Chain, layer, and self-penetrated copper dipyridylamine coordination polymers with conformationally flexible ring-based dicarboxylate ligands

Inorganica Chimica Acta 407 (2013) 297–305

Contents lists available at ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

Chain, layer, and self-penetrated copper dipyridylamine coordination polymers with conformationally flexible ring-based dicarboxylate ligands Sultan H. Qiblawi a, Laura K. Sposato b, Robert L. LaDuca a,⇑ a b

Lyman Briggs College, Department of Chemistry, Michigan State University, East Lansing, MI 48825, USA Department of Chemistry and Physics, King’s College, Wilkes-Barre, PA 18711, USA

a r t i c l e

i n f o

Article history: Received 24 June 2013 Received in revised form 30 July 2013 Accepted 31 July 2013 Available online 13 August 2013 Keywords: Copper Coordination polymer Self-penetration Antiferromagnetism

a b s t r a c t Hydrothermal reaction of copper salts, 4,40 -dipyridylamine (dpa), and a conformationally flexible ring-based dicarboxylic acid has afforded a series of four new crystalline coordination polymer solids with different topologies and dimensionalities. {[Cu(c-14cdc)(dpa)]H2O}n (1, c-14cdc = cis-1,4-cyclohexanedicarboxylate) shows interdigitated paddlewheel-dimer based [Cu(c-14cdc)]n chains with pendant dpa ligands. [Cu2(t-14cdc)2(dpa)]n (2, t-14cdc = trans-1,4-cyclohexanedicarboxylate) possesses {Cu4(OCO)6} linear tetrameric clusters linked into a very rare 8-connected ilc self-penetrated 3D net with 4245.63 topology. The chain coordination polymer {[Cu(t-14cdc)(dpa)(Hdpa)](NO3)3.5H2O}n (3) was obtained as a minor product in the synthesis of 2. Use of a related dicarboxylate ligand with a rigid central ring and flexible carboxylate arms resulted in [Cu(1,4-phda)(dpa)]n (4, 1,4-phda = 1,4-phenylenediacetate), which manifests a rippled (4,4) grid 2D net. Thermal properties of all new materials are presented. A variable temperature magnetic susceptibility study on 2 showed strong antiferromagnetic superexchange between the inner and outer copper atoms within the tetrameric clusters (g = 2.19(2), J1 = 154(2) cm1, J2  0 cm1). Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction As a result of their potential applications [1–5] and aesthetic appeal alike, the synthesis, characterization, and physical property studies of coordination polymers has received an intense research focus in recent years. By judicious choice of divalent metal precursors and either or both dicarboxylate and neutral dipodal nitrogenbased ligands, significant structural diversity can be accessed among a series of related coordination polymer crystalline solids [6–8]. Aromatic dicarboxylates with rigid arms such as terephthalate (1,4-benzenedicarboxylate, bdc) have been the most commonly chosen anionic component in coordination polymers because of their ability to impart substantial structural rigidity, essential for the retention of porous networks and beneficial sorptive behavior [9–12]. Yaghi’s seminal study of the non-interpenetrated 3D phase [Zn4O(bdc)3]n caused an explosion of interest in coordination polymers as gas storage substrates [9]. Even interpenetrated 3D phases can exhibit relevant sorptive properties; [Zn(bdc)(4,40 -bpy)0.5]n (4,40 -bpy = 4,40 -bipyridine) is an efficacious stationary phase for ⇑ Corresponding author. Address: Lyman Briggs College, E-30 Holmes Hall, 919 East Shaw Lane, Michigan State University, East Lansing, MI 48825, USA. Tel.: +1 6168369968. E-mail address: [email protected] (R.L. LaDuca). 0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2013.07.059

the chromatographic separation of branched from linear hydrocarbons [10]. In comparison to coordination polymers containing bdc ligands, those with related ligands bearing either conformationally flexible rings (cis-1,4-cyclohexanedicarboxylate, c-14cdc; trans-1,4-cyclohexanedicarboxylate, t-14cdc) [13–18] or conformationally flexible arms (1,4 phenylenediacetate, 1,4-phda) [19–22] can display different structural topologies or multifunctional chemical properties. For example, {[Cu2(t-14cdc)2]H2O}n can act either as a heterogeneous catalyst for the selective oxidation of alcohols [18a] or as a reversible toluene absorbent [18b]. In some cases these more flexible ligands provided access to higher nuclearity cluster units, which in turn serve as higher connectivity nodes for the construction of complex or self-penetrated coordination polymer topologies. As an example, {[Cu(1,4-phda)3(OH)2(dpp)2]2H2O}n (dpp = 1,3-di-(4-pyridyl)propane) exhibits antiferromagnetically coupled divalent copper tetramers linked into an 8-connected self-penetrated ilc network [19]. The complex multifunctional 3D coordination polymer [Co5(OH)8(t-14cdc)4H2O]n [20] shows reversible dehydration and strong ferromagnetic behavior with a very high Tc of 60 K. In contrast to para aromatic dicarboxylates, 14cdc ligands also present the possibility of different stereochemistry for the two carboxylate termini, allowing the isomeric forms cis-1,4-cyclohexanedicarboxylate (c-14cdc) and trans-1,4-cyclohexanedicarboxylate

298

S.H. Qiblawi et al. / Inorganica Chimica Acta 407 (2013) 297–305

(t-14cdc). In the case of t-14cdc, either the equatorial-equatorial or axial-axial conformation can be trapped in the crystalline state, providing different metal–metal contact distances or preferential binding modes. For c-14cdc, while the ring itself is conformationally flexible (e.g. chair, boat), its carboxylate arms are restricted to an axial-equatorial disposition (Scheme 1). Thus it is possible to deliberately investigate the effect of cyclohexanedicarboxylate stereochemistry on the topology of any resulting coordination polymers with the same metal and nitrogen coligand, although such studies are few to date. Cao et al. reported the synthesis of {[Cd2(c-14cdc)2(phen)2]3.5H2O}n and {[Cd4(Ht-14cdc)2(t-14cdc)3 (phen)4](H2t-14cdc)6H2O}n, which are both 2D phases but with differing topologies [23]. We thus sought to expand the scope of divalent copper coordination polymers containing the conformationally flexible ring dicarboxylate ligands c-14cdc, t-14cdc, and 1,4-phda, by inclusion of the simple, neutral, dipodal 4,40 -dipyridylamine (dpa) coligand (Scheme 1). Through this effort we hoped to probe the specific structural effects imparted by cyclohexanedicarboxylate stereochemistry and conformation, as well as the conformation of flexible acetate arms on an otherwise rigid aromatic dicarboxylate ligand. Unlike the standard rigid-rod tether 4,40 -bpy, the dpa ligand presents a kinked disposition of its nitrogen donors, and possesses a supramolecular structure direction locus through its hydrogen-bonding capable central amine group [24]. Previous work in our group has shown that the kinked nature of the dpa ligand is especially beneficial towards the construction of dicarboxylate coordination polymers with self-penetrated topologies. For instance, {[Cd2(phthalate)2 (dpa)]2H2O}n is the unique example of a 4-connected selfpenetrated 7482 yyz topology [25], and {[Ni(dpa)2(succinate)0.5]Cl}n manifests a 5-connected regular 610 self-penetrated network [26]. Herein we report the synthesis, structural characterization, and topological comparison, of {[Cu(c-14cdc)(dpa)]H2O}n (1), [Cu2(t14cdc)2(dpa)]n (2), {[Cu(t-14cdc)(dpa)(Hdpa)](NO3)3.5H2O}n (3), and [Cu(1,4-phda)(dpa)]n (4). Given the presence of a multi-nuclear cluster node within the structure of 2, its variable temperature magnetic properties were also investigated. Thermal properties were also probed. 2. Experimental 2.1. General considerations Copper nitrate and the dicarboxylic acids used were purchased commercially. 4,40 -Dipyridylamine was prepared by a published procedure [27]. Water was deionized above 3 MX-cm in-house. IR spectra were recorded on powdered samples using a Perkin Elmer Spectrum One instrument. Thermogravimetric analysis was performed under flowing N2 on a TA Instruments TGA 2050 Thermogravimetric Analyzer with a heating rate of 10 °C/min up to 900 °C. Variable temperature magnetic susceptibility data for 2 (2–300 K) were collected on a Quantum Design MPMS SQUID mag-

Scheme 1. Ligands in this study.

netometer at an applied field of 0.1 T. After each temperature change the sample was kept at the new temperature for five minutes before magnetization measurement to ensure thermal equilibrium. The susceptibility data was corrected for diamagnetism using Pascal’s constants [28], and for the diamagnetism of the sample holder. 2.2. Preparation of {[Cu(c14cdc)(dpa)]H2O}n (1) Cu(NO3)24H2O (43 mg, 0.19 mmol), cis-1,4-cyclohexanedicarboxylic acid (32 mg, 0.19 mmol), and dpa (32 mg, 0.19 mmol) were placed in 5 mL of distilled H2O in a distilled H2O in a 15 mL glass vial. An aliquot of NaOH solution (1.0 M, 0.25 mL, 0.25 mmol) was added to basicify the solution. The vial was sealed and heated at 90 °C for 24 h, and then cooled to ambient temperature. Green plates of 1 (38 mg, 48% yield based on Cu) were isolated after washing with distilled water and acetone and drying in air. Anal. Calc. for C18H21CuN3O5 1: C, 51.12; H, 5.01; N, 9.94. Found: C, 50.84 H, 4.95; N, 9.74%. IR (cm–1): 3350 (w), 3263 (w), 3160 (w), 3064 (w), 2966 (m), 2943 (m), 2931(m), 2901 (w), 2860 (w), 2473 (w), 2191 (w), 1932 (w), 1607 (s), 1587 (s), 1514 (s), 1453 (s), 1484 (m), 1433 (s), 1411 (s), 1365 (m), 1340 (s), 1311 (m), 1270 (m), 1208 (m), 1186 (m), 1150 (m), 1100 (m), 1064 (m), 1042 (w), 1014 (m), 1014 (s), 998 (m), 979 (w), 954 (w), 943 (w), 933 (w), 917 (w), 900 (m), 854 (m), 841 (m), 819 (s), 789 (m), 773 (m), 666 (m). 2.3. Preparation of [Cu2(t14cdc)2(dpa)]n (2) Cu(NO3)24H2O (86 mg, 0.37 mmol), trans-1,4-cyclohexanedicarboxylic acid (64 mg, 0.37 mmol), and dpa (63 mg, 0.37 mmol) were placed in 10 mL of distilled H2O in a 23 mL Teflon-lined Parr acid digestion bomb. An aliquot of NaOH solution (1.0 M, 0.5 mL, 0.5 mmol) was added to basicify the solution. The bomb was sealed and heated at 120 °C for 24 h, and then cooled to ambient temperature. Green plates of 2 (71 mg, 60% yield based on Cu) were isolated after washing with distilled water and acetone and drying in air. A few crystals of 3 were manually removed, distinguished by their purple color. Nevertheless, the presence of a very small amount of 3 in the final sample cannot be ruled out, because of the slightly lower than expected carbon analysis value. Anal. Calc. for C52H58Cu4N6O16 2: C, 48.90; H, 4.58; N, 6.58. Found: C, 48.15 H, 4.40; N, 5.92%. IR (cm–1): 2948 (w), 2857 (w), 1708 (m), 1575 (s), 1505 (m), 1458 (m), 1414 (s), 1371 (m), 1327 (m), 1296 (m), 1259 (w), 1219 (m), 1198 (w), 1150 (w), 1118(w), 1086 (w), 1045 (w), 1004 (w), 976 (w), 927 (m), 905 (m), 894 (m), 856 (w), 828 (w), 783 (s), 725 (m), 667 (m). 2.4. Preparation of {[Cu(t14cdc)(dpa)(Hdpa)](NO3)3.5H2O}n (3) Cu(NO3)24H2O (86 mg, 0.37 mmol), trans-1,4-cyclohexanedicarboxylic acid (64 mg, 0.37 mmol), and dpa (126 mg, 0.74 mmol) were placed in 10 mL of distilled H2O in a 23 mL Teflon-lined Parr acid digestion bomb. An aliquot of NaOH solution (1.0 M, 0.5 mL, 0.5 mmol) was added to basicify the solution. The bomb was sealed and heated at 100 °C for 24 h, and then cooled to ambient temperature. Purple plates of 3 (34 mg, 13% yield based on Cu) were isolated after washing with distilled water and acetone and drying in air. Some green crystals of 2 were observed as well; this would cause the observed carbon analysis to be higher than expected, and the observed nitrogen analysis lower than expected. Anal. Calc. for C28H36CuN7O10.5 3: C, 47.83; H, 5.16; N, 13.94. Found: C, 48.41 H, 4.71; N, 13.42%. IR (cm1): 3479 (w), 2854 (w), 1632 (m), 1594 (s), 1524 (s), 1446 (m), 1403 (m), 1358 (s), 1278 (m), 1258 (m), 1213 (m), 1068 (w), 1011 (s), 906 (m), 815 (s), 773 (m).

299

S.H. Qiblawi et al. / Inorganica Chimica Acta 407 (2013) 297–305

2.5. Preparation of [Cu(1,4-phda)(dpa)]n (4)

4. Results and discussion

Cu(NO3)24H2O (86 mg, 0.37 mmol), 1,4-phenylenediacetic acid (71 mg, 0.37 mmol), and dpa (63 mg, 0.37 mmol) were placed in 10 mL of distilled H2O in a 23 mL Teflon-lined Parr acid digestion bomb. An aliquot of NaOH solution (1.0 M, 0.5 mL, 0.5 mmol) was added to basicify the solution. The bomb was sealed and heated at 120 °C for 24 h, and then cooled to ambient temperature. Purple blocks of 4 (102 mg, 65% yield based on Cu) were isolated after washing with distilled water and acetone and drying in air. Anal. Calc. for C20H17CuN3O4 4: C, 56.27; H, 4.01; N, 9.84. Found: C, 56.01 H, 3.74; N, 9.63%. IR (cm1): 3049 (w, br), 1737 (w), 1585 (s), 1523 (m), 1488 (w), 1444 (m), 1424 (w), 1357 (s), 1210 (m), 1165 (w), 1106 (w), 1070 (w), 1056 (w), 1020 (m), 910 (w), 851 (m), 822 (m), 812 (m), 767 (m), 727 (w), 702 (w), 669 (m), 661 (m).

4.1. Synthesis and spectral characterization Compounds 1–4 were prepared as crystalline products by hydrothermal reaction of copper nitrate, 4,40 -dipyridylamine, and the requisite carboxylic acid, along with sodium hydroxide to effect acid deprotonation. Compound 3 was initially identified as a very minor component in the reaction that produced compound 2; these were manually removed. Compound 3 could be prepared as a single phase by adjusting the molar ratios of the reactants. The infrared spectra of 1–4 were consistent with their structural characteristics as determined by single-crystal X-ray diffraction. Medium intensity bands in the range of 1600 to 1200 cm1 can be ascribed to stretching modes of the pyridyl rings within the dpa ligands and the phenyl rings of the dicarboxylate ligand in 4. Puckering modes of the pyridyl rings are evident in the region between 820 and 600 cm–1. Asymmetric and symmetric C–O stretching modes of the deprotonated dicarboxylate ligands were visible at 1587 and 1433 cm1 (for 1), 1575 and 1414 cm1 (for 2), 1594 and 1358 cm1 (for 3), and 1585 and 1357 cm1 (for 4). Broad features in the vicinity of 3200 cm1 in all cases represent the O–H stretching bands for any water molecules of crystallization (1, 3) and N–H groups within the dpa ligands (1–4). Consistent with previously identified trends [33], the Dm gap between the asymmetric and symmetric C–O stretching bands is larger for monodentate carboxylate groups; this is seen in the infrared spectra of 3 and 4.

3. X-ray crystallography Single crystal X-ray diffraction was performed using a BrukerAXS ApexII CCD instrument at 173 K. Reflection data were acquired using graphite–monochromated Mo Ka radiation (k = 0.71073 Å). The data was integrated via SAINT [29]. Lorentz and polarization effect and empirical absorption corrections were applied with SADABS [30]. The structures were solved using direct methods and refined on F2 using SHELXTL [31]. The crystal of 2 was non-merohedrally twinned; its twin law was found using CELL NOW [32]; the structure was refined using reflections both twin components. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon atoms were placed in calculated positions and refined isotropically with a riding model. The hydrogen atoms bound to the amine and amide nitrogen atoms and water molecule oxygen atoms were found by Fourier difference map where possible, and refined with restrained distances and isotropic thermal parameters. Relevant crystallographic data for 1–4 are listed in Table 1.

4.2. Structural description of {[Cu(c-14cdc)(dpa)]H2O}n (1) The asymmetric unit of compound 1 contains a divalent copper atom on a general position, a full axial–equatorial conformation c-14cdc ligand, a dpa ligand, and a water molecule of crystallization. A Jahn–Teller distorted square pyramidal {CuO4N} coordination environment (s = 0.002) [34] is seen at copper, with single

Table 1 Crystal and structure refinement data for 1–4. Data

1

2

3

4

Empirical Formula Formula Weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) Minimum/maximum transition hkl ranges

C18H21CuN3O5 422.93 triclinic  P1

C52H58Cu4N6O16 1277.24 triclinic  P1

C28H36CuN7O10.5 702.18 triclinic  P1

8.443(7) 8.843(9) 12.705(12) 98.12(2) 94.77(3) 105.328(16) 898.4(15) 2 1.563 1.251 0.7755/0.9175 10 6 h 6 10, 10 6 k 6 10, 15 6 l 6 15 14177 3250 0.0931 253/4 0.0796 0.0613 0.1651 0.1493 1.253/1.619 1.018

10.584(2) 11.339(3) 13.522(3) 65.887(3) 87.068(3) 63.399(2) 1307.0(5) 1 1.623 1.683 0.7221/0.8172 12 6 h 6 12, 12 6 k 6 13, 0 6 l 6 16 38175 9110 0.1092 356/0 0.1823 0.0817 0.2086 0.1643 0.894/0.831 0.989

10.6851(8) 11.4951(9) 14.6721(12) 66.984(1) 87.590(1) 68.951(1) 1537.8(2) 2 1.516 0.781 0.8396/0.9445 12 6 h 6 12, 13 6 k 6 13, 17 6 l 6 17 25638 5658 0.0467 454/9 0.0509 0.0379 0.0956 0.0881 0.616/0.423 1.039

C20H17CuN3O4 426.91 monoclinic P21/n 7.5165(1) 15.2093(2) 15.9472(3) 90 99.080(1) 90 1800.25(5) 4 1.575 2.003 0.5647/0.7530 9 6 h 6 9, 16 6 k 6 18, 16 6 l 6 19 12180 3208 0.0296 259/0 0.0372 0.0351 0.1037 0.1018 0.317/0.782 1.054

Total reflections Unique reflections R(int) Parameters/restraints R1 (all data) R1 (I > 2r(I)) wR2 (all data) wR2 (I > 2r(I)) Max/min residual (e/Å3) Goodness-of-fit

300

S.H. Qiblawi et al. / Inorganica Chimica Acta 407 (2013) 297–305

oxygen atom donors from four different c-14cdc ligands in the basal plane; the apical site is taken up by a dpa pyridyl nitrogen donor (Fig. 1a). Among the four ligating c-14cdc carboxylates, two in trans positions belong to an axial position carboxylate and two in the other trans positions belong to an equatorial position carboxylate. Bond lengths and angles within the coordination sphere are standard for this geometry, and are listed in Table 2. The dpa ligands in 1 serve as simple monodentate donors, with unprotonated, pendant pyridyl rings. This is a rarely encountered binding mode in dpa coordination chemistry [35]. The c-14cdc ligands in 1 adopt an exotetradentate l4-j4O:O0 :O00 :O000 binding mode, which constructs {Cu2(OCO)4} paddlewheel dimers as seen in copper(II) acetate and other copper carboxylate coordination polymers [36]. These show a Cu  Cu distance of 2.685(3) Å, with crystallographic inversion centres at the dimer centroids. The cis orientation of the c-14cdc carboxylate groups produces a staplelike ‘‘U’’ conformation, allowing connection of each {Cu2(OCO)4} paddlewheel dimer to two others by pairs of c-14cdc ligands and thus forming [Cu(c-14cdc)(dpa)]n coordination polymer chains oriented along the a crystal direction (Fig. 1b). Pendant pyridyl rings from dpa ligands in neighboring chain motifs engage in pp interactions (centroid–centroid distance = 4.120(5) Å), stabilizing supramolecular layers of interdigitated [Cu(c-14cdc)(dpa)]n chains (Fig. S1). Adjacent supramolecular layers aggregate into the full crystal structure of 1 via hydrogen bonding patterns involving the central dpa amine groups, unligated water molecules, and equatorial c-14cdc carboxylate groups (Fig. S2). The isolated water molecules lie between the layers in small regions occupying only 4.0% of the unit cell volume, according to PLATON [37]. Metrical parameters for the hydrogen bonding interactions in 1 are listed in Table S1. 4.3. Structural description of [Cu4(t-14cdc)4(dpa)2]n (2) The asymmetric unit of compound 2 contains two divalent copper atoms (Cu1, Cu2), one full t-14cdc ligand (t-14cdc-A), two halves of crystallographically distinct t-14cdc ligands (t-14cdc-B, t-14cdc-C) whose cyclohexane ring centroids rest on inversion centers, and one dpa ligand. Both Cu1 and Cu2 have five-coordinate {CuO4N} environments, with bond lengths and angles listed in Table 3. The Cu1 atoms have a Jahn–Teller distorted square pyramidal coordination environment with long apical bonds (s = 0.045), while the Cu2 atoms have a distorted coordination sphere virtually midway between idealized square pyramidal and trigonal bipyramidal (s = 0.60). The coordination environments are depicted in Fig. 2a. As Cu2 possesses four short bonds and one long bond, it is best chemically to consider this a square pyramidal environment with the long bond in the apical position as consistent with the

Table 2 Bond length (Å) and angle (°) data for the coordination environment in 1. Cu1–O2#1 Cu1–O1 Cu1–O3#2 Cu1–O4#3 Cu1–N1 O2#1–Cu1–O1 O2#1–Cu1–O3#2 O1–Cu1–O3#2

1.965(3) 1.966(3) 1.998(3) 2.010(3) 2.181(4) 167.32(12) 90.07(14) 89.78(14)

O2#1–Cu1–O4#3 O1–Cu1–O4#3 O3#2–Cu1–O4#3 O2#1–Cu1–N1 O1–Cu1–N1 O3#2–Cu1–N1 O4#3–Cu1–N1

88.25(14) 89.19(14) 167.64(12) 98.90(14) 93.78(13) 90.07(14) 102.29(14)

Symmetry codes: #1 x + 1, y + 2, z; #2 x, y + 2, z; #3 x + 1, y, z.

Table 3 Bond length (Å) and angle (°) data for the coordination environments in 2. Cu1–O3 Cu1–O7 Cu1–O5#1 Cu1–N1#2 Cu1–O5#3 Cu2–O8 Cu2–O1 Cu2–O4 Cu2–N3 Cu2–O6#1 O3–Cu1–O7 O3–Cu1–O5#1 O7–Cu1–O5#1 O3–Cu1–N1#2 O7–Cu1–N1#2

1.926(4) 1.953(5) 1.953(4) 2.011(6) 2.276(5) 1.912(5) 1.958(5) 1.971(4) 1.977(6) 2.462(5) 91.1(2) 177.5(2) 88.67(19) 89.5(2) 174.8(2)

O5#1–Cu1–N1#2 O3–Cu1–O5#3 O7–Cu1–O5#3 O5–Cu1–O5#3 N1#2–Cu1–O5#3 O8–Cu2–O1 O8–Cu2–O4 O1–Cu2–O4 O8–Cu2–N3 O1–Cu2–N3 O4–Cu2–N3 O8–Cu2–O6#1 O1–Cu2–O6#1 O4–Cu2–O6#1 N3–Cu2–O6#1

91.0(2) 97.77(18) 90.35(19) 79.73(18) 94.7(2) 86.6(2) 94.3(2) 136.8(2) 173.0(2) 91.9(2) 91.5(2) 94.30(19) 110.29(17) 112.66(19) 79.8(2)

Symmetry codes: #1 x  1, y, z; #2 x + 1, y, z + 1; #3 x + 2, y, z + 2.

Jahn–Teller effect formalism placing the elongation along the z axis. At Cu1, the basal positions of the square pyramid are taken up by trans oxygen donors belonging to two different t-14cdc-A ligands, a pyridyl nitrogen donor from a dpa ligand, and an oxygen donor of a t-14cdc-C ligand; the apical site is occupied by an oxygen donor from a third t-14cdc-A ligand. At Cu2, the basal positions are filled by a dpa pyridyl nitrogen donor and a t-14cdc-C ligand carboxylate oxygen donor in two trans positions, and by t-14cdcA and t-14cdc-B oxygen donors. The apical site at Cu2 contains an oxygen atom donor from another t-14cdc-A ligand. Dimeric units containing Cu1 and Cu2 are formed by a pair of bridging carboxylate groups, from t-14cdc-A and t-14cdc-C ligands, that span basal positions of the respective coordination spheres. The Cu1  Cu2 distance within these dimers is 3.227 Å. Pairs of these dimeric units are connected into {Cu4(OCO)6} tetrameric clusters (Fig. 2b) by a third bridging carboxylate, provided by oxygen atoms belonging to t-14cdc-A ligands. These carboxylate groups contain an oxygen atom that is shared between basal and apical

Fig. 1. (a) Coordination environment of 1, with thermal ellipsoids at 50% probability and partial atom numbering scheme. The symmetry codes are as in Table 1. Most hydrogen atoms have been omitted. A complete {Cu2(OCO)4} paddlewheel dimer is shown. (b) [Cu(c-14cdc)(dpa)]n coordination polymer chain in 1.

S.H. Qiblawi et al. / Inorganica Chimica Acta 407 (2013) 297–305

301

Fig. 2. (a) Coordination environments of 2, with thermal ellipsoids at 50% probability and partial atom numbering scheme. The symmetry codes are as in Table 2. Most hydrogen atoms have been omitted. (b) {Cu4(OCO)6} tetranuclear cluster in 2.

Fig. 3. (a) View down a of the 3D [Cu4(t-14cdc)4(dpa)2]n coordination polymer network in 2. (b) View down [1 1 0].

coordination sites on neighboring Cu1 atoms, resulting in a Cu1  Cu1 distance of 3.253 Å. Pairs of these shared oxygen atoms create rhombic {Cu1(l-O)2Cu1} units at the centers of the tetrameric clusters. The Cu1 and Cu2 atoms are positioned in the interior and at the periphery of the tetrameric clusters, respectively. Each {Cu4(OCO)6} tetramer connects to eight others, forming a complex [Cu4(t-14cdc)4(dpa)2]n 3D coordination polymer network (Fig. 3). A close-up of the tetramer connectivity is shown in Fig. 4a and b. Each {Cu4(OCO)6} tetramer connects to two other tetramers

along the a crystal direction by pairs of t-14cdc-A ligands (purple in Fig. 4), which span Cu1  Cu1 and Cu2  Cu2 distances of 10.584 Å. The t-14cdc-A ligands adopt an exopentadentate l5-j4O:O0 :O00 :O000 :O000 binding mode, joining to three Cu1 atoms and two Cu2 atoms. Each {Cu4(OCO)6} tetramer also connects to two other tetramers by bis(monodentate) t-14cdc-B ligands (red in Fig. 4), which connect Cu2 atoms at a distance of 10.932 Å. Additionally, each {Cu4(OCO)6} tetramer connects to two other tetramers via exotetradentate l4-j4O:O0 :O00 :O00 0 binding mode

302

S.H. Qiblawi et al. / Inorganica Chimica Acta 407 (2013) 297–305

Fig. 4. Connectivity of one {Cu4(OCO)6} tetranuclear cluster (denoted as green spheres) to eight other tetrameric clusters in 2. (a) highlighting the equatorial–equatorial conformation t-14cdc-A (purple) and t-14cdc-B (red) connections. (b) highlighting the axial–axial conformation t-14cdc-C (orange) and dpa (blue) connections (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

t-14cdc-C ligands (orange in Fig. 4), with a Cu1  Cu2 intercluster distance of 9.354 Å. This shorter distance is caused by the axial– axial conformation of the t-14cdc-C ligands, in contrast with the equatorial–equatorial conformation of the t-14cdc-A and t-14cdcB ligands. Finally, each {Cu4(OCO)6} tetramer connects to two others by pairs of dipodal dpa ligands (blue in Fig. 4), which span a Cu1  Cu2 distance of 11.603 Å. A topological analysis of the 3D coordination polymer net of 2 was undertaken using TOPOS [38], treating the {Cu4(OCO)6} tetramers as 8-connected nodes. The underlying topology of 2 was determined to be a very rare example of a self-penetrated 4245.63 ilc topology (Fig. 5). To the best of our knowledge, this net has only been reported twice previously, in [Zn5(l3-OH)2(bdc)4(phen)2]n [39] and in {[Cu4(1,4-phda)3(OH)2(dpp)2]2H2O}n [40]. Notably all three known ilc nets are based on high nuclearity clusters, which can provide the necessary large numbers of connections. The stability of the self-penetrated net is supplemented by hydrogen bonding donation from dpa N–H groups to unligated oxygen atoms of the bis(monodentate) t-14cdc-B ligands (Table S1). 4.4. Structural description of {[Cu(t-14cdc)(dpa)(Hdpa)] (NO3)3.5H2O}n (3) Compound 3 was first obtained as a minor product during the synthesis of 2. Adjustment of the stoichiometric ratio of starting

materials allowed preparation of 3 in phase-pure form. Its asymmetric unit contains a divalent copper atom, an equatorialequatorial conformation t-14cdc ligand, a monodentate unprotonated dpa ligand, and a pendant protonated Hdpa ligand, along with an unligated nitrate counteranion and a total of threeand-one-half water molecules of crystallization. The copper atom shows a square planar {CuO2N2} environment, with trans pyridyl nitrogen donor atoms, one from an unprotonated dpa ligand and one from a protonated Hdpa ligand (Fig. 6a). Single carboxylate oxygen atoms from two t-14cdc ligands occupy the other two trans coordination sites. Bond lengths and angles within the coordination environment are listed in Table 4. Adjacent [Cu(dpa)(Hdpa)]3+ units are bridged by bis(monodentate) (l2-j2O:O00 binding mode) t-14cdc ligands to afford 1D [Cu(t14cdc)(dpa)(Hdpa)]+ cationic coordination polymer chains (Fig. 6b). The Cu  Cu distance through the t-14cdc ligands is 10.685 Å. Neighboring chains form supramolecular layers via strong N–H+00 N hydrogen bonding interactions [41] between protonated Hdpa pyridyl rings and unprotonated, unligated pyridyl rings belonging to the unprotonated dpa ligands (Fig. 6c). Charge-balancing nitrate anions are held within the supramolecular layers by hydrogen bonding acceptance from Hdpa central amine groups. Hydrogen bonding patterns between unligated t14cdc carboxylate oxygen atoms, pairs of unligated water molecules, and dpa N–H groups serve to aggregate the supramolecular layers into the 3D crystal structure of 3 (Fig. S3). Hydrogen bonding information is listed in Table S1.

4.5. Structural description of [Cu(1,4-phda)(dpa)]n (4)

Fig. 5. Schematic perspective of the 4245.63 ilc self-penetrated network of 2, treating the tetramers as 8-connected nodes. Tetramer centroids are represented as golden spheres. The purple and blue rods represent t-14cdc-A pairs and dpa pairs, respectively. The red and orange rods represent t-14cdc-B and t-14cdc-C connections, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

The asymmetric unit of compound 4 contains two divalent copper atoms on crystallographic inversion centers (Cu1, Cu2), a fully deprotonated 1,4-phda ligand, and a dpa ligand. Different coordination environments are seen at the two crystallographically distinct copper atoms (Fig. 7a). A Jahn–Teller distorted {CuO4N2} octahedral environment is present at Cu1, with carboxylate groups from two 1,4-phda ligands occupying an axial and an equatorial coordination site each. Pyridyl nitrogen donor atoms from two dpa ligands fill the remaining trans equatorial positions. A simple {CuO2N2} square planar environment exists at Cu2, with trans dpa pyridyl nitrogen donor atoms and trans carboxylate oxygen atom donors from two 1,4-phda ligands. Bond lengths and angles within the different coordination spheres are listed in Table 5. The 1,4-phda ligands adopt a chelating/monodentate l2-j3O,O0 :O00 binding mode, connecting Cu1 and Cu2 atoms into undulated [Cu(1,4-phda)]n coordination polymer chains oriented along the c crystal axis (Fig. 7b). The staple-like conformation of the 1,4-phda ligands, with both carboxylate groups pointing

S.H. Qiblawi et al. / Inorganica Chimica Acta 407 (2013) 297–305

303

Fig. 6. (a) Coordination environment of 3, with thermal ellipsoids at 50% probability and partial atom numbering scheme. The symmetry codes are as in Table 3. Most hydrogen atoms have been omitted. (b) Cationic [Cu(t-14cdc)(dpa)(Hdpa)]nn+ chain in 3. (c) Hydrogen-bonded supramolecular layer in 3, with unligated nitrate ions and water molecules shown.

Table 4 Bond length (Å) and angle (°) data for the coordination environment in 3. Cu1–O3#1 Cu1–O1 Cu1–N2 Cu1–N1 O3#1–Cu1–O1

1.9245(16) 1.9265(17) 2.041(2) 2.042(2) 177.41(8)

O3#1–Cu1–N2 O1–Cu1–N2 O3#1–Cu1–N1 O1–Cu1–N1 N2–Cu1–N1

89.30(8) 91.48(8) 90.45(8) 88.90(8) 177.11(8)

Symmetry code: #1 x  1, y, z.

towards the same side of the aromatic ring plane, provides a Cu  Cu internuclear contact distance of 7.974 Å. Parallel sets of [Cu(1,4-phda)]n chains are connected into [Cu(1,4-phda)(dpa)]n (4,4) grid layers (Fig. 7c) by tethering dpa ligands, which span a Cu1  Cu2 distance of 11.790 Å. The noticeably pinched grid aper-

tures show Cu  Cu  Cu angles of 39.0° and 140.1°, and throughspace Cu  Cu distances of 7.52 and 18.67 Å. Hydrogen bonding between dpa central amine groups and unligated 1,4-phda carboxylate oxygen atoms stabilizes the layer motif (Table S1). Weak C– H  O interactions between unligated 1,4-phda carboxylate oxygen atoms and sp3 hybridized peripheral carbon atoms of other 1,4phda ligands provide the impetus for stacking of neighboring [Cu(1,4-phda)(dpa)]n layers (Fig. S4). 4.6. Magnetic properties As compound 1 contains isolated {Cu2(OCO)4} paddlewheel dimers with likely strong antiferromagnetic coupling between the face-on dx2–y2 orbitals, a magnetic study was not undertaken because it would in all likelihood simply prove a verification of

Fig. 7. (a) Coordination environment of 4, with thermal ellipsoids at 50% probability and partial atom numbering scheme. The symmetry codes are as in Table 4. Most hydrogen atoms have been omitted. (b) [Cu(1,4-phda)]n chain in 4. (c) [Cu(1,4-phda)(dpa)]n (4,4) grid layer in 4, with [Cu(1,4-phda)]n chains shown in red (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

304

S.H. Qiblawi et al. / Inorganica Chimica Acta 407 (2013) 297–305

Table 5 Bond length (Å) and angle (°) data for the coordination environments in 4. Cu1–O4#1 Cu1–N3 Cu1–O3#1 Cu2–O1 Cu2–N1 O4#2–Cu1–O4#1 O4#1–Cu1–N3#3 O4#1–Cu1–N3 N3#3–Cu1–N3 O4#2–Cu1–O3#1 O4#1–Cu1–O3#1 N3#3–Cu1–O3#1 N3–Cu1–O3#1 O3–Cu1–O3#1 O1#4–Cu2–O1 O1#4–Cu2–N1 O1–Cu2–N1 N1#4–Cu2–N1

2.0080(13) 2.0136(15) 2.4575(18) 1.9295(13) 2.0212(15) 180.0 90.82(6) 89.18(6) 180.00(4) 121.94(5) 58.06(5) 86.82(6) 93.18(6) 180.0 180.0 90.58(6) 89.42(6) 180.0

Symmetry codes: #1 x  1, y, z; #2 x + 1, y + 1, z + 1; #3 x, y + 1, z + 1; #4 x + 2, y + 1, z.

the well-known trend. For compound 2, the vmT value was 1.013 cm3-K mol1 at 300 K, less than that predicted for four uncoupled S = 1/2 spins per formula unit and thus portending strong antiferromagnetic coupling. The vmT value decreased rapidly as the sample was cooled, reaching 0.604 cm3-K mol1 at 200 K, 0.153 cm3-K mol1 at 100 K, and 0.021 cm3-K mol1 at 2 K. This behavior is clearly indicative of strong antiferromagnetic coupling within the tetrameric copper clusters. Attempts were made to model the system as a fully interacting tetrameric S = 1/2 system using two different J values for peripheral Cu2  Cu1 (J1) and central Cu1  Cu1 (J2) spin interactions; these proved unsuccessful unless the J2 value was set to zero. Stronger antiferromagnetic coupling is expected between the basal-basal carboxylate bridged peripheral Cu2  Cu1 pairs within the tetramer because of better overlap of the magnetic dx2–y2 orbitals. Magnetic interactions within the central Cu1  Cu1 linkages in this tetramer unit, whether antiferromagnetic or ferromagnetic, is expected to be far weaker in comparison, as a result of the apical-basal bridging mode of the two carboxylate l-O atoms. Thus, the variable temperature magnetic susceptibility behavior of 2 was modeled as a linear tetramer [42] with J2 = 0 (Eq. 1), along with a term (q) accounting for the mole fraction of any residual monomeric paramagnetic impurity (likely some crystals of 3). The best fit values (Fig. 8) were g = 2.19(2),

J1 = 154(2) cm1, J2 = 0 cm1; q = 0.025(1), with R = 3.3  10–4 = {R[(vmT)obs – (vmT)calc]2/R[(vmT)obs]2}, and where the variables have their usual meanings. This analysis confirms very strong antiferromagnetic coupling between the basal-basal bridged Cu1 and Cu2 atoms within the tetramer. In comparison, no appreciable coupling is observed between the between the apical-basal Cu1 atoms at the center of the tetramer.

! E1 E2 E3 E4 Ng 2 b2 10e kT þ 2e kT þ 2e kT þ 2e kT ð1  qÞ vm T ¼ E1 E2 E3 E4 E5 E6 k 5e kT þ 3e kT þ e kT þ e kT þ e kT þ e kT þ

Ng 2 b2 ðqÞ k

  J with E1 ¼ J 1  2 ; 2   J2 ; E2 ¼ J 1  2   qffiffiffiffiffiffiffiffiffiffiffiffiffiffi J E3 ¼ 2 þ J 21 þ J 22 ; 2   qffiffiffiffiffiffiffiffiffiffiffiffiffiffi J2  J 21 þ J 22 ; E4 ¼ 2   qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi J E5 ¼ J 1 þ 2 þ 4J 21  2J 2 J 2 þ J 22 ; 2   qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi J2  4J 21  2J 2 J 2 þ J 22 ; E6 ¼ J 1 þ 2

ð1Þ

4.7. Thermogravimetric analysis Compound 1 underwent dehydration between 165 and 180 °C, as evidenced by a 3.3% mass loss. As the predicted mass loss for one molar equivalent of water was 4.3%, it is apparent that some dehydration occurred on long-term storage of the sample. The mass remained constant until 275 °C, whereupon ligand combustion ensued. The total mass loss between 275 and 475 °C was 70.2%, matching well with the 69.0% expected for ejection of the dpa ligands and the cdc ligands if one equivalent of carboxylate moiety remained. The 27.6% mass remnant at 475 °C likely represents a mixture of CuO (18.8% remnant predicted) and CuCO3 (29.2% remnant predicted). Compound 2 showed no mass loss until 230 °C. A multi-step series of ligand losses occurred between this temperature and 400 °C. The final mass remnant of 30.5% is consistent with a deposition of a mixture of CuO (24.9% calc’d) and CuCO3 (38.7% calc’d). The mass of compound 4 remained steady between 25 and 270 °C, whereupon ligand combustion occurred. The final mass remnant of 35.6% is consistent with a deposition of CuCO3 (28.9% calc’d) along with some uncombusted organic material. The TGA traces for 1, 2, and 4 are given in Figs. S5–S7, respectively. 5. Conclusions

Fig. 8. Plot of vmT vs. T for 2. The best fit to Eq. (1) is shown as a thin line.

Three divalent copper 1,4-cyclohexanedicarboxylate coordination polymers containing the kinked, hydrogen-bonding capable dipodal tethering ligand dpa were prepared and structurally characterized. The resulting topology within these materials depends crucially on the specific 1,4-cyclohexanedicarboxylate isomer used, and also the stoichiometric ratio of the starting materials. The kinked axial–equatorial disposition of the carboxylate groups in the cis 1,4-cyclohexanedicarboxylate isomer resulted in a 1D [Cu(c-14cdc)]n chain motif in 1 comprising commonly encountered {Cu2(OCO)4} paddlewheel dimeric units, decorated by pendant dpa ligands in an uncommon j1 binding mode. Moving to the trans 1,4cyclohexanedicarboxylate isomer afforded antiferromagnetically

S.H. Qiblawi et al. / Inorganica Chimica Acta 407 (2013) 297–305

coupled tetrameric secondary building units linked into a rare highly connected self-penetrated 3D coordination polymer network in 2. The torsional flexibility of the t-14cdc allowed different metal–metal distances to be spanned, via equatorial–equatorial or axial–axial conformations. Additional dpa in the synthetic conditions permitted yield optimization and isolation of a 1D chain variant of 2 with pendant dpa ligands preventing aggregation into higher dimensionality. Imposing flexibility in the pendant dicarboxylate arms by employing the 1,4-phenylenediacetate ligand, while simultaneously restricting flexibility in the ring component, resulted in a layered coordination polymer in 4. Hydrogen bonding mechanisms provided by the central amine group of the dpa ligands plays an important role in the supramolecular aggregations. Subtle conformational or isomeric effects within the dicarboxylate ligands has an extremely significant effect in instilling the final coordination polymer topology in this system. Acknowledgments We acknowledge the donors of the American Chemical Society Petroleum Research Fund and Lyman Briggs College for funding this work. Appendix A. Supplementary material CCDC 929527, 929529, 929528, and 929526 contains the supplementary crystallographic data for 1–4. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2013.07.059. References [1] (a) L.J. Murray, M. Dinca, J.R. Long, Chem. Soc. Rev. 38 (2009) 1294 (and references therein); (b) S.S. Han, J.L. Mendoza-Cortés, W.A. Goddard, Chem. Soc. Rev. 38 (2009) 1460 (and references therein). [2] J.R. Li, R.J. Kuppler, H.C. Zhou, Chem. Soc. Rev. 38 (2009) 1477 (and references therein). [3] (a) Q.-R. Fang, G.-S. Zhu, M. Xue, J.-Y. Sun, S.-L. Qiu, Dalton Trans. (2006) 2399; (b) X.-M. Zhang, M.-L. Tong, H.K. Lee, X.-M. Chen, J. Solid State Chem. 168 (2001) 118; (c) O.M. Yaghi, H. Li, T.L. Groy, Inorg. Chem. 36 (1997) 4292. [4] (a) J.Y. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Chem. Soc. Rev. 38 (2009) 1450 (and references therein); (b) L. Ma, C. Abney, W. Lin, Chem. Soc. Rev. 38 (2009) 1248 (and references therein); (c) T. Uemara, N. Yanai, S. Kitagawa, Chem. Soc. Rev. 38 (2009) 1228 (and references therein). [5] (a) S. Zang, Y. Su, Y. Li, Z. Ni, Q. Meng, Inorg. Chem. 45 (2006) 174; (b) L. Wang, M. Yang, G. Li, Z. Shi, S. Feng, Inorg. Chem. 45 (2006) 2474; (c) S. Wang, Y. Hou, E. Wang, Y. Li, L. Xu, J. Peng, S. Liu, C. Hu, New J. Chem. 27 (2003) 1144. [6] H.W. Roesky, M. Andruh, Coord. Chem. Rev. 36 (2003) 91.

305

[7] O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Acc. Chem. Res. 31 (1998) 474. [8] K. Biradha, M. Sarkar, L. Rajput, Chem. Commun. (2006) 4169. [9] H. Li, M. Eddaoudi, M. O’Keeffe, O.M. Yaghi, Nature 402 (1999) 276. [10] (a) C.G. Silva, I. Luz, F.L. Xamena, A. Corma, H. Garcia, Chem. Eur. J. 16 (2010) 11133; (b) S.J. Garibay, S.M. Cohen, Chem. Commun. 46 (2010) 7700; (c) M. Kandiah, S. Usseglio, S. Svelle, U. Olsbye, K.P. Lillerud, M. Tilset, J. Mater. Chem. 20 (2010) 9848; (d) J.H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga, K.P. Lillerud, J. Am. Chem. Soc. 130 (2008) 13850. [11] W. Mori, S. Takamizawa, J. Solid State Chem. 152 (2000) 120. [12] D.N. Dybtsev, H. Chun, K. Kim, Angew. Chem., Int. Ed. 43 (2004) 5033. [13] Z. Lin, M.L. Tong, Coord. Chem. Rev. 255 (2011) 421. [14] X. Wang, W. Yao, Y. Qi, M. Luo, Y. Wang, H. Xie, Y. Yu, R. Ma, Y. Li, CrystEngComm 13 (2011) 2542. [15] M. Du, H. Cai, X. Zhao, Inorg. Chim. Acta 358 (2005) 4034. [16] M. Kurmoo, H. Kumagai, M. Akita-Tanaka, K. Inoue, S. Takagi, Inorg. Chem. 45 (2006) 1627. [17] Y. Gong, J. Li, J. Qin, T. Wu, R. Cao, J. Li, Cryst. Growth Des. 11 (2011) 1662. [18] (a) M. Inoue, M. Moriwaki, T. Atake, H. Kawaji, T. Tojo, W. Mori, Chem. Phys. Lett. 365 (2002) 509; (b) C.N. Kato, M. Hasegawa, T. Sato, A. Yoshizawa, T. Inoue, W. Mori, J. Catal. 230 (2005) 226. [19] L.K. Sposato, J.H. Nettleman, R.L. LaDuca, CrystEngComm 12 (2010) 2374. [20] M. Kurmoo, H. Kumagai, S.M. Hughes, C.J. Kepert, Inorg. Chem. 42 (2003) 6709. [21] P.C.R. Soares-Santos, L. Cunha-Silva, F.A. Almeida Paz, R.A.S. Ferreira, J. Rocha, T. Trindade, L.D. Carlos, H.I.S. Nogueira, Cryst. Growth Des. 8 (2008) 2505. [22] G.A. Farnum, C.Y. Wang, C.M. Gandolfo, R.L. LaDuca, J. Mol. Struct. 998 (2011) 62. [23] W. Bi, R. Cao, D. Sun, D. Yuan, X. Li, Y. Wang, X. Li, M. Hong, Chem. Commun. (2004) 2104. [24] R.L. LaDuca, Coord. Chem. Rev. 253 (2009) 1759. [25] E. Shyu, R.M. Supkowski, R.L. LaDuca, Inorg. Chem. 48 (2009) 2723. [26] M.R. Montney, S. Mallika Krishnan, N.M. Patel, R.M. Supkowski, R.L. LaDuca, Cryst. Growth Des. 7 (2007) 1145. [27] P.J. Zapf, R.L. LaDuca, R.S. Rarig, K.M. Johnson, J. Zubieta, Inorg. Chem. 37 (1998) 3411. [28] O. Kahn, Molecular Magnetism, VCH Publishers, New York, 1993. [29] SAINT, Software for Data Extraction and Reduction, Version 6.02; Bruker AXS, Inc., Madison, WI, 2002. [30] SADABS, Software for Empirical Absorption Correction. Version 2.03; Bruker AXS, Inc., Madison, WI, 2002. [31] G.M. Sheldrick, SHELXTL, Program for Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997. [32] G.M. Sheldrick, CELLNOW, University of Göttingen, Göttingen, Germany, 2003. [33] K. Nakamoto, Infrared Spectra and Raman Spectra of Inorganic and Coordination Compounds, John Wiley & Sons, New York, 1986. [34] A.W. Addison, T.N.J. Rao, J. Reedijk, J. van Rijn, G.C. Verschoor, J. Chem. Soc., Dalton Trans. (1984) 1349. [35] R.S. Rarig, R.L. La Duca, J. Zubieta, Inorg. Chim. Acta 292 (1999) 131. [36] D.L. Reger, A. Debreczeni, B. Reinecke, V. Rassolov, M.D. Smith, R.F. Semeniuc, Inorg. Chem. 48 (2009) 8911. [37] A.L. Spek, PLATON; A Multipurpose Crystallographic Tool, Utrecht University, Utrecht, The Netherlands, 1998. [38] V.A. Blatov, A.P. Shevchenko, V.N. Serezhkin, J. Appl. Crystallogr. 33 (2000) 1193 (TOPOS software is available for download at: ). [39] X.L. Wang, C. Qin, E.B. Wang, Z.M. Su, L. Xu, S.R. Batten, Chem. Commun. (2005) 4789. [40] J.J. Perry, J.A. Perman, M.J. Zaworotko, Chem. Soc. Rev. 38 (2009) 1400 (and references therein). [41] T. Steiner, Angew. Chem., Int. Ed. 41 (2002) 48. [42] G.V. Rubenacker, J.E. Drumheller, K. Emerson, R.D. Willett, J. Magn. Magn. Mater. 54–57 (1986) 1483.