Hepta-coordinated heteroleptic derivatives of zirconium(IV): Synthesis, structural characterization and ring opening polymerization of ε-caprolactone

Article abstract of DOI:10.1016/j.poly.2016.01.035

A series of new hepta-coordinated heteroleptic derivatives of zirconium(IV) of the types [(CH₃COCHCOCH₃)₂ZrL] (1, 3, 5 and 7) and [(C₆H₅COCHCOC₆H₅)₂ZrL] (2, 4, 6 and 8) (L?=?a dianionic [ONO]-tridentate ancillary ligand) was synthesized quantitatively by reacting Zr(OPrⁱ)₄·PrⁱOH with the tridentate Schiff's base ligands H₂L^x(H₂L¹?=?C₁₃H₁₁NO₂; H₂L²?=?C₁₃H₁₀BrNO₂; H₂L³?=?C₁₄H₁₃NO₃; H₂L⁴?=?C₁₇H₁₃NO₂) and β-diketones (acetylacetone/dibenzoylmethane) in a 1:1:2 stoichiometry using dry benzene and ethanol as solvents. All these newly synthesized solid derivatives were fairly soluble in common organic solvents. They were characterized by elemental analyses, FTIR and NMR (¹H and¹³C) spectral studies. Single crystal XRD data of complexes 1, 2, 3 and 5 indicated the hepta-coordination and monomeric nature of these complexes. Furthermore, all these derivatives were tested for the ring opening polymerization of ε-caprolactone. The dibenzoylmethane derivatives were inactive in polymerizing ε-caprolactone unlike the acetylacetonate complexes. The molecular weight and polydispersity index values of polycaprolactone were obtained by GPC analysis. Kinetic studies for the rate of polymerization of ε-caprolactone by these catalytically active zirconium(IV) complexes authenticated the first order dependence of the polymerization rate on the monomer concentration. Among these, complex 3, with an electronegative bromine atom on the Schiff's base moiety, exhibited the best catalytic activity with a conversion of >95% in 2?h.

Full text of DOI:10.1016/j.poly.2016.01.035

Polyhedron 107 (2016) 163–171
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Hepta-coordinated heteroleptic derivatives of zirconium(IV): Synthesis,
structural characterization and ring opening polymerization of
e
-caprolactone
Vuppalapati Giri Prasanth a, Ravindranath S. Rathore b, Madhvesh Pathak a,
,
⇑
Kulathu Iyer Sathiyanarayanan a,1
a School of Advanced Sciences, VIT University, Vellore 632014, Tamilnadu, India
b BIF, School of Life Sciences, University of Hyderabad, Hyderabad 500046, Telangana, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A
series of new hepta-coordinated heteroleptic derivatives of zirconium(IV) of the types
Received 23 October 2015
Accepted 22 January 2016
Available online 1 February 2016
[(CH3COCHCOCH3)2ZrL] (1, 3, 5 and 7) and [(C6H5COCHCOC6H5)2ZrL] (2, 4, 6 and 8) (L = a dianionic
[ONO]-tridentate ancillary ligand) was synthesized quantitatively by reacting Zr(OPri)4ꢀPriOH with the
tridentate Schiff’s base ligands H2Lx (H2L1 = C13H11NO2; H2L2 = C13H10BrNO2; H2L3 = C14H13NO3;
H2L4 = C17H13NO2) and b-diketones (acetylacetone/dibenzoylmethane) in a 1:1:2 stoichiometry using
dry benzene and ethanol as solvents. All these newly synthesized solid derivatives were fairly soluble
in common organic solvents. They were characterized by elemental analyses, FTIR and NMR (1H and
13C) spectral studies. Single crystal XRD data of complexes 1, 2, 3 and 5 indicated the hepta-coordination
and monomeric nature of these complexes. Furthermore, all these derivatives were tested for the ring
opening polymerization of
izing -caprolactone unlike the acetylacetonate complexes. The molecular weight and polydispersity
index values of polycaprolactone were obtained by GPC analysis. Kinetic studies for the rate of polymer-
ization of -caprolactone by these catalytically active zirconium(IV) complexes authenticated the first
Keywords:
Zirconium
Metallo-organic complexes
e-Caprolactone (e-CL)
Ring opening polymerization (ROP)
Polycaprolactone (PCL)
e-caprolactone. The dibenzoylmethane derivatives were inactive in polymer-
e
e
order dependence of the polymerization rate on the monomer concentration. Among these, complex 3,
with an electronegative bromine atom on the Schiff’s base moiety, exhibited the best catalytic activity
with a conversion of >95% in 2 h.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Polycaprolactone (PCL) is biodegradable, biocompatible and
eco-friendly polyester [14,15]. It is extensively used in packaging
In chemical history, metallo-organic derivatives of zirconium
(IV) have been documented emphatically as noteworthy precur-
sors of nano-structured zirconia [1,2] giving substantial yields with
high purity by the sol–gel synthetic route. Very recently, we have
materials, tissue engineering scaffolds, controlled drug delivery
systems and in biocompatible implants [16–19]. PCL can be
produced by the ring-opening polymerization of
e-caprolactone
(e-CL) using suitable metal complexes as initiators [20,21]. Instead
carried out
a
unique investigation on the wettability of
a
of exploiting the rapidly depleting petrochemical resources to pro-
cure conventional polymers and in order to replenish our deterio-
rating ecosystem, the production of polymeric materials like PCL
and polylactide could be carried out by utilizing their relevant
monomers, which in turn could be outsourced from tremendous
renewable natural capital [8,22].
polyvinylidene fluoride-nano zirconia composite material from a
newly synthesized metallo-organic complex of zirconium(IV) [3].
Apart from this application, these heteroleptic derivatives of zirco-
nium(IV) have also been employed as initiators in the ring-opening
polymerization of cyclic esters [4–13].
Although there are numerous metal complexes reported as cat-
alysts for the ring opening polymerization (ROP) of cyclic esters,
the low toxicity and great activity of metallo-organic complexes
of zirconium(IV) in producing high molecular weight polymers
with good polydispersity index (PDI) values motivated us to enter
this field [12]. Nowadays, zirconium complexes incorporated with
⇑
Corresponding author. Tel.: +91 416 2202553.
E-mail addresses: madhveshpathak@vit.ac.in (M. Pathak), sathiya_kuna@
hotmail.com (S. Kulathu Iyer).
1
Co-corresponding author. Tel.: +91 416 2202081.
http://dx.doi.org/10.1016/j.poly.2016.01.035
0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.

164
G.P. Vuppalapati et al. / Polyhedron 107 (2016) 163–171
ligands such as amine phenolates [7], bis(imino) phenoxides [10],
phenols, benzyl alcohols [9], pyrrolyl N-donors [11], benzotriazole
phenoxides [23] and salen [8] have been well documented for the
polymerization of cyclic esters and the effect of substituents on the
ligands in the polymerization activity has also been interpreted
clearly. However, the unavoidable hydrolytic instability (high
moisture sensitivity), which is a major problem of these com-
plexes, cannot be ignored.
fractionation. The yellow to brown colored solid products obtained
were washed with n-hexane repeatedly and have been character-
ized by FTIR, elemental analysis, 1H and 13C NMR spectra. While
efforts to obtain crystals in suitable solvents, like a DCM–hexane
mixture, were unsuccessful, diffraction quality crystals of 1, 2, 3
and 5 were finally grown in a benzene–hexane mixture, and they
were structurally characterized by single crystal XRD.
Substitution of easily hydrolysable alkoxy groups on metal
alkoxides with non-hydrolysable chelating ligands, such as schiffs
bases, b-diketonates and b-ketoesters will result in the production
of complexes with increased stability against hydrolysis [3,24,25].
Thus, we were inspired to synthesize new zirconium(IV) deriva-
tives associated with non-hydrolysable ligands and with good
ROP activity; herein we report the synthesis of eight hepta-coordi-
nated metallo-organic zirconium(IV) complexes by treating zirco-
nium isopropoxide with Schiff’s bases and b-diketonates in a
stoichiometry of 1:1:2, followed by a detailed study of these
2.1.1. FTIR spectra
The characteristic bands in the FTIR spectra of the complexes
were assigned by comparing the spectra of the free ligands with
the new derivatives. The absence of a strong broad peak around
3400 cmꢁ1 in all the complexes indicated the deprotonation of
the ligands. A strong peak observed at around 750 cmꢁ1 in all the
derivatives confirmed the formation of ZrAO bonds. The appear-
ance of a strong band at around 540 cmꢁ1 indicated the coordina-
tion of a nitrogen atom to zirconium [26]. Shifting of the C
O
stretching frequency of acetylacetone and dibenzoylmethane from
1740 to 1590 cmꢁ1 and the appearance of a strong intense peak at
derivatives on e-CL polymerization activity.
around 1520 cmꢁ1 due to C
C revealed the quasi-aromatic nat-
ure and bidentate coordination pattern of these b-diketonate moi-
eties. Ar-H and CAH stretching frequencies are observed at around
3050 and 2950 cmꢁ1, respectively.
2. Results and discussion
2.1. Synthesis and characterization
The complexes 1-8 were synthesized by the reaction of Zr
(OPri)4ꢀPriOH with the corresponding Schiff’s bases and b-diketo-
nates in a 1:1:2 molar ratio in an anhydrous solvent mixture of
benzene and ethanol (Scheme 1). The isopropanol liberated during
the progress of these one pot alcohol elimination reactions was
removed along with benzene and ethanol by continuous
2.1.2. 1H NMR
In the 1H NMR spectra of the complexes, the disappearance of
the OH peaks corresponding to the Schiff’s bases and b-diketonate
(enolic OH) ligands confirm the deprotonation of the OH groups
and ZrAO bond formation. Coordination of the nitrogen atom to
the zirconium metal center was evidenced by the slight downfield
Scheme 1. Synthesis of the zirconium(IV) derivatives 1–8.

G.P. Vuppalapati et al. / Polyhedron 107 (2016) 163–171
165
shift in the N@CH peaks of the Schiff’s base ligands. Appearance of
two individual singlets in the 1H NMR spectrum (with high and low
proton integration values) for the methyl and methine protons
(respectively) of the acetylacetonate moiety in complex 1, indi-
cated the effect of the Schiff’s base ligand on the chemical equiva-
lence of these protons. Further, a similar type of pattern was
observed in the 13C NMR spectrum of the same complex for the
methyl, methine and carbonyl carbons of the acetylacetonate moi-
ety. Almost, an identical trend of NMR (1H and 13C) spectra were
noticed in the remaining analogous complexes 3, 5 and 7 to
authenticate their plausible structures.
residues, the Oꢀ ꢀ ꢀN bite distances and OAZrAN bite angles were
in the 2.32(3)–2.991(4) Å and 65.2(9)–81.30(12)° ranges
respectively. The coordination bond distances were observed to
be as follows: ZrAO (acac/dbm) 2.121(3)–2.215(2) Å, ZrAO
(H2L1/derivatives) 1.829(14)–2.216(15) Å and ZrAN 2.323(3)–
2.393(8) Å. The observed geometries are in good agreement with
their average values [27]. In all the ligands, the six-membered che-
late rings formed by the O, N atoms are slightly puckered. Among
the metal complexes, slight puckering of the chelate rings in the
flexible ligands arises due to packing forces and to accommodate
weak hydrogen bonds, which is a common observation in coordi-
nation compounds [28,29].
Compounds 1 and 2, having the H2L1 ligand, possess crystallo-
graphically-imposed two-fold symmetry with Z0 = 0.5 and the Zr
atom located at the two-fold axis. The Schiff’s base H2L1 ligand
does not possess any point-group symmetry. However, the mole-
cules in the unit cell occupy (in a 0.5:0.5 ratio) two different orien-
tations related by two-fold rotation, leading to C2 point group
symmetry with the metal lying at the two-fold axis. In 1 and 2,
the asymmetric unit comprises of one acac/dbm ligand, and half
of the H2L1 ligand with half the occupancy of the Schiff’s base
(AC@NA).
2.1.3. Structure of the zirconium(IV) complexes
The structures of the four zirconium(IV) complexes were estab-
lished by collecting intensity data from their single crystal XRD. In
the hepta-coordinated derivatives, six O atoms and one N atom are
attached to the metal center Zr(IV) in a pentagonal bipyramidal
geometry (10 faces, 15 edges), possessing D5h symmetry as shown
in Fig. 1. The Zr-coordination spheres are formed by chelating
bidentate b-diketonates [acetylacetonate (acac)/dibenzoylmethane
(dbm)] along with the tridentate Schiff’s bases in complexes 1, 2, 3
and 5. In the acac and dbm residues, the Oꢀ ꢀ ꢀO bite distances and
the OAZrAO bite angles varied in the ranges 2.620(5)–2.658(3) Å
and 75.09(9)–75.49(12)°, while in H2L1 and its substituent
In the monoclinic crystals (1, 3, 5), four molecules are packed
via weak CAHꢀ ꢀ ꢀO/
p and van der Waals interactions and the
( i ) [(CH3COCHCOCH3)2ZrL1] 1
( ii) [(C6H5COCHCO C6H5)2ZrL1] 2
( iii) [(CH3COCHCOCH3)2ZrL2] 3
( iv) [(CH3COCHCOCH3)2ZrL3] 5
Fig. 1. A perspective view of the sevenfold coordination polyhedron, possessing a pentagonal bipyramid geometry in the metal complexes (i) 1, (ii) 2, (iii) 3 and (iv) 5.
Displacement ellipsoids are drawn at the 35% probability level. Color scheme: C-gray, H-white, Br-brown, N-blue, O-red, Zr-green. Solvent benzene and positional disorder in
1 and 2 are not shown for clarity. In complex 2, the two different orientations of the H2L1 residues, related by a two-fold axis are distinguished by dashed bond connectivity at
C7 and N1. (Colour online.)

166
G.P. Vuppalapati et al. / Polyhedron 107 (2016) 163–171
Fig. 2. 1H NMR of complex 1: (a) unexposed to air. (b) Exposed to air.
central hydrophobic channels running along b-axis are occupied by
the benzene solvent. On the other hand, the packing mode in the
orthorhombic crystal 2 is slightly different from this common pat-
tern due to the presence of a bulky phenyl group in dbm. In this
case, the hydrophobic channel is occupied by the benzene solvent
along the c-axis. Packing diagrams are shown in the Supplemen-
tary Fig. S17 and weak intermolecular interactions are listed in
the Supplementary Table S1.
reactions were performed under a dry nitrogen environment in
DCM at 50 °C. Dibenzoylmethane derivatives 2, 4, 6 and 8 were
inactive in polymerizing
lacetonate derivatives 1, 3, 5 and 7 were quite active in polymeriz-
ing -CL. The temperature dependence of these polymerization
reactions was studied while performing -CL polymerization using
e-CL due to steric effects, whereas acety-
e
e
complex 1 at 25, 50 and 70 °C (Table 2, entries 1–3). The monomer
conversion was found to be negligible at room temperature, but it
was better at higher temperatures. Molecular weight and PDI val-
ues of the PCL produced were determined by gel permeation chro-
matography (GPC) and the PDI values were found to be in a narrow
2.2. Stability studies
The moisture stability of these complexes was studied by com-
paring the 1H NMR spectra recorded for complex 1 in exposed as
well as in unexposed conditions. Despite exposing complex 1 to
air for 2 weeks, no new peaks appeared nor did any existing signals
disappear in the 1H NMR spectra (Fig. 2). Similar behavior of the
same derivative (air exposed and unexposed) in DMSO was also
observed in the UV–Vis investigations. No shift/decay in the char-
acteristic imine absorption peak at 420 nm was noticed in the
spectra. Further, the UV–Vis spectra of complex 1 were recorded
periodically thrice (with a time interval of 24 h) using a solvent
mixture of water and DMSO in a 1:9 ratio. There was no variation
(shift/decay) at all in the position of the band at 420 nm. Thus, the
1H NMR and UV–Vis spectral studies evidently reveal the apprecia-
ble air stability of derivative 1 as well as its analogous complexes.
range from 1.17 to 1.31. This indicates that the
tion occurred in a controlled manner. Systematic
e
e
-CL polymeriza-
-CL polymeriza-
tion studies of
3
with increasing monomer to initiator
concentrations [M]o:[Zr]o resulted in polymers with increasing
average molecular weights Mn and a narrow range of PDI values
(1.17–1.29) revealed that the polymerizations proceeded in a con-
trolled fashion. The linear relationship between Mn and [M]o:[Zr]o
(Fig. 3) showed by complex 3 (Table 2, entries 6–10) specified
the ‘‘living” character of polymerization process. The experimental
results revealed that complexes 1, 3, 5 and 7 are highly active in
polymerizing e-CL with >90% conversion in 2 h. However, the supe-
rior polymerization activity of complex 3 (monomer conversion
>95%) could be due to the electron withdrawing bromine sub-
stituent on the benzene ring of the Schiff’s base moiety. The 1H
NMR spectrum of PCL ([M]o:[Al]o = 100; Table 2, entry 3; where
the end groups were found to be benzyloxy and hydroxyl groups)
(Fig. 4) indicates that the polymerization takes place through a
‘‘coordination insertion” pathway. The integral ratio between Hg
(CH2 protons of PCL from the hydroxyl group end) at 3.58 ppm
and Hb (CH2 protons of PCL from t benzyloxy group end) at
2.3. Polymerization studies
All the derivatives were trialed for the ROP of e-CL in the pres-
ence of external benzyl alcohol as the polymerization did not pro-
ceed without addition of benzyl alcohol. The polymerization

G.P. Vuppalapati et al. / Polyhedron 107 (2016) 163–171
167
Table 1
Crystal data for all the four Zr-complexes.
Complex
1
2
3
5
Crystal data
Chemical formula
Co-crystallizing solvent
Molecular weight
Crystal size (mm)
Morphology
Crystal system
Space group
Unit cell parameters
a (Å)
C
23H23NO6Zr, (C6H6)
C43H31NO6Zr, 2(C6H6)
benzene
905.12
0.35 ꢂ 0.30 ꢂ 0.30
block, orange
orthorhombic
Pbcn
C23H22BrNO6Zr, 1/2(C6H6)
benzene
618.60
0.38 ꢂ 0.25 ꢂ 0.22
rectangular, Yellow
monoclinic
C24H25NO7Zr, (C6H6)
benzene
608.78
0.25 ꢂ 0.16 ꢂ 0.10
square, yellow
monoclinic
C2/c
benzene
578.75
0.38 ꢂ 0.26 ꢂ 0.19
prism, yellow
monoclinic
P2/c
C2/c
11.8514(3)
8.1243(2)
15.9914(4)
117.738(2)
1362.78(6)
2/0.5
12.2550(3)
19.1718(6)
19.3124(6)
40.8686(12)
8.0987(2)
16.0193(4)
103.4146(11)
5157.4(2)
8/1
23.8636(7)
16.4613(5)
16.0004(4)
116.0474(12)
5647.0(3)
8/1
b (Å)
c (Å)
b (°)
V (Å3)
4537.5(2)
4/0.5
Z/Z0
Cell measuring reflections
h-range (°)
Absorption correction,
8293
9928
8597
7596
2.9–26.4
multi-scan, 0.446
596
2.2–26.0
multi-scan, 0.295
1872
2.6–25.6
multi-scan, 2.016
2488
2.3–25.9
multi-scan, 0.437
2512
l )
(mmꢁ1
F (000)
Dx (calculated) (g cmꢁ3
)
1.410
1.325
1.593
1.432
Data collection
Radiation (Å)
T (°K)
0.71073 (Mo K
296
a)
0.71073 (Mo K
296
a)
0.71073 (Mo K
296
a)
0.71073 (Mo K
296
a)
h range (°)
Indices
2.5–25
2.1–25
2.0–25
1.8–25.0
h = ꢁ14 ? 13
k = ꢁ9 ? 9
l = ꢁ15 ? 19
h = ꢁ14 ? 14
k = ꢁ22 ? 22
l = ꢁ22 ? 22
h = ꢁ43 ? 48
k = ꢁ9 ? 9
l = ꢁ19 ? 14
h = ꢁ28 ? 28
k = ꢁ19 ? 19
l = ꢁ15 ? 19
Scan type
Independent reflections
Observed reflections [I > 2r(I)]
/ and
2399
2274
x
scans
/ and
4004
2523
x
scans
/ and
4542
3926
x
scans
/ and
4980
4061
x scans
Refinement
Final indices (Robs, wRall
Goodness-of-fit (S)
)
R = 0.0288, wR = 0.0809
1.644
R = 0.0552, wR = 0.1875
1.167
R = 0.0384, wR = 0.0877
1.128
R = 0.0426, wR = 0.1156
1.070
Extinction coefficient
0.0013(9)
0.0009(4)
nil
nil
(
D
D
/
r
)
0.000
0.002
0.001
0.002
max
qmax and
D
qmin (e Åꢁ3
)
0.705, ꢁ0.518
1.120, ꢁ0.650
0.626, ꢁ0.647
1.133, ꢁ0.331
Data/restraints/parameter
2399/5/181
4005/459/377
4542/0/321
4980/0/359
Weight w = 1/[r
2(F20) + (aP)2 + bP] where P = (F20 + 2Fc2)/3, parameters a and b are: 0.0367 and 0.5310 (1), 0.0779 and 8.0725 (2), 0.0332 and 14.8582 (3), 0.0445 and, 18.3655
(5), respectively.
Table 2
Polymerization of e
-CL catalyzed by complexes 1, 2, 3, 5 and 7.a
Conv.b (%)
Mn (calcd)
Mn (obsd)
Yielde (%)
PDId
c
d
Entry
Cat.
[M]o:[Zr]o:[ROH]o
T (°C)
Time (h)
1
2
3
4
5
6
7
8
9
1
1
1
2
2
3
3
3
3
3
5
7
200:1:2
200:1:2
200:1:2
200:1:2
100:1:2
100:1:2
200:1:2
300:1:2
400:1:2
500:1:2
200:1:2
200:1:2
25
50
70
50
50
50
50
50
50
50
50
50
24
2
2
24
24
2
2
2
2
2
12
94
94
–
–
–
–
–
10800
10800
–
11500
9700
–
93
94
–
1.31
1.24
–
–
–
–
–
–
98
98
96
98
96
91
93
5700
11300
16900
22000
27500
10500
10700
5400
11000
16200
20000
24000
13500
11900
91
95
95
91
93
90
92
1.25
1.17
1.27
1.25
1.29
1.24
1.29
10
11
12
2
2
a
b
c
All polymerization reactions were carried out in 15 mL of DCM.
Measured from 1H NMR analysis.
Mn (calcd) = [114.14 ꢂ [M]o:[Zr]o ꢂ conversion yield/([ROH]eq)] + M(ROH).
e
d
Isolated yield.
Obtained from GPC analysis.
5.00 ppm is close to 1, evidence that the polymerization is initiated
by the insertion of a benzyloxy group into -CL, forming an zirco-
nium alkoxide intermediate that consequently reacts with the
excess lactones, giving polyesters [30–33].
A keen literature survey enabled to compare the reported as
well as present work on e-CL polymerization judiciously. The con-
aryloxy [9], bis(imino) phenoxide [10,34] and bis(aminophenolate)
[13] ligands. Although these newly synthesized zirconium(IV)
complexes have better activity than zirconium complexes sup-
ported by pyrrolyl-linked N-donor [11], aminebisphenolate [7],
diamine [35], pyrrole and phenolic [36] ligands, they show a bit
lower activity than the previously existing zirconium derivatives
incorporated with salen [8,12], salan type diamine bis(phenolato)
[37], salicylaldiminato [38], N-alkyl substituted amine biphenolate
e
cerned activity of these new zirconium(IV) derivatives are almost
at a par with the zirconium complexes associated with benzyloxy,

168
G.P. Vuppalapati et al. / Polyhedron 107 (2016) 163–171
3.0
2.5
2.0
1.5
1.0
3. Experimental
25000
20000
15000
10000
5000
0
3.1. General
All the complexes were synthesized in a moisture-free environ-
ment. Solvents and reagents were made anhydrous by conven-
tional methods [42]. Zirconium(IV) isopropoxide, acetylacetone
and dibenzoylmethane were purchased from Sigma Aldrich. The
Schiff’s base ligands were prepared according to the literature.
The zirconium content was estimated gravimetrically as ZrO2
[26]. Melting points were examined on an Elchem digital melting
point apparatus. FTIR spectra [4000–400 cmꢁ1] were recorded on
a SHIMADZU IR affinity 1 spectrometer with anhydrous KBr pellets.
1H and 13C NMR spectra were obtained from a Bruker ADVANCE III
400 spectrometer in CDCl3 solution at 400 and 100.65 MHz fre-
quencies using TMS as an internal standard. Weight average
molecular weight (Mw), number average molecular weight (Mn),
and molecular weight distributions were determined against poly-
0
100
200
300
[M]0/[Zr]0
400
500
600
Fig. 3. Catalytic polymerization of e-CL by 3 at 50 °C in DCM.
[39], imino phenoxide [40] and catecholate [41] ligands. Thus, the
present investigations also appear to contribute in the field of
polymerization significantly.
e-CL
styrene as a standard by GPC using a PL gel 5 lm MLXED-C column
on an Agilent 1100 series liquid chromatography system with THF
as the eluent. Elemental analyses of complexes were carried out on
an Elementar Vario EL III instrument.
2.4. Kinetic studies for the polymerization of
e-caprolactone
3.2. Synthesis of the Schiff’s bases H2L1, H2L2, H2L3 and H2L4
Kinetic studies [9,37] for the polymerization of
e
-caprolactone
using complexes 1, 3, 5 and 7 as catalysts were also carried out
in the presence of benzyl alcohol in a [M]o/[Zr]o/[BnOH] ratio of
200:1:2 at 50 °C. The experimental results are displayed in
Figs. S21 and 5. The graph plotted between the % conversion of
The Schiff’s base ligands H2L1, H2L2, H2L3 and H2L4 were
prepared by the reaction of 2-aminophenol with salicylaldehyde,
5-bromosalicylaldehyde,
3-methoxysalicylaldehyde
and
2-hydroxy-1-napthaldehyde in a 1:1 molar ratio in refluxing
methanol [43–48].
e
-CL and time was sigmoidal. Four linear curves obtained by plot-
ting ln ([ -CL]o/[ -CL]t) versus time clearly exhibited the well
established pattern of first order kinetics for the polymerization
of -CL. The rate constants for these -CL polymerization reactions
e
e
e
e
3.3. Synthesis of the complexes
catalyzed by complexes 1, 3, 5 and 7 have been determined as
2.12 ꢂ 10ꢁ2
,
3.19 ꢂ 10ꢁ2
,
1.99 ꢂ 10ꢁ2 and 2.07 ꢂ 10ꢁ2 minꢁ1
,
3.3.1. Synthesis of the complex [(CH3COCHCOCH3)2ZrL1] 1
Zirconium(IV) isopropoxide (3.87 g, 10 mmol), the ethanolic
Schiff’s base H2L1 (2.13 g, 10 mmol) and acetylacetone (2.00 g,
20 mmol) were taken as reactants in a stoichiometry of 1:1:2 using
respectively. Thus the rate of polymerization by these complexes
could be arranged in the order 3 > 1 > 7 > 5 by exploiting the com-
puted values of the rate constants.
Fig. 4. The 1H NMR spectrum of PCL catalyzed by complex 3 (entry 6, Table 2).

G.P. Vuppalapati et al. / Polyhedron 107 (2016) 163–171
169
Fig. 5. Semi-logarithmic plots of e-CL conversion in time initiated by 1, 3, 5 and 7: [M]o/[Zr]o/[BnOH] = 200:1:2 at 50 °C.
benzene as a solvent. The resulting reaction mixture was refluxed
for 2 h and the liberated isopropanol was fractionated out, along
with benzene and ethanol. A yellow colored product, obtained after
the removal of excess solvent under vacuum, was washed twice
with n-hexane. (Yield: 4.87 g, 97.4%. Mp: 218–220 °C. 1H NMR
(400 MHz, CDCl3, 25 °C) d (ppm): 1.83 (s, 9H, CH3), 1.97 (s, 3H,
CH3), 5.43, 5.60 (s, 2H, CH), 6.59–6.66 (m, 3H, Ar-H), 6.71–6.74 (t,
1H, J = 8.0 Hz, Ar-H), 6.99–7.02 (t, 1H, J = 8.0 Hz, Ar-H), 7.24–7.33
(m, 3H, Ar-H), 9.52 (s, 1H, N@CH). 13C NMR (100.65 MHz, CDCl3,
25 °C) d (ppm): 24.86, 26.61, 100.45, 104.05, 114.53, 117.4 6,
117.65, 117.95, 120.07, 122.84, 129.30, 133.70, 134.72, 138.64,
NMR (400 MHz, CDCl3, 25 °C) d (ppm): 1.84 (s, 11H, CH3), 1.96 (s,
1H, CH3), 5.42, 5.60 (s, 2H, CH), 6.50–6.52 (d, 1H, J = 8.0 Hz, Ar-
H), 6.57–6.59 (d, 1H, J = 8.0 Hz, Ar-H), 6.61–6.65 (t, 1H, J = 8.0 Hz,
Ar-H), 6.99–7.03 (t, 1H, J = 8.0 Hz, Ar-H), 7.22–7.24 (d, 1H,
J = 8.0 Hz, Ar-H), 7.29–7.30 (d, 1H, J = 4.0 Hz, Ar-H), 7.40–7.41 (d,
1 H J = 4.0 Hz, Ar-H), 8.41 (s, 1H, N@CH). 13C NMR (100.65 MHz,
CDCl3, 25 °C) d (ppm): 24.86, 26.62, 100.46, 104.17, 108.95,
114.68, 117.71, 117.79, 122.06, 124.40, 129.82, 135.28, 137.05,
138.30, 154.34, 162.18, 162.42, 190.68, 191.20. FTIR (solid KBr)
m
(cmꢁ1): 3064.05, 2982.53, 1583.20, 1526.44, 1019.74, 736.92,
618.45, 524.31. Anal. Calc. for C23H22BrNO6Zr: C, 47.67; H, 3.83;
N, 2.42; Zr, 15.74. Found: C, 47.71; H, 3.80; N, 2.39; Zr, 15.69%.
155.86, 162.34, 163.17, 190.56, 191.20. FTIR (solid KBr)
m
(cmꢁ1):
3059.10, 2960.73, 1581.63, 1525.69, 1024.20, 754.17, 655.8,
615.29, 518.85. Anal. Calc. for C23H23NO6Zr: C, 55.18; H, 4.63; N,
2.80; Zr, 18.22. Found: C, 55.20; H, 4.67; N, 2.81; Zr, 18.25%.
As the patterns of these reactions are almost similar, the
remaining seven complexes (2-8) were also synthesized by follow-
ing the above-said procedure.
3.3.4. Synthesis of the complex [(C6H5COCHCOC6H5)2ZrL2] 4
Zirconium(IV) isopropoxide (3.87 g, 10 mmol), Schiff’s base
H2L2 (2.92 g, 10 mmol) and dibenzoylmethane (4.48 g, 20 mmol).
Stoichiometry: 1:1:2. (Yield: 8.05 g, 97.3%. Mp: 225–227 °C. 1H
NMR (400 MHz, CDCl3, 25 °C)
d (ppm): 6.49–6.52 (d, 1H,
J = 8.0 Hz, Ar-H), 6.59–6.61 (d, 1H, J = 8.0 Hz, Ar-H), 6.68–6.71 (t,
1H, J = 8.0 Hz, Ar-H), 7.02–7.06 (t, 1H, J = 8.0 Hz, Ar-H), 7.10 (s,
2H, C-H), 7.25–7.29 (t, 9H, J = 8.0 Hz, Ar-H), 7.36–7.40 (t, 4H,
J = 8.0 Hz, Ar-H), 7.42–7.44 (d, 1H, J = 8.0 Hz, Ar-H), 7.47–7.48 (d,
1H, J = 4.0 Hz, Ar-H), 7.76–7.78 (d, 7H, J = 8.0 Hz, Ar-H), 7.91–7.93
(d, 1H, J = 8.0 Hz, Ar-H), 8.45 (s, 1H, N@CH). 13C NMR
(100.65 MHz, CDCl3, 25 °C) d (ppm): 93.18, 96.78, 109.07, 115.08,
115.08, 117.87, 117.94, 122.36, 125.14, 127.19, 128.07, 128.42,
128.71, 130.14, 132.12, 132.47, 135.31, 135.59, 137.23, 137.41,
3.3.2. Synthesis of the complex [(C6H5COCHCO C6H5)2ZrL1] 2
Zirconium(IV) isopropoxide (3.87 g, 10 mmol), Schiff’s base
H2L1 (2.13 g, 10 mmol) and dibenzoylmethane (4.48 g, 20 mmol).
Stoichiometry: 1:1:2. (Yield: 7.35 g, 98.1%. Mp: 258–260 °C. 1H
NMR (400 MHz, CDCl3, 25 °C)
d (ppm): 6.59–6.61 (d, 2H,
J = 8.0 Hz, Ar-H), 6.66–6.70 (t, 1H, J = 8.0 Hz, Ar-H), 6.73–6.77 (t,
1H, J = 8.0 Hz, Ar-H), 7.00–7.04 (t, 1H, J = 8.0 Hz, Ar-H), 7.08 (s,
2H, C-H), 7.22–7.26 (t, 8H, J = 8.0 Hz, Ar-H), 7.28–7.30 (d, 1H,
J = 8.0 Hz, Ar-H), 7.34–7.37 (t, 5H, J = 8.0 Hz, Ar-H), 7.41–7.43 (d,
1H, J = 8.0 Hz, Ar-H), 7.76–7.78 (d, 7H, J = 8.0 Hz, Ar-H), 7.90–7.92
(d, 1H, J = 8.0 Hz, Ar-H), 8.54 (s, 1H, N@CH). 13C NMR
138.82, 154.35, 162.45, 162.79, 184.04, 185.79. FTIR (solid KBr)
m
(cmꢁ1): 3053.21, 1591.27, 1525.69, 1072.42, 750.31, 675.09,
607.58, 518.85. Anal. Calc. for C43H30BrNO6Zr: C, 62.39; H, 3.65;
N, 1.69; Zr, 11.02. Found: C, 62.42; H, 3.69; N, 1.70; Zr, 11.10%.
(100.65 MHz, CDCl3, 25 °C)
d (ppm): 96.68, 115.02, 117.71,
118.13, 120.30, 123.51, 127.20, 128.10, 128.36, 128.71, 129.64,
131.96, 132.48, 133.83, 134.98, 137.57, 139.22, 155.91, 162.72,
3.3.5. Synthesis of the complex [(CH3COCHCOCH3)2ZrL3] 5
163.32, 183.97. FTIR (solid KBr)
1523.76, 1024.20, 754.17, 680.87, 620.05, 516.92. Anal. Calc. for
43H31NO6Zr: C, 68.96; H, 4.17; N, 1.87; Zr, 12.18. Found: C,
m
(cmꢁ1): 3059.10, 1591.27,
Zirconium(IV) isopropoxide (3.87 g, 10 mmol), Schiff’s base
H2L3 (2.43 g, 10 mmol) and acetylacetone (2.00 g, 20 mmol). Stoi-
chiometry: 1:1:2. (Yield: 5.22 g, 98.4%. Mp: 214–216 °C. 1H NMR
(400 MHz, CDCl3, 25 °C) d (ppm): 1.85 (s, 11H, CH3), 1.97 (s, 1H,
CH3), 3.07 (s, 3H, CH3), 5.42, 5.60 (s, 2H, CH), 6.57–6.66 (m, 3H,
Ar-H), 6.87–6.89 (d, 1H, J = 8.0 Hz, Ar-H), 6.95–6.97 (d, 1H,
J = 8.0 Hz, Ar-H), 6.98–7.02 (t, 1H, J = 8.0 Hz, Ar-H), 7.23–7.25 (d,
1H, Ar-H), 8.51 (s, 1H, N@CH). 13C NMR (100.65 MHz, CDCl3,
25 °C) d (ppm): 24.85, 26.60, 57.10, 100.45, 104.15, 114.52,
C
68.89; H, 4.12; N, 1.85; Zr, 12.13%.
3.3.3. Synthesis of the complex [(CH3COCHCOCH3)2ZrL2] 3
Zirconium(IV) isopropoxide (3.87 g, 10 mmol), Schiff’s base
H2L2 (2.92 g, 10 mmol) and acetylacetone (2.00 g, 20 mmol).
Stoichiometry: 1:1:2. (Yield: 5.70 g, 98.4%. Mp: 208–210 °C. 1H

170
G.P. Vuppalapati et al. / Polyhedron 107 (2016) 163–171
117.27, 117.42, 117.67, 118.71, 123.17, 126.23, 128.35, 129.35,
138.63, 149.92, 154.56, 156.01, 162.41, 163.17, 190.65, 191.20.
FTIR (solid KBr)
1026.78, 755.15, 637.67, 516.31. Anal. Calc. for C24H25NO7Zr: C,
54.32; H, 4.75; N, 2.64; Zr, 17.19. Found: C, 54.19; H, 4.68; N,
2.67; Zr, 17.25%.
duration for each set of reactions (Table 2). Since screw-capped
reaction tubes were employed to carry out all the polymerization
reactions, a bit of pressure was experienced due to overheating
of the dichloromethane solvent in the tubes concerned, but the
process got completed safely and successfully. After quenching
the polymerization by addition of methyl alcohol, the reaction
mixture was poured into ca. 50 mL of methanol for precipitation
of PCL as a white solid. The obtained polymer was purified by
dissolving it in DCM and by re-precipitating it with methanol.
Afterwards, the polymer was dried in a vacuum oven at 50 °C until
a constant weight was obtained.
m
(cmꢁ1): 3042.02, 2978.43, 1595.17, 1521.34,
3.3.6. Synthesis of the complex [(C6H5COCHCOC6H5)2ZrL3] 6
Zirconium(IV) isopropoxide (3.87 g, 10 mmol), Schiff’s base
H2L3 (2.43 g, 10 mmol) and dibenzoylmethane (4.48 g, 20 mmol).
Stoichiometry: 1:1:2. (Yield: 7.58 g, 97.4%. Mp: 225–227 °C. 1H
NMR (400 MHz, CDCl3, 25 °C) d (ppm): 3.50 (s, 3H, CH3), 6.59–
6.61 (d, 1H, J = 8.0 Hz, Ar-H), 6.66–6.69 (t, 2H, J = 8.0 Hz, Ar-H),
6.86–6.88 (d, 1H, J = 8.0 Hz, Ar-H), 7.00–7.05 (m, 2H, Ar-H), 7.09
(s, 2H, C-H), 7.22–7.26 (t, 8H, J = 8.0 Hz, Ar-H), 7.34–7.50 (m, 5H,
Ar-H), 7.77–7.79 (d, 7H, J = 8.0 Hz, Ar-H), 7.91–7.92 (d, 1H,
J = 4.0 Hz, Ar-H), 8.55 (s, 1H, N@CH). 13C NMR (100.65 MHz, CDCl3,
25 °C) d (ppm) 57.44, 96.77, 115.05, 117.58, 117.68, 117.73, 120.27,
124.04, 126.69, 127.20, 128.12, 128.35, 128.71, 129.68, 131.95,
137.55, 139.23, 149.97, 154.87, 156.09, 162.80, 184.02. FTIR (solid
3.5. Single crystal XRD
Single crystals of the new zirconium(IV) complexes were
obtained by keeping a solution of the concerned derivatives in a
benzene–DCM mixture overnight. Intensity data were collected
on a single-crystal Bruker SMART APEX2 diffractometer and pro-
cessed using Saint-Plus [49]. The structure was solved by the appli-
cation of the direct phase-determination technique using SHELXS-97,
and anisotropically refined by full-matrix least-square on F2 using
KBr)
m
(cmꢁ1): 3046.04, 1592.24, 1524.14, 1039.87, 751.28, 618.58,
514.52. Anal. Calc. for C44H33NO7Zr: C, 67.84; H, 4.27; N, 1.80; Zr,
SHELXL-2014/7 [50]. All structural calculations were performed with
the WinGX suit (Version 2013.3) [51]. Hydrogens were placed in
geometrically expected positions and refined with riding options.
The torsion angles for the methyl group hydrogen atoms were
set with reference to a local difference Fourier calculation. The dis-
tances to hydrogen atoms are: aromatic CAH = 0.93 Å, methyl
CAH = 0.96 Å and Uiso = 1.2 Ueq(Car)/1.5 Ueq(CH3). Unlike 3 and 5
(asymmetrical methoxy and bromo-substitutions in H2L1), the
structures of 1 and 2 are disordered. There are two different orien-
tations of the H2L1 residues, related by a two-fold axis passing
through the Zr(IV) center. The two alternative orientations of the
molecule with equal occupancy led to C2 point group symmetry
and the asymmetric unit comprises half of the molecule, with
one acac/dbm and half of H2L1 (with half occupancy of Schiff’s base
AC@NA). The phenol ring in H2L1 of 2 possesses an additional posi-
tional disorder of equal occupancy. The refinement involved a ser-
ies of trials to arrive at the optimum refinement and in the final
cycle notable restraints were applied. In complex 1, for the Schiff’s
base bond distances, the restraints were as follows: AC@NA, 1.28
(1) Å, ACarACA, 1.47(1) Å and ACarANA, 1.45(1) Å. In structure 2,
other than Schiff’s base bond restrains, additional CarACar, 1.39
(1) Å bond restraints for the benzene and the phenol rings were
applied [52]. To deal with the positional disorder in the phenol ring
(in 2), the same distance restrains (SADI) was applied for the CarAO
bond, and ring atoms were restrained with an effective standard
deviation (0.02 Å) to have the same Uij components (SIMU
restraints) and approximating to isotropic behavior (ISOR
restraint). In the final cycle of refinement, reflections, (100) in 1,
(020) in 2 and (200) in 3 were omitted, which were affected by
the beam-stop. Essential crystal data are listed in Table 1.
11.71. Found: C, 67.69; H, 4.15; N, 1.73; Zr, 11.67%.
3.3.7. Synthesis of the complex [(CH3COCHCOCH3)2ZrL4] 7
Zirconium(IV) isopropoxide (3.87 g, 10 mmol), Schiff’s base
H2L4 (2.63 g, 10 mmol) and acetylacetone (2.00 g, 20 mmol). Stoi-
chiometry: 1:1:2. (Yield: 5.41 g, 98.3%. Mp: 225–227 °C. 1H NMR
(400 MHz, DMSO d6, 25 °C) d (ppm): 1.68 (s, 8H, CH3), 1.84 (s,
1H, CH3), 2.03 (s, 2H, CH3), 2.14 (s, 1H, CH3), 5.55, 5.63, 5.70 (s,
2H, CH), 6.42–6.57 (m, 1H, Ar-H), 6.67–6.80 (m, 1H, Ar-H), 6.92–
7.08 (m, 2H, Ar-H), 7.55–7.59 (t, 1H, J = 8.0 Hz, Ar-H), 7.78–7.94
(m, 3H, Ar-H), 8.48–8.57 (m, 2H, Ar-H), 9.61, 9.70 (s, 1H, N@CH).
13C NMR (100.65 MHz, CDCl3, 25 °C)
d (ppm): 24.87, 26.62,
100.46, 103.97, 114.53, 117.59, 117.63, 120.01, 123.17, 127.62,
128.57, 129.14, 135.44, 139.78, 151.05, 162.07, 164.79, 190.53,
191.21. FTIR (solid KBr)
m
(cmꢁ1): 3043.12, 2964.65, 1582.14,
1525.28, 1024.07, 751.64 615.27, 519.45. Anal. Calc. for C27H25NO6-
Zr: C, 58.89; H, 4.58; N, 2.54; Zr, 16.56. Found: C, 58.72; H, 4.55; N,
2.46; Zr, 16.62%.
3.3.8. Synthesis of complex [(C6H5COCHCO C6H5)2ZrL4] 8
Zirconium(IV) isopropoxide (3.87 g, 10 mmol), Schiff’s base
H2L4 (2.63 g, 10 mmol) and dibenzoylmethane (4.48 g, 20 mmol).
Stoichiometry: 1:1:2. (Yield: 7.81 g, 97.7%. Mp: 258–260 °C. 1H
NMR (400 MHz, CDCl3, 25 °C) d (ppm): 6.74–6.79 (m, 3H, Ar-H),
6.89–6.91 (d, 1H, J = 8.0 Hz, Ar-H), 7.08 (s, 2H, C-H), 7.14–7.16 (d,
6H, J = 8.0 Hz, Ar-H), 7.29–7.30 (d, 3H, J = 4.0 Hz, Ar-H), 7.34 (s,
1H, Ar-H), 7.42–7.48 (m, 4H, Ar-H), 7.72–7.74 (d, 9H, J = 8.0 Hz,
Ar-H), 7.91–7.93 (d, 2H, J = 8.0 Hz, Ar-H), 8.10–8.13 (d, 1H,
J = 12.0 Hz, Ar-H), 9.50 (s, 1H, N@CH). 13C NMR (100.65 MHz,
CDCl3, 25 °C) d (ppm): 96.64, 114.91, 115.05, 117.57, 117.87,
120.03, 123.12, 123.31, 127.19, 127.64, 127.86, 128.06, 128.29,
128.70, 128.94, 129.06, 131.94, 132.45, 133.99, 135.66, 137.46,
4. Conclusion
140.29, 151.22, 164.94, 183.99, 185.79. FTIR (solid KBr)
m
(cmꢁ1):
3056.10, 1614.42, 1590.96, 1524.31, 1038.73, 750.85, 669.62,
623.47, 520.26. Anal. Calc. for C43H31NO6Zr: C, 68.96; H, 4.17; N,
1.87; Zr, 12.18. Found: C, 68.89; H, 4.12; N, 1.85; Zr, 12.13%.
Hepta-coordination around zirconium(IV) with a pentagonal
bipyramidal geometry for the new derivatives was evident from
the single crystal XRD of complexes 1, 2, 3 and 5. Derivative 1
was confirmed to be moisture stable from UV–Vis and 1H NMR
spectroscopy. Further, all the synthesized complexes were tested
3.4. Polymerization of e-caprolactone
for the ROP of
e-CL. It could be considered as an encouraging obser-
Typically, to a solution of the catalyst (0.2 mmol) in DCM
(5 mL), another solution of benzyl alcohol (0.4 mmol) in 5 mL
DCM was added. After stirring the solution for 10 min, a measured
vation that the polymerization of
e
-CL occurred exclusively by
complexes 1, 3, 5 and 7, incorporating acetylacetone, while the
derivatives 2, 4, 6 and 8, associated with dibenzoylmethane, exhi-
bit their reluctance towards this activity. The ‘‘living” character of
amount of
e-CL dissolved in 5 mL of DCM was transferred and kept
in an oil bath for stirring at 50 °C for the required individual
complex 3 in polymerizing
e-CL was evidenced by the linear

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CCDC 1402390, 1402391, 1402389 and 1402388 contains the
supplementary crystallographic data for 1, 2, 3 and 5. These data
can be obtained free of charge via http://www.ccdc.cam.ac.
uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary
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