V(iv), Fe(ii), Ni(ii) and Cu(ii) complexes bearing 2,2,2-tris(pyrazol-1-yl)ethyl methanesulfonate: application as catalysts for the cyclooctane oxidation

Abstract

New water-soluble scorpionate complexes were synthesized and applied as selective catalysts for cyclooctane oxidation in presence of basic or acid additives.

C-Homoscorpionate Oxidation Catalysts—Electrochemical and Catalytic Activity

“…Their measured redox potentials in volts vs. saturated calomel electrode (V vs. SCE) and the eventual reversibility of the redox process, are indicated in Table 1. All authors found that C-homoscorpionate ligands are electrochemically inert in the potential range of −2.0 V to 2.0 V vs. SCE, at the used experimental conditions [46,[49][50][51][52][53][54]89,[91][92][93], thus no ligand centered oxidation or reduction has been reported to date.…”

“…In addition, the tripodal functionalized coordination flexibility (e.g., N 3 -or N 2 O-coordination modes for sulfonated derivatives, Figure 5d) involving the functionalized methine carbon [7,8,14,45,49,50] is also tailored by such co-ligands, an important pre-requisite for their catalytic activity.…”

“…The main applications of C-homoscorpionate complexes as catalysts for oxidative reactions, where the involvement of metal redox processes is crucial for the catalytic activity, are the important and challenging single-pot oxidation of gaseous (e.g., direct oxidation of methane to carboxylic acids, Scheme 1) [47,62]) and liquid (e.g., cyclohexane to the correspondent alcohol-ketone mixture, Scheme 2) alkanes [7,8,13,43,[46][47][48][49]53,57,58,[63][64][65][66][67][68]. Indeed, oxidation of alkanes has been the object of considerable research [69][70][71][72][73][74][75][76][77][78], but still constitutes a serious challenge to modern chemistry, owing to the high inertness of these substrates.…”

“…A systematic investigation of the coordination behavior of new C-scorpionates, as well as some of the known ones, toward a variety of transition metals (e.g., V [44,[46][47][48][49], Mo [17,19,50], Re [51,52], Fe [44,46,49,53], Ru [54], Co [12,13], Ni [44,49,55], Pd [44], Cu [43,45,46,56,57], Ag [14], Au [58] or Zn [44,56]) followed, leading to new classes of complexes exhibiting different types of coordination modes (Figures 4 and 5). Like the pincer of a scorpion, these versatile tripodal ligands bind metal centers with nitrogen atoms from two pyrazolyl rings attached to the central carbon atom; the third pyrazolyl attached to carbon rotates forward like a scorpion's tail to "sting" the metal; hence the name of "scorpionates" (Figure 4).…”

“…Scheme 2. Peroxidative oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA oil) in aqueous medium, catalyzed by C-scorpionate catalysts [7,8,13,43,[46][47][48][49][57][58][59][60][61][62][63][64][65][66][67][68].…”