Synthesis of a Neutral Mononuclear Four-Coordinate Co(II) Complex

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Synthesis of a Neutral Mononuclear Four-Coordinate Co(II) Complex Having Two Halved Phthalocyanine Ligands That Shows Slow Magnetic Relaxations under Zero Static Magnetic Field Toshiharu Ishizaki,† Takamitsu Fukuda,*,† Mitsuru Akaki,‡ Akira Fuyuhiro,† Masayuki Hagiwara,‡ and Naoto Ishikawa*,† Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/01/19. For personal use only.

†

Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Center for Advanced High Magnetic Field Science (AHMF), Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

‡

S Supporting Information *

ABSTRACT: Syntheses of a novel pseudotetrahedral fourcoordinate mononuclear Co(II) complex that has two halved phthalocyanine moieties as the ligands, [Co(half-Pc)2] (1), and its magnetic properties as a single molecule magnet (SMM) are reported. A one pot reaction of phthalonitrile and lithium methoxide followed by the coordination to a Co(II) ion gave 1 as an orange solid in a moderate yield. X-ray crystallography on 1 reveals tetragonally distorted coordination geometry around the Co(II) ion. The M−HT−1 plots suggest that 1 has large axial magnetic anisotropy. The ac magnetic susceptibility data of the magnetically diluted 1 (dil.1) clearly show that the complex acts as an SMM even in the absence of the external static magnetic field (Hdc). The influence of intermolecular and intramolecular interactions for the magnetic relaxation behaviors has been discussed by comparison of the magnetic data of 1 and dil.1. The Orbach process is suggested as the predominant mechanism of magnetic relaxations in the high-temperature range, and the Arrhenius plots provide the effective relaxation energy barrier and preexponential factor of Ueff = 54.0 cm−1 and τ0 = 3.17 × 10−10 s, respectively. The direct estimation of the axial anisotropic parameter of 1 was successfully performed by high-field, multifrequency ESR measurements up to 55 T and 2.5 THz. The evaluated axial zero-field splitting (ZFS) energy of 57.0 cm−1 is comparable to the Ueff energy, confirming that the magnetic relaxations are initiated by the thermal excitation from the ground |MS⟩ = |±3/2⟩ states to the |±1/2⟩ states in the hightemperature range. The results of the ab initio calculations based on the CAS(7,5) SCF wave functions indicate that the ground states of 1 consist mainly of |MS⟩ = |±3/2⟩ states, while the first excited states are the mixture of |MS⟩ = |+1/2⟩ and |−1/2⟩.

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INTRODUCTION The past two decades have witnessed marked development in molecular magnetism, and various types of the so-called single molecule magnets (SMMs) have come into being.1 The SMMs have attracted much attention as a candidate for cutting-edge materials for ultra-high-density information storage,2 quantum computing,3 and molecular devices,4 due to their unique magnetic and quantum mechanical properties. Molecules that function as SMMs have a significant energy barrier between the up and down spin states and, therefore, can retain the direction of their magnetic moments for a certain amount of time depending on the height of the barrier even in the absence of the external static magnetic field (Hdc). Although a free paramagnetic metal ion is magnetically isotropic, anisotropic character appears if the ion is subject to an appropriate ligand field. The easy-axis or Ising anisotropy causes the uniaxial preference of the magnetic moment, while the easy-plane anisotropy results in an arbitrary direction of the © XXXX American Chemical Society

magnetic moment within a plane. Aside from a couple of exceptions, the former is required to observe the slow magnetic relaxation behaviors.5 In 1993, Sessoli et al. reported the first example of the SMM, namely, [Mn12O12(OAc)16(H2O)4], which had the ground spin states of MS = 10 and the Ising magnetic anisotropy due to the four Jahn−Teller distortions around the Mn(III) ions.6 Although a large number of the related metal clusters have been prepared since then, it appeared to be difficult to achieve a very high energy barrier between the two distinct spin states because the axial anisotropic parameter (D) is inversely proportional to the ground spin multiplicity.7 The discovery of the first lanthanide-based SMMs, [TbPc2]− and [DyPc2]− where Pc = doubly deprotonated phthalocyanine, opened a new way of developing SMMs.8 The electronic Received: January 30, 2019

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DOI: 10.1021/acs.inorgchem.9b00286 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry states of lanthanide ions can be well-described by using the total angular momentum quantum number, J, rather than the individual L and S due to the large spin−orbit couplings. In the case of [TbPc2]−, the ground and second lowest sublevels were found to be |Jz⟩ = |±6⟩ and |±5⟩, respectively, and the energy gap between these was estimated to be more than 400 cm−1.9 Since the large |Jz| for the ground states is equivalent to the Ising anisotropy, while the large energy gap between the neighboring |Jz⟩ states is related to the energies required to flip the magnetic moments, [TbPc2]− shows slow magnetic relaxations at temperatures even higher than 40 K. In the case of dysprosium complexes, a metallocene complex (J = 15/ 2) has been reported to show slow magnetic relaxations at as high as 100 K with an estimated energy barrier of 1261 cm−1, as well as the observation of the magnetic hysteresis at ca. 60 K,10,11 and more recently, an energy barrier as large as 1541 cm−1 with the magnetic blocking temperature (TB) of 80 K has been achieved by Layfield et al.12 Similar to the lanthanide complexes, mononuclear 3d transition metal complexes also are able to show the SMM properties if the mixing of higher wave functions to the ground states induces the splitting of the originally degenerate spin multiplet.13 Long et al. reported the slow magnetic relaxation behaviors of a low-coordinate trigonal-pyramidal four-coordinate Fe(II) complex, [(tpaMes)Fe]−, in the presence of Hdc, and ascribed them to its large axial magnetic anisotropy arising from the SOC at the iron site.14 A number of mononuclear transition metal SMMs including V(IV),15 Cr(II),16 Mn(III),17−19 Mn(IV),20 Fe(I),21 Fe(II),14,22 Fe(III),23 Co(I),24 Co(II),25−37 Ni(I),38,39 Ni(II),40,41 Ni(III),42 Ru(III),43 and Re(IV)44,45 complexes have been reported to date, although most of these need the Hdc to exhibit the slow magnetic relaxations. Recently, a linear Co(II) complex has been reported to possess a maximum orbital angular momentum to give J = 9/2, and thus shows a magnetic relaxation barrier at a temperature as high as 70 K.46 In 2013, a tetrahedral four-coordinate Co(II) complex [Co(SPh) 4] 2− was reported to exhibit slow magnetic relaxations even in the absence of Hdc.47 So far, trigonal prismatic Co(II),48 pseudo-octahedral Co(II),49−51 pseudotetrahedral Co(II),47,52−58 and linear two-coordinate Fe(I)21a and Co(II) complexes46,59 have been known as the SMMs that require no static magnetic fields. Interestingly, the relaxation energy barriers of these mononuclear SMMs are considerably smaller than the axial zero-field splitting (ZFS) energy of the corresponding molecules, implying the presence of competing relaxation paths. For example, the absolute value of the axial ZFS parameter, |D|, of a series of four-coordinate Co(II) complexes, (PPh4)2[Co(XPh)4], where X = O, S, or Se, increases with increasing the size of the X (−11, −62, and −83 cm−1 for X = O, S, and Se, respectively), although the experimentally estimated relaxation energy barriers are less sensitive to the | D| value (ca. 20 cm−1 for any X’s).52 However, the relationship between the |D| value and the relaxation energy barrier seems still unclear due partially to the lack of sufficient examples of this type of SMMs. In this paper, we report the synthesis of a novel fourcoordinate pseudotetrahedral mononuclear Co(II) complex, Co(half-Pc)2 (1) that shows slow magnetic relaxation behaviors in the absence of Hdc. In compound 1, two halved phthalocyanine ligands, half-Pc, coordinate to the Co(II) ion (Scheme 1).60,61 Since the half-Pc ligand has a charge of −1

Scheme 1. Synthesis of 1

and prefers to have a planar conformation due to its conjugated structure, 1 is neutral and rigid. The ZFS energy of 1 has been determined by using the magnetization fittings and high-field, multifrequency ESR techniques.62−64 We have also examined the effect of the Hdc on the slow magnetic relaxations of 1 for a magnetically diluted 1 by using the diamagnetic zinc congener of 1, demonstrating that the effect of external magnetic field is not equivalent to that resulting from the solid dilution.

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RESULTS AND DISCUSSION Synthesis and Structure. The first step of the preparation of 1 is to generate the half-Pc ligand by the reaction of 1,2dicyanobenzene and lithium methoxide in dry methanol at 70 °C (Scheme 1). Since the free half-Pc ligand was unstable, cobalt(II) chloride was added to the reaction mixture directly without isolation of the half-Pc. The orange residue containing 1 was chromatographed by using silica gel to give the pure complex. The resultant pure 1 is stable in the air and in organic solvents. The compound has been characterized by mass spectroscopy (Figure S2) and elemental analysis, and its structure has been determined crystallographically. Figure 1 shows the ORTEP representation of the single crystals of 1 grown by slow diffusion of hexane into the

Figure 1. ORTEP representation of 1 (50% ellipsoids). Hydrogen atoms are omitted for clarity. The selected numbering system used in the analysis is depicted.

CH2Cl2 solution of 1. The molecule is composed of a cobalt ion and two half-Pc ligands, in which the half-Pc consists of two isoindole moieties bridged by a nitrogen atom. The terminal part of the ligand has either a nitrogen atom or two methoxy groups. The cobalt is coordinated to the four isoindole nitrogens of the two half-Pc ligands, and the two half-Pc ligands are arranged almost perpendicular to each other to render the pseudotetrahedral coordination geometry. The two half-Pcs are crystallographically equivalent, and 1 has the 2-fold rotation axis. No counterions are found in the crystals, suggesting the cobalt ion is divalent, and thus, the complex is neutral in total. The Co1−N1 and Co1−N3 bond lengths are B


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Inorganic Chemistry

|μ0H | < 1 T. Although the slope decreases gradually beyond this region, no saturations of the magnetization appear even at ±7 T. In the higher-temperature range, the magnetization increases at a much slower pace. The M−HT−1 plots based on the variable-temperature magnetization data collected between 1.8 and 10 K are displayed as colored dots in Figure 2b. The dispersion of the dots in the figure indicates the presence of large magnetic anisotropy on the cobalt site. In order to extract the anisotropic parameters, the experimental data were subject to a fitting protocol by using the following simplified Hamiltonian:

1.9773(12) and 1.9750(15) Å, respectively. The N1−Co1−N3 angle of 91.02(5)° is smaller than that of the ideal tetrahedral coordination (109.5°), while the N1−Co1−N1′ and N3− Co1−N3′ angles are wider (129.66(6)° and 110.01(6)°, respectively), indicating that the rigid nature of the half-Pc ligand effectively distorts the coordination geometry from the ideal tetrahedron to give pseudotetragonal ligand fields. As a consequence, the coordination sites around the cobalt are elongated along the bisector of the N1−Co1−N3 angle. The intermolecular π−π interactions are observed between the benzene rings in the crystals. The shortest Co−Co distance in the crystals is 8.1997(2) Å. Magnetic Properties. The temperature dependence of the dc magnetic susceptibility data of 1 (powder) is given in Figure 2a. In the temperature range between 300 and ca. 100 K, the

2 2 2 Ĥ = gμB SH + DSẑ + E(Sx̂ − Sŷ )

The g value was assumed to be isotropic in order to avoid overparameterization. The symbol μB represents the Bohr magneton. D and E are the axial and rhombic ZFS parameters, respectively. H; S; and Sx̂ , Sŷ , and Sẑ are magnetic field; spin multiplicity; and spin angular momentum operators, respectively. The best fit was achieved by employing the following set of parameters: g = 2.30, D = −27.9 cm−1, and |E| = 0.002 cm−1 (Figure 2b, solid lines). That is, the cobalt site in compound 1 has a large axial magnetic anisotropy, while the rhombic anisotropy is negligible. It should be noted, however, that these results are based on the isotropic g , and therefore, another set of parameters with a positive D value may also satisfy the experimental results. Hence, the direct determination of the ZFS energy by using the high-field, multifrequency ESR techniques will be discussed in the subsequent section. Figure 3a,b shows the temperature dependencies of χ″ components of the ac susceptibilities for 1. At Hdc = 0 Oe (Figure 3a), the χ″ signals are significant only if the frequencies of the applied ac magnetic field (f) are higher than 10 Hz. The resultant temperature dependencies lack clear structures, and the ambiguous shoulders appear at ca. 5.6 and 4.8 K for f = 1000 and 100 Hz, respectively. In the temperature range higher than 8 K, practically no χ″ signals were observed even at f = 1000 Hz. With decreasing the temperature at f = 1000 Hz, both the χ′ and χ″ values start to rise gradually and take the highest values at 1.8 K (Figure S10a). In contrast, distinct temperature dependencies were observed even at f = 1 Hz when the static magnetic field of 1000 Oe was applied (Figure 3b). At 1.8 K, both the χ′ and χ″ values are almost zero at f = 1000 and 100 Hz, while those at f = 1 Hz are still nonnegligible. These observations indicate that although the magnetic relaxations are effectively slowed down by the external magnetic field, other decay paths seem to be still significant (Figure S10b). Figure S9 demonstrates the effects of Hdc on the χ′ and χ″ values at 1.8 K (f = 1 Hz). It is clear that an Hdc-sensitive magnetic relaxation path dominates at 1.8 K. On the other hand, the temperature-dependent relaxations take control in the high-temperature range (Figures S5 and S6). Although the presence of the Hdc drastically changes the relaxation behavior, it is difficult to distinguish the intermolecular and intramolecular interactions that affect the magnetic relaxations by the simple comparison of the results shown in Figure 3a,b. In order to separate the effects caused by the intermolecular interactions, a magnetically diluted solid solution, [Co0.03Zn0.97(half-Pc)2] (dil.1), was prepared by using the diamagnetic zinc congener of 1 (see Supporting Information for the synthesis, and Figure S3 for the crystal structure of [Zn(half-Pc)2]). As demonstrated in Figure 3c, the

Figure 2. Plots of (a) χMT against temperature T and (b) magnetization vs magnetic field (a, inset) for 1. (b) Plots of magnetization vs HT−1 at the indicated temperatures (dots) and results of the simulations (lines) for 1.

χMT values remain almost constant (2.37 cm3 K mol−1 at 300 K). As the temperature decreases from 100 down to 1.8 K, the χMT values decrease gradually and reach 1.61 cm3 K mol−1 at 1.8 K. Assuming the spin only angular momentum for highspin tetrahedral Co(II) (S = 3/2) and isotropic g, the observed χMT value corresponds to g = 2.25. Although this value is relatively smaller than those reported in the literature for related compounds,13 the g that is larger than that of free electrons (g = 2.0023) indicates that the additional factors other than the electron spins contribute to the magnetic moments of 1. The drop of the χMT values in the lowtemperature range implies the anisotropic nature of the cobalt ion in complex 1, although the contribution of the intermolecular antiferromagnetic exchange couplings between the cobalt ions also cannot be excluded. The magnetization vs magnetic field (M−μ0H) curves show no hysteresis even at 1.8 K (Figure 2a, inset), and therefore, no remnant magnetizations are recognized at 0 T. At 1.8 K, the magnetization is proportional to the μ0H in the region of ca. C


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Inorganic Chemistry

temperature-dependent relaxation behaviors of dil.1 differ drastically compared to that of undiluted 1. The clear peaks appeared even in the absence of the Hdc. The peak temperatures approximately coincide with those observed in Figure 3b. It is, therefore, conceivable that the magnetic relaxations that obscure the structure of the χ″ plot in Figure 3a can be ascribed mainly to the intermolecular interactions among the 1 molecules. Unlike Figure 3b, however, the χ″ values in Figure 3c do not converge to zero at 1.8 K even when the f is higher than 10 Hz. The corresponding χ′ plots also ascend gradually with lowering the temperature instead of converging to zero (Figure S10c). These facts suggest that the magnetic relaxations dominating in the low-temperature range are not suppressed by reducing the interactions among the Co(II) ions; i.e., the observed relaxations are of intramolecular origin. Although we have no direct evidence that these relaxations are promoted by the mixing of the wave functions, the observed Hdc-dependencies strongly suggest that these magnetic relaxations arise from the QTMs. Application of the Hdc of 1000 Oe to dil.1 gives no significant alterations of the peak temperature, while it leads to clear convergence of both the χ′ and χ″ values at 1.8 K (Figure 3d and Figure S10d). Taken together, our results indicate that multiple magnetic relaxation mechanisms coexist in the experimental temperature range. In the high-temperature range, the temperaturedependent magnetic relaxations of 1 can be extracted by increasing the average distance among the magnetic centers. In the low-temperature range, on the other hand, the magnetic dilution alone is insufficient to suppress the magnetic relaxation; i.e., the relaxation based on the intramolecular origin is dominant. In this case, the presence of the external static magnetic field, in addition to the magnetic dilution, effectively slows down the magnetic relaxation. In order to evaluate the temperature-dependent magnetic relaxations of dil.1 quantitatively, the frequency dependence of the ac magnetic susceptibility at Hdc = 0 was collected. At temperatures higher than 2 K, clear peak frequencies are recognized as depicted in Figure 4. The peaks shift to the highfrequency side with an increase in the temperature and transcend the experimental window (10 000 Hz) at 7.5 K. No

Figure 3. Plots of χ″ against temperature for 1 (a, b) and dil.1 (c, d) at Hdc = 0 (a, c) and 1000 (b, d) Oe. Alternating current magnetic fields oscillating at 1000 (red), 100 (yellow), 10 (pale blue), and 1 (blue) Hz were employed. See Figure S10 for the corresponding χ′ vs T plots.

Figure 4. Plots of ac magnetic susceptibilities vs ac frequencies of dil.1 in the absence of Hdc. The low-frequency range (0.1−100 Hz) was measured using the MPMS, while the PPMS was used to obtain the data for the higher-frequency range (100−10 000 Hz). D


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Inorganic Chemistry clear peaks appear at 1.8 and 2.0 K, due to the Hdc-sensitive relaxations as discussed above. The experimental data were fitted by using the generalized Debye model (see Supporting Information, Figures S7, S8, S12, and S13 and Tables S6−S15). Since the presence of the two components is clearly recognized for 1 at Hdc’s of 100 and 300 Oe (Figures S7e,d, respectively), these are assumed to have two independent relaxation times (τ1 and τ2), while the others are fitted by using a single τ component. The dispersion coefficients, α, for dil.1 are less than ca. 0.30 in the hightemperature range at Hdc = 0 Oe, while the values increase rapidly up to 0.83 when the temperature is lowered from 3 K (Table S13), which is consistent with the fact that the Hdcsensitive relaxation path becomes significant under these conditions. In the presence of Hdc of 1000 Oe, on the other hand, the α is 0.30 even at 1.8 K (Table S15). These relationships are clearly demonstrated in the distribution profiles of the relaxation times as a function of τ (Figure S14). The natural logarithm of the reciprocal of the relaxation time, τ, is plotted against the inverse of the temperature for dil.1 at Hdc = 0 and 1000 Oe as displayed in Figure 5 and Figure S15,

processes contribute to the whole effect in this temperature range. High-Field Multifrequency ESR. In order to evaluate the energy gap between the two doublets of 1, high-field multifrequency ESR measurements have been performed.62−64 Figure 6a,b shows the high-field ESR spectra of 1 monitored at

Figure 5. Natural logarithm of the magnetization relaxation time of dil.1 against the inverse of temperature in the absence of static external magnetic field. The regression line obtained in the temperature range 8.0−5.2 K is shown. The barrier (Ueff) and preexponential factor (τ0) are also given.

Figure 6. High-field ESR spectra of 1 measured at (a) 0.584 and (b) 1.838 THz at 4.2 K. The sweep direction of the pulsed magnetic fields is shown by the black arrows. The signals appearing at both the ascending and descending processes are assigned as the sample origin (blue arrows). (c) Frequency−magnetic field plot of the experimental results. Signals arising from 1 at 4.2 K are plotted by the circles, while the signals appearing at high-temperature (77 K) only are denoted by the squares. The triangles and diamonds correspond to g ≈ 2, and these are ascribed to the internal standard (DPPH) or organic impurities. The simulated field−frequency relationship is shown by the solid lines. Transitions associated with the magnetic fields parallel to the z, x, and y axes of the ZFS tensor are shown by the black, red, and blue lines, respectively. See Figure S19 for the corresponding Zeeman diagrams.

respectively. In Figure 5, the plots fit a straight line in the hightemperature range. According to the regression in the temperature range between 8.0 and 5.2 K, the effective energy barrier (Ueff) of 54.0 cm−1 with the pre-exponential factor τ0 = 3.17 × 10−10 s was obtained. As anticipated from Figure 3, the Ueff value practically remains constant (53.9 cm−1) by applying the Hdc = 1000 Oe (Figure S15), suggesting that the temperature-dependent Orbach mechanism dominates in the high-temperature range irrespective of the suppression of the QTMs. See ref 1g for the table that collects the Ueff values of related Co(II) complexes found in the literature. Since the high-spin Co(II) ion consists of four spin wave functions, i.e., | MS⟩ = |±1/2⟩ and |±3/2⟩, it is conceivable that the observed Orbach process is initiated by the thermal excitations from the ground states having the |±3/2⟩ characters to the excited states arising from the |±1/2⟩ states. The plots deviate from the regression lines in the low-temperature range, suggesting that other relaxation mechanisms such as the direct and/or Raman

0.584 and 1.838 THz at 4.2 K, respectively. Since we used a pulsed magnet, both the field ascending and descending processes were recorded and the signals appearing at only one of these were excluded as the noise. In Figure 6a, the signals arising from DPPH (ESR standard) appear at ca. 20.8 T, while the signals at 45.9 T are clearly different from that of the typical organic radicals; therefore, these are assigned as the cobalt origin. The signals were observed in the low-field range E


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Inorganic Chemistry

the five active 3d-based orbitals, were calculated, which was followed by the N-electron valence state perturbation theory (NEVPT2) calculations in order to recover possible dynamic correlations. By including 10 quartets and 35 doublets for state interaction calculations, magnetic parameters of 1 have been predicted as follows: gx = 2.107, gy = 2.115, gz = 2.566 (giso = 2.263), D = −38.2 cm−1, |E/D| < 0.01 (Table S16). Inspection of the individual contributions of the excited states to the D tensor has clarified that the large negative D value is arising from the first excited quartet (−55.7 cm−1), while the second and third excited quartets impose the opposite effects on the D value (5.0 and 6.3 cm−1, respectively), suggesting that the mixing of the higher excited states is non-negligible for the magnetic anisotropy of this type of mononuclear SMMs (Table S18). Although some excited states contribute to the E value to a certain degree, these compensate for each other, giving rise to the small total |E|. According to the results of the calculations, the ground SOC state arising from the quartet is the doublet which is mainly composed of the |MS⟩ = |±3/2⟩ (Table S19); that is, the magnetic relaxations through the mixed wave functions are unlikely preferable in the ground states. On the contrary, the first excited SOC state at 76.4 cm−1 in energy is composed of the mixture of |±1/2⟩. Consequently, the magnetization is easily reversed at the thermally excited states through the mixing of the |±1/2⟩ wave functions, consistent with the experimental results. The calculated direction of the anisotropic axis is depicted in Figure 7. The easy-axis approximately bisects the molecule through the N2−Co− N2′ direction, although the axis slightly inclines to the methoxy-substituted sites.

(1.62 and 3.02 T) when measured at 1.838 THz (Figure 6b). The spectra obtained at various frequencies are collected in Figures S16 and S17, and the resonance fields are plotted in the frequency−magnetic field plane (Figure 6c), in which the plots are converged to 1.71 THz (57.0 cm−1) at the zero magnetic field. These experimental results were fitted by assuming gx = 2.1, gy = 2.1, gz = 2.6, D = −28.5 cm−1, and E = 0.2 cm−1 (solid lines in Figure 6c). The signals observed at the magnetic fields less than 30 T can be attributed either to the |−3/2⟩ → |+1/2⟩ (ΔMS = 2, black circles), |−3/2⟩ → |−1/2⟩ (green circles), |+3/2⟩ → |+1/2⟩ (blue circle), or |+3/2⟩ → |−1/2⟩ (ΔMS = 2, blown circles) transitions, while the signals appearing near 50 T correspond to the transitions between the lowest two states for H∥x or y (purple circles). It should be noted that the three green circles at less than 10 T possibly arise from the transitions between the lowest two states to the highest states for H∥x or y, because the black and red (or blue) lines in Figure 6c almost overlap in this region (Figure S19). Although no signal was detected corresponding to the square black symbols in Figure 6c at 1.4 K, the appreciable signals were detected at ca. 13 and 21 T when the temperature was raised to 77 K (ca. 54 cm−1, see Figure S18). These signals can be assigned as the transitions between the upper two states, i.e., |−1/2⟩ → |+1/2⟩ (Figure S19), indicating the Ising anisotropic electronic structures of 1. The simulated M−HT−1 curves by using the parameters determined by the ESR measurements are given in Figure S20. To the best of our knowledge, this is the first example that unambiguously confirms the negative sign of the D value by observing the direct transition derived from the ground ±3/2 states to the excited ±1/2 states on the basis of the ESR measurements for cobalt mononuclear SMMs. Electronic Structures. The ground and the second lowest crystal field terms arising from the 4F term for a high-spin Co(II) ion in the ideal tetrahedral crystal field are 4A2 and 4T2, respectively. The former remains degenerated and transforms as the 4B1 term of D2d symmetry when the action of the tetragonal distortion is exerted on the system, while the latter splits into the 4E and 4B2 terms. The second-order spin−orbit coupling perturbations allow the mixing of the 4B2 and 4E terms with the ground 4B1 term, giving rise to a splitting of the ground term into the two Kramers doublets. These are denoted by the Γ6 and Γ7 irreducible representations of the D2d′ double group, of which the former inherits the |±3/2⟩ characters. According to the perturbation theory, the energy gap between the Γ6 and Γ7 states can be written as

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CONCLUSIONS In the present study, we have demonstrated that a novel fourcoordinate mononuclear Co(II) complex, 1, prepared from the reaction of 1,2-dicyanobenzene with lithium methoxide followed by the metal coordination, exhibits clear SMM behaviors even in the absence of the external static magnetic field. Magnetic studies have clarified that the complex has a large axial magnetic anisotropy, and the magnetic relaxations are largely affected by the presence of the intramolecular and intermolecular factors in the low-temperature range, while the Orbach process is dominant in the high-temperature range. The effective relaxation energy barrier (Ueff) of 54.0 cm−1 and the axial anisotropic parameter (D) of −28.5 cm−1 were obtained from the ac magnetic susceptibility and high-field multifrequency ESR data, respectively. The zero-field splitting (ZFS) energy is evaluated to be 57.0 cm−1, which is comparable to the estimated Ueff energy. The importance of the second-order SOC couplings to the ground 4B1 term has been discussed to understand the relationship between the electronic structures and observed dynamic magnetism of 1. The results of the ab initio quantum chemical calculations indicate that 1 is an Ising-type mononuclear SMM, in which the ground Kramers doublet is composed mainly of the |MS⟩ = |±3/2⟩ wave functions. To the best of our knowledge, this is the first example that determines the ZFS energy of the Ising mononuclear SMM by direct observation of the electron spin resonance in the terahertz range.

1 1 ji zyz zz − E(Γ7) − E(Γ6) = 8λ 2jjj 4 4 4 j Δ( B → 4 B ) Δ( B1 → E) z{ 1 2 k = −2D

where λ is the spin−orbit coupling (SOC) constant of the cobalt, and Δ parameters represent the energy gap between the specified states. When the Γ7 states are higher in energy than the Γ6 states, the D value has a negative sign and vice versa. For a molecule possessing the large |D| value, however, these perturbation procedures are no longer applicable, and instead, more sophisticated methods such as the quasidegenerate perturbation theory (QDPT) are appropriate in order to discuss the ZFS quantitatively.65 Multiconfiguration ab initio calculations for 1 were performed by using the ORCA 4.0 program package.66,67 The molecular geometry was taken from the X-ray structure. The CAS(7,5) level SCF wave functions, i.e., seven electrons in

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EXPERIMENTAL SECTION

All reagents except for methanol were used as supplied. Methanol was distilled from calcium hydride prior to use. Absorption spectra were recorded on a SHIMAZU UV-1650PC spectrophotometer. Elemental F


Inorganic Chemistry

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analysis was performed by a YANACO CHN Corder MT-6 analyzer. Mass spectra were obtained using a Thermo Fisher Scientific Orbitrap XL (ESI-LIT-orbitrap) spectrometer. Single-crystal X-ray data were collected with a Rigaku R-AXIS VII diffractometer using filtered Mo Kα (λ = 0.71075 Å) radiation. The refinement with full-matrix leastsquares techniques was carried out with SHELXL-2014/7.68 Magnetization and magnetic susceptibility data were collected on a Quantum Design MPMS-XL7AC SQUID magnetometer. The sample was prepared by wrapping 6.96 mg of 1 and eicosane in a piece of aluminum foil. Diamagnetic components were estimated using the Pascal constants,69 and the contribution from the aluminum foil was corrected on the basis of the blank measurement. Quantum Design MPMS-XL7AC and PPMS-9 magnetometers were used for the low (0.1−1340 Hz)- and high (100−10 000 Hz)-frequency range ac measurements, respectively. Oscillating magnetic fields of 3.9 (MPMS) and 5 Oe (PPMS) were employed. The undiluted sample was prepared by fixing 18.8 mg of 1 in a gelatin capsule using eicosane. The diluted sample was prepared by fixing 133.4 mg of dil.1 in a sample tube using eicosane. High-field, multifrequency ESR measurements were performed on a locally developed system at the Center for Advanced High Magnetic Field Science (AHMF), Osaka University. The system is composed of a 55 T short pulsed magnet and an Edinburgh far-infrared laser equipped with a magnetically tuned InSb detector and a homemade transmission-type ESR cryostat. In our setup, the InSb detector employed for the far-infrared radiations causes the mixing of the intensity and phase factors. Measurements were performed in the frequency range between 0.584 and 2.522 THz in pulsed magnetic fields of up to 55 T with a pulsed duration of 6 ms. The sample was prepared by adding powder of 1 into a cylindrical Teflon container (2 mm in diameter and 3 mm in length). 1,1-Diphenyl-2-picrylhydrazyl (DPPH) radical was employed as an ESR standard (g = 2.0036). Ab initio quantum chemical calculations were performed by using the ORCA 4.0 program package. The resolution of identity (RI) approximation technique was employed with the def2-TZVPP basis set and the def2/JK auxiliary basis set implemented in ORCA. The complete active space self-consistent field (CASSCF) calculations were performed on specified seven active electrons in five Co-based 3d orbitals. There were 10 quartets and 35 doublets included for the state interaction calculations. Effects of dynamic correlations were included by performing the N-electron valence state perturbation theory (NEVPT2) calculations. Synthesis of Co(half-Pc)2, 1. Lithium (250 mg) was dissolved in dry methanol (70 mL) with stirring under an argon atmosphere. 1,2Dicyanobenzene (2.0 g, 15.6 mmol) was added to the reaction mixture, which reacted at 70 °C for 10 min. Cobalt(II) dichloride (420 mg, 3.24 mmol) was added to the reaction mixture, and the reaction was continued for an additional 40 min while maintaining the temperature. The resultant orange precipitate was collected by filtration and purified by column chromatography (silica, CH2Cl2/ MeOH = 40:1 (v/v)) to give 1 as an orange powder in 11.3% yield. MS (ESI): m/z: 698.18018 [M + H]+. Anal. Calcd (%) for C36H30N8O4Co: C 61.98, H 4.33, N 16.06. Found: C 61.87, H 4.34, 15.91. Preparation of Co0.03Zn0.97(half-Pc)2, dil.1. A mixture of 1 and the diamagnetic isostructural [Zn(half-Pc)2] in a molar ratio of 3:97 was dissolved in dichloromethane and passed through a short Celite column. The eluent was collected, concentrated in vacuo, and dried under reduced pressure at 50 °C for 1 day to give dil.1 as a pale yellow powder. See Supporting Information for the synthesis of [Zn(half-Pc)2]. CCDC 1826040 (1) and 1826039 [Zn(half-Pc)2] contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00286. Absorption spectrum, ESI mass spectrum of 1, synthesis of [Zn(half-Pc)2], crystal structures of 1 and [Zn(halfPc)2], the results of ac measurements, Cole−Cole plots and fitting parameters, Arrhenius plots of 1 and dil.1, high-field ESR spectra, corresponding Zeeman diagrams, and the results of ab initio calculations of 1 (PDF) Accession Codes

CCDC 1826039−1826040 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Takamitsu Fukuda: 0000-0002-5615-870X Naoto Ishikawa: 0000-0002-3490-4222 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Dr. Takashi Kajiwara for his help in the PPMS measurements at the Department of Chemistry, Faculty of Science, Nara Women’s University. A part of this work was carried out at the Center for Advanced High Magnetic Field Science in Osaka University under the Visiting Researcher’s Program of the Institute for Solid State Physics, The University of Tokyo.

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REFERENCES

(1) (a) Layfield, R. A. Organometallic Single-Molecule Magnets. Organometallics 2014, 33, 1084−1099. (b) Feltham, H. L. C.; Brooker, S. Review of purely 4f and mixed-metal nd-4f single-molecule magnets containing only one lanthanide ion. Coord. Chem. Rev. 2014, 276, 1−33. (c) Liu, K.; Shi, W.; Cheng, P. Toward heterometallic single-molecule magnets: Synthetic strategy, structures and properties of 3d−4f discrete complexes. Coord. Chem. Rev. 2015, 289−290, 74− 122. (d) Demir, S.; Jeon, I.-R.; Long, J. R.; Harris, T. D. Radical ligand-containing single-molecule magnets. Coord. Chem. Rev. 2015, 289−290, 149−176. (e) Lu, J.; Guo, M.; Tang, J. Recent Developments in Lanthanide Single-Molecule Magnets. Chem. Asian J. 2017, 12, 2772−2779. (f) Woodruff, D. N.; Winpenny, R. E. P.; Layfield, R. A. Lanthanide Single-Molecule Magnets. Chem. Rev. 2013, 113, 5110−5148. (g) Feng, M.; Tong, M.-L. Single Ion Magnets from 3d to 5f: Developments and Strategies. Chem. - Eur. J. 2018, 24, 7574−7594. (2) Sessoli, R. Single-atom data storage. Nature 2017, 543, 189−190. (3) Ladd, T. D.; Jelezko, F.; Laflamme, R.; Nakamura, Y.; Monroe, C.; O’Brien, J. L. Quantum computers. Nature 2010, 464, 45−53. G


Article

Inorganic Chemistry

(18) Chen, L.; Wang, J.; Liu, Y.-Z.; Song, Y.; Chen, X.-T.; Zhang, Y.Q.; Xue, Z.-L. Slow Magnetic Relaxation in Mononuclear Octahedral Manganese(III) Complexes with Dibenzoylmethanide Ligands. Eur. J. Inorg. Chem. 2015, 271−278. (19) Pascual-Á lvarez, A.; Vallejo, J.; Pardo, E.; Julve, M.; Lloret, F.; Krzystek, J.; Armentano, D.; Wernsdorfer, W.; Cano, J. Field-Induced Slow Magnetic Relaxation in a Mononuclear Manganese(III)Porphyrin Complex. Chem. - Eur. J. 2015, 21, 17299−17307. (20) Ding, M.; Cutsail, G. E., III; Aravena, D.; Amoza, M.; Rouzières, M.; Dechambenoit, P.; Losovyj, Y.; Pink, M.; Ruiz, E.; Clérac, R.; Smith, J. M. A low spin manganese(IV) nitride single molecule magnet. Chem. Sci. 2016, 7, 6132−6140. (21) (a) Zadrozny, J. M.; Xiao, D. J.; Atanasov, M.; Long, G. J.; Grandjean, F.; Neese, F.; Long, J. R. Magnetic blocking in a linear iron(I) complex. Nat. Chem. 2013, 5, 577−581. (b) Werncke, C. G.; Bunting, P. C.; Duhayon, C.; Long, J. R.; Bontemps, S.; Sabo-Etienne, S. Two-Coordinate Iron(I) Complex [Fe{N(SiMe3)2}2]−: Synthesis, Properties, and Redox Activity. Angew. Chem., Int. Ed. 2015, 54, 245− 248. (c) Samuel, P. P.; Mondal, K. C.; Amin Sk, N.; Roesky, H. W.; Carl, E.; Neufeld, R.; Stalke, D.; Demeshko, S.; Meyer, F.; Ungur, L.; Chibotaru, L. F.; Christian, J.; Ramachandran, V.; van Tol, J.; Dalal, N. S. Electronic Structure and Slow Magnetic Relaxation of LowCoordinate Cyclic Alkyl(amino) Carbene Stabilized Iron(I) Complexes. J. Am. Chem. Soc. 2014, 136, 11964−11971. (22) (a) Zadrozny, J. M.; Atanasov, M.; Bryan, A. M.; Lin, C.-Y.; Rekken, B. D.; Power, P. P.; Neese, F.; Long, J. R. Slow magnetization dynamics in a series of two-coordinate iron(II) complexes. Chem. Sci. 2013, 4, 125−138. (b) Feng, X.; Hwang, S. J.; Liu, J.-L.; Chen, Y.-C.; Tong, M.-L.; Nocera, D. G. Slow Magnetic Relaxation in Intermediate Spin S = 3/2 Mononuclear Fe(III) Complexes. J. Am. Chem. Soc. 2017, 139, 16474−16477. (23) Mossin, S.; Tran, B. L.; Adhikari, D.; Pink, M.; Heinemann, F. W.; Sutter, J.; Szilagyi, R. K.; Meyer, K.; Mindiola, D. J. A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover. J. Am. Chem. Soc. 2012, 134, 13651−13661. (24) Meng, Y.-S.; Mo, Z.; Wang, B.-W.; Zhang, Y.-Q.; Deng, L.; Gao, S. Observation of the single-ion magnet behavior of d8 ions on twocoordinate Co(I)−NHC complexes. Chem. Sci. 2015, 6, 7156−7162. (25) Gómez-Coca, S.; Urtizberea, A.; Cremades, E.; Alonso, P. J.; Camón, A.; Ruiz, E.; Luis, F. Origin of slow magnetic relaxation in Kramers ions with non-uniaxial anisotropy. Nat. Commun. 2014, 5, 4300. (26) Ruamps, R.; Batchelor, L. J.; Guillot, R.; Zakhia, G.; Barra, A.L.; Wernsdorfer, W.; Guihéry, N.; Mallah, T. Ising-type magnetic anisotropy and single molecule magnet behaviour in mononuclear trigonal bipyramidal Co(II) complexes. Chem. Sci. 2014, 5, 3418− 3424. (27) Palion-Gazda, J.; Klemens, T.; Machura, B.; Vallejo, J.; Lloret, F.; Julve, M. Single ion magnet behaviour in a two-dimensional network of dicyanamide-bridged cobalt(II) ions. Dalton Trans 2015, 44, 2989−2992. (28) Chen, L.; Chen, S.-Y.; Sun, Y.-C.; Guo, Y.-M.; Yu, L.; Chen, X.T.; Wang, Z.; Ouyang, Z. W.; Song, Y.; Xue, Z.-L. Slow magnetic relaxation in mononuclear seven-coordinate cobalt(II) complexes with easy plane anisotropy. Dalton Trans 2015, 44, 11482−11490. (29) Smolko, L.; Č ernák, J.; Dušek, M.; Miklovič, J.; Titiš, J.; Boča, R. Three tetracoordinate Co(II) complexes [Co(biq)X2] (X = Cl, Br, I) with easy-plane magnetic anisotropy as field-induced singlemolecule magnets. Dalton Trans 2015, 44, 17565−17571. (30) Shao, F.; Cahier, B.; Guihéry, N.; Rivière, E.; Guillot, R.; Barra, A.-L.; Lan, Y.; Wernsdorfer, W.; Campbell, V. E.; Mallah, T. Tuning the Ising-type anisotropy in trigonal bipyramidal Co(II) complexes. Chem. Commun. 2015, 51, 16475−16478. (31) Vaidya, S.; Upadhyay, A.; Singh, S. K.; Gupta, T.; Tewary, S.; Langley, S. K.; Walsh, J. P. S.; Murray, K. S.; Rajaraman, G.; Shanmugam, M. A synthetic strategy for switching the single ion anisotropy in tetrahedral Co(II) complexes. Chem. Commun. 2015, 51, 3739−3742.

(4) Bogani, L.; Wernsdorfer, W. Molecular spintronics using singlemolecule magnets. Nat. Mater. 2008, 7, 179−186. (5) (a) Gómez-Coca, S.; Aravena, D.; Morales, R.; Ruiz, E. Large magnetic anisotropy in mononuclear metal complexes. Coord. Chem. Rev. 2015, 289−290, 379−392. (b) Zadrozny, J. M.; Liu, J.; Piro, N. A.; Chang, C. J.; Hill, S.; Long, J. R. Slow magnetic relaxation in a pseudotetrahedral cobalt(II) complex with easy-plane anisotropy. Chem. Commun. 2012, 48, 3927−3929. (6) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Magnetic bistability in a metal-ion cluster. Nature 1993, 365, 141−143. (7) Waldmann, O. A Criterion for the Anisotropy Barrier in SingleMolecule Magnets. Inorg. Chem. 2007, 46, 10035−10037. (8) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level. J. Am. Chem. Soc. 2003, 125, 8694−8695. (9) (a) Ishikawa, N.; Sugita, M.; Okubo, T.; Tanaka, N.; Iino, T.; Kaizu, Y. Determination of Ligand-Field Parameters and f-Electronic Structures of Double-Decker Bis(phthalocyaninato)lanthanide Complexes. Inorg. Chem. 2003, 42, 2440−2446. (b) Fukuda, T.; Shigeyoshi, N.; Yamamura, T.; Ishikawa, N. Magnetic Relaxations Arising from Spin−Phonon Interactions in the Nonthermally Activated Temperature Range for a Double-Decker Terbium Phthalocyanine Single Molecule Magnet. Inorg. Chem. 2014, 53, 9080−9086. (10) Ding, Y.-S.; Chilton, N. F.; Winpenny, R. E. P.; Zheng, Y.-Z. On Approaching the Limit of Molecular Magnetic Anisotropy: A Near-Perfect Pentagonal Bipyramidal Dysprosium(III) Single-Molecule Magnet. Angew. Chem., Int. Ed. 2016, 55, 16071−16074. (11) (a) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Molecular magnetic hysteresis at 60 K in dysprosocenium. Nature 2017, 548, 439−442. (b) Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A. A Dysprosium Metallocene Single-Molecule Magnet Functioning at the Axial Limit. Angew. Chem., Int. Ed. 2017, 56, 11445−11449. (12) Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A. Magnetic hysteresis up to 80 K in a dysprosium metallocene single-molecule magnet. Science 2018, 362, 1400−1403. (13) Frost, J. M.; Harriman, K. L. M.; Murugesu, M. The rise of 3-d single-ion magnets in molecular magnetism: towards materials from molecules? Chem. Sci. 2016, 7, 2470−2491. (14) Freedman, D. E.; Harman, W. H.; Harris, T. D.; Long, G. J.; Chang, C. J.; Long, J. R. Slow Magnetic Relaxation in a High-Spin Iron(II) Complex. J. Am. Chem. Soc. 2010, 132, 1224−1225. (15) (a) Atzori, M.; Tesi, L.; Benci, S.; Lunghi, A.; Righini, R.; Taschin, A.; Torre, R.; Sorace, L.; Sessoli, R. Spin Dynamics and Low Energy Vibrations: Insights from Vanadyl-Based Potential Molecular Qubits. J. Am. Chem. Soc. 2017, 139, 4338−4341. (b) Atzori, M.; Tesi, L.; Morra, E.; Chiesa, M.; Sorace, L.; Sessoli, R. Room-Temperature Quantum Coherence and Rabi Oscillations in Vanadyl Phthalocyanine: Toward Multifunctional Molecular Spin Qubits. J. Am. Chem. Soc. 2016, 138, 2154−2157. (c) Tesi, L.; Lucaccini, E.; Cimatti, I.; Perfetti, M.; Mannini, M.; Atzori, M.; Morra, E.; Chiesa, M.; Caneschi, A.; Sorace, L.; Sessoli, R. Quantum coherence in a processable vanadyl complex: new tools for the search of molecular spin qubits. Chem. Sci. 2016, 7, 2074−2083. (d) Atzori, M.; Morra, E.; Tesi, L.; Albino, A.; Chiesa, M.; Sorace, L.; Sessoli, R. Quantum Coherence Times Enhancement in Vanadium(IV)-based Potential Molecular Qubits: the Key Role of the Vanadyl Moiety. J. Am. Chem. Soc. 2016, 138, 11234−11244. (16) Deng, Y.-F.; Han, T.; Wang, Z.; Ouyang, Z.; Yin, B.; Zheng, Z.; Krzystek, J.; Zheng, Y.-Z. Uniaxial magnetic anisotropy of squareplanar chromium(II) complexes revealed by magnetic and HF-EPR studies. Chem. Commun. 2015, 51, 17688−17691. (17) Sato, R.; Suzuki, K.; Minato, T.; Shinoe, M.; Yamaguchi, K.; Mizuno, N. Field-induced slow magnetic relaxation of octahedrally coordinated mononuclear Fe(III)-, Co(II)-, and Mn(III)-containing polyoxometalates. Chem. Commun. 2015, 51, 4081−4084. H


Article

Inorganic Chemistry (32) Ś witlicka-Olszewska, A.; Palion-Gazda, J.; Klemens, T.; Machura, B.; Vallejo, J.; Cano, J.; Lloret, F.; Julve, M. Single-ion magnet behaviour in mononuclear and two-dimensional dicyanamidecontaining cobalt(II) complexes. Dalton Trans 2016, 45, 10181− 10193. (33) Ziegenbalg, S.; Hornig, D.; Gorls, H.; Plass, W. Cobalt(II)Based Single-Ion Magnets with Distorted Pseudotetrahedral [N2O2] Coordination: Experimental and Theoretical Investigations. Inorg. Chem. 2016, 55, 4047−4058. (34) Shao, D.; Zhang, S.-L.; Shi, L.; Zhang, Y.-Q.; Wang, X.-Y. Probing the Effect of Axial Ligands on Easy-Plane Anisotropy of Pentagonal-Bipyramidal Cobalt(II) Single-Ion Magnets. Inorg. Chem. 2016, 55, 10859−10869. (35) Roy, S.; Oyarzabal, I.; Vallejo, J.; Cano, J.; Colacio, E.; Bauza, A.; Frontera, A.; Kirillov, A. M.; Drew, M. G. B.; Das, S. Two Polymorphic Forms of a Six-Coordinate Mononuclear Cobalt(II) Complex with Easy-Plane Anisotropy: Structural Features, Theoretical Calculations, and Field-Induced Slow Relaxation of the Magnetization. Inorg. Chem. 2016, 55, 8502−8513. (36) Díaz-Torres, R.; Menelaou, M.; Roubeau, O.; Sorrenti, A.; Brandariz-de-Pedro, G.; Sañudo, E. C.; Teat, S. J.; Fraxedas, J.; Ruiz, E.; Aliaga-Alcalde, N. Multiscale study of mononuclear CoII SMMs based on curcuminoid ligands. Chem. Sci. 2016, 7, 2793−2803. (37) Zhang, Y.-Z.; Gómez-Coca, S.; Brown, A. J.; Saber, M. R.; Zhang, X.; Dunbar, K. R. Trigonal antiprismatic Co(II) single molecule magnets with large uniaxial anisotropies: importance of Raman and tunneling mechanisms. Chem. Sci. 2016, 7, 6519−6527. (38) Poulten, R. C.; Page, M. J.; Algarra, A. G.; Le Roy, J. J.; López, I.; Carter, E.; Llobet, A.; Macgregor, S. A.; Mahon, M. F.; Murphy, D. M.; Murugesu, M.; Whittlesey, M. K. Synthesis, Electronic Structure, and Magnetism of [Ni(6-Mes)2]+: A Two-Coordinate Nickel(I) Complex Stabilized by Bulky N-Heterocyclic Carbenes. J. Am. Chem. Soc. 2013, 135, 13640−13643. (39) Lin, W.; Bodenstein, T.; Mereacre, V.; Fink, K.; Eichhofer, A. Field-Induced Slow Magnetic Relaxation in the Ni(I) Complexes [NiCl(PPh3)2]·C4H8O and [Ni(N(SiMe3)2)(PPh3)2]. Inorg. Chem. 2016, 55, 2091−2100. (40) Miklovič, J.; Valigura, D.; Boča, R.; Titiš, J. A mononuclear Ni(II) complex: a field induced single-molecule magnet showing two slow relaxation processes. Dalton Trans 2015, 44, 12484−12487. (41) Marriott, K. E. R.; Bhaskaran, L.; Wilson, C.; Medarde, M.; Ochsenbein, S. T.; Hill, S.; Murrie, M. Pushing the limits of magnetic anisotropy in trigonal bipyramidal Ni(II). Chem. Sci. 2015, 6, 6823− 6828. (42) Bhowmick, I.; Roehl, A. J.; Neilson, J. R.; Rappé, A. K.; Shores, M. P. Slow magnetic relaxation in octahedral low-spin Ni(III) complexes. Chem. Sci. 2018, 9, 6564−6571. (43) Wu, S.-Q.; Miyazaki, Y.; Nakano, M.; Su, S.-Q.; Yao, Z.-S.; Kou, H.-Z.; Sato, O. Slow Magnetic Relaxation in a Mononuclear Ruthenium(III) Complex. Chem. - Eur. J. 2017, 23, 10028−10033. (44) Martínez-Lillo, J.; Mastropietro, T. F.; Lhotel, E.; Paulsen, C.; Cano, J.; De Munno, G.; Faus, J.; Lloret, F.; Julve, M.; Nellutla, S.; Krzystek, J. Highly Anisotropic Rhenium(IV) Complexes: New Examples of Mononuclear Single-Molecule Magnets. J. Am. Chem. Soc. 2013, 135, 13737−13748. (45) Pedersen, K. S.; Sigrist, M.; Sørensen, M. A.; Barra, A.-L.; Weyhermüller, T.; Piligkos, S.; Thuesen, C. Aa.; Vinum, M. G.; Mutka, H.; Weihe, H.; Clérac, R.; Bendix, J. [ReF6]2‑: A Robust Module for the Design of Molecule-Based Magnetic Materials. Angew. Chem., Int. Ed. 2014, 53, 1351−1354. (46) Bunting, P. C.; Atanasov, M.; Damgaard-Møller, E.; Perfetti, M.; Crassee, I.; Orlita, M.; Overgaard, J.; van Slageren, J.; Neese, F.; Long, J. R. A linear cobalt(II) complex with maximal orbital angular momentum from a non-Aufbau ground state. Science 2018, 362, eaat7319. (47) Zadrozny, J. M.; Long, J. R. Slow Magnetic Relaxation at Zero Field in the Tetrahedral Complex [Co(SPh)4]2‑. J. Am. Chem. Soc. 2011, 133, 20732−20734.

(48) (a) Novikov, V. V.; Pavlov, A. A.; Nelyubina, Y. V.; Boulon, M.E.; Varzatskii, O. A.; Voloshin, Y. Z.; Winpenny, R. E. P. A Trigonal Prismatic Mononuclear Cobalt(II) Complex Showing Single-Molecule Magnet Behavior. J. Am. Chem. Soc. 2015, 137, 9792−9795. (b) Pavlov, A. A.; Savkina, S. A.; Belov, A. S.; Nelyubina, Y. V.; Efimov, N. N.; Voloshin, Y. Z.; Novikov, V. V. Trigonal Prismatic Tris-pyridineoximate Transition Metal Complexes: A Cobalt(II) Compound with High Magnetic Anisotropy. Inorg. Chem. 2017, 56, 6943−6951. (49) Zhu, Y.-Y.; Cui, C.; Zhang, Y.-Q.; Jia, J.-H.; Guo, X.; Gao, C.; Qian, K.; Jiang, S.-D.; Wang, B.-W.; Wang, Z.-M.; Gao, S. Zero-field slow magnetic relaxation from single Co(II) ion: a transition metal single-molecule magnet with high anisotropy barrier. Chem. Sci. 2013, 4, 1802−1806. (50) Zhu, Y.-Y.; Zhang, Y.-Q.; Yin, T.-T.; Gao, C.; Wang, B.-W.; Gao, S. A Family of CoIICoIII3 Single-Ion Magnets with Zero-Field Slow Magnetic Relaxation: Fine Tuning of Energy Barrier by Remote Substituent and Counter Cation. Inorg. Chem. 2015, 54, 5475−5486. (51) Rigamonti, L.; Bridonneau, N.; Poneti, G.; Tesi, L.; Sorace, L.; Pinkowicz, D.; Jover, J.; Ruiz, E.; Sessoli, R.; Cornia, A. A PseudoOctahedral Cobalt(II) Complex with Bispyrazolylpyridine Ligands Acting as a Zero-Field Single-Molecule Magnet with Easy Axis Anisotropy. Chem. - Eur. J. 2018, 24, 8857−8868. (52) Zadrozny, J. M.; Telser, J.; Long, J. R. Slow magnetic relaxation in the tetrahedral cobalt(II) complexes [Co(EPh)4]2‑ (E = O, S, Se). Polyhedron 2013, 64, 209−217. (53) Fataftah, M. S.; Zadrozny, J. M.; Rogers, D. M.; Freedman, D. E. A Mononuclear Transition Metal Single-Molecule Magnet in a Nuclear Spin-Free Ligand Environment. Inorg. Chem. 2014, 53, 10716−10721. (54) Carl, E.; Demeshko, S.; Meyer, F.; Stalke, D. Triimidosulfonates as Acute Bite-Angle Chelates: Slow Relaxation of the Magnetization in Zero Field and Hysteresis Loop of a CoII Complex. Chem. - Eur. J. 2015, 21, 10109−10115. (55) Rechkemmer, Y.; Breitgoff, F. D.; van der Meer, M.; Atanasov, M.; Hakl, M.; Orlita, M.; Neugebauer, P.; Neese, F.; Sarkar, B.; van Slageren, J. A four-coordinate cobalt(II) single-ion magnet with coercivity and a very high energy barrier. Nat. Commun. 2016, 7, 10467. (56) Sottini, S.; Poneti, G.; Ciattini, S.; Levesanos, N.; Ferentinos, E.; Krzystek, J.; Sorace, L.; Kyritsis, P. Magnetic Anisotropy of Tetrahedral CoII Single-Ion Magnets: Solid-State Effects. Inorg. Chem. 2016, 55, 9537−9548. (57) Tu, D.; Shao, D.; Yan, H.; Lu, C. A carborane-incorporated mononuclear Co(II) complex showing zero-field slow magnetic relaxation. Chem. Commun. 2016, 52, 14326−14329. (58) Mitsuhashi, R.; Hosoya, S.; Suzuki, T.; Sunatsuki, Y.; Sakiyama, H.; Mikuriya, M. Hydrogen-bonding interactions and magnetic relaxation dynamics in tetracoordinated cobalt(II) single-ion magnets. Dalton. Trans. 2019, 48, 395−399. (59) Yao, X.-N.; Du, J.-Z.; Zhang, Y.-Q.; Leng, X.-B.; Yang, M.-W.; Jiang, S.-D.; Wang, Z.-X.; Ouyang, Z.-W.; Deng, L.; Wang, B.-W.; Gao, S. Two-Coordinate Co(II) Imido Complexes as Outstanding Single-Molecule Magnets. J. Am. Chem. Soc. 2017, 139, 373−380. (60) Fukuda, T.; Shigeyoshi, N.; Fuyuhiro, A.; Ishikawa, N. First example of a hexadentate bicyclic phthalocyanine analogue containing a divalent metal center. Dalton Trans 2013, 42, 16486−16489. (61) Fukuda, T.; Kikukawa, Y.; Fuyuhiro, A.; Kobayashi, N.; Ishikawa, N. Formation and Crystal Structure of a Novel Zinc Phthalocyanine Analogue Having an Axial Isoindole Ligand Obtained by Lithium-mediated Reactions under Mild Conditions. Chem. Lett. 2014, 43, 925−927. (62) Krzystek, J.; Zvyagin, S. A.; Ozarowski, A.; Fiedler, A. T.; Brunold, T. C.; Telser, J. Definitive Spectroscopic Determination of Zero-Field Splitting in High-Spin Cobalt(II). J. Am. Chem. Soc. 2004, 126, 2148−2155. (63) Idešicová, M.; Titiš, J.; Krzystek, J.; Boča, R. Zero-Field Splitting in Pseudotetrahedral Co(II) Complexes: a Magnetic, HighI


Article

Inorganic Chemistry Frequency and -Field EPR, and Computational Study. Inorg. Chem. 2013, 52, 9409−9417. (64) Schweinfurth, D.; Krzystek, J.; Atanasov, M.; Klein, J.; Hohloch, S.; Telser, J.; Demeshko, S.; Meyer, F.; Neese, F.; Sarkar, B. Tuning Magnetic Anisotropy Through Ligand Substitution in Five-Coordinate Co(II) Complexes. Inorg. Chem. 2017, 56, 5253−5265. (65) Maganas, D.; Sottini, S.; Kyritsis, P.; Groenen, E. J. J.; Neese, F. Theoretical Analysis of the Spin Hamiltonian Parameters in Co(II)S4 Complexes, Using Density Functional Theory and Correlated ab initio Methods. Inorg. Chem. 2011, 50, 8741−8754. (66) (a) Neese, F. ORCAAn ab Initio, Density Functional and Semiempirical Program Package, version 4.0; 2017. (b) Neese, F. Software Update: The ORCA Program System, version 4.0. WIREs Comput. Mol. Sci. 2018, 8, e1327. (c) Neese, F. ORCA Program System. WIREs Comput. Mol. Sci. 2012, 2, 73−78. (67) Vaidya, S.; Singh, S. K.; Shukla, P.; Ansari, K.; Rajaraman, G.; Shanmugam, M. Role of Halide Ions in the Nature of the Magnetic Anisotropy in Tetrahedral CoII Complexes. Chem. - Eur. J. 2017, 23, 9546−9559. (68) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (69) Bain, G. A.; Berry, J. F. Diamagnetic Corrections and Pascal’s Constants. J. Chem. Educ. 2008, 85, 532−536.

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