Direct Observation of Reduction of Cu(II) to Cu(I) by P–H Compounds

Article pubs.acs.org/Organometallics

Direct Observation of Reduction of Cu(II) to Cu(I) by P−H Compounds using XAS and EPR Spectroscopy Hong Yi,† Dali Yang,† Yi Luo,† Chih-Wen Pao,‡ Jyh-Fu Lee,‡ and Aiwen Lei*,†,§ †

College of Chemistry and Molecular Sciences, the Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072, People’s Republic of China ‡ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan § State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China S Supporting Information *

ABSTRACT: Recently, transition-metal-catalyzed radical phosphorylations have provided a direct and useful way to P-substituted compounds. Although a variety of methodologies involving phosphinoyl radicals have been developed, the mechanism of the P−H compounds in the redox process is still unclear. In this work, a mechanistic study on the reduction of Cu(II) by P−H compunds through XAS and EPR spectroscopy has been demonstrated. Two commonly used P−H compounds, diphenylphosphine oxide and dialkyl phosphites, have been selected in this reduction system. The structure of formed Cu(I) species is evidenced through fitting results of the EXAFS spectrum. Furthermore, the halide ion can be a mediator to promote the reduction of Cu(II) by P−H compounds. These spectroscopic investigations provide useful insights into the reactions of P−H compounds, which would be helpful for an understanding of the mechanism and the future design of reactions.

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INTRODUCTION

variety of reactions involving phosphinoyl radicals have been developed, their mechanism is still unclear. X-ray absorption spectroscopy serves as a powerful tool for the determination of the local structure of metal species through intense and tunable X-ray beams provided by synchrotron-radiation sources.10 The capability of this technique to analyze the structure of metal species provides a valuable potential for application in identifying important intermediates in catalytic reactions.11 Recently, we have successfully evidenced the reduction of Cu(II) by alkynes with tetramethylethylenediamine (TMEDA) utilizing X-ray absorption spectroscopy (XAS) and electron paramagnetic resonance (EPR).12 In addition, we have also observed the reduction of Cu(II) in the presence of 1,3-diketone, which has shed light on the redox process in copper catalysis.13 Therefore, XAS spectroscopy can be a powerful tool for determining the structures of intermediates in a real reaction system. In this work, we have provided direct evidence of the reduction of Cu(II) to Cu(I) species by P−H compounds through XAS and EPR spectroscopy (Scheme 1). The commonly used P−H compounds diphenylphosphine oxide and dialkyl phosphites

Organophosphorus compounds occupy an important role in organic chemistry due to their wide application in pharmaceuticals and agrochemicals, organic synthesis, and materials science.1 Intense research interest have been directed to developing new synthetic strategies for C−P bond formation, which is highly important in the synthesis of modified nucleosides, nucleotides, and other phosphine-containing ligands.2 The phosphination of aryl halides or triflates mediated by transition metals has served as one of the most powerful methods for aromatic organophosphorus compound synthesis.3 In these transition-metal-catalyzed reactions, the P−H compounds serve solely as nucleophiles and a non-redox process is involved. The P−H compounds could also be transformed into phosphinoyl radicals [R2 P(O) •], and reactions involving organophosphorus radicals have a long history.4 Since a phosphorus radical exhibits high reactivity with unsaturated bonds, it provides an alternative method to construct C−P bonds. In recent years, transition-metalcatalyzed radical C−H phosphorylations, which provide a direct method to phosphinoyl-substituted compounds, have been widely studied.5 Especially, it has been reported that several metal salts such as silver,6 manganese,7 and copper8 can directly react with R2P(O)H to form a phosphorus radical that further promoted phosphorus radical chemistry.9 Although a © XXXX American Chemical Society

Special Issue: Organometallics in Asia Received: January 17, 2016

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DOI: 10.1021/acs.organomet.6b00034 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

When CuCl2 and diphenylphosphine oxide were mixed in DMF at 80 °C, a species with an edge energy of 8981.7 eV was detected, which is the same as that for the reaction between CuBr2 and diphenylphosphine oxide (Figure 1A, black line). Meanwhile, we also used EXAFS spectra to determine the structures of the formed Cu(I) complexes. The EXAFS spectra in Figure 1B revealed that the halide ion may coordinate with formed Cu(I) species. As shown in Figure 2A, the fitting results

Scheme 1. Single Electron Transfer (SET) between P−H Compounds and Cu(II) Species

are selected for this reduction system. The structure of generated Cu(I) species is also confirmed through the fitting results of EXAFS spectra. Furthermore, the halide ion can be a promoter for the reduction of Cu(II) by P−H compounds.

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RESULTS AND DISCUSSION Diphenylphosphine oxide is a common P−H compound and is widely used in radical reactions.14 Therefore, we initially used XAS spectroscopy to investigate the reaction between CuBr2 and diphenylphosphine oxide (HOPh2) under a N2 atmosphere (Figure 1). The XANES spectrum of the solution of CuBr2 in

Figure 2. (A) k2-weighted R-space EXAFS spectrum of the reaction between CuBr2 and diphenylphosphine oxide in DMF: FT range, 2.30−14.5 Å−1; fiiting range, 1.59−2.23 Å. (B) EPR spectra (X band, 9.4 GHz, room temperature): red line, CuBr2 (0.5 mmol) in DMF; black line, CuBr2 (0.5 mmol) and diphenylphosphine oxide (1.0 mmol) in DMF at 80 °C.

of the EXAFS spectrum indicated that two bromide atoms coordinate to the formed Cu(I) species. The fitting results also revealed the Cu−Br bond length was 2.24 Å. Therefore, we assigned the obtained Cu(I) complex as a (CuBr2)− ate complex. An electron paramagnetic resonance (EPR) experiment was also conducted to gain insight into this reduction process. In the EPR spectra (Figure 2B), an EPR-silent species was formed after adding the diphenylphosphine oxide to the CuBr2 solution, further confirming this reduction process. XAS spectroscopy was also used to investigate the reaction between CuX2 (X = Br, Cl) and dialkyl phosphites, which are also very common P−H compounds used in many radical reactions (Figure 3A).15 When CuCl2 and dialkyl phosphites were mixed in DMF at 80 °C under a nitrogen atmosphere, we clearly observed that a new Cu species was formed, the edge energy of which is 8981.3 eV (Figure 3A, black line). The XANES spectrum of the reaction between CuBr2 and dialkyl phosphites showed the same edge energy as for the reaction between CuCl2 and dialkyl phosphites (Figure 3A, red line). The EXAFS spectra in Figure 3B revealed that the halide ion may coordinate with formed Cu(I) species. In addition, the 31P

Figure 1. XANES and EXAFS spectra of reactions between CuX2 and diphenylphosphine oxide. (A) XANES spectra; black line, CuCl2 (0.5 mmol) and diphenylphosphine oxide (1.0 mmol) in DMF at 80 °C; red line, CuBr2 (0.5 mmol) and diphenylphosphine oxide (1.0 mmol) in DMF at 80 °C; blue line, CuBr2 (0.5 mmol) in DMF at room temperature. (B) EXAFS spectra: red line, CuBr2 (0.5 mmol) and diphenylphosphine oxide (1.0 mmol) in DMF at 80 °C; black line, CuCl2 (0.5 mmol) and diphenylphosphine oxide (1.0 mmol) in DMF at 80 °C.

DMF gave a pre-edge at 8976.6 eV (Figure 1A, blue line). After the addition of diphenylphosphine oxide in CuBr2 solution, a new Cu species with an edge energy of 8981.7 eV was clearly detected (Figure 1A, red line). In comparison with the edge energy of a reported CuCl sample,12 this result revealed that a Cu(I) species was formed in the reaction system. B

DOI: 10.1021/acs.organomet.6b00034 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 4. XANES spectra of various Cu species: black line, Cu(OAc)2 (0.5 mmol) and dialkyl phosphites (1.0 mmol) in DMF at 80 °C; red line, Cu(OAc)2 (0.5 mmol), dialkyl phosphites (1.0 mmol), and LiCl (1.0 mmol) in DMF at 80 °C.

insights into the reactions of P−H compounds, which would be useful for an understanding of the mechanism for phosphorus radical chemistry and the future design of reactions.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

X-ray absorption measurements were acquired on the beamline of the beanline 17C1 of the National Synchrotron Radiation Research Center (NSSRC) in Taiwan. The data were collected in transmission mode. Detailed procedures of the XAS experiments are given in the Supporting Information. EPR spectra were measured on a Bruker X-band A200 spectrometer. The sample was placed in a thin small tube and then recorded with a EPR spectrometer at the indicated temperature and parameters.

Figure 3. XANES and EXAFS spectra of reactions between CuX2 and dialkyl phosphites, (A) XANES spectra: black line, CuCl2 (0.5 mmol) and dialkyl phosphites (1.0 mmol) in DMF at 80 °C; red line, CuBr2 (0.5 mmol) and dialkyl phosphites (1.0 mmol) in DMF at 80 °C. (B) EXAFS spectra: black line, CuCl2 (0.5 mmol) and dialkyl phosphites (1.0 mmol) in DMF at 80 °C; red line, CuBr2 (0.5 mmol) and dialkyl phosphites (1.0 mmol) in DMF at 80 °C.

NMR spectrum has also been utilized to study the reaction between CuBr2 and HP(O)(OEt)2 in DMF. A peak at −12.7 ppm was detected, which was assigned as Br−P(O)(OEt)2. Br− P(O)(OEt)2 may be formed from a phosphine radical, which was generated through single electron transfer (SET) between Cu(II) and the P−H compound. Therefore, these above results reveal that the dialkyl phosphites can act as reducing agents to reduce Cu(II) species. On the basis of our previous reports, the halide may have a large effect on the reduction process.16 We also utilized X-ray absorption spectroscopy to reveal the halide effect for the Cu(II) reduction process in this reaction system. A mixture of Cu(I) and Cu(II) was generated, and only some of the Cu(II) was reduced for the reaction between Cu(OAc)2 and dialkyl phosphites (Figure 4, black line). The red line in Figure 4 demonstrated that Cu(OAc)2 could be quickly reduced by dialkyl phosphites and the Cu(I) species was generated in the presence of 2 equiv of LiCl. Therefore, these results revealed that LiCl can be a mediator to promote the reduction of Cu(II) by P−H compounds.

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00034. Experimental procedures, XAFS spectra, and EPR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.L.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (2012CB725302), the National Natural Science Foundation of China (21390400, 21520102003, 21272180, and 21302148), the Hubei Province Natural Science Foundation of China (2013CFA081), the Research Fund for the Doctoral Program of Higher Education of China (20120141130002), and the M i n i s t r y o f S c i e n ce a n d T e c h n o l o g y o f C h i n a (2012YQ120060). The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also appreciated. The X-ray absorption spectroscopy studies were carried out at beamline 17C1 of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. We also thank all the team members at BL14W1 of the Shanghai Synchrotron Radiation Facilities (SSRF).

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CONCLUSIONS In conclusion, we have directly observed the reduction of Cu(II) by P−H compounds utilizing XAS and EPR spectroscopy. Two commonly used P−H compounds, diphenylphosphine oxide and dialkyl phosphite, have been selected for this reduction system. The structure of generate Cu(I) species is revealed through the fitting result of XAS spectrum. Furthermore, the halide ion can promote the reduction of Cu(II). These spectroscopic investigations provide useful C

DOI: 10.1021/acs.organomet.6b00034 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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REFERENCES

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DOI: 10.1021/acs.organomet.6b00034 Organometallics XXXX, XXX, XXX−XXX