Dinuclear complexes, absorption spectra

Fig. 6). The dinuclear complex displays an intense absorption band in its visible spectra. [The Mn(II) is not expected to contribute significantly to the visible spectrum, as Mn(II)(phenoxide)2 is an amorphous, white polymer (144)]. These two complexes represent the first well-characterized high-valent Mn-phenoxide complexes. Note also that in the structures of the biphenoxide complexes, the benzene rings of each half of the biphenoxide ligands are oriented such that no conjugation is present and each half of the ligand is similar, electronically, to phenoxide. 

One of the most powerful methods of evaluating the properties of mixed-valence complexes is spectroelectrochemistry. A key experiment is to reversibly cycle a dinuclear complex through fully oxidized, mixed-valence and fully reduced states. A near IR absorption band that appears only in the electronic spectrum of the mixed-valence state is strong evidence for an intervalence transition. Determination of the nature of the mixed-valence state can also be achieved by spectroelectrochemical methods. 

When manganese(iii) is mixed with the [VO(dtpa)] " ion in an acetate buffer medium, a reaction occurs to form a complex that retains the characteristic visible absorptions of Mn i and but slowly bleaches to Mn and V. The disappearance of and formation of Mn can also be detected by the e.s.r. spectrum. It is proposed that the initial product is the dinuclear complex (5), which slowly undergoes electron transfer and rearrangement to form (6). 

Three Mn catalases have been purified and characterized, and all appear to have similar Mn structures (17). The Mn stoichiometry is ca. 2 Mn/subunit, suggesting a dinuclear Mn site. The optical spectrum of the as-isolated enzyme has a broad weak absorption band at ca. 450-550 nm in addition to the protein absorption at higher energies. This spectrum is similar to those observed for Mn(III) superoxide dismutase and for a variety of Mn(III) model complexes, thus implying that at least some of the Mn in Mn catalase is present as Mn(III). In particular, the absorption maximum at ca. 500 nm is similar in energy and intensity to the transitions seen for oxo-carboxylato-bridged Mn dimers, suggesting that a similar core structure may be seen for Mn catalase (18). 

The photophysical properties of another trinuclear silver(I) complex [Ag3([i3-dppnt)3] (38a) have also been studied [115]. The complex shows absorption bands at 238 and 367 nm in CH2CI2. The emission spectrum of the complex in CH3CN shows an emission band at 550 nm (T6 = 5.0 ps) at room temperature. The phosphorescent state has been assigned to be ligand-centered in nature. The X-ray crystal structure of the dinuclear gold(I) counterpart shows that a potassium ion is encapsulated in the macrocyclic cavity, forming the complex [Au2K(p3-dppnt)j + (38b) [115]. 

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