Vol.:(0123456789) 1 3 Transition Metal Chemistry https://doi.org/10.1007/s11243-018-0278-5 Homogeneous oxidation reactions catalysed by in situ-generated triazolylidene copper(I) complexes Siyabonga G. Mncube 1  · Muhammad D. Bala 1 Received: 28 August 2018 / Accepted: 4 October 2018 © Springer Nature Switzerland AG 2018 Abstract Four new Cu complexes bearing triazolylidene ligands 1-(R)-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium-5-yl: R = phenyl (2a), mesitylenyl (2b), propyl (2c), hexyl (2d) (NHC) were synthesised in high yields. Characterisation by spectroscopic and analytical methods confrmed the molecular composition of the complexes as NHC–Cu-I. The complexes 2(a–d) bearing NHC wingtip variations were tested as in situ-generated catalysts for homogeneous oxidation catalysis with H 2 O 2 as oxidant. The in situ technique was adopted for ease of application and to circumvent the poor stability of the complexes in solution. The results showed that the NHC–Cu-I complexes are capable of initiating oxidation reactions, yielding ketones/aldehydes as dominant products for the oxidation of alkanes under optimised reaction conditions, with complexes bearing aliphatic N-substituents showing the highest catalytic activities. Oxidation of toluene with 2c resulted in a mixture of benzaldehyde and benzyl alcohol as the main products. Also, 2c catalysed the oxidation of n-octane, yielding a mixture of mainly C-8 oxi- dation products with over 75% selectivity for the isomeric octanones. Analysis of regioselectivity indicated that the internal C sp 3–H bonds of n-octane [especially C(2)] are more reactive than the terminal ones. Introduction The oxidation of saturated substrates via activation of unre- active bonds under mild catalytic conditions is still a chal- lenging global problem to both industry and academia [1]. Over the years, frst-row transition metal complexes have emerged as cheap and efective oxidation catalysts that are capable of operating under mild reaction conditions [2]. In particular, copper complexes bearing bulky organic ligands have shown great potential as homogeneous catalysts for the efcient oxidation of a wide range of substrates [3]. With regard to choice of ligand, N-heterocyclic carbenes (NHC) have become very prominent in organometallic chemistry and their metal complexes have made impor- tant contributions in catalysis. This is due to their ability to form stable covalent metal–ligand bonds and their ease of steric and electronic tuning through variation of the wingtip N-substituents of the azole ring [4]. Since the frst cationic di-carbene (NHC) 2 –Cu complex was reported by Arduengo [5] and later application of NHC–Cu complexes in cataly- sis by Woodward et al. [6], a great deal of catalytic and structural work in this area has continued to build on those foundations [7]. In spite of all the interest, the handling of NHC–Cu complexes is non-trivial, and concerns on the rela- tive stability of “free” NHC–Cu have resulted in the develop- ment of in situ catalytic approaches as alternative methods. This technique was frst reported in 2001 by Woodward, [6] from which a number of reports soon followed, all signify- ing that copper salts make better catalysts in the presence of NHC ligands [8–10]. Hence, in this report we present new NHC–Cu I catalysts generated in situ for the oxidation of a range of substrates under mild reaction conditions. Results and discussion Synthesis of the triazolylidene Cu(I) complexes We adopted the method of Son et al. [11] for the synthesis of the NHC–Cu complexes (2, Scheme 1). This is a very simple method akin to the well-established Ag 2 O route for Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11243-018-0278-5) contains supplementary material, which is available to authorized users. * Muhammad D. Bala bala@ukzn.ac.za 1 School of Chemistry and Physics, University of KwaZulu- Natal, Private Bag X54001, Durban 4000, South Africa