Tweezer Amino Alcohol Complexes - ACS Publications

Article Cite This: Inorg. Chem. 2017, 56, 15203−15215

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Metal-Center-Driven Supramolecular Chirogenesis in Tweezer Amino Alcohol Complexes: Structural, Spectroscopic, and Theoretical Investigations Avinash Dhamija, Bapan Saha, and Sankar Prasad Rath* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India

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S Supporting Information *

ABSTRACT: An apparently rigid dibenzothiophene-bridged zinc(II)/magnesium(II) bisporphyrin host (1M) has been explored for an accurate determination of the absolute configuration of a large series of amino alcohols. At lower substrate concentration, a 1:1 sandwich complex is formed which, upon addition of excess of substrate, converts to the 1:2 host−guest complex with complete inversion of the CD exciton couplet. The intensities of the couplet vary widely just by changing the metal ion (Zn vs Mg) and also vary between 1:1 and 1:2 host−guest complexes. Crystallographic characterizations are reported here for both 1:1 sandwich and 1:2 host−guest complexes using the same pair of host and guest, for the first time, which enable us to scrutinize the structural and geometrical changes systematically in rationalizing their optical properties. The intensity of the CD couplet is largely dependent on how strongly the substrate binds with the host and also their mode of binding. No CD couplet is observed in the spectral region of porphyrin absorption when substrate binds in either exo-endo or exo-exo fashion in the 1:2 host−guest complex. However, intermolecular H bonding between two encapsulated substrates in the 1:2 host−guest complex stabilizes the endo-endo conformer in which two porphyrin macrocycles are forced to be oriented in a clockwise/anticlockwise direction to produce an intense CD couplet. Such an endoendo binding of (S)-2-aminobutan-1-ol (S-AB) has resulted in a highly intense CD couplet with 1Mg, while no chiroptic response was observed upon changing the metal to zinc, since S-AB would then bind in an exo-endo form. With an increase in the bulk of the substrate, the endo-endo form first transforms into an exo-endo form which, upon further increase in the bulk of the substrate, converts into an exo-exo complex.

■

INTRODUCTION Supramolecular chirogenesis, one of the most important interdisciplinary fields because of its occurrences in many natural and artificial systems, is a smart combination of supramolecular chemistry and chirality science which deals with different processes such as asymmetry transfer, induction, modulation, and amplification that are solely controlled by various noncovalent forces.1−5 A detailed understanding of various influencing factors is very important to control the process for a variety of useful applications. Exciton-coupled circular dichroism (ECCD) is a very effective, popular, and nonempirical technique for stereochemical identification of chiral substrates.1−4 Porphyrinoids are very attractive chromophores for studying the events mainly because of their interesting photophysical properties that can easily be tunable out of versatile modifications at their periphery and also by metalations at the core.2,3 When an achiral bismetalloporphyrin host and a chiral guest form a host−guest supramolecular complex, a stereospecific twist between the two chromophores is generated upon transfer of chirality from the chiral guest to an achiral host. As a result, a bisignate CD signal (so-called exciton couplet) is produced in the porphyrin spectral region with a sign controlled by the electric transition moment of the interacting chromophores.1−3 © 2017 American Chemical Society

Studies of supramolecular chirality induction in the bisporphyrin tweezer upon complexation with a chiral guest are well reported in the literature.2,3,6−11 The presence of the bulky substituent, weak coordination of the guest, and/or addition of excess substrates often leads to a change in host− guest stoichiometry from 1:1 to 1:2. Multiple coordination sites are available in the host that cumulatively result in the formation of different types of binding motifs even in 1:2 complexes. Some of these conformations would diminish the CD intensity, while others lead to inversion of the CD signal.7,9 Moreover, the influence of the metal center in supramolecular chirogenesis and stoichiometry-controlled chirality inversion are seldom reported. In the present investigation, an apparently rigid dibenzothiophene-bridged zinc(II)/magnesium(II) bisporphyrin host have been used to determine absolute configuration of a series of amino alcohols while the effect of the metal ions has also been thoroughly scrutinized. Synthesis, structure, and spectroscopic properties of series of chiral 1:1 sandwich and 1:2 host−guest complexes of zinc(II) and magnesium(II) bisporphyrin have been discussed, and the origin of chirality has been scrutinized systematically. X-ray Received: October 6, 2017 Published: November 30, 2017 15203

DOI: 10.1021/acs.inorgchem.7b02569 Inorg. Chem. 2017, 56, 15203−15215

Article

Inorganic Chemistry structure determination and density functional theory (DFT) studies permitted us to systematically rationalize the origin of optical activity in the complexes. Detailed investigations pertaining to the role of the central metal ion in controlling the supramolecular chirogenesis would be helpful in the development of efficient chiral sensors for various classes of chiral molecules.

Scheme 1. Synthetic Outline of 1:1 sandwich and 1:2 Host− Guest Complexation with Chiral Amino Alcohols

■

RESULTS AND DISCUSSION A dibenzothiophene-bridged bisporphyrin was synthesized according to the literature procedure.12 Zinc and magnesium were introduced into the free base bisporphyrin cavity following the reported procedures, which afforded zinc(II) bisporphyrin 1Zn and magnesium(II) bisporphyrin 1Mg, respectively, in excellent yields.13 Interactions of the amino alcohol L with metallobisporphyrin host 1M were monitored using UV−visible spectroscopy. Addition of (R)-2-amino-1-phenylethanol (R-A1PE) to 1Zn in dichloromethane results in a red shift (Figure 1A) of the Soret

Chart 1. Achiral Guest (L) Used

1Zn·S-A3MB, 1Zn·S-A3MP, and 1Zn·S-AB, respectively (Figure S2 in the Supporting Information). However, the addition of (S)-2-amino-3-methylbutan-1-ol (S-A3MB) to a dichloromethane solution of 1Mg results in the gradual decrease and hypsochromic shift of the Soret (407 to 406 nm) and Q bands (547 to 546 nm) due to the formation of 1Mg·S-A3MB (Figure S3 in the Supporting Information). Similar spectral changes are also observed with 1Mg upon using other amino alcohols. Job’s continuous variation plot is very helpful in determining the stoichiometry of the host−guest complexes in solution (Figure S4 in the Supporting Information). The maximum changes in the UV−visible spectra of host−guest complexes (1Zn·L and 1Mg·L) were observed at 0.5 molar ratio at a low concentration of the substrate, confirming the 1:1 host−guest complexation. ESI mass spectroscopy reveals a peak at m/z 1350.5410 which is assigned to [1Zn·S-AB + H]+; the isotopic distribution patterns of the experimental mass also correlated well with the theoretical pattern (Figure S5 in the Supporting Information). All of the complexes have been isolated as solids and characterized spectroscopically, including structural characterizations for 1Zn·R-A1PE, 1Zn·S-A3MB, 1Zn·A2MP, and 1Zn· AE. Upon addition of excess guest L, the UV−visible spectra of the 1:1 sandwich complex 1Zn·L changes again! For example, upon further addition of R-A1PE, the Soret (411 to 413 nm) and Q bands (543 to 545 nm and 575 to 578 nm) are more red shifted as the host−guest ratio changes from 1:50 to 1:1550 to form the 1:2 complex 1Zn·(R-A1PE)2 (Figure 1B). Similar spectral changes are also observed when a large excess of S-AB

Figure 1. (A) UV−visible spectral changes of 1Zn (2 × 10−6 M) in dichloromethane upon addition of R-A1PE as the host−guest molar ratio changes from 1:0 to 1:1540 at 295 K. (B) Portion of UV−visible spectra (in dichloromethane at 295 K) comparing 1Zn (blue), 1Zn·RA1PE (brown), and 1Zn·(R-A1PE)2 (red).

(401 to 411 nm), shoulder (413 to 423 nm), and Q bands (534 to 543 nm and 570 to 575 nm) as the host−guest molar ratio changes from 1:0 to 1:50 due to the formation of host−guest complex 1Zn·R-A1PE (Scheme 1). The spectrum of the complex looks very similar to that of the authentic 1:1 sandwich complexes 1Zn·AE and 1Zn·A2MP (Figure S1 in the Supporting Information) prepared separately with achiral 2-aminoethanol (AE) and 2-amino-2-methylpropane-1,3-diol (A2MP) (Chart 1), respectively. Other chiral guests such as (S)-2-amino-2phenylethanol (S-A2PE), (S)-2-amino-3-methylbutan-1-ol (SA3MB), (2S,3S)-2-amino-3-methylpentan-1-ol (S-A3MP), and (S)-2-aminobutan-1-ol (S-AB) have also shown similar spectral changes at their lower concentration to produce 1Zn·S-A2PE, 15204


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Inorganic Chemistry is used as a guest to form 1Zn·(S-AB)2. However, other than intensity no significant shift of the Soret band was observed upon addition of a large excess of (S)-2-amino-2-phenylethanol (S-A2PE) during the conversion of 1:1 sandwich to 1:2 host− guest complex 1Zn·(S-A2PE)2 (Figure S6 in the Supporting Information). Here, metal binds through the alcoholic oxygen of S-A2PE due to steric crowding around the amino group and negligible shifting of the Soret band is thus expected.9b,d Similar spectral changes are also observed for 1Zn upon addition of a large excess of S-A3MB and S-A3MP due to the formation of 1Zn·(S-A3MB)2 and 1Zn·(S-A3MP)2, respectively. Moreover, addition of excess L to the dichloromethane solution of 1Mg·L results in red shifts of the Soret (406 to 408 nm) and Q bands (546 to 548 nm) due to the formation of 1Mg·(L)2. Further addition of chiral guest L produces no change in the UV−vis spectral pattern. ESI-mass spectroscopy reveals peak at m/z 1467.6569 which is assigned to [1Zn·(S-A3MB)2 + H]+; the isotopic distribution patterns of the experimental mass also correlated well with the theoretical pattern (Figure S7 in the Supporting Information). Synthetic outlines of all the complexes along with their abbreviations are displayed in Schemes 1 and 2, while detailed Scheme 2. Synthetic Outline of 1:2 Host−Guest Complexation with Chiral Amino Alcohols

Figure 2. 1H NMR spectra (at 295 K in CDCl3) of (A) 1Zn, (B) 1Zn·SA3MB, and (C) S-A3MB. The inset shows the proton numbering scheme of S-A3MB.

(trace B) displays a large change from that of 1Zn and free SA3MB. Due to encapsulation of the guest within the bisporphyrin cleft, large upfield shifts of the guest protons are observed.8 The most influenced protons are the coordinated NH2/OH group, which are shifted by Δδ = 7.62 ppm as they are lying closest to the porphyrin ring due to its binding with the metal center. Methyl protons of the guest are least influenced with the appearance of two resonances and are shifted by Δδ = 3.09 and 3.33 ppm. The porphyrin ring current is affecting two methyl groups to different degrees, as one methyl group is dipped more inside into the bisporphyrin cleft than the other. The identical 10,20-meso protons are split into two resonances as a result of their different exposure to the ring current effect due to the twisting of two porphyrin moieties, which influences two identical meso protons differently. 1H NMR of other host−guest complexes also displays similar upfield shifts of the guest’s protons (Figures S8−S12 in the Supporting Information). However, the extent of such an upfield shift is less in the magnesium(II) complex because of weaker binding of the guest in the 1:1 sandwich complex. Complete assignments of the guest’s proton resonances are based on their relative intensities, distances from the porphyrin ring, line widths, and comparison with the spectra of similar 1:1 complexes reported in the literature.9b,d,e Crystallographic Characterization. Dark red crystals of 1Zn·AE and 1Zn·A2MP are grown via slow diffusion of acetonitrile into chloroform solution of the respective complexes at room temperature. Perspective views of the molecules are shown in Figure 3A,B, while molecular packings are shown in Figures S13 and S14 in the Supporting Information. The complexes have two zinc centers, each in a five-coordinate square-pyramidal geometry. Zn−Oax and Zn− Nax distances are 2.157(6) and 2.147(6) Å, respectively, in 1Zn· AE, while the metal ions are displaced by 0.28 and 0.31 Å from the mean planes of the C20N4 porphyrinato core. However, the

synthetic procedures and their spectral characterizations are presented in the Experimental Section. While all of the complexes are new, complexation of 1Mg with R-A1PE, SA2PE, S-A3MP, and S-AB has been reported by us earlier.9b 1 H NMR. 1H NMR spectroscopy plays an important role in establishing the formation of 1:1 sandwich complexes in solution. Figure 2 shows the 1H NMR spectra in CDCl3 at 295 K coming from the reaction between 1Zn and S-A3MB, as a representative example. Trace A shows the well-resolved spectrum of 1Zn, while trace B displays the 1H NMR spectrum after the addition of 1 equiv of S-A3MB to form the 1:1 sandwich complex 1Zn·S-A3MB. A similar spectrum was also obtained using the polycrystalline sample of the complex in CDCl3. Trace C, however, shows the spectrum of the free SA3MB. As can be seen, the 1H NMR spectrum of 1Zn·S-A3MB 15205


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Figure 3. Perspective views of (A) 1Zn·AE, (B) 1Zn·A2MP, and (C) 1Zn·S-A3MB showing 50% thermal contours for all non-hydrogen atoms at 100 K (H atoms have been omitted for clarity).

1Zn host can bind with 2-amino-2-methylpropane-1,3-diol (A2MP) through either O,O or O,N coordination to form a 1:1 host−guest complex. Since the zinc(II) ion has a greater binding affinity toward nitrogen, only O,N coordination is formed. This also has resulted shorter Zn−Nax distance than that of Zn−Oax in 1Zn·A2MP. Dark red crystals of 1Zn·S-A3MB are grown by slow diffusion of acetonitrile into a solution of 1:1 mixture of 1Zn and S-A3MB in dichloromethane in air at room temperature. A perspective view of the crystal structure is depicted in Figure 3C, while the molecular packing is displayed in Figure S15 in the Supporting Information. Both of the zinc(II) centers have five-coordinate square-pyramidal coordination, and the metal ions are displaced toward the guest by ∼0.32 Å. The Zn−Nax and Zn−Oax distances are 2.164(5) and 2.187(5) Å, respectively. The Zn···Zn nonbonding distance in the molecule is 7.12 Å. Slow diffusion of n-hexane into the dichloromethane solution of 1Zn and R-A1PE in air at room temperature produces dark red crystals. One such crystal yielded two structurally and geometrically different molecules of 1Zn·R-A1PE (molecule I) and 1Zn·(R-A1PE)2 (molecule II) in the asymmetric unit. For the first time, crystallographic characterizations were done for both 1:1 sandwich and 1:2 host−guest complexes using the same pair of host and guest, which enabled us to scrutinize the structural and geometrical changes systematically to rationalize the complete inversion of optical activities between the complexes. Perspective views are shown in Figure 4A, while the molecular packing is demonstrated in Figure 4B. In molecule I, 1Zn holds the guest inside the cavity by ditopic binding to form the 1:1 sandwich complex 1Zn·R-A1PE, while RA1PE binds to the zinc centers in an endo-endo fashion to form the 1:2 host−guest complex in 1Zn·(R-A1PE)2 (molecule II). Each zinc(II) center acquires a five-coordinate squarepyramidal geometry, and the Zn···Zn nonbonding distances are 7.16 and 7.20 Å for molecules I and II, respectively. The average metal ion displacements from the least-squares plane of the C20N4 porphyrinato core are different: 0.37 and 0.29 Å for molecules I and II, respectively. The Zn−Oax distance of

Figure 4. (A) Perspective views of 1Zn·R-A1PE (molecule I) and 1Zn· (R-A1PE)2 (molecule II) present in the asymmetric unit showing 50% thermal contours for all non-hydrogen atoms at 100 K (H atoms have been omitted for clarity). Bent arrows on molecules I and II represent the direction of the interporphyrin twist. (B) Diagram illustrating the molecular packing in the unit cell (H atoms have been omitted for clarity).

2.177(5) Å is larger than the Zn−Nax distance of 2.131(6) Å observed in the sandwich complex, while an average Zn−Nax distance of 2.140 Å was observed in the 1:2 host−guest complex. As can be seen in Figure 4, the binding of the chiral guest has forced two porphyrin rings to reorganize in a stereospecific orientation in order to minimize the host−guest interactions, which results in a unidirectional screw arrangement in either a clockwise or an anticlockwise direction with torsion angle Φ (M1−C33−C33A−M2) as schematically demonstrated in Figure 5. It is also interesting to note the complete inversion of the torsion angle (Φ) on going from 1Zn· R-A1PE (molecule I, −5.77°) to 1Zn·(R-A1PE)2 (molecule II, +0.70°), which eventually justifies the complete inversion of the sign of the CD couplet observed experimentally (vide infra). Dark red crystals of 1Mg·(S-A3MB)2, 1Mg·(S-A2PE)2, and 1Zn· (S-A2PE)2 were grown by slow diffusion of acetonitrile into solution of the respective complexes in dichloromethane in air at room temperature. All molecules crystallize in chiral space groups. Perspective views of the crystal structures are depicted in Figure 6A, while the molecular packings are displayed in Figures S16−S18 in the Supporting Information. As seen in the crystal structures, the guest ligand coordinates to the metal center through the alcoholic oxygen only, irrespective of the metal ion (Zn/Mg) in the endo-endo conformers. Due to 15206


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C33A−M2) of +3.65° (1Mg·(S-A3MB)2), +2.38° (1Mg·(SA2PE)2), and +1.47° (1Zn·(S-A2PE)2). Dark purple crystals of 1Zn·(MeCN)2 and 1Zn·(S-AB)2 were grown via slow diffusion of acetonitrile into a solution of the complex in dichloromethane at room temperature in air ,and both of them stabilize in exo-endo forms. Perspective views are depicted in Figure 7, while the molecular packings are shown in

Figure 5. Schematic representation showing the torsion angle M− C33−C33A−M in (A) the host (1M) and (B) the host−guest complex where rings are twisted in either anticlockwise or clockwise directions. Bent arrows represent the direction of interporphyrin helicity.

Figure 7. Perspective views of (A) 1Zn·(MeCN)2 and (B) 1Zn·(S-AB)2 showing 50% thermal contours for all non-hydrogen atoms at 100 K (H atoms have been omitted for clarity).

Figures S19 and S20 in the Supporting Information. The metal ions in the complexes have five-coordinate square-pyramidal geometry. The average Zn−Np and Zn−Nax distances in 1Zn·(SAB)2 are found to be 2.077(4) and 2.170(4) Å, respectively, while the Zn···Zn nonbonding distance is 7.10 Å. Two porphyrin rings are nearly planar in 1Zn·(S-AB)2 with an interplanar angle of 15.0° between two cores, whereas the same interplanar angle is 41.7° in 1Zn·(MeCN)2. It would be interesting to compare the molecular structure of 1Zn·(S-AB)2 with its Mg analogue 1Mg·(S-AB)2, which has been reported earlier by our group.9b In 1Mg·(S-AB)2, two guests bind to the Mg(II) center in an endo-endo fashion coordinated through the alcoholic oxygen. Strong OH···NH2 interligand H bonding interlocks two porphyrin rings in a stereospecific way which eventually transfers the chirality information from the guest to the host. However, in 1Zn·(S-AB)2, S-AB binds with zinc(II) ion through the nitrogen of the amino group in an exoendo fashion, which does not produce any chirality. Several geometrical and structural parameters are also different between two molecules, although both of them are 1:2 host− guest complexes with the same S-AB guest. While the M···M nonbonding distance and torsion angle (Φ) are significantly larger in the Mg(II) complex, displacement of the central metal ion from the mean porphyrin plane (ΔM24) and average axial M−N/Oax distance are greater in the zinc(II) complex. Crystal data and data collection parameters for all of the complexes reported here are given in Tables S1 and S2 in the Supporting Information, whereas selected bond lengths and angles are enlisted in Tables S3 and S4 in the Supporting Information. 1Mg·(S-A3MB)2, 1Mg·(S-A2PE)2, and 1Zn·(SA2PE)2 crystallize in chiral space groups. However, the structures of 1 Zn ·S-A3MB and 1 Zn ·(S-AB)2 are highly disordered and the compounds crystallized in achiral space groups. The salient structural features of 1:2 host−guest complexes consisting of zinc(II)/magnesium(II) bisporphyrin host (1M) and guest ligand (L) are compared in Table 1. In the exo-endo complexes 1Zn·(S-AB)2 and 1Zn·(MeCN)2 (Figure 7),

Figure 6. Perspective views of (A) 1Mg·(S-A3MB)2, (B) 1Mg·(SA2PE)2, and (C) 1Zn·(S-A2PE)2 showing 50% thermal contours for all non-hydrogen atoms at 100 K (H atoms have been omitted for clarity).

accommodation of two guest ligands inside the bisporphyrin cavity, the metal centers are separated by a greater distance in the 1:2 endo-endo complex in comparison to the 1:1 sandwich complex. The presence of two strong OH···NH2 interligand H bonds prevents free movement of the chiral guests within the bisporphyrin cavity and thereby interlocks two porphyrin rings in a stereospecific manner to minimize the host−guest repulsion, which resulted in torsional angles (Φ) (M1−C33− 15207


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Inorganic Chemistry Table 1. Selected Structural and Geometrical Parameters of the 1:2 Complexes 1Zn·(MeCN)2 1Zn·(S-AB)2 1Zn·(S-A2PE)2 1Zn·(R-A1PE)2 1Mg·(S-A2PE)2 1Mg·(S-A3MB)2 1Mg·(S-AB)2 1Mg·(S-A3MP)2 molecule I molecule II 1Mg·(L)2f a

conformation

M−Npa

M−N/Oaxa

M···Mb

O(H)···Nc

dihedral angled (θ)

torsion anglee (Φ)

exo-endo exo-endo endo-endo endo-endo endo-endo endo-endo endo-endo

2.055(5) 2.077(4) 2.067(6) 2.068(6) 2.083(7) 2.084(5) 2.083(4)

2.513(12) 2.171(4) 2.178(4) 2.140(6) 2.068(5) 2.064(4) 2.069(3)

8.71 7.10 8.31 7.16 8.10 7.98 7.99

2.78 3.07 2.76 2.72 2.74

41.70 14.96 46.22 24.64 45.88 43.88 44.9

+8.39 −5.85 +1.47 +0.69 +2.38 +3.65 +3.94

this this this this this this 9b

endo-endo endo-endo endo-endo

2.084(8) 2.073(8) 2.087(3)

2.066(6) 2.074(6) 2.069(3)

7.98 8.00 8.02

2.72 2.70 2.84

42.8 42.8 40.2

−0.72 +1.52 −3.84

9b

b

c

ref work work work work work work

9d

d

Average value in Å. M denotes Mg/Zn. Nonbonding distance in Å. H-bonding distance in Å. Angle between two least-squares planes of the C20N4 porphyrinato core. eTorsion angle M−C33−C33A−M (see Figure 5 for schematic representation). fL denotes (1S,2S,3R,5S)-2,6,6trimethylbicyclo[3.1.1]heptane-2,3-diol.

porphyrin rings can also move more freely around the thiophene bridge in comparison to the case for the endoendo complex 1Zn·(L)2, where the presence of interligand H bonding restricts the free movement of two rings. Among the complexes (Table 1), the maximum and minimum M···M nonbonding distances are found to be 8.71 and 7.02 Å for 1Zn· (MeCN)2 and 1Zn·AE, respectively (Figures 7A and 3A), which indicates that the bisporphyrin host has a vertical flexibility of ∼2 Å. In addition, larger M···M separations are found in the 1:2 endo-endo complex in comparison to the 1:1 sandwich complex because of two encapsulated guests within the bisporphyrin cavity in the former. Binding Constant Determination. The binding constants between the metallobisporphyrin host (1M) and chiral guest (L) were determined by both UV−visible and CD spectroscopic titrations. For this purpose, a solution of micromolar concentration of 1M (2 × 10−6 M) was titrated by adding an increasing amount of chiral amino alcohol (10−6 to 10−3 M) in dichloromethane. Spectral data were investigated to obtain the binding constant values by using the fitting procedure provided by the HypSpec computer program14a (Protonic Software, U.K.), and species distribution plots were calculated by the HySS200914b program. Simultaneous fitting of the absorbance data at all wavelengths as a function of chiral amino alcohol concentrations by using software provides more accurate binding constant calculations in comparison with conventional methods based on a single wavelength monitoring. The metallobisporphyrin host 1M was dried carefully under high vacuum for several hours before the titration, and only dry solvents were used. Each metallobisporphyrin unit binds with the amino alcohol guest in a 1:1 sandwich complex initially which, upon an increase in the concentration of guest, transforms to a 1:2 host−guest complex. Three sets of spectral data were analyzed considering a binding model with the three colored stoichiometric states 1M, 1:1 host−guest complex 1M·L, and 1:2 complex 1M·(L)2. For the complexation between 1Zn and R-A1PE, the binding constants K1 and K2 are found to be (7.2 ± 0.3) × 103 M−1 and (6.8 ± 0.1) × 103 M−1, respectively (Figure 8). Similarly, binding constant for all complexes have been calculated and shown in Figures S21−S27 in the Supporting Information and Table 2. Magnesium(II) bisporphyrin 1Mg was kept under high vacuum to obtain an amorphous solid before such titration. However, some water may still be weakly ligated to the magnesium(II) center but it does not greatly affect the binding constant values.

Figure 8. (A) Calculated UV−visible spectra of 1Zn (red), 1Zn·R-A1PE (blue), and 1Zn·(R-A1PE)2 (brown). The green dotted line represents the observed UV−visible spectra of 1Zn in dichloromethane at 295 K. (B) Fits of the titration data of 1Zn with R-A1PE to the theoretical binding isotherm at selected wavelengths of 401 and 413 nm. (C) Species distribution plots of 1Zn, 1Zn·R-A1PE, and 1Zn·(R-A1PE)2.

As seen in Table 2, the binding constant depends upon the relative size of the substituent at the stereogenic center of the guest. Increasing the size of the substituent of the substrates decreases the stability for both 1:1 and 1:2 host−guest complexes. The strongest binding is observed with the least sterically crowded guest S-AB. However, substrates such as SA3MB, S-A3MP, and S-A2PE bind weakly toward 1Zn in their 1:2 complexes in comparison to the Mg analogues due to the lower binding affinity of the alcoholic oxygen toward zinc. In 15208


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Inorganic Chemistry Table 2. CD Spectral Data and Binding Constants in Dichloromethane at 295 K 1:1 sandwich complex CD data, λ (nm) [Δε (M−1 cm−1)] host and guest 1Zn and R-A1PE 1Mg and RA1PE 1Zn and SA2PE 1Mg and SA2PE 1Zn and SA3MB 1Mg and SA3MB 1Zn and SA3MP 1Mg and SA3MP 1Zn and SAB 1Mg and SAB

1:2 complex CD data, λ (nm) [Δε (M−1 cm−1)]

FCa

SCa

Acalb

binding constant K1 (M−1)c,d

421 [−45]

410 [+33]

−78

(7.2 ± 0.3) × 103 [(6.3 ± 0.2) × 103]

FCa

SCa

Acalb

423 [+107]

412 [−54]

+161

413 [+16]

403 [−32]

+48

426 [−11]

416 [+13]

−24

422 [−31]

408 [+46]

−77

423 [−30]

412 [+33]

−63

416 [−79]

408 [+87]

−166

426 [−13]

416 [+20]

−33

423 [−42]

407 [+36]

−78

na

(1.4 ± 0.2) × 104 [(1.9 ± 0.1) × 104] (9.3 ± 0.2) × 103 [(1.0 ± 0.2) × 104] (9.4 ± 0.2) × 103 [(8.9 ± 0.1) × 103] (7.5 ± 0.3) × 103 [(8.2 ± 0.2) × 103] (2.2 ± 0.1) × 104

na

(2.7 ± 0.2) × 104

417 [−87]

na 423 [+68]

414 [−48]

+116

(7.8 ± 0.2) × 103 [(8.6 ± 0.2) × 103]

na 421 [+80]

410 [−50]

+130

416 [+17]

406 [−23]

+40

419 [+66]

407 [−35]

+101

416 [+19]

407 [−11]

+30

na 406 [+80]

−167

binding constant K2 (M−1)c,d

ref

(6.8 ± 0.1) × 10 [(6.6 ± 0.2) × 103] (2.2 ± 0.2) × 103 [(2.5 ± 0.1) × 103] (1.1 ± 0.2) × 103 [(1.6 ± 0.3) × 103] (2.9 ± 0.2) × 103 [(3.1 ± 0.3) × 103] (2.9 ± 0.3) × 103 [(2.8 ± 0.3) × 103] (5.4 ± 0.3) × 103 [(4.9 ± 0.1) × 103] (3.6 ± 0.3) × 103 [(3.9 ± 0.3) × 103] (1.6 ± 0.2) × 103 [(1.8 ± 0.1) × 103] (5.6 ± 0.2) × 103

this work

(3.5 ± 0.2) × 103 [(3.6 ± 0.1) × 103]

9b

3

9b this work 9b this work this work this work 9b this work

Abbreviations: FC, first Cotton effect; SC, second Cotton effect. bAcal = (Δε1 − Δε2) represents the total amplitude of the calculated CD couplets. Calculated from UV−visible spectral measurements. dValues shown within square brackets are calculated from CD spectral measurements.

a c

contrast, 1Zn binds strongly with R-A1PE and S-AB because of the stronger binding affinity of the zinc toward amino nitrogen. Circular Dichroism (CD). The interactions between chiral amino alcohol (guest) and metallobisporphyrin (host) have also been examined in dichloromethane at 295 K by using CD spectroscopic titration (Figures S28−S33 in the Supporting Information). Figure 9 compares the CD spectra of some representative complexes, while Table 2 summarizes the spectral parameters observed. As observed in the UV−visible spectra, two-step binding processes can also be clearly visualized through the CD spectral titration of 1Zn with an enantiomerically pure guest such as R-A1PE. Formation of a 1:1 sandwich complex is clearly evident by the appearance of a negative CD couplet at low substrate concentration and attains a maximum value of −78 cm−1 M−1 at 50 equiv of guest. However, at higher substrate concentration, the CD signal becomes completely inverted to produce a positive CD couplet with a much larger amplitude of +161 cm−1 M−1 due to the formation of a 1:2 endo-endo complex (Figure 9A). The presence of two strong interligand H bonds (OH···NH2) between two encapsulated amino alcohols and strong binding of the substrate through the Zn−Nax bond are responsible for an intense CD couplet in the 1:2 host−guest complex. On the other hand, gradual addition of R-A1PE to a dichloromethane solution of 1Mg produces only the 1:2 host−guest complex 1Mg· (R-A1PE)2, which shows a low-intense positive CD couplet (Figure 9C). Here, the weaker binding of substrates through a Mg−Nax bond in the endo-endo complex results in such a lowintense CD couplet. Addition of a large excess of S-AB to a dichloromethane solution of 1Zn results in no detectable CD response even for the 1:2 exo-endo complex 1Zn·(S-AB)2. In sharp contrast, addition of excess of S-AB to 1Mg produces a high-intense bisignate CD couplet due to the formation of the 1:2 endoendo complex 1Mg·(S-AB)2 (Figure 9E). Moreover, in the 1:1 sandwich complexes 1Zn·S-AB and 1Mg·S-AB, the extents of

Figure 9. CD spectra (in dichloromethane at 295 K) of (A) 1Zn·RA1PE (red) and 1Zn·(R-A1PE)2 (blue), (B) 1Zn·S-A3MB (red) and 1Zn·(S-A3MB)2 (blue), (C) 1Mg·(R-A1PE)2 (red) and 1Zn·(R-A1PE)2 (blue), (D) 1Mg·(S-A2PE)2 (red) and 1Zn·(S-A2PE)2 (blue), (E) 1Mg· (S-AB)2 (red) and 1Zn·(S-AB)2 (blue), and (F) 1Zn·(R-A1PE)2 (red) and 1Zn·(S-A3MB)2 (blue).

interactions between the substrate and the bisporphyrin moiety are insufficient to generate an effective stereospecific twist in the rigid dibenzothiophene spacer for any chiroptical responses. As observed in the X-ray structure of 1Zn·(S-AB)2 (Figure 7), 15209


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Inorganic Chemistry Zn(II) bisporphyrin (1Zn) binds to S-AB in an exo-endo fashion, which does not produce any CD response. In sharp contrast, 1Mg·(S-AB)2 stabilizes in the endo-endo form due to interligand H bonding between two encapsulated substrates in the 1:2 host−guest complex and thereby generates a firm unidirectional twist, as observed in the molecular structure. The presence of such an interchromophoric twist in the endo-endo conformer results in a high-intense CD signal. Similarly, upon addition of S-A3MB to a dichloromethane solution of zinc(II)/magnesium(II) bisporphyrin, a positive sign of the CD couplet has been observed at a low concentration of the guest due to the formation of a 1:1 sandwich complex (Figure 9B) which, at greater substrate concentration, becomes inverted to negative for the 1:2 endoendo complex. However, the intensities of the CD couplet are widely different just by varying the metal ion (Zn vs Mg) and also by changing between 1:1 and 1:2 complexes. For example, S-A3MB produces an intense CD couplet with 1Zn for the 1:1 complex while the intensity falls sharply on going to a 1:2 complex at higher substrate concentration. The response was reversed with the magnesium analogue, where the intensity of the CD couplet increases by more than 4-fold on going from a 1:1 to a 1:2 complex. Moreover, the CD intensity of 1:1 complex decreases on changing the metal from zinc(II) to magnesium(II) while the intensity increases in the same direction for 1:2 host−guest complex. Similar CD spectral behaviors are also observed using S-A2PE and S-A3MP as guest ligands (Figure 9D). As seen in Table 2, substrates having an S chiral center produce a positive CD couplet while R ligands give a negative couplet in 1:1 sandwich complexes irrespective of the metal ion used. Thus, the nature of the chiral center dictates the directions of twist between two porphyrin units in order to minimize the host−guest steric interaction. It has been also observed that CD intensity largely depends upon the bulk of substituent of the substrate. When the bulk of the substituent at the stereogenic center of the chiral guest is less, it is unable to generate a stereospecific twist in the bisporphyrin; therefore, no chiroptical response was observed in 1:1 sandwich complexes. When the bulk of the substituent is increased, the CD intensity increases, although after a threshold value, such intensity again decreases because of increased steric crowding between the host and the guest which thereby weakens the stability of the 1:1 sandwich complex.8a Two porphyrin rings in 1Zn·(R-A1PE)2 are twisted clockwise with a torsional angle (Φ) of +0.69° around the rigid dibenzothiophene due to the projection of the binding sites at the chiral center of R-A1PE which also resulted in the positive sign of the CD couplet in solution. However, the observation of positive torsional angles in the X-ray structures of some other 1:2 endo-endo complexes having a substrate with S configuration (Table 1) such as S-A2PE, S-A3MB, S-A3MP, and S-AB fails to support the negative CD couplets produced in solution. DFT calculations, however, demonstrate preferential stabilization of the anticlockwise twisted conformation for these complexes which eventually support the negative sign of the CD couplets observed in solution. In fact, solid-state CD spectra of 1Zn·(S-A2PE)2 and 1Mg·(S-A3MB)2 have been found to be inverted completely in solution (Figure S34 in the Supporting Information). Thus, the crystal packing effect that arises out of a variety of interactions, namely π−π, CH−π, etc. interactions between two adjacent and nearly coplanar rings in the crystal lattice, is possibly responsible for such a small

positive twist observed in the molecular structure in the solid, which would not contribute in solution and, therefore, yield a negative CD couplet. Generally, weak or negligible CD intensity in 1:2 complexes has been observed due to the monotopic binding between the host and the guest. In contrast, high-intense CD couplets are obtained for the 1:2 endo-endo complexes, due to the formation of unidirectional twist stabilized by the interligand H bonding between two encapsulated guests. Moreover, CD intensity depends upon the mode of binding between the guest and the host. Three possible modes of binding are possible for 1:2 complexes (Scheme 3).15 Exo-endo is always the preferred Scheme 3. Possible Binding Motifs for 1:2 Host−Guest Complexation and Their Chiroptic Responses

binding motif for 1:2 complexes, which can be seen even for a small ligand such as MeCN. However, intermolecular H bonding between the substrates actually stabilizes the endoendo conformer. Increasing the steric bulk at the stereogenic center close to the bound functionality causes a low amplitude of the CD exciton couplet because substrates then increase overall steric crowding of the complex. When the repulsive steric interaction in the 1:2 endo-endo conformers overcomes the stabilization coming out of an attractive H-bonding interaction, the endo-endo form transforms into the exo-endo conformer. Further increase in the bulk of the substrate again converts the exo-endo form into an exo-exo form, as observed earlier with a zinc(II) pyrrole bridged bisporphyrin.15b For 1:2 endo-endo complexes, strong complexation of the substrates inside the bisporphyrin cavity rigidify the host−guest complex in which two porphyrin macrocycles have been forced to be oriented in a clockwise/anticlockwise direction to minimize the host−guest steric interactions. As can be seen in Table 2, the intensity of the CD couplet is largely dependent on how strongly the substrate binds with the metal ions. It has been found9b that when the stereogenic center is at the bound functionality, an S substrate gives a positive sign while an R substrate produces a negative CD sign. However, when the stereogenic center is at the unbound functionality, the sign of the CD couplet, is opposite, e.g. S gives a negative CD couplet while R produces a positive couplet, which is observed here also (Figure 9F). Complete reversal of the bisignate CD signals has been observed just by changing the handedness of the enantiomeric substrate, which demonstrates full and unambiguous rationalization of the chirality transfer processes from the chiral substrate to the achiral host. A substrate that can be fitted well within the bisporphyrin cavity for 1:2 endo-endo complexation would lead us to an accurate stereochemical determination. The sign of the CD couplet also establishes the 15210


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Inorganic Chemistry effective binding sites of the substrate (N-bound vs O-bound) with the metal ion in solution. Furthermore, the sign of the couplet also conclusively demonstrates the type of complexation between host and guest, since the sign becomes completely inverted on moving from 1:1 sandwich to 1:2 host−guest complexes. DFT and TDDFT Calculations. A series of DFT calculations have been performed using the Gaussian 09, revision B.01, package16 to uncover the reasons behind the stabilization of various conformations of 1:2 host−guest complexes. There are two binding sites available for possible binding of the guest: one is inside and the other is outside the bisporphyrin cavity. This leads to the formation of three types of binding motifs: viz., endo-endo, exo-endo, and exo-exo (Scheme 3). In addition, there are two different functional groups (−NH2 and − OH) for possible coordination in each of the amino alcohols. A zinc(II) ion has greater binding affinity toward an amino nitrogen while magnesium(II) has the same affinity for alcoholic oxygen. In order to get more insight into the selective stabilizations of exo-endo and endo-endo conformers for 1Zn·(S-AB)2 and 1Mg· (S-AB)2, respectively, geometry optimizations have been performed using B3LYP17−19 and B97D20 functionals. Figure 10 and Figures S35 and S36 in the Supporting Information

Figure 11. Dependence of the energy difference (ΔE) between the endo-endo and exo-endo conformations of 1Zn·(L)2 (■) upon changing the bulk of the substituents in L.

done by acquiring the coordinates from their corresponding Xray structures. The B97D functional and 6-31G+(d,p) basis set for C, H, N, and O and LANL2DZ for Zn and S were used in such calculations. No imaginary frequencies were found in the frequency calculations of the optimized geometries. For each complex, the energy difference between the endo-endo and exo-endo conformers is denoted ΔE. The presence of two strong interligand H bonds (OH···NH2) between two encapsulated substrates favor the stabilization of the endoendo conformer, while increasing the steric bulk in the substrate would diminish such stability, which after a threshold value converts to the exo-endo conformer. The results are summarized in Figure 11. However, when the substituent is close to the bound functionality (here −NH2), the overall steric effect is relatively large in comparison to that for the substituent at the unbound functionality. Moreover, when the bulk of the substituent is too large, the host can no longer hold the guest inside the cavity. In that case, the exo-endo form further converts to the exo-exo conformation, as also observed earlier by us experimentally using a zinc(II) pyrrole bridged bisporphyrin.15b DFT calculations have also been performed for 1Zn·(RA1PE)2 and 1Zn·(S-A2PE)2 in three possible conformations: i.e., endo-endo (N-bound), exo-endo (N-bound), and endoendo (O-bound) (Figure 12). R-A1PE and S-A2PE guests are very similar; only the position of phenyl group at the stereogenic center is shifted by one carbon atom. DFT calculation shows that the endo-endo (N-bound) conformer of 1Zn·(R-A1PE)2 is more stable than the endo-endo (Obound) and exo-endo (N-bound) conformers by 5.9 and 9.7 kcal/mol, respectively (Figure 12A). However, in 1Zn·(SA2PE)2, the endo-endo (O-bound) conformer is more stable than the endo-endo (N-bound) and exo-endo (N-bound) conformers by 21.2 and 25.2 kcal/mol, respectively (Figure 12B). Interestingly, S-A2PE binds through the alcoholic oxygen only as an endo-endo complex in 1Zn·(S-A2PE)2 to avoid the steric interaction of the phenyl group with the nearby porphyrin ring. DFT and TDDFT calculations have also been performed on both 1:1 and 1:2 host−guest complexes of 1Zn and R-A1PE.

Figure 10. Relative energies of exo-endo and endo-endo conformers of 1Zn·(S-AB)2 and 1Mg·(S-AB)2 using B3LYP and B97D functionals in DFT.

systematically represent their relative energies. Calculations using the B3LYP functional show that the endo-endo conformation is energetically more stable than the exo-endo conformation by 5.9 and 15.9 kcal/mol for 1Zn·(S-AB)2 and 1Mg·(S-AB)2, respectively. However, when dispersion correction is included (B97D functional), the energy gap between endoendo and exo-endo conformations is reduced to 1.2 kcal/mol for 1Zn·(S-AB)2 while it is increased to 30.1 kcal/mol for 1Mg· (S-AB)2. Such a low energy difference between endo-endo and exo-endo conformers of 1Zn·(S-AB)2 suggests that the exo-endo conformer can be stabilized under suitable conditions, which has also been observed experimentally. The larger energy gap between endo-endo and exo-endo conformers in 1Mg·(S-AB)2 suggests that the endo-endo form, as observed in experiments, would be highly stabilized (Figure 10). In order to get more insights into the influences of the H bonding and steric crowding toward the relative stabilities of endo-endo and exo-endo conformers, a series of DFT studies have been performed. A large number of complexes have been optimized in both endo-endo and exo-endo conformations, with variation in the relative bulk of the substituent at various positions as shown in Figure 11. Geometry optimizations were 15211


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substrate, converts to a 1:2 host−guest complex. For the first time, crystallographic characterizations have been done for both 1:1 sandwich and 1:2 host−guest complexes, 1Zn·R-A1PE and 1Zn·(R-A1PE)2 respectively, using the same pair of host and guest, which have enabled us to scrutinize the structural and geometrical changes systematically to rationalize their optical activities. The complete inversion of the sign of the interporphyrin torsional angle (Φ) on going from 1Zn·RA1PE (Φ −5.77°) to 1Zn·(R-A1PE)2 (Φ +0.70°) unambiguously justifies the complete inversion of the CD couplet observed experimentally. A substrate having an S chiral center produces a positive CD couplet, while an R substrate gives a negative couplet in the 1:1 sandwich complex irrespective of the metal ion used. The intensities of the CD couplet have been widely changed just by varying the metal ion (Zn vs Mg) and by changing between 1:1 and 1:2 complexes. In general, the CD intensity decreases for a 1:1 complex with a chiral amino alcohol on changing the metal ion from zinc(II) to magnesium(II), while it increases in the same direction for 1:2 host−guest complexes with other factors kept similar. For the 1:2 complex, the CD intensity depends upon the mode of binding between the guest and the host. Exo-endo is always the preferred binding motif; however, no chirooptic response has been observed in the spectral region of the porphyrin absorption. However, intermolecular H bonding between the two encapsulated substrates actually stabilizes the endo-endo conformer, which produces high-intensity CD couplets. Increasing the steric bulk of the substrate at the stereogenic center close to the bound functionality causes low amplitude of the CD exciton couplet, since it would increase the overall steric crowding in the endo-endo complex. When the repulsive steric interaction overcomes the stabilization coming out of an attractive H-bonding interaction, the endoendo form thereby transforms into the exo-endo conformer. Further increase in the bulk of the substrate again converts the exo-endo form into an exo-exo complex, which is again a chiroptically inactive form. For the 1:2 endo-endo complex, when the stereogenic center is at the bound functionality, an S substrate gives a positive CD couplet while an R substrate produces a negative couplet. However, when the stereogenic center is at the unbound functionality, the sign of the CD couplet is just opposite: e.g. S gives a negative CD couplet while R produces a positive couplet. The sign of the CD couplet also establishes the effective binding sites of the substrate (N-bound vs O-bound) with the metal ion. Furthermore, the sign of the couplet becomes completely inverted on moving from 1:1 sandwich to 1:2 host−guest complexes. It is also interesting to note here that the dibenzothiophene-bridged bisporphyrin host 1M has been able to host a series of amino alcohols for 1:2 endo-endo complexation which would lead us to an accurate stereochemical determination of the substrate.

Figure 12. Relative energies of B3LYP/6-31G+(d,p)-optimized geometries of endo-endo (O-bound), endo-endo (N-bound), and exo-endo (N-bound) conformers of (A) 1Zn·(R-A1PE)2 and (B) 1Zn· (S-A2PE)2 in kcal/mol.

The TD-DFT method is used to produce theoretical CD spectra by SpecDis software,21 which have signs identical with those of the experimental CD signals (Figure 13).

Figure 13. TD-DFT calculated CD spectra (green line) and experimental CD spectra (red line) of (A) 1Zn·R-A1PE and (B) 1Zn· (R-A1PE)2..

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■

CONCLUSIONS An apparently rigid dibenzothiophene-bridged zinc(II)/ magnesium(II) bisporphyrin host (1M) has been explored to determine the absolute configuration of a series of amino alcohols (L), while the effect of the metal ions has been thoroughly scrutinized. At lower substrate concentration, a 1:1 sandwich complex is formed which, upon addition of a large

EXPERIMENTAL SECTION

Materials. Chiral amino alcohols were purchased from SigmaAldrich. The synthesis of 4,6-bis[zinc(II) 5-(3,7,13,17-tetraethyl2,6,12,16-tetramethylporphyrinyl)]dibenzothiophene (1Zn) and 4,6bis[magnesium(II) 5-(3,7,13,17-tetraethyl-2,6,12,16tetramethylporphyrinyl)]dibenzothiophene (1Mg) were accomplished using the literature method.9b,d The magnesium(II) bisporphyrin 1Mg, in particular, was kept under high vacuum for several hours to remove coordinated water molecules. Reagents and solvents were purchased 15212


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Inorganic Chemistry from commercial sources and purified by standard procedures before use. Synthesis. The 1:1 sandwich and 1:2 endo-endo complexes reported in this work were prepared using a general procedure; details are given below for one representative case. Synthesis of 1Zn·R-A1PE. R-A1PE (5 mg, 0.036 mmol) was added to a distilled dichloromethane solution (5 mL) of 1Zn (50 mg, 0.040 mmol), and the mixture was stirred for 30 min in air at room temperature. The solution thus obtained was then filtered off to remove any solid residue and carefully layered with dry n-hexane solvent. After this mixture stood undisturbed for 4−5 days in air at room temperature, brick red crystalline solids precipitated out, which were then isolated by filtration, washed with acetonitrile, and dried under vacuum. Yield: 35 mg (63%). λmax, nm (ε, M−1 cm−1): 411 (4.54 × 105), 423 (1.57 × 105), 543 (2.94 × 104), 575 (1.67 × 104). 1H NMR (CDCl3, 295 K): δ, 9.56 (s, 2H, 10-meso-H); 9.52 (s, 2H, 20meso-H); 9.44 (s, 2H, 15-meso-H); 8.82 (d, J = 8 Hz, 2H, Ar-H); 8.07 (d, J = 6.8 Hz, 2H, Ar-H); 8.00 (t, J = 7.6 Hz, 2H, Ar-H); 6.20 (br, 1H, Ph-H(L)); 5.91 (br, 2H, Ph-H(L)); 4.04 (br, 2H, Ph-H(L)); 3.79− 3.66 (m, 16H, −CH2); 3.24 (s, 12H, −CH3); 2.30 (s, 6H, −CH3); 2.28 (s, 6H, −CH3); 1.62−1.45 (m, 24H, −CH3); −1.11 (br, 1H, Hc); −3.12 (br, 1H, Hb); −3.52 (br, 1H, Ha); −6.18 (br, 3H, -NH2/ OH) ppm. Synthesis of 1Zn·S-A2PE. Yield: 36 mg (65%). λmax, nm (ε, M−1 cm−1): 411 (4.1 × 105), 423 (1.31 × 105), 543 (2.43 × 104), 575 (1.50 × 104). 1H NMR (CDCl3, 295 K): δ, 9.64 (s, 2H, 10-meso-H); 9.60 (s, 2H, 20-meso-H); 9.57 (s, 2H, 15-meso-H); 8.82 (d, J = 8 Hz, 2H, ArH); 8.06 (d, J = 6.8 Hz, 2H, Ar-H); 8.01 (t, J = 8 Hz, 2H, Ar-H); 6.23 (br, 1H, Ph-H(L)); 5.96 (br, 2H, Ph-H(L)); 3.87 (br, 2H, Ph-H(L)); 3.79−3.73 (m, 16H, −CH2); 3.14 (s, 12H, −CH3); 2.31 (s, 6H, −CH3); 2.30 (s, 6H, −CH3); 1.62−1.45 (m, 24H, −CH3); −1.12 (br, 1H, Hd); −1.43 (br, 1H, Ha); −1.92 (br, 1H, Hb); −4.12 (br, 3H, -NH2/OH) ppm. Synthesis of 1Zn·S-A3MB. Yield: 34 mg (62%). ESI-MS: m/z 1467.6569 ([1Zn·(S-A3MB)2 + H]+). λmax, nm (ε, M−1 cm−1): 411 (4.79 × 105), 423 (1.57 × 105), 543 (2.94 × 104), 575 (1.67 × 104). 1 H NMR (CDCl3, 295 K): δ, 9.64 (s, 4H, 10,20-meso-H); 9.55 (s, 2H, 15-meso-H); 8.80 (d, J = 8 Hz, 2H, Ar-H); 8.16 (d, J = 6.8 Hz, 2H, ArH); 8.00 (t, J = 7.6 Hz, 2H, Ar-H); 3.78 (m, 16H, −CH2); 3.30 (s, 12H, −CH3); 2.25 (s, 12H, −CH3); 1.60 (m, 12H, −CH3); 1.53 (m, 12H, −CH3); −1.96 (br, 1H, Hd); −2.17 (br, 3H, −CH3(L)); −2.41 (br, 3H, −CH3(L)); −2.72 (br, 1H, Ha); −3.46 (br, 1H, Hb); −4.41 (br, 1H, Hc); −5.62 (br, 3H, -NH2/OH) ppm. Synthesis of 1Mg·S-A3MB. Yield: 30 mg (55%). λmax, nm (ε, M−1 cm−1): 406 (5.52 × 105), 422 (1.72 × 105), 546 (3.84 × 104), 584 (2.47 × 104). 1H NMR (CDCl3, 295 K): δ, 9.78 (s, 2H, 10-meso-H); 9.69 (br, 4H, 15, 20-meso-H); 8.84 (d, J = 8 Hz, 2H, Ar-H); 8.07 (d, J = 6.8 Hz, 2H, Ar-H); 8.01 (t, J = 8 Hz, 2H, Ar-H); 3.85 (m, 16H, −CH2); 3.34 (s, 12H, −CH3); 3.36 (s, 6H, −CH3); 2.34 (s, 6H, −CH3); 2.30 (s, 6H, −CH3); 1.64 (m, 24H, −CH3); −1.24 (br, 3H, -CH3); −1.34 (br, 3H, −CH3); −1.55 (br, 1H, Hd); −2.33 (br, 1H, Ha); −3.09 (br, 1H, Hb); −3.56 (br, 1H, Hc); −4.00 (br, 3H, −NH2/ OH) ppm. Synthesis of 1Zn·S-AB. Yield: 35 mg (66%). ESI-MS: m/z 1350.5410 ([1Zn·S-AB + H]+). λmax, nm (ε, M−1 cm−1): 411 (4.21 × 105), 423 (1.39 × 105), 543 (2.47 × 104), 575 (1.56 × 104). 1H NMR (CDCl3, 295 K): δ, 9.72 (s, 2H, 10-meso-H); 9.68 (s, 2H, 15-meso-H); 9.62 (s, 2H, 20-meso-H); 8.82 (d, J = 8 Hz, 2H, Ar-H); 8.05 (d, J = 6.8 Hz, 2H, Ar-H); 7.99 (t, J = 8 Hz, 2H, Ar-H); 3.80 (m, 16H, −CH2); 3.41 (s,12H, −CH3); 3.33 (s, 6H, −CH3); 2.29 (s, 12H, −CH3); 1.58 (m, 12H, −CH3); −1.82 (br, 1H, Hd); −2.17 (br, 1H, He); −2.41 (br, 3H, −CH3(L)); −2.69 (br, 1H, Ha); −3.32 (br, 1H, Hb); −4.22 (br, 1H, Hc) −5.55 (br, 3H, −NH2/OH) ppm. Computational Details. The B3LYP17−19 functional has been used for DFT calculations by employing the Gaussian 09, revision B.01, package.16 The method applied was Becke’s three-parameter hybrid exchange functional, the nonlocal correlation provided by the Lee, Yang, and Parr expression, and the Vosko, Wilk, and Nussair 1980 correlation functional (III) for local correction. No imaginary frequencies were found in the frequency calculations of optimized

geometries. The basis sets were 6-31G+(d,p) for C, H, O, and N and LANL2DZ for Zn, Mg, and S. Coordinates for full geometry optimizations of all the complexes were obtained from the crystal structures of the corresponding molecules, and optimizations were performed in dichloromethane solvent. TDDFT calculations were performed using the ωB97X-D functional,22 the basis set was 6-31G*, and dichloromethane was used for solvent correction. SpecDis software21 was used for processing and comparison of CD calculations with experimental spectra. The self-consistent reaction field (SCRF) method was applied for inclusion of solvent correction in all calculations using dichloromethane as solvent. Visualizations of the optimized geometries and the corresponding diagrams were made by using Chemcraft software.23 Instrumentation. UV−visible and circular dichroism spectra were recorded on a PerkinElmer UV/vis spectrometer and JASCO J-815 spectrometer, respectively. The ESI mass spectra were recorded with a Waters Micromass QuattroMicro triple quadrupole mass spectrometer. 1H NMR spectra were recorded on a JEOL 400 MHz instrument. The residual 1H resonances of the solvents were used as a secondary reference. X-ray Structure Solution and Refinement. Crystals were coated with light hydrocarbon oil and mounted in the 100 K dinitrogen stream of a Bruker SMART APEX CCD diffractometer equipped with a CRYO Industries low-temperature apparatus, and intensity data were collected using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The data integration and reduction were processed with SAINT software.24 An absorption correction was applied.25 Structures were solved by the direct method using SHELXS-97 and were refined on F2 by full-matrix least-squares techniques using the SHELXL-2014 program package.26 Non-hydrogen atoms were refined anisotropically. In the refinement, hydrogens were treated as riding atoms using SHELXL default parameters. 1Zn·(S-A3MB)2, and 1Zn·(S-AB)2 contain several severely disordered solvent molecules which could not be modeled due to the weakly diffracting nature of the crystals, and thus, the SQUEEZE27 routine of PLATON was used to remove such highly disordered solvent molecules. CCDC 1571745−1571753 contain supplementary crystallographic data for this paper. These data are provided free of charge by the Cambridge Crystallographic Data Center.

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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.7b02569. UV−visible, ESI-MS, Job plot, 1H NMR, and CD results, packing diagrams, binding constant determinations, crystal data and data collection parameters, bond distances and angles, and Cartesian coordinates (PDF) Accession Codes

CCDC 1571745−1571753 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 for S.P.R.: [email protected]. ORCID

Sankar Prasad Rath: 0000-0002-4129-5074 Notes

The authors declare no competing financial interest. 15213


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Inorganic Chemistry

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ACKNOWLEDGMENTS We thank the Council of Scientific and Industrial Research (CSIR), New Delhi, India, and the Science and Engineering Research Board (SERB) for financial support. The CARE scheme of IIT Kanpur is gratefully acknowledged for the CD facility. A.D. and B.S. thank the CSIR and UGC, respectively, for their fellowships.

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DEDICATION Dedicated to Professor S. Sarkar on the occassion of his 70th birthday.

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