Electrochemical Synthesis and Properties of Organonickel Ï

Review pubs.acs.org/Organometallics

Electrochemical Synthesis and Properties of Organonickel σ‑Complexes Dmitry G. Yakhvarov,*,†,‡ Aliya F. Khusnuriyalova,‡ and Oleg G. Sinyashin† †

A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, Arbuzov str. 8, 420088 Kazan, Russian Federation ‡ Kazan Federal University, Kremlyovskaya str. 18, 420008 Kazan, Russian Federation ABSTRACT: The organonickel complexes are organometallic compounds containing a Ni−C σ-bond (σ-complexes). These species are very reactive and have been mainly characterized as the intermediates of catalytic processes of cross coupling and homocoupling involving organic and elementoorganic substrates such as organic halides, chlorophosphines, unsaturated hydrocarbons, etc. Thus, only a limited number of these complexes have been isolated and characterized as the free stable species. Although the organonickel complexes have been known since the 1960s, the chemistry of these species is currently at the beginning stages of development. The interest of the researchers in this class of compounds has significantly increased over the past decade, resulting in a plethora of scientific papers published on this topic. At the same time, electrochemical methods have become more and more popular in modern synthetic chemistry, due to easy access to high reactive intermediates, including organometallic species, which can be selectively generated in situ and used for subsequent synthetic processes. This review summarizes the elaborated electrochemical approaches for the preparation of organonickel complexes, including a discussion of the important role of the electrochemical cell construction and the influence of the electrode material nature on the electrochemical process. In order to give more insight into the importance of organonickel complexes in synthetic chemistry and introduce the reader to this problem of organometallic chemistry, focused on the development of new synthetic protocols for preparation of stable organonickel complexes, an overview of the most important catalytic processes proceeding with participation of these highly reactive intermediates and the main types of organonickel complexes are presented. However, in this review organonickel complexes will be limited by examples in which the organic fragment is singly bonded to the nickel center, because these species are responsible for the catalytic reactions.

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INTRODUCTION The development of modern chemical science and technology has proceeded in several directions, including the development of effective and ecologically safe methods for the preparation of important and useful chemical compounds and materials.1 The combination of transition-metal catalysis2,3 and organic electrosynthesis4 has attracted increasing attention due to the high selectivity and efficiency of this approach in the synthetic preparation of various compounds bearing carbon−carbon and carbon−element bonds.5,6 The mild conditions, single-stage process, cyclic regeneration of the catalyst, and convenient and relatively inexpensive form of the energy used are the main advantages of electrochemical methods. Application of electrochemical processes to large-scale production (macroscale synthesis) has led to significant development of the chemical technologies of the 21st century, due to easy access to highly reactive intermediates and tuning of the reactivity of the substrate used during the synthetic process by simple adjustment of the electrode potential.5−7 Currently, different electrocatalytic processes occurring with participation of transition-metal complexes have been elaborated.5−7 It was established that highly reactive organonickel complexes containing a metal−carbon bond are key inter© 2014 American Chemical Society

mediates in the process of homocoupling and cross-coupling with organic halides, chlorophosphines, and other organic and organometallic compounds.7 Moreover, electrochemical methods were successfully applied to the preparation of organonickel complexes bearing nickel−carbon σ-bonds due to the easy in situ generation of highly reactive nickel(0) complexes stabilized by πacceptor ligands.8 This technique does not require chemical preparation and isolation of poorly stable coordinatively unsaturated transition-metal complexes in low degrees of oxidation and flammable organometallic reagents based on magnesium and lithium. Organonickel complexes as well as other types of organometallic species can be obtained by the preliminary stabilization of highly reactive Ni−C σ-bonds.9,10 Recently published data include well-known famous works of Tamaru11 and Campora,12 where organonickel complexes are considered as intermediates of various catalytic processes occurring with nickel catalysts. The first stable organonickel complexes bearing a Ni−C σbond were reported in the early 1960s,13 and derivatives Special Issue: Organometallic Electrochemistry Received: January 27, 2014 Published: May 2, 2014 4574

dx.doi.org/10.1021/om500100q | Organometallics 2014, 33, 4574−4589

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The first examples of stable organic σ-complexes of nickel(I) with an alkyl substituent have been recently obtained by the research group of Hillhouse.30 The stabilization of the complexes was performed by using the sterically hindered chelate diphosphine ligand dtbpe (1,2-bis(di-tert-butylphosphino)ethane) (Scheme 1).

containing the bpy ligand were described about 20 years later.14,15 Chatt and Shaw13 had shown that ortho substituents in a σ-bonded aryl substituent can stabilize the complex by preventing free rotation about the nickel−carbon σ-bond. Since then, the synthesis and reactivity of σ-aryl−nickel complexes bearing diimine ligands have been the focus of several reports.16−19 Results obtained over the last yfew ears allow concluding that also the reactivity of the nickel−carbon σ-bond strongly depends on the nature of the organic substituent and the ligand environment at the nickel center. Stabilization of Ni−C σbonds can be achieved by the use of ortho substituents in the aromatic moiety bonded to the nickel center. The introduction of such ortho-substituted aromatic groups has the effect of shielding the metal center and creates an additional energetic barrier retarding free rotation of the aromatic moiety around the nickel− carbon σ-bond, leading to stabilization of the complex. Thus, attempts to apply electrochemical techniques to the preparation of such kinds of species were made on substrates bearing orthosubstituted aryl groups such as Xyl (2,6-dimethylpenyl), Mes (2,4,6-trimethylphenyl), and Tipp (2,4,6-triisopropylphenyl).8,20,21

Scheme 1. Preparation of Stable Organonickel Complexes with σ-Bonded Alkyl Substituentsa

a

From ref 30.

The reactivity of organonickel complexes bearing a σ-bonded methyl group toward unsaturated compounds and carboxylic acids was investigated.31,32 The authors investigated the process of the coordination of unsaturated hydrocarbons and carbonyl compounds to monoalkyl- and dialkylnickel complexes and made some conclusions concerning the mechanism of C−C bond formation under these conditions. The influence of the ligand nature on the stability of nickel−carbon σ-bonds in alkylnickel complexes and the mechanism of C−H bond activation leading to organonickel complexes containing a methyl substituent at the nickel center was also studied.33 It should be noted that alkylnickel σ-complexes are widely distributed in biological systems.34 They were found to be intermediates in many biological processes that are catalyzed by enzymes. Examples of nickel−alkyl bond cleavage and the formation of thioethers35 and the reactivity of an alkylnickel derivative generated in the reaction of methyl coenzyme M reductase with bromo derivatives of some carboxylic acids have been investigated.36 Some binuclear organonickel complexes containing a nickel−methyl σ-bond have been detected as intermediates in catalytic processes involving acetyl coenzyme synthase.37 Organonickel Complexes Bearing a σ-Bonded Aromatic Group. Complexes with an aryl group σ-bonded to the nickel atom are more stable in comparison with the corresponding alkyl derivatives.10−12 The main approach for their stabilization is the use of ortho-substituted aromatic groups.13 The main interest in research of species of this type has been devoted to arylnickel compounds stabilized by αdiimine ligands, due to their importance in catalysis.38 The main synthetic pathways for the preparation of organometallic aryl complexes and their properties are summarized in ref 39, while some special aspects of σ-bond stabilization and the synthesis of organonickel complexes of the type [NiBr(aryl)(N− N)], where aryl = 2,4,6-trimethylphenyl and N−N = α-diimine ligand, were recently described by Klein et al.40−42 An example of organonickel σ-complexes of the type [NiX(Mes)(bpy)], where X = F, Cl, Br, I, OMe, SCN, and the effect of the halide anion on the stability and reactivity of these compounds were studied in detail.41 It was found that the best leaving group is the iodide anion and the most stable complexes are formed with chloride or fluoride anions. Moreover, recent discoveries include data that some organonickel complexes containing an aryl substituent at nickel atom exhibit luminescent properties, as exemplified by [NiBr(Mes)(dppz)], where dppz = dipyridophenazine.42

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MAIN TYPES OF ORGANONICKEL COMPLEXES A sufficiently large number of organonickel complexes containing single nickel−carbon σ-bonds has currently been synthesized.10−12 The organonickel complexes discussed in this review containing ordinary nickel−carbon σ-bonds are the most common and most studied class of these organonickel species, while complexes with double and triple bonds are also wellknown and are presented in a review.22 This class is represented by organonickel compounds containing alkyl, aryl ,or alkynyl substituents at the nickel atom where the formal oxidation state of the metal center is considered as equal to 2, taking into account a negatively charged carbanion of the bonded organic radical. Organonickel Complexes with σ-Bonded Alkyl Substituents. The first examples of stable alkylnickel(II) complexes of the type [Ni(R)2(L)], where R = CH3, C2H5 and L = 2 PEt3, 2 PBu3, dppe (1,2-bis(diphenylphosphino)ethane), were prepared by the research group of Yamamoto by the reaction of [Ni(acac)2], where acac = acetylacetone, with AlR2(OC2H5) in the presence of the corresponding phosphine ligand.23 Some other types of thermally stable organonickel complexes containing two σ-bonds with sp3-hybridized carbon atoms are described in ref 24. The complex [NiCl(CH3)(PMe3)2], formed by strong phosphine ligands, is an example of a stable methylnickel compound with one alkyl−nickel σ-bond.25 Introduction of a second methyl group reduces the stability and leads to the decomposition of the molecule to form ethane and the corresponding coordinatively unsaturated complex [Ni0(PMe3)2].26 Other types of organonickel complexes containing a σ-bonded alkyl group are known as intermediates of the oxidative cyclization of alkenes and ketones in the coordination sphere of nickel(0) complexes.27 Some recent examples include alkynyl nickel complexes of the type [Ni(CCH)2(PR3)],28 which are able to catalyze the reaction polycondensation of unsaturated hydrocarbons in the presence of copper salts in basic media. Organonickel σ-complexes containing a C(O)NR2 group are represented by the complex [NiI(Et2NH)2C(O)NEt2], which was obtained in the reaction of [(Et2NH)2NiI2] with carbon monoxide.29 4575


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(dimethylaminomethyl)bromobenzene,52 as exemplified in Scheme 3.

The complexes formed by diimine ligands are capable of reacting with a solvent in solution.18 It was found that an acetonitrile molecule can be introduced into the nickel−carbon σ-bond of organonickel complexes [NiBr(Mes)(N−N)], where N−N = diazabutadiene (DAD), pyridinylimine (PIM), 2,2′bipyridine (bpy), 1,10-phenanthroline (phen), resulting in the formation of new complexes with nickel−nitrogen σ-bonds (Scheme 2).43

Scheme 3. Preparation of Pincer-Type Organonickel Complexes by Oxidative Additiona

Scheme 2. Acetonitrile Molecule Insertion into the Ni−C Bond in Organonickel Complexes

a

From ref 52.

It was established that pincer-type organonickel complexes exhibit higher activity in the processes of carbon−halogen bond activation. For example, some organonickel complexes with pincer ligands are capable of activating the C−Cl bond in tetrachloromethane, resulting in the formation of cross-coupling products.53 Some examples of C−H, C−C, and C−O bond activations under the action of the pincer-type organonickel complexes have been described.54 New types of organonickel pincer-type complexes formed by POCN,55 PCP,56,57 CC′C,58 and POCOP59 ligands have been described in recent papers. The reactivity of pincer-type organonickel complexes and their application in synthetic catalytic processes can be obtained from a review.60 Carbene-type organonickel complexes are another important class of organonickel compounds which have attracted increasing attention due to their high selectivity and effectiveness in various catalytic processes.61 The main approach to the preparation of carbene-type organonickel complexes involves the use of highly sterically hindered chelating ligands based on di-62 and triphosphines,63,64 as exemplified in Scheme 4. In addition, some new dicarbene derivatives, based on tetradentate N-heterocyclic carbenes, have been prepared and studied from the viewpoint of their biological properties.65 The synthetic approaches to binuclear carbene-type organonickel complexes based on bis(N-heterocyclic carbenes) are summarized in a report.66 Some other examples include dicarbene nickel complexes bearing independent carbene ligands.67,68 It has been established that carbene-type organonickel complexes exhibit higher reactivity with respect to other organonickel species, which makes them convenient precursors for fine organic synthesis.69 Insertion of the alkynes into the nickel−hydrogen bond of nickel hydride complexes allowed new types of mono- and binuclear nickel vinyl σ-complexes to be obtained.70 Some

Organonickel aryl complexes formed by a chelating diphosphine ligand containing RSSR and PCP chelate centers have been presented.44,45 Compounds of this type containing a benzyl substituent at nickel decompose through a radical mechanism, eliminating benzyl radical, which can be involved in subsequent reactions with organic substrates in solution.46 It is interesting to note that most stable arylnickel complexes are σcomplexes based on naphthalene.47 Organonickel porphyrin48 and benzoporphyrin49 complexes were obtained by the reaction of a Grignard reagent with nickel(II) complexes. In addition, some examples of nickel σ-complexes based on sulfur-containing ligands are currently known.50 Recently, new methods for the preparation of organonickel complexes with aryl substituents by heterolytic C−H bond activation have been developed. These methods were used to obtain various organonickel complexes based on 2-arylpyridine derivatives (Figure 1).51 Other Types of Organonickel σ-Bonded Complexes. Among the different classes of organonickel complexes obtained recently,12 σ-complexes based on pincer ligands occupy a special place. The first examples have been synthesized by the reaction of oxidative addition of nickel(0) complexes to 2,6-bis-

Figure 1. Organonickel complexes based on arylpyridine ligands. Reprinted from ref 51 with permission by Elsevier. 4576


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hydrocarbons catalyzed by organonickel complexes81 and insertion of isonitriles into alkyl−nickel bonds82 were studied. Currently, different processes with the participation of organonickel complexes are known: biaryl synthesis by Suzuki coupling with arylboronic acids,83 cross-coupling with participation of organic iodides, bromides, and aromatic aldehydes,84 2bromopyridines,85 anhydrides of organic acids,86 alkenes, aldehydes, and triflates,87 aromatic chlorides and amines,88 carboxylation of alkynes89 and organometallic reagents,90 and addition of terminal olefins to 1,3-dienes and styrenes.91 Moreover, a number of processes involve the activation of boron−chlorine bonds under the action of organonickel complexes.92 The careful study of these processes and some insights into the mechanisms of the catalytic reactions have been described in recent papers.93 The synthetic potential of the organonickel complexes has attracted much attention from researchers, as exemplified in several dissertations.94 The first report concerning the mechanism of the crosscoupling of organic halides catalyzed by nickel complexes was discussed by Kochi.95 The mechanism of electrocatalytic coupling processes was first proposed by Amatore and Jutand for diphosphine complexes96,97 and later was postulated for Nibpy systems by Perichon’s research group.98 However, the nature of the formed intermediates was proposed on the basis of kinetic studies without isolation of the organonickel complexes. According to the postulated mechanism (Scheme 5) the electrocatalytic cross-coupling process involves the electro-

Scheme 4. Preparation of Carbene-Type Organonickel Complexesa

a

From ref 64.

additional examples of organonickel complexes are presented by scorpionate-type ligands71 and nickel(III) derivatives.72 Finally, nickelocene complexes have became more and more popular. These systems have attracted attention in terms of their high potential in catalytic processes.73 The most complete description of the structure, properties, and reactivity of organonickel σ-complexes containing cyclopentadiene ligands are summarized in a recent review.74

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PROPERTIES OF ORGANONICKEL COMPLEXES The catalytic reactions with participation of organonickel complexes are most interesting part of the chemistry of these species.11,12,75 The presence of the σ-bonded organic group makes these species very promising for catalytic processes based on transition-metal catalysts. Several books and reviews devoted to the catalytic activity of organonickel complexes have been published.11,12,75 From a general point of view, reactions with participation of transition-metal complexes can be divided into two classes. The first class includes non-Faraday reactions such as dimerization and oligomerization of unsaturated organic compounds, where the catalyst is responsible for the kinetics and the selectivity. The second class includes reactions where the transition-metal catalyst is participating in the overall process with a stoichiometric amount of the electrons. Different approaches to the study of catalytic electrochemical reactions were applied more than 20 years ago and have been reported in several reviews.4,76−78 However, recent results allow more insight into the mechanisms of the electrocatalytic processes which describe synthetic and mechanistic aspects of homogeneous electrocatalysis. Special attention is now devoted to the electrocatalytic processes where organonickel complexes participate in the formation of carbon−element bonds. Thus, several processes are described for organic halides, halophosphines, and white phosphorus.6,7 We will focus our attention on more important and new reactions which have been elaborated for the past years. Due to the special focus of this review the described catalytic reaction will be limited by processes where organonickel complexes are used as mediators. Electrochemical Coupling Reactions. The first mention of the use of organonickel complexes in catalytic processes of C− C bond formation by reaction of homocoupling and crosscoupling of organic halides was in 1967, where the coupling processes of organic halides with π-allyl nickel(I) complexes79 and later with aromatic chlorides80 were described. The mechanisms of the CO insertion processes with unsaturated

Scheme 5. Postulated Mechanism of the Electrocatalytic Cross Coupling of Organic Halides

chemical formation of a nickel(0) complex, which is able to react with aromatic halide (aryl-X) with formation of the organonickel complex [NiX(aryl)(bpy)], where bpy = 2,2′bipyridine. The subsequent electrochemical activation of the formed organonickel species in the presence of organic halide (RX) led to the formation of the new organonickel complex [Ni(aryl)(R)(bpy)], bearing two σ-bonded organic groups at the metal center. The formation of the cross-coupling product is a result of the subsequent reductive elimination of the electrochemically activated diorganyl derivative accompanied by the regeneration of the nickel catalyst.5a,98 The formation of organonickel complexes and the coupling products was independently proved by performing the chemical reaction with nickel(0) complexes.99 Several reports have described other possible applications of organonickel complexes in coupling processes. Thus, the addition of polyhalogen to CC unsaturated bonds (Kharash reaction) can be achieved on silicon dendrimers modified by organonickel σ-aryl-complexes.100 However, the role of the organonickel aryl complexes101 and pincer-type organonickel species102 in this process was elucidated some time later. The 4577


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alkyl groups. The step of β-hydride elimination116 leads to the formation of linear olefins and regeneration of the nickel hydride complex. The chain transfer process probably can be realized in these systems by associative olefin exchange between ethylene and the nickel complex accompanied by regeneration of the initial nickel hydride ethylene complex. Due to the high selectivity of these systems, the formation of branched-chain olefins is very limited. Important discoveries of the catalytic activity of new types of transition-metal α-diimine complexes were made by the research group of Brookhart in the 1990s.117 They showed that nickeland palladium-based organic catalysts formed by diimine ligands can polymerize ethylene to polymers of high molecular weight with high activity. Thus, the research of many scientific groups focused on the creation of new synthetic processes based on diimine ligands and the investigation of their mechanisms.117−119 Currently α-diimine complexes constitute a new class of highly reactive nickel catalysts for the preparation of α-olefins in the range C4−C26 with high selectivity to linear α-olefins (>94%). The postulated mechanism of the ethylene oligomerization under the action of Brookhart-type catalysts also involves the formation of organonickel complexes. This type of system is an effective alternative to the organometallic metallocene catalysts.120 It should be noted that the first mechanistic insights into the ethylene reaction with organonickel complexes were established by the research group of Yamamoto more than 40 years ago.31 The authors investigated the process of unsaturated hydrocarbon and carbonyl compound coordination to dialkyl nickel complexes and made some conclusions concerning the mechanism of C−C bond formation. Later, at the beginning of the 1990s, Ceder and co-workers investigated the reaction of organonickel complexes of the type [NiX(Mes)(PPh3)2], where X = Br, BF4, with ethylene.113 It was established that the insertion of the ethylene molecule into the Ni−C bond is available only in the case of preliminary dissociation of the neutral ligand. The formation of vinyl and butenyl derivatives with terminal organic aryl groups belonging to the starting organonickel complex was observed. The insertion of 1,2-dienes (allenes) 121 and acetylene122 into the Ni−C bonds of organonickel complexes was also investigated. A new class of neutral nickel complexes with salicylaldimine ligands was discovered. This type of ligand allows an increase in the stability of the complexes during the catalytic cycles.123 Other efficient catalytic systems based on organonickel complexes for the copolymerization of ethylene with CO124 and dimerization of propylene125 are currently known. Examples of organonickel complexes formed by PCCO, PCCN, and PNCN chelating ligands which are used for catalytic oligomerization and polymerization of unsaturated hydrocarbons have been discussed in reports.126,127 The elaborated oligo- and polymerization processes with participation of organonickel complexes are exemplified by the processes of butadiene polymerization,128 propylene dimerization,129 application of nickel phenanthroline complexes,130 isocyanate ligands,131 olefin polymerization with dimethyliminopentanone as a ligand in the nickel catalyst,132 oligomerization and co-oligomerization of alkynes and nitriles,133 polymerization of substituted methylenecyclopropanes,134 and others. The preparation of polyalkylthiophenes by cross-coupling polymerization of thiophene derivatives is a new catalytic application of organonickel σ-complexes,135 and the first examples of electroconductive polymers were obtained by

intramolecular condensation of allylic esters of 2-haloaryls including the formation of organonickel complexes has been described.103,104 In addition, reports of hydrocarbon ring expansion and the synthesis of new types of cyclic organonickel complexes have been presented.105 It should be noted that organonickel complexes have been found to be highly reactive in the catalytic cross-coupling processes of organic halides106 and chlorophosphines107 and in C−N bond formation reactions.108 The less common processes involve the effective mediator electroreduction of olefins and ketones, carboxylation of bromostyrenes and aziridines, and electrochemical coupling of olefins and organic polyhalides.109 Oligomerization and Polymerization Reactions. Oligomerization and polymerization reactions of unsaturated compounds with the participation of organonickel complexes has received special attention due to the high practical importance of these processes.110 Earlier scientific interest was mainly devoted to the organonickel catalysts which can be stabilized and isolated. However, after the discovery of a new class of highly effective nickel catalysts based on α-diimine ligands, several dissertations were devoted to the investigation of the catalytic activity of organonickel complexes.111 The polymerization and oligomerization processes involve three main stagesinitiation (formation of activated complex), growth of the chain, and a chain termination process. As a rule the postulated active species are organometallic alkyl complexes. Generally, polymers with high molecular weight are formed under the action of early transition metals (groups IV−VI), whereas the catalysts based on the metals of group VIII display favorable β-hydride elimination processes and, as a result, the formation of oligomers and dimers as the final products of the catalytic process. Moreover, late-transition-metal complexes have lower electrophilicity and higher heteroatom tolerance in comparison with the early transition metals. As a result, greater stability toward deactivation by polar impurities is observed.112 The transition-metal complexes can be formed by different heteroatom ligands and form mono- and binuclear complexes. Thus, olefin (ethylene, propene, norbornene, methyl acrylate, etc.) dimerization can be easily performed by nickel complexes with monodentate phosphorus ligands and oligomers and longchain polymers based on ethylene and styrene are available using bidentate ligands and cationic nickel complexes.113 Currently SHOP (Shell Higher Olefin Process) is the main process giving linear α-olefins. The organonickel catalysts industrially used in this process are based on the PCCO chelate bidentate ligands,114 as exemplified in Figure 2.

Figure 2. Main types of organonickel complexes operating in SHOP.

These systems can selectively oligomerize ethylene even in the presence of other olefins. This selectivity attributed to bidentate phosphinocarboxylate ligands explains the formation of pure linear α-olefins with a Flory−Schulz distribution.115 The mixture that is formed contains 96−98% of terminal olefins from C4 to C30. The mechanism proposed for this process involves the formation of organonickel complexes. Thus, the first insertion of the ethylene molecule into the Ni−H bond leads to the formation of an ethyl-nickel complex. Subsequent insertions result in the formation of alkyl-nickel complexes with long-chain 4578


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Figure 3. Amount (in grams) of the formed olefins depending on the type of the organonickel precatalysts used.

Scheme 6. Postulated Mechanism of the Ethylene Oligomerization Process under the Action of the Activated Organonickel Complex [NiBr(Mes)(bpy)]a

a

Reprinted from ref 20 with permission by Elsevier.

regioselective polycondensation of alkylthiophenes.136 A discussion of the main aspects of this research can be found in a review.137 Oligomerization processes catalyzed by new asymmetric bidentate neutral nickel aryl phosphine complexes138 and P,N-substituted ferrocene ligands139 have been reported. Some recent works include new nickel derivatives in the catalytic polymerization of norbornene140 and styrene.141 For most of the elaborated processes the mechanism has been established and key intermediates−organonickel complexes have been isolated as the stable species.142 A recent study concludes that polyorganyl nickel complexes, where the σ-bonded organic group can be used as the ligand for stabilization of the catalytically active form of organonickel catalysts, can be formed in the reaction medium during the catalytic process.143,144 Our recent study of the direct involvement of electrochemically synthesized145 organonickel complexes of the type [NiBr(aryl)(bpy)] in the catalytic processes of ethylene oligomerization showed that these species are highly efficient precatalysts for catalytic processes.8,146 The experiments performed showed that the presence of an organic group attached to the nickel atom greatly increases the catalytic activity

of the nickel complex in comparison with related binary nickel complexes based on diimine ligands. The comparable activity of the organonickel complexes of the type [NiBr(aryl)(bpy)] vs the nickel dibromide complex [NiBr2(bpy)] in the ethylene oligomerization process is illustrated in Figure 3.8,146 Indeed, under comparable experimental conditions, the dibromide precursor was significantly less active (also evidenced by a much less exothermic reaction) but showed a similar selectivity for α-olefins and a slightly higher α value. The oligomerization of ethylene by [NiBr2(bpy)]/MAO (MAO = methylalumoxane) has been previously investigated by Brookhart and co-workers.117b Scheme 6 shows the mechanisms of oligomerization, propagation, chain transfer, and olefin isomerization proposed for a variety of Ni(II) catalysts containing chelating nitrogen ligands.38 The results obtained by using [NiBr(Mes)(bpy)] as the catalyst precursor nicely fit the overall mechanism illustrated in Scheme 6. Indeed, after MAO has created a free coordination site at nickel, the first ethylene insertion(s) into the Ni−mesityl bond, followed by β-H elimination, would give a [(bpy)NiH] moiety. The fact that both the activity and the α factor do not change on 4579


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an increase in the ethylene pressure is actually consistent with a Ni alkyl resting state, while the increase in α-olefin selectivity with monomer pressure can be explained by chain transfer occurring via an associative displacement. Finally, the absence of isomerization of added n-undecene indicates that chain transfer from an internal olefin hydride nickel complex is faster than chain transfer from an α-olefin hydride species. Assuming that [NiBr(aryl)(bpy)] and [NiBr2(bpy)] exhibit the same mechanism of ethylene oligomerization, the lower activity of the dibromide complex may be rationalized in terms of the different solubilities of the two precursors in toluene. The dibromide complex is much less soluble than the monobromide derivative, and its reaction with MAO is initially heterogeneous. A shorter induction period for [NiBr(aryl)(bpy)], evidenced by a rapid exotherm, may also be caused by the presence of a metal− carbon bond in this precursor, which would favor the insertion of the monomer.

Scheme 7. Preparation of Organonickel α-Diimine Complexes with One and Two Ni−C Bondsa

PREPARATION OF ORGANONICKEL COMPLEXES The synthetic approaches for the preparation of organonickel complexes can be divided into two classes: (1) reactions which proceed without formation of a new Ni−C σ-bond (ligand exchange reactions, reactions of another σ-bond, etc.) and (2) reactions which proceed with the formation of a new target Ni− C σ-bond (transmetalation, oxidative addition, etc.).11,12 Ligand Exchange Reactions. The ligand exchange reaction is the simplest way to obtain organonickel complexes. This reaction operates only in the presence of a nickel−carbon σ bond in the starting substrate, and the formation of the desired organonickel complex can be achieved by the substitution of the weak ligand by a stronger ligand. The synthetic possibilities of this method are limited by the nature of the ligands used. Organometallic Reagents. The major route to organonickel σ-complexes is the reaction of nickel halide complexes with organomagnesium or organolithium reagents followed by a ligand exchange reaction. The use of organometallic reagents is the most common way to introduce the σ-bonded organic fragment to the coordination sphere of the nickel complex. This is connected mainly with the accessibility and easy synthesis of magnesium, lithium, or aluminum organic reagents. As a rule the reaction of nickel−carbon bond formation operates under the action of the Grignard reagents followed by a ligand exchange reaction.10−12 Stable organonickel complexes formed by phosphine and α-diimine ligands, bearing a σ-bonded aromatic group, can be easily obtained by this procedure, as exemplified in Scheme 7 for aryl nickel complexes formed by tertiary phosphine and diimine ligands.17 The use of organolithium reagents allows the selective formation of alkyl nickel complexes with two Ni−C σ-bonds. However, the synthetic possibilities of this reaction are limited by the high reactivity of the dialkyl nickel complexes formed, which are stable only at low temperatures. In addition, the production of organonickel complexes by the reaction of [Ni(acac)2], where acac = acetylacetone, with alkyl aluminum in the presence of tertiary phosphines has been described by Yamamoto.23 Thus, the organometallic species allow easy access to organonickel complexes with one, two, and more Ni−C σbonds. However, this technique is limited by the reactivity of the ligand attached to the nickel center toward the organometallic reagents used and the nature of the substituents in the organic fragment. A detailed study of the mechanism of organic group transfer in organometallic complexes by transmetalation reactions can be found in a review.147

Oxidative Addition Reactions. Oxidative addition reactions involving nickel(0) complexes are the most common processes for the preparation of organonickel species.11,12 By analogy to the Grignard reagents, nickel(0) species are capable of activating carbon−halogen bonds and form oxidative addition products. The process includes the activation of the substrate used by insertion of the nickel atom into one of the easily reduced chemical bonds of the substrate. The oxidation degree of the metal center is changed to the positive values. Thus the terminology “oxidative addition” is mainly related to the metal as the metal loses, as a rule, two electrons. The first examples of oxidative addition reactions of the organic halides to nickel(0) complexes have been described by Jones et al.148 The formation of biaryls takes place under these conditions. The use of nickel(0) complexes also allows access to the derivatives of dihalogenated compounds.149 Concerning the mechanism of the oxidative addition, here two types have been proposed. The first one involves three-center addition and SN2 substitution. The second one can be characterized as a multistage process involving paramagnetic intermediates. Thus, high selectivity in the reaction with polyhalides allows the conclusion that the mechanism of the process is analogous to the known nucleophilic aromatic substitution. However, the formation of paramagnetic nickel(I) complexes and the products formed in a radical way in the reaction of some aryl halides with nickel(0) complexes supposes a possibility of the second type of process. It should be noted that the synthesis of organonickel complexes by oxidative addition also can be performed by insertion into the carbon−element bond. Thus, in the desulfurization of dibenzothiophene, the formation of organonickel complexes is realized by the reaction of [Ni(bpy)(COD)], where COD = 1,5-cyclooctadiene, with a sulfur-containing substrate.150 The use of Raney nickel is a typical example of oxidative addition to the C−S and C−O bonds of aromatic esters151 and epoxides.152 The experimental results allow the conclusion that the rate of the oxidative addition reaction is the function of the nature of the halogen and organic group. The rate strongly decreases in the order C−I > C−Br > C−Cl to C−F. Only some nickel(0) complexes stabilized by σ-donor ligands, such as trialkylphosphines and hydrides, can be involved in the reactions of oxidative addition with C−F derivatives bearing sp2- and even sp3hybridized carbon atoms. Thus, special attention has currently been devoted to the elaboration of new processes for the activation of relatively inert C−F and C−CN bonds. Recent

a

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substituted aromatic bromides.8,20,145 Recently, we have shown that diimine organonickel σ-aryl complexes [NiBr(aryl)(N−N)] (N−N = 2,2′-bipyridine, 1,10-phenanthroline) bearing ortho substituents in the σ-bonded aromatic ring can be efficiently synthesized using electrochemical techniques, either in a single electrochemical cell with a sacrificial nickel anode8,145 or in an electrochemical cell supplied with a diaphragm to separate the anodic and cathodic compartments.20 The mechanism of the overall process involves cathodic in situ electrochemical generation of the highly reactive nickel(0) complex [Ni0(bpy)] followed by oxidative addition of ortho-substituted aryl bromides (arylBr), while the anode material (Mg, Zn, Ni, or Al) is oxidized (Scheme 9). For stabilization of the electrochemically generated

examples involve activation of C−F bonds in tetrafluorobenzenes.153 As a variant of the oxidative addition reaction, processes where the generation of nickel(0) complexes takes place in situ by the reductive elimination of two alkyl substituents from the coordination sphere of the nickel complex can be considered. The first example of this reaction was reported by Yamamoto,154 where the reaction of the complex [Ni(CH3)2(bpy)] with chlorobenzene led to the organonickel complex [NiCl(Ph)(bpy)]. The process was accompanied by the dimerization of the methyl groups and formation of butane. The highly reactive, coordinatively unsaturated monochelate [Ni0(bpy)] complex that is formed as an intermediate can react with chlorobenzene via activation of the C−Cl bond and formation of the organonickel complex [NiCl(Ph)(bpy)]. Heating the [NiCl(Ph)(bpy)] complex formed in the protic media leads to quantitative formation of benzene and chlorobenzene, as exemplified in Scheme 8.

Scheme 9. Electrochemical Synthesis of Organonickel σ-Aryl Complexes

Scheme 8. Formation and Decomposition of the Organonickel Complex [NiCl(Ph)(bpy)]

nickel(0) complex, 2 equiv of 2,2′-bipyridine is required. Thus, the complex [NiBr2(bpy)2] has been used as the starting reagent for this synthesis. The mechanism of electrochemical reduction of the starting [NiBr2(bpy)2] complex157 and the chemical reactivity of electrochemically generated nickel(0) bpy species toward organic halides have been described in detail earlier.19 The reduction of the [NiBr2(bpy)2] complex is realized as two consecutive quasi-reversible stages of the transfer of two electrons and then one electron. The first stage includes electrochemical reduction of the [NiBr2(bpy)2] complex to [Ni0(bpy)2] at EpC1 = −1.65 V (vs Ag/AgNO3, 0.01 M in CH3CN). The second stage is the transfer of an additional electron to the [Ni0(bpy)2] complex at EpC2 = −2.28 V (vs Ag/ AgNO3, 0.01 M in CH3CN), leading to the formation of an anion radical complex with localization of the unpaired electron predominantly on the ligand.157 The electrochemical difference between these two processes, 630 mV, allows their separation under the conditions of preparative reduction. The electrochemical synthesis of organonickel σ-complexes is carried out at potentials of the first stage corresponding to the reduction of the nickel(II) to the nickel(0) complex (EpC1). The use of sacrificial anodes in electrochemical processes allows an increase in the efficiency of the electrolysis and simplification of the electrolyzer construction, namely to exclude some technical difficulties due to the use of the diaphragm.158 Moreover, this technique allows exclusion of the anodic oxidation of electrochemically formed nickel(0) complexes and generation of oxidants such as molecular bromine and peroxides capable of oxidizing both the reduced nickel complex and the organonickel species formed. The sacrificial anode nature has a crucial influence on the efficiency of the electrochemical synthesis of organonickel σ-complexes.8 The highest yields of organonickel σ-complexes were obtained in an undivided electrochemical cell159 supplied with a sacrificial nickel anode (Figure 5).

According to Tamaru,11 the main possible oxidative addition reactions to ordinary chemical bonds leading to organonickel complexes can be presented as shown in Figure 4.

Figure 4. Main types of the oxidative addition reactions used for preparation of organonickel complexes. Reprinted from ref 11 with permission by John Wiley and Sons.

It should be noted that oxidative addition reactions are also available for unsaturated compounds. During this interaction the decomposition of the π-bond and the formation of two new Ni− C bonds take place. The oxidative addition can be realized by the π-bonds of different molecules, leading to the formation of new cyclic organometallic compounds. The first example was described more then 60 years ago in the reaction of cyclooctatetraene formation in the reaction of ethylene trimerization (Reppe synthesis).155 The other unsaturated organic substrates as alkynes and dienes are capable of reacting with carbon monoxide and dioxide, aldehydes, ketones, and imines with formation of cyclic products with Ni−C σ-bonds.156

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ELECTROCHEMICAL SYNTHESIS OF ORGANONICKEL COMPLEXES As a variant of the oxidative addition reaction, electrochemical methods have been applied to the preparation of organonickel complexes by reaction of electrochemically generated in situ highly reactive nickel(0) complexes in the presence of ortho4581


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technology is the absence of the high-cost supporting electrolyte, which is not desirable at the stage of the isolation of the electrochemically formed products. The electroconductivity of the electrolyte is constant and is achieved by the anodic generation of the nickel(II) ions in solution, keeping its concentration unchanged. The construction of the created electrochemical cell for the preparation of organonickel complexes in a flow regime is presented in Figure 6.

Figure 5. Electrochemical laboratory cell for the preparation of organonickel complexes.

Figure 6. Electrochemical flow cell for the preparation of organonickel complexes.

In this case, the anodically generated cations are the source of nickel ions for the formation of organonickel complexes, since they are able to form complexes with bpy in solution and take part in the electroreduction process followed by the formation of desirable organonickel species. Thus, 2,2′-bipyridine and aromatic bromide are the main consuming reagents, except for the electrochemically soluble nickel anode, of the electrochemical preparation of organonickel complexes. The reaction undergoes the catalytic cycle with the complete absence of side products (Scheme 10). Our recent interest was focused on the development of a new type of electrolyzer, allowing us to synthesize organonickel complexes in a flow regime that is important from the viewpoint of industrial applications.160 The main advantage of the created

The elaborated electrochemical flow cell supplied with a sacrificial nickel anode serving as the source of nickel ions in solution allows organonickel complexes of the type [NiBr(aryl)(bpy)] to be obtained using aromatic bromides and 2,2′bipyridyl as the starting reagents in high yield under conditions of “green chemistry”, without the formation of byproducts.8 The electrochemically synthesized organonickel complexes of the type [NiBr(aryl)(bpy)], where aryl = Xyl (2,6-dimethylphenyl), Mes (2,4,6-trimethylphenyl), Tipp (2,4,6-triisopropylphenyl), have been isolated from the reaction mixture and fully characterized, including by X-ray structure analysis (Figure 7). According to the X-ray data, the nickel center is coordinated in a distorted-square-planar environment by a chelating bpy ligand and a mesityl group and a bromo ligand are in a cis arrangement. The X-ray crystal structure analysis shows that the aromatic ring is perpendicular to the N,N,Ni plane, apparently due to the steric interaction of the ortho substituents of the aromatic group with the cis bromo ligand and the ortho hydrogen atom of bpy.17,20 Alternatively a related organonickel complex bearing an unsubstituted phenanthroline ligand, [NiBr(Mes)(phen)], was successfully synthesized using a modified electrochemical procedure8 in a single electrochemical cell supplied with a sacrificial nickel anode.21 Although the yield of the organonickel

Scheme 10. Electrochemical Synthesis of Organonickel Complexes with a Sacrificial Nickel Anode

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Figure 7. Molecular structures of electrochemically synthesized organonickel complexes.

σ-complex [NiBr(Mes)(phen)] in the electrochemical process is slightly less than in the case of the classical ligand exchange reaction, this method seems more convenient, because it allows direct access to this species starting from the binary nickel salt without the preparation of flammable Grignard reagents or the relatively unstable [Ni(COD)2] complex.161 The investigation of the electronic stricture of the electrochemically obtained organonickel complexes allows the conclusion that these species are diamagnetic due to the presence of a strong-field ligand (σ-bonded organic group), resulting in dsp2 hybridization of the metal center. The diamagnetic state of the nickel(II) ion in this complex is also supported by magnetic measurements (Figure 8) and the complete absence of an EPR signal in the wide temperature range (4−303 K) measured with an X-band EPR spectrometer.

Figure 9. Geometry of the nickel center as the function of ligand field splitting. Reprinted from ref 11 with permission by John Wiley and Sons.

also leads to the formation of organonickel species.165 Moreover, the formation of organonickel complexes as key intermediates of catalytic coupling of organic halides has been found in an electrochemical cell supplied with a Pt cathode and an electrochemically soluble Ni anode.166 However, this method does not allow isolation of the postulated organonickel species.

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ELECTROCHEMICAL PROPERTIES OF ORGANONICKEL COMPLEXES The use of the electrochemical analytical methods allowed investigation of the processes of electrochemical reduction of organonickel complexes. The first experiments of the detailed study of the electron transfer in organonickel α-diimine complexes have been performed by Klein et al.6,167 The performed experiments were limited to organonickel complexes of the type [NiBr(Mes)(N−N)], where N−N = diimine ligand. It was established that electron transfer to the nickel diimine complexes results in formation of a radical anion, where the electronic density is mainly located on the diimine ligand.18 Later the electrochemical properties of the organonickel σ-complex [NiBr(Mes)(bpy)] were investigated.167 It was established that stabilization of the electrochemically formed radical anion [NiBr(Mes)(bpy)]•− proceeds by bromide anion elimination with formation of the coordinatively unsaturated radical complex [Ni(Mes)(bpy)]• . Thus, for the series of organonickel complexes of the type [NiBr(Mes)(N−N)], where N−N = αdiimine ligand, the mechanism of electroreduction given in Scheme 11 has been proposed.167 According to the postulated mechanism, after the first electron transfer the bromo anion is eliminated from the coordination sphere. The coordinatively unsaturated species that is formed can be stabilized by solvent coordination (C2) or by a dimerization process (C2′). The dimerization reaction leads to the formation of a NiI−NiI bond and results in binuclear nickel complexes. The

Figure 8. Magnetic field dependence of the static magnetization of [NiBr(Mes)(bpy)] (red curve) and [NiBr2(bpy)] (blue curve) at T = 2 K.

The approach to the description of the nickel σ-complexes from the standpoint of electronic structure is that generally these compounds are diamagnetic, containing paired d-electrons at the metal center, providing a square-planar geometry. Indeed, according to Tamaru the hybridization of the nickel center is the function of the ligand field splitting, as is shown in Figure 9.11 Therefore, the electron configuration of the nickel center in organonickel complexes is described as 3d8 and corresponds to divalent nickel, which is in the dsp3 hybridization state that realizes a square-planar geometry of the metal center.162,163 Thus, using Tolman nomenclature164 the formation of stable 16and 18-electron complexes as exemplified for the electronic structure of organonickel complexes makes them favorable in catalysis, since catalytic transformations proceed through the elementary steps involving such intermediates. It should be noted that the electrochemical generation of nickel(I) complexes, as exemplified by nickel salen complexes, 4583


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Scheme 11. Electrochemical Reactions of the Organonickel Complex [NiBr(Mes)(bpy)]a

a

Reprinted from ref 167 with permission by John Wiley and Sons.

Figure 10. Cyclic voltammogram of [NiBr(Mes)(phen)] recorded at the first scan at constant potential scan rate 50 mV s−1: (red curve) from 0.00 V to −2.00 V and then to +0.80 V and back to 0.00 V; (blue curve) from 0.00 V to +0.80 V then to −2.00 V and back to 0.00 V. Peak potentials are referenced to an Ag/AgNO3 0.01 M in CH3CN reference electrode.

reactions of these types are well-known in the literature:168 for example, the formation of RuI−RuI or Re0−Re0 dimers in the electroreduction processes of the corresponding complexes [(bpy)RuII(CO)2X2] and [(L)Re(CO)3X] (L = bpy, phen; X = Cl, CN). Both examples can be attributed to the formal d7−d7 system. As an example of a d9−d9 systems, the Pd1−Pd1 dimer [(phen)(MeCN)Pd−Pd(MeCN)(phen)]2+, obtained from the comproportionation of PdII and Pd0, is currently known. A careful study of electron transfer to an organonickel complex formed by unsubstituted 1,10-phenanthroline, [NiBr(Mes)(phen)], has been recently published.21 The question concerning

the bonding nature of the radical anion of phen that is formed was clarified by performing the macroscale electroreduction of the organonickel complex [NiBr(Mes)(phen)] in an EPR electrochemical cell over a wide range of temperature. It was found that the electrochemical behavior of [NiBr(Mes)(phen)] (Figure 10) is very characteristic for the related α-diimine complexes.6,167 Very recently the electrochemical properties of some cationic organonickel complexes with terpyridine ligands have been investigated.169 It was established that the organonickel complexes [(R′-terpy)Ni(aryl)]X, where R′-terpy = alkyl4584


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Scheme 12. Main Stages of C−P Bond Formation in the Reaction of Electrochemically Activated Organonickel Complexes with P4

electrochemically obtained organonickel species was investigated toward chlorophosphines and white phosphorus.7,171,172 The catalytic cross couplings of organic halides and organic mono- and dichlorophosphines have already been reported by several research groups.5−7,107 The electrochemical reaction proceeds very selectively and results in the formation of tertiary phosphines bearing the organic group of the starting organic halide used in the process. Thus, is was concluded that electrochemically generated in situ organonickel complexes can transfer the organic group to the phosphorus atom by chlorine substitution, as in the case of the well-known organolithium and Grignard reagents. However, the use of phosphorus trichloride in the electrocatalytic process seems difficult, due to the low potential of its electrochemical reduction, which is less negative than the potential of the system NiII/Ni0 used for the generation of organonickel complexes. In order to solve this problem, electrochemically isolated organonickel complexes were used in the reaction with phosphorus trichloride.173 It was experimentally found that addition of a solution containing [NiBr(Mes)(bpy)] to a mixture containing an excess of PCl3 in diethyl ether results in selective formation of MePCl2. Note that in this case the transhalogenation process (the halogen exchange of Cl with Br in the formed MesPCl2 molecule) does not occur, this being a characteristic of MesMgBr usage.174 This is probably associated with the stronger coordinating ability of the nickel atom with regard to Br in the organonickel complex [NiBr(Mes)(bpy)]. This feature is also one of the advantages of using this electrochemical approach for the synthesis of MesPCl2 from phosphorus trichloride. However, the electrochemically synthesized organonickel complexes [NiBr(aryl)(bpy)], where aryl = 2,4,6-trimethylphenyl, 2,4,6-triisopropylphenyl, 2,6-dimethylphenyl, have been found to be absolutely inert toward the P4 molecule. Mixing of the solutions containing white phosphorus and σ-complex in different solvents (DMF, benzene, toluene, THF, etc.) and refluxing of the reaction mixtures did not lead to the formation of any phosphorus products. Moreover, according to 31P NMR data, the P4 molecule remained unchanged during the entire time of the experiment. Surprisingly, total conversion of P4 was immediately observed after one-electron reduction of the σbonded complex.171 A deeper study of this system allowed the conclusion that electrochemically activated, coordinatively unsaturated organonickel complexes are capable of coordinating a white phosphorus molecule and the desirable process of C−P bond formation is realized in the subsequent reductive elimination process accompanied by transfer of the aromatic group to the coordinated P atom and opening of the P4 tetrahedron (Scheme 12): The insoluble organic nickel phosphides that are formed can be easily hydrolyzed by acidic water solutions, resulting in phosphinic acids bearing an aryl−P bond. Thus, the corresponding arylphosphinic acids (aryl)P(O)(OH)H, where aryl = 2,4,6trimethylphenyl, 2,4,6-triisopropylphenyl, 2,6-dimethylphenyl, were obtained in moderate yield (up to 64% by the initial nickel complex).171,172 Moreover, it has been found that the reaction of the acids formed with the starting form of the nickel(II) catalyst

substituted 2,2′;6′,2″-terpyridine, aryl = 2,6-dimethylphenyl (Xyl), 2,4,6-trimethylphenyl (Mes), and X = Br, PF6, are capable of being reversibly reduced with electron transfer to the πorbitals of the nitrogen ligand as the oxidation is a nickelcentered process and proceeds irreversibly.

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ELECTROCHEMICAL ACTIVATION OF ORGANONICKEL COMPLEXES We have found that the reactivity of organonickel complexes is strongly increased by transfer of one electron.7,170,171 In other words, the efficiency of the electrochemical mediator system applied to the preparation of homocoupling and cross-coupling products by electrocatalytic reduction of organic halides, chlorophosphines, and other organic and inorganic substrates proceeds with higher yields by performing the electrochemical synthesis at potentials 100−200 mV more negative in comparison with those used for the system NiII/Ni0. This range of potentials corresponds to the reduction of organonickel complexes which are formed in the reaction medium after the first stage of the electrochemical process: namely, the oxidative addition of the electrochemically generated highly reactive nickel(0) complexes to organic halides. We have shown that electrochemical reduction strongly increases the reactivity of the complexes in cross-coupling reactions, giving products that contain the aromatic group of the organonickel complex used.170,171 The increase in the reactivity of organonickel complexes can be attributed to the formation of coordinatively unsaturated organonickel complexes of the type [Ni(aryl)(N− N)], bearing a radical diimine ligand (N−N) bonded to the nickel atom. The free coordination site at the nickel center allows coordination of a substrate, initiating a new catalytic cycle leading to the coupling product. Reaction of C−C Bond Formation. The oxidative addition reaction to electrochemically activated organonickel complexes is a key stage of the electrocatalytic cross coupling of organic halides.5−7 Thus, an investigation of the reactivity of electrochemically activated organonickel complexes toward organic halides has been performed. The electrochemical behavior of the organonickel complex [NiBr(Mes)(bpy)] in the presence of organic halides was investigated.170 It was found that in the presence of increasing amounts of organic halides (RX) the current of the first peak of reduction is also increased. Moreover, this increase is limited at high concentration of the added RX, while at low concentrations it is proportional to √CRX, where CRX is the molar concentration of the added RX in solution. At the same time the anodic reoxidation peak is decreased and completely disappears. This proves the assumption that the electrochemically generated complex [Ni(Mes)(bpy)]•− reacts with organic halides, leading to the products of the oxidative addition [NiX(Mes)(R)(bpy)]•− or the radical species [Ni(Mes)(R)(bpy)]•, as proposed by Perichon et al. (see Scheme 5).5a Subsequent reductive elimination reactions allow the crosscoupling product MesR to be obtained. Reaction of C−P Bond Formation. In order to estimate the possible synthetic potential of organonickel complexes in the processes of C−P bond formation, the reactivity of the 4585


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complex [NiBr2(bpy)2] leads to selective formation of very interesting, from the viewpoint of magnetic chemistry, dinuclear nickel(II) complexes, formed by bridging μ-O2P(aryl)(OH)H ligands, displaying electron interaction between two nickel centers in one molecule.175

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of Tatarstan Republic in 2006 for the best research work. He has authored over 100 scientific publications and reports.

OUTLOOK

Thus, the preparative electrochemistry can be an effective, highyield, and selective method for the preparation and activation of organonickel complexes, which are key intermediates of different catalytic and electrocatalytic processes with participation of nickel complexes. The investigation of the properties of such species allows more insight into the mechanism of the processes catalyzed by transition-metal complexes. Using the elaborated synthetic protocols, a variety of organonickel complexes could be prepared or generated in situ and successfully employed in synthetic and catalytic processes, where they show improved activity in comparison to traditional precursors. Moreover, the smart combination of electrochemical techniques and classical principles of organometallic synthesis allows creation of new highly effective, resource-saving, and ecologically friendly “green” chemical technologies for the preparation of various chemicals for modern chemistry and materials science.

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Aliya Khusnuriyalova was born in Chishmy, Russian Federation, in 1993. She graduated from secondary school with full marks and was awarded a gold medal. Currently she is a fourth-year student at the Chemical Institute of Kazan Federal University. Since 2012 she has been working at the A. E. Arbuzov Institute of Organic and Physical Chemistry of the Russian Academy of Sciences on experimental studies of the electrochemistry of transition-metal complexes and organometallic compounds. Her main research interests are focused on the synthesis and investigation of mono- and binuclear nickel complexes, involving organonickel compounds with σ-bonded organic fragments. She was awarded a special grant of the Kazan Federal University as the prominent student and young scientist.

AUTHOR INFORMATION

Corresponding Author

*D.G.Y.: e-mail, [email protected]; fax, +7-843-2732253. Notes

The authors declare no competing financial interest. Biographies

Oleg Sinyashin was born in Kazan, USSR (now Russian Federation), in 1956. He obtained his Diploma in chemistry in 1978 and Ph.D. degree in 1981 at Kazan State University. He got his Doctor degree in chemistry in 1990. Since 2006 he has been an Academician of the Russian Academy of Sciences and is the Director of the A. E. Arbuzov Institute of Organic and Physical Chemistry of Russian Academy of Sciences. He is the Head of the Laboratory of Organometallic and Coordination Compounds. His research interests include the chemistry and electrochemistry of white phosphorus and organophosphorus compounds, organometallic complexes, and the chemistry of fullerene and the application of its derivatives to the development of solar cells and drugs and other biologically active materials. He has authored over 400 publications, including one monograph, articles in refereed journals, patents, and conference reports.

Dmitry Yakhvarov was born in 1974 in Kazan, Russian Federation. He graduated from the Department of Physical Chemistry, Kazan State University, in 1996 with excellent marks. He got his Ph.D. (Candidate of Sciences) in February 2000 and became a Dr. habil. (Doctor of Sciences) in 2012. In the period 2000−2013 he joined a number of international research groups in Germany, Italy, and France, applying the elaborated electrochemical methods to actual problems of organic and organoelement chemistry and materials science. His current position is a Leading Scientist of the A. E. Arbuzov Institute of Organic and Physical Chemistry of the Russian Academy of Sciences (Kazan, Russian Federation). His current research interests are focused on the chemistry and electrochemistry of organometallic species, elemental (white) phosphorus, silicon derivatives, homogeneous catalysis, and the chemistry of highly reactive intermediates, including electrochemically generated phosphane oxide, H3PO. He was awarded the Medal of the Russian Academy of Science in 2005 and State permit of the government

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ACKNOWLEDGMENTS Financial support from the Russian Foundation for Basic Research and Tatarstan Academy of Sciences (RFBR project 12-03-97067), the German Research Foundation (DFG project 4586


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PE 771/5-1), and the German Academic Exchange Service (DAAD project A/13/71281) is gratefully acknowledged. The authors personally thank Prof. Evamarie Hey-Hawkins, Prof. Bernd Büchner, Prof. Joachim Heinicke, Dr. Andreas Petr, Dr. Vladislav Kataev, Dr. Giuliano Giambastiani, Dr. Claudio Bianchini, Dr. Maurizio Peruzzini, Dr. Peter Lönnecke, Dr. Yulia Krupskaya and Dr. Yulia Ganushevich for scientific collaboration, invaluable scientific discussions, and assistance in some topics of this research.

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