Slow Magnetic Relaxation in a Series of Mononuclear 8-Coordinate

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

pubs.acs.org/IC

Slow Magnetic Relaxation in a Series of Mononuclear 8‑Coordinate Fe(II) and Co(II) Complexes Xin-Xin Jin,†,# Xiao-Xiang Chen,‡,# Jing Xiang,*,† Yun-Zhou Chen,§ Li-Hui Jia,§ Bing-Wu Wang,*,‡ Shun-Cheung Cheng,∥ Xin Zhou,† Chi-Fai Leung,⊥,* and Song Gao*,‡ †

College of Chemistry and Environmental Engineering, Yangtze University, Jingzhou 434020, HuBei, P. R. China State Key Laboratory of Rare Earth Materials Chemistry and Applications and PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, Peking University, Beijing 100871, P. R. China § School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, P. R. China ∥ Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong 999077, P. R. China ⊥ Department of Science and Environmental Studies, The Education University of Hong Kong, 10 Lo Ping Road, Tai Po, NT, Hong Kong 999077, P. R. China ‡

S Supporting Information *

ABSTRACT: A series of homoleptic mononuclear 8-coordinate FeII and CoII compounds, [FeII(L2)2](ClO4)2 (2), [FeII(L3)2](ClO4)2 (3), [FeII(L4)2](ClO4)2 (4), [CoII(L1)2](ClO4)2 (5), [CoII(L2)2](ClO4)2 (6), [CoII(L3)2](ClO4)2 (7), and [CoII(L4)2](ClO4)2 (8) (L1 and L2 are 2,9-dialkylcarboxylate-1,10-phenanthroline ligands; L3 and L4 are 6,6′dialkylcarboxylate-2,2′-bipyridine ligands), have been obtained, and their crystal structures have been determined by X-ray crystallography. The metal center in all of these compounds has an oversaturated coordination number of 8, which is completed by two neutral homoleptic tetradentate ligands and is unconventional in 3d-metal compounds. These compounds are further characterized by electronic spectroscopy, cyclic voltammetry (CV), and magnetic measurements. CV measurements of these complexes in MeCN solution exhibit rich redox properties. Magnetic measurements on these compounds demonstrate that the observed single-ion magnetic (SIM) behavior in the previously reported [FeII(L1)2](ClO4)2 (1) is not a contingent case, since all of the 8-coordinate compounds 2−8 exhibit interesting slow magnetic relaxation under applied direct current fields.

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INTRODUCTION Single-molecule magnets (SMMs) as prospective candidates for potential applications in ultrahigh density memory components, spintronic devices, and processing technologies have attracted wide interest.1 In particular, SMMs with only one magnetic center, i.e., single-ion magnets (SIMs), have recently attracted much attention as their magnetic anisotropy is designable and their magnetic properties have the potential to be fine-tuned on the basis of ligand field theory.2 The lanthanides and actinides, with their f orbitals nearly unaffected by the ligand field and thus remaining essentially degenerate, are desirable candidates in the design of SIMs as a result of their large spin−orbit coupling constant. A number of lanthanidebased single-molecule magnets have been synthesized by rational design with the aim of achieving high anisotropy energy barriers, understanding the relaxation mechanisms, and tuning magnetic properties.3 Compared with lanthanide-based single-molecule magnets, 3d-based single-molecule magnets are still quite limited, mainly because their orbital angular momentum is readily quenched by the ligand field. One of the effective approaches to obtain the 3d-SIMs is to design a © XXXX American Chemical Society

molecule with a lower oxidation state, higher local symmetry, and lower coordination number. It is well-known that a high local symmetry could suppress the quantum tunneling process, while a low coordination number could result in a weak ligand field and enhanced magnetic anisotropy, as a consequence of the nearly unquenched orbital angular momentum and strong spin−orbit coupling effect similarly found in some lanthanide complexes. Comparatively, a low coordination number is a more important factor that is favorable for designing a 3dmetal-based SIM.4−12 On first glance, 3d-complexes with a coordination number greater than 6 are not considered ideal candidates for SIMs. However, recent studies have shown that a significantly high magnetic anisotropy can be induced in some transition-metal complexes with a coordination number higher than 6.13 Further, 8-coordination is usually confined to the larger metal ions of the heavier f-elements, while 3d-compounds with an oversaturated coordination number of 8 are rare.14 However, Received: December 6, 2017

A

DOI: 10.1021/acs.inorgchem.7b03071 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

range of 1694 to 1719 cm−1. As compared to the v(CO) stretches of L1−L4 in the range of 1723−1738 cm−1, the lower v(CO) stretches in 2−8 indicate the compounds’ coordination to the metal centers via carbonyl O atoms rather than via the alkoxyl O atoms, as the COM interactions could increase the bond polarity and then lower the bond order of CO. The coordination modes of these ligands are further validated by their solid state crystal structures (see below). In all of the compounds, the strong stretching bands observed around 1090 cm−1 are assigned to the v(ClO) stretch of counteranion ClO4−. The electronic absorption spectra of the complexes in MeCN solution have been studied. A brief summary is shown in Table 1, and the overlaid absorption spectra are shown in Figure 2

their highly distorted coordination environments result in a relatively weak ligand field and small energy spacing (Δ) between the ground and excited electronic states. Enhanced second-order spin−orbit coupling (SOC) and zero-field splitting (ZFS) of the central metal can be expected, as the second-order effects depend inversely on the energy spacing. Since the ligand orbitals cannot overlap well with the d-orbitals of metal centers in such a highly distorted local coordination geometry, the orbital angular momentum can be maintained to a larger extent, and thus, a strong spin−orbit coupling can also be expected.14 We recently reported an Fe(II) compound, [FeII(L1)2](ClO4)2 (1) (L1 = 2,9-bis(carbomethoxy)-1,10-phenanthroline), as the first example of an 8-coordinate mononuclear Fe(II) compound that exhibits field-induced SIM behavior.14g The ligand structure and its disposition not only play a key role in maintaining its unusual coordination geometry but also allow for the design of new SIMs.14h To further investigate the effect of the 8-coordinate distorted coordination environment on the SIM behavior, a series of mononuclear 8-coordinate Fe(II) compounds, [FeII(L2)2](ClO4)2 (2), [FeII(L3)2](ClO4)2 (3), and [FeII(L4)2](ClO4)2 (4), bearing the modified tetradentate ligand L1 with different alkyl chains and extended π-conjugation (L1−L4, Figure 1) have been prepared with the aim to suppress

Table 1. UV-Vis Absorption Data for 2−8 in MeCN Solution at 298 K 2 3 4 5 6 7 8

213 (71040), 236 (74000), 252 (73720), 291 (66880), 328 sh (17870), 480 (920) 202 (92300), 255 (26320), 266 (23680), 273 (17970), 304 (21000), 314 (22430), 437 (680) 202 (109700), 255 (31200), 266 (28760), 275 (21300), 302 (27200), 314 (29750), 452 (710) 212 (53890), 236 (57290), 249 (55650), 289 (45580), 318 sh (16190), 571 (70) 213 (60400), 236 (62300), 251 (60600), 292 (51650), 319 sh (17090), 569 (60) 204 (92800), 252 (28200), 298 (18950), 310 (17160), 551 (60) 203 (89850), 254 (24870), 298 (17990), 311 (16310), 557 (50)

and Figure S1 in the Supporting Information. The electronic absorption spectra of compounds 2−8 show intense absorptions with molar extinction coefficients (ε) on the order of 104 dm3 mol−1 cm−1 in the UV region (235−350 nm), which are assigned to the ligand-centered (LC) π → π* transitions. As the phen moieties in L1 and L2 have a πconjugated system more extended than that of bpy, the LC bands in 2, 5, and 6 are slightly red-shifted and are more intense than those of the bpy-based compounds. Apart from the intense UV absorption, the Fe(II) complexes 2−4 show relatively stronger absorption bands in the range of 430−480 nm with molar extinction coefficients on the order of 102 dm3 mol−1 cm−1, which are ascribed to the metal-to-ligand charge transfer (MLCT) band. A broad d−d transition is observed around 1400 nm without any clear splitting for the Fe(II) compounds 2−4. The Co(II) complexes, 5−8, show multifeature d−d bands of low intensity near 560 nm in addition to an absorption at 1200 nm. All of the d−d transitions in these compounds have very low molar absorptivities. In order to support these assignments, the lowest 10 vertical electronic transitions of complexes 3, 5, and 7 were calculated by the time-dependent density functional theory (TD-DFT) method, the results of which are summarized in Tables S1−S3. The calculated electron transitions are similar for the two Co complexes (5 and 7), which show four transitions with wavelengths longer than 1000 nm and are all d−d transitions in nature. Two transitions are found to have wavelengths around 750 nm, which are also d−d in origin. For other transitions in the visible region (with wavelengths longer than 390 nm), the transitions are mainly π → π* transitions of the diimine ligands with mixing of d−d transitions. All of the transitions are predicted to have a low molar absorptivity due to the significant contributions of the d−d transitions. The lowest

Figure 1. Ligand structures of L1−L4.

the dipole−dipole interaction of the isolated spin centers and eliminate intermolecular magnetic coupling. Moreover, a series of 8-coordinate homoleptic Co(II) complexes, [CoII(L1)2](ClO4)2 (5), [CoII(L2)2](ClO4)2 (6), [CoII(L3)2](ClO4)2 (7), and [CoII(L4)2](ClO4)2 (8), have also been prepared, and their magnetic properties are compared with those of their Fe(II) analogues, since the high-spin (HS) Co(II) complexes with a half-integer spin ground state could reduce the possibility of quantum tunneling of magnetization (QTM).7−10 The crystal structures for all of the compounds were determined by X-ray crystallography, and it is clearly shown that all of the metal centers have an [MN4O4] coordination set in a highly distorted triangular dodecahedron. Magnetic measurements show that all of these compounds demonstrate a similar field-induced SIM behavior as in the case of our previously reported compound 1. The coordination environment of the metal center is able to not only maintain this special geometry but also induce a large magnetic anisotropy.

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RESULTS AND DISCUSSION Treatment of ligands L1−L4 with hydrated Fe(ClO4)2 and Co(ClO4)2 in MeOH afforded the air-stable mononuclear compounds 2−8 in high yields. The IR spectra of these compounds show the characteristic v(CO) stretches in the B


Article

Inorganic Chemistry

Figure 2. UV-vis absorption spectra of Fe(II) compounds 2−4 (a) and Co(II) compounds 5−8 (b) in MeCN.

phen-based 2 shows a reversible oxidation wave at E1/2 = 1.16 V vs SCE (ΔEp = 72 mV, ipc/ipa = 0.96) that is tentatively assigned as the metal-centered FeIII/II process. Four reversible reduction couples are observed at potentials around −0.85, −0.97, −1.50, and −1.66 V vs SCE (ΔEp ≈ 39−80 mV, ipc/ipa ≈ 0.97−1.05), which are similarly found for the complex [Fe(L1)2](ClO4)2 (1).14g The metal-centered FeIII/II oxidation couples in bpy-based 3 and 4 shift cathodically by ∼140 mV as compared with that of 1, which is in agreement with the stronger π-accepting ability of the phen moiety which better stabilizes the metal center in the lower oxidation state. Only two reversible reduction couples around −1.02 and −1.20 V vs SCE are observed for 3 and 4. The CVs of 5−8 show an irreversible wave around 1.05 V vs SCE, which is assigned to the irreversible oxidation of CoII to CoIII. The phen-based Co(II) complexes 5 and 6 exhibit four successive reversible redox couples around −0.64, −0.98, −1.43, and −1.64 V vs SCE, similar to those of the Fe(II) analogues. However, only one quasi-reversible wave and one irreversible wave are observed at potentials around −0.69 and −1.51 V vs SCE for bpy-based 7 and 8, as in 3 and 4. A comparison between the Fe and Co phen-based complexes shows that the first reduction wave at around −0.64 V shifts cathodically to −0.85 V upon replacing the Fe(II) ion with the Co(II) ion, while the third reduction wave also shifts accordingly. Thus, these two couples are probably the metal-centered process. However, the positions of the other two couples remain more or less unchanged, indicating that they are more likely to be ligandcentered. Crystal Structures of the 8-Coordinate Fe(II) Compounds 2−4. The X-ray crystal structures of 2−4 are shown in Figure 3, and selected bond parameters are summarized in Table 3. It is noteworthy that all the Fe(II) centers are 8-

calculated electron transitions of the Fe complex 3 are slightly different from the two Co ones. The calculated wavelengths of the d−d transitions are red-shifted to the region longer than 2000 nm, and there are no transitions with wavelengths around 750 nm. For higher-energy transitions in the visible region (with wavelengths longer than 390 nm), the transitions mainly originate from MLCT [d(Fe) → π*(bpy)] transitions, with some mixing of d−d transitions of iron and π → π* transitions of the bpy ligands. These transitions are predicted to have molar absorptivities significantly higher than those of the d−d transitions in the IR region. The lower-energy d−d transitions observed in these compounds are well in agreement with small energy splitting, as expected in complexes with an oversaturated coordination number of 8. The redox properties of compounds 2−8 were studied by cyclic voltammetry (CV) in MeCN containing 0.1 M nBu4NPF6 as a supporting electrolyte (Figure S2), and the related electrochemical data are summarized in Table 2. The CV of Table 2. Electrochemical Data for the Complexes 1−8 in MeCN Solution (0.1 M nBu4NPF6) at 298 K Collected under an Ar Atmosphere oxidation, E1/2 (V vs SCE) (ΔEp) 1 2 3 4 5 6 7 8

1.19 1.16 1.02 1.00 1.05 1.02 1.06 1.05

(70) (72) (68) (95)

reduction, E1/2 (V vs SCE) (ΔEp) −0.83 −0.85 −1.02 −1.03 −0.64 −0.64 −0.69 −0.70

(73), −0.95 (64), (59), −0.97 (39), (83), −1.19 (52) (69), −1.22 (82) (76), −0.98 (67), (81), −0.99 (67), (97), −1.49 (100), −1.53

−1.46 (64), −1.62 (40) −1.50 (60), −1.66 (80)

−1.42 (58), −1.63 (73) −1.44 (59), −1.65 (77)

Figure 3. ORTEP drawing of cationic structures for the 8-coordinate Fe(II) compounds 2 (a), 3 (b), and 4 (c). C


Article

Inorganic Chemistry Table 3. Bond Parameters (Å) for Compounds 2−4 Fe1N1 Fe1N2 Fe1N3 Fe1N4 Fe1O1 Fe1O2 Fe1O3 Fe1O4 N2Fe1N1 O2Fe1O1

2

3

4

2.216(5) 2.157(4) 2.154(4) 2.218(4) 2.630(4) 2.426(4) 2.448(4) 2.669(4) 73.97(17) 149.62(12)

2.192(2) 2.191(2)

2.1950(19) 2.2039(18) 2.2012(19) 2.1823(18) 2.4956(16) 2.3622(16) 2.3957(16) 2.4391(16) 72.76(7) 147.77(5)

2.4453(18) 2.4688(19)

72.87(9) 147.87(7)

coordinated with two neutral tetradentate ligands that have four N and four O atoms. Similar to complex 1, the Fe(II) centers in 2−4 are also situated in a distorted triangular dodecahedron with a pseudo-D2d symmetry. The degree of deviation from the standard triangular dodecahedron (D2d) was estimated from the continuous shape measures (CShMs).15 The deviation parameter (S) values of the Fe(II) centers in 2−4 were calculated to be 1.992, 0.812, and 0.677, respectively. A larger S value usually implies a larger deviation of the central metal ion from the specific symmetry. The S value of 2 is very close to that of 1 (1.286), but both values are significantly larger than those of 3 and 4. This result indicates that the steric constraint in 3 and 4 could be effectively released by rotation above the central C−C bond in the bpy ligand to meet the coordination requirement of Fe(II) centers in 3 and 4. In order to reduce the steric repulsion, the two N2O2 planes of the L1−L4 ligands in these Fe(II) compounds are nearly in a vertical form with interplanar angles of 86.4°, 90.2°, 85.5°, and 87.0° for 1−4, respectively. The Fe−N bond lengths in these compounds are distributed in a narrow range of 2.154(4)−2.218(4) Å, where the lengths are comparable to those in 1 (average 2.17 Å). These bond lengths are typical of high-spin FeII complexes.16 Their Fe−O bond lengths are, however, distributed in a wider range of 2.3634(17)−2.630(4) Å. Although these bond distances are longer than those usually found in HS FeII complexes,16 they are much shorter than the sum of the corresponding van der Waals radii (3.60 Å). It is noticeable that these bond parameters are all significantly affected by the substituents and the nature of the aromatic rings. For instance, 3 has a C2 axis as observed in 1, and the Fe−N and Fe−O bond lengths in these two compounds are essentially the same. However, 2 and 4 have lower symmetry, probably as a result of the steric repulsion of the n-butyl groups. The changes in ligand rigidity and the substituting group are therefore suggested to be associated with the observed change of the local symmetry above the metal centers (Table 3). The rigid phen moiety of L2, together with steric repulsion of the bulky n-butyl group, leads to the largest distortion of the coordination geometry in 2 among these Fe(II) complexes. In the packing diagrams the shortest intermolecular Fe···Fe distances are in the range of 8.83−9.51 Å due to the different alkyl groups and aromatic rings. Such longer distances will lead to negligible intermolecular magnetic interactions. Crystal Structures of the 8-Coordinate Co(II) Compounds 5−8. The perspective views of the cations of 5−8 are shown in Figure 4, and selected bond parameters about the Co(II) centers are listed in Table 4. The coordination environments in the Co(II) centers are very similar to those

Figure 4. ORTEP drawing of cationic structures for the 8-coordinate Co(II) compounds 5 (a), 6 (b), 7 (c), and 8 (d).

of the Fe(II) analogues. All the Co(II) centers are coordinated by four N and four O donors in a highly distorted triangular dodecahedron. The local symmetry of 6 is lowest, while the other Co(II) compounds have a C2 axis, in line with the trend observed in the Fe(II) analogues. The distorted coordination environment of 6 is reflected by the bond parameters. The Co− N2 and Co−N4 bond lengths in 6 are ∼0.1 Å longer than those of the Co−N1 and Co−N3 bonds. The Co−O1 and Co−O3 bond lengths are similar (average 2.45 Å), while they are much shorter than those of Co−O2 and Co−O4 (average 2.89 Å) by ∼0.44 Å. Since the ionic radius of a HS Co(II) ion (0.745 Å) is smaller than that of a HS Fe(II) (0.780 Å) ion by 0.035 Å, the Co−N bond distances (2.057(2)−2.091(2) Å) in these Co(II) compounds are all systematically shorter than the Fe−N bond lengths in 2−4. These bond lengths are comparable to those in related HS Co(II) complexes.17 However, the Co−O bond lengths in 5, 7, and 8 are in the range of 2.412(3)−2.686(1) Å, which are longer than those in related HS Co(II) compounds. In the packing diagrams, the shortest intermolecular Co···Co distances are in the range of 9.02−9.58 Å, a result from the different alkyl groups and aromatic rings. The long Co···Co distance also leads to negligible intermolecular magnetic interactions. The ranges of the M−O and M−N bond distances in the Co(II) complexes are larger than those in their Fe(II) analogues. The coordination environments in the Co(II) compounds are more distorted and are attributed to the smaller ionic radius of the Co(II) ion. The shortening of Co−N bonds results in a lengthening of the Co−O bonds in 5−8. The degree of deviation of these Co(II) centers from the standard triangular dodecahedron (D2d) is also estimated from CShMs,15 where the S values were found to be 1.969, 6.195, 1.563, and 1.315 for compounds 5, 6, 7, and 8, respectively. These S values are systematically larger than those for 1−4 and are consistent with the steric effects imposed by the ligands. In particular, the largest S value for 6 indicates that its local symmetry is the lowest, which is reflected by the two unusually long Co−O bond distances (>2.8 Å) in its solid structure. Similar to those in 1−4, the two N2O2 planes of the L1−L4 ligands in these Co(II) compounds are nearly in vertical form with interplanar angles of 88.1°, 90.3°, 86.1°, and 88.3° for 5−8, respectively. Apart from our Fe(II) compound 1 reported recently, only eight 8-coordinate Fe(II) complexes were characterized by XD


Article

Inorganic Chemistry Table 4. Bond Parameters (Å) for Compounds 5−8 5 Co1N1 Co1N2 Co1N3 Co1N4 Co1O1 Co1O2 Co1O3 Co1O4 N2Co1N1 O2Co1O1

2.0760(14) 2.0559(15)

2.6860(13) 2.5791(13)

80.65(6) 143.47(4)

6 2.038(3) 2.183(3) 2.053(3) 2.149(3) 2.413(3) 2.898(3) 2.491(3) 2.893(3) 78.31(12) 145.76(9)

ray crystallography.14 All of these Fe(II) compounds possess a similar [FeN8] coordination sphere, and their reported average Fe−N bond lengths are slightly longer than those in 1−4, possibly due to the significantly longer Fe−O bond lengths in these compounds.14 Moreover, there is only one reported 8coordinate cobalt(II) complex, [CoII(12C4)2](I3)2(12C4),13b where the Co(II) ion is surrounded by eight O atoms in a distorted square antiprism geometry. The reported Co−O bond lengths are distributed in a narrow range of 2.24(0)− 2.29(4) Å, which are significantly shorter than those in 5−8. Magnetic Properties. Static Magnetic Properties for 8Coordinate Fe(II) Compounds. Direct current (dc) magnetic measurements of 2−4 were conducted between 2 and 300 K under a dc field of 1 kOe. The plot of χMT vs T is shown in Figures 5 and S3. It is clearly shown that the temperature

7

8

2.080(3) 2.078(3)

2.064(2) 2.091(2)

2.604(2) 2.636(2)

2.537(2) 2.598(2)

79.53(12) 142.26(7)

79.10(9) 141.36(6)

expected for high-spin Fe(II) complexes with g = 2.50, 2.27, and 2.40 and are comparable with those in other high-spin Fe(II) compounds with pseudo-D2d symmetric metal centers.14a,c,g These χMT values are obviously larger than the expected spin-only value (3.0 cm3 mol−1 K) for a high-spin Fe(II) ion with S = 2 and g = 2 and reveal the presence of a largely unquenched orbital momentum in these FeII complexes, but they are still lower than the value (4.80 cm3 mol−1 K) observed in Fe(C(SiMe3)3)2.4e As similarly observed in 1,14g the χMT values of 2−4 remain almost constant upon cooling from 300 K to about 45 K and then rapidly decrease to 2.82, 2.38, and 2.97 cm3 mol−1 K, respectively, at 2 K. The decrease of χMT at low temperature for 2−4 is also observed in other high-spin Fe(II) complexes and is attributed to the presence of strong magnetic anisotropy,18 which is further validated by the absence of any close Fe···Fe contacts in their solid states. In order to probe their magnetic anisotropy, reduced magnetization data were collected at low temperature. As shown in Figure 6, the resulting magnetization plots (M vs H/T) are well-separated for these isotherm curves, revealing the presence of the strong magnetic anisotropy in these Fe(II) compounds. These curves were fitted by Anisofit 2.0 (H = D[Sz2 − S(S + 1)/3] + E(Sx2 − Sy2) + μBgiso·S·B), where μB is the Bohr magneton and D, E, S, and B represent the axial and rhombic zero-field splitting (ZFS) parameters, the spin, and the magnetic field vectors, respectively. The fitting gives D = −9.5 cm−1, E = 0.8 cm−1, and g = 2.14 for 2; D = −4.3 cm−1, E = 0.77 cm−1, and g = 2.15 for 3; and D = −3.2 cm−1, E = 0.9 cm−1, and g = 2.22 for 4 (Table 5). All the D values found in these Fe(II) compounds are negative, which are comparable to the reported values of the related 8-coordinated Fe(II) compounds.14 To further analyze the magnetic properties of these Fe(II) compounds, ab initio CASSCF/CASPT2/RASSI/ SINGLE_ANISO calculations were also conducted via the MOLCAS 8.0 program package.19 The ab initio calculations afford axial and transverse zero-field splitting (ZFS) negative

Figure 5. Temperature dependence of χM−1 and χMT vs T for 2 (□), 3 (○), and 4 (△) measured at 1 kOe.

dependence of their molar magnetic susceptibilities is typical of noninteracting mononuclear Fe(II) complexes.4 The χMT values of 2−4 at 300 K are 4.66, 3.84, and 4.28 cm3 mol−1 K, respectively. The measured χMT values correspond to values

Figure 6. Low-temperature magnetization data for 2, 3, and 4 under applied dc fields from 1 to 5 T. Solid lines are fitting curves by Anisofit 2.0. E


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Inorganic Chemistry

skeletons (phen vs bpy). The bulky butyl groups may not only effectively reduce the dipole−dipole interactions of the spin centers but also lead to the lower local symmetry, which would enhance the second-order angular momentum and ZFS in the ground state. Relaxation times (τ) were extracted from these peaks with the Debye model and are plotted as a function of T−1 in Figure S6. A weak temperature dependence of τ at low temperature suggests again the significant influence from the quantum tunneling of magnetization, while the dominant spin relaxation undergo the Orbach mechanism at relatively high temperature. The result reveals a thermally activated slow relaxation of the magnetization, and analysis of these data with the Arrhenius equation τ = τ0 exp(Ueff/kBT) yields an effective spin-reversal barrier of Ueff = 12 K and τ0 = 6.9 × 10−7 s for 2, Ueff = 8 K and τ0 = 1.5 × 10−6 s for 3, and Ueff = 15 K and τ0 = 2.8 × 10−7 s for 4. The τ0 values are well in accordance with those found in related transition-metal SMMs in the range of 10−7 to 10−10 s22 and are typical for an Orbach process. The nonlinear fitting for compounds 2−4, considering the Orbach and Raman processes with equation

Table 5. Selected Experimental and Theoretical Magnetic Data for 2−8

2 3 4 5 6 7 8

CShM

D (E) (cm−1)a

ga

D (|E|) (cm−1)b

gb

Ueffc (K)

1.992 0.812 0.677 1.969 6.195 1.563 1.315

−9.5 (0.82) −4.3 (0.77) −3.2 (0.20) −29.4 (1.70) −40.5 (6.27) −22.0 (3.65) −15.8 (2.76)

2.14 2.15 2.22 2.49 2.61 2.37 2.35

−11.7 (0.6) −3.1 (0.6) −2.7 (0.5) −19.0 (1.64) −46.9 (5.2) −20.8 (2.03) −16.9 (1.04)

2.21 2.01 2.00 2.47 2.74 2.48 2.44

12(0.8) 8(0.8) 15(1.5) 44(1.2) 20(0.9) 18(0.6) 31(0.1)

Values obtained from the fit of magnetic susceptibility. bValues obtained from ab initio calculations. cValues in parentheses are standard error for energy barrier fitting. a

parameters of D = −11.7 cm−1, E = 0.6 cm−1, and g = 2.21 for 2; D = −3.1 cm−1, E = 0.6 cm−1, and g = 2.01 for 3; and D = −2.7 cm−1, E = −0.5 cm−1, and g = 2.00 for 4. These negative D values confirm the uniaxial anisotropy in these Fe(II) compounds, and their orders of magnitude are comparable with those of the related Fe(II) complexes.4 Dynamic Magnetic Properties of Fe(II) Complexes. Variable-frequency alternating current (ac) susceptibility data were obtained at varied temperatures under a 0 kOe field for all Fe(II) complexes, a 1.4 kOe dc field for 2 and 4, and a 3.0 kOe field for 3 (Figure 7). Under the zero field, the slow magnetic relaxation process in the frequency range of 1−10000 Hz is not observed for all of the Fe(II) compounds (Figure S4), which is due to fast quantum tunneling of magnetization (QTM) that allows for the quick relaxation of spins without raising the thermal activation energy barrier for spin reversal20 and is also commonly found in related mononuclear Fe(II) SMMs. When dc fields were applied to these Fe(II) complexes, both in-phase (χ′) and out-of-phase (χ″) susceptibilities show significant frequency dependence (Figures 8 and S5), indicating that QTM can be efficiently suppressed or slowed when the degeneracy of the ±Ms levels is lifted in an external dc magnetic field.21 It is not surprising to observe the slow relaxation of magnetization for all the Fe(II) compounds due to their negative D values. The isothermal frequency-dependent ac magnetic susceptibilities of these Fe(II) compounds are shown in Figure 9, where the maximum χ″ vs v plot shifts from 1103 to 10000 Hz for 2, 2585 to 10000 Hz for 3, and 631 to 1000 Hz for 4 with increasing temperature. Accordingly, the appearance of the maximum of χM″ in 2 and 4 is comparable with that in 1 (>690 Hz). However, the maximum of χM″ in 3 appears at a much higher frequency range. The crystal structures of 1−4 are very similar except for the difference in bulky substituting alkane chains (butyl vs methyl) and rigidity of diimine

((τ

ln(τ ) = −ln

0

−1

( ) + CT )), was also performed

exp

−Ueff T

n

(Figure S7), where C and n are Raman process parameters, and the corresponding results are summarized in Table S4. These Ueff values are obviously lower than the values generated from U = D|S|2, with the D values determined from the dc magnetic data (34.9 cm−1 for 2, 9.2 cm−1 for 3, and 8.1 cm−1 for 4). The disparity of the energy barrier between calculations and fitting arises from the fast quantum tunneling, similar to that of 1 and commonly observed in some transition-metal complexes exhibiting single-molecule magnetic behavior.4−12 Plotting χ″ vs χ′ (Cole−Cole plots) for these Fe(II) complexes in the temperature range of 1.8−4.0 K gives well-defined semicircles, confirming the occurrence of a single relaxation process in the high temperature range (Figure S8). The data can be fitted using a generalized Debye model, giving the values of distribution of relaxation time that are listed in Table S5−S7. The low value of α at high temperature indicates a narrow distribution of the relaxation time. Static Magnetic Properties of 8-Coordinate Co(II) Compounds. The plots of χMT vs T obtained by the dc magnetic measurement of 5−8 under a 1 kOe dc field between 2 and 300 K are shown in Figures 9 and S3. The temperature dependence of their magnetic susceptibility is characteristic of noninteracting mononuclear Co(II) complexes.10 The χMT values of 5−8 at 300 K are 2.71, 2.84, 2.65, and 2.90 cm3 mol−1 K, respectively. These χMT values are significantly larger than the expected spin-only value of 1.875 cm3 mol−1 K for an isotropic HS CoII ion (S = 3/2 and g = 2), which indicates the

Figure 7. Temperature dependence for in-phase (χ′) and out-of phase (χ″) signals of the ac magnetic susceptibility for 2, 3, and 4 (for 2 and 4, Hdc = 1.4 kOe and Hac = 3 Oe; for 3, Hdc = 3 kOe and Hac = 3 Oe). F


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Inorganic Chemistry

Figure 8. Frequency dependence of out-of-phase (χ″) ac magnetic susceptibility for 2−4.

Figure 9. Temperature dependence of χM−1 and χMT vs T for 5 (□), 6 (○), 7 (△), and 8 (▽) measured at 1 kOe.

presence of the considerable unquenched orbital angular momentum for these Co(II) ions.23 These values are quite comparable with those observed in some high-spin lowcoordinate Co(II) complexes with an unquenched orbital angular momentum.24,25 On the basis of the χMT values at 300 K, the g values for 5−8 were calculated to be 2.41, 2.48, 2.39, and 2.50, respectively. As the temperature decreases from 300 K, the χMT values of 5−8 remain more or less constant and then decrease continuously to 1.84, 1.96, 1.67, and 1.70 cm3 mol−1 K at 2 K, respectively, when the temperature is below 60 K. The decreases in the χMT values for 5−8 at low temperature are commonly seen in HS Co(II) complexes.24 The field-dependent magnetizations for these Co(II) complexes were performed in up to a 5 T dc field at 2, 3, 5, 8, and 10 K to probe the magnetic anisotropy, the results of which are shown in Figure 10. At 2 K, the magetizations for compounds 5−8 show the gradual increase and approach the values of ∼2 μB at 50 kOe, which are smaller than the calculated saturation magnetizaiton (Ms) with a value of 3 μB for a single HS Co(II) ion. The magnetization plots (M vs H/T) obtained at different temperatures are nonsuperimposed, and significant separations are observed between the isotherm curves. The above results indicate the presence of strong magnetic anisotropy for these Co(II) compounds. As compared with the Fe(II) complexes, the Co(II) analogues have a relatively small total spin (S = 3/2). The bifurcation of M vs H/T plots appears at a higher field range, implying the existence of higher magnetic anisotropy in these Co(II) complxes in comparison to the above Fe(II) analogues. As suggested by the χMT and variable-field magnetization data, all of these 8-coordinate Co(II) complexes have high anisotropic magnetic moments, possibly resulting from the stronger spin− orbital coupling of the central Co(II) ions in these complexes. The fitting of the M vs H/T plots using Anisofit 2.019 affords D = −29.4 cm−1, E = 1.7 cm−1, and g = 2.49 for 5; D = −40.5 cm−1, E = 6.27 cm−1, and g = 2.61 for 6; D = −22.0 cm−1, E = 3.65 cm−1, and g = 2.37 for 7; and D = −15.8 cm−1, E = 2.76

Figure 10. Low-temperature magnetization data for 5−8 under applied dc fields from 1 to 5 T. Solid lines are fitting curves by Anisofit 2.0.

cm−1, and g = 2.35 for 8. The large g-factor variations and negative D values indicate that these compounds have a large uniaxial anisotropy, which is comparable with the value (−37.6 cm−1) reported in other 8-coordinate Co(II) compounds.13b The ab initio calculations were performed to validate the experimental results and afforded axial and transverse ZFS negative parameters of D = −19.0 cm−1, E = 1.64 cm−1, and g = 2.47 for 5; D = −46.9 cm−1, E = 5.2 cm−1, and g = 2.74 for 6; D = −20.8 cm−1, E = 2.03 cm−1, and g = 2.48 for 7; and D = −16.9 cm−1, E = 1.04 cm−1, and g = 2.44 for 8. The large and negative D values could be attributed to the distortion of the coordination geometries of these compounds. Thus, a correlation is suggested between the lowered local symmetry by the deviation from an ideal D2d symmetry and the increased contribution of transverse anisotropy. 24d The distorted symmetry matches the large value of the E/D quotient. The results also suggest that a ZFS arises from a second-order SOC rather than a first-order SOC. Overall theoretical parameters are close to the calculated results from the fit of the experimental data, further demonstrating the large magnetic anisotropy for these Co(II) compounds. Note that both the experimental and calculated D and E values are quite comparable with those of Co(II) compounds with lower coordination numbers.10,13 Dynamic Magnetic Properties for Co(II) Complexes 5−8. Variable-frequency ac susceptibility data were collected at G


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Inorganic Chemistry

Figure 11. Temperature dependence of in-phase (χ′) and out-of phase (χ″) signals of the ac magnetic susceptibility for 5−8 (Hdc = 0.5 kOe and Hac = 3 Oe).

Figure 12. Frequency dependence of out-of-phase (χ″) ac magnetic susceptibility for 5 (a), 6 (b), 7 (c), and 8 (d).

multiple temperatures under 0 and 0.5 kOe dc fields. Similar to the above Fe(II) complexes, no slow magnetic relaxation process was observed for any of them in the absence of an external field (Figure S9). This situation is also attributed to QTM, which operates through either hyperfine or dipolar coupling.18c On the contrary, strong frequency dependence maxima typical of SIMs could be observed for both in-phase (χ′) and out-of-phase (χ″) susceptibility when a 0.5 kOe dc field was applied (Figure 11). Similar to the Fe(II) complexes mentioned above, the fast zero-field quantum tunneling relaxation of the magnetization is suppressed or diminished in these Co(II) complexes in the presence of an applied dc field. The presence of a QTM effect could be validated by the observation of the increase in χ″ at a low temperature and low

frequency range, and this effect in 7 became more obvious. The difference of the ac magnetic behavior between the Fe(II) and Co(II) compounds is possibly due to not only the different spin−orbit coupling constant λ for the free ions but also the fact that Co(II) ion is a non-integer spin system Kramers ion,7e,26 where the tunneling of the magnetic moment caused by mixing of the ground ±Ms components and the rhombic anisotropy parameter E will be minimized. As a result, a lower applied dc field was used as compared with that for the Fe(II) complexes. As shown in Figure 12, χ″ vs v curves for these 8-coordinate Co(II) compounds in the low temperature range under an applied dc field of 0.5 kOe display obvious frequency dependence. The temperature dependence and frequency H


Article

Inorganic Chemistry

prepared from the neutral ligands L1−L4. These tetradentate ligands could stabilize and maintain the high coordination number 8 for Fe(II) and Co(II) ions, which was confirmed by X-ray analysis. It has been shown that the special ligand design and disposition of these complexes result in a highly distorted geometry for each compound, which has a strong impact on their structural, magnetic, and redox properties. These 8coordinate Fe(II) and Co(II) complexes have almost the same [MN4O4] coordination environments, but they could be finetuned by changing the rigidity of the π-systems and bulkiness of the alkyl chains. The lowering of the local symmetry in their solid structures by deviating from the ideal D2d symmetry will increase the transverse magnetic anisotropy. This study helps to improve our understanding of the effects of structural distortions on the electronic structures and magnetic anisotropy of an 8-coordinate compound. Ab initio calculations show the presence of magnetic anisotropy for all of the compounds, which is validated by dc magnetic measurements. It is interesting to note that all the compounds with such a coordination geometry exhibit field-induced SMM behavior due to the presence of significant axial magnetic anisotropy. Although the magnetic anisotropy is readily influenced by various factors, including molecular symmetry, ligand field strength, spin−orbit coupling, and zero-field splitting, it could be fine-tuned by delicate structural modifications. To the best of our knowledge, this work represents the first example in which the 8-coordinate Fe(II) and Co(II) complexes with essentially identical coordination environments exhibit fieldinduced SIM behavior. Structural modification in the organic ligand can tune the magnetic anisotropy in 8-coordinate Fe(II) and Co(II) complexes, and such a strategy of designing an unconventional coordination geometry is useful in the construction of new SIM compounds. This work will improve our understanding of magneto-structural correlation. We have previously synthesized a series of magnetic materials based on paramagnetic RuIII/OsIII building blocks,28 and further work on the extension of this synthetic strategy on an 8-coordinate Ru/ Os system is in progress.

dependence of the ac susceptibility indicate that they all exhibit magnetic relaxation processes. The maximum χ″ values for all the Co(II) complexes shift to higher frequencies with increasing temperature until they move beyond the highfrequency limit of the instrument. As shown by the results, the dc field effectively removes the state degeneracy and slows the quantum tunneling magnetization. The Co(II) compound [CoII(12C4)2](I3)2(12C4) also exhibits similar field-induced slow magnetic relaxation.13b The reported ZFS parameters of D = −38 cm−1, E = −0.75 cm−1, and g = 2.55 for this Co(II) compound are comparable with those of 5−8. However, the maximum of χ″ for this compound appears at a relatively lower frequency range (>63 Hz), possibly due to the fact that this compound has a relatively small rhombic ZFS parameter (−0.75 cm−1). The relaxation times (τ) extracted from the peaks using the Debye model are shown in Figure S10 as a function of T−1. The observed weak temperature dependence of τ at low temperature for the Co(II) compounds is possibly due to Raman and/ or a direct phonon-based relaxation mechanism, while at the relatively high temperature region, the dominant spin relaxation follows the Orbach relaxation process through the excited Ms = ±1/2 levels. The result reveals a thermally activated slow relaxation of the magnetization. By using the Arrhenius equation τ = τ0 exp(Ueff/kBT), the effective spin-reversal barriers are as follows: Ueff = 44 K and τ0 = 7.4 × 10−9 s for 5; Ueff = 20 K and τ0 = 3.9 × 10−9 s for 6; Ueff = 18 K and τ0 = 2.3 × 10−7 s for 7; and Ueff = 31 K and τ0 = 3.9 × 10−9 s for 8. These τ0 values are also well in accordance with those found in related Co(II)-containing SMMs in the range of 10−7 to 10−10 s22, and the Ueff values are also comparable with that of the related mononuclear 8-coordinate compound [CoII(12C4)2](I3)2(12C4) (Ueff = 24 K).13b These Ueff values are all higher than those of the Fe(II) analogues although a relatively lower dc field is used for the Co(II) systems, mainly due to their noninteger spin and stronger spin−orbit coupling constant λ with larger g-factor variation. In addition, the Ueff for these Co(II) compounds could be estimated using the barrier equation U = | D|(ST2 − 1/4) for the half-integer spin ground state, and the values are 82.3, 63.0, 61.6, and 44.2 K for 5−8, respectively. Obviously, the theoretically predicted values for these Fe(II) and Co(II) complexes are far larger than the corresponding Ueff values observed through ac susceptibility measurements. This kind of discrepancy is also found in the other SMMs because, except for the Orbach process, there are some other possible relaxation processes, including direct, Raman, and the presence of the non-negligible QTM.4a,7e,27 Similar to the Fe(II) compounds 2−4, the nonlinear fitting for compounds 5−8 considering the Orbach and Raman processes with equation

(

( ) + CT )

ln(τ ) = −ln τ0−1exp

−Ueff T

n

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EXPERIMENTAL SECTION

Materials and Physical Measurements. IR spectra were obtained from KBr discs using a Nicolet 360 FT-IR spectrophotometer. Electronic spectra were recorded on a PerkinElmer Lambda 19 spectrophotometer in 1 cm quartz cuvettes. Elemental analysis was carried out by using an Elementar Vario EL Analyzer. Cyclic voltammetry of compounds 2−8 was carried out in a CH Instruments Electrochemical Workstation CHI660C. A glassy carbon electrode, a Pt wire electrode, and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. The powder samples for magnetic analysis were made from their crystal samples. Magnetic properties, including variable-temperature magnetic susceptibility, field dependence of magnetization, and ac magnetic susceptibility, were investigated on a Quantum Design MPMS XL-5 SQUID system. The background corrections were measured experimentally on the corresponding sample holder. The diamagnetism of the constituent atoms (Pascal’s tables) was used to correct the experimental susceptibilities. For simplicity, we use Mw × (0.5 × 10−6).25 Structure Determination for 2−8. Crystals of compounds 2−8 that were suitable for X-ray diffraction analysis were obtained slowly from their methanolic solutions. X-ray diffraction data for 2−8 were collected at a low temperature (100 K) on an Oxford CCD diffractometer (Mo Kα, λ = 0.71073 Å). Their structures were resolved by the heavy-atom Patterson method, refined by full-matrix least-squares using SHELX-97, and expanded using Fourier

was also performed

(Figure S7), and the corresponding results are summarized in Table S4. Plotting χ″ vs χ′ (Cole−Cole plots) of 5−8 in the temperature range 1.8−5.6 K also gives well-defined semicircles, confirming the occurrence of a single relaxation process in the high temperature range in these 8-coordiante Co(II) complexes (Figure S8). The values of distribution of relaxation time are summarized in Tables S5−S7, which are obtained by the fitting using a generalized Debye model.

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CONCLUSIONS In conclusion, we have reported a family of homoleptic Fe(II) complexes, 2−4, and Co(II) complexes, 5−8, which are I


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Inorganic Chemistry

[mol−1 dm3 cm−1]): 213 (60400), 236 (62300), 251 (60600), 292 (51650), 319 sh (17090), 569 (60). [Co(L3)2](ClO4)2 (7). The synthetic method is similar to that of 5 except that L3 (50 mg, 0.18 mmol) was used instead of L1. Yield: 48 mg, 67%. Selected IR (KBr, cm−1): v(CO) 1719, v(ClO) 1099. Elemental analysis of C28H24Cl2CoN4O16: calcd C 41.92, H 3.02, N 6.98; found C 41.90, H 3.15, N 6.70. UV-vis (CH3CN) λmax [nm] (ε [mol−1 dm3 cm−1]): 204 (92800), 252 (28200), 298 (18950), 310 (17160), 551 (60). [Co(L4)2](ClO4)2 (8). The synthetic method is similar to that of 5 except that L4 (50 mg, 0.18 mmol) was used instead of L1. Yield: 53 mg, 78%. Selected IR (KBr, cm−1): v(CO) 1704, v(ClO) 1097. Elemental analysis of C40H48Cl2CoN4O16: calcd C 49.65, H 4.98, N 5.77; found C 49.55, H 5.18, N 5.80. UV-vis (CH3CN) λmax [nm] (ε [mol−1 dm3 cm−1]): 203 (89850), 254 (24870), 298 (17990), 311 (16310), 557 (50).

techniques.29 CCDC 1576946 and 1576997−1577002 contain the supplementary crystallographic data for compounds 2−8, which were deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, U.K. Computational Details. The ab initio calculations on the magnetic properties were done by MOLCAS 8.0, using the X-ray structure of the complexes with counterions omitted. The basis sets are ano-rcc-vtzp for Fe and Co atoms, ano-rcc-vdzp for the coordinated N and O atoms, and ano-rcc-vdz for other atoms, using basis libraries from the MOLCAS package. For the complete active space self-consistent field (CASSCF) calculations, the active space consists of six and seven electrons distributed in the five 3d orbitals of the Fe- and Co-metal centers, respectively. The singlet, triplet, and quintet states of Fe, as well as the doublet and quartet states of Co, were considered in the calculations. CASPT2 calculations were performed after CASSCF. The low-lying spin−orbit coupling states were then computed by RASSI-SO calculations. Finally, local magnetic properties (energy levels, D tensors, g-tensors, main magnetic axes, local magnetic susceptibility, etc.) of the complexes were calculated by the SINGLE_ANISO program. The lowest-energy vertical electronic transitions of the Fe(II) complex (3) and Co(II) complexes (5 and 7) were calculated by GAUSSIAN 09.30a−c An M06 functional30d and a mixed basis set of 6-31+G(d,p)30e (for nonmetals) and LANL2DZ effective core potential30f (for Co and Fe atoms) were employed in the calculations. The solvent effect was taken into account by the polarized continuum model (PCM).30g Frequency calculations were done on the optimized ground state structures, and no imaginary frequencies were found. Synthesis of Complexes. [Fe(L2)2](ClO4)2 (2). A mixture of 2,9bis(carbobutyloxy)-1,10-phenanthroline (L2) (50 mg, 0.13 mmol) and Fe(ClO4)2 (40 mg, 0.15 mmol) in MeOH (20 mL) was heated at reflux for 0.5 h to give a dark red solution after hot filtration. Slow evaporation of the solution for 2 days gave red needle crystals suitable for X-ray crystallography (27 mg, 41%). Selected IR (KBr, cm−1): v(CO) 1694, v(ClO) 1090. Elemental analysis of C44H48Cl2FeN4O16: calcd C 52.04, H 4.76, N 5.52; found C 52.08, H 4.96, N 5.49. UV-vis (CH3CN) λmax [nm] (ε [mol−1 dm3 cm−1]): 213 (71040), 236 (74000), 252 (73720), 291 (66880), 328 sh (17870), 480 (920). [Fe(L3)2](ClO4)2 (3). The synthetic method is similar to that of 2 except that 2,9-bis(carbomethoxy)-2,2′-dipyridine (L3) (50 mg, 0.18 mmol) was used instead of L2. Yield: 47 mg, 66%. Selected IR (KBr, cm−1): v(CO) 1704, v(ClO) 1096. Elemental analysis of C28H24Cl2FeN4O16: calcd C 42.08, H 3.03, N 7.01; found C 42.12, H 3.14, N 6.95. UV-vis (CH3CN) λmax [nm] (ε [mol−1 dm3 cm−1]): 202 (92300), 255 (26320), 266 (23680), 273 (17970), 304 (21000), 314 (22430), 437 (680). [Fe(L4)2](ClO4)2 (4). The synthetic method is similar to that of 2 except that 2,9-bis(carbobutyloxy)-2,2′-dipyridine (L4) (50 mg, 0.18 mmol) was used instead of L2. Yield: 53 mg, 78%. Selected IR (KBr, cm−1): v(CO) 1694, v(ClO) 1096. Elemental analysis of C40H48Cl2FeN4O16: calcd C 49.65, H 5.00, N 5.79; found C 49.70, H 5.12, N 5.69. UV-vis (CH3CN) λmax [nm] (ε [mol−1 dm3 cm−1]): 202 (109700), 255 (31200), 266 (28760), 275 (21300), 302 (27200), 314 (29750), 452 (710). [Co(L1)2](ClO4)2 (5). A mixture of L1 (50 mg, 0.17 mmol) and Co(ClO4)2·6H2O (50 mg, 0.14 mmol) in MeOH (20 mL) was heated at reflux for 0.5 h to give a purple solution after hot filtration. Slow evaporation of the solution for 2 days gave the purple needle crystals suitable for X-ray crystallography. Selected IR (KBr, cm−1): v(CO) 1711, v(ClO) 1090. Yield: 51 mg, 71%. Elemental analysis of C32H24Cl2CoN4O16: calcd C 45.20, H 2.84, N 6.59; found C 45.21, H 2.92, N 6.60. UV-vis (CH3CN) λmax [nm] (ε [mol−1 dm3 cm−1]): 212 (53890), 236 (57290), 249 (55650), 289 (45580), 318 sh (16190), 571 (70). [Co(L2)2](ClO4)2 (6). The synthetic method is similar to that of 5 except that L2 (50 mg, 0.18 mmol) was used instead of L1. Yield: 24 mg, 36%. Selected IR (KBr, cm−1): v(CO) 1701, v(ClO) 1089. Elemental analysis of C44H48Cl2CoN4O16: calcd C 51.88, H 4.75, N 5.50; found C 51.92, H 4.82, N 5.55. UV-vis (CH3CN) λmax [nm] (ε

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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.7b03071. Supplementary electronic absorption spectra; predicted transition energies and the changes in the electron densities for 3, 4, and 6; cyclic voltammograms; and related measurements and fitting parameters on the magnetic properties (PDF) Accession Codes

CCDC 1576946 and 1576997−1577002 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: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. cfl[email protected]. [email protected].

ORCID

Jing Xiang: 0000-0003-3968-1643 Author Contributions #

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21201023 and 21771026) and the Hong Kong Research Grants Council (28300014 and 18300715).

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REFERENCES

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