Structural diversity and properties of a series of dinuclear and mononuclear copper(II) and copper(I) carboxylato complexes

Article abstract of DOI:10.1039/b203334h

The syntheses, crystal structures, magnetic and photoluminescence properties of a series of dinuclear and mononuclear copper(II) and copper(I) N-carbazolylacetate [N-carbazolylacetic acid = Hcabo] with different carboxylato coordination modes are reported. Although the carboxylato group has different coordination modes, the same carboxylate ligand binding to copper ion via four coordinating modes is rare. The crystal (structure of complex [Cu₂(Cabo)₄(dmf)₂]·2dmf (1) consists of a symmetrical dimeric Cu(II) carboxylato paddle-wheel core and oxygen atoms from dmf at the apical positions. The dinuclear complex [Cu)₂(Cabo)₃(phen)₂ClO₄·H ₂O·C₂H₅OH (2) (phen = 1,10-phenanthroline) consists of an unusual dimeric core with two copper atoms bridged by three carboxylates one of which is in the η:η:μ₂ bridging mode and the other two are in the rarer monoatomic bridging mode. To our knowledge, the present bridging mode has not been reported hitherto. Magnetic susceptibilities were measured in the temperature range 2-300 K. It is found that paddle-wheel copper(II) ions in 1 are strongly coupled antiferromagnetically with 2J = 356.4(6) cm^-1, whereas complex 2 shows weak antiferromagnetic interaction with a 2J value of -12.8(4) cm^-1. Copper(I) N-carbazolylacetate with strong fluorescence in the solid state as well as high thermal stability was obtained by reduction of the copper(II) N-carbazolylacetate using PPh₃ (triphenylphosphine) in dmf solution.

Full text of DOI:10.1039/b203334h

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Structural diversity and properties of a series of dinuclear and
mononuclear copper(II) and copper(I) carboxylato complexes
Yu-Peng Tian,*abc Xuan-Jun Zhang,a Jie-Ying Wu,a Hoong-Kun Fun,d Min-Hua Jiang,b
d
Zhi-Qiang Xu,e Anwar Usman,d Suchada Chantraprommay and Laurence K. Thompsone
a
Department of Chemistry, Anhui University, Hefei, Anhui 230039, People’s Republic of China.
E-mail: yptian@mars.ahu.edu.cn; Fax: 86-551-5107342
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100
State Key Laboratory of Coordination Chemistry, Nanjing University, 210093,
b
c
People’s Republic of China
X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM,
d
Penang, Malaysia
Department of Chemistry, Memorial University of Newfoundland, St. John’s,
Newfoundland A1B 3X7, Canada
e
Received (in London, UK) 29th March 2002, Accepted 22nd July 2002
First published as an Advance Article on the web 28th August 2002
The syntheses, crystal structures, magnetic and photoluminescence properties of a series of dinuclear and
mononuclear copper(II) and copper(I) N-carbazolylacetate [N-carbazolylacetic acid ¼ Hcabo] with different
carboxylato coordination modes are reported. Although the carboxylato group has different coordination
modes, the same carboxylate ligand binding to copper ion via four coordinating modes is rare. The crystal
structure of complex [Cu2(Cabo)4(dmf)2]ꢀ2dmf (1) consists of a symmetrical dimeric Cu(II) carboxylato paddle-
wheel core and oxygen atoms from dmf at the apical positions. The dinuclear complex
[Cu2(Cabo)3(phen)2]ClO4ꢀH2OꢀC2H5OH (2) (phen ¼ 1,10-phenanthroline) consists of an unusual dimeric core
with two copper atoms bridged by three carboxylates one of which is in the Z:Z:m2 bridging mode and the other
two are in the rarer monoatomic bridging mode. To our knowledge, the present bridging mode has not been
reported hitherto. Magnetic susceptibilities were measured in the temperature range 2–300 K. It is found that
paddle-wheel copper(II) ions in 1 are strongly coupled antiferromagnetically with 2J ¼ ꢁ356.4(6) cmꢁ1
,
whereas complex 2 shows weak antiferromagnetic interaction with a 2J value of ꢁ12.8(4) cmꢁ1. Copper(I) N-
carbazolylacetate with strong fluorescence in the solid state as well as high thermal stability was obtained by
reduction of the copper(II) N-carbazolylacetate using PPh3 (triphenylphosphine) in dmf solution.
tions in electroluminescent (EL) devices17,18 due to their
Introduction
good hole transporting and luminescent properties. However,
most of the materials are organic compounds or polymers with
low melting points or decomposition temperatures that inhibit
the applications of these materials. As far as we know, no car-
bazole containing metal–organic compound has been crystal-
lographically documented hitherto. We have initiated studies
on N-carbazolylacetic acid with Cu(II) and d10 ions in an
attempt to examine their modes of binding and possible prop-
erties. In the present paper, we report on the syntheses, crystal
structures and properties of a series of complexes with different
carboxylato coordination modes.
The coordination chemistry of copper complexes with various
carboxylates has been investigated for a long time. Dinuclear
copper(II) carboxylates with a pair of copper atoms held by
carboxylato-O,O0 bridges are of interest toward understanding
intramolecular magnetic exchange phenomena and for deriv-
ing magneto-structural correlations.1–16 In general, dinuclear
copper(II) carboxylates exhibit a paddle-wheel cage structure,
but due to the various coordination modes of the carboxylato
ligand, additional interesting structures have been confirmed
by X-ray crystallography. The isolation of mononuclear and
other dinuclear carboxylates has also confirmed the existence
of monodentate and didentate coordinating modes and the
coordination modes are dramatically sensitive to ligand steric
effects, solvent effects, the medium, participation of counter
ions and other factors. Therefore, much work is required to
extend the knowledge of the relevant structural types and
establish proper synthetic strategies leading to the desired
architectures.
Experimental
Materials
The chemicals for the syntheses of the compounds were used as
purchased (reagent grade) without further purification.
It’s well known that the carbazole molecule has relatively
efficient charge transfer properties. Recently, carbazole-con-
taining compounds have been extensively studied for applica-
Physical measurements
Elemental analyses were performed with a Perkin-Elmer
240 instrument. I.R. spectrum was recorded with a Nicolet
FT-IR 170SX instrument (KBr discs) in the 4000–400 cmꢁ1
y Permanent Address: Department of Chemistry, Prince of Songkla
University, Hat Yai, Songhkla 90112, Thailand.
1468
New J. Chem., 2002, 26, 1468–1473
DOI: 10.1039/b203334h
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002

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region. UV-vis spectra were recorded with a UV-265 spectro-
photometer and corrected by subtracting solvent backgrounds.
TGA analyses were recorded with a Perkin-Elmer Pris-1
DMDA-V1 analyzer in a atmosphere of nitrogen at a heating
rate of 5 ꢂC minꢁ1. The photoluminescence measurements were
carried out on powder samples. The spectrum of Cu(I) N-car-
bazolylacetate was collected with a Perkin-Elmer LS50B spec-
trofluorimeter using a 357 nm excitation source. Electrospray
mass spectra (ES-MS) were determined with a Finnigan
LCQ mass spectrograph; the concentration of the samples
was about 1.0 mmol Lꢁ1. The diluted solution was electro-
sprayed at a flow rate of 5 ꢃ 10ꢁ6 L minꢁ1 with a needle vol-
tage of 4.5 kV. The mobile phase was an aqueous solution of
methanol (1:1, v/v). The samples were run in the positive-ion
mode.
analysis were formed from the filtrate after three days standing
at room temperature. Anal. Calc. (found) for C66H46ClCu2-
N7O10H2OC2H5OH: C, 61.70(61.72); H, 4.11 (4.27); N, 7.41
(7.11%). IR (KBr, pellet)
n : nas(COO) 1643.9,
max/cmꢁ1
1600vs; ns(COO) 1401.8vs, 1380vs. UV-vis (dmf solution): l/
nm (log e): 266 (4.89), 292 (4.79), 310.6 (4.03), 326 (4.03) and
339 (4.00)
Cu(Cabo)2phen (3). 1 (0.11 g, 0.2 mmol) was dissolved in
dmf (20 mL) at 90 ꢂC and phen (0.04 g, 0.2 mmol) was added
and stirred for 5 min. After cooling of the solution to room
temperature, well developed black crystals formed. Anal. Calc.
(found) for CuC40H28O4N4 : C, 69.40 (69.02); H, 4.08 (4.35);
N, 8.09 (8.10%). IR (KBr, pellet) nmax/cmꢁ1: nas(COO)
1651vs, 1646.3vs; ns(COO) 1425.6vs. UV-vis (dmf solution).
l/nm (log e): 265.2 (4.77), 282.0 (4.57), 292.4 (4.65), 326.4
(3.92), 340 (3.90)
Room-temperature magnetic susceptibilities were measured
by the Faraday method using a Cahn 7600 Faraday magnetic
balance. Variable temperature magnetic data (2–300 K) were
obtained using
a Quantum Design MPMS5S SQUID
magnetometer using a field strength of 0.1 T. Background
corrections for the sample holder assembly and diamagnetic
components of the complexes were applied. Calibrations were
carried out with a standard palladium cylinder, and tempera-
ture errors were determined with [tmen][CuCl4] (tmen ¼
(CH3)2HNCH2CH2NH(CH3)22+).
Cu(Cabo)(PPh3)2 (4). To a solution of 1 (0.11 g, 0.2 mmol)
in dmf (10 mL) was added PPh3 (0.16 g, 0.6 mmol). After reac-
tion at 100 ꢂC for 20 min, a colorless solution was obtained.
The mixture was cooled to room temperature, and then set
aside for 4 h, during which time colorless prism crystals
formed. These were collected by filtration and washed with
cold dmf and air-dried. Anal. Calc. (found) for
CuC50H40O2NP2 : C, 73.93 (74.23); H, 4.96 (5.18); N, 1.72
(1.97%). IR (KBr, pellet) nmax/cmꢁ1: nas(COO) 1585.6vs,
1434.0vs; ns(COO) 1401.4vs. UV-vis (dmf solution). l/nm
(log e): 266 (4.87), 291.6 (4.55), 325.8 (3.59), 340.2 (3.59)
Preparations
Ligand. The N-carbazolylacetic acid had been synthesized by
several methods before,19,20 but we used a new method to give
the product in a high yield. Carbazole (16.7 g, 0.1 mol) and
NaOH (12 g, 0.3 mol) were dissolved in dimethyl sulfoxide
(40 mL) and heated to 85 ꢂC to yield a dark-brown solution.
After 30 min of stirring, BrCH2COOH (16.68 g, 0.12 mol)
was added in portions during a 30 min time span. The resulting
solution was stirred overnight, and then poured into 400 mL
cold water. After filtration under vacuum, the pH of the filtrate
was adjusted to 3–4, during which time a large quantity of a
white product was precipitated. The mixture was filtered under
vacuum and the product was washed exhaustively with water
and dried in air. Yield: 92%. Anal. Calc. (found) for
C14H11O2N: C, 74.65 (74.38); H, 4.92 (4.93); N, 6.22
(6.20%). ES-MS, m/z 224.3.
X-Ray crystal structure determinations
A green prismatic crystal of complex 1, with approximate
dimensions 0.46 ꢃ 0.40 ꢃ 0.34 mm3, a dark green prismatic
crystal of complex 2, with approximate dimensions
0.40 ꢃ 0.32 ꢃ 0.30 mm3, a black prismatic crystal of complex
3, with approximate dimensions 0.48 ꢃ 0.40 ꢃ 0.40 mm3, and
a colorless prismatic crystal of complex 4 with approximate
dimensions 0.50 ꢃ 0.14 ꢃ 0.14 mm3 were mounted in air.
The data for the complexes were collected on a Siemens
SMART CCD area detector diffractometer equipped with
Mo-Ka radiation using o-scan mode. The data collection cov-
ered over a hemisphere of reciprocal space by a combination of
three sets of exposures; each set had a different f angle (0.88
and 180ꢂ) for the crystal and each exposure of 10 s for 1 and
30 s for 2, 3 and 4 covered 0.3ꢂ in o. The crystal-to-detector
distance was 4 cm and the detector swing angle was ꢁ35ꢂ. Cov-
erage of the unique set is over 99% complete. Crystal decay
was monitored by repeating thirty initial frames at the end
of data collection and analyzing the duplicate reflections and
was found to be negligible.
The unit cell parameters were determined using SMART
(Siemens, 1996a).21 The three sets of data collected were
reduced using the program SAINT (Siemens, 1996b).22 The
structure was solved with direct methods using the program
SHELXTL (Sheldrick, 1997).23 All the non-hydrogen atoms
were located from the trial structure and then refined anisotro-
pically with SHELXTL using a full-matrix least-squares proce-
dure. The hydrogen atoms were geometrically fixed at
calculated positions attached to their parent atoms, and were
treated as riding atoms.
[Cu2(Cabo)4(dmf)2]ꢀ2dmf (1). HCabo (0.45 g, 2 mmol) and
NaOH (0.08 g, 2 mmol) were reacted in water (20 mL) for
30 min at room temperature, and the resulting solution was fil-
tered to remove undissolved HCabo. Copper nitrate (0.24 g, 1
mmol) in water (20 mL) was added with vigorous stirring, the
resulting purple precipitation was filtered under vacuum,
washed exhaustively with water and dried in air. 0.05 g of
the crude product (chemical formula is Cu(Cabo)2ꢀH2O
according to elemental analysis) was dissolved in dmf (10
mL) to yield a green solution that was filtered into a large test
tube. Ethyl acetate (10 mL) was added slowly to form a sepa-
rated layer above the dmf solution. After two days, well devel-
oped air-stable green crystals suitable for X-ray analysis
formed on the sides and bottom of the tube. Anal. Calc.
(found) for Cu2C62H58O10N6ꢀ2C3H7NO: C, 61.85 (62.12); H,
5.50 (5.68); N, 8.49 (8.52%). IR (KBr, pellet) nmax/cmꢁ1
:
nas(COO) 1667.4, 1634.4vs; ns(COO) 1401.8vs. UV-vis (dmf
solution): l/nm (log e): 265 (4.44), 292 (4.61), 326.8 (3.94),
340 (3.96).
[Cu2(Cabo)3(phen)2](ClO4)ꢀH2OꢀC2H5OH (2). 1 (0.11 g, 0.2
mmol) was suspended in ethanol (20 mL) and phen (0.04 g,
0.2 mmol) was added. The mixture was then heated to 80 ꢂC.
After stirring for 30 min, NaClO4(0.038 g, 0.2 mmol) in water
(10 mL) was added. The resultant mixture was refluxed for 30
min, and then filtered. Dark-green crystals suitable for X-ray
The crystal data and data collection parameters are given
in Table 1. Selected bond distances and angles are given in
Table 2.
CCDC reference numbers 181844–181847.
See http://www.rsc.org/suppdata/nj/b2/b203334h/ for
crystallographic data in CIF or other electronic format.
New J. Chem., 2002, 26, 1468–1473
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Table 1 The crystal data and parameters of 1, 2, 3 and 4
Complex
1
2
3
4
Formula
Formula weight
Colour
C
68H72Cu2N8O12
C68H54ClCu2N7O12
1323.71
Dark-green
Monoclinic
P21/c
C40H28CuN4O4
692.20
Black
C50H40CuNO2P2
812.31
Colorless
1320.42
Green
Crystal system
Space group
Monoclinic
P21/c
Orthorhombic
Pbca
Triclinic
¯
P1
˚
a/A
10.1516(1)
13.3088(2)
24.2596(2)
90
14.2574(4)
24.8155(7)
21.5853(5)
90
16.1794(1)
24.1843(1)
16.4207(2)
90
12.2055(1)
13.5557(2)
14.4757(1)
111.532(1)
92.833(1)
109.639(1)
2057.11(4)
2
˚
b/A
˚
c/A
a/ꢂ
b/ꢂ
g/ꢂ
96.108(1)
90
129.630(1)
90
90
90
3
˚
V/A
3259.00(6)
2
0.720
5881.8(3)
4
0.842
6425.21(9)
8
0.730
Z
m/mmꢁ1
0.650
1.311
Dc/g cmꢁ3
F(000)
1.346
1380
1.495
2820
1.431
2856
844
8338
Unique reflections
Reflections observed
7875
22293
14221
39847
7933
42835
13175
0.0698
R
Rw
0.0638
0.1138
0.718, ꢁ0.602
0.1057
0.2616
0.0402
0.0715
0.561, ꢁ0.551
0.1052
0.799,ꢁ0.703
ꢁ3
˚
Drmax, min/e A
1.472, ꢁ1.643
ꢂ
˚
Table 2 Selected bond distances (A) and angles ( ) of the complexes
1
2
3
4
Cu(1)–O(3) 1.96(2)
Cu(1)–O(4) 1.97(3)
Cu(1)–O(2) 1.97(3)
Cu(1)–O(5) 1.99(2)
Cu(1)–O(1) 2.12(3)
Cu(1)–Cu(1)# 2.66(8)
Cu(1)–O(5) 1.94(3)
Cu(1)–O(2) 1.97(2)
Cu(1)–O(3) 2.29(4)
Cu(2)–O(4) 1.95(3)
Cu(2)–O(3) 2.03(2)
Cu(2)–O(2) 2.35(4)
Cu(1)–O(3) 1.91(14)
Cu(1)–O(1) 1.90(12)
Cu(1)–N(2) 2.01(16)
Cu(1)–N(1) 2.02(15)
O(3)–C(40) 1.28(2)
O(4)–C(40) 1.22(2)
Cu(1)–O(2) 2.19(3)
Cu(1)–P(1) 2.23(11)
Cu(1)–O(1 2.24(3)
Cu(1)–P(2) 2.25(11)
O(1)–C(37) 1.26(4)
O(2)–C(37) 1.25(4)
O(3)–Cu(1)–O(4) 88.3(12)
O(3)–Cu(1)–O(2) 89.7(12)
O(4)–Cu(1)–O(2) 167.5(11)
O(3)–Cu(1)–O(5) 167.7(10)
O(4)–Cu(1)–O(5) 89.9(12)
O(5)–Cu(1)–O(2) 91.3(11)
O(5)–Cu(1)–N(7) 172.2(15)
O(2)–Cu(1)–N(7) 93.2(12)
O(5)–Cu(1)–O(3) 89.9(14)
O(2)–Cu(1)–O(3) 83.2(12)
O(3)–Cu(1)–O(1) 100.0(5)
O(3)–Cu(1)–N(2) 170.5(6)
N(2)–Cu(1)–N(1) 81.5(6)
C(26)–O(1)–Cu(1) 128.3(13)
O(1)–Cu(1)–N(1) 166.8(6)
O(2)–Cu(1)–O(1) 60.0(10)
O(2)–Cu(1)–P(1) 113.0(9)
P(1)–Cu(1)–O(1) 115.2(8)
Results and discussion
Description of the structure
Crystal structure of [Cu2(Cabo)4(dmf)2]ꢀ2dmf (1). An
ORTEP diagram of the complex 1 is shown in Fig. 1. The
structure consists of centrosymmetric dinuclear unit. Each
Cu(II) ion has four oxygen atoms of RCOOꢁ groups in equa-
torial positions, and a dmf molecule is bound trans to the
Cu–Cu vector at the apex in a fashion similar to that of the
dinuclear structure of Cu2(RCOO)4L2 .6,24–26 A further two
dmf molecules are loosely held in the lattice.
The individual Cu atom has a Jahn–Teller-distorted, octahe-
dral geometry, with four short Cu–ORCOO (1.963(2)–1.986(2)
˚
˚
A) bond lengths and a long solvent Cu–O(2.121(3) A) bond
length. The two copper atoms in each dimeric unit are sepa-
˚
rated by a distance of 2.662(1) A. The distance is similar to
that found in other Cu carboxylate dimers.11,27
Crystal structure of [Cu2(Cabo)3(phen)2](ClO4)ꢀH2Oꢀ
C2H5OH (2). An ORTEP diagram of the complex 2 with
atom numbering scheme around the copper atoms is shown
in Fig. 2. The crystal of 2 consists of a dinuclear [Cu2(Cabo)3-
(phen)2]+ cation and a well-separated perchlorate anion. The
copper(II) ions are bridged by three carboxylate groups. Two
of the N-carbazolylacetates are in the rare monoatomic
Fig. 1 ORTEP view of complex 1 with 50% thermal ellipsoids show-
ing the atom-labeling scheme around the Cu(1) atom. Hydrogens and
solvent molecules have been omitted; for dmf only coordinated oxygen
atoms are shown.
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Fig. 3 ORTEP view of complex 3 with 50% thermal ellipsoids show-
ing the atom-labeling scheme. Hydrogen atoms have been omitted for
clarity.
the Cu(1), O(1), O(3) plane. Each carboxylate group acts as
monodentate ligand in an anti manner. The monomeric struc-
ture is extended into an infinite two-dimensional network via
the C–Hꢀ ꢀ ꢀO hydrogen bonds and aromatic p–p stacking
Crystal structure of Cu(Cabo)(PPh3)2 (4). As shown in Fig.
4, the copper atom is in a distorted tetrahedral CuO2P2 geome-
try, being coordinated by two P atoms of two PPh3 ligands,
two oxygen atoms of one chelating didentate N-carbazolylace-
tato group. The C(37), C(38), O(1), O(2) and Cu(1) atoms are
˚
almost in one plane (largest deviation: 0.0176 A) which forms a
Fig. 2 ORTEP view of complex 2 with 50% thermal ellipsoids (top)
and the atom-labeling scheme around the Cu atoms (bottom). Hydro-
gen atoms, anion and solvent molecules have been omitted for clarity.
dihedral angle of 78.1ꢂ with the carbazole ring. The three ben-
zene rings of PPh3 are almost perpendicular to each other
(C(1)–P(1)–C(13) 103.65ꢂ, C(1)–P(1)–C(7): 106ꢂ). Although
the formation of a four-membered Cu–O–C–O ring,5,31,32 with
the high angular strains involved, is not expected to be energe-
tically favorable, this type of structure is more stable than that
in which carboxylato group adopts a monodentate mode with
three bulky triphenylphosphine ligands to complete four-
coordination geometry of the Cu(I) ion.
bridging mode, and the third is in the didentate Z:Z:m2 brid-
ging mode; a terminal phen molecule completes the five-
coordination at each copper(II) ion. Fig. 2 (bottom) shows that
the copper atoms are orthogonally bridged to the monodentate
bridging carboxylates and equatorially bridged to the diden-
tate carboxylate and have significantly longer Cu–O bond
˚
˚
distances [Cu(1)–O(3) 2.290 A, Cu(2)–O(2) 2.345 A] than
those of [Cu2(O2CMe)3(bpy)2](ClO4).28 Although dinuclear
copper carboxylates have been investigated extensively, this
kind of bridging mode has not been reported according to
the summary of Psomas et al.29 Since many five-coordinate
structures with geometries, intermediate between regular tri-
gonal bipyramidal and rectangular pyramidal, a method30
has been developed which attempts to quantify the structure.
The relevant angles in [Cu2(Cabo)3(phen)2]+, y1 ¼ 172.31ꢂ
and f1 ¼ 165.09ꢂ; y2 ¼ 172.14ꢂ and f2 ¼ 160.72ꢂ, yield t
values of 0.12 and 0.19. The data indicate geometries closer
to rectangular pyramidal for Cu(1) and Cu(2) in [Cu2(Cabo)3-
(phen)2]+, respectively. The cations of 2 form iꢁnfinite chains as
a result of H-bonding interaction with ClO4 anions and a
water molecule.
Crystal structure of Cu(Cabo)2(phen) (3). An ORTEP dia-
gram of complex 3 with atom numbering scheme is shown in
Fig. 3. The copper(II) ion is coordinated by two oxygen atoms
of two monodentate carboxylates and two nitrogen atoms of a
phen forming a four-coordinate planar geometry. The N(1),
C(1), C(12) and N(2), atoms are almost in one plane (lꢂargest
Fig. 4 ORTEP view of complex 4 with 50% thermal ellipsoids show-
ing the atom-labeling scheme around the Cu atom. Hydrogen atoms
have been omitted for clarity.
˚
deviation: 0.0048 A) which forms a dihedral angle of 9.6 with
New J. Chem., 2002, 26, 1468–1473
1471

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Fig. 6 The magnetic susceptibility of 2. See the text for fitting para-
meters.
Fig. 5 The magnetic susceptibility of 1. See the text for fitting para-
meters.
indicates that the carboxylate constituents play a key role in
this system, and the large ꢁ2J value may be a consequence
of the efficient charge transfer ability of the carbazole moiety.
This is supported by the fact that the carbazole moiety is a
strong electron donor, having been used in nonlinear optical
applications.44
Magnetic susceptibility
The magnetic susceptibility for 1 decreases with decreasing
temperature from 300 to 65 K, below which there is a rapid
increase in magnetic susceptibility as the temperature is low-
ered further to 2 K (Fig. 5), typical of a dinuclear copper(II)
complex
exhibiting
intramolecular
antiferromagnetic
The variable temperature magnetic susceptibility data for 2
are plotted in Fig. 6 along with the line of best fit. On cooling
from room temperature the molar susceptibility value reaches
a maximum at 8 K, and then decreases rapidly, as the tempera-
ture is lower further to 2 K indicating the antiferromagnetic
nature of the complex. The magnetic data were also fitted to
the modified Bleaney–Bowers susceptibility expression37,38
within MAGMUN4.039 to give g ¼ 2.166(1), 2J ¼ ꢁ12.8(4)
exchange.33–35 The large increase in w at very low temperatures
is attributed to a small fraction (r) of a monomeric S ¼ 1/2
Cu(II) spin impurity being present.36 The magnetic data were
fitted to modified Bleaney-Bowers susceptibility expression37,38
(based on the exchange Hamiltonian Hex ¼ ꢁ2J{S1ꢀS2})
within MAGMUN4.039 to give g ¼ 2.245(3), 2J ¼ ꢁ356.4(6)
cmꢁ1, TIP ¼ 71 ꢃ 10ꢁ6 emu molꢁ1, r ¼ 0.0024, y ¼ ꢁ2.15 K
2 1/2
]
(102R ¼ 0.62); R ¼ [S(wobs ꢁ wcalc)2/Swobs
; r ¼ fraction
cmꢁ1
,
TIP ¼ 150 ꢃ 10ꢁ6 emu molꢁ1
,
r ¼ 0, y ¼ ꢁ6
K
(102R ¼ 1.62), as the best fit result. The low magnitude of
ꢁ2J indicates a weak antiferromagnetic interaction in the
[Cu2(RCOO)3]+ core. As the superexchange interaction is
operative only through the carboxylate bridges, the magnitude
of the antiferromagnetic interaction (ꢁ2J) might be expected
to be dependent on the number of bridging carboxylate
ligands.45 An examination of the structure of complex 2 shows
that only one carboxylate bridge links the copper magnetic
paramagnetic impurity, TIP ¼ temperature independent para-
magnetsim, y ¼ Weiss-like temperature correction), as the best
fit result. The solid line in Fig. 5 was calculated with these
parameters. The fitted parameters for 1 are comparable to
those for [Cu(OAc)2H2O]2 [2J ¼ ꢁ296 cmꢁ1, gCu ¼ 2.16 and
r ¼ 0.05],40,41 but exchange is somewhat stronger. Although
the magneto-structural relations of various copper(II) carboxy-
lates with [Cu2(RCOO)4L2 structure have been studied, the
finer details remain obscure. The exchange integral (ꢁ2J)
depends primarily on the following factors: (i) the electron
donating nature of the R substituent of the carboxylate brid-
ging ligands, (ii) the s-donor ability of the axial ligand, and
(iii) the Cu–Cu distances.
2
orbitals directly. Cu(1) and Cu(2) have dx2 ꢁ y ground states,
but connections to O(2) and O(3) (Cu(1)–O(3) 2.290 A,
˚
Cu(2)–O(2) 2.345 A) are long and axially oriented relative to
˚
the magnetic planes, thus creating orthogonal bridging connec-
tions, which would lead to insignificant antiferromagnetic
exchange. The single syn-syn-carboxylate bridge is therefore
associated with the weak net intramolecular exchange situation
(ꢁ2J ¼ 12.8 cmꢁ1), which is quite reasonable.28,46
The ꢁ2J value obtained for 1 is significantly higher than the
average value of 316 cmꢁ1 for a dinuclear compound with
CuO5 coordination in a similar bridging situation.42 A com-
parison of the magneto-structural data on representative syn,
syn-carboxylato bridged dicopper(II) complexes with a [Cu2-
(RCOO)4L2] core5,6,43 has been made in Table 3. All four
dinuclear complexes in Table 3 have the same axial coordinat-
ing molecules whereas complex [Cu2(Cabo)4(dmf)2]ꢀ2dmf (1)
has the largest Cu–Cu distance and the largest ꢁ2J value. This
Photoluminescence
The strong emission spectrum of 4 in the solid state at room
temperature with two kinds of emissions is shown in Fig. 7.
The maximum is at ca. 376 nm with a shoulder at ca. 390
nm. The other emission is at ca. 415 nm with a shoulder at
439 nm (357-nm excitation), which can be assigned to the exci-
meric emissions due to the weak ligand–ligand interactions
between the molecules in solid state. The emission at 376 nm
can be assigned to the intraligand p* ! p charge transfer simi-
larly to the emission of carbazole in dilute solution.
Table 3 Structural and magnetic parameters of selected dicopper(II)
complexes with a [Cu2(RCOO)m]n+ core (m ¼ 2–4, n ¼ 2–0)
˚
Complex
Cuꢀ ꢀ ꢀCu /A ꢁ2J Ref.
m ¼ 4
Most carbazole compounds emit at wavelengths > 400 nm
in solid state48–50 due to the aromatic p–p stacking. The reason
for the unusual short wavelength emissions for 4 can be
explained according to its structure. As shown by Fig. 4, the
two bulky PPh3 ligands both with three benzene rings almost
perpendicular to one another significantly decrease the symme-
try of the molecule. The strong emission at short wavelength
may be caused by the energy increase resulting from the sym-
metry decrease of 4 compared to that of the ligand and the
high angular strain of the four-membered chelating ring.
Few metal–organic compounds showing intense fluorescence
Cu2(O2CPh)4(dmf)2
Cu2(Indo)4(dmf)2
Cu2(O2CCHCHMe)4(dmf)2
Cu2(Cabo)4(dmf)2
m ¼ 3
2.630
2.630
2.613
2.663
305
286
305
353
5
6
43
This work
[Cu2(O2CC6H4Me-p)3]PF6
m ¼ 2
3.419
81
46
Cu2(O2CMe)2(phen)2(H2O)(NO3)2 3.063
3.103
3.122
86
125
101
11
11
47
Cu2(O2CH)2(phen)2(H2O)(NO3)2
Cu2(O2CEt)2(saln-p-tolyl)2
1472
New J. Chem., 2002, 26, 1468–1473

View Article Online
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Fig. 7 The photoluminescence spectrum of 4 in solid state
or phosphorescence at wavelengths < 450 nm are reported in
the literature.51 Thus complex 4 may be used as an advanced
material for a light emitting diode device owing to its strong
short-wavelength emission and reasonable thermal stability
(no weight loss occurred below 272 ꢂC according to TGA).
In conclusion, a series of N-carbazolylacetates with different
carboxylato coordination modes were synthesized and mag-
netic and/or PL properties have been studied. In this paper,
for the first time, a novel motif of a dinuclear tricarboxylato
copper(II) complex with one Z:Z:m2 bridge and two unusual
monoatomic bridges is reported. Complex 4 shows unusual
emissions in the solid state. The high UV intraligand emission
together with the visible emission arising from intermolecular
ligand–ligand interactions suggest a future in the design of
photoluminescent materials for light-emitter applications by
tailoring their structures.
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Acknowledgements
34 R. J. Doedens, Prog. Inorg. Chem., 1976, 21, 209.
35 B. Bleaney, K. Bowers, Proc. R. Soc, London, Ser. A., 1952, 451.
36 B. N. Figgis and R. L. Martin, J. Chem. Soc., 1956, 3837.
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This work was supported by a grant for State Key Program of
China, the National Nature Science Foundation of China
(29871001; 20071001), and the Malaysian Government and
Universiti Sains Malaysia for research grant R&D No. 305/
PFIZIK/610961. One of the authors, Y. P. T., thanks the Min-
istry of Education of China, and L. K. T. thanks the Natural
Sciences and Engineering Research Council of Canada for
financial support.
38 Z. Xu, L. K. Thompson and D. O. Miller, Inorg. Chem., 1997, 36,
3985.
39 MAGMUN4.0 is available free of charge. It has been developed
by Dr Zhiqiang Xu (zxu@hpower.com) and Kathy Minzhen He
(heminzhen@yahoo.com) in collaboration with Prof. L. K.
Thompson (lthomp@mun.ca), and Dr O. Waldmann (wald-
mann@physik.uni-erlangen.de), who wrote the energy state rou-
tine (Ow01.exe). We do not distribute the source codes. The
programs may be used only for scientific purposes, and economic
utilization is not allowed. If either routine is used to obtain scien-
tific results, which are published, the origin of the programs
should be quoted.
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