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

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, polynuclear 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-methoxy-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 [Co₂^ⅢCo₄^Ⅱ(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^–1 on 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 ℃ 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_r = 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 MoKα radiation (λ = 0.71073 Å); the ω-2θ scan 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 anisotropically. 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

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

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1
Bond	Dist.		Bond	Dist.		Bond	Dist.
Co(1)–O(2)	1.905(4)		Co(2)–O(2)	2.139(4)		Co(4)–O(19)	2.199(4)
Co(1)–O(13)	1.911(4)		Co(3)–O(4)	1.933(4)		Co(5)–O(9)	1.905(4)
Co(1)–O(1)	1.912(4)		Co(3)–O(5)	2.008(4)		Co(5)–O(8)	1.906(4)
Co(1)–N(4)	1.916(5)		Co(3)–N(1)	2.011(5)		Co(5)–N(3)	1.911(5)
Co(1)–O(19)	1.925(4)		Co(3)–O(1)	2.013(4)		Co(5)–O(20)	1.925(4)
Co(1)–O(17)	1.946(4)		Co(3)–O(19)	2.212(4)		Co(5)–O(16)	1.927(4)
Co(2)–O(15)	2.049(4)		Co(4)–O(10)	2.050(4)		Co(5)–O(18)	1.941(4)
Co(2)–O(14)	2.053(4)		Co(4)–O(5)	2.058(4)		Co(6)–O(11)	1.921(5)
Co(2)–O(18)	2.068(4)		Co(4)–O(18)	2.130(4)		Co(6)–O(10)	1.995(4)
Co(2)–O(17)	2.074(4)		Co(4)–O(17)	2.138(4)		Co(6)–N(2)	2.011(5)
Co(2)–O(8)	2.132(4)		Co(4)–O(20)	2.196(4)		Co(6)–O(9)	2.025(4)
Co(6)–O(20)	2.198(4)
Angle	(°)		Angle	(°)		Angle	(°)
O(2)–Co(1)–O(13)	89.07(2)		O(15)–Co(2)–O(17)	171.08(2)		O(4)–Co(3)–O(19)	93.26(2)
O(2)–Co(1)–O(1)	174.59(2)		O(14)–Co(2)–O(17)	89.83(2)		O(5)–Co(3)–O(19)	81.65(2)
O(13)–Co(1)–O(1)	92.04(2)		O(18)–Co(2)–O(17)	82.06(2)		O(10)–Co(4)–O(5)	100.73(2)
O(2)–Co(1)–N(4)	93.94(2)		O(15)–Co(2)–O(8)	84.43(2)		O(10)–Co(4)–O(18)	91.02(2)
O(13)–Co(1)–N(4)	87.72(2)		O(14)–Co(2)–O(8)	108.12(2)		O(5)–Co(4)–O(18)	167.25(2)
O(1)–Co(1)–N(4)	91.40(2)		O(4)–Co(3)–O(5)	135.69(2)		O(10)–Co(4)–O(17)	168.70(2)
O(2)–Co(1)–O(19)	92.43(2)		O(4)–Co(3)–N(1)	92.2(2)		O(5)–Co(4)–O(17)	89.58(2)
O(13)–Co(1)–O(19)	176.07(2)		O(5)–Co(3)–N(1)	91.46(2)		O(18)–Co(4)–O(17)	79.16(2)
O(15)–Co(2)–O(14)	97.96(2)		O(4)–Co(3)–O(1)	115.95(2)		O(10)–Co(4)–O(20)	80.62(2)
O(15)–Co(2)–O(18)	90.58(2)		O(5)–Co(3)–O(1)	105.60(2)		O(5)–Co(4)–O(20)	104.88(2)
O(14)–Co(2)–O(18)	170.12(2)		N(1)–Co(3)–O(1)	104.95(2)		O(18)–Co(4)–O(20)	71.95(2)

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^–1 range (Fig. S1). Contributions from the carboxylic ν_as(CO₂) and ν_s(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^–1 range, 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 -CH₃ and -CH₂ groups makes assignments difficult. Several peaks in the 979~766 cm^–1 range are found. They can be ascribed to the vibrations of C–H bonds.

3.2 Structural description of 1

X-ray single-crystal analysis reveals that complex 1 crystallizes in the triclinic space group P$\overline 1$. The structure consists of six cobalt atoms, four L^2– ligands, two acetate ions, two MeO^– ions and two solvent molecules (Fig. 1). Bond valence calculation^{[27, 28]} indicated that two cobalt atoms (Co(1), Co(5) are in 3+ valence states while the other metal centers (Co(2), Co(3), Co(4) and Co(6)) are in 2+ oxidation states (Table 2). The presence of two Co^Ⅲ ions is possibly due to the aerial oxidation of Co^Ⅱ to Co^Ⅲ. This phenomenon was observed in many polynuclear Co^Ⅱ/Ⅲ clusters^{[29, 30]}. The six cobalt atoms are located at six corners of a defective tetracubane. Two cubanes share one face and each misses one vertex (Fig. 1), and the Co(4) atom is shared by four cubanes. Thus, complex 1 looks like a butterfly (Fig. 1). In this hexanuclear compound, Co(1), Co(2), Co(4) and Co(5) atoms are hexa-coordinated, while Co(3) and Co(6) atoms are penta-coordinated. The six cobalt atoms are held together by six phenolate oxygen atoms from four L^2– ligands, four oxygen atoms from two chelating acetates and four μ₃-O atoms from four MeO^– groups. The coordination environments of Co(3) and Co(6) atoms are identical (NO4), and they are coordinated by two oxygen atoms and one nitrogen atom from one L^2– ligand, one oxygen atom from a MeO^- unit and one phenolate oxygen atom from another L^2– ligand. The coordination environments of Co(1) and Co(5) atoms are the same (NO(5)). The six coordination atoms are from one L^2– ligand (NO(2)), two MeO^– units and one acetate. The coordination spheres of Co(2) and Co(4) atoms are distinct. Co(4) atom is coordinated by four oxygen atoms of four MeO^– units and two oxygen atoms from two L^2– ligands. Co(2) atom is coordinated by two oxygen atoms of two acetates, two oxygen atoms of two MeO^– ions and two oxygen atoms from two L^2– ligands. The L^2– ligand displays μ₂: ɳ², ɳ¹, ɳ¹, ɳ⁰ and μ₃: ɳ⁰, ɳ², ɳ¹, ɳ² chelating modes.

Figure 1

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

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Table 2

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

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Atom	Valence		Atom	Valence
Co(1)	3.57		Co(4)	1.88
Co(2)	2.06		Co(5)	3.29
Co(3)	2.24		Co(6)	2.28

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 Co^Ⅱ/^Ⅲ clusters which exhibit defect tetracubane core are very rare^[29].

There are many Co₇ clusters 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 Co₇ clusters, 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. The χ_MT value 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 S = 3/2 uncoupled spins, probably due to the orbital contributions of the metal ions^{[42, 43]}. As the temperature is lowered, the χ_MT value 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/χ_M and temperature of 2~300 K obeys the Curie-Weiss Law of 1/χ_M = (T – θ)/C. The Curie constant C = 14.28 cm³·mol^–1·K and Weiss constant θ = –49.84 K were obtained. The negative θ value confirms the antiferromagnetic exchange interactions.

Figure 2

Figure 2.  Temperature dependence of magnetic susceptibilities in the forms of χ_MT vs. T (a) and 1/χ_M vs. T (b) for 1 at 1 kOe. The red solid line corresponds to the best fit of the magnetic data

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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). The χ ''susceptibilities do not increase with the decrease of temperature or no peaks were observed, which indicate that complex 1 is not a single-molecule magnet.

Figure 3

Figure 3.  Temperature dependence of the in-phase (χ') (a) and out-of-phase (χ'') (b) susceptibilities for 1 in the range of 2 to 25 K. The susceptibilities at 1000 Hz frequency under zero dc field

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4. CONCLUSION

A mixed-valence hexanuclear cobalt complex [Co₂^ⅢCo₄^Ⅱ(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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