Halide template

The templation of metal-ligand assemblies by anions can occur in two ways a Lewis acid/base interaction between the anion and the metal or via hydrogen bonding between the anion and organic ligands. One system that displays both of these interactions is the halide templated assembly between 19, one of the tautomeric forms of amidinothiourea (Scheme 5), and NiX2 (X=C1, Br) [55]. In the... 

Wang, R., Selby, H.D., Liu, H., Carducci, M.D., Jin, T., Zheng, Z. etal. (2002) Halide-templated assembly of polynuclear lanthanide-hydroxo complexes. Inorganic Chemistry, 41, 278-286. 

For halide ligands, a coordination number of four (/u4-X) is rare. Self-assembly of ds metal centers and halides around a pyramidal halide gives tetrapalladium complexes. In this unprecedented case the metallamacrocycle owes its formation to the halide acting as a template.347 The complexes (PPN)[Pd4(Fmes)4] (Fmes = 2,4,6-tris(trisfluoromethyl)phenyl) display fluxional behavior in solution in noncoordinating solvents.347... 

One of the most successful approaches to the synthesis of metalla-supramolecular systems has been by templated self-assembly processes. Most of the templating agents used by synthetic chemists are either cationic or neutral species. Anions, on the other hand, have only fairly recently been successfully utilized as templates. The new metalla-macrocycles [Pd2Ni2(atu)4(PPh3)4X]3+ (atu = deprotonated amidinothiourea, X = C1, Br, I) have been prepared using anion templated syntheses. Only in the presence of each of these three halides are the metalla-macrocycles... 

Hawthorne and co-workers have also produced a series of macrocyclic Lewis acid hosts called mercuracarborands (156, 157, and 158) (Fig. 84) with structures incorporating electron-withdrawing icosahedral carboranes and electrophilic mercury centers. They were synthesized by a kinetic halide ion template effect that afforded tetrameric cycles or cyclic trimers in the presence or absence of halide ion templates, respectively.163 These complexes, which can bind a variety of electron-rich guests, are ideal for catalytic and ion-sensing applications, as well as for the assembly of supramolecular architectures. 

It is interesting to note that the size of the cluster cage, as reflected by their Ag-Ag and Ag-X distances, follows a trend with 82a < 82b < 82c, which is in line with the increasing size of the halide ion and demonstrates clearly the template effect of the halide ion in the cluster formation process. Analogous reactions using AgOTs or AgNC>3 as... 

Recently, with a view to overcome the difficulty on the preparation of aryl or alkenyl halides or sulfonates, thioamides and their S-alkyl derivatives have been proposed as a new class of electrophilic partners. This palladium cross-coupling methodology was developed by Liebeskind and mostly applied to heteroaromatic templates.118 121... 

Mechanistic studies on this system have demonstrated that the reaction does not follow a simple stepwise condensation pathway as first assumed. Instead, the authors have proposed a divergent mechanism by which the production of 2 and 3 are linked by a common intermediate (see Scheme 3). In such a mechanism, hydrogen-bonding interactions between the donor molecular moieties and the templating halide (acceptor) play an important role. 

Scheme 7 The syntheses of the metalla-cages and metaUa-macrocydes shown in this scheme is highly dependant on the nature of the anions present in solution. Hydrogen bonding interactions with the templating halides play an essential part in the templated process...

Kinetic template effects have been involved in the formation of crown ethers, with respect to the cyclization step involving a nucleophilic displacement of halide or tosylate by alkoxide ions. It has been proposed that cyclization of the linear bifunctional precursor is enhanced by a cyclic conformation in which the alkoxide cation brings the two ends of the molecule into close proximity... 

In this context it is interesting to note that benzonitrile, Ph—C=N, trimerizes to a triazine on a Raney nickel surface. It was assumed that Jt-bonded nitriles were involved in the reaction mechanism.10 This reaction resembles the well-known template synthesis of phthalocyanine complexes from phthalodinitrile. Formation of linear polymers [—C(R)—N—] occurs on heating aryl or alkyl cyanides with metal halides.11... 

The use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

One of the simplest and widely used methods of forming C-S bonds involves nucleophilic attack of a thiolate on a suitable C-centred electrophile such as an alkyl halide (Fig. 5-74). Co-ordinated thiolate ligands behave as nucleophiles in exactly the same manner, and the method has been extensively used for the preparation of thioethers and their metal complexes. The method has been particularly commonly utilised in the formation of macrocyclic ligands in templated syntheses (see Chapter 6). 

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