Synthesis and properties of [CuLCl2] and [CuL(N3)(OClO3)] · H2O (L = α,α'-bis(pyrazolyl)-m-xylene): X-ray structure of [CuLCl2]2

Article abstract of DOI:10.1016/S0277-5387(00)00318-1

Using an m-xylyl based dinucleating ligand α,α'-bis (pyrazolyl)-m-xylene (L), providing only one pyrazole nitrogen coordination to each copper(II) centre, two copper(II)complexes [CuLCl₂] (1) and [CuL(N₃) (OClO₃)] · H

Full text of DOI:10.1016/S0277-5387(00)00318-1

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 719–724
Synthesis and properties of [CuLCl2] and [CuL(N3)(OClO3)]PH2O
(Lsa,a9-bis(pyrazolyl)-m-xylene): X-ray structure of [CuLCl2]2
*
Rajeev Gupta, Rabindranath Mukherjee
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India
Received 21 July 1999; accepted 28 January 2000
Abstract
Using an m-xylyl based dinucleating ligand a,a9-bis(pyrazolyl)-m-xylene (L), providing only one pyrazole nitrogen coordination to each
copper(II) centre, two copper(II) complexes [CuLCl2] (1) and [CuL(N3)(OClO3)]PH2O (2) have been synthesized. Complex 1 has been
structurally characterized, revealing that the coordination sphere of each copper(II) ion is distorted square pyramidal. Two pyrazole nitrogens
from two different ligands and two chloride ions form the equatorial plane and an additional weak bonding interaction by a chloride ion from
an adjacent layer provide axial coordination. The dimeric repeat unit [CuLCl2]2 having a Cu2Cl2 bridging unit gives rise to an extended
network. Based on physicochemical studies, a similar structure has been proposed for 2; however, in 2 one azide ion and a coordinating
perchlorate ion are in the equatorial plane. Magnetic susceptibility measurements (25–300 K) reveal that the two copper(II) centres in each
‘pseudo dicopper(II) unit’ are weakly antiferromagnetically coupled, with the relevant 2J values of fy10 cmy1 for 1 and fy9 cmy1 for
2. The square pyramidal stereochemistry of the copper(II) centres is revealed by absorption and EPR spectral properties.
q2000 Elsevier
Science Ltd All rights reserved.
Keywords: a,a9-Bis(pyrazolyl)-m-xylene; Copper(II) complexes; X-ray structure; Network structures; Absorption spectroscopy; Electron spin resonance
1. Introduction
between CuCl2P2H2O and Cu(ClO4)2P6H2O/NaN3 and L.
Here we present the properties of two dimeric copper(II)
complexes [CuIILCl2]2 (1) and [CuII (L)2(N3)2-
During the past few years we have investigated [1–13] the
coordination chemistry of pyrazole-based chelating ligands.
Recently, we have initiated a programme [14–16] to inves-
tigate the reactivity properties of binuclear Cu(I) complexes
with dioxygen and pinpoint the effect of ligand topology on
arene hydroxylation, using m-xylyl based ligands capable of
providing only two N-coordination to each copper site. As
part of this activity, using 4-methyl-2,6-bis(pyrazol-1-yl-
methyl)phenol (HL9) and its 3,5-dimethyl pyrazole deriv-
ative (HL99), we have synthesized and characterized two
diphenoxo-bridged copper(II) complexes [Cu2(L9/L99)2-
(OClO3)2] and the complex [Cu2(L9)2(OClO3)2] has been
structurally characterized. The copper(II) centres in these
complexes are strongly antiferromagnetically coupled
[17].
2
(OClO3)2]P2H2O (2). We also present the molecular
structure of
1 and comparative spectroscopic and
temperature-dependent magnetic properties of 1 and 2.
2. Experimental
With the aim of synthesizing and studying the magnetic
properties of dichloro- and diazido-bridged copper(II) com-
plexes, similar to diphenoxo-bridged copper(II) complexes
[Cu2(L9/L99)2(OClO3)2], we have investigated thereaction
2.1. Materials
All chemicals and reagents were obtained from commer-
cial sources and used as received, unless otherwise stated.
Purification of methanol was achieved by distillation from
Mg(OMe)2. DMF was stored over BaO and distilled under
* Corresponding author: Tel.: q91-512-597437; fax: q91-512-597436;
e-mail: rnm@iitk.ac.in
0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd All rights reserved.
PII S0277-5387(00)00318-1
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inert atmosphere. Dichloromethane was washed thoroughly
with aqueous solution of Na2CO3 followed by distilled water
before final reflux and distillation over anhydrous CaCl2.
2,6-bis(1-bromomethyl)benzene was prepared following a
reported procedure [18].
Anal. Found: C, 36.6; H, 3.4; N, 20.6. Calc. for C14H16-
N7O5ClCu: C, 36.4; H, 3.5; N, 21.3%. IR (KBr, cmy1
,
selected peaks): 2080 (n[N3y]); 1100, 1060, 630, 625
(n[ClO4y]). Absorption spectrum (DMF solution): lmax
(nm) (´ (dm3 moly1 cmy1)) 417 (2200), 655 (290).
2.2. Synthesis of ligand a,a9-bis(pyrazolyl)-m-xylene (L)
2.5. Measurements
The methodology followed to prepare this ligand (which
does not require phase-transfer catalysed conditions such as
that reported [19]) is adapted from Sorrell et al. [20].
Pyrazole (0.26 g, 3.8 mmol) was added to a suspension of
sodium hydride (0.24 g, 10.0 mmol) in DMF (7 cm3), under
a dry dinitrogen atmosphere. The mixture was then stirred
for 1 h. To this was added 2,6-bis(1-bromomethyl)benzene
in DMF (4 cm3) dropwise, anaerobically. The resulting mix-
ture was stirred magnetically for 4 days at 298 K. Addition
of water (5 cm3) followed by solvent evaporation under
reduced pressure afforded a slurry, to which was added 10%
sodium hydroxide solution (5 cm3). The ligandwasextracted
with CH2Cl2 and the organic layer washed with water, fol-
lowed by drying over anhydrous Na2SO4. Solvent removal
under reduced pressure afforded a thick yellowish-brown oil
(yield ca. 90%). 1H NMR (80 MHz, CDCl3): d 7.5 (2H, s,
aromatic protons), 7.3 (2H, d, aromatic protons), 7.10–6.73
(4H, m, aromatic protons), 6.30 (2H, t, aromatic protons),
5.30 (4H, s, PhCH2) [19].
Elemental analysis was obtained from the Indian Associ-
ation for the Cultivation of Science, Calcutta. Solution elec-
trical conductivity measurements were carried out with an
Elico (Hyderabad, India) type CM-82 T conductivitybridge.
Spectroscopic data were obtained using the following instru-
ments: IR spectra, Perkin-Elmer M-1320; electronic spectra,
Perkin-Elmer Lambda 2; X-band EPR spectra, Varian E-109
C; 1H NMR spectra, Bruker WP-80 (80 MHz) NMR spec-
trometer. Variable-temperature (25–300 K) solid-state mag-
netic susceptibility measurements were recorded by the
Faraday technique using a locally built magnetometer. The
setup [21,22] consists of an electromagnet with constant
gradient pole caps (Polytronic Corporation, Mumbai, India),
Sartorius M25-D/S balance (Germany), a closed cycle
refrigerator and a Lake Shore temperature controller (Cryo
Industries, USA). All measurements were made at a fixed
main field strength of f10 kG. Susceptibility was corrected
for diamagnetic contribution using Pascal constants [23].
2.6. Structure determination and refinement
2.3. Synthesis of [CuIILCl2]
A green crystal of [CuLCl2] (dimensions 0.2=0.1=0.05
mm) was used for data collection. Diffracted intensitieswere
collected on an Enraf Nonius CAD4-Mach diffractometer
using graphite-monochromated Mo Ka radiation (ls0.710
A methanolic solution (8 cm3) of L (0.1 g, 0.42 mmol)
was added to a solution of CuCl2P2H2O (0.072 g, 0.42
mmol) in methanol (4 cm3), under magnetic stirring at 298
K and a green mixture resulted. After 2 h, the solution was
filtered to remove some yellow precipitate. On concentration
of the filtrate a dark green crystalline material precipitated
out. The crystalline product was collected by filtration,
washed with methanol and dried in vacuo (yield ca. 64%).
Single crystals suitable for X-ray diffraction studies were
obtained from the filtrate within a day. Anal. Found: C, 45.4;
H, 3.7; N, 15.3; Cl, 19.3. Calc. for C14H14N4Cl2Cu: C, 45.1;
H, 3.8; N, 15.0, Cl 19.1%. Absorption spectrum (DMF solu-
tion): lmax (nm) (´ (dm3 moly1 cmy1)) 434 (560), 945
(160).
˚
73 A). The data were collected at 298 K using u–2u scan
techniques to a maximum 2u value of 45.08. 5810 reflections
were measured of which 2614 were unique and 1376 reflec-
tions with I)3s(I) were used in the structure refinement.
The cell parameters were obtained from least-squares analy-
ses of 25 machine-centred reflections in the range
16.02F2uF27.188. Three standard reflections were meas-
ured at every hour to monitor instrument and crystal stability.
2.6.1. Crystal data
C14H14N4Cl2Cu, Ms372.55, monoclinic, space group
˚
P21/c, as7.797(1), bs22.328(7), cs8.699(9) A, bs
3
y3
2.4. Synthesis of [CuII(L)(N3)(OClO3)]PH2O
101.77(5)8, Vs1482.8(2) A , Zs4, Dcs1.670 g cm
,
˚
ms18.3 cmy1, F(000)s756.
A methanolic solution (8 cm3) of L (0.1 g, 0.42 mmol)
was added to a solution of Cu(ClO4)2P6H2O (0.156 g, 0.42
mmol) in methanol (4 cm3), followed by solid NaN3 (0.027
g, 0.42 mmol). The resulting brownish-green mixture was
stirred for 2 h at 298 K. The diethyl ether was added to the
filtrate, resulting in the formation of a dark green crystalline
material. The product was collected by filtration, washedwith
methanol and dried in vacuo. Recrystallization from MeCN–
(C2H5)2O afforded microcrystalline solid (yield ca. 77%).
Intensity data were corrected for Lorentz and polarization
effects; analytical absorption corrections were applied. The
structures were solved by the direct method and successive
difference Fourier syntheses. All non-H atoms were refined
anisotropically and H atoms were included at their calculated
positions but not refined. All non-hydrogen atoms were
refined with anisotropic thermal parameters. All refinements
were performed by full-matrix least-squares procedures on F
where the function minimized was 8w(FoyFc)2 where
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Table 1
Selected bond distances (A) and angles (8) for [CuLCl2]2
˚
Cu–N(2)
Cu–N(3)
Cu–Cl(1)
Cu–Cl(2)
Cu–Cl(2a)
2.003(6)
1.987(7)
2.281(2)
2.325(2)
2.778(2)
N(2)–Cu–N(3)
Cl(1)–Cu–Cl(2)
Cl(1)–Cu–N(2)
Cl(1)–Cu–N(3)
Cl(2)–Cu–N(2)
Cl(2)–Cu–N(3)
162.8(2)
178.8(1)
89.5(2)
90.1(2)
91.3(2)
89.5(2)
ws1/s(F). The final cycle of refinement included 190 var-
iable parameters and converged with Rs0.046, Rws0.036
(I)3s(I)), Ss1.892, final difference Fourier synthesis:
y0.92FDrF0.88 e Ay3. All calculations were performed
˚
using the XTAL3.2 crystallographic software package [24].
Fig. 1. View of the repeat unit of [CuLCl2]2. Hydrogen atoms are omitted
for clarity. Unlabelled atoms are related to labelled atoms by the crystallo-
graphic centre of inversion.
Selected bond lengths and angles are given in Table 1.
3. Results and discussion
moiety only one chloride ion is engaged in this type of inter-
action. Hence each copper(II) ion is present in a five-coor-
dinate environment. The Cu∆Cu separation between the two
3.1. Synthesis
˚
successive layers (Cu–Cu(a)) is 3.798(2) A and within a
The ligand L was prepared from the reaction between
2,6-bis(1-bromomethyl)benzene and sodium pyrazolate in
DMF. A straightforward reaction between L and CuCl2P
2H2O (1:1 mole ratio) in methanol afforded a dark green
complex of composition [CuLCl2]. A single-crystal struc-
tural analysis (vide infra) revealed its dimeric nature
[CuLCl2]2 (1). A similar reaction using Cu(ClO4)2P6H2O,
L and NaN3 (1:1:1 mole ratio) afforded a brownish-green
solid of composition [Cu(L)(N3)(OClO3)]PH2O. The IR
spectrum of this complex clearly demonstrated the presence
of azide ion [25], coordinated perchlorateion[17]andwater
of crystallization. The electrical conductivitydataofcomplex
1 in DMF (LMs50 Vy1 cm2 moly1) revealedthatitbehaves
as a 1:1 electrolyte (expected range for a 1:1 electrolyte: 65–
90 Vy1 cm2 moly1 [26]), implying that the chloride ions
are partially dissociated in solution, i.e. in solution the cop-
per(II) centre is bound by only one chloride ion with addi-
tional DMF coordination (vide infra). Complex 2 behaves
almost as a 1:2 electrolyte in DMF (LMs130 Vy1 cm2
moly1; expected range 130–170 Vy1 cm2 moly1 [26]),
implying that both the anions are dissociated in solution.
Based on physicochemical properties (vide infra) the azido
complex has been proposed to be a dimer [Cu(L)-
(N3)(OClO3)]2P2H2O (2). Microanalytical, IR(for2),and
absorption spectral behaviour (vide infra) are in conformity
with the suggested formulations for 1 and 2.
˚
layer (Cu–Cu9) is 8.112(3) A. The separation between
˚
Cl(2) and Cl(2a) is 3.20(6) A. The angles Cu–Cl(2)–
Cu(a) and Cu–Cl(2a)–Cu(a) are 86.78(4) and 75.29(3)8,
respectively. The average Cu–N(pyrazole) distance of 1.995
˚
A is in line with the metric parameters of copper(II) com-
plexes of pyrazole-based chelating ligands [1]. The average
˚
Cu–Cl distance in the equatorial plane is 2.303 A. The N(2)–
Cu–N(3) angle is 162.8(2)8, which is appreciably smaller
than the expected 1808 for a linear arrangement of two pyr-
azole nitrogens to a copper centre having a perfect square
planar geometry. However, the Cl(1)–Cu–Cl(2) angle
(178.77(8)8) is close to 1808. From this equatorial plane the
Cu(II) ion is displaced towards a chloride ion of an adjacent
˚
layer by 0.1357 A. The Addison structural distortion index
parameter t [27] is 0.266. Therefore the coordination envi-
ronment for each copper(II) centre is best described as dis-
torted square pyramidal.
3.3. Electronic properties
3.3.1. Absorption spectroscopy
In the solid state (dispersed in mineral oil) compound 1
exhibits a peak at ;680 nm along with a low energy shoulder
at ;950 nm, typical of square pyramidal geometry around
the copper(II) centre [28]. However, in going from solid to
solution (DMF), a red shift of the main absorption band was
observed, which is known to occur as a result of increased
coordination around the metal centre (tetragonal geometry)
[13]. Thus this complex exhibits a d–d transition at 945 nm
with an additional band at 434 nm. The latter band could be
due to Cly™Cu(II) charge transfer. In the solid state, com-
plex 2 also shows a low energy band centred at 705 nm;
however, a blue shift has been observed in the DMF solution
(655 nm). This result implies that for 2 in the solid state a
3.2. Description of the structure of [CuLCl2]2
Fig. 1 provides a perspective view of 1. The dimeric unit
sits on a crystallographically imposed inversion centre. Each
copper(II) centre is coordinated by two pyrazole nitrogens
from two L and two chloride ions. Interestingly, anadditional
bonding interaction exists at 2.778 A, provided by a Cl atom
of an adjacent layer (Cl(2a)). However, from each CuN2Cl2
˚
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‘pseudo dimeric structure’ exists [13], due to weak coordi-
nation of one azide ion of a Cu(II) centre to a nearby Cu(II)
centre. We note that the extent of DMF coordination is more
in 1 than in 2. For 2 the band at 417 nm could be assigned as
N3y™Cu(II) charge-transfer transition.
3.3.2. EPR spectroscopy
In the polycrystalline state (77 K), the complex 1 exhibits
an axial EPR spectrum with g≤s2.171, gHs2.066 and
A≤s105 G. The position of the high-field signal (g)2.04)
2
2
is consistent with a dx yy ground state [28]. In frozen DMF
solution, superhyperfine interaction with chlorine atoms on
the gH component is clearly observable; however, the reso-
lution is not sufficient to extract any meaningful information.
Otherwise, the spectral parameters remain almost unaltered
with slightly diminished signal intensity. Complex 2 is EPR
silent in the solid state both at 300 K and at 77 K. This could
be due to concentration broadening caused by magneticdipo-
lar interaction. However, in frozen DMF solution it exhibits
a typical axial spectrum with g≤s2.396, gHs2.077 and
A≤s120 G.
3.4. Magnetic properties
To study (i) whether any magnetic exchange interaction
exists between two copper(II) centres in 1 and (ii) to sub-
stantiate the proposed structure of 2 (unsymmetrical azide
bridging), we investigated the temperature-dependent (25–
300 K) magnetic susceptibility behaviour. The meff/Cu val-
ues at 300 K and at 25 K vary from 1.83 to 1.60 mB for 1 and
1.85 to 1.68 mB for 2. The magnetic behaviour (Fig. 2) is
typical of a very weak antiferromagnetic interaction. If we
assume that the observed magnetic curve is intrinsic to the
binuclear entity then xM may be expressed as [15–17]
Fig. 2. Plot of magnetic susceptibility (left scale) per dimer and effective
magnetic moment (right scale) per metal vs. temperature for polycrystalline
samples of (a) [CuLCl2]2 (1) and (b) [CuL(N3)(OClO3)]2P2H2O (2).
The solid line represents theoretical fits using Eq. (1), as described in the
text.
2Nb2g2
xMs
[3qexp(y2J/kT)]y1q2Na
(1)
kT
where the symbols have their usual meanings. Keeping the
temperature-independent paramagnetism (2Na) fixed at
120=10y6 cm3 moly1, the parameters J (the singlet–triplet
energy gap isexpressedintermsof2J)andgweredetermined
fixed), usy0.055 K, and R is then equal to 9.37=10y7
,
which corresponds to an excellent agreement between
observed and calculated magnetic data. The very small u
value, however, indicates that intermolecular exchange
effects are essentially non-existent. The observed behaviour
is again pointing towards the weak nature of antiferromag-
netic coupling betwen two copper centres.
obsd
by looking for the minimum of Rs8(xM yxMcalc)2/
8(xMobsd)2. Non-linear regression analysis of the data using
Eq. (1) gave good data fits (Fig. 2): Jsy4.9 cmy1
gs2.00, Rs1.45=10y6 for 1 and Jsy4.4 cmy1
,
,
gs2.103, Rs1.60=10y6 for 2. Inclusion of a corrective
term for non-coupled copper(II) impurity gave no improve-
ment in the fit.
In the solid state of compound 1 (Fig. 1) one Cl atom at
each Cu site occupies the fifth coordination of the Cu centre
of an adjacent layer, resulting in a long intermolecular
Cu∆Cl contact. However, the optimized 2J parameter for
this compound does not follow the empirical Hodgson cor-
relation between structural parameter f/R and the coupling
constant J [30,31]. Here f is the Cu–Cl(2)–Cu(a) bridging
angle and R is the longer Cu–Cl(2a) separation. The dihedral
angle between the planes Cu–Cl(2)–Cu(a) and Cu–Cl(2a)–
Cu(a) is 87.928, revealing that the four-membered ring Cu–
Incorporation of a corrective term (u) for interdimer inter-
actions [29] in 1 and hence using Eq. (2),
2Nb2g2
xMs
[3qexp(y2J/kT)]y1q2Na
(2)
k(Tyu)
a similar data analysis gave rise to the following parameters:
Jsy4.67 cmy1, gs1.977 (2Nas120=10y6 cm3 moly1
,
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R. Gupta, R. Mukherjee / Polyhedron 19 (2000) 719–724
723
Cl(2)–Cu(a)–Cl(2a) is non-planar. Thiscouldbethereason
behind its deviation from this empirical correlation.
parameters for non-hydrogen atoms as well as hydrogenatom
parameters are available as supplementary material from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: q44-1223-336033; e-mail:
deposit@ccdc.cam.ac.uk), on request (CCDC deposition
number 129652).
With asymmetric end-to-end azido bridges, with a short
and a long Cu–N(azide) bond, it is well documented that the
magnetic interaction is negligible when the geometry around
the copper(II) ion issquarepyramidal[32,33].Theobserved
magnetic behaviour of 2 is in line with this bonding interac-
tion. In fact, in 2 the geometry around the copper centre is
square-based pyramidal (vide absorption spectra). Two
nitrogen atoms from two ligands are bonded to the Cu(II)
ion; one oxygen atom from a perchlorate ion and a bridging
azide ion are coordinated in the basal plane. The two cop-
per(II) atoms are linked via one end-to-end bridging azide.
The situation is very similar to what we have observed in 1.
Thus the structure of compound 2 can be proposed as shown
below.
Acknowledgements
This work is supported by grants from the Council of
Scientific and Industrial Research (CSIR) and Department
of Science and Technology (DST), Government of India,
New Delhi. R.G. gratefully acknowledges the award of SRF
by CSIR. The financial assistance of the National X-ray dif-
fractometer facility by DST at this department is gratefully
acknowledged.
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(OClO3)]2P2H2O have been synthesized. The presence of a
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StyleTag -- Journal: POLY (Polyhedron) Article: 3394