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Chiral [NaMnIIMnIII3] and [Na2MnII2MnIII6] clusters constructed by chiral multidentate Schiff-base ligands: synthesis, structures, CD spectra and magnetic properties† Ye Song,‡a Guonan Zhang,‡a Xiaoting Qin,a Yanfei Gao,a Shuai Ding,a Yanqin Wang,a Chunfang Dua and Zhiliang Liu*a,b Two pairs of novel enantiomerically chiral clusters R/S-[NaMnIIMnIII3L3(μ3-O)] (R/S-1) and R/S-[Na2MnII2MnIII6L6(μ3-O)2] (R/S-2) have been obtained via the self-assembly of R/S-H2L Schiff base ligands and different auxiliary ligands (N3−, dca−) with divalent manganese salt in an air-exposed methanol–ethanol solution. The structures of R/S-1 and R/S-2 were characterized by single-crystal X-ray diffraction analysis and powder X-ray diffraction. When the dicyanamide anion serves as an auxiliary ligand in the assembling reaction system, a pair of enantiomeric clusters R/S-[NaMnIIMnIII3L3(μ3-O)] (R-1 and S-1) with a trigonal bipyramid configuration were obtained, while another pair of enantiomeric clusters R/S-[Na2MnII2MnIII6L6(μ3-O)2] (R-2 and S-2) were formed in the case of the azide. Interestingly, the skeleton

Received 11th July 2013, Accepted 29th November 2013 DOI: 10.1039/c3dt51874d www.rsc.org/dalton

configuration of R/S-2 can be described as a 3-fold EO-azide bridging double trigonal bipyramid of [NaMnIIMnIII3L3(μ3-O)]2 via MnII vertices. Circular dichroism (CD) spectra demonstrated the enantiomeric nature of the two pairs of clusters. Detailed direct current (DC) magnetic susceptibility studies in the temperature range 2–300 K suggested that R-1 and R-2 showed predominantly antiferromagnetic interactions between the manganese centers.

Introduction Chiral coordination compounds are a significant class of coordination compounds with highly intriguing properties that render them attractive candidates for magnetic materials,1–3 optical materials,4–7 chiral recognition,8–10 asymmetric catalysis,11–13 and so on. Among them, chiral magnets, especially chiral single-molecule magnets (SMMs), which display interactions between magnetism and chirality, have become an important kind of multifunctional molecular material.14,15 Although some prominent works on this approach have been performed by other groups,16–18 designing and constructing materials with magnetochiral dichroism a College of Chemistry and Chemical Engineering Inner Mongolia University Hohhot, 010021, P. R. China. E-mail: [email protected]; Fax: +86-471-4992147; Tel: +18686029088 b Key lab of Nanoscience and Nanotechnology Inner Mongolia Hohhot, 010021, P. R. China. E-mail: [email protected]; Fax: +86-471-4992147; Tel: +18686029088 † Electronic supplementary information (ESI) available: Synthesis of Schiff base ligand, BVS values, selected bond distances and bond angles. CCDC 937457 and 937458. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt51874d ‡ These authors contributed equally to this work.

3880 | Dalton Trans., 2014, 43, 3880–3887

(MChD) effects19 continues to be an interesting and challenging issue. In addition, manganese polynuclear clusters, which may have a large ground spin state and negative magnetic anisotropy, are the most common SMMs.20–22 When divalent manganese ions are introduced to construct chiral clusters, the possibility increases of obtaining new multifunctional molecular materials with both chirality and magnetism,23,24 such as chiral manganese SMMs25 and SCMs.26 To date, chiral coordination compounds have been rationally designed and synthesized in three ways: (1) spontaneous resolution, (2) chiral induction, (3) use of enantiopure ligands.27 Among them, using chiral enantiomeric ligands as the chiral source to generate enantiomerically pure chiral compounds is a direct and effective method to control the chirality of the target compounds.28 Moreover, the flexible coordination mode of the multidentate Schiff bases makes it easier to generate multinuclear clusters.29–31 Thus, the condensation of inexpensive chiral amino alcohols with suitable aldehydes has been the most extensively studied strategy for obtaining chiral multidentate Schiff base ligands for constructing chiral clusters.32–34 For the modulating skeleton structure of the clusters, the azide anion (N3−) and dicyanamide anion (dca−), serving as auxiliary ligands, also play an important role in the

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Table 1 Crystal data and structure refinement for compound R-1 and R-2

Scheme 1

Enantiomeric Schiff base ligands R/S-H2L.

cluster self-assembly procedure. Indeed, the N3− anion has been widely used to generate metal–organic complexes owing to the versatile coordination modes of μ1,1-(end-on, EO), μ1,3(end-to-end, EE), and so on.35–38 Furthermore, the dca− anion also acts as a terminal ligand or bridging ligand in metal– organic complexes. In our work, we tried to use chiral multidentate Schiff base ligands (Scheme 1) together with different auxiliary ligands to construct chiral polynuclear clusters. The multidentate Schiff base ligand chelates metallic atoms through hydroxyls and nitrogen atoms to form the skeleton of the clusters. However, auxiliary ligands such as azide and dicyanamide also play a significant role in influencing the structural motif of the clusters. Herein, two pairs of enantiomerically homochiral clusters [NaMnIIMnIII3(μ3-O)(R/S-L)3Cl2(dca)2(EtOH)(H2O)]·(MeOH) (R/S-1) and [Na2MnII2MnIII6(μ3-O)2(μ1,1-N3)7(R/S-L)6(EtOH) (MeOH)]· (ClO4)(EtOH)(MeOH)(H2O)3 (R/S-2) were assembled and their structure, enantiomeric nature and magnetic properties were also reported.

Experimental

Formula Mr Crystal system Space group a (Å) b (Å) c (Å) α, β, γ (°) V (Å3) Z T (K) Dcalc (g cm−3) μ (mm−1) F(000) θ Range (°) Refl collected Unique reflns Rint GOF on F2 R1 a/wR2 b [I > 2σ(I)] R1/wR2 (all data) a

R-1

R-2

C40H51Cl2Mn4N9NaO13 1179.55 Orthorhombic P212121 12.2814(5) 12.5064(5) 31.8402(13) 90, 90, 90 4890.5(3) 4 293(2) 1.602 1.189 2412 1.78 < θ < 25.00 24 885 8604 0.0489 0.993 0.0360, 0.0701 0.0440, 0.0736

C71H102ClMn8N27Na2O31 2350.75 Orthorhombic P212121 14.179(3) 26.633(5) 26.704(5) 90, 90, 90 10 084(3) 4 293(2) 1.548 1.090 4816 3.06 < θ < 50.04 84 600 17 772 0.0471 1.055 0.0561, 0.1681 0.0584, 0.1704

R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = {[∑w(Fo2 − Fc2)2]/[∑w(Fo2)2]}1/2.

X-ray crystallography X-ray crystallography measurements for R-1 and R-2 were determined at 293(2) K on a Bruker Apex II CCD diffractometer with Mo-Kα radiation (λ = 0.701073 Å) operating in a ω–2θ scanning mode for the data collection. The suitable structures of R-1 and R-2 were both solved by direct methods and refined anisotropically by full-matrix least squares techniques on F2 values, using the SHELX-97 program.39,40 Hydrogen atoms were added at appropriate theoretical positions and refined with isotropic thermal parameters riding on those parent atoms. The corresponding crystallographic and refinement data for R-1 and R-2 are listed in Table 1.

Materials and physical measurements

Preparation

Caution! Although no problems were encountered during the preparation of the clusters, as perchlorate salts and sodium azide are potentially explosive, they should be treated with caution and used in small quantities. All the starting chemicals and solvents were of AR grade and used as received. C, H and N analysis was carried out with a Perkin-Elmer 2400 elemental analyzer. The Fourier transform infrared spectra were recorded in the range of 4000–400 cm−1 using pressed KBr tablets on a MAGNA-IR750 type FT-IR spectrometer. Powder X-ray diffraction (PXRD) measurements on R-1, S-1, R-2 and S-2 were performed on a Bruker AXS-D8 X-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm), in which the X-ray tube was operated at 40 kV and 40 mA. The circular dichroism spectra of R-1, S-1, R-2 and S-2 were recorded on a JASCO J-810 Spectropolarimeter in KBr pellets at room temperature. Magnetic measurements for R-1 and R-2 were carried out with a Quantum Design SQUID MPMS-7 magnetometer.

Synthesis of compounds R/S-1. A methanol (10 mL) solution of NaOH (0.2 mmol, 0.0080 g) was added to a mixed solution of R/S-2-amino-1-propanol (1 mmol, 0.0751 g) and 2-hydroxy-3methoxybenzaldehyde (1 mmol, 0.0152 g) slowly (Scheme S1†). After half an hour, an ethanol (10 mL) solution of MnCl2·4H2O (1 mmol, 0.1979 g) and sodium dicyanamide (1 mmol, 0.0890 g) were added to the above methanol solution. The solution was stirred for a further 2 hours under aerobic conditions. The resulting deep brown solution was filtered off and allowed to evaporate slowly in air. Dark brown bulk crystals of R-1 and polycrystalline powders of S-1 were obtained after 8 days. Yield: 57% for R-1 and 59% for S-1 based on MnCl2·4H2O. Elemental analysis calcd (%) for R-1 (C40H51Cl2Mn4N9NaO13): C, 40.73; H, 4.36; N, 10.69; found (%): C, 40.69; H, 4.45; N, 10.76%. FT-IR (cm−1) for R-1: 2289 m, 2149 vs, 1613 vs, 1444 m, 1548 m, 1440 m, 1322 s, 1231 s, 1080 m, 1028 w, 974 w, 748 m, 618 m. Elemental analysis for S-1: found (%): C, 40.58; H, 4.51; N, 10.81%. FT-IR

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(cm−1) for S-1: 2288 m, 2147 vs, 1615 vs, 1443 m, 1550 m, 1441 m, 1323 s, 1229 s, 1079 m, 1029 w, 975 w, 748 m, 618 m. Synthesis of compounds R/S-2. Both were prepared as described for R/S-1, except that Mn(ClO4)2·6H2O (1 mmol, 0.3620 g) and NaN3 (2 mmol, 0.1300 g) were added instead of MnCl2·4H2O (1 mmol, 0.1979 g) and sodium dicyanamide (1 mmol, 0.0890 g), and upon slow evaporation of the reaction solution, dark brown flake-shaped crystals of R-2 and polycrystalline powders of S-2 formed after 5 days. Yield: 65% for R-2 and 68% for S-2 based on Mn(ClO4)2·6H2O. Elemental analysis calcd (%) for R-2 (C72H103ClMn8N27Na2O31): C, 36.59; H, 4.40; N, 15.90; found (%): C, 36.37; H, 4.53; N, 15.98. FT-IR (cm−1) for R-2: 2056 vs, 1618 vs, 1449 s, 1303 w, 1218 m, 1097 w, 738 m, 616 m. Elemental analysis for S-2: found (%): C, 36.46; H, 4.59; N, 15.94. FT-IR (cm−1) for S-2: 2055 vs, 1616 vs, 1448 s, 1305 w, 1217 m, 1096 w, 738 m, 616 m.

Results and discussion

Fig. 1 A perspective view showing the structure of R-1. All the hydrogen atoms and the lattice-bound solvent are omitted for clarity. Colour codes for the atoms: pink (MnII), green (Cl), brown (MnIII), dark blue (N), yellow (Na), red (O).

Synthesis Compounds R-1, S-1, R-2, S-2 were readily obtained by the reaction of a divalent manganese salt with the chiral Schiff-base ligands R/S-H2L (Scheme 1) which are formed by the in situ condensation of o-vanillin with R/S-2-amino-1-propanols, and in the presence of NaOH and auxiliary ligands sodium dicyanamide or sodium azide. During the reaction, the MnII ion is partially oxidized to the MnIII ion in the open air. Dark brown single crystals of R-1 and R-2 suitable for X-ray analysis were obtained by slow evaporation at room temperature. Unfortunately, we were unable to obtain single crystals of S-1 and S-2 suitable for X-ray single-crystal analysis. The results of single crystal structure analysis indicate that when the dicyanamide anion serves as the auxiliary ligand in the self-assembling procedure, a polynuclear compound R-1 was formed, while compound R-2 was obtained using the azide anion instead of the dicyanamide anion, which has a very different skeleton structure from R-1. Description of the structures Single-crystal X-ray diffraction analyses revealed that compound R-1 crystallizes in the orthorhombic space group P212121. A view of the cluster is shown in Fig. 1 and selected bond distances and bond angles are listed in Table S1.† According to the X-ray crystallographic analyses, the skeleton structure of compound R-1 shows a trigonal propeller-like shape. The core of the cluster consists of one MnII, three MnIII and one NaI ions. These MnIII ions adopt different coordination modes. One MnIII ion (Mn3) adopts a tetragonal pyramid configuration while the other two MnIII ions (Mn2 and Mn4) take octahedral coordination structures. There are three Schiff-base ligands in each cluster, which are oriented mutually in parallel. The valences of the manganese metal ions were determined by detection of Jahn–Teller distortion visible for MnII,III and confirmed by bond-valence sum (BVS)

3882 | Dalton Trans., 2014, 43, 3880–3887

calculations,41,42 which are listed in Table S2.† From the crystal structure it can be seen that in compound R-1, a μ3oxido group bridges three MnIII ions. Three Schiff-base ligands present in each molecule act as a tetradentate ligand via the oxygen of the deprotonated phenol, methoxyl, propanol and nitrogen of the imino. Each of the ligands coordinates to three metal ions (MnII, MnIII and NaI) along each of the three blades of the propeller-shaped molecules. Thus, the main skeleton of R-1 is formed as [NaMnIIMnIII3L3(μ3-O)] with a trigonal bipyramid configuration. Furthermore, the MnIII ions within the planar Mn3O moiety of the trigonal bipyramid bridged by μ3oxido group are further linked by auxiliary μ2-Cl− bridges. The planar Mn3O moiety is approximately perpendicular to the mean planes of the L2− ligands. Interestingly, the skeleton structure consisting of the paramagnetic metallic centers can be described as a super tetrahedron with MnII (Mn1) at the vertex and MnIII (Mn2–Mn4) cations in the triangular plane. The MnII (Mn1) ion assumes a distorted octahedral coordination geometry, which is completed by three deprotonated propanol oxygen atoms, a water oxygen atom and two nitrogen atoms of dca− anions, with Mn–O distances of [2.154(2)– 2.199(3) Å] and Mn–N distances of [2.188(3)–2.316(3) Å]. In the Mn3O moiety, two μ2-Cl− bridges connect Mn4 to Mn2 or Mn3. The distances of the adjacent Mn⋯Mn separation through chlorion bridges are 3.1872(7) Å (Mn2⋯Mn4) and 3.1660(7) Å (Mn3⋯Mn4), respectively. The MnII and all MnIII ions are bridged by the deprotonated propanol oxygen atoms and the Mn–O–Mn angles are in the range of 124.45(11)°–126.10(11)°, the MnII⋯MnIII distances connected by the propanediol oxygen atoms are in the range of 3.5708(7)–3.6151(7) Å. The NaI ion above the [Mn3O] plane is six-coordinated by three deprotonated phenolic oxygen atoms and three methoxy oxygen atoms from three Schiff-base ligands, giving a slightly distorted octahedral stereochemistry.

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Fig. 2 (a) View showing the structure of compound R-2 and the polyhedral representation of the double trigonal bipyramid subunits A and B within R-2. (b) The skeleton structure for the cluster. All the hydrogen atoms are omitted for clarity. Colour codes for the atoms: brown (MnIII), pink (MnII), dark blue (N), yellow (Na), red (O).

Compound R-2 consists of a double trigonal bipyramid cluster cation [Na2MnII2MnIII6(μ3-O)2(μ1,1-N3)7(R-L)6(EtOH)(MeOH)]+ and a well-separated perchlorate anion, methanol molecule, ethanol molecule, and water molecule. The coordination environment of R-2 is depicted in Fig. 2(a) and the skeleton structure for the cluster is shown in Fig. 2(b). The selected bond lengths and angles for R-2 are given in Table S3.† According to X-ray crystallographic analysis, the skeleton configuration of R-2 can be described as a 3-fold EOazide bridged double trigonal bipyramid of [NaMnIIMnIII3L3(μ3-O)]2 via a MnII vertex. BVS values for the Mn atoms are listed in Table S4.† In detail, the skeleton structure of R-2 is composed of two asymmetrical subunits A and B, both of which can be seen as trigonal bipyramids and the structure of subunit B is similar to that of R-1. The coordination environments of MnIII ions in the planar [Mn3O] moieties belonging to A and B are very different. In unit A, three six-coordinated MnIII ions are linked to each other via one μ2-EtOH and two EO–N3− bridges. In unit B, one MnIII ion in the triangle plane is coordinated by one terminal MeOH molecule replacement of the μ2-EtOH bridge, which leads to a different coordination number for the MnIII ions. In R-2, Mn6 is five-coordinated adopting a square pyramidal geometry, while all of the other MnIII ions are six-coordinated with distorted octahedral geometries. The coordination environments of both NaI ions and MnII ions in R-2 are the same as those in R-1. Despite no single crystals suitable for X-ray single-crystal analysis of S-1 and S-2 being obtained, the powder X-ray diffraction measurements of the polycrystalline powders suggested that compounds S-1 and R-1 or S-2 and R-2 were isomorphous. The experimental PXRD patterns for R/S-1, R/S-2 and the single crystal simulated patterns are depicted in Fig. 3.

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Fig. 3 (a) PXRD patterns of compounds R-1 and S-1 and the simulated pattern for R-1. (b) PXRD patterns of compounds R-2 and S-2 and the simulated pattern for R-2.

Fig. 4 CD spectra of R-1, S-1, R-2, and S-2 at room-temperature in KBr pellets.

Circular dichroism (CD) spectra The optical activity and enantiomeric nature of R-1, S-1, R-2 and S-2 were demonstrated with their circular dichroism (CD) spectra. As shown in Fig. 4, the solid-state CD measurements confirmed that the bulk materials of R-1 and S-1 were enantiomeric and displayed opposite Cotton effects at very similar

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wavelengths. The spectra of R-1 and S-1 are roughly mirrorimages of each other. The former exhibits a small negative absorption at ∼310 nm and a large negative absorption at ∼460 nm, and a large positive peak at ∼610 nm. The latter displays absorption peaks of the opposite sign in the same wavelength range. The enantiomers of R-2 and S-2 have approximately similar circular dichroism features to R-1 and S-1. Magnetic properties The magnetic properties of compounds R-1 and R-2 were studied to evaluate the magnetic interactions between the paramagnetic centers. Variable temperature magnetic susceptibility measurements between 2 and 300 K using a Quantum Design SQUID magnetometer MPMS-7 under a constant magnetic field of 1000 Oe were carried out. Data were corrected for the gelatine capsule sample holder as well as for diamagnetic contributions. The magnetic properties of compound R-1 as a χMT against T plot is shown in Fig. 5. For compound R-1, as the temperature is lowered, the χMT value decreases continuously from 10.93 cm3 K mol−1 at 300 K to 4.65 cm3 K mol−1 at 2.0 K. The χMT value at 300 K is less than the spin-only (g = 2) value of 13.38 cm3 K mol−1 for the three MnIII and one non-interacting MnII ions, indicating the presence of dominant antiferromagnetic (AF) interactions within R-1. According to the results of the single crystal structure analysis, combined with a magnetic point of view, the magnetic behavior of compound R-1 should mainly depend on the tetrahedron consisting of MnII (Mn1) at the vertex and MnIII (Mn2–Mn4) cations in the triangular plane. Based upon the structural features within the tetrahedron core [MnIIMnIII3L3(μ3-O)], we can classify the magnetic interaction pathways based on the different coordination motifs between adjacent Mn ions and the distribution length of Mn⋯Mn distances. In R-1, there are six different bridging motifs between adjacent Mn ions in the tetrahedron cluster [MnIIMnIII3L3(μ3-O)(μ2-Cl)2(μ2-O)]: MnIII2 (μ2-Cl) (μ3-O) MnIII4, MnIII3 (μ2-Cl) (μ3-O) MnIII4, MnIII2 (μ3-O) MnIII3, MnII1 (μ2-O) MnIII2, MnII1 (μ2-O)

Fig. 5 χMT versus T plot measured at 1000 Oe for R-1. The solid line is the best fit obtained with the model described in the text. Inset: scheme of the spin topology in R-1.

3884 | Dalton Trans., 2014, 43, 3880–3887

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MnIII3 and MnII1 (μ2-O) MnIII4. Considering the MnII⋯MnIII bridging motifs together with the distances of MnIII⋯MnIII, we can use the three-J isotropic Heisenberg model to evaluate the coupling interactions among the spin carriers within compound R-1 in order to avoid over-parameterization. The super exchange scheme can be represented as shown in the inset of Fig. 5. The corresponding spin Hamiltonian is: Ĥ ¼ 2J 1 ðŜ1 Ŝ2 þ Ŝ1 Ŝ3 þ Ŝ1 Ŝ4 Þ  2J 2 ðŜ2 Ŝ4 þ Ŝ3 Ŝ4 Þ  2J 3 Ŝ2 Ŝ3

ð1Þ

The Heisenberg spin Hamiltonian eqn (1) can be converted into the equivalent one in eqn (2) using Kambe vector coupling. Ĥ ¼ 2J 1 Ŝ1 ŜB  2J 2 Ŝ4 ŜA  2J 3 Ŝ2 Ŝ3

ð2Þ

The Kambe equivalent operator method gives the eigenvalue expression in eqn (3) E ¼ J 1 ST ðST þ 1Þ þ J 1 SB ðSB þ 1Þ  J 2 SB ðSB þ 1Þ þ J 2 SA ðSA þ 1Þ  J 3 SA ðSA þ 1Þ

ð3Þ

where SA = S2 + S3, SB = SA + S4, ST = S1 + SB. This eigenvalue expression and the Van Vleck equation were used to derive a theoretical χMT versus T expression for this complex. In view of the ZFS effects and intermolecular interactions at low temperatures (below 25 K), we fit the magnetic susceptibility only in the temperature range of 25–300 K. The best-fit parameters are J1 = −3.22 cm−1, J2 = 0.35 cm−1, J3 = −1.17 cm−1, g = 1.91, with R = 1.78 × 10−3. The small negative value of J1 represents a weak AF coupling between the MnII (Mn1) ion and each MnIII ion of the MnIII3O unit, while the small positive J2 value indicates a weak ferromagnetic (FM) interaction between the Mn2 and Mn4 ions or between the Mn3 and Mn4 ions, whose MnIII–O–MnIII angle is less than 120°. J3 represents a weak AF interaction between the Mn2 and Mn3 with a MnIII–O–MnIII angle larger than 120°. Magnetization data of R-1 were obtained at varying temperatures, as shown in Fig. 6. The non-superposition of the M versus H/T plots reveals the presence of significant magnetic anisotropy

Fig. 6 Magnetization vs. H/T plot for R-1 in the range 2–5 K and 0–1000 kOe.

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Fig. 7 χMT versus T plot measured at 1000 Oe for R-2. The solid line represents a simulation of the data as a best fit. Inset: Scheme of the spin topology in R-2.

and appreciable zero field splitting (ZFS).43 However, we could not obtain a good fit value of ZFS for compound R-1 by assuming that only the ground state was populated. The magnetic properties of compound R-2 as a χMT against T plot are shown in Fig. 7. For compound R-2, the χMT value decreases from 23.17 cm3 K mol−1 at 300 K tending to zero at very low temperatures. The χMT value at 300 K is less than the spin-only (g = 2) value of 26.75 cm3 K mol−1 for the six MnIII and two non interacting MnII ions, indicating the presence of dominant AF interactions within R-2. According to the results of the single crystal structure analysis, the skeleton configuration of R-2 can be described as a 3-fold EO-azide bridged double trigonal bipyramid of [NaMnIIMnIII3L3(μ3-O)]2 via a MnII vertex. Based upon the structural features and combining with a magnetic point of view, R-2 can be seen as a double-tetrahedron structure via a connecting vertex. Thus, we can use the four-J isotropic Heisenberg model to evaluate the coupling interactions among the spin carriers within R-2.44 A super exchange scheme can be represented as shown in the inset of Fig. 7, and the corresponding spin Hamiltonian is: Ĥ ¼ 2J 1 Ŝ4 Ŝ5  2J 2 ðŜ4 Ŝ1 þ Ŝ4 Ŝ2 þ Ŝ4 Ŝ3 þ Ŝ5 Ŝ6 þ Ŝ5 Ŝ7 þ Ŝ5 Ŝ8 Þ  2J 3 ðŜ2 Ŝ3 þ Ŝ6 Ŝ7 Þ  2J 4 ðŜ1 Ŝ2 þ Ŝ1 Ŝ3 þ Ŝ7 Ŝ8 Þ ð4Þ

Table 2

In view of the ZFS effects and intermolecular interactions at low temperatures (below 4 K), we fit the magnetic susceptibility only in the temperature range of 4–300 K. The MAGPACK simulation resulted in the best-fit parameters of J1 = −3.23 cm−1, J2 = −2.20 cm−1, J3 = −6.28 cm−1, J4 = 1.24 cm−1, g = 1.99. The negative values of J1 and J2 represent weak AF coupling of MnII⋯MnII and MnII⋯MnIII. J3 and J4 represent MnIII⋯MnIII coupling interactions within MnIII3O moieties. By analyzing the magnetic data of compounds R-1 and R-2, we found that the magnetic properties of these two compounds were mainly associated with their crystallographic condition, magnetic orbitals of spin centers (Mn ions), bond distances (Mn⋯Mn) and angles (Mn–O–Mn, and Mn–Nazido–Mn). Since both compounds R-1 and R-2 contain similar tetrahedron skeletons [MnIIMnIII3L3(μ3-O)] as well as the predominantly antiferromagnetic interactions within the manganese centers, the Mn–O–Mn super exchange path should be mainly responsible for the magnetic behavior of the clusters. Thus we speculate that the Mn⋯Mn distance is a major factor in determining the magnitude of exchange interactions in these two compounds and the Mn–O–Mn bond angle plays a decisive role in determining the AF or FM interactions. Specifically, as R-1 has a similar structure and similar magnetic behavior to the reported [NaMnIII3MnII(μ3O)L3(N3)2.7(O2CMe)1.3(MeOH)],21 the obtained exchange parameters for the two complexes have been compared. In the model of the two complexes, the couplings within MnII⋯MnIII propagated by μ3-oxido bridges are characterized as J1, the J1 value for R-1 is smaller than the reported compound because the longer MnII⋯MnIII distance in R-1 results in a weaker exchange interaction. A similar reason applies to the larger J3 for R-1. For R-1, the weak FM interaction characterized as J2 may result from the μ2-Cl− bridge and this type of bridging mode does not exist in the reported structure. Additionally, to discuss the role that the Mn–O–Mn bond angles play, we have collected relevant magnetic and structural data for some related trinuclear manganese complexes with [Mn3(μ-O)]7+ cores45–49 in Table 2. According to this previous literature, superexchange interactions can be expected to change from AF to FM at approximately 120°. The Mn–O–Mn angles for our work are distributed around 120°, which coincides with the reported work`s.

Selected magnetic and structural data for manganese compounds containing μ3-oxide-bridge

Complex

Mn⋯Mn/Åa

Mn–O–Mn/°

J1/cm−1

J2/cm−1

g

Ref.

[Mn3(μ3-O)-(phpzMe)3(EtOH)]·EtOH [Mn3(μ3-O)-(phpzMe)3(Me3OH)3(MeCO2)] (HNEt3)2[Mn3(μ3-O)(bta)6F3 [Mn3(μ3-O)(MeCO2)3(ppko)3](ClO4) [Mn3(μ3-O)(PhCO2)3(mpko)3](ClO4) R-[NaMnIIMnIII3L3(μ3-O)] R-[Na2MnII2MnIII6L6(μ3-O)2]

3.261 3.278 3.249 3.198 3.210 3.245 3.238; 3.233

122.73, 124.28, 112.65 120.61, 120.85, 116.10 120.59, 120.06, 119.33 118.03, 115.91, 118.27 117.24, 118.47, 117.42 127.86, 114.42, 114.15 128.46, 115.15, 112.60; 131.26, 112.71, 111.53

−10.3 −6.95 −5.01 +31.1 +18.6 −1.17 −6.28

+10.9 +3.28 +9.16 +6.7 +6.7 +0.35 1.24

1.91 2.17 2.0 1.91 1.92 1.91 1.99

45 46 47 48 49 T. w.b T. w.b

H2phpzMe = 3(5)-methyl-5(3)-(2-hydroxyphenyl)pyrazole, bta = anion of benzotriazole, ppkoH = phenyl 2-pyridyl ketone oxime, mpkoH = methyl 2-pyridyl ketone oxime. a Average. b T. w. = This work.

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Table 3

Structural and magnetic parameters for EO MnII–Nazido–MnII complexes

Complex

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1

[Mn(L )(N3)2]n [Mn4(abpt)4(N3)8(H2O)2] [MnNa(pyzc)(N3)2(H2O)2]n {[M2(bcpe)(N3)4]·H2O}n [Mn2(L1)(N3)4]n (H3O)[Na2MnIII6MnII2(μ4-O)2(μ1,1-N3)7(μ1,3-N3)(H2L)6Cl]·(H2O)3 [Na2MnII2MnIII6(μ3-O)2(μ1,1-N3)7(R-L)6(EtOH)(MeOH)]·(ClO4)(EtOH)(MeOH)(H2O)3

Mn⋯Mn/Åa

Mn–N–Mn/°a

J/cm−1

g

Ref.

3.486 3.460 3.420 3.269 3.260 3.202 3.186

102.4 102.3 100.23 93.98 93.52 88.19 88.3

2.54 +3.09 +2.75 −3.8 −4.7 −7.19 −3.23

2.00 2.02 2.00 2.00 2.00 1.98 1.99

51 52 53 54 55 56 T. w.b

L1 = N-(2-pyridylmethylene)methylamine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, pyzc = pyrazine-2-carboxylate, bcpe = 1,2-bis(N-carboxymethyl-4-pyridinio)ethane. a Average. b T. w. = This work.

Based on theoretical predictions, the crossover angle from antiferromagnetic to ferromagnetic interactions of the Mn(II) complexes containing end-on azido-bridges is predicted to be 98°.50 The general pattern presents an AF interaction with a bridging Mn–Nazido–Mn bond angle of less than 98°. The obtained J1 values of R-2 are comparable with the literature51–56 of Mn(II) compounds containing end-on azidobridges producing antiferromagnetic coupling or ferromagnetic coupling (Table 3). According to the literature, it may be reasonable that the MnII–Nazido–MnII bond angles of R-2 range from 86.25° to 90.28° producing values of J1 < 0 (AF). The compared results confirm that our fitting results are believable.

Conclusions In summary, two pairs of novel enantiomeric clusters R/S[NaMnIIMnIII3L3(μ3-O)] (R/S-1) and R/S-[Na2MnII2MnIII6L6(μ3-O)2] (R/S-2) have been constructed via the self-assembly of chiral Schiff base ligands R/S-H2L and different auxiliary ligands (N3−, dca−) with divalent manganese salt in an airexposed methanol–ethanol solution. According to X-ray crystallographic analysis, the skeleton structures of R/S-1 possess a trigonal bipyramid configuration while the skeleton configuration of R/S-2 can be described as a 3-fold EO-azide bridged double trigonal bipyramid of [NaMnIIMnIII3L3(μ3-O)]2 via a MnII vertex. The structural differences of R/S-1 and R/S-2 mainly depend on the regulating role of the auxiliary ligands. The circular dichroism (CD) spectra demonstrate the enantiomeric nature of the two pairs of clusters. Based upon the structural features within R-1 and R-2, the temperature dependence of the magnetic susceptibility was modeled with eigenvalue expression or MAGPACK using isotropic Heisenberg–Dirac–van Vleck Hamiltonians. The magnetic coupling parameters within the clusters have been evaluated and the results indicate that there exists dominant AF interactions between the magnetic centers.

Acknowledgements This work is supported by NSFC (21061009, 21361016) and the Inner Mongolia Autonomous Region Fund for Natural Science

3886 | Dalton Trans., 2014, 43, 3880–3887

(2013ZD09) is kindly acknowledged. We thank Prof. Licun Li (Nankai University) for stimulating discussions.

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Dalton Trans., 2014, 43, 3880–3887 | 3887

Chiral [NaMnIIMnIII3] and [Na2MnII2MnIII6] clusters constructed by chiral multidentate Schiff-base ligands: synthesis, structures, CD spectra and magnetic properties.

Two pairs of novel enantiomerically chiral clusters R/S-[NaMn(II)Mn(III)3L3(μ3-O)] (R/S-1) and R/S-[Na2Mn(II)2Mn(III)6L6(μ3-O)2] (R/S-2) have been obt...
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