Exploiting the Pyrazole-Carboxylate Mixed Ligand System in the

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Exploiting the pyrazole-carboxylate mixed ligand system in the crystal engineering of coordination polymers Chris S Hawes, Boujemaa Moubaraki, Keith S. Murray, Paul Eric Kruger, David R. Turner, and Stuart Robert Batten Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg501004u • Publication Date (Web): 18 Sep 2014 Downloaded from http://pubs.acs.org on September 28, 2014

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Cover Page

Crystal Growth & Design

Exploiting the pyrazole-carboxylate mixed ligand system in the crystal engineering of coordination polymers Chris S. Hawes,1,2 Boujemaa Moubaraki,1 Keith S. Murray,1 Paul E. Kruger,2* David R. Turner1* and Stuart R. Batten1,3* 1 2

School of Chemistry, Monash University, Clayton, VIC 3800, Australia.

MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand. 3

Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia.

The utility of the pyrazole-carboxylate mixed ligand system has been probed with the synthesis of five new coordination polymers, derived from bis-pyrazole ligand 4,4ʹ-methylenebis(3,5-dimethyl-1Hpyrazole) H2L1, large semirigid dicarboxylate co-ligands, and the deliberately designed and synthesised ligand para-((3,5-dimethyl-1H-pyrazol-4-yl)methylene)benzoic acid H2L4. Complex poly[Co(H2L1)(L2)(OH2)]·1/2H2O 1 features the co-ligand (S,S)-1,4,5,8-naphthalenetetracarboxylicdiimideN,Nʹ-bis-(2-propionate) L2, and comprises chiral 2-dimensional sheets, associating via substantial π-π stacking interactions. Complexes 2 and 3 contain the co-ligand 1,4-bis((3carboxyphenyl)methyl)piperazine L3. Complex poly-[Co(H2L1)2(L3)] 2 is a 2-dimensional polymer containing octahedral Co(II) ions, whereas modifying the synthesis conditions gave complex poly[Co2(HL1)2(L3)] 3, a 3-dimensional α-Po structure containing pyrazolate-bridged dimers of tetrahedral Co(II) ions, displaying weak antiferromagnetic coupling. Magnetic data for complex 3 fitted well to a S = 3/2 Heisenberg (-2JS1.S2) dimer model with J = -3.2 cm-1 and g = 2.25. We then prepared the new heteroditopic ligand para-((3,5-dimethyl-1H-pyrazol-4-yl)methylene)-benzoic acid H2L4, and the complexes poly-[Co(HL4)2]·H2O 4 and poly-[Cu(HL4)2]·2MeOH 5, which demonstrate for the first time the tendency of flexible pyrazole-carboxylate coordination polymers with perfectly commensurate linker dimensions to lead to low-dimensional assemblies. These results give fresh insight into the structural properties of flexible bispyrazole-carboxylic acid systems as a function of co-ligand dimensions, and provide new directions for designed polymeric cluster compounds.

Contact Author: Prof. Stuart Batten School of Chemistry, Monash University Clayton, VIC 3800, Australia Phone: +61 3 9905 4606 Fax: +61 3 9905 4597 Email [email protected] http://monash.edu/science/about/schools/chemistry/staff/sbatten

ACS Paragon Plus Environment


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Exploiting the pyrazole-carboxylate mixed ligand system in the crystal engineering of coordination polymers Chris S. Hawes,1,2 Boujemaa Moubaraki,1 Keith S. Murray,1 Paul E. Kruger,2* David R. Turner1* and Stuart R. Batten1,3* 1

School of Chemistry, Monash University, Clayton, VIC 3800, Australia. Email [email protected]; [email protected]

2

MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand. Email [email protected] 3

Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia.


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Abstract

The utility of the pyrazole-carboxylate mixed ligand system has been probed with the synthesis of five new coordination polymers, derived from bis-pyrazole ligand 4,4ʹ-methylenebis(3,5dimethyl-1H-pyrazole) H2L1, large semirigid dicarboxylate co-ligands, and the deliberately designed and synthesised ligand para-((3,5-dimethyl-1H-pyrazol-4-yl)methylene)benzoic acid H2L4. Complex poly-[Co(H2L1)(L2)(OH2)]·1/2H2O 1 features the co-ligand (S,S)-1,4,5,8naphthalenetetracarboxylicdiimide-N,Nʹ-bis-(2-propionate)

L2,

and

comprises

chiral

2-

dimensional sheets, associating via substantial π-π stacking interactions. Complexes 2 and 3 contain

the

co-ligand

1,4-bis((3-carboxyphenyl)methyl)piperazine

L3.

Complex

poly-

[Co(H2L1)2(L3)] 2 is a 2-dimensional polymer containing octahedral Co(II) ions, whereas modifying the synthesis conditions gave complex poly-[Co2(HL1)2(L3)] 3, a 3-dimensional α-Po structure containing pyrazolate-bridged dimers of tetrahedral Co(II) ions, displaying weak antiferromagnetic coupling. Magnetic data for complex 3 fitted well to a S = 3/2 Heisenberg (2JS1.S2) dimer model with J = -3.2 cm-1 and g = 2.25. We then prepared the new heteroditopic ligand para-((3,5-dimethyl-1H-pyrazol-4-yl)methylene)-benzoic acid H2L4, and the complexes poly-[Co(HL4)2]·H2O 4 and poly-[Cu(HL4)2]·2MeOH 5, which demonstrate for the first time the tendency of flexible pyrazole-carboxylate coordination polymers with perfectly commensurate linker dimensions to lead to low-dimensional assemblies. These results give fresh insight into the structural properties of flexible bispyrazole-carboxylic acid systems as a function of co-ligand dimensions, and provide new directions for designed polymeric cluster compounds.


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Introduction The notion of predictability has long posed a challenge in the field of structural chemistry.1 The myriad of weak interactions which comprise a crystalline material can be extremely difficult to predict, and great effort has been devoted to ab initio prediction of crystal structures, particularly relevant in the field of pharmaceuticals.2-4 Supramolecular chemistry, and coordination polymer chemistry especially, views this problem from a slightly different angle. Prediction of supramolecular structures is of substantial importance in the construction of extended coordination architectures, particularly where properties such as gas sorption or separation are required, which are heavily dependent on the extended solid-state structure of a material.5-7 The design of such systems has been greatly aided by the discovery of reproducible multi-component structural motifs, sometimes referred to as secondary building units (SBUs), which impart well-defined geometrical features to a coordination assembly and greatly simplify the prediction of the network as a whole.8 The most well-known example of this approach came from the use of the basic zinc acetate [Zn4O(RCOO)6] motif as a node in coordination polymer construction, leading to a series of porous coordination polymers displaying isoreticular extended structures.9 As the study of coordination polymer materials continues to grow, the need to continuously explore new ligand classes has also expanded. In particular, many of the early carboxylate-based materials suffer from poor hydrolytic stability, a crucial shortcoming for most applications currently in development, leading several studies to suggest that nitrogen heterocycles or a mixed-ligand approach may lead to more robust materials.10-14 Pyrazoles in particular have recently enjoyed substantial popularity as ligands in coordination polymer chemistry, in both homotopic and heterotopic ligand environments.15-18 As well as demonstrating excellent stability


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when coordinated to transition metal ions, pyrazole possesses great potential for synthetic functionalization, providing an intriguing basis for continued research.19-21

Figure 1 Representative structure of the [M(RCOO)2(HPz)2] motif. Hydrogen atoms not involved in hydrogen bonding omitted for clarity.

In 2009, extending upon their earlier work, Mondal and colleagues reported the discovery of a reproducible structural motif in a series of pyrazole-carboxylate coordination polymers based on Zn(II), and later extended to Co(II), consisting of a single tetrahedral metal ion coordinated by two pyrazole nitrogen atoms and two carboxylate oxygen atoms in which N-H···O hydrogen bonds form around the periphery of the coordination sphere (Figure 1).22-23 Subsequent works have cemented the pyrazole-carboxylate mixed ligand system as a useful synthon in coordination polymer synthesis, where hydrogen bonding interactions have played a pivotal role in determining the local and extended structure.24-28 The flexible bis-pyrazole ligand 4,4′-


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methylenebis(3,5-dimethyl-1H-pyrazole) H2L1 (Figure 2) has been used to great effect in these studies, and the influence of a flexible sp3 methylene bridge between the coordinating groups has been shown to exert a strong influence on the resulting extended structures. Herein we expand upon current knowledge of these systems in two ways; firstly, three new coordination architectures containing H2L1 are presented, in which uncommon carboxylate co-ligands H2L2 and H2L3 (Figure 2) have been used, each containing long backbones, with flexible groups commensurate with that found in H2L1 itself, and revealing previously unknown structural behavior of the H2L1 ligand. Further, we report the synthesis of a new, deliberately designed flexible heterotopic pyrazole-carboxylate ligand, H2L4, which contains the key structural features identified in carboxylate-containing complexes of H2L1, and present two examples of H2L4-derived coordination polymers.

Figure 2 Structures of H2L1, H2L2, H2L3 and H2L4 with hydrogen atom numbering scheme for H2L3 and H2L4.


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Experimental General Considerations All reagents, solvents and starting materials were purchased from Sigma Aldrich, Alfa Aesar or Merck and were used as received. 4,4′-methylenebis-(3,5-dimethyl-1H-pyrazole) H2L1,29 ethyl 4-bromomethyl benzoate30, (S,S)-1,4,5,8-naphthalenetetracarboxylic diimide-N,N′-bis-(2propionic acid) H2L231 and 1,4-bis((3-cyanophenyl)methyl) piperazine32 were prepared according to published procedures. NMR spectra were recorded on a Bruker AVANCE spectrometer operating at 400 MHz for 1H and 100 MHz for

13

C nuclei (H2L3) or a Varian

INOVA instrument operating at 500 MHz for 1H and 126 MHz for 13C (H2L4 and precursors). Melting points were recorded in air on an Electrothermal melting point apparatus, and are uncorrected. Mass spectrometry was carried out using a Micromass Platform II ESI-MS instrument (H2L3) or a Bruker MaXiS 3G UHR-TOF instrument (H2L4 and precursors). Microanalysis was performed by Campbell Microanalytical Laboratory, University of Otago, New Zealand. Infrared spectra were obtained using an Agilent Cary 630 spectrometer equipped with an Attenuated Total Reflectance (ATR) sampler (complexes 1-3 and 5, H2L3) or a PerkinElmer Spectrum One FTIR instrument operating in diffuse reflectance mode with samples diluted in powdered KBr (complex 4, H2L4 and precursors). Bulk phase purity of all crystalline materials was confirmed with X-ray powder diffraction patterns recorded with a Bruker X8 Focus powder diffractometer operating at Cu Kα wavelength (1.5418 Å), with samples mounted on a zero-background silicon single crystal stage. Scans were performed at room temperature in the 2θ range 5 – 55° and compared with predicted patterns based on low temperature single crystal data (Supporting Information). Thermogravimetric analyses were carried out with a Mettler-Toledo STARe TGA/DSC instrument (complexes 1 and 5) or a TA instruments Q600


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DSC/TGA instrument (complex 4). The magnetic susceptibility measurements were carried out using a Quantum Design SQUID magnetometer MPMS-XL 7 operating between 1.8 and 300 K. Crystalline samples were dispersed in Vaseline to avoid torquing of the crystallites. The sample mulls were contained in a calibrated gelatin capsule held at the centre of a drinking straw that was fixed at the end of the sample rod.

Synthesis of 1,4-bis((3-carboxyphenyl)methyl) piperazine H2L3 1,4-bis((3-cyanophenyl)methyl) piperazine (450 mg, 1.4 mmol) was dissolved in 10 mL of 37% hydrochloric acid solution and heated to reflux in air for 48 hours. On cooling, the white solid was filtered and washed with water. This solid was added to 20 mL of 2M potassium hydroxide solution and refluxed for 24 hours. The resulting pale yellow solution was diluted to 50 mL with water, and 3 mL of glacial acetic acid was added. After standing for two hours, colorless crystals of the product were isolated by filtration, washed with water and dried in vacuo. Yield 219 mg (44%). Microanalysis suggested some association of atmospheric water with the solid on extended standing in air. m.p. >300 °C; Found C, 66.65; H, 6.30; N, 7.68; calculated for C20H22N2O4·1/3H2O C, 66.65; H, 6.34; N, 7.77%; δH(400 MHz, d6-DMSO) 2.39 (br s, 8H, H1), 3.52 (s, 4H, H2), 7.43 (t, 2H, J = 7.5 Hz, H5), 7.52 (d, 2H, J = 7.5 Hz, H6), 7.81 (d, 2H, J = 7.7 Hz, H4), 7.87 (s, 2H, H3); δC(100 MHz, d6-DMSO) 52.54, 61.52, 127.88, 128.38, 129.52, 130.83, 133.15, 138.78, 167.35; m/z (ESMS) 355.1 ([M+H+] 100%, calculated for C20H23N2O4 355.2); υmax(ATR)cm-1 2834m, 1672s br, 1590w, 1326m, 1274s sh, 1219m, 1117m, 979m, 746s, 688s.


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Synthesis of ethyl-para-((2,4-pentanedion-3-yl)methylene)benzoate EtL4acac Ethyl 4-bromomethyl benzoate (4.86 g, 20 mmol) was combined with anhydrous [Co(acac)2] (2.4 g, 10 mmol) in 25 mL chloroform, and the mixture stirred until homogeneous. The solution was immersed in an oil bath at 120 °C and the chloroform allowed to evaporate with stirring. The remaining mixture was allowed to stir while maintaining the temperature until a dark green colour was observed (ca. 30 minutes), at which time the mixture was cooled and exhaustively extracted with a water/diethyl ether mixture. The organic layers were combined, washed with dilute hydrochloric acid solution, dried and evaporated to dryness to give a dark brown oil. The oil was purified by flash chromatography (5:1 DCM/hexanes) to give a yellow liquid which was found, by comparing the relative peak integrals in the 1H NMR spectra, to comprise both the diketo and keto-enol tautomers in approximately an 8:7 ratio. Yield 2.45 g (47 %). δH(500 MHz, CDCl3) 1.35-1.39 (overlapping triplets, 6H, J1 = J2 = 7.1 Hz, H1 + H8), 2.03 (s, 6H, H13), 2.12 (s, 6H, H7), 3.17 (d, 2H, J = 7.7 Hz, H5), 3.69 (s, 2H, H12), 4.03 (t, 1H, J = 7.7 Hz, H6), 4.32-4.38 (overlapping quartets, 4H, J1 = J2 = 7.1 Hz, H2 + H9), 7.20-7.22 (overlapping doublets, 4H, J1 = J2 = 8.3 Hz, H4 + H11), 7.95 (overlapping doublets, 4H, J1 = J2 = 8.3 Hz, H3 + H10), 16.83 (s, 1H, H14); δC(125 MHz, CDCl3) 14.53, 23.47, 29.99, 33.22, 34.20, 61.08, 61.12, 69.55, 107.94, 127.63, 128.92, 129.00, 129.29, 130.15, 130.18, 143.65, 145.35, 166.45, 166.55, 192.15, 203.22; m/z (HR-ESMS) 263.1292 ([M+H+], calculated for C15H19O4 263.1283); υmax(KBr)cm-1 2982m sh, 1718s, 1610m, 1278m, 1106s, 1021s, 943m, 736s.

Synthesis of ethyl-para-((3,5-dimethyl-1H-pyrazol-4-yl)methylene)-benzoate EtHL4 Ethyl-para-((2,4-pentanedion-3-yl)methylene)benzoate (1.80 g, 6.9 mmol) was dissolved in 40 mL methanol with stirring. To this mixture was added dropwise a solution of hydrazine


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hydrate (450 µL, 9 mmol) in 5 mL of methanol. The solution was heated at reflux for 24 hours, before cooling and evaporating under reduced pressure, to give the title compound as a pale yellow solid. Yield 1.43 g (80%). m.p. 104-109 °C; δH(500 MHz, CDCl3) 1.36 (t, 3H, J = 7.1 Hz, H1), 2.13 (s, 6H, H6), 3.77 (s, 2H, H5), 4.34 (q, 2H, J = 7.1 Hz, H2), 7.15 (d, 2H, J = 8.3 Hz, H4), 7.92 (d, 2H, J = 8.3 Hz, H3); δC(500 MHz, CDCl3) 11.03, 14.50, 29.19, 61.04, 113.55, 128.24, 128.45, 129.89, 142.71, 146.27, 166.82; m/z (HR-ESMS) 259.1443 ([M+H+], calculated for C15H19N2O2 259.1447); υmax(KBr)cm-1 2986s, 1930m, 1720s, 1611m, 1414m, 1277s, 1177s, 1105s, 1023m, 854m, 736s.

Synthesis of para-((3,5-dimethyl-1H-pyrazol-4-yl)methylene)-benzoic acid H2L4 Ethyl-para-((3,5-dimethyl-1H-pyrazol-4-yl)methylene)-benzoate (1.0 g, 3.8 mmol) was dissolved in 40 mL tetrahydrofuran with stirring. To this mixture was added a solution of lithium hydroxide (5 g, 210 mmol) dissolved in 20 mL water. The resulting suspension was heated at reflux for 24 hours, cooled and concentrated under reduced pressure to remove tetrahydrofuran. The residue was added to 100 mL water and filtered, and the filtrate acidified to pH 3.0 with dilute hydrochloric acid solution, at which point the product precipitated as a white solid, which was filtered and dried in air at 70 °C. Yield 670 mg (76 %). m.p. 242-244 °C; Found C, 67.47; H, 6.14; N, 12.10; Calculated for C13H14N2O2 C, 67.81; H, 6.13; N, 12.17%; δH(500 MHz, d6DMSO) 2.04 (s, 6H, H4), 3.72 (s, 2H, H3), 7.21 (d, 2H, J = 8.3 Hz, H2), 7.84 (d, 2H, J = 8.3 Hz, H1); δC(125 MHz, d6-DMSO) 10.64, 28.48, 112.61, 128.17, 128.27, 129.42, 140.99, 146.80, 167.27; m/z (HR-ESMS) 307.0260 ([M – H+ + 2K+], calculated for C13H13N2O2K2 307.0251); υmax(KBr)cm-1 3287s, 1682s, 1313m, 1291s, 743s, 515s.


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Synthesis of poly-[Co(H2L1)(L2)(OH2)]·1/2H2O 1 To a 45 mL Telfon-lined autoclave was added H2L1 (5 mg, 25 µmol), H2L2 (10 mg, 24 µmol) and cobalt sulfate heptahydrate (30 mg, 100 µmol) dispersed in 2 mL of water. The autoclave was sealed and heated to 130 °C and allowed to dwell at that temperature for 36 hours, followed by cooling to room temperature over 6 hours. The red crystals of 1 formed were isolated by filtration and dried in air. Yield 8.6 mg (48 %). m.p. >300 °C; Found C, 53.34; H, 4.54; N, 11.91; Calculated for C31H31N6O9.5Co1 C, 53.30; H, 4.47; N, 12.03%; υmax(ATR)cm-1 3324w br, 1706m, 1658s, 1576s, 1432w, 1333m, 1247s, 1093w, 890m, 770s.

Synthesis of poly-[Co(H2L1)2(L3)] 2 A mixture of H2L1 (16 mg, 79 µmol), H2L3 (10 mg, 28 µmol) and cobalt sulfate heptahydrate (20 mg, 71 µmol) suspended in 2 mL of 20 mM sodium hydroxide solution was added to a 45 mL Telfon-lined autoclave, which was heated to 130 °C and allowed to dwell for 36 hours. Following this time, the reaction vessel was cooled to room temperature over a 6 hour period, and the pink crystals of the product were isolated by filtration, washed with water and dried in air. Yield 18.6 mg (81 %). m.p. >300 °C; Found C, 61.83; H, 6.47; N, 17.10; Calculated for C42H52N10O4Co C, 61.53; H, 6.39; N, 17.08%; υmax(ATR)cm-1 3240w, 2802m sh, 1605m, 1555s, 1430m, 1378s, 1365s, 1338s, 1273w, 1161m, 1000m, 888s, 789s, 753s, 674s.

Synthesis of poly-[Co2(HL1)2(L3)] 3 A mixture of H2L1 (16 mg, 79 µmol), H2L3 (10 mg, 28 µmol) and cobalt sulfate heptahydrate (40 mg, 140 µmol) suspended in 2 mL of 40 mM sodium hydroxide solution was added to a 45


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mL Teflon-lined autoclave. The reaction vessel was heated to 130 °C and held at that temperature for 36 hours, and then cooled to room temperature over 6 hours. The resulting dark purple crystals were isolated by filtration, washed with water and dried in air. Yield 5.3 mg (43%). m.p. >300 °C; Found C, 57.78; H, 5.87; N, 16.23; Calculated for C42H50N10O4Co2 C, 57.54; H, 5.75; M, 15.98%; υmax(ATR)cm-1 2970m br, 1611w, 1546m, 1500m, 1388m sh, 1338m, 1296m, 1203m, 1009s, 796s, 756s sh, 680s.

Synthesis of poly-[Co(HL4)2]·H2O 4 A mixture of H2L4 (10 mg, 44 µmol) and cobalt sulfate heptahydrate (20 mg, 71 µmol) were added to 2 mL of water in a 45 mL Teflon-lined autoclave, to which was added one drop of 2,4,6-collidine. The vessel was sealed and heated to 80 °C, and then slowly heated to 120 °C at a rate of 1.5 °C/hr, followed by cooling to room temperature at 8 °C/hr. The purple crystalline product was isolated by filtration and dried in air. Yield 4.1 mg (35 %). m.p. >300 °C; Found C, 58.29; H, 5.33; N, 10.29; calculated for C26H28N4O5Co C, 58.32; H, 5.27; N, 10.46%; υmax(ATR)cm-1 3450 w br, 2926s br, 1657w, 1595s, 1544s, 1412m, 1388m, 1371s, 1300m, 1176m, 1059m, 1018m, 856s, 792w, 747s.

Synthesis of poly-[Cu(HL4)2]·2MeOH 5 A slurry of H2L4 (20 mg; 44 µmol) in 5 mL of methanol was added to a solution of copper sulfate pentahydrate (6 mg, 24 µmol) in 14 mL of methanol. The resulting green turbid mixture was allowed to stand for one week, yielding a pure phase of pale purple crystals which were found to lose crystallinity and uptake water on prolonged standing in air. Yield 6.3mg.

m.p.

>300

°C;

Found

C,

53.71;

H,

5.32;

N,

9.05;

calculated

for


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C26H26N4O4Cu·0.75MeOH·3H2O C, 53.53; H, 5.88; N, 9.34 %; υmax(ATR)cm-1 2924m br, 1595s, 1556s, 1355s, 1299m, 1197w, 1174m, 1098m, 1065m, 1016m, 911w, 851m, 805w, 769s, 736s.

X-Ray Crystallography Crystallographic and refinement data are presented in Table 1. Data collections for H2L3, H2L4 and complexes 1 and 2 were performed on a Bruker APEX-II diffractometer, using graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation, while the diffraction data for complex 4 were collected using an Agilent SuperNova instrument with an Atlas area detector using mirror monochromated, focused microsource Cu Kα (λ = 1.54184 Å) radiation. Diffraction data for complexes 3 and 5 were collected at the Australian Synchrotron on the MX1 beamline, operating at 17.4 keV (λ = 0.7108 Å) with data collections conducted using BluIce control software.33 Data for compounds 3 and 5 were collected with a 360° scan in Φ, and the crystal of complex 5 was remounted in a different orientation and a second 360° scan in Φ was collected, and the data merged to provide sufficient completeness. Corrected anomalous dispersion values were calculated where necessary using Brennan and Cowan data.34 Diffraction data for H2L3, H2L4 and complexes 1 and 2 were processed using the Bruker SAINT suite of programs,35 with multi-scan absorption correction carried out with SADABS,36 while the diffraction data for compound 4 were processed with the CrysAlis Pro software suite,37 with multi-scan absorption correction carried out with the SCALE3 ABSPACK algorithm. The remaining diffraction data were processed, reduced and corrected with the XDS software suite.38 All structures were solved using direct methods with SHELXS39 and refined on F2 using all data by full matrix least-squares procedures with SHELXL-9740 within OLEX-2.41 Non-hydrogen atoms were refined with


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anisotropic displacement parameters. Hydrogen atoms were manually located from residual Fourier difference peaks and restrained with D-A distance restraints where appropriate, and otherwise were included in calculated positions. In both cases, hydrogen atom isotropic displacement parameters were fixed at 1.2 or 1.5 times the isotropic equivalent of their carrier atoms dependent on chemical environment. The functions minimized were Σw(F2o-F2c), with w=[σ2(F2o)+aP2+bP]-1, where P=[max(Fo)2+2F2c]/3. CCDC 1010718-1010724.

Table 1 Crystallographic Data for compounds 1 – 5, H2L3 and H2L4 Compound reference 1 2 3 4 5 H2L3 H2L4 Chemical formula C62H60Co2N12O18 C42H52CoN10O4 C21H25CoN5O2 C26H26CoN4O4 C26H26CuN4O4 C20H22N2O4 C13H14N2O2 •H2O •H2O •2(CH4O) Formula Mass 1397.10 819.87 438.39 535.45 586.13 354.40 230.26 Crystal System Orthorhombic Monoclinic Monoclinic Monoclinic Triclinic Monoclinic Orthorhombic a/Å 15.0516(6) 26.6804(14) 10.954(2) 33.0036(8) 8.0390(16) 9.8442(11) 16.9424(5) b/Å 21.3252(7) 9.0407(5) 13.013(3) 8.55033(11) 9.2100(18) 11.2855(16) 7.4436(2) c/Å 9.1395(3) 17.7821(10) 14.968(3) 21.3589(4) 11.042(2) 7.5036(12) 18.1452(6) α/° 90.00 90.00 90.00 90.00 76.54(3) 90.00 90.00 β/° 90.00 107.089(3) 104.00(3) 124.052(3) 72.17(3) 90.709(3) 90.00 γ/° 90.00 90.00 90.00 90.00 67.29(3) 90.00 90.00 Unit cell volume/Å3 2933.58(18) 4099.8(4) 2070.2(7) 4993.79(17) 711.8(2) 833.6(2) 2288.34(12) Temperature/K 123(2) 123(2) 100(2) 120.0(2) 100(2) 123(2) 123(2) Space group P21212 C2/c P21/n C2/c P1¯ P21/c Pbca Z 2 4 4 8 1 2 8 Radiation source Mo Kα Mo Kα Synchrotron Cu Kα Synchrotron Mo Kα Mo Kα Reflections measured 111189 13771 38473 27263 11733 6979 47235 Independent reflections 8630 4745 5852 5213 3378 2453 2643 Observed Reflections 7533 3163 4318 4528 2797 1281 1863 ((I > 2σ(I)) Rint 0.0635 0.0507 0.0813 0.0365 0.0993 0.0932 0.0605 Final R1 values (obs. 0.0395 0.0478 0.0505 0.0456 0.0465 0.0598 0.0363 data) Final wR(F2) values 0.0885 0.0940 0.1144 0.1030 0.1132 0.1119 0.0919 (obs. data) Final R1 values (all 0.0500 0.0916 0.0775 0.0566 0.0591 0.1320 0.0585 data) Final wR(F2) values (all 0.0945 0.1091 0.1275 0.1106 0.1209 0.1396 0.0996 data) CCDC Number 1010718 1010719 1010720 1010721 1010722 1010723 1010724


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Results Synthesis and structure of H2L3 Ligand H2L3 was prepared by hydrolysis of the previously reported dinitrile compound,32 by heating at reflux for 48 hours in concentrated hydrochloric acid, followed by a further 24 hour reflux in 2 mol L-1 potassium hydroxide solution. Acidification of the resulting solution with glacial acetic acid gave a turbid solution which deposited colorless crystals within several hours. The diffraction data obtained were solved and refined in the monoclinic space group P21/c, and the asymmetric unit was found to contain half of one molecule of H2L3, with the remainder generated by an inversion center located within the piperazine ring. The structure shows the expected chair conformation of the piperazine ring with the benzyl substituent occupying an equatorial position from the ring nitrogen atom (Figure 3).


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Figure 3 (Top) Structure of H2L3 with heteroatom labelling scheme. C-H hydrogen atoms omitted for clarity. (Bottom) O-H···N hydrogen bonding within the structure of H2L3 giving rise to a 1-dimensional chain. Hydrogen atoms not involved in hydrogen bonding omitted for clarity. The extended structure of H2L3 is dominated by a strong hydrogen bonding interaction between the carboxylic acid and piperazine nitrogen atom of adjacent molecules, with O···N distance 2.644(2) Å. The hydrogen atom involved in this interaction gave the best crystallographic fit when modelled as bonded to the carboxylic acid oxygen, rather than in a zwitterionic form, although pKa considerations would imply that in the solution or gas phase this proton would most likely be primarily bonded to the piperazine nitrogen atom. When allowed to freely refine, the position of hydrogen atom H1 converged at an O-H distance of 1.10 Å, and in the final model the O-H distance was loosely restrained and converged to an O-H distance of 0.87 Å. Two such interactions occur between each pair of H2L3 units, and extend the structure into a one-dimensional chain running parallel to the crystallographic a axis. These chains also associate via intermolecular π-π interactions between pairs of parallel phenyl rings (mean


16

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interplanar distance 3.32 Å), while a number of weak C-H···O interactions originating from the piperazine and benzyl methylene groups also permeate the structure.

Synthesis and structure of poly-[Co(H2L1)(L2)(OH2)]·1/2H2O 1 Single crystals of complex 1 were isolated from the reaction between cobalt sulfate heptahydrate, H2L1 and H2L2 under hydrothermal conditions. The red block crystals were subjected to single crystal X-ray diffraction, and the data were solved and refined in the chiral orthorhombic space group P21212. The asymmetric unit of poly-[Co(H2L1)(L2)(OH2)]·1/2H2O 1 contains one Co(II) ion, one molecule of H2L1, which remains protonated in its neutral form, and one molecule of L2, with both carboxylate groups deprotonated. The geometry of the Co(II) ion is best described as distorted octahedral, coordinated by two pyrazole nitrogen atoms, one aqua ligand and two carboxylates, with one each in monodentate and bidentate chelate coordination modes (Figure 4). Both the H2L1 and L2 linkers join two Co(II) ions each, to give a polymeric assembly. The asymmetric unit also contains one non-coordinating water molecule disordered across two symmetry-equivalent orientations.


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Figure 4 Metal and ligand environments in the structure of 1. Hydrogen atoms and disordered lattice water molecule omitted for clarity.

Not unexpectedly, the structure of 1 contains extensive hydrogen bonding interactions. The aqua ligand acts as a hydrogen bond donor in two distinct interactions, forming a sevenmembered R (7) hydrogen bonding ring with the adjacent carboxylate ligand through the noncoordinating carboxylate oxygen O19, and forming a nine-membered R (9) ring by forming a hydrogen bond with the imide oxygen atom O41 from the other adjacent L2 species.42 In a marked departure from the expected behavior, neither of the two pyrazole rings forms a hydrogen bonding interaction with another ligand molecule attached to the same metal ion. Instead, one pyrazole N-H group forms a hydrogen bond with non-coordinating carboxylate oxygen atom O19 from a nearby metal center, while the other pyrazole N-H group donates a hydrogen bond towards the disordered lattice water molecule. This water molecule was modelled


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as disordered across two sites straddling a twofold rotation axis, participating in three hydrogen bonding interactions and their symmetry equivalents, accepting a hydrogen bond from pyrazole nitrogen atom N3 and donating hydrogen bonds to carboxylate oxygen atom O45 and imide oxygen atom O40. The hydrogen bonding interactions within 1 are shown in Figures S1 and S2 (Supporting Information). When extended through the H2L1 and L2 links, the structure is revealed as a 2-dimensional sheet running parallel to the ac plane. The (4,4) sheet is defined by one-dimensional zig-zag chains of H2L1-linked metal ions passing parallel to the c axis, which are linked and surrounded by helical chains of L2-linked metal ions which form the outer layers of each sheet. The chirality of the structure, originating from the chiral L2 ligand, enforces a left-handed directionality for these helices throughout the crystal. The helical chains display a pitch of approximately 15 Å, and a diameter of approximately 7 Å, which defines the thickness of the 2-dimensional sheet structure within which the zig-zag H2L1 chains propagate (Figure 5). Naphthalene diimides (NDIs) are well-known for their propensity to undergo π-π stacking interactions,43-47 and the structure of 1 is no exception. The flat faces of the polymeric sheets of 1 associate with one another through a series of offset face-to-face π-π interactions, where the NDI planes are offset by 3.7 ° to one another, and with a minimum interatomic distance of 3.30 Å. No substantial π-π interactions are observed within each sheet, with such interactions presumably restricted by the small π system of the pyrazole rings and steric bulk of the attached methyl substituents.


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Figure 5 (Top) Structure of a single sheet within complex 1. H2L1 ligands coloured red, L2 ligands coloured yellow (Bottom) Interaction of adjacent sheets in the structure of 1 viewed parallel to the H2L1 linkages, showing π-π interactions between naphthalene diimide units. Hydrogen atoms are omitted for clarity.

Synthesis and Structure of poly-[Co(H2L1)2(L3)] 2 Complex 2 was prepared by reacting H2L1 and H2L3 with cobalt sulfate heptahydrate in 20 mM NaOH solution under solvothermal conditions. The pink crystals were analyzed by single crystal X-ray diffraction, and a structure model was generated in the monoclinic space group C2/c. The asymmetric unit of poly-[Co(H2L1)2(L3)] 2 contains one Co(II) ion residing coincident to a crystallographic twofold rotation element, one molecule of H2L1 in the neutral


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protonation state, and half of one molecule of L3 with a crystallographic inversion centre located within the piperazine ring. The cobalt ion displays an octahedral coordination mode, where the equatorial positions are occupied by pyrazole nitrogen atoms and the axial positions are filled by monodentate carboxylate oxygen atoms, shown in Figure 6. The pyrazole rings adopt a staggered propeller-type arrangement in order to accommodate the steric demands of coordination around the equatorial plane. The Co-O bonds, with length 2.150(2) Å, are of a comparable length to the equatorial Co-N bonds (2.133(2) and 2.182(2) Å). Although less common that the tetrahedral [M(HPz)2(RCOO)2]

coordination

mode,

similar

trans-[M(HPz)4(RCOO)2]

coordination

environments incorporating the H2L1 ligand have been reported previously, for M = Ni or Cd.24,25

Figure 6 Coordination geometry and hydrogen bonding environment in the structure of 2. Ligand molecules truncated and hydrogen atoms not involved in hydrogen bonding omitted for clarity. Symmetry codes used to generate equivalent atoms: 1: 1-x, +y, 1/2-z; 2: +x, 1+y, +z; 3: 1-x, 1+y, 1/2-z.


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The four N-H hydrogen bond donors in the vicinity of the metal site instigate two types of hydrogen bonding interactions, both directed towards the carboxylate groups of L3. One interaction resembles the typical N-H···O interaction frequently observed in pyrazolecarboxylate complexes between the pyrazole N-H group and the non-coordinating carboxylate oxygen atom, forming a 7-membered ring. The remaining pyrazole N-H groups donate hydrogen bonds to the coordinating carboxylate oxygen atoms, forming five-membered rings. Although the latter interaction displays a much shorter D···A distance (2.646(3) Å vs 2.714(2) Å), the seven-membered hydrogen bonding ring displays a much more favorable geometry, leading to an N-H···O angle of 174(2) °, cf. 133(2) ° for the less favorable 5-membered ring. No solvent molecules were observed in the asymmetric unit to lend additional hydrogen bonding interactions to the structure. Both the H2L1 and L3 ligands coordinate to two equivalent cobalt ions, leading to a polymeric network. Extension of the structure through H2L1 linkages gives rise to a 1-dimensional chain running parallel to the b axis comprised of Co2(H2L1)2 loops, as shown in Figure 7. These chains are linked into a second dimension by L3 linkages parallel to the [1,0,1] vector, giving a 2dimensional sheet with (4,4) topology. Despite the presence of several aromatic groups within the structure, no substantial π-π interactions were observed either within or between the polymeric sheets, with the exception of a reciprocated C-H···π interaction between pyrazole methyl group C8 and an adjacent pyrazole ring, with carbon – ring centroid distance 3.25 Å. As expected, thermal analysis of 2 showed no substantial mass losses before a slow single-step decomposition process initiated at approximately 270 °C. We found that the synthesis of compound 2 could also be carried out in a tenfold scale-up by linearly scaling the quantities of solvent and reactants, albeit at a slightly lower percentage yield (113mg, 61%).


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Figure 7 (Top) Structure of the Co2(H2L1)2 loops which form part of the extended structure of 2. (Bottom) Extended structure of 2 viewed perpendicular to the 2-dimensional sheets. Hydrogen atoms are omitted for clarity.

Synthesis and Structure of poly-[Co2(HL1)2(L3)] 3 Crystals of 3 were prepared by a similar method to 2, except the concentration of aqueous sodium hydroxide was increased to 40 mM and a larger excess of cobalt sulfate was used, giving dark purple crystals as a pure phase. The structure of poly-[Co2(HL1)2(L3)] 3 was solved and refined in the monoclinic space group P21/n and, similar to 2, the asymmetric unit was found to


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contain one Co(II) ion, one molecule of HL1 and half of one molecule of L3, with a crystallographic inversion centre located within the piperazine ring. The coordination geometry of the Co(II) ion is best described as regular tetrahedral, with a coordination sphere consisting of one carboxylate oxygen atom and three pyrazole nitrogen atoms. Ligand HL1 is present in a singly deprotonated, monoanionic state, while both carboxylate groups on L3 are deprotonated. One pyrazole group of HL1 coordinates in a neutral monodentate binding mode, while the deprotonated ring bridges two equivalent cobalt ions in a µ2-κN:κNʹ coordination mode. The resulting dinuclear Co2 node is overall coordinated by two bridging pyrazolates, two monodentate pyrazoles and two carboxylates, connecting four HL1 ligands and two L3 ligands, as shown in Figure 8. The neutral pyrazole group of HL1 is involved in hydrogen bonding with the non-coordinating carboxylate oxygen atom of an adjacent L3 ligand, forming the frequently observed seven-membered hydrogen-bonded ring around the metal center (Figure 1). The 3connected, singly deprotonated coordination of the HL1 ligand represents a hitherto unknown coordination mode for methylenebis-pyrazole ligands, falling between the doubly deprotonated 4-connecting mode observed by Kruger et al. in 200029 and the commonly observed neutral 2connected behavior.22,23 The [M2(µ2-pz)2(Hpz)2(RCOO)2] cluster has been occasionally observed in discrete systems, most often with zinc,48-49 however, no polymeric examples have been reported to date.


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Figure 8 Metal coordination environment and hydrogen bonding interactions in the structure of 3. Ligands truncated and hydrogen atoms not involved in hydrogen bonding omitted for clarity.

The bimetallic cobalt clusters within compound 3 are linked into an extended 3-dimensional network by bridging through both HL1 and L3 ligands. Considering only the HL1 bridges and taking the cobalt clusters as nodes, a 2-dimensional sheet results, which is linked into a threedimensional array by bridging through the two-connecting L3 ligands. The topological description of the overall structure matches that of the primitive cubic α-Po network, albeit with significant distortion to the inter-nodal distances by the relatively long bridging distance of the L3 species of ca. 15 Å, compared with 10.5 Å for the HL1 bridging distances. Similar to that seen in compound 2, the S-shaped conformation of the L3 ligand compensates for the geometric distortion of the bimetallic node to provide a relatively regular geometry to the overall network (Figure 9). The steric bulk of the two ligands completely fills the space between nodes, and no solvent molecules or void space was observed within the structure. Similar to compound 2, few substantial π-π interactions are observed in the extended structure of 3, which consist largely of C-H···π or edge-to-face π-π interactions. The most substantial interaction of this type occurs between a C-H group from the piperazine phenyl ring and the deprotonated pyrazole ring, with C···C distance 3.66 Å and C-H···C angle 172°. Thermal analysis of compound 3 showed no


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substantial mass loss across the entire temperature range of the experiment (25 – 400 °C), and the sample was visually unchanged by this heating cycle, demonstrating particularly good thermal stability for the material. A tenfold linear scale-up on compound 3 resulted in an improvement of the isolated yield (118 mg, 94%).

Figure 9 Extended structure of compound 3, showing linkages between Co2 dimers through HL1 ligands (Top) and L3 linkages (Middle), and representation of a single α-Po unit of 3 (dashed lines) with chemical linkages shown for two edges (Bottom).

Magnetic properties of poly-[Co2(HL1)2(L3)] 3


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Motivated by the presence of a closely bridged dinuclear CoII environment, we pursued magnetic susceptibility measurements on compound 3 with the expectation of observing coupling behavior between the two CoII nuclei. Magnetic susceptibility measurements of 3 show that χMT decreases gradually from 2.3 cm3 mol-1 K, per Co, at 300 K (µeff = 4.29 µB) to ~2.0 cm3 mol-1 K at ~70 K, then more rapidly to reach ~0.1 cm3 mol-1 K at 2 K (Figure 10). The corresponding χM plot shows a maximum at 18 K indicative of intra-cluster antiferromagnetic coupling, while below ~5 K there is a small increase in χM due to traces of monomer impurity. The data were fitted extremely well to a simple -2JS1.S2 Heisenberg Hamiltonian, for a S = 3/2 dimer model. This is appropriate to tetrahedral Co(II) centers with orbitally non-degenerate 4A2 ground states. The parameter values are g = 2.25, J = -3.2 cm-1, with a θ value (from T – θ) of -0.01 K possibly indicative of very weak inter-dinuclear interactions, the latter expected to be very weak in view of the long L3 and HL1 bridging distances (vide supra). The susceptibility plot is of very similar shape to that obtained for [CoII(3,5-dimethylpyrazolate)2(3,5-dimethylpyrazole)]2, a dinuclear complex containing tetrahedral Co(II) centers bridged by two pyrazolates, and the J value in that work was also very similar viz. -3.0 cm-1.50


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Figure 10 Plots of magnetic susceptibility vs. T for compound 3; χM (per Co, open squares), and χMT (per Co, open circles). The solid lines are the values calculated for best fit using a S = 3/2 dimer Heisenberg model and the parameters given in the text.

Synthesis and Structure of para-((3,5-dimethyl-1H-pyrazol-4-yl)methylene)-benzoic acid H2L4 Observing the metal coordination geometry and extended structures of 1 – 3, as well as those previously reported with H2L1 and various other, more well-known aromatic dicarboxylates,22-28 it seems likely that one reason for the diversity in the extended structures regularly observed is the mismatch in flexibility and/or length of the carboxylate co-ligands employed in comparison to that of H2L1. With only one flexible sp3 linker, two aromatic rings and a typical donor-donor distance of 5-7 Å, it is non-trivial to envisage a dicarboxylate co-ligand containing complementary structural features with which to test this hypothesis. With this in mind, we devised the flexible ligand para-((3,5-dimethyl-1H-pyrazol-4-yl)methylene)-benzoic acid H2L4,


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in order to test the effect of a perfectly complementary ligand set on the extended structure of a pyrazole-carboxylate coordination polymer. The synthesis of H2L4 was achieved in 3 steps from ethyl 4-(bromomethyl)benzoate, utilising a radical reaction between the alkyl halide and [Co(acac)2] first reported by Marquet51 to generate the diketone precursor, which was cyclised and the carboxylic acid deprotected to give the product H2L4 in 29% overall yield. The synthesis of H2L4 is outlined in Scheme 1.

Scheme 1 Synthesis of H2L4 with hydrogen atom labelling scheme for H2L4 and precursor species.


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O

7 5

O

6

4 3 6

O Br

O

N NH

2 1

O

4

N

5

(i)

(ii)

47%

80%

NH 3

(iii)

4

76%

3

2 1

O

O O 12

O

14

H O

HO

2 1

EtHL4 13

11

O

H2L4

10

O

O

9 8

EtL4acac

Reagents and conditions: (i) [Co(acac)2], CHCl3, 120 °C; (ii) H2NNH2·H2O, MeOH, reflux 24hr; (iii) LiOH, THF/H2O, reflux 24 hr, then HCl(aq).

Single crystals of H2L4 for structural analysis were prepared by heating 10 mg of the material to 100 °C in 5 mL of water in an acid digestion bomb, followed by cooling to room temperature. The diffraction data were solved and refined in the orthorhombic space group Pbca, with one molecule of H2L4 in the asymmetric unit. Geometrically, the molecule bears some resemblance to H2L1, with two aromatic rings at an interplanar angle of 69° linked by a methylene bridge, albeit with greater distance between the sets of possible donor atoms of approximately 8 Å. The structure of H2L4 is shown in Figure 11.


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Figure 11 (Top) Structure of H2L4 with heteroatom labelling scheme. Selected hydrogen atoms omitted for clarity. (Bottom) Extended hydrogen-bonded structure of H2L4 viewed perpendicular to a 2-dimensional hydrogen-bonded sheet.

The extended structure of H2L4 is largely defined by hydrogen bonding interactions between the pyrazole and carboxylic acid groups. As would be expected, two types of interactions are observed, of the type N-H···O and O-H···N, connecting each H2L4 molecule to four others. Extending these interactions from each molecule gives rise to an undulating 2-dimensional (4,4) hydrogen-bonded sheet parallel to the bc plane. No other substantial intermolecular interactions were observed within each plane, however, adjacent planes associate by way of parallel offset face-to-face π-π interactions between phenyl rings at a mean interplanar distance 3.40 Å (Supporting Information, Figure S3).


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Synthesis and structure of poly-[Co(HL4)2]·H2O 4 Compound 4 was prepared by reacting H2L4 with cobalt sulfate heptahydrate under hydrothermal conditions in the presence of the weak organic base 2,4,6-collidine. The purple crystals obtained were subjected to single crystal X-ray diffraction analysis, and the data obtained were solved and the structure model refined in the monoclinic space group C2/c. The asymmetric unit of 4 was found to contain one Co(II) ion, two non-equivalent HL4 ligands and one water molecule (Figure 12). Interestingly, while the coordination around the metal ion is the tetrahedral N2-O2 motif seen previously, only one hydrogen bond between the pyrazole N-H hydrogen atoms and deprotonated carboxylate groups was observed. The protonated pyrazole nitrogen atom N3 instead acts as a hydrogen bond donor to the lattice water molecule O36, which itself acts as a hydrogen bond donor to non-coordinating oxygen atoms O18 and O35, the latter of which also accepts a hydrogen bond from pyrazole nitrogen atom N20. The hydrogen bonding environment of 4 is shown in Figure 13.


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Figure 12 (Top) Metal and ligand environments in compound 4 with partial heteroatom labelling scheme. (Bottom) The extended 1-dimensional polymeric structure of 4. Hydrogen atoms not involved in hydrogen bonding omitted for clarity.

Topologically, 4 possesses a one-dimensional polymeric chain structure, with Co(II) atoms linked by zig-zag loops of HL4 molecules parallel to the c axis. However, when considering hydrogen bonding connections, the network must be considered binodal 3,5-connected, where water molecules are 3-connected nodes, and [Co(HPz)2(COO)2] moieties become 5-connected nodes, owing to their connectivity to two other such moieties through coordination bonds and three water molecules through hydrogen bonds. The resulting network adopts a (42·67·8)(42·6) topology, forming a double-layered two-dimensional sheet parallel to the bc plane. A schematic diagram is shown in Figure 13. Aside from these hydrogen bonding interactions, the structure of 4 contains few significant intermolecular interactions, with only very weak partial π-π overlap between sheets, and no other significant interactions within each sheet. Thermogravimetric


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analysis of 4 showed a gradual mass loss of 3.4% from 100 - 250 °C, consistent with the loss of the lattice water molecule (calculated 3.5% mass), leading directly into a gradual decomposition.

Figure 13 (Top) Hydrogen bonding environment in the structure of 4 showing the linkage of four otherwise non-connected chains, with hydrogen bond donors and acceptors labelled. Symmetry codes used to generate equivalent atoms: 1: +x, 1-y, 1/2-z; 2: +x, 1-y, 1/2+z; 3: +x, -y, 1/2+z; 4: 3/2-x, 1/2-y, 2-z; 5: 3/2-x, -1/2+y, 2-z; 6: 3/2-x, -1/2+y, 3/2-z; 7: 3/2-x, 1/2+y, 3/2-z. (Bottom) Topological representation of the resulting double-layer sheet structure. Red spheres


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represent water molecules, and blue spheres represent metal ions, while blue bonds represent linkages through HL4 and red/blue bonds represent hydrogen bonds.

Synthesis and structure of poly-[Cu(HL4)2]·2MeOH 5 Following the unexpected outcome of the reaction of H2L4 with Co(II) under hydrothermal conditions, the possibility of forming coordination networks of H2L4 with other metal ions was explored, with the expectation of variable ligand behavior based on different metal ion geometries and sizes. To this end, the high reactivity and low probability of a stable tetrahedral coordination sphere from CuII made it an excellent candidate for structural studies. Reaction of H2L4 with one equivalent of copper sulfate pentahydrate in methanol at room temperature gave a green turbid suspension which, over the course of one week, deposited pale purple crystals. The crystals were analyzed by single crystal X-ray diffraction, and the data were solved and refined in the triclinic space group P-1. The asymmetric unit of poly-[Cu(HL4)2]·2MeOH 5 contains one HL4 ligand, with the carboxylate group deprotonated, one Cu(II) ion residing on a crystallographic inversion center, and a non-coordinating methanol molecule. As expected from the purple colour of the complex, the copper ion is coordinated in a square planar manner, with the coordination sphere occupied by two pyrazole nitrogen atoms and two carboxylate oxygen atoms in a trans disposition, shown in Figure 14. The geometry of the ligand itself is essentially equivalent to one of the two HL4 fragments in the structure of 4, and differs to the other fragment by a rotation of the phenyl ring and carboxylate group (Supporting Information, Figures S4 and S5).


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Figure 14 Metal and ligand environments in the structure of 5. Hydrogen atoms not involved in hydrogen bonding omitted for clarity. Symmetry codes used to generate equivalent atoms: 1: 1-x, 1-y, 1-z; 2: 1-x, 1-y, -z; 3: +x, +y, -1+z. When extended through the coordination bonds at each end of the HL4 molecule, a onedimensional chain is observed in the structure of 5. The structure is reminiscent of that seen in 4, however, due to the difference in metal coordination geometry and the symmetry relationship of each molecule of HL4, the loops adopt a slightly different conformation. The metal-metal distance of 11.04 Å compares reasonably well to that observed in 4 of 10.81 Å. The chains interact with one another through hydrogen bonding interactions between the non-coordinating pyrazole N-H groups and lattice methanol oxygen atoms. The lattice methanol molecules also engage in hydrogen bonding interactions with the non-coordinating carboxylate oxygen atoms from a separate chain (Figure 15). This series of interactions is reciprocated between chains at each metal ion, forming a series of R (18) hydrogen bonding rings incorporating two metal ions from adjacent chains. Extension of these interactions parallel to the a axis bridges the coordination chains into a 2-dimensional (4,4) hydrogen bonded sheet.


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Figure 15 (Top) Hydrogen bonding behavior in the structure of 5 showing R (18) hydrogenbonded rings linking chains. (Bottom) Extended structure of 5, showing linkages of polymeric [Co(HL4)2] chains by columns of methanol molecules.

The methanol molecules within the lattice are aligned in linear arrangements running parallel to the c axis, which would be expected to facilitate ready solvent loss. Indeed, crystals of 5 were observed to lose crystallinity rapidly on standing in air, evidenced by peak broadening and the emergence of an amorphous region in the X-ray powder diffraction pattern (Supporting Information). Infrared spectroscopy and microanalysis suggest that this process is accompanied by uptake of atmospheric water, suggesting a solvation for the air-dried material of poly[Cu(HL4)2]·0.75(MeOH)·3(H2O). This formula is corroborated by thermogravimetric analysis on the air-dried material (Supporting Information), which showed a two-step mass loss with onset at room temperature, consisting of a rapid 5% loss up to 60 °C, followed by gradual loss of


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a further 9% mass up to 250 °C. These values are in agreement with the expected loss of methanol (calculated 4%) followed by water (calculated 9%).

Discussion Examination of the structures of 1 – 3 provides several new insights into the coordination behavior of the H2L1/HL1 ligand when present in conjunction with carboxylate co-ligands. Largely as expected, the geometry of the bis-pyrazole species was consistent between the three structures, and co-ligands L2 and L3 acted to provide additional structure direction and hydrogen bond acceptor functionality. The clip-like syn conformation of the L2 ligand in complex 1 most likely contributed to the overall 2-dimensional structure observed by conveniently bridging between [CoH2L1] chains, while the [Co(H2L1)2] looped chains in compound 2 were readily bridged into two-dimensional structures by the relatively slender nature of the L3 co-ligand. Surprisingly, none of the structures 1 - 5 contained the well-known doubly hydrogen-bonded tetrahedral [M(HPz)2(RCOO)2] coordination geometry frequently seen in other systems.22,23 The coordination geometry observed in 1 is most likely rationalized by the large steric bulk and backbone rigidity of L2, which may enforce the less sterically demanding geometry on the cobalt ion within the structure and allow for additional hydrogen bonding interactions. For compounds 2 and 3, this discrepancy is to be expected, given the altered reaction stoichiometries in which an excess of H2L1 was employed. Despite our attempts, we were unable to isolate and crystallographically characterize any compounds of the formula poly-[Co(H2L1)(L3)] by altering the reaction stoichiometry. The presence of a pyrazolate-bridged dinuclear node in 3 provided an additional and unexpected outcome in this structural investigation, and represents a rare example of a


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magnetically active cluster species in an L1-carboxylate coordination polymer. Although the coupling interactions observed in 3 are typical of weak antiferromagnetic coupling behaviour in an isolated dimer-type system, the magnetically active [M2(HPz)2(µ-Pz)2(RCOO)2] system represents a promising area for future research. It is expected that using shorter and fully conjugated carboxylate bridges to link such clusters in HL1 systems may generate a new range of magnetically interesting coordination polymers, extending on the design principles which have already been established for flexible bispyrazole-carboxylate coordination polymers. The structures of 4 and 5 display remarkably similar attributes, especially considering the differences in metal ion geometry and synthesis conditions. In these instances, the formation of a low-dimensionality assembly was most likely directed by the self-complementarity of the ligand sphere. Within each complex, all four coordinated ligands possess identical bridging distances and degrees of flexibility, and are therefore uniquely capable of forming homotopic onedimensional chains. This flexibility was also sufficient to overcome the geometric differences caused by the change in coordination geometry from tetrahedral Co(II) to square planar Cu(II). Both compounds contain N-H hydrogen bond donors directed outwards from the metal center, allowing the incorporation of hydrogen-bonded solvent molecules which extend the structures into 2-dimensional arrays.

Conclusion We have prepared and characterized three new pyrazole-carboxylate coordination polymers derived from the ditopic ligand 4,4′-methylenebis(3,5-dimethyl-1H-pyrazole) H2L1, prepared by reaction with Co(II) and co-ligands (S,S)-1,4,5,8-naphthalenetetracarboxylic diimide-N,N′-bis(2-propionic acid) H2L2 and 1,4-bis((3-carboxyphenyl)methyl) piperazine H2L3. Compound 1, a


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chiral 2-dimensional sheet containing strongly π-π stacking outer layers, and compound 2, a densely packed 2-dimensional sheet, displayed the typical neutral bis-monodentate coordination behavior for H2L1, albeit with uncommon metal coordination geometries and hydrogen bonding modes. The emergence of previously unknown coordination behavior for the HL1 ligand in complex 3, a 3-dimensional α-Po type structure containing pyrazolate-bridged Co2 clusters, revealed the potential for the incorporation of coordination clusters and magnetic coupling interactions into materials based on mixed bis-pyrazole-carboxylate ligands. Finally, we have prepared a new flexible heteroditopic ligand ((3,5-dimethyl-1H-pyrazol-4-yl)methylene)-benzoic acid H2L4 and shown in one-dimensional polymers 4 and 5 the tendency for homotopic chain formation. This behaviour is likely driven by the self-complementary dimensions provided by a homotopic ligand sphere, which nonetheless retains the potential for intramolecular hydrogen bonding interactions in the vicinity of the metal site.

Acknowledgements This work is supported by the Science and Industry Endowment Fund. Portions of this work were carried out on the MX1 Macromolecular Crystallography beamline at the Australian Synchrotron, Victoria, Australia. SRB and DRT acknowledge the Australian Research Council for fellowships. KSM thanks the Australian Research Council for a Discovery grant. CSH acknowledges the University of Canterbury for a PhD Scholarship. CSH and PEK acknowledge the Royal Society of New Zealand Marsden Fund for financial support.


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Supporting Information. Thermogravimetric analysis for all coordination compounds, X-ray powder diffraction patterns for all compounds, additional figures and ellipsoid plots, table of hydrogen bonding parameters. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors Email: [email protected]; [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. References 1. Gavezzotti, A. Acc. Chem. Res. 1994, 27, 309-314. 2. Day, G. M. Crystallogr. Rev. 2011, 17, 3-52. 3. Price, S. L. Adv. Drug. Deliv. Rev. 2004, 56, 301-319. 4. Datta, S.; Grant, D. J. W. Nat. Rev. Drug. Discov. 2004, 3, 42-57. 5. Wilmer, C. E.; Leaf, M.; Lee, C. Y.; Farha, O. K.; Hauser, B. G.; Hupp, J. T.; Snurr, R. Q. Nature Chem. 2012, 4, 83-89. 6. Mellot-Draznieks, C.; Dutour, J.; Férey, G. Angew. Chem. Int. Ed. 2004, 43, 6290-6296. 7. Zhang, M.; Bosch, M.; Gentle III, T.; Zhou, H.-C. CrystEngComm 2014, 16, 4069-4083.


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For Table of Contents Use Only

The utility of the burgeoning pyrazole-carboxylate mixed ligand approach to coordination polymer synthesis has been probed with the preparation of five coordination polymer materials displaying varying linker dimensions and geometries. A new flexible heteroditopic pyrazolecarboxylate ligand has been employed to demonstrate the effect of linker dimensions on extended network architecture in pyrazole-carboxylate polymer systems.


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Exploiting the Pyrazole-Carboxylate Mixed Ligand System in the

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