Porphyrin-Based Organoplatinum(II) Metallacycles With Enhanced Photooxidization Reactivity

In recent years, metal coordination macrocycles have obtained great interests due to the fact that they combined the rich host-guest properties of macro-cyclic hosts and the unique optical properties of the organic ligands. In this work, we constructed two porphyrin-based organoplatinum(II) metallacycles (MC1 and MC2) through coordination-driven self-assembly. 1H NMR, 31P NMR, and HRMS technologies were used to characterize the structures of MC1 and MC2. Interestingly, MC1 and MC2 can be used as catalysts for photooxidization under light irradiation with higher efficiency compared with the porphyrin ligand only. We hope that the coordination-driven self-assembly strategy can provide an efficient method to construct photo-active materials.

Porphyrin derivatives, which contain a large π-conjugated aromatic structure, are a class of famous photo-activities (Liang et al., 2011;Ou et al., 2019;Wang et al., 2019b). Porphyrins usually have very intense absorption bands in the UV-visible region. However, due to the strong π-π stacking between the aromatic systems, porphyrins are easily aggregated in solvents, especially in aqueous solution (Zou et al., 2017). Commonly, porphyrins aggregate more seriously as the concentration increased. This aggregation phenomenon greatly decreases the efficiency of porphyrins to generate 1 O 2 and therefore restrained their potentially wide applications . To address the aggregation of porphyrins in water, chemistry and materials scientists usually introduce a large substituent onto the platform of the porphyrin core (Slater et al., 2015). However, these chemical synthesis and purification processes have some other disadvantages, such as being timeconsuming, tedious, and with higher costs of preparation.
Herein we designed and synthesized two new metallacycles (MC1 and MC2) with p-bipyridine-modified porphyrin (Scheme S1, Scheme 1) as organic donor and organoplatinum(II) (2 or 3) as the metal acceptor (Scheme 1). The weak metal-ligand bonds will prevent the π-π stacking of the conjugated aromatic porphyrin units, thus improving the efficiency of generating 1 O 2 under irradiation. Interestingly, compared with ligand 1 (Figure S1), the resultant metallacycle MC1 or MC2 can be used as catalyst for photo-oxidizing phenols much more efficiently.

Materials
All reagents and solvents were commercially available in analytical grade and used as received. Further purification and drying by standard methods were employed and these were distilled prior to use when necessary. Deuterated solvents were purchased from Cambridge Isotope Laboratory (Andover, MA, USA). All evaporations of organic solvents were carried out with a rotary evaporator in conjunction with a water aspirator. Melting point measurements were taken on a hotplate microscope apparatus and are uncorrected. 1 H and 13 C NMR spectra were recorded with an Aviance III 400 MHz or 600 MHz liquid-state NMR spectrometer. 31 P{ 1 H} NMR chemical shifts are referenced to an external unlocked sample of 85% H 3 PO 4 (δ 0.0). Mass spectra were recorded on a Micromass Quattro II triple-quadrupole mass spectrometer using electrospray ionization with a MassLynx operating system. UV-vis spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer.

NMR Studies
The formation of discrete organoplatinum(II) metallacycles MC1 and MC2 were characterized by multinuclear NMR ( 31 P and 1 H) analysis. The 31 P { 1 H} NMR spectra of MC1 and MC2 showed a sharp singlet with concomitant 195 Pt satellites at 9.53 ppm for MC1 and at −5.04 ppm for MC2 (Figures 1B,D) corresponding to a single phosphorous environment, indicating the formation of discrete and symmetric metallacycles (Wei et al., 2014).
At the same time, downshifts were observed for β-pyridyl hydrogen in 1 H NMR spectra. As shown in Figure 2, β-pyridyl hydrogen changed from 9.04 to 9.51 and 9.72 ppm in MC1 and from 9.04 to10.21 ppm in MC2. β-pyridyl hydrogen also showed a downfield chemical shift. These chemical shift changes in 1 H NMR spectra are similar with the previous analogous organoplatinum(II) system, indicating the formation of discrete metallacycles (Yao et al., 2018).

Electrospray Ionization Time of Flight Mass Spectrometry Studies
Electrospray ionization time of flight mass spectrometry (ESI-TOF-MS) provided further evidence for the stoichiometry formation of discrete metallacycles MC1 and MC2. In the mass spectrum of MC1, the peak at m/z = 1,646.77 is consistent with  an intact [M -3OTf] 3+ charge state, which supported a [3 + 3] metallacycle ( Figure 3A). Similarly, for metallacycle MC2, the peak at m/z = 1,316.92 is consistent with an intact [M + 8 CH 3 COCH 3 − 4OTf] 4+ charge state, which is expected only for a [4 + 4] metallacycle ( Figure 3B). All the evidence from 1 H NMR, 31 P NMR, and ESI-TOF-MS confirmed the formation of a discrete structure as the sole assembly product.

Photooxidization Studies
As we all know, porphyrins have the ability to generate 1 O 2 due to the fact that they could be excited into 3 O 2 state under irradiation and the energy transfer process is accompanied with molecular O 2 . However, due to the strong π-π interactions, most porphyrins applied as photosensitizers are easily aggregated in aqueous solution (Figures S4, S5). This aggregation will greatly restrain the ability of the porphyrins to generate reactive oxygen species. For our obtained metallacycles MC1 and MC2, the coordination bonds will decrease the self-quenching of the excited states and improve the photooxidization efficiency. Therefore, metallacycles MC1 and MC2 can be used as an expected catalyst for the photoreaction mediated by 1 O 2 . Herein quinol was selected as a model substrate for detecting the reactivity, and UV-vis spectroscopy was used to monitor the process. As shown in Figure 4, after 20 ml of aqueous solution of quinol (10 −2 mmol L −1 ) was irradiated by a LED lamp (500 nm) under air with MC1 (5 mg) as catalyst, the absorption band  of the phenyl moiety in quinol in 289 nm gradually decreased, and 65% of quinol was consumed after irradiation for 60 min (Figure 4). As expected, MC2 has a similar catalytic efficiency with MC1 ( Figure S2). However, in the control experiments using the ligand 1 as catalyst instead of MC1, only 8% of quinol was reacted after irradiation at 500 for 60 min under the same conditions ( Figure S2). Importantly, the investigation for the recyclability of MC1 showed that they could be recovered by simple filtration and reused without significant loss of catalytic activity (yield loss within 5% for six cycles, Figure S3).

CONCLUSIONS
In this paper, we synthesized two metallacycles, MC1 and MC2, with p-bipyridines modified porphyrin as the ligands through coordination-driven self-assembly. Then, the obtained metallacycles were characterized by 31 P NMR, 1 H NMR, and ESI-TOF-MS methods. Furthermore, the metallacycles MC1 and MC2 can be used as an expected catalyst for the photoreaction mediated by 1 O 2 due to the coordination bonds that will decrease the self-quenching of the excited states of porphyrin units and improve the photooxidization efficiency. Our next study will focus on the application of our metallacycles in photodynamic therapy.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/Supplementary Material.

AUTHOR CONTRIBUTIONS
LW, CH, and ZW prepared the ligands. LW, XW, and FS constructed the metallacycles. ML and QZ did the photooxidization. LW and XJ analyzed the data. LW, QZ, and XJ wrote the paper.