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Article pubs.acs.org/IC

Ferromagnetic Polarization: The Quantum Picture of Switching On/ Off Single-Molecule Magnetism Fan Cao,†,‡ Rong-Min Wei,† Jing Li,† Li Yang,† Yuan Han,† You Song,*,† and Jian-Min Dou*,‡ †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210046, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China S Supporting Information *

ABSTRACT: The mixed 3d−4f pentanuclear complex (Bu4N)[MnIII4YIII(shi)4(OAc)4(CH3OH)4]·CH3OH·H2O (1) (H3shi = salicylhydroxamic acid) was synthesized by the direct reaction of Y(NO3)3·6H2O, Mn(OAc)2·4H2O, and H3shi. When an additional ligand, (NHBu3)3[W(CN)8]·2H2O, was added, the mixed 3d−4f−5d hexanuclear complex (Et4N)5[MnIII4YIII(shi)4(OAc)4WV(CN)8](WO4)0.5 (2) was obtained. X-ray crystallographic analysis shows that the 3d− 4f complex 1 represents a 12-metallacrown-4 (12-MC-4) structure, in which the metallacrown ring [Mn−N−O]4 connection captures one YIII ion with four bridging acetate anions, completing the eight-coordinated environment around YIII ion, while four methanol molecules each coordinate to the MnIII ions on the other side of the YIII ion. After octacyanotungstate is introduced, the [WV(CN)8] group substitutes for four methanol molecules of 1 to form complex 2. Magnetic studies indicate the overall antiferromagnetic coupling present within the MC ring of complex 1. However, interestingly, the dominant ferromagnetic coupling between MnIII ions was observed in complex 2. A susceptibility analysis shows that the natural spin alignments in 12-MC-4 metallacrowns are tuned from overall antiferromagnetic to dominant ferromagnetic fashions by magnetic coupling between MnIII ions and the WV ion. Complex 1 [MnIII4YIII] retains an S = 0 ground state, and complex 2 [MnIII4YIIIWV] shows obvious single-molecule magnet (SMM) behavior with an ST = 11/2 ground state, respectively, before and after introduction of the octacyanotungstate group. The spin frustration geometrical structure constructed by four MnIII ions and one WV ion was considered as the key factor for switching on the SMM properties of the 12-MC-4 system.

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INTRODUCTION High-spin transition-metal molecules should serve as excellent building blocks for single-molecule magnets (SMMs). One major problem in constructing a SMM concerns the engineering of intramolecular interactions leading to ferromagnetic coupling. According to the orbital models of the isotropic interactions proposed by Kahn, orthogonalized magnetic orbitals are difficult to achieve due to the defect of localization, so that most of the natural magnetic orbitals are in general nonorthogonal. Thus, pure ferromagnetic coupling is theoretically difficult to obtain, and the coupling values are relatively small. Therefore, great efforts have been made by chemists to synthesize and characterize heterometallic complexes to obtain SMMs showing ferrimagnet-like behavior which possess a nonzero spin ground state. Meanwhile, spin-canting complexes have attracted attention due to their net magnetic moment below a critical temperature caused by antisymmetric exchanges in the dominant antiferromagnetic system provoking weak ferromagnetism. However, both of the two routine magnetic systems would result exclusively in a low-spin ground state because of the dominant even overall antiferromagnetic coupling. © XXXX American Chemical Society

Differing from the routine magnetic coupling system, complexes based on the triangular topology of metal centers are of great interest because of spin frustration. The geometric constraints bearing on the topological arrangement of spins located at the vertices of the triangles result in a frustrated situation such that each spin cannot simultaneously satisfy the intrinsic antiferromagnetic interactions.1 Many different topologies feature geometrical frustration such as triangle, butterfly, tetrahedron, trigonal bipyramid, tetragonal pyramid, and octahedron.2 All of these topologies could be considered as multitriangle configurations: for example, two triangles in a butterfly form, four in a tetrahedron and tetragonal pyramid, six in a trigonal bipyramid, and eight in an octahedron. Even though the nature (antiferromagnetic or ferromagnetic) and magnitude of magnetic exchange interactions between two metal ions are reasonably well understood in terms of the overlap of “magnetic orbitals”, there could be unusual magnetic exchange effects in these geometrically frustrated polynuclear complexes. Received: February 4, 2016

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DOI: 10.1021/acs.inorgchem.6b00255 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry The 9-MC-3 structure, the simplest metallacrown (MC) system possessing a typical triangular topology, has been extensively studied for its volatile magnetic properties: in particular, for MnIII-based MC complexes. These complexes are well-known for large ground-state spin and a planar topology to allow the individual anisotropy of MnIII centers arranged in parallel. Two parameters, the total spin of the ground state ST and zero-field splitting (ZFS) D, were considered as the key factors to enhance the energy barrier for relaxation of SMMs first recognized in 1993.3,4 Considerable efforts have been devoted to the magnetic behaviors of 9-MC-3 MnIII clusters by Pecoraro,5 Hendrickson,6,7 Christou,8−14 Brechin,10−17 Perlepes,9−14,18 and their co-workers. Pecoraro and co-workers reported the first 9-MC-3 metallacrown structure [VO(shi) (MeOH)]3 (shi = salicylhydroxamic acid) in 1989.5 In 2005, Christou and co-workers used 2-pyridyl ketone oxime (mpkoH) as a chelating ligand to synthesize the initial example of a triangular SMM with [Mn3O(O2CMe)6(py)3](ClO4).8 Oxygen-centered triangular [M3O(O2-CR)6L3]n+ (n = 0, 1) complexes were known as non-SMMs due to the dominant antiferromagnetic coupling mediated by the carboxyl group within the [M3O] core, leading to the small S values.19 However, the product [Mn3O(O2CMe)3(mpko)3](ClO4)· 3CH2Cl2 features typical SMM behaviors as a result of switching antiferromagnetic to ferromagnetic coupling due to the “twisting” of the [MnIII3] plane by the oxime ligand. This hypothesis has been proved through the use of oxime ligands with “bulky” R-saoH2 groups (R = Me, Et, Ph) tuning the Mn− N−O−Mn torsion angle, which is closely related to the magnetic behaviors of the hexanuclear dimer 9-MC-3 SMM [MnIII6O2(sao-R)6(O2C-R′)2(EtOH)x](R = H, Et, R′ = Ph, Ph(Me)2, x = 4, 6).10,11,20 The complex [MnIII6O2(sao)6(O2CPh)2(EtOH)4] was characterized by an S = 4 spin ground state as a result of the ferromagnetic exchange between two antiferromagnetically coupled [MnIII3] units.20 However, the complex [MnIII6O2(Etsao)6(O2CPh)2(EtOH)4(H2O)2]·2EtOH does indeed display ferromagnetic exchange (S = 12 ground state) according to the structural distortion enforced by the steric bulk of the ethyl group on the ligand.10 Furthermore, the replacement of benzoate with 3,5-dimethylbenzoate produced the analogue [MnIII6O2(Et-sao)6(O2CPh(Me)2)2(EtOH)6], the Mn−N−O− Mn torsion angles of which increased further from an average of αv = 36.5° to 39.1°.11 An investigation by Christou and coworkers showed that the two analogous [MnIII3]2 complexes afforded the effective energy barriers Ueff = 53.1 and 86.4 K with a theoretical limit of S2|D| = 89 K. They ascribed the major reason for this obvious difference in magnetic properties to the coupling constant J value of the [MnIII3] unit, in which the larger torsion angles correspond to the larger J values and could result in the depopulation of low-lying exited states. The structural distortion of 9-MC-3 MnIII clusters is of profound interest in the design of SMMs for the controllability of “switching on” the ferromagnetic coupling. By investigating multitriangle-based complexes from a spin-frustration perspective, Henderickson and co-workers suggested that making relatively minor changes in the exchange-coupled complexes could significantly alter the electronic structure of a complex in order to change the ground state.6,7,21−25 Recently, fully ferromagnetic polarization has been achieved by MnIV···MnIV interactions in a triangle topology with a J/J′ ratio of 3.75, much larger than the critical value 2 in the [MnIV3] system.26

Considering the high-nuclearity geometric frustrated framework, the 12-MC-4 configuration is a suitable candidate to construct a amultitriangle-based skeleton by encapsulating one paramagnetic ion in the MC ring. The first 12-MC-4 MnIII cluster, {MnII[MnIII(shi)]4(OAc)2(DMF)6}·2DMF, featured dominant antiferromagnetic exchange interactions, as reported by Pecoraro and co-workers.27,28 ac susceptibility results showed the very weak frequency-dependent behavior. However, the analogous complexes Li{Li(Cl)2[12-MCMn(III)shi-4]} and (NHEt3)2[Mn3Ca(O2CPh)4(shi)4] with LiI and CaII replacing the central MnII have an S = 0 ground state due to overall antiferromagnetic interactions between the ring MnIII ions.28−30 The origin of the SMM behavior could be attributed to both the center metal ion itself and spin frustration raised by the competition between two coupling interactions J and J′. Very recently, the first application of magnetic Compton scattering (MCS) to molecular material was performed on the monomer [Mn4Ln2] complexes to investigate the spin structure.31 The dc data of [Mn4Y2] were simultaneously fitted considering the S = 0 ground state due to very strong antiferromagnetic coupling between MnIII ions. However, the direct MnIII···MnIII coupling was proved by magnetic Compton profiles (MCP) in [Mn4Gd2], which the enhancement of exchange interaction between the MnIII ions could be attributed to their same magnetic coupling conditions, with the inner GdIII ion providing competition between MnIII···GdIII and MnIII··· MnIII interactions. Even so, the detailed frustration-related magnetochemistry studies on complexes of higher nuclearity such as tetragonal pyramids have always been a challenge due to the increased complexity in theoretical treatments of large spin systems. The synthesis and characterization of a tetranuclear manganese complex and its derivative, (Bu4N)[MnIII4YIII(shi)4(OAc)4(CH3OH)4] (1) and (Et 4 N) 5 [Mn III 4 Y III (shi) 4 (OAc) 4 W V (CN) 8 ](WO 4 ) 0.5 (2), prompted us to extend this work to analyze the total spin ground state (ST) together with the accurate ground state of the substructure (Ssquare) (Ssquare represents the spin state of the square substructure in a tetragonal pyramid) for the first time. Herein, we report the synthesis, X-ray single-crystal structure, and magnetic properties with frustration analysis of complexes 1 and 2.

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EXPERIMENTAL SECTION

Materials and Syntheses. All reagents were obtained from commercial sources and were used as received, without further purification. All reactions were carried out under aerobic conditions. (NHBu3)3[W(CN)8]·2H2O was prepared according to the literature.32,33 Caution! Although no problems were encountered in the preparation of the following complexes, under the acidic reaction conditions, suitable precautions should be taken when handling potentially poisonous compounds. It is of the utmost importance that all preparations should be performed and stored in well-ventilated areas. Synthesis of (Bu4N)[MnIII4YIII(shi)4(OAc)4(CH3OH)4] (1). Y(NO3)3·6H2O (0.0192 g, 0.05 mmol) was added to a stirred solution of Mn(OAc)2·4H2O (0.0735 g, 0.3 mmol) and salicylhydroxamic acid (0.0306 g, 0.2 mmol) in MeOH (15 mL). After 10 min, [(C4H9)4N]OH (0.0519 g, 0.2 mmol) was added to adjust the pH value. The solution immediately became green. After continuous stirring at room temperature for an additional 10 min, the solution turned deep green. After 30 min with stirring, the solution was left undisturbed at room temperature for crystal growth. Black lamellate single crystals, suitable for X-ray diffraction analysis, were obtained B


Article

Inorganic Chemistry Table 1. Crystallographic Data and Structure Refinement Details for Complexes 1 and 2 formula Mr cryst size (mm) color cryst syst space group T (K) a (Å) b (Å) c (Å) V (Å3) Z ρcalcd (Mg/m3) λ (Å) μ (mm−1) F(000) no. of collected/unique rflns Rint GOF on F2 R1(I > 2σ(I)); wR2(all data)

complex 1

complex 2

C60H98Mn4N5O29Y 1662.10 0.30 × 0.24 × 0.13 black tetragonal P4/n 123(2) 17.588(5) 17.588(5) 11.615(4) 3593.2(19) 2 1.536 0.71073 1.568 1728 12632/3172 0.1059 1.090 0.0837; 0.2341

C172H262Mn8N36O44W3Y2 4707.07 0.32 × 0.21 × 0.11 red tetragonal I4/m 123(2) 14.9060(8) 14.9060(8) 52.534(3) 11672.4(11) 2 1.803 0.71073 2.447 4796 40292/6835 0.0454 1.043 0.0511; 0.1681

after 2 weeks. These crystals were collected by filtration, washed, and protected by mother liquor. Yield: 8 mg (9.6%, based on Y(NO3)3· 6H2O). Anal. Calcd for C60H98Mn4N5O29Y: C, 43.32; H, 5.90; N, 4.21. Found: C, 42.92; H, 5.61; N, 4.36. IR (KBr, cm−1): 3405 (m), 2965 (w), 1599 (w), 1574 (vs), 1435 (m), 1396 (m), 1317 (m), 1257 (m), 1156 (w) 1033 (w), 937 (m), 868 (m), 755 (w), 691 (s), 619 (s), 484 (m). Synthesis of (Et4N)5[MnIII4YIII(shi)4(OAc)4WV(CN)8] (WO4)0.5 (2). Y(NO3)3·6H2O (0.0192 g, 0.05 mmol) was added to a stirred solution of Mn(OAc)2·4H2O (0.049 g, 0.2 mmol) and salicylhydroxamic acid (0.0459 g, 0.3 mmol) in DMF/MeCN (10 mL with 1/4 volume ratio). After 10 min, [(C2H5)4N]OH (0.0441 g, 0.3 mmol) was added to adjust the pH value. The solution immediately became deep red and was stirred for 30 min at room temperature. Then, (Bu3NH)3[W(CN)8]·4H2O (0.0494 g, 0.05 mmol) was added to the solution and the solution turned dark red. After it was stirred for 6 h, the solution was filtered and was layered with diethyl ether in a test tube in the dark at room temperature. The dark red lamellate single crystals, suitable for X-ray diffraction analysis, were obtained after 2 weeks. These crystals were collected by filtration, washed with cold diethyl ether, and dried in air. Yield: 16 mg (13.6%, based on Y(NO3)3·6H2O). Anal. Calcd for C172H262Mn8N36O44W3Y2. C, 43.85; H, 5.57; N, 10.71. Found: C, 44.15; H, 5.42; N, 10.32 IR (KBr, cm−1): 3424 (m), 2984 (m), 2360 (w), 2139 (w), 1598 (s), 1573 (vs), 1517 (m), 1434 (s), 1390 (s), 1321 (m), 1257 (m), 1172 (w), 1156 (w), 1029 (m), 937 (s), 865 (m), 779 (m), 759 (m), 684 (s), 648 (s), 612 (s), 482 (m). X-ray Crystallography. Single-crystal X-ray diffraction data for complexes 1 and 2 were collected on a Bruker Smart CCD areadetector diffractometer with Mo Kα radiation (λ = 0.71073 Å) by the ω-scan mode operating at room temperature. The program SAINT was used for integration of the diffraction profiles, and semiempirical absorption corrections were applied using SADABS.34,35 All of the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL.36 Metal ions were located from the E maps, and the other non-H atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F2. Generally, C-bound H atoms were determined theoretically and refined with isotropic thermal parameters riding on their parents. H atoms of water and MeOH were first located by difference Fourier E maps and then treated isotropically as riding. Further details of crystallography and selected bond parameters are given in Tables 1 and 2, respectively. Crystallographic data for the structural analysis of

Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes 1 and 2a Complex 1 Mn1−O1 Mn1−O3 Mn1−O5 Y1−O2 Mn1−O2−Y1 W1−C9 Y1−O3 Mn1−O1 Mn1−O4 Mn1−N1 Mn1−O2−Y1

1.837(5) 1.921(5) 2.117(5) 2.364(5) 121.8(2) Complex 2 2.147(5) 2.286(3) 1.860(3) 1.957(3) 1.979(3) 120.32(12)

Mn1−O2 Mn1−N1 Mn1−O6 Y1−O4

W1−C8 Y1−O2 Mn1−O2 Mn1−O5 Mn1−N3 Mn1−N3−C9−W1

1.889(5) 1.952(6) 2.511(8) 2.273(5)

2.177(6) 2.383(3) 1.899(3) 2.165(4) 2.456(5) 1(7)

Symmetry code for complex 1: (A) −y + 1/2, x, z; (B) −x + 1/2, −y + 1/2, z; (C) y, −x + 1/2, z; (D) y − 1/2, −x + 1, −z + 1; (E) −x + 1/2, −y + 3/2, z; (F) −y + 1, x + 1/2, −z + 1. Symmetry code for complex 2: (A) −x, −y, z; (B) −y, x, z; (C) y, −x, z; (D) y, −x + 1, −z + 1; (E) −y + 1, x, z; (F) −x + 1, −y + 1, −z + 1; (G) x, y, −z + 1; (H) −x, −y + 1, z; (I) y − 1/2, −x + 1/2, −z + 1/2; (J) −y + 1/2, x + 1/2, −z + 1/2. a

the compounds have been deposited with the Cambridge Crystallographic Data Centre, under CCDC nos. 1049031−1049032. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (http://www. ccdc.cam.ac.uk; fax +44-1223-762911). Physical Measurements. Elemental analyses were carried out by standard microanalytical methods. The infrared spectra were recorded on a Shimadzu FT-IR 8200PC spectrometer with a paraffin suspension in the 4000−400 cm−1 region. Magnetic measurements on crystalline samples were carried out at an applied field of 0.3−5 kOe on a Quantum Design MPMS-XL7 superconducting quantum interference device (SQUID) magnetometer working in the temperature range of 300−1.8 K. The molar magnetic susceptibilities were corrected for diamagnetism estimated from Pascal’s tables and for the sample holder by previous calibration. C


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Inorganic Chemistry

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RESULTS AND DISCUSSION Synthesis Strategy. A metallacrown is a comparable crown-configuration ring system composed solely of transition metals and other heteroatoms. Similar to ordinary 12-crown-4 structures, the selectivity is the most striking feature of 12-MC4 for capturing specific ions in the central “pocket”. However, the MC-ring center is suitable for a broad range of metal ions involving alkali metals, transition metals, and rare-earth metals.27,29,30,37 Because of the antiferromangetic coupling between metal ions in 12-MC-4 complexes, the fifth captured ion in the central “pocket” is always located at a frustrated site. This is just the precondition that establishes the spin frustration system for investigating the interesting magnetic properties. Methanol was chosen to synthesize the 12-MC-4 square plane because the solvent molecule could coordinate to metal ions and be replaced easily by other ligands. The central ion YIII is diamagnetic to keep the overall antiferromagnetic interactions within the MC ring between MnIII ions, and the triple valence is beneficial for charge balance of the whole structure. Octacyanometalate is a sensible paramagnetic chelating ligand for the outstanding magnetic exchange character and variable spatial configuration such as square antiprism (D4d), dodecahedron (D2d), and bicapped trigonal prism (C2v). Replacement of methanol by acetonitrile promoted the coordination of octacyanometalate. A small amount of DMF was used to construct the 12-MC-4 building block for the first step. Quaternary ammonium hydroxide could deprotonate the ligand as well as provide positive charge for the entire molecule. Crystal Structure Descriptions of Complexes 1 and 2. Single-crystal X-ray structure analysis reveals that complex 1 crystallizes in the space group P4/n, presenting the typical 12MCMnIII(N)shi-4 framework. A C4 axis is located in the MC central cavity, which produces a Mn−N−O repeating unit that occurs four times to complete a square geometry. As shown in Figure 1, four MnIII ions are chelated by four completely

Complex 2 crystallizes in the space group I4/m with a C4 axis in the MC central cavity. The main configuration of the MC frame is very similar to that of complex 1. The central YIII deviates 1.464 Å from the oxime oxygen mean plane (OoxMP) and 1.132 Å from the corresponding acetate oxygen mean plane (OacMP), providing a 0.332 Å displacement away from the metallacrown ring toward the acetate oxygen atoms, which means that the YIII ions in complexes 1 and 2 are nearly in the same distorted-square-antiprismatic geometry. The separation MnIII···MnIII (4.639 Å) or that between the YIII ion and MnMP (1.851 Å) in 2 are almost same as those in 1. However, the Mn−N−O−Mn torsion angle is 169.12(2)°, which is 4.59° larger than that in complex 1. All of these structural information demonstrate that the MC ring of complex 2 is more coplanar than complex 1. Obviously, the introduced octacyanotungaste group [WV(CN)8]3− instead of methanol molecules on the concave side (opposite side of YIII ion) of the MC ring plays an important role. [WV(CN)8]3− adopts a square-antiprismatic geometry, in which the WV ion is located on the C4 axis. Thus, the sites of seven other CN groups are completely the results of symmetrical operations for C1N1 in crystallography. WV−C− N is fairly linear, and the bond angle is 175.9(5)°, but C−N− MnIII exhibits a somewhat large bend for a bond angle of 142.08(5)°, which is attributed to the greater spatial repulsive effect from the MC-ring skeleton. Three tetraethylammonium ions and a half tungstate anion from decomposition of octacyanotungstate are in the unit cell as counterions. We should note that the other two tetraethylammonium ion positions are disordered over the C4 rotation axis and could not be refined suitably. Thus, the program SQUEEZE was used to calculate the solvent disorder area and remove its contribution to the overall intensity data.38 The molecular weight was calculated with an integral formula which could be proved by inductively coupled plasma atomic emission spectrometry (ICP-AES), elemental analysis (EA), and infrared spectroscopy (IR). Upon careful contradistinction of the two structures, it is noted that the main difference between 1 and 2 is the planarity of the MC planes before and after introducing the octacyanotungstate group. Considerable literature concerning magnetic studies on MCs have proved that the planarity of the MC ring is the principal structural factor dominating the magnetic properties of MC complexes. Brechin and co-workers summarized numerous MnIII3-MC complexes and found a “magic area” for the Mn−N−O−Mn torsion angles of less than 1° which leads to the exactly distinct characters of magnetic coupling, which also appears in 14-MC-5 complexes.16,39−41 Thus, the structural difference could give rise to the different magnetic properties. The continuous shape measures (CShMs) relative to the ideal octahedron calculated using the program SHAPE 2.142 were 1.234 (MnO5N) and 1.121 (MnO4N2) for the MnIII centers of 1 and 2, respectively. Both of the values are markedly deviated from Oh symmetry because of JT effects which generate a D4h symmetry. Moreover, a slight rhombic distortion (C2v) occurred in both complexes which was calculated as 1.350 (MnO2O3O6N1) for 1 and 1.192 (MnO2O4N1N3) for 2 (Figure 2). These distortions can result in the magnetic anisotropy of MnIII centers. Magnetic Properties. dc Magnetic Properties. Directcurrent (dc) magnetic susceptibility measurements were performed on polycrystalline samples of complex 1 under an

Figure 1. Single-crystal X-ray structure of complex 1 (left) and complex 2 (right). The hydrogen atoms, lattice counterions, and solvent molecules have been omitted for clarity. Color scheme: green, YIII; pink, MnIII; red, oxygen; blue, nitrogen; gray, carbon.

deprotonated shi3− ligands to build up the primary MC frame, while four carboxylate groups and four methanol molecules coordinate to the MnIII ions from both sides of the ring, respectively. In the equatorial plane, four shi3− ligands adopt a μ3-η1:η1:η2:η1 mode to connect four MnIII ions, forming a 12MC-4 cyclic unit through the [Mn−N−O] connection with the Mn−N−O−Mn torsion angle 164.53(4)°. The oxidation states of Mn ions were deduced from charge balance considerations, which was further confirmed by bond valence sum (BVS) calculations (see Table S1 in the Supporting Information). All MnIII ions exhibit a distorted six-coordinate octahedral geometry with the Jahn−Teller (JT) apical axes. D


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Inorganic Chemistry

To determine the magnetic exchange interactions in complex 1, the magnetic data over the entire temperature range were fitted using the PHI program43 on the basis of the [MnIII4] unit. As shown in Figure 3, the geometrical model of the 12-MC-4 (Mn4) cluster on the basis of its structure was assumed to be a perfect square. J is the exchange constant between the nearest ring MnIII centers (J = J12 = J23 = J34 = J14). This leads to the Heisenberg−Dirac−van Vleck spin Hamiltonian Ĥ iso = −2J(S1̂ Ŝ 2 + Ŝ 2Ŝ3 + Ŝ3S4̂ + S1̂ S4̂ )

The eigenvalues E|S⟩ of this spin Hamiltionian are given by

Figure 2. View of the coordination sphere of MnIII in complexes 1 (left) and 2 (right) of the displacement of the MnIII ion from the center of the polyhedron that surrounds the metal. Green denotes the original octahedra in D4h symmetry, and blue denotes the C2v distorted fragments.

E|ST , S13 , S24 = −JST(ST + 1) + JS13(S13 + 1) + JS24(S24 + 1)

obtained analytically by using the Kambe method with the substitutions ST̂ = Ŝ1 + S2̂ + S3̂ + S4̂ , Ŝ13 = S1̂ + S3̂ , Ŝ24 = S2̂ + Ŝ4, where ST is the total spin of the molecule and E|ST,S13,S24⟩ is the energy of state ST obtained from given S13 and S24. The best parameters obtained by the PHI program with total Hamiltonian Ĥ total = Ĥ iso + Ĥ Zeeman finds that J = −2.88 cm−1, g = 1.96, zj′ = 0.007 cm−1, and R = 0.23 (Ĥ Zeeman = μBgMn(Ŝ1 + S2̂ + S3̂ + Ŝ4)H, where μB is the Bohr magneton, H is the applied 2 field, and R = ∑i points = 1 (χexptl − χcalcd) ). This result indicates the overall antiferromagnetic coupling within the MC ring, and an analogous situation can be found in many 12-MC-4 complexes.5,27−29,44 Similarly, the S = 0 ground state was observed at low temperature when the central ions were diamagnetic, such as LiI, NaI, KI, and CaII.5,27−29,44 That is to say, antiferromagnetic coupling is the usual nature between ring metal ions in standard 12-MC-4 complexes. The dc magnetizations of complex 1 were measured in the ranges of magnetic field from 1 to 7 T at 1.8 K. An M vs H plot also shows the typical overall antiferromagnetic behavior with a steady linear increase within the whole field range (Figure S2 in the Supporting Information). However, a magnetic phenomenon very different from that of complex 1 was observed in complex 2. At room temperature, the χMT value of 2 is 9.91 cm3 K mol−1, lower than the expected value of 12.375 cm3 K mol−1 for four MnIII and one WV uncoupled ions with Landé factor g = 2. The lower roomtemperature χMT value is mainly ascribed to the antiferromagnetic coupling between MnIII and MnIII ions (the evidence from complex 1), and MnIII and WV ions.45−48 The 5d electrons of the WV ion show a far radial distribution much larger than that of 3d ions such as MnIII; therefore, the cyanide ligand can mediate antiferromagnetic coupling between the WV ion and other transition-metal ions even above room temperature. Namely, if a complex exhibits ferrimagnetic behavior, the minimum in the plot of χMT vs T is always above 300 K.49−51 Upon cooling, the χMT value increases steadily as the temperature is lowered to reach a peak value of 16.50 cm3 K mol−1 at 3 K representing a S = 11/2 ground state, surprisingly, which is obviously dominated by ferromagnetism rather than antiferromagnetism. A sharp decrease was found below 3 K which was mainly caused by Zeeman splitting from the applied field and ZFS. Quantum Picture of Spin Frustration within the [MnIII4YIIIWV] Tetragonal Pyramid. It should be an intricate magnetic phenomenon that overall antiferromagnetic interactions induce an intermediate ferrimagnetic ground state. Several examples have been discussed in simple topologies such as triangular and butterfly by Hendrickson, Christou, and co-

applied field of 2 kOe and of complex 2 under 0.3, 0.5, 0.8, 1, 1.5, 2, 2.5, 3, 4, and 5 kOe in the temperature range of 1.8−300 K. The χMT vs T plots are shown in Figures 3 and 4 (and Figure S1 in the Supporting Information) for complexes 1 and 2, respectively. χM is the molar magnetic susceptibility per [MnIII4YIII] or [MnIII4YIIIWV] unit.

Figure 3. Geometrical model of complex 1 (left) and temperature dependence of χMT at 2 kOe, where the solid line is the best-fit curve (right).

Figure 4. Geometrical model of complex 2 (left) and temperature dependence of χMT in 0.8 kOe, where the solid line is the best-fit curve considering local anisotropy of MnIII (right).

The χMT value at room temperature of complex 1 is 10.21 cm3 K mol−1, slightly lower than the expected value of 12 cm3 K mol−1 for four MnIII uncoupled ions with Landé factor g = 2. In most of the MnIII-based complexes, the room-temperature effective magnetic moment is lower than the theoretical value. As a matter of fact, the Landé factor of MnIII is lower than 2. When the temperature is lowered, the value decreased steadily. Below 50 K, the value decreases rapidly and reaches 0.19 cm3 K mol−1 at 1.8 K, which corresponds to a zero ground state. Such behavior indicates the overall antiferromagnetic coupling between the metal centers. E


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Inorganic Chemistry workers.21,23,52 Kahn has concluded that the antiferromagnetic interactions could be polarized in a ferromagnetic fashion: namely, the ferromagnetic polarization takes over when the J′/J ratio reaches a special value in a spin frustrated system with two coupling constants J′ and J. However, a theoretical analysis of ideal larger multitriangle-topology polynuclear systems such as the [MnIII4YIIIWV] tetragonal pyramid of complex 2 has not been carried out. In order to probe the magnetic properties of complex 2 clearly, a preliminary estimate of exchange interactions by neglecting the local anisotropy of MnIII ions is necessary; therefore, the isotropic Heisenberg−Dirac−van Vleck spin Hamiltonian describing the exchange interactions within a [MnIII4YIIIWV] tetragonal pyramid topology is given as Ĥ iso = −2J ′(S1̂ S5̂ + Ŝ 2S5̂ + Ŝ3S5̂ + S4̂ S5̂ ) − 2J(S1̂ Ŝ 2 + Ŝ 2Ŝ3 + Ŝ3S4̂ + S1̂ S4̂ )

where J′ denotes the WV···MnIII coupling between the “point” WV and “square” MnIII, and J denotes the MnIII···MnIII coupling within the “square”. The corresponding eigenvalues E|S⟩ are given by E|ST , Ssquare , S5 , S13 , S24 = −J ′ST(ST + 1) − (J − J ′)Ssquare(Ssquare + 1) + J ′S5(S5 + 1) + J ′S13(S13 + 1) + JS24(S24 + 1) Figure 5. Energy distribution plots of [MnIII4YIIIWV] with J′/J ratio for the (F,AF) combination of J′ and J (top) and the (AF,AF) combination (bottom). Ground states are labeled as |ST,Ssquare⟩.

obtained analytically by using the Kambe method with the substitutions ŜT = S1̂ + S2̂ + S3̂ + S4̂ + S5̂ , Ŝsquare = Ŝ1 + Ŝ2 + Ŝ3 + ̂ = S1̂ + S3̂ , S24 ̂ = S2̂ + S4̂ , where ST is the total spin of Ŝ4, and S13 the molecule and E|ST,Ssquare,S5,S13,S24⟩ is the energy of state ST obtained from given Ssquare, S13, and S24 values. After the substitution of the spin states into the eigenvalues E|S⟩, 165 degenerate energy states are deduced. With so many different states, in order to avoid a possible mistake in identifying the ground state, the accuracy of the obtained J and J′ values is a matter only of fitting. There are four possible combinations of sign for J′ and J, in the format (J′,J) of (F,F), (F,AF), (AF,F), and (AF,AF). Two of them do not lead to spin frustration because J and J′ are not competing: (F,F) leads to a |17/2,8⟩ ground state in the format |ST,Ssquare⟩, and the maximum χMT value corresponds to 40.375 cm3 K mol−1, while (AF,F) leads to a |15/2,8⟩ ground state with a value of 31.875 cm3 K mol−1. Neither of the two values coincides with the observed peak value from χMT vs T plots in Figure 4. Moreover, the coupling in MC complexes is overall antiferromagnetic in nature, as proved by complex 1. Consequently, the (F,AF) and (AF,AF) combinations lead to competition between J′ and J to involve the intermediate spin ground state value 11/2 deduced from the experimental data. In such a case, the resulting spin alignments and ground state become sensitive to the J′/J ratio. Both the (F,AF) and (AF,AF) combinations are induced to calculate the energy distribution plots of all the spin states of [MnIII4YIIIWV] as a function of J′/J, as shown in Figure 5. The multicrossed energy value followed by the changing J′/J ratio value demonstrates the sensitivity of the ground state. From the energy distribution plots we can conclude that the ground state value increases proportionately with the J′/J ratio value in a large range from 0 to 20, which implies that a much stronger J′ is able to ferromagnetically polarize the spins from an antiferromagntic to a ferromagnetic arrangement. Similarly,

we can use these energy distribution plots easily to predict that the J′/J value ranges from −12 to −10 or 12 to 14 in complex 2 on the basis of the χMT peak value in dc magnetic susceptibility at low temperature. Considering the crossed energy levels that could be influenced by Zeeman perturbation under a relatively small field, a series of different fields were applied from 0.3 to 5 kOe for performing the variable-temperature magnetic properties measurement of 2 (Figure S1 in the Supporting Information). The data in the high-temperature range corresponding to paramagnetism under different fields are very close. However, the ferromagnetism in the low-temperature range shows different χMT peak values and peak temperatures Tp (Table S2 in the Supporting Information). Under small applied fields the Tps are all located at about 3 K, indicating that the energy H/kT is small enough and the van Vleck formula with the approximations of magnetization can be used for treating the variable-temperature magnetic data. In the high-field range from 1.5 to 5 k Oe, the Tps increase from 3.5 to 6.0 K with the applied fields because of the magnetization is no longer linear. We note that the van Vleck fomula should be carefully used in the treatment of magnetic data in the high-field range. For the PHI program, the inclusion of the Zeeman Hamiltonnian allows the magnetic properties to be calculated from firstprinciples without resorting to perturbation theory so that the fitting processes in all applied fields are suitably performed over the entire temperature range. The data at 0.8 kOe were selected as the relatively accurate data because the χMT peak value at 0.8 kOe reaches a maximum in comparison to other fields due to the depopulation of the microstates of the ground state below 0.8 kOe such that all energies En are linear in an applied field. F


Article

Inorganic Chemistry To understand the real ground state of complex 2, the fitting processes by the PHI program were performed in both (F,AF) and (AF,AF) modes. With gW fixed to 2.0, the best parameters were obtained to be J′ = 7.75 cm−1, J = −0.68 cm−1, gMn = 1.75, and R = 2.38 for the (F,AF) fitting mode (Figure S3 in the Supporting Information) and J′ = −15.05 cm−1, J = −1.19 cm−1, gMn = 1.89, and R = 9.38 for the (AF,AF) mode (Figure S1 in the Supporting Information). Obviously, the latter parameters for the (AF,AF) mode are reasonable, which make physical sense for the agreeable g value and WV−CN−MnIII exchange constant J′ value, which is usually antiferromagnetic.45−48 In comparison to complex 1, the MnIII···MnIII exchange value in 2 is slightly smaller. The direct influences are the different Mn−N−O−Mn torsion angles 164.53° for 1 and 169.12° for 2, which are generally the primary factors of magnetic coupling in metallacrown complexes. Corresponding to these parameters, the spin ground state of complex 2 is S = 11/2, while the closest excited states are S = 9/2 and 13/2 with energy differences of 1.10 and 1.50 cm−1, respectively. The next set of excited states then lie at 4.79 and 5.60 cm−1 above the ground state with S = 11/2 and 5/2. The lowest 10 low-lying energy levels corresponding to the J′/J = (−15.06)/(−1.19) = 12.66 deduced from the energy distribution plots are identical with the level parameter fitting from the PHI program (Figure 6 and Figure S5 in the Supporting Information). This proves the significance of the predictable energy distribution plots to provide a guidance in the design of SMMs.

The quantum picture is crucial in understanding the ground state of the spin-frustrated system, for which the classical picture is not sufficient. In view of the JT effect with C2v distortion of MnIII, the ZFS parameters DMn should be included in the fitting. The local axial ZFS spin Hamiltonian is given by 2 Ĥ zfs = DMn[Siẑ − Si(Si + 1)/3]

where Siẑ 2 is the easy-axis spin operator of each center with i = 1−4. Thus, the total Hamiltonian can be written as Ĥ total = Ĥ iso + Ĥ zfs + Ĥ Zeeman

The Zeeman term is Ĥ Zeeman = μB(gMnŜ1 + gMnS2̂ + gMnS3̂ + gMnS4̂ + gWS5̂ )H. It is noted that the JT elongated axes for the MnIII ions in the “square” cluster are canted by an angle of 14.92° to the vertical 4-fold rotation axis and by a 90° angle in the plane of the four metals to each other. Then the noncolinearity of the axial directions for each MnIII ion should be described by the sets of Euler angles, {α,β,γ}: {0,14.92,0}, {90,14.92,0}, {180,14.92,0}, and {270,14.92,0}, respectively (Figure S4 in the Supporting Information). These Euler angles were fixed in the fitting process to adjust the ZFS parameters of MnIII. The fitting process with ZFS parameters gave J′ = −21.09 cm−1, J = −1.77 cm−1, gMn = 1.90, DMn = −3.25 cm−1, zj′ = −0.006 cm−1, and R = 0.7 with fixed gW = 2.0. The fitting curve clearly shows that the results with ZFS in Figure 4 are much more agreeable with experimental data in comparison to those without ZFS in Figure S1 in the Supporting Information. It is worth noting that, to preclude unreasonable results due to possible overparametrization, the J′, J, and gMn values of exchange-only fitting with the (AF,AF) mode were used as starting values. Meanwhile, the fittings with ZFS parameters and Euler angles were carried out in a single process such that the susceptibility and magnetization data were simultaneously fitted (the total residual R is the sum of each data set including χ and M). The dc variable-field magnetization of complex 2 was measured in the ranges of magnetic field from 1 to 7 T and temperature from 1.8 to 20 K. As shown in Figure 7, different from the case for 1, complex 2 shows a quick increase of magnetization in low fields at low temperatures and then a slow and linear increase without clear saturation at 7 T. The linear increase in the curves is caused by the nearly populated lowlying states which would cross by each other under applied field to induce Zeeman splitting. The giant spin approximation (GSA) was used by the PHI program to fit M−H data with a set spin value S = 11/2. The corresponding Hamiltonian is given as

Figure 6. Energy spectra of lowest lying states as deduced from the zoomed energy distribution plots for complex 2.

It is worth mentioning that the energy distribution plots offer us the spin ground state E|11/2,6,1/2,4,4⟩ in E| ST,Ssquare,S5,S13,S24⟩ format, from which the spin state Ssquare = 6 of square substructure can be observed. Thus, we can conclude that the spin of 12-MC-4 was polarized from S = 0 to S = 6. Such a prominent change is very rare in SMMs, which would supply numerous possibilities to “switch on” the SMM behavior of the original antiferromagnetic coupling systems by constructing a frustrated system through the ligand and paramagnetic centers featuring stronger magnetic coupling. In addition, the energy distribution plots are indispensable in predicting and analyzing the magnetic properties of the spinfrustrated system. The S = 6 intermediate-spin state for an [Mn4] square is neither a parallel nor antiparallel arrangement, pointing to ferromagnetic S = 8 and antiferromagnetic S = 0 in the classical picture. Actually, the quantum spin can have discrete values from 0 to 8 for Ssquare, as illustrated in Figure 5.

2 Ĥ = D[Sẑ − S(S + 1)/3] + μB gMn ŜH

where D is the axial anisotropy of giant spin S = 11/2. The parameters g = 1.85 and D = −0.36 cm−1 were obtained from fitting. Obviously, the result of GSA slightly deviates from the data under high applied field. In order to understand this unusual phenomenon, we learn from the Brillouin function that the Zeeman effect may provide important information. The Zeeman splitting energy distribution plots was simulated by the PHI program (Figure 8), from which it is easy to find that the ground-state total spin is fielddependent. |11/2,6⟩ is the ground state with the field ranging from 0 to 1.75 T, while |13/2,7⟩ becomes the ground state until G


Article

Inorganic Chemistry

Figure 9. Plots of the in-phase (χ′M) and out-of-phase (χ″M) signals for complex 2 in ac susceptibility studies vs T in a 5.0 Oe field with a zero dc field. The solid lines are guides for the eyes.

Figure 7. Plots of the field-dependent magnetization of complex 2 at the indicated temperatures with all fields 1−7 T. The solid lines are the best-fit curves with ST = 11/2 by giant spin approximation (GSA) (top) and combined fitting process with susceptibility (bottom).

which represents the SMM behavior. The χ″M signals started to become frequency-dependent below 3 K, which points to the peak value representing the ground state found in susceptibility data. The maxima of these signals, however, were not observed above 1.8 K, the lowest limit of the instrument. Consequently, the energy barrier Ueff and τ0 cannot be determined via the conventional Arrhenius plot method. Another method, recently employed by Bartolomé, is to assume that there is only one characteristic relaxation process of the Debye type with one energy barrier and one time constant.53 With this assumption, one obtains the relation ln(χ ″M /χ ′M ) = ln(ωτ0) + Ea /kBT

From this expression and by plotting ln(χ″M/χ′M) vs 1/T at the different frequencies (Figure 10), one can perform linear regressions to obtain the gradients Ea/kB and intercept ln(ωτ0) and then extract an estimate of the activation energy and the characteristic time. For complex 2, the parameters are Ueff ≈ Ea = 17.8(1) K and τ0 = [2.68(1)] × 10−8 s. Because the slow relaxation of magnetization is experimentally observed only over a short range of temperature and no maximum of χ″M is observed at the temperatures technically available, the estimation of these characteristic parameters might not be very accurate, but τ0 is consistent with the expected values (10−6−10−12 s) for an SMM, and the energy barrier conforms to the theoretical deduction from the fitting data. Thus, the ac data indicate complex 2 to be a new example of mixed 3d/4f/ 5d single-molecule magnets.

Figure 8. Zeeman splitting of the lowest mS states for the lowest total spin states from 0 to 7 T.

H ≈ 4.75 T and is crossed by the |15/2,8⟩ state. When the applied field is larger than 1.75 T, the GSA fitting model with a fixed ST = 11/2 is unreasonable. To obtain more accurate fitting results, we fitted the M−H data in combination with χMT−T data simultaneously in a single process, as discussed above. ac Magnetic Properties. To probe the dynamics of magnetization relaxation of complex 2, ac susceptibility measurements were performed in a zero applied dc field with a 5.0 Oe ac field oscillating at frequencies in the range of 1− 1500 Hz and in the temperature range of 1.8−5.0 K (Figure 9). These measurements avoid Zeeman and other effects of a large applied field because of the small ac field. Both in-phase χ′M and out-of-phase χ″M signals show frequency dependence

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CONCLUSION In summary, the utilization of octacyanotungstate in reactions with the 12-MC-4 system results in the formation of a new 3d− H


Article

Inorganic Chemistry Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program (2013CB922102), the National Natural Science Foundation of China (21571097 and 21271097), the Natural Science Foundation of Jiangsu Province of China (BK20130054), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Figure 10. ln(χ″M/χ′M) vs T−1 plots for complex 2 at different frequencies of the 5.0 Oe ac field. The solid lines are the best-fit curves.

(1) Aidoudi, F. H.; Aldous, D. W.; Goff, R. J.; Slawin, A. M. Z.; Attfield, J. P.; Morris, R. E.; Lightfoot, P. Nat. Chem. 2011, 3, 801− 806. (2) Schnack, J. Dalton Trans. 2010, 39, 4677−4686. (3) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Nature 1993, 365, 141−143. (4) Sessoli, R.; Tsai, H. L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1804−1816. (5) Pecoraro, V. L. Inorg. Chim. Acta 1989, 155, 171−173. (6) McCusker, J. K.; Jang, H. G.; Wang, S.; Christou, G.; Hendrickson, D. N. Inorg. Chem. 1992, 31, 1874−1880. (7) Liu, J.; Koo, C.; Amjad, A.; Feng, P. L.; Choi, E. S.; del Barco, E.; Hendrickson, D. N.; Hill, S. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 094443. (8) Stamatatos, T. C.; Foguet-Albiol, D.; Stoumpos, C. C.; Raptopoulou, C. P.; Terzis, A.; Wernsdorfer, W.; Perlepes, S. P.; Christou, G. J. Am. Chem. Soc. 2005, 127, 15380−15381. (9) Stamatatos, T. C.; Foguet-Albiol, D.; Lee, S. C.; Stoumpos, C. C.; Raptopoulou, C. P.; Terzis, A.; Wernsdorfer, W.; Hill, S. O.; Perlepes, S. P.; Christou, G. J. Am. Chem. Soc. 2007, 129, 9484−9499. (10) Milios, C. J.; Vinslava, A.; Wood, P. A.; Parsons, S.; Wernsdorfer, W.; Christou, G.; Perlepes, S. P.; Brechin, E. K. J. Am. Chem. Soc. 2007, 129, 8−9. (11) Milios, C. J.; Vinslava, A.; Wernsdorfer, W.; Moggach, S.; Parsons, S.; Perlepes, S. P.; Christou, G.; Brechin, E. K. J. Am. Chem. Soc. 2007, 129, 2754−2755. (12) Milios, C. J.; Inglis, R.; Vinslava, A.; Bagai, R.; Wernsdorfer, W.; Parsons, S.; Perlepes, S. P.; Christou, G.; Brechin, E. K. J. Am. Chem. Soc. 2007, 129, 12505−12511. (13) Milios, C. J.; Inglis, R.; Bagai, R.; Wernsdorfer, W.; Collins, A.; Moggach, S.; Parsons, S.; Perlepes, S. P.; Christou, G.; Brechin, E. K. Chem. Commun. (Cambridge, U. K.) 2007, 3476−3478. (14) Cano, J.; Cauchy, T.; Ruiz, E.; Milios, C. J.; Stoumpos, C. C.; Stamatatos, T. C.; Perlepes, S. P.; Christou, G.; Brechin, E. K. Dalton Trans. 2008, 234−240. (15) Prescimone, A.; Milios, C. J.; Moggach, S.; Warren, J. E.; Lennie, A. R.; Sanchez-Benitez, J.; Kamenev, K.; Bircher, R.; Murrie, M.; Parsons, S.; Brechin, E. K. Angew. Chem., Int. Ed. 2008, 47, 2828− 2831. (16) Milios, C. J.; Piligkos, S.; Brechin, E. K. Dalton Trans. 2008, 1809−1817. (17) Cremades, E.; Cano, J.; Ruiz, E.; Rajaraman, G.; Milios, C. J.; Brechin, E. K. Inorg. Chem. 2009, 48, 8012−8019. (18) Stamatatos, T. C.; Vlahopoulou, J. C.; Sanakis, Y.; Raptopoulou, C. P.; Psycharis, V.; Boudalis, A. K.; Perlepes, S. P. Inorg. Chem. Commun. 2006, 9, 814−818. (19) Vincent, J. B.; Chang, H. R.; Folting, K.; Huffman, J. C.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1987, 109, 5703− 5711. (20) Milios, C. J.; Raptopoulou, C. P.; Terzis, A.; Lloret, F.; Vicente, R.; Perlepes, S. P.; Escuer, A. Angew. Chem., Int. Ed. 2004, 43, 210− 212.

4f−5d hexanuclear complex. The magnetic properties of this family of complexes were discussed in detail, including the MnIII···MnIII and MnIII···WV coupling. For the original complex 1 12-MC-4 system, the MnIII ions within the MC ring in all complexes are antiferromagnetically coupled. After the coordination of the W(CN)83− unit in complex 2, the overall antiferromagnetic coupling is tuned to dominate the spins in ferromagnetic alignment according to the influence of the spin frustration effect. Complex 1 [MnIII4YIII] with an absolute S = 0 ground state turned into the SMM complex 2 [MnIII4YIIIWV] with S = 11/2 after the coordination of octacyanotungstate. This study proves that the introduction of a paramagnetic center into the 12-MC-4 system with an S = 0 ground state is a valid approach for designing SMM complexes. Even more importantly, the analysis of a frustrated antiferromagnetic system with tetragonal pyramid topology provides energy distribution plots which could accurately predict the spin ground state of a frustrated system. It is of profound interest to switch on the magnetic properties of antiferromagnetically coupled complexes by building or reconstructing a new spinfrustrated system. Furthermore, complex 2 with close low-lying states raised by competing interactions could be a good candidate for understanding the mechanism of quantum tunneling of magnetization (QTM) at very low temperatures. The study of other Ln-based derivatives of complex 2 with more complicated magnetostructures is in process.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00255. Temperature dependence of χMT for complex 2, fielddependent magnetization of complex 1 at 1.8 K, Zeeman splitting of the lowest mS states in the low-field range for complex 2, BVS calculations for complexes 1 and 2, and fitting results of χMT for complex 2 under multiple applied fields (PDF) X-ray crystallographic data for complexes 1 and 2 (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail for Y.S.: [email protected]. *E-mail for J.-M.D.: [email protected]. I


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Inorganic Chemistry (21) McCusker, J. K.; Christmas, C. A.; Hagen, P. M.; Chadha, R. K.; Harvey, D. F.; Hendrickson, D. N. J. Am. Chem. Soc. 1991, 113, 6114− 6124. (22) McCusker, J. K.; Schmitt, E. A.; Hendrickson, D. N. Nato Adv. Sci. I E-App 1991, 198, 297−319. (23) McCusker, J. K.; Vincent, J. B.; Schmitt, E. A.; Mino, M. L.; Shin, K.; Coggin, D. K.; Hagen, P. M.; Huffman, J. C.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1991, 113, 3012−3021. (24) Wang, S. Y.; Tsai, H. L.; Streib, W. E.; Christou, G.; Hendrickson, D. N. J. Chem. Soc., Chem. Commun. 1992, 677−679. (25) Hendrickson, D. N.; Christou, G.; Schmitt, E. A.; Libby, E.; Bashkin, J. S.; Wang, S. Y.; Tsai, H. L.; Vincent, J. B.; Boyd, P. D. W.; Huffman, J. C.; Folting, K.; Li, Q. Y.; Streib, W. E. J. Am. Chem. Soc. 1992, 114, 2455−2471. (26) Mukherjee, S.; Stull, J. A.; Yano, J.; Stamatatos, T. C.; Pringouri, K.; Stich, T. A.; Abboud, K. A.; Britt, R. D.; Yachandra, V. K.; Christou, G. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 2257−2262. (27) Lah, M. S.; Pecoraro, V. L. J. Am. Chem. Soc. 1989, 111, 7258− 7259. (28) Zaleski, C. M.; Tricard, S.; Depperman, E. C.; Wernsdorfer, W.; Mallah, T.; Kirk, M. L.; Pecoraro, V. L. Inorg. Chem. 2011, 50, 11348− 11352. (29) Koumousi, E. S.; Mukherjee, S.; Beavers, C. M.; Teat, S. J.; Christou, G.; Stamatatos, T. C. Chem. Commun. 2011, 47, 11128− 11130. (30) Gibney, B. R.; Wang, H.; Kampf, J. W.; Pecoraro, V. L. Inorg. Chem. 1996, 35, 6184−6193. (31) Deb, A.; Boron, T. T.; Itou, M.; Sakurai, Y.; Mallah, T.; Pecoraro, V. L.; Penner-Hahn, J. E. J. Am. Chem. Soc. 2014, 136, 4889−4892. (32) Leipoldt, J. G.; Bok, L. D. C.; Cilliers, P. J. Z. Anorg. Allg. Chem. 1974, 407, 350−352. (33) Bok, L. D. C.; Leipoldt, J. G.; Basson, S. S. Z. Anorg. Allg. Chem. 1975, 415, 81−83. (34) SAINT v5.0-6.01; Bruker Analytical X-ray Systems Inc., Madison, WI, USA, 1998. (35) Sheldrick, G. M. SADABS; University of Göttingen, Göttingen, Germany. (36) SHELXTL v5.1; Bruker Analytical X-ray Systems Inc., Madison, WI, USA, 1999. (37) Azar, M. R.; Boron, T. T.; Lutter, J. C.; Daly, C. I.; Zegalia, K. A.; Nimthong, R.; Ferrence, G. M.; Zeller, M.; Kampf, J. W.; Pecoraro, V. L.; Zaleski, C. M. Inorg. Chem. 2014, 53, 1729−1742. (38) Van Der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 46, 194−201. (39) Milios, C. J.; Inglis, R.; Vinslava, A.; Bagai, R.; Wernsdorfer, W.; Parsons, S.; Perlepes, S. P.; Christou, G.; Brechin, E. K. J. Am. Chem. Soc. 2007, 129, 12505−12511. (40) Boron, T. T., 3rd; Kampf, J. W.; Pecoraro, V. L. Inorg. Chem. 2010, 49, 9104−916. (41) Cao, F.; Wang, S.; Li, D.; Zeng, S.; Niu, M.; Song, Y.; Dou, J. Inorg. Chem. 2013, 52, 10747−10755. (42) Llunell, M.; Casanova, D.; Cirera, J.; Alemany, P.; Alvarez, S. SHAPE Version 2.1; Universitat de Barcelona: Barcelona, Spain, 2013. (43) Chilton, N. F.; Anderson, R. P.; Turner, L. D.; Soncini, A.; Murray, K. S. J. Comput. Chem. 2013, 34, 1164−1175. (44) Song, Y.; Liu, J.-C.; Liu, Y.-J.; Zhu, D.-R.; Zhuang, J.-Z.; You, X.Z. Inorg. Chim. Acta 2000, 305, 135−142. (45) Choi, S. W.; Kwak, H. Y.; Yoon, J. H.; Kim, H. C.; Koh, E. K.; Hong, C. S. Inorg. Chem. 2008, 47, 10214−10216. (46) Choi, S. W.; Ryu, D. W.; Lee, J. W.; Yoon, J. H.; Kim, H. C.; Lee, H.; Cho, B. K.; Hong, C. S. Inorg. Chem. 2009, 48, 9066−9068. (47) Przychodzeń, P.; Lewiński, K.; Bałanda, M.; Pełka, R.; Rams, M.; Wasiutyński, T.; Guyard-Duhayon, C.; Sieklucka, B. Inorg. Chem. 2004, 43, 2967−2974. (48) Przychodzeń, P.; Rams, M.; Guyard-Duhayon, C.; Sieklucka, B. Inorg. Chem. Commun. 2005, 8, 350−354. (49) Zhong, Z. J.; Seino, H.; Mizobe, Y.; Hidai, M.; Verdaguer, M.; Ohkoshi, S.; Hashimoto, K. Inorg. Chem. 2000, 39, 5095−5101.

(50) Song, Y.; Ohkoshi, S.; Arimoto, Y.; Seino, H.; Mizobe, Y.; Hashimoto, K. Inorg. Chem. 2003, 42, 1848−1856. (51) Qian, J.; Hu, J.; Yoshikawa, H.; Zhang, J.; Awaga, K.; Zhang, C. Eur. J. Inorg. Chem. 2015, 2015, 2110−2119. (52) Papatriantafyllopoulou, C.; Abboud, K. A.; Christou, G. Inorg. Chem. 2011, 50, 8959−8966. (53) Bartolomé, J.; Filoti, G.; Kuncser, V.; Schinteie, G.; Mereacre, V.; Anson, C. E.; Powell, A. K.; Prodius, D.; Turta, C. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 014430.

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