Letter Cite This: Org. Lett. 2017, 19, 5924-5927
pubs.acs.org/OrgLett
Hybrid Macrocycles of Subporphyrins and Triphyrins Ankit Kumar,† M. Rajeswara Rao,‡ Way-Zen Lee,§ and Mangalampalli Ravikanth*,† †
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Department of Chemistry, Indian Institute of Technology Dharwad, Dharwad, Karnataka 580011, India § Instrumentation Center, Department of Chemistry, National Taiwan Normal University, Ting-Chow Road, Taipei 11677, Taiwan ‡
S Supporting Information *
ABSTRACT: The dibenzofuran/dibenzothiophene-based nonaromatic hybrid macrocycles, exhibiting the features of both contracted macrocycles, subporphyrins and triphyrins, have been synthesized under simple reaction conditions using readily available precursors. The monoanionic new macrocyclic ligands with three donor atoms, such as two pyrrole nitrogens and one dibenzofuran oxygen or dibenzothiophene sulfur, readily form Re(I) complexes.
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ubporphyrins1 and triphyrins2,3 belong to a new branch of the porphyrin4,5 family called contracted porphyrins6,7 and have attracted considerable attention in recent years because of their potential applications in a variety of high technological fields as functional materials.8,9 Subporphyrins 1 can be represented as triphyrin(1.1.1)-containing three pyrrolic units connected via three meso (methine) carbons, whereas triphyrrin 2 contains three pyrroles or related rings connected via four or more meso carbons and can be represented as triphyrin(2.1.1) for four meso carbon analogues. Subporphyrins are 14π aromatic conjugated triphyrins(1.1.1) and can only be synthesized as boron complexes10−13 except subpyriporphyrin, which is nonaromatic and obtained in free base form.14 Osuka and co-workers significantly contributed to the development of subporphyrin chemistry.1,10−13,15−18 However, the major limitation of subporphyrins is that they exist only as boron complexes and attempts to isolate free bases by removing B(III) ion leads to decomposition. Thus, it remained as a challenge to synthesize the free base subporphyrins, which can be used as an unique divalent/tridentate ligand with a smaller cavity than porphyrins for co-ordination chemistry (Figure 1). On the other hand,
characteristic features of both contracted macrocycles, subporphyrins, and triphyrins (Figure 1). These hybrid macrocycles contain two pyrroles and one related dibenzo-fused heterocycle connected via three meso carbons. The hybrid macrocycles possess three inner donor atoms, like subporphyrins and triphyrins, but contain only three meso carbons like subporphyrins. However, the dibenzo-fused heterocycle introduces two additional carbons which are not meso carbons as a part of πdelocalization in these hybrid macrocycles, unlike 14π subporphyrins. As a matter of fact, the hybrid macrocycles reported here are closely related to recently reported phenanthriporphyrin25 and dibenzocorrole26 which contain phenanthrene and biphenyl units, respectively, in place of the dibenzofuran/dibenzothiophene unit of hybrid macrocycles. The hybrid macrocycles were readily synthesized as free bases unlike subporphyrins and demonstrated their potentiality to form metal complexes like triphyrins. Because of the additional two carbons, the cavity size of the hybrid macrocycles is large and able to bind 5d transition metal ions such as Re(I) as demonstrated here. Thus, the hybrid macrocycles described are new ligands for co-ordination chemistry. The precursors, dibenzofuran diol 8 and dibenzothiophene diol 9, were synthesized by treating dibenzofuran and dibenzothiophene, respectively, with 2.1 equiv of n-BuLi followed by 2.5 equiv of p-tolualdehyde in n-hexane/THF at 0 °C.27 After workup, the crude compounds were subjected to silica gel column chromatographic purification and afforded the respective diols 8/9 as white solids in 63−66% yields. The identities of diols 8/9 were confirmed by HR-MS and 1H and 13C NMR spectroscopy (Figures S1, S2, and S10−S13). The diols 8/ 9 were treated with excess pyrrole in 1,2-dichloroethane under acid-catalyzed conditions at reflux for 8 h, and the crude compounds were purified by silica gel column chromatography to afford the respective tripyrranes 10/11 in high yields.26 The
Figure 1. Structures of contracted porphyrins.
triphyrins containing four or more meso (methine) carbons that connect three pyrroles or related heterocyclic rings are stable macrocycles and can be synthesized in free base forms.2,3,19,20 The triphyrins are sufficiently flexible and exhibit rich coordination chemistry.21−24 Herein, we describe the synthesis of unique hybrid macrocycles 3−5, which exhibit the novel © 2017 American Chemical Society
Received: September 18, 2017 Published: October 20, 2017 5924
DOI: 10.1021/acs.orglett.7b02919 Org. Lett. 2017, 19, 5924−5927
Letter
Organic Letters
protons resonated at 7.01 ppm. The type a protons at 6.22 ppm were identified because of their cross peak correlation with type b protons at 6.80 ppm. Similarly, the type d protons at 7.35 ppm and type e protons at 8.04 ppm were identified and assigned on the basis of their cross peak correlations in the COSY spectrum. The NMR resonances of macrocycles 3 and 4 were also easily identified and assigned using 1D and 2D NMR spectroscopy (Figures S18−S22). Thus, NMR spectroscopy was very useful in deducing the molecular structures of hybrid macrocycles 3−5. Single crystals of compound 4 and 5 suitable for crystallographic analysis were grown via slow evaporation of n-hexane into CH2Cl2 solution of 4 and 5 over a period of 5 days at room temperature. The crystallographic relevant data of 4 and 5 are given in Tables S2, S3, and S6 (SI). The crystal structures of 4 and 5 (Figure 3) reveal that both macrocycles are contorted,
novel hybrid macrocycles 3−5 were synthesized by condensing 1 equiv of the appropriate tripyrrane 10 or 11 with 1 equiv of pentafluorobenzaldehyde/p-nitrobenzaldehyde in CH2Cl2 in the presence of a catalytic amount of BF3·(OEt2) at room temperature for 1 h under an inert atmosphere and oxidized with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in open air for additional 2 h (Scheme 1). TLC analysis showed Scheme 1. Synthesis of Macrocycles 3−5 and Their Re(I) Metal Complexes
one major dark green spot followed by a minor blue spot. The crude reaction mixtures were subjected to neutral alumina column chromatography and collected the green spot to afford the desired hybrid macrocycles 3−5 in 9−15% yields. No attempts were made to characterize the minor blue spot at this stage. The hybrid macrocycles were characterized by HR-MS, 1D/2D NMR spectroscopy, and X-ray crystallography. The respective molecular ion peak in HR-MS confirmed the identities of the hybrid macrocycles 3−5 (Figures S5−S7).The representative 1H NMR, 1H−1H COSY, and NOESY NMR spectra for hybrid macrocycle 5 are shown in Figure 2. The proton
Figure 3. Single-crystal X-ray structures of 4 and 5: top view (a and c); side view (b and d). The meso aryl groups were omitted for clarity in the side view.
although the distortion is more pronounced in thia-congener 5 due to the steric repulsions caused by the congested core in 5 over 4. However, the rigid dibenzofuran and dibenzothiophene units in 4 and 5, respectively, maintain the planarity but deviate from the mean plane of the macrocycles in order to minimize the steric repulsions with the dipyrrin core. In addition, the dipyrrin component was also slightly distorted in 4 and 5 with dihedral angles of 23.7° and 22.4° with respect to the meso-positions. All three meso-aryl substituents remain orthogonal to the plane of the macrocycle (43−66°) and render minimal contribution to the π-structure. It is important to note that the bonds connecting the two moieties (dibenzofuran/dibenzothiophene and dipyrrin) C10−C11 and C21−C23 were considerably longer (1.50 Å) than the π-conjugated structure, while all the other bond distances of the macrocycle (dibenzothiophene/dibenzofuran or dipyrrin) were in the range of 1.35−1.40 Å. This clearly supports the π-conjugation was restricted to the individual dibenzothiophene/dibenzofuran and dipyrrin moieties, and minimal πinteractions between the moieties emphasize the nonaromatic nature of the macrocycles 4 and 5. It is relevant to mention here the report on 10-heterocorroles28 that contain one heteroatom such as O, S, or Se in place of the meso-carbon of corroles where the authors showed an enhancement in aromaticity that follows the trend O < Se < S. A similar trend in aromaticity has been observed as we move from macrocycle 3 to macrocycle 5 but the effect is less due to high aromatic stabilization of the dibenzofuran/dibenzothiophene units of the macrocycles that force the π-delocalization to restrict to the individual units of dipyrrin and dibenzofuran/dibenzothiophene, resulting in the predominant nonaromatic character of compounds 3−5.
Figure 2. (a) 1H NMR (b) 1H−1H COSY, and (c) NOESY spectra of compound 5 recorded in CDCl3 (δ in ppm).
assignments were made on the basis of their location, integration, coupling constant, and cross peak correlations observed in COSY and NOESY spectra. The six −CH3 protons (type h) of two meso-tolyl groups appeared as singlet at 2.46 ppm showed NOE cross peak correlation with a resonance appeared at 7.28 ppm, which we identified as type g protons of meso-tolyl group. The type g protons showed cross-peak correlation in the COSY with type f protons resonated at 7.39 ppm, which in turn showed NOE correlation with type b protons resonated at 6.80 ppm and type c 5925
DOI: 10.1021/acs.orglett.7b02919 Org. Lett. 2017, 19, 5924−5927
Letter
Organic Letters
suggests that the macrocycles reported are nonaromatic. Thus, the absorption spectral studies indicated the absence of effective π-delocalization in these macrocycles and the spectral profiles were similar to the other reported nonaromatic macrocycles.6,29 The absorption spectra predicted by TD-DFT are in good agreement with the experimentally observed, predicting three transitions at 398 (oscillatory strength f = 0.32), 460 (0.06), 673 nm (0.19) for 4 and 445 (f = 0.04), 472 (0.13), 645 nm (0.21) for 5 (Figure S44). The absorption spectra of protonated macrocycles 5.H+ showed similar features, but the absorption bands were bathochromically shifted, indicating that the electronic properties of hybrid macrocycles were altered upon protonation (Figure S36). The electrochemical studies revealed that the macrocycles 3−5 exhibit two ill-defined irreversible oxidations (∼0.90 V; ∼1.20 V) and one reversible (∼−0.90 V) and one irreversible reduction (∼−1.45 V), supporting the electrondeficient nature of these macrocycles (Figure S39). The three inner core co-ordinating atoms (two Ns and one O/ S) in hybrid macrocycle 4 and 5 with one ionizable proton indicate that these macrocycles are potential ligands toward metal ion coordination. Interestingly, the macrocycles with three co-ordinating atoms arranged in triangular fashion are quite rare. A structurally unique N-fused macrocycles derived either deliberately from N-confused porphyrins or intuitively from ring-inverted expanded porphyrins are some known tridentate ligands, which were found to exhibit superior binding ability toward transition metals such as Mn2+, Ru+, Re+, and Fe2+ions.30−33 Since the hybrid macrocycles 4 and 5 have one ionizable proton, these macrocycles can form neutral complexes with metal ions in the +1 oxidation state. Thus, the co-ordination chemistry of hybrid macrocycles was performed with Re(I) metal salt. The macrocycles 4 and 5 were treated with 2 equiv of Re(CO)5Cl in chlorobenzene at reflux for 24 h. The color of the reaction mixture changed from dark green to yellowish green as the reaction progressed, which was also monitored by TLC analysis and absorption spectroscopy. The crude compounds were subjected to neutral alumina column chromatography and afforded the metal complexes 4-Re and 5Re in 52% and 69% yields, respectively. The formation of 4-Re and 5-Re was confirmed by the corresponding molecular ion peak in HR-MS (Figures S8 and S9), 1D and 2D NMR spectroscopy (Figures S26−S34), and X-ray crystallography. The compounds 4-Re and 5-Re were crystallized in the Pi ̅ and C2/c space groups (Table S4). In both complexes, the Re(I) possesses a perfect octahedral environment by co-ordinating to three inner core atoms of the macrocycle and three carbonyls (Figure 6). The bond lengths of Re−N1 and Re−N2 were ∼2.15 Å and Re−O and Re−S were 2.29 and 2.49 Å respectively (Tables S5 and S6). However, because of the large size of Re(I), the metal was not accommodated in the plane of the macrocycle,
The optimized (B3LYP/6-31G(d)) gas-phase structures of 4 and 5 match closely to those of the crystal structures with regard to conformation, bond angles, and bond distances. Akin to the distortion observed in the crystal structures of 4 and 5, dibenzothiophene macrocycle 5 displays high out-plane deviation over 4 exerted by the relatively large sulfur atom. In addition, C10−C11 and C21−C23 bond distances were also relatively long (1.49 Å), supporting the restricted π-delocalization within the individual fragments of 4 and 5. The calculated frontier orbital energies (HOMO and LUMO) and surface topologies of 4 and 5 are presented in Figure 4. It is clearly
Figure 4. (a) DFT-calculated frontier molecular orbital energies of 4 and 5 and their surface topologies. Dotted lines represent the predicted transitions.
manifested that the HOMOs of both 4 and 5 were delocalized on both dibenzofuran/dibenzothiophene and dipyrrin units. On the other hand, the LUMOs of 4 and 5 were mainly localized on the dipyrrin unit with weak charge density on the dibenzofuran/ dibenzothiophene unit. It supports the feasibility of intramolecular donor and acceptor interactions between dibenzofuran/dibenzothiophene and dipyrrin moieties in 4 and 5, respectively. The oxa-compound 4 exhibit slightly higher HOMO compared to the thia-congener 5, while the LUMOs of both the compounds were unperturbed leading to the reduced band gap for 4. The narrow band gap of 4 could be attributed to its relatively efficient π-delocalization within the planar ring compared to highly ruffled 5, which was also reflected in the optical (Figure 5) and electrochemical properties (Figure S39). The absorption spectra of macrocycles 3−5 showed an intense sharp band in the region of 380−390 nm, and a broad lowintensity band in the 620−660 nm region (Figure S36, Table S1)
Figure 6. Single-crystal X-ray structure of 4-Re and 5-Re: top view (a and b); side view (b and d). The meso aryl groups are omitted for clarity in the side view.
Figure 5. Electronic absorption spectra of 5 (pink line), 5.H+ (red line), 5-Re (blue line) recorded in toluene (2 × 10−5). 5926
DOI: 10.1021/acs.orglett.7b02919 Org. Lett. 2017, 19, 5924−5927
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but placed above the plane by 1.47 Å as observed in other reported Re(I) complexes of core-modified porphyrins.34−36 Interestingly, the macrocyclic cores in 4-Re and 5-Re were relatively planar compared to their free bases 4 and 5, respectively. The angle between the mean plane of dibenzofuran/dibenzothiophene and the mean plane of the dipyrrin in 4 and 5 was reduced in 4-Re and 5-Re from 23.7°/22.4° to 3.04°/ 5.45°. Thus, 4-Re showed slightly ruffled conformation, whereas 5-Re exhibit a nonruffled conformation, which may be because of their smaller core size. The relatively enhanced planarity of the macrocyclic core in the metal complexes could be attributed to their structural rearrangement due to co-ordination with the Re(I) ions. The absorption spectra of 4-Re and 5-Re showed one broad band at ∼800 nm along with a shoulder and a more intense sharp Soret-like band at ∼400 nm. Thus, the absorption bands of 4-Re and 5-Re were bathochromically shifted compared to their free base analogues. The complexes 4-Re and 5-Re showed two oxidations and two reductions, and the redox data suggest that the complexes were easier to reduce than free bases. In conclusion, we prepared stable hybrid dibenzofuran- and dibenzothiophene-based macrocycles under simple reaction conditions using readily available precursors. These unusual macrocycles showed features of both contracted macrocyles, subporphyrins, and triphyrins. The X-ray structures revealed that both macrocyles were significantly distorted. The DFT calculations in conjunction with spectral studies revealed that the macrocycles are nonaromatic and exhibits donor−acceptor interaction between dibenzofuran/diebnzothiohene and dipyrrin units. The hybrid macrocycles with three potential donor atoms are good ligands to form co-ordination complexes as demonstrated here by synthesizing Re(I) complexes. The crystal structures of Re(I) complexes revealed that the Re(I) complexes were more planar than free base macrocycles. Thus, the hybrid macrocycles reported here are new macrocyclic ligands for exciting co-ordination chemistry, and efforts in this direction are currently underway in our laboratory.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02919. Characterization data for all new compounds (PDF) X-ray crystallographic data for 4 (CIF) X-ray crystallographic data for 5 (CIF) X-ray crystallographic data 4-Re (CIF) X-ray crystallographic data for 5-Re (CIF)
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Mangalampalli Ravikanth: 0000-0003-0193-6081 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.R. thanks the Science & Engineering Research Board, Government of India (EMR/2015/002196), for funding the project, and A.K. thanks UGC for the doctoral fellowship. 5927
DOI: 10.1021/acs.orglett.7b02919 Org. Lett. 2017, 19, 5924−5927
