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Enantiomeric Resolution of Asymmetric-Carbon-Free Binuclear Double-Stranded Cobalt(III) Helicates and Their Application as Catalysts in Asymmetric Reactions Rajendran Arunachalam,†,‡ Eswaran Chinnaraja,†,‡ Arto Valkonen,§ Kari Rissanen,§ Shovan K. Sen,‡,∥ Ramalingam Natarajan,‡,∥ and Palani S. Subramanian*,†,‡

Inorg. Chem. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/29/18. For personal use only.

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Inorganic Materials and Catalysis Division, CSIR-Central Salt and Marine Chemicals Research Institute, Bhavnagar, 364021, Gujarat, India ‡ Academy of Scientific and Innovative Research (AcSIR), Bhavnagar, 364021, Gujarat, India § Department of Chemistry, University of Jyvaskyla, Jyväskylä, FI-40014, Finland ∥ Organic & Medicinal Chemistry Division, CSIR-Indian Institute of Chemical Biology, 4, Raja S. C. Mullick Road, Kolkata, 700032, India S Supporting Information *

ABSTRACT: A series of double-stranded binuclear helicates [Co2(H1)2]4+, [Co2(H2)2]4+, and [Co2(H3)2]4+, derived from monodeprotonated bis-pyridyl hydrazine-based ligands of H21, H22, and H23 with one, two, and three −CH2 spacers, were obtained. These asymmetric-carbon-free racemic helicates were separated into their ΔΔ and ΛΛ enantiomers. The resolved helicates were examined for the first time as enantioselective catalysts in asymmetric benzoylation and nitroaldol reactions.

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INTRODUCTION Werner resolved the first asymmetric-carbon-free inorganic chiral complexes of “Hexol” and [Co(NH3)6]3+ more than a century ago.1 Later, a series of other metal complexes, e.g., [Co(en)3]3+, were resolved into their optical isomers.2 In the last three decades, helicates,3,4 which are discrete and intrinsically chiral double-, triple-, and multistranded complexes, have emerged as a fascinating class of complex structures in supramolecular chemistry. When the bis- or tris-chelating ligands establish coordination by wrapping around the metal ions, the resulting helical chirality observed is denoted as P (positive) and M (minus) or alternatively Δ (delta) and Λ (lambda). The corresponding binuclear doublestranded helicates form homochiral helicates (ΔΔ, ΛΛ)5−7 and achiral mesocates (ΔΛ).8 Harris and McKenzie9 have reported a binuclear Cu2L3 complex and suggested the existence of all above-mentioned three isomers. Raymond and co-workers10,11 have reported a series of binuclear triplestranded complexes with rhodotorulic acid and confirmed the chiral ΔΔ and ΛΛ isomers through circular dichroism (CD) and single crystal X-ray structures, and studied the mechanism of interconversion between the isomers. Despite the fact that metal-ion helicates are intrinsically chiral, the spontaneous formation of ΔΔ, ΛΛ, and ΔΛ isomers in the same mixture © XXXX American Chemical Society

results in a situation where the system is CD silent; viz. the chirality of the helicates cannot be observed. Even though there are abundant reports on the synthesis of helical complexes, the reports on their enantiomeric separation are quite rare. The research groups of Knee,12−15 Lacour,16 Hannon,17,18 Lehn,19,20 Williams,21 and Piguet22 have already demonstrated the enantiomeric resolution of helicates, yet none of these resolved helicates have been examined as enantioselective catalysts. Herein, we describe the synthesis of a series of binuclear Co(III) double-stranded helicates and their resolution into the ΔΔ and ΛΛ enantiomers. Further, we demonstrate, for the first time, that the asymmetric-carbon-free helicates can act as enantioselective catalysts in selected organic transformations such as nitroaldol and benzoylation reactions.

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RESULTS AND DISCUSSION

The bis-pyridylhydrazone based multidentate ligands H21− H23 (Scheme 1) with one, two, and three −CH2 spacers were synthesized via a condensation reaction of pyridine-2carboxaldehyde with malonic, succinic, and glutaric dihydraReceived: May 2, 2018

A

DOI: 10.1021/acs.inorgchem.8b01204 Inorg. Chem. XXXX, XXX, XXX−XXX

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S17). Similar observations were made with [Co2(H2)2]Cl4 and [Co2(H3)2]Cl4. The absorption spectra of ligands and their complexes were recorded in DMF, and both the ligand and metal-centered transitions were well-resolved (Figure 1). The band at 310 nm of the ligand represents the ligand-centered transition, and the corresponding signals on the complexes appeared at 380−390 nm, which manifest the ligand to metal charge transfer band. The two well-resolved bands observed from 600 to 675 nm are attributed to the d−d transitions of 4 T1g(F) → 4A2g(F) (610 nm) and 4T1g(F) → 3T2g(F) (675 nm), supportive for a distorted octahedral geometry of the Co(III) ion. X-ray Structures of [Co222]Cl2 and [Co232](NO3)2. Despite our rigorous efforts, we were able to obtain single crystals suitable for XRD only from two of the helicates, namely, [Co222]Cl2 and [Co232](NO3)2. A dark brown single crystal of [Co222]Cl2 was obtained by diffusing acetone into the aqueous solution of the complex. The [Co 2 2 2]Cl2 crystallizes in the centrosymmetric monoclinic system with P21/c space group (Table 1, S18). The complex has two Co(III) centers coordinated by two deprotonated 22− ligands to form a double-stranded [Co222]2+ dication with chloride anions in the lattice. Both ligands coordinate through their N2O site to two Co(III) centers (Figure 2) in [Co222]Cl2 with a Co···Co distance of 6.05 Å. The symmetrically oriented ligands have a torsion angle of 66.15° through the spacer carbons C7-C8-C9-C10 and provide a C-type conformation to define the complex mesocate. The pyridyl nitrogens (N1, N6) and azomethine nitrogens (N2, N5) of opposite strands form the four Co−N bonds, while the hydrazone carbonyl oxygens (O1 and O2) form the two Co−O bonds. Thus, the Co(III) centers have a distorted octahedral geometry with bond distances ranging from 1.845 to 1.923 Å with the N1−Co1− O1 angle of 165.18°, N5−Co1−N2 of 176.14°, and N6− Co1−O2 of 165.63°. The acylhydrazone moieties in the ligand have nearly equal Co−O bond distances [Co1−O1 = 1.902(2) Å, Co1−O2 = 1.893(2) Å] with similar iminic C−N bond distances [1.320 and 1.318 Å], in accordance with the deprotonated enol form of the carbonyl groups. A similar binuclear double-stranded helicate [Co232](NO3)2 was obtained using H23 and Co(NO3)2. The single crystals were obtained by diffusing isopropyl alcohol into the aqueous solution of the helicate. The X-ray structural analysis of the helicate revealed a similar structure with deprotonated dianionic ligands as for [Co222]Cl2. The ligands adopt a similar C-type conformation to possess cis−cis coordination with two cobalt ions and is a mesocate as [Co222]Cl2. In

Scheme 1. Synthesis of Co(III) Helicates from H21 (n = 1), H22 (n = 2), and H23 (n = 3)

zides, respectively, following the procedure recently reported by us.23,24 The molecular structure of the ligands were characterized by mass spectrometry, NMR, IR spectroscopy, and elemental analysis. The negative-mode electrospray ionization (ESI) mass spectrometry of the newly synthesized ligands H21−H23 exhibited the expected signals for (M − H)− ions at m/z = 309.16 (H1−), 323.12 (H2−), 337.33 (H3−), respectively (Figures S1−S3). The 1H NMR spectroscopy confirmed the presence of the characteristic resonances at δ = 8.23 and 8.05 ppm (for H21), δ = 8.18 and 8.05 ppm (for H22), and δ = 8.16 and 8.04 ppm (for H23) for the azomethine protons, thus confirming the successful Schiff base condensation reaction (Figures S4−S9). Also, the IR spectroscopy revealed the azomethine groups at 1550−1580 cm−1 (νCN) (Figure S10). The helicates were prepared by treating the ligands H21− H23 with CoCl2 in a 1:1 ratio, in methanol, providing the expected double-stranded [M2L2]Cl4 type helicates, [Co2(H1)2]Cl4, [Co2(H2)2]Cl4, and [Co2(H3)2]Cl4, in reasonable yields. The basic monoanionic ligands facilitate the oxidation of Co(II) to Co(III) without any additional oxidizing agents during the helicate formation, as reported earlier.25 The positive-mode ESI mass spectra of the helicates showed the presence of the expected peaks for the dicationic (M2+) ions of [Co212]2+, [Co222]2+, and [Co232]2+ at m/z = 367.80, 381.27, and 395.45, respectively (Figures S11−S13), and confirms the dinuclear helicate formation. The 1H NMR spectra of these complexes span over a narrow spectral range, 0−15 ppm, in accordance with the presence of Co(III) ions (Figures S14− S16). The broad IR bands observed around 3400 cm−1 are attributed to the hydroxyl group of water molecules in helicates. The −CO stretching frequency at 1665 cm−1 in H21 is shifted to 1619 cm−1 in the [Co2(H1)2]Cl4 helicate; also a small shift from 1558 to 1544 cm−1 in the azomethine band was observed, in accord with the complexation (Figure

Figure 1. (a) UV−vis spectra of the ligands and the helicates in DMF; (a′) d−d transitions (1 × 10−4 M). B

DOI: 10.1021/acs.inorgchem.8b01204 Inorg. Chem. XXXX, XXX, XXX−XXX

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the present study, we have attempted to separate the enantiomers from each other and from the mesocate through column chromatography. In this challenging process, the choice of the mobile and stationary phases is of crucial importance. The chiral mobile and stationary phases were chosen in such a way that their interaction with one enantiomer will be stronger than the other. In this respect, we have examined as the chiral stationary either cellulose or SP-Sephadex and as the mobile phases L-potassium antimony(III)-tartrate, sodium-D-gluconate, dibasic L-(+)-potassium tartrate, dibasic D-(−)-potassium tartrate, (+)-O,O′-di-ptoluoyl-D-tartaric acid, (−)-O,O′-di-p-toluoyl-L-tartaric acid, dibenzoyl-L-tartaric acid, and (+)-2,3-dibenzoyl-D-tartaric acid. The initial thin layer chromatography (TLC) analysis suggested that the 2:8 ratio of 0.02 M of dibasic L(+)-potassium-tartrate (aq) with acetonitrile would be the suitable mobile phase and cellulose as the suitable stationary phase. Accordingly, the columns were loaded with cellulose using acetonitrile and a slurry made of [Co2L2]Cl4 helicate, [L = H1, H2, H3], were placed on the column. Initially, the columns were eluted with acetonitrile alone after which the polarity of the mobile phase was raised by incremental amounts of 0.2% potassium-tartrate dibasic(aq) (0.01M) with acetonitrile. Due to the selective interaction of tartrate ions with one of the enantiomers, differential elutions of the enantiomeric helicates were observed. We postulate that the chloride ions in the [Co2(H2)2]Cl4 are replaced by the chiral tartrate ions during the elution. The chiral tartrate ion forms two diastereomeric helicates, leading to the separation of the enantiomeric helicates. The separation process is very slow. Thus, with flow rates being low, the elution process consumes a very long elution time of around 25−30 days. If the separation was attempted by increasing the flow rate of the mobile phase, it only led to no or poor separation. Apart from the exorbitant duration for one enantiomer to elute from the column, the other enantiomer was found to bind so strongly to the cellulose stationary phase that it was difficult to isolate. As we wanted to resolve both enantiomers, we were motivated to modify the protocol, which would facilitate to achieve separation of both of the enantiomers. Accordingly, by keeping the choice of cellulose as stationary phase unchanged, the slurry was made by mixing 0.02 M aq. NaCl, cellulose, and the helicate (dissolved in a minimum amount of water). This slurry was placed on the column top for a few minutes and then eluted with fresh acetonitrile. A pale brown band eluted

Table 1. Crystal Data for [Co222]Cl2(H2O)4 and [Co232](NO3)2 parameters

[Co222]Cl2(H2O)4

[Co232](NO3)2

chemical formula formula weight crystal size (mm) temperature (K) crystal system space group a (Å) b (Å) c (Å) α (Å) β (Å) γ (Å) Z ρcalc (g/cm3) μ (mm−1) F(000) reflections collected independent reflections Rint restraints/parameters GOF on F2 final R1/wR2 [I ≥ 2σ(I)] R1/wR2 (all data) largest diff. peak/hole (e Å3) CCDC

C32H28Cl2Co2N12O8 897.42 0.28 × 0.14 × 0.12 100 monoclinic P21/c 12.5772(6) 17.0364(8) 9.0254(5) 90 97.906(2) 90 2 1.556 1.071 912 25637 4437 0.0459 262/305 1.906 0.0547/0.1420 0.0658/0.1420 1.02/−0.65 1835264

C68H64Co4N28O21 1845.19 0.19 × 0.11 × 0.06 123 monoclinic C2/c 16.9666(9) 25.1732(13) 9.1054(5) 90 101.658(5) 90 2 1.609 0.950 1888 6456 3434 0.0187 296/411 1.062 0.0511/0.1342 0.0642/0.1438 0.904/−0.611 1832297

[Co232](NO3)2, the Co···Co distance is 7.32 Å (Figure 3); the longer distance is caused by the additional CH2 group in the spacer. The octahedral coordination geometry in [Co232](NO3)2 [Co−O = 1.893−1.945 Å; 1.902−1.906 Å] and C−O = [1.291, 1.307 Å] is very similar to that of [Co222]Cl2. The Enantiomeric Resolution of [Co2(H1)2]Cl4, [Co2(H2)2]Cl4, and [Co2(H3)2]Cl4. Although the helicates do not have asymmetric carbons, they are intrinsically chiral due to the helical arrangement of ligands and the induced chirality around the metal centers upon complexation. Thus, the synthesis of double-stranded helicates leads to a mixture consisting a pair of chiral helicates (ΔΔ, ΛΛ) and achiral mesocate (ΔΛ), as shown in the Scheme 2. Since the structural differences between the chiral helicates and the mesocate are small, the separation of them is difficult and rarely reported. In

Figure 2. X-ray structure of [Co222]2+. The chloride counterions are omitted for clarity. C

DOI: 10.1021/acs.inorgchem.8b01204 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. X-ray structure of [Co232]2+. The nitrate counterions are omitted for clarity.

Scheme 2. Possible Enantiomers in the Co(III) Helicates (Each Racemic Helicate Contains Two Enantiomers and One Mesocate)a

Figure 4. Photograph of resolved enantiomers (ΛΛ enantiomer, green band, in acetonitrile) and (ΔΔ enantiomer, brown band, in acetonitrile−0.02 M NaCl (aq)). The n indicates the number of −CH2 groups of ligands 1, 2, and 3.

a

out first, and the elemental and MS analyses match well with those of the parent dinuclear helicate. However, the absence of CD for this fraction suggested that this band consists of achiral mesocate (ΔΛ). Following the elution of this band, an additional two intense bands, green and brown in color, appeared, and both were collected separately (Figure 4). Initially, the green band was collected using acetonitrile alone. Subsequently, the brown band was collected by raising the polarity by using 0.02 M NaCl(aq) solution. The isolated complexes from the both green and brown bands exhibited similar elemental analysis data (S19) and MS spectra (Figure S20). The CD spectra recorded revealed positive and negative Cotton effects (Figure 5) and confirm that they are enantiomers. The respective CD spectra in Figure 5 reveal the ligand-centered [Figure 5a−c] and d−d transitions [Figure 5(a′−c′)], which are resolved at lower concentration (≈1 × 10−4 M) and higher concentration (≈5 × 10−2 M), respectively. The opposite optical behavior of the green and the brown bands at 700−600 nm is associated with the opposite helical chiralities of the respective metal centers (ΔΔ and ΛΛ). This observation matches with Hannon’s report18 suggesting that the green and brown bands correspond to the (M)-helicate

Figure 5. CD spectra recorded in DMF for resolved green and brown bands of tetracationic (a) ΛΛ-[Co2(H1)2] and ΔΔ-[Co2(H1)2], (b) ΛΛ-[Co2(H2)2] and ΔΔ-[Co2(H2)2], (c) ΛΛ-[Co2(H3)2] and ΔΔ[Co2(H3)2]. The a′, b′, and c′ represent the d−d band. D

DOI: 10.1021/acs.inorgchem.8b01204 Inorg. Chem. XXXX, XXX, XXX−XXX

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Although the yields are encouraging (80−90%, Table 2), the enantioselectivities observed were only modest. However, the 14% ee obtained (Table 2: entries 5 and 6) in the reactions are certainly due to the chirality associated with the enantiomeric helicate catalyst. The optical nature of the enantiomeric products (Table 2: entries 5 and 6) obtained using the catalytic ΔΔ and ΛΛ enantiomeric helicates of [Co2(H2)2]4+ was analyzed by HPLC using a Lux-cellulose column, and they were confirmed to be enantiomers (Figures S23 and S24). Similar studies with an asymmetric nitroaldol reaction were also carried out using benzaldehyde and nitromethane as a model substrate, with DIPEA as the base in acetonitrile at 0 °C for 16 h, using 2 mol % of the resolved enantiomers as catalysts (S25). The completion of the reaction was monitored by TLC. Subsequently, the catalysts were separated as described in the general procedure (S25), and the products (S26) were purified by flash column chromatography (Figure S27). As shown in the Table 3, the yields are again high, but the respective ee’s

and (P)-helicates, respectively. The ligand centered bands in the CD spectral range of 260−350 nm of all three helicates show an opposite optical pattern of λmax at around 290 nm for the green bands attributable to ΛΛ-[Co2(H1)2], ΛΛ-[Co2(H2)2], and ΛΛ-[Co2(H3)2] helicates and for the brown bands to their ΔΔ-[Co2(H1)2], ΔΔ-[Co2(H2)2], and ΔΔ[Co2(H3)2] enantiomers (Figure 5). Since the cobalt ions are in the +3 oxidation state in all of these complexes, we were inspired to investigate their 1H NMR spectra. The representative 1H NMR recorded for the [Co2(H2)2]Cl4 complex using D2O as solvent shows two well distinguishable doublets corresponding to the CH2 protons of the succinate spacer in the mixture of enantiomers (Figure S21). Upon separation of the enantiomers from the mesocate, they show only a small change in their chemical shift with reference to the racemic mixture. Further, the 1H NMR spectrum of [Co2(H2)2]Cl4 shows the coexistence of two sets of doublets attributable to the enantiomers and one singlet for mesocate; it allowed us to establish the relative ratio of helicate:mesocate population in the complex as 71:29%. Asymmetric Synthesis with the Helicates as Catalysts. Having successfully isolated the asymmetric-carbon-free enantiomeric helicates, we aimed to examine their hitherto unexplored ability to function as enantioselective catalysts. Accordingly, we have chosen two well-established reactions, the desymmetrization of a meso diol26 and the nitroaldol27,28 reactions. Accordingly, using 2 mol % of the above-resolved enantiomers as catalysts, we treated the meso-hydrobenzoin (model substrate) with benzoyl chloride (acylating reagent) with DIPEA as the base in a MeCN−DCM solvent mixture at −30 °C for 8 h. The results are presented in Table 2. The reaction mixtures were purified following the general procedure (S22) by flash column, and their ee’s were determined (S23) using chiral HPLC (Figure S24).

Table 3. Screening of the Helicate Catalysts for the Asymmetric Nitroaldol Reactiona

entry

catalyst

yield (%)

1 2 3 4 5 6 7 8 9 10

blank ΔΔ-[Co2(H1)2] ΛΛ-[Co2(H1)2] rac-[Co2(H1)2] ΔΔ-[Co2(H2)2] ΛΛ-[Co2(H2)2] rac-[Co2(H2)2] ΔΔ-[Co2(H3)2] ΛΛ-[Co2(H3)2] rac-[Co2(H3)2]

56 92 90 90 95 92 92 90 86 92

ee (%) 24 20 26 22 20 18

a

All reactions were carried out using 2 mol % (0.007 mmol) of catalyst, 0.37 mmol of the substrate, 0.4 mmol of DIPEA (di-isopropyl ethylamine), and 3.76 mmol of CH3NO2 at 0 °C, for 16 h. The ee’s were determined using UFLC, Lux-cellulose-1 in an IPA−Hexane system. *The Co(III) in the catalyst is balanced with chloride counterions. The HPLC profile for the nitroaldol product is given in the Supporting Information (Figure S27a−f).

Table 2. Screening of the Helicate Catalyst for the Asymmetric Benzoylation Reactiona

entry

catalyst

yield (%)

1 2 3 4 5 6 7 8 9 10

blank ΔΔ-[Co2(H1)2] ΛΛ-[Co2(H1)2] rac-[Co2(H1)2] ΔΔ-[Co2(H2)2] ΛΛ-[Co2(H2)2] rac-[Co2(H2)2] ΔΔ-[Co2(H3)2] ΛΛ-[Co2(H3)2] rac-[Co2(H3)2]

42 85 88 90 92 90 90 82 86 86

ee (%)

observed are only modest, ranging from 18% to 26%. As in the benzoylation reaction, the enantiomeric excess obtained here also accounts for the chirality associated with the enantiomerically pure helicate catalyst. The poor enantioselectivities of the reactions urged us to revisit the CD experiments of the resolved helicates. Hence, we observed that the CD signal intensities are reducing as a function of time and suggest that, in solution, the enantiomerically pure helicates racemize through the mesocate (ΔΔ ↔ ΛΔ ↔ ΛΛ), and thus results in the low ee’s observed in the asymmetric catalysis. In order to examine the recyclability of the catalyst, we performed the nitroaldol reaction using ΔΔ-[Co2(H2)2] helicate, under the same catalytic conditions. We found that the catalytic activity remained excellent up to the third cycle, but the ee’s were found to be decreasing systematically from 26% to 10%, and finally to zero. This again confirms that the catalyst is undergoing racemization during the course of the reaction, causing the poor enantioselectivity.

major

4 2 14 14

1R, 2S 1S, 2R

2 6

a

All reactions were carried out using 0.004 mmol of catalyst, 0.2 mmol of the substrate, 0.4 mmol of DIPEA (di-isopropyl ethylamine), 0.2 mmol of benzoyl chloride at −30 °C for 8 h. The enantioselectivities were determined using UFLC, Lux-cellulose-1 in an IPA−Hexane system. The Co(III) in the catalyst is balanced with chloride counterions. The HPLC profile for the benzoylation product is given in the Supporting Information (Figure S24a−f). E

DOI: 10.1021/acs.inorgchem.8b01204 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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26.59. IR(KBr) ν = 3440 (OH), 3076 (C-H), 2951, 2902 (N-H), 2362, 1680 (int, CO), 1581 (int, CN), 1448 (ar CC), 1210 (C-N), 1160 (m), 944, 780 (C-H) cm−1. UV−vis [DMF, λ, nm, (ε/ M−1)]: 310 (10682), 300 (10183). Anal. Calcd (%) for C16H16N6O2: C, 59.25; H, 4.97; N, 25.91. Found: C, 58.42; H, 5.36; N, 25.11. ESIMS: m/z (−) found 323.12 [H2]− (Calcd 323.13). H23. Yield 72% (7.60 g). 1H NMR (DMSO-d6, TMS, 500 MHz): δ = 11.64 (s, 1H, NH), 11.50 (s, 1H, −NH), 8.60 (s, 2H, −CH), 8.18− 8.15 (d, J = 6 Hz, 1H, CH), 8.04−8.01 (d, J = 7 Hz, 1H, CH), 7.94− 7.66 (m, 4H, −CH), 7.39 (t, J = 7 Hz, 2H, −CH), 2.77 (t, 2H), 2.36 (m, 2H), 1.94 (t, 2H). 13C NMR (DMSO-d6, 600 MHz): δ = 174.37, 174.23, 168.68, 168.54, 153.30, 153.18, 153.14, 149.43, 146.21, 146.10, 143.16, 143.04, 136.86, 136.82, 136.75, 124.30, 124.26, 124.06, 119.79, 119.47, 119.38, 33.48, 33.36, 31.36, 31.12, 20.62, 20.01, 19.42. IR (KBr) ν = 3435 (OH), 3191 (C-H), 3053, 3005 (N-H), 2365, 1680 (int, CO), 1566 (int, CN), 1388 (CC, ar), 1249 (C-N), 1150(m), 778 (C-H) cm−1. UV−vis [DMF, λ, nm, (ε/ M−1)]: 311 (10590), 300 (10105). Anal. Calcd (%) for C17H18N6O2: C, 60.22; H, 5.34; N, 24.19. Found: C, 60.34; H,5.36; N, 24.84. ESIMS: m/z (−) found 337.33 [H3]− (Calcd 337.14). Synthesis of Helicates. [Co2(H1)2]Cl4 (H2O)5. To a methanolic solution of ligand H21 (1241 mg, 4 mmol), cobalt(II) chloride hexahydrate (952 mg, 4 mmol) in methanol was added dropwise. The colorless solution of the ligand turns into yellow to orange and then blackish red. The reaction mixture is heated to 60 °C for 2 h and then allowed to stir at RT for 14 h. The solution was evaporated under reduced pressure. Black crystalline solid. Yield: 45% (1.70 g). IR (KBr) ν = 3402 (−OH), 1619 (int, CO), 1544 (int, CN), 1470 (ar CC), 1361, 1222 (C-N), 1153, 773 (C-H) cm−1. UV−vis [DMF, λ, nm, (ε/M−1)]: 675 (452), 655 (429), 600 (481), 486 (2082), 368 (8460), 315 (10034). Anal. Calcd (%) for C30H36Cl4Co2N12O9: C, 37.21; H, 3.75; N, 17.36. Found: C, 37.23; H, 3.45; N, 17.36. ESI-MS: m/z (+) found 734.61 [Co212]2+ (Calcd 734.07); and the corresponding M2+/2 found 367.80 (calcd 367.03). [Co2(H2)2]Cl4(H2O)5. A similar procedure adapted by following the above and using H22 in place of H21. Black solid. Yield: 48% (0.456 g). IR (KBr): ν = 3405 (−OH), 3080 (C-H), 2916, 2834 (N-H), 2365, 1616 (int, CO), 1536 (int, CN), 1238 (C-N), 1175 (m), 938, 780 (C-H) cm−1. UV−vis [DMF, λ, nm, (ε/M−1)]: 676 (549), 657 (492), 631 (372), 607 (320), 367 (7590), 316 (10188). Anal. Calcd (%) for C32H40Cl4Co2N12O9: C, 38.57; H, 4.05; N, 16.87. Found: C, 38.35; H, 4.34; N, 15.39. ESI-MS m/z (+) found 762.57 [Co222]2+ (Calcd 762.10), and the corresponding M2+/2 found 381.27 (calcd 381.05). [Co2(H3)2]Cl4(H2O)3. A similar procedure was adapted using H23 in place of H21. Blackish brown crystalline solid. Yield: 40% (0.77 g). IR (KBr): ν = 3395 (br, −OH), 3024, 2941 (−NH), 2360, 1626 (int, CO), 1540 (int, CN), 1350 (ar CC), 1228 (C-N), 1153, 935, 778 (C-H) cm−1. UV−vis [DMF, λ, nm, (ε/M−1)]: 675 (554), 661 (531), 631 (352), 605 (408), 367 (10602), 350 (10280), 308 (11072). Anal. Calcd (%) for C34H40Cl4Co2N12O7: C, 41.32; H, 4.08; N, 17.01. Found: C, 41.47; H, 4.17; N, 17.31. ESI-MS m/z (+) found 789.92 [Co232]2+ (Calcd 790.13) and the corresponding M2+/2 found 395.45 (calcd 395.07). [Co2(H3)2](NO3)4(H2O)6. A similar procedure adapted by following the above and except H23 (465 mg, 1.374 mmol) and cobalt(II) nitrate (400 mg, 1.374 mmol). The crude complex was washed with diethyl ether to remove trace solvent impurities to yield a fine crystalline blackish brown solid. Yield: 56% (0.86 g). IR(KBr): ν = 3418 (br, −OH), 2926, 2855 (−NH), 1622 (int, CO), 1536 (int, CN), 1350 (ar CC), 1151 (C-N), 1016, 776 (C-H) cm−1. UV− vis [DMF, λ, nm, (ε/M−1)]: 675 (1184), 654 (1059), 605 (956), 389 (8624), 356 (8998), 322 (8434). Anal. Calcd (%) for C34H48Co2N16O22: C, 35.49; H, 4.20; N, 19.48. Found: C, 35.29; H, 3.89; N, 19.41. ESI-MS m/z (+) found 790.52 [Co232]2+ (calcd 790.13) and the corresponding M2+/2 found 396.26 (calcd 395.56). Resolution of Metallohelicates. The cellulose of particle size 20 μ was purchased from Sigma Aldrich and used as stationary phase. The cellulose was made into a slurry with 0.02 M aq. sodium chloride and loaded on the column. The metallohelicates which consist of

CONCLUSION In summary, three binuclear Co(III) helicates [Co2(H1)2]Cl4, [Co2(H2)2]Cl4, and [Co2(H3)2]Cl4 were synthesized and resolved into their ΔΔ and ΛΛ enantiomers. The mirror image CD spectra of the column chromatographically separated enantiomers confirm the resolution of the helicates into their respective ΔΔ and ΛΛ enantiomers. The resolved enantiomers were examined as catalysts for asymmetric organic transformation for the first time and led to the expected products in excellent yields, but disappointedly only with moderate enantioselectivities. The results presented here encourage us to look for a better design strategy of the ligands for enantiostable helicates, and upon resolution, for efficient enantioselective catalysis.

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EXPERIMENTAL SECTION

Materials and General Methods. All the chemicals were purchased from Aldrich & Co. IR spectra were recorded using KBr pellets (1% w/w) on a PerkinElmer Spectrum GX FT-IR spectrophotometer. Electronic spectra were recorded on a Shimadzu UV 3101PC spectrophotometer. Electrospray ionization mass spectrometry (ESI MS) measurements were carried out on a Waters QTof-micro instrument for all of these complexes upon dissolving in methanol−water solvents. CHNS analyses were done using a PerkinElmer 2400 CHNS/O analyzer. Single crystal data for [Co222]Cl2(H2O)4 were collected at 100 K on a Bruker Kappa APEX2 CCD diffractometer with Mo−Kα (λ = 0.71073 Å) radiation. The X-ray data for [Co232](NO3)2O were collected on an Agilent SuperNova single-source diffractometer equipped with an Eos CCD detector using mirror-monochromated Mo-Kα (λ = 0.71073 Å) radiation. 1H and 13C NMR spectra were recorded on a Bruker Avance II 500 or 600 MHz FT-NMR spectrometer. Chemical shifts for proton resonances are reported in ppm (δ) relative to tetramethylsilane, and 13C spectra are calibrated with reference to DMSO-d6. All the catalytic products were established based on 1H NMR spectra. The CD spectra were recorded on a JASCO 815 Spectrometer. The enantioselectivity of the Henry product was determined by UFLC (Shimadzu SCL-10AVP) using chiral columns (Phenomenox Lux cellulose-1 and Amylose-2 column). Synthesis of Ligands H21−H23. To a methanolic suspension of malonohydrazide (5.0 g, 0.034 mol), 2-pyridine carboxaldehyde (7.32 g, 0.068 mol) was added, and the mixture was allowed to stir at 0 °C for 8 h. The suspension changed into a clear solution, and a precipitate slowly formed during the course of the reaction. In the case of H21 and H22, the colorless solid obtained was filtered, washed with methanol, and dried. In the case of ligand H23, a pale yellow product was obtained. H21. Yield 84% (9.86 g). 1H NMR (DMSO-d6, TMS, 500 MHz): δ = 11.94, 11.73 (s, 1H each, −NH), 8.58 (s, 2H), 8.26 (s, 1H), 8.04 (s, 2H), 7.87−7.85 (d, J = 8 Hz, 2H), 7.70−7.67 (t, J = 8 Hz, 2H), 7.37− 7.35 (t, J = 6 Hz, 2H), 4.01 (s, 2H). 13C NMR (DMSO-d6, 600 MHz): δ = 169.45, 168.93, 163.40, 162.95, 153.12, 153.01, 149.53, 149.48, 147.18, 146.64, 143.58, 143.31, 136.92, 136.63, 136.58, 124.52, 124.44, 124.26, 124.19, 120.01, 119.90, 119.58, 119.49, 41.19. IR(KBr): ν = 3445 (−OH), 3200, 3068 (−NH), 1665 (int, CO), 1558 (int, −CN), 1465 (ar CC), 1360, 1214 (C-N), 1151, 780 (C-H) cm−1. UV−vis [DMF, λ, nm, (ε/M−1)]: 305 (8614), 296 (8200). Anal. Calcd (%) for C15H14N6O2: C, 58.06; H, 4.55; N, 27.08. Found: C, 57.51; H, 4.74; N, 27.40. ESI-MS: m/z (−) found 309.16 [H1]− (Calcd 309.11). H22. Yield 76% (8.43 g). 1H NMR (DMSO-d6, TMS, 500 MHz): δ = 11.71 (s, 1H, NH), 11.53 (s, 1H, −NH), 8.59 (s, 2H, −CH), 8.19 (s, 1H, CHN), 8.05 (s, 1H, CHN), 7.90 (m, 2H, −CH), 7.88− 7.83 (t, 2H, −CH), 7.40 (t, 2H, J = 7 Hz, −CH), 3.02 (s, 2H, −CH), 2.60−2.57 (s, 2H, −CH). 13C NMR (DMSO-d6, 600 MHz): δ = 173.89, 173.68, 168.46, 168.19, 153.36, 153.31, 153.22, 153.18, 149.54, 146.10, 145.92, 143.30, 143.12, 136.94, 136.90, 124.38, 124.34, 124.22, 124.19, 119.82, 119.60, 119.52, 29.05, 28.52, 27.25, F

DOI: 10.1021/acs.inorgchem.8b01204 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry mixtures of enantiomers (ΔΔ and ΛΛ) and mesocate (ΔΛ) were dissolved in water as a saturated solution and added to the column. The column was eluted with acetonitrile, and the resolution of bands was observed. Two different bands, one with brown and the other with green, were collected separately and isolated. The isolated compounds were made stock solution, and CD spectra were recorded for their enantiopure nature. The clear resolution of metal centered transitions was obatined at high concentrations in DMF. The racemic helicates were resolved into green and brown bands which are showing similar mass and CHNS data, but opposite pattern in CD spectra, confirming that they are enantiomers (ΔΔ and ΛΛ).

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(7) Perez-Garcia, L.; Amabilino, D. B. Spontaneous resolution under supramolecular control. Chem. Soc. Rev. 2002, 31, 342−356. (8) Albrecht, M.; Kotila, S. Formation of a “meso-Helicate” by SelfAssembly of Three Bis(catecholate) Ligands and Two Titanium(IV) Ions. Angew. Chem., Int. Ed. Engl. 1995, 34, 2134−2137. (9) Harris, C. M.; McKenzie, E. D. Five-co-ordinate copper(II) complexes of the quadridentate Schiff-base ligand N N′-bis-(2pyridylmethylene)ethane-1,2-diamine, and some related ligands. J. Chem. Soc. A 1969, 0, 746−753. (10) Carrano, C. J.; Raymond, K. N. Coordination chemistry of microbial iron transport compounds. 10. Characterization of the complexes of rhodotorulic acid, a dihydroxamate siderophore. J. Am. Chem. Soc. 1978, 100, 5371−5374. (11) Scarrow, R. C.; White, D. L.; Raymond, K. N. Ferric ion sequestering agents. 14. 1- Hydroxy-2(1H)-pyridinone complexes: properties and structure of a novel iron-iron dimer. J. Am. Chem. Soc. 1985, 107, 6540−6546. (12) Fletcher, N. C.; Junk, P. C.; Reitsma, D. A.; Keene, F. R. Chromatographic separation of stereoisomers of ligand-bridged diruthenium polypyridyl species. J. Chem. Soc., Dalton Trans. 1998, 133−138. (13) Keene, F. R. Isolation and characterization of stereoisomers in di- and tri-nuclear complexes. Chem. Soc. Rev. 1998, 27, 185−194. (14) Rapenne, G.; Patterson, B. T.; Sauvage, J. P.; Keene, F. R. Resolution, X-ray structure and absolute configuration of a doublestranded helical diiron(II) bis(terpyridine) complex. Chem. Commun. 1999, 1853−1854. (15) Smith, J. A.; Keene, F. R. Separation of stereoisomers of dinuclear metal complexes by binding affinity chromatography using non-duplex DNA. Chem. Commun. 2006, 2583−2585. (16) Jodry, J. J.; Lacour, J. Efficient resolution of a dinuclear triple helicate by asymmetric extraction/precipitation with TRISPHAT anions as resolving agents. Chem. - Eur. J. 2000, 6 (23), 4297−4304. (17) Hannon, M. J.; Meistermann, I.; Isaac, C. J.; Blomme, C.; Aldrich-Wright, J. R.; Rodger, A. Paper: a cheap yet effective chiral stationary phase for chromatographic resolution of metallo-supramolecular helicates. Chem. Commun. 2001, 1078−1079. (18) Kerckhoffs, J. M. C. A.; Peberdy, J. C.; Meistermann, I.; Childs, L. J.; Isaac, C. J.; Pearmund, C. R.; Reudegger, V.; Khalid, S.; Alcock, N. W.; Hannon, M. J.; Rodger, A. Enantiomeric resolution of supramolecular helicates with different surface topographies. Dalton Trans 2007, 734−742. (19) Hasenknopf, B.; Lehn, J. M. Trinuclear double helicates of Iron(II) and Nickel(II): self-assembly and resolution into helical enantiomers. Helv. Chim. Acta 1996, 79, 1643−1650. (20) Kramer, R.; Lehn, J. M.; De Cian, A.; Fischer, J. Self-Assembly, structure, and spontaneous resolution of a trinuclear triple helix from an oligobipyridine ligand and Ni(II) ions. Angew. Chem., Int. Ed. Engl. 1993, 32 (5), 703−706. (21) Charbonniere, L. J.; Bernardinelli, G.; Piguet, C.; Sargeson, A. M.; Williams, A. F. Synthesis, structure and resolution of a dinuclear Co(III) triple helix. J. Chem. Soc., Chem. Commun. 1994, 1419−1420. (22) Cantuel, M.; Bernardinelli, G.; Muller, G.; Riehl, J. P.; Piguet, C. The first enantiomerically pure helical noncovalent tripod for assembling nine-coordinate lanthanide(III) podates. Inorg. Chem. 2004, 43 (6), 1840−1849. (23) Arunachalam, R.; Aswathi, C. S.; Das, A.; Kureshy, R. I.; Subramanian, P. S. Diastereoselective Nitroaldol Reaction Catalyzed by Binuclear Copper(II) Complexes in Aqueous Medium. ChemPlusChem 2015, 80, 209−216. (24) Sahoo, J.; Arunachalam, R.; Subramanian, P. S.; Suresh, E.; Valkonen, A.; Rissanen, K.; Albrecht, M. Coordinatively unsaturated lanthanide(III) helicates: Luminescence sensors for adenosine monophosphate in aqueous media. Angew. Chem., Int. Ed. 2016, 55, 9625−9269. (25) Sutradhar, M.; Alegria, E. C. B. A.; Mahmudov, K. T.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L. Iron(III) and cobalt(III) complexes with both tautomeric (keto and enol) forms of

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01204. Characterization data of all the ligands and the complexes such as ESI-MS, UV−vis, FT-IR, NMR, and crystal data and the HPLC profile and characterization of catalytic product (PDF) Accession Codes

CCDC 1832297 and 1835264 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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AUTHOR INFORMATION

Corresponding Author

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

Kari Rissanen: 0000-0002-7282-8419 Palani S. Subramanian: 0000-0002-9035-0973 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS CSMCRI communication No. 086/2018. R.A. and E.C. acknowledge the CSIR for SRF. A.V. kindly acknowledges the Academy of Finland (grant no. 314343) for financial support. P.S.S. acknowledges the CSIR for Indus Magic Project No. CSC-0123 and DST-SERB New Delhi (project Nos. SR/ S1/IC-23/2011 & SR/S2/RJN-62-2012) for financial support. We are thankful to ADCIF for the instrument support.

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REFERENCES

(1) Constable, E. C. Stereogenic metal centres − from Werner to supramolecular chemistry. Chem. Soc. Rev. 2013, 42, 1637−1651. (2) Ernst, K. H.; Wild, F. R. W. P.; Blacque, O.; Berke, H. Alfred Werner’s Coordination Chemistry: New Insights from Old Samples. Angew. Chem., Int. Ed. 2011, 50, 10780−10787. (3) Piguet, C.; Bernardinelli, G.; Hopfgartner, G. Helicates as Versatile Supramolecular Complexes. Chem. Rev. 1997, 97, 2005− 2062. (4) Albrecht, M. Let’s Twist Again” Double-Stranded, TripleStranded, and Circular Helicates. Chem. Rev. 2001, 101, 3457−3498. (5) Mateos-Timoneda, M. A.; Crego-Calama, M.; Reinhoudt, D. N. Supramolecular chirality of self-assembled systems in solution. Chem. Soc. Rev. 2004, 33, 363−372. (6) Prabaharan, R.; Fletcher, N. C.; Nieuwenhuyzen, M. Selfassembled triple helicates with preferential helicity. J. Chem. Soc., Dalton Trans. 2002, 602−608. G

DOI: 10.1021/acs.inorgchem.8b01204 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry aroylhydrazone ligands: catalysts for the microwave assisted oxidation of alcohols. RSC Adv. 2016, 6, 8079−8088. (26) Arai, T.; Mizukami, T.; Yanagisawa, A. Reaction optimization using solid-phase catalysis-CD HTS: Nb-Imidazoline−Cu(I)-Catalyzed Asymmetric Benzoylation of 1,2-Diols. Org. Lett. 2007, 9 (6), 1145−1147. (27) Nitabaru, T.; Nojiri, A.; Kobayashi, M.; Kumagai, N.; Shibasaki, M. Anti-selective catalytic asymmetric nitroaldol reaction via a heterobimetallic heterogeneous catalyst. J. Am. Chem. Soc. 2009, 131, 13860−13869. (28) Chinnaraja, E.; Arunachalam, R.; Choudhary, M. K.; Kureshy, R. I.; Subramanian, P. S. Binuclear Cu(II) chiral complexes: synthesis, characterization and application in enantioselective nitroaldol (Henry) reaction. Appl. Organomet. Chem. 2016, 30, 95−101.

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DOI: 10.1021/acs.inorgchem.8b01204 Inorg. Chem. XXXX, XXX, XXX−XXX