Copper(II) halide coordination complexes and salts of 3-halo-2- methylpyridines: Synthesis, structure and magnetism

Article abstract of DOI:10.1016/j.ica.2012.01.050

Two families of compounds have been prepared from the reaction of 3-X′-2-methylpyridine with CuX₂ (X, X′ = Cl, Br) in the presence or absence of HX. Four salts, (3-X′-2-methylpyridinium) ₂CuX₄ were prepared, three of which crystallize in the orthorhombic space group Pbcn while the fourth (X = Br and X′ = Cl) crystallizes in the monoclinic space group C2/c. All four neutral compounds of formula (3-X′-2-methylpyridine)₂CuX₂ crystallize in the space group P1?. Of these, three form weakly bridged chains via long Cu?X contacts, while the fourth (X = X′ = Cl) forms centrosymmetric dimers. Most of the complexes exhibit only very weak antiferromagnetic interactions and can be modeled as weak uniform chains. Compound 4, (3-Cl-2-methylpyridinium)₂CuBr₄, shows behavior that models an isolated 2D-Heisenberg antiferromagnetic layer with J = -5.09(2) K, while compound 7, (3-Cl-2-methylpyridine)₂CuCl₂, crystallizes as a bichloride bridged centrosymmetric dimer with J = -29.31(6) K.

Full text of DOI:10.1016/j.ica.2012.01.050

Inorganica Chimica Acta 389 (2012) 66–76
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Copper(II) halide coordination complexes and salts of
3-halo-2-methylpyridines: Synthesis, structure and magnetism
d
Mahmoud Abdalrahman a, Christopher P. Landee b, Shane G. Telfer c, Mark M. Turnbull a, , Jan L. Wikaira
⇑
a Carlson School of Chemistry and Biochemistry, Worcester, MA 01610, United States
b Dept. of Physics, Clark University, 950 Main St., Worcester, MA 01610, United States
c MacDiarmid Institute for Advanced Materials and Nanotechnology, Institute of Fundamental Sciences, Massey University, Palmerston North, New Zealand
d Dept. of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
a r t i c l e i n f o
a b s t r a c t
Two families of compounds have been prepared from the reaction of 3-X0-2-methylpyridine with CuX2 (X,
X0 = Cl, Br) in the presence or absence of HX. Four salts, (3-X0-2-methylpyridinium)2CuX4 were prepared,
three of which crystallize in the orthorhombic space group Pbcn while the fourth (X = Br and X0 = Cl) crys-
tallizes in the monoclinic space group C2/c. All four neutral compounds of formula (3-X0-2-methylpyri-
Article history:
Available online 7 February 2012
Dedicated to Prof. Jon Zubieta
ꢀ
dine)2CuX2 crystallize in the space group P1. Of these, three form weakly bridged chains via long
Keywords:
Copper halides
Crystal structures
Magnetic measurements
Halopyridine complexes
Cuꢀ ꢀ ꢀX contacts, while the fourth (X = X0 = Cl) forms centrosymmetric dimers. Most of the complexes
exhibit only very weak antiferromagnetic interactions and can be modeled as weak uniform chains. Com-
pound 4, (3-Cl-2-methylpyridinium)2CuBr4, shows behavior that models an isolated 2D-Heisenberg anti-
ferromagnetic layer with J = ꢁ5.09(2) K, while compound 7, (3-Cl-2-methylpyridine)2CuCl2, crystallizes
as a bichloride bridged centrosymmetric dimer with J = ꢁ29.31(6) K.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
the substituted pyridine which serves as an organic base or ligand,
and X is bromide or chloride.
The bulk magnetic properties of transition metal complexes are
dependent upon chemical connectivity, local geometry, and non-
bonding contacts between magnetic species in the crystalline lat-
tice. We have been studying magneto-structural correlations in
copper(II) halide coordination complexes and salts in an attempt
to understand the factors that affect the magnetic superexchange
pathways in the compounds and to derive quantitative structure/
function relationships. There are two common superexchange
pathways in cupric halides:bihalide bridges, where a pair of halide
ions bridge a pair of Cu(II) ions, and the two-halide pathway (Cu–
Xꢀ ꢀ ꢀX–Cu) where non-bonding overlaps between the halide ions
mediate the magnetic exchange. In both cases, there are multiple
bond lengths, angles, dihedral angles, geometric distortions, etc.
that may contribute to the overall pathway [1]. Correlations be-
tween the magnetic properties and the structure of compounds
have continually been drawn [2]. Given the large number of poten-
tial controlling factors, a similarly large number of compounds are
necessary to provide the data needed to attempt any degree of
quantitative correlation. To that end, we have been studying fam-
ilies of Cu(II) coordination complexes and salts of substituted pyr-
idines with the general formulae (AH)2CuX4 or (A)2CuX2 where A is
For both the salts and neutral complexes, the size, shape and
location of substituents on the pyridine rings has a major effect on
the crystal packing. Systems with magnetic interactions that form
dimers [3], chains [4], ladders [5], layers [6] and three-dimensional
systems [7] have all been isolated. We have recently reported work
involving the complexes of CuX2 with 2-X-3-methylpyridine (X = Cl,
Br) where the magnetic behavior of the compounds was success-
fully fit to models for dimers or chains [8]. In the present work,
the relative positions of the halogen and methyl group have been
reversed and we report here the synthesis, structure, and magnetic
behavior of the compounds (3-X0-2-methylpyridinium)2CuX4 (1,
X = Cl, X0 = Br; 2, X = X0 = Br; 3, X = X0 = Cl; 4, X = Br, X0 = Cl) and
(3-X0-2-methylpyridine)2CuX2 (5, X = Cl, X0 = Br; 6, X = X0 = Br; 7,
X = X0 = Cl; 8, X = Br, X0 = Cl).
2. Experimental
3-Bromo-2-methylpyridine was purchased from AK Scientific,
Inc. and 3-chloro-2-methylpyridine was purchased from Synchem
OHG and used without further purification. Copper chloride, cop-
per bromide, HCl and HBr were obtained from VWR and used with-
out further purification. IR spectra were recorded on a Perkin Elmer
Spectrum 100 spectrometer. Elemental analyses were carried out
by Marine Science Institute, University of California, Santa Barbara,
CA 93106, USA.
⇑
Corresponding author.
E-mail address: mturnbull@clarku.edu (M.M. Turnbull).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.01.050

M. Abdalrahman et al. / Inorganica Chimica Acta 389 (2012) 66–76
67
2.1. Synthesis of bis(3-bromo-2-methylpyridinium)
tetrachlorocuprate(II) (1)
Blue crystals were harvested by vacuum filtration, washed with 2-
methyl-2-propanol, and allowed to air dry to give 0.056 g (13%).
Recrystallization from dichloromethane gave X-ray quality crys-
tals. IR (KBr): 3060w, 3016w, 1590m, 1435s, 1060s, 806s, 710m
cmꢁ1. Anal. Calc. for C12H12N2Cl2CuBr2: C, 29.99; H, 2.52; N, 5.83.
Found: C, 30.10; H, 2.59; N, 5.79%.
A solution of 3-bromo-2-methylpyridine (0.300 g, 1.74 mmol)
in 0.5 mL conc. HCl(aq) was added to a solution of copper chloride
(0.117 g, 0.873 mmol) in 2.5 mL of conc. HCl(aq) with stirring. One
mL of 6 M HCl(aq) was added. The resulting green solution was left
to evaporate at room temperature. After several days, orange crys-
tals were harvested by vacuum filtration, washed with 2-methyl-
2-propanol, and allowed to air dry to give 0.158 g (26%). Further
evaporation of the filtrate yielded an additional 0.109 g (17%). IR
(KBr): 3250w, 3142w, 3082m, 3011sh, 2983m, 2886m, 2830s,
2796s, 2698s, 1608m, 1529s, 1459m, 809m, 666m cmꢁ1. Anal. Calc.
for C12H14N2Cl4CuBr2: C, 26.04; H, 2.55; N, 5.06. Found: C, 26.01; H,
2.61; N, 5.05%.
2.6. Synthesis of bis(3-bromo-2-methylpyridine)dibromocopper(II) (6)
A solution of 3-bromo-2-methylpyridine (0.201 g, 1.17 mmol) in
0.5 mL of absolute ethanol was added to a solution of copper bro-
mide (0.129 g, 0.578 mmol) in 2.5 mL of absolute ethanol. A dark
green ppt. formed almost immediately. The powder was harvested
by vacuum filtration, washed with 2-methyl-2-propanol, and al-
lowed to air dry to give 0.252 g (77%). Slow evaporation of the fil-
trate gave X-ray quality crystals. IR (KBr): 3067w, 3011w, 1591m,
2.2. Synthesis of bis(3-bromo-2-methylpyridinium)
tetrabromocuprate(II) (2)
1459s, 1432s, 1062s, 802s, 708m cmꢁ1
12H12N2CuBr4: C, 25.22; H, 2.12; N, 4.90. Found: C, 25.30; H, 2.19;
N, 4.80%.
.
Anal. Calc. for
C
A solution of 3-bromo-2-methylpyridine (0.207 g, 1.17 mmol)
in 0.5 mL conc. HBr(aq) was added to a solution of copper bromide
(0.130 g, 0.583 mmol) in 2 mL of conc. HBr(aq) with stirring. The
resulting dark green solution was left to evaporate at room tem-
perature (turning purple as the solution concentrated). After
2 months, dark purple crystals were harvested by vacuum filtra-
tion, washed with 2-methyl-2-propanol, and allowed to air dry
to give 0.283 g (67%). IR (KBr): 3222w, 3137w, 3076m, 3006sh,
2978m, 2881m, 2839s, 2806s, 2688m, 1608m, 1524s, 1457m,
1346m, 797m, 664m cmꢁ1. Anal. Calc. for C12H14N2CuBr6: C,
19.60; H, 1.92; N, 3.81. Found: C, 19.32; H, 1.80; N, 3.61%.
2.7. Synthesis of bis(3-chloro-2-methylpyridine)dichlorocopper(II) (7)
A solution of 3-chloro-2-methylpyridine (0.201 g, 1.58 mmol) in
0.5 mL of absolute ethanol was added to a solution of copper chlo-
ride (0.106 g, 0.791 mmol) in 2 mL of absolute ethanol. A dark blue
ppt. formed almost immediately. The powder was harvested by vac-
uum filtration, washed with 2-methyl-2-propanol, and allowed to
air dry to give 0.238 g (77%). Recrystallization from 1-propanol gave
X-ray quality crystals. IR (KBr): 3059m, 3013w, 1589m, 1438s,
1086s, 1073s, 807s, 709s cmꢁ1. Anal. Calc. for C12H12N2Cl4Cu: C,
36.99; H, 3.10; N, 7.19. Found: C, 36.28; H, 2.81; N, 6.79%.
2.3. Synthesis of bis(3-chloro-2-methylpyridinium)
tetrachlorocuprate(II) (3)
2.8. Synthesis of bis(3-chloro-2-methylpyridine)dibromocopper(II) (8)
A solution of 3-chloro-2-methylpyridine (0.100 g, 0.783 mmol)
in 0.5 mL conc. HCl(aq) was added to a solution of copper chloride
(0.0525 g, 0.392 mmol) in 1.5 mL of conc. HCl(aq) with stirring. The
resulting green solution was left to evaporate at room temperature.
After 3 weeks, orange crystals were harvested by vacuum filtration,
washed with 2-methyl-2-propanol, and allowed to air dry to give
0.141 g (78%). IR (KBr): 3248w, 3160w, 3087m, 2987m, 2894m,
2832s, 2796s, 2701m, 1622m, 1533s, 1461m, 1384m, 1354m,
1293m, 1089m, 809s, 687s cmꢁ1. Anal. Calc. for C12H14N2Cl6Cu: C,
31.16; H, 3.05; N, 6.06. Found: C, 31.28; H, 2.92; N, 5.90%.
A solution of 3-chloro-2-methylpyridine (0.202 g, 1.59 mmol) in
0.75 mL of absolute ethanol was added to a solution of copper bro-
mide (0.176 g, 0.789 mmol) in 4 mL of absolute ethanol. A dark
green ppt. formed almost immediately. The powder was harvested
by vacuum filtration, washed with 2-methyl-2-propanol, and al-
lowed to air dry to give 0.241 g (64%). Recrystallization from abso-
lute ethanol gave X-ray quality crystals. IR (KBr): 3060w, 3016w,
. Anal. Calc. for
C12H12N2Cl2CuBr2: C, 29.99; H, 2.52; N, 5.83. Found: C, 29.40; H,
2.53; N, 5.57%.
1590m, 1435s, 1060s, 806s, 710m cmꢁ1
2.4. Synthesis of bis(3-chloro-2-methylpyridinium)
2.9. X-ray data collection
tetrabromocuprate(II) (4)
In several cases, X-ray quality crystals formed spontaneously
from the reaction mixture. All compounds could be recrystallized
from absolute ethanol, or 1-propanol. Data collections for com-
pounds 1, 2 and 5 were carried out on a Bruker Apex II diffractom-
A solution of 3-chloro-2-methylpyridine (0.206 g, 1.57 mmol) in
0.75 mL conc. HBr(aq) was added to a solution of copper bromide
(0.175 g, 0.785 mmol) in 2.5 mL of conc. HBr(aq) with stirring. The
resulting dark green solution was left to evaporate at room temper-
ature (turning purple as the solution concentrated). After 2 months,
dark purple crystals were harvested by vacuum filtration, washed
with 2-methyl-2-propanol, and allowed to air dry to give 0.382 g
(75%). IR (KBr): 3241w, 3156w, 3022m, 2989m, 2910m, 2846s,
2818s, 2699m, 1617s, 1531s, 1458w, 1386m, 1352m, 1287m,
1088m, 782s, 681m cmꢁ1. Anal. Calc. for C12H14N2Cl6Cu: C, 31.16;
H, 3.05; N, 6.06. Found: C, 31.28; H, 2.92; N, 5.90%.
eter utilizing Mo K
a radiation (k = 0.71073 Å) and a graphite
monochromator. The data collection, cell refinement, and data
reduction were performed using SHELXTL [9]. Data collections for
compounds 3, 4, 6, 7 and 8 were carried out with a Rigaku–Spider
X-ray diffractometer employing Cu Ka radiation (k = 1.5418 Å) and
a Rigaku MM007 microfocus rotating-anode generator focused
with high-flux Osmic multilayer mirror optics and a curved im-
age-plate detector. CRYSTALCLEAR [10] was utilized for data collection
and FSPROCESS in PROCESS-AUTO [11] for cell refinement and data
reduction. The structures were solved by direct methods and re-
fined via least-squares analysis using SHELX97-2 [12]. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
bonded to N atoms were located in the difference maps and their
positions refined using fixed isotropic U values. Hydrogen atoms
bonded to C atoms were refined using a riding model with fixed
2.5. Synthesis of bis(3-bromo-2-methylpyridine)dichlorocopper(II) (5)
A solution of 3-bromo-2-methylpyridine (0.150 g, 0.872 mmol)
in 1 mL of absolute ethanol was added to a solution of copper chlo-
ride (0.060 g, 0.446 mmol) in 4 mL of absolute ethanol. The result-
ing dark green solution was left to evaporate at room temperature.

68
M. Abdalrahman et al. / Inorganica Chimica Acta 389 (2012) 66–76
isotropic U values. Crystallographic data may be found in Tables 1a
and 1b. Selected bond lengths and angles are given in Table 2.
Hydrogen bonds are given in Table 3.
tetrahedron with a mean trans angle of 131.11(3)° [14] and a mean
Cu–Cl bond length of 2.255(1) Å.
The 3-bromo-2-methylpyridinium ions are hydrogen bonded to
the tetrachlorocuprates via N1–H1ꢀ ꢀ ꢀCl1A (see Table 3) and the
pyridinium rings are planar as expected (mean deviation of con-
stituent atoms = 0.0067 Å). The compound crystallizes in layers
parallel to the A-face of the crystal in an ABBABB pattern, with lay-
ers of CuCl42ꢁ anions separated by double layers of pyridinium cat-
2.10. Magnetic susceptibility data collection
Magnetic data were collected using a Quantum Design MPMS-
XL SQUID magnetometer. Finely ground samples of the crystals
were packed in gelatin capsules. The moment was measured as a
function of field from 0 to 50 kOe at 1.8 K. Several data points were
also collected as the field was brought back to 0 kOe to check for
hysteresis; no hysteresis was observed. In all cases the moment
was linear as a function of field to at least 3 kOe. Temperature
dependant magnetization data was collected in a 1 kOe field from
1.8 to 310 K. The contribution from the sample holder was mea-
sured separately and subtracted from all data sets. A temperature
independent paramagnetism (TIP) correction of 60 ꢂ 10ꢁ6 for cop-
per (II) was applied. Diamagnetic corrections were estimated from
Pascal’s constants [13].
ions (see Fig. 2a). This results in weak
p-stacking interactions
between pyridinium ions in adjacent layers (see Fig. 2b). The inter-
planar distance between the mean plane of one pyridinium ring
and the centroid of the second ring is 3.303(2) Å and the angle be-
tween the rings is 5.3(1)°. The distance between the ring centroids
is 3.682(2) Å and the slip angle is 26.2(1)°.
2ꢁ
The CuCl4 ions pack into chains parallel to the c-axis with
pairs of short, symmetry equivalent Clꢀ ꢀ ꢀCl contacts (Fig. 3, Table 4).
These contacts provide a potential magnetic superexchange path.
The chains are well isolated; the shortest interchain Clꢀ ꢀ ꢀCl con-
tacts are greater than 5.2 Å.
Compound 2 (Fig. 4) is isomorphous with 1. Again, the Cu(II) ions
2ꢁ
sit on a crystallographic twofold axis, however the CuBr4 ion is
3. Results
slightly less flattened with a mean trans angle of 129.55(4)° and a
mean Cu–Br bond length of 2.385(1) Å. The pyridinium rings are
again flat (mean deviation = 0.0086 Å). Compound 2 packs in the
same fashion as 1, with weak pi-stacking interactions between pairs
of 3-bromo-2-methylpyridinium ions, the angle between the ring
planes being 4.3(1)°. The mean distance between planes is
3.354(1) Å, slightly larger than in 1, the distance between the ring
3.1. Synthesis
Reaction of CuX2 with 3-X0-2-methylpyridine in aqueous HX
gave crystals of (3-X0-2-methylpyridinium)2CuX4 in 17–78% yield
(see Scheme 1).
Similarly, reaction of CuX2 with 3-X0-2-methylpyridine in alco-
hol (ethanol or 1-propanol), gave the corresponding neutral com-
plexes (3-X0-2-methylpyridinium)2CuX2 in 13 to 77% yield (see
Scheme 2).
2ꢁ
centroids is 3.755(2) Å and the slip angle is 26.7(1)°. The CuBr4
units are linked into chains similar to that seen in 1 (see Table 4) with
a slightly shorter halideꢀ ꢀ ꢀhalide distance.
Compound 3 is also isomorphous with 1 and 2 (see Fig. 5). The
average Cu–Cl bond length is 2.256(6) Å and the mean trans angle
at the Cu(II) ion is 131.2(1)°, comparable to that seen in 1. The
pyridinium rings are flat (mean deviation = 0.0054 Å) with similar
pi-stacking interactions (angle between adjacent rings = 4.1(1)°;
interplanar distance = 3.299(2) Å; centroid distance = 3.635(2) Å;
slip angle = 24.4(1)°). Halide-halide contacts again link the
3.2. Crystal structure analysis - salts
Compounds 1–3 crystallize in the orthorhombic space group
Pbcn. Fig. 1 shows the molecular unit of 1. The Cu(II) ions sit on a
2ꢁ
crystallographic twofold axis and the CuCl4 ion is a flattened
Table 1a
X-ray data for compounds 1–4.
Compound
1
2
3
4
Empirical formula
Molecular weight
T (K)
C
12H14N2Cl4CuBr2
C12H14N2CuBr6
729.25
113(2)
C12H14N2Cl6Cu
462.49
153(2)
C12H14N2Cl2CuBr4
640.33
148(2)
551.41
113(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
orthorhombic
Pbcn
12.7218(4)
12.5397(4)
11.2824(3)
orthorhombic
Pbcn
12.9660(4)
12.8646(4)
11.5493(3)
orthorhombic
Pbcn
12.6158(9)
12.4437(5)
11.2823(2)
monoclinic
C2/c
13.2998(4)
7.8943(2)
18.5364(15)
103.874(8)
1889.41(17)
4
2.251
14.096
1212
b (°)
V (Å)3
1799.85(9)
4
2.035
6.240
1068
1926.45(10)
4
2.514
13.572
1356
1771.18(15)
4
1.734
10.015
924
Z
Dcalc (Mg/m3)
Absorption coefficient (mmꢁ1
F(000)
)
Crystal size (mm)
hmin, hmax (°)
Index ranges
0.70 ꢂ 0.63 ꢂ 0.30
2.91, 30.0
ꢁ18 6 h 6 18
ꢁ18 6 k 6 18
ꢁ16 6 l 6 16
44212
0.60 ꢂ 0.24 ꢂ 0.17
2.84–25.02
ꢁ15 6 h 6 15
ꢁ15 6 k 6 15
ꢁ13 6 l 6 13
33394
0.24 ꢂ 0.18 ꢂ 0.04
7.02–72.01
ꢁ15 6 h 6 15
ꢁ15 6 k 6 12
ꢁ13 6 l 6 13
12778
0.6 ꢂ 0.4 ꢂ 0.25
6.57–71.84
ꢁ15 6 h 6 16
ꢁ8 6 k 6 9
ꢁ22 6 l 6 21
6935
Reflections collected
Independent reflections
Maximum/minimum transaction factors
Restraints/parameters
2623 [R(int) = 0.0478]
0.2561/0.0972
0/100
1700 [R(int) = 0.0641]
0.2062/0.0451
0/100
1726 [R(int) = 0.0316]
0.6901/0.1973
0/100
1797
1.0/0.61
0/102
Final R1/wR2 [I > 2
Goodness-of-fit (GOF) on F2
Largest peak/hole (eꢁ Åꢁ3
r
(I)]
0.0206/0.0496
1.030
0.677/ꢁ0.392
0.0201/0.0468
1.093
0.612 and ꢁ0.496
0.0239/0.0606
1.205
0.397 and ꢁ0.393
0.0306/0.0779
1.183
0.714 and ꢁ0.552
)

M. Abdalrahman et al. / Inorganica Chimica Acta 389 (2012) 66–76
69
Table 1b
X-ray data for compounds 5–8.
Compound
5
6
7
8
Empirical formula
Molecular weight
T (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (°)
b (°)
C
12H12N2Cl2CuBr2
C12H12N2CuBr4
567.40
153(2)
C12H12N2Cl4Cu
389.58
153(2)
C12H12N2Cl2CuBr2
478.50
153(2)
478.50
113(2)
triclinic
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
ꢀ
P1
P1
P1
P1
4.9557(2)
8.0111(4)
10.1482(5)
92.389(3)
94.321(3)
105.862(3)
385.63(3)
1
5.4616(2)
7.6573(2)
10.1865(7)
96.046(7)
92.495(7)
106.966(7)
403.97(3)
1
8.4573(2)
9.7618(2)
10.3933(7)
64.301(4)
88.062(6)
75.569(5)
746.07(6)
2
5.56120(10)
7.5374(2)
9.9611(7)
98.418(7)
92.143(7)
107.561(8)
392.32(3)
1
c
(°)
V (Å)3
Z
Dcalc (Mg/m3)
Absorption coefficient (mmꢁ1
F(000)
2.060
6.929
231
2.332
13.393
267
1.734
8.541
390
2.025
10.956
231
)
Crystal size (mm)
hmin, hmax (°)
Index ranges
0.70 ꢂ 0.33 ꢂ 0.16
4.04, 54.68
ꢁ6 6 h 6 5
ꢁ10 6 k 6 10
ꢁ13 6 l 6 13
7844
0.30 ꢂ 0.20 ꢂ 0.15
7.05, 60.00
ꢁ6 6 h 6 5
ꢁ8 6 k 6 8
ꢁ11 6 l 6 11
4917
0.30 ꢂ 0.22 ꢂ 0.08
6.86, 61.75
ꢁ9 6 h 6 9
ꢁ11 6 k 6 9
ꢁ11 6 l 6 11
13029
0.76 ꢂ 0.60 ꢂ 0.25
7.08, 60.08
ꢁ6 6 h 6 6
ꢁ8 6 k 6 8
ꢁ10 6 l 6 11
4754
Reflections collected
Independent reflections
Maximum/minimum transaction factors
Restraints/parameters
1740 [R(int) = 0.0381]
0.4036/0.0853
0/89
1173 [R(int) = 0.0551]
0.2386/0.1078
0/89
2234 [R(int) = 0.0295]
1.0/0.64
0/174
1147 [R(int) = 0.1011]
0.1704 and 0.0440
0/89
Final R1/wR2 [I > 2
r
(I)]
0.0329/0.0812
1.094
1.248/ꢁ0.823
0.0490/0.1276
1.104
1.688/ꢁ1.052
0.0306/0.0819
1.037
0.479/ꢁ0.606
0.0585/0.1480
1.066
0.965/ꢁ1.079
Goodness-of-fit (GOF) on F2
Largest peak/hole (eꢁ Åꢁ3
)
*
*
Largest peaks and holes all found near halide ions.
Table 2
Bond lengths (Å) and angles (°) for 1–8.
Bond Lengths
1
2
3
4
5
6
7
8
Cu1–X1
Cu1–X2
Cu1–X1A
Cu1–N1
2.2796(4)
2.2277(4)
2.4054(6)
2.3645(5)
2.2840(6)
2.2289(6)
2.4073(6)
2.3640(6)
2.2532(10)
2.4071(8)
2.3098(8)
2.2547(9)
2.7274(9)
2.048(2)
2.059(2)
2.3980(7)
2.010(3)
1.999(6)
1.995(7)
Cu1–N11
N1–C2/N11–C12
N1–C6/N11–C16
C2–C7/C12–C17
C3–X3/C13–X13
N1–H1
1.345(2)
1.345(2)
1.488(2)
1.8813(18)
0.91(2)
1.340(6)
1.342(6)
1.487(6)
1.878(5)
0.81(5)
1.341(3)
1.341(3)
1.485
1.730(3)
0.86(3)
1.346(6)
1.333(7)
1.495(7)
1.723(4)
0.77(5)
1.350(5)
1.338(5)
1.488(5)
1.896(4)
1.326(9)
1.346(11)
1.499(12)
1.901(8)
1.345(4)/1.349(4)
1.347(4)/1.337(4)
1.492(5)/1.489(5)
1.742(3)/1.741(3)
1.324(10)
1.343(8)
1.477(9)
1.739(9)
Bond Angles
X1–Cu–X2
99.742(16)
127.05(3)
99.833(15)
135.17(3)
100.285(15)
126.31(4)
100.550(16)
132.79(4)
99.65(2)
126.67(4)
99.76(2)
135.88(4)
99.699(19)
123.33(4)
100.141(18)
137.44(5)
100.46(3)
92.41(3)
X1–Cu–X1A
X1–Cu–X2A
X2–Cu–X2A
X1A–Cu–Cl2
X1–Cu–N1
171.92(3)
94.27(8)
171.14(11)
119.8(3)/119.8(3)
90.23(9)
121.1(3)
90.21(18)
121.4(7)
90.19(16)
121.6(7)
N1–Cu–N11
C2–N1–C6/C12–N11–C16
124.28(15)
124.8(4)
124.4(2)
124.6(4)
Table 3
Hydrogen bonding for compounds 1–4.
Compound
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
<(DHA)
Symm. op.
1
2
3
4
N(1)–H(1). . .Cl(1)#1
N(1)–H(1). . .Br(1)
N(1)–H(1). . .Cl1#2
N(1)–H(1). . .Br(1)
0.91(2)
0.81(5)
0.86(3)
0.77(5)
2.18(2)
2.46(5)
2.22(3)
2.48(5)
3.0861(15)
3.237(4)
3.076(2)
3.251(4)
174(2)
161(5)
170(3)
177(4)
#1 ꢁx, y, z + 5/2
#2 ꢁx + 0.5, ꢁy + 0.5, z + 0.5
2ꢁ
tetrahalocuprate units into chains (see Table 4), but with the lon-
gest Xꢀ ꢀ ꢀX separation of compounds 1–3.
twofold axis and the CuBr4 ion is a flattened tetrahedron with a
mean trans angle of 130.4(1)° and a mean Cu–Br bond length of
2.386(1) Å. Unlike the pyridinium rings in 1–3, those of 4 are
nearly perpendicular with an angle of 75.5(1)° between their mean
planes.
Compound 4 is unique among the salts and crystallizes in the
monoclinic space group C2/c. Fig. 6 shows the molecular unit of
4. As was the case for 1–3, the Cu(II) ions sits on a crystallographic

70
M. Abdalrahman et al. / Inorganica Chimica Acta 389 (2012) 66–76
X'
X'
HX
H O
2
CuX
+ CuX
4
2
N
CH3
N
H
CH3
2
Scheme 1. Preparation of salts (1, X = Cl, X0 = Br; 2, X = X0 = Br; 3, X = X0 = Cl; 4,
X = Br, X0 = Cl).
X'
X'
ethanol
CuX
+ CuX
2
2
N
CH3
N
CH3
2
Fig. 3. Chain structure of 1 formed by short intermolecular Cu–Cl contacts parallel
to the c-axis. Viewed parallel to the a-axis.
Scheme 2. Preparation of neutral complexes (5, X = Cl, X0 = Br; 6, X = X0 = Br; 7,
X = X0 = Cl; 8, X = Br, X0 = Cl).
contacts [dBr2ꢀ ꢀ ꢀCl3A = 3.591(3) Å, <Cu1–Br2ꢀ ꢀ ꢀCl3A = 164.7(2)°] are with-
in the sum of their van der Waals radii.
3.3. Crystal structure analysis – neutral complexes
Complexes 5–8 have the general formula CuL2X2 and crystalize
ꢀ
in the triclinic space group P1. For compounds 5, 6 and 8, the Cu(II)
ion is sited on a crystallographic inversion center. The molecular
unit of 5 is shown in Fig. 10. All trans angles are 180° as required
by symmetry and the N1–Cu–Cl1 angle is 90.23(9)°, making the
complex nearly square planar.
The pyridinium ring is planar (mean deviation = 0.0074 Å), and
tilted slightly off linear to the Cu1–N1 bond (<6.7(1)° to the
N1ꢀ ꢀ ꢀC4 vector). The ring is canted relative to the Cu(II) coordina-
tion plane (72.1(1)°). Adjacent molecular units are linked into
chains by long Cuꢀ ꢀ ꢀCl contacts parallel to the a-axis forming the
bihalide bridge seen commonly in CuL2X2 complexes (see Fig. 11
and Table 5), providing a potential magnetic superexchange path-
way. The shortest chlorideꢀ ꢀ ꢀchloride contact distance between
chains is greater than 8 Å, suggesting that the chains should be
magnetically well isolated.
Although there are no close Clꢀ ꢀ ꢀCl contacts between chains,
there are halogen–halide contacts which link the chains molecular
units together (Fig. 12a) parallel to the c-axis, forming layers par-
allel to the B-face of the crystal (Fig. 12b). The Br3ꢀ ꢀ ꢀCl1b distance
is 3.558(1) Å, while the C3–Br3ꢀ ꢀ ꢀCl1b angle is 157.0(2)° and the
Br3ꢀ ꢀ ꢀCl1b–Cu1a angle is 126.8(2)°. These are typical for such hal-
ogen–halide contacts where the angle about the organic halogen
(Br3) is nearer to 180° and the angle about the halide ion is nearer
to 90° [15]].
Fig. 1. Thermal ellipsoid plot of the molecular unit of 1 showing 50% probability
ellipsoids. Only atoms in the asymmetric unit, the copper coordination sphere, and
hydrogen atoms whose positions were refined are labeled.
The compound packs in layers parallel to the ab-face of the crys-
2ꢁ
tal. A similar ABBABB pattern forms with sheets of CuBr4 ions
separated by double layers of the organic cations (Fig. 7).
2ꢁ
The layers of CuBr4 ions map onto a square lattice with each
2ꢁ
CuBr4 ion having four identical nearest neighbors (as a result of
the C-centering) via short Brꢀ ꢀ ꢀBr contacts of 4.18 Å (Fig. 8a). The
next shortest Brꢀ ꢀ ꢀBr contact distance within a layer is greater that
6.6 Å, making any secondary interactions within the layers negligi-
2ꢁ
ble. Adjacent layers of CuBr4 ions are not stacked directly above
each other, but offset parallel to the a-axis (see Fig. 8b). As a result,
the closest Brꢀ ꢀ ꢀBr contacts between layers are 5.664(1) Å, signifi-
cantly longer than the intraplane distance. The three dimensional
structure is stabilized by hydrogen bonds (Table 3) and short
halogen–halogen and halogen–halide contacts (see Fig. 9)
which run parallel to the ac-face diagonal. Both the Clꢀ ꢀ ꢀCl contacts
Compound 6 is isostructural with 5 and the molecular unit is
shown in Fig. 13a. The N1–Cu–Br1 angle is 90.21(18)°, once again
nearly square planar, and the pyridinium ring is again planar
[dCl3ꢀ ꢀ ꢀCl3A = 3.547(3) Å,
<
C3–Cl3ꢀ ꢀ ꢀCl3A = 101.3(2)°] and the Clꢀ ꢀ ꢀBr
(a)
(b)
2ꢁ
Fig. 2. (a) Packing diagram of 1 viewed parallel to the b-axis. Dotted lines represent hydrogen bonds. (b) ABBABB stacking of the CuCl4 and pyridinium ion layers.

M. Abdalrahman et al. / Inorganica Chimica Acta 389 (2012) 66–76
71
Table 4
Halide–halide contacts for compounds 1–4.
Compound
dX. . .X
<Cu1–X1. . .X2B
<X1. . .X2B–Cu1A
<Cu1–X1. . .X2B–Cu1A
1
2
3
4
4.403
4.395
4.430
4.181
130.5
131.1
130.7
156.7
101.3
99.7
101.2
122.2
44.4
67.1
66.7
16.5
Fig. 4. Thermal ellipsoid plot of the molecular unit of 2 showing 50% probability
ellipsoids. Only atoms in the asymmetric unit, the copper coordination plane, and
hydrogen atoms whose positions were refined are labeled.
Fig. 7. Packing of compound 4 viewed parallel to the b-axis. Hydrogen atoms have
been removed for clarity. Dashed lines show short Cl. . .Cl and Cl. . .Br contacts.
via that pathway has been observed, and the Cu1–Br1ꢀ ꢀ ꢀBr1A–
Cu1A torsion angle of 180° is optimal [1]. However, the Cu1–
Br1ꢀ ꢀ ꢀBr1b angles (identical to the Cu1a–Br1Bꢀ ꢀ ꢀBr1 angle by sym-
metry), are approaching 90° (105.4(1)°), too small to expect signif-
icant exchange [1].
The three dimensional structure is further stabilized by
weak halogen–halide contacts (dBrꢀ ꢀ ꢀBr = 3.691(2) Å, \C3–Br3ꢀ ꢀ ꢀBr1b
151.8(2)°, \Br3ꢀ ꢀ ꢀBr1b–Cu1a = 114.6(2)°).
=
The molecular unit of 8 is shown in Fig. 13b. The N1–Cu–Br1 an-
gle is 90.19(16)°, again nearly square planar. The pyridinium ring is
nearly planar (mean deviation = 0.0044 Å) and it is tilted much less
with respect to the Cu1–N1 bond (\2.7(1)° to the N1ꢀ ꢀ ꢀC4 vector)
than observed in 5 or 6. The ring is almost perpendicular to the
Cu(II) coordination plane (\84.5(1)°). As was the case in the bro-
mide compound 6, adjacent molecular units are linked into chains
by very long Cuꢀ ꢀ ꢀBr contacts (>4.6 Å), so any magnetic superex-
change via that pathway is expected to be weak. Similar to 6, there
is a potential bromideꢀ ꢀ ꢀbromide overlap pathway (as in Fig. 14).
Although the Brꢀ ꢀ ꢀBr separation is smaller than in 6 (4.68(1) Å),
the Cu1–Br1ꢀ ꢀ ꢀBr1b angle (105.4°) is again close to 90° and signif-
icant exchange is not expected [1]. The three dimensional structure
is again stabilized by halogen–halide contacts (dClꢀ ꢀ ꢀBr = 3.739(2),
Fig. 5. Thermal ellipsoid plot of the molecular unit of 3 showing 50% probability
ellipsoids. Only atoms in the asymmetric unit, the copper coordination plane, and
hydrogen atoms whose positions were refined are labeled.
\
C3–Cl3ꢀ ꢀ ꢀBr1b = 147.3(2)°, \Cl3ꢀ ꢀ ꢀBr1b–Cu1a = 111.2(2)°).
Compound 7 is unique among the neutral complexes. The com-
Fig. 6. Thermal ellipsoid plot of the molecular unit of 4 showing 50% probability
ellipsoids. Only atoms in the asymmetric unit, the copper coordination plane, and
hydrogen atoms whose positions were refined are labeled.
ꢀ
plex crystallizes in the triclinic space group P1 with two molecules
in the unit cell, situated across a crystallographic inversion center
and forming dimers (Fig. 15). Each Cu(II) ion is nearly square pyra-
midal based on the Continuous Symmetry Measure with values of
SC4v = 0.49 (square pyramidal) compared to SD3h = 5.71 (trigonal
bipyramidal) [16]. The Cu(II) ion sits 0.125(8) Å above the mean
coordination plane. The trans N and Cl angles about the copper
ion are both ꢃ171° (Table 2). The chloride bridges provide an
axial/equatorial link between the Cu ions and the Cu–Cl bond
lengths are 2.31(1) and 2.73(1) Å for the equatorial and axial posi-
tions, respectively. The pyridinium rings are planar with deviations
from the mean plane of 0.0066 and 0.0070 Å, for the N1 and N11
rings, respectively, and nearly coplanar with only a 9.1(1)° angle
between them. The dimers stack parallel to the b-axis (Fig. 16).
(mean dev. = 0.0025 Å). It is tilted similarly (compared to 5) with
respect to the Cu1–N1 bond (\5.4 (1)° to the N1ꢀ ꢀ ꢀC4 vector).
The ring is closer to perpendicular to the Cu(II) coordination plane
(\81.0(1)°) than observed in 5.
Adjacent molecular units are linked into chains by very long
Cuꢀ ꢀ ꢀBr contacts (>4.6 Å), suggesting that any potential magnetic
superexchange via that pathway will be exceedingly weak.
However, there is a second pathway which links the unit together
via halide–halide contacts (Fig. 14). The Brꢀ ꢀ ꢀBr separation of
4.809(1) Å is within the range where weak magnetic exchange

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M. Abdalrahman et al. / Inorganica Chimica Acta 389 (2012) 66–76
(a)
(b)
Fig. 8. (a) One layer of CuBr42ꢁ ions in 4. (b) Two layers of CuBr42ꢁ ions in 4 viewed perpendicular to the layers showing the offset of adjacent layers. CuBr42ꢁ ions in top layer
are linked by dashed lines showing the short Br. . .Br contacts.
Fig. 9. Hydrogen bonds and short contacts between chlorine atoms and bromide
ions in 4.
Fig. 11. Chain formation by intermolecular Cu. . .X contacts in 5.
Table 5
Copper–halide contacts for compounds 5–8.
Compound
dCu1. . .X1B (Å)
<
(°)
Cu1–X1. . .Cu1A
5
6
7
8
4.303
4.634
2.727
4.692
92.8
96.6
92.4
98.1
3.4. Magnetic studies
Susceptibility data were collected for compounds 1–8 from 1.8
to 310 K in a 0.1 T field. The data for compounds 1, 2, 3, 6 and 8
were fit to models for the S = 1/2 uniform antiferromagnetic chain
ˆ
ˆ ˆ
using the Hamiltonian H = ꢁRA,B AB
J
SAꢀSB [17]. Data for compound 5
were fit to the S = 1/2 uniform antiferromagnetic chain model with
a Weiss correction. While the Weiss correction improved the qual-
ity of the fit slightly, the fitted value for h indicates that interchain
interactions are negligible. Data for compound 4 were fit to the
square 2D-Heisenberg antiferromagnetic model, while data for
compound 7 were fit to the model for an isolated dimer. Results
of the fittings are shown in Table 6.
The data for compound 1 are shown in Fig. 18 as a representa-
tive example for compounds 1, 2, 3, 6 and 8. All exhibit very weak
antiferromagnetic interactions. The paramagnetic impurity was
fixed at 1% for fitting purposes (powder X-ray diffraction did not
Fig. 10. Thermal ellipsoid plot of the molecular unit of 5 showing 50% probability
ellipsoids. Only atoms in the asymmetric unit and the copper coordination plane are
labeled.
The closest Cuꢀ ꢀ ꢀCl contact between dimers along the b-axis is
greater than 5.6 Å while the closest Clꢀ ꢀ ꢀCl contacts between Cl1
and/or Cl2 ions is greater than 8.8 Å indicating that the dimers
should be very well isolated magnetically. There are short contacts
between the chlorine atoms on the pyridine rings (dCl3–
Cl13 = 3.65(1) Å) and between the chorine atoms and chloride ions
(d ꢃ 3.7–4.0 Å) that stabilize the 3D structure (Fig. 17).

M. Abdalrahman et al. / Inorganica Chimica Acta 389 (2012) 66–76
73
(a)
(b)
Fig. 12. (a) Halogen–halide contacts in 5. (b) Layer formation in 5 resulting from Cu/halide bibridged chains (parallel to a) and halogen halide contacts (parallel to c).
(a)
(b)
Fig. 13. Thermal ellipsoid plots of the molecular units of 6 (a), and 8 (b) showing
50% probability ellipsoids. Only refined atoms in the asymmetric unit and the
copper coordination sphere are labeled.
Fig. 15. Thermal ellipsoid plot of 7 (50% probability ellipsoids) showing the dimer
structure. Only atoms in the asymmetric unit and the Cu2Cl4 moiety are labeled.
Fig. 14. Bromide–bromide contacts in 6.
Fig. 16. Crystal packing of 7 viewed parallel to the b-axis.

74
M. Abdalrahman et al. / Inorganica Chimica Acta 389 (2012) 66–76
Fig. 18.
line is the fit of the
v
m vs. T (4) and
vmT vs. T (s) plots for 1 in a 1 kOe applied field. The solid
T data to the uniform Heisenberg chain model.
v
Fig. 17. Short chlorine–chlorine and chlorine–chloride contacts between dimers
(dashed lines). Hydrogen atoms have been removed for clarity.
reveal the presence of any impurity in the sample, suggesting an
upper limit for the impurity concentration). The fitted values (Ta-
ble 6) show exchange values ranging from ꢁ0.4 to ꢁ1.8 K.
Compound 5 (Fig. 19) showed slightly stronger interactions as
evidenced by the presence of a maximum in the susceptibility near
3 K.
Magnetic data for compound 4 are shown in Fig. 20. The best fit
to the data was obtained using the 2D-Heisenberg model in agree-
ment with the crystal structure. Fits obtained using a Weiss correc-
tion to account for interlayer interactions showed no change
within experimental error, showing the isolation of the layers.
Data for compound 7 are given in Fig. 21. An excellent fit was
obtained using the model for an isolated magnetic dimer. The qual-
ity of the fit, Curie constant, and exchange value were unchanged
within experimental error when adding a Weiss correction to the
model, suggesting that the dimers are very well isolated in the
lattice.
Fig. 19.
vm vs. T for compound 5 in a 1 kOe applied field. The solid line is the fit to
the uniform Heisenberg chain model with a Weiss correction. The inset shows an
expansion of the region from 0 to 20 K.
4. Discussion
Each of the families, salts and neutral compounds, exhibits a
preferred crystal space group. The majority of the salts (com-
pounds 1–3), crystallize in the orthorhombic space group Pbcn
2ꢁ
with the CuX4 ion located on a center of symmetry and the or-
ganic cations packing between layers of the tetrahalocuprates an-
ions (Fig. 2a). For these complexes, however, the potential
2ꢁ
interactions between the CuX4 within a layer are not uniform.
The magnetic data are fit well by the 1D-Heisenberg uniform chain
Table 6
Fitted magnetic exchange parameters for compounds 1–8.
Compound
C
J
p
h
1
2
3
4
5
6
7
8
0.453(2)
0.411(1)
0.437(2)
0.419(1)
0.419(2)
0.416(2)
0.419(1)
0.413(2)
ꢁ0.415(8)
ꢁ2.35(4)
ꢁ0.437(6)
ꢁ5.09(2)
ꢁ4.98(1)
ꢁ1.80(2)
ꢁ29.31(6)
ꢁ0.612(2)
1
1
1
1.07(7)
2.8(1)
1
2.92(2)
1
ꢁ0.23(4)
Fig. 20.
vm vs. T for compound 4 in a 1 kOe applied field. The solid line is the fit to
C, Curie constant; J, exchange strength.
p, percent paramagnetic impurity; h, Weiss constant.
the 2D-Heisenberg layer model. The inset shows an expansion of the region from 0
to 20 K.

M. Abdalrahman et al. / Inorganica Chimica Acta 389 (2012) 66–76
75
(5.74 Å), but the intraplanar distance is also greater (4.223 Å).
The weaker magnetic exchange seen in 4 (ꢁ5.1 K) compared to
the quinolinium complex (ꢁ6.27 K) demonstrates that the interha-
lide separation is not the only factor controlling the exchange via
the two-halide pathway. Experiments at lower temperatures to
locate the 3D-ordering transition in 4 are in progress.
In the neutral compounds, 5, 6, and 8 all crystallize in the tri-
clinic P-1 space group with the Cu(II) ion on an inversion center.
Unit cell translations provide for weak Cuꢀ ꢀ ꢀX contacts forming
halide bibridged chains parallel to the a-axis (Fig. 11). In the
bromide complexes 6 and 8 a second potential superexchange
pathway is available via the two-halide pathway (Fig. 14).
However, of the three only the chloride complex 5 shows signifi-
cant magnetic interactions (J ꢃ ꢁ5 K) in agreement with the large
Cuꢀ ꢀ ꢀBr separations (>4.6 Å).
ꢀ
Compound 7 also crystallizes in P1, but in this case there are two
molecular units per unit cell, related by an inversion center and
forming a bihalide bridged dimer (Fig. 15). The substantially
shorter Cuꢀ ꢀ ꢀCl distance in the bridge (ꢃ2.7 Å) provides a much
stronger interaction pathway and the magnetic behavior was suc-
cessfully modeled as a dimer with J ꢃ ꢁ30 K. Attempts to model
the data using a Curie–Weiss correction for possible interdimer
interactions did not provide a better fit, supporting the expectation
of weak interactions based on the significant distances between
the Cu2Cl4 units in the crystal. A fundamental difference in local
geometry is likely the cause of the difference in packing. In
compounds 5, 6, and 8 the methyl and halogen substituents on the
pyridine rings are oriented in an anti-fashion (opposite sides of
the copper coordination plane), while in 7 the rings are oriented in
a syn-fashion allowing sufficient room on the opposite face of the
Cu(II) for two molecules to approach each other and form the
dimers. This same relationship between the orientation of the
ligands and the formation of chains or dimers has been reported
previously for the 2-halo-3-methylpyridine complexes [8] where
both orientations are observed. In addition, compounds such as
(2-methylpyridine)2CuBr2 [29] and (2-methylpyridine)2CuCl2 [30]
have syn-configurations for the ligands and show dimer formation,
while the related complexes such as (diethyl nicotinamide)2CuCl2
[31] and (3-(3-hydroxylpropyl)pyridine)2CuBr2 [32], where the
ligands have the anti-configuration, show the formation of
bibridged chains.
Fig. 21. vm vs. T for compound 7. The solid line is the fit of the data to the dimer
model and includes a small paramagnetic impurity (ꢃ3%). The inset shows an
expansion of the region from 6 to 40 K.
model in agreement with the crystal structure which shows short
Xꢀ ꢀ ꢀX contacts propagate in only one dimension. For compounds 1
and 3, even these interactions are vanishingly weak in agreement
with the large chlorideꢀ ꢀ ꢀchloride separation being 4.40–4.43 Å in
both. The copper bromide complex, 2, has a similar separation
(4.395 Å), but the larger van der Waals radius for the bromide
ion provide better orbital overlap at that distance and hence
slightly stronger magnetic exchange is observed (ꢁ2.35(4) K).
The copper bromide compound 4 is unique in this group of
compounds. It crystallizes in the monoclinic space group C2/c.
2ꢁ
Again, the crystal comprises layers of CuBr4 anions with double
layers of pyridinium ions separating the layers, but now the Cu(II)
ions are related by the C-centering, resulting in a square magnetic
lattice. (3-Cl-2-MepyH)2CuBr4 (4) thus becomes the newest mem-
ber of a family of tetrahalocuprate complexes which generate such
square layers. Other reported compounds in the family include (5-
methyl-2-aminopyridinium)2CuCl4 [18], (5-methyl-2-aminopyrid-
inium)CuBr4 [18], (5-chloro-2-aminopyridinium)2CuCl4 [18], (5-
chloro-2-aminopyridinium)CuBr4 [18], (5-bromo-2-aminopyridi-
nium)2CuBr4 [19] (quinolinium)2 CuBr4ꢀH2O [20], (5-bromopyridi-
nium)2CuCl4 [21], (pyridinium)2CuCl4 [22] (4-methyl-2-
aminopyridinium)2CuCl4 [23], (4-aminopyridinium)2CuCl4 [24],
(4-aminopyridinium)2CuBr4 [25], (2-methylimidazolium)2CuCl4
[26], and (1-methylcytosinium)2CuCl4 [27]. Compounds 3 and 4,
the chloride and bromide salts respectively, are not isostructural
which, while somewhat surprising, certainly has precedent having
been seen previously with the (5-bromo-2-aminopyridini-
um)2CuBr4 [19] and (5-bromo-2-aminopyridinium)2CuCl4 [28]. In
such complexes, the degree of isolation of the layers is related to
the strength of magnetic interactions within and between the
layers. In the present compound, the intraplane Brꢀ ꢀ ꢀBr separation
is 4.181 Å while the closest interplanar Brꢀ ꢀ ꢀBr contact is greater
than 5.6 Å. This suggests significantly better isolation than seen
previously in compounds such as (5-bromo-2-aminopyridini-
um)2CuBr4 [19], or (5-chloro-2-aminopyridinium)CuBr4 [18]. In
these compounds the intraplanar separations are 4.39 and 4.35 Å
respectively, both greater than the separation in 4 while the
interplanar distances are 4.85 and 4.83 Å respectively, both much
smaller than observed in 4. This suggests the existence of
very highly isolated layers which agrees with the fact that the
2D-Heisenberg model fits the magnetic data down to the lowest
temperatures with no signs of a transition to a 3D-ordered state.
The most comparably magnetically isolated tetrahalocuprate to
date is the (quinolinium)2 CuBr4ꢀH2O [20] compound. Here the
Further work is in progress to identify additional materials in all
categories, but especially on the layer and halide bibridged dimer
complexes to try to quantify the parameters related to the mag-
netic superexchange and determine the relative contributions of
factors such as the halideꢀ ꢀ ꢀhalide or halideꢀ ꢀ ꢀmetal distances, an-
gles, etc. to the overall exchange. While most of these compounds
show exchange that it too weak to provide highly valuable infor-
mation, complexes 4 and 7 show significant interactions and will
be the subject of more detailed work.
Acknowledgments
Financial assistance from the NSF (IMR-0314773), the Kresge
Foundation, and PCISynthesis toward the purchase of the MPMS
SQUID magnetometer and powder diffractometer are gratefully
acknowledged. M.A. is grateful for an Arthur E. Martell – Thomas
T. Sugihara Summer Science Internship. The MacDiarmid Institute
for Advanced Materials and Nanotechnology is thanked for funding
the purchase of the Rigaku Spider X-ray diffractometer.
Appendix A. Supplementary material
CCDC 843482, 843483, 845754, 845755, 843614, 843616,
845756 and 843615 contain the supplementary crystallographic
interplanar separation distance is slightly larger than in
4

76
M. Abdalrahman et al. / Inorganica Chimica Acta 389 (2012) 66–76
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