Nickel Macrocycles

It has square planar coordination (Pd-N 2.010-2.017 A) similar to the value of 2.009 A in the tetraphenylporphyrin analogue, prepared by a similar route. As with nickel, macrocycle complexes can be made by in situ template... 

SlOO proteins, calcium binding, 46 451-456 Spruhtrocken process, 4 26 Square-planar complexes, 4 157-164 octahedral, compared, 4 162-174 in solution, 34 270-271 Square-planar iridium complexes, 44 295, 297 Square-planar nickel macrocyclic complexes equilibrium with octahedral species, 44 116-118... 

In addition, E /2 for the M(II/I) couple of cobalt and nickel macrocycles under CO2 in aprotic media are typically shifted toward cathodic potentials, but remain fully reversible [15]. These results indicate that CO2 binding is fast and reversible for this class of complexes. Similarly, the Fe(0) species formed on reduction of derivatives of Fe fi tetraphenylporphyrins react very rapidly with CO2 [37, 38]. 

CO2 molecule, or Mg + and CO2 play the role of oxide acceptor to form water, carbonate, and MgC03, respectively [38]. The reactions of the iron carboxylate with these Lewis acids are thought to be fast and not rate determining. For the cobalt and nickel macrocyclic catalysts, CO2 is the ultimate oxide acceptor with formation of bicarbonate salts in addition to CO, but it is not clear what the precise pathway is for decomposition of the carboxylate to CO [33]. The influence of alkali metal ions on CO2 binding for these complexes was discussed earlier [15]. It appears the interactions between bound CO2 and these ions are fast and reversible, and one would presume that reactions between protons and bound CO2 are rapid as well. 

The template synthesis has also been successfully employed for the preparation of macrocycles containing mixed donor atoms. Examples which refer to tetra- and bexa-dentate ligands are given in Schemes 42, 47 and 50.2649,2653 2654,2658 Apart from the template synthesis a number of nickel macrocycles have been prepared by direct combination of the appropriate nickel(II) salt with the preformed macrocyclic ligand in alcoholic medium, often MeOH (see also Tables 103, 106-108). 

Oxidation of DNA by nickel macrocycles and KHSOs occurs via base oxidation (8). It has been proposed that the guanine oxidation leads to the formation of 8-oxoguanine, which is known to promote strand scission upon base treatment. It has been reported that one-electron oxidation of guanine leads to a radical cation that reacts with water to... 

The prototypical photochemical system for CO2 reduction contains a photosensitizer (or photocatalyst) to capture the photon energy, an electron relay catalyst (that might be the same species as the photosensitizer) to couple the photon energy to the chemical reduction, an oxidizable species to complete the redox cycle and CO2 as the substrate. Figure 1 shows a cartoon of the photochemical CO2 reduction system. An effective photocatalyst must absorb a significant part of the solar spectrum, have a long-lived excited state and promote the activation of small molecules. Both organic dyes and transition metal complexes have been used as photocatalysts for CO2 reduction. In this chapter, CO2 reduction systems mediated by cobalt and nickel macrocycles and rhenium complexes will be discussed. 

Overview of CO2 Reduction Systems Mediated by Cobalt and Nickel Macrocycles... 

Photochemical carbon dioxide reduction with metal complexes Differences between cobalt and nickel macrocycles... 

PHOTOCHEMICAL CO2 REDUCTION WITH NICKEL MACROCYCLES 4.1 Photochemical CO2 reduction... 

Both NiL+ and NiL(C02) species are formed under CO2 atmosphere by irradiation at 313 nm in acetonitrile solutions containing TEA and NiL +. In order to understand the interesting behavior of these nickel-based systems we have studied the nature of the ground-state complexes, electrochemical CO2 reduction, and the differences in CO2 binding between cobalt and nickel macrocycles. 

The electrocatalytic activity of various nickel macrocycles in aqueous solution was studied. Cyclic voltammograms indicate that 7 / S -NiHTIM2+, NiMTC2+ and NiDMC + are better catalysts than Ni(cyclam)2+ in terms of more positive potentials and/or their larger catalytic currents [26], Bulk electrolyses with 0.5 mM Ni complexes confirm that these complexes are excellent catalysts for the selective and efficient CO2 reduction to CO. The macrocycles with equatorial substituents showed increased catalytic activity over those with axial substituents. These structural factors may be important in determining their electrode adsorption and CO2 binding properties. 

H. Sohrabi, M. Esmaeeli, F. Farzaneh, M. Ghandi, Nickel(macrocycle) complexes immobilized within montmorillonite and MCM-41 as catalysts for epoxidation of olefins, ]. Inclusion Phenom. Macrocyclic. Chem. 54 (2006) 23. 

A similar picture is reflected by the stability constants of the mixed complexes formed in various donor solvents between iodide ions and nickel macrocyclic systems with inner coordination spheres analogous to those of the dimethylglyoxime complexes. 

Rotating-copper-disk electrode techniques have been used to evaluate the efficiency of the nickel macrocycle catalyst for the reduction of CO2 to Studies have been performed using Ni(diazacyclam) + (diazacyclam = 3,10-dimethyl-1,3,5,8,10,12-hexaazacyclophane) (5.6), a complex derived from cy-clam, which appears to be more active than [Ni(cyclam)2] " under the same conditions (see Figure 5.70) . These results are consistent with a mechanism proposed by other authors - . ... 

Smith, C.I., J.A. Crayston, and R.W. Hay (1993). Reduction of carbon dioxide by nickel macrocyclic catalysts adsorbed on a mercury electrode or a copper rotating disc electrode. J. Chem. Soc. Dalton Trans. 21, 3267-3269. 

Trevin, S., F. Bedioui, G.M.G. Villegas, and C. Bied-Charreton (1997). Electropoly-merized nickel macrocyclic complex-based flms Design and electrocatalytic apph-cation. J. Mat. Chem. 7(6), 923—928. 

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