Hydrogen Atom Abstraction and Reduction Study of 21-Thiaporphyrin and 21,23-Dithiaporphyrin

The metal-free porphyrins protonation has gained interest over five decades because its structure modification and hardly monoacid intermediate isolation. Here, upon the hydrogen atom abstraction processes, one step diproptonated H3STTP(BF4)2 (STTP = 5,10,15,20-tetraphenyl-21-thiaporphyrin) (3) and stepwise protonated HS2TTPSbCl6 (5) and diprotonated H2S2TTP(BF4)2 (6) (S2TTP = 5,10,15,20-tetraphenyl-21,23-thiaporphyrin) compounds were obtained using HSTTP and S2TTP with oxidants. The closed-shell protonated compounds were fully characterized using XRD, UV-vis, IR and NMR spectra. In addition, the reduced 19π compounds [K(2,2,2)]HSTTP (2) and [K(2,2,2)]S2TTP (7) were synthesized by the ligands with reductant KC8 in THF solution. These two open-shell compounds were characterized with UV-vis, IR and EPR spectroscopies. The semiempirical ZINDO/S method was employed to analyze the HOMO/LUMO gap lever and identify the electronic transitions of the UV-vis spectra of the closed- and open-shell porphyrin compounds.

The hydrogen atom abstraction (HAA) strategy is a fundamental process in redox [10], carbohydrate functionalization [11,12], and catalysis reactions [13,14].It often reflects the synchronicity of concerted H + /e − transfers.Enzymes with various porphyrin analogues (P450, F430 and vitamin B12) performing the HAA catalysis from an alkyl group is an important concept [15].However, as innocent ligands, the metal-free porphyrins are seldom used for study in the HAA strategy with pairwise H + /e − transfers.Detecting and isolating mono-protonated intermediate porphyrins in the protonation process is very difficult.Until now, only a few mono-protonated structurally or spectroscopically characterized porphyrins have been reported (Scheme 1) [16][17][18][19][20][21][22][23][24][25].The first structure characterized was reported by Takenaka et al. in 1974 using octaethylporphyrin with HI at room temperature (Scheme 1a) [17].Fukuzumi reported the first monoprotonated porphyrin compound with α,β and meso-substituted (Scheme 1b) [18].Latos-Grazynski studied the monoprotonation of N 3 S-porphyrin with spectroscopies, and Uno first structurally characterized mono-protonation N 3 S 23 -benzoporphyrin in 2017 (Scheme 1c) [23,24].In these reported structure characterizations, the strong acids and α, β-substituents porphyrins are essential for synthesized mono-protonated porphyrin complexes.So far, only one structural determi-Before cobalt 21-thiaporphyrin complexes in three different oxidation states and reactivities were explored by us [7], here we were able to undergo one-step HAA strategy results in diprotonation 21-thiaporphyrin and step-wise HAA strategy results in monoand di-protonation 21,23-thiaporphyrin.The protonated porphyrins were characterized with X-ray crystallography and spectroscopies.In addition, the two porphyrin 19π potassium salts were isolated and fully characterized.To the best of our knowledge, this is the first mono-and di-protonated metal-free porphyrins by oxidants through the HAA process.
Before cobalt 21-thiaporphyrin complexes in three different oxidation states and reactivities were explored by us [7], here we were able to undergo one-step HAA strategy results in diprotonation 21-thiaporphyrin and step-wise HAA strategy results in mono-and di-protonation 21,23-thiaporphyrin.The protonated porphyrins were characterized with X-ray crystallography and spectroscopies.In addition, the two porphyrin 19π potassium salts were isolated and fully characterized.To the best of our knowledge, this is the first mono-and di-protonated metal-free porphyrins by oxidants through the HAA process.

Syntheses
Reaction of HSTTP with excess KC8 in the presence of [2,2,2]Cryptand in THF solution results in the color changing from brown-red to cyan immediately.A structurally identical sample of 19π salt [Na(2,2,2)]HSTTP could be generated from the parent ligand HSTTP with Na/NaCl in THF, and petrol diamond-shaped crystals were obtained from pentane and slowly diffused into a THF/CH3CN mixture overnight at rt.The solid structure of sodium salt 2 is shown in Figure 2b.Using the NOBF4 to oxidize HSTTP in a DCM solution resulting in a color change from red the deep green, a 18π dication complex was generated instead of a 17π complex or 18π cation intermediates.The proton for 18π dication salt may generated by H-atom abstraction from a solvent.An alternative way to synthesize this complex was using ligand 1 with excess HBF4 in a DCM solution.Black-green block crystals of [H3STTP](BF4)2 (3) were obtained by layering a CH3CN mixture with Et2O at rt in two days (Scheme 2).

Crystallographic Details
New 19π sodium salt 2 and 18π diacids 3 complexes were characterized using single X-ray crystallography (Figure 2).Compared to the free ligand 1, no obviously changed bond lengths and angles were viewed in 19π complex 2 (Figure 2a,b).And the thiaporphyrin macrocycle ring was almost planar with slightly tile-up sulphur atom in thiophene by 0.12 Å.In the diacids complex 3, the non-planar porphyrin macrocycle in a saddle shape [2] was observed in the solid-state structure.The pyrrole rings and thiophene ring were disoriented with 33.40( 4

Spectra Characterization
UV-Vis spectra of free-base HSTTP 1, potassium salt 2, and diacids 3 are shown in Figure 3a.The HSTTP 1 and 19π salt 2 have almost the same Soret bands at 375 and 427 nm, and several Q-bands were at 512, 546 and 616 nm.Compared with the other 19π porphyrins salts [30,31], no NIR bands around 900 nm were observed in our system.In diacids 3, the Soret band is weakly blue shifted compared with those in the spectrum of HSTTP 1 and salt 2 and appear at 436 nm.New low-energy bands were observed in the spectrum of diacids 3 at 600 and 654 nm.ATR-IR spectra of HSTTP 1, salt 2, and diacids 3 are shown in Figure 3b.In ligand HSTTP and potassium salt 2, the bands characteristic of the NH vibrations are observed at 3330 cm −1 and 3328 cm −1 , respectively.The band is shifted to a lower value (3260 cm −1 ) at the formation of diacids 3. The 1 H NMR spectrum of diacids 3 in CDCl 3 shows two signals at −1.95 and −2.56 ppm for the NH groups (Figure S12).However, these NH signals were not observed with the CD 3 CN used for NMR spectrum (Figure S10).The X-band EPR spectrum of sodium salt 2 in THF solution recorded at rt shows an organic radical signal with the g = 2.003 (Figure S5).It is very close to the free radical (g = 2.0023) and other 19π porphyrins salts [30,31].

Spectra Characterization
UV-Vis spectra of free-base HSTTP 1, potassium salt 2, and diacids 3 are show Figure 3a.The HSTTP 1 and 19π salt 2 have almost the same Soret bands at 375 an nm, and several Q-bands were at 512, 546 and 616 nm.Compared with the other 19π phyrins salts [30,31], no NIR bands around 900 nm were observed in our system.In di 3, the Soret band is weakly blue shifted compared with those in the spectrum of HST and salt 2 and appear at 436 nm.New low-energy bands were observed in the spec of diacids 3 at 600 and 654 nm.ATR-IR spectra of HSTTP 1, salt 2, and diacids 3 are sh in Figure 3b.In ligand HSTTP and potassium salt 2, the bands characteristic of the vibrations are observed at 3330 cm −1 and 3328 cm −1 , respectively.The band is shifted lower value (3260 cm −1 ) at the formation of diacids 3. The 1 H NMR spectrum of diac in CDCl3 shows two signals at −1.95 and −2.56 ppm for the NH groups (Figure S12).H ever, these NH signals were not observed with the CD3CN used for NMR spectrum ure S10).The X-band EPR spectrum of sodium salt 2 in THF solution recorded at rt s an organic radical signal with the g = 2.003 (Figure S5).It is very close to the free radi = 2.0023) and other 19π porphyrins salts [30,31].

Syntheses
Using one equivalent magic blue (Tris(4-bromophenyl)ammoniumyl hexachlo timonte) or two equivalents NOBF4 with S2TTP in a CHCl3 solution resulting in an im diate color change from brown-red to grass green and green precipitates were gene (Scheme 3).The monoacid 5 and diacids compound 6, which had poor solubility in C and methanol and totally deprotonated in a CH3CN solution.Green block crystals of oprotonated complex HS2TTP(SbCl6) (5) were grown from hexane and slowly diff into the THF/CHCl3 mixture solution in a week.And the blue needle-shaped cryst diacids complex 6 for X-ray diffraction were obtained from Et2O diffusion in CHCl3/CH3CN mixture solution in the presence of HBF4•Et2O drops in three days reaction of S2TTP with KC8 in the presence of [2,2,2]cryptand in the THF solution re in a color change from brown-red to greenish.The solubility of the 19π potassium s also poor in THF and CH3CN.Yellowish diamond-shaped crystals of [K(2,2,2)]S2TT were obtained from hexane diffusion into the CH3CN/THF mixture solution.

Crystallographic Details
The monoacid 5, diacids 6, and 19π 7 complexes were characterized by single X-ray crystallography (Figure 4b-d).The two acid complexes were both adopted saddle shaped.In complex 5, the pyrrole rings and thiophene rings were disoriented with 21.  6).The quality and resolution of the complex 7 was limited, because the compound contained cryptand and solvents; the structure of [K(Cryptand)]S2TTP still provided important information for comparison.Two similar crystallographically molecules of complex 7 were found in the asymmetric, and only one is shown.The reduced 19π complex 7 was like the parent ligand S2TTP, again without significant changes (Figure 4d).The C-S bonds were the largest bonds compared to the complexes 4-6.In addition, the N•••N distances in the opposite pyrrole rings were 4.

Crystallographic Details
The monoacid 5, diacids 6, and 19π 7 complexes were characterized by single Xray crystallography (Figure 4b-d).The two acid complexes were both adopted saddle shaped.In complex 5, the pyrrole rings and thiophene rings were disoriented with 21.28( 2 5) and then increased to 1.74 Å (6).The quality and resolution of the complex 7 was limited, because the compound contained cryptand and solvents; the structure of [K(Cryptand)]S 2 TTP still provided important information for comparison.Two similar crystallographically molecules of complex 7 were found in the asymmetric, and only one is shown.The reduced 19π complex 7 was like the parent ligand S 2 TTP, again without significant changes (Figure 4d).The C-S bonds were the largest bonds compared to the complexes 4-6.In addition, the N•••N distances in the opposite pyrrole rings were 4.     a The tilted angle of the five-membered ring toward the mean plane of 24 porphyrinoid atoms.b The average length of the C-S or C-N bonds.

Spectra Characterization
The UV-vis spectra of S 2 TTP ligand 4, monoacid 5, diacids 6, and 19π compound 7 are shown in Figure 5a.Similar absorption at Sort and Q bands were observed in the parent ligand S 2 TTP and open-shell compound 7.In 19π compound 7, still no NIR bands were viewed in the solution state.In monoacid compound 5, the band at 452 nm was blue-shifted compared to the parent S 2 TTP ligand.Two low-energy bands at 612 and 698 nm were observed in the spectrum.In the diprotonated compound 7, a closed-NIR band at 738 nm was viewed compared to all the parent, reduced, and monoprotoned compounds.The IR spectra show the band at 3138 cm −1 for 5 and 3212 cm −1 for 6, which was assigned to the NH stretching vibration of the protonation of 21,23-thiaporphyrin (Figure 5b).Since the monoacid 5 and diacids 6 compounds had bad solubility in the CHCl 3 solution, no signals assigned to the NH peak were observed in the 1 H NMR spectrum (Figure S24).In monoacid compound 5, a C 2V symmetric was observed at room temperature, indicating that the proton in the NH unit had a rapid interconvert at the NMR time scale.The X-band EPR spectrum of potassium salt 7 in 2-MeTHF solution recorded at 106 K shows an organic radical signal with the g = 2.003, which is the same as compound 2 (Figure S29).(7) a The tilted angle of the five-membered ring toward the mean plane of 24 porphyrinoid atoms.b The average length of the C-S or C-N bonds.

Spectra Characterization
The UV-vis spectra of S2TTP ligand 4, monoacid 5, diacids 6, and 19π compound 7 are shown in Figure 5a.Similar absorption at Sort and Q bands were observed in the parent ligand S2TTP and open-shell compound 7.In 19π compound 7, still no NIR bands were viewed in the solution state.In monoacid compound 5, the band at 452 nm was blueshifted compared to the parent S2TTP ligand.Two low-energy bands at 612 and 698 nm were observed in the spectrum.In the diprotonated compound 7, a closed-NIR band at 738 nm was viewed compared to all the parent, reduced, and monoprotoned compounds.The IR spectra show the band at 3138 cm −1 for 5 and 3212 cm −1 for 6, which was assigned to the NH stretching vibration of the protonation of 21,23-thiaporphyrin (Figure 5b).Since the monoacid 5 and diacids 6 compounds had bad solubility in the CHCl3 solution, no signals assigned to the NH peak were observed in the 1 H NMR spectrum (Figure S24).In monoacid compound 5, a C2V symmetric was observed at room temperature, indicating that the proton in the NH unit had a rapid interconvert at the NMR time scale.The X-band EPR spectrum of potassium salt 7 in 2-MeTHF solution recorded at 106 K shows an organic radical signal with the g = 2.003, which is the same as compound 2 (Figure S29).

ZINDO/S Calculation
The electronic structures and transition energies for the X-ray geometries of the 21and 21,23-thiaporphyrins compounds 1-7 were studied using the ZINDO/S method with ORCA program in version 4.2.1 [32][33][34].The HOMO(SOMO)/LUMO energy gaps for compounds 1-7 are shown in Figures 6 and S38-S44.In compounds 1 and 4, the energy gap difference is 4.78 ev and 4.57 ev, respectively, and it is inferred that the electrons are more easily transferred in compound 4.After protonation, the energy gap differences decreased with 4.04 eV (3), 4.45 eV (5), and 3.96 eV (6), respectively.This indicates that the electrons transit more easily from HOMO to LUMO in compounds 3, 5, and 6, and the corresponding absorptions in acid 3, 5, and 6 are more red-shifted.In the reduced salts 2 and 7, the LUMO lever is both positive (>0), and the energy gap between SOMO and LUMO are 5.6 eV and 6.0 eV, respectively.These results combined with the experimental process indicate that the open-shell compound 7 cannot be reduced anymore, even though the CV

ZINDO/S Calculation
The electronic structures and transition energies for the X-ray geometries of the 21and 21,23-thiaporphyrins compounds 1-7 were studied using the ZINDO/S method with ORCA program in version 4.2.1 [32][33][34].The HOMO(SOMO)/LUMO energy gaps for compounds 1-7 are shown in Figure 6 and Figures S38-S44.In compounds 1 and 4, the energy gap difference is 4.78 ev and 4.57 ev, respectively, and it is inferred that the electrons are more easily transferred in compound 4.After protonation, the energy gap differences decreased with 4.04 eV (3), 4.45 eV (5), and 3.96 eV (6), respectively.This indicates that the electrons transit more easily from HOMO to LUMO in compounds 3, 5, and 6, and the corresponding absorptions in acid 3, 5, and 6 are more red-shifted.In the reduced salts 2 and 7, the LUMO lever is both positive (>0), and the energy gap between SOMO and LUMO are 5.6 eV and 6.0 eV, respectively.These results combined with the experimental process indicate that the open-shell compound 7 cannot be reduced anymore, even though the CV measurement demonstrates that compound 7 has the second reversible reduction potential (Figure 1).measurement demonstrates that compound 7 has the second reversible reduction potential (Figure 1).The UV-vis absorption spectra in closed-shell porphyrin derivatives can be assigned S0→S2 transition around 350-550 nm and S0→S1 transition around 600-1000 nm.As can be seen from Figures S43-49, the calculated UV-vis absorption spectra are in close agreement with the observed spectra pattern in compounds 1-7.In the closed-shell compound, the four orbitals labeled as H, H−1, L, and L+1 play an important role in the energy electronic transitions (Table S2).However, in the open-shell compound 2 and 7, the orbits about S−7, S−5, and L+5 et al. also have some contribution in the energy electronic transitions.

Instruments
Schlenk techniques and a nitrogen atmospheric drybox (Vigor Technology Inc, Vancouver, BC, Canada) were used for handling air-sensitive compounds. 1H NMR spectra were measured on a Bruker Avance spectrometer (500 MHz, Billerica, MA, USA).Chemical shifts are expressed in parts per million relative to residual CHCl3 (δH = 7.26 ppm) and CD3CN (δH = 1.94 ppm).IR spectra of crystalline samples were measured with a Cary 630 FTIR spectrometer equipped with a DialPath and Diamond ATR accessory (Agilent, Santa Clara, CA, USA) placed in a glovebox (N2 atmosphere, Vigor Technology Inc, Vancouver, Canada).IR bands were labeled according to their relative intensities with vs. (very strong), s (strong), m (medium), w (weak), and very weak (vw).UV−vis spectra were recorded on an Agilent Cary 60 (Santa Clara, CA, USA).Compounds 2 and 7 were prepared in a glovebox and transferred out of the glovebox prior to the measurement.Cyclic voltammetry (CV) experiments were performed with an Interface 1000B potentiostat (Vesion 4.5, Gamry Instruments, Warminster, PA, USA) using a three-electrode setup consisting of a glassy carbon working electrode, a platinum wire counter electrode, and an Ag reference electrode and were analyzed using Gamry Framework software (Version 7.8.6).CV The UV-vis absorption spectra in closed-shell porphyrin derivatives can be assigned S0→S2 transition around 350-550 nm and S0→S1 transition around 600-1000 nm.As can be seen from Figures S43-S49, the calculated UV-vis absorption spectra are in close agreement with the observed spectra pattern in compounds 1-7.In the closed-shell compound, the four orbitals labeled as H, H−1, L, and L+1 play an important role in the energy electronic transitions (Table S2).However, in the open-shell compound 2 and 7, the orbits about S−7, S−5, and L+5 et al. also have some contribution in the energy electronic transitions.

Instruments
Schlenk techniques and a nitrogen atmospheric drybox (Vigor Technology Inc., Vancouver, BC, Canada) were used for handling air-sensitive compounds. 1H NMR spectra were measured on a Bruker Avance spectrometer (500 MHz, Billerica, MA, USA).Chemical shifts are expressed in parts per million relative to residual CHCl 3 (δH = 7.26 ppm) and CD 3 CN (δH = 1.94 ppm).IR spectra of crystalline samples were measured with a Cary 630 FTIR spectrometer equipped with a DialPath and Diamond ATR accessory (Agilent, Santa Clara, CA, USA) placed in a glovebox (N 2 atmosphere, Vigor Technology Inc., Vancouver, Canada).IR bands were labeled according to their relative intensities with vs. (very strong), s (strong), m (medium), w (weak), and very weak (vw).UV−vis spectra were recorded on an Agilent Cary 60 (Santa Clara, CA, USA).Compounds 2 and 7 were prepared in a glovebox and transferred out of the glovebox prior to the measurement.Cyclic voltammetry (CV) experiments were performed with an Interface 1000B potentiostat (Vesion 4.5, Gamry Instruments, Warminster, PA, USA) using a three-electrode setup consisting of a glassy carbon working electrode, a platinum wire counter electrode, and an Ag reference electrode and were analyzed using Gamry Framework software (Version 7.8.6).CV experiments were performed in deoxygenated DCM containing n Bu 4 PF 6 (0.1 M) as the supporting electrolyte; ferrocene was used as an internal standard.ESI mass spectra were recorded on a Bruker HCT ultra spectrometer (Billerica, MA, USA).

Materials
Solvents were dried by standard methods and freshly distilled prior to use.Dichloromethane and chloroform were dried with calcium hydride and distilled under nitrogen.THF, hexane, and pentane were distilled under nitrogen in the presence of sodium chips using benzophenone ketyl as an indicator.Dried solvents, which were transferred into round-bottom flasks, bubbled with nitrogen for at least 10 min to remove residual dioxygen and then sealed with a J-Young cap and were stored in the nitrogen atmosphere drybox prior to use.Pyrrole was freshly distilled under nitrogen from calcium hydride prior to use.Other starting materials were obtained commercially and used directly without further purification.Silica gel (100-200 mesh) or neutral alumina was used for column chromatography.The starting 5,10,15,20-tetraphenyl-21-thiaporphyrin (HSTTP) and 5,10,15,20-tetraphenyl-21,23-thiaporphyrin (S 2 TTP) [35], Na/NaCl (5%) [36] were prepared according to the literature with modification.

EPR Spectroscopy
Continuous-wave (cw) X-band EPR measurements were performed on a Bruker A200 spectrometer equipped with a high sensitivity cavity (ER4119HS) in conjunction with microwave bridge Bruker A40X (Billerica, MA, USA).For variable temperature control, cryostat (A4131VT) was employed.EPR simulations have been carried out with esim due to Dr. Eckhard Bill at the Max-Planck-Institut für Chemische Energiekonversion [37] and Easyspin program [38].

Single-Crystal X-ray Structure Determinations
Crystal data and details of the data collections are given in Table S1.X-ray data were collected on a STOE IPDS II diffractometer (Darmstadt, Germany) or Rigaku Oxford diffractometer (graphite monochromated Mo−Kα radiation, λ = 0.71073 Å, Tokyo, Japan) by use of scans at 150 K or 100 K.The structures were solved using SHELXT [39] and refined on F 2 using all reflections with SHELXL2014/16 [40] interfaced with Olex2 [41].All Non-hydrogen atoms were refined anisotropically.Most hydrogen atoms were placed in calculated positions and assigned to an isotropic displacement parameter of 1.2/1.5 Ueq(C).All unit cells contained highly disordered solvent molecules for which no satisfactory model for a disorder could be found.The solvent contribution to the structure factors was calculated with PLATON SQUEEZE [42].ISOR, SADI, SAME, and EADP restraints were applied to model the disorder.The CCDC deposition numbers 2348693-2348699 contain the supplementary crystallographic data.This data can be obtained free of charge via The Cambridge Crystallography Data Centre.

Synthesis of [H 3 STTP](BF 4 ) 2 (3)
HSTTP (100 mg, 0.158 mmol, 1 equiv) and NOBF 4 (18.5 mg, 0.138 mmol, 1 equiv) were dissolved in 3 mL of DCM, resulting in a color change from brown-red to black-green immediately.After stirring overnight at rt, the residual was filtered through a patch of celite.Green-block suitable black-block crystals for X-ray diffraction were obtained by
2.1.H-Atoms Abstraction and Reduction in 21-Thiaporphyrin 2.1.1.Syntheses Reaction of HSTTP with excess KC 8 in the presence of [2,2,2]Cryptand in THF solution results in the color changing from brown-red to cyan immediately.A structurally identical sample of 19π salt [Na(2,2,2)]HSTTP could be generated from the parent ligand HSTTP with Na/NaCl in THF, and petrol diamond-shaped crystals were obtained from pentane and slowly diffused into a THF/CH 3 CN mixture overnight at rt.The solid structure of sodium salt 2 is shown in Figure 2b.Using the NOBF 4 to oxidize HSTTP in a DCM solution resulting in a color change from red the deep green, a 18π dication complex was generated instead of a 17π complex or 18π cation intermediates.The proton for 18π dication salt may generated by H-atom abstraction from a solvent.An alternative way to synthesize this complex was using ligand 1 with excess HBF 4 in a DCM solution.Black-green block crystals of [H 3 STTP](BF 4 ) 2 (3) were obtained by layering a CH 3 CN mixture with Et 2 O at rt in two days (Scheme 2).

Figure 6 .
Figure 6.Illustration of the HOMOs/SOMOs and LUMOs for compounds 1-7 from ZINDO/S calculation.The values were obtained from the ZINDO calculations for the X-ray geometries.

Figure 6 .
Figure 6.Illustration of the HOMOs/SOMOs and LUMOs for compounds 1-7 from ZINDO/S calculation.The values were obtained from the ZINDO calculations for the X-ray geometries.

Table 1 .
Lengths and angles for the 1

Table 1 .
Lengths and angles for the 1