Effect of Two Interacting Rings in Metalloporphyrin Dimers upon

Article pubs.acs.org/IC

Effect of Two Interacting Rings in Metalloporphyrin Dimers upon Stepwise Oxidations Soumyajit Dey, Debangsu Sil, Younis Ahmad Pandit, and Sankar Prasad Rath* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India S Supporting Information *

ABSTRACT: The interaction between two porphyrin macrocycles, connected covalently through either a rigid ethylene or a flexible ethane bridge, in the metalloporphyrin dimers (M: 2H, Zn2+) have been investigated upon stepwise oxidations. Upon 1e-oxidation, two porphyrin macrocycles come closer and cofacial to each other while 2e-oxidation forces them to be separated as far as possible. This has resulted in the conversion of the cis isomer to trans for the ethylene bridged porphyrin dimer with the stabilization of an unusual “U” form, which has unique spectral and geometrical features. Detailed ultraviolet−visible−nearinfrared (UV-vis-NIR), infrared (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopic investigations, along with X-ray structure determination of the 2e-oxidized complexes, have demonstrated strong electronic communications between two porphyrin π-cation radicals through the bridging ethylene group. Such extensive π-conjugation also results in strong antiferromagnetic coupling between the radical spins of both of the macrocycles, which generates a diamagnetic compound. The experimental observations are also strongly supported by density functional theory (DFT) calculations.

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INTRODUCTION Electronic interactions between naturally occurring porphyrinoids play an important role in various systems, e.g., photosynthetic reaction center (PRC) and natural-lightharvesting systems, diheme/multiheme proteins, organic conductors, etc.1−4 For example, the photoinduced formation of the 1e-oxidized species in the special pair is the first step in the conversion of light energy to chemical energy.1−3 A large number of diheme enzymes such as MauG4a,b and bacterial diheme cytochrome c peroxidases (bCcP)4c,d are known to catalyze various biological reactions where inter-ring interaction plays a pivotal role. Electronic communication between the two physically separated heme centers is believed to be facilitated by a tryptophan residue, which is positioned midway between the heme centers.4 Covalently linked porphyrin dimers have been investigated as models of multiheme proteins, photosynthetic reaction centers, and so on over the years.5,6 Prudent choice of the spacer will dictate specific control over the spatial arrangement for intermacrocycle interactions and possible electronic communications.6−16 The conformational variations of a bisporphyrin host offer an effective tool for modulating a wide range of chemical and photophysical properties. Consequently, bisporphyrins have been utilized in designing molecular devices for energy and electron transfer, multiredox catalysis, sensors and cooperative molecular recognition.3,5 Oxidation of a metalloporphyrin can occur either at the coordinated metal center or at the aromatic porphyrin © XXXX American Chemical Society

macrocycle. However, when a nonredox active metal ion is used, oxidation produces porphyrin π-cation radicals only. In the present work, we have investigated the effects of possible interactions between two porphyrin rings upon stepwise oxidations in a series of metalloporphyrin dimers (M: 2H, Zn2+). Two porphyrin macrocycles are bridged covalently through either highly flexible ethane or relatively rigid and unsaturated ethylene bridge, which provide ample flexibility to orient the macrocycles for the efficient electronic coupling and thereby allowing significant modulation of their ground and excited-state properties. The inter-ring coupling has been demonstrated to be closely related to the degree of porphyrin ring overlap. Moreover, the bridging group has also been shown to play a key role in modulating inter-ring communication here. Experimental investigations have clearly demonstrated further closeness of two porphyrin macrocycle after 1e-oxidation due to the strong polarization effect. However, 2e-oxidation has forced two macrocycles to move far apart, because of repulsion between two + ve charges. The ethylene bridged porphyrin dimers, which can be stabilized both in cis and trans forms, converts to trans only after the 2e-oxidation. Sundströ m and co-workers have reported two different conformations of the trans isomer, i.e., “P” and “U” forms.16 In one conformation, both porphyrins are completely orthogonal to the bridging alkene group (normal PReceived: September 10, 2015

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DOI: 10.1021/acs.inorgchem.5b02065 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

because of the formation of trans-1·(SbF6)2. The 2e-oxidized complex has also been isolated as a solid in good yield and spectroscopically characterized. Similarly, 2e-oxidation of free base ethane bridged complex, syn-3, has also been performed, which produces anti-32+ (Scheme 1) in solution. However, no such bands at ∼500 nm and in the 600−900 nm region have been observed during the oxidation process. Thus, the intense bands at 508 nm and 750 nm, observed in the case of trans-1· (SbF6)2, are associated with extensive conjugation through the ethylene bridge between two porphyrin π-cation radicals (vide inf ra). Similarly, stepwise 1e- and 2e-oxidations of cis-2 in dichloromethane by AgSbF6 result in the formation of cis-2· SbF6 and trans-2·(SbF6)2, respectively, and later was isolated as a solid in good yield and structurally characterized. The spectral changes during such oxidation processes are depicted in Figure 2. The conversion of cis-2 to cis-2·SbF6 in dichloromethane results in slight decrease in the Soret band intensity at 392 nm, along with the generation of a broad low energy band at 950 nm, which is attributed to the intravalence charge transfer between two porphyrin macrocycles, as a characteristic of the mixed-valence6c,d,18 π-cation radical dimer cis-2·SbF6. However, upon 2e-oxidation, the Soret band decreases in intensity with the generation of new bands at 502, 655, and 1190 nm due to the formation of trans-2·(SbF6)2. In sharp contrast, 2e-oxidation of the ethane-bridged analogue does not show any such bands (see Figure 2B, as well as Figure S1 in the Supporting Information). Therefore, the bands at 502, 655, and 1190 nm are related to the extensive conjugation between two porphyrin π-cation radicals (vide inf ra) in trans-2·(SbF6)2. Moreover, the broad NIR band that is observed at 1190 nm is attributed to the charge resonance (CR)19 stabilization of the spins and charges in the binuclear dication diradical complex.6c,d Intensity of the Soret band has also been drastically reduced in both trans-1· (SbF6)2 and trans-2·(SbF6)2, which is suggestive of reduced aromaticity of the porphyrin rings in the 2e-oxidized complex, because of such extensive conjugation (vide inf ra). Scheme 1 shows the synthetic outline and list of all the complexes reported here, along with their abbreviations used. Synthetic details and characterization of the complexes are given in the Experimental Section. However, it is interesting to note that 2e-oxidations of both cis-2 and trans-2 produce the identical product (Scheme 2). The spectral changes are depicted in Figure S1. Similar results are also obtained by using other oxidizing agents with weakly coordinating counteranions, such as AgBF4, AgClO4, AgPF6, and Fe(ClO4)3, as well as thianthrenium perchlorate. Syn-anti conformational switching is observed during the stepwise oxidation of the highly flexible ethane bridged bisporphyrin analogues, syn-3 and syn-4, and their UV-vis-NIR spectra, along with the corresponding oxidized products, are shown in Figure S2 in the Supporting Information. The 2e-oxidized species, trans-2·(SbF6)2, upon reduction by NaBH4 shows the presence of a shoulder (at 402 nm), along with the Soret band at 419 nm (Figure S3 in the Supporting Information), which are the characteristics of trans conformation.17b Fourier transform infrared (FT-IR) spectroscopy has been a very useful tool for the identification of the porphyrin π-cation radicals. The most-characteristic marker bands of octaethylporphyrin-based π-cation radicals are observed for Cα−Cmeso and Cβ−Cβ bond stretchings (υ̅ ≈ 1500−1600 cm−1).20 The FT-IR spectrum of trans-2·(SbF6)2 showed characteristic Cα−Cmeso and Cβ−Cβ stretching frequencies at 1549 and 1601 cm−1,

type), whereas in the other, the macrocycles are almost on the same plane with the bridging ethylene moiety (U-type). 2eoxidation of cis isomer stabilizes the unusual “U” form of the trans isomer, because of strong hydrogen-bonding interactions between the counteranions and the axially coordinated water molecules, which eventually facilitates strong electronic conjugation between two macrocycles.

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RESULTS AND DISCUSSION The free base cis-1,2-bis(meso-octaethylporphyrinyl)ethene, cis1 and cis-1,2-bis(meso-octaethylporphyrinyl)ethene Zn(II), cis-2 have been synthesized according to the procedures reported in the literature.16,17a Oxidation of the compounds has been performed in a stepwise manner, using chemical oxidants with weakly coordinating counteranions under nitrogen, and have been monitored using ultraviolet−visible−near-infrared (UVvis-NIR) spectroscopy. The addition of 1 equiv of AgSbF6 as an oxidant to a dichloromethane solution of cis-1 results in an immediate color change from dark red to brown, because of the formation of 1eoxidized product cis-1·SbF6, which has been isolated as a solid in good yield and structurally characterized. During such an oxidation process, a 10 nm red shift of the Soret band (from 391 nm to 401 nm) was observed (Figure 1), along with a slight increase in the band intensity. However, addition of two molar equivalents of oxidant to cis-1 leads to a steep decrease of the Soret band intensity with the generations of a new sharp and highly intense band at 508 nm and a broad band at 750 nm,

Figure 1. UV-vis-NIR spectral changes (at 295 K) of cis-1 in dichloromethane (1.8 × 10−5 M) upon gradual addition of AgSbF6 in acetonitrile up to (A) one and (B) two molar equivalents; arrows indicate the increasing or decreasing trend in intensity. (C) UV-visNIR spectra (at 295 K in CH2Cl2) of trans-1·(SbF6)2 (blue line) and anti-3·(SbF6)2 (red line). B


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Inorganic Chemistry Scheme 1. Stepwise Oxidations of Ethene and Ethane Bridged Porphyrin Dimers

respectively (see Figure 3), while, for trans-1·(SbF6)2, the Cα− Cmeso and Cβ−Cβ stretching frequencies are centered at 1519 and 1568 cm−1, respectively (see Figure S4 in the Supporting Information). Thus, FT-IR data suggests the formation of πcation radicals in the 2e-oxidized complex. Crystallographic Characterizations. cis-1 crystallizes in the triclinic crystal system with the P1̅ space group, while cis-1· SbF6 crystallizes in the monoclinic crystal system with the P21/ n space group. Figure 4 displays the perspective views of both cis-1 and cis-1·SbF6 in which the cofacial arrangements of the porphyrin macrocycles can be seen. The average Ct−Np distance of 2.059 Å, obtained for cis-1, increased to 2.072 Å in the 1e-oxidized complex cis-1·SbF6. The mean plane separation between two porphyrinato cores is found to be 3.72 Å for cis-1, while a separation of 3.23 Å has been obtained for the 1e-oxidized complex cis-1·SbF6, which is ∼0.50 Å less in the oxidized complex. Moreover, the dihedral angle between two least-squares planes has also been reduced from 11.4° in cis-1 to 3.9° in cis-1·SbF6. Thus, after 1e-oxidation, two porphyrin macrocycles come even closer and are more cofacial to each other, which favors stronger π−π interactions in cis-1· SbF6. The stabilization of such mixed-valence state seems to be consequences of the very short inter-ring distance that promotes a stronger interaction. Interestingly, all the ethyl substituent are oriented “outward” in the 1e-oxidized complex to reduce the steric hindrance out of peripheral substitution so that two rings can come as close as possible to each other, which further facilitates stronger interactions. Figures S5 and S6 in the Supporting Information display molecular packing diagrams in the crystal lattice of cis-1 and cis-1·SbF6, respectively. Table S1 in the Supporting Information shows

the crystal data and data collection parameters of the molecules reported here, while the selected bond distance and angles are given in Table S2 in the Supporting Information. The molecules of cis-2 and trans-2·phenylalaninol (Chart 1) are crystallized in the triclinic crystal systems with P1̅ space groups, while trans-2·(SbF6)2 is crystallized in the monoclinic crystal system with the C2/c space group. Figure 5 displays the perspective views of cis-2, trans-2·phenylalaninol, and trans-2· (SbF6)2; Figures S7−S9 in the Supporting Information demonstrate the respective molecular packing diagrams. In trans-2·(SbF6)2, two porphyrin macrocycles are oriented in an unusual bowl-shaped conformation, namely, the “U” form, which is facilitated via four strong hydrogen-bonding interactions [O1···F1, 2.863(7) Å, and O1···F5, 2.703(7) Å] between axially ligated water and SbF6− counteranions. The average Zn−Np distance in cis-2 is determined to be 2.036(4) Å, similar to what has been observed in the four coordinate Zn porphyrins.7b,21 However, the average Zn−Np distances for both trans-2·phenylalaninol and trans-2·(SbF6)2 are found to be 2.076(4) Å and 2.069(4), respectively; which fall within the spread range of five-coordinate Zn porphyrin dimers.22 In trans2·phenylalaninol and trans-2·(SbF6)2, the metal centers are displaced toward the axial ligands by 0.46 Å and 0.51 Å, respectively, from the mean porphyrin planes of the C20N4 porphyrinato cores. Substantial π−π stacking interaction between the two porphyrin macrocycles of two neighboring molecules of trans-2·(SbF6)2 results in π-molecular aggregation with an average interplanar separation of 5.11 Å (see Figure S10 in the Supporting Information). The absorbance maxima at 1190 nm was found to have a nonlinear dependence on the concentration of trans-2·(SbF6)2 (Figure S11 in the Supporting C


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Inorganic Chemistry

Figure 3. IR spectra (at 295 K) of polycrystalline samples of cis-2 (blue line) and trans-2·(SbF6)2 (pink line).

Figure 2. (A) UV-vis-NIR spectral changes (at 295 K) of cis-2 in dichloromethane (0.8 × 10−6 M) upon gradual addition of AgSbF6 in acetonitrile from zero to two molar equivalents; arrows indicate the increasing or decreasing trend in intensity. (B) UV-vis-NIR spectra (at 295 K in CH2Cl2) of trans-2·(SbF6)2 (blue line) and anti-4·(SbF6)2 (red line).

Information), which also supports the formation of π-molecular aggregation in solution, as observed in the solid. The salient structural features of the complexes are summarized in Table 1. As can be seen, the almost-planar porphyrin macrocycle has undergone significant ring deformation in trans-2·(SbF6)2. Figure S12 in the Supporting Information defines some of the structural and geometrical

Figure 4. Perspective views (at 100 K) of (A) cis-1 and (B) cis-1·SbF6, showing 50% thermal contours for all non-hydrogen atoms (H atoms and CHCl3 have been omitted for the sake of clarity).

parameters that would be compared here. The average interplanar angle (α) between the least-squares plane of

Scheme 2. Synthesis of 2·SbF6

D


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Inorganic Chemistry Chart 1. Structure of trans-2·phenylalaninol

have also been observed for the corresponding diiron(III) and dicobalt(II) analogues of trans-2 upon 2e- and 4e-oxidations, respectively.6c,d The electrostatic repulsion between two porphyrin π-cation radicals in a cis isomer has forced the macrocycles to move as far as possible to a trans isomer through a facile C−C bond rotation along the bridge. Electrochemistry. Electrochemical studies have been performed at 295 K under nitrogen in CH2Cl2, using 0.1 M tetrabutylammonium hexafluorophosphate (TBAHFP) as a supporting electrolyte. Cyclic voltammograms of cis-1, trans-1, syn-3, and H2OEP, obtained under identical conditions, are compared in Figure S13 in the Supporting Information. Electrochemical data of H2OEP shows only two reversible 1e-oxidative responses, at 0.83 and 1.36 V (trace A in Figure S13). However, when the two OEP units are connected through an ethane bridge in syn-3, four consecutive ringcentered oxidations (at 0.63, 0.91, 1.25, and 1.39 V) are observed (trace B in Figure S13), which demonstrates strong electronic communication between two redox-active porphyrin macrocycles. At 0.63 V, one porphyrin unit is oxidized to generate syn-3• + at a potential much lower than that of monomeric H2OEP, because of the increased electron density in the bisporphyrin architecture. However, because of strong intermacrocyclic interactions with the first 1e-oxidized ring held in close proximity, the second ring is then oxidized at a significantly higher potential (0.91 V) to produce anti-32+. Similar situations arise for the third and fourth electron oxidized species, which also occur at 1.25 and 1.39 V, respectively. The overall electrochemical responses thus demonstrate strong interelectronic communication between two redox-active porphyrin macrocycles in syn-3. However, only two 2e-oxidative responses (trace C in Figure S13) are observed for trans-1, at 0.61 and 1.41 V, indicating no significant interaction between two well-separated porphyrin units in a trans isomer. In contrast, cis-1 reveals two 1eoxidations at 0.51 and 0.63 V, followed by one 2e-oxidative response at 1.43 V (trace D in Figure S13), which indicates significant interaction between two porphyrin macrocycles ,which are closely spaced in cis conformation. Figure 6 compares the cyclic voltammograms of cis-2, trans-2, syn-4, and ZnOEP under identical conditions. Electrochemical data of ZnOEP (trace A in Figure 6) reveals two reversible 1eoxidations at 0.65 and 1.05 V. While, trace B in Figure 6 shows two reversible 1e-oxidations (0.45 and 0.65 V) and one 2eoxidation (0.95 V) for syn-4. A difference of 200 mV between the first and second oxidations in syn-4 demonstrates the strong intermacrocyclic interaction between two porphyrin rings. However, when two ZnOEP units are covalently connected through an ethene bridge, only two oxidative responses are observed at 0.29 and 1.07 V for trans-2 (trace C in Figure 6) and at 0.36 and 1.05 V for cis-2 (trace D in Figure 6), suggesting the almost noninteractive nature of two porphyrin units in the complexes. However, it is interesting to note here the large reduction in the first oxidative response particularly in the case of trans-2 and cis-2, because of the increased global electron density in the bisporphyrin scaffold. However, there is almost no change in the second oxidative response of the porphyrin units, compared to its monomeric complex. Figure 7 shows the UV-vis-NIR spectral change observed during the electrochemical oxidation of cis-2 in CH2Cl2 at a constant applied potential of 0.56 V which, however, displays 2eoxidation. As can be seen, the spectral changes are very similar

Figure 5. Perspective views (at 100 K) of (A) cis-2, (B) trans-2· (SbF6)2 and (C) trans-2·phenylalaninol, showing 50% thermal contours for all non-hydrogen atoms (H atoms and CHCl3 have been omitted for the sake of clarity).

C20N4 porphyrinato core and the C4 plane of bridging ethylene group in cis-2 is 79.9°, while that of trans-2·phenylalaninol is 86.8°. However, the angle α has been reduced to 51.4° in trans2·(SbF6)2, which eventually facilitates π-electronic conjugation between two porphyrin cation radicals through the bridging ethylene group. This is also reflected in the alteration of C20− C37 and C37−C37A bond distances. In cis-2, the C20−C37 distance is found to be 1.509(7) Å, while a distance of 1.324(6) Å has been observed for C37−C37A (Table 1). Furthermore, in trans-2·phenylalaninol, the C20−C37 and C37−C37A bond distances are found to be 1.499(6) and 1.329(6) Å, respectively, which have been interchanged to 1.368(6) and 1.441(4) Å in trans-2·(SbF6)2. Such an alteration in the bond distance demonstrates the effect of strong π-conjugation through the ethylene bridge between two porphyrin cation radicals in trans2·(SbF6)2, which eventually results in exomethylene connectivity between two porphyrin macrocycles. Similar behaviors E


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Inorganic Chemistry Table 1. Selected Structural and Geometrical Parameters of the Complexes complex cis-1 core-I core-II cis-1·SbF6 core-I core-II cis-2 core-I core-II trans-2·(SbF6)2 trans-2·phenylalaninol syn-4

b ΔZn 24 (Å)

Δ24c (Å)

Ct···Ctd (Å)

Cm···Cme (Å)

MPSf (Å)

θg (deg)

αh (deg)

C20−C37a (Å)

C37−C37Aa (Å)

ref

2.058 2.061

NAi NAi

0.07 0.09

4.12

2.85

3.72

11.4

81.7 72.6

1.498(3)

1.331(3)

this work

2.071 2.072

NAi NAi

0.07 0.07

3.75

2.77

3.23

3.9

69.9 69.9

1.491(8)

1.359(9)

this work

2.036(4) 2.036(4) 2.069(4) 2.076(4) 2.037(3)

0.06 0.02 0.51 0.45 0.03

0.06 0.09 0.23 0.08 0.08

3.69

2.82

3.38

3.4

1.509(7)

1.324(6)

this work

8.88 10.55 5.89

3.74 3.87 3.07

6.94 2.48 3.44

77.1 0 3.0

80.5 79.4 51.4 86.8 NAi

1.368(6) 1.499(6) NAi

1.440(8) 1.328(9) NAi

this work this work 21c

Ct/Zn−Npa (Å)

a

Average value. bAverage displacement of Zn from the least-squares plane of the C20N4 porphyrinato core. cAverage displacement of atoms from the least-squares plane of the C20N4 porphyrinato core. dNonbonding distance. eNonbonding distance between two meso carbons that are covalently connected. fAverage distance of two least-squares planes of the C20N4 porphyrinato core. gAngle between two least-squares planes of the C20N4 porphyrinato core in degree. hInterplanar angle between the plane of the bridging ethylene group (C4) and the least-squares plane of the porphyrin (24 atoms); see Figure S12 in the Supporting Information for details. iNA = not applicable.

to the changes observed during the chemical oxidation process using AgSbF6 (vide supra). 1 H NMR. 1H NMR spectral studies were carried out at 295 K in CDCl3. Figure 8 compares well-resolved 1H NMR spectra of polycrystalline samples of cis-1, cis-1·SbF6, trans-1·(SbF6)2, and trans-1. The single set of meso proton resonances in cis-1·SbF6 indicates the equivalent nature of the two porphyrin macrocycles. However, the average signals in cis-1·SbF6 indicates

Figure 6. Cyclic voltammograms (at 295 K in CH2Cl2) of (A) ZnOEP, (B) syn-4, (C) trans-2, and (D) cis-2 (scan rate = 100 mV/s) with 0.1 (M) tetra(n-butyl)ammoniumhexafluorophosphate as a supporting electrolyte. The reference electrode was Ag/AgCl.

Figure 7. UV-vis-NIR spectral change (at 295 K in CH2Cl2), under nitrogen, of cis-2 at a constant potential of 0.56 V, using 0.1 M TBAHFP as a supporting electrolyte; arrows indicate the increasing or decreasing trend in intensity. Figure 8. 1H NMR spectra (in CDCl3 at 295 K) of (A) cis-1, (B) cis-1· SbF6, (C) trans-1·(SbF6)2, and (D) trans-1. F


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Inorganic Chemistry

The substantial decrease in the separation between the two meso resonances in trans-2·(SbF6)2 indicates stabilization of the trans isomer. Furthermore, the methylene proton resonances are observed in the range of 2−3 ppm, along with the methyl resonances at ∼1 ppm for the 2e-oxidized product. Such upfield shifts of the proton signals clearly explains the ring-centered oxidation in trans-2·(SbF6)2, while sharp proton signals indicate the diamagnetic nature of the oxidized complex arising from the strong antiferromagnetic coupling between two radical spins. Electron paramagnetic resonance (EPR) spectroscopy is one of the most-sensitive techniques for detecting paramagnetism. The X-band EPR spectra are recorded by dissolving pure crystals in deaerated dichloromethane at 77 K. Figure S14 in the Supporting Information displays a single line broad signal at g = 2.0041 for cis-1·SbF6, indicating the presence of a radical. Similar EPR spectra are also observed for other 1e-oxidized complexes reported here. The appearance of such a broad signal has been reported earlier for various 1e-oxidized mixedvalent porphyrin π-cation radicals.5i Molecular Modeling. A series of density funcitonal theory (DFT) studies have been carried out to gain more insight into the electronic structure and properties. The optimized geometries of cis-1 and its 1e-oxidized product are shown in Figure S15 in the Supporting Information. As can be seen, two porphyrin macrocycles are coming even closer after 1eoxidation, as compared to the unoxidized one, which has been also observed in the experimental investigations (vide supra). Similar theoretical analysis of the zinc analogue results in the identical trend in the intermacrocyclic distance (Figure S16 in the Supporting Information). The energies for two possible spin states of trans-2·(SbF6)2, viz. singlet and triplet, have been calculated using two different functionals (Figure S17 in the Supporting Information). The singlet state is determined to be stabilized by 21.81 and 13.19 kcal/mol over the triplet state with cam-B3LYP and B97D functionals, respectively. Figure 10 displays the optimized structure, along with key geometrical parameters of trans-2· (SbF6)2 with singlet and triplet spin states. Interestingly, using

rapid intramolecular and intermolecular electron transfer on the NMR time scale and excludes the possibility of any porphyrin structural modification during the oxidation. Moreover, the 0.41 ppm increased separation between two meso proton resonances in cis-1·SbF6, compared to cis-1, also indicates that the two porphyrin macrocycles are still cofacial and come even closer to each other after 1e-oxidation, as observed in the X-ray structure of the complex. The −NH signals for cis-1 are observed at −4.82 and −5.66 ppm, while such signals for cis-1· SbF6 are too broad to be detected, because of the formation of porphyrin π-cation radicals. Figure 8C displays the 1H NMR spectra obtained from the trans-1·(SbF6)2. Here, one set of proton resonances are observed, which indicates that two porphyrin macrocycles are equivalent in the complex. As can be seen, two meso signals, which were earlier found to be wellseparated in cis-1 and cis-1·SbF6, are now almost on top of each other in the 2e-oxidized product, which suggest the stabilization of trans-conformer (Figure 8D). The sharp proton signals for trans-1·(SbF6)2 indicates the diamagnetic nature of the complex, because of strong antiferromagnetic coupling between the two porphyrin π-cation radicals that are conjugated through the bridging ethylene group. Figure 9 shows the well-resolved 1H NMR spectra of polycrystalline samples of cis-2, trans-2·(SbF6)2, and trans-2,

Figure 9. 1H NMR spectra (in CDCl3 at 295 K) of (A) cis-2, (B) trans-2·(SbF6)2, and (C) trans-2 in CDCl3.

taken at 295 K in CDCl3. The 1H NMR of cis-2 (trace A) displays two meso proton resonances at 8.05 and 9.30 ppm, with a 2:1 intensity ratio, indicating the face-to-face arrangement of the two porphyrin macrocycles. The eight methylene proton resonances are observed between 3.00 ppm and 4.55 ppm, followed by four methyl resonances within 1−2 ppm region for cis-2. Additions of one molar equivalent of AgSbF6 in acetonitrile to cis-2 result in a very broad 1H NMR spectrum for cis-2·SbF6. However, for the 2e-oxidized complex trans-2· (SbF6)2, the meso proton resonances are observed at 6.38, 6.41, and 6.54 ppm, which are largely shifted upfield (Figure 9B).

Figure 10. CAM-B3LYP optimized structure of trans-1,32·(SbF6)2 (with bond lengths given in Angstroms), as calculated using the LanL2DZ basis set for the Zn atom and 6-31G+(d,p) for all other atoms. Free energies (expressed in units of kcal mol−1) are relative to the singlet spin state and include solvent, entropical, and thermal corrections to the energy. G


Inorganic Chemistry

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the S = 1 ground state, the calculation also reproduces the C20−C37 and C37−C37A bond distances of the bridging ethylene group in excellent agreement with the experiment. Moreover, LUMO and HOMO−2, which is energetically very close to the HOMO, have been found to have significant wave coefficients (Figure S18 in the Supporting Information) on the bridging ethylene group, suggesting a substantial conjugation through the bridge14 that leads to a strong antiferromagnetic coupling between two radical spins in trans-2·(SbF6)2. In trans-2·(SbF6)2, two counteranions are involved in the hydrogen-bonding interactions with the axially ligated water, which might play a crucial role in the stabilization of the “U”form. To gain more insights, geometry optimizations for both “P”- and “U”-forms are done with and without counteranions fixing the spin state as S = 1 (see Figures S19−S21 in the Supporting Information). The relative energy difference between the “P”- and “U”-forms (Figure S22 in the Supporting Information) is as low as 0.68 kcal/mol (excluding zero-point energy) without a counteranion. Whereas, with the counteranion, the “U”-form is determined to be 22.42 kcal/mol more stable than the corresponding “P”-form, using the same mode of calculation. With the inclusion of zero-point energy (ZPE), the “P”-form has been found to be stabilized by 1.05 kcal/mol, compared to the “U”-form, in the absence of counteranion, while the “U”-form is more stable than the P-form, by 34.48 kcal/mol, in the presence of counteranion. These observations firmly suggest the key role played by the hydrogen bonding in stabilizing such an unusual geometry (“U”-form) for trans-2· (SbF6)2. DFT study of trans-1·(SbF6)2 have also shown similar results (see Figures S23−S27 in the Supporting Information).

Article

EXPERIMENTAL SECTION

Materials. cis-1, cis-2, trans-2, syn-3, and syn-4 have been synthesized following procedures reported in the literature.16,17a,23 Reagents and solvents were purchased from commercial sources and purified by standard procedures before use. Preparation of cis-1·SbF6. cis-1 (50 mg, 0.046 mmol) was dissolved in dry CH2Cl2 (10 mL) and AgSbF6 (17 mg, 0.049 mmol) dissolved in CH3CN was added to it. The mixture was then stirred for 30 min under nitrogen. The reaction mixture was filtered to remove any solid particle and then dried completely. The solid obtained was redissolved in a minimum amount of dry CH2Cl2 and carefully layered with nhexane. Upon standing for 7−8 days, the dark crystalline solid appeared to produce cis-1·SbF6. (Yield: 40 mg, 66%.) UV-vis (CH2Cl2) [λmax, nm (ε, M−1 cm−1)]: 401 (0.85 × 105), 508 (0.05 × 105), 780 (0.05 × 105). 1H NMR (CDCl3, 295 K): −CH (b): 9.41 (2H); meso-H: 10.09 (2H), 8.25 (4H); ethyl −CH2: 4.48 (4H), 4.09 (6H), 3.87 (6H), 3.62 (8H), 3.38 (4H), 3.27 (4H); −CH3: 1.81 (24H), 1.53 (12H), 1.12 (12H). Preparation of trans-1·(SbF6)2. cis-1 (50 mg, 0.046 mmol) was dissolved in dry CH2Cl2 (10 mL). AgSbF6 (34 mg, 0.10 mmol) dissolved in CH3CN was added to it and was stirred for 45 min under nitrogen. The reaction mixture was filtered to remove any solid particles and then dried completely. The solid obtained was redissolved in a minimum amount of dry CH2Cl2 and carefully layered with n-hexane. Upon standing for 7−8 days, the dark amorphous solid appeared to produce trans-1·(SbF6)2. (Yield: 45 mg, 63%.) UV-vis (CH2Cl2) [λmax, nm (ε, M−1 cm−1)]: 401 (0.57 × 105), 508 (0.81 × 105), 780 (0.19 × 105). 1H NMR (CDCl3, 295 K): meso-H: 10.18 (4H), 10.15 (2H); −CH (b): 9.87 (2H); ethyl −CH2: 3.88 (16H), 3.72 (8H), 3.47 (4H), 3.17 (4H); −CH3: 1.69 (24H), 1.45 (12H), 1.10 (12H). Preparation of trans-2·(SbF6)2. cis-2 (50 mg, 0.041 mmol) was dissolved in dry CH2Cl2 (10 mL), and AgSbF6 (30 mg, 0.089 mmol) that was dissolved in CH3CN was added to it. The resulting solution then was stirred for 15 min under nitrogen, during which time a solid precipitate appeared. The reaction mixture was filtered to remove any solid particles and then dried completely. The solid obtained was redissolved in a minimum amount of dry CH2Cl2 and carefully layered with n-hexane. Upon standing for 7−8 days, the dark crystalline solid appeared to produce trans-2·(SbF6)2. (Yield 55 mg, 79%.) UV-vis (CH2Cl2) [λmax, nm (ε, M−1 cm−1)]: 324 (0.26 × 105), 385 (0.31 × 105), 502 (0.45 × 105), 655 (0.05 × 105). 1H NMR (CDCl3, 295 K): −CH (b): 8.47 (2H); meso-H: 6.53 (2H), 6.41 (2H), 6.38 (2H); ethyl −CH2: 2.70 (8H), 2.55 (8H), 2.41 (16H), 3.17 (4H); −CH3: 1.25 (12H), 1.12 (24H), 0.95 (12H). Instrumentation. Cyclic voltammetric studies were performed on a BAS Epsilon electrochemical workstation in dichloromethane with 0.1 M tetrabutylammonium hexafluorophoshate (TBAH) as a supporting electrolyte; the reference electrode was Ag/AgCl, and a platinum wire was used as the auxiliary electrode. The concentration of the compounds was on the order of 10−3 M. The ferrocene/ ferrocenium couple occurs at E1/2 = +0.45 (65) V versus Ag/AgCl, under the same experimental conditions. UV-vis spectra were recorded on a PerkinElmer UV-vis spectrometer. Elemental (C, H, and N) analyses were performed on a PerkinElmer Model 2400II elemental analyzer. 1H NMR spectra were recorded on a 500 MHz instrument (JEOL). The residual 1H resonances of the solvents were used as a secondary reference. Computational Details. All geometry optimizations were initiated from the crystal structure coordinates of trans-2·(SbF6)2, whereas, in order to generate the “P” form, the rings were rotated to a desired orientation and then optimized. All structure optimizations were carried out by employing a CAM-B3LYP hybrid functional, using the Gaussian 09 package.24 The method used was Becke’s three-parameter hybrid exchange functional provided by the Lee, Yang, and Parr expression for the nonlocal correlation,25 and the Vosko, Wilk, and Nussair 1980 correlation functional (III) for local correction.26 The basis set was LANL2DZ for the Co and Sb atoms and 6-31+G(d,p) for the C, N, O, F, and H atoms.

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CONCLUSION Here, we have demonstrated the effects of possible interactions between metalloporphyrin and metalloporphyrin π-cation radical connected through an ethylene bridge and compared with its ethane bridged analogues. The stepwise oxidations of free base cis-1,2-bis(meso-octaethylporphyrinyl)ethene and its Zn(II) analog were performed to investigate the effect of interring interaction on the geometry of the porphyrin dimers. While 1e-oxidation causes both the porphyrin units to come closer in a more cofacial manner, 2e-oxidation forces them to move far apart, which eventually results in the facile conversion of cis isomer to trans isomer. Moreover, 2e-oxidized species of the ethylene bridged porphyrin dimer have been found to be stabilized in an unusual bowl-shaped conformation (“U”-form) due to hydrogen-bonding interactions between axially ligated water and SbF6− counteranions. Through the bridging ethylene group, both the porphyrin macrocycles in trans-2·(SbF6)2 exhibit a substantial conjugation in its unusual “U”-form, leading to a strong antiferromagnetic coupling between two radical spins. This has resulted in the formation of a diamagnetic compound, although two cores are well separated and not cofacial enough to have any through-space interactions. Such an extensive π-conjugation in trans-2·(SbF6)2 is also reflected in the generation of new absorption bands at 502, 655, and 1190 nm, and in the complete reversal of the bridging C− C and CC distances. The 2e-oxidized conformers have unique spectral and geometrical features and thus can be considered as real supramolecular moieties, rather than two interacting discrete porphyrin π-cation radicals. H


Article

Inorganic Chemistry X-ray Structure Solution and Refinement. Crystals are coated with light hydrocarbon oil and mounted in the 100 K dinitrogen stream of Bruker SMART APEX CCD diffractometer equipped with a low-temperature apparatus (CRYO Industries), and intensity data are collected using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The data integration and reduction were processed with SAINT software.27 An absorption correction was applied.28 Structures were solved by the direct method, using SHELXS-97, and were refined on F2 via a full-matrix least-squares technique, using the SHELXL2014 program package.29 Non-hydrogen atoms were refined anisotropically. In the refinement, hydrogens were treated as riding atoms using SHELXL default parameters. In the refinement, hydrogens were treated as riding atoms using SHELXL default parameters. Crystal data and data collection parameters are given in Table S1.

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India and the Council of Scientific and Industrial Research, New Delhi, are gratefully acknowledged for financial support. S.D. and D.S. thank UGC, India, and Y.A.P. thanks CSIR, New Delhi, for their fellowship. We thank Dr. Susovan Bhowmik for the help at the initial stage.

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DEDICATION Dedicated to Professor Mallayan Palaniandavar on the occasion of his 65th birthday. (1) (a) Croce, R.; van Amerongen, H. Nat. Chem. Biol. 2014, 10, 492. (b) Pullerits, T.; Sundström, V. Acc. Chem. Res. 1996, 29, 381. (2) (a) Poulos, T. L. Chem. Rev. 2014, 114, 3919. (b) McLain, J. L.; Lee, J.; Groves, J. T. In Biomimetic Oxidations Catalyzed by Transition Metal Complexes; Meunier, B., Ed.; Imperial College Press: London, 2000; 91 pp. (c) Goldberg, D. P. Acc. Chem. Res. 2007, 40, 626. (d) Meunier, B.; de Visser, S. P.; Shaik, S. Chem. Rev. 2004, 104, 3947. (3) (a) Schultz, D. M.; Yoon, T. P. Science 2014, 343, 1239176. (b) LaVan, D. A.; Cha, J. N. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 5251. (c) Barber, J. Philos. Trans. R. Soc., A 2007, 365, 1007. (4) (a) Jensen, L. M. R.; Sanishvili, R.; Davidson, V. L.; Wilmot, C. M. Science 2010, 327, 1392. (b) Wang, Y.; Graichen, M. E.; Liu, A.; Pearson, A. R.; Wilmot, C. M.; Davidson, V. L. Biochemistry 2003, 42, 7318. (c) Pulcu, G. S.; Frato, K. E.; Gupta, R.; Hsu, H. − R.; Levine, G. A.; Hendrich, M. P.; Elliott, S. J. Biochemistry 2012, 51, 974. (d) Echalier, A.; Goodhew, C. F.; Pettigrew, G. W.; Fülöp, V. Structure 2006, 14, 107. (5) (a) Tanaka, T.; Osuka, A. Chem. Soc. Rev. 2015, 44, 943. (b) Terazono, Y.; Kodis, G.; Chachisvilis, M.; Cherry, B. R.; Fournier, M.; Moore, A.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 2015, 137, 245. (c) Yang, J.; Yoon, M.-C.; Yoo, H.; Kim, P.; Kim, D. Chem. Soc. Rev. 2012, 41, 4808. (d) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2009, 42, 1890. (e) Aratani, N.; Kim, D.; Osuka, A. Acc. Chem. Res. 2009, 42, 1922. (f) Duncan, T. V.; Susumu, K.; Sinks, L. E.; Therien, M. J. J. Am. Chem. Soc. 2006, 128, 9000. (g) Choi, M. S.; Yamazaki, T.; Yamazaki, I.; Aida, T. Angew. Chem., Int. Ed. 2004, 43, 150. (h) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 1537. (i) Le Mest, Y.; L’Her, M.; Hendricks, N. H.; Kim, K.; Collman, J. P. Inorg. Chem. 1992, 31, 835. (6) (a) Sil, D.; Rath, S. P. Dalton Trans. 2015, 44, 16195. (b) Sil, D.; Tuglak Khan, F. S.; Rath, S. P. Inorg. Chem. 2014, 53, 11925. (c) Sil, D.; Dey, S.; Kumar, A.; Bhowmik, S.; Rath, S. P. Chem. Sci. 2016, 7, 1212. (d) Dey, S.; Sil, D.; Rath, S. P. Angew. Chem., Int. Ed. 2016, 55, 996. (7) (a) Chaudhary, A.; Rath, S. P. Chem.Eur. J. 2012, 18, 7404. (b) Chaudhary, A.; Rath, S. P. Chem.Eur. J. 2011, 17, 11478. (c) Mondal, P.; Rath, S. P. Eur. J. Inorg. Chem. 2015, 2015, 4956. (d) Mondal, P.; Chaudhary, A.; Rath, S. P. Dalton Trans. 2013, 42, 12381. (e) Mondal, P.; Rath, S. P. Isr. J. Chem. 2016, 56, 144. (8) (a) Beletskaya, I.; Tyurin, V. S.; Tsivadze, A. Y.; Guilard, R.; Stern, C. Chem. Rev. 2009, 109, 1659. (b) Harvey, P. D.; Stern, C.; Gros, C. P.; Guilard, R. Coord. Chem. Rev. 2007, 251, 401. (9) (a) Rosenthal, J.; Nocera, D. G. Acc. Chem. Res. 2007, 40, 543. (b) Rosenthal, J.; Bachman, J.; Dempsey, J. L.; Esswein, A. J.; Gray, T. G.; Hodgkiss, J. M.; Manke, D. R.; Luckett, T. D.; Pistorio, B. J.; Veige, A. S.; Nocera, D. G. Coord. Chem. Rev. 2005, 249, 1316. (10) (a) Lindsey, J. S.; Bocian, D. F. Acc. Chem. Res. 2011, 44, 638. (b) Borovkov, V. V.; Hembury, G. A.; Inoue, Y. Acc. Chem. Res. 2004, 37, 449. (11) (a) Lott, G. A.; Perdomo-Ortiz, A.; Utterback, J. K.; Widom, J. R.; Aspuru-Guzik, A.; Marcus, A. H. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 16521. (b) Kuimova, M. K.; Botchway, S. W.; Parker, A. W.; Balaz, M.; Collins, H. A.; Anderson, H. L.; Suhling, K.; Ogilby, P. R. Nat. Chem. 2009, 1, 69. (12) (a) Tanaka, T.; Lee, B. S.; Aratani, N.; Yoon, M.-C.; Kim, D.; Osuka, A. Chem.Eur. J. 2011, 17, 14400. (b) Takai, A.; Chkounda, M.; Eggenspiller, A.; Gros, C. P.; Lachkar, M.; Barbe, J.-M.; Fukuzumi,

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02065. UV-vis spectra for trans-2 in dichloromethane (Figure S1); UV-vis spectra for syn-3, syn-3·SbF6, anti-3·(SbF6)2, syn-4, syn-4·SbF6 and anti-4·(SbF6)2 (Figure S2), as well as trans-2·(SbF6)2 and after addition of 2 mol equiv of NaBH4 (Figure S3); IR spectra of cis-1, trans-1·(SbF6)2 (Figure S4); packing diagrams of cis-1, cis-1·SbF6, cis-2, trans-2·phenylalaninol and trans-2·(SbF6)2 (Figures S5− S9); diagram of π-molecular aggregation of trans-2· (SbF6)2 molecules (Figure S10); absorption spectral changes of trans-2·(SbF6)2 at 1190 nm with an increasing concentration (Figure S11); diagram showing one plane containing a C20N4 porphyrinato core (green) and another plane consisting of C4 atoms of bridging ethylene group (Figure S12); comparison of cyclic voltammograms between H2OEP, syn-3, trans-1 and cis1 (Figure S13); EPR spectra of cis-1·SbF6 (Figure S14) and all optimized geometries (Figures S15, S16, S19− S21, S23−26); relative spin-state energies of “U”-forms of trans-1,32·(SbF6)2 (Figure S17); relative energies of the “U”- and “P”-form of trans-12·(SbF6)2 (Figure S22); HOMO, HOMO-2, and LUMO of trans-2·(SbF6)2 (Figure S18); and trans-11·(SbF6)2 (Figure S27), Cartesians of all optimized geometries; selected bond distances (Å) and bond angles (deg) for the complexes (Table S1); and crystal data and data collection parameters (Table S2) (PDF) Crystallographic data of trans 2·(SbF6)2 (CIF) Crystallographic data of cis 2 (CIF) Crystallographic data of trans 2·phenylalaninol(CIF) Crystallographic data of cis 1 (CIF) Crystallographic data of cis 1·SbF6 (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: (+91)-512-259-7251. Fax: (+91)-512-259-7436. E-mail: [email protected]. 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), I


Article

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Effect of Two Interacting Rings in Metalloporphyrin Dimers upon

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