This journal is © The Royal Society of Chemistry 2014 Chem. Commun. Cite this: DOI: 10.1039/c4cc05720a Revealing the halide effect on the kinetics of the aerobic oxidation of Cu(I) to Cu(II)† Yi Deng,‡ ab Guanghui Zhang,‡ ab Xiaotian Qi, c Chao Liu, a Jeffrey T. Miller, b A. Jeremy Kropf, b Emilio E. Bunel, b Yu Lan* c and Aiwen Lei* ab In situ infrared (IR) and X-ray absorption near-edge structure (XANES) spectroscopic investigations reveal that different halide ligands have distinct effects on the aerobic oxidation of Cu(I) to Cu(II) in the presence of TMEDA (tetramethylethylenediamine). The iodide ligand gives the lowest rate and thus leads to the lowest catalytic reaction rate of aerobic oxidation of hydroquinone to benzoquinone. Further DFT calculations suggest that oxidation of CuI–TMEDA involves a side-on transition state, while oxidation of CuCl–TMEDA involves an end-on transition state which has a lower activation energy. Oxidation reactions play an important role in organic chemi- stry, among which aerobic oxidation is considered to be highly atom-economical and environmentally benign because of the abundance of molecular oxygen and the lack of toxic side products. 1–6 However, the high activation energy of molecular oxygen usually requires a transition metal catalyst. 7 Besides the traditionally used precious Pd catalysts, Cu catalysts have received considerable attention during the last few decades. 8–12 Significant progress has been achieved with Cu/O 2 chemistry and a range of selective and mild oxidation processes have been developed. 13–20 Great effort has also been made to develop an in-depth mechanistic understanding. 21–25 In addition, various ligands have been designed as incisive probes of Cu/O 2 chemi- stry. A judicious selection of the ligand coordination environ- ment, the steric effect, and the electronic effect has allowed the characterization of ‘‘trapped’’ intermediates, and thus provided valuable information on the structures and reactivity of various Cu/O 2 species. 7,26 Compared to the great attention paid to these novel ligands, halide ligands within the coordination sphere are usually considered to be less important, although most commercially available catalysts and pre-catalysts are halo-metal complexes. In most cases, the halide ligands are removed from the coordi- nation sphere first and replaced by other ligands. It has been documented that ubiquitous halide ligands are also invaluable for tuning the reactivity and selectivity of a catalyst for a given transformation. 27–30 However, the importance of the halide effect in Cu-catalyzed oxidative polymerization has been observed, 25 but has not been well understood yet. We believe further investigation into the oxidation states and kinetics should be able to provide valuable information for a better understanding of Cu/O 2 chemistry. 31,32 Herein, we communi- cate our in situ spectroscopic investigation into the distinct kinetic behaviour resulting from the halide ligands in Cu/O 2 chemistry in which the CuX/TMEDA (tetramethylethylenedi- amine) catalyst was selected, since TMEDA has been extensively used in Cu oxidation chemistry. Cu-catalyzed aerobic oxidation of hydroquinone to 1,4-benzo- quinone was selected as the model reaction, and various Cu(I) halides were initially tested in the presence of TMEDA 33 under the optimized conditions (Fig. S3–S5, ESI†). 34 Interestingly, excellent yields of benzoquinone (BQ) were obtained using CuCl and CuBr as the pre-catalysts within 20 minutes, while CuI yielded about 10% BQ in the same period. The detailed kinetic profiles were recorded by in situ IR (Fig. 1), in which the utilization of CuCl as the pre-catalyst afforded the highest reaction rate. CuBr performed slightly slower, while the use of CuI led to the lowest rate. These results implied the presence of the halide effect in Cu-catalyzed aerobic oxidation reaction. Notably, the linear time course data imply zero-order depen- dence on the hydroquinone, which suggests that the oxidation of Cu(I) is the rate-determining step (Fig. S8, ESI†). 35 All these clues prompted us to carry out a detailed mechanistic investigation. In order to find the deep rationale behind these interesting experimental findings, we attempted XANES spectroscopy to monitor the oxidation of Cu(I) catalysts in air without the a College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, China. E-mail: aiwenlei@whu.edu.cn b Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass avenue, Argonne, IL 60439, USA c School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, China. E-mail: lanyu@cqu.edu.cn † Electronic supplementary information (ESI) available: Experimental section and data analysis. See DOI: 10.1039/c4cc05720a ‡ Y. Deng and G. Zhang contributed equally to this work. Received 23rd July 2014, Accepted 1st November 2014 DOI: 10.1039/c4cc05720a www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 03 November 2014. Downloaded by Chongqing University on 01/12/2014 13:12:28. View Article Online View Journal