A mixed-valence [CoII4CoIII2] cluster with defect disk-shaped topology.

The employment of the new Schiff base ligand 2-[(4-chloro-2-hydroxybenzylideneamino)methyl]phenol (H2L) bearing O2N donors for the preparation of a novel Co6 cluster is reported. The hexanuclear cobalt complex, namely, di-μ2-acetatotetrakis{μ2-2-[(4-chloro-2-oxidobenzylideneamino)methyl]phenolato}tetra-μ3-methanolato-tetracobalt(II)dicobalt(III), [CoII4CoIII2(C14H10ClNO2)4(CH3COO)2(CH3O)4], was obtained using Co(CH3COO)2·4H2O and H2L as starting materials in MeOH under solvothermal conditions. The six metal ions are linked together by the μ3-O atoms of four deprotonated MeOH molecules, two CH3COO- units and six phenolate O atoms of four L2- ligands to form a defect disk-shaped topology. DC magnetic susceptibility investigations revealed the existence of antiferromagnetic interactions in the Co6 cluster. open access.

Introduction

Polynuclear coordination com­pounds of 3d transition metals have attracted continued attention for several decades due to their structural novelty, inter­esting catalytic (Dastidar & Chattopadhyay, 2022 ▸; Shul’pin & Shul’pina, 2021 ▸; Nesterov & Nesterova, 2018 ▸; Jing et al., 2020 ▸) and biological properties (Hazari et al., 2017 ▸; Amtul et al., 2002 ▸; Azizian et al., 2012 ▸; Tanaka et al., 2003 ▸), and their potential as single-mol­ecule magnets (SMMs) (Radu et al., 2017 ▸; Pattacini et al., 2011 ▸). Among numerous polynuclear 3d com­plexes, cobalt clusters have received particular inter­est because of their pleasing topological aesthetics (Brechin et al., 1997 ▸; Cao et al., 2013 ▸), their relevance to di­oxy­gen reduction (Monte-Pérez et al., 2017 ▸) and their fascinating magnetic properties (Liu et al., 2020 ▸; Sarto et al., 2018 ▸; Li et al., 2020 ▸; Ma et al., 2012 ▸).

Several synthetic methodologies towards polynuclear cobalt clusters were established and one of the most efficient approaches involves the employment of hy­droxy-containing Schiff base ligands. Schiff base ligands are easy to synthesize and their steric properties can be tuned by varying the size of the amine or carbonyl substituents (Qin et al., 2017 ▸, 2018 ▸; Ge et al., 2018 ▸; Li et al., 2021 ▸). More importantly, the hy­droxy moieties of the Schiff base ligands can combine many metal ions with μ-O bridges, resulting in the formation of large polynuclear clusters.

In the present work, we utilized the hy­droxy-containing Schiff base 2-[(4-chloro-2-hy­droxy­benzyl­idene­amino)­meth­yl]phenol (H2 L) (Huang et al., 2019 ▸) as a ligand to assemble a polynuclear cobalt cluster. The hexa­nuclear cobalt com­pound [Co4 IICo2 III(L)4(CH3COO)2(MeO)4] (1) was obtained suc­cessfully and we report its structural diversity and discuss its magnetic properties.

Experimental

Materials and physical measurements

All chemicals were of reagent grade, purchased from commercial suppliers and used without further purification. All manipulations were conducted under aerobic and solvothermal conditions. H2 L was synthesized following the liter­a­ture procedure of Huang et al. (2019 ▸). Elemental analyses for C, H and N were performed with a Carlo-Erba EA1110 CHNO-S analyser. The FT–IR spectrum was determined on a Nicolet MagNa-IR 500 spectrometer using KBr pellets in the range 400–4000 cm−1. DC magnetic susceptibilities were measured in the temperature range 2–300 K in a field of 1000 Oe using a Quantum Design MPMS-7 SQUID magnetometer.

Synthesis and crystallization

To a Pyrex tube (10 ml) was added a mixture of H2 L (0.0291 g, 0.1 mmol), Co(CH3COO)2·4H2O (0.0249 g, 0.1 mmol), Et3N (0.0202 g, 0.2 mmol) and MeOH (1.5 ml). The tube was sealed and heated at 80 °C for 48 h under autogenous pressure. It was then cooled to room temperature and dark-red needle-like crystals were obtained. The crystals were collected, washed with MeOH (2 ml) and dried in air (yield: 0.020 g; 48% based on cobalt). Analysis calculated (%) for C64H58Cl4Co6N4O16: C 47.03, H 3.58, N 3.43; found (%): C 46.18, H 4.048, N 3.280. Selected IR data for 1 (cm−1): 1637 (s), 1590 (s), 1523 (s), 1450 (m), 1419 (m), 1286 (w), 1248 (s), 1185 (s), 1089 (m), 1021 (m), 933 (s), 874 (m), 852 (w), 755 (s).

Structure determination

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. The crystal structure contained disordered solvent that could not be satisfactorily refined. The SQUEEZE (Spek, 2015 ▸) routine of PLATON (Spek, 2020 ▸) was used in the treatment of the crystallographic data. All H atoms were placed in geometrically idealized positions, with C—H = 0.95–0.99 Å. The H atoms of the CH2, aromatic and amide groups were constrained to ride on their parent atoms, with U iso(H) = 1.2U eq(C). The H atoms of CH3 groups were refined as rotating groups, with U iso(H) = 1.5U eq(C).

Table 1

Experimental details

Crystal data
Chemical formula	[Co6(C14H10ClNO2)4(C2H3O2)2(CH3O)4]
M r	1634.52
Crystal system, space group	Orthorhombic, P212121
Temperature (K)	120
a, b, c (Å)	15.4873 (10), 16.2116 (11), 28.1099 (19)
V (Å3)	7057.7 (8)
Z	4
Radiation type	Mo Kα
μ (mm−1)	1.60
Crystal size (mm)	0.4 × 0.2 × 0.2
Data collection
Diffractometer	Bruker SMART APEXII
Absorption correction	Multi-scan (SADABS; Bruker, 2016 ▸)
T min, T max	0.612, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	78040, 16170, 11376
R int	0.094
(sin θ/λ)max (Å−1)	0.650
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.057, 0.174, 1.04
No. of reflections	16170
No. of parameters	853
No. of restraints	12
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.65, −0.78
Absolute structure	Flack x determined using 4303 quotients [(I +) − (I −)]/[(I +) + (I −)] (Parsons et al., 2013 ▸)
Absolute structure parameter	0.011 (8)

Computer programs: SAINT (Bruker, 2016 ▸), APEX2 (Bruker, 2016 ▸), olex2.solve (Bourhis et al., 2015 ▸), SHELXL (Sheldrick, 2015 ▸), OLEX2 (Dolomanov et al., 2009 ▸) and PLATON (Spek, 2020 ▸).

Results and discussion

Synthesis of com­plex 1 and IR spectral analysis

The reaction of H2 L and Co(CH3COO)2·4H2O in MeOH in the presence of NEt3 under solvothermal conditions led to the isolation of 1 in moderate yield. Co(CH3COO)2·4H2O is a good starting material because it not only serves as a convenient metal source, but also provides CH3COO− bridging ligands. In the solid state, com­plex 1 is stable in air and its elemental analysis is consistent with the given mol­ecular formula.

The vibrational bands in the IR spectrum agree well with the formulation of com­plex 1 (see Fig. S1 in the supporting information). The signals of the carb­oxy­l νas(CO2) and νs(CO2) vibrations were found in the 1637–1419 cm−1 range. The vibrations of the C=N bond appear at 1450 cm−1. Several bands in the 1286–1185 cm−1 range were assigned to the vibrations of the aromatic rings. The sharp signals in the 979–766 cm−1 range were ascribed to the vibrations of C—H bonds.

Structure description of 1

Single crystals of 1 were obtained from MeOH under solvothermal conditions. Complex 1 crystallized in the ortho­rhom­bic space group P212121. The structure is shown in Fig. 1 ▸. The structure analysis shows that com­plex 1 is com­posed of six cobalt ions, four 2-[(4-chloro-2-oxido­benzyl­idene­amino)­meth­yl]phenolate (L 2−) ligands, two acetate ligands and four methanol-solvent-derived MeO− ligands. There exists an approximate C 2 symmetry in the mol­ecule. The imine N atom and both phenolate O-atom donors of each L 2− ligand coordinate each cobalt centre. Bond valence calculations (Brese & O’Keeffe, 1991 ▸; Brown & Altermatt, 1985 ▸) gave valence parameters of 1.90, 2.32, 3.60, 2.12, 3.64 and 2.31 for the Co1–Co6 ions, respectively, indicating that the Co3 and Co5 ions are in 3+ valence states, and that the Co1, Co2, Co4 and Co6 ions are in 2+ oxidation states. The formation of four fused defect cubes confirms the involvement of four methanol-solvent-derived μ3-O− groups, giving the Co6O10 structure. Thus, the mol­ecular structure of 1 displays a defect disk-shaped topology [Fig. 1 ▸(b)]. Of the six cobalt centres, the Co1, Co3, Co4 and Co5 ions are six-coordinated, and the Co2 and Co6 ions are five-coordinated. The coordination environments of the Co2 and Co6 ions, and the Co3 and Co5 ions are individually identical. The Co1 centre is present in a distorted octa­hedral O6 coordination environment, among which two O atoms are from two μ2-κ4 O:O,O′,N L 2− ligands and four O atoms are from four μ3-O− MeO− ligands. The Co2 centre is enclosed by the N and O atoms of one μ2-κ4 O:O,O′,N L 2− ligand, one O atom of a μ3-κ5 O:O,N,O′:O′ L 2− ligand and one O atom of one μ3-O− MeO− ligand. The six-coordinate NO5 environment around the Co3 ion is accom­plished by two μ3-O− MeO− groups, one O atom from one acetate bridge and the N and O atoms of one μ3-κ5 O:O,N,O′:O′ L 2− ligand. The six O-donor atoms around the Co6 centre originate from bridging acetate ligands, two μ3-O− MeO− groups and two μ3-κ5 O:O,N,O′:O′ L 2− ligands. The H2 L ligand exhibits two types of coordination mode.

Figure 1

(a) The mol­ecular structure of 1, (b) the defect disk-shaped topology, (c) the coordination polyhedra of the Co atoms, (d) the metal framework, with the H atoms omitted for clarity, and (e) the coordination modes of the H2 L ligand.

The geometries of the five-coordinated Co2 and Co6 atoms were analyzed with the program SHAPE (Version 2.0; Pinsky & Avnir, 1998 ▸). The calculated values revealed trigonal bipyramid (D 3) geometry for both atoms, with a minimum CShM (contunuous shape measure) value of 1.065 for Co2 and 1.172 for Co6.

Complex 1 joins a small family of Co6 clusters. Hexa­nuclear cobalt com­plexes mainly exhibit wheel, cage and ring topologies (Shi et al., 2021 ▸; Zou et al., 2014 ▸; Wang et al., 2013 ▸; Guo et al., 2013 ▸; Lazzarini et al., 2012 ▸; Chen et al., 2010 ▸; Malassa et al., 2010 ▸; Tudor et al., 2010 ▸; Colacio et al., 2009 ▸; Jones et al., 2009 ▸; Shiga & Oshio, 2007 ▸; Alley et al., 2006 ▸; Murrie et al., 2003 ▸; Kumagai et al., 2003 ▸; Gutschke et al., 1999 ▸). Complex 1 is a rare example that displays a defect disk-shaped structure.

Magnetic properties of 1

Magnetic susceptibility data as a function of temperature for com­plex 1 are shown in Fig. 2 ▸. The room temperature χM T value is 10.96 cm3 mol−1 K, which is greater than the value of 7.50 cm3 mol−1 K for four uncoupled S = 3/2 CoII centres, possibly owing to the orbital contributions of the metal ions (Cao et al., 2013 ▸). Upon lowering the temperature, the χM T value drops slightly to a minimum value of 3.49 cm3 mol−1 K at 2 K, which suggests possible anti­ferromagnetic couplings between the unpaired spins. The data of 1/χ in the temperature range 2–300 K were fitted by the Curie–Weiss Law of 1/χM = (T − θ)/C. The Curie constant C = 12.09 cm3 mol−1 K and the Weiss constant θ = −37.24 K were obtained. The negative θ value proves the anti­ferromagnetic inter­actions.

Figure 2

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

The magnetic dynamic behaviour of 1 was also explored. The ac magnetic susceptibilities for 1 at 1000 Hz under a zero-dc field in the temperature range 2–25 K were shown in Fig. S2 (see supporting information). The χ′′ susceptibilities at 1000 Hz did not increase upon lowering the temperature and no peaks were determined. These phenomena revealed that com­plex 1 is not a single-mol­ecule magnet.

Conclusion

A hexa­nuclear cobalt com­plex of com­position [Co2 IIICo4 II(L)4(CH3COO)2(MeO)4] (1), based on the hydroxy-con­taining Schiff base ligand 2-[(4-chloro-2-hy­droxy­benzyl­idene­amino)­meth­yl]phenol (H2 L) was prepared and char­acterized. Complex 1 exhibits a defect disk-shaped top­ol­ogy. Four cobalt ions are six-coordinated and two cobalt ions are five-coordinated. An investigation of the magnetic properties revealed that there exist anti­ferromagnetic inter­actions between the CoII ions.

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S2053229622005885/wv3009sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2053229622005885/wv3009Isup2.hkl

IR spectrum, magnetic susceptibility figure, geometry details and table of CShM values. DOI: 10.1107/S2053229622005885/wv3009sup3.pdf

CCDC reference: 1991979