Oxime-Bridged Mn6 Clusters Inserted in One-Dimensional

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Oxime-Bridged Mn6 Clusters Inserted in One-Dimensional Coordination Polymer Mirela-Fernanda Zaltariov,† Maria Cazacu,*,† Liviu Sacarescu,† Angelica Vlad,† Ghenadie Novitchi,‡ Cyrille Train,‡ Sergiu Shova,† and Vladimir B. Arion§ †

“Petru Poni” Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A, 700487 Iasi, Romania Laboratoire National des Champs Magnétiques Intenses, CNRSUPR 3228, Univ. Grenoble-Alpes, 25 Rue des Martyrs, 38042 Grenoble, France § Institute of Inorganic Chemistry, University of Vienna, Währinger Str. 42, 1090 Vienna, Austria ‡

S Supporting Information *

ABSTRACT: The reaction of MnCl2·4H2O with salicylaldoxime (H2salox) and the sodium salt of 1,3-bis(carboxypropyl)tetramethyldisiloxane (H2L) in a 1:1:1 molar ratio led to the self-assembly of {[Mn6O2(salox)6(H2salox)(H2O)3(μ-L)]H2salox· 1.2H2O}n, a 1D coordination polymer consisting of hexamanganese(III) salicylaldoximate cluster as secondary building unit (SBU) and tetramethyldisiloxane-based dicarboxylate linker, namely, 1,3-bis(carboxypropyl)tetramethyldisiloxane. The structure of the compound was established by single crystal X-ray diffraction. The Mn(III) clusters consist of two staggered μ3-oxo-bridged Mn3 triangles held together by the oxygen atoms of the oxime groups. Because of Jahn−Teller distortion, the Mn−O distances reach 2.5 Å for the oxygen atoms located above and below the triangles mean planes. The compound showed a glass transition peak at around 14 °C in the differential scanning calorimetry (DSC) curve. The magnetic susceptibility data were fitted with a set of three intracluster antiferromagnetic exchange interaction coupling constants: J1 = −0.65 cm−1, J2 = −1.5 cm−1, and J3 = −0.9 cm−1. The ac magnetic susceptibility measurements in the 2−5 K temperature range reveal a frequencydependent behavior indicative of a slow relaxation of magnetization at low temperature. The coexistence of the lypophilic 1,3-bis(propyl)tetramethyldisiloxane moieties and hydrophilic polar SBUs confers to the structure an amphiphilic character. Dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and transmission (TEM) and scanning (SEM) electron microscopies demonstrate that in dimethylformamide (DMF) the coordination polymer organizes as micelles, whereas in chloroform it tends to form inverse micelles and vesicles.

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INTRODUCTION Under normal biological conditions, the manganese can adopt different oxidation states thus being found at the redox-active sites of some enzymes in green plants and cyanobacteria.1 Many types of organic ligands have been used in manganese coordination chemistry, the most versatile being the alcoholates or polyalcoholates, oximes, and dioximes. It is well-known that oxime species can bind to metal adopting different coordination modes. The oximes and their transition metal complexes have found applications in medicine, biology, and catalysis.2 Moreover, polynuclear manganese(III) complexes were found to often have large ground-state spin values. Some of the species formed by their combining with oximate having large and negative magnetic anisotropy show the behavior of single-molecule magnets (SMMs) or single-chain magnets (SCMs), molecular species that can retain magnetization in the absence of any applied magnetic field below a blocking temperature.3−9 The use of salicylaldoxime derivatives in manganese coordination chemistry led to a large number of SMMs with distinct nuclearity and spin ground state. In addition, homologous members of the family with very © XXXX American Chemical Society

different magnetic properties can be separated by chemically induced structural distortion of the [Mnn] core. This allowed the establishment of semiquantitative magneto-structural correlations useful to predict the magnetic properties of new members of the series.10 To fully exploit the remarkable properties of these clusters, suitable bridging/chelating agents and ancillary ligands are used in order to insert the manganese clusters as secondary building units (SBUs) in coordination polymers of higher dimensionality.11,12 For example, a three-dimensional metamagnet built of ferromagnetic chains of trinuclear manganese(II) SBU [Mn3(4aba)6]n from 4-aminobenzoic acid (4-Haba) and MnCl2·4H2O in the presence of NaN3 was reported by Hong et al.,13 and a threedimensional coordination polymer [Mn3(N3)(nicotinate)4(H2O)2]n that exhibits ferromagnetic intercluster couplings was described by Chen et al.14 Christou et al. have prepared the first Received: May 30, 2016 Revised: July 29, 2016

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DOI: 10.1021/acs.macromol.6b01149 Macromolecules XXXX, XXX, XXX−XXX

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concentrations as those used for DLS measurements were spread on the grids and then dried under vacuum overnight. An environmental scanning electron microscope (ESEM) type Quanta 200 substrate (20 kV operating voltage, secondary electrons: SE, low vacuum mode) was used to acquire SEM images for the film born from CHCl3 sample solution on glass. The small-angle X-ray scattering (SAXS) analysis of the samples was performed on a NanostarU system from Bruker. The equipment has a mirror-conditioned three-pinhole collimation and a IμS microsource with copper anode (Cu Kα radiation, 1.54 Å) that allows short measuring times. Detection of the scattered intensity was done with a Vantec 2000, a large 2D detector with a resolution of 68 μm. The detector is positioned at 107 cm distance from the sample. Such a configuration allows measurements of the momentum transfer vector (q) within 0.008−0.2 Å range. Silver behenate standard was used to calibrate the angular scale before each experiment. The film samples were prepared by drop-casting solution of polymer (5% in DMF) on Kapton tape and drying at 60 °C overnight. The measurements were done under vacuum by exposing the samples for 21 900 s at 25 °C. Corrections of the raw data and background subtraction were done using a SAXS Bruker AXS and ATSAS 2.5.1 software. X-ray Crystallography. X-ray data collection was carried out on an Oxford-Diffraction XCALIBUR E CCD diffractometer equipped with graphite-monochromated Mo Kα radiation. The single crystal was positioned at 40 mm from the detector. The CrysAlis package of Oxford Diffraction was used for unit cell determination and data integration.22 The structure was solved by direct methods using Olex223 software with the SHELXS structure solution program. The found structural model was further refined by full-matrix least-squares on F2 with SHELXL-97.24 The positional parameters of oxime oxygen with site occupancy of 0.5 were refined using the restraints imposed for the disordered atom available in SHELXL. All tested crystals revealed small size and quite low diffraction quality; therefore, after the standard solution in SHELXS program, the resolution of final data was limitted up to 1.2 Å. Consequently, due to the low number of the observed reflections the model of the structure anisotropic refinement was applied only for the heavier atoms: Mn and Si. Crystallographic analysis has revealed the structure to contain large accessible voids accommodating severely disordered water molecules. Since attempts to locate and refine the positions of all disordered molecules in the crystal structure were unsuccessful, in order to improve the refined parameters, the “solvent mask routine” available in the Olex2 program was used to account for the scattering from these voids. As the result, the value of R1obs has been reduced from 0.106 to 0.092. The main crystallographic data together with refinement details are summarized in Table 1, CCDC 1481011. Synthesis and Characterization. Synthesis of {[Mn6O2(Salox)6(H2salox)(H2O)3(μ-L)]H2salox·1.2H2O}n (1). To a solution of NaOH (0.04 g, 1.0 mmol) in methanol/water 1:2 (12 mL) 1,3bis(carboxypropyl)tetramethyldisiloxane (0.153 g, 0.5 mmol) was added, and the mixture was stirred at room temperature for 30 min. Then salicylaldoxime (0.085 g, 0.5 mmol) and MnCl2·4H2O (0.098 g, 0.5 mmol) were added. After adjusting the pH to 8, the color turned to green-brown and a precipitate was formed. The mixture was stirred at room temperature for 1 h, and the product was filtered off, washed with methanol/water 1:2 (2 × 3 mL), and dried at 80 °C overnight. This solid product was dissolved in a 15 mL mixture of solvents (methanol/ CHCl3/THF 1:1:1). Brown crystals of 1 have been separated within one month. Yield 0.08 g, 70.1%. Calcd for C68H76.4Mn6N8O27.2Si2 (Mr 1826.79): C, 44.71; H, 4.22; N, 6.13. Found: C, 44.32; H, 3.82; N, 6.47. IR νmax (KBr), cm−1: 3437 m, 3051 vw, 2953 w, 2926 m, 2897 m, 2876 m, 1645 w, 1599 vs, 1585 s, 1537 vs, 1472 m, 1441 s, 1410 s, 1358 vw, 1327 m, 1315 m, 1283 vs, 1252 s, 1202 s, 1153 m, 1124 m, 1045 vs, 1028 vs, 951 w, 918 vs, 898 m, 837 m, 793 m, 752 s, 679 vs, 648 s, 550 w, 534 m, 465 m, 401 m. UV−vis (DMF), λmax, nm (ε, M−1 cm−1): 286 (87 100), 380 (27 474).

1D chain consisting of mixed-valence trinuclear Mn3O or nonanuclear Mn9O7 cluster and 4,4′-bipyridine linker.15 By exploring a similar approach and starting from Mn6O2 clusters, other 1D chain assemblies were synthesized.16,17 The common feature of these studies is the introduction of linkers that favor the exchange interactions between the SBUs. Herein, because the interactions between the SMMs often favor faster relaxation of the magnetization, we follow this synthetic strategy with the opposite goal in mind; e.g., we are using the linker as a spacer able to isolate efficiently the SMMs. Accordingly, in {[Mn6O2(salox)6(H2salox)(H2O)3(μ-L)]H2salox· 1.2H2O}n (1), the hexanuclear manganese(III) salicylaldoximate complexes act as secondary building units (SBUs) connected by dicarboxylate-containing lypophilic and highly flexible tetramethyldisiloxane spacer. The structure and thermal behavior of 1 was established, and its magnetic behavior was investigated. Finally, the influence of the amphiphilic character of 1 on its selforganization in solution and as cast films was also studied.

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EXPERIMENTAL SECTION

Materials. 1,3-Bis(3-carboxypropyl)tetramethyldisiloxane, [HOOC(CH2)3(CH3)2Si]2O, was synthesized as described in the literature.18 Salicylaldoxime (H2salox) was obtained by the reaction of salicylaldehyde and hydroxylamine hydrochloride as reported previously.19 Physical Measurements. Infrared (IR) spectra were recorded on a VERTEX 70 spectrometer (Bruker Optics, Ettlingen, Germany) in the range 4000−400 cm−1 (on KBr pellets in transmission mode, 2 cm−1 resolution, 32 scans, room temperature). Recording of the UV spectra was carried out on sample as solution in DMF, in quartz cuvettes of 1 cm path length, at room temperature, on a Shimadzu UV-1700 spectrophotometer. The thermogravimetric analyses (TGA) were performed in alumina crucible as sample holder on STA 449F1 Jupiter NETZSCH (Germany) equipment in the temperature range 20−700 °C with a heating rate of 10 °C/min under 50 mL/min nitrogen flow. Differential scanning calorimetry (DSC) curves were recorded using about 10 mg of sample pressed and punched in aluminum crucibles on a DSC 200 F3Maia (Netzsch, Germany) at a heating/cooling rate of 10 °C/min under 100 mL/min nitrogen stream flow rate. Water vapor sorption isotherms were recorded on an IGAsorp (Hiden Analytical, Warrington, UK), a fully automated analyzer in the dynamic regime. The sample a priori dried at 25 °C in a nitrogen stream (250 mL/min) at relative humidity (RH) < 1% was exposed to increasing humidity between 0 and 90% in increments of 10% with equilibration time each of them between 10 and 20 min. All the magnetic measurements were performed on slightly crushed single-crystals using a MPMS XL Quantum Design SQUID magnetometer. A field of 0.1 T was applied to measure the dc magnetic susceptibility between 2 and 300 K. Fields up to 5 T were used to probe the field dependence of the magnetization at 2 K. Alternative current (ac) magnetic susceptibility were performed between 2.0 and 10 K with an alternative field amplitude of 3 Oe and frequency ranging between 1 and 1500 Hz. The sample holder contribution as well as the diamagnetic signal of the constituting atoms were substracted from the raw data.20,21 The dynamic light scattering (DLS) technique was used to estimate the size and distribution of aggregates in DMF and CHCl3 on a Malvern Instruments Autosizer Lo-C 7032 Multi-8 correlator (Malvern Instruments, UK) equipped with a HeNe laser (λ = 632.8 nm). Before measurement the solutions of the sample (3.18 × 10 −5 M in DMF and 5.23 × 10 −4 M in CHCl3) were filtered through 0.5 μm filter. In all registrations, the fluctuations in the intensity of the scattered light were analyzed to obtain an autocorrelation function. Transmission electron (TEM) micrographs were taken using a Hitachi HT7700 microscope. The equipment was operated in high contrast mode at 100 kV accelerating voltage. Carbon-coated copper grids, 300 mesh size, were used to support the samples. For this purpose dispersions of the metal-containing polymer in DMF and CHCl3 of the same

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RESULTS AND DISCUSSION Bis(carboxypropyl)tetramethyldisiloxane (H2L) was converted into its sodium salt and allowed to react with salicylaldoxime, B


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The UV−vis spectrum of manganese oximate-containing siloxane in DMF shows two bands at 286 nm (ε = 87 100 M−1 cm−1) and 380 nm (ε = 27 474 M−1 cm−1) attributed to π−π* intraligand transitions and to metal-to-ligand charge transfer, respectively (Figure S2). These spectral data and the elemental analysis results of 1 are in good accordance with the single crystal X-ray diffraction analysis presented thereafter. Crystal Structure. The crystal of 1 consists of [MnIII6(μ3O)2(salox)6(H2salox)(μ-L)(H2O)3] cluster, H-bonded outersphere H2salox, and cocrystallized water molecules in 1:1:1.2 ratio. The view of the asymmetric part of the structure is shown in Figure 1, while selected bond lengths and angles are gathered

Table 1. Crystallographic Data and Details of Data Collection and Refinement for 1 empirical formula formula weight temperature/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dcalc/mg/mm3 μ/mm−1 crystal size/mm3 θmin, θmax (deg) reflections collected independent reflections data/restraints/parameters R1a (I > 2σ(I)) wR2b (all data) GOFc largest diff peak/hole/e Å−3

C68H76.4Mn6N8O27.2Si2 1826.79 244 monoclinic P21/c 19.029(5) 25.005(5) 19.750(5) 90.0 105.620(5) 90.0 9050(3) 4 1.341 0.911 0.07 × 0.05 × 0.05 4.56 to 34.46 16799 5418 [Rint = 0.2420] 5418/36/544 0.1067 0.3069 1.001 0.62/−0.58

R1 = ∑||F0| − |Fc||/∑|F0|. bwR2 = {∑[w (F02 − Fc2)2]/∑[w(F02)2 ]}1/2. GOF = {∑[w(F02 − Fc2)2]/(n − p)}1/2, where n is the number of reflections and p is the total number of parameters refined.

a c

H2salox, and MnCl2·4H2O in 1:1:1 molar ratio in methanol/ water (1:2). The pH was then adjusted to 8 by adding sodium hydroxide. The increase of the pH allowed the self-assembly of the oxo-bridged clusters revealed by the appearance of the green-brown color soon followed by the formation of a solid corresponding to 1 (Scheme 1). The solid was further purified by Scheme 1. Synthesis of {[Mn6O2(Salox)6(H2salox)(H2O)3(μL)]H2salox·1.2H2O}n, 1

Figure 1. View of the asymmetric part of the unit cell of 1 along with selected atom numbering scheme. Noncoordinated water molecules are omitted for clarity. Intramolecular H-bond parameters: O1w−H···O8 [O1w−H 0.85 Å, H···O8 2.34 Å, O1w···O8 3.02(2) Å, O1w−H···O8 137°]; O1w−H···O11 [O1w−H 0.85 Å, H···O11 2.34 Å, O1w···O11 3.04(2) Å, O1w−H···O11 139°]; O2w−H···O20 [O2w−H 0.85 Å, H··· O20 1.70 Å, O2w···O20 2.54(3) Å, O2w−H···O20 168°]; O13−H···N7 [O13−H 0.85 Å, H···N7 1.82 Å, O13···N7 2.63(3) Å, O13−H···N7 159°]; O14−H···O2 [O14−H 0.85 Å, H···O2 2.39 Å, O14···O2 2.93(2) Å, O14−H···O2 121°]; O14−H···O5 [O14−H 0.85 Å, H···O5 2.28 Å, O14···O2 3.12(2) Å, O14−H···O5 166°]; O20−H···N8 [O14−H 0.85 Å, H···ON8 1.83 Å, O20···N8 2.55(4) Å, O20−H···N8 141°].

recrystallization from the methanol/CHCl3/THF 1:1:1 mixture, affording brown crystals of 1 in 70.1% yield. The replacement of monocarboxylate25 by a dicarboxylate led to the formation of 1D coordination polymer where the [Mn6] clusters act as SBU separated by siloxane-containing spacers. The IR spectrum of 1 (Figure S1) reveals the presence of the asymmetric and symmetric carboxylate stretching at 1537 and 1410 cm−1, respectively. The value of Δ = νas(COO−) − νs(COO−) = 127 cm−1 is almost equal to that observed in the sodium salt (Δsodium salt = 129 cm−1), indicating the syn−syn bridging mode of the carboxylate. Moreover, the red-shift of the stretching vibration of the −CHN− group from 1622 cm−1 in salycilaldoxime to 1585 cm−1 in 1 is indicative of its coordination to manganese(III) ions. This is corroborated by the ν(Mn−O) and ν(Mn−N) vibrations in the 534−401 cm−1 region.26

in Table S1. The molecular structure of 1 results from the combination of two {Mn3(μ3-O)} subunits, which are linked by oximato oxygen atoms O2 and O8 from two asymmetric salox2− ligands. Both {Mn3(μ3-O)} fragments form a nearly isosceles triangles. The six intratriangle Mn···Mn distances fall in the range 3.156(6)−3.252(6) Å. They are shorter than the shortest intertriangle distance Mn1···Mn4 of 3.391(6) Å. The μ3-O atoms are displaced from the {Mn3} planes by 0.21(1) Å (for O22) and 0.24(1) Å (for O1) toward each other. In the triangle subunits, each pair of manganese atoms is additionally bridged by tridentate doubly deprotonated salicylaldoxime ligands. Four of the salox2− ligands exhibit η1:η1:η1:μ2 coordination mode being C


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manganese atoms is usually completed by carboxylate8,25,28,29 or phosphinate30 ligands and by solvent molecules leading to discrete coordination entities. In the case of 1, using a dicarboxylate allowed the formation of a 1D coordination polymer where the [Mn6] clusters are bound together by the siloxane-containing dicarboxylate. Thermal Behavior. The thermal stability of 1 was evaluated by differential thermogravimetric (TG-DTG) analysis performed under an inert atmosphere (Figure S3). The compound decomposes in several stages (Table 2). In the first step (T1 = 84.5 °C− 3.1 wt %), the co-crystallized water molecules are removed. The second step (T2 = 160.3 °C−5.7 wt %) corresponds to the release of the coordinated water molecules. The main weight loss (31.1%) occurs between 224 and 500 °C. It is attributed to the degradation of oximate and carboxylate ligands. The decomposition process is not complete at 700 °C, the residual mass being 60% of the original weight. The thermal behavior of 1 was also studied by DSC, the recorded curve being shown in Figure S4. Compound 1 shows a glassy transition at 14 °C in the second scan. Glass transitions were also noticed in the case of other coordination polymers, such as those including metal-chelated terpyridine units within the backbone.31,32 Given that we are dealing with a crystalline compound, this transition indicates that at this temperature portions of the coordination polymer pass from a glassy state where they are frozen to a rubbery state where they begin to move. Following their flexibility, this transition is attributed to the dimethylsiloxane-containing chains between the dicarboxylate. The huge increase of the Tg value (14 °C) for 1 as compared to the free dicarboxylic acid (−72 °C, Figure S5) is due to the stiffness brought about by the coordination to the bulky SBUs at the two ends of the chain and to the intermolecular O−H···O hydrogen bonds revealed by X-ray diffraction data collected at −30 °C, well-below Tg. Dynamic Water Vapor Sorption Investigation. The water vapor sorption/desorption isotherms were recorded in dynamic regime at two temperatures (25 and 55 °C). As can be seen in Figure S6, there are no significant differences between the shapes of the two isotherms, except the sorption capacity value at RH 90%, which is higher at 55 °C. The sorption capacity is rather high for a compound containing lypophilic tetramethyldisiloxane units. It increases from 18.9 wt % at 25 °C up to 26.9 wt % at 55 °C. The presence of large metal coordination units, which are polar, might be an explanation for the sorption observed. Both curves show hysteresis due to slower desorption than the adsorption rate. This difference slightly diminishes as the temperature increases. Magnetic Data. The results of molar magnetic susceptibility χM measurements on the polycrystalline cluster 1 are shown in Figure 5. At room temperature, the χMT product is 17.03 cm3 K mol−1. This value is close to the value of 18.00 cm3 K mol−1 expected for six isolated MnIII ions

almost coplanar with {Mn3} planes. The two remaining salox2− anions coordinate as η1:η1:η2:μ3 bridging ligands, and their oximato oxygen atoms O2 and O8 reveal a significant displacement at 0.87(1) and 0.76(1) Å from Mn1Mn2Mn3 and Mn4Mn5Mn6 planes, respectively, toward the Mn atoms of the opposite {Mn3(μ3-O)} units. This feature explains the difference in the values of the Mn−N−O−Mn torsion angles. Two of them, involving O2 and O8 atoms, are of 30.4° and 25.4°, respectively, while the remaining ones are smaller, ranging from a minimum of 3.5° to a maximum of 14.6°. As shown in Figure 2, four

Figure 2. Structure of the {Mn6(μ3-O)2} polynuclear core showing the coordination environment of the Mn atoms.

of the Mn atoms exhibit distorted octahedral environment with Jahn−Teller elongation axes approximately parallel with each other and perpendicular to the {Mn3(μ3-O)} core. The axial coordination of Mn2 and Mn5 centers is completed by oxygen atoms provided by three water molecules and one nondeprotonated salicylaldoxime as monodentate ligand. In 1, the double deprotonated L2− anion behaves as a tetradentate linear linker as also observed in a number of analogous polymeric structures comprising {MnIII6(μ3-O)2} clusters and bi- or trifunctional carboxylate ligands reported previously.27 Each of the two carboxylate groups of the L2− anion coordinate a pair of manganese atoms from the triangle of the two neighboring hexanuclear fragments {Mn6(μ3-O)2} to form 1D coordination polymer running parallel to the crystallographic a-axis, as shown in Figure 3. In the crystal, the polymeric chains are fused via O−H···O hydrogen bonds into the supramolecular architecture extended as two-dimensional layers parallel to the (101) plane (Figure 4). The topology of hexametallic core in 1 resembles that found for other oxime-based hexamanganese SMM. For the reported complexes, the coordination sphere of the

Figure 3. View of the 1D polymeric chain in the crystal structure of 1. D


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Figure 4. Crystal structure of 1 viewed along the b-axis. The H atoms noninvolved in H-bonds are omitted for clarity. H-bonds parameters: O2w−H··· O12 [O2w−H 0.90 Å, H···O12 1.90 Å, O2w···O12(x, 0.5 − y, 0.5 + z) 2.78(2) Å, O2w−H···O12 165°]; O3w−H···O6 [O3w−H 0.92 Å, H···O6 2.17 Å, O3w···O6(x, 0.5 − y, −0.5 + z) 3.05(3) Å, O3w−H···O6 159°]; O3w−H···O3 [O3w−H 0.92 Å, H···O6 2.16 Å, O3w···O3(x, 0.5 − y, −0.5 + z) 2.78(2) Å, O3w−H···O3 123°].

connected via two μ-oxido bridges between Mn3 and Mn6. This connection is located on Jahn−Teller axis with important distances Mn6−O2 = 2.4 Å and Mn3−O8 = 2.5 Å. In contrast to the treatment of standard [Mn6] clusters,36 the intertriangles interaction between Mn1−Mn6 and Mn4−Mn3 are not considered here because the Mn4−O5 and Mn1−O11 distances are much longer than those previously reported. Because of the exponential dependence of the exchange interaction with distance, this dramatic increase of the distances leads to a vanishingly small exchange interaction along these two pathways. Accordingly, χMT versus T data are simulated using the isotropic spin Hamiltonian:37

Table 2. Main Parameters of the Thermogram of 1 T (°C) peak (step)

Ton

Tmax

Toff

the mass loss (%)

the rest (%)

I II III IV V

66.3 122.6 210.9 260.8 448.6

84.5 160.3 224.4 423.8 476.3

104.3 188.5 229.8 434.5 699.4

3.1 5.7 6.2 11.4 13.5

60.1

H = {−2J1(S1S2 + S2S3) − 2J2 S1S3} + {−2J1(S4S5 + S5S6) − 2J2 S4S6} − 2J3S3S6

where J1 and J2 are the exchange parameters within the triangles, J3 between the triangles, and Si = 2 is the spin of one of the six MnIII ions involved in the cluster. The best match between experimental and theoretical data was obtained for the set of parameters: J1 = −0.65 cm−1, J2 = −1.5 cm−1, J3 = −0.9 cm−1, and g = 1.98. These values are consistent with those previously reported for similar clusters.10,38−40 Nonetheless, the relative influence of the sign and magnitude of three exchange interaction parameters on the magnetic susceptibility was elucidated. It thus appeared that the magnetic susceptibility is very sensitive to the intratriangle interactions J1 and J2. On the contrary, a much weaker influence of J3 was noticed: a variation of the J3 constant from −0.9 to +0.9 cm−1 does not modify essentially the shape of the temperature dependence of the magnetic susceptibility though it is known to have a dramatic influence on the SMM behavior (see below). It can thus be concluded that the magnetic interaction is essentially governed by the two antiferromagnetic interactions within the two triangles. It should be noted that there is no need to introduce any interaction between the SBUs to fit the magnetic susceptibility. As expected, the long aliphatic siloxane spacer insures a good isolation of the SBUs.

Figure 5. Plots of χMT vs T for 1. The solid lines correspond to the theoretical curves according to the model given in the text with green trace: J1 = −0.65, J2 = −1.5, and J3 = −0.9 cm−1. Inset: magnetization measurements at 2.0 K.

(S = 2, g = 2).24,33 By lowering the temperature, the χMT product at 0.1 T continuously decreases and reaches 2.98 cm3 K mol−1 at 2.0 K, which indicates the presence of dominant antiferromagnetic interactions. Taking into account the structural features of 1, the interactions are concentrated within the hexanuclear Mn(III) units. Following previous magneto-structural studies on [Mn6] species,34,35 the cluster can be regarded as two trinuclear units E


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almost negligible. The absence of maximum in out of phase ac susceptibility in this case should be associated with a small energy barrier for the relaxation of the magnetization, while the frequency dependence of out of phase component of ac susceptibility suggests the presence of slow relaxation of magnetization in 1. This is consistent with a low spin of the ground state, in particular with a negative value of J3. By modifying the oxime ligands, we shall be able to trigger the Ji values in order to increase the energy barrier toward the high values previously observed in this family of SMMs.41 Self-Assembling Ability. As noted previously, the structure of the compound consists of 1D chains in which hexanuclear manganese SBUs alternate with permethylated disiloxane moieties connected via axial coordination of the carboxylate groups. While the metal cluster is polar due to donor−acceptor interactions underlying its formation, the siloxane unit is extremely nonpolar and hydrophobic.42 Thus, our 1D coordination polymer can be considered as a block copolymer incorporating highly hydrophobic bis(propyl)tetramethyldisiloxane unit alternating with more polar hexanuclear manganese cluster (Figure 7). Accordingly, 1 is expected to act an amphiphilic polymer able to self-organize. The self-organization of the coordination polymer was tested in two solvents with different polarity, namely DMF and CHCl3 having dipole moments of 3.86 and 1.04 D, respectively. DLS (dynamic light scattering) measurements in DMF solution (3.18 × 10−5 M concentration) revealed the presence of aggregates whose average diameter is centered around 900 nm, the majority of the populations being those with diameter of 825 and 955 nm and size distribution by intensity of 81.2% and 18.8%, respectively. In CHCl3 solution (5.23 × 10−4 M concentration), there are two distinct populations. The first one corresponds to structures with diameters loosely centered at 955 nm (57%). The second one (43%) is composed of much larger particles since their average diameter is centered at 3100 nm (Figure 8). Both in DMF and in CHCl3 solution, midrange values of polydispersity index (PDI) were recorded (0.353 and 0.672, respectively) that ranges in the area where the distribution algorithms best operate. TEM images taken on the film deposited on copper grids from DMF solution reveal the presence of dark spheres with average diameter centered at 100 nm (Figure S8a), which could be associated with aggregates (micelles) having high electron density metal centers on the surface (Figure 9a). As anticipated

The experimental values of magnetization at 2 K for 1 are shown in Figure 5 (inset). The field evolution of magnetization is typical for the presence of magnetic anisotropy generated by manganese(III) with Jahn−Teller distortion and is consistent with the data reported previously for related [Mn6] species.39−41 Weak competing antiferromagnetic interaction indicates the possibility of “intermediate” spin ground states which can vary from 0 to 6. The ac magnetic susceptibility (Hdc = 0) exhibits an important dependence on temperature and frequency of the out-of-phase signal χ″ (Figure 6 and Figure S7). Nevertheless, no maximum is

Figure 6. Temperature dependence of out-of-phase χ″ (down) and inphase χ′ (top) susceptibilities at Hdc = 0 and Hac = 3.0 Oe for indicated frequencies.

observed. Applying a dc magnetic field of 2000 Oe does not affect essentially the ac behavior. This indicates that the quenching of the quantum tunneling of the magnetization by the field is

Figure 7. Schematic representation of self-assembling possibilities for 1 in solvents with different polarity. F


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Figure 8. Size distribution by intensity for aggregates 1 formed in (a) DMF (3.18 × 10−5 M), PDI-0.353, and (b) CHCl3 (5.23 × 10−4 M), PDI-0.672. The numbers close to the histogram bars indicate the percentage for particles or aggregates of a particular size.

Figure 9. TEM images of aggregates arising from (a) DMF and (b) CHCl3 solution by solvent evaporation. (c) SEM images on the film born from CHCl3. (d) Presumed structure of the vesicle.

In addition, there are differences in terms of sample size analyzed. However, the model and hierarchies are preserved. In DMF it appears to be a single population with relatively narrow polydispersity while in chloroform there is a broader polydispersity. The SEM image that is taken on a larger area than in TEM mode better highlights the larger polydispersity of the particles formed in chloroform (Figure S8c). Small-angle X-ray scattering (SAXS) is able to probe features at a smaller scale than the overall size of the micelles and vesicles visualized by TEM. These aggregates are too large to be finely analyzed by SAXS technique. The 1D plots of the scattering intensity I(q) versus momentum transfer vector q have been obtained by integration of the collected 2D patterns (Figure 10). Both the log−normal and log−log plots show the presence of

from DLS measurements, a much different situation arose in the case of CHCl3. Two types of species are observed in TEM (Figure 9b) and SEM (Figure 9c) images and size distribution histograms (Figure S8b,c). The smaller ones correspond to inverse micelles with hydrophobic siloxane fragment at the periphery of the particles. The bigger ones (>100 nm) correspond to vesicles with hydrophobic siloxane fragments at the periphery of the particles. The image taken by combined scanning and transmission electron microscopy (STEM) (inset of Figure 9b) confirms this hypothesis. The micelles or vesicles formation is determined by the ratio between metal−metal or metal−solvent interaction forces. Being dried, the particles observed by microscopies are smaller as those identified by DLS in the swollen state, in solution. G


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Macromolecules

Figure 10. Log−normal (a) and log−log (b) graphical representations of I(q) versus q for compound 1 in DMF solution.

correlation peaks, suggesting the existence of a certain long-range ordering of the crystalline phases. The position of these peaks (q, 2q) along the scattering curve suggests a lamellar structure of the microdomains. Also, the broadness of the peaks reflects a large distribution of the lamellae thicknesses. The TEM images show that the material is structured in large vesicles having dimensions beyond the SAXS measurement range. Thus, this particular lamellar arrangement on a large length scale is more likely to be due to the particular ordering of the crystalline domains within the vesicle walls (Figure 11). On the other hand,

In addition, siloxane systems are known for their extreme conformational flexibility (originated in the special characteristics of siloxane bond),45 allowing easy access of the reactants to the active centers.46 Metallomicelles have been shown to have a biocidal effect on the different types of bacteria.47 Inverse micelles consisting of hydrophilic core and hydrophobic shell are also accessible by such structures in a hydrophobic medium. Reverse micelles allow for the accommodation of the hydrophilic reagents into their core and their subsequent suspension in hydrophobic solvents. This possibility may be important for industrial applications.48 The presence of bis(propyl)tetramethyldisiloxane in our structure might give it increased solubility in nonpolar environment where only few metal complexes can be dissolved and homogeneously catalyze certain reactions. Such an environment is supercritical carbon dioxide (scCO2), in the latest period increasingly regarded as nontoxic, nonflammable, eco-friendly, and relatively cheap solvent.49,50

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CONCLUSIONS A one-dimensional coordination polymer consisting of hexamanganese(III) salicylaldoxymate clusters acting as SBU and linked together by siloxane-based dicarboxylates was synthesized by self-assembly of the precursors in alkaline conditions. In the crystal, the polymeric chains are fused via O−H···O hydrogen bonds between the SBU into a two-dimensional supramolecular architecture. Magnetic studies revealed the presence of dominant antiferromagnetic exchange interaction between the MnIII ions of the cluster. At low temperature, the ac measurements show the presence of slow relaxation of magnetization consistent with the presence of the [MnIII6] clusters.27−29 The coexistence of SBUs and 1,3-bis(propyl)tetramethyldisiloxane sequences confers to the chain an amphiphilic character. Accordingly, a multitechnique approach revealed a self-organization of the coordination polymer in solution or in cast films as either micelles or inverse micelles and vesicles depending on the polarity of the solvent. This behavior would be very useful for applications in catalysis and biology.

Figure 11. A model proposed for lamellar arrangement in vesicles based on SAXS results.

the peaks are disposed on a quite straight part of the curve. The presence of such linear regions confirms the presence of structures with fractal geometry. In such a case the SAXS intensity can be expressed in a power-law form:43 I(q) = I0q−a

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The value of the slope is in the range of mass fractal dimensions. Thus, assembling of the nanocrystals in clusters having an inner lamellar order results in domains that are hierarchically structured as mass fractals. The above presented results support the self-organization ability of the polymer, this being a rare example of amphiphilic metal complexes (metallomicelles) very useful for some applications.44 A number of catalytic reactions that utilize the interfacial organization of amphiphilic metal complexes have been reported.44

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01149. IR spectra of bis(carboxypropyl)tetramethyldisiloxane, salicylaldoxime and compound 1, UV−vis spectrum for 1, table with selected bond distances and angles for compound 1, TG-DTG and DSC curves for 1, DSC curves H


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for 1,3-bis(carboxypropyl)tetramethyldisiloxane, moisture sorption−desorption isotherms for 1, frequency dependence of out-of-phase and in-phase susceptibilities at different temperatures, size distribution histograms for particles of 1 assembled in films (PDF) Structure of C68H76.4Mn6N8O27.2Si2 (CIF) CheckCIF/PLATON report (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the Romanian Ministry of National Education under Grant 53/02.09.2013, Cod: PN-II-ID-PCE2012-4-0261.

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REFERENCES

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Oxime-Bridged Mn6 Clusters Inserted in One-Dimensional

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