Nickel center

From these results, the Institut Fran ais du Petrole (IFF) has developed a biphasic version of its established monophasic Dimersol process , which is offered for licensing under the name Difasol process [98]. The Difasol process uses slightly acidic chloroaluminate ionic liquids with small amounts of allcylaluminiums as the solvent for the catalytic nickel center. In comparison to the established Dimersol process , the new biphasic ionic liquid process drastically reduces the consumption of Ni-cata-lyst and allcylaluminiums. Additional advantages arise from the good performance obtained with highly diluted feedstodcs and the significantly improved dimer selectivity of the Difasol process (for more detailed information see Section 5.3). 

The bipyridyl ligand, with 80 %, gives the best catalyst, probably because it affords the best balance between the a-donor and rt-acceptor characters which favours respectively the oxidative addition of the aryl bromide and the reductive elimination of the aryl ether at the nickel center. 

The adduct between CO and a nickel center in CODH identified by EPR spectroscopy. ... 

Most of the as-isolated [NiFe] hydrogenases are inactive, and the nickel center exhibits an intense rhombic EPR signal termed Ni-A (g = 2.31, 2.26, and 2.02) with variable amounts of a second nickel species, named Ni-B (g = 2.33, 2.16, and 2.02), with slightly different... 

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

With P-donors a variety of different structures are formed. Two complexes (359a,b) with a five-coordinate square pyramidal geometry and one S-donor in the apical position are reported.934,933 In a related complex (360) the nickel center reveals a distorted square planar geometry, because one xanthate ligand switched to a monodentate coordination mode. 6 By reaction of [NiL2] complexes with Ph2P(CH2)2P(Ph)(CH2)2PPh2, compound (362) is formed, which consists of a five-coordinate cation and a hexacoordinate anion. 7... 

In (362), the cation is distorted trigonal pyramidal with the axial positions occupied by one S atom and the central P atom of the P3 ligand. In the anion the nickel center is bonded to six S-donors of three xanthate ligands. Upon reaction with dppe an approximately square planar NiS2P2 complex results.938 A similar coordination geometry is found for the two complexes (361a,b) 939,940... 

Also, the structure of the Ni11 complex (379) with the related ligand dimethyltetrathiomalonate has been determined.997 The geometry at the nickel center is approximately square planar with an inversion center. (379) shows reversible redox properties 998 Oxidation with I2 affords 1,3-bis (methylthio)-1,2-dithiolium triiodide. 

Reduction of both nickel porphyrins and thiaporphyrins to Ni1 species has been studied by EPR and 2H NMR spectroscopy.179, 2 58 The Ni1 complex of 5,10,15,20-tetraphenyl-21-thiaporphyrin has been isolated and characterized. Reaction of this complex with sulfur dioxide produced a paramagnetic five-coordinated Ni1 S02 adduct, while reaction with nitrogenous base ligands (amines, pyridines, imidazoles) yielded five- and six-coordinate complexes. In addition, the crystal structure of Ni1 diphenyldi-p-tolyl-21-thiaporphyrin has been determined. The coordination geometry about the nickel center is essentially square planar with extremely short Ni—N and Ni—S bonds (Ni—N = 2.015(2) A, 2.014(12) A, and 1.910(14) A and Ni—S = 2.143(6) A).2359... 

The cluster catalyzes hydrogenation (20°C and 3 atm) of dialkyl- and diarylacetylenes to the c/s-olefins via unsaturate routes, likely involving Ni4(CNR)6(RC=CR) and Ni4(CNR)4(RC==CR)3 (391, 392). The acetylenes in the latter complex bridge three nickel centers, and increase of the acetylenic carbon-carbon bond distance is considered to enhance reduction by hydrogen (392, 393). 

Bagley, K. A., Duin, E. C., Roseboom, W, Albracht, S. P. J. and Woodruff, W. H. (1995) Infrared-detectable group senses changes in charge density on the nickel center in hydrogenase from Chromatium vinosum. Biochemistry, 34, 5527-35. 

As indicated in Figure 3.2, this is probably just the binding of hydrogen (presumably as -1- H ) to the active site at some distance from nickel. This binding induces the oxidation of the nickel center (Ni Ni ), whereby the electron initially enters the proximal Fe-S cluster. CO can bind to the divalent nickel in the Nia-S state and thereby fixes the active site in the Nia-S-CO state ... 

Whilst C—H bonds represent the most ubiquitous chemical linkage in Nature, they are at the same time some of the most difficult bonds to cleave, although they are not completely inert [5b,c]. For example, in 1963 Kleinman and Dubeck reported the possibility of C—H bond cleavage in azobenzene by the Cp2Ni complex (Scheme 13.1) [1]. The structure originally proposed by Kleinman and Dubeck considered the nickel center to be coordinated T 2 to the N=N n-bond (2) (Scheme 13.2). 

Knochel and co-workers developed the Ni(acac)3-catalyzed cross-coupling reaction between polyfunctional primary iodoalkanes and a variety of primary diorganozinc compounds in the presence of w-trifluoromethylstyrene as a promoter. " The addition of this unsaturated promoter is required in order to coordinate to the nickel center and remove electron density from the metal atom, to facilitate the reductive elimination step. " " The scope of the reaction is extended, when Ni(acac)2 is used in the presence of BU4NI and fluorostyrene (Scheme With these... 

The bi-electronic reduction of this nickel compound generates a zero-valent nickel complex which is stable on the time scale of cyclic voltametry, provided at least three bipyridine ligands per nickel center are present. 

Figure 2.9 Redox-driven translocation of the anion X (e.g., chloride), based on the Nin/Nim change. The nickel center acts both as an engine and as areceptor for the X anion (when in the Ni111 state). Occurrence of the reversible X translocation is afforded by the following sequence of anion affinity Nim > Cu11 > Ni11.

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