Hydroxide complexes structure

The magnesium hydroxide complex [TpAr2]MgOH (Ar = p-C6H4Bul) has been obtained by the reaction of the methyl derivative [TpAr2]MgMe with H20 [Eq. (29)], although it has not been confirmed that the product is monomeric, and a dimeric structure is possible (148). 

The value of the tris(pyrazolyl)hydroborato complexes [TpRR ]ZnOH is that they are rare examples of monomeric four-coordinate zinc complexes with a terminal hydroxide funtionality. Indeed, [TpBut]ZnOH is the first structurally characterized monomeric terminal hydroxide complex of zinc (149). As such, the monomeric zinc hydroxide complexes [TpRR ]ZnOH may be expected to play valuable roles as both structural and functional models for the active site of carbonic anhydrase. Although a limitation of the [TpRR ]ZnOH system resides with their poor solubility in water, studies on these complexes in organic solvents... 

The zinc acetate complex of tris(3-/-butyl-5-methylpyrazol-l-yl)borate was prepared as a structural model for carbonic anhydrase and comparison with the enzyme active site structures confirmed that the complexes are excellent structural models.239 A mononuclear zinc hydroxide complex can also be formed with the tris(pyrazolyl) borate ligand system as a structural model for carbonic anhydrase.240... 

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. 

PbS and PbSe are almost always found in the rocksalt (RS) crystal form. All structural investigations on CD films have shown this form, with one exception PbSe deposited from hydroxide complex at high hydroxide concentrations and at rela-... 

Decarboxylation of carbonate complexes is usually effected by acid hydrolysis with the formation of a C02 free oxide or hydroxide complex.128 All such reactions involve a protonated (bicarbonate) intermediate but there are some useful deferences which, in many instances, may be reconciled with the three main structural types of carbonate complexes. Both unidentate and chelate carbonates readily yield C02 on acidification, while there is a greater resistance to C02 loss when the carbonate is a bridging ligand. Unidentate carbonate complexes decarboxylate with the initial formation of a bicarbonate intermediate and subsequent loss of C02 without rupture of the M—O bond, viz. structure (3). By contrast, in chelate carbonate complexes, cleavage of the M—O bond occurs (with ring opening) with the formation of a bicarbonate aqua ion before the loss of C02, viz. equation (5).29... 

The complex hydrate structure, 1.67 choline hydroxide-tetra-n-propyl-ammonium fluoride 30-33H2O (space group = R-3, a = 12.533, c = 90.525 A) was discovered by Udachin and Ripmeester (1999b). It should be noted that the tetra-n-propylammonium salt will not form a hydrate on its own (Dyadin et al., 1988), even though other tetra-alkylammonium salts will form a variety of hydrate structures. Similar to structures II, H, and IV, this complex structure consists of stacked sequences of layers, CABBCAABCCABBCAAB. That is,... 

In light of the accepted mechanism for cytochrome P-450 (97-100), a superoxo-Cu(II) intermediate is further reduced, leading to dioxygen activation. Accordingly, a monomeric peroxo or hydroperoxo copper(II) complex serves as a synthetic model for these intermediates of copper-containing monooxygenases. However, no well-characterized complexes of these types are available to date. Formation of a monomeric hydroperoxo or acylperoxo complex was reported to occur when a trans-/u-l,2-peroxo complex, [(Cu(TPA))2(02)]z+, was treated with H+ or RC(O)+, but no details of the structures and properties of the complexes were provided (101). A related complex, a monomeric acylperoxo cop-per(II) complex, was synthesized (102). Low-temperature reaction of a dimeric copper(II) hydroxide complex, [Cu(HB(3,5-iPr2pz)3)]2(OH)2, with 2 equivalents of m-CPBA (3-chloroperoxybenzoic acid) yielded a monomeric acylperoxo complex whose structure was characterized by... 

Unfortunately, this reaction is a gross oversimplification of the series of reaction steps that occur during the hydrolysis reaction. Hydrolysis has been shown to proceed via the formation of an alkylaluminum-water complex, which subsequently eliminates methane to form a dimethylaluminum hydroxide complex. This rapidly associates to give dimers or larger oligomers in solution. In the case of f-butylalumoxane, some of the intermediate species have been isolated and characterized structurally (73-76). 

Column cleaning is performed with high concentrations of phosphate buffer, with nonionic detergents, urea, and sodium hydroxide to restore the complex structure of calcium phosphate. 

There have been a few reports of first generation coordination complex structural models for the phosphatase enzyme active sites (81,82), whereas there are some examples of ester hydrolysis reactions involving dinuclear metal complexes (83-85). Kim and Wycoff (74) as well as Beese and Steitz (80) have both published somewhat detailed discussions of two-metal ion mechanisms, in connection with enzymes involved in phosphate ester hydrolysis. Compared to fairly simple chemical model systems, the protein active site mechanistic situation is rather more complex, because side-chain residues near the active site are undoubtedly involved in the catalysis, i.e, via acid-base or hydrogenbonding interactions that either facilitate substrate binding, hydroxide nucleophilic attack, or stabilization of transition state(s). Nevertheless, a simple and very likely role of the Lewis-acidic metal ion center is to... 

The pentafluorophenyl derivative (C6F5)2Cd is hydrolyzed by water to give the tetrameric cadmium hydroxide complex [(C6F5)Cd(/u-OH)]4 (equation 13). The molecular structure (10) of [(C6F5)Cd(/u-OH)]4 is based on a cube with the cadmium and oxygen atoms occupying the vertices. The hydroxy derivative is characterized by observation of a v(OH) band at 3640 cm. ... 

Figure 4.19 The structures of four complexes [Sml(p.-I)(17)3]2, Sm(17)8l3, [(17)4Sm]( x-OH) 3(p.3-OH)2 l4, and [(17)5Sm(p.-OH)]2l4 [34], (Reproduced with permission from WJ. Evans, GW. Rabe and J.W. Ziller, Utility of N-methylimidazole in isolating crystalline lanthanide iodide and hydroxide complexes crystallographic characterization of octasolvated [Sm(N-MeIm)8]I3 and polymetallic [Sml( x-I)(N-Melm)3]2, [(N-MeIm)5Sm( X-OH)]2l4, and [(N-MeIm)4Sm(tJi-OH)]3(tJi3-OH)2 l4, Inorganic Chemistry, 33, 3072, 1994. 1994 American Chemical Society.)...

The most successful synthetic approach to structurally well defined lanthanide hydroxide complexes is the ligand-controlled hydrolysis approach [69-71]. The essence of this methodology is schematically shown in Figure 6.25. It makes use of the high propensity of lanthanide ions toward hydrolysis, but controlled and limited by certain supporting ligands. The scheme starts with a lanthanide complex whose coordination sphere constitutes both organic and aqua... 

Figure 6.31 The structures of (a) dodeca-, (b) pentadeca-, and (c) octadeca-nuclear clusters templated by two IX4-H ions, a X5-X (X = C1, Br) ion, and Xe p r r r r r COj , respectively. Formal assembly of a 60-metal hydroxide complex featuring 26 vertex-sharing [Ln4(ti3-OH)4] cubane units. These cubane building blocks form six dodecanuclear squares and eight octadecanuclear hexagons [102]. (Redrawn from X. Kong et al, A chiral 60-metal sodaUte cage featuring 26 vertex-sharing [Er4(p.3-OH)4] cubanes, Journal of the American Chemical Society, 131, 6918-6919, 2009.)...

Capping the square-like motif of [Ln4(p,4-OH)] with a fifth metal atom results in a square-pyramidal unit of Ln5((jt4-OH), a pentanuclear building block frequently observed in lanthanide hydroxide complexes with diketonate ligands. One such example is [Dy5( X4-OH)( X3-OH)4( x,Ti -Ph2acac)4(Ti -Ph2acac)6] (Ph = phenyl) whose structure is shown in Figure 6.35 [25, 80]. 

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