Ni(II) and Cu(II) complexes of phenoxy-ketimine ligands: Synthesis, structures and their utility in bulk ring-opening polymerization (ROP) of l-lactide

Article abstract of DOI:10.1016/j.poly.2007.04.039

Synthetic, structural and catalysis studies of Ni(II) and Cu(II) complexes of a series of phenoxy-ketimine ligands with controlled variations of sterics, namely 2-[1-(2,6-diethylphenylimino)ethyl]phenol (1a), 2-[1-(2,6-dimethylphenylimino)ethyl]phenol (1b) and 2-[1-(2-methylphenylimino)ethyl]phenol (1c), are reported. Specifically, the ligands 1a, 1b and 1c were synthesized by the TiCl₄ mediated condensation reactions of the respective anilines with o-hydroxyacetophenone in 21-23% yield. The nickel complexes, {2-[1-(2,6-diethylphenylimino)ethyl]phenoxy}₂Ni(II) (2a) and {2-[1-(2,6-dimethylphenylimino)ethyl]phenoxy}₂Ni(II) (2b), were synthesized by the reaction of the respective ligands 1a and 1b with Ni(OAc)₂ · 4H₂O in the presence of NEt₃ as a base in 71-75% yield. The copper complexes, {2-[1-(2,6-diethylphenylimino)ethyl]phenoxy}₂Cu(II) (3a), {2-[1-(2,6-dimethylphenylimino)ethyl]phenoxy}₂Cu(II) (3b) and {2-[1-(2-methylphenylimino)ethyl]phenoxy}₂Cu(II) (3c) were synthesized analogously by the reactions of the ligands 1a, 1b and 1c with Cu(OAc)₂ · H₂O in 70-87% yield. The molecular structures of the nickel and copper complexes 2a, 2b, 3a, 3b and 3c have been determined by X-ray diffraction studies. Structural comparisons revealed that the nickel centers in 2a and 2b are in square planar geometries while the geometry around the copper varied from being square planar in 3a and 3c to distorted square planar in 3b. The catalysis studies revealed that while the copper complexes 3a, 3b and 3c efficiently catalyze ring-opening polymerization (ROP) of l-lactide at elevated temperatures under solvent-free melt conditions, producing polylactide polymers of moderate molecular weights with narrow molecular weight distributions, the nickel counterparts 2a and 2b failed to yield the polylactide polymer.

Full text of DOI:10.1016/j.poly.2007.04.039

Polyhedron 26 (2007) 4033–4044
www.elsevier.com/locate/poly
Ni(II) and Cu(II) complexes of phenoxy-ketimine ligands:
Synthesis, structures and their utility in bulk
ring-opening polymerization (ROP) of L-lactide
a
b
d
c
b
Alex John , Vimal Katiyar , Keliang Pang , Mobin M. Shaikh , Hemant Nanavati ,
a,
*
Prasenjit Ghosh
a
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
b
c
National Single Crystal X-ray Diffraction Facility, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
d
Department of Chemistry, Columbia University, 3000 Broadway, New York, NY 10027, USA
Received 7 December 2006; accepted 13 April 2007
Available online 10 May 2007
Abstract
Synthetic, structural and catalysis studies of Ni(II) and Cu(II) complexes of a series of phenoxy-ketimine ligands with controlled
variations of sterics, namely 2-[1-(2,6-diethylphenylimino)ethyl]phenol (1a), 2-[1-(2,6-dimethylphenylimino)ethyl]phenol (1b) and 2-[1-
(2-methylphenylimino)ethyl]phenol (1c), are reported. Specifically, the ligands 1a, 1b and 1c were synthesized by the TiCl4 mediated
condensation reactions of the respective anilines with o-hydroxyacetophenone in 21–23% yield. The nickel complexes, {2-[1-(2,6-dieth-
ylphenylimino)ethyl]phenoxy}2Ni(II) (2a) and {2-[1-(2,6-dimethylphenylimino)ethyl]phenoxy}2Ni(II) (2b), were synthesized by the reac-
tion of the respective ligands 1a and 1b with Ni(OAc)2 Æ 4H2O in the presence of NEt3 as a base in 71–75% yield. The copper complexes,
{2-[1-(2,6-diethylphenylimino)ethyl]phenoxy}2Cu(II) (3a), {2-[1-(2,6-dimethylphenylimino)ethyl]phenoxy}2Cu(II) (3b) and {2-[1-(2-
methylphenylimino)ethyl]phenoxy}2Cu(II) (3c) were synthesized analogously by the reactions of the ligands 1a, 1b and 1c with
Cu(OAc)2 Æ H2O in 70–87% yield. The molecular structures of the nickel and copper complexes 2a, 2b, 3a, 3b and 3c have been deter-
mined by X-ray diffraction studies. Structural comparisons revealed that the nickel centers in 2a and 2b are in square planar geometries
while the geometry around the copper varied from being square planar in 3a and 3c to distorted square planar in 3b. The catalysis studies
revealed that while the copper complexes 3a, 3b and 3c efficiently catalyze ring-opening polymerization (ROP) of L-lactide at elevated
temperatures under solvent-free melt conditions, producing polylactide polymers of moderate molecular weights with narrow molecular
weight distributions, the nickel counterparts 2a and 2b failed to yield the polylactide polymer.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Ring-opening polymerization; Phenoxy-ketimine; Copper complexes; Nickel complexes; L-Lactide
1. Introduction
chemistry [2], chemical catalysis [3–7], and materials related
applications [8,9]. Equally impressive are their wide-rang-
ing utility in chemical catalysis, namely in olefin polymeri-
zation [10–13], epoxidation reactions [14,15], Suzuki cross-
coupling reactions [16], oxidation reactions [17], to name
but a few. Of the various phenoxy-imine catalysts that
exist, notable are the Fujita’s fluorinated phenoxy-imine
catalyst for olefin polymerization [18] and the Jacobsen’s
[19] Salen based catalyst for epoxidation reactions, both
Phenoxy-imines are a versatile class of ligands that dis-
play a truly impressive range of diverse applications span-
ning from bioinorganic chemistry [1] to coordination
*
Corresponding author. Tel.: +91 22 2576 7178; fax: +91 22 2572 3480.
E-mail address: pghosh@chem.iitb.ac.in (P. Ghosh).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.04.039

4034
A. John et al. / Polyhedron 26 (2007) 4033–4044
of which have contributed significantly to the development
of the respective fields. The successes of phenoxy-imines are
presumably due to the availability of scope for suitable tun-
ing of the sterics and the electronics of the ligand ancillary
and also due to their easy synthetic accessibility, as the
phenoxy-imines are often prepared by Schiff base conden-
sation reactions. Interestingly enough, despite extensive
utility of phenoxy-imines in many important chemical
transformations, their application in ring-opening poly-
merization (ROP) of L-lactide largely remains unexplored
[20].
O
O
3a or 3b or 3c
O
O
O
O
n
O
O
L-lactide
polylactide
Equation 1. Polymerization of L-lactide by 3a, 3b and 3c.
of moderate molecular weights with narrow molecular
weight distributions (Eq. (1)), the nickel complexes (2a
and 2b) failed to yield a polylactide polymer.
Our interest in the ring-opening polymerization (ROP)
of L-lactide arises mainly because of its eco-friendly nature
as the polylactide polymer (PLA) is biodegradable and also
because the lactide monomer can be generated from renew-
able resources like the corn fermentation process or from
agricultural starch wastes [21,22]. The PLAs are biocom-
patible and exhibit good mechanical properties and, hence,
are long known for their use in medical and pharmaceutical
related applications [23,24]. One of our core programs is
centered around designing novel initiators for the bulk
ring-opening polymerization of L-lactide, and in this regard
we have recently reported Ag(I) [25] and Au(I) [26] com-
plexes of N-heterocyclic carbenes (NHC) as initiators for
the bulk polymerization of L-lactide under solvent-free melt
conditions. It is noteworthy that the PLAs can be synthe-
sized by both solution polymerization [27] and bulk poly-
merization [28], however, the solution polymerization
suffers from certain disadvantages like being susceptible
to the impurity levels and to various unwanted reactions
namely racemization, transesterifications, etc. The bulk
polymerization, which does not suffer from such limita-
tions, is thus, often preferred for the large-scale production
of PLAs [29].
2. Results and discussion
A series of monoanionic phenoxy-ketimine ligands,
namely 2-[1-(2,6-diethylphenylimino)ethyl]phenol (1a), 2-
[1-(2,6-dimethylphenylimino)ethyl]phenol (1b) and 2-[1-(2-
methylphenylimino)ethyl]phenol (1c) were synthesized by
a modification of a TiCl4 catalyzed procedure reported in
the literature for the condensation of a-haloketones with
amines [34]. Specifically, the condensation reaction of
o-hydroxyacetophenone with the respective anilines in
presence of a catalytic amount of TiCl4 gave the phenoxy-
With regard to designing new initiators for our ROP
program we observed that the phenoxy-imine ligands,
despite their proven successes in chemical catalysis, remain
to be tried for ring-opening polymerization (ROP) of L-lac-
tide [20] and so we became interested in exploring the scope
of phenoxy-imine based transition metal complexes as cat-
alysts for bulk polymerization of L-lactide. Furthermore,
phenoxy-imine metal complexes, bearing a metal–alkoxide
bond, are natural candidates for the catalysis, as the well
accepted catalysts for the ring-opening polymerization
(ROP) of L-lactide are often the metal–alkoxide complexes
[21,30–33] (see Fig. 1).
Fig. 1. Ni(II) and Cu(II) complexes of a series of phenoxy-ketimine
ligands with controlled variation of sterics.
Here in this contribution, we report the synthesis, struc-
tural characterizations and catalysis studies of a series of
nickel (2a and 2b) and copper (3a, 3b and 3c) complexes
of phenoxy-ketimine ligands with varying steric demands,
namely 2-[1-(2,6-diethylphenylimino)ethyl]phenol (1a),
2-[1-(2,6-dimethylphenylimino)ethyl]phenol (1b) and 2-[1-
(2-methylphenylimino)ethyl]phenol (1c). Furthermore, we
disclose that while the copper complexes (3a, 3b and 3c)
were found to be efficient initiators for ring-opening poly-
merization of L-lactide at elevated temperatures under sol-
vent-free melt conditions, producing polylactide polymers
Scheme 1. Synthesis of phenoxy-ketimine ligands 1a, 1b and 1c.

A. John et al. / Polyhedron 26 (2007) 4033–4044
4035
ketimine ligands, 1a, 1b and 1c [35], in ca. 21–23% yield
(Scheme 1). It is interesting to note that the ketimine
ligands are often synthesized by two different methods,
namely: (i) Schiff base condensation of ketones with amines
in the presence of a catalytic amount of an acid like
HCO2H [36] or p-toluenesulfonic acid [16] and (ii) Fri-
edel-Crafts acylation of the imidoyl chloride with the
ortho-unsubstituted phenol [37]. The compounds 1a, 1b
The paramagnetic copper complexes 3a, 3b and 3c were
characterized by IR, electronic spectroscopy, ESR, elemen-
tal analysis and X-ray diffraction studies. The mC@N stretch-
ing frequencies in the copper complexes, 3a (1599 cmÀ1), 3b
(1601 cmÀ1) and 3c (1599 cmÀ1), are comparable to the
closely related complex, {2-[1-(2-methoxyphenylimino)-
methyl]phenoxy}2Cu(II) [40], (1600 cmÀ1) and, as was
observed in the case of the nickel complexes 2a and 2b,
the mC@N stretching frequencies are all red shifted by ca.
10–13 cmÀ1 relative to the ligands, 1a (1609 cmÀ1), 1b
(1614 cmÀ1) and 1c (1611 cmÀ1).
1
and 1c have been characterized by H NMR, 13C NMR,
IR, UV–Vis and mass spectrometry. The 1H NMR spectra
of 1a, 1b and 1c showed the ketimine methyl (MeC@N) res-
onances as singlets at 2.09–2.17 ppm, while the phenolic
–OH protons appeared as broad peaks at 14.7–14.8 ppm.
The substantially high downfield shifts of the phenolic
–OH protons in 1a, 1b and 1c are attributed to an intramo-
lecular hydrogen bonding interaction, [phenolic –OHÁ Á Á
N@C (imine N)]. A similar downfield shift of the phenolic
–OH proton (16.8 ppm) due to an intramolecular hydrogen
bonding interaction has been reported for the Schiff base
ligand 1-(2-hydroxy-3,5-dichlorophenyl)-2,5-diaza-6-meth-
ylnona-1,6-diene-8-one [38]. Furthermore, the ketimine
moieties (MeC@N) appeared at 171.1–171.9 ppm
(MeC@N) in the 13C NMR spectra and at 1609–
1614 cmÀ1 (mC@N str) in the infrared spectra. The electronic
spectra of the phenoxy-ketimine ligands 1a, 1b and 1c in
CHCl3 exhibited two distinct absorption bands in the UV
region which have been identified as high energy p–p* and
low energy n–p* transitions, based on the assignment made
for a series of related phenoxy-imine ligands, namely 2-[1-
(R)methyl]phenol (where R = –CH3, –C6H11 and –C5H4N)
[39]. Thehighenergybandsduetothep–p* transitionsshowed
absorptions at 256 nm for 1a (e = 17263 L molÀ1 cmÀ1),
256 nm for 1b (e = 17933 L molÀ1 cmÀ1) and 257 nm for 1c
(e = 21873 L molÀ1 cmÀ1), while the low energy n–p* transi-
tion appeared at 323 nm for 1a (e = 5945 L molÀ1 cmÀ1),
322 nm for 1b (e = 5919 L molÀ1 cmÀ1) and 324 nm for 1c
(e = 8776 L molÀ1 cmÀ1). Electrospray mass analysis gave
peaks at m/z = 268 (1a), 240 (1b) and 226 (1c), corresponding
to the respective M+1 ions in 100% abundances.
The electronic spectra of the nickel complexes, 2a and
2b, in CHCl3 showed absorption bands due to d–d transi-
tions at 601 nm (e = 118 L molÀ1 cmÀ1) and 592 nm
(e = 102 L molÀ1 cmÀ1), respectively, which are compara-
ble to those reported for other square planar Ni(II) com-
plexes. For example, square planar Ni(II) complexes of a
series of [N2O2] chelating salen type ligands with varying
spacer lengths showed similar low intensity absorbances
due to d–d transitions at 532–550 nm (e = 240–
260 L molÀ1 cmÀ1) [41]. The Cu complexes, 3a, 3b and 3c,
on the other hand showed the d–d transitions at 696 nm
(e = 89.4 L molÀ1 cmÀ1), 670 nm (e = 130 L molÀ1 cmÀ1
)
and 630 nm (e = 125 L molÀ1 cmÀ1), respectively, which
are also similar to other square planar copper complexes,
like bis-[(N2-(2-pyridylmethyl)-3-thenyl-carboxamido)cop-
per(II)]perchlorate [42] (675 nm). Similar comparisons of
the d–d transitions in 3a, 3b and 3c can also be drawn with
another set of Schiff base copper complexes, {3-methoxy
-2-[1-(2-pyridylmethylimino)methyl]phenoxy}Cu(II)X [43]
(X = Cl, Br), based on a phenolato-pyridine ligand, which
showed weak broad bands in the 600–700 nm region which
were attributed to d–d transitions. The differences in the
absorption bands amongst the copper complexes 3a, 3b and
3c is notable, showing marked shifts of ca. 30 nm with the
decrease in steric bulk of the 2,6-di-substituted phenyl ring
on going from 3a to 3b to 3c. In contrast, the Ni complexes
did not show any appreciable difference in the d–d transitions
on going from 2a to 2b.
The nickel (2a and 2b) and copper (3a, 3b and 3c) com-
plexes were synthesized by the reaction of the ligands with
M(OAc)2 Æ xH2O (M = Ni, Cu) in the presence of NEt3 in
methanol in 71–75% and 70–87% yields, respectively (see
Scheme 2). Both of the nickel complexes, 2a and 2b, were
diamagnetic in nature, consistent with a square planar
geometry of the Ni(II) d8 configuration. The ketimine
methyl moieties (MeC@N) in 2a and 2b appeared as sing-
The EPR spectra of the copper complexes 3a, 3b and 3c,
recorded in the frozen state in CH2Cl2 at 77 K (Table 1),
showed a commonly observed axial pattern consisting of
two well resolved peaks and two overlapping peaks of
low intensities in the low field region (gi) due to hyperfine
1
lets at 1.87–1.94 ppm (MeC@N) in the H NMR spectra
and at 168.6–168.9 ppm (MeC@N) and 19.7–20.6 ppm
(MeC@N) in the 13C{1H} NMR spectra. Comparison of
infrared spectra for the mC@N stretching frequencies for 2a
(1574 cmÀÀ11) and 2b (1571 cmÀ1) showed red shifts of ca.
35–43 cm
relative to the respective parent ligands 1a
(1609 cmÀ1) and 1b (1614 cmÀ1). The mC@N stretching fre-
quency values observed for 2a and 2b are similar to that
of a related complex, {2-[1-(2,6-diisopropylphenylimino)-
methyl]phenoxy}2Ni(II) [36] (1602 cmÀ1).
Scheme 2. Synthesis of the metal complexes.

4036
A. John et al. / Polyhedron 26 (2007) 4033–4044
Table 1
EPR parameters of 3a, 3b and 3c in CH2Cl2 at 77 K
Complex
Ai
A^a
gi
g^
3a
3b
3c
a
143
140
140
NR
NR
NR
2.2517
2.2517
2.2447
2.0399
2.0463
2.0465
NR, not resolved.
splitting by the copper nucleus (I = 3/2) along with a high
intensity peak in the high field region (g^). In this context,
it is worth mentioning that in square planar copper com-
plexes, an unpaired electron when residing in the dx2Ày2
orbital (2B1g ground state) exhibits gi > g^ whereas the
same when present in the other axial dz2 orbital (2A1g
ground state) displays g^ > gi [44–46]. Quite notably, in
all the three copper complexes 3a, 3b and 3c, the observed
gi > g^ is consistent with a square planar geometry at the
metal center with the unpaired electron placed in the
dx2Ày2 orbital [44–46]. Furthermore, the observed g and A
values of 3a, 3b and 3c are comparable to those of other
reported copper complexes. For example, the average g
and A values recorded at 77 K for the square planar com-
plex {2-[1-(N,N-diethylethyleneimino)methyl]phenol)}2Cu-
(ClO4)2 [47] are gi (2.225), g^ (2.070) and Ai (179.7) and
that of {4-[1-(4-methoxyphenylimino)-pent-2-ene-2-oxy]}2-
Cu(II) [45] are gi (2.328), g^ (2.067) and Ai (140).
Fig. 3. ORTEP of 2b. Thermal motion ellipsoids are drawn at the 50%
probability level for the non-hydrogen atoms. Selected bond lengths (A)
and angles (ꢁ): Ni–N 1.9199(11), Ni–N 1.9199(11), Ni–O 1.8248(10), Ni–O
1.8248(10), N–Ni–N 180.00(4), O–Ni–O 180.00(5), N–Ni–O 90.93(5), N–
Ni–O 89.07(5).
˚
The molecular structures of the nickel (2a and 2b) (Figs.
2 and 3) and copper complexes (3a, 3b and 3c) (Figs. 4–6)
have been determined by X-ray diffraction studies, which
revealed the formation of metal complexes having 2:1
ligand to metal stoichiometries. The metal ions in these
complexes reside at the center of a trans-N2O2 core which
is perfectly square planar in case of the nickel (2a and
2b) and two of the copper complexes (3a and 3c), whereas
it is distorted square planar in the other copper complex 3b.
Fig. 4. ORTEP of 3a. Thermal motion ellipsoids are drawn at the 50%
˚
probability level for the non-hydrogen atoms. Selected bond lengths (A)
and angles (ꢁ): Cu2–N2 2.0161(14), Cu2–N2 2.0161(14), Cu2–O2
1.8631(11), Cu2–O2 1.8630(11), N2–Cu2–N2 180.00(12), O2–Cu2–O2
180.00(7), O2–Cu2–N2 90.50(5), O2–Cu2–N2 89.50(5).
Consistent with a square planar geometry, the Ni(II)
centers in 2a and 2b are surrounded by two equidistant
˚
˚
Ni–N [2a: 1.9085(14) A (Fig. 2); 2b: 1.9199(11) A (Fig. 3)]
˚
and two equidistant Ni–O [2a: 1.8258(11) A (Fig. 2); 2b:
1.8248(10) A (Fig. 3)] bond distances. The Ni–N and the
˚
Ni–O bond distances in 2a and 2b are, however, somewhat
longer than the sum of the individual covalent radii of the
Fig. 2. ORTEP of 2a. Thermal motion ellipsoids are drawn at the 50%
˚
probability level for the non-hydrogen atoms. Selected bond lengths (A)
˚
˚
respective atoms (Ni–N = 1.854 A, Ni–O = 1.814 A) [48].
Quite expectedly, the complementary \O–Ni–N and \N–
Ni–O angles at the nickel centers in 2a [91.10(6),
and angles (ꢁ): Ni1–N1 1.9085(14), Ni1–N1 1.9085(14), Ni1–O1
1.8258(11), Ni1–O1 1.8258(11), N1–Ni1–N1 180.00(13), O1–Ni1–O1
180.00(9), N1–Ni1–O1 91.10(6), N1–Ni1–O1 88.90(6).

A. John et al. / Polyhedron 26 (2007) 4033–4044
4037
˚
[49], [Ni–O = 1.861(2), 1.852(2) A; Ni–N = 1.905(2),
˚
1.893(2) A] are in good agreement with those in the com-
plexes 2a and 2b.
The copper complexes 3a, 3b and 3c are isostructural
with their nickel counterparts, 2a and 2b, though certain
variations were observed. The di-ethyl derivative 3a and
the mono-methyl derivative 3c exhibit square planar geom-
etries like the nickel complexes 2a and 2b, whilst a slightly
distorted square planar geometry was observed in case of
the di-methyl derivative 3b. Thus, unlike the square planar
nickel (2a and 2b) and copper (3a and 3c) complexes, in
which two sets of equidistant M–O and M–N (M = Ni,
Cu) bond lengths were observed, the Cu–N and Cu–O
bond lengths of the trans-N2O2 core in 3b are all marginally
unequal, a consequence of the slight distortion of the
square planar geometry around the copper center. For
example, in 3b the Cu1–N11 and Cu1–N12 bond lengths
Fig. 5. ORTEP of 3b. Thermal motion ellipsoids are drawn at the 50%
˚
probability level for the non-hydrogen atoms. Selected bond lengths (A)
˚
˚
are 2.007(5) A and 2.006(4) A, while the Cu1–O11 and
and angles (ꢁ): Cu1–N11 2.007(5), Cu1–N12 2.006(4), Cu1–O11 1.858(4),
Cu1–O12 1.856(4), N11–Cu1–N12 161.58(19), O11–Cu1–O12 159.8(2),
O12–Cu1–N12 89.36(17), O11–Cu1–N12 91.96(18).
˚
˚
Cu1–O12 distances are 1.858(4) A and 1.856(4) A, respec-
tively (Fig. 5). Owing to the presence of the distortion,
the four \N–Cu–O bond angles at the copper center in
3b are also marginally different from the ideal value of
90ꢁ [\N12–Cu1–O12 = 89.36(17)ꢁ and \N12–Cu1–O11 =
91.96(18)ꢁ], while the diagonal angles [\N11–Cu1–N12 =
161.58(19)ꢁ and \O11–Cu1–O12 = 159.8(2)ꢁ] at the copper
in 3b are significantly deviated from the expected value of
180ꢁ. The extent of the distortion of the square planar
structure in 3b becomes apparent upon comparison with
the perfectly square planar nickel analog 2b (Fig. 7b). It
is interesting to note that although significant square pla-
nar distortion (as reflected in the angles, h and /, in
Fig. 7b) is observed in the di-methyl derivative 3b (h =
21.4ꢁ, / = 17.7ꢁ) relative to the square planar nickel coun-
terpart 2b, no such distortion was visible in case of the
di-ethyl copper complex 3a (h = 1.0ꢁ, / = 1.8ꢁ) relative to
the nickel analog 2a (Fig. 7a). The mono-methyl copper
complex 3c also showed no distortion and closely resem-
bled the square planar copper complex 3a.
Fig. 6. ORTEP of 3c. Thermal motion ellipsoids are drawn at the 50%
˚
probability level for the non-hydrogen atoms. Selected bond lengths (A)
The Cu–O and Cu–N bond distances in 3a, 3b and 3c
are comparable to those observed in other analogous cop-
per phenoxy-ketimine complexes, {2-[1-(2-phenylethyl-
and angles (ꢁ): Cu1–N1 2.001(2), Cu1–N1 2.001(2), Cu1–O1 1.872(2),
Cu1–O1 1.872(2), N1–Cu–N1 180.00(16), O1–Cu1–O1 180.00(12), O1–
Cu1–N1 89.16(9), N1–Cu1–O1 90.84(9).
˚
imino)ethyl]phenoxy}2Cu(II) [50] [Cu–O = 1.865(2) A,
˚
Cu–N = 1.977(2) A] and {2-[1-(2-methylpropylimino)ethyl]-
˚
phenoxy}2Cu(II) [51] [Cu–O = 1.891(2) A, Cu–N =
˚
2.003(2) A], as well as in the related copper phenoxy-aldim-
88.90(6)ꢁ] and 2b [89.07(5), 90.93(5)ꢁ] are equal to ca. 90ꢁ.
The trans disposition of the phenyl rings of the 2,6-disubsti-
tuted-phenyl moieties in both complexes 2a and 2b is a con-
sequence of the reduction of steric strain.
inecomplexes,{5-bromo-2-[1-(3-methylphenylimino)methyl]-
˚
phenoxy}2Cu(II) [52] [Cu–O = 1.884(8) A, Cu–N =
˚
2.005(7) A] and {2-[1-(phenylimino)methyl]phenoxy}2-
˚
˚
Cu(II) [53] [Cu–O = 1.878(3) A, Cu–N = 1.993(4) A].
The comparison of the molecular structures of 2a and 2b
with the other reported nickel phenoxy-ketimine and phen-
oxy-aldimine complexes is important. For example, the
Ni–O and Ni–N bond distances in the phenoxy-ketimine
3. Polymerization studies
complex,
{2-[1-(2,6-diisopropylphenylimino)ethyl]phen-
Quite significantly, the copper complexes 3a, 3b and 3c
efficiently catalyzed the ring-opening polymerization
(ROP) of L-lactide at elevated temperatures under sol-
vent-free melt conditions [28]. In stark contrast to the
˚
oxy}2Ni(II) [36] [Ni–O = 1.8081(10) A; Ni–N = 1.9262
˚
(11) A], and in the phenoxy-aldimine nickel complex, {3-
t-butyl-5-methyl-2-[1-(ethylimino)methyl]phenoxy}2Ni(II)

4038
A. John et al. / Polyhedron 26 (2007) 4033–4044
Fig. 7. (a). Overlaid structures of Ni (2a) (black) and Cu (3a) (grey) complexes. (b). Overlaid structures of Ni (2b) (black) and Cu (3b) (grey) complexes.
Table 2
X-ray crystallographic data for 2a, 2b, 3a, 3b and 3c
Compound
2a
2b
3a
3b
3c
Lattice
Formula
triclinic
C36H40NiN2O2
591.41
monoclinic
C32H32NiN2O2
535.31
monoclinic
C36H40CuN2O2
596.24
orthorhombic
C32H32CuN2O2
540.14
P212121
12.0618(8)
12.0625(7)
37.054(3)
90.00
90.00
90.00
5391.2(6)
8
monoclinic
C30H28CuN2O2
512.08
Formula weight
Space group
ꢀ
P1
P21/n
P21/a
P21/n
˚
a (A)
7.5871(9)
9.878(3)
11.720(3)
64.95(3)
74.013(15)
87.642(15)
762.1(3)
1
9.3513(6)
7.8591(5)
18.2914(12)
90.00
93.3590(10)
90.00
8.8749(18)
20.702(5)
16.6067(11)
90.00
97.281(10)
90.00
6.010 (3)
14.886 (3)
13.368 (3)
90.00
91.90 (3)
90.00
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
1341.98(15)
2
3026.6(10)
4
1195.2 (6)
2
Z
Temperature (K)
150(2)
0.71073
1.289
0.671
32.1953
2674
243(2)
0.71073
1.325
0.754
28.32
293(2)
0.71073
1.309
0.756
32.1670
10077
243(2)
0.71073
1.331
0.842
27.893
150 (2)
0.71073
1.423
0.945
29.9988
2095
˚
Radiation (k, A)
q(calcd.) (g cmÀ3
)
l(Mo Ka) (mmÀ1
h max (ꢁ)
)
Number of data
2452
9875
Number of parameters
R1
190
172
0.0266
0.0737
1.058
379
0.0396
0.0989
0.924
681
0.0651
0.1231
1.123
162
0.0388
0.0971
0.930
0.0273
0.0619
1.099
wR2
Goodness-of-fit
copper complexes 3a, 3b and 3c, the nickel complexes 2a
and 2b, however, failed to yield the polylactide polymer.
1.05–1.46 (3a), 1.08–1.43 (3b) and 1.10–1.58 (3c). An inter-
esting correlation between the initiator structure and the
polymer molecular weight that emerges is the structurally
similar square planar complexes 3a and 3c, which differ sig-
nificantly from the distorted square planar complex 3b,
also exhibit a similar resemblance in their polymer molecu-
lar weights [3a (Mw = 18.5 · 103 entry 1, Table 3); 3c
(Mw = 15.9 · 103 entry 1, Table 5)] that are significantly
greater than that observed in the case of 3b
(Mw = 10.4 · 103 entry 1, Table 4) obtained at the 50:1
[M]:[I] ratio. Consistent with the observed molecular
weight distributions, no correlation of the polymer molec-
ular weight with time was seen in the time dependency
studies (3a: entries 1 and 9–12, Table 3, 3b: entries 1 and
9–12, Table 4, 3c: entries 1 and 9–12, Table 5). The temper-
Specifically,
a
typical polymerization experiment
involved heating L-lactide with any of the copper initiators
3a or 3b or 3c, taken in the desired ratio, in a sealed vessel
under vacuum at designated temperatures for stipulated
periods of time. Under these conditions the reaction mix-
ture would form a monomer melt in which the polymeriza-
tion would occur. A study of the variation of monomer to
initiator ratio [M]:[I] ratio (M = monomer, I = initiator) at
160 ꢁC showed that the maximum molecular weights for 3a
(Mw = 18.5 · 103 entry 1, Table 3), 3b (Mw = 10.4 · 103
entry 1, Table 4) and 3c (Mw = 15.9 · 103 entry 1, Table
5) were obtained at ratios of 50:1 in each case. The
observed polydispersity indices (PDIs) were in the range

A. John et al. / Polyhedron 26 (2007) 4033–4044
4039
18.5 · 103, 26.3 · 103) and 3b (Table 4: Mw: 5.3 · 103,
8.2 · 103, 10.4 · 103, 12.0 · 103) whereas a slight decrease
in molecular weight was observed in the case of 3c at higher
temperatures (Table 5: Mw: 8.6 · 103, 11.1 · 103, 15.9 · 103
and 13.8 · 103) at 120, 140, 160 and 180 ꢁC, respectively.
Worth comparing are the copper initiators 3a, 3b and 3c
with other known catalysts for the ring-opening polymeri-
zation of L-lactide under solvent-free melt conditions, par-
ticularly the copper ones. Interestingly enough, even
though very many metals like Mg [54,27a,31c], Ti [55],
Zn [56,20a,33b,30a,30b,30d,32a], Al [57,20b,20d], Sn
[58,30c], etc. have been reported for lactide polymerization,
we are aware of only a single report of lactide polymeriza-
tion by a Cu catalyst, K[LCu(II)]ÆnH2O (H3L = L-aspartic
acid–salicylidene Schiff base) [20c]. The viscosity average
molecular weight (Mv = 4.86 · 103) obtained at a 200:1
[M]:[I] ratio after 24 h at 130 ꢁC for K[LCu(II)] Æ nH2O
(H3L = L-aspartic acid–salicylidene Schiff base [20c] is
slightly smaller than that observed for 3a (Mw:
18.5 · 103), 3b (Mw: 10.4 · 103), 3c (Mw: 15.9 · 103)
obtained at a 50:1 [M]:[I] ratio after 4 h at 160 ꢁC. Similar
comparisons of 3a, 3b and 3c can also be extended to
other metal catalysts. For example, the polymer molecular
weights and the polydispersity indices observed at a 50:1
[M]:[I] ratio for 3a (Mw: 18.5 · 103, PDI = 1.28), 3b
(Mw: 10.4 · 103, PDI = 1.32), 3c (Mw: 15.9 · 103,
PDI = 1.36), obtained after 4 h at 160 ꢁC, are also compa-
rable to several other melt polymerization catalysts, such
as [3-mesityl-1-picolylimidazol-2-ylidene]Zn(Et)I [30a]
(Mw: 29.4 · 103, PDI = 2.45, 100:1 [M]:[I] ratio, 140 ꢁC,
Table 3
Melt polymerization of L-lactide by 3a
Entry L-
Lactide/
Temperature Time Mw
Mw/ Conversion
Mn (%)
(ꢁC)
(h)
(3a) ratio
1
2
3
4
5
6
7
8
9
50
100
150
200
250
50
50
50
50
50
160
160
160
160
160
120
140
180
160
160
160
160
4
4
4
4
4
4
4
4
1
2
3
6
18.5 · 103 1.28 66
6.2 · 103 1.08 51
3.2 · 103 1.09 54
2.2 · 103 1.09 58
2.3 · 103 1.08 52
9.9 · 103 1.07 27
16.5 · 103 1.27 62
26.3 · 103 1.46 57
7.6 · 103 1.05 71
9.8 · 103 1.18 67
10
11
12
50
50
11.3 · 103 1.1
14.6 · 103 1.3
80
86
Table 4
Melt polymerization of L-lactide by 3b
Entry L- Temperature Time Mw
Lactide/
Mw/ Conversion
Mn (%)
(ꢁC)
(h)
(3b) ratio
1
2
3
4
5
6
7
8
9
50
100
150
200
250
50
50
50
50
50
160
160
160
160
160
120
140
180
160
160
160
160
4
4
4
4
4
4
4
4
1
2
3
6
10.4 · 103 1.32 70
8.2 · 103 1.17 72
4.7 · 103 1.3
74
4.5 · 103 1.43 76
4.5 · 103 1.08 33
5.3 · 103 1.22 39
8.2 · 103 1.21 54
12.0 · 103 1.23 64
7.1 · 103 1.19 53
8.5 · 103 1.21 59
10
11
12
50
50
8.5 · 103 1.2
58
11.2 · 103 1.24 72
5 min),
[30a] (Mw: 15.3 · 103, PDI = 2.51, 50:1 [M]:[I] ratio,
140 ꢁC, 5 min), [(1-i-propyl-3-{N-phenylacetamido}
imidazol-2-ylidene)2Ag]+ClÀ 12.2 · 103,
[25] (Mw:
{[3-mesityl-1-picolylimidazol-2-ylidene]2ZnI}I
Table 5
Melt polymerization of L-lactide by 3c
Entry L- Temperature Time Mw
Lactide/
PDI = 1.40, 100:1 [M]:[I] ratio, 160 ꢁC, 4 h), [1-(2-
hydroxy-cyclohexyl)-3-(N-t-butylacetamido)imidazol-2-yli-
dene]AgCl [26] (Mw: 6.27 · 103, PDI = 1.23, 100:1 [M]:[I]
ratio, 160 ꢁC, 4 h) and [1-(2-hydroxy-cyclohexyl)-3-(N-t-
Mw/ Conversion
Mn (%)
(ꢁC)
(h)
(3c) ratio
1
2
3
4
5
6
7
8
9
50
100
150
200
250
50
50
50
50
50
160
160
160
160
160
120
140
180
160
160
160
160
4
4
4
4
4
4
4
4
1
2
3
6
15.9 · 103 1.36 55
14.1 · 103 1.14 25
6.4 · 103 1.58 33
6.5 · 103 1.29 28
butylacetamido)imidazol-2-ylidene]AuCl
[26]
(Mw:
6.31 · 103, PDI = 1.17, 100:1 [M]:[I] ratio, 160 ꢁC, 4 h).
Furthermore, comparable viscosity-average molecular
weights (Mv) were also reported for a series of late-transi-
tion metal based initiators of the type K[LM(II)] Æ nH2O
(H3L = L-aspartic acid–salicylidene Schiff base) [20c] where
M = Co (5.15 · 103), Ni (11.1 · 103) and Zn (5.23 · 103),
obtained at a 200:1 [M]:[I] ratio after 24 h at 130 ꢁC. How-
ever, several other initiators have also been reported, which
produced higher molecular weight polymers, e.g., anhy-
drous Fe(OAc)2 [59] (Mw: 65.9 · 103, PDI = 5.25, 0.24
weight % of catalyst, 170 ꢁC, 10 h), iron isobutyrate [59]
(Mw: 148 · 103, PDI = 1.60, 0.12 weight % of catalyst,
190 ꢁC, 2 h) and [L]TiO(2,6-i-Pr2C6H3) [28c] (H3L = trieth-
anolamine) (Mw: 66.1 · 103, PDI = 1.97, 300:1 [M]:[I]
ratio, 130 ꢁC, 24 h). Thus, the detailed comparison with
other known catalysts revealed that the copper initiators
3a, 3b and 3c are moderately active for lactide
polymerization.
2.9 · 103 1.1
20
8.6 · 103 1.18 54
11.1 · 103 1.11 60
13.8 · 103 1.21 61.5
8.0 · 103 1.16 45.2
8.1 · 103 1.16 48
13.4 · 103 1.31 48
10.2 · 103 1.24 72
10
11
12
50
50
ature dependence study carried out at the 50:1 [M]:[I] ratio
in the range (100–180 ꢁC)1 showed that the molecular
weight of the polymer increased with temperature for the
initiators 3a (Table 3: Mw: 9.9 · 103, 16.5 · 103,
1
The bulk polymerization temperatures were chosen higher than the
melting point of the lactide monomer (96 ꢁC).

4040
A. John et al. / Polyhedron 26 (2007) 4033–4044
4. Experimental
4.1. General procedures
(o-C6H3(CH2CH3)2), 128.9 (p-C6H4(OH)), 126.1 (m-
C6H3(CH2CH3)2), 124.8 (p-C6H3(CH2CH3)2), 119.1 (ipso-
C6H4(OH)), 118.3 (m-C6H4(OH)), 118.0 (m-(C6H4-
(OH)), 24.7 (C6H3(CH2CH3)2), 17.3 ((C@N)CH3), 14.0
(C6H3(CH2CH3)2). LRMS (ES): m/z 268 (M+H)+ 100%.
HRMS (ES): calcd. for [M+H]+ 268.1701, found m/z
268.1696. IR data cmÀ1 KBr pellet: 1609 (s) (mC@N). kmax
(e) CHCl3: 256 nm (e = 17263 L molÀ1 cmÀ1), 323 nm
(e = 5945 L molÀ1 cmÀ1).
All manipulations were carried out using a combination
of a glovebox and standard Schlenk techniques. Solvents
were purified and degassed by standard procedures. 2,6-
Dimethylaniline and 2,6-diethylaniline were purchased
from Sigma–Aldrich, Germany and o-hydroxyacetophe-
none was purchased from SD-fine Chemicals (India), and
1
they were used without any further purification. H and
4.2.2. Synthesis of 2-[1-(2,6-dimethylphenylimino)ethyl]-
phenol (1b)
13C{1H} NMR spectra were recorded on a Varian
400 MHz NMR spectrometer. 1H NMR peaks are labelled
as singlet (s), doublet (d), triplet (t) and multiplet (m).
Infrared spectra were recorded on a Perkin Elmer Spec-
trum One FT-IR spectrometer. The electronic spectra were
recorded in chloroform by using a Perkin–Elmer Lamda 35
UV-Visible spectrophotometer and JASCO V-570 UV/Vis/
NIR spectrophotometer. Mass spectrometry measurements
were done on a Micromass Q-Tof spectrometer. The EPR
measurements were made with a Varian model 109C E-line
X-band spectrometer fitted with a quartz Dewar for mea-
surements at 77 K. The spectra were calibrated by using
tetracyanoethylene (tcne) (g = 2.0037). The polymer
molecular weights were determined using a Waters GPC
(Waters 2414 RI Detector) with PL-gel, 5l Mixed-D
(2 · 300 mm) Column, with polystyrene standards in chlo-
roform, covering molecular weight ranging from l40 to
4 · 105.
o-Hydroxyacetophenone (8.51 g, 62.5 mmol) and 2,6-
dimethylaniline (12.4 g, 102 mmol) were taken in CH2Cl2
(ca. 75 mL) maintained at 0 ꢁC and a solution of TiCl4
(2 mL, 18.2 mmol) in CH2Cl2 (ca. 10 mL) was added.
The reaction mixture was stirred in an ice bath for 1 h, after
which it was allowed to warm up to room temperature over
a period of 3 h. An aqueous NaOH solution (20 mL, 2 M)
was added to the reaction mixture and washed thoroughly.
The organic layer was separated, dried over anhydrous
MgSO4 and the solvent removed under vacuum to get the
crude product as a yellow oil. The crude compound was
purified by column chromatography (4% EtOAc–hexane
mixture) followed by the removal of excess 2,6-dimethylan-
iline by vacuum distillation (180 ꢁC, 0.2 mbar) to obtain
the product as a yellow solid (3.09 g, 21%). 1H NMR
(CDCl3, 400 MHz, 25 ꢁC) d 14.7 (s, 1H, OH), 7.57 (d,
3
3
1H, JHH = 8 Hz, o-C6H4(OH)), 7.32 (t, 1H, JHH = 8 Hz,
3
p-C6H4(OH)), 7.03 (d, 2H, JHH = 8 Hz, m-C6H3(CH3)2),
4.2. Synthesis of the ligands
6.98–6.92 (m, 2H, p-C6H3(CH3)2and m-C6H4(OH) over-
lapped), 6.83 (t, 1H, JHH = 8 Hz, m-C6H4(OH)), 2.09 (s,
3
4.2.1. Synthesis of 2-[1-(2,6-diethylphenylimino)ethyl]-
phe- nol (1a)
3H, CH3(C@N)), 1.99 (s, 6H, C6H3(CH3)2). 13C NMR
(CDCl3, 100 MHz, 25 ꢁC) d 171.7 (C@N), 162.2 (C6H4-
(OH), 145.0 (ipso-C6H3(CH3)2), 132.9 (o-C6H4(OH)),
128.8 (p-C6H4(OH)), 127.9 (m-C6H3(CH3)2), 127.5 (p-
C6H3(CH3)2), 124.3 (ipso-C6H4(OH), 119.1 (m-C6H4-
(OH)), 118.1 (m-C6H4(OH)), 117.9 (o-C6H3(CH3)2), 18.1
(CH3(C@N)), 16.8 (C6H3(CH3)2). LRMS (ES): m/z 240
(M+H)+ 100%. HRMS (ES): calcd. for [M+H]+
240.1388, found m/z 240.1380. IR data cmÀ1 KBr pellet:
1614 (s) (mC@N). kmax (e) CHCl3: 256 nm (e = 17933 L
molÀ1 cmÀ1), 322 nm (e = 5919 L molÀ1 cmÀ1).
o-Hydroxyacetophenone (4.31 g, 31.7 mmol) and 2,6-
diethylaniline (14.3 g, 96.0 mmol) were taken in CH2Cl2
(ca. 75 mL) maintained at 0 ꢁC and a solution of TiCl4
(3 mL, 27.3 mmol) in CH2Cl2 (ca. 10 mL) was added.
The reaction mixture was stirred in an ice bath for 1 h, after
which it was allowed to warm up to room temperature over
a period of 3 h. An aqueous NaOH solution (20 mL, 2 M)
was added to the reaction mixture and washed thoroughly.
The organic layer was separated, dried over anhydrous
MgSO4 and the solvent removed under vacuum to get the
crude product as a yellow oil. The crude compound was
purified by the removal of excess 2,6-diethylaniline and o-
hydroxyacetophenone by vacuum distillation (180 ꢁC, 0.2
mbar) to obtain the product as a yellow solid (1.83 g,
4.2.3. Synthesis of 2-[1-(2-methylphenylimino)ethyl]phenol
(1c)
o-Hydroxyacetophenone (7.25 g, 53.2 mmol) and 2-
methylaniline (8.62 g, 80.5 mmol) were taken in CH2Cl2
(ca. 60 mL) maintained at 0 ꢁC and a solution of TiCl4
(2 mL, 18.2 mmol) in CH2Cl2 (ca. 10 mL) was added.
The reaction mixture was stirred in an ice bath for 2 h,
after which it was allowed to warm up to room tempera-
ture over a period of 3 h. An aqueous NaOH solution
(40 mL, 2 M) was added to the reaction mixture and
washed thoroughly. The organic layer was separated, dried
over anhydrous MgSO4 and the solvent removed under
vacuum to get the crude product as a yellow oil. The crude
1
22%). H NMR (CDCl3, 400 MHz, 25 ꢁC) d 14.8 (s, 1H,
3
OH), 7.64 (d, 1H, JHH = 8 Hz, o-C6H4(OH)), 7.40 (t,
3
1H, JHH = 8 Hz, p-C6H4(OH)), 7.16–7.11 (m, 3H, m/p-
C6H3(CH2CH3)2), 7.04 (d, 1H, 3JHH = 8 Hz, m-
3
C6H4(OH)), 6.91 (t, 1H, JHH = 8 Hz, m-C6H4(OH)),
2.48–2.32 (m, 4H, CH2CH3), 2.17 (s, 3H, CH3(C@N)),
3
1.13 (t, 6H, JHH = 8 Hz, CH2CH3). 13C NMR (CDCl3,
100 MHz, 25 ꢁC) d 171.9 (C@N), 162.4 (C6H4(OH), 143.8
(ipso-C6H3(CH2CH3)2), 133.0 (o-C6H4(OH)), 133.1

A. John et al. / Polyhedron 26 (2007) 4033–4044
4041
compound was purified by the removal of excess 2-methy-
laniline and o-hydroxyacetophenone by vacuum distilla-
tion (180 ꢁC, 0.2 mbar) to obtain the product as a yellow
4.3.2. Synthesis of {2-[1-(2,6-dimethylphenylimino)ethyl]-
phenoxy}2Ni(II) (2b)
Ni(OAc)2 Æ 4H2O (0.543 g, 2.19 mmol) and 2-[1-
1
oil (2.72 g, 23%). H NMR (CDCl3, 400 MHz, 25 ꢁC) d
(2,6-dimethylphenylimino)ethyl]phenol
(1b)
(0.508 g,
3
14.7 (s, 1H, OH), 7.63 (d, 1H, JHH = 7 Hz, o-
2.12 mmol) were taken in methanol (ca. 30 mL) and stirred
at room temperature until a clear solution was obtained.
To the solution was added NEt3 (0.324 g, 3.20 mmol) with
constant stirring. The reaction mixture was stirred at room
temperature for another 6 h when the product precipitated
out as a green solid and was isolated by filtration and air
dried (0.459 g, 75%). Crystals suitable for X-ray structure
determination were obtained by slow evaporation from a
3
C6H4(OH)), 7.37 (t, 1H, JHH = 9 Hz, p-C6H4(OH)), 7.25
3
(d, 1H, JHH = 8 Hz, m-C6H4(CH3)), 7.21 (t, 1H,
3
3JHH = 8 Hz, m-C6H4(CH3)), 7.10 (t, 1H, JHH = 8 Hz,
3
m-C6H4(OH)), 7.02 (d, 1H, JHH = 8 Hz, m-C6H4(OH)),
3
6.89 (t, 1H, JHH = 8 Hz, p-C6H4(CH3)), 6.76 (d, 1H, o-
C6H4(CH3)), 2.26 (s, 3H, C6H4(CH3)), 2.15 (s, 3H,
CH3(C@N)). 13C NMR (CDCl3, 100 MHz, 25 ꢁC) d
171.1 (C@N), 162.0 (C6H4(OH), 145.8 (ipso-C6H4(CH3)),
133.0 (o-C6H4(OH)), 130.5 (o-C6H4(CH3)), 128.8 (p-
C6H4(OH)), 126.4 (m-C6H4(CH3)), 124.8 (p-C6H4(CH3)),
120.5 (o-C6H4(CH3)), 119.5 (ipso-C6H4(OH), 118.1 (m-
C6H4(OH)), 118.0 (m-(C6H4(OH)), 17.9 ((C@N)CH3),
17.0 (C6H4(CH3)). LRMS (ES): m/z 226 (M+H)+ 100%.
HRMS (ES): calcd. for [M+H]+ 226.1232, found m/z
226.1234. IR data cmÀ1 KBr pellet: 1611 (s) (mC@N). kmax
(e) CHCl3: 257 nm (e = 21873 L molÀ1 cmÀ1), 324 nm
(e = 8776 L molÀ1 cmÀ1).
1
saturated benzene solution. H NMR (CDCl3, 400 MHz,
3
25 ꢁC) d 7.26 (d, 1H, JHH = 8 Hz, o-C6H4(OH)), 7.19
(br, 1H, p-C6H4(OH)), 7.02 (br, 2H, m-C6H3(CH3)2), 6.83
3
(t, 1H, JHH = 8 Hz, p-C6H3(CH3)2), 6.35 (t, 1H,
3
3JHH = 8 Hz, m-C6H4(OH)), 5.66 (d, 1H, JHH = 8 Hz,
m-C6H4(OH)), 2.53 (s, 6H, C6H3(CH3)2), 1.87 (s, 3H,
CH3(C@N)). 13C NMR (CDCl3, 100 MHz, 25 ꢁC) d
168.6
(C@N),
163.7
(C6H4(OH),
146.4
(ipso-
C6H3(CH3)2)), 132.3 (o-C6H4(OH)), 130.9 (p-C6H4(OH)),
129.0 (p-C6H3(CH3)2), 127.6 (m-C6H3(CH3)2), 124.9 (ipso-
C6H4(OH), 122.0 (m-C6H4(OH)), 118.2 (m-(C6H4(OH)),
114.1 (o-C6H3(CH3)2), 19.7 (CH3(C@N)), 19.2 (C6H3-
(CH3)2). IR data cmÀ1 KBr pellet: 1571 (s) (mC@N). kmax
(e) CHCl3: 592 nm (e = 102 L molÀ1 cmÀ1). Anal. Calc.
for C32H32NiO2N2: C, 71.80; H, 6.03; N, 5.23. Found: C,
71.14; H, 6.21; N, 4.88%.
4.3. Synthesis of the Ni complexes
4.3.1. Synthesis of {2-[1-(2,6-diethylphenylimino)ethyl]-
phenoxy}2Ni(II) (2a)
Ni(OAc)2 Æ 4H2O (0.449 g, 1.81 mmol) and 2-[1-(2,6-
diethylphenylimino)ethyl]phenol (1a) (0.478 g, 1.79 mmol)
were taken in methanol (ca. 30 mL) and stirred at room
temperature until a clear solution was obtained. To the
solution was added NEt3 (0.274 g, 2.70 mmol) with con-
stant stirring. The reaction mixture was refluxed for 12 h
and then allowed to cool to room temperature, when the
product precipitated out as a green solid and was isolated
by filtration and air dried (0.493 g, 71%). Crystals suitable
for X-ray structure determination were obtained by slow
evaporation from a saturated methanol solution. 1H
4.4. Synthesis of the Cu complexes
4.4.1. Synthesis of {2-[1-(2,6-diethylphenylimino)ethyl]-
phenoxy}2Cu(II) (3a)
Cu(OAc)2 Æ H2O (0.266 g, 1.33 mmol) and 2-[1-(2,6-
diethylphenylimino)ethyl]phenol (1a) (0.709 g, 2.62 mmol)
were taken in methanol (ca. 40 mL) and stirred at room
temperature until a clear solution was obtained. To the
solution was added NEt3 (0.298 g, 2.94 mmol) with con-
stant stirring. The reaction mixture was stirred at reflux
for 24 h and then allowed to cool to room temperature
when the product was obtained as dark brown crystals
which were isolated by filtration and vacuum dried
(0.542 g, 70%). Crystals suitable for X-ray structure deter-
mination were obtained by slow evaporation from a satu-
rated benzene solution. IR data cmÀ1 KBr pellet: 1599 (s)
(mC@N). kmax (e) CHCl3: 696 nm (e = 89.4 L molÀ1 cmÀ1).
Anal. Calc. for C36H40CuO2N2: C, 72.52; H, 6.76; N,
4.70. Found: C, 72.07; H, 7.06; N, 4.39%.
NMR (CDCl3, 400 MHz, 25 ꢁC)
d 7.31 (d, 1 H,
3JHH = 8 Hz, o-C6H4(OH)), 7.21 (t, 1H, p-C6H4(OH)),
3
7.11 (d, 2H, JHH = 8 Hz, m-C6H3(CH2CH3) ), 6.84 (t,
2
3
1H, JHH = 8 Hz, p-C6H3(CH2CH3) 2), 6.38 (t, 1H,
3JHH = 8 Hz, m-C6H4(OH)), 5.50 (d, 1H, JHH = 8 Hz,
3
m-C6H4(OH)), 3.29–3.19 (m, 2H, CH2CH3), 2.90–2.80
(m, 2H, CH2CH3), 1.94 (s, 3H, CH3(C@N)), 1.45 (t, 6H,
3JHH = 8 Hz, CH2CH3). 13C NMR (CDCl3, 100 MHz,
25 ꢁC)
d
168.9 (C@N), 163.6 (C6H4(OH), 145.8
(ipso-C6H3(CH2CH3)2)), 136.1 (o-C6H4(OH)), 132.2
(p-C6H4(OH)), 129.0 (p-C6H3(CH2CH3)2), 125.0 (m-
C6H3(CH2CH3)2 and ipso-(C6H4(OH)), 123.2 (m-C6H4-
(OH), 122.1 (o-C6H3(CH2CH3)2), 113.9 (m-C6H4(OH)),
24.6 (C6H3(CH2CH3)2), 20.6 (CH3(C@N)), 13.7
(C6H3(CH2CH3)2). IR data cmÀ1 KBr pellet: 1574 (s)
(mC@N). kmax (e) CHCl3: 601 nm (e = 118 L molÀ1 cmÀ1).
Anal. Calc. for C36H40NiO2N2: C, 73.11; H, 6.82; N,
4.74. Found: C, 72.78; H, 7.06; N, 3.87%.
4.4.2. Synthesis of {2-[1-(2,6-dimethylphenylimino)-
ethyl]phenoxy}2Cu(II) (3b)
Cu(OAc)2 Æ H2O (0.418 g, 2.14 mmol) and 2-[1-(2,6-di-
methylphenylimino)ethyl]phenol (1b) (0.512 g, 2.14 mmol)
were taken in methanol (ca. 30 mL) and stirred at room
temperature until a clear solution was obtained. To the
solution was added NEt3 (0.282 g, 2.78 mmol) with con-
stant stirring. The reaction mixture was stirred at room

4042
A. John et al. / Polyhedron 26 (2007) 4033–4044
temperature for another 6 h when the product precipitated
out as a dark brown solid and was isolated by filtration and
vacuum dried (0.505 g, 87%). Crystals suitable for X-ray
structure determination were obtained by slow evaporation
from a saturated benzene solution. IR data cmÀ1 KBr pel-
let: 1601 (s) (mC@N). kmax (e) CHCl3: 670 nm (e = 130 L
molÀ1 cmÀ1). Anal. Calc. for C32H32CuO2N2: C, 71.15;
H, 5.97; N, 5.19. Found: C, 71.18; H, 6.41; N, 5.34%.
5. Conclusion
In summary, a series of new nickel (2a, 2b) and copper
(3a, 3b, 3c) complexes, bearing a 2:1 ligand:metal stoichi-
ometry, has been synthesized from the respective ligands,
2-[1-(2,6-diethylphenylimino)ethyl]phenol (1a), 2-[1-(2,6-
dimethylphenylimino)ethyl]phenol (1b) and 2-[1-(2-methyl-
phenylimino)ethyl]phenol (1c), by the reactions with the
corresponding M(OAc)2 Æ xH2O (M = Ni, Cu) in the pres-
ence of NEt3 as a base in moderate to high yields. The com-
plexes 2a, 2b, 3a, 3b and 3c have been structurally
characterized by X-ray diffraction studies, which revealed
that the geometry around the metal center is square planar
in the nickel (2a, 2b) and copper (3a, 3c) complexes whilst it
is distorted square planar in the copper 3b complex. The
catalysis studies revealed that while the copper complexes
3a, 3b and 3c, efficiently catalyzed the ring-opening poly-
merization of L-lactide under solvent-free melt conditions,
producing polylactide polymers of moderate molecular
weights and having narrow molecular weight distributions,
the nickel counterparts 2a and 2b failed to yield any poly-
lactide polymer.
4.4.3. Synthesis of {2-[1-(2-methylphenylimino)ethyl]-
phenoxy}2Cu(II) (3c)
Cu(OAc)2 Æ H2O (0.582 g, 2.92 mmol) and 2-[1-(2-meth-
ylphenylimino)ethyl]phenol (1c) (0.677 g, 3.00 mmol) were
taken in methanol (ca. 40 mL) and stirred at room temper-
ature until a clear solution was obtained. To the solution
was added NEt3 (0.314 g, 3.10 mmol) with constant stir-
ring. The reaction mixture was stirred at reflux for 12 h
and then allowed to cool to room temperature when the
product was obtained as reddish-brown solid which was
isolated by filtration and vacuum dried (0.573 g, 75%).
Crystals suitable for X-ray structure determination were
obtained by slow evaporation from a saturated benzene
solution. IR data cmÀ1 KBr pellet: 1599 (s) (mC@N). kmax
(e) CHCl3: 630 nm (e = 125 L molÀ1 cmÀ1). Anal. Calc.
for C30H28CuO2N2: C, 70.36; H, 5.51; N, 5.47. Found: C,
69.84; H, 5.55; N, 5.40%.
Acknowledgements
We thank Council of Scientific and Industrial Research
(CSIR), New Delhi, for financial support of this research.
We are grateful to the National Single Crystal X-ray Dif-
fraction Facility at IIT Bombay as well as to Professor
Gerard Parkin, Columbia University for the crystallo-
graphic results. PG thanks Professor Goutam Kumar Lah-
iri, IIT Bombay, for valuable discussion and Professor M.
Ravikanth, IIT Bombay, for help with the electronic stud-
ies. AJ thanks CSIR, New Delhi for a research fellowship.
4.5. Polymerization experiments
Typical polymerization experiments: L-lactide (1.000 g,
6.94 mmol) and 3a, 3b and 3c, in a stipulated molar ratio,
were charged in an ampoule inside a glove box. The
ampoule was taken out and put under vacuum at 50 ꢁC
for 30 min, after which the ampoule was sealed under vac-
uum and heated at 160 ꢁC for 4 h. Under these conditions
the reaction mixture turned into a monomer melt in which
polymerization occurred. Subsequently, the molten reac-
tive polymer mixture was cooled by immersing the sealed
ampoule in liquid nitrogen to stop the polymerization.
Conversion of L-lactide monomer was determined using
the size exclusion chromatography method. The analyses
were performed on the crude reaction mixture and no pre-
cipitation was executed to avoid fractionation of the sam-
ple in order to not influence the results.
Appendix A. Supplementary material
CCDC 609004, 605462, 607766, 605461 and 609526 con-
tain the supplementary crystallographic data for 2a, 2b, 3a,
3b and 3c. 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
data associated with this article can be found, in the online
version, at doi:10.1016/j.poly.2007.04.039.
4.6. X-ray analyses
References
X-ray diffraction data were collected either on a Bruker
P4 diffractometer equipped with a SMART CCD detector
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collection and refinement parameters are summarized in
Table 2. The structures were solved using direct methods
and standard difference map techniques, and were refined
by full-matrix least-squares procedures on F2 with SHELXTL
(Version 6.10).
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