One-Pot Self-Assembly of Dinuclear, Tetranuclear, and H-Bonding-Directed Polynuclear Cobalt(II), Cobalt(III), and Mixed-Valence Co(II)/Co(III) Complexes of Schiff Base Ligands with Incomplete Double Cubane Core

The reaction of 2,6-diformyl-4-methylphenol (DFMF) with 1-amino-2-propanol (AP) and tris(hydroxymethyl)aminomethane (THMAM) was investigated in the presence of Cobalt(II) salts, (X = ClO4−, CH3CO2−, Cl−, NO3−), sodium azide (NaN3), and triethylamine (TEA). In one pot, the variation in Cobalt(II) salt results in the self-assembly of dinuclear, tetranuclear, and H-bonding-directed polynuclear coordination complexes of Cobalt(III), Cobalt(II), and mixed-valence CoIICoIII: [Co2III(H2L−1)2(AP−1)(N3)](ClO4)2 (1), [Co4(H2L−1)2(µ3-1,1,1-N3)2(µ-1,1-N3)2Cl2(CH3OH)2]·4CH3OH (2), [Co2IICo2III(HL−2)2(µ-CH3CO2)2(µ3-OH)2](NO3)2·2CH3CH2OH (3), and [Co2IICo2III (H2L12−)2(THMAM−1)2](NO3)4 (4). In 1, two cobalt(III) ions are connected via three single atom bridges; two from deprotonated ethanolic oxygen atoms in the side arms of the ligands and one from the1-amino-2-propanol moiety forming a dinuclear unit with a very short (2.5430(11) Å) Co-Co intermetallic separation with a coordination number of 7, a rare feature for cobalt(III). In 2, two cobalt(II) ions in a dinuclear unit are bridged through phenoxide O and μ3-1,1,1-N3 azido bridges, and the two dinuclear units are interconnected by two μ-1,1-N3 and two μ3-1,1,1-N3 azido bridges generating tetranuclear cationic [Co4(H2L−1)2(µ3-1,1,1-N3)2(µ-1,1-N3)2Cl2(CH3OH)2]2+ units with an incomplete double cubane core, which grow into polynuclear 1D-single chains along the a-axis through H-bonding. In 3, HL2− holds mixed-valent Co(II)/Co(III) ions in a dinuclear unit bridged via phenoxide O, μ-1,3-CH3CO2−, and μ3-OH− bridges, and the dinuclear units are interconnected through two deprotonated ethanolic O in the side arms of the ligands and two μ3-OH− bridges generating cationic tetranuclear [Co2IICo2III(HL−2)2(µ-CH3CO2)2(µ3-OH)2]2+ units with an incomplete double cubane core. In 4, H2L1−2 holds mixed-valent Co(II)/Co(III) ions in dinuclear units which dimerize through two ethanolic O (μ-RO−) in the side arms of the ligands and two ethanolic O (μ3-RO−) of THMAM bridges producing centrosymmetric cationic tetranuclear [Co2IICo2III (H2L1−2)2(THMAM−1)2]4+ units which grow into 2D-sheets along the bc-axis through a network of H-bonding. Bulk magnetization measurements on 2 demonstrate that the magnetic interactions are completely dominated by an overall ferromagnetic coupling occurring between Co(II) ions.


Introduction
Using simple building blocks, living organisms use self-assembly processes to fabricate symmetrical biomolecules (proteins, DNA, lipids, enzymes), with varied levels of structural complexities [1][2][3]. Inspired by nature, during the last few decades, chemists have used self-assembly processes successfully [4][5][6] to generate a variety of supramolecular architectures (organic materials [7][8][9], metalacyclic polygons and polyhedral [10], and nanoscale systems [11,12]) with desired size, shape, and function. The self-assembly process often utilizes a variety of cooperative and noncovalent interactions such as hydrogen bonding, strong electrostatic, and van der Walls forces, π-π stacking, hydrophobic, hydrophilic, and metal-ligand interactions to produce complex supramolecular architectures which are difficult or in some cases impossible to make otherwise. Self-assembly processes have many advantages over the stepwise synthesis of large supramolecular assemblies, and lead to the formation of desired products from the predetermined building blocks spontaneously and more efficiently while minimizing/eliminating the formation of side products. In most of the self-assembly processes, metal ions or hydrogen ions (H + ) are used as a template for producing supramolecular coordination complexes with 1D-, 2D-, and 3D-network structures and grid systems [13][14][15][16][17][18][19][20][21][22][23].
In continuation of our interest in the self-assembly, structural characterization, and magnetic properties of polynuclear transition metal and lanthanide coordination complexes exhibiting ferromagnetic and antiferromagnetic spin exchange interactions, we have explored the coordination versatility of the Schiff base ligands derived from the condensation of 2,6-diformyl-4-methylphenol (DFMP) with 2-aminoethanol, 1-amino-2-propanol, 2-amino-1,3-propanediol, and tris(hydroxymethyl)aminomethane with Copper(II) [52,54,55], Cobalt(II)/Cobalt(III) [53], and Nickel(II) [53,55,56]. These ligands have a high degree of conformational flexibility and the potential to coordinate in a convergent (directing the formation of the dinuclear units), and a divergent fashion (ability to extend coordination beyond the primary coordination mode), creating one-dimensional single chains, two-dimensional sheets, or three-dimensional network structures utilizing one, two, or three protonated/deprotonated hydroxymethyl/hydroxyethyl groups present in each side arm.
In this publication, we want to report the synthesis, structural characterization, spectroscopic studies, and magnetic properties of dinuclear Cobalt(III), tetranuclear Cobalt(II), and mixed-valence Cobalt(II)/Cobalt(III) complexes with an incomplete double cubane core, which grow into beautiful 1D-single chains and 2D-sheet structures through H-bonding-directed polymerization involving coordinated/uncoordinated hydroxy groups in the side arms of the ligands and methanol/water molecules/NO 3 − ions in the crystal lattice.

Physical Measurements
Exactly same as reported in recently published paper [56].

Materials
2,6-Diformyl-4-methylphenol (DFMP) was isolated by the reported method [63], and 1-amino-2-propanol (AP) and tris(hydroxymethyl)aminomethane (THMAM) were supplied by Aldrich. All other chemicals (solvents and metal salts) used were analytical or reagent grade and were used without further purification. Schiff base ligands used in this investigation have not been isolated and structurally characterized. They have been generated in situ by reacting DFMP with AP (H 3 L) and THMAM (H 4 L1) in the presence of the metal salts.

X-ray Crystallography
Suitable single crystals for X-ray diffraction studies were obtained for 1-4. Crystal data for the compounds were collected by the same method by mounting a crystal onto a thin glass fiber from a pool of Fluorolube TM and immediately placing it under a liquid N 2 cooled stream, on a Bruker AXS diffractometer (Bruker AXS, Inc.: Madison, WI, USA) upgraded with an APEX II CCD detector (Bruker AXS, Inc.: Madison, WI, USA). The radiation used is graphite monochromatized Mo Kα radiation (λ = 0.7107 Å). The lattice parameters are optimized from a least-squares calculation on carefully centered reflections. Lattice determination, data collection, structure refinement, scaling, and data reduction were carried out using the APEX3 Version 2018.11 software (Bruker AXS, Inc.: Madison, WI, USA) package [64,65]. The data were corrected for absorption using the SCALE program within the APEX3 software (Version 6.45A, Bruker AXS, Inc.: Madison, WI, USA) package [64,65]. The structures were solved using SHELXT [66]. This procedure facilitated the assignment of C, N, Co, O, and Cl atoms. Subsequent Fourier synthesis yielded the remaining atom positions. The hydrogen atoms are fixed in positions of ideal geometry (riding model) and refined within the XSHELL software (Version 6.12, Bruker AXS, Inc.: Madison, WI, USA) package [67]. The final refinement of each compound included anisotropic thermal parameters on all non-hydrogen atoms and was performed using OLEX2-1.2 [68]. The crystal data for compounds 1-4 are given in Table 1. Crystallographic data for the structures have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos: CCDC-1948320-1948323. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax, +44-(0)1223-336033; or e-mail, deposit@ccdc.cam.ac.uk). Selected interatomic distances and angles for compounds 1-4 are listed in the Tables S1-S4.

Synthesis of Complexes
Caution: Azide and perchlorate complexes of metal ions involving organic ligands are potentially explosive. Only small quantities of the complexes should be prepared and should be handled with care.
In some cases, there is a difference between the most reasonable formula based on the elemental analysis (analytical formula) and that obtained from X-ray crystallography. This is because the analysis was carried out on air-dried samples due to their explosive nature. For consistency, the x-ray formulae will be used throughout this report.

[Co 2
III (H 2 L −1 ) 2 (AP −1 )(N 3 )](ClO 4 ) 2 (1) 1-amino-2-propanol (AP) (2.0 mmol, 0.15 g) was dissolved in methanol (20 mL), and a solution of DFMP (1.0 mmol, 0.16 g) dissolved in hot methanol (20 mL) was added to it. The resultant yellow solution was stirred under reflux for 30 min. A solution of Co(ClO 4 ) 2 ·6H 2 O (2.2 mmol, 0.82 g) dissolved in methanol (20 mL) was added to it while stirring under reflux dropwise over a period of 5 min. The reddish brown solution formed was refluxed further for 30 min, and a solution of sodium azide (2.0 mmol, 0.13 g) dissolved in a mixture of methanol:water (5:1 mL) was added dropwise. The dark red clear solution formed was stirred under reflux for 1 h and filtered while hot. A total of 5 mL of ethanol was added to the filtrate and was kept at room temperature for slow evaporation. After four weeks, reddish brown crystals suitable for X-ray analysis were obtained. The crystals were separated and washed with ethanol (2 × 2 mL). Yield, 0.14 g, 27 Complex 3 was obtained from a reaction carried out with the intention of getting a mixed metal (Gd/Co) complex but instead only a mixed-valence cobalt complex 3 was formed. To a methanolic solution (5 mL) of 1-amino-2-propanol (0.50 mmol, 0.040 g), a solution of DFMP (0.25 mmol, 0.040 g) dissolved in hot methanol (10 mL) was added. The resultant yellow solution of the Schiff base was stirred under reflux for 30 min. To it, a solution of Co(NO 3 ) 2 ·6H 2 O (0.70 mmol, 0.200 g) in methanol (5 mL) was added dropwise, while stirring under reflux. This was followed by the addition of a solution of Gd(CH 3 CO 2 ) 3 ·XH 2 O (0.60 mmol, 0.200 g) in 5 mL of methanol. To the orange-red solution formed, a solution of triethylamine (TEA) (0.80 mmol, 0.080 g) was added dropwise, and the reaction mixture was stirred under reflux for 40 min. It was filtered while hot and the filtrate was left at room temperature for slow evaporation. After two weeks, the reddish brown residue left after the evaporation of the solvent was treated with 10 mL of ethanol. Most of the solid dissolved leaving behind some white solid which was discarded. After leaving the filtrate at room temperature for 10 weeks, reddish brown crystals suitable for X-ray analysis were formed. Some of the crystals were kept in the mother liquor for X-ray analysis and the rest were filtered off and washed with ethanol (2 × 1 mL). In this reaction, the presence of both CH 3 Tris(hydroxymethyl)aminomethane (THMAM) (1.0 mmol, 0.12 g) was dissolved in 10 mL of methanol and a solution of DFMP (0.50 mmol, 0.080 g) in10 mL of hot methanol (10 mL) was added to it. The reaction mixture was stirred under reflux for 30 min. A solution of Co(NO 3 ) 2 ·6H 2 O, (1.2 mmol, 0.365 g) in 10 mL of methanol was added to it dropwise while stirring under reflux. The reddish brown solution formed was stirred under reflux for 15 min, and 10 drops of triethylamine (TEA) were added to it. A dark red solution formed was stirred under reflux for about 2.0 h and filtered while hot. The filtrate was left at ambient temperature for slow evaporation. After two weeks, reddish brown crystals suitable for X-ray analysis were formed. The crystals were filtered and washed with methanol (2 × 1 mL). Yield: 0.095 g, 34.4%. IR spectrum: 3333 cm −1 , 3179 cm −1 (υ(OH/NH 2 ) H 2 L1 2− /THMAM), 1647cm −1 , 1638 cm −1 υ(C=N/C=O). UV-Vis Spectrum: 440, 415, and 320 nm (Co-

Synthesis of the Complexes
In this publication, we wish to report the self-assembly, structural characterization, and magnetic properties of cobalt(II) (2), cobalt(III) (1), and mixed-valence (Co 2 II /Co 2 III ) (3 and 4) complexes of two very versatile Schiff base: H 3 L (a double Schiff base, potentially pentadentate (N 2 O 3 ) trianionic ligand) and H 4 L1 (a single Schiff base, potentially hexadentate (NO 5 ), tetraanionic ligand) with a high degree of conformational flexibility ( Figure 1). In complex 2, H-bonding directs the polymerization of ferromagnetically coupled tetranuclear cobalt(II) units with an incomplete double cubane core to 1D-single chains. In complex 4, H-bonding directs the formation of a polynuclear complex in which tetranuclear mixed-valence (Co 2 II Co 2 III ) units are interconnected by a network of H-bonding along the bc-axis to produce a beautiful 2D-sheet structure.

Synthesis of the Complexes
In this publication, we wish to report the self-assembly, structural characterization, and magnetic properties of cobalt(II) (2), cobalt(III) (1), and mixed-valence (Co2 II /Co2 III ) (3 and 4) complexes of two very versatile Schiff base: H3L (a double Schiff base, potentially pentadentate (N2O3) trianionic ligand) and H4L1 (a single Schiff base, potentially hexadentate (NO5), tetraanionic ligand) with a high degree of conformational flexibility ( Figure 1). In complex 2, H-bonding directs the polymerization of ferromagnetically coupled tetranuclear cobalt(II) units with an incomplete double cubane core to 1D-single chains. In complex 4, H-bonding directs the formation of a polynuclear complex in which tetranuclear mixed-valence (Co2 II Co2 III ) units are interconnected by a network of Hbonding along the bc-axis to produce a beautiful 2D-sheet structure. Earlier [23,[52][53][54][55][56]69], we have seen that the nature of the anions, nature of the metal ions, and the reaction conditions (presence/absence of TEA) have remarkable effects on the formation of the ligand, self-assembly of dinuclear, tetranuclear, pentanuclear, hexanuclear, heptanuclear, or decanuclear coordination compounds, in which H-bondings direct the formation of 1D-single chains, 2D-sheets, and 3D-network structures.

Scheme 2.
Reactions between DFMP and THMAM in the presence of Co Reactions between DFMP and AP in the presence of Co(II) salts form  [52,55], is presumably due to either cobalt(II) assisted partial hydrolysis of one of the two arms of an initially formed double Schiff base ligand (H 7 L) as we reported earlier [55] in hexanuclear nickel(II) complexes or due to the preferential stereochemical requirements of mixed-valence tetranuclear [Co 2 II Co 2 III ] complex 4 [53,70,71]. Recently, we [53,55,56] and Ray's group [72] have reported the partial hydrolysis of one of the side arms of H 7 L3/H 5 L2 (Figure 1) or similar double Schiff base ligand (formed by 1 + 2 condensation of DFMP and 2-aminoethanol) in hexanickel (Ni II 6 ) clusters. Based on present and earlier investigations [52,53,55,56,69] with Schiff base ligands derived from DFMP, we can conclude that the formation of a single Schiff base ligand (H 4 L1) instead of a double Schiff base ligand (H 7 L3) is the result of either metal-catalyzed partial hydrolysis or preferential 1 + 1 condensation in case of hexanuclear Nickel(II) or mixed-valence tetranuclear cobalt complexes due to stereochemical requirements of these metals.
condensation of DFMP and 2-aminoethanol) in hexanickel (Ni II 6) clusters. Based on present and earlier investigations [52,53,55,56,69] with Schiff base ligands derived from DFMP, we can conclude that the formation of a single Schiff base ligand (H4L1) instead of a double Schiff base ligand (H7L3) is the result of either metal-catalyzed partial hydrolysis or preferential 1 + 1 condensation in case of hexanuclear Nickel(II) or mixed-valence tetranuclear cobalt complexes due to stereochemical requirements of these metals.  (1). The cationic core in the structure of 1 is shown in Figure 2.
Bond angles and distances relevant to the cobalt(III) coordination core are given in Table S1. Complex 1 crystallizes in triclinic system space group P-1 and is comprised of two Co 3+ ions, two H 2 L −1 ligands, one deprotonated AP −1 , and one terminal azide ion. H 3 L, potentially a pentadentate trianionic ligand, in 1 behaves as a tridentate (NO 2 ) monoanionic (H 2 L −1 ) ligand which is contrary to its behavior as a pentadenate/tetradentate di-anionic ligand in Cu(II) [69], Ni(II) [56], and other cobalt(II) complex (2), and mixed-valence Co 2 II /Co 2 III complex (3). The stereochemistry at Co (1)   The disordered oxygen atom has been modeled with partial occupancies and is located where the hydrogen atom would be located. We cannot assign a hydrogen atom to that carbon atom due to this issue. Thus, the sum formula is short a hydrogen atom. The disordered oxygen atom has been modeled with partial occupancies and is located where the hydrogen atom would be located. We cannot assign a hydrogen atom to that carbon atom due to this issue. Thus, the sum formula is short a hydrogen atom.
In 4, Schiff base ligand (H 4 L1), formed from 1 + 1 condensation of DFMP and THMAM, acts in a pentadenate [NO 4 ] dianionic (H 2 L 2− ) capacity binding two metal centers (Co II (Table S4). The Co-N and Co-O distances are very similar to the distances reported for similar mixed-valence cobalt complexes with simiar Schiff base ligands [53,70,71]. The Co-Co distances in the doubly bridged dinuclear and tetranuclear units are in the range 2.9521 (9)-3.055 (1) Å which are slightly shorter than the distances reported in similar mixed-valence cobalt complexes [53]. There are four uncoordinated NO 3 − ions per tetranuclear unit in the lattice. In each dinuclear unit, the phenoxide bridge angle (Co (1)

Magnetic Properties
The variable temperature magnetic properties of 2 are illustrated in Figure 8 as a plot of chi.T vs T. The characteristic maximum at low temperature suggests intramolecular exchange, dominated by ferromagnetic coupling as reported in ferromagnetically coupled tetranuclear Cobalt(II) complexes involving µ 1,1-N3 and µ -O bridges [86] or pseudo halide bridges [87]. The RT moment per metal is 5.46 muB, which is in the upper range for high spin Co(II). The small rise on lowering temperature is the result of spin orbital coupling effects. While the Co-O-Co angle of 100.4° is consistent with antiferromagnetic exchange in general, the small Co-N-Co angle (93.9°) is in the realm expected for ferromagnetic exchange. In this case, the ferromagnetic component appears to dominate. Fitting of the exchange data to an appropriate Hamiltonian did not give a satisfactory fit, as anticipated for Co(II), due to spin-orbit effects.

Magnetic Properties
The variable temperature magnetic properties of 2 are illustrated in Figure 8 as a plot of chi.T vs T. The characteristic maximum at low temperature suggests intramolecular exchange, dominated by ferromagnetic coupling as reported in ferromagnetically coupled tetranuclear Cobalt(II) complexes involving µ 1,1 -N 3 and µ-O bridges [86] or pseudo halide bridges [87]. The RT moment per metal is 5.46 muB, which is in the upper range for high spin Co(II). The small rise on lowering temperature is the result of spin orbital coupling effects. While the Co-O-Co angle of 100.4 • is consistent with antiferromagnetic exchange in general, the small Co-N-Co angle (93.9 • ) is in the realm expected for ferromagnetic exchange. In this case, the ferromagnetic component appears to dominate. Fitting of the exchange data to an appropriate Hamiltonian did not give a satisfactory fit, as anticipated for Co(II), due to spin-orbit effects.
The variable temperature magnetic properties of 2 are illustrated in Figure 8 as a plot of chi.T vs T. The characteristic maximum at low temperature suggests intramolecular exchange, dominated by ferromagnetic coupling as reported in ferromagnetically coupled tetranuclear Cobalt(II) complexes involving µ 1,1-N3 and µ -O bridges [86] or pseudo halide bridges [87]. The RT moment per metal is 5.46 muB, which is in the upper range for high spin Co(II). The small rise on lowering temperature is the result of spin orbital coupling effects. While the Co-O-Co angle of 100.4° is consistent with antiferromagnetic exchange in general, the small Co-N-Co angle (93.9°) is in the realm expected for ferromagnetic exchange. In this case, the ferromagnetic component appears to dominate. Fitting of the exchange data to an appropriate Hamiltonian did not give a satisfactory fit, as anticipated for Co(II), due to spin-orbit effects.

IR and UV-Vis Spectroscopy
In the IR spectra of 1 and 2, which have terminal azide (N 3 ) ions and intra and inter-dimer EO (µ-1,1-N 3 or µ 3 -1,1,1-N 3 ) bridging azide ions, one band is observed at 2227 (1)/2084 cm −1 , typical for υ as (N 3 ). In the IR spectra of 1-4, a band in the region 3445-3333 cm −1 and one or two bands in the region 1651-1637 cm −1 are due to υ(OH) (H 2 O and CH 3 OH) and υ(C=N) of coordinated imine, respectively. In the IR spectra of 1, three bands in the regions at 3294, 3243, 3142 cm −1 , and in 4, one band at 3179 cm −1 are due to (υ(NH 2 ) of the coordinated amino group of AP −1 and THMAM −1 , respectively.
In the UV-Vis spectra of 1-2 compounds, a strong band at 430-390 nm and a high energy band or shoulder at 320-270 nm are assigned to metal-azide and metal-ligand charge transfer transitions. In the visible spectra of the 1-4 complexes, one to three broad bands/shoulders in the region 680-525 nm are due to d-d transitions.