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Cite this: Dalton Trans., 2017, 46,9088

Received 24th May 2017,Accepted 13th June 2017

DOI: 10.1039/c7dt01893b

rsc.li/dalton

Two-dimensional frameworks formed bypentagonal bipyramidal cobalt(II) ions andhexacyanometallates: antiferromagnetic ordering,metamagnetism and slow magnetic relaxation†

Dong Shao, Yan Zhou, Qian Pi, Fu-Xing Shen, Si-Run Yang, Shao-Liang Zhangand Xin-Yi Wang *

We herein report the syntheses, structures, and magnetic properties of two isostructural two-dimensional

(2D) coordination polymers based on a pentagonal bipyramidal CoII unit [Co(TODA)]2+ and two hexacyano-

metallates, namely [MIII(CN)6]2[CoII(TODA)]3·9H2O (M = Cr (1), Co (2), TODA = 1,4,10-trioxa-7,13-diaza-

cyclopentadecane). Structure analyses show that both complexes have 2D honeycomb structures where

the [Co(TODA)]2+ units are bridged by the [MIII(CN)6]3− groups through three cyano groups in the facial

positions. Magnetic investigation reveals ferromagnetic coupling between the CrIII and CoII centres

through cyanides in 1. Due to the antiferromagnetic interaction between the layers, compound 1 exhibits

an antiferromagnetic ordering below 11.4 K, and shows a metamagnetic phase transition under an exter-

nal dc field. Due to the disorder of the TODA ligands, compound 1 shows a spin glass behavior, which

leads to slow magnetic relaxation in 1. A butterfly-shaped hysteresis loop at 1.8 K can be observed with a

coercive field of 720 Oe, which is quite large for cyano-bridged Cr–Co molecular magnets. For com-

pound 2 containing the diamagnetic [CoIII(CN)6]3− unit, field-induced slow magnetic relaxation was also

verified, which makes compound 2 a rare example of an SIM assembled in a 2D network. An easy-plane

magnetic anisotropy with a positive D value (29.9 cm−1 by PHI and 26.5 cm−1 by Anisofit2.0) was deduced

for hepta-coordinated CoII centers. These results show the efficiency of the strategy of combining cyano-

metallates and pentagonal bipyramidal precursors for novel molecular magnetic materials.

Introduction

Metal ions of the pentagonal bipyramidal (PBP) geometry areof specific importance as they can have a very large magneticanisotropy for many transition metal and lanthanide metalcenters, established by both theoretical1 and experimentalstudies.2–6 Thus, PBP metal ions are of particular interest forthe construction of single-molecule magnets (SMMs) andsingle-ion magnets (SIMs, specifically SMMs of a single metalcenter), which have been studied extensively due to theirpotential applications in high density information storage andspintronics.7 Remarkably, the axial symmetry of the PBP geo-

metry can efficiently suppress the quantum tunnelling of mag-netization (QTM) frequently observed in lanthanide SIMs,which leads to new records of both the energy barrier andblocking temperature of SMMs in DyIII complexes and novelmagnetic properties of a HoIII complex.5a,c,d

Another important feature of these PBP metal centers relieson the fact that many of these systems are constructed usingsuitable pentadentate macrocyclic ligands such as thoseshown in Scheme 1, leaving the two axial coordination sitesaccessible for further coordination. This feature makes thesePBP complexes excellent building blocks for the constructionof magnetic materials of a higher nuclearity and dimensional-ity. Along this line, a number of clusters,8 chains,9 and higherdimensional magnetic materials10 based on 3d PBP metalcenters have been reported.

In this vein, recent studies by our group have been focusingon PBP 3d and 4d metal ions, especially highly anisotropicFeII, CoII and MoIII ions.2,3 For example, using the PBP[MoIII(CN)7]

4− unit and suitable organic ligands (Scheme 1),we have reported a series of low dimensional materials, includ-ing a docosanuclear Mo8Mn14 cluster with a very high ground

†Electronic supplementary information (ESI) available: Structure information indetail and additional magnetic data. CCDC 1549280 and 1549281. For ESI andcrystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt01893b

State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of

Advanced Microstructures, School of Chemistry and Chemical Engineering,

Nanjing University, Nanjing, 210023, China. E-mail: wangxy66@nju.edu.cn;

Fax: +86-25-83314502

9088 | Dalton Trans., 2017, 46, 9088–9096 This journal is © The Royal Society of Chemistry 2017

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state spin value,3a a series of trinuclear MoIIIMnII2 complexes

including the first MoIII-based SMM,3b and the first chain com-pound based on [MoIII(CN)7]

4−.3c For 3d metal ions on theother hand, we reported the first field-induced SIM of a PBPgeometry in 2014.2e In addition, using different axial coordi-nation atoms (C, N, O, and S), we managed to tune the mag-netic anisotropy and the relaxation barriers for a series of PBPCoII compounds.2a

Furthermore, by connecting the FeII or CoII PBP centersusing a variety of bridging ligands, we prepared a series ofone-dimensional (1D) magnetic materials.2b–d As the bridgingligands between the PBP centers can transfer different mag-netic exchange interactions, the magnetic properties of these1D systems can be finely tuned by different bridges and alsothe PBP centers of different magnetic anisotropies. We foundthat a negligible magnetic interaction favours the field-induced SIM,2b a weak magnetic interaction can suppress theslow magnetic relaxation,2b while a strong magnetic inter-action leads to single-chain magnet (SCM) behaviour.2c,d

In this work, along our research interest to construct mag-netic materials of higher nuclearity and dimensionality usingthe PBP metal centers, we chose cyanometallates as bridgesbetween the PBP Co2+ metal centers. The synthetic strategyusing the cyanometallates as building blocks was very success-ful and resulted in numerous magnetic materials of novelmagnetic properties, such as high-temperature magneticordering,11 spin crossover (SCO),12 photo-magnetism,13

magneto-chirality,14 SMMs,15 SCMs16 and so on. Furthermore,to our knowledge, there are only a handful of magneticmaterials containing both PBP Co centers and cyanometal-lates, including four pentanuclear clusters([MIII(CN)6]2[Co

II(Hdapb)]3 (M = Cr, Fe), [WV(CN)8]2[CoII(Hdapb)]3(H2O)2 and [WV(CN)8]2[Co

II(LN3O2biPh)3]2(H2O)2,see Scheme 1 for Hdapb and LN3O2biPh),

8a a 1D nanotube([Fe(CN)6]4[Co(LN3O2)]6·19H2O),

10e four 2D honeycomb networks({[FeIII(CN)6]2[Co

II(LN3O2)]3}n, [Fe(CN)6]2[Co(LN3O2

)]3·14H2O·2CH3OH, [Fe(CN)6]2[Co(LN3O2

)]3·6H2O, and {[CrIII(CN)6]2[CoII(LN5

)]3}n), and a 3D helical ferromagnet {[CrIII(CN)6][CoII(LN3O2

)]2[ClO4]}n.10a,b,e For these Cr–Co complexes, the

clusters show paramagnetism while the high-dimensionalcompounds exhibit simple ferromagnetic long-range ordering.

Herein, we reported two isostructural 2D layered coordi-nation polymers [MIII(CN)6]2[Co

II(TODA)]3·18H2O (M = Cr (1),Co (2), TODA = 1,4,10-trioxa-7,13-diazacyclopentadecane,Scheme 1). Magnetic studies revealed that compound 1 exhi-bits antiferromagnetic (AF) ordering and a metamagneticphase transition under an external dc field. Interestingly at lowtemperature, both compounds 1 and 2 show slow magneticrelaxation, which is ascribed to the spin glass behavior for 1and field-induced SIM behavior for 2. The spin glass behaviorof 1 is caused by the disorder of the TODA ligands, while theSIM behavior of 2 is due to the magnetic anisotropy of the PBPCo2+ centers.

Experimental

All preparations and manipulations were performed underaerobic conditions. All reagents were obtained from commer-cially available sources and used as received unless otherwisenoted.

Caution! Cyanides are highly toxic and dangerous. Theyshould be handled in small quantities with great care.

Physical measurements

Infrared spectra (IR) data were measured on KBr pellets usinga Nexus 870 FT-IR spectrometer in the range of4000–400 cm−1. Elemental analyses were performed on anElementar Vario Micro analyzer. Powder X-ray diffraction data(PXRD) were recorded at 298 K on a Bruker D8 Advanced diffr-actometer with a Cu Kα X-ray source (λ = 1.54056 Å) operatedat 40 kV and 40 mA. Thermal gravimetric analysis (TGA) wasperformed in Al2O3 crucibles using a PerkinElmer ThermalAnalyser in the temperature range of 20–700 °C under a nitro-gen atmosphere.

All magnetic data were collected using a Quantum DesignSquid VSM magnetometer on the ground single crystalsamples from 2 to 300 K at applied dc fields ranging from 0 to7 T. Alternating current (ac) magnetic susceptibility data werecollected in the temperature range of 2–10 K, under an ac fieldof 2 Oe, oscillating at frequencies in the range of 1–1000 Hz.All magnetic data were corrected for the diamagnetic contri-butions of the sample holder and of core diamagnetism of thesample using Pascal’s constants.

Synthesis of compounds 1 and 2

[CrIII(CN)6]2[CoII(TODA)]3·18H2O (1). 3 mL aqueous solution

of K3Cr(CN)6 (13 mg, 0.05 mmol) was added to one side of aH-shaped tube, and 3 mL aqueous solution of Co(ClO4)2·6H2O(18 mg, 0.05 mmol) and TODA (21.2 mg, 0.1 mmol) was addedto another side. Water was added onto the top of the solutionsas a buffer. The tube was then sealed and left to stand at roomtemperature. Pink block crystals were formed after one week,which were collected, washed with water, and dried in air.Yield: 22 mg, ∼55% based on K3Cr(CN)6. Elemental analysis

Scheme 1 Representative pentadentate macrocyclic ligands for theconstruction of complexes of a pentagonal bipyramidal geometry andthe ligand TODA used in this work.

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(%) for C42H102Cr2Co3N18O27: C, 32.08; H, 6.53; N, 16.03.Found: C, 32.10, H, 6.26; N, 15.65. IR (KBr, cm−1): 3399(vs),3277(vs), 2932(s), 2877(s), 2160(s), 2133(vs), 1646(w), 1461(s),1347(w), 1301, 1270(w), 1237(w), 1217(w), 1173(w), 1132(s),1102(vs), 1035(s), 964(s), 847(w), 662(w), 561(w), 417(s).

[CoIII(CN)6]2[CoII(TODA)]3·18H2O (2). By using K3Co(CN)6, 2

was also obtained as pink hexagonal block crystals accordingto the same synthetic procedure as 1. Yield: ∼19 mg, 47%based on K3Co(CN)6. Elemental analysis (%) forC42H102Co5N18O27: C, 31.81; H, 6.48; N, 15.89. Found: C, 31.96;H, 6.26; N, 15.53. IR (KBr, cm−1): 3400(vs), 3278(vs), 2937(s),2879(s), 2154(w), 2129(w), 1635(w), 1462(s), 1347(w), 1302(w),1270(w), 1237(w), 1218(w), 1175(w), 1132(s), 1102(vs), 1062(s),1036(s), 983(w), 956(s), 847(s), 667(w), 464(s).

X-ray crystallography

Single crystal X-ray diffraction data were collected on a BrukerAPEX Duo diffractometer with a CCD area detector (Mo-Kαradiation, λ = 0.71073 Å) at 123 K. The APEX II program wasused to determine the unit cell parameters and for data collec-tion. The data were integrated and corrected for the Lorentzand polarization effects using SAINT.17 Absorption correctionswere applied with SADABS.18 The structures were solved bydirect methods and refined by a full-matrix least-squaresmethod on F2 using the SHELXTL crystallographic softwarepackage.19 All the non-hydrogen atoms were refined aniso-tropically. The hydrogen atoms of the organic ligands wererefined as riding on the corresponding non-hydrogen atoms.Additional details of the data collections and structural refine-ment parameters are provided in Table 1. Selected bondlengths and angles of 1 and 2 are listed in Table S1 (ESI†).

CCDC 1549280 and 1549281 contain the supplementary crys-tallographic data for this paper.†

Results and discussionCrystal structure description

Complexes 1 and 2 are isostructural and crystallize in themonoclinic space group P21/c. Both complexes have a cyanobridged layer structure (Fig. 1). The asymmetric unit containsnine lattice water molecules and one MIIICoII1:5 core formed byone [MIII(CN)6]

3− and one and a half [Co(TODA)]2+ units. Allthe CoII centres in 1 and 2 reside in the slightly distortedpentagonal bipyramid consisting of three O and two N atomsfrom TODA and two N atoms from the bridging CN− groups inthe axial position (Fig. 1a). The continuous shape measures(CShMs)20 of the Co2+ centers were calculated to be 0.867 and0.685 for 1, and 0.903 and 0.499 for 2, which are close to zeroof an ideal D5h symmetry. These pentagonal bipyramids are allaxially pressed with the axial Co–N bond lengths being theshortest in all the Co–N and Co–O bonds (Table S1, ESI†).Although all the TODA ligands in both complexes are dis-ordered in two positions, one [Co(TODA)]2+ (Co1) unit is in thespecial position (0.5, 1, 0) and the site occupancy rate is strictly50%. For the other TODA ligand (coordinated to Co2) in ageneral position, the site occupancy of each disordered part isrefined and the SAME command is used to restrain the non-hydrogen atoms in the two disordered parts of TODA ligands.This severe disorder of the TODA ligands is very important forthe magnetic properties of compound 1 and should be thecause for the observed spin glass behaviour (vide post ).

Table 1 Crystallographic data and structure refinement parameters forcomplexes 1 and 2

1 2

Formula C42H102Cr2Co3N18O27 C42H102Co5N18O27Formula weight [g mol−1] 1577.2 1586.07Crystal system Monoclinic MonoclinicSpace group P21/c P21/ca [Å] 9.1509(4) 9.0980(3)b [Å] 15.6851(7) 15.3049(6)c [Å] 25.6844(1) 25.3271(8)α [°] 90 90β [°] 90.0040(1) 90.4180(1)γ [°] 90 90V [Å3] 3686.6(3) 3526.6(2)Z 2 2T [K] 123 123ρcalcd [g cm−3] 1.421 1.494μ(Mo-Kα) [mm−1] 1.029 1.238F(000) 1656 1662Rint 0.0472 0.0335Data/restraints/parameters

8448/2527/623 8027/2623/623

R1a/wR2

b (I > 2σ(I)) 0.0847/0.2429 0.0763/0.2146R1/wR2 (all data) 0.1206/0.2671 0.0853/0.2241GOF on F2 1.046 1.068

a R1 = ∑||Fo| − |Fc||/∑|Fo|.bwR2 = {∑[w(Fo

2 − Fc2)2]/∑[w(Fo

2)2]}1/2.

Fig. 1 (a) The pentagonal bipyramidal CoII ions in 1 and 2; (b) theCrIIICoII

1:5 core of 1 in the asymmetric unit; (c) hexagonal 1D channelalong the a axis; (d) the packing diagram showing the 2D honeycomblayer structure. All H atoms and solvent water molecules were omittedfor clarity and only one component of the disordered TODA ligands isshown in (c) and (d).

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Each [M(CN)6]3− unit connects three pentagonal Co2+

centers via three CN− groups in the fac-positions (Fig. S2†),giving rise to a 2D honeycomb layer structure along the bcplane (Fig. 1d). The repeating unit is the twelve-metallic M6Co6hexagonal ring of a size of ca. 12.9 × 19.3 Å (Fig. 1c, Fig. S3,ESI†). Connected to each other by weak hydrogen bonds, thesehoneycomb layers are further stacked along the a axis to forma 3D supramolecular structure. The shortest interlayer Co–Codistances are of 7.64 and 7.58 Å for 1 and 2, respectively(Fig. S4, ESI†). Furthermore, one-dimensional hexagonal chan-nels occupied by the lattice water molecules were formedalong the a axis (Fig. S5, ESI†). The potential void space perunit volume was calculated to be 23.2% and 24.5% for 1 and 2,respectively. Thermal gravimetric analyses (TGA) showed aweight loss of 19.7% and 20.0% for 1 and 2 in the temperaturerange of 20–140 °C, corresponding to the removal of all latticewater molecules (calc. 20.6% and 20.4% for 1 and 2, Fig. S6,ESI†). Upon further heating, a negligible mass loss wasobserved up to approximately 350 °C, suggesting the highthermal stability of the 2D frameworks of 1 and 2.

Magnetic properties

The variable-temperature magnetic susceptibility data of 1measured under a dc field of 1 kOe between 2 and 300 K aredepicted in Fig. 2. At 300 K, the χMT value is 12.8 cm3 mol−1 K,higher than the spin-only value of 9.375 cm3 mol−1 K expectedfor two isolated CrIII centers (S = 3/2, g = 2.0) and three isolatedCoII centers (S = 3/2, g = 2.0), indicating the large magnetic an-isotropy of PBP CoII and the possible ferromagnetic coupling.Upon cooling, the χMT values increase slowly until about 25 Kand then increase abruptly to a maximum of 231.2 cm3 mol−1

K at 12 K with a rapid decrease to a value of 40.4 cm3 mol−1 Kat 2 K. Fitting of the data above 100 K according to the Curie–Weiss law gives the Weiss constant θ = 26.6 K, suggesting con-siderable ferromagnetic coupling between the cyano bridgedCrIII⋯CoII centres. This is consistent with the Goodenough–

Kanamori rule that the orthogonal metal orbitals (t2g + eg) leadto ferromagnetic interactions. The sharp peak of χMT at lowtemperature indicates the long-range magnetic ordering below12 K and the sharp decrease of χMT could be due to the fieldsaturation of the magnetic moment and/or the interlayer AFinteractions.

To probe the magnetic ordering of 1, zero-field-cooled(ZFC) and field-cooled (FC) magnetizations under a weak fieldof 10 Oe were measured and depicted in Fig. 3a. The ZFC/FC

Fig. 2 Variable-temperature magnetic susceptibility data of 1 measuredunder a dc field of 1 kOe. The red solid line represents the Curie–Weiss fit.

Fig. 3 (a) Field-cooled (FC) and zero-field-cooled (ZFC) magnetizationsof 1 under a dc field of 0 Oe. (b) Temperature dependence of the in-phase (χ’) and out-of-phase (χ’’) parts of the ac magnetic susceptibility of1 under 0 Oe (a) and 1500 (b) Oe dc fields.

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curves show a sharp peak at 11.4 K, and then decrease onfurther cooling with a bifurcation near 8 K. The sharp peak at11.4 K is consistent with the AF ordering, while the unexpectedbifurcation between ZFC and FC suggests the blocking of themagnetization, which is possibly due to spin glass behaviourtriggered by the disorder of the TODA ligands in 1.21,22

The AF magnetic ordering and also the spin glass behaviourwas further confirmed by the ac magnetic susceptibility datacollected at Hdc = 0 Oe and Hac = 2 Oe (Fig. 3b). Very sharp fre-quency independent peaks at 11.4 K in the χ′ component andalso frequency dependent peaks below 10 K in the χ″ com-ponent were observed. The shift of the peak temperature of χ″is rather small (6.0 to 7.0 K from 1 to 999 Hz) and can bemeasured by using the Mydosh parameter φ = (ΔTp/Tp)/Δ(log f ) = 0.05, which is in the normal range of spin glasses.22

Furthermore, the isothermal field dependence of the mag-netization of 1 at 1.8 K with fields up to 70 kOe was alsomeasured. The magnetization curve showed a pronouncedsigmoid shape (Fig. 4a). The magnetization increases slowly in

the low field region and shows an abrupt upturn in the regionof 1.5–3 kOe, before increasing again slowly up to the highestvalue of 14.0μB at 70 kOe, which is close to the saturation mag-netization value of 15μB per CrIII2 CoII3 unit. This behaviour istypical for a metamagnet with two ferromagnetic subnetworksweakly coupled antiferromagnetically. The weak AF couplingcan be overcome by the external magnetic field and theground state can be switched from an antiferromagnet to aferromagnet. The critical field HC is deduced to be about 1499Oe at 1.8 K from the dM/dH curve (Fig. 4a). To check themagnetic hysteretic behaviour of 1 suggested by the irreversibilityof the ZFC/FC curves below 10 K, a hysteresis loop at 1.8 K wasmeasured and depicted in Fig. 4b. As we can see, a butterfly-shaped hysteresis loop can be observed with a coercive field of720 Oe, which is quite large for a cyano-bridged Cr–Co mole-cule magnet.10a,b,23

All these results stated above suggest that compound 1 hasa complex magnetic ordering state. First of all, the ferro-magnetic interactions between the cyano-bridged Cr3+ andCo2+ centres lead to the formation of a ferromagnetic layeralong the bc plane. These layers were then AF coupled by theweak interlayer AF coupling, mostly due to the dipole–dipoleinteraction, resulting in the AF ground state. This weak AFcoupling can be overcome easily by the external dc field,leading to the metamagnetic transition. In addition, the spinglass behaviour originating from the severe disorder in thestructure causes the slow magnetic relaxation, which is respon-sible for the bifurcation in the ZFC/FC curves and also the hys-teresis effect at low temperatures.22

These results prompted more M(T ) measurements atdifferent fields and more M(H) measurements at differenttemperatures in order to obtain the H–T magnetic phasediagram for 1. As can be seen from Fig. 5, the peaks of the χ(T )curves of 1 show a clear field-dependency and shift to lowertemperatures until the dc field reaches 1200 Oe, above whichthe peaks disappear and the curves reach a plateau. As for theseries of M(H) curves in the range of 2–12 K from 0–6 kOe, wecan see that the S-shaped abnormality of the curves becomesless pronounced upon increasing the temperature and dis-appears completely at about 10 K. From the differentials ofthese curves (Fig. S7, ESI†), a set of (H, T ) points wereobtained, from which the H–T magnetic phase diagram of 1was constructed (Fig. 6). This phase diagram is consistent withan antiferromagnet with a field-induced metamagnetic phasetransition below its ordering temperature.24

For compound 2, since the paramagnetic Co2+ centres areseparated by the diamagnetic [CoIII(CN)6]

3− units, it shouldbehave as isolated Co2+ centres. In addition, although thereare two crystallographic unique Co2+ centres in 2, it is imposs-ible to distinguish them in the magnetic data. Thus, we treatthem equally and the magnetic data for compound 2 are calcu-lated according to the formulae containing one CoII centre. Atroom temperature, the χMT value is 2.48 cm3 mol−1 K (Fig. 7a),which is larger than the spin-only value of 1.875 cm3 mol−1 Kfor the CoII ion (S = 3/2, g = 2.0). Upon cooling, the χMT valuesdecrease monotonically down to 1.37 cm3 mol−1 K at 2 K. The

Fig. 4 (a) Field dependent magnetization data at 1.8 K for 1. Inset: TheM(H) curve and the derivative of the magnetization (dM/dH) at lowfields. The field sweep rate is 300 Oe s−1. (b) Hysteresis loop of 1measured at 1.8 K.

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large χMT value at 300 K and also the decrease of the χMT curveboth reflect the intrinsic magnetic anisotropy of the PBP Co2+

centre, as the magnetic coupling through the diamagnetic[CoIII(CN)6]

3− should be negligible. Furthermore, the fielddependent magnetization of 2 was measured at 2, 3, and 5 K(Fig. 7a). The magnetization values at 70 kOe (M = 2.17, 2.15,and 2.09μB) are consistent with the reported values.2,4c

To determine the zero-field splitting parameters of the Co2+

centre in 2, the magnetic susceptibility and the magnetizationdata of 2 were fitted simultaneously using the PHI25 programwith the following spin Hamiltonian:

H ¼ D½Sz2 � SðSþ 1Þ=3� þ EðSx

2 � Sy2Þ þ μBg�S�B ð1Þ

where D, E, S, B, and μB represent the axial and rhombic ZFSparameters, the spin operator, magnetic field vectors, and the

Fig. 5 (a) A series of variable-temperature susceptibility data of 1measured under various applied dc fields. (b) Field dependent magneti-zation data for 1 at various temperatures.

Fig. 7 (a) Temperature dependent magnetic susceptibility data for 2measured at 1 kOe. Inset: The magnetization curve for 2 measured at 2,3, and 5 K. The solid lines represent the best fits by PHI. (b) Reducedmagnetization data of 2 collected in the temperature range of 2–10 Kunder dc fields of 1–7 T. The solid lines correspond to the best fitsobtained with Anisofit2.0.

Fig. 6 The H–T magnetic phase diagram of 1. The circles and diamondsrepresent the M(H) and M(T ) curves, respectively.

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Bohr magneton, respectively. The best fit values are D = 29.9(5)cm−1, E = −0.1(3) cm−1, and g = 2.221(1). These values indicatethe easy-plane magnetic anisotropy of the PBP Co2+ center,which is consistent with the reported similar systems.2

Furthermore, the reduced magnetization data collected atdifferent magnetic fields of 1–7 T in the temperature range of2–10 K (Fig. 7b) show significant separation between the iso-field curves, indicating also the large magnetic anisotropy. Thebest fit using Anisofit2.0 26 gave D = 26.5 cm−1, E = 0.8 × 10−3

cm−1 and g = 2.34 for 2, which agrees well with the fittingresults by PHI.

To investigate the magnetic dynamics in 2, ac susceptibilitydata were measured at 2.0 K under external dc fields of 0–4500Oe, oscillating in 1–1000 Hz (Fig. S8, ESI†). No out-of-phase(χ″) signals were found in the absence of a dc field due to theoccurrence of fast QTM, which has been observed in mostCoII-based SIMs.27 Application of a small dc field efficientlysuppresses the QTM and results in the nonzero χ″ signals(Fig. S8, ESI†). The χ″ signals increase with the increase of thedc field until 1 kOe and decrease upon higher dc fields. Fromthese data, Cole–Cole plots were constructed (Fig. S9, ESI†)and fitted by the generalized Debye model28 to give the field-dependent relaxation time τ (Fig. S10, ESI†). The relaxationtime shows a peak at around 1.5 kOe, which was then chosenas the optimum field for further ac measurements of 2 (Fig. 8).The Cole–Cole plots at different temperatures (1.8–3 K, Fig. 9a)were used to extract the temperature dependent relaxationtimes (τ). The isothermal susceptibility (χT), adiabatic suscepti-bility (χS), τ, and α parameters are listed in Table S2 (ESI†). Theobtained α parameters are in the range of 0.13–0.33,suggesting the narrow distribution of the relaxation time. Asdepicted in Fig. 9b, the ln τ vs. 1/T plot is almost linear in thetemperature range of 2.0–3.0 K. From the fitting of these datato the Arrhenius law τ = τ0 exp(Ueff/kBT ), the effective energybarrier Ueff was estimated to be 12.6 cm−1 (16 K), with the pre-

exponential factor τ0 being 3.2 × 10−6 s (Fig. 9b). Note that atthe lowest temperatures of 1.8 and 1.9 K, the logarithm ofrelaxation times obviously deviates from the linear tendency,which is likely owing to QTM, as has also been observed in arecently reported 2D SIM.29 Compared to other reported PBPCoII SIMs,18,30 the Ueff of compound 2 is much lower (16 K vs.25–50 K). Such phenomena are also observed in other 2D CoII

SIMs with an octahedral local geometry. For example, althoughthe D value is about 76 cm−1, the Ueff is only 11.36 cm−1 in the2D CoII coordination polymer [Co(ppad)2]n.

30d

Conclusions

In summary, we reported the syntheses, crystal structures, andmagnetic behaviour of two isostructural 2D coordination poly-mers based on two hexacyanometallate anions [MIII(CN)6]

3−

(M = Cr, Co) and a pentagonal bipyramidal CoII ion. The long-range antiferromagnetic ordering and metamagnetism wereverified in the CrIII–CoII system. Interestingly, slow magneticrelaxation was observed in both compounds, albeit withdifferent causes. For the CrIII–CoII compound, the slow mag-netic relaxation originates from the spin glass behaviour,which is most likely due to the severe structural disorder. Onthe other hand, for the CoIII–CoII compound, slow magneticrelaxation is due to the field-induced SIM behaviour of thepentagonal bipyramidal CoII ion. The present work enrichesthe molecular magnetic material based on PBP buildingblocks and provides a new approach to construct coordinationpolymers for the exploration of SIM behaviour in frameworksof higher dimensionalities.

Acknowledgements

We thank the Major State Basic Research DevelopmentProgram (2013CB922102), NSFC (21522103, 21471077,91622110) and the NSF of Jiangsu province (BK20150017). Thiswork was also supported by the Nanjing University Innovationand Creative Program for PhD candidates.

Fig. 8 Ac susceptibility data of 2 under the 1500 Oe dc field at differentfrequencies and temperatures.

Fig. 9 (a) Cole–Cole plots of 2 at 1.8–3.0 K at the 1500 Oe dc field. Thesolid lines represent the best fits according to the generalized Debyemodel; (b) the ln(τ) vs. 1/T plot for 2. The line is the Arrhenius law fit ofthe data.

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9094 | Dalton Trans., 2017, 46, 9088–9096 This journal is © The Royal Society of Chemistry 2017

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