catalysts Review ReviewC‐Homoscorpionate Oxidation Catalysts— C-Homoscorpionate Oxidation Electrochemical and Catalytic Activity Catalysts—Electrochemical and Catalytic Activity Luísa M. D. R. S. Martins 1,2 Luísa M. D. R. S. Martins 1,2 1 Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa, 1 InstitutoChemical Politécnico Engineering de Lisboa, Department, R. Conselheiro Instituto SuperiorEmídio Navarro, de Engenharia 1959‐007 de Lisboa, Portugal; [email protected];Instituto Politécnico de Tel.: Lisboa, +351 R.‐21 Conselheiro‐8317226 Emídio Navarro, 1959-007 Lisboa, Portugal; 2 [email protected]; de Química Estrutural, Tel.: +351-21-8317226Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 2 1049–001Centro de Lisboa, Química Portugal Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049–001 Lisboa, Portugal Academic Editor: Georgiy B. Shulʹpin AcademicReceived: 18 Editor: November Georgiy 2016; B. Shul’pin Accepted: 23 December 2016; Published: 1 January 2017 Received: 18 November 2016; Accepted: 23 December 2016; Published: 1 January 2017 Abstract: A survey of the electrochemical properties of homoscorpionate tris(pyrazol‐1‐yl)methane Abstract: A survey of the electrochemical properties of homoscorpionate tris(pyrazol-1-yl)methane complexes is presented. The relationship between structural features and catalytic efficiency toward complexes is presented. The relationship between structural features and catalytic efficiency toward the oxidative functionalization of inexpensive and abundant raw‐materials to added‐value products the oxidative functionalization of inexpensive and abundant raw-materials to added-value products is also addressed. is also addressed. Keywords: C‐scorpionate; cyclic voltammetry; redox potential; catalyst; oxidation; electrochemical Keywords: C-scorpionate; cyclic voltammetry; redox potential; catalyst; oxidation; electrochemical parameter; alkane; alkene; alcohol; ketone parameter; alkane; alkene; alcohol; ketone

1. Introduction 1. Introduction 1 2 Scorpionate compounds (Figure 1), in particular, poly(pyrazol‐1‐yl)borates, 1R BXn(R2 pz)3−n Scorpionate compounds (Figure1), in particular, poly(pyrazol-1-yl)borates, R BXn(R pz)3−n 1 2 (pz = pyrazol‐1‐yl, n = 0 or 1), and poly(pyrazol‐1‐yl)methanes, R1 CXn(R2 pz)3−n (n = 0 or 1), are (pz = pyrazol-1-yl, n = 0 or 1), and poly(pyrazol-1-yl)methanes, R CXn(R pz)3−n (n = 0 or 1), are undoubtedly among the most important N‐donor ligands in coordination chemistry [1–8]. The latter undoubtedly among the most important N-donor ligands in coordination chemistry [1–8]. The latter are considerably less well studied than the analogous borate species [3]. However, in the last two are considerably less well studied than the analogous borate species [3]. However, in the last two decades, mainly driven by improved syntheses [9,10], coordination behavior and physicochemical decades, mainly driven by improved syntheses [9,10], coordination behavior and physicochemical properties of poly(pyrazol‐1‐yl)methanes have attracted considerable interest [5–8] in order to properties of poly(pyrazol-1-yl)methanes have attracted considerable interest [5–8] in order to perform perform the precise tuning of target scorpionates towards a desired function [3,6,7]. Applications of the precise tuning of target scorpionates towards a desired function [3,6,7]. Applications of this highly this highly versatile class of metal compounds range from organic synthesis, analytical, bio‐inorganic versatile class of metal compounds range from organic synthesis, analytical, bio-inorganic or catalytic or catalytic chemistry to material sciences [3,6–8,11–18]. chemistry to material sciences [3,6–8,11–18].

Figure 1. 1. GeneralGeneral scorpionate scorpionate structure: structure: poly(pyrazol poly(pyrazol-1-yl)borates‐1‐yl)borates for Z = B; poly(pyrazol for Z =‐ B;1‐ poly(pyrazol-1-yl)methanesyl)methanes for Z = C. for Z = C.

The development of of sustainable sustainable efficient efficient catalytic catalytic processes processes for for the the activation activation of abundant of abundant and andinexpensive inexpensive raw‐materials raw-materials into high into‐ high-added-valueadded‐value products products remains remains a great a greatchallenge challenge for both for bothacademic academic and industrial and industrial purposes. purposes. In this respect, In this the respect, use of metal the use complexes of metal bearing complexes C‐scorpionate bearing C-scorpionatepoly(pyrazol‐1 poly(pyrazol-1-yl)methane‐yl)methane ligands as catalysts ligands is ascurrently catalysts experiencing is currently significant experiencing development significant development [6–8,16]. Transition metals are important in this topic participating e.g., in redox Catalysts 2017, 7, 12; doi:10.3390/catal7010012 www.mdpi.com/journal/catalysts

Catalysts 2017, 7, 12; doi:10.3390/catal7010012 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 12 2 of 21 processes, which can be applied in specific steps. The electronic interaction between transition metals and scorpionate ligands can play a key role in improving the redox process, and the type of scorpionate ligand can be determinant in achieving the desired properties in such complexes. Thus, one advantage of this catalytic system (over e.g., the metallocene based one) is the ease of modifying the scorpionate ligand to change the steric and electronic properties of the metal complex and therefore, its catalytic performance. Industrially important reactions catalyzed by C-scorpionate complexes include [6–8,17–22] (i) mild partial oxidation of alkanes to alkyl hydroperoxides, alcohols and ketones; (ii) epoxidation of alkenes; (iii) oxidation of primary or secondary alcohols to aldehydes or ketones, respectively; (iv) the Baeyer-Villiger oxidation of linear or cyclic ketones to the corresponding esters and lactones, respectively; (v) the single pot carboxylation of gaseous alkanes into the corresponding Cn+1 carboxylic acids; (vi) the stereo-selective nitroaldol Henry C–C coupling reaction; and (vii) olefin polymerization. Whereas the main catalytic applications of metal complexes with tris(pyrazol-1-yl)borates or heteroscorpionate ligands based on the bis(pyrazol-1-yl)methane moiety are found in olefin polymerization reactions [1,6], tris(pyrazol-1-yl)methane-type complexes of several transition metals are mainly used as catalysts or catalyst precursors for alkane, alkene, alcohol, and ketone oxidation reactions directed toward single-pot organic synthesis [7]. Their use as catalysts for the C–C coupling Henry reaction (a non-redox process) [8] has also proved to be a very promising strategy, in particular for those metals (e.g., Zn) that exhibit no redox flexibility but can behave as Lewis acid catalysts. Moreover, tris(pyrazol-1-yl)methane metal complexes can exhibit remarkable versatile catalytic activity for oxidation reactions [16]. It is believed that the interchange between bidentate and tridentate coordination modes of the C-scorpionate ligands is at the core of the structural and chemical versatility of many metal complexes of this kind and is essential for their catalytic applications. Electron transfer plays a fundamental role in governing the pathway of most of the above chemical reactions. In fact, the activity of metal-based catalysts depends largely on their ligand environment and coordination geometry, which also rule their oxidation/reduction properties, with the redox potential as a determining parameter. Thus, quantification of the net electron donation of the ligands to a metal center would allow predicting metal-centered redox potentials, and vice-versa, providing a powerful tool for the design of metal-based catalysts within a desired redox window. Determination of redox potentials can be conveniently done by e.g., the easy and fast cyclic voltammetry technique, provided the redox signals lie within the available solvent/electrolyte potential window and the species have a sufficient lifetime for signal detection. However, to date, the useful information associated to the redox potential of a metal complex has not yet found a common application as a characterization or identification tool [23]. Moreover, a survey of the redox properties of known C-homoscorpionate metal complexes is missing. Systematic approaches to establish redox potential/structure relationships, following the recognition of full additive ligand (L) effects on that potential have been proposed [24–28]. For example, Lever’s parametrization approach (Equation (1)) [27,28] allows for the prediction of an Mn+1/n redox potential (E) of a six-coordinate metal complex in V vs. SHE (standard hydrogen electrode), where EL is an additive ligand parameter obtained by a statistical analysis on the known redox potentials of a n+1/n high number of M complexes [27,28]. The slope, SM, and intercept, IM, are dependent upon the metal and redox couple, the polygon of the complex, the spin state, and the stereochemistry [23].

E = SM (Σ EL) + IM/V vs. SHE (1)

On the other hand, Equation (1) can be applied to estimate the EL value of a ligand (L) provided one knows the redox potential of a complex with that ligand L bound to a Mn+1/n metal redox couple with known IM and SM parameters, and the EL values of the co-ligands. Herein, the electrochemical properties of homoscorpionate tris(pyrazol-1-yl)methane metal complexes that act as catalysts for the above industrial oxidation reactions are presented. Catalysts 2017, 7, 12 3 of 21

Moreover, the EL ligand parameter, a measure of the electron donor character of ligand L (the lower its value, the stronger that character), is used to establish redox/structure as well as redox/catalytic activity relationships, a very important tool for the design of improved catalysts to address some of the problems presented by current large‐scale industrial partial oxidation processes.

2. C‐Homoscorpionates and Their Metal Catalysts CatalystsFirstly2017, 7reported, 12 by Trofimenko (1966) [29,30] as “a new and fertile field of remarkable scope”,3 of 21 B‐scorpionate tris(pyrazol‐1‐yl)borates (Tp, Figure 2a) indeed were revealed to be a class of compounds that became a precious ligand system in modern coordination chemistry [1,2]. Moreover, the EL ligand parameter, a measure of the electron donor character of ligand L (the lower its value,Although the stronger discovered that character), earlier by is usedHückel to establishet al. (1937) redox/structure [31], the asanalogous well as redox/catalytic C‐scorpionate activitytris(pyrazol relationships,‐1‐yl)methanes a very (Tpm, important Figure tool 2b) for theremained design ofdormant improved with catalysts respect to addressto coordination some of thechemistry problems until presented 1966 [32], by mainly current due large-scale to synthetic industrial difficulties partial and oxidation usually processes.very low yields associated with the preparation of functionalized tris(pyrazol‐1‐yl)methanes where substituents on the 2.pyrazolyl C-Homoscorpionates rings are larger and than Their methyl. Metal In fact, Catalysts until Elguero’s report (1984) of an improved synthetic strategy [33] and its subsequent application in the formation of functionalized derivatives bearing Firstly reported by Trofimenko (1966) [29,30] as “a new and fertile field of remarkable scope”, bulky substituents [34,35], only few (less than 20) reports appeared pertaining to first‐row transition B-scorpionate tris(pyrazol-1-yl)borates (Tp, Figure2a) indeed were revealed to be a class of compounds metal complexes of tris(pyrazol‐1‐yl)methanes, mostly homoleptic ones. that became a precious ligand system in modern coordination chemistry [1,2].

Figure 2.2. StructuralStructural general general representation representation of of tris(pyrazol-1-yl)borate tris(pyrazol‐1‐yl)borate (a); (a tris(pyrazol-1-yl)methane); tris(pyrazol‐1‐yl)methane (b); and(b); and cyclopentadienyl cyclopentadienyl (c) ligands. (c) ligands.

AlthoughIt is commonly discovered agreed earlierto compare by Hückel [2,3,30,36–38] et al. the (1937) main [ 31characteristics], the analogous of tris(pyrazol C-scorpionate‐1‐yl) tris(pyrazol-1-yl)methanestype scorpionate ligands with (Tpm, other Figure face2‐b)capping remained ligands. dormant In particular, with respect the to parallel coordination between chemistry Tp and untilcyclopendadienyl 1966 [32], mainly (Cp, Figure due to 2c) synthetic ligands difficultiesis established and in usually that both very are lowmononegative, yields associated six‐electron with the(ionic preparation model) or offive functionalized‐electron donor tris(pyrazol-1-yl)methanes (covalent model) ligands. whereThey are substituents also formally on theisolobal pyrazolyl [2,3]. ringsThe former are larger are weak than‐field methyl. hard σ‐ InN fact, donors until which Elguero’s tend to report behave (1984) as fac of‐capping an improved chelating synthetic ligands strategy(i.e., occupy [33] three and its coordination subsequent positions), application while in the Cp formation are typically of functionalized 5‐fold π‐donors derivatives and tend bearingto form bulkytetrahedral substituents complexes [34, 35[3,38].], only few (less than 20) reports appeared pertaining to first-row transition metalImportantly, complexes of it tris(pyrazol-1-yl)methanes,has been shown that there is mostly no systematic homoleptic trend ones. in comparative electron donor abilityIt of is Tp commonly relative to agreed Cp [39]. to Their compare electron [2,3,‐30donating,36–38] theabilities main are characteristics dependent upon of tris(pyrazol-1-yl) the identity and typeoxidation scorpionate state of ligands the metal with center other as face-capping well as the properties ligands. In of particular, the other theligands parallel in the between complex Tp [40]. and cyclopendadienylTris(pyrazol‐1‐yl)borates (Cp, Figure are also2c) ligands bulkier is than established the formally in that analogous both are mononegative, Cp and, in six six-electron‐coordinate (ionicenvironments, model) or enforce five-electron nearly donoroctahedral (covalent coordination model) ligands.to the metal They with are N also‐M formally‐N bite angles isolobal close [2, 3to]. Thethe ideal former value are weak-field(90°). This hardhas beenσ-N donorssuggested which to be tend the to primary behave source as fac-capping of the different chelating reactivity ligands (i.e.,between occupy comparable three coordination Tp and Cp positions), complexes while [40,41]. Cp areFor typically example, 5-fold the greaterπ-donors steric and profile tend to of form Tp tetrahedralligands has complexes permitted [3 ,the38]. isolation of molecular species whose pentamethylcyclopentadienyl congenersImportantly, proved it too has reactive been shown [42]. thatIn addition, there is noTp systematic is coordinatively trend in flexible, comparative presenting electron κ2‐ donoror κ3‐ abilitycoordination of Tp relative modes to (i.e., Cp [the39]. scorpionate Their electron-donating feature). The abilities carbon are analogues dependent to upon Tp, the tris(pyrazol identity and‐1‐ oxidationyl)methanes, state maintain of the metal the tripodal center asface well capping as the aspect properties and the of the same other electro ligands‐donor in theability, complex but differ [40]. Tris(pyrazol-1-yl)boratesfrom Tp and Cp in the charge are alsothey bulkierhold (Figure than 2). the formally analogous Cp and, in six-coordinate environments, enforce nearly octahedral coordination to the metal with N-M-N bite angles close to the ideal value (90◦). This has been suggested to be the primary source of the different reactivity between comparable Tp and Cp complexes [40,41]. For example, the greater steric profile of Tp ligands has permitted the isolation of molecular species whose pentamethylcyclopentadienyl congeners proved too reactive [42]. In addition, Tp is coordinatively flexible, presenting κ2- or κ3-coordination modes (i.e., the scorpionate feature). The carbon analogues to Tp, tris(pyrazol-1-yl)methanes, maintain the tripodal face capping aspect and the same electro-donor ability, but differ from Tp and Cp in the charge they hold (Figure2). Catalysts 2017, 7, 12 4 of 21 Catalysts 2017, 7, 12 4 of 21 Catalysts 2017, 7, 12 4 of 21

Since 2005 a considerable interest in the development of a fast an efficient synthetic route for SinceSince 20052005 aa considerableconsiderable interest in thethe developmentdevelopment of a fast anan efficientefficient syntheticsynthetic route forfor hydrotris(pyrazol‐1‐yl)methane, HC(pz)3 (pz = pyrazol‐1‐yl), Tpm [10], as well as on the design and hydrotris(pyrazol-1-yl)methane,hydrotris(pyrazol‐1‐yl)methane, HC(pz) HC(pz)3 (pz(pz = =pyrazol pyrazol-1-yl),‐1‐yl), Tpm Tpm [10], [10 as], well as well as on as the on design the design and 3 1 2 synthesis of brand new poly‐functionalized C‐homoscorpionates, R1C(R 2pz)31, overcoming2 the lack in andsynthesis synthesis of brand of brand new poly new‐functionalized poly-functionalized C‐homoscorpionates, C-homoscorpionates, R C(R pz) R3,C(R overcomingpz)3, overcoming the lack in the chemistry of such species (Figure 3), has been found [43–45]. For example, new tris(pyrazol‐1‐ thethe lackchemistry in the of chemistry such species of such (Figure species 3), has (Figure been3 found), has been[43–45]. found For [example,43–45]. new For example,tris(pyrazol new‐1‐ yl)methanes functionalized at the methine carbon atom (in order to vary the coordination behavior tris(pyrazol-1-yl)methanesyl)methanes functionalized functionalized at the methine at thecarbon methine atom carbon (in order atom to (in vary order the to coordination vary the coordination behavior and physicochemical properties) were successfully prepared: CH3SO3CH2C(pz)3 [43] or behaviorand physicochemical and physicochemical properties) properties) were weresuccessfully successfully prepared: prepared: CH CH3SO3SO3CH3CH2C(pz)2C(pz)3 3[43][43 ] oror PyCH2OCH2C(pz)3 (Py = pyridine) [44]. The functionalization of pyrazol‐1‐yl rings (to modulate the PyCHPyCH22OCH2C(pz)3 (Py(Py = = pyridine) pyridine) [44]. [44]. The The functionalization functionalization of pyrazol pyrazol-1-yl‐1‐yl rings (to modulate thethe coordination properties) was also achieved, as well as those derivatives that combine the two types coordinationcoordination properties)properties) was was also also achieved, achieved, as as well well as as those those derivatives derivatives that that combine combine the the two two types types of − of functionalization: e.g., SO3C(3‐Phpz)−3− [45], HOCH2C(3‐Phpz)3 [44], or PyCH2OCH2C(3‐Phpz)3 [44]. functionalization:of functionalization: e.g., e.g., SO SO3C(3-Phpz)3C(3‐Phpz)3 3[45 [45],], HOCH HOCH2C(3-Phpz)2C(3‐Phpz)3 3[ [44],44], or or PyCH PyCH22OCHOCH22C(3-Phpz)C(3‐Phpz)33 [44].[44].

Figure 3.3. StructuresStructures of of functionalized functionalized tris(pyrazol tris(pyrazol-1-yl)methanes:‐1‐yl)methanes: (a) (aCH) CH3SO33SOCH32CHC(pz)2C(pz)3; (b3); Figure 3. Structures of functionalized tris(pyrazol‐1‐yl)methanes: (a) CH3SO3CH2C(pz)−3; (b) (PyCHb) PyCH2OCH2OCH2C(pz)2C(pz)3 (Py3 = (Pypyridine) = pyridine) or PyCH or2OCH PyCH2C(32‐OCHPhpz)23C(3-Phpz); (c) SO3C(33;(‐Phpz)c) SO3− and3C(3-Phpz) (d) HOCH3 2C(3and‐ PyCH2OCH2C(pz)3 (Py = pyridine) or PyCH2OCH2C(3‐Phpz)3; (c) SO3C(3‐Phpz)3− and (d) HOCH2C(3‐ (Phpz)d) HOCH3. 2C(3-Phpz)3. Phpz)3.

AA systematicsystematic investigationinvestigation ofof thethe coordinationcoordination behaviorbehavior ofof newnew C-scorpionates,C‐‐scorpionates, asas wellwell asas somesome ofof thethe knownknown ones, toward a variety of transition transition metals (e.g., (e.g., V [44,46–49], [[44,46–49],44,46–49], Mo [17,19,50], [[17,19,50],17,19,50], Re [51,52], [[51,52],51,52], FeFe [44,46,49,53], [[44,46,49,53],44,46,49,53], Ru Ru [54],[54], [54], Co Co [12,13],[12,13], [12,13], Ni Ni [44,49,55],[44,49,55], [44,49,55 ], Pd Pd [44],[44], [44 ],Cu Cu [43,45,46,56,57],[43,45,46,56,57], [43,45,46,56,57 Ag], Ag [14],[14], [14 Au], Au [58][58] [ 58 or] Zn or Zn[44,56])[44,56]) [44,56 followed,followed,]) followed, leadingleading leading toto tonew new classes classes of of complexes complexes exhibiting exhibiting different different typestypes types of coordinationcoordination modesmodes (Figures(Figures4 4 and and5 ).5). Like Like the thethe pincer pincer of of a a scorpion, scorpion, these thesethese versatile versatile tripodal tripodaltripodal ligands ligandsligands bind bind metal metal centerscenters withwith nitrogennitrogen atomsatoms fromfrom twotwo pyrazolylpyrazolyl ringsrings attachedattached toto thethe centralcentral carboncarbon atom;atom; thethe thirdthird pyrazolylpyrazolyl attachedattached to toto carboncarbon rotatesrotates forward forwardforward like likelike a a scorpion’s scorpionʹʹs tailtail toto “sting”“sting” thethe metal; hencehence thethe name ofof “scorpionates”“scorpionates” (Figure(Figure4 4).).

2 3 Figure 4. κ 2–κ 3 interchange coordination modes of a tris(pyrazol‐1‐yl)methane scorpionate ligand and FigureFigure 4. κκ2––κ3 interchangeinterchange coordination coordination modes of a a tris(pyrazol tris(pyrazol-1-yl)methane‐1‐yl)methane scorpionate ligandligand andand comparison with a scorpion. comparisoncomparison withwith aa scorpion.scorpion.

Catalysts 2017, 7, 12 5 of 21 Catalysts 2017, 7, 12 5 of 21

(a)(b)

ReO4 O O N C O N N X O N N N M' (BF ) Re N M N 4 2n N X N N N N N N N P O C N N N N N N n C N M = Fe(II) M’ = Pd(II) H X = NO3, Cl

(c)(d)

SO3 N N

C C N N N N SO N N 2 N N N Ph N O Ph Ph Ag Ag

PCy L 3 L = PTA or PPh3

FigureFigure 5.5. Selected C C-homoscorpionate‐homoscorpionate complexes complexes exhibiting: exhibiting: (a) ( atetradentate) tetradentate coordination coordination ability ability of ofthe the scorpionate scorpionate ligand; ligand; (b) κ (b2‐)coordinationκ2-coordination of the of scorpionate the scorpionate ligand ligand at an octahedral at an octahedral geometry; geometry; (c) κ2‐ 2 (ccoordination) κ -coordination of theof scorpionate the scorpionate ligand ligand at a square at a square planar planar geometry geometry (d) N3‐ (dor)N N32O- or‐coordination N2O-coordination of the ofscorpionate the scorpionate ligand. ligand.

Some of the above new C‐homoscorpionates (with three identical pyrazol‐1‐yl rings), such as Some of the above new C-homoscorpionates (with three identical pyrazol-1-yl rings), such as e.g., PyCH2OCH2C(pz)3 (Figure 5a) exhibit extended coordination ability, including tetradentate e.g., PyCH OCH C(pz) (Figure5a) exhibit extended coordination ability, including tetradentate characteristics2 [44],2 where3 the extra coordination moiety has different affinity towards metal centers. characteristics [44], where the extra coordination moiety has different affinity towards metal centers. Therefore, it leads to a sort of metal‐supported scorpionate ligand that forms easily heterobimetallic Therefore, it leads to a sort of metal-supported scorpionate ligand that forms easily heterobimetallic species, opening to a large variety of applications (such as catalysis or supramolecular chemistry). species, opening to a large variety of applications (such as catalysis or supramolecular chemistry). On the other hand, tris(pyrazol‐1‐yl)methane derivatives bearing bulky substituents at the On the other hand, tris(pyrazol-1-yl)methane derivatives bearing bulky substituents at the pyrazol‐1‐yl rings (especially at the 3‐position), when ligating a metal center, such a bulky species pyrazol-1-yl rings (especially at the 3-position), when ligating a metal center, such a bulky species provides a steric control on the other coordination position(s) of the complex, selecting the suitable provides a steric control on the other coordination position(s) of the complex, selecting the suitable ligands on the opposite side, namely preventing the formation of full‐sandwich complexes (with two ligands on the opposite side, namely preventing the formation of full-sandwich complexes (with two such scorpionate ligands) [45]. Moreover, they also offer the opportunity to tailor the coordination suchbehavior scorpionate toward ligands)different [ 45metal]. Moreover, centers. theyThis alsoimportant offer the feature opportunity is directly to tailorcorrelated the coordination to further behaviorresearch towardin catalytic different synthetic metal chemistry. centers. This important feature is directly correlated to further research in catalyticIt was synthetic also found, chemistry. from electrochemical experiments [57,59], that changes on the functionalized methineIt was group also found, of tris(pyrazol from electrochemical‐1‐yl) scorpionates experiments have a [ 57much,59], thatsmaller changes influence on the on functionalized the ligand methineproperties group than of when tris(pyrazol-1-yl) performed at scorpionates the pyrazol have‐1‐yl rings a much (see smaller below). influence on the ligand properties than whenThe coordination performed at versatility the pyrazol-1-yl of tris(pyrazol rings (see‐1‐ below).yl)methanes, namely the interchange between bidentateThe coordination and tridentate versatility coordination of tris(pyrazol-1-yl)methanes, modes (Figure 5b,c for κ2‐coordination) namely the was interchange found to be between tuned 2 bidentateby metal andcenter tridentate as well coordinationas by the electronic modes (Figureproperties5b,c of for theκ -coordination)co‐ligands present was foundat the tocoordination be tuned by metalsphere. center In addition, as well as the by tripodal the electronic functionalized properties coordination of the co-ligands flexibility present (e.g. at N the3‐ or coordination N2O‐coordination sphere. Inmodes addition, for thesulfonated tripodal functionalizedderivatives, Figure coordination 5d) involving flexibility the (e.g., functionalized N3- or N2O-coordination methine carbon modes for[7,8,14,45,49,50] sulfonated derivatives, is also tailored Figure 5byd) such involving co‐ligands, the functionalized an important methine pre‐requisite carbon for [ 7, 8their,14,45 catalytic,49,50] is alsoactivity. tailored by such co-ligands, an important pre-requisite for their catalytic activity. OneOne might might consider consider thatthat thethe coordinationcoordination behavior of the tris(pyrazol tris(pyrazol-1-yl)methane‐1‐yl)methane complexes complexes wouldwould mirror mirror thatthat exhibitedexhibited byby thethe correspondingcorresponding tris(pyrazol-1-yl)boratetris(pyrazol‐1‐yl)borate compounds, compounds, the the major major differencedifference beingbeing inin thethe chargecharge betweenbetween thethe methanemethane and the borate counterpart. However, However, large large differences appear in some cases [60]: for example, the RC(pz)3 ligands react with Group 6 metal

Catalysts 2017, 7, 12 6 of 21

Catalysts 2017, 7, 12 6 of 21 differencesCatalysts 2017 appear, 7, 12 in some cases [60]: for example, the RC(pz)3 ligands react with Group6 of 21 6 metal hexacarbonyls to afford insoluble and non‐volatile species, whereas [M{RB(pz)3}(CO)3] are very hexacarbonyls to afford insoluble and non-volatile species, whereas [M{RB(pz)3}(CO)3] are very soluble soluble and sublime easily. The tripodal ligand HC(pz)3 produces a relatively strong ligand field, andhexacarbonyls sublime easily. to Theafford tripodal insoluble ligand and HC(pz)non‐volatile3 produces species, a whereas relatively [M{RB(pz) strong ligand3}(CO)3] field, are very consistent consistent with the rather short metal–nitrogen bond lengths in the complexes. The pyrazol‐1‐yl withsoluble the rather and sublime short metal–nitrogen easily. The tripodal bond ligand lengths HC(pz) in the3 produces complexes. a relatively The pyrazol-1-yl strong ligand group field, acts as group acts as moderately strong σ donor and a weak out‐of‐plane π donor, with the π interaction in moderatelyconsistent strong with theσ donor rather and short a weakmetal–nitrogen out-of-plane bondπ donor,lengths with in the the complexes.π interaction The inpyrazol the plane‐1‐yl of the the plane of the amine ligand probably being close to zero [61]. aminegroup ligand acts as probably moderately being strong close σ todonor zero and [61 a]. weak out‐of‐plane π donor, with the π interaction in theThe plane main of theapplications amine ligand of probably C‐homoscorpionate being close to zerocomplexes [61]. as catalysts for oxidative reactions, The main applications of C-homoscorpionate complexes as catalysts for oxidative reactions, where theThe involvement main applications of metal of Credox‐homoscorpionate processes is complexescrucial for asthe catalysts catalytic for activity, oxidative are reactions, the important where the involvement of metal redox processes is crucial for the catalytic activity, are the important andwhere challenging the involvement single‐pot of oxidationmetal redox of processes gaseous is (e.g., crucial direct for theoxidation catalytic of activity, methane are tothe carboxylic important acids, and challenging single-pot oxidation of gaseous (e.g., direct oxidation of methane to carboxylic Schemeand challenging 1) [47,62]) andsingle liquid‐pot oxidation (e.g., cyclohexane of gaseous (e.g.,to the direct correspondent oxidation of alcoholmethane‐ ketoneto carboxylic mixture, acids, Scheme acids,Scheme Scheme 1) [47,62])1)[ 47 and,62]) liquid and liquid(e.g., cyclohexane (e.g., cyclohexane to the correspondent to the correspondent alcohol‐ketone alcohol-ketone mixture, Scheme mixture, 2) alkanes [7,8,13,43,46–49,53,57,58,63–68]. Indeed, oxidation of alkanes has been the object of Scheme2) alkanes2) alkanes [7,8,13,43,46–49,53,57,58,63–68]. [ 7,8,13,43,46–49,53,57,58 ,63Indeed,–68]. Indeed,oxidation oxidation of alkanes of has alkanes been hasthe beenobject the of object considerable research [69–78], but still constitutes a serious challenge to modern chemistry, owing to of considerableconsiderable research research [69–78], [69–78 but], but still still constitutes constitutes a serious a serious challenge challenge to modern to modern chemistry, chemistry, owing to owing the high inertness of these substrates. Currently, alkanes are mainly applied as fuels but it would be to thethe high high inertnessinertness of of these these substrates. substrates. Currently, Currently, alkanes alkanes are mainly are mainly applied applied as fuels asbut fuels it would but itbe would desirable to direct their application to the synthesis of organic products of a high added value. The be desirabledesirable to to direct direct their their application application to the to synthesis the synthesis of organic of organic products products of a high of added a high value. added The value. feasibility of this approach is supported by the industrial application of cyclohexane in the production Thefeasibility feasibility of this of this approach approach is supported is supported by the industrial by the application industrial of application cyclohexane of in the cyclohexane production in the of cyclohexanoneof cyclohexanone and and cyclohexanol cyclohexanol (KA(KA oil,oil, Scheme Scheme 2), 2), with with dioxygen dioxygen as oxidizing as oxidizing agent agentand and production of cyclohexanone and cyclohexanol (KA oil, Scheme2), with dioxygen as oxidizing catalystscatalysts based based on on cobalt cobalt [79]. [79]. However,However, thisthis industrial industrial process process has has a very a very low lowyield yield to ensure to ensure an an agent and catalysts based on cobalt [79]. However, this industrial process has a very low yield to acceptableacceptable selectivity. selectivity. Another Another casecase is thethe industrialindustrial production production of aceticof acetic acid, acid, a known a known commodity commodity ensure an acceptable selectivity. Another case is the industrial productionTM of acetic acid, a known of large‐scale demand. Currently mainly obtained by the improved Cativa methanolTM carbonylation of large‐scale demand. Currently mainly obtained by the improved Cativa methanol carbonylationTM commodityprocess [69,71,80], of large-scale it nevertheless demand. requires Currently three mainly steps obtained from natural by the gas improved and considerably Cativa harsh,methanol process [69,71,80], it nevertheless requires three steps from natural gas and considerably harsh, carbonylationpollutant rich process and costly [69, 71conditions.,80], it nevertheless The above examples requires explain three steps the interest from naturalin finding gas more and efficient considerably pollutant rich and costly conditions. The above examples explain the interest in finding more efficient harsh,processes pollutant and richin understanding and costly conditions. the involved The mechanisms above examples [81–83]. explain the interest in finding more processes and in understanding the involved mechanisms [81–83]. efficient processes and in understanding the involved mechanisms [81–83].

Scheme 1. One‐pot carboxylation of methane to acetic acid catalyzed by C‐scorpionate complexes Scheme[7,8,47,62]. 1. One-pot carboxylation of methane to acetic acid catalyzed by C-scorpionate complexes [7,8,47,62]. Scheme 1. One‐pot carboxylation of methane to acetic acid catalyzed by C‐scorpionate complexes [7,8,47,62].

SchemeScheme 2. 2.Peroxidative Peroxidative oxidationoxidation of of cyclohexane cyclohexane to tocyclohexanone cyclohexanone and andcyclohexanol cyclohexanol (KA oil) (KA in oil) in aqueousaqueous medium, medium, catalyzed catalyzed by by C-scorpionateC‐scorpionate catalysts catalysts [7,8,13,43,46–49,57–68]. [7,8,13,43,46–49,57– 68].

Scheme 2. Peroxidative oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA oil) in AmongAmong the the new new catalysts catalysts recently recently found found for for the the above above reactions reactions are areC‐homoscorpionate C-homoscorpionate metal metal complexesaqueous medium, which have catalyzed been successfully by C‐scorpionate applied catalysts either as[7,8,13,43,46–49,57–68]. catalysts for oxygenations, with H2O2, to complexes which have been successfully applied either as catalysts for oxygenations, with H2O2, to produce the respective alcohols and ketones or, with K2S2O8, to directly yield carboxylic acids [7,8]. produceAmongThe the reactions respective the new leading catalysts alcohols to the recently and above ketones oxygenatedfound or, for with the species K above2S2 Oare8 , reactionsbelieved to directly to are proceed yield C‐homoscorpionate carboxylic mainly via acids both [metal7,8]. The reactions leading to the above oxygenated species are believed to proceed mainly via both complexescarbon‐ andwhich oxygen have‐centered been successfully radicals. Interestingly, applied either although as catalystsoccurring forvia theoxygenations, formation of withreactive H 2O2, to carbon- and oxygen-centered radicals. Interestingly, although occurring via the formation of reactive produceradicals, the such respective reactions alcohols are rather and selective ketones [7,8,16]. or, with The K C2S‐scorpionate2O8, to directly catalyst yield initially carboxylic activates acids not [7,8]. radicals,theThe alkane suchreactions but reactions reacts leading arewith ratherto another the selectiveabove reactant, oxygenated [7 ,usually8,16]. The the species C-scorpionateoxidant are (e.g., believed hydrogen catalyst to ‐proceedperoxide) initially mainly activates [7,8]. The via not both the alkanecarbonformed‐ butand reactsreactive oxygen with species‐centered another (e.g., radicals. reactant,hydroxyl radical)Interestingly, usually attacks the oxidant althoughthe alkane (e.g., occurringmolecule hydrogen-peroxide) without via the any formation participation [7,8]. The of reactive formed in the latter process of the metal complex. Thus, the metal catalyst does not take part in the direct reactiveradicals, speciessuch reactions (e.g., hydroxyl are rather radical) selective attacks [7,8,16]. the alkane The C‐ moleculescorpionate without catalyst any initially participation activates in not the “activation” of the carbon‐hydrogen bond by the radical. latterthe alkane process but of reacts the metal with complex. another Thus,reactant, the usually metal catalyst the oxidant does not (e.g., take hydrogen part in the‐peroxide) direct “activation” [7,8]. The of the carbon-hydrogenA possible mechanism bond [69,70] by the for radical. the generation of carboxylic acids is represented in Scheme 3 formedfor an reactive oxo‐V complex: species (e.g.,a potassium hydroxyl peroxodisulfate radical) attacks salt (K the2S2 Oalkane8) is essential molecule for thewithout formation any ofparticipation alkyl in theA latter possible process mechanism of the [metal69,70 ]complex. for the generation Thus, the of metal carboxylic catalyst acids does is represented not take part in Schemein the direct3 for an“activation” oxo-V complex: of the carbon a potassium‐hydrogen peroxodisulfate bond by the saltradical. (K2S 2O8) is essential for the formation of alkyl A possible mechanism [69,70] for the generation of carboxylic acids is represented in Scheme 3 for an oxo‐V complex: a potassium peroxodisulfate salt (K2S2O8) is essential for the formation of alkyl

Catalysts 2017, 7,7, 12 7 of 21 radicals whereas the scorpionate catalyst is not needed for this purpose. Nevertheless, no carboxylic radicals whereas the scorpionate catalyst is not needed for this purpose. Nevertheless, no carboxylic acid is detected in the absence of the catalyst. Under the experimental conditions, peroxodisulfate acid is detected in the absence of the catalyst. Under the experimental• conditions, peroxodisulfate undergoes thermolysis into sulfate (or its protonated form HSO4• if in acidic medium) radicals which undergoes thermolysis into sulfate (or its protonated form• HSO4 if in acidic medium) radicals which are known alkane hydrogen abstractors, leading • to R . Further conversion of alkyl radical to are known alkane hydrogen abstractors, leading to R . Further conversion of alkyl radical to• carboxylic carboxylic acid includes carbonylation of the former by CO to form the acyl radical• RCO . The latter acid includes carbonylation of the former by CO to form the acyl radical RCO . The latter may then• may then be converted, in the presence of the metal catalyst, by its oxygenation to •give RCOO , be converted, in the presence of the metal catalyst, by its oxygenation to give RCOO , involving− a involving a peroxo metal species (Scheme 3) derived from the reaction of catalyst with− HS2O8 or with peroxo metal species (Scheme3) derived from the reaction of catalyst with HS 2O8 or with H2SO5 H2SO5 (peroxomonosulfuric acid) formed upon reaction of HS2O8− with TFA or hydrolysis by traces (peroxomonosulfuric acid) formed upon reaction of HS O − with TFA or hydrolysis by traces of water. of water. Then RCOOC• abstracts a hydrogen atom from,2 8 for example, excess TFA or alkane to afford Then RCOOC• abstracts a hydrogen atom from, for example, excess TFA or alkane to afford the desired the desired carboxylic acid. carboxylic acid.

Scheme 3. Proposed mechanism for the carboxylation of an alkane to the corresponding carboxylic acid catalyzed by a C C-homoscorpionate‐homoscorpionate oxo oxo-V‐V complex.

In thethe casecase of of the the oxidation oxidation of of alkanes alkanes with with hydrogen hydrogen peroxide, peroxide, detailed detailed investigation investigation of the effectsof the effectsof various of various experimental experimental parameters parameters in this reaction, in this reaction, the use of the radical use of traps, radical kinetic traps, and kinetic selectivity and selectivitystudies complemented studies complemented with theoretical with calculations theoretical [ 73calculations,84–86] indicated [73,84–86] the interest indicated of using the interest controlled of • usingamounts controlled of water amounts and acid, of and water assured and acid, the involvement and assured of the hydroxyl involvement (OH• )of and hydroxyl alkyl (R (OH•) radicals) and • alkylin a radical (R ) radicals type mechanism in a radical (see type Equations mechanism (2)–(10) (see and Equations Scheme 4(2)–(10) for a more and detailedScheme formation4 for a more of detailedhydroxyl formation and hydroperoxyl of hydroxyl radicals). and hydroperoxyl radicals). The proposed route for the metal metal-catalyzed‐catalyzed decomposition decomposition of of hydrogen hydrogen peroxide peroxide (Haber–Weiss (Haber–Weiss mechanism) [70,87] [70,87] includes the following two two key stages (Equations (Equations (2) (2) and (3)) (3)) of formation of the • • oxygenoxygen-centered‐centered radicals HOO • and HO •::

Mn+ + H2O2 → HO• + M(n+1)+ + HO− (2) n+ • (n+1)+ − M + H2O2 → HO + M + HO (2) M(n+1)+ + H2O2 → HOO• + H+ + Mn+ (3) (n+1)+ • + n+ M + H2O2 → HOO + H + M (3)

Catalysts 2017, 7, 12 8 of 21 Catalysts 2017, 7, 12 8 of 21

SchemeScheme 4.4. ProposedProposed mechanismmechanism forfor thethe formationformation ofof OH OH•• andand HOOHOO•• radicalsradicals inin thethe oxidationoxidation ofof anan alkanealkane withwith hydrogenhydrogen peroxide,peroxide, catalyzed catalyzed by by a a C-homoscorpionate C‐homoscorpionate oxo-V oxo‐V complex. complex.

It is the hydroxyl radical (derived from the reduction of H2O2 by the reduced form of the metal It is the hydroxyl radical (derived from the reduction of H2O2 by the reduced form of the metal catalyst,catalyst, EquationEquation (2)(2) andand SchemeScheme4 4 for for a a C-homoscorpionate C‐homoscorpionate oxo-V oxo‐V complex) complex) that that reacts reacts with with the the • alkanealkane generatinggenerating the the alkyl alkyl radical radical R• (EquationR (Equation (4)) which, (4)) which, in turn, in reacts turn, with reacts dioxygen with (Equationdioxygen • (5))(Equation to form (5)) the to alkylperoxyl form the alkylperoxyl radical ROO radical•. The ROO latter. The gives latter rise gives to the rise alkyl-hydroperoxide to the alkyl‐hydroperoxide (ROOH) (Equation(ROOH) (Equation (6)) which, (6)) in which, the presence in the presence of both the of reducedboth the andreduced oxidized and oxidized forms of forms the metal of the catalyst, metal decomposescatalyst, decomposes (Equations (Equations (7)–(10)) to(7)–(10)) the ketone to the and/or ketone the and/or alcohol. the alcohol.

• → 2 • HO• + RH H O + R• (4) HO + RH → H2O + R (4) R•+ O2 → ROO• (5) • • R + O2 → ROO (5) ROO• + H2O2 → ROOH + HOO• (6) • → • ROOROOH+ + H M2nO+ 2→ ROROOH• + HO +− HOO+ M(n+1)+ (7)(6) n+ • − (n+1)+ ROOHROOH + M + M(→n+1)+RO → ROO+ HO• + H++ M + Mn+ (8)(7) (n+1)+ • + n+ ROOH +RO M • + RH→ →ROO ROH+ + HR• + M (9)(8) • • 2ROORO • +→ RH ROH→ ROH+ R‐H = + O R + O2 (10)(9) • The use of C‐homoscorpionate2ROO complexes→ ROH to + catalyze R-H = O +epoxidation O2 of alkenes, a very useful(10) synthetic transformation to produce fine chemicals [79,80], is also significant. Selective catalytic The use of C-homoscorpionate complexes to catalyze epoxidation of alkenes, a very epoxidation of cis‐cyclooctene to 1,2‐epoxy‐cyclooctane was achieved in the presence of tris(pyrazol‐ useful synthetic transformation to produce fine chemicals [79,80], is also significant. Selective 1‐yl)methane Mo(VI) complexes [19,20,88], in particular if water is rigorously excluded from the catalytic epoxidation of cis-cyclooctene to 1,2-epoxy-cyclooctane was achieved in the presence reaction mixture. Other alkene substrates such as R‐(+)‐limonene, 1‐octene, trans‐2‐octene, of tris(pyrazol-1-yl)methane Mo(VI) complexes [19,20,88], in particular if water is rigorously cyclododecene, 3‐carene, and 4‐vinyl‐1‐cyclohexene are also selectively converted into the excluded from the reaction mixture. Other alkene substrates such as R-(+)-limonene, 1-octene, corresponding epoxides. trans-2-octene, cyclododecene, 3-carene, and 4-vinyl-1-cyclohexene are also selectively converted In the presence of the sacrificial oxidant PhI(OAc)2, aqua Ru(II) tris(pyrazol‐1‐yl)methane into the corresponding epoxides. compounds catalyze the aerobic epoxidation of a wide variety of alkenes [89]. The mechanistic pathway for the epoxidation (Scheme 5) proceeds via the formation of an active metal–oxo intermediate through the mediation of iodobenzene diacetate. The active catalytic species is a

Catalysts 2017, 7, 12 9 of 21

In the presence of the sacrificial oxidant PhI(OAc)2, aqua Ru(II) tris(pyrazol-1-yl)methane compounds catalyze the aerobic epoxidation of a wide variety of alkenes [89]. The mechanistic pathway forCatalysts the epoxidation2017, 7, 12 (Scheme5) proceeds via the formation of an active metal–oxo intermediate through9 of 21 the mediation of iodobenzene diacetate. The active catalytic species is a formally Ru(IV)=O one, resultingformally Ru(IV)=O from the oxidation one, resulting of the from aqua the Ru(II) oxidation complex of by the PhI(OAc) aqua Ru(II)2. The complex electrophilic by PhI(OAc) metal bound2. The oxoelectrophilic group subsequently metal bound interacts oxo group with subsequently the incoming interacts olefinic with double the incoming bond with olefinic transfer double of the bond oxo group.with transfer The involvement of the oxo group. of a concerted The involvement transition of state a concerted for the transfer transition of state the oxygen for the atomtransfer from of the metal-oxidooxygen atom complex from the to metal the olefinic‐oxido complex double bond to the is olefinic suggested double [89,90 bond]. is suggested [89,90].

Scheme 5.5. Proposed mechanism for the epoxidation of alkenes catalyzed by aa C-homoscorpionateC‐homoscorpionate aqua-Ruaqua‐Ru complex.

From the above, it turns out that an effective catalyst for the above oxidation reactions requires From the above, it turns out that an effective catalyst for the above oxidation reactions requires the the ability to undergo reversible redox processes involving electron transfer at accessible potentials. ability to undergo reversible redox processes involving electron transfer at accessible potentials. This redox potential—oxidative catalytic activity relationship will be addressed in detail in the This redox potential—oxidative catalytic activity relationship will be addressed in detail in the following section. following section.

3. Electrochemical Properties of C-ScorpionateC‐Scorpionate Metal Complexes The electrochemical approach is a very powerful tool for fundamental chemical characterization The electrochemical approach is a very powerful tool for fundamental chemical characterization of of species that can be oxidized or reduced. By continuously changing the working potential, its species that can be oxidized or reduced. By continuously changing the working potential, its cycling or cycling or keeping constant, enables not only the determination of the respective oxidation or keeping constant, enables not only the determination of the respective oxidation or reduction potentials reduction potentials but also revelation of the reversibility of the redox processes, the nature, kinetics but also revelation of the reversibility of the redox processes, the nature, kinetics and equilibrium and equilibrium constants of the follow‐up reactions, the stability and structure of intermediates, the constants of the follow-up reactions, the stability and structure of intermediates, the type and yield type and yield of products, etc. In fact, an electron transfer in a coordination compound can induce of products, etc. In fact, an electron transfer in a coordination compound can induce very diverse very diverse chemical reactivity, ultimately with catalytic significance. chemical reactivity, ultimately with catalytic significance. Some C‐homoscorpionate complexes underwent systematic electrochemical investigation Some C-homoscorpionate complexes underwent systematic electrochemical investigation usually usually by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) techniques, at by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) techniques, at platinum platinum working electrodes (disk or gauze, respectively). Glassy carbon working electrodes for CV working electrodes (disk or gauze, respectively). Glassy carbon working electrodes for CV were were also used [89]. Experiments were performed in a three‐electrode system whose potential was also used [89]. Experiments were performed in a three-electrode system whose potential was controlled vs. a Luggin capillary connected to a silver wire pseudo‐reference electrode and a Pt auxiliary electrode. The complexes were added to a 0.1–0.2 M nBu4N[X] (X = BF4, PF6 or ClO4) or [Et4N][ClO4]/aprotic non‐aqueous medium (e.g., CH2Cl2, NCMe, DMF or DMSO), at room temperature, under dinitrogen [46,49–54,89,91–93]. Their measured redox potentials in volts vs. saturated calomel electrode (V vs. SCE) and the eventual reversibility of the redox process, are indicated in Table 1.

Catalysts 2017, 7, 12 10 of 21 controlled vs. a Luggin capillary connected to a silver wire pseudo-reference electrode and a Pt n auxiliary electrode. The complexes were added to a 0.1–0.2 M [ Bu4N][X] (X = BF4, PF6 or ClO4) or [Et4N][ClO4]/aprotic non-aqueous medium (e.g., CH2Cl2, NCMe, DMF or DMSO), at room temperature, under dinitrogen [46,49–54,89,91–93]. Their measured redox potentials in volts vs. saturated calomel electrode (V vs. SCE) and the eventual reversibility of the redox process, are indicated in Table1.

Table 1. Cyclic voltammetric data a for metal C-homoscorpionate complexes.

Redox Potential/V vs. SCE C-Scorpionate Compound I ox I ox I red I red II red (II red) Ref. Ep ( E1/2 ) Ep ( E1/2 ) Ep E1/2 3 [VCl3{κ -SO3C(pz)3}] (1.14) - - [91] 3 b [VO2{κ -SO3C(pz)3}] - −0.46 −1.82 [47] 3 c [VOCl2{κ -CH3SO2OCH2C(pz)3}] (1.35) −0.78 - [49] 3 b [VO2{κ -HC(pz)3}][BF4] - −0.28 −1.70 [47] 3 [VO2{κ -HC(3,5-Me2pz)3}][BF4] - −0.37 −1.75 [47] 3 Li[Mo{κ -SO3C(pz)3}(CO)3] (0.18) - - [50] 3 [Mo{κ -SO3C(pz)3}I(CO)3] 0.44 - - [50] 3 [Mo{κ -SO3C(pz)3}H(CO)3] 0.09 - - [50] 3 d [ReCl2{κ -HC(pz)3}(PPh3)][BF4] (0.54) (−0.74) - [51] 3 [ReCl3{κ -HC(pz)3}] 1.14 −0.62 −1.70 [52] 3 [ReCl3{κ -HC(3,5-Me2pz)3}] (1.25) (−0.13) (−0.72) [52] 2 [ReCl4{κ -HC(pz)3}] 1.79 (−0.06) −1.50 [52] 3 [ReO3{κ -SO3C(pz)3}] - −0.83 - [52] 3 b [ReO{κ -SO3C(pz)3}(HMT)] (0.86) −0.83 - [21] 3 [ReOCl{κ -SO3C(pz)3}(PPh3)]Cl 1.45 (−0.94) (−1.41) [52] 2 b [ReO3{κ -HC(pz)3}(PTA)][ReO4] -(−0.62) - [21] b [ReO3(Hpz)(HMT)][ReO4] -(−0.33) - [21] 3 c [FeCl2{κ -CH3SO2OCH2C(pz)3}] (1.06) −0.38 - [49] 3 d [FeCl3{κ -HC(pz)3}] (−0.11) - - [92] 3 d [FeCl3{κ -HC(3,5-Me2pz)3}] (−0.20) - - [92] 3 d [FeCl3{κ -HC(3-iPrpz)3}] (−0.04) - - [92] 3 [Ru(p-cymene){κ -SO3C(pz)3}]Cl (0.95) (−0.97) - [54] 3 [Ru(p-cymene){κ -SO3C(pz)3}][BF4] (0.96) (−0.97) - [54] 3 [Ru(p-cymene){κ -SO3C(3-Phpz)3}]Cl 1.02 (−1.00) - [54] 3 [Ru(benzene){κ -SO3C(pz)3}]Cl (1.07) (−0.87) - [54] 3 [Ru(benzene){κ -SO3C(3-Phpz)3}]Cl (1.37) (−0.92) - [54] 3 [Ru(HMB){κ -SO3C(pz)3}]Cl 0.95 (−1.11) - [54] 3 [Ru(cod)Cl{κ -SO3C(pz)3}] (0.96) (−1.10) - [54] 3 [Ru(cod)Cl{κ -SO3C(3-Phpz)3}] (0.99) (−1.27) - [54] 3 c [RuCl{κ -HC(pz)3}(bqdi)][ClO4] (0.82) (−0.79) −1.39 [89] 3 c [Ru(H2O){κ -HC(pz)3}(bqdi)][ClO4]2 (0.44) - - [89] 3 c [Ru{κ -HC(3,5-Me2pz)3}(NCCH3)3][BF4]2 (0.42) - - [93] 3 c [Ru{κ -HC(3,5-Ph2pz)3}(NCCH3)3][BF4]2 (0.71) - - [93] 3 b [Co(OSO3H)(OCH3)(HOCH3){κ -HC(pz)3}] 1.03 −0.40 [12] 3 [Co{κ -HOCH2C(pz)3}2](NO3)2 (0.58) −0.68 [12] [Co{κ3-HOCH C(pz) } ]·[Co{κ3-HOCH C(pz) } 2 3 2 2 3 (0.60) −0.67 −1.21 [12] (H2O)3]2(Cl)6·6H2O 3 [CoCl2(H2O){κ -PyCH2OCH2C(pz)3}] 1.28 −0.60 - [12] 3 c [CoCl2(H2O){κ -CH3SO2OCH2C(pz)3}] 1.10 −0.64 - [12] 3 c [CuCl2{κ -CH3SO2OCH2C(pz)3}] - −0.70 - [49] 2 c [AuCl2{κ -HC(pz)3}]Cl - −0.02 −0.60 [57] 2 c [AuCl2{κ -HOCH2C(pz)3}]Cl - −0.01 −0.58 [57] 2 c [AuCl2{κ -HC(3,5-Me2pz)3}]Cl - −0.11 −0.69 [57] a −1 Values in V ± 0.02 relative to SCE; in CH2Cl2; scan rate of 200 mV·s . Values for reversible waves are given in brackets. bqdi = o-benzoquinonediimine; 3-iPr = iso-propyl group; b In dimethyl sulfoxide (DMSO); c In acetonitrile (NCMe); d In dimethylformamide (DMF). SCE = saturated calomel electrode. Catalysts 2017, 7, 12 11 of 21

All authors found that C-homoscorpionate ligands are electrochemically inert in the potential range of −2.0 V to 2.0 V vs. SCE, at the used experimental conditions [46,49–54,89,91–93], thus no ligand centered oxidation or reduction has been reported to date. Most of the metallic compounds bearing tris(pyrazol-1-yl)methane ligands exhibit at least a single-electron (determined by exhaustive CPE) oxidation wave, assigned to the dn → dn−1 metal oxidation. Exceptions are, as expected, V(V), Re(VII), Ni(II), Cu(II), Au(III), and Zn(II) complexes. The said oxidation waves can meet the reversibility criteria [94] or be irreversible due to chemical reactions that follow the electron-transfer process (Table1). Most of the C-homoscorpionate complexes also exhibit (Table1) a reduction wave which usually is followed, at a lower potential, by a second one. These waves often (e.g., for V, Re, Fe, Ru or Co complexes) correspond to single-electron processes, being assigned to the dn → dn+1 and dn+1 → dn+2 metal reductions. The highest known first oxidation potential of all C-scorpionate metal complexes is shown by 2 I ox the 15-electron Re(IV) complex [ReCl4{κ -HC(pz)3}] ( Ep = 1.79 V vs. SCE, Table1) per its electron deficiency. Such oxidation potential value is even higher than the one of the oxo-Re(V) 16-electron 3 I ox complex [ReOCl{κ -SO3C(pz)3}(PPh3)]Cl ( Ep = 1.45 vs. SCE, Table1) in spite of the higher metal oxidation state of the latter. The presence of the strong electron-donor oxo-ligand provides another 2 reason for the lower oxidation potentials of this oxo-complex. [ReCl4{κ -HC(pz)3}] is also the one I ox that exhibits the most favorable (highest) reduction potential ( E1/2 = −0.06 V vs. SCE, Table1) in 3 accord with its low electron-count. Harder to reduce are the oxo-Re species [ReO3{κ -SO3C(pz)3}] 3 and [ReOCl{κ -SO3C(pz)3}(PPh3)]Cl, in agreement with the presence of the strong electron-donor oxide ligand and with their higher electron count. Among the rhenium complexes, Re(III) 16-electron 3 3 complexes [ReCl3{κ -HC(pz)3}] and [ReCl3{κ -HC(3,5-Me2pz)3}] are those that present the lowest I ox I ox oxidation potential ( Ep = 1.14 and E1/2 = 1.25 V vs. SCE, Table1), consistent with the lower metal 3 oxidation state. In contrast with the measured values, complex [ReCl3{κ -HC(pz)3}] would be expected 3 to have a higher oxidation potential than the analogous [ReCl3{κ -HC(3,5-Me2pz)3}], on account of the weaker electron-donor character of HC(pz)3 in the former in comparison with HC(3,5-Me2pz)3 in the latter. However, the irreversible character of the oxidation wave of the former (indicative of a chemical reaction following the electron-transfer step, with a resulting shift of the oxidation potential) preclude a reliable comparison between the measured potentials for these complexes. In the case of Mo(0 or II) complexes a second single-electron oxidation process is detected (not 3 shown in Table1) in the potential range of 0.18 to 0.6 V vs. SCE. In Li[Mo{ κ -SO3C(pz)3}(CO)3] yields 3 + the 16-electron Mo(II) complex [Mo{κ -SO3C(pz)3}(CO)3] , its irreversibility being associated to fast coordination of a solvent molecule, leading to an electronically saturated product. 3 The irreversibility of the first oxidation wave of compounds [MoI{κ -SO3C(pz)3}(CO)3] and 3 [MoH{κ -SO3C(pz)3}(CO)3] signals the instability of the resulting cationic Mo(III) complexes, which then rapidly decompose with probable CO loss [95] and, for the hydride compound, by deprotonation [96–98]. The first oxidation potentials of all these tricarbonyl complexes are much lower than that of the parent hexacarbonyl compound, on account of the replacement of three carbonyls in the latter by the more electron-donating C-scorpionate ligands [24,27,28]. Moreover, the first oxidation 3 3 potential of [MoH{κ -SO3C(pz)3}(CO)3] in comparison with [MoI{κ -SO3C(pz)3}(CO)3] reflects the stronger electron-donor character of the hydride relatively to the iodide ligand [28]. These Mo (0 or II) compounds have not yet been used for catalytic oxidation reactions. Nevertheless, their low oxidation potentials (first oxidation wave in the range 0.09–0.44 V vs. SCE, Table1) and the detected easy coordination/decoordination of substrates are promising features for a possible good oxidative catalytic performance. The interest in electron transfer induced reactivity of C-scorpionate metal compounds is demonstrated in the following catalytic systems where such complexes provide unprecedented examples. Catalysts 2017, 7, 12 12 of 21

3.1. Oxidation of Alkanes to Alcohols and Ketones In the case of the oxidation of alkanes with peroxides, the availability of reducible metal species, easily detectable by electrochemical experiments, was found very important for the catalytic performance of C-homoscorpionate complexes. As previously mentioned, the formation of RO• and ROO• radicals (Equations (2) and (3), and Scheme4) involves the reaction of both reduced and oxidized forms of the metal catalyst and is a key step for the occurrence of the C–H abstraction from the alkane. Therefore, C-homoscorpionate complexes that undergo redox processes at accessible potential values are expected to display better oxidative catalytic performance than those harder to oxidize or reduce. 3 3 In fact, V(V) complexes [VO2{κ -SO3C(pz)3}] and [VO2{κ -HC(pz)3}][BF4], whose accessible potential values for the first single-electron [V(V) to V(IV)] reduction process are −0.46 and −0.48 V vs. SCE, lead to quite similar (19% and 18.6%, respectively [47]) KA oil yields (among the highest values obtained for this class of catalysts) by catalytic oxidation of cyclohexane. The turnover number (TON, moles of product per mole of catalyst) values also follow the trend: 117 and 112, respectively, 3 3 for [VO2{κ -SO3C(pz)3}] and [VO2{κ -HC(pz)3}][BF4]. 3 A further example comes from V(III or IV) complexes. [VCl3{κ -SO3C(pz)3}] is easier to oxidize 3 than [VOCl2{κ -CH3SO2OCH2C(pz)3}] (1.14 vs. 1.35 V, Table1) and thus yields higher KA oil amounts 3 (13% (TON = 121) [46] vs. 7% (TON = 89) in the presence of [VOCl2{κ -CH3SO2OCH2C(pz)3}] [49]). 3 Moreover, trichlorovanadium(III) [VCl3{κ -HC(pz)3}] leads to higher yield (18%) and TON (167) 3 values [47] than the related [VCl3{κ -SO3C(pz)3}] (13% yield and a TON of 121, [46,47]), in accordance 3 with its lower oxidation state and the neutral scorpionate ligand in[VCl3{κ -HC(pz)3}]. 3 3 Likewise, for the Co(II) complexes [CoCl2(H2O){κ -PyCH2OCH2C(pz)3}] and [CoCl2(H2O){κ - CH3SO2OCH2C(pz)3}]; the latter presents lower oxidation potential (1.10 V vs. SCE, Table1) and thus exhibits better catalytic performance: 10.5% vs. 3.2% yield of KA oil in the presence of 3 [CoCl2(H2O){κ -PyCH2OCH2C(pz)3}] [12] which is oxidized at 1.28 V vs. SCE (Table1). 2 A similar behavior is found for the chloro-Au(III) complexes: [AuCl2{κ -HC(pz)3}]Cl and 2 [AuCl2{κ -HOCH2C(pz)3}]Cl which present very close reduction potentials for the first irreversible two electrons Au(III) → Au(I) reduction process (Table1) and are the most active, leading to 8.1% and 10.3%, respectively, of KA oil [57]. The hardest to reduce (−0.11 V vs. SCE, Table1) yields only 7.5% of the oxygenated mixture under the same conditions [64]. The lower reduction potential of 2 2 [AuCl2{κ -HC(3,5-Me2pz)3}]Cl in comparison with the one of [AuCl2{κ -HC(pz)3}]Cl is consistent 2 with the stronger electron-donor ability of the methyl-substituted κ -HC(3,5-Me2pz)3 ligand than that 2 of κ -HC(pz)3 [24]. However, an accurate comparison cannot be established due to the irreversibility of the reduction waves (the reduction potential is not the thermodynamic one). Moreover, whereas the CH2OH substituent at the apical methine carbon appears to have limited influence on the redox potential of the gold complexes (Table1), the replacement of hydrogens by an electron donor group (Me) at the pyrazolyl rings of the C-scorpionate leads to an electronically richer Au(III) center, resulting in a measurable (ca. 0.1 V) cathodic shift of the potential. A second irreversible reduction (Table1) assigned to the Au(I) → Au(0) reduction leads to the appearance of gold metal at the platinum electrodes surface after exhaustive controlled potential electrolysis and an irreversible anodic wave (in the range 0.44–0.50 V vs. SCE) observed upon scan reversal after the second reduction wave, corresponding to the oxidation of the Au(0) species formed in the second reduction process.

3.2. Oxidation of Alkanes to Carboxylic Acids

3 The catalytic activity of the only to date tested [51] Re(III) complexes, [ReCl3{κ -HC(pz)3}] and 3 [ReCl2{κ -HC(pz)3}(PPh3)][BF4], for the for the direct oxidation of ethane to acetic acid follows their 3 oxidation behavior (Table1 and Figure6): [ReCl 2{κ -HC(pz)3}(PPh3)][BF4] presents considerably lower ox oxidation potential (E1/2 = 0.54 V vs. SCE) and leads to higher acetic acid yield (16%) and TON (8) 3 ox values than [ReCl3{κ -HC(pz)3}] (E1/2 = 1.14 V vs. SCE; 5% acetic acid yield and TON = 2) [51,52]. CatalystsCatalysts 20172017, ,7,7 ,12 12 1313 of of 21 21

Figure 6. Yields of: () acetic acid produced from the one-pot oxidation of ethane catalyzed by Figure 6. Yields of: () acetic acid produced from the one‐pot oxidation of ethane catalyzed by the the Re(III) complexes vs. their Re(III) oxidation potentials; (•) 6-methylhexanolide obtained from Re(III) complexes vs. their Re(III) oxidation potentials; () 6‐methylhexanolide obtained from Baeyer‐ Baeyer-Villiger (BV) oxidation of 2-methylcyclohexanone, vs. their Re(VII) oxidation potentials; and (•) Villiger (BV) oxidation of 2‐methylcyclohexanone, vs. their Re(VII) oxidation potentials; and () 6‐ 6-methylhexanolide from BV oxidation of 2-methylcyclohexanone, vs. their Re(III) oxidation potentials. methylhexanolide from BV oxidation of 2‐methylcyclohexanone, vs. their Re(III) oxidation potentials. 3.3. Baeyer-Villiger Oxidation of Ketones 3.3. Baeyer‐Villiger Oxidation of Ketones The regioselective Baeyer-Villiger (BV) oxidation of 2-methylhexanone to 6-methylhexanolide (as The regioselective Baeyer‐Villiger (BV) oxidation of 2‐methylhexanone to 6‐methylhexanolide a result of the formal insertion of the oxygen atom between the carbonyl and the more substituted (as a result of the formal insertion of the oxygen atom between the carbonyl and the more substituted Cα atom) was found [21] to be favored by strong Lewis acid Re catalysts (Table1 and Figure6). Cα atom) was found [21] to be favored by strong Lewis acid Re catalysts (Table 1 and Figure 6). The The non-radical mechanism of BV ketone oxidation proceeds via the activation of the ketone, upon non‐radical mechanism of BV ketone oxidation proceeds via the activation of the ketone, upon coordination to the metal catalyst, to nucleophilic attack of the peroxide oxidant, followed by heterolytic coordination to the metal catalyst, to nucleophilic attack of the peroxide oxidant, followed by peroxo-bond cleavage and carbanion migration. In fact, for the same Re oxidation state (III (•) or heterolytic peroxo‐bond cleavage and carbanion migration. In fact, for the same Re oxidation state VII (•), Figure6) higher lactone yields are obtained when the catalyst presents a higher (more positive) (III () or VII (), Figure 6) higher lactone yields are obtained when the catalyst presents a higher reduction potential. The electron deficiency of the catalyst (stronger Lewis acid character) activates to (more positive) reduction potential. The electron deficiency of the catalyst (stronger Lewis acid a greater extent the carbonyl group of the ketone for the nucleophilic attack by hydrogen peroxide. character) activates to a greater extent the carbonyl group of the ketone for the nucleophilic attack by hydrogen3.4. Oxidation peroxide. of 1,2-Diols

3 3.4. OxidationThe Fe(III)/Fe(II) of 1,2‐Diols redox potentials of the chloro-iron(III) complexes follow the trend [FeCl3{κ - 3 HC(3-iPrpz)3}] (3-iPr = iso-propyl group on the 3-position of pyrazole rings) > [FeCl3{κ -HC(pz)3}] 3 The Fe(III)/Fe(II)3 redox potentials of the chloro‐iron(III) complexes follow the trend [FeCl3{κ ‐ > [FeCl3{κ -HC(3,5-Me2pz)3}] (Table1), which represents a decrease in Lewis acidity of the iron(III) 3 HC(3‐iPrpz)3}] (3‐iPr = iso‐propyl group3 on the 3‐position of pyrazole rings) > [FeCl3{κ ‐HC(pz)3}] > center along this series. In [FeCl3{κ -HC(3-iPrpz)3}], the sterically hindering iso-propyl group weakens [FeCl3{κ3‐HC(3,5‐Me2pz)3}] (Table 1), which represents a decrease in Lewis acidity of the iron(III) the coordination of nitrogen of pyrazol-1-yl ring conferring an enhanced Lewis acidity of the iron(III) 3 center along this series. In [FeCl3{κ ‐HC(3‐iPrpz)3}], the sterically hindering iso‐propyl3 group weakens center. The electron-releasing methyl groups on the pyrazol-1-yl ring in [FeCl3{κ -HC(3,5-Me2pz)3}] the coordination of nitrogen of pyrazol‐1‐yl ring conferring an enhanced Lewis acidity of the iron(III) increase the electron density on pyrazol-1-yl nitrogen and hence decreases the Lewis acidity of the center. The electron‐releasing methyl groups on the pyrazol‐1‐yl ring in [FeCl3{κ3‐HC(3,5‐Me2pz)3}] iron(III) center [92]. increase the electron density on pyrazol‐1‐yl nitrogen and hence decreases the Lewis acidity of the The catechol dioxygenase activity of the above iron(III) complexes was tested and the iron(III) center [92]. electrochemical properties of the catecholate adducts of the complexes reveal that a systematic variation The catechol dioxygenase activity of the above iron(III) complexes was tested and the in the ligand donor atom type significantly influences the Lewis acidity of the iron(III) center and electrochemical properties of the catecholate adducts of the complexes reveal that a systematic hence the interaction of the complexes with simple and substituted catechols. variation in the ligand donor atom type significantly influences the Lewis acidity of the iron(III) The rate of oxygenation increases upon increasing the Lewis acidity of the iron(III) center by center and hence the interaction of the complexes with simple and substituted catechols. modifying the ligand substituents. One of the pyrazolyl arms in the catecholate adducts is sterically The rate of oxygenation increases upon increasing the Lewis acidity of the iron(III) center by constrained by the 6,6,6-chelate ring system and appears to dissociate from the coordination sphere modifying the ligand substituents. One of the pyrazolyl arms in the catecholate adducts is sterically upon binding to the catecholate substrate, which is followed by dioxygen attack at the equatorial plane constrained by the 6,6,6‐chelate ring system and appears to dissociate from the coordination sphere leading to the formation of benzoquinone [92]. upon binding to the catecholate substrate, which is followed by dioxygen attack at the equatorial plane leading to the formation of benzoquinone [92].

Catalysts 2017, 7, 12 14 of 21 Catalysts 2017, 7, 12 14 of 21

3.5.3.5. Carboxylation Carboxylation of of Alkanes Alkanes TheThe catalytic activity of C C-homoscorpionate‐homoscorpionate V(V) V(V) complexes complexes for for the one one-pot‐pot carboxylation of of methanemethane to aceticacetic acid acid (Scheme (Scheme1) was1) was found found [ 47 ][47] to be to in be accordance in accordance with thewith order the oforder their of V(V) their→ V(V)V(IV) → reduction V(IV) reduction potentials, potentials, which follows which the follows electron-donor the electron characters‐donor characters of the scorpionate of the scorpionate ligands ligandsand the and charge the charge of the complexof the complex (see Table (see1 Tableand Figure 1 and 7Figure). The 7). stronger The stronger vanadium(V) vanadium(V) Lewis Lewis acid acid(the easiest(the easiest to reduce) to reduce) favors favors the carboxylationthe carboxylation mechanism mechanism represented represented in Scheme in Scheme3 and 3 and allows allows the thehighest highest product product yield yield (under (under the samethe same experimental experimental conditions conditions of the of other the other two V(V)two V(V) complexes) complexes) to be toachieved. be achieved. Moreover, Moreover, turnover turnover number number values values follow follow the yields the yields trend trend [47]. [47].

FigureFigure 7. YieldsYields of acetic acid produced from the one one-pot‐pot carboxylation of methane catalyzed by by the 33 3 33 V(V) complexes [VO[VO22{{κκ -HC(pz)‐HC(pz)33}][BF44], [VO[VO22{κ ‐-HC(3,5-MeHC(3,5‐Me2pz)pz)33}][BF}][BF44] ]and and [VO 2{{κκ ‐-SOSO33C(pz)C(pz)3}]3}] vs. vs. theirtheir V(V) V(V) to to V(IV) reduction reduction potentials.

3.6.3.6. Epoxidation Epoxidation of of Alkenes Alkenes

For the o‐benzoquinonediimine (bqdi) Ru(II) complexes [RuCl{κ33‐HC(pz)3}(bqdi)][ClO4] and For the o-benzoquinonediimine (bqdi) Ru(II) complexes [RuCl{κ -HC(pz)3}(bqdi)][ClO4] and [Ru(H2O){κ33‐HC(pz)3}(bqdi)][ClO4]2, electrochemical and DFT calculations established [89] that the [Ru(H2O){κ -HC(pz)3}(bqdi)][ClO4]2, electrochemical and DFT calculations established [89] that the redoxredox non non-innocent‐innocent bqdi was stabilized stabilized in in its its fully fully oxidized oxidized quinone quinone state state in in both both the the chloro chloro complex complex ([Ru{κ3‐HC(pz)3}(bqdi)(Cl)]++) and the aqua ([Ru){κ33‐HC(pz)3}(bqdi)(H2O)]2+2+) derivative. The chloro ([Ru{κ -HC(pz)3}(bqdi)(Cl)] ) and the aqua ([Ru){κ -HC(pz)3}(bqdi)(H2O)] ) derivative. The chloro complexcomplex exhibits metal based Ru(II)/(III) oxidation oxidation and and bqdi bqdi centered centered reduction. reduction. The aqua complex [Ru(H2O){κ3‐HC(pz)3}(bqdi)][ClO4]2 exhibits two one electron oxidations at The aqua complex [Ru(H2O){κ -HC(pz)3}(bqdi)][ClO4]2 exhibits two one electron oxidations at pH 7, suggesting the formation of a {Ru(IV)=O} species, species, the the supposed supposed active active species species of of the alkene epoxidation catalytic cycle (see Scheme 5). Thus, [Ru(H2O){κ33‐HC(pz)3}(bqdi)]2+2+ functions as an epoxidation catalytic cycle (see Scheme5). Thus, [Ru(H 2O){κ -HC(pz)3}(bqdi)] functions as an efficientefficient pre pre-catalyst‐catalyst for for the the selective epoxidation epoxidation of of a a wide wide variety variety of of alkenes alkenes in in the the presence of iodobenzeneiodobenzene diacaetate as the the sacrificial sacrificial oxidant.

3.7.3.7. Redox Redox Potential Potential Parametrization Parametrization TheThe values of the Ru(II/III) oxidation oxidation potential potential (in (in the the range range of of 0.95–1.37 0.95–1.37 V V vs. vs. SCE, SCE, Table Table 11)) ofof [Ru(L)(L[Ru(L)(L′0)]X)]X complexes complexes [L [L = = pp‐-cymene,cymene, benzene, benzene, hexamethylbenzene hexamethylbenzene (HMB), (HMB), or or cyclooctadiene cyclooctadiene (cod), (cod), 0 LL′ == tris(pyrazol tris(pyrazol-1-yl)methanesulfonate‐1‐yl)methanesulfonate or or the the 3‐phenylpyrazolyl 3-phenylpyrazolyl-substituted‐substituted derivative, derivative, X = XCl = or Cl BF or4] reflectBF4] reflect [54] [the54] electron the electron-donor‐donor characters characters of their of their ligands: ligands: for for the the cationic cationic complexes, complexes, with with the the 3 ++ commoncommon [Ru{ κ ‐-SOSO33C(pz)C(pz)33}]}] center,center, the the order order of of the the oxidation oxidation potentials potentials follows follows that that (in (in the the opposite opposite direction)direction) ofof the electron-releasingthe electron‐releasing character character of the corresponding of the variablecorresponding ligand (cymenevariable > benzene)ligand (cymeneas measured > benzene) by the electrochemical as measured by Lever the electrochemicalEL ligand parameter Lever (+1.48 EL ligand and +1.59parameter V vs. NHE(+1.48 for and cymene +1.59 Vand vs. benzene, NHE for respectively) cymene and [benzene,54]. respectively) [54]. As mentioned in the introduction, EL is a measure of the electron‐donor character of the ligand, the stronger this character, the lower is EL. Moreover, the experimental oxidation potentials are in accordance with those predicted from the knowledge of EL values for cymene [54], benzene [54] and

Catalysts 2017, 7, 12 15 of 21

As mentioned in the introduction, EL is a measure of the electron-donor character of the ligand, the stronger this character, the lower is EL. Moreover, the experimental oxidation potentials are in accordance with those predicted from the knowledge of EL values for cymene [54], 3 benzene [54] and κ -SO3C(pz)3 (Table2)[ 52] by applying the Lever method. Accordingly, the 3 3 higher oxidation potentials of [Ru{κ -SO3C(3-Phpz)3}(benzene)]Cl or [RuCl{κ -SO3C(3-Phpz)3}(cod)], bearing the 3-phenyl substituted tris(pyrazol-1-yl)methanesulfonate ligand, than those of the 3 3 analogous [Ru(benzene){κ -SO3C(pz)3}]Cl or [RuCl(cod){κ -SO3C(pz)3}] reflect the expected weaker 3 − 3 − electron-donor character of {κ -SO3C(3-Phpz)3} ligand in comparison with that of {κ -SO3C(pz)3} . Hence, the former ligand should present a higher EL value than the latter [52] (Table2).

Table 2. Electrochemical EL Lever ligand parameter for C-homoscorpionate ligands.

a Tris(pyrazol-1-yl)methane EL/V vs. SHE

HC(pz)3 0.14 − {SO3C(pz)3} −0.09 − {SO3C(3-Phpz)3} −0.05 a for each coordinated pyrazol-1-yl group.

The electrochemical EL Lever parameters for tris(pyrazol-1-yl)methane ligands [23,51,52,54] to date, found possible to estimate from the oxidation potential values of C-homoscorpionate complexes, by applying the linear (valid for octahedral complexes) relationship (1) and considering its extension to square-planar coordination and to full- and half-sandwich complexes [28,99–103], are presented in Table2. These values correspond to partial EL parameters assigned to each metal ligated arm (2-electron-donor) of the scorpionate ligand. Thus, the overall EL value of a scorpionate ligand will depend on its coordination mode to the metal center in a complex. Since the EL parameter is a measure of the electron donor character of a ligand (the lower − the parameter value, the stronger is that character), each ligated pyrazol-1-yl arm in {SO3C(pz)3} (EL = −0.09 V vs. SHE, Table2) is clearly a stronger electron donor than in HC(pz) 3 (EL = 0.14 V vs. − − SHE), indicating a much stronger electron-releasing ability of the anionic CSO3 group at {SO3C(pz)3} than the methine HC group in the neutral HC(pz)3. That is consistent with the above reported electrochemical behavior for V, Re, and Au complexes. Moreover, the value of –0.05 V vs. SHE (for each coordinating pyrazolyl arm) agrees with the expected slightly weaker electron-donor character of − − SO3C(3-Phpz)3 relative to SO3C(pz)3 due to the phenyl substituent at the pyrazol-1-yl rings in the former ligand. The above two series Ru(II) complexes, bearing the tris(pyrazol-1-yl)methanesulfonate ligand and its 3-phenyl substituted derivative, have not yet been tested as catalysts for partial oxidation reactions. Nevertheless, based on the reported electrochemical studies [54] we would expect a better oxidative catalytic performance for the tris(pyrazol-1-yl)methanesulfonate complexes 3 3 [Ru(p-cymene){κ -SO3C(pz)3}]Cl and [Ru(cod)Cl{κ -SO3C(pz)3}]. A comparison of the effect of HC(pz)3 or HB(pz)3 ligands on the redox potential of a metal complex 3 was reported for acetonitrile-Ru(II) complexes [93]. [Ru{κ -HB(3,5-Me2pz)3}(NCCH3)3][OTf] exhibits 3 a higher Ru(III)/(II) potential than its carbon analogue [Ru{κ -HC(3,5-Me2pz)3}(NCCH3)3][BF4]2 3 − (0.59 and 0.42 V vs. SCE, respectively), indicating that the charged {κ -HB(3,5-Me2pz)3} ligand 3 stabilizes Ru(III) relative to Ru(II) compared to the neutral {κ -HC(3,5-Me2pz)3}. In contrast, the 3 Ru(III)/(II) reduction potential observed for the [Ru{κ -HB(3,5-Ph2pz)3}(NCCH3)3][BF4] complex 3 (0.57 V vs. SCE) is lower than the same potential for the [Ru{κ -HC(3,5-Ph2pz)3}(NCCH3)3][BF4]2 complex (0.71 V vs. SCE, Table1), indicating that ligand charge is not as significant a factor as steric in determining the stability of ruthenium oxidation states for complexes with these bulky 3 ligands. Since the redox potential of [Ru{κ -HB(3,5-Ph2pz)3}(NCCH3)3][BF4] complex is smaller 3 than that of the [Ru{κ -HB(3,5-Me2pz)3}(NCCH3)3][OTf] complex, but the reverse is true for the 3 3 [Ru{κ -HC(3,5-Ph2pz)3}(NCCH3)3][BF4]2 and [Ru{κ -HC(3,5-Me2pz)3}(NCCH3)3][BF4]2 complexes, Catalysts 2017, 7, 12 16 of 21

both electronic and steric factors of these ligands affect the redox potentials of their Ru(II) complexes. 3 − Overall, Ru(II) coordination by negatively charged {κ -HB(3,5-R2pz)3} (R = Me or Ph) or neutral 3 {κ -HC(3,5-R2pz)3} ligands with varying steric bulk alters the Ru(III)/(II) potential by over 400 mV. The ability to alter the stability of ruthenium +2 or +3 oxidation states may be used to tune catalytic reactions. The electronic and structural properties of scorpionate ligands, such as poly(pyrazol-1-yl)methane ligands, play an important role in the ability of several transition metal complexes to mediate C–H activation and functionalization as well as other partial oxidations. Thus, the knowledge of the redox behavior of a certain C-homoscorpionate catalyst, as well as its relationship with the structure of the catalyst, may allow tailoring of its structural design to present a favorable value of potential to enhance its catalytic performance. Moreover, tris(pyrazol-1-yl)methane ligands may act as more than simple spectators during chemical reactions experienced by their metal complexes, and have an important influence on their reactivity by means of temporary changes of denticity. Of course, other factors are involved in the catalytic activity exhibited by the metal complex. Importantly, the tripodal C-scorpionate ligand, bearing three pyrazol-1-yl moieties (via their N atoms) is found to assist proton-transfer steps (see Scheme4) that are involved in key catalytic oxidation processes. Such factors should additionally be considered in the design of a catalyst with expected improved activity for the above oxidation reactions.

Acknowledgments: The author gratefully acknowledges all the co-authors cited in the joint publications. The work in this area has been partially supported by the Fundação para a Ciência e a Tecnologia (FCT), Portugal, and its projects PTDC/QEQ-ERQ/1648/2014 and UID/QUI/00100/2013. Conflicts of Interest: The author declares no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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