Dinuclear hydroxo-bridged complexes

Bimetallic zinc complexes formed with hexaazamacrocycles were studied in the hydrolysis of activated carboxyesters. Potentiometric titration demonstrated the dominant presence of a dinuclear hydroxo bridged species at pH >7. /)-Nitrophenyl acetate is hydrolyzed with no loss of catalytic activity for at least 2.7 catalytic cycles 4... 

Fig. 11. Intermolecular hydrogen-bonded pairs of aqua hydroxo complexes have been found in several crystal structures and are probably always present in those salts of the type cis-[ML4(H20)(0H)lX2, which yield dinuclear dihydroxo-bridged complexes upon heating in the solid state.

The hydroxide ion is a well-established bridging ligand, and hydroxo-bridged complexes of iridium(III) have been reviewed. Among the dinuclear iridium(I) complexes that have structures supported by 0H ligands is [Cp Ir(/u.-0H)3lrCp ]+, and closely related to this is (cod)Ir(/u.-OPh)2lr(cod). The presence of the three OH... 

DMSO to yield the bicarbonato species 39. The same product is formed on reaction of the dinuclear C02-bridged complex 38 with carbon dioxide. Complexes 37 and 39 react in DMSO to yield 40. The latter reacts with [Re(CO)3(4,4 -Me2bpy)(OTf)] to yield 41. [Re(CO)3(bpy)(OTf)] interacts with aqueous potassium hydroxide to give [Re(CO)3(bpy)(OH)]. Complex [Re(CO)3(4,4,-Me2bpy)(CO)4](OTf) reacts with the latter hydroxo-complex to yield [(4,4,-Me2bpy)(0C)3Re(p-C02)Re(C0)3(bpy)]. 

The di- i,-hydroxo bridged complexes, the preparations of which are described in Sections D and H, have several chemical and physical properties in common. The dithionates are only sparingly soluble in water, whereas the bromides are quite soluble. The dinuclear structures of the cations, as well as the A,A configuration of the 1,2-ethanediamine compounds have been established by the close similarity of the Guinier X-ray powder diffraction patterns of [(NH3)4Rh(0H)2Rh(NH3)JBr4-4H20 and [(en)2Rh(OH)2Rh(en)2] (8205)2 to those of the well-characterized cobalt(III) and chromium(III) analogs. 

The dimeric itaconic acid chloro-hydroxo ester tin complex, [(MeOOCCH2-(COOMe)CH2Sn(/t-OH)Cl]2-2dioxane also contains a dinuclear hydroxo-bridged nucleus [404], The dioxane molecules are hydrogen-bonded to the OH bridges. 

A structurally characterized example of a dinuclear zinc complex with a bridging phosphate monoester was provided by Kitajima and co-workers using the tris(pyrazolyl)borate ligand system. The P—O bond in a tris- or bis-phosphate ester is cleaved by a hydroxo zinc complex resulting in the monoester compound.443... 

The bridging hydroxo ligand is a weak acid as well as an extremely weak base. The acid properties are by far the best investigated, and acid dissociation constants have been reported for singly and doubly bridged dinuclear complexes containing one or two hydroxo bridges (cf. Tables XV and XVI). 

Finally, it should be noted that this chemistry may have biological relevance. Several metalloenzymes are believed to contain more than one metal ion bound at the active site. One relevant example is the glucose tolerance factor (GTF) which is important for the metabolic degradation of glucose (398-401). GTF is a low-molecular-weight protein which contains chromium(III). Its structure is not known, but it has been suggested that the active site contains a dinuclear chromium(III) complex (401). The fact that hydroxo-bridged dinuclear chromium(III) complexes exhibit reactions which are often very fast compared with those observed for the parent mononuclear species seems to support such a proposal. 

A different type of bridging occurs in hydrolysis complexes of tho-rium(IV) (219) and uranium(IV) (130). Here a distinct peak at 3.94(2) A in the hydrolyzed solutions can be ascribed to the metal-metal distances in the hydrolysis complexes. Discrete dinuclear complexes with a very similar metal-metal distance, 3.988(2) A, in which the metal atoms are joined by double hydroxo bridges have been found in crystals ofTh2(OH2)(N03)6(H20)8 (229). The same type of bridging, therefore, must occur in solution. When hydrolysis is increased, however, the number of metal-metal distances per metal atom increases beyond a value of 0.5, valid for a dinuclear complex, and larger hydrolysis complexes are obviously formed. These structures are unknown but an extensive X-ray investigation of highly hydrolyzed thorium(IV) solutions has shown that there is probably no close relation between the structures of the hydrolysis complexes in solution and the structure of thorium dioxide, which is the ultimate product of the hydrolysis process (230). 

Wei, D., Zheng, Y, and Lin, J. (2002) Two hydroxo bridged dinuclear lanthanide-phen complexes [Ln2(phen)4(H20]4(0H]2](phen)2(N03)4 withLn = TmYb. Zeitschrift fuer Natutforschung, B Chemical Sciences, 57, 625-630. 

The phenol substituted [9]aneN3 derivative H2L22 (Fig. 24) also forms a dinuclear zinc complex ion with the formula [Zn2(HL22)20H], in which one phenol from each ligand is deprotonated. The two zinc atoms are linked by one /ii-hydroxo bridge supported by two hydrogen bonds between the phenol on one macrocycle and the phenolate of the second. The Zn—Zn distance is 3.55 A (106). 

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