Supramolecular Structures from Three New Molecular Building Blocks

Supramolecular Structures from Three New Molecular Building Blocks Based on the Protonation/Deprotonation of 3,3-Bis(2-imidazolyl)propionic Acid (HBIP): [H3BIP](C4O4)‚H2O, [Cu(HBIP)2](HC4O4)2, and [Cu(BIP)2]‚2H2O

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 5 1124-1133

Youness Akhriff,† Juan Server-Carrio´,† Julia Garcıá-Lozano,† Jose´ Vicente Folgado,† Amparo Sancho,† Emilio Escriva`,† Pablo Vitoria,‡ and Lucıá Soto*,† Departament de Quı´mica Inorga` nica, UniVersitat de Vale` ncia, c/Vicent Andre´ s Estelle´ s s/n, 46100 Burjassot, Vale` ncia, Spain, and Departamento de Quı´mica Inorga´ nica, Facultad de Ciencia y Tecnologıá, UniVersidad del Paı´s Vasco, Barrio Sarriena s/n, 48080 Leioa, Bizkaia, Spain ReceiVed October 14, 2005; ReVised Manuscript ReceiVed February 16, 2006

ABSTRACT: This paper reports the synthesis, structure solution, and electronic and electron paramagnetic resonance (EPR) characterization of three new HBIP-containing compounds (HBIP ) 3,3-bis(2-imidazolyl)propionic acid): [H3BIP](C4O4)‚H2O (1), [Cu(HBIP)2](HC4O4)2 (2), and [Cu(BIP)2]‚2H2O (3). The structure of 1, resulting from the cocrystallization of [H3BIP]2+ and SQ2-, is made up of squarate anions, diprotonated HBIP, and water molecules packed together by multiple-path H-bonding into a supramolecular 3D network. Compounds 2 and 3 constitute the first cases of structurally characterized Cu(II)-HBIP/BIP complexes in which the ligand acts in a tridentate chelating fashion. They consist of Jahn-Teller elongated octahedral mononuclear Cu(II) units packed together into a 3D network driven in each case by a different set of multiple H-bonding paths. These differences in the supramolecular crystal packing of 2 and 3 arise from the different nature of the uncoordinated counterparts, HSQ- in 2 vs H2O in 3, which play a critical role in the set of H-bonding interactions. The use of simple anion templation to modify the network topology is discussed. Electronic and EPR spectra are indicative of an essentially dx2-y2 ground state for the copper(II) ions in 2 and 3. Introduction The design and synthesis of specific crystalline architectures are the subject of current interest in connection with the development of novel materials with potential applications based on their optical, electrical, magnetic, and catalytic properties, as well as for their applications as host systems in inclusion processes.1 An attractive strategy to create such aggregates is based on the use of polyfunctional molecules capable of providing not only binding sites for coordination or semicoordination to the metal centers but also chemical groups capable of acting as H-donor/acceptors to interlink the coordination molecules, that is, the molecular building blocks (MBBs), via hydrogen-bond interactions.2 Squaric acid (H2SQ) and squarate anions appear particularly attractive, from the point of view of crystal engineering, as building blocks with template capacity to control, drive, and modulate the assembly process of stable one-, two-, and threedimensional crystalline materials.3 The squarate group is a versatile ligand that can adopt a variety of coordination modes toward metal ions. It can be used to prepare polymeric complexes with novel structural topologies through which to propagate a variety of magnetic interactions.4 In addition, these anions seem particularly suited to interact with molecules with high H-bond donating capacities. The combination of such pairs as squaric acid and N-heterocyclic moieties (bipyridine and imidazole-type ligands, among others) has recently attracted much attention because of their capacity to interact through a combination of hydrogen bonds and ionic interactions.5 In this context, HBIP [3,3-bis(2-imidazolyl)propionic acid] and its deprotonated form, BIP, have proven particularly useful systems in the construction of transition-metal-containing * To whom correspondence should be addressed. Fax: 0034963544960. E-mail: [email protected]. † Universitat de Vale ` ncia. ‡ Universidad del Paı´s Vasco.

networks of a variety of dimensionalities.6 HBIP has several advantages. It is a versatile polyfunctional ligand combining the donor function of two imidazole groups with that of a carboxylic group, and in addition, it has available many H-bond donor/acceptor centers.7 It is well-known that imidazole and carboxylate groups, as a result of these properties, play a crucial role in the catalytic activity of many metallo-enzymes.8 We have recently shown that the particular combinations of the donoracceptor capacity of HBIP and squarate ligands with the reactivity of Cu(II) may be exploited to favor a particular outcome. For example, depending on the molar ratio of the reagents and on other experimental conditions, reaction between squaric acid, HBIP, and copper(II) yields crystalline materials of different structures and dimensionalities, such as [Cu(HBIP)(BIP)](C4O4)1/2‚2H2O and [{Cu(BIP)(OH2)}4(µ-C4O4)](ClO4)2‚ 4H2O.9 These results moved us to explore the possibility of obtaining HBIP/H2SQ and HBIP/H2SQ/Cu(II) novel systems with different crystalline structures. We herein report the preparation and structural characterization of three new compounds built from protonation and deprotonation of the HBIP molecule. [H3BIP](C4O4)‚H2O (1) results from cocrystalization of [H3BIP]2+ cations and SQ2- anions. This compound exhibits a structure in which squarate anions, diprotonated HBIP, and water molecules are extensively hydrogen-bonded into a supramolecular 3D structure. Also reported herein are two new Cu(II) complexes with three-dimensional networks formed from copper coordination monomers interconected by extensive H-bonding, namely, [Cu(HBIP)2](HC4O4)2 (2) and [Cu(BIP)2]‚2H2O (3). Compounds 2 and 3 are the first examples of structurally characterized Cu(II)-HBIP/BIP complexes in which HBIP and BIP act as tridentate ligands. The octahedral copper fragments show significantly different crystal packings in 2 and 3 as an example of the important key role played by H-bonding to drive the resulting 3D structure.

10.1021/cg050543q CCC: $33.50 © 2006 American Chemical Society Published on Web 03/29/2006

Supramolecular Structures from HBIP Building Blocks

Experimental Section Materials. Synthesis of HBIP was carried out according to the method described by Joseph et al.,10 and HBIP was characterized by 1 H NMR, 13C NMR, IR spectroscopy, and powder X-ray diffraction. All other reagents were purchased from commercial sources and used without any further purification. Syntheses. [H3BIP](C4O4)‚H2O (1). This compound was prepared by adding an aqueous solution of HBIP (10-2 M) to a solution of squaric acid in a 1:1 molar ratio. The resulting crystals were separated by filtration and washed with water and ethanol. Anal. Calcd for C13H14N4O7: C, 46.13; H, 4.18; N, 16.57. Found: C, 45.96; H, 4.21; N, 16.39. [Cu(HBIP)2](HC4O4)2 (2). Aqueous solutions of HBIP (0.5 mmol, 50 mL) and CuCl2‚2H2O (0.25 mmol, 2.5 mL) were added to an aqueous solution of H2C4O4 (0.5 mmol, 5 mL) resulting in a green solution with a final pH of 2.3. The solution was allowed to stand at room temperature for about 1 month, yielding violet thin crystals of compound 2, which were separated by filtration and washed with water and ethanol. Anal. Calcd for C26H22N8O12Cu (2): C, 44,45; H, 3.16; N, 15.97; Cu, 9.05. Found: C, 44.76; H, 3.21; N, 15.69; Cu, 9.10. This compound can also be obtained from compound 1 by the following procedure: A solution of CuCl2‚2H2O (0.05 mmol, 0.5 mL) was added to an aqueous solution of 1 (0.1 mmol, 20 mL) with stirring (pH after mixing, 2.7). Slow evaporation at room temperature for about 2 weeks afforded violet crystals of complex 2. [Cu(BIP)2]‚2H2O (3). An aqueous solution of CuCl2‚2H2O (0.25 mmol, 2.5 mL) was added to an alkaline solution of the ligand (0.5 mmol of HBIP and 0.5 mmol of KOH, 50 mL), giving a final pH of 5.5. The resulting purple powder was separated by filtration, washed with water and ethanol, and dried to constant weight at 70 °C. Elemental analysis fits the formulation given for 3. The remaining blue solution was allowed to stand at room temperature for about 2 months to produce the complex 3 as purple crystals. Anal. Calcd for C18H20N8O5Cu (3): C, 42.39; H, 4.36; N, 21.98; Cu, 12.46. Found: C, 42.71; H, 4.24; N, 22.10; Cu, 12.23. It should be pointed out that compounds 1-3 show very different solubilities. While compound 1 dissolves in water, as expected for a typical ionic compound with electrostatic interactions, 2 and 3 are insoluble in most solvents, except in strong acids, indicating a significant stability of their networks. Physical Measurements. The IR spectra were recorded on a Pye Unicam SP 2000 spectrophotometer as KBr pellets in the 4000-300 cm-1 region. Diffuse reflectance spectra were obtained using a PerkinElmer Lambda 9 UV/Vis/IR spectrophotometer. Polycrystalline powder electron paramagnetic resonance (EPR) spectra were recorded at room temperature on a Bruker ESP-300. Magnetic susceptibility was measured by means of a commercial SQUID magnetometer, Quantum Design model MPMS7, down to 1.8 K. X-ray Data Collection and Structure Determinations. X-ray single-crystal diffraction data for compounds 1-3 were collected on an Enraf-Nonius CAD4 diffractometer, and intensity measurements were carried out at room temperature (293 K) using graphitemonochromated Mo KR radiation (λ ) 0.710 69 Å). The unit cell dimensions were determined from the angular settings of 25 reflections. The intensity data were measured between the limits 1° < θ < 25° using the ω/(2θ) scan technique. Data reduction was performed with X-RAY76.11 Empirical absorption corrections, following the procedure DIFABS,12 were applied in compounds 2 and 3. All the structures were solved by direct methods using the program SIR97.13 In each case, all non-hydrogen atoms were anisotropically refined by least-squares on F2 with SHELXL97.14 Hydrogens bonded to C atoms were placed at calculated positions, and hydrogens bonded to O and N atoms were located by difference synthesis. All of them were kept fixed in the final refinement with isotropic temperature factors related to their parent atom. Graphical representations were produced with ORTEP3 for Windows15 and Mercury 1.2.16 in Table 1. Other data relevant to the crystal structure study are listed in Table 1. Compound 1. The unit cell dimensions of the selected colorless crystal, with approximate dimensions 0.15 × 0.30 × 0.35 mm3, were determined in the range 7° < θ < 14°. A total of 3325 reflections were measured in the hkl ranges 0 to 9, 0 to 19, and -14 to 14. From the 2892 independent reflections, 1537 were considered observed with I > 2σ(I). There were 232 refined parameters. After the final refinement,

Crystal Growth & Design, Vol. 6, No. 5, 2006 1125 Table 1. Crystallographic Data for [H3BIP](C4O4)‚H2O (1), [Cu(HBIP)2](HC4O4)2 (2), and [Cu(BIP)2]‚2H2O (3) chemical formula fw space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z D(calcd), g cm-1 µ, mm-1 R1a wR2b

1

2

3

C13H14N4O7 338.28 P21/n 8.040(1) 15.799(2) 12.103(1) 90 105.44(1) 90 1481.9(3) 4 1.516 0.125 0.0483 0.1223

C26H22CuN8O12 702.06 P1h 7.588(1) 9.085(1) 11.739(1) 68.41(1) 71.15(1) 75.84(1) 704.8(1) 1 1.654 0.857 0.0294 0.0780

C18H22CuN8O6 509.98 P1h 8.978(1) 10.047(1) 12.531(1) 90.29(1) 106.54(1) 105.85(1) 1038.1(2) 2 1.631 1.108 0.0488 0.1563

a R1 ) ∑||F | - |F ||/∑|F | for reflections with I > 2σ(I). b wR2 ) o c o {∑[w(Fo2 - Fc2)2]/[w(Fc2)2]}1/2 for all reflections; w ) 1/[σ2(Fo)2 + (aP)2 + bP], where P ) [2Fc2 + Fo2]/3 and a and b are constants set by the program.

goodness of fit on F2 ) 0.959 and largest difference peak and hole ) 0.228 and -0.286 e Å-3, respectively. Compound 2. The unit cell dimensions of the selected violet crystal, with approximate dimensions 0.15 × 0.25 × 0.30 mm3, were determined in the range 8° < θ < 16°. A total of 2722 reflections were measured in the hkl ranges 0 to 9, -10 to 10, and -12 to 12. From the 2466 independent reflections, 2181 were considered observed with I > 2σ(I). There were 214 refined parameters. After the final refinement, goodness of fit on F2 ) 1.049 and largest difference peak and hole ) 0.356 and -0.463 e Å-3, respectively. Compound 3. The unit cell dimensions of the selected purple crystal, with approximate dimensions 0.10 × 0.15 × 0.20 mm3, were determined in the range 7° < θ < 15°. A total of 4034 reflections were measured in the hkl ranges 0 to 10, -11 to 11, and -14 to 14. From the 3643 independent reflections, 2726 were considered observed with I > 2σ(I). There were 301 refined parameters. After the final refinement, goodness of fit on F2 ) 1.015 and largest difference peak and hole ) 0.432 and -0.471 e Å-3, respectively.

Computational Details All the quantum calculations have been carried out using the Gaussian03 program17 running on computers with GNU/Linux operating systems. Density functional theory (DFT), specifically Becke’s hybrid method with three parameters18 based on nonlocal exchange and correlation funtionals, as implemented in Gaussian03 (B3LYP), has been used in all calculations. The standard 6-31G(d)19 basis set has been chosen for all atoms. This basis contains polarization functions20 in all atoms, except hydrogen. Experimental data were used as the starting point in the global optimization of the [H3BIP]2+, [Cu(HBIP)2]2+, and [Cu(BIP)2], with the copper complexes in Ci symmetry. Results and Discussion Crystal Structure of [H3BIP](C4O4)‚H2O (1). Control deprotonation of squaric acid (H2SQ) (pK1 ) 1.2-1.7; pK2 ) 3.2-3.5) by H-acceptors easily generates monohydrogensquarate (HSQ-) and squarate (SQ2-) anions. The fully protonated form, H2SQ, has two donor O-H groups and two carbonyl H-bond acceptors, while the HSQ- form has one donor O-H and three proton acceptor groups, and SQ2- is a powerful acceptor. All these three species show a certain degree of delocalization, although this effect is most pronounced in SQ2-, which is considered to be a typical 2π aromatic system, and are ideal building blocks for controlling and fine-tuning the orientation and architecture of polarizable cations during self-

1126 Crystal Growth & Design, Vol. 6, No. 5, 2006

Akhriff et al.

Chart 1

assembly processes. In fact, their proton-transfer ability based on their acid-base properties and the capacity for the formation of highly directional H-bonds plays a key role on such template effects. In this respect, the HBIP system is specially attractive as a squaric acid partner given the presence of two polarizable imidazole basic nitrogens and one carboxylic group in its side chain, all of which can participate in acid-base reactions and, at the same time, act as simultaneous H-donors and acceptors in hydrogen bond formation. In addition, the location of the proton donor and acceptor sites within its structure make the HBIP molecule very suited to act as a cross-linker in response to the input of external information, mainly in the form of pH changes.21 In its solid state, the HBIP molecule [pK1 ) 2.79 (carboxylic group); pK2 ) 4.62 and pK3 ) 6.90 (imidazole nitrogens)]22 adopts the zwitterion form, with one (imidazole)H+ (imidazolium) and one carboxylate anion (-COO-),7 (see Chart 1). Upon mixing of H2SQ and HBIP, it can be expected that squaric acid would donate either one or two protons to the aromatic nitrogens or the carboxylate group or both, leading to the formation of H2BIP+ or H3BIP2+ cationic fragments. Attemps to obtain the H2BIP+/HSQ- ion pair by modulating or adjusting the pH of the starting solution mixture have been thus far unsuccessful. Instead, we have always obtained the H3BIP2+/SQ2- ion pair as compound 1, with a positive net charge of +1 on both imidazole rings (adopting the imidazolium form), while the carboxylate group remains in its protonated (carboxylic acid) form. These aromatic N+H functions can then participate in H-bond formation of the N+H‚‚‚O- type, often called (() CAHB (positive/negative charge-assisted hydrogen bonds), whose strength depends on the correlation between the pKa values of squaric acid and the base involved in the H-bonding.5e,23 The crystal structure of the asymmetric unit of 1, made up of one H3BIP2+, one SQ2-, and a water molecule, is shown in Figure 1, together with the numbering scheme for the crystallographically unique atoms. Details of H-bonding within the structure are given in Table 2. Interatomic bond distances and angles are listed in Table 3. The crystal structure clearly shows how the N+H imidazolium moieties and squarate oxygens control and drive the selfassembly process. The facts that each protonated imidazolium cation has a rigid structure with the H-bond donors located at almost opposite sites, providing cross-linkages for adjacent SQ2anions, and that each of the two imidazolium groups are located in different planes within the H3BIP2+ moiety can be considered as the main factors responsible for the build-up of the twodimensional network shown in Figure 2. One important building block of such network structure is a pair of non-bonded-toeach-other centrosymmetric SQ2- held in their positions through Osquarate‚‚‚H-Nimidazolium interactions with four adjacent H3BIP2+ cations. That is, each SQ2- pair is connected to four H3BIP+ cations through O3 [N1-H‚‚‚O3 and N2-H‚‚‚O3], O4 [N3H‚‚‚O4], and O5 [N4-H‚‚‚O5]. It should be pointed out that the ribbons making up the 2D array are not identical as a result

Figure 1. Molecular structure of 1 and the atomic numbering scheme. Table 2. Hydrogen Bonds in the Crystal Structuresa of Compounds 1-3 and DFT Optimized [H3BIP]2+ X-H‚‚‚Y

X‚‚‚Y, Å

∠X-H‚‚‚Y, deg

Compound 1 2.585(4) 2.728(3) 2.966(4) 2.906(3) 2.686(3) 2.644(3) 2.632(3)

166 160 176 143 171 157 164

DFT-Optimized [H3BIP]2+ 2.706

138

O(2)-H(20)‚‚‚O(3)i O(4)-H(40)‚‚‚O(5)ii N(1)-H(1)‚‚‚O(3) N(4)-H(4)‚‚‚O(5)iii

Compound 2 2.631(2) 2.542(2) 2.805(2) 2.867(2)

176 165 143 155

O(3)-H(3A)‚‚‚O(11) O(3)-H(3B)‚‚‚O(21)i N(11)-H(11)‚‚‚O(22) N(41)-H(41)‚‚‚O(4) O(4)-H(4A)‚‚‚O(11) O(4)-H(4B)‚‚‚O(22)ii N(12)-H(12)‚‚‚O(21)iii N(42)-H(42)‚‚‚O(3)iii

Compound 3 2.814(5) 2.807(5) 2.736(4) 2.898(5) 2.806(5) 2.832(5) 2.730(4) 2.794(5)

175 178 149 139 164 173 155 145

O(2)-H(20)‚‚‚O(7)i O(7)-H(7A)‚‚‚O(6)ii O(7)-H(7B)‚‚‚O(4) N(1)-H(1)‚‚‚O(3)iii N(2)-H(2)‚‚‚O(3)iv N(3)-H(3)‚‚‚O(4)v N(4)-H(4)‚‚‚O(5) N(3)-H(3)‚‚‚O(1)

Symmetry codes for each compound are as follows: for 1, (i) -x + y + 1/2, -z + 3/2; (ii) -x, -y, -z + 1; (iii) -x + 1, -y, -z + 1; (iv) -x + 1/2, y + 1/2, -z + 1/2; (v) x - 1/2, -y + 1/2, z - 1/2; for 2, (i) x - 1, y, z; (ii) -x + 1, -y - 2, -z + 1; (iii) x, y + 1, z; for 3, (i) -x, -y - 1, -z; (ii) -x, -y, -z + 1; (iii) x, y, z + 1. a

1/

2,

of the different orientations of the H3BIP2+ moieties and the relative positions of SQ2- pairs. These differences are responsible for the ABAB-type of ribbon packing observed. Such 2D arrays are stacked one on top of the other with the water molecules providing, by means of extensive hydrogen bonding, the necessary cross-links between them to lead to the supramolecular three-dimensional (3D) network shown in Figure 3. Stacking distances between the water-cross-linked two-dimensional network are 4.44 Å. It should be pointed out that the stacking of the undulated sheets results in a novel framework with extended channels running along the [1 0 1] direction. A characteristic of this structure is that, except for the carboxylate O(1), all the available hydrogen bond donors and acceptors are engaged in a fully supramolecular assembly.


Crystal Growth & Design, Vol. 6, No. 5, 2006 1127

Table 3. Selected Bond Distances (Å) and Angles (deg)a Copper Coordination Sphere Cu-N(2) Cu-N(3) Cu-O(1)

Compound 2 1.989(2) [2.017] N(2)-Cu-N(3) 1.987(2) [2.015] N(2)-Cu-O(1) 2.480(2) [2.450] N(3)-Cu-O(1)

87.7(1) [87.4] 85.3(1) [84.7] 85.9(1) [84.7]

Cu(1)-N(21) Cu(1)-N(31) Cu(1)-O(11) Cu(2)-N(22) Cu(2)-N(32) Cu(2)-O(12)

Compound 3 1.980(3) [1.996] N(21)-Cu(1)-N(31) 2.005(3) [2.073] N(21)-Cu(1)-O(11) 2.511(3) [2.267] N(31)-Cu(1)-O(11) 1.988(3) N(22)-Cu(2)-N(32) 2.020(3) N(22)-Cu(2)-O(12) 2.487(3) N(32)-Cu(2)-O(12)

87.4(1) [85.2] 83.9(1) [86.5] 90.6(1) [92.8] 87.5(1) 84.3(1) 90.3(1)

Squarate Anion C(10)-C(11) C(11)-C(12) C(12)-C(13) C(13)-C(10) C(11)-C(10)-O(3) C(11)-C(10)-C(13) C(13)-C(10)-O(3) C(10)-C(11)-O(4) C(10)-C(11)-C(12) C(12)-C(11)-O(4)

Compound 1 1.431(4) C(10)-O(3) 1.444(4) C(11)-O(4) 1.474(4) C(12)-O(5) 1.466(4) C(13)-O(6) 134.5(3) C(11)-C(12)-O(5) 90.5(3) C(11)-C(12)-C(13) 134.9(3) C(13)-C(12)-O(5) 134.2(3) C(10)-C(13)-O(6) 91.1(3) C(10)-C(13)-C(12) 134.6(3) C(12)-C(13)-O(6)

1.270(3) 1.265(3) 1.248(3) 1.239(3) 133.2(3) 89.7(3) 137.0(3) 135.9(3) 88.5(3) 135.6(3)

C(10)-C(11) C(11)-C(12) C(12)-C(13) C(13)-C(10) C(11)-C(10)-O(3) C(11)-C(10)-C(13) C(13)-C(10)-O(3) C(10)-C(11)-O(4) C(10)-C(11)-C(12) C(12)-C(11)-O(4)

Compound 2 1.427(3) C(10)-O(3) 1.411(3) C(11)-O(4) 1.484(3) C(12)-O(5) 1.504(3) C(13)-O(6) 136.2(2) C(11)-C(12)-O(5) 88.8(2) C(11)-C(12)-C(13) 135.1(2) C(13)-C(12)-O(5) 129.9(2) C(10)-C(13)-O(6) 93.5(2) C(10)-C(13)-C(12) 136.6(2) C(12)-C(13)-O(6)

1.243(3) 1.314(3) 1.260(3) 1.209(3) 137.7(2) 90.2(2) 132.1(2) 137.6(2) 87.5(2) 134.9(2)

a

DFT-optimized values are given in brackets.

In a similar structural study concerning histidinium hydrogen squarate (a 1:1 salt) and dihistidinium squarate (2:1 salt), reported by Karle et al.,5f the cross-linking mode is significantly different from that reported herein. In fact, the presence of one single imidazole group per histidine molecule leads to the formation of layer motifs as a result of the self-assemblies of both the squarate (1:1 salt) and the HSQ (2:1 salt). In both cases, the layered structures are formed by cross-linking of the cationic self-assemblies (histidine-histidine infinite helices in the 1:1 salt and histidine-histidine zigzag ribbons in the 2:1 salt) by HSQ-1 and SQ2-, respectively. The squarate dianions are essentially planar, the largest deviation from the mean plane being 0.0146 Å. Its “quasi”-D4h geometry suggests certain delocalization in the C-C and C-O bonds, which can be considered as intermediate between double and single bonds, although the C(10)-C(11) and C(13)-O(6) bond distances are more compatible with a certain degree of double bond character. The average carbon-carbon and carbon-nitrogen bond distances within the H3BIP2+ cation are in good agreement with the values reported for related structures6,7,9 and with the calculated values at the B3LYP-optimized geometry. The flexibility of the H3BIP2+ cations is well reflected by the significant differences observed in some dihedral and torsion angles. Compounds 1, 2, and 3 of this work can be better rationalized and compared by means of three geometrical parameters: θ1, the dihedral angle between the two imidazoles, and θ2 and θ3, the dihedral angles formed between carboxylate

and each imidazole group. Table 4 lists the values of θ1, θ2, and θ3 and other useful geometrical parameters for compounds 1-3. Complexes [Cu(HBIP)2](HC4O4)2 (2) and [Cu(BIP)2]‚ 2H2O (3). Compounds 2 and 3 were prepared by mixing copper(II) chloride and HBIP in a 1:2 molar ratio. In addition, for the synthesis of compound 3, 1 equiv of KOH per equivalent of HBIP was also added to afford the ligand in its deprotonated form, as required for the formation of the neutral [Cu(BIP)2]‚ 2H2O (3) complex. Involvement of anions in an extended network requires adjusting the pH value to introduce positive charges leading to [Cu(HBIP)2]2+ moieties. This was achieved by addition of 2 equiv of squaric acid per equivalent of HBIP, yielding [Cu(HBIP)2](HC4O4)2 (2). Complex 2 contains the carboxy group in its protonated form, squarate acting as an anion template. To the best of our knowledge, complexes 2 and 3 constitute the first examples of structurally characterized Cu(II)HBIP/BIP compounds in which HBIP and BIP act as tridentate chelating ligands through two imidazole nitrogen atoms and one monodentate carboxylic/carboxylato group. To a first approximation, the asymmetric units of compounds 2 and 3 are very similar. In fact, the only difference in the copper coordination is the extra proton in 2, [Cu(HBIP)2]2+ versus Cu(BIP)2, as shown in Figures 4 and 7. Yet, the different nature of the uncoordinated counterparts, HSQ- in 2 vs H2O in 3, which are critically involved in strong H-bond interactions, leads to different supramolecular architectures for 2 and 3. We herein report the modification of the network topology by simple anion templation.The structures of compounds 2 and 3 are discussed below with special emphasis on their differences. Crystal Structure of [Cu(HBIP)2](HC4O4)2 (2). The structure of this complex is made up of centrosymmetric [Cu(HBIP)2]2+ complex units and two nonbonded monohydrogensquarate counterions (HC4O4)-, which are linked together through extensive hydrogen bonding into a 3D network. A perspective view of the mononuclear fragment and the atom numbering scheme is shown in Figure 4. Selected bond distances and angles are listed in Table 3. Two HBIP ligands are bonded to the copper(II) ion, leading to a CuN4O2 chromophore. The geometry around Cu(II) shows the typical Jahn-Teller distortion of an elongated octahedral coordination. The equatorial plane is defined by four imidazole nitrogen atoms from two different HBIP ligands. The Cu-N(2) and Cu-N(3) bond lengths are 1.989(2) and 1.987(2) Å, respectively, in good agreement with Cu-N bond distances found in other copper complexes with N-heterocyclic ligands such as pyridines, imidazoles, or benzimidazoles in equatorial positions.4i,6,9,24 The axial sites are occupied by oxygen atoms of monodentate carboxylic groups from two different chelated HBIP molecules at longer distances of 2.480(2) Å, within the range of those found for carboxylic acid coordinated in axial position, such as that in the [Cu(HCOO)2(HCOOH)2]n compound.25 The equatorial N-Cu-N angles deviate by less than 3° from 90°, while the N-Cu-O angles ranging from 85.3(1)° to 94.7(1)°, show a greater distortion from octahedral symmetry, probably originating from internal strain in the ligand to allow the interaction between Cu ions and carboxylate oxygen. A detailed analysis of the crystal structure of compounds 1 and 2 reveals that the molecular conformation of the H3BIP2+ and HBIP moieties are very different. The relative orientation of the two imidazole and carboxylic groups stems from the necessary rotations around the single bonds of the chain linking the three groups to allow their interaction with copper ions and formation of H-bonding. This leads to significant differences


Akhriff et al.

Figure 2. Two-dimensional assembly in the crystal structure of 1 viewed perpendicular to the (1 0 -1) plane, showing only hydrogen bonds between N+H imidazolium and squarate oxygens.

Figure 3. Stacking of the two-dimensional network through cross-linkage by water molecules leading to the 3D assembly of 1, viewed perpendicular to [1 0 1] direction.

between 1 and 2 in θ1, θ2, and θ3 dihedral angles. These differences are also reflected in the torsion angles (see Table 4). With regard to the carboxylic group of HBIP ligand, the C-O bond distances satisfy the expected trend of C-O hydroxylic bond [1.319(3) Å] > C-O ketonic bond [1.205(3) Å], and these values are similar to those found in other related systems.25,26 The ketonic C-O bond is greater in compound 2 than in compound 1, as expected from the polarization of the charge density toward the metal-bonded oxygen atom. The monohydrogensquarate ion is almost planar, the largest deviation from the mean plane being 0.0088 Å. Complete topological parameters are listed in Table 3. The carbon-carbon bond lengths correspond to a delocalized three-centered double C-C bond around C(11) and primarily single bonds around C(13). A similar bond-length distribution has been found in L-argininium and phenylglycinium hydrogen squarate compounds.27 The C(11)-O(4) hydroxylic bond agrees well with those previously found in HC4O4- ion.3b Furthermore, the C(13)-O(6) bond distance is significantly shorter, 1.209 Å, than the other C-O distances and close to the usual value for a Cd O double bond.3b The hydrogen bonds within the structure are summarized in Table 2. The most remarkable feature in the assembly of the

crystal structure of 2 is the presence of a ten-membered cyclic head-to-head hydrogen-bonded dimer of hydrogensquarate formed by a pair of O-H‚‚‚O hydrogen bonds. These HSQdimers are connected by their opposite sides bridging adjacent copper complexes through O‚‚‚H-N and O‚‚‚H-O bonds in which the squarate O(3) is the donor atom. Thus, O(3) forms bifurcated H-bonds with the noncoordinated carboxylic oxygen of a copper complex and with N-imidazole of the adjacent complex displaced in the x direction, forming a layered network in the (0 1 2) plane as shown in Figure 5a. This net can be described as formed from two alternating different ladders whose rungs are centrosymmetric dimers of HSQ- in one and [Cu(HBIP)2]2+ cations in the other. The copper-copper distance within each ladder is 7.588(1) Å. Figure 5b shows a lateral view of the undulated sheets. These layers are stacked one above the other with an interlayer shift so that the squarate dimers of one sheet overlap with the [Cu(HBIP)2]2+ cations of the next one in an ABAB sequence, leading to the 3D supramolecular assembly shown in Figure 6.As reported in previous studies,3f the monohydrogensquarate anions generally align to form either a head-to-tail chain or a head-to-head cyclic dimer or even a tetramer through strong H-O‚‚‚O hydrogen bonds. The monohydrogen squarate dimer reported herein is rare and has been



Table 4. Comparison of Dihedral Angles, Torsion Angles, and Bond Angles (deg) of Compounds 1-3a θ1b θ2c θ3d

1

2

3A

3B

97.5 (97.4) 139.4 (125.8) 103.9 (133.7)

133.0 (128.8) 104.4 (115.4) 122.6 (115.7) 84.5 (90.0) 5.1 (0.0) 8.0 (0.0) 111.0 (110.5) 126.0 (126.0) 136.2 (138.2)

132.1 (124.8) 19.9 (16.7) 54.1 (44.9) 27.1 (25.1) 15.3 (17.5) 78.2 (89.0) 117.0 (116.8) 118.1 (113.9) 111.7 (111.9)

121.2

θ4e H(3)-C(3)-C(2)-C(1)

54.3 (42.1)

C(2)-C(1)-O(1)-Cu C(2)-C(1)-O(2) C(2)-C(1)-O(1) C(1)-O(1)-Cu

111.8 (110.8) 124.6 (123.9)

17.4 59.3 27.3 16.8 78.2 117.7 118.6 111.2

a DFT-optimized values in parentheses. b Dihedral angle between the two imidazole rings of the same ligand. c Dihedral angle between the carboxylate moiety and imidazole ring (N1-C4-N2-C6-C5 of the same ligand). d Dihedral angle between the carboxylate moiety and imidazole rings (N4C7-N3-C8-C9) of the same ligand. e Dihedral angle between the carboxylate moiety and the basal plane of Cu(II).

Figure 4. Molecular structure of 2 with the atomic numbering scheme.

found in only a few other cases.3f,5b,5e,5f,23b,28 According to Karle et al.,5f this mode of linking is reminiscent of the typical headto-head cyclic dimeric motif found in monocarboxylic acids, in support of the idea that squaric acid can be regarded as an expanded carboxylate assembly. The short bond distance observed in the monohydrogensquarate dimer herein reported by us [O(4)-H(40)‚‚‚O(5)ii ) 2.542(2) Å) is consistent with a strong negative-charge-assisted (-)CAHB29 O(4)-H-O(5)

hydrogen bond and is in agreement with those found in 2-aminopyrimidinium hydrogen squarate complexes.5f Crystal Structure of [Cu(BIP)2]‚2H2O (3). The title compound is composed of neutral Cu(BIP)2 entities and solvated water molecules. The mononuclear units are linked together by extensive hydrogen bonds, leading to the topology of a threedimensional framework. The asymmetric unit consists of two independent centrosymmetric [Cu(BIP)2]‚2H2O molecules, hereafter referred to as molecules 3A and 3B. Relevant bond lengths and angles are listed in Table 3. As illustrated in Figure 7, each Cu(BIP)2 fragment exhibits an elongated octahedral geometry, similar to that observed for compound 2. The distortion from octahedral symmetry is slightly greater in 3 than in 2, as shown by the N-Cu-O angles. Such differences in the distortion of the coordination sphere between 2 and 3 probably stems from a different H-bond scheme, which allows a bifurcated H-bond sequence involving the carboxylate coordinate O(2) in compound 3 but not in compound 2. A close examination of the geometrical parameters shown in Table 4 confirms the strong influence of the coordination of the metal center, as well as of the H-bonding scheme, on the internal geometry of the BIP moiety. The carboxylate group bonds to Cu(II) as a monodentate ligand, and thus, it should be expected that the C-O distances follow the trend C-Ocoord > C-Ouncoord. Instead, both distances are very similar. This similarity is probably the result of the weakness of Cu-Ocoord interaction and the involvement of the uncoordinated oxygen atoms in hydrogen bonding (see below). Table 2 lists the most relevant H-bond parameters. Figure 8 shows how the copper ions are bridged by water molecules leading to infinite (H2O)2-Cu(BIP)2-(H2O)2 chains in the y direction. Each chain, labeled in Figure 8, as chain 1 and chain 2, contains a different type of copper ion (either Cu(1) or Cu(2)) at a Cu-Cu distance of 10.047(1) Å. These parallel chains are distributed in an alternated sequence at 6.266(1) Å and displaced one with respect to the next one by half unit cell along the b axis. Chain 1 is formed by lattice water O(3) acting as donor in bifurcated H-bonds with the coordinated carboxylate O(11) of the 3A molecule and with the uncoordinated carboxylate O(21) of the other adjacent 3A molecule in the y direction. Chain 2 is formed in a similar manner, except that it is the water O(4) atom that is involved in the Cu(2)-(H2O)2-Cu(2) bridge. Neighboring chains are interconnected in the ABAB fashion, forming layers in the bc plane through NH imidazole bridges. The resulting layers are stacked one above the other along the a axis, with an interplanar distance of 8.246(1) Å (see Figure 9). The three-dimensional network structure results from stacking of these sheets driven by aromatic-aromatic interactions between the bulky groups in BIP. Three different modes of these noncovalent interactions can be observed, namely, two edgeto-face (EF1 and EF2) and one offset face-to-face (OFF). The centroid-to-centroid distances for EF1, EF2, and OFF interactions are 5.125, 4.783, and 4.119 Å, respectively. Furthermore, in the OFF interaction, the plane-to-plane distance is 3.572 Å, with a γ value of 29.9° (γ ) angle between the centroidcentroid vector and the perpendicular to the imidazole nucleus plane) indicating significant σ,π-contribution to this interaction. It is noteworthy that the two carboxylic/carboxylate groups in compounds 1-3 use different H-bonding schemes. While the carboxylic residue defined by O(1), O(2), C(1), and C(2) in compounds 1 and 2 uses only the protonated O(2) in H-bonding, the carboxylate moiety in compound 3 makes use of both the coordinated O(1) and the pendant oxygen atoms O(2). This, in


Akhriff et al.

Figure 5. (a) Layered network in the crystal structure of 2 viewed perpendicular to (0 1 2) plane. For simplicity, only hydrogen bonds between O(3) squarate and N-H imidazole and carboxylic oxygen are shown. Adjacent copper complexes are aligned along the a axis. (b) Lateral view of the undulated sheets.

Figure 6. Stacking of the layered network through cross-linkage by N(4) imidazole and O(5) squarate atoms in 2.

turn, is probably responsible for the torsion angles of the carboxylate side chain, as well as for the differences among compounds 1-3 in those angles in which the same carboxylate is involved (see Table 4). Such a H-bonding scheme, together with the coordination of carboxylic/carboxylate groups to the metal center, leads to the differences in the resulting topologies of this group. DFT Optimization of the BIP Containing Species in Compounds 1-3. To better understand the observed influence of the hydrogen bonding scheme on the internal conformation of the BIP moiety and its coordination to Cu(II), the geometries of the isolated [H3BIP]2+, [Cu(HBIP)2]2+, and [Cu(BIP)2]

species have been optimized using DFT calculations. As can be seen in Tables 3 and 4 (and Supporting Information), the optimized geometries are very similar to the experimental ones, and in the following, only the significant differences, most of them due to the unavailability of neighboring molecules in the calculations, will be highlighted. The optimized [H3BIP]2+ cation only differs significantly in a rotation of an imidazolium ring (∆θ3 ) 30°) to establish a rather short but bent intramolecular N-H‚‚‚O hydrogen bond with the nonprotonated O of the carboxylic group (Table 2), which keeps the group in a very similar conformation to that found in the crystal structure of compound 1.



Figure 8. Sheets in the bc plane of 3 formed by parallel chains in ABAB fashion, running along the y direction. Chain 1 contains Cu(1), and chain 2 contains Cu(2).

Figure 7. Molecular structure of 3 with the atomic numbering scheme.

The calculations of [Cu(HBIP)2]2+ and [Cu(BIP)2] nicely reproduce the experimental conformation of the BIP moiety in the complexes, both of the imidazole rings and of the carboxylate group. Although the distortion from octahedral symmetry of the copper(II) coordination spheres in the calculated complexes is lower than that observed in compounds 2 and 3, the only significant difference is the axial Cu-O bond length of [Cu(BIP)2], 2.27 (calculated) vs 2.51 (3A) and 2.49 (3B) Å. The shorter calculated Cu-O bond in [Cu(BIP)2] must be due to the stronger coordination to a negatively charged carboxylate group in BIP as compared to a neutral carboxylic one in HBIP. But the involvement of the carboxylate group in strong hydrogen bonding through both the coordinated and pendant oxygen atoms in the crystal structure of compound 3 must have the opposite effect, leading to a similar Cu-O bond distance to both BIP and HBIP in compounds 2 and 3. It should also be noticed that the experimental similarity between the C-Ocoord and C-Ouncoord bond distances in 3 is not observed in the calculated geometry of [Cu(BIP)2], which follows the expected trend in the absence of hydrogen bonding, C-Ocoord > C-Ouncoord. All these facts lend support to the strong influence of the hydrogen bonding scheme in the internal geometry of the copper complexes.

Figure 9. (a) Supramolecular network of 3 formed by stacking of the sheets along the a axis. Projection viewed on the ca plane. (b) Schematic representation of the π-π interactions in compound 3 (see text).

Vibrational and Electronic Spectra. The IR spectra of compounds 1 and 3 show a sharp band at 3490 cm-1 and a shoulder at 3450 cm-1 assigned to ν(OH) stretching vibrations of water molecules.30 The N-H stretching vibrations of the three complexes appear in the 3200-3000 cm-1 region, and their frequencies are consistent with the existence of H-bonding between imidazole N-H and other groups. The spectrum of compound 1 exhibits a broad band centered at ca. 2520 cm-1


and a weak band at 1955 cm-1, both assignable to ν(NH+) stretching vibrations.31 In the 1820-800 cm-1 region, the spectra display a large number of absorptions that correspond to the coexistence of squarate, imidazole, and carboxylate moieties. Compounds 1 and 2 exhibit one band at 1725 and 1710 cm-1, respectively, assigned to un-ionized COOH groups.6b,9,26 This vibration is replaced by two carboxylato bands [νas(COO-) and νs(COO-)] at 1600 and 1400 cm-1 for 3, which are consistent with a monodentate coordination of the carboxylate group.6b,9,30,32 The IR bands associated with the squarate group appear in the 1810-1500 cm-1 region.33 Compounds 1 and 2 exhibit a very strong and extremely broad band centered at ca. 1500 cm-1, assigned to a mixture of C-C and C-O stretching vibrations. In addition, only the IR spectrum of compound 2 exhibits moderate bands at 1810 and 1676 cm-1 assigned to ν(CdO) and ν(CdC) stretching modes. Neither of these bands are present in the complex [{Cu(BIP)(OH2)}4(µ-C4O4)](ClO4)2‚4H2O9 in which the squarate is coordinated through all four oxygen atoms, in agreement with the existence of C4O4 groups with roughly D4h symmetry. Altogether, these spectral results are in good agreement with the crystal structure analyses of the complexes reported herein. EPR Spectroscopy and Magnetic Behavior. The roomtemperature polycrystalline powder EPR spectrum at X-band shows a wide axial-type signal with g| (ca. 2.23) greater than g⊥ (ca. 2.06) and hyperfine 63Cu coupling partially resolved in the parallel part of the spectrum. Both, parallel hyperfine coupling and g factors are better resolved in the Q-band spectrum. The experimental features can be nicely reproduced with g| ) 2.237, g⊥ ) 2.057, |A|| ) 180 10-4 cm-1, and line widths of ca. 60 G. These values are indicative of a dx2-y2 ground state for the unpaired electron of the Cu(II) ion in an elongated octahedral environment, in full agreement with the observed CuN4O2 chromophores present in the compound. The magnetic study of both compounds shows that in the studied temperature range (2-300 K) the χMT product is nearly constant, and the whole susceptibility data can be nicely fitted to the Curie-Weiss expression χM ) C/(T - θ), affording C ) 0.414 cm3 mol-1, g ) 2.10, and θ ) -0.05 K for compound 2 and C ) 0.422 cm3 mol-1, g ) 2.12, and θ ) -0.10 K for compound 3. The very small absolute value of the Weiss correction confirms the lack of any antiferromagnetic interaction between the copper(II) ions in both compounds. Acknowledgment. We are grateful to the Direccioń General de Investigacioń Cientı´fica y Tećnica (DGICYT) (Project BQU2003-01476) for financial support. Supporting Information Available: CIF file giving crystallographic data and DFT optimized geometries of [H3BIP]2+, [Cu(HBIP)2]2+, and [Cu(BIP)2] species in MDL Molfile (.mol) format. This material is available free of charge via the Internet at http://pubs.acs.org.

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(2)

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(4)

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(8)

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