inorganics

Review Exploring Mechanisms in Ni Terpyridine Catalyzed C–C Cross-Coupling Reactions—A Review

Yulia H. Budnikova 1,2,* ID , David A. Vicic 3,* and Axel Klein 4,* ID 1 A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center of Russian Academy of Sciences, 8, Arbuzov Str., 420088 Kazan, Russia 2 A. M. Butlerov Chemistry Institute, Kazan Federal University, Kremlevskaya str. 18, 420008 Kazan, Russia 3 Department of Chemistry, Lehigh University, 6 E. Packer Ave., Bethlehem, PA 18015, USA 4 Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln, Greinstraße 6, D-50939 Köln, Germany * Correspondence: [email protected] (Y.H.B.); [email protected] (D.A.V.); [email protected] (A.K.); Tel.: +49-221-470-4006 (A.K.)

Received: 9 November 2017; Accepted: 18 January 2018; Published: 23 January 2018

Abstract: In recent years, nickel has entered the stage for catalyzed C–C cross-coupling reactions, replacing expensive palladium, and in some cases enabling the use of new substrate classes. Polypyridine ligands have played an important role in this development, and the prototypical tridentate 2,20:60,200-terpyridine (tpy) stands as an excellent example of these ligands. This review summarizes research that has been devoted to exploring the mechanistic details in catalyzed C–C cross-coupling reactions using tpy-based nickel systems.

Keywords: nickel; 2,20:60,200-terpyridine; catalysis; mechanism; electrosynthesis; cross-coupling

1. Introduction Polypyridine ligands such as 2,20-bipyridine (bpy) and 2,20:60,200-terpyridine (tpy) are common ligands in coordination chemistry and molecular catalysis, and they generally generate stable well-defined complexes [1–3]. Tpy and the first nickel(II) complexes [Ni(tpy)2]X2·H2O (X = Br or I) were prepared by Morgan and Burstall in the 1930s [4,5]. The terpyridine molecule, because of its three pyridine nitrogen donor atoms, can form stable complexes of the [M(tpy)]n+ type with transition metals. Nickel(II) complexes of the composition [Ni(tpy)X2] (X = halides and pseudohalides) [6–10] have been structurally characterized by X-ray crystallographic analysis as monomeric trigonal–bipyramidal complexes [8–10], or as dimeric X-bridged species containing octahedrally configured nickel(II) [10,11], depending on the nature of the X coligand. In dilute solutions, monomeric species can be expected [6], and they can be pentacoordinate or hexacoordinate if further ligands are available [8,10,12]. For example, the recrystallization of [Ni(tpy)Cl2]·3H2O from a water/ethanol mixture gave the isomeric [Ni(tpy)Cl(H2O)2]Cl·H2O in 2+ single crystalline form [12] nickel(II) complexes of the composition [Ni(tpy)2] , where all the X type coligands are outer sphere, and show octahedral coordination [3,13]. Nickel(II) complexes have received high attention as (pre)catalysts in the field of transition metal catalyzed C–C cross-coupling reactions [14–27]. This area has been dominated by palladium catalysts in the last 20 years, which became manifest in the 2010 Nobel prize for R. Heck, E. Negishi, and A. Suzuki [28,29]. Importantly, the introduction of nickel-based catalysts to C–C cross-coupling catalysis has broadened the substrate scope due to its different reactivity, and lowered the cost relative to palladium-catalyzed reactions [21–25,30,31]. Although many C–C cross-coupling reactions can be catalyzed by palladium and nickel compounds, there are often large mechanistic differences between the two metals under similar reaction conditions. Palladium catalysts normally shuttle

Inorganics 2018, 6, 18; doi:10.3390/inorganics6010018 www.mdpi.com/journal/inorganics Inorganics 2018, 6, x FOR PEER REVIEW 2 of 18 InorganicsInorganics 20182018,, 66,, x 18 FOR PEER REVIEW 2 of 18 of the three even electron oxidation states (0, +II, and +IV) during C–C coupling reactions, and two- of the three even electron oxidation states (0, +II, and +IV) during C–C coupling reactions, and two- betweenelectron processes two of the such three as evenoxidative electron addition oxidation and reductive states (0, elimination +II, and +IV) are duringcommon C–C mechanistic coupling electron processes such as oxidative addition and reductive elimination are common mechanistic reactions,steps [14,15,24, and two-electron25]. For nickel, processes single such electron as oxidative pathways addition between and the reductive oxidation elimination states (0, are +I, common +II, +III, steps [14,15,24,25]. For nickel, single electron pathways between the oxidation states (0, +I, +II, +III, mechanisticand +IV) often steps occur [14, 15in ,addition24,25]. For to nickel,oxidative single addition electron and pathways reductive between elimination the oxidation [24–26,31,32]. states (0, +I, and +IV) often occur in addition to oxidative addition and reductive elimination [24–26,31,32]. +II, +III,Out and of the +IV) many often applications occur in addition of nickel(II) to oxidative catalysts addition in C–C and or reductive C–E cross-coupling elimination [reactions,24–26,31, 32we]. Out of the many applications of nickel(II) catalysts in C–C or C–E cross-coupling reactions, we will Outfocus of in the this many contribution applications on ofthe nickel(II) Negishi-type catalysts reactions in C–C [15,18,19,25,26,33] or C–E cross-coupling using reactions, organometallic we will will focus in this contribution on the Negishi-type reactions [15,18,19,25,26,33] using organometallic focuszinc reagents, in this contribution and on electrochemically on the Negishi-type driven reactions C–C [15cross-coupling,18,19,25,26,33 reactions.] using organometallic Also, instead zinc of zinc reagents, and on electrochemically driven C–C cross-coupling reactions. Also, instead of reagents,providing and a literature on electrochemically overview of driventhe many C–C tran cross-couplingsformation that reactions. have been Also, achieved instead of using providing these providing a literature overview of the many transformation that have been achieved using these amethods, literature we overview will focus of the on many selected transformation studies that that havehave beenprovided achieved mechanistic using these insight methods, into we these will methods, we will focus on selected studies that have provided mechanistic insight into these focusreactions. on selected studies that have provided mechanistic insight into these reactions. reactions. Non-innocentNon-innocent ligandsligands [[34–39]34–39] suchsuch asas tpytpy contributecontribute stronglystrongly toto thethe electronelectron inventoryinventory ofof suchsuch Non-innocent ligands [34–39] such as tpy contribute strongly to the electron inventory of such nickelnickel complexes,complexes, and and redox redox chemistry chemistry may often may occur often at theoccur ligand at inthe addition ligand to thein abovementionedaddition to the nickel complexes, and redox chemistry may often occur at the ligand in addition to the oxidationabovementioned states (0, oxidation +I, +II, +III,states and (0, +IV).+I, +II, For +III, instance, and +IV). since For tpy inst isance, an excellent since tpy electron is an acceptorexcellent abovementioned oxidation states (0, +I, +II, +III, and +IV). For instance, since tpy is an excellent throughelectron acceptor its extended throughπ system, its extended a species π system, resulting a species from resulting the reduction from ofthe a reduction nickel(II)-tpy of a nickel(II)- complex electron acceptor through its extended π system, a species resulting from the reduction of a nickel(II)- cantpy complex be described can be in described two different in two resonance differentforms: resonanc ase a forms: divalent as a nickel divalent bound nickel to bound a radical to a anionic radical tpy complex can be described in two different resonance forms: as a divalent nickel bound to a radical (tpy)anionic•− ligand(tpy)•− ligand [Ni(II)(tpy [Ni(II)(tpy•−)](n−1)+•−)]((nA−1)+), (A), or asor aas monovalent a monovalent nickel nickel [Ni(I)(tpy)] [Ni(I)(tpy)](n−(1)+n−1)+complex complex ((B)B) anionic (tpy)•− ligand [Ni(II)(tpy•−)](n−1)+ (A), or as a monovalent nickel [Ni(I)(tpy)](n−1)+ complex (B) (Scheme(Scheme1 1))[ 40[40,41].,41]. (Scheme 1) [40,41].

Scheme 1. Two resonance forms for mono-reduced nickel 2,2′:60′,20′′-terpyridine00 (tpy) complexes. SchemeScheme 1. 1.TwoTwo resonance resonance forms forms for for mono-reduced mono-reduced nickel nickel 2,2 2,2′:6′:6,2′′,2-terpyridine-terpyridine (tpy) (tpy) complexes. complexes. The fundamental studies on the electronic structures of terpyridine nickel complexes have also The fundamental studies on the electronic structuresstructures ofof terpyridineterpyridine nickelnickel complexescomplexes have alsoalso prompted others to explore details of related pybox complexes (pybox = pyridine-2,6-bisoxazolidine). promptedprompted othersothers to explore detailsdetails of related pybox complexes (pybox = pyridine-2,6-b pyridine-2,6-bisoxazolidine).isoxazolidine). Nickel pybox complexes are known to display high activity for alkyl–alkyl cross-coupling reactions, Nickel pybox complexes are known to display high activity for alkyl–alkyl cross-coupling reactions, and can even promote stereoconvergent cross-couplings of racemic alkyl halides [26,42,43]. Recently, and can even promote stereoconvergent cross-couplingscross-couplings of racemic alkyl halides [26,42,43]. [26,42F ,43]. Recently, Recently, Fu et al. found that upon reduction of the nickel(II) complex [Ni(i-Pr-pybox)(Ph)](BArF 4), the reduced Fu et al. found that upon reduct reductionion of the nickel(II) complex [Ni(ii-Pr-pybox)(Ph)](BAr-Pr-pybox)(Ph)](BArF4), the reduced species [Ni(i-Pr-pybox)(Ph)]• 2 could be isolated and characterized (Scheme 2) [26].4 Spectroscopic species [Ni([Ni(ii-Pr-pybox)(Ph)]-Pr-pybox)(Ph)]• 2 could be isolatedisolated andand characterizedcharacterized (Scheme(Scheme2 2))[ [26].26]. SpectroscopicSpectroscopic studies of this complex in analogy to what was previously observed for related terpyridine complexes studies of of this this complex complex in analogy in analogy to what to was what pr waseviously previously observed observed for related for terpyridine related terpyridine complexes [44] indicated that the proper electronic structure consisted of a nickel(II) center bearing a reduced complexes[44] indicated [44 ]that indicated the proper that theelectronic proper structure electronic consisted structure of consisted a nickel(II) of a center nickel(II) bearing center a reduced bearing pybox ligand [Ni(II)(i-Pr-pybox•−)(Ph)] [26]. apybox reduced ligand pybox [Ni(II)( ligandi-Pr-pybox [Ni(II)(i•-Pr-pybox−)(Ph)] [26].•− )(Ph)] [26].

Scheme 2. Reduction of [Ni(II)(i-Pr-pybox)(Ph)])BArF4) (1) using pentamethyl cobaltocene, adapted Scheme 2. Reduction of [Ni(II)(i-Pr-pybox)(Ph)])BArFF4) (1) using pentamethyl cobaltocene, adapted Schemefrom ref. 2. [26].Reduction Copyright of [Ni(II)( (2014)i-Pr-pybox)(Ph)])BAr American Chemical4 )(Society.1) using Further pentamethyl permissions cobaltocene, related adapted to the from ref. [26]. Copyright (2014) American Chemical Society. Further permissions related to the frommaterial ref. [excerpted26]. Copyright must (2014)be directed American to the Chemical ACS. Society. Further permissions related to the material excerptedmaterial excerpted must be directedmust be todirected the ACS. to the ACS. 2. Nickel–tpy Catalyzed C–C Cross-Coupling 2. Nickel–tpy CatalyzedCatalyzed C–C Cross-Coupling The simple organometallic low-valent species [Ni(tpy)(Me)] was reported to catalyze the cross- The simple organometallic low-valent species [Ni(tpy)(Me)] was reported to catalyze the cross- couplingThe simpleof unactivated organometallic alkyl halides low-valent with species primary [Ni(tpy)(Me)] alkylzinc reagents was reported about to 10 catalyze years ago the coupling of unactivated alkyl halides with primary alkylzinc reagents about 10 years ago cross-coupling(Scheme 3) [44–47]. of unactivated alkyl halides with primary alkylzinc reagents about 10 years ago (Scheme(Scheme3 3))[ 44[44–47].–47].

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Scheme 3. Negishi-type cross-coupling of C–C bonds with [Ni(tpy)(Me)] as catalyst, adopted from Scheme 3. Negishi-typeNegishi-type cross-couplingcross-coupling ofof C–CC–C bondsbonds withwith [Ni(tpy)(Me)][Ni(tpy)(Me)] asas catalyst,catalyst, adoptedadopted fromfrom Schemeref.Scheme [45]. 3. Negishi-type 3. Negishi-type cross-coupling cross-coupling of C–C of C–C bonds bonds with with [Ni(tpy)(Me)] [Ni(tpy)(Me)] as catalyst, as catalyst, adopted adopted from ref.from [45 ]. ref.ref. [45].[45]. ref. [45]. When propargylic secondary bromides and chlorides were used as the electrophiles, secondary When propargylic secondarysecondary bromides andand chlorideschloridess were were used as as the the electrophiles, electrophiles, secondary secondary alkylzincWhen reagents propargylic could secondary be coupled, bromides producing and chloridetwo adjacents were secondary used as the carbon electrophiles, centers secondaryusing the alkylzinc reagents couldcould bebe coupled,coupled, producingproducing twotwo adjacentadjacent secondarysecondary carboncarbon centerscenters usingusing thethe nickel–terpyridinealkylzinc reagents catalystcould be system coupled, (Scheme producing 4) [48]. two adjacent secondary carbon centers using the nickel–terpyridine catalystcatalyst systemsystem (Scheme(Scheme4 4))[ 48[48].]. nickel–terpyridine catalyst system (Scheme 4) [48].

Scheme 4. Cross-coupling of a secondary alkyl bromide with a secondary alkylzinc reagent, adopted Scheme 4. Cross-coupling of a secondary alkyl bromide with a secondary alkylzinc reagent, adopted SchemeScheme 4. Cross-coupling 4. Cross-coupling of a secondary of a secondary alkylbromide alkyl bromide with a secondarywith a secondary alkylzinc alkylzi reagent,nc reagent, adopted adopted from ref. [48]. fromScheme ref. 4.[48]. Cross-coupling of a secondary alkyl bromide with a secondary alkylzinc reagent, adopted fromfrom ref.ref. [48].[48]. from ref. [48]. 2.1. First Mechanistic Investigations on the [[Ni((tpytpy)()(Me)])] Catalyst 2.1. First Mechanistic Investigations on the [[Ni((tpytpy)()(Me)])] CatalystCatalyst 2.1. FirstThe Mechanistic reaction of aInvestigations nickel(0)nickel(0) precursor on the [Ni with(tpy MeI)(Me in)] Catalyst the presence of aa tpytpy derivativederivative providedprovided the The reaction of a nickel(0) precursor with MeI in the presence of a tpy derivative provided the cationicThe methylnickel(II) reactionmethylnickel(II) of a nickel(0) complex complex precursor3 (Scheme 3 (Scheme with5). In MeI a related5). in theIn reaction, presencea related the of treatmentareaction, tpy derivative ofthe [Ni(tmeda)(Me) treatmentprovided theof2] cationiccationic methylnickel(II)methylnickel(II) complexcomplex 3 (Scheme(Scheme 5).5). InIn aa relatedrelated reaction,reaction, thethe treatmenttreatment ofof with[Ni(tmeda)(Me) the same terpyridine2] with the ligand same gaveterpyridine the neutral ligand complex gave the4 [45 neutral,46]. Reaction complex of 4 the [45,46]. cationic Reaction nickel(II) of cationic methylnickel(II)2 complex 3 (Scheme 5). In a related reaction, the treatment of [Ni(tmeda)(Me)[Ni(tmeda)(Me)2]] withwith thethe samesame terpyridineterpyridine ligandligand gavegave thethe neutralneutral complexcomplex 4 [45,46].[45,46]. ReactionReaction ofof complexthe[Ni(tmeda)(Me) cationic3 with nickel(II)2 an] with alkylzinc complex the same reagent 3 with terpyridine gavean alkylzinc only ligand a 8% reagent gave yield thegave of theneutral only cross-coupled a complex 8% yield 4 ofproduct [45,46]. the cross-coupled (SchemeReaction5 of). thethe cationiccationic nickel(II)nickel(II) complexcomplex 3 withwith anan alkylzincalkylzinc reagentreagent gavegave onlyonly aa 8%8% yieldyield ofof thethe cross-coupledcross-coupled Onproductthe thecationic other (Scheme nickel(II) hand, 5). the complexOn neutral the other3 methylnickel(I)with hand, an alkylzinc the neutral complex reagent methylnickel(I)4 gavereacted only with a 8% complexn yield-heptyliodide of 4 the reacted cross-coupled to givewith the n- product (Scheme 5). On the other hand, the neutral methylnickel(I) complex 4 reactedreacted withwith n-- correspondingheptyliodideproduct (Scheme to coupling give 5). theOn product correspondingthe other in 90% hand, yield coupling the (Scheme neutral product6). Thesemethylnickel(I) in results 90% yield suggest complex (Scheme that the4 6).reacted Ni-tpy Thesecatalytic withresults n- heptyliodide to give the corresponding coupling product in 90% yield (Scheme 6). These results systemsuggestheptyliodide does that notthe to Ni-tpy includegive the catalytic a simplecorresponding system Ni(0)/Ni(II) does coupling not mechanism include product a comprising simple in 90% Ni(0)/Ni(II) yield of an oxidative(Scheme mechanism 6). addition These comprising ofresults alkyl suggestsuggest thatthat thethe Ni-tpyNi-tpy catalyticcatalytic systemsystem doesdoes notnot includeinclude aa simplesimple Ni(0)/Ni(II)Ni(0)/Ni(II) mechanismmechanism comprisingcomprising halidesofsuggest an oxidativeat that nickel(0), the addition Ni-tpy transmetalation catalytic of alkyl systemhalides of a doesat nickel nickel(0), not alkyl include halidetransmetalation a simple complex, Ni(0)/Ni(II) andof a nickel lastly, mechanism reductivealkyl halide eliminationcomprising complex, of an oxidative addition of alkyl halides at nickel(0), transmetalation of a nickel alkyl halide complex, ofandof carbon–carbonan lastly, oxidative reductive addition bonds elimination (Schemeof alkyl halides 7of). carbon–carbon at nickel(0), bonds transmetalation (Scheme 7). of a nickel alkyl halide complex, and lastly, reductive elimination of carbon–carbon bonds (Scheme 7). and lastly, reductive elimination of carbon–carbon bonds (Scheme 7).

Scheme 5. Control experiment of a cationic nickel(II) complex 1 with an alkylzinc reagent [[46].46]. Scheme 5. ControlControl experimentexperiment ofof aa cationiccationic nickel(II)nickel(II) complexcomplex 11 withwith anan alkylzincalkylzinc reagentreagent [46].[46]. Scheme 5. Control experiment of a cationic nickel(II) complex 1 with an alkylzinc reagent [46].

Scheme 6. Intermediacy of a neutral nickel(I) complex 2 with an alkyl iodide, adopted from ref. [46]. SchemeScheme 6. 6. IntermediacyIntermediacyIntermediacy ofof aa neutralneutral nickel(I)nickel(I) complexcomplex 22 withwith anan alkylalkyl iodide, iodide, adopted adopted from from ref. ref. [46]. [[46].46]. Scheme 6. Intermediacy of a neutral nickel(I) complex 2 with an alkyl iodide, adopted from ref. [46].

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Scheme 7. Proposed catalytic cycle of the nickel–tpy ca catalytictalytic system in an alkyl–alkyl cross-coupling reaction, adopted from ref. [[46],46], the oxidation states shown in red were the results of studies outlined in Sections 2.2 and 2.32.3..

A plausible catalytic cycle for the nickel–terpyridine-catalyzed cross-coupling reaction of alkyl A plausible catalytic cycle for the nickel–terpyridine-catalyzed cross-coupling reaction of alkyl iodides with alkylzinc reagents is shown in Scheme 7. The in situ-formed low-valent nickel complex iodides with alkylzinc reagents is shown in Scheme7. The in situ-formed low-valent nickel complex 5 5 undergoes transmetalation with organozinc reagents to give the corresponding organonickel undergoes transmetalation with organozinc reagents to give the corresponding organonickel species 6. species 6. Single electron transfer (SET) from the nickel complex 6 to alkyl iodides forms alkyl radicals Single electron transfer (SET) from the nickel complex 6 to alkyl iodides forms alkyl radicals and the and the nickel(II) complex 7. These radicals add oxidatively [46,49] to the Ni(II) complex, giving a nickel(II) complex 7. These radicals add oxidatively [46,49] to the Ni(II) complex, giving a nickel(III) nickel(III) species 8 as an intermediate addition, which then undergoes reductive elimination to yield species 8 as an intermediate addition, which then undergoes reductive elimination to yield coupling coupling products with generation of a nickel(I) species 5 to complete the catalytic cycle. products with generation of a nickel(I) species 5 to complete the catalytic cycle. Studies were performed on [Ni(tpy)(Me)] in order to determine its electronic structure. Electron Studies were performed on [Ni(tpy)(Me)] in order to determine its electronic structure. Electron paramagnetic resonance (EPR) spectroscopy, in combination with density functional theory (DFT) paramagnetic resonance (EPR) spectroscopy, in combination with density functional theory (DFT) calculations on the electronic nature of the neutral complex [Ni(tpy)(Me)], revealed that the species calculations on the electronic nature of the neutral complex [Ni(tpy)(Me)], revealed that the species should be described as divalent nickel bound to a radical anionic (tpy)•− ligand [Ni(II)(tpy•−)(Me)], should be described as divalent nickel bound to a radical anionic (tpy)•− ligand [Ni(II)(tpy•−)(Me)], rather than the monovalent resonance structure [Ni(I)(tpy)(Me)] (Scheme 1) [44,47]. rather than the monovalent resonance structure [Ni(I)(tpy)(Me)] (Scheme1)[44,47].

2.2. Investigations of the Low-Valent Systems [Ni(tpy)(aryl)], [Ni(tpy)I], [Ni(tpy)Br], and [Ni(tpy)Cl] After initial characterization of the reduced/low valent [Ni(tpy)(Me)] complex, further After initial characterization of the reduced/low valent [Ni(tpy)(Me)] complex, further mechanistic work was devoted to understanding the nature of the so far proposed reduced species mechanistic work was devoted to understanding the nature of the so far proposed reduced species [Ni(tpy)I] [45–47], [Ni(tpy)Br] [50–53], and aryl derivatives [Ni(tpy)(aryl)]. Cationic complexes [Ni(tpy)I] [45–47], [Ni(tpy)Br] [50–53], and aryl derivatives [Ni(tpy)(aryl)]. Cationic complexes [Ni(II)(R-tpy)(aryl)]+ (R-tpy = substituted terpyridines) have been prepared from suitable precursors [Ni(II)(R-tpy)(aryl)]+ (R-tpy = substituted terpyridines) have been prepared from suitable precursors and studied structurally and spectroscopically in detail [51]. Their completely reversible first and studied structurally and spectroscopically in detail [51]. Their completely reversible first electrochemical reductions are centered at potentials near −1.5 V, and by using spectroelectrochemical electrochemical reductions are centered at potentials near −1.5 V, and by using spectroelectrochemical (UV–Vis and EPR) methods, the reduced species [Ni(tpy)(aryl)] could be generated and studied. As (UV–Vis and EPR) methods, the reduced species [Ni(tpy)(aryl)] could be generated and studied. found before for the Me derivate [Ni(tpy)(Me)], the aryl complexes [Ni(tpy)(aryl)] were best As found before for the Me derivate [Ni(tpy)(Me)], the aryl complexes [Ni(tpy)(aryl)] were best described as divalent nickel bound to a radical anionic (tpy)•− ligand [Ni(II)(tpy•−)(aryl)], and not as described as divalent nickel bound to a radical anionic (tpy)•− ligand [Ni(II)(tpy•−)(aryl)], and not as [Ni(I)(tpy)(aryl)] [50,51]. [Ni(I)(tpy)(aryl)] [50,51]. [Ni(tpy)Br] was prepared in a comproportionation reaction from [Ni(II)(dme)Br2] (dme = 1,2- [Ni(tpy)Br] was prepared in a comproportionation reaction from [Ni(II)(dme)Br2] (dme = 1,2- dimethoxy-ethane) and [Ni(0)(COD)2] (COD = 1,5-cyclooctadiene) in the presence of the tpy ligand, dimethoxy-ethane) and [Ni(0)(COD)2] (COD = 1,5-cyclooctadiene) in the presence of the tpy ligand, and its structure was studied in detail. EPR and quantum chemical calculations clearly point to a and its structure was studied in detail. EPR and quantum chemical calculations clearly point to complex containing monovalent nickel [Ni(I)(tpy)Br] and a neutral tpy ligand, in marked contrast to a complex containing monovalent nickel [Ni(I)(tpy)Br] and a neutral tpy ligand, in marked contrast to the methyl and aryl derivatives, as described above [52]. the methyl and aryl derivatives, as described above [52]. The low-temperature solid-state powder EPR spectrum of [Ni(tpy)Br]• exhibits an axial signal The low-temperature solid-state powder EPR spectrum of [Ni(tpy)Br]• exhibits an axial signal with g║= 2.256 and g⊥ = 2.091 consistent with a metal-centered d ground state [52]. In DMF with gk= 2.256 and g⊥ = 2.091 consistent with a metal-centered dx2−y2 ground state [52]. In DMF solution, an isotropic signal can be observed with giso = 2.139. Thus, both in the crystaline form and in solution, an isotropic signal can be observed with giso = 2.139. Thus, both in the crystaline form and • solution,in solution, signals signals for fora radical a radical with with substantial substantial metal metal character character are areobserved observed for for[Ni(tpy)Br] [Ni(tpy)Br] [52].• [52 In]. contrast to this, [Ni(tpy)(Me)]•, exhibits an isotropic signal at 298 K with giso = 2.021, and a rhombic spectrum with g1 = 2.056, g2 = 2.021, and g3 = 1.999 at 77 K in a glassy frozen solution [44]. The spectra

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• InorganicsIn contrast 2018 to, 6 this,, x FOR [Ni(tpy)(Me)] PEER REVIEW , exhibits an isotropic signal at 298 K with giso = 2.021, and a rhombic5 of 18 spectrum with g1 = 2.056, g2 = 2.021, and g3 = 1.999 at 77 K in a glassy frozen solution [44]. The spectra of • of[Ni(tpy)(Mes)] [Ni(tpy)(Mes)](Mes• (Mes = 2,4,6-trimethylphenyl) = 2,4,6-trimethylphenyl) are veryare very similar similar with gwithiso = g 2.0006,iso = 2.0006,g1 = 2.009, g1 = 2.009,g2 = 2.002 g2 =, 2.002,and g3 and= 1.991 g3 = at1.991 298 at K 298 or 110 K or K, 110 respectively K, respectively (Table (Table1). 1).

Table 1. Selected EPR data of reduced nickelnickel tpytpy complexescomplexes aa.

Complex giso (298 K) gav (LT)b b g1 (LT) g2 (LT) g3 (LT) ∆g (LT)c c Complex giso (298 K) gav (LT) g1 (LT) g2 (LT) g3 (LT) ∆g (LT) • [Ni(tpy)Br][Ni(tpy)Br]• 2.139 2.139 2.146 2.146 2.2562.256 2.0912.091 2.0912.091 0.1650.165 [Ni(tpy)(Me)][Ni(tpy)(Me)]•• 2.021 2.021 2.025 2.025 2.0562.056 2.0212.021 1.9991.999 0.0260.026 [Ni(tpy)(Mes)][Ni(tpy)(Mes)]•• 2.0006 2.0006 2.0006 2.0006 2.0092.009 2.0022.002 1.9911.991 0.0180.018 • d [(([((t-Bu)t-Bu)3tpy)Ni(Xyl)]3tpy)Ni(Xyl)] • d 2.00062.0006 2.0007 2.0007 2.0082.008 2.0032.003 1.9911.991 0.0170.017 a b c a From references references [51 [51]] and and [52], [52], LT measurement LT measurement at 77 K or at 110 77 K,K respectively;or 110 K, respectively;gav = (g1 + g 2b +ggav3 )/3;= (g1 ∆+g g=2 +g 1g−3)/3;g3; d Xyl = 2,6-dimethylphenyl. c ∆g = g1 − g3; d Xyl = 2,6-dimethylphenyl.

Two parameters in Table1 1 are are most most revealing revealing for for the the charactercharacter ofof thethe complexcomplex radicals.radicals. BothBoth thethe • high gisoiso valuevalue and and the the high high g ganisotropyanisotropy ∆g∆ gobservedobserved for for [Ni(tpy)Br] [Ni(tpy)Br]• characterizecharacterize this this complex complex as havingas having the unpaired the unpaired electron electron largely largely centered centered at the metal at the in metal contrast in to contrast the Me toand the aryl Me derivatives and aryl [50,51].derivatives [50,51]. Recently, the the Cl Cl derivative derivative [Ni(tpy)Cl] [Ni(tpy)Cl] was was added added to this to this list. list. The Themolecular molecular structure structure from fromXRD XRDclearly clearly points points to a toNi(I) a Ni(I) description description [Ni(I)(tpy)Cl] [Ni(I)(tpy)Cl] with with neutral neutral tpy, tpy, as asfor for the the Br Br derivative derivative [41]. [41]. Unfortunately, EPR spectra were not reported. For [Ni(tpy)I], quantum chemical calculations show similar electron distribution as as that found for the Br derivative [52]. [52]. In In summary, it can be assumed that the largely superior superior σσ-donor-donor character character of of the the Me Me or or aryl aryl coligands coligands raise raise the the ddx2−y2 orbital at the nickel above the lowest π* orbitalsorbitals of thethe tpytpy ligand,ligand, andand reductionreduction leadsleads toto aa (d(d88)(π**11) configuredconfigured 88 1 species, while for the weaker ligands Cl, Br, andand I,I, aa (d(d )(d)(dx2−y2 )) configuration configuration is is found found [41,52]. [41,52].

+ 2.3. Studies on the Trivalent Species [[Ni(tpy)()(CnnFm))22]+](C(CnFnmF m= CF= CF3 or3 orC2F C52) F5) Vicic showed that trifluoromethyltrifluoromethyl ligands can support the five-coordinate five-coordinate nickel(II) species species 99,, which can be chemically oxidized with [ferrocenium][PF 66] to generate generate the the transient transient nickel(III) nickel(III) species species 10 (Scheme8 8))[ [54].54]. Once formed, however, 10 undergoes a reductive homolysis of a trifluoromethyltrifluoromethyl ligand to afford the cationiccationic nickel(II)nickel(II) species 11. Spectroelectrochemical EPR studies supported the intermediacy ofof10 10,, but but its its short-lived short-lived nature nature precluded precluded any any fundamental fundamental studies studies of its of reactivity. its reactivity. Quite • Quiteclearly, clearly, a CF3 aradical •CF3 radical is cleaved is cleaved from from10, which 10, which was shown was shown through through spin-trapping spin-trapping studies studies using using PBN (PBNN-t-Bu- (N-αt-Bu--phenylnitrone)α-phenylnitrone) [54]. [54].

SchemeScheme 8. OxidationOxidation of of the the isolated isolated complex and follo follow-upw-up reactions, adopted from ref. [[54].54].

+ 2.4. Electrochemistry and Character of the Trivalent Species [Ni(tpy)(C4F8)] + 2.4. Electrochemistry and Character of the Trivalent Species [Ni(tpy)(C4F8)] It was found that the addition of terpyridine to [Ni(MeCN)2(C4F8)] 12 led cleanly to the formation It was found that the addition of terpyridine to [Ni(MeCN)2(C4F8)] 12 led cleanly to the formation of [Ni(tpy)(C4F8)] 14 (Scheme 9) [55]. Surprisingly, in the solid state, the terpyridine ligand in 14 of [Ni(tpy)(C4F8)] 14 (Scheme9)[ 55]. Surprisingly, in the solid state, the terpyridine ligand in 14 coordinates to nickel in a η2-fashion. However, solutions of 14 are paramagnetic, supporting an coordinates to nickel in a η2-fashion. However, solutions of 14 are paramagnetic, supporting an equilibrium with the η3-binding mode. Interestingly, when 0.5 equiv. of terpyridine is added to 12, the bimetallic species 13 can be isolated reproducibly, which is a testament to the lability of the acetonitrile ligands. Terpyridine is known to bind to more than one metal center [56–61], but 13 represented the first such adduct with nickel.

Inorganics 2018, 6, 18 6 of 18

equilibrium with the η3-binding mode. Interestingly, when 0.5 equiv. of terpyridine is added to 12, the bimetallic species 13 can be isolated reproducibly, which is a testament to the lability of the acetonitrile ligands. Terpyridine is known to bind to more than one metal center [56–61], but 13 Inorganics 2018, 6, x FOR PEER REVIEW 6 of 18 representedInorganics 2018,the 6, x firstFOR PEER such REVIEW adduct with nickel. 6 of 18

Scheme 9. Synthesis of tpy nickel complexes bearing a readily attached [C4F82−] ligand, adopted from Scheme 9. Synthesis of tpy nickel complexes bearing a readily attached [C2−4F82−] ligand, adopted from Scheme 9. Synthesis of tpy nickel complexes bearing a readily attached [C4F8 ] ligand, adopted from ref. [55]. ref.ref. [55].[55]. 3 Oxidation of the [Ni(tpy)(C[Ni(tpy)(C44F8)])] complex complex 14 was explored. The η3-binding-binding of the terpyridineterpyridine Oxidation of the [Ni(tpy)(C4F8)] complex 14 was explored. The η3-binding of the terpyridine ligand generates a high-spinhigh-spin nickelnickel complexcomplex which,which, uponupon additionaddition ofof Ag[BFAg[BF44], affords the ligand generates a high-spin nickel complex which, upon addition of Ag[BF4], affords the bluish/purple nickel(III) species 15 (Scheme 10). Unlike complex 10, which readily loses bluish/purple nickel(III) species 15 (Scheme(Scheme 10).10). UnlikeUnlike complexcomplex 10,, whichwhich readilyreadily losesloses aa a trifluoromethyl radical, there is no evidence that 15 loses a perfluoroalkyl radical, even upon trifluoromethyltrifluoromethyl radical,radical, therethere isis nono evidenceevidence thatthat 15 losesloses aa perfluoroalkylperfluoroalkyl radical,radical, eveneven uponupon standingstanding standing for hours in MeCN solution. The stability of 15 facilitated its characterization by X-ray forfor hourshours inin MeCNMeCN solution.solution. TheThe stabilitystability ofof 15 facilitatedfacilitated itsits characterizationcharacterization byby X-rayX-ray crystallography, which identified the addition of an acetonitrile to the coordination sphere. crystallography, which identified the addition of anan acetonitrileacetonitrile toto thethe coordinationcoordination sphere.sphere.

Scheme 10. StableStable terpyridineterpyridine perfluoroalkylated perfluoroalkylatedperfluoroalkylated Ni(III) Ni(III) species, adopted fromfrom ref.ref. [[55].[55].55].

Variable temperature EPR spectra and magnetic moment data were obtained for 15, confirming Variable temperaturetemperature EPREPR spectraspectra andand magneticmagnetic momentmoment data were obtained for 15,, confirmingconfirming its unpaired electron. Cyclic voltammetry experiments on 6 identifies a redox potential of +1.34 V for itsits unpairedunpaired electron.electron. CyclicCyclic voltammetryvoltammetry experimentsexperiments onon 66 identifiesidentifies aa redoxredox potentialpotential ofof +1.34+1.34 V for the Ni(III)/Ni(IV) couple. So, the [C4F82−] ligand is exceptionally suitable for stabilizing high valent the Ni(III)/Ni(IV) couple. So, the [C4F822−−] ligand is exceptionally suitable for stabilizing high valent the Ni(III)/Ni(IV) couple. So, the [C4F8 ] ligand is exceptionally suitable for stabilizing high valent nickel. Unlike [Ni(tpy)(CF3)2]+ complexes, which readily lose [•CF3] radicals [54], the [Ni(tpy)(C4F8)]+ nickel. Unlike [Ni(tpy)(CF3)2]+ complexes, which readily lose [••CF3] radicals [54], the [Ni(tpy)(C4F8)]++ nickel. Unlike [Ni(tpy)(CF3)2] complexes, which readily lose [ CF3] radicals [54], the [Ni(tpy)(C4F8)] analogue is solution stable, and can also be isolated in the solid state. The unique stability afforded analogue is solution2− stable, and can also be isolated in the solid state. The unique stability afforded by by the [C4F82−] ligand now provides a family of related fluoroalkyl nickel complexes spanning the +II theby the [C [CF 24F−8] ligand] ligand now now provides provides a familya family of of related related fluoroalkyl fluoroalkyl nickel nickel complexes complexes spanning spanning the the +II +II to to +IV4 oxidation8 states for future fundamental studies [55]. +IVto +IV oxidation oxidation states states for for future future fundamental fundamental studies studies [55 ].[55]. 2.5. Further Ni–tpy Catalyzed C–C Cross-Coupling Reactions 2.5. Further Ni–tpy Catalyzed C–C Cross-Coupling ReactionsReactions In the last 10 years, nickel–tpy systems have been used successfully for a number of C–C cross- InIn the the last last 10 10 years, years, nickel–tpy nickel–tpy systems systems have have been been used used successfully successfully for a fornumber a number of C–C of cross- C–C couplings and related reactions. cross-couplingscouplings and related and related reactions. reactions. Recently, Han et al. reported on Ni-catalyzed reductive cross-coupling reactions between two Recently, HanHan etet al.al. reportedreported onon Ni-catalyzedNi-catalyzed reductivereductive cross-couplingcross-coupling reactionsreactions betweenbetween twotwo electrophiles, amides and aryl iodides [62]. The nickel–tpy catalysts system turned out to be superior electrophiles, amidesamides and and aryl aryl iodides iodides [ 62[62].]. The The nickel–tpy nickel–tpy catalysts catalysts system system turned turned out out to to be be superior superior to to catalysts containing the bidentate ligands 2,2′-bipyridine, 2,2′-biquinoline, 1,10-phenanthroline, catalyststo catalysts containing containing the bidentatethe bidentate ligands ligands 2,20-bipyridine, 2,2′-bipyridine, 2,20-biquinoline, 2,2′-biquinoline, 1,10-phenanthroline, 1,10-phenanthroline, and its and its 5,6-dione. In a mechanistic proposal (Scheme 11), the starting NiI2 is reduced in the presence 5,6-dione.and its 5,6-dione. In a mechanistic In a mechanistic proposal proposal (Scheme (Scheme11), the starting 11), the NiIstarting2 is reduced NiI2 is inreduced the presence in the ofpresence tpy (L) of tpy (L) to [Ni(0)(L)] by the added metallic Zn, and the encompassing C–N bond activation of the toof [Ni(0)(L)]tpy (L) to [Ni(0)(L)] by the added by the metallic added Zn, metallic and the Zn, encompassing and the encompassing C–N bond C–N activation bond activation of the amide of the is amide is described as an oxidative addition to yield an [Ni(II)(L)(acyl) (amide)] species 16. This describedamide is described as an oxidative as an addition oxidative to yieldaddition an [Ni(II)(L)(acyl) to yield an [Ni(II)(L)(acyl) (amide)] species (amide)]16. This species complex 16 reacts. This complex reacts with an aryl radical stemming from the aryl iodide substrate, and forms the withcomplex an aryl reacts radical with stemming an aryl fromradical the stemming aryl iodide from substrate, the aryl and iodide forms thesubstrate, [Ni(III)(L)(acyl)(amide) and forms the [Ni(III)(L)(acyl)(amide) (aryl)] species 17 through a radical oxidative addition. This species cleaves (aryl)][Ni(III)(L)(acyl)(amide) species 17 through (aryl)] a radical species oxidative 17 through addition. a radical This species oxidative cleaves addition. the C–C This coupling species product, cleaves thethe C–CC–C couplingcoupling product,product, leavingleaving anan [Ni(I)(L)(amide)][Ni(I)(L)(amide)] complexcomplex 18.. TheThe Ni(I)Ni(I) speciesspecies reactsreacts withwith arylaryl iodideiodide toto produceproduce thethe aforementionedaforementioned arylaryl radical, and the resulting [Ni(II)(L)(amide)I] 19 isis subsequently reduced to [Ni(0)(L)] by metallic Zn to restartrestart thethe cycle.cycle. InIn additionaddition toto thethe productionproduction of the aryl radical through complex 18,, 16 couldcould alsoalso reactreact withwith Ph-IPh-I toto yieldyield thisthis radicalradical inin anan initiationinitiation stepstep formingforming anotheranother Ni(III)Ni(III) speciesspecies 20.. AsAs aa proofproof forfor thethe radicalradical oxidativeoxidative additionaddition reaction, the addition of TEMPO (TEMPO = 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) inhibited the

Inorganics 2018, 6, 18 7 of 18 leaving an [Ni(I)(L)(amide)] complex 18. The Ni(I) species reacts with aryl iodide to produce the aforementioned aryl radical, and the resulting [Ni(II)(L)(amide)I] 19 is subsequently reduced to [Ni(0)(L)] by metallic Zn to restart the cycle. In addition to the production of the aryl radical through complex 18, 16 could also react with Ph-I to yield this radical in an initiation step forming another Ni(III) species 20. As a proof for the radical oxidative addition reaction, the addition of TEMPO Inorganics 2018, 6, x FOR PEER REVIEW 7 of 18 (TEMPO = 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) inhibited the reaction and yielded the acyl–TEMPO adduct.reaction Furthermore, and yielded [Ni(COD) the acyl–TEMPO2] was also adduct. successfully Furthermore, used [Ni(COD) as the catalyst2] was also precursor, successfully underpinning used the roleas the of thecatalyst metallic precursor, zinc [underpinning62]. the role of the metallic zinc [62].

SchemeScheme 11. Proposed 11. Proposed reaction reaction mechanisms mechanismsfor for thethe reductive cross-coupling cross-coupling reaction reaction of amides of amides and and aryl iodides, adapted with permission from [62]. Copyright (2017) American Chemical Society. aryl iodides, adapted with permission from [62]. Copyright (2017) American Chemical Society.

An earlier study using [Ni(glyme)Cl2] (glyme = ethyleneglycoldimethylether) and various bi- Anand earlier tridentate study aromatic using [Ni(glyme)Climine ligands2 as] (glyme catalyst =s ethyleneglycoldimethylether)systems for the Negishi-type cross-coupling and various bi-of and tridentatesecondary aromatic alkylzinc imine halides ligands and as aryl/heteroaryl catalysts systems iodides for also the found Negishi-type terpyridines cross-coupling superior to bidentate of secondary alkylzincbpy or halides phen ligands, and aryl/heteroaryl but also slightly iodidesbetter than also the found tridentate terpyridines 2,6-di(1H-pyrazol-1-yl)pyridine, superior to bidentate with bpy or phenthe ligands, exception but of also the slightly4′-NO2– and better 4′-CF than3–tpy the derivatives tridentate [63]. 2,6-di(1 This indicatesH-pyrazol-1-yl)pyridine, that the redox potentials with the of the various Ni–tpy species are important, which is in line with our observations. exception of the 40-NO – and 40-CF –tpy derivatives [63]. This indicates that the redox potentials of Elemental manganese2 has been3 used in effective Ni-catalyzed reductive dimerization of alkyl the various Ni–tpy species are important, which is in line with our observations. halides, tosylates, and trifluoroacetates applying a [Ni(glyme)Cl2]/4,4′,4′′-(t-Bu)3tpy catalyst system Elemental[64]. Interestingly, manganese the addition has of been NaI usedpromotes in the effective reaction, Ni-catalyzed but the reason reductive for this is not dimerization yet clear. of 0 00 alkyl halides,An interesting tosylates, andNi-catalyzed trifluoroacetates reductive applyingCsp3–Csp3 ahomocoupling [Ni(glyme)Cl using2]/4,4 elemental,4 -(t-Bu) zinc3tpy was catalyst systemreported [64]. Interestingly,to yield an alkyl the chain addition axle [2]rotaxane of NaI promotes[65]. The authors the reaction, firstly optimized but the the reason homocoupling for this is not yet clear.reaction of 1-bromo-3-phenoxypropane, and found the Ni-(R,R)-Ph-pybox system slightly superior Anto Ni–tpy interesting (pybox Ni-catalyzed = pyridine-2,6-bisoxazolidine, reductive Csp3–C seesp3 alsohomocoupling introduction). using Then, elemental they coordinated zinc was Ni(II) reported to yieldto a pybox-containing an alkyl chain macrocycle axle [2]rotaxane (*pybox), [65 and]. added The authors1-bromo-3-O–C firstly6H optimized4–C(t-Bu–Ph) the3-propyl. homocoupling With reactionthe addition of 1-bromo-3-phenoxypropane, of Zn, the thread (t-Bu–Ph) and3C–C found6H4–O–(CH the Ni-(2)6–O–CR,R6)-Ph-pyboxH4–C(t-Bu–Ph system3) was formed slightly at superior the catalyst Ni center, and interlocked with the macrocycle. Mechanistically, it was proposed that Zn to Ni–tpy (pybox = pyridine-2,6-bisoxazolidine, see also introduction). Then, they coordinated reduces the initial Ni(II) complex to Ni(0), which undergoes oxidative addition to a Ni(II) to a pybox-containing macrocycle (*pybox), and added 1-bromo-3-O–C H –C(t-Bu–Ph) -propyl. [Ni(II)(*pybox)Br(propyl-OR)] species. This is reduced to [Ni(I)(*pybox)(propyl-OR)],6 4 cleaving3 With the addition of Zn, the thread (t-Bu–Ph) C–C H –O–(CH ) –O–C H –C(t-Bu–Ph ) was formed bromide. This complex undergoes oxidative3 addi6tion,4 yielding2 a6 [Ni(III)6 (*pybox)Br((propyl-OR)4 3 2], at thewhich catalyst subsequently Ni center, cleaves and interlockedthe C–C coupling with theproduct, macrocycle. the [2]rotaxane Mechanistically, and a Ni(I) itspecies was proposedin a thatreductive Zn reduces elimination. the initial This Ni(II) not further complex described to Ni(0), Ni(I) whichspecies undergoesis reduced by oxidative Zn to Ni(0) addition and to a [Ni(II)(*pybox)Br(propyl-OR)]coordinates to *pybox to restart species.the cycle.[65] This is reduced to [Ni(I)(*pybox)(propyl-OR)], cleaving bromide.When This complexusing [Ni(tpy)(Py)(MeCN) undergoes oxidative2](PF6) addition,as the catalyst yielding precursor, a [Ni(III)(*pybox)Br((propyl-OR) Vannucci et al. were able to 2], whichperform subsequently efficient cleaves photoredox-assisted the C–C coupling reductive product, coupling the [2]rotaxane of two carbon and aelectrophiles Ni(I) species [66]. in a Their reductive elimination.mechanistic This proposal not further starts describedwith two subsequent Ni(I) species reductions is reduced of the by catalyst Zn to precursor Ni(0) and to coordinatesNi(I)- and to Ni(0)–tpy species by the iridium/TEOA photosystem (TEOA = triethanolamine). The Ni(0) complex *pybox to restart the cycle. [65] undergoes oxidative addition of aryl halides to yield a Ni(II)Br(aryl) complex. At the same time, this When using [Ni(tpy)(Py)(MeCN) ](PF ) as the catalyst precursor, Vannucci et al. were able to perform species is supposed to react with2 alkyl6 halides through single electron transfer (SET) under the efficientformation photoredox-assisted of an alkyl radical reductive and a halide, coupling thus recove of tworing carbon the Ni(I) electrophiles species. Very[ similar66]. Their to the mechanistic report by Han [62], and our findings [44], the next step comprises a radical oxidative addition of an alkyl radical [49] stemming from the second substrate to the Ni(II) complex. The resulting Ni(III) species

Inorganics 2018, 6, 18 8 of 18 proposal starts with two subsequent reductions of the catalyst precursor to Ni(I)– and Ni(0)–tpy species by the iridium/TEOA photosystem (TEOA = triethanolamine). The Ni(0) complex undergoes oxidative addition of aryl halides to yield a Ni(II)Br(aryl) complex. At the same time, this species is supposed to react with alkyl halides through single electron transfer (SET) under the formation of an alkyl radical and a halide, thus recovering the Ni(I) species. Very similar to the report by Han [62], and our findings [44], theInorganics next 2018 step, 6 comprises, x FOR PEER a REVIEW radical oxidative addition of an alkyl radical [49] stemming from the second8 of 18 substrateInorganics 2018 to the, 6, x Ni(II) FOR PEER complex. REVIEW The resulting Ni(III) species undergoes reductive elimination to yield8 of the 18 C–Cundergoes coupling reductive product elimination and a Ni(I) species,to yield which the C–C is then coupling reduced product by the and photosystem a Ni(I) species, to recover which the is active then undergoes reductive elimination to yield the C–C coupling product and a Ni(I) species, which is then Ni(0)reduced species by the [66 photosystem]. to recover the active Ni(0) species [66]. reduced by the photosystem to recover the active Ni(0) species [66]. In 2010, C–H alkylations of 1,3-azoles with alkyl halides were reported by Hirano and MiuraMiura In 2010, C–H alkylations of 1,3-azoles with alkyl halides were reported by Hirano and Miura (Scheme 1212))[ [67].67]. C–H functionalizations using alkyl halides as a coupling partner are relatively rare (Scheme 12) [67]. C–H functionalizations using alkyl halides as a coupling partner are relatively rare compared with using unsaturated reactants, becausebecause of the difficultydifficulty in suppressing the β-hydrogen compared with using unsaturated reactants, because of the difficulty in suppressing the β-hydrogen elimination of the alkylmetalalkylmetal intermediates.intermediates. The nickel–terpyridinenickel-terpyridine combination, combination, in in the presence of elimination of the alkylmetal intermediates. The nickel-terpyridine combination, in the presence of LiOt-Bu, was effective atat suppressingsuppressing suchsuch ββ-hydrogen eliminations.eliminations. LiOt-Bu, was effective at suppressing such β-hydrogen eliminations.

Scheme 12. C–H alkylation of 1,3-azoles and alkyl halideshalides (diglyme(diglyme == bis(2-methoxyethyl)ether), Scheme 12. C–H alkylation of 1,3-azoles and alkyl halides (diglyme = bis(2-methoxyethyl)ether), adopted from ref. [[67].67]. adopted from ref. [67]. Gagné et al. achieved a total synthesis of a family of salmochelins, which are metabolites of the GagnGagnéé et al. al. achieved achieved a atotal total synt synthesishesis of ofa family a family of ofsalmochelins, salmochelins, which which are aremetabolites metabolites of the of ferric-binding siderophores produced by E. coli and S. enterica. A key step in the synthesis was the theferric-binding ferric-binding siderophores siderophores produced produced by E. by coli E. coliandand S. enterica. S. enterica. A key A keystep step in the in thesynthesis synthesis was was the nickel-catalyzed cross-coupling reaction of a bromoglucose derivative with an arylzinc reagent thenickel-catalyzed nickel-catalyzed cross-coupling cross-coupling reaction reaction of ofa abromoglucose bromoglucose derivative derivative with with an an arylzinc reagent employing the terpyridine ligand on nickel [68]. The key coupling reaction proceeded with excellent employing the terpyridine ligand on nickel [[68].68]. The key coupling reaction proceeded with excellent diastereoselectivity in a good yield to give 21 (Scheme 13). The C-aryl glycoside 21 was successfully diastereoselectivity in a good yield to give 21 (Scheme(Scheme 1313).). The C-aryl glycoside 21 was successfully converted into salmochelin S1 23 and S2 24 by subsequent condensation and deprotection processes converted into salmochelin S1 23 and S2 24 by subsequent condensation and deprotection processes (Scheme 13) [69]. Moreover, Gagné also found that the combination of terpyridine and [Ni(0)(COD)2] (Scheme(Scheme 1313))[ [69].69]. Moreover, GagnGagnéé alsoalso foundfound thatthat the combination ofof terpyridine andand [Ni(0)(COD)[Ni(0)(COD)2] (COD = 1,5 cyclooctadiene) can catalyze the reductive coupling of glycosyl bromides with activated2 (COD(COD == 1,5 cyclooctadiene) can catalyze the reductivereductive coupling of glycosyl bromides with activatedactivated alkenes (Scheme 14) [70]. alkenes (Scheme 1414))[ [70].70].

Scheme 13. Total synthesis of salmochelins, adopted from ref. [69]. Scheme 13. Total synthesis of salmochelins, adopted from ref.ref. [[69].69].

Scheme 14. Reductive coupling of glycosyl bromides with activated alkenes, adopted from ref. [70]. Scheme 14. Reductive coupling of glycosyl bromides with activated alkenes, adopted from ref. [70]. 3. Electrochemical Cross-Coupling Using Ni–tpy Systems 3. Electrochemical Cross-Coupling Using Ni–tpy Systems In recent years, the benefits of electrochemistry in nickel-catalysed reductive C–C cross-coupling In recent years, the benefits of electrochemistry in nickel-catalysed reductive C–C cross-coupling reactions has been demonstrated [71–77]. In the same way, electrochemically induced P–C cross- reactions has been demonstrated [71–77]. In the same way, electrochemically induced P–C cross- coupling under the action of Ni complexes with α-diimines has been successful, and was described coupling under the action of Ni complexes with α-diimines has been successful, and was described

Inorganics 2018, 6, x FOR PEER REVIEW 8 of 18 undergoes reductive elimination to yield the C–C coupling product and a Ni(I) species, which is then reduced by the photosystem to recover the active Ni(0) species [66]. In 2010, C–H alkylations of 1,3-azoles with alkyl halides were reported by Hirano and Miura (Scheme 12) [67]. C–H functionalizations using alkyl halides as a coupling partner are relatively rare compared with using unsaturated reactants, because of the difficulty in suppressing the β-hydrogen elimination of the alkylmetal intermediates. The nickel-terpyridine combination, in the presence of LiOt-Bu, was effective at suppressing such β-hydrogen eliminations.

Scheme 12. C–H alkylation of 1,3-azoles and alkyl halides (diglyme = bis(2-methoxyethyl)ether), adopted from ref. [67].

Gagné et al. achieved a total synthesis of a family of salmochelins, which are metabolites of the ferric-binding siderophores produced by E. coli and S. enterica. A key step in the synthesis was the nickel-catalyzed cross-coupling reaction of a bromoglucose derivative with an arylzinc reagent employing the terpyridine ligand on nickel [68]. The key coupling reaction proceeded with excellent diastereoselectivity in a good yield to give 21 (Scheme 13). The C-aryl glycoside 21 was successfully converted into salmochelin S1 23 and S2 24 by subsequent condensation and deprotection processes (Scheme 13) [69]. Moreover, Gagné also found that the combination of terpyridine and [Ni(0)(COD)2] (COD = 1,5 cyclooctadiene) can catalyze the reductive coupling of glycosyl bromides with activated alkenes (Scheme 14) [70].

Inorganics 2018, 6, 18 9 of 18 Scheme 13. Total synthesis of salmochelins, adopted from ref. [69].

Scheme 14. Reductive coupling of glycosyl bromides with activated alkenes, adopted from ref. [[70].70].

3. Electrochemical Cross-Coupling Using Ni–tpy Systems

InorganicsIn recent 2018, 6 , years, x FOR PEERthe benefits benefits REVIEW of electrochemistry electrochemistry in nickel-catalysed redu reductivective C–C cross-coupling 9 of 18 reactions has has been been demonstrated demonstrated [71–77]. [71–77 ].In th Ine same the same way, way,electrochemically electrochemically induced induced P–C cross- P–C cross-couplingcouplingin detail [50,78–83]. under underthe Nickelaction the of actioncomplexes Ni complexes of Ni of complexesα-diimines with α-diimines within highα-diimines oxidation has been hasstatessuccessful, been catalyze successful, and ligand-directed was described and was described C–H functionalization in detail [50 under,78–83 ].electrochemical Nickel complexes conditions of α-diimines [84]. in high oxidation states catalyze ligand-directedThe most important C–H functionalization features of such under electrocatal electrochemicalytic reactions conditions include [84]. the possibility of using stable,The simple, most commercially important features available of precatalysts, such electrocatalytic their compatibility reactions with include organic the halides, possibility and high of usingfunctional stable, group simple, tolerances. commercially In addition, available compared precatalysts, with theirthe more compatibility conventional with organicchemical halides, cross- andcoupling high methods, functional electrochemical group tolerances. reactions In addition, have the compared advantage with of theproviding more conventional an additional chemical driving cross-couplingforce (cathodic potential) methods, electrochemicalfor the reaction processes, reactions havewhich the thus advantage limits the of need providing for additional an additional means drivingof activation, force (cathodicsuch as heating potential) of the for reaction the reaction medium processes, [85–89]. which thus limits the need for additional means of activation, such as heating of the reaction medium [85–89]. 3.1. Electrochemical Properties of Ni–tpy Complexes 3.1. Electrochemical Properties of Ni–tpy Complexes Knowledge of mechanism and key intermediates is important for the rationalization and control of catalyticKnowledge processes. of mechanism In Ni–tpy and catalyzed key intermediates reactions, isthe important nickel oxidation for the rationalization states +IV, +III, and +II, control +I and of 0 catalytichave to processes.be considered In Ni–tpy in combination catalyzed reactions, with neutral the nickel (tpy) oxidationor reduced states forms +IV, of +III, the +II, ligand +I and (tpy 0 have•− or •− − totpy be2−). considered in combination with neutral (tpy) or reduced forms of the ligand (tpy or tpy2 ). As outlinedoutlined in SectionSection 2.22.2,, reducedreduced [Ni(tpy)(R)][Ni(tpy)(R)] complexescomplexes with RR == alkyl or arylaryl clearlyclearly showshow •− structural andand spectroscopicspectroscopic propertiesproperties inin lineline withwith aa ligand-reducedligand-reduced charactercharacter [Ni(II)(tpy[Ni(II)(tpy•−)(R)], rather thanthan withwith aa monovalentmonovalent nickelnickel atomatom [Ni(I)(tpy)(Me)][Ni(I)(tpy)(Me)] [[44,45,48,49].44,45,48,49]. In In contrast contrast to to this, monovalent halide halide complexes complexes [Ni(tpy)(X)] [Ni(tpy)(X)] with with X = X Cl, = Br, Cl, or Br, I are or best I are described best described as Ni(I) as(d9 Ni(I)) systems (d9) systems[41–43,45,49,50]. [41–43,45 ,49,50]. 2+ The complexcomplex [Ni(II)(tpy)[Ni(II)(tpy)2]]2+ isis reversibly reversibly reduced reduced in in three three one-electron one-electron steps, steps, and can also bebe 3+ reversiblyreversibly oxidizedoxidized (Scheme(Scheme 15 15))[ [40,41,52,940,41,52,900–93].–93]. ForFor thethe oxidizedoxidized speciesspecies a a [Ni(III)(tpy) [Ni(III)(tpy)2]2]3+ character has recentlyrecently beenbeen assumedassumed fromfrom calculationscalculations [[41];41]; however, furtherfurther spectroscopicspectroscopic evidence is notnot 3+ available fromfrom this this report. report. The controlledThe controlled potential potential oxidation ofoxidation [Ni(tpy) 2](ClOof [Ni(tpy)4)2 to the2](ClO [Ni(III)(tpy)4)2 to 2the] complex[Ni(III)(tpy) was2]3+ tried complex in 1977 was [93 tried], and inthe 1977 visible [93], absorptionand the visible spectrum absorption and magnetic spectrum moment and magnetic of the obtainedmoment of material the obtained assumed material to be assumed [Ni(tpy) 2to](ClO be [Ni(tpy)4)3 were2](ClO in line4)3 were with in a Ni(III)line with species. a Ni(III) However, species. theseHowever, spectroscopic these spectroscopic results were results devaluated were bydevaluat the elementaled by the analysis, elemental which analysis, was far which off the was calculated far off 3+ values.the calculated The authors values. ascribed The authors this to theascribed high reactivitythis to the of high [Ni(III)(tpy) reactivity3] ofin [Ni(III)(tpy) air, decomposing3]3+ in theair, isolateddecomposing compound the isolated upon workupcompound and upon isolation. workup and isolation.

Scheme 15. Proposed redox steps for [Ni(tpy)2 n]n++. Scheme 15. Proposed redox steps for [Ni(tpy)2] .

Most of the studies agree that the reduction sequence is largely ligand centered: Most of the studies agree that the reduction sequence is largely ligand centered: [Ni(II)(tpy) ]2+/ 2+ •− + •− 0 2− •− − 2 [Ni(II)(tpy)•−2] /[Ni(II)(tpy+ )(tpy)]•− /[Ni(II)(tpy0 )22]−/[Ni(II)(tpy•− − )(tpy )] [40,41,52,90–93]. A recent [Ni(II)(tpy )(tpy)] /[Ni(II)(tpy )2] /[Ni(II)(tpy )(tpy )] [40,41,52,90–93]. A recent study revealed study revealed that this sequence is true for six coordinated [Ni(tpy)2]n complexes [41]. Presumably, that this sequence is true for six coordinated [Ni(tpy) ]n complexes [41]. Presumably, the octahedral the octahedral coordination through two tpy ligands2 (2 × η3 coordinate tpy) favors a Ni(II) d8 coordination through two tpy ligands (2 × η3 coordinate tpy) favors a Ni(II) d8 configuration over configuration over a (probably Jahn-Teller distorted) Ni(I) d9 system, and details of quantum a (probably Jahn-Teller distorted) Ni(I) d9 system, and details of quantum chemical calculations reveal chemical calculations reveal a significant interaction of the ligand (tpy) π radical anions and the a significant interaction of the ligand (tpy) π radical anions and the unpaired electrons at the Ni(II) [41]. unpaired electrons at the Ni(II) [41]. On the other hand, a four-coordinate species [Ni(tpy)2]0 with two η2 coordinating tpy ligands has been proposed for the doubly-reduced state [41,94]. Quantum chemical calculations reveal two isomers with a pseudo-tetrahedral structure and an [Ni(I)(tpy•−)(tpy0)] electron configuration with an S = 0 ground state. This is due to an antiferromagnetic coupling of the Ni(I), and the tpy•− radical and the calculations are in agreement with magnetic data [41]. Thus, the number of tpy ligands and the way they coordinate to Ni determines the redox potentials, the character of the reduced species, and therefore the reactivity of catalytically relevant complex species [52].

Inorganics 2018, 6, 18 10 of 18

0 2 On the other hand, a four-coordinate species [Ni(tpy)2] with two η coordinating tpy ligands has been proposed for the doubly-reduced state [41,94]. Quantum chemical calculations reveal two isomers with a pseudo-tetrahedral structure and an [Ni(I)(tpy•−)(tpy0)] electron configuration with an S = 0 ground state. This is due to an antiferromagnetic coupling of the Ni(I), and the tpy•− radical and the calculations are in agreement with magnetic data [41]. Thus, the number of tpy ligands and the way they coordinate to Ni determines the redox potentials, the character of the reduced species, and therefore the reactivity of catalytically relevant complex speciesInorganics [ 522018]., 6, x FOR PEER REVIEW 10 of 18 Inorganics 2018, 6, x FOR PEER REVIEW 10 of 18 3.2.3.2. Aromatic PerfluoroalkylationPerfluoroalkylation UsingUsing Ni–tpyNi-tpy Under Electrocatalytic Conditions 3.2. Aromatic Perfluoroalkylation Using Ni-tpy Under Electrocatalytic Conditions InIn recentrecent years,years, extensiveextensive efforts havehave ledled toto newnew methodsmethods forfor thethe introductionintroduction ofof thethe trifluoromethyltrifluoromethylIn recent andyears, and perfluoroalkyl perfluoroalkextensive efforts groupsyl groups have into aromaticledinto toaromatic andnew heteroaromaticmethods and heteroaromatic for ringthe systems.introduction ring [Ni(II)(tpy)] systems. of the complexestrifluoromethyl[Ni(II)(tpy)] werecomplexes and found perfluoroalk were to found be effectiveyl to begroups effective catalyst into catalyst precursorsaromatic precursors and in olefinheteroaromaticin olefin fluoroalkylationns fluoroalkylationns ring systems. andand cross-couplings[Ni(II)(tpy)]cross-couplings complexes ofof fluoroalkylhalidesfluoroalkylhalides were found to andbeand effective arylhalides.arylhalides. catalyst precursors in olefin fluoroalkylationns and cross-couplingsAA one-stepone-step of catalyticcatalytic fluoroalkylhalides methodmethod forfor and aromaticaromatic arylhalides. perfluoroalkylationperfluoroalkylation catalyzedcatalyzed byby electrochemicallyelectrochemically reducedreducedA one-step metalmetal complexes, complexes,catalytic method includingincluding for aromatic Ni(tpy),Ni(tpy), hasperfhas recentlyluoroalkylationrecently beenbeen developed developedcatalyzed [by72[72].]. electrochemically Cross-couplingCross-coupling proceedsreducedproceeds metal underunder complexes, mildmild conditionsconditions including withwith thetheNi(tpy), decisivedecisive has rolerole recently ofof thethe sacrificialsacrificialbeen developed anodeanode metal.metal.[72]. Cross-coupling TheThe reactionreaction isis successfulproceedssuccessful under withwith bromo-bromo-mild conditions andand iodobenzeneiodobenzene with the andanddecisive perfluoroalkylperf luoroalkylrole of the iodides iodidessacrificial asas substratesanodesubstrates metal. (Scheme(Scheme The reaction 16 16).). is successful with bromo- and iodobenzene and perfluoroalkyl iodides as substrates (Scheme 16).

SchemeScheme 16. ElectrocatalyticalElectrocatalytical aromatic aromatic perfluoroalkylation perfluoroalkylation catalyzed catalyzed by by Ni(tpy), Ni(tpy), adopted adopted from from ref. ref. [72]. [72 ]. Scheme 16. Electrocatalytical aromatic perfluoroalkylation catalyzed by Ni(tpy), adopted from ref. [72]. The most effective catalysts systems in this study turned out to be Ni(II), Co(II), and Cu(II) The most effective catalysts systems in this study turned out to be Ni(II), Co(II), and Cu(II) complexesThe most of bpy effective and tpy catalysts (and relatedsystems diimines), in this stud in conjunctiony turned out with to bea copperNi(II), Co(II),anode, andand Cu(II)Cu(II) complexes of bpy and tpy (and related diimines), in conjunction with a copper anode, and Cu(II) complexescomplex species of bpy wereand tpy therefore (and related considered diimines), in inthe conjunction proposed withmechanistic a copper cycle anode, (Scheme and Cu(II) 17). complex species were therefore considered in the proposed mechanistic cycle (Scheme 17). Furthermore, complexFurthermore, species the successwere therefore of the catalytic considered process in de thepends proposed on the reductionmechanistic potential cycle of(Scheme the catalyst. 17). theFurthermore, success of the the success catalytic of processthe catalytic depends process on de thepends reduction on the potential reduction of potential the catalyst. of the The catalyst. more The more negative reduction potential, the more effective the catalyst. Since [Ni(tpy)Br2] is reduced negative reduction potential, the more effective the catalyst. Since [Ni(tpy)Br ] is reduced at the lowest Theat the more lowest negative potential reduction (−1.30 V),potential, compared the withmore the effective other metal the catalyst. complexes Since 2studied [Ni(tpy)Br in this2] iswork reduced (e.g., potentialat the lowest (−1.30 potential V), compared (−1.30 V), with compared the other with metal the complexesother metal studied complexes in this studied work (e.g.,in this (− work1.12 V(e.g., for (−1.12 V for [Ni(bpy)Br2]), the yields of the cross-coupling product with [Ni(tpy)Br2] participation are [Ni(bpy)Br ]), the yields of the cross-coupling product with [Ni(tpy)Br ] participation are relatively (relatively−1.12 V for higher2 [Ni(bpy)Br (up to2 ]),73%) the [72].yields of the cross-coupling product with 2[Ni(tpy)Br2] participation are higherrelatively (up higher to 73%) (up [72 to]. 73%) [72].

Scheme 17. Catalytic cycle of Cu-assisted aromatic perfluoroalkylation with electrochemically Schemereduced 17.Ni–tpy17.Catalytic Catalytic complex, cycle cycle adopted of Cu-assistedof Cu-assisted from ref. aromatic [72]. aromatic perfluoroalkylation perfluoroalkylation with electrochemically with electrochemically reduced Ni–tpyreduced complex, Ni–tpy complex, adopted fromadopted ref. from [72]. ref. [72]. The proposed catalytic cycle (Scheme 17) involves a monovalent species [Ni(I)(L)] 25 (L = tpy or The proposed catalytic cycle (Scheme 17) involves a monovalent species [Ni(I)(L)] 25 (L = tpy or bpy), which is generated at the cathode. Oxidative addition leads to complexes [Ni(III)(L)(RF)X]+ 26; bpy), which is generated at the cathode. Oxidative addition leads to complexes [Ni(III)(L)(RF)X]+ 26; two electron reductions yield [Ni(I)(L)(RF)X]− 27. The oxidative addition of aryl-X substrates leads − twothen electron to formal reductions Ni(III) species yield 28[Ni(I)(L)(R, which areF)X] reduced 27. The again oxidative to yield addition the stable of aryl-X so-called substrates σ-complexes leads then to formal Ni(III) species 28, which are reduced again to yield the stable so-called σ-complexes [Ni(II)(L)(aryl)(RF)] 29. Importantly, neither the Ni(III) species [Ni(III)(L)(RF)X(aryl)] 28 nor the Ni(II) [Ni(II)(L)(aryl)(RF)] 29. Importantly, neither the Ni(III) species [Ni(III)(L)(RF)X(aryl)] 28 nor the Ni(II) complexes [Ni(II)(L)(aryl)(RF)] 29 undergo reductive elimination to yield cross-coupling products complexes [Ni(II)(L)(aryl)(RF)] 29 undergo reductive elimination to yield cross-coupling products aryl–RF [95,96]. Instead, the Ni(II) σ-complexes 29 transmetalate copper halide species to yield aryl–RF [95,96]. Instead, the Ni(II) σ-complexes 29 transmetalate copper halide species to yield [Cu(II)(L)(RF)(aryl)] 32. They finally cleave the cross-coupling products through reductive [Cu(II)(L)(RF)(aryl)] 32. They finally cleave the cross-coupling products through reductive elimination [72]. The halide complexes [Ni(L)X2] 30 were reductively reconverted to the initial Ni(I) eliminationspecies 25. To [72]. support The halide the described complexes mechanism, [Ni(L)X2] 30detailed were reductively cyclic voltammetry reconverted (CV) to experiments the initial Ni(I) and species 25. To support the described mechanism, detailed cyclic voltammetry (CV) experiments and stoichiometric reactions were performed. It was shown that Cu(II) complexes such as [Cu(bpy)Cl2], stoichiometric reactions were performed. It was shown that Cu(II) complexes such as [Cu(bpy)Cl2],

Inorganics 2018, 6, 18 11 of 18

The proposed catalytic cycle (Scheme 17) involves a monovalent species [Ni(I)(L)] 25 (L = tpy or + bpy), which is generated at the cathode. Oxidative addition leads to complexes [Ni(III)(L)(RF)X] 26; − two electron reductions yield [Ni(I)(L)(RF)X] 27. The oxidative addition of aryl–X substrates leads then to formal Ni(III) species 28, which are reduced again to yield the stable so-called σ-complexes [Ni(II)(L)(aryl)(RF)] 29. Importantly, neither the Ni(III) species [Ni(III)(L)(RF)X(aryl)] 28 nor the Ni(II) complexes [Ni(II)(L)(aryl)(RF)] 29 undergo reductive elimination to yield cross-coupling products aryl–RF [95,96]. Instead, the Ni(II) σ-complexes 29 transmetalate copper halide species to yield [Cu(II)(L)(RF)(aryl)] 32. They finally cleave the cross-coupling products through reductive elimination [72]. The halide complexes [Ni(L)X2] 30 were reductively reconverted to the initial Ni(I) Inorganicsspecies 252018., To6, x supportFOR PEER theREVIEW described mechanism, detailed cyclic voltammetry (CV) experiments11 of 18 and stoichiometric reactions were performed. It was shown that Cu(II) complexes such as

[Cu(dmphen)Cl[Cu(bpy)Cl2], [Cu(dmphen)Cl2] (dmphen = 22,9-dimethyl-1,10-phenanthroline),] (dmphen = 2,9-dimethyl-1,10-phenanthroline), or [Cu(dppe)Cl or2] [Cu(dppe)Cl(dppe = 1,2-2] (dppebis(diphenylphosphano)ethane = 1,2-bis(diphenylphosphano)ethane)) under reductive under reductive conditions conditions do not doreact not reactwith withthe theRF–I R Fwith–I with a noticeablea noticeable rate, rate, and and their their reduction reduction behavior behavior (potenti (potentials,als, reversibility) reversibility) is is not not changed changed by by the the addition addition of R F–I.–I. In In contrast contrast to to this, this, the the CVs CVs of of the corresponding nickel and co cobaltbalt complexes are sensitive to the presence of RF–I.–I. The The reduction wave of M(II) to M(I) for both Ni and Co complexes experienced a twofold increase in current with a loss of reversibility [[52,72],52,72], which indicates the occurrence of oxidative addition addition R RFF–X–X to to [M(I)(L)] [M(I)(L)] without without the the cyclic cyclic regene regenerationration of of the the catalyst. catalyst. Regeneration Regeneration of theofthe catalyst catalyst would would have have led to led a strong to a strong catalytic catalytic current current in the presence in the presence of substrate of substrate [97,98], but [97 ,this98], wasbut thisnot observed. was not observed.Thus, the observed Thus, the changes observed in CVs changes can be indescribed CVs can by bean describedECE (electrochemical- by an ECE (electrochemical-chemical-electrochemical)chemical-electrochemical) scheme [50,52]. scheme [50,52]. The advantages of this Cu-assisted method are as follows: it it is a one-step catalytic reaction carrying out under mild conditions, at room temper temperature,ature, and the cross-couplings are successful not only with iodobenzene, but also with bromobenzenebromobenzene and perfluoroalkylperfluoroalkyl iodides.iodides. Later, thethe methodmethod was was extended extended to to various various aromatic aromatic halides halides (Scheme (Scheme 18)[ 18)71]. [71]. So, theSo, single-stagethe single- stagesynthesis synthesis of perfluoroalkylated of perfluoroalkylated arenes arenes via the via cross-coupling the cross-coupling of bromo of bromo (in some (in cases, some chloro)cases, chloro) arenes arenesor heteroarenes or heteroarenes (derivatives (derivatives of benzene, of benzene, pyridine, pyridine, and furan) and andfuran) organic and organic perfluoroalkyl perfluoroalkyl halides halidesinvolving involving [Ni(tpy)] [Ni(tpy)] complexes complexes (and other (and metalother andmetalα -diiminesand α-diimines ligands) ligands) in a low in a oxidation low oxidation state stateunder under mild conditionsmild conditions was achieved. was achieved. Perfluoroalkylated Perfluoroalkylated products products are obtained are obtained in good in yieldsgood yields in the presencein the presence of the of [Ni(I)(L)] the [Ni(I)(L)] catalyst catalyst (1–10%) (1–10%) that that was was electrochemically electrochemically generated generated from from [Ni(II)(L)] [Ni(II)(L)] at atroom room temperature temperature without without chemical chemical reductant. reductant. In the In general the general case, itcase, has beenit has found been thatfound the that success the successof the catalytic of the catalytic process dependsprocess de onpends the reduction on the reduction potential potential of the catalyst. of the Thecatalyst. more The negative more reduction negative potential,reduction thepotential, more effective the more the effective catalyst. the catalyst.

SchemeScheme 18. Ni-catalyzed18. Ni-catalyzed cross-coupling cross-coupling of perfluoroalkyl of perfluoroalkyl halides halides and and aromatic aromatic halides, halides, adopted adopted from from ref. [ 71]. ref. [71].

Very effective cross-coupling process was found for 4-bromobenzonitrile, 2-bromofuran, 2- bromopyridine, 2-chloropyridine, or 3-bromopyridine substrates, and perfluoro-n-hexyl iodide, perfluoro-n-hexyl bromide, or 1-bromo-1H,1H,2H,2H-perfluorodecan partners (Scheme 18). Thus, the method is suitable for different arenes, except thiophene and derivatives with NH2– and OCH3– groups. The most effective catalysts are complexes characterized by negative reduction potentials, and [Ni(tpy)Br2] is one of the best catalysts. There is no simple explanation for this observation, but perhaps a more negative reduction potential determines the greater force of the reducing agent and a faster oxidative addition step rate, which is a determining factor in the catalytic cycle. In any case, these catalysts were effective for mediating the assumed steps of the catalytic cycle: oxidative addition and reductive elimination [71].

Inorganics 2018, 6, 18 12 of 18

Very effective cross-coupling process was found for 4-bromobenzonitrile, 2-bromofuran, 2-bromopyridine, 2-chloropyridine, or 3-bromopyridine substrates, and perfluoro-n-hexyl iodide, perfluoro-n-hexyl bromide, or 1-bromo-1H,1H,2H,2H-perfluorodecan partners (Scheme 18). Thus, the method is suitable for different arenes, except thiophene and derivatives with NH2– and OCH3– groups. The most effective catalysts are complexes characterized by negative reduction potentials, and [Ni(tpy)Br2] is one of the best catalysts. There is no simple explanation for this observation, but perhaps a more negative reduction potential determines the greater force of the reducing agent and a faster oxidative addition step rate, which is a determining factor in the catalytic cycle. In any case, these catalysts were effective for mediating the assumed steps of the catalytic cycle: oxidativeInorganics 2018 addition, 6, x FOR and PEER reductive REVIEW elimination [71]. 12 of 18 Inorganics 2018, 6, x FOR PEER REVIEW 12 of 18 3.3.3.3. ElectrochemicalElectrochemical Nickel-InducedNickel-Induced FluoroalkylationFluoroalkylation ofof OlefinsOlefins 3.3. Electrochemical Nickel-Induced Fluoroalkylation of Olefins ElectrochemicalElectrochemical one one electronelectron reduction reduction of of [Ni(tpy)Br [Ni(tpy)Br2]2] leadsleads toto anan activeactive speciesspecies thatthat cancan catalyzecatalyze thethe additionadditionElectrochemical ofof perfluoroalkylperfluoroal one electronkyl radicalsradicals reduction toto olefinsolefins of [Ni(tpy)Br [[53,99,100].53,99,1002]]. leads TheThe to electrocatalyticelectrocatalytic an active species generationgeneration that can of ofcatalyze aa Ni(I)Ni(I) catalystthecatalyst addition byby reduction reductionof perfluoroal ofof aakyl Ni(II)–tpyNi(II)–tpy radicals precursorprecursorto olefins [53,99,100]. complexcomplex inin The thethe electrocatalytic presencepresence ofof olefinicolefinic generation substratessubstrates of a Ni(I) andand fluoroalkylcatalystfluoroalkyl by reduction halideshalides leadsleads of a totoNi(II)–tpy newnew organicorganic precursor productsproducts complex derivedderived in the fromfrom presence addition-dimerizationaddition-dimerization of olefinic substrates processesprocesses and (Schemefluoroalkyl(Scheme 1919), ),halides showingshowing leads aa to directdirect new evidenceevidenceorganic products ofof thethe importance importancederived from ofof theaddition-dimerizationthe four-coordinatefour-coordinate lowlow processes valentvalent [Ni(tpy)Br](Scheme[Ni(tpy)Br] 19), complex.complex. showing a direct evidence of the importance of the four-coordinate low valent [Ni(tpy)Br] complex.

SchemeScheme 19.19. Addition-dimerizationAddition-dimerization catalyzedcatalyzed byby aa NiNi complex,complex, adoptedadopted fromfrom refs.refs. [[53,99,100].53,99,100]. Scheme 19. Addition-dimerization catalyzed by a Ni complex, adopted from refs. [53,99,100]. Due to the presence of two chiral carbon atoms in the dimerization products, three isomers were Due to the presence of two chiral carbon atoms in the dimerization products, three isomers were characterizedDue to the by presence a variety of oftwo analytical chiral carbon techniques, atoms inincluding the dimerization multi-dimensional products, threeNMR isomersmethods were and characterized by a variety of analytical techniques, including multi-dimensional NMR methods and X-ray single crystal diffraction [71,101]. The cyclic voltammetry study of the [Ni(tpy)Br2] complex, X-ray single crystal diffraction [71,101]. The cyclic voltammetry study of the [Ni(tpy)Br ] complex, X-rayalong singlewith fluoroalkyl crystal diffraction halides, [71,101].demonstrated The cyclic that [Ni(I)(L)Br]voltammetry is studythe active of the form [Ni(tpy)Br of the catalyst.2] complex, The along with fluoroalkyl halides, demonstrated that [Ni(I)(L)Br] is the active form of the catalyst. alongcyclic withvoltammetry fluoroalkyl and halides, preparative demonstrated electrolysis that da[Ni(I)(L)Br]ta give experimental is the active formsupport of theto thecatalyst. reactions The Thecyclic cyclic voltammetry voltammetry and and preparative preparative electrolysis electrolysis da datata give give experimental experimental support support to to the the reactions highlighted in Scheme 20 [53]. The cyclic voltammograms of [Ni(tpy)Br2] under the reaction highlighted inin Scheme Scheme 20 [2053 ].[53]. The cyclicThe cyclic voltammograms voltammograms of [Ni(tpy)Br of [Ni(tpy)Br2] under2 the] under reaction the conditions reaction conditions are shown in Figure 1. The addition of a C6F13I substrate increases the current of the first are shown in Figure1. The addition of a C F I substrate increases the current of the first reduction conditionsreduction wavesare shown in the in complex, Figure 1. suggesting The addition6 that13 of the a C Ni(I)6F13I speciessubstrate are increases the active the forms current of the of thecatalyst. first wavesreduction in the waves complex, in the suggestingcomplex, suggesting that the Ni(I) that species the Ni(I) are species the active are formsthe active of the forms catalyst. of the catalyst.

Scheme 20. Proposed reaction sequence in the electrocatalytic fluoroalkylation of α-methyl styrene Scheme 20. Proposed reaction sequence in the electrocatalytic fluoroalkylation of α-methyl styrene Schemeusing [Ni(tpy)Br 20. Proposed2], adopted reaction from sequence ref. [53]. in the electrocatalytic fluoroalkylation of α-methyl styrene using [Ni(tpy)Br2], adopted from ref. [53]. using [Ni(tpy)Br2], adopted from ref. [53].

Figure 1. Cyclic voltammograms of [Ni(tpy)Br2] (0.01 M) in the presence of increasing quantities of Figure 1. Cyclic voltammograms of [Ni(tpy)Br2] (0.01 M) in the presence of increasing quantities of С6F13I. Ratios of complexes to RFI are 1:0 (red); 1:3 (blue); 1:6 (violet); 1:12 (dark blue); 1:30 (black). СReproduced6F13I. Ratios fromof complexes ref. [53] with to R permissiFI are 1:0on (red); form 1:3 The (blue); Royal 1:6 Society (violet); of Chemistry.1:12 (dark blue); 1:30 (black). Reproduced from ref. [53] with permission form The Royal Society of Chemistry.

The proposed mechanism of the process involves electrochemical reduction of the [Ni(II)(L)Br2] complexThe proposedto [Ni(I)(L)Br], mechanism followed of the byprocess an oxidation involves electrochemicalby the fluoroorganic reduction halide of the to[Ni(II)(L)Br regenerate2] complex[Ni(II)(L)BrX] to [Ni(I)(L)Br], (Scheme 20). followed The perfluoroalkyl by an oxidation radicals byproduced the fluoroorganic by these redox halide events to react regenerate with α- [Ni(II)(L)BrX] (Scheme 20). The perfluoroalkyl radicals produced by these redox events react with α-

Inorganics 2018, 6, x FOR PEER REVIEW 12 of 18

3.3. Electrochemical Nickel-Induced Fluoroalkylation of Olefins

Electrochemical one electron reduction of [Ni(tpy)Br2] leads to an active species that can catalyze the addition of perfluoroalkyl radicals to olefins [53,99,100]. The electrocatalytic generation of a Ni(I) catalyst by reduction of a Ni(II)–tpy precursor complex in the presence of olefinic substrates and fluoroalkyl halides leads to new organic products derived from addition-dimerization processes (Scheme 19), showing a direct evidence of the importance of the four-coordinate low valent [Ni(tpy)Br] complex.

Scheme 19. Addition-dimerization catalyzed by a Ni complex, adopted from refs. [53,99,100].

Due to the presence of two chiral carbon atoms in the dimerization products, three isomers were characterized by a variety of analytical techniques, including multi-dimensional NMR methods and X-ray single crystal diffraction [71,101]. The cyclic voltammetry study of the [Ni(tpy)Br2] complex, along with fluoroalkyl halides, demonstrated that [Ni(I)(L)Br] is the active form of the catalyst. The cyclic voltammetry and preparative electrolysis data give experimental support to the reactions highlighted in Scheme 20 [53]. The cyclic voltammograms of [Ni(tpy)Br2] under the reaction conditions are shown in Figure 1. The addition of a C6F13I substrate increases the current of the first reduction waves in the complex, suggesting that the Ni(I) species are the active forms of the catalyst.

Scheme 20. Proposed reaction sequence in the electrocatalytic fluoroalkylation of α-methyl styrene Inorganics 2018, 6, 18 13 of 18 using [Ni(tpy)Br2], adopted from ref. [53].

Figure 1. Cyclic voltammograms of [Ni(tpy)Br2] (0.01 M) in the presence of increasing quantities of Figure 1. Cyclic voltammograms of [Ni(tpy)Br2] (0.01 M) in the presence of increasing quantities of 6 13 F СC6FF13I.I. Ratios Ratios of of complexes complexes to to R RFI Iare are 1:0 1:0 (red); (red); 1:3 1:3 (blue); (blue); 1:6 1:6 (violet); (violet); 1:12 1:12 (dark (dark blue); blue); 1:30 1:30 (black). (black). Reproduced from ref. [[53]53] with permissionpermission form The Royal SocietySociety ofof Chemistry.Chemistry.

The proposed mechanism of the process involves electrochemical reduction of the [Ni(II)(L)Br2] The proposed mechanism of the process involves electrochemical reduction of the [Ni(II)(L)Br2] complex to [Ni(I)(L)Br], followed by an oxidation by the fluoroorganic halide to regenerate complex to [Ni(I)(L)Br], followed by an oxidation by the fluoroorganic halide to regenerate [Ni(II)(L)BrX] (Scheme 20). The perfluoroalkyl radicals produced by these redox events react with α- [Ni(II)(L)BrX] (Scheme 20). The perfluoroalkyl radicals produced by these redox events react with α-methyl styrene to form an addition product, which undergoes either a dimerization process or forms the monomer product under the treatment of tributyltin hydride. An alternative and more complex mechanism would involve coordination of the olefinic substrate [53].

Acknowledgments: This work was partially supported by the grant of the Russian Science Foundation No. 14-23-00016 (Y.H.B., electrosynthesis), the German Science Foundation KL1194/5-1, and KL1194/6-1 (A.K.), and the Office of Basic Energy Sciences of the U.S. Department of Energy (DE-SC0009363) (D.A.V.). Author Contributions: All three authors have contributed equally to the writing of this manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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