**1. Introduction**

The oxidation of cyclic olefins is a reaction that has become of interest for many industries, such as the agrochemical, pharmaceutical, and perfumery industries, as well as the manufacture of adhesives [1–4], because the products of this reaction such as epoxides, alcohol, ketones, and aldehydes are used in commodities. However, it has become necessary to find alternative routes of production for these compounds, in order to decrease the carbon footprint and the generation of environmentally harmful sub-products [5].

Catalysis is an important tool in many industrial processes when it comes to reducing the energy cost, as well as the reaction time and the sub-product generation, enhancing the selectivity. In this sense, homogeneous and supported catalysts are classically used in the oxidation of cycloalkenes [6,7]. However, they present well known problems such as the inability to separate and recover the homogeneous catalyst, and on the other hand, the leaching of the catalyst from the support, preventing its reusability. Heterogeneous catalysts, particularly inorganic polymers, present a good alternative, as these are insoluble and stable under the usual cycloalkene oxidation conditions. The corresponding catalytic activity of inorganic polymers can be found in the literature in several reviews [8–14]. These inorganic polymers are based on metallic cores coordinated with organic linkers, with the metal centres being the catalytically active species. Transition metal ions are a good choice when designing these systems as their cost is low and their abundance is high. In this context, cobalt (II) containing molecular sieves show activity in the epoxidation with molecular oxygen of styrene at 100 ◦C, presenting a conversion of 45% with a selectivity of 62% to the desired epoxide [15]. The same research group reported a family of cobalt (II) exchanged zeolite X catalysts, which permitted, for the studied catalytic systems, an almost complete conversion of styrene [16].

Copper (I) species have been used as a catalyst in the aerobic oxidation of different amines to imines, with a conversion ranging in most cases from 85% to 95% using CuCl [17]. A different biomimetic Cu (I) species, [Cu(CH3CN)4]PF6, was reported for the aerobic oxidation of secondary alcohols, yielding over a 90% conversion in most experiments [18]. A Cu(II) compound, used as a catalyst for an aerobic oxidation was Cu2(OH)PO4, and this catalyst produced a 30% conversion for styrene and 47% for cyclohexene [19]. Two catalysts, based on Cu(II) using N-benzylethanolamine or triisopropanolamine as ligands, were also used in the oxidation of cycloalkanes assisted by H2O2; a conversion of 23% was achieved for cycloheptane [20].

However, owing to the lower catalytic activity of some of these systems, as compared with that of the homogeneous ones, it is sometimes necessary to add co-oxidants such as TEMPO [21], isobutyraldehyde [22], TBHP [23,24], or H2O2 [25]. For example, Cu3(BTC)2 (BTC, 1,3,5-benzenetricarboxylate) is a Cu(II) complex assisted by TEMPO, used as a catalyst for the oxidation of benzylic alcohols, yielding 89% of conversion [5]. However, these co-catalysts sometimes produce harmful by-products, such as tert-butanol, in the case of TBHP. Therefore, aerobic oxidation using only molecular oxygen as the oxidizing agen<sup>t</sup> becomes an ideal goal to achieve.

Metal organic frameworks (MOFs) can be modified by changing either the metal ions or the organic linkers, thus modifying the catalytic properties or the capability of adsorbing gases. Monometallic MOFs based on copper (II) [26,27], cobalt (II) [28,29], vanadium (IV) [30,31], or iron (III) ions [32–34] have been used in heterogeneous catalytic systems. The use of MOFs as catalysts in the aerobic oxidation of olefins has been reported by Fu et al. [35]. These researchers used catalysts based on copper (II) and cobalt (II) with 2,5-dihydroxyterephthalic acid (DOBDC), [M2(DOBDC) (H2O)2]·8H2O (M= CuII or CoII ), and molecular oxygen to oxidize cyclohexene at 80 ◦C. Under these conditions and after 15 h, the reaction only produced 14.6% conversion for the copper catalyst and 10.5% for the cobalt catalyst. Tuci et al. found better results using a different cobalt (II) catalyst, [Co(L-RR)(H2O)]·H2O (L-RR = (R,R)-thiazolidine-2,4-dicarboxylate), at 70 ◦C. However, molecular oxygen pressure was increased from 1 to 5 bar. A conversion of 37% with a selectivity of 49% to 2-cyclohexen-1-one was achieved [36].

Heterometallic MOFs also present activity for the oxidation of different substrates. For example, copper (II)-based MOFs with adsorbed palladium or gold nanoparticles have been used in the oxidation of benzylic alcohols [37,38], and a bimetallic catalyst of Pd–Au nanoparticles supported on an aluminium (III) MOF was used in the aerobic oxidative reaction of carbonylation of amines [39]. However, as mentioned, these systems use supported catalysts, which may not be optimal.

Our group has worked with heterogeneous CuII- 4f MOF catalysts, tuning the organic linkers [40], the *4f* lanthanide ion [41], and using different substrates for the aerobic oxidation reaction [42]. We now report the study of the effect of changing the *3d* transition metal ion of two MOFs used as catalysts in a

solvent-free system, with molecular oxygen as an oxidant in the oxidation of cyclohexene, without the use of a co-catalyst. Cobalt (II) and copper (II) were chosen as the *3d* redox transition metal ions, while the *4f* ion was lanthanum (III). The studied catalysts were {[La2Cu3(μ-H2O)(ODA)6(H2O)3]·3H2O}n (LaCuODA) (1) and {[La2Co3(ODA)6(H2O)6]·12H2O}n (LaCoODA) (2) (H2ODA = oxydiacetic acid).
