One-, Two-, and Three-Dimensional Heterospin Complexes

Dec 24, 2015 - Graduate School of Pharmaceutical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-Ku, Fukuoka 812-8582, Japan ... The 1:2:2 mixed ...
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One‑, Two‑, and Three-Dimensional Heterospin Complexes Consisting of 4‑(N-tert-Butyloxylamino)pyridine (4NOpy), Dicyanamide Ion (DCA), and 3d Metal Ions: Crystal Structures and Magnetic Properties of [MII(4NOpy)x(DCA)y(CH3CN)z]n (M = Mn, Co, Ni, Cu, Zn) Hiraku Ogawa,† Koya Mori,† Kensuke Murashima,† Satoru Karasawa,*,†,‡ and Noboru Koga*,† †

Graduate School of Pharmaceutical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-Ku, Fukuoka 812-8582, Japan PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan

‡

S Supporting Information *

ABSTRACT: Solutions of 3d metal ion salts, M(NO3)2, 4(N-tert-butyloxylamino)pyridine (4NOpy), and dicyanamide (DCA) in CH3CN were mixed to afford single crystals of the polymeric complexes [MII(4NOpy)x(DCA)y(CH3CN)z]n (MII = Mn (1), Co (2), Ni (3), Cu (4a and 4b), Zn (5)). X-ray crystallography revealed that the crystal structures are a threedimensional (3-D) network for 1, 2-D networks for 2, 3, 4a, and 5, and a 1-D chain for 4b. Crystals of 2, 3, 4a, and 5 contained CH3CN molecules as crystal solvents, which were readily desorbed in the ambient atmosphere. After desorption of the CH3CN molecules, the crystal structures of 2 and 3 were confirmed to be slightly shrunk without destruction of the crystal lattice. Crystals of 2, 3, 4a, and 5 after desorption of crystal solvents were used for investigations of the magnetic properties. Complex 1 showed antiferromagnetic interactions to form a ferrimagnetic chain and exhibited the magnetic behavior of a 2-D (or 3-D) spin-canted antiferromagnet with TN = 12 K. Complex 2 containing anisotropic CoII ions also showed the behavior of a 1-D (or 2-D) spin-canted antiferromagnet with TN = 6 K. In 3, 4a, and 4b, the aminoxyl of 4NOpy ferromagnetically interacted with the metal ion with coupling constants of JM−NO/kB = 45, 45, and 43 K, respectively. In 5, the magnetic couplings between the aminoxyls in 4NOpy through the diamagnetic ZnII ion were weakly antiferromagntic (JNO−NO = −1.2 K). DCA might be a weak antiferromagnetic connector for the metal chains.

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the π spin of organic unpaired electrons. The characteristic point of our heterospin system is that the organic spin is far from the binding site with the metal ion. Therefore, various combinations of metal ions and the aromatic ligands with organic spin can be used for the study of molecule-based magnets. Especially, NOpy2 and C1py2 having two pyridines function as magnetic couplers. For example, the combinations of D1py2 with MnII(hfac)2 and CuII(hfac)2 (hfac = hexafluoroacetylacetonato) gave ferri- and ferromagnetic chains, respectively, after irradiation.3 Furthermore, these heterospin systems were applied to construct single-molecule magnets (SMMs) and single-chain magnets (SCMs) exhibiting slow magnetic relaxation, and heterospin SMMs and SMM chains were successfully obtained.4 The combinations of anisotropic cobalt ions CoIIX2 (X = NCS−, NCO−, Cl−) with 4NOpy (or C1py) and chelated cobalt complexes CoII(hfpip)2 (hfpip = 1,1,1,5,5,5-hexafluoro-4-(phenylimino)-2-pentanoate) with NOpy2 (or C1py2) provided heterospin SMM complexes and

INTRODUCTION As a useful strategy for the construction of molecule-based magnets, a heterospin system1 consisting of the 3d or 4f spins of the metal ions and the 2p spins of the organic ligands has been studied by many groups, including our own. In our heterospin systems,2 carbene generated by photolysis of a diazo precursor and stable aminoxyl were used as organic spins and were attached to aromatic pyridine ligands for coordination with the metal ions. The typical aromatic ligands prepared for the construction of heterospin complex are shown in Scheme 1. The organic spins having π character magnetically interact with the metal ions through the aromatic ligands by delocalization of Scheme 1. Pyridine Ligands Carrying an Organic Spin for Heterospin Complexes

Received: September 19, 2015

© XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.5b02159 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Formulas, Numbers of Contained CH3CN Molecules, Space Groups, and Crystal Classes for All of the Complexes complex 1 2α 2β 3α 3β 4aα 4aβa 4b 5α 5βa 5′α 5′β a

formula

number of CH3CN moleculesb

space group

crystal class

[Mn (4NOpy)(μ2-1,5-DCA)2]n [CoII(4NOpy)2(μ2-1,5-DCA)2(CH3CN)2]n [CoII(4NOpy)2(μ2-1,5-DCA)2]n [NiII(4NOpy)2(μ2-1,5-DCA)2(CH3CN)2]n [NiII(4NOpy)2(μ2-1,5-DCA)2]n [CuII(4NOpy)(μ2-1,5-DCA)(μ3-1,3,5-DCA) (CH3CN)]n [CuII(4NOpy) (μ2-1,5-DCA) (μ3-1,3,5-DCA)]n [CuII(4NOpy)2(μ2-1,5-DCA) (NO3)]n [ZnII(4NOpy)2(μ2-1,5-DCA)2(CH3CN)2]n [ZnII(4NOpy)2(μ2-1,5-DCA)2]n [ZnII(4NOpy)2(μ2-1,5-DCA)2(CH3CN)2]n [ZnII(4NOpy)2(μ2-1,5-DCA)2]n

0 8 0 8 0 2 0 0 8 0 8 0

Pbca Pca21 Pca21 Pca21 Pca21 P1̅ − P21/c P21/c − Pca21 Pca21

orthorhombic orthorhombic orthorhombic orthorhombic orthorhombic triclinic − monoclinic monoclinic − orthorhombic orthorhombic

metal ion II

Mn CoII NiII CuII CuII ZnII ZnII

II

No X-ray crystallographic data. bIn a unit cell.

the solution of NaDCA was added, the single-crystal copper complex 4b was obtained as green blocks. The zinc complex was prepared in a manner similar to the procedure for 1−4a by using an aqueous solution of ZnII(NO3)2·6H2O in place of the solution of MII(NO3)2·6H2O in CH3CN. The zinc complex 5 was obtained as red bricks together with a small amount of another zinc complex, 5′. In ambient atmosphere, the obtained crystals of 1 and 4b were stable, while the crystals of 2, 3, 4a, 5, and 5′ gradually became opaque upon standing. The observed alteration for the latter crystals suggested the desorption of crystal solvents. Furthermore, the elemental analyses suggested that the desorbed crystals absorbed water molecules. Molecular and Crystal Structure Analysis of 1, 2, 3, 4a, 4b, 5, and 5′. In X-ray analyses of 2, 3, 4a, 5, and 5′, two kinds of samples (the α form and the β form) were prepared for each crystal. The single crystals immediately after removal from the mother liquor for the recrystallization were denoted as α, while crystals left at room temperature for 200 min were denoted as β. The molecular and crystal structure analyses for the 12 single crystals (1, 4b, and the α and β forms of 2, 3, 4a, 5, and 5′) were performed by X-ray crystallography at 90 K. For the crystals of 4aβ and 5β, results with R values less than 8% could not be obtained. The full crystal parameters and structure refinement information for the 10 obtained complexes are shown in Tables S1 and S2. The molecules were crystallized in the space groups Pbca for 1, Pca21 for 2α, 2β, 3α, 3β, 5′α, and 5′β, P1̅ for 4aα, and P21/c for 4b and 5α. Crystals 2, 3, 5, and 5′ were isomorphous, and crystals 5α and 5′α, were polymorphous. Crystals 1 and 4b had no crystal solvent, while 4aα had two CH3CN molecules per unit cell. In crystals of 2, 3, 5, and 5′, the α forms contained eight CH3CN molecules per unit cell, and the β forms had no solvent molecule. The results of X-ray analyses for 2, 3, and 5′ indicated that at ambient temperature the α form gradually desorbed the solvent molecules and converted to the β form without destruction of the crystal structure. The complexes were formulated as [MnII(4NOpy)(μ2-1,5-DCA)2]n for 1, [MII(4NOpy)2(μ2-1,5-DCA)2(CH3CN)2]n for the α forms of 2, 3, 5, and 5′, [MII(4NOpy)2(μ2-1,5-DCA)2]n for the β forms of 2, 3, and 5′, [CuII(4NOpy)(μ2-1,5-DCA)(μ3-1,3,5-DCA)(CH3CN)]n for 4aα, and [CuII(4NOpy)2(μ2-1,5-DCA)(NO3)]n for 4b. All of the complexes characterized by X-ray crystallography are listed in Table 1 together with their formulas, numbers of contained CH3CN molecules, space groups, and crystal classes.

SMM chains, respectively, showing relatively high activation barriers for reorientation of the magnetism and hysteresis loops with large coercive force values. In these studies, bidentate monoanionic ligands such as hfac and hfpip were used as diamagnetic ligands, and heterospin discrete complexes and one-dimensional (1-D) complexes were prepared. To increase the dimensionality of a metal complex, this time, dicyanamide ion (DCA)5 was selected and used as a monoanionic bridging ligand. DCA has often been used as the flexible ligand in host− guest complexes and as an organic ligand for metal−organic frameworks (MOFs),6,7 and many functional metal complexes containing DCA have been reported. DCA has three nitrogen atoms: the two terminal ones and the center one. These can coordinate with the metal ion and provide the complexes with a high-dimensional structure. Therefore, the combination of the heterospin complexes with DCA was expected to lead to new MOFs exhibiting unique magnetic behavior. In this study, DCA was added as a bridging ligand to combinations of 4NOpy with 3d metal ions MII (M = Mn, Co, Ni, Cu, Zn) in the DCA:4NOpy:3d metal ratio of 2:2:1. Polymeric complexes formulated as [MII(4NOpy)2(DCA)2]n were expected to result. Especially, the magnetic behavior of the complex containing the large anisotropic CoII ion was interesting. Herein we report the crystal structures of these polymeric complexes and their magnetic properties.

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RESULTS AND DISCUSSION Preparation of [MII(4NOpy)x(DCA)y]n (M = Mn (1), Co (2), Ni (3), Cu (4), Zn (5)). 4NOpy was prepared by the procedure reported previously and used immediately after preparation.8 The 3d metal ions MnII, CoII, NiII, CuII, and ZnII were selected and used as the metal spin source in heterospin systems. Since the FeII ion was oxidized by aminoxyl in 4NOpy, it could not be used. Solutions of MnII(ClO4)2·6H2O and MII(NO3)2·6H2O (M = Co, Ni, Cu, Zn) in CH3CN were mixed with a solution of sodium dicyanamide (NaDCA) in CH3CN and then a solution of 4NOpy in CH3CN in the MII(NO3)2/6H2O:NaDCA:4NOpy ratio of 1:2:2 to afford polymeric metal complexes with various dimensions. The ZnII complex was obtained by addition of H2O and ether to the above mixture. Single crystals of the complexes of MnII (1), CoII (2), NiII (3), and CuII (4a) were obtained as red plates, red bricks, red bricks, and reddish-brown plates, respectively. It is interesting that in the case of the copper complex, a different crystal was obtained by changing the mixing order. When the solutions of CuII(NO3)2 and 4NOpy were mixed first and then B

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Figure 1. ORTEP drawings (50% probability) of partial structures of the metal complex units of (a) 1, (b) 2α, (c) 4aα, and (d) 4b. Crystal solvent CH3CN molecules in 2α and 4aα have been omitted.

Table 2. Selected Bond Lengths and Dihedral Angles for All Complexes 1 rM−Npy/Å rM−NDCA/Å

2.240 2.174, 2.186, 2.187, 2.211

rM−O/Å ∠py−NO/deg ∠py−XY/deg a

2α (2β) 2.149, 2.162

3α (3β)

4aα

Bond Lengths (Å) 2.095, 2.121 2.026, 2.028

(2.156, 2.167) 2.106, 2.107,

(2.096, 2.121) 2.066, 2.067,

2.110, 2.112 (2.097, 2.100, 2.102, 2.107)

2.073, 2.085 (2.048, 2.059, 2.066, 2.071)

1.991, 2.382,

1.998, 2.020 1.974, 2.225

5α 2.132 2.189, 2.193

1.992,a 1.997,a 2.410a

5′α (5′β) 2.107, 2.125 (2.146, 2.186) 2.084, 2.090, 2.092, 2.096 (2.081, 2.105, 2.111, 2.115)

2.018,b 2.635b

2.228 25.46

4b

22.27, 29.20 (12.38, 20.41) −

Dihedral Angles (deg) 22.47, 29.76 3.540 (16.09, 17.33) − 52.72

5.95, 19.85 28.05, 59.43

15.81 −

20.04, 28.19 (14.06, 17.76) −

Bond lengths of the octahedral Cu2 unit coordinated with DCA only. bOxygen atom of NO3−.

(i) Structure of Local Complex Units. In all of the complexes, the coordination structures of local metal complex units are octahedral with two 4NOpy ligands coordinated to the metal ions in a trans configuration. The metal ions are surrounded by the DCA nitrogen atoms (denoted as NtDCA and NcDCA for the terminal and center nitrogen atoms, respectively) and 4NOpy pyridine nitrogen (Npy) and aminoxyl oxygen (ONO) atoms. In complex 1, the Mn(II) ion is coordinated with four DCA NtDCA atoms and Npy and ONO atoms of two 4NOpy

molecules. In 2, 3, 5, and 5′, the metal ions are coordinated with two 4NOpy Npy atoms and four DCA NtDCA atoms. In the copper complexes, 4a has two kinds of CuII ions, Cu1 and Cu2. Cu1 is coordinated with two Npy atoms and four NtDCA atoms, while Cu2 is coordinated with four NtDCA atoms and two NcDCA atoms, which bridge Cu1 and Cu2. In contrast, the CuII ion for 4b is coordinated with two Npy atoms, two NtDCA atoms, and the two oxygen atoms of one NO3− ion (ONO3). C

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Figure 2. Illustration of high-dimensional network structures: (a) 3-D for 1; (b) 2-D for 2, 3, 5, and 5′; (c) 2-D for 4a; (d) 1-D for 4b.

oxygen atom according to an HSAB rule,11 leading to the formation of a partial 1:1 MnII−4NOpy chain in 1. (ii) Crystal Structures. Both 4NOpy and DCA connectors effectively operate as bridging ligands to form polymeric complexes having 1-D to 3-D structures. For the bridging ligand DCA, five coordination modes have been reported to date; μ-1-, μ2-1,3-, μ2-1,5-, μ3-1,3,5-, and μ4-1,3,5,5-DCA (Scheme S1),12 In these complexes, DCA coordinates to the metal ions in a μ2-1,5-DCA mode in all cases except 4aα, which has μ2-1,5- and μ3-1,3,5-DCA modes. All of the complexes have polymeric structures, with a 3-D structure for 1, 2-D structures for 2, 3, 4a, 5, and 5′, and a 1-D structure for 4b through ligation with 4NOpy and DCA. Interestingly, only the complexes having 2-D structures contained crystal solvents. The 1-D to 3-D structures for the complexes are illustrated in Figure 2. (A) [MnII(4NOpy)(μ2-1,5-DCA)2]n (1). The crystal structure of 1 is shown in Figure 3. The Npy and ONO atoms in a 4NOpy coordinate with the different MnII ions to form chain structures along the a axis. The chains, which are alternately arranged in opposite directions (violet and sky blue chains in Figure 3), are connected with DCA1 to form 2-D planes (the ab plane). Subsequently, the MnII ions in the ac plane are connected with DCA2 to form the 3-D structure. Accordingly, 1 has two kinds of bridging ligands that coordinate to the MnII ions, DCA1 and DCA2, and the distances rMn−Mn between the MnII ions are 7.86 and 7.13 Å through DCA1 and DCA2, respectively. It was important to understand the magnetic properties of 1, in which the crystal lattice has acentric symmetry within the ab plane and centric symmetry between the ab planes. No short contacts within 4.0 Å relating to the aminoxyl center (NO) were observed. (B) [Co I I (4NOpy) 2 (μ 2 -1,5-DCA) 2 (CH 3 CN) 2 ] n (2α), [NiII(4NOpy)2(μ2-1,5-DCA)2(CH3CN)2]n (3α), [ZnII(4NOpy)2(μ2-1,5-DCA)2(CH3CN)2]n (5α and 5′α), and Their β Forms (2β, 3β, and 5′β). The complexes of 2, 3, 5, and 5′ have two-dimensional (2-D) structures. The local complex units, MII(4NOpy)2(μ2-1,5-DCA)2, comprising two 4NOpy and two DCA ligands, are coordinated to each other with NtDCA atoms of DCA to form the 2-D structures. The 2-D

ORTEP drawings of the metal complex units are shown in Figure 1 for 1, 2α, 4aα, and 4b and Figure S1 for 3α and 5α. The observed octahedral structures for all of the complexes were slightly elongated. In the complexes other than the copper complexes, the bond lengths between the metal ion and Npy of 4NOpy (rM−Npy) are longer by 0.04−0.06 Å compared with those between the metal ion and NtDCA of DCA (rM−NDCA), suggesting that 4NOpy is coordinated on the z axis of the metal complex. Noticeably, the cobalt complex 2 having 4NOpy coordinated on the z axis is the first example of a heterospin CoII complex, and its magnetic properties are of interest. In contrast, the bond lengths rM−NDCA for the copper complexes 4a and 4b are longer by ca. 0.2 Å than rM−NO, which are affected by the Jahn−Teller effect,9 indicating that 4NOpy is coordinated in the xy plane of the CuII complex. In 4a having two CuII ions in a unit cell, Cu1 and Cu2 are coordinated by the NtDCA and NcDCA atoms of the bridging DCA, and their bond lengths are longer; rM−NDCA for Cu1 and Cu2 are 2.38 and 2.41 Å, respectively. The observed long bond lengths suggest that the magnetic interaction between Cu1 and Cu2 through DCA might be weak. In all of the complexes, the dihedral angles between the pyridine and the aminoxyl planes (∠py−NO) are 4− 30°, indicating that the spin of the aminoxyl in 4NOpy can be delocalized to interact effectively with the metal ion through the pyridine ring. In the comparison of the α forms with the β forms in 2, 3, and 5′, subtle alterations of the bond lengths and dihedral angles were observed. In 2, 3, and 5′, however, the differences between the bond lengths rM−Npy in the α and β forms are insignificant for the magnetic interaction. Selected bond lengths and dihedral angles for all of the complexes are summarized in Table 2. The bond lengths rM−Npy became shorter in the order of MnII < CoII < NiII < CuII ion, and that for the ZnII complex was longer than that for CuII complex. This result is consistent with the order of the Irving−Williams series.10 The 4NOpy ligands in 1 coordinate to the MnII ion with Npy and ONO, while in the remaining complexes 4NOpy coordinates to the metal ion with Npy. Generally, the MnII ion favorably coordinates with the D

DOI: 10.1021/acs.inorgchem.5b02159 Inorg. Chem. XXXX, XXX, XXX−XXX

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reported previously,10 might cause the intraplane antiferromagnetic interaction. In the crystals of 2, 3, and 5′, no short distance within 4 Å between the planes relating to the magnetic interaction was observed. In contrast, 5α has short rNO−C3 contacts of 3.35 and 3.39 Å that are not in the plane but between the planes (Figure S4b). The two CH3CN molecules, which form a head-to-tail dimer, are located at the center of a square network within the 2D plane, as shown in Figure 4. In the crystal packing of 2α, 3α, and 5′α, the channels occupied by the CH3CN molecules are directed toward the a and c axes. After desorption, vacant channels are present in 2β, 3β, and 5′β, producing subtle alterations of the crystal structure. The rM−M distance between metal ions connected with DCA becomes shorter by 0.2−0.3 Å, and the spaces formerly occupied by CH3CN molecules shrink. In addition, the distances between the ab planes become longer by 0.1 Å, corresponding to the elongation of the c axis in a cell after desorption (see Tables S1 and S2). The values of the cell volume (V) and calculated density (Dcalc) for 2α, 3α, and 5′α and the corresponding β forms are listed in Table 1 in addition to the numbers of CH3CN molecules. The V and Dcalc values for the β forms are reduced to 92−94% and 93−94% of those for the α forms, respectively, indicating that the crystals shrank and produced the vacant space (void) as a result of the desorption of crystal solvents. Such desorptions of crystal solvents from the flexible crystal lattice have often been observed in MOF complexes.7 The selected distances and the V and Dcalc values before and after desorption of CH3CN in 2, 3, and 5′ are summarized in Table 3. (C) [Cu II (4NOpy)(μ 2 -1,5-DCA)(μ 3 -1,3,5-DCA)(CH 3 CN)] n (4aα) and [CuII(4NOpy)2(μ2-1,5-DCA)(NO3)]n (4b). The two copper complexes had different dimensional structures. Crystal 4aα consists of two kinds of local CuII complex units, formulated as CuII(4NOpy)2(μ2-1,5-DCA)2 and CuII(μ3-1,3,5DCA)2. The complex units are linked with DCA to form chain structures. The chains of CuII(4NOpy)2(μ2-1,5-DCA)2 and CuII(μ3-1,3,5-DCA)2 are alternately coordinated with the center nitrogen of DCA (Ncdca) in CuII(4NOpy)2(μ2-1,5-DCA)2 to form [CuII(4NOpy)(μ 2-1,5-DCA)(μ3-1,3,5-DCA)(CH3CN)]n with a 2-D plane of the interpenetrated structure. 4aα has one molecule of CH3CN as a crystal solvent, which is located between the interpenetrated planes. In contrast, the local CuII complex unit for 4b is CuII(4NOpy)2(μ2-1,5-DCA)(NO3), and these are connected by DCA units to form the 1-D [CuII(4NOpy)2(μ2-1,5-DCA)(NO3)]n structure. The 2-D structure for 4aα and 1-D structure for 4b are shown in Figure 5. In 4aα, short interplane contacts between the aminoxyl centers (rO−O = 3.9 Å) were observed (dotted red ovals in Figure 5b), while no short contacts within the chains were observed. Complex 4b had short contacts of rNO−C3 = 3.17 and 3.31 Å within the chain (dotted dark-green lines in Figure 5d). Similar short contacts were observed in 2α, 3α, and 5′α. Magnetic Properties. Crystals were roughly crushed, and the resulting microcrystals were used as the samples for SQUID measurements. Before performing the magnetic experiments, the microcrystalline samples of 2α, 3α, 4aα, and 5α for SQUID measurements were investigated by powder X-ray diffraction (PXRD) measurements, whether or not CH3CN molecules were present. Microcrystalline samples left at room temperature were subjected to PXRD measurements. The PXRD patterns of 2, 3, 4a, and 5 left for 8 and 200 min are shown in Figure S6 together with the simulation patterns obtained from single-

Figure 3. Crystal structure of 1 viewed along the (a) b, (b) c, and (c) a axes using a ball and stick model (color code: manganese, violet or sky blue; carbon, gray; oxygen, red; nitrogen, blue) with colored octahedra of Mn ions and pyridine planes. The two chains consisting of units of Mn and 4NOpy along the +a and −a directions are indicated with violet and sky-blue colors, respectively. Hydrogen atoms and tert-butyl groups of 4NOpy have been omitted for the sake of clarity.

structures for 2α and 2β obtained at 90 K are shown in Figure 4. The corresponding structures are shown in Figure S2 for 3α and 3β, Figure S3 for 5′α and 5′β, and Figure S4 for 5α. In the α and β forms of 2, 3, and 5′, short contacts between the aminoxyl center and the carbon atom at the 3-position of pyridine (C3) for the neighboring complex unit were observed within the plane (dotted lines in Figure 4). The rNO−C3 distances are 3.15 (3.14) and 3.23 (3.17) Å for 2α (2β), 3.14 (3.12) and 3.23 (3.18) Å for 3α (3β), and 3.15 (3.16) and 3.22 (3.19) Å for 5′α (5′β). The short rNO−C3 distance, which has often been observed in aminoxyl heterospin complexes E

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Figure 4. Crystal structures for (left) 2α and (right) 2β viewed along the (a) c, (b) a, and (c) b axes using a ball and stick model (color code: cobalt, blue; carbon, gray; oxygen, red; nitrogen, light blue). CH3CN molecules and DCA chains are shown using space-filling and wire-frame models, respectively. Dotted lines indicate short contacts of 3.15 and 3.23 Å. Hydrogen atoms and tert-butyl groups of 4NOpy have been omitted for the sake of clarity.

Table 3. Values of rNO−C3, rM−M, rplane−plane, V, and Dcalc before and after Desorption in 2, 3, and 5′ rNO‑C3/Å rM‑M/Å rplane−plane/Å V/Å3 Dcalc/g·cm−3

2α

2β

3α

3β

5′α

5′β

3.15, 3.24 8.13, 8.16 11.78 3112.6(10) 1.288

3.14, 3.17 7.82, 7.84 11.89 2865.9 (7) 1.208

3.14, 3.23 8.09, 8.12 11.76 3081(2) 1.301

3.12, 3.18 7.81, 7.84 11.82 2857.1(15) 1.212

3.15, 3.22 8.10, 8.13 11.77 3088.5(11) 1.312

3.16, 3.19 7.81, 7.84 11.89 2864.17(2) 1.224

Figure 5. Crystal structures for (a−c) 4aα viewed along the (a) c, (b) a, and (c) b axes and (d) 4b viewed along the b axis. CH3CN molecules and DCA chains are shown using space-filling and wire-frame models, respectively (color code: copper, orange; carbon, gray; oxygen, red; nitrogen, light blue). Dotted red ovals in (b) and dotted dark-green lines in (d) indicate short rO−O contacts of 3.9 Å and short rNO−C3 contacts of 3.17 and 3.31 Å, respectively. Hydrogen atoms and tert-butyl groups in 4NOpy have been omitted for the sake of clarity.

F

DOI: 10.1021/acs.inorgchem.5b02159 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry crystal X-ray diffraction (SXRD) results (90 K) for the α and β forms. The PXRD patterns for the microcrystalline samples left for 8 and 200 min were different from those simulated from the SXRD results for the corresponding α forms, indicating that the CH3CN molecules were readily desorbed. Furthermore, the PXRD patterns for the samples of 2 and 3 were close to the simulation patterns for 2β and 3β, respectively. From the obtained PXRD patterns, the microcrystalline samples of 2, 3, 4a, and 5 for the SQUID measurements were determined to be the β forms after desorption of CH3CN molecules. The magnetic properties of microcrystalline samples of 1, 2β, 3β, 4aβ, 4b, and 5β were investigated by SQUID magneto/ susceptometry. (i) [MnII(4NOpy)(μ2-1,5-DCA)2]n (1) and [CoII(4NOpy)2(μ21,5-DCA)2]n (2β). Values of the direct current (dc) molar magnetic susceptibility (χmol) were collected at constant fields of 5.0 and 1.0 kOe at temperatures of 300−2.0 K and below 100 K, respectively. In addition, the samples were measured at fields of 0.1 and 0.5 kOe in the region 50−2.0 K. The obtained χmol values were plotted as a function of temperature. In regard to the temperature dependence of χmolT, complexes 1 and 2β showed different thermal profiles of χmolT. The plots of χmolT versus T for 1 and 2β are shown in Figure 6.

Scheme 2. Magnetic Interactions in a MnII−4NOpy Chain

reported to be antiferromagnetic: the signs of J1Mn−NO/kB and J2Mn−NOpy/kB are negative, and their magnitudes are related as | J1Mn−NO|/kB > |J2Mn−NOpy|/kB. The absolute values of |J1Mn−NO|/ kB for the direct coordination of the aminoxyl center to the MnII ion are larger than 100 K (|J1Mn−NO|/kB > 100 K),13 while the value of J2Mn−NOpy/kB is −12 K for the discrete [MnII(hfac)2(4NOpy)2],14 in which 4NOpy is coordinated to the MnII ion through the pyridine nitrogen. In the χmolT versus T plot (Figure 6a), the χmolT value at 300 K and a dc field of 5.0 kOe was 3.2 cm3·K·mol−1, which was close to the theoretical value of 3.0 cm3·K·mol−1 calculated for a spin-only equation with S = 4/2 rather than the value of 4.8 cm3· K·mol−1 for the isolated MnII ion with S = 5/2 (4.37 cm3·K· mol−1) and aminoxyl with S = 1/2 (0.38 cm3·K·mol−1). This result indicated that the direct coordination of the aminoxyl with the MnII ion was intensely antiferromagnetic (J1Mn−NO/kB < 0). Upn cooling, the χmolT value was nearly constant until 100 K, gradually increased to reach sharp maximum (5.3 cm3·K· mol−1) at ca. 15 K, and then steeply decreased below 10 K. The observed increase in χmolT below 100 K suggested the formation of a ferrimagnetic chain due to the antiferromagnetic interaction (J2Mn−NO/kB < 0) between the aminoxyl spin center and the MnII ion through the pyridine ring. The abrupt decrease below 15 K suggested that the interchain antiferromagnetic interaction through DCA might operate to form a 2-D (or 3-D) magnetic network. Interestingly, the χmolT values below 15 K depended on the applied dc field, and a new maximum χmolT value appeared at 10 K in a 0.1 kOe dc field (see the inset in Figure 6a). To understand the magnetic properties of chains in 1, a single JMn−NO parameter was assumed for the chains and Seiden’s ferrimagnetic chain model,13,15 H = −J∑(Si·Si+1)Si (eq 1 in section S9 in the Supporting Information) with the interchain interaction zj′/kB (z = 4), was applied to this spin system. The theoretical equations are shown in section S9 together with the schemes illustrating the spin systems. The theoretical equation was fitted to the experimental data above 30 K by a least-squares method. The best fitting results for JMn−NO/kB and zj′/kB were −72 and −0.1 K, respectively. The theoretical curve is shown as a solid line in the χmolT versus T plot (Figure 6a). (i-2) [CoII(4NOpy)2(μ2-1,5-DCA)2]n (2β). In the χmolT vs T plot (Figure 6b), the χmolT value of 3.4 cm3·K·mol−1 at 300 K was close to the value of 3.3−3.8 cm3·K·mol−1 obtained as 0.76 cm3·K·mol−1 for two aminoxyls + 2.5−3 cm3·K·mol−1 for the CoII ion with S = 3/2, indicating that the CoII ion and aminoxyls were magnetically isolated. Upon cooling, the χmolT value gradually decreased, reached a plateau of 2.5 cm3·K·mol−1 at ca. 30 K, and then steeply decreased below 15 K. The observed thermal profile of χmolT in the range of 300−30 K suggested that the χmolT value was intensely affected by the zero-field splitting (zfs) parameter |D|/kB, caused by the spin−orbit coupling in the anisotropic CoII ion, and that the magnetic coupling of 4NOpy with the CoII ion, JCo−NO/kB, was negligible. Actually, the observed plateau value of 2.5 cm3·K·mol−1 at 30− 22 K was close to the value of 2.58−2.78 cm3·K·mol−1 obtained

Figure 6. Plots of χmolT vs T for (a) 1 and (b) 2β. The insets show the plots of χmolT vs T at the given dc fields below 30 K.

(i-1) MnII(4NOpy)(μ2-1,5-DCA)2]n (1). According to the results of X-ray crystallography, the heterospin chain system for 1 could be expressed in terms of magnetic interactions between the MnII ions and 4NOpy with two exchange coupling parameters, J1Mn−NO/kB and J2Mn−NOpy/kB, corresponding to the interactions between the MnII ion and the aminoxyl center directly and through the pyridine ring, respectively (Scheme 2). The former and latter magnetic interactions were already G

DOI: 10.1021/acs.inorgchem.5b02159 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry as 0.78 for two aminoxyls + 1.8−2.0 for the CoII ion with effective spin S′ = 1/2.16 To estimate the values of JCo−NOpy/kB and |D|/kB for 2β quantitatively, a three-spin compound, CoII(4NOpy)2(μ2-1,5-DCA)2, with the JCo−NO/kB value was assumed as a model complex for 2β, and a three-spin model, H = −J∑(S1·S2 + S2·S3) + D[S22 − 1/3S2(S2 + 1)] for the JULX program,17 was applied. The theoretical equation (eq 2 in section S9) was fitted to the experimental data above 50 K by a least-squares method. The best fitting results for JCo−NOpy/kB and |D|/kB were −0.8 and 185 K, respectively, suggesting that the observed decrease of χmolT from 300 K is mainly due to the effect of the zfs parameter |D|/kB and that the magnetic interaction with JCo−NO/kB is weak. The obtained theoretical curve is shown as a solid line in the χmolT versus T plot (Figure 6b). The decrease in χmolT below 15 K might be caused by the antiferromagnetic interaction through the NO−C3 short contacts and/or through DCA, suggesting that antiferromagnetic chains (or 2-D planes) formed below 10 K. Again, the thermal profile of χmolT below 10 K depended on the applied dc field, and a new maximum χmolT value appeared at 3.5 K in a 0.1 kOe dc field (see the inset in Figure 6b). Determination of Spin-Canted Antiferromagnets. In 1 and 2β, the observed thermal profiles of χmolT dependent on the applied constant field were characteristic of spin-canted antiferromagnets, suggesting a phase transition between the paramagnetic phase (P) and the antiferromagnetic phase (AF) in the low-temperature region. In order to understand the magnetic properties of the complexes of 1 and 2β in the lowtemperature region in more detail, the following dc and ac magnetic measurements were carried out. (A) Sequence of ZFCM, FCM, and RM measurements. The dc magnetic measurements were performed using a sequence of zero-field-cooled magnetization (ZFCM), field-cooled magnetization (FCM), and remnant magnetization (RM) over the temperature range of 1.9−15 K for 1 and 1.9−10 K for 2β. For ZFCM and FCM measurements, a constant dc field of 4.7 Oe was used. The plots of Mmol vs T for 1 and 2β in the sequence of ZFCM, FCM, and RM are shown in Figure 7. As shown in the ZFCM, FCM, and RM measurements for 1 (Figure 7a), in ZFCM the Mmol value was 1.4 cm3·Oe·mol−1 at 2 K, slightly increased upon warming until 10 K, and then decreased. In FCM, upon cooling the Mmol value steeply increased below 12 K, reached a maximum of 17.8 cm3·Oe· mol−1 at 9.0 K, and then decreased below 9.0 K. In RM, the Mmol value was 1.4 cm3·Oe·mol−1 at 2 K, gradually increased upon warming, reached a maximum of 15.5 cm3·Oe·mol−1 at 9.5 K, and then disappeared at 11.5 K. The large discrepancy between the Mmol values for the FCM and ZFCM measurements and the large RM values indicated long-range magnetic ordering of the magnetic spin arising from spin canting. The decrease in Mmol below 9 K in FCM might be due to the interplane antiferromagnetic interaction. From the divergent point of the FCM and ZFCM measurements, the critical temperature (TN) was determined to be 12 K. In the thermal profile of Mmol for 2β in ZFCM, FCM, and RM cycles, a similar discrepancy between the ZFCM and FCM values was observed (Figure 7a). In ZFCM, the Mmol value was 5.2 cm3·Oe·mol−1 at 2 K and slightly decreased upon warming until 10 K, while in FCM, the Mmol value gradually increased upon cooling until 4 K and then steeply increased below 4 K. In RM, the Mmol value was 39.1 cm3·Oe·mol−1 at 2 K, steeply decreased upon warming until 4 K, gradually decreased, and then disappeared at around

Figure 7. Plots of Mmol vs T in a sequence of ZFCM (black), FCM (red), and RM (blue) at a field of 4.7 Oe for (a) 1 and (b) 2β.

6 K. From the divergent point of the FCM and ZFCM measurements, the TN value of 6 K was obtained. These ZFCM, FCM, and RM measurements for 1 and 2β suggested the formation of 2-D (or 3-D) canted antiferromagnets below TN = 12 and 6 K, respectively. However, the origins of the high-dimensional canted antiferromagnets in these two crystals were different. The MnII and CoII complexes were magnetically isotropic and anisotropic, respectively, and in the crystal lattices revealed by X-ray crystallography, the centric Pbca space group of 1 had acentric symmetries within the ab planes and between the ac planes, while 2β had an acentric space group of Pca21. In 1, the antiferromagnetic interactions took place between the ferrimagnetic chains below 12 K to form a 2-D (or 3-D) canted antiferromagnet.18 In contrast, 2β might show the antiferromagnetic interaction due to the NO− C3 short contacts and/or through DCA below 6 K in addition to the weak magnetic interaction between the CoII ion and the aminoxyl of 4NOpy, leading to a 2-D (or 1-D) canted antiferromagnet. Many MnII and CoII complexes showing behaviors of canted antiferromagnets have been reported and investigated in detail.19,20 (B) Mmol versus H Plots. The field dependence of Mmol for 1 and 2β was investigated at various temperatures over the field ranges of 0−70 and 0−50 kOe, respectively. The plots of Mmol/ NμB versus H and dM/dH versus H at various temperatures are shown in Figure 8 for 1 and Figure S7 for 2β. In the Mmol/NμB versus H plots at 2.0 K, the Mmol values increased in a sigmoid shape at low field ( 0). The maximum values for 3β, 4aβ, and 4b were 2.63, 1.00, and 1.72 cm3·K·mol−1, respectively. These values are smaller than those of 3.0, 1.1, and 1.9 cm3·K·mol−1 for single units of the NO−MII−NO complex with S = 4/2 and 3/2, respectively, indicating that highdimensional spin networks could not be established. The decrease in χmolT observed in the low-temperature region might be caused by the antiferromagnetic interactions due to the NO−C3 short contacts (NO−NO contacts for 4aβ) and through DCA. The thermal profiles of χmolT in the χmolT versus T plots for 3β, 4aβ, and 4b were similar, especially for 4aβ and 4b, in spite of the different-dimensional structures, suggesting that the major magnetic interaction might be the metal−NO interactions. In contrast, the thermal profile of χmolT for the diamagnetic metal complex 5β was different from those for 3β, 4aβ, and 4b. Upon cooling, χmolT for 5β was constant until 10 K and then decreased. The thermal profile for 5β might indicate that the magnetic interaction between the 4NOpy units through the ZnII ion was extremely weak. The decrease

Table 4. Magnetic Coupling Constants JM−NOpy/kB and j′/kB for 1, 2β, 3β, 4aβ, 4b, and 5β JM−NOpy/kB (K) j′/kB (K) g

1

2β

3β

4aβ

4b

5β

−72a −0.03e 2.0

−0.8b

45 −0.4e 2.1

45 (−2.3)c −2.2 (fixed) 2.2

43 −2.2 2.2

−1.2d −0.03e 2.0

2.4a

Assumed to be a single magnetic coupling. b|D|/kB = 185 K. cJCu−Cu/kB. dJNO−NO/kB through the ZnII ion. eObtained as the value of zj′/kB divided by 4.

a

J

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Inorganic Chemistry

heterospin complexes, attempts to use NOpy2 (Scheme 1) in place of 4NOpy are in progress.

2β, 3β, 4aβ, 4b, and 5β are summarized in Table 4. Complex 1 showed Mn II −NO antiferromagnetic interactions with JMn−NOpy/kB = −72 K, which was estimated to be a single magnetic coupling parameter between the aminoxyl and the manganese ion to form a ferrimagnetic chain, and behaved as a spin-canted antiferromagnet with TN = 12 K in the lowtemperature region. Complex 2β containing the CoII ion with a large magnetic anisotropy showed the magnetic behavior of a spin-canted antiferromagnet with TN = 6 K. It is worthy of note that the CoII−NO magnetic interaction from the z axis of the CoII ion in 2β was extremely weak. In contrast, isomorphous 3β showed paramagnetic behavior with the NiII−NO ferromagnetic interaction of JNi−NOpy/kB = 45 K. The complexes 4aβ and 4b showed paramagnetic behavior similar to that of 3β, and the CuII−NO magnetic interactions were ferromagnetic with JCu−NOpy/kB = 45 and 43 K, respectively. The obtained JM−NOpy/kB values for 3β, 4aβ, and 4b are comparable to those for the corresponding discrete complexes [MII(hfac)2(4NOpy)2] (47 and 60 K for the NiII and CuII complexes, respectively2) In 3β, 4aβ, and 4b, antiferromagnetic interactions between the local complex units operated in the lowtemperature region, which were simulated as j′/kB. The obtained j′/kB values of −0.4 and −2.2 K for 3β and 4b, respectively, were due to the antiferromagnetic interactions through DCA and/or the short NO−C3 contacts, which could not be distinguished. However, the magnetic behavior of the antiferromagnet observed in 1 having no short NO−C3 contacts suggested the existence of an antiferromagnetic interaction through DCA. In 5β, the magnetic coupling between the aminoxyls through the diamagnetic ZnII ion might be weakly antiferromagnetic, and the value of JNO−NO/kB = −1.2 K was estimated using the Bleaney−Bowers model.23

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02159. Full crystallographic data (CCDC 1413262, 1413263, 1424111, 14132624, 1415031, 1413265, 1413266, 1413267, 1413268, and 1413269 for 1, 2α, 2β, 3α, 3β, 4aα, 4b, 5α, 5′α, and 5′β, respectively) have been deposited at the Cambridge Crystallographic Database Center and are available on request from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (Fax: + 44-1223-336-033; E-mail: [email protected]; Web: http://www.ccdc.cam.ac.uk). Crystallographic data for 1, 2α, 2β, 3α, 3β, 4aα, 4b, 5α, 5′α, and 5′β (CIF) Supplementary data for experiments, X-ray crystallography, and magnetic properties (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.K.). *E-mail: [email protected] (S.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Platform for Drug Discovery, Informatics, and Structural Life Science from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. S.K. appreciates Grants-in-Aid for Scientific Research (B)(2) (25288038) from the Japan Society for the Promotion of Science (JSPS) and the PRESTO Program on Molecular Technology from the Japan Science Technology Agency (JST).

CONCLUSION

Dicyanamide ion (DCA) was used as a bridging anion, and polymeric heterospin complexes consisting of 3d metal ions, DCA, and 4NOpy were prepared. Polymeric complexes [MII(4NOpy)x(DCA)y(CH3CN)z]n (M = Mn (1), Co (2), Ni (3), Cu (4), Zn (5)) were obtained as single crystals. X-ray crystallography revealed that the crystal structures included a 3D network for 1, 2-D networks for 2, 3, 4a, and 5, and 1-D chains for 4b. Crystals 2, 3, 4a, and 5 having 2-D structures contained CH3CN molecules as crystal solvents, and the solvents gradually desorbed in the ambient atmosphere. Crystals 1 and 2 showed the magnetic behavior of highdimensional spin-canted antiferromagnets (weak ferromagnets) with TN = 12 and 6 K, respectively. In crystals 3, 4a, and 4b, the magnetic interactions between the aminoxyl and the metal ion through the pyridine ring were ferromagnetic with JM−NOpy/kB = 45, 45, and 43 K, respectively. In crystal 5, the magnetic interaction between the aminoxyl centers through the diamagnetic ZnII ion was weakly antiferromagnetic with JNO−NO/kB = −1.2 K. Although the magnetic couplings between the metal ions through DCA were extremely weak and might be antiferromagnetic, DCA effectively functioned as a bridging anion to afford high-dimensional heterospin metal complexes having relatively stiff structures. The observed desorption of crystal solvents without destruction of the crystal structures would be expected to apply for a new functional MOF material24 exhibiting unique magnetic behavior in a heterospin system. To improve and extend the magnetic coupling network of the

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REFERENCES

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