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Mar 29, 2016 - nonlinear least-squares program SQUAD.27 The detailed fitting graphs are provided in Figure S6. Binding constants K1, K2, and. K3 are f...
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Crystallographic and Spectroscopic Studies of a Host−Guest Complex Consisting of a Novel Zinc Trisporphyrinate and a Chiral Monoamine Zhen Han,† Li Li,† Bo Shi,‡ Xianshi Fang,† Yong Wang,*,† and Chuanjiang Hu*,† †

Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China ‡ Jiangsu Key Laboratory of Technology for Polyamine, Polyol and Derived Polymers, Jiangsu Feixiang Group of Companies, Suzhou, P. R. China S Supporting Information *

ABSTRACT: We have designed and synthesized a novel zinc trisporphyrinate with a benzene tricarboxamide as the linker. In the presence of a large excess of 1-phenylethylamine, single crystals of the corresponding 1:3 host−guest complex were obtained, which provide the crystallographic structure of a host−guest complex consisting of an achiral porphyrin and a chiral monoamine. The structure reveals the 1-phenylethylamines adopt the “inside” binding mode that is stabilized by intramolecular hydrogen bonds. The NH2 of the 1-phenylethylamine is involved in both coordination and hydrogen bonding interactions. Circular dichroism (CD) and ultraviolet−visible spectra revealed that the 1:3 host−guest complex is dominant in the presence of a large excess of 1-phenylethylamine. The crystal structure shows there are two diastereomers of the 1:3 host−guest complexes. Density functional theory and TDDFT calculations suggest that one of the diastereomers is more energetically favorable, which dominates the CD signals.

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INTRODUCTION Because of distinct spectroscopic properties, such as a redshifted and intense ultraviolet−visible (UV−vis) absorption, porphyrins have been used as hosts for chirality-related studies, such as chirality induction, chirality transfer, chirality recognition, etc., in recent decades.1−4 Many porphyrin hosts and chiral guests have been studied. For the hosts, bisporphyrins have been intensively studied.5−7 For guests, they range from multidentate ligands, such as sugars and peptides, to monodentate ligands, such as monoamines,8−15 monoalcohols,9,10,16,17 etc. However, monodentate ligands are usually more difficult to study because there is only one binding site. When monoamines were used as guests, only limited systems have been reported.8−15 For example, Berova, Nakanishi, and co-workers have developed a pentanediolbridged bisporphyrin that can assign the absolute configuration of chiral monoamines through their derivatives.8−10 Inoue, Borovkov, and co-workers developed an ethylene-bridged bisporphyrin system, in which the short linkage causes the © XXXX American Chemical Society

steric interactions between the 3,7-ethyl groups of the porphyrin and the substituents of the ligand, resulting in the induced supramolecular chirality.11−14 Borhan and co-workers have recently developed the biphenol-bridged metal-free bisporphyrin system, in which the hydrogen bonding and steric interactions between the monoamines and the host molecules lead to the stereodifferentiation.15 One important part of these studies is finding the binding mode of the host−guest complexes. An accurate binding mode can reveal the interactions between host and guest molecules, help us understand the corresponding spectroscopic results and structure−function relationship, and even lead to the development of new materials. Crystal structures of the host−guest complexes could provide direct binding information, which makes them very useful in these studies. However, it is usually Received: October 5, 2015

A

DOI: 10.1021/acs.inorgchem.5b02295 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthetic Route for Compound 1, [Zn3-1], and [Zn2Ni-1]

difficult to obtain the crystal structures of the complicated host−guest complexes. For porphyrin systems, only a few crystallographically characterized examples have been reported. For instance, Aida and co-workers have developed a chiralitymemory molecule and crystallographically characterized the mandelate complex of a D2-symmetric saddle-shaped porphyrin that showed that two mandelate anions were hydrogen bonded to the pyrrole NH moieties in a monodentate fashion.18 Inoue, Borovkov, and co-workers have reported the first crystallographic structure of a host−guest complex between an achiral bisporphyrin host and a chiral diamine guest.19 Rath and coworkers have recently crystallographically characterized several host−guest complexes formed between bisporphyrin hosts and diamines or diols.20 Our group has focused on chirality-related studies in recent years. We have developed a series of amide-linked bisporphyrins from 2-aminophenyl-substituted monoporphyrin.21 For these porphyrins, zinc can provide a coordination bonding site and the amide can provide a hydrogen bonding site. Our previous studies showed the m-phthalic diamide-linked zinc bisporphyrinates have the ability to assign the absolute configuration of chiral monoamines.21 We were not able to obtain the crystal structures of the host−guest complexes for the m-phthalic diamide-linked zinc bisporphyrinates, so we have tried to find a more rigid system that could not only have similar interactions with monodentate guests but also be much easier to crystallize. We have designed and synthesized a novel zinc trisporphyrinate [Zn3-1] as shown in Scheme 1. We have examined the circular dichroism (CD) spectra of the complexes between this host and a series of chiral monoamines as shown in Figure 1. When we used one chiral monoamine, R-1phenylethylamine (PEA), as the guest, and single crystals of the 1:3 host−guest complex, [Zn3-1]·(R-PEA)3, were obtained. Herein, we report the first crystallographic structure of a host− guest complex consisting of an achiral porphyrin host and a chiral monoamine guest, in which the NH2 group of the 1phenylethylamine is involved in both coordination and

Figure 1. Structural formulas for the ligands used in this work.

hydrogen bonding interactions. On the basis of the structural data, the CD spectra were also rationalized by density functional theory (DFT) calculations.

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EXPERIMENTAL SECTION

Material and Physical Methods. All reagents were obtained from commercial sources without further purification unless otherwise noted. Triethylamine (Et3N) was distilled over potassium hydroxide, and methylene chloride was treated with CaH2 before being used. All monoamine used was purchased from commercial sources. The zinc 5(2-aminophenyl)-10,15,20-triphenylporphyrinate was synthesized according to the reported methods.22 Elemental analyses (C, H, and N) were performed with an Elementar Vario EL III analytical instrument. 1H nuclear magnetic resonance (NMR) spectra were recorded at room temperature, using a Bruker AVANCE 400 MHz spectrometer in CDCl3 with tetramethylsilane (TMS) as the internal standard; chemical shifts are expressed in parts per million relative to TMS (0 ppm). Twodimensional NMR spectra were recorded on an Agilent 600 MHz NMR instrument. UV−vis spectra were recorded with a Shimadzu UV-3150 spectrometer. Infrared (IR) spectra were recorded with a Bruker vertex-70 spectrometer. Mass spectra were recorded with an Agilent 6220 Accurate-Mass TOF LC/MS system. CD spectra were recorded on an AVIV model 410 spectropolarimeter at 295 K. Scanning conditions were as follows: wavelength step, 1.00 nm; bandwidth, 2 nm; response time, 0.1 s; averaging time, 0.100 s; settling time, 0.333 s. B

DOI: 10.1021/acs.inorgchem.5b02295 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 1 H NMR, CD, and UV−Vis Measurements. The UV−vis and CD experiments were conducted as follows. Measurements have been performed by adding different aliquots of the optically active monoamine solution to the [Zn3-1] or [Zn2Ni-1] solution in methylene chloride at room temperature. Then UV−vis and CD spectra were recorded after each addition. The resultant CD spectra recorded in millidegrees were normalized on the basis of the concentration of [Zn3-1] or [Zn2Ni-1]. 1 H NMR measurements were taken as follows. A solution of 1phenylethylamine in CDCl3 was added to the solution of zinc triporphyrinate (500 μL) in CDCl3 in a 5 mm outside diameter NMR tube, and 1H NMR spectra were recorded after each addition. Preparation of Free Base Trisporphyrin 1. The reaction was conducted under anaerobic conditions. 5-(2-Aminophenyl)-10,15,20triphenylporphyrinate (0.50 g, 0.78 mmol) was dissolved in anhydrous methylene chloride (50 mL). Et3N (150 μL, 1.07 mmol) was added to the solution described above and the mixture stirred for 15 min in an ice bath, and then trimesoyl chloride (0.08 g, 0.30 mmol) was added. The mixture was slowly warmed to room temperature and monitored by thin-layer chromatography (TLC). After 8 h, the reaction was complete. The solution was rotorevaporated to dryness under vacuum. The purple solid was obtained and purified by column chromatography (silica, 2:1 CH2Cl2/petroleum ether) (0.15 g, 28% yield). 1H NMR (400 MHz, CDCl3): δ 8.85 (6H, d), 8.81 (6H, d) 8.24 (6H, d), 8.20 (6H, t), 8.10 (6H, d), 8.05 (12H, t), 7.82 (9H, m), 7.69 (15H, m), 7.51 (6H, t), 7.40 (3H, t), 7.31 (3H, t), 7.16 (3H, d), 6.62 (3H, s), 5.17 (3H, s), −3.02 (6H, s). Anal. Calcd for C141H93N15O3: C, 82.80; H, 4.58; N, 10.27. Found: C, 82.81; H, 4.56; N, 10.28. MS (ESI) (C141H93N15O3): calcd [M + H]+ m/z 2044.76, found [M + H]+ m/z 2044.77. IR (ν): 3408 (w), 3315 (w), 3354 (w), 1687 (m), 1511 (m), 1442 (m), 1350 (m), 1238 (m), 1222 (m), 965 (s), 800 (s), 726 (s), 700 (s) cm−1. Preparation of Zinc Trisporphyrinate [Zn3-1]. The free base trisporphyrin 1 (0.30 g, 0.14 mmol) was dissolved in a mixture of CHCl3 (150 mL) and CH3OH (50 mL). Zn(CH3COO)2 (0.25 g, 1.4 mmol) was added to the solution described above and refluxed for 2 h. Then it was extracted with water, and the organic layer was collected and evaporated to dryness under vacuum. A purple solid was obtained and purified by silica gel chromatography (1:1 CH2Cl2/petroleum ether) (0.31 g, 98% yield).1H NMR (400 MHz, CDCl3): δ 9.01 (6H, d), 8.91 (6H, d) 8.45 (6H, d), 8.39 (3H, d), 8.15 (3H, d), 8.08 (12H, m), 7.85 (15H, m), 7.77 (3H, t), 7.67 (12H, m), 7.47 (6H, t), 7.37 (6H, m), 6.83 (3H, s), 5.90 (3H, d), 3.56 (3H, s). Anal. Calcd for C141H87N15O3Zn3: C, 75.76; H, 3.92; N, 9.40. Found: C, 75.75; H, 3.94; N, 9.39. MS (ESI) (C141H87N15O3Zn3): calcd [M + H]+ m/z 2230.50, found [M + H]+ m/z 2230.50. IR (ν): 3392 (w), 3052 (w), 1668 (m), 1518 (m), 1440 (m), 1338 (m), 1204 (m), 1069 (m), 993 (s), 796 (s), 752 (s), 700 (s) cm−1. Preparation of Crystals of [Zn3-1]·(R-PEA)3. [Zn3-1] (5 mg, 0.002 mmol) was dissolved in a CH2Cl2/CHCl3 solution (1:1) (1 mL); 3.6 mg (0.03 mmol) of R-1-phenylethylamine was added to it and the mixture stirred for ∼3 min. Then it was transferred into 8 mm × 250 mm glass tubes. n-Hexane was added as a nonsolvent at room temperature. After three months, purple crystals were obtained, which were then isolated by filtration, washed with n-hexane, and dried under vacuum. (1.2 mg, 24% yield). Anal. Calcd for C330H240N36O6Zn6: C, 76.25; H, 4.65; N, 9.70. Found: C, 76.23; H, 4.66; N, 9.68. IR (ν): 3400 (w), 1673 (m), 1515 (m), 1442 (m), 1338 (m), 1307 (m), 1067 (m), 991 (s), 795 (s), 699 (s) cm−1. Preparation of [Zn2Ni-1]. The free base triporphyrin 1 (0.30 g, 0.14 mmol) was dissolved in a mixture of CH2Cl2 (100 mL) and CH3OH (50 mL). Zn(CH3COO)2 (0.03 g, 0.17 mmol) was added to the solution described above and refluxed. Frequent TLC monitoring allowed us to establish optimized conditions for a maximal yield of the bismetalated trimer [Zn2-1]. Then it was extracted with water, and the organic layer was evaporated to dryness under vacuum. The resulting purple solid was purified by silica gel chromatography. [Zn2-1] (0.15 g, 0.07 mmol) was obtained and dissolved in DMF (15 mL). NiCl2· 6H2O (0.08 g, 0.56 mmol) was added to the solution described above and refluxed for 2 h. Then it was extracted with water, and the organic

layer was evaporated to dryness under vacuum. A purple solid was obtained and purified by silica gel chromatography (1:2 CH2Cl2/ petroleum ether) (0.14 g, 90% yield). 1H NMR (400 MHz, CDCl3): δ 8.99 (4H, s), 8.87 (4H, s), 8.72 (4H, d), 8.42 (8H, s), 8.13 (6H, d), 8.06 (4H, s), 7.92 (7H, d), 7.84 (8H, s), 7.73 (7H, s), 7.63 (12H, m), 7.52 (5H, s), 7.41 (5H, t), 7.32 (4H, s), 6.73 (2H, s), 6.60 (2H, s), 6.44 (1H, s), 5.79 (1H, s), 4.27 (2H, s), 3.65 (1H, s). Anal. Calcd for C141H87N15NiO3Zn2: C, 75.98; H, 3.93; N, 9.43. Found: C, 75.96; H, 3.94; N, 9.42. MS (ESI) (C141H87N15NiO3Zn2): calcd [M + H]+ m/z 2224.51, found [M + H]+ m/z 2224.51. IR (ν): 3393 (w), 3053 (w), 1656 (m), 1513 (m), 1439.72 (m), 1338 (m), 1301 (m), 1202 (m), 992 (s), 794 (s), 751 (s), 699 (s) cm−1. X-ray Structure Determination. The measurements of a single crystal of [Zn3-1]·(R-PEA)3 were taken on an Agilent Xcalibur diffractometer with an Atlas (Gemini Ultra Cu) detector by using graphite monochromated Cu Kα radiation (λ = 0.154178 nm) at 223(2) K. The structure was determined by direct methods and refined on F2 using full matrix least-squares methods with SHELXTL version 97.23 The asymmetric unit contains two molecules of [Zn3-1]· (R-PEA)3. All non-hydrogen atoms were refined anisotropically; all the hydrogen atoms were theoretically added and riding on their parent atoms. One phenyl ring of R-1-phenylethylamine was found disordered over two positions. The final refinement gave an occupancy of the major component of 77%. The asymmetric unit contains badly disordered solvent molecules. SQUEEZE24 was used to model all disordered solvate. The electron count within the inter-porphyrin voids is 152 e (corresponding to roughly two molecules of CH2Cl2 per compound [Zn3-1]). Details of the crystal parameters, data collection, and refinement are summarized in Table 1. Complete crystallographic details, atomic coordinates, anisotropic thermal parameters, and fixed

Table 1. Crystal Data and Structural Refinement Data of [Zn3-1]·(R-PEA)3·2CH2Cl2 chemical formula formula weight wavelength (Å) temperature (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z density (Mg/m3) absorption coefficient (mm−1) F(000) data collection θ range (deg) index ranges

no. of reflections collected Rint no. of independent reflections data/restraints/parameters goodness of fit on F2 R1a [I > 2σ(I)] wR2b residual peak/hole (e/Å3) Flack parameter a

C

C167H124Cl4N18O3Zn3 2768.75 1.54178 223(2) triclinic P1 13.0878(4) 17.9670(7) 31.6685(8) 82.813(3) 88.295(2) 69.030(3) 6898.0(4) 2 1.333 1.825 2868 3.15−67.50 −15 ≤ h ≤ 15 −21 ≤ k ≤ 21 −24 ≤ l ≤ 37 48517 0.0527 28815 28815/771/3317 1.195 0.0715 0.1871 1.391/−0.818 0.04(3)

R1 = (F0 − Fc)/F0. bwR2 = [w(F02 − Fc2)2/w(F02)2]1/2. DOI: 10.1021/acs.inorgchem.5b02295 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry hydrogen atom coordinates are given in the CIF file in the Supporting Information. Computational Methods. Full optimizations of diastereomers A and B were performed by DFT with the gradient-corrected Perdew− Wang 91 correlation functional (B3PW91) at the B3PW91/6-31G* level using the Gaussian09 suite of programs.25 We employed DFT with no symmetry constraints to investigate the optimized geometries. Finally, ECD spectra were calculated by the TDDFT method. Both electronic excitation energies (nanometers) and rotational strengths (Δε) were calculated. To cover the range of 400−600 nm, 40 transitions were selected. As shown in Figure S14, the simulated spectrum is in good agreement with the experimental spectral data.

For S-PEA, the spectra show similar shapes but opposite signs (see Figure S4). Besides 1-phenylethylamine, four other pairs of chiral monoamines were also examined. Their spectra are shown in Figure 3. In the presence of an excess of guests, these spectra

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RESULTS AND DISCUSSION CD and UV−Vis Spectral Studies. The CD titration spectra were recorded by addition of optically pure PEA into the solution of [Zn3-1] or [Zn2Ni-1]. CD spectra in Figure 2

Figure 2. CD titration spectra of [Zn3-1] (9.95 × 10−7 M) (a−c) and [Zn2Ni-1] (9.90 × 10−7 M) (d and e) by R-PEA in methylene chloride at 295 K. The host:guest ratios are (a) from 1:0 to 1:130, (b) from 1:130 to 1:390, (c) from 1:390 to 1:1500, (d) from 1:0 to 1:30, and (e) from 1:30 to 1:1500.

clearly show strong signals in the Soret band region. The titration can be divided into three steps. In the first step, when the amount of the guest is from 0 to 130 equiv, CD spectra show a positive peak at 421 nm and a negative peak at 410 nm. Intensities of both absorptions increase with an increasing concentration of the guest. In the second step, when the amount of R-PEA changes from 130 to 390 equiv, the intensities of the two peaks mentioned above start to decrease, and a new negative peak occurs at 425 nm. At 390 equiv, the negative peak at 410 nm disappears, and the signal shows a typical bisignate shape. In the third step, when the amount changes from 390 to 1500 equiv, the peak at 425 nm continues to increase and a new positive peak appears at 411 nm. When the amount is >1500 equiv, the spectrum remains unchanged.

Figure 3. Circular dichroism spectra of a solution of [Zn3-1] (1.00 × 10−6 mol L−1) and 1500 equiv of (A) 2S (---) and 2R (), (B) 3S (---) and 3R (), (C) 4S (---) and 4R (), and (D) 5S (---) and 5R () in methylene chloride at 25 °C.

showed a similar feature: when R-type amines were used, the spectra showed a negative peak at longer wavelengths and a positive peak at shorter wavelengths. For guests 3−5, their spectra also show a weak peak (or shoulder) in the middle position. For S-type amines, the spectra show opposite signs but similar intensities. K1

[Zn3‐1] + L ⇌ [Zn3‐1]·L D

(1) DOI: 10.1021/acs.inorgchem.5b02295 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry K2

[Zn3‐1]·L + L ⇌ [Zn3‐1]·(L)2

disappears. That suggests all four-coordinate zinc porphyrinates are converted into five-coordinate species and the 1:3 complex is the major species under such conditions. If we treat the spectral changes as the three equilibria mentioned above, the corresponding binding constants can be calculated with the nonlinear least-squares program SQUAD.27 The detailed fitting graphs are provided in Figure S6. Binding constants K1, K2, and K3 are found to be (1.4 ± 0.3) × 104, (1.1 ± 0.2) × 104, and (1.8 ± 0.3) × 103, respectively. We also obtained the corresponding binding constants by fitting the CD spectral changes as shown in Figure S8, which give similar values with slightly larger errors (Table S1). K1 is similar to K2 but larger than K3. K1 and K2 are larger than the corresponding binding constant (∼2 × 103) for [Zn(OEP)]13 (OEP = octaethylporphyrin), which suggests there could be extra factors to stabilize the coordination complex, such as hydrogen bonds shown in the crystal structure (vide inf ra). For comparison, we also performed titration experiments with [Zn2Ni-1]. As shown in Figure S5, the spectra are similar to those of [Zn3-1]. The fitting by SQUAD gives the corresponding binding constants: K1 = (2.6 ± 0.2) × 104, and K2 = (5.3 ± 0.5) × 103. These values are also similar to those for [Zn3-1]. That fact further confirms there are three equilibria in solution for [Zn3-1], but two for [Zn2Ni-1]. However, as we also notice, the UV−vis spectral change does not clearly show three steps like the CD spectra. The difference could be related to the conformational issue. For example, for the 1:3 complex, there could be multiple conformers, but their contributions to CD spectra are generally not proportional to their contributions to UV−vis spectra. Our calculations of binding constants were based on UV−vis titration, which could be simplified. The real solution behavior may be more complicated, but it is certain that the 1:3 host−guest complex is the dominant species for [Zn3-1] when the guests are in a large excess (high concentration). Crystal Structure and Solid CD Spectrum of the Host− Guest Complex [Zn3-1]·(R-PEA)3. To obtain the binding model of the 1:3 complex, we have tried to determine its crystal structure. We used many methods to attempt to grow crystals from the mixture of [Zn3-1] and a large excess of R-PEA. Suitable single crystals were finally obtained by diffusion of hexane into the CH2Cl2/CHCl3 solution (1:1). The structure of [Zn3-1]·(R-PEA)3 was determined in chiral space group P1. One asymmetric unit contains two independent trizinc trisporphyrinate molecules. Both of them are shown in Figure 5. As expected, the trisporphyrin host molecules have three zinc porphyrinate subunits that are linked by a benzene tricarboxamide group. Each zinc is five-coordinated with R-1-phenylethylamine as the axial ligand, which leads to the 1:3 host− guest complex. As we notice, all ligands adopt the “inside” binding mode (the ligand is facing the linker). In the structure, there are also interesting hydrogen bonding interactions as shown in Figure 5 (vide supra). Because the structural data were not good enough to find H atoms in the difference Fourier map, all hydrogen atoms were theoretically added and riding on their parent atoms. Even so, the corresponding N···O distances (ranging from 2.92 to 3.32 Å) indicate there are weak hydrogen bonds between the carboxylic oxygens and the coordinated NH2. In Inoue and Borovkov’s system, the NH2 of monoamine is involved in coordination interaction.14 In Borhan’s system, the NH2 is involved in hydrogen bonds.15 Different from their systems, in our case, the NH2 is involved in both coordination and hydrogen bonding interactions. These interactions could

(2)

K3

[Zn3‐1]·(L)2 + L ⇌ [Zn3‐1]·(L)3

(3)

How do we understand the titration CD spectra in Figure 2? For comparison, we also measured the CD spectra of the mixture between chiral monoamines and free base trisporphyrin, which did not show any observable signal. That suggests the coordination compounds are responsible for the resulting CD. Because there are three zinc ions in [Zn3-1], when the 1phenylethylamine is coordinated to [Zn3-1], the equilibria described above should exist in solution. There will be three possible complexes with host:guest molar ratios of 1:1, 1:2, and 1:3. The overall titration spectral changes for [Zn3-1] could be explained as follows. In step 1 (0−130 equiv), the dominant species is the 1:1 complex; in step 2 (130−390 equiv), the dominant species in solution is the 1:2 complex, and in step 3 (>390 equiv), the dominant species is the 1:3 complex. To confirm this explanation, we also took CD measurements with [Zn2Ni-1] as the host. Because inertness toward most of the conventional host−guest binding modes was welldocumented for nickel porphyrins,26 [Zn2Ni-1] can be treated as only two coordination sites. The spectral changes (Figure 2d,e) of the mixture of [Zn2Ni-1] and R-PEA clearly indicate the formation of two different optically active species, which most likely correspond to the 1:1 and 1:2 complexes. The titration spectra in Figure 2d are similar to those in Figure 2a. In Figure 2e, the peak at 407 nm disappears when 1phenylethylamine reaches ∼1500 equiv, so the signal shows a typical bisignate shape, which is also similar to the case of [Zn31] at 390 equiv of 1-phenylethylamine as shown in Figure 2b. Therefore, the corresponding spectral change of [Zn3-1] should be caused by the formation of the 1:1 and 1:2 complexes. For [Zn2Ni-1], the CD titration spectra did not show the third step as [Zn3-1] in Figure 2c, which suggests the third step should be dominated by the 1:3 complex for [Zn3-1]. We also took the UV−vis titration measurements. As shown in Figure 4, initially, [Zn3-1] shows a strong peak at 420 nm.

Figure 4. UV−vis spectral change of [Zn3-1] (9.95 × 10−7 M) upon addition of S-1-phenylethylamine as the host:guest molar ratio changes from 1:0 to 1:500.

During the titration, the intensity of such a peak decreases, and a new peak appears at 427 nm. The intensity of the peak at 427 nm keeps increasing until reaching saturation. Such a red shift is generally due to the coordination of a nitrogen-containing ligand to the zinc complex. The original peak at 420 nm is caused by four-coordinate zinc porphyrinate, while the peak at 427 nm is caused by five-coordinate zinc porphyrinate. When the amount of guest is ∼500 equiv, the peak at 420 nm almost E

DOI: 10.1021/acs.inorgchem.5b02295 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. CD spectra of two single crystals of [Zn3-1]·(R-PEA)3 in the KBr pellet.

Figure 7. 1H NMR spectra in CDCl3 (at 295 K) of (A) [Zn3-1], (B) the mixture of [Zn3-1] (6.3 × 10−3 M) and S-PEA (0.6 equiv), and (C) S-PEA. The inset shows the proton numbering scheme of S-PEA. The arrows show the shifts of the relevant protons.

ring causes such shifts.28 The titration spectra were also recorded when the concentrations of S-1-phenylethylamine changed from 0 to 11 equiv. As shown in Figure S11 of the Supporting Information, when the concentration increases, the proton signals of the guest become broader. In solution, when a large excess of S-1-phenylethylamines is added, the two diastereomers of the 1:3 host−guest complexes mentioned above could form. It is most likely that one of the diastereomers is more energetically favorable, which dominates the CD signals in solution. To observe the diastereomeric excess, we also performed low-temperature 1H NMR measurements. We chose 4 equiv as the amount of guest for the following reasons. (1) According to the equilibrium constant, such an amount of 1-phenylethylamine is enough to guarantee the dominant species is a 1:3 complex. (2) According to the titration spectra (Figure S11), 4 equiv of 1-phenylethylamine can still provide clear NMR signals. The 1H NMR spectra at variable temperatures are shown in Figure 8. It is clear that all signals became broadened when the temperature was lowered. That indicates there may be multiple conformers in solution. Unfortunately, even down to −58 °C, the lowest temperature we can reach in our experiment, there was no clear splitting. It suggests the exchange between diastereomers could be much faster than the NMR time scale. Computational Studies. To gain further insight into this system, we also performed investigations via DFT calculations. The optimized structures for diastereomers A and B were based on the crystal structure data for molecules A and B, respectively. The results suggest that diastereomer B is more energetically favorable (5.95 kcal/mol lower) than diastereomer

Figure 5. ORTEP view for [Zn3-1]·(R-PEA)3 (top, molecule A; bottom, molecule B) with 30% probability thermal ellipsoids. Hydrogen bonds are shown as dashed lines. Some phenyl rings and some hydrogens have been omitted for the sake of clarity. N1···O013, 2.919 Å; N2···O011, 2.931 Å; N3···O012, 3.160 Å; N4···O022, 2.987 Å; N5···O023, 2.922 Å; N6···O021, 3.320 Å.

be the major factors stabilizing the “inside” binding mode for the 1:3 complex. Notably, in the asymmetric unit of the 1:3 complex, two zinc trisporphyrinate host molecules form a pair of quasienantiomers, while the guest molecule is a single enantiomer, which led to two diastereomers of the host−guest complexes in the crystal structure. To confirm the chirality of single crystals, we took CD measurements on several single-crystal samples. For these measurements, the signals were all weak, but they did show CD signals in the Soret band region. Two of them are given in Figure 6. These CD spectra show the same sign as the solution CD spectra in the presence of a large excess of R-PEA and a similar shape. It suggests that chirality is maintained in the single crystals. 1 H NMR Studies. 1H NMR experiments have been performed between [Zn3-1] and S-1-phenylethylamine. The assignments of the signals are based on 1H−1H gCOSY spectra (Figure S10). As shown in Figure 7, upon addition of 1phenylethylamine, the resonances of the ligand protons show remarkable upfield shifts, which indicates the coordination of the ligand to the zinc, and the shielding effect of the porphyrin F

DOI: 10.1021/acs.inorgchem.5b02295 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

bisporphyrin systems. There are also reports that this method can be used to explain the CD spectra for systems containing three chromophores in high symmetry,30−33 but for systems containing three chromophores in low symmetry, it is very difficult. In our case, in which the structure of 1:3 complex has only P1 symmetry, we cannot directly relate the geometry of the trisporphyrinate to the sign of CD signals, so we performed further studies by TDDFT calculations. On the basis of the optimized structure, TDDFT calculations gave the calculated electronic CD spectrum shown in Figure S14. The major feature of the calculated CD spectrum is in agreement with the experimental result. Compared with the observed CD bands, the calculated CD bands are red-shifted. Such cases have also been reported in other DFT-based calculations.22,34 Moreover, TDDFT calculations reveal more information about the corresponding optical transitions as summarized in Table S3 and Figure S15. According to DFT and TDDFT calculations, the lowest-energy band experimentally found for diastereomer B around 425 nm mainly originates from HOMO → LUMO, HOMO−1 → LUMO+2, and HOMO−2 → LUMO+1 transitions, which can be assigned to πII → π*II, πIII → π*III, πIII → π*I, and πI → π*I, πI → π*III due to the orbital characters of the corresponding starting and arriving states (I, II, and III refer to three porphyrin moieties). The experimental absorption band at 421 nm originates from HOMO → LUMO +2, HOMO−1 → LUMO+1, HOMO → LUMO+6, HOMO → LUMO+4, HOMO−2 → LUMO+6, and HOMO → LUMO+7 transitions, which also show π → π* character.

Figure 8. 1H NMR spectra for the mixture of [Zn3-1] (6.3 × 10−3 M) and S-1-phenylethylamine (4 equiv) at variable temperatures. The inset shows the proton numbering scheme of S-PEA. The arrows show the shifts of the relevant protons.

A (Figure S13). As shown in Figure 9, in their optimized structures, a noteworthy difference is the orientation of the

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CONCLUSION We have designed and synthesized a novel benzene tricarboxamide-linked zinc trisporphyrinate. CD and UV−vis spectra revealed that the 1:3 host−guest complex is dominant in the presence of a large excess of 1-phenylethylamine. The crystal structure of [Zn3-1]·(R-PEA)3 reveals there are two diastereomers of the 1:3 host−guest complexes; the 1phenylethylamines adopt the “inside” binding mode in both diastereomers that is stabilized by multiple intramolecular hydrogen bonds. The NH2 of 1-phenylethylamine is involved in both coordination and hydrogen bonding interactions. On the basis of structural data, DFT and TDDFT calculations suggest that diastereomer B is more energetically favorable, which dominates the CD signals. These studies could also help us understand the binding interactions in other amide-linked porphyrin systems. Further investigation of the 1:1 and 1:2 complexes and other chiral guests is ongoing.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 9. Space-filling view for optimized diastereomers A and B of [Zn3-1]·(R-PEA)3. The 1-phenylethylamine guests are colored red.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02295. MS spectra, CD spectra of a solution of [Zn3-1] with SPEA, 1H NMR titration spectra, 1H−1H gCOY spectrum, and DFT calculation results (PDF) Crystallographic data (CCDC 1055417) (CIF)

guests. Two guest molecules are away from each other in diastereomer B, while they are facing toward each other in diastereomer A. Such an orientation may cause less steric repulsion between the 1-phenylethylamine guests for diastereomer B and, hence, lower its energy. Therefore, diastereomer B should be the major contributor to the CD spectra. It is well-known that exciton-coupled circular dichroism (ECCD) spectroscopy29 is a useful nonempirical method for relating the sign of the observed CD with the dihedral angles of the transition dipoles of the chromophores. This method is very useful for systems containing two chromophores, such as

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AUTHOR INFORMATION

Corresponding Authors

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

DOI: 10.1021/acs.inorgchem.5b02295 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Author Contributions

(22) Goudriaan, P. E.; Jang, X.-B.; Kuil, M.; Lemmens, R.; Van Leeuwen, P. W. N. M.; Reek, J. N. H. Eur. J. Org. Chem. 2008, 2008, 6079. (23) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112. (24) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, G. H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision C.01; Gaussian, Inc.: Wallingford, CT, 2010. (26) Sanders, J. K. M.; Bampos, N.; Clyde-Watson, Z.; Darling, S. L.; Hawley, J. C.; Kim, H.-J.; Mak, C. C.; Webb, S. J. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 3, pp 1. (27) Leggett, D. J. Computational Methods for the Determination of Formation Constants; Plenum: New York, 1985. (28) Medforth, C. J. In The Porphyrin Handbook; Kadish, K., Smith, K., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 5, Chapter 35. (29) Berova, N., Nakanishi, K., Woody, R. W., Eds. Circular Dichroism: Principles and Applications, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2000. (30) Mason, S. F. Molecular Optical Activity & the Chiral Discriminations; Cambridge University Press: Cambridge, U.K., 1982. (31) Mason, S. F. Inorg. Chim. Acta, Rev. 1968, 2, 89. (32) Bosnich, B. Acc. Chem. Res. 1969, 2, 266. (33) Dragna, J. M.; Pescitelli, G.; Tran, L.; Lynch, V. M.; Anslyn, E. V.; DiBari, L. J. Am. Chem. Soc. 2012, 134, 4398. (34) (a) Alonso-Gomez, J. L.; Petrovic, A. G.; Harada, N.; RiveraFuentes, P.; Berova, N.; Diederich, F. Chem. - Eur. J. 2009, 15, 8396. (b) Wang, Y.; Wang, Y.; Wang, J.; Liu, Y.; Yang, Y. J. Am. Chem. Soc. 2009, 131, 8839. (c) Šebek, J.; Bouř, P. J. Phys. Chem. A 2008, 112, 2920.

Z.H. and L.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Nature Science Foundation of China for financial support (21271133 and 21531006), the State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). We also thank W. R. Scheidt (University of Notre Dame, Notre Dame, IN) for help with manuscript preparation.

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DOI: 10.1021/acs.inorgchem.5b02295 Inorg. Chem. XXXX, XXX, XXX−XXX