Oxo-bridged dimer

The iron protein has been found in animals, viruses and some prokaryotic organisms. The best studied example is from Escherichia coli.S19 The enzyme consists of two subunits, Bi and B2. Subunit B, has a molecular weight of 160 000 and is made up of two polypeptide chains. It contains redox-active SH groups, which play a role in the reaction, and has two binding sites for substrates and sites for effector molecules. It can bind any of the four ribonucleotide substrates. Subunit B2 has a molecular weight of 78 000 and also has two polypeptide chains. It contains a dimeric oxo-bridged iron site associated with a remarkably stable tyrosine radical. The formation of the active enzyme from B, and B2 requires the presence of magnesium ions. 

A major contaminant of many of the earlier preparations of K3[Mn(CN)6],575 and often described as a different form of this, was a golden brown dimeric oxo-bridged species [(CN)5Mn—O— Mn(CN)5]6-. It is also formed when KMn04 is reacted with KCN, and an X-ray analysis of the crystalline compound K7[Mn2O(CN)I0l,CN showed a linear Mn—O—Mn bridge, with Mn— O = 1.72 A and Mn—CN = 1.97-2.06 A. It is almost diamagnetic because of antiferromagnetism. 

Dimeric oxo-bridged V111 complexes with bipy or phen as supportive ligands (186) were found to cleave DNA without the addition of exogenous oxidants or reductants.740 The dinuclear V111 complex formed from 2-(dimethylamino)ethanethiol has a linear V—O—V 41 unit (187).793 This compound was the first example of a class of dinuclear complexes containing NS donors.793... 

Tridentate red dimeric / -oxo-bridged complexes, [ MoOjlLXDjfAz-O)], prepared in high yield and purity, are used as catalysts in the epoxidation of several alkenes with TBHP as the oxidant. The epoxides are obtained in moderate to good yields and high... 

Cp derivatives (Scheme 185) to form -oxo-bridged dimeric complexes. The stabilities of the Af,0-bound carbamate intermediates depend mainly on the cyclopentadienyl ring substituents, but there is also a small dependence on the nature of the length of the (dimethylamino)alkyl chain for the C5H4Me systems. ... 

It appears that Cluster C catalyzes the chemistry of CO oxidation and transfers electrons to Cluster B, which donates electrons to external acceptors such as ferredoxin. Since a crystal structure of this protein does not exist, the proposed structure of Cluster C is based on spectroscopic measurements. In some cases, the EPR spectrum of a metal center is diagnostic of the type of center. However, the EPR spectra of Cluster C are unusual. The paramagnetic states of Cluster C (Credi and Cred2) have g-values that are atypical of standard [4Fe-4S] clusters (Table III) and are similar to those in a variety of structurally unrelated systems including a t-oxo bridged ion dimer), a [Fe4S4] ... 

A strong decrease in relaxivity (from 12.8mM-1s-1 to 2mM-1s-1) between pH 6 and 11 has been reported for a positively charged macrocyclic Gdm complex (Scheme 10), which was explained by the successive deprotonation of the coordinated water molecules.167 Luminescence lifetime measurements of a Yb111 analogue proved that the complex possesses three bound waters at pH 5.5. Above pH 11, a di-oxo-bridged dimer is formed that has no more bound water or OH groups. 

Figure 1.35 The structure of the oxo-bridged dimer Ru20(0Ac)4(PPh3)2.

The EXAFS of Mn in photosystem II looks very much like that of a mixed-valence di-/j-oxo-bridged Mn dimer model compound (33,35). In particular, the Mn-Mn distance of 2.7 A in photosystem II is characteristic of a di-/j-oxo-bridged structure. Recent EXAFS data have indicated that a second Mn-Mn distance of 3.3 A may also be present (37-38). Although a clear picture of all of the Mn-Mn distances is not yet available, the EXAFS results are consistent with a structure in which two di-/j-oxo bridged Mn dimers are present in close proximity. 

The Mn-Mn distance of 2.7 A determined by EXAFS is diagnostic of a di-/i-oxo-bridged Mn dimer in photosystem II (33,35). However, one could imagine a number of arrangements of the four Mn ions in photosystem II which contain a di-/i-oxo-bridged structure. Moreover, the observation of two different Mn-Mn distances by EXAFS, could only be accounted for by one of the following three arrangements of Mn a Mn trimer plus one isolated (more than 3.3 A away) Mn mononuclear center, two isolated inequivalent Mn dimers, or a Mn tetramer. 

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