Synthesis, magnetic behaviour, and X-ray structures of dinuclear copper complexes with multiple bridges. Efficient and selective catalysts for polymerization of 2,6-dimethylphenol

Article abstract of DOI:10.1039/b618559b

The use of a potentially tridentate mono-anionic bridging ligand, 1,3-bis(3,5-dimethylpyrazol-1-yl)propan-2-ol (bdmpp-H), in assembling new dimeric copper complexes with interesting magnetic properties has been investigated. The reaction of copper hydroxide or copper acetate with phenyl phosphinic acid or diphenyl phosphinic acid in the presence of bdmpp-H produces the dinuclear complexes [Cu(bdmpp)(ppi)]₂ (1) and [Cu(bdmpp)(dppi-H)]₂(dppi)₂ (2) (ppi-H = phenylphosphinic acid; dppi-H = diphenylphosphinic acid), respectively. The products have been characterized with the help of analytical, thermal, and spectroscopic (IR, UV-vis, and EPR) techniques. Single crystal X-ray diffraction studies of 1 and 2 reveal that the two bdmpp ligands hold together the dimeric copper unit in each complex through -O alkoxide and the pyrazolyl nitrogen ligating centers. Two phenyl phosphinate ligands additionally bridge the dicopper core in 1 to result in octahedral coordination geometry around each metal ion. The diphenyl phosphinic acid acts as a terminal ligand in 2, and thus completes a square pyramidal geometry around each copper ion. Both complexes show a very short Cu...Cu separation (3.001 and 3.065 A for 1 and 2, respectively). The investigation of the magnetic properties reveals the efficiency of the double alkoxide bridge between the two paramagnetic copper ions to mediate strong antiferromagnetic interactions [J/k_B = -620(5) K (-431(4) cm ^-1) and -685(5) K (-476(4) cm^-1) for 1 and 2, respectively]. Compounds 1 and 2, along with a few other copper phosphate complexes, were tested as catalysts for the oxidative polymerization of 2,6-dimethylphenol; 1 and 2 were found to be efficient catalysts with an increased selectivity for the formation of the polyphenylene ether. However a related mononuclear octahedral copper complex [Cu(imz)₄(dtbp) ₂] (dtbp-H = di-tert-butylphosphate) was found to be more efficient. The Royal Society of Chemistry.

Full text of DOI:10.1039/b618559b

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Synthesis, magnetic behaviour, and X-ray structures of dinuclear copper
complexes with multiple bridges. Efficient and selective catalysts for
polymerization of 2,6-dimethylphenol†‡
Ramaswamy Murugavel,*a Ramasamy Pothiraja,a Nayanmoni Gogoi,a Rodolphe Cle´rac,b Lollita Lecren,b
Ray J. Butcherc and Munirathinam Nethajid
Received 20th December 2006, Accepted 21st March 2007
First published as an Advance Article on the web 5th April 2007
DOI: 10.1039/b618559b
The use of a potentially tridentate mono-anionic bridging ligand, 1,3-bis(3,5-dimethylpyrazol-1-yl)-
propan-2-ol (bdmpp-H), in assembling new dimeric copper complexes with interesting magnetic
properties has been investigated. The reaction of copper hydroxide or copper acetate with phenyl
phosphinic acid or diphenyl phosphinic acid in the presence of bdmpp-H produces the dinuclear
complexes [Cu(bdmpp)(ppi)]2 (1) and [Cu(bdmpp)(dppi-H)]2(dppi)2 (2) (ppi-H = phenylphosphinic
acid; dppi-H = diphenylphosphinic acid), respectively. The products have been characterized with the
help of analytical, thermal, and spectroscopic (IR, UV-vis, and EPR) techniques. Single crystal X-ray
diffraction studies of 1 and 2 reveal that the two bdmpp ligands hold together the dimeric copper unit
in each complex through l-O alkoxide and the pyrazolyl nitrogen ligating centers. Two phenyl
phosphinate ligands additionally bridge the dicopper core in 1 to result in octahedral coordination
geometry around each metal ion. The diphenyl phosphinic acid acts as a terminal ligand in 2, and thus
completes a square pyramidal geometry around each copper ion. Both complexes show a very short
˚
Cu · · · Cu separation (3.001 and 3.065 A for 1 and 2, respectively). The investigation of the magnetic
properties reveals the efficiency of the double alkoxide bridge between the two paramagnetic copper
ions to mediate strong antiferromagnetic interactions [J/kB = −620(5) K (−431(4) cm−1) and −685(5)
K (−476(4) cm−1) for 1 and 2, respectively]. Compounds 1 and 2, along with a few other copper
phosphate complexes, were tested as catalysts for the oxidative polymerization of 2,6-dimethylphenol; 1
and 2 were found to be efficient catalysts with an increased selectivity for the formation of the
polyphenylene ether. However a related mononuclear octahedral copper complex [Cu(imz)4(dtbp)2]
(dtbp-H = di-tert-butylphosphate) was found to be more efficient.
around a dicopper core, bridged by either l-oxo or l-hydroxo
Introduction
ligands, the use of multidentate ligands containing alkoxy or
Assembling dimeric, tetrameric, and other oligomeric copper
phenoxy groups in multi-metal compounds has continued to
complexes with the aid of suitably designed polydentate ligands
attract considerable attention among synthetic inorganic chemists
with multiple functional groups has been of great interest in
in recent years.
recent times for a variety of reasons.1 In particular, there have been
We have been interested for a while in the synthesis of a variety
of copper and other transition metal phosphate complexes,4
containing structurally diverse auxiliary ligands, which are useful
as open framework solids or models to framework solids,4a
complexes having interesting magnetic properties,4b and model
compounds for phosphate diester hydrolysis.4c In the present study
we have used a pyrazole based tridentate N2O− ligand, bdmpp,
and synthesized two new l-alkoxo bridged dicopper complexes
with interesting structural, magnetic, and catalytic properties. The
results of this investigation are reported in this paper.
several studies reporting synthesis of dinuclear copper complexes,
with short non-bonded contacts between the metal centers, as
structural models for DNA cleavage reactions through phosphate
ester hydrolysis.2 Dinuclear copper compounds have also been
implicated as active catalysts for the polymerization of substituted
phenols to yield poly(phenylene ether) (PPE).3 While both these
aspects have already been investigated with a variety of ligands
aDepartment of Chemistry, Indian Institute of Technology-Bombay, Powai,
Mumbai, 400076, India. E-mail: rmv@iitb.ac.in; Fax: +91-25723480
bUniversite´ Bordeaux 1; CNRS, Centre de Recherche Paul Pascal-UPR8641,
115 av. du Dr. Albert Schweitzer, 33600, Pessac, France
Results and discussion
cDepartment of Chemistry, Howard University, Washington DC, 20059, USA
Synthesis
dDepartment of Inorganic and Physical Chemistry, Indian Institute of
Science, Bangalore, 560012, India
Methylene bridged (–(CH2)n–) bis-pyrazolyl and bis-imidazolyl
compounds have been used as convenient bridging or chelating
ligands in the coordination chemistry of first row transition metal
ions.5 While a variety of complexes have been synthesized with
† The HTML version of this article has been enhanced with colour images.
‡ Electronic supplementary information (ESI) available: Details of X-ray
structure determination and figures showing the characterization data for
PPE. See DOI: 10.1039/b618559b
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this type of ligand, it is often difficult to predict the type of
structure (e.g. chelate or bridging mode of coordination) a priori
due to the free rotation of the C–C or C–N bonds in the ligand
backbone. In order to evaluate any such conformational changes in
the presence of additional chelating ligands, we have investigated
the reactions of Cu(II) salts with bdmpp-H in the presence of
phosphinic acids. Thus, the reaction between equimolar amounts
of copper hydroxide, phenylphosphinic acid, Ph(H)PO2H and
bdmpp-H results in the formation of [Cu(bdmpp)(ppi)]2 (1) in
good yield (Scheme 1). [Cu(bdmpp)(dppi-H)]2(dppi)2 (2) has been
synthesized by a similar reaction from copper acetate and dppi-H
in the presence of bdmpp-H (Scheme 1).
Molecular structure of 1·CH2Cl2·2H2O. Compound 1 was
◦
crystallized from a CH2Cl2/toluene mixture (1 : 1) at 5 C. Blue
single crystals of 1·CH2Cl2·2H2O were obtained after one week by
slow evaporation of the solvent in air. The copper phosphinate
1 crystallizes in the monoclinic C2/m space group with one
molecule of CH2Cl2 and two molecules of water. The molecule
has a crystallographically imposed 2/m symmetry. In the dimeric
structure of 1, each copper ion is firmly coordinated by two
nitrogen atoms and two oxygen atoms of two bdmpp in a distorted
square-planar environment (Fig. 1). The two phenylphosphinate
anions symmetrically bridge the two copper ions on either side of
the square plane formed by the donor atoms of the bdmpp ligands
to result in a tetragonally elongated octahedral geometry for the
copper ions.
Scheme 1 Syntheses of 1 and 2.
Characterization
Complexes 1 and 2 have been characterized with the aid of
elemental analysis, IR and UV-visible spectroscopic studies
(Table 1). Both the compounds yielded correct elemental analysis.
Strong IR bands observed at 1157, 1132, and 1018 cm−1 for 1
and 1197, 1127 and 1044 cm−1 for 2 indicate the presence of
POO− stretching and M–O–P vibrations. The characteristic P–
H stretching vibration of the phenyl phosphinate in 1 appears as
a strong band at 2284 cm−1. The UV-visible spectra of both 1 and
2 show a strong absorption in the ultraviolet region (∼345 cm−1)
and a weak absorption in the visible region (∼630 cm−1). While
the 345 cm−1 absorption is readily assignable to the p–p* or n–p*
transition arising out of the pyrazole groups in bdmpp, the weak
absorption in the visible region is attributable to the symmetry
forbidden d–d transition of the Cu(II) ion.
Fig. 1 ORTEP of [Cu(ppi)(bdmpp)]2 (1) (hydrogen atoms removed for
clarity).
The central Cu2O2 ring is planar and there are two types of Cu–
˚
O distances in the molecule. The Cu(1)–O(1) distance (1.931(2) A)
˚
is considerably shorter than the Cu(1)–O(2) bond (2.767(4) A) due
to the Jahn–Teller distortion. The Cu(1)–O(1) distance within the
four-membered ring is slightly longer compared to similar Cu–O
distances found in [Cu2(bdpo)2](ClO4)2·2MeOH (bdpo = 1,3-
6
˚
bis(2-benzimidazolyl)-propan-2-oxide) (1.930(2) and 1.923(2) A).
˚
Likewise the Cu(1)–O(2) distance (2.767(4) A) is one of the longest
Cu–O distances known in the literature,4d which can be ascribed to
the combined effect of steric crowding around the copper ions and
the Jahn–Teller elongation along the z-axis of the d9-Cu(II) ion.
Table 1 Comparison of selected structural parameters of dicopper complexes with a Cu2O2 core
Cu–O–Cu/◦
g
J/K
J/cm−1
Reference
˚
Compounds
Cu · · · Cu/A
[Cu(bdmpp)(ppi)]2 (1)
3.001
3.065
3.018(2)
2.990(2)
3.055
3.035(2)
3.033(4)
101.95(1)
103.50(1)
104.3(4)
103.9(2), 98.8(2)
105.4, 104.2
102.6(5)
2.08
2.08
2.25
2.10
—
2.15
2.24
−620(5)
−431(4)
−476(4)
−375
This work
This work
7
6
8
9
9
[Cu(bdmpp)(dppi-H)]2(dppi)2 (2)
−685(5)
−539
[Cu2(N-(2-hydroxymethylphenyl)salicylamide)2](NBu4)2
[Cu2(bdmpp)2][Cu(MeOH)Cl3]2
[Cu(nhep)Cl]2 (Hnhep is 1-(2-hydroxyethyl)-pyrazole)
[PBu4]2[Cu2(N-(2-hydroxyphenyl)salicylamide)2]
[NPr4]2[Cu2(N-(2-hydroxyphenyl)-3-methoxycarbonyl-
4,6-dimethylsalicylamide)2]
−556
−386
<−800 K
−322
<−500
−224
103.7(3)
−272
−189
2406 | Dalton Trans., 2007, 2405–2410
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˚
The Cu · · · Cu distance in 1 (3.001 A) is comparable to those found
in other hydroxo or alkoxo bridged dinuclear complexes,5,7–10 but
is considerably shorter compared to the mono-hydroxo or mono-
alkoxo bridged dinuclear complexes [Cu2(L1)(O2P(OAr))](PF6)2
11
12
˚
˚
(3.773(4) A), [(L2)Cu2(O2P(OCH2Ph)2](ClO4)2 (3.67 A) and
[Cu2(L3)((O2P(OAr))](ClO4)2 (3.779(1) A)13 (HL1 = 2,6-bis[bis(2-
˚
pyridylethyl)aminomethyl]phenol; Ar = 4-nitrophenyl; HL2 = 2,
6-bis[bis(2-benzimidazolylmethyl)aminomethyl]-4-methylphenol),
HL3 = N,N,Nꢀ,Nꢀ-tetrakis{(6-methyl-2-pyridyl)methyl}-1,3-di-
aminopropan-2-ol).
The bdmpp ligands chelate the copper atoms through their
alkoxide oxygen and the pyrazolyl nitrogen to form two six-
membered rings. Thus the core structure is stabilized by the forma-
tion of one four-membered Cu2O2 and four six-membered Cu–O–
C–C–N–N metallocycles (Fig. 1). The intermolecular hydrogen
bonding between the lattice water molecules and phosphinate
oxygen atoms (O–H · · · O 168◦) leads to the formation of a chain-
like structure as shown in Fig. 2.
Fig. 3 ORTEP of the cationic part of [Cu(dppi-H)(bdmpp)]2(dppi)2 (2)
(C–H hydrogen atoms removed for clarity).
the phosphinic acid ligand is coordinated to the metal. While the
phenyl phosphinate in 1 bridges the two copper ions in a symmetric
fashion, the diphenyl phosphinic acid in 2 prefers to coordinate to
the metal in unidentate fashion, that too through its phosphoryl
˚
=
oxygen (P O) (Cu(1)–O(2) 2.292(2) A) on only one side of the
square plane formed by the alkoxide oxygen and pyrazolyl nitrogen
atoms. This leads to a square pyramidal geometry around the
metal, and leaves the complex with a di-positive (+2) charge. For
the charge neutrality, one uncoordinated dppi anion is additionally
present for each copper atom as shown in Fig. 3. It is also of interest
to note that the elongation of the axial site in 2 is considerably
small, compared to 1.
Apart from the fact that the hydrogen atom on O(3) was located
from the difference map and refined, the observed difference in
the P–O distances involving O(2) and O(3)14 (P(1)–O(2) 1.498(2)
Fig.
2
Hydrogen bonding aided polymeric chain formation in
[Cu(ppi)(bdmpp)]2CH2Cl2·2H2O (1).
Molecular structure 2. Compound 2 was crystallized from
methanol at room temperature. Blue single crystals were obtained
after one week by slow evaporation of the solvent. The compound
¯
crystallizes in the triclinic P1 space group with a crystallographi-
cally imposed inversion centre. Each copper ion in 2 is coordinated
by two nitrogen atoms and two oxygen atoms of two bdmpp
ligands in a distorted square-planar environment. A phosphoryl
oxygen atom at the axial position completes the square pyramidal
geometry around each of the copper ions in the molecule. As in
the case of 1, the two copper atoms and two alkoxy oxygen atoms,
which form the central Cu2O2 unit, are lying in the same plane.
There are two types of Cu(1)–O(1) distances in the structure of
2 (Cu–O(1)#1 1.938(2) A and Cu–O(1) 1.966(2) A), which are
longer than those observed for 1. The larger difference in the
internal angles (Cu–O–Cu 103.50(1) and O–Cu–O 76.49(7)◦) and
the distances account for the deviation of the central ring from a
regular square shape. The Cu–O–Cu angle in 2 (103.50(1)◦) is one
of the largest observed for this type of compound.6–10 Although the
˚
and P(1)–O(3) 1.562(2) A), supports the inference that the metal
−
=
is bound to the P O group rather than the P–O terminal in
2 (Fig. 3). Additional support for this result comes from the
observation that the two P–O distances in the uncoordinated
˚
phosphinate are indeed similar (P(2)–O(4) 1.510(2) A and P(2)–
˚
O(5) 1.527(2) A). The phosphinate anions, apart from providing
the charge stabilization, also bridge the dimeric cations through
O–H · · · O hydrogen bonding on one side (O–H · · · O 173.7◦) and
C–H · · · O hydrogen bonding on the other side (C–H · · · O 165.9◦)
as shown in Fig. 4, to form an extended structure. The implications
of the observed structural similarities and differences between the
structure of 1 and 2 on the observed magnetic behaviour of these
complexes are described below.
˚
˚
˚
Cu · · · Cu distance in 2 (3.065 A) is comparable to other dinuclear
complexes with a Cu2O2 core, (including 1), it definitely lies on
the upper side of the range observed for compounds with a Cu2O2
core.
Magnetic properties of 1 and 2. The temperature dependence
of the susceptibility v (where v is M/H) for 1 and 2 are shown
in Fig. 5. The magnetic behaviors of both compounds are very
similar in spite of the observed structural differences. At room
temperature, these compounds possess a very weak paramagnetic
susceptibility that highlights the presence of strong antiferro-
magnetic interaction between copper centers. On lowering the
temperature, the susceptibilities slowly decrease until 150 K where
they reach a minimum. Below 150 K, the susceptibilities increase
Each bdmpp ligand in 2 chelates the two copper atoms through
its alkoxide oxygen and the pyrazolyl nitrogen to form two six-
membered rings as in 1. Thus the core structure is again stabilized
by the formation of one four-membered Cu2O2 and four six-
membered Cu–O–C–C–N–N metallocycles (Fig. 3). The major
difference between the structures of 1 and 2 is the mode in which
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fitted to the following expression:
gimpNl2BSimp(Simp + 1)
v = (1 − q)vdimer + q
+ vdia
(2)
3kBT
where vdia is a fixed diamagnetic contribution calculated from the
Pascal’s constant,16 q is the fraction of paramagnetic impurity
(with gimp = 2 and Simp = 1/2). The experimental data fit very
well to eqn (2) (Fig. 5, solid line) and the best set of parameters
obtained is J/kB = −620(5) K (431(4) cm−1) and −685(5) K
(476(4) cm−1), g = 2.08(1) and 2.08(1), q = 0.010 and 0.015, for 1
and 2, respectively. These results show that these complexes possess
a diamagnetic ground state, i.e. S = 0. Hence the enhancement of
the susceptibility above 150 K is due to the thermal population
of the triplet excited spin state (S = 1). Within this type of Cu(II)
dimer unit, the magnetic interaction is governed by the overlap
Fig. 4 Hydrogen bonding pattern in [Cu(dppi-H)(bdmpp)]2(dppi)2 (2).
2
2
between the dx
orbital of the copper ions and the p orbitals
−y
of the oxygen linkers. Therefore the magnitude of the Cu · · · Cu
interaction depends strongly on the Cu–O–Cu angles and the co-
planarity of Cu–O2–Cu bridge.17 Therefore the contribution of
the O–P–O bridges to J in 1 is difficult to evaluate due to the
difference on the Cu–O–Cu angles between 1 and 2. Nevertheless,
it is important to note that the obtained values of the magnetic
interactions are close to those reported for related compounds
(Table 1).
Polymerization of 2,6-dimethylphenol using 1 and 2 as cata-
lysts. The Cu(II) catalyzed polymerization of 2,6-dimethylphenol
(DMP) leading to the formation of poly(phenylene ether) (PPE),
an industrial high performance thermoplastic, and small amount
of a dimeric product, 4-(3,5-dimethyl-4-oxo-2,5-cyclohexadienyl-
idene)-2,6-dimethyl-2,5-cyclohexadienone (DPQ), has been
known for over four decades.18 Historically, a varying combination
of copper salts and amines viz. tertiary alkylamines, pyridine,
or imidazole derivatives have been employed for the industrial
scale production of PPE.19 Recent mechanistic proposals suggest
a dinuclear copper-phenoxo species as the key intermediate in
the polymerization reaction20 and therefore it is expected that
dinuclear copper complexes like 1 and 2 can be better catalysts
and give higher selectivity in PPE over the dimer DPQ.
Fig. 5 Temperature dependence of the magnetic susceptibility for 1 (ꢀ)
and 2 (᭺) at 1000 G. The solid lines show the best fits obtained. Inset:
Zoom v vs T data in the 100–300 K range for 1 (ꢀ) and 2 (᭺) at 1000 G.
obeying the Curie law. On the basis of the structures described
above, the magnetic properties of these complexes should be
described as a dimer of S = 1/2. The theoretical expression for
the magnetic susceptibility of an antiferromagnetic coupled S =
1/2 dimer is given by the Bleaney–Bowers equation (eqn (1)).15
2Ng2l2
kBT(3 + exp(−B2J/kBT))
vdimer
=
(1)
where N is the Avogadro number, lB is the magnetic moment and
kB is the Boltzmann constant, g is the Lande´ factor of the given
Cu2 complex, and J is the magnetic exchange constant between
paramagnetic ions (S = 1/2) in the dimer (H = −2JS1·S2). A
second superposed magnetic contribution is observed (Fig. 5);
the increase in the susceptibility value in the lowest temperature
region is often seen in these dimer systems (that present a singlet
ground state). This Curie type behavior (second part of eqn (2)) is
associated with the presence of small amounts of paramagnetic
impurities. Therefore the experimental susceptibility has been
The catalytic activity of both the new complexes
[Cu(bdmpp)(ppi)]2 (1) and [Cu(bdmpp)(dppi-H)]2(dppi)2 (2)
in the oxidative polymerization reaction of 2,6-dimethylphenol,
has been investigated (Scheme 2). For the sake of comparison,
we also investigated the catalytic activity of two other related
mononuclear copper complexes previously prepared in our
laboratory.4d The results of polymerization studies carried out are
summarized in Table 2. Both the complexes 1 and 2 are found to
be excellent catalysts yielding high molecular weight PPE along
with a very small amount of undesired DPQ (1% for complex 1
Table 2 Polymerization of 2,6-dimethylphenol catalyzed by different copper complexes
Catalyst
Molar ratio Substrate : Cat.
Time/h
%PPE
%DPQ
Mw
14 700
19 100
271 000
11 900
Mn
7100
10 600
56 000
2320
PDI
[Cu2(bdmpp)2(ppi)]2·CH2Cl2.2H2O
[Cu(bdmpp)(dppi-H)]2(dppi)2
[Cu(imz)4(dtbp)2]
1 : 0.05
1 : 0.05
1 : 0.05
1 : 0.05
5
5
5
5
99.0
97.5
94.6
95.8
1.0
2.5
5.4
4.2
2.0
1.8
4.8
5.1
[Cu(dpp)2(bpy)(H2O)]a
a dpp = diphenylphosphate
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after one week. Yield: 0.35 g (70%). Mp: 189–191 ◦C. Anal. Calcd
for C39H56Cl2Cu2N8O8P2 (Mr = 1024.9): C, 46.52; H, 5.41; N,
11.13. Found: C, 46.0; H, 5.4; N, 11.3. IR (KBr/cm−1): 3400 (br),
3078 (w), 2989 (m), 2929 (m), 2284 (m), 1648 (m), 1552 (s), 1469
(m), 1437 (m), 1396 (m), 1350 (w), 1303 (w), 1263 (w), 1205 (w),
1157 (vs), 1132 (vs), 1096 (m), 1032 (s), 1018 (vs), 994 (s), 912
(s), 805 (w), 753 (m), 705 (m), 646 (m), 629 (w). UV-vis (CH3OH,
nm), (e/cm−1M−1): 348 (3388), 629 (64). TGA: Temp. range/◦C
(% weight loss): 30–212 (10.2); 213–502 (48.9); 503–1188 (15.5).
DSC/◦C: 130 (endo); 223 (exo); 315 (endo).
Scheme 2 Oxidative polymerization of 2,6-dimethylphenol using Cu(II)
catalysts.
and 2.4% for complex 2). Thus the formation of DPQ has been
suppressed significantly while other polymerization methods give
DPQ in 3–5% yield.21 The products obtained have been identified
as PPE by 1H, 13C NMR and IR spectroscopy. The glass transition
temperature (Tg) and thermal decomposition temperature (Td) of
PPE were determined to be 225 ◦C and 417–481 ◦C, respectively22
by TG-DSC measurements. These data are in good agreement
with the values reported for commercially available samples of
PPE.23
Quite surprisingly, in the case of the mononuclear complex
Cu(dtbp)2(imz)4, PPE of a larger molecular weight has been
obtained, as compared to those obtained in the case of the
dinuclear complexes 1 and 2. The observed marginal selectivity
in DMP conversion, when 1 and 2 were used as catalysts, seems to
indicate that a dinuclear copper species, as proposed earlier, could
indeed be a key intermediate in the DMP polymerization reaction.
On the contrary, the isolation of higher molecular weight polymers
when Cu(dtbp)2(imz)4 is used as a catalyst (Table 2) clearly suggests
that further work involving other types of copper complexes is
necessary before any generalization can be arrived at relating to the
nuclearity of the copper complex and the polymerization activity.
We are currently investigating this aspect.
Synthesis of 2. Cu(OAc)2·H2O (199 mg, 1 mmol) was dissolved
in methanol (35 mL) and dppi-H (436 mg, 2 mmol) was added.
To the resulting turbid solution, bdmpp-H (248 mg, 1 mmol)
was added, filtered and kept for crystallization to obtain X-ray
diffraction quality crystals after one week. Yield: 0.67 g (90%). Mp:
196–198 ◦C. Anal. Calcd for C74H80Cu2N8O10P4 (Mr = 1492.5):
C, 59.55; H, 5.40; N, 7.51. Found: C, 58.1; H, 5.5; N, 7.5. IR
(KBr, cm−1): 3397 (br), 3053 (m), 2925 (m), 1690 (m), 1548 (m),
1440 (s), 1398 (m), 1366 (s), 1197 (m), 1127 (s), 1044 (m), 940 (m),
912 (m), 697 (s). UV-vis (CH3OH/nm), (e/cm−1M−1): 344 (6131),
635 (186). TGA: Temp. range/◦C (% weight loss): 50–193 (6.4);
194–302 (39.4); 303–525 (36.5). DSC/◦C: 138 (endo); 203 (endo);
356 (exo).
Magnetic studies. The magnetic susceptibility measurements
were obtained with the use of a Quantum Design SQUID
magnetometer MPMS-XL operating between 1.8 and 300 K. Mea-
surements were performed on finely ground crystalline samples of
1 (26.57 mg) and 2 (20.30 mg). Magnetic data were corrected for
the sample holder.
Experimental
X-Ray structure determination of 1 and 2
The crystals for X-ray diffraction studies for compounds 1 and 2
were grown using the procedures described above. A suitable single
crystal of each compound was used for cell determination and
intensity data collection on a Bruker AXS CCD diffractometer (at
293 K for 1 and 103 K for 2). The structures of both the compounds
were solved using direct methods as implemented in SHELXS-97
and the structures were refined using the full-matrix least square
method on F2 using SHELXL-97. All non-hydrogen atoms were
refined with anisotropic thermal parameters. The hydrogen atoms
were fixed at calculated positions and were refined subsequently
using a riding model. The crystal data and selected details of
the refinement are listed in Table 3. The details of final refined
structures are provided as ESI‡
General
Elemental analyses were performed on a Carlo Erba Model 1106
Elemental Analyzer. Infrared spectra were recorded on a Perkin
Elmer Spectrum One FT-IR spectrometer as KBr diluted discs.
UV-vis spectra were obtained on Shimadzu UV-260 and UV-
160A spectrophotometers. Thermal analyses were carried out on
a Perkin Elmer Pyris Diamond thermal analysis system. The
molecular weight determination of the PPE polymers were carried
out on a Merck La Chrom instrument with a UV detector and PL
gel mixed D column calibrated with linear polystyrene standards
eluting with chloroform.
All reagents, purchased from commercial sources, were used as
received. 3,5-Dimethylpyrazole was prepared from hydrazine and
purified by recrystallization from n-hexane. The synthesis of 1,3-
bis(3,5-dimethylpyrazol-1-yl)propan-2-ol was performed under an
inert atmosphere of argon, but its purification was commonly
performed in air.5 Sodium methoxide was prepared freshly by
reacting sodium with methanol under dinitrogen atmosphere.
CCDC reference numbers 631558 and 631559.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b618559b
Polymerization of 2,6-dimethylphenol
In a typical experiment, the substrate solution is prepared freshly
by mixing a methanol (10 mL) solution of NaOMe (129 mg,
2.4 mmol) and a toluene (20 mL) solution of 2,6-dimethylphenol
(244 mg, 2 mmol). The resulting yellow color solution is added
dropwise to a methanol (25 mL) solution of the complex 2
(118 mg, 0.1 mmol) and stirred at 50 ◦C, under a continuous
current of air until all the phenol has reacted. The reaction
mixture is slowly added to a 100 mL methanol solution containing
Synthesis of 1. Cu(OH)2 (98 mg, 1 mmol) was suspended in
water (20 mL) and bdmpp-H (248 mg, 1 mmol) in MeOH (30 mL)
was added. To the resulting solution, ppi-H (142 mg, 1 mmol) was
added and stirred for 1 h. The solvent was removed under vacuum
and the remaining compound was dissolved in CH2Cl2/toluene
(3 : 1) mixture and filtered. The filtrate was kept at 5 ◦C for
crystallization. X-Ray diffraction quality crystals were obtained
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Table 3 Crystal data for 1 and 2
2 (a) D. S. Sigman, A. Mazumder and D. M. Perrin, Chem. Rev., 2003, 93,
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1
2
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Gamez and J. Reedijk, Catal. Today, 2004, 96, 259.
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C. N. R. Rao, Acc. Chem. Res., 2004, 37, 763; (b) R. Murugavel,
M. Sathiyendiran, R. Pothiraja, M. G. Walawalkar, T. Mallah and
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Sathiyendiran, R. Pothiraja and R. J. Butcher, Chem. Commun., 2003,
2546; (d) R. Murugavel and M. Sathiyendiran, Chem. Lett., 2001, 84;
(e) R. Murugavel, M. Sathiyendiran and M. G. Walawalkar, Inorg.
Chem., 2001, 40, 427; (f) M. Sathiyendiran and R. Murugavel, Inorg.
Chem., 2002, 41, 6404; (g) R. Pothiraja, M. Sathiyendiran, R. J. Butcher
and R. Murugavel, Inorg. Chem., 2004, 43, 7585; (h) R. Pothiraja, M.
Sathiyendiran, R. J. Butcher and R. Murugavel, Inorg. Chem., 2005, 44,
6314; (i) R. Murugavel and S. Kuppuswamy, Angew. Chem., Int. Ed.,
2006, 45, 7022; (j) R. Murugavel, S. Kuppuswamy, R. Boomishankar
and A. Steiner, Angew. Chem., Int. Ed., 2006, 45, 5536–5540.
5 P. Gamez, J. Harra, O. Roubeau, W. L. Driessen and J. Reedijk, Inorg.
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Empirical formula
FW
C39H56Cl2Cu2N8O8P2
1095.74
293(2)
0.71073
Monoclinic
C2/m
16.360(5)
17.733(5)
9.758(5)
—
125.619(5)
—
2301(2)
2
1.581
1.286
C74H80Cu2N8O10P4
1492.42
103(2)
0.71073
Triclinic
¯
P1
Temp/K
Wavelength/A
˚
Crystal system
Space group
˚
a/A
13.206(2)
13.439(2)
13.497(2)
71.321(3)
61.987(2)
69.552(3)
1945.8(5)
1
1.274
0.688
778
0.56 × 0.5 × 0.12
1.65 to 28.41
9035/0/448
1.005
0.0476
0.1249
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
V/A
Z
3
˚
D(calcd)/Mg m−3
Abs. coeff./mm−1
F(000)
1132
Crystal size/mm3
0.53 × 0.29 × 0.07
1.91 to 28.04
2813/0/210
0.825
0.0446
0.1257
h range/◦
Data/restraints/params
Goodness-of-fit on F2
R1 [I > 2r(I)]
R2 [I > 2r(I)]
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Inorg. Chem., 1996, 35, 3409.
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6, 157.
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Inorg. Chim. Acta, 2003, 355, 458.
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Kohzuma, J. Chem. Soc., Dalton Trans., 1989, 2089.
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32, 1633.
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Suzuki, Chem. Commun., 2001, 375.
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Bleaney and K. D. Bowers, Proc. R. Soc. London, Ser. A, 1952, 214,
451.
2 mL concentrated HCl so that the catalyst gets destroyed. The
precipitated polymer is filtered out and washed several times with
methanol. The re-precipitation from a chloroform solution in
methanol yielded the polymer as an off-white powder. Yield =
0.21 g (86%). 1H NMR (400 MHz, CDCl3) d 2.08 (s, 6H), 6.46(s,
2H); 13C NMR (100 MHz, CDCl3): d 16.92, 114.62, 132.71, 145.62,
154.90; IR (KBr) cm−1: 3430, 2922, 1604, 1471, 1305, 1188, 1020,
857.
At the end of the polymerization reaction, 1 mL of the reaction
mixture was diluted to 10 mL with toluene. This solution gave a
UV-visible absorption with a maximum at 421 nm, which was used
to determine the amount of DPQ formed. The molar extinction
coefficient of DPQ at 421 nm is determined to be 54 000 M−1 cm−1.
The amount of DPQ formed is reported as a percentage with
respect to the initial amount of 2,6-dimethylphenol.
16 E. A. Boudreaux and L. N. Mulay, Theory and Applications of
Molecular Paramagnetism, John Wiley & Sons, New York, 1976.
17 (a) P. J. Hay, J. C. Thibeault and R. Hoffmann, J. Am. Chem. Soc.,
1975, 97, 4884; (b) O. Kahn, Molecular Magnetism, Wiley-VCH, New
York, 1993.
Acknowledgements
18 A. S. Hay, H. S. Stafford, G. F. Endres and J. W. Eustance, J. Am. Chem.
This work was supported by the DST, New Delhi (through a
Swarnajayanti Fellowship to RM and a SERC project) and by
the CNRS, the University of Bordeaux 1, the Conseil Re´gional
d’Aquitaine. We also thank the SAIF, IIT-Bombay, for character-
ization data and Prof. Anil Kumar for GPC data. R. P. and N. G.
thank the CSIR, New Delhi for a research fellowship.
Soc., 1959, 81, 6336.
19 (a) A. S. Hay, J. Polym. Sci., Part A: Polym. Chem., 1998, 36, 505; (b) P.
Gamez, C. Simons, G. Aromi, W. L. Driessen, G. Challa and J. Reedijk,
Appl. Catal., A, 2001, 214, 187.
20 P. Gamez, C. Simons, R. Steensma, W. L. Driessen, G. Challa and J.
Reedijk, Eur. Polym. J., 2001, 37, 1293.
21 F. J. Viersen, J. Renkema, G. Challa and J. Reedijk, J. Polym. Sci., Part
A: Polym. Chem., 1992, 30, 901.
22 The Tg and Td correspond to the polymer prepared using 2 as the
catalyst.
References
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2410 | Dalton Trans., 2007, 2405–2410
This journal is
The Royal Society of Chemistry 2007
©