Zinc complexes of piperazinyl-derived aminephenolate ligands: Synthesis, characterization and ring-opening polymerization activity

Article abstract of DOI:10.1002/ejic.201100703

A series of zinc complexes was prepared from 4,6-disubstituted (R,R′) o-[(4-methyl-1-piperazinyl)methyl]phenols ([ONN^R′,R]H) and characterized by elemental analysis, ¹H and ¹³C{ ¹H} NMR spectroscopy, and X-ray crystallography. Reaction of these products with diethylzinc gave Zn[ONN^tBu,tBu]₂ (1) as a monometallic complex and {[μ-ONN^R′,R]ZnEt}₂ [R′ = Me, R = tBu, (2); R′ = R = tBu (3); R′ = R = tAm (4)] bimetallic species that have distorted tetrahedral environments about the Zn centres. Reaction of 3 and 4 with alcohols gave {[ONN^tAm,tAm]Zn(μ- OR″)}₂ [R″ = Bn (5); R″ = Et (6)] bimetallic species in which the Zn centres are bridged by benzyl alkoxide and ethoxide groups, respectively. A morpholinyl derived ligand was also synthesized and characterized (L4H), and its 1:1 stoichiometric reaction with ZnEt₂ resulted in complex 7, {[μ-ONO^tBu,tBu]ZnEt}₂. The reactivity of complexes 2-7 in the ring-opening polymerization of rac-lactide (LA) and Iμ-caprolactone (Iμ-CL) was studied. Reactions of carbon dioxide with cyclohexene oxide in the presence of 6 or 7/ROH (R = Bn, Et) afforded cyclohexene carbonate.

Full text of DOI:10.1002/ejic.201100703

FULL PAPER
DOI: 10.1002/ejic.201100703
Zinc Complexes of Piperazinyl-Derived Aminephenolate Ligands: Synthesis,
Characterization and Ring–Opening Polymerization Activity
Nduka Ikpo,[a] Lisa N. Saunders,[a] Jillian L. Walsh,[a] Jennifer M. B. Smith,[a]
Louise N. Dawe,[b] and Francesca M. Kerton*[a]
Keywords: Ring-opening polymerization / Polymerization / Zinc / Carbon dioxide / Lactide / Caprolactone
A series of zinc complexes was prepared from 4,6-disub-
stituted (R,RЈ) o-[(4-methyl-1-piperazinyl)methyl]phenols
([ONNRЈ,R]H) and characterized by elemental analysis, 1H
and 13C{1H} NMR spectroscopy, and X-ray crystallography.
Reaction of these products with diethylzinc gave
Et (6)] bimetallic species in which the Zn centres are bridged
by benzyl alkoxide and ethoxide groups, respectively. A mor-
pholinyl derived ligand was also synthesized and charac-
terized (L4H), and its 1:1 stoichiometric reaction with ZnEt2
resulted in complex 7, {[μ-ONOtBu,tBu]ZnEt}2. The reactivity
of complexes 2–7 in the ring-opening polymerization of rac-
lactide (LA) and ε-caprolactone (ε-CL) was studied. Reac-
tions of carbon dioxide with cyclohexene oxide in the pres-
ence of 6 or 7/ROH (R = Bn, Et) afforded cyclohexene carbon-
ate.
Zn[ONNtBu,tBu
]
2
(1) as a monometallic complex and {[μ-
ONNRЈ,R]ZnEt}2 [RЈ = Me, R = tBu, (2); RЈ = R = tBu (3); RЈ =
R = tAm (4)] bimetallic species that have distorted tetrahedral
environments about the Zn centres. Reaction of 3 and 4 with
alcohols gave {[ONNtAm,tAm]Zn(μ-ORЈЈ)}2 [RЈЈ = Bn (5); RЈЈ =
Introduction
the current study, Carpentier and co-workers recently de-
Aminephenolate and related ligands that possess a mixed scribed the synthesis of zinc and magnesium complexes sup-
set of N- and O-donor atoms have attracted a great deal of ported by ether-aminephenolate ligands, including those
interest over the past decade due to their ability to coordi- bearing morpholinyl side-arms, that are effective initiators
nate to a range of metal centres and the ease of systematic for immortal ROP of cyclic esters.[25] Lappert and co-
control of their steric properties by variation of their back- workers have reported zinc complexes stabilized by a piper-
bone and phenol substituents.[1–23] Various ligands of this azinylphenolate ligand,[26] and one such complex was found
type have been used in main group and early transition to uptake some CO2 when cyclohexene oxide–CO2 copoly-
metal chemistry (including lithium,[2–3] magnesium,[4–10] merizations were attempted although no polymer product
calcium,[11] rare-earths,[12,13] zinc,[1,14–17,19] aluminium,[18,20] was isolated.
zirconium[21,22] and titanium[23]). Many of these complexes
In this paper, we describe the synthesis and structural
have been reported to be excellent initiators for the ring-
opening polymerization (ROP) of cyclic esters such as LA.
The quest is ongoing for new systems that can advance our
understanding of this reaction, and ultimately lead to well
controlled initiator performance in this and other polymeri-
zation reactions.
Recently, Issenhuth et al. reported the synthesis of cat-
ionic aluminium complexes supported by morpholine-de-
rived aminephenolate ligands that act as efficient initiators
for ROP of propylene oxide.[24] Of particular relevance to
characterization of a group of piperazinyl–phenol ligands
and a morpholinyl phenol ligand, their coordination chem-
istry with Zn, and their use as single and/or binary compo-
nent initiators for the ROP of LA and ε-CL under various
conditions, including under microwave (MW) irradiation.
MW irradiation has not been used extensively in ROP of
lactide facilitated by metal complexes.[27,28] In the two ex-
amples reported to date simple tin initiators, e.g. stannous
octoate, were used and the resulting polymers have broad
polydispersities. High reaction temperatures are normally
detrimental to achieving controlled polymerization, how-
ever, MW methods allow rapid heating of reaction mix-
tures. Sluggish initiators, e.g. those bearing ligands with ste-
rically demanding groups, normally require heating to en-
able a worthwhile reaction rate to be obtained. In such
cases, MW heating may lead to a reduction in the number
of side reactions e.g. transesterification by reducing the
overall reaction time. Also, MW heating could lead to more
rapid screening of potential ROP initiators.
[a] Department of Chemistry, Memorial University of
Newfoundland,
St. John’s, NL, A1B 3X7, Canada
Fax: +1-709-864-3702
E-mail: fkerton@mun.ca
[b] X-ray Crystallography Laboratory, Centre for Chemical
Analysis, Research and Training, Memorial University of
Newfoundland,
St. John’s, NL, A1B 3X7, Canada
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100703.
Eur. J. Inorg. Chem. 2011, 5347–5359
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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F. M. Kerton et al.
FULL PAPER
As many Zn phenolate and alkoxide complexes have
shown good activity towards either copolymerization of
CO2 with epoxides or the formation of cyclic carbonates in
this article we also describe our initial studies of the reactiv-
ity of 6 and 7/ROH with carbon dioxide..[29,30]
Results and Discussion
The protio ligands were synthesized from the appropriate
phenol, formaldehyde, and 1-methylpiperazine via a modi-
fied Mannich condensation reaction, as shown in Scheme 1.
They were isolated in excellent yields. L2H has previously
been prepared through a slightly different procedure that
involved reduction with hydrobromic acid and neutraliza-
tion with aqueous NaHCO3,[26] however structural data for
L2H have not been reported. The preparation of L4H
through a Mannich condensation reaction in 1,4-dioxane
under reflux has been reported previously.[25]
Figure 1. Molecular structure of L2H. The thermal ellipsoids are
drawn at the 50% probability level; H1 atom included. Selected
bond lengths [Å] and bond angles [°]: O(1)–C(1), 1.366(2); N(1)–
C(15), 1.472(2); C(14)–C(15), 1.516(2); O(1)–C(1)–C(14),
119.73(14); O(1)–C(1)–C(2), 119.76(14); N(1)–C(15)–C(14),
113.72(14).
Synthesis and Characterization of Zinc Aminephenolate
Complexes
Reactions of protio ligands [ONNR,RЈ]H with ZnEt2 in
toluene under ambient conditions led to the isolation of
molecular zinc complexes Zn[ONNtBu,tBu
] (1) and {[μ-
2
ONNR,RЈ]ZnEt}2 (2–4) in 56–96% yield (Scheme 2). Treat-
ment of complexes 3 and 4 with two equivalents of benzyl
alcohol and ethanol, respectively, in toluene at ambient
Scheme 1. Sythesis procedure for amine-phenolate ligand precur-
sors.
The recently reported benign route (reaction in
water)[31,31,32] to amine-phenol ligands reduces the amount
of reagents required relative to more traditional reaction
routes and afforded high yields of L1H–L4H. These were
recrystallized from methanol (cooled to 0 °C). Their charac-
1
terization by H- and 13C{1H} NMR techniques afforded
well resolved resonances for all proton and carbon environ-
ments, while elemental analyses showed that the com-
pounds were obtained in pure form. Single crystals of L2H
suitable for X-ray crystallography were grown via slow
evaporation of a saturated methanol solution of L2H at
ambient temperature. The solid state structure of L2H
shown in Figure 1 and reveals intramolecular hydrogen
bonding between the phenol and the proximal amine
groups. The N-methylpiperazine ring adopts the normal
chair conformation. Tables 5 and 6 summarize crystal data
for all the structures reported in this article.
Scheme 2. Synthetic procedure for amine-phenolate zinc complexes.
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Zinc Complexes of Aminephenolate Ligands
temperature afforded 5 and 6. An equimolar reaction be- lowering the temperature, the methylene signals became
tween L4H and ZnEt2 afforded {[μ-ONOtBu,tBu]ZnEt}2 (7) broader for all the complexes, which implied that at high
in 81% yield.
temperature there is fast exchange on the NMR time scale
The synthesis of a monometallic zinc complex related to of a coordinated pyridine or coordinated piperazine ligand
1 has been reported by Lappert and co-workers, the synthe- and the free pyridine solvent. It should also be noted that
sis proceeded via the indirect route of reacting a bimetallic in solution the bimetallic species (analogous to their solid
phenolate-bridged ethylzinc complex with 2 equiv. of meth- state structures) are unlikely to exist in the presence of such
anol.[26] Another similar monometallic zinc complex was re- a large quantity of Lewis base (C5D5N). However, use of
ported by Sobota and co-workers, and was obtained from this solvent was essential for gaining sufficient solubility for
the reaction of 2 equiv. of an amine-phenol ligand with all samples studied, and to allow for direct comparison of
1 equiv. of ZnEt2.[33] In our case both the monometallic (1) their spectra.
and bimetallic (2 and 4) zinc complexes were synthesized
by reaction of 1:1 ratio of ZnEt2 with the corresponding
X-ray Crystallographic Analyses of Zinc Complexes
ligands. Although all reactions were carried out under rig-
orous air- and moisture-free conditions, we cannot rule out
the potential role of adventitious water in the preparation
of 1. In this regard, the preparation of 1 would proceed by
a similar route to that reported by Lappert who used meth-
anol as a proton source. Nevertheless, the reaction of L2H
with an excess of ZnEt2 in toluene did lead to the formation
of the desired bimetallic zinc complex 2. Treatment of com-
plexes 3 and 4 with 2 equiv. of ethanol and benzyl alcohol,
respectively, afforded alkoxy-bridged complexes 5 and 6.
The zinc complexes 1–7 were characterized by elemental
Crystal structures of complexes 1–7 were determined by
X-ray diffraction analysis. The molecular structure of com-
plex 1 is shown in Figure 3 along with selected bond lengths
and angles. The central zinc atom adopts a distorted tetra-
hedral coordination geometry and is coordinated by four
donor atoms from two (methylpiperazinyl)phenolate li-
gands. The fact that one of the amine nitrogen atoms re-
mains pendant can be attributed to the restriction of the
chair conformation adopted by the piperazinyl unit of the
ligand that orientates N(2) away from the central metal
atom. Future studies will involve investigating the chemistry
at this nitrogen centre. The structure of 1 is centrosymmet-
ric (the zinc ion is located on a crystallographic inversion
centre) with Zn–O and Zn–N distances within the normal
ranges for related complexes. The more acute bond angles
around Zn, O(1)–Zn(1)–N(1) 96.82(5)°, can most probably
be associated with the bite of the six-membered C3NZnO
chelate ring. This ring adopts a legless chair[34] or slight
sofa[1] conformation with the C(6) atom forming the back-
rest and situated ca. 0.76 Å out of Zn(1)–O(1)–C(20)–C(7)–
N(1) plane.
1
1
analyses, and H and 13C{1H} NMR spectroscopy. The H
NMR spectra of 1–7 in C5D5N solvent recorded at room
temperature showed broad resonances in the methylene
(PhCH2N) and amine methyl regions, which prompted vari-
able-temperature NMR studies of the complexes. A portion
of a representative variable-temperature spectrum of 4 is
shown in Figure 2. At elevated temperature (343 K), sharp
signals were discernable in the spectrum that were very in-
formative for establishing the formation of the complexes
1–7. The spectrum of complex 1 exhibits three sharp sing-
lets associated with the aromatic and methylene protons,
these signals occur at δ = 7.58, 7.11 and 4.07 ppm and are
shifted from 7.26, 6.93 and 3.75 ppm where they are ob-
served in the spectrum of free ligand (L2H). No Zn-ethyl
resonances were observed in the spectrum of 1. In contrast,
the signals from the methylene (PhCH2N) protons in 3 are
observed at 3.65 ppm, and additional signals were observed
at δ = 0.59 (quartet) and 1.57 ppm (triplet) that correspond
to the ethyl group bound to the zinc ion. Similar resonances
are observed in the spectra of complexes 2, 4 and 7. Upon
Figure 3. Molecular structure of 1. The thermal ellipsoids are
drawn at the 50% probability level; H atoms are excluded for clar-
ity. The two halves of the molecule are symmetry related by inver-
sion. Selected bond lengths [Å] and bond angles [°]: Zn(1)–O(1),
1.9008(11); Zn(1)–N(1), 2.1198(13); O(1)–C(20), 1.3357(17); N(1)–
C(1), 1.4885(19); N(1)–C(5), 1.4909(19); N(1)–C(6), 1.4982(19);
N(2)–C(2), 1.453(2); N(2)–C(3), 1.454(2); N(2)–C(4), 1.463(2);
C(1)–C(2), 1.520(2); C(4)–C(5), 1.510(2); C(15)–C(20), 1.422(2);
O(1)–Zn(1)–O(1), 124.94(7); O(1)–Zn(1)–N(1), 110.52(5); O(1)–
Zn(1)–N(1Ј), 96.82(5); C(20)–O(1)–Zn(1), 125.45(10); C(1)–N(1)–
C(6), 108.82(12); C(1)–N(1)–Zn(1), 114.91(9); C(6)–N(1)–Zn(1),
102.40(9); N(1)–C(1)–C(2), 111.18(13).
1
Figure 2. The methylene region of variable temperature H NMR
spectra of 4 in C5D5N.
Eur. J. Inorg. Chem. 2011, 5347–5359
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F. M. Kerton et al.
FULL PAPER
An ORTEP drawing of 2 is presented in Figure 4 along olate ligands. The coordination geometry at the zinc atom
with selected bond lengths and angles. The structure exhib- is a distorted tetrahedron. It should be noted that the bond
its a butterfly-like arrangement with the two ethyl groups angle O(1)–Zn(1)–O(1Ј) [83.78(13)°] is wider than that in
positioned on the same side with respect to the Zn2O2 core Bochmann’s [EtZn(μ-OC6F5)(py)]2 (py = pyridine) complex
plane, while the two piperazine units are situated on oppos- [78.00(5)°], which contains a trans-alkylzinc centre with a
ing sides of the Zn2O2 plane. This cisoid conformation of phenolate ligand and a nitrogen donor (in this case from
ethyl groups is rare in zinc ethyl chemistry, although an- pyridine).[42] The similarities in the types of donors in the
other example has been reported.[35] Each zinc ion adopts coordination spheres of Zn in each complex allows the dif-
a distorted tetrahedral coordination geometry consisting of ference in the bond angles to be attributed to the chelate
two bridging phenolate oxygen atoms, one nitrogen donor bite angle of L2 in complex 3, whereas the donors in Boch-
and one ethyl group. The central metal ring, Zn2O2, dis- mann’s complex are not linked and are free to orientate
plays a twisted geometry with O(1) and O(2) ca. 0.54 and themselves with minimal constraint. In 3, C(15) and O(1)
0.55 Å from the O(2)–Zn(1)–Zn(2) and O(1)–Zn(1)–Zn(2) lie ca. 0.75 and 0.45 Å from the Zn(1)–N(1)–C(6)–C(1)
planes, respectively. The angles at the central atoms Zn(1) mean plane, inducing the chelate ring to adopt a chair con-
[O(1)–Zn(1)–O(2), 80.47(8)°] and Zn(2) [O(1)–Zn(2)–O(2), formation, while the zinc atoms lie ca. 0.58 Å above the
80.89(8)°] are narrower than the corresponding angles for plane defined by the coordinated N(1), C(21) and O(1)
the bridging oxygen atoms O(1) [Zn(2)–O(1)–Zn(1), atoms. The central Zn2O2 ring is planar with the angle at
97.75(9)°] and O(2) [Zn(1)–O(2)–Zn(2), 96.55(9)°]. The Zn(1) being narrower than that at O(1). The Zn–C, Zn–N
acute angles are likely a result of the six-membered chelate and Zn–O bond lengths are within the ranges previously
ring that is enforced by the ligand. The two C3OZnN che- reported for zinc complexes.[1,26,39,43] The ethyl groups on
late rings adopt boat conformations with C(6) and O(1) ca. each of the two metal centres lie trans to each other. Pre-
0.62 and 0.71 Å from the Zn(1)–C(17)–C(7)–N(1) plane, sumably, the large tert-butyl group in the 4-position of the
while C(23) and O(2) are ca. 0.64 and 0.74 Å from the phenolate donor in 3, although distant from the metal coor-
Zn(2)–C(34)–C(24)–N(2) plane. The Zn–C bond lengths in dination sphere, causes the ethyl groups to orientate them-
2 are comparable to those in related complexes reported in selves in a trans arrangement in contrast to the syn-cis ar-
the literature,[36] where the mean Zn–C bond lengths in such rangement in 2, in which the 4-position of the phenolate
species are 1.978 Å. Within those complexes that are bime- ligand is occupied by a methyl group.
tallic, the ethyl groups are transoidal.[21,37,38] The Zn–O
bond lengths in 2 are similar to the mean distance [2.039 Å]
determined from a literature survey, and are comparable to
those within related zinc complexes that are reported in the
literature.[26,35,38–41] The Zn(1)–Zn(2) distance [3.0527(6) Å]
is close to that reported by Gibson [3.0276(5) Å], while the
O(1)–O(2) separation distance [2.634 Å] is shorter
[2.817(3) Å].[1]
Figure 5. Molecular structure of 3. The thermal ellipsoids are
drawn at the 50% probability level; H atoms are excluded for clar-
ity. The two halves of the molecule are symmetry related by inver-
sion. Selected bond lengths [Å] and bond angles [°]: Zn(1)–C(21),
1.977(4); Zn(1)–O(1), 2.052(3); Zn(1)–N(1), 2.185(4); Zn(1)–Zn(1),
3.068(5); O(1)–C(1), 1.362(3); N(1)–C(20), 1.479(4); N(1)–C(16),
1.483(4); N(1)–C(15), 1.498(4); N(2)–C(17), 1.457(4); N(2)–C(18),
1.458(4); N(2)–C(19), 1.460(4); C(1)–C(2), 1.416(4); C(21)–Zn(1)–
O(1), 124.87(11); O(1)–Zn(1)–O(1Ј), 83.78(13); C(21)–Zn(1)–N(1),
118.53(16); O(1)–Zn(1)–N(1), 92.53(10); O(1)–Zn(1)–N(1Ј),
103.06(11); C(1)–O(1)–Zn(1), 117.95(18); C(1)–O(1)–Zn(1Ј),
123.86(17); Zn(1)–O(1)–Zn(1Ј), 96.22(13); C(20)–N(1)–C(16),
Figure 4. Molecular structure of 2. The thermal ellipsoids are
drawn at the 50% probability level; H atoms are excluded for clar-
ity. Selected bond lengths [Å] and bond angles [°]: Zn(1)–C(35),
1.981(3); Zn(1)–O(1), 2.037(2); Zn(1)–O(2), 2.043(2); Zn(1)–N(1),
2.148(3); Zn(1)–Zn(2), 3.0527(6); Zn(2)–C(37), 1.991(3); Zn(2)–
O(1), 2.015(2); Zn(2)–O(2), 2.047(2); Zn(2)–N(3), 2.143(3); O(1)–
C(17), 1.361(3); O(2)–C(34), 1.353(3); C(35)–Zn(1)–O(1),
117.75(13); C(35)–Zn(1)–O(2), 124.72(12); C(35)–Zn(1)–N(1),
106.4(2);
C(20)–N(10)–C(15), 107.5(2);
C(16)–N(1)–C(15),
123.31(12);
105.03(9); C(37)–Zn(2)–O(1), 123.41(12); C(37)–Zn(2)–O(2),
115.93(12); O(1)–Zn(2)–O(2), 80.89(8); Zn(2)–O(1)–Zn(1),
97.75(9); Zn(1)–O(2)–Zn(2), 96.55(9).
O(1)–Zn(1)–N(1),
93.77(9);
O(2)–Zn(1)–N(1),
110.1(2); C(20)–N(1)–Zn(1), 113.99(18); C(16)–N(1)–Zn(1),
114.17(18).
The single crystal structure of complex 3 depicted in Fig-
The single-crystal X-ray data for 4 were of poor quality
ure 5 reveals a centrosymmetric bimetallic complex with the due to merohedral twinning of the crystals and their weak
zinc centres bridged through the oxygen atoms of the phen- diffraction, however the structure and connectivity of 4 was
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Zinc Complexes of Aminephenolate Ligands
confirmed and authenticates the theory that it is structur-
Crystals of complex 7 were obtained by cooling a satu-
ally analogous to 3. A ball and stick model of 4 is available rated toluene solution to –35 °C. The solid state structure
in the Supporting Information. of complex 7 (Figure 7), like the related piperazinyl complex
Reaction of the alkyl complexes with alcohols afforded 3, contains zinc atoms coordinated by the phenolate oxygen
alkoxy-bridged complexes. Suitable single crystals of these atom and the proximal nitrogen atom of the ligand to form
complexes were grown by the slow evaporation of a hexane/ a planar Zn2O2 core. The bond angles and distances are in
toluene mixture (5) and from a saturated toluene solution the range known for bimetallic zinc phenolate complexes,
at –35 °C (6). The molecular structures of 5 and 6 reveal and comparable to those in 3.[1,26,39,43]
the bimetallic nature of these species. An ORTEP drawing
of 6 is shown in Figure 6 and an illustration of the structure
of 5 is available in the Supporting Information. The zinc
centres in these complexes adopt distorted tetrahedral geo-
metries and are bridged by the oxygen atoms of the eth-
oxide or benzyl alkoxide groups to form planar Zn2O2
cores. Comparison of the angles within the Zn2O2 planar
cores of complexes 5 and 6 reveals that the O–Zn–O bond
angle in 6 is slightly narrower than that in complex 5, while
the Zn–O–Zn bond angle is wider in complex 6 than in
complex 5. The bond lengths within the two complexes are
Figure 7. Molecular structure of 7. The thermal ellipsoids are
similar, and 6 contains bond lengths comparable to those drawn at the 50% probability level; H atoms are excluded for clar-
in a related ethoxide-bridged zinc species.[44]
ity. The two halves of the molecule are symmetry related by inver-
sion. Selected bond lengths [Å] and bond angles [°]: Zn(1)–C(20),
1.956(6); Zn(1)–O(1), 2.071(4); Zn(1)–N(1), 2.177(5); Zn(1)–Zn(1),
3.0608(14); O(1)–C(1), 1.354(7); O(2)–C(17), 1.393(8); C(20)–
Zn(1)–O(1), 128.8(2); C(20)–Zn(1)–O(1Ј), 115.6(2); O(1)–Zn(1)–
O(1), 82.87(17); C(20)–Zn(1)–N(1), 121.1(2); O(1)–Zn(1)–N(1),
92.70(18); C(1)–O(1)–Zn(1), 119.1(3).
Polymerization of Cyclic Esters
Ring-Opening Polymerization of rac-Lactide
The activities of complexes 2–5 and 7 as initiators for
ROP of LA in toluene (Table 1 and Figure 8) and neat ester
(Table 2) were examined. Compound 7 has been studied
previously by Carpentier and co-workers.[25] It is included
here for comparative purposes and afforded a polymer with
relatively narrow Mw/Mn (Mw = weight averaged molecular
weight, Mn = number average molecular weight). For the
Figure 6. Molecular structure of 6. The thermal ellipsoids are
drawn at the 50% probability level; H atoms are excluded for clar-
ity. Selected bond lengths [Å] and bond angles [°]: Zn(1)–N(1),
2.088(6); Zn(1)–Zn(1), 2.954(2); O(1)–C(1), 1.330(8); O(2)–C(21),
1.404(8); O(2)–Zn(1), 1.957(5); O(2) –Zn1–O(2), 82.0(2); O(1)–
Zn(1)–N(1), 100.8(2); O(2)–Zn(1)–N(1), 117.4(2); C(21)–O(2)–
Zn(1), 131.9(4); Zn(1)–O(2)–Zn(1Ј), 98.0(2).
Table 1. Details of the ring-opening polymerizations of LA initiated by complexes 2–5 and 7 in toluene.
[f]
Entry Initiator
[LA]0:[Zn]0:[ROH]0
t [min]
T [°C]
Conv. [%][d] Mncal[e] ϫ 103
Mn[f] ϫ 103
Mw/Mn
Pr[g]
1
2
3
4
5
6
7
8
2
2
2
3
3
3
3
3
3
3
4
5
7
100:1:1[a]
100:1:1[a]
200:1:1[a]
100:1:1[a]
100:1:1[b]
100:1:0
60
10
15
90
120
120
120
120
5
60
60
60
70
70
70
70
70
97.1
96.0
96.3
98.0
96.4
0.1
98.0
98.3
90.0
93.6
97.0
94.9
97.6
13.9
13.8
27.7
14.1
13.9
–
28.2
42.5
12.9
13.5
11.4
13.7
13.9
10.9
7.99
11.8
13.6
12.0
–
13.3
15.3
12.0
9.19
12.3
10.0
9.56
1.25
1.23
1.39
1.32
1.60
–
1.76
1.87
1.33
1.42
1.42
1.37
1.25
0.49
0.49
0.53
0.45
0.53
–
0.55
0.49
0.47
0.47
0.51
0.47
0.49
200:1:1[b]
300:1:1[b]
100:1:1[b]
100:1:1[a]
100:1:1[a]
100:1:0
9
120[c]
120[c]
70
70
60
10
11
12
13
5
60
15
30
100:1:1[a]
[a] In the presence of one equivalent of benzyl alcohol. [b] In the presence of one equivalent of tert-butyl alcohol. [c] Microwave reaction
(MW). [d] Determined by 1H NMR spectroscopy. [e] The Mncal value of the polymer was calculated with Mncal = ([rac-LA]0/
[Zn]0)ϫ144.13ϫconv. %/100. [f] The Mn value was calculated according to Mn = 0.58MnGPC, where MnGPC is the value of Mn determined
by GPC (chloroform) and is relative to polystyrene standards. [g] Pr is the probability of racemic enchainment of the monomers units,
1
as determined by homodecoupled H NMR spectroscopy according to the method reported by Coates et al.[51]
Eur. J. Inorg. Chem. 2011, 5347–5359
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5351

F. M. Kerton et al.
FULL PAPER
Table 2. Details of the bulk polymerizations of LA initiated by complexes 5 and 6 and preformed at 130 °C.
[c]
Entry
Initiator
[LA]0:[I]0
t [min]
Conv. [%][a]
Mncal[b] ϫ103
Mn[c] ϫ103
Mw/Mn
Pr[d]
1
2
3
4
5
6
5
5
5
6
6
6
100:1
100:1
100:1
100:1
100:1
100:1
15
60
120
15
60
120
54.8
74
91.1
41.4
83.9
95.2
7.90
10.7
13.2
4.72
9.58
10.9
10.8
12.7
20.2
1.28
3.39
3.96
1.39
1.67
1.65
1.18
1.54
1.66
0.53
0.53
0.51
0.53
0.53
0.53
1
[a] Determined by H NMR spectroscopy. [b] The Mncal value of the polymer was calculated with Mn = ([LA]0/[Zn]0)ϫ144.13ϫconver-
GPC
sion %/100. [c] The Mn value was calculated according to Mn = 0.58MnGPC, where Mn
is the value of Mn determined by GPC
(chloroform) and is relative to polystyrene standards. [d] Pr is the probability of racemic enchainment of monomers units, as determined
1
by homodecoupled H NMR spectroscopy according to the method reported by Coates et al.[51]
piperazinyl-derived initiators, Mw/Mn values for polylactide ture.[48–49] The use of BnOH and tBuOH as coinitiators
(PLA) were between 1.25 and 1.87. Reaction rates generally with 3 was studied in reactions conducted with conven-
correlated with both the steric demands of the ligand and tional heating and MW irradiation. It was noted that con-
coinitiator (alcohol), with 2 and BnOH affording near versions of 90% and 93% could be reached in 5 min with
quantitative conversions within shorter times than its tAm MW for tBuOH and BnOH, respectively (Table 1, entries 9
and tBu analogues, and with activity comparable to that of and 10), but similar conversions with conventional heating
compound 7. Overall the Mn values of the polymer samples required significantly longer reaction times, particularly for
were lower than the expected values based on % conversion. tBuOH when compared with BnOH (Table 1, entries 4 and
This deviation likely implies that some transesterification 5). The Mn of the PLA prepared via a MW method showed
reactions and/or a slow initiation step relative to propaga- good agreement with the theoretical values. These data im-
tion occurred. Both the broadening of the molecular weight ply that MW irradiation can be used to assist in the control
distribution and the deviation from expected Mn increased of ROP when bulky coinitiators are used, as there is a
with increasing [LA]0:[Zn]0 ratio (Table 1, entries 2 and 3, smaller apparent difference in the initial reaction rate under
entries 6–8), and with increasing reaction time and tempera- such conditions compared with conventional heating meth-
ture. These trends are frequently observed in ROP of cyclic ods. This could be useful particularly in the preparation of
esters.[45–47] In an attempt to distinguish between less con- star-polymers and other systems where a sterically con-
trolled polymerization due to slow initiation vs. transesteri- gested coinitiator is required. It should be noted that in the
fication, the single component initiator 5 was studied conventionally heated reactions, solutions of monomer and
(Table 1, entry 12). The Mw/Mn for the resulting polymer initiator were heated separately and then mixed once the
was not significantly different to that obtained for PLA desired reaction temperature was achieved.
with 4/BnOH under similar conditions(Table 1, entry 11).
Microstructural analyses of the resulting PLA were per-
However, as expected, 5 did result in more rapid polymeri- formed through inspection of the methine region of the
zation than 4/BnOH. The difference in the rate of polymeri- homodecoupled 1H NMR spectra. Pr values (probability of
zation could be attributed to the time lapse required for the racemic enchainment of the monomer units) were generally
transformation of the precursor alkyl species, by close to 0.5 indicating that an atactic polymer was pro-
alcoholysis, to the active alkoxide complex.
duced. Reactions with good initiation rates (Table 1, entries
4, 9, 10 and 12) afforded polymers with a very slight isotac-
tic bias (Pr = 0.45–0.47) whereas long reaction times and
high temperatures (excluding MW reactions) gave polymers
with a very slight heterotactic bias (Pr = 0.51–0.55). The
latter polymers had, on average, broader molecular weight
distributions than those with Pr Ͻ 0.5. Therefore, the ap-
parent increase in heterotacticity is likely to be an artefact
of transesterification reactions, and the small degree of iso-
tacticity is probably induced through a chain-end control
mechanism. Occurrence of transesterification was con-
firmed by matrix-assisted laser desorption ionisation time-
of-flight mass spectrometry (MALDI-TOF MS) and 1H
NMR end-group analysis of the polymers. In the mass spec-
tra, two main series of signals were observed that were sepa-
rated by a difference of 72 in m/z. These were assigned to
open chain (even mass) polymers and oligomers clustered
with Na+ and K+ ions. Two less intense series of signals
(80% less intense than the main series signals), separated
by 72 in m/z, were also observed. These weaker signals were
Figure 8. Plots of Mn and PDI (PDI = Mw/Mn and is determined
by GPC) vs. monomer conversion for 3/tBuOH initiated ROP of
LA. Conditions: [LA]:[Zn]:[tBuOH] = 50:1:1, [Zn]0 = 18.2 mm, tol-
uene, 60 °C, 0.1 mL aliquots taken at the given intervals. Diamonds
correspond to Mw/Mn and circles to Mn.
For the Zn-alkyl species the presence of alcohol was nec-
essary to generate an efficient initiator system for the poly-
merization of LA, as has been reported in the litera-
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 5347–5359

Zinc Complexes of Aminephenolate Ligands
assigned to cyclic (odd mass) polymers and oligomers clus-
1
tered with Na+ and K+ ions. H NMR spectra of the poly-
mers reported in this paper all contained resonances attrib-
utable to OH end groups. However, integration of either
the –OH or RO– end group resonances relative to the meth-
ine protons generally afforded higher molecular weights
than those obtained through either gel permeation
chromatography (GPC) or MALDI-TOF MS analysis, the
values were up to three times higher for polymers with the
broadest Mw/Mn values. Therefore, the 1H NMR end group
analyses confirmed that intramolecular transesterification
had occurred, which produced cyclic species and led to less
intense end group resonances than predicted based on the
Mn data. Our results contrast with the previously reported
work of Carpentier and coworkers where no transesterifica-
tion was apparent (as indicated by narrower molecular
weight distributions than seen for our polymers and by
MALDI-TOF MS studies) and a heterotactic bias was ob-
served in the resulting polymers (Pr = 0.60)[25] although to
achieve this THF was used as the solvent, while in toluene
a slight isotactic bias was observed (Pr = 0.46), in line with
our results obtained with piperazinyl systems. For the single
morpholinyl derived initiator in our study, we found Pr =
0.49 for the PLA formed in toluene (Table 1, entry 13). The
first-order plot for solution polymerization with this initia-
tor (Figure 9) was linear with kapp = 0.263 min–1. The rates
observed for the piperazinyl-derived species in this study
were between 0.010 and 0.030 min–1, both in toluene and
in molten LA. These data suggest that the morpholinyl de-
rived systems are superior initiators when compared with
their piperazinyl analogues in terms of fast reaction rates
and greater degree of control. We propose that the outer
sphere ether group in these systems may play a role in di-
recting the incoming monomer for reaction at the metal
Figure 9. First-order plot of LA consumption in a reaction with 7/
BnOH conducted at 60 °C. Conditions: [LA]:[Zn]:[ROH] = 100:1:1,
10 mL of toluene, [Zn]0 = 32.0 mm.
of 0.0179 and 0.0247 min–1, respectively. Bulk polymeriza-
tion of LA in the presence of 6 (an ethoxide complex) is
moderately faster than in the presence of 5 (a benzyl alk-
oxide species) due to the differences in the steric demands
of the initiating alkoxide ligands. Complex 5 afforded
higher molecular weight polymers than 6. The Mn values
of these polymers were also significantly higher than the
predicted values (Mncal), which might be attributed to de-
composition of the initiators at the high temperatures re-
quired for melt polymerization. Initiator decomposition
leading to higher than expected Mn values has been ob-
served previously by Drouin et al.[50] Both systems afforded
predominantly atactic polymers under these reaction condi-
tions due to the high temperatures used.
Ring-Opening Polymerization of ε-Caprolactone
Polymerization of ε-CL by complexes 2–5 and 7 was also
centre, and that the outer sphere tertiary amine group is assessed (Table 3). Similar trends in behaviour can be seen
less efficient in this process. in the results from these reactions when compared to those
ROP reactions with the single component initiators 5 and of the ROP of LA study, e.g. slow initiation and reaction
6 were also performed under solventless conditions at rates when tBuOH was used as the coinitiator (Table 3, en-
130 °C, Table 2. Conversions reached more than 90% over tries 3 and 4). Molecular weight distributions were between
2 h (Table 2, entries 1 and 6) suggesting slow initiation com- 1.22 and 1.82. Conversions were generally higher when
pared with analogous solution phase polymerizations. The BnOH, which is less sterically demanding, was used in place
first-order plots for the bulk polymerization of LA with 5 of tert-butyl alcohol. Of the complexes studied those con-
and 6 as initiators (Figure S7) were linear with kapp values taining the tAm substituted ligands afforded initiators with
Table 3. Details of the ring-opening polymerizations of ε-CL initiated by complexes 2–5 and 7 in toluene.
Conv. [%][d]
Mncal[e] ϫ 103
Mn[f] ϫ 103
Mw/Mn
[f]
Entry
Initiator
[ε-CL]0:[Zn]0:[ROH]0
t [min]
T [°C]
1
2
3
4
5
6
7
8
9
2
2
3
3
3
3
4
5
7
100:1:1[b]
200:1:1[b]
100:1:1[a]
200:1:1[a]
100:1:1[a]
100:1:1[b]
100:1:1[b]
100:1:0
15
60
120
270
5
60
60
70
74.4
94.5
17.1
76.3
50.0
98.3
99
8.49
21.4
–
17.4
5.71
11.2
11.3
10.8
20.2
5.69
9.72
–
14.6
12.2
7.70
8.56
9.03
17.5
1.27
1.34
–
1.82
1.37
1.46
1.22
1.30
1.21
70
130[c]
130[c]
70
5
40
60
30
70
70
95
88.5
100:1:1[b]
[a] In the presence of one equivalent of tert-butyl alcohol. [b] In the presence of one equivalent of benzyl alcohol. [c] Microwave reaction
cal
(MW). [d] Determined by 1H NMR spectroscopy. [e] The Mncal value of the polymer was calculated with Mn
= ([ε-CL]0/
GPC
[Zn]0)ϫ114.14ϫconv. %/100. [f] The Mn value was calculated according to Mn = 0.56MnGPC, where Mn
was determined by GPC
(chloroform), and is relative to polystyrene standards.
Eur. J. Inorg. Chem. 2011, 5347–5359
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5353

F. M. Kerton et al.
Table 4. Details of the reactions of cyclohexene oxide (CHO) with CO2 in the presence of Zn complexes and co-catalysts.
FULL PAPER
Entry
1
Catalyst
–
Cocatalyst
Bu4NBr
[CHO]0:[Zn]0:[Co-cat]0
500:0:1
t [h]
P [bar]
T [°C]
Conv. [%][a]
24
65
100
39.0
(100% cis)
2
3
4
5
6
6
6
6
–
500:1:0
500:1:1
500:1:1
500:1:1
18
16
18
24
45
45
45
65
70
70
70
0
0
4.8
DMAP
Bu4NBr
Bu4NBr
100
57.1
(96.2% cis)
8.3
45.1
(86.4% cis)
6
7
7 + BnOH
7 + EtOH
Bu4NBr
Bu4NBr
500:1:1
500:1:1
18
24
45
65
70
100
1
[a] Determined by the relative intensities of the methine protons in the H NMR spectra.[58]
the greatest degree of reaction control for ROP of ε-CL, conversion increases by 18.1% when it is present in the reac-
in that 4 and 5 afforded ε-polycaprolactone [P(ε-CL)] with tion mixture, and a small amount of trans-cyclic carbonate
narrow molecular weight distributions and molecular was observed. A 7/EtOH/Bu4NBr combination under the
weights close to the expected values (Table 3, entries 7 and same conditions, entry 7, produced slightly less carbonate
8). First-order plots of the consumption of ε-CL in ROP but with a larger amount of the trans-isomer than when
reactions conducted at 70 °C in the presence of 7/BnOH only Bu4NBr is present in the reaction mixture. The cis-
and 2/BnOH are shown in Figure 10.
and trans-isomers are likely formed through different
mechanisms, with the trans-isomer probably formed
through a back-biting reaction at the metal centre. Al-
though conversions were modest and high temperatures and
pressures were required, compared to state-of-the-art cata-
lysts reported in the literature these results can act as a
starting point for further studies, and also give some indica-
tion as to the reason for CO2 uptake by the related Zn com-
plexes reported by Lappert and co-workers.[26]
Conclusions
Piperazinyl-derived amine-phenolate protio ligands can
be prepared in excellent yields in water via a modified Man-
nich condensation reaction. As expected, these ligands
readily react with diethylzinc and generally afford phenolate
bridged Zn-Et dimers. These complexes readily undergo
alcoholysis to yield alkoxide bridged species. A range of
these complexes has been structurally characterized. The
Zn-Et species in the presence of an alcohol, and the alk-
oxide complexes as single component initiators, are able to
facilitate ring-opening polymerization of cyclic esters, how-
ever, no control of stereoselectivity is observed in ROP of
LA and this is probably due to the solvent employed in
Figure 10. First-order plots of ε-CL consumption in ROP reactions
conducted at 70 °C with 7/BnOH (hollow circles) and 2/BnOH
(filled circles). Conditions: [ε-CL]:[Zn] = 200:1, 1 equiv. BnOH,
[Zn]0 = 22.5 mm (7) and 18.8 mm (2).
Reactivity of Complexes Towards CO2 and Cyclohexene
Oxide Coupling
Amongst catalytic coordination complexes for CO2/ep- these reactions. In comparison with morpholinyl derived
oxide couplings and copolymerizations, zinc is central to complexes, the piperazinyl-derived species show slower
many of the species studied.[29,30,52–55] Therefore, studies ROP reaction rates. MW irradiation was used in some of
were performed with 6 and 7/ROH, with 4-(dimethylamino)- the ROP reactions and was useful in reducing reaction
pyridine (DMAP) and Bu4NBr as cocatalysts (Table 4), to times whilst maintaining control of the reaction (i.e. in-
evaluate their activities in the reaction of CO2 with cyclo- creased polydispersities that are usually due to the high
hexene oxide. A control reaction was performed with temperatures used in ROP reactions were not observed).
Bu4NBr alone as ionic salts such as this are known to cata- Both classes of complex show slight activity towards cyclic
lyze the formation of cyclic carbonates (Table 4, entry carbonate formation involving cyclohexene oxide and car-
1).[56–57] Compounds 6 and 7 exhibit some activity towards bon dioxide. These studies suggest that an outer sphere li-
cyclic carbonate formation under high temperature and gand group or base (ether or amine) within an initiating or
pressure conditions and in the presence of an ionic cocata- catalytic species can affect the outcome of ROP and poten-
lyst. Comparison of entries 1 and 5 (Table 4) indicates that tially other reactions. Further studies utilizing these inter-
the zinc complex does play a role in this reaction as the esting ligands are underway in our laboratory.
5354
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 5347–5359

Zinc Complexes of Aminephenolate Ligands
of the homonuclear decoupled 1H NMR spectra (CDCl3). The
equations used to calculate Pr were as described by Coates et al.[51]
The conversions were determined by integration of the relative in-
tensities of the OCHMe resonances corresponding to the residual
LA and poly(rac-lactide), and integration of the ε-methylene sig-
nals due to the residual ε-CL and poly(ε-caprolactone).
Experimental Section
General Considerations: All experiments involving metal complexes
were performed in a nitrogen atmosphere with standard Schlenk
and glovebox techniques. THF was distilled under nitrogen over
sodium/benzophenone. Toluene and hexane were purified by an
MBraun Solvent Purification System. Deuterated solvents (C6D6,
CDCl3, C5D5N) were purchased from Cambridge Isotope
Laboratories, Inc. and purified and dried before use. All solvents
were degassed by freeze-vacuum-thaw cycles prior to use. 2,4-bis-
(tert-butyl)phenol, 2-tert-butyl-4-methylphenol, 2,4-bis(tert-amyl)-
phenol, n-butyllithium, 1-methyl piperazine, morpholine, dieth-
ylzinc (15 wt.-% solution in hexane), LA, ε-CL and cyclohexene
oxide were purchased from Sigma–Aldrich or Alfa Aesar. Mono-
mers were dried and degassed prior to use. Elemental analyses were
performed by Canadian Microanalytical Service Ltd., Delta, BC,
Canada. 1H and 13C{1H} NMR spectra were recorded on a Bruker
Avance 500 MHz spectrometer at 25 °C (unless otherwise stated)
and were referenced internally with respect to the residual proton
and 13C resonances of the solvent. 13C signals were assigned by
heteronuclear single quantum correlation spectroscopy (HSQC) ex-
periments. MALDI-TOF MS measurements were performed with
an Applied Biosystems Voyager DE-PRO equipped with a re-
flectron, delayed ion extraction and high performance nitrogen la-
ser (337 nm). Anthracene was the matrix for the ligands and com-
plexes.[59–60] Mass spectra were also obtained by atmospheric pres-
sure chemical ionization mass spectrometry (AP-CI MS) in positive
mode (70 eV) with an Agilent 1100 LC mass spectrometer. GPC
data were collected on a Viscotek GPCMax System equipped with
Microwave heated polymerizations were performed with a Biotage
InitiatorTM eight microwave synthesizer.
Crystallographic Procedures: All crystals were mounted with para-
tone oil onto a low temperature diffraction loop. Measurements
were made on a Rigaku AFC8-Saturn 70 single-crystal X-ray dif-
fractometer equipped with an X-stream 2000 low temperature sys-
tem and a SHINE optic. Data were collected with Mo-Kα radia-
tion. Data were collected and processed with the CrystalClear soft-
ware (Rigaku).[61a,61b] Numerical absorption corrections were ap-
plied and the data were corrected for Lorentz and polarization ef-
fects. The structures were solved by direct methods (L1H, 1, 3, 4,
5, 6, 7),[61c] (2)[61d] and refined by Fourier techniques.[61e] Neutral
atom scattering factors were taken from those provided by Cromer
and Waber.[61f] Anomalous dispersion effects were included in
[61g]
Fcalc
;
the values for ΔfЈ and ΔfЈЈ were those of Creagh and
McAuley.[61h] The values for the mass attenuation coefficients were
those of Creagh and Hubbell.[61i] All calculations were performed
with the CrystalStructure[61j,61k] crystallographic software package
except for refinement, which was performed with SHELXL-97.[61c]
Hydrogen atoms were introduced at calculated positions and re-
fined with a riding model, unless otherwise indicated. All non-hy-
drogen atoms were refined anisotropically. For L2H, 1, 3, 5 and
a Refractive Index Detector, and columns were purchased from 6, data collections, structural solutions, and refinements proceeded
Phenomenex (Phenogel 5 μ Linear/mixed bed 300ϫ4.60 mm col- normally. For L2H, atom H1 was introduced into the refinement
umn in series with a Phenogel 5 μ, 100 Å, 300ϫ4.60 mm column). model according to its position in the difference map,and initially
Samples were run in chloroform at a concentration of 1 mg/mL,
and at 35 °C. The instrument was calibrated against polystyrene
standards (Viscotek) to determine the molecular weights (Mn and
Mw) and the polydispersity index (Mw/Mn) of the polymers. For
PLA samples, Pr was determined by analysis of the methine region
the atomic x, y, z coordinated were refined, but finally the atom
was refined with a riding model. Friedel mates were averaged and
the absolute configuration was not determined. The asymmetric
unit of 1 contained a half-occupancy toluene molecule that was
disordered over two sites. The toluene protons could not be located
Table 5. Summary of crystal data for compounds L2H, 1–5.
L2H
1
2
3
5
Formula
C20H34N2O
318.50
C47H74N4O2Zn
792.51
monoclinic
C2/c
10.0658(11)
17.945(2)
25.003(3)
90
99.377(3)
90
4455.8(9)
153
C38H64N4O2Zn2
739.71
C58H92N4O2Zn2
1008.15
C79H112N4O4Zn2
1312.54
monoclinic
P2/c
24.314(4)
10.9529(17)
28.023(4)
90
94.232(4)
90
7442.7(20)
153
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
triclinic
triclinic
triclinic
¯
¯
¯
P1
P1
P1
6.1799(14)
8.4415(18)
10.081(2)
67.242(7)
78.354(11)
89.180(12)
473.76(17)
153
9.6308(9)
11.0488(11)
19.6721(19)
100.023(5)
97.146(5)
108.791(4)
1914.6(3)
153
9.206(14)
11.540(19)
13.902(20)
92.493(13)
93.888(18)
111.02(3)
1372(4)
153
α [°]
β [°]
γ [°]
V [Å3]
T [K]
Z
1
4
2
1
4
Dc [g/cm3]
F(000)
1.116
176
1.181
1720
1.283
792
1.220
544
1.171
2824
μ(Mo-Kα)
Total reflections
Unique reflections
Rint
Observations
Reflections I Ͼ 2σI
R (I Ͼ 2σI)
wR2 (all reflections)
GOF
0.68
3883
1914
0.0167
1904
5.92
28248
4605
0.0216
4605
4503
0.0338
0.0924
1.038
12.89
20078
20078
0.000
20078
18150
0.0905
0.2538
1.144
9.18
14165
5600
0.0314
5600
6.94
96494
15414
0.0563
15414
209
5239
14312
0.0348
0.0961
1.049
0.0540
0.1280
1.120
0.0643
0.1574
1.172
Eur. J. Inorg. Chem. 2011, 5347–5359
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5355

F. M. Kerton et al.
FULL PAPER
from difference maps, and were omitted from the model. For 5, one
disordered amyl group was present (PART 1: [C32–35, C39] at 0.65
occupancy; PART 2: [C32A, C36–38, C40] at 0.35 occupancy). Hy-
drogen atoms on C33 and C36 were omitted from the model.
(br, 8 H, NCH2CH2N), 2.31 (s, 3 H, NCH3), 2.23 (s, 3 H, ArCCH3)
1.40 [s,
9
H, ArC–C(CH3)3] ppm. 13C{1H} NMR (CDCl3,
125 MHz, 298 K): δ = 154.7 (ArC–OH), 136.6 (ArC–CH2–N),
127.7 (ArCH), 127.5 (ArCH), 127.0 [ArC–C(CH3)3], 121.6
(ArCCH3), 62.1 (ArC–CH2–N), 55.3 (N–CH2–CH2–N), 52.7 (N–
CH2–CH2–N), 46.3 (CH3–N), 35.0 [ArC–C(CH3)3], 30.0 [ArC–C–
(CH3)]3, 21.2 (ArC–CH3) ppm.
Complexes 2 and 7 were refined as non-merohedral twins with
component #1 comprising 47.99% and 64.81% of the crystal,
respectively (Tables 5 and 6). For 2, a large negative residual density
peak was present in the difference map (1.82 Å from H5A) and is
likely a result of twinning that was not accounted for; overlap from
the second twin domain caused errors in the intensities of some
reflections. Complex 4 was refined as a merohedral twin, with com-
ponent #1 comprising 19.60% of the crystal. The merohedral twin
law was identified by the TwinRotMat function in the PLATON
software.[61l]
L2H: This compound was prepared in the same manner as de-
scribed above for L1H with 2,6-di-tert-butylphenol (12.69 g,
61.5 mmol), formaldehyde 37% solution in water (5 mL,
61.5 mmol), and 1-methyl piperazine (6.8 mL, 61.5 mmol) as start-
ing materials. The product was recrystallized to yield a colourless
crystalline solid; yield 17.65 g, 90%. m.p. 119 °C. MS (MALDI,
anthracene matrix): m/z = 381 (M+, 100%). C20H34N2O (318.50):
calcd. C 75.42, H 10.76, N 8.80; found C 75.40, H 10.65, N 8.79.
1H NMR (CDCl3, 500 MHz, 298 K): δ = 11.49 (s, 1 H, ArOH),
7.26 (s, 1 H, ArH), 6.93 (s, 1 H, ArH), 3.75 (s, 2 H, Ar–CH2–N),
2.98 (d, J = 10.2 Hz, 4 H, NCH2CH2N), 2.86 (d, J = 10.3 Hz, 4
H, NCH2CH2N), 2.35 (s, 3 H, NCH3), 1.42 [s, 9 H, C(CH3)3], 1.29
[s, 9 H, C(CH3)3] ppm. 13C{1H} NMR (CDCl3, 125 MHz, 298 K):
δ = 154.5 (ArCOH), 140.9 [ArCC(CH3)3], 135.8 [ArC–C(CH3)3],
123.8 (ArCH), 123.3 (ArCH), 120.9 (ArC–CH2), 62.5 (CH3–N),
55.3 (N–CH2–CH2–N), 52.7 (N–CH2–CH2–N), 46.3 (ArC–C–
CH2), 35.2 [ArC–C(CH3)3], 34.5 [ArC–C(CH3)3], 32.1 [ArC–C–
(CH3)]3, 30.0 [ArC–C–(CH3)]3 ppm.
Table 6. Summary of crystal data for compounds 6–7.
Compounds
6
7
Formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C44H76N4O4Zn2
855.87
C56H86N2O4Zn2
982.07
triclinic
triclinic
¯
¯
P1
P1
8.960(3)
9.099(3)
14.779(6)
81.33(3)
76.04(2)
76.04(2)
1123.8(7)
153
10.9384(15)
11.4289(15)
11.9898(16)
102.923(8)
106.541(7)
107.637(8)
1288.1(3)
153
α [°]
β [°]
γ [°]
L3H: Water (100 mL), 2,6-di-tert-amylphenol (18.99 g, 81.0 mmol)
and formaldehyde 37% solution in water (6.6 mL, 81.0 mmol) were
added to a 500 mL round-bottom flask equipped with a stir bar
and a condenser. 1-Methyl piperazine (8.1 g, 81.0 mmol) was added
dropwise to the stirred solution. The resulting mixture was heated
at reflux for 18 h. Upon cooling to room temperature a two phase
mixture formed. The organic material was extracted with diethyl
ether and dried with magnesium sulfate. The solvent was removed
by rotary evaporation to give a pale brown oil from which colour-
less crystals were obtained upon standing for several days under
ambient conditions; yield 23.02 g, 82%. m.p. 82 °C. MS (AP-CI,
MeOH): m/z = 347 (M+, 100%). C22H38NO2 (348.55): calcd. C
V [Å3]
T [K]
Z
1
1
Dc [g/cm3]
F(000)
1.265
460
1.266
528
μ(Mo-Kα)
Total reflections
Unique reflections
Rint
11.11
8979
4141
0.0939
9.77
15846
15846
0.000
Reflections I Ͼ 2σ(I) 2984
14915
Parameters
245
290
R [I Ͼ 2σ(I)]
wR2 (all reflections)
GOF
0.1060
0.2132
1.149
0.1061
0.2854
1.248
1
76.25, H 11.05, N 8.08; found C 76.36, H 10.80, N 8.04. H NMR
(CDCl3, 500 MHz, 298 K): δ = 10.77 (s, 1 H, ArOH), 7.07 (d, J =
2.2 Hz, 1 H, ArH), 6.75 (d, J = 2.2 Hz, 1 H, ArH), 3.67 (s, 2 H,
ArC–CH2–N), 2.53 (br, 8 H, N–C2H4–N), 2.30 (s, 3 H, NCH3),
1.88 (q, J = 7.4 Hz, 2 H, CCH2CH3), 1.56 (q, J = 7.4 Hz, 2 H,
CCH2CH3), 1.36 [s, 6 H, ArC–C(CH3)2], 1.23 [s, 6 H, ArC–
C(CH3)2], 0.65 (t, J = 7.4 Hz, 3 H, CH2CH3), 0.64 (t, J = 7.4 Hz,
3 H, CH2CH3) ppm. 13C{1H} NMR (CDCl3, 500 MHz, 298 K): δ
= 154.2 (ArC–OH), 139.0 [ArCC(CH3)3], 134.1 [ArC–C(CH3)3],
125.3 (ArCH), 124.4 (ArCH), 120.6 (ArC–CH2–N), 62.5 (N–CH2–
C), 55.3 (N–CH2–CH2–N), 52.3 (N–CH2–CH2–N), 46.3 {ArC–
C[(CH3)2–CH2]}, 38.7 (N–CH3), 37.6 {C[(CH3)2–CH2]}, 37.5 {C–
[(CH3)2–CH2]}, 33.4 {ArC–C[(CH3)2–CH2]}, 28.9 {C[(CH3)2–
CH2]}, 28.0 {C[(CH3)2–CH2]}, 9.8 (CH2–CH3), 9.5 (CH2–CH3)
ppm.
CCDC-762217 (for L2H), -762221 (for 1), -762220 (for 2), -762222
(for 3), -833271 (for 4), -762223 (for 5), -833272 (for 6), -833273
(for 7) contain the supplementary crystallographic data for this pa-
per. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Synthetic Procedures
L1H: Water (80 mL), 2-tert-butyl-4-methylphenol (10.10 g,
61.5 mmol) and formaldehyde 37% solution in water (5 mL,
61.5 mmol) were added to 250 mL round-bottom flask equipped
with a stir bar and a condenser. 1-Methylpiperazine (6.8 mL,
61.5 mmol) was added dropwise to the stirred solution. The re-
sulting mixture was heated at reflux for 18 h. Upon cooling to
room temperature a two phase mixture was formed. The organic
layer (solid) was isolated, washed with methanol and dried under
vacuum to remove any unreacted organic components. The re-
sulting pale grey solid was recrystallized from methanol to give
colourless crystals; yield 16.71 g, 98%. m.p. 90 °C. MS (AP-CI,
MeOH): m/z = 277 (M+, 100%). C17H28N2O (276.42): calcd. C
73.87, H 10.21, N 10.13; found C 73.81, H 10.10, N 10.66. 1H
NMR (CDCl3, 500 MHz, 298 K): δ = 10.90 (s, 1 H, ArOH), 6.99
(s, 1 H, ArH), 6.67 (s, 1 H, ArH), 3.65 (s, 2 H, Ar–CH2–N), 2.51
L4H: Water (80 mL), 2-tert-butyl-4-methylphenol (16.51 g,
80.0 mmol) and formaldehyde 37% solution in water (6.50 mL,
80.0 mmol) were added to 250 mL round-bottom flask equipped
with a stir bar and a condenser. Morpholine (6.97 mL, 80.0 mmol)
was added dropwise to the stirred solution. The resulting mixture
was heated at reflux for 18 h. Upon cooling to room temperature
a two phase mixture was formed. The organic layer (solid) was
isolated, washed with methanol and dried under vacuum to remove
any unreacted organic components. The resulting brown gel-like
solid was recrystallized from methanol to give a colourless solid;
5356
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 5347–5359

Zinc Complexes of Aminephenolate Ligands
yield 21.23 g, 87%. m.p. 112 °C. MS (MALDI, anthracene matrix):
(s, 2 H, Ar–CH2–N), 2.64 (br, 4 H, NCH2CH2N), 2.54 (s, 4 H,
NCH2CH2N), 2.27 (s, 3 H, NCH3), 1.81 [s, 9 H, C(CH3)3], 1.57 (t,
m/z = 305 (M+, 100%). C19H31NO2 (305.46): calcd. C 74.71, H
1
10.23, N 4.59; found C 74.66, H 10.27, N 4.52. H NMR (CDCl3, J = 8.0 Hz, 3 H, CH3CH2), 1.50 [s, 9 H, C(CH3)3], 0.59 (q, J =
500 MHz, 298 K): δ = 10.66 (s, 1 H, ArOH), 7.23 (d, J = 2.2 Hz,
1 H, ArH), 6.84 (d, J = 2.2 Hz, 1 H, ArH), 3.75 (s, 2 H, Ar–CH2–
8.0 Hz, 2 H, CH3CH2) ppm. 13C{1H} NMR (C5D5N, 125 MHz,
298 K): δ = 165.3 (ArC–O), 138.3 (ArC–CH2N), 134.5 [ArC–
N), 3.68 (br, 4 H, OC2H4C2H4N) 2.56 (br, 4 H, OC2H4C2H4N), C(CH3)3], 126.6 (ArCH), 126.0 (ArCH), 122.1 [ArC–C(CH3)3],
1.41 [s, 9 H, C(CH3)3], 1.28 [s, 9 H, C(CH3)3] ppm. 13C{1H} NMR
64.6 (ArC–CH2–N), 55.2 (N–CH2–CH2–N), 53.5 (N–CH2–CH2–
(CDCl3, 125 MHz, 298 K): δ = 154.4 (ArC–OH), 141.2 [ArC– N), 46.1 (CH3–N), 36.0 [C(CH3)3], 34.4 [C(CH3)3], 32.6 [C(CH3)3],
C(CH3)3], 136.0 [ArC–C(CH3)3], 124.0 (ArCH), 123.6 (ArCH), 30.5 [C(CH3)3], 14.1 (CH3CH2), 1.5 (CH3CH2) ppm.
120.5 (ArC–CH2), 67.2 (O–C2H4–C2H4-N), 63.1 (ArCCH2N), 53.2
{[μ-ONNtAm,tAm]ZnEt}2 (4): To a 50 mL flask containing dieth-
(O–C2H4–C2H4–N), 35.3 [ArC–C(CH3)3], 34.6 [ArC–C(CH3)3],
ylzinc (6.2 mL, 5.02 mmol, 10 wt.-% in hexane) a solution of L3H
32.1 [ArC–C–(CH3)]3, 30.0 [ArC–C–(CH3)]3 ppm.
(1.74 g, 5.02 mmol) in toluene (2.0 mL) that was cooled to –35 °C
Zn[ONNtBu,tBu
manner as described below for
]
2
(1): This compound was prepared in the same
was added dropwise. The colourless mixture was warmed, while
being stirred, to room temperature and then held at this tempera-
ture for 18 h. Volatiles were removed under vacuum to give a white
residue, which was washed with hexane (2ϫ 4 mL) and dried in
vacuo. The crude product was recrystallized from a saturated tolu-
ene solution at –35 °C to give 4 as a colourless crystalline solid;
yield 1.92 g, 87%. C48H84N4O2Zn2 (879.98): calcd. C 65.51, H 9.62,
N 6.37; found C 65.47, H 10.89, N 6.28. 1H NMR (C5D5N,
2 with diethylzinc (1.7 mL,
2.0 mmol, 15 wt.-% in hexane) and L2H (0.64 g, 2.0 mmol) as start-
ing materials. 1 was obtained as a colourless crystalline solid; yield
1.07 g, 77%. Crystals suitable for X-ray crystallography could be
grown by slow evaporation, or by cooling at –35 °C, of a saturated
toluene/hexane solution. C47H74N4O2Zn(C7H8) (792.5 + 92.1 =
884.6): calcd. C 71.23, H 9.41, N 7.07; found C 71.10, H 9.49, N
7.05. 1H NMR (C5D5N, 500 MHz, 343 K): δ = 7.58 (s, 1 H, ArH), 500 MHz, 358 K): δ = 7.49 (s, 1 H, ArH), 7.04 (s, 1 H, ArH), 3.65
7.11 (s, 1 H, ArH), 4.07 (s, 2 H, Ar–CH2–N), 3.07 (br, 4 H, (s, 2 H, ArC–CH2–N), 2.64 (br, 4 H, NCH2CH2N), 2.56 (s, 4 H,
NCH2CH2N), 2.83 (s, 4 H, NCH2CH2N), 2.29 (s, 3 H, NCH3),
NCH2CH2N), 2.41 (q, J = 7.4 Hz, 2 H, CCH2CH3), 2.27 (s, 3 H,
NCH3), 1.80 (q, J = 7.4 Hz, 2 H, CCH2CH3), 1.74 [s, 6 H,
1.60 [s, 9 H, C(CH3)3], 1.44 [s, 9 H, C(CH3)3] ppm. 13C{1H} NMR
(C5D5N, 125 MHz, 298 K): δ = 165.3 (ArC–O), 137.8 (ArC– C(CH3)2], 1.59 (t, J = 8.0 Hz, 3 H, CH3CH2), 1.46 [s, 6 H,
CH2N), 134.5 [ArC–C(CH3)3], 129.6 (CH), 128.9 (CH), 122.1 C(CH3)2], 0.98 (t, J = 7.4 Hz, 3 H, CH2CH3), 0.91 (t, J = 7.4 Hz,
[ArC–C(CH3)3], 64.6 (ArCCH2N), 55.1 (N–CH2–CH2–N), 53.5 3 H, CH2CH3), 0.61 (q, J = 8.0 Hz, 2 H, CH3CH2) ppm. 13C{1H}
(N–CH2–CH2–N), 46.1 (CH3–N), 36.0 [ArC–C(CH3)3], 34.4 [ArC–
C(CH3)3], 32.6 [C(CH3)3], 30.5 [C(CH3)3] ppm.
NMR (C5D5N, 125 MHz, 298 K): δ = 165.2 (ArC–O), 132.4 (ArC–
CH2–N), 127.3 (ArCH), 126.4 (ArCH), 121.8 {ArC–C[(CH3)2–
CH2]}, 64.5 (ArC–CH2–N), 55.1 (N–CH2–CH2–N), 53.5 (N–CH2–
CH2–N), 46.1 (N–CH3), 39.42 {C[(CH3)2–CH2]}, 37.9 {C[(CH3)2–
CH2]}, 37.5 {C[(CH3)2–CH2]}, 33.3 {C[(CH3)2–CH2]}, 29.6–28.5
{C[(CH3)2–CH2]}, 14.1 (CH3CH2) 10.7–9.9 [(CH3)2–CH2–CH3],
1.6 (CH3CH2) ppm.
{[μ-ONNMe,tBu]ZnEt}2 (2): To a 50 mL flask containing diethylzinc
(6.2 mL, 5.02 mmol, 15 wt.-% in hexane) a solution of L1H (1.39 g,
5.02 mmol) in toluene (2.0 mL) that was cooled to –35 °C was
added dropwise. The colourless mixture was warmed, while being
stirred, to room temperature and then held at this temperature for
18 h. Volatiles were removed under vacuum to give a white residue,
which was washed with hexane (2ϫ 4 mL) and dried in vacuo. The
crude product was recrystallized from a saturated toluene solution
held at –35 °C to give 2 as a colourless crystalline solid; yield 1.43 g,
77%. C38H64N4O2Zn2 (739.71): calcd. C 61.70, H 8.72, N 7.57;
found C 62.02, H 9.13, N 7.98. 1H NMR (C5D5N, 500 MHz,
343 K): δ = 7.33 (s, 1 H, ArH), 6.84 (s, 1 H, ArH), 4.00 (s, 2 H,
{[ONNtAm,tAm]Zn(μ-OBn)}2 (5): To
a solution of 4 (0.80 g,
0.91 mmol) in toluene (5 mL) that was cooled to –35 °C BnOH
(190 μL, 1.8 mmol) was added dropwise. The reaction mixture was
warmed, while being stirred, to room temperature and then held at
this temperature for 3 h. The solvent was removed under vacuum
to give a white residue, which was washed with cold hexane (2ϫ
4 mL) and dried in vacuo. The crude product was recrystallized
Ar–CH2–N), 3.04 (br, 4 H, N CH2CH2N), 2.83 (br, 4 H, from toluene/hexane to give 5 as a colourless crystalline solid; yield
NCH2CH2N), 2.35 (s, 3 H, NCH3), 2.30 (s, 3 H, ArC–CH3), 1.78 0.84 g, 89%. C58H88N4O4Zn2 (1036.12): calcd. C 67.23, H 8.56, N
1
(s, 2 H, CH2CH3), 1.58 [q, 9 H, ArC–C(CH3)3], 0.91 (t, 3 H,
5.41; found C 67.78, H 9.12, N 5.63. H NMR (C5D5N, 500 MHz,
CH2CH3) ppm. 13C{1H} NMR (C5D5N, 125 MHz, 298 K): δ = 343 K): δ = 7.69 (s, 1 H, ArH), 7.51 (s, 2 H, ArH), 7.43 (s, 1 H,
164.4 (ArC–O), 138.6 (C–CH2N), 130.4 (ArCH), 128.7 (ArCH), ArH), 7.37 (s, 1 H, ArH), 7.05 (s, 2 H, ArH), 5.11 (br, 2 H,
121.9 [C–C(CH3)3], 120.8 [C(CH3)], 64.6 (N–CH2–C), 55.4 (N– OCH2Ar), 4.09 (s, 2 H, ArC–CH2–N), 2.60–3.05 (br, 8 H, overlap-
CH2–CH2–N), 53.0 (N–CH2–CH2–N), 45.9 (Ar–CH2–N), 35.5 ping, N–C2H4–N), 1.77 (m, 4 H, CCH2CH3), 1.58 (s, 3 H, NCH3),
(CH3–N), 30.6 [C(CH3)3], 30.4 [C(CH3)3], 21.4 (CH3CH2), 21.2 1.41 [s, 6 H, C(CH3)2], 1.38 [s, 6 H, C(CH3)2], 0.87 (m, 6 H,
[C(CH3)], 14.1 (CH3CH2) ppm.
CH2CH3) ppm. 13C{1H} NMR (C5D5N, 125 MHz, 298 K): δ =
163.8 (ArC–O), 144.5 (ArC–CH2), 133.6 (ArC–CH2–N), 129.1
(ArC), 129.0 (ArC), 128.0 (ArCH), 127.3 (ArCH), 126.8 (ArC),
119.9 {ArC–C[(CH3)2–CH2]}, 69.4 (ArC–CH2), 65.0 (ArC–CH2–
N), 55.3 (N–CH2–CH2–N), 54.2 (N–CH2–CH2–N), 45.9 (N–CH3),
39.1 {C[(CH3)2–CH2]}, 37.8 {C[(CH3)2–CH2]}, 37.5 {C[(CH3)2–
CH2]}, 33.4 {C[(CH3)2–CH2]}, 29.4–28.5 {C[(CH3)2–CH2]}, 10.4–
9.9 [(CH3)2–CH2–CH3] ppm.
{[μ-ONNtBu,tBu]ZnEt}2 (3): To a 50 mL flask containing diethylzinc
(3.3 mL, 4.0 mmol, 15 wt.-% in hexane) a solution of L2H (0.64 g,
2.0 mmol) in toluene (2.0 mL) that was cooled to –35 °C was added
dropwise. The colourless mixture was warmed, while being stirred,
to room temperature and then held at this temperature for 18 h.
Volatiles were removed under vacuum to give a white residue, which
was washed with hexane (2ϫ 4 mL) and dried in vacuo; yield
0.92 g, 56%. Crystals suitable for X-ray crystallography could be
grown by slow evaporation, or by cooling at –35 °C, of a saturated
toluene/hexane solution. C44H76N4O2Zn2 (823.87): calcd. C 64.14,
{[ONNtBu,tBu]Zn(μ-OEt)}2 (6): To a 50 mL flask containing dieth-
ylzinc (7.8 mL, 6.3 mmol, 15 wt.-% in hexane) a solution of L2H
(2.00 g, 6.3 mmol) in toluene (2.0 mL) cooled to –35 °C was added
H 9.30, N 6.80; found C 63.72, H 9.21, N 6.10. 1H NMR (C5D5N, dropwise. The colourless mixture was warmed, while being stirred,
500 MHz, 358 K): δ = 7.63 (s, 1 H, ArH), 7.10 (s, 1 H, ArH), 3.65
to room temperature and then held at this temperature for 3 h.
Eur. J. Inorg. Chem. 2011, 5347–5359
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5357

F. M. Kerton et al.
FULL PAPER
1
Volatiles were removed under vacuum to give an off-white residue,
which was washed with pentane (2ϫ 4 mL) and redissolved in tolu-
ene (2.0 mL). To this solution EtOH (366 μL, 6.3 mmol) was added
dropwise, and the reaction mixture was stirred at room temperature
for 18 h. The resulting mixture was filtered and the filtrate was
concentrated under vacuum and cooled to –35 °C to afford 6 as
a colourless crystalline solid; yield 1.65 g, 61%. C44H76N4O4Zn2
(855.87): calcd. C 61.75, H 8.95, N 6.55; found C 61.69, H 9.19, N
7.00. 1H NMR (C5D5N, 500 MHz, 358 K): δ = 7.72 (s, 1 H, ArH),
7.66 (s, 1 H, ArH), 4.04 (br, 2 H, OCH2CH3), 3.83 (s, 2 H, Ar–
CH2–N), 3.00 (br, 4 H, NCH2CH2N), 2.79 (br, 4 H, NCH2CH2N),
2.53 (s, 3 H, NCH3), 1.92 [s, 9 H, C(CH3)3], 1.58 [s, 9 H, C(CH3)3],
1.55 (t, 3 H, OCH2CH3) ppm. 13C{1H} NMR (C5D5N, 125 MHz,
298 K): δ = 164.2 (ArC–O), 138.1 (C–CH2N), 126.4 (ArCH), 126.1
(ArCH), 119.9 [C–C(CH3)3], 119.7 [C–C(CH3)3], 65.6 (N–CH2–C),
55.3 (N–CH2–CH2–N), 54.1 (N–CH2–CH2–N), 46.1 (CH3–N), 45.8
(OCH2CH3) 36.0 [C(CH3)3], 35.6 [C(CH3)3], 34.3 (OCH2CH3) 32.4
[C(CH3)3], 30.5 [C(CH3)3] ppm.
time an aliquot (ca 0.3 mL) was withdrawn from the flask for H
NMR analysis to determine the monomer conversion. The reaction
was quenched with methanol and the resulting solid was dissolved
in dichloromethane, precipitated with excess cold methanol, then
filtered and dried under reduced pressure.
Typical Microwave-Assisted Ring-Opening Polymerization: To a mi-
crowave vial charged with LA (576 mg, 5.27 mmol) or ε-CL
(601 mg, 5.27 mmol) in toluene (10 mL),
a toluene solution
(2.00 mL) of the initiator containing a prescribed amount of benzyl
or tert-butyl alcohol was added. The vial was sealed with a septum
and the mixture was irradiated to a constant required temperature
for the desired amount of time. After irradiation, the vial was co-
oled to room temperature, an aliquot of the product was taken
for NMR spectroscopic analysis, and the reaction was quenched
immediately by the addition of methanol. The resulting solid was
dissolved in dichloromethane and the polymer precipitated with
excess cold methanol. A colourless product was collected by fil-
tration, washed with methanol to remove any unreacted monomer,
and dried under reduced pressure.
{[μ-ONOtBu,tBu]ZnEt}2 (7): To a 50 mL flask containing diethylzinc
(6.2 mL, 5.02 mmol, 15 wt.-% in hexane) a solution of L4H (1.53 g,
5.02 mmol) in toluene (2.0 mL) cooled at –35 °C was added drop-
wise. The colourless mixture was warmed, while being stirred, to
room temperature and then held at this temperature for 18 h. Vola-
tiles were removed under vacuum to give a white residue, which
was washed with hexane (2ϫ 4 mL) and dried in vacuo; yield
1.62 g, 81%. Crystals suitable for X-ray crystallography could be
grown by slow evaporation, or by cooling at –35 °C, of a saturated
toluene solution. C42H70N4O4Zn2 (825.80): calcd. C 63.23, H 8.84,
N 3.51; found C 63.16, H 8.55, N 3.60. 1H NMR (C5D5N,
500 MHz, 313 K): δ = 7.63 (s, 1 H, ArH), 7.10 (s, 1 H, ArH), 3.81
(br, 4 H, OC2H4C2H4N), 3.59 (s, 2 H, Ar–CH2–N), 2.50 (br, 4 H,
OC2H4C2H4N), 1.79 [s, 9 H, C(CH3)3], 1.59 (t, J = 8.0 Hz, 3 H,
CH3CH2), 1.48 [s, 9 H, C(CH3)3], 0.58 (q, J = 8.0 Hz, 2 H,
CH3CH2) ppm. 13C{1H} NMR (C5D5N, 125 MHz, 298 K): δ =
165.35 (ArC–O), 141.2 [ArC–C(CH3)3], 137.93 (ArC–CH2N),
134.70 [ArC–C(CH3)3], 126.73 (ArCH), 124.44 (ArCH), 121.57
[ArC–C(CH3)3], 65.55 (O–C2H4–C2H4–N), 65.45 (O–C2H4–C2H4–
N), 65.13 (ArCCH2N), 36.01 [ArC–C(CH3)3], 34.41 [ArC–
C(CH3)3], 32.60 [ArC–C–(CH3)]3, 30.48 [ArC–C–(CH3)]3, 13.98
(CH3CH2), 0.76 (CH3CH2) ppm.
CO2-Cyclohexene Oxide Reactions: The complex (0.2 mol-%) was
dissolved in cyclohexene oxide (2.27 g, 23.2 mmol) that was held in
a glovebox. The mixture and cocatalyst (0.2 mol-%) was placed in
a predried autoclave equipped with a pressure gauge. The autoclave
was sealed, removed from the glovebox and attached to an over-
head magnetic stirrer. The system was pressurized to approx. 40 bar
with CO2 and heated (as with all processes under high pressure,
appropriate safety precautions must be taken). The mixture was
stirred for the desired amount of time and then the pressure was
slowly released whilst the pressure vessel was cooled in an ice-bath.
A small amount of the residue in the vessel was removed for 1H
NMR analysis.
Supporting Information (see footnote on the first page of this arti-
1
cle): H NMR and MALDI-TOF MS spectra of L2H, representa-
tive homodecoupled 1H NMR spectra of PLA, diagrams of the
molecular structures of 4 and 5, and ln([LA]0/[LA]t) vs. time plot
for the bulk polymerization of LA.
Acknowledgments
We thank Natural Sciences and Engineering Research Council of
Canada (NSERC) of Canada, the Canada Foundation for Innova-
tion, the Provincial Government of Newfoundland and Labrador,
and Memorial University for financial support. J. M. B. S. thanks
Women in Science and Engineering High-School Summer Student
Employment Program. N. I. thanks Dr. Pawel Horeglad (Univer-
sity of Warsaw) for advice on tacticity calculations.
Typical Solution Polymerization Procedure: The reaction mixtures
were prepared in a glovebox and subsequent operations were per-
formed with standard Schlenk techniques. A monomer:initiator ra-
tio of 100:1 was employed. A sealable Schlenk tube containing a
stir bar and the monomer (0.05 mmol) in toluene (8.00 mL) was
heated to the desired temperature, then the initiator solution
(26.3 mm), which was taken from a stock solution containing a pre-
scribed amount of ROH, was added to the tube. The mixture was
stirred for the allotted time. An aliquot of the reaction solution
was taken for NMR spectroscopic analysis, and the reaction was
quenched immediately by the addition of methanol. The resulting
solid was dissolved in dichloromethane and the polymer precipi-
tated with excess cold methanol. The polymer was collected by fil-
tration, washed with methanol to remove any unreacted monomer,
and dried under reduced pressure.
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Received: July 8, 2011
Published Online: September 16, 2011
Eur. J. Inorg. Chem. 2011, 5347–5359
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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