Doubly N-confused and ring-contracted [24]hexaphyrin Pd-complexes as stable antiaromatic N-confused expanded porphyrins

As isomers of the regular porphyrins, N-confused porphyrins have attracted extensive attention of chemists because of their unique chemical structures, chemical reactivities, and physical properties, which result in their promising applications in the fields of catalytic chemistry, biochemistry and material science. Typically, N-confused porphyrins are synthesized via acid catalyzed condensation and following oxidation during which lactams are often formed as the byproducts. Here we report doubly N-confused and ring-contracted [24]hexaphyrin(1.1.0.1.1.0) mono- and bis-Pd-complexes as stable antiaromatic N-confused expanded porphyrins, which are synthesized through Pd-catalyzed Suzuki-Miyaura coupling of 1,14-dibromotripyrrin. These macrocycles show a paratropic ring currents, an ill-defined Soret band, a red-shifted weak absorption tail, and a small HOMO-LUMO gap. NBS bromination of the bis Pd-complex give its mono- and dibromides regioselectively, which are effectively used to synthesize a [24]hexaphyrin dimer and a NiII porphyrin-[24]hexaphyrin-NiII porphyrin triad, respectively.

Doubly N-confused and ring-contracted [24]  hexaphyrin Pd-complexes as stable antiaromatic N-confused expanded porphyrins Fuying Luo 1,2 , Le Liu 1,2 , Han Wu 1 , Ling Xu 1 , Yutao Rao 1 , Mingbo Zhou 1 , Atsuhiro Osuka 1 & Jianxin Song 1 As isomers of the regular porphyrins, N-confused porphyrins have attracted extensive attention of chemists because of their unique chemical structures, chemical reactivities, and physical properties, which result in their promising applications in the fields of catalytic chemistry, biochemistry and material science.Typically, N-confused porphyrins are synthesized via acid catalyzed condensation and following oxidation during which lactams are often formed as the byproducts.Here we report doubly N-confused and ring-contracted [24] hexaphyrin(1.1.0.1.1.0)mono-and bis-Pd-complexes as stable antiaromatic N-confused expanded porphyrins, which are synthesized through Pdcatalyzed Suzuki-Miyaura coupling of 1,14-dibromotripyrrin.These macrocycles show a paratropic ring currents, an ill-defined Soret band, a red-shifted weak absorption tail, and a small HOMO-LUMO gap.NBS bromination of the bis Pd-complex give its mono-and dibromides regioselectively, which are effectively used to synthesize a [24]hexaphyrin dimer and a Ni II porphyrin- [24]  hexaphyrin-Ni II porphyrin triad, respectively.

Results
With the aim to synthesize m-benziporphyrin(1.1.0.0) 5, we attempted the cyclization reaction of α,α'-diboryltripyrrane 3a with 1,3-dibromo-4-methoxybenzene under the usual Suzuki-Miyaura coupling conditions (Pd 2 (dba) 3 , X-phos, Cs 2 CO 3 , CsF, and toluene/DMF).In addition to the target 5 16 which was actually obtained in 5% yield, we isolated dark green product 6 in ca.1% (Fig. 2).Fortunately, the structure of 6 has been revealed by X-ray analysis to be a symmetric and planar [18]  triphyrin(5.1.1)with a small mean-plane deviation (MPD) of 0.26 Å (Fig. 3a, b).The carbonyl oxygen atom is hydrogen bonded with the two adjacent pyrrolic NH groups and the C=O bond length is 1.288(7) Å, being distinctly longer than those of usual ketones (1.23 Å) 17,18 .This structural feature suggests a significant contribution of a dipolar resonance state.The 1 H NMR spectrum of 6 shows a singlet at 9.74 ppm due to the vinylic proton, two doublets at 8.36 and 8.08 ppm (J = 4.0 Hz) and a singlet at 8.31 ppm due to the pyrrolic protons, and signals due to the NH protons at 3.89 (2H) and −4.13 (1H) ppm, suggesting a distinct diatropic ring current, reflecting its 18π-electronic circuit.The absorption spectrum of 6 shows a split Soret-like band at 419 and 444 nm and vibronic-structured Q-like bands as characteristic features of aromatic porphyrinoids (Fig. 4).Further, 6 displays vibronic-structured fluorescence at 652 and 721 nm with a high quantum yield of Φ F = 0.48.These data strongly indicate that 6 is an aromatic tripyrrolic porphyrinoid.Actually, the calculated NICS values are negative (−12.15 ppm) inside the macrocycle.

NH
The structure of 9 has been determined by X-ray diffraction analysis to be a DNCRC keto tautomer, in which one α-position of the N-confused pyrrole is oxidized.Unfortunately, the serious disorder impeded a detailed analysis of the structure.The 1 H NMR spectrum of 9 displays two signals due to the inner NH protons at 21.89 and 19.85 ppm and those in the range of 5.22-3.79ppm due to the pyrrolic β-protons, clearly indicating its paratropic ring current and thus antiaromatic character.Judging from these chemical shifts, it is conceivable that the antiaromatic character of 9 is slightly larger as compared with 8.The NICS value of 9 was calculated to be 10.43, which is larger than that of 8. Importantly, 8 and 9 are stable antiaromatic N-confused expanded porphyrins.
Cyclic voltammetry (CV) and differential-pulse voltammetry (DPV) experiments were conducted and the redox potentials are summarized in Table 1.
Further fabrications of 8 were attempted.Monobromide 8a was obtained in 78% yield by treating 8 with 1 equiv.NBS at 0 °C (Fig. 8).The parent ion peak of 8a was observed at m/z = 1196.1615(calcd for (C 64 H 53 BrN 6 Pd 2 ) + = 1196.1601([M] + )) and its 1 H NMR spectrum showed nine peaks at 5.23-4.05ppm, indicating an asymmetric substitution.Subsequently, directly β-to-β linked DNCRC [24]hexaphyrin dimer 10 was successfully obtained by reductive coupling of 8a with Ni(cod) 2 in 61% yield.The parent ion peak of 10 was observed at m/z = 2234.4843(calcd for (C 128 H 106 N 12 Pd 4 ) + = 2234.4862([M] + )).The 1 H NMR spectrum of 10 exhibited three singlets at 4.30, 3.91, and 3.74 ppm and three pairs of doublets in the range of 5.13-4.69ppm.The structure of 10 has been confirmed by X-ray analysis as shown in Fig. 9.The dihedral angle between the two hexaphyrins is 54.2°and the bond length of the connecting C-C is 1.48 (1) Å.The first oxidation potentials of 10 were split at 0.31 and 0.15 V, indicating the electronic interaction between the two DNCRC [24]hexaphyrin units.
Dibromide 8b was obtained in 86% yield by treating 8 with 2 equiv.NBS at 0 °C.The parent ion peak of 8b was observed at m/ z = 1274.0660(calcd for (C 64 H 52 Br 2 N 6 Pd 2 ) + = 1274.0701([M] + )) and its 1 H NMR spectrum showed two doublets at 5.22 and 4.84 ppm (J = 4.7 Hz) and two singlets at 5.20 and 4.23 ppm, indicating a symmetric substitution.The single-crystal structure of 8b was also determined by X-ray diffraction analysis, which shows its centrosymmetric dibromide feature.Finally, the Suzuki-Miyaura

Materials and characterization
1 H NMR spectra (500 MHz) were taken on a Bruker ADVANCE-500 spectrometer, and chemical shifts were reported as the delta scale in ppm relative to CHCl 3 (δ = 7.260 ppm) as an internal reference.UV/Vis absorption spectra were recorded on a Shimadzu UV-3600 spectrometer.MALDI-TOF mass spectra were obtained with a Bruker ultrafleXtreme MALDI-TOF/TOF spectrometer with trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as a matrix.X-ray data were taken on an Agilent Supernova X-ray diffractometer equipped with a large area CCD detector.Redox potentials were measured by cyclic voltammetry on a CHI900 scanning electrochemical microscope.Toluene and THF were distilled after refluxing with Na and benzophenone ketyl indicator under an argon atmosphere and stored over 3 Å molecular sieves in a glovebox for at least 12 h prior to use.DMF was refluxed with CaH 2 under an argon atmosphere for at least 3 h and distilled prior to use.Unless otherwise noted, materials obtained from commercial suppliers were used without further purification.

Synthesis of 8a
A solution of NBS (3.2 mg, 0.018 mmol) in CHCl 3 (10 mL) was added to a solution of 8 (20.0 mg, 0.018 mmol) in CHCl 3 (10 mL) dropwise at 0 °C.After the consumption of 8 was confirmed by TLC monitoring, the reaction mixture was diluted with CHCl 3 .The organic layer was separated and washed with water, and dried over anhydrous sodium sulfate, evaporation of the solvent was followed by silica-gel column chromatography (eluent: CH 2 Cl 2 /n-hexane = 1:4, v/v) and recrystallization with CH 2 Cl 2 /MeOH.8a was obtained as grass-green solids (16.7 mg, 0.014 mmol, 78% yield).

Synthesis of 8b
A solution of NBS (6.4 mg, 0.036 mmol) CHCl 3 (10 mL) was added to a solution of 8 (20.0 mg, 0.018 mmol) in CHCl 3 (20 mL) dropwise at 0 °C.After the consumption of 8 and 8a was confirmed by TLC monitoring, the reaction mixture was diluted with CHCl 3 .The organic layer was separated and washed with water, and dried over anhydrous sodium sulfate, Evaporation of the solvent was followed by silica-gel column chromatography (eluent: CH 2 Cl 2 /n-hexane = 1:4, v/v) and recrystallization with CH 2 Cl 2 /MeOH gave 8b as grass green solids (19.7 mg, 0.016 mmol, 86% yield). 8b:

Fig. 3 |
Fig. 3 | X-ray crystal structures of 6 and 7. a Top view of 6, b side view of 6, c top view of 7, d side view of 7. Ellipsoids are drawn at the 30% probability level.All hydrogen atoms except those connected to N and O atoms are omitted for clarity.Carbon atom, black ellipsoid; nitrogen atom, blue; palladium atom, orange; oxygen atom, red; hydrogen atom small black ball (These instructions are omitted in the following figures for clarity).

Fig. 6 |
Fig. 6 | X-ray crystal structures of 8, 9 and 9Pd. a Top view of 8, b side view of 8, c top view of 9, d side view of 9, e top view of 9Pd, f side view of 9Pd.Ellipsoids are drawn at the 30% probability level.All hydrogen atoms except those connected to N atoms are omitted for clarity.

Fig. 9 |
Fig. 9 | X-ray crystal structures of 10 and 12. a Top view of 10, b side view of 10, c top view of 12, and d side view of 12. Ellipsoids are drawn at the 30% probability level.All hydrogen atoms are omitted for clarity.Meso-substituents of porphyrin parts are omitted for clarity.Nickel atom, green.