Green, homogeneous oxidation of alcohols by dimeric copper(II) complexes

Abstract

Three pyrazole derivatives, 3,5-dimethyl-1H-pyrazole (DMPz) (I), 3-methyl-5-phenyl-1H-pyrazole (MPPz) (II), and 3,5-diphenyl-1H-pyrazole (DPPz) (III), were prepared via reacting semicarbazide hydrochloride with the acetylacetone, 1-phenylbutane-1,3-dione, and 1,3-diphenylpropane-1,3-dione, respectively. Complexes 1–3 were isolated by reacting CuCl₂·2H₂O with I–III, respectively, and characterized by CHNS elemental analyses, FT-IR, UV-Vis, ¹H and ¹³C NMR, EPR spectra, and TGA/DTA. Molecular structures of the pyrazole derivatives I–III and copper(II) complexes 2 and 3 were studied through single-crystal XRD analysis to confirm their molecular structures. Overlapping of hyperfine splitting in the EPR spectra of the dimeric copper(II) complexes 1–3 indicates that both copper centers do not possess the same electronic environment in solution. The copper(II) complexes are dimeric in solid state as well as in solution and catalyze the oxidation of various primary and secondary alcohols selectively. Catalysts 1–3 show more than 92% product selectivity toward ketones during the oxidation of secondary alcohols. Surprisingly primary alcohols, which are relatively difficult to oxidize, produce carboxylic acid as a major product (48%–90% selectivity) irrespective of catalytic systems. The selectivity for carboxylic acid rises with decreasing the carbon chain length of the alcohols. An eco-friendly and affordable catalytic system for oxidation of alcohols is developed by the utilization of H₂O₂, a green oxidant, and water, a clean and greener solvent, which is a notable aspect of the study.

Graphical abstract

Keywords:

• Homogeneous catalysis
• copper complexes
• pyrazole derivatives
• EPR
• alcohol oxidation

Acknowledgements

C.H. is very thankful to SAIF Mumbai for recording the TGA/DTA and EPR spectra. The authors also acknowledge CRF, IIT Ropar for ESI-MS data. The authors would like to thank CIF, IISER Bhopal, for the 400 MHz NMR facility. C.H. acknowledges CFR IIT(ISM) Dhanbad for providing single-crystal XRD analysis. A.M. is thankful to IIT (ISM) for fellowship. The GC and GC-MS used in this study were procured from the grant given by the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), grant no. SB/FT/CS-027/2014 and grant no. SB-EMEQ-055/2014, respectively, Government of India, New Delhi, India to C.H.

Disclosure statement

There are no conflicts of interest to declare.

Additional information

Funding

C.H. thanks the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), the Government of India, New Delhi for financial support [grant No. SB-EMQ-055/2014].