NH Tautomerism of a Quadruply Fused Porphyrin: Rigid Fused

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NH Tautomerism of a Quadruply-Fused Porphyrin — Rigid Fused Structure Delays the Proton Transfer Yuta Saegusa, Tomoya Ishizuka, Yoshihito Shiota, Kazunari Yoshizawa, and Takahiko Kojima J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b10945 • Publication Date (Web): 10 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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The Journal of Physical Chemistry

NH Tautomerism of a Quadruply-Fused Porphyrin —Rigid Fused Structure Delays the Proton Transfer Yuta Saegusa,† Tomoya Ishizuka,*,† Yoshihito Shiota,‡ Kazunari Yoshizawa‡ and Takahiko Kojima*,† †

Department of Chemistry, Graduate School of Pure and Applied Sciences, University of

Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan, ‡ Institute for Materials Chemistry and Engineering and IRCCS, Kyushu University, Motooka, Nishi-Ku, Fukuoka 819-0395, Japan.

ABSTRACT: We report herein NH tautomerism of a freebase derivative of a quadruply fused porphyrin (H2QFP-Mes, 3), which has one mesityl group at one of the β-positions of the nonfused pyrroles to lower the structural symmetry, allowing us to observe the NH tautomerism with 1

H NMR spectroscopy. Compound 3 was revealed to have the two inner NH protons on the two

non-fused pyrroles and the NH tautomerism of 3 was evidenced by variable-temperature (VT) 1H NMR experiments in various deuterated solvents. The VT-NMR studies revealed that the activation barrier for the NH tautomerism of 3 was larger than that of tetraphenylporphyrin. The positive activation entropy (∆S‡ = 89 J mol–1 K–1), determined for the NH tautomerism, can be explained by dissociation of the π-π stacked dimer structure of 3 in the ground state as evidenced by the crystal structure and NMR measurements. Based on the kinetic studies and DFT

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calculations, the stability of intermediates in the NH tautomerism of 3 and the transitions states have been discussed in detail.

Introduction Intramolecular hydrogen-bonding plays important roles to regulate molecular structures and to control physical properties of organic compounds.1-4 For example, intramolecular hydrogen bonding contributes to structural planarization of an oligoarene having hydrogenbonding substituents to induce bathochromic shifts of the absorption bands by extension of the πconjugation.5 Intramolecular hydrogen bonding is also found between the inner pyrrolic NH protons and imino nitrogen atoms of freebase porphyrins6 and the analogues,7 and the hydrogen bonding contributes to stabilization of the planar structures of porphyrins and their analogues and controls their physical properties.8 Proton transfer between intramolecular hydrogen-bonded sites causing NH tautomerism has attracted much attention because of interests in controlling the electronic structures of heteroaromatic compounds in the excited states.9,10 Also, proton transfer in imine derivatives forming intramolecular hydrogen bonds with hydroxyl groups have been intensively studied as model compounds of condensation intermediates of pyridoxal-5’-phosphate as a cofactor with amino acids, in which an intramolecular O-H···N hydrogen bond is formed in the course of enzymatic transformation of amino acids.11-13 NH tautomerism of freebase porphyrins, in which two inner pyrrolic NH protons move very fast among the four nitrogen atoms, has been intensively studied due to the interest in proton-transfer reactions of strongly hydrogen-bonded protons locating inside of an aromatic current.14-66 Structural anisotropy caused by the positions


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of the inner NH protons in freebase forms of porphyrin derivatives and the dynamics of the NH tautomerism significantly affect their physical and chemical properties.62,63 Furthermore, thermally and vibrationally induced proton transfer in the inner cavity of a porphyrin analogue has been utilized for applications to a single-molecule switch, which is an essential component in single-molecule-based devices.64-66 Therefore, the NH tautomerism of porphyrin and its analogues has been well investigated experimentally14-50 and theoretically51-61 and has been regarded as an important issue in porphyrin chemistry toward development of porphyrin-based molecular devices.62-66 In recent years, remarkable efforts have been dedicated to synthesis and characterization of novel π-expanded porphyrins having ring-fused structures at the periphery.67-96 Among them, the derivatives having fused five-membered rings have attracted considerable attention due to the interest in their unique aromaticity.97-109 However, the effects of the ring fusion on the properties

Scheme 1. NH Tautomerism of a Freebase QFP


3


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of the freebase derivatives of the ring-fused porphyrins, such as NH tautomerism, have yet to be well explored. We have recently synthesized and characterized a ZnII complex of a quadruplyfused porphyrin (ZnIIQFP) having four five-membered fused rings.110-112 A freebase derivative of the quadruply-fused porphyrin (1-R) has been also synthesized from ZnIIQFP and characterized; the imino-nitrogen atoms of the fused pyrroles in H2QFP exhibit high proton acceptability.113 Thus, the NH tautomerism of 1-R, which involves intermediates having a proton on the imino nitrogen atoms of the fused pyrroles, is expected to be observed. Herein, we describe the NH tautomerism of a freebase QFP derivative (Scheme 1) on the basis of kinetic analysis and DFT calculations.

Experimental Section General. Chemicals and solvents were used as received from commercial sources unless otherwise mentioned. CH2Cl2 used for the UV-vis spectral measurements was distilled over CaH2 before use. DMA for the synthesis was distilled before use. Compound 2 was synthesized according to the literature procedure.113 Mesitylboronic acid was also synthesized in accordance with a reported procedure.111,114 1

H NMR measurements were performed on Bruker AVANCE-400, 500 and 600

spectrometers. UV-vis absorption spectra were measured in CH2Cl2 on a Shimadzu UV-3600 spectrophotometer. MALDI-TOF-MS spectrometry was performed on an AB SCIEX TOF/TOF 5800 spectrometer by using dithranol as a matrix. Gel permeation chromatography (GPC) was


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performed in 0.5% triethylamine/CHCl3 utilizing a LC-9110 NEXT Liquid Chromatography system equipped with two JAIGEL-40 columns.

Synthesis. 2-Mesityl-24,30,36,42-tetrakis(tert-butyl)-quadruply-fused porphyrin (3). To a solution of 2 (27.8 mg, 33.4 µmol) in CHCl3 (50 mL) was added recrystallized N-bromosuccinimide (NBS) (11.2 mg, 64.7 µmol) and the reaction mixture was refluxed for 10 h. After cooling to room temperature, the solvent was removed under vacuum and the residual solid was washed with H2O and dried under vacuum to obtain black powder. The black powder was recrystallized from THF/MeOH (1:3, v/v) and black precipitate obtained was dried under vacuum to obtain crude mono-brominated 2. To a solution of the crude mono-brominated 2 (26 mg) in DMA (1 mL) was added mesitylboronic acid111,114 (47.3 mg, 288 µmol), K2CO3 (172 mg, 1.24 mmol), and Pd(PPh3)4 (2.0 mg, 1.7 µmol). The reaction mixture was stirred at 130 °C for 9.5 h and the solvent was removed under vacuum. The residual solid was dissolved in CHCl3 (50 mL) and filtered. TFA (5.0 mL, 65 mmol) was added to the filtrate and the reaction mixture was stirred for 1 h at room temperature. The mixture was washed with Na2CO3 aq and water, and dried over Na2SO4. The solvent was removed under vacuum and black solids obtained were chromatographed on a silica gel column using toluene/hexane (1:2, v/v) as an eluent. The second moving fraction was collected and the solvent was removed under vacuum. The residual solid was recrystallized from CH2Cl2/MeOH (1:3, v/v) to give black crystals of 3 (1.6 mg, 1.7 µmol, 5% (3 steps)). 1H NMR (CDCl3, 298 K): δ 7.80 (dd, J = 4.7, 2.1 Hz, 1H, β-H), 7.75 (dd, J = 4.7, 2.0 Hz, 1H, β-H), 7.68 (d, J = 2.4 Hz, 1H, 3-β-H), 7.20 – 7.14 (m, 3H, 26,32,38-Ph-H), 7.11 (s,


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2H, m-mesityl-H), 6.90 – 6.81 (m, 6H, 23,25,29, 31,35,37-Ph-H), 6.76 (d, J = 2.0 Hz, 1H, 41-PhH), 6.19 (dd, J = 8.2, 2.0 Hz, 1H, 43-Ph-H), 6.06 (br s, 1H, inner NH), 5.99 (br s, 1H, inner NH), 5.20 (d, J = 8.2 Hz, 1H, 44-Ph-H), 2.47 (s, 3H, mesityl-p-CH3), 2.10 (s, 6H, mesityl-o-CH3), 1.32 (s, 9H, t-Bu-CH3), 1.31 (s, 9H, t-Bu-CH3), 1.30 (s, 9H, t-Bu-CH3), 1.22 (s, 9H, t-Bu-CH3). UVvis (CHCl3): λ [nm] (log ε) = 820 (3.52), 745 (4.00), 667 (4.10), 605 (4.84), 478 (4.34), 417 (4.60), 331 (4.56). MS (MALDI-TOF, dithranol matrix): m/z = 949.5 (calcd. for [M]+: 949.3). Anal. Calcd for C60H54N4·0.5H2O·2CH2Cl2·1.5C6H14: C 76.42, H 7.21, N 4.46; Found: C 76.64, H 7.46, N 4.24.

X-ray Diffraction Analysis. A single crystal of 3 was obtained by recrystallization from the chloroform solution with vapor deposition method using acetonitrile as a poor solvent. A single crystal was mounted on a mounting loop. All diffraction data were collected on a Bruker APEXII diffractometer at – 153 °C with graphite-monochromated Mo Kα (λ = 0.71073 Å) irradiation by the ω-scan. The structures were solved by direct methods using SIR97 and SHELX-97.115 The co-crystallized MeCN molecule in the crystal of 3 were severely disordered and deleted with use of the SQUEEZE program.116 Crystallographic data for 3 are summarized in Table S2. CCDC-1434622 contains the supplementary crystallographic data.

Kinetic Studies on NH Tautomerism.


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The determination of the rate constants, k, for the NH tautomerism of 3 was performed on a Bruker AVANCE-500 spectrometer at various temperatures. Porphyrin 3 (1.1 mM) was dissolved in CDCl3 or one of three kinds of mixed deuterated solvents, 50%C6D6/CDCl3, 10%CD3CN/CDCl3, and 25%(CD3)2CO/CDCl3.117 The k values were determined by line-shape analyses118 of the 1H NMR signals for the inner NH protons and the values determined at various temperatures were used to provide the corresponding Eyring plots.

Determination of a Dimerization Constant of 3. Concentration of 3 was changed in CDCl3, 50%C6D6/CDCl3, or C6D6 at 298 K and the 1H NMR spectral changes at an appropriate chemical shifts was fitted with eq 1.

δobs =

(δmonomer – δdimer)(1 + √1 + 8K[3] 4K[3]

+ δdimer

(1)

Here, δobs, δmonomer, δdimer, K, and [3] refer to the observed chemical shift, the chemical shift of the monomer of 3, the chemical shift of the dimer of 3, the dimerization constant, and the concentration of 3, respectively.

Computational Method. We optimized local minima and saddle points on potential energy surfaces using the B3LYP functional119,120 combined with the 6-31G(d,p) basis set.121,122 After geometry optimizations, we performed vibrational analyses to confirm that an optimized geometry


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corresponds to a local minimum that has no imaginary frequency or a transition state that has only one imaginary frequency. An appropriate connection between a reactant and a product was confirmed by quasi-IRC calculations. In the quasi-IRC calculation, the geometry of a transition state was at first shifted by perturbing the geometries very slightly along the reaction coordinate and released for equilibrium optimization. This approach provides qualitatively identical results with IRC calculations at considerably lower computational cost. The B3LYP method has been reported to provide excellent descriptions of various reaction profiles, particularly in geometries, heats of reaction, barrier heights, and vibrational analyses.123 The Gaussian 09 program package124 was used for all DFT calculations.

Results Synthesis and Characterization. For exploration of the NH tautomerism of a freebase QFP, a substituent was introduced at one of the β-positions of a non-fused pyrrole to lower the structural symmetry, since the structure of 1-R is in a D2h symmetry to make the two inner NH protons identical in the 1H NMR spectrum and the NH tautomerism cannot be clarified. To increase the solubility in common organic solvents, a tert-butyl derivative of ZnIIQFP, 2,111 was employed as a starting material. Bromination of one of the β-positions of the non-fused pyrroles in 2 was performed with N-bromosuccinimide (NBS) in CHCl3 (Scheme 2). When 1.9 equiv of NBS was used to brominate 2, a mixture of monobrominated (Br) and dibrominated (Br2) derivatives was obtained, together with unreacted 2. The ratio of the three species was estimated to be Br : Br2 : 2 = 7 : 1 : 2, on the basis of the GPC (gel permeation chromatography)-HPLC analysis. Without


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Scheme 2. Synthesis of Mesityl-QFP, 3.

Figure 1. 1H NMR spectrum of 3 (1.1 × 10–3 M) in CDCl3 at 298 K.


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isolation of the monobrominated species, a Suzuki-cross-coupling reaction was performed with use of mesitylboronic acid in the presence of Pd(PPh3)4 as a catalyst and K2CO3 as a base in dimethylacetamide (DMA). The reaction mixture was separated by column chromatography on silica gel and subsequent demetallation with trifluoroacetic acid (TFA) in CHCl3 gave the target mono-mesityl derivative, 3. The 1H NMR spectrum of 3 in CDCl3 exhibited a larger number of independent signals in comparison to that of 1-t-Bu, due to the lower symmetry of 3 (Figure 1). As a feature of the 1H NMR spectrum of 3, the fused meso-aryl groups nearest to the mesityl group showed the signals in a relatively up-field region due to the ring-current effect of the mesityl group; i.e. the signal for the o-C-H remaining after the ring fusion appeared at 5.20 ppm as a doublet and that for the m-C-H adjacent to the o-carbon was observed at 6.19 ppm as a double-doublet. The 1H NMR signal of the inner NHs was observed at 6.02 ppm as a broad singlet at 298 K. Crystal Structure. Introduction of the mesityl group at one of the β-positions of the non-fused pyrroles in 3 was explicitly confirmed by the X-ray diffraction analysis. In the triclinic lattice with the space group of P1, two independent molecules of 3 were included in the unit cell. ORTEP drawings of one of the independent molecules are shown in Figure 2. Judging from the pyrrolic Cα-N-Cα bond angles (Table S1), the inner NHs should be located at the non-fused pyrroles, same as the case of 1-t-Bu.113 The mesityl group was almost perpendicular to the porphyrin plane; the dihedral angle between the mean plane of the porphyrin core consisting of 48 atoms and the benzene ring of the mesityl group was estimated to be 82°. One of the fused meso-aryl groups, nearest to the mesityl group, formed edge-to-face π-π interaction with the


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(a)

(b) C56

N1

N2

N4 N3

Figure 2. Top (a) and side views (b) for ORTEP drawings of 3. Thermal ellipsoids are depicted at the 40% probability level. t-Bu groups are omitted for clarity in (b).

mesityl group and the distance of one of the o-carbon (C56) from the benzene ring of the mesityl group was 3.19 Å. Due to the overlapping, the o-C-H proton exhibited a large upfield shift in the 1

H NMR spectrum (See above). Reflecting the expanded π-conjugated plane of 3, two molecules of 3 formed a π-π

stacking dimer in the crystal packing with a head-to-tail manner to avoid steric repulsion caused by the mesityl group (Figure S1) and the interplane distance in the dimer was estimated to be 3.43 Å. This π-stacked dimer can be observed even in solution. Upon increasing the concentration of 3 in CDCl3 from 4.89 × 10–4 to 1.71 × 10–3 M at 298 K, the 1H NMR signals of the inner NH protons and the β-protons of N3-pyrrole showed upfield shifts from 6.1 to 6.0 ppm


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and from 7.8 to 7.7 ppm, respectively (Figure S2). The upfield shifts by increasing the concentration are probably due to ring-current effects induced by the formation of the π-π stacking dimer. Actually, the signals showing significant shifts are assigned to the proton covered by the other molecule in the dimer as observed in the crystal packing. Based on the shifts of the signals for the β-proton by increasing the concentration, the equilibrium constant of the dimerization of 3 (K) was determined to be 46 ± 6 M–1 in CDCl3 at 298 K (Figure S3a). In less polar solvents such as 50% C6D6/CDCl3 and 100% C6D6, similar shifts of the 1H NMR signals of 3 were observed upon increasing the concentration of 3 due to formation of the π-π stacked dimer, and the K values were also determined at 298 K as (1.0 ± 0.1) × 102 M–1 in 50% C6D6/CDCl3 and (1.6 ± 0.1) × 102 M–1 in C6D6 (Figure S3b and c). On the other hand, in more polar mixed solvents such as 10% CD3CN/CDCl3 and 25% (CD3)2CO/CDCl3, which were used for the studies on the NH tautomerism of 3 (see below), the shifts of the 1H NMR signals upon increasing the concentration of 3 were too small, and thus, the dimerization constants could not be determined properly by 1H NMR spectroscopy. Based on the observations that the K values increase in accordance with lowering the polarity of the solvents, it is evident that the dimerization of 3 is promoted in a less polar solvent. In general, π-π interaction mainly induced by solvophobic effect is promoted in a more polar solvent;125-127 however, π-π stacking mainly driven by dipole-dipole interaction is strengthened in a less polar solvent.128,129 In fact, the DFTcalculated dipole moment of 3 is large (0.798 debye, Figure S4) and the dimer of 3 in the crystal showed a head-to-tail structure, matching the dipole of one molecule to the opposite direction of the dipole of the other molecule (Figure S1). Thus, we can conclude that the π-π stacking observed for 3 should be derived from the dipole-dipole interaction, rather than solvophobic effects.


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1

H NMR Studies on the NH Tautomerism of 3. As mentioned above, the 1H NMR signal of

the inner NH protons for 3 was observed as a broad singlet in CDCl3 at 298 K, indicating that the exchange process of the two inner NH protons (i.e. NH tautomerism) is too fast for the two protons to be distinguished in the NMR timescale. Thus, to observe the NH tautomeric behavior of 3, the 1H NMR spectra of 3 were measured at lower temperatures (Figure 3). At 293 K, the signal of the inner NH protons was observed as a slightly split signal, which indicates that motion of the two protons involved in the NH tautomerism is slow enough to distinguish them in

Figure 3. Experimental (left, black) and simulated 1H NMR spectra (right, red) of 3 (1.1 mM) in CDCl3.


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the NMR timescale; the signals of the two inner NH protons appeared at different chemical shifts, reflecting the different magnetic circumstances. Upon cooling the sample below 293 K, the split of the signal became evident.130 Line-shape analysis118 of the 1H NMR signals was adopted to obtain rate constants of the inner-NH exchange at various temperatures (Figure 3). Based on the rate constants, an Eyring plot for the NH tautomerism of 3 was provided (Figure S5) to obtain the activation parameters (Table 1). On the basis of the Eyring plot, the activation enthalpy, ∆H‡, and activation entropy, ∆S‡, were determined to be 95 kJ mol–1 and 89 J mol–1 K– 1

, respectively. In comparison with the activation parameters for the NH tautomerism of

tetraphenylporphyrin (H2TPP) in CDCl3 (∆H‡ = 38.6 kJ mol–1 and ∆S‡ = –42 J mol–1 K–1),16 the ∆H‡ value for 3 was 2.5 times larger, indicating that the proton transfer reaction in 3 is energetically hard to occur relative to H2TPP, despite the fact that the inter-nitrogen distances are shorter in 3 than those in H2TPP (see Table S1). The large ∆H‡ for 3 is probably derived from the steric repulsion between the two protons in an intermediate formed in the course of NH t a u t o m e r i s m ,

w h i c h

h a s

a n

i n n e r

N H

Table 1. Activation Parameters of the NH Tautomerism of 3 in Various Solvents 50% C6D6 d

CDCl3

10% CD3CN d

25% (CD3)2CO d

3.7

4.9

8.0

9.0

∆G‡298 b

79 1

68 3

70 2

75 2

∆H‡ b

121 1

95 2

52 1

50 1

∆S‡ c

144 1

89 2

–64 1

–83 2

solvent a


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Static permittivity (ε) was calculated with the following equation: ε = x1ε1 + x2ε2; xn: mole fraction of solvent n, εn: permittivity of solvent n. b In kJ mol–1. c In J mol–1 K–1. d The symbol indicates the percentage of the solvent in a mixed solvent with CDCl3. a

‡ N ‡ N

H

N H N

N HH N N

N Mes

Mes

N

H

N H N

N

Mes

tautomer A

N Entropy Gain

N H N H Mes N N Mes N H H N N N

N HH N N Mes

cis-form

N N HH N N Mes

tautomer B

dimer

Figure 4. A proposed energy diagram of the NH tautomerism of 3.

proton at a non-fused pyrrole and the other at a fused pyrrole. It should be noted that the ∆S‡ value was large positive for 3, whereas that for H2TPP was negative.16 The large positive ∆S‡ value for the NH tautomerism of 3 in CDCl3 should derive from dissociation of the π-stacked dimer structure observed even in the solution (see above). The π-stacked dimer structure observed in the ground state is assumed to dissociate in the transition state to afford the large positive ∆S‡ value (Figure 4). The dissociation is probably caused by the distortion of 3 in the transition state due to the steric repulsion of the inner NH protons (see below).


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Solvent Effects on the NH Tautomerism of 3. Solvent effects on the NH tautomerism of 3 were investigated using four kinds of mixture of deuterated solvents, 50% C6D6/CDCl3, 10%

Figure 5. Plots of ∆G‡298 (a), ∆H‡ (b) and ∆S‡ (c) for the NH tautomerism of 3 against calculated permittivity (ε).

CD3CN/CDCl3, and, 25% (CD3)2CO/CDCl3, by 1H NMR spectroscopy.117,131 The rate constants of the NH tautomerism were determined at various temperatures in each mixed solvent (Figure S6 – S8). Based on the temperature dependence of the rate constants, an Eyring plot for 3 in each mixed solvent was provided (Figure S9); the activation enthalpy and entropy, and the activation free energies at 298 K, ∆H‡, ∆S‡, and ∆G‡298, respectively, were obtained for the NH tautomerism as listed in Table 1. To elucidate the solvent effects on the NH tautomersim, we focused on static permittivity, ε, as a solvent parameter. Since the permittivity values of the four solvents employed in this study are not so different (4.9 for chloroform, 2.4 for benzene, 36 for acetonitrile, and 21 for acetone), we assumed the additivity of the permittivity of each solvent in the mixed solvents.132 Then, the activation parameters, ∆G‡298, ∆H‡ and ∆S‡, were plotted against the calculated ε of the mixed solvent (Figure 5). As a result, ∆H‡ and ∆S‡ exhibited linearity in


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the plots against ε (Figure 5b and c), and upon increasing the solvent polarity, both ∆H‡ and ∆S‡ decreased. In contrast, ∆G‡298 seems independent of the solvent polarity (Figure 5a). The negative correlations of ∆H‡ and ∆S‡ to ε indicate that the transition state of the NH tautomerism of 3 is more strongly solvated in a more polar solvent, and thus, the molecular arrangement of the species involved in the transition state is relatively more polar than the ground state. Especially, the negative correlation of ∆S‡ to ε strongly invokes more organized solvation of the species in the transition state in a more polar solvent. Therefore, the solvation to cause both the entropy loss and the enthalpy gain should afford the independency of ∆G‡ on the solvent polarity.133,134 In addition, the ∆S‡ values are largely positive in a less polar solvent (50% C6D6/CDCl3 and CDCl3), which can be explained by dissociation of the π-stacked dimer in the transition state (see below). Supporting this, the solvents, CDCl3 and 50% C6D6/CDCl3, affording a large dimerization constant (K) (see above), indicate a large positive ∆S‡ value for the NH tautomerism of 3 (Table 1). In contrast, in the two polar solvents, 10% CD3CN/CDCl3 and 25% (CD3)2CO/CDCl3, the dimerization constants (K) of 3 could not be determined due to the small shifts of the 1H NMR signals (see above). In addition, the ∆S‡ values for the NH tautomerism of 3 were also negative in these two polar solvents.

Discussion Two kinds of reaction mechanisms have been proposed for the NH tautomerism of porphyrins: stepwise mechanism,23,55 which involves an asynchronous transfer of the two N-H protons through a cis-type tautomer as the intermediate, and concerted one,17 in which the two N-H protons transfer synchronously. Long-term debates have been devoted to answering the


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question; which is the mechanism plausible in the NH tautomerism on the basis of experiments and theoretical studies. The stepwise mechanism is now generally accepted to be the true mechanism for the NH tautomersim of porphyrins.59 Judging from the crystal structure of 3 (see above), the most stable NH tautomer of 3 should be tautomer A, having the two inner NH protons on the two non-fused pyrroles as shown in Figure 4. The NH tautomerism of 3 observed in the 1H NMR studies described above is a position-exchanging reaction of the two inner NH protons (HA and HB in Scheme 1). In the NH tautomerism of 3, the stepwise mechanism can be expected; as the first step, one of the two inner NH protons moves to one the adjacent fused pyrroles to give the cis-type tautomer. Then, the other inner NH proton moves to the other fused pyrrole to give the other trans-type NH tautomer (tautomer B in Figure 4). From tautomer B, the proton migration proceeds again to form tautomer A again, in which the two inner NH protons exchange the positions each other. Optimized structures and relative energies of the intermediate and the two transition states of the NH tautomerism described above were calculated for 1-t-Bu not for 3 to reduce the calculation costs for DFT calculations at the B3LYP/6-31G** level of theory (Figure 6). The first step of the NH tautomerism is proton transfer from an N atom of the fused pyrroles to an inner N atom of the non-fused pyrroles. TS1 is a transition state, in which proton transfer occurs to connect tautomer A with the cis-form in an imaginary frequency mode of 1533i cm–1. At TS1, the vibrational mode associated with the imaginary frequency is the reaction coordinate corresponding to the proton transfer to form the cis-form, and verification of the atomic motions indicates that it is the “correct” TS. Moreover, we performed the quasi-IRC calculations to obtain a minimum energy path from tautomer A to the cis-form through TS1. The activation energy was calculated to be 84 kJ mol–1 (Figure 6a) and the N-H distances of the breaking and forming N-H


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bonds were computed reasonably well to be 1.410 and 1.255 Å, respectively (Figure 6b). The distances between the two NH protons were calculated to be 2.540 Å in tautomer A and 2.335 Å

‡

(a)

‡

TS2 +105 kJ mol –1

TS1 +84 kJ mol –1

tautomer B +52 kJ mol –1

cis-form +47 kJ mol –1

tautomer A

(b) 4 2.

1.2 55

61

1.025 1. 87 1 2.335 2.5

1.373 1.2 8 91 1.

67

1.017

1.410

TS1

73

1.023

2.540 2.

13

1.025

7

61

2.022

TS2

1.019 1. 91 9

1 .8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.409

1.005

2.432 1.761

1.005

2.103 1.023

Tautmer A

cis-form

Tautmer B

Figure 6. (a) Optimized structures and calculated energies obtained by DFT calculations on the transition states and intermediates in the NH tautomerism of 1-t-Bu and (b) selected interatomic distances in the porphyrin cores. The peripheral t-Bu groups are omitted for clarity in (a). To


19


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focus on the H and N atoms of the porphyrin ring, other atoms are omitted in (b). Units are in angstrom in (b). in TS1, whereas the distance in the cis-form is shortened to be 1.873 Å (Figure 6b). Thus, the proton transfer results in increase of the steric repulsion between the two NH protons. The geometrical changes in the distances between NH protons are completely consistent with the computed relative energy of the cis-form as 47 kJ mol–1 against tautomer A. The proton transfer gives a structure with lower symmetry compared to tautomer A and distorted N-H bonds due to formation of new hydrogen bonds and also due to the increased steric repulsion between the two NH protons. In the next step, the cis-form undergoes another proton transfer to form tautomer B through the transition state, TS2, in which the proton transfer occurs to connect cis-form with tautomer B in an imaginary frequency mode of 1543i cm–1. At TS2, we confirmed atomic motions of the imaginary frequency mode and also performed the quasi-IRC calculations. The NH distances of the breaking and forming N-H bonds were computed to be 1.373 and 1.255 Å, respectively. The distances between the two NH protons were calculated to be 1.918 Å in TS2, whereas 1.761 Å in the cis-form. Thus, steric repulsion between the two NH protons remains unchanged in the course of the second step. The activation energy for TS2 is computed to be 105 kJ mol–1. The reason why TS2 lies above TS1 is explained by the steric repulsion: Tautomer B is less stable than the cis-form due to the lack of hydrogen bonds and also due to the severe steric repulsion between the two inner NH protons. DFT calculations suggest that the steric repulsion of the two inner NH protons in the cisintermediate results in distortion of the core structure of 3 from the original rhombic structure of tautomer A (see Table S1)110-113 into a parallelogram-shaped structure (Figure 6). Additionally, calculated dipole moments in debye are 0.075 for tautomer A, 0.019 for tautomer B, 1.245 for


20

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the cis-form, 0.628 for TS1, and 0.622 for TS2 (Figure S10). Therefore, the TS states bear more polar characters than tautomer A, which is fully consistent with the stronger solvation to lower values of both ∆H‡ and ∆S‡ in more polar solvents as mentioned above (Figure 5). In summary, the π-π stacked dimer of tautomer A formed in a less-polar solvent such as C6D6 dissociates in the initial step of the NH tautomerism, which causes the large positive ∆S‡. And then, the inner NH tautomerism is accomplished with passing through unstable and more polar transition states. The low stability of the transition states brings the large positive ∆H‡ for the NH tautomerism of 3, compared to that for the NH tautomerism of H2TPP.16

Conclusion We have synthesized and characterized a freebase derivative of a symmetry-broken H2QFP, 3, having one mesityl group at the one of the β-positions of non-fused pyrroles. The NH tautomerism of 3 was kinetically investigated to determine the activation parameters. The kinetic analysis indicated that the ∆G‡298 value for the NH tautomerism of 3 was larger than that of H2TPP due to severer steric congestion of the NH protons in the deformed QFP core. Based on the kinetic and thermodynamic analysis, we conclude that the NH tautomerism in 3 proceeds through dissociation of a π-stacked dimer of 3 formed by dipole-dipole interaction and subsequent polar transition states bearing larger dipole moments than that of 3 in the ground state. The large positive ∆S‡ values, observed for the NH tautomerism of 3 in the less polar solvents, are very unique for the porphyrin NH tautomerism and should be derived from the dissociation of the dimeric structure into the monomeric transition state. Finally, the present


21


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study provides a support for a generally accepted stepwise mechanism of NH tautomerism of the porphyrin core.

ASSOCIATED CONTENT Supporting Information. Experimental and computational details, crystal packing, 1H NMR spectra, Eyring plots and complete author list (PDF). Crystallographic data for 3 (CIF). This information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Yuta Saegusa: 0000-0003-3365-2462 Tomoya Ishizuka: 0000-0002-3897-026X Yoshihito Shiota: 0000-0003-2054-9845 Kazunari Yoshizawa: 0000-0002-6279-9722 Takahiko Kojima: 0000-0001-9941-8375


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Grants-in-Aid (Nos. 25410033, 15K13710, 16K05739, 17H03117, 24109014, and 24245011) and a Grant-in-Aid for JSPS Fellows (16J05288 to Y. Saegusa) from Japan Society for the Promotion of Science (JSPS). T. I. also appreciates financial support from The Tokuyama Science Foundation.

REFERENCES (1) The Hydrogen Bond; Schuster, P.; Zundel, G.; Sandorfy, C. Eds.; North-Holland Publ. Co.: Amsterdam, 1976. (2) Kovács, A.; Szabó, A.; Hargittai, I. Structural Characteristics of Intramolecular Hydrogen Bonding in Benzene Derivatives. Acc. Chem. Res. 2002, 35, 887. (3) Itoh, M.; Fujiwara, Y. Transient Absorption and Two-Step Laser Excitation Fluorescence Studies

of

Photoisomerization

in

2-(2-Hydroxyphenyl)benzoxazole

and

2-(2-

Hydroxyphenyl)benzothiazole. J. Am. Chem. Soc. 1985, 107, 1561. (4) Chan-Huot, M.; Dos, A.; Zander, R.; Sharif, S.; Tolstoy, P. M.; Compton, S.; Fogle, E.; Toney, M. D.; Shenderovich, I.; Denisov, G. S., et al. NMR Studies of Protonation and Hydrogen Bond States of Internal Aldimines of Pyridoxal 5’-Phosphate Acid-Base in


23


Page 24 of 52

Alanine Racemase, Aspartate Aminotransferase, and Poly-l-lysine. J. Am. Chem. Soc. 2013, 135, 18160. (5) Monkman, A. P.; Pålsson, L.-O.; Higgins, R. W. T.; Wang, C.; Bryce, M. R.; Batsanov, A. S.; Howard, J. A. K. Protonation and Subsequent Intramolecular Hydrogen Bonding as a Method to Control Chain Structure and Tune Luminescence in Heteroatomic Conjugated Polymers. J. Am. Chem. Soc. 2002, 124, 6049. (6) Somma, M. S.; Medforth, C. J.; Nelson, N. Y.; Olmstead, M. M.; Khoury, R. G.; Smith, K. M. Evidence for Unusually Strong Intramolecular Hydrogen Bonding in Highly Nonplanar Porphyrins. Chem. Commun. 1999, 1221. (7) Vogel, E.; Köcher, M.; Schmickler, H.; Lex, J. Porphycene—a Novel Porphin Isomer. Angew. Chem., Int. Ed. Engl. 1986, 25, 257. (8) Bhyrappa, P.; Arunkumar, C.; Varghese, B. Influence of Mixed Substituents on the Macrocyclic Ring Distortions of Free Base Porphyrins and Their Metal Complexes. Inorg. Chem. 2009, 48, 3954. (9) Obi, M.; Sakuragi, H.; Arai, T. Adiabatic Intramolecular Hydrogen Atom Transfer in 2-[2(2-Pyrrolyl)ethenyl]pyridine. Chem. Lett. 1998, 27, 169. (10)

Park, S.; Kwon, O.-H.; Kim, S.; Park, S.; Choi, M.-G.; Cha, M.; Park, S. Y.; Jang, D.-J.

Imidazole-Based Excited-State Intramolecular Proton-Transfer Materials: Synthesis and Amplified Spontaneous Emission from a Large Single Crystal. J. Am. Chem. Soc. 2005, 127, 10070.


24

Page 25 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11)

Sharif, S.; Denisov, G. S.; Toney, M. D.; Limbach, H.-H. NMR Studies of Solvent-

Assisted Proton Transfer in a Biologically Relevant Schiff Base: Toward a Distinction of Geometric and Equilibrium H-Bond Isotope Effects. J. Am. Chem. Soc. 2006, 128, 3375. (12)

Shenderovich, I. G.; Tolstoy, P. M.; Golubev, N. S.; Smirnov, S. N.; Denisov, G. S.;

Limbach, H. H. Low-Temperature NMR Studies of the Structure and Dynamics of a Novel Series of Acid-Base Complexes of HF with Collidine Exhibiting Scalar Couplings Across Hydrogen Bonds. J. Am. Chem. Soc. 2003, 125, 11710. (13)

Benedict, H.; Limbach, H.-H.; Wehlan, M.; Fehlhammer, W. P.; Golubev, N. S.;

Janoschek, R. Solid State

15

N NMR and Theoretical Studies of Primary and Secondary

Geometric H/D Isotope Effects on Low-Barrier NHN-Hydrogen Bonds. J. Am. Chem. Soc. 1998, 120, 2939. (14)

Medforth, C. J. The Porphyrin Handbook, Vol. 5, Kadish, K. M., Smith, K. M., Guilard,

R., Eds.; Academic Press: San Diego, 2000; Chap. 35, pp. 61-65 and references therein. (15)

J. Waluk, Ground- and Excited-State Tautomerism in Porphycenes. Acc. Chem. Res.

2006, 39, 945. (16)

Abraham, R. J.; Hawkes, G. E.; Smith, K. M. N-H Tautomerism in Porphyrins: An NMR

Study. Tetrahedron Lett. 1974, 15, 1483. (17)

Eaton, S. S.; Eaton, G. R. Kinetic Isotope Effect on Proton Tautomerism in

Tetraarylporphyrins. J. Am. Chem. Soc. 1977, 99, 1601.


25


(18)

Page 26 of 52

Butcher, R. J.; Jameson, G. B.; Storm, C. B. Crystal Structure of meso-

Tetratolylporphyrin: Implications for the Solid-State 15N CPMAS NMR. J. Am. Chem. Soc. 1985, 107, 2978. (19)

Seidel, R. W.; Goddard, R.; Hoch, C.; Breidung, J.; Oppel, I. M. On the Structure of

Unsolvated Free-Base 5,10,15,20-Tetra(3-pyridyl)porphyrin. J. Mol. Struct. 2011, 985, 307. (20)

Hennig, J.; Limbach, H.-H. Localization and Transfer of Protons between Nitrogen-15

Atoms of meso-Tetraphenylporphine Probed by Nuclear Overhauser Effects and DipoleDipole Relaxation Times. J. Am. Chem. Soc. 1984, 106, 292. (21)

Limbach, H.-H.; Hennig, J.; Kendrick, R.; Yannoni, C. S. Proton-Transfer Kinetics in

Solids: Tautomerism in Free Base Porphines by 15N CPMAS NMR. J. Am. Chem. Soc. 1984, 106, 4059. (22)

Braun, J.; Schlabach, M.; Wehrle, B.; Köcher, M.; Vogel, E.; Limbach, H.-H. NMR

Study of the Tautomerism of Porphyrin Including the Kinetic HH/HD/DD Isotope Effects in the Liquid and the Solid State. J. Am. Chem. Soc. 1994, 116, 6593. (23)

Braun, J.; Limbach, H.-H.; Williams, P. G.; Morimoto, H.; Wemmer, D. E. Observation

of Kinetic Tritium Isotope Effects by Dynamic NMR. The Tautomerism of Porphyrin. J. Am. Chem. Soc. 1996, 118, 7231. (24)

Braun, J.; Schwesinger, R.; Williams, P. G.; Morimoto, H.; Wemmer, D. E.; Limbach,

H.-H. Kinetic H/D/T Isotope and Solid State Effects on the Tautomerism of the Conjugate Porphyrin Monoanion. J. Am. Chem. Soc. 1996, 118, 11101.


26

Page 27 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25)

Schlbach, M.; Wehrle, B.; Limbach, H.-H.; Bunnenberg, E.; Knierzinger, A.; Shu, A. Y.

L.; Tolf, B. R.; Djerassi, C. Evidence of Tautomerism in a Triply

15

N Labeled

Monoacetylporphyrin by NMR: Kinetic HH/HD Isotope Effects and Thermodynamics in CD2Cl2. J. Am. Chem. Soc. 1986, 108, 3856. (26)

Schlabach, M.; Rumpel, H.; Limbach, H.-H. Investigation of the Tautomerism of

15

N-

Labeled Hydroporphyrins by Dynamic NMR Spectroscopy. Angew. Chem., Int. Ed. Engl. 1989, 28, 76. (27)

Schlabach, M.; Scherer, G.; Limbach, H.-H. Kinetic HH/HD/DH/DD Isotope Effects on

Nondegenerate Stepwise Reversible Double Proton Transfer Reactions. NMR Study of the Tautomerism of meso-Tetraphenylchlorin. J. Am. Chem. Soc. 1991, 113, 3550. (28)

Schlbach, M.; Limbach, H.-H.; Bunenberg, E.; Shu, A. Y. L.; Tolf, B. R.; Djerassi, C.

NMR Study of Kinetic HH/HD/DH/DD Isotope Effects on the Tautomerism of Acetylporphyrin: Evidence for a Stepwise Double Proton Transfer. J. Am. Chem. Soc. 1993, 115, 4554. (29)

Ghosh, A.; Almliif, J. Structure and Stability of cis-Porphyrin. J. Phys. Chem. 1995, 99,

1073. (30)

Thomas, K. E.; McCormick, L. J.; Vazquez-Lima, H.; Ghosh, A. Stabilization and

Structure of the cis Tautomer of a Free-Base Porphyrin. Angew. Chem., Int. Ed. 2017, 56, 10088. (31)

Crossley, M. J.; Harding, M. M.; Sternhell, S. Tautomerism in 2-Substituted 5,10,15,20-

Tetraphenylporphyrins. J. Am. Chem. Soc. 1986, 108, 3608.


27


(32)

Page 28 of 52

Crossley, M. J.; Field, L. D.; Harding, M. M.; Sternhell, S. Kinetics of Tautomerism in 2-

Substituted 5,10,15,20-Tetraphenylporphyrins: Directionality of Proton Transfer between the Inner Nitrogens. J. Am. Chem. Soc. 1987, 109, 2335. (33)

Wacker, P.; Dahms, K.; Senge, M. O.; Kleinpeter, E. Influence of the Core Conformation

on the NH-Tautomerism in Porphyrins: A Study of meso-(1,3-Dithian-2-yl)porphyrins. J. Org. Chem. 2008, 73, 2182. (34)

Ermilov, E. A.; Büge, B.; Jasinski, S.; Jux, N.; Röder, B. Spectroscopic Study of NH-

Tautomerism in Novel Cycloketo-Tetraphenylporphyrins. J. Chem. Phys. 2009, 130, 134509. (35)

Tohara, A.; Sato, M. Inner-Hydrogen Tautomerism in Some 5,15-Unsymmetrically

Disubstituted and β-Unsubstituted Porphyrins. Chem. Pharm. Bull. 2008, 56, 1041. (36)

Urbani, M.; Torres, T. Tautomerism and Atropisomerism in Free-Base (meso)-Strapped

Porphyrins: Static and Dynamic Aspects. Chem.–Eur. J. 2014, 20, 16337. (37)

García-Ortega, H.; Crusats, J.; Feliz, M.; Ribo, J. M. Modulation of the Porphyrin Inner

Proton Exchange Rates by the Steric Effects of Bridge Substitution. J. Org. Chem. 2002, 67, 4170. (38)

Nakamura, Y.; Aratani, N.; Furukawa, K.; Osuka, A. Synthesis and Characterizations of

Free Base and Cu(II) Complex of a Porphyrin Sheet. Tetrahedron 2008, 64, 11433. (39)

Crossley, M. J.; Harding, M. M.; Sternhell, S. Direct Observation of Tautomeric Forms

of Deuteroporphyrin Derivatives by 1H NMR Spectroscopy: Substituent Effects and Structure Implications. J. Org. Chem. 1992, 57, 1833.


28

Page 29 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40)

Helaja, J.; Montforts, F.-P.; Kilpeläinen, I.; Hynninen, P. H. NH Tautomerism in the

Dimethyl Ester of Bonellin, a Natural Chlorin. J. Org. Chem. 1999, 64, 432. (41)

Helaja, J.; Stapelbroek-Möllmann, M.; Kilpeläinen, I.; Hynninen, P. H. J. Org. Chem.

2000, 65, 3700. (42)

Meier, B. H.; Storm, C. B.; Earl, W. L. Two-Dimensional Chemical Exchange NMR in

the Solid: Proton Dynamics in Phthalocyanine. J. Am. Chem. Soc. 1986, 108, 6072. (43)

Wehrle, B.; Limbach, H.-H.; Köchner, M.; Ermer, O.; Vogel, E.

15

N-CPMAS-NMR

Study of the Problem of NH Tautomerism in Crystalline Porphine and Porphycene. Angew. Chem., Int. Ed. Engl. 1987, 26, 934. (44) 15

Frydman, B.; Fernandez, C. O.; Vogel, E. Variable-Temperature Solid-State

13

C- and

N-CPMAS NMR Analyses of Alkyl-Substituted Porphycenes. J. Org. Chem. 1998, 63,

9385. (45)

Langer, U.; Hoelger, C.; Wehrle, B.; Latanowicz, L.; Vogel, E.; Limbach, H.-H.

15

N

NMR Study of Proton Localization and Proton Transfer Thermodynamics and Kinetics in Polycrystalline Porphycene. J. Phys. Org. Chem. 2000, 13, 23. (46)

Gil, M.; Dobkowski, J.; Wiosna-Sałyga, G.; Urbanska, N.; Fita, P.; Radzewicz, C.;

Pietraszkiewicz, M.; Borowicz, P.; Marks, D.; Glasbeek, M., et al. Unusual, Solvent Viscosity-Controlled Tautomerism and Photophysics: Meso-Alkylated Porphycenes. J. Am. Chem. Soc. 2010, 132, 13472.


29


(47)

Page 30 of 52

Ciącka, P.; Fita, P.; Listkowski, A.; Kijak, M.; Nonell, S.; Kuzuhara, D.; Yamada, H.;

Radzewicz, C.; Waluk, J. Tautomerism in Porphycenes: Analysis of Rate-Affecting Factors. J. Phys. Chem. B 2015, 119, 2292. (48)

Rachlewicz, K.; Latos-Grażyński, L.; Gebauer, A.; Vivian, A.; Sessler, J. L. NH

Tautomerization of 2,7,18,23-Tetramethyl-3,8,12,13,17,22-Hexaethylsapphyrin. J. Chem. Soc., Perkin Trans. 2 1999, 2189. (49)

Furuta, H.; Ishizuka, T.; Osuka, A.; Dejima, H.; Nakagawa, H.; Ishikawa, Y. NH

Tautomerism of N-Confused Porphyrin. J. Am. Chem. Soc. 2001, 123, 6207. (50)

Ostapko, J.; Nawara, K.; Kijak, M.; Buczynska, J.; Lesniewska, B.; Pietrzak, M.;

Orzanowska, G.; Waluk, J. Parent, Unsubstituted Hemiporphycene: Synthesis and Properties. Chem.–Eur. J. 2016, 22, 17311. (51)

Smedarchina, Z.; Zerbetto, F. Comparison of Synchronous and Asynchronous Hydrogen

Transfer Mechanisms in Free-Base Porphyrins. Chem. Phys. 1989, 136, 285. (52)

Smedarchina, Z.; Zgierski, M. Z.; Siebrand, W.; Kozlowski, P. M. Dynamics of

Tautomerism in Porphine: An Instanton Approach. J. Chem. Phys. 1998, 109, 1014. (53)

Boronat, M.; Ortí, E.; Viruela, P. M.; Tomás, F. New Hydrogen-Bonded Molecular

Crystals with Nonlinear Second-Order Optical Properties. J. Mol. Struct. 1997, 390, 149. (54)

Sarai, A. Conformation-Coupled Proton Migration in Tetraphenylporphin. Chem. Phys.

Lett. 1981, 83, 50.


30

Page 31 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(55)

Sarai, A. Dynamics of Proton Migration in Free Base Porphines. J. Chem. Phys. 1982,

76, 5554. (56)

Ghosh, A.; Almlöf, J. The Ultraviolet Photoelectron Spectrum of Free-Base Porphyrin

Revisited. The Performance of Local Density Functional Theory. Chem. Phys. Lett. 1993, 213, 519. (57)

Reimers, J. R.; Lü, T. X.; Crossley, M. J.; Hash, N. S. The Mechanism of Inner-

Hydrogen Migration in Free Base Porphyrin: Ab Initio MP2 Calculations. J. Am. Chem. Soc. 1995, 117, 2855. (58)

Maity, D. K.; Bell, R. L.; Truong, T. N. Mechanism and Quantum Mechanical Tunneling

Effects on Inner Hydrogen Atom Transfer in Free Base Porphyrin: A Direct ab Initio Dynamics Study. J. Am. Chem. Soc. 2000, 122, 897. (59)

Maity, D. K.; Bell, R. L.; Truong, T. N. Mechanism and Quantum Mechanical Tunneling

Effects on Inner Hydrogen Atom Transfer in Free Base Porphyrin: A Direct ab Initio Dynamics Study. J. Am. Chem. Soc. 2000, 122, 897. (60)

Cybulski, H.; Pecul, M.; Helgaker, T.; Jaszuñski, M. Theoretical Studies of Nuclear

Magnetic Resonance Parameters for the Proton-Exchange Pathways in Porphyrin and Porphycene. J. Phys. Chem. A 2005, 109, 4162. (61)

Lu, X.; He, Y.; Chen, J.; Wang, J.; Shi, H. Theoretical Study of the Inner Hydrogen

Migration in the β-Substituted 5,10,15,20-Tetraphenylporphyrins. J. Phys. Chem. A 2010, 114, 12731.


31


(62)

Page 32 of 52

Liljeroth, P.; Repp, J.; Meyer, G. Current-Induced Hydrogen Tautomerization and

Conductance Switching of Naphthalocyanine Molecules. Science 2007, 317, 1203. (63)

Auwärter, W.; Seufert, K.; Bischoff, F.; Ecija, D.; Vijayaraghavan, S.; Joshi, S.;

Klappenberger, F.; Samudrala, N.; Barth, J. V. A Surface-Anchored Molecular Four-Level Conductance Switch Based on Single Proton Transfer. Nat. Nanotech. 2012, 7, 41. (64)

Kumagai, T.; Hanke, F.; Gawinkowski, S.; Sharp, J.; Kotsis, K.; Waluk, J.; Persson, M.;

Grill, L. Thermally and Vibrationally Induced Tautomerization of Single Porphycene Molecules on a Cu(110) Surface. Phys. Rev. Lett. 2013, 111, 246101. (65)

Kumagai, T.; Hanke, F.; Gawinkowski, S.; Sharp, J.; Kotsis, K.; Waluk, J.; Persson, M.;

Grill, L. Controlling Intramolecular Hydrogen Transfer in a Porphycene Molecule with Single Atoms or Molecules Located Nearby. Nat. Chem. 2014, 6, 41. (66)

Ladenthin, J. N.; Frederiksen, T.; Persson, M.; Sharp, J. C.; Gawinkowski, S.; Waluk, J.;

Kumagai, T. Force-Induced Tautomerization in a Single Molecule. Nat. Chem. 2016, 8, 935. (67)

Fox, S.; Boyle, R. W. Synthetic Routes to Porphyrins Bearing Fused Rings. Tetrahedron

2006, 62, 10039. (68)

Lewtak, J. P.; Gryko, D. T. Synthesis of π-Extended Porphyrins via Intramolecular

Oxidative Coupling. Chem. Commun. 2012, 48, 10069. (69)

Mori, H.; Tanaka, T.; Osuka, A. Fused Porphyrinoids as Promising Near-Infrared

Absorbing Dyes. J. Mater. Chem. C 2013, 1, 2500.


32

Page 33 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(70)

Stępień, M.; Gońka, E.; Żyła, M.; Sprutta, N. Heterocyclic Nanographenes and Other

Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications. Chem. Rev. 2017, 117, 3479. (71)

Lash, T. D.; Novak, B. H. Tetraphenanthro[9,10-b:9,10-g:9,10-l:9,10-q]-Porphyrin, a

New Highly Conjugated Porphyrin System. Angew. Chem., Int. Ed. Engl. 1995, 34, 683. (72)

Ito, S.; Murashima, T.; Uno, H.; Ono, N. A New Synthesis of Benzoporphyrins Using

4,7-Dihydro-4,7-Ethano-2H-Isoindole as a Synthon of Isoindole. Chem. Commun. 1998, 1661. (73)

Deshpande, R.; Jiang, L.; Schmidt, G.; Rakovan, J.; Wang, X.; Wheeler, K.; Wang, H. A

Concise Approach to the Synthesis of opp-Dibenzoporphyrins through the Heck Reaction. Org. Lett. 2009, 11, 4251. (74)

Jiang, L.; Engle, J. T.; Sirk, L.; Hartley, C. S.; Ziegler, C. J.; Wang, H. Triphenylene-

Fused Porphyrins. Org. Lett. 2011, 13, 3020. (75)

Banala, S.; Rühl, T.; Wurst, K.; Kräutler, B. “Blackening” Porphyrins by Conjugation

with Quinones. Angew. Chem., Int. Ed. 2009, 48, 599. (76)

Aihara, H.; Jaquinod, L.; Nurco, D. J.; Smith, K. M. Multicarbocycle Formation

Mediated by Arenoporphyrin 1,4-Diradicals: Synthesis of Picenoporphyrins. Angew. Chem., Int. Ed. 2001, 40, 3439.


33


(77)

Page 34 of 52

Nath, M.; Huffman, J. C.; Zaleski, J. M. Ambient Temperature Activation of

Haloporphyrinic-Enediynes: Electronic Contributions to Bergman Cycloaromatization. J. Am. Chem. Soc. 2003, 125, 11484. (78)

Nath, M.; Pink, M.; Zaleski, J. M. Controlling Both Ground- and Excited-State Thermal

Barriers to Bergman Cyclization with Alkyne Termini Substitution. J. Am. Chem. Soc. 2005, 127, 478. (79)

Cammidge, A. N.; Scaife, P. J.; Berber, G.; Hughes, D. L. Cofacial Porphyrin-Ferrocene

Dyads and a New Class of Conjugated Porphyrin. Org. Lett. 2005, 7, 3413. (80)

Tanaka, M.; Hayashi, S.; Eu, S.; Umeyama, T.; Matano, Y.; Imahori, H. Novel

Unsymmetrically π-Elongated Porphyrin for Dye-Sensitized TiO2 Cells. Chem. Commun. 2007, 2069. (81)

Jiao, C.; Huang, K.-W.; Guan, Z.; Xu, Q.-H.; Wu, J. N-Annulated Perylene Fused

Porphyrins with Enhanced Near-IR Absorption and Emission. Org. Lett. 2010, 12, 4046. (82)

Gill, H. S.; Harmjanz, M.; Santamaría, J.; Finger, I.; Scott, M. J. Facile Oxidative

Rearrangement of Dispiro-Porphodimethenes to Nonplanar and Sheetlike Porphyrins with Intense Absorptions in the Near-IR Region. Angew. Chem., Int. Ed. 2004, 43, 485. (83)

Kurotobi, K.; Kim, K. S.; Noh, S. B.; Kim, D.; Osuka, A. A Quadruply Azulene-Fused

Porphyrin with Intense Near-IR Absorption and a Large Two-Photon Absorption Cross Section. Angew. Chem., Int. Ed. 2006, 45, 3944.


34

Page 35 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(84)

Jiao, C.; Zhu, L.; Wu, J. BODIPY-Fused Porphyrins as Soluble and Stable Near-IR Dyes.

Chem.–Eur. J. 2011, 17, 6610. (85)

Davis, N. K. S.; Pawlicki, M.; Anderson, H. L. Expanding the Porphyrin π-System by

Fusion with Anthracene. Org. Lett. 2008, 10, 3945. (86)

Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. Bis-Anthracene Fused Porphyrins:

Synthesis, Crystal Structure, and Near-IR Absorption. Org. Lett. 2010, 12, 2124. (87)

Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. A Porphyrin Fused to Four

Anthracenes. J. Am. Chem. Soc. 2011, 133, 30. (88)

Sendt, K.; Johnston, L. A.; Hough, W. A.; Crossley, M. J.; Hush, N. S.; Reimers, J. R.

Switchable Electronic Coupling in Model Oligoporphyrin Molecular Wires Examined through the Measurement and Assignment of Electronic Absorption Spectra. J. Am. Chem. Soc. 2002, 124, 9299. (89)

Paolesse, R.; Jaquinod, L.; Della, F. S.; Nurco, D. J.; Prodi, L.; Montalti, M.; Natale, C.

D.; D’Amico, A.; Carlo, A. D.; Lugli, P., et al. β-Fused Oligoporphyrins: A Novel Approach to a New Type of Extended Aromatic System. J. Am. Chem. Soc. 2000, 122, 11295. (90)

Uno, H.; Masumoto, A.; Ono, N. Hexagonal Columnar Porphyrin Assembly by Unique

Trimeric Complexation of a Porphyrin Dimer with π-π Stacking: Remarkable Thermal Behavior in a Solid. J. Am. Chem. Soc. 2003, 125, 12082. (91)

Akita, M.; Hiroto, S.; Shinokubo, H. Oxidative Annulation of β-Aminoporphyrins into

Pyrazine-Fused Diporphyrins. Angew. Chem., Int. Ed. 2012, 51, 2894.


35


(92)

Page 36 of 52

Tsuda, A.; Nakano, A.; Furuta, H.; Yamochi, H.; Osuka, A. Doubly meso-β-Linked

Diporphyrins from Oxidation of 5,10,15-Triaryl-Substituted NiII- and PdII-Porphyrins. Angew. Chem., Int. Ed. 2000, 39, 558. (93)

Tsuda, A.; Furuta, H.; Osuka, A. Completely Fused Diporphyrins and Triporphyrin.

Angew. Chem., Int. Ed. 2000, 39, 2549. (94)

Tsuda, A.; Furuta, H.; Osuka, A. Syntheses, Structural Characterizations, and Optical and

Electrochemical Properties of Directly Fused Diporphyrins. J. Am. Chem. Soc. 2001, 123, 10304. (95)

Tsuda, A.; Osuka, A. Fully Conjugated Porphyrin Tapes with Electronic Absorption

Bands That Reach into Infrared. Science 2001, 293, 79. (96)

Nakamura, Y.; Aratani, N.; Shinokubo, H.; Takagi, A.; Kawai, T.; Matsumoto, T.; Yoon,

Z. S.; Kim, D. Y.; Ahn, T. K.; Kim, D., et al. A Directly Fused Tetrameric Porphyrin Sheet and Its Anomalous Electronic Properties That Arise from the Planar Cyclooctatetraene Core. J. Am. Chem. Soc. 2006, 128, 4119. (97)

Fox, S.; Boyle, R. W. First Examples of Intramolecular Pd(0) Catalyzed Couplings on

ortho-Iodinated meso-Phenyl Porphyrins. Chem. Commun. 2004, 1322. (98)

Shen, D.-M.; Liu, C.; Chen, Q.-Y. A Novel and Facile Zn-Mediated Intramolecular Five-

Membered Cyclization of β-Tetraarylporphyrin Radicals from β-Bromotetraarylporphyrins. Chem. Commun. 2005, 4982.


36

Page 37 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(99)

Shen, D.-M.; Liu, C.; Chen, Q.-Y. A General and Efficient Palladium-Catalyzed

Intramolecular Cyclization Reaction of β-Brominated Porphyrins. J. Org. Chem. 2006, 71, 6508. (100) Hayashi, S.; Matsubara, Y.; Eu, S.; Hayashi, H.; Umeyama, T.; Matano, Y.; Imahori, H. Fused Five-membered Porphyrin for Dye-sensitized Solar Cells. Chem. Lett. 2008, 37, 846. (101) Lash, T. D.; Smith, B. E.; Melquist, M. J.; Godfrey, B. A. Synthesis of Indenoporphyrins, Highly Modified Porphyrins with Reduced Diatropic Characteristics. J. Org. Chem. 2011, 76, 5335. (102) Pereira, A. M. V. M.; Neves, M. G. P. M. S.; Cavaleiro, J. A. S.; Jeandon, C.; Gisselbrecht, J.-P.; Choua, S.; Ruppert, R. Diporphyrinylamines: Synthesis and Electrochemistry. Org. Lett. 2011, 13, 4742. (103) Mitsushige, Y.; Yamaguchi, S.; Lee, B. S.; Sung, Y. M.; Kuhri, S.; Schierl, C. A.; Guldi, D. M.; Kim, D.; Matsuo, Y. Synthesis of Thieno-Bridged Porphyrins: Changing the Antiaromatic Contribution by the Direction of the Thiophene Ring. J. Am. Chem. Soc. 2012, 134, 16540. (104) Fliegl, H.; Özcan, N.; Mera-Adasme, R.; Pichierri, F.; Jusélius, J.; Sundholm, D. Aromatic Pathways in Thieno-Bridged Porphyrins: Understanding the Influence of the Direction of the Thiophene Ring on the Aromatic Character. Mol. Phys. 2013, 111, 1364. (105) Nakano, A.; Aratani, N.; Furuta, H.; Osuka, A. Chem. Commun. 2001, 1920. (106) Sahoo, A. K.; Mori, S.; Shinokubo, H.; Osuka, A. Angew. Chem., Int. Ed. 2006, 45, 7972.


37


Page 38 of 52

(107) Fukui, N.; Yorimitsu, H.; Lim, J. M.; Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2014, 53, 4395. (108) Fukui, N.; Cha, W.-Y.; Lee, S.; Tokuji, S.; Kim, D.; Yorimitsu, H.; Osuka, A. Oxidative Fusion Reactions of meso-(Diarylamino)porphyrins. Angew. Chem., Int. Ed. 2013, 52, 9728. (109) Ota, K.; Tanaka, T.; Osuka, A. meso-β Dibenzo[a,g]corannulene-Fused Porphyrins. Org. Lett. 2014, 16, 2974. (110) Ishizuka, T.; Saegusa, Y.; Shiota, Y.; Ohtake, K.; Yoshizawa, K.; Kojima, T. MultiplyFused Porphyrins—Effects of Extended π-Conjugation on the Optical and Electrochemical Properties. Chem. Commun. 2013, 49, 5939. (111) Saegusa, Y.; Ishizuka, T.; Komamura, K.; Shimizu, S.; Kotani, H.; Kobayashi, N.; Kojima, T. Ring-fused Porphyrins: Extension of π-Conjugation Significantly Affects the Aromaticity and Optical Properties of the Porphyrin π-Systems and the Lewis Acidity of the Central Metal Ions. Phys. Chem. Chem. Phys. 2015, 17, 15001. (112) Saegusa, Y.; Ishizuka, T.; Kojima, T.; Mori, S.; Kawano, M.; Kojima, T. Supramolecular Interaction of Fullerenes with a Curved π-Surface of a Monomeric Quadruply Ring-Fused Porphyrin. Chem.–Eur. J. 2015, 21, 5302. (113) Saegusa, Y.; Ishizuka, T.; Shiota, Y.; Yoshizawa, K.; Kojima, T. Acid-Base Properties of a Freebase Form of a Quadruply Ring-Fused Porphyrin—Stepwise Protonation Induced by Rigid Ring-Fused Structure. J. Org. Chem. 2017, 82, 322.


38

Page 39 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(114) Leermann, T.; Leroux, F. R.; Colobert, F. Highly Efficient One-Pot Access to Functionalized Arylboronic Acids via Noncryogenic Bromine/Magnesium Exchanges. Org. Lett. 2011, 13, 4479. (115) Sheldrick, G. M. SIR97 and SHELX97, Programs for Crystal Structure Refinement, University of Göttingen, Göttingen (Germany), 1997. (116) Sluis, P. V. D.; Spek, A. L. BYPASS: An Effective Method for the Refinement of Crystal Structures Containing Disordered Solvent Regions. Acta Crystallogr. 1990, A46, 194. (117) Selection of the solvents is highly limited, because the 1H NMR signals of the inner NH protons need to coalesce between the melting and boiling points of the solvent. The four kinds of the mixed solvents used here satisfied the criteria. (118) Gutowsky, H. S.; Holm, C. H. Rate Processes and Nuclear Magnetic Resonance Spectra. II. Hindered Internal Rotation of Amides. J. Chem. Phys. 1956, 25, 1228. (119) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098. (120) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785. (121) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-Consistent Molecular-Orbital Methods. IX. An Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724.


39


Page 40 of 52

(122) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 1972, 56, 2257. (123) Tirado-Rives, J.; Jorgensen, W. L. Performance of B3LYP Density Functional Methods for a Large Set of Organic Molecules. J. Chem. Theory Comput. 2008, 4, 297. (124) Gaussian 09, Revision D.01, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian, Inc., Wallingford CT, 2009. (125) Hunter, C. A.; Sanders, J. K. M. The Nature of π-π Interactions. J. Am. Chem. Soc. 1990, 112, 5525. (126) Hunter, C. A. Quantifying Intermolecular Interactions: Guidelines for the Molecular Recognition Toolbox. Angew. Chem., Int. Ed. 2004, 43, 5310. (127) Biedermann, F.; Nau, W. M.; Schneider, H.-J. The Hydrophobic Effect Revisited–Studies with Supramolecular Complexes Imply High-Energy Water as a Noncovalent Driving Force. Angew. Chem., Int. Ed. 2014, 53, 11158. (128) Würthner, F.; Yao, S.; Debaerdemaeker, T.; Wortmann, R. Dimerization of Merocyanine Dyes. Structural and Energetic Characterization of Dipolar Dye Aggregates and Implications for Nonlinear Optical Materials. J. Am. Chem. Soc. 2002, 124, 9431.


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(129) Chen, Z.; Fimmel, B.; Würthner, F. Solvent and Substituent Effects on Aggregation Constants of Perylene Bisimide π-Stacks – A Linear Free Energy Relationship Analysis. Org. Biomol. Chem. 2012, 10, 5845. (130) Only one porphyrin derivative having one mesityl group at the pyrrolic β-positions has been reported (Tse, M. K.; Zhou, Z.-y.; Mak, T. C. W.; Chan, K. S. Tetrahedron 2000, 56, 7779.); however, the NH tautomerism of the derivative has never been investigated. On the other hand, mono-substitution with cyano-group or nitro-group, etc. at the β-positions of porphyrin to lower the structural symmetry have been applied to study the NH tautomerism of the β-substituted derivatives in detail (see ref. 31 and 32). (131) 100% C6D6 employed for the dimerization studies (see above) was not used for the studies on the NH tautomerism of 3 (see below), since the 1H NMR signals of the NH protons showed downfield shifts in C6D6 compared to those in other solvents and overlapped with other signals of 3. In consequence, the coalescence of the NH proton signals could not be clearly observed in 100% C6D6. (132) Plowas, I.; Swiergiel, J.; Jadzyn, J. Relative Static Permittivity of Dimethyl Sulfoxide + Water Mixtures. J. Chem. Eng. Data 2013, 58, 1741. (133) Scherer, G.; Limbach, H.-H. Observation of a Stepwise Double Proton Transfer in Oxalamidine Which Involves Matched Kinetic HH/HD/DD Isotope and Solvent Effects. J. Am. Chem. Soc. 1989, 111, 5946.


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(134) Scherer, G.; Limbach, H.-H. Dynamic NMR Study of the Tautomerism of Bicyclic Oxalamidines: Kinetic HH/HD/DD Isotope and Solvent Effects. J. Am. Chem. Soc. 1994, 116, 1230.


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TOC Graphics


43


Scheme 1. NH Tautomerism of a Freebase QFP 75x37mm (300 x 300 DPI)


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Scheme 2. Synthesis of Mesityl-QFP, 3. 98x36mm (300 x 300 DPI)



Figure 1. 1H NMR spectrum of 3 (1.1 × 10–3 M) in CDCl3 at 298 K. 139x118mm (200 x 200 DPI)


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Figure 2. Top (a) and side views (b) for ORTEP drawings of 3. Thermal ellipsoids are depicted at the 40% probability level. t-Bu groups are omitted for clarity in (b). 101x44mm (600 x 600 DPI)




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‡ N ‡ N

H

N H N

N HH N N

N Mes

Mes

N

H N H N N Mes

tautomer A

N Entropy Gain

N HH N N

N H N H Mes N N Mes N H H N N N

Mes

cis-form

dimer


N N HH N N Mes

tautomer B



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Figure 6. (a) Optimized structures and calculated energies obtained by DFT calculations on the transition states and intermediates in the NH tautomerism of 1-t-Bu and (b) selected interatomic distances in the porphyrin cores. The peripheral t-Bu groups are omitted for clarity in (a). To focus on the H and N atoms of the porphyrin ring, other atoms are omitted in (b). Units are in angstrom in (b). 271x303mm (200 x 200 DPI)




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NH Tautomerism of a Quadruply Fused Porphyrin: Rigid Fused

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