Bridging alkoxides

In order to provide a dinucleating scaffold for proper orientation of two proximite Ni11 ions, compartmental ligands based on bridging alkoxide, phenolate, pyrazolate, or phtalazine units have been employed.422,2066-2082 Selected examples are discussed below. 

Recently, we discovered that phosphate P-O bond cleavage is promoted by an alkox-ide bridged dizinc(II) complex with an hydroxyloctaazacryptand. In this case weakly coordinating secondary nitrogen attacks the substrate phosphomonoester, and the bridging alkoxide () anion between two zinc(II) ions has no nucleophilic activity, which is somewhat similar to our dinuclear complex 13 T. Koike, M. Inoue, E. Kimura, and M. Shiro, J. Am. Chem. Soc., 118 3091-3099 (1996). 

The ligand contributes a bridging alkoxide and the metal coordination sphere is completed with one bridging and two terminal chloride anions. Incorporation of a carboxylate moiety in the system resulted in new compounds,... 

The pyridyl complex Cp 2Y(2-pyridyl) reacts both with excess (1 bar) and 1 equivalent of CO to yield a n-r)2 t/2-dipyridylketone fragment by unexpected CO insertion (Eq. 43) [283]. The Y-O bond distances are in the range of bridging alkoxide ligands (Table 18). 

A second method that has been described as a means to deal with the limited accessibility issue involves a copolymerization route. In this approach, the porous matrix is copolymerized with the template. Burleigh and coworkers, for example, described the preparation of imprinted polysilsesquioxanes for the recognition of metal ions.79 Polysilsesquioxanes are hybrid porous materials synthesized from bridged alkoxide precursors such as shown in Figure 20.2.80 In this example, the bridged silsesquioxane precursors were copolymerized with the metal ion complex in the presence of surfactant. Once the surfactant and metal ion were removed, a porous network was formed that showed a high affinity for the metal ion.79... 

Without doubt, ligands based on bridging alkoxide or phenolate groups are most prominent in biomimetic or bioinspired chemistry with bimetallic sites. Even before the X-ray crystal structure of urease became known, first model compounds using this ligand type were developed. Complex 3, reported in 1989 by Buchanan,... 

Figure 52 shows the structure of [Mn4" (HL33)(L33)(NCS)4], which can also be viewed as either a dimer-of-dimers or a severely distorted butterfly. The links between the two macrocycles are two triply bridging alkoxide donors, one from each macrocycle. The wings of the butterfly have closed to bring the two outer manganese ions quite close to one another an extension of this movement would result in formation of a cubane. 

The chlorine atoms on each titanium adopt a cis orientation, with 0(1) being trans to the coordinated alcohol and 0(2) being trans to the bridging alkoxide. The central structural feature of the structure shown in Fig. 52 is the planar Ti202 core with unsymmetrical Ti(l)—0(1) and Ti(l) —0(1) bond distances of 1.956 and 2.087 A, respectively. As expected, the coordinated alcohol Ti(l)—0(2) bond length (2.102 A) is longer than the titanium—alkoxide Ti(l)—0(3) distance of 1.746 A. 

CH30)i6] [105], and Dsj again in the spectacular 18-membered ring recently reported by Lippard s group [106]. All these species contain coordination centres linked by bridging alkoxides or carboxylates. 

The bridged alkoxide complexes (RO)2Al—O—Cr—O—Al(OR)2 have been prepared. In solution the complexes are associated to an extent dependent on the nature of R. ... 

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