A Benzaldehyde Derivative as a Chelating Ligand: Helical

Article pubs.acs.org/crystal

A Benzaldehyde Derivative as a Chelating Ligand: Helical Manganese(II) Coordination Polymers Assembling into a Porous Solid Galina Dulcevscaia,† Shi-Xia Liu,*,† Jürg Hauser,† Karl W. Kram ̈ er,† Gabriela Frei,† Andreas Möller,‡ and Silvio Decurtins† †

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Institut für Nichtklassische Chemie e.V., Permoserstrasse 15, D-04318 Leipzig, Germany

‡

S Supporting Information *

ABSTRACT: A molecular, porous crystalline material constructed from neutral helical coordination polymers incorporating manganese(II) ions and two types of bridging ligands, namely the deprotonated form of 2-hydroxy-5-methoxy-3-nitrobenzaldehyde (HL) and isobutyrate (iB−), has been obtained and structurally characterized. Structural analysis reveals that within the coordination polymer each benzaldehyde derivative ligates two manganese ions in 6-membered chelating rings, and the isobutyrate ligands cooperatively chelate either two or three manganese ions. The solid state assembly of the resulting polymeric chains of formula [Mn4(L)2(iB)6]n (1), described in the polar space group R3c, is associated with tubular channels occupied by MeCN solvent molecules (1·xMeCN; x ≤ 9). TGA profiles and PXRD measurements demonstrate that the crystallinity of the solid remains intact in its fully desolvated form, and its stability and crystallinity are ensured up to a temperature of 190 °C. Gas adsorption properties of desolvated crystals were probed, but no remarkable sorption capacity of N2 and only a limited one for CO2 could be observed. Magnetic susceptibility data reveal an antiferromagnetic type of coupling between adjacent manganese(II) ions along the helical chains with energy parameters J1 = −5.9(6) cm−1 and J2 = −1.8(9) cm−1.

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INTRODUCTION Aldehyde complexes have often been postulated as key intermediates in metal-mediated organic synthesis and a major perspective in that context is the elucidation of the structural nature and reactivity of such types of compounds. The prevailing view is that most main group, transition metal, and lanthanide Lewis acids form η1(σ)-type cationic complexes1 with carbonyl-containing compounds; however, electron-rich transition metals, with a few exceptions,2 show η2(π)-bonding to carbonyls, as has been demonstrated for many isolable and neutral organometallic aldehyde complexes.3 Leading references to aldehyde and ketone complexes may be found in a review by Gladysz et al.4 As for the structural features of aldehyde complexes, it is noteworthy that in a recent experimental and theoretical study by Darensbourg and coworkers,5 a chelating binding mode of a formylpyrrolyl ligand was shown within a tetracarbonylrhenium complex. Notably, the presence of the weak aldehyde Re−O link, that can easily dissociate to form the intermediate, allows the opening of a coordination site on the metal for an incoming ligand, which demonstrates the versatility of the aldehyde bond. In order to further investigate the coordination behavior of aldehydes, but not necessarily on the basis of weak ligation, we probed as a © 2013 American Chemical Society

new ligand an extensively substituted benzaldehyde moiety, namely 2-hydroxy-5-methoxy-3-nitrobenzaldehyde (HL), which in its deprotonated form can potentially offer two 6membered chelating binding sites to metal ions (Chart 1). Alternatively, from a structural point of view, this ligand could also be compared with salicylaldehyde, which is further substituted by a nitro group in an ortho position to the hydroxy group. Chart 1. The Benzaldehyde Derivative, 2-Hydroxy-5methoxy-3-nitrobenzaldehyde (HL), Which Acts As a Chelating Ligand in Its Deprotonated Form L−

Received: June 28, 2013 Revised: July 23, 2013 Published: July 24, 2013 4138

dx.doi.org/10.1021/cg400972k | Cryst. Growth Des. 2013, 13, 4138−4144

Crystal Growth & Design

Article

measurement of the skeletal density with helium at the same temperature. Before each measurement, the samples were degassed for 12 h at 373 K. During this time, the pressure was monitored and reached a value below 0.1 Pa. Detailed procedures for volumetric and gravimetric measurements can be found in the literature.10,11 X-ray Crystallography. A rod-shaped crystal of [Mn4(L)2(iB)6]n·xMeCN (1·xMeCN; x ≤ 9), directly taken from the MeCN solution, was mounted with Paratone on a glass needle and used for X-ray structure determination at 150 K. All measurements were made on an Oxford Diffraction SuperNova area-detector diffractometer12 using mirror optics monochromated Mo Kα radiation (λ = 0.71073 Å). The unit cell constants and an orientation matrix for data collection were obtained from a least-squares refinement of the setting angles of 188177 reflections in the range of 2.22° < θ < 23.88°. A total of 948 frames were collected using ω scans, 160 s exposure time, and a rotation angle of 0.5° per frame and a crystal-detector distance of 65.0 mm. Data reduction was performed using CrysAlisPro12. The intensities were corrected for Lorentz and polarization effects, and an absorption correction based on the multiscan method using SCALE3 ABSPACK in CrysAlisPro12 was applied. Data collection and refinement parameters are given in Table 1.

Herein, we report the ligation behavior of such a multiply functionalized benzaldehyde ligand while employing a manganese precursor in the form of a manganese isobutyrate complex, [Mn(iB)2] (iB− = isobutyrate). This specific synthetic protocol leads to the assembly of neutral one-dimensional (1D) polymeric chains of formula [Mn4(L)2(iB)6]n (1), which in the crystalline solid state (polar space group R3c) are associated with large tubular channels occupied by MeCN solvent molecules (1·xMeCN; x ≤ 9) when grown from a MeCN solution. Besides addressing the issue of the ligating property of the benzaldehyde derivative, it is also interesting to examine to what extent the manganese(II) isobutyrate precursor, [Mn(iB)2], affects the cooperative formation of an extended coordination entity.6 Furthermore, and in a broader context, it should be stressed that this study presents a porous crystalline compound that, in a strict sense, is neither classified as a metal−organic framework (MOF)7 nor as a covalent organic framework (COF).7f Whereas, MOFs with their intriguing structural topologies and promising applications as functional materials express extended iono-covalent bonding patterns, this compound 1 constitutes a molecular crystal, nevertheless, with a fascinating crystal structure. Moreover, coordination polymers comprising paramagnetic centers are also an interesting class of compounds in the context of molecular magnetism.8 In the current case, the strength of magnetic exchange interactions between closely linked manganese(II) spin centers (S = 5/2) along the polymeric chains will be discussed.

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Table 1. Crystal Data and Structure Refinements Parameter of [Mn4(L)2(iB)6]n·xMeCN (1·xMeCN; x ≤ 9) formula of (1) M (g mol−1) of (1) T (K) space group, Z a (Å) b (Å) c (Å) V (Å3) ρcalc (g cm−3) of (1) μ(Mo Kα) (mm−1) crystal size (mm3) index ranges reflections collected independent reflections completeness to θ = 25° absorption correction max and min transmission refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) absolute structure parameter largest difference peak and hole (e Å−3)

EXPERIMANTAL SECTION

Synthesis. All chemicals were purchased from commercial sources and used without additional purification. [Mn(iB)2] (iB− = isobutyrate) was prepared in analogy to the literature procedure.9 A solution of [Mn(iB)2] (120 mg, 0.52 mmol), pyrazine (80 mg, 0.1 mmol), and 2-hydroxy-5-methoxy-3-nitrobenzaldehyde (HL) (80 mg, 0.02 mmol) in MeCN (10 mL) was refluxed for 30 min; the resulting red solution did not contain any precipitation. A red crystalline product was formed while cooling the solution. The product was filtered off, washed with ether, and then dried to get a desolvated product (1). Yield: 66 mg (55%). Anal. Calcd. for C40H54Mn4N2O22 (%): C, 42.34; H, 4.80; N, 2.47. Found (%): C, 41.95; H, 4.77; N, 2.38. IR (1, selected bands): ν/cm−1 = 2966, 1668, 1595, 1543, 1500, 1416, 1360, 1275, 1268, 1243, 1051, 822. The compound decomposes at temperatures > 190 °C. Single crystals suitable for X-ray measurements were obtained by slow evaporation of a MeCN solution of 1. The crystalline product is soluble in MeOH, EtOH, 1PrOH, acetone, DMSO, DMF, THF, MeCN, CH2Cl2, and CHCl3 but not or only slightly soluble in toluene, hexane, CCl4, benzene, pxylene, and m-xylene. Physical Measurements. DSC and thermogravimetric measurements were performed using a Mettler Toledo TGA/SDTA 851 in dry N2 (60 mL min−1) at varying heating rates. Elemental analysis was performed on a Carlo Erba Instruments EA 1110 Elemental Analyzer CHN. Infrared spectrum was obtained on a FT/IR-460 plus spectrometer. Powder X-ray diffraction (PXRD) patterns were recorded on a STOE spellmann generator type DF4 with a Cu anode. Magnetic susceptibility data were recorded using a Quantum design MPMS-5XL SQUID magnetometer as a function of temperature (1.9−300 K) at a magnetic field of 0.1 T. Experimental data were corrected for sample holder and diamagnetic contributions calculated from tabulated values. Measurement of Gas Adsorption. The nitrogen physisorption was carried out on a volumetric measurement system BelSorp-Max (Bel Japan Inc., Japan) at 77 K. For carbon dioxide sorption experiments, a magnetic suspension balance (Rubotherm GmbH, Germany) was used, and a buoyancy correction was performed by

C40H54Mn4N2O22 1134.61 150(2) R3c, 18 46.2653(5) 46.2653(5) 15.95404(18) 295741(6) 1.15 0.811 0.361 × 0.065 × 0.056 −57 ≤ h ≤ 56, −57 ≤k ≤ 57, −19 ≤l ≤ 19 66922 13414 [R(int) = 0.0458] 100.0% semiempirical from equivalents 1 and 0.72649 full-matrix least-squares on F2 13414/48/614 1.091 R1 = 0.0503, wR2 = 0.1294 R1 = 0.0623, wR2 = 0.1355 0.112(15) 0.534 and −0.371

The structure was solved by direct methods using SIR97,13 which revealed the positions of all not disordered nonhydrogen atoms. The nonhydrogen atoms were refined anisotropically. All H atoms were placed in geometrically calculated positions and refined using a riding model where each H atom was assigned a fixed isotropic displacement parameter with a value equal to 1.2 Ueq of its parent atom (1.5 Ueq for methyl groups). The remaining electron density in the channels along the c direction could not be assigned to individual solvent molecules and was therefore accounted for with the SQUEEZE14 technique, which calculated the solvent accessible void to 8110 Å3 (i.e., 27.4% of the unit cell volume). Assuming as a rough estimate a nonhydrogen atom volume of 18 Å3, the solvent accessible void would allow for about 9 acetonitrile molecules per formula unit; this value compares well with the thermogravimetric analysis (see below). 4139



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Figure 1. Coordination geometry of one of the two chelating terdentate benzaldehyde ligands (L−) (left). Selected bond lengths (Å) and angles (deg): Mn1−O1 = 2.216(4), Mn1−O2 = 2.147(3), Mn4−O2 = 2.133(3), Mn4−O3 = 2.295(4), C7−O1−Mn1 = 126.4(3). Coordination geometries of the six bridging isobutyrate ligands (iB−) from the asymmetric unit (right). Color code: Mn (green), N (blue), O (red), C (gray), and H (white). Symmetry code: (i) −x + y + 2/3, −x + 1/3, z + 1/3.

Figure 2. Chain fragments of [Mn4(L)2(iB)6]n. View of a Mn4−Mn1−Mn2−Mn3 sequence (left). View of a Mn1−Mn2−Mn3−Mn4 sequence (right). Color code: Mn (green), N (blue), O (red), and C (gray). Refinement of the structure was carried out on F2 using full-matrix least-squares procedures, which minimized the function Σw(Fo2 − Fc2)2. The weighting scheme was based on counting statistics and included a factor to downweight the intense reflections. All calculations were performed using the SHELXL-97 program.15

neutral, helical polymeric chains with the formula [Mn4(L)2(iB)6]n. Because of the very complex bonding pattern of the eight multiply chelating ligands, the structural description is organized such as first addressing the “ligand view” and only second describing the metal coordination. Thus, the discussion will begin by outlining the structural features of the asymmetric unit, whereby a central theme is the coordination behavior of the benzaldehyde-type of ligand. The two deprotonated 2hydroxy-5-methoxy-3-nitrobenzaldehyde (L−) ligands bind in each case in a terdentate chelating mode two Mn(II) ions into close proximity. Figure 1 (left) illustrates the coordination geometry for one of the benzaldehydes (L−) of the asymmetric unit (see Figure S2 of the Supporting Information for the analogous geometry of the other L− of the asymmetric unit). Clearly, the chelating bonding of Mn1 by the carbonyl oxygen atom [O1: η1(σ)-type] and central oxygen atom (O2) is demonstrated, and this binding configuration can be compared with that of a chelating salicylaldehyde, where [e.g., in the case of a corresponding Mn(III) complex]16 the carbonyl oxygen atom lies on the Jahn−Teller axis of the d4 metal ion and thus exhibits a larger bond length [Mn(III)−O(carbonyl) = 2.2444(17) Å] than in the current case. Notably, through the presence of the nitro group bound to C3 of L−, Mn4 is chelated as a second metal ion since the central oxygen atom (O2)

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RESULTS AND DISCUSSION Crystal Structure. The title compound with the formula [Mn4(L)2(iB)6]n·xMeCN (1·xMeCN; x ≤ 9) is an ordered assembly of neutral manganese(II) coordination polymers associated with tubular channels occupied by MeCN solvent molecules. It crystallizes from a MeCN solution as red rodshaped crystals in the trigonal noncentrosymmetric polar space group R3c, z = 18 (Table 1). The crystal morphology is shown with a scanning electron micrograph in Figure S1 of the Supporting Information. The crystals exhibit a dichroitic property when viewed with polarized light: red color with light polarized along the long crystal rod axis (c axis) and yellow color with a perpendicular orientation to the c axis. The asymmetric unit of the crystal structure of 1 (omitting solvent molecules) consists of four Mn(II) ions, two L−, and six iB− anionic ligands. All metal ions and ligand atoms lie on general positions: Wyckoff position b. Basically, both types of ligands bridge in an intricate manner the Mn(II) ions and a symmetry expansion of the asymmetric unit shows the formation of 4140



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adopts a μ2-binding mode. Consequently, Mn4 and Mn1, and analogously Mn2 and Mn3, are linked by two 6-membered chelating rings into close proximity through both ligands L−. Next, six isobutyrate (iB−) ligands, which are introduced in the synthesis through the manganese precursor complex, [Mn(iB)2], further support the formation of a coordination polymer. Essentially, they express two different binding modes [Figure 1 (right)] and thus Mn1 are triply bridged to Mn2 through three iB− ligands: once through iB− exhibiting a 1,3chelating mode (μ2-η1:η1) and twice through iB− ligands expressing an overall μ3-η1:η2 mode, while the latter additionally bridge to the adjacent Mn4 and Mn3 ions, respectively. The analogous linking pattern holds for the Mn3−Mn4 pair, and also, here there are two iB− bridges to the adjacent Mn2 and Mn1 ions, respectively. In contrast, the Mn4−Mn1 and Mn2− Mn3 pairs, which are already bridged in a tridentate manner with one L− ligand each, are also bridged through two iB− ligands expressing 1,3-chelating modes (iB−:μ2-η1:η1). In total, tightly bridged Mn(II) ions with the Mn1−Mn2−Mn3−Mn4 sequence as a repetitive entity (Figure 2) form 1D coordination polymers. The Mn···Mn separations are 3.250, 3.512, 3.238, and 3.498 Å along the bonding sequence starting with Mn1···Mn2. The Mn−O distances of the MO6 coordination spheres range from 2.086(3) to 2.330(4) Å, whereby the larger values stem from bonds to oxygen atoms of the aldehyde and nitro groups of L−. The observed distortions from octahedral geometries around the manganese centers are caused by the multiple chelating modes of both types of the bridging ligands, but the values of the bond angles lie in the expected range.6 In summary, both types of ligands cooperatively bridge in an intricate manner the Mn(II) ions while equally forming leftand right-handed infinite helical chains (Figure 3). A turn of the

Figure 4. View of a helical chain of [Mn4(L)2(iB)6]n along the c axis, emphasizing the MnO6 coordination spheres.

a regular manner, exhibiting a 3-fold symmetry in the ab plane. The helical chains run parallel to the c axis, whereby alternatingly left- and right-handed chains form the vertices of a honeycomb pattern while building up van der Waals contacts with each other (Figure 5); the center-to-center distance

Figure 5. Structural view along the c axis of the trigonal unit cell of [Mn4(L)2(iB)6]n (1) emphasizing the open framework with the wide tubular channels resulting from the honeycomb-type packing arrangement of the helices.

Figure 3. View of a helical chain fragment of [Mn4(L)2(iB)6]n perpendicular to the c axis at a length of twice the c axis (left). View along the c axis, specifically along a 3-fold screw axis (right). Color code: Mn (green), N (blue), O (red), C (gray), H (white).

between the helices amounts to 15.4 Å. The crystal lattice as a whole is achiral because enantiomeric pairs of helices are present. As a consequence of this packing arrangement, an open structure with large tubular channels emerges, whose centers are located on 3-fold rotation axes and are separated by 26.7 Å. The channel walls, taken per perimeter and length of the c axis, are mainly formed by twelve deprotonated 2hydroxy-5-methoxy-3-nitrobenzaldehyde ligands (L−) and the distance across the channel is up to 19.3 Å, excluding the van der Waals radius of carbon or oxygen atoms (Figure S3 of the Supporting Information). In addition, for each channel at a length of the c axis, six isobutyl groups protrude into the open space and they are arranged in two sets of three, related by a

helices contains three times the asymmetric unit, hence 12 Mn(II) ions, and this arrangement gives a pitch of 15.95404(18) Å, which is the c axis length. To put it another way, we reiterate that all four manganese ions of the asymmetric unit possess distorted O6 coordination spheres. By analogy, a description of these helices based on polyhedra representing the MnO6 octahedrons shows that adjacent metal centers are alternatingly edge-shared (Mn1−Mn2 and Mn3− Mn4) or corner-shared (Mn2−Mn3 and Mn4−Mn1) while forming the helical 1D coordination polymers (Figure 4). In the crystal structure, both left- and right-handed helical chains, which are centered on 3-fold screw-axes, are arranged in 4141



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Figure 6. Thermal variation of χmT for [Mn4(L)2(iB)6]n (1) (the solid line is a fit according to eq 2) (left). Inverse magnetic susceptibility versus temperature for [Mn4(L)2(iB)6]n (1) (the solid line represents a Curie−Weiss fit in the temperature range of 50−300 K) (right).

shift of c/2 and a staggered configuration. Notably, for these porous crystals of 1, the free accessible volume calculated with the SQUEEZE14 technique is 27.4% of the unit cell volume, which would correspond to a diameter of 14.6 Å taken for an ideal cylindric shape. A valid point is also that the desolvated compound exhibits a low density of 1.15 g cm−3 (calcd). Thermogravimetric Analysis. In order to examine the thermal stability of the crystalline compound 1 and the amount of solvent molecules being taken up in its solvated state (1·xMeCN), a thermogravimetric (TG) measurement was carried out. The stepwise TG curve of 1·xMeCN, prepared directly from its MeCN solution, resulted in a weight loss of 25%, corresponding to x = 9 MeCN molecules per stoichiometric formula unit. Thereby, the sample was slowly warmed up to 80 °C, then kept at that temperature until constant weight was reached, and with further heating a plateau at constant weight was observed. The desolvated compound 1 is then stable up to 190 °C; at higher temperatures, there is a constant weight loss indicating its decomposition. Gas Adsorption Measurements. The accessibility of the network was tested by nitrogen adsorption at 77 K and carbon dioxide adsorption at 273 K. The obtained adsorption isotherms are shown in Figure S4 of the Supporting Information. The isotherms of nitrogen are of type III according to IUPAC classification.11 The strong slope at relative higher pressures associated by a fully reversible adsorption−desorption branch generally indicates a condensation step between the particles. This interparticular volume is mainly influenced by particle size and particle size distribution. Besides, to exclude a size exclusion effect, carbon dioxide adsorption at higher temperatures was measured. These measurements were done if the pore aperture is too small for nitrogen molecules to enter the pore due to slow kinetics at cryogenic temperatures.17 In reference to the crystal structure of 1, we can assume a type I isotherm according to the IUPAC classification with a steep increase at low pressures and a plateau at higher pressures.11 Contrary to that expectation, the obtained carbon dioxide adsorption isotherm at 273 K (Figure S4 of the Supporting Information) is more a type II isotherm with a slow increase in loading and no significant plateau at higher pressures. From the crystal structure, it is known that the material consists of a density of 1.15 g cm−3 and a porosity of 27.4%. Thus, a theoretical pore volume of 0.23 cm3 g−1 can be estimated, which principally could result in a higher amount of

adsorption. Importantly, the PXRD pattern of [Mn4(L)2(iB)6]n (1) taken after the gas adsorption measurements corresponds well with the calculated diffractogram with RT parameters a = b = 46.58(7), c = 16.118(20) Å (Figure S5 of the Supporting Information). This observation shows that the crystallinity of 1 remains intact during the gas adsorption measurements. The fact that, despite a stable framework, little N2 sorption and some but limited selective sorption of CO2 was noticed has also been reported in similar cases.18 The authors ascribed this behavior to strong interactions of N2 molecules with the pore windows which subsequently block the pore openings. Moreover, due to the specific morphology of the crystals of the actual compound, it also has to be taken into account that only the small end facets of the crystals expose the channel system to the outside; there are no open channels along the a and b axes. At this stage of the experimental study, one may speculate as well that the isobutyl groups which protrude into the open space of the channels influence the gas adsorption behavior in the case of the CO2 molecules. Magnetic Properties. The magnetic susceptibility data for a polycrystalline sample of [Mn4(L)2(iB)6]n (1) are displayed as plots of χmT versus T and 1/χm versus T in Figure 6 and as χm versus T in Figure S6 of the Supporting Information. Upon cooling, the χmT versus T plot shows decreasing χmT values starting with 17.2 cm3 K mol−1 (300 K) and reaching 1.0 cm3 K mol−1 at 1.9 K, which indicates (i) overall antiferromagnetic exchange interactions between the paramagnetic Mn(II) centers and (ii) a diamagnetic ground state of the compound. The experimental χmT value at room temperature is due to the antiferromagnetic interactions slightly lower than the theoretical χmT value (17.5 cm3 K mol−1) for four magnetically isolated manganese(II) ions representing the formula unit. The inverse magnetic susceptibility curve shows a linear behavior in the temperature range from 50−300 K, which results in a Weiss constant Θ = −61 K, in good agreement with the negative slope of the χmT versus T curve. Given the 1D connectivities of the Mn(II) ions along the polymeric chains, however, with basically two different and alternating bridging modes, the system can magnetically be treated with a 1D Heisenberg model with alternating nearest neighbor J1−J2 exchange interactions described by the spin Hamiltonian H = −J1 ∑ S2iS2i + 1 − J2 ∑ S2i + 1S2i + 2 4142

(1)



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ACKNOWLEDGMENTS Financial support for this research by the Swiss National Science Foundation (Grant 200021-147143) is gratefully acknowledged. G. D. acknowledges the Swiss government scholarship program.

where J1 and J2 stand for the alternating exchange constants, and the S’s are classical spin operators. Consequently, following the theory of Rojo et al.,19 one can deduce the analytical expression for χm as χm = Ng 2β 2 /3kT[(1 + μ1 + μ2 + μ1μ2 )/(1 − μ1μ2 )]

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(2)

with μ1 = coth (J1/kT) − kT/J1 and μ2 = coth(J2/kT) − kT/J2 Using eq 2, the experimental data of χmT versus T were fitted satisfactorily in the temperature range of 300−1.9 K and best fit parameters are J1 = −5.9(6) cm−1 and J2 = −1.8(9) cm−1 with g fixed at 2.0, which is the expected value for Mn(II). The occurrence of two different J parameters along the chain can be ascribed to the different exchange pathways, which are essentially of the type Mn4−Mn1 and Mn1−Mn2. While there is no question that the overall magnetic exchange interaction is of antiferromagnetic nature, it is also true that this is due to a combination of factors rather than solely attributable to one mechanistic pathway. While taking a glance at Figure 2, one notes the complexity of the multiple exchange pathways for this case. Additionally, as it was demonstrated with another case study (4Γ instead of 6Γ spin ground state),20 it requires a much simpler and more symmetric molecular geometry to elucidate and rationalize in detail a manifold of magnetic exchange pathways.

CONCLUSIONS In summary, we have prepared and characterized a porous molecular crystal based on helical coordination polymers. A benzaldehyde derivative and isobutyrate act cooperatively as multiply bridging ligands for manganese(II) ions and orchestrate the formation of helical chains with intricate binding modes. Special attention needs to be paid to the ligating behavior of the benzaldehyde type of ligand, which despite its simplicity is not often encountered in extended coordination frames. In addition, the strength of the magnetic exchange interactions between the paramagnetic centers, which is of an antiferromagnetic nature, has been revealed. Interestingly, the helical coordination polymers assemble into a robust, up to 190 °C, highly symmetric crystalline packing pattern, which is based on a polar space group. Thereby, tubular channels filled with solvent molecules emerge, and the crystallinity of the solid remains intact upon desolvation. Despite the stable crystalline packing arrangement, which maintains a porous structure in the desolvated state, no substantial gas sorption (N2 and CO2) has been observed. ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF format, CCDC ref no. 946803. Scanning electron micrograph of 1·xMeCN (x ≤ 9), PXRD patterns of 1, nitrogen sorption isotherms and carbon dioxide adsorption on 1, additional figures for the single crystal structure analysis and magnetic properties of 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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H.; Vuong, G.-T.; Fontaine, F.-G.; Do, T.-O. Cryst. Growth Des. 2012, 12, 3091. (19) Cortés, R.; Drillon, M.; Solans, X.; Lezama, L.; Rojo, T. Inorg. Chem. 1997, 36, 677. (20) (a) Decurtins, S.; Güdel, H. U.; Pfeuti, A. Inorg. Chem. 1982, 21, 1101. (b) Decurtins, S.; Güdel, H. U. Inorg. Chem. 1982, 21, 3598.

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