Hydroxide-bridged complexes

Fig. 26. (a) A stereoview of the hydroxide-bridged complex and (b) the crystal structure (93) of the complex cation CudIXOHXO-BISTREN), showing the hydrogen bonding that appears to promote formation of the bridged hydroxy complex. 

Using a new tripodal ligand, Sorrell and co-workers synthesized compound (61).83 The dioxygen reactivity of the pseudotetrahedral complex of this ligand of composition [(L)Cu(MeCN)][BF4] (not structurally characterized) was examined. In MeOH at —78 °C, the thermal decomposition of the side-on peroxodicopper(II) adduct in the presence of water yields a bis(hydroxide)-bridged... 

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. 

Dinuclear complexes with hydroxide bridged dizinc centers were synthesized with a ligand based on the phthalazine framework.455 The dizinc complex of 1,4-bis(2,2 -dipyridylmethyl)phtha-lazine (49) is bridged by the phthalazine moiety, a water molecule, and a hydroxide ion. Both centers are pseudooctahedral with two pyridine donors and a water molecule occupying the remaining coordination sites. 

Synthesis of functional models of carbonic anhydrase has been attempted with the isolation of an initial mononuclear zinc hydroxide complex with the ligand hydrotris(3-t-butyl-5-methyl-pyrazolyl)borate. Vahrenkamp and co-workers demonstrate the functional as well as the structural analogy to the enzyme carbonic anhydrase. A reversible uptake of carbon dioxide was observed, although the unstable bicarbonate complex rapidly forms a dinuclear bridged complex. In addition, coordinated carbonate esters have been formed and hydrolyzed, and inhibition by small ions noted.462 A number of related complexes are discussed in the earlier Section 6.8.4. 

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]. 

Binuclear iron(II) complexes in which a hydroxide bridge is supported by the dinucleating bis-carboxylate ligand dibenzofuran-4,6-bis(diphenylacetate), (217), have proved useful models for hemerythrin. The nature of the binuclear iron center in hemerythrin itself, and in other metalloproteins, has been reviewed, the binding of O2, NO, N3, and NCS to the iron of hemerythrin discussed, " and the volume profile for hemerythrin reacting with O2 established. Bulky tolyl-substituted carboxylate ligands, both bridging and terminal, and... 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

The magnetic susceptibilities of solutions of hydrolysis products were consistent with the binuclear complex in such solutions having a double hydroxide bridge [V(OH)2V]4+ rather than an oxo bridge [V—O—v]4+.230... 

Hydroxy-bridged complexes [Pt2(,u-OH)2(PEt3)4]2+ can also be prepared. The structure consists of two square planar platinum(II) centers bridged by hydroxide ligands with an angle of 36.4° between the mean plane normals.1569 A useful method to prepare these complexes involves the use of phase-transfer catalysis with crown ethers to facilitate the reaction of KOH with platinum(II) chloro complexes.1570... 

As mentioned above, both cis-p-1,2 and p-rj2 172 remain vital candidates as the coordination mode of the peroxide in oxy-Hc and oxy-Tyr. The p-172 172 structure does seem, however, more likely because its particular structure gives a simpler and more straightforward account of all the unusual features of oxy-Hc. In order to distinguish these two possibilities conclusively, the synthesis and characterization of a cis-p-1,2 peroxo complex with a hydroxide bridge presents a serious challenge for inorganic chemists. On the other hand, a single-crystal X-ray analysis for oxy-Hc is currently underway and may provide a definitive answer (234). 

Often, with precipitation reactions the starting materials are limited to whatever salts are soluble in the solvent of choice. For water systems this is often limited to metal salts of halides, nitrates, and some sulfates and phosphates. Halides, in particular chlorides, have a pronounced effect on precipitation reactions. Chlorine is able to form bridged complexes much like the hydroxides or oxides of the desired compounds. In addition, acidic environments make possible the oxidation of chloride to chlorine gas, which can further complicate the synthesis. Sulfates and phosphates are typically easier to work with since they do not have the complicated redox behavior of the halides, but they typically have reduced solubilities. Nitrates, although they do not have the solubility concerns of sulfates and phosphates, do have redox complications, which typically result in oxidation of cations. So, the anion, which is expected to act solely as a spectator, in many cases is actually acting as a catalyst. 

Aluminum binds to nucleoside phosphates mainly through the basic terminal phosphate groups. Nucleosides mono-, di-, and triphosphates demonstrate similar phosphate basicity. Aqueous solutions of Al3+ and nucleoside phosphates have a tendency to form ternary complexes with hydroxide in a pH-dependent manner. In addition, there is a possibility of Al3+-bridged complexes being formed. Fig. 3 shows the species distribution for the A13+-ATP system. At physiological pH the merged hydroxo mono complexes predominate [9, 18]. 

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