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Dec 6, 2016 - CD couplet of the overall complex have been analyzed. ... The observation of a weak positive CD couplet between (1R,2R)-DPEA guest and ...
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Induction and Rationalization of Supramolecular Chirality in the Tweezer−Diamine Complexes: Insights from Experimental and DFT Studies Avinash Dhamija, Sk Asif Ikbal, and Sankar Prasad Rath* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India S Supporting Information *

ABSTRACT: A series of supramolecular chiral 1:1 sandwich complexes (1M·L and 2M·L) consisting of diphenylether/ethane bridged metallobisporphyrin host (1M and 2M; M: Zn/ Mg) and chiral diamine guest (L) have been presented. The host−guest complexes are compared just upon changing the metal ion (Mg vs Zn) or the bridge (highly flexible ethane vs rigid diphenylether) keeping other factors similar. The factors that would influence the chirality induction process along with their contributions toward the sign and intensity of the CD couplet of the overall complex have been analyzed. Larger CD amplitude was observed in the host−guest complex with the more flexible ethane bridge as compared to the rigid diphenylether bridged one, irrespective of the metal ion used. Also, Zn complexes have displayed larger CD amplitude because of their stronger binding with the chiral diamines. A fairly linear dependence between the binding constant (K) and CD amplitude has been observed. Moreover, the amplitude of the CD couplet has been correlated with the relative steric bulk of the substituent at the stereogenic center: with increasing the bulk, CD intensity gradually increases. However, large increase of steric hindrance, after a threshold value, has diminished the intensity. The observation of a weak positive CD couplet between (1R,2R)-DPEA guest and Zn-bisporphyrin hosts indicates that the clockwise-twisted (steric-controlled) conformer is more populated as compared to the anticlockwise (chirality-controlled) one. In contrast, amplitude of the positive CD couplets is larger with Mg-bisporphyrin hosts, suggesting almost exclusive contribution of the clockwise-twisted conformer guided solely by sterics. DFT calculations support the experimental observations and have displayed the possible interconversion between clockwise and anticlockwise twisted conformers just upon changing the bulk of the substituent irrespective of the nature of chirality at the stereogenic center.

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INTRODUCTION Supramolecular chirogenesis has attracted special interest in the field of chemical research, in connection with its direct relevance with a wide range of natural (heme proteins, secondary α-helix structure of proteins, DNA double helix, etc.) and artificial systems.1−7 Hence, it is essential to have a clear understanding about the various controlling factors and mechanism of chirality transfer processes which apart from promoting the fundamental science also expand its wide applicability in different areas of science. In order to study the various parameters of the supramolecular chirality induction, porphyrinoids have been shown to be well suited mostly due to their distinct spectroscopic features and ability of versatile modification by metalations of the porphyrin core or at their periphery.6,7 Supramolecular complexation between the metalloporphyrin tweezer as host and chiral substrates as guest through axial coordination has been studied extensively.6−13 Stereocontrolled ditopic binding of the bidentate chiral guest to the porphyrin metal centers leads to the formation of a 1:1 host−guest sandwich complex which displays an intense CD couplet due to the transfer of chirality to the host from the chiral guest through generation of unidirectional chiral twist between two porphyrin rings. The sign of the Soret CD couplet is dictated © XXXX American Chemical Society

by the twist between the electric transition moments of interacting porphyrin chromophores. Clockwise orientation of the two-porphyrin unit results in positive first Cotton effect while anticlockwise orientation produces negative first Cotton effect of the CD couplet. On the other hand, chiral ligands’ preexisting chirality along with the differences in the effective size of the substituents around the chiral center of the ligand dictates the direction of interporphyin twist. However, the intensity of the CD signal has been shown to be dependent upon various factors such as interchromophoric distance, intramolecular interaction, the extent of interchromophoric twist, etc.6,7 The fundamental reason for the transfer of chirality depends upon the mechanism of the molecular recognition during the complexation process, and a complete understanding of the origin of chirality transfer information at the electronic and molecular levels would help us to determine absolute configuration of natural products and synthetic compounds. Therefore, understanding of the various influencing factors is extremely important for controlling chirality induction phenomena. In this report, synthesis, structure, and spectroReceived: October 20, 2016

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DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry scopic properties of a wide range of supramolecular chiral tweezer−diamine complexes composed of diphenylether/ ethane bridged bisporphyrin host and chiral diamine guests have been discussed along with very rare examples of crystallographic characterization of the chiral host−guest complexes. For the same chiral guest, two different bisporphyrin systems including two different metal ions (Mg and Zn) have been used in the present investigation. We have then analyzed various essential factors that would influence the chirality induction process along with their contributions toward the sign and intensity of the CD couplet of the tweezer−diamine complexes reported here and elsewhere.8a,b,10f,g Extensive theoretical calculations are conducted to scrutinize the various factors that control the chirogenic process. These systematic studies will lead to the development of efficient chirality sensors for various classes of chiral molecules and would open up further perspectives for the design of smart chiroptical devices.

Figure 1. UV−visible spectral changes of 2Mg (3 × 10−6 M) in dichloromethane upon addition of (1R,2R)-CHDA as the host:guest molar ratio changes from 1:0 to 1:250 at 295 K; inset shows the expanded Soret band region.

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changes to form 2Mg·DAP(R), 2Mg·PEDA(S), 2Mg·PPDA(S), and 2Mg·DPEA(R,R), respectively. On the contrary, 2Zn shows a large red shift of the Soret band by 12 nm (398 to 410 nm) and Q bands by 4 nm (553 to 557 and 597 to 601 nm) upon similar addition of (1R,2R)-CHDA (Figure S4). A synthetic outline of the complexes and their abbreviations are shown in Scheme 1.

RESULTS AND DISCUSSION Diphenylether- and ethane-bridged bisporphyrins were prepared according to the literature procedures.14,15 Metalation of the free base bisporphyrin using MgBr2·OEt2 followed by column chromatography in basic alumina afforded Mg(II) bisporphyrin complexes, 1Mg and 2Mg, in excellent yields.16 The ESI-mass spectrum of 1Mg reveals a peak at m/z 1167.61 which is assigned for [1Mg + H]+; the isotopic distribution pattern of the experimental mass also nicely correlated with the theoretical pattern (Figure S1). Metalation of Zn(II) was accomplished by reported procedures,8a,10f,g yielding Zn(II) bisporphyrins, 1Zn and 2Zn. Interactions of metallobisporphyrin host with chiral diamine L have been studied by UV−visible spectroscopy. UV−visible spectrum of 1Mg displays a Soret band at 407 nm and Q bands at 547 and 585 nm while 1Zn shows an intense Soret band at 393 nm along with two Q bands at 536 and 571 nm. Addition of (1R,2R)-diaminocyclohexane (CHDA) to 1Mg (3 × 10−6 M) in dichloromethane causes small red shifts of the Soret band by 2 nm (407 to 409 nm) and Q bands by 1 nm (547 to 548 nm and 585 to 586 nm) (Figure S2). In sharp contrast, 1Zn, upon addition of (1R,2R)-CHDA shows a large red shift of the Soret band by 19 nm (393 to 412 nm) and Q band by 11 nm (536 to 546 and 571 to 581 nm). ESI-mass spectroscopy has revealed peaks at m/z 1280.5922 and 1252.8100 which are assigned for [1Mg·CHDA(R,R)]+ and [2Mg·CHDA(R,R)]+, respectively. The isotopic distributions of the experimental mass were also correlated nicely with the theoretical pattern (Figure S3), thus confirming formation of 1:1 host−guest complexes. Addition of enantiomerically pure (1R,2R)-CHDA to the dichloromethane solution of 2Mg (3 × 10−6 M) at room temperature results in red shifts in the Soret (from 404 to 406 nm), shoulders (416 to 418 nm and 435 to 437 nm), and Q bands (555 to 557 nm and 599 to 601 nm) due to the formation of 1:1 sandwich complex 2Mg·CHDA(R,R) in solution (Figure 1), which was then isolated in pure form and characterized. Further addition of chiral ligand results in no change in the UV−visible spectral pattern, suggesting high stability of the 1:1 sandwich complex. Even at the very high concentration of the guest, there is no evidence of formation of 1:2 host−guest complex. Other guests such as (R)diaminopropane (DAP), (S)-phenyl ethylenediamine (PEDA), (S)-phenylpropane diamine (PPDA), and (1R,2R)diphenylethylene diamine (DPEA) gave similar spectral

Scheme 1

Figure 2 compares UV−visible spectra between 1M (M: Zn, Mg) and 1:1 sandwich complexes, 1Zn·CHDA(R,R) and 1Mg· CHDA(R,R). However, similar additions of the chiral monoamines, such as (S)-2-aminobutane, to the same metallobisporphyrin hosts (1M and 2M) have produced only 1:2 host−guest complexes, which result in large red shifts of the B

DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX

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inside the bisporphyrin cavity. For example, NH2 and H5 protons are upfield shifted in the complex by 9.62 and 8.91 ppm, respectively, from its resonance in the free state. Similar shifts of the guest’s proton held within two porphyrin subunits have also been observed for other complexes (Figure S8) and also in their Zn analogues. However, due to the weaker binding of the DPEA substrate, the relevant host−guest complexes show very broad signals. Resonances are assigned on the basis of relative intensities, line widths, and distances from the porphyrin ring and also by comparing the 1H NMR spectra of a large number of complexes varying the substituent on the guest ligand. A similar spectral pattern has also been observed in the previously reported 1:1 sandwich complexes.10f,g Crystallographic Characterization. Dark purple colored crystals of 1Mg·CHDA(R,R) are obtained by slow diffusion of acetonitrile into a solution of the complex in dichloromethane at room temperature in air. The complex crystallizes in the triclinic crystal system with P1 chiral space group. Two molecules, which are structurally and geometrically similar, are present in the asymmetric unit; a perspective view is shown in Figure 4. CHDA binds within the bisporphyrin cavity via

Figure 2. UV−visible spectra (in CH2Cl2 at 295 K) of 1Zn (black), 1Zn· CHDA(R,R) (green), 1Mg (blue), and 1Mg·CHDA(R,R) (red).

Soret band in the UV−vis spectra (Figures S5 and S6). In the present investigation, 1Mg·L and 2Mg·L complexes along with three 2Zn·L complexes (2Zn·DAP(R), 2Zn·PPDA(S), 2Zn·PEDA(S)) have been studied while 1Zn·L, 2Zn·CHDA(R,R), and 2Zn· DPEA(R,R) have been reported previously.8a,b,10f,g Stoichiometry of the host−guest complexes can easily be determined by Job’s continuous variation plot (Figure S7). It has been found that maximal changes in UV−visible spectra of the host−guest complexes (1M·L and 2M·L) were found at 0.5 molar ratio, confirming the 1:1 complexation. The formation of a 1:1 sandwich complex in solution can be established by the 1 H NMR spectra in CDCl3 at 295 K. Figure 3 displays 1H

Figure 4. Perspective view of 1Mg·CHDA(R,R) showing 50% thermal contours for all non-hydrogen atoms at 100 K (H atoms have been omitted for clarity).

coordination of two equatorial amino groups of the guest to the Mg(II) center. Chair conformation of the cyclohexane ring allows CH−π interactions with the porphyrin ring, which further help to stabilize the 1:1 sandwich complex. Mg atoms are displaced by ∼0.41 Å from the mean porphyrin plane of the C20N4 porphyrinato core. The two porphyrin rings are oriented in a unidirectional screw arrangement directed by the coordination of chiral (1R,2R)-CHDA guest in order to minimize the host−guest steric clash which ultimately transfers the chirality information to the achiral host 1Mg from enantiomeric guest (1R,2R)-CHDA. As a result, the porphyrin rings in 1Mg·CHDA(R,R) are twisted anticlockwise around the diphenylether bridge, with torsional angles Φ (Mg1−C33− C83−Mg2) of −35.5° and −32.6° for molecules I and II, respectively. Chirality information transfers from chiral diamine ligand to the achiral host can be visualized from the direction of the screw observed in the bisporphyrin moiety. Molecular packing of the complex in the crystal lattice is shown in Figure S9. Crystal data and data collection parameters are given in Table 1 while selected bond distances and angles are given in Table S1. It would be interesting to compare 1Mg·CHDA(R,R) with its zinc analogue10f 1Zn·CHDA(R,R) here. While the M−Np and M− Nax distances are similar, several structural and geometrical

Figure 3. 1H NMR spectra of (A) 2Mg, (B) 2Mg·CHDA(R,R), and (C) (1R,2R)-CHDA at 295 K in CDCl3. The inset shows the proton numbering scheme of CHDA.

NMR spectra coming from the reaction between 2Mg and CHDA, as a representative example. Trace A shows the wellresolved spectrum of 2Mg in CDCl3, while trace B shows the 1H NMR spectrum of the crystalline sample of 2Mg·CHDA(R,R) in CDCl3. Trace C, however, shows the 1H NMR spectrum of free CHDA guest in the same solvent. As can be seen, the 1H NMR spectrum of 2Mg·CHDA(R,R) (trace B) shows a large shift of peaks from that of 2Mg and free CHDA ligand. The meso protons, which are otherwise identical, become nonequivalent in the encapsulated complex due to twisting of the two porphyrin subunits. As a result, the 15-meso proton is upfield shifted by Δδ = 0.12 ppm while CHDA protons are found to be large upfield shifted, which supports the ligand encapsulation C

DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX

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asymmetric unit; the (S) isomer of CHDA twisted two porphyrin units in the clockwise direction (molecule I) while the (R) isomer twisted anticlockwise (molecule II). Perspective views are shown in Figure 5 while Figure S10 shows the packing of the molecule in the crystal lattice. Two porphyrin units open up its binding pocket to accommodate the CHDA ligand inside its cavity, which makes dihedral angles of 39.1° and 28.0° between two porphyrin rings in molecules I and II, respectively. The metal ion has been displaced from the leastsquares plane of the C20N4 porphyrinato core by 0.42 and 0.44 Å for molecules I and II, respectively. Interestingly, the projection of binding sites at the chiral centers compels two porphyrin rings to reorganize the bisporphyrin framework in a stereospecific orientation in order to minimize the host−guest steric interactions. As a result, two porphyrin rings are aligned in the unidirectional screw arrangement with torsional angle Φ (Mg1−C37−C37A−Mg1A) of 58.02° and −56.96° for molecules I and II, respectively. Mg···Mg nonbonding distances are also different between the two molecules, which are 5.743(2) Å and 5.716(2) Å, respectively, for molecules I and II. However, M···M nonbonding distance and torsional angle (Φ) are relatively larger in the Mg complex compared to its Zn analogue 2Zn·CHDA(R,R). The prominent structural features of the 1:1 sandwich complexes consisting of the chiral diamine guests and Zn(II)/ Mg(II)bisporphyrin host are compared in Table 2. (R) guest produces anticlockwise twist of the two porphyrin units (with negative sign of Φ) around the bridge in the host−guest complex while (S) guest produces clockwise twist (with positive sign of Φ). The effect can, however, be seen clearly with 2Mg·CHDA where the asymmetric unit contains two molecules that are found structurally very similar besides two Mg-porphyrins twisted in either clockwise or anticlockwise direction with torsional angle of +58.02° and −56.96°, respectively, due to the presence of both (1S,2S)-CHDA and (1R,2R)-CHDA guests (racemic mixture) during the crystallization process. As seen in Table 2, the M−Nax distance increases upon increasing the size of the substituent at the chiral diamine guest. Also, the average displacement of atoms from the mean porphyrin plane (Δ24) and the dihedral angle (θ) between the two porphyrin rings gradually increases with increasing bulk of the substituent at the diamine guest in order to accommodate the same within the bisporphyrin cavity with minimum host−guest clash. However, in 1Zn·DPEA(R,R), two

Table 1. Crystal Data and Data Collection Parameters 1Mg·CHDA(R.R) formula T, (K) formula weight crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 radiation (λ, Å) Z dcalcd, g.cm−3 F(000) μ, mm−1 No. of unique data No. of parameters, refined GOF on F2 R1a [I > 2σ(I)] R1a (all data) wR2b (all data) largest diff. peak and hole

C80H104N10Mg2 100(2) 1254.35 orthorhombic Pna21 24.472(4) 14.682(2) 39.942(6) 90 90 90 14351(4) Mo Kα (0.71073) 8 1.161 5424 0.084 32801 1689

1.020 0.0705 0.1220 0.1654 0.287 and −0.471 e·Å−3

0.986 0.0838 0.2108 0.1812 0.316 and −0.310 e·Å−3

R1 = ∑||F o | − |F c ||/∑|F o |. ∑[w(Fo2)2]]1/2. a

2Mg·CHDA

C82H92N10OMg2 100(2) 1282.27 triclinic P1 13.5777(9) 14.3089(10) 22.2385(15) 84.186(2) 72.6580(10) 63.2910(10) 3681.4(4) Mo Kα (0.71073) 2 1.157 1372 0.085 20897 1750

b

wR2 = [∑[w(F o 2 − F c 2 ) 2 ]/

parameters are different between the two molecules. For example, displacement of the central metal ion from the mean porphyrin plane (ΔM24), average atom displacement (Δ24), M··· M nonbonding distance, angle between two least-squares planes of porphyrin (θ), and torsional angle (Φ) are all significantly larger in the Mg complex compared to its Zn analogue. 2Mg·CHDA crystallizes by slow diffusion of acetonitrile into a chloroform solution of 2Mg and a racemic mixture of CHDA at room temperature in air. The complex crystallized in the orthorhombic crystal system with Pna21 space group. Two molecules which are structurally very similar but twisted in the opposite direction around the ethane bridge are present in the

Figure 5. Perspective views of 2Mg·CHDA of (A) molecule I (clockwise) and (B) molecule II (anticlockwise) showing 50% thermal contours for all non-hydrogen atoms at 100 K (H atoms have been omitted for clarity). D

DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Selected Structural and Geometrical Parameters of the Chiral 1:1 Sandwich Complexes M−Npa

M−Naxa

ΔM24b

Δ24c

M···Md

dihedral anglee (θ)

torsional anglef (Φ)

ref

2.077(6) 2.071(4) 2.074(4) 2.074(6) 2.066(4)

2.202(5) 2.205(5) 2.188(6) 2.203(6) 2.218(4)

0.41 0.36 0.35 0.39 0.33

0.16 0.14 0.14 0.16 0.15

5.966(2) 5.908(2) 5.992(2) 5.989(2) 6.724(2)

31.6 28.7 28.6 32.5 39.5

−35.48(5) 30.9(5) −33.4 (5) 32.7(5) 35.2(5)

this work 10f 10f 10f 10f

I II

2.053(8) 2.073(8)

2.209(8) 2.232(8)

0.36 0.43

0.16 0.18

6.372(3) 5.879(3)

40.3 38.7

34.5(7) −33.1(7)

10f,g

I II

2.078(5) 2.080(5) 2.076

2.206(5) 2.196(5) 2.178

0.42 0.44 0.44

0.14 0.15 0.11

5.743(2) 5.716(2) 5.604(5)

39.1 28.0 39.5

58.02(5) −56.96(5) −54.1(5)

this work

complex 1Mg·CHDA(R,R) 1Zn·CHDA(S,S) 1Zn·DAP(R) 1Zn·PPDA(S) 1Zn·PEDA(S) 1Zn·DPEA(R,R) molecule molecule 2Mg·CHDA molecule molecule 2Zn·CHDA(R,R)

8a

a

Average value in Å. M: Mg/Zn. bDisplacement (in Å) of M from the least-squares plane of the C20N4 porphyrinato core. cAverage displacement (in Å) of atoms from the least-squares plane of the C20N4 porphyrinato core. dNonbonding distance in Å. eAngle between two least-squares plane of the C20N4 porphyrinato core. fTorsional angle (M1−C33−C83−M2 for 1M·L and M1−C37−C37A−M1A for 2M·L).

structurally different molecules are present due to the presence of two bulky phenyl substituents in the DPEA ligand. Interestingly, Zn−Np and Zn−Nax distances are relatively shorter in molecule I with clockwise interporphyrin twist while the distances are remarkably longer in molecule II with anticlockwise twist. In 1Zn·PEDA(S) and 1Zn·DPEA(R,R), there are one and two bulky phenyl substituents, respectively, on the chiral guest ligand which, however, imposes large steric interaction with the nearby porphyrin ring. The effect of such interactions can also be visible clearly looking at two different Zn−Nax distances of 2.189(4) Å and 2.247(4) Å observed in 1Zn·PEDA(S): the −NH2 group far from the phenyl group binds strongly compared to the closer one. It would be interesting now to compare the structure and geometrical parameters between tweezer−diamine complexes just upon changing the metal ion (Mg vs Zn) or the linker (highly flexible ethane vs rigid diphenylether) keeping other factors similar. As can be seen, Mg−Nax distance is relatively weaker while the displacement (in Å) of Mg from the leastsquares plane of the C20N4 is larger compared to its Zn analogue, which, however, reflects the weaker binding of diamine guests with the magnesium ion. However, torsional angle (Φ) is larger with Mg(II) bisporphyrin host. Moreover, while the average M−Np and M−Nax distances are similar in both 1M·L and 2M·L, various geometrical parameters such as dihedral angle between two porphyrin planes (θ), torsional angles (Φ), and displacement of metal from the mean porphyrin plane (ΔM24) are larger in the complex with the highly flexible ethane bridge, which also results in lower M···M separation. It appears as if flexibility of the host molecule facilitates the guest ligand binding, which has also been reflected in the larger binding constant value (K) with 2M (vide inf ra). Binding Constant Determination. Binding constants between metallobisporphyrin host and chiral diamine guest are determined by CD spectroscopic titration method using HypSpec computer program (Protonic Software, U.K.), and HySS200917 program was used to calculate the species distribution plots. Binding constants of the complexes have been calculated and are given in Table S2. Two sets of CD titration data were analyzed considering a binding model with two-colored stoichiometric states of 1M and 1:1 sandwich complex, 1M·L. For complexation between 1Mg and (1R,2R)CHDA, the binding constant (K) found to be (9.2 ± 0.2) × 104

(Figure 6). Similarly, for the complexation between 2Mg and (1R,2R)-CHDA, K is found to be (5.0 ± 0.1) × 105 (Figure S11). In general, Zn complexes have been found to bind more

Figure 6. (A) Calculated CD spectra of 1Mg (black) and 1Mg· CHDA(R,R) (red). Green line represents the observed CD spectrum in dichloromethane at 295 K. (B) Fits of the titration data of 1Mg with (1R,2R)-CHDA to the theoretical binding isotherm at selected wavelengths of 407 and 421 nm. (C) Species distribution plots of 1Mg and 1Mg·CHDA(R,R). E

DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX

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Close examination of the molecular structure has revealed that the two porphyrin units are orientated in an anticlockwise direction to achieve minimum host−guest repulsions which introduce negative sign of the first Cotton effect of the CD couplet. Similarly, the −NH2 binding site of the (S)stereocenter will twist two porphyrin moieties in a clockwise direction and hence produce positive sign of the first Cotton effect. Thus, the absolute configuration at the chiral center dictates the overall direction of the twist between two porphyrin units in these molecules. Interestingly, enantiopure monoamine such as (S)-2-aminobutane binds to the metallobisporphyrin hosts reported here to produce a 1:2 complex (Figure S18), which, however, produces either negligible or very weak CD response. Thus, the above observations establish a firm correlation between the interporphyrin twist and the sign of the CD signal. More rigid nature of (1S,2S)-CHDA in comparison to (S)PPDA and (R)-DAP may be responsible for the comparatively high CD amplitude of the CHDA complex. Moreover, larger CD amplitude was observed (Figure 7B) with the more flexible ethane bridge as compared to the diphenylether-bridged one, irrespective of the metal ion used. The highly flexible ethanebridged bisporphyrin binds more strongly with the chiral guest, which might be responsible for such enhancement in the CD amplitude. Also, Zn complexes, in general, produce relatively large CD amplitude because of their stronger binding with the chiral diamines (Table S2). In sharp contrast, 1M·DPEA(R,R) and 2M·DPEA(R,R) show very low amplitude bisignate positive CD signal. Observations of such low-intense positive CD couplet for a (R)-ligand were not “expected”. According to the pre-existing chirality of the (1R,2R)-DPEA, a structure with exclusive left-handed screw should be observed. However, in the X-ray structure of 1Zn· DPEA(R,R), both right- and left-handed conformers are present, which contributes toward the intensity in the opposite directions.10f,g The observation of positive CD signals for 1M· DPEA(R,R) and 2M·DPEA(R,R) suggests that the right-handed conformer is present in excess. Unlike other chiral ligands in the series, DPEA contains two bulky phenyl groups, which increase the steric interactions between host and guest, and the effect can be seen in the low binding constant value for the host−guest complex. To avoid such steric clash, two porphyrin macrocycles also twisted in a clockwise direction in which the bulky phenyl group is placed as anti to the porphyrin ring. Figures 7C and 7D compare the CD spectra between Zn(II) and Mg(II) complexes with DPEA in each case using diphenylether- and ethane-bridged bisporphyrin hosts, respectively. As can be seen, CD amplitude is more in the Mg complex, which is just opposite to the trends observed in other diamine complexes. Thus, there are two factors that regulate the direction of interporphyrin helicity in the tweezer−diamine complexes. The preorganized projection of the NH2 group in (1R,2R)-DPEA dictates anticlockwise twist between two porphyrin units (Figure 8A) leading to negative CD signal. On the other hand, clockwise twisting between two porphyrin rings is favored due to steric reasons which provide positive CD signal (Figure 8B). Observation of the positive CD signal of the complex indicates that the clockwise-twisted structure has relatively greater contribution than the left-handed one, which eventually reduces overall intensity to a significant extent, particularly with Zn-bisporphyrin hosts. In sharp contrast, CD amplitude is higher in the case of 1Mg·DPEA(R,R), which

strongly compared to their Mg analogues with the same host and guest ligands (Figure S12). Also, a more flexible ethanebridged bisporphyrin host (2M), compared to the rigid diphenylether bridge, binds more strongly with the chiral diamine guest. Moreover, the binding constant is highest with CHDA guest.

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CIRCULAR DICHROISM (CD) The interactions between chiral diamine guest and metallobisporphyrin host have also been investigated by CD spectroscopy. The sign of the CD couplet in the Soret region is directly related to the absolute configuration of the chiral substrate. Upon addition of increasing amount of (1R,2R)CHDA to the dichloromethane solution of 1Mg (3 × 10−6 M), CD intensity gradually increases. CD amplitude becomes maximum (Aobs, −425 M−1 cm−1) with addition of 50 equiv of the chiral ligand. However, no change in CD intensity upon further additions of guest up to 400 equiv suggests exclusive formation of 1:1 sandwich complex; the spectral changes have been shown in Figure S13. Thus, even the large concentrations of guest do not support the formation of 1:2 host−guest complexes (which would result in some lowering in CD couplet intensity) due to the restricted rotation of the bisporphyrin around the bridging oxygen atom. Similarly, with addition of increasing amount of (1R,2R)-CHDA to the dichloromethane solution of 2Mg (3 × 10−6 M), CD intensity gradually increases to a maximum value of Aobs, −560 M−1 cm−1 (Figure S14). Figures S15−S17 display CD and UV−vis spectral change for the complexes reported here while Table S2 summarizes the spectral parameters observed. Figure 7 compares the CD

Figure 7. CD spectra (in dichloromethane at 295 K) of (A) 1Mg· CHDA(R,R) (blue) and 1Mg·CHDA(S,S) (red), (B) 1Mg·PPDA(S) (blue) and 2Mg·PPDA(S) (red), (C) 1Mg·DPEA(R,R) (red) and 1Zn·DPEA(R,R) (blue), and (D) 2Mg·DPEA(R,R) (red) and 2Zn·DPEA(R,R) (blue).

spectra of relevant complexes recorded in dichloromethane at 295 K. Complete reversals of the bisignate CD signals between 1Mg·CHDA(R,R) and 1Mg·CHDA(S,S) have been observed (trace A) just by changing the handedness of the enantiomeric substrate. Such highly enhanced CD amplitude in 1M· CHDA(R,R) and 2M·CHDA(R,R) can be attributed to the high stability of the complex along with the formation of a unidirectional left-handed screw. Direction of twist between two porphyrin units is solely guided by the projection of the −NH2 group at the chiral center (i.e., absolute configuration). F

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Figure 8. Binding mechanism between 1Zn and (1R,2R)-DPEA resulting (A) anticlockwise and (B) clockwise twisted conformers.

suggests almost exclusive contribution of the right-handed conformer guided solely by steric considerations (vide inf ra). DFT and TDDFT Calculations. The Gaussian 09, revision B.01, package18 was used for DFT studies in which B3LYP hybrid functional19 was employed. For local correction, Vosko, Wilk, and Nussair 1980 correlation functional (III) was used.20,21 Frequency calculations were done to ensure that there are no imaginary frequencies in the optimized geometries. The self-consistent reaction field (SCRF) method was applied for inclusion of solvent correction in all the calculations using dichloromethane as solvent. Visualizations of the optimized geometries, and the corresponding diagrams were made by using Chemcraf t software.22 In order to get more insight about the contributions of steric hindrance in the host−guest complexes with DPEA, geometry optimizations of 1Zn·DPEA(R,R), 1Mg·DPEA(R,R), 2Zn·DPEA(R,R), and 2Mg·DPEA(R,R) have been performed in both clockwise and anticlockwise twisted conformers. It has been observed that right-handed conformers are more stable compared to the lefthanded one by 2.7 and 3.2 kcal mol−1 (in gas phase), respectively, for 1Zn·DPEA(R,R) and 1Mg·DPEA(R,R) (Figure 9). These results are in good agreement with the experiment. The TDDFT method is used to calculate excitation energies and rotational strength in order to obtain calculated CD spectra. CD calculations are processed and compared with the experimental spectra using SpecDis software.23 As can be seen, the calculated CD spectra are in good agreement with the experimental CD spectra (Figure S19). CD spectra of clockwise and anticlockwise conformations of 1Mg·DPEA(R,R) and 1Zn· DPEA(R,R) have also been calculated separately and are displayed in Figure 10; the experimental CD spectra are also in good match with the calculated one using the clockwise conformer (vide supra). Two bulky phenyl groups of the DPEA guest increase significant steric crowding within the host−guest complex, which subsequently stabilizes the clockwise over the anticlockwise conformation. In order to understand more on the effect of sterics, large numbers of complexes in both their clockwise and anticlockwise conformations have been optimized varying

Figure 9. Relative energies of B3LYP/6-31G+(d,p)-optimized geometries of clockwise and anticlockwise conformers of (A) 1Zn· DPEA(R,R) and (B) 1Mg·DPEA(R,R).

Figure 10. TDDFT-calculated CD spectra of clockwise (green line) and anticlockwise (blue line) conformations and experimental CD spectra (red line) of (A) 1Mg·DPEA(R,R) and (B) 1Zn·DPEA(R,R).

the bulk of the substituents at the chiral center of the diamine guests. The complexes (1Zn·L and 1Mg·L), presented in Figure 11, are made by taking coordinates directly from the conformers obtained in the X-ray structure of 1Zn·DPEA(R,R). The optimizations were executed using the B3LYP functional and 6-31G+(d,p) basis set for C, H, O, N and LANL2DZ for Zn and Mg. Frequency calculations were done to ensure that there are no imaginary frequencies in the optimized geometries. For each complex, energy difference between clockwise and anticlockwise conformers is called “ΔE”. As demonstrated in Figure 11, substrates containing smaller substituents at the chiral center stabilize the anticlockwise conformation whereas, upon increasing the bulk, the clockwise conformation is found to be increasingly more stable. The observation of a weak positive CD couplet between (1R,2R)-DPEA guest and Znbisporphyrin hosts indicates that the clockwise-twisted (stericcontrolled) conformer is more populated as compared to the anticlockwise (chirality-controlled) one. In contrast, amplitude of the positive CD couplet is larger in the case of 1Mg· G

DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 11. Dependence of energy difference (ΔE) between clockwise and anticlockwise twisted conformation of 1Zn·L (■) and 1Mg·L (red ▲) upon changing the bulk of the substituents at the stereogenic center.

DPEA(R,R), suggesting almost exclusive contribution of the clockwise-twisted conformer guided solely by steric considerations. Such a trend can be seen clearly in Figure 11 where the clockwise-twisted conformer of 1Mg·DPEA(R,R) is relatively more stable as compared to the same in 1Zn·DPEA(R,R). DFT calculations, therefore, support the experimental observations and have displayed the possible interconversion between clockwise and conterclockwise twisted conformers just upon changing the bulk of the substituent irrespective of the nature of chirality at the stereogenic center. It would be interesting now to find out other factors that contribute toward the overall CD amplitude of the tweezer− diamine complexes. A fairly linear dependence between the binding constant (K) and CD amplitude has been observed varying a large number of substrates in both diphenylether (Figure 12A) and ethane (Figure 12B) bridged complexes 1M·L and 2M·L. However, a large deviation from linearity has been observed for the Zn(II) complexes of the DPEA ligand only. This is quite expected since the CD amplitude was exceptionally low in the Zn(II) complexes with DPEA guest due to the presence of both left- and right-handed conformers with unequal populations (vide supra). It is interesting to note here that, due to exclusive population of the clockwise conformer, 1Mg·DPEA(R,R) and 2Mg·DPEA(R,R) are close to the line along with other host−guest complexes. We also have tried to plot the torsional angle with the CD amplitude (Figure S20) for a large number of structurally characterized 1:1 sandwich complexes; however, no correlation could be obtained. The amplitude of the CD exciton couplets have also been correlated here with the relative steric size of substituents at the stereogenic center. A nonequivalent steric environment was generated upon the host and chiral guest complexation in which two porphyrins undergo stereodifferentiation and adopt

Figure 12. Dependence of the binding constant on the CD intensity in (A) 1M·L and (B) 2M·L complexes.

a chiral conformation that minimizes the host−guest steric interaction. The relative bulk of the substituents at the stereogenic center decides the overall direction and extent of the interporphyrin helicity leading to the sign and intensity of the CD couplets. Over the years, conformational energies or A values, as defined by Winstein and Holness,24 constitute an invaluable resource for quick and quantitative prediction of the relative steric demand of a variety of substituents which are also similar to effective steric sizes (ESS’s) proposed by Borovkov et al.25 Therefore, the A values are extensively used to correlate between the stereochemistry of the substrate and the interporphyrin twist.26 Here, A values of the substituent have been correlated with the relative CD intensity which has been displayed in Figure 13. A values for methyl, benzyl, cyclohexyl, and phenyl groups are 1.70, 1.81, 2.15, and 3.0 kcal/mol, respectively.24 As can be seen, the CD intensity gradually increases to a maximum value before it further decreases upon increasing the “A” value. Large increase of steric bulk (and hence “A” value), after a threshold value, would further destabilize the 1:1 sandwich complex, and hence the CD intensity decreases. Complexes containing DPEA ligand have not been included here since the “A” value out of two phenyl groups is not known yet. When the steric hindrance is too large, a 1:1 sandwich complex would convert to a 1:2 host−guest complex since it would not be possible to encapsulate the substrate within the two porphyrin rings then.10,27 A similar trend has been observed irrespective of the nature of host, guest, and metal ion used. H

DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX

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stereogenic center. The projection of binding sites at the chiral center compels two porphyrin rings to reorganize in a stereospecific orientation in order to minimize the host− guest steric interactions. Consequently, (S) isomer twisted two porphyrin units in a clockwise direction while the (R) isomer directed anticlockwise. With increasing the bulk of the substituent at the stereogenic center, the CD intensity also gradually increases. However, large increase of steric hindrance, after a threshold value, has diminished the intensity of the CD couplet. This has been the situation with (1R,2R)-diphenylethylene diamine (DPEA) guest which, after complexation with 1M (M: Zn/Mg), has shown a very low amplitude bisignate positive CD signal that is unexpected for a (R)-guest. The preorganized projection of the NH2 groups in (1R,2R)-DPEA dictates anticlockwise twist (and hence negative CD couplet) between two porphyrin units, which, however, increases significant steric crowding in the host−guest complex due to the presence of two bulky phenyl groups. To reduce such crowding, two porphyrin rings are also twisted clockwise which, however, produces a positive CD couplet. Both clockwise and anticlockwise twisted forms are present in the X-ray structure of 1Zn·DPEA(R,R), which, however, contributed opposite toward the intensity of the CD couplet. The observation of a weak positive CD couplet with Zn-bisporphyrin hosts indicates that the clockwise-twisted conformer is more populated as compared to the anticlockwise one. In contrast, amplitude of the positive CD couplet is larger in the case of 1Mg·DPEA(R,R), suggesting almost exclusive contribution of the clockwisetwisted conformer guided solely by steric considerations, which is also supported further by DFT calculations.

Figure 13. Dependence of the effective size (“A” value) of the bulkiest substituent on the CD amplitude of (A) 1M·L and (B) 2M·L complexes.

■

■

EXPERIMENTAL SECTION

Materials. 4,6-Bis[Zn(II) 5-(3,7,13,17-tetraethyl-2,6,12,16tetramethylporphyrinyl)]diphenylether, 1Zn, 1,2-bis(Zn(II) mesooctaethylporphyrinyl)ethane, 2 Zn , and 1,2-bis(Mg(II) mesooctaethylporphyrinyl)ethane, 2Mg, were prepared using the previously reported methods.14−16 Reagents and solvents were purchased from commercial sources and purified by standard procedures before use. Syntheses. 1Mg. 50 mg (0.044 mmol) of free base 1 was taken in 20 mL of freshly distilled dichloromethane. 250 μL of anhydrous triethylamine followed by MgBr2·OEt2 (227 mg, 0.88 mmol) was added to it, and stirred under inert atmosphere at room temperature for 1 h. The reaction mixture was washed with distilled water once, and solvent was dried over anhydrous Na2SO4, filtered, and evaporated to dryness. Column chromatography was done over basic alumina using dry CH2Cl2:acetone (1:1) as eluant. Yield: 49 mg (94%) ESIMS: m/z 1167.61 ([M + H]+). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 407 (2.42 × 105), 423sh (5.01 × 104), 547 (2.15 × 104), 585 (1.13 × 104). 1H NMR (CDCl3, 295 K): δ, 9.60 (s, 2H, 15meso-H); 9.36 (br, 4H, 10, 20-meso-H); 8.38 (d, 2H, Ar−H); 7.88 (t, 2H, Ar−H); 7.28 (t, 2H, Ar−H); 7.20 (s, 4H, 10,20-meso-H); 7.19 (d, 2H, Ar−H); 3.83−3.37 (m, 24H, −CH3); 2.52 (q, 16H, −CH2); 0.99 (t, 24H, −CH3) ppm. 1:1 sandwich complexes reported in the present work were prepared using the general procedure; details for one representative case have been described below. 1Mg·CHDA(R,R). 1Mg (50 mg, 0.042 mmol) was dissolved in distilled dichloromethane, and (1R,2R)-CHDA (5.4 mg, 0.047 mmol) was added. The mixture was stirred for 15 min, and then the resulting solution was filtered off to remove any solid residue and layered with acetonitrile carefully. After 7−8 days, dark-purple crystalline solid was formed, which was then isolated by filtration, washed well with nhexane, and dried in vacuum. Yield: 42 mg (76%). ESI-MS: m/z 1280.5922 ([1Mg·CHDA(R,R)]+). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 409 (2.39 × 105), 425sh (6.01 × 104), 549 (2.24 × 104), 587 (1.22 × 104). 1H NMR (CDCl3, 295 K): δ 9.88 (s, 2H, 10-

CONCLUSIONS A series of 1:1 sandwich complexes consisting of diphenylether/ethane-bridged bisporphyrin host and chiral diamine guests have been synthesized and characterized by various spectroscopic techniques. For the same chiral guest, two different bisporphyrin systems including two different metal ions (Mg and Zn) have been utilized. We have then scrutinized various factors that possibly contribute toward the sign and intensity of the CD couplet of the host−guest complex. The molecular structures between tweezer−diamine complexes have been compared just upon changing the metal ion (Mg vs Zn) or the linker (highly flexible ethane vs rigid diphenylether) keeping other factors similar. While the M−Np and M−Nax distances are similar in both 1M·L and 2M·L, several structure and geometrical parameters such as torsional angle (Φ), dihedral angle between two porphyrin planes (θ), and displacement of metal from the mean porphyrin plane (ΔM24) are larger in the complex with the highly flexible ethane bridge. Moreover, larger CD amplitude was observed in the host−guest complex with more flexible ethane bridge as compared to the rigid diphenylether-bridged one, irrespective of the metal ion used. Such an increase in the CD amplitude can be ascribed to the highly flexible nature of ethane-bridged bisporphyrin, which also binds more strongly with the guest. Moreover, Zn complexes have displayed, in general, relatively larger CD amplitude because of their stronger binding with the chiral diamines. A fairly linear dependence between the binding constant (K) and CD amplitude has been observed. The intensity of the CD exciton couplet has also been correlated with the relative steric size of substituent at the I

DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX

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(m, 32H, −CH2); 2.47−0.69 (m, 48H, −CH3); −4.43 (br, 1H, H1, PEDA); −6.43 (br, 1H, H3, PEDA); −6.86 (br, 2H, H2, PEDA); −7.12 (br, 2H, NH2); −7.51 (br, 2H, −NH2). 2Mg·DAP(R). Yield: 43 mg (81%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 406 (2.91 × 105), 418sh (2.98 × 105), 436sh (9.22 × 104), 557sh (2.62 × 104), 601 (1.38 × 104). 1H NMR (CDCl3, 295 K): δ 9.96 (s, 2H, 10-meso-H); 9.47 (s, 2H, 15-meso-H); 8.78 (s, 2H, 20-meso-H); 6.32, 6.22 (br, 4H, −CH2(b)); 4.12−3.11 (m, 32H, −CH2); 2.37−0.86 (m, 48H, −CH3); −4.62 (m, 3H, CH3, DAP); −5.88 (br, 1H, H1, DAP); −6.52 (br, 1H, H3, DAP); −6.82 (br, 1H, H2, DAP); −7.88 (m, 2H, −NH2); −7.98 (m, 4H, −NH2). 2Mg·DPEA(R,R). Yield: 43 mg (73%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 406 (1.58 × 105), 418sh (1.88 × 105), 436sh (5.31 × 104), 557sh (2.23 × 104), 601 (1.31 × 104). 1H NMR (CDCl3, 295 K): δ 9.91 (s, 2H, 10-meso-H); 9.37 (s, 2H, 15-meso-H); 8.68 (s, 2H, 20meso-H); 6.68 (m, 2H, −CH, DPEA); 6.38 (m, 4H, −CH, DPEA); 6.12, (br, 4H, −CH2(b)); 5.22 (m, 4H, −CH, DPEA); 4.33−3.21 (m, 32H, −CH2); 2.47−0.69 (m, 48H, −CH3);); −1.12 (br, 2H, CH, DPEA); −2.54 (br, 4H, NH2). 2Zn·PPDA(S). Yield: 45 mg (80%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 411 (2.32 × 105), 419sh (1.88 × 105), 438sh (6.71 × 104), 557sh (2.56 × 104), 601 (1.33 × 104). 1H NMR (CDCl3, 295 K): δ 9.82 (s, 2H, 10-meso-H); 9.27 (s, 2H, 15-meso-H); 8.66 (s, 2H, 20-meso-H); 6.84 (m, 1H, −CH, PPDA); 6.68 (m, 2H, −CH, PPDA); 6.77, 6.02 (m, 4H, −CH2(b)); 4.86 (m, 2H, −CH, PPDA); 4.72−3.21 (m, 32H, −CH2); 1.98−0.96 (m, 48H, −CH3); −3.49 (br, 1H, H4, PPDA); −3.83 (br, 1H, H5, PPDA); −5.48 (br, 1H, H1, PPDA); −5.72 (m, 1H, H3, PPDA); −6.42 (m, 1H, H2, PPDA); −7.81 (m, 2H, −NH2); −8.06 (m, 2H, −NH2). 2Zn·PEDA(S). Yield: 43 mg (77%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 410 (2.33 × 105), 418sh (2.12 × 105), 438sh (8.88 × 104), 557sh (2.78 × 104), 601 (1.39 × 104). 1H NMR (CDCl3, 295 K): δ 9.78 (s, 2H, 10-meso-H); 9.28 (s, 2H, 15-meso-H); 8.70 (s, 2H, 20-meso-H); 6.98 (br, 1H, −CH, PEDA); 6.81 (br, 2H, −CH, PEDA); 6.62, 6.22 (m, 4H, −CH2(b)); 4.88 (br, 2H, −CH, PEDA); 4.33−3.21 (m, 32H, −CH2); 2.47−0.69 (m, 48H, −CH3); −4.83 (br, 1H, H1, PEDA); −5.87 (br, 1H, H3, PEDA); −6.42 (br, 2H, H2, PEDA); −7.22 (br, 2H, NH2); −7.88 (br, 2H, −NH2). 2Zn·DAP(R). Yield: 45 mg (79%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 410 (2.72 × 105), 418sh (2.41 × 105), 438sh (1.12 × 105), 557sh (3.28 × 104), 601 (2.36 × 104). 1H NMR (CDCl3, 295 K): δ 9.86 (s, 2H, 10-meso-H); 9.37 (s, 2H, 15-meso-H); 8.68 (s, 2H, 20-meso-H); 6.18, 6.08 (br, 4H, −CH2(b)); 4.12−3.11 (m, 32H, −CH2); 2.37−0.86 (m, 48H, −CH3); −3.22 (m, 3H, CH3, DAP); −5.88 (br, 1H, H1, DAP); −6.52 (br, 1H, H3, DAP); −6.86 (br, 1H, H2, DAP); −8.02 (m, 2H, −NH2); −8.19 (m, 4H, −NH2). Computational Details. All optimizations were executed using functional B3LYP in combination with using the Gaussian 09, revision B.01, package.18 The basis sets were 6-31G+(d,p) for C, H, O, N and LANL2DZ for Zn and Mg. Coordinates for full geometry optimizations of all the complexes were obtained from the crystal structure of corresponding molecules, and optimizations were performed in dichloromethane solvent. TDDFT calculations were performed using ωB97X-D functional,28 the basis set was 6-31G*, and dichloromethane was used for solvent correction. SpecDis software23 was used for processing and comparison of CD calculations with experimental spectra. The following UV shift and σ values were used: 0.08 eV and 85 nm for 1Mg·CHDA(R,R), 2Mg·CHDA(R,R) and 0.1 eV and 88 nm for 1Mg·CHDA(R,R), 1Zn·CHDA(R,R). Instrumentation. Circular dichroism and UV−visible spectra were recorded on a JASCO J-815 spectrometer and PerkinElmer UV/vis spectrometer, respectively. 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. Slow diffusion of acetonitrile into the dichloromethane solutions of 1Mg·CHDA(R,R) and 2Mg·CHDA in air at room temperature gave dark red crystals. A Bruker SMART APEX CCD diffractometer equipped with CRYO Industries low-temperature apparatus was utilized to collect the data at 100 K

meso-H); 9.48 (s, 2H, 15-meso-H); 8.79 (s, 2H, 20-meso-H); 8.54 (d, 2H, Ar−H); 7.90 (t, 2H, Ar−H); 7.25 (t, 2H, Ar−H); 6.96 (d, 2H, Ar−H); 3.96−2.93 (m, 24H, −CH3); 2.39 (q, 8H, −CH2); 1.82 (m, 8H, −CH2); 1.22−0.85 (m, 24H, −CH3); −0.62 (br, 2H, H1, CHDA); −2.13 (br, 2H, H2, CHDA); −3.33 (br, 2H, H3, CHDA); −6.19 (br, 2H, H4, CHDA); −6.95 (m, 2H, H5, CHDA); −7.98 (m, 4H, −NH2). 1Mg·PPDA(S). Yield: 45 mg (80%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 409 (2.46 × 105), 425sh (6.11 × 104), 549 (2.29 × 104), 587 (1.21 × 104). 1H NMR (CDCl3, 295 K): δ 9.87 (s, 2H, 10meso-H); 9.46 (s, 2H, 15-meso-H); 8.77 (s, 2H, 20-meso-H); 8.54 (d, 2H, Ar−H); 7.91 (t, 2H, Ar−H); 7.27 (t, 2H, Ar−H); 6.97 (d, 2H, Ar−H); 6.77 (m, 1H, −CH, PPDA); 6.71 (m, 2H, −CH, PPDA); 4.22 (m, 2H, −CH, PPDA); 4.00−2.68 (m, 24H, −CH3); 2.33 (q, 16H, −CH3); 2.04−1.02 (m, 24H, −CH3); −3.02 (br, 1H, H4, PPDA); −3.79 (br, 1H, H5, PPDA); −5.19 (br, 1H, H3, PPDA); −5.49 (br, 1H, H2, PPDA); −6.32 (m, 1H, H1, PPDA); −7.36 (m, 2H, −NH2); −7.49 (m, 2H, −NH2). 1Mg·PEDA(S). Yield: 47 mg (87%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 409 (2.89 × 105), 425sh (6.32 × 104), 550 (2.44 × 104), 588 (1.42 × 104). 1H NMR (CDCl3, 295 K): δ 9.79 (s, 2H, 10meso-H); 9.46 (s, 2H, 15-meso-H); 8.79 (s, 2H, 20-meso-H); 8.53 (d, 2H, Ar−H); 7.91 (t, 2H, Ar−H); 7.24 (t, 2H, Ar−H); 6.97 (d, 2H, Ar−H); 6.34 (br, 1H, −CH, PEDA); 5.98 (br, 2H, −CH, PEDA); 4.28 (br, 2H, −CH, PEDA); 3.98−2.65 (m, 24H, −CH3); 2.38 (q, 16H, −CH3); 1.94−0.82 (m, 24H, −CH3); −4.93 (br, 1H, H1, PEDA); −5.23 (br, 1H, H2, PEDA); −6.51 (br, 1H, H3, PEDA); −6.68 (br, 2H, NH2); −6.96 (br, 2H, −NH2). 1Mg·DAP(R). Yield: 44 mg (82%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 409 (2.55 × 105), 425sh (6.05 × 104), 549 (2.26 × 104), 587 (1.20 × 104). 1H NMR (CDCl3, 295 K): δ 9.86 (s, 2H, 10meso-H); 9.45 (s, 2H, 15-meso-H); 8.75 (s, 2H, 20-meso-H); 8.54 (d, 2H, Ar−H); 7.90 (t, 2H, Ar−H); 7.25 (t, 2H, Ar−H); 6.96 (d, 2H, Ar−H); 3.96−2.63 (m, 24H, −CH3); 2.39 (m, 16H, −CH3); 1.96− 0.85 (m, 24H, −CH3); −4.32 (m, 3H, CH3, DAP); −5.41 (br, 1H, H3, DAP); −5.82 (br, 1H, H2, DAP); −6.25 (br, 1H, H1, DAP); −7.62 (m, 2H, −NH2); −7.88 (m, 4H, −NH2). 1Mg·DPEA(R,R). Yield: 45 mg (76%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 409 (2.78 × 105), 425sh (6.11 × 104), 549 (2.28 × 104), 587 (1.22 × 104). 1H NMR (CDCl3, 295 K): δ 9.87 (s, 2H, 10-meso-H); 9.77 (s, 2H, 15-meso-H); 8.77 (s, 2H, 20-meso-H); 8.53 (d, 2H, Ar−H); 7.90 (t, 2H, Ar−H); 7.25 (t, 2H, Ar−H); 6.96 (d, 2H, Ar−H); 6.68 (m, 2H, −CH, DPEA); 6.38 (m, 4H, −CH, DPEA); 5.22 (m, 4H, −CH, DPEA); 3.96−2.61 (m, 24H, −CH3); 2.39 (q, 16H, −CH3); 1.96−0.85 (m, 24H, −CH3); −0.48 (br, 2H, CH, DPEA); −1.50 (br, 4H, NH2). 2Mg·CHDA(R,R). Yield: 43 mg (78%). ESI-MS: m/z 1252.8100 ([2Mg· CHDA(R,R)]+). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 406 (2.55 × 105), 418sh (2.12 × 105), 436sh (7.12× 104), 557sh (2.58 × 104), 601 (1.36 × 104). 1H NMR (CDCl3, 295 K): δ 9.91 (s, 2H, 10meso-H); 9.37 (s, 2H, 15-meso-H); 8.68 (s, 2H, 20-meso-H); 6.82, 6.33 (m, 4H, −CH2(b)); 4.33−3.21 (m, 32H, −CH2); 2.47−0.69 (m, 48H, −CH3); −0.64 (br, 2H, H1, CHDA); −1.97 (br, 2H, H2, CHDA); −3.12 (br, 2H, H3, CHDA); −5.76 (br, 2H, H4, CHDA); −6.64 (br, 2H, H5, CHDA); −7.79 (br, 4H, −NH2) ppm. 2Mg·PPDA(S). Yield: 43 mg (76%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 406 (2.33 × 105), 418sh (2.58 × 105), 436sh (8.44 × 104), 557sh (2.59 × 104), 601 (1.37 × 104). 1H NMR (CDCl3, 295 K): δ 9.92 (s, 2H, 10-meso-H); 9.41 (s, 2H, 15-meso-H); 8.67 (s, 2H, 20-meso-H); 7.14 (m, 2H, −CH, PPDA); 6.87 (m, 1H, −CH, PPDA); 6.11 (br, 4H, −CH2(b)); 5.01 (m, 2H, −CH, PPDA); 4.81−3.31 (m, 32H, −CH2); 1.95−0.97 (m, 48H, −CH3); −3.29 (br, 1H, H4, PPDA); −3.89 (br, 1H, H5, PPDA); −5.23 (br, 1H, H1, PPDA); −5.77 (br, 1H, H3, PPDA); −6.20 (br, 1H, H2, PPDA); −7.60 (br, 2H, −NH2); −7.76 (m, 2H, −NH2). 2Mg·PEDA(S). Yield: 43 mg (75%). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 406 (1.78 × 105), 418sh (2.23 × 104), 557sh (2.32 × 105), 436sh (4.82 × 104), 601 (1.33 × 104). 1H NMR (CDCl3, 295 K): δ 9.90 (s, 2H, 10-meso-H); 9.27 (s, 2H, 15-meso-H); 8.86 (s, 2H, 20-meso-H); 7.04 (br, 1H, −CH, PEDA); 6.91 (br, 2H, −CH, PEDA); 6.62, 6.12 (m, 4H, −CH2(b)); 4.88 (br, 2H, −CH, PEDA); 4.33−3.21 J

DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). SAINT29 software was used to process the data. An absorption correction was applied.30 SHELXS-97 was used to solve the structures by using direct methods. The structures were then refined on F2 by the full-matrix least-squares technique using the SHELXL-2014 program package.31 The asymmetric unit contains two molecules in both complexes. All the non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included at the calculated positions and treated as riding atoms using SHELXL default parameters.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02544. ESI-MS, UV−visible, 1H NMR, and CD results, Job’s plot, packing diagram, binding constant determination, bond distances and angles, and Cartesian coordinates (PDF) X-ray crystallographic details for 1Mg·CHDA(R,R)(CIF) X-ray crystallographic details for 2Mg·CHDA (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank IIT Kanpur for providing all the facilities and support. Science and Engineering Research Board (SERB), India, and the Council of Scientific and Industrial Research (CSIR), New Delhi, are gratefully acknowledged for financial support. A.D. and S.A.I. thank CSIR, New Delhi, for the fellowship.

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DEDICATION Dedicated to Professor Harkesh B. Singh on the occasion of his 60th birthday. REFERENCES

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DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02544 Inorg. Chem. XXXX, XXX, XXX−XXX