Nickel complexes deprotonated

Transition metal hydroxyoxime complexes have been reviewed very recently.2507 Their use in both analytical chemistry and extraction metallurgy is well known. The square planar structure of the bis chelate complex NiL (347) with the deprotonated 2-hydroxybenzaldoxime (HL) is typical of this series of nickel complexes.2508 Their bis adducts, NiLJ, with bases such as py, substituted pyridines and cyclomethyleneimines, are six-coordinate.2509 The acyl oxime (H2L) complexes are similar to the aforementioned complexes being either square planar bis chelates Ni(HL)2 (348) or octahedral bis adducts, Ni(HL)2B2.2507 When the acyl oxime acts as a dibasic ligand L, the corresponding (NiL) complexes are insoluble and involve extensive polymerization. 

Nickel(lll) deprotonated peptide complexes can be easily obtained in solution by chemical or electrochemical oxidation of the corresponding nickel(U) complexes. They are moderately stable in aqueous solutions and have been widely characterized by EPR, electron spectroscopy and cyclic voltammetry.3047,3058,3087,3088 Some selected examples of nickel(III) peptide complexes are reported in Table 115. 

Table 115 Selected Examples of Nickel(IU) Deprotonated Peptide Complexes... 

Taddol has been widely used as a chiral auxiliary or chiral ligand in asymmetric catalysis [17], and in 1997 Belokon first showed that it could also function as an effective solid-liquid phase-transfer catalyst [18]. The initial reaction studied by Belokon was the asymmetric Michael addition of nickel complex 11a to methyl methacrylate to give y-methyl glutamate precursors 12 and 13 (Scheme 8.7). It was found that only the disodium salt of Taddol 14 acted as a catalyst, and both the enantio- and diastereos-electivity were modest [20% ee and 65% diastereomeric excess (de) in favor of 12 when 10 mol % of Taddol was used]. The enantioselectivity could be increased (to 28%) by using a stoichiometric amount of Taddol, but the diastereoselectivity decreased (to 40%) under these conditions due to deprotonation of the remaining acidic proton in products 12 and 13. Nevertheless, diastereomers 12 and 13 could be separated and the ee-value of complex 12 increased to >85% by recrystallization, thus providing enantiomerically enriched (2S, 4i )-y-methyl glutamic add 15. 

Figure 5-13. The linking together of two pseudo-macrocyclic nickel complexes by electrophilic attack of an aldehyde on a diazadiketonate. Both the product and the starting material are neutral and contain nickel(n) centres. Each ring is doubly deprotonated - first at the 1,3-diazadiketonate and second at the dioxime.

Macrocycle Vila readily forms the doubly deprotonated nickel complex upon addition of Ni(OAc)2. The methylene chloride soluble complex was unreactive under reaction conditions similar to those employed in the nickel cyclam/PhIO system. However, oxidation of - 8-methylstyrene occurred readily when the hypochlorite phase transfer conditions were used. Epoxidation represented about 50% of the reaction pathway with the remainder yielding a mixture of benzaldehyde and other over-oxidation products. 

Treatment of the amino-phosphine complex with H2 generates a hydrido nickel complex with a pendant ammonium substituent. Catalysis, which was established electrochemicaUy, is proposed to involve oxidation of the Ni(ll) hydride to a Ni(III) hydride, a process which enhances the acidity of the hydrido ligand sufficiently to allow its deprotonation by the pendant amine (Fig. 12.12). This proton transfer gives an easily oxidized Ni(I) intermediate. Catalysis of the H2 oxidation by mononuclear nickel complexes foreshadows the preparation of related bimetallic species exhibiting hydrogenase reactivity. 

Methyl-3-formylpyrazole A-phenylthiosemicarbazone, 45, coordinates as a deprotonated NNS tridentate ligand to form the diamagnetic, spin-paired [Ni(45-H)NCS] complex [200]. However, the complexes formed with other nickel(II) salts have stoichiometry [Ni(45)2]A2, where A = Cl, Br, NO3, CIO4, and BF4. These octahedral complexes NNS coordinated neutral hgands have Dq values between 1086 and 1150cm" ... 

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