Nickel complexes, electron-transfer reactions

Macartney DH, Sutin N (1983) Electron-exchange rates of polypyridine complexes electron-transfer reactions involving the tiis(polypyridine)nickel(II/III) couple in acidic aqueous media. Inorg Chem 22 3530-3534... 

Dynamic voltammetry is currently employed in studying nickel(III) and nickel(IV) complexes, particularly for the characterization of short-lived species and of the kinetic and thermodynamic behaviour of the electron transfer reactions.3026-3028... 

As has been noted (29, 31), nickel(III) poly(pyridine) complexes may be prepared. Electron-exchange rates have been determined (15) from a series of cross-reactions. The rate of the electron transfer reaction has... 

A comparison of Eq. 54 and Scheme 36 reveals that the nickel center performs two functions. First, the nickel center acts as mediator of the two electron-transfer reactions during which S—C bonds are cleaved and formed (36 —> 37 and 39 —> 40). Second, it facilitates the formation of an acyl group from an alkyl group and CO (38 —> 39). This reaction is expected to be favored when the nickel center has a coordination number lower than 5. A low coordination number of nickel also facilitates the final release of the thioester in the consecutive reaction of 39 with excess CO, because intermediates such as complex 42 can readily form. 

The remarkable hexanuclear complex [ NiCp ] (81), prepared by the sodium naphthalenide reduction of nickelocene, undergoes an extensive series of reversible one-electron transfer reactions cyclic voltammetry shows waves relating the six species [ NiCp 6] (Z = -2 to 3). Chemical oxidation of 81, with Ag, gave the monocation whose structure shows only a small tetragonal distortion from the octahedral array of nickel atoms in the neutral precursor (198). 

Stereoselective synthesis of organometallic complexes has been achieved in the oxidative addition of aryl halides to triethylphosphine nickel(O) complexes, leading to the exclusive formation of trans-2ivy nickel(II) halide complexes [383]. Electron-transfer reactions on the Fe of cis- and ra/w-[7/-C5H5Fe(CO)SR]2 occur stereospecifically with no stereoisomerization on changing the oxidation state of the Fe [384,385]. In the electrochlorination of a ligand (R) of the //-C5H5Fe(CO)R complex, the stereochemistry is retained [386]. 

Sulfur nucleophiles give reduced products consistent with either nucleophilic attack at a sulfur atom or electron transfer reactions <90PS(53)425>. The dithiolate anions formed by these reactions can be characterized as nickel complexes <89JAP(K)63222188>. Thiocyanate ion attacks at S(2) in 1,2-dithiole-3-thiones <88jhci223>. 

The Marcus analysis of the rate constants for electron transfer reactions of the [Ni((-)-(R)-Me[9]aneN3)2] couple with nickel and cobalt complexes yields a self-exchange rate constant of 1.2 x 10" The oxidations of... 

The cationic nickel(I) macrocycle, [Ni(tmc)], reacts with 1,4-dihaloalkanes to form ethylene with no evidence for the formation of cyclobutane. The mechanism is thought to proceed via an initial electron transfer to give the monoalkyl radical and free halide. The radical species then reacts with the nickel cation to give an organonickel complex, which undergoes a further electron transfer reaction with another [Ni(tmc)] to provide the products as in equations (66)-(68). 

The kinetic data for a series of outer-sphere electron transfer reactions between the [Rh2(02CCH3)4(CH3CN)2] couple and nickel tetraaza macrocycles and iron and ruthenium tris(polypyridine) complexes in acetonitrile have been correlated in terms of the Marcus relationship, yielding a [Rh2] electron exchange rate constant of 3.0 1.7 x 10 M A somewhat smaller value of 5.3 1.3 x... 

Therefore, the catalytic cycle is found to proceed with a succession of elementary steps involving paramagnetic and diamagnetic nickel complexes. The catalytic cycle is a succession of single electron transfer reactions (whose half-wave potentials have been characterized) and chemical steps (whose rate constants have been determined by transient electrochemical technics) . The first oxidative addition step and the fixation of CO2 by ArNi (dppe) are fast reactions while the last chemical step, e.g. the insertion step of CO2 is the rate determining step of the catalytic process. 

An example of reversible intramolecular electron transfer has now been reported though not for a metal-metal system. It had previously been shown that the complexes [Ni (TPP)] (TPP =tetraphenylporphinate) when electrochemically oxidized in benzonitrile solution yield first the brown-coloured nickel(m) complex, which then decays at a measurable rate by a process of ligand-to-metal electron transfer [reaction (7), forward step (TPP )"=radical-ion] ... 

Nickel(IV) complexes react with dimethyl sulphoxide in acidic solution to give the sulphone and nickel(II) ions. The kinetics of this reaction have been studied and found to be very complex in nature. The reaction probably proceeds by initial complexation of the dimethyl sulphoxide to the nickel(IV) species followed by electron transfer and oxygen atom transfer producing the observed products149. 

However, there is evidence that reactions of aluminium hydride produced in situ involve single-electron-transfer (SET) processesThe reactions described by Trost and Ghadiri have most likely not been studied in sufficient detail to permit an adequate description of the reaction mechanism to be given at this stage. It is, however, quite likely that the Grignard reactions catalyzed by copper(II) and nickel(II) complexes , as developed by julia - and by Masaki , do involve SET processes, although, if this is so, the preservation of stereochemistry in some of the examples described by these workers is quite remarkable. (In this context, the reader s attention is drawn to Reference 196, end of this section.)... 

It was first suggested that the reaction of an alkyl halide with a nickel(I) Schiff base complex yields an alkylnickel(III) intermediate (Equation (56)). Homolytic cleavage of RBr to give an alkyl radical R and a nickel(II) complex (Equation (57)) or, alternatively, one-electron dissociative reduction leading to R (Equation (58)) are possible pathways.254 A mechanism based on the formation of R via dissociative electron transfer of Ni -salen to RX (Equation (59)) has also been proposed.255... 

Few studies have systematically examined how chemical characteristics of organic reductants influence rates of reductive dissolution. Oxidation of aliphatic alcohols and amines by iron, cobalt, and nickel oxide-coated electrodes was examined by Fleischman et al. (38). Experiments revealed that reductant molecules adsorb to the oxide surface, and that electron transfer within the surface complex is the rate-limiting step. It was also found that (i) amines are oxidized more quickly than corresponding alcohols, (ii) primary alcohols and amines are oxidized more quickly than secondary and tertiary analogs, and (iii) increased chain length and branching inhibit the reaction (38). The three different transition metal oxide surfaces exhibited different behavior as well. Rates of amine oxidation by the oxides considered decreased in the order Ni > Co >... 

A similar reaction can be written for the [Fe] hydrogenases with a Fe-[4Fe-4S] complex replacing the nickel. Note that the nickel atom in the NiFe cluster, and the Fe-[4Fe-4S] sites are nearest to the electron carrier [4Fe-4S] clusters, indicating that electron transfer occurs through these atoms. The other atom in each of the centres is an iron atom with -CN and -CO ligands, and it seems likely that this is a binding site for hydride (Fig. 8.1). 

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

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