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Jun 29, 2016 - vitamin B12, and these molecules are known as the emeralds of expanded porphyrins because of their intense green color.3. However ...
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Synthesis and Quantum Mechanical Studies of a Highly Stable Ferrocene-Incorporated Expanded Porphyrin Tamal Chatterjee,† G. G. Theophall,‡ K. Ishara Silva,‡ K. V. Lakshmi,*,‡ and Mangalampalli Ravikanth*,† †

Department of Chemistry Indian Institute of Technology Bombay, Mumbai 400 076, India Department of Chemistry and Chemical Biology and The Baruch ’60 Center for Biochemical Solar Energy Research, Rensselaer Polytechnic Institute, Troy, New York 12180, United States

‡

S Supporting Information *

ABSTRACT: We present the first evidence for an unusual stable metallocene-containing expanded porphyrinoid macrocycle that was synthesized by condensing one equivalent of 1,1′-bis[phenyl(2-pyrroyl)methyl]ferrocene with one equivalent of 5,10-di(ptolyl)-16-oxa-15,17-dihydrotripyrrane under acid-catalyzed conditions. The formation of ferrocene-incorporated expanded porphyrin macrocycle was confirmed by HR-MS and 1D/2D NMR spectroscopy. The macrocycle was nonaromatic and displayed absorption bands in the region of 420−550 nm. The molecular and electronic structure of the ferrocene-incorporated expanded porphyrin was investigated by DFT methods. The DFT calculations indicated a partially twisted structure of the molecule, and the extent of torsional distortion was larger than previously observed for ruthenocenoporphyrinoids and ferrocenothiaporphyrin. The HOMO and LUMO states that were obtained from the DFT calculations indicated partial charge density on all four pyrrole nitrogen atoms and the furanyl oxygen atom in the HOMO state and partial charge density on the α and β carbon atoms in the LUMO state. In addition, the ferrocene moiety displayed the presence of partial charge density on the Fe atom and the cp rings in both the HOMO and LUMO states. Moreover, DFT studies of the diprotonated form of macrocycle indicated that the diprotonated form also retained a synclinal conformation and that its torsional strain was slightly higher than its free base form.

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INTRODUCTION Smaragdyrins are expanded porphyrin macrocycles with 22 conjugated π electrons that are composed of three meso-carbon atoms and two direct bonds that connect five pyrrole/ heterocycle rings.1 Smaragdyrins were discovered by Woodward and co-workers2 in 1967 during their pioneering work on vitamin B12, and these molecules are known as the emeralds of expanded porphyrins because of their intense green color.3 However, smaragdyrins are very unstable and tend to readily decompose.4 Due to the inherently unstable nature of these compounds, knowledge of their chemistry has remained elusive. In fact, to the best of our knowledge, there are no synthetic protocols available to obtain stable meso-aryl pentaazasmaragdyrins containing five pyrrole rings that are directly bonded or connected by methine bridges. Chandrashekar and coworkers have previously reported the synthesis of the first stable meso-triaryl 25-oxasmaragdyrins containing four pyrrole rings and one furan ring by [3 + 2] condensation of meso-aryl dipyrromethane and 16-oxatripyrrane under mild acid-catalyzed conditions.5 The meso-triaryl 25-oxasmaragdyrins possess novel photophysical and electrochemical properties and readily coordinate both metal and nonmetal atoms.6 Recently, we demonstrated that BF27-, B(OR)28-, and PO29-complexation of 25-oxasmaragdyrin renders the macrocycle more robust with © XXXX American Chemical Society

significant enhancement of the electronic properties. During the course of our investigation on smaragdyrin macrocycles, we encountered an interesting class of porphyrinoids where a ferrocene moiety is incorporated into the macrocycle in an ansa-type orientation (Figure 1). Srinivasan et al.10 and Grazynski et al.11 have previously reported the preparation of ansa-metallocene porphynoids 1−3 by using 1,1′-bis[phenyl(2pyrroyl)methyl]ferrocene as a key precursor that was condensed with either aldehydes or 2,5-bis(hydroxymethyl)aryl thiophene/furan under mild acid-catalyzed conditions. These ansa-metallocene porphyrinoids exhibited interesting structural and physicochemical properties. However, there are very few reports of ansa-metallocene porphyrinoids, and with the exception of one report,10b there is no information available on metallocene-incorporated expanded porphyrinoids. At this juncture, we thought of introducing the ferrocene moiety into the backbone of the smaragdyrin macrocycle to understand its physicochemical properties, stability, and suitability for various applications. Because smaragdyrins were prepared by [3 + 2] condensation of meso-aryl dipyrromethane with 16-oxatripyrrane, we incorporated the ferrocene moiety in dipyrromethane Received: January 13, 2016

A

DOI: 10.1021/acs.inorgchem.6b00092 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Structure of a variety of ansa-metallocene porphyrinoids.

Scheme 1. Synthetic Scheme for Ferrocene-Incorporated Expanded Porphyrin 4

of the ferrocene unit were arranged in a staggered conformation (Supporting Information S2). Because the X-ray structure has an R-factor of 0.11, (Supporting Information S3), we do not use this structure to describe the atomic details of compound 4. However, we have used the X-ray structure for the assignment of the resonances that were observed in the 1H NMR spectrum. The 1H NMR spectrum of 4 at room temperature revealed significant broadening of the peaks (Figure 2a) due to the effects of the ferrocene unit and the expanded macrocyclic skeleton. In contrast, the 1H NMR spectrum of 4 at −40 °C (233 K) displayed narrow peaks as shown in Figure 2b which led to higher resolution at low temperature. We did not observe any resonances in the negative region of the 1H NMR spectrum from the inner NH protons due to the nonaromatic nature of compound 4. The 1H NMR spectrum of compound 4 at −40 °C (233 K) displayed two closely spaced singlet peaks at 2.41 and 2.45 ppm due to the methyl protons of the meso-tolyl groups; eight singlet peaks in the region of 3.60−6.82 ppm due to the eight protons of the cyclopentadienyl rings (cp1 and cp2) of the ferrocene moiety; ten peaks in the region of 5.12−7.60 ppm from the protons corresponding to the four pyrrole and one furan ring; and two peaks at 8.17 and 7.84 ppm from the inner NH protons. We also recorded 2D NMR spectra (both the COSY and ROESY spectra) at −40 °C to assign all of the resonances that were observed in the 1D 1H NMR spectrum of compound 4. We observed a sharp resonance for the NH proton at 8.17 ppm although the macrocycle contains two inner NH protons. The inner NH resonance that is observed in the 1 H NMR spectrum is present on the pyrrole N3 atom (type-k), which is less likely to be involved in tautomerism due to the trans orientation of proton on the pyrrole N4 with respect to the proton on the pyrrole N3. In contrast, the other NH proton that is present on the pyrrole N1 (type-l) can be involved in rapid tautomerism with the pyrrole N2 proton. However, the

followed by condensation with 16-oxatripyrrane under mild acid-catalyzed conditions. The objective is to prepare a nonaromatic ferrocene-incorporated expanded porphyrin macrocycle with interesting structural features and properties. Herein, we report the successful synthesis of 1,1′-bis[phenyl(2pyrroyl)methyl]ferrocene 5 which upon condensation with 5,10-di-(p-tolyl)-16-oxa-15,17-dihydrotripyrrane (16-oxatripyrrane) 6 under mild acid-catalyzed conditions afforded a stable nonaromatic ferrocene-incorporated expanded porphyrin macrocycle 4. The molecular and electronic structure of the ferrocene-incorporated expanded porphyrinoid was studied by mass spectrometry, optical and magnetic resonance spectroscopy, and quantum mechanical calculations.

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RESULTS AND DISCUSSION The 1,1′-bis[phenyl(2-pyrroyl)methyl]ferrocene11b 5 and 5,10di(p-tolyl)-16-oxa-15,17-dihydrotripyrrane12 (16-oxatripyrrane) 6 precursors were prepared by previously published procedures. The condensation of 1,1′-bis[phenyl(2-pyrroyl)methyl]ferrocene 5 with 16-oxatripyrrane 6 in CH2Cl2 was carried out in the presence of catalytic amount of trifluoroacetic acid (TFA) under N2 atmosphere at room temperature for 2 h. This was followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ) and chromatographic purification using a basic alumina column that yielded the desired ferroceneincorporated expanded porphyrin macrocycle 4 as a redcolored solid in 10% yield (Scheme 1). The formation of the macrocycle 4 has been confirmed by high-resolution mass spectrometry (HR-MS) and 1D/2D nuclear magnetic resonance (NMR) spectroscopy. We observed a M+1 peak at 893.2940 in the mass spectrum, which confirmed the formation of compound 4 (Supporting Information S1). The molecular structure of 4 that was obtained from single crystal X-ray diffraction revealed a partially twisted structure where an inverted pyrrole ring (N4) and the cyclopentadienyl (cp) ring B

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Figure 2. Comparison of the 1H NMR spectra of ferroceneincorporated expanded porphyrin 4 recorded in CDCl3 at (a) room temperature and (b) −40 °C (233 K) (* and # denote the residual solvent peaks, and cp denotes the peaks from the cyclopentadienyl ring protons).

observed broad resonance at 7.84 ppm, which did not show proper correlations, was tentatively assigned as the type-l resonance of pyrrole 3. In the 1H−1H COSY spectra of 4, the NH resonance at 8.17 ppm was correlated with the resonances at 6.48 and 7.53 ppm that were identified as type-h and type-g protons, respectively (Figure 3a). The inner NH proton at 8.17 ppm also displayed a ROE correlation with the doublet peaks at 5.14 ppm that was assigned to the type-i proton of the inverted pyrrole ring, N4. The type-i resonance at 5.14 ppm correlated with the resonance at 5.84 ppm that was assigned as type-j proton of the inverted pyrrole ring, N4. Compound 4 contains a total of 18 aryl protons, which appeared as a broad multiplet in the 7.21−7.68 ppm region of the spectrum. The methyl protons of meso-tolyl groups at 2.41 and 2.45 ppm correlated with the aryl peaks in the 7.20−7.30 ppm region of the 1H spectrum. However, the aryl peaks in the 7.31−7.42 ppm region of the spectrum correlated with the cp signals (at 4.39, 4.49, and 4.60 ppm) as well as the two β-pyrrole signals at 6.17 and 5.84 ppm (Figure 3b). The resonance at 6.17 ppm was assigned to a type-a β-pyrrole proton. The 6.17 ppm signal correlated with the doublet peak at 6.92 ppm, which was identified as the type-b β-pyrrole proton. We observed one more closely spaced doublet at 6.90 ppm and a multiplet in the 6.79−6.84 ppm region of the spectrum. Although the peaks in this region were overlapping with each other, the correlations were clearly resolved in the 2D spectrum. On the basis of the integrated intensity of the overlapping peaks, the peaks in the 6.79−6.84 ppm region of the spectrum were assigned the two β-pyrrole protons (type-c and type-d) and the two β-furan protons (type-e and type-f) of 4. Furthermore, we assigned the resonances of the cyclopentadienyl ring protons, cp1 and cp2, of the ferrocene unit. The eight protons of ferrocene unit appeared as eight sets of resonances in 1H NMR spectrum which were identified on the basis of their correlations in 2D NMR spectra (Supporting Information S4 and S5). Further

Figure 3. (a) Partial 1H−1H COSY and (b) ROESY NMR spectra of compound 4 recorded in CDCl3 at −40 °C (233 K) (* and # denote peaks from the residual solvent protons and cp denotes the peaks from the cyclopentadienyl ring protons).

examination of the cross-peak correlations indicated that the four resonances at 4.33, 4.39, 4.49, 6.80 ppm corresponded to the cyclopentadienyl ring (cp1), whereas the resonances at 3.60, 4.60, 4.79, and 4.98 ppm corresponded to the second pentadienyl ring (cp2). The most upfield shifted resonance at 3.60 ppm was assigned as type-m proton of the ferrocene moiety as this proton experiences maximum ring current effect due to its close proximity and orientation to the meso-aryl group. The type-m proton resonance displayed a correlation with type-n proton resonance at 4.60 ppm, which in turn was correlated with the type-o proton resonance at 4.79 ppm. The type-p proton resonance at 4.98 ppm was similarly identified as this resonance displayed a correlation with the type-o proton resonance. Similarly, the most downfield resonance of cp1 ring at 6.80 ppm was identified as the type-q proton as this resonance displayed an ROE correlation with the type-m proton (Supporting Information S5). The resonance at 4.39 ppm was identified as the type-r proton on the basis of its correlation with the type-q proton. The resonances at 4.33 and 4.49 ppm were assigned as the type-s and type-t protons, C

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Figure 4. (a) 1H NMR spectrum of 4.2H2+ recorded in CDCl3 at 298 K (* and # denote residual solvent peaks). (b) 1H−1H COSY NMR spectra of compound 4.2H2+. The spectrum was recorded in CDCl3 at room temperature (* and # denote residual solvent peaks).

ferrocene unit over the macrocycle skeleton significantly alters the electronic structure of compound 4.2H2+. Interestingly, the 1 H NMR spectrum of 4.2H2+ displayed four broad peaks at 11.71, 10.80, 8.14, and 8.04 ppm due to the presence of the four inner NH protons (Figure 4a). These inner NH proton resonances were confirmed on the basis of their correlation with the corresponding β-pyrrole protons in 1H−1H COSY NMR spectrum (Figure 4b). The downfield resonance at 11.71 ppm that was identified as the type-v pyrrole NH proton (pyrrole 4) was correlated with the resonances at 3.59 and 3.93 ppm from the type-i and type-j pyrrole (pyrrole 4) protons, respectively. The inner NH resonance of pyrrole 1 at 10.80 ppm (type-u proton) was correlated with the resonances at 3.49 and 2.37 ppm in the 1H−1H COSY NMR spectrum (Figure 4b), which were identified as the type-a and type-b protons of pyrrole 1. The inner NH proton of pyrrole ring 3 (type-k) at 8.14 ppm was correlated with the type-g resonance at 6.97 ppm and the type-h resonance at 5.85 ppm. Similarly, the inner NH

respectively, on the basis of similar correlations in the 2D NMR spectra. Thus, we were successful in determining the highresolution molecular structure of compound 4 from the 1D and 2D NMR spectra that were acquired at −40 °C (233 K). We also recorded the 1D and 2D NMR spectra for the protonated species 4.2H2+ by the addition of trifluoroacetic acid (TFA) (60 μL) to compound 4 in CDCl3 at variable temperature (233−313 K) (Supporting Information S8 and S9). During transformation of 4 to 4.2H2+ upon treatment with TFA, the formation of monocationic intermediate 4.H+ is also detected in the 1H NMR and UV−vis spectral titration (Figure 5 and Supporting Information S9). However, the 1H NMR resonances are broad and similar to the diprotonated species 4.2H2+. Hence, we do not identify the resonances of monocationic intermediate species 4.H+ in variable temperature 1 H NMR spectra. The 1H NMR and 1H−1H COSY spectra of 4.2H2+ recorded at room temperature are presented in Figure 4. Upon protonation, transference of the d electrons of D

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0.33, 0.57, and 0.79 V and one irreversible oxidation at 1.22 V along with two irreversible reductions at −1.30 and −1.78 V (Supporting Information S10). The ease of oxidation of compound 4 supports the electron-rich nature of the macrocycle and probably the transfer of electron density from ferrocene moiety to the macrocyclic skeleton. We used density functional theory (DFT) to further probe compound 4. Shown in Figure 6a is the molecular structure

proton of pyrrole ring 2 (type-l) at 8.04 ppm was correlated with the type-c and type-d pyrrole protons at 4.72 and 7.09 ppm, respectively (Supporting Information S6). The resonances at 4.92 and 6.20 ppm, which were correlated with each other, were identified as the type-e and type-f furan ring protons, respectively. The resonances of the ferrocenyl unit of 4.2H2+ were identified from the correlations in the NOESY spectrum (Supporting Information S7). In the NOESY spectrum, the type-v pyrrole NH proton at 11.71 ppm displayed an NOE correlation with the resonance at 3.60 ppm, which was assigned as the type-m proton of cyclopentadienyl ring (cp2). The resonances at 4.32, 6.06, 7.01 ppm were identified as the type-n, o, and p protons of the (cp2) ring, respectively, based on their cross-peak correlations (Supporting Information S6). Similarly, the resonance at 2.75 ppm was identified as the type-t proton of the cp1 ring based on its NOE correlation with the type-a proton of pyrrole ring 1. The resonances at 4.25, 4.76, 6.30 ppm were assigned to the types-s, -r, and q protons of the (cp1) ring, respectively, on the basis of cross-peak correlations in the 1 H−1H COSY NMR (Supporting Information S6). Thus, all of the protons of 4.2H2+ displayed upfield and downfield shifts upon protonation that were assigned by 1D and 2D NMR studies. We studied the absorption properties of the neutral compound 4 and its diprotonated form, 4.2H2+, which was generated by systematic addition of increasing amounts of TFA, as shown in Figure 5. The absorption spectrum of the maroon-

Figure 5. Change in the absorption spectra of compound 4 (1 × 10−5 M) upon systematic addition of TFA solution (0−20 equiv) in CH2Cl2 solution at room temperature. A color change is observed upon the protonation of compound 4.

colored compound 4 in CHCl3 displayed bands at 423, 460, and 547 nm, whereas the violet-colored dicationic compound 4.2H2+ displayed two relatively sharp bands at 500 and 658 nm. However, the titration curve also revealed a relatively broad band around at ∼475 nm, which indicates the formation of intermediate monocationic species 4.H+ during the conversion of 4 to 4.2H2+. The compound 4 did not display any emission spectrum in solution. The electrochemical properties of compound 4 were investigated by cyclic voltammetry in CH2Cl2 containing TBAP (0.1 M) as supporting electrolyte. The compound 4 exhibited three quasi-reversible oxidations at

Figure 6. Structure of compound (a) 4 and (b) 4.2H2+ that was obtained from the DFT calculations.

that was obtained from DFT calculations of compound 4. We observed that the dihedral angle between the pyrrolic N1 and N2 heterocycles was the smallest in comparison with the dihedral angle between the N2−O and O−N3 heterocycles (Supporting Information S11). This was expected as the mediating meso-carbon atom in the structure would allow for increased flexibility of rotation of the N2−O and O−N3 heterocycles. In contrast, the dihedral angle between the E

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unoccupied molecular orbital (LUMO) of compound 4 are shown in Figure 7. In the HOMO state, we observed that there

pyrrolic N3 and N4 heterocycles was the largest in compound 4, and this was due to the orientation of the N4 nitrogen atom, which is out of plane with respect to the center of the molecule. This is in excellent agreement with the partially twisted structure of this compound that was observed in the X-ray crystal structure (Supporting Information S2). This could be the genesis of the asymmetric charge density distribution that was observed in the electronic structure of compound 4 that is discussed below. The angle between the plane of the two cp rings of compound 4 was ∼45°, invoking a torsional strain within the partially twisted structure of the molecule. The extent of strain (∼29°) that was observed for compound 4 was larger than the torsional distortion of 5° to 13° and −1° to 13.8° that was previously observed for ruthenocenoporphyrinoids11c and ferrocenothiaporphyrin,11b respectively. We observed that the cp rings of compound 4 were in a staggered conformation that was consistent with the preferred geometry of a ferrocene moiety. Moreover, the structure of compound 4 indicated an inversion of the N4 pyrrole ring that provided further confirmation of the peaks that were observed in the 1H NMR spectrum. We also used DFT to probe the molecular structure and electronic properties of the diprotonated form of compound 4, 4.2H2+. Shown in Figure 6b is the structure that was obtained from the DFT calculations of compound 4.2H2+. The dihedral angle between the pyrrolic N1 and N2 heterocycles of 4.2H2+ was slightly larger than the corresponding dihedral angle that was observed in compound 4 (Supporting Information S11). The increase in the N1−N2 dihedral angle of 4.2H2+ was attributed to the increased planarity of the aryl substituents that perturb the geometry of neighboring pyrrolic heterocycles. However, the presence of a rigid ferrocene moiety in close proximity likely constrained thermodynamic relaxation of the pyrrolic N1 and N2 heterocycles leaving little room for major conformational changes to minimize steric interactions. In comparison with the N1 and N2 heterocycles, the N2−O and O−N3 heterocycles of 4.2H2+ have mediating meso-carbon atoms with increased flexibility of rotation that rendered larger conformational changes in these heterocycles upon diprotonation of compound 4. The N2−O dihedral angle was observed to decrease by ∼5° while the O−N3 dihedral angle increased by ∼9° in compound 4.2H2+. Moreover, the protonation of the inverted N4 heterocycle caused an unfavorable steric repulsion with the bottom cp ring that led to an upward displacement of the nitrogen atom of the N4 (and N3) heterocycle. This effect was not observed with the N1 heterocycle as the pyrrolic group was not inverted, and hence, there was no steric repulsion with the corresponding cp ring. The largest dihedral angle was observed between the N3−N4 heterocycles of compound 4.2H2+ due to the presence of the inverted N4 heterocycle. We observed that the diprotonated form of compound 4, 4.2H2+, retained a synclinal conformation. The angle between the two cp rings of compound 4.2H2+ was ∼45° due to a torsional strain within the helical structure of the molecule. The torsional strain of compound 4.2H2+ was ∼31°, which was slightly increased from the strain of 29° that was observed for compound 4. The torsional strain of compound 4.2H2+ was also well above the values of 5° to 13° that were previously reported in literature.11c We used DFT to probe the electronic structure of compound 4 and 4.2H2+. The partial charge density distribution in the highest occupied molecular orbital (HOMO) and the lowest

Figure 7. Theoretical view of the (a) HOMO and (b) LUMO state of compound 4 that was obtained from the DFT calculations.

was partial charge density on all four pyrrole nitrogen atoms and the furanyl oxygen atom of compound 4. In contrast, we observed partial charge density on the α- and β-carbon atoms of compound 4 in the LUMO state. The partial charge density distribution on the heteroatoms and the α/β carbon atoms in the HOMO and LUMO state of compound 4 was similar to the charge density distribution that was previously observed in other smaragdyrin macrocycles. However, the partial charge density distribution on the meso-carbon atoms in the HOMO state of compound 4 was distinctly higher (Figure 7 and Supporting Information S12). The presence of increased charge density was most pronounced for the meso-carbon atoms between the cp ring and the pyrrole heterocycle. The other meso-carbon atoms also displayed the presence of increased charge density as these atoms conjugate proficiently with the α-carbon of the adjacent furanyl moiety. The partial charge density was more localized on the meso-carbon atoms that were proximal to the pyrrolic N1 and N4 heterocycles. This could be due to the interaction of these meso-carbon atoms with π system of the cp ring and the Fe atom. The aryl substituent groups, A and D, displayed very similar partial charge density distribution in the sigma-bond network in the HOMO state of compound 4. This was expected as compound 4 is nonaromatic. In addition, the two aryl groups, B and C, which were bonded to the meso-carbon atoms proximal to the furanyl ring, displayed an asymmetric charge density distribution. This was likely due to the difference in the dihedral angle of the aryl B and C substituents (Supporting Information S13). The ferrocene moiety displayed partial charge density on the Fe atom and the cp rings in the HOMO and LUMO state, respectively. The total contribution from the orbitals of the Fe atom was estimated as 7−10% of the partial charge density in the HOMO and LUMO states. We analyzed the participation of the individual atomic orbitals of the Fe atom that suggested maximum contribution from the dyz orbital (with small contribution from the other d oribtals) in the HOMO state. The dxz and dyz orbitals are expected to be degenerate in ferrocene; however, the degeneracy is lifted in compound 4 due to the bonding anisotropy of the cp ring and the nonparallel arrangement of the cp rings. In comparison with the HOMO state, the LUMO state of compound 4 displayed more partial charge density on the ferrocene moiety, cp rings and the N1 and N4 heterocycles. The N1 heterocycle had partial charge density on all of the constituent atoms of the pyrrole ring. In contrast, the N4 F

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and effects from the altered configuration of the cp rings of the ferrocene group in compound 4.2H2+. The DFT structure of compound 4 supports the NMR spectral interpretation. As mentioned above, the N1−N2 dihedral of compound 4 was ∼6° with a N2−H···N1distance and angle of 2.8 Å and 90°, respectively, which was indicative of tautomerization between the N1−N2 heterocycles. This is in agreement with the NMR results as the resonance from the type-l proton that was absent at room temperature (Figure 2a) displayed a broad peak in the NMR spectrum at 233 K (Figure 2b). Moreover, the well-defined NMR resonance from the typek proton is in agreement with the presence of an inverted N4 heterocycle and the distance between the N3−N2 heterocycles that was too large for tautomerization. The improved resolution of the low temperature NMR spectra led to the detection of the resonances from the cp protons of compound 4. The cp proton signals displayed eight peaks with increased dispersion that indicated significant anisotropy. The distance between the center of the rings A and D to the proximal type-q and -m cp protons in the DFT structure was 3.2 and 3.4 Å, respectively, which suggested inequivalent steric packing effects. Moreover, the orientation of the inverted N4 heterocycle relative to the N1 heterocycle also suggested that the type-p and type-t protons were inequivalent. The DFT and X-ray crystal structure of compound 4 indicated a helical geometry that was previously shown to influence the proton chemical shifts of metallocenoporphyrinoids.11 Kleinpeter18 et al. performed Gauge-Independent Atomic Orbital (GIAO) calculations on conjugated helical systems that estimated the magnetic fields, which were represented by iso-chemical-shif t-surfaces that were produced from magnetic ring currents in the plane of the helix. Moreover, these magnetic ring currents diminished upon approaching the discontinuity of the helical system. In independent observations, Simkowa11b et al. investigated the magnetic field contributions from metallocene moieties in aromatic, antiaromatic and ferrocene-conjugated helical systems. These studies suggested a strong correlation with the π system of metallocene moieties. Although major changes were observed for both aromatic and antiaromatic species, the helical structure of the π system was shown to affect the nonaromatic species. In the present case, the helical nature of compound 4 suggested that these effects will produce signature chemical shifts of the cp protons. The chemical shifts of the type-p, -o, -n, -m or type-q, -r, -s, -t protons depend on the relative orientation of the cp rings, which is in excellent agreement with previous experimental11b and theoretical18 observations. The chemical shift of the type-t proton of compound 4 breaks this trend. This can be attributed to steric packing effects of the N1 β-type-a proton distance of 2.1 Å. However, there is no break in this trend for the type-p due to the inversion of the N4 heterocycle. The HOMO state of compound 4, shown in Figure 7a and Supporting Information S12, depicts significant density on meso-carbons of ring A and D, which suggested conjugation between the ferrocene and the heterocycle π systems. This could lead to the anisotropies that were observed in the NMR spectra (Figure 2a,b). Moreover, the partial charge density in the HOMO state was localized near N1 and N4 heterocycle, which suggested increased shielding, as was observed in the chemical shifts of the β-protons with the trend N4 > N1 > N3 ∼ N2 ∼ O. The DFT structures were also analyzed to understand the NMR spectra of compound 4.2H2+. The addition of two

nitrogen displayed an inversion of the charge density distribution. We also observed that the LUMO contained charge density on both the Fe atom and the cp ligand with maximum contribution from the dxz and dx2−y2 atomic orbitals of the Fe atom. The partial charge density distribution was asymmetric in both the HOMO and LUMO state of compound 4. This asymmetry suggested the presence of multiple bands in the absorption spectrum of compound 4, which was in agreement with the experimental observation (Figure 5). The partial charge density distribution in the HOMO and LUMO state of compound 4.2H2+ that were obtained from DFT calculations are shown in Figure 8. In the HOMO state of

Figure 8. Theoretical view of the (a) HOMO and (b) LUMO state of compound 4.2H2+ that was obtained from DFT calculations.

compound 4.2H2+, once again we observed that there was partial charge density on all four pyrrole nitrogen atoms and the furanyl oxygen atom and there was partial charge density on the α- and β-carbon atoms in the LUMO state. The larger dihedral angles of compound 4.2H2+ resulted in increased partial charge density on the aryl substituent groups, A and C. A small increase in the partial charge density was also observed on the aryl substituent group, B. Moreover, the partial charge density on the β-carbon atoms proximal to the N2 and N3 heterocycles was diminished while there was an increase in the partial charge density on the β carbon atoms proximal to the N1 and N4 heterocycles. The LUMO state of 4.2H2+ (Figure 8 and Supporting Information S14) displayed changes due to the diprotonation of the pyrrolic nitrogen atoms. We observed a large shift of partial charge density from the aryl substituent groups, B and C, to the ferrocene moiety in the LUMO state of compound 4.2H2+. Overall, the partial charge density distribution in the LUMO state of compound 4.2H2+ was more symmetric in comparison with the LUMO state of compound 4, which confirmed the decreased number of peaks that were observed in the absorption spectrum. The absorption spectrum of 4.2H2+ displayed two bands that were red-shifted relative to compound 4 with an increase in the corresponding extinction coefficients (Figure 5). Examination of the HOMO and LUMO state of compound 4.2H2+ revealed an increase of partial charge density on the pyrrolic nitrogen atoms and aryl substituent groups to facilitate delocalization of the cationic charge that decreased the energy gap leading to a bathochromic shift. Although there was a decrease in the partial charge density on the furan ring, there was an increase in the charge density in the proximity of the ferrocene group of compound 4.2H2+. This was in agreement with the NMR spectrum that indicated d-orbital interactions G

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protons in compound 4.2H2+ inhibited tautomerization between the N1 and N2 heterocycle, which led to a narrow signal from the type-l proton. There were two NMR signals that were observed from the type-u and type-v protons with a chemical shift difference of 1 ppm. This was likely due to an increase in the distance from type-p proton as evidenced by a 10° increase in the dihedral angle between the of cp ring and the N4 heterocycle. The protonation of compound 4 caused large changes in the NMR signals from the cp rings due to spatial confinement and the decrease in the dihedral angles of the A and D rings (Supporting Information S13 and S15). The steric crowding effect between the type-p and type-v protons was reciprocal which was seen in the ∼2 ppm increase in the chemical shift of the type-p proton. As observed in the case of compound 4, the chemical shift of the cp protons in compound 4.2H2+ decreased around the ring which was consistent with previous observations in aromatic helical species.18,11b Moreover, there was no break in this trend for the type-t proton due to a minimal change in the dihedral angle between the cp ring and the N1 heterocycle. In addition, the A and D rings displayed the largest decrease in dihedral angles that led to increased delocalization of the partial charge density, which resulted in a larger change in the chemical shift of the βhydrogens of N1 and N4 heterocycle in comparison with the N2 and N3 heterocycle.

Article

EXPERIMENTAL METHODS

General. The compounds, 1,1′-bis(aryl(2-pyrolyl)methyl)ferrocene11b 5 and 16-oxatripyrrane12 6, were synthesized by following the literature methods. The 1H and 13C NMR spectra were recorded in CDCl3 using tetramethylsilnae (Si(CH3)4) as internal standard. The HR-MS spectra were recorded on a Bruker microTOF-Q spectrometer using the electrospray technique. The electronic spectra were recorded on a Cary series UV−vis NIR spectrophotometer. The cyclic voltammetric (CV) studies were carried out with BAS electrochemical system utilizing the three-electrode configuration consisting of a glassy carbon (working electrode), platinum wire (auxiliary electrode), and saturated calomel (reference electrode) electrodes in dry dichloromethane using 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The solution was deaerated by bubbling argon gas, and during the acquisition, argon was slowly flowed above the solution. Synthesis of Ferrocenooxasmaragyridyrin (4). 1,1′-Bis(aryl(2pyrolyl)methyl)ferrocene 5 (1.25 mmol) and 16-oxatripyrrane 6 (1.25 mmol) were taken in a 500 mL round-bottom flask and dissolved in 300 mL of dichloromethane (DCM) solution. The solution was stirred under nitrogen atmosphere for 10 min, trifluoroacetic acid (TFA) (0.1 equiv) was added to the reaction mixture, and the mixture was further stirred for 2 h. Subsequently, DDQ (3 equiv) was added to the reaction mixture and was stirred for an additional 2 h. The formation of compound 4 was confirmed by the appearance of a red-colored spot in the TLC. The crude compound was subjected to chromatography with a basic alumina column, and the desired reddish-colored band was eluted by DCM/Pet ether (60:40) solvent. The solvent was evaporated using rotary evaporator and recrystallization in a chloroform/n-hexane mixture afforded pure compound 4 as brownish solid in 10% yield. The 1H NMR at 500 MHz in CDCl3 at −40 °C yielded the following resonances: δ 8.17 (s, 1H, NH, type-k), 7.84 (br, 1H, NH, type-l), 7.83−7.67 (br, 2H, Ar), 7.59 (d, 1H, J (H,H) = 7.3 Hz, Ar), 7.53 (m, 1H, β-pyrrole H, type-g), 7.46 (t, 2H, J (H,H) = 7.0 Hz, Ar), 7.39−7.30 (br, 9H, Ar), 7.21 (2H, Ar), 6.97−6.90 (m, 4H, Ar + β-pyrrole H’s, type-b and type-d), 6.84−6.79 (m, 4H, β-pyrrole H, type-c; β-furan H’s, type-e and type-f; type-q, cp1), 6.48 (m, 1H, βpyrrole H, type-h), 6.17 (d, 1H, J (H,H) = 4.0 Hz, β-pyrrole H, type-a), 5.84 (d, 1H, J (H,H) = 4.4 Hz, β-pyrrole H, type-j), 5.14 (d, 1H, J (H,H) = 4.4 Hz, β-pyrrole H, type-i), 4.98 (s, 1H, type-p, cp2), 4.79 (s, 1H, type-o, cp2), 4.60 (s, 1H, type-n, cp2), 4.49 (s, 1H, type-t, cp1), 4.39 (s, 1H, type-r, cp1), 4.33 (s, 1H, type-s, cp1), 3.60 (s, 1H, type-m, cp2), 2.45 (s, 3H, tolyl-CH3), 2.41 (s, 3H, tolyl-CH3) ppm. The UV−vis spectrum (λmax nm (log ε), CH2Cl2) contained the following peaks: 423 (5.70), 473 (5.21), 547 (4.85). HR-MS calcd for C60H45N4OFe (M+H)+ m/z 892.2939; found, 893.2940. Density Functional Theory (DFT) Calculations. Density functional theory calculations were performed using the Vienna Abinitio software package13 (VASP) (Kresse, G. Software VASP, Vienna), The PBE functional14 and the projected augmented wave15 (PAW) potential method embedded in the VASP code were used to describe the core−valence electron interaction. Due to the strong correlation effect between 3d electrons of the Fe atom, we employed DFT with the Hubbard U approach.16 The Hubbard U parameter17 that was used in the calculations accounts for the on-site Coulombic repulsions. The initial positions of the atoms were adapted from the X-ray crystal structure of compound 4, and the structure was relaxed prior to the DFT calculations. For compound 4.2H2+, the initial positions of the atoms were taken from the X-ray crystal structure of compound 4, two protons were added to the basic pyrrole groups, and the resulting structure was relaxed prior to performing DFT calculations. The coordinates were centered into a periodic box in order to perform quantum mechanical calculations, and a vacuum space of ∼5 Å was provided to prevent electronic interaction between periodic cells. The wave functions were decomposed into separate energy bands for the valence electrons, which were then analyzed to find the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states.

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CONCLUSIONS In conclusion, we successfully synthesized a highly stable ferrocene-incorporated expanded porphyrin 4 by 3 + 2 condensation of the 1,1′-bis[phenyl(2-pyrroyl)methyl]ferrocene with 16-oxatripyrrane under TFA catalyzed conditions. This is the first example of metallocene incorporated into an expanded porphyrinoid macrocycle. We determined the molecular structure by HR-MS, 1D and 2D NMR spectroscopy and observed that compound 4 is nonaromatic with broad absorption peaks, but it was not fluorescent in solution. We performed DFT calculations on compound 4. The molecular structure that was obtained from the DFT calculations was in excellent agreement with the partially twisted structure of compound 4 that was observed in the X-ray crystal structure with limited resolution. There was partial charge density on all four pyrrole nitrogen atoms and the furanyl oxygen atom in the HOMO state of compound 4, and we observed partial charge density on the α- and β-carbon atoms of compound 4 in the LUMO state. In particular, the partial charge density distribution on the meso-carbon atoms in the HOMO state of compound 4 was distinctly higher than the typical charge density distribution patterns of other smaragdyrins. The ferrocene moiety in compound 4 displayed the presence of charge density on the Fe atom and the cp rings in the HOMO and LUMO state, respectively. The DFT studies of the diprotonated form of compound 4 indicated that 4.2H2+ retained a synclinal conformation and its torsional strain were slightly higher than compound 4. An examination of the HOMO and LUMO state of compound 4.2H2+ revealed an increase of partial charge density on the pyrrolic nitrogen atoms and aryl substituent groups to facilitate delocalization of the cationic charge. This was accompanied by a decrease in the partial charge density on the furan ring and an increase of charge density in the proximity of the ferrocene group of compound 4.2H2+. H

DOI: 10.1021/acs.inorgchem.6b00092 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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(16) (a) Jiang, H. Int. J. Quantum Chem. 2015, 115, 722−730. (b) Himmetoglu, B.; Floris, A.; de Gironcoli, S.; Cococcioni, M. Int. J. Quantum Chem. 2014, 114, 14−49. (17) Anisimov, V. I.; Aryasetiawan, F.; Lichtenstein, A. I. J. Phys.: Condens. Matter 1997, 9, 767−808. (18) Kleinpeter, E.; Klod, S.; Koch, A. J. Mol. Struct.: THEOCHEM 2007, 811, 45−60.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00092. HR-MS mass spectrum, X-ray structure, crystal data, NMR spectra, cyclic voltagram, dihedral angles, structural side views (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.R. thanks Department of Science & Technology, Government of India for funding the project and TC thanks CSIR for the SRF fellowship. K.V.L. acknowledges support by the Photosynthetic Systems Program, Office of Basic Energy Sciences, United States Department of Energy (DOE) under Grant No. DE-FG02-07ER15903. We thank the Center for Computational Innovations (CCI) at Renselaer for computational time.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00092 Inorg. Chem. XXXX, XXX, XXX−XXX