A. Graham Lappin

The Redox Chemistry of Nickel

Publisher Summary The discovery of nickel(III) and nickel(I) in methanogenic bacteria and in other biological systems has focused attention on the redox chemistry of nickel. This chapter discusses the redox chemistry of nickel basing on the most recent advances. This chapter presents a discussion on the structural and electronic requirements of the four oxidation states—I, II, III, and IV— and of the primary physical techniques and probes used. A description of the most commonly used ligand types follows, together with an indication of recent trends in the area. Discussions conclude with a review of the recent mechanistic chemistry of the oxidation states in turn. The nickel(II) ion has a d 8 electronic configuration and, with weak-field ligands such as H 2 O, it forms a six-coordinate ion with approximately octahedral symmetry and a paramagnetic (two unpaired electrons) 3 A 2 ground state. The X-ray crystal structures of a variety of high- and low-oxidation-state nickel complexes are now known. The technique most widely applied as a structural probe in the chemistry of both nickel(III) and nickel(I) is electron paramagnetic resonance. The mechanistic chemistry of nickel(IV) is dominated by substitution inert complexes and outer-sphere electron transfer reactions.

Oxygenation of Cobalt Porphyrinates: Coordination or Oxidation?

The X-ray characterization of the five-coordinate picket-fence porphyrin complex, [Co(TpivPP)(2-MeHIm)], is reported. The complex has the displacement of cobalt from the porphyrin plane = 0.15 A, and Co-N(Im) = 2.145(3) and (Co-N(p))(av) = 1.979(3) A. This five-coordinate complex, in the presence of dioxygen and excess 2-methylimidazole, undergoes an unanticipated, photoinitiated atropisomerization of the porphyrin ligand, oxidation of cobalt(II), and the formation of the neutral cobalt(III) complex [Co(alpha,alpha,beta,beta-TpivPP)(2-MeHIm)(2-MeIm(-)]. Two distinct examples of this complex have been structurally characterized, and both have structural parameters consistent with cobalt(III). The two new Co(III) porphyrin complexes have axial Co-N(Im) distances ranging from 1.952 to 1.972 A, but which allow for the distinction between imidazole and imidazolate. An interesting intermolecular hydrogen bonding network is observed that leads to infinite helical chains. UV-vis spectroscopic study suggests that [Co(TpivPP)(2-MeHIm)(O(2))] is an intermediate state for the oxidation reaction and that the atropisomerization process is photocatalyzed. A reaction route is proposed based on the spectroscopic studies.

Driving force effects in proton coupled electron transfer

The rates of reduction of the nickel(III) oxime‐imine complexes, [Ni III (Me2L)] and [Ni III (Me2LH)] 2 by the corresponding iron(II) species are determined. Comparisons with rates calculated with the use of Marcus theory reveal that reactions of [Fe II (Me2LH)], where the electron transfer product is the highly acidic [Fe III (Me2LH)] 2 , show rate enhancements of several orders of magnitude. These are ascribed to a strong coupling of the electron transfer with the concomitant, thermodynamically favorable proton transfer. A free energy correlation supports this conclusion.

The oxidation of [Co(edta)]2− by [Co(phen)3]3+

Abstract [(Co(phen)3][(Sb(R, R)-tartrate)2] · 8H2O, C44H44N6O20CoSb2, crystallizes in the trigonal space group P3221 (No. 154) with Z = 3, a = 18.861(4), c = 11.917(3) A and R = 0.035 for 1800 reflections. The structure has been determined and the absolute configuration of (+)589-[Co(phen)3](ClO4)3 · 2H2O formed from the compound by Cl2 oxidation is confirmed as Λ. Reduction of [Co(phen)3]3+ by [Co(edta)2− is outer-sphere with a second-order rate constant of 9.6 x 10−3 M−1 s−1 at 25 °C and 0.10 M ionic strength. When Λ-[Co(phen)3]3+ is used as oxidant, chiral induction is observed and the [Co(edta)]− product shows a enantiomeric excess of 20% of the Λ isomer. This homochiral (ΛΛ) preference contrasts with the heterochiral (ΔΛ) preference found previously for reactions of [Co(edta)]2− with [Fe(phen)3]3+ and [Os(phen)3]3+. Interpretation of the results is aided by 1H NMR relaxation studies of the ion pair {[Co(phen)3]3+, [Co(edta)]−}.

Unfolding of the [Cu2(1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane)2]2+ Helicate. Coupling of the Chlorocarbon Dehalogenation to the Unfolding Process

A new helical dimeric copper(I) complex [Cu(2)(mphenpr)(2)](ClO(4))(2) where mphenpr is 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane has been prepared and characterized by X-ray crystallography and NMR. In the solid state, the metal centers are 6.42 A apart, and the electronic structure has been investigated with use of density functional theory (DFT) calculations. In solution the dimer equilibrates with a monomeric form [Cu(mphenpr)](ClO(4)), and the mechanism of unfolding of the dimer into monomer has been studied. In the presence of CCl(4), formation of the monomer is coupled to the reductive dehalogenation of the halocarbon. The mechanism of this process has been probed by the study of short-lived potential reaction intermediates using fast kinetic pulse radiolysis techniques and comparisons with DFT calculations. The copper(II) product [Cu(mphenpr)Cl](ClO(4)) and an analogue [Cu(mphenpr)](ClO(4))(2) have been isolated and characterized by X-ray crystallography.

Kinetics and mechanisms of reduction of a sexidentate di(oxime–imine) complex of nickel(IV) with two-electron reductants, 1,2-di-hydroxybenzene and 1,4-dihydroxybenzene

The kinetics and mechanisms of reduction of a nickel(IV) di(oxime–imine) complex, [NiIVL]2+(H2L = 3,14-dimethyl-4,7,10,13-tetra-azahexadeca-3,13-diene-2,15-dione dioxime), by 1,2-dihydroxybenzene (H2Z) and 1,4-dihydroxybenzene (H2Q) are reported. Both reactions proceed by consecutive one-electron transfers and kinetic traces are biphasic with quantitative formation of nickel(III) intermediates, [NiIIIL]+ and [NiIII(HL)]2+ depending on the pH. The dominant pathways for reduction of both nickel(IV) and nickel(III) involve the reductant anions HZ– and HQ–. Reactions of nickel(IV) are outer-sphere in nature while those of nickel(III) proceed by an inner-sphere pathway. Preliminary results on the oxidation of the corresponding nickel(II) complex by thallium(III) indicate that no detectable intermediate oxidation-state species is formed in this reaction.

Resolution and characterization of helicate dimer and trimer complexes of 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane with copper(I)

Complexation of copper(I) with the binucleating ligand, 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane, mphenpr, result in formation of helical dimers, [Cu2(mphenpr)2](2+). The resolution of the enantiomeric forms of the dimers has been carried out with Δ-[As(cat)3](-) as resolving agent and X-ray structures for two compounds, P-[Cu2(mphenpr)2](Δ-[As(cat)3])2 and P-[Cu2(mphenpr)2](Δ-[As(cat)3])2·4(CH3CN), are reported. The rate of racemization in poorly-coordinating solvents has been examined by (1)H NMR, and is slow. At saturating concentrations of [[Cu2(mphenpr)2](2+)] in acetonitrile, crystals of the helical trimeric complex [Cu3(mphenpr)3](ClO4)3 are obtained. The X-ray structure of the trimer is reported. This species has also been resolved. As with the helical dimer, racemization in poorly-coordinating solvents is slow, and circular dichroism and (1)H NMR spectra are reported. The absolute configuration of the resolved complex, P-[Cu3(mphenpr)3](Δ-[As(cat)3])3, has been determined by X-ray crystallography.

Structure and magnetic properties of [Cr(en)2(ox)][Cr(en)(ox)2]·2H2O,Δ-[Cr(en)3]Δ-[Cr(ox)3] and Δ-[Co(en)3]Δ-[Cr(ox)3]

Abstract Interactions between metal ion complexes in the compounds Δ-[Cr(en)3]Δ-[Cr(ox)3] and Δ-[Co(en)3]Δ-[Cr(ox)3] have been investigated by X-ray crystallography and magnetic susceptibility measurements. Crystals of both compounds are hexagonal and are isostructural with the known Δ-[Co(en)3]Δ-[Rh(ox)3] [Inorg. Chem. 30 (1991) 4954] with chains of alternating anions and cations parallel to the crystallographic c-axis. The susceptibility behavior suggests that magnetic interactions are stronger in Δ-[Cr(en)3]Δ-[Cr(ox)3] where both anion and cation are paramagnetic than in Δ-[Co(en)3]Δ-[Cr(ox)3] where only the anion is paramagnetic. Single crystal studies with the former compound reveal that magnetic interactions along the c-axis chains, where the Cr–Cr distances are shortest, are weaker than those involving off-axis interactions. Magnetic measurements were also made on [Cr(en)2(ox)][Cr(en)(ox)2]·2H2O. The structure of this compound has been previously reported.

Effects of electronic structure on chiral induction in outer-sphere electron transfer reactions of complexes with the C3 symmetric ligand, 1,4,7-triazacyclononane-1,4,7-tris[2′(R)-2′-propionate](-3)

Abstract The ligand 1,4,7-triazacyclononane-1,4,7-tris[2′( R )-2′-propionate](-3)(( R )-tacntp 3− ), binds stereospecifically to transition metal ions. The structures of the complexes [Cr(( R )-tacntp)]·NaBr and [Fe(( R )-tacntp)]·H 2 O have been determined by X-ray crystallography. Both complexes have the Λ-configuration but the conformation of the chelate rings in Λ-[Cr(( R )-tacntp)] is (λ,λ,λ) with a geometry close to octahedral while in Λ-[Fe(( R )-tacntp)] it is (δ,δ,δ) and the geometry is closer to that of a trigonal prism. Chiral induction in the electron transfer reactions of Λ-[Co(( R )-tacntp)], Λ-[Fe(( R )-tacntp)] and Λ-[Mn(( R )-tacntp)] with [Co(( RR,SS )-chxn) 3 ] 2+ has been investigated. All three reactions are outer-sphere and four isomeric [Co(( RR,SS )-chxn) 3 ] 3+ products are identified in each case. The oxidants Λ-[Fe(( R )-tacntp)] and Λ-[Mn(( R )-tacntp)] show very similar selectivities, quite different from those of Λ-[Co(( R )-tacntp)]. Reasons for this behavior are discussed.

STEREOSELECTIVITY IN THE REDUCTION OF Λ-[1,4,7-TRIAZACYCLONONANE-1,4,7-TRIS[2′(R)-2′-PROPIONATE]COBALT(III)]

Abstract [Co((R)-tacntp)] · NaBr, C15H24BrCoNaN3O6 crystallizes in the trigonal space group R3(No. 146) with Z = 3, a = 9.843(2) A, c = 15.781(4) A and R = 0.053 for 859 reflections. The structure has been determined and the absolute configuration about the complex is M(C3) (Λ). The second-order rate constant for outer-sphere reduction of Λ-[Co((R)-tacntp)] by [Co(en)3]2+ is 6.3 × 10−2 M−1s−1 at 25.0°C and 0.10 M ionic strength. The product, [Co(en)3]3+, is formed with 11% enantiomeric excess of the ▵ isomer. The corresponding reduction of Λ-[Fe((R)-tacntp)] has a rate constant 5 × 103 M−1s−1 and results in formation of [Co(en)3]3+ which shows 3% enantiomeric excess of the ▵ isomer.