Synthesis and structures of tridentate ketoiminate zinc complexes that act as l-lactide ring-opening polymerization catalysts

Article abstract of DOI:10.1021/om200865w

A series of NNO tridentate Schiff base ligands were used to prepare zinc amide and zinc phenoxide complexes that were shown to be efficient l-lactide ring-opening polymerization (ROP) catalysts. The complexes were prepared from ketoimines bearing a pendant quinoline donor, zinc bis(trimethylsilyl)amide, and 2,6-di-tert-butylphenol. They were characterized with ¹H and ¹³C NMR, absorbance spectroscopy, microanalysis, and X-ray crystallography. The zinc amide and zinc phenoxide structures showed mononuclear complexes with tridentate coordination by the ketoiminate ligands. ROP of l-lactide with the zinc amides and phenoxide complexes gave isotactic poly-l-lactide with generally low molecular weight distributions. As compared to their amide counterparts, the zinc phenoxide complexes showed superior lactide ROP behavior in terms of percent conversion as a function of time, measured molecular weights closer to the predicted values, and lower polydispersity index values. Increasing size of the substituent at the 2-position on quinoline (H, Me, Ph) improved the synthesis of the complexes but adversely affected the ROP.

Full text of DOI:10.1021/om200865w

Article
pubs.acs.org/Organometallics
Synthesis and Structures of Tridentate Ketoiminate Zinc Complexes
That Act As L-Lactide Ring-Opening Polymerization Catalysts
Courtney C. Roberts, Brandon R. Barnett, David B. Green, and Joseph M. Fritsch*
Department of Chemistry, Pepperdine University, Malibu, California 90263, United States
S
* Supporting Information
ABSTRACT: A series of NNO tridentate Schiff base ligands were used to prepare zinc amide and zinc phenoxide complexes
that were shown to be efficient L-lactide ring-opening polymerization (ROP) catalysts. The complexes were prepared from
ketoimines bearing a pendant quinoline donor, zinc bis(trimethylsilyl)amide, and 2,6-di-tert-butylphenol. They were
1
characterized with H and 13C NMR, absorbance spectroscopy, microanalysis, and X-ray crystallography. The zinc amide and
zinc phenoxide structures showed mononuclear complexes with tridentate coordination by the ketoiminate ligands. ROP of L-
lactide with the zinc amides and phenoxide complexes gave isotactic poly-L-lactide with generally low molecular weight
distributions. As compared to their amide counterparts, the zinc phenoxide complexes showed superior lactide ROP behavior in
terms of percent conversion as a function of time, measured molecular weights closer to the predicted values, and lower
polydispersity index values. Increasing size of the substituent at the 2-position on quinoline (H, Me, Ph) improved the synthesis
of the complexes but adversely affected the ROP.
catalysts are inorganic metal complexes that include a
supporting ligand and a ROP-initiating ligand.
INTRODUCTION
■
Significant interest and research efforts have been expended on
the development of polymers from 100% renewable resources
as alternatives to petrochemical-based polymers.1 Aliphatic
polyesters have drawn wide attention, and, specifically,
polylactide (PLA) has seen intense interest due to its
biocompatibility for use in biomedical devices (sutures, drug
delivery systems, etc.).2 The uses of PLA have expanded into
consumer products as the price of manufacture has declined1d
and the attractiveness of biodegradable polymers has increased
in response to increasing emphasis on the environmental fate of
such objects.
Lactide is a cyclic dimer of lactic acid that is derived from
100% renewable resources, e.g., corn or sugar beets. The
presence of two chiral centers on each lactide monomer
generates three isomers, L-lactide (SS-configurations), D-lactide
(RR-configurations), and meso-lactide (RS-configurations).
Lactide monomers are converted to PLA by ring-opening
polymerization (ROP) catalysts, which utilize a coordination−
insertion chain growth mechanism to produce, in the case of L-
and D-lactide, highly crystalline isotactic PLA.3 Many of these
Reviews of the field detailing the metal centers and
supporting ligands utilized for lactide ROP have appeared in
the literature in recent years.3,4 Some trends among complexes
employed as ROP catalysts include the use of highly Lewis
acidic metal centers and the use of multidentate supporting
ligands to the metal center during polymerization. In addition,
several common ROP initiator ligands that have seen broad use
include alkyl (Et), alkoxides (OBn, OSiPh3, O(tBu)2Ph), and
amides (N(SiMe3)2, NPr2).
Recent work has focused on the use of biocompatible metal
centers such as Mg,5 Al,6 and Zn.6d,7 For Zn, bidentate
complexes of β-diketiminate5g,j−o,8 and salicylaldiminate
ligands9 have been widely used as lactide ROP catalysts.
Tridentate Schiff bases have garnered significant interest where
salicylaldiminates bearing a pendant donor group such as
ethylenediamine, cyclohexylamine, pyridine, and quinoline have
found purchase.5i,6g,10 In many cases bis-trimethylsilyl amide or
Received: September 14, 2011
© XXXX American Chemical Society
A
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benzyl alkoxide groups have been included as ROP-initiating
substituents. Ketoiminates with pendant donor groups have
been widely used in the stabilization of metal centers from
across the periodic table;11 however despite the similarities to
the salicylaldiminates, these ligands have found little use in
lactide ROP catalysts.5h,6k
Scheme 1
Whether the supporting ligand system is a β-diketiminate,
salicylaldiminate, or ketoiminate, the formation of bis-ligated
zinc complexes has been regularly reported in studies
concerning lactide ROP catalysts and in other setting-
s.5i,6d,g,8c,9,10,10j,12 In lactide settings, a few of the bis-ligated
zinc complexes in either tetrahedral or octahedral coordination
geometries were shown to be effective ROP catalysts.6d,7a,12c
However, the trend is for generally inactive bis-ligated zinc
complexes. The introduction of bulky groups on the ROP-
initiating ligand (bis(trimethylsilyl)amide, di-tert-butyl phen-
oxide) or strategically positioned groups on the supporting
ligands (tBu on salicylaldiminate) has helped to mitigate this
challenge.9a,10b−d,g,j,13 In contrast to the salicylaldiminates,
ketoiminates do not offer the same opportunity to introduce
such a sterically bulky group on the supporting ligand.
spectrometry (HR-MS), and elemental analysis. The 2-phenyl-
8-hydroxyquinoline, I, was converted to the amine at elevated
temperature in a steel reactor over one week. Compound II was
isolated in 31% yield with column chromatography and was
1
characterized with H and 13C NMR, HR-MS, and elemental
analysis. The 2-phenyl-8-aminoquinoline, II, was combined
with 2,4-pentanedione under the Schiff base condensation
reaction conditions to prepare L4-H (Scheme 2).
Scheme 2
Here we report on the synthesis and characterization of zinc
bis(trimethylsilyl)amide and zinc di-tert-butyl phenoxide
complexes with ketoiminates bearing quinoline pendant donors
and their use as L-lactide ROP catalysts. The use of a flexible
architecture on the Schiff base allowed for differing substituents
on the ketoimine backbone and groups of increasing steric bulk
(H, Me, Ph) to be incorporated at the 2-position on the
quinoline pendant to mitigate the formation of bis-ligated zinc
complexes. In addition, the steric demands of bis-
(trimethylsilyl)amide and 2,6-di-tert-butylphenol were required
for the synthesis of the mononuclear zinc complexes. The zinc
amide and phenoxide complexes were characterized spectro-
scopically, and several were characterized crystallographically.
All complexes were assessed for their L-lactide ROP efficiency,
as well. The phenoxide complexes were both more efficient and
better behaved catalysts than their amide counterparts.
L4-H was isolated in 68% yield with column chromatography
1
and was characterized by H and 13C NMR, HR-MS, and
elemental analysis.
Zinc Amide Complexes. Zinc amide complexes, 1a−4a,
were synthesized by combining 1.1 to 1.3 equivalents of zinc
bis(trimethylsilyl)amide (prepared with a method adapted from
the literature17) and L1-H−L4-H under an inert atmosphere
(Scheme 3).
Reaction of zinc bis(trimethylsilyl)amide with L1-H−L4-H
gave zinc amide complexes that were isolated by precipitation
from Et2O and hexanes in 70−73% yield. A slight excess of zinc
bis(trimethylsilyl)amide was required due to the formation of
bis-ligated zinc complexes as an impurity when equimolar ratios
RESULTS AND DISCUSSION
■
1
Synthesis and Structural Studies. Synthesis of Bulky
Ketoimine Ligand. The three ketoimines, L1-H, L2-H, and L3-
H, were synthesized according to the literature method and
isolated in 88%, 90%, and 81% yields, respectively.11f When
these ketoimines were previously combined with magnesium
benzyl alkoxide, bis-ligated magnesium complexes formed
despite the incorporation of a methyl group at the 2-position
on the quinoline pendant donor.11f These complexes were
inactive toward lactide ROP. Because many zinc bis-ligated
complexes have been reported,5i,6d,g,8c,9,10,12,14 we anticipated
the need to include a larger group to prevent the formation of
bis-ligated zinc complexes here. Thus, our group synthesized a
new Schiff base ligand, L4-H, with a more bulky phenyl group at
the 2-position on the quinoline pendant donor to inhibit the
formation of the bis-ligand complex.
were used. The H and 13C NMR spectra of 1a−4a show the
incorporation of a single ketoiminate ligand and a single
1
bis(trimethylsilyl)amide in the zinc complexes. The H NMR
spectra of 1a−4a show sharp signals, indicating there is no
fluctuation in the zinc coordination environment. For example,
the methine proton on the ketoimine backbone of 1a is a sharp
singlet at 5.08 ppm, indicating the oxygen and nitrogen are
coordinated to the zinc. Additionally, the proton at the 2-
position on the quinoline pendant is a sharp doublet of
doublets at 8.53 ppm, indicating no fluxional coordination of
the quinolyl nitrogen to zinc. Despite the incorporation of the
bulky bis(trimethylsilyl)amide group, complexes 1a−4a were
thermally unstable when not in the presence of zinc
bis(trimethylsilyl)amide and formed bis-ligated complexes in
solution at room temperature over the course of hours.
To use the synthetic methods employed for L1-H−L3-H, the
synthesis of L4-H required the preparation of 2-phenyl-8-
aminoquinoline. The previously reported synthetic method for
this compound required the use of arsenic pentoxide.15 An
alternate synthetic pathway was developed starting from 8-
hydroxyquinoline with methods adapted from the literature
(Scheme 1).16
Zinc Phenoxide Complexes. Complexes 1a−4a were
converted to the phenoxide analogues by addition of 2,6-di-tert-
butylphenol, yielding 1b−4b (Scheme 3). Compounds 1b−4b
were insoluble in hexanes and isolated by filtration in 59−76%
yield. Attempts to prepare zinc alkoxide complexes with L1-H
and L2-H in a 1:1 ratio of zinc amide (1a and 1b) to alcohol
with benzyl alcohol, 4-methylbenzyl alcohol, and 2,6-
dimethylphenol were unsuccessful, yielding bis-ligated com-
Compound I was isolated in 75% yield by sublimation and
1
1
characterized with H and 13C NMR, high-resolution mass
plexes as shown in the H NMR spectra. The presence of the
B
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Scheme 3
tert-butyl groups on 2,6-di-tert-butylphenol prevented forma-
tion of the bis-ligand complex.
The 1H and 13C NMR spectra of 1b−4b show the
incorporation of one ketoiminate and a single phenoxide.
The chemical shifts of the 1H NMR signals for the ketoiminate
in the spectra of 1b−4b are similar to those observed in 1a−4a,
suggesting a similar coordination mode between the amide and
phenoxide complexes. The methyl peaks of the amide complex
are at 1.75 and 2.07 ppm for 1a, and the methyl peaks for 1b
are at 1.65 and 2.30 ppm, indicating a similar chemical
environment. The proton at the 2-position on quinoline in L1-
H appears as a sharp doublet of doublets at 8.53 and 8.20 ppm,
respectively, for the amide and phenoxide complexes. Because
the signals in the NMR spectra of 1b−4b are sharp, there is no
indication of a fluctuating zinc coordination number as was
observed in other studies where dinuclear zinc complexes were
reported.5j
Figure 2. X-ray crystal structure of 4a with thermal ellipsoids drawn at
the 50% probability level. The hydrogen atoms are omitted for clarity.
Structures of Zinc Amide and Phenoxide Complexes.
The crystal structures of 1a, 2a, 4a, and 4b confirm the
tridentate coordination of zinc by the ketoiminate and the
coordination of one bis(trimethylsilyl)amide or di-tert-butyl
phenoxide to the zinc center, Figures 1, 2, and 3 and
Figure 3. X-ray crystal structure of 4b with thermal ellipsoids drawn at
the 50% probability level. One molecule of 4b in the asymmetric unit,
the hydrogen atoms, and the cocrystallized molecule of dichloro-
methane are omitted for clarity. Selected bond lengths (Å) and angles
(deg): Zn1−N1 2.013 (2), Zn1−N2 2.099 (2), Zn1−O1 1.937 (2),
Zn1−O2 1.887 (1), N1−Zn1−N2 80.21 (6), N1−Zn1−O2 129.84
(6), N2−Zn1−O2 105.84 (6), O1−Zn1−N1 95.77 (6), O1−Zn1−N2
131.58 (6), O1−Zn1−O2 112.77 (6).
Figure 1. X-ray crystal structure of 1a with thermal ellipsoids drawn at
the 50% probability level. The hydrogen atoms are omitted for clarity.
Supporting Information SI Figure 1. Crystals suitable for X-
ray crystallography for 1a, 2a, and 4a were grown by the slow
evaporation of hexanes and dichloromethane under an inert
atmosphere at −35 °C, while 4b was prepared by cooling a
dichloromethane solution to −35 °C. Crystallographic and
refinement data can be seen in Supporting Information SI
Table 1, while selected bond lengths and angles of interest are
shown in Table 1. The zinc amide complexes did not contain
cocrystallized solvent molecules, while the structure of 4b
contained dichloromethane.
These four-coordinate zinc complexes adopt distorted
tetrahedral geometries, with only a few angles being close to
ideal (e.g., in 1a O1−Zn1−N3 112.2°, in 2a N2−Zn1−N3
110.0° and O1−Zn1−N3 111.7°, and in 4b O1−Zn1−O2
112.77°). The rigidity of the ketoiminate backbone and the
pendant quinolyl donor restrict the bite angles to zinc to be
C
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shorter than the 1.89 Å Zn−O bond distance reported here.9a
Zinc−oxygen bonds of similar length were reported in zinc
phenoxide complexes in the absence of multidentate supporting
ligands.17
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Zn
Complexes 1a, 2a, and 4a
1a
Bond Lengths
2a
4a
Comparison to structures in which ketoiminates bear
pendant donors is possible for metals other than zinc.
Structures of bis-ligated magnesium complexes with L2-H and
L3-H were reported to form octahedral magnesium complexes
with meridional coordination by the two ligands.11f Zinc and
magnesium have very similar ionic radii and often yield similar
metal−oxygen bond distances.18 Here where L2-H was
employed in 2a the Zn−O bond distance was 1.97 Å, while
an average Mg−O distance of 2.01 Å was previously reported.
The slighter longer bond distances for the magnesium complex
can be attributed to the crowding from the two ligands around
magnesium. The bond distances between the metal center and
the quinoline nitrogen are also longer in the magnesium
complexes, where an average Mg−N distance 2.26 Å was
observed compared to 2.10−2.11 Å for Zn−N(2) in 2a, 4a, and
4b. Without the presence of a second NNO ligand in 2a, the
zinc center is positioned closer to the quinolyl moiety, yielding
a wider N−Zn−N bite angle of 79.9° than in N−Mg−N, where
the angle was 73.6°.
Zn1−N1
Zn1−N2
Zn1−O1
Zn1−N3
2.0138(11)
2.2260(12)
1.9618(10)
1.9059(11)
2.034(2)
2.108(2)
1.970(2)
1.925(2)
2.0329(9)
2.1144(9)
1.9752(8)
1.9201(9)
Bond Angles
N1−Zn1−N2
N1−Zn1−N3
N2−Zn1−N3
O1−Zn1−N1
O1−Zn1−N2
O1−Zn1−N3
77.57(5)
142.26(5)
102.23(5)
94.36(4)
130.20(4)
112.16(5)
79.88(9)
79.78(3)
129.05(4)
118.25(4)
92.07(3)
116.65(3)
115.02(4)
135.65(10)
109.95(8)
93.36(8)
125.57(8)
111.72(8)
much smaller than the ideal bond angles for tetrahedral
coordination (e.g., N1−Zn1−N2 77.6° in 1a, 79.9° in 2a, 79.8°
in 4a, and 80.2° in 4b). The angles between the ketoiminate
and the amide or phenoxide ligand are much larger than the
ideal 109.5° (e.g., the angle N1−Zn1−N3 in 1a 142.3°, in 2a
135.7°, in 4a 129.1° or the angle N1−Zn1−O2 in 4b 129.8°).
As a result, an open site for coordination of a lactide monomer
for the initiation of ROP is present. Despite the incorporation
of different groups on the quinolyl substituent, the bond
lengths between the four complexes are quite similar, with the
exception of Zn1−N2, being slightly longer for 1a (2.23 Å) as
compared to 2a, 4a, and 4b (2.10−2.11 Å), which have
substituents on the quinoline ring. The Zn−N3 bond is slightly
longer (1.92 Å) in the amide complexes than the Zn−O2 bond
in 4b (1.89 Å).
Bis-ketoiminate Zinc Complex. A side product was
observed during the synthesis of 1a and 1b when less than a
slight excess of zinc bis(trimethylsilyl)amide was used in the
preparation of 1a or when benzyl alcohol, 4-methylbenzyl
alcohol, or 2,6-dimethylphenol was used in attempts to prepare
1b (Scheme 4). The side product was identified as a bis-
Scheme 4
Only one other set of zinc amide complexes analogous to 1a,
2a, and 4a has been reported where a tridentate NNO
salicylaldiminate ligand was employed in addition to the
bis(trimethylsilyl)amide.10d There are many similarities be-
tween the literature structures and 1a, 2a, and 4a including
distorted tetrahedral coordination geometry around zinc and
bond lengths and angles that are in close agreement. For
example, the average Zn−N(amide) bond distance for 1a, 2a,
and 4a is 1.92 Å, and that for the salicylaldiminate complexes
reported is 1.91 Å. The average N1−Zn−N2 angle for the
salicylaldiminates and for the ketoiminates in 1a, 2a, and 4a
differs by only 0.2°. Other bis(trimethylsilyl)amide zinc
complexes have been prepared with bidentate NN β-
diketiminate (BDI) or bidentate NO salicylaldiminate
ligands.5j,k,9a For BDI complexes, the Zn−N(amide) bond
distances were slightly shorter, at 1.88 and 1.89 Å, than the
bond distances reported here.5j,k For salicylaldiminate NO zinc
amide complexes, the Zn−N(amide) bond distances was 1.87
Å.9a
The structure of 4b joins a small group of mononuclear zinc
alkoxide complexes where, as in this structure, sterically
encumbered groups were required.5m,9a,17 More often the
reported structures of zinc alkoxide complexes show dinuclear
complexes with bridging alkoxides with BDI,8b,c salicylaldimi-
nate,5f,10a,d,g or ketoiminate ligands.12c For the tridentate NNO
dinuclear zinc complexes longer Zn−N1, Zn−N2, Zn−O1, and
Zn−O2 bond lengths were reported than those in 4b, reflecting
the influence of the bridging alkoxide on those zinc
coordination geometries.5f,10a,d,g In the case of a zinc complex
with a bidentate ketoiminate and a di-tert-butyl phenoxide
moiety the Zn−O bond distance was 1.82 Å, which is slightly
ketoiminate zinc complex through its intentional synthesis.
Compound 1c was synthesized by adding 2 equivalents of L1-H
to zinc bis(trimethylsilyl)amide at ambient temperatures in
Et2O under an inert atmosphere. An immediate color change
from faint yellow to bright yellow was observed when the first
aliquot of ligand solution was added to the zinc amide solution.
The bis-ketoiminate complex precipitated from solution and
was isolated by filtration in 72% yield.
1
Compound 1c was characterized by H and 13C NMR, HR-
1
MS, absorbance spectroscopy, and elemental analysis. The H
NMR spectrum of 1c revealed the absence of the hydroxyl
proton signal (δ ∼13.5 ppm) in L1-H and the bis-
(trimethylsilyl)amide peak (δ ∼0.3 ppm) as compared to 1a.
The proton at the 2-position on the quinolyl moiety was
observed to shift upfield by ∼0.7 ppm, while the methine
proton shifted upfield by ∼0.2 ppm. Both changes suggest
coordination of the ketoimine to the zinc center. The coupling
constants for 1c were consistent with those for L1-H. In
addition, the spectrum showed an average symmetric metal
coordination by two ketoiminate ligands with no apparent
fluctuations in coordination. Mass spectra of 1c were collected
D
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from a 100% acetonitrile solution with ESI-MS, where
[(C14H13N2O)2Zn + H]+ was observed at m/z 515.14 with
an isotope pattern consistent with C, H, N, O, and Zn
incorporation. The composition was confirmed with high-
resolution ESI-MS by comparison to leucine enkephalin, where
the observed mass and the calculated mass differed by 1.9 ppm.
While crystals suitable for X-ray diffraction analysis could not
be prepared, an octahedral coordination geometry around zinc
in 1c can be inferred based on the octahedral structures
reported for bis-salicylaldiminate zinc complexes bearing
pyridyl or quinolyl pendant donors and from the previous
use of L1-H in octahedral bis-ligated magnesium complex-
es.11f,14 Unfortunately, bis-ligated zinc complexes were formed
with L4-H when zinc bis(trimethylsilyl)amide was not in slight
after 24 h, ROP experiments with 3a and 4a had only modest
conversions of monomer to PLA, entries 3 and 4. On the other
hand, 1b−4b showed much higher catalytic activity toward the
ROP of lactide. This follows the trend that alkoxide initiators
generally have higher activities than the corresponding amide
complexes.5m
Studies were also conducted to compare the polymerization
rates of different ketoiminate complexes with the same
phenoxide initiator. Entries 7 and 9−11 in Table 2 show that
catalytic activity decreases from 1b to 4b. For example, entries
9 and 11 show that catalytic activity is drastically decreased
from 97% to 35% conversion in 1 h when the methyl group at
the 2-position on quinoline is replaced by a phenyl group. The
presence of the phenyl group likely interferes with lactide
monomer coordination to the zinc center during ROP.
Additionally, entries 9 and 10 show that catalytic activity is
again significantly reduced from 97% to 44% conversion in 1 h
upon the addition of the phenyl groups added to the ketoimine
backbone. Electron-donating groups have been found to
increase polymerization rates, and the electron-withdrawing
phenyl substituents in 3b and 4b showed diminished ROP
activity resulting from both steric and electronic effects.10a,e
NMR spectroscopy was used to study the ROP of L-lactide
with the zinc amide and phenoxide complexes. 1H NMR
spectra demonstrated the conversion of L-lactide to PLA
(Supporting Information Figures 14 and 16 for 1a and 1b,
respectively). Homonuclear decoupled 1H NMR spectra
(Supporting Information Figures 15 and 17 for 1a and 1b,
respectively) show that the isolated polymeric materials are
isotactic PLA where the stereochemistry of L-lactide was
retained, yielding poly-L-lactide (PLLA) with both the amide
and phenoxide initiating ligands.19 An NMR study of
percentage conversion and molecular weight for [L-lactide}/
[1b] = 100 was conducted in CDCl3 (Supporting Information
SI Figures 18 and 19).
1
excess but to a lesser extent than with L1-H−L3-H. The H
NMR spectrum of the bis-ligated complex from L4-H showed
an average symmetric metal coordination by ketoiminate
ligands but with broad signals suggesting a fluxional and
different coordination around zinc. It is likely that the phenyl
group on quinoline was too large to allow metal coordination
by the quinoline nitrogen, possibly leading to a tetrahedral
coordination geometry in a similar manner to literature
reports.5i,6g
L-Lactide ROP Studies. Polymerization studies were
conducted to compare percent conversions, molecular weights,
and polydispersity index (PDI) values between and among the
amide and phenoxide complexes. All ROP reactions were
carried out at ambient temperatures in CH2Cl2 for 24 h for
complexes 1a−4a and for 1 h for 1b−4b. The total reaction
volume was kept at 3 mL except for 2b due to lower solubility
in CH2Cl2. Preliminary studies conducted using THF as the
polymerization solvent yielded lower percent conversion with
longer reaction times. This is consistent with the literature,
where the phenomenon was attributed to competition between
THF and lactide monomers for coordination.5b
The disparity between the observed and calculated molecular
weights for the isolated PLLA varied, Table 2. In general,
polymerization with the zinc amide complexes had the largest
differences. This likely resulted from a number of processes.
The primary process in the case of the amide complexes would
seem to be catalyst decomposition to bis-ligated complexes.
The long polymerization times where the amides were in
solution at ambient temperature resulted from the low
nucleophilicity and slow ROP initiation of the amide ligand.
As a result, the number of active catalyst sites was reduced.
Transesterification has explained the molecular weight disparity
in catalytic systems incorporating zinc alkoxide20 or aluminum
alkoxide complexes;21 however, such processes are often
accompanied by a broadening of the molecular weight
distribution, which does not appear to be significant in this
system.22
For 1b and 2b, molecular weights and PDI values are close to
the predicted values. With the same catalyst:lactide ratio, the
PDI values of the phenoxides shown in entries 7 and 9−11 are
low, which is in agreement with the living polymerization
mechanism. Additionally, Figure 4 shows that as the monomer
percentage conversion increases, the polymer molecular weight
increases in a linear fashion, further suggesting the living
character of ROP with these complexes.
The activities of the amide and phenoxide complexes with
the same Schiff bases were compared to each other, as shown in
Table 2. In all cases, the phenoxide complexes showed higher
catalytic activity than their amide counterparts. Polymerizations
with 1a−4a were shown to require longer reaction times. Even
Table 2. Polylactides Produced from the ROP of L-Lactide in
CH2Cl2 at Ambient Temperatures
PDI
(Mw/
Mn)
b
c
d
lactide/ conv
Mn,calc (103, Mn,obs (103,
a
entry complex
Zn
(%)
g/mol)
g/mol)
1
1a
2a
3a
4a
1b
1b
1b
1b
2b
3b
4b
250
250
250
250
50
100
100
67
36.0
36.0
24.1
12.6
7.2
156.0
33.6
118.6
77.6
15.1
24.0
50.7
74.7
38.9
93.7
61.7
1.07
1.23
1.10
1.18
1.15
1.10
1.11
1.18
1.11
1.14
1.20
2
3
4
35
5
100
100
100
100
97
6
100
250
500
250
250
250
14.4
36.0
72.0
34.9
15.8
12.6
7
8
9
10
11
44
35
a
All reactions were carried out at in CH2Cl2 at ambient temperature.
b
c
Lactide conversion as determined by 1H NMR. Mn,calc = (M/I) × (%
A catalytic amount of 1c and 2,6-disubstituted phenol were
added to L-lactide, and the mixture was stirred for 24 h. No
PLA was observed by 1H NMR or GPC. It has been previously
reported that in some instances bis-ligated complexes are active
d
conv) × (mol wt of lactide). Mn,obs values were determined by GPC
in THF vs polystyrene standards and were corrected with a Mark−
Houwink factor of 0.58.23
E
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400 spectrometer. 1H (500 MHz) NMR spectra were collected with a
Varian VNMR S500 spectrometer. Electronic spectra were collected
with a Hewlett-Packard 8453e photodiode array spectrometer (200−
1100 nm) with a 4 mL sealed quartz cuvette. Mass spectrometry data
were collected with a Waters LCT Premier XE time-of-flight
instrument controlled by MassLynx 4.1 software. Samples were
infused using direct loop injection from a Waters Acquity UPLC into
the multi-mode ionization source. The lock mass standard for accurate
mass determination was leucine enkephalin (Sigma L9133). Gel
permeation chromatography to determine polymer molecular weights
(Mn and Mw) used a Thermo Separation Products SpectraSystem
P4000 HPLC pump equipped with Phenomenex Phenogel columns
(30.0 × 7.8 cm, 5 μm) with molecular weight ranges of 1−75 kDa
(#00H-04440K0) or 5−500 kDa (#00H-0445-K0). Detection was
performed with a Polymer Laboratories PL-ELS1000 evaporative light
scattering detector. Continuously vacuum degassed THF (HPLC
grade, uninhibited) was used as mobile phase at ambient temperature
with a flow rate of 1.0 mL min−1. Linear polystyrene standards were
used as calibration standards, and a Mark−Houwink factor of 0.58 was
applied.
X-ray Crystallography. Crystal data for compounds 1a, 2a, 4a,
and 4b were collected at 100(2) K using a Bruker Smart 1000 APEX2
CCD diffractometer. Data collection was carried out with Mo Kα
radiation (λ = 0.71073 Å) with a frame time of 30 s for 1a, 2a, 4a, and
4b. Frames were collected with 0.30° steps in ω at four different φ
settings and a detector position of −28° in 2 h. The intensity data were
corrected for absorption and decay with SADABS (Bruker).24 The
data were integrated with SAINT (Bruker).25 The structure was solved
and refined using SHELXTL.26 Crystallographic data for compounds
1a, 2a, 4a, and 4b are given in SI Table 1 in the Supporting
Information.
Synthesis of 2-Phenyl-8-hydroxyquinoline, I. The synthesis of
compound I was completed using methods adapted from the
literature.16a Under an inert atmosphere, bromobenzene (9.1 g, 28
mmol) was added dropwise to lithium granules (850 mg, 122 mmol)
suspended in 40 mL of Et2O and stirred for 1 h. A solution of 8-
hydroxyquinoline (4.0 g, 58.0 mmol) in Et2O (70 mL) was added
dropwise to the reaction and stirred at 35 °C for 1 h. The flask was
cooled to room temperature, and air was bubbled through the reaction
for 2 h. Diethyl ether (80 mL) and water (40 mL) were added, and the
solution was neutralized with 6 M HCl and 1 M Na2CO3. The
aqueous layer was extracted with Et2O (2 × 75 mL), and the organic
layer was filtered and dried in vacuo. The white solid was isolated by
sublimation (4.60 g, 20.7 mmol, 75.4%). 1H NMR (400 MHz;
CDCl3): δ 7.22 (d, J = 7.0 Hz, 1H, ArH), 7.36 (d, J = 8.0 Hz, 1H,
ArH), 7.46 (t, J = 8.0 Hz, 1H, ArH), 7.50 (t, J = 7.0 Hz, 1H, ArH),
7.56 (t, J = 7.5 Hz, 2H, ArH), 7.93 (d, J = 9.0 Hz, 1H, ArH), 8.17 (d, J
= 8.0 Hz, 2H, ArH), 8.24 (d, J = 9.0 Hz, 1H, ArH), 8.45 (s, br, 1H,
−OH). 13C NMR (100 MHz; CDCl3): δ 110.63, 117.79, 119.74,
127.49, 127.53, 127.74, 129.03, 129.82, 137.36, 137.92, 138.69 (Ar),
152.36 (−C-OH), 155.09. ESI-HRMS observed m/z = 222.0926 ([M
+ H]+), calculated exact mass C15H12NO = 222.0919, error: 3.2 ppm.
Anal. Calcd for C15H11NO: C, 81.43; H, 5.01; N, 6.33. Found: C,
81.71; H, 4.93; N, 6.26.
Figure 4. Linear relationship observed between Mn and monomer/
initiator ratio of PLA produced from the ROP of L-lactide by 1b in
CH2Cl2. PDI values are provided in parentheses.
ROP catalysts in the presence of a phenol, but that was not
observed here.6d,12c
CONCLUSIONS
■
A series of zinc amide and zinc phenoxide complexes were
prepared with tridentate ketoiminate ligands. The zinc amide
structures are among the few reported with monoanionic NNO
Schiff bases, and the steric bulk of di-tert-butyl phenoxide
yielded an uncommon mononuclear zinc phenoxide. While the
incorporation of increasingly bulky groups minimized the
formation of bis-ligated zinc complexes, the byproduct was not
completely eliminated. Preliminary studies of L-lactide ROP
with the zinc phenoxide complexes showed them to be more
efficient and, as expected, yielded more complete ROP than the
corresponding amide complexes. The addition of larger groups
at the quinoline 2-position led to lower percentage conversion
of L-lactide to isotactic PLLA during ROP.
EXPERIMENTAL SECTION
■
Materials. L1-H, L2-H, and L3-H were prepared with literature
methods.11f Zinc bis(trimethylsilyl)amide was prepared with a
modified procedure from the literature.17 The following chemicals
were purchased from Acros and used without further purification
unless otherwise stated: 8-aminoquinoline, 2,6-di-tert-butylphenol, 2,6-
dimethylphenol, p-toluenesulfonic acid, lithium granules, anhydrous
zinc chloride, sodium bis(trimethylsilyl)amide, triethylamine, dichloro-
methane, and chloroform. The following chemicals were purchased
from Aldrich and used without further purification unless otherwise
stated: 2,4-pentandione, dibenzoylmethane, L-lactide, 4-methylbenzyl
alcohol, calcium hydride, benzophenone, ammonia (33%), sodium
metal, ethyl acetate, toluene, tetrahydrofuran, hexanes, benzene-d6,
dichloromethane-d2, and chloroform-d1. 8-Hydroxyquinoline was
purchased from Fisher. Bromobenzene was purchased from J.T.
Baker. Ammonium sulfite monohydrate was purchased from Alfa
Aesar. 8-Amino-2-methylquinoline was purchased from TCI America.
Solvents used in the preparation and characterization of the zinc
complexes were dried with calcium hydride and sodium benzophenone
ketyl and stored under an inert atmosphere (inert atmosphere
glovebox). Chloroform-d1 and dichloromethane-d2 were degassed and
dried over 4 Å molecular sieves. L-Lactide was purified by
recrystallization from toluene and held in vacuo at 40 °C overnight
prior to use.
Synthesis of 2-Phenyl-8-aminoquinoline, II. The conversion of I to
2-phenyl-8-aminoquinoline was performed with a method adapted
from the literature.16b Compound I (1.1 g, 5.0 mmol) was added to
ammonium sulfite monohydrate, (NH4)2SO3·H2O (6.7 g, 50 mmol),
and 20 mL of NH3 (33%). The reactants were sealed in a steel reactor
and heated to 175 °C for 7 days. The reactor was cooled and washed
with CH2Cl2 and water. The aqueous layer was extracted with CH2Cl2
(3 × 50 mL), and the solvent was removed in vacuo. The bright yellow
solid was isolated with column chromatography (silica, hexanes/
1
EtOAc, 12:1, 3% Et3N) (0.340 g, 1.45 mmol, 31%). H NMR (400
MHz; CDCl3): δ 5.11 (s, br, 2H, −NH2), 6.95 (dd, J = 1.2, 7.5 Hz, 1H,
ArH), 7.16 (dd, J = 1.2, 8.0 Hz, 1H, ArH), 7.33 (t, J = 7.8 Hz, 1H,
ArH), 7.48 (t, J = 7.5 Hz, 1H, ArH), 7.52 (m, 2H, ArH), 7.86 (d, J =
8.6 Hz, 2H, ArH), 8.13 (d, J = 8.6 Hz, 1H, ArH), 8.21 (d, J = 7.0 Hz,
2H, ArH). 13C NMR (100 MHz; CDCl3): δ 110.36, 115.85, 118.99,
127.32, 127.42, 127.80, 128.83, 129.18, 136.90, 138.19, 139.75, 144.29
1
General Methods and Instrumentation. H (400 MHz) and
13C (100 MHz) NMR spectra were collected with a Bruker Avance III
F
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(Ar), 154.23 (C-OH). ESI-HRMS observed m/z = 221.1076 ([M +
H]+), calculated exact mass C15H13N2 = 221.1079, error: 1.4 ppm.
Anal. Calcd for C15H12N2: C, 81.79; H, 5.49; N, 12.72. Found: C,
81.92; H, 5.77; N, 12.63.
26.28 (Ar-CH3), 101.52 (−CH), 120.44, 121.29, 123.64, 126.89,
127.11, 127.61, 127.84, 127.91, 128.10, 128.53, 129.34, 129.55, 131.12,
140.09, 140.69, 140.79, 141.16, 142.12, 159.24 (Ar), 171.82 (−N
C−), 182.78 (−OC−). UV (Et2O) λmax: 263, 355, 434 nm.
Synthesis of 4a. Following procedure A, L4-H (0.170 g, 0.56
mmol) was dissolved in Et2O and added dropwise to zinc
bis(trimethylsilyl)amide (0.257 g, 0.66 mmol) dissolved in Et2O.
The pale orange solid was isolated by filtration (0.190 g, 0.36 mmol,
64.6%). 1H NMR (400 MHz; CD2Cl2): δ −0.54 (s, 18H, −Si(CH3)3),
2.11 (s, 3H, −CH3), 2.24 (s, 3H, −CH3), 5.27 (s, 1H, −CH)), 7.6 (m,
6H, ArH), 7.87 (d, J = 8.5 Hz, 1H, ArH), 7.99 (dd, J = 1.7, 8.2 Hz, 2H,
ArH), 8.40 (d, J = 8.5 Hz, 1H, ArH). 13C NMR (100 MHz; CD2Cl2):
δ 5.08 (−Si(CH3)3), 23.95 (−CH3), 28.63 (−CH3), 101.54 (−CH),
122.45, 122.72, 123.32, 126.08, 127.32, 128.53, 128.65, 129.52, 130.12,
130.65, 139.49, 140.07, 142.46, 143.56, 159.17 (Ar), 169.35 (−N
C−), 186.47 (−OC−). UV (Et2O) λmax: 263, 312 nm.
Synthesis of L4-H. The synthesis of L4-H was conducted with a
similar method to previously reported.11f A toluene solution (20 mL)
containing 2,4-pentanedione (1.500 g, 15.0 mmol) and p-toluene-
sulfonic acid (catalytic amount) was sparged with N2 for 20 min.
Compound 2 (0.680 g, 3.09 mmol) was added to the flask, which was
heated to reflux for azeotropic distillation for 20 h. Water produced by
the reaction was removed at regular intervals. The pale yellow solid
was isolated with column chromatography (silica, hexanes/EtOAc, 7:1,
3% Et3N) (0.633 g, 2.10 mmol, 68%). 1H NMR (400 MHz; CDCl3): δ
2.21 (s, 3H, −CH3), 2.31 (s, 3H, −CH3), 5.34 (s, 1H, −CH), 7.40 (m,
3H, ArH), 7.48 (t, J = 7.2 Hz, 1H, ArH), 7.58 (t, J = 8.0 Hz, 2H, ArH),
7.98 (d, J = 8.8 Hz, 1H, ArH), 8.16 (d, J = 8.8 Hz, 1H, ArH), 8.52 (d, J
= 7.2 Hz, 2H, ArH), 13.68 (s, 1H, −OH). 13C NMR (100 MHz;
CDCl3): δ 21.57 (−CH3), 29.77 (−CH3), 100.80 (−CH), 118.70,
119.05, 122.13, 126.01, 127.83, 127.90, 129.06, 129.71, 136.95, 137.20,
138.92, 140.76, 155.89 (Ar), 157.09 (−CN−), 196.24 (−C−OH).
ESI-HRMS observed m/z = 303.1508 ([M + H]+), calculated exact
mass C20H19N2O = 303.1497, error: 3.6 ppm. Anal. Calcd for
C20H18N2O: C, 79.44; H, 6.00; N, 9.26. Found: C, 79.91; H, 6.01; N,
9.20. UV (Et2O) λmax: 288, 384 nm.
Preparation of Zinc Phenoxide Complexes with Ketominiate
Ligands, Procedure B. Under an inert atmosphere, the zinc amide
complex, 1a−4a (1.0 equivalent), was dissolved in Et2O (5 mL), and a
Et2O solution (5 mL) of 2,6-di-tert-butylphenol (1.3−1.5 equivalents)
was added to the stirring solution of zinc amide complex. The reaction
was stirred for 2 h. After 30 min a precipitate was observed. The
reaction solution volume was reduced to 2 mL. Et2O and 2 mL of
hexanes were added. The flask was placed into the −35 °C freezer for
1 h, and the solid was isolated by filtration and washed with 3 mL of
cold hexanes. The solid was dried in vacuo. Compounds 1b−4b were
insufficiently stable to obtain suitable microanalysis results. NMR
spectra of 1b−4b are provided in the Supporting Information.
Synthesis of 1b. Following procedure B, 2,6-di-tert-butylphenol
(0.190 g, 0.92 mmol) was dissolved in Et2O and added to a solution of
1a (0.296 g, 0.66 mmol) in Et2O. The bright yellow solid was isolated
by filtration (0.191 g, 0.39 mmol, 58.5%). 1H NMR (400 MHz;
CD2Cl2): δ 1.44 (s, 18H, −C(CH3)3), 1.65 (s, 3H, −CH3), 2.30 (s,
3H, −CH3), 5.07 (s, 1H, −CH), 6.79 (t, J = 7.8 Hz, 1H, ArH), 7.15 (d,
J = 7.8 Hz, 2H, ArH), 7.20 (dd, J = 4.4, 8.2 Hz, 1H, ArH), 7.32 (dd, J =
1.2, 7.6 Hz, 1H, ArH), 7.37 (dd, J = 1.2, 8.2 Hz, 1H, ArH), 7.50 (t, J =
8.0 Hz, 1H, ArH), 8.08 (dd, J = 1.6, 8.2 Hz, 1H, ArH), 8.20 (dd, J =
1.7, 4.4 Hz, 1H, ArH). 13C NMR (100 MHz; CD2Cl2): δ 22.94
(−CH3), 28.50 (−CH3), 30.48 (−C(CH3)3), 34.61 (−C(CH3)3),
100.45 (−CH), 120.05, 120.18, 121.22, 121.37, 125.28, 127.38, 129.18,
136.43, 137.61, 141.44, 145.15, 146.76, 154.30 (Ar), 168.50 (−N
C−), 188.30 (−OC−). Anal. Calcd for C28H34N2O2Zn: C, 67.80; H,
6.91; N, 5.65. Found: C, 66.32; H, 6.53; N, 5.53. UV (Et2O) λmax: 267,
345, 397 nm.
Synthesis of 2b. Following procedure B, 2,6-di-tert-butylphenol
(0.260 g, 1.26 mmol) was dissolved in Et2O and added to a solution of
2a (0.436 g, 0.94 mmol) in Et2O. The bright yellow solid was isolated
by filtration (0.260 g, 0.72 mmol, 75.7%). 1H NMR (400 MHz;
CD2Cl2): δ 1.15 (s, 18H, −C(CH3)3), 2.09 (s, 3H, −CH3), 2.26 (s,
3H, −CH3), 3.00 (s, 3H, Ar-CH3), 5.26 (s, 1H, −CH), 6.33 (t, J = 7.6
Hz, 1H, ArH), 6.95 (d, J = 7.6 Hz, 2H, ArH), 7.51 (d, J = 8.4 Hz, 1H,
ArH), 7.62 (m, 3H, ArH) 8.32 (d, J = 8.4 Hz, 1H, ArH). 13C NMR
(100 MHz; CD2Cl2): δ 23.82 (−CH3), 27.16 (Ar-CH3), 28.57
(−CH3), 30.62 (−C(CH3)3), 35.14 (−C(CH3)3), 101.75, (−CH)
113.47, 122.79, 123.19, 124.78, 127.09, 127.72, 138.97, 140.35, 141.07,
141.81, 159.70, 164.91 (Ar), 170.24 (−NC−), 186.86 (−OC−).
Anal. Calcd for C29H36N2O2Zn: C, 68.29; H, 7.11; N, 5.49. Found: C,
67.42; H, 6.52; N, 5.15. UV (Et2O) λmax: 270, 333, 402 nm.
Preparation of Zinc Bis(trimethylsilyl)amide Complexes with
Ketoimines, Procedure A. Under an inert atmosphere, Et2O (10 mL)
was added to a Schlenk flask containing zinc bis(trimethylsilyl)amide
(1.1−1.2 equivalents). The ketoimines (1.0 equivalent) were dissolved
or slurried in Et2O (5 mL) and added dropwise to the flask. An
immediate color change from pale yellow to orange was observed. The
reaction was stirred for 1 h. The reaction volume was reduced in vacuo
to approximately 1 mL, and 1 mL of cold hexanes was added. The flask
was placed in the freezer at −35 °C for 1 h to reduce solubility.
Compounds 1a−4a were isolated by filtration, washed with cold
hexanes, and dried in vacuo. Compounds 1a−4a were insufficiently
stable to obtain suitable microanalysis results. NMR spectra of 1a−4a
are provided in the Supporting Information.
Synthesis of 1a. Following procedure A, L1-H (0.212 g, 0.94
mmol) was dissolved in Et2O and added dropwise to zinc
bis(trimethylsilyl)amide (0.430 g, 1.11 mmol) dissolved in Et2O.
The yellow solid was isolated by filtration (0.296 g, 0.65 mmol,
1
70.0%). H NMR (500 MHz; C6D6): δ 0.29 (s, 18H, −Si(CH3)3),
1.75 (s, 3H, −CH3), 2.07 (s, 3H, −CH3), 5.08 (s, 1H, −CH), 6.58 (dd,
J = 4.0, 8.0 Hz, 1H, ArH), 6.94 (dd, J = 2.0, 7.0 Hz, 1H, ArH), 7.04 (m,
2H, ArH), 8.53 (dd, J = 1.5, 2.5 Hz, 1H, ArH). 13C NMR (100 MHz;
CD2Cl2): δ 5.04 (−Si(CH3)3), 24.36 (−CH3), 28.17 (−CH3), 101.78
(−CH), 122.37, 122.57, 122.68, 127.88, 129.62, 139.59, 141.52, 149.14
(Ar), 169.78 (−NC−), 187.29 (−OC−). UV (Et2O) λmax: 241,
269, 340, 408 nm.
Synthesis of 2a. Following procedure A, L2-H (0.228 g, 0.95
mmol) was dissolved in Et2O and added dropwise to zinc
bis(trimethylsilyl)amide (0.443 g, 1.15 mmol) dissolved in Et2O.
The yellow solid was isolated by filtration (0.324 g, 0.70 mmol,
73.3%). 1H NMR (500 MHz; C6D6): δ 0.24 (s, 18H, Si(CH3)3), 1.76
(s, 3H, −CH3), 2.13 (s, 3H, −CH3), 2.67 (s, 3H, Ar-CH3), 5.10 (s, 1H,
−CH), 6.51 (d, J = 8.0, 1H, ArH), 6.99 (d, J = 8.0, 1H, ArH), 7.06 (m,
2H, ArH), 7.33 (d, J = 8.0 Hz, 1H, ArH). 13C NMR (100 MHz;
CD2Cl2): δ 5.33 (−Si(CH3)3), 24.80 (−CH3), 26.22 (−CH3), 28.59
(Ar-CH3), 102.11 (−CH), 122.52, 123.66, 126.87, 127.74, 139.85,
141.37, 142.79, 159.22 (Ar), 169.79 (−NC−), 187.59 (−OC−).
UV (Et2O) λmax: 238, 269, 334, 398 nm.
Synthesis of 3b. Following procedure B, 2,6-di-tert-butylphenol
(0.113 g, 0.55 mmol) was dissolved in Et2O and added to a solution of
3a (0.240 g, 0.41 mmol) in Et2O. The bright yellow solid was isolated
by filtration (0.174 g, 0.27 mmol, 66.7%). 1H NMR (400 MHz;
CD2Cl2): δ 1.29 (s, 18H, −C(CH3)3), 2.99 (s, 3H, Ar-CH3), 6.08 (s,
1H, −CH), 6.38 (t, J = 7.6 Hz, 1H, ArH), 6.61 (d, J = 8.0 Hz, 1H,
ArH), 7.00 (d, J = 7.7 Hz, 2H, ArH), 7.16 (t, J = 8.0 Hz, 1H, ArH),
7.43 (m, 10H, ArH), 8.00 (d, J = 7.4 Hz, 2H, ArH), 8.28 (d, J = 8.4 Hz,
1H, ArH). 13C NMR (100 MHz; CD2Cl2): δ 27.07 (Ar-CH3), 30.93
(−C(CH3)3), 35.34 (−C(CH3)3), 100.66 (−CH), 121.42, 122.40,
123.64, 124.84, 127.00, 127.44, 127.78, 128.03, 128.51, 129.32, 129.40,
131.17, 139.17, 140.45, 140.50, 140.58, 140.64, 141.08, 159.79, 165.40
Synthesis of 3a. Following procedure A, L3-H (0.136 g, 0.37
mmol) was dissolved in Et2O and added dropwise to zinc
bis(trimethylsilyl)amide (0.159 g, 0.41 mmol) dissolved in Et2O.
The yellowish-orange solid was isolated by filtration (0.160 g, 0.27
mmol, 73.4%). 1H NMR (500 MHz; C6D6): δ 0.33 (s, 18H,
−Si(CH3)3), 2.79 (s, 3H, Ar-CH3), 6.23 (s, 1H, −CH), 6.53 (d, J = 8.5
Hz, 1H, ArH), 6.66 (d, J = 8.0 Hz, 1H, ArH), 6.72 (t, J = 8.0 Hz, 1H,
ArH), 6.81 (d, J = 8.0 Hz, 1H, ArH), 7.2 (m, 9H, ArH), 8.17 (d, J = 7.0
Hz, 2H, ArH). 13C NMR (100 MHz; CD2Cl2): δ 5.35 (−Si(CH3)3),
G
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(Ar), 171.86 (−NC−), 182.31 (−OC−). Anal. Calcd for
C39H40N2O2Zn: C, 73.86; H, 6.36; N, 4.42. Found: C, 73.86; H,
6.34; N, 4.27. UV (Et2O) λmax: 263, 353, 433 nm.
ACKNOWLEDGMENTS
■
This work was funded by a Research Corporation for Science
Advancement Cottrell College Science Award (#10728),
Tooma Undergraduate Research Fellowships, and the generous
support of Pepperdine University undergraduate research
programs. We would like to thank Daphne Green and Allison
Zorn for their laboratory support. Instrumentation was made
available through the support of Phil Hampton (California
Synthesis of 4b. Following procedure B, 2,6-di-tert-butylphenol
(0.094 g, 0.46 mmol) was dissolved in Et2O and added to a solution of
4a (0.190 g, 0.36 mmol) in Et2O. The bright yellow solid was isolated
by filtration (0.135 g, 0.24 mmol, 65.6%). 1H NMR (400 MHz;
CD2Cl2): δ 0.85 (s, 18H, −C(CH3)3), 2.15 (s, 3H, −CH3), 2.30 (s,
3H, −CH3), 5.32 (s, 1H, −CH), 6.23 (t, J = 7.7 Hz, 1H, ArH), 6.82 (d,
J = 7.6 Hz, 2H, ArH), 7.66 (m, 6H, ArH), 8.00 (d, J = 8.6 Hz, 1H,
ArH), 8.08 (d, J = 7.0 Hz, 2H, ArH), 8.51 (d, J = 8.6 Hz, 1H, ArH).
13C NMR (100 MHz; CD2Cl2): δ 23.51 (−CH3), 28.78 (−CH3),
30.16 (−C(CH3)3), 34.70 (−C(CH3)3), 101.65 (−CH), 122.23,
123.05, 123.22, 124.47, 127.63, 128.53, 129.21, 130.39, 131.36,
138.84, 139.13, 140.96, 141.98, 142.57, 158.88, 164.46 (Ar), 169.94
(−NC−), 187.04 (−OC−). Anal. Calcd for C34H38N2O2Zn: C,
71.38; H, 6.70; N, 4.90. Found: C, 69.89; H, 6.67; N, 4.73. UV (Et2O)
λmax: 264, 313 nm.
Synthesis of Bis-ligated Zinc Complex 1c. Under an inert
atmosphere, Et2O (10 mL) was added to a Schlenk flask, and zinc
bis(trimethylsilyl)amide (0.130 g, 0.34 mmol) was added. A solution
of L1-H (0.134 g, 0.59 mmol) in Et2O (5 mL) was added dropwise. An
immediate color change from colorless to yellow was observed, and a
precipitate formed after 20 min. The reaction was stirred for 24 h. The
reaction solution volume was reduced to approximately 2 mL.
Compound 1c was isolated by filtration, washed with cold hexanes,
and dried in vacuo (0.109 g, 0.21 mmol, 71.7%). 1H NMR (400 MHz;
CD2Cl2): δ 1.64 (s, 6H, −CH3), 2.30 (s, 6H, −CH3), 5.07 (s, 2H,
−CH), 7.21 (dd, J = 4.4, 8.2 Hz, 2H, ArH), 7.33 (d, J = 7.6 Hz, 2H,
ArH), 7.38 (d, J = 8.1 Hz, 2H, ArH), 7.28 (t, J = 7.7 Hz, 2H, ArH),
8.09 (d, J = 8.3 Hz, 2H, ArH), 8.21 (d, J = 4.4 Hz, 2H, ArH). 13C
NMR (100 MHz; CD2Cl2): δ 22.91 (−CH3), 28.46 (−CH3), 100.42
(−CH3), 120.15, 121.20, 121.34, 127.36, 129.16, 137.58, 141.42,
145.13, 146.75 (Ar), 168.49 (−NC−), 188.28 (−OC−). ESI-
HRMS observed m/z = 515.1410 ([M + H]+), calculated exact mass
C28H27N4O2Zn = 515.1420, error: 1.94 ppm. Anal. Calcd for
C28H26N4O2Zn: C, 65.18; H, 5.08; N, 10.86. Found: C, 64.91; H,
5.08; N, 10.15. UV (Et2O) λmax: 244, 271, 342, 421 nm. Compound 1c
was insufficiently stable to obtain suitable microanalysis results. NMR
spectra of 1c are provided in the Supporting Information.
1
State University, Channel Islands, 500 MHz H NMR), Saeed
Khan (University of California, Los Angeles, X-ray crystallog-
raphy of 1a, 2a, 4a, and 4b), and Greg Khitrov (University of
California, Los Angeles Molecular Instrumentation Center−
Mass Spectrometry Facility in the Department of Chemistry).
The mass spectrometry data were collected through a project
described and supported by Grant Numbers S10-RR024605
and S10-RR025631 from the National Center for Research
Resources. The content is solely the responsibility of the
authors and does not necessarily represent the official views of
the National Center for Research Resources or the National
Institutes of Health.
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atmosphere, a CH2Cl2 solution of catalyst was dispensed into a 25 mL
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1
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ASSOCIATED CONTENT
* Supporting Information
■
S
NMR spectra of the reported compounds and X-ray crystallo-
graphic data for the crystal structure determination of 1a, 2a,
4a, and 4b are available in CIF and table format and are
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone: 310-506-6705. Fax: 310-506-4875. E-mail: Joseph.
fritsch@pepperdine.edu.
■
Notes
The authors declare no competing financial interest.
H
dx.doi.org/10.1021/om200865w | Organometallics XXXX, XXX, XXX−XXX

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