Dimer hydroxide-bridged

M(0H)2SOi, H2O where M=Zr (8), Hf (12) also have been determined and reveal the presence of almost planar zigzag chains of metal atoms joined by double hydroxide bridges. The single exception to this trend toward formation of double hydroxy-bridged metal dimers or chains is the compound which is best described as CeOSOif,H20 (17). However, even in this structure the cerium ions form chains which are linked by bridging oxide ions. 

Dopamine /3-hydroxylase (D/3H) is a copper-containing glycoprotein that hydroxylates dopamine at the benzylic position to norepinephrine.84 During the attempted crystallization of the bis(hydroxide)-bridged dicopper(II) dimer, a side product was subsequently isolated (complex (63)), revealing intramolecular hydroxylation at a formally benzylic position of the tris(imidazo-lyl)phosphine ligand.85 The copper(II) center has an axially compressed TBP structure. 

Hydrolysis reactions catalyzed by zinc centers and the formation of hydroxide bridged dimeric species have been the subject of much recent study, and reproduction of the activity of hydrolytic zinc enzymes has been the goal of many research groups. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

The dihydroxide is new on this list di-/-butyltin dihydroxide exists in the crystal as a hydroxide-bridged dimer 35,361 and the diprotonated form has been isolated as the triflate, again as a dimer 36. 6... 

Monovalent copper salts were initially found to be better catalyst precursors than divalent copper salts. The latter needed the addition of base. In the presence of dioxygen the copper(I) salts are oxidised to copper(II) hydroxides forming hydroxide bridged dimers. The hydroxides can be replaced by phenolates, thus producing the key-intermediate [23] (Figure 15.16). From this equilibrium we understand that water concentration should be kept low in order to have a maximum amount of phenoxides coordinated to the copper dimers. 

The dimeric tetraacetato bridged Rh2(OCOCH3)4 has been obtained by the interaction of ammonium chlororhodate(III) or rhodium (III) hydroxide with acetic acid.1-3 Other (car-boxylato)rhodium(II) compounds were prepared directly in a similar way or from the acetate by exchange.2,3 Halo car-boxylates (RCOO, R = CC13, CF3, CH2C1, etc.) were prepared also by interaction of rhodium trichloride with the appropriate sodium salt in ethanol.4 The carboxylatcs are normally first isolated as a solvent adduct, e.g., [Rh(OCOR)2-C2H5OH]2 but are easily converted to the unsolvated complex. The acetate is readily prepared in a modification of this last procedure. A similar method is satisfactory for the preparation of other lower carboxylates as well as halo carboxylates. 

Figure 1 Model of a hydroxide bridged copper dimer...

Thus, in solution, trimethyltin hydroxide exists as a hydroxide-bridged dimer (12-1). A complete crystal structure has not been reported, but it is clear that it is a hydroxide-bridged polymer.24-25... 

The reaction rate of the reduction of these carbonyl compounds showed a sharp maximum at pH 3.2, which coincides with the pKA value of HCOOH in the studied concentration, and there was no reaction above pH 5. The lack of reactivity at higher pH can be attributed to the formation of the catalytically inactive hydroxide-bridged trimer, [(IrCp )2(p-OH)3]+ which, however, is in equilibrium with the starting catalyst precursor at the optimum pH of the reaction. The active form of the catalyst is most probably the dimeric [(IrCp )2(p-H)(p-HCOO)(p-OH)]+ which happens to form to the... 

Type 3 has two copper centres closely adjacent, forming a hydroxide-bridged dimer with each copper ion in an approximately square planar geometry. 

Figure 19, A hydroxide-bridged platinum dimer bound to two guanine molecules co-stacked as in a dinucleotide.

It has been shown that hydroxide bridged dimeric palladium complexes can form and impede catalysis, see Hartwig, J.F. Palladium-catalyzed amination of aryl halides mechanism and rational catalyst design. Synlett 1997, 4, 329-340. 

We are faced, therefore, with the puzzling problem of why it is not possible to isolate any aquo-species and why all hydroxo-species are polymerized through hydroxide bridges. It is accepted that aquo-species exist in solution Raman spectroscopy (Figure 8) shows that the coordinated nitrate groups are replaced immediately on solution in water (32), and n.m.r. studies have shown that these aquo-species are more slowly converted into the polymerised dimer and trimer hydroxo-bridged species(30). 

Thus we see that the interactions of the hydroxide bridged dimer with DNA bases are markedly different from those of cis-PtCJl2(NH2)2 with the same bases. 

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