Inorg. Chem. 2016, 55, 10859

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Oct 7, 2016 - macrocyclic ligand tdmmb, the CoII centers in all of these ...... Coordinate Cyclic Alkyl(amino) Carbene Stabilized Iron(I) Com- plexes.


Probing the Effect of Axial Ligands on Easy-Plane Anisotropy of Pentagonal-Bipyramidal Cobalt(II) Single-Ion Magnets Dong Shao,† Shao-Liang Zhang,† Le Shi,† Yi-Quan Zhang,*,‡ and Xin-Yi Wang*,† †

State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China ‡ Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, China S Supporting Information *

ABSTRACT: We herein reported the synthetic, structural, computational, and magnetic studies of four air-stable heptacoordinated mononuclear cobalt(II) complexes, namely, [CoII(tdmmb)(H2O)2][BF4]2 (1), [CoII(tdmmb)(CN)2]·2H2O (2), [CoII(tdmmb)(NCS)2] (3), and [CoII(tdmmb)(SPh)2] (4) (tdmmb = 1,3,10,12-tetramethyl-1,2,11,12-tetraaza[[3](2,6)pyridino[3](2,9)-1,10-phenanthrolinophane-2,10-diene; SPh− = thiophenol anion). Constrained by the rigid pentadentate macrocyclic ligand tdmmb, the CoII centers in all of these complexes are in the heptacoordinated pentagonal-bipyramidal geometry. While the equatorial environments of these complexes remain very similar to each other, the axial ligands are systematically modified from C to N to O to S atoms. Analyses of the magnetic data and the ab initio calculations both reveal large easy-plane magnetic anisotropy (D > 0) for all four complexes. While the experimentally obtained D values do not show any clear tendency when the axial coordinated atoms change from C to N to O atoms (complexes 1−3), the largest value is for the heavier and softer S-atom-coordinated complex 4. Because of significant magnetic anisotropy, all four complexes are field-induced single-ion magnets. This work represents a delicate modification of the magnetic anisotropy by tuning the chemical environment of the metal centers.

INTRODUCTION Single-molecule magnets (SMMs) have attracted considerable attention in the field of molecular magnetism for their potential applications in high-density information storage and quantum computing.1,2 As a consensus, high magnetic anisotropy is considered to be most critical for high-performance SMMs. However, fine-tuning of the magnetic anisotropy has been proven to be extremely difficult because it is influenced by multiple factors such as molecular symmetry, ligand-field strength, spin−orbit coupling (SOC), zero-field splitting (ZFS), and so on.3 The pursuit for better control of the magnetic anisotropy has inspired intensive studies, including numerous reports of mononuclear SMMs, also known as single-ion magnets (SIMs).4 Besides the easier preparation of mononuclear complexes compared to multinuclear clusters, the principles of ligand-field theory make the prediction and design of the magnetic anisotropy, and thus the SMM properties, much more feasible. For the SIMs with 3d metal centers (3d-SIMs), since the report of the first FeII SIM by Long et al. in 2010,5 a number of SIMs containing CrII,6 FeI/II/III,7 CoI/II,8−11 NiI/II/III,12 and MnIII centers,13 in different coordination numbers (2−8) and molecular symmetries have been reported. Considering their magnetic anisotropy, although the majority of © 2016 American Chemical Society

the 3d-SIMs have large easy-axis magnetic anisotropy with a negative ZFS parameter D (D < 0), the observation of fieldinduced slow magnetic relaxation in complexes with easy-plane anisotropy (D > 0) has promoted a great deal of studies.10,11 Actually, Ruiz, Luis, and co-workers proposed that, for a Kramers ion of strong magnetic anisotropy, field-induced slow magnetic relaxation can be observed regardless of the sign of the D values.11e Along this line, a number of CoII-based SIMs with positive D values were reported in the literature.10,11 To manipulate the magnetic anisotropy of a SIM, efforts have been mainly focused on the fine-tuning of the electronic configurations of the metal centers. Synthetically, this can be achieved by variation of the coordination atoms, the coordination numbers, and thus the molecular symmetry of the metal centers.9b−d,14 As an illustrative example, Saber and Dunbar have investigated the effect of ligands with different donor atoms (N, P, and As) on the magnetic anisotropy of the pseudotetrahedral cobalt complexes.9b Long et al. have also reported a family of tetrahedral complexes, [Co(EPh)4]2− (E = O, S, and Se), where the magnetic anisotropy can be adjusted by varying the donor atoms from O to the heavier and softer Received: April 7, 2016 Published: October 7, 2016 10859

DOI: 10.1021/acs.inorgchem.6b00854 Inorg. Chem. 2016, 55, 10859−10869


Inorganic Chemistry

For example, in the above-mentioned Co-H2dapb system,15f both the pentagonal (neutral or deprotonated) and axial (O or N atoms) ligands were altered, which results in several possible factors on the change of the magnetic anisotropy. To investigate the influence of the axial ligands on the magnetic anisotropy of the pentagonal-bipyramidal cobalt(II) compounds, we selected another pentadentate macrocyclic ligand, 1,3,10,12-tetramethyl-1,2,11,12-tetraaza[3](2,6)pyridino[3](2,9)-1,10-phenanthrolinophane-2,10-diene (tdmmb; Scheme 1). This neutral ligand with a closed pentadentate ring is much more rigid than ligands LN5 and H2dapb and offers an ideal ligand for the aforementioned study. As a matter of fact, this ligand was reported to yield a series of transition-metal complexes where the axial ligand can be replaced by a large number of different ligands, such as water, pyridine, imidazole, and so on.17 In this work, from the reported compound [CoII(tdmmb)(H2O)2][BF4]2 (1),18 we successfully synthesized and structurally characterized three closely related compounds, [CoII(tdmmb)(CN)2]·2H2O (2), [CoII(tdmmb)(NCS)2] (3), and [CoII(tdmmb)(SPh)2] (4). All of these compounds have very similar pentagonal-bipyramidal CoII centers. While the equatorial environments of these complexes remain very similar to each other, the axial ligands were systematically modified from C to N to O to S atoms. Magnetic measurements revealed that all complexes are of easy-plane magnetic anisotropy and show field-induced slow magnetic relaxation. Ab initio CASSCF/RASSI calculations were performed, confirming their easy-plane magnetic anisotropy. The present results revealed that the sign of the D value (nature of the magnetic anisotropy) is largely unaffected by axially coordinated atoms, while heavier and softer axially coordinated atoms can significantly enhance the magnitude of the easyplane magnetic anisotropy.

atoms S and Se, leading to observation of the SIM behavior under a zero direct-current (dc) field for the [Co(SPh)4]2− complex.9c,d Furthermore, the magnetic anisotropy was found to be enhanced via heavy halide ligand effects, as reported for a series of CrII/III,14a MnII,14b and NiII 14c compounds by different groups. These results show that modification of the coordination environment indeed plays a critical role in the enhancement of the magnetic anisotropy. To better understand the influence of the coordination environment on the magnetic anisotropy, a delicate design of the ligand field of the metal center is desired. Ideally, the variables upon ligand substitution should be as few as possible in order to establish a conclusive trend of the magnetic anisotropy upon these variables. For such studies, the coordination environment of the metal center should be able to not only induce the large magnetic anisotropy but also maintain the coordination geometry upon ligand substitution with different donor atoms. In this regard, compounds with pentagonal-bipyramidal geometry constructed from pentadentate macrocyclic ligands are of special interest. These complexes usually have large magnetic anisotropy, being easy-axis or easyplane depending on the metal centers. Furthermore, the axial weakly coordinated atoms can be easily replaced, which enables systematic modification of the coordination environment and magnetic anisotropy.7a,10,15 In addition, we have to emphasize here that lanthanide compounds with pentagonal-bipyramidal geometry is also of special interest.16 In fact, very recently, dysprosium(3+) compounds of pentagonal-bipyramidal geometry have been reported to have very high energy barriers and blocking temperatures, showing the great potential of the pentagonal-bipyramidal geometry for the construction of highperformance SIMs.16a,b Recently, our group reported the first observation of fieldinduced SIM behavior in a series of pentagonal-bipyramidal cobalt(II) compounds constructed from the pentagonal ligands LN5 and H2dapb (Scheme 1).10 Easy-plane magnetic anisotropy


Materials and Synthesis. All reagents were obtained from commercially available sources and used as received unless otherwise noted. 2,9-Dichloro-1,10-phenanthroline and 2,9-di(L-methylhydrazino)-1,10-phenanthroline were synthesized following the previously reported method.19 The powder of the starting material 1 was synthesized according to a literature procedure.17 The crystals of 1−4 for single-crystal X-ray analyses and property measurements were prepared by the following methods. Caution! Cyanides and methylhydrazine are highly toxic and dangerous. They should be handled in small quantities with great care. [CoII(tdmmb)(H2O)2][BF4]2 (1). The obtained orange powder (33 mg, 0.05 mmol) synthesized according to the literature17 was dissolved in a mixed solvent containing 4 mL of acetonitrile and 2 mL of water. Orange block single crystals formed after slow evaporation in a dark place for 3 days. The crystals were filtered and washed with water. Yield: ∼19 mg, 57%. Elem anal. Calcd for C23H25CoB2F8N7O2: C, 41.60; H, 3.79; N, 14.76. Found: C, 41.61, H, 3.82; N, 14.78. IR (KBr, cm−1): 3388 (s), 3039 (s), 3002 (w), 1593 (vs), 1576 (vs), 1553 (vs), 1500 (vs), 1477 (vs), 1426 (vs), 1416 (vs), 1363 (vs), 1307 (vs), 1258 (s), 1187 (s), 1150 (s), 1095 (vs), 1034 (vs), 873 (s), 796 (s). [Co(tdmmb)(CN)2]·2H2O (2). A solution of 1 (33 mg, 0.05 mmol) in 3 mL of water/acetonitrile (1:2, v/v) was placed at the bottom of one arm of an H-shaped tube, and a solution of KCN (10 mg, 0.15 mmol) in 3 mL of the same solvent system was placed on the other side. A total of 10.0 mL of water/acetonitrile (1:3, v/v) was layered on the top of the solutions on both sides to provide a diffusion pathway. Dark-orange block crystals of 2 formed in 2 weeks, which were washed with water and dried in air. Yield: ∼15 mg, 55%. Elem anal. Calcd for C25H25CoN9O2: C, 55.35; H, 4.64; N, 23.24. Found: C, 55.95, H, 4.69; N, 23.68. IR (KBr, cm−1): 3404 (vs), 3075 (w), 2095 (vs), 1592 (vs), 1544 (vs), 1495 (vs), 1472 (s), 1407 (s), 1350 (vs), 1300 (vs), 1252

Scheme 1. Structures of the Pentadentate Macrocyclic Ligands for Constructing Pentagonal-Bipyramidal Complexes and Synthesis of the Starting Material Compound 1

was verified experimentally and theoretically15 and further confirmed by high-field electron paramagentic resonance in a very recent study by Chen et al.11b As a matter of fact, very recently, as we were preparing this paper, Gogoi et al. reported their study on the alteration of the magnetic anisotropy by modulation of the coordination environment of CoII-H2dapb complexes.15f Although these compounds offer the opportunity to probe the influence of the axial donor atoms on their easyplane magnetic anisotropy, these ligands are not rigid enough to maintain the same equatorial geometry of the CoII centers. 10860

DOI: 10.1021/acs.inorgchem.6b00854 Inorg. Chem. 2016, 55, 10859−10869


Inorganic Chemistry (vs), 1180 (vs), 1148 (vs), 1089 (vs), 1032 (vs), 849 (s), 796 (s), 746 (s), 657 (m). [Co(tdmmb)(NCS)2] (3). 1 (33 mg, 0.05 mmol) was dissolved in 2 mL of water/acetonitrile (2:1, v/v) and added to an 8 mL test tube. Another 2 mL of the same mixed-solvent system was carefully added to the top of the solution of 1 as a buffer layer. Finally, 2 mL of a solution of NaSCN [81 mg, 0.1 mmol, in water/acetonitrile (1:1, v/ v)] was layered on the top of the buffer layer. Over the course of several days, orange single crystals formed at the interface. Yield: ∼18 mg, 63%. Elem anal. Calcd for C25H21CoN9S2: C, 52.62; H, 3.71; N, 22.09. Found: C, 52.59, H, 3.75; N, 21.94. IR (KBr, cm−1): 2080 (vs), 1591 (vs), 1472 (s), 1354 (s), 1302 (vs), 1254 (s), 1183 (s), 1151 (vs), 1091 (vs), 1034 (vs), 797 (m). [Co(tdmmb)(SPh)2] (4). Upon replacement of NaSCN with NaSPh, 4 was also obtained as dark block crystals according to the same synthetic procedure as that of 3. Yield: ∼21 mg, 57%. Elem anal. Calcd for C35H31CoN7S2: C, 62.49; H, 4.64; N, 14.57. Found: C, 62.30, H, 4.67; N, 14.44. IR (KBr, cm−1): 1591 (vs), 1568 (vs), 1496 (vs), 1465 (vs), 1360 (s), 1303 (s), 1187 (s), 1147 (s), 1079 (s), 1033 (s), 844 (s), 790 (s), 746 (s), 694 (s). Physical Measurements. IR spectral data were measured on KBr pellets using a Nexus 870 Fourier transform infrared spectrometer in the range of 4000−400 cm−1. Elemental analyses of C, H, and N were performed with an Elementar Vario Micro analyzer. Powder X-ray diffraction (PXRD) data were recorded at 298 K on a Bruker D8 Advance diffractometer with a Cu Kα X-ray source (λ = 1.54056 Å) operated at 40 kV and 40 mA. Thermal gravimetric analysis (TGA) was measured in Al2O3 crucibles using a PerkinElmer thermal analyzer in the temperature range of 20−700 °C under a nitrogen atmosphere. Magnetic measurements from 2 to 300 K with an applied dc field of up to 7 T were performed using a Quantum Design SQUID vibrating sample magnetometer on the crushed samples from single crystals of the compounds. Alternating-current (ac) magnetic susceptibility data were collected in a zero dc field or different applied dc fields in the temperature range of 2−10 K, under an ac field of 2 Oe, oscillating at frequencies in the range of 1−950 Hz. All magnetic data were corrected for the diamagnetic contributions of the sample holder and of core diamagnetism of the sample using Pascal’s constants. X-ray Crystallography. Single-crystal X-ray crystallographic data were collected on a Bruker APEXII or APEX Duo diffractometer with a CCD area detector (Mo Kα radiation, λ = 0.71073 Å) at room temperature or 123 K. The APEXII program was used to determine the unit cell parameters and for data collection. The data were integrated and corrected for Lorentz and polarization effects using SAINT.20 Absorption corrections were applied with SADABS.21 The structures were solved by direct methods and refined by a full-matrix least-squares method on F2 using the SHELXTL crystallographic software package.22 All non-H atoms were refined anisotropically. H atoms of the organic ligands were refined as riding on the corresponding non-H atoms. Additional details of the data collection and structural refinement parameters are provided in Table 1. Selected bond lengths and angles of 2−4 are listed in the Supporting Information (Table S1). CCDC 1444971−1444973 are the supplementary crystallographic data for this paper. They can be obtained free of charge from the Cambridge Crystallographic Data Center via www.

Table 1. Crystallographic Data and Structure Refinement Parameters for Compounds 2−4 formula Mr [g mol−1] cryst syst space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z T [K] ρcalcd [g cm−3] μ(Mo Kα) [mm−1] F(000) Rint Tmax/Tmin R1a/wR2b [I > 2σ(I)] R1/wR2 (all data) GOF on F2 max/min [e Å−3]




C25H25CoN9O2 542.47 monoclinic P21/c 8.7090(4) 15.0672(6) 18.6382(7) 90 104.019(2) 90 2372.86(1) 4 123(2) 1.518 0.768

C25H21CoN9S2 570.56 orthorhombic Fdd2 34.7559(2) 9.6439(5) 14.2612(8) 90 90 90 4780.1(4) 8 123(2) 1.395 0.929

C35H31CoN7S2 672.72 monoclinic P21/c 12.311(3) 19.802(5) 11.859(3) 90 93.549(4) 90 2885.5(1) 4 123(2) 1.549 0.781

1124 0.0532 0.7912/0.6864 0.0307/0.0769

1440 0.0170 0.8967/0.6799 0.0504/0.1189

1396 0.0546 0.8347/0.6778 0.0426/0.1090




1.030 0.364/−0.363

1.102 1.250/−1.128

1.038 1.204/−0.773

R1 = ∑||F o| − |Fc ||/∑|F o |. ∑[w(Fo2)2]}1/2. a


wR2 = {∑[w(F o 2 − Fc 2 ) 2]/

published method.17 Using compound 1 as the starting material, compounds 2−4 were synthesized by the metathetical reaction with the appropriate anions. The phase purity of the bulk samples was confirmed by PXRD spectra and elemental analysis (Supporting Information, Figure S1). The thermal stability of the four compounds was probed by TGA under a nitrogen atmosphere (Supporting Information, Figure S2). The results indicated that 3 and 4 exhibited negligible mass loss up to approximately 350 and 250 °C, respectively, suggesting the high thermal stability of 3 and 4. The 6.8% weight loss of 2 below 120 °C can be attributed to the release of lattice water molecules, corresponding to the removal of two crystallized water molecules. We also tried to replace the neutral water molecules in 1 with the phenolate anion (PhO−), hoping to synthesize a neutral [CoII(tdmmb)X2] structure similar to that in compounds 2−4. However, in the same synthetic procedure as that in 2−4, the seemingly strongly coordinated PhO− anion cannot substitute for the axial water molecules. As confirmed by the X-ray structure, the PhO− anions just serve as counteranions to the [CoII(tdmmb)(H2O)2]2+ cation, and a hydrogen-bond-bridged 1D chain compound was formed instead (Supporting Information, Figure S3). Crystal Structure Descriptions. Although the structure of 1 was reported before, we remeasured its structure for the sake of consistency. The crystal systems and space groups for 1−4 are monoclinic P2 1/c for 1, monoclinic P21 /c for 2, orthorhombic Fdd2 for 3, and monoclinic P21/c for 4 (Table 1). As has been reported for 1, all complexes are mononuclear compounds, with the Co2+ center residing in a slightly distorted pentagonal bipyramid. The equatorial plane is formed by the five N atoms from the pentadentate tdmmb ligand, while the

RESULTS AND DISCUSSION Synthesis. The pentadentate macrocyclic ligands reported in the literature were usually synthesized by the condensation reaction of 2, 6-diacetylpyridine with different amines in the presence of metal ions as templates. For better coplanarity of the ligand, we chose 2,9-di(L-methylhydrazino)-1,10-phenanthroline as the “diamine” for the condensation reaction, resulting in the rigid ligand tdmmb with five N atoms as donor atoms. When the reaction was performed in the presence of the “innocent” BF4− anion, compound 1 with two water molecules as the axial ligands was synthesized according to the 10861

DOI: 10.1021/acs.inorgchem.6b00854 Inorg. Chem. 2016, 55, 10859−10869


Inorganic Chemistry

Figure 1. Crystal structures of 1−4. H atoms are omitted for clarity.

pentagonal-bipyramidal cobalt(II) compounds (Figure 2 and Supporting Information, Figure S5).10 At 300 K, the χMT values

axial positions are occupied by different donor groups: H2O for 1, CN− for 2, SCN− for 3, and PhS− for 4 (Figure 1). For compound 2, we have to point out that, although it is very difficult to distinguish the C and N atoms from the singlecrystal X-ray data, we believe that the coordination atom in 2 is the C atom. Because the N atom is more electronegative than the C atom so that its lone pair is less available for σ bonding, the bond between the terminal CN− group and the transition metal is preferably formed between the C atom and the metal rather than from the N atom. As a matter of fact, a search in the CCDC database led to only a few examples containing the Co− NC motifs except when the CN− group acted as a bridge and had its C atom coordinated to another metal center. As designed, the [CoII(tdmmb)]2+ motifs in these four compounds have almost the exact same geometry, thanks to the rigid ligand. The Co−Neq bond lengths, the Neq−Co−Neq bond angles, and the distances between the Co2+ ion and the equatorial N5 plane are all very close to each other (Supporting Information, Table S1). The Xaxial−Co−Xaxial (X = O, C, N, S) bond angles are very close to linearity, being 178.1(4), 179.3(8), 175.5(3), and 176.50(2)° for 1−4, respetively. As for the Co−Xaxial bond lengths, the Co−Saxial lengths in 4 [2.520(1) and 2.546(1) Å] are significantly longer than the Co−Oaxial lengths in 1 [2.123(4) and 2.123(4) Å], the Co−Caxial lengths in 2 [2.151(2) and 2.141(2) Å], and the Co−Naxial lengths in 3 [2.171(5) and 2.173(5) Å], resulting a significantly axially elongated pentagonal-bipyramidal geometry in 4. To evaluate the degree of deviation from the ideal pentagonal bipyramid of D5h symmetry, the continuous shape measures (CShMs) were calculated.23,24 The CShM values related the pentagonal bipyramid for 1−4 are 0.101, 0.109, 0.195, and 0.667, respectively. Obviously for 1−3, the small CShM values indicate the closeness of their coordination geometry to the ideal pentagonal bipyramid. For 4, the relatively large value is consistent with the long Co−Saxial bonds and elongated environment. To study the SIM behavior, we hope that the spin centers are well-isolated from each other and the possible intermolecular magnetic interaction can be eliminated. As can be seen in the packing diagrams in the Supporting Information (Figure S4), the mononuclear units in 1−4 are indeed well-isolated from each other, with the shortest Co···Co distances being 8.929(1), 8.510(1), 8.607(9), and 8.342(2) Å for 1−4, respectively. These long distances make the intermolecular dipole−dipole interactions ignorable and leave the axial donor atoms as the main variable in the easy-plane magnetic anisotropy. Magnetic Properties. Variable-temperature magnetic susceptibility data of 1−4 in the temperature range of 2−300 K were measured on ground single-crystal samples under a dc field of 1 kOe. The shapes of the χMT versus T curves of 1−4 are found to be quite similar to those of our previously reported

Figure 2. Temperature-dependent magnetic susceptibility data for 4 measured at 1 kOe. Inset: Magnetization curve for 4 measured at 2 K. The red solid lines represent the best fits by PHI, and the blue line is for the ab initio calculation result.

of 1−4 are 2.31, 2.33, 2.66, and 2.52 cm3 mol−1 K, respectively. These values are clearly larger than the spin-only value of 1.875 cm3 mol−1 K for a high-spin CoII ion (S = 3/2 and g = 2), indicating the significant orbital angular momentum of CoII ions. Upon cooling from 300 K, the χMT values decreased monotonously to 2 K, reaching 1.37, 1.22, 1.34, and 1.38 cm3 mol−1 K for 1−4, respectively. Similar to the reported compounds, the decrease of the χMT curves at low temperature should be mainly due to the intrinsic magnetic anisotropy of the CoII ions.25 In addition, the magnetization data for 1−4 were measured at 2.0 K with a field of up to 70 kOe (inset of Figure 2 and Supporting Information, Figure S6). The magnetization values at 70 kOe (M = 2.10, 2.09, 2.07, and 2.04 μB for 1−4, respectively) are consistent with the reported values.8−11,15 To evaluate the magnetic anisotropy of compounds 1−4, both the magnetic susceptibility data over the full temperature range and the magnetization data at 2 K of 1−4 were fitted simultaneously using the PHI26 program with the following spin Hamiltonian: k

Ĥ =

∑ ∑ k = 2,4,6 q =−k


Bkq θkÔk + μB g ·S·B (1)

where are the crystal-field parameters in Steven’s notation, θk are the operator equivalent factors, Ô qk are the operator equivalents, and S, B, and μB are the spin operator, magnetic field vector, and Bohr magneton, respectively. The two terms correspond to the crystal-field and Zeeman effects. This Hamiltonian is closely related to the commonly used Hamiltonian H = D[Sz2 − S(S + 1)/3] + E(Sx2 − Sy2) + μBg· S·B, where the D and E are the axial and rhombic ZFS Bqk


DOI: 10.1021/acs.inorgchem.6b00854 Inorg. Chem. 2016, 55, 10859−10869


Inorganic Chemistry

Table 2. Magnetic Parameters (D, E, g, gx, gy, gz, and Ueff) Obtained by the Magnetic Fitting and Ab Initio Calculation for Complexes 1−4 D (E) (cm−1) compound 1 2 3 4

Anisof it2.0 32.7 22.8 22.6 39.9

(6.8) (0) (0) (1.1)

PHI 25.6 17.4 26.3 34.5

(−1.0) (−0.6) (−0.1) (−1.8)

calcda 34.5 35.1 37.7 39.7

g for Anisofit2.0

(−2.7) (−3.4) (−3.6) (−6.8)

2.244 2.207 2.372 2.238

gx, gy, gzb for PHI 2.228, 2.126, 2.260, 2.239,

2.0 2.0 2.0 2.0

gx, gy, gzc for calcd 2.303, 2.307, 2.349, 2.413,

2.238, 2.224, 2.264, 2.252,

1.998 1.998 1.996 1.993

Ueff/K (cm−1) 42.2 48.9 49.2 54.7

(29.3) (34.0) (34.2) (38.0)


CASPT2/RASSI/SINGLE_ANISO calculation using the MOLCAS 7.8 program (see the computational details in the Supporting Information). bgz was fixed at 2.0 in the fitting progress. cThese g values were obtained from both the ground and first excited states.

positive D value.15a To increase the D value, all excited states, especially the first excited state, should have a small energy difference between the ground state. Chemically, this could be achieved by using apical ligands with weaker σ-donor ability. Clearly, the σ-donor ability of the S atom is weaker than that of the C, N, and O atoms and leads to the evidently large D value for compound 4. To probe the slow magnetic relaxation, frequency-dependent ac susceptibility measurements were performed under zero (Supporting Information, Figure S8) and nonzero applied dc fields (Figures 4 and 5 and the Supporting Information, Figures S9−S12). No out-of-phase ac signal (χ″) was observed under a zero applied dc field for all complexes (Supporting Information, Figure S8). This phenomenon is probably due to the direct relaxation or fast quantum tunneling of magnetization (QTM) between the Ms = ±1/2 levels and is consistent with the majority of the CoII-based SIMs. As for QTM, although it should be suppressed at zero dc field due to spin-parity effects for the Kramers ion, other effects such as the dipolar or hyperfine interactions might mediate this fast relaxation. In order to determine the optimum dc field to suppress the QTM effect, ac measurements were performed on all four compounds under various dc fields up to 4500 Oe at 1.8 K (Supporting Information, Figure S9). As can be seen, frequency-dependent ac signals were observed upon application of a 200 Oe dc field, indicating efficient suppression of the QTM effect and fieldinduced slow magnetic relaxation of 1−4. The peak maxima move to lower frequencies as the field increases until around 1500 Oe and then move to higher frequencies when the field increases further to 4500 Oe, which is likely due to the increased relaxation rate by direct relaxation procession. These frequency-dependent ac data were drawn as Cole−Cole plots (Supporting Information, Figure S10) and fitted by the generalized Debye model, 28 giving the field-dependent relaxation time τ (Supporting Information, Figure S13). As can be seen, the relaxation time shows peaks at around 1500 Oe for compounds 1, 3, and 4 and 2000 Oe for compound 2. From these results, comprehensive ac measurements in the temperature range of 1.8−10 K at different temperatures for all four compounds were performed under a dc field of 1500 Oe for the sake of consistency (Figure 4 and the Supporting Information, Figure S11). These data were used to construct the Cole−Cole plots for 1−4 at different temperatures (Figure 5a and the Supporting Information, Figure S12). These curves displayed typical semicircles at low temperatures. Fitting the data using the generalized Debye model28 gave the values and distribution of the relaxation time (τ and α; Supporting Information, Table S2). The obtained α values are in the ranges of 0.07−0.22, 0.04−0.20, 0.05−0.18, and 0.005−0.21 for 1−4, respectively, which suggest the relatively narrow distribution of the relaxation time. In addition, the Arrhenius plots [ln(τ) vs

parameters and can be calculated by the following equation: D = 3B02θ2 and E = B22θ2. The obtained D values (Table 2) were not sensitive to the initial value of g. Furthermore, the negative initial values of D would give terribly unmatched fitting curves, indicating the correct choice of the positive sign of D. According to the ab initio calculations (vide post), the gz values were fixed at 2.0. The best fits of the data gave D = 25.6(0) cm−1, E = −1.0(4) cm−1, and gx,y = 2.228(2) for 1, D = 17.3(9) cm−1, E = −0.6(3) cm−1, and gx,y = 2.126(0) for 2, D = 26.3(5) cm−1, E = −0.0(8) cm−1, and gx,y = 2.259(7) for 3, and D = 34.5(8) cm−1, E = −1.79(4) cm−1, and gx,y = 2.238(7) for 4. These large and positive D values are consistent with the values (25−35.8 cm−1) reported for other pentagonalbipyramidal cobalt(II) complexes, which can be explained as a consequence of the SOC through second-order perturbation involving the electronic ground and excited states.10,11b,15 Furthermore, the reduced magnetization curves for 1−4 were measured at different magnetic fields of 1−7 T in the temperature range of 2−10 K (Figure 3 and Supporting

Figure 3. Reduced magnetization data of 4 collected in the temperature range of 2−10 K under dc fields of 1−7 T. The solid lines correspond to the best fits obtained with Anisof it2.0.

Information, Figure S7), and these isothermal curves were well fitted by Anisofit 2.027 (H = D[Sz2 − S(S + 1)/3] + E(Sx2 − Sy2) + μBg·S·B), giving the D, E, and g values as listed in Table 2. These values are comparable to the fitting values by PHI and further confirmed the validity of the obtained results. As can be seen in Table 2, there seems to be no any obvious correlation between the experimentally obtained D values and the axial donor atoms (O, C, N, and S) in 1−4. However, we did notice that when the axial ligand is SPh−, compound 4 has the largest D value in all four compounds. The softness of the S atom should lead to a weaker axial ligand-field strength, which is known to be able to enhance the magnitude of the D value.9a−d As a matter of fact, Mallah and co-workers have pointed out in their work that, for the CoII ion in pentagonalbipyramidal geometry, all of the excited states contribute to a 10863

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Inorganic Chemistry

Figure 4. Frequency dependence of the in-phase (χ′) and out-of-phase (χ″) parts of the ac magnetic susceptibilities for 1−4 collected under a 1.5 kOe dc field over the temperature range of 1.8−6.0 K.

Information, Figure S13). Fitting the high-temperature data using the Arrhenius law τ = τ0 exp(Ueff/kBT) affords the effective spin-reversal energy barrier (Ueff) and preexponential factor τ0. Thus, Ueff was obtained as 42.2, 48.9, 49.2, and 54.7 K (29.3, 34.0, 34.2, and 38.0 cm−1) with τ0 = 1.1 × 10−6, 3.2 × 10−7, 1.0 × 10−6, and 2.1 × 10−7 s for 1−4, respectively. Apparently, the above-obtained efficient energy barriers for 1−4 are only based on the high-temperature data considering only the thermally assisted Orbach process and are significantly lower than the estimated 2D values. As can be seen in Figure 5b and the Supporting Information (Figure S13), the plots of ln(τ) versus T−1 show clear curvature, especially at low temperatures. This behavior suggests the coexistence of multiple relaxation pathways, such as the direct, QTM, Raman, and Orbach relaxation processes.7g These processes can be described by eq 2 including the sum of four terms: τ −1 = AH2T +

B1 1 + B2 H2

+ CT n + τ0−1 exp( −Ueff /kBT ) (2)

where A, B1, B2, C, and n are coefficients and H, T, τ0, Ueff, and kB have their usual meanings. The first two terms correspond to the direct and QTM processes, which are field-dependent, while the latter two terms represent the contributions of the two-phonon Raman and Orbach mechanisms. To avoid overparameterization, the coefficients related to the direct and QTM relaxation processes (A, B1, and B2) can be determined from the field dependence of the relaxation time at the lowest temperature of 1.8 K, where the contribution of the twophonon Raman and Orbach relaxation should be negligible. As can be seen from the τ versus H plots at 1.8 K (insets of Figure 5b and the Supporting Information, Figure S13), the fits of these curves by the first two terms of eq 2 are generally very good. The obtained values were listed in Table 3. We can see that the QTM is dominant at low field and is suppressed upon application of a dc field. Under higher dc field, the relaxation becomes rapid because the direct relaxation becomes dominant. Then, parameters A, B1, and B2 were fixed, and the Arrhenius plots of 1−4 were fitted using eq 2. The obtained values of C, n, τ0, and Ueff are also listed in Table 3. The observed magnetization reversal barriers [Ueff = 85.2, 76.4, 82.2, and 98.6 K (59.2, 53.1, 57.1, and 68.5 cm−1) for 1−4] are significantly larger than the barriers considering only the Orbach process, which are also closer to the estimated 2D values. As for the n coefficients corresponding to the Raman process, the values that we obtained for all four compounds were around 5, which is smaller than the expected value of 9 for

Figure 5. (a) Cole−Cole plots for 4 under 1500 Oe dc field. The solid lines are the best fits to the experiments with the generalized Debye model. (b) Arrhenius plot with ln(τ) versus T−1 for 4. The red line shows the fit of the data in the range of 4.6−6 K to the Arrhenius law τ = τ0 exp(Ueff/kBT), assuming the Orbach relaxation mechanism. The green line represents the fit to the data using eq 2. Inset: Field dependence of the magnetic relaxation time at 1.8 K for 4 and its approximation by τ−1 = AH2T + B1/(1 + B2H2).

T−1] were constructed from the above-obtained relaxation time at different temperatures (Figure 5b and the Supporting 10864

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Inorganic Chemistry Table 3. Summary of the Obtained Parameters of the Magnetic Relaxation for Complexes 1−4 compound 1 2 3 4

A (s−1 K−1) 45.2 14.6 27.5 17.8

B1 (s−1) 160 560 854 390

B2 (kOe−2)


0.029 1.87 12.06 4.01

4.3 5.1 4.8 5.5

a Raman relaxation in Kramers ions, suggesting probably an optical acoustic Raman process involving a virtual state, where both acoustic and optical phonons are considered.11e,29,30 Nevertheless, we noticed that the magnitudes of the Ueff values from eq 2 for all complexes 1−4 have a trend similar to that of their experimental D values, which suggests that larger magnetic anisotropy is advantageous for the construction of SIMs with larger energy barriers, even for an easy-plane magnetic system. All of the obtained D (both experimentally and theoretically) and Ueff values are the highest for compound 4, indicating the effect of the softer and heavier S atoms on the highperformance SIMs. Theoretical Calculations. Further insight into the magnetic anisotropy in the four complexes was obtained using ab initio calculations. The axial and rhombic ZFS parameters (D and E), the g tensor from both the ground Kramers doublet and first excited states, and the local magnetic susceptibility of complexes 1−4 were calculated using complete-active-space second-order perturbation theory (CASPT2) considering the effect of the dynamical electronic correlation based on complete-active-space self-consistent field (CASSCF) with the MOLCAS 7.8 program package.31 Further computational details are given in the Supporting Information. Tables S3 and S4 in the Supporting Information show that the energy differences between the lowest two spin-free states for four complexes are all much larger than those between the lowest two spin−orbit states. Then, we can use the ZFS parameters D and E to depict their magnetic anisotropies. The calculated D and E (cm−1) and g values (x, y, z) for the four complexes are listed in Table 2. Using these parameters, the magnetic susceptibilities of these four complexes were calculated. These calculated curves reproduced the experimental dc data nicely, which make the extracted ZFS parameters more credible (Figure 2 and the Supporting Information, Figure S5). Importantly, the calculated D values of all four compounds are positive, which agrees with the experimental analyses and further confirmed the easy-plane magnetic anisotropy of complexes 1−4. Notably, the D values of 4 from both experiment and computation exhibit the largest positive values in these compounds. As shown in Figure 6, the orientations of gx, gy, and gz on CoII of complexes 1−4 were successfully calculated. All of the hard axes (gz) are almost collinear with the axial ligands, while the easy axes (gx and gy) distribute vertically in the planes of the macrocyclic ligands. Inspecting the gz values of 1−4, we can consider that varying the axial ligands nearly cannot change the gz values. Thus, monoanionic and neutral ligands are also not influential on the easy-plane anisotropy in these systems. The transverse g values (gx and gy in Table 2) are significantly larger than gz, which further confirmed the easy-plane anisotropy of these heptacoordinated CoII ions in distorted pentagonalbipyramidal complexes. To deeply analyze the magnetic anisotropy of complexes 1− 4, ORCA 3.03 calculations32 were performed with a differencededicated configuration interaction (DDCI3)33 method. The

C (s−1 K−n) 15.8 2.9 60.5 0.8

Ueff/K (cm−1)

τ0 (s) 5.2 7.4 8.2 6.5

× × × ×


10 10−10 10−8 10−8

85.2 76.4 82.2 98.6

(59.2) (53.1) (57.1) (68.5)

Figure 6. Orientations of gx, gy, and gz (hard axis) on CoII of complexes 1−4.

SOC operator used was the efficient implementation of the multicenter spin−orbit mean-field (SOMF) concept developed by Hess et al.34 The spin−spin contributions to the D values were also included, although they are very small for our complex. The first CASSCF calculations with 7 3d electrons in the 10 Co 3d-based orbitals [CAS(7,10)] were performed on complexes 1−4, and then DDCI333 on top of the CAS(7,10) reference states was carried out. Our previous calculations13a showed that DDCI3 performed well on calculations of the ZFS parameters D and E for transition-metal-based magnets. In the calculations, the orbitals were determined for the average of 10 S = 3/2 and 40 S = 1/2 roots. All calculations were performed with triple ζ with one polarization function TZVP35 basis set for all atoms. Tight convergence criteria were used in order to ensure that the results are well converged with respect to the technical parameters. The effective Hamiltonian implemented in ORCA was used to extract the ZFS parameters D and E. The calculated values of the ZFS parameters of 1−4, and the first, third, and fourth quartet excited-state contributions are shown in Table 4. Also, the spin-free states before the SOC and also the determinants that compose the wave function of the different states are provided in the Supporting Information (Tables S3 and S4). Table 4 shows that the calculated D and E values using DDCI3 are close to those calculated using CASPT2. From observation of the output of the DDCI3 calculation, only the first, third, and fourth excited states play an important role in the sign and magnitude of D and E. Because the spin−spin coupling has a smaller contribution to the magnetic anisotropy, we only use the spin−orbit operator to analyze the sign and magnitude of D. The 3d ground-state energy levels of the four complexes calculated using B3LYP (see the computational details in the Supporting Information) are shown in the Supporting Information (Figure S14). Table 4 indicates that the first quartet excited state has a small contribution to D for 1 and 2, while the fourth quartet excited state has a small one for 3. Only for 4, all three quartet excited states have the same positive contributions to D, which leads to the largest overall D 10865

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Inorganic Chemistry

Table 4. Calculated D and E (cm−1) of Complexes 1−4 and the Main Perturbative Contributions of the Quartet Excited States to D and E Using DDCI3 1 total first quartet third quartet fourth quartet










35.7 1.7 15.0 12.9

4.2 2.2 15.1 −12.9

40.6 5.3 15.5 15.2

5.2 5.3 15.5 −15.2

40.6 15.7 11.5 7.5

4.2 15.7 −8.3 −2.1

45.4 14.7 12.1 15.5

10.6 14.7 11.9 −15.3

21522103 and 21471077). This work was also supported by the Natural Science Foundation of Jiangsu Province of China (Grants BK20150017 and BK20151542) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

value for compound 4. However, as can be seen in the Supporting Information (Figure S15), the lowest three spinfree excited states (first, third, and fourth quartets) are very multiconfigurational, preventing us from further rationalizing the contributions of the excited states to the total D only using the 3d orbitals.

CONCLUSIONS In summary, we reported here the systematic investigation on the effect of the axial ligands on the magnetic anisotropy of the CoII center in the pentagonal-bipyramidal geometry. To eliminate other factors influencing the magnetic anisotropy, we chose a rigid pentadentate microcyclic ligand to constrain the equatorial coordination environment. Following this strategy, four air-stable cobalt(II) complexes were synthesized and characterized. These four complexes have almost the same equatorial coordination environments, leaving the axial ligands as the sole variable on the magnetic anisotropy. For all four complexes, the easy-plane magnetic anisotropy with large positive D values was confirmed experimentally and theoretically. Because of the presense of the significant magnetic anisotropy, all four complexes show field-induced SIM behavior. Our results show that the easy-plane magnetic anisotropy of these cobalt(II) complexes of pentagonalbipyramidal geometry is maintained upon modification of the axial ligands. However, the magnitude of the D value can be increased by heavier and softer donor atoms. Further attempts are made in order to incorporate such large single-ion anisotropy into larger clusters, chains, and high-dimensional complexes.


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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00854. Tables of selected bond distances, bond angles, and other structural parameters of compounds 1−4, TGA of compounds 2−4, PXRD spectra of 1−4, and additional magnetic figures and tables (PDF) X-ray crystallographic data in CIF format (CIF)




Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the Major State Basic Research Development Program (Grant 2013CB922102) and NSFC (Grants 10866

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Inorganic Chemistry

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