Design and Enantioresolution of Homochiral Fe(II)-Pd(II) Coordination

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Design and Enantioresolution of Homochiral Fe(II)-Pd(II) Coordination Cages from Stereolabile Metalloligands: Stereochemical Stability and Enantioselective Separation Ya-Jun Hou, Kai Wu, Zhang-Wen Wei, Kang Li, Yu-Lin Lu, Cheng-Yi Zhu, Jing-Si Wang, Mei Pan, Ji-Jun Jiang, Guangqin Li, and Cheng-Yong Su J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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Journal of the American Chemical Society

Design and Enantioresolution of Homochiral Fe(II)-Pd(II) Coordination Cages from Stereolabile Metalloligands: Stereochemical Stability and Enantioselective Separation Ya-Jun Hou,† Kai Wu,† Zhang-Wen Wei,† Kang Li,† Yu-Lin Lu,† Cheng-Yi Zhu,† Jing-Si Wang† Mei Pan,† Ji-Jun Jiang,† Guang-Qin Li,† and Cheng-Yong Su*,†,‡ † MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China ‡ State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China. Supporting Information Placeholder

ABSTRACT: The stereochemistry of chiral-at-metal complexes is much more abundant, albeit complicated, than chiral-at-carbon compounds, but how to make use of stereolabile metal-centers remains a formidable challenge due to highly versatile coordination geometry of metal ions and racemization/epimerization problem. We demonstrate herein a stepwise assembly of configurationally stable [Pd6(FeL3)8]28+ (/-MOCs-42) homochiral octahedral cages from unstable D3-symmetry tris-chelate-Fe type metalloligands via strong face-directed stereochemical coupling and facile chiral-induced resolution processes based on stereodifferentiating hostguest dynamics. Kinetic studies reveal that the dissociation rate of MOC-42 cages is 100-fold slower than Fe-metalloligands and the racemization is effectively inhibited, making the cages retain their chirality over extended periods of time (> 5 months) at room temperature. Recyclable enantioseparation of atropisomeric compounds has been successfully achieved, giving up to 88% ee.

INTRODUCTION Supramolecular chirality in coordination chemistry is plentiful because of not only numerous chiral information brought about by chiral ligands in the coordination sphere, but also enormous stereogenic metal-centers generated by a large variety of stereoconfigurations from versatile coordination geometry.1-9 However, synthesis and predetermination of chiral-atmetal complexes from achiral ligands are very cumbersome as many possible stereoisomers have to be separated and notorious racemization or epimerization phenomena occur for stereolabile metal-centers.10-16 As a keystone of coordination chemistry, the D3-symmetry tris-chelate-M complexes of octahedral geometry with chirality simply originated from propeller-like arrangement of bidentate ligands are vigorously studied in chiral induction/recognition,17-21 enantioseparation,22-24 asymmetric catalysis,25-27 and pharmaceuticals.28,29 Nevertheless, in contrast to the kinetically inert stereogenic metal-centers like in Ru(phen)32+, the chiral complexes of first-row transition metals which is abundant and easy access to synthesis are often highly liable to racemize, hampering their application towards enantioseparation, catalysis and pharmaceuticals. The chiral tris-chelate-Fe complexes are known for their good chemical but low stereoconfigurational stability, and earlier research disclosed a racemization mechanism comprising relatively faster intramolecular racemization and slower ligand dissociation paths.30-34 Much efforts were made on stabilization of configurationally labile Fe-centers, including introduction of optically active counterions,35,36 modification of ligands with chiral groups,37,38 utilization of connecters between adjacent ligands,39 and integration of chiral centers into

supramolecular architectures.11,27,28,40-42 On the hand, formation of metal-organic cages (MOCs) has been proven an unique strategy for effective chirality communication/transformation and stereochemical control owing to mechanical coupling between chiral vertices/edges.3,9,24,43-47 However, feasible enantioresolution process for spontaneously formed cage enantiomers and their potential application are still demanding.43-46 We previously reported stepwise assembly of heterometallic Pd(II)-Ru(II) MOCs-1652 and succeeded in synthesis of homochiral /-MOCs-16 based on the kinetic inertness of /-Rumetalloligands.24 Herein, we expand this stepwise strategy to the stereolabile tris-chelate-Fe type metalloligand which is much more convenient for synthesis, demonstrating its excellent stabilization when incorporated into Fe(II)-Pd(II) octahedral MOCs-42 (Scheme 1). A strong face-directed stereochemical coupling is verified from racemization kinetic studies, and practical chiral-induced resolution processes are established, giving enantiopure /-MOCs-42 with substantial stereochemical stability competent for reusable enantioseparation of racemic atropisomers. RESULTS AND DISCUSSION Stepwise assembly of racemic MOC-42. The FeL32+ metalloligand was prepared by a fast and mild reaction between Fe(BF4)26H2O and 2-(pyridin-3-yl)-1H-imidazo[4,5f][1,10]phenanthroline (L) in DMSO solution at room temperature (RT, molar ratio Fe:L = 1:3). Without tedious purification, Pd(CH3CN)4(BF4)2 was directly added into above reaction solution (Fe:Pd = 4:3), and the assembly converged on the formation of sole heteronuclear [Pd6(FeL3)8](BF4)28

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Scheme 1. Synthetic route for stereolabile FeL32+ racemate and stereostable Pd6(FeL3)828+ /-MOCs-42. Two chiral-induced enantioresolution processes by addition of R/S-BINOLs are illustrated, through a post-assembly process after rac-MOCs-42 cages formation and an in situ-assembly process during secondary assembly, leading to enantiomeric or diastereoisomeric products following distinct kinetic and thermodynamic resolution mechanisms.

(desinated as MOC-42 neglecting anions) coordination cage either at 80 oC for 5 hr or at RT for 22 hr (Scheme 1). The solution structure of the cage has been well established by 1H NMR, 1H-1H COSY, 1H DOSY, MS spectra and 1H NMR titration experiments52 (Figures 1a and S1-4), in which the cage size was estimated by DOSY analysis as 32 Å in diameter (Figure S2, lg D = -10.04), and the cage formula was identified by the high resolution electrospray ionization time of flight mass spectrometry (HR ESI-TOF-MS) with a series of successively valent {[Pd6(FeL3)8]28++nBF4-}(28-n)+ species (Figure S4). The stepwise assembly manner of MOC-42 was testified by the controlled experiments via altering the feeding order of Fe2+, Pd2+ salts and L ligand (Figure S5). It is evident that the primary assembly of FeL32+ metalloligand before the secondary assembly with Pd2+ is prerequisite. In case Pd2+ has a chance to react with the free L ligand either before adding Fe2+ or simultaneously with Fe2+, the product will diverse from MOC-42 and cannot convert to MOC-42 even by heating. This indicates a preferential directing effect of FeL32+ metalloligand in the assembly of heterometallic Fe(II)-Pd(II) cage, implying that the good thermodynamic stability of MOC-42 relies mainly on retarded Fe-L dissociation rate rather than stonger competition over Pd-L coordination (vide infra). Homoconfiguration of individual cages in rac-MOCs-42. The definite Pd6(FeL3)828+ cage structure was unveiled from the single-crystal diffraction analysis of the crystals obtained by vapor diffusion of THF into the water solution of MOC-42, which displays a similar truncated octahedon as our earlier reported Pd6(RuL3)828+ MOC-16 (Figures 2 and S6),52 in full agreement with above solution structural characterization. The structural analysis discloses that each individual cage in racMOCs-42 comprises eight same handed FeL32+ metalloligands. This is indicative of effective chirality communication among eight labile /-FeL32+ stereogenic centers,3,24,48 revealing that the specific stereochemical information of one face-FeL32+ centers can be exactly transformed to others via the bridging Pd2+-vertices, thus controlling the homoconfiguration of the whole cage. Since coordination of L ligand with Fe2+ in lack of any chiral factor should always lead to equal amount of - and -FeL32+ to form rac-FeL32+ racemate, the stereoselective secondary assembly with Pd2+ is expected to give equivalent

homochiral -MOC-42 (abbr. -MOC-42) and -MOC-42 (abbr. -MOC-42) as racemic racMOCs-42 (Scheme 1 and Figure S6).

Figure 1. (a) 1H NMR spectra of L and FeL32+ in DMSO-d6, and MOC-42, R-BINOL@-MOC-42 and R-BINOL in DMSOd6/D2O (1/5, v/v, 400 MHz, 298 K). Proton shifts shown by the arrows. (b) 1H NMR enantiodifferentiation experiments for capture of R/S-BINOLs by /-MOCs-42 (DMSO-d6/D2O, 1/5, v/v, 400 MHz, 298 K). Red circles denote signals of encapsulated guests.


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Journal of the American Chemical Society

Figure 2. Crystal structures of -MOC-42 showing internal void by yellow ball (left) and S-BINOL@-MOC-42 with each cage (in space-filling mode) capturing eight S-BINOL guests (in capped-sticks mode) onto the window pockets.

Chiral-induced resolution of /-MOCs-42. To verify stereocontrol and stablization of labile - and -FeL32+ in the cage formation, we added enantiopure R- or S-BINOLs in a solution of rac-MOCs-42 to drive slow cocrystallization via vapor diffusion of diethyl ether, and single-crystals of SBINOL@-MOC-42 and R-BINOL@-MOC-42 enantiomers were successfully obtained, respectively. This means a postassembly process for enantiomeric resolution of rac-MOCs-42 by virtue of chiral R- or S-BINOL guests is feasible as illustrated in Scheme 1, which is based on the chiral-induced slow cocrystallization of thermodynamic S-BINOL@-MOC42 or R-BINOL@-MOC-42, respectively (Figure S6 and Table S1-4). In this way, chiral BINOLs act not only as cocrystallizing reagents, but also as enantioselectors. The single-crystal analysis confirms the absolute self-organization of eight - or -FeL32+ in either -MOC-42 or -MOC-42 cages, and each homochiral cage encapsulate eight S- or RBINOLs guests into its pocket of windows, respectively

(Figure 2 and S6). Furthermore, the obtained chiral crystals were washed with diethyl ether for several times and redissolved in CH3CN to perform UV-Vis absorption and CD spectra measurements. As shown in Figure 3a, two enantiomers exhbit mirrored CD signals with the maximum peak at 300 nm reflecting -* transition of L at 283 nm. The first Cotton effect around 550 nm corresponds to the 1MLCT absorption band of FeL32+ metalloligand, showing positive signal for -MOC-42 and opposite one for -MOC-42, which is in accordance with analogous chiral tris-chelate-Fe complexes.23,38 The above post-assembly method is applied for resolution of homochiral /-MOCs-42 after stable racemic MOCs-42 are formed. Considering the stereoconfigurational lability of free FeL32+ itself and steady interconversion between - and FeL32+ isomers,30-34 we are interested to know if it is possible to control the in situ stereoselective assembly of MOC-42 by addition of chiral auxiliary.35 Thus, we introduced enantiopure R- or S-BINOLs to rac-FeL32+ and Pd2+ mixture before the secondary assembly step (designated as in situ-assembly process shown in Scheme 1), and CD spectra were employed to monitor configuration transformation. As seen from Figure 3b, upon addition of excess R- or S-BINOLs and stoichiometric Pd2+ in together to a solution of rac-FeL32+ to form initial mixture, the first Cotton effect resembling - or -MOC-42 appeared, indicating chiral-induced intermediate formation of enantioexcess - or -MOC-42, respectively. In contrast, no CD signals can be detected in absence of enantiopure BINOLs during rac-MOC-42 assembly. The CD signals remarkably increased along reaction time, suggesting an effective templating assembly of one enantiomer of the cage from a racemic mixture of rac-FeL32+ induced by enantiopure BINOL, and the best CD profile was obtained for precipitating product by adding diethyl ether. It is also noticeable that, during this in situ-

Figure 3. (a) CD and UV-Vis absorption spectra of homochiral /-MOCs-42 obtained by different resolution processes (2.0×10-6 M in CH3CN, 298 K). “Post” or “in situ” represent post- or in situ-assembly processes, “R or S” refer to R- or S-BINOLs, “crys” or “ppt” means crystallization or precipitation, respectively. (b) CD spectra recorded during in situ-assembly process (2.0×10-6 M, CH3CN, 298 K). (c,d) Stereochemical stability of /-Fe(phen)3(PF6)2 (c, 1.6×10-5 M in CH3CN) and /-MOCs-42 (d, 2.0×10-6 M in D2O) depending on time at RT. (e,f) UV-Vis absorption spectra showing dissociation of FeL32+ (0.8 mM, 1 eq.) and rac-MOC-42 (0.1 mM, 0.125 eq. vs. FeL32+) in 25% CH3CN aqueous solution containing 1 M H2SO4 (2,500 eq. H+ vs. FeL32+) at 35.0(±0.1) oC. Spectra were measured after 20 times dilution of the original solution.



assembly process, complex CD signals appeared in UV region, implying presence of intermediate stereoisomers possible for configuration transformation to a major enantioexcess product, owing to a reason that rac-FeL32+ becomes biased towards an enantiomeric excess of one hand of FeL32+ induced by the chiral BINOL, especially during the secondary assembly with Pd2+. Upon fast precipitation, the homochiral /-MOCs-42 were obtained as major product, justifying effectiveness of this in situ-assembly method. Different from above post-assembly cocrystallization process, the CD signals of homochiral /-MOCs-42 in situinduced by R/S-BINOLs display opposite guest-host correlation, namely, R-BINOLs inducing formation of -MOC-42, while S-BINOLs inducing -MOC-42, in reverse to the cocrystallization of S-BINOL@-MOC-42 and R-BINOL@MOC-42 enantiomers (Figure 3a). This means the postassembly and in situ-assembly processes show different formation mechanism of homochiral /-MOCs-42. To further understand enantiomer selection between /-MOCs-42 and R/S-BINOLs, slow single-crystal cultivation from the in situassembly reaction mixture was carried out. Structural analysis revealed that, similar with the post-assembly cocrystallization from rac-MOCs-42, the R- or S-BINOLs cocrystallized with - or -MOCs-42, respectively, despite the solution is dominated by the opposite cage stereoconfigurations. Moreover, the in situ chiral-induced cage enantiomers, either from precipitation or cocrystallization, show significantly enhanced Cotton effects when compared to the resolved products from postassembly process (Figure 3a), suggesting the in situ-assembly process represents a better approach with superior enantiopurity of homochiral product than the post-assembly process. We attribute the fast precipitation by virtue of chiral-induced in situ-assembly to kinetic resolution, while the slow cocrystallization with chiral BINOLs in both post- and in situ-assembly processes to thermodynamic resolution (Scheme 1). The distinct stereoselectivity between /-MOCs-42 and R/SBINOLs in kinetic and thermodynamic resolution processes can be easily understood from our previous study on the hostguest interaction dynamics between analogous RuL32+-based /-MOCs-16 and R/S-BINOLs.24 In solution, the -MOC-42 cage may capture R-BINOL via a faster and low energy path than S-BINOL, thus templated formation of -MOC-42 is kinetically favored, and vice versa for the chiral-induced formation of -MOC-42 by S-BINOL. On the contrary, slow cocrystallization may be thermodynamically favored for the SBINOL@-MOC-42 and R-BINOL@-MOC-42 enantiomers which demands high activation energy. This speculation is supported by the distinct solution dynamics between a pair of host-guest diastereoisomers whereas the similarity between a pair of host-guest enantiomers as revealed by the 1H NMR enantiodifferentiation experiments (Figure 1b).24,53,54 An estimation of guest binding constants by 1H NMR titration experiments and Hill equation analysis also confirm a stronger hostguest interaction for the thermodynamically favored pairs than the kinetic preferred ones (Figure S7-14). Optimization of the in situ-assembly precipitation conditions disclosed that, the excess amount of chiral BINOLs and reaction temperature impact the enantiopurity of the resolved homochiral /MOCs-42, addition of 72 eq. enantiopure BINOLs at 80 oC leading to the highest optical purity thereof best enantioresolution (Figure S15).

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Cage stabilization effect. Stereochemical stability of the resulting homochiral /-MOCs-42 was investigated by CD spectra with comparison to the classic /-Fe(phen)3(PF6)2 enantiomers freshly resolved55 (Figure S16). As seen from Figure 3d, the CD profile of /-MOCs-42 are maintained in D2O at RT even after 5 months, while a rapid decay of CD signals is observed for /-Fe(phen)32+ with a half-life period less than 3 min (Figure 3c), evidently proving the stereoconfiguration around Fe-centers is strongly fixed by cage integrity. The specific optical rotation tests disclose less than 6% decay even after heating at 60 oC for one day (/MOCs-42: []25D (c = 0.04, CH3CN), , +1316.6, , -1281.5; decrease after heating, []25D, , -79.0, , -54.1), also verifying the stereostability of the homochiral cages. In order to explore the racemization process of /-MOCs-42 in contrast to /-FeL32+, an acidic solution has to be used to accelerate dissociation,31 and the kinetics experiments were carried out with UV-Vis spectral measurements under guidance of pseudo first-order reaction theory (see details in SI). We firstly checked the chemical stability of MOC-42 (0.1 mM) in CD3CN/D2O (1/4, v/v) mixture containing different amount of D2SO4 by 1H NMR (Figure S17). The proton signals became well resolved and retained the overal partern up to 8 M D+ concentration when measured immediately, suggesting strong chemical stability against acid. When keep the acidic solution for 6 days, dissociation of a few free FeL32+ was observed for the solutions above 4 M D+. Thus a solution containing 2 M D+ (2500 eq. D+) was chosen for stability study. As shown in Figure 3e and 3f, the maximum absorption peak of 1MLCT at 520 nm nearly disappears for FeL32+ after 1 d’s standing at 35.0(±0.1) oC, yet only slightly dropped for MOC-42 under the same condition even after 21 d. 1H NMR spectra confirmed that the cage structure remained unchanged after 21 d’s acidic treatment (Figure S18). Table 1. Kinetic and thermodynamic parameters of racemizationa

FeL32+

21.0 25.0 30.0 35.0 -

kd 10-4 min-1 2.42 6.43 14.0 31.2 -

MOC-42

21.0 25.0 30.0 35.0

-b 0.065 0.171 0.384

Compds

T oC

cFe(phen) 2+25.0 3

42

dFe(PBS) 43

16.8e

25.0

Ea kcal mol-1 30.3 ± 0.9 32.5 ± 1.4 32.1 ± 0.5 31.1e

≠G

≠H

≠S

kJ mol-1 101.5

kJ mol-1 124.4 ± 3.7 133.3 ± 6.0 -

J K-1 mol-1 76.9

112.7 -

130 ± 20

69.4 90 ± 15

a

In 20% CH3CN aqueous solution containing 0.5 M H2SO4. Initail concentration: FeL32+ (0.04 mM); MOCs-42 (0.005 mM). b Spectral change is too tiny to distinguish within observation time. c In 1 M HCl, Ref 31. dIn water containing 0.2 mM of Ni(ClO4)2, Ref 22. eCalculated from Ref 22.

To obtain detailed kinetic information for the dissociation (kd) and non-dissociative intramolecular racemization (kr) rate


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Journal of the American Chemical Society constants, more extreme conditions were applied with dilute FeL32+ (0.04 mM, 1 eq.) and MOCs-42 (0.005 mM, 0.125 eq.) in 25% CH3CN aqueous solution containing 1 M H2SO4 (25000 eq. H+) at variable temperatures (Figures S19-22). By virtue of time- and temperature-dependent decrease of the characteristic 1MLCT absorption intensity around 520 nm, the kd values were easily calculated, as well as the apparent activation energy (Ea) and Gibbs energy of activation ≠G according to Arrhenius and Erying--Polanyi equations (see SI for detailed data processing). The effort to obtain kr of MOC42 from CD measurement failed because the contribution of intramolecular racemization in MOC-42 is trivial, which is in stark contrast to Fe(phen)32+ in which kr is about 1 order of magnitude faster than kd.30-34 As listed in Table 1 and S5, the dissociation rates kd of FeL32+ are comparable with those of known tris-chelate-Fe compounds (Table S5), amounting to nearly 2 order of magnitude larger than those of MOC-42 under the same conditions, indicating 100-fold slower dissociation of FeL32+ after incorporated in cage structure. The Ea of MOC-42 is a little higher than that of FeL32+, suggesting the stabilization effect arises from kinetic factor rather than lower energy barrier of FeL32+ than MOC-42. Furthermore, the higher Gibbs energy ≠G of MOC-42 is both contributed from increased enthalpy ≠H and decreased entropy ≠S in compraison with those of FeL32+, revealing that dissociative racemization of MOC-42 is also thermodynamically disfavored than FeL32+. These kinetic and thermodynamic results account for intrinsic stereostability of chiral /-MOCs-42, in which the mechanical coupling among eight face-FeL32+ stereogenic centers are synergistically enhanced through six bridging Pdvertices, and racemization of homochiral /-MOCs-42 has to involve multiple synchronous configuration transformation,48 thus entirely inhibited in normal conditions.

Figure 4. CD enantiodifferentiation experiments for interactions between /-MOCs-42 (2.0×10-6 M) and R/S-6-Br-BINOLs (CH3CN/H2O, 1/100, v/v, 298 K). MOC-42+6-Br-BINOL represents superposition of individual spectra of each components for comparison. Local zoom spectra show difference of Cotton effect of 1MLCT absorption at 550 nm.

Enantioseparation of atropisomers. The facial resolution and sufficient stereostability of enantiopure /-MOCs-42 pave the way for practical application. Based on our previous finding that homochiral /-MOCs-16 analogues can separate atropisomeric enantiomers of C2-symmetry,24 we carried out enantioseparation experiments using the excellent optical /-

MOCs-42 obtained from in situ-process precipitation. Three racemic atropisomers were tested: rac-BINOL, rac-6-BrBINOL (6,6’-dibromo-1,1’-bi-2-naphthol) and racbifendatatum. Before enantioseparation experiments, homochiral /-MOC-42 samples were washed with diethyl ether for several times until no signals of BINOLs in the filtrate can be detected on HPLC spectra. Firstly, the hostguest stereochemical interactions between the pairs of /MOCs-42 and R/S-BINOLs was examined by 1H NMR enantiodifferentiation experiments (Figure 1b and S23), of which two pairs of S-BINOL@-MOC-42, R-BINOL@MOC-42 and S-BINOL@-MOC-42, R-BINOL@-MOC-42 diastereomers and two pairs of R-BINOL@-MOC-42, SBINOL@-MOC-42 and S-BINOL@-MOC-42, RBINOL@-MOC-42 enantiomers are formed. The results obviously display distinguishable solution dynamics between the host-guest diastereomeric pairs and enantiomers pairs.24 In addition, CD spectral enantiodifferentiation experiments for /-MOCs-42 and R/S-BINOLs or R/S-6-Br-BINOLs also confirmed the distinct enantioselective interactions between the host-guest enantiomers and diastereoisomers (Figures 4 and S24, Table S6), implying specific stereochemical recognition dynamics as seen from the difference in the first Cotton effect. Under above guidance of recognition effect between homochiral /-MOCs-42 and atropisomeric enantiomers with specific configuration matching,24 either a homogeneous or a heterogeneous methods were employed for enantioseparation of racemic atropisomers (see details in SI). A kinetically driven separation procedure is found the same as homochiral /-MOCs-16 analogues do.24 The enantiomeric excess (ee%) values were determined by high-performance liquid chromatography (HPLC, Figures S25-29) and listed in Table 2. All pairs of atropisomers were successfully resolved with the highest ee values reaching to 66% in one cycle of enantioseparation. A relatively low performance in resolving R/S-bifendatatum enantiomers was found, which were known to gradually racemize in solution at RT.56,57 As an efficient antidote for liver, R-bifendatatum has high biological activities while S-bifendatatum has no pharmacological activities at all. Therefore, further effort is underway to enantioseparate this pharmacologically active molecule with chirality enrichment and stabilization by the aid of cage effect. In order to further improve enantiomeric purity, successive separation of R/SBINOLs and R/S-6-Br-BINOLs was applied by recycle of /-MOCs-42, which increase the ee values up to 88% for R/S-6-Br-BINOLs in just two cycles while 86% for R/SBINOLs in four cycles (Table 2). The stereochemical stability of /-MOCs-42 during separation processes was tested by CD measurements (Figure S30), and the little CD signals change proves reusability of enantiomeric /-MOCs-42. Table 2. Enantioselective separation resultsa -MOC-42 -MOC-42 Compds

rac-BINOL rac-6-Br-BINOL rac-bifendatatum rac-BINOLb rac-BINOLc rac-BINOLd rac-6-Br-BINOLb aee,

R,S ratio 65:35 80:20 60:40 78:22 88:12 93:7 80:20

enantiomeric excess, %. respectively.


ee 30 60 20 56 76 86 80 b-dSecond,

R,S ratio 35:65 17:83 39:61 21:79 13:87 8:92 6:94

ee -30 -66 -22 -58 -74 -84 -88

third and fourth cycles,


CONCLUSIONS In summary, we present an easy strategy to stabilize stereolabile tris-chelate-Fe type stereogenic metal-centers via stepwise assembly of homochiral Fe-Pd heterometallic cages for practical application purpose. Excellent enantiopurity of the /-MOCs-42 is achieved through an in situ-assembly process for enantioresolution of rac-MOCs-42, of which the chiralinduced precipitation leads to kinetic resolution while cocrystallization leads to thermodynamic resolution, both are superior to the normal post-assembly process. The intrinsic stereochemical stability of homochiral /-MOCs-42 are thoroughly studied, unveiling strong mechanical coupling among chiral face-FeL32+ centers to lead to 00-fold slow of dissociation rate and entire inhibition of configurational racemization. Successful enantioselective separation is accomplished for racemic atropisomers, giving good ee value via a successive enantioseparation process by recycling /-MOCs-42. This work provide a new avenue to make use of economic albeit stereolabile metal ions in stereochemistry and relevant applications. Moreover, owing the MLCT absorption in visible region and redox behavior of Fe3+/Fe2+, the present homochiral cages may have potential in asymmetric photocatalysis and photoelectronic devices. EXPERIMENTAL DETAILS General. All reagents and solvents were used as commercially purchased without additional purification. 2-(pyridin-3yl)-1H-imidazo[4,5-f][1,10]phenanthroline (L) was prepared as described in the literature.58 1H NMR, 1H-1H COSY and 1H DOSY spectra were recorded on Bruker AVANCE III 400 (400 MHz). Chemical shifts were quoted in parts per million (ppm) referenced to the appropriate solvent peak. The data were processed by MestReNova software. HR ESI-TOF-MS spectra were measured on Bruker maXis 4G ESI-Q-TOF with Tunningmix purchased from Agilent as calibrating agent. Data analysis was processed on Bruker DataAnalysis program. Single crystal reflection intensity data were collected at 150 K on an Agilent SuperNova single crystal diffractometer using Cu radiation (λ = 1.5418 Å). The structures were solved by direct methods and refined using the SHELXTL program package. UV-Vis and CD spectra were measured by a Shimadzu UV3600 UV-Vis-NIR spectrophotometer and a JASCO J-810 spectropolarimeter equipped with temperature controllers, respectively. Optical rotations were recorded on an Anton Paar MCD-200 polarimeter. HPLC was performed by Agilent-2000 equipped with Daicel chiral chromatographic columns and a UV detector with detection wavelength of 254 nm. Synthesis of metalloligands FeL32+. Fe(BF4)26H2O (101.3 mg, 0.3 mmol) and L (267.6 mg, 0.9 mmol) were mixed in a 25 mL round-bottom flask equipped with a magnetic stir bar. After addition of 9 mL DMSO (Superdry, J&K Seal), the mixture turned claret color as the reactants were dissolved, which was then stirred at RT for 1 hr. Precipitation by adding ethyl acetate yield a scarlet solid as FeL32+ metalloligands (yield 318.0 mg, 94.5%), which were confirmed by 1H NMR spectra as well as HR ESI-TOF-MS. 1H NMR (400 MHz, DMSO-d6, 298 K): 14.58 (s, 3H), 9.50 (s, 3H), 9.14 (d, J = 7.7 Hz, 3H), 9.07 (d, J = 9.2 Hz, 3H), 8.79 (d, J = 4.4 Hz, 3H), 8.63 (d, J = 8.3 Hz, 3H), 7.82 (d, J = 19.6 Hz, 6H), 7.77-7.67 (m, 9H). HR ESI-TOF-MS: m/z calcd. for [C54H33N15Fe]2+ (FeL32+) 473.6192, found 473.6189. Synthesis of rac-MOC-42. To a 50 mL round-bottom flask containing a magnetic stir bar was added 6 mL DMSO solu-

tion of FeL32+ (224.3 mg, 0.2 mmol). Then 2 mL DMSO containing Pd(CH3CN)4(BF4)2 (66.7 mg, 0.15 mmol) was added into the above solution with stirred. Either after heated at 80 o C for 5 hr or stirred at RT for 22 hr, 30 mL ethyl acetate was added to precipitate the product, which was then centrifuged, washed with ethyl acetate, and dried in vacuum at RT to yield a carmine powder confirmed to be the target rac-MOC-42, [Pd6(FeL3)8](BF4)28 (160 mg, 60%). 1H NMR (400 MHz, DMSO-d6/D2O, 1/5, v/v, 298 K): 10.01 (s, 24H), 9.17 (s, 24H), 8.82 (d, J = 8.4 Hz, 24H), 8.71 (m, 24H), 7.74 (m, 24H), 7.56 (s, 24H), 7.46 (m, 48H), 7.37 (m, 72H). HR ESI-TOFMS: m/z calcd. for [C432H264N120B19F76Pd6Fe8]9+ 28+ {[Pd6(FeL3)8] +19BF4 } 1096.6022, found 1096.5987; for [C432H264N120B18F72Pd6Fe8]10+ {Pd6(FeL3)8]28++18BF4-} 978.2416, found 978.2382; for [C432H264N120B17F68Pd6Fe8]11+ {[Pd6(FeL3)8]28++17BF4-} 881.4003, found 881.3992. Syntheses of homochiral /-MOC-42. The homochiral - or -MOCs-42 were obtained through two chiral-induced enantioresolution processes by addition of chiral R- or SBINOLs. One is post-assembly resolution of cages via slow cocrystallization with R- or S-BINOLs after the secondary assembly of rac-MOCs-42 enantiomeric pairs, and the other is an in situ-assembly process during secondary assembly of enantiomeric - or -MOCs-42 as major products templated by R- or S-BINOLs, followed by fast precipitation or slow cocrystallization. The slow cocrystallization proceeds via a thermodynamic resolution mechanism, while the fast precipitation follows a kinetic resolution mechanism. (1) Post-assembly process. Slow cocrystallization of /MOCs-42 with R/S-BINOLs. To 1 mL CH3CN solution of rac-MOC-42 (5.3 mg, 1 eq.) was added powder of enantiopure R- or S-BINOLs (3.4 mg, 24 eq.). After filtration, the solution was transferred to several 1 mL glass tubes and placed in diethyl ether atmosphere. After 2 weeks’ standing, red block single-crystals suitable for single-crystal X-ray diffraction were obtained. The crystals were collected and washed by diethyl ether for several times until no BINOLs in the filtrate were detectable by HPLC, and then dried in vacuum at RT (yield 0.5 mg, 9%). (2) In situ-assembly process. (a) Slow cocrystallization of /-MOCs-42 with R/S-BINOLs. FeL32+ (112 mg, 0.1 mmol) and enantiopure R- or S-BINOLs (258 mg, 0.9 mmol) were mixed up in advance and dissolved by 10 mL CH3CN in a 50 mL round-bottom flask equipped with a magnetic stir bar. Then 2 mL CH3CN containing Pd(CH3CN)4(BF4)2 (33.3 mg, 0.075 mmol) was added into the above solution with stirring. The solution was heated to 80 oC and reacted for 22 hr, after which the resulting solution was cooled to RT, filtered and transferred to several 1 mL glass tubes, then placed in diethyl ether atmosphere. After 2 weeks’ standing, red block single crystals suitable for structural analysis were obtained, collected and washed by diethyl ether for several times until no BINOLs in the filtrate were detectable by HPLC, and then dried in vacuum at RT (yield 11.5 mg, 8.6%). (b) Fast precipitation of /-MOC-42 with R/S-BINOLs. The procedure was similar to above slow cocrystallization except for direct precipitation by addition of 40 mL diethyl ether to the resulting solution. The precipitate was isolated by centrifugation and washed by diethyl ether for several times until no BINOLs in the filtrate were detectable by HPLC, and then dried in vacuum at RT to yield a carmine powder (121 mg, 91%).

ASSOCIATED CONTENT


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Journal of the American Chemical Society Supporting Information Details of syntheses, characterization, spectra, resolution, separation, kinetics and crystallography (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected].

ACKNOWLEDGMENT This work was supported by the NSFC (21821003, 21720102007, 21573291, 21890380), LIRT Project of GPRTP (2017BT01C161) and the FRF for the Central Universities for funding.

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Journal of the American Chemical Society SYNOPSIS TOC


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Design and Enantioresolution of Homochiral Fe(II)-Pd(II) Coordination

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