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

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Influence of the Coordination Environment on Easy-Plane Magnetic Anisotropy of Pentagonal Bipyramidal Cobalt(II) Complexes Amit Kumar Mondal, Arpan Mondal, Bijoy Dey, and Sanjit Konar*

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/08/18. For personal use only.

Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal, 462066 Madhya Pradesh, India S Supporting Information *

ABSTRACT: A rational approach of modulating the easy-plane magnetic anisotropy of mononuclear pentagonal bipyramidal CoII single molecule magnets (SMMs) has been revealed in this paper. A class of three new pentagonal-bipyramidal complexes with formulas [Co(H2daps)(MeOH)2] (1), [Co(H4daps)(NCS)(MeOH)]·(ClO4)·(MeOH) (2), and [Co(H4daps)(NCS)2]· (MeOH)2 (3) (H4daps = 2,6-bis(1-salicyloylhydrazonoethyl) pyridine) were studied. In these complexes, the axial positions are successively replaced by different O and N donar ligands in a systematic way. Detailed magnetic measurements disclose the existence of large easy-plane magnetic anisotropy and field-induced slow magnetic relaxation behavior. Both experimental and ab initio theoretical calculations display that easy-plane magnetic anisotropy is maintained upon variation of coordination environments. Nevertheless, the magnitude of the D value was found to be increased in the case of weaker axially coordinated σdonor ligands and a more symmetrical equatorial ligand. Additionally, the detailed investigation of field and temperature dependence of relaxation time revealed that quantum tunnelling of magnetization is the predominant process for slow magnetic relaxation and the Raman process is significant which explicates the thermal dependence. Magnetic dilution experiments have been performed to eliminate the possible influence of intermolecular interactions on magnetic behaviors of adjacent CoII centers.

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INTRODUCTION

the magnetic anisotropy can be improved by restraining the coordination number around the metal center.9 Nevertheless, low coordinate complexes are stable only under an inert atmosphere which restricts their possible usefulness in diverse applications. In this situation, a strategy to tune the single ion anisotropy in high coordinate metal complexes is much appreciated. Among mononuclear transition metal based SMMs, CoII based complexes are mostly remarkable as they have a noninteger spin ground state10a that decreases the possibility of quantum tunnelling of magnetization (QTM).10b However, from the theoretical investigations it has been established that pentagonal bipyramidal metal complexes exhibited very large magnetic anisotropy.11,12 The value of zero-field splitting (ZFS) parameter (D) for pentagonal bipyramidal CoII center was positive.9d A very few examples

1

The design of single molecule magnets (SMMs) has attracted remarkable attention in last two decades, mainly motivated by their potential applications in high-density data storage, quantum computing, and molecular spintronics.2 Recently, efforts have been given to search for the smallest possible SMMs where a single ion can act as an SMM, termed mononuclear SMMs. The first mononuclear lanthanide (Ln) complex which exhibits slow magnetic relaxation behavior was reported by Ishikawa et al.3 The extensive research have been done on both mononuclear Ln-based4 and as well as transition metal based SMMs.5−8 In view of this, efforts to judiciously control the anisotropy of mononuclear SMMs have intensified during recent years.5k Nevertheless, parameters controlling magnetic anisotropy are unstated and its intricate control has endured the most challenging task. For all reported mononuclear 3d-based SMMs, one common feature is the low coordinated metal centers, and in this case, © XXXX American Chemical Society

Received: April 27, 2018

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DOI: 10.1021/acs.inorgchem.8b01162 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Molecular structures of complexes 1−3 (a−c); hydrogen atoms are omitted for clarity.

crystal structure analysis reveals that the axial bond distances are shorter Co1−N1 (2.070(5) Å) and Co1−N2 (2.062(5) Å) in complex 3 (Table S2). This gives a greater distortion from ideal geometry around CoII in complex 3. In complex 1, the ligand coordinates to the CoII center in anionic form (enolform), whereas in complexes 2 and 3, it coordinates in neutral form (keto-form) which results in the different CO bond distances in equatorial positions in the respective complexes. The CO bond distances in complex 1 (1.257(2)−1.265(6) Å) are found to be larger than the CO bond distances in complexes 2 and 3 (1.230(4)−1.237(1) Å). The structural features of complex 1 are similar to those of the previously reported seven-coordinated MII complexes (M = Sn, Mn, Cd).15a−c All these studied complexes comprise similar equatorial CoN3O2 cores, while the different axial and equatorial coordination leads to diverse coordination surroundings around the CoII centers. In studied complexes, the central CoII ion is seven-coordinated and displays distorted pentagonal bipyramid coordination geometry (Figure S1). Detailed investigation of the CoII centers using SHAPE 2.115d divulges that all of the CoII centers adopt pentagonal bipyramid coordination geometries. The calculated minimum CShM values were 0.235, 0.211, and 0.464 for 1−3, respectively, which are close to zero for the ideal D5h symmetry. Complete results of geometric analyses have been shown in Table S4. Substantial π···π interaction and intermolecular H-bonding are present in all complexes which supports the construction of different supramolecular arrangements. In complex 1, there is intermolecular hydrogen bonding (Table S5) among axially coordinated methanol molecules and phenoxy groups of neighboring molecules. This strong hydrogen bonding network gives rise to supramolecular two-dimensional arrangement (Figure S2). Intermolecular hydrogen bonding among axially coordinated methanol molecules and thiocyanate anions of neighboring molecules are present in complex 2 which leads to formation of a supramolecular one-dimensional arrangement (Figure S3). Additionally, lattice methanol molecule and perchlorate anion participated in intermolecular hydrogen bonding between them and with the phenoxy groups (Figure S4 and Table S6). Furthermore, π···π interactions are observed in 2 with centroid to centroid distance of 4.371(2) Å. Lattice methanol molecules in complex 3 formed intermolecular Hbonding (Table S7) with sulfur and phenoxy oxygen atoms which assists the construction of 2D supramolecular arrangement (Figure S5−S6). Additionally, strong CH···π interactions are also observed with CH to centroid (of phenyl rings) distance of 2.642(2) Å for 3. Magnetic Property Studies. Phase purity of all the complexes were confirmed by the good agreement of the bulk

of mononuclear transition metal based SMMs have been reported in the literature with a positive D value which is different from conventional SMM behavior.5g,9b,c,13 In general, it is possible to affect the D parameter by changing the mixing of ground electronic state with the excited electronic states. Thus, it can be expected that subtle variation of the coordination surroundings can act as an auspicious method to manipulate the D parameter. Nevertheless, without complete understanding of the electronic states involved in spin−orbit coupling, it is not feasible to commence such studies. Thus, investigations on coordination environment arbitrated control of magnetic anisotropy are very scarce.14 In this work, an attempt has been made to establish the effect of coordination environment on the easy-plane magnetic anisotropy of pentagonal-bipyramidal CoII complexes [Co(H2daps)(MeOH)2] (1), [Co(H4daps)(NCS)(MeOH)]· (ClO4)·(MeOH) (2), and [Co(H4daps)(NCS)2]·(MeOH)2 (3). Both experimental and ab initio theoretical calculations show that the sign of the D value is mostly unaffected by the change in coordination environments, whereas the axially coordinated weaker σ-donor ligand and a more symmetrical equatorial ligand can considerably increase the magnitude of the easy-plane magnetic anisotropy.

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RESULTS AND DISCUSSIONS Structural Description. Single-crystal X-ray analysis revealed that both complexes 1 and 2 crystallized in the monoclinic P21/c space groups and that complex 3 crystallized in the triclinic P1̅ space group (Table S1). The asymmetric unit of complex 1 consists of one CoII ion, one H2daps ligand, and two coordinated methanol molecules (Figure 1). The charge of the CoII center is balanced by anionic H2daps ligand. The equatorial positions of the pentagonal bipyramidal coordination environment around CoII are fullfilled by H2daps ligand and the axial positions are occupied by methanol molecules. Unlike complex 1, in complex 2, the axial positions are occupied by the one thiocyanate and a methanol molecule. The charge of the metal center is balanced by a perchlorate counterion, and one molecule of solvent methanol is also found in the crystal lattice. Complex 3 was synthesized by a similar procedure. The asymmetric unit of 3 contains one CoII ion coordinated with one H4daps ligand, two coordinated thiocyanate anions, and two noncoordinated methanol molecules. Similar to complex 1, in complex 3, the CoII center is in pentagonal bipyramidal geometry where H4daps coordinates equatorially and two thiocyanate anions occupy the axial positions. However, the equatorial coordination has little difference [ligand bite angles from 69.30° (for Npyridyl−Co−N) to 79.34° (for O−Co−O), compared to 68.04−78.89° for the equivalent angles in 1]. More detailed B


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Inorganic Chemistry

Figure 2. χMT vs T plots measured at 0.1 T for complex 1 (a) and 2 (b) 1/χM vs T plots shown in the inset; M/NμB vs H plots for complexes 1 (c) and 2 (d) at the indicated temperatures. The red lines are the best fit.

Figure 3. Frequency dependency of the in-phase (χM′) (a and b) and out-of-phase (χM″) (c and d) ac magnetic susceptibility plots for complexes 1 and 2 under 1000 Oe dc field.

Figure 4. Cole−Cole plots for complexes 1 (a) and 2 (b). Solid lines represent the best fit. Temperature dependency of the average relaxation time for complexes 1 (c) and 2 (d) at 0.1 T dc field. The red lines are the best fits obtained according to eq 3.

highly anisotropic nature of the CoII centers (Figure S10). The spin Hamiltonian of eq 1 has been used to quantify the anisotropy:

phase powder X-ray diffraction patterns which matches well with the simulated ones based on crystal structure data (Figure S7−S8). Direct current (dc) magnetic susceptibility measurements were performed under an applied field of 1000 Oe in the temperature range of 2−300 K. The χMT (χM = molar magnetic susceptibility) values at 300 K for complex 1−3 are 2.97, 2.93, and 2.89 cm3 K mol−1 respectively. These values are higher than spin-only value of 1.875 cm3 mol−1 K for a isolated CoII high spin system (S = 3/2). The χMT values matches well with the other reported CoII complexes (2.1−3.4 cm3 mol−1 K) with high magnetic anisotropy and a significant orbital contribution.16 The χMT value of complex 1 decreases monotonously upon cooling from room temperature attaining a value of 1.73 cm3 mol−1 K at 2 K (Figure 2), whereas for complexes 2 and 3, there is no change in the χMT value up to 100 K, below which it decreases and reaches to a values of 1.86 and 1.79 cm3 mol−1 K, respectively, at 2 K (Figures 2 and S9). The χMT value decreases, owing to the inherent anisotropy of the CoII centers. Field dependence of magnetization data (M/ NμB vs H) achieved the highest values of 2.3, 2.14, and 2 NμB for 1−3, respectively, at 2 K and 7 T (Figures 2 and S9). The experimental magnetization values are lower than the theoretical value of 3.3 and do not saturate at the maximum measured field. All the isotherm plots in M/NμB versus H/T data sets do not fall on the same curve which represents the

H = gμB S ·B + D[Sz 2 − S(S + 1)/3] + E(Sx 2 − Sy 2) (1)

where μB, S, and B represent the Bohr magneton, spin, and magnetic vectors, respectively; D and E indicate the axial and rhombic D parameters. To estimate the D parameters of CoII centers, PHI code17 was used by concomitant fitting of the χMT versus T and M/NμB versus H plots, and g tensor was kept isotropic during the fitting process. The best fits produced D = 43.1(7) cm−1, |E| = 3.3(2) cm−1, and g = 2.27(1) for 1; D = 41.5(6) cm−1, |E| = 1.5(4) cm−1, and g = 2.21(2) for 2; D = 38.8(2) cm−1, |E| = 2.1(7) cm−1, and g = 2.18(0) for 3. Additionally, to justify the correct sign of D, we also have done the fitting of the magnetization and susceptibility data by using a negative initial D value, and we got another set of values of D ≈ −17.5(3) cm−1 and E ≈ −9.1(5) cm−1 for 1 with much poorer fit. This result also conflicts with the computational results and hence indicates the correct choice of a positive sign of D. The ZFS parameters of previously reported mononuclear pentagonal-bipyramidal CoII complexes have been shown in Table S8, and it has been found that the obtained large and positive D values for 1−3 agree well with the reported values C


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Inorganic Chemistry for other mononuclear pentagonal-bipyramidal CoII complexes. The interaction between the ground and excited electronic states coupled through spin−orbit coupling produced the positive sign of D parameter. The quartet excited state with utmost contribution to ZFS parameter epitomizes the electronic configuration where an electron moves from dxz or dyz orbital to the dz2 orbital. Furthermore, the energy of the dz2 orbital can be altered by adjusting the axial donations, which affects the contribution of second-order perturbation in spin orbit coupling. In the present case of 1−3, the axial ligands (i.e., metal−donor atom bond distances) are different as compared to those of the previously reported pentagonal-bipyramidal CoII complexes by Wang and coworkers,6b and that might be the reason for obtaining different D parameters for the studied complexes. Alternating current (ac) magnetic susceptibility measurements have been performed to investigate the relaxation dynamics of all studied complexes with an oscillating ac field of 3.5 Oe and static dc field of 0 and 1000 Oe. There was no frequency dependency in the out of phase ac data (χM″) under zero applied dc field, but under a field of 1000 Oe, all the complexes displayed ac signals (Figures 3 and S11−S13). Frequency dependent in-phase (χM′) and out-of-phase (χM″) ac data were used to construct the Cole−Cole plots (Figures 4 and S14) and χM″ versus χM′ data were fitted by using a generalized Debye model18 which gave α values (the width of distribution of relaxation times have been described by α parameter) within the ranges 0.04−0.26 (1), 0.08−0.31 (2), and 0.05−0.28 (3) suggesting the narrow distribution of relaxation time.19 To check field dependence, the relaxation time was studied at 2 K. Equation 2 has been used to quantify the parameters and first term represents the field-dependent processes (QTM process), and the second term contains the weakly fielddependent processes (Raman and Orbach processes) which has been kept as constant, C.20 In the present case, the direct term contribution was not considered as the τ value increases at higher fields. The relaxation time has been well-defined by this way (τQTM was calculated as 1.67 × 10−4 s), and it demonstrates that QTM has significant effect in the slow relaxation process. τ −1 = B1/(1 + B2 H2) + C

Additionally, to examine the influence of dipolar interactions on magnetic behavior, the effect of magnetic dilution was performed. A diluted sample was prepared by using a mixture of Co(ClO 4 ) 2 ·6H2 O and Zn(ClO 4 ) 2 ·6H2 O in a 5:95 percentage ratio. Energy-dispersive X-ray spectroscopy (EDS) has been performed to verify the doped level in the diluted sample, and it indicates the presence of Zn, Co, O, S, and N elements in the diluted sample (Figure S15). No difference was found in ac susceptibility studies for the diluted complex with the undiluted one (Figure S16). Therefore, any kind of dipolar interactions are unimportant in this case; the relaxation originates from the single CoII center. Ab Initio Investigation. To go inside the electronic and magnetic properties for complexes 1−3, we have performed ab initio multireference methodology by ORCA 4.022 and MOLCAS 8.2 software packages. All the calculations have been performed on experimentally determined X-ray structure without optimization. For all the complexes, calculations were CASSCF/RASSI-SO/SINGLE_ANISO type in MOLCAS and CASSCF/NEVPT2 in ORCA. ORCA provides two types of results: CASSCF and CASSCF+NEVPT2, whereas only CASSCF results have been calculated by MOLCAS. Due to the limitation in computational facility, we are unable to perform the CASPT2 calculation on the converged CASSCF wave function. All atoms were described by the ANO-RCC basis sets of functions ([ANO-RCC-VTZP] for Co, [ANORCC-VTZ] for S, O, and N, and [ANO-RCC-VDZ] for C and H), including the relativistic effects within the Douglas−Kroll− Hess Hamiltonian.23 The Cholesky decomposition for two electron integral is employed throughout the calculations to save the disk space. Active space of the CASSCF method included all electrons spanning the 3d shell of the main metal site and a set of empty orbitals of the same angular momentum, accounting for the double shell effect. Spin− orbit coupling was included by means of the RASSI program24 by mixing all available spin states optimized in previous calculations (for 1−3: 10 spin quartet and 40 spin doublet states). The resulting low-lying part of the spin−orbit spectrum is given in Table S9. The parameters of the pseudospin Hamiltonians describing ZFS and magnetic anisotropy of lowlying states were computed within the SINGLE_ANISO program,25 using the ab initio wave function and spin−orbit eigenstates. We notice that the ground doublet state of the complexes are relatively well-separated from the excited state (Table S9). This means that the effects of the low-symmetry of the ligand environment corroborated with relatively strong spin−orbit coupling on CoII ions lead to sizable effects of the ZFS in this series. The ZFS of the two low-lying Kramers doublets for 1− 3, as interpreted within the s̃ = 3/2 formalism leads to easyplane anisotropy, with gx and gy components of the g tensor larger than the gz value. This is perfectly in line with the positive sign of the axial anisotropy D, as shown in Table 1 and agrees with the ones extracted from the fitting of measured magnetism. The easy-plane type magnetic anisotropy is also reflected in the anisotropy of the ground Kramers doublet (KD) (s̃ = 1/2), as shown in Table S9. As well-known, the axial anisotropy acting on the S = 3/2 quartet splits the latter in two KDs: ±1/2 doublet and a ±3/2 doublet. In the case of positive D, the doublet of ±1/2 is the ground, while the ±3/2 doublet becomes lower in energy when D is negative. The g tensor of the ground KD state for 1−3 is very close to the ideal values (gx = 6, gy = 4, gz = 2) for the ±1/2 type of the Kramers

(2)

Additionally, to check the temperature dependence, the relaxation time was studied at 0.1 T (Figure 4), and different thermally active processes (Orbach and Raman processes) have been included.20 The relaxation time is quite well described by eq 3 (where Ueff and τ0 are the energy barrier to magnetization reversal and relaxation times, respectively). Best fitting gave Ueff = 33.5 K, τ0 = 7.4 × 10−6 s, and n = 4.7 for 1; Ueff = 28.4 K, τ0 = 5.6 × 10−6 s, and n = 4.2 for 2; Ueff = 23.6 K, τ0 = 4.8 × 10−6 s, and n = 3.7 for 3 (Figure 4). The obtained n values are found similar to the reported values for CoII−YIII complex by Colacio et al.21 τ −1 = τQTM −1 + bT n + τ0−1 exp( −Ueff /kBT )

(3)

Thus, from the detailed analysis of the relaxation time, it has been revealed that QTM is the predominant process for slow magnetic relaxation and the Raman process is also important for relaxation process which actually elucidates its thermal dependence. D


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Inorganic Chemistry

values obtained from the ac susceptibility measurements are lower than the computed magnetization blocking barrier; this is may be because of the presence of strong QTM between the ground state KDs that is not fully suppressed under dc field. The relaxations of magnetization through the direct process, the Raman process, and QTM usually follow shortcut path and have different temperature dependencies which significantly reduced the Ueff. The Orbach process is not applicable for the studied complexes as the direct relaxation between the ground Ms = 1/2 level is allowed, and the first excited KDs are relatively higher energy (91−94 cm−1). Thus, it is expected that other processes like Raman process involving virtual states, provide the sufficient contribution to the relaxation mechanism for the studied complexes. The magnetization reversal mechanism does not follow the second excited KDs as they are in much higher energy (1500−2900 cm−1) as compared to the ground state and first excited states. In addition, in case of the complexes the matrix elements of the transition magnetic moment between the states +1 and −1 is 1.6 μB which is much higher than the 0.1 required for the efficient relaxation mechanism (Figure 6). The presence of such strong tunnelling between the ground state KDs quench the magnetization completely and results the absence of zero field slow magnetic relaxation behaviors for complexes 1−3. Thus, application of external dc field suppress the fast relaxation process and show slow magnetic relaxation in all complexes, although QTM cannot be ignored completely even in presence of the external field as this process is also facilitated by some other factors like hyperfine interaction, dipolar interaction, and so on. Magneto−Structural Correlation. The obtained ZFS parameters for studied complexes are larger than the other mononuclear pentagonal-bipyramidal CoII SMMs reported in the literature (Table S8). Moreover, the value of the D parameter for complex 1 is the highest among the reported seven-coordinate CoII system. In this system the value of easyplane magnetic anisotropy parameter should increase with weaker axially coordinated σ-donor ligands. From the d orbital splitting of the complexes it has been revealed that the energy gap between dz2 and (dx2−y2 and dyz) or (dx2−y2 and dxy) or (dx2−y2 and dxz) sets of orbital increases as we move from 1−3. This is mostly because of the increase of axial ligand field from 1 (both O-donors) to 2 (O and N-donors) to 3 (both Ndonors), which results that the presence of weak donor ligand in axial position increase the positive D value. In complex 1, the axial ligands are weaker σ-donors, and consequently, it results in substantial spin−orbit mixing of the ground quartet

Table 1. ORCA/CASSCF, ORCA/CASSCF+NEVPT2, and MOLCAS/CASSCF+RASSI-SO Computed D, |E|, and Main Values of gx,y,z from the Pseudospin Hamiltonian w.r.t. s̃ = 3/2 for Complexes 1−3 complex

Dexpt (cm−1)

Dcalc (cm−1) b

1

43.1(7)a

41.5(6)a 2 3

38.8(2)a

41.9 38.0c 46.2d 42.1b 38.0c 45.7d 42.3b 37.9c 45.4d

|E| (cm−1)

gx, gy, gzd

a

3.3(2) 3.26c 4.3d 1.5(4)a 1.32c 2.1d 2.1(7)a 2.49c 3.6d

(2.48, 2.38, 1.98)d

(2.45, 2.41, 1.99)d (2.48, 2.40, 2.00)d

a

Experimental values. b ORCA/CASSCF. c ORCA/CASSCF +NEVPT2. dMOLCAS/CASSCF+RASSI-SO/Single_Aniso.

doublet, in line with the positive D of the entire s̃ = 3/2 (Table S9). It is worth mentioning that the anisotropy of first excited Kramers doublet is much closer to the ideal values for the ±3/ 2 type doublet (gx = 0, gy = 0, gz = 6, Table S10). The electronic excitation between same and different ml value results a negative and positive D respectively. The obtained d orbital splitting from the ab initio ligand field theory (AILFT) approach26 also confirms the positive D value. All the complexes show that the electronic transition occurs between the different ml value of the d orbital which results in the positive nature of the anisotropy parameter D (Figure 5), and the spin-conserved transition has major contribution to overall positive D. It has been observed that the positive D contribution from the first excited state decreases from complexes 1−3 as the energy difference between the ground state and first excited state decreases in the same order and leads to the highest D value for complex 1. For the rhombic parameter E, third and forth excited states has the significant contribution for all complexes. However, the larger positive contribution from the first excited state leads to a large overall E value for 1, while that is not the case for 2 and 3 (Table S11). The orientation of the computed g-tensors for complexes 1−3 have been given in Figure S17. The energy of lowest lying KD and relaxation process for 1− 3 has been computed with the SINGLE_ANISO code as implemented in MOLCAS (Figure 6). It has been observed that the first excited state lies above the ground at 93.8, 91.83, and 91.74 cm−1 for complexes 1−3, respectively. The Ueff

Figure 5. AILFT computed d-orbital splitting of complexes 1−3 (a−c). Orbital relative energies are given in cm−1. E


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Inorganic Chemistry

Figure 6. Ab initio computed relaxation mechanism with two lowest Kramers’ doublets (KDs) in complexes 1-3 (a−c). KDs, as a function of their magnetic moment along the main anisotropy axis, are indicated by thick black lines. The blue line indicates the probable relaxation pathways for magnetization reversal mechanism and purple lines show the possible pathways of the Orbach process. The dotted red lines represents the ground state QTM and TA-QTM (thermally assisted-QTM) via first excited KDs. The numbers close to each arrow designate matrix elements of transition magnetic moments.5k EMPYREAN instrument using Cu Kα radiation. IR spectrum (4000− 400 cm−1) was recorded on KBr pellets with a PerkinElmer spectrometer. Magnetic measurements were performed using a Quantum Design SQUID-VSM magnetometer. The measured values were corrected for the experimentally measured contribution of the sample holder, while the derived susceptibilities were corrected for the diamagnetic contribution of the sample, estimated from Pascal’s tables.27 Caution! Perchlorate salts are potentially hazardous, and caution should be exercised when dealing with such salts. Crystal Data Collection and Structure Determination. Bruker APEX-II CCD diffractometer was used for mounting suitable single crystals of 1−3 using a graphite monochromated Mo Kα radiation (α = 0.71073 Å). φ and ω scan were used for intensity data collection. The structure solution was achieved using direct methods followed by full matrix least-squares refinements against F2 (all data HKLF 4 format) using SHELXTL.28 Remaining non-hydrogen atoms were positioned properly using difference Fourier synthesis and leastsquares refinement. Established procedures were used for the determination of the exact crystal system, orientation matrix, and cell dimensions followed by Lorentz polarization and multiscan absorption correction. Hydrogen atoms were placed geometrically and refined using the riding model, and non-hydrogen atoms were refined with independent anisotropic displacement parameters. All calculations were carried out using SHELXL 97,29 PLATON 99,30 and WinGX system Version 1.64.31 Crystallographic data for complexes 1−3 were summarized in Table S1. In complexes 1−3, phenoxy groups of the ligand and lattice methanol molecules are found to be highly disordered and became anisotropic displacement parameters (ADPs) upon anisotropic refinement. This results in few A and B level alerts. Attempts have been taken care to resolve those disorder atoms but remain unsuccessful. This might be because of the presence of various-low-resolution reflections at lower angle during data collections. Computational Details (ORCA). Both the first principles and ZFS calculations have been performed with scalar relativistic effect with ZORA (zeroth-order regular approximation). We have used the def2-TZVP basis set for Co, S, N, and O and def2-SV(P) for other atoms with the auxiliary basis set def2/JK for all atoms. In the active space, we have considered 7 electrons in 5 d orbitals (CAS 5, 7) and computed 10 quartet and 40 doublets in Configuration Interaction (CI) procedure. To introduce the effect of dynamic correlation, we employed N-electron valence perturbation theory (NEVPT2) on top of the CASSCF wave function. The spin−orbit coupling effects were incorporated by using quasi-degenerate perturbation theory (QDPT) approach.32,26 The ZFS parameters (D) and E were calculated both from second-order perturbation theory and an effective Hamiltonian approach (EHA).33

with the excited quartet level and therefore larger D value for complex 1 along the series. Another important influence of easy-plane magnetic anisotropy parameter in pentagonalbipyramidal CoII complexes arise from spin−orbit mixing of the ground quartet with an excited doublet state. In complex 1, the H4daps ligand is in dianionic form, and both the anionic charges are conjugated. Therefore, the equatorial coordination surroundings are more symmetrical as compared to other complexes, which results in the ability to increase the positive contribution to the D parameter. Thus, the combined effects of the weaker axially coordinated σ-donors and more symmetrical ligand in the equatorial plane can considerably increase the magnitude of the D parameter in the case of complex 1.

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CONCLUSION In conclusion, both experimental and ab initio theoretical calculations show that the second-order spin−orbit perturbation mediated control of easy-plane magnetic anisotropy of seven-coordinate CoII system is possible by suitable variation of the coordination surroundings around CoII center. There is no delicate design for unravelling the combined effects of both axial and equatorial ligands on easy-plane magnetic anisotropy of pentagonal bipyramidal CoII complexes. Herein, we for the first time reported the systematic investigation of combined effects of the weaker axially coordinated σ-donors and symmetrical ligand in the equatorial plane can considerably increase the magnitude of the easy-plane magnetic anisotropy, which in overall results the highest D value for complex 1 reported so far among seven-coordinate CoII system. The fieldinduced slow magnetic relaxation behaviors have been observed for all studied complexes. Additionally, the detailed experimental and theoretical investigations of relaxation pathways have been studied for the first time in sevencoordinate CoII system. It can be concluded from field and temperature dependence of relaxation time that QTM is the predominant process for the slow magnetic relaxation and that the Raman process is also important for relaxation process which actually elucidates its thermal dependence.

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EXPERIMENTAL SECTION

Materials and General Procedure. The elemental analyses were performed on Elementar Microvario Cube Elemental Analyzer. Powder X-ray diffraction (PXRD) data was collected on a PANalytical F


Inorganic Chemistry

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Synthesis of H4daps (2,6-Bis(1-salicyloylhydrazonoethyl)pyridine). H4daps was prepared according to the reported procedure by condensation reaction of 1 equiv 2,6-diacetylpyridine with 2 equiv of 2-salicyloylhydrazide.34 Synthesis of [Co(H2daps)(MeOH)2] (1). H4daps (43 mg, 0.1 mmol) was dissolved in 5 mL of MeOH; then, 5 mL methanolic solution of Co(ClO4)2·H2O (36 mg, 0.1 mmol) was added and the reaction mixture stirred. Then, triethylamine (20 mg, 0.2 mmol) was added dropwise to the above reaction mixture. The whole mixture forms a red color and stirred for 1 h. Then, the solution was filtered and kept for slow evaporation for 2 days and gave X-ray quality red crystals of [Co(H2daps)(MeOH)2] (1). These were washed with ether, and yield was calculated as 80%. Anal. Calcd for C25H27CoN5O6: C, 54.36; H, 4.93; N, 12.67%. Found: C, 54.45; H, 5.03; N, 12.73%. IR (KBr pellet, 4000−400 cm−1) ν/cm−1: 3420, 3064, 2918, 2788, 1528, 1374, 1332, 1291, 1074, 1034, 715. Synthesis of [Co(H 4daps)(NCS)(MeOH)]·(ClO4)·(MeOH) (2). H4daps (43 mg, 0.1 mmol) was dissolved in 5 mL of MeOH; then, methanolic solution of Co(ClO4)2·6H2O (36 mg, 0.1 mmol) was added and stirred. Then, an aqueous solution of KSCN (10 mg, 0.1 mmol and 5 mL of water) was added to the above reaction mixture. The final solution turns red and was refluxed for 2 h at 80 °C. Then, the solution was filtered and kept for slow evaporation for 3 days, giving X-ray quality red crystals of [Co(H4daps)(NCS)(MeOH)]· (ClO4)·(MeOH) (2). Finally, sample was washed with ether, and yield was measured as 72%. Anal. Calcd for C26H29ClCoN6O10S: C, 43.87; H, 4.11; N, 11.80; S, 4.49%. Found: C, 43.96; H, 4.21; N, 11.87; S, 4.55%. IR (KBr pellet, 4000−400 cm−1) ν/cm−1: 3422, 3073, 2917, 2818, 2175, 1529, 1384, 1318, 1289, 1076, 1028, 713. Synthesis of [Co(H4daps)(NCS)2]·(MeOH)2 (3). H4daps (43 mg, 0.1 mmol) was dissolved in 5 mL of MeOH; then, 5 mL methanolic solution of Co(ClO4)2·6H2O (36 mg, 0.1 mmol) was added to the above solution and the reaction mixture stirred for 1 h. Then, an aqueous solution of KSCN (20 mg, 0.2 mmol and 10 mL of water) was added to above reaction mixture. The final solution turns red and was refluxed for additional 2 h at 80 °C. Then, the solution was filtered and kept for slow evaporation for 4 days, giving X-ray quality red crystals of [Co(H4daps)(NCS)2]·(MeOH)2 (3). These were washed with ether, and yield was calculated as 65%. Anal. Calcd for C27H29CoN7O6S2: C, 48.37; H, 4.36; N, 14.62; S, 9.54%. Found: C, 48.44; H, 4.44; N, 14.69; S, 9.61%. IR (KBr pellet, 4000−400 cm−1) ν/cm−1: 3429, 3052, 2921, 2838, 2165, 1525, 1363, 1327, 1295, 1070, 1052, 718. Preparation of Diluted Sample. The diluted sample has been prepared by a similar method to that for compound 3; however a mixture of Zn(ClO4)2·6H2O and Co(ClO4)2·6H2O in a 95:5 percentage ratio has been used. Energy dispersive X-ray spectroscopy (EDS) has been performed to verify the doped level in the diluted sample and it indicates the presence of Zn, Co, O, S and N elements in the diluted sample (Figure S15).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91-755-6691313. ORCID

Amit Kumar Mondal: 0000-0001-5187-9949 Bijoy Dey: 0000-0003-4185-4240 Sanjit Konar: 0000-0002-1584-6258 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K.M. thanks UGC, India, for the SRF fellowship. S.K. thanks DAE BRNS(project no. No. 37(2)/14/09/2015-BRNS), Government of India, and IISER Bhopal for generous financial support. S.K. and A.M. thank Dr Shu-Qi Wu from Kyushu University, Japan, and Mr. Arup Sarkar and Prof. Gopalan Rajaraman from IIT Bombay for their helpful scientific discussion. The high-performance computing (HPC) facility at IISER Bhopal is gratefully acknowledged for the computational work.

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DEDICATION Dedicated to Professor R. N. Mukherjee on the occasion of his 65th birthday REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01162. Additional data and plots, PXRD, and magnetic characterizations (PDF) Accession Codes

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