Aryloxy and benzyloxy compounds of zirconium: Synthesis, structural characterization and studies on solvent-free ring-opening polymerization of ε-caprolactone and δ-valerolactone

Article abstract of DOI:10.1016/j.jorganchem.2010.09.026

A series of aryloxy compounds and benzyloxy derivatives of Zr(IV) were synthesized in good yields and purity, employing the alcoholysis route. They were completely characterized with different spectroscopic techniques and single crystal X-ray diffraction.

Full text of DOI:10.1016/j.jorganchem.2010.09.026

Journal of Organometallic Chemistry 696 (2011) 572e580
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Aryloxy and benzyloxy compounds of zirconium: Synthesis, structural
characterization and studies on solvent-free ring-opening polymerization
of
3
-caprolactone and
d-valerolactone
*
Ravikumar R. Gowda, Debashis Chakraborty , Venkatachalam Ramkumar
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, 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 aryloxy compounds and benzyloxy derivatives of Zr(IV) were synthesized in good yields and
purity, employing the alcoholysis route. They were completely characterized with different spectroscopic
techniques and single crystal X-ray diffraction. A high degree of fluxional behavior of these compounds
was understood through variable-temperature NMR studies. X-ray diffraction studies prove that these
compounds exist as dimers in the solid state. They are potent catalysts for the ring-opening polymeri-
Received 21 July 2010
Received in revised form
6 September 2010
Accepted 9 September 2010
Available online 17 September 2010
zation of 3-caprolactone (CL) and d-valerolactone (VL) resulting in high polymers with good number
average molecular weights (Mn) and molecular weight distributions (MWDs). The degree of control in
these polymerizations was found to be superior with the zirconium compounds when compared with
the titanium analogues. The rate of polymerization was found to be slower for the zirconium compounds,
as realized from kinetic studies. Analysis of the MALDI-TOF and 1H NMR spectra of low molecular weight
oligomers of CL synthesized using these compounds reflect that these polymerizations proceed by the
activated monomer mechanism.
Keywords:
Zirconium(IV)
3-Caprolactone
d-Valerolactone
Polymerization
Activated monomer mechanism
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
narrow MWDs [1,13e27]. The polymerization initiates rapidly and
there are minimal risks of unwanted transesterification reactions.
The rapid depletion of petrochemical resources underlines the
need for research in search of new polymeric materials that are
derived from renewable feed stock [1e5]. As a consequence, there
is a need to study the polymerization of cyclic ester monomers and
lactide. These polymers are derived from completely renewable
natural resources such as corn and sugar-beet [6e10]. At the
moment, these polymers have found vital applications in the
biomedical and pharmaceutical sectors such as drug delivery,
fabrication of implants and scaffolds for tissue engineering [11]. In
addition, the film and fiber industry have also benefited by
employing such polymers in suitable applications [6]. Such impact
is as a result of their permeability, biocompatibility and biode-
gradability [7,12]. One of the most common and rapidly used
strategies in the synthesis of such polymers is the catalytic ring-
opening polymerization of suitable cyclic ester monomers and
lactide that are mediated by metal catalysts [11,13,14]. Amongst
several options available for ring-opening polymerization, the
coordinationeinsertion polymerization has been extensively
popular because of its capability in producing high polymers with
The synthesis of new metal catalysts containing rigid and poly-
dentate ancillary ligands with suitable pendant substituents as
initiating groups has been the common methodology of choice for
catalyst construction. It is understood that the nuclearity of the
metal catalyst is controlled by this ancillary ligand and that the
catalyst nuclearity plays a vital role on the coordination-insertion
growth mechanism.
A reasonable controlled polymerization may be attained by
employing suitable catalysts containing group 4 metals, leading to
their popular utility. Numerous organometallic complexes con-
taining group 4 metals and have been reported in the recent liter-
ature which include different ligands such as chiral phenoxyimine
[28], tris(phenoxy)amine [29], bis(
b-ketoamidate) [30], bis(imino-
phenoxy) [19,31], N-heterocyclic carbine [32], bis(phenoxy)amine
[33e35], pyrrolylamine [36], tris(alkoxy) [37,38], tris(alkoxy)amine
[39e41], bis(amido) [42], chalcogen-bridged bis(aryloxy) [43] and
methylene-bridged bis(phenoxy) [44]. Commercially available
reagents such as titanium n-butoxide and zirconium n-propoxide
[45] have been reported to be active catalytically in ring-opening
polymerization of cyclic esters and lactide. In addition, the viability
of titanium phenoxide as a suitable catalyst was reported [46].
A titanium alkoxide based metal organic framework was recently
* Corresponding author. Tel.: þ91 44 2257 4223; fax: þ91 44 2257 4202.
E-mail address: dchakraborty@iitm.ac.in (D. Chakraborty).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.09.026

R.R. Gowda et al. / Journal of Organometallic Chemistry 696 (2011) 572e580
573
shown to possess activity for such polymerizations [47]. The use of
metallocene ester enolates as viable catalysts for the polymeriza-
tion of lactide is reported recently [48]. New complexes of titanium
and zirconium containing the phenylenediamine bis(phenolate)
ligand [49] and sulphonamide supported backbone [50] have
shown potency for lactide polymerization. Titanium complex of
isoquinolinyl-naptholate [51] and zirconium complex of trans-
substituted diamido/diamine cyclam are active for CL polymeriza-
tion [52].
The key step of the coordination-insertion mechanism is the
cleavage of the acyl-oxygen bond of the monomer which may be
attributed to the electronic environment provided by the initiating
group(s) [1]. Hence, the electronic environment around the central
metal provided by the initiating group(s) has a decisive role. Since
the polymerization of lactones proceeds in a non-stereospecific
manner, it is natural to conclude that the ancillary ligand envi-
ronment has limited or no role in such polymerizations apart from
controlling the nuclearity of the metal complex.
The activated monomer polymerization [53e59] uses
a Brønsted acid catalyst to activate the acyl-oxygen bond of the
monomer followed by the ring-opening brought about by suitable
alcohol initiators. We surmised using such a mechanism in
a productive manner by replacing the Brønsted acid by a Lewis acid
catalyst. The proof of such a concept was the outcome of employing
iron and ruthenium chloride catalysts for the polymerization of
lactones [58]. The polymerization results were comparable to the
ones reported for iron compounds containing bulky ligand envi-
ronment [23]. These results proved our contentions that metal
complexes containing elaborate ligands may not be necessary for
developing a viable technology for lactone polymerization. We
were able to confirm our assessment by developing suitable tita-
nium aryloxy and benzyloxy compounds that effectively catalyze
the polymerization of CL and VL [59]. Having obtained encouraging
results from this work, we were inspired to evaluate the role of
zirconium analogues to understand the effect of other group 4
metals in such polymerizations. Polymerization of VL with zirco-
nium compounds has not been reported till date.
standards. For CL, molecular weights (Mn) were corrected according
to Mark-Houwink corrections [60].
2.2. Starting materials
All phenols and benzyl alcohol derivatives used in this study
along with Zr(O-iPr)4(HO-iPr) were purchased from Aldrich and
used without subsequent purification. CL and VL were purchased
from Aldrich, dried over CaH2 overnight and distilled fresh prior to
use. CDCl3 used for NMR spectral measurements was purchased
from Aldrich.
2.3. Syntheses
2.3.1. [Zr(O-2-MeC6H4)4(HO-iPr)]2 (1)
Under an argon atmosphere, to
a stirred solution of
2-MeC6H4OH (0.11 g, 1.03 mmol) in 2.5 mL toluene at ꢂ25 ꢁC, was
added a solution of Zr(O-iPr)4(HO-iPr) (0.10 g, 0.26 mmol) in 2.5 mL
toluene. The reaction mixture was allowed to attain ambient
temperature and stirred for 24 h. The solvent was removed under
reduced pressure to yield a white solid which was purified by
crystallization from toluene at ꢂ25 ꢁC. The crystallized solid was
recovered by filtration and dried in vacuum; yield: 0.26 g (87%);
1
m.p. 173 ꢁC. H NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d 7.05e6.50
(16H, ortho, meta, para), 4.33 (br, 1H, CHMe2), 2.18 (s, 12H, ArMe),
13
1.09 (br, 6H, CHMe2). C NMR (100 MHz, CDCl3, 55 ꢁC, ppm):
d
159.79 (Ar-O), 153.95 (Ar-C), 131.13 (Ar-C), 127.20 (Ar-CH3),
120.81 (Ar-C), 114.97 (Ar-C), 72.46 (CHMe2), 24.62 (CHMe2), 16.91
(Ar-CH3). ESI-MS: m/z ¼ 1075 {(Zr(O-2-MeC6H4)4(HO-iPr))2 ꢂ (O-2-
MeC6H4) þ Na}þ. Anal. Calc. for C31H36O5Zr (579.84): C, 64.21; H,
6.26. Found: C, 64.34; H, 6.31%.
2.3.2. [Zr(O-2-FC6H4)4(HO-iPr)]2 (2)
2-FC6H4OH (0.12 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.28 g (90%); m.p. 179 ꢁC. 1H
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
6.89e6.71 (16H, ortho, meta,
para), 4.64 (br, 1H, CHMe2), 1.27 (br, 6H, CHMe2). 13C NMR
(100 MHz, CDCl3, 55 ꢁC, ppm):
155.04 (Ar-F),149.20 (Ar-O), 123.99
2. Experimental
d
(Ar-C), 123.07 (Ar-C), 120.25 (Ar-C), 115.73 (Ar-C), 72.71 (CHMe2),
23.76 (CHMe2). ESI-MS: m/z ¼ 1103 {(Zr(O-2-FC6H4)4(HO-
iPr))2 ꢂ (O-2-FC6H4) þ Na}þ. Anal. Calc. for C27H24F4O5Zr (595.69):
C, 54.44; H, 6.23. Found: C, 54.68; H, 6.31%.
2.1. General methods and instrumentation
All reactions were performed under dry argon atmosphere using
standard Schlenk techniques or in a glove box with rigorous
exclusion of moisture and air. Toluene was dried by heating under
reflux over sodium and benzophenone and distilled fresh prior to
use. CDCl3 used for NMR spectral measurements was dried over
calcium hydride, distilled and stored in a glove box. 1H and 13C NMR
spectra were recorded with a Bruker Avance 400 instrument.
Chemical shifts for 1H and 13C NMR spectra were referenced to
residual solvent resonances and are reported as parts per million
relative to SiMe4. ESI-MS spectra of the samples were obtained
from Waters Q-Tof micro mass spectrometer. MALDI-TOF
measurements were performed on a Bruker Daltonics instrument
in dihydroxy benzoic acid matrix. Elemental analyses were done
with a Perkin Elmer Series 11 analyzer.
2.3.3. [Zr(O-2-ClC6H4)4(HO-iPr)]2 (3)
2-ClC6H4OH (0.13 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.30 g (88%); m.p. 179 ꢁC. 1H
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
7.26e6.71 (16H, ortho, meta,
para), 5.52 (br, 1H, OH), 4.84 (br, 1H, CHMe2), 1.33 (br, 6H, CHMe2).
13C NMR (100 MHz, CDCl3, 55 ꢁC, ppm):
156.72 (Ar-O), 129.58
d
(Ar-C), 127.67 (Ar-C), 127.29 (Ar-Cl), 123.51 (Ar-C), 121.50
(Ar-C), 73.55 (CHMe2), 24.80 (CHMe2). ESI-MS: m/z ¼ 1218 {(Zr
(O-2-ClC6H4)4(HO-iPr))2 ꢂ (O-2-ClC6H4) þ Na}þ. Anal. Calc. for
C27H24Cl4O5Zr (661.51): C, 49.02; H, 3.66. Found: C, 49.35; H, 3.58%.
Molecular weights and the polydispersity indices of the poly-
mers were determined by GPC instrument with Waters 510 pump
and Waters 410 Differential Refractometer as the detector. Three
columns namely WATERS STRYGEL-HR5, STRYGEL-HR4 and STRY-
GEL-HR3 each of dimensions (7.8 ꢀ 300 mm) were connected in
series. Measurements were done in THF at 27 ꢁC. Number average
molecular weights (Mn) and molecular weight distributions
(MWDs) of polymers were measured relative to polystyrene
2.3.4. [Zr(O-2-BrC6H4)4]2 (4)
2-BrC6H4OH (0.18 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.35 g (85%); m.p. 176 ꢁC. 1H
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
7.51e6.79 (16H, ortho, meta,
152.45 (Ar-O),
13
para). C NMR (100 MHz, CDCl3, 55 ꢁC, ppm):
d
132.16 (Ar-C), 129.33 (Ar-C), 121.95 (Ar-C), 116.33 (Ar-C), 110.44 (Ar-
Br). ESI-MS: m/z ¼ 1409 {(Zr(O-2-BrC6H4)4)2 ꢂ (O-2-BrC6H4) þ

574
R.R. Gowda et al. / Journal of Organometallic Chemistry 696 (2011) 572e580
Na}þ. Anal. Calc. for C24H16Br4O4Zr (779.22): C, 36.99; H, 2.07.
Found: C, 36.78; H, 2.23%.
7.52e6.25 (16H, ortho, meta), 4.32 (br, 1H, CHMe2), 2.36 (s, C7H8),
13
1.16 (br, 6H, CHMe2). C NMR (100 MHz, CDCl3, 55 ꢁC, ppm):
d
160.47 (Ar-O), 139.22 (Ar-C), 138.28 (C7H8), 129.16 (C7H8), 128.49
2.3.5. [Zr(O-3-FC6H4)4(HO-iPr)]2 (5)
(C7H8), 122.51 (Ar-C), 121.21 (C7H8), 83.45 (Ar-I), 72.46 (CHMe2),
24.59 (CHMe2), 21.68 (C7H8). ESI-MS: m/z ¼ 1858 {(Zr(O-4-
3-FC6H4OH (0.12 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.27 g (87%); m.p. 180 ꢁC. 1H
IC6H4)4(HO-iPr))2 ꢂ (O-4-IC6H4) þ Na}þ.
Anal.
Calc.
for
C27H24I4O5Zr (1027.32): C, 31.57; H, 2.35. Found: C, 31.70; H, 2.28%.
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
7.17e6.09 (16H, ortho, meta,
para), 4.42 (br, 1H, CHMe2), 1.23 (br, 6H, CHMe2). 13C NMR
(100 MHz, CDCl3, 55 ꢁC, ppm):
164.82 (Ar-F), 162.29 (Ar-O), 130.13
2.3.11. [Zr(OC6F5)4(HO-iPr)]2 (11)
d
C6F5OH (0.19 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.40 g (87%); m.p. 189 ꢁC. 1H
(Ar-C), 114.86 (Ar-C), 108.22 (Ar-C), 106.51 (Ar-C), 72.72 (CHMe2),
24.56 (CHMe2). ESI-MS: m/z ¼ 1103 {(Zr(O-3-FC6H4)4(HO-
iPr))2 ꢂ (O-3-FC6H4) þ Na}þ. Anal. Calc. for C27H24F4O5Zr (595.69):
C, 54.44; H, 4.06. Found: C, 54.57; H, 4.18%.
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
4.63 (br, 1H, CHMe2), 1.39 (br,
6H, CHMe2). 13C NMR (100 MHz, CDCl3, 55 ꢁC, ppm):
d
141.46 (Ar-F),
139.37 (Ar-F), 136.89 (Ar-F), 136.09 (Ar-O), 72.74 (CHMe2), 24.38
(CHMe2). ESI-MS: m/z ¼ 1606 {(Zr(O-C6F5)4(HO-iPr))2 ꢂ (O-C6F5) þ
Na}þ. Anal. Calc. for C27H8F20O5Zr (883.54): C, 36.70; H, 0.91. Found:
C, 36.59; H, 1.17%.
2.3.6. [Zr(O-3-CF3C6H4)4(HO-iPr)]2 (6)
3-CF3C6H4OH (0.17 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.36 g (88%); m.p. 191 ꢁC. 1H
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
7.61e6.24 (16H, ortho, meta,
para), 4.48 (br, 1H, CHMe2), 1.25 (br, 6H, CHMe2). 13C NMR
(100 MHz, CDCl3, 55 ꢁC, ppm):
160.44 (Ar-O), 132.53 (Ar-CF3),
2.3.12. [Zr(OC6Cl5)4(HO-iPr)]2 (12)
C6Cl5OH (0.27 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.57 g (91%); m.p. 195 ꢁC. 1H
d
130.28 (Ar-C), 125.25 (-CF3), 122.13 (Ar-C), 118.27 (Ar-C), 115.60 (Ar-
C), 73.24 (CHMe2), 24.40 (CHMe2). ESI-MS: m/z ¼ 1453 {(Zr(O-3-
CF3C6H4)4(HO-iPr))2 ꢂ (O-3-CF3C6H4) þ Na}þ. Anal. Calc. for
C31H24F12O5Zr (795.72): C, 46.79; H, 3.04. Found: C, 46.90; H, 2.89%.
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
7.25-7.17 (m, C7H8), 4.60 (br,
1H, CHMe2), 2.36 (s, C7H8), 1.36 (br, 6H, CHMe2). 13C NMR (100 MHz,
CDCl3, 55 ꢁC, ppm):
153.89 (Ar-O), 138.16 (C7H8), 131.36 (Ar-Cl),
d
129.18 (C7H8), 128.37 (C7H8), 125.45 (Ar-Cl), 124.17 (C7H8), 123.78
(Ar-Cl), 73.10 (CHMe2), 24.95 (CHMe2), 21.59 (C7H8). ESI-MS:
m/z ¼ 2183 {(Zr(O-C6Cl5)4(HO-iPr))2 ꢂ (O-C6Cl5) þ Na}þ. Anal. Calc.
for C27H8Cl20O5Zr (1212.63): C, 26.74; H, 0.66. Found: C, 26.90; H,
0.82%.
2.3.7. [Zr(O-4-MeC6H4)4]2 (7)
4-MeC6H4OH (0.11 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.24 g (89%); m.p. 165 ꢁC. 1H
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
7.04 (d, 8H, meta), 6.73 (d, 8H,
ortho), 2.28 (s, 12H, ArMe). 13C NMR (100 MHz, CDCl3, 55 ꢁC, ppm):
153.35 (Ar-O), 130.21 (Ar-CH3), 130.11 (Ar-C), 115.23 (Ar-C), 20.61
2.3.13. [Zr(2, 4, 6-F3C6H2)4(HO-iPr)]2 (13)
2, 4, 6-F3C6H2OH (0.15 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr)
(0.10 g, 0.26 mmol) were reacted and a procedure identical to that
d
(Ar-Me).
ESI-MS:
m/z ¼ 955
{(Zr(O-4-MeC6H4)4)2 ꢂ (O-4-
MeC6H4) þ Na}þ. Anal. Calc. for C28H28O4Zr (519.74): C, 64.70; H,
used for the synthesis of (1) was followed; yield: 0.33 g (87%); m.p.
1
5.43. Found: C, 64.61; H, 5.61%.
168 ꢁC. H NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
7.25-7.17 (m,
C7H8), 6.47 (8H, meta), 4.71 (br, 1H, CHMe2), 2.36 (s, C7H8), 1.33 (br,
2.3.8. [Zr(O-4-tBuC6H4)4]2 (8)
6H, CHMe2). 13C NMR (100 MHz, CDCl3, 55 ꢁC, ppm):
d
154.91 (Ar-F),
4-tBuC6H4OH (0.15 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.32 g (89%); m.p. 183 ꢁC. 1H
152.55 (Ar-F), 138.16 (C7H8), 135.30 (Ar-O), 129.18 (C7H8), 128.38
(C7H8), 125.45 (C7H8), 99.74 (Ar-C), 73.77 (CHMe2), 23.27 (CHMe2),
21.52 (C7H8). ESI-MS: m/z ¼ 1355 {(Zr(2, 4, 6-F3C6H2)4(HO-
iPr))2 ꢂ (O-2, 4, 6-F3C6H2) þ Na}þ. Anal. Calc. for C27H16F12O5Zr
(739.62): C, 43.85; H, 2.18. Found: C, 43.97; H, 2.04%.
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d 7.25 (8H, br, meta), 6.76 (8H,
13
br, ortho), 1.28 (s, 36H, CMe3). C NMR (100 MHz, CDCl3, 55 ꢁC,
ppm): d 154.04 (Ar-O), 143.58 (Ar-t-Bu),125.58 (Ar-C), 114.92 (Ar-C),
34.23 (CMe3), 31.69 (CMe3). ESI-MS: m/z ¼ 1250 {(Zr(O-4-
tBuC6H4)4)2 ꢂ (O-4-tBuC6H4) þ Na}þ. Anal. Calc. for C40H52O4Zr
(688.06): C, 69.82; H, 7.62. Found: C, 69.70; H, 7.78%.
2.3.14. [Zr(O-CH2-4-MeC6H4)4]2 (14)
4-MeC6H4CH2OH (0.13 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr)
(0.10 g, 0.26 mmol) were reacted and an identical procedure used
for the synthesis of (1) was employed. Colorless viscous oil was
1
2.3.9. [Zr(O-4-FC6H4)4(HO-iPr)]2 (9)
obtained; yield: 0.28 g (93%). H NMR (400 MHz, CDCl3, 55 ꢁC,
4-FC6H4OH (0.12 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.28 g (90%); m.p. 184 ꢁC. 1H
ppm):
d
7.09e6.80 (16H, ortho, meta), 4.67 (br, 8H, -OCH2Ar), 2.32
140.32
(br, 12H, ArCH3). 13C NMR (100 MHz, CDCl3, 55 ꢁC, ppm):
d
(-OCH2Ar), 135.31 (Ar-CH3), 128.78 (Ar-C), 127.10 (Ar-C), 72.35
(-OCH2Ar), 21.27 (Ar-CH3). ESI-MS: m/z ¼ 1053 {(Zr(O-CH2-4-
MeC6H4)4)2 ꢂ (O-CH2-4-MeC6H4) þ Na}þ. Anal. Calc. for C32H36O4Zr
(575.85): C, 66.74; H, 6.30. Found: C, 66.59; H, 6.38%.
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
7.18e6.19 (16H, ortho, meta),
4.35 (br, 1H, CHMe2), 1.20 (br, 6H, CHMe2). 13C NMR (100 MHz,
CDCl3, 55 ꢁC, ppm):
158.20 (Ar-F), 157.07 (Ar-O), 119.56 (Ar-C),
d
115.99 (Ar-C), 71.95 (CHMe2), 24.64 (CHMe2). ESI-MS: m/z ¼ 1103
{(Zr(O-4-FC6H4)4(HO-iPr))2 ꢂ (O-4-FC6H4) þ Na}þ. Anal. Calc. for
C27H24F4O5Zr (595.69): C, 54.44; H, 4.06. Found: C, 54.65; H, 4.12%.
2.3.15. [Zr(O-CH2-4-OMeC6H4)4]2 (15)
4-MeOC6H4CH2OH (0.14 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr)
(0.10 g, 0.26 mmol) were reacted and an identical procedure used
for the synthesis of (1) was employed. Colorless viscous oil was
2.3.10. [Zr(O-4-IC6H4)4(HO-iPr)]2 (10)
1
4-IC6H4OH (0.23 g, 1.03 mmol) and Zr(O-iPr)4(HO-iPr) (0.10 g,
0.26 mmol) were reacted and an identical procedure used for the
synthesis of (1) was employed; yield: 0.48 g (91%); m.p. 198 ꢁC. 1H
obtained; yield: 0.29 g (88%). H NMR (400 MHz, CDCl3, 55 ꢁC,
ppm):
-OCH2Ar), 3.83 (s, 12H, ArOCH3). 13C NMR (100 MHz, CDCl3, 55 ꢁC,
ppm): 159.33 (Ar-OCH3), 133.28 (-OCH2Ar), 128.77 (Ar-C), 114.08
d 7.39 (br, 8H, ortho), 6.92 (br, 8H, meta), 4.63 (s, 8H,
NMR (400 MHz, CDCl3, 55 ꢁC, ppm):
d
7.25e7.18 (m, C7H8),
d

R.R. Gowda et al. / Journal of Organometallic Chemistry 696 (2011) 572e580
575
Table 1
(Ar-C), 65.19 (-OCH2Ar), 55.42 (Ar-OCH3). ESI-MS: m/z ¼ 1165 {(Zr
Crystal data for the structures 2, 3 and 5.a
(O-CH2-4-OMeC6H4)4)2 ꢂ (O-CH2-4-OMeC6H4) þ Na}þ. Anal. Calc.
for C32H36O8Zr (639.85): C, 60.07; H, 5.67. Found: C, 60.13; H, 5.51%.
Compound
2
3
5
Empirical formula
Formula weight
Crystal system
Space group
C
54H48F8O10Zr2
C27H24Cl4O5Zr
661.51
Triclinic
P1
173
C54H48F8O10Zr2
1191.38
Triclinic
P1
1191.38
Triclinic
P1
2.4. General procedure for the bulk polymerization of CL and VL
For CL polymerization, 23.6 mmol of the aryloxy compound or
Temp, K
173
173
ꢀ
benzyloxy derivative of zirconium were taken in a flask under an
argon atmosphere. The contents were stirred at 80 ꢁC and 0.50 mL
CL (0.54 g, 4.71 mmol) was added neat. The mixture was rapidly
stirred at the given temperature. A rise in viscosity was observed
Wavelength/A
0.71073
10.7276(3)
11.1626(3)
12.2289(4)
95.6600(10)
112.9840(10)
102.2700(10)
1290.21(7)
1
1.528
16,244
5853
1.083
R1 ¼ 0.0374
wR2 ¼ 0.1077
R1 ¼ 0.0447
wR2 ¼ 0.1268
P
0.71073
0.71073
10.0708(4)
10.9001(4)
13.0320(5)
110.842(2)
104.282(2)
94.737(2)
1272.50(8)
1
1.552
16,490
5858
0.774
ꢀ
a/A
10.6878(2)
11.2615(2)
12.9477(3)
103.9590(10)
94.4410(10)
102.9870(10)
1362.56(5)
2
1.607
18,274
6475
1.036
ꢀ
b/A
ꢀ
c/A
ꢁ
ꢁ
a
/
and finally the stirring ceased. For VL polymerization, 26.9 mmol
b
/
ꢁ
/
g
was used for 0.50 mL (0.54 g, 5.38 mmol) of monomer and a similar
procedure was followed. The progress of the polymerization was
followed by monitoring the disappearance of the monomer using
TLC technique [58,59]. The polymerizations were quenched by
pouring the contents into cold heptane. The polymer was isolated
by subsequent filtration and dried till a constant weight was
attained.
ꢀ
V/A
Z
Dcalc/g cmꢂ3
Reflns collected
No. of indep reflns
GoF
Final R indices [I>2
s
(I)]
R1 ¼ 0.0614
wR2 ¼ 0.1612
R1 ¼ 0.0903
wR2 ¼ 0.1839
R1 ¼ 0.0272
wR2 ¼ 0.0851
R1 ¼ 0.0347
wR2 ¼ 0.0991
R indices (all data)
2.5. General procedure for the bulk polymerization of CL and VL
in toluene
P
P
P
a
R1
¼
jF0j ꢂ jFcj/ jF0j, wR2 ¼ [ (F20 ꢂ F2c)2/ w(F02)2]1/2
.
For CL polymerization, 23.6 mmol of the aryloxy compound or
Reaction of the different phenols with Zr(O-iPr)4(HO-iPr) in 4:1
benzyloxy derivative of zirconium and 2 mL of dry toluene were
taken in a flask under a nitrogen atmosphere. The contents were
stirred at 80 ꢁC and 0.5 mL CL (0.54 g, 4.71 mmol) was added neat.
stoichiometric ratio in toluene under ambient conditions afforded
the corresponding compounds as crude product which were
subsequently purified by crystallization from toluene and isolated
as colorless solids. These compounds possess appreciably high
melting points. The benzyloxy derivatives namely Zr(O-CH2-4-
MeC6H4)4 (14) and Zr(O-CH2-4-OMeC6H4)4 (15) were synthesized
by using a similar procedure and isolated as colorless viscous oils.
The synthetic route is viable under ambient conditions in sharp
contrast to the titanium analogues where several reaction mixtures
were heated [59]. These compounds were thoroughly characterized
employing 1H and 13C NMR spectroscopy, electrospray ionization
mass spectrometry (ESI-MS) and X-ray diffraction studies. They
were subjected to elemental analysis for assurance of purity.
The 1H NMR of these compounds under ambient conditions was
found to contain several signals, in addition to the expected. The
spectra were found to simplify considerably at 55 ꢁC. The signals
appear broad, suggesting a fluxional behavior exhibited by these
compounds in solution. The degree of broadening was found to be
much higher than the corresponding titanium analogues [59]. The
The mixture was rapidly stirred. For VL polymerization, 26.9 mmol
was used for 0.50 mL (0.54 g, 5.38 mmol) of monomer and a similar
procedure was followed. The progress of the polymerization was
followed by monitoring the disappearance of the monomer using
TLC technique [58,59]. The polymerizations were quenched by
pouring the contents into cold heptane. The polymer was isolated
by subsequent filtration and dried till a constant weight was
attained.
2.6. X-ray structure determination of compounds 2, 3 and 5
Single crystals suitable for structural studies were obtained by
crystallization of these compounds from toluene at ꢂ25 ꢁC. X-ray
data collection was performed with Bruker AXS (Kappa Apex 2)
CCD diffractometer equipped with graphite monochromated Mo
ꢀ
(Ka) (
l
¼ 0.7107 A) radiation source. The data were collected with
100% completeness for
collect the data. The frame width for
q
up to 25ꢁ.
u
and
f
scans was employed to
u
was set to 0.5ꢁ for data
collection. The frames were integrated and data were reduced for
Lorentz and polarization corrections using SAINT-NT. The multi-
scan absorption correction was applied to the data set. All struc-
tures were solved using SIR-92 and refined using SHELXL-97 [61].
The crystal data are summarized in Table 1. The non-hydrogen
atoms were refined with anisotropic displacement parameter. All
the hydrogen atoms could be located in the difference Fourier map.
The hydrogen atoms bonded to carbon atoms were fixed at
chemically meaningful positions and were allowed to ride with the
parent atom during refinement.
toluene
Zr(OAr)4(HO-iPr)n
+ (5-n) (HO-iPr)
Zr(O-iPr)4(HO-iPr)
+ 4 HOAr
Compound
1
Ar
n
1
1
1
0
1
1
0
0
1
1
1
1
1
Yield (%)
87
2-MeC6H4
2-FC6H4
2-ClC6H4
2-BrC6H4
3-FC6H4
3-CF3C6H4
4-MeC6H4
4-tBuC6H4
4-FC6H4
4-IC6H4
90
2
88
3
85
4
87
5
88
6
89
7
3. Results and discussion
89
8
90
9
3.1. Synthesis and spectral characterization of compounds
91
10
11
12
13
87
C6F5
91
The alcoholysis route [62,63] was used to synthesize a large
number of zirconium aryloxy compounds 1e13 respectively in high
yields and purity starting from Zr(O-iPr)4(HO-iPr) along with
phenols with different substituents on the aromatic ring (Scheme 1).
C6Cl5
87
2, 4, 6-F3C6H2
Scheme 1. Synthesis of aryloxy derivatives of zirconium.

576
R.R. Gowda et al. / Journal of Organometallic Chemistry 696 (2011) 572e580
expected multiplicity for each signal was found to disappear
completely. However, the signals appeared at their expected posi-
tions with the correct ratio of integration (see Supplementary data).
In compounds 10, 12 and 13, there is a molecule of residual toluene
which could not be removed completely by drying these
compounds under high vacuum. Similar observation was noted
with a few titanium analogues [59]. For the zirconium aryloxy
compounds 1e13, the eOH signal of the coordinated HO-iPr
remains silent with the exception of 3 where it appears at
d
¼ 5.52 ppm as a broad peak. The eCH signal contributed by the
coordinated HO-iPr appears quite deshielded (
¼ 4.32e4.84 ppm)
in comparison to that of the free alcohol (
¼ 4.04 ppm) which may
d
d
be attributed to its coordination with the zirconium center. It is
pertinent to mention here that fluxionality and oligomer formation
is a characteristic feature with group 4 metal alkoxides [64e66].
The scope of ligand scrambling or external ligand exchange is due
to the zirconium centers which provide a relatively easy associative
route. Such observations of signal broadening or disappearance of
signal multiplicities is not observed for the benzyloxy derivatives
14 and 15 respectively. We have performed variable-temperature
1H NMR studies with compounds 2 and 13. The results for 13 are
depicted in Fig. 1 (see Supplementary data for 2).
A number of additional signals are seen corresponding to each
moiety as the temperature is gradually lowered. However at 55 ꢁC,
considerable simplification in the spectral pattern was noticed.
These observations suggest fluxionality and complicated dynamics
possessed by these molecules in solution [64e66]. The 13C NMR of
the aryloxy compounds 1e13 is quite complicated under ambient
conditions, characterized by the appearance of extra signals for each
moiety. Again, considerable simplification at 55 ꢁC was seen. The
ipso carbon of the phenyl ring is considerably deshielded in these
aryloxy compounds and no significant change was noticed with
respect to the appearance of signals contributed by other carbon
atoms of the phenyl ring. In 1e13, the eCH peak of the coordinated
Fig. 2. Molecular structure of 2; thermal ellipsoids were drawn at 30% probability
ꢁ
ꢀ
level. Selected bond lengths (A) and angles ( ): Zr(1)eO(5), 2.178(14); Zr(1)eO(5a),
2.187(14); Zr(1)eZr(1a), 3.573(3); Zr(1)eO(1), 1.989(3); Zr(1)eO(2), 1.950(15); Zr(1)eO
(3), 2.259(15); Zr(1)eO(4), 1.937(3); Zr(1)eO(5a)eZr(1a), 109.89(6); O(5)eZr(1)eO(5a),
70.11(6); O(2)eZr(1)eO(3), 89.46(7); O(2)eZr(1)eO(4), 103.72(7); O(1)eZr(1)eO(4),
99.66(7); O(1)eZr(1)eO(5), 88.61(6); O(2)eZr(1)eO(5a), 94.57(6); O(4)eZr(1)eO(5a),
159.71(6).
a prominent peak thereby indicating the presence of these mole-
cules predominantly in the dimeric state.
3.2. X-ray diffraction studies
Suitable single crystals of 2, 3 and 5 for X-ray diffraction studies
were grown in a glove box at ꢂ25 ꢁC over a three week period from
dilute solutions of the respective compounds in toluene. The
molecular structures of 2 and 3 are depicted in Figs. 2 and 3
respectively (see Supplementary data for 5).
HO-iPr is deshielded considerably (
d
¼ 71.95e73.76 ppm) when
compared with the free alcohol (
d
¼ 64.50 ppm) as a result of
coordination with the zirconium center. The signals appear in their
respective positions for the benzyloxy derivatives 14 and 15
respectively. ESI-MS performed on all the compounds indicates
the presence of {M2-(OAr) þ Na}þ[M ¼ Zr(OAr)4 (4, 7, 8), Zr
(OAr)4(HO-iPr) (1e3, 5, 6, 9e13), Zr(OCH2Ar)4 (14, 15) as
Fig. 3. Molecular structure of 3; thermal ellipsoids were drawn at 30% probability
ꢁ
ꢀ
level. Selected bond lengths (A) and angles ( ): Zr(1)eO(5), 2.192(3); Zr(1)eO(5a),
2.199(3); Zr(1)eZr(1a), 3.607(8); Zr(1)eO(1), 1.989(3); Zr(1)eO(2), 1.948(3); Zr(1)eO
(3), 2.268(3); Zr(1)eO(4), 1.945(3); Zr(1)eO(5a)eZr(1a), 110.51(2); O(5)eZr(1)eO(5a),
69.49(12); O(2)eZr(1)eO(3), 86.71(14); O(2)eZr(1)eO(4), 103.51(14); O(1)eZr(1)eO
(4), 97.40(15); O(1)eZr(1)eO(5), 92.54(13); O(2)eZr(1)eO(5a), 94.43(13); O(4)eZr(1)e
O(5a), 159.52(12).
Fig. 1. Variable-temperature 1H NMR of 13 in CDCl3.

R.R. Gowda et al. / Journal of Organometallic Chemistry 696 (2011) 572e580
577
Table 2
These compounds appear as centrosymmetric dimers in the
solid state. Each metal center is in an edge shared distorted octa-
hedral environment coordinated to a molecule of HO-iPr. The
various bond lengths and angles of these two compounds are
almost identical with the two zirconium centers separated by ca.
Results of CL polymerization using various aryloxy compounds and benzyloxy
derivatives of zirconium in 200:1 ratio at 80 ꢁC.
Entry Compound Timea (min) % Yieldb 103 Mnc (g/mol) MWD (Mw/Mn)
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
45
20
30
67
70
15
95
100
90
18
62
55
110
47
50
96
96
95
97
95
96
95
95
96
95
96
96
97
98
95
10.80
8.50
1.31
1.31
1.34
1.27
1.32
1.31
1.33
1.34
1.30
1.33
1.31
1.30
1.26
1.11
1.39
ꢀ
3.5 A. This is longer than the distance found in the corresponding
21.58
28.94
16.25
65.37
61.99
59.62
51.26
46.92
9.26
titanium analogue [59] and explains the greater degree of flux-
ionality seen for these compounds as compared to that observed for
the corresponding titanium compounds. All the bond lengths and
angles match well with literature precedents [33,36]. A stable
dimeric core is formed as a result of nearly identical ZreO bonds
constituting the core, suggesting almost equal covalent and dative
contributions. There are two different types of ZreO bonds
contributed by the phenoxy groups. The ones constituting the
terminal ZreO bonds were found to be slightly shorter than those
contributing to the core. The distorted octahedral environment
around each zirconium center is also evident from the bond angles.
In 2, 3 and 5, one of the terminal phenoxy ligand forms hydrogen
bonding with the coordinated HO-iPr of the neighboring zirconium
9
10
11
12
13
14
15
48.57
51.93
6.33
21.88
a
Time of polymerization measured by quenching the polymerization reaction
when all monomer was found consumed.
b
Isolated yield.
ꢀ
ꢀ
c
Measured by GPC at 27 ꢁC in THF relative to polystyrene standards with
center. These have been estimated to be 2.215(4) A in 2, 2.531(6) A
ꢀ
MarkeHouwink corrections for Mn.
in 3, and 2.102(2) A in 5 respectively. The crystal data for 2, 3 and 5
are given in Table 1.
Space-filling models for 2, 3 and 5 (see Supplementary data)
suggest that the environment around the zirconium center is
sterically congested with the zirconium center embedded deeply
within the aryloxy periphery. Such a situation becomes extremely
important for compounds having substituents at 2-position of the
phenyl ring. With halogens at this position, there is an overall
increase in acidity of the zirconium centers which facilitates HO-iPr
coordination. This is inhibited completely if sterically bulky
substituent like Br is present at the 2-position of the phenyl ring.
As a consequence, HO-iPr does not coordinate in 4. The sterical
constraints do not play a major role with substituents at 3- and
4-positions of the phenyl ring. Hence, it is expected that
compounds with electron-withdrawing substituents at these
positions shall facilitate HO-iPr coordination. This rationalization is
indeed observed for 5, 6, 9 and 10. With electron-withdrawing
substituents at all the positions of the phenyl ring, the Lewis acidity
of the zirconium centers is increased to a maximum extent. This
forces the coordination of HO-iPr, even though it is not sterically
favored. These observations are seen in 11 and 12.
substituents on the aryloxy moiety results in comparatively faster
transesterification reactions leading to the formation of low to
modest Mn polymers. It is well known that for polymerizations
with slow initiation and rapid transesterification, the observed Mn
is lower than the calculated estimate [67e70]. The observed Mn is
found to be much higher than the expected with most of our
catalysts. The absence of an ideal ligand periphery is realized here
as the observed Mn are far greater than the theoretical. This
suggests that the ligand play a role in controlling the extent of
propagation. Our system is simple and deserved special mention
since we have achieved good Mn with such a simple initiator
system. The benzyloxy derivatives 14 and 15 were not as effective in
carrying out such polymerizations as compared to the aryloxy
compounds. In comparison to the polymerization initiated by
analogous titanium compounds [59], there is a decrease in Mn
while control in MWDs was noticed. The variation of Mn with [CL]o/
[Zr]o ratio using 6, 12, 13 and 15 for CL polymerizations were
studied next. The plots are linear and there is a continuous increase
in Mn with [CL]o/[Zr]o ratio. Similar studies were carried out with VL
(see Supplementary data).
3.3. Polymerization activity and characteristics
To understand the ligandemetal relationship for construction of
new catalysts for ring-opening polymerization, we understood that
the electronic environment around the metal plays a decisive role
in the polymerization mechanism concerning lactones [58,59]. The
activated monomer mechanism can be made productive in
synthesizing polymers with good Mn and MWDs. The ligand has
a vital role in controlling the extent of chain propagation [1]. The
idea of synthesizing a plethora of different aryloxy compounds with
different central metal and different substituents on the aryoxy
periphery is basically to construct a catalyst library. We surmised
that a variation of electronic environment around the metal must
Table 3
Results of VL polymerization using various aryloxy compounds and benzyloxy
derivatives of zirconium in 200:1 ratio at 80 ꢁC.
Entry Compound Timea (min) % Yieldb 103 Mnc (g/mol) MWD (Mw/Mn)
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
55
38
40
79
85
96
95
97
95
95
95
95
96
95
95
95
96
97
98
98
14.09
23.52
71.67
138.93
147.78
156.39
14.89
1.31
1.32
1.28
1.23
1.34
1.34
1.32
1.30
1.32
1.33
1.34
1.31
1.29
1.16
1.11
20
120
130
100
30
75
68
125
58
62
40.27
have
a consequence in polymerization results. Alternatively,
9
122.86
133.97
113.36
66.48
89.88
12.79
a variation in the metal must produce impact on the polymeriza-
tion. Our initial success has been in this direction through the
synthesis of different aryloxy and benzyloxy compounds of tita-
nium [59]. Polymerization under solvent-free (bulk) conditions at
ambient temperatures yielded low molecular weight oligomers.
These polymerizations were conducted at 80 ꢁC under bulk
conditions. The results are depicted in Tables 2 and 3 respectively.
A detailed inspection of the results depicted in Tables 2 and 3
reveals that with compounds with electron-withdrawing
10
11
12
13
14
15
10.54
a
Time of polymerization measured by quenching the polymerization reaction
when all monomer was found consumed.
b
Isolated yield.
c
Measured by GPC at 27 ꢁC in THF relative to polystyrene standards.

578
R.R. Gowda et al. / Journal of Organometallic Chemistry 696 (2011) 572e580
Y = -0.00736 + 0.04387 X
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Y = -0.00105 + 0.01911 X
Y = -0.00145 + 0.03012 X
Y = -0.01478 + 0.03652 X
Zr(O-2,4,6-F
C
H (HO-iPr) (13)
)
3
6 2 4
Zr(O-CH -4-OMeC H
) (15)
2
6
4 4
Zr(OC Cl
) (HO-iPr) (12)
6
5 4
Zr(O-3-CF
C
H (HO-iPr) (6)
)
3
6 4 4
0
20
40
60
80
100
120
140
160
Time (min)
Fig. 4. Semilogarithmic plots of CL conversion in time initiated by 6, 12, 13 and 15:
[CL]o/[Zr]o ¼ 1000 at 80 ꢁC.
Fig. 6. 1H NMR spectrum of the crude product obtained from a reaction between CL
and 2 in 20:1 ratio.
3.4. Determination of polymerization kinetics
The kinetic studies for the polymerization of CL and VL with 6,
12, 13 and 15 as catalysts were performed. The kinetic studies for
the polymerization of CL in ratio [CL]o/[Zr]o ¼ 1000 were carried out
at 80 ꢁC (see Supplementary data). The results are depicted in Fig. 4.
The plots indicate that there is complete absence of induction
period. There is a first-order dependence of the rate of polymeri-
zation on monomer concentration. Similar observations were
noticed for VL polymerization (see Supplementary data).
3.5. View of polymerization mechanism
Suitable spectroscopic methods like MALDI-TOF and 1H NMR are
viable tools which can be used in providing insight into the poly-
merization mechanism. Our compounds 1e15 may be character-
ized into two broad groups, one with alcohol coordination and one
without. We have chosen 2 and 8 as prototype representatives for
compounds with alcohol coordination and one without. Low
molecular weight oligomers were synthesized by reacting neat CL
with 2 or 8 in 20:1 stoichiometric ratio at 80 ꢁC. The reaction
mixture was quenched with heptane. The residues remaining after
removal of volatiles were subjected to MALDI-TOF and 1H NMR
spectroscopy.
Analysis of the oligomer synthesized from 2 using MALDI-TOF
and 1H NMR spectroscopy suggests that the product iPrO[CO
(CH2)5O]nH is formed. This obviously implies that HO-iPr is the
active initiator in this polymerization and that it proceeds by the
activated monomer mechanism [53e59]. These results are depicted
in Figs. 5 and 6 respectively.
The ln[CL]o/[CL]t versus time plot as depicted in Fig. 4 show linear
variation. The values of the apparent rate constant (kapp) for CL poly-
merizations initiated by 6, 12, 13 and 15 were determined from the
slope of these plots and were found to be 4.38 ꢀ 10ꢂ2 minꢂ1
,
3.65 ꢀ10ꢂ2 minꢂ1, 1.91 ꢀ10ꢂ2 minꢂ1 and 3.01 ꢀ10ꢂ2 minꢂ1 respec-
tively. Similarly for VL polymerization the trends in the plots were
found to be similar and apparent rate constant (kapp) for VL polymer-
izations initiateꢂd1 by 6, 12, 13 and 15 were determined to be
3.39 ꢀ 10ꢂ2 min
,
3.19 ꢀ 10ꢂ2 minꢂ1
,
1.79 ꢀ 10ꢂ2 minꢂ1
and
2.58 ꢀ 10ꢂ2 minꢂ1 (see Supplementary data). These rates were slower
than the rates of polymerization initiated by the corresponding tita-
nium analogues [59].
iPrO[CO(CH ) O] H•Na+
2
5
n
2000
1500
1000
500
0
n
mass
7
8
881.519
995.583
9
1109.688
1223.766
1337.835
1451.894
1565.945
1679.987
1794.032
1908.082
2022.090
2136.095
2250.054
2364.304
2479.125
2593.186
2708.001
2821.151
2935.296
3050.557
3164.933
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
1000
1250
1500
1750
2000
2250
2500
2750
3000
3250
m/z
Fig. 5. MALDI-TOF of the crude product obtained from a reaction between CL and 2 in 20:1 ratio.

R.R. Gowda et al. / Journal of Organometallic Chemistry 696 (2011) 572e580
579
polymerizations in the presence of toluene as a suitable solvent
(Table 4). The control in such polymerizations in the presence of
toluene was found to be better. Interestingly, these results are
comparable or much improved than the literature reports [34,45,52].
Zr(OAr)
(iPrOH)
4
iPr
O
H
iPr
OZr(OAr)4
O
O
O
O
Zr(OAr)4
O
OH
4. Conclusion
The alcoholysis route [62,63] was followed to synthesize several
zirconium aryloxy compounds and benzyloxy derivatives. These
compounds were isolated in extremely good yields and purity and
were thoroughly characterized by different spectroscopic methods
and their purity was assured by the correct elemental analysis
values. ESI-MS studies suggest that these compounds exist
predominantly as dimers in solution. These compounds exhibit
fluxional behavior, the detailed study of which has been carried out
using variable-temperature 1H NMR spectroscopy. The presence of
coordinatively unsaturated zirconium provides a relatively easy
associative route for either internal ligand scrambling or external
ligand exchange. These compounds exhibit greater propensity for
such behavior as compared to the titanium compounds [59].
Analysis of the spectra indicate complicated solution dynamics.
The unambiguous characterization was performed using single
crystal X-ray diffraction studies on a few aryloxy compounds. These
compounds are centrosymmetric dimers in the solid state. The
M-M (M ¼ Ti, Zr) distance increases from titanium [59] to zirco-
nium. This explains for a stronger propensity of fluxional behavior
exhibited by the zirconium compounds.
These compounds are potent initiators for the bulk ring-opening
polymerization of lactones (CL and VL). The titanium derivatives
yield high Mn polymers but rather poor MWDs [59]. In the presence
of HO-iPr as initiator, the resulting Mn for the polymers are signif-
icantly enhanced with control in the MWDs. The zirconium
compounds yield better polymerization results in terms of control
of MWDs. In general the benzyloxy, derivatives were found to much
lower in activity as compared to the aryloxy compounds. Kinetic
studies reveal that the polymerization rates are fastest for the
titanium compounds [59] followed by the zirconium analogues. For
compounds which are HO-iPr adducts or in the presence of HO-iPr,
the polymerization proceeds by the activated monomer mecha-
nism. Hence, mild Lewis acids can be used effectively for the
polymerization of cyclic ester monomers. This study has ques-
tioned the popular notion of using initiators based on elaborate
ligands for lactone polymerization.
O
O
H
OZr(OAr)4
O
iPr
O
OH
O
iPr
O
O
O
iPr
O
H
n
Scheme 2. Activated monomer mechanism.
Meticulous analysis of these spectra also suggests that the
alternative product namely 2-FC6H4O[CO(CH2)5O]nH resulting from
the competing coordinationeinsertion mechanism of polymeriza-
tion is completely absent. This unambiguously suggests that the
system has a natural preference to adopt the activated monomer
mechanism where ever a choice is possible with the competing
coordinationeinsertion mechanism. On the basis of these results
the prevailing mechanism is represented in Scheme 2.
Activation of the acyl-oxygen bond leads to ring-opening and
the resulting product initiates the subsequent cycle and the prop-
agation continues in this manner. Compound 8 leads to the
formation of 4-tBuC6H4O[CO(CH2)5O]nH as the major product
which is totally as a result of the well anticipated coordination-
insertion mechanism. These conclusions were drawn upon the
analysis of MALDI-TOF and 1H NMR spectroscopy (see
Supplementary data). Again, addition of 1 equiv. of HO-iPr along
with 8 in the reaction mixture, results in the formation of iPrO[CO
(CH2)5O]nH exclusively as understood from MALDI-TOF and 1H
NMR spectroscopy. This proves our theory that given a choice, the
system has a natural tendency to adopt the activated monomer
mechanism [53e59].
Acknowledgements
3.6. Comparison of polymerization with the literature
The authors thank the Department of Science and Technology,
New Delhi for a research grant. The services of the NMR instrument
at the Department of Chemistry, Indian Institute of Technology
Madras, purchased under the IRPHA scheme are gratefully
acknowledged.
For a direct comparison of the polymerization results obtained
with our compounds and the reported ones, we resorted to perform
Table 4
Results of CL polymerization with representative compounds in 200:1 ratio at 80 ꢁC
in toluene.
Appendix A. Supplementary data
Entry Compound Timea (min) % Yieldb 103 Mnc (g/mol) MWD (Mw/Mn)
CCDC 769553, CCDC 769554 and CCDC 769555 contain the
crystallographic data for 2, 3 and 5. These can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
1
2
3
4
5
6
8
12
13
15
38
170
115
195
104
98
99
98
99
98
42.59
31.59
14.86
34.20
9.36
1.24
1.19
1.21
1.18
1.22
Supplementary data consists of spectroscopic characterization
spectra of all compounds described along with space-filling models,
kinetic data and characterization of low molecular weight oligo-
mers. Supplementary data associated with this article can be found
in the online version, at doi:10.1016/j.jorganchem.2010.09.026.
a
Time of polymerization measured by quenching the polymerization reaction
when all monomer was found consumed.
b
Isolated yield.
c
Measured by GPC at 27 ꢁC in THF relative to polystyrene standards with
MarkeHouwink corrections for Mn.

580
R.R. Gowda et al. / Journal of Organometallic Chemistry 696 (2011) 572e580
[34] A.J. Chmura, M.G. Davidson, M.D. Jones, M.D. Lunn, M.F. Mahon, A.F. Johnson,
References
P. Khunkamchoo, S.L. Roberts, S.S.F. Wong, Macromolecules 39 (2006)
7250e7257.
[35] A.J. Chmura, M.G. Davidson, M.D. Jones, M.D. Lunn, M.F. Mahon, Dalton Trans.
(2006) 887e889.
[36] K.-C. Hsieh, W.-Y. Lee, L.-F. Hsueh, H.M. Lee, J.-H. Huang, Eur. J. Inorg. Chem.
(2006) 2306e2312.
[37] S.K. Russell, C.L. Gamble, K.J. Gibbins, K.C.S. Juhl, W.S. Mitchell, A.J. Tumas,
G.E. Hofmeister, Macromolecules 38 (2005) 10336e10340.
[38] Y. Kim, J.G. Verkade, Macromol. Rapid Commun. 23 (2002) 917e921.
[39] Y. Kim, J.G. Verkade, Macromol. Symp. 224 (2005) 105e117.
[40] Y. Kim, G.K. Jnaneshwara, J.G. Verkade, Inorg. Chem. 42 (2003) 1437e1447.
[41] Y. Kim, J.G. Verkade, Organometallics 21 (2002) 2395e2399.
[42] Y. Takashima, Y. Nakayama, T. Hirao, H. Yesuda, A. Harada, J. Organomet.
Chem. 689 (2004) 612e619.
[43] Y. Takashima, Y. Nakayama, K. Watanabe, T. Itono, N. Ueyama, A. Nakamura,
H. Yesuda, A. Harada, Macromolecules 35 (2002) 7538e7544.
[44] D. Takeuchi, T. Nakamura, T. Aida, Macromolecules 33 (2000) 725e729.
[45] H.R. Kricheldorf, M. Berl, N. Scharnagl, Macromolecules 21 (1988) 286e293.
[46] J. Cayuela, V. Bounor-Legaré, P. Cassagnau, A. Michel, Macromolecules
39 (2006) 1338e1346.
[47] C.J. Chuck, M.G. Davidson, M.D. Jones, G. Kociok-Köhn, M.D. Lunn, S. Wu,
Inorg. Chem. 45 (2006) 6595e6597.
[48] Y. Ning, Y. Zhang, A.R. Delgado, E.Y.-X. Chen, Organometallics 27 (2008)
5632e5640.
[49] A.L. Zelikoff, J. Kopilov, I. Goldberg, G.W. Coates, M. Kol, Chem. Commun.
(2009) 6804e6806.
[50] A.D. Schwarz, A.L. Thompson, P. Mountford, Inorg. Chem. 48 (2009) 10442e10454.
[51] R.H. Howard, C. Alonso-Moreno, L.M. Broomfield, D.L. Hughes, J.A. Wright,
M. Bochmann, Dalton Trans. (2009) 8667e8682.
[1] O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 104 (2004)
6147e6176.
[2] G.W. Coates, J. Chem. Soc., Dalton Trans. (2002) 467e475.
[3] Z. Hou, Y. Wakatsuki, Coord. Chem. Rev. 231 (2002) 1e22.
[4] R.E. Drumright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841e1846.
[5] T. Aida, S. Inoue, Acc. Chem. Res. 29 (1996) 39e48.
[6] E. Chiellini, R. Solaro, Adv. Mater. 8 (1996) 305e313.
[7] Y. Ikada, H. Tsuji, Macromol. Rapid Commun. 21 (2000) 117e132.
[8] M. Stefan, Angew. Chem., Int. Ed. 43 (2004) 1078e1085.
[9] C.K. Williams, M.A. Hillmyer, Polym. Rev. 48 (2008) 1e10.
[10] E.S. Place, J.H. George, C.K. Williams, M.M. Stevens, Chem. Soc. Rev. 38 (2009)
1139e1151.
[11] K.E. Uhrich, S.M. Cannizzaro, R.S. Langer, K.M. Shakesheff, Chem. Rev. 99
(1999) 3181e3198.
[12] O. Coulembier, P. Dege’e, J.L. Hedrick, P. Dubois, Prog. Polym. Sci. 31 (2006)
723e747.
[13] H.R. Kricheldorf, Chemosphere 43 (2000) 49e54.
[14] J. Wu, T.L. Yu, C.T. Chen, C.C. Lin, Coord. Chem. Rev. 250 (2006) 602e626.
[15] F.G. Hutchinson, B.J.A. Furr, in: A.H. Fawcett (Ed.), High Value Polymers, The
Royal Society of Chemistry, Science Park, Cambridge, 1991.
[16] C.M. Silvernail, L.J. Yao, L.M.R. Hill, M.A. Hillmyer, W.B. Tolman, Inorg. Chem.
46 (2007) 6565e6574.
[17] H. Du, X. Pang, H. Yu, X. Zhuang, X. Chen, D. Cui, X. Wang, X. Jing, Macro-
molecules 40 (2007) 1904e1913.
[18] L.E. Breyfogle, C.K. Williams, V.G. Young Jr., M.A. Hillmyer, W.B. Tolman,
Dalton Trans. (2006) 928e936.
[19] C.K.A. Gregson, I.J. Blackmore, V.C. Gibson, N.J. Long, E.L. Marshall, A.J.P. White,
Dalton Trans. (2006) 3134e3140.
[20] H.R. Kricheldorf, H. Hachmann-Thiessen, G. Schwarz, Macromolecules
37 (2004) 6340e6345.
[21] C.K. Williams, L.E. Breyfogle, S.K. Choi, W. Nam, V.G. Young Jr., M.A. Hillmyer,
W.B. Tolman, J. Am. Chem. Soc. 125 (2003) 11350e11359.
[22] L.M. Alcazar-Roman, B.J. O’Keefe, M.A. Hillmyer, W.B. Tolman, Dalton Trans.
(2003) 3082e3087.
[23] B.J. O’Keefe, L.E. Breyfogle, M.A. Hillmyer, W.B. Tolman, J. Am. Chem. Soc.
124 (2002) 4384e4393.
[24] M.S. Lindblad, Y. Liu, A.-C. Albertsson, E. Ranucci, S. Karlsson, Adv. Polym. Sci.
157 (2002) 139e161.
[25] B.J. O’Keffe, M.A. Hillmyer, W.B. Tolman, J. Chem. Soc., Dalton. Trans. (2001)
2215e2224.
[26] B.M. Chamberlain, M. Cheng, D.R. Moore, T. Ovitt, E.B. Lobkovsky, G.W. Coates,
J. Am. Chem. Soc. 123 (2001) 3229e3238.
[27] W.M. Stevels, M.J.K. Ankone, P.J. Dijkstra, J. Feijen, Macromolecules 29 (1996)
8296e8303.
[28] A.J. Chmura, D.M. Cousins, M.G. Davidson, M.D. Jones, M.D. Lunn, M.F. Mahon,
Dalton Trans. (2008) 1437e1443.
[29] A.J. Chmura, M.G. Davidson, C.J. Frankis, M.D. Jones, M.D. Lunn, Chem. Com-
mun. (2008) 1293e1295.
[30] F. Gornshtein, M. Kapon, M. Botoshansky, M. Eisen, Organometallics 26 (2007)
497e507.
[31] R.C.J. Atkinson, K. Gerry, V.C. Gibson, N.J. Long, E.L. Marshall, L.J. West,
Organometallics 26 (2007) 316e320.
[32] D. Patel, S.T. Liddle, S.A. Mungur, M. Rodden, A.J. Blake, P.L. Arnold, Chem.
Commun. (2006) 1124e1126.
[33] S. Gendler, S. Segal, I. Goldberg, Z. Goldschmidt, M. Kol, Inorg. Chem. 45 (2006)
4783e4790.
[52] R.F. Munha, L.G. Alves, N. Maulide, M.T. Duarte, I.E. Marko, M.D. Fryzuk,
A.M. Martins, Inorg. Chem. Commun. 11 (2008) 1174e1176.
ꢁ
[53] M. Basko, P. Kubisa, J. Polym. Sci., Part A: Polym. Chem. 44 (2006) 7071e7081.
[54] D. Bourissou, B. Martin-Vaca, A. Dumitrescu, M. Graullier, F. Lacombe,
Macromolecules 38 (2005) 9993e9998.
[55] M.S. Kim, K.S. Seo, G. Khang, H.B. Lee, Macromol. Rapid Commun. 26 (2005)
643e648.
[56] P. Kubisa, J. Polym. Sci., Part A: Polym. Chem. 41 (2003) 457e468.
[57] S. Penczek, J. Polym. Sci., Part A: Polym. Chem. 38 (2000) 1919e1933.
[58] R.R. Gowda, D. Chakraborty, J. Mol. Catal. A: Chem. 301 (2009) 84e92.
[59] R.R. Gowda,D.Chakraborty,V.Ramkumar, Eur.J.Inorg.Chem. (2009)2981e2993.
[60] I. Barakat, P. Dubois, R. Jérôme, P. Teyssié, J. Polym. Sci., Part A: Polym. Chem.
31 (1993) 505e514.
[61] G.M. Sheldrick, SHELXL97, Program for Crystal Structure Refinement, Göt-
tingen, Germany, 1997.
[62] S. Mishra, A. Singh, Transit. Met. Chem. 30 (2005) 163e169.
[63] D.C. Bradley, R.C. Mehrotra, D.P. Gaur, Metal Alkoxides. Academic Press,
London, 1978.
[64] C.E. Holloway, J. Chem. Soc., Dalton Trans. (1976) 1050e1054.
[65] D.C. Bradley, Prog. Inorg. Chem. 2 (1960) 303e361.
[66] D.C. Bradley, C.E. Holloway, J. Chem. Soc., A (1968) 1316e1319.
[67] N. Spassky, V. Simic, M.S. Montaudo, L.G. Hubert-Pfalzgraf, Macromol. Chem.
Phys. 201 (2000) 2432e2440.
[68] J. Baran, A. Duda, A. Kowalski, R. Szymanski, S. Penczek, Macromol. Symp.
123 (1997) 93e101.
[69] J. Baran, A. Duda, A. Kowalski, R. Szymanski, S. Penczek, Macromol. Rapid
Commun. 18 (1997) 325e333.
[70] W.J. Evans, M.A. Ansari, J.W. Ziller, Inorg. Chem. 38 (1999) 1160e1164.