(Benzimidazolylmethyl)amine ZnII and CuII carboxylate complexes: Structural, mechanistic and kinetic studies of polymerisation reactions of ε-caprolactone

Article abstract of DOI:10.1002/ejic.201402049

Compounds N-(1H-benzimidazol-2-ylmethyl)aniline (L1), N-(1H-benzimidazol-2- ylmethyl)-2-bromoaniline (L2), and N-(1H-benzimidazol-2-ylmethyl)-2- aminothiophenol (L3) react with Zn^II and Cu^II carboxylates to form complexes [Zn₂(L1)₂(OBn)₄] (1), [Zn₂(L2)₂(OBn)₄] (2), [Zn₂(L3) ₂(OBn)₄] (3), [Cu₂(L2)₂(OBn) ₄] (4), [Zn(L1)₂(OAc)₂] (5), [Zn(L2) ₂(OAc)₂] (6) and [Cu₂(L1)₂(OAc) ₄] (7). Structures of 2, 4 and 6 revealed that L1-L3 are monodentate, binding through the imidazolyl N-atom. The X-band EPR spectrum of 4 in the solid state is consistent with an antiferromagnetically-coupled (singlet) ground state and a low-lying EPR-active triplet excited state characterised by two main transitions. In dimethyl sulfoxide (DMSO) solution, a single resonance confirmed the retention of the dinuclear paddlewheel core. Complexes 1-7 formed active catalysts towards ring-opening polymerisation of ε-caprolactone. The polymerisation reactions follow first-order kinetics with respect to the monomer and occur through a coordination-insertion pathway. Copyright

Full text of DOI:10.1002/ejic.201402049

FULL PAPER
DOI:10.1002/ejic.201402049
II
II
(
Benzimidazolylmethyl)amine Zn and Cu Carboxylate
Complexes: Structural, Mechanistic and Kinetic Studies of
Polymerisation Reactions of ε-Caprolactone
[a]
[a]
[a]
Nelson W. Attandoh, Stephen O. Ojwach,* and Orde Q. Munro
Keywords: Zinc / Copper / Ligand design / Ring-opening polymerization / Polymerization / Kinetics
Compounds N-(1H-benzimidazol-2-ylmethyl)aniline (L1), N-
is consistent with an antiferromagnetically-coupled (singlet)
ground state and a low-lying EPR-active triplet excited state
characterised by two main transitions. In dimethyl sulfoxide
(DMSO) solution, a single resonance confirmed the retention
of the dinuclear paddlewheel core. Complexes 1–7 formed
active catalysts towards ring-opening polymerisation of ε-ca-
prolactone. The polymerisation reactions follow first-order
kinetics with respect to the monomer and occur through a
coordination–insertion pathway.
(
(
1H-benzimidazol-2-ylmethyl)-2-bromoaniline (L2), and N-
1H-benzimidazol-2-ylmethyl)-2-aminothiophenol (L3) react
II
II
with Zn and Cu carboxylates to form complexes [Zn
(OBn) (1), [Zn (L2) (OBn) (2), [Zn (L3) (OBn)
Cu (L2) (OBn) ] (4), [Zn(L1) (OAc) ] (5), [Zn(L2) (OAc)
and [Cu (L1) (OAc) ] (7). Structures of 2, 4 and 6 revealed
2
(L1)
2
4
]
2
2
4
]
2
2
4
]
(3),
[
2
2
4
2
2
2
2
] (6)
2
2
4
that L1–L3 are monodentate, binding through the imidazolyl
N-atom. The X-band EPR spectrum of 4 in the solid state
Introduction
preciable attention as catalysts in the ROP of lactides and
lactones due to their low toxicity, lower costs, and ease of
Transition-metal alkoxides have been employed as cata-
lyst initiators for the ring-opening polymerisation (ROP) of
cyclic monomers for decades.[ Fine-tuning both the steric
and electronic properties is essential for controlling both
the activity and the nature of the polymers produced. For
instance, increasing steric bulk of the ligand motif limits
side reactions such as transesterification reactions. These
considerations have led to the design of a plethora of li-
gands containing single or multiple donors that are either
monodentate, bidentate, or multidentate, depending on the
[10,11]
[12]
synthesis.
For instance, Garces et al.
synthesised
good molecular weight polymers (21000–34600 Da) with
moderate polydispersity index (PDI; 1.09–1.53) by using
heteroscorpionate amide zinc complexes, whereas Li et
1]
[13]
al. employed a series of air-stable copper complexes sup-
ported on benzotriazole phenoxide ligands to synthesise po-
lylactides with molecular weight as high as 28000 Da and
molecular weight distribution in the range of 1.09–1.75.
As part of our contribution in this area, we recently re-
[
2]
II
II
ported the use of (pyrazolylmethyl)pyridine Zn and Cu
[
3]
ligand architecture and the identity of the metal atom.
A
[14]
complexes as catalysts in the ROP of ε-caprolactones. In
number of reviews detailing the use of metal complexes
with different ligand architectures as catalyst initiators for
the ROP of lactides and lactones have recently appeared in
this context, we investigated the viability of using (benz-
II
II
imidazolyl)amine Zn and Cu complexes to catalyse ROP
of ε-caprolactones. These ligands have recently attracted
[
4]
literature. In addition to the ligand design, the choice of
the metal centre has become an integral aspect in the ROP
of cyclic esters due to their potential applications in bio-
[15]
significant interest due to the nucleophilic imide N-atom,
[16]
in addition to their biocompatible nature. Herein, we re-
II
II
port the coordination chemistry of Zn and Cu carboxyl-
ate complexes with (benzimidazolyl)amine ligands and their
applications in the ROP of ε-caprolactone. Detailed struc-
tural, kinetics and mechanistic studies of these polymerisa-
tion reactions have been performed and are discussed.
[
5]
medical and pharmaceutical fields. As a result, the use of
more biocompatible metals that display comparable activity
to those of commercial tin-based catalysts[ and other
heavy metals is gaining momentum. Examples of such
6]
[
7]
[8]
[1a]
metal complexes include Ca, Mg, Fe, and alkali met-
als.[ More recently, Zn and Cu metals have received ap-
9]
II
II
Results and Discussion
[
a] School of Chemistry and Physics, University of
KwaZulu-Natal, Pietermaritzburg Campus,
Private Bag X01 Scottsville, 3209, South Africa
E-mail: ojwach@ukzn.ac.za
Synthesis and Structures of Zn and Cu Complexes
http://chemistrypmb.ukzn.ac.za/StaffProfiles/
DrStephenOjwach.aspx
The (benzimidazolylmethyl)amine ligands, N-(1H-benz-
imidazol-2-ylmethyl)aniline (L1), N-(1H-benzimidazol-2-yl-
methyl)-2-bromoaniline (L2), and N-(1H-benzimidazol-2-
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402049.
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ylmethyl)-2-aminothiophenol (L3) were synthesised by
The identities of all the new compounds were established
[
17]
using a modified literature procedure (Scheme 1).
The by using elemental analyses, mass spectrometry, IR spec-
compounds were obtained in moderate to good yields (51– troscopy, and single-crystal X-ray crystallography for com-
71%) as pale-yellow solids.
plexes 2, 4 and 6. In addition, Zn complexes and paramag-
netic Cu compounds were characterised by H NMR spec-
1
troscopy and magnetic moment measurements, respectively.
1
A notable feature in the H NMR spectra of the ligands
was the appearance of methylene protons as a doublet in
L2 as opposed to the singlet peaks recorded for L1 and
1
1
L3. H– H COSY spectrum of L2 (Figure S1) revealed the
coupling of the methylene protons to the adjacent amine
proton. This could occur because of the slow rate of ex-
change of N–H proton in the presence of the electron-with-
drawing Br atom. The H NMR spectra of the Zn com-
plexes exhibited slight shifts relative to those of the respec-
tive ligands, which was useful in deducing complex forma-
Scheme 1. Synthesis of ligands.
[
18]
1
The corresponding bimetallic ZnII and CuII benzoate
complexes [Zn (L1) (OBn) ] (1), [Zn (L2) (OBn) ] (2),
2
2
4
2
2
4
[
Zn (L3) (OBn) ] (3), and [Cu (L2) (OBn) ] (4) were syn-
2 2 4 2 2 4
tion. For example, CH signals for L2 and 2 were observed
thesised by a one-pot, two-step reaction (Scheme 2). Gener-
ation of the metal benzoate salts was achieved by reacting
the appropriate metal acetate with two equivalents of
benzoic acid. This was followed by addition of the appro-
priate ligand (L1–L3) in situ to afford the corresponding
binuclear compounds in low to good yields (21–87%). The
corresponding acetate complexes were prepared by reacting
the Zn and Cu acetate salts with one equivalent of the ap-
propriate ligand to produce complexes [Zn(L1) (OAc) ] (5),
2
at = 4.59 and 4.69 ppm, respectively.
Typical IR spectra of L1–L3 and their corresponding
complexes showed νC=N shifts to higher frequencies in the
complexes. For example, the ν
in L3 shifted from 1595
C=N
to 1687 cm–1 in the corresponding complex 3. This was di-
agnostic of coordination of the metal atoms to the nitrogen
atom of the benzimidazolyl ring, consistent with previous
[
19]
reports.
The measured magnetic moments of the para-
2
2
magnetic complexes 4 and 7 were 1.86 and 2.12 BM, respec-
[
Zn(L2) (OAc) ] (6), and [Cu (L1) (OAc) ] (7) in moderate
2 2 2 2 4
II
tively. The values fall within the range of Cu complexes
yields (Scheme 3). Whereas the acetate Zn complexes 5 and
were monometallic in the solid state, the Cu complex 7
[
20]
containing Cu–Cu interaction.
The microanalyses data
6
of all the complexes were consistent with the proposed
structures in Schemes 1 and 2 and confirmed the purity of
the compounds.
was bimetallic. The zinc and copper complexes were iso-
lated as pale-yellow and pale-blue solids, respectively.
Solid-State Structures of Complexes 2, 4 and 6
Single crystals of 2, 4 and 6 that were suitable for X-ray
diffraction analyses were obtained by slow evaporation of
methanol solutions of 2 and 4 and a dichloromethane solu-
tion of 6 at room temperature. Crystallographic data and
structure refinement parameters are presented in Table 1,
and Figures 1, 2 and 3 show the molecular structures of
complexes 2, 4 and 6, respectively. The solid-state structures
of 2 and 4 are dinuclear and exhibit inversion symmetry.
The asymmetric unit in 2 and 4 comprises one metal centre,
two benzoate anions, and one benzimidazolyl ligand. In
Scheme 2.
Scheme 3.
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Table 1. Crystal data collection and structural refinement for complexes 2, 4 and 6.
2
4
6
Chemical formula
Mr
Crystal system
Space group
Temperature [K]
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C
56
H
44Br
2
N
6
O
8
Zn
2
·2(CH
3
OH)
C
56
H
44Br
2
Cu
2
N
6
O
8
·2(CH
3
OH)
C
787.81
triclinic
P1
100
9.4041(6)
12.1658(8)
14.7332(9)
103.670(3)
102.615(3)
94.692(3)
1582.42(17)
2
32 2 6 4
H30Br N O Zn
1283.62
triclinic
P1
1279.96
triclinic
P1
¯
¯
¯
100
100
10.7836(15)
11.6897(16)
11.6997(16)
74.925(8)
82.395(8)
75.853(8)
1377.2(3)
1
10.5718(7)
11.6594(7)
11.6631(7)
75.481(3)
75.859(3)
82.766(3)
γ [°] 3
V [Å ]
1346.32(14)
1
Z
Radiation
μ [mm ]
Mo-K
2.39
α
Mo-K
2.34
α
Mo-K
α
3.35
–
1
Crystal size [mm]
Data collection
Diffractometer
Absorption correction
0.38ϫ0.35ϫ0.25
0.20ϫ0.20ϫ0.15
0.10ϫ0.10ϫ0.10
Bruker Apex II Duo
Multi-scan
SADABS2012/1 (Bruker, 2012)
0.771, 1.000
Bruker Apex II Duo
Multi-scan
SADABS2012/1 (Bruker, 2012) SADABS2012/1 (Bruker, 2012)
Bruker Apex II Duo
Multi-scan
T
min., Tmax.
0.623, 0.746
0.654, 0.745
Measured, independent and
11291, 4843, 4391
23779, 6531, 5905
23107, 6113, 5574
observed [IϾ2σ(I)] reflections
R
int
0.021
0.018
0.018
Refinement
2
2
2
R[F Ͼ 2σ(F )], wR(F ), S
Number of reflections
Number of parameters
Number of restraints
0.023, 0.052, 1.05
4843
365
1
0.034, 0.094, 1.07
6531
365
0
0.021, 0.051, 1.06
6311
424
0
3
Δ͘max, Δ͘min [eÅ ]
0.31, –0.37
0.61, –1.21
0.37, –0.28
Figure 1. (a) View of the solid-state structure of 2 with thermal ellipsoids drawn at the 50% probability level. H atoms are rendered as
spheres of arbitrary radii. Selected symmetry-unique atom labels are shown. Selected bond lengths [°] and angles [Å]: Zn1–Zn1[i] 3.0311(5),
Zn1–O1 2.1164(14), Zn1–O2 2.0856(14), Zn1–O3 2.0268(14), Zn1–O4 2.0273(14), Zn1–N3 2.0195(16), N3–Zn1–O1 98.51(6), N3–Zn1–O2
1
02.90(6), N3–Zn1–O3 101.64(6), N3–Zn1–O4 101.30(6), O3–Zn1–O4 157.03(6), O3–Zn1–O2 87.84(6). (b) View of the one-dimensional
hydrogen-bonded chain in the crystal structure of 2 showing two adjacent dinuclear complexes bridged by a pair of methanol solvent
molecules; the chain axis is collinear with the c-axis. Symmetry codes: [i] 1 – x, 1 – y, 2 – z; [ii] x, 1 + y, z.
each molecule, the two metal centres are bridged by two inversion symmetry. The asymmetric unit contains the
pairs of benzoate ligands to form a paddlewheel structure. metal centre, two benzimidazolyl ligand units, and two
Complex 6, on the other hand, is mononuclear and lacks metal-bound monodentate acetate ions.
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Figure 2. (a) View of the solid-state structure of 4 with thermal ellipsoids drawn at the 50% probability level. H atoms are rendered as
spheres of arbitrary radii. Selected symmetry-unique atom labels are shown. Selected bond lengths [°] and angles [Å]: Cu(1)–N(1)
2
.1390(19), Cu(1)–O(1) 1.9624(16), Cu(1)–O(2) 1.9970(16), Cu(1)–O(3) 1.9549(16), Cu(1)–O(4) 1.9891(17), Cu(1)–Cu(2) 2.6947(4), O(1)–
Cu(1)–N(1) 96.62(7), O(2)–Cu(1)–N(1) 94.47(7), O(3)–Cu(1)–N(1) 96.84(7), O(4)–Cu(1)–N(1) 98.56(7), O(3)–Cu(1)–O(1) 166.48(7), O(1)–
Cu(1)–O(2) 90.64(7). (b) View of the 1D hydrogen-bonded chain in the crystal structure of 4; adjacent dinuclear complexes bridged by a
pair of methanol solvent molecules are highlighted. The chain axis is collinear with the b-axis. Symmetry codes: [i] 1 – x, y, 2 – z; [ii] x,
1
+ y, z.
Figure 3. (a) View of the solid-state structure of 6 with thermal ellipsoids rendered at the 50% probability level. H atoms are depicted as
spheres of arbitrary radii. Selected symmetry-unique atom labels are shown. Selected bond lengths [°] and angles [Å]: Zn(1)–O(3)
1
1
.9708(13), Zn(1)–N(4) 2.0346(16), Zn(1)–N(1) 2.0277(17), Zn(1)–O(1) 1.9493(14), O(1)–Zn(1)–O(3) 117.84(6), O(1)–Zn(1)–N(4)
05.66(6), O(1)–Zn(1)–N(1) 118.81(6), N(1)–Zn(1)–N(4) 100.27(7). (b) View of the one-dimensional hydrogen-bonded chain in the crystal
structure of 8; both the intra- and intermolecular hydrogen bonds are highlighted. The chain axis is collinear with the [1,0,1] plane
diagonal (i.e., the bisector of angle aôc). Symmetry codes: [i] – x, 2 – y, – z; [ii] 1 – x, 2 – y, 1 – z.
In the molecular and crystal structure of 2 (Figure 1), functionality linking the aromatic ring systems of the ligand
II
each Zn ion of the dinuclear compound is coordinated is noncoordinating. Interestingly, the trigonal planar nature
equatorially by four O-atoms of the bridging benzoate li- of the secondary amine group (–CH –NH–Ar) suggests
2
gands and one axial nitrogen atom from L2 to give a five- conjugation of the amine lone pair with the π-electron sys-
coordinate geometry. Although we initially anticipated a bi- tem of the aryl ring, possibly accounting for its apparently
dentate coordination mode for L2, the ligand in compound rather poor σ-donor strength and, hence, inability to facili-
2
coordinates in a monodentate fashion through the benz- tate chelation of the metal ion. The N–H groups of the
imidazolyl N-atom, whereas the secondary amine N–H benzimidazole ligands are, furthermore, hydrogen bonded
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to the two solvent molecules (CH OH). The interaction drogen bonding between 4 and the solvent are: N2–H2,
3
[
ii]
[ii]
metrics of the primary hydrogen bonding between 2 and 0.77(3) Å; H2···O5, 2.01(3) Å; N2–H2···O5, 158(3)°;
the solvent are: N2–H2, 0.81(2) Å; H2···O5, 1.97(2) Å; N2– and O5 –H5A, 0.74(5) Å; H5 ···O4, 2.12(5) Å; O5[ii]–
[
ii]
[ii]
[ii]
[ii]
[
ii]
[ii]
H2···O5, 157(2)°. The solvent hydroxy groups are further H5A ···O4, 175(5)° (symmetry code [ii]: x, 1 + y, z).
involved in bridging adjacent molecules in the lattice by do- The four symmetry-unique Cu–O bond lengths range
nation of a hydrogen bond to a metal-bound benzoate oxy- from 1.9549(16) to 1.9970(16) Å, averaging 1.976(20) Å.
gen atom (O2) of the neighbouring molecule. Because two The imine-type nitrogen atoms of the pair of axial mono-
such interactions exist for each molecule in the unit cell, dentate benzimidazole ligands effectively cap the paddle-
one-dimensional hydrogen-bonded chains are formed in wheel core; the symmetry-unique Cu–N bond length mea-
which pairs of methanol molecules effectively bridge pairs sures 2.1390(19) Å. The mean Cu–O and Cu–N distances
of dinuclear complexes (as shown in Figure 1, b). The chain within the coordination group of 4 compare favourably
II
[28]
axis is collinear with the c-axis of the unit cell. Because the with other related Cu complexes.
The O–Cu–O bond
chain propagates via the centre of inversion in the centre of angles average 89(1)° and 166.7(3)° for the cis- and trans-
the unit cell, it runs down the unit cell centre. The metrics angles, respectively. The Cu–Cu bond length of 2.6946(5) Å
of the secondary hydrogen bonds permitting this type of is within the normal range of 2.40–2.70 Å reported for re-
[
ii]
[29]
II
extended structure are: O5–H5A, 0.81(2) Å; H5A···O2,
1
lated complexes.
The Cu ions of the paddlewheel core
.98(2) Å; O5–H5A···O2,[ 177(2)° (symmetry code [ii]: x, are displaced by 0.23 Å above the mean plane of the four
ii]
1
+ y, z). The one-dimensional chains are additionally stabi- metal-bound benzoate oxygen atoms. Collectively, the coor-
II
lised by π–π interactions between stacked benzimidazole dination geometry of each Cu ion is best described as a
rings of adjacent pairs of dinuclear complexes (the in- mildly distorted octahedron (taking into account the Cu–
terplanar spacing measures 3.38 Å).
The Zn–O bond lengths in the equatorial plane remain
in the range 2.0269(14) – 2.1164(14) Å, whereas the axial state structure of the Zn derivative 6 is a mononuclear
Cu bond).
In distinct contrast to the structures of 2 and 4, the solid-
II
II
Zn–N bond length is 2.0195(16) Å; both are similar to those species in which the four-coordinate Zn ion binds two
II
[21]
reported for other Zn complexes. For example, the Zn– monodentate benzimidazole ligands (L2) and two mono-
N bond length of complex 2 compares well with the dis- dentate acetate ions, thereby adopting a distorted tetrahe-
tance of 2.02 Å reported for a single axially-coordinated N- dral coordination geometry (Figure 3). Each ligand is coor-
[
22]
II
atom, but is shorter than the average Zn–N bond length dinated to the Zn ion through the benzimidazole imine-
of 2.10–2.11 Å reported for four-coordinate ZnII[23] and the type nitrogen atom as observed in 2 and 4. The Zn–N and
II [24]
2.15–2.19 Å bond length typical of six-coordinate Zn .
Zn–O bond lengths average 2.031(5) and 1.960(15) Å,
II
This is attributed to the extent of coordination around the respectively, and compare well with other Zn complexes
[
25]
[30]
metal centre. The N–Zn–O bond angles subtended at the that display a similar coordination geometry. The coordi-
II
metal ion are inequivalent, averaging 101(2)°, and are con- nation group bond angles subtended at the Zn ion deviate
sistent with a somewhat distorted square-pyramidal coordi- significantly from the regular tetrahedral angle of 109.5°,
II
nation geometry. The symmetry-related Zn ions are each consistent with a distorted tetrahedral geometry about the
displaced from the plane of their four coordinated oxygen metal centre.
atoms by 0.40 Å. The Zn···Zn distance between the two Zn
atoms bridged by the four carboxylate groups is oxygen atom (O1 and O3) is involved in an intramolecular
.0311(5) Å, similar to literature reports of other dimeric hydrogen bond with the nearest aryl-appended N–H group
In the solid-state structure of 6, each coordinated acetate
3
II
[26]
Zn complexes. The observed metal-to-metal distance is of the neighbouring benzimidazolyl ligand, effectively lock-
greater than the sum of the van der Waal radii of Zn ing in the observed orientation of the brominated aryl ring,
[
27]
(1.39 Å),
consistent with the absence of any meaningful at least in the solid state. The mean interaction metrics for
II
Zn–Zn interatomic metal bond. Each Zn ion is therefore the pair of intramolecular N–H···O hydrogen bonds are: N–
five-coordinate with a distorted square pyramidal geometry. H, 0.82(1) Å; NH···O, 2.284(7) Å; N–H···O, 139(4)°. The
II
In the dinuclear structure of 4 (Figure 2), each Cu ion approximately trigonal planar geometry of the N–H donor
is equatorially bonded to four oxygen atoms of the bridging groups of the ligands suggests substantial conjugation of
carboxylate groups that form the paddlewheel core in a sim- the amine lone pair of electrons with the π electrons of the
ilar fashion to 2. The structure of 4 is, likewise, centrosym- parent aryl ring and might contribute to the strength of the
metric about an inversion centre located midway along the NH group as a hydrogen-bond donor in this case. Hydrogen
metal-to-metal interaction vector and a bis(methanol) solv- bonding is, furthermore, significant in the supramolecular
ate. As illustrated in Figure 2 (b), the extended structure of structure of 6. Thus, although 6 does not form a solvated
4
is similar to that seen for 2, with pairs of methanol mol- lattice, it forms one-dimensional hydrogen bonded chains
ecules bridging dinuclear complexes by hydrogen bonds. akin to those in 2 and 4, primarily as a result of centrosym-
The one-dimensional hydrogen bonded chains in 4, how- metric N–H···O hydrogen bonding involving the benzimid-
ever, run collinear with the b-axis rather than the c-axis (as azole N–H donor in one molecule and the uncoordinated
found in 2). Taken together, the molecular structures of 2 acetate oxygen atom of the neighbouring molecule. A com-
and 4 are essentially isomorphous, at both the molecular plementary interaction completes the pair of centrosymmet-
and supramolecular level. The interaction metrics of the hy- ric hydrogen bonds and generates an inversion pair, or H-
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xy
z
bonded dimer. Repetition of the hydrogen-bonding pattern
g
Ͻ g for both transitions reflects axial compression of
n n
along the cell diagonal (the line bisecting aôc) generates the the g-tensors.
1D chain.
The grinding of polycrystalline 4 to record the powder
The interaction metrics of the hydrogen bonding within EPR spectrum evidently caused some paddlewheel cleavage
[
i]
II
the 1D chains of 6 are: N2–H2, 0.79(3) Å; H2···O2,
and the formation of a mononuclear Cu species (S = 1/2)
[i]
imp
1
.97(3) Å; N2–H2···O2, 160(3)°; and N5–H5, 0.84(2); characterised by the signal at g
H5···O4, 1.86(2); N5–H5···O4, 171(3) (symmetry code called “impurity” signal is commonly observed in the pow-
ii]: x, 1 + y, z). Interestingly, the intermolecular H···O dis- der EPR spectra of dinuclear Cu paddlewheel derivatives
= 2.09 (3372 G). This so-
[
ii]
[ii]
II
[
tances in 6 are consistent with data reviewed by Steiner for and may be ignored when analysing the triplet state EPR
N–H···O hydrogen bonds involving water as the acceptor spectrum. The powder EPR spectrum of 4 is very similar
and a range of N–H functional group donors (i.e., H···O to those of the acetate-acetamido paddlewheel derivative
interactions spanning the range 1.71–2.16 Å).[
31]
{Cu (μ -O CCH ) }(OCNH CH ) recently studied in detail
2
2
2
3 4
2
3
[
32]
by Paredes-García et al.,
the dinuclear guanidinoacetic
acid-bridged paddlewheel system reported by de Miranda
Electron Paramagnetic Resonance (EPR) Analysis
[
33]
et al.,
and the antiferromagnetically coupled dinuclear
II
Cu paddlewheel-like compounds with 4-azabenzimidazole
ligands reported by van Albada et al. Consistent with the
triplet-state spectra of other Cu paddlewheel derivatives,
The 295 K powder and solution phase (dimethyl sulfox-
ide, DMSO) X-band EPR spectra of the structurally-char-
[
34]
II
II
acterised Cu paddlewheel complex 4 are shown in Fig-
hyperfine coupling to the Cu nuclei (I = 3/2) within dinu-
clear 4 is not observed.
ure 4. The solid-state EPR spectrum of 4 is consistent with
a dinuclear complex having two antiferomagnetically cou-
II
pled Cu ions and a diamagnetic (singlet) ground state (S
In solution, the EPR spectrum of 4 is almost perfectly
=
0) with a thermally accessible low-lying excited triplet isotropic and is characterised by a single line (g = 2.14),
state (S = 1). Because the g-tensors for the two excitations consistent with averaging of the zero-field splitting tensor,
are of tetragonal symmetry (components x,y ϶ z), the two D, to zero as a result of random tumbling of the paddle-
spin-allowed Δm = Ϯ1 transitions involving the excited wheel complex in solution. Such a situation renders the ms
s
triplet state levels (unpaired spins either ȆȆ, m = +1, or = –1, 0, and +1 triplet-state levels degenerate in the absence
s
ȇȇ, m = –1, within the two Cu d
x2–y2
orbitals) are ob- of an applied magnetic field. In an applied magnetic field,
s
xy
xy
served at 462 G (B1 = 15.3) and 4878 G (B2 = 1.45) as the Zeeman splitting is symmetric, leading to equal spac-
the primary (more intense) spectral features. The z-compo- ings for the upper m = +1 and lower m = –1 states from
s
s
nents of the two signals are more difficult to assign pre- the m = 0 state (barycentre). The spin-allowed Δm = Ϯ1
s
s
cisely, but possibly correspond to the features discernible at transitions are thus degenerate and occur at a resonant fre-
z
z
iso
iso
1
930 G (B ) and 6308 G (B ), completing the picture for quency governed by g . In the case of 4, g = 2.14; the
1 2
the allowed triplet state transitions. The observed relation significant g-shift relative to g (2.0023) reflects spin-orbit
e
Figure 4. (a) Room temperature (295 K) powder EPR spectrum of 4 (microwave frequency, 9.870 GHz). (b) Room temperature EPR
spectrum of 4 dissolved in DMSO (microwave frequency, 9.786 GHz). The inset spectra in both cases are the absorption spectra over the
range encompassing the primary spectral features.
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coupling effects in the paddlewheel complex. It is important the reaction profile. Thus, the rate of ε-CL polymerisation
to note that the absorption envelope for the signal in Fig- could be represented as depicted in Equation (1).
ure 4 (b) is slightly anisotropic, or non-Lorentzian, with
d[CL]
some asymmetry on the high-field side of the signal. This
–
= kapp[CL]
(1)
dt
is possibly caused by the presence of the mononuclear im-
purity observed in the powder spectrum. That said, its con-
tribution to the absorption profile in solution is essentially
insignificant. The fact that a solution-phase signal for the
triplet state of 4 is easily observed at room temperature sug-
gests that the two spins are well-separated spatially and
deftly confirms the vanishingly small magnitude of the zero-
field splitting, D.
x
where kapp = k [I] , and where k is the chain propagation
rate constant.
p
p
Polymerisation of ε-Caprolactone (ε-CL)
The catalytic activities of complexes 1–7 in the ROP of
ε-caprolactone (ε-CL) were investigated in bulk at 110 °C.
Table 2 contains a summary of the polymerisation data for
complexes 1–7. From preliminary investigations, it was evi-
dent that all the complexes exhibited significant catalytic
activities in the ROP of ε-CL, achieving maximum conver-
sion of about 95% between 48–96 h (Figure S2). These find-
ings prompted us to perform detailed kinetics and mechan-
istic studies of these polymerisation reactions.
Figure 5. Plot of ln[CL]
induction period for catalysts 1–4.
0
/[CL]
t
vs. time showing linear fits after
The apparent rate constants were extracted from the
plots of In[CL] /[CL] vs. time taking into account the in-
0
t
–1
–1
–1
duction period as 0.062 h (1), 0.054 h (2), 0.076 h (3),
Table 2. ROP of ε-CL catalysed by complexes 1–5 and 7.[a]
.045 h–1 (4), 0.092 h (5) and 0.027 h (7). Thus Zn
–1
–1
II
0
Conv.
[%]
M
n
n
M (exp)
complexes exhibited higher catalytic activities compared
with the corresponding Cu complexes. For example, rate
Entry
Cat. t [h]
(NMR)[
b]
[c]
PDI[c] I*[d]
II
1
2
3
4
5
6
1
2
3
4
5
7
36
48
32
52
32
48
96
95
96
95
97
96
10906
10857
10960
10843
11078
10983
3701
3993
2701
4599
4538
3046
1.52
1.50
1.43
1.49
3.64
2.29
0.34
0.37
0.25
0.42
0.41
0.28
–1
constants of 0.054 and 0.045 h were recorded for the anal-
II
II
ogous Zn and Cu complexes 2 and 4, respectively. The
II
higher catalytic activities of Zn complexes relative to their
II
corresponding Cu complexes could be due to the higher
II
electrophilicity of Zn metals making them better Lewis
[
36]
[a] Reaction conditions: 110 °C, [M]/[I]
=
100, [ε-CL]
o
=
acids.
The ligand architecture also played a role in the
5
.41 mmol, polymerisation of ε-CL in bulk. [b] Calculated from catalytic activities of the complexes. For instance, complex
of monomer)ϫ[M]/[I]ϫ(% conv). [c] M (exp)
.56ϫM (GPC). [d] Initiator efficiency = M (exp)/M (NMR).
–
(M
w
n
=
–1
3
(0.076 h ) containing an –SH group was more active than
0
n
n
n
–1
the corresponding complex 2 (0.045 h ) bearing a Br
group. Similar trends have been reported by Hodgson et
[
37]
al.
who found that replacement of an isopropyl group
with a phenyl group on bis(thiophosphinic amine) resulted
Kinetics of ε-Caprolactone Polymerisation Reactions
–3
in a decrease in the rate constant from 9.50ϫ10 to
The kinetics of the ROP of ε-CL catalysed by complexes 1.57ϫ10–3 s . The identity of the carboxylate anion also
–1
1
–7 were investigated by monitoring the reaction profiles had an effect on catalyst activity. Generally, acetate com-
1
by H NMR spectroscopy. The rates of the reaction were plexes afforded more active catalysts than the correspond-
determined by plotting semilogarithm graphs of In[CL]0/ ing benzoate counterparts (Table 1, entries 1 vs. 5). This
[
CL] vs. time (Figure 5). In all cases, induction periods were trend could be attributed to steric factors imposed by the
t
observed between 8–26 h. Reactions with copper catalysts bulky benzoate groups that limit monomer access to the
[
38]
showed longer induction periods compared with the analo- metal centre. We also observed greater catalytic activities
gous zinc complexes (Figure S3). Induction periods are usu- for monomeric complexes in comparison to the correspond-
ally associated with structural rearrangement/aggregation ing dinuclear analogues (Table 2, entries 1 and 5). These
of the reacting species to form the active sites.[ In this results contrast with literature reports whereby multinuclear
case, longer induction periods could imply significant dif- complexes generally form more active catalysts than mono-
ferences between the active species and the catalyst precur- nuclear species.[1a] Our observation is consistent with
sors. However, linear relationships consistent with pseudo- stronger M–O bonds in the paddlewheel structures limiting
first-order dependency on the monomer were observed af- dissociation to facilitate lactone monomer coordination.
ter the induction periods (Figure 5), which is an indication EPR data confirmed retention of the dimeric structure in
that the generated active sites remained unchanged during solution and supports this hypothesis. Comparatively, the
35]
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observed rate constants of our complexes were lower than (Table 3, entries 4–6). The use of methanol had a profound
those of the most active zinc complexes reported in the lit- effect on the catalytic activity of complex 4, affording a rate
[
39]
–1
–1
erature. However, they displayed comparable catalytic ac- constant of 0.147 h compared with a value of 0.045 h
[
40]
tivities to zinc complexes reported by Chamberlain et al.
obtained in the bulk experiment. Significantly, the activity
and aluminium alkoxide systems investigated by Zhong et recorded in toluene solvent was comparable to that in the
al.[
41]
bulk experiment (Table 3, entries 1–3). The significant in-
To gain insight into the order of reaction with respect to crease in the catalytic activity of 4 in the presence of meth-
the catalyst initiator and the subsequent overall order of the anol could be due to generation of Zn-alkoxides (Zn-
reaction, further kinetic studies were performed by using OCH ) in situ, which are known to form highly active spe-
3
complex 1. The order of the reaction was deduced from the cies in the polymerisation of cyclic esters as compared to
gradient of the line of best fit of a plot of lnkapp vs. In[1] metal acetates.[
21c,43]
(
Figure 6). From the plot, the order of the reaction with
Table 3. Effect of temperature and solvent on the polymerisation
respect to catalyst 1 was obtained as 1.0085 ≈ 1. The overall
order of the reaction could thus be described as second or-
der according to Equation (2).
of ε-CL by 2 and 4.[
a]
–1 [b]
Entry
[I] Solvent
T [°C]
M
n
[g/mol] PDI
k
app [h ]
1
2
3
4
5
6
4
4
4
2
2
2
bulk
110
110
110
110
90
4 599
1.49
1.58
1.29
1.50
1.42
–
0.045
0.053
0.147
0.056
0.030
0.007
d[CL]
toluene
methanol
bulk
bulk
bulk
5 102
1 530
3 993
2 554
–
–
= k
p
[CL][1]
(2)
dt
60
[
a] Reaction conditions: [M]/[I] = 100, [ε-CL] = 4.4 mmol. [b] kapp
for the overall reaction including the induction period.
Molecular Weight and Molecular Weight Distribution
The molecular weight and molecular weight distribution
(
7
(
PDI) for PCL obtained by ROP catalysed by complexes 1–
were determined by gel permeation chromatography
GPC) analyses and compared with their theoretical values
Figure 6. Plot of lnkapp vs. ln[1] for the determination of order of
reaction with respect to catalyst 1.
1
computed on the basis of their H NMR spectra. The mo-
lecular weight of the polymers ranged from 1847 to 4599 g/
mol, corresponding to initiator efficiencies of between 25
and 42%. As an illustration, an experimentally determined
molecular weight of 4538 g/mol was obtained for catalyst 5
at 97% compared with the expected theoretical molecular
weight of 11078 g/mol. The polymers produced also exhib-
ited moderate to broad PDI values in the range of 1.29 to
Stability of Catalysts
The stability of the catalysts was investigated by the se-
quential addition of an equivalent amount of the monomer
without addition of the catalyst, using complex 5. This was
done by addition of 100 equiv. of ε-CL after complete con-
sumption of the monomer in the first cycle without adding
the catalyst. The polymerisation kinetics proceeded to near
completion, achieving 98% after 49 h (ε-CL) in the second
3.97. The broad molecular weight distributions coupled
with low molecular weights of the polymer are indicative of
[
44]
transesterification reactions.
The poor initiating tenden-
cycle. The recorded k
for the polymerisation kinetics
app
1
–
–1
dropped from 0.092 h (1st cycle) to 0.073 h (2nd cycle)
corresponding to a 21% drop (Figure S4). It is therefore
apparent that although the complexes remain significantly
active in the second cycle, a loss of activity occurs. This is
likely to originate from catalyst deactivation promoted by
[2c]
build-up of monomer impurities and/or thermal decom-
position of the complex.[
42]
Effect of Temperature and Solvent on the Polymerisation
The influence of temperature and solvent on the catalytic
activities of complexes 2 and 4 are presented in Table 3.
Decreasing the temperature from 110 to 60 °C resulted in a
Figure 7. Plot of experimental and theoretical molecular weight
against %conversion, showing the living polymerisation nature of
drastic decrease in rate constant from 0.056 to 0.007 h–1 catalyst 5.
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Figure 8. ESI-MS spectrum of crude polymer obtained by using catalyst 7 at 110 °C for 8 h. The presence of -OH end groups suggests
hydrolysis of the acetate end groups.
cies of the carboxylate groups in relation to alkoxides may Conclusion
account for the lack of controlled polymerisation reac-
[
3b,23]
We have demonstrated that (benzimidazolylmethyl)amine
ligands adopt a monodentate coordination mode when re-
acted with zinc or copper carboxylates. Mononuclear and
binuclear complexes are formed depending on the type of
metal centre and the nature of the carboxylate ligand. EPR
studies confirm that the paddle-wheel binuclear copper
complexes are retained in solution. All the complexes form
active catalysts for the ROP of ε-CL to produce low to
moderate molecular weight polymers. The activities of these
catalysts are dictated to a great extent by the nature of the
metal centre and by the ligand architecture. The polymeri-
sation kinetics were first order with respect to both mono-
mer and catalyst and proceeded through a coordination-
insertion mechanism. Despite the inferior catalytic behav-
iour of these complexes when compared with established
zinc systems, they provide a convenient synthetic route to
very stable catalysts. Moreover, the N–H functionality can
be utilised to prepare discrete metal-alkoxide initiators that
could offer better control of the polymerisation reactions.
tions.
However, the reactions exhibited living poly-
merisation behaviour as demonstrated by the increase in
molecular weight with percentage conversion in addition to
the independence of PDI on percentage conversions (Fig-
ure 7).
Mechanism of ε-CL Polymerisation
The polymerisation of cyclic esters using ZnII and CuII
complexes is known to proceed through either a coordina-
tion insertion mechanism (CIM) or an activated-monomer
mechanism (AMM).[
45]
To establish the mechanism of the
1
ROP of ε-CL by catalysts 1–7, a combination of H NMR
spectroscopy and mass spectroscopy was used to analyse
the polymers obtained. The first-order dependency of the
rates of polymerisation reactions on catalyst concentration
(
Figure 6) points to a coordination insertion mechanism.
1
However, H NMR spectra of the polymers obtained from
did not exhibit any signals associated with either the acet-
ate or complex motifs (Figure S5). ESI-MS spectra of the
7
polymers (Figure 8) showed fragments consistent with the Experimental Section
+
presence of an –OH functionality [HO(C H O) H·Na ].
6
10
n
Materials and Measurements: All chemicals and solvents were pur-
chased from Sigma–Aldrich and used as received. The monomer,
ε-caprolactone was dried with CaH , vacuum distilled, and stored
2
under inert conditions prior to use. Toluene was distilled from so-
dium, whereas methanol was purified by distillation from magne-
This could be due to the hydrolysis of the acetate and com-
[43,46]
plex end-groups by adventitious water molecules
might explain their absence in the H NMR spectra. Thus,
and
1
the polymerisation reactions of ε-CL by complexes 1–7
could be said to occur through a coordination-insertion sium. NMR spectra were recorded with a Bruker 400 UltraShield
1
13
pathway followed by hydrolysis of the acetate end groups.
NMR (400 MHz for H and 100 MHz for C) spectrometer. All
Eur. J. Inorg. Chem. 2014, 3053–3064
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the chemical shifts are recorded in δ (ppm) relative to tetramethyl-
(30 mL) was heated to reflux at 80 °C for 5 h followed by dropwise
addition of L1 (0.317 g, 1.42 mmol) in methanol (10 mL). The solu-
tion was heated to reflux for an additional 24 h, then the mixture
was cooled to room temperature, filtered, and the solvent was re-
moved under reduced pressure to afford a sticky yellow precipitate.
The resulting yellow precipitate was dissolved in dichloromethane
1
13
silane. The H and C NMR spectra are referenced using residual
CDCl and [D ]DMSO solvent peaks, and the coupling constants
J) are reported in Hertz [Hz]. Elemental analyses were carried out
3
6
(
with a Flash 2000 thermoscientific analyser. IR spectra were re-
corded with a Perkin–Elmer spectrum 100 series FTIR spectrome-
ter. Mass spectra of the analytes were obtained with a micromass
LCT premier mass spectrometer. The magnetic moments of para-
magnetic copper complexes were determined with an Evans bal-
ance.
and the solvent was removed in vacuo to give 1 (0.577 g, 38%) as
1
a pale-yellow solid. H NMR (400 MHz, [D
J
6
]DMSO): δ = 7.96 (d,
3
H,H = 7.4 Hz, 7 H, ArH), 7.50–7.39 (m, 13 H, ArH), 7.20 (br., 4
H, ArH), 7.01 (br., 2 H, ArH), 6.62–6.56 (m, 3 H, ArH), 6.36 (br.,
1
3
1
H, ArH), 4.65 (s, 2 H, CH
2 3
) ppm. C NMR (100 MHz, CDCl ):
General Procedure for the Synthesis of Ligands: Equimolar amounts
of 2-(chloromethyl)benzoimidazole, KI and amine were dissolved
in ethanol (40 mL) and heated to reflux for 6 h at 80 °C. This was
followed by addition of an equimolar amount of KOH and reflux
was continued for a further 2 h. The reaction mixture was cooled
to room temperature and poured into ice-cold water to give precipi-
tates that were filtered and dried.
δ = 174.0 (CO), 148.3 (C), 134.5 (C), 131.8 (CH), 129.9 (CH), 129.3
(
(
1
8
CH), 128.5 (CH), 123.2 (C), 122.4 (C), 117.5 (C), 113.1 (C), 22.0
CH ) ppm. IR (KBr): ν˜ = 3034 (w), 1700 (br), 1602 (w), 1558 (br),
403 (m), 1317 (w), 1220 (s), 1176 (w), 1070 (w), 1025 (w), 935 (w),
2
–1
46 6 8 2 2 2
41 (w), 755 (s), 714 (m), 687 (m) cm . C56H N O Zn ·0.5CH Cl :
calcd. C 61.45, H 4.29, N 7.61; found C 61.69, H 4.40, N 7.17.
Zn (L2) (OBn) (2): Zn(OAc) ·2H (0.091 g, 0.415 mmol),
COOH (0.101 g, 0.826 mmol) and L2 (0.125 g, 0.415 mmol).
[
2
2
4
]
2
2
O
Ligand L1: 2-(Chloromethyl)benzoimidazole (1.67 g, 10.00 mmol),
KI (1.66 g, 10.00 mmol), aniline (0.94 g, 0.92 mL, 10.00 mmol) and
KOH (0.40 g, 10.00 mmol) were reacted to give L1 (1.59 g, 71%)
6 5
C H
Pale-yellow solid (0.319 g, 63%). Slow evaporation of methanol
solution of 2 at room temperature gave pale-yellow crystals for X-
1
as a pale-yellow solid. H NMR (400 MHz, CDCl
3
): δ = 7.53 (dd,
1
ray diffraction analysis. H NMR (400 MHz, DMSO): δ = 7.98–
3
3
J
7
H,H = 3.2 Hz, 2 H, ArH), 7.23 (dd, JH,H = 3.1 Hz, 2 H, ArH),
7
7
.96 (m, 2 H, ArH), 7.64 (s, 1 H, ArH), 7.50–7.39 (m, 5 H, ArH),
.12 (dd, 3JH,H = 7.6 Hz, 2 H, ArH), 6.73 (t, JH,H = 7.3 Hz, 1 H,
3
3
.15 (s, 2 H, ArH), 7.06 (s, 1 H, ArH), 6.65 (d, JH,H = 7.9 Hz, 2
3
ArH), 6.60 (d, JH,H = 7.7 Hz, 2 H, ArH), 4.62 (s, 2 H, CH
2
) ppm.
): δ = 153.2 (C), 147.2 (C), 137.9 (C),
29.5 (CH), 122.8 (CH), 118.9 (CH), 114.9 (C), 113.2 (CH), 43.0
CH ) ppm. IR (KBr): ν˜ = 3405 (s), 3050 (w), 1604 (s), 1510 (s),
3
H, ArH), 6.53 (t, JH,H = 7.1 Hz, 1 H, ArH), 5.99 (t, 1 H, NH),
1
3C NMR (100 MHz, CDCl
3
) ppm. 13C NMR (100 MHz, [D
]DMSO): δ =
2 6
4
1
.69 (s, 2 H, CH
45.1 (C), 132.8 (CH), 129.9 (CH), 129.0 (CH), 128.4 (CH), 118.4
) ppm. IR (KBr): ν˜ = 3034
w), 1687 (w), 1597 (w), 1558 (w), 1509 (w), 1453 (1383 w br),
1
(
2
(
CH), 112.1 (CH), 109.4 (C), 42.2 (CH
2
1
1
8
455 (m), 1421 (s), 1347 (w), 1318 (s), 1269 (s), 1183 (w), 1153 (w),
106 (w), 1075 (w), 1011 (w), 996 (w), 928 (w), 875 (w), 875 (w),
(
–
1
1
220 (s), 1022 (w), 931 (w), 843 (w), 772 (s), 710 (w), 684 (w) cm .
–1
38 (w), 745 (s), 692 (m) cm . HRMS (ESI): m/z calcd for
C
56
H44Br
2
N
6
O
8
Zn : calcd. C 55.15, H 3.64, N 6.89; found C 55.43,
2
+
14 13 3
C H N [M – H ] 222.111; found 222.104.
H 3.83, N 7.06.
Ligand L2: 2-(Chloromethyl)benzoimidazole (1.52 g, 9.14 mmol),
KI (1.52 g, 9.14 mmol), 2-bromoaniline (1.57 g, 1.03 mL,
[
C
Zn
2
(L3)
2
(OBn)
4 2 2
] (3): Zn(OAc) ·2H O (0.095 g, 0.433 mmol),
6
H
5
COOH (0.105 g, 0.859 mmol) and L3 (0.110 g, 0.431 mmol).
9
6
=
.14 mmol) and KOH (0.52 g, 9.14 mmol) afforded L2 (1.77 g,
1
Pale-green solid (0.425 g, 87%). H NMR (400 MHz, CDCl
8
ArH), 7.36 (t, JH,H = 7.4 Hz, 1 H, ArH), 7.19–7.13 (m, 1 H, ArH),
6
(
(
1
(
3
): δ =
1
4%) as a pale-yellow solid. H NMR (400 MHz, [D
6
]DMSO): δ
3
3
.07 (d, JH,H = 7.3 Hz, 1 H, ArH), 7.49 (t, JH,H = 7.4 Hz, 1 H,
3
7.44 (dd, JH,H = 1.7 Hz, 3 H, ArH), 7.14–7.11 (m, 3 H, ArH),
3
3
6.66 (dd, JH,H = 1.5 Hz, 1 H, ArH), 6.57–6.53 (m, 1 H, ArH), 4.59
3
.98 (t, JH,H = 5.6 Hz, 1 H, ArH), 6.73–6.70 (m, 1 H, ArH), 6.58
ddd, JH,H = 1.3, 1.4, 1.4 Hz, 1 H, ArH), 6.52 (s, 1 H, NH), 4.41
3
13
(d, JH,H = 5.7 Hz, 2 H, CH
2 3
) ppm. C NMR (100 MHz, CDCl ):
3
δ = 153.3 (C), 145.2 (C), 132.7 (CH), 129.1 (CH), 118.3 (CH), 112.1
1
3
s, 2 H, CH
48.6 (C), 136.8 (CH), 132.6 (CH), 132.3 (C), 131.6 (CH), 130.3
CH), 128.2 (CH), 124.0 (C), 118.8 (C), 118.3 (CH), 115.3 (CH),
2.1 (CH ) ppm. IR (KBr): ν˜ = 3061 (br), 1705 (m), 1600 (s), 1559
s), 1478 (w), 1448 (m), 1400 (s), 1314 (m), 1281 (m), 1250 (m),
175 (m), 1158 (m), 1070 (m), 1048 (m), 1024 (m), 936 (w), 916
2 3
) ppm. C NMR (100 MHz, CDCl ): δ = 173.4 (CO),
(
(
1
(
(
CH), 109.3 (C), 42.2 (CH
w), 1595 (m), 1510 (m), 1456 (m), 1424 (m), 1399 (w), 1324 (w),
311 (m), 1271 (m), 1220 (m), 1163 (w), 1097 (w), 1021 (m), 997
2
) ppm. IR (KBr): ν˜ = 3426 (m), 2916
2
(
1
2
–1
w), 929 (w), 903 (w), 845 (w), 772 (s), 736 (s), 665 (w) cm . HRMS
+
12 3
ESI): m/z calcd for C14H N Br [M ] 302.169; found 302.029.
–
1
Ligand L3: 2-(Chloromethyl)benzimidazole (1.26 g, 7.55 mmol), 2-
aminothiophenol (0.81 mL, 7.55 mmol), KI (1.25 g, 7.55 mmol)
and KOH (0.43 g, 7.63 mmol) gave L3 (1.20 g, 62.3%) as a pale-
(w), 842 (m), 820 (w), 772 (m), 747 (m), 684 (m) cm .
C H N O S Zn ·0.5CH Cl : calcd. C 57.45, H 4.03, N 6.09;
5
6
46
6
8
2
2
2
2
found C 57.48, H 4.34, N 6.32.
Cu (L2) (OBn) (4): Compound L2 (0.308 g, 1.02 mmol),
Cu(OAc) ·2H O (0.205 g, 1.03 mmol) and C COOH (0.250 g,
.05 mmol). Pale-green solid (0.512 g, 41%). IR (KBr): ν˜ = 2831
w), 2554 (w), 1684 (s), 1601 (m), 1573 (m), 1496 (w), 1453 (m),
400 (s), 1324 (m), 1291 (s), 1178 (m), 1128 (w), 1072 (w), 1026
1
3
yellow solid. H NMR (400 MHz, CDCl
3
): δ = 7.52 (dd, JH,H
=
[
2
2
4
]
3
3
.2 Hz, 2 H, ArH), 7.31 (dd, JH,H = 1.5 Hz, 1 H, ArH), 7.23 (dd,
2
2
6 5
H
3
J
H,H = 3.2 Hz, 2 H, ArH), 7.14–7.10 (m, 1 H, ArH), 6.72 (dd,
3JH,H = 1.2 Hz, 1 H, ArH), 6.66–6.63 (m, 1 H, ArH), 4.21 (s, 2 H,
CH ): δ = 151.7 (C), 148.4 (C),
) ppm. 13C NMR (100 MHz, CDCl
38.5 (C), 136.3 (CH), 130.8 (CH), 122.6 (CH), 119.1 (CH), 116.6
C), 115.4 (CH), 115.0 (CH), 32.9 (CH ) ppm. IR (KBr): ν˜ = 3358
w), 3053 (w), 2743 (w), 1604 (s), 1531 (w), 1477 (s), 1432 (s), 1308
m), 1272 (s), 1227 (m), 1139 (w), 1023 (m), 999 (w), 909 (w), 841
w), 768 (m), 737 (s) cm . HRMS (ESI): m/z calcd for C14
M – H ] 254.338; found 254.075.
Synthesis of ZnII and CuII Complexes
Zn (L1) (OBn) ] (1): A solution of Zn(OAc)
.42 mmol) and C COOH (0.348 g, 2.85 mmol) in methanol
2
(
1
2
3
1
–
1
(
C
w), 933 (m), 843 (w), 805 (w), 745 (m), 684 (m), 667 (m) cm .
Cu ·0.5CH Cl : calcd. C 53.93, H 3.60, N 6.68;
found C 54.07, H 4.10, N 6.51. μeff = 1.86 BM.
(
(
(
(
2
56
H
44Br
2
2
N
6
O
8
2
2
–1
H
13
N
3
S
[Zn(L1) (OAc) ] (5): To a solution of L1 (0.24 g, 1.08 mmol) in
methanol (5 mL) was added Zn(OAc) ·2H O (0.24 g, 1.08 mmol)
and the mixture was stirred at room temperature for 24 h. The solu-
tion was then evaporated under vacuum and the crude product
was recrystallised from dichloromethane/hexane solvent mixture to
2
2
+
[
2
2
[
2
2
4
2 2
·2H O (0.312 g,
1
1
6
H
5
afford complex 5 (0.27 g, 62%) as a pale-yellow solid. H NMR
Eur. J. Inorg. Chem. 2014, 3053–3064
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
(
400 MHz, [D
CH ), 1.8 (s, 6 H, CH
46.04 (60) [M+ – C
Zn·1.5CH Cl
C 52.98, H 4.83, N 11.17.
Zn(L2) (OAc) ] (6): To a solution of L2 (0.13 g, 0.42 mmol) in
methanol (5 mL), was added Zn(OAc) ·2H O (0.092 g, 0.42 mmol)
6
]DMSO): δ = 6.3–7.7 (m, 9 H, ArH), 4.4 (s, 2 H,
reaction by freezing in liquid nitrogen. The quenched aliquots were
+
1
2
3
) ppm. MS (ESI): m/z (%) = 405 (5) [M ],
dissolved in CDCl
ratio of initial monomer concentration to monomer concentration
at time t, [CL] /[CL] , was determined based on the peak intensities
of ε-CL and PCL from the H NMR spectrum.
3
and analysed by H NMR spectroscopy. The
+
3
C
2
H
3
O
2
4 6 4
], 223.11 (15) [M – C H O Zn].
32
H
32
N
6
O
4
2
2
: calcd. C 53.12, H 4.66, N 11.10; found
o
t
1
[51]
The signal
around 4.2 ppm and 4.0 ppm corresponds to ε-CL and PCL,
respectively.
[
2
2
2
2
and the mixture was stirred at room temperature for 24 h. The solu-
tion was then evaporated under vacuum and the crude product
was recrystallised from dichloromethane/hexane solvent mixture to
Gel Permeation Chromatography: The molecular weight (M
w
) and
number average molecular mass (M ) of the polymers were deter-
n
mined by Size Exclusion Chromatography (SEC) at Stellenbosch
University. The SEC instrument consist of a Waters 1515 isocratic
HPLC pump, Waters 717plus autosampler, Waters 600E system
controller (run by Breeze version 3.30 SPA), a Waters in-line De-
gasser AF and a Waters 2414 differential refractometer (operated
at 30 °C) in series with a Waters 2487 dual wavelength absorbance
UV/Vis detector operating at variable wavelength. The polymers
1
afford complex 6 (0.13 g, 62%) as a pale-yellow solid. H NMR
(
(
400 MHz, CDCL
s, 2 H, Ar-H), 7.0 (s, 1 H, Ar-H), 6.5 (s, 2 H, Ar-H), 4.7 (s, 2 H,
), 2.0 (s, 6 H, CH ) ppm. C32 Zn·CH Cl : calcd. C
5.41, H 3.70, N 9.63; found C 45.92, H 3.73, N 9.67.
Cu (L1) (OAc) ] (7): This complex was prepared by dissolving L1
0.18 g, 0.82 mmol) and Cu(OAc) ·2H
3
): δ = 7.6 (s, 2 H, Ar-H), 7.4 (s, 2 H, Ar-H), 7.3
CH
4
2
3
H30Br
2
N
6
O
4
2
2
[
(
2
2
4
O (0.16 g, 0.82 mmol) in were dissolved in BHT-stabilised THF (2 mg/mL), filtered through
2
2
methanol (10 mL) and stirring the mixture for 24 h at room tem-
perature. The resulting precipitate was collected by filtration and
washed with methanol to give 7 (0.22 g, 66%) as a pale-blue solid.
0.45 μm nylon filters and eluted through two sets of PLgel (Polymer
laboratories) 5 μm Mixed-C (300ϫ7.5 mm) column and a precol-
umn (PLgel 5 μm Guard, 50ϫ7.5 mm) at a flow rate of 1 mL/ min.
The column oven was kept at 30 °C and injection volume was
100 μL. THF (HPLC grade stabilised with 0.125% BHT) was used
+
+
ESI (MS): m/z (%) = 404 (20) [M ], 345 (10) [M – C
19CuN (404.91): calcd. C 53.39, H 4.73, N 10.38; found C
3.16, H 4.99, N 10.45. μeff = 2.12 BM.
2 3 2
H O ].
C
18
H
3 4
O
5
as the eluent. Narrow polystyrene standards ranging from 580 to
6
2
ϫ10 g/mol was used for calibration, hence, molecular weights
Single Crystal X-ray Crystallography: Crystallographic data were
collected with a Bruker APEX-II Duo CCD X-ray diffractometer
were measured as polystyrene equivalents.
with Mo-K radiation (λ = 0.7107 Å) at 100(2) K. Suitable crystals Supporting Informationormation (see footnote on the first page of
α
1
1
were selected and mounted in cryoprotectant oil with a 200 micron
cryoloop (MiTEGen) for data collection. Data collection and pro-
cessing employed the standard routines of the APEX-II program
this article): Supplementary Figure S1 represents H– H COSY
spectra of L2; graphs of percentage conversions of monomer to
PCL for catalysts 1–7 and induction periods are shown in Fig-
ures S2 and S3, respectively. A plot of ln[CL] /[CL] vs. time for
[
47]
suit. The data were corrected for absorption effects with SAD-
o
t
[
48]
[49]
ABS. Using Olex2, the structure was solved with the ShelXS
structure solution program by direct methods and refined with the
ShelXL refinement package using cycles of least-squares minimi-
the first and second cycle polymerisation reactions of ε-CL using
complex 5 is given in Figure S4, and Figure S5 represents the H
NMR spectrum of crude polymers obtained from catalyst 5 after
1
[
50]
sation. All non-hydrogen atoms were refined anisotropically; H
atoms involved in hydrogen bonds were located experimentally by
using difference Fourier synthesis and refined isotropically.
8 h and 32 h.
Acknowledgments
CCDC-986363 (for 2), -986364 (for 4), and -986365 (for 6) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
The authors would like to thank the University of KwaZulu-Natal
for financial support.
EPR Experiments: EPR spectra were recorded with a Bruker
EMX-plus X-band spectrometer at 295 K. Solid crystalline 4 was
ground into a fine powder and the spectrum was recorded by using
the following parameters: microwave frequency: 9.870185 GHz;
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim