Nitrogen-containing ligands pyrroles

Although Li[B(NHMe)4] exists,most tetraamidoborates contain ligands derived from five-membered heterocyclics such as pyrrole, indole, imidazole, pyrazole and indazole. The thermal and hydrolytic stabilities of these borates are fairly hi. When the heterocycle contains two nitrogens the borates can exist in acidic form and the tetraimidazol-l-ylborate can be used for gravimetric determination of the proton. Another type of borate containing a BN4 unit is represented by bicyclic derivatives formed with iV,iV",iV"-trimethylbiuret. ... 

Examples of palladium- and rhodium-catalyzed hydroaminations of alkynes are shown in Equations 16.90-16.92 and Table 16.9. The reaction in Equation 16.90 is one of many examples of intramolecular hydroaminations to form indoles that are catalyzed by palladium complexes. The reaction in Equation 16.91 shows earlier versions of this transformation to form pyrroles by the intramolecular hydroamination of amino-substituted propargyl alcohols. More recently, intramolecular hydroaminations of alkynes catalyzed by complexes of rhodium and iridium containing nitrogen donor ligands have been reported, and intermolecular hydroaminations of terminal alkynes at room temperature catalyzed by the combination of a cationic rhodium precursor and tricyclohexylphosphine are known. The latter reaction forms the Markovnikov addition product, as shown in Equation 16.92 and Table 16.9. These reactions catalyzed by rhodium and iridium complexes are presumed to occur by nucleophilic attack on a coordinated alkyne. 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

Fig. 3. Schematic illustration of the iron linked groups in hemoproteins. The porphyrin ring containing the four pyrrole nitrogen atoms is shown as an elipsoid. The iron atom is 0.5 A out of the prophyrin plane in the direction of X (54). The axial ligands confirmed by crystallography (55, 56) are ...

The nitrogen atom contained in a neutral pyrrole molecule (184, R = Alk, Ar) is practically deprived of its donor properties [8,302]. In this respect, for nonsubstituted pyrrole (184, R — II). the exchange reactions of the proton of the NH group with a metal (187) are typical, as well as its participation in the complex formation as an anion r 5(ji)-ligand system (see above, Sec. 2.2.4.1) ... 

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