Single bridge dimers

The aqueous solution chemistry of Ir in its higher oxidation states III, IV, and V has been explored by Sykes et al.41,48 Chemical and electrochemical oxidation of Ir(H20)6]3+ gives a brown-green Irv product, which undergoes chemical and electrochemical reduction to a blue and a purple IrIV complex. 170 NMR studies are consistent with double- and single-bridged dimeric structures, with likely formulas [(H20)4Ir(/i-0H)2Ir(H20)4]6+ for the blue complex and [(H20)5Ir(/r-0)Ir(H20)5]6+ for the purple one. 

Singly bridged dimers resulting from C—C coupling had been produced by in vitro oxidation of benzylisoquinoline monomers, but this is the first report of their natural occurrence. Pisopowine-type dimers may be biogenetic intermediates in the formation of alkaloids of the funiferine (71) and tiliacorine (65) groups (566). 

From anon-linear least-squares fit to the data at two concentrations near 21 °C and 0.5m ionic strength, for the first reaction, pKa = 7.74(1), and for the dimerization reaction, Kd = 132(12) m. This singly-bridged dimer is much weaker than the doubly-bridged one in the (en)Pd11 complex above. The maximum concentration of dimer occurs at the half-equivalence point, 0.57 mM for a solution that is 5 mM in total Pd11. This weak dimerization does not affect the results with nucleic bases mentioned below [36],... 

Now consider an induced reaction for proton-assisted dissociation of a singly bridged dimer (see 15, 17) where the reaction products retain the proton. The first step, as before, is equilibrium association of a proton to the Li2-OH site between the metals, thereby forming a weakly bridging water molecule between two Fe(III) atoms ... 

With single-bridge bonding the shape of the OH absorption band remains essentially sharp [38], and observations on limited groups of materials indicate that within any one class, at least, the extinction coefficients are reasonably constant and that Beer s Law is obeyed [29, 35]. In the case of single-bridge dimers the dimeric forms contribute to both the free and bonded OH absorption. 

This was the first unexpected example of the combined acids system for asymmetric synthesis (Scheme 10) [20]. It is known that coordinatively unsaturated monomers are far more Lewis acidic than doubly bridged coordinatively saturated dimers [21]. A mono-coordinated complex, however, can be generated in some cases and is even more Lewis acidic than the monomer through the formation of a singly bridged dimer. This species is the combined acid catalyst. 

Rate-enhancing protons and ligands are commonly stable constituents in the inner co-ordination sphere of a dissociation complex. Consider how a single-bridged dimer dissociates to release monomers. 

Table IV lists the redox potentials of conjugated ferrocene oligomers (mainly dimers with a single bridge). Potential values are denoted against different reference electrodes as given in the references. The values can be primarily compared using the relationship mentioned in the footnote of the table, although care should be taken with some errors derived from junction potentials which depend on experimental conditions. There have been several reports on the quantitative estimation of structural factors affecting internuclear electron delocalization.

Sequential steps are proposed (75- 77) with two monomeric units combining to form first a single hydroxo-bridged dimer, which in turn closes to give the double hydroxo-bridged dimer. This latter species [(H20)4Cr(0H)2Cr(0H2)4]4+ is one of the major products from the 02 oxidation of [Cr(OH2)6]2+ (11) (Section I,A), and the crystal structure of the p-toluenesulfonate salt has been determined (78). 

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