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Sep 27, 2017 - Competitive Transmetalation of First-Row Transition-Metal Ions between Trinuclear Triple-Stranded Side-by-Side Helicates. Barun Jana,. ...
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Competitive Transmetalation of First-Row Transition-Metal Ions between Trinuclear Triple-Stranded Side-by-Side Helicates Barun Jana,† Luca Cera,‡,§ Bidyut Akhuli,† Sourenjit Naskar,† Christoph A. Schalley,*,‡ and Pradyut Ghosh*,† †

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Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata 700032, India ‡ Institut für Chemie und Biochemie der Freien Universität Berlin, Takustr. 3, 14195 Berlin, Germany S Supporting Information *

ABSTRACT: A hybrid tris-bidentate neutral ligand (L) composed of a central 2,2′-bipyridine and two terminal triazolyl-pyridine chelating units connected by methylene spacers is employed to synthesize trinuclear triple-stranded side-by-side helicates of first-row transition-metal(II) ions. Three such new homometallic helicates L3M3(OTf)6 [ M = Cu2+ (4); Ni2+ (5); Co2+ (6)], along with our recently reported helicates L3Fe3(OTf)6 (1), L3Zn3(OTf)6 (2), and L3Fe2Zn(OTf)6 (3) are taken into consideration for competitive formation and transmetalation studies. Single-crystal X-ray structures of L3Cu3(OTf)6 (4) and L3Ni3(OTf)6 (5) show the formation of trinuclear triple-stranded side-by-side helicates with alternating Λ and Δ chiralities at the metal ions as earlier observed in cases of L3Fe3(OTf)6 (1), L3Zn3(OTf)6 (2), and L3Fe2Zn(OTf)6 (3). ESI-FTICR mass spectrometry and UV−vis spectroscopy studies show that helicates L3Fe3(OTf)6 (1), L3Zn3(OTf)6 (2), L3Fe2Zn(OTf)6 (3), and L3Co3(OTf)6 (6) can easily be transmetalated to helicate L3Cu3(OTf)6 (4) in the presence of Cu(OTf)2. On the other hand, only a trace amount of heterometallic helicate L3Ni2Cu(OTf)6 forms even after several days, when Cu(OTf)2 is added to a the solution of homometallic helicate L3Ni3(OTf)6 (5). Further, we have demonstrated the formation of a heterometallic helicate L3Ni2Co(OTf)6 (7) from a 1:1:1 reaction mixture of L, Ni(OTf)2, and Co(OTf)2, which can also be prepared from homometallic helicate L3Co3(OTf)6 (6) by transmetalation with Ni(OTf)2.

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INTRODUCTION Metal-assisted self-assembly of multidentate ligands through coordinative interactions is a powerful approach for the construction of discrete and well-defined three-dimensional structures.1−7 Supramolecular architectures, such as helicates,8−16 grids,17 and wheels18 have been prepared utilizing this approach, and interesting structure-specific properties, such as chirality, 19−24 magnetism, 25−29 redox behavior, and applications in molecular recognition,30−33 have been studied. Among them metallo-helicates are particularly important as sensors,34,35 DNA binders,21,36,37 and as anticancer agents.38,39 Several bi- and trinuclear homo- and heterometallic helicates for both transition metals40−51 and f-block elements32,34,52−57 have been reported following the isolation of the first homometallic helicates by Lehn et al. in 1987.58 In addition, there are mesocates composed of an even number of metal centers with alternating chirality having a mirror plane at the middle.59−62 In those cases, in which an odd number of metal centers with alternating chirality are present, the ligands bind in a parallel zigzag fashion instead of wrapping around the helix.8,63 Such assemblies are known as side-by-side helicates. Recently, we have introduced a neutral hybrid tris-bidentate © 2017 American Chemical Society

ligand (L), in which a central 2,2′-bipyridine and two terminal triazolyl-pyridine chelating coordination units are connected by methylene spacers. This ligand has been utilized for the construction of homometallic trinuclear triple-stranded side-byside helicates with general formula L3M3(OTf)6 [M = Fe (1), Zn (2)]. Also, a heterometallic helicate L3Fe2Zn(OTf)6 (3) selectively formed, which was found to be the thermodynamically most stable among these three helicates.13 In fact, a large number of coordination equilibria are possible in such a selfassembly process; however, the system finally converges to the most stable structure. Therefore, many different processes occur simultaneously in the reaction mixture. This makes it difficult to follow using most conventional spectroscopic techniques. However, the understanding of this multicomponent metal−ligand self-assembly is very important in constructing targeted metallo-supramolecular compounds. At this juncture, we include other competitive metal ions (Cu2+, Ni2+, and Co2+) along with Fe2+ and Zn2+ for self-assembly, selectivity and transmetalation studies of L. Herein, we Received: August 4, 2017 Published: September 27, 2017 12505

DOI: 10.1021/acs.inorgchem.7b01980 Inorg. Chem. 2017, 56, 12505−12513

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Inorganic Chemistry demonstrate the selective formation of trinuclear triplestranded side-by-side metallo-helicates of L individually by Fe2+, Cu2+, Ni2+, Co2+, and Zn2+. In addition, competitive transmetalation reactions are carried out to understand their formation mechanism and relative thermodynamic stability.

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RESULTS AND DISCUSSION Trinuclear Triple-Stranded Side-by-Side Helicates of Ligand L. In a procedure similar to that of our recently reported synthesis of helicates L3Fe3(OTf)6 (1), L3Zn3(OTf)6 (2), and L3Fe2Zn(OTf)6 (3), complexes of Cu2+ (4), Ni2+ (5), and Co2+ (6) with ligand L are synthesized (Scheme 1). Clear Scheme 1. Synthesis of Complexes 1−6 Figure 1. ESI-FTICR mass spectra of 20 μM methanol solutions of (a) 4, (b) 5, and (c) 6; Insets: calculated and experimental isotope patterns for the triply charged ions.

nonplanar C2-symmetric tris-bidentate ligands around the three octahedral M2+ centers lead to a racemic mixture of heterochiral triple helicates with alternating Λ and Δ chiralities (Figure 2).

green (Cu2+, 4) and light yellow (Ni2+, 5; Co2+, 6) solutions are obtained after 4 to 12 h. The synthesized compounds are purified by crystallization either from a 3:1 methanol-benzene solvent mixture (4 and 5) or from a concentrated solution of methanol (6). The CHN analyses of the crystallized solids indicate the presence of metal and ligand in 1:1 ratio. NMR spectroscopic data of these complexes cannot be obtained because of the paramagnetic nature of the metal ions in this particular geometry. UV−vis spectra of isolated 4−6 show almost identical absorption band with ∼λmax = 300 nm, that is, for the ligand-centered n−π* transitions (Figure 1S in SI). In addition, at higher concentration, complex 4 shows a broad d− d absorption band with ∼λmax = 690 nm, whereas no such absorption band is observed for complexes 5 and 6 (Figure 2S in SI). Electrospray-ionization Fourier-transform ion cyclotron resonance (ESI-FTICR) mass spectrometry clearly confirms the formation of complexes containing metal ions and ligand in 3:3 ratios as observed in our earlier reported Fe2+ and Zn2+ complexes of ligand L. All three complexes generate cations by the loss of two or more counterions and appear in different charge states as [L3M3(OTf)4]2+ [m/z: 1101.6 (4, Figure 1a), 1093.1 (5, Figure 1b), 1094.6 (6, Figure 1c)], [L3M3(OTf)3]3+ [m/z: 684.7 (4), 679.1 (5), 680.1 (6)] and [L3M3(OTf)2]4+ [m/z: 475.8 (4), 472.1(5), 472.8 (6)]. Experimental isotope patterns match nicely with those calculated on the basis of natural abundance (Figure 1). Single-crystal X-ray diffraction studies of the complexes 4 and 5 show the formation of trinuclear triple-stranded side-by-side helicates resembling our recently reported helicates 1−3. The metal ions are located in a distorted octahedral geometry and coordinated with either three bipyridine units (central metal ion) or with three triazolyl-pyridine units (terminal metal ions). Six triflate counterions balance the six positive charges of the helicate-framework. The side-by-side binding mode of the three

Figure 2. Single-crystal X-ray structure of cation of L3Cu3(OTf)6 (4) as a representative example of trinuclear triple-stranded side-by-side metallo-helicates, generated via self-assembly of the ligand L and Cu2+ ions. Hydrogen atoms and counterions are not shown for clarity.

Helicates 4 and 5 crystallize either in monoclinic P2/c (4) or in orthorhombic Pbcn (5) space groups (Table 3). In helicate 4, the internuclear distances of Cu1−Cu2 (7.708 Å) and Cu2− Cu2 (15.415 Å) are comparable with the internuclear distances of helicates 1 (15.33 Å) and 2 (15.39 Å) with the same ligand (Figure 23S in SI).13 The average metal−ligand bond distance for the terminal Cu2 atoms in 4 are Cu2−N triazole = 1.92(4) Å and Cu2−N pyridine = 1.99(4) Å, while one obtains Cu1−N5 = 1.97(4) Å, Cu1−N10 = 1.96(4) Å, and Cu1−N11 = 1.98(4) Å for the central Cu1 (Table 1S in SI). These metal−ligand bond lengths are comparable with other literature known complexes of Cu(II) with triazolyl-pyridine and bipyridine units.64−67 Compound 5 (Figure 23S in SI) also has a helical structure, similar to 1−4.13 Here, the internuclear distances of Ni1−Ni2 and Ni2−Ni2 are of 7.584 and 15.167 Å respectively (Figure 23S in SI), which is only marginally shorter than the 12506

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Inorganic Chemistry Table 1. Average M−N Bond Lengths (Å) at Different Coordination Centers in Complexes 1−5 central entrya Fe3L3(OTf)6 (1) Zn3L3(OTf)6 (2) Fe2Zn(OTf)6 (3) Cu3L3(PF6)6 (4) Ni3L3(OTf)6 (5) a

terminal

M−N (bipyridine) Fe−N Zn−N Zn−N Cu−N Ni−N

1.971 2.158 2.132 2.018 2.076

M−N (pyridine) Fe−N Zn−N Fe−N Cu−N Ni−N

M−N (triazolyl) 1.995 2.163 2.010 2.054 2.110

Fe−N Zn−N Fe−N Cu−N Ni−N

1.922 2.144 1.936 2.143 2.061

A table with all the separate bond lengths is added in the Supporting Information (Table 3S).

internuclear distances of helicates 1−4. The average Ni−N bond distance of Ni1−N bipyridine, Ni2−N pyridine, Ni2−N triazole is comparable to other literature known complexes of Ni(II) with triazolyl-pyridine and bipyridine units (Table 2S in SI).68−70 For comparison, the average metal−nitrogen bond lengths at different coordination centers of complexes 1− 5 are summarized in Table 1, which are in good agreement with the ionic radii of metal ions. Mechanistic Studies. According to the Irving−Williams series, in an octahedral ligand environment, the thermodynamic stability of Cu(II) complex is highest among the first-row transition-metal ion complexes. Thus, ESI mass spectrometry was used to follow the transmetalation reactions between helicates 1−6. Further, in-depth time-dependent ESI-FTICR mass spectrometry analysis is done to follow the reactions. Selectivity Studies of Ligand L toward Cu2+ Metal Ion over Fe2+ and Zn2+ Metal Ion. When a 2:2:1 mixture of Cu2+, Fe2+, and L is allowed to react in acetonitrile, the thermodynamically more favorable Cu-helicate (4) is formed within 1 min. The ESI-FTICR mass spectra do not show any sign for the formation of helicate L3Fe3(OTf)6 (1) (Figure 3S in SI). The higher thermodynamic stability of L3Cu3(OTf)6 (4) is further confirmed, when L3Fe3(OTf)6 (1) is fully converted to L3Cu3(OTf)6 (4) upon addition of three equivalents of Cu(OTf)2. However, this process is slow, and it requires a period of ca. 8 days at room temperature for completion (Figure 3). Further, the transmetalation reaction studied by UV−vis absorption spectroscopy shows a gradual diminution of MLCT absorption band of L3Fe3(OTf)6 (1) at 422 nm (Figure 4a). The above set of reactions performed with Zn2+ instead of Fe2+ also establishes the higher thermodynamic stability of L3Cu3(OTf)6 (4) over homometallic helicate L3Zn3(OTf)6 (2) (Figure 5S in SI). Expectedly, in the case of Zn2+, the transmetalation reaction occurs much faster than in the case of Fe2+ and transmetalation completes within 35 min. So, unlike Fe2+, which only transmetallates the two terminal Zn2+ ions to give heterometallic helicate L3Fe2Zn(OTf)6 (3) at higher temperature,13 Cu2+ can transmetallate all three Zn2+ ions of L3Zn3(OTf)6 (2) at room temperature. The time-dependent UV−vis studies of the Zn2+-to-Cu2+ transmetalation shows the characteristics d-d absorption band of L3Cu3(OTf)6 (4) at ∼ λmax = 690 nm increases over time, which is at a relatively higher concentration (∼1 × 10−1 M solution) (Figure 4b). Our subsequent transmetalation reaction between heterometallic helicate L3Fe2Zn(OTf)6, (3), and Cu(OTf)2 also leads to the formation of helicate L3Cu3(OTf)6 (4) within 26 h at room temperature (Figure 5). Isotopic patterns analysis reveals the presence of only three species, that is, L3Fe2Zn(OTf)6, (3), L3FeZnCu(OTf)6, and L3Cu3(OTf)6 (4), during the whole reaction time, while L3Fe2Cu(OTf)6, L3Cu2Zn(OTf)6, and L3Cu2Fe(OTf)6 are not observed with significant abundance (Figure 6S in SI). Furthermore, the final product, that is,

Figure 3. Time-dependent changes in the ESI-FTICR mass spectra of a mixture of L3Fe3(OTf)6 (1) and Cu(OTf)2 (1:3) solution [20 μM in L3Fe3(OTf)6] in acetonitrile. (a) At the beginning; (b) the gradual changes of the composition as monitored for the doubly charged helicate ions; (c) the final mass spectrum recorded after 8 days.

L3Cu3(OTf)6 (4), becomes visible after ca. 1 h, and its intensity rises above the intensity of L3FeZnCu(OTf)6 after 5 h (Figure 5b). This observation indicates that the first event is an iron− copper exchange at one of the ends of the helicate, which generates L3FeZnCu(OTf)6 at m/z = 1098.58. Then, the insertion of a second Cu2+ ion, which has a lower coordination number than Fe2+ ion, in the terminal position, may allow a fast exchange of the central Zn2+ ion, as the whole helicate scaffold becomes more flexible. Time-dependent UV−vis studies of this transmetalation reaction also shows the gradual decrease of the absorption band at ∼λmax = 422 nm, which is characteristic of the Fe2+-triazolyl-pyridine MLCT transition of L3Fe2Zn(OTf)6 (3) (Figure 4c). The above studies unambiguously establish that Cu-helicate (4) is also thermodynamically more stable than Fe2Zn-helicate (3). Selectivity Studies of Ligand L toward Cu2+ Metal Ion over Co2+ and Ni2+ Metal Ion. Further, a 1:3 reaction between L3Co3(OTf)6 (6) and Cu(OTf)2 unambiguously establishes the higher thermodynamic stability of homometallic helicate L3Cu3(OTf)6 (4) over its Co2+ analogue (6) (Figure 6). This transmetalation reaction gets almost completes within 1 h. Surprisingly, almost no transmetalation reaction occurs when helicate L3Ni3(OTf)6 (5) is treated with Cu(OTf)2. The isotopic distribution pattern at m/z = 1095.6 shows only a partial conversion of L3Ni3(OTf)6 (5) to L3Ni2Cu(OTf)6 as the equilibrium state after 78 h (Figure 7S in SI). Very slow or no 12507

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Inorganic Chemistry

Figure 4. Time-dependent changes in the UV−vis spectra upon addition of three equivalents Cu(OTf)2 to the solution of (a) L3Fe3(OTf)6 (4); (b) L3Zn3(OTf)6 (5) (∼1 × 10−1 M solution) at room temperature; (c) L3Fe2Zn(OTf)6 (6) in acetonitrile at room temperature.

lead to any transmetalation product even after 24 h at 65 °C (Figure 8S in SI). Synthesis of Heterometallic Helicate L3Ni2Co(OTf)6 (7). The hybrid nature of the ligand L was established from the fact that it selectively chooses Fe2+ and Zn2+ metal ion for the triazolyl-pyridine and bipyridine chelating units respectively from a mixture of Fe2+ and Zn2+ to form L3Fe2Zn(OTf)6 (3), the first trinuclear triple-stranded side-by-side heterometallic helicate.13 To further establish its true hybrid nature, ligand L is treated with a mixture of Co2+ and Ni2+ metal ions. ESI-FTICR mass spectrometry analysis of the solution shows presence of [L 3Ni2Co(OTf) 4]2+, [L3Ni2Co(OTf)3 ]3+, and [L 3Ni2 Co(OTf)2]4+ molecular ion peaks at m/z = 1093.6, 679.4, and 472.3, respectively, after 12 h (Figure 7). Peaks at m/z = 435.1 Figure 5. Time-dependent changes in the ESI-FTICR mass spectra of a mixture of L3Fe2Zn(OTf)6 (3) and Cu(OTf)2 (1:3) in acetonitrile [20 μM in L3Fe2Zn(OTf)6], (a) after 0 min; (b) the gradual changes of the composition as monitored for the doubly charged helicate ions; (c) the final mass spectrum after 26 h.

Figure 7. ESI-FTICR-MS spectra of 20 μM methanol solutions of 7; insets: calculated and experimental isotope patterns for the triply charged ions.

and m/z = 629.4 correspond to the molecular ions [L3Ni2Co(OTf)]4+ and [L3Ni2Co(OTf)2]3+, respectively, and they are generated via one-electron reduction and single-ligand-loss processes occurring at the ESI needle tip of the mass spectrometer. Such reactivity has also been observed in the case of helicates having different metal centers. Timedependent ESI-FTICR mass spectrometry shows that initially L3Co3(OTf)6 (6) is formed as the kinetically controlled product that via stepwise Co2+-to-Ni2+ transmetalation of the terminal two Co2+ metal ions generates the thermodynamically controlled product L3Ni2Co(OTf)6 (7) (Figure 9S in SI). This L3Co3(OTf)6 (6) to L3Ni2Co(OTf)6 (7) conversion reaction is further established via transmetalation reaction between L3Co3(OTf)6 (6) and Ni(OTf)2 (Figure 10S in SI). However, the reverse reaction between L3Ni3(OTf)6 (5) and Co(OTf)2 does not lead to the formation of heterometallic helicate L3Ni2Co(OTf)6 (7) . This may be due to some structural barrier to replace the central Co2+ metal ion. Unambiguous spectroscopic discrimination of Ni2+ and Co2+ metal centers in 7 is difficult. However, by comparing the structural features of

Figure 6. Time-dependent changes in the ESI-FTICR mass spectra of a mixture of L3Co3(OTf)6 (6) and Cu(OTf)2 (1:3) solution [20 μM in L3Co3(OTf)6] in acetonitrile (a) after 10 s; (b) the gradual changes of the composition as monitored for the doubly charged helicate ions; (c) the final mass spectrum after 16 h at room temperature.

transmetalation reaction indicates comparable stabilities of the two helicates. As a proof of concept, additional control reaction between L3Ni3(OTf)6 (5) and Fe(OTf)2 in 1:3 ratios does not 12508

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Inorganic Chemistry

FESEM Studies of the Ligand and the Self-Assembled Helicates. Single-crystal X-ray diffraction and formation/ transmetalation mechanism studies establish that all the firstrow transition-metal ions form discrete trinuclear triplestranded side-by-side helical assembly with ligand L, among which the Cu-helicate is the thermodynamically most stable. Such discrete supramolecular assemblies may further interact with each other in solution through different supramolecular interactions that may lead to interesting macroscopic properties. To, observe any such macroscopic behavior, FESEM studies are carried out for ligand L (5 × 10−3 M in DMF) and for the Cu-helicates (4.2 × 10−3 M in acetonitrile). Formation of morphologically pure nanosheets is observed in case of the free ligand which may be generated via π−π stacking interaction among the pyridine and triazole units present in the ligand (Figure 8a). Interestingly, the Cu-helicate does not

heterometallic helicate L3Fe2Zn(OTf)6 (3) with 7, we assume that the Ni2+ centers are coordinated to the terminal triazolylpyridine units and that the Co2+ is positioned at the central bipyridine unit. Selective formation of heterohelicates 3 and 7 establish that the ligand L can be utilized for the synthesis of large number of such heterometallic complexes with new properties. At this juncture, it is important to mention that CrErCr and CrYbCr heterometallic helicates generated upon utilizing both d-block and f-block elements have shown interesting properties for applications in nonlinear optics (NLO),71 in energy upconversion processes,71,72 and NIRbased materials.73 Compound 7 was crystallized from a 3:1 benzene-methanol solvent mixture. The isolated crystals are analytically pure as observed in the experimental CHN analysis values, which closely matched the calculated data. Unfortunately, all attempts to obtain a molecular structure of this complex via single-crystal X-ray diffraction studies failed due to very weak diffraction of the isolated crystals. Transmission electron microscopy-energy dispersive X-ray (TEM-EDX) experiments support the presence of both Ni and Co in the isolated crystals (Figure 11S in SI). Similar to helicates 5 and 6, complex 7 also shows only absorbance for ligand centered transitions at λmax = ∼300 nm, and no absorption band is detected for the d-d transition (Figure 2S in SI). Calculations of Association Constant (Ka) of Complexes 1, 2, and 4−6 Using UV−vis Titration Method. Further, the association constant of complexes 1, 2, 4−6 were determined using UV−vis titration. UV−vis titration experiments of the ligand L (∼5 × 10−5 M) solutions in 1:1 CHCl3/ MeOH were carried out with Fe2+, Zn2+, Cu2+, Ni2+, and Co2+ (∼5 × 10−4 M) solution in MeOH at room temperature. Upon addition of metals ion, in all the experiments, a red-shifted absorption band at ∼275 nm [305 nm (1), 315 nm (2), 315 nm (4), 310 nm (5), and 310 nm (6)] of L in the UV region was observed. An isosbestic point at ∼295 nm in all the cases indicated a single equilibrium that supports the formation of one type of complex in the solution, which could be due to the formation of the M3L3 helicate [Figures 14S (4), 17S (5), 20S (6) in SI]. The nonlinear 1:1 curve fitting method is utilized to evaluate the association constant values (Ka) for each helicatecomplex [Figures 12S (1), 13S (2), 16S (4), 19S (5), 22S (6) in SI]. The analyses of the calculated values show that Cuhelicate has the maximum association constant (4.97 × 105 M−1) among all the helicates of ligand L under consideration, which is marginally higher than the association constant of Nihelicate (4.88 × 105 M−1). Further, in agreement with the ESIFTICR-MS data, Co-helicate, Fe-helicate, and Zn-helicates have relatively lower association constant compare to the Cu- and Ni-helicates (Table 2).

Figure 8. FESEM images of a) ligand L (5 × 10−3 M in DMF) and b) Cu-helicate (4.2 × 10−3 M in acetonitrile).

show such sheet-like assembly; rather appears as nanocrystallike assembly. However, other helicates do not show any definite higher order assembly (Figure 8b).

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CONCLUSIONS We have developed a series of rare trinuclear triple-stranded side-by-side homo- and hetero- metallic helicates of ligand L with different first-row transition-metal ions having the general formula (L3M3)6+ [M = Fe (1), Zn (2), Fe2Zn (3), Cu (4), Ni (5) Co (6), and Ni2Co (7)]. A detailed transmetalation study among these helicates suggests L3Cu3(OTf)6 (4) as the thermodynamically most stable product. Further, macroscopic studies confirm the conversion of a morphologically pure nanosheet of a ligand to discrete crystalline materials upon incorporation of metal ion. Importantly, we have identified the formation of trinuclear heterometallic helicate with three different metal centers, L3FeZnCu(OTf)6 as an intermediate during the transmetalation reaction of L3Fe2Zn(OTf)6 (3) to L3Cu3(OTf)6 (4). To the best of our knowledge, this represents the first example of its kind. Finally, the selective formation of heterometallic helicate L3Ni2Co(OTf)6 (7) from a mixture of L, Ni2+ and Co2+ further establishes the true hybrid nature of the ligand. Overall, this work goes beyond the general synthesis and characterization of supramolecular architectures toward the understanding of their formation mechanism, thermodynamic and kinetic data.

Table 2. Association Constant Data of Complexes 1, 2, 4−6 Determined via UV−vis Titration Methoda ligand L

a

metal ion 2+

Fe Zn2+ Cu2+ Ni2+ Co2+

association constant (Ka, M−1) in 1:1 model 7.78 7.65 4.97 4.88 3.34

× × × × ×

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104 104 105 105 105

EXPERIMENTAL SECTION

Materials and Methods. All the reactions were carried out in dry argon/nitrogen gas atmosphere. The following workup procedures were carried out at ambient condition. Acetonitrile was refluxed over CaH2 and was collected prior to use. Methanol was dried over magnesium granules and was distilled prior to use. Helicates

In all cases, the error is within ±10% error limit. 12509

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Inorganic Chemistry Table 3. Crystal Data and Structure Refinement for the Compounds 4 and 5 compd reference

4

5

chemical formula formula mass cryst syst a/Å b/Å c/Å α/° β/° γ/° unit cell vol/Å3 temp/K space group no. of formula units per unit cell, Z radiation type no. of reflns measured no. of independent reflns Rint final R1 values (I > 2σ(I)) final wR(F2) values (I > 2σ(I)) final R1 values (all data) final wR(F2) values (all data) CCDC number

C84H72Cu3F36N30O2P6 2594.16 monoclinic 13.0116(12) 13.4224(13) 35.194(3) 90.00 95.066(2) 90.00 6122.6(10) 150(2) P2/c 2 Mo Kα 47106 8575 0.0791 0.0918 0.2684 0.1160 0.2860 1514774

C82H60F12N30Ni3O12S4 2189.97 orthorhombic 17.2208(14) 19.6574(16) 34.035(3) 90 90 90 11521.4(16) 150(2) Pbcn 4 Mo Kα 77084 6743 0.0732 0.0724 0.1998 0.0929 0.2120 1514775

Fe3L3(OTf)6 (1), Zn3L3(OTf)6 (2), Fe2ZnL3(OTf)6 (3), and ligand L were synthesized according to our recently published procedures.13 Cu(OTf)2, Ni(OTf)2 and Co(BF4)2·6H2O, NaOTf were purchased from Sigma-Aldrich and were used as received. Deuterated solvents and all other chemicals were used as received from the supplier without further purification. Electrospray-ionization Fourier-transform ion-cyclotron-resonance mass spectrometry (ESI-FTICR-MS) experiments were done on a Varian/IonSpec QFT-7 instrument. UV−vis absorption spectra were recorded in a PerkinElmer Lambda 900 UV−vis−NIR spectrometer (with a quartz cuvette of path length 1 cm). Synthesis of Helicate Cu3L3(OTf)6 (4). A solution of Cu(OTf)2 (18.1 mg, 0.050 mmol) in dry methanol was added to a suspension of L (23.6 mg, 0.050 mmol) in the same solvent (3 mL) in a roundbottom flask at room temperature. The reaction mixture was allowed to stir in inert gas atmosphere; a clear green solution was obtained after 4 h. The reaction mixture was stirred for an additional 24 h and was dried under vacuum. The residual solid was redissolved in 2 mL of dry methanol and was filtered through a pad of Celite. The filtrate was layered over benzene (2 mL) in a test tube. Clear green needle-shaped crystals were obtained at the interlayer between the two solvents in 12 h. The solvents were removed using a pipet, and the residue was dried under vacuum to get 4 in analytically pure form. Single crystals suitable for X-ray measurements were obtained after anions exchange to hexafluorophosphate. Yield: 32.5 mg (78%). ESI-FTICR-MS: m/z calcd for [L3Cu3(OTf)4]2+, 1101.5786; found, 1101.5789; calcd for [L 3 Cu 3 (OTf) 3 ] 3 + , 684.7349; found, 684.7353; calcd for [L3Cu3(OTf)2]4+, 475.8130; found, 475.8138. Elemental analysis for C84H60F18N30O18S6Cu3: Found: C, 40.34; H, 2.49; N, 16.63. Calcd: C, 40.31; H, 2.42; N, 16.79 Synthesis of Helicate Ni3L3(OTf)6 (5). A solution of Ni(OTf)2 (17.85 mg, 0.050 mmol) in dry methanol was added to a suspension of L (23.6 mg, 0.050 mmol) in the same solvent (3 mL) in a roundbottom flask at room temperature. The reaction mixture was allowed to stir in inert gas atmosphere; a clear light yellow solution was obtained after 12 h. The reaction mixture was stirred for additional 12 h and was dried under vacuum. The residual solid was redissolved in 2 mL of dry methanol and was filtered through a pad of Celite. The filtrate was layered over benzene (2 mL) in a test tube. Yellow blockshaped crystals were obtained at the interlayer between the two solvents in 2 days. The solvents were removed using a pipet and the

residue was dried under vacuum to get 5 in analytically pure form. Yield: 22 mg (53%). ESI-FTICR-MS: m/z calcd for [L3Ni3(OTf)4]2+, 1093.0873; found, 1093.0878; calcd for [L3Ni3(OTf)3]3+, 679.0740; found, 679.0744; calcd for [L3Ni3(OTf)2]4+, 472.0674; found, 472.0676. Elemental analysis for C84H60F18N30O18S6Ni3: Found: C, 40.69; H, 2.50; N, 16.79. Calcd: C, 40.55; H, 2.43; N, 16.89 Synthesis of Helicate Co3L3(OTf)6 (6). A solution of Co(BF4)2.6H2O (17 mg, 0.050 mmol) in dry methanol was added to a suspension of L (23.6 mg, 0.050 mmol) in the same solvent (3 mL) in a round-bottom flask at room temperature. The reaction mixture was allowed to stir in inert gas atmosphere; a light yellow solution was obtained after 0.5 h. The reaction mixture was stirred for additional 12 h and was dried under vacuum. The residual solid was redissolved in 0.5 mL of methanol and was poured in a solution of NaOTf (35 mg, 0.20 mmol) in 10 mL of water to get a clear very light yellow solution. The solvent mixture was allowed to evaporate slowly at room temperature. Light yellow needle-shaped crystals were obtained at the bottom of the beaker in 3 days. The solvents were removed using a pipet and the residue was first dried under vacuum and then stored in a vacuum desiccators to get 6 in analytically pure form. Yield: 16.5 mg (42%). ESI-FTICR-MS: m/z calcd for L3Co3(OTf)4]2+, 1094.5841; found, 1094.5843; calcd for [L3Co3(OTf)3]3+, 680.0719; found, 680.0719; calcd for [L3Co3(OTf)2]4+, 472.8157; found, 472.8158. Elemental analysis for C84H60F18N30O18S6Co3: Found: C, 40.51; H, 2.51; N, 16.72. Calcd: C, 40.54; H, 2.43; N, 16.88 Synthesis of Helicate Ni2CoL3(OTf)6 (7). A mixture of Ni(OTf)2 (17.8 mg, 0.050 mmol), Co(BF4)2.6H2O (17 mg, 0.050 mmol), and L (23.6 mg, 0.050 mmol) was taken in a round-bottom flask in 1:1:1 ratio. Five milliliters of dry methanol was added to the reaction mixture at room temperature and was allowed to stir in inert atmosphere for another 16 h to generate a clear light yellow solution. The reaction mixture was dried in reduce pressure. The residual solid was redissolved in 2 mL of dry methanol, and 35 mg of NaOTf was added and was filtered through a pad of Celite. The filtrate was layered over benzene (2 mL) in a test tube and was sealed. Needle-shaped crystals were obtained at the interlayer between the two solvents in 2 h. The solvents were removed using a pipet and the residue was dried under vacuum to get 7 in analytically pure form. Yield: 24.1 mg (58%). ESI-FTICR-MS: m/z calcd for [L3Ni2Co(OTf)4]2+, 1093.5863; found, 1093.5865; calcd for [L3Ni2Co(OTf)3]3+, 679.4066; found, 679.4067; calcd for [L3Ni2Co(OTf)2]4+, 472.3168; found, 472.3171. Elemental 12510

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analysis for C84H60F18N30O18S6Ni2Co: Found: C, 40.63; H, 2.52; N, 16.88. Calcd: C, 40.55; H, 2.43; N, 16.89 Calculation of Association Constant (Ka). Upon addition of metal ions (guest) to the solution of hybrid ligand L (host), absorption spectra changed gradually due to the binding of the ligand with the metal ions. The association constant (Ka) values of these bindings were determined by nonlinear fitting of the curves obtained by plotting the absorbance changes (ΔA) at a fixed λ value with the guest concentration. All the absorption titration data were fitted to the below equation:

§

L.C.: Disease Biophysics Group, John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.G. gratefully acknowledges the Science and Engineering Research Board (SERB), New Delhi (Project SR/S1/IC-39/ 2012) for financial support. B.J. acknowledges SERB (Project no SB/FT/CS-095/2013). C.A.S. thanks The Deutsche Forschungsgemeinschaft and the Freie Universität Berlin for financial support. Single-crystal X-ray diffraction data were collected at the DBT-funded CEIB program (Project No. BT/ 01/CEIB/11/V/13) awarded to the Department of Organic Chemistry IACS Kolkata, India.

ΔA = A{([H] + [X−] + (1/K a)) − {([H] + [X−] + (1/K a))2 − (4[H][X−])}1/2 }/(2/[H])

A = absorbance value upon each addition of the guest, ΔA = (A − A0), [H] = concentration of the host and [X−] = concentration of the guest. X-ray Crystallographic Refinement Details. Crystals suitable for X-ray diffraction studies were selected from the mother liquor and immersed in paratone oil and then mounted on the tip of a glass fiber using epoxy resin. Intensity data for the crystals 4 and 5 were collected using Mo Kα (λ = 0.7107 Å) radiation on a Bruker SMART APEX II diffractometer equipped with CCD area detector at 150 K. The data integration and reduction were processed with SAINT software74 provided with the software package of SMART APEX II. An empirical absorption correction was applied to the collected reflections with SADABS.75 The structures were solved by direct methods using SHELXL76 and were refined on F2 by the full-matrix least-squares technique using the SHELXL-9777 program package. Graphics were generated using PLATON78 and MERCURY 3.3.79 Non-hydrogen atoms were refined anisotropically until convergence was reached. In complex 5, out of three, one triflate counteranion was not observed. In the cases of complexes 4 and 5, the hydrogen atoms were geometrically fixed at idealized positions. For both complexes 4 and 5, we are unable to assign electron density for some solvent molecules in the unit cell, even though the data was collected at 150 K for several times. The routine SQUEEZE80 was applied to intensity data of both complexes 4 and 5 to take into account the disordered solvent molecules (Table 3).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01980. Details of characterization data and crystallographic information files with CCDC 1514774 (4) and 1514775 (5) (PDF) Accession Codes

CCDC 1514774−1514775 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Sourenjit Naskar: 0000-0003-4533-8809 Pradyut Ghosh: 0000-0002-5503-6428 12511

DOI: 10.1021/acs.inorgchem.7b01980 Inorg. Chem. 2017, 56, 12505−12513

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DOI: 10.1021/acs.inorgchem.7b01980 Inorg. Chem. 2017, 56, 12505−12513

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DOI: 10.1021/acs.inorgchem.7b01980 Inorg. Chem. 2017, 56, 12505−12513