WO2024251660A1 - Process for the photocatalytic oxidation of alkanes and aromatic hydrocarbons - Google Patents

Abstract

The present invention relates to a process for the photocatalytic oxidation of a substrate selected from: an alkane, a cycloalkane, an aromatic hydrocarbon or an alkyl aromatic hydrocarbon comprising contacting the substrate with a θ-type metallacarborane catalyst, and an oxidising agent selected from air, O₂ or Na₂S₂O₈ in water, at room temperature and under UV radiation.

Description

Process for the

:ic oxidation of alkanes and aromatic

The invention relates to a process for oxidation alkanes and aromatic hydrocarbons to form alcohols. More particularly, the invention relates to a process for the photocatalytic oxidation of the said substrates in presence of 0-type metallacarborane catalysts in water and mild conditions.

Thus, this invention could be framed in the field of synthetic chemistry.

BACKGROUND ART

The development of methods for the oxidative transformation of inert C-H bonds into more reactive functional groups or involving changes in physical properties, such as generating alcohols, esters, ethers, amines, halides, carbon-carbon bonds is a key aspect of current research and necessary for sustainable development, avoiding as much as possible the extraction of hydrocarbons. It should be noted that C-H bonds are typically strong and kinetically inert, therefore, strong conditions are often required to achieve the activation of such bond, and this causes chemoselective oxidation to be challenging because overoxidation is highly thermodynamically favourable.

Current research is focused on addressing all these challenges and developing new chemical methods for the chemo-, regio- and stereoselective functionalization of C-H bonds in the context of complex organic molecules, in which transition metals are used as catalysts.

Regarding cases of high industrial interest, such as the oxidation of the C-H bond of methane, benzene or mixed alkyl/aromatic hydrocarbons, it is observed that the oxidation of simple alkanes and arenes (such as methane and benzene), in a single step, is a problem of strategic importance worldwide, as natural gas (which is >90% methane) is becoming the precursor for carbon-containing chemicals and liquid fuels. Currently, methane derivatization is achieved by steam reforming to obtain synthesis gas, followed by the Fisher Tropsch process (to access higher alkanes) or methanol synthesis. However, many large chemical companies have found this two-step sequence too costly and inefficient for long-term application. Direct conversion of benzene to phenol is another important industrial goal, as the currently practiced phenol synthesis (the "cumene process") is energy intensive and low yielding.

Some strategies proposed in the state of the art to oxidise C-H bonds, are found, for example, in next document: Joel Rosenthal et al. J. Am. Chem. Soc. 2006, 128, 20, 6546-6547, which discloses the catalytic oxidation of C-H bonds of hydrocarbons by the electron-deficient Pacman porphyrin, (DPDF)Fe2O, using visible light and molecular oxygen as the terminal oxidant and oxygen atom source. Also, next document: Junghyun Hong et al. ACS Catal. 2013, 3, 9, 2154-2157, it is disclosed a new selective hydrocarbon oxidation heterogeneous catalyst obtained by tethering an iron-coordinated cavitand to the surfaces of a SBA-15 mesoporous material. The resulting material was shown to catalyse the oxidation of cyclic hydrocarbons at room temperature and to be quite robust and easily recyclable.

Taking into account the usefulness and necessity of a method to efficiently oxidise inert C-H bonds in mild conditions, the present invention propose a new process for the transformation of alkanes and aromatic hydrocarbons into their respective alcohols using 0-type metal lacarboranes as catalysts. This new process means an advantageous alternative to the known methods of the state of the art.

The compounds 0-type metallacarboranes (also called, metallabisdicarbollides) are anionic compounds with formula M’[M(m,n-C2B9Hn._yXy)2], where M’ is the counterion, M is a transition metal (e.g. Cr, Fe, Co, Ni, Ru, among others) or a main group metal (e.g. Al, among others); X is halogen, alkyl, chalcogen, or any other substituent; m and n are numbers indicating the position of the carbon atoms in the cluster; and y is the number of non-hydrogen substituents.

Metallacarboranes are part of a broad family of carborane species that contain in their structures one or more transition metals or lanthanides (R. N. Grimes. Coord. Chem. Rev. 2000, 200, 773-81 1 ; N. Hosmane and J. Maguire. Comprehensive Organometallic Chemistry III. 2007, 3, 175-264). The metallabisdicarbollides are the metallacarboranes more studied. They are formed by two dianionic dicarbollide clusters with formula [7,8-C2B₉Hn]^2_ as ligands with a central transition metal which is located endo-cluster. The first metallabisdicarbollide was synthesized by M. F Hawthorne et al. (M. F. Hawthorne, D. C. Young and P. A. Wegner. J. Am. Chem. Soc., 1965, 87, 1818-1819), [3,3’-Fe(1 ,2-C2B9Hii)₂]^_, named ferrabis(dicarbollide), [o-FESAN]- or simply FESAN (Figure 1 b). Sometime later, the same authors synthetized a similar complex but based on cobalt, [3,3’-Co(1 ,2-C2B₉Hn)2]^_, named cobaltabis(dicarbollide), [o-COSAN]- or simply COSAN (Figure 1 a)( M. F. Hawthorne and T. D. Andrews. Chem. Commun, , 1965, 443-444).

In COSAN and FESAN the Co³⁺ and Fe³⁺ metal ions are linked to two dicarbollide ligands each one with two negative charges, resulting in a global negative charge delocalized in the overall molecule. This negative charge is delocalized throughout the volume of the molecule leading to a low charge density (C. Masalles, S. Borros, C. Vihas and F. Teixidor. Adv. Mater., 2000, 12, 1199-1202) and thanks to the high molecular volume allows to the molecule have a high thermal and chemical stability (C. Vihas, S. Gomez, J. Bertran, F. Teixidor, J.F. Dozol, H. Rouquette, H, Chem. Commun. 1998, 2, 191 -192; C. Vihas, S. Gomez, J. Bertran, F. Teixidor, J.F. Dozol, H. Rouquette, H, Inorg. Chem. 1998, 37(14), 3640-3643), global aromaticity (J. Poater, C. Vihas, I. Bennour, s.E. Gordils, M. Sola, F. Teixidor, J. Am. Chem. Soc. 2020, 142(20), 9396-9407) as well as amphiphilic properties (P. Bauduin, S. Prevost, P. Farras, F. Teixidor, O. Diat and T. Zemb. Angew. Chem. Int. Ed., 201 1 , 50, 5298- 5300), and diverse conformational water/polar medium behaviour (E. J. Juarez-Perez, R. Nunez, C. Vihas, R. Sillanpaa and F. Teixidor. Eur. J. Inorg. Chem, 2010, 2385- 2392; D. C. Malaspina, C. Vihas, F. Teixidor and J. Faraudo. Angew. Chem. Int. Ed., 2020, 59, 3088-3092).

Isabel Guerrero et al. (CHEMISTRY- A EUROPEAN JOURNAL, vol. 26, no. 22 (2020), 5027-5036) disclose a process using metallacarborane catalysts, more precisely Na[3,3'-Co(1 ,2-C2B₉HII)₂ to further oxidise alcohols to ketones or aldehydes with a very high selectivity in water.

SUMMARY OF THE INVENTION

The inventors have found that the 0-type metallacarboranes are useful as photoredox catalysts for the generation of alcohols or phenols from the corresponding alkane/benzene derivative in a single step, by UV light activation of the metallacarborane. The reaction takes place in water, at room temperature, the yields are between 90-100% and the catalyst can be in molecular form or heterogenized on magnetic particles or adsorbed on solids.

Then, a first aspect of the invention refers to a process (process of the invention) for the photocatalytic oxidation of a set of substrates selected from: alkanes, cycloalkanes, aromatic hydrocarbons, alkyl aromatic hydrocarbons and phenol comprising: contacting the substrate with a 0-type metallacarborane catalyst of formula M’[M(C2B9Hn._yX_y)2], wherein:

M’ is a counterion selected from: Na⁺, Li⁺, H⁺, K⁺ and [NH4]⁺ that make the metallacarborane soluble in water (as described in I. Fuentes, A. Andrio, F. Teixidor, C. Vihas, V. Compah, Phys. Chem. Chem. Phys. 2017, 19, 15177-151869; A. Zaulet, F. Teixidor, P. Bauduin, O. Diat, P. Hirva, A. Ofori, C. Vihas J. Organomet. Chem. 2018, 865, 214-225; M. Tarres, C. Vihas, P. Gonzalez-Cardoso, M. M. Hanninen, R. Sillanpaa, V. Dordovic, M. llchman, F. Teixidor, P. Matejcek, Chem. Eur. J. 2014, 20, 6786 - 6794),

M is a trivalent cation of a metal selected from Cr, Fe, Co, Ni, Ru and Al,

X is H, halogen, chalcogen-R or alkyl, preferably a C1-C10 alkyl, wherein R is H or C1- C alkyl, y is an integer selected from 0 to 1 1 , and an oxidising agent selected preferably from air, O2 or Na2S20s, in water, at room temperature and under UV radiation.

An embodiment of the present invention refers to a process (for the photocatalytic oxidation of a set of substrates selected from: alkanes, cycloalkanes, aromatic hydrocarbons or alkyl aromatic hydrocarbons comprising: contacting the substrate with a 0-type metallacarborane catalyst of formula M’[M(C2B9Hn._yX_y)2], wherein:

M’ is a counterion selected from: Na⁺, Li⁺, H⁺, K⁺and [NH4]⁺

M is a trivalent cation of a metal selected from Cr, Fe, Co, Ni, Ru and Al,

X is H, halogen or alkyl, preferably a C1-C10 alkyl, y is an integer selected from 0 to 11 , and an oxidising agent selected preferably from air, O2 or Na2S2O₈, in water, at room temperature and under UV radiation.

In the formula M’[M(C2B₉Hn.yXy)2], the position of the trivalent cation M in the 0-type metallacarborane as well as the position of the carbon atoms in the cluster of the 0- type metallacarborane could be indicated. The position could be indicated by adding the corresponding position numbers just before M and C, respectively, in the formula. In the vast majority of the situations, the position of M is 3,3’ or 2,2’ depending on the position of C atoms in the cluster that could be from 1 ,2 to 1 ,11. That is, one C atom would occupy position 1 if it is adjacent to M and the other carbon atom would occupy any position from 3 to 11. What it is clear is that M merges the two icosahedra that share one vertex occupied by M. In the odd case that the metal and at least one carbon are not adjacent the numbering changes.

Herein, room temperature is considered a temperature between 19 to 25^eC.

In a preferred embodiment, the catalyst is selected from Na[3,3’-Co(1 ,2-C2B₉Hn)2] (also referred as Na[o-COSAN]) ,Na[3,3’-Fe(1 ,2-C2B₉HI 1)2] (also referred as Na[o- FESAN]), Na[3,3’-Co(8,9,12-Cl3-1 ,2-C2B₉H₈)2 (also referred as Na[C/e-COSAN]) and H[3,3’-Co(8,9,12-Cl3-1 ,2-C2B₉H₈)2] (also referred as H[C/e-COSAN]). The numbers “3,3”’ of the formulas indicate the position of the metal (Co or Fe) in the 0-type metallacarborane. The numbers “1 ,2” indicates the position of the carbon atoms in the cluster of the 0-type metallacarborane. The numbers “8,9,12” has been included to indicate the position of Cl atoms bonded to boron atoms of the 0-type metallacarborane.

The metallabisdicarbollides Na[o-COSAN]) and Na[o-FESAN]), are prepared by metathesis reaction, e.g. cation-exchange resin as described in the prior art (I. Fuentes, A. Andrio, F. Teixidor, C. Vihas, V. Compah, Phys. Chem. Chem. Phys. 2017, 19, 15177-151869), from the water-insoluble Cs[o-COSAN] that is commercially available from i.e Katchem or Cs[o-FESAN] that is synthesised (M. F. Hawthorne, D. C. Young and P. A. Wegner. J. Am. Chem. Soc., 1965, 87, 1818-1819), respectively. In addition, recently, C. Vihas et al. have published a fast and very efficient green synthesis for this kind of metallacarboranes that does not require any solvent (I. Bennour, A. M. Cioran, F. Teixidor and C. Vihas. Green Chem. 2019, 21, 1925-1928).

H[Cl6-COSAN]) was prepared by a chlorination reaction, as described in the prior art (I. Fuentes, J. Pujols, C. Vihas, S. Ventura and F. Teixidor. Chem. Eur. J. 2019, 25, 12820), from the water-insoluble Cs[o-COSAN] that is commercially available from i.e Katchem. The subsequent methatesis reaction, e.g. cation-exchange resin that is described in the literature (I. Fuentes, A. Andrio, F. Teixidor, C. Vihas, V. Compa , Phys. Chem. Chem. Phys. 2017, 19, 15177-151869) was used to obtain the Na[Cle- COSAN]).

Then, metallacarboranes used in the present invention can be synthetized by following any of the procedures described in the prior art to obtain them.

In a preferred embodiment, when air or O2 are the oxidising agents, they are used at a pressure of 1 atm. However, higher pressures could be used, for example between 1 and 5 atm, or even higher.

In a preferred embodiment, the UV radiation has a wavelength (A) between 200 and 400 nm. More preferably, the A of the UV radiation is selected from: 253.7, 300, 352 or 368 nm and even more preferably, 300 or 352 nm.

In a preferred embodiment, the power of each lamp for the UV radiation is of 2.2 W.

In a preferred embodiment, the concentration of the substrate in the water solution is between 1.5x1 O'² and 0.3 M, more preferably, between 1.5x1 O'² and 2.5x10'¹M. Even more preferred concentrations of the substrate are selected from 1 ,5x10'²M, 4.5x1 O' ²M and 2.25x10 ¹ M.

In a preferred embodiment, the molar ratio catalyst: substrate is between 1 :1000 and 1 :30000. More preferably, the molar ratio catalyst: substrate is selected from 1 :1000, 1 :10000 and 1 :30000. In a preferred embodiment, if the solid oxidant Na2S20s is used, the molar ratio catalyst: substrate: oxidant is between 1 :1000:2000 and 1 :30000:60000. More preferably, the molar ratio catalyst: substrate: oxidant is selected from 1 :1000:2000, 1 :10000:20000 and 1 :30000:60000.

In a preferred embodiment, the process is carried out at pH 7. A base, such as K2CO3, NaOH or EtsN can be added to maintain said pH. Compound K2CO3 is the most preferred since it is more economical and easier to handle.

A “base”, a chemical species that donates electrons, accepts protons, or releases hydroxide (OHj ions in aqueous solution.

In a preferred embodiment, additives, e.g. surfactants among other can be added to the mixture of reaction.

In a preferred embodiment, the reaction time is between 1 h and 8h.

The alcohols obtained in the procedure of the present inventions can be monoalcohols, dialcohols or even polyalcohols. The skilled person could make variations within the scope of the described process with no difficulty, for example in the time of reaction, the concentration of the substrate or the catalyst used, to obtain the desired product.

The product of reaction can be isolated by extracting the reaction mixture with an organic solvent, such as dichloromethane or diethyl ether and then, evaporating the organic solvent under reduced pressure.

The term “alkane” refers to a branched or straight hydrocarbon chain, containing only single carbon-carbon bonds. Preferably the alkane used in the process of the present invention as starting material has a chain having 4 to 20 carbons (C4-C20 alkane). The alkane can be (including branched or straight isomers when possible), for example, methane, propane, butane, pentane, hexane, heptane, octane and so on. In a preferred embodiment, the alkane is n-hexane. The term “cycloalkane”, as used herein, refers to saturated cyclic hydrocarbons having from 3 to about 10 carbon atoms (C3-C10 cycloalkane), more usually from about 5 to about 8 carbon atoms (Cs-Cs cycloalkane). Non-limiting examples of cycloalkanes include cyclopentane, cyclohexane, cycloheptane, and cyclooctane. The term “cycloalkane” also includes, according to the present invention, saturated cyclic hydrocarbons having from 3 to about 10 carbon atoms, more usually from about 5 to about 8 carbon atoms, having one or more alkyl substituents with a number of carbon atoms preferably between 1 to 8. In a preferred embodiment, the cycloalkane is cyclohexane.

The term “aromatic hydrocarbon” is intended to mean an organic compound consisting of one or several aromatic cycles, that is, unsaturated cycles, having 4n+2 delocalized pi electrons (fused or linked together by a covalent bond). Examples are benzene, naphthalene and anthracene. In a preferred embodiment, the aromatic hydrocarbon is benzene.

The term “alkyl aromatic hydrocarbon” is intended to mean an “aromatic hydrocarbon” as defined above and having one or several alkyl substituents, preferably one or several C1-C5 alkyl substituents, also referred to as side chains. Examples of “alkyl aromatic hydrocarbons” are o-, m- or p-xylene, hemimellitene, mesitylene, prehnitene, isodurene, durene, ethylbenzene, cumene and o-,m- or p-cymene In a preferred embodiment, the alkyl aromatic hydrocarbon is toluene.

The term "alkyl" refers to a branched, unbranched, and saturated hydrocarbon radical, including, but not limited to, methyl, ethyl, propyl, butyl, isopropyl, isobutyl, tertiary butyl, pentyl, hexyl and the like.

The term "halogen" used herein refers to fluorine, chlorine, bromine or iodine.

The term “chalcogen” used herein refers to oxygen, selenium or sulfur. Preferably, the chalcogen is oxygen.

In a preferred embodiment R is H.

In another preferred embodiment “chalcogen-R” is -OH. In a preferred embodiment, the substrate is an alkane, more preferably, n-hexane, and the reaction is carried out contacting the substrate with Na[3,3’-Co(1 ,2-C2B₉Hn)2] and an oxidizing agent selected from air, O2 or Na2S20s in water, at room temperature and under LIV radiation for 4h to 8h. More preferably, the concentration of the substrate is between 1 .5x1 O'² and 2.5x10^-1 M. Under these conditions, when n-hexane is used, the majority product is a monoalcohol if the reaction time is 4 h, while the majority product is a dialcohol is the reaction time is 8h.

In a preferred embodiment, the substrate is an alkane, preferably n-hexane, and the reaction is carried out contacting the substrate with Na[3,3’-Fe(1 ,2-C2B₉Hn)2] and an oxidizing agent selected from air, O2 or Na2S20s, preferably O2, in water, at room temperature and under UV radiation for 8h. More preferably, the concentration of the substrate is between 1 .5x10^-2 and 2.5x10^-1 M. Under these conditions, when n-hexane is used, the majority product is a dialcohol.

In a preferred embodiment, the substrate is a cycloalkane, preferably cyclohexane, and the reaction is carried out contacting the substrate with Na[3,3’-Co(1 ,2-C2B₉Hn)₂] or Na[3,3’-Fe(1 ,2-C2B₉Hn)₂] and an oxidizing agent selected from O2 or Na2S20s in water, at room temperature and under UV radiation for 8h. More preferably, the concentration of the substrate is between 1 .5x1 O'² and 2.5x10^-1 M. Depending on the conditions used, the main product is cyclohexanol or 1 ,4-ciclohexanediol.

In a preferred embodiment of the method of the present invention, the substrate is an aromatic hydrocarbon, preferably benzene.

In a preferred embodiment, the substrate is an aromatic hydrocarbon, preferably benzene, and the reaction is carried out contacting the substrate with Na[3,3’-Co(1 ,2- C2B₉HH)₂] and an oxidizing agent selected from air, O2 or Na2S20s in water, at room temperature and under UV radiation for 1 to 4 h. This reaction allows to obtain phenol with high yield and selectivity. Preferably, if the reaction is carried out with O2 as oxidising agent (preferably, at 1 atm) and the concentration of the substrate used is 2.25x10^-1 M, phenol can be obtained in 1 h with a yield of 98% and a selectivity above 99%. Other applicable concentrations are in the range 1 ,5x10^-2 M-0.3M. When Na2S2C>8 is used and/or concentrations of the substrate between 1 .5x1 O’² and 4.5x1 O’ ² M are used, phenol is obtained in 4h.

In a preferred embodiment, the substrate is an aromatic hydrocarbon, preferably benzene, and the reaction is carried out contacting the substrate with Na[3,3’-Co(1 ,2- C2B₉HII)₂] and an oxidizing agent selected from air, O2 or Na2S2O₈ in water, at room temperature and under UV radiation for 8h. These conditions lead to di- or polyalcohols derived from benzene as majority products.

In a preferred embodiment, the substrate is an aromatic hydrocarbon, preferably benzene, and the reaction is carried out contacting the substrate with Na[3,3’-Fe(1 ,2- C2B₉HII)₂] and an oxidizing agent selected form, O2 or Na2S2O₈ in water, at room temperature and under UV radiation for 4 h to 8h. These conditions lead to di- or polyalcohols derived from benzene as majority products.

In a preferred embodiment, the substrate is an alkyl aromatic hydrocarbon, preferably toluene, wherein reaction is carried out contacting the substrate with Na[3,3’-Co(1 ,2- C2B₉HI 1 )₂] or Na[3,3’-Fe(1 ,2-C2B₉Hn)₂] and an oxidizing agent is Na2S2O₈ in water, at room temperature and under UV radiation for 4h to 8 h. These conditions lead to products such as benzyl alcohol, benzoic acid, phenylmethanediol and 4- hydroxybenzoic acid.

In a preferred embodiment, the substrate is an alkane, more preferably, n-hexane, and the reaction is carried out contacting the substrate with [3,3’-Co(8,9,12-CI₈-1 ,2- C2B₉H₈)2] and an oxidizing agent selected from air, O2 or Na2S2O₈ in water, at room temperature and under UV radiation for 4h to 8h. More preferably, the concentration of the substrate is between 1 .5x1 O’² and 4.5x1 O’² M. Under these conditions, when n- hexane is used, the majority product is a monoalcohol if the reaction time is 4 h.

In a preferred embodiment, the substrate is a cycloalkane, preferably cyclohexane, and the reaction is carried out contacting the substrate with H[3,3’-Co(8,9,12-CI₈-1 ,2- C₂B₉H₈)2] or Na[3,3’-Co(8,9,12-Ch-l ,2-C2B₉H₈)₂] and an oxidizing agent selected from O2 or Na2S2O₈ in water, at room temperature and under UV radiation for 4h to 8h. More preferably, the concentration of the substrate is between 1.5x10^-2 and 4.5x1 O' ²M. The main product is cyclohexanol.

In a preferred embodiment, the substrate is an aromatic hydrocarbon, preferably benzene, and the reaction is carried out contacting the substrate with H[3,3’- Co(8,9,12-Cls-1 ,2-C2B₉H₈)2] and an oxidizing agent selected from air, O2 or Na2S2O₈ in water, at room temperature and under UV radiation for 4 to 8 h. More preferably, the concentration of the substrate is between 1.5x10^-2 and 4.5x10^-2M. These conditions lead to phenol or di- or polyalcohols derived from benzene as majority products.

In a preferred embodiment, the substrate is phenol, the photooxidation of which under the conditions of the process of the invention leads to dihydroxylated derivatives, such as, pyrocatechol, resorcinol hydroquinone. Preferably, the reaction is carried out contacting the substate (i.e.: phenol) with Na[3,3’-Co(1 ,2-C2B₉HII)₂] or Na[3,3’- Fe(1 ,2-C2B₉Hn)₂] and an oxidizing agent selected from O2 and Na2S2O₈ in water, at room temperature and under UV radiation for 4 to 8 h. More preferably, the concentration of the substrate is between 1 .5x1 O'² and 4.5x10^-2M.

The process of the present invention presents many advantages with respect to the known methods of the state of the art to oxidise alkanes and aromatic hydrocarbons. Some of the advantages are highlighted below:

• It does not require high temperatures, high pressures, the use of organic solvents or high-cost catalysts. Contrary to that, the process of the present invention requires mild conditions, such as room temperature and atmospheric pressure, and the reaction solvent/medium is water. The process of the present invention is considered highly efficient, sustainable and environmentally friendly.

• The compound Cs[3,3’-Co(1 ,2-C2B₉Hn)2] (immediate precursor of the catalyst Na[3,3’-Co(1 ,2-C2B₉Hn)2] used in the examples of the present invention) can be purchased since it is commercially available, which gives the security of using a pure catalyst with guarantees. It should be noted that the catalyst is dissolved in water and is capable of dispersing the organic molecules, something totally new.

• The direct oxidation of the C-H bond in benzene with an oxidising agent such as O2 to produce phenol is the "dream" reaction for the chemical industry, and this oxidation can be achieved in a few hours with the process of the present invention.

• It can be used for the production of methanol, which is currently an alternative as a transportation fuel. For example, the process can be adapted to carry out onsite oxidation of farm-generated methane to methanol, producing an additional benefit to the farmer, greatly reducing methane emission to the atmosphere and producing a liquid fuel.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. Throughout the description and claims the word "comprise" and its variations are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention.

DESCRIPTION OF THE DRAWINGS

Fig. 1. a) [o-COSAN]^_ and b) [o-FESAN]^_ anion structures, B-H (light grey), C-H (dark grey) c) metallabisdicarbollide with the numbered atom vertexes.

Fig. 2. ¹H NMR spectrum (CDCh) corresponding to the photooxidation of benzene, using Na[3,3’-Co(1 ,2-C2BgHn)2] as catalyst. Conditions: Na[3,3’-Co(1 ,2-C2BgHn)2] (7.5x10^-6 M), substrate (2.25x10^-1 M), O2 (1 atm), 5 mL water. Light irradiation 1 h.

Fig, 3. ¹H NMR spectrum (CDCh) corresponding to the photooxidation of benzene, using Na[3,3’-Fe(1 ,2-C2BgHn)2] as catalyst. Conditions: Na[3,3’-Fe(1 ,2-C2BgHn)2] (7.5x10^-6 M), substrate (2.25x10^-1 M), NagSgOs (4.50x10^-1 M), 5 mL water. Light irradiation 8h.

Fig. 4: [C/e-COSAN] anion structures, B-H (light grey), C-H (dark grey). Examples

The following examples are provided by way of illustration and are not intended to be limiting of the present invention.

Example 1 : Small scale general process:

In a quartz tube, a water solution (5 mL at pH 7) containing the corresponding photoredox catalyst, e.g. Na[3,3’-Co(1 ,2-C2BgHn)2] or Na[3,3’-Fe(1 ,2-C2BgHn)2], the substrate (alkane, arene or alkyl aromatic compound) and the oxidant (e.g. NagSgOs, air or O2) was exposed to UV light (2.2 W, A= 253.7 or 300 or 352 or 368 nm) for different times ranging from one to eight hrs. In the set of experiments, a base (e.g. K2CO3) was added to water in order to control the pH = 7. For each experiment, light illumination was supplied by a light reactor with fourteen lamps that produce UVA light at room temperature. The resulting solutions were extracted with dichloromethane or diethyl ether three times. The solution was dried with anhydrous sodium sulfate and the solvent was evaporated under reduced pressure. To check the reproducibility of the reactions all experiments were carried out three times. The reaction products were quantified and characterized by ¹H NMR spectroscopy using tetramethylsilane (TMS) as internal standard.

Compounds Na[3,3’-Co(1 ,2-C2BgHn)₂] or Na[3,3’-Fe(1 ,2-C2BgHn)₂] were synthetized by following the method described in the prior art (I. Fuentes, A. Andrio, F. Teixidor, C. Vihas, V. Compah, Phys. Chem. Chem. Phys. 2017, 19, 15177-151869; T. Garcia- Mendiola, V. Bayon-Pizarro, A. Zaulet, F. Pariente, I. Fuentes, C. Vihas, F. Teixidor and E. Lorenzo. Chem. I Sci., 2016, 7, 5786-5797). Briefly, around 200 mg of the starting compound (Cs[3,3’-Co(1 ,2-C2B₉Hn)₂]) was dissolved in a minimum volume of acetonitrile/water (50:50) and passed repeatedly (4 times) through the previously prepared cationic resin. Before collecting the solution containing the metallacarborane, 50mL of fresh acetonitrile/water (50:50) was added to the column. In a flask, 50 mL were collected, then the solvent was evaporated and the compound was dried in a vacuum. The cationic resin was prepared following the procedure described in the same reference. Briefly, approximately 2/3 of the volume of the column (30cm) was filled with the strongly acidic cationic exchange resin. Before starting, the cationic resin was kept at 24h in 3M HCI to hydrate it. Then, a 150mL solution of HCI 3M was slowly passed through the column to load it with H⁺. T 0 remove the excess HCI, distilled water was quickly passed through the column until neutral pH was reached. When the desired cation was sodium, a solution of 3M NaCI was passed slowly through the column to exchange H+ with Na+ until neutral pH was reached (the change produced HCI). Distilled water was used to rinse the excess NaCI through the column. To know if NaCI was removed, 3 drops of a solution of 100 mM AgNOs was added to a small fraction of solution coming out of the column, until a clear solution was observed. Then, 30 mL of acetonitrile/water (50:50) mixture was allowed to flow through the column to set the column’s liquid composition.

Example 1.1 Detailed description for the oxidation of n-HEXANE. A mixture of 1 .5x1 O'² M of n-hexane, 1 .5x10^-6 M of Na[o-COSAN] as catalyst, and a cat/substrate ratio of 1 :10000 , bubbling O2 at 1 atm in 5 mL of H₂O (pH = 7) was irradiated under LIV light (A= 352 nm). After 4 hours of irradiation, 1 -hexanol was produced in 68% yield (99% of selectivity). When Na[o-FESAN] was used as catalyst and UV light (A= 300 nm) in the same proportion, 1 ,1 -hexanediol was obtained in 89% yield (99% of selectivity) after 8 hours.

Example 1.2 Detailed description for the oxidation of CYCLOHEXANE. A mixture of 1.5x10^-2 M of cyclohexane, 1.5x10^-6 M of Na[o-COSAN] as catalyst and a cat/substrate ratio of 1 :10000, bubbling O₂ at 1 atm in 5 mL of H₂O (pH = 7) was irradiated under UV light (A= 352 nm). After 8 hours of irradiation, the monosubstituted cyclohexanol was produced in 92% yield (99% of selectivity). On the other hand, when Na[o-FESAN] was used as catalyst, Na2S₂O8 as oxidant and UV light (A= 300 nm), a diol, 1 ,4-cyclohexanediol was produced in 74% yield (76 % of selectivity) after 8 hours of reaction.

Example 1.3. Detailed description for the oxidation of BENZENE. When benzene (1.5x10^-2 M) was used as substrate, 1.5x10^-6 M of Na[o-COSAN] as catalyst and 3.0x10^-2 M of Na₂S2O8 as oxidant. The ratio cat/substrate ratio of 1 :10000:20000 was used. Phenol was obtained in 96% yield (99% selectivity) after 4 hours or reaction, whereas resorcinol was obtained as the only product in a 99% yield after extension of the reaction to 8 hours, using only 1.5x10^-6 M of catalyst. Using the same conditions but O2 as oxidant, 72% of phenol after 4 hours was obtained. Besides, when Na[o-FESAN] was used as catalyst, hydroquinone was formed in 67% yield and high selectivity (99%) with O2 as oxidant and 4 hours of reaction. However, with longer times of reaction up to 8 hours, polyhydroxilation was achieved.

Example 1.4. Detailed description for the oxidation of TOLUENE. Using 1 .5x10² M of toluene as substrate, 1.5x10^-5 M of Na[o-COSAN] as catalyst and 3.0x10^-2 M of Na2S2C>8 as oxidant. The ratio cat/substrate ratio of 1 :1000:2000 was used. The formation of benzyl alcohol in 76% yield and high selectivity (96%) after 8 hours was achieved. In the case of using Na[o-FESAN], benzoic acid was produced in a 59% yield with the same selectivity after 8 hours of reaction. Remarkably, polyalcohols as phenylmethanediol was obtained in 87% yield and 88 % of selectivity after 4 hours of reaction using 1.5x10^-6 M of Na[o-FESAN], In addition, 4-hydroxybenzoic acid* was produced in 73% yield and the same selectivity using Na[o-COSAN] and 8 hours of reaction.

* This result indicated that the toluene has been hydroxylated in one aromatic carbon of the phenyl ring after the methyl group has been oxidized to acid.

Example 2. Scaling up the substrate concentration experiments

For scaling up the reactions, a higher concentration of substrate (e.g. 4.5x10^-2 M or 2.25x10^-1M) and a cat/substrate/oxidant ratio of 1 :30000:60000 were used. In these experiments Na[o-FESAN] and Na[o-COSAN], and Na2S20s, O2 or air as oxidants were used. C>2 or air are used in excess without controlling the proportion.

Example 2.1. Detailed description for the oxidation of ALKANES. Using hexane 4.5x10^-2 M as substrate and Na[o-COSAN] in 1.5x10^-6 M and Na2S20s in 9.0x10^-2 M as oxidant, 1 -hexanol in 43% yield was obtained after 4 hours, but at 8 hours, the 1 ,1 - hexanediol was produced in 67% yield with the same selectivity. When cyclohexane was used as substrate in the same conditions, 83% of 1 ,4-cyclohexanediol with a high selectivity of 87% was obtained after 8 hours.

Example 2.2. Detailed description for the oxidation BENZENE. Starting from benzene 4.5x10^-2 M as substrate and Na[o-COSAN] as catalyst in 1 .5x10^-6 M, phenol was obtained in a 72% yield and the same selectivity after 4 hours of reaction and using Na2S20s as oxidant. Whereas resorcinol was produced in 57% with moderate selectivity after 8 hours of reaction.

By increasing the concentration of substrate (e.g. 2.25x10^-1 M), phenol was formed in 47% yield and 66% selectivity after 4 hours of reaction and using Na[o-COSAN] in 5 7.5x10^-6 M as catalyst and Na2S20s in 4.5x10^-1 M as oxidant. On the contrary, when the catalyst was changed to Na[o-FESAN], resorcinol was obtained in 79% yield (82% selectivity) after 8h of reaction.

Finally, for Benzene as substrate, the most prominent results were obtained using Na[o-COSAN] as catalyst and O2 or air as oxidants, from where phenol was obtained 0 as the only product after 1 hour in a 98% yield (99% selectivity) using O2 as oxidant or pyrocatechol as the only product after 4 hours in a 96 yield (99 % selectivity) using air as oxidant. If the reaction is maintained for 4 hours, resorcinol is formed in 76% yield and the same selectivity, and 68% after 8 hours, using O2 as oxidant and other products (pyrocatechol and hydroquinone) are released in lower yield in these 5 conditions.

In following Table 1 , the more remarkable results described above are also outlined.

Table 1. Targeted syntheses to produce specific alcohols from alkanes and aromatic hydrocarbons

The concentrations of catalyst used in the catalytic experiments were: *1 .5x10^-6 M; **7.5x10^-6 M or ***1 .5x10^-5 M Example 3. Procedure of invention using the catalyst H[C/6-COSAN] or Na [C/₆- COSAN] for each substrate.

Compounds [Ch-COSAN] or Na[Cle-COSAN] were synthetized as described below: The hexachloro cobaltabis(dicarbollide), [3,3’-Co(8,9,12-Cl3-1 ,2-C2B₉H8)2]' was prepared by a chlorination reaction, as described in the prior art (I. Fuentes, J. Pujols, C. Vihas, S. Ventura and F. Teixidor. Chem. Eur. J. 2019, 25, 12820), from the waterinsoluble Cs[o-COSAN] that is commercially available from i.e Katchem. Briefly, to a solution of 50 mg of the starting compound (Cs[3,3’-Co(1 ,2-C2B₉Hn)2]) in 1.5 mL of acetonitrile was added 1.5 mL of SO2CI2, drop by drop. The mixture was heated for 2h at 70°C. Then, volatiles were removed under reduced pressure, the resulting product was submitted to a liquid-liquid extraction with 15 mL of diethyl ether and 15 mL of 3M HCI to remove the impurities. The washing procedure was done two more times with 3M HCI (2 x 15 mL) and then, the organic layer was dried with MgSC . After filtration, the liquid was evaporated under vacuum. The described procedure allowed us to obtain the proton salt of hexacloro cobaltabis(dicarbollide), H[3,3’- Co(8,9,12-Cl3-1 ,2-C2B₉H₈)2]. The subsequent methatesis reaction, e.g. cationexchange resin that is described in the literature (I. Fuentes, A. Andrio, F. Teixidor, C. Vihas, V. Compa , Phys. Chem. Chem. Phys. 2017, 19, 15177-151869) was used to obtain the sodium salt of the hexachloro cobaltabis(dicarbollide), Na[3,3’-Co(8,9,12- CI3-1 ,2-C₂B₉H₈)2].

For hexane as substrate and a concentration of catalyst of 1 .5- 10⁶ M, it was possible to achieve a yield of 86 % of 1 -hexanol after 4 hours of reaction, that was 18 % higher with respect to the Na[o-COSAN] (68 %) in the same reaction conditions, and the same selectivity (>99%). The reduction of the catalyst concentration (1 .5- 10⁶ M) with respect to the substrate concentration and the use of O2 as oxidant, leads to an increase of the yield (89% or 91%, respectively) while maintaining the selectivity (>99). However, secondary or polysubtituted alcohols were released in lower yield when H[Cle-COSAN] was used in comparison to the Na[o-COSAN],

For cyclohexane as substrate and concentrations of the catalyst 1 .5- 10⁵ - 1 .5- 10^-6 M the cyclohexanol was obtained in a very similar yield (93 %) to that obtained with Na[o-COSAN] (92 %), either by using NagSgOs or O2 as oxidants. Nevertheless, it is worth mentioning that the change of the cation in H[Cle-COSAN] and Na[Cle-COSAN] does not affect the high yield values and both catalysts showed comparable values, while maintaining the high selectivity values (see Table 2).

Using benzene as substrate, there are several trends subjected to change with the use of the different catalysts: The use of H[C/e-COSAN] allowed to obtain higher yield and selectivity of phenol than that obtained with Na[o-COSAN] at the same reaction conditions. With respect to polyhydroxylated alcohols, the formation of pyrocatechol was favored for H[C/e-COSAN] and O2 or Na2S20s as oxidants after 4h of reaction when compared to the Na[o-COSAN], In the same way, the meta polyalcohol derivative, resorcinol, experienced a decrease of yield (72%) using H[C/e-COSAN] with respect the obtained with Na[o-COSAN] with less concentration of catalyst, 1.5-1 O’⁵ M vs. 1.5-1 O’⁶ M, respectively, at the same reaction time. In addition, using H[C/e-COSAN], the formation of trihydroxylated alcohols with low to moderate yields was observed, whereas the formation of this derivative using Na[o-COSAN] was negligible. Nevertheless, the formation of the tetrahydroxylated derivative 1 ,2,4,5- benzenetetraol was not observed when H[C/e-COSAN] was used .

In following Table 2 the results of table 1 and the results of the additional example 3 are included. The results of example 3 are highlighted in bold type.

Table 2. Targeted syntheses to produce specific alcohols from alkanes and aromatic hydrocarbons

The concentrations of catalyst used in the catalytic experiments were: *1 .5x10^-6 M; **7.5x10^-6 M or

***1 .5x10-⁵ M

Example 4: Procedure of invention using phenol as a substrate. Different metallabisdicarbollides, such as Na[3,3’-Co(1 ,2-C2BgHn)2] or Na[o-COSAN], Na[3,3’-Fe(1 ,2-C2B₉Hn)2] or Na[o-FESAN], were tested as photoredox catalysts in the oxidation of phenol as substrate.

A quartz tube containing an aqueous solution (5 mL) at pH 7 with the corresponding catalyst, phenol as substrate, and NagSgOs or a molecular oxygen stream as sacrificial acceptor was exposed to LIV light 2.2 W, A=352 or 300 nm) for different times (The use of irradiating wavelength of 352 nm and 300 nm are corresponding to the use of Na[o-O-COSAN] and Na[o-FESAN], respectively). The catalyst: substrate: sacrificial oxidant ratios used (1 :1000:2000, 1 :10000:20000, 1 :30000:60000) corresponding to concentrations of (1.5x1 O'⁵: 1.5x1 O'²: 3x1 O'²) M; (1.5x10-⁶: 1.5x10^-2: 3x1 O'²) M and (1.5x1 O'⁶: 4.5x1 O'²: 9.0x1 O'²) M, respectively, were varied according to the study. For each experiment, light illumination was supplied by a light reactor with fourteen lamps that produce UVA light at room temperature. The resulting solutions were extracted with diethyl ether three times. The solution was dried with anhydrous sodium sulfate and the solvent was evaporated under reduced pressure. T o check the reproducibility of the reactions all the experiments were carried out three times. The reaction products were quantified and characterized by ¹H NMR spectroscopy using tetramethylsilane (TMS) as internal standard,

The following table (Table 3) shows the results of the experiments carried out:

The concentrations of catalyst used in the catalytic experiments were: *1 .5x10^-6 M; **7.5x10^-6 M In the case of the use of phenol as substrate and a concentration of catalyst 1 .5x10^-6 M, it was possible to achieve a yield of 99 % in pyrocatechol after 8h of reaction using Na[o-COSAN] as catalyst, with the achievement of the same selectivity. In the case of using Na[o-FESAN] the best result with respect of the pyrocatechol product was of 65% but in half time of reaction.

In the case of resorcinol, the best yield was found for Na[o-FESAN] as catalyst, with a 76 % yield with high selectivity, using a concentration of catalyst of 1.5x10^-5M and 8 h of reaction.

The obtaining of hydroquinone was exceptionally obtained by the use of Na[o-FESAN] as catalyst, and with a lower yield in comparison to the other products obtained.

As a conclusion, the photooxidation of phenol to their dihydroxylated derivative is feasible using the catalyst mentioned in the present invention, such as, Na[o-COSAN] and Na[o-FESAN], under similar conditions to those used from the benzene.

Claims

1 . Process for the photocatalytic oxidation of a substrate selected from: an alkane, a cycloalkane, an aromatic hydrocarbon, an alkyl aromatic hydrocarbon and phenol, comprising: contacting the substrate with a 0-type metallacarborane catalyst of formula M’[M(C2B₉Hii-_yX_y)2], wherein:

M’ is a counterion selected from: Na⁺, Li⁺, H⁺K⁺, and [NH4]⁺

M is a trivalent cation of a metal selected from Cr, Fe, Co, Ni, Ru and Al,

X is H, halogen, chalcogen-R or alkyl, preferably C1-C10 alkyl, wherein R is H or C1- C alkyl, y is an integer selected from 0 to 11 , and an oxidising agent in water, at room temperature and under UV radiation.

2. The process, according to claim 1 , wherein the catalyst is selected from Na[3,3’- CO(1 ,2-C₂B₉HII)2], Na[3,3’-Fe(1 ,2-C2B₉Hii)₂], Na[3,3’-Co(8,9,12-CI₃-1 ,2-C₂B₉H₈)2] and H[3,3’-Co(8,9,12-Ch-l ,2-C₂B₉H₈)2].

3. The process, according to any of previous claims 1 or 2, wherein the concentration of the substrate in the water solution is between 1 .5x1 O'² M and 0.3 M.

4. The process, according to any of previous claims 1 to 3, wherein the molar ratio catalyst:substrate is between 1 :1000 and 1 :30000.

5. The process, according to any of previous claims 1 to 4, wherein the oxidising agent is selected from air, O2 or Na2S2O₈.

6. The process, according to any of previous claims 1 to 5, wherein the oxidising agent is Na2S2O₈ and the molar ratio catalyst:substrate:oxidant is between 1 :1000:2000 and 1 :30000:60000.

7. The process, according to any of previous claims 1 to 6, wherein reaction is carried out at pH 7.

8. The process, according to any of previous claims 1 to 7, wherein the reaction time is between 1 h and 8h.

9. The process, according to any of previous claims 1 to 8, wherein the substrate is an alkane, and the reaction is carried out contacting the substrate with Na[3,3’-Co(1 ,2- C₂B₉Hn)₂] and an oxidizing agent selected from air, O₂ or Na₂S₂0s in water, at room temperature and under UV radiation for 4h or 8h.

10. The process, according to any of previous claims 1 to 8, wherein the substrate is an alkane, and the reaction is carried out contacting the substrate Na[3,3’-Fe(1 ,2- C₂B₉HH)₂] and an oxidizing agent selected from air, O₂ or Na₂S₂0s in water, at room temperature and under UV radiation for 8h.

11 . The process, according to any of previous claims 1 to 8, wherein the substrate is a cycloalkane and the reaction is carried out contacting the substrate with Na[3,3’- Co(1 ,2-C₂B₉HH)₂] or Na[3,3’-Fe(1 ,2-C₂B₉Hn)₂] and an oxidizing agent selected from air, O₂ or Na₂S₂0s in water, at room temperature and under UV radiation for 8h.

12. The process, according to any of previous claims 1 to 8, wherein the substrate is an aromatic hydrocarbon, preferably benzene.

13. The process, according to claim 12, wherein reaction is carried out contacting the substrate with Na[3,3’-Co(1 ,2-C₂B₉Hn)₂] and an oxidizing agent is air, O₂ or Na₂S₂0s in water, at room temperature and under UV radiation for 1 to 4 h.

14. The process, according to claim 12, wherein reaction is carried out contacting the substrate with Na[3,3’-Fe(1 ,2-C₂B₉Hn)₂] and an oxidizing agent is air, O₂ or Na₂S₂0s in water, at room temperature and under UV radiation for 4 or 8h.

15. The process, according to any of previous claims 1 to 8, wherein the substrate is alkyl aromatic hydrocarbon, preferably toluene, wherein reaction is carried out contacting the substrate with Na[3,3’-Fe(1 ,2-C₂B₉Hn)₂] or Na[3,3’-Co(1 ,2-C₂B₉Hn)₂] and an oxidizing agent is Na₂S₂0s in water, at room temperature and under UV radiation for 4h or 8 h.

16.The process according to any of previous claims 1 to 8, wherein the substrate is an alkane, and the reaction is carried out contacting the substrate with H[3,3’- Co(8,9,12-Cl3-1 ,2-C2B₉H₈)2] and an oxidizing agent selected from air, O2 or Na2S2O₈ in water, at room temperature and under UV radiation for 4h to 8h.

17. The process according to any of previous claims 1 to 8, wherein the substrate is a cycloalkane, the reaction is carried out contacting the substrate with H[3,3’- CO(8,9,1 2-CI₈-1 ,2-C2B₉H₈)₂] or Na[3,3’-Co(8,9,12-Cl3-1 ,2-C2B₉H₈)₂] and an oxidizing agent selected from O2 or Na2S2O₈ in water, at room temperature and under UV radiation for 4h to 8h.

18. The process according to any of previous claims 1 to 8, wherein the substrate is an aromatic hydrocarbon, the reaction is carried out contacting the substrate with H[3,3’-CO(8,9,1 2-CI₈-1 ,2-C2B₉H₈)₂] and an oxidizing agent selected from air, O2 or Na2S2O₈ in water, at room temperature and under UV radiation for 4 to 8 h.

19. The process, according to any of previous claims 1 to 8, wherein the substrate is phenol.

20. The process, according to claim 19, wherein the reaction is carried out contacting the phenol with Na[3,3’-Co(1 ,2-C2B9H11)2] or Na[3,3’-Fe(1 ,2-C2B9H11 )2] and an oxidizing agent selected from O2 or Na2S2O₈ in water, at room temperature and under UV radiation for 4 to 8 h.