Nickel complexes electrochemical preparation

The electrochemical preparation of organozinc compounds obtained from the corresponding aromatic halides and with the use of a nickel complex as catalyst is only efficient in dimethylformamide as solvent. Moreover, in most cases and as described previously, the reaction requires the presence of excess 2,2 -bipyridine (five molar equivalents with respect to nickel) to achieve the transmetallation reaction leading to the organozinc compound and to avoid the formation of biaryl, Ar-Ar (equation 53). 

A number of nickel(I) complexes were prepared in situ by electrochemical reduction of the corresponding nickel(II) complexes, mainly with macrocyclic tetraaza ligands and dithiolenes. 

The above study has been extended to the synthesis of 4 which was prepared in a similar fashion to 1 and 3 starting from tris(l,4,7-triazacyclonon-l-ylmethyl)benzene [12], The X-ray structure of the [Ni2(4)(H20)3]4+ cation shows that both nickel ions adopt distorted octahedral coordination geometries, with one nickel being sandwiched between two tacn residues while the other is coordinated to the third tacn residue with the coordination sphere being completed by three water ligands. Electrochemical studies indicate that the sandwiched Ni(II) centres in this and the related nickel complexes mentioned above may the be reversibly oxidised to the Ni(III) state. In the case of the dinuclear bis-sandwich complex, [Ni2(4)](C104)4, the electrochemical results indicate that the two nickel centres behave in an essentially independent manner. 

With the aid of NMR spectroscopy ( H- H NOESY) for PDDT and its copolymer with 3-methylthiophene in deuterated chloroform, the configurational structures were determined [90,91]. The 3-dodecylthiophene monomer is proved to be attacked predominantly at the p position of the thiophene ring (head) with a probability of 82% during the electrochemical polymerization [90]. PHT prepared by using a zero-valent nickel complex contains a larger proportion of HH units than HT units [92]. Chemically polymerized poly-(3-cyclohexylthiophene) contains HT and HH-TT coupled components in the ratio of 7 3 [93]. 

Then, the use of nickel(II) or cobalt(II) complexes as catalyst associated to the sacrificial anode process allows synthesis of functionalized mono- or diorganozinc species in a simple and efficient manner. Alternating Jt-conjugated copolymers, based on this electrochemical preparation of intermediate aryldizinc species and their subsequent palladium-catalyzed coupling with unsaturated dihalogenated compounds, can be synthesized. Furthermore, aromatic ketones are synthesized efficiently via cobalt-catalyzed cross-coupling reaction between arylzinc bromides and acid chlorides. 

This oxidation state which resembles Cu(II) may be prepared by electrochemical, photochemicalor pulse radiolytic reduction of nickel(II). Nickel(I) macrocycles are powerful reductants and their spectra and redox potentials have been measured. The reactions of the Ni(I) complexes Ni(tmc) 10 and 11 with RX are similar. 

The first structural report on a phthalocyanine complex concerned [Ni(pc)J (Table 110 I).2878 In the crystal lattice of this compound the square planar macrocycles are arrayed in slipped stacks such that the distance between the molecular planes along the perpendicular direction is 388 pm. [Ni(pc)] may be prepared by a variety of methods 2873,2871 2880 a convenient one is heating a foil of elemental nickel in o-cyanobenzamide at 270 °C (Scheme 60).2881 [Ni(pc)] is insoluble in the most common organic solvents, but soluble in concentrated sulfuric acid from which it is reprecipitated unchanged upon dilution. This complex is thermally very stable and may be sublimed in vacuo. The reduction of [Ni(pc)] can be accomplished by chemical or electrochemical methods and results in ligand-based reduced anions [Ni(pc)]" ( = 1, 2). Analogously, the electrochemical oxidation results in the oxidized ligand. 3... 

In the most important series of polymers of this type, the metallotetraphenylporphyrins, a metalloporphyrin ring bears four substituted phenylene groups X, as is shown in 7.19. The metals M in the structure are typically iron, cobalt, or nickel cations, and the substituents on the phenylene groups include -NH2, -NR2, and -OH. These polymers are generally insoluble. Some have been prepared by electro-oxidative polymerizations in the form of electroactive films on electrode surfaces.79 The cobalt-metallated polymer is of particular interest since it is an electrocatalyst for the reduction of dioxygen. Films of poly(trisbipyridine)-metal complexes also have interesting electrochemical properties, in particular electrochromism and electrical conductivity.78 The closely related polymer, poly(2-vinylpyridine), also forms metal complexes, for example with copper(II) chloride.80... 

Among the electrochemical syntheses related to the change of metal oxidation number, we emphasize obtaining acetylacetonates of divalent iron, cobalt, and nickel [551,623]. The method of alternating-current electrochemical synthesis was applied to isolate Ji-complexes of monovalent copper with allylamines, allylimines, and ally-lurea from the salts of divalent copper [624-628], We note that the same method was used for preparation of analogous ji-complexes with copper(II) halides (X = Cl, Br) [629a]. Other electrochemical syntheses with participation of metal salts and complexes are described in monographs [201,202] and literature cited therein. 

With a view to preparing polymerizable complexes, thiophene-substituted nickel-dithiolene complexes [Ni(L)(L )] have been synthesized and used to prepare films by electrochemical polymerization. The features of the complexes and of the polymers depend on the number of thiophene substituents. In particular, the complex with four thiophene substituents (L = L = thpdt, 12) shows a narrower HOMO-LUMO gap as compared to complexes with two thiophene and two phenyl groups or four phenyl groups [Xmax, nm (e, M- cm- ) 976 (38800) L = L = 12 931 (37700) L = 12, L = 9, R = Ph 866 (30900) L = L = 9, R = Ph] and gave a polymer whose electrochemical features are similar to those of poly[l,2-di(2,5-thienylene)ethane], suggesting that similar extended chains are formed"". 

Salts of [NKbpyXs] can be prepared by electrochemical oxidation of the corresponding nickel(II) complex (93, 962) the ion is a powerful... 

Substrates 1-4, containing Rp substituents (Cf. eqs. 1-3) have been prepared according to literature procedures [13]. Their electrochemical behaviour and in particular, the CO2 fixation under mild conditions have been examined in the presence of several nickel(II) complexes used as catalyst precursors. Ni(II) derivatives associated to polydentate nitrogen ligands have been reported as efficient catalysts in the electrochemical carboxylation of atkynes [14] and diynes... 

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