Please cite this article in press as: S.G. Mncube, M.D. Bala, Application of 1,2,3-triazolylidene nickel complexes for the catalytic oxidation of n-octane, Mol. Catal. (2017), http://dx.doi.org/10.1016/j.mcat.2017.03.005 ARTICLE IN PRESS G Model MCAT-107; No. of Pages 8 Molecular Catalysis xxx (2017) xxx–xxx Contents lists available at ScienceDirect Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat Editor’s choice paper Application of 1,2,3-triazolylidene nickel complexes for the catalytic oxidation of n-octane Siyabonga G. Mncube, Muhammad D. Bala ∗ School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa a r t i c l e i n f o Article history: Received 7 December 2016 Received in revised form 26 February 2017 Accepted 6 March 2017 Available online xxx Keywords: Paraffin oxidation n-Octane Nickel complexes Triazolium N-heterocyclic carbenes a b s t r a c t Half-sandwich nickel complexes bearing a variety of mesoionic N-heterocyclic carbene lig- ands ( 5 -cyclopentadienyl)-iodo-{1-(R)-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium-5-yl}nickel; where R = phenyl (3a); 2-ethoxy-2-oxoethyl (3b); propyl (3c); benzyl (3d) were synthesized by the reaction of nickelocene with the respective triazolium salts. The complexes were characterized by HRMS and multi- nuclear NMR, and the solid state structures of 3c and 3d were elucidated by single crystal X-ray diffraction analysis in which both complexes displayed a trigonal planar geometry. As catalysts for the oxidation of n-octane in the presence of oxidants under mild reaction conditions, all the complexes showed activity for the substrate yielding a range of oxygenated products. Under optimized reaction conditions, catalyst 3c with lighter substituents on the triazolium ring exhibited the highest catalytic activity of 15% total conversion to products. With H 2 O 2 as the more productive oxidant, the preferential activation of internal carbons led to the observation of a mixture of octanones as the dominant product stream of the oxidation reaction. © 2017 Elsevier B.V. All rights reserved. Introduction N-Heterocyclic carbenes (NHC) have become one of the most popular supporting ligands in organometallic chemistry and homo- geneous catalysis, due mainly to unique binding properties that make them superior to related two-electron donor ligands such as the ubiquitous phosphines [1–3]. The Arduengo-type imidazol-2- ylidenes dominate NHC chemistry because they are comparatively easy to prepare from readily available and affordable starting mate- rials and have better stability than most phosphines [4,5]. However, limitations in their donating ability to transition metals, especially to highly reactive base metals (Fe, Co, Ni) has led to moves toward other heterocyclic azolium scaffolds that include triazolylidenes or so-called mesoionic carbene (mNHC) ligands [6]. This is related to the fact that 1,2,3-triazolylidenes are strong -donors that stabilise and provide access to highly reactive metal centres especially dur- ing catalysis [7,8]. In addition, the ease with which substituents around the triazole moiety may be fine-tuned to yield a myriad of ancillary mNHC ligands that display a wide variety of requi- site properties has also added to the intense interest in the use of their metal complexes in catalysis [9,10]. In spite of all the interest, the isolation of metal-NHC complexes is non-trivial, and there are ∗ Corresponding author. E-mail addresses: bala@ukzn.ac.za, mdmdbala@gmail.com (M.D. Bala). many synthetic techniques available for their preparation, of which the earliest and most utilised involves the preparation of “free” car- benes [11] that are subsequently reacted with metal precursors. However, concerns on the relative stability of “free” carbene lig- ands have resulted in the development of in situ approaches as alternative methods. Thus, the direct substitution of a labile basic metal bound ligand by a mNHC has been used as an effective strat- egy for the synthesis of triazolylidene transition metal complexes [12]. An example of this route developed by Cowley and co-workers [13] is the direct reaction of nickelocene with azolium salts to yield NHC-nickel complexes. Later, Albrecht and co-workers [14] adapted the method to the synthesis of mNHC-Ni complexes used in catalysis. In general, base metal complexes of this nature have demonstrated catalytic activity in a variety of reactions that include cross-coupling reactions [15,16], hydroarylation of alkynes [17] and enantioselective cyclopentane synthesis [18]. In much broader terms, NHC-metal complexes have been utilised in cross coupling reactions [19–21] and ring-closing metathesis [22] to mention just a few of their numerous applications. However, due to the rela- tive inertness of saturated paraffin hydrocarbon C sp 3 –H bonds, the area of alkane oxidation has remained a challenge that is relatively unexplored [23,24]. Hence, as a continuation of our contributions [25–27] to the subject of paraffin oxidation using non-precious base metal complexes, we herein present the first use of mNHC-Ni com- plexes prepared via the direct (ligand substitution) route for the oxidation of n-octane under mild reaction conditions. http://dx.doi.org/10.1016/j.mcat.2017.03.005 2468-8231/© 2017 Elsevier B.V. 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