2
1.166
C33H38MgN4.C7H8
607.12
180(2)
C30H32N4Zn
513.97
180(2)
0.71073
hexagonal
P32
9.7099(3)
9.7099(3)
23.409(2)
90
90
120
1911.4(2)
3
1.340
0.989
810
0.43 ꢀ 0.34 ꢀ 0.30
-9 e h e 9
-9 e k e 9
-23 e l e 19
6887
C36.50 H44 N4 Si Zn
632.24
180(2)
0.71073
0.71073
monoclinic
P21/c
9.152(1)
21.851(3)
17.090(2)
monoclinic
P21/n
9.715(6)
26.118(2)
14.081(9)
95.141(2)
104.79(1)
γ, deg
volume, A3
Z
3403.9(8)
4
1.185
0.086
1304
0.81 ꢀ 0.75 ꢀ 0.68
-7 e h e 8
-20 e k e 20
-16 e l e 16
11188
3454(4)
4
1.213
0.775
1334
0.26 ꢀ 0.17 ꢀ 0.12
-11 e h e 11
-31 e k e 31
-16 e l e 16
19976
density (calculated), g/cm3
absorption coefficient, mm-1
F(000)
0.123
584
crystal size, mm3
index ranges
0.53 ꢀ 0.45 ꢀ 0.36
-11 e h e 3
-15 e k e 15
-5 e l e 16
7900
reflections collected
independent reflections
data/restraints/parameters
goodness-of-fit on F2
2085 [R(int) = 0.1454]
2085/0/359
0.972
2936 [R(int) = 0.0678]
2936/0/414
1.044
2588 [R(int) = 0.0662]
2588/1/321
0.829
6077 [R(int) = 0.1170]
6077/48/390
0.973
Final R indices [I > 2σ(I)]
R1 = 0.0656
wR2 = 0.1551
R1 = 0.1594
wR2 = 0.2099
0.180 and -0.153
R1 = 0.0509
wR2 = 0.1174
R1 = 0.0771
wR2 = 0.1338
0.276 and -0.230
R1 = 0.0467
wR2 = 0.1171
R1 = 0.0596
wR2 = 0.1288
0.424 and -0.281
R1 = 0.0742
wR2 = 0.1867
R1 = 0.1684
wR2 = 0.2462
0.640 and -0.423
R indices (all data)
largest diff. peak and hole, e A-3
the analogous amidinate-based alkyl magnesium20 com-
plexes (entry 8) in the L-LA ROP process, probably as a
result of the lower tendency to undergo possible sym-
metrical (Schlenk)40 equilibrium competition than the
alkyl amidinate-based heteroscorpionate and, therefore,
the lower formation of sandwich species versus catalytic
performance. In contrast, zinc complexes 10, 13, and 14
were found to be much less active than the magnesium
derivative 5 (Table 4, entries 9-11), as previously ob-
served for the production of PCL. Thus, after 30 h in
toluene at 90 °C, 81% of the monomer was converted in
the case of 10, and the material had a narrow poly-
dispersity index (Mw/Mn = 1.18, entry 9). In contrast,
the aliphatic bis(trimethylsilyl)amide derivative 13 did
not produce any polymeric materials after 72 h (entry 10)
under otherwise identical conditions, as recently reported
by our group,22 when an amidinate-based heteroscorpio-
nate zinc derivative was tested with this aliphatic amide.
Notably, the aryloxo zinc derivative 14 showed higher
polymerization activity than the alkyl derivative 10 and in
18 h 92% of the monomer was converted with a slightly
broader molecular weight distribution (Mw/Mn = 1.19,
entry 11).
Figure 5. Plot of PLA Mn as a function of monomer conversion (%) for
the polymerization of L-LA initiated by 5; [L-LA]0/[5]0 = 200, toluene,
90 °C. (Polydispersity values have been included in parentheses).
22
optical activity remains almost constant, [R]D
145-148°.
=
The high level of control afforded by this initiator in the
polymerization of L-lactide is further exemplified by the
narrow molecular weight distributions and linear correla-
tions between Mn and percentage conversion (Figure 5).
These results are characteristic of well-controlled living
propagations as well as the existence of a single type of
reaction site, a situation similar to the living behavior
previously observed in the amidinate-based alkyl magne-
sium derivatives.20 As expected, when the polymerization
was carried out in THF (entry 6) the activity of 5 dropped
and after 5 h only 70% of the monomer was transformed as
a result of the competition reaction between THF and the
monomer moiety for the metal center. In contrast, when the
polymerization was carried out under bulk conditions, 81%
of the polymer was recovered after 0.5 h, and the product
had a broader polydispersity (Mw/Mn = 1.38, entry 7).
The new hybrid scorpionate/cyclopentadienyl alkyl
magnesium derivatives proved to be more active than
Initiator 5 was also tested for the polymerization of
rac-lactide in toluene at 90 °C (Table 4, entries 12-14).
Derivative 5 gave 32% conversion of 200 equiv after 4 h
(entry 12) and produced a low molecular weight material
with a very narrow polydispersity (Mn = 8 800, Mw/Mn =
1.07). Extension of the reaction time to 8 h increased
the activity, with conversion rising to 59%, and led to a
(40) (a) Ashby, E. C. Pure Appl. Chem. 1980, 52, 545–569. (b) Ashby, E. C.;
Laemmle, J.; Neumann, H. M. Acc. Chem. Res. 1974, 7, 272–280. (c) Ashby,
E. C. Quart. Rev., Chem. Soc. 1967, 259–285. (d) Kharasch, M. S.; Reinmuth, O.
Grignard Reactions of Nonmetallic Substances; Prentice-Hall: New York, 1954.

ꢀ
Garces et al.
2868 Inorganic Chemistry, Vol. 49, No. 6, 2010
significant increase in the polymer molecular weight and a
slight increase in polydispersity (Mn = 15 500, Mw/Mn =
1.12, entry 13). Under bulk conditions 41% of the poly-
mer was recovered after 2 h, and this had a broader
polydispersity (Mw/Mn = 1.41, entry 14). We attri-
bute this broad molecular weight distribution to gradual
catalyst decomposition on increasing the temperature
and a small degree of transesterification and/or slow
and incomplete initiation relative to propagation. In all
cases (entries 12-14), low melting materials were ob-
tained with the Tm ranging from 126 to 135 °C. As far
as we are aware, very few examples of magnesium alkyl
initiators20,21,37a,38,41,42 have been reported to act as
active single-site catalysts for the ROP of rac-lactide.
PLA end-group analysis showed that in all cases
(entries 1-7 and 9-14), as for PCL initiated by alkyl
magnesium and zinc initiators, the polymerization was
initiated by nucleophilic attack of the alkyl group on
lactide. Microstructural analysis of the poly(rac-lactide)
coordination mode for the cyclopentadienyl fragment,
zinc derivatives (7-10) feature a η1(π) arrangement in the
solid state. Furthermore, we have explored the reactivity
of this hybrid lithium salt with ZnCl2 to yield a zinc
chloride complex [ZnCl{κ2-η1(π)-bpzcp}] (11) that is an
attractive starting material for the successful preparation
of the aromatic amide [Zn(NH-4-MeC6H4){κ2-η1(π)-
bpzcp}] (12). In contrast, the use of 11 to prepare aliphatic
amide derivatives proved fruitless. Alternatively, proto-
nolysis of a zinc and magnesium metal amide/alkyl by the
hybrid ligand bpzcpH leads to aliphatic amide and alk-
oxide complexes [M(R)(bpzcp)] (13-15).
The alkyl and alkoxide nucleophilicity of the corres-
ponding magnesium and zinc complexes means that they
can act as highly effective single-site living initiators for
the well-controlled polymerization of polar monomers
over a wide range of temperatures. ε-Caprolactone is
polymerized within minutes to give high molecular weight
polymers with narrow polydispersities. Not surprisingly,
the polymerization of LA occurs more slowly than that of
CL but offers good control. L-Lactide afforded highly
crystalline, isotactic PLA materials with medium mole-
cular weights, polydispersities as narrow as Mw/Mn =
1.02, and very high melting temperatures (Tm = 165 °C).
Propagation occurs without observable epimerization
with both families of initiators. Polymerization of rac-
lactide did not result in appreciable levels of selectivity
and produced atactic PLA. End group analysis suggests
that magnesium and zinc catalysts initiate the polymeri-
zation process by alkyl/alkoxide transfer to the monomer.
The slow initiation rate and the lack of stereoselectivity in
lactide polymerizations have prompted us to reevaluate
the ligand design and incorporate groups that are more
sterically challenging than the cyclopentadienyl group in
an effort to produce high levels of stereocontrol during
the propagations.
1
by H NMR spectroscopy revealed that 5 exerts a low
degree of stereoselectivity. This conclusion is based on the
fact that the heterotactic tetrads isi and sis were not
greatly enhanced as a result of the preference of the
consecutive alternate insertion of the L- and D-lactide
units into the growing chain. This behavior during
the propagation is most probably the result of the low
steric demand of the methyl subtituents in the two
pyrazole rings. This leads to sterically less congested
and more flexible (and therefore less selective) active
centers to the incoming lactide, even in the permanent
presence of the sterically hindered cyclopentadienyl sub-
stituent in the position cis to the alkyl leaving group.
These results parallel the previous observations reported
by Chisholm, Bochmann, and our group on magnesium
alkoxides [(BDI)Mg(OtBu)(THF)]43 and [(η3-trispyrazolyl-
borate)MgOR],17d and magnesium alkyls [(BDI)Mg-
(η1-C3H5)(THF)]37a and [Mg(CH2SiMe3)(tbpamd)],20 res-
pectively, which were found to polymerize rac-lactide to
atactic poly(lactide). However, the results differ from the
appreciable preference for the heterotactic poly(lactide)
from rac-lactide (Pr = 0.68) previously observed in our
group21 on using amidinate-based heteroscorpionate
alkyl zinc initiators with the bulky tert-butyl substituent
in the pyrazole rings.
In conclusion, we report here a facile synthesis of low-
coordinative neutral monoalkyl magnesium complexes
[Mg(R)(κ2-η5-bpzcp)] (1-6) through the reaction of a
series of commercially available Grignard reagents and
the lithium salt [Li(bpzcp)(THF)]. Alternatively, hydro-
lysis of this hybrid lithium salt and subsequent reaction
with ZnR2 cleanly affords the monoalkyl zinc derivatives
[Zn(R){κ2-η1(π)-bpzcp}] (7-10) through alkane elimina-
tion reactions. Interestingly, single-crystal X-ray diffrac-
tion studies on derivatives 4, 5, 7, and 10 reveal that
whereas alkyl magnesium derivatives (1-6) present a η5
Experimental Section
General Procedures. All manipulations were performed under
nitrogen, using standard Schlenk techniques. Solvents were
predried over sodium wire (toluene, n-hexane, THF, diethyl
ether) or calcium hydride (dichloromethane) and distilled
under nitrogen from sodium (toluene), sodium-potassium alloy
(n-hexane), sodium-benzophenone (THF, diethyl ether), or cal-
cium hydride (dichloromethane). Deuterated solvents were stored
over activated 4 A molecular sieves and degassed by several
freeze-thaw cycles. Microanalyses were carried out with a Per-
1
kin-Elmer 2400 CHN analyzer. H and 13C NMR spectra were
recorded on a Varian Mercury FT-400 spectrometer and refer-
enced to the residual deuterated solvent. The NOESY-1D spectra
were recorded on a Varian Inova FT-500 spectrometer with the
following acquisition parameters: irradiation time 2 s and number
of scans 256, using standard VARIANT-FT software. Two-
dimensional NMR spectra were acquired using standard VAR-
IANT-FT software and processed using an IPC-Sun computer.
ZnMe2, ZnEt2, ZnCl2, ClMgMe, ClMgEt, ClMgtBu, MgBu2,
ClMgCH2SiMe3, ClMgBz, and 2,4,6-trimethylphenol were pur-
chased from Aldrich. [Mg{N(SiMe3)2}2],44a [Zn{N(SiMe3)2}2],44b
[Li(CH2SiMe3)],44c [LiNH-4-MeC6H4],44d [Li(bpzcp)(THF)]23,24b
and bpzcpH25 were prepared according to the literature procedures.
(41) 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.
(42) Duda, A.; Penczek, S. Polymers from Renewable Resources: Biopoly-
esters and Biocatalysts; Scholz, C., Gross, R. A., Eds.; ACS Symposium Series
Vol.764; American Chemical Society: Washington, DC, 2000; p 160.
(43) Chisholm, M. H.; Huffman, J. C.; Phomphrai, K. J. Chem. Soc.,
Dalton Trans. 2001, 222–224.
(44) (a) Westerhausen, M. Inorg. Chem. 1991, 30, 96–101. (b) B€urger,
H.; Sawodny, W.; Wannagat, U. J. Organomet. Chem. 1965, 3, 113–120.
(c) Hartwell, F. E.; Brown, T. L. Inorg. Chem. 1966, 5, 1257–1263. (d) Talaleva,
T. V.; Kocheschkov, K. A. Doklady Akademi Nauk SSSR 1995, 104, 260–263.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2869
ε-Caprolactone was dried by stirring over fresh CaH2 for 48 h, then
distilled under reduced pressure and stored over activated 4 A
molecular sieves. L-Lactide and rac-lactide were sublimed twice,
recrystallized from THF and finally sublimed again prior to use.
Gel permeation chromatography (GPC) measurements were per-
formed on a Polymer Laboratories PL-GPC-220 instrument
equipped with a TSK-GEL G3000H column and an ELSD-LTII
light-scattering detector. The GPC column was eluted with THF at
45 °C at 1 mL/min and was calibrated using eight monodisperse
polystyrene standards in the range 580-483000 Da. PLA melting
temperatures were measured using a melting point Block SMP 10.
The sample was heated up to 100 °C and then heated at a rate of
1 °C/min up to 165 °C. The specific rotation [R]D22 was measured
at a concentration of 10 mg/mL in CHCl3 at 22 °C on a Perkin-
Elmer 241 Polarimeter equipped with a Na lamp operating at 589
nm with a light path length of 10 cm.
THF) (1.95 cm3, 1.95 mmol), [Li(bpzcp)(THF)] (1.00 g,
1.95 mmol). Yield: 0.89 g, 89%. Anal. Calcd for C33H38MgN4:
C, 76.96; H, 7.44; N, 10.88. Found: C, 77.09; H, 7.57; N, 10.65.
1H NMR (C6D6, 298 K), δ 7.23-6.86 (m, 11H, CH, Phenyl
groups), 6.76 (m, 2H, Hd - C5H4), 5.98 (m, 2H, Hc - C5H4), 5.22
(s, 2H, H4), 2.12 (s, 6 H, Me3), 1.51 (s, 9H, MgtBu), 1.46 (s, 6H,
Me5). 13C-{1H} NMR (C6D6, 298 K), δ 149.8, 147.5 (C3 or 5),
141.2-128.4 (Phenyl groups), 112.6 (Cc - C5H4), 108.2 (Cb
-
C5H4), 106.8 (Cd - C5H4), 105.3 (C4), 72.1 (CH), 62.5 (Ca), 35.1
(MgC(CH3)3), 21.7 (MgC(CH3)3), 13.8 (Me3), 11.0 (Me5).
Synthesis of [Mg(CH2SiMe3)(K2-η5-bpzcp)] (5). The synthesis
of 5 was carried out in an identical manner to 1. Me3SiCH2MgCl
(1.0 M in Et2O) (1.95 cm3, 1.95 mmol), [Li(bpzcp)(THF)]
(1.00 g, 1.95 mmol). Yield: 0.94 g, 88%. Anal. Calcd for
C33H40MgN4Si: C, 72.71; H, 7.40; N, 10.28. Found: C, 72.91;
1
H, 7.77; N, 10.51. H NMR (C6D6, 298 K), δ 7.29-6.86 (m,
11H, CH, Phenyl groups), 6.74 (m, 2H, Hd - C5H4), 6.10 (m, 2H,
Hc - C5H4), 5.18 (s, 2H, H4), 2.09 (s, 6 H, Me3), 1.45 (s, 6H, Me5),
Preparation of Compounds (1-16). Synthesis of [Mg(Me)-
(K2-η5-bpzcp)] (1). In a 250 cm3 Schlenk tube, [Li(bpzcp)(THF)]
(1.00 g, 1.95 mmol) was dissolved in dry toluene (70 cm3) and
cooled to -70 °C. A solution of MeMgCl (3.0 M in THF) (0.65
cm3, 1.95 mmol) was added, and the mixture was allowed to
warm up to room temperature and stirred during 1 h. The
solvent was evaporated to dryness under reduced pressure to
yield a sticky yellow product. The product was washed with
hexane and recrystallized from toluene at -26 °C to give
compound 1 as yellow crystals. Yield: 0.71 g, 77%. Anal. Calcd
for C30H32MgN4: C, 76.19; H, 6.82; N, 11.85. Found: C, 76.29;
H, 6.59; N, 11.79. 1H NMR (C6D6, 298 K), δ 7.24-6.78 (m, 11
H, CH, Phenyl groups), 6.62 (m, 2H Hd - C5H4), 6.06 (m, 2H,
Hc - C5H4), 5.10 (s, 2H, H4), 1.99 (s, 6H, Me3), 1.42 (s, 6H, Me5),
-0.96 (s, 3 H, MgCH3). 13C-{1H} NMR (C6D6, 298 K), δ 149.7,
147.6 (C3 or 5), 140.8-125.6 (Phenyl groups), 113.9 (Cc - C5H4),
109.2 (Cb - C5H4), 106.6 (Cd - C5H4), 105.3 (C4), 72.1 (CH), 62.4
(Ca), 13.2 (Me3), 11.1 (Me5), -10.8 (MgCH3).
0.59 (s, 9H, MgCH2SiMe3), -1.31 (s, 2H, MgCH2SiMe3).
or
13C-{1H} NMR (C6D6, 298 K), δ 149.7, 147.5 (C3
5),
-
140.9-126.5 (Phenyl groups), 113.9 (Cc - C5H4), 109.2 (Cb
C5H4), 106.5 (Cd - C5H4), 105.8 (C4), 72.1 (CH), 62.3 (Ca), 13.8
(Me3), 11.0 (Me5), 5.15 (MgCH2SiMe3), -4.24 (MgCH2SiMe3).
Synthesis of [Mg(Bz)(K2-η5-bpzcp)] (6). The synthesis of 6 was
carried out in an identical manner to 1. BzMgCl (2.0 M in THF)
(0.975 cm3, 1.95 mmol), [Li(bpzcp)(THF)] (1.00 g, 1.95 mmol).
Yield: 0.87 g, 81%. Anal. Calcd for C36H36MgN4: C, 78.76; H,
1
6.61; N, 10.21. Found: C, 78.69; H, 6.57; N, 10.33. H NMR
(C6D6, 298 K), δ 7.38-6.86 (m, 16H, CH, Phenyl groups), 6.64
(m, 2H, Hd - C5H4), 6.00 (m, 2H, Hc - C5H4), 5.15 (s, 2H, H4),
1.99 (s, 6H, Me3), 1.44 (s, 6H, Me5), 0.29 (s, 2H, MgCH2C6H5).
or
13C-{1H} NMR (C6D6, 298 K), δ 149.9, 147.3 (C3
5),
-
141.1-127.8 (Phenyl groups), 114.2 (Cc - C5H4), 108.6 (Cb
C5H4), 106.7 (Cd - C5H4), 104.9 (C4), 72.1 (CH), 62.5 (Ca), 13.5
(Me3), 11.0 (Me5) 1.35 (MgCH2C6H5).
Synthesis of [Mg(Et)(K2-η5-bpzcp)] (2). The synthesis of 2 was
carried out in an identical manner to 1. EtMgCl (2.0 M in THF)
(0.98 cm3, 1.95 mmol), [Li(bpzcp)(THF)] (1.00 g, 1.95 mmol).
Yield: 0.87 g, 92%. Anal. Calcd for C31H34MgN4: C, 76.46; H,
Synthesis of [Zn(Me){K2-η1(π)-bpzcp}] (7). In a 250 cm3
Schlenk tube, bpzcpH (1.00 g, 2.30 mmol) was dissolved in
dry toluene (70 cm3) and cooled to -70 °C. A solution of ZnMe2
(2.0 M in toluene) (1.15 cm3, 2.30 mmol) was added, and the
mixture was allowed to warm up to room temperature and
stirred during 16 h. The solvent was evaporated to dryness under
reduced pressure to yield a sticky yellow product. The product
was washed with hexane and recrystallized from toluene at
-26 °C to give compound 7 as yellow crystals. Yield: 0.97 g,
82%. Anal. Calcd for C30H32N4Zn: C, 70.10; H, 6.28; N, 10.90.
Found: C, 69.98; H, 6.44; N, 10.79. 1H NMR (C6D6, 298 K), δ
7.14-6.90 (m, 10H, Phenyl groups), 6.95 (m, 2H, Hd - C5H4)
6.79 (s, 1H, CH), 6.16 (m, 2H, Hc - C5H4), 5.18 (s, 2H, H4), 1.97
1
7.04; N, 11.51. Found: C, 76.61; H, 6.89; N, 11.42. H NMR
(C6D6, 298 K), δ 7.33-6.89 (m, 11H, CH, Phenyl groups), 6.76
(m, 2H Hd -C5H4), 6.12 (m, 2H Hc-C5H4), 5.20 (s, 2 H, H4), 2.11
(s, 6H, Me3), 1.92 (t, 3H, 3JH-H = 8.4 Hz, MgCH2CH3), 1.50
3
(s, 6H, Me5), -0.09 (q, 2 H, JH-H = 8.4 Hz, MgCH2CH3).
or
13C-{1H} NMR (C6D6, 298 K), δ 149.6, 147.6 (C3
5),
-
140.8-128.5 (Phenyl groups), 113.9 (Cc - C5H4), 108.9 (Cb
C5H4), 106.5 (Cd - C5H4), 104.8 (C4), 72.1 (CH), 62.5 (Ca), 13.8
(MgCH2CH3), 12.9 (Me3), 11.0 (Me5), 1.44 (MgCH2CH3).
Synthesis of [Mg(nBu)(K2-η5-bpzcp)] (3). In a 250 cm3 Schlenk
tube, bpzcpH (1.00 g, 2.30 mmol) was dissolved in dry toluene
(70 cm3) and cooled to -70 °C. A solution of MgBu2 (1.0 M in
heptane) (2.30 cm3, 2.30 mmol) was added, and the mixture was
allowed to warm up to room temperature and stirred during
16 h. The solvent was evaporated to dryness under reduced
pressure to yield a sticky yellow product. The product was
washed with hexane and recrystallized from toluene at -26 °C
to give compound 3 as yellow crystals. Yield: 0.94 g, 88%. Anal.
Calcd for C33H38MgN4: C, 76.96; H, 7.44, N, 10.88. Found: C,
76.91; H, 7.29; N, 10.71. 1H NMR (C6D6, 298 K), δ 7.32-6.87
(m, 11H, CH, Phenyl groups), 6.74 (m, 2H, Hd - C5H4), 6.12 (m,
2H, Hc- C5H4), 5.20 (s, 2H, H4), 2.11 (s, 6H, Me3), 1.89 (m, 4H,
(s, 6 H, Me3), 1.58 (s, 6H, Me5), -0.20 (s, 3 H, ZnCH3). 13C-{1H}
or
NMR (C6D6, 298 K), δ 149.9, 146.9 (C3
5), 140.8-126.6
(Phenyl groups), 117.6 (Cb - C5H4), 115.4 (Cc - C5H4), 108.0 (Cd -
C5H4), 106.3 (C4), 70.8 (CH), 63.5 (CPh2), 13.0 (Me3), 11.3
(Me5), -12.9 (ZnCH3).
Synthesis of [Zn(Et){K2-η1(π)-bpzcp}] (8). The synthesis of 8
was carried out in an identical manner to 7. ZnEt2 (1.0 M in
hexane) (2.30 cm3, 2.30 mmol), bpzcpH (1.00 g, 2.30 mmol).
Yield: 1.04 g, 86%. Anal. Calcd for C31H34N4Zn: C, 70.51; H,
1
6.49; N, 10.61. Found: C, 70.27; H, 6.33; N, 10.34. H NMR
(C6D6, 298 K), δ 7.13-6.91 (m, 10H, Phenyl groups), 7.02 (m,
2H, Hd - C5H4), 6.80 (s, 1H, CH), 6.14 (m, 2H, Hc - C5H4),
5.20 (s, 1H, H4), 1.99 (s, 6H, Me3), 1.76 (t, 3H, 3JH-H = 6.8 Hz,
ZnCH2CH3), 1.58 (s, 6H, Me5), -0.61 (q, 2H, 3JH-H = 6.8 Hz,
ZnCH2CH3). 13C-{1H} NMR (C6D6, 298 K), δ 149.7, 146.9
MgCH2CH2CH2CH3), 1.49 (s, 6H, Me5), 1.34 (t, 3H, 3JH-H
=
7.6 Hz, MgCH2CH2CH2CH3), -0.09 (m, 2 H, 3JH-H = 6.0 Hz,
MgCH2CH2CH2CH3). 13C-{1H} NMR (C6D6, 298 K), δ 149.6,
147.6 (C3 or 5), 140.7-118.5 (Phenyl groups), 113.7 (Cc - C5H4),
108.9 (Cb - C5H4), 106.5 (Cd - C5H4), 104.9 (C4), 72.0 (CH), 62.4
(Ca), 33.2, 32.6 (MgCH2CH2CH2CH3), 14.6 (MgCH2CH2CH2-
CH3), 12.9 (Me3), 11.2 (MgCH2CH2CH2CH3), 10.9 (Me5).
Synthesis of [Mg(tBu)(K2-η5-bpzcp)] (4). The synthesis of 4
was carried out in an identical manner to 1. tBuMgCl (1.0 M in
(C3 or 5), 140.8-127.3 (Phenyl groups), 117.6 (Cb - C5H4), 114.4
(Cc - C5H4), 107.8 (Cd - C5H4), 106.2 (C4), 70.8 (CH), 63.5 (CPh2),
13.7 (ZnCH2CH3), 12.9 (Me3), 11.2 (Me5), 0.4 (ZnCH2CH3).
Synthesis of [Zn(tBu){K2-η1(π)-bpzcp}] (9). Compound 9 was
made by a one-pot reaction. In a 250 cm3 Schlenk tube, ZnCl2

ꢀ
Garces et al.
2870 Inorganic Chemistry, Vol. 49, No. 6, 2010
(0.45 g, 3.30 mmol) was dissolved in dry diethyl ether (70 cm3)
and cooled to -70 °C. A solution of tBuLi (1.7 M in pentane)
(3.88 cm3, 6.60 mmol) was added, and the mixture was allowed
to warm up to room temperature and stirred for 1 h. An
increasing turbidity developed, and this finally led to the for-
mation of a white suspension. The suspension was filtered, and
the filtrate corresponding to ZntBu2 was added to a precooled
(-20 °C) solution of bpzcpH (1.43 g, 3.30 mmol) in diethyl ether.
The mixture was stirred at this temperature for 16 h. The
volatiles were removed, and the resulting sticky yellow solid
was washed with hexane (30 cm3) and recrystallized from
toluene to give the title compound 9 as yellow crystals. Yield:
1.45 g, 79%. Anal. Calcd for C33H38N4Zn: C, 71.28; H, 6.89; N,
10.08. Found: C, 70.99; H, 6.57; N, 10.22. 1H NMR (C6D6, 298
K), δ 7.13-6.83 (m, 10 H, Phenyl groups), 6.92 (m, 2H, Hd -
C5H4), 6.67 (s, 1H, CH), 6.26 (m, 2H, Hc- C5H4), 5.20 (s, 2H,
Synthesis of [Zn(NH-4-MeC6H4){K2-η1(π)-bpzcp}] (NH-4-
MeC6H4 = p-toluidine) (12). In a 250 cm3 Schlenk tube,
compound 11 (0.92 g, 1.72 mmol) was dissolved in dry THF
(70 cm3) and cooled to -70 °C. LiNHAr (NHAr = para-
toluidine) (0.19 g, 1.72 mmol) was dissolved in THF (30 cm3)
and added dropwise to the solution. The mixture was allowed to
warm up and stirred at room temperature during 1 h. The
solvent was evaporated to dryness under reduced pressure to
yield a sticky yellow product. The product was extracted from
toluene and recrystallized at -26 °C to give compound 12 as
yellow crystals. Yield: 0.71 g, 68%. Anal. Calcd for
C36H37N5Zn: C, 71.45; H, 6.16; N, 11.57. Found: C, 71.10; H,
6.46; N, 11.91. 1H NMR (Toluene-d8, 298 K), δ 7.15-7.01 (m,
11H, CH, Phenyl groups), 6.92 (m, 2H, Hd - C5H4), 6.78, 6.69
(2 m, 2H each, H - HNC6H4Me), 5.77 (m, 2H, Hc - C5H4), 5.18
(s, 2H, H4), 3.22 (s, 1H, HNC6H4Me), 2.13 (s, 3H, HNC6H4Me),
1.95 (s, 6 H, Me3), 1.55 (s, 6H, Me5), 13C-{1H} NMR (Toluene-
d8, 298 K), δ 149.8, 146.7 (C3 or 5), 140.9-126.5 (Phenyl groups,
HNC6H4Me), 117.6 (Cb - C5H4), 114.7 (Cc - C5H4), 108.2 (Cd -
C5H4), 106.3 (C4), 70.8 (CH), 63.5 (CPh2), 25.4 (HNC6H4Me),
13.1 (Me3), 11.4 (Me5).
Synthesis of [Zn{N(SiMe3)2}{K2-η1(π)-bpzcp}] (13). In a 250
cm3 Schlenk tube, Zn[N(SiMe3)2]2 (0.88, 2.30 mmol) was dis-
solved in dry toluene (70 cm3) and cooled to -70 °C. A solution
of bpzcpH (1.00 g, 2.30 mmol) in toluene (30 cm3) was added,
and the mixture was allowed to warm up to room temperature
and stirred during 4 h. The solvent was evaporated to dryness
under reduced pressure to yield a sticky yellow product. The
product was washed with hexane and recrystallized from to-
luene at -26 °C to give compound 13 as white crystals. Yield:
1.14 g, 74%. Anal. Calcd for C35H47N5Si2Zn: C, 63.75; H, 7.18;
N, 10.62. Found: C, 63.45; H, 7.23; N. 10.75. 1H NMR
(Toluene-d8, 298 K), δ 7.28-6.87 (m, 11H, CH, Phenyl groups),
6.83 (m, 2H, Hd - C5H4), 5.70 (m, 2H, Hc - C5H4), 5.36 (s, 2H,
H4), 2.22 (s, 6H, Me3), 1.88 (s, 6H, Me5), 0.15 (s, 18H, ZnN-
(SiMe3)2). 13C-{1H} NMR (Toluene-d8, 278 K), δ 151.3, 147.2
(C3 or 5), 141.3-126.2 (Phenyl groups), 125.5 (Cc - C5H4), 117.74
(Cd - C5H4), 106.5 (C4), 106.0 (Cb - C5H4), 71.2 (CH), 64.8 (Ca),
13.9 (Me3), 11.4 (Me5), 2.5 (ZnN(SiMe3)2).
H4), 2.11 (s, 6H, Me3), 1.57 (s, 9H, ZntBu), 1.50 (s, 6H, Me5).
or
13C-{1H} NMR (C6D6, 298 K), δ 149.3, 146.7 (C3
5),
141.3-126.2 (Phenyl groups), 117.1 (Cb - C5H4), 115.5 (Cc -
C5H4), 109.9 (Cd - C5H4), 106.7 (C4), 70.8 (CH), 65.2 (CPh2),
34.6 (ZnC(CH3)3), 32.1 (ZnC(CH3)3), 14.0 (Me3), 11.3 (Me5).
Synthesis of [Zn(CH2SiMe3){K2-η1(π)-bpzcp}] ꢀ 0.5 C7H8
(10). Compound 10 was made by a one-pot reaction. A solution
of [Li(CH2SiMe3)] (0.62 g, 6.60 mmol) in diethyl ether was
added to a cooled (-40 °C), stirred solution of ZnCl2 (0.45 g,
3.30 mmol) in diethyl ether in a 250 mL Schlenk tube. The
mixture was allowed to warm up to room temperature and
stirred for 1 h. An increasing turbidity developed, and this
finally led to the formation of a white suspension. The suspen-
sion was filtered, and the filtrate corresponding to Zn(CH2Si-
Me3)2 was added to a precooled (-20 °C) solution of bpzcpH
(1.43 g, 3.30 mmol) in diethyl ether. The mixture was stirred at
this temperature for 16 h. The volatiles were removed, and the
sticky yellow solid obtained was washed with hexane (30 cm3)
and recrystallized from toluene to give the title compound 10 as
yellow crystals. Yield: 1.61 g, 77%. Anal. Calcd for C33H40N4-
SiZn ꢀ 0.5 C7H8 C, 69.35; H, 6.96; N, 8.86. Found: C, 69.53; H,
1
6.89; N, 8.91. H NMR (C6D6, 298 K), δ 7.13-6.88 (m, 10H,
Phenyl groups), 6.95 (m, 2H, Hd - C5H4), 6.78 (s, 1H, CH), 6.09
(m, 2 H, Hc- C5H4), 5.19 (s, 2H, H4), 2.08 (s, 6H, Me3), 1.56 (s,
Synthesis of [Zn(2,4,6-Me3C6H2O){K2-η1(π)-bpzcp}] (2,4,6-
Me3C6H2O = mesityloxy) (14). Compound 14 was prepared in
a one-pot reaction. A solution of 2,4,6-trimethylphenol (0.31 g,
2.30 mmol) in THF (30 cm3) was added to a cooled (-40 °C),
stirred solution of ZnEt2 (0.28 g, 2.30 mmol) in THF (30 cm3) in
a 250 mL Schlenk tube. The mixture was allowed to warm up to
room temperature and stirred for 1 h at room temperature.
A solution of bpzcpH (1.00 g, 2.30 mmol) in THF (30 cm3) was
added, and the mixture was heated to 50 °C over 2 h. The
volatiles were removed, and the resulting sticky white solid was
washed with hexane (60 cm3) and recrystallized from toluene to
give the title compound 14 as white crystals. Yield: 1.30 g, 89%.
Anal. Calcd for C38H40N4OZn: C, 71.97; H, 6.36; N, 8.83.
6H, Me5), 0.51 (ZnCH2SiMe3), -0.57 (ZnCH2SiMe3). 13C-{1H}
or
NMR (C6D6, 298 K), δ 149.9, 146.9 (C3
5), 140.8-126.6
(Phenyl groups), 117.6 (Cb - C5H4), 116.4 (Cc - C5H4), 107.5 (Cd -
C5H4), 106.2 (C4), 70.8 (CH), 63.6 (CPh2), 13.7 (Me3), 11.4
(Me5), 3.96 (ZnCH2SiMe3), -6.4 (ZnCH2SiMe3).
Conversion from [Mg(R)(K2-η5-bpzcp)] (1, 2, 4, and 5) into
[Zn(R){κ2-η1(π)-bpzcp}] (7-10). As an example, in a 250 cm3
Schlenk tube, [Mg(CH2SiMe3){κ2-η5-bpzcp}] (5) (1.00 g, 1.83 mmol)
was dissolved in dry toluene (60 cm3) and reacted with an ether
(20 cm3) solution of ZnCl2 (0.25 g, 1.83 mmol). The mixture was
stirred during 24 h. The solvent was evaporated to dryness under
reduced pressure, and the product was extracted and recrystallized from
toluene to give compound (10) as white crystals. Yield: 0.925 g, 86%.
Synthesis of [ZnCl{K2-η1(π)-bpzcp}] (11). In a 250 cm3
Schlenk tube, [Li(bpzcp)(THF)] (1.17 g, 2.30 mmol) was dis-
solved in dry THF (70 cm3) and cooled to -70 °C. A solution of
ZnCl2 (0.31, 2.30 mmol) was added, and the mixture was
allowed to warm up to room temperature and stirred during
16 h. The solvent was evaporated to dryness under reduced
pressure, and the product was extracted and recrystallized from
CH2Cl2 to give compound 11 as white crystals. Yield: 0.92 g,
75%. Anal. Calcd for C29H29ClN4Zn: C, 65.17; H, 5.47; N,
10.48. Found: C, 65.38; H, 5.34; N, 10.11. 1H NMR (CDCl3, 298
K), δ 7.25-7.12 (m, 11H, Phenyl groups, CH), 6.48 (m, 2H, Hd -
C5H4), 5.53 (m, 2H, Hc - C5H4), 5.84 (s, 2H, H4), 2.29 (s, 6 H,
Me3), 2.28 (s, 6H, Me5). 13C-{1H} NMR (CDCl3, 298 K), δ
151.8, 145.9 (C3 or 5), 141.6-126.6 (Phenyl groups), 115.7 (Cc -
C5H4), 112.6 (Cb - C5H4), 107.0 (Cd - C5H4), 101.5 (C4), 72.0
(CH), 65.4 (Ca), 13.5 (Me3), 12.1 (Me5).
1
Found: C, 72.11; H, 6.61; N, 8.72. H NMR (CDCl3, 298 K),
δ 7.23-7.00 (m, 11H, CH, Phenyl groups), 6.66 (s, 2H, H3,5
-
OR), 6.48 (m, 2H, Hd - C5H4), 5.81 (s, 2H, H4), 5.56 (m, 2H, Hc-
C5H4), 2.20 (s, 6H, Me3), 2.15 (s, 3H, Me4 - OR), 2.03 (s, 6H,
Me5), 1.93 (s, 6H, Me2,6 - OR). 13C-{1H} NMR (CDCl3, 298
K), δ 151.9 (Cb - OR), 146.3, 141.8 (C3 or 5), 129.6-126.5
(Phenyl groups, Zn-O-C6H3(Me)3), 122.3 (Cb - C5H4), 115.4
(Cc - C5H4), 106.9 (Cd - C5H4), 102.0 (C4), 72.3 (CH), 62.2
(CPh2), 20.7 (Me4 - OR), 18.4 (Me2,6 - OR), 13.2 (Me3), 12.4
(Me5).
Synthesis of [Mg{N(SiMe3)2}(K2-η5-bpzcp)] (15). In a 250 cm3
Schlenk tube, [Mg{N(SiMe3)2}2]2 (0.79, 2.30 mmol) was dis-
solved in dry toluene (70 cm3) and cooled to -70 °C. A solution
of bpzcpH (2.00 g, 4.60 mmol) in toluene (30 cm3) was added
dropwise, the mixture allowed to warm up to room tempera-
ture and stirred during 16 h. The solvent was evaporated to
dryness under reduced pressure to yield a sticky yellow product.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2871
The product was recrystallized from toluene at -26 °C to give
compound 15 as white crystals. Yield: 2.15 g, 81%. Anal. Calcd
for C35H47N5Si2Mg: C, 67.99; H, 7.66; N, 11.33. Found: C,
67.71; H, 7.33; N. 11.15. 1H NMR (Toluene-d8, 298 K), δ
initiator was dissolved in the appropriate amount of solvent
and temperature equilibration was ensured by stirring the
solution for 15 min in a temperature-controlled bath. ε-CL
was injected and polymerization times were measured from
that point. Polymerizations were terminated by addition of
acetic acid (5 vol-%) in methanol. Polymers were precipitated
in methanol, filtered, dissolved in THF, reprecipitated in
methanol, and dried in vacuo to constant weight.
7.32-6.86 (m, 11H, CH, Phenyl groups), 6.61 (m, 2H, Hd
-
C5H4), 5.93 (m, 2H, Hc - C5H4), 5.32 (s, 2H, H4), 2.28 (s, 6H,
Me3), 1.60 (s, 6H, Me5), 0.08 (s, 18H, MgN(SiMe3)2). 13C-{1H}
or
NMR (Toluene-d8, 328 K),
δ
151.3, 147.2 (C3
5),
141.1-126.42 (Phenyl groups), 125.3 (Cc - C5H4), 116.13 (Cd -
C5H4), 106.2 (C4), 105.7 (Cb - C5H4), 71.2 (CH), 64.7 (Ca), 14.1
(Me3), 11.3 (Me5), 2.7 (MgN(SiMe3)2).
Polymerizations of L-lactide and rac-lactide (LA) were
performed on a Schlenk line in a flame-dried round-bot-
tomed flask equipped with a magnetic stirrer. The Schlenk
tubes were charged in the glovebox with the required amount
of rac-lactide and initiator, separately, and then attached to
the vacuum line. The initiator and monomer were dissolved
in the appropriate amount of solvent, and temperature
equilibration was ensured in both Schlenk flasks by stirring
the solutions for 15 min in a bath. The appropriate amount of
initiator was added by syringe, and polymerization times
were measured from that point. Polymerizations were
stopped by injecting a solution of acetic acid (5 vol-%) in
methanol. Polymers were precipitated in methanol, filtered,
dissolved in THF, reprecipitated in methanol, and dried in
vacuo to constant weight.
X-ray Crystallographic Structure Determination for Com-
plexes 4, 5, 7, and 10 ꢀ 0.5C7H8. A summary of crystal data
collection and refinement parameters for all compounds is given
in Table 5.
Single crystals of 4, 5, 7, and 10 ꢀ 0.5C7H8 were mounted on a
glass fiber and transferred to a Bruker X8 APEX II CCD-based
diffractometer equipped with a graphite monochromated
Mo-KR radiation source (λ = 0.71073 A). Data were inte-
grated using SAINT,45 and an absorption correction was per-
formed with the program SADABS.46 The software package
SHELXTL version 6.1247 was used for space group determina-
tion, and structure solution and refinement were carried out by
full-matrix least-squares methods based on F2. All non-hydro-
gen atoms were refined with anisotropic thermal parameters.
Hydrogen atoms were placed using a “riding model” and
included in the refinement at calculated positions. The complex
10 ꢀ 0.5C7H8 crystallizes with half a molecule of toluene in the
asymmetric unit. The toluene solvent is highly disordered about
an inversion center and was refined with soft restraints and
constraints, and their carbon atoms were refined with isotropic
displacement parameters.
Acknowledgment. We gratefully acknowledge finan-
ꢀ
cial support from the Ministerio de Educacion y Ciencia
(Direccion General de Investigacion), Spain (Grants
ꢀ
ꢀ
CTQ2008-05892/BQU, CTQ2008-00318/BQU, Consolider-
a de
Ingenio 2010 ORFEO CSD2007-00006), Consejerı
´
Educacion, Comunidad de Madrid, Spain (Grant
ꢀ
Polimerization Procedures
ꢀ
S-0505/PPQ/0328) and Consejerıa de Educacion y
´
Polymerizations of ε-caprolactone (CL) were carried out
on a Schlenk line in a flame-dried round-bottomed flask
equipped with a magnetic stirrer. In a typical procedure, the
Ciencia, Comunidad de Castilla-La Mancha, Spain
(Grant PCI08-0010).
Supporting Information Available: Full crystallographic data
for 4, 5, 7, and 10. This material is available free of charge via the
Internet at http://pubs.acs.org.
(45) SAINTþ v7.12a, Area-Detector Integration Program; Bruker-Nonius
AXS: Madison, WI, 2004.
(46) Sheldrick, G. M. SADABS version 2004/1, A Program for Empirical
€
€
Absorption Correction; University of Gottingen: Gottingen, Germany, 2004.
(47) SHELXTL-NT version 6.12, Structure Determination Package;
Bruker-Nonius AXS: Madison, WI, 2001.
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(48) Barakat, I.; Dubois, Ph.; Jerome, R.; Teyssie, Ph. J. Polym. Sci.,
Part A: Polym. Chem. 1993, 31, 505–514.