Cytosine Nucleobase Ligand: A Suitable Choice for Modulating

Article pubs.acs.org/IC

Cytosine Nucleobase Ligand: A Suitable Choice for Modulating Magnetic Anisotropy in Tetrahedrally Coordinated Mononuclear CoII Compounds Rosaria Bruno,† Julia Vallejo,‡ Nadia Marino,† Giovanni De Munno,† J. Krzystek,§ Joan Cano,*,‡ Emilio Pardo,*,‡ and Donatella Armentano*,† †

Dipartimento di Chimica e Tecnologie Chimiche (CTC), Università della Calabria, 87036 Cosenza, Italy Departament de Química Inorgànica/Instituto de Ciencia Molecular (ICMol), Universitat de València, 46980 Paterna, València, Spain § National High Magnetic Field Laboratory (NHMFL), Florida State University, Tallahassee, Florida 32310, United States ‡

S Supporting Information *

ABSTRACT: A family of tetrahedral mononuclear CoII complexes with the cytosine nucleobase ligand is used as the playground for an in-depth study of the effects that the nature of the ligand, as well as their noninnocent distortions on the Co(II) environment, may have on the slow magnetic relaxation effects. Hence, those compounds with greater distortion from the ideal tetrahedral geometry showed a larger-magnitude axial magnetic anisotropy (D) together with a high rhombicity factor (E/D), and thus, slow magnetic relaxation effects also appear. In turn, the more symmetric compound possesses a much smaller value of the D parameter and, consequently, lacks single-ion magnet behavior.

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INTRODUCTION

work is still required to understand correctly the mechanisms involved in these slow relaxation processes. Among transition metal ions, those metal complexes with low coordination numbers and low symmetries have exhibited excellent performances so far.23−26 For example, tetrahedral Co(II)-based complexes are particularly attractive for blocking the magnetization as they possess a half-integer spin ground state, which sometimes reduces the possibility of quantum tunnelling of magnetization (QTM).29−47 In previous papers, we have used biomolecules, such as pyrimidine nucleobase derivatives,48−52 as ligands to construct tetrahedral cobalt(II) complexes.48 These ligands can also play a major role in understanding the driving forces that govern the presence or lack of a slow magnetic relaxation in cobalt(II) complexes, which allows improvement of the design of new CoII-based SIMs. We have thus performed an experimental and theoretical study of a new family of tetrahedral Co(II) complexes with cytosine nucleobase ligands, covering two

Single-molecule magnets (SMMs) have attracted a great deal of interest because of their exciting potential applications in highdensity magnetic information storage and quantum computation devices.1 Initially, following the discovery of the SMM behavior of the Mn12 cluster,2 most efforts were devoted to developing multinuclear complexes with large magnetic ground spin states to achieve a large energy barrier for magnetic relaxation.3−6 This strategy was not always successful because the magnetic anisotropy parameter (D), which must be large to observe superparamagnetic behavior, is not easily controlled. The use of simpler systems was mandatory for monitoring the magnetic anisotropy, and thus, the first report of slow magnetic relaxation in a mononuclear lanthanoid complex constituted a milestone in the field of molecular magnetism.7 Since then, a substantial number of single-ion magnets (SIMs) using lanthanides,8−12 actinides,13−16 and first-row17−26 and third-row27,28 transition metals have been reported. In this respect, an exact design of the coordination geometry of the metal cation is crucial for observing SIM behavior, and further © XXXX American Chemical Society

Received: October 7, 2016

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DOI: 10.1021/acs.inorgchem.6b02448 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

highly distorted pseudo-octahedral (1 and 2) and cubic (3) symmetries. As a result, the orientation of the nucleobase rings in these compounds is such that the carbonyl group points toward the cobalt atom, with its oxygen atom occupying elongated positions (Figure 1b). In such cases, the Co−N bond lengths [mean values of 2.075 Å (1), 2.169 Å (2), and 2.175 Å (3)] increase to maintain the Co−O interactions [2.793 Å (1), 2.823 Å (2), and 2.744 Å (3)]. This lengthening of the Co−N bond is more marked (mean values of 2.169, 2.158, and 2.175 Å, respectively) when the Co−O interactions are stronger, and the Co−O bond lengths are shorter (mean values of 2.229, 2.222, and 2.207 Å, respectively).55,56 The [Co(Cyt)2(NCS)2] units in 1 {like [Co(Cyt)2(NCO)2] in 2 (see the Supporting Information for packing details)} are joined together by hydrogen bonds that involve the N(1), N(4), and O(2) atoms from the Cyt nucleobase [O(2)···N(1b) = 2.867(1) Å, (b) = −1 + x, y, z; O(21)···N(4a) = 2.817(1) Å, (a) = 1 − x, −y, −z]. Other and further weak symmetric Hbonds through sulfur (1) (or oxygen in 2) terminal atoms of a coordinated NCS− or NCO− group with N(1) atoms of the ligand coexist [S(1C)···N(11d) = 3.626(3) Å; d = x, 1 + y, z in 1]. Auxiliary π···π interactions (interplanar and centroid··· centroid distances d and h in 1 of 3.51 and 3.93 Å, respectively) contribute to stabilize the network [Co(1)···Co1(1a) = 7.862(1) Å, (a) = 1 − x, −y, −z; Co(1)···Co(1b) = 7.752(1) Å, (b) = 1 + x, y, z; Co(1)···Co(1c) = 7.142(1) Å, (c) = −x, −y, −z; Co(1)···Co(1d) = 8.790(1) Å; Co(1)···Co(1e) = 7.753(1) Å, (e) = 1 − x, 1 − y, 1 − z] (Figure 2 and Figure S2 for 1 and Figures S3 and S4 for 2). In any case, the mononuclear units are well isolated from each other, exhibiting

novel simple compounds with the general formula [Co(Cyt)2X2], where Cyt = cytosine and X = NCS− (1) and NCO− (2) and a previously reported related compound with the formula [Co(1-MeCyt)4][ClO4]2 (3) (1-MeCyt = 1methylcytosine) (Figure 1a).48 As an extension of a previously

Figure 1. Perspective (left) and side (right) views of (a) 3 and (b) 1 and 2 with the atom numbering scheme.

reported work by Boča et al. on a cytosine−chloride complex,37 their magnetic properties have been compared to shed light on the effect of the nature of such ligands on the cobalt(II) environment and, ultimately, on the magnetic properties.

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RESULTS AND DISCUSSION Synthesis and X-ray Crystal Structures. Compounds 1 and 2 were obtained, in a one-pot reaction, by mixing stoichiometric amounts of [CoX2] and Cyt (1:1 molar ratio) in a water solution, whereas compound 3 has been prepared using a method reported in the literature.48 After slow evaporation at room temperature (see Experimental Section), violet and blue crystals of 1 and 2 were obtained in excellent yields. The molecular structures of 1 and 2 (Figure 1b and Figure S1) display cobalt(II) ions in a highly distorted fourcoordinated tetrahedral environment, CoN4. Despite compounds being isostructural, there are differences in their Co− NCyt and Co−NX bond lengths [mean values of 2.033(1) and 1.989(1) Å in 1 and 2.042(2) and 1.980(2) Å in 2] and interbond angles at the cobalt atoms [97.89(6)−113.56(5)° in 1 and 103.0(1)−117.07(8)° in 2] (see Tables S2 and S3). Meanwhile, in 3, CoII ions linked to four N(3) atoms of the nucleobase derivative (Figure 1a) exhibit a less distorted tetrahedral arrangement with all Co−N distances equal to 2.076(2) Å and with related bond angles of 101.2(5)° and 113.2(5)° (Figure S1). This situation contrasts with that seen for other tetrahedral cobalt(II) compounds with four pyridines and a CoN4 chromophore,53,54 where the Co−N bond lengths are slightly shorter (varying in the range of 1.996−2.014 Å). This difference is mainly linked to the exocyclic O(2) atom, which favors a weak electronic interaction with Co2+ ions leading to

Figure 2. (a) View along the a crystallographic axis of a fragment of packing in 1. (b) Details of a portion of crystal packing of 1 showing the [Co(Cyt)2(NCS)2] units in 1 joined by hydrogen bonds and π···π interactions. (c) Details of further weak H-bonds involving sulfur atoms of NCS− groups and nitrogen atoms of the Cyt nucleobase ligand [S1C···N4 = 3.293(2) Å and Co(1)···Co(1f) = 9.244(2) Å, (f) = 2 − x, 1 − y, 1 − z]. The carbon, nitrogen, oxygen, sulfur, and hydrogen atoms of the ligands are shown as gray, blue, red, yellow, and white sticks, respectively, whereas the cobalt atoms are depicted as violet spheres. B


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Inorganic Chemistry 2

rather long intermetallic separations, the shortest intermolecular Co···Co distances being 6.778(1) and 7.032(1) Å (Figure 2b and Figure S3b). Similarly, the Co(II) ions are well separated in 3 with a shortest intermetallic distance of 7.965(1) Å. Magnetic Properties. The direct current (dc) magnetic properties of 1−3 in the form of the χMT versus T plot (χM being the dc magnetic susceptibility per CoII ion) were first investigated (Figure 3). The χMT values of 2.36 cm3 mol−1 K

2

2

H = D[Sẑ − 1/3S(S + 1)] + E[Sx̂ − Sŷ ] + μB BgS ̂ (1)

which takes into account both the axial (D) and rhombic (E) zfs parameters of the tetrahedral CoII ions. This simultaneous treatment permits a better estimation of the g factors, the E/D quotients, and the sign of the D parameters. Intermolecular interactions were not considered because the mononuclear units in 1−3 are well isolated and the theoretical curves with the parameters in Table 1 match very well with the experimental data over the whole temperature range (solid lines in Figure 3 and Figure S5). The relatively large and negative D values obtained by magnetometry for the distorted tetrahedral CoII complexes 1 and 2 contrast with the small and positive value found for 3, which is most likely due to its quasiideal tetrahedral symmetry. In the same way, the values of the E/D quotient match those we can expect from the molecular geometry: large values for distorted symmetry and close to zero for ideal tetrahedral symmetries. However, magnetometry has limitations in the quantitative evaluation of E/D. Also, geometrical and electronic symmetries occasionally are not equivalent. We therefore resorted to high-frequency and -field electron paramagnetic resonance (HFEPR) and theoretical studies to establish more solid conclusions. The D parameter of the 4A1 ground state relates to energies (Ei) for the three closest quartet excited states (Qi) originating from the 4T2 term through the equation:

Figure 3. Temperature dependence of χMT under an applied dc field of 10 mT (T < 30 K) and 500 mT (T ≥ 30 K) for 1 (black), 2 (blue), and 3 (red). The solid lines are the best-fit curves (see the text).

D = λ′2 (3/E1 + 3/E2 − 6/E3)

(2)

where λ′ is the multielectronic soc constant for the complex. According to this equation, while two of these excited states contribute to a positive D value, one of them does so to a negative value. Furthermore, in an ideal tetrahedral symmetry, where Q1−Q3 states are degenerate, both positive and negative contributions to D cancel each other. However, this value is non-zero when there is a loss of electronic symmetry that originates from both structural distortions and the presence of ligands of a different nature, which occurs in 1 and 2, where two 1-MeCyt residues were replaced with pseudohalide ligands. Moreover, the stronger ligand field provided by the NCS− and shorter Co−N distances in 1 and the slightly different value for the N−Co−N angle are probably at the origin of the various D values between 1 and 2. HFEPR Spectroscopy. The magnetometric results were checked by HFEPR measurements on 1 and 2 that provided more accurate information about spin Hamiltonian parameters. In both cases, the spectra showed patterns typical for an S = 3/2 spin state with a moderate zfs. The spin Hamiltonian parameters (see Table 1) were best fit to the two-dimensional maps of resonance fields versus frequency (Figure S6). Simulations of single-frequency HFEPR spectra, such as the

(1), 2.21 cm3 mol−1 K (2), and 2.25 cm3 mol−1 K (3) at room temperature are higher than that expected for an isolated S = 3 /2 compound with no orbital momentum contribution (χMT = 1.9 cm3 mol−1 K with g = 2.0). When the samples are cooled, the χMT values for 1−3 remain constant until 50 K (1 and 2) and 20 K (3) and then continuously decrease to reach values of 1.52, 1.25, and 2.02 cm3 mol−1 K at 2.0 K, respectively (Figure 3). On the other hand, the M versus H/T plots of 1−3 (M being the reduced dc molar magnetization per mononuclear unit and H the applied magnetic dc field) do not superimpose (Figure S5), particularly for 1 and 2. These behaviors explicitly discard the occurrence of a first-order spin−orbit coupling (soc) but suggest a zero-field splitting (zfs) arising from a second-order soc, which is weaker in 3 as a consequence of its higher molecular symmetry. The experimental magnetic susceptibility and magnetization data of 1−3 were simultaneously analyzed through the VPMAG program57 by using the appropriate spin Hamiltonian for a mononuclear model

Table 1. Selected Experimental and Theoretical Magnetic Data for 1−3 Da (cm−1) 1 2 3

−6.1 −7.4 +1.4

E/Da 0.125 0.170 0.026

ga 2.154 2.198 2.189

F × 106b

Dc (cm−1)

32 12 1.1

−6.81(1) +8.10(1)e ±1.50 e

E/Dc

gxc

0.24 0.29 0.31

2.204(5) 2.27(4)e 2.20

gyc e

gzc e

2.16(1) 2.26(3)e 2.20

2.262(5) 2.32(3)e 2.20

e

Dd (cm−1)

E/Dd

gd

−6.3 −8.5 +0.5

0.199 0.304 0.064

2.235 2.252 2.281

Values obtained from the fit of the magnetic susceptibility data. A temperature-independent paramagnetism (+8.9 × 10−6 + 190 × 10−6 cm3 mol−1 for 1 and 2) was also considered. bAgreement factor defined as ∑[(P)exp − (P)calcd]2/∑[(P)exp]2, where P is the physical property under study. c Values obtained from the fit of multifrequency HFEPR data. dValues obtained from NEVPT2 calculations. eStandard deviations are given in parentheses. a

C


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Inorganic Chemistry

reason, no useful two-dimensional map of resonances could be plotted, and spin Hamiltonian parameters were estimated by comparing single-frequency spectra with simulations. An example of such spectra and simulations that best reproduce them is shown in Figure S9. The small magnitude of D (1.5 cm−1) obtained from HFEPR is basically in agreement with the results of magnetometry (Table 1), but the E/D value (0.31) is not and does not intuitively agree with the high symmetry of complex 3. The HFEPR spectra are, however, deficient in that the simulations show a strong inter-Kramers transition, whereas it appears very weak only in the experiment. For that reason, it can only be stated that HFEPR confirms the small magnitude of |D| in 3 but cannot deliver robust information about the other spin Hamiltonian parameters. Theoretical Calculations. To further confirm the validity of the experimental results, we performed NEVPT2 calculations (see Table 1 and Table S4 and computational details). Overall theoretical parameters agree well in both sign and magnitude with those obtained from the fit of the experimental data, but the quantitative agreement with the results from HFEPR spectroscopy is remarkable for both zfs tensor and Landé factors. In addition, NEVPT2 calculations also confirm that the quadruplet excited states from the 4T2 term are the closest ones to the 4A2 ground state (GS) and responsible for the axial zfs in 1−3 through a second-order soc (Figure 5 and Table S4). As eq

one shown in Figure 4 and Figures S7 and S8, served the purpose of establishing the sign of D. For example, the two

Figure 4. EPR spectrum of complex 1 as a pellet at 20 K and 634 GHz (black trace) along with its simulations (colored traces) assuming a powder distribution of the crystallites and spin Hamiltonian parameters as in Table 1. The blue trace is for negative D and the red trace for positive D. The bottom panel shows the analogous spectrum of complex 2 at 10 K and the same frequency, and its simulations.

peaks at ∼8.3 and ∼10.6 T in 1 and ∼7.7 and ∼8.5 T in 2 correspond to transitions originating from different Kramers doublets (see Figure S14). The intensity ratio between them is thus a direct indication of which doublet lies lower on the energy scale, i.e., the sign of D. Simulations prove that D is negative in 1, confirming the results of magnetometry. In 2, a positive D value is, however, established. In both complexes, rhombicity factor E/D is very large (0.24 and 0.29, respectively), approaching the limit of 0.33. This means that the sign of D, obtained from either magnetometry or HFEPR has very little physical meaning because in such a case there is almost no difference between the properties of the two Kramers doublets. While the agreement between D magnitudes obtained from the magnetometry and HFEPR (Table 1) is very good, that between the E/D ratios is much less so. The rhombicity factor is very difficult to estimate accurately by magnetometry; hence, the determination of the sign of D by the latter technique can be inaccurate in the presence of a large rhombicity. Complex 3 produced an HFEPR response weaker by at least an order of magnitude than that of 1 and 2, suggesting faster relaxation rates, particularly in the inter-Kramers transitions that are crucial for determining the zfs parameters. For that

Figure 5. (a) Perspective views of the calculated 3d metal orbitals for the quartet (S = 3/2) spin ground state considering an ideal Td geometry. (b) Distribution of the quadruplet states in 1−3, including those coming from the terms 4F (blue) and 4P (pink) and noted according to an ideal Td geometry.

2 shows, the negative contribution to D is larger than the two remaining positive ones, which leads to a value close to zero in 3 and a non-negligible negative value in 1 and 2 (Table S4). Moreover, from these contributions and the energies of Q1− Q3, it was possible to estimate the λ′ values for 1−3 [−138.0(9), −133(4), and −141.2(2) cm−1, respectively], which were satisfactory. The negative D value found for 2 contrasts with that provided by HFEPR spectroscopy, but both theoretical and experimental techniques agree with the presence of a high rhombicity factor (E/D) close to its limit, where the sign of D lacks meaning. In any case, the z-axis of the D tensor is equally oriented into the molecule in 1−3 (Figure S10). Whereas the contribution to parameter D from a given excited state should be zero when this was far from the GS, it is assumed that it will take a finite but non-zero value when this state was degenerate with the GS. D


Article

Inorganic Chemistry Dynamic (ac) Magnetic Properties. Finally, from calculations it is logically observed that greater splits of the 4 T2 term lead to larger E/D ratios. The alternating current (ac) magnetic susceptibility of 1−3 in the form of the plots of χM′ and χM″ versus T (χM′ and χM″ being the in-phase and out-ofphase ac magnetic susceptibilities per mononuclear unit, respectively) was then investigated under applied static fields of 0, 50 and 100 mT (Figure 6 and Figures S11 and S12).

behavior should appear below 2 K because of the low D value or should not appear because of the positive D value. We think both causes brought about the different magnetic dynamic behavior of 3. The values of the pre-exponential factor (τ0) [3.41 × 10−8 (1) and 4.17 × 10−8 s (2)] and the activation energy (Ea) (13.0 and 19.1 cm−1 for 1 and 2, respectively), which were obtained from the fit of the Arrhenius plots (Figure S12 and Table S5), suggest a single relaxation process characteristic of an Orbach mechanism. The obtained D and Ea values for 1−3 are coherent [Ea = |D| (S2 − 1/4)], and they are consistent with those previously reported for other tetrahedral CoII SIMs.32,33 The agreements between D and Ea values and between the sign of Dor failing that, the role played by the rhombicity E/D ratioand the emergence of a SIM behavior are a consequence of the fact that the zfs in these tetrahedral complexes comes from a secondorder instead of a first-order soc, as occurs in the equivalent octahedral complexes, where these rules are not obeyed. Finally, the values of the α parameter obtained for 1 and 2 from the Cole−Cole plots (Table S5 and Figure S13) further support single relaxation processes.

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CONCLUSIONS In conclusion, we report on a family of low-coordinate Co(II) complexes (1−3) prepared with the cytosine nucleobase ligand, which is used as a playground to study the effects of structural distortions in magnetic anisotropy and, ultimately, in the slow magnetic relaxation effects. Hence, those complexes exhibiting both very distorted tetrahedral geometries and the presence of ligands of different natures (1 and 2) showed higher values of the axial magnetic anisotropy, which was reflected in the appearance of SIM behavior. In contrast, the more symmetric compound, 3, lacks slow magnetic relaxation effects, in total agreement with its small and positive D parameter. Overall, our results suggest that a good structure-based ligand design for constructing new appealing compounds may improve our essential understanding of matter and thus could contribute to acceleration of the successful generation and perfection of materials with potential for development.

Figure 6. Temperature dependence of χM′ (left) and χM″ (right) of (a) 1, (b) 2, and (c) 3 in a 100 mT applied static field and under a ±0.5 mT oscillating field in the frequency range of 0.1−10 kHz (see the legend).

Although χM″ signals were not observed for any of them in a zero dc magnetic field (Figure S11), strong frequencydependent maxima typical of SIMs could be observed for both χM′ and χM″ below 5 K for 1 and 2 when applying static dc fields of 50 and 100 mT (Figure 6 and Figure S12). This fact is indicative of the presence of a fast zero-field quantum tunneling relaxation of the magnetization, which is suppressed upon application of a dc magnetic field.58 Therefore, 1, exhibiting a moderate and negative D value, shows the typical slow magnetic relaxation effects of a field-induced SIM. However, 2 also displays this particular dynamic behavior, despite its positive D value suggested by HFEPR spectroscopy. Although this circumstance could be explained as an electronuclear spin entanglement coming from hyperfine interactions, the relaxation time at 2.0 K is not dependent on the applied magnetic field in this case, and therefore, a direct relaxation process is discarded.59 In light of that, we think the dynamic behavior in 2 is possible only because of the large E/D ratio mixes the pure functions of two Kramers doublets, removing the selection rules that prevent the transition between the resultant Kramers doublets. Therefore, an energy barrier appears, supporting the possibility that a slow magnetic relaxation occurs. This situation contrasts with that of 3 that, having a small and positive D value, does not behave as a SIM above 2 K. In such a case, it is not clear whether the SIM

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EXPERIMENTAL SECTION

Materials. All chemicals were of reagent grade quality. They were purchased from commercial sources and used as received. Preparation of [Co(Cyt)2(NCS)2] (1), Where Cyt = Cytosine. 1 was prepared in very good yields from the reaction of stoichiometric amounts of Co(NCS) 2 and the corresponding Cyt ligand, commercially available (1:2 metal:ligand molar ratio) in water solutions. X-ray quality purple rhombus prisms of 1 were obtained by slow evaporation of H2O/EtOH [2:1 (v/v)] mixtures. Yield: 70%. Elemental analysis calcd (%) for C10H10S2CoN8O2 (1): C, 30.23; H, 2.54; N, 28.20. Found: C, 30.12; H, 2.72; N, 28.92. IR (KBr): ν = 1681vs, 1637vs, and 1383s cm−1 (CO) from Cyt and 2043 cm−1 (CN) from NCS. Preparation of [Co(Cyt)2(NCO)2] (2). Co(NCO)2 has been generated in situ by mixing stoichiometric amounts of Co(ClO4)2· 6H2O and KNCO (1:2 molar ratio) in water solutions with an aqueous solution containing the corresponding Cyt ligand, commercially available (1:2 metal:ligand molar ratio). X-ray quality blue rhombus prisms of 1 were obtained by slow evaporation of H2O/ EtOH [2:1 (v/v)] mixtures. Yield: 70%. Elemental analysis calcd for C10H10CoN8O4 (2): C, 32.89; H, 2.76; N, 30.69. Found: C, 33.01; H, 2.86; N, 29.78. IR (KBr): ν = 1688vs, 1641vs, and 1370s cm−1 (C O) from Cyt and 2205 and 2190 cm−1 (CN) from N-bonded NCO. Physical Techniques. Elemental analyses (C, H, N) were performed by the microanalysis service of the Dipartimento di E


Article

Inorganic Chemistry Chimica e Tecnologie Chimiche of the Università della Calabria. The FTIR spectra were recorded on a Nicolet-6700 spectrophotometer as KBr pellets. Crystal Structure Data Collection and Refinement. X-ray crystallographic data for 1 and 2 were collected with a Bruker-Nonius X8 APEXII CCD area detector diffractometer using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) at low temperatures, performing φ- and ω-scans. Suitable crystals with approximate dimensions of 0.22 mm × 0.18 mm × 0.18 mm (1) and 0.20 mm × 0.16 mm × 0.16 mm (2) were selected for data collection. Data reduction and multiscan absorption corrections were performed using SAINT60 and SADABS.61 The structures were determined by direct methods using SHELXS and refined against F2 on all data by full-matrix least squares with SHELXL-2013.62 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms of the Cyt ligand were set in calculated positions and refined using a riding model. Graphical manipulations were performed with the XP utility of the SHELXTL63 system and the CrystalMaker software.64 Crystal data for 1 and 2 are summarized in Table S1, whereas selected bond lengths and angles are listed in Tables S2 and S3. CCDC reference numbers are 1474777 and 1474778 for 1 and 2, respectively. Magnetic Measurements. Variable-temperature (2.0−300 K) direct current (dc) magnetic susceptibility measurements under an applied field of 10 mT (T < 30 K) and 0.5 T (T ≥ 30 K) and variablefield (0−5.0 T) magnetization measurements at low temperatures in the range of 2.0−10.0 K were performed for 1−3 with a Quantum Design SQUID magnetometer. Variable-temperature (2.0−8.0 K) alternating current (ac) magnetic susceptibility measurements were taken for 1 and 2 with a Quantum Design Physical Property Measurement System (PPMS). The dc and ac magnetic measurements were taken by powdering and restraining the sample to prevent any displacement caused by its magnetic anisotropy. The susceptibility data were corrected for the diamagnetism of both the constituent atoms and the sample holder. Computational Details. To evaluate the parameters that determine the axial (D) and rhombic (E) zfs, calculations were based on a second-order N-electron valence state perturbation theory (NEVPT2) on mononuclear cobalt(II) complexes 1−3. Calculations were performed with version 3.0 of the ORCA program65 using the TZVP basis set proposed by Ahlrichs66,67 and the auxiliary TZV/C Coulomb fitting basis sets.68−70 The second-order contributions to zfs from 10 quartet and 20 doublet excited states generated from an active space with seven electrons in five d orbitals were included. The g tensors were calculated for the ground Kramer’s pair using Multireference Configuration Interaction (MRCI) wave functions with a first-order perturbation theory on the SOC matrix.71 Spectroscopic Measurements. HFEPR spectra of 1−3 were recorded on polycrystalline samples (50−100 mg) by using a homodyne spectrometer associated with a 15/17 T superconducting magnet in a frequency range from 52 to 634 GHz.72 Whereas 3 was studied on both loose and constrained polycrystalline samples, the presence of strong field-torquing effects forced us to prepare pellets from 1 and 2. Detection was provided with an InSb hot electron bolometer (QMC Ltd., Cardiff, U.K.). The magnetic field was modulated at 50 kHz for detection purposes. A Stanford Research Systems SR830 lock-in amplifier converted the modulated signal to dc voltage. The single-frequency spectra were simulated, and the spin Hamiltonian parameters were fitted with SPIN.

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complexes 1−3 and PXRD data for 1 and 2 (Figure S15) (PDF) Crystallographic details for 1 (CIF) Crystallographic details for 2 (CIF)

AUTHOR INFORMATION

Corresponding Authors

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

Donatella Armentano: 0000-0002-8502-8074 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministero dell’Istruzione, dell’Università e della Ricerca (Italy), the mineco (Spain) (Projects CTQ2013-46362-P, CTQ2013-44844-P, and MDM2015-0538), and the Generalitat Valenciana (Spain) (Project PROMETEOII/2014/070). Thanks are also extended to the Ramón y Cajal Program and the “Convocatoria 2015 de Ayudas Fundación BBVA a Investigadores y Creadores Culturales” (E.P.). HFEPR studies were supported by the National High Magnetic Field Laboratory (NHMFL), which is funded by the National Science Foundation through Cooperative Agreement DMR 1157490, the State of Florida, and the U.S. Department of Energy. We are grateful to Dr. Andrew Ozarowski (NHMFL) for the EPR simulation and fitting software SPIN.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02448. Selected structural (Tables S1−S3 and Figures S1−S4) high-frequency and -field EPR (HFEPR) and magnetic (Tables S4 and S5 and Figures S5−S14) data for F


Article

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