Synthesis of a xanthene-based macrocyclic imine ligand and two-step planarization by metalation with Ni²⁺ and Na⁺

Abstract

A xanthene-based macrocyclic π-containing imine ligand, H₄L², was newly designed and synthesized in order to obtain a series of metal-containing planar structures. Due to the xanthene framework with the methylene bridge, the macrocyclic molecule, H₄L², had a more planar structure than the corresponding diaryl-ether-type analogue, H₄L¹. This macrocyclic molecule, H₄L², was converted into the dinuclear nickel(II) complex, L²Ni₂, which was characterized by spectroscopic techniques as well as crystallography. The planarity of the macrocyclic ligand, H₄L², was greatly improved by the introduction of two Ni²⁺ ions. Furthermore, the L²Ni₂ molecule became more planar by incorporation of a Na⁺ ion in the central O₆ binding cavity. Thus, the xanthene-based macrocycle, H₄L², allowed complexation with two different metal ions, Ni²⁺ and Na⁺, to demonstrate a two-step improvement in the planarity.

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Introduction

Compounds containing π-planes have attracted much attention as functional molecules based on their electronic and optical properties. The planarity of such π-containing molecules has a significant effect on their electronic and photophysical properties derived from the π-conjugation of the molecules. For example, various physicochemical properties, such as optical properties,¹ electrochemical properties,² efficiency of photovoltaics,³etc., are influenced by the molecular planarity, which can be changed by introducing or changing the substituents.⁴ The dynamic change in the molecular planarity can also cause a significant change in the fluorescent behavior.⁵

The planarity of some macrocyclic π-containing molecules can be easily controlled by the incorporation of guest species into the cavity, because the macrocyclic compounds generally show a good host–guest binding affinity. For example, the properties of porphyrin derivatives can be altered by protonation⁶ and metalation⁷ in the binding pocket, axial functionalization on the metal ions,⁸etc., although the porphyrin derivatives are generally less flexible to allow a rather smaller conformational change. If a macrocyclic molecule has a larger cavity with two or more binding sites to accommodate multiple guest species, the planarity could be more drastically controlled, perhaps in a multistep manner by using two or more different types of guest species.

In this respect, the macrocyclic imine ligand, H₄L¹ (Scheme 1a (i)), which we have previously reported, exhibited a unique two-step planarity enhancement, because this molecule has two types of binding sites inside the macrocyclic structure containing six benzene rings, i.e., two H₂saloph‡ coordination pockets doubly connected by diaryl ether linkages and one central O₆ binding site.^9–13 The structure of H₄L¹ in the absence of metal ions is far from flat, but when Ni²⁺ ions are introduced into the saloph sites of this molecule, the two square planar [Ni(saloph)] structures are formed to make the L¹Ni₂ macrocycle relatively more planar (Scheme 1b (i)).^9,10 However, this L¹Ni₂ is still significantly deviated from an ideal planar structure, although the parent [Ni(saloph)] motif is known to be highly planar.¹⁴ Particularly, the two [Ni(saloph)] substructures in the L¹Ni₂ macrocycle are curved in opposite directions to each other mainly due to the steric hindrance of the ortho hydrogen atoms in the diaryl ether moieties (Scheme 1b (i)).

Scheme 1 (a) Molecular structures of bis(saloph) macrocycles: (i) diaryl-ether-based macrocycle H₄L¹ and (ii) xanthene-based macrocycle H₄L², and (b) their nickel(II) complexes (i) L¹Ni₂ and (ii) L²Ni₂. (c) Schematic drawing of two-step planarization of a macrocycle by metalation with two different kinds of metal ions, (i) the first metalation with Ni²⁺ and (ii) the second metalation with Na⁺.

If these two neighboring ortho hydrogen atoms are replaced with a bridging methylene group to make a xanthene motif, the resultant macrocycle is expected to be more planar. This structural modification would lead to unique properties originating from the more effective π-conjugation and facilitate the formation of higher-order stacking structures.^9–11,15 In this study, we synthesized the macrocyclic imine ligand, H₄L², based on the xanthene motif (Scheme 1a (ii)), which can be converted into a planar dinickel(II) complex, L²Ni₂ (Scheme 1b (ii)). We found that the planarity of the macrocyclic ligand, H₄L², was greatly improved by introduction of two Ni²⁺ ions. Furthermore, the L²Ni₂ molecule became more planar by incorporation of Na⁺ ion into the O₆ binding cavity, demonstrating a two-step improvement in the planarity (Scheme 1c).

Results and discussion

Synthesis

For the synthesis of the precursor 4 for the macrocyclic ligand, H₄L² (Scheme 2), we first prepared 9,9-dimethylxanthene-4,5-diol (1) from xanthone according to a literature method.¹⁶ The MOM protection of the hydroxy groups in this diol 1 gave the MOM ether 2 (Fig. S1, and S2†), which was then converted to dialdehyde 3 (Fig. S3, and S4†) by dilithiation followed by the reaction with DMF. Removal of the MOM groups by acid hydrolysis gave the dialdehyde 4 (Fig. S5, and S6†).

Scheme 2 Synthesis of xanthene-based macrocyclic ligand H₄L² and the corresponding metallohost L²Ni₂.

The [2 + 2] macrocyclization of the dialdehyde 4 with o-phenylenediamine readily proceeded in a chloroform/acetonitrile (1 : 1) mixed solvent (Scheme 2). The macrocyclic ligand, H₄L², was obtained in 89% yield as orange crystals (Fig. S7–S11†). The product showed one singlet each for the imine proton at 8.99 ppm and the hydroxy proton at 13.82 ppm, indicative of the formation of a symmetric macrocyclic molecule. The mass spectrum showing m/z = 741.3 for [H₄L² + H]⁺ confirmed the [2 + 2] macrocyclic structure of H₄L² (Fig. S12†). This compound was almost insoluble in pure chloroform or acetonitrile, while slightly soluble in DMSO.

The reaction of H₄L² with nickel(II) acetate in chloroform/methanol then afforded the dinuclear nickel(II) metallohost, L²Ni₂, in 56% yield (Scheme 2; Fig. S13†). This metallohost showed a very low solubility in common organic solvents, and only very slightly soluble in DMSO.

Crystallography and theoretical studies

The H₄L² macrocycle crystallized in the monoclinic system, space group P2₁/m, and the crystallographic analysis clearly demonstrated the [2 + 2] macrocyclic structure (Fig. 1a and b). The unit cell contains two crystallographically independent molecules having different conformations, which are referred to as H₄L²-A and H₄L²-B, hereafter. Both conformations are found to be nonplanar as seen in the diaryl ether analogue, H₄L¹,^9,10 which is presumably due to the repulsion of the oxygen lone pairs. The H₄L²-A molecule has a crystallographicaly imposed mirror plane and adopted an approximate C_2v symmetry. The two xanthene planes are bent by 39.12(4) deg to the same side with respect to the O₆ mean plane of the macrocycle resulting in a V-shaped structure (Fig. 1a). The other molecule, H₄L²-B, has a crystallographically imposed inversion center and adopted an approximate C_2h symmetry. In contrast to molecule H₄L²-A, the two xanthene planes are bent to the opposite side resulting in a step structure (Fig. 1b).

Fig. 1 X-ray crystal structures of H₄L² and L²Ni₂ showing two crystallographically independent molecules. (a) H₄L²-A with a V-shaped conformation; (b) H₄L²-B with a step conformation; (c) L²Ni₂-A with a shallow V-shaped conformation; (d) L²Ni₂-B with a step conformation (ORTEP, 50% probability). Solvent molecules are not shown.

The relative stability of the two conformations of H₄L² was investigated by DFT calculations. The geometry optimizations were performed starting from the crystal structures, H₄L²-A and H₄L²-B, to obtain the corresponding optimized structures, H₄L²-a and H₄L²-b, respectively (Fig. S14†). The molecule of H₄L²-a, which has a V-shaped structure, was more stable by 7.03 kJ mol⁻¹ than H₄L²-b having a step conformation.

In contrast to the metal-free macrocycle, H₄L², the corresponding nickel(II) complex has a more planar structure. The nickel(II) complex, L²Ni₂, crystallized in the monoclinic system, space group P2₁/n, and the crystal structural analysis clearly demonstrated a macrocyclic structure containing two square planar nickel(II) ions in the saloph coordination pockets (Fig. 1c and d). The unit cell contains two crystallographically independent molecules, L²Ni₂-A and L²Ni₂-B hereafter, in a 2 : 1 ratio. The L²Ni₂-A molecule had no crystallographically-imposed symmetry and was slightly bent to form a shallow V-shaped structure (Fig. 1c). The L²Ni₂-B molecule had a crystallographically-imposed inversion center (Fig. 1d). The two xanthene moieties are slightly bent in the opposite directions to form a step-like conformation.

The geometry optimizations of L²Ni₂ starting from the two structures, L²Ni₂-A and L²Ni₂-B, gave the corresponding energy-minimized structures, L²Ni₂-a and L²Ni₂-b, respectively, but both optimized structures were significantly curved unlike the nearly planar structures found in the crystalline state (Fig. S15†). L²Ni₂-a is more stable than L²Ni₂-b by only 2.97 kJ mol⁻¹, suggesting that both conformations as well as those in the crystalline state are present in the solution.

The macrocyclic metallohost, L²Ni₂, was expected to show a binding affinity to cationic guest species, because it has a crown ether-like cavity. Indeed, the diaryl ether analog, L¹Ni₂, and related compounds showed an excellent binding affinity to alkali metal ions, etc., mainly due to the more polarized phenoxo–Ni bonds.^9–11,17 In addition, Na⁺ was the best metal to be accommodated in the binding cavity of L¹Ni₂ in a monomeric 1 : 1 stoichiometry. However, the guest binding behavior of L²Ni₂ could not be investigated due to its very low solubility. Nevertheless, single crystals of the guest-inclusion complex, [L²Ni₂Na(DMSO)₂](OTf), was directly obtained by mixing H₄L², Ni(OAc)₂, and NaOTf in a methanol/DMSO solution followed by vapor-phase diffusion of diethyl ether.

The X-ray crystallographic analysis (Fig. 2) showed that a Na⁺ ion was situated at the center of the O₆ cavity, exactly on the O₆ mean plane. The six Na–O distances are almost the same within a range of 2.531–2.665 Å, approximately forming a regular hexagon. This metallohost–guest complex, L²Ni₂Na, adopted an almost planar but slightly wavy structure, and two DMSO molecules coordinated to the Na⁺ ion from above and below the macrocyclic plane. In the packing structure, the planar [Ni(saloph)] substructures in this complex were stacked on top of each other in such a way that they avoided these coordinating DMSO molecules. The counter anion did not directly coordinate to any of the metal centers, Ni²⁺ or Na⁺.

Fig. 2 X-ray crystal structure of [L²Ni₂Na(DMSO)₂]⁺ (ORTEP, 50% probability). Hydrogen atoms are omitted for clarity. Noncoordinating DMSO molecule and the TfO⁻ counter anion are not shown.

The DFT calculations gave the optimized structure of the inclusion complex, [L²Ni₂Na]⁺, with the coordinating DMSO molecules omitted (Fig. S16†). The calculated host–guest interaction energy between L²Ni₂ and Na⁺ using the optimized geometry was −528.5 kJ mol⁻¹. The destabilization by deformation of the L²Ni₂ macrocycle caused by the formation of the Na⁺ complex was calculated to be only 17.1 kJ mol⁻¹, indicating that the uncomplexed L²Ni₂ already has a structure suitable for Na⁺ binding. As a result, the stabilization energy upon the formation of [L²Ni₂Na]⁺ from L²Ni₂ and Na⁺ was determined to be −528.5 + 17.1 = −511.4 kJ mol⁻¹, which corresponds to the binding energy in the gas phase. In solution, the formation of [L²Ni₂Na]⁺ requires desolvation of Na⁺; therefore, the binding energy in solution is expected to be much smaller (less negative). From the viewpoint of the small deformation energy associated with complex formation, along with favorable geometrical matching, this L²Ni₂ macrocycle appears to be well preorganized for the Na⁺ binding.

Planarity of the complexes

Since we have obtained the crystal structures of a series of H₄L², L²Ni₂, and L²Ni₂Na, their structural features, such as the planarity and cavities sizes/shapes, were summarized for comparison with the corresponding diaryl ether analogues (Table 1). Based on the X-ray crystal structures, we defined three parameters in order to discuss the planarity of the macrocyclic molecules; deviation d_dev as the average RMS deviation of the 40 skeletal carbon and 4 nitrogen atoms from the O₆ mean plane (Fig. 3a); macrocycle bent angle θ_a as the dihedral angle between the xanthene mean plane and the O₆ mean plane (Fig. 3b); and xanthene bent angle θ_b as the dihedral angle between the two benzene rings in each xanthene unit (Fig. 3c). In addition, the size/shape of the O₆ guest binding site is discussed in terms of the diagonal O⋯O distances, d_OO(X) and d_OO(P), as well as the continuous shape measure (CShM) value,¹⁸ which represents the deviation from ideal regular hexagon.

Fig. 3 Definition of planarity parameters (a) d_dev, (b) θ_a, and (c) θ_b. (d) Comparison of the planarity parameters for H₄L¹, L¹Ni₂, L^1′Ni₂Na, H₄L², L²Ni₂, and L²Ni₂Na.

Table 1 Structural parameters for H₄L², L²Ni₂, L²Ni₂Na, H₄L¹, L¹Ni₂, and L^1′Ni₂Na

Structure	d OO(X) ^a (Å)	d OO(P) ^b (Å)	d dev ^c (Å)	θ a ^d (°)	θ b ^e (°)	CShM^f
a The diagonal O⋯O distance between the xanthene oxygen atoms. b The diagonal O⋯O distance between the phenoxo oxygen atoms. c Average RMS deviation of the 40 skeletal carbon and 4 nitrogen atoms from the O6 mean plane (Fig. 3a). d The dihedral angle between the xanthene mean plane and the macrocyclic O6 mean plane (Fig. 3b). e The dihedral angle between the two benzene rings in each xanthene unit (Fig. 3c). f Continuous shape measure (CShM) value for the geometry calculated with the program SHAPE 2.1.^18 g [L^2Ni2Na(DMSO)2](OTf). h [L^1′Ni2Na](OTf).
H4L^2-A	6.612(3)	5.683(2)	1.36	39.12(4)	27.28(10)	3.486
H4L^2-B	6.597(3)	5.489(3), 5.865(3)	1.67	37.51(5)	30.41(10)	3.226
L^2Ni2-A	5.261(12)	5.078(3), 5.151(3)	0.49	7.60(16)	12.64(12)	0.705
L^2Ni2-B	5.3048(17)	5.0983(17), 5.0538(17)	0.48	6.38(5), 6.27(4)	23.21(9), 16.41(9)	0.221
L^2Ni2Na^g	5.319(4)	5.075(4), 5.077(4)	0.25	4.28(13), 5.86(13)	9.8(3), 7.6(3)	0.153
H4L^1 [ref. 9 and 10]	6.354(2)	5.270(2), 5.956(2)	1.88	38.12(3), 75.28(4)	72.91(5)	4.624
L^1Ni2 [ref. 9 and 10]	5.238(4)	5.167(4), 5.193(4)	0.80	29.29(11), 15.34(16)	42.24(13)	0.027
L^1′Ni2Na-A [ref. 11]^h	5.091(5)	5.070(5), 5.205(5)	0.82	25.41(11), 21.76(12)	44.46(10)	0.115
L^1′Ni2Na-B [ref. 11]^h	5.072(5)	5.189(5), 5.079(5)	0.85	22.60(13), 26.00(11)	45.66(10)	0.125

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As already mentioned, the metal-free macrocycles, H₄L²-A and H₄L²-B, have a non-planar conformation in which the two xanthene planes are bent at an angle θ_a of 37–39 deg with respect to the O₆ mean plane. The difference in the bending orientations of the two xanthene planes resulted in two types of conformations, i.e., V-shaped (H₄L²-A) and step-shaped (H₄L²-B). In addition, each xanthene moiety was also bent at the angle θ_b of about 27–30 deg. This bending makes the molecular conformation far from planar, with the average deviation d_dev of 1.36 and 1.67 Å for H₄L²-A and H₄L²-B, respectively.

It was clear that the conversion of H₄L² into L²Ni₂ by introducing two Ni²⁺ ions improved the overall planarity of the macrocyclic molecule; both molecules, L²Ni₂-A and L²Ni₂-B, showed the average deviations d_dev of 0.48–0.49, which are much smaller than those of H₄L². The nickel(II) complex, L²Ni₂, also had bent angles θ_a (6.3–7.6 deg) and θ_b (12–23 deg) that are significantly smaller than those of H₄L². This planarization also changed the arrangement of the six oxygen atoms; in particular, the diagonal distance between the two xanthene oxygen atoms, d_OO(X), was shortened from 6.6 Å to 5.3 Å upon the conversion of H₄L² to L²Ni₂. As a result, all the diagonal O⋯O distances within the O₆ binding site became almost similar (d_OO(P) of 5.05–5.15 Å), approximately making a regular hexagon, which is also evident from the significant decrease in the CShM values (from 3.2–3.5 to 0.2–0.7).

The incorporation of Na⁺ ion into L²Ni₂ further improved the planarity of the macrocycle. The deviation d_dev of [L²Ni₂Na(DMSO)₂](OTf) was 0.25, which was approximately half that for L²Ni₂ (d_dev = 0.48–0.49). The bent angles θ_b became smaller (7.6–9.8 deg), which are also smaller than those of L²Ni₂. Therefore, the planarity of this host H₄L² was clearly improved in two steps by introducing two different kinds of metals, Ni²⁺ and Na⁺. Considering the molecular curvature found in the DFT calculated structures, the crystal packing may also contribute to the observed highly-planar structures of L²Ni₂ and L²Ni₂Na.

While there is a noticeable difference in the conformational planarity, the geometrical features of the O₆ sites of L²Ni₂ and L²Ni₂Na are quite similar. Whereas the CShM value for L²Ni₂Na was slightly smaller than that for L²Ni₂, it was particularly noteworthy that there is almost no difference in the diagonal O⋯O distances, d_OO(X) and d_OO(P), between the inclusion complex L²Ni₂Na and the guest-free L²Ni₂ macrocycle, indicating that Na⁺ binding occurs almost without significant deformation of the host framework of L²Ni₂. This was also demonstrated from the viewpoint of the deformation energy in the computational investigations as already discussed. Thus, the L²Ni₂ metallohost has a well preorganized binding site that is particularly suitable for Na⁺ binding.

A similar preorganization effect of the O₆ binding site was observed for the corresponding diaryl ether analogues, H₄L¹, L¹Ni₂,^9,10 and L^1′Ni₂Na.¹¹ However, the xanthene analogues presented here generally exhibit greater planarity than these diaryl ether analogues, particularly in the case of the metal-free H₄L². This enhanced planarity contributes to its stronger binding to Na⁺ (K_a = 182 M⁻¹; Fig. S17†) compared to H₄L¹ (K_a = 13 M⁻¹),⁹ owing to its well-preorganized binding site. Moreover, the xanthene analogues showed a more pronounced increase in planarity upon stepwise binding with Ni²⁺ and Na⁺, demonstrating the advantage of employing the xanthene motif to construct metal-containing planar structures.

Conclusion

A novel xanthene-based macrocyclic imine ligand, H₄L², was designed and synthesized to make a planar metallohost molecule in which the aromatic organic macrocyclic moiety and the square planar metals are likely to form a coplanar structure. The use of the xanthene motif instead of the diaryl ether in H₄L¹ lead to a more planar structure of the resultant dinickel(II) metallohost without suffering the steric repulsion between the neighboring hydrogen atoms found in the diaryl ether analogues. From the viewpoint of the structural parameters, we demonstrated that the planarity of the H₄L² macrocycle was improved in a two-step manner by the introduction of Ni²⁺ into the H₂saloph coordination pockets followed by Na⁺ in the O₆ guest binding site. Also, a detailed structural investigation clearly demonstrated that this L²Ni₂ is well preorganized for the binding with Na⁺ in the O₆ binding site. However, the present macrocycle, H₄L², and its nickel(II) complex, L²Ni₂, showed a significantly low solubility in common organic solvents, which prevented us from investigating their behavior in solution. Nevertheless, the host–guest binding behavior and the possible formation of aggregated species would be of interest due to their highly-planar structures. Further investigations are currently underway to synthesize more soluble analogues by introducing alkyl groups into the phenylenediamine subunit, aiming to facilitate studies in solution chemistry.

Experimental section

General

9,9-Dimethylxanthene-4,5-diol was synthesized according to the literature.¹⁶ The reagents and solvents were purchased from commercial sources and used without further purification. The ¹H and ¹³C NMR spectra were recorded on a JEOL JNM-ECS 400 (¹H, 400 MHz; ¹³C, 100 MHz) or a Bruker Avance 600 (¹H, 600 MHz). The chemical shifts were referenced with respect to tetramethylsilane (0 ppm) as an internal standard. The ESI-TOF mass spectra were recorded on a Bruker Daltonics micrOTOF II.

4,5-Bis(methoxymethoxy)-9,9-dimethylxanthene (2)

Under a nitrogen atmosphere, NaH (60% in oil, 5.63 g, 0.141 mol) was washed with petroleum ether to remove the oil and then a solution of 9,9-dimethylxanthene-4,5-diol (1) (8.72 g, 36.0 mmol) in dehydrated DMF (150 mL) was introduced into a flask containing the NaH. The resultant mixture was stirred for 4 h at rt, then chloromethyl methyl ether (8.20 mL, 0.108 mol) was added to the mixture at 0 °C, which was further stirred overnight at rt. After the addition of water, the mixture was extracted several times with chloroform. The combined organic layer was washed with brine, dried over anhydrous sodium sulfate, filtered, and concentrated to dryness under reduced pressure. The crude product was purified by column chromatography on silica gel (hexane/EtOAc, 9 : 1 → 4 : 1) to obtain MOM ether 2 (11.7 g, 35.3 mmol, 98%) as a pale yellow oil; ¹H NMR (400 MHz, CDCl₃) δ 1.62 (s, 6H), 3.57 (s, 6H), 5.30 (s, 4H), 7.00 (t, J = 7.8 Hz, 2H), 7.04 (dd, J = 7.8, 1.9 Hz, 2H), 7.09 (dd, J = 7.8, 1.9 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 31.81, 34.46, 56.30, 96.10, 115.78, 119.35, 122.77, 131.61, 141.49, 145.12. Anal. calcd for C₁₉H₂₂O₅: C, 69.07; H, 6.71. Found: C, 68.76; H, 6.69.

4,5-Bis(methoxymethoxy)-9,9-dimethylxanthene-3,6-dicarbaldehyde (3)

Under a nitrogen atmosphere, n-butyllithium (1.6 M in hexane, 6.0 mL, 9.6 mmol) was added to a solution of 2 (0.994 g, 3.01 mmol) in dehydrated THF (20 mL) at 0 °C. After the mixture was stirred for 1 h at 0 °C, dehydrated DMF (1.20 mL, 15.5 mmol) was added at 0 °C to the mixture, which was further stirred overnight at rt. After the addition of diluted hydrochloric acid (1.0 M, 20 mL), the mixture was extracted several times with chloroform. The combined organic layer was washed with brine, dried over anhydrous sodium sulfate, filtered, and concentrated to dryness under reduced pressure. The crude product was purified by column chromatography on silica gel (hexane/EtOAc, 9 : 1 → 4 : 1) to obtain dialdehyde 3 (0.692 g, 1.79 mmol, 60%) as pale yellow crystals; ¹H NMR (400 MHz, CDCl₃) δ 1.67 (s, 6H), 3.64 (s, 6H), 5.41 (s, 4H), 7.29 (dd, J = 8.4, 0.7 Hz, 2H), 7.62 (d, J = 8.4 Hz, 2H), 10.44 (d, J = 0.7 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 31.36, 35.65, 58.20, 100.08, 121.38, 122.27, 128.97, 137.33, 143.02, 147.96, 189.40. Anal. calcd for C₂₁H₂₂O₇: C, 65.28; H, 5.74. Found: C, 65.01; H, 5.75.

4,5-Dihydroxy-9,9-dimethylxanthene-3,6-dicarbaldehyde (4)

Hydrochloric acid (4.0 M, 9.0 mL) was added to a solution of 3 (2.25 g, 5.82 mmol) in methanol/chloroform (3 : 1, 48 mL) and the mixture was stirred overnight at rt. After the addition of water, the mixture was extracted with chloroform, and the combined organic layer was dried over anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to a minimal volume. The addition of hexane to the resultant solution produced precipitates, which were collected on a filter to yield 4 (1.62 g, 5.43 mmol, 93%) as yellow crystals; ¹H NMR (400 MHz, CDCl₃) δ 1.67 (s, 6H), 7.06 (d, J = 8.4 Hz, 2H), 7.33 (d, J = 8.4 Hz, 2H), 9.91 (s, 2H), 11.21 (s, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 31.38, 35.62, 116.34, 119.52, 126.87, 136.81, 138.56, 150.33, 195.77. Anal. calcd for C₁₇H₁₄O₅·0.4H₂O: C, 66.84; H, 4.88. Found: C, 66.86; H, 4.70.

Macrocyclic ligand H₄L²

A solution of 4 (0.298 g, 1.00 mmol) in dehydrated chloroform (5 mL) was mixed with a solution of o-phenylenediamine (0.108 g, 1.00 mmol) in dehydrated acetonitrile (5 mL) and the resultant solution was allowed to stand for 2 d at rt. The reaction mixture was then filtered using a membrane filter and the filtrate was concentrated under reduced pressure to a minimal volume. The addition of hexane to the resultant solution gave precipitates, which were collected on a filter to yield H₄L² (0.365 g, 0.443 mmol, 89%) as orange crystals; ¹H NMR (400 MHz, DMSO-d₆) δ 1.64 (s, 12H), 7.17 (d, J = 8.4 Hz, 4H), 7.41 (d, J = 8.4 Hz, 4H), 7.46–7.50 (m, 4H), 7.56–7.60 (m, 4H), 8.99 (s, 4H), 13.82 (s, 4H); ¹³C NMR (100 MHz, DMSO-d₆) δ 31.48, 34.93, 115.76, 117.68, 119.65, 126.74, 128.39, 133.93, 138.03, 141.82, 149.70, 164.56. ESI-TOF MS observed m/z = 741.2737 ([H₄L² + H]⁺), calcd for C₄₆H₃₆N₄O₆H m/z = 741.2713. Anal. calcd for C₄₆H₃₆N₄O₆·0.7CHCl₃: C, 68.04; H, 4.49, N, 6.80. Found: C, 67.82; H, 4.78, N, 6.82.

Macrocyclic metallohost L²Ni₂

A solution of H₄L² (21.0 mg, 28.3 μmol) in chloroform/methanol (1 : 1, 4 mL) was mixed with a solution of nickel(II) acetate tetrahydrate (21.1 mg, 84.8 μmol) in methanol (1.5 mL). After stirring for 6 h at rt, the resultant precipitates were collected on a filter to yield L²Ni₂ (14.6 mg, 15.9 μmol, 56%) as brown crystals; ¹H NMR (400 MHz, DMSO-d₆) δ 1.61 (s, 12H), 6.88 (d, J = 8.6 Hz, 4H), 7.33–7.38 (m, 4H), 7.40 (d, J = 8.6 Hz, 4H), 8.15–8.19 (m, 4H), 8.85 (s, 4H). Anal. calcd for C₄₆H₃₂N₄O₆Ni₂·3.5H₂O: C, 60.24; H, 4.29, N, 6.11. Found: C, 60.26; H, 4.14, N, 6.16.

X-ray crystallography

The intensity data were collected at 93 K on a Rigaku Mercury diffractometer (Mo Kα, λ = 0.71073 Å) or a Bruker APEX diffractometer (Cu Kα, λ = 1.54178 Å). The data were corrected for Lorentz and polarization factors, and for absorption by using semiempirical methods based on symmetry-equivalent and repeated reflections. The structures were solved by direct methods (SHELXS 97 or SIR 97)¹⁹ and refined by full-matrix least-squares on F² using SHELXL 2014.²⁰ The crystallographic data are summarized in Table 2.

Table 2 Crystallographic data

	H4L^2·0.5CHCl3·2.5MeCN	L^2Ni2·0.67H2O·0.67MeOH·1.33DMSO	[L^2Ni2Na(DMSO)2](OTf)·DMSO
Formula	C51.5H44Cl1.5N6.5O6	C49.33H44N4Ni2O8.67S1.33	C53H50F3N4NaNi2O12S4
a R 1 = ∑||Fo| − |Fc||/|Fo|; wR2 = {∑w(Fo^2 − Fc^2)^2/∑[w(Fo^2)^2]}^1/2.
Formula weight	903.10	991.72	1260.62
Crystal system	Monoclinic	Monoclinic	Monoclinic
Space group	P21/m	P21/n	Pn
a (Å)	10.9940(14)	16.3754(5)	10.7194(4)
b (Å)	23.218(3)	21.6479(6)	9.1083(3)
c (Å)	17.421(3)	19.6275(6)	26.4196(9)
α (°)	90	90	90
β (°)	99.151(4)	111.4120(10)	99.796(2)
γ (°)	90	90	90
V (Å^3)	4390.2(10)	6477.6(3)	2541.88(15)
Z	4	6	2
D calcd (g cm^−3)	1.366	1.525	1.647
μ (mm^−1)	0.178	2.217	3.239
Collected/unique reflections	41 360/7892	49 196/11 574	19 130/7753
2θmax	50.00	134.24	136.73
R int	0.0460	0.0271	0.0484
Limiting indices	−13 ≤ h ≤ 12	−19 ≤ h ≤ 19	−12 ≤ h ≤ 12
	−27 ≤ k ≤ 27	−23 ≤ k ≤ 25	−10 ≤ k ≤ 10
	−20 ≤ l ≤ 19	−23 ≤ l ≤ 23	−31 ≤ l ≤ 30
Parameters/restraints	660/53	1044/277	796/136
GOF (F^2)	1.078	1.014	1.006
R 1 (I > 2σ(I))^a	0.0647	0.0334	0.0363
R 1 (all data)^a	0.0749	0.0384	0.0472
wR2 (I > 2σ(I))^a	0.1650	0.0854	0.0735
wR2 (all data)^a	0.1804	0.0886	0.0777
CCDC No.	2448855	2448856	2448857

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Computational methods

The Gaussian 16 program²¹ was used for the DFT calculations. Geometries of the ligand (H₄L²) and the metal complexes (L²Ni₂ and [L²Ni₂Na]⁺) were optimized at the B3LYP/6-311G**²² level with Grimme's D3 dispersion correction.²³ The relative energies of the different conformations were calculated at the same level using the optimized geometries. Interaction energies were calculated by the supermolecule method at the B3LYP/6-311G** level with Grimme's D3 dispersion correction. The basis set superposition error (BSSE)²⁴ was corrected by the counterpoise method.²⁵

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for H₄L²·0.5CHCl₃·2.5MeCN, L²Ni₂·0.67H₂O·0.67MeOH·1.33DMSO, and [L²Ni₂Na(DMSO)₂](OTf)·DMSO have been deposited at the CCDC under 2448855, 2448856, and 2448857† and can be obtained from https://www.ccdc.cam.ac.uk/.

Acknowledgements

We thank Dr Kenji Yoza (Bruker AXS) for the X-ray data collection of L²Ni₂·0.67H₂O·0.67MeOH·1.33DMSO and [L²Ni₂Na(DMSO)₂](OTf)·DMSO. The research was partly supported by the Japan Society for the Promotion of Science (KAKENHI Grants JP21H05477 and JP23H04021 [Condensed Conjugation] as well as JP16H06510 [Coordination Asymmetry], JP18H03913, JP20K21206, JP23K17928 and JP23H01972) and the World Premier International Research Initiative, Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2448855–2448857. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5dt01183c

‡ H₂saloph = N,N′-disalicylidene-o-phenylenediamine.

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