Reactions of (E)-N-[2-(benzyliminomethyl)phenyl]-2,6-diisopropylaniline with diethylzinc: Synthesis, characterization and catalytic studies for ring-opening polymerization of ε-caprolactone

Article abstract of DOI:10.1016/j.inoche.2011.05.046

Zinc complexes supported by NN-bidentate AA^Bn ligand (AA ^Bn-H = (E)-N-[2-(benzyliminomethyl)phenyl]-2,6-diisopropylaniline) are synthesized and fully characterized. The reaction of diethyl zinc (ZnEt ₂) with AA^Bn-H (1.1 mol equiv) yields three-coordinated mono-adduct zinc complex [(AA^Bn)ZnEt] (1). However, treatment of AA^Bn-H with ZnEt₂ in the same stoichiometric proportion in the presence of 4-dimethylaminopyridine (DMAP, one equiv) affords [(AA ^Bn)Zn(DMAP)Et] (2) as the tetra-coordinated monomeric zinc complex. Bis-adduct complex [(AA^Bn)₂Zn] (3) results from treatment of ZnEt₂ with AA^Bn-H (two equiv) in refluxing toluene in high yield. Complexes 1 and 3 are efficient catalysts for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) in the presence of alcohol. The "controlled" character of Zn complex 1 also enabled preparation of the PEG-b-PCL copolymer.

Full text of DOI:10.1016/j.inoche.2011.05.046

Inorganic Chemistry Communications 14 (2011) 1456–1460
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Inorganic Chemistry Communications
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Reactions of (E)-N-[2-(benzyliminomethyl)phenyl]-2,6-diisopropylaniline with
diethylzinc: Synthesis, characterization and catalytic studies for ring-opening
polymerization of ε-caprolactone
⁎
Chao-Hsiang Wang, Chen-Yu Li, Chia-Her Lin, Yi-Chang Liu, Bao-Tsan Ko
Department of Chemistry, Chung Yuan Christian University, Chung-Li 32023, Taiwan
a r t i c l e i n f o
a b s t r a c t
Zinc complexes supported by NN-bidentate AABn ligand (AABn-H=(E)-N-[2-(benzyliminomethyl)phenyl]-
2,6-diisopropylaniline) are synthesized and fully characterized. The reaction of diethyl zinc (ZnEt2) with AABn
Article history:
Received 21 April 2011
Accepted 25 May 2011
Available online 2 June 2011
-
H (1.1 mol equiv) yields three-coordinated mono-adduct zinc complex [(AABn)ZnEt] (1). However, treatment
of AABn-H with ZnEt2 in the same stoichiometric proportion in the presence of 4-dimethylaminopyridine
(DMAP, one equiv) affords [(AABn)Zn(DMAP)Et] (2) as the tetra-coordinated monomeric zinc complex. Bis-
adduct complex [(AABn)2Zn] (3) results from treatment of ZnEt2 with AABn-H (two equiv) in refluxing toluene
in high yield. Complexes 1 and 3 are efficient catalysts for the ring-opening polymerization (ROP) of ε-
caprolactone (ε-CL) in the presence of alcohol. The “controlled” character of Zn complex 1 also enabled
preparation of the PEG-b-PCL copolymer.
Keywords:
Zinc complex
Catalyst
NN-bidentate
Anilido-aldimine
Ring-opening polymerization
ε-caprolactone
© 2011 Elsevier B.V. All rights reserved.
Over the past decade, considerable attention has been devoted to
the development of anilido-aldimine (AA) metal complexes [1],
mainly due to their applications in the CO2/epoxide copolymerization
[2] and ring-opening polymerization (ROP) [3]. These AA ligands can
be designed to achieve the criteria of single-site catalysts for
minimizing the side reaction. Moreover, metal complexes supported
by AA derivatives also provide great catalytic activities with living
properties. For instance, a series of NNN-tridentate AA rare-earth
metal complexes has reported that the pendant arm of the quinolinyl
group can coordinate with the metal to create a single active site
nature to catalyze the living polymerization of ε-caprolactone (ε-CL)
[4]. Most recently, we have synthesized and characterized well-
defined magnesium and zinc complexes incorporated by novel NNN-
tridentate AA ligands with the N-donor pendant arm on the imine
nitrogen [5]. These complexes have shown high performance for the
ROP of cyclic esters in the presence of alcohol and catalyze the
polymerization in a controlled fashion, giving polyesters with a
narrow polydispersity index.
diisopropylaniline (AABn-H) with benzyl group attached on the imine
nitrogen was successfully prepared in our group [6], and no zinc
complex of AABn ligand has been isolated to date. Herein, we describe
the synthesis, crystal structure and ROP catalytic studies of novel zinc
derivatives based on a NN-bidentate AA ligand of this kind.
Mono-adduct zinc compound [(AABn)ZnEt] (1) [7] is synthesized
in high yield (81%) by the reaction of AABn-H with ZnEt2 (1.1 mol
equiv) in hexane, as shown in Scheme 1. Compound 1 further reacted
with a stoichiometric amount of 4-dimethylaminopyridine (DMAP),
yielding the tetra-coordinated monomeric zinc complex [(AABn)Zn
(DMAP)Et] (2) [8]. Alternatively, 2 could also be prepared directly by
the reaction of AABn-H with 1.1 mol equiv of ZnEt2, followed by the
addition of DMAP (one equiv) in toluene in 70% yield. The formations
of expected complexes 1 and 2 were demonstrated by the disappear-
ance of the N―H signal of AABn-H (~10.60 ppm) and the appearance
of the resonance for methylene protons of Zn–Et in the region of
−0.02 ppm for 1 and −0.21 ppm for 2. Furthermore, treatment of
AABn-H (two equiv) with ZnEt2 in refluxing toluene gave the four-
coordinated bis-adduct zinc complex [(AABn)2Zn] (3) in 80% yield [9].
All these compounds are isolated as yellow crystalline solids and
characterized by spectroscopic studies as well as microanalyses. The
molecular structures of complexes 1–3 are further confirmed by X-ray
structure determinations.
Encouraged by the excellent catalytic systems derived from NNN-
tridentate AA ligands, our current interest has been to synthesize and
to characterize other AA ligands bearing aluminum, magnesium and
zinc derivatives and to apply these complexes as catalysts for CO2/
epoxide coupling reaction and ROP of cyclic esters. In particular, the
NN-bidentate AA ligand, (E)-N-[2-(benzyliminomethyl)phenyl]-2,6-
Single crystals of complexes 1–3 suitable for X-ray structural
analysis were obtained from concentrated hexane or concentrated
toluene solutions. Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawings
displaying selected bond distances and angles of the molecular
structures of 1[10], 2[11] and 3[12] are shown in Figs. 1, 2 and 3,
⁎
Corresponding author. Tel.: +886 3 2653327; fax: +886 3 2653399.
E-mail address: btko@cycu.edu.tw (B.-T. Ko).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.05.046

C.-H. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1456–1460
1457
Scheme 1. Synthetic routes for complexes 1–3.
respectively. Complex 1 crystallizes in monoclinic space group P21/c
with two molecules of [(AABn)ZnEt] in the asymmetric unit. The Zn
center of complex 1 assumes a trigonal planar geometry with the
metal center coordinated by amido and imine nitrogen atoms of the
bidentate AABn ligand, forming a six-membered chelating ring. This
ring is nearly coplanar with mean deviation of 0.0146 Å and the Zn
atom lies out of the plane by 0.0174 Å. The Zn―N(1) (amido) distance
(1.950(3) Å) is shorter than theZn―N(2)(imine) distance(2.015(3) Å),
which is similar to those found in Zn complexes with bidentate anilido-
aldimine ligands [1d,3b]. The solid-state structure of 2 is a monomeric
feature coordinated by the NN-bidentate ligand in an unsymmetrical
fashion; the geometry around Zn is distorted from tetrahedral with
Fig. 2. ORTEP drawing of complex
2
with probability ellipsoids drawn at level
20%. Hydrogen atoms are omitted for clarity. Selected bond lengths/Å and angles/deg:
Zn―N(1) 2.0067(13), Zn―N(2) 2.0519(15), Zn―N(3) 2.1525(16), Zn―C(27) 1.972(2),
N(1)―Zn―N(2) 91.53(6), N(1)―Zn―N(3) 105.09(6), N(2)―Zn―N(3) 93.44(6),
C(27)―Zn―N(1) 119.79(10), C(27)―Zn―N(2) 128.44(12), C(27)―Zn―N(3) 113.44(11).
bond lengths Zn―N(1)=2.0067(13) Å, Zn―N(2)=2.0519(15) Å,
Zn―N(3)=2.1525(16) Å and Zn―C(27)=1.972(2) Å, which are all
longer than the bond lengths observed for three-coordinated Zn
Fig. 3. ORTEP drawing of complex 3 with probability ellipsoids drawn at level 50%.
Hydrogenatomsare omittedforclarity. Selectedbondlengths/Å and angles/deg:Zn―N(1)
2.0147(11), Zn―N(2) 1.9750(11), Zn―N(3) 1.9757(11), Zn―N(4) 2.0043(11), N(2)―
Zn―N(3) 128.34(4), N(2)―Zn―N(4) 110.18(4), N(3)―Zn―N(4) 94.77(4), N(2)―
Zn―N(1) 92.51(4), N(3)―Zn―N(1) 107.12(4), N(4)―Zn―N(1) 127.72(5).
Fig. 1. ORTEP drawing of complex 1 with probability ellipsoids drawn at level 60%.
Selected bond lengths/Å and angles/deg: Zn(1)―N(1) 1.950(3), Zn(1)―N(2) 2.015(3),
Zn(1)―C(27) 1.950(4), N(1)―Zn(1)―N(2) 92.98(12), N(1)―Zn(1)―C(27) 138.67(16),
C(27)―Zn(1)―N(2) 127.79(16).

1458
C.-H. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1456–1460
complex 1. The six-membered ring consisting of a Zn center chelated
with amido and imine nitrogen atoms of the ligand is almost coplanar
with mean deviation of 0.0906 Å. The torsion angles between the six-
membered chelating ring and the aromatic ring attached to the amido
nitrogen atom are 95.4° for 1 and 83.2° for 2. The nitrogen atom of DMAP
is coordinated to zinc with a Zn―N(3) bond distance of 2.1525(16) Å,
which is shorter than the Zn―N(quinolyl) distances (2.266(3), 2.2290
(4) and 2.313(4) Å) found in the NNN-tridentate AA zinc complex [1e].
The molecular structure of compound 3 exhibits the homoleptic
and monomeric feature with a four-coordinated zinc center supported
by two NN-bidentate AABn ligands, forming two six-membered che-
lating rings. The bond angles around Zn in 3 ranging from 92.51(4)° to
128.34(4)° all show a distorted tetrahedral geometry. The bond
distances between the Zn atom and atoms N(1), N(2), N(3) and N(4)
are 2.0147(11), 1.9750(11), 1.9757(11) and 2.0043(11) Å, respec-
tively. These distances are similar with the bond distances observed
for the other homoleptic zinc complex with NN-bidentate AA ligand
[1f].
Fig. 4. Polymerization of ε-CL catalyzed by 1 in toluene at 55 οC. The relationship
between Mn(■)(PDI(□)) of polymer and the initial molar ratio [CL]0/[9-AnOH]0 is
shown.
On the basis of lactone polymerizations catalyzed by tridentate
anilido-aldimine magnesium/zinc derivatives [5], it was expected that
Zn complexes incorporating AABn ligand can behave as efficient
catalysts toward the ROP of ε-CL in the presence of alcohol. The results
of the polymerization of ε-CL catalyzed by 1–3 in the presence of 9-
anthracenemethanol (9-AnOH) or poly(ethylene glycol) methyl ether
(PEG-OH) are listed in Table 1. Similar conditions following our
previous studies [13] were first applied to examine the catalytic
activities of ε-CL polymerizations using 1/9-AnOH as the catalytic
system under dry N2. Optimized conditions were a 9-AnOH con-
centration of 0.01 M with a zinc catalyst (1 mol equiv) in toluene
(10 mL) at 55 °C. Experimental results indicated that complex 1 and 3
exhibit efficient activity within 50 min, whereas complex 2 can
reach the similar conversion, however with times up to 90 min
(Table 1, entries 1–3). As a result, polymerizations catalyzed by 1 were
systematically studied the “controlled” character under optimized
conditions (Table 1, entries 1, 4–6). The conversion yields go to
completion within 35 min with the ratio [ε-CL]0/[9-AnOH]0 in a range
of 25–200. As demonstrated in Fig. 4, a linear relationship between
actual molecular weight Mn (Mn (GPC) value corrected by a factor of
0.56) and [ε-CL]0/[9-AnOH]0 as well as narrow polydispersity indices
(PDI=1.07–1.17) displayed the “controlled” character of the poly-
merization. High molecular weight PCL (Mn(GPC)=63,400) with
remaining narrow PDI (~1.08) is accomplished by polymerization of
ε-CL with [ε-CL]0/[9-AnOH]0 =1000 at 55 °C for 60 min (Table 1,
entry 7). However, the actual molecular weight (Mn =36,800) [14] is
smaller than the molecular weight calculated from [ε-CL]/[9-AnOH]
ratio. It is believed that a small amount of water from a great quantity
of ε-CLs will deactivate the Zn catalyst and also behave as the chain
transfer agent, resulting only 58% conversion and the lower molecular
weight in comparison with the calculated molecular weight.
To realize the capability of producing PEG-PCL block copolymer
with a narrow PDI, polymerizing ε-CL in the presence of PEG-OH
catalyzed by Zn complex 1 was carried out (Table 1, entry 8). It is
found that the catalytic activity and the controlled character are not
drastically affected when PEG-OH is employed as the initiator. The
synthesized PEG-b-PCL copolymer was verified by 1H NMR spectrum
and GPC profile. In the 1H NMR spectrum (Fig. 5) of PEG-b-PCL, a
sharp singlet of methylene protons (He, 3.60 ppm) for the PEG
polymer and a triplet signal of methylene protons (Hf, 4.02 ppm) for
the PCL polymer show the block nature of the PEG-b-PCL copolymer.
For the GPC profile as shown in Fig. 6, peak A corresponds to
PEG (Mn =500, PDI=1.34) and peak B to PEG-b-PCL after copoly-
merization with ε-CL at 200 equiv. A unimodal peak with increased
Table 1
Ring-opening polymerization of ε-caprolactone (ε-CL) catalyzed by complexes 1–3 in the presence of 9-AnOH or PEG-OH.
Entry
Cat.
[ε-CL]0/[Zn]0/[ROH]0
t (min)
Conv. (%)d
Mn(calcd.)e
Mn(obsd.)f
Mn(NMR)g
PDIh
1a
2a
3a
4a
5a
6a
7b
8c
1
2
3
1
1
1
1
1
200/1/1
200/1/1
200/1/1
25/1/1
35
90
50
20
30
30
60
60
99
95
97
99
99
99
58
95
22,800
21,900
22,300
3000
29,900(16,700)
32,600(18,300)
33,700(18,900)
4700(2600)
25,900
23,300
24,800
3200
1.09
1.72
1.09
1.17
1.14
1.07
1.08
1.13
50/1/1
5900
8800(4900)
6900
100/1/1
1000/1/1
200/1/1
11,500
66,100
22,400
17,300(9700)
63,400(36,800)
34,100
14,200
i
–
23,700
a
ROH=9-AnOH, 0.1 mmol complexes, 10 mL toluene, 55 °C.
b
c
d
e
f
ROH=9-AnOH, 0.1 mmol complexes, 15 mL toluene, 55 °C.
ROH=PEG-OH(Mw =750), 0.1 mmol complexes, 10 mL toluene, 55 °C.
Obtained from 1H NMR determination.
Calculated from the molecular weight of ε-CL times [ε-CL]0/[ROH]0 times conversion yield plus the molecular weight of ROH.
Obtained from GPC analysis and calibrated by polystyrene standard. Values in parentheses are the values obtained from GPC times 0.56 [14].
g
h
i
Obtained from 1H NMR analysis.
Obtained from GPC analysis.
Not available.

C.-H. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1456–1460
1459
Fig. 5. 1H NMR spectra of PEG-b-PCL (Table 1, entry 8) in CDCl3.
molecular weight (Mn =34,100, PDI=1.13) was observed (peak B),
Acknowledgment
indicating the formation of the block copolymer.
In conclusion, three novel zinc complexes of NN-bidentate AABn
ligands are synthesized and structurally characterized by spectro-
scopic studies and X-ray single crystal measurements. Experimental
results indicate that complex 1 catalyzes the ROP of ε-CL in the
presence of 9-AnOH with the highest activity in a controlled manner.
In an investigation of AA-relative complexes for catalytic ROP of cyclic
esters and CO2/epoxide coupling reaction, different pendant arms of
AA derivatives and various metal systems are now in progress in our
group.
We gratefully acknowledge the financial support from the National
Science Council, Taiwan (NSC99-2113-M-033-007-MY2).
Appendix A. Supplementary material
Supporting informationavailable: experimental details forsynthesis,
polymerization procedures and X-ray crystallographic files in CIF format
for structural determinations of complexes 1–3 are listed. Supplemen-
tary data to this article can be found onlineat doi:10.1016/j.inoche.2011.
05.046.
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Fig. 6. GPC profiles of copolymerization of PEG-b-PCL: (peak A) the prepolymer
(Mn =500, PDI=1.34); (peak B) the block copolymer of PEG-PCL (Mn =34,100,
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(1H, d, PhH), 3.96 (2H, q, CH2Ph), 3.21 (2H, m, CH(CH3)2), 1.16 (3H, d, CH(CH3)2),
0.93 (3H, d, CH(CH3)2), 0.87 (3H, d, CH(CH3)2), 0.52 (3H, d, CH(CH3)2). 13C NMR
(CDCl3, ppm): δ 169.03 (HC=N), 160.64, 147.45, 145.63, 143.83, 137.10, 136.74,
132.30, 129.08, 128.71, 127.74, 124.79, 124.06, 123.30, 120.51, 115.91, 112.09
(Ar), 61.24 (HC=NCH2), 28.42, 27.78 (CH(CH3)2), 26.14, 24.98, 24.91, 22.53 (CH
(CH3)2).
[10] Crystal data for complex 1: C28H34N2Zn, M=463.96, monoclinic, space group P21/
c, a=10.9955(3) Å, b=18.8155(4) Å, c=24.0070(6) Å, α=90.00° β=99.8050
(10)° γ=90.00° V=4894.2(2) Å3, T=100(2)K, Z=8, Dc=1.259 Mg/m3, F(000)
=1968, 2θmax =56.54° 43902 reflections collected, 12027 unique [R(int)
= 0.0528], no. of observed reflections 8642 (I N2σ(I)); R1 = 0.0625,
wR2=0.1362.
[6] Y.-C. Liu, C.-H. Lin, H.-L. Chen, B.-T. Ko, (E)-N-[2-(Benzyliminomethyl)phenyl]-
2,6-diisopropylaniline, Acta Cryst. E65 (2009) o2791.
[7] [(AABn)ZnEt] (1): Yield: 0.75 g (81%). Anal. calc. for C28H34N2Zn: N, 6.04; C, 72.48;
H, 7.39. Found: N, 5.98; C, 72.47; H, 7.68%. 1H NMR (CDCl3, ppm): δ 8.45 (1H, s,
HC=N), 7.39–6.32 (10H, m, PhH), 6.49 (1H, t, PhH), 6.34 (1H, d, PhH), 4.93 (2H, s,
CH2Ph), 3.08 (2H, m, CH(CH3)2), 1.13 (12H, m, CH(CH3)2), 0.72 (3H, t, ZnCH2CH3),
–0.02 (2H, q, ZnCH2CH3). 13C NMR (CDCl3, ppm): δ 169.69 (HC=N), 157.45,
144.84, 143.68, 138.69, 137.26, 133.90, 128.95, 127.92, 127.71, 125.04, 123.81,
116.27, 114.04, 112.70 (Ar), 64.87 (HC=NCH2), 27.98 (CH(CH3)2), 24.33, 24.17
(CH(CH3)2), 11.74 (ZnCH2CH3), –1.84 (ZnCH2CH3).
[11] Crystal data for complex 2: C35H44N4Zn, M=586.11, triclinic, space group P−1,
a=10.8647(19) Å, b=11.0194(19) Å, c=15.094(3) Å, α=101.644(9)°
β = 90.420(9)° γ = 112.568(8)° V = 1627.2(5) Å3, T = 299(2)K, Z = 2,
Dc=1.196 Mg/m3, F(000)=624, 2θmax =56.62° 28445 reflections collected,
8024 unique [R(int)=0.0401], no. of observed reflections 6528 (IN2σ(I));
R1=0.0357, wR2=0.0944.
[12] Crystal data for complex 3: C52H58N4Zn, M=804.39, triclinic, space group P−1,
a=10.6506(2) Å, b= 12.0858(3) Å, c=18.3447(4) Å, α=78.5620(10)°
β=84.5310(10)° γ=68.6750(10)° V=2155.29(8) Å3, T=100(2)K, Z=2,
Dc=1.239 Mg/m3, F(000)=856, 2θmax =56.66° 38127 reflections collected,
10650 unique [R(int)=0.0250], no. of observed reflections 9483 (I N2σ(I));
R1=0.0307, wR2=0.0778.
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(1-phenyl-3-methylpyrazol-5-one) with trimethylaluminum: synthesis, struc-
ture and catalysis for the ring-opening polymerization of ε-caprolactone, Inorg.
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[14] The Mn (GPC) value is multiplied by a factor of 0.56, giving the actual Mn of the
poly(ε-caprolactone), see: M. Save, M. Schappacher, A. Soum, Controlled ring-
opening polymerization of lactones and lactides initiated by lanthanum
isopropoxide, 1. general aspects and kinetics, Macromol. Chem. Phys. 203
(2002) 889–899.
[8] [(AABn)Zn(DMAP)Et] (2): Yield: 0.82 g (70%). Anal. calc. for C35H44N4Zn: N, 9.56;
C, 71.72; H, 7.57. Found: N, 9.15; C, 72.05; H, 7.90%. 1H NMR (CDCl3, ppm): δ 8.31
(1H, s, HC=N), 7.98 (1H, d, NCHCH), 7.26–7.10 (11H, m, PhH), 6.92 (1H, t, PhH),
6.38 (2H, d, NCHCH), 6.30 (1H, t, PhH), 6.15 (1H, d, PhH), 4.72 (2H, s, CH2Ph), 3.07
(2H, m, CH(CH3)2), 3.00 (6H, s, NCH3), 0.97 (6H, d, CH(CH3)2), 0.92 (6H, d, CH
(CH3)2), 0.65 (3H, t, ZnCH2CH3), –0.21 (2H, q, ZnCH2CH3). 13 C NMR (CDCl3, ppm):
δ 169.69 (HC=N), 158.00, 154.43, 149.09, 146.04, 143.88, 138.50, 137.15, 132.97,
128.48, 127.88, 127.36, 124.09, 123.48, 116.81, 114.22, 110.99, 106.45 (Ar), 64.62
(HC=NCH2), 39.05 (NCH3), 27.71 (CH(CH3)2), 24.39, 23.92 (CH(CH3)2), 12.24
(ZnCH2CH3), –2.97 (ZnCH2CH3).
[9] [(AABn)2Zn] (3) : Yield: 0.64 g (80%). Anal. calc. for C52H58N4Zn: N, 6.96; C, 77.64;
H, 7.27. Found: N, 6.37; C, 77.43; H, 6.99%. 1H NMR (CDCl3, ppm): δ 7.70 (1H, s,
HC=N), 7.22–7.05 (6H, m, PhH), 6.87–6.76 (4H, m, PhH), 6.25 (1H, t, PhH), 6.14