A family of 1,1,3,3-tetraalkylguanidine (H-TAG) solvated zinc aryloxide precatalysts for the ring-opening polymerization of rac-Lactide

Article abstract of DOI:10.1002/ejic.200900986

Reaction of [Zn(μ-TAG){N(SiMe₃)₂}]₂ {TAG = N=C[N(CH₂CH₃)₂N(CH₃) ₂] (DEDMG), N=C{[NCH₂CH₂CH₂C ^dH₂(N-C^d)]N(CH₂CH₃) ₂} (DEPYRG) and N=C{[NCH₂CH₂CH ₂CH₂C^e H₂(N-C^e)]N(CH ₂CH₃)₂} (DEPIPG)} with 2 equiv. of ethanol (EtOH) and 2 equiv. of HOAr {OAr = OC₆H₃(CMe ₃)-2-(CH₃)-6 (BMP) or OC₆H ₂[C(CH₃)₃]₂-2,6-(CH₃)-4 (4MeDBP)} results in dizinc alkoxides with the general formula [Zn(μ-OEt)-(OAr)(H-TAG)]₂ (1-3). Et₂Zn was additionally treated with 2 equiv. of 1,1,3,3-tetramethylguanidine (H-TMG) and H-BMP or HOC₆H₃(C₆H₅)₂-2,6 to yield [Zn(BMP)₂(H-TMG)₂] (4) and [Zn{OC₆H ₃(C₆H₅)₂-2,6}₂(H-TMG) ₂] (5). Complexes 1, 2, 4, and 5 were characterized by single-crystal X-ray diffraction. Polymerization of rac-lactide with 1-5 and [Zn(μ-OMe)(4MeDBP)(H-TMG)]₂ (6) were found to generate polylactide (PLA). The bulk powders for all complexes were found to be in agreement with the crystal structures based on elemental analyses, FTIR spectroscopy, and ¹H and ¹³C NMR spectroscopic studies.

Full text of DOI:10.1002/ejic.200900986

FULL PAPER
DOI: 10.1002/ejic.200900986
A Family of 1,1,3,3-Tetraalkylguanidine (H-TAG) Solvated Zinc Aryloxide
Precatalysts for the Ring-Opening Polymerization of rac-Lactide
Julia J. Ng,[a] Christopher B. Durr,[a] Jacob M. Lance,[a] and Scott D. Bunge*[a]
Keywords: Guanidine / Polymerization / Zinc / Catalysts / Aryl oxide
Reaction of [Zn(µ-TAG){N(SiMe3)2}]2 {TAG = N=C[N(CH2-
BMP or HOC6H3(C6H5)2-2,6 to yield [Zn(BMP)2(H-TMG)2]
CH3)2N(CH3)2] (DEDMG), N=C{[NCH2CH2CH2CdH2(N–Cd)]- (4) and [Zn{OC6H3(C6H5)2-2,6}2(H-TMG)2] (5). Complexes 1,
N(CH2CH3)2} (DEPYRG) and N=C{[NCH2CH2CH2CH2Ce
H2(N–Ce)]N(CH2CH3)2} (DEPIPG)} with 2 equiv. of ethanol
(EtOH) and 2 equiv. of HOAr {OAr = OC6H3(CMe3)-2-(CH3)-
6 (BMP) or OC6H2[C(CH3)3]2-2,6-(CH3)-4 (4MeDBP)} results
in dizinc alkoxides with the general formula [Zn(µ-OEt)-
(OAr)(H-TAG)]2 (1–3). Et2Zn was additionally treated with
2 equiv. of 1,1,3,3-tetramethylguanidine (H-TMG) and H-
2, 4, and 5 were characterized by single-crystal X-ray diffrac-
tion. Polymerization of rac-lactide with 1–5 and [Zn(µ-OMe)-
(4MeDBP)(H-TMG)]2 (6) were found to generate polylactide
(PLA). The bulk powders for all complexes were found to be
in agreement with the crystal structures based on elemental
analyses, FTIR spectroscopy, and 1H and 13C NMR spectro-
scopic studies.
Ideally, to shape the microenvironment, chelating multiden-
tate ligands are used to generate dinuclear and mononuclear
Introduction
Lactide (LA), the cyclic dimer formed by condensation compounds with a single reactive site.[9–11] This is a difficult
of lactic acid, is an efficient precursor for the production of challenge with respect to designing Zn systems, where the
the biodegradable and recyclable polymer, polylactide d10 shell imparts a constrained coordination geome-
(PLA).[1,2] The synthesis of PLA poses environmental bene- try.[6,10–13] To overcome this challenge, the use of bulky aryl-
fits, and thus its use for various packaging applications has oxide ligands in conjunction with an auxiliary Lewis base
been industrially pursued. Additionally, interest in this has been proven useful.[11,14] Recently, we reported a de-
commodity has expanded to biomedical applications such tailed effort to outline the stoichiometric reactivity of het-
as medical implants and drug delivery systems.[3] The physi- ero-ligated zinc systems involving aryloxide and guanidine
cal properties of PLA are often compared to polystyrene; ligand sets.[15] In this previous investigation, the use of the
it has similar modulus and strength.[4] Notably, the crystal- bulky aryloxide ligand 4MeDBP, in conjunction with
linity of this renewable polymer is linked to the stereochem- 1,1,3,3-tetramethylguanidine (H-TMG) was found to assist
istry of the monomer units. Isotactic PLA is a crystalline in producing a well-defined metal aryloxide with a potential
thermoplastic with a relatively high glass transition tem- single site for reactions. The complexes were subsequently
perature, Tg, whereas atactic PLA is an amorphous mate- used in the ring-opening polymerization (ROP) of rac-LA
rial.[1]
One method for producing PLA is through a metal alk-
and exhibited only modest stereoselectivity.[15]
It was therefore of interest to examine the effect that vari-
ation in the OAr/H-TAG ligand set (Figure 1) might have
on the resultant stereoselectivity of the Zn-catalyzed ROP.
Toward this end, the synthesis of three dinuclear 1,1,3,3-
tetraalkylguanidine solvated zinc complexes with the gene-
ral formula [Zn(µ-OEt)(OAr)(H-TAG)]2 (1–3) has been per-
formed and is reported herein. Additionally, in an attempt
to examine the possibility of isolating alternative H-TMG
solvated zinc aryloxides, reactions of Et2Zn with EtOH, H-
TMG and HOC6H3(CMe3)-2-(CH3)-6 (H-BMP) or
HOC6H3(C6H5)2-2,6 were examined. However, instead of
isolating dinuclear ethoxide complexes, two monomeric
four-coordinate Zn aryloxide complexes were isolated,
[Zn(BMP)2(H-TMG)2] (4) and [Zn{OC6H3(C6H5)2-2,6}2-
(H-TMG)2] (5).
oxide catalyzed ring-opening polymerization (ROP).[5] Lith-
ium, aluminum, titanium, zirconium, zinc and yttrium have
all been proven effective as catalysts for this process.[6] The
desire to produce PLA with tailorable physical characteris-
tics from rac-LA has led to the development of single-site
precatalysts for the stereocontrolled ROP.[7,8] Achieving de-
sired characteristics is leveraged through controlled con-
struction of the ligand architecture around the metal center.
[a] Department of Chemistry, Kent State University,
Kent, OH 44242, USA
Fax: +1-330-6723816
E-mail: sbunge@kent.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900986.
1424
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 1424–1430

Precatalysts for the Ring-Opening Polymerization of rac-Lactide
(H-TAG)]2.[19] It therefore seemed reasonable to utilize this
approach for the synthesis of additional H-TAG solvated
Zn aryloxides.
Due to a lack of commercial availability, a structurally
diverse series of lithium 1,1,3,3-tetraalkylguanidinato [Li-
(TAG)] complexes are obtained by the addition of a lithium
dialkylamide to a dialkylcyanamide. Derivatives of Li-
(TAG) may be subsequently used in conjunction with
Li[N(SiMe3)2] and ZnCl2 to generate the correspond-
ing hetero-ligated Zn(TAG) complex [Zn(µ-TAG){N-
(SiMe3)2}]2.[20]
Upon successful isolation of [Zn(µ-TAG){N(SiMe3)2}]2,
the synthesis of 1–3 may be performed as shown in
Scheme 1; 1 equiv. of [Zn(µ-TAG){N(SiMe3)2}]2 is added to
hexanes solution containing 2 equiv. of EtOH and HOAr
(H-BMP or H-4MeDBP); 2 equiv. of HN(SiMe3)2 are pre-
sumably eliminated, serving to drive the reaction forward
and result in the rapid precipitation of a powder. The pow-
der was re-dissolved by dropwise addition of thf and heat-
ing. Crystals of 1 and 2, suitable for X-ray structural analy-
sis, were obtained by concentrating the solutions and then
cooling the respective saturated solutions to –35 °C for
24 h. Microcrystalline solid of 3 was obtained according to
this approach, and attempts to generate larger single crys-
tals were unsuccessful. The isolated solids of 1–3 were
slightly air-sensitive and soluble in hexanes, toluene, diethyl
ether, and thf.
Figure 1. 1,1,3,3-Tetraalkylguanidine (H-TAG) ligands.
Single-crystal X-ray diffraction, elemental analysis,
1
FTIR, H and 13C NMR spectroscopy were performed to
characterize the guanidine solvated zinc complexes. To as-
certain their utility as precatalysts, complexes 1–5 and the
previously reported [Zn(µ-OMe)(4MeDBP)(H-TMG)]2 (6)
were subsequently treated with excess rac-LA. 1H NMR
spectroscopy was utilized to examine the resultant tacticity
of the PLA.
Results and Discussion
Synthesis
In an attempt to examine the possibility of isolating al-
ternative H-TMG solvated Zn aryloxides, reactions similar
to those that generated complexes 1–3 were also performed
Previous investigations have demonstrated that the OAr/ with Et2Zn, EtOH, H-BMP and HOC6H3(C6H5)2-2,6.
H-TAG ligand set exists as a robust system for isolating However, instead of isolating dinuclear complexes, two mo-
main group, d-block, lanthanide, and actinide complexes nomeric complexes were serendipitously isolated,
with low coordination numbers and discrete reactivity.[15–18] [Zn(BMP)2(H-TMG)2] (4) and [Zn{OC6H3(C6H5)2-2,6}2-
The driving force for the synthesis of these complexes has (H-TMG)2] (5). Upon determination of the structures of 4
typically been the elimination of a volatile alkane or amine. and 5, the complexes were synthesized with the appropriate
In a recent report, the reaction of [Mn(µ-TAG){N- stoichiometry, and complete characterization was per-
(SiMe3)2}]2 with EtOH and HOC6H3(CMe3)2-2,6 (H-DBP) formed (Scheme 2). Crystals suitable for X-ray structural
resulted in retention of the protonated H-TAG and the for- analysis were obtained by slow concentration of solutions
mation of a dinuclear MnII alkoxide, [Mn(µ-OEt)(DBP)- in hexanes/thf (1:1). Apparently, there is a steric require-
Scheme 1. Synthesis of 1–3.
Eur. J. Inorg. Chem. 2010, 1424–1430
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1425

J. J. Ng, C. B. Durr, J. M. Lance, S. D. Bunge
FULL PAPER
Scheme 2. Synthesis of 4 and 5.
ment for the use of a “one-pot” approach involving the
OAr/H-TMG ligand set. This requirement was also found
for similar Mg systems.[16]
Structural Descriptions
The structures of compounds 1, 2, 4, and 5 are illustrated
by the thermal ellipsoid plots depicted in Figures 2, 3, 4,
and 5. The data collection parameters are presented in
Table 1.
Complexes 1 and 2 crystallized in the triclinic and mono-
clinic space groups, respectively. Each complex contains a
planar Zn2O2 core with a distorted tetrahedral Zn atom (τ4
= 0.8) connected to an adjacent Zn atom by two bridging
ethoxide ligands.[21] The Zn···Zn distances are ca. 3 Å for
both compounds. The tetrahedral coordination is fulfilled
through additional coordination of the Zn atom to one ter-
minal aryloxide ligand and one terminal H-TAG ligand. In
both complexes, the Zn–O distances fall within the expected
Figure 3. Thermal ellipsoid plot of 2. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity. Selected in-
teratomic distances [Å] and angles [°]: Zn(1)–O(1) 1.930(3), Zn(1)–
O(2) 1.963(3), Zn(1)–N(1) 2.000(4), O(1)–C(1) 1.339(6), O(2)–C(16)
1.432(8); O(1)–Zn(1)–O(2) 125.61(19), O(1)–Zn(1)–N(1) 102.0(2),
C(1)–O(1)–Zn(1) 120.1(3), O(2)–Zn(1)–N(1) 111.38(19), C(12)–
O(2)–Zn(1) 127.4(4), C(14)–N(1)–Zn(1) 137.0(5).
range of 1.928–1.978 Å, whereas the terminal Zn–N dis-
tances are ca. 2.0 Å.[22,23] A distinguishing feature of both
compounds is the terminal Zn–O–Ar angles ranging from
120 to 138°. The relatively small Zn–O–Ar angle indicates
the absence of π-donation from the aryl ring; a structural
feature common in Zn aryloxide compounds.[19] Com-
pounds 1 and 2 are similar to the previously reported
zinc ethoxide, [Zn(µ-OEt)(OAr)(H-TMG)]2 (OAr
=
4MeDBP).[15] The C=N bond lengths for the H-TAG in 1
and 2 vary from 1.309 to 1.320 Å, whereas the C–N bonds
are longer, ranging from 1.334 to 1.369 Å.
Compounds 4 and 5 contain a distorted tetrahedral zinc
core coordinated to two H-TMG ligands and two aryloxide
groups (τ4 = 0.83 for 4 and 0.90 for 5).[21] The Zn–O dis-
Figure 2. Thermal ellipsoid plot of 1. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity. Selected in-
teratomic distances [Å] and angles [°]: Zn(1)–O(1) 1.928(4), Zn(1)–
O(2) 1.960(5), Zn(1)–N(1) 1.974(5); O(1)–Zn(1)–O(2) 125.61(19),
O(1)–Zn(1)–N(1) 102.0(2), C(1)–O(1)–Zn(1) 120.1(3), C(2)–Zn(1)–
N(1) 111.38(19), C(12)–O(2)–Zn(1) 127.4(4), C(14)–N(1)–Zn(1)
137.0(5).
tances are comparable to [Zn(DBP)2(thf)2] [Zn–O(avg.)
=
1.87 Å].[23] The Zn–O(avg.) distance is 1.942 Å for 4 and
1.932 Å for 5. Both complexes are quite similar to the pre-
viously reported magnesium complexes, [Mg(BMP)2(H-
TMG)2] (τ4 = 0.85) and [Mg{OC6H3(C6H5)2-2,6}2(H-
1426
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 1424–1430

Precatalysts for the Ring-Opening Polymerization of rac-Lactide
Figure 5. Thermal ellipsoid plot of 5. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity. Selected in-
teratomic distances [Å] and angles [°]: Zn(1)–O(2) 1.9287(19),
Zn(1)–O(1) 1.934(2), Zn(1)–N(1) 1.998(3), Zn(1)–N(2) 2.021(3);
O(2)–Zn(1)–O(1) 116.67(9), O(2)–Zn(1)–N(1) 102.01(9), O(1)–
Zn(1)–N(1) 114.43(8), O(2)–Zn(1)–N(2) 112.10(11), O(1)–Zn(1)–
N(2) 96.50(11), N(1)–Zn(1)–N(2) 116.30(11), C(11)–O(2)–Zn(1)
133.31(19) C(29)–O(1)–Zn(1) 133.9(2).
Figure 4. Thermal ellipsoid plot of 4. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity. Selected in-
teratomic distances [Å] and angles [°]: Zn(1)–O(1) 1.9465(18),
Zn(1)–O(2) 1.9372(18), Zn(1)–N(1) 2.015(2), Zn(1)–N(2) 2.018(2);
O(2)–Zn(1)–O(1) 124.90(8), O(2)–Zn(1)–N(1) 94.36(9), O(1)–
Zn(1)–N(1) 114.43(8), O(2)–Zn(1)–N(2) 118.51(9), O(1)–Zn(1)–
N(2) 91.48(8), N(1)–Zn(1)–N(2) 114.94(9), C(22)–O(1)–Zn(1)
139.35(18), C(11)–O(2)–Zn(1) 135.90(18).
Spectroscopy
Crystals of 1–5 were dried in vacuo, and their bulk pow-
ders were isolated for spectroscopic analysis. Compounds
1–3 were found to be sparingly soluble in toluene, and com-
TMG)2] (τ4 = 0.88).[16] Notably, a structurally similar pounds 4 and 5 were soluble in chloroform. All compounds
cadmium complex, [Cd{µ-OC6H3(C6H5)2-2,6}{OC6H3- exhibited the expected 1H and 13C NMR spectra. In the 1H
(C6H5)2-2,6}]2, was reported by Darensbourg and co- NMR spectrum, a broad singlet corresponding to the N–H
workers.[24] In the Cd structure, the phenyl substituent on hydrogen atom was found for each compound and ranged
the diphenylphenoxide group was shown to interact with from δ = 3.7 to 5.0 ppm. Several resonances between δ =
the Cd center. The Cd–C bond was reported to be ca. 2.6 Å. 3.7 and 1.0 ppm are assigned to the alkyl substituents of
In 6, the closest Zn–C distance is ca. 3.0 Å and thus does the guanidine ligands. The existence of multiple resonances
not indicate an interaction between the phenyl moiety and for each alkyl substituent of the H-TAG moiety may be
the Zn center.
tentatively attributed to the potential zwitterionic resonance
Table 1. Data collection parameters for compounds 1, 2, 4, and 5.
1
2
4
5
Empirical formula
Formula mass
Temperature [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C40H74N6O4Zn2
833.79
100(2)
C52H94N6O4Zn2
998.07
100(2)
monoclinic
P21/n
9.307(2)
C32H56N6O2Zn
622.20
160(2)
orthorhombic
Pbca
16.967(3)
16.548(3)
24.687(4)
C50H60N6O3Zn
858.41
160(2)
monoclinic
P21/n
10.4637(12)
12.0936(14)
38.482 (4)
triclinic
¯
P1
9.027(9)
9.629(10)
14.338(15)
72.276(13)
74.194(15)
74.248(14)
1118(2)
1
18.965(4)
16.150(4)
104.375(3)
93.923 (2)
γ [°]
V [Å3]
2761.3(10)
2
1.200
0.914
4886
6931(2)
8
1.193
0.744
6138
4858.3(10)
4
1.174
0.551
8602
Z
Dcalcd. [Mg/m3]
µ (Mo-Kα) [mm–1]
Number of reflections
obsd.
1.239
1.116
3915
R(int)
0.0899
1.099
7.73 (13.20)
17.29 (21.60)
0.0689
1.025
5.92 (10.29)
15.51 (18.82)
0.0476
1.069
3.76 (6.09)
11.95 (14.71)
0.0871
0.930
4.96 (7.78)
14.43 (15.81)
GOF on F2
[a]
R1 (%) (all data)
[b]
wR2 (%) (all data)
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|ϫ100. [b] wR2 = [Σw(Fo – Fc ) /Σ(w |Fo|2)2]1/2 ϫ100.
2
2 2
Eur. J. Inorg. Chem. 2010, 1424–1430
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1427

J. J. Ng, C. B. Durr, J. M. Lance, S. D. Bunge
FULL PAPER
of the ligand. Rotation about the C–N bond results in the range from 1180 to 6803. Overall, the tacticity is not signifi-
possibility of both isomers present in solution. The central cantly influenced by the substituents of the guanidine do-
carbon atom “CN3” of the H-TAG ligand was confirmed nor. Interestingly, variation in the substituents on the ary-
through the presence of a weak downfield peak ranging loxide ring also does not significantly result in altering the
from δ = 168 to 161 ppm in the 13C NMR spectrum.[25]
tacticity. This differs with results reported for salen ligand
The FTIR spectra of 1–5 exhibited an absence of sets, where dramatic differences in stereocontrol have been
stretches associated with –OH ligands, indicative of com- found through alternation of the aryloxide architec-
plete substitution. The expected alkyl and aryl stretches for ture.[13,27]
the aryloxide, alkoxide and guanidine ligands are present
for each sample. In 1, 4 and 5 a characteristic stretch at
Table 2. Polymerization data for 1–6.
750 cm–1 may be assigned to the γ(CH) vibration of the
Conversion[a]
[%]
Mn
Pr/Pm
[a]
[b]
Catalyst
Time
[h]
OAr ring. This has been observed previously in the spectra
of lanthanide aryloxide systems.[17,26] For 1–5, the presence
of ν(N–H) and ν(C=N) were confirmed by peaks around
3300 and 1580 cm–1, respectively.[25] Assignment of the Zn–
O bands in aryloxides is often difficult owing to the cou-
pling of the C–O and Zn–O modes.[23] Comparison of the
data for 1–5 with data for HOAr indicates that bands lo-
cated between 540 and 450 cm–1 are most likely associated
with the Zn–O bonds.
1
2
3
4
5
6
18
18
18
18
18
18
99
100
100
99
97
99
4394
1180
4391
1412
6803
1844
0.39:0.61
0.40:0.60
0.34:0.66
0.37:0.63
0.47:0.53
0.42:0.58
1
[a] Conversion and Mn determined by H NMR spectroscopy. [b]
Pr and Pm are the probability of hetero- and isotactic enchainment
1
based on H NMR spectroscopy. [LA]/[Zn] = 100, toluene, 25 °C.
In a recent report examining guanidine solvated lantha-
nide aryloxides, disassociation of H-TMG was found to be
possible under catalytic conditions of increased temperature
and excess substrate.[17] As shown in Table 2, 4 and 5 were
successful in initiating the ROP of rac-LA. Presumably, due
to the steric constraints around the Zn center of 4 and 5,
de-solvation of H-TMG is a necessary prerequisite prior to
coordination of lactide. This indicates that OAr/H-TAG is
perhaps not a robust system for tailoring the microenviron-
ment of a Zn center. In support of this finding, in a few
cases, complexes that were transferred from cold solution
and placed under vacuum hindered obtaining satisfactory
elemental analysis. This is consistent with possible de-sol-
vation of H-TAG.
rac-Lactide Polymerization
An ideal ROP polymerization catalyst would be expected
to exhibit high activity, generate a defined molecular
weight, provide a polymer with a narrow molecular weight
distribution, and demonstrate stereoselectivity in the poly-
merization of a stereochemically diverse set of cyclic es-
ters.[6] Although this combination of characteristics has
been partially realized through the use of single-site alumi-
num alkoxides, such conversions are generally attained only
at elevated temperatures and over the course of days.[8,27]
Therefore, notwithstanding these reports, understanding
the origin of stereocontrol and high activity in LA polyme-
rizations by metal alkoxides is necessary and improvements
in catalyst versatility warranted.
Although less versatile from the standpoint of tacticity,
Zn complexes have generally shown increased activity. As
part of an ongoing effort to address stereocontrol issues,
we have synthesized Zn complexes 1–6 with coordinated H-
TAG, a unique ligand set with readily tuned steric and elec-
Conclusions
Five Zn aryloxide complexes have been synthesized and
tronic properties. The ROP polymerization of rac-LA was structurally characterized by a facile “one-pot” synthetic
carried out by dissolving 0.025 mmol of Zn complex in ca. approach. The straightforward reaction of a set of amido
10 mL of toluene. After adding 100 equiv. of rac-LA, the Zn guanidinate complexes with H-4-MeDBP or H-BMP
reaction mixture was stirred at 25 °C for 18 h. The solvent and EtOH results in the liberation of HN(SiMe3)2, reten-
was removed under vacuum and PLA was isolated. Mono- tion of the coordinated H-TAG, and the formation of dinu-
1
mer conversion was monitored by H NMR spectroscopy clear Zn ethoxides 1–3. The two monomeric H-TMG sol-
by integrating the relative intensities of the methine reso- vated complexes 4 and 5 were also synthesized by using H-
nances attributable to the monomer and polymer; 1H- BMP or 2,6-diphenylphenol. By utilizing complexes 1–3
homonuclear-decoupled spectra were acquired in order to and 6, the ROP of rac-lactide was performed, and the re-
determine the tacticity.
sultant PLA was found to exhibit a slight isotactic bias.
The results for complexes 1–6 are summarized in Table 2. Notably, alternation of the ligand set did not impart signifi-
Based upon previous reports, a proposed reaction scheme cant variation in stereochemical control. In addition, the
for the formation of PLA with 1–3 and 6 is shown in Fig- coordinatively saturated monomeric Zn complexes 4 and 5
ure S3 (Supporting Information).[15] An examination of Pr/ also successfully polymerized rac-lactide, indicative of pos-
Pm reveals a slight bias toward isotactic enchainment for sible H-TAG labiality and a potential drawback in using
complexes 1–4 and 6 (66% for 3).[28] The MW(avg.) values this combination of ligands.
1428
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 1424–1430

Precatalysts for the Ring-Opening Polymerization of rac-Lactide
CH3), 34.8 {HN=C[N(CH2CH3)2N(C4H8)]}, 34.2 {H=C[N(CH2-
CH3)2N(C4H8)]}, 32.1 {OC6H2[C(CH3)3]2-2,6-(CH3)-4}, 31.8
{OC6H2[C(CH3)3]2-2,6-(CH3)-4}, 30.3 {HN=C[N(CH2CH3)2N-
(C4H8)]}, 22.9 {OC6H2[C(CH3)3]2-2,6-(CH3)-4}, 21.2 (OCH2CH3),
Experimental Section
General Considerations: All syntheses were handled with rigorous
exclusion of water and air by using standard glove-box techniques.
All anhydrous solvents were stored under argon and used as re-
ceived in sure-seal bottles. The following chemicals were acquired
from commercial suppliers: LiNEt2, LiNMe2, LiN(SiMe3)2, nBuLi
(1.6  in hexanes), diethylcyanamide, pyrrolidine, ZnCl2, (Et)2Zn
(1.0  in hexanes), H-TMG, EtOH, MeOH, H-BMP, H-4MeDBP,
and HOC6H3(C6H5)2-2,6. [Zn(µ-DEDMG){N(SiMe3)2}]2, [Zn(µ-
DEPYRG){N(SiMe3)2}]2, [Zn(µ-DEPIPG){N(SiMe3)2}]2, and
[Zn(µ-OMe)(4MeDBP)(H-TMG)]2 (6) were synthesized as pre-
viously reported in the literature.[15,20] FTIR data were obtained
with a Bruker Tensor 27 instrument by using KBr pellets under
flowing nitrogen. All NMR spectroscopy samples were prepared
from synthesized compounds under argon and dissolved in either
[D8]toluene (1–3) or CDCl3 (4–5). All solution spectra were ob-
14.1 {HN=C[N(CH CH ) N(C H )]} ppm. FTIR (KBr): ν = 3363
˜
2
3 2
4
8
(w), 3304 (w), 2960 (s), 2872 (s), 2720 (w), 2181 (w), 2108 (m), 1742
(w), 1558 (s), 1522 (s), 1443 (s), 1420 (s), 1382 (s), 1358 (m), 1315
(w), 1264 (m), 1230 (m), 1215 (m), 1199 (w), 1160 (w), 1103 (m),
1059 (s), 932 (w), 919 (w), 887 (m), 860 (m), 838 (w), 821 (w), 803
(w), 793 (w), 774 (w), 754 (w), 576 (w), 525 (s) cm–1.
C52H94N6O4Zn2 (998.07): calcd. C 55.00, H 8.53, N 11.66; found
C 57.02, H 8.67, N 7.42.
[Zn(µ-OEt)(4MeDBP)(DEPIPG)]2 (3): From [Zn(µ-DEPIPG)-
{N(SiMe3)2}]2 (0.11 g, 0.14 mmol), H-4MeDBP (0.06 g, 0.3 mmol),
and EtOH (0.01 g, 0.3 mmol). Yield 0.106 g (76.3%). 1H NMR
(400 MHz, [D8]toluene): δ = 7.00 {s, 4 H, OC6H2[C(CH3)3]2-2,6-
(CH3)-4}, 4.77 [s, 2 H, HN=CN(CH2CH3)2N(C5H10)], 3.33 {t, 20
H, HN=C[N(CH2CH3)2N(C5H10)]}, 2.22 {s, 6 H, OC6H2[C-
(CH3)3]2-2,6-(CH3)-4}, 1.79 {t, 12 H, HN=C[N(CH2CH3)2N-
(C5H10)]}, 1.35 {s, 36 H, OC6H2[C(CH3)3]2-2,6-(CH3)-4}, 1.07 (t, 6
H, 12 H, OCH2CH3), 0.79 {t, 12 H, HN=C[N(CH2CH3)2N-
(C5H10)]} ppm. 13C{1H} NMR (100.5 MHz, [D8]toluene): δ =
166.8 {HN=C[N(CH2CH3)2N(C5H10)]}, 136.4, 128.8, 127.9, 126.2,
{OC6H2[C(CH3)3]2-2,6-(CH3)-4}, 36.2 (OCH2CH3), 34.7 {HN=C-
[N(CH2CH3)2N(C5H10)]}, 30.9 {HN=C[N(CH2CH3)2N(C5H10)]},
26.5 {OC6H2[C(CH3)3]2-2,6-(CH3)-4}, 25.2 {OC6H2[C(CH3)3]2-2,6-
tained with
a Bruker DRX400 spectrometer at 400.1 and
100.5 MHz for 1H and 13C NMR spectroscopy experiments, respec-
tively.
Synthesis of 1, 2, and 3: [Zn(µ-TAG){N(SiMe3)}2]2, was dissolved
in hexanes, and 2 equiv. of EtOH and H-OAr (H-BMP or H-
4MeDBP) were added; thf was added to dissolve the precipitate
that formed. After evaporation of the volatile components from the
reaction mixture over 24 h, colorless crystals of 1, 2, and 3 were
isolated.
[Zn(µ-OEt)(BMP)(DEDMG)]2 (1): From [Zn(µ-DEDMG)- (CH3)-4}, 23.5 {HN=C[N(CH2CH3)2N(C5H10)]}, 21.8 {OC6H2-
{N(SiMe3)2}]2 (0.25 g, 0.34 mmol), H-BMP (0.11 g, 0.68 mmol), [C(CH3)3]2-2,6-(CH3)-4}, 14.8 {HN=C[N(CH2CH3)2N(C5H10)]},
and EtOH (0.03 g, 0.7 mmol). Yield 0.116 g (41%). M.p. 118 °C.
13.6 (OCH2CH3) ppm. FTIR (KBr): ν˜ = 3308 (w), 2956 (s), 2869
(s), 1638 (m), 1560 (s), 1515 (s), 1443 (s), 1382 (m), 1361 (m), 1316
1H NMR (400 MHz, [D8]toluene):
δ
=
7.45 {d, H,
2
OC6H3[C(CH3)3]-2-(CH3)-6}, 7.23 {m, 2 H, OC6H3[C(CH3)3]-2- (w), 1246 (m), 1215 (w), 1196 (w), 1159 (m), 1122 (m), 1105 (m),
(CH3)-6}, 6.77 {d, 2 H, OC6H3[C(CH3)3]-2-(CH3)-6}, 4.48 {s, 6 H, 1060 (w), 1029 (w), 887 (w), 860 (m), 819 (m), 804 (w), 774 (w)
OC6H3[C(CH3)3]-2-(CH3)-6}, 4.08 {s, 2 H, HN=C[N(CH3)2][N- cm–1.
(CH2CH3)2]}, 3.60 (d, 4 H, OCH2CH3), 2.83 {q, 8 H, HN=C-
[N(CH3)2][N(CH2CH3)2]}, 2.55 {s, 12 H, HN=C[N(CH3)2][N-
(CH2CH3)2]}, 1.81 {s, 18 H, OC6H3[C(CH3)3]-2-(CH3)-6}, 1.50 (t,
Synthesis of 4 and 5: To a solution of 2 equiv. each of H-TMG and
H-OAr [H-BMP or HO-C6H3(C6H5)2-2,6] in hexanes was added
1 equiv. of (Et)2Zn. The resulting precipitate was dissolved in thf.
After evaporation of the volatile components from the reaction
6 H, OCH2CH3), 0.78 {t, 12 H, HN=C[N(CH3)2][N(CH2CH3)2]}
ppm. 13C{1H} NMR (100.5 MHz, [D8]toluene): δ = 167.7 {HN=C-
mixture over 24 h, colorless crystals of 4 and 5 were isolated.
[N(CH3)2][N(CH2CH3)2]}, 142.2, 138.7, 138.1, 137.5, 126.0, 120.1
[Zn(BMP)2(H-TMG)2] (4): From (Et)2Zn (1.24 g, 1.5 mmol), H-
TMG (0.35 g, 3.0 mmol), and H-BMP (0.50 g, 3.0 mmol). Yield
0.612 g (57.5%). M.p. 202 °C. 1H NMR (400 MHz, CDCl3): δ =
{OC6H3[C(CH3)3]-2-(CH3)-6}, 68.0 (OCH2CH3), 39.9 {HN=C-
[N(CH3)2][N(CH2CH3)2]}, 35.9 {HN=C[N(CH3)2][N(CH2CH3)2]},
31.2 {OC6H3[C(CH3)3]-2-(CH3)-6}, 30.9 {OC6H3[C(CH3)3]-2-
(CH3)-6}, 26.2 (OCH2CH3), 19.4 {HN=C[N(CH3)2][N(CH2-
7.28 {m,
6 H, OC6H3[C(CH3)3]-2-CH3-6}, 5.00 {s, 2 H,
HN=C[N(CH3)2]2}, 2.63 {s, 6 H, OC6H3[C(CH3)3]-2-CH3-6}, 2.25
{s, 18 H, OC6H3[C(CH3)3]-2-CH3-6}, 1.88 {s, 24 H, HN=C[N-
(CH3)2]2} ppm. 13C{1H} NMR (100.5 MHz, CDCl3): δ = 168.25
{HN=C[N(CH3)2]2}, 165.32, 154.52, 138.41, 128.93, 120.83, 116.44
{OC6H3[C(CH3)3]-2-(CH3)-6}, 41.56 {HN=C[N(CH3)2]2}, 39.54
{OC6H3[C(CH3)3]-2-(CH3)-6}, 38.44 {OC6H3[C(CH3)3]-2-(CH3)-
CH ) ]}, 12.6 {OC H [C(CH ) ]-2-(CH )-6} ppm. FTIR (KBr): ν
˜
3 2
6
3
3 3
3
= 3345 (w), 2961 (s), 2130 (w), 1577 (s), 1526 (m), 1459 (m), 1416
(s), 1381 (m), 1269 (m), 1251 (m), 1183 (w), 1126 (m), 1098 (m),
1060 (m), 1005 (w), 933 (m), 885 (m), 849 (m), 798 (w), 748 (m),
668 (w), 656 (w), 535 (w), 505 (w), 441 (w) cm–1. C40H74N6O4Zn2
(833.79): calcd. C 57.62, H 8.95, N 10.08; found C 56.17, H 8.64,
N 9.41.
6}, 31.49 {OC H [C(CH ) ]-2-(CH )-6} ppm. FTIR (KBr): ν =
˜
6
3
3 3
3
3371 (s), 2947 (s), 2806 (m), 1577 (s), 1545 (s), 1458 (m), 1417 (s),
1341 (w), 1279 (m), 1225 (s), 1197 (w), 1128 (m), 1096 (m), 1066
(m), 1034 (w), 996 (w), 903 (m), 861 (s), 797 (w), 748 (m), 721 (m),
661 (w), 559 (w), 531 (w) cm–1. C32H56N6O2Zn (622.20): calcd. C
61.77, H 9.07, N 13.51; found C 61.57, H 9.38, N 14.50.
[Zn(µ-OEt)(4MeDBP)(DEPYRG)]2 (2): From [Zn(µ-DEPYRG)-
{N(SiMe3)2}]2 (0.10 g, 0.13 mmol), H-4MeDBP (0.06 g, 0.3 mmol),
and EtOH (0.01 g, 0.3 mmol). Yield 0.037 g (29%). M.p. 162 °C.
1H NMR (400 MHz, [D8]toluene): δ = 7.02 {s, 4 H, OC6H2[C-
(CH3)3]2-2,6-(CH3)-4}, 4.92 [s, 2 H, HN=CN(CH2CH3)2N(C4H8)],
4.13 (t, 4 H, OCH2CH3), 3.17 [q, 8 H, HN=CN(CH2CH3)2-
[Zn{OC6H3(C6H5)2-2,6}2(H-TMG)2]·thf (5): From (Et)2Zn (0.73 g,
N(C4H8)], 2.66 [t, 8 H, HN=CN(CH2CH3)2N(C4H8)], 2.41 {s, 6 H, 1.0 mmol), H-TMG (0.23 g, 2.0 mmol), and HOC6H3(C6H5)2-2,6
OC6H2[C(CH3)3]2-2,6-(CH3)-4}, 2.24 (m, 4 H, OCH2CH3), 1.76 [m, (0.50 g, 2.0 mmol). Yield 0.77 g (98%). M.p. 213 °C. 1H NMR
8 H, HN=CN(CH2CH3)2N(C4H8)], 1.38 {s, 32 H, OC6H2[C-
(400 MHz, CDCl3): δ = 8.09 [m, 10 H, OC6H3(C6H5)2-2,6], 7.49,
(CH3)3]2-2,6-(CH3)-4}, 1.06 (q, 6 H, OCH2CH3), 0.79 [m, 6 H, 6.85 [m, 6 H, OC6H3(C6H5)2-2,6], 7.41 [m, 10 H, OC6H3(C6H5)2-
HN=CN(CH2CH3)2N(C4H8)] ppm. 13C{1H} NMR (100.5 MHz,
2,6], 3.51 {s, 2 H, HN=C[N(CH3)2]2}, 2.07 {s, 24 H,
[D8]toluene): δ = 161.4 {HN=C[N(CH2CH3)2N(C4H8)]}, 136.0, HN=C[N(CH3)2]2} ppm. 13C{1H} NMR (100.5 MHz, CDCl3): δ =
128.9, 128.6, 125.6 [OC6H2[C(CH3)3]2-2,6-(CH3)-4}, 35.5 (OCH2- 163.88 {HN=CN[(CH3)2]2}, 149.90, 133.03, 131.00, 130.34, 129.39,
Eur. J. Inorg. Chem. 2010, 1424–1430
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1429

J. J. Ng, C. B. Durr, J. M. Lance, S. D. Bunge
FULL PAPER
[5] J. Wu, T.-L. Yu, C.-T. Chen, C.-C. Lin, Coord. Chem. Rev.
2006, 250, 602–626; M. H. Chisholm, Z. P. Zhou, J. Mater.
Chem. 2004, 14, 3081–3092.
[6] C. A. Wheaton, P. G. Hayes, B. J. Ireland, Dalton Trans. 2009,
4832–4846.
127.66, 125.43, 114.69 [OC6H3(C6H5)2-2,6], 39.17 {HN=C[N-
(CH ) ] } ppm. FTIR (KBr): ν = 3351 (m), 3050 (m), 3021 (m)
˜
3 2 2
2927 (m), 2806 (m), 1886 (w), 1595 (s), 1569 (s), 1543 (s), 1494 (m),
1455 (m), 1407 (m), 1308 (m), 1290 (m), 1247 (m), 1224 (m), 1177
(m), 1126 (m), 1088 (m), 1069 (m), 1029 (m), 992 (m), 904 (m), 854
(m), 805 (m), 751 (m), 729 (m), 701 (w), 620 (w), 596 (w), 585 (w),
560 (w), 512 (w), 464 (w) cm–1. C50H60N6O3Zn (858.41): calcd. C
70.26, H 6.67, N 10.69; found C 70.53, H 6.38, N 10.55.
[7] a) K. Majerska, A. Duda, J. Am. Chem. Soc. 2004, 126, 1026–
1027; b) T. R. Jensen, L. E. Breyfogle, M. A. Hillmyer, W. B.
Tolman, Chem. Commun. 2004, 2504–2505; c) C. X. Cai, A.
Amgoune, C. W. Lehmann, J. F. Carpentier, Chem. Commun.
2004, 330–331; d) Z. H. Tang, X. S. Chen, Y. K. Yang, X. Pang,
J. R. Sun, X. F. Zhang, X. B. Jing, J. Polym. Sci., Part B: Po-
lym. Chem. 2004, 42, 5974–5982.
[8] P. Hormnirun, E. L. Marshall, V. C. Gibson, A. J. P. White,
D. J. Williams, J. Am. Chem. Soc. 2004, 126, 2688–2689.
[9] G. Labourdette, D. J. Lee, B. O. Patrick, M. B. Ezhova, P.
Mehrkhodavandi, Organometallics 2009, 28, 1309–1319.
[10] a) C. K. Williams, L. E. Breyfogle, S. K. Choi, W. Nam, V. G.
Young, M. A. Hillmyer, W. B. Tolman, J. Am. Chem. Soc. 2003,
125, 11350–11359; b) B. M. Chamberlain, M. Cheng, D. R.
Moore, T. M. Ovitt, E. B. Lobkovsky, G. W. Coates, J. Am.
Chem. Soc. 2001, 123, 3229–3238; c) M. B. Dinger, M. J. Scott,
Inorg. Chem. 2001, 40, 1029–1036.
Polymerization Procedure: Under argon, complexes 1–6 were
treated with rac-lactide (LA). The catalyst (0.025 mmol) and LA
(100 equiv.) were dissolved in toluene (10 mL). The reaction mix-
ture was stirred for 18 h, at the completion of which a drop of
glacial acetic acid was added to terminate the reaction. The solvent
was removed under vacuum, and a colorless solid was isolated. The
1
solid was analyzed by H NMR spectroscopy.
Crystallography: X-ray crystallography was performed, by mount-
ing each crystal onto a thin glass fiber from a pool of Fluorolube
and immediately placing it under a liquid N2 stream, with a Bruker
AXS diffractometer. Graphite-monochromatized Mo-Kα radiation
(λ = 0.7107 Å) was used. The lattice parameters were optimized
from a least-squares calculation on carefully centered reflections.
Lattice determination, data collection, structure refinement, scal-
ing, and data reduction were carried out by using the APEX2 ver-
sion 1.0-27 software package.[29] Each structure was determined by
using direct methods. This procedure yielded the Zn atoms along
with a number of C, N, and O atoms. Subsequent Fourier synthesis
yielded the remaining atom positions. The hydrogen atoms were
fixed in positions of ideal geometry and refined within the
XSHELL software.[30] These idealized hydrogen atoms had their
isotropic temperature factors fixed at 1.2 or 1.5 times the equivalent
isotropic U value of the C atoms to which they were bonded. The
final refinement of each compound included anisotropic thermal
parameters on all non-hydrogen atoms. CCDC-745545, -745546,
-745547, -745548 for compounds 1, 2, 4 and 5, respectively, contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[11] M. H. Chisholm, J. C. Gallucci, H. H. Zhen, J. C. Huffman,
Inorg. Chem. 2001, 40, 5051.
[12] a) J. Börner, S. Herres-Pawlis, U. Flörke, K. Huber, Eur. J. In-
org. Chem. 2007, 5645–5651; b) J. Börner, U. Flörke, K. Huber,
A. Döring, D. Kuckling, S. Herres-Pawlis, Chem. Eur. J. 2009,
15, 2362–2376; c) M. H. Chisholm, N. W. Eilerts, J. C. Huff-
man, S. S. Iyer, M. Pacold, K. Phomphrai, J. Am. Chem. Soc.
2000, 122, 11845–11854; d) M. P. Coles, P. B. Hitchcock, Eur.
J. Inorg. Chem. 2004, 2662–2672; e) R. Eberhardt, M. Allmend-
inger, G. A. Luinstra, B. Rieger, Organometallics 2003, 22, 211–
214.
[13] J.-C. Wu, B.-H. Huang, M.-L. Hsueh, S.-L. Lai, C.-C. Lin,
Polymer 2005, 46, 9784–9792.
[14] H. Kawaguchi, T. Matsuo, J. Organomet. Chem. 2004, 689,
4228–4243.
[15] S. D. Bunge, J. M. Lance, J. A. Bertke, Organometallics 2007,
26, 6320–6328.
[16] J. D. Monegan, S. D. Bunge, Inorg. Chem. 2009, 48, 3248–3256.
[17] T. E. Janini, R. Rakosi, C. B. Durr, J. A. Bertke, S. D. Bunge,
Dalton Trans., in press.
[18] J. A. Bertke, S. D. Bunge, Dalton Trans. 2007, 4647–4649.
[19] S. D. Bunge, J. A. Bertke, T. L. Cleland, Inorg. Chem. 2009, 48,
8037–8043.
[20] T. L. Cleland, S. D. Bunge, Polyhedron 2007, 26, 5506–5512.
[21] L. Yang, D. R. Powell, R. P. Houser, Dalton Trans. 2007, 955–
964.
Supporting Information (see footnote on the first page of this arti-
1
cle): H NMR spectra and a reaction scheme are provided in Fig-
ures S1–S3.
[22] D. J. Darensbourg, M. W. Holtcamp, G. E. Struck, M. S. Zim-
mer, S. A. Niezgoda, P. Rainey, J. B. Robertson, J. D. Draper,
J. H. Reibenspies, J. Am. Chem. Soc. 1999, 121, 107–116.
[23] T. J. Boyle, S. D. Bunge, N. L. Andrews, L. E. Matzen, K. Sieg,
M. A. Rodriguez, T. J. Headley, Chem. Mater. 2004, 16, 3279–
3288.
[24] D. J. Darensbourg, J. R. Wildeson, S. J. Lewis, J. C. Yarbrough,
J. Am. Chem. Soc. 2002, 124, 7075–7083.
[25] M. P. Coles, Dalton Trans. 2006, 985–1001.
[26] G. B. Deacon, T. C. Feng, S. Nickel, M. I. Ogden, A. H. White,
Aust. J. Chem. 1992, 45, 671–683.
Acknowledgments
Jennifer D. Rossi is acknowledged for helpful assistance. We
acknowledge Kent State University for financial support of this
work. Funds to initiate this project were provided by the National
Science Foundation through the Ohio Consortium for Under-
graduate Research: Research Experiences to Enhance Learning
(REEL) Award 0532250.
[27] Z. Y. Zhong, P. J. Dijkstra, J. Feijen, Angew. Chem. Int. Ed.
2002, 41, 4510–4513.
[28] H. R. Kricheldorf, C. Boettcher, K. U. Tonnes, Polymer 1992,
33, 2817–2824.
[29] Bruker, Analytical X-ray Systems, AXS Inc., 5465 East Cheryl
Parkway, Madison, WI 53711-5373, 2005.
[30] G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
[1] R. H. Platel, L. M. Hodgson, C. K. Williams, Polym. Rev.
2008, 48, 11–63.
[2] O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev.
2004, 104, 6147–6176.
[3] M. Vert, Biomacromolecules 2005, 6, 538–546; A.-C. Al-
bertsson, I. K. Varma, Biomacromolecules 2003, 4, 1466–1486.
[4] R. E. Drumright, P. R. Gruber, D. E. Henton, Adv. Mater.
2000, 12, 1841–1846.
Received: October 8, 2009
Published Online: February 18, 2010
1430
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 1424–1430