Mixed-Valent Trinuclear CoIII-CoII-CoIII Complex with 1,3-Bis(5-chlorosalicylideneamino)-2-propanol

A mixed-valent trinuclear complex with 1,3-bis(5-chlorosalicylideneamino)-2-propanol (H3clsalpr) was synthesized, and the crystal structure was determined by the single-crystal X-ray diffraction method at 90 K. The molecule is a trinuclear CoIII-CoII-CoIII complex with octahedral geometries, having a tetradentate chelate of the Schiff-base ligand, bridging acetate, monodentate acetate coordination to each terminal Co3+ ion and four bridging phenoxido-oxygen of two Schiff-base ligands, and two bridging acetate-oxygen atoms for the central Co2+ ion. The electronic spectral feature is consistent with the mixed valent CoIII-CoII-CoIII. Variable-temperature magnetic susceptibility data could be analyzed by consideration of the axial distortion of the central Co2+ ion with the parameters Δ = –254 cm−1, λ = –58 cm−1, κ = 0.93, tip = 0.00436 cm3 mol−1, θ = –0.469 K, gz = 6.90, and gx = 2.64, in accordance with a large anisotropy. The cyclic voltammogram showed an irreversible reduction wave at approximately −1.2 V·vs. Fc/Fc+, assignable to the reduction of the terminal Co3+ ions.


Synthesis of the Trinuclear Cobalt Complex
The present complex was prepared by the reaction of 1,3-bis(5-chlorosalicylideneamino)-2-propanol (H3clsalpr) and cobalt(II) acetate tetrahydrate in acetonitrile at ambient temperature ( Figure 3). As the cobalt salt, we selected cobalt(II) acetate tetrahydrate, aiming at the bridging property of acetate ions to form a trinuclear species. For a favorable condition for trinuclear formation, we reacted H2clsalpr with Co(CH3COO)2·4H2O in a 1:3 molar ratio under aerobic conditions, although we could isolate the same complex with a lower yield when the reaction was performed in a 1:1 or 1:2 molar ratio. The elemental analysis data of the obtained complex are in agreement with the trinuclear formulation of [Co3(Hclsalpr)2(CH3COO)4]. The oxidation of the two Co 2+ ions to Co 3+ ions may be accomplished by atmospheric oxygen acting as an oxidant, as usually observed for the synthesis of the related trinuclear Co II Co III 2 complexes [40,51,[53][54][55][56]. The synthetic method is similar to that of [Co3(Hsalpr)2(CH3COO)4] [40]. However, the reported method was slightly complicated with the further addition of an aqueous solution of NaN(CN)2 to the reaction solution.

Synthesis of the Trinuclear Cobalt Complex
The present complex was prepared by the reaction of 1,3-bis(5-chlorosalic amino)-2-propanol (H3clsalpr) and cobalt(II) acetate tetrahydrate in acetonitrile ent temperature ( Figure 3). As the cobalt salt, we selected cobalt(II) acetate tetra aiming at the bridging property of acetate ions to form a trinuclear species. For a fa condition for trinuclear formation, we reacted H2clsalpr with Co(CH3COO)2·4H2O molar ratio under aerobic conditions, although we could isolate the same comple lower yield when the reaction was performed in a 1:1 or 1:2 molar ratio. The el analysis data of the obtained complex are in agreement with the trinuclear formu [Co3(Hclsalpr)2(CH3COO)4]. The oxidation of the two Co 2+ ions to Co 3+ ions may be plished by atmospheric oxygen acting as an oxidant, as usually observed for the s of the related trinuclear Co II Co III 2 complexes [40,51,[53][54][55][56]. The synthetic method i to that of [Co3(Hsalpr)2(CH3COO)4] [40]. However, the reported method was sligh plicated with the further addition of an aqueous solution of NaN(CN)2 to the solution. (a-e) Trinuclear cobalt complexes with a linear array of Co III -Co II -Co III or Co II -Co II -Co II .

Synthesis of the Trinuclear Cobalt Complex
The present complex was prepared by the reaction of 1,3-bis(5-chlorosalicylideneamino)-2-propanol (H 3 clsalpr) and cobalt(II) acetate tetrahydrate in acetonitrile at ambient temperature ( Figure 3). As the cobalt salt, we selected cobalt(II) acetate tetrahydrate, aiming at the bridging property of acetate ions to form a trinuclear species. For a favorable condition for trinuclear formation, we reacted H 2 clsalpr with Co(CH 3 COO) 2 ·4H 2 O in a 1:3 molar ratio under aerobic conditions, although we could isolate the same complex with a lower yield when the reaction was performed in a 1:1 or 1:2 molar ratio. The elemental analysis data of the obtained complex are in agreement with the trinuclear formulation of [Co 3 (Hclsalpr) 2 (CH 3 COO) 4 ]. The oxidation of the two Co 2+ ions to Co 3+ ions may be accomplished by atmospheric oxygen acting as an oxidant, as usually observed for the synthesis of the related trinuclear Co II Co III 2 complexes [40,51,[53][54][55][56]. The synthetic method is similar to that of [Co 3 (Hsalpr) 2 (CH 3 COO) 4 ] [40]. However, the reported method was slightly complicated with the further addition of an aqueous solution of NaN(CN) 2 to the reaction solution.

Infrared Spectra of the Trinuclear Cobalt Complex
In the infrared spectrum of the complex, the C=N stretching band w 1634 cm -1 due to the presence of the Schiff-base ligand. The lower energy s with that of the free Schiff-base ligand (H3clsalpr: νC=N at 1646 cm -1 ) sugge nation of the imino-nitrogen atom of the Schiff-base ligand in the cobalt complex shows two sets of antisymmetric stretching νas(COO) and symme νs(COO) bands at 1590 and 1389 cm -1 , respectively, with a ∆ value of 201 cm and 1415 cm -1 , respectively, with a ∆ value of 147 cm -1 . The former and the ascribed to the typical IR spectral features of the monodentate and bridgi ands, respectively [62,63]. These spectral features are similar [Co3(Hsalpr)2(CH3COO)4] [40].

Electronic Spectra of the Trinuclear Cobalt Complex
The solid-state diffuse reflectance spectra exhibit a broad band with a side shoulder at 354 nm, which may be ascribed to the CT transition band of to metal as shown in Figure 4 [64]. The bands at 568 and 640 nm may be a transitions ( 1 A1g → 1 B2g, 1 A1g → 1 A2g, 1 A1g → 1 Eg) of an octahedral Co III with a [64,65]. Furthermore, the spectra show a broad band at approximately 1260 be ascribed to the d-d transition ( 4 T1g → 4 T2g) due to an octahedral Co II wi state [65]. The complex dissolves in THF. The solution spectra are similar solid-state spectra, showing a d-d absorption band (ε = 538 dm 3 cm −1 mol −1 ) a shoulder (ε = 320 dm 3 cm −1 mol −1 ) at 632 nm, although absorption in the could not be detected. Similar absorption spectra were [Co3(Hsalpr)2(CH3COO)4] [40].

Infrared Spectra of the Trinuclear Cobalt Complex
In the infrared spectrum of the complex, the C=N stretching band was observed at 1634 cm −1 due to the presence of the Schiff-base ligand. The lower energy shift compared with that of the free Schiff-base ligand (H 3 clsalpr: νC=N at 1646 cm −1 ) suggests the coordination of the imino-nitrogen atom of the Schiff-base ligand in the cobalt complex. The complex shows two sets of antisymmetric stretching ν as (COO) and symmetric stretching ν s (COO) bands at 1590 and 1389 cm −1 , respectively, with a ∆ value of 201 cm −1 , and at 1562 and 1415 cm −1 , respectively, with a ∆ value of 147 cm −1 . The former and the latter may be ascribed to the typical IR spectral features of the monodentate and bridging acetate ligands, respectively [62,63]. These spectral features are similar to those of [Co 3 (Hsalpr) 2 (CH 3 COO) 4 ] [40].

Electronic Spectra of the Trinuclear Cobalt Complex
The solid-state diffuse reflectance spectra exhibit a broad band with a lower-energy side shoulder at 354 nm, which may be ascribed to the CT transition band of the phenolate to metal as shown in Figure 4 [64]. The bands at 568 and 640 nm may be ascribed to d-d of an octahedral Co III with a low-spin state [64,65]. Furthermore, the spectra show a broad band at approximately 1260 nm, which can be ascribed to the d-d transition ( 4 T 1g → 4 T 2g ) due to an octahedral Co II with a high-spin state [65]. The complex dissolves in THF. The solution spectra are similar to those of the solid-state spectra, showing a d-d absorption band (ε = 538 dm 3 cm −1 mol −1 ) at 564 nm with a shoulder (ε = 320 dm 3 cm −1 mol −1 ) at 632 nm, although absorption in the near-IR region could not be detected. Similar absorption spectra were reported for [Co 3 (Hsalpr) 2

Crystal Structure of the Trinuclear Cobalt Complex
Single crystals of the complex suitable for X-ray crystal structure analysis w by the slow evaporation of the THF solution of the complex. Crystallograph collected in Table 1. Selected bond distances and angles are given in Table 2. Th crystallized in the monoclinic system. A perspective drawing of the structure in Figure 5. The molecule is a centrosymmetric trinuclear cobalt complex, whe atom is located at the crystallographical inversion center. The two Schiff-ba work as anionic tetradentate ligands Hclsalpr 2-to the terminal two cobalt atom The between the bond distances around the Co1 and Co2 (Co2 i ) atoms suggests th atom is in a high-spin state of Co 2+ ion and that the Co2 and Co2 i atoms are in Co 3+ ion state [67]. The bond valence sum calculation supports the mixed-valen Co III state [68,69]. This is in agreement with the spectral feature in the diffuse spectra of the present complex. The alcoholate H atom of O2 is hydrogen bon monodentate acetate-O atom O7 [O2-H…O7 2.659(2) Å]. In the crystal, there ar molecules in the asymmetric unit, and these molecules are oriented around the molecule ( Figure 6). The trinuclear structure is similar to that of the reported cobalt complex [Co3(Hsalpr)(CH3COO)4], which lacks a center of symmetry,

Crystal Structure of the Trinuclear Cobalt Complex
Single crystals of the complex suitable for X-ray crystal structure analysis were grown by the slow evaporation of the THF solution of the complex. Crystallographic data are collected in Table 1. Selected bond distances and angles are given in Table 2. The complex crystallized in the monoclinic system. A perspective drawing of the structure is depicted in Figure 5. The molecule is a centrosymmetric trinuclear cobalt complex, where the Co1 atom is located at the crystallographical inversion center. The two Schiff-base ligands work as anionic tetradentate ligands Hclsalpr 2to the terminal two cobalt atoms, Co2 and Co2 i , where the superscript i denotes the equivalent position (1 − x, 1 − y, 1 − z), and the alcoholate hydrogen atom is not deprotonated, but two phenolate H atoms of each Schiff-base ligand are deprotonated. The Co1 atom is coordinated by two sets of two phenoxido-O atoms of Hclsalpr 2ligands (O1, O3, O1 i , O3 i ;) and µ-acetato-O atoms (O4 and O4 i ) to form an octahedral geometry with Co-O distances of 2.0493(16)-2.1318(15) Å. It should be noted that the axial bond lengths (2.1318(15) Å) are longer than the equatorial bond lengths (2.0493(16) and 2.0851(16) Å), showing an axial distortion around the Co1 atom. The Co2 atom is coordinated by two phenoxido-O atoms (O1 and O3) and two imino-N atoms (N1 and N2) of the tetradentate Schiff-base ligand in trans geometry [66] to occupy the equatorial site. The axial site is occupied by the O atoms of the µ-bridging acetate (O5) and monodentate acetate (O6). The Co-O and Co-N bond distances are in the range of 1.8943(15)-1.9263(16) Å, significantly shorter than those of the Co1 atom. The difference between the bond distances around the Co1 and Co2 (Co2 i ) atoms suggests that the Co1 atom is in a high-spin state of Co 2+ ion and that the Co2 and Co2 i atoms are in a low-spin Co 3+ ion state [67]. The bond valence sum calculation supports the mixed-valent Co III -Co II -Co III state [68,69]. This is in agreement with the spectral feature in the diffuse reflectance spectra of the present complex. The alcoholate H atom of O2 is hydrogen bonded to the monodentate acetate-O atom O7 [O2-H . . . O7 2.659(2) Å]. In the crystal, there are four THF molecules in the asymmetric unit, and these molecules are oriented around the trinuclear molecule ( Figure 6). The trinuclear structure is similar to that of the reported trinuclear cobalt complex [Co 3 (Hsalpr)(CH 3 COO) 4 ], which lacks a center of symmetry, where the distortion around the Co 2+ ion is more distorted compared with the present complex [40]. In these complexes, the two bridging acetate groups and four µ-phenoxido-O atoms of the Schiff-base ligands play an important role in connecting the two tetradendate Co(Hclsalpr) 2 moieties. This motif was also found in the trinuclear zinc(II) complex [Zn 3 (Hsalpr) 2

Magnetic Properties of the Trinuclear Cobalt Complex
The present complex is expected to be paramagnetic because of the presence high-spin Co 2+ ion at the central position of the trinuclear cobalt molecule, althou terminal two Co 3+ ions are in a diamagnetic low-spin state. The magnetic suscep data for the complex are depicted in Figure 7 as the temperature variation of th product. The effective magnetic moment at 300 K is 5.73 µB per trinuclear molecule, corresponds to the theoretical value of 5.20 µB for a magnetically isolated S = 3/2 spi the contribution of orbital angular momentum (L = 3). The magnetic moment gra

Magnetic Properties of the Trinuclear Cobalt Complex
The present complex is expected to be paramagnetic because of the presence high-spin Co 2+ ion at the central position of the trinuclear cobalt molecule, althou terminal two Co 3+ ions are in a diamagnetic low-spin state. The magnetic suscep data for the complex are depicted in Figure 7 as the temperature variation of th product. The effective magnetic moment at 300 K is 5.73 µB per trinuclear molecule, corresponds to the theoretical value of 5.20 µB for a magnetically isolated S = 3/2 spi the contribution of orbital angular momentum (L = 3). The magnetic moment gra decreases with decreasing temperature, reaching a value of 3.82 µB at 4.5 K. This ma

Magnetic Properties of the Trinuclear Cobalt Complex
The present complex is expected to be paramagnetic because of the presence of the high-spin Co 2+ ion at the central position of the trinuclear cobalt molecule, although the terminal two Co 3+ ions are in a diamagnetic low-spin state. The magnetic susceptibility data for the complex are depicted in Figure 7 as the temperature variation of the χ M T product. The effective magnetic moment at 300 K is 5.73 µ B per trinuclear molecule, which corresponds to the theoretical value of 5.20 µ B for a magnetically isolated S = 3/2 spin with the contribution of orbital angular momentum (L = 3). The magnetic moment gradually decreases with decreasing temperature, reaching a value of 3.82 µ B at 4.5 K. This magnetic behavior is similar to that of the related linear Co III -Co II -Co III complex with 1,3-bis(salicylideneamino)-2-propanol [40]. The decrease in the magnetic moments may be ascribed to the axial distortion around the Co 2+ ion, which was observed in the crystal structure. The axial splitting parameter ∆ was defined as the splitting of the local 4 T 1g state of the octahedral Co 2+ ion in the absence of spin-orbit coupling and introduced to the magnetic data analysis [70][71][72]. The magnetic data were simulated with the axial splitting parameter ∆, the spin-orbit coupling parameter λ, the orbital reduction factor κ for the Co 2+ ion (H = ∆(L z 2 − 2/3) − (3/2)κλL·S + β[-(3/2)κL u + g e S u ]·H u (u = x, z)), the temperature-independent paramagnetism tip for the Co centers, and the Weiss constant θ for intermolecular magnetic interactions by using the MagSaki(A)W1.0.11 program [72].
Magnetic susceptibility equations are shown below (Equations (1)-(6)), where E (0) n , E (1) u,n , and E (2) u,n (n = ±1 − ±6, u = x, z) represent the zero-field energies, first-order Zeeman coefficients, and second-order Zeeman coefficients of the local 4 T 1 ground state for the octahedral Co 2+ ion. From this, the anisotropic g-factors, g z and g x , could be simulated using these parameters [72]. The simulation gave the following parameter values: ∆ = -254 cm −1 , λ = -58 cm −1 , κ = 0.93, tip = 0.00436 cm 3 mol −1 , and θ = -0.469 K. A large value of the tip may be ascribed to the presence of three cobalt atoms in the molecule. The g values were simulated as g z = 6.90 and g x = 2.64. This result suggests that the magnetic behavior of the present complex can be interpreted by the axial distortion of the central Co 2+ ion and thus proposed to be considerably anisotropic. If we apply the present magnetic analysis to the reported magnetic data of [Co 3 (Hsalpr) 2 (CH 3 COO) 4 ] [40], we obtain the following parameter values: ∆ = -950 cm −1 , λ = -131 cm −1 , κ = 0.93, tip = 0.00082 cm 3 mol −1 , θ = -0.67 K, g z = 7.71, and g x = 1.94, as shown in Figure 8. The magnetic analysis suggests that a considerable anisotropic character may also be found in [Co 3 (Hsalpr) 2 (CH 3 COO) 4 ] and that the larger negative ∆ and λ values may reflect a greater degree of axial distortion around the Co 2+ ion in the crystal structure [40]. (1)

Cyclic Voltammogram of the Trinuclear Cobalt Complex
The redox behavior of the complex was studied by cyclic voltammetry. The cyclic voltammogram ( Figure 9) showed an irreversible reduction wave at approximately -1.2 V vs. Fc/Fc + , which may be assigned to the reduction of the terminal Co 3+ ions in the reduction of the Co III -Co II -Co III species. The corresponding oxidation wave can be observed at approximately -0.3 V vs. Fc/Fc + . No oxidation wave was observed until +1.0 V vs. Fc/Fc + on the oxidation side. This result suggests that the trinuclear complex may not be maintained in the redox reaction, meaning that the stable form of the trinuclear species should be the Co III -Co II -Co III mixed-valent state. A similar irreversible reduction wave was observed in [Co II {Co III (µ-L 1 )X 2 } 2 ] [51] and [Co III 2 (nitrosalpr) 2 (CH 3 OH)] (H 3 nitrosalpr = 1,3-bis(5-nitrosalicylideneamino)-2-propanol) [29].   [40]. The solid lines were calculated and drawn with the parameter values described in the text.

Cyclic Voltammogram of the Trinuclear Cobalt Complex
The redox behavior of the complex was studied by cyclic voltammetry. The cyclic voltammogram ( Figure 9) showed an irreversible reduction wave at approximately -1.2 V vs. Fc/Fc + , which may be assigned to the reduction of the terminal Co 3+ ions in the reduction of the Co III -Co II -Co III species. The corresponding oxidation wave can be observed at approximately -0.3 V vs. Fc/Fc + . No oxidation wave was observed until +1.0 V vs. Fc/Fc + on the oxidation side. This result suggests that the trinuclear complex may not be maintained in the redox reaction, meaning that the stable form of the trinuclear species should be the Co III -Co II -Co III mixed-valent state. A similar irreversible reduction wave was observed in [Co II {Co III (µ-L 1 )X2}2] [51] and [Co III 2(nitrosalpr)2(CH3OH)] (H3nitrosalpr = 1,3bis(5-nitrosalicylideneamino)-2-propanol) [29].   [40]. The solid lines were calculated and drawn with the parameter values described in the text.

Cyclic Voltammogram of the Trinuclear Cobalt Complex
The redox behavior of the complex was studied by cyclic voltammetry. The cyclic voltammogram ( Figure 9) showed an irreversible reduction wave at approximately -1.2 V vs. Fc/Fc + , which may be assigned to the reduction of the terminal Co 3+ ions in the reduction of the Co III -Co II -Co III species. The corresponding oxidation wave can be observed at approximately -0.3 V vs. Fc/Fc + . No oxidation wave was observed until +1.0 V vs. Fc/Fc + on the oxidation side. This result suggests that the trinuclear complex may not be maintained in the redox reaction, meaning that the stable form of the trinuclear species should be the Co III -Co II -Co III mixed-valent state. A similar irreversible reduction wave was observed in [Co II {Co III (µ-L 1 )X2}2] [51] and [Co III 2(nitrosalpr)2(CH3OH)] (H3nitrosalpr = 1,3bis(5-nitrosalicylideneamino)-2-propanol) [29].

Materials and Methods
All reagents and metal salts were obtained from commercial sources and used without further purification.

Materials and Methods
All reagents and metal salts were obtained from commercial sources and used without further purification.
The Schiff-base ligand H 3 clsalpr was prepared by the methods described in the literature [13,15,16]. An amount of 1,3-Diamino-2-propanol (2.177 g, 0.024 mol) and 5chlorosalicylaldehyde (7.566 g, 0.048 mol) were dissolved in methanol (45 cm  Analytical data of C, H, and N were obtained on a Thermo Finnigan FLASH EA1112 series CHNO-S analyzer (Thermo Finnigan, Milan, Italy). IR spectra were obtained by KBr discs of samples on a JASCO MFT-2000 FT-IR spectrometer (JASCO, Tokyo, Japan). Powder reflectance spectra were obtained on a Shimadzu Model UV-3100 UV-vis-NIR spectrophotometer (Shimadzu, Kyoto, Japan). Magnetic susceptibility measurements were obtained on a Quantum Design SQUID susceptometer (MPMS-XL7, Quantum Design North America, San Diego, CA, USA) with a magnetic field of 0.5 T over a temperature range of 4.5-300 K. The magnetic susceptibility χ M is the molar magnetic susceptibility per mole of [Co 3 (Hclsalpr) 2 (CH 3 COO) 4 ] unit and was corrected for the diamagnetic contribution calculated from Pascal's constants [73]. Cyclic voltammograms were measured in THF solutions containing tetra-n-butylammonium perchlorate (TBAP) on a BAS 100BW Electrochemical Workstation (Bioanalytical Systems, West Lafayette, IN, USA) with a glassy carbon electrode, a platinum wire counter electrode, and an Ag/Ag + reference electrode. Ferrocene (Fc) was used as an internal standard. All the potentials are quoted relative to Fc + /Fc. X-ray crystallographic data were collected on a Bruker Smart APEX CCD diffractometer (Bruker, Billerica, MA, USA) using graphite monochromated Mo-Kα radiation. The structures were solved by intrinsic phasing methods and refined by full-matrix least-squares methods. The hydrogen atoms were included at their geometrical positions calculated geometrically. All of the calculations were carried out using the SHELXTL software package [74]. Crystallographic data have been deposited with Cambridge Crystallographic Data Centre: Deposit number CCDC-2175785. Copies of the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (accessed on 30 May 2022) (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; Fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).

Conclusions
In this study, new trinuclear cobalt complex was synthesized by the reaction of 1,3bis(5-chlorosalicylideneamino)-2-propanol (H 3 clsalpr) with cobalt(II) acetate tetrahydrate. The X-ray structure analysis revealed that a linear trinuclear Co III -Co II -Co III complex was formed with two partially deprotonated Schiff-base ligands Hclsalpr 2-, two bridging acetate ligands, and two monodentate acetate ligands. The electronic absorption spectra and cyclic voltammetry data suggest that the mixed-valent oxidation state is stable. The temperature dependence of the magnetic susceptibilities is in accordance with the magnetic property of the central Co 2+ ion becoming considerably anisotropic due to the axial distortion of the coordination geometry. This anisotropic property could also be found in the related trinuclear complex [Co 3 (Hsalpr) 2 (CH 3 COO) 4 ]. The anisotropic magnetic behavior of the mixed-valent Co III -Co II -Co III complexes is interesting as a potential application for singlemolecule magnets. Further study to pursue such magnetic relaxation properties is planned in our laboratories.