**3. Discussion**

To understand these results, it is important to remark that the catalysts present a few di fferences, mentioned in the structural description of the environment and geometry of the *3d* metal ion, being **1** square pyramidal and **2** octahedral. The di fference between them is the number of water molecules in the axial positions (one and two, respectively). Taking into account the data obtained in the TGA measurements, it was assumed that, under the reaction conditions, all water molecules were removed, and thus both compounds could be considered isostructural. The crystal structure shows channels along the *c axis* for both catalysts, with water molecules inside. The TGA analyses permit to infer that, for the used reaction conditions, these water molecules could leave the channels, making the channels partially free for the entry of the substrate and the oxidant. Besides, the CO2 absorption measurements complement the crystallographic results, showing that the BET and Langmuir surface areas are bigger for LaCoODA as compared with LaCuODA. Thus, it is possible to suppose that the catalyst based on CoII has a greater amount of active sites, favoring in this way the interaction between the catalyst and the substrate/oxidant. Moreover, even though both catalysts permit to obtain the same major product (2-cyclohexen-1-one), the distribution of the products is completely di fferent. Table 3 shows that the cobalt (II) MOF with better catalyst performance also permits to obtain a higher selectivity for 2-cyclohexen-1-one.

However, the catalysts present additional di fferences that could permit to explain the obtained results. Both transition metal ions have di fferent chemical properties, such as the redox potential or the Lewis acidity. These properties can modulate the reaction mechanism and the activation of the oxidant [45]. The implied redox properties during the oxidation reaction are di fferent because, for LaCoODA, the CoII/CoIII couple must be active, while for LaCuODA, the redox couple is CuII/CuI. For LaCoODA, the activation will occur with the oxidation of the cobalt(II) centres through a single electron transfer to molecular oxygen and the formation of the superoxide anion [35]. In the case of LaCuODA, we recently reported the generation of a Cu−O2 adduct at the beginning of the catalytic cycle of the oxidation reaction [41]. In the formation of this adduct, the Lewis acidity plays an important role, with the interaction between the catalyst and the oxidant being an acid–base interaction. [45] When we compare the acid sites and the amount of these sites in the catalyst, it is possible to conclude that the LaCuODA has stronger acid sites than LaCoODA. Apparently, the greater surface area of LaCoODA and the redox properties of the cobalt (II) ion predominate over the acid properties of the copper (II) ion, thus LaCoODA has a better catalytic performance for the oxidation reaction of cyclohexene. It is possible to conclude that the combination of the structural and physicochemical properties of the studied catalytic systems is determinant for the catalytic behavior.

As LaCoODA showed the best performance, this catalyst was used for the optimization of some catalytic parameters. The first parameter to be studied was the thermal dependence of the conversion. Figure 2 (Table S1) shows a linear dependence between the conversion and the reaction temperature. As mentioned above, the increase of the conversion with temperature can be explained by assuming that the channels present in the catalyst start to release the encapsulated water molecules, and the presence of free space in the channels facilities the interaction between the metal centres and the oxidant/substrate.

**Figure 2.** Thermal dependence of conversion, using LaCoODA as catalyst.

To test this assumption, we compared the catalytic performance of non-activated LaCoODA and thermally activated LaCoODA. Figure 3 (Table S2) summarizes the results, which shows that both the as prepared catalyst and the activated catalyst display a similar time dependence in their activity. As expected at short times of reaction, the oxidation process increases significantly as the reaction evolves, but it is possible to observe that, after 12 h, the activity starts to approach an asymptotic value, increasing only slightly for a sample of 24 h of reaction. The overlap of the curves indicates that the catalytic activity is similar within the experimental error for both the activated and non-activated catalyst. Apparently, the rehydration process of the catalyst is so fast that, practically, it is not possible to enhance the catalytic performance by thermal activation.

**Figure 3.** Time dependence of conversion using LaCoODA as catalyst. Non-activated (-); activated (•).

If we compare the results obtained with other previously reported works with similar catalysts, it is possible to find interesting aspects to discuss. For example, our group reported in 2017 [46] the catalytic performance of LaCuODA in the oxidation reaction of cyclohexene, using *tert*-butylhydroperoxide (TBHP) as an oxidant in DCE/water as a reaction medium. The achieved catalytic performance under the studied conditions in this work, that is, solvent free and O2 as oxidant, was better in conversion and selectivity than using TBHP and the biphasic medium (Table 4). We propose that, considering that, in the DCE/water medium, the channels are fully occupied with water molecules, there must be many water molecules obstructing the interaction of the active metal centres of the catalyst and the substrate/oxidant.


**Table 4.** Catalytic results using LaCuODA after 24 h of oxidation of cyclohexene at 75 ◦C.

\* When using *tert*-butylhydroperoxide (TBHP) as oxidant, 1,2-cyclohexenediol was also detected as a minor reaction product (8%).

Table 5 shows the comparison between different catalytic results for CoII-based MOF catalysts. All four catalysts have a distorted octahedral geometry around the cobalt(II) ion. However, one position of the octahedron is occupied by a water molecule for (**III**) and by two water molecules for **2**, **I**, and **II**. [35,36,43,44,47] Besides, all the compounds have channels with water molecules inside them, but the crystallographic diameter of the pores varies among them. Compounds **2**, **I** and **II** have a diameter *ca.* 11 Å, while compound **III** has only 5.7 Å. Despite the structural similarities of the pores of **2**, **I**, and **II**, the catalytic results reveal significant differences. Moreover, from the comparison using two sets of data (one set corresponds to 24 h of reaction, that is, catalysts **2** and **III**, and the second one to 10 and 12 hours of reaction corresponds to data for **2**, **I**, and **II**, shown in Table 5), it becomes evident that **2** has by far the best catalytic performance.

**Table 5.** Catalytic results of different CoII-based metal organic frameworks (MOFs) using solvent free conditions.


**Flow**: continuous flow of oxygen at 1 bar of pressure. **Balloon:** oxygen atmosphere, using a balloon fully filled with oxygen. **Charged**: the reactor is pressurized with oxygen at 1 bar of pressure.

However, the catalytic systems present some differences that could permit to explain the obtained results. As the structures do not clarify these differences, maybe the reaction conditions can give some light on the obtained data. As the temperature is quite similar for all the reported systems, the oxygen pressure is an interesting parameter to analyze. Even though the oxygen pressure used is the same for all the catalytic systems, the way of supplying the oxidant to the reacting substrate is not the same. Thus, the amount of oxygen in the reactor varies depending on the source used. That is, the pressurized oxygen in the reaction vessel has an initial finite concentration [36], while a continuous oxygen flow maintains the concentration of the oxidant in the reaction vessel. On the other hand, the oxygen concentration provided by a balloon is variable [35,47].

Therefore, it is possible to conclude that oxygen concentration is determinant in the reaction mechanism. Depending on the amount of oxygen in the reaction medium, the chance to obtain the interaction between the active site and oxidant/substrate will be modified [42], and thus the onset of the chain reactions that will form the products.

#### **4. Materials and Methods**
