Modulating Porosity through Conformer-Dependent Hydrogen

May 11, 2015 - This seemingly small difference arising from variation in synthetic conditions influences the extended structures of each network throu...
1 downloads 8 Views 3MB Size
Download PDF
Article pubs.acs.org/crystal

Modulating Porosity through Conformer-Dependent Hydrogen Bonding in Copper(II) Coordination Polymers Chris S. Hawes,† Gregory P. Knowles,† Alan L. Chaffee,† David R. Turner,*,† and Stuart R. Batten*,†,‡ †

School of Chemistry, Monash University, Clayton, Victoria 3800, Australia Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

‡

S Supporting Information *

ABSTRACT: A new divergent ligand, N,N′-bis(4carboxyphenylmethylene)ethane-1,2-diamine (H4L1), has been prepared in high yield and used to generate two copper(II) coordination polymer materials, poly-[Cu(H2L1)(OH2)]·H2O (1) and poly-[Cu(H2L1)(OH2)]·H2O·DMF (2). Both networks possess (4,4) sheet topologies and have almost identical compositions and coordination modes. The only major difference between the compounds lies with the conformation of the chelating ethylenediamine cores; compound 1 adopts a trans-(R,R/S,S) conformation, while compound 2 exhibits a cis-(R,S) conformation. This seemingly small difference arising from variation in synthetic conditions influences the extended structures of each network through hydrogen bonding interactions, resulting in the formation of a close packed 2-fold 2D → 2D parallel interpenetrated network for 1, while the extended, non-interpenetrated structure of 2 contains aligned one-dimensional solvent channels. After solvent exchange and evacuation, compound 2 was found to adsorb approximately 35 cm3(STP)/g of CO2 at atmospheric pressure at 273 K, with a zero-loading enthalpy of adsorption of −33 kJ/ mol, while adsorbing only minimal quantities of N2. These findings are a rare example of conformer-dependent porosity in otherwise geometrically similar frameworks and highlight the importance of understanding weak and fluxional secondary interactions in framework and ligand design.

■

INTRODUCTION Research into the synthesis and analysis of new solid-phase carbon dioxide separation and capture materials is rapidly gaining momentum, due to the globally acknowledged need for urgent action to mitigate industrial CO2 emissions.1−4 Metal− organic frameworks (MOFs) have been widely touted as ideal candidates for this purpose, largely due to the unprecedented level of micropore and/or mesopore surface customizability which is possible in these materials.5−8 In particular, the ability to generate porous materials with monodisperse pore sizes and functionality tailored to a specific application means that such materials can be readily adapted to carbon dioxide capture applications in a wide variety of real-world settings.9,10 The self-assembly process which forms crystalline supramolecular materials from solution is well-known as a delicate interplay of a large number of competing interactions.11−14 As such, while the formation of MOF materials from rigid ligands containing few sites of strong secondary interaction is now relatively well understood, the same cannot be said for the case of highly flexible ligands and/or those ligands containing additional hydrogen bonding sites or regions of other strong intermolecular interactions. Indeed, the structure directing ability of outer sphere hydrogen bonding interactions within MOFs is well-known to be a powerful force in driving the formation and topology of extended assemblies.15−18 Herein, © XXXX American Chemical Society

we report the synthesis of a new, readily available divergent ligand N,N′-bis(4-carboxyphenylmethylene)ethane-1,2-diamine (H4L1) and its use in the formation of copper(II) coordination polymers, where hydrogen bonding interactions dictate either the formation or interpenetration of solvent-accessible void space capable of carbon dioxide uptake.

■

EXPERIMENTAL SECTION

General Considerations. All solvents, reagents, and starting materials were of reagent grade or better and were used as received from Sigma-Aldrich, Alfa-Aesar, or Merck. NMR spectra were recorded on a Bruker AVANCE spectrometer operating at 400 MHz for 1H and 100 MHz for 13C nuclei. 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 ESIMS instrument. 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. 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 Received: April 13, 2015 Revised: May 5, 2015

A

DOI: 10.1021/acs.cgd.5b00502 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Table 1. Crystallographic Refinement Data for Compounds H4L1·2H2O, 1, and 2 H4L1·2H2O empirical formula formula wt temp/K cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg Volume/Å3 Z ρcalc g/cm3 μ/mm−1 F(000) 2Θ range for data collection/deg index ranges reflns collected indep reflns reflns obsd [I ≥ 2σ(I)] data/restraints/ params goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)] final R indexes [all data] CCDC no.

1

2

C18H24N2O6

C18H22CuN2O6

C21H28CuN3O6.5

364.39 100 triclinic P1̅ 6.238(2) 8.098(2) 18.502(4) 78.40(3) 86.55(3) 78.71(3) 897.6(4) 2 1.348 0.102 388.0 2.24 to 55.86

425.92 100 monoclinic P21/c 9.5050(19) 18.014(4) 11.178(2) 90.00 106.04(3) 90.00 1839.4(6) 4 1.538 1.225 884.0 4.52 to 55.86

490.00 100 monoclinic C2/c 19.824(4) 17.458(4) 14.066(3) 90.00 114.58(3) 90.00 4426.9(15) 8 1.470 1.032 2048.0 3.24 to 55.92

−8 ≤ h ≤ 8, −10 ≤ k ≤ 10, −24 ≤ l ≤ 24 29711 4243 [Rint = 0.0619, Rsigma = 0.0365] 3882

−12 ≤ h ≤ 12, −23 ≤ k ≤ 23, −14 ≤ l ≤ 14 30386 4396 [Rint = 0.0456, Rsigma = 0.0235] 4034

−26 ≤ h ≤ 26, −22 ≤ k ≤ 22, −18 ≤ l ≤ 18 36912 5220 [Rint = 0.0653, Rsigma = 0.0305] 4474

4243/8/259

4396/8/262

5220/5/300

1.068

1.075

1.032

R1 = 0.0442, wR2 = 0.1316

R1 = 0.0440, wR2 = 0.1098

R1 = 0.0595, wR2 = 0.1722

R1 = 0.0470, wR2 = 0.1349

R1 = 0.0476, wR2 = 0.1119

R1 = 0.0672, wR2 = 0.1792

1058126

1058127

1058128

(1.28 g, 34 mmol) portionwise over 1 h with stirring. The reaction mixture was then stirred at room temperature overnight and then poured onto 300 mL of ice water. The resulting white solid was filtered, washed with water, and air-dried: yield 2.94 g (88%); mp 74− 76 °C; δH (400 MHz, d6-DMSO) 2.58 (s, 4H, H1), 3.74 (s, 4H, H2), 3.83 (s, 6H, H5), 7.45 (d, 4H, 3J = 8.3 Hz, H3) 7.89 (d, 4H, 3J = 8.3 Hz, H4); δC (100 MHz, d6-DMSO) 48.40, 51.97, 52.54, 127.85, 128.00, 129.01, 147.03, 166.22; m/z (ESMS) 357.1 (100%, [M + H+], calculated for C20H25N2O4 357.2); νmax(ATR)/cm−1 3293m, 2817m, 1716s, 1609m, 1451m, 1432m, 1412m, 1367w, 1282s, 1187m, 1175m, 1127s, 1111s, 1015m, 987m, 954m, 849m sh, 795s, 748s, 690m. Synthesis of N,N′-Bis(4-carboxyphenylmethylene)ethane1,2-diamine Dihydrochloride (H4L1·2HCl). To a mixture of potassium hydroxide (12 g, 210 mmol) in 75 mL of water and 20 mL of tetrahydrofuran was added Me2H2L1 (2.94g, 8.3 mmol). The resulting mixture was heated at reflux for 48 h and then allowed to cool to room temperature. The clear colorless solution was concentrated by rotary evaporation to remove tetrahydrofuran and diluted to 200 mL volume with water. The mixture was then taken to pH 2 with concentrated hydrochloric acid, and the resulting white suspension was stirred at room temperature for 2 h, followed by vacuum filtration. The white solid obtained was dried in vacuo: yield 2.27 g (69%); mp >300 °C; found C 53.58, H 5.68, N 6.98, calculated for C18H22N2O4Cl2 C 53.87, H 5.52, N 6.98%; δH (400 MHz, d6DMSO) 3.39 (s, 4H, H1), 4.28 (s, 4H, H2), 7.70 (d, 4H, 3J = 8.2 Hz, H3) 7.98 (d, 4H, 3J = 8.2 Hz, H4) 9.87 (br s, 4H, H5), 13.09 (br s, 2H, H6); δC (100 MHz, d6-DMSO) 42.80, 49.67, 129.49, 130.09, 131.26, 136.44, 166.83; m/z (ESMS) 329.1 (100%, [H5L1+], calculated for C18H21N2O4 329.2); νmax(ATR)/cm−1 2926m, 2751s, 2700s br, 2693m, 2577m, 2412m, 1682s, 1615m, 1579m br, 1450m, 1427s sh, 1270s sh, 1191m, 1125m, 1023m, 928m, 862s, 783m, 759s, 699s.

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. Thermogravimetric analyses were carried out with a Mettler-Toledo STARe TGA/DSC instrument. Gas adsorption analyses were carried out using a Micromeritics TriStar 3020 volumetric analyzer, with samples prepared by heating under high vacuum overnight using a Micromeritics Vacprep 061 station. Ultrahigh purity gases were used for all analyses. Temperature control was achieved using insulated ice water (273 K) or liquid nitrogen (77 K) baths, or a temperature controlled water recirculator (298 K, 308 K). Enthalpy of adsorption calculations were performed by fitting a virial-type equation to the adsorption data at three temperatures (Supporting Information). Synthesis of N,N′-Bis(methyl 4-carboxyphenylmethylidene)ethane-1,2-diamine (Me2L1-imine). Ethane-1,2-diamine (270 mg, 4.5 mmol) was added dropwise with stirring to a solution of methyl 4formylbenzoate (1.50 g, 9.2 mmol) in 40 mL of ethanol. The mixture was heated at reflux for 2 h and cooled to room temperature, and the resulting white solid was filtered, washed with a small amount of cold ethanol, and dried in air: yield 1.45 g (91%); mp 180−181 °C; δH (400 MHz, CDCl3) 3.93 (s, 6H, H5), 4.03 (s, 4H, H1), 7.76 (d, 4H, 3J = 8.3 Hz, H3), 8.06 (d, 4H, 3J = 8.3 Hz, H4), 8.33 (s, 2H, H2); δC (100 MHz, CDCl3) 52.22, 61.52, 127.93, 129.83, 131.86, 139.90, 161.78, 166.61; m/z (ESMS) 353.1 (100%, [M + H+], calculated for C20H21N2O4 353.2), 375.1 (49%, [M + Na+], calculated for C20H20N2O4Na 375.1); νmax(ATR)/cm−1 2894w, 2839m, 1719s, 1640s sh, 1570m, 1434m, 1411m, 1364m, 1271s, 1195m, 1099s, 1015s, 954s, 858m, 846m, 816m, 764s, 695s. Synthesis of N,N′-Bis(methyl 4-carboxyphenylmethylene)ethane-1,2-diamine (Me2H2L1). To a slurry of Me2L1-imine (3.0 g, 8.5 mmol) in 150 mL of methanol was added sodium borohydride B

DOI: 10.1021/acs.cgd.5b00502 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Scheme 1. Synthesis of H4L1·2HCl Showing 1H NMR Numbering Schemea

a

Reagents and conditions: (i) EtOH, reflux 2 h; (ii) NaBH4, MeOH, 16 h; (iii) KOH, THF/H2O, reflux 48 h, HCl(aq).

Synthesis of N,N′-Bis(4-carboxyphenylmethylene)ethane1,2-diamine Dihydrate H4L1·2H2O. To 4 mL of a 1:1 DMF:H2O mixture was added H4L1·2HCl (20 mg; 50 μmol), and the mixture was sealed and heated to 100 °C. Colorless single crystals deposited within 3 h and were isolated by filtration, washed with DMF and water, and dried in air: yield 8.9 mg (49%). The extremely low solubility of this material under standard conditions prevented meaningful solution-phase characterization: mp >300 °C; found C 59.75, H 6.70, N 7.86, calculated for C18H20N2O4·1.75H2O (loss of 0.25H2O upon drying in transit) C 60.07, H 6.58, N 7.78%; νmax(ATR)/cm−1 3406m br, 3022m br, 2817m br, 1612s, 1530m, 1481m, 1368s br, 1297m, 1260m, 1211m, 1103s, 1044s, 1017s, 961m, 865m, 844s, 793s, 766s, 708s. Phase purity supported by X-ray powder diffraction (Supporting Information). Synthesis of poly-[Cu(H2L1)(OH2)]·H2O (1). A mixture of H4L1· 2HCl (10 mg, 25 μmol), copper(II) chloride dihydrate (18 mg, 100 μmol), and 40 mM aqueous sodium hydroxide solution (4 mL) was added to a 45 mL capacity Teflon lined stainless steel autoclave. The vessel was heated to 130 °C, allowed to dwell for 36 h, and cooled to room temperature over 6 h. The blue crystals of 1 were isolated by filtration and washed with water: yield 5.6 mg (53%); mp 274−275 °C (dec); found C 50.65, H 5.22, N 6.58, calculated for C18H22N2O6Cu C 50.76, H 5.21, N 6.58%; νmax(ATR)/cm−1 3387w br, 3153m, 2873w, 1640w, 1587s sh, 1541s, 1410m, 1350s sh, 1266w, 1172m, 1156m, 1131m, 1102w, 1051m, 1017s sh, 971s, 936m, 867, 844s, 803m, 768s, 714s. Phase purity supported by X-ray powder diffraction (Supporting Information). Synthesis of poly-[Cu(H2L1)(OH2)]·H2O·DMF (2). To 4 mL of a 1:1 DMF:H2O mixture were added H4L1·2HCl (20 mg, 50 μmol) and copper(II) chloride dihydrate (10 mg, 55 μmol), and the mixture was sealed and heated at 100 °C for 2 days. The resulting dark blue crystals were isolated by filtration, washed with DMF, and dried in air: yield 23 mg (94%); mp 282−284 (dec); found C 50.64, H 5.48, N 8.29, calculated for C18H29N3O7Cu C 50.54, H 5.86, N 8.42%; νmax(ATR)/ cm−1 3440m br, 3211m br, 2937m br, 1658s, 1605s, 1563s, 1433m, 1409m, 1353s, 1253m, 1175m, 1137m, 1087s, 1016s, 959m, 937m, 867m, 839s, 803s, 765s, 709m. Phase purity supported by X-ray powder diffraction. Synthesis of poly-[Cu(H2L1)(OH2)]·H2O (1) Using Compound 2 as Feedstock. Compound 2 (15 mg; 31 μmol) was combined with copper(II) chloride dihydrate (20 mg; 110 μmol) in 4 mL of water, and the mixture was sealed and heated in a 45 mL capacity stainless steel autoclave at 130 °C for 36 h. On cooling to room temperature, blue crystals of 1 were obtained as a pure phase: yield 5.4 mg (42%). Purity was confirmed by X-ray powder diffraction. X-ray Crystallography. Refinement and structural information is presented in Table 1. All data collections were carried out on the MX1 beamline at the Australian Synchrotron, Victoria, Australia, operating at 17.4 keV (λ = 0.7109 Å) with data collections conducted using

BluIce control software.19 The diffraction data were processed using the XDS software suite,20 and anomalous dispersion corrections for the nonstandard wavelength were made in the final refinement using Brennan and Cowan data.21 All data sets were solved using direct methods with SHELXS-9722 and refined on F2 using all data by full matrix least-squares procedures with SHELXL-9723 within the OLEX2 GUI.24 Non-hydrogen atoms were refined with anisotropic displacement parameters, while most hydrogen atoms were included in calculated positions with isotropic displacement parameters either 1.2 or 1.5 times the isotropic equivalent of their carrier atoms. Where appropriate, hydrogen atoms involved in hydrogen bonding interactions were located on the basis of Fourier residuals and restrained with standard distance and Uiso values. Disordered water molecules within the structure of 2 were modeled as isolated anisotropic oxygen atoms in the absence of any sensible hydrogen bond acceptors; however, the associated hydrogen atoms were included in the crystallographic formula to allow for accurate determination of density and absorption coefficient. The functions minimized were ∑w(Fo2 − Fc2), with w = [σ2(Fo2) + aP2 + bP]−1, where P = [max(Fo)2 + 2Fc2]/3.

■

RESULTS

Synthesis and Structure of H4L1. The ligand N,N′-bis(4carboxyphenylmethylene)ethane-1,2-diamine (H4L1) was prepared as its dihydrochloride salt H4L1·2HCl in three steps from ethylenediamine using straightforward Schiff base chemistry followed by reduction and ester hydrolysis, with an overall yield of 55% (Scheme 1). The white solid obtained by precipitation from the hydrolysis mixture with an excess of concentrated hydrochloric acid was shown by NMR spectroscopy to contain both COOH and R2NH2+ proton environments, consistent with a dihydrochloride salt. This observation was corroborated by microanalysis and infrared spectroscopy (localized CO stretch 1682 cm−1, N−H and O−H stretches 2900−2400 cm−1). While this material displayed favorable solubility properties and was used in the subsequent preparations, we were able to prepare single crystals of the neutral species H4L1· 2H2O by heating the dihydrochloride salt in a 1:1 DMF:H2O mixture for several hours, which produced a highly insoluble crystalline sample of H4L1·2H2O after deprotonation by dimethylamine formed by in situ DMF hydrolysis.25 The crystals of H4L1·2H2O were subjected to analysis by single crystal X-ray diffraction, which provided a structural model in the triclinic space group P1̅. The asymmetric unit contains one complete molecule of H4L1 and two lattice water molecules. The diffraction data were best fitted by treating C

DOI: 10.1021/acs.cgd.5b00502 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

geometry of the copper ion is best described as intermediate between trigonal bipyramidal and square pyramidal (τ5 = 0.4),27 with the major axis O2−Cu1−N12, and pseudo basal plane occupied by a chelating diaminoethane moiety and two monodentate carboxylate groups. Consistent with a pseudo square pyramidal geometry, the apical aqua ligand O26 resides at a long bond distance of 2.328(2) Å, compared with the remaining coordination bonds, which lie in the range 1.979(2)−2.042(2) Å. The structure of compound 1 is shown in Figure 2.

H4L1 as a zwitterionic form, with the amine nitrogen atoms doubly protonated and both carboxylate groups deprotonated. Two arrangements of hydrogen bonding interactions are present within the structure. Two ammonium groups and two carboxylate groups interact in a cyclic R44(12) motif,26 linking together four molecules of H4L1, and propagating a hydrogen bonded network in two dimensions (Figure 1).

Figure 2. Coordination environment in the structure of complex 1 with selected heteroatom labeling scheme. Selected hydrogen atoms omitted for clarity.

The extended structure of 1 consists of a two-dimensional polymer sheet containing copper ions bound to three separate molecules of H2L1, each of which coordinates to three unique copper ions. Topologically, the best simplification of the network is arrived at by considering the copper ion and the inner sphere donor atoms as a node and the linking benzoate arms as links, which provides a (4,4) network description, consistent with the molecular geometry. These sheets occupy the ac plane with undulation in the b direction. The square windows formed within each sheet are occupied by another interpenetrating network in a parallel 2D → 2D fashion, shown in Figure 3. The structure of 1 contains several hydrogen bonding interactions. The N−H groups of the ligand are oriented in a trans (R,R/S,S) orientation and form hydrogen bonds in opposite directions; nitrogen atom N15 acts as a hydrogen bond donor to the noncoordinating carboxylate oxygen atom O4 from an adjacent ligand within the same coordination sphere, while N12 engages in hydrogen bonding with the lattice water molecule O27. This water molecule links noncoordinating carboxylate atoms from both interpenetrating sheets via hydrogen bonding interactions, providing an additional stabilization to the high density interpenetrated structure. Interaction between adjacent pairs of interlocked sheets takes the form of reciprocated R22(6) hydrogen bonding loops between the aqua ligand and two coordinated carboxylate oxygen atoms of each metal site. These interactions bring the two metal centers together to a distance of 5.001(2) Å supported by four hydrogen bonding interactions in total. The hydrogen bonding interactions within 1 are shown in Figure 4. No substantial π···π or other supramolecular interactions were observed either between or within pairs of interpenetrated sheets. The structure of compound 1 displays no continuous

Figure 1. (Top) A section of the structure of H4L1·2H2O with partial atom labeling scheme, showing the R44(12) motif between H4L1 molecules. (Bottom) The interaction of the lattice water molecules with carboxylate groups in H4L1. Selected hydrogen atoms and lattice water molecules omitted for clarity.

These sheets are linked in the perpendicular direction by hydrogen bonding between the lattice water molecules and the carboxylate oxygen atoms of adjacent sheets. Some degree of π···π interaction is also evident within the structure, with a good overlap of phenyl rings between the two-dimensional sheets displaying mean interplanar distances of 3.47 and 3.39 Å for the two crystallographically unique pairs. Synthesis and Structure of poly-[Cu(H2L1)(OH2)]·H2O (1). The reaction of H4L1 in its neutral, zwitterionic form with transition metal ions tended to lead to the formation of amorphous polymeric materials under aqueous conditions, so the dihydrochloride form was used in combination with dilute aqueous sodium hydroxide for in situ deprotonation and coordination of the ligand species. Reaction of H4L1·2HCl with excess copper(II) chloride in 40 mM NaOH solution under solvothermal conditions gave a light blue crystalline material. Surprisingly, we were only able to form this material as a pure phase when an excess of copper(II) chloride was employed in the reaction; the use of stoichiometric amounts of the metal salt in the reaction gave rise to amorphous black solids. These crystals were analyzed by single crystal X-ray diffraction, which provided a structural model in the monoclinic space group P21/ c. The asymmetric unit of 1 contains one complete molecule of H2L1, coordinating through both carboxylate and both amine groups to symmetry related Cu(II) ions, with one aqua ligand and one noncoordinating water molecule. The coordination D

DOI: 10.1021/acs.cgd.5b00502 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

the loss of both the aqua ligand and lattice water molecule (calculated 8.5%). Synthesis and Structure of poly-[Cu(H2L1)(OH2)]·H2O· DMF (2). Having observed the in situ deprotonation of H4L1· 2HCl in 1:1 DMF:H2O to crystallize H4L1·2H2O, the same conditions were employed in a reaction with copper(II) chloride. No dissolution or reaction of H4L1·2HCl was observed in neat DMF in the presence of copper(II) chloride after 72 h, necessitating the addition of water for solubility purposes. The reaction of H4L1·2HCl with copper chloride under these conditions gave large blue crystals, which were isolated by filtration. Analysis by single crystal X-ray diffraction gave a structural model in the monoclinic space group C2/c, where the asymmetric unit contains one molecule of H2L1, one copper ion, an aqua ligand, and lattice water and dimethylformamide molecules. Similar to complex 1 above, each H2L1 ligand coordinates to three unique copper ions, by chelation through the diamine domain and through the two carboxylate groups in a monodentate fashion. The geometry of the copper ion is regular square pyramidal (τ5 = 0.1), with the amine and carboxylate donors occupying the basal plane and the aqua ligand occupying the apical position at an elongated bond length of 2.339(3) Å. The structure of 2 is shown in Figure 5. Figure 3. (Top) Structure of a single sheet of 1, with one molecule of H2L1 highlighted in green. Hydrogen atoms and lattice water molecules omitted for clarity. (Bottom) Topological representation of the extended structure and interpenetration mode between interlocked (4,4) sheets in the structure of 1.

Figure 5. Metal site environment of compound 2 with partial heteroatom labeling scheme. Selected hydrogen atoms omitted for clarity.

The extended structure of 2 is also similar to that observed in 1, where H2L1 units each connect three copper ions through chelation and the monodentate coordination of carboxylate groups, forming a (4,4) two-dimensional sheet parallel to the bc plane. Interestingly, however, the sheets formed in 2 are planar, rather than undulating as was the case in 1. This difference in geometry can be rationalized by examination of the copper coordination geometry and, importantly, the orientation of the chelating ethylenediamine moiety. In 1, the combination of coordination sphere distortion and the trans-orientation of the substituents from the ethylenediamine group enforces a noncoplanar geometry for the four diverging groups from each copper site. In 2, the substituents attached to the chelating ethylenediamine group are oriented in a cis (R,S) orientation, and supported by the planar arrangement of ligands around the metal ion, a highly planar polymeric assembly is the result. Likely due in part to this planarity, no interpenetration is observed in the structure of 2; rather, the square windows within each channel are occupied by solvent molecules. The extended structure of 2 is shown in Figure 6.

Figure 4. Primary modes of hydrogen bonding interactions in the structure of 1: (Top) The linkage between metal sites via hydrogen bonding between aqua ligands and carboxylates, linking noninterlocked sheets. (Bottom) Hydrogen bonding interactions of the lattice water molecules involving amine and carboxylate groups, joining interlocking sheets. Selected hydrogen atoms and backbone atoms omitted for clarity.

solvent channels or void space. Thermogravimetric analysis measurements were carried out on the material and showed the loss of approximately 8.4% mass in the temperature range 50− 120 °C, followed by a broad plateau leading to an abrupt decomposition at 250 °C. This mass loss corresponds well to E

DOI: 10.1021/acs.cgd.5b00502 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

are observed between sheets but are restricted due to the orientation of the phenyl rings largely perpendicular to the network windows. The hydrogen bonding linkages between sheets of 2 limit the potential offset for the packing of each layer, and as a result the windows from each network align with one another, forming one-dimensional solvent channels parallel to the [1,0,1] vector. These square-walled channels have minimum edge-to-edge interatomic dimensions of approximately 9 × 9 Å, measured between the carbon atoms of edgeon aromatic groups, and are bordered by the faces of the aromatic rings of H2L1, as well as the central aliphatic ethyl fragments. Crystallographic modeling suggested channel occupancy of one DMF molecule and half of one water molecule per copper ion. This water molecule was modeled as disordered over two sites, with no obvious hydrogen bonding interactions observed. Microanalysis was consistent with channel occupancy of one DMF molecule and one water molecule per copper ion, implying the presence of slightly more water in the lattice than could be located crystallographically. Thermal and Gas Adsorption Studies of Compound 2. Unlike complex 1, complex 2 displayed a clear potential for permanent porosity from the single crystal structure, with the substantial hydrogen bonding within the structure expected to reinforce the three-dimensional packing arrangement. Thermal analysis was carried out on the freshly isolated sample (Figure 8) which showed a multistep desolvation profile characterized by a sharp onset of solvent loss at approximately 100 °C which slowly flattens to a very brief plateau at approximately 200 °C before the onset of decomposition at approximately 250 °C. The total mass loss observed in the range 25−250 °C was approximately 21.1%, consistent with the loss of the lattice DMF and water molecules detected by microanalysis and the coordinated aqua ligand (calculated 21.9%). Attempts to directly desolvate this material were unsuccessful, as heating the material to 200 °C under dynamic vacuum overnight led to slow decomposition. To provide a more gentle route to the desolvated framework, the material was immersed in either acetonitrile or methanol as exchange solvents, to afford a lower desolvation temperature. Shown in Figure 8, both solvents appear to completely replace the lattice solvent molecules, and are rapidly removed from the pores with room temperature onset of approximately 3% mass loss, and leaving a ca. 4% mass loss completed by 100 °C consistent with the loss of the aqua ligand. Of these two solvents, acetonitrile was chosen as the exchange solvent for further studies, due to the lower potential for interference with the metal site and hydrogen bonding network. The solvent exchange could also be followed by infrared spectroscopy (Figure 8), which showed the loss of the prominent DMF amide carbonyl absorbance at 1658 cm−1 upon acetonitrile exchange and the growth of a small peak at 2320 cm−1 corresponding to the acetonitrile cyano group absorbance. Both peaks, and the absorbances due to the aqua ligand, were completely removed by evacuation under high vacuum at 100 °C. X-ray powder diffraction on the acetonitrileexchanged material showed retention of bulk crystallinity with some deviation in peak positions from the as-synthesized phase, consistent with retention of the pore structure in the exchanged material. Surprisingly, X-ray powder diffraction analysis of the fully desolvated material showed that, despite the loss of the structure-anchoring aqua ligands, the material retained partial crystallinity upon desolvation (Figure 8), evidenced by the retention of the major peaks at 2θ = 8.1, 16.4, 18.3, and 21.7°.

Figure 6. (Top) Topological representation of 2 showing slightly offset AABB stacking behavior. (Bottom) The pore structure of 2. Hydrogen atoms and noncoordinating solvent molecules omitted for clarity.

Individual sheets of 2 interact with one another primarily through hydrogen bonding interactions. The coordination sphere of each copper ion contains four sites of hydrogen bond donation, through the secondary amine and aqua ligands, and hydrogen bond acceptor functionality in the form of coordinating and noncoordinating carboxylate oxygen atoms. Direct interaction between sheets results in the pairing of adjacent metal sites by the formation of four complementary hydrogen bonding interactions between the cis-oriented N−H groups and the noncoordinating carboxylate oxygen atoms. This interaction separates adjacent copper ions by 4.213(2) Å in the direction of the vacant axial coordination site (angle O26−Cu1−Cu1′ 175.61(7)°). The pairs of sheets then interact with adjacent pairs through hydrogen bonding interactions between the aqua ligands and the coordinating carboxylate oxygen atom O2, while the aqua ligand also donates a hydrogen bond to a lattice DMF molecule (Figure 7). These hydrogen bonding interactions give rise to an AABB-type stacking arrangement. Additional weak π···π and C−H···π interactions

Figure 7. Hydrogen bonding modes between four adjacent sheets of compound 2, showing both close contacts through amine−carboxylate bridges and longer aqua−carboxylate links. Ligand molecules truncated for clarity. F

DOI: 10.1021/acs.cgd.5b00502 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

crystallites on solvent exchange and high temperature evacuation (Supporting Information). N2 adsorption measurements at 273 K show that the material displays low uptake (