Oxidation of Organic Compounds with Peroxides Catalyzed by Polynuclear Metal Compounds

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Review Oxidation of Organic Compounds with Peroxides Catalyzed by Polynuclear Metal Compounds

Georgiy B. Shul’pin 1,* and Lidia S. Shul’pina 2

1 Federal Research Center for Chemical Physics, N.N. Semenov Russian Academy of Sciences, Ulitsa Kosygina 4, 119991 Moscow, Russia 2 A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Ulitsa Vavilova 28, 119991 Moscow, Russia; [email protected] * Correspondence: [email protected] or [email protected]; Tel.: +7-916-384-7912

Abstract: The review describes articles that provide data on the synthesis and study of the properties of catalysts for the oxidation of alkanes, oleﬁns, and alcohols. These catalysts are polynuclear complexes of iron, copper, osmium, nickel, manganese, cobalt, vanadium. Such complexes for exam- 1 ple are: [Fe2(HPTB)(m-OH)(NO3)2](NO3)2·CH3OH·2H2O, where HPTB- 4 N,N,N0,N0-tetrakis(2- benzimidazolylmethyl)-2-hydroxo-1,3-diaminopropane; complex [(PhSiO1,5)6]2[CuO]4[NaO0.5]4 [dppmO2]2, where dppm-1,1-bis(diphenylphosphino)methane; (2,3-η-1,4- diphenylbut-2-en-1,4- dione)undecacarbonyl triangulotriosmium; phenylsilsesquioxane [(PhSiO1.5)10(CoO)5(NaOH)]; bi- 2 3 and tri-nuclear oxidovanadium(V) complexes [{VO(OEt)(EtOH)}2(L )] and [{VO(OMe)(H2O)}3(L )]· 2 3 2H2O (L = bis(2-hydroxybenzylidene)terephthalohydrazide and L = tris(2-hydroxybenzylidene) benzene-1,3,5-tricarbohydrazide); [Mn2L2O3][PF6]2 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane). For comparison, articles are introduced describing catalysts for the oxidation of alkanes and alcohols

with peroxides, which are simple metal salts or mononuclear metal complexes. In many cases, polynuclear complexes exhibit higher activity compared to mononuclear complexes and exhibit increased

Citation: Shul’pin, G.B.; Shul’pina, regioselectivity, for example, in the oxidation of linear alkanes. The review contains a description L.S. Oxidation of Organic of some of the mechanisms of catalytic reactions. Additionally presented are articles comparing Compounds with Peroxides the rates of oxidation of solvents and substrates under oxidizing conditions for various catalyst Catalyzed by Polynuclear Metal structures, which allows researchers to conclude about the nature of the oxidizing species. This Compounds. Catalysts 2021, 11, 186. review is focused on recent works, as well as review articles and own original studies of the authors. https://doi.org/10.3390/catal 11020186 Keywords: hydrogen peroxide; alkanes; alcohols; alkyl hydroperoxides; mechanisms of oxidation

Academic Editor: Ken-ichi Fujita Received: 29 September 2020 Accepted: 23 January 2021 1. Introduction Published: 31 January 2021 Hydrocarbons, in particular, saturated hydrocarbons (alkanes) are the main con-

Publisher’s Note: MDPI stays neutral stituents of petroleum and natural gas. These compounds are raw materials for the chem- with regard to jurisdictional claims in ical industry in production of polymers, pharmaceuticals, fragrances, fuels, etc. One of published maps and institutional afﬁl- the ways of converting hydrocarbons is catalytic oxygenation with formation of alcohols, iations. ketones, aldehydes, carboxylic acids, and alkyl peroxides. Usually these processes are carried out under severe conditions (high temperature and pressure using heterogeneous catalysts). New catalytic systems for the oxidation of organic compounds with peroxides were described in recent decades (see reviews) [1–13]. Hydrogen peroxide, alkyl peroxides, peroxy acids, Oxone, as well as molecular oxygen were used in these reactions as oxidants. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Derivatives of transition metals are widely employed in such reactions. These reactions This article is an open access article are carried out under mild conditions (at low temperatures, even at room temperature; distributed under the terms and atmospheric pressure and often using environmentally friendly solvents such as water and conditions of the Creative Commons alcohols). Polynuclear complexes are especially interesting catalysts [14,15]. Indeed, these Attribution (CC BY) license (https:// compounds can in many cases exhibit higher catalytic activity in comparison with simple creativecommons.org/licenses/by/ mononuclear complexes and they oxygenate long and branched alkanes with enhanced 4.0/). selectivity [16,17]. Most frequently such reactions proceed with the formation of radical

Catalysts 2021, 11, 186. https://doi.org/10.3390/catal11020186 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 186 2 of 37

intermediate species such as hydroxyl radicals [1,2]. A comparison of such catalytic systems for the oxidation of hydrocarbons with simple catalysts—mononuclear complexes or metal salts, is interesting, because the comparison can shed light on the features of oxidation mechanisms.

2. Oxidations Catalyzed by Soluble Polynuclear Compounds This Chapter describes oxidation of organic compounds with peroxides catalyzed by di- and polynuclear metal complexes and (as comparison) by some mononuclear complexes of transition metals. Polynuclear complexes are of particular interest both from a practical and an academic point of view, since such compounds can have increased catalytic activity and exhibit unique selectivity in the oxidation of organic compounds, in particular, alkanes.

2.1. Oxidation Catalyzed by Iron Complexes Iron ions have been known for a long time as initiators and catalysts for the decompo- +2 sition of hydrogen peroxide [1,18]. Thus, the reaction of H2O2 with Fe gives hydroxyl +3 +3 radicals and Fe( ) (stoichiometric Fenton reaction). The interaction of Fe with H2O2 causes formation of hydroxyl radicals and Fe+2. The resulting Fe+2 ion is further involved into a catalytic cycle of generating hydroxyl radicals with an intermediate participation of the Fenton reaction. Hydroxyl radicals attack organic compounds, for example, with the abstraction of a hydrogen atom. The abstraction of a hydrogen atom from a saturated hydrocarbon (RH) gives rise to an unstable alkyl radical(R•). The latter attaches an O2 molecule from atmosphere, ultimately forming a relatively stable alkyl hydroperoxide (ROOH) [1]. The formation of alkyl hydroperoxide in the reaction of the binuclear complex 1 0 0 [Fe2(HPTB)(µ-OH)(NO3)2](NO3)2·CH3OH·2H2O [HPTB = N,N,N ,N -tetrakis(2-benzimidazolylmethyl)-2-hydroxo-1,3-diaminopropane] [19] containing the 1,4,7-triazacyclononane ligand turned out to be almost inactive as a catalyst in the cyclohexane oxidation with H2O2 at room temperature. However, the addition of a comparatively small amount of 2-pyrazinecarboxylic acid (PCA), to the reaction solution causes intense alkane oxidation (Figure1) to form cyclohexyl hydroperoxide which decomposes during the reaction to produce the corresponding cyclohexanone and cyclohexanol. Total turnover number (TON) attained was 240 after 24 h. In kinetic measurements the authors determined only the concentrations of the ketone and the alcohol after the reduction with triphenylphosphine. The iron (III) complex 1 was used as catalyst. Catalysts 2021, 11, 186 3 of 37 Catalysts 2021, 11, x FOR PEER REVIEW 3 of 39

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FUL L FigureFigure 1. Compound 1. Compound 1 in the1 in absence the absence (curves (curves a) and a) in and the inpresence the presence of PCA; of curves PCA; (curvesb) in MeCN b) in at MeCN ◦ 25 °C.at catalyzed 25 C. catalyzed the oxidat theion oxidation of cyclohexane of cyclohexane by H2O2 by(0.59 H2 M)O2 Accumulati(0.59 M) Accumulationon of cyclohexanol of cyclohexanol (curves(curves 1b) and 1b) cyclohexanone and cyclohexanone (curves (curves 2b) (concent 2b) (concentrationsrations were weremeasured measured after afterthe reduction the reduction with with PPh3,PPh used3, methodused method by Shul’pin, by Shul’pin, which whichis in comparison is in comparison with the with concentrations the concentrations of alcohol of and alcohol ke- and tone ketonein solution in solution samples samples before beforeand after and adding after adding PPh3) PPhare shown.3) are shown. The data The are data adapted are adapted from [19], from [19], CopyrightCopyright (2004) (2004) with withpermission permission of Wiley. of Wiley. FUL L AnotherAnother binuclear binuclear iron iron complexcomplex2 [202] also[20] requires also therequires presence the of 2-pyrazinecarboxy-presence of Figure 1. Compound 1 in the absence (curves a) and in the presence of PCA; curves b) in MeCN at 2-pyrazinecarboxyliclic acid (PCA) for theacid oxidation (PCA) for of alkanes.the oxidation The accumulation of alkanes. of The products accumulation during oxidation of 25 °C. catalyzed the oxidation of cyclohexane by H2O2 (0.59 M) Accumulation of cyclohexanol (curvesproductswith 1b) and hydrogenduring cyclohexanone oxidation peroxide (curves with catalyzed hydrogen 2b) (concent by peroxide complexrations were catalyzed2 is measured shown by in aftercomplex Figure the reduction2 .2 Asis shown you with can in see, PPhFigure3, used2-pyrazinic 2. method As you by acid can Shul’pin, has see, a dramatic2-pyrazinic which is in effect comparison acid on has the a reaction withdramatic therate concentrations effect and producton the of reaction alcohol yield.As and rate can ke- and be seen toneproduct inin solution the yield. Figure samples As2 ,can other before be acids seen and areafterin the much adding Figure less PPh effective2,3) otherare shown. inacids accelerating The are data much are the lessadapted reaction effective from compared [19],in ac- to Copyrightcelerating2-pyrazinecarboxylic (2004) the withreaction permission compared acid. of Wiley. to 2-pyrazinecarboxylic acid. In works [21–36] other polynuclear iron complexes have been described. The oxidation Anotherof cyclohexane binuclear by H2ironO2 andcomplextert-butyl 2 hydroperoxide[20] also requires under catalysisthe presence by binuclear of iron 2-pyrazinecarboxyliccomplexes [(SO 4acid)(L)Fe( (PCA)µ-O)Fe(L)(SO for the 4ox)]·idation6H2O, 3of, and alkanes. [Cl(L)Fe( Theµ-O)Fe(L)Cl]Cl accumulation2· 2Hof 2O, 4; products1-(bis-pyridin-2-ylmethylamino)-3-chloropropan-2-ol during oxidation with hydrogen peroxide catalyzed (L) by have complex been described2 is shown [ 21in]. The Figureelectrochemical 2. As you can see, analysis 2-pyrazinic showed acid that has the dinucleara dramatic species effect ison more the reaction stable under rate and reduction productin compoundyield. As can3 thanbe seen in compound in the Figure4. It2, was other found acids that are compound much less 4effectiveis more in active ac- than celeratingcompound the reaction3 in oxidation compared cyclohexane to 2-pyrazinecarboxylic with hydrogen acid. peroxide in acetonitrile at 50 degrees.

Figure 2. Cont.

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Figure Figure2. Accumulation 2. Accumulation of sum of of oxygenates sum of oxygenates (predominantly (predominantly cyclohexyl cyclohexyl hydroperoxide) hydroperoxide) in the in the reactionreaction of cyclohexane of cyclohexane with hy withdrogen hydrogen peroxide peroxide (in acetonitrile (in acetonitrile at 25 at°C 25 catalysed◦C catalysed by complex by complex 2 in 2 in the the absenceabsence of additives of additives (curve (curve 1) and1) and in the in the presence presence of aminoacids: of aminoacids: PCA PCA (2), ( 2pyra-), pyrazine-2,3-dicarboxylic zine-2,3-dicarboxylicacid (3), picolinic acid acid (3), ( 4picolinic), and pyridine-2,6-dicarboxylic acid (4), and pyridine-2,6-dicarboxylic acid (5). Adapted acid from (5). [20 Adapted] Copyright (2002), from [20] Copyright (2002), with permission of Elsevier. with permission of Elsevier.

In worksOxidation [21–36] ofother alkanes polynuclear with m-chloroperoxybenzoic iron complexes have been acid described. catalyzed byThe binuclear oxida- iron tion of cyclohexane by H2O2 and tert-butyl hydroperoxide under catalysis by binuclear complexes non-heme µ-oxo-bridged diiron(III) complexes [Fe2(µ-O)(L1)2], where H2(L1) 0 4 4 • 2 0 iron is N,Ncomplexes-o-phenylenebis(salicylideneimine),[(SO )(L)Fe(μ-O)Fe(L)(SO [Fe2(µ-O)(L)] 6H2)2O,]·2H 2O, where3, H2(Land2) is N,N - 2• 2 [Cl(L)Fe(o-phenylenebis(3,5-di-tert-butylsalicylideneimine),μ-O)Fe(L)Cl]Cl 2H O, 4.; 1-(bis-pyridin-2-ylmethylamino)-3-chloropropan-2-ol and [Fe2(µ-O)(L3)2], where H2 (L3) = (L) have1,4-bis(2-hydroxybenzyl)-1,4-diazepane, been described [21]. The electrochemical occurs analysis with theshowed intermediate that the formationdinuclear of the speciescomplex is more [Festable2 (O) under (L2) 2reduction(OOR)] and in compound leads mainly 3 than to the in formation compound of 4. alcohols It was found and a small that compoundamount of 4 ketonesis more [active22]. than compound 3 in oxidation cyclohexane with hydrogen peroxideRemarkable in acetonitrile results at 50 were degrees. obtained in the oxidations of alkanes by hydrogen peroxide Oxidationcatalyzed of by alkanes N-bridged with diironm-chloroperoxybenzoic phthalocyanine complex acid catalyzed ((FePc tBuby binuclear4)2N[23], iron especially complexesvery non-heme inert methane μ-oxo-bridged and ethane. diiron(III) Reactions complexes were performed [Fe2(μ-O)(L in1 MeCN.)2], where Formic H2(L1) acid is was N,N′-o-phenylenebis(salicylideneimine),the main product of the oxidation of[Fe methane.2(μ-O)(L2 Kinetic)2]·2H2O, analysis where showed H2(L that2) theis ratio N,N′-o-phenylenebis(3,5-di-tert-butylsalicylideneimine),of the rates of oxidation of acetonitrile and methane and is incompatible [Fe2(μ-O)(L with3)2], thewhere concept of H2(L3) the= 1,4-bis(2-hydroxybenzyl)-1,4-diazepane, participation of the hydroxyl radical in occurs this process. with the Water intermediate was also formation investigated in of the complexthis reaction [Fe2 as(O) a (L solvent.2) 2 (OOR)] m-nitrido and leads complex mainly was to supportedthe formation onto of silica. alcohols Heterogeneous and a small amountoxidations of ketones of CH4 were [22]. performed in pure H2O. In this case, the following mechanism was Remarkabletentatively proposed.results were In theobtained ﬁrst stage, in the (FePBu oxidations4)2N coordinates of alkanes Hby2 Ohydrogen2 to form perox- hydroperoxo IV III ide catalyzedcomplex by Fe N-bridgedNFe OOH diiron which phthalocyanine is probably complex in equilibrium ((FePc tBu with4)2N the [23], deprotonated especially form − very inertFeIV methaneNFeIIIOO and. The ethane. heterolytic Reactions cleavage were ofperformed the O–O bondin MeCN. in Fe IVFormicNFeIII acidOOH was complex the and main productthe formation of the ofoxidation very strong of methane. oxidizing Ki FeneticIVNFe analysisV=O species showed was that favored the ratio in the of presence the of rates ofacid oxidation by the protonation of acetonitrile of peroxide and methan oxygen.e is incompatible with the concept of the ◦ participationµ-nitrido of the hydroxyl diiron phthalocyanine radical in this activates process. HWater2O2 to was oxidize also investigated CH4 in water in at this 25–60 C reactionto as methanol, a solvent. formaldehyde m-nitrido complex and formic was acid supported as evidenced. onto silica. The similar Heterogeneous binuclear porphyrinoxi- dationsiron of CH complex4 were wasperformed also used in pure in the H2 oxidationO. In this ofcase, methane the following with m-chloroperoxybenzoic mechanism was tentativelyacid [proposed.24]. It is assumed In the first that oxidationstage, (FePBu with4) binuclear2N coordinates complex H2O proceeds2 to form with hydroper- the intermedi- oxo complexate formation FeIVNFe N-bridgedIIIOOH which high-valent is probably diiron–oxo in equilibrium species. with the deprotonated form FeIVNFeTheIIIOO diiron–oxo‒−. The heterolytic complex demonstrated cleavage of the signiﬁcantly O–O bondstronger in FeIVNFe oxidizingIIIOOH propertiescomplex than and thesimilar formation mononuclear of very strong complex oxidizing [(TMP† þ)Fe(IV)(O)]þFeIVNFeV=O species (TMP 1/4tetramesitylporphyrin)was favored in the pres- [25]. ence of acidIt by is notthe uninterestingprotonation of to peroxide note that oxygen. iron mononuclear complex [FeII(Me3NTB)(CH3CN)] (CF SO ) , (Me NTB = tris(N-methylbenzimidazol-2-ylmethyl)amine, exhibits a rather μ-nitrido3 3diiron2 phthalocyanine3 activates H2O2 to oxidize CH4 in water at 25–60 °C to methanol,high activity formaldehyde in the oxidation and formic of cyclohexane acid as evidenced. [26]. It cannotThe similar be ruled binuclear out that porphy- the mononu- rin ironclear complex complex was in also solution used in forms the oxidation polynuclear of particlesmethane thatwith are m-chloroperoxybenzoic involved in the oxidation of

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acid [24]. It is assumed that oxidation with binuclear complex proceeds with the intermediate formation N-bridged high-valent diiron–oxo species. The diiron–oxo complex demonstrated significantly stronger oxidizing properties than similar mononuclear complex [(TMP†þ)Fe(IV)(O)]þ (TMP ¼tetramesitylporphyrin)[25]. It is not uninteresting to note that iron mononuclear complex [FeII(Me3NTB)(CH3CN)](CF3SO3)2, (Me3NTB = tris(N-methylbenzimidazol-2-ylmethyl)amine, exhibits a rather high activity in the oxi- Catalysts 2021, 11, 186 5 of 37 dation of cyclohexane [26]. It cannot be ruled out that the mononuclear complex in solution forms polynuclear particles that are involved in the oxidation of alkane. Other compounds appearing on the oxidation of organic compounds with peroxides catalyzed byalkane. polynuclear Other compounds iron derivatives appearing have been on the also oxidation published oforganic in works compounds [27–36]. with peroxides catalyzedIn recent years, by polynuclear polynuclear iron silsesquioxane derivatives have complexes been also of publishediron have been in works synthesized [27–36]. and theirIn recent catalytic years, activity polynuclear in the silsesquioxaneoxidation of alkanes complexes and ofalcohols iron have with been peroxides synthesized and oxygenand their has catalytic been investigated activity in the [37–40]. oxidation Such of complexes alkanes and are alcohols usually with highly peroxides active and contributeoxygen has to beenthe formation investigated of oxidation [37–40]. produc Such complexests in high yields are usually at low highlytemperatures active and atmosphericcontribute to pressure. the formation Examples of oxidation of structures products of such in high complexes yields at are low presented temperatures in Figure and 3atmospheric. pressure. Examples of structures of such complexes are presented in Figure3.

FigureFigure 3. AllAll structures structures adapted from Ref. [40], [40], Copyright (2016) with permissions of Wiley.

As mentioned above, mononucl mononuclearear iron derivatives are often less active in alkane oxidation reactions,reactions, which which proceed proceed according according to Fenton-liketo Fenton-like mechanism mechanism involving involving hydroxyl hy- droxylradicals radicals [41–59]. [41–59].

2.2. Oxidation Oxidation Catalyzed Catalyzed by Copper Complexes The large number of works are devoted to catalysts containing copper. Usually these complexes contain N-ligands [60–88]. [60–88]. Formulae Formulae and and structures structures of some of the catalysts used in the reactions are shown in Figures4 4–9.–9. Complex 5 (Figure(Figure4 )[4) 63[63]] efﬁciently efficiently catalyzes catalyzes the the oxygenation oxygenation of alkanesof alkanes with withtert- tertbutyl-butyl hydroperoxide hydroperoxide (TBHP) (TBHP) in in acetonitrile acetonitrile solution solution under under relatively relatively mildmild conditionsconditions ◦ (temperature 50–7050–70 °C,C, underunder normal normal pressure pressure of of air). air). Concentrations Concentrations of of the the ﬁnal final products products(alcohols (alcohols and ketones)and ketones) in the in oxidation the oxidation of cyclohexane of cyclohexane gives gives after after 8.5 h 8.5 cyclohexanol h cyclohexanol and andcyclohexanone cyclohexanone in 17% in yield17% yield (after (after addition addition of PPh of3), PPh and3), the and turnover the turnover number number (TON) (TON)attains attains 800. Complex 800. Complex6 does not6 does exhibit not catalytic exhibit catalytic activity inactivity the oxidation in the oxidation of alkanes of with al- kanesTBHP, with which TBHP, can be which associated can be with associated its relatively with rapid its relatively decomposition rapid decomposition or transformation or transformationinto a catalytically into inactive a catalytically species under inactive the reactionspecies conditions.under the reaction It should conditions. be noted that It shouldthe oxidation be noted of that alcohols the oxidation with TBHP of alcohols is effectively with catalyzedTBHP is effectively by both complexes catalyzed 5byand both6. In the case of the cyclohexanol oxidation the maximum values of TON and yield of cyclohexanone attained 820 and 78%, respectively. However, in oxidation of alcohol the activity of compound 6 is a bit less than activity of compound 5. Both compounds 5 and 6 catalyze the hydrocarboxylation of cycloalkanes to the corresponding cycloalkanecarboxylic acids. In all cases, catalyst 6 shows a slightly higher activity over the dicopper (II) complex 6. 2 0 02 Dinuclear [Cu2(1κNO :2κN O -H2L)(NO3)2(H2O)2](7) (Figure5) and the tetranuclear 2 0 2 [Cu4(µ-1κNO :2κN O -H2L)2(µ-NO3)2(H2O)4]·2C2H5OH (8) (Figure6) complexes were used as catalysts for oxidation of alcohols and alkanes [66]. The catalytic activity of both 7 and 8 has been screened toward the solvent-free microwave-assisted oxidation of alcohols and the peroxidative oxidation of alkanes under mild conditions. Complex 7 (Figure5) exhibits the highest activity for both oxidation reactions, leading selectively to a maximum Catalysts 2021, 11, x FOR PEER REVIEW 6 of 39

complexes 5 and 6. In the case of the cyclohexanol oxidation the maximum values of TON and yield of cyclohexanone attained 820 and 78%, respectively. However, in oxidation of alcohol the activity of compound 6 is a bit less than activity of compound 5. Both compounds 5 and 6 catalyze the hydrocarboxylation of cycloalkanes to the correspond- Catalysts 2021, 11, 186 ing cycloalkanecarboxylic acids. In all cases, catalyst 6 shows a slightly higher activity6 of 37 over the dicopper (II) complex 6.

product yield of 99% (for the 1-phenylethanol oxidation after 1 h without any additive) and 13% (for the cyclohexane oxidation to cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone after 3 h).

Figure 4. Formulae Complex 5(left) and 6 (right), which have been used in oxidations with peroxides adapted from Ref. [64], Copyright (2011) with permission of Elsevier.

Dinuclear [Cu2(1κNO2:2κN′O′2-H2L)(NO3)2(H2O)2] (7) (Figure 5) and the tetranuclear [Cu4(μ-1κNO2:2κN′O2-H2L)2(μ-NO3)2(H2O)4]·2C2H5OH (8) (Figure 6) complexes were used as catalysts for oxidation of alcohols and alkanes [66]. The catalytic activity of both 7 and 8 has been screened toward the solvent-free microwave-assisted oxidation of alcohols and the peroxidative oxidation of alkanes under mild conditions. Complex 7 (Figure 5) exhibits the highest activity for both oxidation reactions, leading selectively to a maximum product yield of 99% (for the 1-phenylethanol oxidation after 1 h without any Figure 4. Formulae Complexadditive)5 (left) and and6 (right), 13% which (for havethe cyclohexane been used in oxidations oxidation with to peroxidescyclohexyl adapted hydroperoxide, from Ref. [64 ],cyclo- Copyright (2011) with permissionhexanol, of Elsevier. and cyclohexanone after 3 h).

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FigureFigure 5. 5.Complex Complex7 7Adapted Adapted fromfrom Ref. Ref. [ 67[67]],, Copyright Copyright (2018), (2018), Molecules Molecules (MDPI), (MDPI), Open Open Access. Access.

Figure 6. Complex 8 Adapted from Ref. [[67],67], Copyright (2018), Molecules (MDPI).

2 2 The catalyticcatalytic activity ofactivity a hexanuclear of Cu(II)a complexhexanuclear [Cu3 (µ2-1kNOCu(II) ,2kNOcomplex-L)(µ- [CuCl)23(Cl)(MeOH)(DMF)(μ2-1kNO2,2kNO2-L)(2]2)μ (Figure-Cl)2(Cl)(MeOH)(DMF)7) for the microwave-assisted2]2) (Figure neat 7) oxidation for the of alcoholsmicro- wave-assistedwas explored. neat This oxidation Cu (II) complexof alcohols was was found explored. to exhibit This Cu high (II) activitycomplex under was found milder to exhibit high activity under milder reaction conditions of oxidation of 1-Phenylethanol by tert-butyl hydroperoxide. Yield 95% of acetophenone in the presence of catalyst was found [70].

Figure 7. Complex tested in oxidations with peroxides adapted from Ref. [70], Copyright (2020), Int. J. Mol. Sci. (MDPI), Open Access.

The copper complexes shown in Figure 8 [72] were tested in catalytic oxidation. It should be noted that in the case of binuclear complex 15 (Figure 10), the initial rate of accumulation of oxidation products is higher than for complex 9 (Figure 9), however, the maximum product yield is somewhat lower than in catalysis with mononuclear complex 9. The authors proposed the following explanation for this fact. This phenomenon is probably due to a higher rate of interaction of 15 with the resulting hydroperoxide in comparison with the situation found for the complex 9 or the dimeric complex 15 effectively decomposes hydrogen peroxide via a catalase pathway that is not associated with the generation of intermediate species of an oxidizing nature, and therefore the yield of

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Figure 6. Complex 8 Adapted from Ref. [67], Copyright (2018), Molecules (MDPI).

Catalysts 2021, 11, 186 The catalytic activity of a hexanuclear Cu(II) complex7 of 37 [Cu3(μ2-1kNO2,2kNO2-L)(μ-Cl)2(Cl)(MeOH)(DMF)2]2) (Figure 7) for the microwave-assisted neat oxidation of alcohols was explored. This Cu (II) complex was found to exhibit high activity under milder reaction conditions of oxidation of 1-Phenylethanol by reactiontert-butyl conditions hydroperoxide. of oxidation Yield of 95% 1-Phenylethanol of acetophenone by tert in-butyl the hydroperoxide.presence of catalyst Yield 95%was of acetophenone in the presence of catalyst was found [70]. found [70].

FigureFigure 7.7. Complex tested in in oxidations oxidations with with peroxides peroxides adapted adapted from from Ref. Ref. [70], [70 ],Copyright Copyright (2020), (2020), Int.Int. J.J. Mol.Mol. Sci.Sci. (MDPI),(MDPI), OpenOpen Access.Access.

TheThe coppercopper complexes shown in in Figure Figure 88[ [72]72] were were tested tested in in catalytic catalytic oxidation. oxidation. It Itshould should be be noted noted that that in in the the case case of of binuclear binuclear complex 15 (Figure 1010),), thethe initialinitial raterate ofof accumulationaccumulation ofof oxidationoxidation productsproducts isis higherhigher thanthan forfor complexcomplex9 9(Figure (Figure9 ),9), however, however, the the maximummaximum product product yield yield is is somewhat somewhat lower lower than than in in catalysis catalysis with with mononuclear mononuclear complex complex9. The9. The authors authors proposed proposed the following the following explanation explan foration this for fact. this This fact. phenomenon This phenomenon is probably is dueprobably to a higher due to rate a higher of interaction rate of ofinteraction15 with the of resulting15 with the hydroperoxide resulting hydroperoxide in comparison in withcomparison the situation with found the situation for the complex found for9 or the the complex dimeric complex9 or the 15dimericeffectively complex decomposes 15 effec- hydrogentively decomposes peroxide hydrogen via a catalase peroxide pathway via a that catalase is not pathway associated that with is not the associated generation with of intermediatethe generation species of intermediate of an oxidizing species nature, of an andoxidizing therefore nature, the yieldand therefore of oxidizing the speciesyield of decreases. The lowest activity was found for compound 17 (see Figure 11), in which the copper ion is strongly screened by methyl groups in the ligand.

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Catalysts 2021, 11, 186 8 of 37 oxidizing species decreases. The lowest activity was found for compound 17 (see Figure 11), in which the copper ion is strongly screened by methyl groups in the ligand.

FigureFigure 8. 8.Complexes Complexes studied studied as as catalysts, catalysts, adapted adapted from from Ref. Ref. [72 [72],], Copyright Copyright (2020) (2020) with with permission permission ofof Elsevier. Elsevier.

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FigureFigure 9.9.Oxidation Oxidation of of cyclohexane cyclohexane with with hydrogen hydrogen peroxide peroxide (50% (50% aqueous) aqueous) catalyzed catalyzed by compoundby com- 9 pound 9 in MeCN at 50 °C (Graphs (A,B)); total volume of the reaction solution was 5 mL. Con- inpound MeCN 9 in at 50MeCN◦C (Graphs at 50 °C ( A(Graphs,B)); total (A volume,B)); total of volume the reaction of the solution reaction was solution 5 mL. was Concentrations 5 mL. Con- of centrations of cyclohexanone and cyclohexanol were determined before (Graph A) and after cyclohexanonecentrations of cyclohexanone and cyclohexanol and were cyclohex determinedanol were before determined (Graph A before) and after(Graph (Graphs A) andB )after reduction (Graphs B) reduction of the aliquots with solid PPh3. Total yield of products 27% after 2 h, TON = (Graphs B) reduction of the aliquots with solid PPh3. Total yield of products 27% after ◦2 h, TON = of the aliquots with solid PPh3. Total yield of products 27% after 2 h, TON = 250 at 50 C and 30% 250 at 50 °C and 30% after 1 h, TON = 280 at 60 °C. adapted from [72], Copyright (2020) with per- after 1 h, TON = 280 at 60 ◦C. adapted from [72], Copyright (2020) with permission of Elsevier. mission of Elsevier.

Figure 10. Figure 10. Oxidation of of cyclohexane cyclohexane with with hydrogen hydrogen peroxide peroxide (50% (50% aqueous;) aqueous;) catalyzed catalyzed by bycom- com- ◦ poundpound15 15in in MeCN MeCN at at 60 60 C.°C. Concentrations Concentrations of of cyclohexanone cyclohexanone and and cyclohexanol cyclohexanol were were after after reduction reduction of the aliquots with solid PPh3. Points for cyclohexanol (red) and cyclohexanone (blue) are oftion the of aliquots the aliquots with solidwith PPhsolid3. PPh Points3. Points for cyclohexanol for cyclohexanol (red) and (red cyclohexanone) and cyclohexanone (blue) are (blue) marked are by numbersmarked by 1 and numbers 2, respectively. 1 and 2, respectively. Total yield of Total products yield 20%of products after 1 h, 20% TON after = 180. 1 h, AdaptedTON = 180. from [72], CopyrightAdapted from (2020) [72], with Copyright permission (2020) of Elsevier. with permission of Elsevier.

Catalysts 2021, 11, 186 10 of 37 Catalysts 2021, 11, x FOR PEER REVIEW 10 of 39

FigureFigure 11.11. OxidationOxidation of of cyclohexane cyclohexane with with hydrogen hydrogen peroxide peroxide (50% (50% aqueous) aqueous) catalyzed catalyzed by com- by com- poundpound17 17in in MeCN MeCN at at 60 60◦ C;°C; Concentrations Concentrations of of cyclohexanone cyclohexanone and and cyclohexanol cyclohexanol were were after after reduction reduc-

oftion the of aliquots the aliquots with with solid solid PPh3 .PPh Points3. Points for cyclohexanol for cyclohexanol (red) (red and) cyclohexanoneand cyclohexanone (blue) (blue) are marked are bymarked numbers by numbers 1 and 2, respectively.1 and 2, respectively. Total yield Total of productsyield of products 2.5% after 2.5% 2 h, after TON 2 h, = 22.TON Adapted = 22. from Ref.Adapted [72], Copyrightfrom Ref. [72], (2020) Copyright with permission (2020) with of Elsevier. permission of Elsevier.

AsAs inin the the case case of ironof iron ions, ions, a large a numberlarge number of works of have works been have devoted been to silsesquiox-devoted to anesilsesquioxane complexes withcomplexes copper with and theircopper use and in catalytic their use oxidation in catalytic [73 –oxidation88]. A few [73–88]. structures A few of thestructures catalysts of used the catalysts in the most used recent in the years most are recent shown years in Figuresare shown 12– in15 .Figures 12–15. FirstFirst examplesexamples ofof heptanuclearheptanuclear cage cage silsesquioxanes, silsesquioxanes, (PhSiO (PhSiO1.51.)514)14(CuO)(CuO)77 18 and (Me-(Me- SiOSiO1.51.5))1414(CuO)(CuO)77 1919,, were were obtained obtained [81]. [81]. It It should be noted thatthat inin Ref.Ref. [[81]81] (Figure(Figure 1212),),the the complexescomplexes18 18and and19 19used used in in the the catalysis catalysis of alkaneof alkane oxidation oxidation exhibited exhibited somewhat somewhat different dif- activity.ferent activity. Compound Compound18 gave 18 a highergave a yield higher of oxidationyield of oxid products.ation products. Product accumulation Product accu- curvesmulation are curves shown are in Figure shown 13 in. TheFigure maximum 13. The observedmaximum in observed the Figure in 13 theB and Figure the decrease13 B and inthe the decrease alcohol in yield the can alcohol be explained yield can by be the explained over-oxidation by the of over-oxida alcohol withtion the of formationalcohol with of unidentiﬁablethe formation oxidationof unidentifiable products. oxidation It is important products. that theIt is oxidation important requires that the the oxidation presence re- of nitricquires acid. the presence Under conditions of nitric acid. of the Under experiment conditions shown of inthe Figure experiment 13 the yieldshown of in oxygenates Figure 13 wasthe yield 39%. Thisof oxygenates is a high value was 39%. taking This into is accounta high value the pronounced taking into inertnessaccount the of alkanes. pronounced The oxidationinertness of n-heptanealkanes. The allowed oxidation to measure of n-hept theane regioselectivity allowed to measure parameters the regioselectivity for positions 1,parameters 2, 3 and 4: for C (1):positions C (2): C1, (3): 2, 3 C and (4) =4: 1.0: C (1): 6.0: C 6.0: (2): 5.5. C (3): This C data (4) = also 1.0: indicates 6.0: 6.0: 5.5that. This the reactiondata also proceeds indicates with that the the participation reaction proceeds of free hydroxyl with the radicals participation generated of from free hydrogenhydroxyl peroxide.radicals generated However, from these hydrogen parameters peroxide. are higher However, than those these usually parameters observed are in higher oxidation than withthose hydroxyl usually observed radicals(1: in 2:oxidation 2: 2). Such with an hydroxyl increase radicals in regioselectivity (1: 2: 2: 2).can Such be an explained increase byin theregioselectivity fact that oxidation can be in ex theplained case ofby complex the fact 18thatoccurs oxidation in the in cavities the case formed of complex by bulky 18 ligandsoccurs in surrounding the cavities the formed reaction by centers bulky ofliga thends catalyst surrounding molecule. the reaction centers of the catalystComplex molecule.20 [ 82] (see Figure 14) was found to be a good catalyst in oxidations of alcohols and alkanes with TBHP and H2O2, respectively. 1-Phenylethanol and heptanol-2 could be converted into corresponding ketones in yields up to 94% and 50%, respectively. Hydroperoxidation of alkanes by H2O2 was found when complex 20 is used as a catalyst. Cyclohexane has been converted into cyclohexyl hydroperoxide which was gradually transformed in a mixture of cyclohexanol and cyclohexanone over the course of the oxidation. Catalysts 2021, 11, x FOR PEER REVIEW 11 of 39

Catalysts 2021, 11, 186 11 of 37

FigureFigure 12. Molecular 12. Molecular structures structures of 18 ofand18 19and. Color19. Color code: code: Si—yellow, Si—yellow, O—red, O—red, Cu—green, Cu—green, N—blue, N—blue, adaptedadapted from from [81] Copyright [81] Copyright (2018) (2018) withwith permission permission from from Wiley. Wiley.

Not only silsesquioxane complexes, but germsesquioxanes, which are precatalysts in the oxidation of organic compounds, were obtained and described [86–88]. An example of structure of such catalysts is given in Figure 15. Germanium-based sesquioxane copper complex exhibits an extremely high nuclearity (Cu42Ge24Na4) and unusual encapsula- tion features. This compound is a very active catalyst in the oxidation of alkanes and alcohols [87].

CatalystsCatalysts2021 2021,,11 11,, 186 x FOR PEER REVIEW 1212 of 3739

concentration (M) B 0.12

cyclohexanol

0.08

after PPh3

0.04

cyclohexanone 0

concentration (M) A cyclohexanol 0.02

before PPh 0.01 3

0 cyclohexanone

0 1 2 time (h)

◦ FigureFigure 13.13. OxidationOxidation ofof cyclohexanecyclohexane withwith HH22O22 (50%) catalyzed by complex 18 at 60 °C.C. Concentra-Concen- Catalysts 2021, 11, x FOR PEER REVIEWtrations of cyclohexanol and cyclohexanone were measured both before (A)and after reduction13 of (B) 39 tions of cyclohexanol and cyclohexanone were measured both before (A) and after reduction (B) with with PPh3 (used method of Shul’pin). Adapted from [81], Copyright (2018) with permission of PPh3 (used method of Shul’pin). Adapted from [81], Copyright (2018) with permission of Wiley. Wiley.

Complex 20 [82] (see Figure 14) was found to be a good catalyst in oxidations of alcohols and alkanes with TBHP and H2O2, respectively. 1-Phenylethanol and heptanol-2 could be converted into corresponding ketones in yields up to 94% and 50%, respectively. Hydroperoxidation of alkanes by H2O2 was found when complex 20 is used as a catalyst. Cyclohexane has been converted into cyclohexyl hydroperoxide which was gradually transformed in a mixture of cyclohexanol and cyclohexanone over the course of the oxidation.

Figure 14. Scheme synthes ComplexComplex 2020 usedused in in oxidation oxidation alkanes alkanes with with peroxides peroxides is ispresented presented on on this ﬁgure,figure, adaptedadapted fromfrom Ref.Ref. [[82],82], CopyrightCopyright (2018)(2018) withwith permissionpermission ofof Elsevier.Elsevier.

Not only silsesquioxane complexes, but germsesquioxanes, which are precatalysts in the oxidation of organic compounds, were obtained and described [86–88]. An example of structure of such catalysts is given in Figure 15. Germanium-based sesquioxane copper complex exhibits an extremely high nuclearity (Cu42Ge24Na4) and unusual encap- sulation features. This compound is a very active catalyst in the oxidation of alkanes and alcohols [87].

Figure 15. The copper complex with germsesquioxane surroundings. Adapted from Ref. [87], Copyright (2018), Catalysts, MDPI.

Catalysts 2021, 11, x FOR PEER REVIEW 13 of 39

Figure 14. Scheme synthesComplex 20 used in oxidation alkanes with peroxides is presented on this figure, adapted from Ref. [82], Copyright (2018) with permission of Elsevier.

Not only silsesquioxane complexes, but germsesquioxanes, which are precatalysts in the oxidation of organic compounds, were obtained and described [86–88]. An example of structure of such catalysts is given in Figure 15. Germanium-based sesquioxane copper complex exhibits an extremely 42 24 4 Catalysts 2021, 11, 186 high nuclearity (Cu Ge Na ) and unusual encap-13 of 37 sulation features. This compound is a very active catalyst in the oxidation of alkanes and alcohols [87].

FigureFigure 15.15. The copper complex with germsesquioxane surroundings. Adapted from Ref. [[87],87], Copyright (2018), Catalysts, MDPI.MDPI.

A recent publication [89] described the synthesis and catalytic properties of complex 21 (see Figure 16). The kinetic study allowed authors to propose the mechanism of the oxidation reaction of alkanes and alcohols with hydrogen peroxide and tert-butyl hydroperoxide. Complex 21 exhibited high catalytic activity in the oxidation of cyclohexane and other

alkanes with H2O2 in acetonitrile in the presence of nitric acid. The following selectivity parameters were obtained for the oxidation of n-heptane: C(1):C(2):C(3):C(4) = 1.0:5.6:5.6:5.0. These data as well as the character of dependence of the initial cyclohexane oxidation rate on the initial hydrocarbon concentration (approaching a plateau at [cyclohexane]0 > 0.3 M) (see Figure 17) indicate that the reaction occurs with the participation of hydroxyl radicals and alkyl hydroperoxides are formed as the main primary products. However, the regioselectivity in the oxidation of n-heptane (see above) is noticeably higher than the regioselectivity usually observed for oxidation with hydroxyl radicals (1: 2: 2: 2). The increased regioselectivity in the case of catalysis by complex 21 can be explained by the fact that steric hindrances arise in the molecule of this complex around the copper-containing reaction center. Previously, for most of the catalytic systems studied, based on the data on the selectivity of alkane oxidation, it was concluded that the oxidizing species is a hydroxyl radical. This conclusion has received kinetic conﬁrmation see Ref. [20]. On the one hand, the calculated ratio of the constants of the rates of reactions of the interaction of an oxidizing species with acetonitrile and cyclohexane k2/k3 is much higher than for the reactions ratio characterized the reactions involving hydroxyl radicals [20,86]. The obtained value (k2/k3 = 0.033) is very different from the 0.006–0.01. Catalysts 2021, 11, x FOR PEER REVIEW 14 of 39

A recent publication [89] described the synthesis and catalytic properties of complex 21 (see Figure 16). The kinetic study allowed authors to propose the mechanism of the oxidation reaction of alkanes and alcohols with hydrogen peroxide and tert-butyl hydroperoxide. Complex 21 exhibited high catalytic activity in the oxidation of cyclohexane and other alkanes with H2O2 in acetonitrile in the presence of nitric acid. The following selectivity parameters were obtained for the oxidation of n-heptane: C(1):C(2):C(3):C(4) = 1.0:5.6:5.6:5.0. These data as well as the character of dependence of the initial cyclohexane oxidation rate on the initial hydrocarbon concentration (approaching a plateau at [cyclohexane]0 > 0.3 M) (see Figure 17) indicate that the reaction occurs with the participation of hydroxyl radicals and alkyl hydroperoxides are formed as the main primary products. However, the regioselectivity in the oxidation of n-heptane (see above) is noticeably higher than the regioselectivity usually observed for oxidation with hydroxyl radicals (1: 2: 2: 2). The increased regioselectivity in the case of catalysis by complex 21 can be explained by the fact that steric hindrances arise in the molecule of this complex around the copper-containing reaction center. Previously, for most of the catalytic systems studied, based on the data on the selectivity of alkane oxidation, it was concluded that the oxidizing species is a hydroxyl radical. This conclusion has received kinetic confirmation see Ref. [20]. On the one hand, the calculated ratio of the constants of the rates of reactions of the interaction of an oxidizing species with acetonitrile and cyclohexane k2/k3 is much higher than for the reactions ratio characterized the reactions involving hydroxyl radicals [20,86]. The obtained value (k2/k3 = 0.033) is very different from the 0.006‒0.01. On the other hand, the low selectivity of the effect of oxidizing species (Z = HO•) is close to the selectivity found for reactions involving hydroxyl radicals, indicates that hydroxyl radicals are generated in the catalytic system studied in the work [89]. Similar results (a decrease in the relative reactivity of the oxidizing species for cyclohexane in comparison with acetonitrile) were obtained earlier [86]. Thus, based on the kinetic data and selectivity parameters in alkane oxidation, authors came to different conclusions about the nature of the oxidizing species in the 21/H2O2 catalytic system. This contradiction can be resolved if it is assumed that the concentrations of acetonitrile and cyclohexane near the reaction center (that is, at the place where the oxidizing species originated) differ from their concentrations in the volume. In this model, the results obtained indicate that the ratio of acetonitrile and cyclohexane concentrations inside the cluster of the Catalysts 2021, 11, 186 catalytic species (near the reaction center) exceeds their ratio in volume. Such a differ-14 of 37 ence may occur due to a higher concentration of acetonitrile and hydrogen peroxide inside a cavity in the catalyst cage.

Catalysts 2021, 11, x FOR PEER REVIEW 15 of 39

B FigureFigure 16. Molecular 16. Molecular structure structure of complex of complex 21. A: 21side.( Aview.): side B: view. top. View. (B): top. Color View. code: Color Ge: code: grey, Ge:O: grey, O: red, Cu:red, green, Cu: green,N: blue. N: Adapted blue. Adapted from [89], from Copyright [89], Copyright (2019) with (2019) permission with permission of Elsevier. of Elsevier.

On the other hand, the low selectivity of the effect of oxidizing species (Z = HO•) is close to the selectivity found for reactions involving hydroxyl radicals, indicates that hydroxyl radicals are generated in the catalytic system studied in the work [89]. Similar results (a decrease in the relative reactivity of the oxidizing species for cyclohexane in comparison with acetonitrile) were obtained earlier [86]. Thus, based on the kinetic data and selectivity parameters in alkane oxidation, authors came to different conclusions about the nature of the oxidizing species in the 21/H2O2 catalytic system. This contradiction can be resolved if it is assumed that the concentrations of acetonitrile and cyclohexane near the reaction center (that is, at the place where the oxidizing species originated) differ from their concentrations in the volume. In this model, the results obtained indicate that the ratio of acetonitrile and cyclohexane concentrations inside the cluster of the catalytic species (near the reaction center) exceeds their ratio in volume. Such a difference may occur due to a higher concentration of acetonitrile and hydrogen peroxide inside a cavity in the catalyst cage. As can be seen from the above, polynuclear copper complexes have great potential as catalysts in the oxidation of alkanes and alcohols. For comparison, we present several references to articles devoted to mononuclear copper complexes in oxidative catalysis [90–96]. These works investigated mononuclear complexes that exhibit signiﬁcant activity.

CatalystsCatalysts 2021,2021 11, x, 11FOR, 186 PEER REVIEW 16 of 1539 of 37

Figure 17. Cont.

Catalysts 2021, 11, 186 16 of 37 Catalysts 2021, 11, x FOR PEER REVIEW 17 of 39

Concentration (M) Concentration (M)

FigureFigure 17. Accumulation 17. Accumulation of cyclohexanol of cyclohexanol and cyclohexanone and cyclohexanone in oxidation in oxidation of cyclohexane of cyclohexane with hy- with drogenhydrogen peroxide peroxide catalyzed catalyzed by compound by compound 21 in MeCN21 in MeCN at 50 °C. at 50Dependence◦C. Dependence of the initial of the initialrate of rate of oxygenate formation W0 on initial concentration of cyclohexane is shown in graph (A). Lineariza- oxygenate formation W0 on initial concentration of cyclohexane is shown in graph (A). Linearization tion of curve from graph A in coordinates W0−11—[CH3CN]/[C6H12] is presented in graph (B). Con- of curve from graph (A) in coordinates W0 —[CH3CN]/[C6H12] is presented in graph (B). Concen- centrations of cyclohexanone and cyclohexanol were determined after reduction of the aliquots trations of cyclohexanone and cyclohexanol were determined after reduction of the aliquots with with solid PPh3. Accumulation of cyclohexanol and cyclohexanone (under conditions given above) is shownsolid PPhwhen3. Accumulationtheir concentrations of cyclohexanol were measured and cyclohexanonebefore (graph (C (under)) and after conditions treating given the reac- above) is C tionshown sample when with theirPPh3 concentrations(graph (D)). Adapted were measured from [89], before Copyright (graph (2019) ( )) andwith after permission treating of the Else- reaction vier.sample with PPh3 (graph (D)). Adapted from [89], Copyright (2019) with permission of Elsevier. 2.3. Polymanganese Complexes in Oxidations with H O As can be seen from the above, polynuclear copper2 2 complexes have great potential as catalystsManganese in the oxidation complexes of alkanes are activeand alcohols. in the For oxidation comparison, of we hydrocarbons present several and referencesalcohols to[97 articles–120]. Carbonyldevoted to manganese mononucl complexesear copper with complexes chelating in oroxidative bridging catalysis mesoionic [90–96].di(1,2,3-triazolylidene) These works investigated ligands weremononuclea synthesizedr complexes and characterized that exhibit ofsignificant a bimetallic activ- man- ity.ganese(0) complex 23 [99]. (Figure 18). This complex exhibited high activity (yields up to 99%) and selectivity in the catalytic oxidation of secondary alcohols and benzyl alcohol

with tert-butyl hydroperoxide). Comparison of the mono- and bimanganese complexes

Catalysts 2021, 11, x FOR PEER REVIEW 18 of 39

2.3. Polymanganese Complexes in Oxidations with H2O2 Manganese complexes are active in the oxidation of hydrocarbons and alcohols [97– 120]. Carbonyl manganese complexes with chelating or bridging mesoionic di(1,2,3-triazolylidene) ligands were synthesized and characterized of a bimetallic manganese(0) complex 23 [99]. (Figure 18). This complex exhibited high activity (yields up to Catalysts 2021, 11, 186 99%) and selectivity in the catalytic oxidation of secondary alcohols and benzyl alcohol17 of 37 with tert-butyl hydroperoxide). Comparison of the mono- and bimanganese complexes 22 and 23 showed that the binuclear complex is more active in the oxidation of spites by tert-butyl hydroperoxide. The yield of acetophenone in the reaction with complex 23 at- tained22 99%and at23 40showed °C after that 2 h. theA similar binuclear yield complex was obtained is more by active carrying in the out oxidation the reaction of spites withoutby solvent.tert-butyl hydroperoxide. The yield of acetophenone in the reaction with complex 23 attained 99% at 40 ◦C after 2 h. A similar yield was obtained by carrying out the reaction without solvent.

Figure 18. Carbonyl manganese compounds as a catalysts in oxidation with peroxides, Adapted from Ref. [99] Copyright (2019) with permission from Royal Society of Chemistry.

In 1998, the effective catalytic action of the binuclear manganese (IV) complex 24

(Figure 19) was described in the oxidation of organic compounds with hydrogen peroxide in acetonitrile at low temperatures [100,101]. This system works efﬁciently only in the presence of an added organic acid. This acid was acetic acid, and later it was found that oxalic acid is even more effective [112–117]. Thus, for example, the system complex 24/oxalic acid/H2O2/O2 gives products in oxidation cyclohexane with yield up to 50% (TON = 2700) [115]. Other organic acids also accelerate the oxidation reaction [102–120]. Catalysts 2021, 11, x FOR PEER REVIEW 19 of 39

Figure 18. Carbonyl manganese compounds as a catalysts in oxidation with peroxides, Adapted from Ref. [99] Copyright (2019) with permission from Royal Society of Chemistry.

In 1998, the effective catalytic action of the binuclear manganese (IV) complex 24 (Figure 19) was described in the oxidation of organic compounds with hydrogen peroxide in acetonitrile at low temperatures [100,101]. This system works efficiently only in the presence of an added organic acid. This acid was acetic acid, and later it was found that Catalysts 2021, 11, 186 oxalic acid is even more effective [112–117]. Thus, for example, the system complex18 of 37 24/oxalic acid/H2O2/O2 gives products in oxidation cyclohexane with yield up to 50% (TON = 2700) [115]. Other organic acids also accelerate the oxidation reaction [102–120]. TheThe work work [109] [109] on on the the oxidation oxidation of of organic organic co compoundsmpounds in in water water is is of of particular particular interest. interest. InIn oxidation oxidation of of n-heptane n-heptane TON TON was was attained attained 160 160 at at 50 50 °C◦C after after 3 3 h. h. That That study study revealed revealed a a remarkableremarkable regio-selectivity regio-selectivity in in the the oxidation oxidation of of higher higher nn-alkanes.-alkanes.

FigureFigure 19. 19. BinuclearBinuclear manganese manganese complex 24 which catalyzescatalyzesa a very very efﬁcient efficient oxidation oxidation with with peroxides, peroxides,Adapted Adapted from [120 from], Copyright [120], Copyright (2017) with(2017) permission with permission of Elsevier. of Elsevier.

Further,Further, this this system system was was investigated investigated in in more more detail detail [119,120]. [119,120]. The The kinetic kinetic study study led led toto the the conclusion conclusion about about the the oxidation oxidation mechanism mechanism proposed proposed in in [120], [120], which which includes includes the the formationformation of of intermediate intermediate complexes. complexes. A A catalytic catalytic cycle cycle shown shown in in Figure Figure 20 20 (see (see below). below). ManyMany catalytic catalytic reactions reactions of alkane oxidationoxidation intointoalkyl alkyl hydroperoxides hydroperoxides contain contain as as a cruciala cru- cialstep step the the abstraction abstraction of a of hydrogen a hydrogen atom atom from from the alkanethe alkane with with subsequent subsequent fast interactionfast inter- of the formed alkyl radical with atmospheric dioxygen (Route 1 in Figure 20). The data action of the formed alkyl radical with atmospheric dioxygen (Route 1 in Figure 20). The obtained in the present study show that, on the one hand, the transformation of the data obtained in the present study show that, on the one hand, the transformation of the alkane R0R00R000C–H into the corresponding alkyl hydroperoxide R0R00R000C–OOH proceeds alkane R′R″R′′′C–H into the corresponding alkyl hydroperoxide R′R″R′′′C–OOH pro- with the formation of alkyl radical R0R00R000C• which rapidly reacts with atmospheric ceeds with the formation of alkyl radical R′R″R′′′C• which rapidly reacts with atmos- molecular oxygen. On the other hand, alkyl radicals can be generated via hydrogen pheric molecular oxygen. On the other hand, alkyl radicals can be generated via hydro- abstraction from the alkane not by hydroxyl radicals but most likely by manganyl Mn=O gen abstraction from the alkane not by hydroxyl radicals but most likely by manganyl fragments. A catalytic cycle shown in Figure 20 is in good agreement with all experimental Mn=O fragments. A catalytic cycle shown in Figure 20 is in good agreement with all ex- data and allowed authors to explain all features of the reaction. In the ﬁrst step of the perimental data and allowed authors to explain all features of the reaction. In the first process carboxylic acid protonates one of the oxygen bridges between two manganese step of the process carboxylic acid protonates one of the oxygen bridges between two (IV) centers, resulting in the formation of a vacant site at one Mn (IV). The generated manganese (IV)IV centers, resultingIV 3+in the formation of a vacant site at one Mn (IV). The complex [Mn (µ-O)2Mn (OH)] then adds one hydrogen peroxide molecule and the generated complex [MnIV(μ-O)2MnIV(OH)]3+IV then adds oneV hydrogen2+ peroxide molecule formed hydroperoxo derivative [(HOO)Mn (µ-O)2MnI (OH)] eliminates hydroperoxyl and the formed hydroperoxo derivative [(HOO)MnIV(μ-O)2MnIV(OH)]III 2+ eliminatesIV hy-2+ radical to afford the catalytically active Mn(III)Mn(IV) species [Mn (µ-O) 2Mn (OH)] droperoxyl radical to afford the catalytically active Mn(III)Mn(IV) species [MnIII(μ-O) shown in Figure 20. The interaction with a second H2O2 molecule leads to the formation IV 2+ 2Mnof the (OH)] dihydroperoxo shown in complex Figure 20. of Mn(III)Mn(IV).The interaction Thewith protonation a second H of2O the2 molecule MnOOH leads ligand to theand formation subsequent of the evolution dihydroperoxo of the water complex molecule of Mn(III)Mn(IV). gives the high-valent The protonation oxomanganese of the MnOOHspecies whichligand abstractsand subsequent a hydrogen evolution atom of from the thewater alkane molecule R0R00R gives000C–H the to high-valent produce an oxomanganesealkyl radical. Thespecies recombination which abstracts in the a hydrogen solvent cage atom of from this radicalthe alkane and R either′R″R′′′C–H HO– to or produceHOO– ligand an alkyl (oxygen radical. rebound The recombination or hydroperoxyl in the rebound) solvent produces cage of this either radical alkanol and or either alkyl HO–hydroperoxide, or HOO– ligand respectively. (oxygen rebound or hydroperoxyl rebound) produces either al- kanolR or0R alkyl00R000C hydroper• + HO–Mnoxide,→ respectively.R0R00R000C–OH + Mn (Route 2). RR′R0R″00RR′′′000C•C• + +HO HOO–‒MnMn → R→′RR″R0R′′′00CR‒000OHC–OOH + Mn +(RouteMn• (Route2). 3) where Mn is a fragment of the binuclear intermediate species. Reactions of the manganyl fragment formation and hydrogen atom abstraction are rate-limiting stages. Thus, this mechanism is a crypto-radical one but not a radical-chain process. Catalysts 2021, 11, x FOR PEER REVIEW 20 of 39

R′R″R′′′C• + HOO‒Mn → R′R″R′′′C‒OOH + Mn• (Route 3)

Catalysts 2021, 11, 186 where Mn is a fragment of the binuclear intermediate species. Reactions of19 ofthe 37 manganyl fragment formation and hydrogen atom abstraction are rate-limiting stages. Thus, this mechanism is a crypto-radical one but not a radical-chain process.

H2O2 H2O HO O HO O + HA, + H O 2+ 2 2 2+ IV 2+ MnIV MnIV MnIII MnIV MnIII Mn O O K O O 1 O O 18 18 Route 1 H O O−CR'R"R'" H16O16O−CR'R"R'" H2O2 − 16 18 A H O−CR'R"R'" + O2 16 K2 + O2 CR'R"R'" AH + Route 2 + 16 16 H O OH 16O Route 3 HO HO H16O16O−CR'R"R'" O O 2+ IV IV + Mn Mn III IV O Mn Mn O O H−CR'R"R'" AH O A− k1

K3 HO H2O HO O H O O 2 O O 2+ 2+ MnV MnIV MnIII MnIV O O O O 18 16 Figure 20. A catalytic cyclecycle proposedproposed forfor thethe alkanealkane hydroperoxidationhydroperoxidation inin thethepresence presenceof of 18OO22 (in(in anan atmosphereatmosphere 16O2 + 18 OO22).). AH AH is is a a carboxylic carboxylic acid acid where where A A is is its its anion. anion. Adapted Adapted from from [120], [120], Copyright Copyright (2017) (2017) with with permission permission of of Elsevier. Elsevier.

2.4. Oxidation Oxidation Catalyzed Catalyzed by Polyvanadate Ions Vanadium ions play play an an important important role role in in biological biological systems systems and and are are effective effective cata- cat- lystsalysts for for the the oxidation oxidation of oforganic organic compounds compounds with with peroxides peroxides (See (See recent recent papers papers and and re- viewsreviews Refs. Refs. [5,121–155]). [5,121–155 ]).Mono Monovanadatevanadate anion anion forms forms polyvanadate polyvanadate ions ions under under the theaction ac- oftion strong of strong proton proton acids. acids. Such Such polyvanadates polyvanadates are arecapable capable of catalyzing of catalyzing the the oxidation oxidation of organicof organic compounds compounds with with peroxides peroxides [121–157]. [121–157 Examples]. Examples of such of such compounds compounds the di- the and di- tetra-nuclearand tetra-nuclear complexes complexes of vanadium of vanadium are are desc describedribed in inpaper paper Ref. Ref. [156]. [156]. The The catalytic properties ofof mono-(25),mono- (25), di-(26) di- (26) and and tetranucleovanadium tetranucleovanadium (27) (27) complexes complexes Figure Figure 21 have 21 havebeen been studied studied [156]. [156]. Interestingly, Interestingly, the threethe three complexes complexes exhibit exhibit catalytic catalytic activity activity only only in inthe the presence presence of pyrazine-2-carboxylicof pyrazine-2-carboxylic acid acid (PCA). (PCA). The The vanadium vanadium complexes complexes differ differ sig- significantlyniﬁcantly in in their their catalytic catalytic behavior, behavior, the the most most efﬁcient efficient catalyst catalyst being being complex complex25 25.. This compound catalyzes oxidation of of cyclohexane with with high initial rate and high TON after 24 h ((TONTON = 14001400).). The binuclear complex 26 catalyzes the reaction with approximately the samesame activity,activity, but but the the behavior behavior of of complex complex27 containing27 containing strongly strongly complexing complexing chelating che- latingand voluminous and voluminous ligands ligands is remarkable is remarkable because itbecause gives rise it togives the formationrise to the of formation almost pure of almostcyclohexyl pure hydroperoxide. cyclohexyl hydroperoxide.

Catalysts 2021, 11, x FOR PEER REVIEW 21 of 39 Catalysts 2021, 11, 186 20 of 37

Figure 21. Structures of vanadium complexes 25–27, Adapted from [156], Copyright (2009) with permission of Elsevier. Figure 21. Structures of vanadium complexes 25–27, Adapted from [156], Copyright (2009) with permission of Elsevier. The formation of oligovanadates in the presence of protons from strong acids was describedThe formation in Ref. [158 of]. oligovanadates In reactions with in hydrogenthe presence peroxide of protons catalyzed from bystrong oligovanadate, acids was describedthe adjacent in Ref. vanadate [158]. In fragment reactions is with the carrierhydrogen of Hperoxide+ to the catalyzed vanadate by ion. oligovanadate, Similarly to thehow adjacent PCA works vanadate in reactions fragment catalyzed is the carrier by monovanadates, of H+ to the vanadate the scheme ion. Similarly for generating to how PCAcatalytically works in active reactions species catalyzed is shown by below. monova Thenadates, possible the catalytic scheme cyclesfor generating for generating cata- lyticallyradicals active HOO •speciesand HO is •shownwith thebelow. participation The possible of model catalytic divanadate cycles for (Figure generating 22A) radi- and calsmonovanadate HOO• and (Figure HO• 22withB) havethe participation been analyzed. of Themodel activity divanadate of the polyvanadate (Figure 22A) ion, and in monovanadatecontrast to monovanadate, (Figure 22B) can have be been explained analyzed. by the The fact activity that, in of the the presence polyvanadate of an adjacent ion, in contrastV=O fragment to monovanadate in polyvanadate,, can be the explained formation by of the a hydroperoxyl fact that, in the complex presence form of a complexan adja-

Catalysts 2021, 11, x FOR PEER REVIEW 22 of 39

Catalysts 2021, 11, 186 21 of 37

cent V=O fragment in polyvanadate, the formation of a hydroperoxyl complex form a with coordinatedcomplex with H2 Ocoordinated2 molecule H with2O2 themolecule participation with the of participation a six-membered of a transition six-membered state tran- is facilitatedsition state (Figure is facilitated 23)[158]. (Figure 23) [158].

H O OH2 2 O O O O V V H O O 2 2 O O H2O H2O OH2 H OH O OH2 O O OH O O H O O HO 2 V V 2 HO• O O O HO V V O O O O H OH H2O OH2 O OH OH2 OOOH O 2 OH O O O OH2 V V OH O • O O O HO O V V O O H2O OH2 O O H2O OH2 OH H2O H O OOH HO 2 2 HOO• HO V V O H2O2 O O OH O O H O OH OH 2 OH 2 OOH O 2 O O 2 V O O V V O O V O O O O O H2O OH2 H2O OH2 H2O2 OOH OH2 OH HO O O H OH V V O OH O O O OH O 2 O O H O OH2 V V 2 O O O

H OH H O OH2 O OH 2 O HO 2 O V V O A O O O H2O OH2 O HO O O HOO• O O HO HO V V OH OH H O H2O 2

H2O2

H H HO O HO O O O O O V HO V O OH OH H2O H2O

HO•

H H2O HO O O O O O O V O V HO OH HO OH H2O H2O H2O H2O2

Figure 22. Catalytic cyclesFigure of 22. radicalCatalytic generation cycles of involving radical generation divanadate involving (A) and divanadatemonovanadate (A) and(B). Adapted monovanadate from [158], (B). Copyright (2011) withAdapted permission from of [158 American], Copyright Chem (2011)ical Society. with permission of American Chemical Society.

Catalysts 2021, 11, 186 22 of 37 Catalysts 2021, 11, x FOR PEER REVIEW 23 of 39

FigureFigure 23.23. ProtonProton transfertransfer betweenbetween twotwo vanadium-containingvanadium-containing fragments. Adapted from from Ref. Ref. [158], [158], CopyrightCopyright(2011) (2011)with with permission permission of of American American Chemical Chemical Society. Society.

TheThe paperpaper Ref.Ref. [[159]159] (Figure(Figure 2424)) describesdescribes inin detaildetail thethe preparationpreparation ofof mono-,mono-, di-,di-, tri-nucleartri-nuclear complexescomplexes ofof vanadiumvanadium ((2828––3030)) (Figure(Figure 25 25).). CompoundsCompounds 2828,, 2929,, andand 3030were were testedtested asas homogeneoushomogeneous catalystscatalysts forfor microwavemicrowave oxidationoxidation ofof cyclohexanecyclohexane withwith hydrogenhydrogen peroxideperoxide to to produce produce cyclohexyl cyclohexyl hydroperoxide hydroperoxide (CyOOH), (CyOOH), cyclohexanol, cyclohexanol, and and cyclohexanone cyclohexa- (seenone Figure (see Figure 25). Advantages 25). Advant ofages these of catalyticthese catalytic systems systems are mildness are mildness and environmental and environ- friendliness.mental friendliness. The formation The formation of CyOOH of Cy wasOOH shown was by shown the Shulpin by the method.Shulpin GCmethod. analysis GC ofanalysis the products of the products showed showed a noticeable a noticeable increase increase in the amount in the amount of cyclohexanol of cyclohexanol (after the(af- reductionter the reduction of CyOOH of CyOOH by PPh 3by) and PPh a3) decrease and a decrease of cyclohexanone of cyclohexanone amount amount in comparison in com- withparison the with nonreduced the nonreduced sample. Insample. the case In ofthe the case catalysis of the by catalysis complex by30 complexthe maximum 30 the yieldmaximum of all productsyield of all after products the reduction after the of reduction PPh3 attained of PPh 39%3 attained based on39% initial based amount on initial of cyclohexane.amount of cyclohexane. In this work, In a this reaction work, mechanism a reaction was mechanism proposed. was The proposed. authors proposed The authors the mechanismproposed the which mechanism is a bit differentwhich is froma bit thosedifferent usually from accepted those usually for the accepted oxidizing for systems the ox- containingidizing systems H2O2 andcontaining an metal H ion.2O2 and The mechanisman metal ion. proposed The mechanism in Ref. [159 proposed] includes in at Ref. the ﬁrst[159] step includes coordination at the offirs Ht2 Ostep2 to coordination the catalyst molecule, of H2O2 protonto the transfercatalyst from molecule, ligated proton H2O2 totransfer the methoxy from ligated ligand H and2O2 elimination to the methoxy of the ligand formed and methanol elimination molecule of the and formed coordination metha- • ofnol the molecule second and H2O coordination2 molecule followedof the second by HO–OH H2O2 molecule bond cleavage followed to by give HO–OH HO . bond The activationcleavage to of give the H HO•.2O2 towards The activation this cleavage of the is associatedH2O2 towards with this the redoxcleavage active is associated nature of thewith ligand the redox in the active catalyst nature molecule, of the whichligand acts in the (instead catalyst of themolecule, metal) which as the reducingacts (instead agent of of the H O ligand. Generated hydroxyl radicals attack the substrate molecules. the metal)2 2as the reducing agent of the H2O2 ligand. Generated hydroxyl radicals attack the substrateTheoretical molecules. DFT calculations were in agreement with previous works on oxidation involving vanadium complexes (Ref. [145]), and led to the conclusion that the global radical type mechanism involves the formation of RO• radicals (or HO • if H2O2 is used as an oxidizing agent). Then this radical oxidizes the alkane, a hydrogen atom is removed from the R–H molecule to form the corresponding alkyl radical R•. The latter species react with molecular oxygen from the atmosphere, ultimately forming the alkyl hydroperoxide ROOH. The trinuclear vanadium complexes shown in Figure 26 were investigated as catalysts for the oxidation of dopamine by hydrogen peroxide [160]. Kinetic studies showed that the reaction follows a Michaelis-Menten like kinetics. The conversion of dopamine to aminochrome with different catalysts showing high activity under mild conditions with good conversions (Figure 27).

CatalystsCatalysts2021 2021, 11,, 18611, x FOR PEER REVIEW 2423 of of 39 37

Figure 24. Vanadium Compounds tested as catalysts in oxidation with peroxides. Adapted from [159 ], Copyright (2019) with permission of Royal Society of Chemistry.

Catalysts 2021, 11, x FOR PEER REVIEW 25 of 39

CatalystsFigure2021 24., Vanadium11, 186 Compounds tested as catalysts in oxidation with peroxides. Adapted from [159], Copyright (2019) 24 of 37 with permission of Royal Society of Chemistry.

Review

Oxidation of OrganicFigure 25. Effect Compounds of the heating mode (MWwith irradiation Peroxides or oil bath) on theCatalyzed total yield of cyclohexanol by Figure 25. Effect of the heating mode (MW irradiation or oil bath) on the total yield of cyclohexa- and cyclohexanone obtained by neat oxidation of cyclohexane with aq. H2O2 catalyzed by complex 30, Polynuclear Metalnol and cyclohexanone Compounds obtained by neat oxidation of cyclohexane with aq. H2O2 catalyzed by complexadapted 30, fromadapted Ref. from [159 ],Ref. Copyright [159], Copyright (2019) with (2019) permission with permission of Royal Societyof Royal of Society Chemistry. of Chem- istry.

Theoretical DFT calculations were in agreement with previous works on oxidation involving vanadium complexes (Ref. [145]), and led to the conclusion that the global radical type mechanism involves the formation of RO• radicals (or HO • if H2O2 is used as an oxidizing agent). Then this radical oxidizes the alkane, a hydrogen atom is removed from the R‒H molecule to form the corresponding alkyl radical R•. The latter species react with molecular oxygen from the atmosphere, ultimately forming the alkyl hydroperoxide ROOH. The trinuclear vanadium complexes shown in Figure 26 were investigated as catalysts for the oxidation of dopamine by hydrogen peroxide [160]. Kinetic studies showed that the reaction follows a Michaelis-Menten like kinetics. The conversion of dopamine to aminochrome with different catalysts showing high activity under mild conditions with good conversions (Figure 27).

1, 2 CatalystsGeorgiy 2021 B., 11 Shul’pin, x FOR PEER * and REVIEW Lidia S. Shul’pinaIV 26 of 39 Figure 26. Structural formulae of the V O-complexes prepared in this work. Adapted from Ref. [160], Copyright (2020) with permission of Royal Socaity of Chemistry. 1 Federal Research Center for Chemical Physics, N.N. Semenov Russian Academy of Sciences, Ulitsa Kosygina 4, 119991 Moscow, Russia 2 A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Ulitsa Vavilova 28, 119991 Moscow, Russia; [email protected] * Correspondence: [email protected] or [email protected]; Tel.: Citation: Shul’pin, G.B.; Shul’pina, +7-916-384-7912 L.S. Oxidation of Organic Compounds with Peroxides Abstract: The review describes articles that provide data on the synthesis and study of the proper- Catalyzed by Polynuclear Metal ties of catalystsFigureFigure for 27. 27. the SchemeScheme oxidation of of oxidation oxidation of alkanes, of of dopamine.olefins, dopamine. and alcohols. These catalysts are polynuclear Compounds. Catalysts 2021, 11, x. complexes of iron, copper, osmium, nickel, manganese, cobalt, vanadium. Such complexes for https://doi.org/10.3390/xxxxx IV Figure 26. Structural formulae exampleof theFigureFigure V O-complexes 2828 showsshowsare: prepared the structure [Fein 2this(HPTB)(m-OH)(NO work. of binuclearbinuclear Adapted vanadiumvanadium 3from)2](NO Ref.3)2·CH [160], complexcomplex3OH·2H Copyright2 O,31 .. Although Although(2020) where the the with permission of Royal SocaitymoleculemoleculeHPTB-¼N,N,N0,N0-tetrakis(2-ben of Chemistry of of the the. complexcomplex-3131 containscontainszimidazolylmethyl)-2-hydroxo-1 two two fragments fragments inclus inclusiveive,3-diaminopropane; vanadium, vanadium, these these fragments fragmentscomplex Received: 29 September 2020 are[(PhSiO separated1,5)6]2[CuO] by a4[NaO large0.5 bridge]4[dppmO and2]2 are, locatedwhere at adppm-1,1-bis(diphenylphosphino)methane; long distance from each other. Therefore, Accepted: 23 January 2021 are separated by a large bridge and are located at a long distance from each other. it(2,3- isη more-1,4-diphenylbut-2-en-1,4-dione)un reasonable to attribute compounddecacarbonyl31 totriangulotriosmium mononuclear complexes.; phenylsilsesquioxane In the work Published: date Therefore, it is more reasonable to attribute compound 31 to mononuclear complexes. In theRef.[(PhSiO work [1501.5) ]10Ref.(CoO) was [150] studied5(NaOH)]; was in studied detailsbi- in the anddetails oxidation tri-nuclearthe oxidation of cyclic, oxidovanadium(V) of linear cyclic, and linear branched andcomplexes branched alkanes 2 3 2 Publisher’s Note: MDPI stays neu- alkanes[{VO(OEt)(EtOH)} with the 231(L /PCA/H)] 2Oand2 system in [{VO(OMe)(Hacetonitrile solution2O)}3(L )]·2H (typical2O temperature(L of =50 3 tral with regard to jurisdictional °C).bis(2-hydroxybenzylidene)terephthalohydrazide Oxidation of cyclohexane is shown in Fi gure 29. Theand accumulati on Lof oxygenates =at claims in published maps and insti- differenttris(2-hydroxybenzylidene)benzen concentrations of complexe-1,3,5-tricarbohydrazide); 31 is shown in the presence[Mn2L of2O and3][PF 6in]2 the absence(L =of tutional affiliations. PCA.1,4,7-trimethyl-1,4,7-triazacyclon onane). For comparison, articles are introduced describing catalysts for the oxidation of alkanes and alcohols with peroxides, which are simple metal salts or mononuclear metal complexes. In many cases, polynuclear complexes exhibit higher activity compared to mononuclear complexes and exhibit increased regioselectivity, for example, in the Copyright: © 2021 by the authors. oxidation of linear alkanes. The review contains a description of some of the mechanisms of cata- Submitted for possible open access lytic reactions. Additionally presented are articles comparing the rates of oxidation of solvents and publication under the terms and substrates under oxidizing conditions for various catalyst structures, which allows researchers to conditions of the Creative Commons Attribution (CC BY) license conclude about the nature of the oxidizing species. This review is focused on recent works, as well (http://creativecommons.org/licenses as review articles and own original studies of the authors. /by/4.0/).

Catalysts 2021, 11, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/catalysts

Figure 28. Structure of complex 31 (C30H36N4O10V2) Adapted from Ref. [150], Copyright (2013) with permission of Royal Society of Chemistry.

Catalysts 2021, 11, x FOR PEER REVIEW 26 of 39

Figure 27. Scheme of oxidation of dopamine.

Figure 28 shows the structure of binuclear vanadium complex 31. Although the molecule of the complex-31 contains two fragments inclusive vanadium, these fragments Catalysts 2021, 11, 186 are separated by a large bridge and are located at a long distance from each other.25 of 37 Therefore, it is more reasonable to attribute compound 31 to mononuclear complexes. In the work Ref. [150] was studied in details the oxidation of cyclic, linear and branched alkanes with the 31/PCA/H2O2 system in acetonitrile solution (typical temperature of 50 ◦ °C).with Oxidation the 31/PCA/H of cyclohexane2O2 system is shown in acetonitrile in Figure 29. solution The accumulati (typical temperatureon of oxygenates of 50 atC). differentOxidation concentrations of cyclohexane of complex is shown 31 is in shown Figure in 29 the. Thepresence accumulation of and in ofthe oxygenates absence of at PCA.different concentrations of complex 31 is shown in the presence of and in the absence of PCA.

Catalysts 2021, 11, x FOR PEER REVIEW 27 of 39 FigureFigure 28. 28.StructureStructure of complex of complex 31 31(C(C30H3036HN364ON104VO210) AdaptedV2) Adapted from from Ref. Ref. [150], [150 Copyright], Copyright (2013) (2013) with with permissionpermission of Royal of Royal Society Society of Chemistry. of Chemistry.

Complex 1 + PCA 0.14 A 1a 0.12 1 0.08

2a Complex 1 0.04 Concentration (M) 2 0 0 1 2 3 4 Time (h)

0.04 3 B

0.03 3a

0.02 4a 4 0.01 Concentration (M)

0 0 400 800 1200 1600

Time (min)

Figure 29. Oxidation of cyclohexaneFigure 29. withOxidation the H2O of2/31/PCA/CH cyclohexane3CN–H with the2O (curve H2O2 /31/PCA/CH1). Curve of accumula3CN–H2tionO (curve of oxygenates1). Curve of (predominantly cyclohexyl accumulationhydroperoxide) of in oxygenates the absence (predominantly of PCA (curve 2 cyclohexyl) are shown. hydroperoxide) For Graph A: [catalyst in the absence 31]0 = 2.0 of × PCA 10–5 M; [PCA]0 = 0.005 M; [H2O2]0 = 2.2 M (50% aqueous), [H2O]total = 4.2 M; [cyclohexane]–5 0 = 0.46 M; MeCN; 50 °C. For (curve 2) are shown. For Graph (A): [catalyst 31]0 = 2.0 × 10 M; [PCA]0 = 0.005 M; [H2O2]0 = 2.2 M Graph B: [catalyst 31]0 = 1.0 × 10–6 M (curve 3) or 5.0 × 10–7 M (curve 4); [PCA]0 = 6 × 10–4 M; [H2O2]0 =◦ 0.4 M (50% aqueous), (50% aqueous), [H2O]total = 4.2 M; [cyclohexane]0 = 0.46 M; MeCN; 50 C. For Graph (B): [catalyst [cyclohexane]0 = 0.4 M; MeCN; 50 °C. Maximum initial rates W0 were determined from the slopes of tangents (two ex- 31] = 1.0 × 10–6 M (curve 3) or 5.0 × 10–7 M (curve 4); [PCA] = 6 × 10–4 M; [H O ] = 0.4 M (50% amples are depicted with straight0 dotted lines 1a, 2a, 3a and 4a) to the kinetic curves in the0 intervals where the2 2rate0 attains aqueous), [cyclohexane] = 0.4 M; MeCN; 50 ◦C. Maximum initial rates W were determined from maximum (in these cases in the beginning of the 0oxidation reaction). Adapted from Ref. [150] Copyright0 (2013) with permission of Royal Societythe of slopesChemistry. of tangents (two examples are depicted with straight dotted lines 1a, 2a, 3a and 4a) to the kinetic curves in the intervals where the rate attains maximum (in these cases in the beginning of the oxidationIt has been reaction). shown Adapted earlier from [6–11] Ref. that [150 a] Copyrightvanadium(2013) complex with as permission catalysts of and Royal PCA Society as a ofcocatalyst Chemistry. is a very efficient catalytic system if H2O2 is used as an oxidant. The addition of strong acids to the monovanadate ion solution leads to an increase the catalyst activity. This phenomenon is due to the formation of polyvanadate species. (see Section 2.4). The kinetic study of alkane oxidation by the 31/PCA/H2O2 CH3CN–H2O system the dependences of the initial rate of oxygenate accumulation W0. Examples are presented by the dotted straight lines in Figure 29. The follow kinetic scheme was proposed for the oxidation reaction catalyzed by vanadium complexes: 31 + PCA 31 (PCA) (1)

31 (PCA) + PCA 31 (PCA)2 (2)

31 (PCA) + H2O2 → Z (3) Reactions (1) and (2) are the equilibria stages of adduct formation between compound 31 and one or two PCA molecules, respectively. A rate-limiting stage is reaction (3) in the sequence of transformations which are induced by the interaction of 31. (PCA) and H2O2. These transformations lead to the generation of species Z (in many cases Z = HO•) and regeneration of the catalyst active form. The rate WZ of the Z generation in reaction (3) is equal to

WZ = k3[31.(PCA)][H2O2] (4)

Catalysts 2021, 11, 186 26 of 37

It has been shown earlier [6–11] that a vanadium complex as catalysts and PCA as a cocatalyst is a very efﬁcient catalytic system if H2O2 is used as an oxidant. The addition of strong acids to the monovanadate ion solution leads to an increase the catalyst activity. This phenomenon is due to the formation of polyvanadate species. (see Section 2.4). The kinetic study of alkane oxidation by the 31/PCA/H2O2 CH3CN–H2O system the dependences of the initial rate of oxygenate accumulation W0. Examples are presented by the dotted straight lines in Figure 29. The follow kinetic scheme was proposed for the oxidation reaction catalyzed by vanadium complexes:

31 + PCA −−−−→←−−−− 31 (PCA) (K1) (1) ←−−−− 31 (PCA) + PCA −−−−→ 31 (PCA)2 (K2) (2)

31 (PCA) + H2O2 → Z (k3) (3) Reactions (1) and (2) are the equilibria stages of adduct formation between compound 31 and one or two PCA molecules, respectively. A rate-limiting stage is reaction (3) in the sequence of transformations which are induced by the interaction of 31. (PCA) and H2O2. These transformations lead to the generation of species Z (in many cases Z = HO•) and regeneration of the catalyst active form. The rate WZ of the Z generation in reaction (3) is equal to WZ = k3[31.(PCA)][H2O2] (4) The data on the kinetics of cyclohexane oxidation and further calculations based on conﬁrmed the assumption about the participation of hydroxyl radicals as key species Z in the oxidation catalyzed by compound 31. Basing on the results of all studies of vanadium complexes, both mononuclear and polynuclear, we can conclude that they all have approximately the same activity in the oxidation of organic compounds by peroxides. Of particular note is the simplest and − cheapest system VO3 /PCA/H2O2. The system was discovered in 1993. This reaction takes place at low temperatures in a solution of acetonitrile [122–124]. Later, this system was studied in detail. As substrates were used: alkanes, oleﬁns, arenes, and alcohols and as oxidants: hydrogen peroxide, tert-butyl hydroperoxide, and other peroxides [125–149].

2.5. Oxidation Catalyzed by Other Metal Complexes Complexes of other metals have been reported as catalysts in oxidations by peroxides. Some of these catalysts are demonstrated in Figures 30–33. Nickel complexes were used in the oxidation of alkanes most often with the use of m-chloroperoxybenzoic acid–(m-CPBA) turned out to be a good oxidant [161–166]. An example of oxidation catalyzed by the nickel complex was described in Ref. [161] Complex oxidized cyclohexane with m-CPBA at room temperature in a mixture of methylene chloride and acetonitrile. The reaction gave cyclohexanol and cyclohexanone in ratio 8:1 with total TON = 612. Another example of nickel catalyst is presented by compound 32 (Figure 30, Ref. [164]). Complex 32 catalyzes the oxidation of cyclohexane m-CPBA to afford cyclohexanol and cyclohexanone. The ketone/alcohol ratio is not changed in the chromatograms made before and after reduction of samples with triphenylphosphine. This indicates that cyclohexyl hydroperoxide is not formed in the course of the oxidation. The yield of oxygenates was 24%. The oxidation of n-octane (0.12 M) with (0.13 M) m-CPBA in the presence of −4 ◦ compound 32 (5 × 10 M) and co-catalyst HNO3 (0.05 M) at 60 C during 3 h gave rise to the formation of a mixture of 2-, 3-, and 4-octanones (0.009, 0.009, and 0.008 M, respectively; yield 22%). The oxidation of methylcyclohexane under similar conditions gave predominantly isomeric ketones and tert-alcohol. Alkyl hydroperoxides have not been formed in this reaction. Catalysts 2021, 11, x FOR PEER REVIEW 28 of 39

The data on the kinetics of cyclohexane oxidation and further calculations based on confirmed the assumption about the participation of hydroxyl radicals as key species Z in the oxidation catalyzed by compound 31. Basing on the results of all studies of vanadium complexes, both mononuclear and polynuclear, we can conclude that they all have approximately the same activity in the oxidation of organic compounds by peroxides. Of particular note is the simplest and cheapest system VO3−/PCA/H2O2. The system was discovered in 1993. This reaction takes place at low temperatures in a solution of acetonitrile [122–124]. Later, this system was studied in detail. As substrates were used: alkanes, olefins, arenes, and alcohols and as oxidants: hydrogen peroxide, tert-butyl hydroperoxide, and other peroxides [125–149].

2.5. Oxidation Catalyzed by Other Metal Complexes Complexes of other metals have been reported as catalysts in oxidations by peroxides. Some of these catalysts are demonstrated in Figures 30–33. Nickel complexes were used in the oxidation of alkanes most often with the use of m-chloroperoxybenzoic acid ‒ (m-CPBA) turned out to be a good oxidant [161–166]. An example of oxidation catalyzed by the nickel complex was described in Ref. [161] Complex oxidized cyclohexane with m-CPBA at room temperature in a mixture of methylene chloride and acetonitrile. The reaction gave cyclohexanol and cyclohexanone in ratio 8:1 with total TON = 612. Another example of nickel catalyst is presented by compound 32 (Figure 30, Ref. [164]) Complex 32 catalyzes the oxidation of cyclohexane m-CPBA to afford cyclohexanol and cyclohexanone. The ketone/alcohol ratio is not changed in the chromatograms made before and after reduction of samples with triphenylphosphine. This indicates that cyclohexyl hydroperoxide is not formed in the course of the oxidation. The yield of oxygenates was 24%. The oxidation of n-octane (0.12 M) with (0.13 M) m-CPBA in the presence of compound 32 (5 × 10−4 M) and co-catalyst HNO3 (0.05 M) at 60 °C during 3 h gave rise to the formation of a mixture of 2-, 3-, and 4-octanones (0.009, 0.009, and 0.008 M,

Catalysts 2021, 11, 186 respectively; yield 22%). The oxidation of methylcyclohexane under similar conditions27 of 37 gave predominantly isomeric ketones and tert-alcohol. Alkyl hydroperoxides have not been formed in this reaction.

Figure 30. MolecularMolecular structure structure of of compound compound 3232 (A((A—side)—side view, view, B—top (B)—top view). view). Solvating Solvating ligands ligands and counterand counter ion Na ion+ Naare+ omittedare omitted for clarity. for clarity. Adapted Adapted from from Ref. Ref. [164], [164 Copyright], Copyright (2016) (2016) from from Molecules Molecules MDPI (Open Access).

Complexes containing containing cobalt cobalt and and zinc zinc MOF MOF [166] [166 can] can also also exhibit exhibit catalytic catalytic activity activ- in Catalysts 2021, 11, x FOR PEER REVIEW 29 of 39 theity inoxidation the oxidation of alcohols of alcohols and and alkanes. alkanes. A Apentanuclear pentanuclear “cylinder”-like “cylinder”-like cobalt(II) phenylsilsesquioxane [(PhSiO 11.5.5))1010(CoO)(CoO)5(NaOH)]5(NaOH)] [165] [165 ]((3333) )(see (see Figure Figure 31)31) exhibits a high catalytic activity and stereoselectivity in in the the oxidation of of alkanes and and alcohols. ComplexComplex 3333efﬁciently efficiently (yield (yield 62%) 62%) catalyzes catalyzes stereoselective stereoselective (trans/cis (trans/cis ratio ratio = 0.04) = 0.04) cis- 1,2-dimethylcyclohexanecis-1,2-dimethylcyclohexane oxidation oxidation with withmeta -chloroperoxybenzoicmeta-chloroperoxybenzoic acid (m-CPBA)acid (m-CPBA) and 1-phenylethanol’sand 1-phenylethanol’s oxidation oxidation (yield (yield 99%) 99%) with TBHP.with TBHP.

FigureFigure 31. 31.Crystal Crystal structure structure of of complex complex33 33.. Color Color code: code: Co, Co, blue; blue; Na, Na, orange; orange; Si, Si, light light yellow; yellow; O, O, red; C,red; grey; C, grey; S, yellow; S, yellow; N, light N, ligh blue.t blue. Adapted Adapted from from Ref. Ref. [165 ],[165], Copyright Copyright (2016) (2016) with with permission permission from Royalfrom Royal Society Society of Chemistry. of Chemistry.

TheThe heterogeneousheterogeneous catalytic catalytic activity activity of of compounds compounds34 and34 and35 (Figure 35 (Figure 32), under32), under eco- friendlyeco-friendly conditions, conditions, was assessedwas assessed in benzyl in benzyl alcohol alcohol oxidation oxidation [166]. Complex[166]. Complex34 has 34 good has activitygood activity in the solvent-free in the solvent-free microwave-assisted microwave-assisted oxidation oxidation of benzyl of alcohol benzyl to alcohol benzaldehyde to ben- usingzaldehydetert-butyl using hydroperoxide tert-butyl hydroperoxide (t-BuOOH, TBHP)(t-BuOOH, as oxidizing TBHP) as agent oxidizing (yields agent up to (yields 89%). Althoughup to 89%). with Although a lower with activity, a lower MOF activity,35 with MOF a redox 35 with inactive a redox Zn(II) inactive site, also Zn(II) catalyzes site, al- suchso catalyzes alcohol oxidationsuch alcohol (yields oxidation(yields up 27%). up 27%).

Catalysts 2021, 11, 186 28 of 37

Figure 32. Structures complexes cobalt and zinc used as catalyst in oxidation with peroxides, adapted from Ref. [166], Copyright (2020) with permission from Elsevier.

Three Os (0)—carbonyl complexes [167–170] (see Figure 33) are active in the oxidation of alkanes and alcohols by peroxides in acetonitrile. Thus, oxidation, for example, upon catalysis of alkanes by triosmium dodecacarbonyl, Os3(CO)12 with hydrogen peroxide, gave product yields above 60% and TON above 60,000 [167]. In this review, other complexes of osmium, cobalt, and nickel are not described in detail. Catalysts 2021, 11, 186 29 of 37 Catalysts 2021, 11, x FOR PEER REVIEW 31 of 39

Figure 33.33. Osmium carbonyls derivatives which extremely efficien efﬁcientlytly catalyzed the oxidation of alkanes and alcoholsalcohols with peroxides.

3. Conclusions In many cases polynuclear complexes of transitiontransition metals exhibit higher catalyticcatalytic activity in oxidations of organic compounds by peroxides in comparisoncomparison with simple mononuclear derivatives.derivatives. Moreover,Moreover, such such complexes complexes oxidize oxidize long-chain long-chain and and branched branched sat- saturatedurated hydrocarbons hydrocarbons with with non-conventional non-conventional selectivity. selectivity. These These features features can be successfullycan be suc- cessfullyuse in ﬁne use chemical in fine technology.chemical technology. Examination Examination of the oxidation of the reactionsoxidation withreactions peroxides with peroxidesshows that shows both monothat both and mono polynuclear and polynucl complexesear complexes of transition of transition metals are metals often are active of- tencatalysts active in catalysts these reactions. in these However,reactions.they Howeve all requirer, they either all require the presence either the of apresence redox-active of a redox-activeligand in the ligand molecule in the or amine-containingmolecule or amine-containing organic compounds organic addedcompounds to the added solution, to whichthe solution, changes which the mechanismchanges the of mechanism the reaction of ofthe hydrogen reaction peroxideof hydrogen with peroxide a metal (inwith the a metalabsence (in of the such absence ligands of or such additives, ligands only or theadditives, reaction only of hydrogen the reaction peroxide of hydrogen decomposition perox- withide decomposition the formation with of oxygen the formation and water oftakes oxygen place). and water takes place).

Author Contributions: G.B.S. and L.S.S.L.S.S. wrotewrote thethe paper.paper. AllAll authorsauthors havehave readread andand agreedagreed to the published version of the manuscript. Funding: This researchresearch was was funded funded by by the the Russian Russian Foundation Foundation for Basicfor Basic Research Research (Grant (Grant Nos. 19-03-Nos. 19-03-00142),00142), the Ministry the Ministry of Education of Education and Science and of theScienc Russiane of Federationthe Russian (project Federation code RFMEFI61917X- (project code RFMEFI61917X0007),0007), as well as by the as Initiativewell as Programby the Initiative in the frames Program of the in State the Taskframes 0082-2014-0007, of the State “Fun-Task 0082-2014-0007,damental regularities “Fundamental of heterogeneous regularities and of homogeneous heterogeneous catalysis.” and homogeneou and alsos bycatalysis.” the Program and al- of soFundamental by the Program Research of Fundamental of the Russian Research Academy of the of Russian Sciences Academy for 2013–2020 of Sciences on the for research 2013–2020 issue on of theIChP research RAS No. issue 47.16. of State IChP registration RAS No. number47.16. State of Center registration of Information number Technologies of Center andof Information Systems for TechnologiesExecutive Power and Authorities Systems for (CITIS): Executive AAAA-A17-117040610283-3; Power Authorities (CITIS): A.N. AAAA-A17-117040610283-3; Nesmeyanov Institute RAS. A.N. Nesmeyanov Institute RAS. Data Availability Statement: All links are given to publications that are in the open press and are Dataeasily Availability available. Statement: All links are given to publications that are in the open press and are easily available. Acknowledgments: Authors thank for support RFBR according to Research Projects Grant No. 19-03- Acknowledgments:00142; the Min-istry ofAuthors Education thank and for Science support of the RFBR Russian accord Federationing to Research (project code Projects RFMEFI61917X- Grant No. 19-03-00142; the Min-istry of Education and Science of the Russian Federation (project code RFMEFI61917X0007), as well as by the Initiative Program in the frames of the State Tasks

Catalysts 2021, 11, 186 30 of 37

0007), as well as by the Initiative Program in the frames of the State Tasks 0082-2014-0004 and 0082- 2014-0007 “Fundamental regularities of heterogeneous and homogeneous catalysis”; the Program of Fundamental Research of the Russian Academy of Sciences for 2013–2020 on the re-search issue of IChP RAS No. 47.16. Access to electronic scientific resources was provided by INEOS RAS with the support of Ministry of Science and Higher Education of the Russian Federa-tion. Conflicts of Interest: The authors declare no conflict of interest.

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