A Hexanuclear Cobalt Cluster with Tetracubane-like Topology: Synthesis, Structure and Magnetic Properties①

SHI Zhen-Hi HUANG Yun WU Yu-Ze CHEN Xio-Li YANG Hu

A Hexanuclear Cobalt Cluster with Tetracubane-like Topology: Synthesis, Structure and Magnetic Properties①

SHI Zhen-Haia, bHUANG YuancWU Yu-ZeaCHEN Xiao-LiaYANG Huaa②

a(716000)b(721013)c(215123)

hexanuclear, cobalt, tetracubane, topology, magnetic property;

1 INTRODUCTION

Polynuclear transition-metal clusters have attracted increasing interest due to their aesthetically pleasing architectures[1-5]and diverse applications in magnetism[6-8], luminescence[9-11], and catalysis[12,13].Particularly, polynu-clear cobalt clusters have gathered tremendous attention in recent years especially for the following two important reasons: (i) some cobalt clusters behave as single-molecule magnets (SMMs)[14-17], and could be potentially utilized in high-density information storage devices[18,19]; (ii) these clusters are also successfully used in many catalysis reactions, such as dioxygen reduction[20-22]and epoxidation of alkenes[23].

A fertile route for the preparation of polynuclear cobalt clusters involves employing polydentate ligands containing several oxygen donors that could incorporate many cobalt ions into one molecular entity.In this work, we employed polydentate Schiff base H2L (H2L = 2-((2-hydroxy-4-me- thoxy-benzylideneamino)methyl)phenol)[24]as a ligand to assemble polynuclear cobalt compound.The existence of two -OH groups renders H2L as a good ligand to generate polynuclear clusters.A hexanuclear cobalt compound with formula [Co2IIICo4II(L)4(CH3COO)2(MeO)4]·MeOH (1)was prepared.Herein, we report the synthesis, structure and magnetic properties of complex 1.

2 EXPERIMENTAL

2.1 Materials and physical measurements

All manipulations were performed under aerobic and solvothermal conditions using reagents and solvents as received.The H2L ligand (H2L = 2-((2-hydroxy-4-methoxy- benzylideneamino)methyl)phenol) was prepared according to the literature procedure[24].

The C, H and N microanalyses were carried out with a Carlo-Erba EA1110 CHNO-S elemental analyser.FT-IR spectrum was recorded from KBr pellets in the range of 400～4000 cm–1on a Nicolet MagNa-IR 500 spectrometer.Variable-temperature dc magnetic susceptibility data were collected using a Quantum Design MPMS-7 SQUID magnetometer.

2.2 Synthesis of complex 1

A mixture of H2L (0.0257 g, 0.1 mmol), Co(OAc)2·4H2O (0.0249 g, 0.1 mmol) and MeOH (1.5 mL) was sealed in a Pyrex-tube (10 mL).The tube was heated at 80 °C for 2 days under autogenous pressure.Cooling of the resultant solution to room temperature gave dark-red needle-like crystals.The crystals were collected by filtration, washed with MeOH (2 mL) and dried in air.Yield: 0.019 g (45% based on cobalt).Anal.Calcd.(%) for C70H78N4O22Co6(M= 1672.94):C, 50.02; H, 4.68; N, 3.33.Found (%): C, 49.97; H, 4.65; N, 3.49.Selected IR data for 1 (cm?1): 1606 (s), 1529 (m), 1479 (m), 1446 (m), 1299 (w), 1251 (s), 1219 (s), 1165 (m), 1144 (m), 1122 (m), 1031 (m), 979 (m), 874 (w), 851 (w), 766 (w).

2.3 Structure determination

The data collection for 1 were carried out on a Bruker Smart ApexII diffractometer equipped with a graphite monochromator utilizing Moradiation (= 0.71073 ?); the-2scan technique was applied.The crystal structure of complex 1 was solved with the Olex2 solve solution program[25]using Intrinsic Phasing and refined by full-matrix least-squares minimization with the ShelXL refinement package[26].All non-hydrogen atoms were refined aniso- tropically.The collected crystal data for 1 are shown in Table S1.Selected bond lengths and bond angles of 1 are listed in Table 1 and Table S2.

Table 1.Selected Bond Lengths (?) and Bond Angles (°) for 1

3 RESULTS AND DISCUSSION

3.1 Synthesis and IR spectrum of 1

Complex 1 is prepared by mixing H2L and Co(OAc)2·4H2O in 1:1 ratio in MeOH under solvothermal conditions.This solvothermal reaction mode allows to crystallize complex 1 directly from the reaction solution.

Several bands appear in the 1605～1445 cm–1range (Fig.S1).Contributions from the carboxylicas(CO2) ands(CO2) vibrations would be expected in this region.The vibration of the C=N bonds is at 1445 cm–1.Several bands appear in the 1299～1121 cm–1range, whilst the contributions from the vibrations of aromatic rings would be expected in this region.The overlap of the signals of aromatic rings with the vibrations of -CH3and -CH2groups makes assignments difficult.Several peaks in the 979～766 cm–1range are found.They can be ascribed to the vibrations of C–H bonds.

3.2 Structural description of 1

Table 2.Oxidation States of Cobalt Atoms Obtained by Bond Valence Calculations[27, 28]

Fig.1.(a) Molecular structure of 1 looking like a butterfly.(b) Coordination polyhedralof cobalt atoms.(c)Defect tetracubane structure of metal core.H atoms are omitted for clarity

Complex 1 is a member of a big family of hexanuclear cobalt clusters.These complexes display various structures including cage[31], hexameric ring[2, 21], and giant wheel[22].The examples of hexanuclear CoII/IIIclusters which exhibit defect tetracubane core are very rare[29].

There are many Co7clusters that show disc-like configuration, in which the seven cobalt centers are almost coplanar and occupy the vertexes of the six defect cubanes[32-41].Compared the structure of complex 1 with those of Co7clusters, complex 1 can be regarded as obtained by removing one of seven vertexes of the six cubanes.

3.3 Magnetic properties of 1

Direct current (dc) magnetic susceptibilities for complex 1 were determined at an applied magnetic field of 1000 Oe in the temperature range of 2～300 K.TheχTvalue of 1 at 300 K is 12.08 cm3·mol–1·K (Fig.2), which is much larger than the spin-only value of 7.50 cm3·mol–1·K expected for four= 3/2 uncoupled spins, probably due to the orbital contributions of the metal ions[42, 43].As the temperature is lowered, theχTvalue decreases gradually to a minimum value of 4.53 cm3·mol–1·K at 2 K.This behavior is indicative of the presence of antiferromagnetic exchange interactions between the metal ions.The relationship between 1/χand temperature of 2～300 K obeys the Curie-Weiss Law of 1/χ= (–)/.The Curie constant= 14.28 cm3·mol–1·K and Weiss constant= –49.84 K were obtained.The negativevalue confirms the antiferromagnetic exchange interactions.

Fig.2.Temperature dependence of magnetic susceptibilities in the forms of χT.(a) and 1/χ.(b) for 1 at 1 kOe.The red solid line corresponds to the best fit of the magnetic data

Fig.3.Temperature dependence of the in-phase () (a) and out-of-phase () (b) susceptibilities for 1 in the range of 2 to 25 K.The susceptibilities at 1000 Hz frequency under zerofield

In order to study the magnetic dynamic behavior of 1, the ac magnetic susceptibilities for complex 1 at 1000 Hz under a zero dc field were determined (Fig.3).Thesusceptibilities do not increase with the decrease of temperature or no peaks were observed, which indicate that complex 1 is not a single-molecule magnet.

4 CONCLUSION

A mixed-valence hexanuclear cobalt complex [Co2IIICo4II(L)4(CH3COO)2(MeO)4]·2MeOH (1) supported by a Schiff base ligand H2L was synthesized.Complex 1 exhibits defect tetracubane-type architecture.Four cobalt atoms are hexa-coordinated and two cobalt atoms are penta-coordinated.The dc magnetic property measurements reveal the existence of antiferromagnetic interactions.

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20 July 2020;

12 October 2020 (CCDC 1991978)

① Supported by the Natural Science Foundation of Yan’an University (YDY2017-08), Innovation and Entrepreneurship Training Program of College Students of China (S202010719031), the Natural Science Foundation of Yulin (CXY-2020-065) and the National Natural Science Foundation of China (21763028)

.Born in 1979, associate professor, majoring in organometallic chemistry.E-mail: yanghua_08@163.com

10.14102/j.cnki.0254–5861.2011–2942