Cofacial ligands

Figure 67 Cofacial disposition of the metals allowing for the incorporation of negatively charged ligands...

Unsymmetrical zinc phthalocyanine analogs with three 15-crown-5 ether ligands appended at the 3,4-positions were synthesized and characterized. Introduction of alkali metal ions results in cofacial dimer formation with evidence for this dimerization from NMR.841... 

Cofacial ruthenium and osmium bisporphyrins proved to be moderate catalysts (6-9 turnover h 1) for the reduction of proton at mercury pool in THF.17,18 Two mechanisms of H2 evolution have been proposed involving a dihydride or a dihydrogen complex. A wide range of reduction potentials (from —0.63 V to —1.24 V vs. SCE) has been obtained by varying the central metal and the carbon-based axial ligand. However, those catalysts with less negative reduction potentials needed the use of strong acids to carry out the catalysis. These catalysts appeared handicapped by slow reaction kinetics. 

In order to prepare a dirhodium compound, Rh2(DPB), the cofacial bispor-phyrin, H4(DPB), which was introduced in Sect. 3.2.1, was metallated to yield dirhodium(III) compounds with iodide and/or ethoxycarbonyl as axial ligands [279], While iodide is supposed to be able to reside inside the diporphyrin cavity, the ester moiety is located outside the cavity, since the compound (RhCOOEt)2(DPB) does not add triethylphosphane, contrary to the diodide. Photolysis of all Rh(III) derivatives obtained yielded the wanted product,... 

Figure 12.6 (a) The structure of Pacman-type cofacial bisporphyrin and (b) simplified illustration of the Pacman effect in a photocatalytic cycle. LMCT, ligand-to-metal charge transfer... 

The stracture of hemerythrin in a variety of derivatives (oxy, azido, met, and deoxy) is now well-characterized. With three bridging ligands, a distinctive cofacial bioctahedral stereochemistry is seen (Figure... 

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