(trans-Dihydroxo)Sn(IV)-[5,10,15,20-tetrakis(2-pyridyl)porphyrin]

Abstract

Sn(IV)-porphyrin complex with trans-dihydroxo axial-ligands and 2-pyridyl peripheral substituents, namely (trans-dihydroxo)[5,10,15,20-tetrakis(2-pyridyl)porphyrinato]tin(IV) was synthesized and fully characterized by various techniques such as elemental analysis, ¹H NMR spectroscopy, ESI-MS spectrometry, UV-visible spectroscopy, and fluorescence spectroscopy.

1. Introduction

Sn(IV)-porphyrins are attractive motifs for constructing self-assembled metalloporphyrin nanostructures because they can exhibit unique features by adopting functional ligands in axial positions [1,2,3]. The role of hydroxo ligands in Sn(IV)-porphyrins has been investigated and exploited from various chemical perspectives. The X-ray structural analysis of (trans-dihydroxo)[5,10,15,20-tetrakis(4-pyridyl)porphyrinato]tin(IV) revealed that the hydroxo ligands act as a hydrogen bonding acceptor and the Sn(IV)-porphyrins are supramolecularly assembled into a two-dimensional network through hydrogen bonding [4]. The hydroxo ligand can be readily converted to alcoholic or carboxylic ligands by acidolysis, resulting in various hexacoordinate Sn(IV)-porphyrin complexes that exhibit interesting functions [5,6,7,8,9,10,11,12,13]. Sn(IV)-porphyrins with additional binding sites enable the creation of sophisticated multiporphyrin arrays [14,15,16] and nanostructures for application in photocatalysis [17,18,19]. Here, we report a tin(IV) porphyrin complex with trans-dihydroxo axial-ligand and 2-pyridyl peripheral substituents, namely, (trans-dihydroxo)[5,10,15,20-tetrakis(2-pyridyl)porphyrinato]tin(IV). Cooperativities between axial ligands and peripheral functional groups in six-coordinated Sn(IV)-porphyrins can lead to various assembled structures for the development of interesting porphyrin materials [20,21,22,23,24,25,26,27,28,29,30].

2. Results and Discussion

The synthetic route used for preparing free-base 5,10,15,20-tetrakis(2-pyridyl)porphyrin (H₂T(2-Py)P) and its Sn(IV) complex (1) is shown in Scheme 1. In brief, H₂T(2-Py)P was reacted with tin(II) chloride in pyridine under aerobic conditions followed by hydrolysis to afford 1 in high yield. 1 was fully characterized using various techniques, including elemental analysis, electrospray ionization mass (ESI-MS) spectrometry, ¹H NMR, UV–vis absorption, and fluorescence spectroscopy. The ¹H NMR spectrum of 1 in CDCl₃ is shown in Figure S2. The resonance at −7.34 ppm of much higher field by the strong shielding of porphyrin ring current obviously represents the proton at the axial Sn-OH. On the other hand, the β-pyrrole and α-pyridyl protons of 1 are in the deshielding zone, and their resonances appear at 9.05−9.15 ppm. The remaining protons of the pyridyl group appear at 7.79, 8.17, and 8.35 ppm. ¹³C NMR confirmed the aromatic as well as porphyrinic carbon centers present at 1 as shown in Figure S3. In the FT-IR spectra of 1, the absorption peaks at 1025 cm⁻¹ and 795 cm⁻¹ belong to the bending vibration of C-H and the out-of-plane bending vibration of C-H in the aromatic ring, respectively. The peaks at 3050 cm⁻¹, 1580 cm⁻¹, and 1430 cm⁻¹ are attributed to the stretching vibrations of C-H, C=C, and C-N in the pyrrole ring, respectively. The peaks at 3605 cm⁻¹ are assigned to the stretching vibrations of the OH signal of the axial hydroxyl group present in 1. As shown in Figure S6, a peak at 771.12 is observed in the ESI-MS spectrum of 1, corresponding to the molecular ion peak [1 + H] ⁺. The UV-vis absorption of 1 at 427 nm represents the strong Soret band, while the Q-bands appear at 522, 560, and 600 nm (Figure S7). This UV-vis absorption spectral pattern is quite similar to that of 4-pyridyl and 3-pyridyl analogues [5]. The fluorescence of 1 is observed as a two-band emission at 601 and 652 nm, as shown in Figure S8.

3. Materials and Methods

All chemicals were purchased from TCI (Tokyo Chemical Industry Co. LTD, Tokyo, Japan) and used without further purification. Elemental analysis was carried out using an EA 1110 Fisons analyzer (Used Lab Machines Limited, London, England). ¹H NMR spectra were obtained using a Bruker BIOSPIN/AVANCE III 400 spectrometer at 293 K (Bruker BioSpin GmbH, Silberstreifen, Rheinstetten, Germany). Electrospray ionization mass (ESI-MS) spectra were recorded using a Thermo Finnigan linear ion trap quadrupole mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Steady-state UV–Vis and fluorescence spectra were recorded using a Shimadzu UV-3600 and Shimadzu RF-5301PC fluorescence spectrophotometer, respectively (Shimadzu, Tokyo, Japan). FT-IR (Fourier transform infrared spectroscopy) spectra (KBr) were obtained using a Shimadzu FTIR-8400S spectrophotometer (Shimadzu, Tokyo, Japan).

3.1. 5,10,15,20-tetrakis(2-pyridyl)porphyrin H₂T(2-Py)P

Freshly distilled pyrrole (1.10 mL, 16 mmol) was added dropwise to a solution of 2-pyridinecarboxaldehyde (1.52 mL, 16 mmol) in propionic acid (300 mL) under reflux. After 4 h, the solvent was evaporated to dryness under reduced pressure. Then, the resulting oily residue was dissolved in 10 mL of N,N’-dimethylformamide (DMF) and neutralized by adding 1 mL of triethylamine. Subsequently, the reaction mixture was kept in a refrigerator for 12 h. The solid precipitate was filtered, washed with hot water, and then dried in an oven. The crude compound was purified by column chromatography (SiO₂, eluent: CHCl₃/EtOH = 95:5) to afford 5,10,15,20-tetra(2-pyridyl)porphyrin H₂T(2-Py)P. The product was recrystallized from CHCl₃/n-hexane to give violet purple crystals. Yield: 0.544 g (22%). Anal. calculated for C₄₀H₂₆N₈: C, 77.65; H, 4.24; N, 18.11. Found: C, 77.48; H, 4.57; N, 17.95. ¹H NMR (400 MHz, CDCl₃, ppm): δ −2.84 (s, 2H, NH), 7.71 (m, 4H, H4-Py), 7.89 (m, 4H, H3-Py), 8.21 (m, 4H, H5-Py), 8.85 (s, 8H, β-pyrrole), 9.14 (s, 4H, H2-Py). UV-vis (CHCl₃): λ_nm (log ε), 419 (5.73), 516 (4.60), 552 (4.43), 593 (4.15), 650 (4.07). Emission (CHCl₃): λ_nm 653, 712.

3.2. Synthesis of (trans-dihydroxo)[5,10,15,20-tetrakis(2-pyridyl)porphyrinato]tin(IV) (1)

H₂T(2-Py)P (0.31 g, 0.5 mmol) was dissolved in 40 mL of pyridine, and SnCl₂∙2H₂O (0.225 g, 1 mmol) was added to the above solution and the mixture was refluxed for 12 h. After removal of all the volatiles in vacuo, the residue was dissolved in CHCl₃ and filtered through a celite pad to remove the excess SnCl₂. The filtrate was evaporated, and the solid residue was re-dissolved in a 3:1 mixture of THF and H₂O (80 mL). K₂CO₃ (0.276 g, 2 mmol) was added to the reaction mixture, which was then refluxed for 12 h. After removal of THF under reduced pressure, the mixture was cooled to 10 °C until precipitation of a reddish compound. The filtered solid compound was dried in an oven. The crude product was recrystallized from CHCl₃/CH₃CN to give Sn(OH)₂T(2-Py)P (1) as a red crystalline compound. Yield: 0.27 g (70%). Anal. calculated for C₄₀H₂₆N₈O₂Sn: C, 62.44; H, 3.41; N, 14.56; R, 19.59. Found: C, 62.18; H, 3.76; N, 14.43; R, 19.63. ¹H NMR (400 MHz, CDCl₃, ppm): δ −7.34 (s, 2H, Sn-OH), 7.79 (m, 4H, H4-Py), 8.17 (m, 4H, H3-Py), 8.38 (m, 4H, H5-Py), 9.05–9.15 (m, 12H, β-pyrrole + H2-Py). ¹³C NMR (400 MHz, CDCl₃, ppm): δ 122.97, 123.02, 130,79, 132.68, 135.23, 146.60, 149.07, 159.31, 159.63. FT-IR (KBr): 3605, 3050, 1630, 1580, 1565, 1510, 1460, 1430, 1345, 1260, 1210, 1075, 1025, 990, 845, 795, 765, 725, 680, 555 cm⁻¹. UV-vis (CHCl₃): λ_nm (log ε), 427 (5.70), 522 (3.65), 560 (4.30), 600 (4.10). Emission (CHCl₃): λ_nm 601, 652.

4. Conclusions

A tin(IV) porphyrin complex with trans-dihydroxo axial-ligands and 2-pyridyl peripheral substituents was synthesized and characterized by elemental analysis, ¹H and ¹³C NMR spectroscopy, FT-IR spectroscopy, ESI-MS spectrometry, UV-visible spectroscopy, and fluorescence spectroscopy. This work can contribute to the design and synthesis of novel macrocyclic compounds for recognizing important biomolecules.