Mixed-valent complexes, dinuclear

The synthesis, structure, and electrochemistry of Ir1 dinuclear complexes [Ir(/i-L)(cod)]2 (L = 2-aminopyridinato (ap) and 2-anilinoj)yridinato (anp)) are reported, in which the square-planar Ir1 centers are 3.0998 A and 3.0681 A apart, respectively.511 Both complexes may be reversibly oxidized to the mixed-valent species. The frozen EPR spectrum of the mono-oxidized anp species shows a rhombic signal with no resolved hyperfine splitting. 

For dinuclear Cu complexes, several pathways are possible as summarized in Scheme 15 [182]. In addition, plausible alternatives involve mixed-valent Cu Cu species where only one of the Cu ions is directly involved in the electron transfer. The latter seems most hkely in cases where the substrate binds to only one of the two copper ions, and H2O2 may then form upon oxidation of the Cu Cu -semiquinone intermediate [195]. Different coordination modes of the DTBC substrate appear to be indeed possible, depending on the particular dicopper scaffold [133,196,197]. Unfortunately, detailed mechanistic studies are still quite scarce [198-203] and most proposed catalytic pathways are rather speculative. 

Since it is known that the tetranuclear mixed-valent platinum blue and tan complexes such as 1 and 2 undergo disproportionation and reduction by water as Eqns. 1-3 and 7-9 show [54] [66], all the species appearing in Eqns. 1-3 and 7-9 are present in the solution. However, only one or several of the four species in the solution may in fact be active during catalytic olefin oxidation. To clarify this point, the effects of the Pt oxidation state in the platinum complexes were compared. The results are summarized in Table 2. It clearly shows that the dinuclear Pt111 complex is most effective, and is likely to be a true catalyst. Compound 1 also exhibits high activity, whereas the dinuclear Pt11 complex is ineffective. All other factors expected to affect the catalytic efficiency, including the presence of 02, the surfactant and the choice of solvent, have been examined and the results are summarized... 

The reversible FeTe1 FeTe11 couple of an N-heterocyclic carbene dinuclear FeTe1 complex, (p-pdt) [FeI(CO)2(PMe3)][FeI(CO)2(IMes)] (IMes= l,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), complex D, has led to the isolation of the mixed-valent cationic complex D as a biomimetic of the 2Fe2S subsite of the oxidized H cluster in... 

Several dinuclear units have been assembled to form linear Au5 + and Ane complexes. However, they were shown to be Au An mixed-valent compounds. The normal Au -Au distances are in the range 2.5-2.7A. The only binuclear gold(II) complex (15) with unsupported Au Au bond has been shown to possess interesting photophysical and thermal redox properties." ... 

The electrochemistry of dinuclear vanadium o-A-salicylideneamino-ethylphenyl complexes were investigated.372 Conversion of Viv—Viv dimers to Vv—Vv dimers was reported and serves as a representative model system for oxidation reactions.294 296 A related class of these dinuclear complexes form mixed valence vanadium(IV/V) complexes upon oxidation and exhibit an unusual electron delocalization over the V203 3+ core.310 A V111—Vlv mixed valent, dinuclear bis(salicy-lidene)ethylenediamine (salen) complex underwent multielectron oxidation with dioxygen to yield a Vv—Viv mixed valence complex.302... 

Similar reactions of dinuclear complexes sometimes give rise to mixed-valent intermediates, for example, diamond core Fera( a,-0)2FeIV species obtained from diiron(III) TPA complexes and H202.85... 

Krogmann et al. (516) have also reported the preparation of the isoelectron-ic mixed valent Ir2(C0)4Cl2(02CCH3) species from a mixture of hexachloro-iridate(IV) and formic and acetic acids. Preliminary powder x-ray data indicate that the dinuclear species possess extremely short Ir-Ir spacings of 2.78 A and a helical superstructure of 0.7 A in diameter. Further work is necessary to clarify the physical and chemical properties of these Ir - complexes in solution and the solid state. 

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