Implementation of Chirality into High-Spin Ferromagnetic CoII9WV6

Article pubs.acs.org/crystal

Implementation of Chirality into High-Spin Ferromagnetic CoII9WV6 and NiII9WV6 Cyanido-Bridged Clusters Szymon Chorazy,†,§ Mateusz Reczyński,† Robert Podgajny,*,† Wojciech Nogaś,† Szymon Buda,† Michał Rams,‡ Wojciech Nitek,† Beata Nowicka,† Jacek Mlynarski,† Shin-ichi Ohkoshi,§ and Barbara Sieklucka† †

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland Institute of Physics, Jagiellonian University, Łojasiewicza 11, 30-348 Kraków, Poland § Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ‡

S Supporting Information *

ABSTRACT: The synthesized chiral (R)- and (S)-2-(1-hydroxyethyl)pyridine ligands (R/S-mpm) were introduced to self-assembled CoII-[WV(CN)8] and NiII[WV(CN)8] magnetic systems giving a remarkable series of four enantiopure cyanido-bridged clusters, {MII[MII(R/S-mpm)(MeOH)]8[WV(CN)8]6}·14MeOH (M = Co, 1-R and 1-S; M = Ni, 2-R and 2-S). They consist of 15 metal centers, 9 CoII or NiII ions, and 6 [WV(CN)8]3− ions, embedded in a 6-capped bodycentered cube topology. Bidentate enantiopure mpm ligands coordinated to eight external CoII or NiII sites induce their chiral character, which results in the strong natural optical activity in the broad UV−vis range of 200−700 nm. All (1-R/S) and (2-R/S) clusters reveal cyanido-mediated ferromagnetic exchange interaction giving high-spin ground states of 15/2 (1-R/S) and 12 (2-R/S). For (2-R/S) forms of {Ni9W6}, the exchange constant J = +16.1 cm−1 was obtained using exact diagonalization of the exchange Hamiltonian. Because of the significant magnetic anisotropy, (1-R/S) forms of {Co9W6} cluster reveal the low temperature onset of the slow magnetic relaxation characteristic of single-molecule magnets (SMMs). Thus, they can be considered as a rare example of chiral SMM molecules.

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magnetic materials based on molecules,21,22 coordination chains,23−25 layered frameworks,15,17,20 and three-dimensional extended networks have been reported.16,19,26,27 In this regard, construction of chiral high-spin clusters appears to be a promising route toward multifunctional molecular materials. A few examples of chiral polynuclear clusters, including polyoxometalates and hydroxido-bridged assemblies, have been presented, but they do not reveal highspin due to diamagnetic character, or strong antiferromagnetic coupling.28−33 The chiral magnetic clusters have been constructed almost exclusively by mixed-valence Mn II/III centers, and they exhibit high-nuclearity and high-spin ground state, sometimes combined with SMM and ferroelectric behavior.34−39 Slow magnetic relaxation for chiral clusters was also found for some DyIII-containing molecules.40,41 Surprisingly, only one report presents a heterobimetallic chiral highspin molecule, that is, a {NiII6FeIII4} cyanido-bridged cage revealing a ferromagnetic ground state with S = 8.42 In this context, we focused on the undeveloped area of enantiopure heterobimetallic high-spin clusters, in the construction of which we have applied an octacyanidotungstate(V) building block.

INTRODUCTION Over the past decade, nanosized polynuclear clusters have been attracting intense research interest from the viewpoint of valuable electronic, magnetic, and optical properties.1−4 The great effort is focused on the design and synthesis of novel high-nuclearity clusters consisting of paramagnetic metal centers which are magnetically coupled to give a well-defined, often high-spin ground state. Such magnetic molecules can show a magnetocaloric effect, enabling technological applications as low-temperature magnetic coolers.5−7 They are also considered as good candidates for the components of a quantum computer.8,9 Some polynuclear magnetic clusters built of metal centers with high uniaxial magnetic anisotropy reveal slow relaxation of magnetization below blocking temperature, and this special group of single-molecule magnets (SMMs) is now intensively studied due to potential applications in highdensity information storage and spintronics.10−12 The introduction of chirality into molecular materials, which induces a variety of optical functionalities, including natural optical activity,13 magnetochiral dichroism,14,15 nonlinear effects, e.g., second-harmonic generation,16−18 and ferroelectricity,18−20 has been also intensively investigated. When these phenomena are combined with magnetic ordering, the cooperative effects, such as magnetization induced second harmonic generation,16,17 or multiferroicity are observed.19,20 Following these findings, a considerable number of chiral © XXXX American Chemical Society

Received: August 29, 2014 Revised: May 25, 2015

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DOI: 10.1021/acs.cgd.5b00321 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Photos of the single crystals of 1-R, 1-S, 2-R, and 2-S, and the structural views of cyanido-bridged clusters of 1-R and 1-S along the W3− Co1−W5 and Co3−Co1−Co9 directions. Solvent molecules, and most of hydrogen atoms, are omitted for clarity. 2-R and 2-S are constructed of the analogous cyanido-bridged clusters (Figure S3).

The octacyanidometalates [MIV/V(CN)8]4−/3− (M = Nb, Mo, W) have been shown to be the efficient metalloligands for bimetallic chiral magnetic chains,43,44 layers,13,45 and threedimensional (3-D) networks,27 but they were not yet used for the synthesis of enantiopure high-spin clusters. Our approach comprises the exploration of a unique family of pentadecanuclear {MII9[MV(CN)8]6} (MII = 3d metal ion, MV = Mo, W, Re) clusters which exhibit various magnetic functionalities, including cyanido-mediated magnetic coupling leading to the high-spin ground states with the record value of SGS = 39/2 in the case of {Mn9W6}.46−52 Some clusters of this family have also been shown to reveal SMM behavior,48,53,54 as well as the thermally induced charge transfer and spin crossover.50−52 As an additional advantage, these clusters contain labile coordinated solvent molecules, which gives the opportunity of further functionalization of the cluster surface by bidentate capping or bridging ligands.53−60 To introduce chirality into pentadecanuclear {MII9[MV(CN)8]6} molecular topology, we followed the route of chirality transfer from enantiopure organic ligand to metal complexes.61,62 We have synthesized chiral N,Obidentate (R)- and (S)-2-(1-hydroxyethyl)pyridine, alternatively named as α-methyl-2-pyridinemethanol (mpm), and

combined them with the self-assembled {Co9W6} and {Ni9W6} clusters. Here, we report the syntheses, structure, and properties of chiral heterobimetallic {MII[MII(R/S-mpm)(MeOH)]8[WV(CN)8]6}·14MeOH (M = Co, 1-R and 1-S; M = Ni, 2-R and 2-S) clusters combining natural optical activity with the high spin ferromagnetic ground state, and slow magnetic relaxation phenomenon for CoII-based 1-R and 1-S.

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EXPERIMENTAL SECTION

Materials. The reagents CoIICl2·6H2O, NiII(NO3)2·6H2O, and methanol used during the synthesis were purchased from commercial sources (Sigma-Aldrich, Idalia) and used without further purification. The compound Na3[WV(CN)8]·4H2O was prepared following the literature procedure.63 The organic ligands, (R)- and (S)-2-(1hydroxyethyl)pyridine, alternatively named as (R)- and (S)-αmethyl-2-pyridinemethanol (R- and S-mpm), were synthesized by the modification of the published procedure (see Supporting Information).64 The related organic reagents, 2-acetylopyridine, formic acid, triethylamine, and the catalysts, [RuCl(p-cymene)][(R,R)-TsDPEN] (Ru-L(R,R)) and [RuCl(p-cymene)][(S,S)-Ts-DPEN] (RuL(S,S)) (Scheme S1), were purchased from Sigma-Aldrich and used without further purification. B



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Table 1. Crystal Data and Structure Refinement of 1-R, 1-S, 2-R, and 2-S compound formula formula weight [g·mol−1] T [K] λ [Å] crystal system space group unit cell a [Å] b [Å] c [Å] β [deg] V [Å3] Z calculated density [g·cm3] absorption coefficient [cm−1] F(000) crystal size [mm × mm × mm] θ range [deg] limiting indices

collected reflections symmetry−independent reflections Rint completeness [%] max and min transmission refinement method data/restrains/parameters GOF on F2 Flack parameter final R indices largest diff peak/hole [e·Å−3]

1-R

1-S

2-R

C126H84Co9N56O30W6 4430.87 100(2) 0.71073 monoclinic P21

C124H75Co9N56O28W6 4430.87 100(2) 0.71075 monoclinic P21

16.8892(7) 30.0190(12) 18.7067(9) 113.3600(10) 8864.1(3) 2 1.660 4.760 4244 0.21 × 0.14 × 0.12 1.2−27.5 −22 < h < 20 −29 < k < 39 −24 < l < 24 61708 31067 0.075 96.3 0.678 and 0.564

17.0220(3) 16.8310(7) 30.1570(5) 29.8285(11) 18.7570(3) 18.6310(8) 112.985(1) 113.6880(10) 8706.8(7) 8565.5(6) 2 2 1.690 1.717 4.850 5.050 4244 4262 0.13 × 0.1 × 0.08 0.15 × 0.09× 0.08 3−27.5 3.0−27.5 −21< h < 21 −21 < h < 21 −38 < k < 38 −38 < k < 38 −24< l < 24 −24 < l < 24 84709 85045 39429 38570 0.085 0.097 99.6 98.1 0.698 and 0.571 0.668 and 0.586 full-matrix least-squares on F2 39429/409/1993 38570/315/1992 1.090 1.047 0.029(11) 0.060(8) R[F2 > 2σ(F2)] = 0.068 R[F2 > 2σ(F2)] = 0.074 2 wR(F ) = 0.184 wR(F2) = 0.163 2.45/−2.39 1.59/−2.49

31067/221/1992 1.090 0.039(8) R[F2 > 2σ(F2)] = 0.052 wR(F2) = 0.159 2.62/−2.30

Synthesis of 1-R. Methanol solutions of CoIICl2·6H2O (2 mL, 23.8 mg, 0.1 mmol) and (R)-mpm (2 mL, 12.3 mg, 0.1 mmol) were stirred together for several minutes. Then, a solution of Na3[WV(CN)8]·4H2O (35.7 mg, 0.067 mmol) in 2 mL of methanol was added, and the resulting deep red solution was left in the dark for crystallization. Red block crystals (Figure 1) of {CoII[CoII(Rmpm)(MeOH)]8[WV(CN)8]6}·14MeOH, 1-R appeared after a few days. The composition of a single crystal of 1-R was determined on the basis of X-ray structure analysis. The crystalline product was suitable for X-ray characterization and other physical measurements only in the mother liquor or dispersed in Apiezon N grease. Washing crystals with methanol, or exposition to the air, leads to the breakdown of crystals. Elemental analysis (CHN) and TGA indicated that washing with methanol and air-drying lead to the exchange of methanol for water, which results in the air-stable hydrated form with the formula CoII9[WV(CN)8]6(R-mpm)8(H2O)26. Yield: 41.4 mg, 70%. Elemental analysis. Found: C, 28.6%; H, 2.53%; N, 18.5%. Calculated for Co9W6C104H124N56O34 (M = 4335.9 g·mol−1): C, 28.8%; H, 2.88%; N, 18.1%. IR (dried crystals, ATR-Diamond, Figure S1): 2177m, 2160m, 2155m cm−1, ν(CN); 1574w, 1487m, 1439m, 1377vw, 1335w, 1308vw cm−1, ν(CC) and ν(CN); 1280w, 1247vw, 1226w, 1159w cm−1, δ(C−H); 904w, 791w, 762m cm−1, γ(C−H) and aromatic ring deformations; 1099m, 1085m, 1056m, 1019m cm−1 ν(C−O); 1609s cm−1, δ(H2O). TGA (Figure S2): loss of 26 H2O, calcd. 10.8%, found 10.4%. Synthesis of 1-S. The synthetic route was identical as described for 1-R, except that (S)-mpm was used instead of (R)-mpm. Red block crystals (Figure 1) of {CoII[CoII(S-mpm)(MeOH)]8[WV(CN)8]6}· 14MeOH, 1-S appeared after a few days of crystallization. The

C124H75Ni9N56O28W6 4428.89 100(2) 0.71075 monoclinic P21

2-S C124H75Ni9N56O28W6 4428.80 100(2) 0.71075 monoclinic P21 16.9144(6) 29.9433(9) 18.6540(5) 113.6360(10) 8655.2(5) 2 1.699 4.996 4262 0.19 × 0.12× 0.11 3.011−27.484 −21 < h < 21 −38 < k < 38 −24 < l < 24 83872 39330 0.0586 99.8 0.577 and 0.492 39330/118/1992 1.050 0.044(5) R[F2 > 2σ(F2)] = 0.0515 wR(F2) = 0.123 2.867/−2.616

composition of a single crystal of 1-S was determined on the basis of X-ray structure analysis. The crystalline product was suitable for X-ray characterization and other physical measurements only in the mother liquor, or dispersed in Apiezon N grease. The crystals of 1-S were filtrated, washed with methanol, and dried in air, which gave a red airstable solid with the formula CoII9[WV(CN)8]6(S-mpm)8(H2O)26, as determined by elemental analysis (CHN) and TGA measurements. It is worth noticing that single crystals of 1-S grow a bit more slowly than 1-R. Yield: 37.6 mg, 65%. Elemental analysis. Found: C, 28.6%; H, 2.51%; N, 18.4%. Calculated for Co9W6C104H124N56O34 (M = 4335.9 g·mol−1): C, 28.8%; H, 2.88%; N, 18.1%. IR (dried crystals, ATRDiamond, Figure S1): 2177m, 2160m, 2155m cm−1, ν(CN); 1574w, 1487m, 1439m, 1377vw, 1335w, 1308vw cm−1, ν(CC) and ν(C N); 1281w, 1248vw, 1226w, 1159w cm−1, δ(C−H); 904w, 791w, 762m cm−1, γ(C−H) and aromatic ring deformations; 1099m, 1085m, 1056m, 1019m cm−1 ν(C−O); 1608s cm−1, δ(H2O). TGA (Figure S2): loss of 26 H2O, calcd. 10.8%, found 10.4%. Synthesis of 2-R. Methanol solutions of NiII(NO3)2·6H2O (2 mL, 29.0 mg, 0.1 mmol) and (R)-mpm (2 mL, 12.3 mg, 0.1 mmol) were mixed carefully. After the resulting solution was stirred for several minutes, a methanol solution of Na3[WV(CN)8]·4H2O (2 mL, 35.7 mg, 0.067 mmol) was slowly added. The spontaneous crystallization in the dark from the resulting light orange solution gave yellow block crystals (Figure 1) of {NiII[NiII(R-mpm)(MeOH)]8[WV(CN)8]6}· 14MeOH, 2-R after a few days. The composition of single crystal of 2R was determined on the basis of X-ray structure analysis. The crystalline product was suitable for X-ray characterization and other measurements only in the mother liquor, or dispersed in Apiezon N grease. Suction filtration, washing with methanol, and air-drying led to C



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the quick breakdown of crystals. The resulting air-stable yellow solid is characterized by the formula NiII9[WV(CN)8]6(R-mpm)8(H2O)26, as determined by elemental analysis (CHN) and TGA. Yield: 30.1 mg, 62%. Elemental analysis. Found: C, 28.4%; H, 2.77%; N, 17.9%. Calculated for Ni9W6C104H124N56O34 (M = 4333.7 g·mol−1): C, 28.8%; H, 2.88%; N, 18.1%. IR (dried crystals, ATR-Diamond): 2185m, 2154m cm−1, ν(CN); 1575w, 1488m, 1441m, 1380w, 1335w, 1308vw cm−1, ν(CC) and ν(CN); 1282w, 1249vw, 1228w, 1160w cm−1, δ(C−H); 907w, 793w, 766m cm−1, γ(C−H) and aromatic ring deformations; 1099m, 1086m, 1058m, 1022m cm−1 ν(C−O); 1609s cm−1, δ(H2O). TGA (Figure S2): loss of 26 H2O, calcd. 10.8%, found 10.7%. Synthesis of 2-S. The synthesis was conducted as described for 2R, but using (S)-mpm instead of (R)-mpm. Yellow block crystals (Figure 1) of {NiII[NiII(S-mpm)(MeOH)]8[WV(CN)8]6}·14MeOH, 2-S appeared after a few days of crystallization in the darkness. The composition of single crystal of 2-R was determined on the basis of Xray structure analysis. The crystalline product was suitable for X-ray characterization and other physical measurements only in the mother liquor, or dispersed in Apiezon N grease. The crystals of 2-S were filtrated, washed with methanol, and dried in air, giving a yellow solid with the formula NiII9[WV(CN)8]6(S-mpm)8(H2O)26. It is important to notice that single crystals of 2-S grow more slowly than 2-R. Yield: 28.1 mg, 62%. Elemental analysis. Found: C, 28.4%; H, 2.79%; N, 18.0%. Calculated for Ni9W6C104H124N56O34 (M = 4333.7 g·mol−1): C, 28.8%; H, 2.88%; N, 18.1%. IR (dried crystals, ATR-Diamond, Figure S1): 2185m, 2153m cm−1, ν(CN); 1575w, 1488m, 1441m, 1380w, 1337w, 1311vw cm−1 ν(CC) and ν(CN); 1282w, 1249vw, 1227w, 1161w cm−1, δ(C−H); 907w, 792w, 766m cm−1, γ(C−H) and aromatic ring deformations; 1099m, 1086m, 1058m, 1022m cm−1 ν(C−O); 1610s cm−1, δ(H2O). TGA (Figure S2): loss of 26 H2O, calcd. 10.8%, found 10.7%. Crystal Structure Determination and Powder Diffraction Study. X-ray diffraction data of 1-R were collected on a Nonius KappaCCD diffractometer with graphite monochromated Mo Kα radiation, while the measurements for 1-S, 2-R, and 2-S were performed on a Rigaku R-AXIS RAPID imaging plate area detector with graphite monochromated Mo Kα radiation. To prevent the loss of solvent molecules, single crystals were dispersed with Apiezon N grease, and measured at low temperature of T = 100 K. The crystal structure was solved by direct methods using SHELXS-97 and refined by a full-matrix least-squares technique using SHELXH-97 (1-R and 1S), and SHELXL-2014/7 (2-R and 2-S).65 Calculations were performed using a WinGX (ver. 1.80.05) integrated system.66 Most of the non-hydrogen atoms were refined anisotropically, while atoms of some disordered solvent molecules were refined isotropically. Because of a significant structural disorder, the positions of hydrogen atoms could not be found independently, so the calculations of ideal positions with the refinements using a riding model were applied. Despite the measurement conducted at low temperature, the crystal structure of all compounds includes a number of disordered atoms. Thus, in order to maintain the proper geometries, and to ensure the convergence of the refinement process, a number of restraints on the bond lengths, angles (DFIX), and thermal ellipsoids (SIMU, DELU, ISOR) were applied. In particular, the set of used restraints were connected with (i) the bond lengths between carbon and oxygen atoms in solvent methanol molecules, (ii) the bond lengths, angles, and thermal ellipsoids of atoms of strongly disordered mpm ligand coordinated to Co3/Ni3, (iii) the bond lengths and thermal ellipsoids of carbon and nitrogen atoms of disordered terminal cyanides coordinated to W6. In addition, a few other restraints on thermal ellipsoids (mainly DELU, SIMU) of atoms of some terminal cyanide ligands and mpm ligands were also applied. It is important to note that the structural disorder was observed in the similar parts of crystal structures of all compounds, 1-R/S and 2-R/S, and thus, analogous refinement procedures were applied. Structural diagrams were prepared using Mercury 2.3 software.67 CCDC reference numbers 1001660 (1-R), 1051300 (1-S), 1051301 (2-R), and 1051300 (2-S). Powder X-ray diffraction patterns of 1-R, 1-S, 2-R, and 2-S sealed in a glass capillary (0.5 mm) were collected on a PANalytical X’Pert PRO

MPD diffractometer with Debye−Scherrer geometry using CuKα radiation (λ = 1.54187 Å; 2θ range: 4−50°; room temperature). The most important crystal data and structure refinement details are presented in Table 1. Physical Techniques. Elemental analyses of C, H, N were performed using EuroEA EuroVector elemental analyzer. Thermogravimetric data in the temperature range 25−400 °C were collected on a Mettler Toledo TGA/SDTA 851e microthermogravimeter at a heating rate of 10 °C/min in Ar atmosphere. Infrared spectra were recorded on dried crystals in the range of 4000−550 cm−1 using a Thermo Scientific Nicolet iS5 spectrometer equipped with iD5 ATR-Diamond. The UV/vis absorption spectra for both solid state (crystals dispersed in Apiezon N grease, and put on BaSO4 pellet), and mother solutions were measured by a PerkinElmer Lambda 35 spectrophotometer. For solid state measurements, the spectrum of pure Apiezon N grease put on BaSO4 pellet was used as the background. Natural circular dichroism (NCD) spectra for both solid state (crystals dispersed in Apiezon N grease, and introduced between two CaF2 plates), and mother solutions were collected using JASCO J-810 spectropolarimeter. Magnetic measurements were performed using a Quantum Design MPMS-XL magnetometer and a Quantum Design MPMS-3 EVERCOOL magnetometer. The magnetic data were collected for crystalline samples in the frozen mother solution and were corrected for the diamagnetic contribution using Pascal constants.68

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CALCULATIONS Continuous Shape Measure for the coordination spheres of eight-coordinated WV centers, and Continuous Chirality Measurement Analysis for chiral fragments was performed using SHAPE software ver. 2.1, 69−71 and Continuous Symmetry Measures (CoSyM) software72−74 available free of charge.

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RESULTS AND DISCUSSION

Crystal Structures. Single crystals of 1-R, 1-S, 2-R, and 2-S compounds were characterized by X-ray diffraction analysis, and the resulting structures and crystal data are presented in Figures 1, S3−S6, and Tables S1−S3. The crystal structures consist of bimetallic pentadecanuclear cyanido-bridged {MII[MII(R-mpm)(MeOH)]8[WV(CN)8]6}, {M9W6} (M = Co or Ni) clusters of a six-capped body-centered cube topology, and intercluster solvent molecules. The cluster core is analogous to previously shown {Co9W6} or {Ni9W6} molecules crystallizing solely with solvent molecules as well as those incorporating chelating or bridging ligands.47,48,53−60 The unique structural feature of 1-R, 1-S, 2-R, and 2-S is the presence of chiral (R)-mpm ligands coordinated by external Co and Ni centers (M2−M9, Figures 1 and S3, Table 1). This breaks the high symmetry of the cluster core, and results in the lack of any nontrivial intrinsic symmetry element of the molecule giving the asymmetric unit consisting of the whole cluster (Figures S4 and S5). This is in a great contrast with all reported achiral {M9W6} clusters which are always constructed of two symmetrically dependent {M4.5W3} molecular fragments, sometimes even combined by the inversion point placed exactly in the center of the cluster.46−58 As a second-order consequence of the coordination of chiral mpm ligand, 1-R/S and 2-R/S reveal the non-centrosymmetric P21 space group different from the set of three centrosymmetric space groups detected so far in this 0D compounds’ family: (i) C2/c observed in purely methanol solvated {M9W6} clusters,47,48,51,52 (ii) R-3 found in purely ethanol solvated {Mn9W6} molecules as well as in {Ni9W6} clusters with coordinated bulky 3,4,7,8tetramethyl-1,10-phenanthroline,46,53 and (iii) P1̅ characteristic D



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of all other {M 9W6} clusters with bidentate blocking ligands.55,57,58 The asymmetric units contain the whole cluster, comprising nine M II (M = Co, Ni) centers combined with six [WV(CN)8]3− complexes, and several uncoordinated methanol molecules (Figures S4 and S5). Each W center forms five cyanide bridges, while the remaining three CN− ligands are free. The geometry of [WV(CN)8]3− units were assigned on the basis of Continuous Shape Measures analysis.69−71 In all enantiomers, W1 and W2 centers adopt a geometry close to an ideal dodecahedron (DD-8), while the geometry of W3, W4, and the disordered W6 is intermediate between a dodecahedron (DD-8) and a bicapped trigonal prism (BTP-8, Table S3). On the contrary, the W5 geometry is described as an intermediate between BTP-8, and a square antiprism (SAPR8). M1, located in the center of a cluster core, coordinates six cyanides, while each of the external M2−M9 coordinates facially three cyanides, one bidentate (R)-mpm, and one MeOH. All external MII complexes noticeably differ in the structural parameters and in spatial arrangement of mpm, which results in non-centrosymmetric character of {M9W6} molecule. The 3-D supramolecular networks are controlled by intercluster π−π stacking between aromatic rings of mpm, and hydrogen bonds involving MeOH, terminal CN−, and OH groups of mpm. These interactions lead to the chain-like alignment of M9W6 clusters along the a and c directions, while the supramolecular layers are observed along the b direction, within the (101̅) plane (Figure S6). The shortest intermetallic contacts between the neighboring clusters are in the range 7.0− 7.5 Å, which is mainly controlled by a large quantity of intercluster methanol. Within the experimental errors, all these structural features are identical for both pairs of enantiomers, 1-R/S and 2-R/S, proving their perfect isostructurality (Tables 1, S1 and S2). The small differences in metric parameters between enantiomorphs of 1 and 2 is only the natural consequence of slightly smaller ionic radius of NiII in comparison to CoII. Moreover, the measured and calculated powder X-ray diffraction (PXRD) patterns for polycrystalline samples of 1-R, 1-S, 2-R, and 2-S indicate that their isostructural models from single-crystal XRD are valid for the relevant bulk samples used in other physical measurements (Figure 2). The elemental analyses, TGA measurements, and IR spectroscopy also show the same results for 1-R, 1-S and 2-R, 2-S pairs (see Experimental Section and Figures S1−S2). Optical Properties. The optical properties of 1-R, 1-S, 2-R, and 2-S were investigated by solid state UV−vis-NIR absorption and natural circular dichroism spectra (Figure 3). The strong absorption of 1-R and 1-S ranges from UV, through the visible region up to 680 nm, with maxima at 255, 360, and 520 nm. The spectra were deconvoluted into seven components with maxima at 245 (peak 1), 310 (2), 360 (3), 415 (4), 495 (5), 560 (6), and 605 nm (7). Peaks 1 and 2 are related to π−π* transitions of mpm, as observed for pure ligand (Figure S7). The [WV(CN)8]-centered components are also expected for peaks 1 and 2. These are metal-to-ligand charge transfer (MLCT) from W V to CN − [E″( 2 B 1 ) → E″(2B2);b2(π*) and E″(2E);e(π*)] to peak 1, and ligand-tometal charge transfer (LMCT) from CN− to WV [E″(2B1); [b1(x2−y2)]1 → E″(2B1);b1[b1(x2−y2)]2] to peak 2. The peaks 3 and 4 can be assigned only to [WV(CN)8]3−, revealing the transitions of a LMCT character: E″(2B1) → E″(2E);e3[b1(x2− y2)]2 (peak 3), E″( 2B 1) → E′( 2E);e3 [b 1(x2 −y2 )]2 and

Figure 2. Experimental and calculated (from single-crystal XRD models) powder X-ray diffraction patterns for 1-R, 1-S, 2-R, and 2-S in the representative 5−30° range of 2θ angle.

E′(2A2);a2[b1(x2−y2)]2 (peak 4).75 The UV part of the spectrum contains also the contribution from ligand-field transitions of WV. Of different origins are peaks 5−7, which can be attributed to metal-to-metal charge transfer (MMCT) from CoIIHS to WV (peak 5), and d−d transitions in high-spin CoII: 4T1g(4F) → 4T1g(4P) (6), and 4T1g(4F) → 4A2g(4F) (7).76 The other d−d transition of CoII, 4T1g(4F) → 4T2g(4F) causes weak absorption in the NIR range above 900 nm (Figure S8). In NCD spectrum, 1-R exhibits a positive Cotton effect for 200−220 and 300−690 nm, while negative bands are in the range 220−300 nm. The NCD spectrum of 1-S is a mirror image of pattern for 1-R, indicating the enantiopure character of both compounds. NCD signal appears for the whole absorption region proving an efficient transfer of chirality from mpm to CoII and WV.61,62 In contrast to CoII-based 1-R and 1-S, NiII-containing 2-R and 2-S reveal strong absorption in the narrower range, from UV to 550 nm, with a weak tail to 650 nm, and maxima at 250, 315, and 420 nm. The deconvolution of the spectra gave five components with maxima at 250 (peak 1), 310 (2), 365 (3), 420 (4), and 565 nm (5). Peaks 1−4 can be ascribed similarly as for 1-(R/S), as they are connected with transitions of mpm and [WV(CN)8]3−, present in the structures of both 1 and 2. Peak 5 can be assigned to d−d transition of NiII, that is 3A2(3F) → 3T1(3F).77 Other expected d−d transitions of NiII, 3A2(3F) → 1E(1D), and 3A2(3F) → 3T2(3F) result in the additional weak absorption in the NIR range above 700 nm (Figure S9). 2-R exhibits a positive Cotton effect in the region 325−380 nm, while the ranges 200−325 and 380−570 nm are covered by the negative signal. The NCD spectrum of 2-S is a mirror image of the that for 2-R, indicating the enantiopure character of these compounds, and an efficient transfer of chirality from mpm to NiII and WV. The natural optical activity was also found for the mother solutions of 1-R, 1-S, 2-R, 2-S, and NCD patterns are comparable to those presented for solid state samples, which suggests that chiral {Co9W6} and {Ni9W6} clusters probably exist also in solution (Figures S10 and S11). The mother solution absorption and NCD spectra can be interpreted in an analogous way to solid state properties, and thus, they support the postulated transfer of chirality from mpm ligands to metal centers, and the whole coordination network. The ligand-to-metal chirality transfer in all enantiomers is additionally illustrated by the results of continuous chirality E



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dodecahedron (W1−W4, and W6, Table S4). The crucial role seems to be played by the intrinsic asymmetry of distorted eight-coordinated complexes of octacyanidometallates revealing various geometries, and the CCM analysis cannot prove the chirality transfer to WV centers. Magnetic Properties. The magnetic data of 1-R, 1-S, 2-R, and 2-S are presented in Figures 4−5, and S12−S16. As

Figure 4. Magnetic properties of 2-R and 2-S: χMT(T) measured at H = 1 kOe, and field dependences of magnetization at T = 1.8 K (the inset). The magnetic data of the related compound 3 are presented for comparison (see text for details).53

Figure 3. Solid state UV−vis absorption spectra and the related natural circular dichroism (NCD) spectra for (top) 1-R and 1-S, (bottom) 2-R and 2-S.

measures (CCM) calculations (Table S4).72−74 The strong chirality was proved for all six crystallographically distinguishable external fac-[M(μ-NC)3(R-/S-mpm)(MeOH)] (M = Co2−Co6, 1-R/S; M = Ni2−Ni6, 2-R/S) units revealing CCM parameters in the range of 13−16. In comparison, the central [M1(μ-NC)6] moieties of obviously nonchiral nature gave the CCM parameters close to 0.01. Nonzero but very small CCM parameters in the range 0.05−0.3 were found for [WV(CN)8]3− ions. The relatively higher values were observed for octacyanide complexes of the geometry closer to a

Figure 5. Magnetic properties of 1-R, 1-S: (top) χMT(T) at H = 1 kOe with field dependences of magnetization at T = 1.8 K (inset), (bottom) ac χM″(T) and ln(χM″/χM′)(T−1) plots of 1-R without dc field (Hac = 3 Oe). F



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{NiII9WV6} clusters capped by bidentate ligands were already reported,53,55−58 magnetism of NiII-containing 2-R and 2-S is presented first, and compared with the previous findings (Figure 4 and Figure S13). This is followed by the description for 1-R and 1-S consisting of {CoII9WV6} molecules bearing bidentate ligands which is reported for the first time (Figure 5 and Figures S14−S16). For 2-R and 2-S, the room temperature χMT products are 15.3 and 15.6 cm3 mol−1 K, respectively, which is slightly higher than 13.2 cm3 mol−1 K calculated for the uncoupled six WV (SW = 1/2, gW = 2.0), and nine NiII (SNi = 1, gNi = 2.2). With decreasing temperature, the χMT gradually increases to a broad maximum at 12 K (Figure 4). The maximum values of 83.7 (2-R) and 83.1 (2-S) cm3 mol−1 K correspond well to 86.0 cm3 mol−1 K expected for S = 12 with the average g-value of 2.1, which proves the ferromagnetic ground-state for Ni9W6 clusters in 2-R and 2-S. The decrease of χMT below 10 K can be ascribed to the zero-field splitting of NiII78 and intercluster interactions. The ferromagnetic nature of 2-R and 2-S is confirmed by H dependence of the magnetization at T = 1.8 K, showing the fast increase of the signal to 25.3 (1-R) and 25.2 (1-S) Nβ at H = 50 kOe, which corresponds well to the expected 25.8 Nβ. Because of low magnetic anisotropy of octahedral NiII, 2-R and 2-S do not show, even with the applied dc field, significant χM″ signal, which excludes the slow magnetic relaxation above 2 K. To compare the properties of {NiII9WV6} clusters of 2-R/S with already reported results, χMT(T) was simulated using the MAGPACK program.79 The same Heisenberg exchange interaction was assumed for all cyanide bridges:

coordination geometry of [W(CN)8]3−, mainly dodecahedral in the case of 2-R and 2-S (see Table S3), and mainly bicapped trigonal prismatic in the case of 3 as well as (ii) visible differences in details of coordination sphere of the external NiII moieties including different chelating ligands (Figure S12). These contributions are expected to influence the configuration of 3d and 5d magnetic orbitals, and spin density transfer toward the orbitals of CN− bridges, which was observed in the case of paramagnetic [M(CN)8]n− moieties.81−83 The use of a simplified single J model is justified by the fact that rather a mixed character of [W(CN)8]3− polyhedra is encountered along the molecular structure of {Ni9W6} clusters in 1-R and 1S. Room temperature χMT for {CoII9WV6} unit of 1-R and 1-S is 30.2 and 30.4 cm3 mol−1 K, respectively, which is within the range 27−33 cm3 mol−1 K expected for combined contributions from the uncoupled six WV (SW = 1/2, gW = 2.0), and nine CoII (SCo = 3/2, gCo = 2.4−2.7).84 On cooling, the χMT gradually increases to the maximum of 79.6 (1-R) and 80.5 (1S) cm3 mol−1 K at 5 K, indicating the CoII − WV ferromagnetic coupling (Figure 5). This is followed by the decrease of χMT to 70 cm3 mol−1 K, related to antiferromagnetic intercluster interactions and zero-field splitting of CoII states. The maximum χMT values can be correlated with the ground-state spin of Co9W6 molecules, when applying the effective spin approach for CoII with the effective spin of SCo,LT = 1/2, and the average g-value of gCo,LT = 13/3 at low T.80 This, together with SW = 1/2, gW = 2.0, and the ferromagnetic CoII − WV coupling, gives the ground state of Co9W6 with a spin SF = 15/2 and an average g-value of 3.4. The resulting theoretical maximum χMT of 92 cm3 mol−1 K correlates well with the experimental values, proving the high-spin ferromagnetic character of Co9W6 clusters in 1-R and 1-S. This is supported by the M(H) plot at T = 1.8 K, showing the increase to 25.8 (1R) and 25.9 (1-S) Nβ at H = 50 kOe, which is only slightly higher than the expected 25.5 Nβ (Figure 5). Because of the presence of anisotropic CoII, 1-R exhibits the frequencydependent ac susceptibility, suggesting the onset of slow magnetic relaxation typical for SMM behavior. However, it happens at very low temperatures, and the maxima of χM″(T), even with the applied dc field, are not observed above 1.8 K, hampering the standard analysis of the spin dynamics. Despite this, the low-temperature frequency-dependent ac susceptibility were successfully analyzed using a generalized Debye model to fit the Cole−Cole plot (Figure S14 bottom). The obtained parameter of 0.45(5) suggests a broad distribution of relaxation times. Assuming a single Arrhenius relaxation to roughly estimate the average energy barrier ΔE, we used the linear part of ln(χ″M/χ′M) versus T−1 plots (Figure 5), which slope is equal ΔE(1-α)/kB (kB = Boltzmann constant). The resulting values of ΔE/kB = 19(4) K and τ0 = 4(2) × 10−9 s are within the range characteristic of single-molecule magnets.43,85 The additional proof of an uniaxial type of anisotropy in {CoII9WV6} clusters of 1-(R) might be obtained by fitting magnetization curves below 6 K, which gives a negative value of the zero-field splitting parameter D/kB = −0.61(4) K and a g-value of 3.50, when the effective spin of 15/2 is assumed (Figure S16). The negative value of D/kB can be reasonably explained by the highly unsymmetrical distribution of ligands around CoII centers, and the decreased symmetry of a cluster core in 1-R, which have been predicted theoretically to be crucial factors for the observation of uniaxial anisotropy of {CoII9WV6} clusters.86 The related energy barrier of 34 K, calculated from simple equation:

H = −2J[S Ni1(S W1 + S W2 + S W3 + S W4 + S W5 + S W6) + S Ni2(S W1 + S W2 + S W5) + S Ni3(S W1 + S W2 + S W3) + S Ni4(S W1 + S W3 + S W4) + S Ni5(S W1 + S W4 + S W5) + S Ni6(S W2 + SW 5 + S W6) + S Ni7(S W2 + S W3 + S W6) + S Ni8(S W3 + S W4 + S W6) + S Ni9(S W4 + S W5 + S W6)]

and the same average gav value for all spins. To account for the decrease of χMT below 12 K, the intercluster interaction was introduced in the molecular field approximation. From the fit to 2-R data, the parameters J = +16.2(1) cm−1, gav = 2.14(1), and zJ′ = −0.0012 cm−1 were obtained. The decrease of χMT below 12 K can also be partially caused by zero-field splitting of NiII states. To estimate this effect we measured magnetization M(H,T) at high fields and low temperatures and used these data to fit D, using a single spin approach for S = 12, and Hamiltonian H = DSz2. Two different values, D = 0.066 cm−1 and D = −0.050 cm−1, give almost the same quality of the fit, and it is not possible to obtain clearly the sign of D from the powder magnetization data (Figure S12). This ambiguity may interfere with the obtained value of zJ′, but it does not influence the obtained value of J. The magnitude of J is at the highest limit of the values calculated for the relevant ferromagnetic Ni9W6 clusters, from magnetic data and from DFT calculations. 47,53,80 For comparison, we applied our procedure to the χMT(T) data measured for [NiII9(tmphen)6(CH3OH)6(H2O)6WV6(CN)48]· 6DMF (3).53 The obtained parameters set J = +12.5(1) cm−1, gav = 2.12(1), and zJ = −3.4(2)·10−3 cm−1 conforms well to those calculated by Hilfiger et al.53 The observed difference for 2 and 3 is not dramatic and could be correlated with the structural differences between both clusters. This involves (i) G



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ΔE/kB = |D/kB|(S2 − 1/4) is higher than 19 K obtained from the analysis of ac signals.87 All these features indicate the SMM character of 1-R. Identical characteristics of ac susceptibility were found for 1-S (Figure S15). The addition of dc field shifts the ac signals into the lower temperature range (not shown), and the maxima on χ″M(T) plots cannot be observed, precluding the more precise analysis of slow magnetic relaxation in 1-R/S. Such behavior was also observed in other CoII−WV magnetic low-dimensional materials.43,54

00716, and by the JSPS within Grant-in-Aid for Specially Promoted Research (Grant Number 15H05697). S.C. acknowledges the START Fellowship of the Foundation for Polish Science (2014 edition). M.R. acknowledges the research grant under the “Diamond Grant” program, 0195/DIA/2013/42 of Polish Ministry of Science and Higher Education.

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CONCLUSIONS In summary, we have prepared the first chiral representatives of [M(CN)8]-based polynuclear clusters. They were obtained by the successful implementation of chiral organic ligand into the pentadecanuclear {CoII9WV6} (1-R, 1-S) and {NiII9WV6} (2-R, 2-S) molecules. Realizing the idea of chirality transfer from enantiopure ligand to metal ions, the resulting compounds reveal the natural optical activity in the broad UV−vis range. This is combined with the cyanido-mediated CoII/NiII −WV ferromagnetic coupling leading to the high spin ground state of 15/2 and 12 for 1-R, 1-S, and 2-R, 2-S respectively. As first chiral heterobimetallic clusters, CoII-based 1-R and 1-S show slow magnetic relaxation characteristic of SMM. Because of their chiral structure, 1-R, 1-S, and 2-R, 2-S are promising molecular platform for the observation of magnetochiral dichroism, second-harmonic generation, and ferroelectricity phenomena. These properties, and their potential interactions with the ferromagnetic nature of clusters, are currently under investigation in our laboratory. It would be also interesting to introduce chirality into other members of {MII9[MV(CN)8]6} (MII = Fe, Fe/Co; M = W, Re) clusters family, which have recently been shown by us to reveal extraordinary charge transfer and spin transitions.50−52 The research along this line is now in progress.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization of organic ligands, IR spectra, TGA analysis, structural views, detailed structure parameters, results of continuous shape and chirality measure analyses, additional solid-state and solution UV−vis-NIR absorption spectra, additional magnetic characteristics. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00321.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Marcin Kozieł for PXRD measurements, and Prof. Z. Tomkowicz for the access to NCD spectrometer purchased with the support of European Regional Development Fund within the Polish Innovation Economy Operational Program (POIG.02.01.00-12-023/08). This work was supported by the International Ph.D. Studies Program at the Faculty of Chemistry, Jagiellonian University, within the Foundation for Polish Science MPD Program, cofinanced by EU European Regional Development Fund, by the Polish National Sci. Centre within Project DEC-2011/01/B/ST5/ H



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