The Rule Rather than the Exception: Structural Flexibility of [Ni(cyclam

Article pubs.acs.org/crystal

The Rule Rather than the Exception: Structural Flexibility of [Ni(cyclam)]2+-Based Cyano-Bridged Magnetic Networks Beata Nowicka,*,† Mateusz Reczyński,† Maria Bałanda,‡ Magdalena Fitta,‡ Bartłomiej Gaweł,§ and Barbara Sieklucka† †

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland § Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway ‡

S Supporting Information *

ABSTRACT: Changes in structure and magnetic properties accompanying guest removal, inclusion, or exchange in two CN-bridged 2D networks of honeycomb topology: {[Ni(cyclam)]3[Fe(CN)6]2}n (1) and {[Ni(cyclam)]3[Cr(CN)6]2}n (2) (cyclam = 1,4,8,11-tetraazacyclotetradecane) were studied by PXRD and magnetic measurements. For each compound four pseudopolymorphic forms differing in structure and magnetic characteristics were identified: fully hydrated form stable in water, partly hydrated form stable in the air at ambient conditions, anhydrous form, and MeOH-modified form. All forms can be reversibly transformed into one another by several interconversion pathways, which fully correspond between Fe and Cr compounds. All forms of 1 and 2 are metamagnetic-like with varied Tc and critical field Hcr. For several forms, differently shaped magnetic hysteresis loops can be observed. For the partly hydrated and MeOH modified forms structure models are proposed on the basis of PXRD data. Correlations between structural features and magnetic properties are discussed.

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INTRODUCTION The possibility of inducing structural and magnetic changes by the sorption of guest molecules is one of the important features of molecular magnetic materials that make them different from classical magnets. Solvatomagnetic materials comprise magnetic sponges, where weakly coordinated water can be reversibly detached,1,2 as well as networks with elastic coordination frameworks, where removal of crystallization solvent causes changes in bond lengths and angles.3−8 Such materials may find potential applications as molecular switches or sensors. Moreover, their study provides valuable insight into the relation between magnetic superexchange and subtle structural features, like bonding geometry and supramolecular interactions. We have earlier reported some structurally flexible networks based on [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) and different cyanometallates. A 2D microporous honeycomb-like {[NiII(cyclam)]3[WV(CN)8]2}n network,9 featuring parallel empty channels in its structure (Figure 1), showed rare reversible single-crystal-to-single-crystal transformation upon water sorption, accompanied by the alteration in low temperature magnetic behavior. This compound could also be modified by sorption of methanol,10 which caused deeper structural changes with the loss of crystalline form, increase in Tc, and appearance of magnetic hysteresis. Altogether, we have obtained and fully structurally and magnetically characterized four different pseudopolymorphic © XXXX American Chemical Society

Figure 1. Channels in the honeycomb-like networks based on octaand hexacyanometallates: {[Ni(cyclam)]3[W(CN)8]2·16H2O}n, left, and {[Ni(cyclam)]3[Cr(CN)6]2·22.5H2O}n (2·22.5H2O), right; water molecules and hydrogen atoms removed for clarity.

forms of {[NiII(cyclam)]3[WV(CN)8]2}n,11 which made an unprecedented example of structural flexibility among CNbridged coordination polymers.12 We have subsequently used the Ni-cyclam complex in combination with other polycyanometallates and obtained {[NiII(cyclam)]2[MIV(CN)8]}n (M = W, Nb) 3D networks13 and a {[NiIII(cyclam)][FeII(CN)6]}n chain,14 both of which exhibited partly or fully reversible dehydration resulting in structural and magnetic changes. In the Received: May 26, 2016 Revised: June 28, 2016

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DOI: 10.1021/acs.cgd.6b00800 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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{[Ni(cyclam)]3[Cr(CN)6]2·16H2O}n (2·16H2O). The form stable in the air was obtained by drying 2·22.5H2O at 20 °C. (Found: C, 34.15; H, 7.04; N, 22.94; calc. for C42H104N24O16Ni3Cr2: C, 34.05; H, 7.08; N, 22.69.) {[Ni(cyclam)]3[Cr(CN)6]2}n (2). Anhydrous sample was obtained by drying 2·16H2O at 75 °C under vacuum for 3 h. {[Ni(cyclam)]3[Cr(CN)6]2·nMeOH}n (2·nMeOH). MeOH-modified sample was obtained by immersion of anhydrous 2 in dry MeOH. Structure Determination. The single-crystal diffraction data for 2·22.5H2O were collected on a Bruker Apex-II CCD equipped with a Mo Kα radiation source. The structures were solved by direct methods using SHELXT.21 Refinement and further calculations were carried out using SHELXL.21 The non-H atoms were refined anisotropically using weighted full-matrix least-squares on F2. Some O atoms of disordered crystallization water were refined isotropically. The C−H and N−H H atoms positions were calculated from geometrical conditions and refined isotropically using constraints. H atoms were not considered for disordered water molecules; however, they were considered in the calculation of the molecular weight. The structure and crystallographic data of 2·22.5H2O are presented in Figure S1 and Table S1. Powder XRD patterns were measured at room temperature between 3° and 70° 2θ angle on a PANalytical X’Pert PRO MPD diffractometer with a capillary spinning add-on using Cu Kα radiation (λ = 1.54187 Å). Samples in the form of dry powder or thick suspensions were sealed in 0.5 mm glass capillaries. The reference powder patterns were generated using Mercury 3.3 software.22 The unit cell parameters of 2· 16H2O, 1·nMeOH, and 2·nMeOH were determined using Winplotr and DICVOL06 indexing software.23,24 The structure determination, however, was only possible for 2·16H2O and 1·nMeOH on the basis of the crystal structure model of 1·16H2O. Refinement of the structures was carried out using Jana2006 software.25 The structure models were refined without solvent molecules using the rigid body approach where the positions and the orientation of the rigid [M(CN)6]3− and two [Ni(cyclam)]2+ fragments were only refined. TLS model was used to calculate isotropic temperature factors. The structure and crystallographic data, including the final agreement factors and the comparison between calculated and experimental diffractograms, are presented in Figures S2−S4 and Table S2. Solvent accessible volume was calculated by SOLV option in Platon.26 Graphics were created with Mercury 3.3.22 Measurements. Elemental analyses were performed on an ELEMENTAR Vario Micro Cube CHNS analyzer. Magnetic measurements were performed using a Quantum Design MPMS magnetometer. For magnetic measurements partly hydrated samples of 1 and 2 were packed in paper, placed in straw holders, and inserted into the magnetometer cavity at 275 K to avoid dehydration. Anhydrous samples were obtained in the procedure described above directly in the magnetometer cavity from partly hydrated samples. Fully hydrated and MeOH-modified samples were sealed in glass tubes under solvent. After measurements the tubes were opened and left in the air at 20 °C in order to evaporate the solvent and transform the samples into stable partly hydrated forms. In that form the samples were weighed and molecular weights of 1·12H2O and 2·16H2O, respectively, were used to calculate their molar amount and thus molar magnetic susceptibility. The temperature independent contribution to magnetic susceptibility, which includes the diamagnetic correction for sample holders was estimated and subtracted. Thermogravimetric analysis was measured on a Mettler Toledo TGA/SDTA 851e equipped with a QMS Thermostar GSD 300 T Balzers detector in the temperature range 35−250 °C with a heating rate of 2 K/min, under Ar atmosphere.

case of Ni−Fe chain a remarkable dehydration-induced chargetransfer in the unusual NiII/III−FeII/III redox pair occurred. These results, together with our earlier experiences in the synthesis of [Ni(cyclam)]2+-based coordination polymers, which indicated possibilities of polymorphism and reversible dehydration,15,16 suggested that this particular linear cationic building block is in general likely to give structurally flexible networks. It prompted us to take a closer look at two CNbridged magnetic assemblies: {[Ni(cyclam)]3[Fe(CN)6]2· nH 2 O} n (1·nH 2 O) 17 and {[Ni(cyclam)] 3 [Cr(CN) 6 ] 2 · 20H2O}n (2·20H2O),18 which were reported for the first time over 15 years ago by the groups of Colacio and Verdaguer, respectively. Both of these 2D compounds show honeycomb topology with channels going across the layered structure similar to the NiII−WV network9−11 (Figure 1). The structures of two different hydrates with n = 22.5 or 12 had been reported for 1, however, with only one set of magnetic data. Later another group presented the structure of yet another hydrate of 1 obtained under different conditions with n = 16.19 We also knew from our previous study that the powder form of 1 undergoes reversible dehydration.15 All this suggested that flexibility of the coordination skeleton similar to that observed for NiII−WV may also characterize hexacyanometallate-based networks, despite the rigidity of the octahedral coordination sphere of the [M(CN)6]3− anions. Moreover, from all the reports on magnetic properties of 1, including our own,15,17,19 it was not clear for which pseudopolymorphic form they were measured, as the samples were presumably not protected from dehydration, which may occur under reduced pressure in the magnetometer cavity. The molecular magnetism was at that time an emerging field and the impact of subtle structural changes upon solvent loss on magnetic superexchange was not yet fully appreciated. For these reasons we decided to reexamine [Ni(cyclam)]2+-based networks with FeIII and CrIII hexacyanometallates, and in this work we present a thorough study of magnetic and structural changes in 1 and 2 induced by guest molecule removal, inclusion, or exchange.

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EXPERIMENTAL SECTION

Syntheses. [Ni(cyclam) (NO3)2] was obtained by the literature method.20 Other reagents and solvents were commercially available and used as supplied. {[Ni(cyclam)]3[Fe(CN)6]2·22.5H2O}n (1·22.5H2O). Crystalline sample was obtained by slow diffusion of water solutions of [Ni(cyclam) (NO3)2] (77.6 mg, 0.2 mmol in 20 mL of H2O) and K3[Fe(CN)6] (42.8 mg, 0.13 mmol in 20 mL of H2O) in an H-tube. After 1 week, brown hexagonal plates were obtained. The same compound in the form of fine brown powder was obtained by quick mixing of the above solutions. Structure and phase purity was checked by PXRD. {[Ni(cyclam)]3[Fe(CN)6]2·12H2O}n (1·12H2O). The form stable in the air was obtained by drying 1·22.5H2O at 20 °C. (Found: C, 35.91; H, 6.59; N, 23.81; calc. for C42H96N24O12Ni3Fe2: C, 35.60; H, 6.83; N, 23.72.) {[Ni(cyclam)]3[Fe(CN)6]2}n (1). Anhydrous sample was obtained by drying 1·12H2O at 75 °C under vacuum for 3 h. {[Ni(cyclam)]3[Fe(CN)6]2·nMeOH}n (1·nMeOH). MeOH-modified sample was obtained by immersion of anhydrous 1 in dry MeOH. {[Ni(cyclam)]3[Cr(CN)6]2·22.5H2O}n (2·22.5H2O). The compound was prepared in a similar way to 1, using 2-fold diluted solutions for slow diffusion experiments: [Ni(cyclam) (NO3)2] (38.8 mg, 0.2 mmol in 20 mL of H2O) and K3[Cr(CN)6] (21.8 mg, 0.067 mmol in 20 mL of H2O). After 1 week yellow block-shaped crystals were obtained. Quick precipitation afforded yellow powder. Structure and phase purity was checked by PXRD.

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RESULTS AND DISCUSSION Synthesis and Structural Transformations. The reaction between the respective building blocks [Ni(cyclam)]2+ and [Fe(CN)6]3− or [Cr(CN)6]3− carried out in water solutions by both slow diffusion or fast precipitation methods resulted in pure fully hydrated forms: {[Ni(cyclam)] 3 [Fe(CN) 6 ]2 · 22.5H2O}n (1·22.5H2O) and {[Ni(cyclam)]3[Cr(CN)6]2· 22.5H2O}n (2·22.5H2O), respectively. The structures of these B



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Table 1. Crystallographic Data for Different Pseudopolymorphic Forms of 1 and 2a compound

1·22.5H2O

1·12H2O

1·16H2O

2·16H2O

1·nMeOH

2·nMeOH

space group a/Å b/Å c/Å β/° V/Å3 Z V/Z/Å3 layer area per Ni3M2 unitc/Å2 interlayer distanced/Å layer thicknesse/Å Ni−N−C bridge angles/°

C2/cb 25.76 16.79 17.94 91.18 7758 4 1939 216.3 8.97 4.63 154.91 165.41 170.51 2.134 5.134

C2/c 25.72 17.02 18.00 91.47 7873 4 1968 218.8 9.00 4.93 155.44 167.08 171.69 2.109 5.231

C2/m 27.38 14.31 8.48 90.18 3323 2 1661 196.0 8.48 5.11 153.01 169.77 169.77 2.047 5.056

P21/c 8.75 15.53 26.19 91.21 3557 2 1778 203.3 8.75 5.04 154.43 165.74 176.83 2.095 5.115

P21/c 8.63 15.86 26.37 91.82 3608 2 1804 209.1 8.63 5.09

P21/c 9.02 15.66 27.10 90.02 3826 2 1913 212.2 9.02 4.94

P21/c 8.99 15.91 27.38 90.44 3913 2 1957 217.7 8.99

5.170

5.160

108.20 97.31 97.83 303.34 ref 17

107.41 95.24 95.75 298.4 this work

88.91 100.33 100.33 289.57 ref 17

98.00 101.01 94.36 293.37 ref 19

99.20 94.43 99.89 293.52 this work

99.16 98.07 99.44 296.67 this work

average Ni−N bridge bonds/Å average M−Ni distances/Å intermetallic angles/° Ni1-M1-Ni3 Ni2-M1-Ni3 Ni1-M1-Ni2 sum source

2·22.5H2O

this work

a

For clarity of presentation, the corresponding values are given with the same precision based on the highest uncertainty; the standard deviations are given in Tables S1 and S2. bSpace group transformed to standard setting for easier comparison. cCalculated as the area of the unit cell face to which the layers are parallel divided by Z/2 for C2/c or Z for other space groups. dCorresponding to a for P21/c, c for C2/m, and c/2 for C2/c space groups. eThickness of corrugated honeycomb-like layers composed of metal ions and bridging ligands (see Figure S8) calculated as a displacement of Fe or Cr from the plane defined by Ni ions.

Figure 2. PXRD patterns of different pseudopolymorphs of 1 and 2.

forms have been reported before.17,18 However, we have reexamined the structure of chromium-based network at 120 K and found that, similarly to iron-based network, at low temperature all cyclam rings are ordered (Figure S1) and both compounds are isostructural (Table 1). The fully hydrated forms are stable only under solvent; thus, the amount of crystallization water could only be estimated from crystallographic data. In low temperature measurements 22.5 water molecules per formula unit fit both 1 and 2. The structural changes upon dehydration and guest sorption, as well as the reversibility of these processes, were studied by PXRD. These results were corroborated by parallel magnetic studies, from which reversibility of the transformations and

purity of the phases could be discerned. The experimental PXRD patterns and their comparison to those calculated from known SC-XRD data are shown in Figures 2 and S5. For both Fe- (1) and Cr- (2) based networks we have characterized four different pseudopolymorphic forms: fully hydrated, party hydrated, anhydrous, and MeOH-modified. The interconversion pathways between these forms are identical for both 1 and 2 and they are shown in Figure 3. The structures of 2·16H2O and 1·nMeOH were solved from powder diffraction patterns. The discussion of structural changes and their correlation with magnetic properties presented below is based on selected parameters listed in Table 1. C



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For the Fe-based network one more pseudopolymorph 1· 16H2O is known,19 which differs in structure from all other forms of 1 (Figure 4). It was obtained in the reaction between

Figure 3. Interconversion pathways between different forms of 1 and 2.

When the crystals of 1·22.5H2O and 2·22.5H2O are removed from water they lose part of the crystallization H2O molecules, upon which we observed loss of monocrystalline form. Under ambient conditions (19−24 °C, 40−60% relative humidity) well-defined partly hydrated 1·12H2O and 2·16H2O forms can be obtained, as proven by reproducible PXRD, magnetic, and EA measurements. The PXRD patterns (Figure 2), which are very similar for fully hydrated pseudopolymorphs of 1 and 2, differ significantly for air-stable forms. 1·12H2O is isostructural with the monocrystalline dodecahydrate described earlier as crystallizing simultaneously with 1·22.5H2O17 (Figure S5). Although we observed formation of pure 1·22.5H2O phase and collapse of crystals upon drying, it is possible that crystalline partly hydrated form can be obtained under different conditions as reported by Colacio et al.,17 or that single-crystalto-single-crystal transformation takes place when the crystals of 1·22.5H2O are slowly dried. The structure of Cr-based 2· 16H2O network, obtained from PXRD data (Figure S2, Tables 1 and S2), is similar to that of Fe-based hexadecahydrate 1· 16H2O.19 Both 1·12H2O and 2·16H2O can be transformed back to fully hydrated forms by immersion in water, although in the case of 1 the presence of small amount of another phase can be detected (Figure S5). The appearance of several additional low intensity peaks (e.g., 2θ = 6.6°, 10.0°, 13.2°, 16.5°) suggests that the impurity may be 1·16H2O. Anhydrous 1 and 2 networks can be obtained by drying 1· 12H2O and 2·16H2O at 75 °C. The thermogravimetric analyses (Figure S6) show that loss of water starts below 35 °C and the anhydrous compounds are stable within a wide temperature range, with the decomposition and loss of CN ligands starting above 150 °C. The diffraction patterns (Figure 2) show that the anhydrous forms are different from other pseudopolymorphs and indicate structural similarity between 1 and 2, although the structures could not be established, due to poor quality of the PXRD data. When exposed to humid air 1 and 2 absorb moisture to recreate partly hydrated 1·12H2O and 2·16H2O, respectively (Figure S5). Immersion of anhydrous 1 and 2 in dry MeOH results in the formation of methanol-modified forms. The PXDR patterns of 1·nMeOH and 2·nMeOH are very similar to each other and distinctly different from those of other pseudopolymorphs (Figure 2). The structure of 1·nMeOH, solved from PXRD data (Figure S3, Table S1), shows the same symmetry as 1· 16H2O, but significantly larger unit cell volume. 1·nMeOH and 2·nMeOH are stable only under MeOH; therefore, the amount of absorbed MeOH could not be established. From the solvent accessible volume in the structure of 1·nMeOH it can be roughly estimated at 13 MeOH molecules.26 Upon removal from solvent and drying at 20 °C the MeOH guest molecules are replaced by water from humid air, and partly hydrated 1· 12H2O and 2·16H2O are formed.

Figure 4. Structures of different pseudopolymorphs of 1: Ni, green balls; Fe, orange balls; N, blue sticks; C, gray sticks; H atoms and solvent molecules omitted for clarity.

[Ni(cyclam)]2+ and [Fe(CN)6]4− carried out at slightly elevated temperature under aerial conditions as a result of oxidation of FeII to FeIII. However, we found that the pure 1· 16H2O phase could not be reproduced by water sorption, and therefore it is not included in the study of guest-induced transformations. We have also tested the possibility of sorption of other small molecules into anhydrous 1 and 2, including MeCN and CH2Cl2. For these guest molecules, changes in diffraction patterns and magnetic properties were observed; however, pure solvent-modified forms could not be obtained. Most probably, due to the larger size of the tested guest molecules, diffusion into the channels was limited to the area close to the surface. Structure. Analysis of Structural Differences between Pseudopolymorphs. Selected structural parameters for different forms of 1 and 2 are listed in Table 1. For the sake of clarity we have changed the space group setting of 1·22.5H2O17 from A2/n to the standard one of C2/c. Data for the 1·16H2O19 form are included for comparison. All structures belong to the monoclinic system, but show different symmetry. In the fully hydrated forms (C2/c) the asymmetric unit consists of one anion and three halves of Ni cations located in the centers of symmetry. In 1·nMeOH and 2·16H2O (P21/c) only Ni2 lays in the inversion center, while two other Ni cations are symmetry dependent, but occupy general positions. The partly hydrated Fe compound 1·12H2O (C2/m) shows the highest symmetry of the network, with the anion located in the mirror plane and only two symmetry independent positions of Ni, both in the inversion centers. In all structures the honeycomb-like layers are parallel either to (001) or (100) planes, while the channels run along [001] and [100] directions, respectively. Thus, they cross the layers at the β angle, which deviates from 90° by no more than 2°. Ni2 ion in all structures occupies an inversion center, while the symmetry of Ni1 and Ni3 differs between pseudopolymorphs. D



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coordination sphere of [W(CN)8]3−, the layers in the Ni3W2 network are only slightly undulating. This is reflected in the layer thickness (defined and listed in Table 1), which is in the range of 4.6−5.1 Å for 1 and 2, while it changes from 0.9 to 2.5 Å for different pseudopolymorphs of Ni3W2.11 It is also visible in the increased layer separation for 1 and 2 (8.5−9.0 Å) as compared to Ni3W2 (7.4−8.1 Å). Moreover, the presence of eight CN ligands with small and changeable angles between them in Ni3W2 allows the formation of direct hydrogen bonds between NH groups of cyclam and N atoms of terminal CN groups, which in different pseudopolymorphs form either interor intralayer connections. Conversely, in 1 and 2 all hydrogen bonds are mediated by solvent. An important consequence of the above listed differences is the relative positions of the layers. In 1 and 2 they are placed nearly directly above one another and the channels cross them almost perpendicularly. In Ni3W2 the layers are significantly skidded and the channels cross them at 55−60°. Therefore, the channel diameter in 1 and 2 is similar to that in Ni3W2 (Figure 1), despite smaller diameter of the anions and stronger layer corrugation. Magnetic Properties. For all forms of 1 and 2, temperature dependence of DC magnetic susceptibility at the applied field of 1000 Oe (Figures S9 and S10) and AC susceptibility at the frequency of 125 Hz (Figure S11), as well as magnetization vs field at 2 K (Figure S12), were measured. The temperature dependences of DC magnetic susceptibility for all pseudopolymorphic forms of Fe- and Cr-based networks obey the Curie−Weiss law in the range of ∼70−300 K, with the exception of 2·16H2O where the Curie−Weiss law is fulfilled in the range of 120−300 K. The Curie constant values conform to the presence of three NiII (s = 1, g = 2.2) and two FeIII (s = 1/2, g = 2.0) or CrIII (s = 3/2, g = 1.95) centers per formula unit, with the average g values usually observed for the respective ions. The θC−W values are all positive (Table 2), indicating the

The comparison between isostructural Fe and Cr compounds shows a difference in M−Ni distances (ca. 0.6−0.9 Å), which is consistent with the average difference in atomic radii27 between CrIII and LS FeIII and translates into the difference in volume per formula unit. This disparity may account for the fact that the stable-in-air partly hydrated form of 1 (1·12H2O) accommodates less water and differs in structure from the stable hydrate of 2 (2·16H2O). Otherwise 1 and 2 show similar structure parameters with parallel changes between pseudopolymorphic forms. The most pronounced difference between pseudopolymorphs of 1 and 2 is the change in volume per formula unit, which reflects expansion of the structure. The largest volume characterizes fully hydrated forms, though for the MeOHmodified 1·nMeOH it is only slightly lower (1.4%) than for 1· 22.5H2O. The decrease of 14.3% and 8.4% is observed in the partial dehydration from 1·22.5H2O to 1·12H2O and 2· 22.5H2O to 2·16H2O, respectively. Similar maximum decrease in volume (15.6%) was observed for Ni3W2 network, although the MeOH and mixed-solvent modified forms were much larger than hydrated and anhydrous forms which differed from one another only by 2.5%. The volume may be considered as the product of two factors: interlayer distance and layer area per formula unit, both of which contribute to the observed changes. There is a marked decrease in layer separation upon partial dehydration. It is connected with the fact that in fully hydrated forms some water molecules are located between the layers, while in partly hydrated forms water remains only in the channels (Figure S7). The slightly larger layer separation in the MeOH-modified 1·nMeOH form, as compared to fully hydrated 1·22.5H2O, indicates that MeOH also fits in between the layers. The decrease in layer area per formula unit upon dehydration marks shrinkage of the channels. The layers at the same time become more corrugated, which is reflected in the increase of the layer thickness and lower sum of intermetallic angles. Upon the MeOH sorption the hexagonal rings of the network do not regain the maximum size observed for 1· 22.5H2O, but this effect is partly compensated by the change in the β angle, which in 1·nMeOH becomes almost right. Surprisingly, despite all the previously mentioned structural changes, the Ni−N−C angles are very similar in all pseudopolymorphs (we do not include in the comparison CN−bridge angles in 1·nMeOH and 2·16H2O, as the positions of the light atoms obtained from PXRD data are less reliable). This is in contrast with our earlier observations of the Ni3W2 network, where marked variations in the CN-bridge angles were the crucial factor determining changes in magnetic properties.9−11 In the hexacyanometallate-based networks the expansion of the structure results in increasing bond lengths between CN-bridges and Ni cations and consequently in longer intermetallic distances. Comparison between Topology-Related Hexa- and OctaCyanometallate-Based Networks. Compounds 1 and 2 show 2D honeycomb-like topology and the presence of solventaccessible channels similarly to {[NiII(cyclam)]3[WV(CN)8]2}n (Ni3W2) network, for which we have previously reported rich structural dynamics.9−11 There are, however, marked differences between octa- and hexacyanometallate based networks arising mainly from different geometry of the anionic building blocks. In the rigid octahedral [M(CN)6]3− anions three CN ligands in facial arrangement form bridges to Ni cations. In effect, the honeycomb layers in 1 and 2 are corrugated in a stair-like fashion (Figure S8). In contrast, due to flexible

Table 2. Magnetic Data for Different Pseudopolymorphic Forms of 1 and 2a compound

Tc/K

Hcr/Oe

Hc/Oe

Mrem/Nβ

θC−W/K

1·22.5H2O 1·12H2O 1 1·nMeOH 2·22.5H2O 2·16H2O 2 2·nMeOH

8.4 8.3 8.6 4.8 11.8 18.0 11.0 14.8

4460 5700 1650 1900 1220 10200 8180 3010

3080 470 1050 650 170

4.8 0.7 0.2 4.2 0.2

8.6 8.0 5.2 10.6 29.3 31.6 20.3 36.7

a Tc − temperature of magnetic ordering, Hcr − critical field for metamagnetic transition, Hc − coercive field, Mrem − magnetic remanence, θC−W − Curie-Weiss temperature.

net ferromagnetic coupling in the compounds. The temperature dependence plots of the χT products (Figure S10) are constant down to about 100 K, and then increase due to the predominantly ferromagnetic interactions within the layers. After reaching a maximum the χT values decrease, marking the onset of interlayer antiferromagnetic interactions. Magnetic ordering temperatures Tc (Table 2) were determined from the minima of the temperature derivatives of the AC susceptibility, dχAC/dT. All forms of 1 and 2 are metamagnetic-like, with characteristic inflection point visible in the initial M(H) curves (Figure S12). The most pronounced change in the slope of the E



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Three pseudopolymorphs 1·22.5H2O, 1·12H2O, and 1 show similar Tc values of long-range magnetic ordering of about 8.5 K. For these forms, differently shaped magnetic hysteresis are present, among which the one for fully hydrated forms is significantly larger (Figure 6). Such extensive irreversibility with

magnetization curve is observed for 1·12H2O. This behavior is often encountered in layered structures28−30 and was also observed for Ni3W2 network.9−11 However, there are marked differences in low-temperature magnetic properties between different pseudopolymorphs. First, different values of the critical field estimated from the maxima of the dM/dH curves are observed (Table 2). The highest Hcr values characterize partly hydrated forms, for which the interlayer distance is shorter than in the fully hydrated and MeOH-modified forms. They are also higher for 1·12H2O and 2·16H2O than for the anhydrous 1 and 2, in which similar or even shorter interlayer distance can be expected. Stronger interlayer interactions in the partly hydrated forms may be attributed to the presence of Hbonds mediated by the water molecules which link the layers. Among the forms of Cr-based network, for the fully hydrated and MeOH-modified ones the magnetization saturates very quickly above Hcr reaching 11.92 Nβ (Figure S12), which is only slightly lower than the expected value of 12.45 Nβ. For the partly hydrated and anhydrous forms the magnetization rises slowly with increasing field and reaches saturation above 30−40 kOe. This behavior indicates that strong ferromagnetic interactions within the layers are present for 2·22.5H2O and 2·nMeOH, where the layers are stretched (large value of layer area per formula unit) and became weaker for more contracted layers in 2·16H2O and (presumably) 2. It is also reflected in the AC magnetic susceptibility (Figure S11), which rises more steeply upon cooling from high temperatures and reaches higher values for 2·22.5H2O and 2·nMeOH. Additionally, antiferromagnetic coupling between the layers with some uncompensated moment in those two forms results in the appearance of narrow magnetic hysteresis (Figure 5), with the characteristic metamagnetic shape in the case of 2·nMeOH. Similar correlations were also observed for the Ni 3 W 2 network.9−11

Figure 6. Magnetization vs field at 2 K for different pseudopolymorphs of 1.

large remnant magnetization may also occur for metamagnets. It may originate from large single ion anisotropy of the 3d center30−32 or from different pinning mechanisms of the magnetic moment.33 The AC susceptibility peak for 1·22.5H2O (Figure S11) shows slight asymmetry, probably due to the presence of a small amount of 1·16H2O phase. This impurity is not detectable in other measurements, but its traces are visible in PXRD of the rehydrated phase (Figure S5). The analysis of the presented data shows that, despite isostructurallity of most pseudopolymorphic forms, the changes in magnetic behavior are not parallel between Fe- and Cr-based networks, due to different electronic structure of metal centers in the anionic building blocks. Some correlations between certain structural features, like layer contraction or interlayer separation and magnetic properties can be identified; however, they are not as clearly pronounced as in the case of Ni3W2 network.9−11 Most probably hydrogen bonds play an important part in mediating interlayer interactions; however, their role can only be roughly established due to disorder of guest solvent molecules or incompleteness of structural data. On the basis of the magnetic characterization of the pseudopolymorphs of 1 and 2 and their comparison with the literature data, we can conclude that all the previous reports on the Fe-based network15,17,19 as well as the original report on the Cr-based network18 present magnetic data for the partly hydrated forms 1·12H2O and 2·16H2O, respectively. Within the scope of this work we have supplemented the magnetic data for the fully hydrated forms that were structurally characterized before,17,18 and in addition, we have explored two previously unknown forms of each network: anhydrous and MeOH-modified.

Figure 5. Magnetization vs field at 2 K for different pseudopolymorphs of 2.

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In contrast to the Cr-based network, for the pseudopolymorphs of 1 the magnetization does not saturate below 50 kOe (Figure S12). The M(H) curves above Hcr rise at first steeply, then slowly. For the guest-containing forms, 1·22.5H2O, 1· 12H2O, and 1·nMeOH, the expected saturation is 8.6 Nβ. For the anhydrous 1 the magnetization reaches only 5.3 Nβ at 50 kOe, which indicates that some intralayer interactions through CN-bridges are antiferromagnetic, analogously to anhydrous Ni3W2 network.9 Although for the anhydrous form no structure data are available, these results strongly suggest that CNbridges in 1 are more bent than in the guest-containing forms.

SUMMARY AND CONCLUSIONS We have characterized four pseudopolymorphic forms of two [Ni(cyclam)]2+-based CN-bridged networks of honeycomb-like topology: {[Ni(cyclam)]3[M(CN)6]2}n, where M = Fe (1) or Cr (2). All the different forms can be reversibly converted into one another. The structures and interconversion pathways are parallel for both Fe- and Cr-based networks, apart from the partly hydrated form, stable under ambient conditions, which in the case of Cr-based assembly accommodates more water F



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molecules. There are marked differences in magnetic properties between different pseudopolymorphs, which confirms our earlier observation that even relatively small structural changes influence magnetic exchange in CN-bridged networks. The structural flexibility of the hexacyanometallate-based honeycomb-like networks is similar to that of the octacyanometallate-based analogue {[Ni(cyclam)]3[W(CN)8]2}n, that we reported previously,9−11 despite the rigidity of the octahedral [M(CN)6]3− building blocks. Together with our earlier observations 9−11,13−15 these results show that [Ni(cyclam)]2+/3+, and most probably other cyclam complexes, are prone to form structurally flexible solvent-sensitive CNbridged networks. This effect is most pronounced in the microporous 2D structures, which can accommodate large amounts of different guest molecules, but to a lesser extent it is also present in other nonporous 3D and 1D assemblies.13−15 The flexibility of structures based on cyclam complexes may be attributed to the linear geometry and relatively small size of the cations, as well as weak intermolecular interactions afforded by the aliphatic chains that form the outer shell of the building block. It is probable that other CN-bridged assemblies based on complexes of cyclam,34,35 or perhaps even those of cyclam derivatives,36−40 show structural flexibility, which went unnoticed in their original studies. Structurally flexible magnetic networks are perfect model systems to study the effects of CN-bridge geometry and intermolecular interactions on magnetic properties. Moreover, such materials, which react to external stimuli in the form of the presence of guest molecules or changes in humidity and temperature, are potential molecular sensors and switches, especially that in addition to magnetic changes other sorptioninduced effects can be observed, as we have previously shown.14 Cyclam complexes of transition metals have been widely studied, as documented by nearly 500 structures deposited in the CCDC database, including more than 70 of CN-bridged bimetallic assemblies, and that is not counting innumerable structures based on different cyclam derivatives. However, apart from our own reports,9−11,13,14 structural flexibility of [M(cyclam)]n+-based networks was not investigated. Therefore, we believe that the potential of these building blocks in the field of molecular materials deserves further exploration.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Christian Näther for inspiring discussions and Prof. Wiesław Łasocha and Dr. Marcin Kozieł for PXRD measurements. This work was supported by the Polish National Science Centre (research project 2014/15/B/ ST5/04465). M.R. acknowledges the support of the Polish Ministry of Science and Higher Education under the ’Diamond Grant’ program (0195/DIA/2013/42). Some measurements were carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00800. Crystallographic tables and structure diagrams; additional PXRD patterns; thermogravimetric analysis; magnetic data for different pseudopolymorphs of 1 and 2 including field dependence of magnetization at 2K and temperature dependence of AC and DC magnetic susceptibility plots (PDF) Accession Codes

CCDC 1455658−1455659 and 1460834 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. G



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The Rule Rather than the Exception: Structural Flexibility of [Ni(cyclam

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