Bridging fragments

Preparation of the bridging fragment 150 (Scheme 23) followed a similar pathway. In this case, stereoselective reduction of the cyclic enol ether 149 formed by the RCM of 148 was achieved using Et3SiH and TFA, leading after des-ilylation to the WVU ring system 150 [34b]. 

Sequence analysis is one of the most useful tools for determining disulfide connectivities by detecting the phenylthiohydantoins (PTH) of either cystine or cysteine derivatives. Sequencing can be carried out on the intact peptide, although most frequently disulfide-bridged fragments are subjected to this type of analysis. 

Di- and polynuclear complexes with the p2-SR ligand are very common. The following conformers [1, vol.2] are four samples of these bridge fragments ... 

The interaction (3.70) between 2-aminopyrimidine with Cu(C104)2 6H20 results in a binuclear complex 631, containing a two hydroxo-bridge fragment [82] ... 

Other examples of binuclear structures with oxygen, chalcogen, and mixed-element metal-bridge fragments are reported in Refs. 93, 127, and 129. 

A change of salt anions allows us to carry out controlled syntheses of definite types of coordination compounds. Thus, metal halides are widely used to prepare molecular (Sec. 3.1.1.1) and ti-complexes (Sec. 3.1.1.3). Metal acetates are mostly applied in the syntheses of metal chelates and, especially, inner-complex compounds (Sec. 3.1.1.2). Di- and polynuclear structures are formed under use of the anions mentioned above, which, in some cases, determine (see Sec. 3.1.1.4) a type and donor centers of bridge fragments. At the same time, the mentioned approach to choice of salts of metal complex-formers has many exceptions, although it is useful. 

It was recently shown [435] that the mixed-valent complex compounds with N-coordinated bridge fragment can be obtained on the basis of the nitride rhenium complex as a result of the transformation (3.202) ... 

Developing the study of binuclear copper complexes with NCS bridge fragments [48], the transformation (4.3) was carried out [49] ... 

A pecularity of the anion structure in the tritin complex 54c214 is that the central tin atom does not deviate from the equatorial plane defined by the carbon atoms. The Sn(central)—Fbr—Sn(terminal) fragment is more asymmetrical than in the related bridged fragment in 52b, X = F. 

The simplest dinuclear cyanide-bridged fragments are represented by the homo-leptic cyanides (TEA)5[Mo2° CN)n] (30a) and (TEA)g[Cr°(CN)5][Cr2 (CN)9] (30b) (Eig. 2). The former is obtained when Mo OTQa is reacted with (TEA)CN in DMF in approximately a 1 5.4 ratio. As suggested by Beauvais and Long, the limited quantity of cyanide favors the formation of the dinuclear species over the mononuclear species [Mo CN)6]. The conformation of the dinuclear anion [Mo2 °(CN)n] is nearly eclipsed. 

This compound was isolated as a polymeric gel. Macrobicyclic fragments in the polymer chain are linked by Si-O-Si bridging fragments. It was claimed [39] that this clathrochelate may be either a low-spin cobalt(II) compound or a mixture of cobalt(II) and cobalt(III) compounds. However, taking into account the polymeric nature of the complex and the difficulties encountered in its isolation and investigation, one may conclude that the data reported [39] are not sufficient for its unambiguous identification. 

Polymer 139 is another example of a mixed-ligand 1-D polymer. The difference with 137 and 138 is that bromide ions replace the diisocyanides.124 The X-ray structure reveals two Ag-Br distances (2.7431(13) and 2.9453(14) A), and the dimension indicates ionic interactions. The structure also exhibits a staircase geometry as previously and the nonbonding Ag -Ag separations are 3.605(2) and 3.916(2) A for the dmpm- and Br-bridged fragments, respectively. Raman spectra exhibit v(Ag -Ag) of 48 cm-1, giving a weak approximate force constant of 0.03 mdyn A-1. 

---