A Benzaldehyde Derivative as a Chelating Ligand: HelicalManganese(II) Coordination Polymers Assembling into a PorousSolidGalina Dulcevscaia,† Shi-Xia Liu,*,† Jurg Hauser,† Karl W. Kramer,† Gabriela Frei,† Andreas Moller,‡
and Silvio Decurtins†
†Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland‡Institut fur Nichtklassische Chemie e.V., Permoserstrasse 15, D-04318 Leipzig, Germany
*S Supporting Information
ABSTRACT: A molecular, porous crystalline material con-structed from neutral helical coordination polymers incorporatingmanganese(II) ions and two types of bridging ligands, namely thedeprotonated form of 2-hydroxy-5-methoxy-3-nitrobenzaldehyde(HL) and isobutyrate (iB−), has been obtained and structurallycharacterized. Structural analysis reveals that within thecoordination polymer each benzaldehyde derivative ligates twomanganese ions in 6-membered chelating rings, and theisobutyrate ligands cooperatively chelate either two or threemanganese ions. The solid state assembly of the resultingpolymeric chains of formula [Mn4(L)2(iB)6]n (1), described inthe polar space group R3c, is associated with tubular channelsoccupied by MeCN solvent molecules (1·xMeCN; x ≤ 9). TGAprofiles and PXRD measurements demonstrate that the crystallinity of the solid remains intact in its fully desolvated form, and itsstability and crystallinity are ensured up to a temperature of 190 °C. Gas adsorption properties of desolvated crystals wereprobed, but no remarkable sorption capacity of N2 and only a limited one for CO2 could be observed. Magnetic susceptibilitydata reveal an antiferromagnetic type of coupling between adjacent manganese(II) ions along the helical chains with energyparameters J1 = −5.9(6) cm−1 and J2 = −1.8(9) cm−1.
■ INTRODUCTION
Aldehyde complexes have often been postulated as keyintermediates in metal-mediated organic synthesis and amajor perspective in that context is the elucidation of thestructural 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-richtransition metals, with a few exceptions,2 show η2(π)-bondingto carbonyls, as has been demonstrated for many isolable andneutral organometallic aldehyde complexes.3 Leading refer-ences to aldehyde and ketone complexes may be found in areview by Gladysz et al.4 As for the structural features ofaldehyde complexes, it is noteworthy that in a recentexperimental and theoretical study by Darensbourg and co-workers,5 a chelating binding mode of a formylpyrrolyl ligandwas shown within a tetracarbonylrhenium complex. Notably,the presence of the weak aldehyde Re−O link, that can easilydissociate to form the intermediate, allows the opening of acoordination site on the metal for an incoming ligand, whichdemonstrates the versatility of the aldehyde bond. In order tofurther investigate the coordination behavior of aldehydes, butnot necessarily on the basis of weak ligation, we probed as a
new ligand an extensively substituted benzaldehyde moiety,namely 2-hydroxy-5-methoxy-3-nitrobenzaldehyde (HL),which in its deprotonated form can potentially offer two 6-membered chelating binding sites to metal ions (Chart 1).Alternatively, from a structural point of view, this ligand couldalso be compared with salicylaldehyde, which is furthersubstituted by a nitro group in an ortho position to thehydroxy group.
Received: June 28, 2013Revised: July 23, 2013Published: July 24, 2013
Chart 1. The Benzaldehyde Derivative, 2-Hydroxy-5-methoxy-3-nitrobenzaldehyde (HL), Which Acts As aChelating Ligand in Its Deprotonated Form L−
Herein, we report the ligation behavior of such a multiplyfunctionalized benzaldehyde ligand while employing a man-ganese precursor in the form of a manganese isobutyratecomplex, [Mn(iB)2] (iB
− = isobutyrate). This specific syntheticprotocol leads to the assembly of neutral one-dimensional (1D)polymeric chains of formula [Mn4(L)2(iB)6]n (1), which in thecrystalline solid state (polar space group R3c) are associatedwith large tubular channels occupied by MeCN solventmolecules (1·xMeCN; x ≤ 9) when grown from a MeCNsolution. Besides addressing the issue of the ligating property ofthe benzaldehyde derivative, it is also interesting to examine towhat extent the manganese(II) isobutyrate precursor, [Mn-(iB)2], affects the cooperative formation of an extendedcoordination entity.6 Furthermore, and in a broader context,it should be stressed that this study presents a porouscrystalline compound that, in a strict sense, is neither classifiedas a metal−organic framework (MOF)7 nor as a covalentorganic framework (COF).7f Whereas, MOFs with theirintriguing structural topologies and promising applications asfunctional materials express extended iono-covalent bondingpatterns, 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 thecontext of molecular magnetism.8 In the current case, thestrength of magnetic exchange interactions between closelylinked manganese(II) spin centers (S = 5/2) along thepolymeric chains will be discussed.
■ EXPERIMANTAL SECTIONSynthesis. 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 Asolution of [Mn(iB)2] (120 mg, 0.52 mmol), pyrazine (80 mg, 0.1mmol), and 2-hydroxy-5-methoxy-3-nitrobenzaldehyde (HL) (80 mg,0.02 mmol) in MeCN (10 mL) was refluxed for 30 min; the resultingred solution did not contain any precipitation. A red crystallineproduct was formed while cooling the solution. The product wasfiltered off, washed with ether, and then dried to get a desolvatedproduct (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 decomposesat temperatures > 190 °C. Single crystals suitable for X-raymeasurements were obtained by slow evaporation of a MeCNsolution of 1. The crystalline product is soluble in MeOH, EtOH, 1-PrOH, acetone, DMSO, DMF, THF, MeCN, CH2Cl2, and CHCl3 butnot or only slightly soluble in toluene, hexane, CCl4, benzene, p-xylene, and m-xylene.Physical Measurements. DSC and thermogravimetric measure-
ments were performed using a Mettler Toledo TGA/SDTA 851 in dryN2 (60 mL min−1) at varying heating rates. Elemental analysis wasperformed on a Carlo Erba Instruments EA 1110 Elemental AnalyzerCHN. Infrared spectrum was obtained on a FT/IR-460 plusspectrometer. Powder X-ray diffraction (PXRD) patterns wererecorded on a STOE spellmann generator type DF4 with a Cuanode. Magnetic susceptibility data were recorded using a Quantumdesign MPMS-5XL SQUID magnetometer as a function of temper-ature (1.9−300 K) at a magnetic field of 0.1 T. Experimental data werecorrected for sample holder and diamagnetic contributions calculatedfrom 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 sorptionexperiments, a magnetic suspension balance (Rubotherm GmbH,Germany) was used, and a buoyancy correction was performed by
measurement of the skeletal density with helium at the sametemperature. Before each measurement, the samples were degassedfor 12 h at 373 K. During this time, the pressure was monitored andreached a value below 0.1 Pa. Detailed procedures for volumetric andgravimetric measurements can be found in the literature.10,11
X-ray Crysta l lography. A rod- shaped cry s t a l o f[Mn4(L)2(iB)6]n·xMeCN (1·xMeCN; x ≤ 9), directly taken fromthe MeCN solution, was mounted with Paratone on a glass needle andused for X-ray structure determination at 150 K. All measurementswere made on an Oxford Diffraction SuperNova area-detectordiffractometer12 using mirror optics monochromated Mo Kα radiation(λ = 0.71073 Å). The unit cell constants and an orientation matrix fordata collection were obtained from a least-squares refinement of thesetting 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 exposuretime, and a rotation angle of 0.5° per frame and a crystal-detectordistance of 65.0 mm. Data reduction was performed usingCrysAlisPro12. The intensities were corrected for Lorentz andpolarization effects, and an absorption correction based on themultiscan method using SCALE3 ABSPACK in CrysAlisPro12 wasapplied. Data collection and refinement parameters are given in Table1.
The structure was solved by direct methods using SIR97,13 whichrevealed the positions of all not disordered nonhydrogen atoms. Thenonhydrogen atoms were refined anisotropically. All H atoms wereplaced in geometrically calculated positions and refined using a ridingmodel where each H atom was assigned a fixed isotropic displacementparameter with a value equal to 1.2 Ueq of its parent atom (1.5 Ueqfor methyl groups). The remaining electron density in the channelsalong the c direction could not be assigned to individual solventmolecules 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 anonhydrogen atom volume of 18 Å3, the solvent accessible void wouldallow for about 9 acetonitrile molecules per formula unit; this valuecompares well with the thermogravimetric analysis (see below).
Table 1. Crystal Data and Structure Refinements Parameterof [Mn4(L)2(iB)6]n·xMeCN (1·xMeCN; x ≤ 9)
formula of (1) C40H54Mn4N2O22
M (g mol−1) of (1) 1134.61T (K) 150(2)space group, Z R3c, 18a (Å) 46.2653(5)b (Å) 46.2653(5)c (Å) 15.95404(18)V (Å3) 295741(6)ρcalc (g cm−3) of (1) 1.15μ(Mo Kα) (mm−1) 0.811crystal size (mm3) 0.361 × 0.065 × 0.056index ranges −57 ≤ h ≤ 56, −57 ≤k ≤ 57, −19 ≤l ≤ 19reflections collected 66922independent reflections 13414 [R(int) = 0.0458]completeness to θ = 25° 100.0%absorption correction semiempirical from equivalentsmax and min transmission 1 and 0.72649refinement method full-matrix least-squares on F2
data/restraints/parameters 13414/48/614goodness-of-fit on F2 1.091final R indices [I > 2σ(I)] R1 = 0.0503, wR2 = 0.1294R indices (all data) R1 = 0.0623, wR2 = 0.1355absolute structureparameter
Refinement of the structure was carried out on F2 using full-matrixleast-squares procedures, which minimized the function Σw(Fo2 −Fc
2)2. The weighting scheme was based on counting statistics andincluded a factor to downweight the intense reflections. Allcalculations were performed using the SHELXL-97 program.15
■ RESULTS AND DISCUSSION
Crystal Structure. The title compound with the formula[Mn4(L)2(iB)6]n·xMeCN (1·xMeCN; x ≤ 9) is an orderedassembly of neutral manganese(II) coordination polymersassociated with tubular channels occupied by MeCN solventmolecules. It crystallizes from a MeCN solution as red rod-shaped crystals in the trigonal noncentrosymmetric polar spacegroup R3c, z = 18 (Table 1). The crystal morphology is shownwith a scanning electron micrograph in Figure S1 of theSupporting Information. The crystals exhibit a dichroiticproperty when viewed with polarized light: red color withlight polarized along the long crystal rod axis (c axis) and yellowcolor with a perpendicular orientation to the c axis. Theasymmetric unit of the crystal structure of 1 (omitting solventmolecules) consists of four Mn(II) ions, two L−, and six iB−
anionic ligands. All metal ions and ligand atoms lie on generalpositions: Wyckoff position b. Basically, both types of ligandsbridge in an intricate manner the Mn(II) ions and a symmetryexpansion of the asymmetric unit shows the formation of
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 organ-ized such as first addressing the “ligand view” and only seconddescribing the metal coordination. Thus, the discussion willbegin by outlining the structural features of the asymmetricunit, whereby a central theme is the coordination behavior ofthe benzaldehyde-type of ligand. The two deprotonated 2-hydroxy-5-methoxy-3-nitrobenzaldehyde (L−) ligands bind ineach case in a terdentate chelating mode two Mn(II) ions intoclose proximity. Figure 1 (left) illustrates the coordinationgeometry for one of the benzaldehydes (L−) of the asymmetricunit (see Figure S2 of the Supporting Information for theanalogous geometry of the other L− of the asymmetric unit).Clearly, the chelating bonding of Mn1 by the carbonyl oxygenatom [O1: η1(σ)-type] and central oxygen atom (O2) isdemonstrated, and this binding configuration can be comparedwith that of a chelating salicylaldehyde, where [e.g., in the caseof a corresponding Mn(III) complex]16 the carbonyl oxygenatom lies on the Jahn−Teller axis of the d4 metal ion and thusexhibits a larger bond length [Mn(III)−O(carbonyl) =2.2444(17) Å] than in the current case. Notably, through thepresence of the nitro group bound to C3 of L−, Mn4 is chelatedas a second metal ion since the central oxygen atom (O2)
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). Coordinationgeometries of the six bridging isobutyrate ligands (iB−) from the asymmetric unit (right). Color code: Mn (green), N (blue), O (red), C (gray), andH (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).
adopts a μ2-binding mode. Consequently, Mn4 and Mn1, andanalogously Mn2 and Mn3, are linked by two 6-memberedchelating 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 coordinationpolymer. Essentially, they express two different binding modes[Figure 1 (right)] and thus Mn1 are triply bridged to Mn2through three iB− ligands: once through iB− exhibiting a 1,3-chelating mode (μ2-η
1:η1) and twice through iB− ligandsexpressing an overall μ3-η
1:η2 mode, while the latter additionallybridge to the adjacent Mn4 and Mn3 ions, respectively. Theanalogous linking pattern holds for the Mn3−Mn4 pair, andalso, here there are two iB− bridges to the adjacent Mn2 andMn1 ions, respectively. In contrast, the Mn4−Mn1 and Mn2−Mn3 pairs, which are already bridged in a tridentate mannerwith 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−Mn4sequence as a repetitive entity (Figure 2) form 1D coordinationpolymers. The Mn···Mn separations are 3.250, 3.512, 3.238,and 3.498 Å along the bonding sequence starting withMn1···Mn2. The Mn−O distances of the MO6 coordinationspheres range from 2.086(3) to 2.330(4) Å, whereby the largervalues stem from bonds to oxygen atoms of the aldehyde andnitro groups of L−. The observed distortions from octahedralgeometries around the manganese centers are caused by themultiple 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 anintricate manner the Mn(II) ions while equally forming left-and right-handed infinite helical chains (Figure 3). A turn of the
helices contains three times the asymmetric unit, hence 12Mn(II) ions, and this arrangement gives a pitch of15.95404(18) Å, which is the c axis length. To put it anotherway, we reiterate that all four manganese ions of theasymmetric unit possess distorted O6 coordination spheres.By analogy, a description of these helices based on polyhedrarepresenting the MnO6 octahedrons shows that adjacent metalcenters are alternatingly edge-shared (Mn1−Mn2 and Mn3−Mn4) or corner-shared (Mn2−Mn3 and Mn4−Mn1) whileforming 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
a regular manner, exhibiting a 3-fold symmetry in the ab plane.The helical chains run parallel to the c axis, wherebyalternatingly left- and right-handed chains form the vertices ofa honeycomb pattern while building up van der Waals contactswith each other (Figure 5); the center-to-center distance
between the helices amounts to 15.4 Å. The crystal lattice as awhole is achiral because enantiomeric pairs of helices arepresent. As a consequence of this packing arrangement, anopen structure with large tubular channels emerges, whosecenters are located on 3-fold rotation axes and are separated by26.7 Å. The channel walls, taken per perimeter and length ofthe c axis, are mainly formed by twelve deprotonated 2-hydroxy-5-methoxy-3-nitrobenzaldehyde ligands (L−) and thedistance across the channel is up to 19.3 Å, excluding the vander Waals radius of carbon or oxygen atoms (Figure S3 of theSupporting Information). In addition, for each channel at alength of the c axis, six isobutyl groups protrude into the openspace and they are arranged in two sets of three, related by a
Figure 3. View of a helical chain fragment of [Mn4(L)2(iB)6]nperpendicular to the c axis at a length of twice the c axis (left). Viewalong the c axis, specifically along a 3-fold screw axis (right). Colorcode: Mn (green), N (blue), O (red), C (gray), H (white).
Figure 4. View of a helical chain of [Mn4(L)2(iB)6]n along the c axis,emphasizing the MnO6 coordination spheres.
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 widetubular channels resulting from the honeycomb-type packing arrange-ment of the helices.
shift of c/2 and a staggered configuration. Notably, for theseporous crystals of 1, the free accessible volume calculated withthe SQUEEZE14 technique is 27.4% of the unit cell volume,which would correspond to a diameter of 14.6 Å taken for anideal cylindric shape. A valid point is also that the desolvatedcompound 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 amountof solvent molecules being taken up in its solvated state(1·xMeCN), a thermogravimetric (TG) measurement wascarried out. The stepwise TG curve of 1·xMeCN, prepareddirectly from its MeCN solution, resulted in a weight loss of25%, corresponding to x = 9 MeCN molecules perstoichiometric formula unit. Thereby, the sample was slowlywarmed up to 80 °C, then kept at that temperature untilconstant weight was reached, and with further heating a plateauat constant weight was observed. The desolvated compound 1is then stable up to 190 °C; at higher temperatures, there is aconstant weight loss indicating its decomposition.Gas Adsorption Measurements. The accessibility of the
network was tested by nitrogen adsorption at 77 K and carbondioxide adsorption at 273 K. The obtained adsorptionisotherms are shown in Figure S4 of the SupportingInformation. The isotherms of nitrogen are of type IIIaccording to IUPAC classification.11 The strong slope atrelative higher pressures associated by a fully reversibleadsorption−desorption branch generally indicates a condensa-tion step between the particles. This interparticular volume ismainly influenced by particle size and particle size distribution.Besides, to exclude a size exclusion effect, carbon dioxideadsorption at higher temperatures was measured. Thesemeasurements were done if the pore aperture is too small fornitrogen molecules to enter the pore due to slow kinetics atcryogenic temperatures.17 In reference to the crystal structureof 1, we can assume a type I isotherm according to the IUPACclassification with a steep increase at low pressures and aplateau at higher pressures.11 Contrary to that expectation, theobtained carbon dioxide adsorption isotherm at 273 K (FigureS4 of the Supporting Information) is more a type II isothermwith a slow increase in loading and no significant plateau athigher pressures. From the crystal structure, it is known that thematerial consists of a density of 1.15 g cm−3 and a porosity of27.4%. Thus, a theoretical pore volume of 0.23 cm3 g−1 can beestimated, 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 correspondswell with the calculated diffractogram with RT parameters a = b= 46.58(7), c = 16.118(20) Å (Figure S5 of the SupportingInformation). This observation shows that the crystallinity of 1remains intact during the gas adsorption measurements. Thefact that, despite a stable framework, little N2 sorption andsome but limited selective sorption of CO2 was noticed has alsobeen reported in similar cases.18 The authors ascribed thisbehavior to strong interactions of N2 molecules with the porewindows which subsequently block the pore openings.Moreover, due to the specific morphology of the crystals ofthe actual compound, it also has to be taken into account thatonly the small end facets of the crystals expose the channelsystem to the outside; there are no open channels along the aand b axes. At this stage of the experimental study, one mayspeculate as well that the isobutyl groups which protrude intothe open space of the channels influence the gas adsorptionbehavior in the case of the CO2 molecules.
Magnetic Properties. The magnetic susceptibility data fora polycrystalline sample of [Mn4(L)2(iB)6]n (1) are displayedas plots of χmT versus T and 1/χm versus T in Figure 6 and asχm versus T in Figure S6 of the Supporting Information. Uponcooling, the χmT versus T plot shows decreasing χmT valuesstarting with 17.2 cm3 K mol−1 (300 K) and reaching 1.0 cm3 Kmol−1 at 1.9 K, which indicates (i) overall antiferromagneticexchange 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 theantiferromagnetic interactions slightly lower than the theoreti-cal χmT value (17.5 cm3 K mol−1) for four magnetically isolatedmanganese(II) ions representing the formula unit. The inversemagnetic susceptibility curve shows a linear behavior in thetemperature range from 50−300 K, which results in a Weissconstant Θ = −61 K, in good agreement with the negative slopeof the χmT versus T curve.Given the 1D connectivities of the Mn(II) ions along the
polymeric chains, however, with basically two different andalternating bridging modes, the system can magnetically betreated with a 1D Heisenberg model with alternating nearestneighbor J1−J2 exchange interactions described by the spinHamiltonian
∑ ∑= − −+ + +H J S S J S Si i i i1 2 2 1 2 2 1 2 2 (1)
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 versustemperature 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).
where J1 and J2 stand for the alternating exchange constants,and the S’s are classical spin operators. Consequently, followingthe theory of Rojo et al.,19 one can deduce the analyticalexpression for χm as
χ β μ μ μ μ μ μ= + + + −Ng kT/3 [(1 )/(1 )]m2 2
1 2 1 2 1 2(2)
with μ1 = coth (J1/kT) − kT/J1 and μ2 = coth(J2/kT) − kT/J2Using eq 2, the experimental data of χmT versus T were fitted
satisfactorily in the temperature range of 300−1.9 K and best fitparameters are J1 = −5.9(6) cm−1 and J2 = −1.8(9) cm−1 with gfixed at 2.0, which is the expected value for Mn(II). Theoccurrence of two different J parameters along the chain can beascribed to the different exchange pathways, which areessentially of the type Mn4−Mn1 and Mn1−Mn2. Whilethere is no question that the overall magnetic exchangeinteraction is of antiferromagnetic nature, it is also true that thisis due to a combination of factors rather than solely attributableto one mechanistic pathway. While taking a glance at Figure 2,one notes the complexity of the multiple exchange pathways forthis case. Additionally, as it was demonstrated with another casestudy (4Γ instead of 6Γ spin ground state),20 it requires a muchsimpler and more symmetric molecular geometry to elucidateand rationalize in detail a manifold of magnetic exchangepathways.
■ CONCLUSIONSIn summary, we have prepared and characterized a porousmolecular crystal based on helical coordination polymers. Abenzaldehyde derivative and isobutyrate act cooperatively asmultiply bridging ligands for manganese(II) ions andorchestrate the formation of helical chains with intricatebinding modes. Special attention needs to be paid to theligating behavior of the benzaldehyde type of ligand, whichdespite its simplicity is not often encountered in extendedcoordination frames. In addition, the strength of the magneticexchange interactions between the paramagnetic centers, whichis of an antiferromagnetic nature, has been revealed.Interestingly, the helical coordination polymers assemble intoa robust, up to 190 °C, highly symmetric crystalline packingpattern, which is based on a polar space group. Thereby,tubular channels filled with solvent molecules emerge, and thecrystallinity of the solid remains intact upon desolvation.Despite the stable crystalline packing arrangement, whichmaintains a porous structure in the desolvated state, nosubstantial gas sorption (N2 and CO2) has been observed.
■ ASSOCIATED CONTENT*S Supporting InformationCrystallographic data in CIF format, CCDC ref no. 946803.Scanning electron micrograph of 1·xMeCN (x ≤ 9), PXRDpatterns of 1, nitrogen sorption isotherms and carbon dioxideadsorption on 1, additional figures for the single crystalstructure analysis and magnetic properties of 1. This material isavailable free of charge via the Internet at http://pubs.acs.org.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSFinancial support for this research by the Swiss NationalScience Foundation (Grant 200021-147143) is gratefullyacknowledged. G. D. acknowledges the Swiss governmentscholarship program.
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