Ion, bridged

K2O with Al2Mc6 in the presence of dibenzo-18-crown-6 (p. 96) yields the surprisingly stable anion [(pt, ) -in which one O of the superoxo ion bridges the 2 A1 atoms (angle Al-O-Al 128°) ... 

The structure of / -alumina is shown in Fig. 5. The aluminum and oxygen ions (green and red, respectively) form spinel blocks. The mobile sodium ions (blue) are located in layers between them. The spinel blocks are connected to each other by oxygen ion bridges within the conducting layer. 

The alternative mechanism (Fig. 18.16, mechanism B) is based on the fully reduced [(dipor)Co2] state as the redox-active form of the catalyst. The redox equilibrium between the mixed-valence and fully reduced forms is shifted toward the catalytically inactive mixed-valence state, and hence controls the amount of catalytically active species in the catalytic cycle and contributes to the — 60 mV/pH dependence. The fully reduced form is known to bind O2 (probably reversibly) in organic solvents [LeMest et al., 1997 Fukuzumi et al., 2004], and the resulting diamagnetic adducts are typically viewed as a pair of Co ions bridged by a peroxide, which are of course quite common in the O2 chemistry of nonporphyrin Co complexes. To obtain the —60 mV/pH dependence of the catalytic turnover rate, a protonation step is required either prior to the TDS or as the TDS. Mechanism B cannot be extended to monometallic cofacial porphyrins or heterometallic porphyrins with a redox-inert ion, but there is no reason to assume that the two classes of cofacial porphyrin catalysts, with rather different catalytic performance (Fig. 18.15), must follow the same mechanism. 

Intramolecular reactions usually dominate intermolecular addition for favorable ring sizes. Semiempirical (AMI) calculations found the intramolecular TS favorable to a comparable intermolecular reaction.68 (See Figure 4.1) The intramolecular TS, which is nearly 4 kcal/mol more stable, is quite productlike with a C—O bond distance of 1.6 A, and a bond order of 0.62. The bromonium ion bridging is unsymmetrical and fairly weak. The bond parameters for the intra- and intermolecular TSs are quite similar. 

The system (23)/SnCl2, an active intermediate in the catalytic hydroformylation of 1-hexene, has been investigated by 31P NMR spectroscopy and two species are observed at low temperature, in equilibrium with the starting Pt complex (23). One is complex (27), and the other is a species which does not show Sn-P coupling and which has been tentatively attributed to a complex having chloride ions bridging the Pt and Sn metal centers. Formation of the complex (27) does not occur when EtOH is added to the CD2C12 or acetone solutions.91... 

Some representative systems. A further type of binuclear species incorporates two metal ions bridged by one or two small groups within the macrocyclic ring. Such complexes differ from the previous type in that the bridges between the metal ions do not involve donor groups which are directly attached to the macrocycle - see (122) and (123). A variety of... 

This compound can be considered as a derivative of the hydrolysis product, [(H20)3Be(0H)Be(H20)3]3+, in which the bidentate 3-methyl-salicylate ligands each replace two water molecules, the other two water molecules being replaced by a carbonate ion bridging between two beryllium atoms. Similar carbonate bridges have been proposed previously (132). 

The reaction in Equation (102)115 produces a paramagnetic compound in which a Tl2+ ion bridges between two [Pt(C6F5)4]2 groups (Structure 122).115 The Pt-Tl-Pt angle is 179(1)°. The Pt-Tl distances are considerably shorter than those in the related Tl+ Pt/G6F5 complexes mentioned above. 

The related reaction shown in Equation (104)117 leads to a butterfly arrangement with two thallium ions bridging between two gold atoms, 127. Here, the Tl-Tl distance is 360.27 pm and is thought to contribute significantly to the physical properties of the complex. The compound shows solvent-dependent luminescent behavior in solution as well as in the solid state. 

Figure 18 Molecular structure of IBP collagen (PDB accession number 1Q7D) showing a direct intrahelical ion bridge and a water-mediated interaction between Glu and Arg of different chains. The figure was generated using the UCSF Chimera package.

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