Bpytrisaloph: A Triangular Platform That Spatially

May 2, 2019 - Complexes that possess multiple metals in their proximity exert useful functions such as multivalent capture and catalytic activities. H...
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Article Cite This: Inorg. Chem. 2019, 58, 7863−7872

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Bpytrisalen/Bpytrisaloph: A Triangular Platform That Spatially Arranges Different Multiple Labile Coordination Sites Takashi Nakamura, Yuto Kawashima, Eiji Nishibori, and Tatsuya Nabeshima* Graduate School of Pure and Applied Sciences and Tsukuba Research Center for Energy Materials Science (TREMS), University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan

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ABSTRACT: Complexes that possess multiple metals in their proximity exert useful functions such as multivalent capture and catalytic activities. However, it is often challenging to arrange multiple metals with regular distances and to maintain the reactive coordination sites. We have now designed and synthesized organic macrocyclic ligands, bpytrisalen and bpytrisaloph, as platforms to spatially arrange different kinds of metals and their coordination sites. Metals coordinating to the bpy N2 units of the triangular ligands are assembled in the cavity with their labile coordination sites directed inward. Meanwhile, the metals at the salen/saloph N2O2 units have axial coordination sites that are vertically pointing out of the triangle. As a result, planar homo- and heterohexanuclear complexes with different kinds of coordination sites are obtained. Furthermore, a double-decker structure was selectively constructed upon coordinating a ditopic linker ligand to the axial coordination sites of the salen units, which shows the orthogonality of the coordination sites at the bpy and salen units. The consistency in the array of metal centers and their coordination sites irrespective of the elements is a desirable feature in the pursuit of applications for the multinuclear complexes.

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particular, trisaloph, a triangular macrocycle first reported in 2001,32 is formed by the condensation of the 1,4-diformyl-2,3dihydroxybenzene linker and o-phenylenediamine.32−48 The trisaloph has three saloph N2O2 units at the vertex and a central cavity surrounded by six phenolic oxygen atoms. The diameter of the cavity is 5.6−5.8 Å (diagonal O−O),32 which is suitable for the capture of one metal ion, such as a lanthanide33,37,38,40 and an alkali metal,43 by trinuclear trisaloph complexes. The O6 cavity of the trisaloph complexes was also utilized to capture the M3 or M4 cluster (M = Zn or Cd) assisted by external bridging ligands such as acetates, and it was reported that subtle factors (metal, ligand, etc.) changed the geometry of the clusters dynamically.33,36,39,44−48 With these backgrounds, we considered that the direct installation of strong chelating moiety into expanded, larger trisalen/trisaloph frameworks would realize the rigid fixation of multiple metals in its cavity. Such a macrocycle should become a versatile platform that provides multiple labile coordination sites on metals in an ordered manner. Furthermore, a heterometallic arrangement could be also achieved by utilizing the difference in geometry and hardness/softness between the newly introduced chelating unit and the salen/saloph N2O2 unit. Compared to the heteronuclear complexes formed by the multicomponent self-assembly of metal ions and organic ligands,49−57 the rigid covalently bonded macrocyclic scaffold

INTRODUCTION Complexes that possess multiple metals in their proximity exert useful functions. Such complexes can bind small molecules via multipoint coordination bonds and serve as synthetic receptors that display unique selectivity1,2 and responsiveness to stimuli.3−5 Furthermore, the homo- and heteromultinuclear catalysts6−11 allow reactions to proceed efficiently by coordinating the substrate to more than one metal center7,9,11 and by the multistep redox.6,10 To regulate and understand their performances, it is essential to control not only the numbers and positions of the metals but also those of the coordination sites on the metals at which a target substrate binds.12 However, it is often challenging to arrange multiple metals with regular distances and to maintain the reactive coordination sites. Difficulty in controlling the nuclearity and coordination geometries is seen, for instance, in the cluster complexes whose structures are affected by subtle synthetic conditions and modifications of the external ligands.13,14 More challenging is the spatial arrangement of different kinds of metals and their labile coordination sites, which can lead to cooperative and synergistic functions.15,16 Such an arrangement can be achieved by the judicious design of a large organic ligand that fixes the metals at the desired sites and that is rigid enough to determine the relative positions of the metal coordination sites without structural fluctuation. The formation of an imine bond between a diamine derivative and a salicylaldehyde analogue to form the N2O2 coordination units has been utilized to construct a rigid covalent scaffold and its multinuclear complexes.15−31 In © 2019 American Chemical Society

Received: February 25, 2019 Published: May 2, 2019 7863

DOI: 10.1021/acs.inorgchem.9b00549 Inorg. Chem. 2019, 58, 7863−7872

Article

Inorganic Chemistry is suitable for maintaining the relative positions of metal centers irrespective of the kind of metals and counteranions. We have now designed organic macrocyclic ligands, H61a and H61b, as a platform for spatially arranging different kinds of metals and their coordination sites (Figure 1). These

Figure 1. Chemical structure and schematic presentation of a macrocyclic hexanuclear complex [1M6Xn], which possesses two kinds of spatially arranged labile coordination sites.

triangular macrocycles were synthesized by Schiff base formation of a novel 2,2′-bipyridyl (bpy) linker H22 and a diamine 3 (Figure 3). H61a/H61b possesses three bpy units at the edges and three salen/saloph units at the vertices. On the basis of this structural feature, these macrocycles are called bpytrisalen (H61a) and bpytrisaloph (H61b). Metals coordinating to the bpy N2 units are assembled in the cavity with their labile coordination sites directed inward. Meanwhile, the metals at the salen/saloph N2O2 units have axial coordination sites that are vertically pointing out of the triangle. As a result, planar homo- and heterohexanuclear complexes with different kinds of coordination sites are obtained (Figures 5 and 6). Furthermore, a double-decker structure 5 was constructed upon selectively coordinating a ditopic linker ligand 4 to the axial coordination sites of the salen-Zn units, which shows the orthogonality of the coordination sites at the bpy and salen units (Figure 7).

Figure 2. (A) Synthesis of bpy linker H 22 (Mes = 2,4,6trimethylphenyl). Conditions: (a) (1) lithium N,N,N′-trimethylethylenediamide, DME, −50 °C, 30 min; (2) n-BuLi, −50 °C, 2 h; (3) I2, −72 °C, 1.5 h; (b) ethylene glycol, p-TsOH·H2O, CH(OEt)3, toluene, room temperature, 18 h, 37% (two steps from 6); (c) 2,4,6trimethylphenylboronic acid, Pd(PPh3)4, Cs2CO3, DMF, 80 °C, 22 h, 93%; (d) Ni(cod)2, 1,5-COD, 2,2′-bpy, DMF, room temperature, 22 h, 86%; (e) HCl, THF/H2O, reflux, 1 h, 97%; (f) BBr3, CH2Cl2, room temperature, 1 h, 97%. (B) Equilibrium of H22 in a CDCl3 solution between 6,6′-dihydroxybipyridine H22 and unsymmetrical hydroxypyridine-pyridone H22′. (C) Intermolecular hydrogen bonds of 2,2′bipyridone H22″ in the crystal.

Next, the homocoupling reaction using the Ni(0) catalyst was conducted to produce bipyridine derivative 10 in 86% yield. The subsequent deprotection reactions of the formyl and hydroxy groups yielded bipyridine linker H22. H22 was characterized by 1H NMR, 13C NMR, two-dimensional (2D) NMR (NOESY, HSQC, HMBC), HRMS (ESI), IR, and a single-crystal X-ray diffraction analysis (Figure 4A and Figures S9−S14). 6,6′-Dihydroxy-2,2′-bipyridine60−62 is part of an important class of organic ligands that adopt multiple redox and protonation states, 63−66 and its equilibrium with the bipyridone form was reported.63,64 Here, we have found an interesting tautomerization behavior of the 5,5′-diformyl-6,6′dihydroxy-4,4′-dimesityl-2,2′-bipyridine H22 among three isomers. The 1H NMR spectrum of H22 in CDCl3 was broad at 298 K but became sharp and showed two different sets of signals at 225 K (Figure 4A). The 1H NMR spectrum at 225 K revealed that H22 is in equilibrium between a bipyridine form H22 and an unsymmetric pyridine-pyridone form H22′ (Figure 2B). The ratio of these two tautomers (bipyridine H22:pyridine-pyridone H22′) was 36:64 (225 K, CDCl3). The preference for unsymmetric pyridine-pyridone H22′ is explained by the intramolecular hydrogen bond between the N−H of the pyridone and the N of the adjacent pyridine. A single crystal suitable for an X-ray diffraction analysis was obtained by the slow diffusion of hexane into a chloroform solution of H22. In contrast to the equilibrium in the chloroform solution, bpy linker H22 took a bipyridone form H22″ in the crystalline state (Figure 2C). The pyridone form

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RESULTS AND DISCUSSION Bpy Linker H22. In this study, the targeted bpytrisalen H61a/bpytrisaloph H61b ligands were designed to be synthesized by the condensation of three units each of a bpy linker H22 (Figure 2) and a diamine 3a [(1R,2R)-(−)-cyclohexanediamine]/3b (o-phenylenediamine). Bpy linker H22 is a 2,2′-bipyridine derivative with two hydroxy groups at the 6 positions, two formyl groups at the 5 positions, and two mesityl groups at the 4 positions. The two mesityl groups were introduced to increase the solubility of bpy linker H22 and the following macrocyclic products. Meanwhile, direct introduction of the formyl and hydroxy groups onto the pyridine ring makes it possible to precisely define the relative positions of the two coordination units, i.e., the bpy N2 unit and the salen/ saloph N2O2 unit. Bipyridine H22 was synthesized as in Figure 2A (see also Schemes S1−S5 and Figures S1−S14). The 4 position of 6chloro-3-formyl-2-methoxypyridine (6) was selectively iodinated via deprotonative lithiation to produce 758 using lithium N,N,N′-trimethylethylenediamide according to a reported procedure.59 The iodopyridine derivative 8 with 1,3-dioxolane was obtained in 37% yield in two steps from 6. A mesityl group was introduced by the Suzuki−Miyaura coupling reaction with 2,4,6-trimethylphenylboronic acid, which gave 9 in 93% yield. 7864

DOI: 10.1021/acs.inorgchem.9b00549 Inorg. Chem. 2019, 58, 7863−7872

Article

Inorganic Chemistry was confirmed by the short carbon−oxygen distance at the 6 position [1.238(5) Å], which is a typical bond length for a CO bond of a pyridone.67,68 Furthermore, H22″ was found to form the walls of the channel structure in the crystalline state via one-dimensional intermolecular hydrogen bonds between the pyridone rings of the neighboring H22″ (Figure S14). Macrocyclic Ligand Bpytrisalen H61a. Macrocyclic bpytrisalen ligand H61a was synthesized by mixing H22 with (1R,2R)-(−)-cyclohexanediamine (3a) in CHCl3 at 60 °C for 1 h (Figure 3A). Targeted cyclic trimer H61a was obtained in

Figure 3. (A) Synthesis of bpytrisalen ligand H61a via 3:3 macrocyclization reaction of bpy linker H22 and diamine 3a (Mes = 2,4,6-trimethylphenyl). (B) Synthesis of hexanuclear zinc complexes [1Zn6Xn]. Conditions: (a) Zn(OAc)2·2H2O, 1:1 (v/v) CHCl3/ CH3OH, room, temperature, 30 min; (b) Zn(OAc)2·2H2O, Zn(OTf)2, 1:1 (v/v) CHCl3/CH3OH, room temperature, 1 h (Tf = CF3SO2); (c) Zn(OAc)2·2H2O, 5:1 (v/v) CHCl3/CH3OH, 60 °C, 20 h.

Figure 4. Characterization of macrocyclic ligand H61a and its zinc complex 1aZn6(OAc)6. (A−C) 1H NMR spectra (600 MHz, CDCl3). (A) Bpy linker H22 (225 K). Red circles denote signals of 6,6′dihydroxybipyridine H22, and blue circles denote those of unsymmetrical hydroxypyridine-pyridone H22′. See Figure S9 for the complete assignment. (B and C) Macrocyclic ligand H61a and zinc complex 1aZn6(OAc)6, respectively (298 K). See Figure 3A for the assignment of the NMR signals. (D) ESI-TOF mass spectrum of H61a (positive, CH3CN). The simulated and observed isotope patterns of [H61a + 2H]2+ are shown on the right. (E and F) Structures of bpytrisalen ligand H61a obtained by PM6 calculation. The red arrow indicates a pair of transannular 1H−1H atoms (k and g′) between which an ROE cross-peak was observed (Figure S18), which supports the twisted macrocyclic structure with the anti-periplanar conformation in the bipyridyl unit. (G and H) Absorption and circular dichroism spectra, respectively, of H61a (black) and 1aZn6(OAc)6 (red) (10 μM, CHCl3).

85% yield. The high yield of H61a can be ascribed to the reversibility of the imine bond21 and to the rigidity of the macrocyclic framework. Macrocycle H61a showed good solubility in common organic solvents such as CHCl3, CH2Cl2, AcOEt, and THF. The 1H NMR spectrum of H61a is shown in Figure 4B. A singlet peak assigned to imine bond proton f was observed at 7.85 ppm. In the far-downfield region, a peak attributed to hydroxy proton k was observed at 14.05 ppm, which is characteristic of a salen unit possessing an intramolecular O−H···NC hydrogen bond. The formation of a single macrocyclic Schiff base with time-averaged D3 symmetry was also supported by the 13C NMR and 2D NMR measurements (Figures S16−S20). Furthermore, the ESI-TOF mass spectrum showed that the product is a pure cyclic timer and no other oligomers or larger macrocycles were detected (Figure 4D). Intriguingly, it was found that H61a took a twisted structure with bipyridine in the anti-periplanar conformation (Figure 4E,F). The conformation of macrocycle H61a was investigated by 1H NMR, ROESY, and PM6 calculations. One of the proton signals of the mesityl groups, g′, was observed at 1.64 ppm, in the upfield region for an aromatic methyl group. This proton experiences a shielding effect from the macrocyclic

framework, because the methyl group was positioned inside of the macrocycle (Figure 4F). Other structural information was obtained from proton j (3 position of bipyridine), which was observed relatively downfield at 7.84 ppm. This suggests the hydrogen bonding of proton j to the neighboring nitrogen, thus indicating the anti-periplanar conformation of the bipyridyl. Furthermore, a transannular 1H−1H ROE was observed between inner methyl proton g′ and hydroxy proton k in the ROESY measurement (Figure S18), which also 7865

DOI: 10.1021/acs.inorgchem.9b00549 Inorg. Chem. 2019, 58, 7863−7872

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Inorganic Chemistry

Figure 5. Molecular structure of [1bZn6(H2O)6(OAc)4]2+, determined by single-crystal X-ray crystallography. Solvents, hydrogens, and Mes groups have been omitted for the sake of clarity. (A and B) Ellipsoidal model (50% probability). (C) Least-squares plane (red) defined by six Zn atoms. (D) [1bZn6(H2O)6(OAc)4]2+. (E) Labile coordination sites (magenta) of Zn that were not occupied by macrocyclic chelating ligand 1b6− (the ligands coordinating to Zn were omitted except for 1b6−). The Zn atoms are described by a space-filling model, and the other atoms are shown as sticks. (F) Distances between zinc centers in angstroms.

conformation of macrocycle 1a6− with bipyridines in a synperiplanar conformation. A titration experiment with Zn(OAc)2 against a CDCl3 solution of ligand H61a was conducted to investigate whether the complexation to the bpy and salen units occurs in a stepwise manner or concurrently. As a result, only metal-free ligand H61a and hexanuclear complex 1aZn6(OAc)6 were present during the titration, while the other intermediate species were not observed in the 1H NMR spectra (Figure S25). Thus, the complexation reaction proceeds in a highly cooperative manner. It is suggested that the first metal binding resulted in the planarization of the initially twisted H61a scaffold, which alters the conformation of the other bpy and salen chelating units in favor of the following complexation. A red shift in the absorption was observed for H61a after complexation to form 1aZn6(OAc)6 [for H61a, λabs = 360 nm; for 1aZn6(OAc)6, λabs = 418 nm (CHCl3)], which can be explained by the planarization of the macrocycle and the intraligand charge transfer (Figure 4G). In contrast, the emission showed a blue shift upon zinc complexation [for H61a, λem = 495 nm (ΦF = 0.16); for 1aZn6(OAc)6, λem = 470 nm (ΦF = 0.12)]. The relatively red-shifted emission of H61a might be ascribed to the excited-state intramolecular proton transfer (ESIPT), that is, the photoinduced tautomerization from the phenol to keto form by the transfer of the hydroxyl proton to the imine nitrogen.69 The intensity of the circular dichroism of H61a is remarkably high, with Δε = 332 M−1 cm−1 at 355 nm and Δε = −608 M−1 cm−1 at 386 nm (Figure 4H). This large negative Cotton effect is explained by the twisted structure of the metal-free ligand. The CD intensity showed a decrease upon zinc complexation, which reflects the planarization of the macrocycle.

supports the twisted macrocyclic structure obtained by a PM6 calculation (the red arrow in Figure 4E,F). Hexanuclear Zinc Complexes 1aZn6(OAc)6. Zinc was chosen as a metal to introduce into H61a for its labile coordination bonds, which is an advantage for pursuing the molecular recognition and catalytic ability. Macrocyclic hexanuclear zinc complex 1aZn6(OAc)6 was obtained in 90% yield by adding 6 equiv of Zn(OAc)2·2H2O to ligand H61a in a chloroform/methanol mixed solution, followed by recrystallization (Figure 3B). Elemental analysis of the product indicated that the product is the acetate salt of the hexanuclear Zn complex with 11 H2O molecules as aqua ligands and noncoordinating solvents {Calcd for C120H148N12O29Zn6 [1aZn6(H2O)11(OAc)6]: C, 55.12; H, 5.71; N, 6.43. Found: C, 55.26; H, 5.52; N, 6.53}. An ESI-TOF mass measurement of a methanol solution of 1aZn6(OAc)6 showed a set of strong signals assigned to [1aZn6(CH3O)3]3+ (m/z 718.5), thus supporting the formation of the targeted Zn complex (Figure S24). The conformational change in the macrocyclic scaffold of bpytrisalen H61a upon metal complexation was investigated by 1 H NMR measurements (Figure 4B,C). In the 1H NMR spectrum of 1aZn6(OAc)6, the chemical shift differences of the protons in the mesityl groups [Δδ values of 0.03 ppm (g/g′) and