Three-Dimensional Mixed-Ligand Coordination Polymers with

ARTICLE pubs.acs.org/crystal

Three-Dimensional Mixed-Ligand Coordination Polymers with Ferromagnetically Coupled Cyclic Tetranuclear Copper(II) Units Bonded by Weak Interactions Susana Balboa,† Joaquín Borr
as,‡ Pilar Brandi,|| Rosa Carballo,§ Alfonso Casti~neiras,*,† Ana B. Lago,§ Juan Nicl
os-Guti
errez,|| and Jose A. Real# †

)

Departamento de Química Inorg
anica, Facultad de Farmacia, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain ‡ Departamento de Química Inorg
anica, Facultad de Farmacia, Universidad de Valencia, E-46100, Burjassot, Spain § Departamento de Química Inorg
anica, Facultad de Química, Universidad de Vigo, E-36310 Vigo, Spain Departamento de Química Inorg
anica, Facultad de Farmacia, Universidad de Granada, E-18071 Granada, Spain # Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, E-46980 Paterna, Spain

bS Supporting Information ABSTRACT: The complexes {[Cu(HGLYO)(NO3)(bpy)] 3 H2O}4 (1), {[Cu(HGLYO)(NO3)(phen)] 3 H2O}4 (2), and [Cu(HLACO)(ClO4)(phen)]4 (3) (bpy is 2,20 -bipyridine, phen is 1,10-phenanthroline, HGLYO is monoanionic glycolate, and HLACO is monoanionic lactate) have been synthesized and their crystal structures were determined by X-ray diffraction analysis. The thermal behavior and IR, electronic and electron paramagnetic resonance (EPR) spectra are also reported. The structures of compounds 1
3 consist of cyclic tetrameric units in which the copper(II) ions are connected by carboxylate groups in an anti-syn conformation. The hydroxycarboxylates, diimine ancillary ligands, and crystallization water molecules, when present, have a significant influence on the crystal structures through weak interactions such as hydrogen bonding and π
π stacking. These interactions mainly contribute to the stabilization of the overall supramolecular structures, which contain rectangular voids of different dimensionality. Magnetic measurements in the temperature range 1.8
300 K showed that compounds 1 and 2 are ferromagnetic. The J values obtained were 3.2 cm
1 for 1 and 3.3 cm
1 for 2. An electrochemical study for complexes 1 and 2 by cyclic voltammetry in dimethylformamide solutions showed the irreversible nature of the electrode process.

’ INTRODUCTION Research in metal
organic frameworks in recent years has been given impetus by the growth of two other closely related areas: crystal engineering and metallosupramolecular chemistry.1 Crystal engineering seeks to understand why molecules pack in the ways that they do and to use that knowledge to deliberately engineer the arrangements of molecules in new materials.2 This is important because the properties of materials are often governed by the way in which their constituent molecules are arranged. Control over this arrangement gives control over the properties of the material. The aim of supramolecular chemistry is similar, that is, to create assemblies of molecules. The process does not involve creating structures one atom at a time, but the design of molecules that, when combined, spontaneously self-assemble in a predetermined fashion to give larger architectures.3 Coordination compounds with appropriate metal
ligand combinations are used to create molecular building blocks for self-organized systems with diverse topologies and potential applications.4 In addition to covalent linkages between the r 2011 American Chemical Society

building blocks, noncovalent intermolecular interactions ranging in strength from very strong hydrogen bonding to weak C
H 3 3 3 A hydrogen bonds,5 π interactions,6 and, ultimately, van der Waals forces direct the formation of self-assembled structures of variable dimensions.7 In particular, there has been considerable interest in the properties of polynuclear copper(II) complexes in order to understand the relationship between structure and magnetic properties.8 Copper(II) complexes are also of interest because of their relevance as active site structures of metalloproteins.9 It is well-known that a carboxylate group can bridge two metal ions to give rise to a wide variety of polynuclear complexes ranging from discrete entities to three-dimensional (3D) systems.10 In these complexes, a carboxylate group can assume many types of bridging conformations, and, as expected, the magnetic properties Received: December 9, 2010 Revised: August 17, 2011 Published: August 18, 2011 4344

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Crystal Growth & Design

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Table 1. Crystal Data and Structure Refinement for Compounds 1
3 compound

1

2

3

empirical formula

C48H52Cu4N12O28

C56H52Cu4N12O28

formula weight

1499.18

1595.26

1729.06

crystal system

tetragonal

tetragonal

tetragonal

space group

I41/a (No. 88)

P421c (No. 114)

P421c (No. 114)

a/Å

27.794(1)

20.968(1)

21.083(1)

b/Å

27.794(1)

20.968(1)

21.083(1)

c/Å volume/Å
3

7.508(1) 5799.6(3)

7.239(2) 3182.7(9)

8.489(1) 3773.6(7)

Z

4

2

2

calc density/Mg/m3

1.717

1.665

1.522

absorp coefc/mm
1

2.539

2.359

3.295

F(000)

3056

1624

1752

crystal size

0.56  0.20  0.12

0.60  0.06  0.06

0.40  0.32  0.28

C60H52Cl4Cu4N8O28

unit cell dimensions

θ range/


4.50
73.05

2.98
72.94

2.96
73.94

limiting indices/h, k, l refl. collect/unique (Rint)


34/0, 0/34, 0/9 3033/2902 (0.0425)

0/26, 0/25,
8/0 3541/1810 (0.0668)


26/0,
26/0,
10/0 4210/2150 (0.0615)

completeness θ/
(%)

73.05 (99.8)

72.98 (99.9)

73.94 (99.4)

absorp correct

Psi-scan

Psi-scan

Psi-scan

max/min transm

0.7504/0.3305

0.8714/0.3319

0.4589/0.3524

data/parameters

2902/236

1810/227

2150/237

goodness-of-fit on F2

1.021

1.099

1.038

final R indices

R1 = 0.0563

R1 = 0.0590

R1 = 0.0549

R indices (all data)

wR2 = 0.1424 R1 = 0.1091

wR2 = 0.1528 R1 = 0.1124

wR2 = 0.1468 R1 = 0.0593

wR2 = 0.1698 Flack parameter largest diff peak/hole

0.290/
0.483

are closely related to the carboxylate bridging conformation in these polynuclear systems.11 One particular example is represented by the α-hydroxycarboxylates. The coordination chemistry of metal complexes containing α-hydroxycarboxylic acids has been extensively studied in recent years due to their unusual structural features and various physical and chemical properties.12
15 These carboxylate systems show denticity, owing to the presence of both carboxylic acid and hydroxyl groups, and this can lead to mono- or dianionic species and consequently to a wide variety of coordinative possibilities.14e In addition, different coordination patterns were reported in the same compound, as well as bridging behavior between two metal centers
as observed for simple carboxylate ligands. The coordination mode eventually adopted by the α-hydroxycarboxylate anions depends not only on the metal ion and coligands used in the synthesis of the complexes but also on the molar ratio of reagents and the α-hydroxycarboxylic acid used. Furthermore, the monoanionic α-hydroxycarboxylates can act as a source of hydrogen bonding and are suitable for C
H 3 3 3 π interactions with nitrogen donor ligands and, where applicable, for π 3 3 3 π stacking interactions. Our previous investigations about the Cu(II)/glycolate or lactate/2,20 -bypidine or 1,10-phenanthroline systems showed that the use of CuCO3 3 Cu(OH)212e,13a,13b as copper(II) precursor in an ethanolic media and in the presence of an excess of α-hydroxycarboxylate leads to the formation of monomeric or dimeric discrete complexes. When Cu(AcO)213i was used in the same media in a copper(II)/α-hydroxycarboxylate molar ratio

wR2 = 0.1763

wR2 = 0.1521

0.00(11)

0.00(6)

0.908/
0.468

0.892/
0.412

1:2, a monomeric discrete compound was isolated. The present work shows how the combination of different copper(II) precursors such as Cu(NO3)2 and Cu(ClO4)2 in a copper(II)/ α-hydroxycarboxylate molar ratio 1:1 affords tetrameric compounds. These results suggest that in the Cu(II)/glycolate or lactate/2,20 -bypidine or 1,10-phenanthroline systems the choice of a particular copper(II) precursor and a copper(II)/αhydroxycarboxylate molar ratio can be used as tools for control the nuclearity of the resulting copper(II) species which serves as a building block of the corresponding supramolecular network. Also, in a previous study we reported a structural examination of the reactions of copper(II) ions and mandelic acid in basic aqueous solutions in the presence of diimine ancillary ligands. These processes gave rise to the hydroxo-bridged tetranuclear Cu(II) complexes of formulas [Cu4(μ3-OH)2(μ2-MANO)2(bpy)4](phglyo)2 3 8H2O and [Cu4(μ3-OH)2(μ2-OH)2(OH2)2(phen)4] (Bza)2(OH)2 3 5H2O (bpy is 2,20 -bipyridine, phen is 1,10-phenanthroline, MANO is dianionic mandelato, phglyo is phenylglyoxylato, and Bza is benzoate), which have chairlike stepped cubane structures.15b As a continuation of these studies, we report here new results on the reactions of copper(II) ions with glycolic (H2GLYCO) and lactic (H2LACO) acids in the presence of bpy and phen, which led to the synthesis of cyclic tetranuclear copper(II) complexes of formulas {[Cu(HGLYO)(NO3)(bpy)] 3 H2O}4 (1), {[Cu(HGLYO)(NO3)(phen)] 3 H2O}4 (2), and [Cu(HLACO)(ClO4)(phen)]4 (3). The crystal structures, 4345

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Crystal Growth & Design

ARTICLE

Figure 1. Molecular structures of {[Cu(NO3)(HGLYO)(bpy)].H2O}4 (1) and [Cu(ClO4)(HLACO)(phen)]4 (3). The H atoms have been omitted for clarity.

Table 2. Selected Distances and Bond Angles for Compounds 1
3 compound

1

2

3

Distances [Å] Cu(1)
O(11) Cu(1)O(12)a

1.952(3) 1.968(3)

1.954(7) 1.976(6)

1.976(4) 1.978(3)

Cu(1)
O13)

2.436(4)

2.343(7)

2.304(4)

Cu(1)
N(11)

1.992(4)

2.003(7)

2.002(4)

Cu(1)
N12)

2.006(5)

2.029(7)

2.0221(4)

Cu(1)
O(21)

2.434(13)

2.783(9)

2.569(5)

Cu(1)
Cu(1)a

5.0286(10)

5.0077(17)

5.0877(9)

Cu(1)
Cu(1)b

5.0286(10)

5.0077(17)

5.0877(9)

O(11)
Cu(1)
O(12)a O(11)
Cu(1)
O(13)

90.64(15) 74.81(13)

91.5(3) 76.9(2)

89.73(15) 76.35(13)

Angles [
]

O(11)
Cu(1)
N(11)

171.76(17)

174.2(3)

176.68(16)

O(11)
Cu(1)
N(12)

93.6(2)

94.5(3)

94.68(17) 83.24(19)

O(11)
Cu(1)
O(21)

96.2(3)

80.1(3)

O(12)a
Cu(1)
O(13)

106.13(13)

106.6(2)

102.12(14)

O(12)a
Cu(1)
N(11)

95.61(19)

92.9(3)

93.24(17)

O(12)a
Cu(1)
N(12)

167.26(17)

162.2(3)

170.53(17)

O(12)a
Cu(1)
O(21) O(13)
Cu(1)
N(11)

82.2(3) 98.22(15)

83.8(2) 98.2(3)

90.35(187) 104.42(16)

O(13)
Cu(1)
N(12)

86.59(16)

91.1(3)

87.08(16)

O(13)
Cu(1)
O(21)

167.6(2)

154.9(2)

155.89(17)

N(11)
Cu(1)
N(12)

81.4(2)

82.4(3)

82.17(18)

N(11)
Cu(1)
O(21)

90.0(3)

104.1(3)

95.6(2)

N(12)
Cu(1)
O(21)

85.4(3)

80.7(3)

81.87(19)

a: y + 1/4,
x
1/4,
z
1/4

a:
y, x,
z + 1

b:
y
1/4, x
1/4,
z
1/4

b: y,
x,
z + 1

thermal behavior, spectroscopic properties, and magnetism of these complexes are presented here.

The magnetic behaviors of 1 and the isostructural compounds 2 and 3 were found to be the same and the magnetic properties of 4346

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Crystal Growth & Design 2,12g which was reported to show ferrimagnetic behavior,12f were confirmed. In addition, the electrochemistry of complexes 1 and 2 was studied by cyclic voltammetry.

’ EXPERIMENTAL SECTION General. Elemental analyses were carried out on a Carlo Erba EA1108 microanalyzer. Melting points were determined with a B€uchi apparatus. Infrared spectra in the 4000
400 cm
1 range were recorded using KBr pellets on a Perkin-Elmer 597 spectrophotometer, and in the 700
100 cm
1 range using polyethylene-sandwiched Nujol mulls on a Bruker IFS-66v spectrophotometer. The electronic spectra of microcrystalline samples were obtained by the diffuse reflectance method on Shimadzu UV-3101PC spectrophotometers over the range 900
250 nm. TGA analyses (pyrolytic decomposition) of the compounds (20
950
C, 10
C/min) were carried out with a constant air flow (100 mL/min) using a Shimazu Thermobalance TGA-DTG-50H instrument, and between 21 and 24 FT-IR spectra of evolved gases were recorded in a strategically time-spaced manner in each experiment, using a coupled FT-IR Nicolet Magna 550 spectrophotometer. Solid state EPR spectra were recorded on a Bruker EMX instrument (band X) at room temperature and 120 K. X-ray powder diffraction patterns were recorded on a Siemens D5005 diffractometer using a CuKα radiation source. The variable temperature magnetic susceptibility measurements were carried out on a microcrystalline sample using a Quantum Design MPMS2 SQUID susceptometer equipped with a 55 KG magnet and operating at 10 KG in the range 1.8
300 K. Pascal constants were used to correct for the diamagnetism of the compounds. Cyclic voltammograms were obtained on a 273 EG&gG Princeton Applied Research electrochemical analyzer. The measurements were carried out at 25.0 ( 0.2
C in dimethylformamide (DMF) solutions using 0.1 M NBun4ClO4 (TBAP) as supporting electrolyte, at 50 mV s
1 under oxygen-free conditions using a three-electrode cell with a glassy carbon electrode. All the chemicals and solvents were of analytical grade and were used as received. The TBAP used as supporting electrolyte in electrochemical measurements was purchased from Fluka and recrystallized from hot methanol. (Caution! Perchlorates are potentially explosive and care should be taken in handling these compounds.) {[Cu(HGLYO)(NO3)(bpy)] 3 H2O}4 (1). 2,20 -bpy (1.28 mmol) was dissolved in ethanol (10 mL), and a solution of Cu(NO3)2 3 3H2O (1.24 mmol) in ethanol (10 mL) was added dropwise. To the resulting solution was added an aqueous solution (10 mL) of H2GLYO (1.31 mmol) and NaHCO3 (1.19 mmol). The reaction mixture was stirred at room temperature for 7 days. The resulting solution was filtered and was allowed to concentrate slowly by evaporation at room temperature. One week later, blue crystals were obtained. The crystals were picked out and dried in air (yield ca. 50% based on CuII). Mp 179
C. Anal. Calc. for C48H52Cu4N12O28 (1499.18): C, 38.5; H, 3.5; N, 11.2; Cu, 16.9%. Found: C, 38.5; H, 3.3; N, 11.3; Cu, 16.2%. IR/(KBr, ν/cm
1): 3401 m,br, ν(OH); 1602 m, ν(CC); 1573 m, νasym(CO2); 1495w, 1448 m, ν(CC + CN); 1421 m, νsym(CO2); 1384vs, ν3(NO3
); 1065 m, ν(C
O); 814vw, ν2(NO3
); 394w, ν(CuO); 289wd, ν(CuN). UV
vis (ν/cm
1): 14430. EPR: g0 = 2.13. {[Cu(HGLYO)(NO3)(phen)] 3 H2O}4 (2).12f. The compound was isolated as blue crystals using 1,10-phen in place of 2,20 -bpy (yield ca. 75% based on CuII). Mp 188
C. Anal. Calc. for C56H52Cu4N12O28 (1595.28): C, 42.2; H, 3.3; N, 10.5; Cu, 15.9%. Found: C, 41.8; H, 3.3; N, 10.3; Cu, 15.7%. IR/(KBr, ν/cm
1): 3406 m,br, ν(OH); 1602 m, ν(CC); 1575 m, νasym(CO2); 1520 m, 1459v, ν(CC + CN); 1426 m, νsym(CO2); 1384s, ν3(NO3
); 1067 m, ν(CO); 395w, ν(CuO); 296 m, ν(CuN). UV
vis (ν/cm
1): 15150, 13980. EPR: g0 = 2.12. [Cu(HLACO)(ClO4)(phen)]4 (3). The complex was synthesized according to the procedure described for complex 2 except H2LACO

ARTICLE

was used instead of glycolic acid. The solution was stirred for 2 days and the solvent was slowly evaporated to give blue crystals suitable for X-ray analysis (yield ca. 20% based on CuII). Mp 168
C. Anal. Calc. for C60H52Cl4Cu4N8O28 (1729.06): C, 41.7; H, 3.0; N, 6.5; Cu, 14.7%. Found: C, 42.3; H, 3.2; N, 6.4; Cu, 14.3%. IR/(KBr, ν/cm
1): 3369 m,br, ν(OH); 1618s, ν(CC); 1594 m, νasym(CO2); 1519 m, 1430 m, ν(CC + CN); 1383 m, νsym(CO2); 1049 m, ν(CO); 1147s, ν3(OClO3); 928w, ν4(OClO3); 393w, ν(CuO); 296w, ν(CuN). UV
vis (ν/cm
1): 16110, 14750. EPR: g0 = 2.10 X-ray Crystallography. Suitable single crystals were mounted on a glass fiber and these samples were used for data collection. Data were collected at room temperature and with CuKα radiation (λ = 1.54184 Å) on Enraf Nonius CAD4 (1) and MACH3 diffractometers (2 and 3). The data were corrected for Lorentz and polarization effects16 and for absorption using ψ-scans.17 The structures were solved by direct methods,18,19 which revealed the positions of all non-hydrogen atoms. These atoms were refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters.18 In 1 the oxygen atoms of the NO3 are disordered over two positions; the occupancy factor for each was refined, resulting in a value of 0.53(6) for O(a) and 0.47(6) for O(b). Hydrogen atoms were located in difference maps (1 and 3) or placed geometrically (2), but the OH and H2O hydrogen atoms were located unambiguously from difference Fourier maps (2). All hydrogen atoms were included as fixed contributions riding on attached atoms with isotropic atomic displacement parameters constrained to be 1.2 times those of their carrier atoms. The absolute structures for compounds 2 and 3 were chosen according to the Flack parameter.20 Criteria for a satisfactory complete analysis were the ratios of rms shift to standard deviation being less than 0.001 and no significant features in final difference maps. Atom scattering factors were taken from the International Tables for Crystallography.21 The crystal data and structural refinement parameters are displayed in Table 1.

’ RESULTS AND DISCUSSION Structural Description of {[Cu(NO3)(HGLYO)(bpy)] 3 H2O}4 (1), {[Cu(NO3)(HGLYO)(phen)] 3 H2O}4 (2), and [Cu(ClO4)(HLACO)(phen)]4 (3). The crystal structures of 1
3 are based

on neutral tetranuclear molecules (Figure 1), but the structure of 2 contains nitrates coordinated at a greater distance than in compound 1. The structure of 2 was reported elsewhere,12f but it is summarized here for the sake of comparison. Selected bond lengths and angles are presented in Table 2. In the three tetranuclear compounds, the Cu(II) ions are connected by the bridging carboxylate group of the glycolato (1 and 2) and lactato (3) ligands. In all cases the carboxylate bridge adopts the anti-syn conformation usually observed in similar tetranuclear systems.22 In 2 the copper(II) ion displays a 4 + 1+1 octahedral coordination geometry, which is very similar to those in other tetranuclear complexes,22 with the nitrate anion at the long distance of 2.783(9) Å (sum of van der Waals radii, 2.92 Å). In 1 and 3 each copper atom has a distorted elongated octahedral environment (type 4 + 2 and 4 + 1+1, respectively). The basal plane around the metal center, with bond distances in the range 1.952
2.021 Å, is formed by two nitrogen atoms of one 2,20 -bpy or 1,10-phen ligand and by two oxygen carboxylate atoms of monoanionic chelating and bridging glycolato or lactato ligands. The apical positions are occupied by the hydroxyl group of the corresponding α-hydroxycarboxylato ligand, at 2.436(4) Å in 1 and at 2.304(3) Å in 3, and by one oxygen atom of the corresponding oxoanion, at 2.434(13) Å in 1 and at 2.567(5) Å in 3. The Cu(II) centers are 0.652 Å (in 1 and 2) and 0.617 Å (in 3) above and below their mean plane, respectively, and thus almost 4347

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Table 3. Hydrogen Bond Parameters [Å,
] for Compounds 1
3 D3 3 3A

— DHA

2.05

2.711(12)

120.8

2.12

3.020(17)

147.4

0.65

2.12

2.753(5)

168.6

0.86

2.12

2.948(18)

159.9

0.93

2.14

3.049(12)

165

D
H 3 3 3 A

D
H

O(13)
H(13A) 3 3 3 O(22)a O(13)
H(13A) 3 3 3 O(11)a O(1)
H(1A) 3 3 3 O(1)c

1.01 1.01

O(1)
H(1B) 3 3 3 O(23) C(17)
H(17) 3 3 3 O(22)d

H3 3 3A Compound 1

a: y + 1/4,
x
1/4,
z
1/4; c: y + 1/4,
x
3/4, z + 1/4; d:
3/4
y,
1/4 + y,
1/4 + z Compound 2 O(13)
H(13A) 3 3 3 O(22)a O(13)
H(13A) 3 3 3 O(11)a O(13)
H(13A) 3 3 3 O(21)a O(1)
H(1A) 3 3 3 O(23)c C(11)
H(11) 3 3 3 O(12)a

0.82 0.82

2.23 2.35

2.905(11) 2.888(9)

139.6 124.2

0.82

2.53

3.219(12)

141.9

0.82

2.53

2.98(2)

116.3

0.99

2.53

3.095(11)

116

a:
y, x,
z + 1; c:
x + 1/2, y + 1/2,
z + 1/2 Compound 3 O(13)
H(13B) 3 3 O(13)
H(13B) 3 3

b 3 O(4) b 3 O(1)

C(12)
H(12) 3 3 3 O(4)b b: y,
x,
z + 1

0.97

3.34

3.98(2)

124.5

0.97

2.60

3.505(9)

154.6

0.93

2.53

3.121(19)

121

Figure 2. 3D supramolecular assembly in 1 showing helical water and marking the π
π stacking region.

define a square with Cu
Cu distances 5.0286(10) Å (1), 5.0077(17) Å (2), and 5.0879(9) Å (3), and with the Cu
Cu distance across the square between 6.84 and 6.98 Å. The angles at the corners of these squares are around 86
in all three compounds, a value close to the ideal square-planar angle of 90
. In analogous systems, the square planar angles are reported as ranging from 82 to 88
.23 The three cyclic tetramers are further stabilized by intramolecular hydrogen bonds (Table 3) that always involve the hydroxyl group of the α-hydroxycarboxylato as a hydrogen donor and the coordinated oxygen atom of the carboxylate group and one oxygen atom from nitrate or perchlorate anions as acceptors, with OH 3 3 3 O distances in the range 2.71
3.02 Å. In compounds 1 and 3 weak intramolecular CH 3 3 3 π contacts are found between an ortho-hydrogen atom of a pyridine moiety from 2,20 -bipy or 1,10-phen and the chelate

Figure 3. 3D supramolecular organization in 2 showing water molecules in the voids and π
π stacking interactions.

ring, with CH 3 3 3 chelate ring centroid distances of 3.176 (1) and 3.207 Å (3). The discrete tetramers are linked by means of weak CHbpy/phen 3 3 3 O hydrogen bonds (Table 3), with distances always greater than 3 Å, and by π
π stacking interactions, giving rise in all three cases to 3D supramolecular organizations that contain rectangular voids, as shown in Figures 2
4. In this way the tetranuclear molecules are stacked parallel to each other along the tetragonal c axis (Figures 3 and 4) with interplanar distances between the involved phen or bipy rings ranging from 3.543 Å in 4348

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Figure 4. 3D supramolecular organization in 3 showing the empty voids and a detail of π
π stacking interactions.


,
)a Table 4. Intermolecular π 3 3 3 π Interaction Parameters (Å π3 3 3π

compound 1

i

Cg(1) 3 3 3 Cg(1) Cg(1) ring (N11/C11/C12/ C13/C14/C15)

Cg(I) 3 3 3 Cg(J)

α

3.543

0.00

3.692

2.35

3.627

0.03

3.789

2.06

i =
x,
y, 1
z 2

Cg(3) 3 3 Cg(5) 3 3

i

3 Cg(5) i 3 Cg(5)

Cg(3) ring (N11/C11/C12/ C13/C14/C22) Cg(5) ring (C14/C15/C16/ C17/C21/C22) i = 1/2
y, 1/2
x, 1/2 + z 3

Cg(3) 3 3 3 Cg(5)i Cg(3) ring (N11/C11/C12/ C13/C14/C22) Cg(5) ring (C14/C15/C16/ C17/C21/C22) i = 1/2
y, 1/2
x,
1/2 + z

Cg(I) 3 3 3 Cg(J): Distance between ring centroids; α: dihedral angle between planes I and J.

a

1 to 3.789 Å in 3 (Table 4). The strength of the aromatic interactions decreases on going from 1 to 3, and, following this sequence, the dimensions of the generated voids increase (5.4  6.3 Å in 1, 7.5  10.7 Å in 2 and 7.6  11.8 Å in 3). These voids have a different occupancy in each case. In 2 the water molecule of crystallization is located in the void (Figure 3), and it can be considered as isolated, with only a weak hydrogen bond (Table 3) involving one oxygen atom of the nitrate anion as the acceptor. This situation means that this water molecule does not contribute to the supramolecular organization of the tetranuclear units. The resulting crystal packing is very efficient, as evidenced by the Kitaigorodskii packing index24 of 71.4%. Compound 3 has empty voids (Figure 4) and as a consequence this has the lowest calculated density of the three compounds (Table 1) and also the lowest value for the Kitaigorodskii packing index with a value of 61.9%. In this case the free solvent area corresponds to a volume of 671.2 Å3, which represents 17.8% of the unit cell.

In 1 rather an infinite chain of hydrogen-bonded water molecules weaves through the channels (Figure 2). The most fascinating structural feature is the alternate left- and right-handed one-dimensional (1D) helical water chains in the 1D channels of the 3D architecture. The complete water chain structure is built up from only one crystallographically independent water molecule O1. The O1 3 3 3 O1 distance of 2.753(5) Å is within the range reported for other water chains.25,15a The pitch of each water helix is 7.508 Å and this corresponds to the c axis unit cell length. The helical water chain is further supported and stabilized by hydrogen bonds to one oxygen atom of the nitrate anion. The resulting crystal packing is very efficient, as shown by the Kitaigorodskii packing index of 72.1%, which is the highest value of the three metallosupramolecular structures investigated here. Thermogravimetric Analysis. Thermogravimetric analysis was carried out in order to examine the thermal stability of compounds 1 and 2 (Table 5). Compound 1 lost a proportion of its coordinated water in a flow of dry air, with [Cu(HGLYO)(NO3)(bpy)]4 3 1.36H2O remaining. The presence of nitrate means that after the initial water loss (essentially between 56 and 110
C), a rapid pyrolytic decomposition process begins and this represents the loss of more than 50% of the sample to produce CO2, H2O, H2CO, CO, N2O, NO, and NO2. It is worth highlighting the formation of formaldehyde and three oxides of nitrogen in this stage. The weight loss determined experimentally (50.156%) is greater than that calculated for the pyrolysis of the HGLYO and nitrate ligands (estimated to be 41.902%). This suggests some overlap of these processes with pyrolysis of the bpy ligands. The residue obtained is CuO, although this also contained some nitrate (∼6.7%). Complex 2 lost only a minimal amount of water in a stream of dry air, indicating the presence of [Cu(HGLYO)(NO3)(phen)]4 3 3.85H2O. In the first step (25
185
C), only water was lost (found 4.36%, calc. for 3.85 H2O 4.36%), as evidenced by the corresponding FT-IR spectra. The second stage (185
255
C) corresponds to the pyrolysis of HGLYO and the decomposition of nitrate, in overlapping processes, with a significant evolution of nitrogen oxides (N2O, NO, and NO2) along with CO2, H2O, and CO, as shown by the FT-IR spectra. This situation is in contrast to the formation of only CO2, H2O, CO, and N2O (stages III and IV) or only CO2, H2O, and CO (stage V). The final residue (at 700
C) is believed to be CuO (found 18.40%, calc. 19.98%), which may be formed via another residue that could not be identified but pyrolysis of which leads to the formation of CuO. In this process, the weight of residue obtained is significantly less than that calculated. In any case, it is worth noting that pyrolytic processes that give rise to CuO require very high temperatures. Spectral Studies. The IR spectra of the Cu(II) complexes contain broad bands in the range 3410
3365 cm
1 and these correspond to ν(OH) from hydroxyl groups and crystallization water molecules. Bands are also observed between 1520 and 1430 cm
1 and these are due ring stretching of diimines. In all complexes the νas(COO) and νs(COO) are assigned to the bands at around 1580 and 1400 cm
1, which indicates bridging tridentate chelate coordination of the hydroxycarboxylato ligand in a k2O,O00 :kO0 fashion. The infrared spectra of compounds 1 and 2 show a strong band at 1384 cm
1 and this is assigned to ν3(NO3
), a finding in contrast to the behavior expected for a monodentate nitrate group. This fact could be associated with weak coordination of this group to the metal center, an interpretation consistent with the occupancy of this ligand in axial positions in a highly elongated octahedral geometry. This effect is 4349

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Table 5. Data for the Thermogravimetric Analysis of Compounds 1 and 2 weight loss temp (
C)

t (min)

exp (%)

calc (%)

I

60
165

4
13

1.687

1.688

II

165
285

13
27.5

52.156

III

285
515

27.5
50

21.835

389

residue

515

stage

DTG (
C)

identified gases

observations

82 and 95

H2O


1.36H2O

225.7

CO2, H2O, H2CO, CO

HGLYO, NO3


Compound 1

N2O, NO, NO2 24.168

CO2, H2O, N2O, NO (traces)

bpy

21.916

CuO

23.903

14CuO
Cu(NO3)2

36.800

CuO
Cu(NO3)2

Compound 2 I

25
185

0
18

4.358

4.357

50

H2O


3.852

II

185
255

18
25

40.211

34.363

221.4

CO2, H2O, CO

HGLYO, NO3


III

255
335

25
33

26.568

298.6

CO2, H2O, CO, N2O

phen

IV

335
500

33
48

8.898

∼450

CO2, H2O, CO, N2O

phen

V

500
700

48
68

1.410

559.2

CO2, H2O, CO

phen

Residue

700

N2O, NO, NO2

18.403

19.909

Figure 5. Thermal dependence of the χMT product (cm3 mol
1 K) (χM is the magnetic susceptibility per four Cu(II) ions) for 1 (a) and 2 (b). The solid line represents least-squares fit eq 2 and the parameters described in the text. The inset shows the χMT plot in the low temperature region.

less pronounced than in complex 1, which also gave a weak band at 814 cm
1 assignable to ν2(NO3
). In complex 3 the bands corresponding to ν3(ClO4) and ν4(ClO4) appear at 1147 and 928 cm
1, respectively. These values are consistent with the literature values quoted for ClO4 units acting as weak monodentate ligands.26 In the far-infrared region the bands corresponding to Cu
Ndiimine and Cu
O appear at around 400 and 300 cm
1, indicating coordination of these atoms to the metal center. The diffuse reflectance spectra of the complexes reported here show a band in the low energy region (14430
16125 cm
1) that has two components (15150
13980 cm
1 in 2 and 16110
14750 cm
1 in 3) and these bands correspond to a d-d transition.27 The X-band EPR spectra of the three complexes were measured at room temperature and the spectra of complexes 1 and 2 are shown as Supporting Information. The spectrum exhibits an isotropic signal with g0 = 2.127 and a weak signal at 1558 G attributed to a ΔMS = ( 2 half field. The EPR spectra of 2 and 3 are also isotropic, with g0 = 2.122 and g0 = 2.102, respectively, that is, similar to the other complex. However, the half field signal does not appear in these spectra.

CuO

Magnetic Properties. The temperature dependence of the product χMT (where χM is the molar magnetic susceptibility and T is the temperature) was studied in the range 2
300 K for 1 and 2 and the behavior is shown in Figure 5, panels a and b, respectively. At 300 K, χMT is around 1.75 cm3 K mol
1
a value consistent with that expected for four magnetically uncoupled copper(II) ions with g = 2.16. The χMT values remain constant upon cooling down to 100 K and then increase rapidly from 2.58 and 2.68 cm3 K mol
1 on further cooling to 3.25 K for 1 and 2, respectively. Finally, χMT decreases between 3.25 and 2 K, reaching values of 2.47 cm3 K mol
1 (1) and 2.54 cm3 K mol
1 (2), respectively, at 2 K. The increase in χMT indicates the occurrence of ferromagnetic interactions between copper atoms in the tetranuclear species. For four ferromagnetically coupled copper(II) ions, an S = 2 ground state with χMT ≈ 3 cm3 K mol
1 (g ≈ 2) would be expected. The lower experimental values observed for both derivatives may be the result of competition between weak ferromagnetic intramolecular and antiferromagnetic intermolecular interactions, with both effects being operative at low temperatures. According to the crystal structure data, the tetranuclear entity would strictly require two coupling constants, one representing the coupling constant, J, between pairs of consecutive copper atoms and other, J0 , considering coupling between pairs of alternate copper centers through the diagonals of the [Cu4] square frame. We did not consider the “diagonal” coupling constant as it is expected to be considerably smaller than J and also as this treatment avoids overparametrization in the fitting procedure. For this system the appropriate Hamiltonian is

H ¼
2JðS1 S2 þ S2 S3 þ S3 S4 þ S4 S1 Þ þ gβHS

ð1Þ

The corresponding expression for χM is χM ¼

  2Nβ2 g 2 5 expð6J=kTÞ þ 2 expð4J=kTÞ þ expð2J=kTÞ kðT
θÞ 7 expð4J=kTÞ þ 5 expð6J=kTÞ þ 3 expð2J=kTÞ þ 1 þ Nα

4350

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Crystal Growth & Design

ARTICLE

Table 6. Reduction Peak Potentials for Cu(II) and the Corresponding Peaks for the Reoxidation of Cu(I) (V vs ECS) Using a Glassy Carbon Electrode compound 1 2

V s
1

(Ep)c

(Ep)a

(Ep)a
(Ep)c

0.02


0.192

0.034

0.226


0.079

0.20


0,200

0.023

0.223


0.089

0.02


0.156

0.042

0.198


0.057

0.20


0.152

0.017

0.169


0.068

[(Ep)a + (Ep)c]/2

Nα represents the temperature-independent paramagnetism and θ represents the intermolecular interactions. The parameters J, g, θ, and Nα were determined by a least-squares fit minimizing R = .[(χMT)obsd
(χMT)calcd]2/.[(χMT)obsd]2. The values obtained were J = 3.2 cm
1, g = 2.08, θ =
0.6 cm
1 and Nα = 373  10
6 cm3 mol
1 and R = 2  10
5 for 1 and J = 3.3 cm
1, g = 2.11, θ =
0.6 cm
1 and Nα = 212  10
6 cm3 mol
1 and R = 9  10
5 for 2. The solid line in Figure 5 represents the leastsquares curve generated with the calculated parameters. The good quality of the fit, together with reasonable parameters obtained for both compounds, supports our previous hypothesis. Cyclic Voltammetry. Complexes 1 and 2 have very similar cyclic voltammetric responses as they have the same arrangement of donor atoms and vary only by eight atoms per molecule. The reduction peak potentials for Cu(II) are shown in Table 6 along with the corresponding reoxidation peaks for Cu(I). It can be seen that the more negative the cathodic peak, the more stable the complex in solution (for a variation in potential of 0.20 V s
1). In all cases a well-defined cathodic peak was observed, and, on the basis of the position of the peak on the potential scale (
0.8 to
0.4 V vs. SCE), this can be attributed to the reduction of copper(II) according to the equation: Cu2þ þ e
a Cuþ In both complexes the corresponding anodic peak is also observed (Ea), although in some cases the appearance of the peak depends on the experimental conditions (e.g., potential range and length of time after preparation of the solution). The peak shapes and the separation between the cathodic and anodic peaks demonstrate the irreversible nature of the electrode process occurring.

’ CONCLUSIONS In summary, we report the preparation of a set of squarelike copper(II) tetranuclear compounds formed by the appropriate combinations of diimine ancillary ligands with glycolic and lactic acids and different counterions. The choice of nitrate and perchlorate as copper(II) precursors and the 1:1 copper(II)/ α-hydroxycarboxylate molar ratio used seem to be the key to the formation of these tetrameric building blocks. The crystal packing of the three complexes shows 3D polymers formed through hydrogen bonding and by heteroaromatic π 3 3 3 π stacking interactions. These arrangements have voids with sizes ranging from 6.3 to 11.8 Å. Magnetic results indicate that ferromagnetic interactions take place in the compounds. The lower experimental values observed for χMT may be the result of a competition between weak ferromagnetic intramolecular and antiferromagnetic intermolecular interactions, both of which are operative at low temperatures.

’ ASSOCIATED CONTENT

bS

Supporting Information. X-ray crystallographic data in CIF format, packing diagram of unit cells of compounds 1
3, thermogravimetric analysis plots for 1 and 2, variable temperature EPR spectra for 1 and 2, and cyclic voltammograms for 1 and 2. This information is available free of charge via the Internet at http://pubs.acs.org. In addition, crystallographic data for structures 1
3 have been deposited with the following Cambridge Crystallographic Data Centre codes: CCDD-803830 {[Cu(HGLYO)(NO3)(bpy)] 3 H2O}4 (1); CCDC-803831 {[Cu(HGLYO)(NO3)(phen)] 3 H2O}4 (2); CCDC-803832, [Cu(HLACO)(ClO4)(phen)]4 (3).

’ AUTHOR INFORMATION Corresponding Author

*Telephone: (+34)-8-8181 4951. Fax: (+34)-9-8154 7163. E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from ERDF (EU)-DGI-MEyC (Spain) (Projects CTQ2006-15329-C02/BQU and 10TMT314002PR) is gratefully acknowledged. We thank Prof. J. C. GarcíaMonteagudo (Univ. Santiago de Compostela, Spain) for the use of facilities for the cyclic voltammetry measurements. ’ REFERENCES (1) (a) Abd-El-Asiz, A. S.; Carraher, Ch. E., Jr.; Pittman, Ch. U., Jr.; Zeldin, M., Ed.; Macromolecules Containing Metal and Metal-Like Elements. In Metal-Coordination Polymers; John Wiley & Sons, Inc.: Hoboken, USA, 2005; Vol. 5. (b) Batten, S. R.; Neville, S. M.; Turner, D. R. Coordination Polymers. Design, Analysis and Application; RSC Publishing: Cambridge, UK, 2009. (2) (a) Desiraju, G. R. Crystal Engineering. The Design of Organic Solids; Elsevier: Amsterdam, The Netherlands, 1989. (b) Seddon, K. R.; Zaworotko, M., Ed. Crystal Engineering: The Design and Application of Functional Solids; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1999. (3) (a) Lehn, J.-M. Supramolecular Chemistry. Concepts and Perspectives; VCH: Weinheim, Germany, 1995. (b) Lindoy, L. F.; Atkinson, I. M. Self-Assembly in Supramolecular Systems; RSC, Cambridge, UK, 2000. (4) (a) Janiak, C. Dalton Trans. 2003, 2781–2804. (b) Hong, M.-Ch.; Chen, L., Ed. Design and Construction of Coordination Polymers; John Wiley & Sons, Inc.: Hoboken, USA, 2009. (c) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry, 2nd ed.; John Wiley & Sons: Chichester, 2009. (5) (a) Desiraju, G. R. Acc. Chem. Res. 1996, 29, 441–449. (b) Nishio, M.; Hirota, M.; Umezawa, Y. The CH/π Interaction. Evidence, Nature, and Consequences; Wiley-VCH: New York, USA, 1998. (c) Desiraju, G. R., Ed. Crystal Design: Structure and Function (Perspectives in Supramolecular Chemistry); John Wiley & Sons: Chichester, 2003; Vol. 7. (d) Alajarin, M.; Aliev, A. E.; Burrows, A. D.; Harris, K. D. M.; Pastor, A.; Steed, J. W.; Turner, D. R. Structure and Bonding; Supramolecular Assembly Via Hydrogen; Bonds, I; Mingos, D. M. P., Ed.; Springer Verlag: Berlin, 2004; Vol. 108. (6) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885–3896. (7) (a) Custelcean, R.; Moyer, B. A. Eur. J. Inorg. Chem. 2007, 1321–1340. (b) Shimomura, S.; Horike, S.; Kitagawa, S. Stud. Surf. Sci. Catal. 2007, 170, 1983–1990. (c) Liu, Y.-Y.; Ma, J.-F.; Yang, J.; Ma, J.-Ch; Ping, G.-J. CrystEngComm 2008, 10, 565–572. (8) (a) Gatteschi, D.; Kahn, O.; Willet, R. D. Magnetostructural Correlations in Exchange Coupled Systems; Reidel: Dordrecht, The Netherlands, 1984. (b) Kahn, O. Molecular Magnetism; VCH Publishers: New York, USA, 4351

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